Functional analysis of a recombinant glycoprotein Ia/IIa (integrin α₂β₁) I domain that inhibits platelet adhesion to collagen and endothelial matrix under flow conditions

The interaction of platelets with collagen plays an important role in primary hemostasis. Glycoprotein Ia/IIa (GPIa/IIa, integrin α₂β₁) is a major platelet receptor for collagen. The binding site for collagen has been mapped to the I domain within the α₂ subunit (GPIa). In order to assess the role of the α₂-I domain structure in GPIa/IIa binding to collagen, a recombinant I domain (amino acids 126-337) was expressed in Escherichia coli. The a₂-I protein bound human types I and III collagen in a saturable and divalent cation-dependent manner and was blocked by the α₂β₁ function blocking antibody 6F1. The α₂-I protein inhibited collagen-induced platelet aggregation (IC₅₀ = 600 nM). Unexpectedly, 6F1, an antibody that fails to inhibit platelet aggregation in platelet-rich plasma, blocked the inhibitory effect of the α2-I protein. The α2-I protein was able to prevent platelet adhesion to a collagen surface exposed to flowing blood under low shear stress. Interestingly, it inhibited platelet adhesion to extracellular matrix at high shear stress. These results, taken together, provide firm evidence that GPIa/IIa directly mediates the first contact of platelets with collagen under both stirring and flow conditions.