Platelet-derived microparticles express high affinity receptors for factor VIII

Factor VIII is a cofactor in the tenase enzyme complex which assembles on the membrane of activated platelets. A critical step in tenase assembly is membrane binding of factor VIII. Platelet membrane factor VIII-binding sites were characterized by flow cytometry using either fluorescein maleimide-labeled recombinant factor VIII or a fluorescein-labeled monoclonal antibody against factor VIII. Following activation by thrombin, most platelets bound factor VIII within 90 s. In addition, over the course of several minutes, membranous vesicles (microparticles) were shed from the platelet plasma membrane and each microparticle bound as much factor VIII as a stimulated platelet. Over 30 min, stimulated platelets (but not microparticles) lost the capacity to bind factor VIII. Factor VIII bound saturably to microparticles from platelets stimulated with thrombin, thrombin plus collagen, or the complement proteins C5b-9. The binding of factor VIII was compared to factor V, a structurally homologous coagulation cofactor. Analysis of microparticle binding kinetics yielded similar on and off rates for factor VIII and factor Va and KD values of 2-10 nM. In the presence of 20 nM factor Va, the binding of factor VIII to microparticles was increased, and there was a comparable increase in platelet tenase activity. At higher factor Va concentrations, factor VIII binding and tenase activity were inhibited. Conversely, factor VIII had a similar dose-dependent effect on factor Va binding and platelet prothrombinase activity. Synthetic phospholipid vesicles containing phosphatidylserine competed with microparticles for binding of factor VIII and factor Va. These studies indicate that activated platelets express a transient increase in high affinity receptors for factor VIII, whereas platelet-derived microparticles express a sustained increase in receptors. The binding characteristics of platelet membrane receptors for factor VIII are similar to those for factor Va.