Complement proteins C5b-9 initiate secretion of platelet storage granules without increased binding of fibrinogen or von Willebrand factor to newly expressed cell surface GPIIb-IIIa.
The functional and conformational activation of cell surface glycoproteins IIb-IIIa (GPIIb-IIIa) was probed in platelets stimulated to secrete by complement proteins C5b-9. Gel-filtered human platelets exposed to the purified human C5b-9 proteins exhibited non-lytic secretory release of both alpha- and dense granule storage pools with only a small increase in total binding of 125I-fibrinogen (less than 3000 molecules/cell) to the cell surface. By contrast to ADP- or thrombin-activated platelets, increased 125I-fibrinogen bound to C5b-9 platelets was not inhibited by Arg-Gly-Asp-containing peptides, suggesting that the high affinity membrane receptor for fibrinogen is not expressed under these conditions. C5b-9-stimulated platelets also failed to bind 125I-von Willebrand factor (less than 1 ng/10(8) platelets), confirming that the adhesive protein receptor function of cell surface GPIIb-IIIa is not expressed in these cells. Although specific binding of 125I-fibrinogen or 125I-von Willebrand factor did not significantly increase after C5b-9 assembly, these proteins elicited de novo expression of the GPIIb-IIIa activation-associated epitope recognized by monoclonal antibody PAC-1, and binding of this antibody to C5b-9 platelets was fully competed by Arg-Gly-Asp-containing peptides. These data suggest that the metabolic events which trigger granule secretion after C5b-9 insertion into the plasma membrane cause cell surface GPIIb-IIIa to be expressed in an activation-associated but functionally incompetent conformation.
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Cardiovascular Biology Research Program, Oklahoma Medical Research Foundation, Oklahoma City 73104.
Wiedmer
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Hamilton
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Sims
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PJ
D000244:Adenosine Diphosphate / Q000494:pharmacology; D000911:Antibodies, Monoclonal / Q000502:physiology; D001666:Binding Sites, Antibody; D001667:Binding, Competitive; D001792:Blood Platelets / Q000187:drug effects / Q000648:ultrastructure*; D002850:Chromatography, Gel; D015938:Complement Membrane Attack Complex; D003165:Complement System Proteins / Q000502:physiology*; D003594:Cytoplasmic Granules / Q000502:physiology*; D005246:Feedback; D005340:Fibrinogen / Q000378:metabolism*; D009842:Oligopeptides / Q000378:metabolism; D010980:Platelet Membrane Glycoproteins / Q000276:immunology / Q000378:metabolism*; D011487:Protein Conformation; D013917:Thrombin / Q000494:pharmacology; D014841:von Willebrand Factor / Q000378:metabolism*
D000911:Antibodies, Monoclonal; D015938:Complement Membrane Attack Complex; D009842:Oligopeptides; D010980:Platelet Membrane Glycoproteins; D014841:von Willebrand Factor; D000244:Adenosine Diphosphate; D005340:Fibrinogen; D003165:Complement System Proteins; D013917:Thrombin