Cytoskeletal reorganization is crucial for platelet adhesion and thrombus formation to avoid excessive bleeding. Major regulators of cytoskeletal dynamics are small GTPases of the Rho family. Rho GTPases become activated by G-protein coupled receptor activation, downstream of immunoreceptor tyrosine-based activation motif (ITAM)-coupled receptors and by outside-in signaling of integrins. They act as molecular switches and cycle between active and inactive states. GTPase activating proteins (GAPs) stimulate the hydrolysis of GTP to GDP to terminate Rho signaling. Nadrin is a RhoGAP that was recently identified in platelets. Five Nadrin isoforms are known consisting of a unique GAP and an N-terminal BAR domain responsible for the selective regulation of RhoA, Cdc42 and Rac1. Besides BAR domain mediated regulation of Nadrin GAP activity nothing is known about the regulation of Nadrin and the impact on cytoskeletal reorganization. Here we show that Nadrin becomes tyrosine phosphorylated upon platelet activation. We found Src family proteins (Src, Lyn, Fyn) to be responsible to control Nadrin GAP activity by phosphorylation. Interestingly, phosphorylation of Nadrin leads to tightly regulated Rho activation that was found to be Nadrin isoform- and (Rho) target-specific. Src-phosphorylation of Nadrin5 mediated inactivation of Cdc42 while RhoA and Rac1 became activated upon Src-mediated phosphorylation of Nadrin2. Our results suggest a critical role for spatial and temporal regulation of Nadrin and thus for the control of Rho GTPases in platelets.
Keywords: Cytoskeleton; GTPase activating protein; Nadrin; Platelets; Rho GTPases; Tyrosine phosphorylation.
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