Summary
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