Tissue transglutaminase drives fibrin β-chain cross-linking: a novel fibrin modification observed in trauma patients.

Covalent crosslinking of fibrin by the plasma transglutaminase coagulation factor XIII (FXIII) is a key determinant of blood clot stability and function. FXIII-catalyzed formation of ε-N-(γ-glutamyl)-lysyl crosslinks is restricted to the fibrin γ- and α-chains and follows thrombin driven fibrin polymerization. Fibrinogen is also crosslinked by tissue transglutaminase (TG2) in a reaction favoring intra- and intermolecular a-g crosslinking. Emerging evidence points to fibrinogen as a relevant substrate of TG2 in conditions of acute tissue damage. Remarkably, beyond detection of prototypical FXIII-directed crosslinks (i.e., a-a, g-g), we identified entirely novel covalent crosslinks involving the fibrinogen β chain (i.e., b-a, via FGB-Q82). Addition of TG2 to in vitro clotting reactions and analysis of fibrin(ogen) in reducing conditions revealed loss of β chain polypeptide paired with formation of high-molecular weight β chain species. Mass spectrometry-based crosslinking proteomic analysis of in vitro clots recapitulated the precise TG2-directed β chain crosslinks observed in clots made using the plasma of trauma patients. The results are the first to document in vitro and ex vivo crosslinking of the fibrin β chain and highlight a novel example of TG2 emerging as a relevant plasma transglutaminase.