Elevated TGFβ–Smad signalling in experimental Pkd1 models and human patients with polycystic kidney disease

Autosomal dominant polycystic kidney disease (ADPKD) is a common inherited renal disease characterized by many fluid‐filled cysts and interstitial fibrosis in the kidneys, leading to chronic renal failure. During cystogenesis the renal tubules undergo extensive structural alterations that are accompanied by altered cellular signalling, directly and/or indirectly regulated by the PKD1 and PKD2 proteins. Since transforming growth factor (TGF)‐β signalling modulates cell proliferation, differentiation, apoptosis, adhesion and migration of various cell types, we studied the activation of this signalling pathway in Pkd1‐mutant mouse models at different stages of the disease. Therefore, we analysed expression of the TGFβ–Smad signalling pathway and its target genes in different Pkd1 mutant mouse models in various stages of polycystic disease. Nuclear accumulation of P‐Smad2 in cyst lining epithelial cells was not observed in the initiation phase but was observed at mild and more advanced stages of PKD. This coincides with mild fibrosis and increased mRNA levels of TGFβ target genes, such as fibronectin, collagen type I, plasminogen activator inhibitor 1 and matrix metalloproteinase‐2. At this stage many interstitial fibroblasts were found around cysts, which also showed nuclear localization for P‐Smad2. However, bone morphogenetic protein (BMP) signalling, which can antagonize TGFβ signalling, is not affected, since nuclear expression of P‐Smad1/5/8 and expression of the BMP target gene, inhibitor of DNA binding/differential‐1 (ID‐1) is not altered compared to wild‐type controls. Also, human kidneys with progressive ADPKD showed increased nuclear localization of P‐Smad2, while in general expression of P‐Smad1/5/8 was weak. These results exclude TGFβ signalling at the initiation of cystogenesis, but indicate an important role during cyst progression and in fibrogenesis of progressive ADPKD. Copyright © 2010 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.