Key Points• Factor XIII-A is exposed in protruding caps on the activated platelet surface.• Platelet FXIII-A exerts antifibrinolytic function by cross-linking a 2 AP to fibrin.Factor XIII (FXIII) stabilizes thrombi against fibrinolysis by cross-linking a 2-antiplasmin (a 2 AP) to fibrin. Cellular FXIII (FXIII-A) is abundant in platelets, but the extracellular functions of this pool are unclear because it is not released by classical secretion mechanisms. We examined the function of platelet FXIII-A using Chandler model thrombi formed from FXIIIdepleted plasma. Platelets stabilized FXIII-depleted thrombi in a transglutaminase-dependent manner. FXIII-A activity on activated platelets was unstable and was rapidly lost over 1 hour. Inhibiting platelet activation abrogated the ability of platelets to stabilize thrombi. Incorporating a neutralizing antibody to a 2 AP into FXIII-depleted thrombi revealed that the stabilizing effect of platelet FXIII-A on lysis was a 2 AP dependent. Platelet FXIII-A activity and antigen were associated with the cytoplasm and membrane fraction of unstimulated platelets, and these fractions were functional in stabilizing FXIII-depleted thrombi against lysis. Fluorescence confocal microscopy and flow cytometry revealed exposure of FXIII-A on activated membranes, with maximal signal detected with thrombin and collagen stimulation. FXIII-A was evident in protruding caps on the surface of phosphatidylserine-positive platelets. Our data show a functional role for platelet FXIII-A through exposure on the activated platelet membrane where it exerts antifibrinolytic function by cross-linking a 2 AP to fibrin. (Blood. 2014;124(26):3982-3990)
IntroductionFactor XIII (FXIII) plays an essential role in normal hemostasis where it contributes to the regulation of fibrinolysis, 1-3 the maintenance of pregnancy, 4 wound healing, and angiogenesis. 5 The function of FXIII in hemostasis is further emphasized in its deficiency, which results in hemorrhage 6,7 and slow wound healing. 4,7 FXIII circulates in plasma as a heterotetramer (FXIII-A 2 B 2 ) where the catalytic A (FXIII-A 2 ) subunits are almost exclusively in complex with the inhibitory carrier B (FXIII-B 2 ) subunits. 8 FXIII is activated by the combined action of thrombin and Ca 21 to form the active transglutaminase (TG) FXIIIa. 9,10 FXIIIa confers stability to the fibrin matrix by cross-linking fibrin, substantially altering its rheologic properties.11,12 TG catalyzes formation of covalent e-(g-glutamyl) lysyl bonds 13 in which the lysine e-amino group is cross-linked to the glutamine g-carboxymide group.
14The cross-linking of g-chains confers a degree of rigidity to the fibrin network, which is further stabilized by the cross-linking of a-chains 15 to generate high-molecular-weight polymers. 10,16 FXIIIa also cross-links inhibitors of fibrinolysis to fibrin, further increasing its insolubility and resistance to plasmin. These inhibitors include a 2 -antiplasmin (a 2 AP), 17 thrombin activatable fibrinolysis inhibitor, 18 and plasminogen ac...