Collagen-stimulated activation of Syk but not c-Src is severely compromised in human platelets lacking membrane glycoprotein VI

Abstract

Activation of circulating platelets by subendothelial collagen is an essential event in vascular hemostasis. In human platelets, two membrane glycoprotein (GP) abnormalities, integrin α2β1 deficiency and GPVI deficiency, have been reported to result in severe hyporesponsiveness to fibrillar collagen. Although it has been well established that integrin α2β1, also known as the GPIa-IIa complex, functions as a primary platelet adhesion receptor for collagen, the mechanism by which GPVI contributes to collagen-platelet interaction has been ill defined to date. However, our recent observation that GPVI cross-linking couples to cyclic AMP-insensitive activation of c-Src and Syk tyrosine kinases suggested a potential role for GPVI in regulating protein-tyrosine phosphorylation by collagen (Ichinohe, T., Takayama, H., Ezumi, Y., Yanagi, S., Yamamura, H., and Okuma, M. (1995) J. Biol. Chem. 270, 28029-28036). To further investigate this hypothesis, here we examined the collagen-induced protein-tyrosine phosphorylation in GPVI-deficient platelets expressing normal amounts of α2β1. In response to collagen, these platelets exhibited α2β1-dependent c-Src activation accompanied by tyrosine phosphorylation of several substrates including cortactin. In contrast, severe defects were observed in collagen-stimulated Syk activation and tyrosine phosphorylation of phospholipase C-γ2, Vav, and focal adhesion kinase, implicating a specific requirement of GPVI for recruiting these molecules to signaling cascades evoked by collagen-platelet interaction.