The talin head domain reinforces integrin-mediated adhesion by promoting adhesion complex stability and clustering.

Talin serves an essential function during integrin-mediated adhesion in linking integrins to actin via the intracellular adhesion complex. In addition, the N-terminal head domain of talin regulates the affinity of integrins for their ECM-ligands, a process known as inside-out activation. We previously showed that in Drosophila, mutating the integrin binding site in the talin head domain resulted in weakened adhesion to the ECM.

Up-regulation of thrombospondin-2 in Akt1-null mice contributes to compromised tissue repair due to abnormalities in fibroblast function.

Vascular remodeling is essential for tissue repair and is regulated by multiple factors, including thrombospondin-2 (TSP2) and hypoxia/VEGF-induced activation of Akt. In contrast to TSP2 knock-out (KO) mice, Akt1 KO mice have elevated TSP2 expression and delayed tissue repair. To investigate the contribution of increased TSP2 to Akt1 KO mice phenotypes, we generated Akt1/TSP2 double KO (DKO) mice.

Dynamin 2 regulation of integrin endocytosis, but not VEGF signaling, is crucial for developmental angiogenesis.

Here we show that dynamin 2 (Dnm2) is essential for angiogenesis in vitro and in vivo. In cultured endothelial cells lacking Dnm2, vascular endothelial growth factor (VEGF) signaling and receptor levels are augmented whereas cell migration and morphogenesis are impaired. Mechanistically, the loss of Dnm2 increases focal adhesion size and the surface levels of multiple integrins and reduces the activation state of β1 integrin.

Zasp regulates integrin activation.

Integrins are heterodimeric adhesion receptors that link the extracellular matrix (ECM) to the cytoskeleton. Binding of the scaffold protein, talin, to the cytoplasmic tail of β-integrin causes a conformational change of the extracellular domains of the integrin heterodimer, thus allowing high-affinity binding of ECM ligands. This essential process is called integrin activation.

A conserved lipid-binding loop in the kindlin FERM F1 domain is required for kindlin-mediated αIIbβ3 integrin coactivation.

The activation of heterodimeric integrin adhesion receptors from low to high affinity states occurs in response to intracellular signals that act on the short cytoplasmic tails of integrin β subunits. Binding of the talin FERM (four-point-one, ezrin, radixin, moesin) domain to the integrin β tail provides one key activation signal, but recent data indicate that the kindlin family of FERM domain proteins also play a central role. Kindlins directly bind integrin β subunit cytoplasmic domains at a site distinct from the talin-binding site, and target to focal adhesions in adherent cells.

Talin and signaling through integrins.

Integrin adhesion receptors are essential for the development and functioning of multicellular animals. Integrins mediate cell adhesion to the extracellular matrix and to counter-receptors on adjacent cells, and the ability of integrins to bind extracellular ligands is regulated in response to intracellular signals that act on the short cytoplasmic tails of integrin subunits. Integrin activation, the rapid conversion of integrin receptors from low to high affinity, requires binding of talin to integrin β tails and, once bound, talin provides a connection from activated integrins to the actin cytoskeleton.

The E3 ubiquitin ligase specificity subunit ASB2α targets filamins for proteasomal degradation by interacting with the filamin actin-binding domain.

Filamins are an important family of actin-binding and crosslinking proteins that mediate remodeling of the actin cytoskeleton and maintain extracellular matrix connections by anchoring transmembrane proteins to actin filaments and linking them to intracellular signaling cascades. We recently found that filamins are targeted for proteasomal degradation by the E3 ubiquitin ligase specificity subunit ASBα and that acute degradation of filamins through this ubiquitin-proteasome pathway correlates with cell differentiation. Specifically, in myeloid leukemia cells retinoic-acid-induced expression of ASB2α triggers filamin degradation and recapitulates early events crucial for cell differentiation.

Structure of a double ubiquitin-like domain in the talin head: a role in integrin activation.

Talin is a 270-kDa protein that activates integrins and couples them to cytoskeletal actin. Talin contains an N-terminal FERM domain comprised of F1, F2 and F3 domains, but it is atypical in that F1 contains a large insert and is preceded by an extra domain F0. Although F3 contains the binding site for beta-integrin tails, F0 and F1 are also required for activation of beta1-integrins.

The structure of the N-terminus of kindlin-1: a domain important for alphaiibbeta3 integrin activation.

The integrin family of heterodimeric cell adhesion molecules exists in both low- and high-affinity states, and integrin activation requires binding of the talin FERM (four-point-one, ezrin, radixin, moesin) domain to membrane-proximal sequences in the beta-integrin cytoplasmic domain. However, it has recently become apparent that the kindlin family of FERM domain proteins is also essential for talin-induced integrin activation. FERM domains are typically composed of F1, F2, and F3 domains, but the talin FERM domain is atypical in that it contains a large insert in F1 and is preceded by a previously unrecognized domain, F0.

Kindlin-1 and -2 directly bind the C-terminal region of beta integrin cytoplasmic tails and exert integrin-specific activation effects.

Integrin activation, the rapid conversion of integrin adhesion receptors from low to high affinity, occurs in response to intracellular signals that act on the short cytoplasmic tails of integrin beta subunits. Talin binding to integrin beta tails provides one key activation signal, but additional factors are likely to cooperate with talin to regulate integrin activation. The integrin beta tail-binding proteins kindlin-2 and kindlin-3 were recently identified as integrin co-activators.

The N-terminal domains of talin cooperate with the phosphotyrosine binding-like domain to activate beta1 and beta3 integrins.

The activation of integrin adhesion receptors from low to high affinity in response to intracellular cues controls cell adhesion and signaling. Binding of the cytoskeletal protein talin to the beta3 integrin cytoplasmic tail is required for beta3 activation, and the integrin-binding PTB-like F3 domain of talin is sufficient to activate beta3 integrins. Here we report that, whereas the conserved talin-integrin interaction is also required for beta1 activation, and talin F3 binds beta1 and beta3 integrins with comparable affinity, expression of the talin F3 domain is not sufficient to activate beta1 integrins.

Distinct signaling pathways are involved in leukosialin (CD43) down-regulation, membrane blebbing, and phospholipid scrambling during neutrophil apoptosis.

Although leukosialin (CD43) membrane expression decreases during neutrophil apoptosis, the CD43 molecule, unexpectedly, is neither proteolyzed nor internalized. We thus wondered whether it could be shed on bleb-derived membrane vesicles. Membrane blebbing is a transient event, hardly appreciated during the asynchronous, spontaneous apoptosis of neutrophils.

TNF-induced beta2 integrin activation involves Src kinases and a redox-regulated activation of p38 MAPK.

We previously demonstrated that the TNF-alpha-induced inside-out signaling leading to beta(2) integrin activation is redox regulated. To identify kinases involved in this pathway, the effects of kinase inhibitors on the expression of beta(2) integrin activation neoepitope (clone 24) were investigated. We show that both p38 MAPK (inhibited by SB203580) and Src kinases (inhibited by PP2) are involved in beta(2) integrin activation by TNF and oxidants in human neutrophils.