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I have been involved with investigations in the field of thrombosis and hemostasis since joining the Brass lab at the University of Pennsylvania as a post-doctoral fellow in 2003. Since that time, the goal of my research studies has been to gain a better understanding of the mechanisms responsible for hemostasis and thrombosis, with a particular emphasis on how multiple signaling inputs present at a site of vascular injury are integrated to regulate platelet activation in vivo. The following is a brief summary of major ongoing projects performed in association with members of the Brass lab. In addition to these projects, I am involved in a number of collaborative studies with investigators at Penn, CHOP and other institutions across the country.Project 1: Spatio-temporal regulation of platelet activation following vascular injury in vivoWe recently determined that hemostatic plugs formed following vascular injury in vivo are composed of discrete regions with variable degrees of platelet activation. Ongoing studies are investigating how multiple components of the platelet signaling network are integrated to produce this heterogeneous hemostatic plug architecture. To accomplish these goals, we make extensive use of multiple systems for examination of thrombosis in vitro and in vivo, including a spinning disk confocal intravital microscopy system for visualization of thrombosis in the microcirculation of mice. Intravital microscopy approaches also involve the use of established and novel fluorescent probes for visualizing various aspects of the hemostatic response in vivo, including fluorescently labeled antibodies, fluorescent biochemical activity sensors and genetically encoded fluorescent indicators.Project 2: The influence of local microenvironments on hemostasis and thrombosis in vivoIn conjunction with the studies in Project 1, we have become interested in how local microenvironments within a platelet aggregate help to shape the movement and overall distribution of soluble plasma components that regulate platelet activation and coagulation. These studies couple in vivo imaging approaches measuring solute transport with in vitro and computational approaches to model and analyze the physical characteristics of the microenvironment between adjacent platelets as they become tightly packed in a hemostatic plug.Project 3: Platelet function in the setting of trauma and other pathologic statesPlatelet function is known to be perturbed in a number of pathologic settings and may contribute to the morbidity and mortality associated with these pathologies. One example is in the setting of trauma, where a subset of severe trauma patients develop a coagulopathic state characterized by abnormal blood clotting and excessive hemorrhage. We are using animal models to study platelet function in the setting of trauma-induced coagulopathy as part of a multi-institution consortium funded by the NHLBI (TACTIC).Project 4: The role of junctional adhesion molecules in platelet biologyAnother longstanding research project involves examination of the role of a family of cell adhesion molecules found on the surface of platelets, including ESAM and JAM-A, which are found at the tight junctions of other cell types (e.g. endothelial cells and epithelial cells). As platelets do not form tight junctions the role of these proteins in platelet biology is rather unclear, but they appear to be negative regulators of platelet plug formation in vivo as genetic deletion of these proteins in mice leads to a pro-thrombotic phenotype. Current studies are investigating the mechanisms by which this family of proteins influences platelet functions, as well as their role in platelet-leukocyte and platelet-endothelial cell crosstalk.
Welsh JD, Stalker TJ, Voronov R, Muthard RW, Tomaiuolo M, Diamond SL, Brass LF.: A systems approach to hemostasis: 1. The interdependence of thrombus architecture and agonist movements in the gaps between platelets. Blood 124 (11): 1808-15,2014.
Tomaiuolo M, Stalker TJ, Welsh JD, Diamond SL, Sinno T, Brass LF.: A systems approach to hemostasis: 2. Computational analysis of molecular transport in the thrombus microenvironment. Blood 124 (11): 1816-23,2014.
Stalker TJ, Welsh JD, Tomaiuolo M, Wu J, Colace TV, Diamond SL, Brass LF.: A systems approach to hemostasis: 3. Thrombus consolidation regulates intrathrombus solute transport and local thrombin activity. Blood 124 (11): 1824-31,2014.
Min S, Suzuki, A, Stalker, TJ, Zhao, L, Wang, Y, McKennan, C, Riese, M, Guzman, J, Zhang, S, Lian, L, Joshi, R, Meng, R, Seeholzer, SH, Choi, J, Koretzky, G, Marks, MS, Abrams, CS: Loss of PIKfyve in platelets causes a lysosomal disease leading to inflammation and thrombosis in mice Nat Commun 5 : 4691,2014.
Stalker TJ, Welsh JD, Brass LF.: Shaping the platelet response to vascular injury. Curr Opin Hematol 21 (5): 410-7,2014.
Stalker, TJ, Traxler, EA, Wu, J, Wannemacher, KM, Cermignano, SL, Voronov, R, Diamond, SL, Brass, LF: Hierarchical organization in the hemostatic response and its relationship to the platelet-signaling network Blood 121 (10): 1875-85,2013.
Welsh J D, Colace T V, Muthard R W, Stalker T J, Brass L F, Diamond S L: Platelet-targeting sensor reveals thrombin gradients within blood clots forming in microfluidic assays and in mouse. J Thromb Haemost. 10 (11): 2344-53,2012.
Naik MU, Stalker TJ, Brass LF, Naik UP.: JAM-A protects from thrombosis by suppressing integrin αIIbβ3-dependent outside-in signaling in platelets. Blood 119 (14): 3352-3360,2012.
Ma P, Cierniewska A, Signarvic R, Cieslak M, Kong H, Sinnamon AJ, Neubig RR, Newman DK, Stalker TJ, Brass LF.: A newly identified complex of spinophilin and the tyrosine phosphatase, SHP-1, modulates platelet activation by regulating G protein-dependent signaling. Blood 119 (8): 1935-45,2012.
Brass Lawrence F, Tomaiuolo Maurizio, Stalker Timothy J: Harnessing the platelet signaling network to produce an optimal hemostatic response. Hematol Oncol Clin North Am. 27 (3): 381-409,2013.
Schmaier AA, Stalker TJ, Runge JJ, Lee D, Nagaswami C, Mericko P, Chen M, Cliche S, Gariépy C, Brass LF, Hammer DA, Weisel JW, Rosenthal K, Kahn ML.: Occlusive thrombi arise in mammals but not birds in response to arterial injury: evolutionary insight into human cardiovascular disease. Blood 118 (13): 3661-3669,2011.
Stalker, TJ, Skvarka, CB, Scalia, R: A novel role for calpains in the endothelial dysfunction of hyperglycemia FASEB J 17 (11): 1511-13,2003.
Stalker TJ, Gong YL, Scalia R: The calcium-dependent protease calpain causes endothelial dysfunction in type 2 diabetes Diabetes 54 (4): 1132-1140,2005.
Prevost, N, Woulfe, DS, Jiang, H, Stalker, TJ, Marchese, P, Ruggeri, ZM, Brass, LF: Eph kinases and ephrins support thrombus growth and stability by regulating integrin outside-in signaling in platelets Proc Natl Acad Sci U S A. 102 (28): 9820-9825,2005.
Zhu, L, Bergmeier, W, Wu, J, Jiang, H, Stalker, TJ, Cieslak, M, Fan, R, Boumsell, L, Kumanogoh, A, Kikutani, H, Tamagnone, L, Wagner, DD, Milla, ME, Brass, LF: Regulated surface expression and shedding support a dual role for semaphorin 4D in platelet responses to vascular injury Proc Natl Acad Sci U S A 104 (5): 1621-1626,2007.
Wang, YF, Chen, XS, Lian, LR, Tang, T, Stalker, TJ, Sasaki, T, Brass, LF, Choi, JK, Hartwig, JH, Abrams, CS: Loss of PIP5KI beta demonstrates that PIP5KI isoform-specific PIP2 synthesis is required for IP3 formation Proc Natl Acad Sci U S A 105 (37): 14064-14069,2008.
Zhu, L, Stalker, TJ, Fong, KP, Jiang, H, Tran, A, Crichton, I, Lee, EK, Neeves, KB, Maloney, SF, Kikutani, H, Kumanogoh, A, Pure, E, Diamond, SL, Brass, LF: Disruption of SEMA4D Ameliorates Platelet Hypersensitivity in Dyslipidemia and Confers Protection Against the Development of Atherosclerosis Arterioscler Thromb Vasc Biol. 29 (7): 1039-1045,2009.
Stalker TJ, Wu J, Morgans A, Traxler EA, Wang L, Chatterjee MS, LeeD, Quertermous T, Hall RA, Hammer DA, Diamond SL, Brass LF: Endothelial cell specific adhesion molecule (ESAM) localizes to platelet-platelet contacts and regulates thrombus formation in vivo J Thromb Haemost. 7 (11): 1886-1896,2009.
Wannemacher, KM, Zhu, L, Jiang, H, Fong, KP, Stalker, TJ, Lee, D, Tran, AN, Neeves, KB, Maloney, S, Kumanogoh, A, Kikutani, H, Hammer, DA, Diamond, SL, Brass, LF.: Diminished contact-dependent reinforcement of Syk activation underlies impaired thrombus growth in mice lacking Semaphorin 4D Blood 116 (25): 5707-5715,2010.
Signarvic, RS, Cierniewska, A, Stalker, TJ, Fong, KP, Chatterjee, MS, Hess, PR, Ma, PS, Diamond, SL, Neubig, RR, Brass, LF: RGS/G(i2 alpha) interactions modulate platelet accumulation and thrombus formation at sites of vascular injury Blood 116 (26): 6092-6100,2010.
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