Interferometric fringe fluorescence photobleaching recovery interrogates entire cell surfaces

FRAP, FLIM, and FRET: Detection and analysis of cellular dynamics on a molecular scale using fluorescence microscopy

The combination of fluorescent-probe technology plus modern optical microscopes allows investigators to monitor dynamic events in living cells with exquisite temporal and spatial resolution. Fluorescence recovery after photobleaching (FRAP), for example, has long been used to monitor molecular dynamics both within cells and on cellular surfaces. Although bound by the diffraction limit imposed on all optical microscopes, the combination of digital cameras and the application of fluorescence intensity information on large-pixel arrays have allowed such dynamic information to be monitored and quantified. Fluorescence lifetime imaging microscopy (FLIM), on the other hand, utilizes the information from an ensemble of fluorophores to probe changes in the local environment. Using either fluorescence-intensity or lifetime approaches, fluorescence resonance energy transfer (FRET) microscopy provides information about molecular interactions, with Ångstrom resolution. In this review, we summarize the theoretical framework underlying these methods and illustrate their utility in addressing important problems in reproductive and developmental systems.

Recent applications of fluorescence recovery after photobleaching (FRAP) to membrane bio-macromolecules

This review examines some recent applications of fluorescence recovery after photobleaching (FRAP) to biopolymers, while mainly focusing on membrane protein studies. Initially, we discuss the lateral diffusion of membrane proteins, as measured by FRAP. Then, we talk about the use of FRAP to probe interactions between membrane proteins by obtaining fundamental information such as geometry and stoichiometry of the interacting complex. Afterwards, we discuss some applications of FRAP at the cellular level as well as the level of organisms. We conclude by comparing diffusion coefficients obtained by FRAP and several other alternative methods.

T cell adhesion mechanisms revealed by receptor lateral mobility

Cell surface receptors mediate the exchange of information between cells and their environment. In the case of adhesion receptors, the spatial distribution and molecular associations of the receptors are critical to their function. Therefore, understanding the mechanisms regulating the distribution and binding associations of these molecules is necessary to understand their functional regulation. Experiments characterizing the lateral mobility of adhesion receptors have revealed a set of common mechanisms that control receptor function and thus cellular behavior. The T cell provides one of the most dynamic examples of cellular adhesion. An individual T cell makes innumerable intercellular contacts with antigen presenting cells, the vascular endothelium, and many other cell types. We review here the mechanisms that regulate T cell adhesion receptor lateral mobility as a window into the molecular regulation of these systems, and we present a general framework for understanding the principles and mechanisms that are likely to be common among these and other cellular adhesion systems. We suggest that receptor lateral mobility is regulated via four major mechanisms-reorganization, recruitment, dispersion, and anchoring-and we review specific examples of T cell adhesion receptor systems that utilize one or more of these mechanisms.

Fluorescence photobleaching recovery using total internal reflection interference fringes

Lateral diffusion measurements on cell membrane molecules, most commonly accomplished through fluorescence photobleaching recovery (FPR or FRAP), provide information on such molecules' size, environment, and participation in intermolecular interactions. However, difficulties arise in FPR measurements of lateral dynamics of materials, such as visible fluorescent protein (VFP) fusion proteins, where fluorescent intracellular species contribute to the fluorescence recovery signal and thus distort measurements intended to reflect surface molecules only. A new method helps eliminate these difficulties. In total internal reflection interference fringe FPR, interfering laser beams enter a 1.65-numercial aperture (NA) Olympus objective at the periphery of the back focal plane where the NA exceeds 1.38. This creates an extended interference pattern totally internally reflected at the coverslip-medium interface which excites fluorescence only from fluorescent molecules located where the cell contacts the coverslip. The large illuminated area interrogates many more membrane receptors than spot methods and hence obtains more diffusion information per measurement while rejecting virtually all interfering intracellular fluorescence. We report successful measurements of membrane dynamics of both VFP-containing and conventionally labeled molecules by this technique and compare them with results of other FPR methods.

High probe intensity photobleaching measurement of lateral diffusion in cell membranes

Lateral diffusion measurements, most commonly accomplished through Fluorescence Photobleaching Recovery (FPR or FRAP), provide important information on cell membrane molecules' size, environment and participation in intermolecular interactions. However, serious difficulties arise when these techniques are applied to weakly expressed proteins of either of two types: fusions of membrane receptors with visible fluorescent proteins or membrane molecules on autofluorescent cells. To achieve adequate sensitivity in these cases, techniques such as interference fringe FPR are needed. However, in such measurements, cytoplasmic species contribute to the fluorescence recovery signal and thus yield diffusion parameters not properly representing the small number of surface molecules. A new method helps eliminate these difficulties. High Probe Intensity (HPI)-FPR measurements retain the intrinsic confocality of spot measurements to eliminate interference from fluorescent cytoplasmic species. However, HPI-FPR methods lift the previous requirement that FPR procedures be performed at probe beam intensities low enough to not induce bleaching in samples during measurements. The high probe intensities now employed provide much larger fluorescence signals and thus more information on molecular diffusion from each measurement. We report successful measurement of membrane dynamics by this technique.

Interactions of the mast cell function-associated antigen with the type I Fcepsilon receptor

Clustering the mast cell function-associated antigen (MAFA), a membrane glycoprotein expressed on 2H3 cells, by its specific monoclonal antibody G63 substantially inhibits secretion normally triggered by aggregating these cells' Type I Fcepsilon receptor (FcepsilonRI). To explore possible MAFA-FcepsilonRI interactions giving rise to this inhibition, we have studied by time-resolved phosphorescence anisotropy the rotational behavior of both MAFA and FcepsilonRI as ligated by various reagents involved in FcepsilonRI-induced degranulation and MAFA-mediated inhibition thereof. From 4 to 37 degrees C the rotational correlation times (mean+/-S.D.) of FcepsilonRI-bound, erythrosin-conjugated IgE resemble those observed for MAFA-bound erythrosin-conjugated G63 Fab, 82+/-17 micros and 79+/-31 micros at 4 degrees C, respectively. Clustering the FcepsilonRI-IgE complex by antigen or by anti-IgE increases the phosphorescence anisotropy of G63 Fab and slows its rotational relaxation. Lateral diffusion of G63 Fab is also slowed by antigen clustering of the receptor. Taken together, these results suggest that unperturbed MAFA associates with clustered FcepsilonRI. They are also consistent with its interaction with the isolated receptor, a situation also suggested by FRET measurements on the system.

Fluorescence pattern photobleaching recovery for samples with multi-component diffusion

The translational mobility of proteins and lipids in phospholipid bilayers is often not well described as ideal self diffusion. One of the best methods for characterizing such non-ideal diffusion is to use fluorescence pattern photobleaching recovery. In this method, the spatial gradient of the monitoring and bleaching intensity is created by using epi-fluorescence and an expanded Gaussian-shaped laser beam which passes though a Ronchi ruling placed at the back image plane of a microscope. A difficulty arises when the fluorescence recovery from the exchange of slowly diffusing molecules between illuminated and non-illuminated stripes temporally overlaps with the recovery from the exchange of more rapidly diffusing molecules through the gradient produced by the broad Gaussian shape of the illumination. In the work presented here, a general theory is developed that describes the shape of the resulting fluorescence recovery curve for these typical experimental conditions. Approximate expressions amenable to non-linear curve fitting are also given. The new theoretical formalism has been demonstrated on data for the translational mobility of a fluorescent lipid probe in phospholipid bilayers deposited on planar-fused silica substrates.

The mast cell function-associated antigen and its interactions with the type I Fcepsilon receptor

Rat mucosal-type mast cells of the RBL-2H3 line express a glycoprotein termed the MAst cell Function-associated Antigen (MAFA). When MAFA is clustered by its specific monoclonal antibody G63, secretion normally triggered by aggregating these cells' type I Fcepsilon receptor (FcepsilonRI) is substantially inhibited. The nature of MAFA-FcepsilonRI interactions giving rise to this inhibition remains unclear. Rotational diffusion of a membrane protein is a sensitive probe of its involvement in intermolecular interactions. We have therefore studied by time-resolved phosphorescence anisotropy the rotational behavior of both MAFA and FcepsilonRI as ligated by various reagents involved in FcepsilonRI-induced degranulation and MAFA-mediated inhibition thereof. From 4 to 37 degrees C, the rotational correlation times (mean +/- SD) of FcepsilonRI-bound, erythrosin-conjugated IgE resemble those observed for MAFA-bound, erythrosin-conjugated G63 Fab, 82 +/- 17 and 79 +/- 31 micros at 4 degrees C, respectively. Clustering the FcepsilonRI-IgE complex by antigen or by anti-IgE increases the phosphorescence anisotropy of G63 Fab and slows its rotational relaxation. Lateral diffusion of G63 Fab is also slowed by antigen clustering of the receptor. Taken together, these results indicate that unperturbed MAFA associates with clustered FcepsilonRI. They are also consistent with its interaction with the isolated receptor.

Anomalous subdiffusion in fluorescence photobleaching recovery: a Monte Carlo study

Anomalous subdiffusion is hindered diffusion in which the mean-square displacement of a diffusing particle is proportional to some power of time less than one. Anomalous subdiffusion has been observed for a variety of lipids and proteins in the plasma membranes of a variety of cells. Fluorescence photobleaching recovery experiments with anomalous subdiffusion are simulated to see how to analyze the data. It is useful to fit the recovery curve with both the usual recovery equation and the anomalous one, and to judge the goodness of fit on log-log plots. The simulations show that the simplest approximate treatment of anomalous subdiffusion usually gives good results. Three models of anomalous subdiffusion are considered: obstruction, fractional Brownian motion, and the continuous-time random walk. The models differ significantly in their behavior at short times and in their noise level. For obstructed diffusion the approach to the percolation threshold is marked by a large increase in noise, a broadening of the distribution of diffusion coefficients and anomalous subdiffusion exponents, and the expected abrupt decrease in the mobile fraction. The extreme fluctuations in the recovery curves at and near the percolation threshold result from extreme fluctuations in the geometry of the percolation cluster.

Molecular dynamics of point mutated I-A(k) molecules expressed on lymphocytes

We have recently reported the lateral and rotational diffusion parameters for I-A(k) molecules expressing various cytoplasmic truncations (Int. Immunol. 12 (2000) 1319). We now describe the membrane dynamics of I-A(k) with various mutations in the presumed contact region between alphabeta-heterodimers in an (alphabeta)2 dimer of dimers structure. Such mutations are known to strongly affect the antigen presentation ability of these molecules (Int. Immunol. 10 (1998) 1237-1249) but cause relatively small changes in the molecular dynamics of I-A(k). Lateral diffusion coefficients of I-A(k) wild-type molecules and mutants obtained via fringe fluorescence photobleaching recovery (FPR) ranged from 1.1 to 2.3x10(-10)cm2/s at room temperature while fractional mobilities averaged 75+/-6%. For all cell types examined, treatment with either hen egg lysozyme 46-61 peptide or db-cAMP reduced the I-A(k) mobile fraction by about 10% relative to untreated cells, suggesting that these treatments may increase lateral confinement of class II in lipid rafts or cytoskeletal interactions of the molecules. Wild-type I-A(k) and mutants capable of normal or partial antigen presentation exhibited, as a group, slightly longer rotational correlation times (RCT) at 4 degrees C than did mutants inactive in antigen presentation, 14+/-4 versus 10+/-1 micros, respectively. Moreover, peptide, cAMP and anti-CD40 mAb treatment all increased rotational correlation times for fully- and partially-functional I-A(k) but not for non-functional molecules. For example, 16 h peptide treatment yielded average RCTs of 28+/-12 and 10+/-1 micros for the groups of functional and non-functional molecules, respectively. Such modulation of the dynamics of functional class II molecules is consistent with these treatments' stabilization of class II or induction of new gene expression. Measurements of fluorescence resonant energy transfer between I-A(k), though complicated by cellular autofluorescence, averaged 6+/-7% over 15 cells or treatments, a result consistent with the presence of a small fraction of I-A(k) as a dimer of dimers species. In summary, our results suggest subtle changes in the molecular motions of class II molecules correlate with a significant impact on class II function. Molecules active in antigen presentation exhibit more restricted motion in the membrane, and thus presumably more extensive intermolecular interactions, than non-functional molecules. Further, treatments, such as db-cAMP and anti-CD40, which rescue antigen presentation by partially defective mutants, appear to increase such interactions, several types of which have already been reported for class II. A more detailed understanding of these phenomena will require both more sensitive biophysical tools and a more refined model of the role of class II intermolecular interactions in antigen presentation.

Combined laser tweezers and dielectric field cage for the analysis of receptor-ligand interactions on single cells

A new technique based on the combination of optical and chip-based dielectrophoretical trapping was developed and employed to manipulate cells and beads with micrometer precision. The beads were trapped with optical tweezers (OT) and brought into contact for defined times with cells held in the dielectrophoretic field cage (DFC). The well-defined ligand-receptor system biotin-streptavidin was used to study the multiple interaction between biotinylated live cells and streptavidin-coated beads. The biotin density on the cell surface was varied down to a few single bonds (3 +/- 2 bonds/microm2) to control the valency of the binding. The quantitative relationship between the contact area, ligand density and its diffusion rate in the outer membrane of the cell could be demonstrated. The increase of the strength of the cell-bead adhesion was strictly dependent on the increase of individual bond numbers in the contact area. This is in part due to accumulation of ligands (D approxiamtely (0.5 +/- 0.1) 10(-8) cm2/s) in the contact area as seen by confocal laser scanning microscopy. Individual receptor-ligand rupture forces were evaluated and are compatible with values obtained by biomembrane force probe techniques. To summarize, the combination leads to a new powerful microsystem for cell handling and pN-force measurements on the single-cell level.

Rotational and lateral dynamics of I-A(k) molecules expressing cytoplasmic truncations

Rotational and lateral diffusion of I-A(k) molecules with various alpha and beta chain cytoplasmic truncations known to affect class II function were measured to assess the role of cytoplasmic domains in regulating I-A(k) molecular motions. Deletion of all 12 alpha chain C-terminal residues and all 18 corresponding beta chain residues (alpha-12/beta-18) is known to abrogate translocation of protein kinase C to the nucleus upon class II cross-linking. Similarly, truncation of the entire cytoplasmic alpha chain domain and the 10 C-terminal residues of the beta chain impairs presentation of antigenic peptides to T cells. The rotational correlation time of the wild-type molecule, 11.9 +/- 2.6 micros as measured by time-resolved phosphorescence anisotropy, decreased to 7. 2 +/- 3.7 micros in the fully truncated alpha-12/beta-18 protein. Other truncated class II molecules exhibited only small changes in molecular rotation rates relative to the wild-type. The rate of lateral diffusion of the fully truncated molecule, measured with two independent methods, 2.3 x 10(-10) cm(2)/s, was comparable with that of the wild-type molecule. Thus, it appears that the alpha and beta chain cytoplasmic domains regulate the molecular motions of unperturbed I-A(k) molecules only modestly, despite the known involvement of these regions in class II signaling. Various explanations for this behavior are discussed, e.g. the possibility that class II membrane complexes are sufficiently large that association and dissociation of specific signaling proteins during antigen presentation do not significantly perturb the apparent molecular motions of the complex.

Translational diffusion of globular proteins in the cytoplasm of cultured muscle cells

Modulated fringe pattern photobleaching (MFPP) was used to measure the translational diffusion of microinjected fluorescein isothiocyanate (FITC)-labeled proteins of different sizes in the cytoplasm of cultured muscle cells. This technique, which is an extension of the classical fluorescence recovery after photobleaching (FRAP) technique, allows the measurement of the translational diffusion of macromolecules over several microns. Proteins used had molecular masses between 21 and 540 kDa. The results clearly indicated that the diffusivity of the various proteins is a decreasing function of their hydrodynamic radius. This decrease is more rapid with globular proteins than with FITC-labeled dextrans (, Biophys. J. 70:2327-2332), most likely because, unlike globular proteins, dextrans are randomly coiled macromolecules with a flexible structure. These data do not exclude the possibility of a rapid diffusion over a short distance, unobservable with our experimental set-up, which would take place within the first milliseconds after bleaching and would correspond to the diffusion in restricted domains followed by impeded diffusion provoked by the network of microtubules, microfilaments, and intermediate filaments. Thus our results may complement rather than contradict those of Verkman and collaborators (, J. Cell Biol. 138:1-12). The biological consequence of the size-dependent restriction of the mobility of proteins in the cell cytoplasm is that the formation of intracellular complexes with other proteins considerably reduces their mobility.