Brass Lab

Tim Stalker
Research Assistant Professor
 tstalker@mail.med.upenn.edu

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 vivo

We 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 vivo

In 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 states

Platelet 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 biology

Another 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.