Circadian regulation of kidney function: finding a role for Bmal1

Abstract

Mounting evidence suggests that there is an internal molecular “clock” within the kidney to help maintain normal renal function. Disturbance of the kidney circadian rhythm may pose a threat to water and electrolyte homeostasis and blood pressure regulation, among many other problems. The identification of circadian genes facilitated a more comprehensive appreciation of the importance of “keeping the body on time”; however, our knowledge is very limited with regard to how circadian genes regulate kidney function. In this brief review, we summarize recent progress in circadian control of renal physiology, with a particular focus on aryl hydrocarbon receptor nuclear translocator-like protein (Arntl1; also called Bmal1).

INTRODUCTION

Nighttime blood pressure (BP) among healthy individuals drops 10–20% from the daytime average, which is known as nocturnal dipping. Diminishment or absence of nocturnal BP dipping (nondipping) is associated with a poor prognosis and severe end organ damage in patients (42). In fact, several studies have suggested that nighttime BP may be the most essential prognostic marker for morbidity and mortality of cardiovascular disease (23, 29). As a result, the idea of treating patients suffering from hypertension and other cardiovascular diseases with specific time-of-day dosing, otherwise known as chronotherapy, has gained considerable attention in recent years. In nondipping hypertensive patients with chronic kidney disease, bedtime dosing of antihypertensive medications may help reduce nocturnal BP and restore a rhythmic BP pattern, which also correlates with improved long-term prognosis (21, 25, 43). A study performed in 2,156 hypertensive subjects from a Spanish cohort showed that bedtime therapy with antihypertensive medications improved BP control and decreased the prevalence of nondipping BP (16). However, it was suggested that antihypertensive chronotherapy needed to be performed in larger, more diverse populations and that different regimens should be tested more rigorously (1, 40). Nevertheless, the causes of nocturnal hypertension as well as mechanisms for circadian regulation of BP still remain to be elucidated. The extent to which the renal molecular clock may account for derangements in BP rhythms needs to be explored.

The mammalian circadian clock can generally be categorized into two parts. The central clock is located in the suprachiasmatic nucleus (SCN) of the brain, which is also known as the pacemaker for circadian rhythms. This clock is largely responsive to light as the zeitgeber, or timing cue. The peripheral clocks exist in most every other part of the body and are regulated by a variety of zeitgebers such as activity, food intake, and other environmental cues that have not been resolved (6). On the molecular level, major circadian genes form a transcription-translation oscillating loop. Aryl hydrocarbon receptor nuclear translocator-like protein (Arntl1; also called Bmal1) and circadian locomotor output cycles kaput (CLOCK) heterodimerize and bind to promoter regions of circadian target genes, which include the genes for period (Per) and cryptochrome (Cry). Once translated, Per and Cry associate and then translocate into the nucleus to inhibit the expression of CLOCK and Bmal1 (30). Although we have learned a good deal from the generation of global knockout mice about the specific activity of these genes, we know very little with regard to how peripheral clocks work within different organs. In particular, clock gene regulation of kidney function remains largely unknown. In this review, we briefly summarize current progress in this field.

KIDNEY FUNCTION FOLLOWS A CIRCADIAN PATTERN

In otherwise healthy laboratory animals or humans, variations in food and water intake periodicity do not have a significant impact on BP or renal excretory rhythm (8, 26, 27), which suggests that rhythmicity within the kidney is under strict regulation according to time of day. Growing evidence suggests that disruptions in circadian renal function control can contribute to hypertension and kidney disease (31). The renin-angiotensin-aldosterone system (RAAS) plays a key role in regulating BP and ion homeostasis through various mechanisms in the kidney, and most components of the RAAS exhibit robust circadian rhythms. For example, plasma renin activity as well as plasma concentration and urinary excretion of aldosterone display a diurnal rhythm that can be completely inverted by reversing the light-dark cycle (17). These findings suggest that the RAAS is controlled by the central clock in the superchiasmatic nucleus. In addition, the responsiveness of the RAAS can vary according to the time of day when activated by traditional mechanisms such as sympathetic activity, neural input, and so forth (12, 17, 22). There is also evidence suggesting that the circadian rhythm of intrarenal RAAS activity contributes to the genesis of salt-sensitive and nondipping BP in patients with IgA nephropathy (10, 24). Moreover, several groups showed that the expression of sodium transporters along the nephron exhibit a circadian rhythm and might be under the control of circadian genes (13, 15, 34). Saifur Rohman et al. (34) observed that the mRNA expression profile of the sodium-hydrogen exchanger 3 in rats displays a cyclical pattern over a 24-h period with a peak expression level at zeitgeber time 16 (ZT16; or 4 h into the dark/active phase) and was regulated by CLOCK:Bmal1 heterodimers. Gumz and colleagues (13, 15) and Stow et al. (39) showed that epithelial sodium channel (ENaC) was expressed in a circadian pattern in the collecting duct of the mouse and that the pattern was altered in mice lacking circadian genes. These observations suggest that molecular clock mechanisms may control the normal oscillations in Na⁺ excretion (higher during the active phase).

Bmal1: A CORE CLOCK GENE

Disruption of circadian rhythms contributes to a multitude of pathological states. The central molecular clock, sometimes referred to as a “pacemaker,” responds to signals derived from the optic nerve and serves to focus the periodicity of the peripheral clocks but is often not the specific zeitgeber for the peripheral clocks. It has been clear for decades that kidney function varies according to the time of day and is independent of sleep, activity, and intake of food and water (20). How this may impact various kidney-related disorders has only recently gained attention. One area that may be relevant to this topic is nocturnal hypertension that may be at least in part caused by impaired capacity of the kidney to excrete sodium during daytime, leading to elevated extracellular fluid volume at night (2, 9). The idea is that BP tends to be maintained at higher levels at night to keep 24-h sodium balance. However, solid evidence for this idea is lacking.

Bmal1 and CLOCK are often referred to as being a part of the positive arm of the core clock because of their ability to stimulate transcription of a wide variety of genes. Work by Nikolaeva et al. (28) and Tokonami et al. (41) has provided the most evidence to date that Bmal1 may be involved in maintaining normal renal function. These authors showed that loss of Bmal1 in renal collecting duct and juxtaglomerular cells caused increased urine volume and decreased plasma aldosterone levels in mice (41). Furthermore, they generated a whole nephron-specific Bmal1-knockout mouse and showed compromised mitochondrial function and impaired drug pharmacokinetics in the knockout mice (28). The latter finding is most likely due to observed loss of regulation of the organic acid transporter 3 in the kidneys of these mice (28). Ongoing preliminary work from our laboratory shows impaired diurnal control of renal excretion of sodium and water in global Bmal1-knockout mice (7). These findings coordinate with the observed lack of circadian BP rhythms in these mice, as reported by Curtis et al. (4). Further resolution of diurnal control of BP may lie in finding the mechanism(s) for Bmal1 involvement in regulating BP and electrolyte and water homeostasis.

There has been focus on exploring how the circadian gene period 1 (Per1) regulates renal excretory function. Several investigators found that Per1 is a target gene of aldosterone and thus regulates several renal transporters, including the epithelial sodium channel (ENaC) (3, 13, 15, 32, 33). Their recent work showed that desoxycorticosterone pivalate-salt treatment results in a nondipping BP pattern in Per1-knockout mice (35). Thus, it is clear that we know very little about specific mechanisms that control renal function in a circadian manner. Given the complexities of the peripheral molecular clock, this will require a great deal of investigation in the coming years.

RELATIONSHIP OF ENDOTHELIN-1 AND THE KIDNEY CLOCK

Endothelin-1 (ET-1) is a master regulator of sodium balance and thus BP through activation of its ET_A and ET_B receptors (36). Within the kidney, ET-1 increases sodium excretion and lowers BP through ET_B receptor-mediated inhibition of Na⁺ reabsorption along the renal tubules, predominantly in the collecting duct (5). ET-1 is under aldosterone-induced modulation and may be involved in Per1-mediated aldosterone function (14, 38). Richards et al. (33) and Stow et al. (39) showed that renal ET-1 levels were increased in Per1-knockout mice and Per1-heterozygous mice. In addition, ET-1 gene expression and protein levels were significantly greater after Per1 knockdown in cortical-collecting duct cells (39). These data suggest that ET-1 may be involved in circadian regulation of renal function. Our laboratory recently showed that loss of ET_B receptor function in rats impaired Na⁺ excretion in response to an acute load in a time-of-day-dependent manner (20). When the salt load was given at the beginning of the inactive period [zeitgeber time (ZT) 0], the impaired natriuretic response caused by dysfunctional ET_B receptors was much more severe compared with a salt load given at ZT12. Also, the difference in the diurnal responses in female rats was not as severe as male rats, indicating that ET-1 mediated diurnal control of sodium excretion exhibits sex differences (20). Furthermore, recent data from our laboratory suggests that the circadian expression pattern of Bmal1 mRNA is regulated by ET_B receptor activity in a salt-dependent manner (37). These data suggest that Bmal1 and the endothelin system may work collaboratively to regulate renal function (Fig. 1). Future studies will need to determine how the other clock components contribute to ET-1-dependent control of sodium handling by the kidney. In addition, the involvement of other specific nephron segments and, importantly, the potential role of circadian changes in GFR on diurnal control will need to be explored.

Fig. 1.

Proposed interaction between clock genes and the renal endothelin system in regulating sodium homeostasis and blood pressure. Dotted lines represent the hypothetical scheme based on our preliminary data. ET-1, endothelin 1; ENaC, epithelial sodium channel; BP, blood pressure.

PERSPECTIVE

Many pathophysiological states are associated with disturbances of tissue/organ-specific circadian rhythms, including hypertension and metabolic disease (11, 19, 31). We know that the kidney follows a strong circadian rhythm that is tightly regulated and that clock genes are active in control of various kidney functions; however, much remains unknown. What triggers the disruption of kidney rhythms? Eating behavior? Salt content? Hormones? Could other dietary factors such as fat content play a role? Signals from the microbiome have also been shown to impact various rhythms (18). Moreover, understanding how circadian genes are involved in regulating renal function is important in helping us better appreciate potential chronotherapy for cardiovascular and renal diseases.

GRANTS

This work was supported by a grant from the National Heart, Lung, and Blood Institute P01-HL-127178.

DISCLOSURES

No conflicts of interest, financial or otherwise, are declared by the authors.

AUTHOR CONTRIBUTIONS

D.Z. prepared figures; D.Z. drafted manuscript; D.Z. and D.M.P. edited and revised manuscript; D.Z. and D.M.P. approved final version of manuscript.