Image cross-correlation spectroscopy: A new experimental biophysical approach to measurement of slow diffusion of fluorescent molecules

Colocalization by cross-correlation, a new method of colocalization suited for super-resolution microscopy

BACKGROUND

A common goal of scientific microscopic imaging is to determine if a spatial correlation exists between two imaged structures. This is generally accomplished by imaging fluorescently labeled structures and measuring their spatial correlation with a class of image analysis algorithms known as colocalization. However, the most commonly used methods of colocalization have strict limitations, such as requiring overlap in the fluorescent markers and reporting requirements for accurate interpretation of the data, that are often not met. Due to the development of novel super-resolution techniques, which reduce the overlap of the fluorescent signals, a new colocalization method is needed that does not have such strict requirements.

RESULTS

In order to overcome the limitations of other colocalization algorithms, I developed a new ImageJ/Fiji plugin, Colocalization by cross-correlation (CCC). This method uses cross-correlation over space to identify spatial correlations as a function of distance, removing the overlap requirement and providing more comprehensive results. CCC is compatible with 3D and time-lapse images, and was designed to be easy to use. CCC also generates new images that only show the correlating labeled structures from the input images, a novel feature among the cross-correlating algorithms.

CONCLUSIONS

CCC is a versatile, powerful, and easy to use colocalization and spatial correlation tool that is available through the Fiji update sites. Full and up to date documentation can be found at https://imagej.net/plugins/colocalization-by-cross-correlation . CCC source code is available at https://github.com/andmccall/Colocalization_by_Cross_Correlation .

Dynamic Heterochromatin States in Anisotropic Nuclei of Cells on Aligned Nanofibers

The cancer cell nucleus deforms as it invades the interstitial spaces in tissues and the tumor microenvironment. While alteration of the chromatin structure in a deformed nucleus is expected and documented, the chromatin structure in the nuclei of cells on aligned matrices has not been elucidated. In this work we elucidate the spatiotemporal organization of heterochromatin in the elongated nuclei of cells on aligned nanofibers with stimulated emission depletion nanoscopy and fluorescence correlation spectroscopy. We show that the anisotropy of nuclei is sufficient to drive H3K9me3-heterochromatin alterations, with enhanced H3K9me3 nanocluster compaction and aggregation states that otherwise are indistinguishable from diffraction-limited microscopy. We interrogated the higher-order heterochromatin structures within major chromatin compartments in anisotropic nuclei and discovered a wider spatial dispersion of nanodomain clusters in the nucleoplasm and condensed larger nanoclusters near the periphery and pericentromeric heterochromatin. Upon examining the spatiotemporal dynamics of heterochromatin in anisotropic nuclei, we observed reduced mobility of the constitutive heterochromatin mark H3K9me3 and the associated heterochromatin protein 1 (HP1α) at the nucleoplasm and periphery regions, correlating with increased viscosity and changes in gene expression. Since heterochromatin remodeling is crucial to genome integrity, our results reveal an unconventional H3K9me3 heterochromatin distribution, providing cues to an altered chromatin state due to perturbations of the nuclei in aligned fiber configurations.

Revealing Plasma Membrane Nano-Domains with Diffusion Analysis Methods

Nano-domains are sub-light-diffraction-sized heterogeneous areas in the plasma membrane of cells, which are involved in cell signalling and membrane trafficking. Throughout the last thirty years, these nano-domains have been researched extensively and have been the subject of multiple theories and models: the lipid raft theory, the fence model, and the protein oligomerization theory. Strong evidence exists for all of these, and consequently they were combined into a hierarchal model. Measurements of protein and lipid diffusion coefficients and patterns have been instrumental in plasma membrane research and by extension in nano-domain research. This has led to the development of multiple methodologies that can measure diffusion and confinement parameters including single particle tracking, fluorescence correlation spectroscopy, image correlation spectroscopy and fluorescence recovery after photobleaching. Here we review the performance and strengths of these methods in the context of their use in identification and characterization of plasma membrane nano-domains.

Spatially Selective Dissection of Signal Transduction in Neurons Grown on Netrin-1 Printed Nanoarrays via Segmented Fluorescence Fluctuation Analysis

Axonal growth cones extend during neural development in response to precise distributions of extracellular cues. Deleted in colorectal cancer (DCC), a receptor for the chemotropic guidance cue netrin-1, directs F-actin reorganization, and is essential for mammalian neural development. To elucidate how the extracellular distribution of netrin-1 influences the distribution of DCC and F-actin within axonal growth cones, we patterned nanoarrays of substrate bound netrin-1 using lift-off nanocontact printing. The distribution of DCC and F-actin in embryonic rat cortical neuron growth cones was then imaged using total internal reflection fluorescence (TIRF) microscopy. Fluorescence fluctuation analysis via image cross-correlation spectroscopy (ICCS) was applied to extract the molecular density and aggregation state of DCC and F-actin, identifying the fraction of DCC and F-actin colocalizing with the patterned netrin-1 substrate. ICCS measurement of spatially segmented images based on the substrate nanodot patterns revealed distinct molecular distributions of F-actin and DCC in regions directly overlying the nanodots compared to over the reference surface surrounding the nanodots. Quantifiable variations between the populations of DCC and F-actin on and off the nanodots reveal specific responses to the printed protein substrate. We report that nanodots of substrate-bound netrin-1 locally recruit and aggregate DCC and direct F-actin organization. These effects were blocked by tetanus toxin, consistent with netrin-1 locally recruiting DCC to the plasma membrane via a VAMP2-dependent mechanism. Our findings demonstrate the utility of segmented ICCS image analysis, combined with precisely patterned immobilized ligands, to reveal local receptor distribution and signaling within specialized subcellular compartments.

Three-dimensional intracellular transport in neuron bodies and neurites investigated by label-free dispersion-relation phase spectroscopy

Due to the limitations of fluorescence imaging techniques, the study of intracellular cargo is typically restricted to two-dimensional analyses. To overcome low light levels and the risk of phototoxicity, we employ quantitative phase imaging, a family of full-field imaging techniques that measure the optical path length shift introduced by the specimen. Specifically, we use spatial light interference microscopy (SLIM) to study the transport of mass in whole tomographic volumes and show that a time-correlation technique, dispersion-relation phase spectroscopy (DPS), can be used to simultaneously assay the horizontal and vertical traffic of mass through a cell. To validate our method, we compare the traffic inside cell bodies and neuronal extensions, showing that the vertical transport of mass may prove a more sensitive and interesting metric than similar measurements limited to a 2D, horizontal plane. © 2017 International Society for Advancement of Cytometry.

Netrin-1-Regulated Distribution of UNC5B and DCC in Live Cells Revealed by TICCS

Netrins are secreted proteins that direct cell migration and adhesion during development. Netrin-1 binds its receptors deleted in colorectal cancer (DCC) and the UNC5 homologs (UNC5A-D) to activate downstream signaling that ultimately directs cytoskeletal reorganization. To investigate how netrin-1 regulates the dynamic distribution of DCC and UNC5 homologs, we applied fluorescence confocal and total internal reflection fluorescence microscopy, and sliding window temporal image cross correlation spectroscopy, to measure time profiles of the plasma membrane distribution, aggregation state, and interaction fractions of fluorescently tagged netrin receptors expressed in HEK293T cells. Our measurements reveal changes in receptor aggregation that are consistent with netrin-1-induced recruitment of DCC-enhanced green fluorescent protein (EGFP) from intracellular vesicles to the plasma membrane. Netrin-1 also induced colocalization of coexpressed full-length DCC-EGFP with DCC-T-mCherry, a putative DCC dominant negative that replaces the DCC intracellular domain with mCherry, consistent with netrin-1-induced receptor oligomerization, but with no change in aggregation state with time, providing evidence that signaling via the DCC intracellular domain triggers DCC recruitment to the plasma membrane. UNC5B expressed alone was also recruited by netrin-1 to the plasma membrane. Coexpressed DCC and UNC5 homologs are proposed to form a heteromeric netrin-receptor complex to mediate a chemorepellent response. Application of temporal image cross correlation spectroscopy to image series of cells coexpressing UNC5B-mCherry and DCC-EGFP revealed a netrin-1-induced increase in colocalization, with both receptors recruited to the plasma membrane from preexisting clusters, consistent with vesicular recruitment and receptor heterooligomerization. Plasma membrane recruitment of DCC or UNC5B was blocked by application of the netrin-1 VI-V peptide, which fails to activate chemoattraction, or by pharmacological block of Src family kinase signaling, consistent with receptor recruitment requiring netrin-1-activated signaling. Our findings reveal a mechanism activated by netrin-1 that recruits DCC and UNC5B to the plasma membrane.

Lipid Diffusion in Supported Lipid Bilayers: A Comparison between Line-Scanning Fluorescence Correlation Spectroscopy and Single-Particle Tracking

Diffusion in lipid membranes is an essential component of many cellular process and fluorescence a method of choice to study membrane dynamics. The goal of this work was to directly compare two common fluorescence methods, line-scanning fluorescence correlation spectroscopy and single-particle tracking, to observe the diffusion of a fluorescent lipophilic dye, DiD, in a complex five-component mitochondria-like solid-supported lipid bilayer. We measured diffusion coefficients of DFCS ~ 3 um2 * s-1 and DSPT ~ 2 um2 * s-1, respectively. These comparable, yet statistically different values are used to highlight the main message of the paper, namely that the two considered methods give access to distinctly different dynamic ranges: D sup or approximatively 1um2 * s-1 for FCS and D inf or approximatively 5 um2 s-1 for SPT (with standard imaging conditions). In the context of membrane diffusion, this means that FCS allows studying lipid diffusion in fluid membranes, as well as the diffusion of loosely-bound proteins hovering above the membrane. SPT, on the other hand, is ideal to study the motions of membrane-inserted proteins, especially those presenting different conformations, but only allows studying lipid diffusion in relatively viscous membranes, such as supported lipid bilayers and cell membranes.

Imaging fluorescence fluctuation spectroscopy: new tools for quantitative bioimaging

Fluorescence fluctuation spectroscopy (FFS) techniques provide information at the single-molecule level with excellent time resolution. Usually applied at a single spot in a sample, they have been recently extended into imaging formats, referred to as imaging FFS. They provide spatial information at the optical diffraction limit and temporal information in the microsecond to millisecond range. This review provides an overview of the different modalities in which imaging FFS techniques have been implemented and discusses present imaging FFS capabilities and limitations. A combination of imaging FFS and nanoscopy would allow one to record information with the detailed spatial information of nanoscopy, which is ∼20 nm and limited only by fluorophore size and labeling density, and the time resolution of imaging FFS, limited by the fluorescence lifetime. This combination would provide new insights into biological events by providing spatiotemporal resolution at unprecedented levels.

Lipid-coated gold nanoparticles promote lamellar body formation in A549 cells

Gold nanoparticles (GNPs) have been applied as diagnostic and therapeutic agents because they can be targeted, localized, and be heated to cause cell death. However, their use has been limited by their relatively low biocompatibility. In this work, we coated the GNPs' surface by a biocompatible phospholipid bilayer composed of 1-stearoyl-2-oleoyl-sn-glycero-3-phospho-(1'-rac-glycerol) (SOPG). We tested their interaction with A549 cells to investigate their uptake and intracellular fate as well as the response of the cells to the presence of the GNPs. We used flow cytometry and confocal microscopy to show that the SOPG coated GNPs were readily taken up by the A549 cells. Transmission electron microscopy (TEM) images and fluorescence images further showed that the number of granular structures in the cells was increased following exposure to the lipid coated GNPs. Co-localization experiments demonstrated that SOPG coated GNPs localize in acidic compartments in a time dependent manner and that the number of these increase as the cells are exposed to the GNPs suggesting that they induce formation of lamellar bodies (LBs) which in A549 cells in turn can serve as a means of exporting the GNPs.

STICCS reveals matrix-dependent adhesion slipping and gripping in migrating cells

Two-color spatio-temporal image cross-correlation spectroscopy (STICCS) is a new, to our knowledge, image analysis method that calculates space-time autocorrelation and cross-correlation functions from fluorescence intensity fluctuations. STICCS generates cellular flow and diffusion maps that reveal interactions and cotransport of two distinct molecular species labeled with different fluorophores. Here we use computer simulations to map the capabilities and limitations of STICCS for measurements in complex heterogeneous environments containing micro- and macrostructures. We then use STICCS to analyze the co-flux of adhesion components in migrating cells imaged using total internal reflection fluorescence microscopy. The data reveal a robust, time-dependent co-fluxing of certain integrins and paxillin in adhesions in protrusions when they pause, and in adhesions that are sliding and disassembling, demonstrating that the molecules in these adhesions move as a complex. In these regions, both α6β1- or αLβ2-integrins, expressed in CHO.B2 cells, co-flux with paxillin; an analogous cotransport was seen for α6β1-integrin and α-actinin in U2OS. This contrasts with the behavior of the α5β1-integrin and paxillin, which do not co-flux. Our results clearly show that integrins can move in complexes with adhesion proteins in protrusions that are retracting.

Fluorescence correlation spectroscopy: past, present, future

In recent years fluorescence correlation spectroscopy (FCS) has become a routine method for determining diffusion coefficients, chemical rate constants, molecular concentrations, fluorescence brightness, triplet state lifetimes, and other molecular parameters. FCS measures the spatial and temporal correlation of individual molecules with themselves and so provides a bridge between classical ensemble and contemporary single-molecule measurements. It also provides information on concentration and molecular number fluctuations for nonlinear reaction systems that complement single-molecule measurements. Typically implemented on a fluorescence microscope, FCS samples femtoliter volumes and so is especially useful for characterizing small dynamic systems such as biological cells. In addition to its practical utility, however, FCS provides a window on mesoscopic systems in which fluctuations from steady states not only provide the basis for the measurement but also can have important consequences for the behavior and evolution of the system. For example, a new and potentially interesting field for FCS studies could be the study of nonequilibrium steady states, especially in living cells.

Dissecting chromatin interactions in living cells from protein mobility maps

The genome of eukaryotes is organized into a dynamic nucleoprotein complex referred to as chromatin, which can adopt different functional states. Both the DNA and the protein component of chromatin are subject to various post-translational modifications that define the cell's gene expression program. Their readout and establishment occurs in a spatio-temporally coordinated manner that is controlled by numerous chromatin-interacting proteins. Binding to chromatin in living cells can be measured by a spatially resolved analysis of protein mobility using fluorescence microscopy based approaches. Recent advancements in the acquisition of protein mobility data using fluorescence bleaching and correlation methods provide data sets on diffusion coefficients, binding kinetics, and cellular concentrations on different time and length scales. The combination of different techniques is needed to dissect the complex interplay of diffusive translocations, binding events, and mobility constraints of the chromatin environment. While bleaching techniques have their strength in the characterization of particles that are immobile on the second/minute time scale, a correlation analysis is advantageous to characterize transient binding events with millisecond residence time. The application and synergy effects of the different approaches to obtain protein mobility and interaction maps in the nucleus are illustrated for the analysis of heterochromatin protein 1.

Probe-independent image correlation spectroscopy

Conventional image correlation spectroscopy (ICS) analysis assumes point-like probe particles whose sizes are much smaller than the beam focus. This assumption yields erroneous results when the particle size is larger than a certain threshold. Here, a formalism is presented to study image correlation spectroscopy for particles of arbitrary geometries, sizes, and fluorophore distributions. We demonstrate the usefulness of this method by analyzing simulated image sequences of diffusing fluorescent point sources, disks, and randomly oriented rigid rods of various sizes. In addition, we also perform ICS analysis on confocal images of fluorescent microspheres of different diameters diffusing in a medium of known viscosity to experimentally validate the method. The new method, which we call template analysis, yields excellent agreement with theoretical predictions, thus extending the capability of ICS for studying dynamic processes in a probe-independent manner.

Diffusion, transport, and cell membrane organization investigated by imaging fluorescence cross-correlation spectroscopy

Cell membrane organization is dynamic and is assumed to have different characteristic length scales. These length scales, which are influenced by lipid and protein composition as well as by the cytoskeleton, can range from below the optical resolution limit (as with rafts or microdomains) to far above the resolution limit (as with capping phenomena or the formation of lipid "platforms"). The measurement of these membrane features poses a significant problem because membrane dynamics are on the millisecond timescale and are thus beyond the time resolution of conventional imaging approaches. Fluorescence correlation spectroscopy (FCS), a widely used spectroscopic technique to measure membrane dynamics, has the required time resolution but lacks imaging capabilities. A promising solution is the recently introduced method known as imaging total internal reflection (ITIR)-FCS, which can probe diffusion phenomena in lipid membranes with good temporal and spatial resolution. In this work, we extend ITIR-FCS to perform ITIR fluorescence cross-correlation spectroscopy (ITIR-FCCS) between pixel areas of arbitrary shape and derive a generalized expression that is applicable to active transport and diffusion. ITIR-FCCS is applied to model systems exhibiting diffusion, active transport, or a combination of the two. To demonstrate its applicability to live cells, we observe the diffusion of a marker, the sphingolipid-binding domain (SBD) derived from the amyloid peptide Abeta, on live neuroblastoma cells. We investigate the organization and dynamics of SBD-bound lipid microdomains under the conditions of cholesterol removal and cytoskeleton disruption.

Cellular dynamics of Ku: characterization and purification of Ku-eGFP

Ku is a predominantly nuclear protein that functions as a DNA double-strand-break (DSB) binding protein and regulatory subunit of the DNA-dependent protein kinase (DNA-PK). DNA-PK is involved in synapsis and remodeling of broken DNA ends during nonhomologous end-joining (NHEJ) of DNA DSBs. It has also recently been demonstrated that Ku plays roles in cytoplasmic and membrane processes, namely: interaction with matrix metalloproteinase 9, acting as a co-receptor for parvoviral infection, and also interacting with cell polarity protein, Par3. We present a method for creating stable expression of Ku-eGFP in CHO cells and extend the procedure to purify Ku-eGFP for in vitro assaying. We demonstrated that Ku-eGFP localizes to the nucleus of HeLa cells upon microinjection into the cytoplasm as well as localizing to laser induced DNA damage. We also characterized the diffusional dynamics of Ku in the nucleus and in the cytoplasm using fluorescence correlation spectroscopy (FCS). The FCS data suggest that whereas the majority of Ku (70%) in the nucleus is mobile and freely diffusing, in a cellular context, there also exists a significant slow process fraction (30%). Strikingly, in the cytoplasm, this immobile/slow moving fraction is even more pronounced (45%).

Studies of distribution, location and dynamic properties of EGFR on the cell surface measured by image correlation spectroscopy

In this work, we have studied the distribution and dynamic properties of Epidermal Growth Factor (EGF) receptors in the plasma membrane of fixed and live cells as well as the extent of co-localization of this transmembrane protein with proteins specific for three-membrane microdomains: membrane rafts, caveolae and clathrin-coated pits. This was achieved using a family of image-processing tools called image correlation spectroscopy (ICS), image cross-correlation spectroscopy (ICCS) and dynamic image correlation spectroscopy (DICS). Our results indicate that EGFR is diffusely distributed on the cell surface at 37 degrees C and aggregates as the temperature is lowered to 4 degrees C. This aggregation takes place within 15 min and is reversible. Changes in temperature also affect the diffusion of EGFR by two orders of magnitude. The dynamic properties of EGFR are similar to the dynamic properties of a GPI-anchored protein known to be present in membrane rafts, which motivated us to explore the extent of co-localization of EGFR with this membrane raft protein using ICCS. Our results indicate that more than half of the EGFR population is present in membrane rafts and smaller percentages are present in caveolae and clathrin-coated pits.

Characterizing the spatio-temporal behavior of cell populations through image auto-and cross-correlation microscopy

We propose two methods for characterizing the spatio-temporal behavior of cell populations in culture. The first method, image auto-correlation microscopy (IACM), allows us to characterize the variation in the number of objects as a function of time, thus enabling the quantification of the clustering properties of cell populations to be performed. The second method, image cross-correlation microscopy (ICCM), allows us to characterize the migration properties of cell populations. The latter method does not require estimation or measurement of the trajectories of individual cells, which is very demanding when populations of >100 cells are examined. The capabilities of the two methods are demonstrated with simulated cell populations, and their usefulness is illustrated with experiments involving invasive and noninvasive tumor cell populations.

Sampling effects, noise, and photobleaching in temporal image correlation spectroscopy

We present an extensive investigation of the accuracy and precision of temporal image correlation spectroscopy (TICS). Using simulations of laser scanning microscopy image time series, we investigate the effect of spatiotemporal sampling, particle density, noise, sampling frequency, and photobleaching of fluorophores on the recovery of transport coefficients and number densities by TICS. We show that the recovery of transport coefficients is usually limited by spatial sampling, while the measurement of accurate number densities is restricted by background noise in an image series. We also demonstrate that photobleaching of the fluorophore causes a consistent overestimation of diffusion coefficients and flow rates, and a severe underestimation of number densities. We derive a bleaching correction equation that removes both of these biases when used to fit temporal autocorrelation functions, without increasing the number of fit parameters. Finally, we image the basal membrane of a CHO cell with EGFP/alpha-actinin, using two-photon microscopy, and analyze a subregion of this series using TICS and apply the bleaching correction. We show that the photobleaching correction can be determined simply by using the average image intensities from the time series, and we use the simulations to provide good estimates of the accuracy and precision of the number density and transport coefficients measured with TICS.

Detecting microdomains in intact cell membranes

Current models for cellular plasma membranes focus on spatial heterogeneity and how this heterogeneity relates to cell function. In particular, putative lipid raft membrane domains have been postulated to exist based in large part on the results that a significant fraction of the membrane is detergent insoluble and that molecules facilitating key membrane processes like signal transduction are often found in the detergent-resistant membrane fraction. Yet, the in vivo existence of lipid rafts remains extremely controversial because, despite being sought for more than a decade, evidence for their presence in intact cell membranes is inconclusive. In this review, a variety of experimental techniques that have been or might be used to look for lipid microdomains in intact cell membranes are described. Experimental results are highlighted and the strengths and limitations of different techniques for microdomain identification and characterization are assessed.

Quantitating intracellular transport of polyplexes by spatio-temporal image correlation spectroscopy

Quantitatively understanding how nonviral gene delivery vectors (polyplexes) are transported inside cells is essential before they can be optimized for gene therapy and medical applications. In this study, we used spatio-temporal image correlation spectroscopy (ICS) to follow polymer-nucleic acid particles (polyplexes) of various sizes and analyze their diffusive-like and flow behaviors intracellularly to elucidate the mechanisms responsible for their transport. ICS is a quantitative imaging technique that allows the assessment of particle motion in complex systems, although it has not been widely used to date. We find that the internalized polyplexes are able to use microtubule motors for intracellular trafficking and exhibit different transport behaviors for short (<10 s) versus long (approximately 60 s) correlation times. This motion can be explained by a memory effect of the microtubule motors. These results reveal that, although microtubule motor biases may be present for short periods of time, resulting in a net directional velocity, the overall long-term motion of the polyplexes is best described as a random walk-like process. These studies suggest that spatio-temporal ICS is a powerful technique for assessing the nature of intracellular motion and provides a quantitative tool to compare the transport of different objects within a living cell.

An internalization-competent influenza hemagglutinin mutant causes the redistribution of AP-2 to existing coated pits and is colocalized with AP-2 in clathrin free clusters

Image correlation spectroscopy and cross correlation spectroscopy were used to demonstrate that approximately 25% of the internalization-competent influenza virus hemagglutinin mutant, HA+8, is colocalized with clathrin and AP-2 at the plasma membrane of intact cells, while wild-type HA (which is excluded from coated pits) does not colocalize with either protein. Clathrin and AP-2 clusters were saturated when HA+8 was overexpressed, and this was accompanied by a redistribution of AP-2 into existing coated pits. However, de novo coated pit formation was not observed. In nontreated cells, the number of clusters of clathrin or AP-2 colocalized with HA+8 was always comparable. Hypertonic treatment which disperses the clathrin lattices resulted in more clusters containing AP-2 and HA+8 than clathrin and HA+8. Less colocalization of HA+8 with clathrin was also observed after cytosol acidification, which causes the formation of deeply invaginated pits, where the HA+8 may be inaccessible to extracellular labeling by antibodies, and blocks coated vesicle budding. However, cytosol acidification elevated the number of clusters containing both HA+8 and AP-2, suggesting an increase in their level of association outside of the deep invaginations. Our results imply that AP-2 and HA+8 can colocalize in clusters devoid of clathrin, at least in cells treated to alter the clathrin lattice structure. Although we cannot ascertain whether this also occurs in untreated cells, we propose that AP-2 binding to membrane proteins carrying internalization signals can occur prior to the binding of AP-2 to clathrin. While such complexes can in principle serve to recruit clathrin for the formation of new coated pits, the higher affinity of the internalization signals for clathrin-associated AP-2 [Rapoport, I., et al. (1997) EMBO J. 16, 2240-2250] makes it more likely that once the AP-2-membrane protein complexes form, they are quickly recruited into existing coated pits.

Looking at lipid rafts?

The notion that microdomains enriched in certain specialized lipids exist in membranes has been both attractive and controversial since it was first proposed that such domains, termed rafts, might act as apical sorting devices in epithelial cells. The observation that certain lipids are not extractable in cold nonionic detergent supports the raft concept, but the nature of the in vivo correlate of such detergent-resistant membranes remains enigmatic. In principle, microscopy should be able to determine whether the postulated rafts exist. This article focuses on recent microscopy experiments addressing this question. Several, but not all, results support the raft concept, but further definition of the structure, dynamics and function of lipid domains in various biological contexts is urgently required.

Membrane organization in immunoglobulin E receptor signaling

The structure and dynamics of the plasma membrane are proposed to be critical for the initial steps of signal transduction by the high-affinity immunoglobulin E receptor. Recent experimental advances indicate that interactions between the high-affinity immunoglobulin E receptor and the tyrosine kinase Lyn with cholesterol- and sphingolipid-rich regions within the plasma membrane are important for receptor function. This accumulating evidence points to spatio-temporal control of immunoglobulin E receptor signaling by the organization of the plasma membrane; an attractive hypothesis is that ligand-dependent receptor aggregation causes the segregation of Lyn-containing ordered regions of the plasma membrane from disordered regions.

Image correlation spectroscopy. II. Optimization for ultrasensitive detection of preexisting platelet-derived growth factor-beta receptor oligomers on intact cells

Previously we introduced image correlation spectroscopy (ICS) as an imaging analog of fluorescence correlation spectroscopy (FCS). Implementation of ICS with image collection via a standard fluorescence confocal microscope and computer-based autocorrelation analysis was shown to facilitate measurements of absolute number densities and determination of changes in aggregation state for fluorescently labeled macromolecules. In the present work we illustrate how to use ICS to quantify the aggregation state of immunolabeled plasma membrane receptors in an intact cellular milieu, taking into account background fluorescence. We introduce methods that enable us to completely remove white noise contributions from autocorrelation measurements for individual images and illustrate how to perform background corrections for autofluorescence and nonspecific fluorescence on cell population means obtained via ICS. The utilization of photon counting confocal imaging with ICS analysis in combination with the background correction techniques outlined enabled us to achieve very low detection limits with standard immunolabeling methods on normal, nontransformed human fibroblasts (AG1523) expressing relatively low numbers of platelet-derived growth factor-beta (PDGF-beta) receptors. Specifically, we determined that the PDGF-beta receptors were preaggregated as tetramers on average with a mean surface density of 2.3 clusters micrometer(-2) after immunolabeling at 4 degreesC. These measurements, which show preclustering of PDGF-beta receptors on the surface of normal human fibroblasts, contradict a fundamental assumption of the ligand-induced dimerization model for signal transduction and provide support for an alternative model that posits signal transduction from within preexisting receptor aggregates.

Diffusion of transferrin receptor clusters

Two dimensional motion of membrane receptors provides a mechanism for interaction among receptors in the plane of the membrane. In some cases the lateral diffusion leads to formation of clusters which may also be mobile. We have used image cross-correlation (ICCS) spectroscopy technique to measure the translational motion of transferrin receptors in the membrane of 3T3 fibroblasts and HEp2 carcinoma cells. The technique is based on the measurement and analysis of fluctuations in the intensity observed in fluorescence confocal microscope images measured as a function of time. The fluorescence fluctuations arise from stochastic concentration fluctuations about the equilibrium concentration caused by movement of receptors. The amplitude of the fluctuations depend on the number of fluorescent molecules in the observation volume and the dynamics provide the rate of movement. The diffusion observed by this analysis is orders of magnitude slower than that measured by conventional photobleaching techniques. The slower motion corresponds to the diffusion of receptor clusters which provide the more dominant fluctuations.

Fusion of Sendai virus and individual host cells and inhibition of fusion by lipophosphoglycan measured with image correlation spectroscopy

Fusion between Sendai virus (SV) and individual host cells was investigated with confocal laser scanning microscopy (CLSM) and image correlation spectroscopy (ICS). SV was labeled with the fluorescent probe 7-octadecylamino-4-nitrobenz-2-oxa-1,3-diazole (NBD-NH-C18) and was allowed to bind to host cells (HEp-2, BALB-3T3) at 4 degrees C. The effect of lipophosphoglycan (LPG), isolated from Leishmania donovani, on virus fusion was investigated by incorporation of LPG (0, 5, 10 or 20 microM) into the host cell membrane (HEp-2) before addition of SV. LPG did not affect the number of SV bound per cell. After incubation at 37 degrees C for 15 min without LPG, CLSM revealed a redistribution of NBD-NH-C18 from the SV envelope to the host cell membrane and an increase in average fluorescence intensity, indicating dequenching. ICS analysis of images obtained after incubation at 37 degrees C showed an increased mean cluster density to 260% of the value at 4 degrees C, reflecting the disappearance of labeled SV from the cell surface and diffusion of NBD-NH-C18 into the host cell membrane. Preincubation of the cells with LPG inhibited the temperature-induced redistribution and dequenching of NBD-NH-C18 in a concentration-dependent manner, with a total inhibition of fusion at 20 microM LPG. Together, the results demonstrate that CLSM combined with ICS is a powerful tool for studies of fusion of enveloped viruses with individual host cells and that LPG inhibits the fusion process at or before the hemifusion (lipid mixing) stage of SV interaction with cells.

Fluorescence correlation spectroscopy: diagnostics for sparse molecules

The robust glow of molecular fluorescence renders even sparse molecules detectable and susceptible to analysis for concentration, mobility, chemistry, and photophysics. Correlation spectroscopy, a statistical-physics-based tool, gleans quantitative information from the spontaneously fluctuating fluorescence signals obtained from small molecular ensembles. This analytical power is available for studying molecules present at minuscule concentrations in liquid solutions (less than one nanomolar), or even on the surfaces of living cells at less than one macromolecule per square micrometer. Indeed, routines are becoming common to detect, locate, and examine individual molecules under favorable conditions.