Nonspecific Repulsion Research Articles

Fluctuation-Induced Interactions between Membrane-Bound Proteins

The spatial organization of membrane-bound proteins is in part determined by interactions that originate from long-range correlations due to the membrane's elastic behavior. Even basic geometric mechanisms, such as the suppression of membrane height fluctuations near protein binding sites, can lead to nontrivial interactions between proteins that might result in their aggregation. To study the effect of such membrane-induced interactions, we devise a simple model that captures (a) a nonspecific repulsion between proteins, (b) elastic properties of the membrane, and (c) a local harmonic coupling between proteins and membrane shape. The model's dynamics is governed by Langevin equations to faithfully capture entropic effects and the importance of rare fluctuations. We find that the membrane induces an attractive interaction between the proteins, which aggregate to mitigate the entropic cost of suppressing membrane fluctuations. This generic mechanism might help explain the spatial patterns induced by membrane sculpting proteins.

Mobile linkers on DNA-coated colloids: valency without patches.

Colloids coated with single-stranded DNA (ssDNA) can bind selectively to other colloids coated with complementary ssDNA. The fact that DNA-coated colloids (DNACCs) can bind to specific partners opens the prospect of making colloidal "molecules." However, in order to design DNACC-based molecules, we must be able to control the valency of the colloids, i.e., the number of partners to which a given DNACC can bind. One obvious, but not very simple approach is to decorate the colloidal surface with patches of single-stranded DNA that selectively bind those on other colloids. Here we propose a design principle that exploits many-body effects to control the valency of otherwise isotropic colloids. Using a combination of theory and simulation, we show that we can tune the valency of colloids coated with mobile ssDNA, simply by tuning the nonspecific repulsion between the particles. Our simulations show that the resulting effective interactions lead to low-valency colloids self-assembling in peculiar open structures, very different from those observed in DNACCs with immobile DNA linkers.

Diffusing Colloidal Probes of Protein–Carbohydrate Interactions

We present diffusing colloidal probe measurements of weak, multivalent, specific protein-polysaccharide interactions mediated by a competing monosaccharide. Specifically, we used integrated evanescent wave and video microscopy methods to monitor the three-dimensional Brownian excursions of conconavilin A (ConA) decorated colloids interacting with dextran-functionalized surfaces in the presence of glucose. Particle trajectories were interpreted as binding lifetime histograms, binding isotherms, and potentials of mean force. Binding lifetimes and isotherms showed clear trends of decreasing ConA-dextran-specific binding with increasing glucose concentration, consistent with expectations. Net potentials were accurately captured by superposition of a short-range, glucose-independent ConA-dextran repulsion and a longer-range, glucose-dependent dextran bridging attraction modeled as a harmonic potential. For glucose concentrations greater than 100 mM, the net ConA-dextran potential was found to have only a nonspecific repulsion, similar to that of bovine serum albumin (BSA) decorated colloids over dextran determined in control experiments. Our results demonstrate the first use of optical microscopy methods to quantify the connections between potentials of mean force and the binding behavior of ConA-decorated colloids on dextran-functionalized surfaces.

Influence of substrate stiffness on cell–substrate interfacial adhesion and spreading: A mechano-chemical coupling model

Cell interactions with extracellular matrix, such as cell adhesion and spreading, are crucial for many biological functions and processes. Recent experimental progresses have demonstrated that substrate rigidities exert a remarkable influence on cell–substrate interfacial adhesion and spreading behaviors. The underlying biophysical mechanism, however, remains elusive. Based on the classical Bell-Dembo’s theory, this paper develops a mechano-chemical coupling model to physically describe cell adhesion and spreading mediated by substrate stiffness. By investigating the competitive nature between cell–substrate specific attraction and non-specific repulsion, the kinetic relation of receptor–ligand interplay is established, in which the influences of receptor–ligand separation, substrate elasticity and non-specific repulsion on cell adhesions are especially addressed. According to mechanical equilibrium conditions between cell membranes and underlying elastic substrates, an analytical expression is then deduced to relate the cell–substrate interfacial adhesion strength to the substrate rigidity. Moreover, by means of the conventional wetting theory, the dependence of steady-state cell spreading on substrate stiffness is also quantitatively studied. Comparisons with the existing experimental data show that the proposed model can be used to explore cell–substrate interactions regulated by substrate rigidities.

Finite Element Modeling of Cell-Matrix Adhesive Interaction

Finite Element Modeling of Cell-Matrix Adhesive InteractionWhen cells adhere to extracellular matrix (ECM) or other bioactive surfaces (substrates), the bond formation is mediated by the bindings of cell receptors, which can diffuse along the cell membrane surface, to immobilized ligands in ECM. Cells spread and the adhesion zone grows as bond formation at the adhesion front increases to a critical level. This process consists of multiple physical and chemical mechanisms and involves the coupling of reaction-diffusion and mechanical contact between cells and ECM. In this study, we have developed a finite element code to incorporate the kinetics of receptor-ligand interaction into the mass diffusion of cell receptor. For the mechanical interactions attributed to the cell adhesion development and spreading, this code is implemented in a commercial finite element program, through features of user subroutine provided, using its coupled diffusion-displacement solver. This can take into account the fully coupling of reaction-diffusion with mechanics of cell/substrate contact and deformation. In the finite element model, interaction forces between cell and substrate include the specific attraction due to the receptor-ligand binding and the nonspecific repulsion due to glycocalyx proteins associated with cell surface. Parametric studies are also performed to investigate effect of various system parameters, such as ligand density and mechanical properties of the receptor-ligand interaction, on the kinetics and mechanics of cell adhesion process. More importantly, this study provides a computational framework, with multi-scales and multi-physics, that can be extended for better controlling of cell interactions at the cell-biomaterial interface and for modeling the cell motility.

Mechanical modeling of the rolling phenomenon at the cell scale

Rolling is an important manifestation of cell adhesion [Viallat, A., Akarian, M., Campillo, C., Faivre, M., Pepin-Dona, B., 2005. Dynamique de vésicules géantes et de globules rouges sous écoulement de cisaillement. In: 17 e`me Congrès Français de Mécanique, Troyes], especially for the leukocyte cell in the immune process [Bell, G.I., Dembo, M., Bongrand, P., 1984. Cell adhesion: competition between non-specific repulsion and specific bonding. Biophys. J. 45, 1051–1064]. The purpose of this work is the mechanical description of the kinetic of adhesion of a single cell adhering to an extracellular wall, in terms of the failure and creation of connections during the rolling of a single adherent cell subjected to a fluid flow. A 2D model is developed in the present contribution, which describes the local behavior of the contact zone between the cell and the wall. The contact zone is assimilated to two rigid rectilinear beams linked by elastic springs, subjected to the fluid flow and to interaction forces, namely Van der Waals (attractive) and electrostatic (repulsive) forces. The first aspect being investigated concerns the mechanism of failure of the existing connections: the distribution of the failure limits of these fibers is described as a random gaussian field. Moreover, we use a spatial and temporal discretization, thereby assuming that the mass of the cell is concentrated in nodal points, which correspond to the junction points between the cell and the wall. Using the principle of virtual work and a failure criterion, the vibration-failure behavior of the contact interface is described, neglecting mechanical damping in a first step [Mefti, N., Haussy, B., Ganghoffer, J.F., 2005. Mechanical modeling of the rolling phenomenon at the cell scale. Euromech Colloquium 463, Gronnigen, Netherland]. Accounting for the maturation of the fibbers properties results in a pseudo-periodical response due to the competition between the increase of the fibbers stiffness and the increase of the applied pulling effort. The creation of new connections on a part of the cell boundary located opposite to the plasma flow, is integrated into the modeled, and analyzed. A set of free adhesion molecules – ligands and receptors – is distributed on the wall and cell external surfaces. As the ligands are subjected to the Brownian motion of the fluid particles, the orientation and the amplitude of this movement are assume to be described by gaussian random processes. The effects of the chemical affinity are accounted for as molecular Van der Waals interactions. Numerical simulations emphasize the rolling phenomenon, the mechanism of which being the creation and the rupture of the bonds of the contact zone, which involve the rotation of the cell and the relaxation effects of the elastic properties of these connections.

Influence of surface charges on cell adhesion: difference between static and dynamic conditions.

We tested the hypothesis that nonspecific repulsion, as a result of electrostatic forces and (or) steric stabilization effects, impaired adhesion more efficiently under dynamic than under static conditions. Cells from the human monocytic line THP1 were plated on a glass surface. Spherical particles bearing monoclonal antibodies specific for antigens expressed by THP1 cells (CD11b, CD18, CD35, CD64) were then added and adhesion was quantified. The effect of neuraminidase treatment of THP1 cells was also studied. Adhesion was then measured in a flow chamber under low shear flow (wall shear rate was 11 or 22 s-1), allowing a quantitative determination of cell adhesion frequency. The following conclusions were obtained: (i) under static conditions, neuraminidase treatment had little effect on adhesion (only CD18-mediated interaction was significantly increased at 4 degrees C after enzyme treatment); (ii) under dynamic conditions, neuraminidase treatment significantly increased binding; (iii) surprisingly, there was no clear relationship between the length of adhesion molecules involved in the interaction and binding efficiency; and (iv) such parameters as cell shape and topographical distribution of adhesion molecules may strongly influence adhesion under flow. It is concluded that a dynamic reorganization of the pericellular matrix following intercellular contact may play an important role in regulating adhesion.

Morphology of cell-substratum adhesion. Influence of receptor heterogeneity and nonspecific forces.

Many cell types modulate growth, differentiation, and motility through changes in cell substrate adhesion, including regulation of focal contact formation. Clustering of cell surface adhesion receptors is an essential early step in the development of focal contacts, and thus may influence cell physiology. In this paper, we present a theoretical framework to examine how cell surface chemistry affects receptor clustering. Our one-dimensional tape-peeling model couples the equations of mechanical equilibrium for a cell membrane with kinetic receptor-ligand binding relations. We considered two distinct model scenarios: Adhesion mediated by multiple receptor-ligand interactions of different length and specific binding of a single receptor type occurs in the presence of van der Waals attraction and nonspecific repulsion. In each case, nonuniform (wave-like) membrane morphologies are observed in certain parameter ranges that support the clustering of adhesion receptors. The formation of these morphologies is described in terms of a balance of membrane stresses; when cell-surface potential as a function of separation distance is symmetric between two potential energy minima, nonuniform morphologies are obtained. Increases in the chemical binding energy between receptor and ligand (e.g., increases in ligand density) or decreases in the membrane rigidity result in smaller wavelengths for nonuniform interfaces. Additionally, we show wave-like geometries appear only when the mechanical compliance of receptor-ligand bonds is within an intermediate range, and examine how the mobility of "repellers"--glycocalyx molecules that exert a nonspecific repulsive force--influences membrane morphology. We find fully mobile repellers always redistribute to prevent nonuniform morphologies.

Yeast flocculation: competition between nonspecific repulsion and specific bonding in cell adhesion.

Yeast flocculation is governed by the competition between electrostatic repulsion (nonspecific interaction) and polysaccharide-protein bonds (specific interaction). The electrical surface potential, which is mainly due to phosphodiester linkages (of the cell wall phosphomannan), maintains the cells dispersed. Polysaccharides and proteins of the cell surface can readily penetrate the potential barrier and may establish specific bonds. The specific inhibition of flocculation by various mannosyl derivatives suggested that the protein receptor binds to the group Man alpha----3 Man alpha----PO4- ----6 Man alpha----2 Man alpha... of phosphomannan. Calcium, which is required for flocculation, could act as a bridge between the negatively charged groups of phosphomannan and those of the protein receptor. The role of calcium, however, cannot be restricted only to charge neutralization because other divalent cations inhibit flocculation; our results show that cation binding is governed by strong stereochemical constraints. Studies on protein-polysaccharide interactions have shown that electrical charges may remain uncompensated at short distance, but can be stabilized by hydrogen bonds. Calcium could induce a "locked" conformation of the receptor; this conformation is the only one capable of binding phosphomannan strongly enough to make cell adhesion possible.

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.