Platelet activation plays a major role in hemostasis and thrombosis. Platelet agonists cause shape change, fibrinogen receptor activation, dense granule release, and thromboxane A2 (TXA2) generation, leading to the activation of other platelets. The mechanisms regulating these platelet physiological events have not been completely understood. All the platelet agonists, either directly or indirectly, depend on G protein pathways to cause platelet activation. We propose to further understand the downstream events in the G protein pathways and their second messengers in platelet activation using complementary biochemical, pharmacological, and gene knockout approaches. ADP and thrombin differ in their ability to activate platelets. ADP fails to cause dense granule release in aspirin-treated platelets, whereas thrombin can. ADP depends on integrin signaling to activate phospholipase A2, whereas thrombin does not. Whereas both these agonists activate Gq-phospholipase C pathways, only thrombin stimulates G12/13 pathways. This grant application is built upon our recent studies demonstrating important roles for a) G12/13 pathways in platelet fibrinogen receptor activation, b) protein kinase C delta isoform in thromboxane generation, and c) Gi pathways in Akt phosphorylation in platelets. We will test the hypothesis that G12/13 pathways contribute to dense granule release, TXA2 generation, and Akt phosphorylation and activation, using pharmacological approaches complemented with platelets from Galpha12 and Galpha13 gene knockout mice and constitutively active Galpha12 and Galpha13 transgenic mice. We will evaluate the relative contributions of Gq/PLC pathways and G12/13 pathways to agonist-induced dense granule release, TXA2 generation, and Akt phosphorylation. We also hypothesize that HAX-1, HS-1, and Src family kinases are activated downstream of G12/13 pathways, which play an important role in platelet activation. We will delineate some of these signaling molecules downstream of the G12/13 pathways, as this pathway is the least understood in platelets. Finally, we will evaluate the functional role of HS1 in platelets using mice-deficient in HS1 in ex vivo platelet functional studies and in vivo thrombosis models. We have strong preliminary data supporting each of the above specific aims. These studies will enhance our understanding of the signaling pathways and their role in platelet activation, and might identify potential newer targets for the treatment of thrombosis.