Mechanisms regulating factor V endocytosis by megakaryocytes

Model of factor V endocytosis by megakaryocytes
Endocytosis of factor V by megakaryocytes


I received my Ph.D. in the Cell & Molecular Biology Program at the University of Vermont in 1996. My dissertation research focused on the regulation of procoagulant enzyme complex assembly and function on cells. During a Postdoctoral Fellowship with Barbara Furie, Ph.D. at Harvard Medical School, I continued with my research interests in coagulation studying the vitamin K-dependent gamma-glutamyl carboxylase, the enzyme responsible for catalyzing an important post-translational modification of the blood clotting proteins. In 2000, I returned to UVM and am currently an Assistant Professor of Biochemistry.

Research Description

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.

Model of factor V endocytosis by megakaryocytes

Model of factor V endocytosis by megakaryocytes

Endocytosis of factor V by megakaryocytes

Endocytosis of factor V by megakaryocytes

Highlighted Publications

Bouchard, B.A., Meisler, N.T., Nesheim, M.E., Liu, C.X., Strickland, D.K., and Tracy, P.B. A unique function for LRP-1: a component of a two-receptor system mediating specific endocytosis of plasma-derived factor V by megakaryocytes. J Thromb Haemosta 6(4):638-44, 2008.

Bouchard, B.A., Butenas, S., Mann K.G. and Tracy, P.B. Interactions between platelets and the coagulation system. In: Platelets, Michelson AD (2nd Ed), Academic Press, San Diego, CA, pp. 377-402, 2007.

Bouchard, B.A., Williams, J.L., Meisler, N.T., Long, M.W. and Tracy, P.B. Endocytosis of plasma factor V by megakaryocytes occurs via a clathrin-dependent, specific receptor-mediated event. J Thromb Haemosta 3:541-551, 2005

Damron, D.P., Bouchard, B.A., Shapiro, R.E., Schonberg, A.L. and Bernstein, I.M. Platelet activation, sympathetic tone, and plasma volume in nulligravid women of reproductive age. Obstet Gynecol 103:931-936, 2004

Bouchard, B., Paradis, A., and Brummel-Ziedins, K. Measurement of procoagulant platelet subpopulations in whole blood: Development of an assay for population-based studies. Thromb Res. 2011 Jan;127(1):62-4. Epub 2010 Jun 11.

View all Bouchard publications here.

* indicates equal contribution

Recent Publications

Bouchard BA, Chapin J, Brummel-Ziedins KE, Durda P, Key NS, Tracy PB (2015) Platelets and platelet-derived factor Va confer hemostatic competence in complete factor V deficiency. Blood 125(23): 3647-50.

Gertz JM, Bouchard BA (2015) Mechanisms Regulating Acquisition of Platelet-Derived Factor V/Va by Megakaryocytes. J Cell Biochem in press.

Sharma D, Brummel-Ziedins KE, Bouchard BA, Holmes CE (2014) Platelets in tumor progression: a host factor that offers multiple potential targets in the treatment of cancer. J Cell Physiol 229(8): 1005-15.

Bouchard BA, Gissel MT, Whelihan MF, Mann KG, Butenas S (2014) Platelets do not express the oxidized or reduced forms of tissue factor. Biochim Biophys Acta 1840(3): 1188-93.

Bouchard BA, Abdalla S, Tracy PB (2013) The factor V light chain mediates the binding and endocytosis of plasma-derived factor V by megakaryocytes. J Thromb Haemost 11(12): 2181-3.

Ayombil F, Abdalla S, Tracy PB, Bouchard BA (2013) Proteolysis of plasma-derived factor V following its endocytosis by megakaryocytes forms the platelet-derived factor V/Va pool. J Thromb Haemost 11(8): 1532-9.

Haynes LM, Bouchard BA, Tracy PB, Mann KG (2012) Prothrombin activation by platelet-associated prothrombinase proceeds through the prethrombin-2 pathway via a concerted mechanism. J Biol Chem 287(46): 38647-55.

View all Bouchard publications here.

Beth Bouchard, Ph.D.

Bouchard, Ph.D.

Assistant Professor
Department of Biochemistry


Office: Given C440A
Lab: Given C440

Lab Members

   Jacqueline Gertz, Biochem Grad Student

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