Department of Biomedical Engineering, School of Medicine, Oregon Health & Science University, Portland, Oregon.
Division of Hematology-Medical Oncology, Knight Cancer Institute, Oregon Health & Science University, Portland, Oregon.
Am J Physiol Cell Physiol. 2021 Mar 1;320(3):C365-C374. doi: 10.1152/ajpcell.00056.2020. Epub 2021 Jan 20.
Factor XI (FXI) has been shown to bind platelets, but the functional significance of this observation remains unknown. Platelets are essential for hemostasis and play a critical role in thrombosis, whereas FXI is not essential for hemostasis but promotes thrombosis. An apparent functional contradiction, platelets are known to support thrombin generation, yet platelet granules release protease inhibitors, including those of activated FXI (FXIa). We aim to investigate the secretory and binding mechanisms by which platelets could support or inhibit FXIa activity. The presence of platelets enhanced FXIa activity in a purified system and increased coagulation Factor IX (FIX) activation by FXIa and fibrin generation in human plasma. In contrast, platelets reduced the activation of FXI by activated coagulation factor XII (FXIIa) and the activation of FXII by kallikrein (PKa). Incubation of FXIa with the platelet secretome, which contains FXIa inhibitors, such as protease nexin-II, abolished FXIa activity, yet in the presence of activated platelets, the secretome was not able to block the activity of FXIa. FXIa variants lacking the anion-binding sites did not alter the effect of platelets on FXIa activity or interaction. Western blot analysis of bound FXIa [by FXIa-platelet membrane immunoprecipitation] showed that the interaction with platelets is zinc dependent and, unlike FXI binding to platelets, not dependent on glycoprotein Ib. FXIa binding to the platelet membrane increases its capacity to activate FIX in plasma likely by protecting it from inhibition by inhibitors secreted by activated platelets. Our findings suggest that an interaction of FXIa with the platelet surface may induce an allosteric modulation of FXIa.
因子 XI（FXI）已被证明能与血小板结合，但这一观察结果的功能意义尚不清楚。血小板对于止血至关重要，在血栓形成中起着关键作用，而 FXI 对于止血并非必需，但却促进血栓形成。这是一个明显的功能矛盾，因为血小板已知能支持凝血酶的生成，但血小板颗粒释放蛋白酶抑制剂，包括激活的 FXI（FXIa）的抑制剂。我们旨在研究血小板支持或抑制 FXIa 活性的分泌和结合机制。在一个纯化系统中，血小板的存在增强了 FXIa 活性，并增加了人血浆中凝血因子 IX（FIX）被 FXIa 激活和纤维蛋白生成。相比之下，血小板减少了激活的凝血因子 XII（FXIIa）对 FXI 的激活和激肽释放酶（PKa）对 FXII 的激活。将 FXIa 与含有 FXIa 抑制剂（如蛋白酶 nexin-II）的血小板分泌组孵育，可消除 FXIa 活性，但在激活的血小板存在下，分泌组无法阻止 FXIa 的活性。缺乏阴离子结合位点的 FXIa 变体不会改变血小板对 FXIa 活性或相互作用的影响。通过 FXIa-血小板膜免疫沉淀法进行的结合 FXIa 的 Western blot 分析表明，与血小板的相互作用依赖于锌，并且与 FXI 与血小板的结合不同，不依赖于糖蛋白 Ib。FXIa 与血小板膜的结合增加了其在血浆中激活 FIX 的能力，这可能是通过防止其被激活的血小板分泌的抑制剂抑制。我们的研究结果表明，FXIa 与血小板表面的相互作用可能诱导 FXIa 的变构调节。
