Involvement of activated integrin alpha(2)beta(1) in the firm adhesion of platelets onto a surface of immobilized collagen under flow conditions

Recently, we demonstrated that. agonist-induced activation of the platelet surface collagen-receptor integrin alpha(1)beta(2) converts it to an active form that can bind soluble collagen with high affinity (Jung, SM, Moroi, M : J Biol Chem 1998; 273: 14827-37). Here, the involvement of alpha(2)beta(1 ) activation and the high affinity binding property of activated alpha(2)be ta(1) in platelet adhesion to a collagen surface under flow conditions were analyzed. Platelet adhesion to immobilized collagen was measured in the pr esence of TS2/16, an activating anti-integrin alpha(2)beta(1) antibody, and inhibiting antibodies, Gi9 and 6F1. TS2/16 decreased the moving velocity o f platelets on the collagen surface, but Gi9 and 6F1 increased it, indicati ng that alpha(2)beta(1) activation induces the tight binding of platelets t o immobilized collagen under flow. Platelet adhesion, expressed as the surf ace area occupied by adhered platelets, in the presence of TS2/16 was simil ar to that in its absence. In contrast, adding Gi9 or 6F1 caused biphasic a dhesion composed of a first phase, a lag phase whose length differed in eac h experiment, and a second phase adhesion with a rate similar to that of th e control. This biphasic adhesion indicates that alpha(2)beta(1) activity i s inhibited and also suggests that some other factor(s) may contribute to t he adhesion under flow. At concentrations where neither 6F1 nor Gi9 affecte d collagen-induced aggregation, these antibodies inhibited soluble collagen binding to thrombin-activated platelets. Only at much higher concentration did 6F1 inhibit collagen-induced aggregation. TS2/16 had no effect on the aggregation. The present results are evidence against the major involvement of integrin alpha(2)beta(1) in platelet aggregation; instead, they indicat e that integrin alpha(2)beta(1), would be mainly associated with the tight binding of platelets to collagen.