Characterization of coagulation factor-platelet interactions: role of FXI

描述(由申请人提供)：凝血因子XI(FXI)水平升高是深静脉血栓形成、缺血性中风和心肌梗死的独立危险因素。遗传性FXI缺乏会导致轻微的出血倾向，但也被发现对缺血性中风具有保护作用。FXI已被证明在实验性血栓形成中起关键作用，在动物血栓形成模型中，FXI的基因缺失或药物抑制可以防止血管闭塞。虽然这些发现暗示了FXI在血栓形成中的重要作用，以及在止血方面的可能作用，但它们并没有暗示FXI对(病理性)生理性凝血的不同贡献的分子机制。血小板是主要止血的基本细胞成分。FXI已被证明通过血小板膜糖蛋白(GP)Ib-IX-V复合体与血小板表面特异性结合；然而，这种相互作用的功能意义尚不清楚。此外，尚不清楚血流引起的剪切力是否在调节FXI-血小板结合中发挥作用。有人认为GPIB通过凝血酶调节FXI的激活；然而，围绕这一假说有相当大的争议。此外，虽然凝血酶、激活的FXI(FXIa)或激活的因子XII(FXIIa)已被证明在体外纯化系统中激活FXI，但这些FXI激活剂在体外或体内对血小板表面的相对重要性尚未确定。我们提出的研究的目的是阐明FXI-血小板相互作用的机制，并进一步深入了解FXI在正常止血和病理性凝血中的生理作用。我们假设在剪切流动条件下，FXI与血小板的相互作用促进了血栓的形成。我们已经确定FXI是血小板载脂蛋白E受体2(ApoER2)的配体。我们推测，ApoER2在介导FXI-血小板结合和启动血流下的凝血过程中起着关键作用。这些假说将通过以下具体目标进行检验：目标1：确定血小板-FXI相互作用的分子机制和后果。目的2：确定FXI与血小板结合在体外血栓形成中的作用。目的3：确定FXI与血小板结合在体内血栓形成中的作用。我们致力于新型抗血栓FXI抑制剂的设计和开发。我们认为，鉴于与FXI缺乏相关的强大的抗血栓疗效和轻微的出血素质，治疗性抑制FXI轴是对抗病理性血栓形成的一种有前景的治疗策略。这一研究的最终目的是建立关于FXI-血小板相互作用的有价值的机制信息，并进一步深入了解FXI在正常止血和病理性凝血中的生理作用。

DESCRIPTION (provided by applicant): Elevated coagulation factor XI (FXI) level is an independent risk factor for deep vein thrombosis, ischemic stroke, and myocardial infarction. Inherited FXI-deficiency causes mild bleeding tendency, yet it has also been found to be protective against ischemic stroke. FXI has been shown to play a critical role in the formation of experimental thrombi, as evidenced by the fact that genetic deletion or pharmacological inhibition of FXI prevents vascular occlusions in animal thrombosis models. While these findings implicate an important role for FXI in thrombosis, and a possible role in hemostasis, they do not suggest molecular mechanisms by which FXI differentially contributes to (patho)physiological coagulation. Blood platelets are the essential cellular components of primary hemostasis. FXI has been shown to bind specifically to the platelet surface via the platelet glycoprotein (GP) Ib-IX-V complex; however, the functional significance of this interaction is unclear. Moreover, it is unknown whether shear forces due to blood flow play a role in regulating FXI-platelet binding. It has been suggested that GPIb orchestrates the activation of FXI through the protease thrombin; however, there is considerable controversy surrounding this hypothesis. Moreover, while thrombin, activated FXI (FXIa), or activated factor XII (FXIIa), has been shown to activate FXI in purified systems in vitro, the relative importance of these FXI activators on the platelet surface ex vivo or in vivo has not been established. The objectives of our proposed studies are to elucidate the mechanisms of FXI-platelet interactions and to provide further insight into the physiological role of FXI in normal hemostasis and pathologic coagulation. We hypothesize that FXI-platelet interactions promote clot formation under shear flow conditions. We have identified FXI as a ligand for the platelet apolipoprotein E receptor 2 (ApoER2). We hypothesize that ApoER2 plays a critical role in mediating FXI-platelet binding and in initiating coagulation under flow. These hypotheses will be tested through the following specific aims: Aim 1: Determine the molecular mechanisms and consequences of platelet-FXI interactions. Aim 2: Determine the role of FXI-platelet binding in ex vivo thrombus formation. Aim 3: Determine the role of FXI-platelet binding in thrombus formation in vivo. We are committed to the design and development of novel antithrombotic FXI inhibitors. We believe that therapeutic inhibition of the FXI axis is a promising therapeutic strategy to combat pathological thrombus formation, in light of the strong antithrombotic efficacy and mild bleeding diathesis associated with FXI deficiency. The ultimate goal of this line of research is to establish valuable mechanistic information concerning FXI-platelet interactions and to provide further insight into the physiological role of FXI in normal hemostasis and pathologic coagulation.