An Important Role for Rac1 in Glycoprotein Ib-IX Induced Platelet Activation,

Abstract 3250

The platelet receptor for von Willebrand factor, the glycoprotein Ib-IX (GPIb-IX) complex, not only plays a critical role in mediating platelet adhesion at sites of vascular injury, but also transmits signals leading to platelet activation. GPIb-IX mediated activation of the ligand binding function of integrin αIIbβ3 is required for stable platelet adhesion under high shear rate conditions. Previous studies have suggested that ligand occupancy of GPIb-IX leads to activation of the Src family kinase Lyn, phosphoinositide 3-kinase (PI3K), Akt, cGMP-dependent protein kinase, and the MAPK and ITAM signaling pathways. However, it remains unclear how the PI3K/Akt pathway is activated during GPIb-IX dependent signal transduction. Using platelet-specific conditional Rac1^−/− mice and the Rac1 specific inhibitor NSC23766 (NSC), we examined the role of Rac1 in GPIb-IX induced platelet activation. Platelet-specific conditional Rac1^−/− mice were generated by crossing Pf4-Cre transgenic mice with mice containing the Rac1 conditional allele. Rac1^−/− mouse platelets had a defect in VWF/botrocetin induced aggregation, TXA2 synthesis, and ATP secretion. Rac1^−/− mouse platelets were also defective in stable adhesion to VWF under shear stress and in spreading on immobilized VWF, as compared to WT. The defects observed in Rac1^−/− mouse platelets were recapitulated in human platelets treated with the Rac1 inhibitor NSC23766. Importantly, we show that ligand binding to GPIb-IX induced Rac1 activation, which was abolished in Lyn^−/− mouse platelets. We also show that Rac1^−/− (or NSC-treated) platelets have a defect in GPIb-IX induced Akt and P38 MAPK activation. Altogether, this study reveals that Rac1 plays an important role in GPIb-IX dependent platelet activation and stable adhesion to VWF under shear stress. Rac1 is activated downstream of Lyn and is important for GPIb-IX and Lyn-dependent activation of Akt and MAPK signaling pathways.