Myocardin-related transcription factor (MRTF) mediates epithelial fibrogenesis in polycystic kidney disease
Background: Autosomal dominant polycystic kidney disease (PKD) is characterized by extensive cyst formation and progressive fibrosis. However, the molecular mechanisms whereby the loss/loss-of-function of polycystin 1 or 2 (PC1/2) provokes fibrosis are largely unknown. The small GTPase RhoA has been implicated in cystogenesis, while we have shown that the RhoA/cytoskeleton/myocardin-related transcription factor (MRTF) pathway is a key inducer of epithelium-induced fibrogenesis. Therefore, we hypothesized that MRTF 1) is activated by PC1/2 loss, and 2) plays a critical role in fibrogenic repogramming of the epithelium and the subsequent induction of fibrosis. Methods: PC1/2 loss was achieved by gene silencing in tubular cells (in vitro) and in PKD1 (RC/RC) and PKD2 (ws25/-) mice (in vivo). RhoA activation was measured by pull-down assays and immunofluorescence. MRTF localization (nuclear/cytosolic ratio, intensity) was quantified in large cell populations and in renal tissue using automated image analysis. PC1/PC2 loss-promoted, MRTF-dependent gene expression was followed by qPCR, RNAseq and RNAscope. MRTF-dependent paracrine effects were assessed by a bioassay. Results:  Loss of PC1 or PC2 activated RhoA in vitro and in vivo, resulting in cytoskeletal remodelling and robust nuclear MRTF translocation, accompanied by increased MRTF expression. Nuclear translocation occurred predominantly in dilated tubules, while overexpression in the cyst-lining epithelium. A large cohort of PC1/PC2 downregulation-induced genes was MRTF-dependent, including cytoskeletal, integrin-related, and matricellular/fibrogenic proteins. Epithelial MRTF was necessary for paracrine priming of fibroblast-myofibroblast transition.   Conclusion: The Rho/MRTF pathway is a critical novel mediator of PC1/2 loss-induced acquisition of the profibrotic epithelial phenotype and the ensuing fibrosis. Overall design: The porcine proximal tubule cell line LLC-PK1 was subjected to treatment with siRNAs (siPC1, siPC2, and in combination with siMRTFA) along with a non-related control siRNA (siNR). This was carried out to assess the impact of MRTFA in the context of polysystic kidney disease, particularly when the Polycystin (PC1 or PC2) gene is downregulated.