A Potential Regulatory Role of Xanthine Dehydrogenase (XDH) in the Kidney Development and Damage

Abstract

There is a clear association of uric acid (UA) dysregulation in cardiovascular and chronic kidney diseases elucidated in the literature. Xanthine dehydrogenase (XDH) encodes for XDH enzyme that catalyzes hypoxanthine oxidation to xanthine and xanthine to UA, the end-product of purine catabolism in humans. UA can act as both an antioxidant scavenging reactive oxygen species (ROS) and conditionally a pro-oxidant. The lack of understanding of UA metabolism and its impact on CKD progression reflects the current controversy regarding the clinical management of the related pathologies. To explore mechanistic insights into UA affecting the kidney, we created a genetic ablation of the Xdh gene in the Dahl salt-sensitive rat background (SSXdh-/-). The absence of Xdh protein expression in SSXdh-/- was confirmed by Western blotting and immunohistochemistry analyses. The knockout rat is underdeveloped (179 ± 6 and 70 ± 7 g: N=9 for both male SSXdh+/+ and SSXdh-/-) and hypouricemic (UA in plasma below the level of detection). RNA-Seq analysis of kidney tissue of the SSXdh-/- and SSXdh+/+ 6-week-old rats revealed 4738 differentially expressed genes. The functional analyses of the obtained data were performed using the ingenuity pathway analysis (IPA) software. The changes in oxidative phosphorylation and the presence of mitochondrial dysfunction were among the top canonical pathways associated with the differentially expressed genes. The cardiac hypertrophy signaling pathway was enhanced the most (z-score 6.5). IPA toxic function analysis suggested the presence of renal damage. Among the upregulated genes were transforming growth factor-β (Tgf-β) and connective tissue growth factor (Ccn2), previously reported to drive the tissue fibrosis. Indeed, SSXdh-/- rats show increased levels of fibrosis in the kidneys. Western blot analysis revealed Ctgf lacking in the SSXdh-/- rats despite the increased mRNA levels. Ctgf is essential for kidney development, and the lack of this protein might explain the underdeveloped kidneys seen in the SSXdh-/- rat. Furthermore, we analyzed the expression of the primary transporters for UA in the kidney: Urat1 and Glut9, encoded by Slc22a12 and Slc2a9 genes, respectively. Western blot analysis revealed that Urat1 is not detectable and Glut9 expression is decreased in SSXdh-/- rats; potential glycosylation of Glut9 in SSXdh-/- rats is also observed. Catalase enzyme encoding Cat gene is also downregulated (log FC=-3.05) in SSXdh-/- rats, indicating the potential ROS production increase. These results provide insights into the regulatory role of the Xdh gene beyond purine catabolism that involves kidney tissue development and kidney damage.