Role of transglutaminases in model thrombus stability
The effect of transglutaminases in fibrin stability was studied in model thrombus system. Model thrombi were formed under flow from whole blood and FITC-fibrinogen was incorporated. A specific inhibitor of transglutaminases, R283, was included in some thrombi. Lysis was monitored as fluorescence release. The rate of lysis by tPA or uPA present at pharmacological concentrations was significantly increased when transglutaminases were inhibited. Effect was more pronounced with uPA lysis. The model thrombi were divided into cell-rich head and fibrin-rich tail. The effect of transglutaminases was mainly observed in tail, where lysis was increased significantly. It has been previously shown that endogenous lysis is due to uPA and localised in to the head. Inhibition of transglutaminases caused minor increase in rate of endogenous lysis. A patient deficient in FXIII was also studied and model thrombi were found to be markedly less stable. FXIII therapy increased stability of thrombus. These data show that transglutaminases are important in stabilization of thrombi to thrombolysis, but that endogenous fibrinolytic activity is not affected by the degree of cross-linking. Potential role for TG2 in thrombus was also studied. TG2 was found to be present in model and arterial thrombi by immunohistochemistry and also by western blotting. Addition of purified TG2 from guinea pig to forming thrombi stabilised model thrombi, whereas additional FXIII had no effect using normal blood. Inhibition of TG2 in model thrombi was attempted using neutralising antibody and allosteric inhibitor of TG2, the GTPγS. FXIIa is the main transglutaminase responsible for stabilisation of fibrin in model thrombus system.