Blood, 1 October 2000, Vol. 96, No. 7, pp. 2487-2495
HEMOSTASIS, THROMBOSIS, AND VASCULAR BIOLOGY
Active GPIIb-IIIa conformations that link ligand interaction
with cytoskeletal reorganization
Traci Heath Mondoro,
Melanie McCabe White, and
Lisa K. Jennings
From the Vascular Biology Program, University of
Tennessee, Memphis, TN; Center for Biologics Evaluation and Research,
Food and Drug Administration, Bethesda, MD.
Glycoprotein (GP) IIb-IIIa plays a critical role in platelet
aggregation and platelet-mediated clot retraction. This study examined
the intramolecular relationship between GPIIb-IIIa activation and
fibrinogen binding, platelet aggregation, and platelet-mediated clot
retraction. To distinguish between different high-affinity activation
states of GPIIb-IIIa, the properties of an antibody (D3) specific for
GPIIIa that induces GPIIb-IIIa binding to adhesive protein molecules
and yet completely inhibits clot retraction were used. Clot retraction
inhibition by D3 was not due to altered platelet-fibrin interaction;
however, combination treatments of D3 and adenosine diphosphate (ADP)
inhibited full-scale aggregation and decreased the amounts of
GPIIb-IIIa and talin incorporated into the core cytoskeletons.
Morphologic evaluation of the D3/ADP aggregates showed platelets that
were activated but to a lesser extent when compared to ADP only. ADP
addition to platelets caused an increase in the number of D3 binding
sites indicating that ligand had bound to the GPIIb-IIIa receptor.
These data suggest that high-affinity GPIIb-IIIa- mediated ligand
binding can be separated mechanistically from GPIIb-IIIa-mediated clot
retraction and that clot retraction requires additional signaling
through GPIIb-IIIa after ligand binding. The conformation recognized by D3 represents the expression of a GPIIb-IIIa activation state that
participates in full-scale platelet aggregation, cytoskeletal reorganization, and clot retraction.
