Demonstration of catch bonds between an integrin and its ligand

Type 1 fimbrial adhesin FimH elicits an immune response that enhances cell adhesion of Escherichia coli

Escherichia coli causes about 90% of urinary tract infections (UTI), and more than 95% of all UTI-causing E. coli express type 1 fimbriae. The fimbrial tip-positioned adhesive protein FimH utilizes a shear force-enhanced, so-called catch-bond mechanism of interaction with its receptor, mannose, where the lectin domain of FimH shifts from a low- to a high-affinity conformation upon separation from the anchoring pilin domain. Here, we show that immunization with the lectin domain induces antibodies that exclusively or predominantly recognize only the high-affinity conformation. In the lectin domain, we identified four high-affinity-specific epitopes, all positioned away from the mannose-binding pocket, which are recognized by 20 separate clones of monoclonal antibody. None of the monoclonal or polyclonal antibodies against the lectin domain inhibited the adhesive function. On the contrary, the antibodies enhanced FimH-mediated binding to mannosylated ligands and increased by severalfold bacterial adhesion to urothelial cells. Furthermore, by natural conversion from the high- to the low-affinity state, FimH adhesin was able to shed the antibodies bound to it. When whole fimbriae were used, the antifimbrial immune serum that contained a significant amount of antibodies against the lectin domain of FimH was also able to enhance FimH-mediated binding. Thus, bacterial adhesins (or other surface antigens) with the ability to switch between alternative conformations have the potential to induce a conformation-specific immune response that has a function-enhancing rather than -inhibiting impact on the protein. These observations have implications for the development of adhesin-specific vaccines and may serve as a paradigm for antibody-mediated enhancement of pathogen binding.

Interstitial flow influences direction of tumor cell migration through competing mechanisms

Interstitial flow is the convective transport of fluid through tissue extracellular matrix. This creeping fluid flow has been shown to affect the morphology and migration of cells such as fibroblasts, cancer cells, endothelial cells, and mesenchymal stem cells. A microfluidic cell culture system was designed to apply stable pressure gradients and fluid flow and allow direct visualization of transient responses of cells seeded in a 3D collagen type I scaffold. We used this system to examine the effects of interstitial flow on cancer cell morphology and migration and to extend previous studies showing that interstitial flow increases the metastatic potential of MDA-MB-435S melanoma cells [Shields J, et al. (2007) Cancer Cell 11:526-538]. Using a breast carcinoma line (MDA-MB-231) we also observed cell migration along streamlines in the presence of flow; however, we further demonstrated that the strength of the flow as well as the cell density determined directional bias of migration along the streamline. In particular, we found that cells either at high seeding density or with the CCR-7 receptor inhibited migration against, rather than with the flow. We provide further evidence that CCR7-dependent autologous chemotaxis is the mechanism that leads to migration with the flow, but also demonstrate a competing CCR7-independent mechanism that causes migration against the flow. Data from experiments investigating the effects of cell concentration, interstitial flow rate, receptor activity, and focal adhesion kinase phosphorylation support our hypothesis that the competing stimulus is integrin mediated. This mechanism may play an important role in development of metastatic disease.

Probing mechanical principles of focal contacts in cell-matrix adhesion with a coupled stochastic-elastic modelling framework

Cell-matrix adhesion depends on the collective behaviours of clusters of receptor-ligand bonds called focal contacts between cell and extracellular matrix. While the behaviour of a single molecular bond is governed by statistical mechanics at the molecular scale, continuum mechanics should be valid at a larger scale. This paper presents an overview of a series of recent theoretical studies aimed at probing the basic mechanical principles of focal contacts in cell-matrix adhesion via stochastic-elastic models in which stochastic descriptions of molecular bonds and elastic descriptions of interfacial traction-separation are unified in a single modelling framework. The intention here is to illustrate these principles using simple analytical and numerical models. The aim of the discussions is to provide possible clues to the following questions: why does the size of focal adhesions (FAs) fall into a narrow range around the micrometre scale? How can cells sense and respond to substrates of varied stiffness via FAs? How do the magnitude and orientation of mechanical forces affect the binding dynamics of FAs? The effects of cluster size, cell-matrix elastic modulus, loading direction and cytoskeletal pretension on the lifetime of FA clusters have been investigated by theoretical arguments as well as Monte Carlo numerical simulations, with results showing that intermediate adhesion size, stiff substrate, cytoskeleton stiffening, low-angle pulling and moderate cytoskeletal pretension are factors that contribute to stable FAs. From a mechanistic point of view, these results provide possible explanations for a wide range of experimental observations and suggest multiple mechanisms by which cells can actively control adhesion and de-adhesion via cytoskeletal contractile machinery in response to mechanical properties of their surroundings.

The bacterial fimbrial tip acts as a mechanical force sensor

The subunits that constitute the bacterial adhesive complex located at the tip of the fimbria form a hook-chain that acts as a rapid force-sensitive anchor at high flow.

There is increasing evidence that the catch bond mechanism, where binding becomes stronger under tensile force, is a common property among non-covalent interactions between biological molecules that are exposed to mechanical force in vivo. Here, by using the multi-protein tip complex of the mannose-binding type 1 fimbriae of Escherichia coli, we show how the entire quaternary structure of the adhesive organella is adapted to facilitate binding under mechanically dynamic conditions induced by flow. The fimbrial tip mediates shear-dependent adhesion of bacteria to uroepithelial cells and demonstrates force-enhanced interaction with mannose in single molecule force spectroscopy experiments. The mannose-binding, lectin domain of the apex-positioned adhesive protein FimH is docked to the anchoring pilin domain in a distinct hooked manner. The hooked conformation is highly stable in molecular dynamics simulations under no force conditions but permits an easy separation of the domains upon application of an external tensile force, allowing the lectin domain to switch from a low- to a high-affinity state. The conformation between the FimH pilin domain and the following FimG subunit of the tip is open and stable even when tensile force is applied, providing an extended lever arm for the hook unhinging under shear. Finally, the conformation between FimG and FimF subunits is highly flexible even in the absence of tensile force, conferring to the FimH adhesin an exploratory function and high binding rates. The fimbrial tip of type 1 Escherichia coli is optimized to have a dual functionality: flexible exploration and force sensing. Comparison to other structures suggests that this property is common in unrelated bacterial and eukaryotic adhesive complexes that must function in dynamic conditions.

Noncovalent biological interactions are commonly subjected to mechanical force, particularly when they are involved in adhesion or cytoskeletal movements. While one might expect mechanical force to break these interactions, some of them form so-called catch bonds that lock on harder under force, like a nanoscale finger-trap. In this study, we show that the catch-bond forming adhesive protein FimH, which is located at the tip of E. coli fimbriae, allows bacteria to bind to urinary epithelial cells in a shear-dependent manner; that is, they bind at high but not at low flow. We show that isolated fimbrial tips, consisting of elongated protein complexes with FimH at the apex, reproduce this behavior in vitro. Our molecular dynamics simulations of the fimbrial tip structure show that FimH is shaped like a hook that is normally rigid but opens under force, causing structural changes that lead to firm anchoring of the bacteria on the surface. In contrast, the more distal adaptor proteins of the fimbrial tip create a flexible connection of FimH to the rigid fimbria, enhancing the ability of the adhesin to move into position and form bonds with mannose on the surface. We suggest that the entire tip complex forms a hook-chain, ideal for rapid and stable anchoring in flow.

Actomyosin-generated tension controls the molecular kinetics of focal adhesions

Focal adhesions (FAs) have key roles in the interaction of cells with the extracellular matrix (ECM) and in adhesion-mediated signaling. These dynamic, multi-protein structures sense the ECM both chemically and physically, and respond to external and internal forces by changing their size and signaling activity. However, this mechanosensitivity is still poorly understood at the molecular level. Here, we present direct evidence that actomyosin contractility regulates the molecular kinetics of FAs. We show that the molecular turnover of proteins within FAs is primarily regulated by their dissociation rate constant (k(off)), which is sensitive to changes in forces applied to the FA. We measured the early changes in k(off) values for three FA proteins (vinculin, paxillin and zyxin) upon inhibition of actomyosin-generated forces using two methods - high temporal resolution FRAP and direct measurement of FA protein dissociation in permeabilized cells. When myosin II contractility was inhibited, the k(off) values for all three proteins changed rapidly, in a highly protein-specific manner: dissociation of vinculin from FAs was facilitated, whereas dissociation of paxillin and zyxin was attenuated. We hypothesize that these early kinetic changes initiate FA disassembly by affecting the molecular turnover of FAs and altering their composition.

Dissociation of bimolecular αIIbβ3-fibrinogen complex under a constant tensile force

The regulated ability of integrin αIIbβ3 to bind fibrinogen plays a crucial role in platelet aggregation, adhesion, and hemostasis. Employing an optical-trap-based electronic force clamp, we studied the thermodynamics and kinetics of αIIbβ3-fibrinogen bond formation and dissociation under constant unbinding forces, mimicking the forces of physiologic blood shear on a thrombus. The distribution of bond lifetimes was bimodal, indicating that the αIIbβ3-fibrinogen complex exists in two bound states with different mechanical stability. The αIIbβ3 antagonist, abciximab, inhibited binding without affecting the unbinding kinetics, whereas Mn²(+) biased the αIIbβ3-fibrinogen complex to the strong bound state with reduced off-rate. The average bond lifetimes decreased exponentially with increasing pulling force from ∼5 pN to 50 pN, suggesting that in this force range the αIIbβ3-fibrinogen interactions are classical slip bonds. We found no evidence for catch bonds, which is consistent with the known lack of shear-enhanced platelet adhesion on fibrinogen-coated surfaces. Taken together, these data provide important quantitative and qualitative characteristics of αIIbβ3-fibrinogen binding and unbinding that underlie the dynamics of platelet adhesion and aggregation in blood flow.

Polymer-based catch-bonds

Catch-bonds refer to the counterintuitive notion that the average lifetime of a bond has a maximum at a nonzero applied force. They have been found in several ligand-receptor pairs and their origin is still a topic of debate. Here, we use coarse-grained simulations and kinetic theory to demonstrate that a multimeric protein, with self-interacting domain pairs, can display catch-bond behavior. Our model is motivated by one of the largest proteins in the human body, the von Willebrand Factor, which has been found to display this behavior. In particular, our model polymer consists of a series of repeating units that self-interact with their nearest neighbors along the chain. Each of the units mimics a domain of the protein. Apart from the short-range specific interaction, we also include a linker chain that will hold the domains together if unbinding occurs. This linker molecule represents the sequence of unfolded amino acids that connect contiguous domains, as is typically found in multidomain proteins. The units also interact with an immobilized ligand, but the interaction is masked by the presence of the self-interacting neighbor along the chain. Our results show that this model displays all the features of catch-bonds because the average lifetime of a binding event between the polymer and the immobilized receptor has a maximum at a nonzero pulling force of the polymer. The effects of the energy barriers for detaching the masking domain and the ligand from the binding domain, as well as the effects of the properties of the polypeptide chain connecting the contiguous domains, are also studied. Our study suggests that multimeric proteins can engage in catch-bonds if their self-interactions are carefully tuned, and this mechanism presumably plays a major role in the mechanics of extracellular proteins that share a multidomain character. Furthermore, our biomimetic design clearly shows how one could build and tune macromolecules that exhibit catch-bond characteristics.

Fibronectin fragment activation of ERK increasing integrin α₅ and β₁ subunit expression to degenerate nucleus pulposus cells

Fibronectin fragments (Fn-f), which are the breakdown products of fibronectin, accumulate in the disc during degeneration and are proved to induce the degeneration of intervertebral disc. The goal of this investigation was to determine the functional role of integrin α₅ β₁, extracellular signal-regulated kinase (ERK), and protein kinase C (PKC) in the process of Fn-f degeneration nucleus pulposus (NP) cells. We found that Fn-f (100 nM, 30 kDa) exposure led to degeneration of NP cells, up-regulation of integrin α₅ β₁ expression and phosphorylation of the ERK(½) . After the expression of integrin α₅ β₁ was silenced in NP cells, the phosphorylation of ERK(½) and the expression of MMP9, MMP13, and collagen II had no difference with control under the treatment of Fn-f. Finally, when the inhibitor of ERK(½) and the inhibitor of PKC were added into the medium of NP cells; we found these two inhibitors could eliminate the effect of Fn-f on NP cells. It is concluded that Fn-f had the potential to enhance the NP cell degeneration in a vicious circle. And the integrin α₅ β₁ subunit, ERK, and PKC were all included in this loop.

Focal adhesions in osteoneogenesis

As materials technology and the field of tissue engineering advance, the role of cellular adhesive mechanisms, in particular, interactions with implantable devices, becomes more relevant in both research and clinical practice. A key tenet of medical device technology is to use the exquisite ability of biological systems to respond to the material surface or chemical stimuli in order to help to develop next-generation biomaterials. The focus of this review is on recent studies and developments concerning focal adhesion formation in osteoneogenesis, with an emphasis on the influence of synthetic constructs on integrin-mediated cellular adhesion and function.

Dynamic adhesion of umbilical cord blood endothelial progenitor cells under laminar shear stress

Late outgrowth endothelial progenitor cells (EPCs) represent a promising cell source for rapid reendothelialization of damaged vasculature after expansion ex vivo and injection into the bloodstream. We characterized the dynamic adhesion of umbilical-cord-blood-derived EPCs (CB-EPCs) to surfaces coated with fibronectin. CB-EPC solution density affected the number of adherent cells and larger cells preferentially adhered at lower cell densities. The number of adherent cells varied with shear stress, with the maximum number of adherent cells and the shear stress at maximum adhesion depending upon fluid viscosity. CB-EPCs underwent limited rolling, transiently tethering for short distances before firm arrest. Immediately before arrest, the instantaneous velocity decreased independent of shear stress. A dimensional analysis indicated that adhesion was a function of the net force on the cells, the ratio of cell diffusion to sliding speed, and molecular diffusivity. Adhesion was not limited by the settling rate and was highly specific to α(5)β(1) integrin. Total internal reflection fluorescence microscopy showed that CB-EPCs produced multiple contacts of α(5)β(1) with the surface and the contact area grew during the first 20 min of attachment. These results demonstrate that CB-EPC adhesion from blood can occur under physiological levels of shear stress.

Molecular dynamics simulations of forced unbending of integrin α(v)β₃

Integrins may undergo large conformational changes during activation, but the dynamic processes and pathways remain poorly understood. We used molecular dynamics to simulate forced unbending of a complete integrin α_Vβ₃ ectodomain in both unliganded and liganded forms. Pulling the head of the integrin readily induced changes in the integrin from a bent to an extended conformation. Pulling at a cyclic RGD ligand bound to the integrin head also extended the integrin, suggesting that force can activate integrins. Interactions at the interfaces between the hybrid and β tail domains and between the hybrid and epidermal growth factor 4 domains formed the major energy barrier along the unbending pathway, which could be overcome spontaneously in ∼1 µs to yield a partially-extended conformation that tended to rebend. By comparison, a fully-extended conformation was stable. A newly-formed coordination between the α_V Asp457 and the α-genu metal ion might contribute to the stability of the fully-extended conformation. These results reveal the dynamic processes and pathways of integrin conformational changes with atomic details and provide new insights into the structural mechanisms of integrin activation.

Proteins can regulate their functions via conformational changes. One example is integrins, which are transmembrane receptors mediating cell-cell and cell-matrix adhesions. Inactive integrins may assume a bent conformation with low affinities for ligands unable to support adhesions. Intracellular or extracellular stimuli induce large scale changes from the bent to an extended conformation, resulting in active integrins with high affinities for ligands to mediate strong adhesions. We used molecular dynamics simulations to reveal the dynamics and pathways of integrin unbending in atomic details. Critical interactions in this process were identified. This study not only sheds light on the structural mechanisms of integrin activation, but also exemplifies allosteric regulations of protein functions.

Biophysics of selectin-ligand interactions in inflammation and cancer

Selectins (L-, E- and P-selectin) are calcium-dependent transmembrane glycoproteins that are expressed on the surface of circulating leukocytes, activated platelets, and inflamed endothelial cells. Selectins bind predominantly to sialofucosylated glycoproteins and glycolipids (E-selectin only) present on the surface of apposing cells, and mediate transient adhesive interactions pertinent to inflammation and cancer metastasis. The rapid turnover of selectin-ligand bonds, due to their fast on- and off-rates along with their remarkably high tensile strengths, enables them to mediate cell tethering and rolling in shear flow. This paper presents the current body of knowledge regarding the role of selectins in inflammation and cancer metastasis, and discusses experimental methodologies and mathematical models used to resolve the biophysics of selectin-mediated cell adhesion. Understanding the biochemistry and biomechanics of selectin-ligand interactions pertinent to inflammatory disorders and cancer metastasis may provide insights for developing promising therapies and/or diagnostic tools to combat these disorders.

Overview: assays for studying integrin-dependent cell adhesion

Interaction of the integrin receptors with ligands determines the molecular basis of integrin-dependent cell adhesion. Integrin ligands are typically large proteins with relatively low binding affinities. This makes direct ligand-binding kinetic measurements somewhat difficult. Here we examine several real-time methods, aimed to overcome these experimental limitations and to distinguish the regulation of integrin conformation and affinity. This chapter includes: the use of a small ligand-mimetic probe for studies of inside-out regulation of integrin affinity and unbending, real-time cell aggregation and disaggregation kinetics to probe integrin conformational states and the number of integrin-ligand bonds, as well as the real-time monitoring of ligand-induced epitopes under signaling through G-protein-coupled receptors, and others. Experimental data obtained using these novel methods are summarized in terms of the current model of integrin activation.

Catch-bond behavior of bacteria binding by slip bonds

It is shown that multipili-adhering bacteria expressing helix-like pili binding by slip bonds can show catch-bond behavior. When exposed to an external force, such bacteria can mediate adhesion to their hosts by either of two limiting means: sequential or simultaneous pili force exposure (referring to when the pili mediate force in a sequential or simultaneous manner, respectively). As the force is increased, the pili can transition from sequential to simultaneous pili force exposure. Since the latter mode of adhesion gives rise to a significantly longer bacterial adhesion lifetime than the former, this results in a prolongation of the lifetime, which shows up as a catch-bond behavior. The properties and conditions of this effect were theoretically investigated and assessed in some detail for dual-pili-adhering bacteria, by both analytical means and simulations. The results indicate that the adhesion lifetime of such bacteria can be prolonged by more than an order of magnitude. This implies that the adhesion properties of multibinding systems cannot be directly conveyed to the individual adhesion-receptor bonds.

Integrins: versatile receptors controlling melanocyte adhesion, migration and proliferation

From the onset of melanocyte specification from the neural crest, throughout their migration during embryogenesis and until they reside in their niche in the basal keratinocyte layer, melanocytes interact in dynamic ways with the extracellular environment of the growing embryo. To recognize and to adhere to their environment, melanocytes depend on heterodimeric cell surface receptors of the family of integrins. In addition to the control of adhesive interactions between melanocytes and the extracellular matrix scaffold secreted by fibroblasts and keratinocytes, the integrin receptors allow cells also to sense the mechanical condition of the extracellular environment, responding by intracellular signaling, triggering cell survival, proliferation or migration events. In this review, we summarize the recently emerged concepts that explain integrin-dependent adhesion and how this adhesion system interfaces with integrin-dependent signaling events. The gained information will help to understand melanocyte behavior in pathological situations such as melanoma growth and metastasis formation.

Visualization of allostery in P-selectin lectin domain using MD simulations

Allostery of P-selectin lectin (Lec) domain followed by an epithelial growth factor (EGF)-like domain is essential for its biological functionality, but the underlying pathways have not been well understood. Here the molecular dynamics simulations were performed on the crystallized structures to visualize the dynamic conformational change for state 1 (S1) or state 2 (S2) Lec domain with respective bent (B) or extended (E) EGF orientation. Simulations illustrated that both S1 and S2 conformations were unable to switch from one to another directly. Instead, a novel S1' conformation was observed from S1 when crystallized B-S1 or reconstructed “E-S1” structure was employed, which was superposed well with that of equilibrated S1 Lec domain alone. It was also indicated that the corresponding allosteric pathway from S1 to S1' conformation started with the separation between residues Q30 and K67 and terminated with the release of residue N87 from residue C109. These results provided an insight into understanding the structural transition and the structure-function relationship of P-selectin allostery.

Forcing switch from short- to intermediate- and long-lived states of the alphaA domain generates LFA-1/ICAM-1 catch bonds

Binding of lymphocyte function-associated antigen-1 (LFA-1) to intercellular adhesion molecule-1 (ICAM-1) mediates leukocyte adhesion under force. Using a biomembrane force probe capable of measuring single bond interactions, we showed ICAM-1 binding to LFA-1 at different conformations, including the bent conformation with the lowest affinity. We quantify how force and conformations of LFA-1 regulate its kinetics with ICAM-1. At zero-force, on-rates were substantially changed by conditions that differentially favor a bent or extended LFA-1 with a closed or open headpiece; but off-rates were identical. With increasing force, LFA-1/ICAM-1 bond lifetimes (reciprocal off-rates) first increased (catch bonds) and then decreased (slip bonds). Three states with distinct off-rates were identified from lifetime distributions. Force shifted the associated fractions from the short- to intermediate- and long-lived states, producing catch bonds at low forces, but increased their off-rates exponentially, converting catch to slip bonds at high forces. An internal ligand antagonist that blocks pulling of the α(7)-helix suppressed the intermediate-/long-lived states and eliminated catch bonds, revealing an internal catch bond between the αA and βA domains. These results elucidate an allosteric mechanism for the mechanochemistry of LFA-1/ICAM-1 binding.

Mechanical integration of actin and adhesion dynamics in cell migration

Directed cell migration is a physical process that requires dramatic changes in cell shape and adhesion to the extracellular matrix. For efficient movement, these processes must be spatiotemporally coordinated. To a large degree, the morphological changes and physical forces that occur during migration are generated by a dynamic filamentous actin (F-actin) cytoskeleton. Adhesion is regulated by dynamic assemblies of structural and signaling proteins that couple the F-actin cytoskeleton to the extracellular matrix. Here, we review current knowledge of the dynamic organization of the F-actin cytoskeleton in cell migration and the regulation of focal adhesion assembly and disassembly with an emphasis on how mechanical and biochemical signaling between these two systems regulate the coordination of physical processes in cell migration.

Rolling cell adhesion

Rolling adhesion on vascular surfaces is the first step in recruiting circulating leukocytes, hematopoietic progenitors, or platelets to specific organs or to sites of infection or injury. Rolling requires the rapid yet balanced formation and dissociation of adhesive bonds in the challenging environment of blood flow. This review explores how structurally distinct adhesion receptors interact through mechanically regulated kinetics with their ligands to meet these challenges. Remarkably, increasing force applied to adhesive bonds first prolongs their lifetimes (catch bonds) and then shortens their lifetimes (slip bonds). Catch bonds mediate the counterintuitive phenomenon of flow-enhanced rolling adhesion. Force-regulated disruptions of receptor interdomain or intradomain interactions remote from the ligand-binding surface generate catch bonds. Adhesion receptor dimerization, clustering in membrane domains, and interactions with the cytoskeleton modulate the forces applied to bonds. Both inside-out and outside-in cell signals regulate these processes.

Specificities of β1 integrin signaling in the control of cell adhesion and adhesive strength

Cells exert actomyosin contractility and cytoskeleton-dependent force in response to matrix stiffness cues. Cells dynamically adapt to force by modifying their behavior and remodeling their microenvironment. This adaptation is favored by integrin activation switch and their ability to modulate their clustering and the assembly of an intracellular hub in response to force. Indeed integrins are mechanoreceptors and mediate mechanotransduction by transferring forces to specific adhesion proteins into focal adhesions which are sensitive to tension and activate intracellular signals. α(5)β(1) integrin is considered of major importance for the formation of an elaborate meshwork of fibronectin fibrils and for the extracellular matrix deposition and remodeling. Here we summarize recent progress in the study of mechanisms regulating the activation cycle of β(1) integrin and the specificity of α(5)β(1) integrin in mechanotransduction.

The catch bond mechanism between von Willebrand factor and platelet surface receptors investigated by molecular dynamics simulations

The multi-domain protein von Willebrand factor is crucial in the blood coagulation process at high shear. The A1 domain binds to the platelet surface receptor glycoprotein Ibalpha (GpIb alpha) and this interaction is known to be strengthened by tensile force. The molecular mechanism behind this observation was investigated here by molecular dynamics simulations. The results suggest that the proteins unbind through two distinct pathways depending whether a high-tensile force is applied or whether unbinding happens through thermal fluctuations. In the high-force unbinding pathway the A1 domain was observed to rotate away from the C-terminus of GpIb alpha. In contrast, during thermal unbinding the A1 domain rotated in the opposite direction as in the high-force pathway and the distance between the terminii of A1 and the GpIb alpha C-terminus shortened. This shortening was reduced and the lifetime of the bond extended if a moderate tensile force was applied across the complex. This suggests that the thermal unbinding pathway is inhibited by a moderate tensile force which is in agreement with the catch bond property shown previously in single molecule experiments. A designed mutant of GpIb alpha is suggested here in order to test in vitro the thermal unbinding pathway observed in silico.

Spatial organization of adhesion: force-dependent regulation and function in tissue morphogenesis

Integrin- and cadherin-mediated adhesion is central for cell and tissue morphogenesis, allowing cells and tissues to change shape without loosing integrity. Studies predominantly in cell culture showed that mechanosensation through adhesion structures is achieved by force-mediated modulation of their molecular composition. The specific molecular composition of adhesion sites in turn determines their signalling activity and dynamic reorganization. Here, we will review how adhesion sites respond to mecanical stimuli, and how spatially and temporally regulated signalling from different adhesion sites controls cell migration and tissue morphogenesis.

Regulation of catch binding by allosteric transitions

An allosteric model is used to describe changes in lifetimes of biological receptor-ligand bonds subjected to an external force. Force-induced transitions between the two states of the allosteric site lead to changes in the receptor conformation. The ligand bound to the receptor fluctuates between two different potentials formed by the two receptor conformations. The effect of the force on the receptor-ligand interaction potential is described by the Bell mechanism. The probability of detecting the ligand in the bound state is found to depend on the relaxation times of both ligand and allosteric sites. An analytic expression for the bond lifetime is derived as a function of force. The formal theoretical results are used to explain the anomalous force and time dependences of the integrin-fibronectin bond lifetimes measured by atomic force microscopy (Kong, F.; et al J. Cell Biol. 2009, 185, 1275-1284). The analytic expression and model parameters describe very well all anomalous dependences identified in the experiments.

Stretchy proteins on stretchy substrates: the important elements of integrin-mediated rigidity sensing

Matrix and tissue rigidity guides many cellular processes, including the differentiation of stem cells and the migration of cells in health and disease. Cells actively and transiently test rigidity using mechanisms limited by inherent physical parameters that include the strength of extracellular attachments, the pulling capacity on these attachments, and the sensitivity of the mechanotransduction system. Here, we focus on rigidity sensing mediated through the integrin family of extracellular matrix receptors and linked proteins and discuss the evidence supporting these proteins as mechanosensors.

The non-equilibrium thermodynamics and kinetics of focal adhesion dynamics

Background

We consider a focal adhesion to be made up of molecular complexes, each consisting of a ligand, an integrin molecule, and associated plaque proteins. Free energy changes drive the binding and unbinding of these complexes and thereby controls the focal adhesion's dynamic modes of growth, treadmilling and resorption.

Principal Findings

We have identified a competition among four thermodynamic driving forces for focal adhesion dynamics: (i) the work done during the addition of a single molecular complex of a certain size, (ii) the chemical free energy change associated with the addition of a molecular complex, (iii) the elastic free energy change associated with deformation of focal adhesions and the cell membrane, and (iv) the work done on a molecular conformational change. We have developed a theoretical treatment of focal adhesion dynamics as a nonlinear rate process governed by a classical kinetic model. We also express the rates as being driven by out-of-equilibrium thermodynamic driving forces, and modulated by kinetics. The mechanisms governed by the above four effects allow focal adhesions to exhibit a rich variety of behavior without the need to introduce special constitutive assumptions for their response. For the reaction-limited case growth, treadmilling and resorption are all predicted by a very simple chemo-mechanical model. Treadmilling requires symmetry breaking between the ends of the focal adhesion, and is achieved by driving force (i) above. In contrast, depending on its numerical value (ii) causes symmetric growth, resorption or is neutral, (iii) causes symmetric resorption, and (iv) causes symmetric growth. These findings hold for a range of conditions: temporally-constant force or stress, and for spatially-uniform and non-uniform stress distribution over the FA. The symmetric growth mode dominates for temporally-constant stress, with a reduced treadmilling regime.

Significance

In addition to explaining focal adhesion dynamics, this treatment can be coupled with models of cytoskeleton dynamics and contribute to the understanding of cell motility.

Monitoring ligand-receptor interactions by photonic force microscopy

We introduce a method for the acquisition of single molecule force measurements of ligand-receptor interactions using the photonic force microscope (PFM). Biotin-functionalized beads, manipulated with an optical trap, and a streptavidin-functionalized coverslip were used to measure the effect of different pulling forces on the lifetime of individual streptavidin-biotin complexes. By optimizing the design of the optical trap and selection of the appropriate bead size, pulling forces in excess of 50 pN were achieved. Based on the amplitude of three-dimensional (3D) thermal position fluctuations of the attached bead, we were able to select for a bead-coverslip interaction that was mediated by a single streptavidin-biotin complex. Moreover, the developed experimental system was greatly accelerated by automation of data acquisition and analysis. In force-dependent kinetic measurements carried out between streptavidin and biotin, we observed that the streptavidin-biotin complex exhibited properties of a catch bond, with the lifetime increasing tenfold when the pulling force increased from 10 to 20 pN. We also show that silica beads were more appropriate than polystyrene beads for the force measurements, as tethers, longer than 200 nm, could be extracted from polystyrene beads.

Transient frictional slip between integrin and the ECM in focal adhesions under myosin II tension

BACKGROUND

The spatiotemporal regulation of adhesion to the extracellular matrix is important in metazoan cell migration and mechanosensation. Although adhesion assembly depends on intracellular and extracellular tension, the biophysical regulation of force transmission between the actin cytoskeleton and extracellular matrix during this process remains largely unknown.

RESULTS

To elucidate the nature of force transmission as myosin II tension is applied to focal adhesions, we correlated the dynamics of focal adhesion proteins and the actin cytoskeleton to local traction stresses. Under low extracellular tension, newly formed adhesions near the cell periphery underwent a transient retrograde displacement preceding elongation. We found that myosin II-generated tension drives this mobility, and we determine the interface of differential motion, or "slip," to be between integrin and the ECM. The magnitude and duration of both adhesion slip and associated F-actin dynamics is strongly modulated by ECM compliance. Traction forces are generated throughout the slip period, and adhesion immobilization occurs at a constant tension.

CONCLUSIONS

We have identified a tension-dependent, extracellular "clutch" between integrins and the extracellular matrix; this clutch stabilizes adhesions under myosin II driven tension. The current work elucidates a mechanism by which force transmission is modulated during focal adhesion maturation.

Harnessing traction-mediated manipulation of the cell/matrix interface to control stem-cell fate

Stem cells sense and respond to the mechanical properties of the extracellular matrix. However, both the extent to which extracellular-matrix mechanics affect stem-cell fate in three-dimensional microenvironments and the underlying biophysical mechanisms are unclear. We demonstrate that the commitment of mesenchymal stem-cell populations changes in response to the rigidity of three-dimensional microenvironments, with osteogenesis occurring predominantly at 11-30 kPa. In contrast to previous two-dimensional work, however, cell fate was not correlated with morphology. Instead, matrix stiffness regulated integrin binding as well as reorganization of adhesion ligands on the nanoscale, both of which were traction dependent and correlated with osteogenic commitment of mesenchymal stem-cell populations. These findings suggest that cells interpret changes in the physical properties of adhesion substrates as changes in adhesion-ligand presentation, and that cells themselves can be harnessed as tools to mechanically process materials into structures that feed back to manipulate their fate.

Model for how retrograde actin flow regulates adhesion traction stresses

Cells from animals adhere to and exert mechanical forces on their surroundings. Cells must control these forces for many biological processes, and dysfunction can lead to pathologies. How the actions of molecules within a cell are coordinated to regulate the adhesive interaction with the extracellular matrix remains poorly understood. It has been observed that cytoplasmic proteins that link integrin cell-surface receptors with the actin cytoskeleton flow with varying rates from the leading edge toward the center of a cell. Here, we explore theoretically how measurable subcellular traction stresses depend on the local speed of retrograde actin flow. In the model, forces result from the stretching of molecular complexes in response to the drag from the flow; because these complexes break with extension-dependent kinetics, the flow results in a decrease in their number when sufficiently large. Competition between these two effects naturally gives rise to a clutch-like behavior and a nonmonotonic trend in the measured stresses, consistent with recent data for epithelial cells. We use this basic framework to evaluate slip and catch bond mechanisms for integrins; better fits of experimental data are obtained with a catch bond representation. Extension of the model to one comprising multiple molecular interfaces shifts the peak stress to higher speeds. Connections to other models and cell movement are discussed.

Force-induced destabilization of focal adhesions at defined integrin spacings on nanostructured surfaces

Focal adhesions are the anchoring points of cells to surfaces and are responsible for a large number of surface sensing processes. Nanopatterning studies have shown physiological changes in fibroblasts as a result of decreasing density of external binding ligands. The most striking of these changes is a decreased ability to form mature focal adhesions when lateral ligand distances exceed 76 nm. These changes are usually examined in the context of protein signaling and protein interactions. We show a physical explanation based on the balance between the forces acting on individual ligand connections and the reaction kinetics of those ligands. We propose three stability regimes for focal adhesions as a function of ligand spacing and applied stress: a stable regime, an unstable regime in which a large fraction of unbound protein causes adhesion disintegration, and a regime in which the applied force is too high to form an adhesion structure.

Pericellular fibronectin is required for RhoA-dependent responses to cyclic strain in fibroblasts

To test the hypothesis that the pericellular fibronectin matrix is involved in mechanotransduction, we compared the response of normal and fibronectin-deficient mouse fibroblasts to cyclic substrate strain. Normal fibroblasts seeded on vitronectin in fibronectin-depleted medium deposited their own fibronectin matrix. In cultures exposed to cyclic strain, RhoA was activated, actin-stress fibers became more prominent, MAL/MKL1 shuttled to the nucleus, and mRNA encoding tenascin-C was induced. By contrast, these RhoA-dependent responses to cyclic strain were suppressed in fibronectin knockdown or knockout fibroblasts grown under identical conditions. On vitronectin substrate, fibronectin-deficient cells lacked fibrillar adhesions containing alpha5 integrin. However, when fibronectin-deficient fibroblasts were plated on exogenous fibronectin, their defects in adhesions and mechanotransduction were restored. Studies with fragments indicated that both the RGD-synergy site and the adjacent heparin-binding region of fibronectin were required for full activity in mechanotransduction, but not its ability to self-assemble. In contrast to RhoA-mediated responses, activation of Erk1/2 and PKB/Akt by cyclic strain was not affected in fibronectin-deficient cells. Our results indicate that pericellular fibronectin secreted by normal fibroblasts is a necessary component of the strain-sensing machinery. Supporting this hypothesis, induction of cellular tenascin-C by cyclic strain was suppressed by addition of exogenous tenascin-C, which interferes with fibronectin-mediated cell spreading.

Transcriptional response of E. coli upon FimH-mediated fimbrial adhesion

Functionalities which may be genetically programmed into a bacterium are limited by its range of possible activities and its sensory capabilities. Therefore, enhancing the bacterial sensory repertoire is a crucial step for expanded utility in potential biomedical, industrial or environmental applications. Using microarray and qRT-PCR analyses, we have investigated transcription in E. coli (strain CSH50) following FimH-mediated adhesion to biocompatible substrates. Specifically, wild-type FimH-mediated adhesion of E. coli to mannose agarose beads and His-tagged FimH-mediated adhesion to Ni²⁺-NTA beads both led to induction of ahpCF, dps, grxA and marRAB genes among bound cells relative to unbound cells. The strongly-induced genes are known to be regulated by OxyR or SoxS cytoplasmic redox sensors. Some differentially altered genes also overlapped with those implicated in biofilm formation. This study provides an insight into transcriptional events following FimH-mediated adhesion and may provide a platform for elucidation of the signaling circuit necessary for engineering a synthetic attachment response in E. coli.

Focal adhesions are sites of integrin extension

FRET experiments model integrin conformation changes in adherent cells.

Integrins undergo global conformational changes that specify their activation state. Current models portray the inactive receptor in a bent conformation that upon activation converts to a fully extended form in which the integrin subunit leg regions are separated to enable ligand binding and subsequent signaling. To test the applicability of this model in adherent cells, we used a fluorescent resonance energy transfer (FRET)–based approach, in combination with engineered integrin mutants and monoclonal antibody reporters, to image integrin α5β1 conformation. We find that restricting leg separation causes the integrin to adopt a bent conformation that is unable to respond to agonists and mediate cell spreading. By measuring FRET between labeled α5β1 and the cell membrane, we find extended receptors are enriched in focal adhesions compared with adjacent regions of the plasma membrane. These results demonstrate definitely that major quaternary rearrangements of β1-integrin subunits occur in adherent cells and that conversion from a bent to extended form takes place at focal adhesions.

The Effects of Load on E-Selectin Bond Rupture and Bond Formation

Molecular dissociation rates have long been known to be sensitive to applied force. We use a laser trap to provide evidence that rates of association may also be force-dependent. We use the thermal fluctuation assay to study single bonds between E-selectin and sialyl Lewis(a) (sLe(a)), the sugar on PSGL-1 to which the three selectins bind. Briefly, an E-selectin-coated bead is held in a laser trap and pressed with various compressive loads against the vertical surface of a bead coated with sLe(a). The time it takes for a bond to form is used to calculate a specific two-dimensional on-rate, kono. We observe an increase in kono with increasing compressive force, providing single molecule evidence that on-rate, in addition to off-rate, is influenced by load. By measuring bond lifetimes at known tensile loads, we show that E-selectin, like its family members L- and P-selectin, is capable of forming catch bonds. Our data support a reverse Bell model, in which compressive forces lower the activation energy for binding. Load-dependent on-rates may be a general feature of all intermolecular bonds.

Molecular Biomechanics: The Molecular Basis of How Forces Regulate Cellular Function

Recent advances have led to the emergence of molecular biomechanics as an essential element of modern biology. These efforts focus on theoretical and experimental studies of the mechanics of proteins and nucleic acids, and the understanding of the molecular mechanisms of stress transmission, mechanosensing and mechanotransduction in living cells. In particular, single-molecule biomechanics studies of proteins and DNA, and mechanochemical coupling in biomolecular motors have demonstrated the critical importance of molecular mechanics as a new frontier in bioengineering and life sciences. To stimulate a more systematic study of the basic issues in molecular biomechanics, and attract a broader range of researchers to enter this emerging field, here we discuss its significance and relevance, describe the important issues to be addressed and the most critical questions to be answered, summarize both experimental and theoretical/computational challenges, and identify some short-term and long-term goals for the field. The needs to train young researchers in molecular biomechanics with a broader knowledge base, and to bridge and integrate molecular, subcellular and cellular level studies of biomechanics are articulated.

Design rules for biomolecular adhesion: lessons from force measurements

Cell adhesion to matrix, other cells, or pathogens plays a pivotal role in many processes in biomolecular engineering. Early macroscopic methods of quantifying adhesion led to the development of quantitative models of cell adhesion and migration. The more recent use of sensitive probes to quantify the forces that alter or manipulate adhesion proteins has revealed much greater functional diversity than was apparent from population average measurements of cell adhesion. This review highlights theoretical and experimental methods that identified force-dependent molecular properties that are central to the biological activity of adhesion proteins. Experimental and theoretical methods emphasized in this review include the surface force apparatus, atomic force microscopy, and vesicle-based probes. Specific examples given illustrate how these tools have revealed unique properties of adhesion proteins and their structural origins.

Material properties of the cell dictate stress-induced spreading and differentiation in embryonic stem cells

Growing evidence suggests that physical microenvironments and mechanical stresses, in addition to soluble factors, help direct mesenchymal-stem-cell fate. However, biological responses to a local force in embryonic stem cells remain elusive. Here we show that a local cyclic stress through focal adhesions induced spreading in mouse embryonic stem cells but not in mouse embryonic stem-cell-differentiated cells, which were ten times stiffer. This response was dictated by the cell material property (cell softness), suggesting that a threshold cell deformation is the key setpoint for triggering spreading responses. Traction quantification and pharmacological or shRNA intervention revealed that myosin II contractility, F-actin, Src or cdc42 were essential in the spreading response. The applied stress led to oct3/4 gene downregulation in mES cells. Our findings demonstrate that cell softness dictates cellular sensitivity to force, suggesting that local small forces might have far more important roles in early development of soft embryos than previously appreciated.

Force-induced cleavage of single VWFA1A2A3 tridomains by ADAMTS-13

A disintegrin and metalloprotease with a thrombospondin type 1 motifs 13 (ADAMTS-13) regulates hemostasis by cleaving the folded A2 domain of von Willebrand factor (VWF). The cleavage is regulated by forces as it occurs in flowing blood. We tested the hypothesis that force-induced A2 domain unfolding facilitates cleavage using atomic force microscopy to pull single VWF A1A2A3 tridomain polypeptides by platelet glycoprotein Ibalpha or antibodies to measure time, distance, and force. Structural destabilization of A1A2A3 was induced by 5- to 80-pN forces, manifesting as an abrupt molecular length increase distributed around 20 and 50 nm, probably because of uncoupling A1A2A3 (or partially unfolding A2) and fully unfolding A2, respectively. Time required to destabilize A1A2A3 first increased (catch), reaching a maximum of 0.2 seconds at 20pN, then decreased (slip) with increasing force, independent of ADAMTS-13. The time required to rupture A1A2A3 exhibited a similar catch-slip behavior when pulled by glycoprotein Ibalpha but only slip behavior when pulled by antibody, which was progressively shortened by increasing concentration of ADAMTS-13 after (but not before) structural destabilization, indicating that cleavage of A2 requires the force-induced A2 unfolding. Analysis with a model for single-substrate trimolecular enzymatic kinetics estimated a cleavage rate k(cat) of 2.9 (+/- 59) seconds and a K(d) of 5.6 (+/- 3.4) nM for ADAMTS-13/A1A2A3 binding. These findings quantify the mechanical regulation of VWF cleavage by ADAMTS-13 at the level of single A1A2A3 tridomain.

The myofibroblast: paradigm for a mechanically active cell

Tissues lose mechanical integrity when our body is injured. To rapidly restore mechanical stability a multitude of cell types can jump into action by acquiring a reparative phenotype-the myofibroblast. Here, I review the known biomechanics of myofibroblast differentiation and action and speculate on underlying mechanisms. Hallmarks of the myofibroblast are secretion of extracellular matrix, development of adhesion structures with the substrate, and formation of contractile bundles composed of actin and myosin. These cytoskeletal features not only enable the myofibroblast to remodel and contract the extracellular matrix but to adapt its activity to changes in the mechanical microenvironment. Rapid repair comes at the cost of tissue contracture due to the inability of the myofibroblast to regenerate tissue. If contracture and ECM remodeling become progressive and manifests as organ fibrosis, the outcome of myofibroblast activity will have more severe consequences than the initial damage. Whereas the pathological consequences of myofibroblast occurrence are of great interest for physicians, their mechano-responsive features render them attractive for physicists and bioengineers. Their well developed cytoskeleton and responsiveness to a plethora of cytokines fascinate cell biologists and biochemists. Finally, the question of the myofibroblast origin intrigues stem cell biologists and developmental biologists-what else can you ask from a truly interdisciplinary cell?

Clustering of alpha(5)beta(1) integrins determines adhesion strength whereas alpha(v)beta(3) and talin enable mechanotransduction

A key molecular link between cells and the extracellular matrix is the binding between fibronectin and integrins alpha(5)beta(1) and alpha(v)beta(3). However, the roles of these different integrins in establishing adhesion remain unclear. We tested the adhesion strength of fibronectin-integrin-cytoskeleton linkages by applying physiological nanonewton forces to fibronectin-coated magnetic beads bound to cells. We report that the clustering of fibronectin domains within 40 nm led to integrin alpha(5)beta(1) recruitment, and increased the ability to sustain force by over six-fold. This force was supported by alpha(5)beta(1) integrin clusters. Importantly, we did not detect a role of either integrin alpha(v)beta(3) or talin 1 or 2 in maintaining adhesion strength. Instead, these molecules enabled the connection to the cytoskeleton and reinforcement in response to an applied force. Thus, high matrix forces are primarily supported by clustered alpha(5)beta(1) integrins, while less stable links to alpha(v)beta(3) integrins initiate mechanotransduction, resulting in reinforcement of integrin-cytoskeleton linkages through talin-dependent bonds.

Integrin activation dynamics between the RGD-binding site and the headpiece hinge

Integrins form mechanical links between the extracellular matrix and the cytoskeleton. Although integrin activation is known to be regulated by an allosteric conformational change, which can be induced from the extracellular or intracellular end of the molecule, little is known regarding the sequence of structural events by which signals propagate between distant sites. Here, we reveal with molecular dynamics simulations of the FnIII(10)-bound alpha(V)beta(3) integrin headpiece how the binding pocket and interdomain betaA/hybrid domain hinge on the distal end of the betaA domain are allosterically linked via a hydrophobic T-junction between the middle of the alpha1 helix and top of the alpha7 helix. The key results of this study are: 1) that this T-junction is induced by ligand binding and hinge opening, and thus displays bidirectionality; 2) that formation of this junction can be accelerated by ligand-mediated force; and 3) how formation of this junction is inhibited by Ca(2+) in place of Mg(2+) at the site adjacent to the metal ion-dependent adhesion site ("ADMIDAS"). Together with recent experimental evidence that integrin complexes can form catch bonds (i.e. become strengthened under force), as well as earlier evidence that Ca(2+) at the ADMIDAS results in lower binding affinity, these simulations provide a common structural model for the dynamic process by which integrins become activated.

The regulation mechanism for the auto-inhibition of binding of human filamin A to integrin

The ability of adhesion receptors to transmit biochemical signals and mechanical force across cell membranes depends on interactions with the actin cytoskeleton. Human filamins are large actin cross-linking proteins that connect integrins to the cytoskeleton. Filamin binding to the cytoplasmic tail of beta integrins has been shown to prevent integrin activation in cells, which is important for controlling cell adhesion and migration. The molecular-level mechanism for filamin binding to integrin has been unclear, however, as it was recently demonstrated that filamin undergoes intramolecular auto-inhibition of integrin binding. In this study, using steered molecular dynamics simulations, we found that mechanical force applied to filamin can expose cryptic integrin binding sites. The forces required for this are considerably lower than those for filamin immunoglobulin domain unfolding. The mechanical-force-induced unfolding of filamin and exposure of integrin binding sites occur through stable intermediates where integrin binding is possible. Accordingly, our results support filamin's role as a mechanotransducer, since force-induced conformational changes allow binding of integrin and other transmembrane and intracellular proteins. This observed force-induced conformational change can also be one of possible mechanisms involved in the regulation of integrin activation.