The role of activated coagulation factor XII in overall clot stability and fibrinolysis

Activated coagulation factor XII (alpha-FXIIa) is able to bind to fibrin(ogen) and increases the density and stiffness of the fibrin clot. Conversely, proteins of the contact system and the fibrinolytic system show a high degree of homology and alpha-FXIIa can convert plasminogen into plasmin resulting in fibrin degradation. Therefore, we studied the contribution of alpha-FXIIa to overall clot stability and plasmin driven fibrinolysis in the absence and presence of tissue plasminogen activator (tPA). We observed that alpha-FXIIa directly converted plasminogen into plasmin and reduced clot lysis time at all tPA concentrations tested (15-1500 pM). Simultaneous assessment of plasmin generation (chromogenic substrate S-2251) and fibrin formation and degradation (absorbance at 405 nm), showed an earlier onset of fibrinolysis and plasmin formation in the presence of alpha-FXIIa. Fibrinolysis of clots formed under flow conditions, revealed that incorporation of alpha-FXIIa accelerated clot breakdown (fluorescence release of labeled fibrin) by additional plasmin generation on top of formation by tPA. Scanning electron microscopy (SEM) revealed that the surface area pore size increased in the presence compared with the absence of alpha-FXIIa when fibrinolysis was initiated by the conversion of plasminogen with tPA during clot formation. alpha-FXIIa enhances fibrinolysis in the presence of plasminogen, irrespective of whether tPAwas present during clot formation or was added afterwards to initiate fibrinolysis. We postulate that FXIIa first strengthens the clot structure during clot formation and thereafter contributes towards fibrinolysis.