Active GPIIb-IIIa conformations that link ligand interaction with cytoskeletal reorganization

Glycoprotein (GP) IIb-IIIa plays a critical role in platelet aggregation an d platelet-mediated clot retraction. This study examined the intramolecular relationship between GPIIb-IIIa activation and fibrinogen binding, platele t aggregation, and platelet-mediated clot retraction. To distinguish betwee n different high-affinity activation states of GPIIb-IIIa, the properties o f an antibody (D3) specific for GPIIIa that induces GPIIb-IIIa binding to a dhesive protein molecules and yet completely inhibits clot retraction were used. Clot retraction inhibition by D3 was not due to altered platelet-fibr in interaction; however, combination treatments of D3 and adenosine diphosp hate (ADP) inhibited full-scale aggregation and decreased the amounts of GP IIb-IIIa and talin incorporated into the core cytoskeletons. Morphologic ev aluation of the D3/ADP aggregates showed platelets that were activated but to a lesser extent when compared to ADP only. ADR addition to platelets cau sed an increase in the number of D3 binding sites indicating that ligand ha d bound to the GPIIb-IIIa receptor. These data suggest that high-affinity G PIIb-IIIa-mediated ligand binding can be separated mechanistically from GPI Ib-IIIa-mediated clot retraction and that clot retraction requires addition al signaling through GPIIb-IIIa after ligand binding. The conformation reco gnized by D3 represents the expression of a GPIIb-IIIa activation state tha t participates in full-scale platelet aggregation, cytoskeletal reorganizat ion, and clot retraction.(Blood. 2000; 96:2487-2495) (C) 2000 by The Americ an Society of Hematology.