Expression of the platelet receptor GPVI confers signaling via the Fc receptor gamma-chain in response to the snake venom convulxin but not to collagen

The mechanism of signal transduction underlying the activation of platelets by collagen has been actively investigated for over 30 years, but the rece ptors involved remain incompletely understood. Studies of human platelets, which are unresponsive to collagen, mouse knockout models, and platelet bio chemical studies support the hypothesis that the recently cloned platelet s urface protein GPVI functions as a signaling receptor for collagen. To dire ctly test this hypothesis, we have expressed wild-type and mutant forms of GPVI in RBL-2H3 cells, which express the Fee receptor gamma -chain (Fc R ga mma), the putative signaling co-receptor for GPVI in platelets, but lack GP VI itself. Expression of GPVI in RBL-2H3 cells confers strong adhesive and signaling responses to convulxin (a snake venom protein that directly binds GPVI) and weak responsiveness to collagen-related peptide but no responsiv eness to collagen, To elucidate the mechanism of GPVI intracellular signali ng, mutations were introduced in the receptor's transmembrane domain and C- terminal tail. Unlike reported studies of other Fc R gamma partners, these studies reveal that both the GPVI transmembrane arginine and intracellular C-tail are necessary for coupling to Fc R gamma and for signal transduction . To our knowledge, these studies are the first to demonstrate a direct sig naling role for GPVI and the first to directly test the role of GPVI as a c ollagen receptor. Our results suggest that GPVI may be necessary but not su fficient for collagen signaling and that a distinct ligand-binding collagen receptor such as the alpha (2)beta (1) integrin is likely to play a necess ary role for collagen signaling as well as adhesion in platelets.