Increased release and activity of transglutaminase-2 (TG2) in the tubulointerstitial space has been correlated with the progression of kidney fibrosis during chronic kidney disease (CKD), a condition for which there is no cure. TG2 extracellular activity contributes to kidney fibrosis by determining matrix deposition and resistance to degradation, through calcium dependent protein transamidation and matrix recruitment of latent TGF-β1, however, the mechanism of TG2 secretion from kidney cells is still unknown. This study aims at elucidating the mechanism of TG2 trafficking in CKD as a way to control its release and ultimately limit fibrosis progression. The murine unilateral ureteric obstruction (UUO) model of renal fibrosis was employed. Firstly, the UUO proteome was resolved and the effect of TG2-KO elucidated by quantitative proteomics of kidney homogenates. This led to the identification of markers of CKD strongly upregulated at an advanced stage of UUO, some of which were dysregulated in TG2-null kidneys, and allowed to set a background for subsequent analyses. Secondly, the TG2 interactome was generated by developing an original approach, based on quantitative mass spectrometry of TG2 immunoprecipitates. This highlighted a significant association of TG2 with a large cluster of vesicular proteins which increased post-UUO, forming the hypothesis that TG2 could be secreted by extracellular vesicles (EV) in CKD. The hypothesis was tested in a model of established tubular epithelial cells (TEC), which revealed TG2 in EV mainly of intraluminal origin (exosomes). The involvement of cell surface heparan sulfate proteoglycan (HSPG) syndecan-4 in the secretion pathway of TG2 was also investigated, and a dual role was ascribed to the proteoglycan, supporting both TG2 secretion and extracellular retention of the enzyme, with promotion of TGF-β1 activation.
In conclusion, this study has shown a novel Sdc4-dependent secretion pathway for TG2 by TECs via extracellular vesicles, which is relevant to CKD condition
