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

Non-invasive in vivo measurement of the tear film using spatial autocorrelation in a live mammal model

Tear film stability and its interaction with the corneal surface play an important role in maintaining ocular surface integrity and quality of vision. We present a non-invasive technique to quantify the pre-corneal tear film thickness. A cMOS camera is used to record the interference pattern produced by the reflections from multiple layers of the tear film Principles of spatial autocorrelation are applied to extract the frequency of the periodic patterns in the images. A mathematical model is developed to obtain the thickness of the tear film from the spatial autocorrelation image. The technique is validated using micro-fabricated thin parylene films. We obtained repeatable and precise measurement on a live rabbit model (N = 6). We obtained an average value of 10.2µm and standard deviation of, SD = 0.3 (N = 4). We measured one rabbit infected with HSV-1 virus that had a baseline tear film thickness of 4.7µm.

Distribution and dynamics of rat basophilic leukemia immunoglobulin E receptors (FcepsilonRI) on planar ligand-presenting surfaces

There is considerable interest in the signaling mechanisms of immunoreceptors, especially when triggered with membrane-bound ligands. We have quantified the spatiotemporal dynamics of the redistribution of immunoglobulin E-loaded receptors (IgE-FcepsilonRI) on rat basophilic leukemia-2H3 mast cells in contact with fluid and gel-phase membranes displaying ligands for immunoglobulin E, using total internal reflection fluorescence microscopy. To clearly separate the kinetics of receptor redistribution from cell spreading, and to precisely define the initial contact time (+/-50 ms), micropipette cell manipulation was used to bring individual cells into contact with surfaces. On ligand-free surfaces, there are micron-scale heterogeneities in fluorescence that likely reflect regions of the cell that are more closely apposed to the substrate. When ligands are present, receptor clusters form with this same size scale. The initial rate of accumulation of receptors into the clusters is consistent with diffusion-limited trapping with D approximately 10(-1) microm2/s. These results support the hypothesis that clusters form by diffusion to cell-surface contact regions. Over longer timescales (>10 s), individual clusters moved with both diffusive and directed motion components. The dynamics of the cluster motion is similar to the dynamics of membrane fluctuations of cells on ligand-free fluid membranes. Thus, the same cellular machinery may be responsible for both processes.

Spatial and spectral detection of protein monolayers with deterministic aperiodic arrays of metal nanoparticles

Light scattering phenomena in periodic systems have been investigated for decades in optics and photonics. Their classical description relies on Bragg scattering, which gives rise to constructive interference at specific wavelengths along well defined propagation directions, depending on illumination conditions, structural periodicity, and the refractive index of the surrounding medium. In this paper, by engineering multifrequency colorimetric responses in deterministic aperiodic arrays of nanoparticles, we demonstrate significantly enhanced sensitivity to the presence of a single protein monolayer. These structures, which can be readily fabricated by conventional Electron Beam Lithography, sustain highly complex structural resonances that enable a unique optical sensing approach beyond the traditional Bragg scattering with periodic structures. By combining conventional dark-field scattering micro-spectroscopy and simple image correlation analysis, we experimentally demonstrate that deterministic aperiodic surfaces with engineered structural color are capable of detecting, in the visible spectral range, protein layers with thickness of a few tens of Angstroms.

Image correlation spectroscopy to define membrane dynamics

Fluorescent imaging techniques are powerful tools that aid in studying protein dynamics and membrane domains and allow for the visualization and data collection of such structures as caveolae and clathrin-coated pits, key players in the regulation of cell communication and signaling. The family of image correlation spectroscopy (FICS) provides a unique way to determine details about aggregation, clustering, and dynamics of proteins on the plasma membrane. FICS consists of many imaging techniques which we will focus on including image correlation spectroscopy, image cross-correlation spectroscopy and dynamic image correlation spectroscopy. Image correlation spectroscopy is a tool used to calculate the cluster density, which is the average number of clusters per unit area along with data to determine the degree of aggregation of plasma membrane proteins. Image cross-correlation spectroscopy measures the colocalization of proteins of interest. Dynamic image correlation spectroscopy can be used to analyze protein aggregate dynamics on the cell surface during live-cell imaging in the millisecond to second range.

Quantitative determination of spatial protein-protein correlations in fluorescence confocal microscopy

To quantify spatial protein-protein proximity (colocalization) in paired microscopic images of two sets of proteins labeled by distinct fluorophores, we showed that the cross-correlation and the autocorrelation functions of image intensity consisted of fast and slowly decaying components. The fast component resulted from clusters of proteins specifically labeled, and the slow component resulted from image heterogeneity and a broadly-distributed background. To better evaluate spatial proximity between the two specifically labeled proteins, we extracted the fast-decaying component by fitting the sharp peak in correlation functions to a Gaussian function, which was then used to obtain protein-protein proximity index and the Pearson's correlation coefficient. We also employed the median-filter method as a universal approach for background reduction to minimize nonspecific fluorescence. We illustrated our method by analyzing computer-simulated images and biological images.

Observing protein interactions and their stoichiometry in living cells by brightness analysis of fluorescence fluctuation experiments

A single fluorescently labeled protein generates a short burst of light whenever it passes through a tiny observation volume created within a biological cell. The average amplitude of the burst is related to the stoichiometry of the fluorescently labeled protein complex. Fluorescence fluctuation spectroscopy quantifies the burst amplitude by introducing the brightness parameter. Brightness provides a spectroscopic marker for observing protein interactions and their stoichiometry directly inside cells. Not all fluorescent proteins are suitable for brightness experiments. Here we discuss how brightness properties of the fluorophore influence brightness measurements and how to identify a well-behaved fluorescent protein. Protein interactions and stoichiometry are determined from a brightness titration. Experimental details of brightness titration measurements are described together with the necessary calibration and control experiments.

Fluorescence correlation spectroscopy and fluorescence cross-correlation spectroscopy

This article focuses on methods based on fluctuation correlation spectroscopy to determine the formation of protein complexes in living cells. We present the principles of the fluctuation method applied to cells. We discuss the novelty and the promises of this approach. The emphasis is in the discussion of the underlying statistical assumptions of the image correlation spectroscopy analysis rather than in reviewing applications of the method. Although one example of the application of the fluctuation method is given, this article also contains simulations that are better suited to illustrate and support the basic assumptions of the method.

Oligomerized Tie2 localizes to clathrin-coated pits in response to angiopoietin-1

The tyrosine kinase receptor Tie2 is expressed on endothelial cells, and together with its ligand angiopoietin-1 (Ang1), is important for angiogenesis and vascular stability. Upon activation by Ang1, Tie2 is rapidly internalized and degraded, a mechanism most likely necessary to attenuate receptor activity. Using immunogold electron microscopy, we show that on the surface of endothelial cells, Tie2 is arranged in variably sized clusters containing dimers and higher order oligomers. Clusters of Tie2 were expressed on the apical and basolateral plasma membranes, and on the tips of microvilli. Upon activation by Ang1, Tie2 co-localized with the clathrin heavy chain at the apical and basolateral plasma membranes and within endothelial cells indicating that Tie2 internalizes through clathrin-coated pits. Inhibiting cellular endocytosis by depleting cellular potassium or by acidifying the cytosol blocked the internalization of Tie2 in response to Ang1. Our results suggest that one pathway mediating the internalization of Tie2 in response to Ang1 is through clathrin-coated pits.

Multiparameter fluorescence image spectroscopy to study molecular interactions

Multiparameter Fluorescence Image Spectroscopy (MFIS) is used to monitor simultaneously a variety of fluorescence parameters in confocal fluorescence microscopy. As the photons are registered one by one, MFIS allows for fully parallel recording of Fluorescence Correlation/Cross Correlation Spectroscopy (FCS/FCCS), fluorescence lifetime and pixel/image information over time periods of hours with picosecond accuracy. The analysis of the pixel fluorescence information in higher-dimensional histograms maximizes the selectivity of fluorescence microscopic methods. Moreover it facilitates a statistically-relevant data analysis of the pixel information which makes an efficient detection of heterogeneities possible. The reliability of MFIS has been demonstrated for molecular interaction studies in different complex environments: (I) detecting the heterogeneity of diffusion properties of the dye Rhodamine 110 in a sepharose bead, (II) Förster Resonance Energy Transfer (FRET) studies in mammalian HEK293 cells, and (III) FRET study of the homodimerisation of the transcription factor BIM1 in plant cells. The multidimensional analysis of correlated changes of several parameters measured by FRET, FCS, fluorescence lifetime and anisotropy increases the robustness of the analysis significantly. The economic use of photon information allows one to keep the expression levels of fluorescent protein-fusion proteins as low as possible (down to the single-molecule level).

Fluctuations as a source of information in fluorescence microscopy

Fluctuations in fluorescence spectroscopy and microscopy have traditionally been regarded as noise—they lower the resolution and contrast and do not permit high acquisition rates. However, fluctuations can also be used to gain additional information about a system. This fact has been exploited in single-point microscopic techniques, such as fluorescence correlation spectroscopy and analysis of single molecule trajectories, and also in the imaging field, e.g. in spatio-temporal image correlation spectroscopy. Here, we discuss how fluctuations are used to obtain more quantitative information from the data than that given by average values, while minimizing the effects of noise due to stochastic photon detection.

Spatial diffusivity and availability of intracellular calmodulin

Calmodulin (CaM) is the major pathway that transduces intracellular Ca2+ increases to the activation of a wide variety of downstream signaling enzymes. CaM and its target proteins form an integrated signaling network believed to be tuned spatially and temporally to control CaM's ability to appropriately pass signaling events downstream. Here, we report the spatial diffusivity and availability of CaM labeled with enhanced green fluorescent protein (eGFP)-CaM, at basal and elevated Ca2+,quantified by the novel fluorescent techniques of raster image scanning spectroscopy and number and brightness analysis. Our results show that in basal Ca2+ conditions cytoplasmic eGFP-CaM diffuses at a rate of 10 microm(2)/s, twofold slower than the noninteracting tracer, eGFP, indicating that a significant fraction of CaM is diffusing bound to other partners. The diffusion rate of eGFP-CaM is reduced to 7 microm(2)/s when a large (646 kDa) target protein Ca2+/CaM-dependent protein kinase II is coexpressed in the cells. In addition, the presence of Ca2+/calmodulin-dependent protein kinase II, which can bind up to 12 CaM molecules per holoenzyme, increases the stoichiometry of binding to an average of 3 CaMs per diffusive molecule. Elevating intracellular Ca2+ did not have a major impact on the diffusion of CaM complexes. These results present us with a model whereby CaM is spatially modulated by target proteins and support the hypothesis that CaM availability is a limiting factor in the network of CaM-signaling enzymes.

Accurate measurements of protein interactions in cells via improved spatial image cross-correlation spectroscopy

The sensitive detection of protein interactions in living cells is an important first step toward understanding each of the multitude of cellular processes that are regulated by such interactions. Spatial image cross-correlation spectroscopy (ICCS) is one method used to measure protein-protein interactions from the analysis of two-channel fluorescence microscopy images. In spatial ICCS, cross-correlation of fluctuations in fluorescence intensity recorded as images from two independent wavelength detection channels in a fluorescence microscope is used to determine the average number of interacting particles in the imaged region. Even in situations where the particle number density is relatively high, ICCS provides an accurate measure of molecular interactions. However, it was shown previously that the method suffers from relatively high detection limits of interacting particles (approximately 20%) and can be perturbed by heterogeneous spatial distributions of the fluorescent particles within the images. Here, we demonstrate new approaches to circumvent some of the limitations of ICCS. Spatial scrambling of pixel blocks within fluorescence images was investigated as a way of extending the detection of spatial ICCS to measure lower interaction fractions as well as colocalization within cells. We also show that 'mean-intensity-padding' of regions of interest within fluorescence images is a feasible method of applying ICCS to arbitrarily selected areas of the cell with boundaries or edge morphologies that would be impossible to analyze with conventional ICCS. Using these newly developed strategies we were able to measure the fraction of actin that interacts with alpha-actinin in the leading edge of a migrating cell.

Fluorescence fluctuation spectroscopy in the presence of immobile fluorophores

Fluorescence contributions from immobile sources present a challenge for fluorescence fluctuation spectroscopy (FFS) because the absence of signal fluctuations from stationary fluorophores leads to a biased analysis. This is especially of concern for cellular FFS studies on proteins that interact with immobile structures. Here we present a method that correctly analyzes FFS experiments in the presence of immobile sources by exploiting selective photobleaching of immobile fluorophores. The fluorescence decay due to photobleaching of the immobile species is modeled taking into account the nonuniform illumination volume. The experimentally observed decay curve serves to separate the mobile and immobile fluorescence contribution, which is used to calculate the molecular brightness from the FFS data. We experimentally verify this approach in vitro using the fluorescent protein EGFP as our immobilized species and a diffusing dye of a different color as the mobile one. For this special case, we also use an alternative method of determining the brightness by spectrally resolving the two species. By conducting a dilution study, we show that the correct parameters are obtained using either technique for a wide range of mobile fractions. To demonstrate the application of our technique in living cells, we perform experiments using the histone core protein H2B fused with EGFP expressed in COS-1 cells. We successfully recovered the brightness of the mobile fraction of H2B-EGFP.

Regulation of parathyroid hormone type 1 receptor dynamics, traffic, and signaling by the Na+/H+ exchanger regulatory factor-1 in rat osteosarcoma ROS 17/2.8 cells

The effects of the expression of the Na+/H+ exchanger regulatory factor-1 (NHERF1) on the distribution, dynamics, and signaling properties of the PTH type 1 receptor (PTH1R) were studied in rat osteosarcoma cells ROS 17/2.8. NHERF1 had a dramatic effect on the subcellular distribution of PTH1R, promoting a substantial relocation of the receptor to regions of the plasma membrane located in very close proximity to cytoskeletal fibers. Direct interactions of NHERF1 with the PTH1R and the cytoskeleton were required for these effects, because they were abolished by 1) PTH1R mutations that impair NHERF1 binding, and 2) NHERF1 mutations that impair binding to the PTH1R or the cytoskeleton. NHERF1 reduced significantly the diffusion of the PTH1R by a mechanism that was also dependent on a direct association of NHERF1 with the PTH1R and the cytoskeleton. NHERF1 increased ligand-dependent production of cAMP and induced ligand-dependent rises in intracellular calcium. These effects on calcium were due to increased calcium uptake, as they were blocked by calcium channel inhibitors and by the addition of EGTA to the medium. These calcium effects were abolished by protein kinase A inhibition but phospholipase C inhibition was without effect. Based on these analyses, we propose that, in ROS cells, the presence of NHERF1 induces PTH-dependent calcium signaling by a cAMP-mediated mechanism that involves local protein kinase A-dependent activation of calcium channels.

Image correlation spectroscopy

Membrane domains, such as caveolae and clathrin-coated pits, regulate cell signaling and protein internalization in the plasma membrane. Fluorescence imaging and microscopy provide an opportunity to determine the receptor protein dynamics of membrane microdomains. The family of image correlation spectroscopy (ICS) techniques provides powerful tools with which to measure the aggregation, clustering, and dynamics of proteins in the plasma membrane. ICS is used to calculate the cluster density and the degree of aggregation of plasma membrane proteins, whereas image cross-correlation spectroscopy (ICCS) measures the fraction of colocalization of two proteins. Dynamic image correlation spectroscopy (DICS) can be used to analyze protein dynamics on the cell surface during live-cell imaging.

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.

NHERF-1 and the cytoskeleton regulate the traffic and membrane dynamics of G protein-coupled receptors

The sodium-hydrogen exchange regulatory factor 1 (NHERF-1/EBP50) interacts with the C terminus of several G protein-coupled receptors (GPCRs). We examined the role of NHERF-1 and the cytoskeleton on the distribution, dynamics, and trafficking of the beta(2)-adrenergic receptor (beta(2)AR; a type A receptor), the parathyroid hormone receptor (PTH1R; type B), and the calcium-sensing receptor (CaSR; type C) using fluorescence recovery after photobleaching, total internal reflection fluorescence, and image correlation spectroscopy. beta(2)AR bundles were observed only in cells that expressed NHERF-1, whereas the PTH1R was localized to bundles that parallel stress fibers independently of NHERF-1. The CaSR was never observed in bundles. NHERF-1 reduced the diffusion of the beta(2)AR and the PTH1R. The addition of ligand increased the diffusion coefficient and the mobile fraction of the PTH1R. Isoproterenol decreased the immobile fraction but did not affect the diffusion coefficient of the beta(2)AR. The diffusion of the CaSR was unaffected by NHERF-1 or the addition of calcium. NHERF-1 reduced the rate of ligand-induced internalization of the PTH1R. This phenomenon was accompanied by a reduction of the rate of arrestin binding to PTH1R in ligand-exposed cells. We conclude that some GPCRs, such as the beta(2)AR, are attached to the cytoskeleton primarily via the binding of NHERF-1. Others, such as the PTH1R, bind the cytoskeleton via several interacting proteins, one of which is NHERF-1. Finally, receptors such as the CaSR do not interact with the cytoskeleton in any significant manner. These interactions, or the lack thereof, govern the dynamics and trafficking of the receptor.

Spatially Resolved Total Internal Reflection Fluorescence Correlation Microscopy Using an Electron Multiplying Charge-Coupled Device Camera

A spatially resolved total internal reflection fluorescence correlation microscopy (TIR-FCM) system is constructed with an electron multiplying charge-coupled device (EMCCD) camera. The system was used to determine diffusion coefficients of lipid molecules in a planar lipid bilayer, and lipids and epidermal growth factor receptor (EGFR) proteins on cell membranes of Chinese Hamster Ovary (CHO) cells. The evaluation of the "cross talk" between neighboring pixels suggests that a higher degree of multiplexing can be achieved than was previously proposed [Kannan, B. et al. Anal. Chem. 2006, 78, 3444-51] using the same camera with a focused laser excitation. The best time resolution possible with this system is 4 ms for a region of interest comprising 20 lines in the CCD and is good enough to determine membrane diffusion in lipid bilayers and of membrane proteins in living cells. In this work, using a TIR-FCM setup, 1600 autocorrelation functions were measured simultaneously with a time resolution of 4.8 ms. This area corresponds to a 40 x 40 pixel region of interest with a dimension of 11.3 x 11.3 microm2 and is sufficiently large to allow the measurement of the lower membrane of a whole cell simultaneously.

Investigating membrane protein dynamics in living cells

Live cell imaging is a powerful tool for understanding the function and regulation of membrane proteins. In this review, we briefly discuss 4 fluorescence-microscopy-based techniques for studying the transport dynamics of membrane proteins: fluorescence-correlation spectroscopy, image-correlation spectroscopy, fluorescence recovery after photobleaching, and single-particle and (or) molecule tracking. The advantages and limitations of each approach are illustrated using recent studies of an ion channel and cell adhesion molecules.

A guide to accurate fluorescence microscopy colocalization measurements

Biomolecular interactions are fundamental to the vast majority of cellular processes, and identification of the major interacting components is usually the first step toward an understanding of the mechanisms that govern various cell functions. Thus, statistical image analyses that can be performed on fluorescence microscopy images of fixed or live cells have been routinely applied for biophysical and cell biological studies. These approaches measure the fraction of interacting particles by analyzing dual color fluorescence images for colocalized pixels. Colocalization algorithms have proven to be effective, although the dynamic range and accuracy of these measurements has never been well established. Spatial image cross-correlation spectroscopy (ICCS), which cross-correlates spatial intensity fluctuations recorded in images from two detection channels simultaneously, has also recently been shown to be an effective measure of colocalization as well. Through simulations, imaging of fluorescent antibodies adsorbed on glass and cell measurements, we show that ICCS performs much better than standard colocalization algorithms at moderate to high densities of particles, which are often encountered in cellular systems. Furthermore, it was found that the density ratio between the two labeled species of interest plays a major role in the accuracy of the colocalization analysis. By applying a direct and systematic comparison between the standard, fluorescence microscopy colocalization algorithm and spatial ICCS, we show regimes where each approach is applicable, and more importantly, where they fail to yield accurate results.

Use of wavelet and fast Fourier transforms in pharmacodynamics

Progress has been made in the development and application of mechanism-based pharmacodynamic models for describing the drug-specific and physiological factors influencing the time course of responses to the diverse actions of drugs. However, the biological variability in biosignals and the complexity of pharmacological systems often complicate or preclude the direct application of traditional structural and nonstructural models. Mathematical transforms may be used to provide measures of drug effects, identify structural and temporal patterns, and visualize multidimensional data from analyses of biomedical signals and images. Fast Fourier transform (FFT) and wavelet analyses are two methodologies that have proven to be useful in this context. FFT converts a signal from the time domain to the frequency domain, whereas wavelet transforms colocalize in both domains and may be utilized effectively for nonstationary signals. Nonstationary drug effects are common but have not been well analyzed and characterized by other methods. In this review, we discuss specific applications of these transforms in pharmacodynamics and their potential role in ascertaining the dynamics of spatiotemporal properties of complex pharmacological systems.

Measurement of monomer-oligomer distributions via fluorescence moment image analysis

We present higher-order moment analysis of fluorescence intensity fluctuations from individual laser scanning microscopy images applied to study monomer-oligomer distributions. We demonstrate that the number densities and brightness ratios of a mixed population of monomers and oligomers can be determined by analyzing higher-order moments of the fluorescence intensity fluctuations from individual images for specific ranges of densities and particle brightness ratios. Computer simulations and experiments with fluorescent microspheres and cells were performed to illustrate the detection limits and accuracy of this statistical approach. The simulation results show that the concentration of the dimer or oligomer population should be less than or equal to the monomeric concentration for the method to provide accurate results, and that the upper density detection limit of the population of monomers is one order-of-magnitude higher than the concentration of the oligomers. We implemented this technique to resolve two populations of fluorescent microspheres with different brightness ratios and we also applied the moment-analysis method to examine the distribution of aggregation states of PDGF-beta receptors in human fibroblast cells. The method was able to resolve a tetrameric population of the PDGF-beta receptors relative to the background distribution of nonspecifically bound fluorophore.

k-Space image correlation spectroscopy: a method for accurate transport measurements independent of fluorophore photophysics

We present the theory and application of reciprocal space image correlation spectroscopy (kICS). This technique measures the number density, diffusion coefficient, and velocity of fluorescently labeled macromolecules in a cell membrane imaged on a confocal, two-photon, or total internal reflection fluorescence microscope. In contrast to r-space correlation techniques, we show kICS can recover accurate dynamics even in the presence of complex fluorophore photobleaching and/or "blinking". Furthermore, these quantities can be calculated without nonlinear curve fitting, or any knowledge of the beam radius of the exciting laser. The number densities calculated by kICS are less sensitive to spatial inhomogeneity of the fluorophore distribution than densities measured using image correlation spectroscopy. We use simulations as a proof-of-principle to show that number densities and transport coefficients can be extracted using this technique. We present calibration measurements with fluorescent microspheres imaged on a confocal microscope, which recover Stokes-Einstein diffusion coefficients, and flow velocities that agree with single particle tracking measurements. We also show the application of kICS to measurements of the transport dynamics of alpha5-integrin/enhanced green fluorescent protein constructs in a transfected CHO cell imaged on a total internal reflection fluorescence microscope using charge-coupled device area detection.

Accuracy and dynamic range of spatial image correlation and cross-correlation spectroscopy

We present a comprehensive study of the accuracy and dynamic range of spatial image correlation spectroscopy (ICS) and image cross-correlation spectroscopy (ICCS). We use simulations to model laser scanning microscopy imaging of static subdiffraction limit fluorescent proteins or protein clusters in a cell membrane. The simulation programs allow us to control the spatial imaging sampling variables and the particle population densities and interactions and introduce and vary background and counting noise typical of what is encountered in digital optical microscopy. We systematically calculate how the accuracy of both image correlation methods depends on practical experimental collection parameters and characteristics of the sample. The results of this study provide a guide to appropriately plan spatial image correlation measurements on proteins in biological membranes in real cells. The data presented map regimes where the spatial ICS and ICCS provide accurate results as well as clearly showing the conditions where they systematically deviate from acceptable accuracy. Finally, we compare the simulated data with standard confocal microscopy using live CHO cells expressing the epidermal growth factor receptor fused with green fluorescent protein (GFP/EGFR) to obtain typical values for the experimental variables that were investigated in our study. We used our simulation results to estimate a relative precision of 20% for the ICS measured receptor density of 64 microm(-2) within a 121 x 98 pixel subregion of a single cell.

Position-sensitive scanning fluorescence correlation spectroscopy

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.

Spatiotemporal image correlation spectroscopy (STICS) theory, verification, and application to protein velocity mapping in living CHO cells

We introduce a new extension of image correlation spectroscopy (ICS) and image cross-correlation spectroscopy (ICCS) that relies on complete analysis of both the temporal and spatial correlation lags for intensity fluctuations from a laser-scanning microscopy image series. This new approach allows measurement of both diffusion coefficients and velocity vectors (magnitude and direction) for fluorescently labeled membrane proteins in living cells through monitoring of the time evolution of the full space-time correlation function. By using filtering in Fourier space to remove frequencies associated with immobile components, we are able to measure the protein transport even in the presence of a large fraction (>90%) of immobile species. We present the background theory, computer simulations, and analysis of measurements on fluorescent microspheres to demonstrate proof of principle, capabilities, and limitations of the method. We demonstrate mapping of flow vectors for mixed samples containing fluorescent microspheres with different emission wavelengths using space time image cross-correlation. We also present results from two-photon laser-scanning microscopy studies of alpha-actinin/enhanced green fluorescent protein fusion constructs at the basal membrane of living CHO cells. Using space-time image correlation spectroscopy (STICS), we are able to measure protein fluxes with magnitudes of mum/min from retracting lamellar regions and protrusions for adherent cells. We also demonstrate the measurement of correlated directed flows (magnitudes of mum/min) and diffusion of interacting alpha5 integrin/enhanced cyan fluorescent protein and alpha-actinin/enhanced yellow fluorescent protein within living CHO cells. The STICS method permits us to generate complete transport maps of proteins within subregions of the basal membrane even if the protein concentration is too high to perform single particle tracking measurements.

Analyzing protein-protein interactions in cell membranes

Interactions among membrane proteins regulate numerous cellular processes, including cell growth, cell differentiation and apoptosis. We need to understand which proteins interact, where they interact and to which extent they interact. This article describes a set of novel approaches to measure, on the surface of living cells, the number of clusters of proteins, the number of proteins per cluster, the number of clusters or membrane domains that contain pairs of interacting proteins and the fraction of one protein species that interacts with another protein within these domains. These data can then be interpreted in terms of the function of the protein-protein interactions.

Protein localization in living cells and tissues using FRET and FLIM

Interacting proteins assemble into molecular machines that control cellular homeostasis in living cells. While the in vitro screening methods have the advantage of providing direct access to the genetic information encoding unknown protein partners, they do not allow direct access to interactions of these protein partners in their natural environment inside the living cell. Using wide-field, confocal, or two-photon (2p) fluorescence resonance energy transfer (FRET) microscopy, this information can be obtained from living cells and tissues with nanometer resolution. One of the important conditions for FRET to occur is the overlap of the emission spectrum of the donor with the absorption spectrum of the acceptor. As a result of spectral overlap, the FRET signal is always contaminated by donor emission into the acceptor channel and by the excitation of acceptor molecules by the donor excitation wavelength. Mathematical algorithms are required to correct the spectral bleed-through signal in wide-field, confocal, and two-photon FRET microscopy. In contrast, spectral bleed-through is not an issue in FRET/FLIM imaging because only the donor fluorophore lifetime is measured; also, fluorescence lifetime imaging microscopy (FLIM) measurements are independent of excitation intensity or fluorophore concentration. The combination of FRET and FLIM provides high spatial (nanometer) and temporal (nanosecond) resolution when compared to intensity-based FRET imaging. In this paper, we describe various FRET microscopy techniques and its application to protein-protein interactions.

Kinetic analysis of platelet-derived growth factor receptor/phosphoinositide 3-kinase/Akt signaling in fibroblasts

Isoforms of the serine-threonine kinase Akt coordinate multiple cell survival pathways in response to stimuli such as platelet-derived growth factor (PDGF). Activation of Akt is a multistep process, which relies on the production of 3'-phosphorylated phosphoinositide (PI) lipids by PI 3-kinases. To quantitatively assess the kinetics of PDGF receptor/PI 3-kinase/Akt signaling in fibroblasts, a systematic study of this pathway was performed, and a mechanistic mathematical model that describes its operation was formulated. We find that PDGF receptor phosphorylation exhibits positive cooperativity with respect to PDGF concentration, and its kinetics are quantitatively consistent with a mechanism in which receptor dimerization is initially mediated by the association of two 1:1 PDGF/PDGF receptor complexes. Receptor phosphorylation is transient at high concentrations of PDGF, consistent with the loss of activated receptors upon endocytosis. By comparison, Akt activation responds to lower PDGF concentrations and exhibits more sustained kinetics. Further analysis and modeling suggest that the pathway is saturated at the level of PI 3-kinase activation, and that the p110alpha catalytic subunit of PI 3-kinase contributes most to PDGF-stimulated 3'-PI production. Thus, at high concentrations of PDGF the kinetics of 3'-PI production are limited by the turnover rate of these lipids, while the Akt response is additionally influenced by the rate of Akt deactivation.

Isolation of bright aggregate fluctuations in a multipopulation image correlation spectroscopy system using intensity subtraction

Image correlation spectroscopy allows sensitive measurement of the spatial distribution and aggregation state of fluorescent membrane macro molecules. When studying a single population system (i.e., aggregates of similar brightness), an accurate measure can be made of the aggregate number per observation area, but this measurement becomes much more complex in a distributed population system (i.e., bright and faint aggregates). This article describes an alternate solution that involves extraction of the bright aggregate population information. This novel development for image correlation spectroscopy, termed intensity subtraction analysis, uses sequential uniform intensity subtraction from raw confocal images. Sequential intensity subtraction results in loss of faint aggregate fluctuations that are smaller in magnitude than fluctuations due to the brightest aggregates. The resulting image has correlatable fluctuations originating from only the brightest population, permitting quantification of this population's distribution and further cross-correlation measurements. The feasibility of this technique is demonstrated using fluorescent microsphere images and biological samples. The technique is further used to examine the spatial distribution of a plasma-membrane-labeled fluorescent synthetic ganglioside, and to cross-correlate this probe with various membrane markers. The evidence provided demonstrates that bright aggregates of the fluorescent ganglioside are associated with clathrin-coated pits, membrane microvilli, and detergent-resistant membranes.

Imaging protein-protein interactions in living cells

The complex organization of plant cells makes it likely that the molecular behaviour of proteins in the test tube and the cell is different. For this reason, it is essential though a challenge to study proteins in their natural environment. Several innovative microspectroscopic approaches provide such possibilities, combining the high spatial resolution of microscopy with spectroscopic techniques to obtain information about the dynamical behaviour of molecules. Methods to visualize interaction can be based on FRET (fluorescence detected resonance energy transfer), for example in fluorescence lifetime imaging microscopy (FLIM). Another method is based on fluorescence correlation spectroscopy (FCS) by which the diffusion rate of single molecules can be determined, giving insight into whether a protein is part of a larger complex or not. Here, both FRET- and FCS-based approaches to study protein-protein interactions in vivo are reviewed.

Seeing the light: preassembly and ligand-induced changes of the interferon gamma receptor complex in cells

Our experiments were designed to test the hypothesis that the cell surface interferon gamma receptor chains are preassembled rather than associated by ligand and to assess the molecular changes on ligand binding. To accomplish this, we used fluorescence resonance energy transfer, a powerful spectroscopic technique that has been used to determine molecular interactions and distances between the donor and acceptor. However, current commercial instruments do not provide sufficient sensitivity or the full spectra to provide decisive results of interactions between proteins labeled with blue and green fluorescent proteins in living cells. In our experiments, we used the blue fluorescent protein and green fluorescent protein pair, attached a monochrometer and charge-coupled device camera to a modified confocal microscope, reduced background fluorescence with the use of two-photon excitation, and focused on regions of single cells to provide clear spectra of fluorescence resonance energy transfer. In contrast to the prevailing view, the results demonstrate that the receptor chains are preassociated and that the intracellular domains move apart on binding the ligand interferon gamma. Application of this technology should lead to new rapid methods for high throughput screening and delineation of the interactome of cells.

Nerve growth factor signals via preexisting TrkA receptor oligomers

Nerve growth factor (NGF) promotes neuronal survival and differentiation by activating TrkA receptors. Similar to other receptor tyrosine kinases, ligand-induced dimerization is thought to be required for TrkA receptor activation. To study this process, we expressed TrkA receptors in Xenopus laevis oocytes and analyzed their response to NGF by using a combination of functional, biochemical, and structural approaches. TrkA receptor protein was detected in the membrane fraction of oocytes injected with TrkA receptor cRNA, but not in uninjected or mock-injected oocytes. Application of NGF to TrkA receptor-expressing oocytes promoted tyrosine phosphorylation and activated an oscillating transmembrane inward current, indicating that the TrkA receptors were functional. Freeze-fracture electron microscopic analysis demonstrated novel transmembrane particles in the P-face (protoplasmic face) of oocytes injected with TrkA cRNA, but not in uninjected or mock injected oocytes. Incubating TrkA cRNA-injected oocytes with the transcriptional inhibitor actinomycin D did not prevent the appearance of these P-face particles or electrophysiological responses to NGF, demonstrating that they did not arise from de novo transcription of an endogenous Xenopus oocyte gene. The appearance of these particles in the plasma membrane correlated with responsiveness to NGF as detected by electrophysiological analysis and receptor phosphorylation, indicating that these novel P-face particles were TrkA receptors. The dimensions of these particles (8.6 x 10 nm) were too large to be accounted for by TrkA monomers, suggesting the formation of TrkA receptor oligomers. Application of NGF did not lead to a discernible change in the size or shape of these TrkA receptor particles during an active response. These results indicate that in Xenopus oocytes, NGF activates signaling via pre-formed TrkA receptor oligomers.

Adaptor protein-2 exhibits alpha 1 beta 1 or alpha 6 beta 1 integrin-dependent redistribution in rhabdomyosarcoma cells

Downregulation of several signaling pathways, such as those stimulated by growth factor receptors, occurs by internalization of signaling receptors through clathrin-coated pits. The first step in internalization or endocytosis is interaction with AP-2, which results in coated pit formation by assembly of clathrin to AP-2. Changes in endocytosis are reflected in the distribution of AP-2 molecules at the cell surface. Integrins are receptors which mediate attachment to the extracellular matrix and also stimulate numerous intracellular signaling pathways; however, it is not known how signaling through integrins is terminated or downregulated. Endocytosis through clathrin-coated pits offers an attractive mechanism for this. This work explores the relationship between AP-2 and beta(1) integrins. RD cells grown for 24 h on collagen or laminin exhibit a redistribution of AP-2 to the cell periphery relative to those grown on fibronectin or polylysine. The total AP-2 protein levels in the cells are unaffected. Blocking alpha(1)beta(1) integrin ligand binding on collagen prevents this redistribution fully. On laminin where alpha(1)beta(1) and alpha(6)beta(1) integrins are engaged, both receptors must be simultaneously blocked to prevent AP-2 redistribution, confirming that the redistribution depends on the specific engagement of the receptors. Immunofluorescence reveals that the majority of alpha(1)beta(1) integrins colocalize with alpha(6)beta(1) integrins in linear structures identified as focal adhesions. A separate fraction of alpha(1)beta(1) integrins colocalize with AP-2 in coated pits. Interestingly, alpha(6)beta(1) integrins are not located in coated pits, demonstrating that integrin colocalization with AP-2 is not necessary to induce redistribution of AP-2.

The Sendai virus membrane fusion mechanism studied using image correlation spectroscopy

The mechanism of Sendai virus membrane fusion to cultured cell membranes was studied. Viral lipids were labeled with the lipophilic dye, 4-(4-(dihexadecylamino)styryl-N-methylquinolinium iodine) (DiQ), and viral proteins were labeled using fluorescein isothiocyanate (FITC). The redistribution of these probes from the virus to cultured cells was followed using the technique of image correlation spectroscopy. This technique assayed the intensity change and the redistribution of these probes as fusion progressed from a more to less aggregated state. The lipid probe DiQ dispersed into the membrane of the target membrane at both 22 and 37 degrees C, while the FITC-labeled proteins dispersed only at 37 degrees C. Simultaneous labeling of virus with both of these probes showed that at 37 degrees C their redistribution proceeded at different rates. These data were consistent with the formation of a hemifusion intermediate during the fusion process.

Total internal reflection with fluorescence correlation spectroscopy: combined surface reaction and solution diffusion

Total internal reflection with fluorescence correlation spectroscopy (TIR-FCS) is a method for measuring the surface association/dissociation rates and absolute densities of fluorescent molecules at the interface of solution and a planar substrate. This method can also report the apparent diffusion coefficient and absolute concentration of fluorescent molecules very close to the surface. An expression for the fluorescence fluctuation autocorrelation function in the absence of contributions from diffusion through the evanescent wave, in solution, has been published previously (N. L. Thompson, T. P. Burghardt, and D. Axelrod. 1981, Biophys. J. 33:435-454). This work describes the nature of the TIR-FCS autocorrelation function when both surface association/dissociation kinetics and diffusion through the evanescent wave contribute to the fluorescence fluctuations. The fluorescence fluctuation autocorrelation function depends in general on the kinetic association and dissociation rate constants, the surface site density, the concentration of fluorescent molecules in solution, the solution diffusion coefficient, and the depth of the evanescent field. Both general and approximate expressions are presented.

How does the plasma membrane participate in cellular signaling by receptors for immunoglobulin E?

Accumulating evidence strongly supports the view that the plasma membrane participates in transmembrane signaling by IgE-receptors (IgE-Fc epsilon RI) through the formation of lipid-based domains, also known as rafts. Ongoing biochemical and biophysical experiments investigate the composition, structure, and dynamics of the corresponding membrane components and how these are related to functional coupling between Fc epsilon RI and Lyn tyrosine kinase to initiate signaling in mast cells.

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.

Free clathrin triskelions are required for the stability of clathrin-associated adaptor protein (AP-2) coated pit nucleation sites

In this study image correlation spectroscopy was used to demonstrate the presence of two populations of clathrin in situ, on intact cells. In the periphery of the cell ~35% of the clathrin triskelions are free within the cytosol while ~65% are in large aggregates, presumably coated pits. Although endocytosis is inhibited at low temperature, free clathrin triskelions are still present and small AP-2 aggregates (of ~20 proteins), or coated pit nucleation sites, are still observed. Following hypertonic treatment, or cytoplasmic acidification, free clathrin triskelions within the cytosol are depleted and all of the clathrin becomes associated with the membrane. Under these conditions coated pit associated AP-2 remains while the smaller AP-2 aggregates, or coated pit nucleation sites, dissociate. This indicates that the stabilization of AP-2 coated pit nucleation sites requires the presence of free clathrin triskelions within the cytosol. Furthermore, this indicates that free clathrin is required for the early stages of coated pit formation and presumably the continuation of the clathrin-mediated endocytic process. We also provide indirect evidence that AP-2 binding to the membrane in coated pit nucleation sites may be regulated in part by binding to internalization-competent membrane receptors.Key words: adaptor protein (AP-2), clathrin, distribution, nucleation sites, endocytosis.