Antifibrotic Actions of SARA

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Fibrogenesis plays a major role in chronic kidney disease (CKD) progression. Our laboratory has identified Smad anchor for receptor activation (SARA) as a candidate anti-fibrotic molecule. Originally described as a TGF-β receptor adaptor molecule that facilitates Smad signaling, SARA has a broader role as a scaffolding protein that assembles molecules in complexes to direct cell signaling events. We have shown how SARA participates in endosomal signaling, and that its expression helps to maintain cells in a differentiated state. SARA is downregulated in cells that are subjected to fibrogenic stimuli, concomitant with the cells presenting a fibrogenic phenotype. Overexpressing SARA in these cells prevents the phenotypic switch associated with fibrosis; conversely, knocking down SARA amplifies collagen production in response to TGF-β. In preliminary studies, we show: 1) SARA expression is markedly reduced in kidneys undergoing fibrosis, 2) aristolochic acid-induced tubulointerstitial fibrosis is reduced in mice that overexpress SARA in an inducible fashion, and 3) overexpressed SARA prevents fibrosis in a Drosophila larval heart-kidney tube model where fibrogenesis is induced by knockdown of histone-modifying genes. Together, these data support the hypothesis that SARA maintains cells in a fibrosis-resistant state. By regulating the expression of SARA and/or molecules that mediate SARA actions, we can modify the events leading to renal fibrosis. Here, we propose three specific aims to test our hypothesis and will determine: (1) How SARA expression changes during renal fibrosis in mice, and if increasing or blocking SARA expression using transgenic approaches ameliorates or accelerates disease, respectively. (2) How SARA expression is regulated during fibrosis in vitro and in vivo. We have found that the transcriptional repressor, BHLHE40, inhibits SARA expression; and the anti-fibrotic protein, soluble Klotho, stimulates SARA expression. We will determine how these molecules act through SARA to influence the outcome of fibrotic renal disease. (3) How SARA signals downstream to protect cells from the fibrogenic `switch.' We will examine how SARA affects Wnt/β- catenin signaling and seek new effector mechanisms of SARA through RNA-Seq of TGF-β-treated mouse cells, and of Drosophila larvae undergoing pericardial nephrocyte-associated fibrosis. SIGNIFICANCE: Completion of these studies will elucidate the mechanism by which SARA modulates cellular fibrogenic activity, with a goal of identifying novel therapeutic targets to prevent or slow CKD-associated fibrosis.