COLLAGEN RECEPTOR SIGNALING IN PLATELETS

在动脉粥样硬化斑块破裂和正常血管损伤期间，血小板暴露于胶原蛋白。暴露的胶原蛋白不仅作为直接的血小板激动剂，而且还为血小板提供粘附表面，从而促进血栓形成。血小板上的两种主要胶原受体是糖蛋白（GP） VI和α -2 β -1整合素。GPVI和α -2 β -1都是完全胶原介导的血小板粘附和激活所必需的，可能涉及GPVI介导的α ？2 β -1，我们的实验室和其他人已经证明了这一点。然而，GPVI或其他激动剂受体激活α -2 β -1的信号尚不清楚。与此相关，小gtpase Rap1和R-Ras都被认为是整合素激活的正调节因子，而Ras被认为是负调节因子。我们最近证明，血小板中的GPVI信号以依赖于P2Y12受体分泌的ADP激活以及P2Y12独立途径的方式导致Rap1激活。我们还利用可转导的原代小鼠巨核细胞系统提供了初步证据，表明Rap1可能促进α -2

Platelets become exposed to collagen during rupture of the atherosclerotic plaque and during normal vascular injury. The exposed collagen serves not only as a direct platelet agonist but also provides an adhesive surface to platelets, thus contributing to thrombosis. Two of the major collagen receptors on platelets are glycoprotein (GP) VI and the Alpha-2 Beta-1 integrin. GPVI and Alpha-2 Beta-1 are both required for full collagen mediated platelet adhesion and activation, likely involving a GPVI-mediated activation of Alpha?2 Beta-1 as demonstrated by our lab and others. However, signals leading to Alpha-2 Beta-1 activation by GPVI or other agonist receptors are not well understood. Related to this, the small GTPases Rap1 and R-Ras have each been proposed as positive regulators of integrin activation whereas Ras has been proposed as a negative regulator. We recently demonstrated that GPVI signaling in platelets leads to Rap1 activation in a manner dependent upon secreted ADP activation of the P2Y12 receptor as well as a P2Y12-independent pathway. We also provide preliminary evidence using a transducible primary mouse megakaryocyte system, that Rap1 may promote Alpha-2 Beta-1 activation. In separate studies we found that R-Ras promotes several Alpha-2 Beta-1 mediated events, and that Ras is present in platelets and activated by agonist stimulation. However, the interrelationship of Rap1 to R-Ras or Ras is not understood. In the present proposal, we aim to further define the communication between GPVI and Alpha-2 Beta-1 by first, clarifying the roles of Rap1 and R-Ras in GPVI-induced Alpha-2 Beta-1 activation, second, mapping upstream pathways leading to Rap1 activation with regard to the potential role of R-Ras and other molecules in this event, third, mapping signaling pathways downstream of activated Rap1 leading to Alpha-2 Beta-1 integrin activation, and finally, determining the role of Ras in regulating Alpha-2 Beta-1 integrin activation. Results from these studies will provide fundamental information on how these important collagen receptors communicate with one another via these small G-proteins in platelets and megakaryocytes.