Characterization of the intracellular signalling capacity of natural FXa mutants with reduced pro-coagulant activity

IntroductionFactor X (FX) is a serine-protease playing a crucial role in the blood coagulation pathway and triggering intracellular signalling in a variety of cells via protease-activated receptors (PARs). By exploiting naturally occurring variants (V342A and G381D, catalytic domain; E19A, γ-carboxyglutamic acid (GLA)-rich domain), we investigated the relationship between the pro-coagulant activity and the signal transduction capacity of FX.Materials and methodsRecombinant FX (rFX) variants were expressed in Human Embryonic Kidney cells and purified by immunoaffinity chromatography. Activated rFX (rFXa) variants were characterized for pro-coagulant, amidolytic and thrombin generation activity. rFXa signalling was assessed through evaluation of extracellular signal-regulated kinase 1 and 2 (ERK1/2) phosphorylation in C2C12 myoblasts.Results and conclusionsrFX variants showed reduced (rFX-342A, 29%; rFX-19A, 12%) or not detectable (rFX-381D) amidolytic activity. Thrombin generation activity in a plasma system was also decreased either upon activation by Russell's viper venom (rFX-342A, 38%; rFX-19A, 7%; rFX-381D, not detectable) or by the extrinsic pathway (rFX-342A, 36%; rFX-19A, rFX-381D, not detectable). The rFXa-381D mutant displayed little or no enzymatic activity, and did not induce any appreciable signal transduction capacity. The rFXa-342A mutant induced a dose-dependent signalling with a 50% reduced signalling capacity. At the highest concentration (174 nM), signalling progressed with a time course similar to that of rFXa-wt. Zymogen rFX-19A showed defective and incomplete activation resulting in strongly reduced enzymatic activity and signalling. Taken together our data are consistent with a close correlation between pro-coagulant activity and intracellular signalling capacity.