INTERACTION OF INTEGRIN ALPHA(IIB)BETA(3) WITH MULTIPLE FIBRINOGEN DOMAINS DURING PLATELET-ADHESION

We have investigated how modulation of integrin alpha(IIb)beta(3) function influences the mechanisms that initiate platelet thrombus formation onto surface-bound fibrinogen and isolated fibrinogen domains, Under stationary conditions and with full activation of platelets blocked by prostaglandin E(1), the carboxyl-terminal gamma(400-411) sequence is necessary for establishing initial contact with the immobilized substrate. Molecules containing a single copy of this sequence, like the plasmin-generated fibrinogen fragment D, support platelet spreading, but the resulting attachment to the surface is loose and disrupted by minimal peeling force, In contrast, platelets adhere firmly to intact fibrinogen under the same conditions, suggesting that recognition of contact sites outside a single D domain can secure the firm interaction not supported by a single gamma(400-411) sequence. If platelets are activated, the gamma(400-411) sequence is no longer necessary to initiate the adhesion process but becomes sufficient, even as a single copy, to mediate stable surface attachment in the absence of shear stress, Under conditions of flow, however, intact fibrinogen but not fragment D can support adhesion, regardless of whether platelets have the potential to become activated or not, These results indicate the functional relevance of multiple fibrinogen domains during the initial stages of the platelet adhesion process.