HEMOSTASIS, THROMBOSIS, AND VASCULAR BIOLOGY Cytoskeletal regulation of the platelet glycoprotein Ib/V/IX–von Willebrand factor interaction

Shear-induced binding of von Willebrand factor (vWf) to the platelet glycoprotein (GP) Ib/V/IX complex plays a key role in initiating platelet adhesion and aggregation at sites of vascular injury. This study demonstrated that pretreating human platelets with inhibitors of actin polymerization, cytochalasin D or latrunculin B, dramatically enhances platelet aggregation induced by vWf. The effects of these inhibitors were specific to the vWf-GPIba interaction because they enhanced vWfinduced aggregation of Glanzmann thrombasthenic platelets and Chinese hamster ovary (CHO) cells transfected with GPIb/V/ IX. Moreover, cytochalasin D enhanced the extent of platelet aggregation induced by high shear stress (5000 s21) and also lowered the shear threshold required to induce aggregation from 3000 s21 to as low as 500 s21. Studies of CHO cells expressing GPIba cytoplasmic tail truncation mutants that failed to bind actinbinding protein-280 (deletion of residues 569-610 or 535-568) demonstrated that the linkage between GPIb and actin-binding protein-280 was not required for vWf-induced actin polymerization, but was critical for the enhancing effects of cytochalasin D on vWf-induced cell aggregation. Taken together, these studies suggest a fundamentally important role for the cytoskeleton in regulating the adhesive function of GPIb/V/IX. (Blood. 2000;96:3480-3489)