Transglutaminase-2: Nature's Glue in Lung Fibrosis?

Glaesserella parasuis induces tissue transglutaminase-mediated fibrinogen crosslinking through NF-κB activation

Glaesserella parasuis (GPS) is the causative agent of Glässer's disease, leading to significant economic losses in the global swine industry. During post-mortem inspection, the main observation in pigs affected by Glässer's disease is the presence of serofibrinous or fibrinopurulent exudate on the mucosal surface. Nevertheless, the mechanism by which fibrinogen is converted into a fibrin clot during Glässer's disease is not fully understood. In this study, we discovered in the liver, lung, and kidney, that GPS infection upregulates the expression of tissue transglutaminase (tTG) and promotes the co-localization of tTG with fibrin. In porcine aortic endothelial cells, knockdown of tTG significantly reduced fibrinogen cross-linking and pro-inflammatory factor production after GPS infection. In addition, in investigating the mechanism of tTG upregulation by GPS infection, inhibitor assays revealed the involvement of the NF-κB signaling pathway in the upregulation of tTG expression during GPS infection. Further data from dual-luciferase assays and chromatin immunoprecipitation confirmed that phosphorylated p65 binding to the tTG promoter sequences increased tTG expression and the specific binding site was discovered at GACCTTCCCT (-1082 to -1072 bp), AGGGAAATTG (-807 to -797 bp), and TAAGTTCCCC (+22 to +32 bp). The above results indicate that GPS infection may promote the cross-linking of fibrinogen by tTG, thereby mediating exudative fibrinous inflammation, providing new insights into the pathogenesis of GPS infection, and suggesting potential molecular targets for therapeutic intervention.

Biomechanical Force and Cellular Stiffness in Lung Fibrosis

Lung fibrosis is characterized by the continuous accumulation of extracellular matrix (ECM) proteins produced by apoptosis-resistant (myo)fibroblasts. Lung epithelial injury promotes the recruitment and activation of fibroblasts, which are necessary for tissue repair and restoration of homeostasis. However, under pathologic conditions, a vicious cycle generated by profibrotic growth factors/cytokines, multicellular interactions, and matrix-associated signaling propagates the wound repair response and promotes lung fibrosis characterized not only by increased quantities of ECM proteins but also by changes in the biomechanical properties of the matrix. Importantly, changes in the biochemical and biomechanical properties of the matrix itself can serve to perpetuate fibroblast activity and propagate fibrosis, even in the absence of the initial stimulus of injury. The development of novel experimental models and methods increasingly facilitates our ability to interrogate fibrotic processes at the cellular and molecular levels. The goal of this review is to discuss the impact of ECM conditions in the development of lung fibrosis and to introduce new approaches to more accurately model the in vivo fibrotic microenvironment. This article highlights the pathologic roles of ECM in terms of mechanical force and the cellular interactions while reviewing in vitro and ex vivo models of lung fibrosis. The improved understanding of the fundamental mechanisms that contribute to lung fibrosis holds promise for identification of new therapeutic targets and improved outcomes.