PROJECT SUMMARY/ABSTRACT Venous thrombosis (VT) and its major complication, pulmonary embolism (PE), are often grouped together as venous thromboembolism (VTE). This disease affects nearly 600,000 individuals per year and is the leading cause of end-stage cardiovascular disease and death. VT is promoted by three major factors, known as Virchow’s triad: hypercoagulability, endothelial cell dysfunction, and stasis of blood flow. It is now recognized that platelets play a critical initiating role, and the mechanism is just now being uncovered. Hypercoagulation and thrombin generation are major risk factors for VT, and thrombin-activated protease activated receptor 4 (PAR4) promotes procoagulant platelets, phosphatidyl serine (PS) exposure, and subsequent thrombin generation. I propose that platelet PAR4 is an important contributor to VTE. The scientific premise of this proposal is based my preliminary data showing that a hypo-reactive PAR4 single nucleotide polymorphism (SNP) (PAR4- P310L) located in extracellular loop 3 of PAR4 is associated with a lower risk for VTE in a GWAS meta-analysis. Further, mouse platelets with a homologous PAR4-P322L polymorphism exhibit decreased aggregation in response to thrombin activation and ultimately show decreased PAR4 receptor reactivity. The goals of this proposal are 1) to determine how PAR4 contributes to VT initiation and propagation using complementary mouse models, 2) validate PAR4 as a target to treat VT 3) define how PAR4 modulates platelet function in VT. I hypothesize that PAR4 signaling on platelets is a driver of VT, and reduced PAR4 signaling or pharmacological inhibition will offer protection from VTE. I aim to determine the role of PAR4 in the initiation and propagation of venous thrombosis. I will use 3 complementary mouse models of VT to establish PAR4 as a major contributor of clot development under different thrombotic conditions. In order to determine the level of PAR4 reactivity necessary for thrombus propagation, I will use 3 mouse models with differing levels of PAR4 activity: wild-type, PAR4-P322L mimicking the human SNP, and PAR4 -/- to completely remove PAR4. Additionally, this proposal aims to define how PAR4 activation modulates platelet response in VTE. I will use human blood in microfluidic chambers as well as mouse models with varying levels of PAR4 reactivity described above to determine how PAR4 influences the development of VT. Upon completion, this proposal will identify an undetermined mechanism behind PAR4-mediated platelet procoagulant activity in VTE, which can provide a novel therapeutic target for patients with VTE.