Mechanisms regulating factor V endocytosis by megakaryocytes
Platelet-derived factor Va is critical for blood clot formation by serving as the essential cofactor for thrombin generation via the prothrombinase complex assembled on activated platelets. The entire platelet-derived factor Va pool originates via endocytosis of the procofactor, factor V, from plasma by megakaryocytes, the platelet precursor cells. Factor V endocytosis by megakaryocytes is specific, clathrin-dependent, and appears to be mediated by a two receptor system. In this model, factor V initially binds to a specific factor V receptor expressed only on megakaryocytes able to endocytose factor V. This binding event facilitates an interaction between a second factor V molecule and LDL receptor-related protein-1 (LRP-1), an endocytic receptor, which subsequently mediates the endocytosis of bound factor V. These combined observations represent a unique role for LRP-1 in endocytosis of a coagulation protein not destined for lysosomal degradation. Rather, subsequent to its endocytosis, factor V is functionally modified, trafficked to, and stored in alpha-granules.
The overall goal of our laboratory is to identify and characterize the proteins that form the megakaryocyte two receptor system involved in the binding and endocytosis of plasma-derived factor V.
Some specific projects include: (1) Identification of the specific, factor V receptor expressed on human megakaryocytes; (2) Determination of the fates of the factor V receptor and LRP-1 subsequent to factor V endocytosis, and during megakaryocyte development and platelet formation; (3) Identification of the minimum factor V amino acid sequence(s) involved in its interactions with the factor V receptor, as well as LRP-1 expressed on megakaryocytes; and (4) Definition of the intracellular signaling events that regulate factor V binding and endocytosis by megakaryocytes.
Our laboratory is also involved in large population-based studies of thrombophilic and hemophilic individuals. These studies, being performed in collaboration with Dr. Kathleen Brummel-Ziedins, use a flow cytometry-based assay of prothrombinase complex assembly on activated platelets in whole blood developed by our laboratory to predict an individual’s propensity to bleed or clot.