Department of Cardiology and Cardiovascular Medicine, University Clinic of Tübingen, Germany.
Cell Signal. 2013 Jan;25(1):236-46. doi: 10.1016/j.cellsig.2012.09.005. Epub 2012 Sep 10.
Cytoskeletal reorganization of activated platelets plays a crucial role in hemostasis and thrombosis and implies activation of Rho GTPases. Rho GTPases are important regulators of cytoskeletal dynamics and function as molecular switches that cycle between an inactive and an active state. They are regulated by GTPase activating proteins (GAPs) that stimulate GTP hydrolysis to terminate Rho signaling. The regulation of Rho GTPases in platelets is not explored. A detailed characterization of Rho regulation is necessary to understand activation and inactivation of Rho GTPases critical for platelet activation and aggregation. Nadrin is a RhoGAP regulating cytoplasmic protein explored in the central nervous system. Five Nadrin isoforms are known that share a unique GAP domain, a serine/threonine/proline-rich domain, a SH3-binding motif and an N-terminal BAR domain but differ in their C-terminus. Here we identified Nadrin in platelets where it co-localizes to actin-rich regions and Rho GTPases. Different Nadrin isoforms selectively regulate Rho GTPases (RhoA, Cdc42 and Rac1) and cytoskeletal reorganization suggesting that - beside the GAP domain - the C-terminus of Nadrin determines Rho specificity and influences cell physiology. Furthermore, Nadrin controls RhoA-mediated stress fibre and focal adhesion formation. Spreading experiments on fibrinogen revealed strongly reduced cell adhesion upon Nadrin overexpression. Unexpectedly, the Nadrin BAR domain controls Nadrin-GAP activity and acts as a guidance domain to direct this GAP to its substrate at the plasma membrane. Our results suggest a critical role for Nadrin in the regulation of RhoA, Cdc42 and Rac1 in platelets and thus for platelet adhesion and aggregation.
激活血小板的细胞骨架重排在止血和血栓形成中起着至关重要的作用，并暗示了 Rho GTPases 的激活。Rho GTPases 是细胞骨架动力学的重要调节剂，作为分子开关，在非活性和活性状态之间循环。它们受 GTPase 激活蛋白（GAPs）的调节，GAPs 刺激 GTP 水解以终止 Rho 信号。血小板中 Rho GTPases 的调节尚未得到探索。详细描述 Rho 调节对于理解血小板激活和聚集所必需的 Rho GTPases 的激活和失活是必要的。Nadrin 是一种在中枢神经系统中探索的调节细胞质蛋白的 RhoGAP。已知有 5 种 Nadrin 同工型，它们共享独特的 GAP 结构域、丝氨酸/苏氨酸/脯氨酸丰富的结构域、SH3 结合基序和 N 端 BAR 结构域，但在 C 端不同。在这里，我们在血小板中鉴定出 Nadrin，它与富含肌动蛋白的区域和 Rho GTPases 共定位。不同的 Nadrin 同工型选择性地调节 Rho GTPases（RhoA、Cdc42 和 Rac1）和细胞骨架重排，表明 - 除了 GAP 结构域外 - Nadrin 的 C 端决定了 Rho 的特异性并影响细胞生理学。此外，Nadrin 控制 RhoA 介导的应力纤维和焦点粘连的形成。在纤维蛋白原上进行的扩展实验表明，Nadrin 过表达后细胞黏附能力显著降低。出乎意料的是，Nadrin BAR 结构域控制 Nadrin-GAP 活性，并作为导向结构域将该 GAP 导向质膜上的其底物。我们的结果表明，Nadrin 在调节血小板中的 RhoA、Cdc42 和 Rac1 中起着关键作用，因此在血小板黏附和聚集中起着关键作用。
