Contribution of PAR-1, PAR-4 and GPIbα in intracellular signaling leading to the cleavage of the β3 cytoplasmic domain during thrombin-induced platelet aggregation

Abstract

SummaryIntegrin αIIbβ3 plays a pivotal role in platelet aggregation by binding to fibrinogen. The β3 cytoplasmic domain of αIIbβ3 interacts with cytoskeletal and signaling proteins and is cleaved by µ-calpain, a calcium regulated cysteine protease. In the present study, we have investigated in more detail the cleavage of the β3 cytoplasmic domain during platelet aggregation induced by thrombin, TRAP-1 and TRAP-4. Our data show that β3 is cleaved in all three cases. The time course of β3 cleavage and the amount of cleaved β3 depends on the way platelets are activated and on the complete activation of µ-calpain, with a maximum of 90% of cleaved β3 obtained when thrombin is used. Furthermore, our results also show that the cleaved αIIbβ3 is mainly distributed in the Triton soluble fraction, indicating its inability to bind to the cytoskeleton. Interestingly, in the absence of GPIbα or following inhibition of thrombin binding to GPIbα, there is a reduction in the thrombin-induced calcium flux, β3 cleavage and µ-calpain activation. These results suggest that cleavage of the β3 cytoplasmic domain by µ-calpain might be an important step regulating the link between the cytoskeleton and αIIbβ3 during platelet aggregation, and that GPIbα could function as a cofactor for the complete activation of platelets by thrombin.