PB0592 - Evaluation of the Role of Skeletal Muscle Myosin as a Cofactor for Factor XI Activation of Factor XII

Background: We recently showed that the procoagulant activity of skeletal muscle myosin (SkM) includes binding of coagulation factor (F) XI to enhance FXI activation by thrombin. It is unknown whether the procoagulant activities of SkM include additional mechanisms for potentiating FXII or FXI autoactivation or FXII activation by FXIa, as has been observed with polyphosphates.

Aims: Determine if SkM serves as a cofactor for FXII or FXI autoactivation, or for FXII activation by FXIa

Methods: Citrated platelet-poor plasmas collected from healthy volunteers were tested in clotting time assays in the absence or presence of different concentrations of SkM (25-100nM) and specific contact pathway inhibitors (corn trypsin inhibitor [CTI], anti-FXII antibody 5C12, and anti-FXI antibodies 1A6 and 14E11). Whole blood thromboelastography was performed in the absence or presence of 100nM SkM. Fibrin generation was analyzed by measuring turbidity with a spectrophotometer. FXII and FXI autoactivation and FXII activation by FXIa was measured by chromogenic assays. Data were analyzed by paired Wilcoxon Rank Sign tests.

Results: SkM shortened the clotting time of recalcified plasma in a concentration-dependent manner. Moreover, SkM exhibited procoagulant activity even when clotting was initiated with 0.3pM tissue factor, a condition sensitive to FXI activation and activity. The procoagulant effect of 50nM SkM on recalcified plasma was abrogated by the contact pathway inhibitors CTI, 5C12, 1A6, and 14E11. SkM reduced the initiation time and increase of fibrin generation, as well as decreased the initiation R time for thromboelastography. While SkM did not support FXII or FXI autoactivation, we found that SkM enhanced the rate of FXII activation by FXIa.

Conclusion(s): SkM may serve as a cofactor in the contact activation pathway by enhancing FXI activation by thrombin and FXII activation by FXIa. Future work will examine the role of the procoagulant activity of SkM in the setting of hemostasis and acute trauma.