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

Introduction: Factor 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 methods: Recombinant 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 conclusions: rFX 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.