Patients with proteinuric kidney diseases often experience symptoms of salt and water retention. It has been hypothesized that the dysregulated Na+ absorption is due to increased proteolytic cleavage of epithelial sodium channel (ENaC) and increased Na,K-ATPase expression. Microarray analysis identified a reduced corin mRNA expression in kidneys from rat models of puromycin aminonucleoside-induced nephrotic syndrome (PAN) and acute anti-Thy1 glomerulonephritis (GN). Corin has been shown to convert pro-atrial natriuretic peptide (ANP) to ANP. Because ANP resistance has been assumed to be a mechanism accounting for volume retention, experiments were undertaken to analyze the renal expression and function of corin. Immunohistochemistry revealed that corin co-localized with ANP. In PAN and GN, kidneys exhibited concomitant increased pro-ANP and decreased ANP protein expression levels consistent with low corin levels. Importantly, kidneys from corin −/− mice showed increased levels of renal β-ENaC, phosphodiesterase 5 (PDE5) and protein kinase G II (PKGII) when compared to wild-type mice. Similar expression profile was observed in cell culture experiments suggesting that the increase in PDE5 and PKGII could account for the increase in β-ENaC as observed in PAN and GN.
To conclude, our data provide novel insights into the mechanisms of volume retention in renal disease with corin as an important new mediator that acts through PKGII induction and ENaC activation.
In many kidney diseases, the original insult primarily involves the glomerulus and may then pass onto the tubulointerstitium. Several hypotheses link glomerular disease to tubular injury; perhaps the foremost hypothesis involves chronic tubular hypoxia. The reported effects of hypoxia and consecutive stabilization of hypoxia-inducible factors (HIFs), however, are controversial. Hypoxia induces interstitial fibrosis but also has beneficial effects on renal disease progression when HIF is activated pharmacologically. To analyze the impact of HIF on tubulointerstitial disease development in primary glomerular disease, transgenic von Hippel Lindau (VHL)-knockout mice were generated and null expression was induced before the onset of autoimmune IgG-mediated anti-glomerular basement membrane glomerulonephritis (GN). Tubular VHL knockout and, thus, local HIF-α stabilization increased renal production of vascular endothelial growth factor, tumor growth factor-β(1), and platelet-derived growth factor-B, resulting in augmented formation of capillaries and interstitial matrix, and conversion of fibroblasts to myofibroblasts. Within the glomerular disease, VHL knockout reduced the glomerular damage and attenuated tubulointerstitial injury. Likewise, proteinuria, plasma urea concentration, and tubulointerstitial matrix were decreased in VHL knockout with GN. These findings shown that tubular HIF-α stabilization in glomerular disease is beneficial for disease outcome. In comparison with VHL knockout alone, GN is a much stronger activator of fibrosis such that stimuli other than hypoxia may be considered important for renal disease progression.
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