Inner Medullary Urea Transporters Contribute to Development of Renal Fibrosis in Mice With Unilateral Ureteral Obstruction

Abstract

Fourteen percent of the population of the United States is affected by chronic kidney disease (CKD). Although the mechanisms that are responsible for CKD remain unclear, there are many clinical and animal studies showing that low protein diets reduce the long term effects of CKD. One possible explanation for these benefits is the restricted urea load of the low protein diet suggesting a role for the urea transporters in this process. Unilateral ureteral obstruction (UUO) is a well known model to produce fibrogenic responses such as increased capillary macrophages, pro‐fibrogenic proteins, interstitial matrix, myofibroblasts, etc. in CKD rodents. We hypothesized that the urea transporters contribute to the development of renal fibrosis. We used UUO mice receiving a maximal urea load (high protein diet) to investigate contributions of urea transporters to renal fibrosis. C57Bl6 mice ± genetic ablation of UT‐A1 and UT‐A3 urea transporters (UT‐A1/A3 KO) underwent UUO surgery (ligation of the left proximal ureter). Four days after surgery mice were given a high (40%) protein diet for 10 further days. Mice were killed and obstructed kidneys collected for protein and histochemical analysis. All of the obstructed kidneys showed evidence of hydronephrosis after 14 days. BUN was unchanged by UUO: WT control: 44.2 mg/dl, WT UUO: 46.8 mg/dl, KO control: 43.4, KO UUO: 47.1. TGF‐b was increased in both WT and UT‐A1/A3 KO mice with UUO (302% and 389% respectively). WT with UUO showed a 320% increase in α‐SMA vs mice without UUO. UT‐A1/A3 KO mice with UUO showed a blunted 271% increase in α‐SMA vs unobstructed UT‐A1/A3 KO mice. Vimentin, another marker of fibrosis, was significantly increased 555% by UUO (vs no UUO). UUO in UT‐A1/A3 KO mice did not increase vimentin. H&E staining revealed similar infiltration of inflammatory cells in both UUO WT and UUO UT‐A1/A3 KO mice. Trichrome staining showed higher collagen levels in UUO WT vs UUO UT‐A1/A3 KO mice. Immunohistochemical staining for αSMA and vimentin revealed higher levels in UUO WT vs UUO UT‐A1/A3 KO mice. These data suggest that urea transporters do not prevent inflammation caused by UUO. However, the overall development of fibrosis in UUO mice fed a high protein diet is attenuated by the absence of urea transporters suggesting that urea reabsorption by urea transporters contributes to renal fibrosis. If urea transport inhibitors replicate the effects of genetic knock out, they may represent a promising future therapy.Support or Funding InformationRelypsa Nephrology and Cardiology Fellowship Grant;NIH RO1 DK41707This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.