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M Dembo

Publications and source records attributed to M Dembo.

At least 55 records · Page 3Linked to original sources
PubMed1986

The mechanics of motility in dissociated cytoplasm.

We stimulate the dynamical behavior of dissociated cytoplasm using the Reactive Flow Model (Dembo, M., and F. Harlow, 1986, Biophys. J., 50:109-121). We find that for the most part the predicted dynamical behavior of the cytoplasm is governed by three nondimensional numbers. Several other nondimensional parameters, the initial conditions, and boundary conditions are found to have lesser effects. Of the three major nondimensional parameters, one (D#) controls the percentage of ectoplasm, the second (C#) controls the sharpness of the endoplasm-ectoplasm boundary, and the third (R#) controls the topological complexity of the endoplasm-ectoplasm distribution. If R# is very small, then the cytoplasm contracts into a single uniform mass, and there is no bulk streaming. If R# is very large, then the cytoplasmic mass breaks up into a number of clumps scattered throughout the available volume. Between these clumps the solution undergoes turbulent or chaotic patterns of streaming. Intermediate values of R# can be found such that the mass of cytoplasm remains connected and yet undergoes coherent modes of motility similar to flares (Taylor, D.L., J.S. Condeelis, P.L. Moore, and R.D. Allen, 1973, J. Cell Biol., 59:378-394) and rosettes (Kuroda, K., 1979, Cell Motility: Molecules and Organization, 347-362).

Algorithms↗
PubMed1986

Thermodynamics of cell adhesion. II. Freely mobile repellers.

The equilibrium adhesion of a cell or vesicle to a substrate is analyzed in a theoretical model in which two types of mobile molecules in the cell membrane are of interest: receptors that can form bonds with fixed ligands in the substrate and repellers that repel the substrate. If the repulsion between the repeller molecule and substrate is greater than kT, there is substantial redistribution of the repellers from the contact area. Coexisting equilibrium states are observed having comparable free energies (a) with unstretched bonds and repeller redistribution and (b) with stretched bonds and partial redistribution.

Cell Adhesion↗
PubMed1985

Test of a theory relating to the cross-linking of IgE antibody on the surface of human basophils.

Recent mathematical models of bivalent hapten-induced histamine release from basophils predict that under appropriate conditions histamine release is maximum when cross-link formation is maximum, at a hapten concentration equal to 1/(2Ka), where Ka is the average affinity constant of the hapten for a single IgE binding site. To test this prediction we sensitized human basophils with a monoclonal anti-dinitrophenol IgE and generated histamine release dose-response curves with a bivalent hapten, alpha, epsilon-DNP-lysine. The monoclonal IgE has a published affinity constant of 7.1 X 10(7) M-1 for epsilon-DNP-lysine as determined by equilibrium dialysis. From the position of the maximum of the histamine dose-response curves, both in the presence and in the absence of monovalent DNP hapten, we determine that the sensitizing IgE has an intrinsic affinity constant of 6.9 +/- 0.5 X 10(7) M-1 for epsilon-DNP-lysine and 1.2 +/- 0.6 X 10(6) M-1 for alpha-DNP-lysine. The agreement between the two estimates of the epsilon-DNP-lysine affinity constant, one from histamine release experiments involving surface bound IgE and one from binding experiments involving IgE free in solution, 1) is consistent with a central prediction of the theory of cross-linking and 2) indicates that the hapten-binding properties of the IgE are unaffected by its being bound to Fc epsilon receptors on the basophil surface.

Basophils↗
PubMed1984

Interaction between anions and the reduced folate/methotrexate transport system in L1210 cell plasma membrane vesicles: directional symmetry and anion specificity for differential mobility of loaded and unloaded carrier.

The effect of various anions on the mediated influx and efflux of [3H]methotrexate by L1210 cell plasma membrane vesicles in a HEPES buffer system was studied. Our results show that flux is stimulated to the same extent in either direction when SO4, Pi, or folate compounds (1,L5-CHO-folate-H4, methotrexate), but not Cl- was present in the opposite compartment. This implies the property of directional symmetry, a condition in which differential mobility of loaded and unloaded carriers occurs in both directions. We also observed a similarity in the specificity of the interaction between various anions and carrier in each orientation of the membrane, in the order, Cl- much less than Pi approximately equal to SO2-4 much less than methotrexate less than 1,L5-CHO-folate-H4. Also, the absolute differential in mobility of loaded and unloaded carrier (assumed from the extent of transstimulation obtained) varied substantially among the anions examined. No stimulation was obtained with Cl-, and stimulation was twofold with Pi, SO2-4 and methotrexate and fourfold with 1,L5-CHO-folate-H4. Transstimulation of flux from either external or internal compartment only occurred when a positive gradient of total anions was maintained in the opposite compartment. Also, no stimulation occurred when the same equivalence of two different anions are present in opposing compartments. The concentration of anions required to transstimulate [3H]methotrexate influx was increased four- to 10-fold when vesicles were equilibrated in 145 mM NaCl.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1984

Effects of metabolic deprivation on methotrexate transport in L1210 leukemia cells: further evidence for separate influx and efflux systems with different energetic requirements.

Measurements of methotrexate transport in L1210 cells in the presence and absence of D-glucose reveal that both influx and efflux are depressed in the absence of D-glucose, whereas the steady-state accumulation of drug is enhanced. The reason for the increase in steady state is that the relative decline in efflux is greater than the decline in influx. Analysis of the concentration dependence of steady-state methotrexate accumulation in D-glucose-deprived cells indicates a linear relationship consistent with a one-carrier active transport model. Similar data in nondeprived cells is highly nonlinear and strongly supports the postulate that under physiological conditions influx and efflux of methotrexate are mediated by separate carrier systems. These results indicate that the efflux system, preferentially transporting methotrexate under normal conditions, cannot operate in the absence of D-glucose, whereas the influx system is only partially inhibited under conditions of glucose deprivation.

Animals↗
PubMed1984

Cell adhesion. Competition between nonspecific repulsion and specific bonding.

We develop a thermodynamic calculus for the modeling of cell adhesion. By means of this approach, we are able to compute the end results of competition between the formation of specific macromolecular bridges and nonspecific repulsion arising from electrostatic forces and osmotic (steric stabilization) forces. Using this calculus also allows us to derive in a straightforward manner the effects of cell deformability, the Young's modulus for stretching of bridges, diffusional mobility of receptors, heterogeneity of receptors, variation in receptor number, and the strength of receptor-receptor binding. The major insight that results from our analysis concerns the existence and characteristics of two phase transitions corresponding, respectively, to the onset of stable cell adhesion and to the onset of maximum cell-cell or cell-substrate contact. We are also able to make detailed predictions of the equilibrium contact area, equilibrium number of bridges, and the cell-cell or cell-substrate separation distance. We illustrate how our approach can be used to improve the analysis of experimental data, by means of two concrete examples.

Animals↗
PubMed1983

Relationships between carrier-mediated transport of folate compounds by L1210 leukemia cells: evidence for multiplicity of entry routes with different kinetic properties expressed in plasma membrane vesicles.

Various independent kinetic criteria for indicating multiplicity of carrier-mediated entry of folate compounds into L1210 cell plasma membrane vesicles are studied. We find a marked inconsistency between values for influx Km and Ki in reciprocal experiments measuring competition between various folate compounds as well as inconsistent effects of transloading shown for 5-formyltetrahydrofolate influx, but not folic acid influx. These results argue strongly against a one-carrier model for transport of folate compounds. The most straightforward interpretation of our data is that two distinct transport systems mediate entry of folate compounds in L1210 plasma membrane vesicles. If a two-carrier model is correct, then our data indicate that one of the carriers has low capacity and high affinity for folate coenzymes and methotrexate. This system is apparently negligible as a transport route for folic acid. Transtimulation of initial influx by substrates of the low capacity system is obtained following transloading with coenzymes but not by transloading with folic acid. Our data indicate that the second folate transport system postulated by the two-carrier model has a low affinity for all the folate compounds studied. Nevertheless, the putative second system is significant, especially for folic acid transport, because it has a much higher capacity than the first transport system. In contrast to the first system, transloading with any of the folate compounds studied had no effect on initial influx mediated by the second folate transport system. The two systems are also differentially inhibited by pCMBS, DIDS and SITS and the influx Vmax for the high-affinity/low-capacity system was altered in a vesicle preparation derived from a methotrexate resistant L1210 cell line.

Animals↗
PubMed1982

Two-compartment behavior during transport of folate compounds in L1210 cell plasma membrane vesicles.

The transport of [3H] 1,L 5-formyltetrahydrofolate, [3H] folic acid, and [3H]methotrexate by L1210 cell plasma membrane vesicles exhibited multicompartmental behavior. Two separate vesicular compartments (parallel relationship) of approximately equal volume were revealed during measurements of influx and efflux. Flux in one compartment was rapid, saturable, highly temperature-sensitive, and inhibited by pCMBS. Flux in the other compartment exhibited all of the characteristics of passive diffusion. These results imply that our plasma membrane vesicle preparations consist of a mixture of two functional species. Transport of folate into one of these species occurs by passive diffusion alone, whereas transport into the other kind of vesicle occurs by both passive diffusion and carrier-facilitated transport.

Animals↗
PubMed1982

Kinetic analysis of histamine release due to covalently linked IgE dimers.

We present a kinetic model of histamine release from human basophils due to covalently linked IgE dimers. Comparison of theory with experiment shows that the model gives a good description of histamine release by IgE dimers and allows a number of the parameters of the model to be determined. Comparison with previous models of release by conventional antigens indicates that despite their covalent structure, IgE dimers are subject to the same laws governing inactivation as are antigen produced crosslinks. In addition, the kinetic equation which relates the rate of histamine release to the number of crosslinked Fc epsilon receptors per cell is the same for crosslinks formed by IgE dimers as for antigen induced crosslinks. Quantitative fitting of histamine release data also yields a value for the rate constant for crosslink formation by IgE dimer on the cell surface (rx approximately 5 x 10(-10) cm2/sec). This rate constant is remarkably high and indicates that the reaction is diffusion controlled.

Basophils↗
PubMed1981

Motion of polymorphonuclear leukocytes: theory of receptor redistribution and the frictional force on a moving cell.

As a cell moves over a surface, the distribution of membrane proteins that adhere to the surface will be changed relative to the distribution of these molecules on a static cell. Observations of this redistribution offer, in principle, evidence as to the mechanisms of membrane dynamics during cell locomotion. Toward extracting such information we present and analyze a mathematical model of receptor transport in the membrane by diffusion and convection, as affected by the making and breaking of the bonds between the receptors and the surface as the cell moves. We show that the disruption of receptor-surface bonds at the tail of the cell provides a mechanism by which the frictional force opposing a cell's motion is exerted, and calculate the magnitude of this force as a function of cell velocity. Assuming this to be the major contribution to the frictional force, we show that when the shear force on a cell is above a critical value it is no longer possible for the cell to slide across the surface. For such large forces, it is still possible for the cell to roll; alternatively the cell can be torn free of the surface. Our analysis of existing data on movement of polymorphonuclear leukocytes indicates that cell motion is not accompanied by a bulk flow of membrane from the front to the back of the cell. The data also indicate that cells do not tend to roll as they move over a surface under normal conditions. The data are most consistent with a model where the membrane as a whole is stationary but where receptors that bind to the surface become coupled to submembrane contractile proteins.

Animals↗
PubMed1981

Motion of particles adhering to the leading lamella of crawling cells.

Time-lapsed films of particle motion on the leading lamella of chick heart fibroblasts and mouse peritoneal macrophages were analyzed. The particles were composed of powdered glass or powdered aminated polystyrene and were 0.5-1.0 micrometer in radius. Particle motions were described by steps in position from one frame to the time-lapse movies to the next. The statistics of the step-size distribution of the particles were consistent with a particle in Brownian motion subject to a constant force. From the Brownian movement, we have calculated the two-dimensional diffusion coefficient of different particles. These vary by more than an order of magnitude (10(-11)-10(-10) cm2/s) even for particles composed of the same material and located very close to each other on the surface of the cell. This variation was not correlated with particle size but is interpretable as a result of different numbers of adhesive bonds holding the particles to the cells. The constant component of particle movement can be interpreted as a result of a constant force acting on each particle (0.1-1.0 x 10(-8) dyn). Variations in the fractional coefficient for particles close to each other on the cell surface do not yield corresponding differences in velocity, suggesting that the frictional coefficient and the driving force vary together. This is consistent with the hypothesis that the particles are carried by flow of the membrane as a whole or by flow of some submembrane material. The utility of our methods for monitoring cell motile behavior in biologically interesting situations, such as a chemotactic gradient, is discussed.

Animals↗
PubMed1981

Qualitative characteristics of histamine release from human basophils by covalently cross-linked IgE.

We have studied the effects of permanent oligomers of human IgE produced using the cross-linking reagent, dimethyl suberimidate, on histamine release from human basophils. IgE dimers were found to be sufficient stimuli for both release and desensitization of these cells; monomeric IgE had no effect. Histamine release was augmented by deuterium oxide (D2O) in the medium, but D2O was not an absolute requirement to observe release. Desensitization by the dimeric IgE was specific in that the response to anti-IgE was not affected by preincubation of the leukocytes with the IgE dimer under suboptimal releasing conditions. IgE trimers and higher oligomers of IgE also caused both release and desensitization. IgE trimers were 3- to 4-fold more effective than IgE dimers with regard to the amount required for 50% histamine release. Dilution studies with monomeric IgE suggested that the difference was due to the presence of more "active" dimers in the trimeric IgE fractions. We conclude that dimeric IgE, by juxtaposing 2 receptors on the basophil membrane, is the "unit signal" for both release and desensitization of these cells.

Animals↗
PubMed1980

A model of cell activation and desensitization by surface immunoglobin: the case of histamine release from human basophils.

We present a model for the control of immunoglobulin E (IgE)-mediated histamine release from human basophils. We suggest that there is a calcium gating factor which interacts with crosslinked IgE to form a short-lived open calcium channel. After formation of the channel the activated gating factor rapidly decays to an inactive form. It is the loss of the active gating factor which causes the basophil to desensitize nonspecifically. We propose that the crosslinked IgE molecules are deactivated by a mechanism, such as endocytosis or shedding, which is independent of the mechanism which inactivates the calcium gating factor. This loss of functional IgE leads to specific desensitization. The mathematical formulation of the model explains the relationship of specific and nonspecific desensitization to the amount of specific IgE on the basophil surface; explains why there are two types of antigen excess inhibition; explains the relationship between antigen excess inhibition and desensitization; explains why, for a fixed antigen concentration, increasing the concentration of cell surface IgE increases histamine release until an optimal concentration is reached, then decreases histamine release; predicts the effects that changing the external calcium will have on the dose response curve; and predicts that increasing the amount of specific IgE on the cell surface will cause the dose response curve to undergo a transition from a curve with a single maximum to a curve with two maxima.

Basophils↗
PubMed1979

A method for measuring membrane microviscosity using pyrene excimer formation. Application to human erythrocyte ghosts.

In order to determine the microviscosity of human erythrocyte membrane suspensions, a method has been developed which is based on pyrene excimer formation. First, measurements of partitioning of pyrene into membranes, in conjunction with known values for the volume of the lipid compartment of erythrocyte ghosts are used to determine the concentration of pyrene in the membrane lipid. Secondly, reported measurements of the diffusion constants of aromatic hydrocarbons similar in structure to pyrene, are used to derive an empirical equation relating solvent viscosity and the diffusion constant of pyrene. Then, measurements of pyrene excimer formation in a series of solvents ranging up to several poise in viscosity are used to determine that the interaction diameter of the excimer formation reaction is 3 +/- 1 A. Finally all these data are brought together in order to conclude that the viscosity of the lipid in the human erythrocyte ghost is 8.0, 4.0 and 1.6 P at 10, 25 and 40 degrees C, respectively.

Erythrocyte Membrane↗
PubMed1979

Some quantitative aspects of the passive sensitization of human basophils.

We review the theory for the binding of IgE to Fc receptors on basophil surfaces. We then use the theory to analyze binding experiments of Malveaux et al. and T. Ishizaka on the passive sensitization of basophils from a patient with chronic myelocytic leukemia and 75% basophilia. From their data we determine that the forward rate constant for the binding of human IgE to receptors on these human basophils is (3.0 +/- 1.0) x 10(4) M-1 sec-1.

Basophils↗