Role for ADAP in shear flow-induced platelet mechanotransduction

Binding of platelets to fibrinogen via integrin alpha IIb beta 3 stimulates cytoskeletal reorganization and spreading. These responses depend on tyrosine phosphorylation of multiple proteins by Src family members and Syk. Among Src substrates in platelets is adhesion- and degranulation-promoting adapter protein (ADAP), an adapter with potential binding partners: SLP-76, VASP, and SKAP-HOM. During studies of platelet function under shear flow, we discovered that ADAP(-/)-mouse platelets, unlike ADAP(+/+) platelets, formed unstable thrombi in response to carotid artery injury. Moreover, fibrinogen-adherent ADAP(-/)-platelets in shear flow ex vivo showed reduced spreading and smaller zones of contact with the matrix. These abnormalities were not observed under static conditions, and they could not be rescued by stimulating platelets with a PAR4 receptor agonist or by direct alpha IIb beta 3 activation with MnCl(2), consistent with a defect in outside-in alpha IIb beta 3 signaling. ADAP(-/)-platelets subjected to shear flow assembled F-actin-rich structures that colocalized with SLP-76 and the Rac1 exchange factor, phospho-Vav1. In contrast, platelets deficient in ADAP, but not those deficient in VASP or SKAP-HOM, failed to form these structures. These results establish that ADAP is an essential component of alpha IIb beta 3-mediated platelet mechanotransduction that promotes F-actin assembly and enables platelet spreading and thrombus stabilization under fluid shear stress. (Blood. 2010;115:2274-2282)