Tick Tock
Authors: Michelle L. Gumz
Journal: Journal of the American Society of Nephrology (2014)

A variety of physiologic functions related to the kidney, including BP, GFR, renal blood flow, and urinary sodium excretion, exhibit a circadian pattern of variation (reviewed by Stow and Gumz1). Whereas these clinical observations are well established, the underlying molecular mechanisms are not completely characterized. On the molecular level, the circadian clock consists of a complex series of transcriptional, translational, and post-translational feedback loops.2 More simply, the four core circadian proteins—Bmal1, CLOCK, Period (Per1, Per2, Per3) and Cryptochrome (Cry1, Cry2)—regulate expression of thousands of target genes via a transcriptional mechanism to perpetuate rhythms in physiologic function.3 There has long been a debate in the circadian field regarding the relationship between the central clock, located in the suprachiasmatic nucleus (SCN) of the brain, and the peripheral clocks, located in nearly every cell type and tissue of the body. Zeitgebers, or "time-givers," act as inputs to the circadian clock and these cues include light and food (reviewed by Richards and Gumz4). The prevailing model at the present time is that the central clock in the SCN, entrained by light, acts as a "conductor" to coordinate the physiologic functions of the "orchestra" made up of peripheral clocks located throughout the body (reviewed by Richards and Gumz4). Neuronal and humoral signaling is involved in the function of the conductor to synchronize the peripheral clocks in the orchestra. Generation of cell type–specific knockouts (KOs) has recently begun to shed light on the extent to which the peripheral clocks may independently contribute to physiologic function. For example, studies by Young and colleagues in the cardiomyocyte-specific clock mutant mouse have demonstrated a clear role for the CLOCK protein in the metabolic function of the heart (reviewed by Richards and Gumz4). Specific deletion of Bmal1 in pancreatic β cells demonstrated a role for the circadian clock in glucose-stimulated insulin secretion and oxidative stress–induced β-cell failure.5 Until now, studies like these have been lacking for the kidney. In a groundbreaking report presented in this issue of JASN, Tokonami et al. demonstrate a role for a kidney-specific peripheral clock in the regulation of renal function and BP.6 The first report of a circadian clock–controlled gene in the kidney came from Saifur Rohman et al., with the demonstration that the Na/H exchanger NHE3 was directly regulated by Bmal1 and CLOCK via a transcriptional mechanism.7 Our subsequent reports showed that Per1 regulates the expression of the α subunit of the epithelial sodium channel and the activity of epithelial sodium channel.8,9 Consistent with these mechanistic molecular findings in renal models, studies in circadian KO mice have consistently demonstrated a role for each of the core clock proteins in BP control.10–13 An important role for the kidney in these BP phenotypes has often been proposed, but the lack of renal cell type–specific KO models of circadian genes has prevented the use of a genetic model to directly test this hypothesis. In the report by Tokonami et al., floxed Bmal1 mice were crossed with Ren1dCre mice to generate mice lacking Bmal1 expression in renin-producing cells of the kidney. Specifically, Bmal1 expression was significantly reduced in the renin-secreting granular cells of the juxtaglomerular apparatus and in principal cells of the cortical collecting duct and the outer medullary collecting duct. Less dramatic decreases in Bmal1 expression were also observed in the medullary thick ascending limb. Reduction of Bmal1 expression in these specific vascular and tubular kidney cell types was associated with significant changes in several parameters, including increases in GFR and urine volume and decreases in plasma aldosterone and both systolic and diastolic BP. These landmark findings clearly demonstrate the importance of the kidney clock in the control of BP and renal function. Moreover, because this study was performed in mice with an intact clock mechanism in the SCN, these results suggest that the function of the kidney clock may be somewhat independent of the central clock. Interestingly, the overall phenotype of the kidney-specific Bmal1 KO is very similar to that of the global KO of CLOCK12,14 and that of Per1.8,13,15 The features of these phenotypes include reduced plasma aldosterone, mild polyuria, altered urinary sodium excretion, and reduced BP. These shared features between global loss of CLOCK or Per1 and a kidney-specific Bmal1 KO are consistent with the notion that the kidney significantly contributes to the phenotypes observed in the global KO mice. An intriguing feature of the study by Tokonami et al. is the finding that although loss of Bmal1 from specific cell types in the kidney was associated with lower BP, the circadian rhythm of BP did not appear to be altered. This is in contrast with the phenotype of the global Bmal1 KO, which included disruption of the normal dipping pattern of BP.10 Indeed, time-independent effects of circadian rhythm proteins were previously observed and may be attributable to the general function of these proteins as transcription factors.16 Future studies combining cell type conditional KO of circadian proteins together with alteration of entraining signals such as food and light may help dissect time-dependent and independent effects of circadian proteins in a manner that can be correlated with tissue-specific function. Another explanation for the maintenance of the dipping pattern in the kidney-specific Bmal1 KO could be related to the function of nitric oxide in the vasculature; it has been proposed that vascular nitric oxide signaling may be a key determinant of the dipping pattern of BP.17 Nitric oxide action may explain the other provocative finding in this study, that GFR is increased in the kidney-specific Bmal1 KO in the absence of increased protein or glucose in the urine. The authors speculate that increased nitric oxide produced by tubular cells could explain the altered GFR. Future studies using additional cell type–specific Cre mice in combination with floxed circadian gene models will certainly help elucidate the role of clock proteins in nephron segments beyond those investigated by Tokonami et al. The work of Tokonami et al. represents a significant milestone in our understanding and perception of the kidney clock. Knockout of Bmal1 expression in a subset of specific renal cell types resulted in alteration of urinary Na excretion, GFR, and BP. These findings clearly and specifically demonstrate a role for circadian clock proteins, localized in the kidney, in the regulation of renal function and BP control. Disclosures None.