› InstitutsangabenGrant support: Supported by grant 42450 from the Canadian Institutes of Health Research (C.P.M.H.). C.P.M.H is supported by a Career Investigator Award from the Heart and Stroke Foundation of Ontario, a Canada Research Chair in Molecular Hemostasis from the Government of Canada and a Premier's Research Excellence Award from the Government of Ontario. D.K.V. is the recipient of a Focus on Stroke Doctoral Research Award from the Heart and Stroke Foundation and Canadian Institutes of Health Research.
Factor V is an essential coagulation cofactor that circulates in plasma and platelet α-granules where it is stored complexed to multimerin 1 (MMRN1). To gain insights into the origin and processing of human platelet factor V, and factor V-MMRN1 complexes, we studied factor V in cultured megakaryocytes. Factor V mRNA was detected in all megakaryocyte cultures. However, like albumin, IgG and fibrinogen, factorV protein was detectable only in megakaryocytes cultured with exogenous protein. The amount of factor V associated with megakaryocytes was influenced by the exogenous factorV concentration. Similar to platelet factor V, megakaryocyte factor V was proteolyzed and complexed with megakaryocyte-synthesized MMRN1. With secretagogues, megakaryocytes released factorV, IgG, fibrinogen and MMRN1. Immunofluorescent and electron microscopy confirmed factorV uptake by endocytosis and its trafficking to megakaryocyte α-granules. These data provide direct evidence that human megakaryocytes process plasma-derived factor V into α-granules and generate factor V-MMRN1 complexes from endogenously and exogenously synthesized proteins.
Keywords
Factor VIII -
factor V -
platelet activation markers -
endocytosis -
megakaryocytes / thrombopoiesis
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