Importance of force linkage in mechanochemistry of adhesion receptors

The C-terminal αI domain linker as a critical structural element in the conformational activation of αI integrins

The activation of α/β heterodimeric integrins is the result of highly coordinated rearrangements within both subunits. The molecular interactions between the two subunits, however, remain to be characterized. In this study, we use the integrin α(L)β(2) to investigate the functional role of the C-linker polypeptide that connects the C-terminal end of the inserted (I) domain with the β-propeller domain on the α subunit and is located at the interface with the βI domain of the β chain. We demonstrate that shortening of the C-linker by eight or more amino acids results in constitutively active α(L)β(2) in which the αI domain is no longer responsive to the regulation by the βI domain. Despite this intersubunit uncoupling, both I domains remain individually sensitive to intrasubunit conformational changes induced by allosteric modulators. Interestingly, the length and not the sequence of the C-linker appears to be critical for its functionality in α/β intersubunit communication. Using two monoclonal antibodies (R7.1 and CBR LFA-1/1) we further demonstrate that shortening of the C-linker results in the gradual loss of combinational epitopes that require both the αI and β-propeller domains for full reactivity. Taken together, our findings highlight the role of the C-linker as a spring-like element that allows relaxation of the αI domain in the resting state and controlled tension of the αI domain during activation, exerted by the β chain.

Mesenchymal stem cells exhibit firm adhesion, crawling, spreading and transmigration across aortic endothelial cells: effects of chemokines and shear

Mesenchymal stem cells (MSCs) have anti-inflammatory and immunosuppressive properties and may be useful in the therapy of diseases such as arteriosclerosis. MSCs have some ability to traffic into inflamed tissues, however to exploit this therapeutically their migratory mechanisms need to be elucidated. This study examines the interaction of murine MSCs (mMSCs) with, and their migration across, murine aortic endothelial cells (MAECs), and the effects of chemokines and shear stress. The interaction of mMSCs with MAECs was examined under physiological flow conditions. mMSCs showed lack of interaction with MAECs under continuous flow. However, when the flow was stopped (for 10min) and then started, mMSCs adhered and crawled on the endothelial surface, extending fine microvillous processes (filopodia). They then spread extending pseudopodia in multiple directions. CXCL9 significantly enhanced the percentage of mMSCs adhering, crawling and spreading and shear forces markedly stimulated crawling and spreading. CXCL9, CXCL16, CCL20 and CCL25 significantly enhanced transendothelial migration across MAECs. The transmigrated mMSCs had down-regulated receptors CXCR3, CXCR6, CCR6 and CCR9. This study furthers the knowledge of MSC transendothelial migration and the effects of chemokines and shear stress which is of relevance to inflammatory diseases such as arteriosclerosis.

Biomechanics of leukocyte rolling

Leukocyte rolling on endothelial cells and other P-selectin substrates is mediated by P-selectin binding to P-selectin glycoprotein ligand-1 expressed on the tips of leukocyte microvilli. Leukocyte rolling is a result of rapid, yet balanced formation and dissociation of selectin-ligand bonds in the presence of hydrodynamic shear forces. The hydrodynamic forces acting on the bonds may either increase (catch bonds) or decrease (slip bonds) their lifetimes. The force-dependent 'catch-slip' bond kinetics are explained using the 'two pathway model' for bond dissociation. Both the 'sliding-rebinding' and the 'allosteric' mechanisms attribute 'catch-slip' bond behavior to the force-induced conformational changes in the lectin-EGF domain hinge of selectins. Below a threshold shear stress, selectins cannot mediate rolling. This 'shear-threshold' phenomenon is a consequence of shear-enhanced tethering and catch bond-enhanced rolling. Quantitative dynamic footprinting microscopy has revealed that leukocytes rolling at venular shear stresses (>0.6 Pa) undergo cellular deformation (large footprint) and form long tethers. The hydrodynamic shear force and torque acting on the rolling cell are thought to be synergistically balanced by the forces acting on tethers and stressed microvilli, however, their relative contribution remains to be determined. Thus, improvement beyond the current understanding requires in silico models that can predict both cellular and microvillus deformation and experiments that allow measurement of forces acting on individual microvilli and tethers.

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.

The power of AAA-ATPases on the road of pre-60S ribosome maturation--molecular machines that strip pre-ribosomal particles

The biogenesis of ribosomes is a fundamental cellular process, which provides the molecular machines that synthesize all cellular proteins. The assembly of eukaryotic ribosomes is a highly complex multi-step process that requires more than 200 ribosome biogenesis factors, which mediate a broad spectrum of maturation reactions. The participation of many energy-consuming enzymes (e.g. AAA-type ATPases, RNA helicases, and GTPases) in this process indicates that the expenditure of energy is required to drive ribosome assembly. While the precise function of many of these enzymes remains elusive, recent progress has revealed that the three AAA-type ATPases involved in 60S subunit biogenesis are specifically dedicated to the release and recycling of distinct biogenesis factors. In this review, we will highlight how the molecular power of yeast Drg1, Rix7, and Rea1 is harnessed to promote the release of their substrate proteins from evolving pre-60S particles and, where appropriate, discuss possible catalytic mechanisms. This article is part of a Special Issue entitled: AAA ATPases: structure and function.

Biology and physics of von Willebrand factor concatamers

Structural specialisations enable von Willebrand factor (VWF) to assemble during biosynthesis into helical tubules in Weibel-Palade bodies (WPB). Specialisations include a pH-regulated dimeric bouquet formed by the C-terminal half of VWF and helical assembly guided by the N-terminal half that templates inter-dimer disulphide bridges. Orderly assembly and storage of ultra-long concatamers in helical tubules, without crosslinking of neighboring tubules, enables unfurling during secretion without entanglement. Length regulation occurs post-secretion, by hydrodynamic force-regulated unfolding of the VWF A2 domain, and its cleavage by the plasma protease ADAMTS13 (a disintegrin and metalloprotease with a thrombospondin type 1 motif, member 13). VWF is longest at its site of secretion, where its haemostatic function is most important. Moreover, elongational hydrodynamic forces on VWF are strongest just where needed, when bound to the vessel wall, or in elongational flow in the circulation at sites of vessel rupture or vasoconstriction in haemostasis. Elongational forces regulate haemostasis by activating binding of the A1 domain to platelet GPIbα, and over longer time periods, regulate VWF length by unfolding of the A2 domain for cleavage by ADAMTS13. Recent structures of A2 and single molecule measurements of A2 unfolding and cleavage by ADAMTS13 illuminate the mechanisms of VWF length regulation. Single molecule studies on the A1-GPIb receptor-ligand bond demonstrate a specialised flex-bond that enhances resistance to the strong hydrodynamic forces experienced at sites of haemorrhage.

Kindlin-3 is required for the stabilization of TCR-stimulated LFA-1:ICAM-1 bonds critical for lymphocyte arrest and spreading on dendritic cells

Kindlin-3 is a key lymphocyte function–associated antigen-1 (LFA-1) coactivator deleted in leukocyte adhesion deficiency-III (LAD-III). In the present study, we investigated the involvement of this adaptor in lymphocyte motility and TCR-triggered arrest on ICAM-1 or on dendritic cells (DCs). Kindlin-3–null primary T cells from a LAD-III patient migrated normally on the major lymph node chemokine CCL21 and engaged in normal TCR signaling. However, TCR activation of Kindlin-3–null T lymphocytes failed to trigger the robust LFA-1–mediated T-cell spreading on ICAM-1 and ICAM-1–expressing DCs that is observed in normal lymphocytes. Kindlin-3 was also essential for cytoskeletal anchorage of the LFA-1 heterodimer and for microclustering of LFA-1 within ventral focal dots of TCR-stimulated lymphocytes spread on ICAM-1. Surprisingly, LFA-1 on Kindlin-3–null lymphocytes migrating over CCL21 acquired normal expression of an epitope associated with the conformational activation of the key headpiece domain, β I. This activated LFA-1 was highly responsive to TCR-triggered ICAM-1–driven stop signals in normal T cells locomoting on CCL21, but not in their Kindlin-3–null T-cell counterparts. We suggest that Kindlin-3 selectively contributes to a final TCR-triggered outside-in stabilization of bonds generated between chemokine-primed LFA-1 molecules and cell-surface ICAM-1.

Chemokine triggered integrin activation and actin remodeling events guiding lymphocyte migration across vascular barriers

Chemokine signals activate leukocyte integrins and actin remodeling machineries critical for leukocyte adhesion and motility across vascular barriers. The arrest of leukocytes at target blood vessel sites depends on rapid conformational activation of their α4 and β2 integrins by the binding of endothelial-displayed chemokines to leukocyte Gi-protein coupled receptors (GPCRs). A universal regulator of this event is the integrin-actin adaptor, talin1. Chemokine-stimulated GPCRs can transmit within fractions of seconds signals via multiple Rho GTPases, which locally raise plasma membrane levels of the talin activating phosphatidyl inositol, PtdIns(4,5)P2 (PIP2). Additional pools of GPCR stimulated Rac-1 and Rap-1 GTPases together with GPCR stimulated PLC and PI3K family members regulate the turnover of focal contacts of leukocyte integrins, induce the collapse of leukocyte microvilli, and promote polarized leukocyte crawling in search of exit cues. Concomitantly, other leukocyte GTPases trigger invasive protrusions into and between endothelial cells in search of basolateral chemokine exit cues. We will review here major findings and open questions related to these sequential guiding activities of endothelial presented chemokines, focusing mainly on lymphocyte-endothelial interactions as a paradigm for other leukocytes.

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.

Nanoengineering of Immune Cell Function

T lymphocytes are a key regulatory component of the adaptive immune system. Understanding how the micro- and nano-scale details of the extracellular environment influence T cell activation may have wide impact on the use of T cells for therapeutic purposes. In this article, we examine how the micro- and nano-scale presentation of ligands to cell surface receptors, including microscale organization and nanoscale mobility, influences the activation of T cells. We extend these studies to include the role of cell-generated forces, and the rigidity of the microenvironment, on T cell activation. These approaches enable delivery of defined signals to T cells, a step toward understanding the cell-cell communication in the immune system, and developing micro/nano- and material- engineered systems for tailoring immune responses for adoptive T cell therapies.

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.

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.

A mechanically stabilized receptor-ligand flex-bond important in the vasculature

Haemostasis in the arteriolar circulation mediated by von Willebrand factor (VWF) binding to platelets is an example of an adhesive interaction that must withstand strong hydrodynamic forces acting on cells. VWF is a concatenated, multifunctional protein that has binding sites for platelets as well as subendothelial collagen. Binding of the A1 domain in VWF to the glycoprotein Ib alpha subunit (GPIbalpha) on the surface of platelets mediates crosslinking of platelets to one another and the formation of a platelet plug for arterioles. The importance of VWF is illustrated by its mutation in von Willebrand disease, a bleeding diathesis. Here, we describe a novel mechanochemical specialization of the A1-GPIbalpha bond for force-resistance. We have developed a method that enables, for the first time, repeated measurements of the binding and unbinding of a receptor and ligand in a single molecule (ReaLiSM). We demonstrate two states of the receptor-ligand bond, that is, a flex-bond. One state is seen at low force; a second state begins to engage at 10 pN with a approximately 20-fold longer lifetime and greater force resistance. The lifetimes of the two states, how force exponentiates lifetime, and the kinetics of switching between the two states are all measured. For the first time, single-molecule measurements on this system are in agreement with bulk phase measurements. The results have important implications not only for how platelets bound to VWF are able to resist force to plug arterioles, but also how increased flow activates platelet plug formation.

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.

Tethering of intercellular adhesion molecule on target cells is required for LFA-1-dependent NK cell adhesion and granule polarization

Alpha(L)beta(2) integrin (LFA-1) has an important role in the formation of T cell and NK cell cytotoxic immunological synapses and in target cell killing. Binding of LFA-1 to ICAM on target cells promotes not only adhesion but also polarization of cytolytic granules in NK cells. In this study, we tested whether LFA-1-dependent NK cell responses are regulated by the distribution and mobility of ICAM at the surface of target cells. We show that depolymerization of F-actin in NK-sensitive target cells abrogated LFA-1-dependent conjugate formation and granule polarization in primary NK cells. Degranulation, which is not controlled by LFA-1, was not impaired. Fluorescence recovery after photobleaching experiments and particle tracking by total internal reflection fluorescence microscopy revealed that ICAM-1 and ICAM-2 were distributed in largely immobile clusters. ICAM clusters were maintained and became highly mobile after actin depolymerization. Moreover, reducing ICAM-2 mobility on an NK-resistant target cell through expression of ezrin, an adaptor molecule that tethers proteins to the actin cytoskeleton, enhanced LFA-1-dependent adhesion and granule polarization. Finally, although NK cells kept moving over freely diffusible ICAM-1 on a lipid bilayer, they bound and spread over solid-phase ICAM-1. We conclude that tethering, rather than clustering of ICAM, promotes proper signaling by LFA-1 in NK cells. Our findings suggest that the lateral diffusion of integrin ligands on cells may be an important determinant of susceptibility to lysis by cytotoxic lymphocytes.

Structural basis for mechanical force regulation of the adhesin FimH via finger trap-like beta sheet twisting

The Escherichia coli fimbrial adhesive protein, FimH, mediates shear-dependent binding to mannosylated surfaces via force-enhanced allosteric catch bonds, but the underlying structural mechanism was previously unknown. Here we present the crystal structure of FimH incorporated into the multiprotein fimbrial tip, where the anchoring (pilin) domain of FimH interacts with the mannose-binding (lectin) domain and causes a twist in the beta sandwich fold of the latter. This loosens the mannose-binding pocket on the opposite end of the lectin domain, resulting in an inactive low-affinity state of the adhesin. The autoinhibition effect of the pilin domain is removed by application of tensile force across the bond, which separates the domains and causes the lectin domain to untwist and clamp tightly around the ligand like a finger-trap toy. Thus, beta sandwich domains, which are common in multidomain proteins exposed to tensile force in vivo, can undergo drastic allosteric changes and be subjected to mechanical regulation.

Chemokine arrest signals to leukocyte integrins trigger bi-directional-occupancy of individual heterodimers by extracellular and cytoplasmic ligands

Integrin heterodimers acquire high affinity to endothelial ligands by extensive conformational changes in both their alpha and beta subunits. These heterodimers are maintained in an inactive state by inter-subunit constraints. Changes in the cytoplasmic interface of the integrin heterodimer (referred to as inside-out integrin activation) can only partially remove these constraints. Full integrin activation is achieved when both inter-subunit constraints and proper rearrangements of the integrin headpiece by its extracellular ligand (outside-in activation) are temporally coupled. A universal regulator of these integrin rearrangements is talin1, a key integrin-actin adaptor that regulates integrin conformation and anchors ligand-occupied integrins to the cortical cytoskeleton. The arrest of rolling leukocytes at target vascular sites depends on rapid activation of their alpha4 and beta2 integrins at endothelial contacts by chemokines displayed on the endothelial surface. These chemotactic cytokines can signal within milliseconds through specialized Gi-protein coupled receptors (GPCRs) and Gi-triggered GTPases on the responding leukocytes. Some chemokine signals can alter integrin conformation by releasing constraints on integrin extension, while other chemokines activate integrins to undergo conformational activation mainly after ligand binding. Both of these modalities involve talin1 activation. In this opinion article, I propose that distinct chemokine signals induce variable strengths of associations between talin1 and different target integrins. Weak interactions of the integrin cytoplasmic tail with talin1 (the cytoplasmic integrin ligand) dissociate unless the extracellular ligand can simultaneously occupy the integrin headpiece and transmit, within milliseconds, proper allosteric changes across the integrin heterodimer back to the tail-talin1 complex. The fate of this bi-directional occupancy of integrins by both their extracellular and intracellular ligands is likely to benefit from immobilization of both ligands to cortical cytoskeletal elements. To properly anchor talin1 onto the integrin tail, a second integrin partner, Kindlin-3 may be also required, although an evidence that both partners can simultaneously bind the same integrin heterodimer is still missing. Once linked to the cortical actin cytoskeleton, the multi-occupied integrin complex can load weak forces, which deliver additional allosteric changes to the integrin headpiece resulting in further bond strengthening. Surface immobilized chemokines are superior to their soluble counterparts in driving this bi-directional occupancy process, presumably due to their ability to facilitate local co-occupancy of individual integrin heterodimers with talin1, Kindlin-3 and surface-bound extracellular ligands.

Structure of an integrin with an alphaI domain, complement receptor type 4

We report the structure of an integrin with an alphaI domain, alpha(X)beta(2), the complement receptor type 4. It was earlier expected that a fixed orientation between the alphaI domain and the beta-propeller domain in which it is inserted would be required for allosteric signal transmission. However, the alphaI domain is highly flexible, enabling two betaI domain conformational states to couple to three alphaI domain states, and greater accessibility for ligand recognition. Although alpha(X)beta(2) is bent similarly to integrins that lack alphaI domains, the terminal domains of the alpha- and beta-legs, calf-2 and beta-tail, are oriented differently than in alphaI-less integrins. Linkers extending to the transmembrane domains are unstructured. Previous mutations in the beta(2)-tail domain support the importance of extension, rather than a deadbolt, in integrin activation. The locations of further activating mutations and antibody epitopes show the critical role of extension, and conversion from the closed to the open headpiece conformation, in integrin activation. Differences among 10 molecules in crystal lattices provide unprecedented information on interdomain flexibility important for modelling integrin extension and activation.

Dynamics of beta3 integrin I-like and hybrid domains: insight from simulations on the mechanism of transition between open and closed forms

The conformational dynamics of the I-like and Hybrid domains from the beta3 integrin headpiece were studied by molecular dynamics simulation and normal mode analysis. Crystallographic structures of integrins show that the integrin headpiece can exist in largely different conformations manifested by a significant difference in the angle between the I-like and Hybrid domains. The relative orientation of these two domains is believed to be a crucial element of integrin function, as it may relate local structural modifications induced by ligand binding into large-scale conformational changes. To investigate the detailed mechanisms responsible for this coupling, we carried out molecular dynamics simulations of the I-like/Hybrid system and employed quasi-harmonic and normal mode analyses to characterize the large-scale motions. Our results show that the conformational transition of I-like and Hybrid domains inferred from crystallographic data is contained in the low-frequency dynamics of the system. Using targeted molecular dynamics simulations, we investigated the roles played by two structural elements of the I-like domain, the alpha7 and alpha1 helices, in the interdomain transition. From our results, we propose that these two helices function in tandem to initiate large-scale, interdomain conformational transition apparent in integrin activation and signaling.

Mechanochemical removal of ribosome biogenesis factors from nascent 60S ribosomal subunits

The dynein-related AAA ATPase Rea1 is a preribosomal factor that triggers an unknown maturation step in 60S subunit biogenesis. Using electron microscopy, we show that Rea1's motor domain is docked to the pre-60S particle and its tail-like structure, harboring a metal ion-dependent adhesion site (MIDAS), protrudes from the preribosome. Typically, integrins utilize a MIDAS to bind extracellular ligands, an interaction that is strengthened under applied tensile force. Likewise, the Rea1 MIDAS binds the preribosomal factor Rsa4, which is located on the pre-60S subunit at a site that is contacted by the flexible Rea1 tail. The MIDAS-Rsa4 interaction is essential for ATP-dependent dissociation of a group of non-ribosomal factors from the pre-60S particle. Thus, Rea1 aligns with its interacting partners on the preribosome to effect a necessary step on the path to the export-competent 60S subunit.

Do membrane undulations help cells probe the world?

Cells sense physical properties of their environment including substratum rigidity, roughness, and topography of recognition sites. The cell surface displays continuous deformations of nanometer-scale amplitude and Hz frequency. Recent results support the hypothesis that these surface undulations constitute a powerful strategy for the rapid acquisition of environmental cues: transient contact with surroundings generates forces of piconewton intensity as a result of rapid formation and dissociation of intermolecular bonds. The combination of binding and steric forces is expected to drive conformational changes and lateral reorganization of membrane biomolecules, thus generating signaling cascades. We propose that spontaneous membrane mobility shapes the initial information generated by cell-to-surface contacts, and thereby biases later consequences of these interactions.

Integrins

Integrins are cell adhesion receptors that are evolutionary old and that play important roles during developmental and pathological processes. The integrin family is composed of 24 αβ heterodimeric members that mediate the attachment of cells to the extracellular matrix (ECM) but that also take part in specialized cell-cell interactions. Only a subset of integrins (8 out of 24) recognizes the RGD sequence in the native ligands. In some ECM molecules, such as collagen and certain laminin isoforms, the RGD sequences are exposed upon denaturation or proteolytic cleavage, allowing cells to bind these ligands by using RGD-binding receptors. Proteolytic cleavage of ECM proteins might also generate fragments with novel biological activity such as endostatin, tumstatin, and endorepellin. Nine integrin chains contain an αI domain, including the collagen-binding integrins α1β1, α2β1, α10β1, and α11β1. The collagen-binding integrins recognize the triple-helical GFOGER sequence in the major collagens, but their ability to recognize these sequences in vivo is dependent on the fibrillar status and accessibility of the interactive domains in the fibrillar collagens. The current review summarizes some basic facts about the integrin family including a historical perspective, their structure, and their ligand-binding properties.

Demonstration of catch bonds between an integrin and its ligand

Binding of integrins to ligands provides anchorage and signals for the cell, making them prime candidates for mechanosensing molecules. How force regulates integrin-ligand dissociation is unclear. We used atomic force microscopy to measure the force-dependent lifetimes of single bonds between a fibronectin fragment and an integrin alpha(5)beta(1)-Fc fusion protein or membrane alpha(5)beta(1). Force prolonged bond lifetimes in the 10-30-pN range, a counterintuitive behavior called catch bonds. Changing cations from Ca(2+)/Mg(2+) to Mg(2+)/EGTA and to Mn(2+) caused longer lifetime in the same 10-30-pN catch bond region. A truncated alpha(5)beta(1) construct containing the headpiece but not the legs formed longer-lived catch bonds that were not affected by cation changes at forces <30 pN. Binding of monoclonal antibodies that induce the active conformation of the integrin headpiece shifted catch bonds to a lower force range. Thus, catch bond formation appears to involve force-assisted activation of the headpiece but not integrin extension.

Loss of Kindlin-3 in LAD-III eliminates LFA-1 but not VLA-4 adhesiveness developed under shear flow conditions

Leukocyte adhesion deficiency (LAD)-III is associated with homozygous stop codon mutations in Kindlin-3, the hematopoietic member of the Kindlin family of integrin coactivators. In addition, a subgroup of LAD-III patients has a homozygous splice junction mutation in and reduced expression of the Rap-1 guanine nucleotide exchange factor, CalDAG-GEFI (CDGI). In this study, we compared the adhesive properties of the leukocyte function-associated antigen-1 (LFA-1) and very late activation antigen-4 (VLA-4) integrins in both primary and activated leukocytes derived from these 2 LAD-III subgroups. Primary lymphocytes lacking both Kindlin-3 and CDGI lost all firm T-cell receptor-stimulated LFA-1 adhesiveness, in contrast to LAD-III lymphocytes deficient in Kindlin-3 alone. Effector T cells expanded from all tested LAD-III variants expressed normal CDGI, but lacked Kindlin-3. These Kindlin-3-null effector T cells exhibited total loss of inside-out LFA-1 activation by chemokine signals as well as abrogated intrinsic LFA-1 adhesiveness. Surprisingly, VLA-4 in Kindlin-3-null resting or effector lymphocytes retained intrinsic rolling adhesions to vascular cell adhesion molecule-1 and exhibited only partial defects in chemokine-stimulated adhesiveness to vascular cell adhesion molecule-1. Deletion of the putative beta(1) Kindlin-3 binding site also retained VLA-4 adhesiveness. Thus, our study provides the first evidence that Kindlin-3 is more critical to LFA-1 than to VLA-4-adhesive functions in human lymphocytes.

Molecular mechanisms of mechanosensing and their roles in fungal contact sensing

Numerous fungal species respond to contact with a surface by undergoing differentiation. Contact between plant pathogenic fungi and a surface results in the elaboration of the complex structures that enable invasion of the host plant, and for the opportunistic human pathogen Candida albicans, contact with a semi-solid surface results in invasive growth into the subjacent material. The ability to sense contact with an appropriate surface therefore contributes to the ability of these fungi to cause disease in their respective hosts. This Review discusses molecular mechanisms of mechanosensitivity, the proteins involved, such as mechanosensitive ion channels, G-protein-coupled receptors and integrins, and their putative roles in fungal contact sensing.

LFA-1 affinity regulation is necessary for the activation and proliferation of naive T cells

The activation of LFA-1 (lymphocyte function-associated antigen) is a critical event for T cell co-stimulation. The mechanism of LFA-1 activation involves both affinity and avidity regulation, but the role of each in T cell activation remains unclear. We have identified antibodies that recognize and block different affinity states of the mouse LFA-1 I-domain. Monoclonal antibody 2D7 preferentially binds to the low affinity conformation, and this specific binding is abolished when LFA-1 is locked in the high affinity conformation. In contrast, M17/4 can bind both the locked high and low affinity forms of LFA-1. Although both 2D7 and M17/4 are blocking antibodies, 2D7 is significantly less potent than M17/4 in blocking LFA-1-mediated adhesion; thus, blocking high affinity LFA-1 is critical for preventing LFA-1-mediated adhesion. Using these reagents, we investigated whether LFA-1 affinity regulation affects T cell activation. We found that blocking high affinity LFA-1 prevents interleukin-2 production and T cell proliferation, demonstrated by TCR cross-linking and antigen-specific stimulation. Furthermore, there is a differential requirement of high affinity LFA-1 in the activation of CD4(+) and CD8(+) T cells. Although CD4(+) T cell activation depends on both high and low affinity LFA-1, only high affinity LFA-1 provides co-stimulation for CD8(+) T cell activation. Together, our data demonstrated that the I-domain of LFA-1 changes to the high affinity state in primary T cells, and high affinity LFA-1 is critical for facilitating T cell activation. This implicates LFA-1 activation as a novel regulatory mechanism for the modulation of T cell activation and proliferation.

Linking integrin conformation to function

Integrins are alphabeta heterodimeric adhesion receptors that relay signals bidirectionally across the plasma membrane between the extracellular matrix and cell-surface ligands, and cytoskeletal and signalling effectors. The physical and chemical signals that are controlled by integrins are essential for intercellular communication and underpin all aspects of metazoan existence. To mediate such diverse functions, integrins exhibit structural diversity, flexibility and dynamism. Conformational changes, as opposed to surface expression or clustering, are central to the regulation of receptor function. In recent years, there has been intense interest in determining the three-dimensional structure of integrins, and analysing the shape changes that underpin the interconversion between functional states. Considering the central importance of the integrin signalling nexus, it is perhaps no surprise that obtaining this information has been difficult, and the answers gained so far have been complicated. In this Commentary, we pose some of the key remaining questions that surround integrin structure-function relationships and review the evidence that supports the current models.

Lymphocyte crawling and transendothelial migration require chemokine triggering of high-affinity LFA-1 integrin

Endothelial chemokines are instrumental for integrin-mediated lymphocyte adhesion and transendothelial migration (TEM). By dissecting how chemokines trigger lymphocyte integrins to support shear-resistant motility on and across cytokine-stimulated endothelial barriers, we found a critical role for high-affinity (HA) LFA-1 integrin in lymphocyte crawling on activated endothelium. Endothelial-presented chemokines triggered HA-LFA-1 and adhesive filopodia at numerous submicron dots scattered underneath crawling lymphocytes. Shear forces applied to endothelial-bound lymphocytes dramatically enhanced filopodia density underneath crawling lymphocytes. A fraction of the adhesive filopodia invaded the endothelial cells prior to and during TEM and extended large subluminal leading edge containing dots of HA-LFA-1 occupied by subluminal ICAM-1. Memory T cells generated more frequent invasive filopodia and transmigrated more rapidly than their naive counterparts. We propose that shear forces exerted on HA-LFA-1 trigger adhesive and invasive filopodia at apical endothelial surfaces and thereby promote lymphocyte crawling and probing for TEM sites.

Enhanced focal adhesion assembly reflects increased mechanosensation and mechanotransduction at maternal–conceptus interface and uterine wall during ovine pregnancy

The integrity of the fetal–maternal interface is critical for proper fetal nourishment during pregnancy. Integrins are important adhesion molecules present at the interface during implantation; however,in vivoevidence for integrin activation and focal adhesion formation at the maternal–conceptus interface is limited. We hypothesized that focal adhesion assembly in uterine luminal epithelium (LE) and conceptus trophectoderm (Tr) results from integrin binding of extracellular matrix (ECM) at this interface to provide increased tensile forces and signaling to coordinate utero-placental development. An ovine model of unilateral pregnancy was used to evaluate mechanotransduction events leading to focal adhesion assembly at the maternal–conceptus interface and within the uterine wall. Animals were hysterectomized on days 40, 80, or 120 of pregnancy, and uteri immunostained for integrins (ITGAV, ITGA4, ITGA5, ITGB1, ITGB3, and ITGB5), ECM proteins (SPP1, LGALS15, fibronectin (FN), and vitronectin (VTN)), cytoskeletal molecules (ACTN and TLN1), and a signal generator (PTK2). Focal adhesion assembly in myometrium and stroma was also studied to provide a frame of reference for mechanical stretch of the uterine wall. Large focal adhesions containing aggregates of ITGAV, ITGA4, ITGA5, ITGB1, ITGB5, ACTN, and PTK2 were detected in interplacentomal uterine LE and Tr of gravid but not non-gravid uterine horns and increased during pregnancy. SPP1 and LGALS15, but not FN or VTN, were present along LE and Tr interfaces in both uterine horns. These data support the idea that focal adhesion assembly at the maternal–conceptus interface reflects adaptation to increasing forces caused by the growing fetus. Cooperative binding of multiple integrins to SPP1 deposited at the maternal–conceptus interface forms an adhesive mosaic to maintain a tight connection between uterine and placental surfaces along regions of epitheliochorial placentation in sheep.

Mechanisms of chemokine and antigen-dependent T-lymphocyte navigation

T-lymphocyte trafficking is targeted to specific organs by selective molecular interactions depending on their differentiation and functional properties. Specific chemokine receptors have been associated with organ-specific trafficking of memory and effector T-cells, as well as the recirculation of naïve T-cells to secondary lymphoid organs. In addition to the acquisition of tissue-selective integrins and chemokine receptors, an additional level of specificity for T-cell trafficking into the tissue is provided by specific recognition of antigen displayed by the endothelium involving the TCRs (T-cell antigen receptors) and co-stimulatory receptors. Activation of PI3K (phosphoinositide 3-kinase) is a robust signalling event shared by most chemokine receptors as well as the TCR and co-stimulatory receptors, contributing to several aspects of T-lymphocyte homing as well as actin reorganization and other components of the general migratory machinery. Accordingly, inhibition of PI3K has been considered seriously as a potential therapeutic strategy by which to combat various T-lymphocyte-dependent pathologies, including autoimmune and inflammatory diseases, as well as to prevent transplant rejection. However, there is substantial evidence for PI3K-independent mechanisms that facilitate T-lymphocyte migration. In this regard, several other signalling-pathway components, including small GTPases, PLC (phospholipase C) and PKC (protein kinase C) isoforms, have also been implicated in T-lymphocyte migration in response to chemokine stimulation. The present review will therefore examine the PI3K-dependent and -independent signal-transduction pathways involved in T-cell migration during distinct modes of T-cell trafficking in response to either chemokines or the TCR and co-stimulatory molecules.

Cell fate regulation by coupling mechanical cycles to biochemical signaling pathways

Many aspects of cellular motility and mechanics are cyclic in nature such as the extension and retraction of lamellipodia or filopodia. Inherent to the cycles of extension and retraction that test the environment is the production of mechano-chemical signals that can alter long-term cell behavior, transcription patterns, and cell fate. We are just starting to define such cycles in several aspects of cell motility, including periodic contractions, integrin cycles of binding and release as well as the normal oscillations in motile activity. Cycles of local cell contraction and release are directly coupled to cycles of stressing and releasing extracellular contacts (matrix or cells) as well as cytoplasmic mechanotransducers. Stretching can alter external physical properties or sites exposed by matrix molecules as well as internal networks; thus, cell contractions can cause a secondary wave of mechano-regulated outside-in and internal cell signal changes. In some cases, the integration of both external and internal signals in space and time can stimulate a change in cell state from quiescence to growth or differentiation. In this review we will develop the basic concept of the mechano-chemical cycles and the ways in which they can be described and understood.

Structure of a complete integrin ectodomain in a physiologic resting state and activation and deactivation by applied forces

The complete ectodomain of integrin alpha(IIb)beta(3) reveals a bent, closed, low-affinity conformation, the beta knee, and a mechanism for linking cytoskeleton attachment to high affinity for ligand. Ca and Mg ions in the recognition site, including the synergistic metal ion binding site (SyMBS), are loaded prior to ligand binding. Electrophilicity of the ligand-binding Mg ion is increased in the open conformation. The beta(3) knee passes between the beta(3)-PSI and alpha(IIb)-knob to bury the lower beta leg in a cleft, from which it is released for extension. Different integrin molecules in crystals and EM reveal breathing that appears on pathway to extension. Tensile force applied to the extended ligand-receptor complex stabilizes the closed, low-affinity conformation. By contrast, an additional lateral force applied to the beta subunit to mimic attachment to moving actin filaments stabilizes the open, high-affinity conformation. This mechanism propagates allostery over long distances and couples cytoskeleton attachment of integrins to their high-affinity state.

Mechanochemistry of receptor-ligand bonds

When cell receptors bind to immobilized ligands, the resulting bond can be subjected to tensile mechanical force. This might be expected to shorten bond lifetimes. However, cells from bacteria to blood cells express receptors that are activated by tensile force to form longer-lived bonds, referred to as catch bonds. The process of catch bond activation involves non-equilibrium processes that are poorly probed by experimental and computational structural methods alike. However, I argue here that the preponderance of data indicates that force acts on an interdomain region which regulates the conformation of a distal ligand-binding site, in a process closely related to mechanochemistry and allosteric regulation.

Integrins in immunity

A successful immune response depends on the capacity of immune cells to travel from one location in the body to another–these cells are rapid migrators, travelling at speeds of μm/minute. Their ability to penetrate into tissues and to make contacts with other cells depends chiefly on the β2 integrin known as LFA-1. For this reason, we describe the control of its activity in some detail. For the non-immunologist, the fine details of an immune response often seem difficult to fathom. However, the behaviour of immune cells, known as leukocytes (Box 1), is subject to the same biological rules as many other cell types, and this holds true particularly for the functioning of the integrins on these cells. In this Commentary, we highlight, from a cell-biology point of view, the integrin-mediated immune-cell migration and cell-cell interactions that occur during the course of an immune response.

Chapter 14. Real-time in vitro assays for studying the role of chemokines in lymphocyte transendothelial migration under physiologic flow conditions

The mechanisms underlying leukocyte migration across endothelial barriers are still largely elusive. Integrin activation by chemokine signals is a key checkpoint in this process. Most of the current knowledge on transendothelial migration (TEM) of leukocytes has been derived from in vitro modified Boyden-chamber transfilter migration assays. In these assays, leukocyte migration toward chemokine gradients established across an endothelial barrier is measured under shear-free conditions. Consequently, these assays do not address the critical contribution of shear forces to dynamic integrin activation and redistribution at focal lymphocyte-endothelial contacts. Endothelial chemokines are displayed at high levels on blood vessel walls in vivo and play critical roles in both integrin activation and polarization of leukocytes on blood vessels, yet transwell assays do not assess the role of these chemokines in leukocyte TEM. To overcome these two drawbacks, several laboratories, including our group, developed assays based on in vitro live imaging microscopy to follow leukocyte migration across endothelial barriers that display defined compositions of integrin-stimulatory chemokines. These assays not only successfully simulate physiologic TEM processes but also enable the tracking and dissection of leukocyte adhesion, motility, and crossing of endothelial barriers in real time and under physiologic flow conditions. In addition, fluorescent tagging of membranes, adhesion molecules, and cytoskeletal regulatory elements on the endothelial barrier or the leukocyte can provide key spatial and temporal information on the mode of activity of these elements during distinct stages of leukocyte TEM. After fixation, subcellular changes in the redistribution of these key molecules can be further dissected by immunofluorescence tools and by ultrastructural analysis based on scanning and transmission electron microscopy.

Structural basis for selectin mechanochemistry

Selectins are adhesion molecules that resist large tensile forces applied by hydrodynamic forces to leukocytes binding to vessel walls. In crystals, the liganded (high-affinity) and unliganded (low-affinity) conformations differ in orientation between their tandem lectin and EGF domains. I examine how tensile force exerted on a selectin-ligand complex in vivo could favor the more extended, high-affinity conformation. Allostery is transmitted from the EGF-lectin domain interface to the ligand-binding interface on the lectin domain, 30 A away. Trp-1 of the lectin domain and the long axis of the EGF domain form an L-shaped prybar that is welded together by hydrogen bonds to the Trp-1 alpha-amino group. Pivoting of the prybar induced by force demolishes an interface between the Trp-1 side chain and the lectin domain at a switch1 region. These changes are transmitted by rigid body movement of the switch2 region to rearrangements in the switch3 region at the ligand binding site. Another switch region corresponds to a single residue in the EGF domain with large effects on ligand binding and rolling adhesion. Allostery in selectins, and the alignment of tensile force on a selectin-ligand complex with the transition pathway for conformational change, explain much of the structural basis for selectin mechanochemistry.

Transmission of allostery through the lectin domain in selectin-mediated cell adhesion

The selectins are cell adhesion proteins that must resist applied forces to mediate leukocyte tethering and rolling along the endothelium and have 2 conformational states. Selectin-ligand bond dissociation increases only modestly with applied force, and exhibits catch bond behavior in a low-force regime where bond lifetimes counterintuitively increase with increasing force. Both allosteric and sliding-rebinding models have emerged to explain catch bonds. Here, we introduce a large residue into a cleft that opens within the lectin domain to stabilize the more extended, high-affinity selectin conformation. This mutation stabilizes the high-affinity state, but surprisingly makes rolling less stable. The position of the mutation in the lectin domain provides evidence for an allosteric pathway through the lectin domain, connecting changes at the lectin-EGF interface to the distal binding interface.

Catch-bond mechanism of force-enhanced adhesion: counterintuitive, elusive, but ... widespread?

Catch bonds are bonds between a ligand and its receptor that are enhanced by mechanical force pulling the ligand-receptor complex apart. To date, catch-bond formation has been documented for the most common Escherichia coli adhesin, FimH, and for P-/L-selectins, universally expressed by leukocytes, platelets, and blood vessel walls. One compelling explanation for catch bonds is that force-induced structural alterations in the receptor protein are allosterically linked to a high-affinity conformation of its ligand-binding pocket. Catch-bond properties are likely to be widespread among adhesive proteins, thus calling for a detailed understanding of their underlying mechanisms and physiological significance.

Surface-anchored monomeric agonist pMHCs alone trigger TCR with high sensitivity

At the interface between T cell and antigen-presenting cell (APC), peptide antigen presented by MHC (pMHC) binds to the T cell receptor (TCR) and initiates signaling. The mechanism of TCR signal initiation, or triggering, remains unclear. An interesting aspect of this puzzle is that although soluble agonist pMHCs cannot trigger TCR even at high concentrations, the same ligands trigger TCR very efficiently on the surface of APCs. Here, using lipid bilayers or plastic-based artificial APCs with defined components, we identify the critical APC-associated factors that confer agonist pMHCs with such potency. We found that CD4⁺ T cells are triggered by very low numbers of monomeric agonist pMHCs anchored on fluid lipid bilayers or fixed plastic surfaces, in the absence of any other APC surface molecules. Importantly, on bilayers, plastic surfaces, or real APCs, endogenous pMHCs did not enhance TCR triggering. TCR triggering, however, critically depended upon the adhesiveness of the surface and an intact T cell actin cytoskeleton. Based on these observations, we propose the receptor deformation model of TCR triggering to explain the remarkable sensitivity and specificity of TCR triggering.

Using the T cell receptor (TCR) as a sensor, T cells of the immune system constantly migrate in lymphoid organs and probe the surface of antigen-presenting cells (APCs) for foreign antigens, a sign of pathogen infection. Antigen binding by TCRs leads to T cell activation and subsequent immune response to combat the pathogens. Interestingly, although T cells respond well to antigens on APCs, they do not recognize the same antigens in solution. What is it that makes antigens on APCs recognizable? To address this, we used lipid bilayers and plastic surfaces to construct artificial APCs with defined antigen number, composition, and configuration. We found that T cells respond to very few individual foreign antigens on artificial APCs, and contrary to some current opinion, formation of antigen clusters on APCs is not required for antigen recognition by T cells. TCR triggering, however, requires T cell adhesion to the APC surface and then occurs only if the T cells are able to move. We propose that at the dynamic T cell–APC interface, antigen on APCs activates T cells by applying force to the TCR and deforming its structure, which cannot be achieved by soluble antigens due to their lack of anchorage.

Why is it that T cells are blind to antigens in solution but highly sensitive to antigens anchored on a surface? The authors show that this is not due to antigen clustering, but could involve mechanical forces associated with cell locomotion.

Cells on the run: shear-regulated integrin activation in leukocyte rolling and arrest on endothelial cells

The arrest of rolling leukocytes on various target vascular beds is mediated by specialized leukocyte integrins and their endothelial immunoglobulin superfamily (IgSF) ligands. These integrins are kept in largely inactive states and undergo in situ activation upon leukocyte-endothelial contact by both biochemical and mechanical signals from flow-derived shear forces. In vivo and in vitro studies suggest that leukocyte integrin activation involves conformational alterations through inside-out signaling followed by ligand-induced rearrangements accelerated by external forces. This activation process takes place within fractions of seconds by in situ signals transduced to the rolling leukocyte as it encounters specialized endothelial-displayed chemoattractants, collectively termed arrest chemokines. In neutrophils, selectin rolling engagements trigger intermediate affinity integrins to support reversible adhesions before chemokine-triggered arrest. Different leukocyte subsets appear to use different modalities of integrin activation during rolling and arrest at distinct endothelial sites.

What is the biological relevance of the specific bond properties revealed by single-molecule studies?

During the last decade, many authors took advantage of new methodologies based on atomic force microscopy (AFM), biomembrane force probes (BFPs), laminar flow chambers or optical traps to study at the single-molecule level the formation and dissociation of bonds between receptors and ligands attached to surfaces. Experiments provided a wealth of data revealing the complexity of bond response to mechanical forces and the dependence of bond rupture on bond history. These results supported the existence of multiple binding states and/or reaction pathways. Also, single bond studies allowed us to monitor attachments mediated by a few bonds. The aim of this review is to discuss the impact of this new information on our understanding of biological molecules and phenomena. The following points are discussed: (i) which parameters do we need to know in order to predict the behaviour of an encounter between receptors and ligands, (ii) which information is actually yielded by single-molecule studies and (iii) is it possible to relate this information to molecular structure?

A mechanism for antibody-mediated outside-in activation of LFA-1

MEM83 is an inserted domain (I-domain)-specific antibody that up-regulates the interaction of LFA-1 with ICAM-1 through an outside-in activation mechanism. We demonstrate here that there is no change in the affinity of the MEM83 antibody for the I-domain in either its low (wild-type) or high affinity form and that MEM83 does not enhance the binding of the wild-type I-domain to ICAM-1. Furthermore, we show that the antibody acts as an activating agent to induce LFA-1/ICAM-1-dependent homotypic cell aggregation only as an IgG, but not as a Fab fragment. On the basis of these data, we propose an avidity-based mechanism that requires no direct activation of the LFA-1 I-domain by the binding of the antibody; rather, activation is enhanced when there is an interaction with both arms of the IgG. A molecular model of the antibody interaction with LFA-1 illustrates the symmetry and accessibility of the two MEM83 epitopes across the LFA-1/ICAM-1 heterotetramer. We hypothesize that MEM83 stabilizes adjacent LFA-1 molecules in their active form by the free energy that is gained from the binding of the I-domains to each arm of the IgG. This leads to stabilization of the open state of the integrin and outside-in signaling. Our model supports a mechanism in which both affinity and avidity regulation are required in the activation of LFA-1.

Getting to the site of inflammation: the leukocyte adhesion cascade updated

Neutrophil recruitment, lymphocyte recirculation and monocyte trafficking all require adhesion and transmigration through blood-vessel walls. The traditional three steps of rolling, activation and firm adhesion have recently been augmented and refined. Slow rolling, adhesion strengthening, intraluminal crawling and paracellular and transcellular migration are now recognized as separate, additional steps. In neutrophils, a second activation pathway has been discovered that does not require signalling through G-protein-coupled receptors and the signalling steps leading to integrin activation are beginning to emerge. This Review focuses on new aspects of one of the central paradigms of inflammation and immunity--the leukocyte adhesion cascade.

Lymph node chemokines promote sustained T lymphocyte motility without triggering stable integrin adhesiveness in the absence of shear forces

Lymphocyte motility in lymph nodes is regulated by chemokines, but the contribution of integrins to this motility remains obscure. Here we examined lymphocyte migration over CCR7-binding chemokines that 'decorate' lymph node stroma. In a shear-free environment, surface-bound lymph node chemokines but not their soluble counterparts promoted robust and sustained T lymphocyte motility. The chemokine CCL21 induced compartmentalized clustering of the integrins LFA-1 and VLA-4 in motile lymphocytes, but both integrins remained nonadhesive to ligands on lymphocytes, dendritic cells and stroma. The application of shear stress to lymphocytes interacting with CCL21 and integrin ligands promoted robust integrin-mediated adhesion. Thus, lymph node chemokines that promote motility and strongly activate lymphocyte integrins under shear forces fail to stimulate stable integrin adhesiveness in extravascular shear-free environments.

Comparative normal mode analysis of LFA-1 integrin I-domains

The conformational dynamics of the Inserted domain (I-domain) from the lymphocyte function-associated antigen-1 (LFA-1) was investigated by normal mode analysis of multiple structures of the low, intermediate, and high affinity states. LFA-1 is an integrin expressed on leukocytes and is of critical importance in adhesion reactions, like antigen-specific responses, homing, and diapedesis. The main ligand binding site of LFA-1 is the I-domain, which recognizes intercellular adhesion molecules (ICAMs), members of the immunoglobulin superfamily. From experimental crystal structures, a large-scale conformational change of, among others, the alpha7 helix of the I-domain has been observed leading to the proposal that these structural changes are linked to the conformational regulation of LFA-1. The results from the present calculations show that structural changes of the alpha7 helix consistent with those observed in the crystal structures are significantly sampled by the low frequency modes. This was found to be particularly true for the low affinity state of the I-domain, indicating that low frequency motions favor the conformational transition implicated in activation. However, beyond the simple downward shift of the helix implied by the crystal structures, the calculations further show that there is a noticeable swinging-out motion of the helix. The consequences of this motion are discussed in the context of integrin activation and inhibition. Moreover, significant changes in the atomic-level dynamics and in long-range correlated motions of the I-domain were found to occur upon binding of the natural ligand ICAM. These changes were more local upon binding of an allosteric inhibitor. The present study opens the question of how changes in dynamics may contribute to the long-range transmission of signal upon ICAM binding by the LFA-1 I-domain.