A PLCγ2-independent platelet collagen aggregation requiring functional association of GPVI and integrin α2β1

The role of the phospholipase C (PLC)gamma2 isotype in platelet activation was evaluated by studying PLCgamma2 -/- mice. These mice have a prolonged bleeding time but their platelets respond normally to non-collagenous agonists. PLCgamma2-null platelets show residual aggregation response to collagen fibres (6% versus 74% for wild-type) with minimal granule secretion and no shape change. A delayed shape change is observed at later aggregation times. Specific activation by glycoprotein (GP)VI agonists (convulxin, collagen-related peptide and GPVI crosslinking) is, however, abolished. Antibodies against integrin alpha(2)beta(1) and GPVI each inhibit the residual collagen response, implying a role of alpha(2)beta(1) in platelet activation and a functional association with GPVI. These responses are also prevented by blocking integrin alpha(IIb)beta(3) and phosphoinositide 3-kinase, whereas aspirin treatment and ADP receptor blockade only, inhibit shape change. These results provide evidence for a PLCgamma2-independent collagen activation pathway requiring cooperation between GPVI and alpha(2)beta(1) leading to alpha(IIb)beta(3)-dependent aggregation and shape change by released ADP and thromboxane A(2). (C) 2003 Federation of European Biochemical Societies. Published by Elsevier Science B.V. All rights reserved.