Activation of human platelets by
complement proteins C5b-9 is accompanied by the release of small plasma membrane vesicles (microparticles) that are highly enriched in binding sites for
coagulation factor Va and exhibit
prothrombinase activity. We have now examined whether assembly of the
prothrombinase enzyme complex (factors VaXa) is directly linked to the process of microparticle formation. Gel-filtered platelets were incubated without stirring with various agonists at 37 degrees C, and the functional expression of
cell surface receptors on platelets and on shed microparticles was analyzed using specific
monoclonal antibodies and fluorescence-gated flow cytometry. In addition to the
C5b-9 proteins,
thrombin,
collagen, and the
calcium ionophore A23187 were each found to induce formation of platelet microparticles that incorporated plasma membrane
glycoproteins GP Ib, IIb, and IIIa. These microparticles were enriched in binding sites for
factor Va, and their formation paralleled the expression of catalytic surface for the
prothrombinase enzyme complex. Little or no microparticle release or
prothrombinase activity were observed when platelets were stimulated with
epinephrine and
ADP, despite exposure of platelet
fibrinogen receptors by these agonists. When platelets were exposed to
thrombin plus
collagen, the shed microparticles contained activated
GP IIb-IIIa complexes that bound
fibrinogen. By contrast,
GP IIb-IIIa incorporated into
C5b-9 induced microparticles did not express
fibrinogen receptor function. Platelets from a patient with an isolated defect in inducible procoagulant activity (
Scott syndrome) were found to be markedly impaired in their capacity to generate microparticles in response to all platelet activators, and this was accompanied by a comparable decrease in the number and function of inducible
factor Va receptors. Taken together, these data indicate that the exposure of the platelet
factor Va receptor is directly coupled to plasma membrane vesiculation and that this event can be dissociated from other activation-dependent platelet responses. Since a catalytic membrane surface is required for optimal
thrombin generation, platelet microparticle formation may play a role in the normal
hemostatic response to
vascular injury.