A localized pattern photobleaching method for the concurrent analysis of rapid and slow diffusion processes

Methods for measuring rates of protein binding to insoluble scaffolds in living cells: Histone H1-chromatin interactions

Understanding of cell regulation is limited by our inability to measure molecular binding rates for proteins within the structural context of living cells, and many systems biology models are hindered because they use values obtained with molecules binding in solution. Here, we present a kinetic analysis of GFP-histone H1 binding to chromatin within nuclei of living cells that allows both the binding rate constant k(ON) and dissociation rate constant k(OFF) to be determined based on data obtained from fluorescence recovery after photobleaching (FRAP) analysis. This is accomplished by measuring the ratio of bound to free concentration of protein at steady state, and identifying the rate-determining step during FRAP recovery experimentally, combined with mathematical modeling. We report k(OFF) = 0.0131/s and k(ON) = 0.14/s for histone H1.1 binding to chromatin. This work brings clarity to the interpretation of FRAP experiments and provides a way to determine binding kinetics for nuclear proteins and other cellular molecules that interact with insoluble scaffolds within living cells.

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.

Fluorescence recovery after photobleaching of suspensions of vacuoles

In this work we derive theoretical expressions for the FRAP measured on a liquid suspension of vacuoles labelled by a fluorescent probe bound to the surface membrane of these vacuoles. The bleaching laser beam creates an inhomogeneity in the surface concentration of the probe molecules. We consider the case in which the randomization of these probe molecules on the vacuole surface occurs much faster than the fluorescent recovery due to the vacuole diffusion. For a given value of the bleaching parameter K, we found that the bleaching fraction of the fluorescent molecules and the fluorescence recovery rate are decreasing functions of the square ratio of the vacuole to the laser beam radius of the FRAP instrument.

In vitro measurement and screening of monoclonal antibody affinity using fluorescence photobleaching

Antibody screening is a routine in vitro assay in monoclonal antibody development and production. We have recently adapted the fluorescence photobleaching method to quantify antibody mass transport and binding parameters in bulk solution (Kaufman and Jain, 1990, 1991). The present study uses this in vitro method to screen a series of monoclonal antibodies (IgG) developed against the rabbit VX2 carcinoma tumor line. These experiments indicate that the three antibodies recognize distinct epitopes on the tumor, with equilibrium binding constants of 1.3 +/- 0.5, 5.1 +/- 3.6 and 2.0 +/- 1.1 x 10(7) M-1 for the antibodies RVC-184, RVC-626 and RVC-779, respectively. The antibody diffusion coefficient revealed no dependent upon protein concentration or antigen bead volume fraction within the ranges investigated. It was demonstrated experimentally that the interactions conformed to a reaction limited binding model of fluorescence recovery, that the system was at equilibrium, and that non-specific binding due to the fluorescein probe was not significant. Once the non-reactive fraction of antibody is determined, this photobleaching technique does not require perturbation or physical separation of the unbound species. As such, it has many potential applications including in vivo investigation of binding parameters.

Theory of fluorescence recovery after photobleaching measurements on cylindrical surfaces

The theory of fluorescence recovery after photobleaching measurements of isotropic diffusion on a cylindrical surface is developed for Gaussian beam illumination centered perpendicular to an infinitely-long cylinder. A general analytical solution is obtained which is a function of the ratio of the cylindrical radius (r) to the beam exp [-2] radius omega. Numerical analysis of this solution demonstrates that significant deviations from one dimensional recovery are observed for omega less than 3r and from two-dimensional recovery for omega greater than or equal to 0.5r. Numerical data and an algorithm for analysis of recovery data where 0.5r less than or equal to omega less than or equal to 3r is presented.

Lateral diffusion in an archipelago. Distance dependence of the diffusion coefficient

An understanding of the distance dependence of the lateral diffusion coefficient is useful in comparing the results of diffusion measurements made over different length scales, and in analyzing the kinetics of mobile redox carriers in organelles. A distance-dependent, concentration-dependent diffusion coefficient is defined, and it is evaluated by Monte Carlo calculations of a random walk by mobile point tracers in the presence of immobile obstacles on a triangular lattice, representing the diffusion of a lipid or a small protein in the presence of immobile membrane proteins. This work confirms and extends the milling crowd model of Eisinger, J., J. Flores, and W. P. Petersen (1986. Biophys J. 49:987-1001). Similar calculations for diffusion of mobile particles interacting by a hard-core repulsion yield the distance dependence of the self-diffusion coefficient. An expression for the range of short-range diffusion is obtained, and the distance scales for various diffusion measurements are summarized.

Interpretation of fluorescence correlation spectroscopy and photobleaching recovery in terms of molecular interactions

The theoretical basis and experimental implementation of FCS and FPR measurements are now well established. Because of the requirements for system stability and long data acquisition times FCS is relatively rarely used. But FCS can provide unique information, especially about extents of aggregation or polymerization and therefore is a useful supplement to FPR for certain applications. FPR measurements are now carried out routinely in many laboratories in a variety of formats using different beam profiles, optical systems, and analytical schemes. A particular version may be better adapted to a specific application. The spot photobleaching approach, however, seems simplest and most versatile for cellular studies and is now most often used. Important experimental considerations in setting up a spot photobleaching instrument are discussed in detail in Chapter 10 by Wolf (this volume) and elsewhere (Petersen et al., 1986a). In interpreting FPR measurements it is also important to take into account the possibility of systematic errors from a number of sources. In Chapter 10 in this volume, Wolf discusses many factors that must be properly controlled in carrying out FPR measurements. Additional consideration of some of these points is presented by Petersen et al. (1986a). One potentially troublesome type of error arises from the possibility that chemical reactions initiated by the photobleaching pulse or during the measurement of recovery could significantly perturb the system. Evidence from a variety of sources [summarized, for example, in Petersen et al. (1986a)] indicates that photobleaching fluorophores can induce chemical cross-linking of cellular proteins under some conditions. But measurements in a number of different systems have demonstrated that, even if these types of reactions occur in FPR measurements, nevertheless they do not perturb the measured mobilities. If possible, however, this point should be checked for each new system because variations in structure or environmental conditions could enhance the chemical cross-linking reactions mediated by photogenerated free radicals. In practice, the principal difficulty in carrying out FPR measurements on cells is frequently the low intensity of the fluorescent signal which can be obtained from specifically labeled cell surface ligands or microinjected components. This low intensity results from the typically low capacity of an individual cell for the specifically labeled macromolecule. Even in the absence of systematic errors, low emission intensity will reduce the accuracy of measurements due to shot noise. This is an important practical limitation on measuring accuracy. Low measurement accuracy severely limits the extent to which the data can be interpreted mechanistically. Precision can be improved by averaging many recovery experiments.(ABSTRACT TRUNCATED AT 400 WORDS)

Diffusion of dihydropyridine calcium channel antagonists in cardiac sarcolemmal lipid multibilayers

A membrane bilayer pathway model has been proposed for the interaction of dihydropyridine (DHP) calcium channel antagonists with receptors in cardiac sarcolemma (Rhodes, D.G., J.G. Sarmiento, and L.G. Herbette. 1985. Mol. Pharmacol. 27:612-623) involving drug partition into the bilayer with subsequent receptor binding mediated (though probably not rate-limited) by diffusion within the bilayer. Recently, we have characterized the partition step, demonstrating that DHPs reside, on a time-average basis, near the bilayer hydrocarbon core/water interface. Drug distribution about this interface may define a plane of local concentration for lateral diffusion within the membrane. The studies presented herein examine the diffusional dynamics of an active rhodamine-labeled DHP and a fluorescent phospholipid analogue (DiIC16) in pure cardiac sarcolemmal lipid multibilayer preparations as a function of bilayer hydration. At maximal bilayer hydration, the drug diffuses over macroscopic distances within the bilayer at a rate identical to that of DiI (D = 3.8 X 10(-8) cm2/s), demonstrating the overall feasibility of the membrane diffusion model. The diffusion coefficients for both drug and lipid decreased substantially as the bilayers were dehydrated. While identical at maximal hydration, drug diffusion was significantly slower than that of DiIC16 in partially dehydrated bilayers, probably reflecting differences in mass distribution of these probes in the bilayer.

Microscopic imaging of cells

The microworld was revealed to investigators through a glass bead or a hanging water droplet long before optics was understood. The cellular structure of plants was well resolved by such simple magnifying glasses, van Leeuwenhoek, the Dutch merchant and amateur microscopist, was the first to report to the English Royal Society his observations of bacteria with his single-lens microscope in 1665.

High lateral mobility of endogenous and transfected alkaline phosphatase: a phosphatidylinositol-anchored membrane protein

The lateral mobility of alkaline phosphatase (AP) in the plasma membrane of osteoblastic and nonosteoblastic cells was estimated by fluorescence redistribution after photobleaching in embryonic and in tumor cells, in cells that express AP naturally, and in cells transfected with an expression vector containing AP cDNA. The diffusion coefficient (D) and the mobile fraction, estimated from the percent recovery (%R), were found to be cell-type dependent ranging from (0.58 +/- 0.16) X 10(-9) cm2s-1 and 73.3 +/- 10.5 in rat osteosarcoma cells ROS 17/2.8 to (1.77 +/- 0.51) X 10(-9) cm2s-1 and 82.8 +/- 2.5 in rat osteosarcoma cells UMR106. Similar values of D greater than or equal to 10(-9) cm2s-1 with approximately 80% recovery were also found in fetal rat calvaria cells, transfected skin fibroblasts, and transfected AP-negative osteosarcoma cells ROS 25/1. These values of D are many times greater than "typical" values for membrane proteins, coming close to those of membrane lipid in fetal rat calvaria and ROS 17/2.8 cells (D = [4(-5)] X 10(-9) cm2s-1 with 75-80% recovery), estimated with the hexadecanoyl aminofluorescein probe. In all cell types, phosphatidylinositol (PI)-specific phospholipase C released 60-90% of native and transfection-expressed AP, demonstrating that, as in other tissue types, AP in these cells is anchored in the membrane via a linkage to PI. These results indicate that the transfected cells used in this study possess the machinery for AP insertion into the membrane and its binding to PI. The fast AP mobility appears to be an intrinsic property of the way the protein is anchored in the membrane, a conclusion with general implications for the understanding of the slow diffusion of other membrane proteins.

Biophysical consequences of lipid peroxidation in membranes

This article reviews the biophysical consequences of lipid peroxidation in biological membranes. In the lipid domain, lipid peroxidation (a) causes an increase in the order and "viscosity" of the membrane bilayer, particularly at the depth around acyl-carbon 12, (b) changes the thermotropic phase behaviour, (c) decreases the electrical resistance, and (d) facilitates phospholipid exchange between the two monolayers. Upon lipid peroxidation membrane proteins are crosslinked, and their rotational and lateral mobility is decreased. Studies with microsomal cytochrome P-450 suggest protein aggregation but not the increased lipid order to be the major cause of protein immobilization in peroxidized membranes.

The mobility of concanavalin A receptors and surface immunoglobulins on rat hepatocyte plasma membranes

Lateral mobilities of lectin receptors and surface immunoglobulins were measured in plasma membranes of hepatocytes prepared by smearing small pieces of rat liver tissue and then using the fluorescence recovery after photobleaching (FRAP) technique. Smears were treated with various doses of fluorescein isothiocyanate (FITC) conjugated concanavalin A (ConA), succinylated ConA (SConA), wheat germ agglutinin (WGA), and soybean agglutinin (SBA), as well as with rabbit anti-rat IgG (RARa/IgG) and goat anti-rat IgM(Fc) (GARa/IgM(Fc] antisera. 10 micrograms/ml ConA and SConA concentrations and a 55 X dilution of the GARa/IgM(Fc) antiserum were found to be suitable for measuring the lateral mobilities dependent on age. Diffusion constant and mobile fractions of receptor complexes were measured in different age groups of female Fisher rats (from 1 to 26 month-old). The FRAP measurements revealed that at least two major receptor sites can be distinguished in cell membranes of compact tissue (similar to the cultured and isolated cells), forming a mobile and an immobile fraction. The mobile fractions of both the lectin receptors and the surface immunoglobulins tended to decrease with age, while the age differences of the diffusion constants were not statistically significant. The observed alterations could be due to the covalent crosslinking of the mobile receptors to immobile patches and/or to the retardation of free diffusion by the cytoskeleton, dependent on age.

Asymmetric lateral mobility of phospholipids in the human erythrocyte membrane

The fluorescent phospholipid 1-acyl-2-[12-(7-nitrobenz-2-oxa-1,3-diazol-4- yl)aminododecanoyl]phosphatidylcholine (NBD-phosphatidylcholine) and the corresponding aminophospholipid derivatives (NBD-phosphatidylethanolamine and NBD-phosphatidylserine) were introduced in the human erythrocyte membrane by a nonspecific phospholipid exchange protein purified from corn. The lateral mobility of the fluorescent phospholipids was measured by using an extension of the classical photobleaching recovery technique that takes advantage of a modulated fringe pattern and provides a high sensitivity. In intact erythrocytes and in ghosts resealed in the presence of ATP, the fluorescence-contrast curves after photobleaching decayed biexponentially corresponding to two lateral diffusion constants. With NBD-phosphatidylcholine, the majority of the signal corresponded to a "slow" component (1.08 X 10(-9) cm2/sec at 20 degrees C), whereas with the amino derivatives the majority of the signal corresponded to a "fast" component (5.14 X 10(-9) cm2/sec at 20 degrees C). If the ghosts were resealed without ATP, the fast component of the aminophospholipids disappeared. We interpret these results as follows: (i) Provided the cells or the ghosts contain ATP, the three fluorescent phospholipids distribute spontaneously between inner and outer leaflets as endogenous phospholipids, namely NBD-phosphatidylcholine is located in the outer leaflet, while both aminophospholipids are preferentially located in the inner leaflet. (ii) The viscosity of the inner leaflet of human erythrocyte membranes is lower than that of the outer leaflet.

Normal-mode analysis of lateral diffusion on a bounded membrane surface

The normal-mode analysis of fluorescence redistribution after photobleaching, introduced for the characterization of lateral diffusion on spherical membrane surfaces, has been generalized and extended to other surface geometries. Theoretical expressions are derived for the characteristic values and orthogonal characteristic functions of the diffusion equations for cylindrical surfaces, ellipsoids of revolution and dimpled discoidal surfaces. On the basis of these results, a simple analytical function is proposed as an empirical solution for the analysis of photobleaching data on a variety of discoidal surfaces. Special experimental and computational methods for determining the surface-diffusion coefficient are described, and demonstrated with data for lipid diffusion in erythrocyte membranes.

A localized surface protein of guinea pig sperm exhibits free diffusion in its domain

Using the technique of fluorescence redistribution after photobleaching, we are studying the cellular mechanisms involved in localizing surface molecules to particular domains. A number of antigens localized to discrete surface regions have been identified with monoclonal antibodies on guinea pig sperm cells ( Primakoff , P., and D. G. Myles , 1983, Dev. Biol., 98:417-428). One of these monoclonal antibodies, PT-1, binds exclusively to the posterior tail region of the sperm cell surface. PT-1 recognizes an integral membrane protein that in complex with n-octyl-beta-D-glucopyranoside has a sedimentation coefficient of 6.8S in sucrose density gradients. Fluorescence redistribution after photobleaching measurements reveal that within its surface domain the PT-1 antigen diffuses rapidly (D = 2.5 X 10(-9) cm2/s) and completely (greater than 90% recovery after bleaching). These results rule out for this membrane protein all models that invoke immobilization as a mechanism for maintaining localization. We propose that the mechanism for localization of the PT-1 antigen may be a barrier to diffusion at the domain boundary.