Transglutaminase transcription and antigen translocation in experimental renal scarring

It was recently demonstrated that renal tissue transglutaminase (tTg) prote in and its catalytic product the epsilon(gamma-glutamyl) lysine protein cro ss-link are significantly increased in the subtotal (5/6) nephrectomy model (SNx) of renal fibrosis in rats. It was proposed that the enzyme had two i mportant physiologic functions in disease development; one of stabilizing t he increased extracellular matrix (ECM) by protein crosslinking, the other in a novel form of tubular cell death. This study, using the same rat SNx m odel, demonstrates first by Northern blotting that expression of tTg mRNA w hen compared with controls is increased by day 15 (+70% increase, P < 0.05) , then rises steadily, peaking at day 90 (+391%, P < 0.01), and remains ele vated at 120 d (+205%, P < 0.05) when compared with controls. In situ hybri dization histochemistry demonstrated that the tubular cells were the major site of the additional tTg synthesis. Immunohistochemistry on cryostat sect ions revealed a sixfold increase (P < 0.001) in ECM-bound tTg antigen at 90 -d post-SNx, whereas in situ transglutaminase activity demonstrated by the incorporation of fluorescein cadaverine into cryostat sections indicated a 750% increase (P < 0.001) on day 90 in SNx animals. This increased activity was extracellular and predominantly found in the peritubular region. These results indicate that increased tTg gene transcription by tubular cells un derlies the major changes in renal tTg protein reported previously in SNx r ats, and that the presence of the epsilon(gamma-glutamyl) lysine cross-link s in the extracellular environment is the result of the extracellular actio n of tTg. These changes may be in response to tubular cell injury during th e scarring process and are likely to contribute to the progressive expansio n of the ECM in renal fibrosis.