Structures of integrin domains and concerted conformational changes in the bidirectional signaling mechanism of alphaIIbbeta3

Extracellular Redox Regulation of α7β Integrin-Mediated Cell Migration Is Signaled via a Dominant Thiol-Switch

While adhering to extracellular matrix (ECM) proteins, such as laminin-111, cells temporarily produce hydrogen peroxide at adhesion sites. To study the redox regulation of α7β1 integrin-mediated cell adhesion to laminin-111, a conserved cysteine pair within the α-subunit hinge region was replaced for alanines. The molecular and cellular effects were analyzed by electron and atomic force microscopy, impedance-based migration assays, flow cytometry and live cell imaging. This cysteine pair constitutes a thiol-switch, which redox-dependently governs the equilibrium between an extended and a bent integrin conformation with high and low ligand binding activity, respectively. Hydrogen peroxide oxidizes the cysteines to a disulfide bond, increases ligand binding and promotes cell migration toward laminin-111. Inversely, extracellular thioredoxin-1 reduces the disulfide, thereby decreasing laminin binding. Mutation of this cysteine pair into the non-oxidizable hinge-mutant shows molecular and cellular effects similar to the reduced wild-type integrin, but lacks redox regulation. This proves the existence of a dominant thiol-switch within the α subunit hinge of α7β1 integrin, which is sufficient to implement activity regulation by extracellular redox agents in a redox-regulatory circuit. Our data reveal a novel and physiologically relevant thiol-based regulatory mechanism of integrin-mediated cell-ECM interactions, which employs short-lived hydrogen peroxide and extracellular thioredoxin-1 as signaling mediators.

B cell mechanosensing: A mechanistic overview

B cells are essential to the adaptive immune system for providing the humoral immunity against cohorts of pathogens. The presentation of antigen to the B cell receptor (BCR) leads to the initiation of B cell activation, which is a process sensitive to the stiffness features of the substrates presenting the antigens. Mechanosensing of the B cells, potentiated through BCR signaling and the adhesion molecules, efficiently regulates B cell activation, proliferation and subsequent antibody responses. Defects in sensing of the antigen-presenting substrates can lead to the activation of autoreactive B cells in autoimmune diseases. The use of high-resolution, high-speed live-cell imaging along with the sophisticated biophysical materials, has uncovered the mechanisms underlying the initiation of B cell activation within seconds of its engagement with the antigen presenting substrates. In this chapter, we reviewed studies that have contributed to uncover the molecular mechanisms of B cell mechanosensing during the initiation of B cell activation.

Platelet-Activation Mechanisms and Vascular Remodeling

This overview article for the Comprehensive Physiology collection is focused on detailing platelets, how platelets respond to various stimuli, how platelets interact with their external biochemical environment, and the role of platelets in physiological and pathological processes. Specifically, we will discuss the four major functions of platelets: activation, adhesion, aggregation, and inflammation. We will extend this discussion to include various mechanisms that can induce these functional changes and a discussion of some of the salient receptors that are responsible for platelets interacting with their external environment. We will finish with a discussion of how platelets interact with their vascular environment, with a special focus on interactions with the extracellular matrix and endothelial cells, and finally how platelets can aid and possibly initiate the progression of various vascular diseases. Throughout this overview, we will highlight both the historical investigations into the role of platelets in health and disease as well as some of the more current work. Overall, the authors aim for the readers to gain an appreciation for the complexity of platelet functions and the multifaceted role of platelets in the vascular system. © 2017 American Physiological Society. Compr Physiol 8:1117-1156, 2018.

The hydrophobic contacts between the center of the βI domain and the α1/α7 helices are crucial for the low-affinity state of integrin α4 β7

Integrin α4 β7 mediates both rolling and firm adhesion of lymphocytes by modulating its affinity to the ligand: mucosal addressin cell adhesion molecule-1 (MAdCAM-1). Integrin activation is associated with allosteric reshaping in the β subunit I (βI) domain. A prominently conformational change comprises displacement of the α1 and α7 helices in the βI domain, suggesting that the location of these helices is important for the change in integrin affinity. In the present study, we report that the hydrophobic contacts between the center of the β7 I domain and the α1/α7 helices play critical roles in keeping α4 β7 in a low-affinity state. Using molecular dynamics simulation, we identified nine hydrophobic residues that might be involved in the critical hydrophobic contacts maintaining integrin in a low-affinity state. Integrin β7 I domain exhibited a lower binding free energy for ligand after disrupting these hydrophobic contacts by substituting the hydrophobic residues with Ala. Moreover, these α4 β7 mutants not only showed high-affinity binding to soluble MAdCAM-1, but also demonstrated firm cell adhesion to immobilized MAdCAM-1 in shear flow and enhanced the strength of the α4 β7 -MAdCAM-1 interaction. Disruption of the hydrophobic contacts also induced the active conformation of α4 β7 . Thus, the findings obtained in the present study reveal an important structural basis for the low-affinity state of integrin.

Role of extracellular matrix and YAP/TAZ in cell fate determination

The mechanical signals transduced from cellular microenvironment can regulate cell shape and affect cell fate determination. However, how these mechanical signals are transduced to regulate biological processes of cells has remained elusive. Recent studies had elucidated a novel mechanism through which the interactions between mechanical signals from extracellular matrix and cell behavior regulation converged on the function of core components in Hippo signaling pathway, including YAP and TAZ in mammals. Moreover, several very recent studies have found a new crosstalk between Wnt and Hippo signaling in the regulation of cell fate determination. Such mechanism may explain how mechanical signals from microenvironment can regulate cell behavior and determine cell fate.

"INTEGRINating" the connexin hemichannel function in bone osteocytes through the action of integrin α5

Mechanical loading influences skeletal structural integrity and bone remodeling. Application of a mechanical stimulus such as fluid flow shear stress to the bone osteocytes activates the cascade of mechanotransduction mediated by multiple signaling molecules. Hemichannels formed by connexin molecules are emerging as a candidate mechanosensor. Connexin 43 (Cx43) hemichannels open in response to mechanical stimulation to release bone modulators which influence bone remodeling. Our study identified a direct interaction between integrin α5 and Cx43 which was essential for hemichannels to open. Uncoupling the interaction blocked the hemichannels and shear stress enhanced the interaction between the two proteins to promote channel opening. More importantly, integrin α5, independent of its association with fibronectin, was activated upon shear stress through a PI3K signaling pathway. These results suggest a critical regulatory mechanism for Cx43 hemichannel opening through the association of integrin α5, resulting in release of bone anabolic factors required for bone development.

Endothelial mechanosensors of shear stress as regulators of atherogenesis

PURPOSE OF REVIEW

Differences in local blood flow patterns along the endothelium may trigger abnormal vascular responses which can have profound pathophysiological consequences. While endothelial cells exposed to laminar blood flow (high shear stress) are protected from atherosclerosis formation, turbulent or disturbed blood flow, which occurs at bends and bifurcations of blood vessels, facilitates atherosclerosis formation. Here, we will highlight the endothelial cell mechanisms involved in detecting shear stress and their translation into downstream biochemical signals.

RECENT FINDINGS

Prior evidence supports a role for integrins as mechanotransducers in the endothelium by promoting phosphorylation of different targets through the activation of focal adhesion kinase. Our recent findings show that integrins contact integrin-linked kinase and regulate vasomotor responses by an endothelial nitric oxide synthase-dependent mechanism, which stabilizes the production of vasoactive factor nitric oxide. In addition, different structures of endothelial cells, mainly primary cilia, are investigated, as they can explain the differential responses to laminar versus disturbed flow.

SUMMARY

The discovery of a connection between endothelial cell structures such as cilia, integrin, extracellular matrix, and signaling events opens today a new chapter in our understanding of the molecular mechanisms regulating vascular responses to the changes in flow.

Solution structure, dynamics and thermodynamics of the three SH3 domains of CD2AP

CD2 associated protein (CD2AP) is an adaptor protein that plays an important role in cell to cell union needed for the kidney function. It contains three N-terminal SH3 domains that are able to interact among others with CD2, ALIX, c-Cbl and Ubiquitin. To understand the role of the individual SH3 domains of this adaptor protein we have performed a complete structural, thermodynamic and dynamic characterization of the separate domains using NMR and DSC. The energetic contributions to the stability and the backbone dynamics have been related to the structural features of each domain using the structure-based FoldX algorithm. We have found that the N-terminal SH3 domain of both adaptor proteins CD2AP and CIN85 are the most stable SH3 domains that have been studied until now. This high stability is driven by a more extensive network of intra-molecular interactions. We believe that this increased stabilization of N-terminal SH3 domains in adaptor proteins is crucial to maintain the necessary conformation to establish the proper interactions critical for the recruitment of their natural targets.

Glycoprotein IIb/IIIa inhibitors: an update on the mechanism of action and use of functional testing methods to assess antiplatelet efficacy

The human glycoprotein (GP)IIb/IIIa belongs to a large family of cation-dependent adhesion molecules known as integrins, which share a common heterodimeric structure. The primary function of GPIIb/IIIa is to aid platelet aggregation by transmitting bidirectional signals across the plasma membrane. Since the GPIIb/IIIa receptor is among the key integrins involved in platelet aggregation and, therefore, thrombus formation, the development of GPIIb/IIIa antagonists (e.g., abciximab, eptifibatide and tirofiban) has become an attractive strategy for antiplatelet therapy with an expected strong and specific effect. All three drugs are administered intravenously, and large-scale clinical trials have demonstrated a clear clinical benefit and good safety profile in high-risk patients, especially those undergoing percutaneous coronary intervention. However, the adverse events related to thrombosis or bleeding are still reported in patients undergoing therapy with GPIIb/IIIa antagonists and reflect a variable interindividual responsiveness. Therefore, some form of laboratory monitoring is required to optimize the effects of a drug or to indicate that it needs replacing with other antithrombotic agents, as well as for identifying and enhancing the platelet inhibition in this subgroup of patients to improve the clinical outcome and reduce bleeding complications. As such, the aim of this article is to provide an update on the mechanism of action and use of functional testing methods to assess antiplatelet efficacy in patients undergoing therapy with GPIIb/IIIa antagonists.

Integrins and extracellular matrix in mechanotransduction

Integrins bind extracellular matrix fibrils and associate with intracellular actin filaments through a variety of cytoskeletal linker proteins to mechanically connect intracellular and extracellular structures. Each component of the linkage from the cytoskeleton through the integrin-mediated adhesions to the extracellular matrix therefore transmits forces that may derive from both intracellular, myosin-generated contractile forces and forces from outside the cell. These forces activate a wide range of signaling pathways and genetic programs to control cell survival, fate, and behavior. Additionally, cells sense the physical properties of their surrounding environment through forces exerted on integrin-mediated adhesions. This article first summarizes current knowledge about regulation of cell function by mechanical forces acting through integrin-mediated adhesions and then discusses models for mechanotransduction and sensing of environmental forces.

Key interactions in integrin ectodomain responsible for global conformational change detected by elastic network normal-mode analysis

Integrin, a membrane protein with a huge extracellular domain, participates in cell-cell and cell-extracellular-matrix interactions for metazoan. A group of integrins is known to perform a large-scale structural change when the protein is activated, but the activation mechanism and generality of the conformational change remain to be elucidated. We performed normal-mode analysis of the elastic network model on integrin alpha(V)beta(3) ectodomain in the bent form and identified key residues that influenced molecular motions. Iterative normal-mode calculations demonstrated that the specific nonbonded interactions involving the key residues work as a snap to keep integrin in the bent form. The importance of the key residues for the conformational change was further verified by mutation experiments, in which integrin alpha(IIb)beta(3) was used. The conservation pattern of amino acid residues among the integrin family showed that the characteristic pattern of residues seen around these key residues is found in the limited groups of integrin beta-chains. This conservation pattern suggests that the molecular mechanism of the conformational change relying on the interactions found in integrin alpha(V)beta(3) is unique to the limited types of integrins.

Purification, analysis, and crystal structure of integrins

Integrins are large modular cell-surface receptors that regulate almost every aspect of cellular function through bidirectional signals transmitted across the lipid bilayer. Regulation of integrin activity is accomplished by complex and still incompletely understood biochemical pathways that modify integrin ligand binding, clustering, trafficking, and signaling functions. The dynamic tertiary and quaternary changes required to channel some of these activities have hampered, until recently, the crystal structure determination of these heterodimeric receptors. In this chapter, we review the methods used to purify and characterize these proteins biophysically and functionally, and to derive their three-dimensional structures.

Occupation of alphavbeta3-integrin by endogenous ligands modulates IGF-I receptor activation and proliferation of human intestinal smooth muscle

We have previously shown that endogenous IGF-I regulates growth of human intestinal smooth muscle cells by stimulating proliferation and inhibiting apoptosis. In active Crohn's disease, expression of IGF-I and the alpha(v)beta(3)-integrin receptor ligands fibronectin and vitronectin is increased. The aim of the present study was to determine whether occupation of the alpha(v)beta(3)-receptor influences IGF-I receptor tyrosine kinase activation and function in human intestinal smooth muscle cells. In untreated cells, IGF-I elicited time-dependent tyrosine phosphorylation of its cognate receptor that was maximal within 2 min and sustained for 30 min. In the presence of the alpha(v)beta(3)-ligand fibronectin, IGF-I-stimulated IGF-I receptor activation was augmented. Conversely, in the presence of the alpha(v)beta(3)-specific disintegrin echistatin, IGF-I-stimulated IGF-I receptor tyrosine kinase phosphorylation was inhibited. IGF-I-stimulated IGF-I receptor activation was accompanied by recruitment of the adapter protein IRS-1, activation of Erk1/2, p70S6 kinase, and proliferation. These effects were augmented by fibronectin and attenuated by echistatin. IGF-I also elicited time-dependent recruitment of protein tyrosine phosphatase SHP-2 that coincided with dephosphorylation of the tyrosine phosphorylated IGF-I receptor tyrosine kinase. The alpha(v)beta(3)-disintegrin echistatin accelerated the rate of SHP-2 recruitment and deactivation of the IGF-I receptor tyrosine kinase. The results show that occupancy of the alpha(v)beta(3)-integrin receptor modulates IGF-I-induced IGF-I receptor activation and function in human intestinal muscle cells. We hypothesize that the concomitant increases in the expression of alpha(v)beta(3)-ligands and of IGF-I in active Crohn's disease may contribute to muscle hyperplasia and stricture formation by acting in concert to augment IGF-I-stimulated IGF-I receptor tyrosine kinase activity and IGF-I-mediated muscle cell growth.

Integrins as a distinct subtype of dependence receptors

In the absence of their cognate ligand, dependence receptors trigger programmed cell death. This function is the defining feature of dependence receptors, which include members of several different protein families. The integrins are a family of heterodimeric receptors for extracellular matrix (ECM) proteins, mediating cell anchorage and migration. Integrins share characteristics with dependence receptors, and integrin binding to substrate ECM ligands is essential for cell survival. Although integrins do not conform in all characteristics to the established definitions of dependence receptors, alterations in the expression of integrins and their ligands during physiological and pathological events, such as wound healing, angiogenesis and tumorigenesis, do regulate cell fate in a ligand-dependent manner. This biosensory function of integrins fits well with our current concept of dependence receptor action, and thus integrins may rightly be considered to comprise a distinct subclass of dependence receptor.

The response of VEGF-stimulated endothelial cells to angiostatic molecules is substrate-dependent

Background

The microenvironment surrounding cells can exert multiple effects on their biological responses. In particular the extracellular matrix surrounding cells can profoundly influence their behavior. It has been shown that the extracellular matrix composition in tumors is vastly different than that found in normal tissue with increased amounts of certain matrices such as collagen I. It has been previously demonstrated that VEGF stimulation of endothelial cells growing on type I collagen results in the induction of bcl-2 expression and enhanced endothelial cell survival. We sought to investigate whether this increased endothelial cell survival resulted in the failure of angiostatic molecules to inhibit angiogenesis.

Results

We now demonstrate that VEGF-induced survival on collagen I impairs the ability of three known angiostatic molecules, TSP-1, IP-10 and endostatin to inhibit endothelial cell proliferation. Apoptosis of endothelial cells, growing on collagen I, induced by TSP-1 and IP-10 was also inhibited following VEGF stimulation. In contrast, endostatin induced apoptosis in these same cells. Further analysis determined that endostatin did not decrease the expression of bcl-2 nor did it increase activation of caspase-3 in the presence of VEGF. Alternatively, it appeared that in the presence of VEGF, endostatin induced the activation of caspase-8 in endothelial cells grown on collagen I. Furthermore, only endostatin had the ability to inhibit VEGF-induced sprout formation in collagen I gels.

Conclusion

These data suggest that TSP-1, IP-10 and endostatin inhibit endothelial cells via different mechanisms and that only endostatin is effective in inhibiting angiogenic activities in the presence of collagen I. Our results suggest that the efficacy of angiostatic treatments may be impaired depending on the context of the extracellular matrix within the tumor environment and thus could impede the efficacy of angiostatic therapies.

Distinct mechanisms of integrin binding by Yersinia pseudotuberculosis adhesins determine the phagocytic response of host macrophages

The enteropathogenic yersiniae express two outer membrane adhesins, invasin and YadA, that contribute to pathogenesis. While invasin binds directly to beta1 integrin receptors with high affinity, YadA binds indirectly through extracellular matrix (ECM) components. In this study, Yersinia pseudotuberculosis inv and yadA mutants were used to investigate how these distinct binding mechanisms compare and potentially compete in activating signalling pathways and promoting bacterial uptake by host macrophages. The efficiency of adhesin-mediated phagocytic responses was found to be dependent on the relative expression of invasin and YadA on the bacterial surface as well as the expression of ECM proteins in the extracellular milieu. Under conditions of low concentrations of ECM, invasin was found to be the dominant adhesin, promoting high levels of phagocytosis coincident with robust and sustained activation of the protein tyrosine kinases Fak and Pyk2, phosphorylation of the adaptor molecule Cas and activation of the small GTPase Rac1. In the presence of higher concentrations of ECM, YadA became the dominant functional adhesin through its ability to engage integrin receptors via an ECM bridge. We propose a model whereby invasin promotes robust and prolonged activation of phagocytic signalling cascades by inducing a 'high-affinity' integrin conformation as well as integrin clustering. We postulate that YadA-ECM promotes phagocytosis through a more transient activation of signalling cascades that arises from integrin clustering in the context of a cross-linked fibrillar ECM network.

Calcium- and magnesium-dependent interactions between calcium- and integrin-binding protein and the integrin alphaIIb cytoplasmic domain

Calcium- and integrin-binding protein (CIB) is a small EF-hand calcium-binding protein that is involved in hemostasis through its interaction with the alphaIIb cytoplasmic domain of integrinalphaIIbbeta(3). We have previously demonstrated that CIB lacks structural stability in the absence of divalent metal ions but that it acquires a well-folded conformation upon addition of Ca(2+) or Mg(2+). Here, we have used fluorescence spectroscopy, NMR spectroscopy, and isothermal titration calorimetry to demonstrate that both Ca(2+)-bound CIB (Ca(2+)-CIB) and the Mg(2+)-bound protein (Mg(2+)-CIB) bind with high affinity and through a similar mechanism to alphaIIb cytoplasmic domain peptides, but that metal-free CIB (apo-CIB) binds in a different manner. The interactions are thermodynamically distinct for Ca(2+)-CIB and Mg(2+)-CIB, but involve hydrophobic interactions in each case. Since the Mg(2+) concentration inside the cell is sufficient to saturate CIB at all times, our results imply that CIB would be capable of binding to the alphaIIb cytoplasmic domain independent of an intracellular Ca(2+) stimulus in vivo. This raises the question of whether CIB can act as a Ca(2+) sensor in alphaIIbbeta(3) signaling or if other regulatory mechanisms such as fibrinogen-induced conformational changes in alphaIIbbeta(3), post-translational modifications, or the binding of other accessory proteins mediate the interactions between CIB and alphaIIbbeta(3). Differences in NMR spectra do suggest, however, that Ca(2+)-binding to the Mg(2+)- CIB-alphaIIb complex induces subtle structural changes that could further modulate the activity of alphaIIbbeta(3).