Endocytosis and storage of plasma factor V by human megakaryocytes

Factor V is an essential coagulation cofactor that circulates in plasma and platelet a-granules where it is stored complexed to multimerin I (MMRNI). To gain insights into the origin and processing of human platelet factor V, and factor V-MMRNI complexes, we studied factor V in cultured megakaryocytes. Factor V mRNA was detected in all megakaryocyte cultures. However, like albumin, IgG and fibrinogen, factor V protein was detectable only in megakaryocytes cultured with exogenous protein. The amount of factor V associated with megakaryocytes was influenced by the exogenous factor V concentration. Similar to pla telet factor V, megakaryocyte factor V was proteolyzed and complexed with megakaryocyte-synthesized MMRNI . With secretagogues, megakaryocytes released factor V, IgG, fibrinogen and MMRNI. Immunofluorescent and electron microscopy confirmed factor V 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 factor V-MMRNI complexes from endogenously and exogenously synthesized proteins.