Postnatal Deletion of Bmal1 in Mice Protects Against Obstructive Renal Fibrosis Via Suppressing Gli2 Transcription

Background: Circadian clock is involved in regulating most renal physiological functions, including water and electrolyte balance and blood pressure homeostasis. However, the role of circadian clock in renal pathophysiology remains largely unknown. Methods: The inducible postnatal Bmal1 knockout mice (iKO) were used in the study. The diurnal oscillation of blood pressure and urinary water and electrolyte excretion was compared between iKO and controls. Unilateral ureteral obstruction (UUO) was performed to induce renal fibrosis. ChIP, luciferase reporter assay, and the in vitro cell culture experiments were used to elucidate the transcriptional activation of Bmal1 on Gli2. Findings: The iKO mice exhibit increased urinary volume and electrolyte excretion but abolished diurnal variation. Upon UUO, the iKO mice displayed significantly attenuated tubulointerstitial fibrosis and decreased expression of the hedgehog pathway transcriptional effectors Gli1 and Gli2. ChIP and luciferase reporter assays revealed that BMAL1 bound to the promoter of and activate the transcription of Gli2, but not Gli1, suggesting that the involvement of Bmal1 in renal fibrosis was possibly mediated via Gli2-dependent mechanisms. Furthermore, treatment with TGF-β increased Bmal1 in cultured murine proximal tubular cells, and knockdown of Bmal1 abolished, while overexpression of Bmal1 increased, Gli2 and the expression of fibrosis related genes. Interpretation: Our results revealed a prominent role of the circadian clock in renal fibrosis. Moreover, we identified Gli2 as a novel target of Bmal1, which may mediate the adverse effect of Bmal1 in obstructive nephropathy.