Specific Receptor-ligand Bonds Research Articles

E-selectin-mediated rolling facilitates pancreatic cancer cell adhesion to hyaluronic acid.

Tumor cell extravasation is a multistep process preceded by cell rolling and arrest on the vessel wall via the formation of specific receptor-ligand bonds. The strength, availability, and number of receptor-ligand bonds regulate the rate by which tumor cells tether, roll, and adhere to vascular walls. Although the mechanics of selectin-mediated rolling have been extensively studied, little is known regarding how tumor cell rolling on selectins facilitates adhesion to a distinct substrate-bound protein with different kinetic properties. By using multicomponent protein patterning and a microfluidic system, we evaluated how E-selectin-dependent rolling modulates hyaluronic acid (HA) adhesion as a function of fluid shear, contact time, and the spacing between E-selectin and HA regions patterned on the substrate. We show that tumor cells rolling on E-selectin were ∼40-fold more likely to bind to HA than nonrolling cells in shear flow. Furthermore, E-selectin-dependent rolling promotes adhesion to HA by both physically slowing cells and enabling them to position proximal to the surface, thereby increasing the on rate of adhesion. A better understanding of tumor cell adhesion under physiologic shear would lead to the development of new diagnostic assays and pave the way to clinical approaches aimed ultimately to halt metastasis.-Shea, D. J., Li, Y. W., Stebe, K. J., Konstantopoulos, K. E-selectin-mediated rolling facilitates pancreatic cancer cell adhesion to hyaluronic acid.

Effects of functionally graded materials on dynamics of molecular bond clusters

Unlike nonspecific adhesion of conventional hard materials in engineering commonly described by JKR and DMT type models, the molecular adhesion via specific receptor-ligand bonds is stochastic by nature and has the feature that its strength strongly depends on the medium stiffness surrounding the adhesion. In this paper, we demonstrate in a stochastic-elasticity framework that a type of materials with linearly graded elastic modulus can be designed to achieve “equal load sharing” and enhanced cooperative rebinding among interfacial molecular bonds. Upon modulus gradation, multiple molecular bonds can be elastically decoupled but statistically cooperative. In general, uniform molecular adhesion can be accomplished by two strategies: homogeneous materials with sufficient stiffness higher than a threshold or heterogeneous materials satisfying the criterion on modulus gradation. These results not only provide a theoretical principle for possible applications of functionally graded materials in quantitatively controlling cell-matrix adhesion, but also have general implications on adhesion between soft materials mediated by specific molecular binding.

Effective free energy for pinned membranes

We consider membranes adhered through specific receptor-ligand bonds. Thermal undulations of the membrane induce effective interactions between adhesion sites. We derive an upper bound to the free energy that is independent of interaction details. To the lowest order in a systematic expansion we obtain two-body interactions that allow us to map the free energy onto a lattice gas with a constant density. The induced interactions alone are not strong enough to lead to a condensation of individual adhesion sites. A measure of the thermal roughness is shown to depend on the inverse square root of the density of adhesion sites, which is in good agreement with previous computer simulations.

Adhesion dynamics of circulating tumor cells under shear flow in a bio-functionalized microchannel

The adhesion dynamics of circulating tumor cells in a bio-functionalized microchannel under hydrodynamic loading is explored experimentally and analyzed theoretically. EpCAM antibodies are immobilized on the microchannel surface to specifically capture EpCAM-expressing target breast cancer cells MDA-MB-231 from a homogeneous cell suspension in shear flow. In the cross-stream direction, gravity is the dominant physical mechanism resulting in continuous interaction between the EpCAM cell receptors and the immobilized surface anti-EpCAM ligands. Depending on the applied shear rate, three dynamic states have been characterized: firm adhesion, rolling adhesion and free rolling. The steady-state velocity under adhesion- and free-rolling conditions as well as the time-dependent velocity in firm adhesion has been characterized experimentally, based on video recordings of target cell motion in functionalized microchannels. A previously reported theoretical model, utilizing a linear spring to represent the specific receptor–ligand bonds, has been adopted to analyze adhesion dynamics including features such as the cell–surface binding force and separation gap. By fitting theoretical predictions to experimental measurements, a unified exponential decay function is proposed to describe the target cell velocity evolution during capture; the fitting parameters, velocity and time scales, depend on the particular cell–surface system.

Cell receptor and surface ligand density effects on dynamic states of adhering circulating tumor cells

Dynamic states of cancer cells moving under shear flow in an antibody-functionalized microchannel are investigated experimentally and theoretically. The cell motion is analyzed with the aid of a simplified physical model featuring a receptor-coated rigid sphere moving above a solid surface with immobilized ligands. The motion of the sphere is described by the Langevin equation accounting for the hydrodynamic loadings, gravitational force, receptor-ligand bindings, and thermal fluctuations; the receptor-ligand bonds are modeled as linear springs. Depending on the applied shear flow rate, three dynamic states of cell motion have been identified: (i) free motion, (ii) rolling adhesion, and (iii) firm adhesion. Of particular interest is the fraction of captured circulating tumor cells, defined as the capture ratio, via specific receptor-ligand bonds. The cell capture ratio decreases with increasing shear flow rate with a characteristic rate. Based on both experimental and theoretical results, the characteristic flow rate increases monotonically with increasing either cell-receptor or surface-ligand density within certain ranges. Utilizing it as a scaling parameter, flow-rate dependent capture ratios for various cell-surface combinations collapse onto a single curve described by an exponential formula.

Formation of specific receptor–ligand bonds between liquid interfaces

Bi-dimensional (2D) specific interactions are crucial in numerous biological processes. Attachment of molecules to a surface provides several distinct properties absent in a three-dimensional (3D) configuration, such as type of surface molecular anchorage, nature of the surface and sensitivity to local mechanical stress. While rules governing formation and stability of specific molecular interactions in 3D have been extensively described, much less is known about 2D. Here, we devised a model experiment seeking to clarify the main features of these interactions when molecules are attached to liquid interfaces. We prepared two complementary micrometric liquid droplet populations exhibiting either streptavidin as the receptor or biotin as the ligand. We brought the grafted interfaces into contact under different experimental configurations and analyzed the molecular interfacial contact formed. We measured contact formation characteristic time, molecular density, molecular dynamics and geometry. We observed formation of an irreversible liquid film concentrated in ligands and receptors and driven by specific molecular interactions. Molecular contact, although sterically saturated, was not frozen and the bonds themselves were capable of lateral diffusion in the confined space of interdroplet film. While bond diffusion was limited to the interfacial contact zone, unbound molecules were free to be exchanged between droplet contact and contour. We concluded that formation of specific receptor–ligand bonds between liquid interfaces resulted from the subtle interplay of universal surface forces and specific molecular interactions. This shed light on an interesting system of functional colloidal interactions in which molecular composition and physical properties of interfacial contact could be independently tuned.

Mechanics of Molecular Bond Clusters between Elastic Media: Stochastic-Elastic Coupling in Cell-Matrix Adhesion

Focal adhesions are clusters of specific receptor-ligand bonds that link an animal cell to an extracellular matrix. A capability to control focal adhesions, for which a quantitative description of the collective behavior of multiple molecular bonds is a critical step, is essential for tissue and cellular engineering. While the behavior of single molecular bonds is governed by statistical mechanics at small scale, continuum mechanics should be valid at large scale. How can this transition be modeled and can this tell us something about the mechanics of cell adhesion? Here we develop a stochastic-elasticity model of a periodic array of adhesion clusters between two dissimilar elastic media subjected to an inclined tensile stress, in which stochastic descriptions of molecular bonds and elastic descriptions of interfacial traction are unified in a single modeling framework. A fundamental scaling law of interfacial traction distribution is established to govern the transition between uniform and cracklike singular distributions of the interfacial traction within molecular bonds. Guided by this scaling law, we perform Monte Carlo simulations to investigate the effects of cluster size, cell/matrix modulus and loading direction on lifetime and strength of the adhesion clusters. The results show that intermediate adhesion size, stiff substrate, cytoskeleton stiffening, and low-angle pulling are factors that contribute to the stability of focal adhesions. The predictions of our model provide feasible explanations for a wide range of experimental observations and suggest possible mechanisms by which cells can modulate adhesion and deadhesion via cytoskeletal contractile machinery.

Lifetime and Strength of Periodic Bond Clusters between Elastic Media under Inclined Loading

Focal adhesions are clusters of specific receptor-ligand bonds that link an animal cell to an extracellular matrix. To understand the mechanical responses of focal adhesions, here we develop a stochastic-elasticity model of a periodic array of adhesion clusters between two dissimilar elastic media subjected to an inclined tensile stress, in which stochastic descriptions of molecular bonds and elastic descriptions of interfacial traction are unified in a single modeling framework. We first establish a fundamental scaling law of interfacial traction distribution and derive a stress concentration index that governs the transition between uniform and cracklike singular distributions of the interfacial traction within molecular bonds. Guided by this scaling law, we then perform Monte Carlo simulations to investigate the effects of cluster size, cell/extracellular matrix modulus, and loading direction on lifetime and strength of the adhesion clusters. The results show that intermediate adhesion size, stiff substrate, cytoskeleton stiffening, and low-angle pulling are factors that contribute to the stability of focal adhesions. The predictions of our model provide feasible explanations for a wide range of experimental observations and suggest possible mechanisms by which cells can modulate adhesion and deadhesion via cytoskeletal contractile machinery and sense mechanical properties of their surroundings.

Dynamic states of cells adhering in shear flow: From slipping to rolling

Motivated by rolling adhesion of white blood cells in the vasculature, we study how cells move in linear shear flow above a wall to which they can adhere via specific receptor-ligand bonds. Our computer simulations are based on a Langevin equation accounting for hydrodynamic interactions, thermal fluctuations, and adhesive interactions. In contrast to earlier approaches, our model not only includes stochastic rules for the formation and rupture of bonds, but also fully resolves both receptor and ligand positions. We identify five different dynamic states of motion in regard to the translational and angular velocities of the cell. The transitions between the different states are mapped out in a dynamic state diagram as a function of the rates for bond formation and rupture. For example, as the cell starts to adhere under the action of bonds, its translational and angular velocities become synchronized and the dynamic state changes from slipping to rolling. We also investigate the effect of nonmolecular parameters. In particular, we find that an increase in viscosity of the medium leads to a characteristic expansion of the region of stable rolling to the expense of the region of firm adhesion, but not to the expense of the regions of free or transient motion. Our results can be used in an inverse approach to determine single bond parameters from flow chamber data on rolling adhesion.

Mechanisms for Flow-Enhanced Cell Adhesion

Cell adhesion is mediated by specific receptor-ligand bonds. In several biological systems, increasing flow has been observed to enhance cell adhesion despite the increasing dislodging fluid shear forces. Flow-enhanced cell adhesion includes several aspects: flow augments the initial tethering of flowing cells to a stationary surface, slows the velocity and increases the regularity of rolling cells, and increases the number of rollingly adherent cells. Mechanisms for this intriguing phenomenon may include transport-dependent acceleration of bond formation and force-dependent deceleration of bond dissociation. The former includes three distinct transport modes: sliding of cell bottom on the surface, Brownian motion of the cell, and rotational diffusion of the interacting molecules. The latter involves a recently demonstrated counterintuitive behavior called catch bonds where force prolongs rather than shortens the lifetimes of receptor-ligand bonds. In this article, we summarize our recently published data that used dimensional analysis and mutational analysis to elucidate the above mechanisms for flow-enhanced leukocyte adhesion mediated by L-selectin-ligand interactions.

Spreading and peeling dynamics in a model of cell adhesion

To understand how viscous and elastic membrane forces mediate the adhesion of fluid-borne cells to biological surfaces under the action of specific receptor–ligand bonds, we consider a model problem in which a two-dimensional cell interacts with a plane adhesive surface. The cell is modelled as an extensible membrane under tension containing fluid of constant volume. Assuming rapid binding kinetics, molecular binding forces are described through a contact potential that is long-range attractive but short-range repulsive. Using lubrication theory to describe the thin-film flow between the cell and the plane, we model sedimentation of the cell onto the plane under adhesive forces, followed by removal of the cell from the plane under the action of an external force. Numerical simulations show how these events are dominated respectively by quasi-steady spreading and peeling motions, which we capture using an asymptotic analysis. The analysis is extended to model a cell tank-treading over an adhesive wall in an external shear flow. The relation between cell rolling speed and shear rate is determined: at low speeds it is linear and independent of the viscosity of the suspending fluid; at higher speeds it is nonlinear and viscosity-dependent.