The Role of Rac1 in Platelet Signal Transduction and Function

Von Willebrand factor (VWF)/glycoprotein Ib-IX (GPIb-IX) interaction mediates platelet adhesion at sites of vascular injury and transmits signals leading to platelet activation and thrombosis. Ligand occupancy of GPIb-IX sequentially activates Lyn, PI3K and Akt, leading to activation of integrin αIIbβ3 and integrin-dependent stable platelet adhesion and aggregation. However, how Lyn activates the PI3K/Akt pathway remains unclear. We discovered that Rac1 is required for GPIb-induced platelet activation, as Rac1−/− mouse platelets and Rac1 inhibitor (NSC23766)-treated human platelets were defective in GPIb-dependent stable adhesion to VWF under shear, integrin activation, thromboxane A2 synthesis and platelet aggregation. GPIb-induced activation of Rac1 and the guanine nucleotide exchange factor for Rac1, Vav, was abolished in both Lyn−/− and Lyn inhibitor-treated platelets but was unaffected by inhibition of PI3K, indicating that Lyn mediates activation of Vav and Rac1 independently of PI3K. Furthermore, GPIb-induced activation of Akt was abolished in Rac1-deficient platelets, suggesting that Rac1 is upstream of the PI3K/Akt pathway. Thus, Lyn activates the PI3K/Akt pathway through Vav and Rac1 to promote GPIb-IX–dependent platelet activation. Platelets facilitate blood coagulation via phosphatidylserine (PS) exposure and microvesicle (MV) release. Although thrombin and collagen are potent agonists, they poorly induce PS exposure. To obtain a greater procoagulant response, thrombin is commonly used in combination with glycoprotein (GP) VI agonists, but still only a percentage of platelets express procoagulant activity. To date, it remains unclear why platelets poorly expose PS and release MVs even when stimulated with multiple agonists. We discovered that physiological levels of shear present in blood significantly enhance agonist-induced platelet PS exposure and MV release, enabling low doses of a single agonist to induce a full-scale procoagulant response. PS exposed on the platelet surface was immediately released as MVs, revealing a tight coupling between the 2 processes under shear. Using platelet-specific Rac1-/- mice, we discovered that Rac1 plays a common role in mediating the low-dose agonist-induced procoagulant response independent of platelet aggregation, secretion and the apoptosis pathway. Platelet-specific Rac1 function was not only important for coagulation in vitro but also for fibrin accumulation in vivo following laser-induced arteriolar injury.