A novel way for transglutaminase 2 to mitigate liver damage Free

In this issue of Blood, Wei et al demonstrate that tissue transglutaminase (TG2) can help mitigate liver damage by dynamically altering the structure of extravascular fibrin(ogen), thus providing hepatoprotective effects.¹ 

Factor XIII (FXIII) and TG2 are transglutaminases capable of introducing cross-links into substrates that involve distinct glutamine and lysine side chains. Plasma FXIII exists as an A₂B₂ dimer that is activated by thrombin in the presence of calcium. TG2 is a monomeric protein under GTP control, and its activation is driven by multiple calcium ions.² TG2 has typically been viewed as a “nonhemostatic” transglutaminase. Under healthy conditions, the plasma levels of TG2 are much lower than those of FXIII. However, TG2 levels can increase significantly with acute liver injuries such as an overdose of acetaminophen (APAP) where heightened extravascular fibrin(ogen) deposition occurs.³ The authors tested the hypothesis that TG2 modifies FXIIIa-directed fibrin(ogen) cross-linking in vitro and within APAP-injured liver. This project provides novel results that build on excellent prior studies from the authors.^4-6 The current study explored how TG2 and FXIII can generate hepatoprotective fibrin(ogen) in an injured liver.

The livers of APAP-challenged mice were found to contain increased levels of fibrinogen (Aα, Bβ, and γ), FXIII, and TG2. The TG2 levels were 7 times higher than those of FXIII. Fibrinogen that is normally aligned with areas of necrosis became far more varied when TG2^−/− mice were subjected to APAP challenge. Such results indicate that TG2 deficiency alters hepatic fibrin(ogen) deposition in APAP-challenged mice. Additional studies revealed that TG2 modifies FXIIIa-catalyzed cross-linking of hepatic fibrin(ogen) in APAP-challenged mice. Although FXIIIa was found to take the lead in forming γ-γ cross-links in APAP-injured levels, such cross-linking could be suppressed by TG2. Finally, TG2 deficiency was observed to occur to injured livers after APAP challenge.

In-depth proteomic studies made it possible to examine protein-protein cross-linking and obtain detailed information on the fibrin(ogen) cross-linking partners. Such studies were done with concentrations of TG2 and FXIII comparable to those found in injured livers. The accompanying mass spectrometry–based studies revealed that FXIIIa generates more robust fibrin γ-γ cross-linking than with the TG2-based reactions.⁷ Furthermore, the presence of TG2 could reduce the amount of FXIIIa-catalyzed fibrin γ-γ. In addition, TG2 production of fibrin α-γ cross-linking was hindered in the presence of FXIIIa. Interestingly, there was a synergistic increase in fibrin α-α cross-links when both transglutaminases were present. This study is the first reported evidence for such synergistic α-α cross-linking involving a combination of TG2 and FXIIIa. Unlike plasma FXIII A₂B₂, TG2 does not exhibit high affinity for the fibrinogen γ chain.⁸ As a result, more TG2 may be available for fibrin α-α cross-links. Another possibility is that TG2 has greater specificity for the unique structure features found in specific ordered or disordered regions of fibrin α.

The FXIII and TG2 systems could be further studied using mice expressing fibrinogen that is resistant to TG2 cross-linking. Duval and coworkers have already shown the utility of working with fibrinogen mutants, where the ability to make FXIIIa-catalyzed fibrin γ-γ cross-links was hindered.⁹ Such exciting studies could similarly be pursued in new projects once a complete map of TG2-cross-linked fibrin(ogen) becomes available. Results could then be compared with the cross-linking maps already available for FXIII cross-links.⁷ From such studies, the exact role of TG2-mediated fibrinogen cross-linking in acute liver conditions could be better addressed.

In this issue of Blood, Wei and colleagues have identified a novel mechanism whereby the fibrin(ogen) cross-linking observed in experimental liver injuries is mediated by TG2. It is important to note that the cross-linking documented in the injured liver is largely comparable to those observed in vitro using pathological ratios of TG2 and FXIII. This project makes highly valuable advancements to the field by demonstrating that the traditionally nonhemostatic TG2 is a critical contributor to fibrin(ogen) clot cross-linking.

The present studies also established a new approach to address a series of research areas in greater depth. The combination of TG2 and FXIII may be retooled to help with treating FXIII deficiency, taking advantage of the synergistic effects. There is also a definite need to further probe new roles of TG2 in trauma and chemical injury. The authors have clearly confirmed their hypothesis that TG2 modifies FXIIIa-directed fibrin(ogen) cross-linking in vitro and within APAP-injured liver. They are well poised to take their TG2 projects into new research directions that go beyond the toxicological basis of liver injuries.

Conflict-of-interest disclosure: M.C.M. declares no competing financial interests.