Biochemistry of Platelet Function


Dr. Tracy received her Ph.D. in Biology, with an emphasis in Biochemistry, from Syracuse University in 1977. Her current interests in how platelets regulate blood clotting reactions developed as a result of her 5 years of post-doctoral training in the Special Coagulation Laboratory, Division of Hematology at the Mayo Clinic in Rochester, MN. Her first university appointment was made in 1983 as an Assistant Professor of Pathology at the University of Rochester, Rochester, NY. In 1985, she moved to the University of Vermont as a Research Assistant Professor of Medicine and Biochemistry and member of the Thrombosis Research Center. She became a tenured Professor of Biochemistry in 1995, and in July 2005, accepted the position of Interim Chairperson of the department. In addition to her research activities and pre- and post-doctoral training activities, Dr. Tracy also has assumed a significant teaching role in the graduate and medical schools. She is a strong proponent of academia and thus spends her time between administration, research, teaching and mentoring her students. As a result of her research and training endeavors, Dr. Tracy serves on the editorial board of several journals related to hemostasis and thrombosis, was a member of the National Institutes of Health Hematology Study Section for eight years and served as Chairperson of several highly specialized meetings, such as the Gordon Research Conference on Hemostasis in 1996, the FASEB Summer Research Conference on Proteases in Vascular Biology, and most recently shared the responsibility for the 22nd Annual Meeting of the International Society of Thrombosis and Haemostasis in Boston, MA, July 2009. .

Research Description

The principle objective of our laboratory is to develop a fundamental understanding of how platelets participate in and regulate the formation of the important bioregulatory effector molecule, thrombin. Thrombin generation is effected by a Ca2+-dependent, membrane-bound complex of the cofactor protein, factor Va, and the serine protease, factor Xa. One specific goal is to provide a quantitative understanding of the integrally related kinetic and binding events regulating the functional interactions of factors Va and Xa with the platelet membrane surface. The membrane receptors, the intracellular signalling pathways and the enzymatic processes controlling these events are being identified using biochemical and molecular biological approaches. A second major goal is to define how megakaryocytes, platelet progenitor cells, developmentally regulate the endocytosis and possible synthesis of the required cofactor factor V(a) and, to determine the cellular events regulating its endocytosis, its intracellular trafficking to storage granules and the phenotypic changes in the factor V molecule resulting from these interactions. Since the platelet-derived factor Va pool is essential for normal blood clotting, defining how platelets acquire this essential protein, process it and express it at their membrane surface is key in regulating thrombin generation. The formation of thrombin at the surface of platelets is pivotal to the physiological and pathophysiological functions they provide as they localize to vascular and extravascular tissue sites.

Student Opportunities:

Laboratory members become proficient in protein purification and several techniques used for protein characterization (SDS-PAGE, Western blotting, fluorography, autoradiography, amino acid analyses, HPLC, FPLC and MALDI-TOF mass spectrometry). Proficiency in the performance and analysis of chromogenic and fluoroscopic methods for assessing enzyme kinetic reactions can also be learned. Sterile technique is practiced as several cell lines are maintained in continuous culture. Platelets are isolated from whole blood donations on a daily basis. Platelet progenitor cells, megakaryocytes, are isolated from bone marrow aspirates as that primary mature cell population, or can be expanded and differentiated ex vivo from CD34+ bone marrow precursors. Other methods can include kinase assays, fluorescence-activated flow cytometric analyses, fluorescence-activated cell sorting, fluorescence microscopy and confocal microscopy.

Highlighted Publications

Wood, J., Silveira, J., Maille, N., Haynes, L., and Tracy, P. Prothrombin activation on the activated platelet surface optimizes expression of procoagulant activity. Blood. 2011 Feb 3;117(5):1710-8. Epub 2010 Dec 3. PMID: 21131592

Fager, A., Wood, J., Bouchard, B., Feng, P.,and Tracy, P. Properties of procoagulant platelets: defining and characterizing the subpopulation binding a functional prothrombinase. Arterioscler Thromb Vasc Biol. 2010 Dec;30(12):2400-7. Epub 2010 Nov 11. PMID: 21071689

Kleiman NS, Freedman J, Tracy PB, Furie BC, Bray PF, Phillips DR, Storey RF, Rusconi CP, French PA, Steinbuhl SR, and Becker RC (2008). Platelets: Developmental biology, physiology and translatable platforms for preclinical investigation and drug development. Platelets 19(4):239-251

Bouchard BA, Meisler NT, Nesheim ME, Liu C-X, Strickland DK and Tracy PB. (2008). A unique function for low-density lipoprotein receptor related protein-1 (LRP-1): A component of a two-receptor system mediating specific endocytosis of plasma-derived factor V by megakaryocytes. J Thromb Haemost. 6:638-644

Bouchard BA, Williams JL, Meisler NT, Long MW, Tracy PB. (2005) Endocytosis of plasma-derived factor V by megakaryocytes occurs via a clathrin-dependent, specific membrane binding event. J Thromb Haemost. 3:541-51.

Gould WR, Simioni P, Silveira JR, Tormene D, Kalafatis M, Tracy PB. (2005) Megakaryocytes endocytose and subsequently modify human factor V in vivo to form the entire pool of a unique platelet-derived cofactor. J Thromb Haemost. 3:450-6.

View all Tracy 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.

Wood JP, Bunce MW, Maroney SA, Tracy PB, Camire RM, Mast AE (2013) Tissue factor pathway inhibitor-alpha inhibits prothrombinase during the initiation of blood coagulation. Proc Natl Acad Sci U S A 110(44): 17838-43.

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.

Wood JP, Silveira JR, Maille NM, Haynes LM, Tracy PB (2011) Prothrombin activation on the activated platelet surface optimizes expression of procoagulant activity. Blood 117(5): 1710-8.

Fager AM, Wood JP, Bouchard BA, Feng P, Tracy PB (2010) Properties of procoagulant platelets: defining and characterizing the subpopulation binding a functional prothrombinase. Arterioscler Thromb Vasc Biol 30(12): 2400-7.

View all Tracy publications here.

Selected Awards

Established Investigator, American Heart Association (1987-1992)

AHA Council on Arteriosclerosis, Thrombosis and Vascular Biology, Special Recognition Award (2001)

University Scholar, University of Vermont (2004)

American Medical Women's Association, “Gender Equity Award,” UVM Chapter (2005)

Councillor, International Society on Thrombosis and Haemostasis (2010-2016)

Paula Tracy, Ph.D.

Tracy, Ph.D.

Interim Chairperson and Professor
Department of Biochemistry


Office: Given C409
Lab: Given C403

Lab Members

   Francis Ayombil, Biochem Grad Student
   Laura Haynes, Postdoctoral Fellow
   Jay R. Silveira, Assistant Professor
   Brittany Todd, Biochem Grad Student

Upcoming Events

  • 7/7/2015 11:30 AM – 12:00 PM
    Davis Auditorium
    Laura Solomon
  • 7/7/2015 12:00 PM – 12:30 PM
    Davis Auditorium
    Phyu Thwe

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