Mechanisms and consequences of agonist-induced talin recruitment to platelet integrin αIIbβ3

Platelet aggregation requires agonist-induced alpha IIb beta 3 activation, a process mediated by Rap1 and talin. To study mechanisms, we engineered alpha IIb beta 3 Chinese hamster ovary (CHO) cells to conditionally express talin and protease-activated receptor ( PAR) thrombin receptors. Human PAR1 or murine PAR4 stimulation activates alpha IIb beta 3, which was measured with antibody PAC-1, indicating complete pathway reconstitution. Knockdown of Rap1 guanosine triphosphate-interacting adaptor molecule (RIAM), a Rap1 effector, blocks this response. In living cells, RIAM overexpression stimulates and RIAM knockdown blocks talin recruitment to alpha IIb beta 3,which is monitored by bimolecular fluorescence complementation. Mutations in talin or beta 3 that disrupt their mutual interaction block both talin recruitment and alpha IIb beta 3 activation. However, one talin mutant (L325R) is recruited to alpha IIb beta 3 but cannot activate it. In platelets, RIAM localizes to filopodia and lamellipodia, and, in megakaryocytes, RIAM knockdown blocks PAR4-mediated alpha IIb beta 3 activation. The RIAM-related protein lamellipodin promotes talin recruitment and alpha IIb beta 3 activity in CHO cells but is not expressed in megakaryocytes or platelets. Thus, talin recruitment to alpha IIb beta 3 by RIAM mediates agonist-induced alpha IIb beta 3 activation, with implications for hemostasis and thrombosis.