2005 FCs papers |
Fluorescence
correlation spectroscopy, a tool to investigate supramolecular dynamics: inclusion
complexes of pyronines with cyclodextrin. Al-Soufi, W., Reija, B., Novo, M., Felekyan, S.,
Kuhnemuth, R., and Seidel, C.A., J Am Chem Soc, 2005. 127(24):
pp. 8775-84
The control of
supramolecular systems requires a thorough understanding of their dynamics on a
molecular level. We present fluorescence correlation spectroscopy (FCS) as a
powerful spectroscopic tool to study supramolecular dynamics with single
molecule sensitivity. The formation of a supramolecular complex between
beta-cyclodextrin (beta-CD) as host and pyronines Y (PY) and B (PB) as guests
is studied by FCS. Global target analysis of full correlation curves with a
newly derived theoretical model yields in a single experiment the fluorescence
lifetimes and the diffusion coefficients of free and complexed guests and the
rate constants describing the complexation dynamics. These data give insight
into the recently published surprising fact that the association equilibrium
constant of beta-CD with PY is much lower than that with the much bulkier guest
PB. FCS shows that the stability of the complexes is dictated by the
dissociation and not by the association process. The association rate constants
are very similar for both guests and among the highest reported for this type
of systems, although much lower than the diffusion-controlled collision rate
constant. A two-step model including the formation of an encounter complex
allows one to identify the unimolecular inclusion reaction as the rate-limiting
step. Simulations indicate that this step may be controlled by geometrical and
orientational requirements. These depend on critical molecular dimensions which
are only weakly affected by the different alkyl substituents of PY and PB.
Diffusion coefficients of PY and PB, of their complexes, and of rhodamine 110
are given and compared to those of similar molecules.
Evidence for major
structural changes in subunit C of the vacuolar ATPase due to nucleotide
binding.
Armbruster, A., Hohn, C., Hermesdorf, A., Schumacher, K., Borsch, M., and
Gruber, G., FEBS Lett, 2005. 579(9): pp. 1961-7
The ability of subunit C
of eukaryotic V-ATPases to bind ADP and ATP is demonstrated by photoaffinity
labeling and fluorescence correlation spectroscopy (FCS). Quantitation of the
photoaffinity and the FCS data indicate that the ATP-analogues bind more weakly
to subunit C than the ADP-analogues. Site-directed mutagenesis and N-terminal
sequencing of subunit C from Arabidopsis (VHA-C) and yeast (Vma5p) have been
used to map the C-terminal region of subunit C as the nucleotide-binding site.
Tryptophan fluorescence quenching and decreased susceptibility to tryptic
digestion of subunit C after binding of different nucleotides provides evidence
for structural changes in this subunit caused by nucleotide-binding.
Quantitative
measurement of the resolution and sensitivity of confocal microscopes using
line-scanning fluorescence correlation spectroscopy. Balaji, J. and Maiti, S., Microsc
Res Tech, 2005. 66(4): pp. 198-202
Spatial resolution and
the sensitivity to detect a fluorophore are the two most important optical
parameters that characterize a confocal microscope. However, these are rather
difficult to estimate quantitatively. We show that fluorescence correlation
spectroscopy (FCS) provides an easy and reliable measure of these quantities.
We modify existing schemes for performing FCS on a commercial confocal
microscope to carry out these measurements, and provide an analysis routine
that can yield the relevant quantities. Our method does not require any
modification of the confocal microscope, yet it yields a robust measure of the
resolution and sensitivity of the instrument.
Anomalous diffusion
of proteins due to molecular crowding. Banks, D.S. and Fradin, C., Biophys J, 2005. 89(5):
pp. 2960-71
We have studied the
diffusion of tracer proteins in highly concentrated random-coil polymer and
globular protein solutions imitating the crowded conditions encountered in
cellular environments. Using fluorescence correlation spectroscopy, we measured
the anomalous diffusion exponent alpha characterizing the dependence of the
mean-square displacement of the tracer proteins on time, r(2)(t) approximately
t(alpha). We observed that the diffusion of proteins in dextran solutions with
concentrations up to 400 g/l is subdiffusive (alpha < 1) even at low
obstacle concentration. The anomalous diffusion exponent alpha decreases
continuously with increasing obstacle concentration and molecular weight, but
does not depend on buffer ionic strength, and neither does it depend strongly
on solution temperature. At very high random-coil polymer concentrations, alpha
reaches a limit value of alpha(l) approximately 3/4, which we take to be the
signature of a coupling between the motions of the tracer proteins and the
segments of the dextran chains. A similar, although less pronounced,
subdiffusive behavior is observed for the diffusion of streptavidin in
concentrated globular protein solutions. These observations indicate that
protein diffusion in the cell cytoplasm and nucleus should be anomalous as well,
with consequences for measurements of solute diffusion coefficients in cells
and for the modeling of cellular processes relying on diffusion.
Fluorescence
correlation spectroscopy for flow rate imaging and monitoring--optimization,
limitations and artifacts. Brister, P.C., Kuricheti, K.K., Buschmann, V., and Weston, K.D., Lab
Chip, 2005. 5(7): pp. 785-91
We recently demonstrated
a new method for mapping fluid velocities in 3 dimensions and with
exceptionally high spatial resolution for the characterization of flow in
microfluidic devices. In the method, a colloidal suspension containing
fluorescent tracer particles, dye doped polymer spheres, is pumped through a
microchannel and confocal microscopy combined with fluorescence correlation
spectroscopy is used to measure fluid velocities. In this report, we further
characterize the technique and report on optimizations that allow a 5-fold
increase in speed of single point velocity measurements. This increase in
measurement speed will yield a 25 fold reduction in the time needed to collect
a complete velocity image. The precision of measured velocities was
characterized as a function of tracer particle concentration, measurement time,
and fluid velocity. In addition, we confirm the linearity of the measurement
method (velocity vs. applied pressure) over a range of velocities spanning four
orders of magnitude. Furthermore, we demonstrate that an artifact in velocity
measurements using fluorescence correlation spectroscopy (FCS) that was
interpreted by others as being caused by optical trapping forces is actually an
artifact caused by detector saturation and can be avoided by careful choice of
experimental conditions.
Four-color
fluorescence correlation spectroscopy realized in a grating-based detection
platform.
Burkhardt, M., Heinze, K.G., and Schwille, P., Opt Lett, 2005. 30(17):
pp. 2266-8
We have developed a
filterless multicolor detection unit for fluorescence correlation spectroscopy
(FCS). This grating-based setup is continuously tunable for multicolor separation
and is thus a powerful alternative to the classical cascade of dichroic mirrors
and filters. Our tailored platform allows for accommodation of up to 15
detection channels covering the entire visible spectral range. As a proof of
principle, we successfully demonstrate simultaneous FCS of four distinct
fluorescent quantum dot species being mixed in aqueous solution. Grating-based
detection allows for spectral high-resolution FCS in a stable and compact setup
and is a feasible tool for quantitative investigation of complexbiomolecular
dynamics on a single molecule level.
Local mobility in
lipid domains of supported bilayers characterized by atomic force microscopy
and fluorescence correlation spectroscopy. Burns, A.R., Frankel, D.J., and Buranda, T., Biophys
J, 2005. 89(2): pp. 1081-93
Fluorescence correlation
spectroscopy (FCS) is used to examine mobility of labeled probes at specific
sites in supported bilayers consisting of
1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) lipid domains in 1,2-dioleoyl-sn-glycero-3-phosphocholine
(DOPC). Those sites are mapped beforehand with simultaneous atomic force
microscopy and submicron confocal fluorescence imaging, allowing
characterization of probe partitioning between gel DPPC and disordered liquid
DOPC domains with corresponding topography of domain structure. We thus examine
the relative partitioning and mobility in gel and disordered liquid phases for
headgroup- and tailgroup-labeled GM1 ganglioside probes and for headgroup- and
tailgroup-labeled phospholipid probes. For the GM1 probes, large differences in
mobility between fluid and gel domains are observed; whereas unexpected
mobility is observed in submicron gel domains for the phospholipid probes. We
attribute the latter to domain heterogeneities that could be induced by the
probe. Furthermore, fits to the FCS data for the phospholipid probes in the
DOPC fluid phase require two components (fast and slow). Although proximity to
the glass substrate may be a factor, local distortion of the probe by the fluorophore
could also be important. Overall, we observe nonideal aspects of phospholipid
probe mobility and partitioning that may not be restricted to supported
bilayers.
Reactant
concentrations from fluorescence correlation spectroscopy with tailored fluorescent
probes. An example of local calibration-free pH measurement. Charier, S., Meglio, A., Alcor,
D., Cogne-Laage, E., Allemand, J.F., Jullien, L., and Lemarchand, A., J Am
Chem Soc, 2005. 127(44): pp. 15491-505
The present account is
concerned with the measurement of local reactant concentrations by observing
specific fluorescent probes in fluorescence correlation spectroscopy (FCS). The
Theoretical Analysis section revisits the photophysical, thermodynamic, and
kinetic information that is contained in the corresponding FCS correlation
curves. In particular, we examine the conditions under which FCS is revealed as
a superior tool to measure concentrations of reactive species. Careful
molecular engineering of the specific fluorescent probes that simultaneously
integrates photophysical, thermodynamic, and kinetic constraints will be
required to benefit most from FCS. We illustrate the FCS titration approach
with a series of fluorescent probes that we tailored to measure pH at around
4-6 by FCS after two-photon excitation. We show that an optimal design allows
one to access pH without any preliminary calibrations such as the determination
of the protonation constant or the photophysical properties of the fluorescent
probe.
The kinetics of
conformational fluctuations in an unfolded protein measured by fluorescence
methods.
Chattopadhyay, K., Elson, E.L., and Frieden, C., Proc Natl Acad Sci U S A,
2005. 102(7): pp. 2385-9
The simplest dynamic
model for an unfolded protein is a statistical coil that continually undergoes
substantial conformational fluctuations. A growing number of studies indicate
that the unfolded protein is not a simple random coil but rather forms
transient structures. We have directly measured the rate of conformational
fluctuations of unfolded intestinal fatty acid binding protein (131 aa, 15 kDa)
by using fluorescence self-quenching in combination with fluorescence
correlation spectroscopy. The conformational fluctuations in this state have an
apparent relaxation time, tauR, of 1.6 microsec in 3 M guanidine-HCl at pH 7
and 20 degrees C. The value of tauR increases with increasing solution
viscosity, suggesting a diffusive process. In the molten globule state at pH 2,
tauR is 2.5 microsec, increasing further with the formation of salt-induced
secondary structure. These measurements, which should be widely applicable to
other systems, can provide important information about the still incompletely
understood conformational properties of unfolded proteins and the mechanism of
protein folding.
Measuring unfolding
of proteins in the presence of denaturant using fluorescence correlation
spectroscopy.
Chattopadhyay, K., Saffarian, S., Elson, E.L., and Frieden, C., Biophys J,
2005. 88(2): pp. 1413-22
IFABP is a small (15
kDa) protein consisting mostly of antiparallel beta-strands that surround a
large cavity into which ligands bind. We have previously used FCS to show that
the native protein, labeled with fluorescein, exhibits dynamic fluctuation with
a relaxation time of 35 micros. Here we report the use of FCS to study the
unfolding of the protein induced by guanidine hydrochloride. Although the
application of this technique to measure diffusion coefficients and molecular
dynamics is straightforward, the FCS results need to be corrected for both viscosity
and refractive index changes as the guanidine hydrochloride concentration
increases. We present here a detailed study of the effects of viscosity and
refractive index of guanidine hydrochloride solutions to calibrate FCS data.
After correction, the increase in the diffusion time of IFABP corresponds well
with the unfolding transition monitored by far ultraviolet circular dichroism.
We also show that the magnitude of the 35 micros phase, reflecting the
conformational fluctuation in the native state, decreases sharply as the
concentration of denaturant increases and the protein unfolds. Although FCS
experiments indicate that the unfolded state at pH 2 is rather compact and
native-like, the radius in the presence of guanidine hydrochloride falls well within
the range expected for a random coil.
Podophyllotoxin-loaded
solid lipid nanoparticles for epidermal targeting. Chen, H., Chang, X., Du, D., Liu,
W., Liu, J., Weng, T., Yang, Y., Xu, H., and Yang, X., J Control Release,
2006. 110(2): pp. 296-306
The purpose of this
study was to evaluate solid lipid nanoparticles as the topical carrier for
epidermal targeting of podophyllotoxin (POD). The high pressure homogenization
was employed to prepare drug-loaded solid lipid nanoparticles. The POD-loaded
SLN stabilized by 0.5% poloxamer 188 and 1.5% soybean lecithin (P-SLN) and 2%
polysorbate 80 (T-SLN) was characterized by photon correlation spectroscopy
(PCS). P-SLN showed an average diameter of 73.4 nm and a zeta potential of
-48.36 mV. The imaging of AFM indicated that the P-SLN had a spherical shape.
DSC and X-ray diffraction analysis showed that POD was dispersed in SLN in an
amorphous state. The in vitro permeation study showed that P-SLN increased the
accumulative amount of POD in porcine skin 3.48 times over 0.15% tincture. But
T-SLN with a diameter of 123.1 nm and a zeta potential of -17.4 mV did not show
a high accumulative amount of POD when compared with P-SLN, though both P-SLN
and T-SLN could avoid the systemic uptake of POD. Because of the fluorescence
property of POD, fluorescence microscopy imaging was employed to visualize the
penetration of POD into skin from SLN. The penetration of POD from P-SLN seemed
to follow two pathways along the stratum corneum and hair follicle route. The
imaging revealed that P-SLN had a strong localization of POD within epidermis.
The penetration of P-SLN with low particle size into stratum corneum along the
skin surface 'furrow' and the consequent controlled release of POD might lead
to the epidermal targeting. P-SLN provides a good epidermal targeting effect
and may be a promising carrier for topical delivery of POD.
Dual-color
photon-counting histogram. Chen, Y., Tekmen, M., Hillesheim, L., Skinner, J., Wu, B., and Muller,
J.D., Biophys J, 2005. 88(3): pp. 2177-92
We report on the
development of dual-color photon-counting histogram (PCH) analysis. Dual-color
PCH is an extension of regular PCH and considers the photon counts received in
two detection channels instead of one. Because each detection channel records a
different color, dual-color PCH distinguishes fluorescent species not only by
differences in their brightness, but also according to their color. The
additional discrimination by color increases the sensitivity of PCH in
resolving a mixture of species considerably. Most dual-color fluorescence
fluctuation experiments are performed on fluorophores with overlapping emission
spectra. This overlap results in spectral cross talk between the detector
channels, which reduces resolvability. Here, we demonstrate that dual-color PCH
is able to resolve binary dye mixtures in the presence of cross talk from a
single measurement without any additional information about the sample. We
discuss the effect of sampling time on the fit parameters of dual-color PCH.
Differences between dual-color fluorescence correlation spectroscopy and
dual-color PCH will also be addressed. We quantitatively resolve a mixture of
the two fluorescent proteins CFP and YFP, which is challenging because of the
strong spectral overlap of their emission spectra. Dichroic mirrors are needed
to direct the light into the two detection channels. We quantify the influence
of these filters on dual-color PCH analysis and determine the optimal
transition wavelength of the dichroic mirror for the CFP-YFP pair.
Unraveling
protein-protein interactions in living cells with fluorescence fluctuation
brightness analysis.
Chen, Y., Wei, L.N., and Muller, J.D., Biophys J, 2005. 88(6):
pp. 4366-77
Fluorescence correlation
spectroscopy is a potentially powerful tool for measuring protein-protein
interactions directly in single living cells. We previously reported on the
detection of homodimer formation in cells using molecular brightness analysis.
Here, we extend the technique to detect binding between different proteins. Proteins
are labeled with the fluorescent markers YFP and CFP. We first determine the
coexpression ratio of both proteins by measuring the intensity ratio with a
dual-color setup. The effect of fluorescence resonance energy transfer on the
intensity ratio is explicitly taken into account. The brightness of cells
coexpressing both proteins is measured in a single-color setup. Selecting the
laser wavelength of the two-photon light source allows us to either coexcite
both proteins or to selectively excite YFP-labeled proteins. This approach
enables us to distinguish between homodimer and heterodimer formation. We first
present the theory and then demonstrate experimental feasibility using the
ligand binding domains of retinoic acid receptor (RARLBD) and of retinoid X
receptor (RXRLBD). Both proteins form heterodimers, and RXRLBD also forms
homodimers in the presence of its agonist. We explore binding between these
proteins in the presence and absence of RXR agonist. Our results demonstrate
that brightness analysis offers a quantitative method for determining protein
interactions in cells.
Actin cytoskeleton as
the principal determinant of size-dependent DNA mobility in cytoplasm: a new
barrier for non-viral gene delivery. Dauty, E. and Verkman, A.S., J Biol Chem, 2005. 280(9):
pp. 7823-8
The cytosol of mammalian
cells is a crowded environment containing soluble proteins and a network of
cytoskeletal filaments. Gene delivery by synthetic vectors involves the
endocytosis of DNA-polycation complexes, escape from endosomes, and diffusion
of non-complexed DNA through the cytosol to reach the nucleus. We found
previously that the translational diffusion of large DNAs (>250 bp) in
cytoplasm was greatly slowed compared with that of smaller DNAs (Lukacs, G. L.,
Haggie, P., Seksek, O., Lechardeur, D., Freedman, N., and Verkman, A. S. (2000)
J. Biol. Chem. 275, 1625-1629). To determine the mechanisms responsible for
size-dependent DNA diffusion, we used fluorescence correlation spectroscopy to
measure the diffusion of single fluorophore-labeled DNAs in crowded solutions,
cytosol extracts, actin network, and living cells. DNA diffusion (D) in
solutions made crowded with Ficoll-70 (up to 40 weight percentage) or soluble
cytosol extracts (up to 100 mg/ml) relative to diffusion of the same sized DNAs
in saline (D/D(o)) was approximately independent of DNA size (20-4500 bp),
quite different from the strong reduction in D/D(o) in the cytoplasm of living
cells. However, the reduced D/D(o) with increasing DNA size was closely
reproduced in solutions containing cross-linked actin filaments assembled with
gelsolin, whereas soluble macromolecules of the same size and concentration did
not reduce D/D(o). In intact cells microinjected with fluorescent DNAs and
studied by fluorescence correlation spectroscopy or photobleaching methods,
D/D(o) was reduced by 5-150-fold (20-6000 bp); however, the size-dependent
reduction in D/D(o) was abolished after actin cytoskeleton disruption. Our
results identify the actin cytoskeleton as a major barrier restricting
cytoplasmic transport of non-complexed DNA in non-viral gene transfer.
Imaging molecular
interactions in living cells. Day, R.N. and Schaufele, F., Mol Endocrinol, 2005. 19(7):
pp. 1675-86
Hormones integrate the
activities of their target cells through receptor-modulated cascades of protein
interactions that ultimately lead to changes in cellular function.
Understanding how the cell assembles these signaling protein complexes is
critically important to unraveling disease processes, and to the design of
therapeutic strategies. Recent advances in live-cell imaging technologies,
combined with the use of genetically encoded fluorescent proteins, now allow
the assembly of these signaling protein complexes to be tracked within the
organized microenvironment of the living cell. Here, we review some of the
recent developments in the application of imaging techniques to measure the
dynamic behavior, colocalization, and spatial relationships between proteins in
living cells. Where possible, we discuss the application of these different
approaches in the context of hormone regulation of nuclear receptor
localization, mobility, and interactions in different subcellular compartments.
We discuss measurements that define the spatial relationships and dynamics between
proteins in living cells including fluorescence colocalization, fluorescence
recovery after photobleaching, fluorescence correlation spectroscopy,
fluorescence resonance energy transfer microscopy, and fluorescence lifetime
imaging microscopy. These live-cell imaging tools provide an important
complement to biochemical and structural biology studies, extending the
analysis of protein-protein interactions, protein conformational changes, and
the behavior of signaling molecules to their natural environment within the
intact cell.
Studying biophysical
barriers to DNA delivery by advanced light microscopy. De Smedt, S.C., Remaut, K., Lucas,
B., Braeckmans, K., Sanders, N.N., and Demeester, J., Adv Drug Deliv Rev,
2005. 57(1): pp. 191-210
Advanced light microscopy
(ALM) has been intensively employed by biophysicists to reveal cellular
mechanisms. As described in this review, ALM clearly has potential to enhance
our understanding of the mechanisms that affect macromolecular therapeutics or
nanoscopic drug vectors in biological environments. However, while in recent
years confocal microscopy and related techniques became rather routinely used
in drug delivery it remains challenging to extract reliable information on the
biophysical behaviour of drug delivery systems from ALM measurements. This
review discusses studies in which confocal imaging, fluorescence recovery after
photobleaching (FRAP), fluorescence correlation spectroscopy (FCS) and
fluorescence energy transfer were employed to reveal biophysical properties of
DNA and DNA containing nanoparticles in extra- and intracellular media.
Measuring fast
dynamics in solutions and cells with a laser scanning microscope. Digman, M.A., Brown, C.M.,
Sengupta, P., Wiseman, P.W., Horwitz, A.R., and Gratton, E., Biophys J,
2005. 89(2): pp. 1317-27
Single-point
fluorescence correlation spectroscopy (FCS) allows measurements of fast
diffusion and dynamic processes in the microsecond-to-millisecond time range.
For measurements on living cells, image correlation spectroscopy (ICS) and
temporal ICS extend the FCS approach to diffusion times as long as seconds to
minutes and simultaneously provide spatially resolved dynamic information.
However, ICS is limited to very slow dynamics due to the frame acquisition
rate. Here we develop novel extensions to ICS that probe spatial correlations
in previously inaccessible temporal windows. We show that using standard laser
confocal imaging techniques (raster-scan mode) not only can we reach the
temporal scales of single-point FCS, but also have the advantages of ICS in
providing spatial information. This novel method, called raster image
correlation spectroscopy (RICS), rapidly measures during the scan many focal
points within the cell providing the same concentration and dynamic information
of FCS as well as information on the spatial correlation between points along
the scanning path. Longer time dynamics are recovered from the information in
successive lines and frames. We exploit the hidden time structure of the scan
method in which adjacent pixels are a few microseconds apart thereby accurately
measuring dynamic processes such as molecular diffusion in the
microseconds-to-seconds timescale. In conjunction with simulated data, we show
that a wide range of diffusion coefficients and concentrations can be measured
by RICS. We used RICS to determine for the first time spatially resolved
diffusions of paxillin-EGFP stably expressed in CHOK1 cells. This new type of
data analysis has a broad application in biology and it provides a powerful tool
for measuring fast as well as slower dynamic processes in cellular systems
using any standard laser confocal microscope.
Characterization of
the photoconversion on reaction of the fluorescent protein Kaede on the
single-molecule level.
Dittrich, P.S., Schafer, S.P., and Schwille, P., Biophys J, 2005. 89(5):
pp. 3446-55
Fluorescent proteins are
now widely used in fluorescence microscopy as genetic tags to any protein of
interest. Recently, a new fluorescent protein, Kaede, was introduced, which
exhibits an irreversible color shift from green to red fluorescence after
photoactivation with lambda = 350-410 nm and, thus, allows for specific
cellular tracking of proteins before and after exposure to the illumination
light. In this work, the dynamics of this photoconversion reaction of Kaede are
studied by fluorescence techniques based on single-molecule spectroscopy. By
fluorescence correlation spectroscopy, fast flickering dynamics of the
chromophore group were revealed. Although these dynamics on a submillisecond
timescale were found to be dependent on pH for the green fluorescent Kaede
chromophore, the flickering timescale of the photoconverted red chromophore was
constant over a large pH range but varied with intensity of the 488-nm
excitation light. These findings suggest a comprehensive reorganization of the
chromophore and its close environment caused by the photoconversion reaction.
To study the photoconversion in more detail, we introduced a novel experimental
arrangement to perform continuous flow experiments on a single-molecule scale
in a microfluidic channel. Here, the reaction in the flowing sample was induced
by the focused light of a diode laser (lambda = 405 nm). Original and
photoconverted Kaede protein were differentiated by subsequent excitation at
lambda = 488 nm. By variation of flow rate and intensity of the initiating
laser we found a reaction rate of 38.6 s(-1) for the complete photoconversion,
which is much slower than the internal dynamics of the chromophores. No
fluorescent intermediate states could be revealed.
Distribution, lateral
mobility and function of membrane proteins incorporated into giant unilamellar
vesicles. Doeven,
M.K., Folgering, J.H., Krasnikov, V., Geertsma, E.R., van den Bogaart, G., and
Poolman, B., Biophys J, 2005. 88(2): pp. 1134-42
GUVs have been widely
used for studies on lipid mobility, membrane dynamics and lipid domain (raft)
formation, using single molecule techniques like fluorescence correlation
spectroscopy. Reports on membrane protein dynamics in these types of model
membranes are by far less advanced due to the difficulty of incorporating
proteins into GUVs in a functional state. We have used sucrose to prevent four
distinct membrane protein(s) (complexes) from inactivating during the
dehydration step of the GUV-formation process. The amount of sucrose was
optimized such that the proteins retained 100% biological activity, and many
proteo-GUVs were obtained. Although GUVs could be formed by hydration of lipid
mixtures composed of neutral and anionic lipids, an alternate current electric
field was required for GUV formation from neutral lipids. Distribution, lateral
mobility, and function of an ATP-binding cassette transport system, an
ion-linked transporter, and a mechanosensitive channel in GUVs were determined
by confocal imaging, fluorescence correlation spectroscopy, patch-clamp
measurements, and biochemical techniques. In addition, we show that sucrose
slows down the lateral mobility of fluorescent lipid analogs, possibly due to
hydrogen-bonding with the lipid headgroups, leading to larger complexes with
reduced mobility.
Kinetics of Insulin
Adsorption at the Oil-Water Interface and Diffusion Properties of Adsorbed
Layers Monitored Using Fluorescence Correlation Spectroscopy. Donsmark, J., Jorgensen, L., Mollmann,
S., Frokjaer, S., and Rischel, C., Pharm Res, 2005
The adsorption of
insulin at an oil-water interface was studied with fluorescence correlation
spectroscopy (FCS). FCS is able to measure diffusion properties of insulin at
nanomolar concentrations, making it possible to detect the very early steps in
the adsorption process. Below 20 nM bulk insulin concentration, the insulin
molecules adsorbed to the surface diffuse freely at all times during the
experiment (a few hours). At higher concentrations, a surprisingly abrupt
transition to a slow diffusion phase is observed. Based on the information
about both diffusion times and molecular brightness derived from the FCS
experiments, we suggest that the transition represents the formation of a
fractal network. FCS may be a valuable tool in pharmaceutical formulation
science, because it provides information about concentration buildup and phase
changes at interfaces formed in drug delivery systems.
A close look at
fluorescence quenching of organic dyes by tryptophan. Doose, S., Neuweiler, H., and
Sauer, M., Chemphyschem, 2005. 6(11): pp. 2277-85
Understanding
fluorescence quenching processes of organic dyes by biomolecular compounds is
of fundamental importance for in-vitro and in-vivo fluorescence studies. It has
been reported that the excited singlet state of some oxazine and rhodamine
derivatives is efficiently and almost exclusively quenched by the amino acid
tryptophan (Trp) and the DNA base guanine via photoinduced electron transfer
(PET). We present a detailed analysis of the quenching interactions between the
oxazine dye MR121 and Trp in aqueous buffer. Steady-state and time-resolved
fluorescence spectroscopy, together with fluorescence correlation spectroscopy
(FCS), reveal three contributing quenching mechanisms: 1) diffusion-limited
dynamic quenching with a bimolecular quenching rate constant k(d) of 4.0 x
10(9) s(-1) M(-1), 2) static quenching with a bimolecular association constant
K(s) of 61 M(-1), and 3) a sphere-of-action contribution to static quenching
described by an exponential factor with a quenching constant lambda of 22
M(-1). The latter two are characterized as nonfluorescent complexes, formed
with approximately 30 % efficiency upon encounter, that are stable for tens of
nanoseconds. The measured binding energy of 20-30 kJ mol(-1) is consistent with
previous estimates from molecular dynamics simulations that proposed stacked
complexes due to hydrophobic forces. We further evaluate the influence of
glycerol and denaturant (guanidine hydrochloride) on the formation and
stability of quenched complexes. Comparative measurements performed with two
other dyes, ATTO 655 and Rhodamine 6G show similar results and thus demonstrate
the general applicability of utilizing PET between organic dyes and Trp for the
study of conformational dynamics of biopolymers on sub-nanometer length and
nanosecond time-scales.
Comparison of
photophysical and colloidal properties of biocompatible semiconductor nanocrystals
using fluorescence correlation spectroscopy. Doose, S., Tsay, J.M., Pinaud, F., and Weiss, S., Anal
Chem, 2005. 77(7): pp. 2235-42
A number of different
surface chemistries have been developed in recent years to render semiconductor
nanocrystals (NCs) stable in water and biocompatible. However, most of these
surface modifications affect NCs' photophysical properties, calling for a
method to simultaneously monitor colloidal and fluorescence properties.
Fluorescence correlation spectroscopy (FCS) combined with ensemble
spectroscopic methods and Monte Carlo simulations were used to interpret and
derive photophysical as well as colloidal properties of four different NC
surface treatments. Using a novel FCS scheme with alternating laser excitation
at two different intensities, we first ruled out influences from optical
gradient forces (optical trapping). We then compared concentration of emitting
particles, brightness per particle, saturation intensity, blinking
(intermittency), hydrodynamic radius, and propensity for aggregation of the
different bioconjugated NCs. This approach was successfully applied during the
development and optimization of peptide-coated NCs.
Investigation by
fluorescence correlation spectroscopy of the chaperoning interactions of HIV-1
nucleocapsid protein with the viral DNA initiation sequences. Egele, C., Schaub, E., Piemont,
E., de Rocquigny, H., and Mely, Y., C R Biol, 2005. 328(12): pp.
1041-51
HIV-1 nucleocapsid
protein (NC) exhibits nucleic acid chaperone properties that are important
during reverse transcription. Herein, we review and extend our recent
investigation by fluorescence correlation spectroscopy (FCS) of the NC
chaperone activity on the primer binding site sequences (PBS) of the (-) and
(+) DNA strands, which are involved in the second strand transfer during
reverse transcription. In the absence of NC, the PBS stem-loops exhibited a
fraying limited to the terminal G-C base pair. The kinetics of fraying were
significantly activated by NC, a feature that may favour (-)PBS/(+)PBS
annealing during the second strand transfer. In addition, NC was found to
promote the formation of PBS kissing homodimers through interaction between the
loops. These kissing complexes may favour secondary contacts between viral
sequences and thus, promote recombination and viral diversity.
Comparison of
different fluorescence fluctuation methods for their use in FRET assays:
monitoring a protease reaction. Eggeling, C., Jager, S., Winkler, D., and Kask, P., Curr
Pharm Biotechnol, 2005. 6(5): pp. 351-71
We compare the accuracy
of a variety of Fluorescence Fluctuation Spectroscopy (FFS) methods for the
study of Forster Resonance Energy Transfer (FRET) assays. As an example, the
cleavage of a doubly labeled, FRET-active peptide substrate by the protease
Trypsin is monitored and analyzed using methods based on fluorescence
intensity, Fluorescence Correlation Spectroscopy (FCS) and Fluorescence
Intensity Distribution Analysis (FIDA). The presented fluorescence data are
compared to High-Pressure Liquid Chromatography (HPLC) data obtained from the
same assay. The HPLC analysis discloses general disadvantages of the FRET
approach, such as incomplete labeling and the need for aliquots. However, the
simultaneous use of two photon detectors monitoring the fluorescence signal of
both labels significantly improves the analysis. In particular, the two global
analysis tools Two-Dimensional Fluorescence Intensity Distribution Analysis
(2D-FIDA) and Two-Color Global Fluorescence Correlation Spectroscopy (2CG-FCS)
highlight the potential of a combination of FFS and FRET. While conventional
FIDA and FCS auto- or cross-correlation analysis leaves the user with drawbacks
inherent in two-color and FRET applications, these effects are overcome by the
global analysis on the molecular level. Furthermore, it is advantageous to
analyze the unnormalized as opposed to the normalized correlation data when
combining any fluorescence correlation method with FRET, since the analysis of
the unnormalized data introduces more accuracy and is less sensitive to the
experimental drawbacks.
Rapid analysis of
Forster resonance energy transfer by two-color global fluorescence correlation
spectroscopy: trypsin proteinase reaction. Eggeling, C., Kask, P., Winkler, D., and Jager, S.,
Biophys J, 2005. 89(1): pp. 605-18
In this study we
introduce the combination of two-color global fluorescence correlation
spectroscopy (2CG-FCS) and Forster resonance energy transfer (FRET) as a very
powerful combination for monitoring biochemical reactions on the basis of
single molecule events. 2CG-FCS, which is a new variation emerging from the
family of fluorescence correlation spectroscopy, globally analyzes the
simultaneously recorded auto- and cross-correlation data from two photon
detectors monitoring the fluorescence emission of different colors. Overcoming
the limitations inherent in mere auto- and cross-correlation analysis, 2CG-FCS
is sensitive in resolving and quantifying fluorescent species that differ in
their diffusion characteristics and/or their molecular brightness either in one
or both detection channels. It is able to account for effects that have often
been considered as sources of severe artifacts in two-color and FRET
measurements, the most prominent artifacts comprising photobleaching, cross
talk, or concentration variations in sample preparation. Because of its very
high statistical accuracy, the combination of FRET and 2CG-FCS is suited for
high-throughput applications such as drug screening. Employing beam scanning
during data acquisition even further enhances this capability and allows
measurement times of <2 s. The improved performance in monitoring a FRET
sample was verified by following the protease cleavage reaction of a
FRET-active peptide. The FRET-inactive subpopulation of uncleaved substrate
could be correctly assigned, revealing a substantial portion of inactive or
missing acceptor label. The results were compared to those obtained by
two-dimensional fluorescence intensity distribution analysis.
Molecular
photobleaching kinetics of Rhodamine 6G by one- and two-photon induced confocal
fluorescence microscopy. Eggeling, C., Volkmer, A., and Seidel, C.A., Chemphyschem,
2005. 6(5): pp. 791-804
Under high-excitation
irradiance conditions in one- and two-photon induced fluorescence microscopy,
the photostability of fluorescent dyes is of crucial importance for the
detection sensitivity of single molecules and for the contrast in fluorescence
imaging. Herein, we report on the dependence of photobleaching on the
excitation conditions, using the dye Rhodamine 6G as a typical example. The
different excitation modes investigated include 1) one-photon excitation into
the first-excited singlet state in the range of 500 to 528 nm by continuous
wave and picosecond-pulsed lasers and 2) two- and one-photon excitation to
higher-excited singlet states at 800 and 350 nm, respectively, by femtosecond
pulses. Experimental strategies are presented, which allow resolving the
photophysics. From single-molecule trajectories and fluorescence correlation spectroscopy,
as well as with a simple theoretical model based on steady-state solutions of
molecular rate equation analysis, we determined the underlying photobleaching
mechanisms and quantified the photokinetic parameters describing the dependence
of the fluorescence signal on the excitation irradiance. The comparison with
experimental data and an exact theoretical model show that only minor
deviations between the different theoretical approaches can be observed for
high-pulsed excitation irradiances. It is shown that fluorescence excitation is
in all cases limited by photolysis from higher-excited electronic states. In
contrast to picosecond-pulsed excitation, this is extremely severe for both
one- and two-photon excitation with femtosecond pulses. Furthermore, the
photostability of the higher-excited electronic states is strongly influenced
by environmental conditions, such as polarity and temperature.
Performance of
fluorescence correlation spectroscopy for measuring diffusion and concentration. Enderlein, J., Gregor, I., Patra,
D., Dertinger, T., and Kaupp, U.B., Chemphyschem, 2005. 6(11):
pp. 2324-36
Fluorescence correlation
spectroscopy (FCS) has become an important tool for measuring diffusion,
concentration, and molecular interactions of cellular components. The
interpretation of FCS data critically depends on the measurement set-up. Here,
we present a rigorous theory of FCS based on exact wave-optical calculations.
Six of the most important optical and photophysical factors that influence FCS
are studied: fluorescence anisotropy, cover-slide thickness, refractive index
of the sample, laser-beam geometry, optical saturation, and pinhole adjustment.
Our theoretical framework represents a general attempt to link all relevant
parameters of the experimental set-up with the measured correlation function.
Statistical analysis
of diffusion coefficient determination by fluorescence correlation spectroscopy. Enderlein, J., Gregor, I., Patra,
D., and Fitter, J., J Fluoresc, 2005. 15(3): pp. 415-22
Fluorescence correlation
spectroscopy (FCS) has become an important and widely used technique for many
applications in physics, chemistry, and biology. The parameter most frequently
addressed by FCS is the diffusion of molecules in solution. Due to the highly
non-linear connection between the diffusion coefficient and a measured
autocorrelation function, it is extremely difficult to analyse the accuracy of
the diffusion-coefficient determination in a FCS experiment. Here, we present a
simplified analysis based on some general maximum-likelihood considerations,
and numerical result are given for the dependence of the accuracy of the
diffusion-coefficient determination on sample concentration, brightness, and
measurement time. Optimal concentration values for performing FCS are found.
Fluorescence imaging
reveals the nuclear behavior of peroxisome proliferator-activated
receptor/retinoid X receptor heterodimers in the absence and presence of ligand. Feige, J.N., Gelman, L., Tudor,
C., Engelborghs, Y., Wahli, W., and Desvergne, B., J Biol Chem, 2005. 280(18):
pp. 17880-90
In a global approach
combining fluorescence recovery after photobleaching (FRAP), fluorescence
correlation spectroscopy (FCS), and fluorescence resonance energy transfer
(FRET), we address the behavior in living cells of the peroxisome
proliferator-activated receptors (PPARs), a family of nuclear receptors
involved in lipid and glucose metabolism, inflammation control, and wound
healing. We first demonstrate that unlike several other nuclear receptors,
PPARs do not form speckles upon ligand activation. The subnuclear structures
that may be observed under some experimental conditions result from
overexpression of the protein and our immunolabeling experiments suggest that
these structures are subjected to degradation by the proteasome. Interestingly
and in contrast to a general assumption, PPARs readily heterodimerize with
retinoid X receptor (RXR) in the absence of ligand in living cells. PPAR
diffusion coefficients indicate that all the receptors are engaged in complexes
of very high molecular masses and/or interact with relatively immobile nuclear
components. PPARs are not immobilized by ligand binding. However, they exhibit
a ligand-induced reduction of mobility, probably due to enhanced interactions
with cofactors and/or chromatin. Our study draws attention to the limitations
and pitfalls of fluorescent chimera imaging and demonstrates the usefulness of
the combination of FCS, FRAP, and FRET to assess the behavior of nuclear
receptors and their mode of action in living cells.
How the molecule
number is correctly quantified in two-color fluorescence cross-correlation
spectroscopy: corrections for cross-talk and quenching in experiments. Foldes-Papp, Z., Curr Pharm
Biotechnol, 2005. 6(6): pp. 437-44
Fluorescence correlation
spectroscopy (FCS) and two-color fluorescence cross-correlation spectroscopy
(FCCS) are among the cutting-edge technologies for measuring molecule numbers
at the single-molecule level in liquid phases. Yet, even after single molecule
technologies caught up with theory, the techniques remained tools only for
specialists able to navigate the formulas that give meaning to their
observations. This original article aims at the derivations of relevant and
useful quantification of molecule numbers for researchers with more diverse
backgrounds who have begun probing questions previously unanswerable, except on
the level of the molecule. The quantitation depends on the exact conditions of
measurement. To some extent these are arbitrary, so that standard procedures
are necessary in for valid comparisons of measurements among different data
sets. To agree on and specify such procedures is one of the further aims here.
No matter what fluorophores, which have, of course, to meet photophysical and
photochemical requirements for FCS/FCCS, and optical setups/devices are used,
the primary measurement signal arises from fluctuations of the mean molecule
number in a confocal femtoliter or smaller probe region. Since FCS/FCCS relies
on fluorescence emission measurements of rare events, one is looking for small
signals on essentially zero background. Optical separation by FCCS setups is
usually defined in terms of cross-talk and cross-excitation/cross-emission,
respectively, which can be calculated and minimized by the experimenter from
readily measurable quantities of the absorption/emission scenario for single
labels and multiple labels n and m bound to or incorporated into the two-color
molecules. Furthermore, this article derives relevant formulas for the
quantification of molecule numbers under different experimental conditions with
substantial quenching of the two-color molecules such as single labels and
multiple labels n and m bound to or incorporated into the two-color molecules,
high-density labeling of two-color molecules with multiple n green labels and
one red label. Here, we summarize and extend the formulas to make them more
generally applicable.
Dissociation of
nuclear import cargo complexes by the protein Ran: a fluorescence correlation
spectroscopy study.
Fradin, C., Zbaida, D., and Elbaum, M., C R Biol, 2005. 328(12):
pp. 1073-82
In nucleated cells,
proteins designed for nuclear import form complexes with soluble nuclear
transport receptors prior to translocation across the nuclear envelope. The
directionality of transport is due to the asymmetric distribution of the
protein Ran, which dissociates import cargo complexes only in its nuclear
RanGTP form. Using fluorescence correlation spectroscopy, we have studied the
stability of cargo complexes in solution in the presence and in the absence of
RanGTP. We find that RanGTP has a higher affinity for the major import
receptor, the importin alpha/beta heterodimer, when importin alpha does not
carry a cargo, suggesting that some nuclear transport targets might be
preferentially released.
CD8 kinetically
promotes ligand binding to the T-cell antigen receptor. Gakamsky, D.M., Luescher, I.F.,
Pramanik, A., Kopito, R.B., Lemonnier, F., Vogel, H., Rigler, R., and Pecht,
I., Biophys J, 2005. 89(3): pp. 2121-33
The mechanism of CD8
cooperation with the TCR in antigen recognition was studied on live T cells.
Fluorescence correlation measurements yielded evidence of the presence of two
TCR and CD8 subpopulations with different lateral diffusion rate constants.
Independently, evidence for two subpopulations was derived from the
experimentally observed two distinct association phases of cognate peptide
bound to class I MHC (pMHC) tetramers and the T cells. The fast phase rate
constant ((1.7 +/- 0.2) x 10(5) M(-1) s(-1)) was independent of examined cell
type or MHC-bound peptides' structure. Its value was much faster than that of
the association of soluble pMHC and TCR ((7.0 +/- 0.3) x 10(3) M(-1) s(-1)),
and close to that of the association of soluble pMHC with CD8 ((1-2) x 10(5)
M(-1) s(-1)). The fast binding phase disappeared when CD8-pMHC interaction was
blocked by a CD8-specific mAb. The latter rate constant was slowed down
approximately 10-fold after cells treatment with methyl-beta-cyclodextrin.
These results suggest that the most efficient pMHC-cell association route
corresponds to a fast tetramer binding to a colocalized CD8-TCR subpopulation,
which apparently resides within membrane rafts: the reaction starts by pMHC
association with the CD8. This markedly faster step significantly increases the
probability of pMHC-TCR encounters and thereby promotes pMHC association with
CD8-proximal TCR. The slow binding phase is assigned to pMHC association with a
noncolocalized CD8-TCR subpopulation. Taken together with results of cytotoxicity
assays, our data suggest that the colocalized, raft-associated CD8-TCR
subpopulation is the one capable of inducing T-cell activation.
Diffusion behavior of
gap junction hemichannels in living cells. Gerken, M., Thews, E., Tietz, C., Wrachtrup, J.,
and Eckert, R., Curr Pharm Biotechnol, 2005. 6(2): pp. 151-8
Due to its non-invasive
character, fluorescence correlation spectroscopy (FCS) is particularly suited
for the investigation of diffusion behavior of proteins in living cells. In
this study we have investigated the diffusion properties of CFP-labeled gap
junction hemichannels in the plasma membrane of living HeLa cells. Gap junction
hemichannels or connexons are the precursors for the cell-cell- or gap junction
channels that form large plaques at the contact areas between two adjacent
cells. It has been proposed that new channels are recruited into a gap junction
structure from a pool of hemichannels that can freely diffuse over the entire
plasma membrane. The statistical approach shows that the geometry of the
membrane within the focus is the most important property for the form of the
autocorrelation curve and in turn for the determination of the diffusion
coefficient. On the other hand binding-unbinding events which lead to anomalous
diffusion have only a minor effect to the position and shape of the correlation
curve compared to the geometry of the membrane.
Diffusion of
sphingomyelin and myelin oligodendrocyte glycoprotein in the membrane of OLN-93
oligodendroglial cells studied by fluorescence correlation spectroscopy. Gielen, E., Vercammen, J., Sykora,
J., Humpolickova, J., Vandeven, M., Benda, A., Hellings, N., Hof, M.,
Engelborghs, Y., Steels, P., and Ameloot, M., C R Biol, 2005. 328(12):
pp. 1057-64
Evidence has been
accumulated that the plasma membrane of various mammalian cell types is
heterogeneous in structure and may contain lipid microdomains (lipid rafts).
This study focuses on the membrane organization of living oligodendrocytes,
which are the myelin-producing cells of the central nervous system.
Fluorescence correlation spectroscopy (FCS) was used to monitor the lateral
diffusion of a lipid and of a protein in the oligodendroglial cell line OLN-93.
The lipid was fluorescently labelled sphingomyelin (Bodipy FL-C5 SM). The protein
was the myelin oligodendrocyte glycoprotein (MOG). In order to monitor the
lateral diffusion of MOG, OLN-93 cells were transfected with a MOG-EGFP
(enhanced green fluorescent protein) fusion plasmid. The measurements were
performed at room temperature. FCS data were analyzed for two-dimensional (2D)
diffusion according to three models which all included a triplet fraction: (a)
2D 1 component (2D1C), (b) 2D anomalous diffusion (2D1Calpha), and (c) 2D 2
components (2D2C). Preliminary results indicate that for the lipid case, the
best fits are obtained with 2D2C. In the case of MOG-EGFP, 2D2C and 2D1Calpha
give fits of similar quality. The parameter estimates obtained with 2D1Calpha,
however, have a lower standard deviation. The anomaly parameter for MOG-EGFP is
0.59+/-0.01.
Molecular and
structural characterization of fluorescent human parvovirus B19 virus-like
particles. Gilbert,
L., Toivola, J., White, D., Ihalainen, T., Smith, W., Lindholm, L., Vuento, M.,
and Oker-Blom, C., Biochem Biophys Res Commun, 2005. 331(2): pp.
527-35
Although sharing a T=1
icosahedral symmetry with other members of the Parvoviridae family, it has been
suggested that the fivefold channel of the human parvovirus B19 VP2 capsids is
closed at its outside end. To investigate the possibility of placing a
relatively large protein moiety at this site of B19, fluorescent virus-like
particles (fVLPs) of B19 were developed. The enhanced green fluorescent protein
(EGFP) was inserted at the N-terminus of the structural protein VP2 and assembly
of fVLPs from this fusion protein was obtained. Electron microscopy revealed
that these fluorescent protein complexes were very similar in size when
compared to wild-type B19 virus. Further, fluorescence correlation spectroscopy
showed that an average of nine EGFP domains were associated with these
virus-like structures. Atomic force microscopy and immunoprecipitation studies
showed that EGFP was displayed on the surface of these fVLPs. Confocal imaging
indicated that these chimeric complexes were targeted to late endosomes when
expressed in insect cells. The fVLPs were able to efficiently enter cancer
cells and traffic to the nucleus via the microtubulus network. Finally,
immunoglobulins present in human parvovirus B19 acute and past-immunity serum
samples were able to detect antigenic epitopes present in these fVLPs. In
summary, we have developed fluorescent virus-like nanoparticles displaying a
large heterologous entity that should be of help to elucidate the mechanisms of
infection and pathogenesis of human parvovirus B19. In addition, these B19
nanoparticles serve as a model in the development of targetable vehicles
designed for delivery of biomolecules.
Characterization of
interaction between cationic lipid-oligonucleotide complexes and cellular membrane
lipids using confocal imaging and fluorescence correlation spectroscopy. Gordon, S.P., Berezhna, S.,
Scherfeld, D., Kahya, N., and Schwille, P., Biophys J, 2005. 88(1):
pp. 305-16
Complexes formed by
cationic liposomes and single-strand oligodeoxynucleotides (CL-ODN) are
promising delivery systems for antisense therapy. ODN release from the
complexes is an essential step for inhibiting activity of antisense drugs. We
applied fluorescence correlation spectroscopy and confocal laser scanning
microscopy to monitor CL-ODN complex interaction with membrane lipids leading
to ODN release. To model cellular membranes we used giant unilamellar vesicles
and investigated the transport of Cy-5-labeled ODNs across DiO-labeled
membranes. For the first time, we directly observed that ODN molecules are
transferred across the lipid bilayers and are kept inside the giant unilamellar
vesicles after release from the carriers. ODN dissociation from the carrier was
assessed by comparing diffusion constants of CL-ODN complexes and ODNs before
complexation and after release. Freely diffusing Cy-5-labeled ODN (16-nt) has
diffusion constant D(ODN) = 1.3 +/- 0.1 x 10(-6) cm2/s. Fluorescence
correlation spectroscopy curves for CL-ODN complexes were fitted with two components,
which both have significantly slower diffusion in the range of D(CL-ODN) =
approximately 1.5 x 10(-8) cm2/s. Released ODN has the mean diffusion constant
D = 1.1 +/- 0.2 x 10(-6) cm2/s, which signifies that ODN is dissociated from
cationic lipids. In contrast to earlier studies, we report that
phosphatidylethanolamine can trigger ODN release from the carrier in the full
absence of anionic phosphatidylserine in the target membrane and that
phosphatidylethanolamine-mediated release is as extensive as in the case of
phosphatidylserine. The presented methodology provides an effective tool for
probing a delivery potential of newly created lipid formulations of CL-ODN
complexes for optimal design of carriers.
Fluorescence
correlation spectroscopy of molecular motions and kinetics. Gosch, M. and Rigler, R., Adv
Drug Deliv Rev, 2005. 57(1): pp. 169-90
The foundations for
fluorescence correlation spectroscopy (FCS) were already laid in the early
1970s, but this technique did not become widely used until single-molecule
detection was established almost 20 years later with the use of
diffraction-limited confocal volume element. The analysis of molecular noise
from the GHz- to the Hz-region facilitates measurements over a large dynamic
range covering photophysics, conformational transitions and interactions as
well as transport properties of fluorescent biomolecules. From the Poissonian
nature of the noise spectrum the absolute number of molecules is obtainable.
Originally used for the analysis of molecular interactions in solutions, the
strength of FCS lies also in its applicability to molecular processes at either
the surface or interior of single cells. Examples for the analysis of surface
kinetics including on and off rates of ligand-receptor interactions will be
given. The possibility of obtaining this type of information by FCS will be of
particular interest for cell-based drug screening.
Optical saturation in
fluorescence correlation spectroscopy under continuous-wave and pulsed
excitation. Gregor,
I., Patra, D., and Enderlein, J., Chemphyschem, 2005. 6(1): pp.
164-70
A detailed theoretical
and experimental study of the dependence of fluorescence correlation
measurements on optical excitation power due to optical saturation effects is
presented. It is shown that the sensitivity of a fluorescence correlation
measurement on excitation power becomes increasingly stronger for decreasing
excitation power. This makes exact measurements or diffusion coefficients with
fluorescence correlation spectroscopy rather difficult. A strong difference of
this behavior for continuous-wave and pulsed excitation is found.
Single-molecule
studies on DNA and RNA. Greulich, K.O., Chemphyschem, 2005. 6(12): pp. 2458-71
DNA and RNA are the most
individual molecules known. Therefore, single-molecule experiments with these
nucleic acids are particularly useful. This review reports on recent
experiments with single DNA and RNA molecules. First, techniques for their
preparation and handling are summarised including the use of AFM nanotips and
optical or magnetic tweezers. As important detection techniques, conventional
and near-field microscopy as well as fluorescence resonance energy transfer
(FRET) and fluorescence correlation spectroscopy (FCS) are touched on briefly.
The use of single-molecule techniques currently includes force measurements in
stretched nucleic acids and in their complexes with binding partners,
particularly proteins, and the analysis of DNA by restriction mapping, fragment
sizing and single-molecule hybridisation. Also, the reactions of RNA
polymerases and enzymes involved in DNA replication and repair are dealt with
in some detail, followed by a discussion of the transport of individual nucleic
acid molecules during the readout and use of genetic information and during the
infection of cells by viruses. The final sections show how the enormous
addressability in nucleic acid molecules can be exploited to construct a
single-molecule field-effect transistor and a walking single-molecule robot,
and how individual DNA molecules can be used to assemble a single-molecule DNA
computer.
Diffusion and binding
properties investigated by Fluorescence Correlation Spectroscopy (FCS). Grunwald, D., Cardoso, M.C.,
Leonhardt, H., and Buschmann, V., Curr Pharm Biotechnol, 2005. 6(5):
pp. 381-6
During the last years,
Fluorescence Correlation Spectroscopy (FCS) has proven to be a powerful tool
for basic research in many applications. The combination of a minimal detection
volume in the femtoliter range coupled with very high sensitivity extends the
possibilities to design sensitive homogeneous tests. In this article we
illustrate the analysis of binding processes with FCS based on the changes in
diffusion characteristics of GFP upon binding to an antibody. Problems induced
by highly heterogeneous samples are discussed and differences of GFP binding to
a monoclonal and a polyclonal antibody are shown and analyzed. We stress data
processing, limitations and useful approximations in FCS methodology. Basic
ideas of data acquisition and processing as well as new developments and
applications are presented.
Structural aspects of
histone H1-DNA complexes and their relation to transfection efficiency. Haberland, A., Cartier, R., Heuer,
D., Zaitsev, S., Paulke, B.R., Schafer-Korting, M., and Bottger, M., Biotechnol
Appl Biochem, 2005. 42(Pt 2): pp. 107-17
During transfection,
polycation-DNA complexes are normally diluted by the transfection medium, which
often contains salt in the physiological concentration range and serum. It is
not exactly known to what extent this dilution step influences the properties
of the complexes, which in turn influence the transfection efficiency. In order
to gain more insight into the size-structure-transfection activity
relationship, we prepared histone H1-DNA complexes in NaCl solutions at various
concentrations known to determine the size and structure of the resulting
complexes. We characterized the complexes by physicochemical methods.
Fluorescence correlation spectroscopy enabled relative measurements of complex
sizes even under physiological conditions. The different appearances of the
complexes were correlated with their transfection efficiency. When transfection
was performed by dilution of the complexes in cell-cultivation media, the
initial structure of H1-DNA complexes preformed under distinct salt conditions
had no significant influence on the transfection efficiency. The dilution of
the preformed complexes with cell-cultivation medium resulted in re-formation
and aggregation of the complexes. The addition of the complexes to the cells
without cell-cultivation medium, however, showed a direct correlation between
the size of the complexes and the transfection efficiency (correlation
coefficient 0.91). Small complexes did not contribute to the transfection.
Diffusion in
two-component lipid membranes--a fluorescence correlation spectroscopy and
monte carlo simulation study. Hac, A.E., Seeger, H.M., Fidorra, M., and Heimburg, T., Biophys
J, 2005. 88(1): pp. 317-33
Using fluorescence
correlation spectroscopy, calorimetry, and Monte Carlo simulations, we studied
diffusion processes in two-component membranes close to the chain melting
transition. The aim is to describe complex diffusion behavior in lipid systems
in which gel and fluid domains coexist. Diffusion processes in gel membranes
are significantly slower than in fluid membranes. Diffusion processes in mixed
phase regions are therefore expected to be complex. Due to statistical
fluctuations the gel-fluid domain patterns are not uniform in space and time.
No models for such diffusion processes are available. In this article, which is
both experimental and theoretical, we investigated the diffusion in DMPC-DSPC
lipid mixtures as a function of temperature and composition. We then modeled
the fluorescence correlation spectroscopy experiment using Monte Carlo
simulations to analyze the diffusion process. It is shown that the simulations
yield a very good description of the experimental diffusion processes, and that
predicted autocorrelation profiles are superimposable with the experimental
curves. We believe that this study adds to the discussion on the physical
nature of rafts found in biomembranes.
High count rates with
total internal reflection fluorescence correlation spectroscopy. Hassler, K., Anhut, T., Rigler,
R., Gosch, M., and Lasser, T., Biophys J, 2005. 88(1): pp. L01-3
Application of
fluorescence correlation spectroscopy to hapten-antibody binding. Hazlett, T.L., Ruan, Q., and
Tetin, S.Y., Methods Mol Biol, 2005. 305: pp. 415-38
Two-photon fluorescence
correlation spectroscopy 2P-FCS has received a large amount of attention over
the past ten years as a technique that can monitor the concentration, the
dynamics, and the interactions of molecules with single molecule sensitivity.
In this chapter, we explain how 2P-FCS is carried out for a specific
ligand-binding problem. We briefly outline considerations for proper instrument
design and instrument calibration. General theory of autocorrelation analysis
is explained and straightforward equations are given to analyze simple binding
data. Specific concerns in the analytical methods related to IgG, such as the
presence of two equivalent sites and fractional quenching of the bound
hapten-fluorophore conjugate, are explored and equations are described to
account for these issues. We apply these equations to data on two
antibody-hapten pairs: antidigoxin IgG with fluorescein-digoxin and
antidigitoxin IgG with Alexa488-digitoxin. Digoxin and digitoxin are important
cardio glycoside drugs, toxic at higher levels, and their blood concentrations
must be monitored carefully. Clearly, concentration assays based on IgG rely on
accurate knowledge of the hapten-IgG binding strengths. The protocols for
measuring and determining the dissociation constants for both IgG-hapten pairs are
outlined and discussed.
Deficiency of
disulfide bonds facilitating fibrillogenesis of endostatin. He, Y., Zhou, H., Tang, H., and
Luo, Y., J Biol Chem, 2006. 281(2): pp. 1048-57
Endostatin is an
endogenous inhibitor of tumor angiogenesis and tumor growth. It has two pairs
of disulfide bonds in a unique nested pattern, which play a key role in its
native conformation, stability, and activity. Here, we constructed a
disulfide-deficient variant of endostatin, endo-all-Ala, to examine the effects
of the two disulfide bonds on fibrillogenesis of endostatin under nondenaturing
conditions. Based on thioflavin T fluorescence, atomic force microscopy, far-UV
circular dichroism, and Fourier transform infrared spectroscopy, we found that
endo-all-Ala, which has a higher alpha-helical content compared with wild type,
is prone to forming fibrils in a pH-dependent manner. Subsequently, more
hydrophobic patches with a lower stability of endo-all-Ala were observed when
compared with wild type, which possibly contributes to the propensity of
amyloid formation of endo-all-Ala. To our surprise, the significant increase of
the alpha-helical content in endostatin induced by trifluoroethanol can also
facilitate fibril formation. In addition, the cytotoxicity of fibrillar aggregates
of endo-all-Ala, which were generated at different stages of the fibril
formation process, was evaluated by cell viability assay. The results indicate
that the cytotoxicity is not due to the fibrils but rather due to the granular
aggregates of endo-all-Ala. Moreover, endostatin was interestingly found to be
reduced by glutathione at physiological concentrations. Our present work not
only elucidates the correlation between the existence of disulfide bonds and
the fibril formation of endostatin but also may provide some insights into the
structural and functional basis of endostatin in Alzheimer disease brains.
CK2 phosphorylation
of eukaryotic translation initiation factor 5 potentiates cell cycle
progression. Homma,
M.K., Wada, I., Suzuki, T., Yamaki, J., Krebs, E.G., and Homma, Y., Proc
Natl Acad Sci U S A, 2005. 102(43): pp. 15688-93
Casein kinase 2 (CK2) is
a ubiquitous eukaryotic Ser/Thr protein kinase that plays an important role in
cell cycle progression. Although its function in this process remains unclear,
it is known to be required for the G(1) and G(2)/M phase transitions in yeast.
Here, we show that CK2 activity changes notably during cell cycle progression
and is increased within 3 h of serum stimulation of quiescent cells. During the
time period in which it exhibits high enzymatic activity, CK2 associates with
and phosphorylates a key molecule for translation initiation, eukaryotic
translation initiation factor (eIF) 5. Using MS, we show that Ser-389 and -390
of eIF5 are major sites of phosphorylation by CK2. This is confirmed using eIF5
mutants that lack CK2 sites; the phosphorylation levels of mutant eIF5 proteins
are significantly reduced, relative to WT eIF5, both in vitro and in vivo.
Expression of these mutants reveals that they have a dominant-negative effect
on phosphorylation of endogenous eIF5, and that they perturb synchronous
progression of cells through S to M phase, resulting in a significant reduction
in growth rate. Furthermore, the formation of mature eIF5/eIF2/eIF3 complex is reduced
in these cells, and, in fact, restricted diffusional motion of WT eIF5 was
almost abolished in a GFP-tagged eIF5 mutant lacking CK2 phosphorylation sites,
as measured by fluorescence correlation spectroscopy. These results suggest
that CK2 may be involved in the regulation of cell cycle progression by
associating with and phosphorylating a key molecule for translation initiation.
Cluster formation of
nanoparticles in an optical trap studied by fluorescence correlation
spectroscopy.
Hosokawa, C., Yoshikawa, H., and Masuhara, H., Phys Rev E Stat Nonlin Soft
Matter Phys, 2005. 72(2 Pt 1): p. 021408
We report in situ
observation of cluster growth of nanoparticles confined in an optical trapping
potential by means of fluorescence correlation spectroscopy. When an optical
trapping force caused by a highly focused laser beam acts on nanoparticle
suspensions, the number of nanoparticles increases and an assembly can be
formed at the focal spot. The decay times of fluorescence autocorrelation
curves were investigated as a function of the irradiation time of the laser
beam and the laser power. In the initial stage of the optical assembling, the
decay time increases with the irradiation time of the laser beam. On the other
hand, in the later stage, a decrease of the decay time was observed. This
behavior is explained successfully by using two models of Brownian motion under
weak and strong optical trapping. It was revealed that trapping and clustering
of nanoparticles proceed simultaneously and clusters confined in the focal spot
make larger aggregates spontaneously.
Potential application
of synchronous fluorescence spectroscopy to determine benzo[a]pyrene in soil
extracts. Hua, G.,
Killham, K., and Singleton, I., Environ Pollut, 2006. 139(2): pp.
272-8
Benzo[a]pyrene (BaP) is
a significant environmental pollutant and rapid, accurate methods to quantify
this compound in soil for both research and environmental investigation
purposes are required. In this work, solvent extracts from five contrasting
soils spiked with four different polycyclic aromatic hydrocarbons (PAHs) were
rapidly analysed by using a synchronous fluorescence spectroscopy (SFS) method.
The SFS method was validated using HPLC with ultraviolet detection. A good
correlation for the quantification of BaP in soil extracts by the two methods
was observed. The detection limit of the SFS method was 1.6x10(-9)g/ml in CTAB
micellar medium (7.8mmol/l). The work demonstrates that SFS has potential as a
sensitive, accurate, rapid, simple and economic methodology and an efficient
alternative to HPLC for fast confirmation and quantification of BaP in complex
soil extracts.
Diffusion-time
distribution analysis reveals characteristic ligand-dependent interaction
patterns of nuclear receptors in living cells. Jankevics, H., Prummer, M., Izewska, P., Pick, H.,
Leufgen, K., and Vogel, H., Biochemistry, 2005. 44(35): pp.
11676-83
Nuclear receptors
initiate transcription, interact with regulatory proteins, and are influenced
by hormones, drugs, and pollutants. Herein, we discover ligand-specific
mobility patterns of human estrogen receptor-alpha (ER) in living cells using
diffusion-time distribution analysis (DDA). This novel method, based on
fluorescence correlation spectroscopy (FCS), is especially suited to unraveling
multiple protein interactions in vivo at native expression levels. We found
that ER forms a limited number of distinct complexes with a varying population
by dynamic interaction with other nuclear components. Dose-response curves of
different ligands could be obtained for each receptor interaction. The
potential to identify interacting proteins was demonstrated by comparing DDA of
the ER cofactor SRC-3 attached to yellow fluorescent protein (YFP) with those
of YFP-ER. Our findings open up new routes to elucidating transcription
regulation and to detecting and distinguishing pharmacologically and
toxicologically active compounds in vivo. Moreover, DDA provides a general
approach to monitoring biochemical networks in individual living cells.
Calixarene-coated
water-soluble CdSe-ZnS semiconductor quantum dots that are highly fluorescent
and stable in aqueous solution. Jin, T., Fujii, F., Sakata, H., Tamura, M., and Kinjo, M.,
Chem Commun (Camb), 2005(22): pp. 2829-31
A simple method for the
preparation of highly fluorescent and stable, water-soluble CdSe-ZnS quantum
dots is reported using calix[4]arene carboxylic acids as surface coating
agents; the coating of the surface with the calixarene and the conjugation of
antibodies to the quantum dots are confirmed by fluorescence correlation
spectroscopy.
Raft partitioning and
dynamic behavior of human placental alkaline phosphatase in giant unilamellar
vesicles. Kahya,
N., Brown, D.A., and Schwille, P., Biochemistry, 2005. 44(20):
pp. 7479-89
Much attention has
recently been drawn to the hypothesis that cellular membranes organize in
functionalized platforms called rafts, enriched in sphingolipids and
cholesterol. The notion that glycosylphosphatidylinositol (GPI)-anchored proteins
are strongly associated with rafts is based on their insolubility in nonionic
detergents. However, detergent-based methodologies for identifying raft
association are indirect and potentially prone to artifacts. On the other hand,
rafts have proven to be difficult to visualize and investigate in living cells.
A number of studies have demonstrated that model membranes provide a valuable
tool for elucidating some of the raft properties. Here, we present a model
membrane system based on domain-forming giant unilamellar vesicles (GUVs), in
which the GPI-anchored protein, human placental alkaline phosphatase (PLAP),
has been functionally reconstituted. Raft morphology, protein raft
partitioning, and dynamic behavior have been characterized by fluorescence confocal
microscopy and fluorescence correlation spectroscopy (FCS). Approximately
20-30% of PLAP associate with sphingomyelin-enriched domains. The affinity of
PLAP for the liquid-ordered (l(o)) phase is compared to that of a nonraft
protein, bacteriorhodopsin. Next, detergent extraction was carried out on
PLAP-containing GUVs as a function of temperature, to relate the lipid and
protein organization in distinct phases of the GUVs to the composition of
detergent resistant membranes (DRMs). Finally, antibody-mediated cross-linking
of PLAP induces a shift of its partition coefficient in favor of the l(o)
phase.
Differential lipid
packing abilities and dynamics in giant unilamellar vesicles composed of
short-chain saturated glycerol-phospholipids, sphingomyelin and cholesterol. Kahya, N., Scherfeld, D., and
Schwille, P., Chem Phys Lipids, 2005. 135(2): pp. 169-80
The ability of membrane
components to arrange themselves heterogeneously within the bilayer induces the
formation of microdomains. Much work has been devoted to mimicking
domain-assembly in artificial bilayers and characterizing their
physico-chemical properties. Ternary lipid mixtures composed of unsaturated
phospholipids, sphingomyelin and cholesterol give rise to large, round domains.
Here, we replaced the unsaturated phospholipid in the ternary mixture with
sphingomyelin and cholesterol by saturated glycero-phospholipids of different
chain length and characterized the critical role of cholesterol in sorting
these lipids by confocal imaging and fluorescence correlation spectroscopy
(FCS). More cholesterol is needed to obtain phase segregation in ternary
mixtures, in which the unsaturated phospholipid is replaced by a saturated one.
Finally, lipid dynamics in distinct phases is very low and astonishingly
similar, thereby suggesting the poor ability of cholesterol in sorting
short-chain saturated glycero-phospholipids and sphingomyelin.
Diffusion of spheres
in crowded suspensions of rods. Kang, K., Gapinski, J., Lettinga, M.P., Buitenhuis, J.,
Meier, G., Ratajczyk, M., Dhont, J.K., and Patkowski, A., J Chem Phys,
2005. 122(4): p. 44905
Translational tracer
diffusion of spherical macromolecules in crowded suspensions of rodlike
colloids is investigated. Experiments are done using several kinds of spherical
tracers in fd-virus suspensions. A wide range of size ratios L/2a of the length
L of the rods and the diameter 2a of the tracer sphere is covered by combining
several experimental methods: fluorescence correlation spectroscopy for small
tracer spheres, dynamic light scattering for intermediate sized spheres, and
video microscopy for large spheres. Fluorescence correlation spectroscopy is
shown to measure long-time diffusion only for relatively small tracer spheres.
Scaling of diffusion coefficients with a/xi, predicted for static networks, is
not found for our dynamical network of rods (with xi the mesh size of the
network). Self-diffusion of tracer spheres in the dynamical network of freely
suspended rods is thus fundamentally different as compared to cross-linked
networks. A theory is developed for the rod-concentration dependence of the
translational diffusion coefficient at low rod concentrations for freely
suspended rods. The proposed theory is based on a variational solution of the
appropriate Smoluchowski equation without hydrodynamic interactions. The theory
can, in principle, be further developed to describe diffusion through dynamical
networks at higher rod concentrations with the inclusion of hydrodynamic
interactions. Quantitative agreement with the experiments is found for large
tracer spheres, and qualitative agreement for smaller spheres. This is probably
due to the increasing importance of hydrodynamic interactions as compared to
direct interactions as the size of the tracer sphere decreases.
Fluorescence
fluctuation spectroscopy in subdiffraction focal volumes. Kastrup, L., Blom, H., Eggeling,
C., and Hell, S.W., Phys Rev Lett, 2005. 94(17): p. 178104
We establish
fluorescence fluctuation spectroscopy (FFS) with nanoscale detection volumes generated
by stimulated emission depletion. Our method applies fluorescence correlation
spectroscopy and fluorescence intensity distribution analysis to extract
molecular information about mobilities and fluorescence emission in solution.
The combination of correlation analysis with that of photon intensity
distributions reveals a fivefold squeezing of the detection volume over current
diffraction-limited systems, which is in full agreement with the simultaneously
demonstrated 25-fold reduction in (axial) focal transit time. Our method
significantly extends the potential of far-field FFS, including for the
noninvasive investigation of molecular reactions at higher concentrations.
Fluorescence
correlation microscopy with real-time alignment readout. Kaushalya, S.K., Balaji, J.,
Garai, K., and Maiti, S., Appl Opt, 2005. 44(16): pp. 3262-5
In confocal fluorescence
correlation microscopy (FCM) it is important to ensure that the correlation
measurement is actually performed at the chosen location of the three-dimensional
image of the specimen. We present a confocal FCM design that provides an
automatic real-time readout of the location in the confocal microscopic image,
which is aligned with the detector of the fluorescence correlation
spectrometer. The design accomplishes this without using any special
positioning device. The design is based on an apertured fluorescence detector
placed close to the back aperture of the objective lens and can be easily
incorporated into virtually any confocal microscope. We demonstrate the method
by performing FCM measurements of a dye diffusing on a cell membrane.
The phosducin-like
protein PhLP1 is essential for G{beta}{gamma} dimer formation in Dictyostelium
discoideum. Knol,
J.C., Engel, R., Blaauw, M., Visser, A.J., and van Haastert, P.J., Mol Cell
Biol, 2005. 25(18): pp. 8393-400
Phosducin proteins are
known to inhibit G protein-mediated signaling by sequestering Gbetagamma
subunits. However, Dictyostelium discoideum cells lacking the phosducin-like
protein PhLP1 display defective rather than enhanced G protein signaling. Here
we show that green fluorescent protein (GFP)-tagged Gbeta (GFP-Gbeta) and
GFP-Ggamma subunits exhibit drastically reduced steady-state levels and are
absent from the plasma membrane in phlp1(-) cells. Triton X-114 partitioning
suggests that lipid attachment to GFP-Ggamma occurs in wild-type cells but not
in phlp1(-) and gbeta(-) cells. Moreover, Gbetagamma dimers could not be
detected in vitro in coimmunoprecipitation assays with phlp1(-) cell lysates.
Accordingly, in vivo diffusion measurements using fluorescence correlation
spectroscopy showed that while GFP-Ggamma proteins are present in a complex in
wild-type cells, they are free in phlp1(-) and gbeta(-) cells. Collectively,
our data strongly suggest the absence of Gbetagamma dimer formation in
Dictyostelium cells lacking PhLP1. We propose that PhLP1 serves as a
cochaperone assisting the assembly of Gbeta and Ggamma into a functional
Gbetagamma complex. Thus, phosducin family proteins may fulfill hitherto unsuspected
biosynthetic functions.
Determining protease
activity in vivo by fluorescence cross-correlation analysis. Kohl, T., Haustein, E., and
Schwille, P., Biophys J, 2005. 89(4): pp. 2770-82
To date, most
biochemical approaches to unravel protein function have focused on purified
proteins in vitro. Whereas they analyze enzyme performance under assay
conditions, they do not necessarily tell us what is relevant within a living
cell. Ideally, cellular functions should be examined in situ. In particular, association/dissociation
reactions are ubiquitous, but so far there is no standard technique permitting
online analysis of these processes in vivo. Featuring single-molecule
sensitivity combined with intrinsic averaging, fluorescence correlation
spectroscopy is a minimally invasive technique ideally suited to monitor
proteins. Moreover, endogenous fluorescence-based assays can be established by
genetically encoding fusions of autofluorescent proteins and cellular proteins,
thus avoiding the disadvantages of in vitro protein labeling and subsequent
delivery to cells. Here, we present an in vivo protease assay as a model
system: Green and red autofluorescent proteins were connected by Caspase-3-
sensitive and insensitive protein linkers to create double-labeled protease
substrates. Then, dual-color fluorescence cross-correlation spectroscopy was
employed to study the protease reaction in situ. Allowing assessment of
multiple dynamic parameters simultaneously, this method provided internal
calibration and improved experimental resolution for quantifying protein
stability. This approach, which is easily extended to reversible
protein-protein interactions, seems very promising for elucidating
intracellular protein functions.
Fluorescence
correlation spectroscopy with autofluorescent proteins. Kohl, T. and Schwille, P., Adv
Biochem Eng Biotechnol, 2005. 95: pp. 107-42
Fluorescence correlation
spectroscopy (FCS) is a versatile technique operating at the single-molecule
level, that successfully meets many challenges of modern biological research.
Based on the detection of mobile fluorescent molecules diffusing in and out of
a diffraction-limited laser focus, the method allows to resolve particle
dynamics within cells and their compartments. Previous FCS studies have described
various parameters of protein function, namely mobility, transport and
localization phenomena, enzymatic turnovers of biochemical substrates and
molecular association and dissociation reactions. Recent progress in the
application of FCS to intracellular systems has particularly taken advantage of
detecting autofluorescent proteins and their genetically encoded fusions to
cellular proteins. This review discusses recent applications of FCS analysis
with and on fluorescent proteins, particularly highlighting chemical and
physical properties. Inherent limitations of the presented approaches are
discussed in detail and strategies for optimisation of experimental systems
outlined.
Detection of motional
heterogeneities in lipid bilayer membranes by dual probe fluorescence
correlation spectroscopy. Korlach, J., Baumgart, T., Webb, W.W., and Feigenson, G.W., Biochim
Biophys Acta, 2005. 1668(2): pp. 158-63
We report the detection
of heterogeneities in the diffusion of lipid molecules for the three-component
mixture dipalmitoyl-PC/dilauroyl-PC/cholesterol, a chemically simple lipid
model for the mammalian plasma membrane outer leaflet. Two-color fluorescence
correlation spectroscopy (FCS) was performed on giant unilamellar vesicles
(GUVs) using fluorescent probes that have differential lipid phase partition
behavior--DiO-C18:2 favors disordered fluid lipid phases, whereas DiI-C20:0
prefers spatially ordered lipid phases. Simultaneously-obtained fluorescence
autocorrelation functions from the same excitation volume for each dye showed
that, depending on the lipid composition of this ternary mixture, the two dyes
exhibited different lateral mobilities in regions of the phase diagram with
previously proposed submicroscopic two-phase coexistence. In one-phase regions,
both dyes reported identical diffusion coefficients. Two-color FCS thus may be
detecting local membrane heterogeneities at size scales below the optical
resolution limit, either due to short-range order in a single phase or due to
submicroscopic phase separation.
Trapping,
deformation, and rotation of giant unilamellar vesicles in octode
dielectrophoretic field cages. Korlach, J., Reichle, C., Muller, T., Schnelle, T., and
Webb, W.W., Biophys J, 2005. 89(1): pp. 554-62
The behavior of
freestanding lipid bilayer membranes under the influence of dielectric force
potentials was studied by trapping, holding, and rotating individual giant
unilamellar vesicles (GUVs) inside dielectrophoretic microfield cages. Using
laser scanning confocal microscopy and three-dimensional image reconstructions
of GUVs labeled with fluorescent membrane probes, field strength and
frequency-dependent vesicle deformations were observed which are explained by
calculations of the dielectric force potentials inside the cage. Dynamical membrane
properties under the influence of the field cage were studied by fluorescence
correlation spectroscopy, circumventing potential artifacts associated with
measurements involving GUV immobilization on support surfaces. Lipid transport
could be accelerated markedly by the applied fields, aided by hydrodynamic
fluid streaming which was also studied by fluorescence correlation
spectroscopy.
Propidium iodide and
PicoGreen as dyes for the DNA fluorescence correlation spectroscopy
measurements. Kral,
T., Widerak, K., Langner, M., and Hof, M., J Fluoresc, 2005. 15(2):
pp. 179-83
Many experimental
designs, in which nucleic acid conformational changes are of interest, require
reliable fluorescence labeling. The appropriate fluorescence probe should have
suitable optical properties and, more importantly, should not interfere with
the investigated processes. In order to avoid chemical modifications the
fluorescence label needs to be associated with nucleic acid via weak
non-covalent interactions. There are a number of fluorescent probes that change
their fluorescent properties (i.e. their quantum yield and/or spectral
characteristics) upon association with nucleic acid. Such probes are frequently
used to detect, visualize and follow processes involving nucleic acid and its
conformational changes. In order to obtain reliable data regarding
macromolecule or aggregate topology a detailed knowledge of probe-nucleic acid
interactions on the molecular level is needed. In this paper we show that the
association of propidium iodide with DNA alters its conformation and that it
selectively labels plasmid fragments and/or its subpopulations in a
concentration-dependent meaner. Another dye, PicoGreen, exhibits better
properties. It labels nucleic acid uniformly and without any concentration-dependent
artifacts.
Separating the
contribution of translational and rotational diffusion to protein association. Kuttner, Y.Y., Kozer, N., Segal,
E., Schreiber, G., and Haran, G., J Am Chem Soc, 2005. 127(43):
pp. 15138-44
The association of two
proteins is preceded by a mutual diffusional search in solution. The role of
translational and rotational diffusion in this process has been studied
theoretically for many years. However, systematic experimental verification of
theoretical results is still lacking. We report here measurements of
association rates of the proteins beta-lactamase (TEM) and beta-lactamase
inhibitor protein (BLIP) in solutions of glycerol and poly(ethylene glycol) of
increasing viscosity. We also measured translational and rotational diffusion
in the same solutions, using fluorescence correlation spectroscopy and
fluorescence anisotropy, respectively. It is found that in glycerol both
translational and rotational diffusion rates are inversely dependent on
viscosity, as predicted by the classical Stokes-Einstein relations, while the
association rate depends nonlinearly on viscosity. In contrast, the association
rate depends only weakly on the viscosity of the polymer solutions, which
results in a similar weak dependence of k(on) on viscosity. The data are
modeled using the theory of diffusion-limited association. Deviations from the
theory are explained by a short-range solute-induced repulsion between the
proteins in glycerol solution and an attractive depletion interaction generated
by the polymers. These results open the way to the creation of a unified
framework for all nonspecific effects involved in the protein association
process, as well as to better theoretical understanding of these effects.
Further, they reflect on the complex factors controlling protein association
within the crowded environment of cells and suggest that a high concentration
of macromolecules does not significantly impede protein association.
In situ measurements
of viral particles diffusion inside mucoid biofilms. Lacroix-Gueu, P., Briandet, R.,
Leveque-Fort, S., Bellon-Fontaine, M.N., and Fontaine-Aupart, M.P., C R Biol,
2005. 328(12): pp. 1065-72
Fluorescence correlation
spectroscopy (FCS) under two-photon excitation was used successfully to characterize
the diffusion properties of model virus particles (bacteriophages) in bacterial
biofilm of Stenotrophonas maltophilia. The results are compared to those
obtained with fluorescent latex beads used as a reference. The FCS data clearly
demonstrated the possibility for viral particles to penetrate inside the
exopolymeric matrix of mucoid biofilms, and hence to benefit from its
protective effect toward antimicrobials (antibiotics and biocides). Microbial
biofilms should hence be considered as potential reservoirs of pathogenic
viruses, and are probably responsible for numerous persistent viral infections.
Enhancing the
sensitivity of fluorescence correlation spectroscopy by using time-correlated
single photon counting. Lamb, D.C., Muller, B.K., and Brauchle, C., Curr Pharm Biotechnol,
2005. 6(5): pp. 405-14
Fluorescence correlation
spectroscopy (FCS) and fluorescence cross-correlation spectroscopy (FCCS) are
methods that extract information about a sample from the influence of thermodynamic
equilibrium fluctuations on the fluorescence intensity. This method allows
dynamic information to be obtained from steady state equilibrium measurements
and its popularity has dramatically increased in the last 10 years due to the
development of high sensitivity detectors and its combination with confocal
microscopy. Using time-correlated single-photon counting (TCSPC) detection and
pulsed excitation, information over the duration of the excited state can be
extracted and incorporated in the analysis. In this short review, we discuss
new methodologies that have recently emerged which incorporated fluorescence
lifetime information or TCSPC data in the FCS and FCCS analysis. Time-gated FCS
discriminates between which photons are to be incorporated in the analysis
dependent upon their arrival time after excitation. This allows for accurate
FCS measurements in the presence of fluorescent background, determination of
sample homogeneity, and the ability to distinguish between static and dynamic
heterogeneities. A similar method, time-resolved FCS can be used to resolve the
individual correlation functions from multiple fluorophores through the
different fluorescence lifetimes. Pulsed interleaved excitation (PIE) encodes
the excitation source into the TCSPC data. PIE can be used to perform
dual-channel FCCS with a single detector and allows elimination of spectral
cross-talk with dual-channel detection. For samples that undergo fluorescence
resonance energy transfer (FRET), quantitative FCCS measurements can be
performed in spite of the FRET and the static FRET efficiency can be
determined.
Quantifying Pb and Cd
complexation by alginates and the role of metal binding on macromolecular
aggregation.
Lamelas, C., Avaltroni, F., Benedetti, M., Wilkinson, K.J., and Slaveykova,
V.I., Biomacromolecules, 2005. 6(5): pp. 2756-64
The Pb and Cd binding
capacity of alginates were quantified by the determination of their complex
stability constants and the concentration of complexing sites using H+, Pb2+,
or Cd2+ selective electrodes in both static and dynamic titrations.
Centrifugation filter devices (30 kDa filter cutoff), followed by inductively
coupled plasma mass spectrometry (ICP-MS) measurements of lead or cadmium in
the filtrates, were used to validate the results. The influence of ionic
strength, pH, and the metal-to-alginate ratio was determined for a wide range
of metal concentrations. Because of their polyelectrolytic properties,
alginates may adopt different conformations depending on the physicochemistry
of the medium, including the presence of metals. Therefore, molecular diffusion
coefficients of the alginate were determined by fluorescence correlation
spectroscopy under the same conditions of pH, ionic strength, and
metal-to-alginate ratios that were used for the metal binding studies. The
complexation and conformational properties of the alginate were related within
the framework of the nonideal competitive adsorption isotherm (NICA) combined
with a Donnan approach to account for both intrinsic and electrostatic
contributions.
Temporally resolved
interactions between antigen-stimulated IgE receptors and Lyn kinase on living
cells. Larson,
D.R., Gosse, J.A., Holowka, D.A., Baird, B.A., and Webb, W.W., J Cell Biol,
2005. 171(3): pp. 527-36
Upon cross-linking by
antigen, the high affinity receptor for immunoglobulin E (IgE), FcepsilonRI, is
phosphorylated by the Src family tyrosine kinase Lyn to initiate mast cell
signaling, leading to degranulation. Using fluorescence correlation
spectroscopy (FCS), we observe stimulation-dependent associations between
fluorescently labeled IgE-FcepsilonRI and Lyn-EGFP on individual cells. We also
simultaneously measure temporal variations in the lateral diffusion of these
proteins. Antigen-stimulated interactions between these proteins detected
subsequent to the initiation of receptor phosphorylation exhibit time-dependent
changes, suggesting multiple associations between FcepsilonRI and Lyn-EGFP.
During this period, we also observe a persistent decrease in Lyn-EGFP lateral
diffusion that is dependent on Src family kinase activity. These stimulated
interactions are not observed between FcepsilonRI and a chimeric EGFP that
contains only the membrane-targeting sequence from Lyn. Our results reveal
real-time interactions between Lyn and cross-linked FcepsilonRI implicated in
downstream signaling events. They demonstrate the capacity of FCS
cross-correlation analysis to investigate the mechanism of signaling-dependent
protein-protein interactions in intact, living cells.
Real-time RNA
profiling within a single bacterium. Le, T.T., Harlepp, S., Guet, C.C., Dittmar, K., Emonet,
T., Pan, T., and Cluzel, P., Proc Natl Acad Sci U S A, 2005. 102(26):
pp. 9160-4
Characterizing the
dynamics of specific RNA levels requires real-time RNA profiling in a single
cell. We show that the combination of a synthetic modular genetic system with
fluorescence correlation spectroscopy allows us to directly measure in real
time the activity of any specific promoter in prokaryotes. Using a simple inducible
gene expression system, we found that induced RNA levels within a single
bacterium of Escherichia coli exhibited a pulsating profile in response to a
steady input of inducer. The genetic deletion of an efflux pump system, a key
determinant of antibiotic resistance, altered the pulsating transcriptional
dynamics and caused overexpression of induced RNA. In contrast with population
measurements, real-time RNA profiling permits identifying relationships between
genotypes and transcriptional dynamics that are accessible only at the level of
the single cell.
Functional
conformational motions in the turnover cycle of cholesterol oxidase. Lerch, H.P., Rigler, R., and
Mikhailov, A.S., Proc Natl Acad Sci U S A, 2005. 102(31): pp.
10807-12
Reexamining experimental
data of single-molecule fluorescence correlation spectroscopy for cholesterol
oxidase, we find that the existing Michaelis-Menten models with dynamical
disorder cannot explain strong correlations between subsequent turnover cycles
revealed in the diagonal feature in the joint statistical distribution of
adjacent "on" times of this enzyme. We suggest that functional
conformational motions representing ordered sequences of transitions between a
set of conformational substates are involved, along with equilibrium
conformational fluctuations in the turnover cycle of cholesterol oxidase. A
two-channel model of single-enzyme dynamics, including a slow functional
conformational motion in one of the channels, is proposed that allows us to
reproduce such strong correlations.
Albumin-protamine-oligonucleotide
nanoparticles as a new antisense delivery system. Part 1: physicochemical
characterization.
Lochmann, D., Weyermann, J., Georgens, C., Prassl, R., and Zimmer, A., Eur J
Pharm Biopharm, 2005. 59(3): pp. 419-29
In this paper, a ternary
system of albumin-protamine-oligonucleotide nanoparticles (AlPrO-NP) recently
developed by Vogel et al. [V. Vogel, D. Lochmann, J. Weyermann, G. Mayer, C.
Tziatzios, J.A. van den Broek, W. Haase, D. Wouters, U.S. Schubert, J. Kreuter,
A. Zimmer, D. Schubert, Oligonucleotide-protamine-albumin nanoparticles:
preparation, physical properties and intracellular processing, J. Controlled
Rel. (in press)] which could serve as a potential drug delivery system for
antisense oligonucleotides. Former studies of binary protamine-oligonucleotide
nanoparticles showed two main disadvantages: (i) aggregation of the particles
within a few minutes in the presence of salt; (ii) low intracellular
dissociation between protamine and oligonucleotide, especially
phosphorothioates. To overcome these problems, human serum albumin (HSA) as a
non-toxic, biodegradable macromolecule was introduced as protective colloid.
The assembly process of AlPrO-NP was investigated by small angle X-ray
scattering (SAXS), fluorescence correlation spectroscopy (FCS), photon
correlation spectroscopy (PCS) measurements and scanning electron microscopy
(SEM). 'Initial complexes' of HSA and protamine sulphate with a mean
hydrodynamic diameter (dh) of about 10-14 nm were found. After adding
oligonucleotides (unmodified, phosphorothioate DNA and small interfering RNA),
nanoparticles (NPs) were assembled in water and in isotonic media with a dh in
a range of 230-320 nm for most preparations. The chemical composition of the
particles was investigated by high performance liquid chromatography and
fluorescence spectrometry. The whole amount of oligonucleotides (30 microg) was
entrapped into the particles at a 1:2 mass ratio (oligonucleotide/protamine).
Approximately 7-10% (w/w) of the HSA was bound to the particles. The surface
charge of the particles ranged from about +12 to -60 mV depending on the
protamine concentration and the ionic conditions. The size and the molecular
weight of the components, initial complexes and two model NP preparations were
calculated from FCS data. These data verified the PCS, SEM and SAXS
measurements.
Measuring
protein-protein interactions inside living cells using single color
fluorescence correlation spectroscopy. Application to human immunodeficiency
virus type 1 integrase and LEDGF/p75. Maertens, G., Vercammen, J., Debyser, Z., and Engelborghs,
Y., FASEB J, 2005. 19(8): pp. 1039-41
Recently we described
the interaction of human immunodeficiency virus type 1 (HIV-1)1 integrase (IN)
with a cellular protein, lens epithelium-derived growth factor/transcription
co-activator p75 (LEDGF/p75). We now present the study of the diffusion
behavior of the three independent domains of IN and LEDGF/p75 using
fluorescence correlation microscopy (FCM). We show that diffusion in the cell
of the different enhanced green fluorescent protein (EGFP) fusion proteins is
described by two components with different fractions and that the average
parameters in the nucleus are comparable with those in the cytoplasm. In
addition, we demonstrate that specific interaction between EGFP-fused HIV-1 IN
and LEDGF/p75 results in a shift in diffusion coefficient (D). The opposite
shift was observed in an IN-deletion mutant that does not exhibit LEDGF/p75
binding or in a LEDGF/p75 knock-down experiment using siRNA. We thus
demonstrate that protein-protein interactions can be studied in living cells,
using single-color FCM (scFCM).
Membrane perturbation
effects of peptides derived from the N-termini of unprocessed prion proteins. Magzoub, M., Oglecka, K.,
Pramanik, A., Goran Eriksson, L.E., and Graslund, A., Biochim Biophys Acta,
2005. 1716(2): pp. 126-36
Peptides derived from
the unprocessed N-termini of mouse and bovine prion proteins (mPrPp and bPrPp,
respectively), comprising hydrophobic signal sequences followed by charged
domains (KKRPKP), function as cell-penetrating peptides (CPPs) with live cells,
concomitantly causing toxicity. Using steady-state fluorescence techniques,
including calcein leakage and polarization of a membrane probe (diphenylhexatriene,
DPH), as well as circular dichroism, we studied the membrane interactions of
the peptides with large unilamellar phospholipid vesicles (LUVs), generally
with a 30% negative surface charged density, comparing the effects with those
of the CPP penetratin (pAntp) and the pore-forming peptide melittin. The prion
peptides caused significant calcein leakage from LUVs concomitant with
increased membrane ordering. Fluorescence correlation spectroscopy (FCS)
studies of either rhodamine-entrapping (REVs) or rhodamine-labeled (RLVs)
vesicles, showed that addition of the prion peptides resulted in significant
release of rhodamine from the REVs without affecting the overall integrity of
the RLVs. The membrane leakage effects due to the peptides had the following
order of potency: melittin>mPrPp>bPrPp>pAntp. The membrane
perturbation effects of the N-terminal prion peptides suggest that they form
transient pores (similar to melittin) causing toxicity in parallel with their
cellular trafficking.
Modeling the endosomal
escape of cell-penetrating peptides: transmembrane pH gradient driven
translocation across phospholipid bilayers. Magzoub, M., Pramanik, A., and Graslund, A., Biochemistry,
2005. 44(45): pp. 14890-7
Cell-penetrating
peptides (CPPs) are able to mediate the efficient cellular uptake of a wide
range of cargoes. Internalization of a number of CPPs requires uptake by
endocytosis, initiated by binding to anionic cell surface heparan sulfate (HS),
followed by escape from endosomes. To elucidate the endosomal escape mechanism,
we have modeled the process for two CPPs: penetratin (pAntp) and the N-terminal
signal peptide of the unprocessed bovine prion protein (bPrPp). Large
unilamellar phospholipid vesicles (LUVs) were produced encapsulating either
peptide, and an ionophore, nigericin, was used to create a transmembrane pH
gradient (DeltapH(mem), inside acidic) similar to the one arising in endosomes
in vivo. In the absence of DeltapH(mem), no pAntp escape from the LUVs is
observed, while a fraction of bPrPp escapes. In the presence of DeltapH(mem), a
significant amount of pAntp escapes and an even higher degree of bPrPp escape
takes place. These results, together with the differences in kinetics of
escape, indicate different escape mechanisms for the two peptides. A minimum
threshold peptide concentration exists for the escape of both peptides.
Coupling of the peptides to a cargo reduces the fraction escaping, while
complexation with HS significantly hinders the escape. Fluorescence correlation
spectroscopy results show that during the escape process the LUVs are intact.
Taken together, these results suggest a model for endosomal escape of CPPs:
DeltapH(mem)-mediated mechanism, following dissociation from HS of the
peptides, above a minimum threshold peptide concentration, in a process that
does not involve lysis of the vesicles.
G-protein-coupled
glucocorticoid receptors on the pituitary cell membrane. Maier, C., Runzler, D.,
Schindelar, J., Grabner, G., Waldhausl, W., Kohler, G., and Luger, A., J
Cell Sci, 2005. 118(Pt 15): pp. 3353-61
Rapid, nongenomic
actions of glucocorticoids (GCs) have been well documented, but information
about putative membrane receptors that mediate them is scarce. We used
fluorescence correlation spectroscopy to search for membrane GC-binding on the
mouse pituitary cell line AtT-20. A slowly diffusing fraction (tau3; diffusion
constant 3x10(-10) cm2 s-1) of fluorescein-labeled dexamethasone on the cell
membrane corresponds to fluorescein-dexamethasone binding. Preincubation
experiments were performed to test binding specificity: a 500-fold excess of
unlabeled dexamethasone abolished subsequent fluorescein-dexamethasone membrane
binding from 58+/-2 (control) to 8+/-1 (% of tau3, mean+/-s.e.m.), the natural
ligand corticosterone prevented it partially (29+/-2), while the sex steroids
estradiol (56+/-4) and progesterone (50+/-4) and the GC-receptor antagonist
RU486 (56+/-2) had no effect. Preincubation with pertussis toxin resulted in
disappearance of the slowest diffusion component (11+/-4) suggesting
association of the receptor with a G-protein. Varying the concentration of
fluorescein-dexamethasone showed that membrane binding is highly cooperative
with an apparent Kd of 180 nM and Bmax of 230 nM. Taken together, these results
demonstrate high-affinity GC-binding on the cell membrane of AtT-20 cells with
characteristics distinct from intracellular binding.
Translocation of
dynorphin neuropeptides across the plasma membrane. A putative mechanism of
signal transmission.
Marinova, Z., Vukojevic, V., Surcheva, S., Yakovleva, T., Cebers, G., Pasikova,
N., Usynin, I., Hugonin, L., Fang, W., Hallberg, M., Hirschberg, D., Bergman,
T., Langel, U., Hauser, K.F., Pramanik, A., Aldrich, J.V., Graslund, A.,
Terenius, L., and Bakalkin, G., J Biol Chem, 2005. 280(28): pp.
26360-70
Several peptides,
including penetratin and Tat, are known to translocate across the plasma
membrane. Dynorphin opioid peptides are similar to cell-penetrating peptides in
a high content of basic and hydrophobic amino acid residues. We demonstrate
that dynorphin A and big dynorphin, consisting of dynorphins A and B, can
penetrate into neurons and non-neuronal cells using confocal fluorescence
microscopy/immunolabeling. The peptide distribution was characterized by
cytoplasmic labeling with minimal signal in the cell nucleus and on the plasma
membrane. Translocated peptides were associated with the endoplasmic reticulum
but not with the Golgi apparatus or clathrin-coated endocytotic vesicles. Rapid
entry of dynorphin A into the cytoplasm of live cells was revealed by
fluorescence correlation spectroscopy. The translocation potential of dynorphin
A was comparable with that of transportan-10, a prototypical cell-penetrating
peptide. A central big dynorphin fragment, which retains all basic amino acids,
and dynorphin B did not enter the cells. The latter two peptides interacted
with negatively charged phospholipid vesicles similarly to big dynorphin and
dynorphin A, suggesting that interactions of these peptides with phospholipids
in the plasma membrane are not impaired. Translocation was not mediated via
opioid receptors. The potential of dynorphins to penetrate into cells
correlates with their ability to induce non-opioid effects in animals.
Translocation across the plasma membrane may represent a previously unknown
mechanism by which dynorphins can signal information to the cell interior.
Single-molecule
studies of diffusion by oligomer-bound dyes in organically modified
sol-gel-derived silicate films. Martin-Brown, S.A., Fu, Y., Saroja, G., Collinson, M.M.,
and Higgins, D.A., Anal Chem, 2005. 77(2): pp. 486-94
Single-molecule
fluorescence spectroscopy is used to study dye diffusion within organically
modified silicate (ORMOSIL) films. ORMOSIL films are prepared from sols
containing tetraethoxysilane and isobutyltrimethoxysilane in 2:1 and 1:9 molar
ratios. Nile red and a new silanized form of nile red that can be covalently
attached to the silicate matrix are used as fluorescent probe molecules. The
number and rate of single molecules diffusing through these films increases
dramatically with increasing film organic content. Autocorrelation of the
fluorescence images yields a quantitative measure of the relative populations
of fixed and diffusing species. Surprisingly, both "free" and silicate-bound
nile red exhibit relatively facile translational motions.
Single-molecule/single-point fluorescence correlation spectroscopy (FCS) is
used to measure the dye diffusion coefficients in submicrometer-scale film
regions. The most common diffusion coefficients for "free" and
silicate-bound nile red molecules in the 1:9 films are 3.9 x 10(-10) and 1.6 x
10(-10) cm(2)/s, respectively. The unexpectedly rapid diffusion of
silicate-bound nile red is attributed to the presence of liquidlike silicate
oligomers in the films. A lower bound for the molecular weight of the oligomers
is estimated at 2900. Bulk solution-phase FCS experiments performed on
"free" and silicate-bound nile red species extracted into chloroform
solutions provide valuable support for these conclusions. Comparison of the
results derived from experimental and simulated time transients indicates film
heterogeneity occurs on sub-100-nm-length scales and likely results from the
presence of inorganic- and organic-rich domains.
New fluorescence
correlation spectroscopy enabling direct observation of spatiotemporal
dependence of diffusion constants as an evidence of anomalous transport in
extracellular matrices. Masuda, A., Ushida, K., and Okamoto, T., Biophys J, 2005. 88(5):
pp. 3584-91
The potential of
fluorescence correlation spectroscopy (FCS) is extended to enable the direct
observation of anomalous subdiffusion (ASD) in inhomogeneous media that are of
great importance particularly in many biological systems, such as membranes,
cytoplasm, and extracellular matrices (ECMs). Because ASD can be confirmed by
monitoring the spatiotemporal dependence of observable diffusion coefficients
(D(obs)), the size of the effective confocal volume (V(eff)) for FCS sampling
(sampling volume) was continuously changed on a scale of 300-500 nm using a
motorized variable beam expander through which an illuminating laser beam
passes. This new method, namely, sampling-volume-controlled (SVC)-FCS, was
applied to the analysis of hyaluronan (HA) aqueous solutions where the D(obs)
of light-emitting solute (Alexa 488) markedly changed, corresponding to the
change in V(eff) (220-340 nm in the half-axis), because the network structure
of HA of 7-33 nm (nanostructure) interferes with the material transport within
it. The results indicate that moderate ASD may occur even in the presence of a
small amount ( approximately 0.1 wt %) of HA in ECM. Because the change in
D(obs) along with the traveling distance (the mean-square displacement) can be
identified even in systems with no deformation of the autocorrelation function,
this technique has a great potential for general applications to many
biological systems in which ASD shows complex time and space dependences.
An electrostatic
engine model for autoinhibition and activation of the epidermal growth factor
receptor (EGFR/ErbB) family. McLaughlin, S., Smith, S.O., Hayman, M.J., and Murray, D., J Gen
Physiol, 2005. 126(1): pp. 41-53
We propose a new
mechanism to explain autoinhibition of the epidermal growth factor receptor
(EGFR/ErbB) family of receptor tyrosine kinases based on a structural model
that postulates both their juxtamembrane and protein tyrosine kinase domains
bind electrostatically to acidic lipids in the plasma membrane, restricting
access of the kinase domain to substrate tyrosines. Ligand-induced dimerization
promotes partial trans autophosphorylation of ErbB1, leading to a rapid rise in
intracellular [Ca(2+)] that can activate calmodulin. We postulate the
Ca(2+)/calmodulin complex binds rapidly to residues 645--660 of the
juxtamembrane domain, reversing its net charge from +8 to -8 and repelling it
from the negatively charged inner leaflet of the membrane. The repulsion has
two consequences: it releases electrostatically sequestered
phosphatidylinositol 4,5-bisphosphate (PIP(2)), and it disengages the kinase
domain from the membrane, allowing it to become fully active and phosphorylate
an adjacent ErbB molecule or other substrate. We tested various aspects of the
model by measuring ErbB juxtamembrane peptide binding to phospholipid vesicles
using both a centrifugation assay and fluorescence correlation spectroscopy;
analyzing the kinetics of interactions between ErbB peptides, membranes, and
Ca(2+)/calmodulin using fluorescence stop flow; assessing ErbB1 activation in
Cos1 cells; measuring fluorescence resonance energy transfer between ErbB
peptides and PIP(2); and making theoretical electrostatic calculations on
atomic models of membranes and ErbB juxtamembrane and kinase domains.
Characterizing observation
volumes and the role of excitation saturation in one-photon fluorescence
fluctuation spectroscopy. Nagy, A., Wu, J., and Berland, K.M., J Biomed Opt, 2005. 10(4):
p. 44015
Fluorescence correlation
spectroscopy (FCS) and related distribution analysis techniques have become
extremely important and widely used research tools for analyzing the dynamics,
kinetics, interactions, and mobility of biomolecules. However, it is not widely
recognized that photophysical dynamics can dramatically influence the
calibration of fluctuation spectroscopy instrumentation. While the basic
theories for fluctuation spectroscopy methods are well established, there have
not been quantitative models to characterize the photophysical-induced
variations observed in measured fluctuation spectroscopy data under varied
excitation conditions. We introduce quantitative models to characterize how the
fluorescence observation volumes in one-photon confocal microscopy are modified
by excitation saturation as well as corresponding models for the effect of the
volume changes in FCS. We introduce a simple curve fitting procedure to model
the role of saturation in FCS measurements and demonstrate its accuracy in
fitting measured correlation curves over a wide range of excitation conditions.
Observation volumes
and {gamma}-factors in two-photon fluorescence fluctuation spectroscopy. Nagy, A., Wu, J., and Berland,
K.M., Biophys J, 2005. 89(3): pp. 2077-90
Fluorescence fluctuation
spectroscopy has become an important measurement tool for investigating
molecular dynamics, molecular interactions, and chemical kinetics in biological
systems. Although the basic theory of fluctuation spectroscopy is well
established, it is not widely recognized that saturation of the fluorescence
excitation can dramatically alter the size and profile of the fluorescence
observation volume from which fluorescence fluctuations are measured, even at
relatively modest excitation levels. A precise model for these changes is
needed for accurate analysis and interpretation of fluctuation spectroscopy
data. We here introduce a combined analytical and computational approach to
characterize the observation volume under saturating conditions and demonstrate
how the variation in the volume is important in two-photon fluorescence
correlation spectroscopy. We introduce a simple approach for analysis of
fluorescence correlation spectroscopy data that can fully account for the
effects of saturation, and demonstrate its success for characterizing the
observed changes in both the amplitude and relaxation timescale of measured
correlation curves. We also discuss how a quantitative model for the observed
phenomena may be of broader importance in fluorescence fluctuation
spectroscopy.
A microscopic view of
miniprotein folding: enhanced folding efficiency through formation of an
intermediate.
Neuweiler, H., Doose, S., and Sauer, M., Proc Natl Acad Sci U S A, 2005.
102(46): pp. 16650-5
The role of polypeptide
collapse and formation of intermediates in protein folding is still under
debate. Miniproteins, small globular peptide structures, serve as ideal model
systems to study the basic principles that govern folding. Experimental
investigations of folding dynamics of such small systems, however, turn out to
be challenging, because requirements for high temporal and spatial resolution
have to be met simultaneously. Here, we demonstrate how selective quenching of
an extrinsic fluorescent label by the amino acid tryptophan (Trp) can be used
to probe folding dynamics of Trp-cage (TC), the smallest protein known to date.
Using fluorescence correlation spectroscopy, we monitor folding transitions as
well as conformational flexibility in the denatured state of the 20-residue
protein under thermodynamic equilibrium conditions with nanosecond time
resolution. Besides microsecond folding kinetics, we reveal hierarchical
folding of TC, hidden to previous experimental studies. We show that specific
collapse of the peptide to a molten globule-like intermediate enhances folding
efficiency considerably. A single point mutation destabilizes the intermediate,
switching the protein to two-state folding behavior and slowing down the
folding process. Our results underscore the importance of preformed structure
in the denatured state for folding of even the smallest globular structures. A
unique method emerges for monitoring conformational dynamics and ultrafast
folding events of polypeptides at the nanometer scale.
Dead-time distortion
in fluorescence correlation measurements. Nishimura, G. and Kinjo, M., Appl Opt, 2005. 44(17):
pp. 3458-67
The dead time of the
detector significantly distorts the fluorescence correlation function of
fluorescent particles in solution. This distortion of the correlation function
is similar to the saturation effect of the correlation function in a high-power
excitation region. The correlation amplitude is significantly reduced by the
dead time. The deviations in the number of molecules and the diffusion time are
empirically given by the deviation of the fluorescence intensity linearity. The
empirical curves of the deviations can be applied to the systematic error
estimation of the parameters. The proportionality of the number of molecules to
the concentration of fluorophores is no longer maintained with a large dead
time, although almost all of the proportionality of the diffusion time to the
inverse diffusion constant remains. This fact makes the dead-time effect
different from the saturation effect, which is due to photokinetics. In
practice, these distortions can be reduced by use of a smaller excitation power
in which the proportionality of the fluorescence intensity is maintained.
Monitoring the
interaction between DNA and a transcription factor (MEF2A) using fluorescence
correlation spectroscopy. Octobre, G., Lemercier, C., Khochbin, S., Robert-Nicoud, M., and
Souchier, C., C R Biol, 2005. 328(12): pp. 1033-40
Fluorescence correlation
spectroscopy (FCS) is an analytical method that allows distinguishing different
populations of fluorescent probes in solution and provides data on their
concentrations and their diffusion coefficients. FCS was used to characterize
the interaction of the transcription factor (MEF2A) with its DNA target
sequence. The myocyte enhancer factor 2 (MEF2) belongs to the MADS-box family
and activates transcription of numerous muscle genes during myogenesis.
Measurements were made using TAMRA-labelled oligonucleotide duplexes derived
from a wild type (WT) or a mutated MEF2 target gene. Binding of the protein to
the WT DNA resulted in significant changes of the diffusion. Specificity of the
interaction was confirmed using the mutated DNA. Bound to free probe ratios
were determined at different MEF2A concentrations and the apparent equilibrium
dissociation constant K(D) for the full-length MEF2A was estimated.
Hansenula polymorpha
Pex20p is an oligomer that binds the peroxisomal targeting signal 2 (PTS2). Otzen, M., Wang, D., Lunenborg,
M.G., and van der Klei, I.J., J Cell Sci, 2005. 118(Pt 15): pp.
3409-18
We have cloned and
characterized the Hansenula polymorpha PEX20 gene. The HpPEX20 gene encodes a
protein of 309 amino acids (HpPex20p) with a calculated molecular mass of
approximately 35 kDa. In cells of an HpPEX20 disruption strain, PTS2 proteins
were mislocalized to the cytosol, whereas PTS1 matrix protein import proceeded
normally. Also, the PTS2 proteins amine oxidase and thiolase were normally
assembled and active in these cells, suggesting HpPex20p is not involved in
oligomerization/activation of these proteins. Localization studies revealed
that HpPex20p is predominantly associated with peroxisomes. Using fluorescence
correlation spectroscopy we determined the native molecular mass of purified
HpPex20p and binding of a synthetic peptide containing a PTS2 sequence. The
data revealed that purified HpPex20p forms oligomers, which specifically bind
PTS2-containing peptides.
Calculation of photon
count number distributions via Master equations. Palo, K., Mets, U., Loorits, V.,
and Kask, P., Biophys J, 2005
Fitting of photon count
number histograms is a way of analysis of fluorescence intensity fluctuations,
a successor to fluorescence correlation spectroscopy. First versions of the
theory for calculating photon count number distributions have assumed constant
emission intensity by a molecule during a counting time interval. For a long
time a question has remained unanswered: to what extent is this assumption
violated in experiments. Here we present a theory of photon count number
distributions that takes account of intensity fluctuations during a counting
time interval. Theoretical count number distributions are calculated via a
numerical solution of Master equations (ME), which is a set of differential
equations describing diffusion, singlet-triplet transitions and photon
emission. After-pulsing and dead time corrections are also included. The
ME-theory is tested by fitting a series of photon count number histograms
corresponding to different lengths of the counting time interval. Compared to
the first version of FIMDA theory introduced in 2000, the fit quality is
significantly improved. It is discussed how a theory of photon count number
distributions, which assumes constant emission intensity during a counting time
interval, may also yield a good fit quality. We argue that the spatial
brightness distribution used in calculations of the fit curve is not the
"true" spatial brightness distribution. Instead, a number of dynamic
processes, which cause fluorescence intensity fluctuations, are indirectly
taken into account via the adjustment parameters.
Effect of bin time on
the photon counting histogram for one-photon excitation. Perroud, T.D., Huang, B., and
Zare, R.N., Chemphyschem, 2005. 6(5): pp. 905-12
We have demonstrated
that our photon counting histogram (PCH) model with the correction for
one-photon excitation is valid at multiple bin times. The fitted apparent
brightness and concentration follow the three-dimensional diffusion model. More
importantly, the semi-empirical parameter, F, introduced in the PCH model for
one-photon excitation to correct for the non-Gaussian shape of the observation
volume, shows small variations with different bin times. These variations are
consistent with the physical interpretation of F, and they do not affect the
resolving power of the PCH model for one-photon excitation. Based on these
findings, we extend the time-independent PCH analysis to time-dependent photon
counting multiple histograms (PCMH). This model considers the effect of bin
time on the PCH parameters in a way that is similar to fluorescence intensity
multiple distribution analysis (FIMDA). From the same set of data, PCMH
extracts time-dependent parameters (diffusion time and triplet-state relaxation
time) as well as time-independent parameters (true specific brightness and true
average number of molecules). Given a three- to fourfold experimental
difference in molecular brightness, we find that PCMH can resolve each species
in a two-species sample and extract their respective diffusion times even when
fluorescence correlation spectroscopy cannot.
The nanopore
connection to cell membrane unitary permeability. Peters, R., Traffic, 2005. 6(3):
pp. 199-204
Artificial nanopores
have recently emerged as versatile tools for analyzing and sorting single
molecules at high speed. However, the biological cell has already developed a
large set of sophisticated protein nanopores that are able to selectively
translocate all types of molecules through membranes. Therefore, hybrid devices
combining artifical solid-state with biomimetic protein nanopores appear to us
as a particularly promising approach to the creation of powerful diagnostic,
preparative and therapeutic devices. Here, we discuss a technique, optical
single-transporter recording (OSTR), in which arrays of artificial micropores
and nanopores are employed to analyze protein nanopores of cellular membranes.
After briefly summarizing some salient features of OSTR, the technique is
compared with the electrical patch clamp method and the first results of our
efforts to amalgamate optical and electrical recording are described. Finally,
prospects for combining OSTR with 4Pi microscopy, single-molecule fluorescence
spectroscopy and fluorescence correlation spectroscopy are discussed.
Imaging the secretory
pathway: the past and future impact of live cell optical techniques. Presley, J.F., Biochim Biophys
Acta, 2005. 1744(3): pp. 259-72
Classically, the
secretory pathway has been studied using a combination of electron microscopic,
biochemical and genetic approaches. In the last 20 years with the arrival of
molecular biology and epitope tagging, fluorescence microscopy has become more
important than previously. Moreover, with the common availability of Green
Fluorescent Protein (GFP) and confocal microscopes in the last 10 years, live
cell imaging has become a major experimental approach. This review highlights
the impact of the recent introduction of single-cell quantitative time-lapse
imaging and photobleach techniques on the study of the secretory pathway, and
the potential impact of those optical techniques which may play a significant
future role in the study of the Golgi apparatus and the secretory pathway.
Particular attention is paid to techniques (Fluorescence Resonance Energy
Transfer, Fluorescence Correlation Spectroscopy) which can monitor protein-protein
interactions in living cells.
Fluorescence
correlation spectroscopy studies of diffusion of a weak polyelectrolyte in
aqueous solutions.
Pristinski, D., Kozlovskaya, V., and Sukhishvili, S.A., J Chem Phys,
2005. 122(1): p. 14907
We apply fluorescent
correlation spectroscopy (FCS) to investigate solution dynamics of a synthetic
polyelectrolyte, i.e., a weak polycarboxylic acid in aqueous solutions. The
technique brings single molecule sensitivity and molecular specificity to
dynamic measurements of polyelectrolyte solutions. Translational diffusion of
Alexa-labeled poly(methacrylic acid), PMAA*, chains was studied in very dilute,
10(-4) mg/ml, solutions as a function of solution pH and ionic strength. The
observed changes in diffusion coefficients were consistent with about twofold
expansion of PMAA* coils when pH was changed from 5 to 8, and with chain
contraction for alkaline metal ion concentrations from 0.01 to 0.1 M. The
dependence of the hydrodynamic size of PMAA* chains on the counterion type
followed the sequence: Li(+)>Na(+) approximately equal to Cs(+)>K(+). The
dependence of translational diffusion on polyacid concentration was weak at the
low concentration limit, but chain motions were significantly slower at higher
polymer concentrations when PMAA chains overlapped. Finally, measurements of
dynamics of PMAA* chains in "salt-free" solutions showed that
self-diffusion of PMAA* chains significantly slowed down when PMAA
concentration was increased, probably reflecting the sensitivity of PMAA*
translational motions to the onset of interchain domain formation. These
results illustrate the utility of the FCS technique for studying hydrodynamic
sizes of polyelectrolyte coils in response to variation in solution pH or
concentration of salt and polyelectrolytes. They also suggest that FCS will be
a promising technique for selective observation of the dynamics of
polyelectrolyte components in complex polymer mixtures.
Alpha 1-antitrypsin
polymerization: a fluorescence correlation spectroscopic study. Purkayastha, P., Klemke, J.W.,
Lavender, S., Oyola, R., Cooperman, B.S., and Gai, F., Biochemistry,
2005. 44(7): pp. 2642-9
Alpha(1)-antitrypsin
(AT) is the most abundantly circulating human proteinase inhibitor in the
serpin family. The polymerization of AT, leading to alpha(1)-antitrypsin
deficiency, has been studied extensively in vitro by a variety of ensemble
methods. Here we report the use of fluorescence correlation spectroscopy to
gain further insight into this process. Measurements of the distributions of
diffusion times of polymerizing AT, carried out at 45, 50, and 55 degrees C,
clearly show the existence of a kinetic lag phase, during which short oligomers
are formed, prior to the formation of heterogeneous mixtures of longer
polymers, and suggest that long polymers, which appear to be metastable, are
produced through the condensation of shorter oligomers.
FRET-FCS as a tool to
evaluate the stability of oligonucleotide drugs after intracellular delivery. Remaut, K., Lucas, B., Braeckmans,
K., Sanders, N.N., De Smedt, S.C., and Demeester, J., J Control Release,
2005. 103(1): pp. 259-71
The intracellular
degradation of single-stranded, double-labeled oligonucleotides (ONs) was
studied by following the disappearance of Fluorescence Resonance Energy
Transfer (FRET) between the rhodamine green and Cy5 fluorophores attached to
respectively the 3' and 5' end of the ONs. The green and red fluorescence
intensities upon rhodamine green excitation were monitored using the
ultra-sensitive detectors of a dual-color Fluorescence Correlation Spectroscopy
(FCS) instrument. The ratio of the red to green fluorescence (R/G ratio) as
obtained from such FRET-FCS measurements showed to give accurate information on
the integrity of the ONs, without the need for additional auto- or
cross-correlation analysis of the registered fluorescence intensity
fluctuations. Intracellular measurements revealed that most of the 40mer
phosphodiester ONs were degraded before they entered the nucleus. For the 20mer
phosphodiester ONs, this degradation occurred more slowly, and both intact and
degraded ONs entered the nucleus. For the 20mer phosphorothioate ONs, no
intracellular degradation was observed during the measured time period. The
sensitive detection of the intracellular fluorescence by the FCS setup will be
particularly useful in situations where the expected fluorescence is too low to
be detected by FRET-imaging as may occur after intracellular delivery of ONs by
cationic carriers.
Protection of
oligonucleotides against nucleases by pegylated and non-pegylated liposomes as
studied by fluorescence correlation spectroscopy. Remaut, K., Lucas, B., Braeckmans,
K., Sanders, N.N., Demeester, J., and De Smedt, S.C., J Control Release,
2005. 110(1): pp. 212-26
Antisense phosphodiester
oligonucleotides (ONs), complexed to carriers such as cationic liposomes,
inhibit the production of proteins. The biochemical and biophysical phenomena
that govern the extent of this inhibition are still not fully understood. Major
biological barriers limiting a pronounced antisense effect are the cellular
entry and endosomal escape of the ONs containing liposomes, the release of the
ONs from the liposomes and the extra- and intracellular degradation of the ONs.
In this paper we focus on the latter barrier and evaluate, by fluorescence
correlation spectroscopy (FCS), to what extent phosphodiester ONs complexed to
DOTAP/DOPE liposomes, are protected against degradation by nucleases. Liposomes
studied were either with or without a polyethyleneglycol (PEG) moiety at the
surface. Using non-pegylated liposomes the phosphodiester ONs were initially
adequately protected when exposed to DNase I. Indeed, in the mechanism for
lipoplex formation as suggested by others, the ONs become trapped between lipid
bilayers and are therefore shielded from the environment. However, after a few
hours the phosphodiester ONs no longer stayed intact. This was explained by a
gradual fusion of the lipoplexes in time thereby spontaneously releasing
phosphodiester ONs. Using pegylated liposomes, a substantial fraction of the
phosphodiester ONs degraded immediately after exposing the complexes to DNase
I. Based on experimental evidence we suggest that the presence of the
PEG-chains influences lipoplex formation so that the ONs are not trapped between
lipid bilayers and therefore remain accessible by the DNase I enzyme.
Rapid, Diffusional
Shuttling of Poly(A) RNA between Nuclear Speckles and the Nucleoplasm. Ritland Politz, J.C., Tuft, R.A.,
Prasanth, K.V., Baudendistel, N., Fogarty, K.E., Lifshitz, L.M., Langowski, J.,
Spector, D.L., and Pederson, T., Mol Biol Cell, 2005
Monitoring Editor: Greg
Matera Speckles are nuclear bodies that contain pre-mRNA splicing factors and
polyadenylated RNA. Because nuclear poly(A) RNA consists of both mRNA
transcripts and nucleus-restricted RNAs, we tested whether poly(A) RNA in
speckles is dynamic or rather an immobile, perhaps structural, component.
Fluorescein-labeled oligo(dT) was introduced into HeLa cells stably expressing
a red fluorescent protein chimera of the splicing factor SC35 and allowed to
hybridize. Fluorescence correlation spectroscopy (FCS) showed that the mobility
of the tagged poly(A) RNA was virtually identical in both speckles and at
random nucleoplasmic sites. This same result was observed in
photoactivation-tracking studies in which caged fluorescein-labeled oligo(dT)
was used as hybridization probe and the rate of movement away from either a
speckle or nucleoplasmic site was monitored using digital imaging microscopy
after photoactivation. Furthermore, the tagged poly(A) RNA was observed to
rapidly distribute throughout the entire nucleoplasm and other speckles,
regardless whether the tracking observations were initiated in a speckle or the
nucleoplasm. Finally, in both FCS and photoactivation-tracking studies, a
temperature reduction from 37 degrees C to 22 degrees C had no discernable
effect on the behavior of poly(A) RNA in either speckles or the nucleoplasm,
strongly suggesting that its movement in and out of speckles does not require
metabolic energy.
lambda-Repressor
oligomerization kinetics at high concentrations using fluorescence correlation
spectroscopy in zero-mode waveguides. Samiee, K.T., Foquet, M., Guo, L., Cox, E.C., and
Craighead, H.G., Biophys J, 2005. 88(3): pp. 2145-53
Fluorescence correlation
spectroscopy (FCS) has demonstrated its utility for measuring transport
properties and kinetics at low fluorophore concentrations. In this article, we
demonstrate that simple optical nanostructures, known as zero-mode waveguides,
can be used to significantly reduce the FCS observation volume. This, in turn,
allows FCS to be applied to solutions with significantly higher fluorophore
concentrations. We derive an empirical FCS model accounting for one-dimensional
diffusion in a finite tube with a simple exponential observation profile. This
technique is used to measure the oligomerization of the bacteriophage lambda
repressor protein at micromolar concentrations. The results agree with previous
studies utilizing conventional techniques. Additionally, we demonstrate that
the zero-mode waveguides can be used to assay biological activity by measuring
changes in diffusion constant as a result of ligand binding.
Lipid--protein
interactions revealed by two-photon microscopy and fluorescence correlation
spectroscopy.
Sanchez, S.A. and Gratton, E., Acc Chem Res, 2005. 38(6): pp.
469-77
Cellular processes
involve a multitude of chemical reactions that must be kept in delicate
equilibrium to maintain cellular homeostasis. Powerful biophysical techniques
are needed to measure the localization and concentration of target molecules as
well as to quantify complex molecular processes in model and in vivo systems.
Two-photon microscopy and fluorescence correlation spectroscopy (FCS) can
measure association and dynamics of appropriate molecules under equilibrium
conditions. FCS provides information on motility (diffusion coefficients),
concentration (number of particles), association (molecular brightness), and
localization (image) of the target molecules. All of this information, in
conjunction with computational modeling techniques, can help us to better
understand the network of complex molecular interactions, which are at the
basis of cellular processes. Fluorescence imaging techniques add the beauty of
visualization to the scientific information. Photons emitted by a fluorescent
dye are digitized, and the associated spatial information and intensity can be
translated into different colors and shades providing the researcher not only
with quantitative intensity information but also with spatial resolution and
visual comprehension of two- or three-dimensional images. In this Account, we
review the use of two-photon excitation microscopy and FCS in the study of
lipid-protein interactions. We discuss these new methodologies and techniques,
and we present examples of different complexity from qualitative to
quantitative, from simple model systems to studies in living cells.
Metal-free catalysts
for the hydrolysis of RNA derived from guanidines, 2-aminopyridines, and
2-aminobenzimidazoles.
Scheffer, U., Strick, A., Ludwig, V., Peter, S., Kalden, E., and Gobel, M.W., J
Am Chem Soc, 2005. 127(7): pp. 2211-7
2-aminopyridine and
2-aminobenzimidazole were chosen as structural analogues to substitute
guanidinium groups in receptor molecules designed as phosphoryl transfer
catalysts. Shifting the pKa of the guanidinium analogues toward 7 was expected
to raise catalytic activities in aqueous buffer. Although the pKa's of both
heterocycles are similar (6.2 and 7.0), only 2-aminobenzimidazole led to active
RNA cleavers. All cleavage assays were run with fluorescently labeled
substrates and a DNA sequencer. RNase contaminations would degrade RNA
enantioselectively. In contrast, achiral catalysts such as 9b and 10b
necessarily induce identical cleavage patterns in RNA and its mirror image.
This principle allowed us to safely rule out contamination effects in this
study. The most active catalysts, tris(2-aminobenzimidazoles) 9b and 10b, were
shown by fluorescence correlation spectroscopy (FCS) to aggregate with
oligonucleotides. However, at very low concentrations the compounds are still
active in the nonaggregated state. Conjugates of 10b with antisense
oligonucleotides or RNA binding peptides, therefore, will be promising
candidates as site specific artificial ribonucleases.
The oxidation state
of a protein observed molecule-by-molecule. Schmauder, R., Librizzi, F., Canters, G.W.,
Schmidt, T., and Aartsma, T.J., Chemphyschem, 2005. 6(7): pp.
1381-6
We report the
observation of the redox state of the blue copper protein azurin on the
single-molecule level. The fluorescence of a small fluorophore attached to the
protein is modulated by the change in absorption of the copper center via
fluorescence resonance energy transfer (FRET). In our model system, the
fluorescence label Cy5 was coupled to azurin from Pseudomonas aeruginosa via
cysteine K27C. The Cy5 fluorescence was partially quenched by the absorption of
the copper center of azurin in its oxidized state. In the reduced state,
absorption is negligible, and thus no quenching occurs. We report on
single-molecule measurements, both in solution by using fluorescence
correlation spectroscopy (FCS) combined with fluorescence intensity
distribution analysis (FIDA), and on surfaces by using wide-field fluorescence
microscopy.
Substrate binding
stoichiometry and kinetics of the norepinephrine transporter. Schwartz, J.W., Novarino, G., Piston,
D.W., and DeFelice, L.J., J Biol Chem, 2005. 280(19): pp.
19177-84
The human norepinephrine
(NE) transporter (hNET) attenuates neuronal signaling by rapid NE clearance
from the synaptic cleft, and NET is a target for cocaine and amphetamines as
well as therapeutics for depression, obsessive-compulsive disorder, and
post-traumatic stress disorder. In spite of its central importance in the
nervous system, little is known about how NET substrates, such as NE,
1-methyl-4-tetrahydropyridinium (MPP+), or amphetamine, interact with NET at
the molecular level. Nor do we understand the mechanisms behind the transport
rate. Previously we introduced a fluorescent substrate similar to MPP+, which
allowed separate and simultaneous binding and transport measurement (Schwartz,
J. W., Blakely, R. D., and DeFelice, L. J. (2003) J. Biol. Chem. 278,
9768-9777). Here we use this substrate,
4-(4-(dimethylamino)styrl)-N-methyl-pyridinium (ASP+), in combination with
green fluorescent protein-tagged hNETs to measure substrate-transporter
stoichiometry and substrate binding kinetics. Calibrated confocal microscopy
and fluorescence correlation spectroscopy reveal that hNETs, which are
homomultimers, bind one substrate molecule per transporter subunit. Substrate
residence at the transporter, obtained from rapid on-off kinetics revealed in
fluorescence correlation spectroscopy, is 526 micros. Substrate residence
obtained by infinite dilution is 1000 times slower. This novel examination of
substrate-transporter kinetics indicates that a single ASP+ molecule binds and
unbinds thousands of times before being transported or ultimately dissociated
from hNET. Calibrated fluorescent images combined with mass spectroscopy give a
transport rate of 0.06 ASP+/hNET-protein/s, thus 36,000 on-off binding events
(and 36 actual departures) occur for one transport event. Therefore binding has
a low probability of resulting in transport. We interpret these data to mean
that inefficient binding could contribute to slow transport rates.
DNA length evaluation
using cyanine dye and fluorescence correlation spectroscopy. Shimizu, M., Sasaki, S., and
Tsuruoka, M., Biomacromolecules, 2005. 6(5): pp. 2703-7
To develop a
high-performance method for measuring the length of double-stranded DNA (dsDNA)
fragments, the capability of fluorescence correlation spectroscopy (FCS) was
examined. To omit troublesome and time-consuming labeling operations such as
PCR with fluorescently labeled mononucleotides or primers, intercalation of
dimeric cyanine dye YOYO-1 iodide (YOYO) to dsDNA was utilized as a simple
labeling method. Various lengths of dsDNA fragments were prepared and mixed
with YOYO prior to FCS, and the dependence of the diffusion time of a
dsDNA-YOYO complex on the length of dsDNA fragment and the dsDNA/YOYO ratio was
investigated. It was successfully demonstrated that the dsDNA length can be
measured using YOYO and FCS, and the calibration curve was developed taking
into account the rewinding and expansion of the dsDNA fragment caused by YOYO
intercalation.
Probing vesicle
dynamics in single hippocampal synapses. Shtrahman, M., Yeung, C., Nauen, D.W., Bi, G.Q., and Wu,
X.L., Biophys J, 2005. 89(5): pp. 3615-27
We use fluorescence
correlation spectroscopy and fluorescence recovery after photobleaching to
study vesicle dynamics inside the synapses of cultured hippocampal neurons
labeled with the fluorescent vesicle marker FM 1-43. These studies show that
when the cell is electrically at rest, only a small population of vesicles is
mobile, taking seconds to traverse the synapse. Applying the phosphatase
inhibitor okadaic acid causes vesicles to diffuse freely, moving 30 times
faster than vesicles in control synapses. These results suggest that vesicles
move sluggishly due to binding to elements of the synaptic cytomatrix and that
this binding is altered by phosphorylation. Motivated by these results, a model
is constructed consisting of diffusing vesicles that bind reversibly to the
cytomatrix. This stick-and-diffuse model accounts for the fluorescence
correlation spectroscopy and fluorescence recovery after photobleaching data,
and also predicts the well-known exponential refilling of the readily
releasable pool. Our measurements suggest that the movement of vesicles to the
active zone is the rate-limiting step in this process.
Global analysis of
fluorescence fluctuation data. Skakun, V.V., Hink, M.A., Digris, A.V., Engel, R.,
Novikov, E.G., Apanasovich, V.V., and Visser, A.J., Eur Biophys J, 2005.
34(4): pp. 323-34
Over the last decade the
number of applications of fluorescence correlation spectroscopy (FCS) has grown
rapidly. Here we describe the development and application of a software
package, FCS Data Processor, to analyse the acquired correlation curves. The
algorithms combine strong analytical power with flexibility in use. It is
possible to generate initial guesses, link and constrain fit parameters to
improve the accuracy and speed of analysis. A global analysis approach, which
is most effective in analysing autocorrelation curves determined from fluorescence
fluctuations of complex biophysical systems, can also be implemented. The
software contains a library of frequently used models that can be easily
extended to include user-defined models. The use of the software is illustrated
by analysis of different experimental fluorescence fluctuation data sets
obtained with Rhodamine Green in aqueous solution and enhanced green
fluorescent protein in vitro and in vivo.
Position-sensitive
scanning fluorescence correlation spectroscopy. Skinner, J.P., Chen, Y., and Muller,
J.D., Biophys J, 2005. 89(2): pp. 1288-301
Fluorescence correlation
spectroscopy (FCS) uses a stationary laser beam to illuminate a small sample
volume and analyze the temporal behavior of the fluorescence fluctuations
within the stationary observation volume. In contrast, scanning FCS (SFCS)
collects the fluorescence signal from a moving observation volume by scanning
the laser beam. The fluctuations now contain both temporal and spatial
information about the sample. To access the spatial information we synchronize
scanning and data acquisition. Synchronization allows us to evaluate
correlations for every position along the scanned trajectory. We use a circular
scan trajectory in this study. Because the scan radius is constant, the phase
angle is sufficient to characterize the position of the beam. We introduce
position-sensitive SFCS (PSFCS), where correlations are calculated as a
function of lag time and phase. We present the theory of PSFCS and derive
expressions for diffusion, diffusion in the presence of flow, and for
immobilization. To test PSFCS we compare experimental data with theory. We
determine the direction and speed of a flowing dye solution and the position of
an immobilized particle. To demonstrate the feasibility of the technique for applications
in living cells we present data of enhanced green fluorescent protein measured
in the nucleus of COS cells.
Breakdown of axonal
synaptic vesicle precursor transport by microglial nitric oxide. Stagi, M., Dittrich, P.S., Frank,
N., Iliev, A.I., Schwille, P., and Neumann, H., J Neurosci, 2005. 25(2):
pp. 352-62
The mechanism of axonal
injury in inflammatory brain diseases is still unclear. Increased microglial
production of nitric oxide (NO) is a common early sign in neuroinflammatory
diseases. We found by fluorescence correlation spectroscopy that synaptophysin
tagged with enhanced green fluorescence protein (synaptophysin-EGFP) moves
anterogradely in axons of cultured neurons. Activated microglia focally
inhibited the axonal movement of synaptophysin-EGFP in a NO synthase-dependent
manner. Direct application of a NO donor to neurons resulted in inhibition of
axonal transport of synaptophysin-EGFP and synaptotagmin I tagged with EGFP,
mediated via phosphorylation of c-jun NH2-terminal kinase (JNK). Thus, overt
production of reactive NO by activated microglia blocks the axonal transport of
synaptic vesicle precursors via phosphorylation of JNK and could cause axonal
and synaptic dysfunction.
Single molecule
studies of quantum dot conjugates in a submicrometer fluidic channel. Stavis, S.M., Edel, J.B., Samiee,
K.T., and Craighead, H.G., Lab Chip, 2005. 5(3): pp. 337-43
A microfluidic and
optical system was created for the detection and analysis of single molecules
in solution. Fluidic channels with submicrometer dimensions were used to
isolate, detect and identify individual quantum dots conjugated with organic
fluorophores. The channels were fabricated in fused silica with a 500 nm square
cross section. The resulting focal volume of approximately 500 aL reduced
fluorescent background and increased the signal to noise ratio of single
molecule detection. The channels also enabled the rapid detection of 99% of
quantum dots and organic fluorophores traversing the focal volume. Conjugates
were driven through the channels electrokinetically at 2.3 kV cm(-1), excited
with a single 476 nm wavelength laser and detected with a confocal microscope.
Fluorescence emission was collected simultaneously from green (500-590 nm) and
red (610-680 nm) regions of the spectrum. Signal rejection was minimized by the
narrow and symmetric emission spectra of the quantum dots. To demonstrate
efficient multicolor detection and characterization of single molecule binding,
Qdot 655 Streptavidin Conjugates were bound to Alexa Fluor 488 molecules and
individually detected. Photon counting histogram analysis was used to quantify
coincident detection and degree of binding. Fluorescence correlation
spectroscopy was used to measure the mobility of bound and unbound species. The
union of fluidic channels with submicrometer dimensions and quantum dots as
fluorescent labels resulted in efficient and rapid multiplexed single molecule
detection and analysis.
Reversible
aggregation of polystyrene-block-poly(2-vinylpyridine)-block-poly(ethylene oxide)
block copolymer micelles in acidic aqueous Solutions. Stepanek, M., Matejicek, P.,
Humpolickova, J., and Prochazka, K., Langmuir, 2005. 21(23): pp.
10783-90
Micelles of
polystyrene-block-poly(2-vinylpyridine)-block-poly(ethylene oxide) (PS-PVP-PEO)
were studied in acidic aqueous solutions by static and dynamic light
scattering, alkalimetric titration, fluorescence correlation spectroscopy, and
after deposition on a mica surface by atomic force microscopy. The PS-PVP-PEO
micelles prepared by dialysis in ternary 1,4-dioxane-methanol-acidic water
mixtures have a very low association number and show a strong tendency to form
aggregates. The aggregation, which is promoted at low pH, seems to be fully
reversible. Possible mechanisms of the aggregation are discussed. Atomic force
microscopy scans of PS-PVP-PEO micelles deposited on a mica surface reveal the
formation of micellar aggregates and support the general concept of aggregation
upon changes in conditions and deterioration of the stability of small micelles.
One-step analysis of
protein complexes in microliters of cell lysate. Stoevesandt, O., Kohler, K.,
Fischer, R., Johnston, I.C., and Brock, R., Nat Methods, 2005. 2(11):
pp. 833-5
We present 'mix and
measure' procedures for the analysis of protein complexes in microliters of
crude human and mouse cell lysates using fluorescence correlation and
crosscorrelation spectroscopy. We labeled interacting endogenous proteins by
indirect immunofluorescence with all primary and secondary reagents added in
one step. Especially for the screening of compounds interfering with
interactions that depend on signaling-induced posttranslational modifications,
the approach represents a major advance over existing protocols.
Analysis of cellular
functions by multipoint fluorescence correlation spectroscopy. Takahashi, Y., Sawada, R.,
Ishibashi, K., Mikuni, S., and Kinjo, M., Curr Pharm Biotechnol, 2005. 6(2):
pp. 159-65
The biophysical
investigation of living cells is currently possible by single molecular
detection methods such as fluorescence correlation spectroscopy (FCS). FCS is
applied for measuring the dynamic mobility of target molecules in living cells;
however, the conventional FCS systems still lack quantitative analysis for many
regions of interests (ROI) in real time. To improve this situation, we have
developed a novel multipoint FCS system (M-FCS) that can measure multipoint
correlation functions in the cell simultaneously. To evaluate its performance,
we measured correlation functions for rhodamine 6G (Rh6G) in homogeneous
conditions and for green fluorescence protein (GFP) in HeLa cells. We conclude
that M-FCS possesses reliable performance. As a pharmacological application,
glucocorticoid receptor protein fused GFP (GR-GFP) was transfected in HeLa
cells and FCS measurements were carried out in the cytoplasm and the nucleus
simultaneously. The translocation of GR-GFP from the cytoplasm to the nucleus
by ligand stimulation was observed with laser scanning microscopy (LSM) and M-FCS.
Particularly in the nucleus, the slower diffusion of GR-GFP suggested molecular
interactions after the translocation. These data imply that M-FCS can be
applied for quantitative analysis of kinetic processes in living cells.
Probing single DNA
mobility with fluorescence correlation microscopy. Tatarkova, S.A. and Berk, D.A., Phys
Rev E Stat Nonlin Soft Matter Phys, 2005. 71(4 Pt 1): p. 041913
Fluorescence correlation
spectroscopy combined with microscopy (FCSM) is used to study the mobility of
DNA fragments in aqueous solution and tissue models on the single molecule
level. The effective hydrodynamic radius was measured for various lengths of
ds-DNA chains and obeyed the theoretically inveterate [DNA length](0.5)
relationship. Hindered diffusion of ds-DNA through the gel matrix of various
densities is thought of as an extension of Kramer's problem for a flexible
polymer chain. With increasing DNA length the average barrier crossing time
rises as [DNA length](2) and this agrees with theory predictions for polymer
molecules surmounting an entropic barrier.
Quantitative
fluorescence microscopy of macromolecules in gel and biological tissue. Tatarkova, S.A., Kamra Verma, A.,
Berk, D.A., and Lloyd, C.J., Phys Med Biol, 2005. 50(23): pp.
5759-68
Quantitative
fluorescence microscopy provides valuable insight into drug delivery and
pharmacokinetics. The technique is based on analysis of statistical
fluctuations in fluorescence that arises as fluorophores pass through a small
volume illuminated by a focused laser beam, and has been applied to measure
particle motion and binding interactions in solutions, on surfaces and inside
the cells. We examined the use of fluorescence correlation spectroscopy
combined with a microscope (FCSM) to assess the transport of fluorescent beads
and macromolecules in aqueous solutions, gels and living biological tissue.
Obstructed diffusion of fluorescent beads in gels of various densities was
tested to get a sensible estimate of diffusion in the interstitial tissue matrix
consistent with previous reports. Fluorescently labelled liposomes as an
artificial drug or gene carrying vehicles were used for pharmacokinetic tests
of drug delivery in living tissue. The results indicate that FCS is an accurate
and valuable tool for measuring the physical properties of gene vectors in
vitro and for characterizing interactions with tissue in vivo.
Highlighting protein
kinase CK2 movement in living cells. Theis-Febvre, N., Martel, V., Laudet, B., Souchier, C.,
Grunwald, D., Cochet, C., and Filhol, O., Mol Cell Biochem, 2005. 274(1-2):
pp. 15-22
Protein kinase CK2 has
traditionally been described as a stable heterotetrameric complex (alpha2beta2)
but new approaches that effectively capture the dynamic behavior of proteins,
are bringing a new picture of this complex into focus. To track the
spatio-temporal dynamics of CK2 in living cells, we fused its catalytic alpha
and regulatory beta subunits with GFP and analog proteins. Beside the mostly
nuclear localization of both subunits, and the identification of specific
domains on each subunit that triggers their localization, the most significant
finding was that the association of both CK2 subunits in a stable tetrameric
holoenzyme eliminates their nuclear import (Mol Cell Biol 23: 975-987, 2003).
Molecular movements of both subunits in the cytoplasm and in the nucleus were
analyzed using different new and updated fluorescence imaging methods such as:
fluorescence recovery after photo bleaching (FRAP), fluorescence loss in photo
bleaching (FLIP), fluorescence correlation spectroscopy (FCS), and
photoactivation using a biphoton microscope. These fluorescence-imaging
techniques provide unprecedented ways to visualize and quantify the mobility of
each individual CK2 subunit with high spatial and temporal resolution.
Visualization of CK2 heterotetrameric complex formation could also be recorded
using the fluorescence resonance energy transfer (FRET) technique. FRET imaging
revealed that the assembling of this molecular complex can take place both in the
cytoplasmic and nuclear compartments. The spatio-temporal organization of
individual CK2 subunits and their dynamic behavior remain now to be correlated
with the functioning of this kinase in the complex environment of the cell.
Cross talk free
fluorescence cross correlation spectroscopy in live cells. Thews, E., Gerken, M., Eckert, R.,
Zapfel, J., Tietz, C., and Wrachtrup, J., Biophys J, 2005. 89(3):
pp. 2069-76
Fluorescence correlation
spectroscopy (FCS) is now a widely used technique to measure small ensembles of
labeled biomolecules with single molecule detection sensitivity (e.g., low
endogenous concentrations). Fluorescence cross correlation spectroscopy (FCCS)
is a derivative of this technique that detects the synchronous movement of two
biomolecules with different fluorescence labels. Both methods can be applied to
live cells and, therefore, can be used to address a variety of unsolved
questions in cell biology. Applications of FCCS with autofluorescent proteins
(AFPs) have been hampered so far by cross talk between the detector channels
due to the large spectral overlap of the fluorophores. Here we present a new
method that combines advantages of these techniques to analyze binding behavior
of proteins in live cells. To achieve this, we have used dual color excitation
of a common pair of AFPs, ECFP and EYFP, being discriminated in excitation
rather than in emission. This is made possible by pulsed excitation and
detection on a shorter timescale compared to the average residence time of
particles in the FCS volume element. By this technique we were able to
eliminate cross talk in the detector channels and obtain an undisturbed cross
correlation signal. The setup was tested with ECFP/EYFP lysates as well as
chimeras as negative and positive controls and demonstrated to work in live
HeLa cells coexpressing the two fusion proteins ECFP-connexin and
EYFP-connexin.
Disassembly of
structurally modified viral nanoparticles: characterization by fluorescence
correlation spectroscopy. Toivola, J., Gilbert, L., Michel, P., White, D., Vuento, M., and
Oker-Blom, C., C R Biol, 2005. 328(12): pp. 1052-6
Analysis of the
breakdown products of engineered viral particles can give useful information on
the particle structure. We used various methods to breakdown both a recombinant
enveloped virus and virus-like particles (VLPs) from two non-enveloped viruses
and analysed the resulting subunits by fluorescence correlation spectroscopy
(FCS). Analysis of the enveloped baculovirus, Autographa californica
multicapsid nucleopolyhedrovirus (AcMNPV), displaying the green fluorescent
protein (GFP) fused to its envelope protein gp64 was performed in the presence
and absence of 5 mM SDS and 25 mM DTT. Without treatment, the viral particle
showed a diffusion time of 3.3 ms. In the presence of SDS, fluorescent subunits
with diffusion times of 0.2 ms were observed. Additional treatment with DTT
caused a drop in the diffusion time to 0.1 ms. Changes in the amplitude of the
autocorrelation function suggested a 3-fold increase in fluorescent particle
number when viral particles were treated with SDS, and a further 1.5-fold
increase with additional treatment with DTT. Thus, the data showed that an
average of 4.5 molecules of gp64-GFP was incorporated in the membrane of the
modified baculovirus. Further, this suggests that each fluorescent gp64 trimer
carries on average 1.5 fluorescent units. Similar experiments were carried out
with two non-enveloped fluorescent virus-like particles (fVLPs) that displayed
enhanced green fluorescent protein (EGFP). These, fVLPs of canine and human B19
parvoviruses were treated with 6 M urea and 5 mM SDS, respectively.
Correspondingly, the original hydrodynamic radii of 17 and 14 nm were reduced
to 9 and 5 nm after treatment. Here, the change in the amplitude of the
autocorrelation curve suggested a 10-fold increase in particle number when
viral particles of CPV were treated with 6 M urea at 50 degrees C for 10 min.
For EGFP-B19, there was a decrease in the amplitude, accompanied by a 9-fold
increase in the number of fluorescent units with SDS treatment. The results
showed that approximately 10 and 9 fluorescent units were associated with the
corresponding CPV and B19 VLPs. In summary, we were able to estimate the number
of fluorescent subunits in a baculovirus containing a GFP-fusion with its gp64
envelope protein and in two different parvo-VLPs containing EGFP-fused with
their VP2 capsid proteins.
Study of molecular
events in cells by fluorescence correlation spectroscopy. Vukojevic, V., Pramanik, A.,
Yakovleva, T., Rigler, R., Terenius, L., and Bakalkin, G., Cell Mol Life Sci,
2005. 62(5): pp. 535-50
To understand processes
in a living cell, sophisticated and creative approaches are required that can
be used for gathering quantitative information about large number of components
interacting across temporal and spatial scales without major disruption of the
integral network of processes. A physical method of analysis that can meet
these requirements is fluorescence correlation spectroscopy (FCS), which is an
ultrasensitive and non-invasive detection method capable of single-molecule and
real-time resolution. Since its introduction about 3 decades ago, this until
recently emerging technology has reached maturity. As commercially built
equipment is now available, FCS is extensively applied for extracting
biological information from living cells unattainable by other methods, and new
biological concepts are formulated based on findings by FCS. In this review, we
focus on examples in the field of molecular cellular biology. The versatility
of the technique in this field is illustrated in studies of single-molecule
dynamics and conformational flexibility of proteins, and the relevance of
conformational flexibility for biological functions regarding the
multispecificity of antibodies, modulation of activity of C5a receptors in
clathrin-mediated endocytosis and multiplicity of functional responses mediated
by the p53 tumor suppressor protein; quantitative characterization of
physicochemical properties of the cellular interior; protein trafficking; and
ligand-receptor interactions. FCS can also be used to study cell-to-cell
communication, here exemplified by clustering of apoptotic cells via bystander
killing by hydrogen peroxide.
Fluorescence
correlation spectroscopy diffusion laws to probe the submicron cell membrane
organization.
Wawrezinieck, L., Rigneault, H., Marguet, D., and Lenne, P.F., Biophys J,
2005. 89(6): pp. 4029-42
To probe the complexity
of the cell membrane organization and dynamics, it is important to obtain simple
physical observables from experiments on live cells. Here we show that
fluorescence correlation spectroscopy (FCS) measurements at different spatial
scales enable distinguishing between different submicron confinement models. By
plotting the diffusion time versus the transverse area of the confocal volume,
we introduce the so-called FCS diffusion law, which is the key concept
throughout this article. First, we report experimental FCS diffusion laws for
two membrane constituents, which are respectively a putative raft marker and a
cytoskeleton-hindered transmembrane protein. We find that these two
constituents exhibit very distinct behaviors. To understand these results, we
propose different models, which account for the diffusion of molecules either in
a membrane comprising isolated microdomains or in a meshwork. By simulating FCS
experiments for these two types of organization, we obtain FCS diffusion laws
in agreement with our experimental observations. We also demonstrate that
simple observables derived from these FCS diffusion laws are strongly related
to confinement parameters such as the partition of molecules in microdomains
and the average confinement time of molecules in a microdomain or a single mesh
of a meshwork.
FCS cell surface
measurements--photophysical limitations and consequences on molecular ensembles
with heterogenic mobilities. Widengren, J. and Thyberg, P., Cytometry A, 2005. 68(2):
pp. 101-12
BACKGROUND: Fluorescence
Correlation Spectroscopy is a powerful method to analyze densities and
diffusive behavior of molecules in membranes, but effects of photodegradation
can easily be overlooked. METHOD: Based on experimental photophysical
parameters, calculations were performed to analyze the consequences of
photobleaching in fluorescence correlation spectroscopy (FCS) cell surface
experiments, covering a range of standard measurement conditions. RESULTS:
Cumulative effects of photobleaching can be prominent, although an absolute
majority of the fluorescent molecules would pass the laser excitation beam
without being photo-bleached. Given a distribution of molecules on a cell
surface with different diffusive properties, the fraction of molecules that is
actually analyzed depends strongly on the excitation intensities and
measurement times, as well as on the size of the reservoir of freely diffusing
molecules. Both the slower and the faster diffusing molecules can be
disfavored. CONCLUSIONS: Apart from quantifying photobleaching effects, the
calculations suggest that the effects can be used to extract additional
information, for instance about the size of the reservoirs of free diffusion.
By certain choices of measurement conditions, it may be possible to more
specifically analyze certain species within a population, based on their
different diffusive properties, different areas of free diffusion, or different
kinetics of possible transient binding.
Consensus paper of
the WFSBP Task Force on Biological Markers of Dementia: the role of CSF and
blood analysis in the early and differential diagnosis of dementia. Wiltfang, J., Lewczuk, P.,
Riederer, P., Grunblatt, E., Hock, C., Scheltens, P., Hampel, H.,
Vanderstichele, H., Iqbal, K., Galasko, D., Lannfelt, L., Otto, M., Esselmann,
H., Henkel, A.W., Kornhuber, J., and Blennow, K., World J Biol Psychiatry,
2005. 6(2): pp. 69-84
Aging of population, and
increasing life expectancy result in an increasing number of patients with
dementia. This symptom can be a part of a completely curable disease of the
central nervous system (e.g, neuroinflammation), or a disease currently
considered irreversible (e.g, Alzheimer's disease, AD). In the latter case,
several potentially successful treatment approaches are being tested now,
demanding reasonable standards of pre-mortem diagnosis. Cerebrospinal fluid and
serum analysis (CSF/serum analysis), whereas routinely performed in
neuroinflammatory diseases, still requires standardization to be used as an aid
to the clinically based diagnosis of AD. Several AD-related CSF parameters
(total tau, phosphorylated forms of tau, Abeta peptides, ApoE genotype, p97,
etc.) tested separately or in a combination provide sensitivity and specificity
in the range of 85%, the figure commonly expected from a good diagnostic tool.
In this review, recently published reports regarding progress in neurochemical
pre-mortem diagnosis of dementias are discussed with a focus on an early and
differential diagnosis of AD. Novel perspectives offered by recently introduced
technologies, e.g, fluorescence correlation spectroscopy (FCS) and surface enhanced
laser desorption/ionization time-of-flight mass spectrometry (SELDI-TOF MS) are
briefly discussed.
Scanning fluorescence
correlation spectroscopy: a tool for probing microsecond dynamics of
surface-bound fluorescent species. Xiao, Y., Buschmann, V., and Weston, K.D., Anal Chem,
2005. 77(1): pp. 36-46
In this report, a
combined imaging and fluorescence correlation spectroscopy (FCS) method is
described and its ability to characterize microsecond fluctuations in the
fluorescence emission of a sample is demonstrated. A sample scanning laser
confocal microscope is operated in the customary way while recording the time
that each photon is detected with a time resolution of 50 ns using a low-cost
counting board. The serial data stream of photon detection times allows access
to fluorescence signal fluctuations that can be used to characterize dynamics
using correlation methods. The same data stream is used to generate images of
the sample. Using the technique, we demonstrate that it is possible to
characterize the kinetics of transitions to and from nonemitting or
"dark" states of the fluorescent dyes DiIC(16) and ATTO 520. Results
are similar to, but deviate slightly from, a model that has been frequently
used for extracting singlet-triplet: conversion rates using conventional
solution-based FCS. Like conventional FCS, the concentration, or in our case
the areal density of coverage, of fluorescent species can also be obtained.
Many single-molecule fluorescence experiments aim to extract kinetics from
intensity trajectories; this method may be used as a rapid and convenient
technique for characterization of surface-linked or thin-film samples prior to
performing the more time and effort intensive single-molecule measurements.
Besides the capacity to measure photophysical phenomena, the surface-sensitive
FCS method could also be applied for measuring conformational changes or
interaction kinetics for species immobilized on a surface. One possible
scenario is measurements of the frequency and duration of association of
ligand-receptor pairs where a fluorescently labeled component is freely
diffusing and the other is surface immobilized. Given that microarrays of
custom-designed, surface-immobilized peptides and nucleic acids are now readily
available, the ability to sensitively measure association and dissociation
rates of the surface-linked species with a freely diffusing species could be a
useful extension to what has already become an extremely important tool for
characterizing the interactions of biomolecules.
Blinking and
nonradiant dark fraction of water-soluble quantum dots in aqueous solution. Yao, J., Larson, D.R., Vishwasrao,
H.D., Zipfel, W.R., and Webb, W.W., Proc Natl Acad Sci U S A, 2005. 102(40):
pp. 14284-9
Water-soluble quantum
dots (qdots) are now being used in life sciences research to take advantage of
their bright, easily excited fluorescence and high photostability. Although the
frequent erratic blinking and substantial dark (never radiant) fractions that
occur in all available qdots may interfere with many applications, these
properties of individual particles in biological environments had not been
fully evaluated. By labeling Qdot-streptavidin with organic dyes, we were able
to distinguish individual dark and bright qdots and to observe blinking events
as qdots freely diffused in aqueous solution. Bright fractions were measured by
confocal fluorescence coincidence analysis (CFCA) and two-photon
cross-correlation fluorescence correlation spectroscopy (FCS). The observed
bright fractions of various preparations were proportional to the ensemble
quantum yields (QYs), but the intrinsic brightness of individual qdots was
found to be constant across samples with different QYs but the same emission
wavelengths. Increasing qdots' illuminated dwell time by 10-fold during FCS did
not change the fraction of apparently dark qdots but did increase the detected
fraction of blinking qdots, suggesting that the dark population does not arise
from millisecond blinking. Combining CFCA with wide-field imaging of arrays of
qdots localized in dilute agarose gel, the blinking of qdots was measured
across five orders of magnitude in time: approximately 0.001-100 s. This
research characterizes photophysical pathologies of qdots in biologically
relevant environments rather than adhered on dielectric surfaces and describes
methods that are useful for studying various bioapplicable nanoparticles.
Electrically induced
microflows probed by Fluorescence Correlation Spectroscopy. Ybert, C., Nadal, F., Salome, R.,
Argoul, F., and Bourdieu, L., Eur Phys J E Soft Matter, 2005
We report on the
experimental characterisation of electrically induced flows at the micrometer
scale through Fluorescence Correlation Spectroscopy (FCS) measurements. We
stress the potential of FCS as a useful characterisation technique in
microfluidics devices for transport properties cartography. The experimental
results obtained in a model situation are in agreement with previous
calculations (F. Nadal, F. Argoul, P. Kestener, B. Pouligny, C. Ybert, A. Ajdari,
Eur. Phys. J. E 9, 387 (2002)) predicting the structure and electric-field
dependency of the induced flow. Additionally, the present study evidences a
complex behaviour of the probe nanobeads under electric field whose precise
understanding might prove relevant for situations where nano-objects interact
with an external electric field.
Investigation of a
novel artificial antimicrobial peptide by fluorescence correlation
spectroscopy: an amphipathic cationic pattern is sufficient for selective
binding to bacterial type membranes and antimicrobial activity. Yu, L., Ding, J.L., Ho, B., and
Wohland, T., Biochim Biophys Acta, 2005. 1716(1): pp. 29-39
Fluorescence Correlation
Spectroscopy (FCS) is used to study the interaction of a recently designed
antimicrobial peptide, called V4, with LPS and lipids of varying head and tail
groups. V4 is designed based on a known amphipathic cationic pattern BHPHB (B:
basic; H: hydrophobic; P: polar residue, respectively) and shows a good
combination of high antimicrobial activity, low cytotoxic activity and low
hemolytic activity. It is shown that V4 has high binding affinity for LPS,
which is the major component of the outer membrane of Gram-negative bacteria,
and shows selectivity for negatively charged lipids in contrast to zwitterionic
lipids at a low peptide/lipid ratio. At high peptide/lipid ratio, V4 can
permeabilize vesicles composed of negatively charged lipids and eventually
cause vesicle fusion. The identification of the amphipathic cationic pattern as
the mediator of selectivity and antimicrobial activity could be a first step in
the rational design of better antimicrobial peptides.
Lipid diffusion
compared in outer and inner leaflets of planar supported bilayers. Zhang, L. and Granick, S., J
Chem Phys, 2005. 123(21): p. 211104
The translational
diffusion coefficient (D) of lipids located in the outer and inner leaflets of
planar supported DLPC (1,2-dilauroyl-sn-glycero-3-phosphocholine) bilayers in
the fluid phase was measured using fluorescence correlation spectroscopy of
dye-labeled lipids at the low concentration of 0.001% and using iodide
quenching of dyes in the outer leaflet to distinguish diffusion in the inner
leaflet from that in the outer leaflet. To confirm the generality of these
findings, the bilayers were prepared not only by vesicle fusion but also by
Langmuir-Blodgett deposition. We conclude that regardless of whether the
bilayers were supported on quartz or on a polymer cushion, D in the inner and
outer leaflets was the same within an experimental uncertainty of +/-10% but
with a small systematic tendency to be slower (by <5%) within the inner
leaflet.
Slaved diffusion in
phospholipid bilayers.
Zhang, L. and Granick, S., Proc Natl Acad Sci U S A, 2005. 102(26):
pp. 9118-21
The translational diffusion
of phospholipids in supported fluid bilayers splits into two populations when
polyelectrolytes adsorb at incomplete surface coverage. Spatially resolved
measurements using fluorescence correlation spectroscopy show that a slow mode,
whose magnitude scales inversely with the degree of polymerization of the
adsorbate, coexists with a fast mode characteristic of naked lipid diffusion.
Inner and outer leaflets of the bilayer are affected nearly equally. Mobility
may vary from spot to spot on the membrane surface, despite the lipid
composition being the same. This work offers a mechanism to explain how
nanosized domains with reduced mobility arise in lipid membranes.
Probing strong
adsorption of solute onto C18-silica gel by fluorescence correlation imaging and
single-molecule spectroscopy under RPLC conditions. Zhong, Z., Lowry, M., Wang, G.,
and Geng, L., Anal Chem, 2005. 77(8): pp. 2303-10
Understanding molecular
adsorption at a chromatographic interface is of great interest for addressing
the tailing problem in chemical separations. Single-molecule spectroscopy and
confocal fluorescence correlation imaging are used to study the adsorption
sites of C(18) silica beads under RPLC chromatographic conditions. The
experiments show that cationic molecule rhodamine 6G laterally diffuses through
the chromatographic interface of a C(18) hydrocarbon monolayer and acetonitrile
with occasional reversible strong adsorptions. Fluorescence correlation imaging
extracts the rare strong adsorption events from large data sets, revealing that
the strong adsorption sites are randomly distributed throughout the silica
beads. Virtually every imaging pixel of silica beads adsorbs molecules.
Single-molecule spectroscopy of the 584 strong adsorption events observed
indicates that the strong adsorptions persist on the time scales from several
milliseconds to seconds, having an average desorption time of 61 ms. The strong
adsorption events are rare, comprising 0.3% of the total observation time. The
sizes of strong adsorption sites are within the optical resolution of confocal
imaging.
Fluorescence
quenching by TEMPO: a sub-30 A single-molecule ruler. Zhu, P., Clamme, J.P., and Deniz,
A.A., Biophys J, 2005. 89(5): pp. L37-9
A series of DNA molecules labeled
with 5-carboxytetramethylrhodamine (5-TAMRA) and the small nitroxide radical
TEMPO were synthesized and tested to investigate whether the intramolecular
quenching efficiency can be used to measure short intramolecular distances in
small ensemble and single-molecule experiments. In combination with distance
calculations using molecular mechanics modeling, the experimental results from
steady-state ensemble fluorescence and fluorescence correlation spectroscopy
measurements both show an exponential decrease in the quenching rate constant
with the dye-quencher distance in the 10-30 A range. The results demonstrate
that TEMPO-5-TAMRA fluorescence quenching is a promising method to measure
short distance changes within single biomolecules.
2005 FCS abstracts |