The role of tissue transglutaminase in the extracellular matrix changes associated with kidney scarring

The up-regulation and trafficking of tissue transglutaminase (TG2) has been implicated in the development of kidney scarring and the progression of chronic kidney disease (CKD). TG2 is a calcium dependent enzyme that catalyses the crosslinking of proteins via the formation of highly stable epsilon-(gamma-glutamyl) lysine bonds. It is proposed that the increased formation of these bonds within the ECM contributes to scarring directly by enhancing ECM deposition and impeding proteolytic degradation. To investigate this, ECM metabolism was studied in vitro in tubular epithelial and mesangial cells stably transfected to over-express TG2 and in cells isolated from the TG2 knockout mouse. Further, in vivo TG2 inhibition studies were performed in rats subjected to 5/6ths nephrectomy. Increased expression of TG2 in tubular epithelial cells corresponded to increased extracellular TG2 activity (p<0.05), elevated epsilon(gamma-glutamyl) lysine crosslinking in the ECM and higher levels of ECM collagen. Increased ECM collagen was attributable to elevated collagen III and IV. Elevated TG2 levels were associated with accelerated collagen deposition and reduced ECM breakdown by matrix metalloproteinases (MMPs). In contrast, a lack of tubular TG2 was associated with reduced epsilon(gamma-glutamyl) lysine crosslinking in the ECM, causing reduced ECM collagen levels and lower ECM per cell. TG2 over-expressing mesangial cells differed from tubular cells in their handling of TG2 by retaining the enzyme and failing to show increases in extracellular activity, epsilon(gamma-glutamyl) lysine crosslinking or ECM levels. The in vivo application of site directed irreversible TG2 inhibitors strongly supported a causative role for TG2 in the accumulation of ECM associated with scarring. Inhibition of TG2 was effective at preventing a decline in kidney function by reducing glomerulosclerosis and tubulointerstitial fibrosis through a lowering in the accumulation of interstitial collagen. The data presented here demonstrates a direct pathological role for TG2 in the accumulation of matrix associated with kidney scarring.