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M Dembo

Publications and source records attributed to M Dembo.

At least 37 records · Page 2Linked to original sources
PubMed1996

Numerical simulation of oil-droplet cleavage by surfactant.

We present numerical computations of the deformation of an oil-droplet under the influence of a surface tension gradient generated by the surfactant released at the poles (the Greenspan experiment). We find this deformation to be very small under the pure surface tension gradient. To explain the large deformation of oil droplets observed in Greenspan's experiments, we propose the existence of a phoretic force generated by the concentration gradient of the surfactant. We show that this hypothesis successfully explains the available experimental data and we propose some further tests.

Cell Division↗
PubMed1995

Traction forces in locomoting cells.

A means of determining quantitative maps of the tractions exerted by locomoting cells on a substratum has been developed. This method is similar to the Harris silicone substratum assay [Harris et al., 1980: Science 208:177-179], but uses an improved non-wrinkling film that deforms more predictably in response to traction forces. The method also utilizes a mathematical analysis of rubber deformation to produce the final map of the distribution of tractions. The resulting maps consistently showed that fish keratocytes exert a steady-state "pinching" on the substratum, perpendicular to the cell's direction of locomotion. No significant rearward tractions were detected at or near the front edge of the cell. Likewise, no significant forward tractions associated with peeling of adhesions were found at the back of the cell. A second assay uses deflection of a lightly attached glass microneedle to measure the total force exerted by locomoting cells. Forces of approximately 4.5 x 10(-3) dyn were required to "stall" locomoting keratocytes. The implications of these findings for cell movement are discussed.

Animals↗
PubMed1995

Approximating the effects of diffusion on reversible reactions at the cell surface: ligand-receptor kinetics.

We consider the problem of determining the time dependence of the bound ligand concentration for the reversible binding of a diffusing monovalent ligand to receptors uniformly distributed over the surface of a spherical cell. We start by formulating a boundary value problem that captures the essential physics of this situation. We then introduce a systematic approximation scheme based on the method of weighted residuals. By this means we convert the initial boundary value problem into a simpler problem that requires solving only a small number of ordinary differential equations. We show how, at the lowest order of approximation, the method can be used to obtain modified chemical rate equations where, in place of fundamental rate constants, effective rate coefficients appear. These rate coefficients are functions of the ligand diffusion coefficient, the cell radius, the receptor density and other variables. We compare exact and approximate solutions and discuss under what conditions the approximate equations can be used. We also apply the method of weighted residuals to obtain approximate descriptions of the binding kinetics when (1) there are two different cell surface receptor populations that bind the ligand and (2) the cell secretes a ligand that can bind back to receptors on the cell (autocrine binding).

Biophysical Phenomena↗
PubMed1994

Binding of bivalent ligand to cell surface IgE: can one detect ring formation?

It is well established that aggregation of cell surface immunoglobulin is involved in signal transduction by cells of the immune system. It is less well understood what special properties of these cell surface aggregates are important in initiating the signal cascade. Several authors have proposed that cells respond to the size (Fewtrell and Metzger (1980) J. Immun. 125, 701-710) as well as the stereochemistry (Ortega et al. (1989) Eur. J. Immun. 19, 2251-2256) of receptor aggregates. One approach to arriving at data relevant to this question has been to construct simple bivalent ligands that can bind to surface immunoglobulin. Several authors have suggested that when these bivalent ligands interact with surface immunoglobulin the formation of small stable cyclic complexes is highly favored. In this paper we consider whether it is possible to completely determine the parameters that describe the binding of a bivalent ligand to a bivalent receptor with the available experimental technology. We show that with the appropriate analysis procedure, using a modified equivalent site model, these parameters can be reliably determined from only three experiments even when there is a large amount of ring formation.

Dinitrophenols↗
PubMed1994

Kinetics of cell detachment: peeling of discrete receptor clusters.

Clustering of cell surface adhesion receptors is an essential step in the development of focal contacts, specialized cell-substrate attachment sites where receptors are simultaneously linked to extracellular ligand and cytoskeletal proteins. Previously, we examined the effect of receptor clustering on attachment strength. Here, we employ the numerical methodology developed by Dembo and colleagues (Dembo, M., D.C. Torney, K. Saxman, and D. Hammer. 1988. Proc. R. Soc. Lond. B. 234:55-83) to investigate the kinetics of cell detachment when receptors are clustered into discrete patches. We show that the membrane peeling velocity decreases if receptors are clustered within a patch located inside the contact region. Peeling of clusters is influenced by the chemistry and mechanics of receptor-ligand bonds within the patch. Detachment is also prohibited if the applied tension equals the critical tension of the patch, unless the patch length is small compared with the boundary length over which membrane bending occurs, in which case the patch will peel. Peeling of these short patches only occurs when the mechanical stiffness of clustered bonds is within an optimal range. We compare our model predictions with experimental measurements of T lymphocyte detachment from ICAM-1 substrates. We demonstrate that if discrete patches of receptors are present, detachment occurs through intervals of slow and fast peeling, similar to the dynamics of T lymphocyte peeling, indicating that clustering of LFA-1 receptors is one possible explanation for the observed detachment kinetics in this system.

Animals↗
PubMed1992

Factors underlying spontaneous inactivation and susceptibility to neutralization of human immunodeficiency virus.

To determine the factors governing inactivation and neutralization, physical, chemical, and biological assays were performed on a molecular clone of human immunodeficiency type 1 (HIV-1HXB3). This included quantitative electron microscopy, gp120 and p24 enzyme-linked immunosorbent assays, reverse, transcriptase assays, and quantitative infectivity assays. For freshly harvested stocks, the ratio of infectious to noninfectious viral particles ranged from 10(-4) to 10(-7) in viral stocks containing 10(9) to 10(10) physical particles per milliliter. There were relatively few gp120 knobs per HIV particle, mean approximately 10 when averaged over the total particle count. Each HIV particle contained a mean approximately 5 x 10(-17) g of p24 and approximately 2 x 10(-16) g of RNA polymerase, corresponding to about 1200 and 80 molecules, respectively. The spontaneous shedding of gp120 envelope proteins from virions was exponential, with a half-life approximately 30 hr. The loss of RNA polymerase activity in virons was also exponential, with a half-life approximately 40 hr. The physical breakup of virions and the dissolution of p24 core proteins were slow (half-life greater than 100 hr) compared to the gp120 shedding and polymerase loss rates. The decay of HIV-1 infectivity was found to obey superimposed single- and multihit kinetics. At short preincubation times, the loss of infectivity correlated with spontaneous shedding of gp120 from virions. At longer times, an accelerating decay rate indicated that HIV requires a minimal number of gp120 molecules for efficient infection of CD4+ cells. The blocking activity of recombinant soluble CD4 (sCD4) and phosphonoformate (foscarnet) varied with the number of gp120 molecules and number of active RNA polymerase molecules per virion, respectively. These results demonstrate that the physical state of virions greatly influences infectivity and neutralization. The knowledge gained from these findings will improve the reliability of in vitro assays, enhance the study of wild-type strains, and facilitate the evaluation of potential HIV therapeutics and vaccines.

CD4 Antigens↗
PubMed1992

The auto-regulation model: a unified concept of how HIV regulates its infectivity, pathogenesis and persistence.

The life cycle of HIV can be divided into two distinct stages: intracellular and extracellular. The prevailing view is that the intracellular stage provides the only locus for regulating the virus in response to physiologic stimuli. Such regulation is accomplished by modulating the rates of transcription, translation and viral assembly. The extracellular stage consists of physical processes such as diffusion, adhesion and penetration of cells by viral particles. These latter processes are commonly thought to be "automatic" and not subject to regulation. For the past several years, we have developed means of more carefully measuring and characterizing the extracellular stage of HIV infection, and we have obtained evidence indicating that novel regulatory processes do, in fact, take place during this extracellular stage. We believe that this extracellular regulation permits HIV to adapt to a wide range of physiologic cell densities, to maintain persistent but slow growing infection, and to defeat the protective activity of humoral blockers. The overall purpose of this review is to consider our evidence for this hypothesis.

CD4 Antigens↗
PubMed1991

Blocking of human immunodeficiency virus infection depends on cell density and viral stock age.

Quantitative infectivity assays were used to study how the blocking activity of soluble CD4 (sCD4) is affected by sCD4 concentration, target cell density, and viral stock age. During incubation with 20 nM sCD4, human immunodeficiency virus type 1 (HIV-1) stocks underwent irreversible inactivation. In contrast, inactivation with 2 nM sCD4 was almost entirely reversible. At lower sCD4 concentrations (less than or equal to 2 nM) and target cell densities of 6.25 x 10(4) ml-1, sCD4 blocking activity for HIV-1 gave a gp120-sCD4 association constant (Kassoc) of 1.7 x 10(9) M-1, which agrees with chemical measurements. At the higher density of 1.6 x 10(7) cells ml-1, however, the blocking activity was 20-fold less. During incubation of HIV-1 stock optimized for infectivity by rapid harvest, sCD4 blocking activity increased 20-fold during a 3-h window. These results show that competitive blocking activity depends strongly on target cell density and virion age. Thus, unappreciated variations in HIV stocks and assay conditions may hinder comparisons of blockers from laboratory to laboratory, and the age of HIV challenge stocks may influence studies of drug and vaccine efficacy. The results also suggest that blocking of viral particles in lymphoid compartments will require very high competitive blocker concentrations, which may explain the refractory outcomes from sCD4-based drug trials in humans.

Acquired Immunodeficiency Syndrome↗
PubMed1990

HIV requires multiple gp120 molecules for CD4-mediated infection.

Binding of glycoprotein gp120 to the T cell-surface receptor CD4 is a crucial step in CD4-dependent infection of a target cell by the human immunodeficiency virus (HIV). Blocking some or all gp120 molecules on the viral surface should therefore inhibit infection. Consequently, competitive receptor inhibitors, such as soluble synthetic CD4 (sCD4), synthetic CD4 peptides and immunoglobulins, have been investigated in vitro and in vivo, but little is known about the molecular mechanisms of these inhibitors. We have now quantitatively examined blocking by soluble CD4 in the hope of gaining insight into the complex process of viral binding, adsorption and penetration. At low sCD4 concentrations, the inhibition in three HIV strains is proportional to the binding of gp120. The biological association constant (gp120-sCD4 Kassoc) for HIV-2NIHZ is (8.5 +/- 0.5) x 10(7) M-1, whereas Kassoc for HIV-1HXB3 (1.4 +/- 0.2) and HIV-1MN (1.7 +/- 0.1) x 10(9) M-1 are 15-20-fold larger. For all three viral strains, the biological Kassoc from infectivity assays is comparable to the chemical Kassoc. The inhibitory action of sCD4 at high concentrations, however, is not fully explained by simple proportionality with the binding to gp120. Positive synergy in blocking of infection occurs after about half the viral gp120s molecules are occupied, and is identical for all three viral strains, despite the large differences in Kassoc. Our method of measuring the viral-cell receptor Kassoc directly from infectivity assays is applicable to immunoglobulins, to other viruses and to assays using primary or transformed cell lines.

CD4 Antigens↗
PubMed1990

A thermodynamic model of hemoglobin suitable for physiological applications.

We propose a quantitative model of the thermodynamics of hemoglobin in contact with its five major ligands (O2, CO2, Cl-, 2,3-bisphosphoglycerate, and H+). Our model incorporates the two-state formalism of J. Monod, J. Wyman, and J.P. Changeux (J. Mol. Biol. 12: 88-118, 1965) for treatment of quanternary transitions and also the mean field formalism of K. Linderstrom-Lang (C. R. Trav. Lab. Carlsberg Ser. Chim. 15: 1-30, 1924) for treatment of electrostatic interactions. On the basis of this approach, we develop an algorithm for the efficient computation of observable quantities, such as the occupancy of various ligand binding sites, and an objective statistical procedure for determining both maximum likelihood values and confidence limits of all the intrinsic thermodynamic parameters of hemoglobin. Finally, we show that the predictions of our theory are in good agreement with independent experimental observations.

Carbon Dioxide↗
PubMed1989

Analytic results for quantifying HIV infectivity.

In a separate paper, we developed a mathematical model describing HIV infection and used it to suggest experiments for quantifying characteristic viral parameters. In this paper we generalize the model to any well-mixed assay system. We also present complete and rigorous derivations of fundamental results needed for the design and analysis of HIV infectivity assays. The model is applicable to infectious agents with multiple receptors for their target cell (e.g. HIV, Epstein-Barr virus and Plasmodium), and to blockers (both reversible and irreversible), as long as blocker and target cells are the same diffusion compartment.

Acquired Immunodeficiency Syndrome↗
PubMed1989

Mechanics and control of the cytoskeleton in Amoeba proteus.

Many models of the cytoskeletal motility of Amoeba proteus can be formulated in terms of the theory of reactive interpenetrating flow (Dembo and Harlow, 1986). We have devised numerical methodology for testing such models against the phenomenon of steady axisymmetric fountain flow. The simplest workable scheme revealed by such tests (the minimal model) is the main preoccupation of this study. All parameters of the minimal model are determined from available data. Using these parameters the model quantitatively accounts for the self assembly of the cytoskeleton of A. proteus: for the formation and detailed morphology of the endoplasmic channel, the ectoplasmic tube, the uropod, the plasma gel sheet, and the hyaline cap. The model accounts for the kinematics of the cytoskeleton: the detailed velocity field of the forward flow of the endoplasm, the contraction of the ectoplasmic tube, and the inversion of the flow in the fountain zone. The model also gives a satisfactory account of measurements of pressure gradients, measurements of heat dissipation, and measurements of the output of useful work by amoeba. Finally, the model suggests a very promising (but still hypothetical) continuum formulation of the free boundary problem of amoeboid motion. by balancing normal forces on the plasma membrane as closely as possible, the minimal model is able to predict the turgor pressure and surface tension of A. proteus. Several dynamical factors are crucial to the success of the minimal model and are likely to be general features of cytoskeletal mechanics and control in amoeboid cells. These are: a constitutive law for the viscosity of the contractile network that includes an automatic process of gelation as the network density gets large; a very vigorous cycle of network polymerization and depolymerization (in the case of A. proteus, the time constant for this reaction is approximately 12 s); control of network contractility by a diffusible factor (probably calcium ion); and control of the adhesive interaction between the cytoskeleton and the inner surface of the plasma membrane.

Amoeba↗
PubMed1989

Quantifying the infectivity of human immunodeficiency virus.

We have developed a mathematical model that quantifies lymphocyte infection by human immunodeficiency virus (HIV) and lymphocyte protection by blocking agents such as soluble CD4. We use this model to suggest standardized parameters for quantifying viral infectivity and to suggest techniques for calculating these parameters from well-mixed infectivity assays. We discuss the implications of the model for our understanding of the infectious process and virulence of HIV in vivo.

Acquired Immunodeficiency Syndrome↗
PubMed1989

Reversible sodium pump defect and swelling in the diabetic rat erythrocyte: effects on filterability and implications for microangiopathy.

We have found a defect in the ouabain-sensitive Na+, K+-ATPase (Na+ pump, EC 3.6.1.37) of erythrocytes from streptozocin diabetic rats. This defect was accompanied by an increase in cell volume and osmotic fragility and a decrease in the cytosolic K+/Na+ ratio. There was also a doubling in the time needed for diabetic erythrocytes to pass through 4.7-micron channels in a polycarbonate filter. Our data are consistent with a primary defect in the erythrocyte Na+ pump and secondary changes in cell volume, osmotic fragility, K+/Na+ ratio, and cell filterability. All were reversed or prevented in vivo by insulin or the aldose reductase inhibitor Sorbinil. Protein kinase C agonists (phorbol ester and diacylglycerol) and agonist precursor (myoinositol) reversed the Na+ pump lesion, suggesting that protein kinase C-dependent phosphorylation of the 100-kDa subunit regulates Na+ pump activity and that insulin can influence erythrocyte protein kinase C activity. Ouabain inhibition of the erythrocyte Na+ pump also produced increases in cell size and reductions in rates of filtration. Theoretical treatment of the volume changes also predicts reduction in filterability as a consequence of cell swelling. We suggest that enlarged erythrocytes could play a role in the evolution of the microvascular changes of diabetes mellitus.

Adenosine Triphosphate↗
PubMed1988

The reaction-limited kinetics of membrane-to-surface adhesion and detachment.

Biological adhesion is frequently mediated by specific membrane proteins (adhesion molecules). Starting with the notion of adhesion molecules, we present a simple model of the physics of membrane-to-surface attachment and detachment. This model consists of coupling the equations for deformation of an elastic membrane with equations for the chemical kinetics of the adhesion molecules. We propose a set of constitutive laws relating bond stress to bond strain and also relating the chemical rate constants of the adhesion molecules to bond strain. We derive an exact formula for the critical tension. We also describe a fast and accurate finite difference algorithm for generating numerical solutions of our model. Using this algorithm, we are able to compute the transient behaviour during the initial phases of adhesion and detachment as well as the steady-state geometry of adhesion and the velocity of the contact. An unexpected consequence of our model is the predicted occurrence of states in which adhesion cannot be reversed by application of tension. Such states occur only if the adhesion molecules have certain constitutive properties (catch-bonds). We discuss the rational for such catch-bonds and their possible biological significance. Finally, by analysis of numerical solutions, we derive an accurate and general expression for the steady-state velocity of attachment and detachment. As applications of the theory, we discuss data on the rolling velocity of granulocytes in post-capillary venules and data on lectin-mediated adhesion of red cells.

Algorithms↗
PubMed1986

Cell motion, contractile networks, and the physics of interpenetrating reactive flow.

In this paper we propose a physical model of contractile biological polymer networks based on the notion of reactive interpenetrating flow. We show how our model leads to a mathematical formulation of the dynamical laws governing the behavior of contractile networks. We also develop estimates of the various parameters that appear in our equations, and we discuss some elementary predictions of the model concerning the general scaling principles that pertain to the motions of contractile networks.

Cell Movement↗
PubMed1986

Numerical studies of unreactive contractile networks.

We present a finite difference algorithm for integrating the reactive flow model of contractile biological polymer networks on a fixed Eulerian mesh. We discuss the accuracy and limits of the algorithm. To illustrate the application of the algorithm, we carry out a family of computations involving an unreactive contractile network contained in a two-dimensional square reaction vessel. By numerical experiments using different values of the physical parameters of the model, we find that for this simple sort of system two major dynamical modes of contraction are predicted to occur. There is the squeezing type contraction in which the network contracts to a single small clump with gradual expulsion of solution material, and the rending type contraction in which the network tears itself into a number of separate pieces. We find that to a good approximation the transition between the squeezing mode and the rending mode is controlled by a single nondimensional number (the rending number). We discuss the relevance of these results for the analysis of various experimental observations.

Cell Movement↗