Platelet accumulation is a hallmark of hemostasis and a contributor to heart attacks and strokes. In previous work, we and others have focused on identifying molecules that support platelet activation. Here we attempt a paradigm shift, approaching platelet activation as the product of a flexible signaling network rather than a collection of pathways, and joining testable ideas about the operation of that network to high resolution imaging of platelet activation in vivo. Our goal is to understand how the platelet signaling network shapes the hemostatic response and how the hemostatic response impacts the network. Our initial studies show that, rather than a homogeneous mass of platelets and fibrin, the response to penetrating injuries yields a hierarchical structure comprised of distinct regions that vary in the extent of platelet activation, packing density and porosity. A prominent feature is a core of fully-activated platelets overlaid with an unstable shell of less-activated platelets, but additional domains are present as well. Greater packing in the core allows adjacent platelets to interact, generating contact-dependent signals. Within the core is a region where thrombin activation and fibrin deposition occur. Our hypothesis is that all parts of this structure are subject to the platelet signaling network, with the core providing thrombus stabilit and gradients of soluble agonists determining the extent of thrombus growth and the size of the shell.
Aim #1 will test and extend this model, applying additional activation markers, developing new methods for tracking individual platelets in vivo, and exploiting a newly- developed thrombin biosensor.
Aim #2 will map relationships between the platelet signaling network and the structure of the hemostatic mass, and reassess the impact of anti-platelet agents in the context of a hierarchical model of hemostasis.
Aim #3 will test the novel hypothesis that contact-dependent events are separated spatially and temporally into sets that either promote or restrain the thrombus core. Through these aims, we hope to determine why multiple agonists and signals are needed to shape an optimal platelet response, account for differences in the impact of antiplatelet agents, and show how pathological conditions can subvert normal responses by effects on the platelet signaling network.

Public Health Relevance

Platelet activation is part of the normal response to vascular injury, producing a plug that limits blood loss. Thrombosis occurs when platelets are activated inappropriately, blocking blood flow and damaging tissues such as the heart and brain. The goal of this project is to better understand the molecular basis of platelet activation and translate that understanding into improved methods to prevent thrombosis.

National Institute of Health (NIH)
National Heart, Lung, and Blood Institute (NHLBI)
Research Project (R01)
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Special Emphasis Panel (ZRG1-VH-D (03))
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Link, Rebecca P
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University of Pennsylvania
Internal Medicine/Medicine
Schools of Medicine
United States
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Ivanciu, L; Stalker, T J (2015) Spatiotemporal regulation of coagulation and platelet activation during the hemostatic response in vivo. J Thromb Haemost 13:1949-59
Meng, Ronghua; Wu, Jie; Harper, Dawn C et al. (2015) Defective release of α granule and lysosome contents from platelets in mouse Hermansky-Pudlak syndrome models. Blood 125:1623-32
Muthard, Ryan W; Welsh, John D; Brass, Lawrence F et al. (2015) Fibrin, γ'-fibrinogen, and transclot pressure gradient control hemostatic clot growth during human blood flow over a collagen/tissue factor wound. Arterioscler Thromb Vasc Biol 35:645-54
Welsh, John D; Stalker, Timothy J; Voronov, Roman et al. (2014) A systems approach to hemostasis: 1. The interdependence of thrombus architecture and agonist movements in the gaps between platelets. Blood 124:1808-15
Tomaiuolo, Maurizio; Stalker, Timothy J; Welsh, John D et al. (2014) A systems approach to hemostasis: 2. Computational analysis of molecular transport in the thrombus microenvironment. Blood 124:1816-23
Stalker, Timothy J; Welsh, John D; Tomaiuolo, Maurizio et al. (2014) A systems approach to hemostasis: 3. Thrombus consolidation regulates intrathrombus solute transport and local thrombin activity. Blood 124:1824-31
Stalker, Timothy J; Welsh, John D; Brass, Lawrence F (2014) Shaping the platelet response to vascular injury. Curr Opin Hematol 21:410-7
Stalker, Timothy J; Traxler, Elizabeth A; Wu, Jie et al. (2013) Hierarchical organization in the hemostatic response and its relationship to the platelet-signaling network. Blood 121:1875-85
Voronov, Roman S; Stalker, Timothy J; Brass, Lawrence F et al. (2013) Simulation of intrathrombus fluid and solute transport using in vivo clot structures with single platelet resolution. Ann Biomed Eng 41:1297-307