Key Points• GPVI interaction with polymerized fibrin triggers a new loop amplifying thrombin generation and platelet recruitment at the clot surface.Fibrin, the coagulation end product, consolidates the platelet plug at sites of vascular injury and supports the recruitment of circulating platelets. In addition to integrin aIIbb3, another as-yet-unidentified receptor is thought to mediate platelet interaction with fibrin. Platelet glycoprotein VI (GPVI) interacts with collagen and several other adhesive macromolecules. We evaluated the hypothesis that GPVI could be a functional platelet receptor for fibrin. Calibrated thrombin assays using platelet-rich plasma (PRP) showed that tissue factor-triggered thrombin generation was impaired in GPVI-deficient patients and reduced by the anti-GPVI Fab 9O12. Assays on reconstituted PRP and PRP from fibrinogen-deficient patients revealed a fibrinogen-dependent enhancement of thrombin generation, which relied on functional GPVI. The effect of GPVI was found to depend on fibrin polymerization. A binding assay showed a specific interaction between GPVI-Fc and fibrin, inhibited by the Fab 9O12. This Fab also reduced platelet adhesion to fibrin at low (300 s 21) and high (1500 s 21 ) wall shear rates. Platelets adherent to fibrin displayed shape change, exposure of procoagulant phospholipids, and the formation of small clots. When hirudinated blood was perfused at 1500 s 21 over preformed fibrin-rich clots, the Fab 9O12 decreased the recruitment of platelets by up to 85%. This study identifies GPVI as a platelet receptor for polymerized fibrin with 2 major functions: (1) amplification of thrombin generation and (2) recruitment of circulating platelets to clots. These so-farunrecognized properties of GPVI confer on it a key role in thrombus growth and stabilization. (Blood. 2015;126(5):683-691)

Objective-The immune receptor homologue glycoprotein VI (GPVI)/FcR receptor ␥ chain complex is primarily responsible for platelet activation by collagen. There is growing evidence that optimal binding of GPVI to collagen depends on the assembly of GPVI dimers. The valence of GPVI on resting platelets needs to be clearly established because platelet avidity for collagen would be greater if GPVI is constitutively expressed as a dimer than as a monomer. Methods and Results-Using a monoclonal antibody (9E18) that preferentially binds to GPVI dimers, we found that GPVI was maintained in a monomeric form on human resting platelets under the control of intraplatelet cAMP concentration. Activation by soluble agonists or von Willebrand factor induced a shift toward GPVI dimerization related to increased platelet adhesion to collagen. A correlation between platelet binding of 9E18 and P-selectin exposure was observed in patients experiencing coronary artery disease, and antagonists of the ADP receptor P2Y 12 limited ADP-induced GPVI dimerization. Key Words: antiplatelet drugs Ⅲ collagen Ⅲ monoclonal antibodies Ⅲ platelets Ⅲ glycoprotein VI P latelet adhesion at sites of vascular damage is required for normal hemostasis. Circulating platelets adhere to proteins of the subendothelial matrix exposed by vessel injury in a process involving several receptors. Collagen fibers are highly thrombogenic, and the platelet glycoprotein VI (GPVI) predominantly mediates collagen-induced platelet responses. Conclusion-TheGPVI is a platelet-specific receptor of the immunoglobulin (Ig) superfamily containing 2 extracellular Ig domains, a single transmembrane domain, and a short cytoplasmic tail. [1][2][3] GPVI shares with other receptors of the same family the particularity that the extracellular ligand-binding domain and the intracellular signaling domain are on separate subunits. GPVI signals through the noncovalently associated immune receptor adaptor FcR␥. 4 The receptor is assembled via a transmembrane interaction between Asp11 in the FcR␥ homodimer and Arg273 of GPVI. On stimulation, the Tyr residues of the immunoreceptor tyrosine-based activation motif of FcR␥ are phosphorylated in an early, obligatory event triggering the signaling cascade. See accompanying article on page 552There is growing evidence that optimal binding of GPVI to collagen depends on the formation of GPVI dimers at the platelet surface. Miura et al, 5 using recombinant proteins, were the first to report that collagen binds to the dimeric form but not to the monomeric form of GPVI and that only the former was able to inhibit collagen-induced platelet activation. Crystallographic data showing dimerization of GPVI ectodomains, 6 studies using synthetic peptides with differentially spaced GPVI binding motifs to activate the receptor in platelets, 7 and studies using chemical cross-linking agents 8 have strongly reinforced the notion that GPVI functions as a dimer. However, the valence of GPVI on resting and on activated platelets is still a matter of debate. It was ...