PROJECT SUMMARY/ABSTRACT Venous thrombosis/thromboembolism (VTE) results in approximately 10 million cases worldwide and between 100,000 and 300,000 deaths yearly. Even with treatment, 30-50% of VTE patients develop recurrent VTE or suffer debilitating long-term health complications such as post-thrombotic syndrome and/or pulmonary hypertension. The prevailing clinical problem is that current anticoagulation therapies are associated with a significant risk of hemorrhage, thus the development of targeted anticoagulant agents which reduce thrombosis without increasing the risk of bleeding is necessary. We have recently discovered that factor XIII (FXIII) mediates RBC retention in clots and determines clot size. Importantly, we have shown that inhibiting FXIII activity or blocking FXIII-fibrinogen interactions reduces clot RBC content, and consequently, decreases thrombus size in vitro and in vivo. This observation suggests that inhibiting FXIII activity or blocking FXIII- fibrinogen interactions represents a new approach to reduce VTE risk. The primary goal of this application is to determine the molecular mechanisms by which FXIII mediates VTE in three Specific Aims: 1) Determine the relative contributions of plasma and platelet FXIII to clot composition and weight in vitro and in vivo, 2) Determine the contribution of accelerated FXIII activation by V34L polymorphism to clot composition and weight in vitro and in vivo, and 3) Determine the level of FXIII that reduces thrombus weight without increasing bleeding. Clot formation will be studied in plasma and physiologically-relevant whole blood systems. This study will apply a combination of biochemical and genetic tools to address the role of FXIII activity on RBC retention during whole blood clot formation in in vitro and in vivo models of VTE and hemostasis. This research plan will greatly enrich my technical abilities and train me to be an independent investigator. Through supportive coursework, diverse seminars, and participation at national and international conferences, I will expand my scientific knowledge to address unanswered questions pertinent to coagulation research. Concurrently, the proposed research will significantly advance our understanding of the role of FXIII activity and activation kinetics in thrombus formation and composition. This study is highly innovative because it investigates a novel hypothesis that blocking FXIII activity will reduce thrombus size, and consequently, VTE. This approach is fundamentally different than existing therapies because it would not interfere with factors upstream of the clot fibrin network formation and thus, is not expected to result in bleeding risk as traditional anticoagulants. Finally, these findings will advance the field by providing information on the role of FXIII in VTE and is clinically significant with the potential to yield a novel target to reduce VTE risk.