The platelet receptor for von Willebrand factor (VWF), the glycoprotein (GP) Ib-IX-V complex (GPIb-IX), is vital for initial platelet adhesion to injured blood vessel wall, especially under shear stress. VWF binding to GPIb-IX also initiates signaling resulting in the activation of the platelet integrin aIIbb3, which mediates stable platelet adhesio, platelet aggregation, and thrombus formation. In the current funding periods, we have made significant progress in understanding the signaling mechanisms that regulates the receptor function of GPIb and VWF-induced GPIb signaling leading to platelet activation. In particular, we have demonstrated an important role for Rac1 in mediating VWF/GPIb-IX-mediated early signaling. We also demonstrated an unexpected role of a potential downstream Rac1 effector, LIM kinase 1 (LIMK1) in selectively stimulating GPIb- IX-dependent thromboxane (TX) A2 synthesis and signal amplification. GPIb-IX also interacts with thrombin, and is important in low dose thrombin-induced platelet activation. However, it is unclear whether GPIb-IX serves facilitate more efficient thrombin cleavage of protease-activated receptors (PAR), or GPIb signaling promote thrombin-induced platelet activation. Based on our preliminary data, we hypothesize that the Rac1- and LIMK1-mediated GPIb-IX signaling pathways promotes PAR-dependent platelet response to low dose thrombin. In Aim 1 of this competitive renewal application, we will investigate the role of GPIb-IX signaling in promoting thrombin-induced platelet activation and the Rac1 and LIMK1 signaling pathways that mediates VWF- and thrombin-induced GPIb-IX signaling. Activated platelets play an important role in facilitating coagulation, mainly by providing externalized phosphotidylserine (PS). However, under currently accepted assay conditions, even extremely high concentrations of physiological platelet agonists collagen and thrombin only induce a small fraction of platelets to externalize PS, the reason for this paradox remains unclear. We show that under the shear force relevant to normal blood flow, platelets do not express procoagulant activity (although the extremely high shear stress may induce platelet procoagulant MP release that requires GPIb-IX- VWF interaction). However, physiological levels of shear stress are required for agonists to efficiently induce PS externalization and MP release from platelets. Importantly, we discovered that Rac1 plays a critical role in shear-dependent platelet PS externalization and MP release induced by GPIb-independent platelet agonists. Thus, we hypothesize that activation of platelet procoagulant function requires shear force-induced signaling in which Rac1 plays an important role. Therefore, in Aim 2, we will investigate the role of shear force, Rac1 and GPIb-IX signaling in the activation of platelet procoagulant function. These study should facilitate our understanding how low concentrations of agonists thrombin or initial platelet adhesion will elicit strong platele response leading to not only platelet thrombus formation but also clotting at the site of vascular injury, and help develop new inhibitor for treating thrombotic diseases.