Talin is required for integrin-mediated platelet function in hemostasis and thrombosis.

Department of Medicine, University of California, San Diego, La Jolla, CA 92093, USA.
Journal of Experimental Medicine (Impact Factor: 13.91). 01/2008; 204(13):3103-11. DOI: 10.1084/jem.20071800
Source: PubMed

ABSTRACT Integrins are critical for hemostasis and thrombosis because they mediate both platelet adhesion and aggregation. Talin is an integrin-binding cytoplasmic adaptor that is a central organizer of focal adhesions, and loss of talin phenocopies integrin deletion in Drosophila. Here, we have examined the role of talin in mammalian integrin function in vivo by selectively disrupting the talin1 gene in mouse platelet precursor megakaryocytes. Talin null megakaryocytes produced circulating platelets that exhibited normal morphology yet manifested profoundly impaired hemostatic function. Specifically, platelet-specific deletion of talin1 led to spontaneous hemorrhage and pathological bleeding. Ex vivo and in vitro studies revealed that loss of talin1 resulted in dramatically impaired integrin alphaIIbbeta3-mediated platelet aggregation and beta1 integrin-mediated platelet adhesion. Furthermore, loss of talin1 strongly inhibited the activation of platelet beta1 and beta3 integrins in response to platelet agonists. These data establish that platelet talin plays a crucial role in hemostasis and provide the first proof that talin is required for the activation and function of mammalian alpha2beta1 and alphaIIbbeta3 integrins in vivo.

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    ABSTRACT: IntroductionHemostasis is a rapid response by the body to stop bleeding at sites of vessel injury. Both platelets and fibrin are important for the formation of a hemostatic plug. Mice have been used to uncover the molecular mechanisms that regulate the activation of platelets and coagulation under physiological conditions. However, measuring hemostasis in mice is quite variable and current methods do not quantify platelet adhesion or fibrin formation at the site of injury.Methods We describe a novel hemostasis model that uses intravital fluorescence microscopy to quantify platelet adhesion, fibrin formation, and time to hemostatic plug formation in real-time. Repeated vessel injuries of ~50-100μm in diameter were induced using laser ablation technology in the saphenous vein of mice.ResultsHemostasis in this model was strongly impaired in mice deficient in GPIbα or talin-1, important regulators of platelet adhesiveness. In contrast, the time to hemostatic plug formation was only minimally affected in mice deficient in the extrinsic (TFlow) or the intrinsic (FIX-/-) coagulation pathways, even though platelet adhesion was significantly reduced. A partial reduction of platelet adhesiveness using clopidogrel led to instability within the hemostatic plug, especially when combined with impaired coagulation in TFlow mice.Conclusions In summary, we present a novel, highly sensitive method to quantify hemostatic plug formation in mice. Based on its sensitivity towards platelet adhesion defects and its real-time imaging capability, we propose this model as an ideal tool to study the efficacy and safety of antiplatelet agents.This article is protected by copyright. All rights reserved.
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    ABSTRACT: 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 integrin TMD can transmit these inside out signals.
    BMB reports 11/2014; · 1.99 Impact Factor
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    ABSTRACT: Plasma membrane (PM)-bound GTPase Rap1 recruits the Rap1-interacting-adaptor-molecule (RIAM), which in turn recruits talin to bind and activate integrins. However, it is unclear how RIAM recruits talin and why its close homolog lamellipodin does not. Here, we report that, although RIAM possesses two talin-binding sites (TBS1 and TBS2), only TBS1 is capable of recruiting cytoplasmic talin to the PM, and the R8 domain is the strongest binding site in talin. Crystal structure of an R7R8:TBS1 complex reveals an unexpected kink in the TBS1 helix that is not shared in the homologous region of lamellipodin. This kinked helix conformation is required for the colocalization of RIAM and talin at the PM and proper activation of integrin. Our findings provide the structural and mechanistic insight into talin recruitment by RIAM that underlies integrin activation and explain the differential functions of the otherwise highly homologous RIAM and lamellipodin in integrin signaling. Copyright © 2014 Elsevier Ltd. All rights reserved.

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