Protein diffusion in cell membranes: some biological implications

A Lattice-Boltzmann scheme for the simulation of diffusion in intracellular crowded systems

Background

The intracellular environment is a complex and crowded medium where the diffusion of proteins, metabolites and other molecules can be decreased. One of the most popular methodologies for the simulation of diffusion in crowding systems is the Monte Carlo algorithm (MC) which tracks the movement of each particle. This can, however, be computationally expensive for a system comprising a large number of molecules. On the other hand, the Lattice Boltzmann Method (LBM) tracks the movement of collections of molecules, which represents significant savings in computational time. Nevertheless in the classical manifestation of such scheme the crowding conditions are neglected.

Methods

In this paper we use Scaled Particle Theory (SPT) to approximate the probability to find free space for the displacement of hard-disk molecules and in this way to incorporate the crowding effect to the LBM. This new methodology which couples SPT and LBM is validated using a kinetic Monte Carlo (kMC) algorithm, which is used here as our "computational experiment".

Results

The results indicate that LBM over-predicts the diffusion in 2D crowded systems, while the proposed coupled SPT-LBM predicts the same behaviour as the kinetic Monte Carlo (kMC) algorithm but with a significantly reduced computational effort. Despite the fact that small deviations between the two methods were observed, in part due to the mesoscopic and microscopic nature of each method, respectively, the agreement was satisfactory both from a qualitative and a quantitative point of view.

Conclusions

A crowding-adaptation to LBM has been developed using SPT, allowing fast simulations of diffusion-systems of different size hard-disk molecules in two-dimensional space. This methodology takes into account crowding conditions; not only the space fraction occupied by the crowder molecules but also the influence of the size of the crowder which can affect the displacement of molecules across the lattice system.

Electronic supplementary material

The online version of this article (doi:10.1186/s12859-015-0769-8) contains supplementary material, which is available to authorized users.

Forward transport of proteins in the plasma membrane of migrating cerebellar granule cells

Directional flow of membrane components has been detected at the leading front of fibroblasts and the growth cone of neuronal processes, but whether there exists global directional flow of plasma membrane components over the entire migrating neuron remains largely unknown. By analyzing the trajectories of antibody-coated single quantum dots (QDs) bound to two membrane proteins, overexpressed myc-tagged synaptic vesicle-associated membrane protein VAMP2 and endogenous neurotrophin receptor TrkB, we found that these two proteins exhibited net forward transport, which is superimposed upon Brownian motion, in both leading and trailing processes of migrating cerebellar granule cells in culture. Furthermore, no net directional transport of membrane proteins was observed in nonmigrating cells with either growing or stalling leading processes. Analysis of the correlation of motion direction between two QDs on the same process in migrating neurons also showed a higher frequency of correlated forward than rearward movements. Such correlated QD movements were markedly reduced in the presence of myosin II inhibitor blebbistatin,suggesting the involvement of myosin II-dependent active transport processes. Thus, a net forward transport of plasma membrane proteins exists in the leading and trailing processes of migrating neurons, in line with the translocation of the soma.

Cellular fluid mechanics

The coupling of fluid dynamics and biology at the level of the cell is an intensive area of investigation because of its critical role in normal physiology and disease. Microcirculatory flow has been a focus for years, owing to the complexity of cell-cell or cell-glycocalyx interactions. Noncirculating cells, particularly those that comprise the walls of the circulatory system, experience and respond biologically to fluid dynamic stresses. In this article, we review the more recent studies of circulating cells, with an emphasis on the role of the glycocalyx on red-cell motion in small capillaries and on the deformation of leukocytes passing through the microcirculation. We also discuss flows in the vicinity of noncirculating cells, the influence of fluid dynamic shear stress on cell biology, and diffusion in the lipid bi-layer, all in the context of the important fluid-dynamic phenomena.

Bioelectric potential gradients may initiate cell cycling: ELF and zeta potential gradients may mimic this effect

When a number of experimental studies in bioelectromagnetics were reviewed, those in which weak, exogenous extremely low frequency (ELF) fields were applied in fixed juxtaposition to their target tissues, were found to initiate mitogenesis or mitogenesis-related signals more successfully than when the target tissue moved freely during the irradiation. It is suggested that ELF fields in fixed juxtaposition to their target tissue and implanted foreign bodies or endogenous tissues with a significant zeta potential, mimic bioelectric fields generated at wounds. When the potential is high enough, they assist healing by moving cells into the wound and stimulating quiescent cells at the wound margin to cycle. Electrophoresis may help the initial migration of cells into the wound to form a clot, and migration of fibroblasts and epithelial cells from the wound margin. When exposed for a long time in a fixed juxtaposition to a potential gradient too weak to show in situ microelectrophoresis along the cell membrane surface, surface particles may coalesce to form microclusters, where like-charged surface particles are in close proximity, and growth factor receptor oligomerization and other cycle-initiating reactions are facilitated.

Regulation of protein mobility in cell membranes: a dynamic corral model

We analyze a two-state stochastic corral model for regulation of protein diffusion in a cell membrane. This model could mimic control of protein transport in the membrane by the cytoskeleton. The dynamic corral acts as a gate which when open permits an otherwise trapped protein to escape to a neighboring corral in the cytoskeletal network. We solve for the escape rate over a wide range of parameters of the model, and compare these results with Monte Carlo simulations. Upon introducing measured values of the model parameters for Band 3 in erythrocyte membranes, we are able to estimate the value for one unknown parameter, the average rate at which the corral closes. The ratio of calculated closing rate to measured opening rate is roughly 100:1, consistent with a gating mechanism whereby protein mobility is regulated by dissociation and reassociation of segments of the cytoskeletal network.

Contraction in the smooth muscle cell

This paper advances the hypothesis that the rearrangement of the actin cytoskeleton that takes place during contraction in the SMC is a mechanical reflection of the spatiotemporal pattern of the cell's polarized stimulus. In that sense the cell is responding more like a motile non-muscle cell than like a skeletal muscle cell. The paper reviews how diffusion patterns are generated and modified and suggests how the patterns are detected by the cell and transduced into cytoskeletal movement. Evidence is presented suggesting the actin cytoskeleton is composed of conical-shaped myofibrils (contractile units) measuring half a cell in length and containing filament-free spaces at their centres filled with cell inclusions. It is argued that the SMC contracts by involving variable combinations of the myofibrils in sequence and that the cell takes advantage of that fact to translocate various contractile elements between the myofibrils during contraction, thus economizing on its needs for those elements. Among the elements translocated are thought to be myosin, SR and mitochondria.

Weak dependence of mobility of membrane protein aggregates on aggregate size supports a viscous model of retardation of diffusion

Proteins in plasma membranes diffuse more slowly than proteins inserted into artificial lipid bilayers. On a long-range scale (>250 nm), submembrane barriers, or skeleton fences that hinder long-range diffusion and create confinement zones, have been described. Even within such confinement zones, however, diffusion of proteins is much slower than predicted by the viscosity of the lipid. The cause of this slowing of diffusion on the micro scale has not been determined and is the focus of this paper. One way to approach this question is to determine the dependence of particle motion on particle size. Some current models predict that the diffusion coefficient of a membrane protein aggregate will depend strongly on its size, while others do not. We have measured the diffusion coefficients of membrane glycoprotein aggregates linked together by concanavalin A molecules bound to beads of various sizes, and also the diffusion coefficients of individual concanavalin A binding proteins. The measurements demonstrate at most a weak dependence of diffusion coefficient on aggregate size. This finding supports retardation by viscous effects, and is not consistent with models involving direct interaction of diffusing proteins with cytoskeletal elements.

Temperature-induced differential kinetic properties between an initial burst and the following steady state in membrane-bound enzymes: studies on lathosterol 5-desaturase

The NADH-dependent lathosterol 5-desaturation reaction that forms 7-dehydrocholesterol is biphasic, an initial burst followed by steady state. The steady-state phase is slower than the burst phase, because the latter diffusion of the lathosterol substrate within the microsomal membrane must occur before the next reaction can take place [Y. Takakuwa, H. Nishino, Y. Ishibe, and T. Ishibashi (1994) J. Biol. Chem. 269, 27889-27893]. In the present study, changes in the structure and function of the membrane were examined by measurement of the Arrhenius activation energy of lathosterol 5-desaturase at various temperatures between 2 and 45 degrees C. At the burst phase, there was a lack of discontinuity in the Arrhenius plots at the presumed phase transition temperature for the microsomal membrane. However, the plots of the activities of the steady state showed breaks at around 17 and 32 degrees C. It was concluded that phospholipid phase transition affects the steady-state phase but not the burst phase. Furthermore, treatment of microsomes with low concentrations of deoxycholate, known to perturb the membrane integrity, resulted in a break of the activation energy of the burst phase. These results have revealed further evidence for our previous model suggesting interaction between the substrate and enzyme within the microsomal membrane via lateral diffusion.

Tracking movements of lipids and Thy1 molecules in the plasmalemma of living fibroblasts by fluorescence video microscopy with nanometer scale precision

The lateral diffusion of 100 nm fluorescent latex microspheres (FS) bound to either N-biotinyl-phosphatidyl-ethanolamine or the glycosylphosphatidylinositol-linked protein Thy1 were monitored in the plasmalemma of primary rat fibroblasts by single particle tracking of FS centroids from digital fluorescence micrographs. A silicon intensified target camera was found to be superior to slow scan cooled CCD and intensified interline transfer CCD cameras for monitoring lateral diffusion of rapidly moving FS with nanometer level precision. To estimate the maximum tracking precision, a 4 sec-sequence comprising 120 images of FS fixed to a cover glass was obtained. The mean distance of the centroids from the origin was 7.5 +/- 0.4 nm, and no centroids were beyond 16 nm from the origin. The SIT camera was then used to track FS attached to lipids and Thy1 molecules on the surface of fibroblasts. The lateral diffusion of lipid-bound FS was unconstrained, and the ensemble averaged diffusion coefficient was 0.80 x 10(-9) cm2/sec. Thy1-bound FS existed in two mobility populations, both of which demonstrated constrained mobility. The rapidly moving population, comprising 61% of the total, had an ensemble diffusion coefficient of 6.1 x 10(-10) cm2/sec, and appeared to be restricted to domains with a mean length of about 700 nm. The slowly moving population, comprising about 39% of the total, had a diffusion coefficient of 5.7 x 10(-12) cm2/sec. These results demonstrate that nanovid can be extended to the realm of fluorescence microscopy and support previous studies indicating that while the lateral mobilities of at least some lipids are not constrained to small domains by barriers to lateral diffusion in the fibroblast plasmalemma, a peripheral membrane protein which is bound only by a lipid anchor can be prevented from diffusing freely.

Potentiation by ATP of the postsynaptic acetylcholine response at developing neuromuscular synapses in Xenopus cell cultures

1. Extracellular application of ATP to developing Xenopus neuromuscular synapses in culture resulted in a marked increase in the amplitude and frequency of spontaneous synaptic currents, using whole-cell recording. 2. The postsynaptic action of ATP was examined by studying the response of isolated muscle cells to iontophoretically applied acetylcholine (ACh). ATP enhanced the responses of the muscle membrane to ACh. The order of potency for various nucleotides (ATP = ADP >> AMP, adenosine, GTP) suggests that ATP acts through P2-purinoceptors. The effect of ATP on whole-cell currents was also abolished by the protein kinase inhibitor H-7. 3. Single-channel measurements indicate that ATP increased the mean open time of low-conductance ACh channels. No change in the conductance of ACh channels was observed. 4. Local application of ATP to one region of the elongated myocyte surface resulted in potentiated ACh responses only at the ATP-treated region, suggesting that the cytosolic second messengers were effectively confined within the muscle cytoplasm. 5. The results of the present study suggest that ATP released from the nerve terminals may potentiate the ACh response of developing muscle cells during the early phase of synaptogenesis, and that the action of ATP can be restricted to the subsynaptic region exposed to the secreted ATP.

Carcinogenesis and initiation of cell cycling by charge-induced membrane clusters may be due to mitogen receptors and Na+/H+ antiports

The membrane cluster hypothesis of mitogenesis and carcinogenesis is extended by proposing that much of the Na+ ingress across a cell's plasma membrane at surface charge-induced (SCI) aggregates is due to mitogen-induced activation of Na+/H+ antiports. Intrinsic proteins (including mitogen receptors and antiports) are electrostatically attracted to and become part of the aggregate. In this location, close proximity facilitates antiport activation. Resulting Na+ ingress may cause sustained partial depolarization, cytoplasmic alkalinization, and initiation of cell cycling. Chronic phosphorylation-dephosphorylation at SCI aggregates too weak to induce cycling, may slowly form polyionic bonds between adjacent proteins at the inner lipid layer. These bonds convert the SCI aggregates to 'permanent' clusters that pass to a daughter cell with parental plasma membrane at mitosis, and are associated with malignancy. EGF and PDGF growth factors are used to develop the hypothesis, which is also applied to steroid and dioxin receptors and to oncogene products.

Automated detection and tracking of individual and clustered cell surface low density lipoprotein receptor molecules

We have developed a technique to detect, recognize, and track each individual low density lipoprotein receptor (LDL-R) molecule and small receptor clusters on the surface of human skin fibroblasts. Molecular recognition and high precision (30 nm) simultaneous automatic tracking of all of the individual receptors in the cell surface population utilize quantitative time-lapse low light level digital video fluorescence microscopy analyzed by purpose-designed algorithms executed on an image processing work station. The LDL-Rs are labeled with the biologically active, fluorescent LDL derivative dil-LDL. Individual LDL-Rs and unresolved small clusters are identified by measuring the fluorescence power radiated by the sub-resolution fluorescent spots in the image; identification of single particles is ascertained by four independent techniques. An automated tracking routine was developed to track simultaneously, and without user intervention, a multitude of fluorescent particles through a sequence of hundreds of time-lapse image frames. The limitations on tracking precision were found to depend on the signal-to-noise ratio of the tracked particle image and mechanical drift of the microscope system. We describe the methods involved in (i) time-lapse acquisition of the low-light level images, (ii) simultaneous automated tracking of the fluorescent diffraction limited punctate images, (iii) localizing particles with high precision and limitations, and (iv) detecting and identifying single and clustered LDL-Rs. These methods are generally applicable and provide a powerful tool to visualize and measure dynamics and interactions of individual integral membrane proteins on living cell surfaces.

Protein lateral mobility as a reflection of membrane microstructure

The lateral mobility of membrane lipids and proteins is presumed to play an important functional role in biomembranes. Photobleaching studies have shown that many proteins in the plasma membrane have diffusion coefficients at least an order of magnitude lower than those obtained when the same proteins are reconstituted in artificial bilayer membranes. Depending on the protein, it has been shown that either the cytoplasmic domain or the ectodomain is the key determinant of its lateral mobility. Single particle tracking microscopy, which allows the motions of single or small groups of membrane molecules to be followed, promises not only to reveal new features of membrane dynamics, but also to help explain longstanding puzzles presented by the photobleaching studies, particularly the so-called immobile fraction. The combination of the two complementary technologies should measurably enhance our understanding of membrane microstructure.

The membrane cluster hypothesis of mitogenesis and carcinogenesis

This paper modifies and extends an earlier one on the same subject. It explains why external (but not internal) surface molecules of plasma membrane clusters may be rapidly scattered by any external challenging bioelectrical field. Temporary clusters from challenges may induce mitosis in cells near wounds and in epithelial stem cells. Weak challenges of much longer duration may initiate carcinogenesis by permanent clusters. Basic intracellular ligand/receptors or oncogene products in sufficient concentration at the membrane inner lipid layer may form permanent clusters rapidly. Additive increase of inner surface clusters by initiating agents is equated to promotion; accelerated cluster growth to progression. As a malignant cell grows, its cluster population increases until its membrane becomes permeable enough to stimulate mitosis. A progression mechanism is suggested that is consistent with the known properties of ras p21 proteins. The effect of long term exposure to power transmission line fields on mitosis and carcinogenesis is discussed. An approach to anticancer therapy is suggested, using a hypothesis-based mechanism for the anti-cancer activity of retinoic acid.

Steady-state kinetics of ubiquinol-cytochrome c reductase in bovine heart submitochondrial particles: diffusional effects

In an attempt to establish the relative importance of diffusional and chemical control in the reactivity of the two of the two substrates, ubiquinol and cytochrome c, we have undertaken as extensive characterization of the steady-state kinetics of ubiquinol-cytochrome c reductase (EC 1.10.2.2) when present in open submitochondrial particles from bovine heart. The kinetic pattern follows a Ping Pong mechanism; contrary to the situation found with the isolated enzyme [Speck and Margoliash (1984) J. Biol. Chem. 259, 1064-1072, and confirmed in our laboratory], no substrate inhibition by oxidized cytochrome c was observed with the membrane-bound enzyme. Endogenous oxidized ubiquinone-10 is unable to exert product inhibition under the conditions employed. In the Ping Pong mechanism for this enzyme, the reaction scheme can be clearly divided into two parts, and the Kmin. (kcat./km) value for one substrate is independent of the rate constant for the second substrate. Both ubiquinol-1 and ubiquinol-2 can be used as electron donors reacting with the enzyme from within the lipid bilayer [Fato, Castelluccio, Palmer and Lenaz (1988) Biochim. Biophys. Acta 932, 216-222]; the kmin. values for ubiquinols, when calculated on the basis of their membranous concentrations, are significantly lower than the kmin. for cytochrome c. The temperature-dependence of the kinetic parameters was investigated by titrating each of the substrates under quasi-saturating concentrations of the second substrate. Arrhenius plots of Vmax. extrapolated from both cytochrome c and ubiquinol titrations were linear, when care was taken to verify the quasi-saturating concentrations of the fixed co-substrate. The Arrhenius plots for the kmin. values for both ubiquinol and cytochrome c were linear, but the activation energy was much higher for the former, particularly when calculated for ubiquinol dissolved in the lipid phase; the very low value of activation energy of the kmin. for cytochrome c is strong support for diffusion control being present in the reaction of cytochrome c with the membranous enzyme. In contrast to the soluble enzyme, ubiquinone titrations of submitochondrial particles at low cytochrome c concentrations deviated from hyperbolic behaviour. Changing the medium viscosity with either poly(ethylene glycol) or sucrose had a strong effect on the cytochrome c kmin., whereas the change in the ubiquinol kmin. was much smaller. From the viscosity studies the extent of diffusional control could be calculated, revealing that the reaction with cytochrome c was mostly diffusion-limited. The viscosity of the membrane was changed by incorporating cholesterol; no significant effect on the ubiquinol kmin. ascribable to diffusion control could be recognized.(ABSTRACT TRUNCATED AT 400 WORDS)

Amiloride-sensitive sodium channel is linked to the cytoskeleton in renal epithelial cells

Amiloride-sensitive sodium channels are localized to the microvillar domain of apical membranes in sodium-transporting renal epithelial cells. To elucidate the elements that maintain sodium channel distribution at the apical membrane, we searched for specific proteins associating with the channel. Triton X-100 extraction of A6 epithelial cells reveals that sodium channels are associated with detergent-insoluble and assembled cytoskeleton. Indirect immunofluorescence and confocal microscopy show that sodium channels are segregated to the apical microvillar membrane and colocalize with ankyrin, fodrin, and actin. We document by immunoblot analysis that ankyrin and fodrin remain associated with sodium channels after isolation and purification from bovine renal papillae. 125I-labeled ankyrine can be precipitated by anti-sodium-channel antibodies only in the presence of purified bovine sodium-channel complex. Direct binding of 125I-labeled ankyrin shows ankyrin binds to the 150-kDa subunit of the channel. Fluorescence photobleach lateral-diffusion measurements indicate sodium channels are severely restricted in their lateral mobility. We conclude that ankyrin links the amiloride-sensitive sodium channel to the underlying cytoskeleton and this association may sequester sodium channels at apical microvilli and maintain their polarized distribution in renal epithelial cells.

Quenched flow analysis of exocytosis in Paramecium cells: time course, changes in membrane structure, and calcium requirements revealed after rapid mixing and rapid freezing of intact cells

Synchronous exocytosis in Paramecium cells was analyzed on a subsecond time scale. For this purpose we developed a quenched flow device for rapid mixing and rapid freezing of cells without impairment (time resolution in the millisecond range, dead time approximately 30 ms). Cells frozen at defined times after stimulation with the noncytotoxic secretagogue aminoethyldextran were processed by freeze substitution for electron microscopic analysis. With ultrathin sections the time required for complete extrusion of secretory contents was determined to be less than 80 ms. Using freeze-fracture replicas the time required for resealing of the fused membranes was found to be less than 350 ms. During membrane fusion (visible 30 ms after stimulation) specific intramembranous particles in the cell membrane at the attachment sites of secretory organelles ("fusion rosette") disappear, possibly by dissociation of formerly oligomeric proteins. This hitherto unknown type of rapid change in membrane architecture may reflect molecular changes in protein-protein or protein-lipid interactions, presumably crucial for membrane fusion. By a modification of the quenched flow procedure extracellular [Ca++] during stimulation was adjusted to less than or equal to 3 x 10(-8) M, i.e., below intracellular [Ca++]. Only extrusion of the secretory contents, but not membrane fusion, was inhibited. Thus it was possible to separate both secretory events (membrane fusion from contents extrusion) and to discriminate their Ca++ requirements. We conclude that no Ca++ influx is necessary for induction of membrane fusion.

Binding and mobility of anti-dinitrophenyl monoclonal antibodies on fluid-like, Langmuir-Blodgett phospholipid monolayers containing dinitrophenyl-conjugated phospholipids

The association of a fluorescently labelled anti-dinitrophenyl monoclonal antibody (ANO2) with Langmuir-Blodgett monolayers composed of three different binary mixtures of phosphatidylcholine and dinitrophenyl-conjugated phosphatidylethanolamine has been characterized. Quantitative fluorescence microscopy measurements demonstrated that measurable amounts of antibodies bound to the monolayers only at high molar fractions of dinitrophenyl-conjugated lipid (greater than or equal to 5 mol%). Fluorescence pattern photobleaching recovery measurements showed that the apparent translational diffusion coefficients and mobile fractions of a fluorescent lipid were high for all monolayer compositions and that the antibody translational mobility was measurable but slow and depended on the two-dimensional antibody density. The results demonstrate that the ANO2-binding characteristics of Langmuir-Blodgett monolayers containing dinitrophenyl-conjugated phospholipids are substantially different from those of similar model systems but that the ANO2 antibodies, when bound, display similar diffusive behavior.

Gamete interactions and the fate of sperm organelles in fertilized echinoderm eggs

Investigations of gamete fusion, sperm entry and the fate of the sperm nucleus, plasma membrane, mitochondrion, and axonemal complex in fertilized echinoderm eggs are reviewed. The timing of gamete fusion with respect to the onset of electrical activity characteristic of the activated egg and the affects of fixation conditions on the stability of fusing membranes are discussed. Observations from investigations using cationized ferritin labeled gametes and immunogold cytochemistry to demonstrate the mixing of sperm plasma membrane components within the egg plasma membrane, in particular along the surface of the fertilization cone, are compared with results from studies in somatic cells. Transformations of the sperm nucleus into a male pronucleus, consisting of sperm nuclear envelope breakdown, chromatin dispersion, and formation of a pronuclear envelope, are correlated with recent biochemical observation of similar processes in other cellular systems. Fates of the sperm mitochondrion and axonemal complex are examined.

Solubility as a function of protein structure and solvent components

This review deals with ways of stabilizing proteins against aggregation and with methods to determine, predict, and increase solubility. Solvent additives (osmolytes) that stabilize proteins are listed with a description of their effects on proteins and on the solvation properties of water. Special attention is given to areas where solubility limitations pose major problems, as in the preparation of highly concentrated solutions of recombinant proteins for structural determination with NMR and X-ray crystallography, refolding of inclusion body proteins, studies of membrane protein dynamics, and in the formulation of proteins for pharmaceutical use. Structural factors relating to solubility and possibilities for protein engineering are analyzed.

Slow rotational mobilities of antibodies and lipids associated with substrate-supported phospholipid monolayers as measured by polarized fluorescence photobleaching recovery

Polarized fluorescence photobleaching recovery has been used to monitor slow rotational motions of a fluorescently-labeled anti-dinitrophenyl mouse IgGl monoclonal antibody (ANO2) specifically bound to substrate-supported monolayers composed of a mixture of distearoylphosphatidylcholine (DSPC) and dinitrophenyldioleoylphosphatidylethanolamine (DNP-DOPE). ANO2 antibodies were labeled with a new bifunctional carbocyanine fluorophore that has two amino-reactive groups; steady-state fluorescence anisotropy data confirmed the expected result that the ANO2-conjugated bifunctional probe had less independent flexibility than ANO2-conjugated unifunctional fluorescence labels. Rotational mobilities were also measured for the fluorescent lipid 1,1'-dioctadecyl 3,3,3',3'-tetramethylindocarbocyanine (dil) in DSPC and in mixed DSPC/DNP-DOPE monolayers in the presence and absence of unlabeled ANO2 antibodies. The apparent rotational correlation time and fractional mobility of ANO2 on supported monolayers were approximately 70 and approximately 0.3 s, respectively. These measured parameters of rotational mobility did not depend on the ANO2 surface density or on kinetic factors, but addition of unlabeled polyclonal anti-(mouse IgG) antibodies significantly decreased the apparent mobile fraction. The measured fluorescence recovery curves for dil were consistent with two fluorophore populations with rotational correlation times of approximately 4 and approximately 100 s and a population of immobile fluorescent lipid. No difference in fluorescence recovery and decay curves was measured for dil in DSPC monolayers, DSPC/DNP-DOPE monolayers, and DSPC/DNP-DOPE monolayers treated with unlabeled ANO2 antibodies.

Novel function of the cell adhesion molecule uvomorulin as an inducer of cell surface polarity

Na+,K(+)-ATPase has distinctly different distributions in mesenchymal cells, where it has an unrestricted distribution over the entire cell surface, compared with polarized epithelial cells, where it is restricted to the basal-lateral membrane domain. The generation of this restricted distribution is important in mesenchyme to epithelia conversion in development and the function of transporting epithelia, but the mechanisms involved are unknown. Here we show that expression of the epithelial CAM uvomorulin in transfected fibroblasts is sufficient to induce a redistribution of Na+,K(+)-ATPase to sites of uvomorulin-mediated cell-cell contacts, similar to that in polarized epithelial cells. This restricted distribution of Na+,K(+)-ATPase occurs in the absence of tight junctions but coincides with the reorganization of the membrane cytoskeleton. The results indicate a direct role for CAMs as inducers of cell surface polarity of selective cytoplasmic and membrane proteins.

Small intestinal differentiation in human colon carcinoma HT29 cells has distinct effects on the lateral diffusion of lipids (ganglioside GM1) and proteins (HLA class 1, HLA class 2, and neoplastic epithelial antigens) in the apical cell membrane

We have studied the effect of maturation to small intestinal-like epithelial cells of the human colonic carcinoma cell line HT29 on the lateral mobility of different representative membrane components (lipid, proteins), as assessed with fluorescence recovery after photobleaching (FRAP). Maturation was induced in vitro in the HT29 cells by replacing glucose (Glu) with galactose (Gal) in the growth medium (DMEM) during a 21-day period. Scanning electron microscopy revealed an increased number of microvilli in the apical cell membrane, and enzyme analyses (alkaline phosphatase, aminopeptidase) in combination with aqueous countercurrent distribution, indicated that maturation was induced with DMEM-Gal. In comparison to control cells grown in DMEM-Glu medium, the more small intestinal-like cells grown in DMEM-Gal displayed no alteration of the lateral mobility of either cholera toxin (B subunit)-labelled ganglioside GM1 (diffusion coefficient, D [x 10(8)] = 0.8-0.9 cm2s-1; mobile fraction, R = 50-60%) or antibody-stained Class 2 histocompatibility (HLA-DR) antigen (D [x 10(9)] = 2 cm2s-1; R = 60-70%). However, antibody-labelled beta 2-microglobulin of HLA Class 1 antigen displayed increased mobility in HT29-Gal cells; D was x 1.4 and R x 1.8 larger in the HT29-Gal cells. By contrast, the mobility of a neoplastic antigen was reduced; D and R were x0.60 and x0.69 of the values seen in HT29-Glu cells. It is thus concluded that DMEM-Gal-induced differentiation in confluent HT29 cells is accompanied by specific rather than general effects on the lateral mobility of different membrane components.

Diffusional and electrokinetic redistribution at the synapse: a physicochemical basis of synaptic competition

Coinnervating nerve terminals may compete for "stabilizing factors" confined within the postsynaptic cell. The competition could be achieved through a diffusion-mediated trapping the factor, facilitated by an activity-dependent electrokinetic migration of the factor toward the synaptic site. We have examined the evidence for diffusional and electrokinetic motions of cell surface and cytoplasmic components, the profile and magnitude of the electric field produced by the synaptic current, and the plausibility that these motions underlie the process of synaptic competition.

Role of heterologous and homologous glycoproteins in phenotypic mixing between Sendai virus and vesicular stomatitis virus

Phenotypic mixing between Sendai virus and vesicular stomatitis virus (VSV) or the mutant VSV ts045 was studied. Conditions were optimized for double infection, as shown by immunofluorescence microscopy. Virions from double-infected cells were separated by sequential velocity and isopycnic gradient centrifugations. Two types of particles with mixed protein compositions were found. One type was VSV particles with Sendai virus spikes, i.e., phenotypically mixed particles. A second type was Sendai virus-VSV associations, which in plaque assays also behaved as phenotypically mixed particles. The ratio of VSV G protein to Sendai virus glycoproteins on the cell surface was varied, using the VSV mutant ts045 in double infections. Thus, different amounts of the VSV G protein were allowed to reach the cell surface at 32, 38, and 39 degrees C in Sendai virus-infected cells. However, a fixed number of Sendai virus spikes was always found in the ts045 virions. This represented 12 to 16% of the number of G proteins present in normal VSV. Furthermore, the yield of ts045 virions was radically reduced during double infection when the temperature was raised to block G-protein transport to the cell surface, suggesting that the Sendai virus glycoproteins were not able to compensate for G protein in budding. These results emphasize the role of the G protein in VSV assembly.

Selforganization of a membrane in synaptic geometry

The self-organization of mobile membrane proteins in a synaptic structure is computed. The model of dissipative condensation is used which takes into account the intermolecular interactions due to electrical channel currents and electrophoretic charges as they occur in a leaky membrane. An instability of the homogeneous fluid mosaic of charged channels is observed in the region of the synapse. Periodic accumulations of channels appear which are modulated by the attraction of protein from the perisynaptic membrane. The sensitivity of the process with respect to the geometrical constraints and to electrochemical parameters is discussed.

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.

Membrane specializations associated with the acrosomal complex of sea urchin sperm as revealed by immunocytochemistry and freeze fracture replication

Observations, employing freeze fracture replication and electron microscopic immunochemistry, have been carried out to determine structural correlations of the plasma membrane domain occupied by a 210 kDa protein involved in the acrosomal reaction of sea urchin sperm and recognized by the monoclonal antibody, J10/14 (Trimmer et al.: Cell 40:697-703, 1985; Proceedings of the National Academy of Sciences of the United States of America 83: 9055-9059, 1986). Immunogold-J10/14 staining of acrosome-intact sperm was intense along the flagellum and a narrow collar just posterior to the sperm apex that surrounded the acrosomal complex (acrosomal vesicle and subjacent anterior nuclear fossa containing g-actin). Counts of gold particles revealed a density (average number of particles/micron2 of surface area) eightfold greater along the plasma membrane associated with the acrosomal complex than membrane delimiting the remainder of the sperm head. The collar of J10/14 staining was isomorphic with a dense aggregation of intramembranous particles in the P-face of the plasma membrane and a thin cytoplasmic region that surrounded the acrosomal complex. In acrosome-reacted sperm, intense J10/14 staining was distributed along the flagellum and sperm head; prominent anterior staining was not apparent in all specimens. The density of gold particles associated with plasma membrane delimiting components of the former acrosomal complex, nucleus and mitochondrion, as well as the total average number of particles along the entire sperm surface, were increased in sperm acrosome-reacted with A-23187. Concomitant with this change in staining was the disappearance/reduction of the collars of intramembranous particles and cytoplasm. These observations indicate that plasma membrane components (210 kDa protein and intramembranous particles) and the collar of cytoplasm which are associated with the acrosomal complex are functionally, as well as structurally related. Analyses of particle density distributions along acrosome- and non-acrosome-reacted sperm suggest that the different staining patterns observed may be brought about by the recognition of cryptic sites at the time of the acrosomal reaction.

Mutual diffusion of interacting membrane proteins

The generalized Stokes-Einstein equation is used, together with the two-dimensional pressure equation, to analyze mutual diffusion in concentrated membrane systems. These equations can be used to investigate the role that both direct and hydrodynamic interactions play in determining diffusive behavior. Here only direct interactions are explicitly incorporated into the theory at high densities; however, both direct and hydrodynamic interactions are analyzed for some dilute solutions. We look at diffusion in the presence of weak attractions, soft repulsions, and hard-core repulsions. It is found that, at low densities, attractions retard mutual diffusion while repulsions enhance it. Mechanistically, attractions tend to tether particles together and oppose the dissipation of gradients or fluctuations in concentration, while repulsions provide a driving force that pushes particles apart. At higher concentrations, changes in the structure of the fluid enhance mutual diffusion even in the presence of attractions. It is shown that the theoretical description of postelectrophoresis relaxation and fluorescence correlation spectroscopy experiments must be modified if interacting systems are studied. The effects of interactions on mutual diffusion coefficients have probably already been seen in postelectrophoresis relaxation experiments.

Two stage carcinogenesis by membrane potential changes

The membrane potential theory is modified and extended. It is shown to be applicable to carcinogenesis by prolonged treatment of target tissues with an initiating external carcinogen or by a single sub-threshold exposure to the initiating external carcinogen followed by subsequent treatment with a phorbol ester internal promoter.

Self diffusion of interacting membrane proteins

A two-dimensional version of the generalized Smoluchowski equation is used to analyze the time (or distance) dependent self diffusion of interacting membrane proteins in concentrated membrane systems. This equation provides a well established starting point for descriptions of the diffusion of particles that interact through both direct and hydrodynamic forces; in this initial work only the effects of direct interactions are explicitly considered. Data describing diffusion in the presence of hard-core repulsions, soft repulsions, and soft repulsions with weak attractions are presented. The effect that interactions have on the self-diffusion coefficient of a real protein molecule from mouse liver gap junctions is also calculated. The results indicate that self diffusion is always inhibited by direct interactions; this observation is interpreted in terms of the caging that will exist at finite protein concentration. It is also noted that, over small distance scales, the diffusion coefficient is determined entirely by the very strong Brownian forces; therefore, as a function of displacement the self-diffusion coefficient decays (rapidly) from its value at infinite dilution to its steady-state interaction-averaged value. The steady-state self-diffusion coefficient describes motion over distance scales that range from approximately 10 nm to cellular dimensions and is the quantity measured in fluorescence recovery after photobleaching experiments. The short-ranged behavior of the diffusion coefficient is important on the interparticle-distance scale and may therefore influence the rate at which nearest-neighbor collisional processes take place. The hard-disk theoretical results presented here are in excellent agreement with lattice Monte-Carlo results obtained by other workers. The concentration dependence of experimentally measured diffusion coefficients of antibody-hapten complexes bound to the membrane surface is consistent with that predicted by the theory. The variation in experimental diffusion coefficients of integral membrane proteins is greater than that predicted by the theory, and may also reflect protein-induced perturbations in membrane viscosity.

Direct visualization of the interrelationship between intramembrane and extracellular structures

The apical surface of the toad urinary bladder is covered by an interconnected mesh of glycocalyx, which appears to attach to the plasma membrane bilayer. To evaluate the interrelationship between these extracellular elements and intramembrane structures, a strategy was devised to produce composite replicas that allow the simultaneous visualization of intramembrane particles by freeze-fracture while the glycocalyx mesh is replicated by rotary shadowing of the extracellular surface after freeze-drying. Evaluation of these composite replicas by electron microscopy reveals that contacts occur between extracellular filamentous elements and intramembrane particles. This structural organization may be important for stabilizing intramembrane components and for anchoring extracellular elements to the membrane.

Concentration effects on reactions in membranes: rhodopsin and transducin

The reaction rate of two laterally-diffusing species in a biological membrane shows a maximum at some concentration of reactants, because an increase in the concentration of reactants tends to increase the reaction rate by the law of mass action but decreases the diffusion rate of the reactants. The activation of transducin by rhodopsin in the disk membrane of the rod outer segment is described in terms of a steady-state diffusion model with concentration-dependent diffusion coefficients. The optimum concentrations of reactants are obtained from contour plots of the reaction rate as a function of rhodopsin and transducin concentrations, and the sensitivity of the results to the assumed values of the variables is examined. To determine whether the observed concentrations are in fact those yielding the maximum reaction rate, several variables must be known more accurately.

Lateral diffusion of lectin receptors in fibroblast membranes as a function of cell shape

Anchorage-dependent fibroblasts respond to biochemical growth signals only when attached to and spread on a suitable substrate surface. Attachment of fibroblasts initiates a cytoskeletal assembly process that results in the organization of long actin stress fibers and microtubules which may be required for transmembrane signal transduction. Fibroblasts maintained in suspension, however, remain spherical with no apparent stress fibers or lengthy microtubules. Because of the significant differences in cytoskeletal organisation induced by shape modification, and the resulting possible changes in organization and dynamics of membrane receptors, the technique of fluorescence redistribution after photobleaching (FRAP) was employed to examine the lateral mobility of wheat germ agglutinin (WGA) and succinylated concanavalin A (sCon A) receptors in the plasma membrane of untransformed and Kirsten murine sarcoma virus-transformed Balb/c 3T3 fibroblasts in the spread and spherical state. An examination of FITC-WGA and FITC-sCon A binding to the plasma membrane for both cell lines in a spread or spherical state demonstrated no significant differences in the number of WGA or Con A receptors as a function of shape or transformation. The primary observations from this study are (a) membrane WGA and sCon A receptors in spherical Balb/c 3T3 fibroblasts display mobility 12 times faster than in the spread state, while phospholipid mobility is similar and apparently shape independent, (b) transformed cells in the spread state have WGA and sCon A receptor mobilities similar to those of untransformed cells in the spread state, (c) flat adherent, but not unattached spherical, Balb/c 3T3 fibroblasts are subject to Con A-induced global modulation and (d) transformed cells in the spherical state contain a significant population of cells (approximately 30%) with WGA receptor mobilities faster than those observed in spherical untransformed cells. These observations are discussed in terms of a linked matrix model for membrane protein diffusion.

Morphological, physiological and biochemical parameters associated with cell injury: a review

Various forms of cellular injury, whether induced by immune effector cells, aberrant metabolic processes, chemotherapeutic drugs or temperature shifts, result in common morphological changes consisting of the formation and shedding of membrane vesicles from the injured cell surfaces. This dynamic cell surface membrane behavior appears to be dependent on the disruption of cytoplasmic microtubules. Concomitant with the altered cell surface morphology certain physiological and biochemical events have been found to be associated with cell injury. These include changes in membrane permeability, elevated oxygen consumption rates and nuclear DNA fragmentation. However, it remains to be experimentally established which of these biological changes defines a state of irreparable cell injury. The objective of the present review is to compare and evaluate the cell injury process induced by effector lymphocytes with that caused by low temperature. The latter mimics most, if not all, the currently known criteria of immune effector cell mediated injury of target tumor cells.

Alpha interferon accelerates lateral diffusion of Daudi cell surface differentiation antigens: measurement by fluorescence redistribution after photobleaching

Lateral diffusion coefficients (D) of two surface differentiation antigens (sIgM and Bp35) were determined on interferon-sensitive (-IFs) or resistant (-IFr) Daudi cells by fluorescence photobleaching, using monospecific FITC-anti-IgM or PE-anti-Leu 16 probes. For untreated Daudi -IFs, mean (D) were 5.8 and 5.3 (x10(-10) cm2/sec). These increased, to 11 and 7.9 x 10(-10) cm2/sec (p less than 0.001) within 30 min after binding of recombinant IFN-a (80 to 800 U/10(6) cells), but decreased by up to 4-fold after Con A Mean (D) of identical surface antigens on Daudi-IFr were 8.2 and 9.4 x 10(-10) cm2/sec; and were not altered by IFN-a. Mean (D) of a lipid analog was up to 40-fold higher than for surface proteins and statistically identical in Daudi-IFs and Daudi-IFr. Rapid acceleration by IFN-a of surface protein lateral diffusion in Daudi-IFs obviously could facilitate anti-proliferative signal transduction; by contrast, a baseline increase of (D) in Daudi-IFr was evidently associated with their refractory state.

Luminescence spectroscopic approaches in studying cell surface dynamics

The major elements of membranes, such as proteins, lipids and polysaccharides, are in dynamic interaction with each other (Albertset al.1983). Protein diffusion in the lipid matrix of the membrane, the lipid diffusion and dynamic domain formation below and above their transition temperature from gel to fluid state, have many functional implications. This type of behaviour of membranes is often summarized in one frequently used word membrane fluidity (coined by Shinitzky & Henkart, 1979). The dynamic behaviour of the cell membrane includes rotational, translational and segmental movements of membrane elements (or their domain-like associations) in the plane of, and perpendicular to the membrane. The ever changing proximity relationships form a dynamic pattern of lipids, proteins and saccharide moieties and are usually described as ‘cell-surface dynamics’ (Damjanovichet al.1981). The knowledge about the above defined behaviour originates from experiments performed mostly on cytoplasmic membranes of eukaryotic cells. Nevertheless numerous data are available also on the mitochondrial and nuclear membranes, as well as endo (sarco-)plasmic reticulum (Martonosi, 1982; Slater, 1981; Siekevitz, 1981).

Self-organization of the fluid mosaic of charged channel proteins in membranes

Electrically charged ion channels in a fluid membrane may form dissipative structures driven by a concentration gradient of salt. On a molecular level the effect is due to dissipative attractive forces; the channel currents induce local gradients of the membrane potential that interact with the protein charge. Self-organization by "charged channel condensation" is treated on a phenomenological level: Smoluchowski's equation describing diffusion and drift of the membrane proteins and Kelvin's equation describing the dynamics of the membrane potential are considered as a coupled system of equations. The patterns of the two morphogens, the membrane protein and the membrane potential, are controlled by global parameters--the average density of charged channels, the level of their reversal potential, and the size of the membrane.

Inhomogeneous translational diffusion of monoclonal antibodies on phospholipid Langmuir-Blodgett films

The translational mobility of fluorescent-labeled monoclonal antibodies specifically bound to supported phospholipid bilayers containing hapten-conjugated phospholipids has been measured as a function of the surface concentration of bound antibodies using fluorescence recovery after photobleaching. Fluorescence recovery curves are fit well by a model that assumes the presence of two populations of antibodies with different lateral diffusion coefficients. The larger diffusion coefficient equals 3.5 x 10(-9) cm2/s, the smaller diffusion coefficient ranges from 1.5 x 10(-9) cm2/s to 2.5 x 10(-10) cm2/s, and the fractional fluorescence recovery associated with the smaller coefficient increases from approximately 0 to approximately 0.7 with increasing concentration of bound antibody. These results suggest that complexes of haptenated phospholipids and antibodies in phospholipid Langmuir-Blodgett films form clusters or domains in a concentration-dependent fashion.