Post-Doc Fellowship Binggang Xiang: cGMP regulates platelets activation

Platelets play a central role in physiological process of hemostasis and in development of pathologic thrombus formation. At sites of vascular injury, exposure of platelets to soluble agonists or subendothelial adhesive proteins triggers platelet activation, leading to thrombus formation. However, under physiological conditions, this process is tightly regulated, limiting platelet activation to ensure that the response to injury halts bleeding without causing vascular occlusion. As an important second messenger, cGMP has been shown to contribute to both agonist-induced platelet activation and to nitric oxide (NO) donor-induced platelet inhibition. Therefore, understanding of the role of cGMP signaling in platelets may provide novel insights into how platelet activation is regulated. The overall goal of this proposal is to identify the function of the key element of the cGMP signaling in platelets, the guanylyl cyclases (GC), in regulating platelet activation and in vivo thrombosis and hemostasis. Platelet cGMP is believed to be synthesized predominantly by soluble guanylyl cyclase (sGC) that is activated by NO. Therefore, we generated platelet-specific sGC conditional knockout mice to investigate the role of sGC in platelet activation. Platelets lacking sGC have a defect in aggregation in response to agonists, but are resistant to NO donor-induced inhibition. cGMP production in response to agonists is diminished but not totally abolished in sGC deficient platelets. These results demonstrate an important role of sGC in agonist-induced cGMP production, but also suggest a sGC-independent alternative pathway contributing to the agonist-induced cGMP synthesis in platelets. Furthermore, we found that a membrane-bound GC, the natriuretic peptide receptor-A (NPRA), is functionally expressed in platelets. Therefore, we further hypothesize that both sGC and NPRA contribute to the elevation in intra-platelet cGMP levels and subsequent platelet activation. We will test these hypotheses in the following specific aims: 1. To demonstrate the role of sGC in the regulation of platelet activation and in thrombosis and hemostasis in vivo. 2. To elucidate the role of a membrane-bound GC, NPRA, in platelet activation and thrombosis. Evidence obtained from these studies should provide novel information on the roles of sGC and NPRA in platelet activation in vitro and in thrombosis and hemostasis in vivo.