A new look at TFPI inhibition of factor X activation

Blood coagulation is a vital physiological process involving a complex network of biochemical reactions, which converge to form a blood clot that repairs vascular injury. This process unfolds in three phases: initiation, amplification, and propagation, ultimately leading to thrombin formation. Coagulation begins when tissue factor (TF) is exposed on an injured vessel’s wall. The first step is when activated factor VII (VIIa) in the plasma binds to TF, forming complex TF:VIIa, which activates factor X. Activated factor X (Xa) is necessary for coagulation, so the regulation of its activation is crucial. Tissue Factor Pathway Inhibitor (TFPI) is a critical regulator of the initiation phase as it inhibits the activation of factor X. While previous studies have proposed two pathways—direct and indirect binding—for TFPI’s inhibitory role, the specific biochemical reactions and their rates remain ambiguous. Many existing mathematical models only assume an indirect pathway, which may be less effective under physiological flow conditions. In this study, we revisit datasets from two experiments focused on activated factor X formation in the presence of TFPI. We employ an adaptive Metropolis method for parameter estimation to reinvestigate a previously proposed biochemical scheme and corresponding rates for both inhibition pathways. Our findings show that both pathways are essential to replicate the static experimental results. Previous studies have suggested that flow itself makes a significant contribution to the inhibition of factor X activation. We added flow to this model with our estimated parameters to determine the contribution of the two inhibition pathways under these conditions. We found that direct binding of TFPI is necessary for inhibition under flow. The indirect pathway has a weaker inhibitory effect due to removal of solution phase inhibitory complexes by flow.Author summary: After vascular injury, a large complex network of biochemical reactions leads to clot formation to repair vessel damage. Three vital biochemical components in the early phase of this process are coagulation factor X (X), the activated coagulation factor VII and tissue factor (TF) complex (TF:VIIa) that activates X, and tissue factor pathway inhibitor (TFPI), which regulates X activation. The mechanism of this TFPI regulation remains ambiguous. Here, we develop a mathematical model of the biochemical reactions involved in this process and calibrate our model to published data from two experimental studies of factor X activation. To explain the experimental data, we find that TFPI acts through two distinct biochemical pathways: direct and indirect binding, the latter being a multi-step process involving Xa re-binding to TF:VIIa. We extend our model to study TFPI’s regulatory effects under flow and, in contrast to a previous study, demonstrate that the direct pathway of regulation is essential in the presence of flow. Incorporating our findings into larger models that include later phases of the clotting process could enhance our understanding of clotting disorders and aid in the development of targeted therapies.