Signaling to the mammalian circadian clocks: in pursuit of the primary mammalian circadian photoreceptor

Neuroendocrine-immune interaction: Evolutionarily conserved mechanisms that maintain allostasis in an ever-changing environment

It has now become accepted that the immune system and neuroendocrine system form an integrated part of our physiology. Immunological defense mechanisms act in concert with physiological processes like growth and reproduction, energy intake and metabolism, as well as neuronal development. Not only are psychological and environmental stressors communicated to the immune system, but also, vice versa, the immune response and adaptation to a current pathogen challenge are communicated to the entire body, including the brain, to evoke adaptive responses (e.g., fever, sickness behavior) that ensure allocation of energy to fight the pathogen. This phenomenon is evolutionarily conserved. Hence it is both interesting and important to consider the evolutionary history of this bi-directional neuroendocrine-immune communication to reveal phylogenetically ancient or relatively recently acquired mechanisms. Indeed, such considerations have already disclosed an extensive "common vocabulary" of information pathways as well as molecules and their receptors used by both the neuroendocrine and immune systems. This review focuses on the principal mechanisms of bi-directional communication and the evidence for evolutionary conservation of the important physiological pathways involved.

Ghrelin and the central regulation of feeding and energy balance

Ghrelin was discovered in 1999 as growth hormone secretagouge released from the gut. Soon after it was recognized that ghrelin is a fundamental driver of appetite in rodents and humans and that its mode of action requires alteration of hypothalamic circuit function. Here we review aspects of ghrelin's action that revolve around the central nervous system with the goal to highlight these pathways in integrative physiology of metabolism regulation including ghrelin's cross-talk with the action of the adipose hormone, leptin.

Retinoic acid receptors move in time with the clock in the hippocampus. Effect of a vitamin-A-deficient diet

An endogenous time-keeping mechanism controls circadian biological rhythms in mammals. Previously, we showed that vitamin A deficiency modifies clock BMAL1 and PER1 as well as BDNF and neurogranin daily rhythmicity in the rat hippocampus when animals are maintained under 12-h-light:12-h-dark conditions. Retinoic acid nuclear receptors, retinoic acid receptors (RARs) and retinoid X receptors (RXRs), have been detected in the same brain area. Our objectives were (a) to analyze whether RARα, RARβ and RXRβ exhibit a circadian variation in the rat hippocampus and (b) to investigate the effect of a vitamin-A-deficient diet on the circadian expression of BMAL1, PER1 and retinoic acid receptors (RARs and RXRβ) genes. Holtzman male rats from control and vitamin-A-deficient groups were maintained under 12-h-light:12-h-dark or 12-h-dark:12-h-dark conditions during the last week of treatment. RARα, RARβ, RXRβ, BMAL1 and PER1 transcript and protein levels were determined in hippocampus samples isolated every 4 h in a 24-h period. Regulatory regions of RARs and RXRβ genes were scanned for clock-responsive sites, while BMAL1 and PER1 promoters were analyzed for retinoic acid responsive elements and retinoid X responsive elements. E-box and retinoid-related orphan receptor responsive element sites were found on regulatory regions of retinoid receptors genes, which display an endogenously controlled circadian expression in the rat hippocampus. Those temporal profiles were modified when animals were fed with a vitamin-A-deficient diet. Similarly, the nutritional vitamin A deficiency phase shifted BMAL1 and abolished PER1 circadian expression at both mRNA and protein levels. Our data suggest that vitamin A deficiency may affect the circadian expression in the hippocampus by modifying the rhythmic profiles of retinoic acid receptors.

Entrainment of breast (cancer) epithelial cells detects distinct circadian oscillation patterns for clock and hormone receptor genes

Most physiological and biological processes are regulated by endogenous circadian rhythms under the control of both a master clock, which acts systemically and individual cellular clocks, which act at the single cell level. The cellular clock is based on a network of core clock genes, which drive the circadian expression of non-clock genes involved in many cellular processes. Circadian deregulation of gene expression has emerged to be as important as deregulation of estrogen signaling in breast tumorigenesis. Whether there is a mutual deregulation of circadian and hormone signaling is the question that we address in this study. Here we show that, upon entrainment by serum shock, cultured human mammary epithelial cells maintain an inner circadian oscillator, with key clock genes oscillating in a circadian fashion. In the same cells, the expression of the estrogen receptor α (ER A) gene also oscillates in a circadian fashion. In contrast, ER A-positive and -negative breast cancer epithelial cells show disruption of the inner clock. Further, ER A-positive breast cancer cells do not display circadian oscillation of ER A expression. Our findings suggest that estrogen signaling could be affected not only in ER A-negative breast cancer, but also in ER A-positive breast cancer due to lack of circadian availability of ER A. Entrainment of the inner clock of breast epithelial cells, by taking into consideration the biological time component, provides a novel tool to test mechanistically whether defective circadian mechanisms can affect hormone signaling relevant to breast cancer.

A common origin: signaling similarities in the regulation of the circadian clock and DNA damage responses

Circadian clocks are intrinsic, time-tracking systems that endow organisms with a survival advantage. Studies of animal models and human tumor samples have revealed that the disruption of circadian rhythms is an important endogenous factor that can contribute to mammalian cancer development. The core of the circadian clock mechanism is a cell-autonomous and self-sustained oscillator system mediated by a transcription/translation-based negative feedback loop that relies on positive and negative elements. Recent studies have implicated these core circadian components in the regulation of both the cell cycle and DNA damage responses (DDR). Indeed, the circadian feedback loop controls the timing of cell proliferation by regulating the expression of key cell cycle genes. Conversely, several intracellular signaling cascades and post-translational modifications that play important roles in the cell cycle and DDR are also essential for circadian clock regulation. Importantly, alteration of a cell's reduction-oxidation (redox) state triggers the transduction of photic signals that regulate circadian clock gene transcription, suggesting that cellular responses to photo-oxidative stress may have been the evolutionary origin of the circadian clock. This review describes selected regulatory aspects of circadian machinery that are evidence of a molecular link between the circadian clock and DDR, focusing particularly on the signaling cascades involved in the light entrainment of the zebrafish circadian clock.

PPARs in Rhythmic Metabolic Regulation and Implications in Health and Disease

The circadian rhythm, controlled by a complex network of cellular transcription factors, orchestrates behavior and physiology in the vast majority of animals. The circadian system is comprised of a master clock located in central nervous system with 24-hour rotation and periphery clocks to ensure optimal timing of physiology in peripheral tissues. Circadian expression of peroxisome proliferator-activated receptors (PPARs), members of the nuclear receptor superfamily and key mediators of energy homeostasis and metabolism, is regulated by clock genes. PPARs serve as sensors of nutrient and energy/metabolism status to temporally entrain peripheral clock. Metabolism and circadian clocks are tightly intertwined: clock genes drive metabolism, and various metabolic parameters affect clock genes, producing a reciprocal feedback relationship. Due to PPARs' robust relationship with energy status and metabolism, the aberration of PPARs in the biological clock system leads to abnormal expression of genes in metabolic pathways, thus, contributing to etiology of metabolic syndrome. Studying PPARs' functions under the context of the mammalian circadian system could advance our understanding of how energy and metabolic status are maintained in the body, which may ultimately lead to rhythmic medical treatment against metabolic syndrome.

PPARalpha is a potential therapeutic target of drugs to treat circadian rhythm sleep disorders

Recent progress at the molecular level has revealed that nuclear receptors play an important role in the generation of mammalian circadian rhythms. To examine whether peroxisome proliferator-activated receptor alpha (PPARalpha) is involved in the regulation of circadian behavioral rhythms in mammals, we evaluated the locomotor activity of mice administered with the hypolipidemic PPARalpha ligand, bezafibrate. Circadian locomotor activity was phase-advanced about 3h in mice given bezafibrate under light-dark (LD) conditions. Transfer from LD to constant darkness did not change the onset of activity in these mice, suggesting that bezafibrate advanced the phase of the endogenous clock. Surprisingly, bezafibrate also advanced the phase in mice with lesions of the suprachiasmatic nucleus (SCN; the central clock in mammals). The circadian expression of clock genes such as period2, BMAL1, and Rev-erbalpha was also phase-advanced in various tissues (cortex, liver, and fat) without affecting the SCN. Bezafibrate also phase-advanced the activity phase that is delayed in model mice with delayed sleep phase syndrome (DSPS) due to a Clock gene mutation. Our results indicated that PPARalpha is involved in circadian clock control independently of the SCN and that PPARalpha could be a potent target of drugs to treat circadian rhythm sleep disorders including DSPS.

Clock mutation affects circadian regulation of circulating blood cells

Background

Although the number of circulating immune cells is subject to high-amplitude circadian rhythms, the underlying mechanisms are not fully understood.

Methods

To determine whether intact CLOCK protein is required for the circadian changes in peripheral blood cells, we examined circulating white (WBC) and red (RBC) blood cells in homozygous Clock mutant mice.

Results

Daytime increases in total WBC and lymphocytes were suppressed and slightly phase-delayed along with plasma corticosterone levels in Clock mutant mice. The peak RBC rhythm was significantly reduced and phase-advanced in the Clock mutants. Anatomical examination revealed hemoglobin-rich, swollen red spleens in Clock mutant mice, suggesting RBC accumulation.

Conclusion

Our results suggest that endogenous clock-regulated circadian corticosterone secretion from the adrenal gland is involved in the effect of a Clock mutation on daily profiles of circulating WBC. However, intact CLOCK seems unnecessary for generating the rhythm of corticosterone secretion in mice. Our results also suggest that CLOCK is involved in discharge of RBC from the spleen.

Circadian expression of clock genes is maintained in the liver of Vitamin A-deficient mice

In mammals, circadian oscillators exist not only in the central clock of the suprachiasmatic nucleus (SCN) but also in peripheral tissues such as the liver, heart and kidneys. Peripheral clocks are entrained to the SCN clock by both neural and humoral signals. Vitamin A might be one candidate that synchronizes peripheral clocks by activating its ligand-dependent nuclear receptors in mammals. The present study examines the effect of a Vitamin A deficiency on the circadian expression of clock genes in the mouse liver. Serum Vitamin A levels remained constant throughout the day in control mice, and were significantly reduced in Vitamin A-deficient mice. Northern blots showed that circadian expression of the clock genes mPer1, mPer2, Clock, and BMAL1, and of the clock-controlled output gene D-site binding protein (DBP), was maintained in Vitamin A-deficient mice. Our results suggest that dietary Vitamin A is not essential for generating circadian rhythms of peripheral clocks in mammals.

CLOCK is involved in the circadian transactivation of peroxisome-proliferator-activated receptor alpha (PPARalpha) in mice

PPARalpha (peroxisome-proliferator-activated receptor alpha) is a member of the nuclear receptor superfamily of ligand-activated transcription factors that regulate the expression of genes associated with lipid metabolism. In the present study, we show that circadian expression of mouse PPARalpha mRNA requires the basic helix-loop-helix PAS (Per-Arnt-Sim) protein CLOCK, a core component of the negative-feedback loop that drives circadian oscillators in mammals. The circadian expression of PPARalpha mRNA was abolished in the liver of homozygous Clock mutant mice. Using wild-type and Clock-deficient fibroblasts derived from homozygous Clock mutant mice, we showed that the circadian expression of PPARalpha mRNA is regulated by the peripheral oscillators in a CLOCK-dependent manner. Transient transfection and EMSAs (electrophoretic mobility-shift assays) revealed that the CLOCK-BMAL1 (brain and muscle Arnt-like protein 1) heterodimer transactivates the PPARalpha gene via an E-box-rich region located in the second intron. This region contained two perfect E-boxes and four E-box-like motifs within 90 bases. ChIP (chromatin immunoprecipitation) also showed that CLOCK associates with this E-box-rich region in vivo. Circadian expression of PPARalpha mRNA was intact in the liver of insulin-dependent diabetic and of adrenalectomized mice, suggesting that endogenous insulin and glucocorticoids are not essential for the rhythmic expression of the PPARalpha gene. These results suggested that CLOCK plays an important role in lipid homoeostasis by regulating the transcription of a key protein, PPARalpha.

Sex differences in adult suprachiasmatic nucleus neurons emerging late prenatally in rats

The suprachiasmatic nucleus (SCN) is implicated in the control of circadian rhythms of gonadal function. Although several structures surrounding the SCN are sensitive to the effects of gonadal steroids, similar effects in the SCN remain unclear. For example, there are conflicting data on whether the SCN is sexually differentiated. This study attempted to determine sex differences in the number of SCN cells generated during late gestation, and if testosterone mediates these differences. Pregnant female rats were treated with 5-bromo-2'-deoxyuridine (BrdU; 50 mg/kg) on gestational day 18 (E18), the day when aromatase activity peaks in the developing rat fetus. These animals were also given injections of oil or testosterone propionate (10 mg/0.1 mL peanut oil) from E15 until parturition. Litters were allowed to survive until adulthood and were killed on postnatal day 60 (PN60). Following fixation, brain sections containing the SCN from these rats were processed for BrdU immunocytochemistry. A second set of SCN sections was processed for immunocytochemistry detecting BrdU and some of the cell groups prevalent within the SCN. Data showed that female rats have a higher number of cells labeled with BrdU in the SCN, particularly in the medial and caudal SCN. This sex difference was abolished in animals treated with testosterone during late gestation. Double immunocytochemistry revealed that BrdU-labeled cells were neurons expressing calbindin-D28K, vasoactive intestinal peptide and, to a lesser degree, vasopressin. Our results unveiled a previously unknown effect of gonadal steroids on the developing SCN, which may contribute to the emergence of gender-specific circadian rhythms.

Genome-wide expression analysis of mouse liver reveals CLOCK-regulated circadian output genes

CLOCK is a positive component of a transcription/translation-based negative feedback loop of the central circadian oscillator in the suprachiasmatic nucleus in mammals. To examine CLOCK-regulated circadian transcription in peripheral tissues, we performed microarray analyses using liver RNA isolated from Clock mutant mice. We also compared expression profiles with those of Cryptochromes (Cry1 and Cry2) double knockout mice. We identified more than 100 genes that fluctuated from day to night and of which expression levels were decreased in Clock mutant mice. In Cry-deficient mice, the expression levels of most CLOCK-regulated genes were elevated to the upper range of normal oscillation. Most of the screened genes had a CLOCK/BMAL1 binding site (E box) in the 5'-flanking region. We found that CLOCK was absolutely concerned with the circadian transcription of one type of liver genes (such as DBP, TEF, and Usp2) and partially with another (such as mPer1, mPer2, mDec1, Nocturnin, P450 oxidoreductase, and FKBP51) because the latter were damped but remained rhythmic in the mutant mice. Our results showed that CLOCK and CRY proteins are involved in the transcriptional regulation of many circadian output genes in the mouse liver. In addition to being a core component of the negative feedback loop that drives the circadian oscillator, CLOCK also appears to be involved in various physiological functions such as cell cycle, lipid metabolism, immune functions, and proteolysis in peripheral tissues.

Light-dependent changes in the chick pineal temperature and the expression of cHsp90 alpha gene: a potential contribution of in vivo temperature change to the photic-entrainment of the chick pineal circadian clock

The circadian clock is entrained to the diurnal alteration of environmental conditions such as light and temperature, but the molecular mechanism underlying the entrainment is not fully understood. In the present study, we employed a differential display-based screening for a set of genes that are induced by light in the chick pineal gland, a structure of the central clock entrainable to both light and temperature changes. We found that the level of the mRNA encoding chicken heat shock protein 90 alpha (cHSP90 alpha) was rapidly elevated in the pineal gland within a 5-min exposure of chicks to light. Furthermore, the pineal cHsp90 alpha mRNA was expressed rhythmically under both 12-hr light/12-hr dark (LD) cycles and constant dark (DD) conditions. The total amount of the pineal cHSP90 alpha protein was, however, kept at nearly constant levels under LD cycles, and immunohistochemical analyses of the pineal cHSP90 alpha showed invariable localization at the cytoplasm throughout the day. In vivo measurement of the chick pineal temperature demonstrated its light-dependent and time-of-day-dependent change, and the profile was very similar to that of the pineal cHSP90 alpha mRNA level. These observations suggest that the in vivo temperature change regulates the expression of temperature-responsive genes including cHSP 90 alpha in the pineal gland. The temperature change may induce a phase-shift of the pineal clock, thereby facilitating its efficient entrainment to environmental LD cycles.

Light induction of a vertebrate clock gene involves signaling through blue-light receptors and MAP kinases

The signaling pathways that couple light photoreception to entrainment of the circadian clock have yet to be deciphered. Two prominent groups of candidates for the circadian photoreceptors are opsins (e.g., melanopsin) and blue-light photoreceptors (e.g., cryptochromes). We have previously showed that the zebrafish is an ideal model organism in which to study circadian regulation and light response in peripheral tissues. Here, we used the light-responsive zebrafish cell line Z3 to dissect the response of the clock gene zPer2 to light. We show that the MAPK (mitogen-activated protein kinase) pathway is essential for this response, although other signaling pathways may also play a role. Moreover, action spectrum analyses of zPer2 transcriptional response to monochromatic light demonstrate the involvement of a blue-light photoreceptor. The Cry1b and Cry3 cryptochromes constitute attractive candidates as photoreceptors in this setting. Our results establish a link between blue-light photoreceptors, probably cryptochromes, and the MAPK pathway to elicit light-induced transcriptional activation of clock genes.