Initiation of Blood Coagulation: a Systems Biology Approach
William S. Denney and Scott L. Diamond. Chemical and Biomolecular Engineering, University of Pennsylvania, 1150 Vagelos Labs, 3340 Smith Walk, Philadelphia, PA 19104
Systems biology is the integration of large datasets to form a holistic model of a biological pathway. Through the tools of systems biology we are studying the chemical processes that occur during the initiation stages of thrombosis. Understanding these reactions can assist with the development of new drugs with potential to effectively combat coagulation related disorders. We investigated the concentrations of plasma proteins where coagulation will occur stochastically with both computational simulation using the Mann model and experiment with the dilution of whole human blood. Comparing simulation to experiments, the Mann model was determined to be most accurate between 1 and 15 pM tissue factor (TF). Stochastic simulation was initiated with TF to determine the conditions when blood had a 50% chance of reaching a concentration (0.45 nM) of thrombin that causes irreversible clotting (~0.2 pM TF). To complement the stochastic simulation, a deterministic sensitivity analysis was used to attempt to isolate the rate limiting reactions within the TF initiated coagulation cascade. As expected, reactions involving TF conversion of Factor X (FX) to FXa and inhibition reactions including TF, FXa, and thrombin were found to be rate limiting reactions; one unexpected reaction of importance was the attenuation of coagulation by Factor VII binding to TF. These simulations motivated experiments to identify a dilution of whole blood where there would be a 50% chance of initiating coagulation (as measured by cleavage of the thrombin substrate boc-VPR-AMC). We found that 1:15 to 1:31 (blood:buffer dilution) recalcified whole human blood (containing corn trypsin inhibitor (CTI) to inhibit the contact pathway) without external activators undergoes a bifurcation between coagulation and quiescence. This is the first determination of a reaction condition that results in a bifurcation in coagulation, potentially indicating a stochastic threshold.