Integrin activation.

Integrin-mediated cell adhesion is important for development, immune responses, hemostasis and wound healing. Integrins also function as signal transducing receptors that can control intracellular pathways that regulate cell survival, proliferation, and cell fate. Conversely, cells can modulate the affinity of integrins for their ligands a process operationally defined as integrin activation. Analysis of activation of integrins has now provided a detailed molecular understanding of this unique form of "inside-out" signal transduction and revealed new paradigms of how transmembrane domains (TMD) can transmit long range allosteric changes in transmembrane proteins. Here, we will review how talin and mediates integrin activation and how the integrin TMD can transmit these inside out signals

Spatial restriction of alpha4 integrin phosphorylation regulates lamellipodial stability and alpha4beta1-dependent cell migration.

Integrins coordinate spatial signaling events essential for cell polarity and directed migration. Such signals from alpha4 integrins regulate cell migration in development and in leukocyte trafficking. Here, we report that efficient alpha4-mediated migration requires spatial control of alpha4 phosphorylation by protein kinase A, and hence localized inhibition of binding of the signaling adaptor, paxillin, to the integrin. In migrating cells, phosphorylated alpha4 accumulated along the leading edge. Blocking alpha4 phosphorylation by mutagenesis or by inhibition of protein kinase A drastically reduced alpha4-dependent migration and lamellipodial stability. alpha4 phosphorylation blocks paxillin binding in vitro; we now find that paxillin and phospho-alpha4 were in distinct clusters at the leading edge of migrating cells, whereas unphosphorylated alpha4 and paxillin colocalized along the lateral edges of those cells. Furthermore, enforced paxillin association with alpha4 inhibits migration and reduced lamellipodial stability. These results show that topographically specific integrin phosphorylation can control cell migration and polarization by spatial segregation of adaptor protein binding

Transmission of integrin β7 transmembrane domain topology enables gut lymphoid tissue development.

Integrin activation regulates adhesion, extracellular matrix assembly, and cell migration, thereby playing an indispensable role in development and in many pathological processes. A proline mutation in the central integrin β3 transmembrane domain (TMD) creates a flexible kink that uncouples the topology of the inner half of the TMD from the outer half. In this study, using leukocyte integrin α4β7, which enables development of gut-associated lymphoid tissue (GALT), we examined the biological effect of such a proline mutation and report that it impairs agonist-induced talin-mediated activation of integrin α4β7, thereby inhibiting rolling lymphocyte arrest, a key step in transmigration. Furthermore, the α4β7(L721P) mutation blocks lymphocyte homing to and development of the GALT. These studies show that impairing the ability of an integrin β TMD to transmit talin-induced TMD topology inhibits agonist-induced physiological integrin activation and biological function in development

β7 Integrin Inhibition Can Increase Intestinal Inflammation by Impairing Homing of CD25hiFoxP3+ Regulatory T Cells.

Background & aimsIntegrin α4β7 mediates lymphocyte trafficking to the gut and gut-associated lymphoid tissues, a process critical for recruitment of effector lymphocytes from the circulation to the gut mucosa in inflammatory bowel disease (IBD) and murine models of intestinal inflammation. Antibody blockade of β7 integrins generally is efficacious in IBD; however, some patients fail to respond, and a few patients can experience exacerbations. This study examined the effects of loss of β7 integrin function in murine models of IBD.MethodsIn a mouse IBD model caused by lack of interleukin 10, a cytokine important in CD25hiFoxP3+ regulatory T cell (Treg) function, genetic deletion of β7 integrin or antibody blockade of α4β7-mucosal addressin cell adhesion molecule-1 interaction paradoxically exacerbated colitis.ResultsLoss of β7 impaired the capacity of Tregs homing to the gut and therefore suppress intestinal inflammation in an adoptive T-cell transfer model; however, the intrinsic suppressive function of β7-deficient Tregs remained intact, indicating that the β7 deficiency selectively impacts gut homing. Deletion of β7 integrin did not worsen colitis in an acute dextran sodium sulfate model in which Treg number and function were normal.ConclusionsIn Integrin subunit beta (Itgb)7-/-Il10-/- mice, loss of β7-dependent Treg homing to gut-associated lymphoid tissues combined with loss of intrinsic Treg function exacerbated intestinal inflammation. These results suggest that IBD patients with reduced CD25hiFoxP3+ Treg numbers or function or lack of interleukin 10 could be at risk for failure of α4β7 blocking therapy

Blocking neutrophil integrin activation prevents ischemia-reperfusion injury.

Neutrophil recruitment, mediated by β2 integrins, combats pyogenic infections but also plays a key role in ischemia-reperfusion injury and other inflammatory disorders. Talin induces allosteric rearrangements in integrins that increase affinity for ligands (activation). Talin also links integrins to actin and other proteins that enable formation of adhesions. Structural studies have identified a talin1 mutant (L325R) that perturbs activation without impairing talin's capacity to link integrins to actin and other proteins. Here, we found that mice engineered to express only talin1(L325R) in myeloid cells were protected from renal ischemia-reperfusion injury. Dissection of neutrophil function in vitro and in vivo revealed that talin1(L325R) neutrophils had markedly impaired chemokine-induced, β2 integrin-mediated arrest, spreading, and migration. Surprisingly, talin1(L325R) neutrophils exhibited normal selectin-induced, β2 integrin-mediated slow rolling, in sharp contrast to the defective slow rolling of neutrophils lacking talin1 or expressing a talin1 mutant (W359A) that blocks talin interaction with integrins. These studies reveal the importance of talin-mediated activation of integrins for renal ischemia-reperfusion injury. They further show that neutrophil arrest requires talin recruitment to and activation of integrins. However, although neutrophil slow rolling requires talin recruitment to integrins, talin-mediated integrin activation is dispensable

Recreation of the terminal events in physiological integrin activation.

Increased affinity of integrins for the extracellular matrix (activation) regulates cell adhesion and migration, extracellular matrix assembly, and mechanotransduction. Major uncertainties concern the sufficiency of talin for activation, whether conformational change without clustering leads to activation, and whether mechanical force is required for molecular extension. Here, we reconstructed physiological integrin activation in vitro and used cellular, biochemical, biophysical, and ultrastructural analyses to show that talin binding is sufficient to activate integrin alphaIIbbeta3. Furthermore, we synthesized nanodiscs, each bearing a single lipid-embedded integrin, and used them to show that talin activates unclustered integrins leading to molecular extension in the absence of force or other membrane proteins. Thus, we provide the first proof that talin binding is sufficient to activate and extend membrane-embedded integrin alphaIIbbeta3, thereby resolving numerous controversies and enabling molecular analysis of reconstructed integrin signaling

Heart of glass anchors Rasip1 at endothelial cell-cell junctions to support vascular integrity.

Heart of Glass (HEG1), a transmembrane receptor, and Rasip1, an endothelial-specific Rap1-binding protein, are both essential for cardiovascular development. Here we performed a proteomic screen for novel HEG1 interactors and report that HEG1 binds directly to Rasip1. Rasip1 localizes to forming endothelial cell (EC) cell-cell junctions and silencing HEG1 prevents this localization. Conversely, mitochondria-targeted HEG1 relocalizes Rasip1 to mitochondria in cells. The Rasip1-binding site in HEG1 contains a 9 residue sequence, deletion of which abrogates HEG1's ability to recruit Rasip1. HEG1 binds to a central region of Rasip1 and deletion of this domain eliminates Rasip1's ability to bind HEG1, to translocate to EC junctions, to inhibit ROCK activity, and to maintain EC junctional integrity. These studies establish that the binding of HEG1 to Rasip1 mediates Rap1-dependent recruitment of Rasip1 to and stabilization of EC cell-cell junctions

KRIT-1/CCM1 is a Rap1 effector that regulates endothelial cell–cell junctions

Cerebral cavernous malformation (CCM), a disease associated with defective endothelial junctions, result from autosomal dominant CCM1 mutations that cause loss of KRIT-1 protein function, though how the loss of KRIT-1 leads to CCM is obscure. KRIT-1 binds to Rap1, a guanosine triphosphatase that maintains the integrity of endothelial junctions. Here, we report that KRIT-1 protein is expressed in cultured arterial and venous endothelial cells and is present in cell–cell junctions. KRIT-1 colocalized and was physically associated with junctional proteins via its band 4.1/ezrin/radixin/moesin (FERM) domain. Rap1 activity regulated the junctional localization of KRIT-1 and its physical association with junction proteins. However, the association of the isolated KRIT-1 FERM domain was independent of Rap1. Small interfering RNA–mediated depletion of KRIT-1 blocked the ability of Rap1 to stabilize endothelial junctions associated with increased actin stress fibers. Thus, Rap1 increases KRIT-1 targeting to endothelial cell–cell junctions where it suppresses stress fibers and stabilizes junctional integrity

Enhanced Integrin α4β1-Mediated Adhesion Contributes to a Mobilization Defect of Endothelial Progenitor Cells in Diabetes.

Diabetes is associated with a deficit of circulating endothelial progenitor cells (EPCs), which has been attributed to their defective mobilization from the bone marrow. The basis for this mobilization defect is not completely understood, and we sought to determine if hyperglycemic conditions enhanced EPC adhesion. We found that culturing EPCs in high glucose media increased adhesion to bone marrow stromal cells. This enhanced adhesion was associated with decreased expression of protein kinase A regulatory subunit 1β (PRKAR1β), activation of protein kinase A (PKA), and phosphorylation of α4-integrin on serine 988. This potentiated adhesion was reversed by treatment with a PKA inhibitor, overexpression of PRKAR1β, or expression of a phosphorylation-defective α4-integrin variant (α4[S988A]). Using a model of type 1 diabetes, we showed that α4(S988A)-expressing mice have more circulating EPCs than their wild-type counterparts. Moreover, diabetic α4(S988A) mice demonstrate enhanced revascularization after hind limb ischemia. Thus, we have identified a novel signaling mechanism activating PKA in diabetes (downregulation of an inhibitory regulatory subunit) that leads to deficits of circulating EPCs and impaired vascular repair, which could be reversed by α4-integrin mutation