The Role of Xanthine Dehydrogenase (XDH) and Uric Acid in the Kidney Development and Renal Injury

Abstract

Uric acid is the end product of purine catabolism in humans and is capable of acting as both an antioxidant scavenging reactive oxygen species and conditionally a pro‐oxidant. Pathophysiological changes in serum uric acid levels (hyperuricemia and hypouricemia) are associated with prevalent diseases, including hypertension and chronic kidney disease as well as rare disorders, such as hereditary xanthinuria. Despite the clear association between uric acid dysregulation and pathology, the mechanisms involved remain unclear. XDH and xanthine oxidase (XO) (together known as Xanthine oxidoreductase (XOR)) is a complex enzyme group that catalyzes the oxidation of hypoxanthine to xanthine and xanthine to uric acid. To assess the role of this pathway beyond purine catabolism, we created a genetic ablation of the Xdh gene in the Dahl salt‐sensitive rat background (SSXdh−/−). Sequencing of SSXdh−/− genomic DNA confirmed a 7‐base‐pair‐deletion in exon 4 of the Xdh gene located on chromosome 6. The absence of Xdh protein expression in homozygotes and a reduction in heterozygotes was confirmed by both Western blot and immunohistochemistry analyses. SSXdh−/− rats exhibited early mortality with 7 out of 28 rats not surviving till 6 weeks of age compared to SSXdh+/− and SSXdh+/+ littermates, all of whom survived. Plasma uric acid of 6 weeks old male rats was 0.25±0.03 mg/dl (N=6) in SSXdh+/+ rats and decreased to a level out of the detectable range by the test in the SSXdh−/− group. The 24hr uric acid excretion was also dramatically decreased in SSXdh−/− rats compared to SSXdh+/+ littermates. Masson’s Trichrome staining revealed severe renal damage with an increase in fibrosis (1 and 14% (N=3 and 2 for male SSXdh+/+ and SSXdh−/−, respectively), and dilated tubules in the SSXdh−/− rats. Total body weight was decreased in SSXdh−/− rats, indicating a failure to thrive compared to control rats (190 ± 4 and 58 ± 8 g; N=6 and 4 for male SSXdh+/+ and SSXdh−/−). Additionally, In SSXdh−/− rats at 6 weeks, Western blot analysis showed a lack of connective tissue growth factor (CTGF), which is known to be critical in the nephrogenesis. No apparent sex‐specific differences in development or kidney injury were identified. Our results suggest that the genetic ablation of Xdh caused uric acid depletion and severe renal damage. Therefore, XOR may be essential for the development of the kidney and survival of the rats by regulating purine catabolism.Support or Funding InformationSupported by the Department of Veteran Affairs Grant I01 BX004024 and the National Institutes of Health Grants R35 HL‐135749.