A central role of GPIb-IX in the procoagulant function of platelets that is independent of the 45-kDa GPIbalpha N-terminal extracellular domain.

Activated platelets become procoagulant and efficiently promote the conversion of prothrombin to thrombin. A role of the GPIb-V-IX complex has long been postulated in view of the decreased prothrombin consumption in Bernard-Soulier patients. We evaluated the impact of GPIb-V-IX deficiency and the requirement for the GPIbalpha extracellular domain. In GPIbbeta(-/-) mice, thrombin generation was profoundly decreased in tissue factor- or collagen-related peptide (CRP)-activated platelet-rich plasma and in washed platelets supplemented with normal plasma or with FVa, FXa, and prothrombin. Phosphatidylserine (PS) exposure was similarly decreased in response to thrombin, CRP, or CRP + PAR4 peptide despite a normal platelet phospholipid composition. The hypothesis that these defects originate from lack of the GPIbalpha N-terminal domain was evaluated after its removal from normal mouse and human platelets with Nk protease or O-sialoglycoprotein endopeptidase. Unexpectedly, the treated platelets exhibited normal thrombin generation and PS exposure, indicating that GPIb-V-IX regulates procoagulant activity independently of its GPIbalpha-binding region. These results suggested a more general structuring role through intracellular cytoskeleton-anchoring portions regulating responses leading to PS exposure. This hypothesis was supported by the decreased calcium mobilization observed in GPIbbeta(-/-) platelets in response to several agonists, some acting independently of GPIb, in contrast to the normal calcium responses in Nk protease-treated platelets.