Increased platelet activation and thrombosis in transgenic mice expressing constitutively active P2Y12

. Background: In our previous in vitro study, we reported a constitutively active chimeric P2Y12 (cP2Y12) and found that AR-C78511 is a potent inverse agonist at this receptor. The role of cP2Y12 in platelet activation and thrombosis is not clear. Objectives: To investigate the physiologic implications of cP2Y12 for platelet activation and thrombus formation, and to evaluate the antiplatelet activity of AR-C78511 as an inverse agonist. Methods and Results: We generated transgenic mice conditionally and platelet-specifically expressing cP2Y12. High-level expression of cP2Y12 in platelets increased platelet reactivity, as shown by increased platelet aggregation in response to multiple platelet agonists. Moreover, transgenic mice showed a shortened bleeding time, and more rapid and stable thrombus formation in mesenteric artery injured with FeCl3. The constitutive activity of cP2Y12 in platelets was confirmed by decreased platelet cAMP levels and constitutive Akt phosphorylation in the absence of agonists. AR-C78511 reversed the cAMP decrease in transgenic mouse platelets, and exhibited a superior antiplatelet effect to that of AR-C69931MX in transgenic mice. Conclusions: These findings further emphasize the importance of P2Y12 in platelet activation, hemostasis, and thrombosis, as well as the prothrombotic role of the constitutive activity of P2Y12. Our data also validate the in vivo inverse agonist activity of AR-C78511, and confirm its superior antiplatelet activity over neutral antagonists.