Regular Article PLATELETS AND THROMBOPOIESIS b1 integrin2mediated signals are required for platelet granule secretion and hemostasis in mouse

The rupture of atherosclerotic plaques leads to the exposure of matrix proteins, which in turn trigger arterial thrombosis, leading to stroke and cardiac infarction. Different platelet receptors sense the exposed matrix and induce platelet activation and secretion of prothrombotic molecules, leading to further platelet recruitment and finally thrombus formation. Integrins represent the major family of cell-adhesion receptors on platelets. They are heterodimeric transmembrane molecules consisting of a and b subunits. The aIIbb3 integrin is exclusively expressed on platelets and can bind von Willebrand factor (vWF), fibrinogen, and fibronectin. In addition to aIIbb3, platelets express threeb1 integrins, a2b1,a5b1, anda6b1, albeit at lower levels, which bind to collagen, fibronectin, and laminin, respectively. Circulating platelets express integrins in a conformation with a low affinity for the ligand. At sites of vascular injury, extracellular stimuli, such as collagen binding to glycoprotein VI (GPVI), induce the formation of active integrins with high affinity for ligands. This process is termed integrin activation or integrin inside-out signaling and requires the direct interaction of talin and kindlins with the cytoplasmic domain of the b integrin subunit. Although the fundamental role of the aIIbb3 integrin for plateletmediated hemostasis is indisputable, the in vivo relevance of b1 integrins on platelets remains controversial because of either different approaches to target b1 integrin function in platelets, including the use of a2 or b1 integrin-deficient mice, b1 integrin2blocking antibodies, synthetic collagen-derived peptides, or small-molecule inhibitors, or to the different experimental methods used to model arterial thrombosis and to assay bleeding tendency. Depending on the assay, b1 integrins have been reported to be both crucial and dispensable for platelet accumulation in vivo. Several in vitro studies indicated an important role of a2b1 integrin in the activation of various signaling molecules that are known to induce integrin aIIbb3 activation, platelet spreading, and aggregation. In one study with two patients, investigators reported that the loss of a2b1 integrin is associated with minor bleeding attributable to defective a2b1 integrin function, resulting in impaired platelet adhesion and aggregation to collagen or collagen-derived peptides under shear conditions. Overall, the exact role of b1 integrins remains a matter of debate. In the present study we therefore investigated the role of b1 integrins on platelets during arterial thrombosis by using mouse mutants that express different levels of b1 integrins or an activationdeficient b1 integrin. We report that b1-null mice or an activationdeficient b1 integrin mutant show prolonged bleeding times because of insufficient activation of Rac-1, actin dynamics, granule secretion, and platelet aggregation. Interestingly, platelets expressing 3% of wild-type b1 integrin show reduced adhesion to sites of vascular injury; however, bleeding times, Rac-1 activation, and degranulation are not affected. The relevance of these findings is discussed.