Mechanisms Regulating Acquisition of Platelet-Derived Factor V/Va by Megakaryocytes.

Factor Va serves as the nonenzymatic protein cofactor for the prothrombinase complex, which converts prothrombin to thrombin in the events leading to formation of a hemostatic plug. Several observations support the concept that platelet-derived factor V/Va is physically and functionally distinct and plays a more important role in thrombin generation at sites of vascular injury as compared to its plasma counterpart. Platelet-derived factor V/Va is generated following endocytosis of the plasma-derived molecule by the platelet precursor cells, megakaryocytes, via a two receptor system consisting of low density lipoprotein (LDL) receptor-related protein-1 (LRP-1) and an unidentified specific "binding site". More recently, it was suggested that a cell surface-expressed β-galactoside binding protein, galectin-8, was involved in factor V endocytosis. Endocytosed factor V is trafficked through the cell and retailored prior to its storage in α-granules. Given the essential role of platelet-derived factor Va in clot formation, understanding the cellular and molecular mechanisms that regulate how platelets acquire this molecule will be important for the treatment of excessive bleeding or clotting.