Activated platelets retain and protect most of their factor XIII-A cargo from proteolytic activation and degradation.

Platelet factor (F)XIII-A is a major cytoplasmic protein (~3% of total) representing ~50% of total circulating FXIII. However, mobilization of FXIII-A during platelet activation is not well defined. To determine mechanisms mediating the retention versus release of platelet FXIII-A, platelets from healthy humans and mice (F13a1-/-, Fga-/-, Plg-/-, Stim1fl/fl, Pf4-Cre and respective controls) were stimulated with thrombin, convulxin+thrombin, or calcium ionophore (A23187), in the absence or presence of inhibitors of transglutaminase activity, mRNA translation, microtubule rearrangement, calpain, and Rho GTPase. Platelet releasates and pellets were separated by (ultra)centrifugation. FXIII-A was detected by immunoblotting and immunofluorescence microscopy. Even following strong dual agonist (convulxin+thrombin) stimulation of human platelets, >80% platelet FXIII-A remained associated with the platelet pellet. In contrast, essentially all tissue factor pathway inhibitor, another cytoplasmic protein in platelets, was released to the supernatant. Pellet-associated FXIII-A was not due to de novo synthesis via platelet F13A1 mRNA. The proportion of platelet FXIII-A retained by, versus released from, activated platelets was partly dependent on STIM1 signaling, microtubule rearrangement, calpain, and RhoA activation, but did not depend on the presence of fibrinogen or plasminogen. Immunofluorescence microscopy confirmed the presence of considerable FXIII-A within the activated platelets. Whereas released FXIII-A was cleaved to FXIII-A* and could be degraded by plasmin, platelet-associated FXIII-A remained uncleaved. Retention of substantial platelet-derived FXIII-A by activated platelets, and its reduced susceptibility to thrombin- and plasmin-mediated proteolysis, suggests platelet FXIII-A is a protected pool with biological role(s) that differs from plasma FXIII.