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

Summary. 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.