Endocytosis of plasma-derived factor V by megakaryocytes occurs via a clathrin-dependent, specific membrane binding event

Abstract

Summary. Megakaryocytes were analyzed for their ability to endocytose factor V to define the cellular mechanisms regulating this process. In contrast to fibrinogen, factor V was endocytosed by megakaryocytes derived from CD34 + cells or megakaryocyte-like cell lines, but not by platelets. CD41 + ex vivo-derived megakaryocytes endocytosed factor V, as did subpopulations of the megakaryocyte-like cells MEG-01, and CMK. Similar observations were made for fibrinogen. Phorbol diester-induced megakaryocytic differentiation of the cell lines resulted in a substantial increase in endocytosis of both proteins as compared to untreated cells that did not merely reflect their disparate plasma concentrations. Factor IX, which does not associate with platelets or megakaryocytes, was not endocytosed by any of the cells examined. Endocytosis of factor V by megakaryocytes proceeds through a specific and independent mechanism as CHRF-288 cells endocytosed fibrinogen but not factor V, and the presence of other plasma proteins had no effect on the endocytosis of factor V by MEG-01 cells. Furthermore, as the endocytosis of factor V was also demonstrated to occur through a clathrin-dependent mechanism, these combined data demonstrate that endocytosis of factor V by megakaryocytes occurs via a specific, independent, and most probably receptor-mediated, event.

Novel Role for Galectin-8 Protein as Mediator of Coagulation Factor V Endocytosis by Megakaryocytes

“…1C). These data seem to be consistent with an active receptor-mediated saturable process (20). As FV is known to bind with high affinity phosphatidylserine (PS)-containing phospholipids (30), we investigated whether PS might also contribute to the uptake of FV by the cells.…”

“…Human CD34ϩ bone marrow cells have been shown to endocytose FV only from day 7 to day 10 of the differentiation process into megakaryocytes (20). Compatible with this observation is the notion that platelets completely lack the ability to internalize FV (21).…”

“…The source of platelet FV has been debated for years. Although it was initially suggested that megakaryocytes synthesize FV (19), it has now been established that platelet FV is taken up from plasma by megakaryocytes via receptor-mediated endocytosis (16,20). Megakaryocytes appear to transiently express the proteins that contribute to FV internalization as the uptake process is absent in platelets (21).…”

Genotype-phenotype correlation in combined deficiency of factor V and factor VIII

“…33,34 Recent studies suggest that endocytosis of FV is a specific, clathrin-dependent, and probably receptor-mediated mechanism. 37 Our observations of similarly reduced FV levels in platelets and plasma of F5F8D patients suggest that the receptor for uptake of plasma FV into megakaryocytes is not saturated at physiologic concentrations of plasma FV. Alternatively, a mechanism may exist that regulates the rate of FV uptake into megakaryocytes to match the plasma FV level.…”

Increased expression of urokinase plasminogen activator in Quebec platelet disorder is linked to megakaryocyte differentiation

“…26 Our current study provides indirect evidence that the uptake of plasminogen for costorage with uPA is required to generate sufficient plasmin to trigger ␣-granule protein degradation in QPD, as the trafficking of plasminogen to forming ␣-granules was not recapitulated in cultures with or without added plasma. The mechanism of plasminogen uptake into ␣-granules is unknown, although bulk transport or receptor-mediated endocytosis (as demonstrated for other proteins) [39][40][41][42][43] seems plausible given that plasminogen production by cultured megakaryocytes was undetectable. In vivo, the uptake of plasma proteins (eg, fibrinogen) into ␣-granules occurs late during megakaryopoiesis.…”