COLLAGEN-STIMULATED ACTIVATION OF SYK BUT NOT C-SRC IS SEVERELY COMPROMISED IN HUMAN PLATELETS LACKING MEMBRANE GLYCOPROTEIN-VI

Activation of circulating platelets by subendothelial collagen is an e ssential event in vascular hemostasis. In human platelets, two membran e glycoprotein (GP) abnormalities, integrin alpha(2) beta(1) deficienc y and GPVI deficiency, have been reported to result in severe hyporesp onsiveness to fibrillar collagen. Although it has been well establishe d that integrin alpha(2) beta(1), also known as the GPIa-IIa complex, functions as a primary platelet adhesion receptor for collagen, the me chanism by which GPVI contributes to collagen-platelet interaction has been ill defined to date. However, our recent observation that GPVI c ross-linking couples to cyclic AMP-insensitive activation of c-Src and Syk tyrosine kinases suggested a potential role for GPVI in regulatin g protein-tyrosine phosphorylation by collagen (Ichinohe, T., Takayama , H., Ezumi, Y., Yanagi, S., Yamamura, H., and Okuma, M. (1995) J. Bio l. Chem. 270, 28029-28036). To further investigate this hypothesis, he re we examined the collagen-induced protein-tyrosine phosphorylation i n GPVI-deficient platelets expressing normal amounts of alpha(2) beta( 1). In response to collagen, these platelets exhibited alpha(2) beta(1 )-dependent c-Src activation accompanied by tyrosine phosphorylation o f several substrates including cortactin. In contrast, severe defects were observed in collagen-stimulated Syk activation and tyrosine phosp horylation of phospholipase C-gamma 2, Vav, and focal adhesion kinase, implicating a specific requirement of GPVI for recruiting these molec ules to signaling cascades evoked by collagen-platelet interaction.