Blood coagulation is the process where a cascade of enzymatic reactions generates a clot to stem the flow of blood following injury. Unfortunately, the same cascade can also form pathologic intravenous clots in a process termed venous thrombosis. Current anticoagulant therapies used to prevent thrombosis also carry a risk of bleeding. Further investigation of the basic biochemical and regulatory mechanisms underlying coagulation are required to identify new antithrombotic strategies with reduced risk of bleeding. The studies presented in this dissertation examine the interactions between three key players in the terminal stages of blood coagulation: factor XIII (FXIII), fibrin(ogen), and red blood cells (RBCs).

First, we examined the mechanism of FXIII-mediated RBC retention in clots. We found that FXIII does not promote RBC retention by crosslinking RBCs directly to the clot or by modulating fibrin network density. Instead, specific FXIII-mediated crosslinking of the fibrin alpha-chains is required for normal RBC retention in clots. This finding is the first to reveal a pathophysiologic role of ?-chain crosslinking, and establishes this process as a crucial mediator of venous thrombosis.

Second, we investigated the nature of the FXIII-fibrinogen binding interaction to determine the domains of each protein involved in binding. We found that FXIII binds to fibrinogen alpha-chain residues 390-396 via the FXIII-B subunits. These findings enhance our understanding of this important interaction in both physiologic and pathologic coagulation.

Third, we examined the regulation of the FXIII-A and -B subunits. We found that the FXIII subunits exhibit inter-tissue, reciprocal regulation whereby FXIII-B promotes FXIII-A stability in circulation, while FXIII-A increases FXIII-B stability and production. FXIII-A upregulates liver RNA-binding proteins and may modulate post-transcriptional regulation of FXIII-B synthesis. These data clarify the mechanisms governing FXIII levels in plasma and identify a unique regulatory relationship between two protein subunits synthesized in different tissues. These data also expose a new liver-regulatory function of FXIII-A.

Collectively, these studies greatly extend our understanding of the interactions between FXIII, fibrin(ogen), and RBCs, three crucial mediators of clot dynamics. Importantly, this dissertation also emphasizes the utility of basic mechanistic studies to inform translational research efforts.