University of Vermont

College of Medicine

Department of Biochemistry


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.

Last modified March 28 2014 10:43 AM