title: Endocytosis and storage of plasma factor V by human megakaryocytes.
abstract: Factor V is an essential coagulation cofactor that circulates in plasma and platelet alpha-granules where it is stored complexed to multimerin I (MMRN1). To gain insights into the origin and processing of human platelet factor V, and factor V-MMRN I complexes, we studied factorV 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 factor V, IgG, fibrinogen and MMRN1. Immunofluorescent and electron microscopy confirmed factorV uptake by endocytosis and its trafficking to megakaryocyte alpha-granules. These data provide direct evidence that human megakaryocytes process plasma-derived factor V into alpha-granules and generate factorV-MMRN I complexes from endogenously and exogenously synthesized proteins.
authors.affiliation: Department of Pathology, McMaster University, Hamilton, Ontario, Canada.
authors.forename: Youko
authors.initials: Y
authors.lastname: Suehiro
authors.forename: Dragoslava Kika
authors.initials: DK
authors.lastname: Veljkovic
authors.forename: Nola
authors.initials: N
authors.lastname: Fuller
authors.forename: Yasuaki
authors.initials: Y
authors.lastname: Motomura
authors.forename: Jean Marc
authors.initials: JM
authors.lastname: Massé
authors.forename: Elisabeth M
authors.initials: EM
authors.lastname: Cramer
authors.forename: Catherine P M
authors.initials: CP
authors.lastname: Hayward
mesh_terms: D001792:Blood Platelets / Q000378:metabolism; D001798:Blood Proteins / Q000378:metabolism; D001854:Bone Marrow Cells / Q000378:metabolism; D002478:Cells, Cultured; D003594:Cytoplasmic Granules / Q000378:metabolism; D004705:Endocytosis; D005165:Factor V / Q000235:genetics / Q000378:metabolism*; D005340:Fibrinogen / Q000378:metabolism; D006801:Humans; D007074:Immunoglobulin G / Q000378:metabolism; D008533:Megakaryocytes / Q000378:metabolism*; D010949:Plasma / Q000378:metabolism; D040201:Platelet Membrane Glycoprotein IIb / Q000032:analysis; D012333:RNA, Messenger / Q000378:metabolism*; D013997:Time Factors
chemical_list: D001798:Blood Proteins; D007074:Immunoglobulin G; D040201:Platelet Membrane Glycoprotein IIb; D012333:RNA, Messenger; C069282:multimerin; D005165:Factor V; D005340:Fibrinogen