Thrombin binding to GPIbα induces platelet aggregation and fibrin clot retraction supported by resting αIIbβ3 interaction with polymerized fibrin

We have investigated the mechanisms leading to platelet aggregation following thrombin interaction with the glycoprotein (GP) Ib-IX-V complex. We show that platelets desensitized for the two thrombin receptors, PAR-1 and PAR-4, are still able to aggregate in response to thrombin and that this aggregation can be inhibited by a monoclonal antibody (VM16d) that blocks thrombin binding to GPIbalpha, or by pretreatment of platelets with Mocarhagin, a protease that specifically cleaves GPIbalpha. The thrombin/GPIbalpha-initiated signaling cascade induces platelet shape change through activation of the Rho kinase p160ROCK, independent of calcium mobilization, transient MEK-1 phosphorylation as well as the cleavage of talin through a calcium-independent mechanism. This signaling cascade does not induce the exposure of high affinity alphaIIbbeta3 integrin receptors, nor does it lead to micro -calpain cleavage of filamin or the integrin cytoplasmic tail. In contrast, we provide evidence that binding of thrombin to GPIbalpha induces fibrin binding to resting alphaIIbbeta3 leading to fibrin-dependent platelet aggregation and clot retraction, that can be selectively inhibited by alphaIIbbeta3 antagonists such as RGDS, the dodecapeptide or lamifiban, as well as by the fibrin polymerization inhibitor GPRP-amide.