Functional central rhythmicity and light entrainment, but not liver and muscle rhythmicity, are Clock independent

Running skeletal muscle clocks on time- the determining factors

Circadian rhythms generate 24 h-long oscillations, which are key regulators of many aspects of behavior and physiology. Recent circadian transcriptome studies have discovered rhythmicity at the transcriptional level of hundreds of skeletal muscle genes, with roles in skeletal muscle growth, maintenance, and metabolic functions. These rhythms allow this tissue to perform molecular functions at the appropriate time of the day in order to anticipate environmental changes. However, while the last decade of research has characterized several aspects of the skeletal muscle molecular clock, many still are unexplored, including its functions, regulatory mechanisms, and interactions with other tissues. The central clock is believed to be located in the suprachiasmatic nucleus (SCN) of the brain hypothalamus, providing entrainment to peripheral organs through humoral and neuronal signals. However, these mechanisms of action are still unknown. Conversely, muscle tissue can be entrained through extrinsic, SCN-independent factors, such as feeding and physical activity. In this review, we provide an overview of the recent research about the extrinsic and intrinsic factors required for skeletal muscle clock regulation. Furthermore, we discuss the need for future studies to elucidate the mechanisms behind this regulation, which will in turn help dissect the role of circadian disruption at the onset of aging and diseases.

Role of circadian rhythm and impact of circadian rhythm disturbance on the metabolism and disease

ABSTRACT

Molecular circadian clocks exist in almost all cells of the organism and operate for approximately 24 h, maintain the normal physiological and behavioral body processes and regulate metabolism of many cells related to a variety of disease states. Circadian rhythms regulate metabolism, mainly including neurotransmitters, hormones, amino acids and lipids. Circadian misalignment is related to metabolic syndromes, such as obesity, diabetes and hypertension, which have reached an alarming level in modern society. We reviewed the mechanism of the circadian clock and the interaction between circadian rhythm and metabolism, as well as circadian rhythm disturbance on the metabolism of hypertension, obesity and diabetes. Finally, we discuss how to use the circadian rhythm to prevent diseases. Thus, this review is a micro to macro discussion from the perspective of circadian rhythm and aims to provide basic ideas for circadian rhythm research and disease therapies.

The regulatory role of melatonin in skeletal muscle

Melatonin (N-acetyl-5-methoxy-tryptamine) is an effective antioxidant and free radical scavenger, that has important biological effects in multiple cell types and species. Melatonin research in muscle has recently gained attention, mainly focused on its role in cells or tissue repair and regeneration after injury, due to its powerful biological functions, including its antioxidant, anti-inflammation, anti-tumor and anti-cancer, circadian rhythm, and anti-apoptotic effects. However, the effect of melatonin in regulating muscle development has not been systematically summarized. In this review, we outline the latest research on the involvement of melatonin in the regulation of muscle development and regeneration in order to better understand its underlying molecular mechanisms and potential applications.

Myomaker, and Myomixer-Myomerger-Minion modulate the efficiency of skeletal muscle development with melatonin supplementation through Wnt/β-catenin pathway

Melatonin, a pleiotropic hormone secreted from the pineal gland, has been shown to exert beneficial effects in muscle regeneration and repair due to its functional diversity, including anti-inflammation, anti-apoptosis, and anti-oxidative activity. However, little is known about the negative role of melatonin in myogenesis. Here, using skeletal muscle cells, we found that melatonin promoted C2C12 cells proliferation and inhibits differentiation both in C2C12 cells and primary myoblasts in mice. Melatonin administration significantly down-regulated differentiation and fusion related genes and inhibited myotube formation both in C2C12 cells and primary myoblasts in mice. RNA-seq showed that melatonin down-regulated essential fusion pore components Myomaker and Myomixer-Myomerger-Minion. Moreover, melatonin suppressed Wnt/β-catenin signaling. Inhibition of GSK3β by LiCl rescued the influence of melatonin on differentiation efficiency, Myomaker, but not Myomxier in C2C12 cells. In conclusion, melatonin inhibits myogenic differentiation, Myomaker, and Myomixer through reducing Wnt/β-catenin signaling. These data establish a link between melatonin and fusogenic membrane proteins Myomaker and Myomixer, and suggest the new perspective of melatonin in treatment or preventment of muscular diseases.

Maternal circadian rhythms and the programming of adult health and disease

The in utero environment is inherently rhythmic, with the fetus subjected to circadian changes in temperature, substrates, and various maternal hormones. Meanwhile, the fetus is developing an endogenous circadian timing system, preparing for life in an external environment where light, food availability, and other environmental factors change predictably and repeatedly every 24 h. In humans, there are many situations that can disrupt circadian rhythms, including shift work, international travel, insomnias, and circadian rhythm disorders (e.g., advanced/delayed sleep phase disorder), with a growing consensus that this chronodisruption can have deleterious consequences for an individual's health and well-being. However, the impact of chronodisruption during pregnancy on the health of both the mother and fetus is not well understood. In this review, we outline circadian timing system ontogeny in mammals and examine emerging research from animal models demonstrating long-term negative implications for progeny health following maternal chronodisruption during pregnancy.

Deoxycorticosterone/Salt-Mediated Cardiac Inflammation and Fibrosis Are Dependent on Functional CLOCK Signaling in Male Mice

Activation of the mineralocorticoid receptor (MR) promotes inflammation, fibrosis, and hypertension. Clinical and experimental studies show that MR antagonists have significant therapeutic benefit for all-cause heart failure; however, blockade of renal MRs limits their widespread use. Identification of key downstream signaling mechanisms for the MR in the cardiovascular system may enable development of targeted MR antagonists with selectivity for pathological MR signaling and lower impact on physiological renal electrolyte handling. One candidate pathway is the circadian clock, the dysregulation of which is associated with cardiovascular diseases. We have previously shown that the circadian gene Per2 is dysregulated in hearts with selective deletion of cardiomyocyte MR. We therefore investigated MR-mediated cardiac inflammation and fibrosis in mice that lack normal regulation and oscillation of the circadian clock in peripheral tissues, that is, CLOCKΔ19 mutant mice. The characteristic cardiac inflammatory/fibrotic response to a deoxycorticosterone (DOC)/salt for 8 weeks was significantly blunted in CLOCKΔ19 mice when compared with wild-type mice, despite a modest increase at "baseline" for fibrosis and macrophage number in CLOCKΔ19 mice. In contrast, cardiac hypertrophy in response to DOC/salt was significantly greater in CLOCKΔ19 vs wild-type mice. Markers for renal inflammation and fibrosis were similarly attenuated in the CLOCKΔ19 mice given DOC/salt. Moreover, increased CLOCK expression in H9c2 cardiac cells enhanced MR-mediated transactivation of Per1, suggesting cooperative signaling between these transcription factors. This study demonstrates that the full development of MR-mediated cardiac inflammation and fibrosis is dependent on intact signaling by the circadian protein CLOCK.

Circadian Rhythms in Adipose Tissue Physiology

The different types of adipose tissues fulfill a wide range of biological functions-from energy storage to hormone secretion and thermogenesis-many of which show pronounced variations over the course of the day. Such 24-h rhythms in physiology and behavior are coordinated by endogenous circadian clocks found in all tissues and cells, including adipocytes. At the molecular level, these clocks are based on interlocked transcriptional-translational feedback loops comprised of a set of clock genes/proteins. Tissue-specific clock-controlled transcriptional programs translate time-of-day information into physiologically relevant signals. In adipose tissues, clock gene control has been documented for adipocyte proliferation and differentiation, lipid metabolism as well as endocrine function and other adipose oscillations are under control of systemic signals tied to endocrine, neuronal, or behavioral rhythms. Circadian rhythm disruption, for example, by night shift work or through genetic alterations, is associated with changes in adipocyte metabolism and hormone secretion. At the same time, adipose metabolic state feeds back to central and peripheral clocks, adjusting behavioral and physiological rhythms. In this overview article, we summarize our current knowledge about the crosstalk between circadian clocks and energy metabolism with a focus on adipose physiology. © 2017 American Physiological Society. Compr Physiol 7:383-427, 2017.

The impact of prenatal circadian rhythm disruption on pregnancy outcomes and long-term metabolic health of mice progeny

Animal studies demonstrate that circadian rhythm disruption during pregnancy can be deleterious to reproductive capacity and the long-term health of the progeny. Our previous studies in rats have shown that exposure of pregnant dams to an environment that significantly disrupts maternal circadian rhythms programs increased adiposity and poor glucose metabolism in offspring. In this study, we used mice with a ClockΔ19 mutation to determine whether foetal development within a genetically disrupted circadian environment affects pregnancy outcomes and alters the metabolic health of offspring. Ten female ClockΔ19+MEL mutant mice were mated with 10 wildtype males, and 10 wildtype females were mated with 10 ClockΔ19+MEL mutant males. While genetically identical, the heterozygote foetuses were exposed to either a normal (wildtype dams) or disrupted (ClockΔ19+MEL mutant dams) circadian environment during gestation. Pregnancy outcomes including time to mate, gestation length, litter size and birth weight were assessed. One male and one female offspring from each litter were assessed for postnatal growth, body composition, intraperitoneal glucose tolerance test and intraperitoneal insulin tolerance test at 3 and 12 months of age. There was no effect of maternal genotype on pregnancy outcomes, with days to plug, gestation length, litter size and perinatal mortality not significantly different between wildtype and ClockΔ19+MEL mutant dams. Similarly, there was no effect of maternal genotype on weight of the offspring at birth or at any stage of postnatal growth. While there was an effect of sex on various tissue weights at 3 and 12 months of age, there were minimal effects of maternal genotype. Relative adrenal weight was significantly reduced (-32%) in offspring from ClockΔ19+MEL mutant dams, whereas gastrocnemius muscle was significantly increased (+16%) at 3 months of age only. Intraperitoneal glucose tolerance tests at 3 months of age revealed female offspring from ClockΔ19+MEL mutant dams had significantly reduced area under the curve following glucose administration (-25%), although no differences were found at 12 months of age. There was no effect of maternal genotype on intraperitoneal insulin tolerance at 3 or 12 months of age for either sex. These results demonstrate that foetal growth within a genetically disrupted circadian environment during gestation has no effect on pregnancy success, and only marginal impacts upon the long-term metabolic health of offspring. These results do not support the hypothesis that circadian rhythm disruption during pregnancy programs poor metabolic homeostasis in offspring. However, when maintained on a 12L:12D photoperiod, the ClockΔ19+MEL mutant dams display relatively normal patterns of activity and melatonin secretion, which may have reduced the impact of the mutation upon foetal metabolic programming.

NPAS2 Compensates for Loss of CLOCK in Peripheral Circadian Oscillators

Heterodimers of CLOCK and BMAL1 are the major transcriptional activators of the mammalian circadian clock. Because the paralog NPAS2 can substitute for CLOCK in the suprachiasmatic nucleus (SCN), the master circadian pacemaker, CLOCK-deficient mice maintain circadian rhythms in behavior and in tissues in vivo. However, when isolated from the SCN, CLOCK-deficient peripheral tissues are reportedly arrhythmic, suggesting a fundamental difference in circadian clock function between SCN and peripheral tissues. Surprisingly, however, using luminometry and single-cell bioluminescence imaging of PER2 expression, we now find that CLOCK-deficient dispersed SCN neurons and peripheral cells exhibit similarly stable, autonomous circadian rhythms in vitro. In CLOCK-deficient fibroblasts, knockdown of Npas2 leads to arrhythmicity, suggesting that NPAS2 can compensate for loss of CLOCK in peripheral cells as well as in SCN. Our data overturn the notion of an SCN-specific role for NPAS2 in the molecular circadian clock, and instead indicate that, at the cellular level, the core loops of SCN neuron and peripheral cell circadian clocks are fundamentally similar.

In mammals, circadian clocks are based on a core transcriptional–translational feedback loop. BMAL1 and CLOCK activate the transcription of Per1-3 and Cry1/2. PER and CRY proteins inhibit BMAL1/CLOCK, and thus their own transcription. In Clock^-/- mice, NPAS2 can substitute for CLOCK in the suprachiasmatic nucleus (SCN), the major circadian pacemaker. However, peripheral tissues of Clock^-/- mice were reported to lack circadian rhythms. Since then, the protein CLOCK has been deemed essential for circadian rhythms in peripheral tissues. However, here we show that Clock^-/- peripheral cells and tissues exhibit stable, autonomous circadian rhythms. Furthermore, in Clock^-/- fibroblasts, knockdown of Npas2 leads to arrhythmicity, suggesting that NPAS2 can compensate for the loss of CLOCK in peripheral cells as well as in SCN. Our data overturn the notion of an SCN-specific role for NPAS2, and instead indicate that the core loops of SCN neuron and peripheral cell circadian clocks are fundamentally similar. This finding redefines a basic principle of molecular circadian clock regulation in peripheral organs which are essential for many metabolic processes. Disturbances of these rhythms lead to disorders like diabetes, obesity, and cancer. Thus, understanding the molecular basis of peripheral circadian oscillators is essential to develop treatments against clock-related disorders.

Interactions between endocrine and circadian systems

In most species, endogenous circadian clocks regulate 24-h rhythms of behavior and physiology. Clock disruption has been associated with decreased cognitive performance and increased propensity to develop obesity, diabetes, and cancer. Many hormonal factors show robust diurnal secretion rhythms, some of which are involved in mediating clock output from the brain to peripheral tissues. In this review, we describe the mechanisms of clock-hormone interaction in mammals, the contribution of different tissue oscillators to hormonal regulation, and how changes in circadian timing impinge on endocrine signalling and downstream processes. We further summarize recent findings suggesting that hormonal signals may feed back on circadian regulation and how this crosstalk interferes with physiological and metabolic homeostasis.

Circadian variation in gastric vagal afferent mechanosensitivity

Food intake is coordinated to cellular metabolism by clock gene expression with a master clock in the suprachiasmatic nucleus synchronized by light exposure. Gastric vagal afferents play a role in regulating food intake, but it is unknown whether they exhibit circadian variation in their mechanosensitivity. We aimed to determine whether gastric vagal afferents express clock genes and whether their response to mechanical stimuli oscillates throughout the light/dark cycle. Nodose ganglia were collected from 8-week-old female C57BL/6 mice every 3 h starting at lights off (1800 h) to quantify Bmal1, Per1, Per2, and Nr1d1 mRNA by qRT-PCR. Additionally in vitro single-fiber recordings of gastric vagal mechanoreceptors were taken at all time points. Per1, Per2, Bmal1, and Nr1d1 mRNA is expressed in the nodose ganglia and levels oscillated over a 24 h period. In mice fed ad libitum, gastric content was 3 times higher at 0000 h and 0300 h than 1200 h. The response of tension receptors to 3 g stretch was reduced by up to 70% at 2100 h, 0000 h, and 0300 h compared with 1200 h. Gastric mucosal receptor response to stroking with a 50 mg von Frey hair was 3 times greater at 1200 h and 1500 h than the response at 0000 h. Similar findings were obtained in mice fasted for 6 h or maintained in darkness for 3 d before study. Therefore, these changes do not result from food intake or the light/dark cycle. Thus, gastric vagal mechanoreceptors display circadian rhythm, which may act to control food intake differentially at different times of the day.

Long-term day-and-night rotating shift work poses a barrier to the normalization of alanine transaminase

To evaluate the impact of day-and-night rotating shift work (RSW) on liver health, we performed a retrospective analysis of the association between long-term RSW exposure and the normalization of plasma alanine transaminase (ALT) levels over a five-year period. The data from physical examinations, blood tests, abdominal sonographic examinations, personal histories, and occupational records were collected from a cohort of workers in a semiconductor manufacturing company. The sample population was divided into three subgroups for analysis, according to self-reported shift work status over the five-year interval: persistent daytime workers, workers exposed intermittently to RSW (i-RSW), and workers exposed persistently to RSW (p-RSW). Records were analyzed for 1196 male workers with an initial mean age of 32.5 years (SD 6.0 years), of whom 821 (68.7%) were identified as rotating shift workers, including 374 i-RSW (31.3%) and 447 p-RSW workers (37.4%). At the beginning of the follow-up, 275 were found to have elevated ALT (e-ALT): 25.1% daytime workers, 23.0% i-RSW workers, and 21.3% p-RSW workers (p = 0.098). Of those with e-ALT at the beginning, 101 workers showed normalized serum ALT levels at the end of five-year follow-up: 40 (10.7%) of 375 daytime workers, 32 (8.6%) of 374 i-RSW workers, and 29 (6.5%) of 447 p-RSW workers (p = 0.016). Compared with the workers having persistent e-ALT at the end of follow-up, the workers normalized serum ALT levels had significantly lesser exposures to RSW during follow-up. By performing multivariate logistic regression analyses, and comparing with the persistent daytime co-workers, after controlling for confounding variables (age, occupational factors, educational levels, lifestyle factors, metabolic syndrome, hepatovirus infection, and fatty liver), analysis indicated that the workers exposed to p-RSW were 46% less likely (OR, 0.54; 95% CI, 0.30-0.95; p = 0.03) to attain normal ALT levels within a five-year interval. These observations demonstrate that persistent day-and-night RSW pose a vigorous obstacle to the normalization of e-ALT among workers with preexisting abnormal liver function. We suggest that workers and managers approach with caution the consideration of assigning or accepting long-term day-and-night RSW when an employee health screening shows evidence of abnormal liver function.

Global loss of bmal1 expression alters adipose tissue hormones, gene expression and glucose metabolism

The close relationship between circadian rhythm disruption and poor metabolic status is becoming increasingly evident, but role of adipokines is poorly understood. Here we investigated adipocyte function and the metabolic status of mice with a global loss of the core clock gene Bmal1 fed either a normal or a high fat diet (22% by weight). Bmal1 null mice aged 2 months were killed across 24 hours and plasma adiponectin and leptin, and adipose tissue expression of Adipoq, Lep, Retn and Nampt mRNA measured. Glucose, insulin and pyruvate tolerance tests were conducted and the expression of liver glycolytic and gluconeogenic enzyme mRNA determined. Bmal1 null mice displayed a pattern of increased plasma adiponectin and plasma leptin concentrations on both control and high fat diets. Bmal1 null male and female mice displayed increased adiposity (1.8 fold and 2.3 fold respectively) on the normal diet, but the high fat diet did not exaggerate these differences. Despite normal glucose and insulin tolerance, Bmal1 null mice had increased production of glucose from pyruvate, implying increased liver gluconeogenesis. The Bmal1 null mice had arrhythmic clock gene expression in epigonadal fat and liver, and loss of rhythmic transcription of a range of metabolic genes. Furthermore, the expression of epigonadal fat Adipoq, Retn, Nampt, AdipoR1 and AdipoR2 and liver Pfkfb3 mRNA were down-regulated. These results show for the first time that global loss of Bmal1, and the consequent arrhythmicity, results in compensatory changes in adipokines involved in the cellular control of glucose metabolism.

Endurance training ameliorates the metabolic and performance characteristics of circadian Clock mutant mice

Circadian locomotor output cycles kaput (CLOCK) is a nuclear transcription factor that is a component of the central autoregulatory feedback loop that governs the generation of biological rhythms. Homozygous Clock mutant mice contain a truncated CLOCK(Δ19) protein within somatic cells, subsequently causing an impaired ability to rhythmically transactivate circadian genes. The present study sought to investigate whether the Clock mutation affects mitochondrial physiology within skeletal muscle, as well as the responsiveness of these mutant animals to adapt to a chronic voluntary endurance training protocol. Within muscle, Clock mutant mice displayed 44% and 45% reductions in peroxisome proliferator-activated receptor-γ coactivator 1-α (PGC-1α) and mitochondrial transcription factor-A protein content, respectively, and an accompanying 16% decrease in mitochondrial content, as determined by cytochrome c oxidase enzyme activity. These decrements contributed to a 50% decrease in exercise tolerance in Clock mutant mice. Interestingly, the Clock mutation did not appear to alter subsarcolemmal or intermyofibrillar mitochondrial respiration within muscle or systemic glucose tolerance. Daily locomotor activity levels were similar between wild-type and Clock mutant mice throughout the training protocol. Endurance training ameliorated the decrease in PGC-1α protein expression and mitochondrial content in the Clock mutant mice, eliciting a 2.9-fold improvement in exercise tolerance. Thus our data suggest that a functional CLOCK protein is essential to ensure the maintenance of mitochondrial content within muscle although the absence of a functional CLOCK protein does not impair the ability of animals to adapt to chronic exercise.

The circadian clock in cancer development and therapy

Most aspects of mammalian function display circadian rhythms driven by an endogenous clock. The circadian clock is operated by genes and comprises a central clock in the brain that responds to environmental cues and controls subordinate clocks in peripheral tissues via circadian output pathways. The central and peripheral clocks coordinately generate rhythmic gene expression in a tissue-specific manner in vivo to couple diverse physiological and behavioral processes to periodic changes in the environment. However, with the industrialization of the world, activities that disrupt endogenous homeostasis with external circadian cues have increased. This change in lifestyle has been linked to an increased risk of diseases in all aspects of human health, including cancer. Studies in humans and animal models have revealed that cancer development in vivo is closely associated with the loss of circadian homeostasis in energy balance, immune function, and aging, which are supported by cellular functions important for tumor suppression including cell proliferation, senescence, metabolism, and DNA damage response. The clock controls these cellular functions both locally in cells of peripheral tissues and at the organismal level via extracellular signaling. Thus, the hierarchical mammalian circadian clock provides a unique system to study carcinogenesis as a deregulated physiological process in vivo. The asynchrony between host and malignant tissues in cell proliferation and metabolism also provides new and exciting options for novel anticancer therapies.

Tissue-Specific Interaction of Per1/2 and Dec2 in the Regulation of Fibroblast Circadian Rhythms

In mammals, the molecular circadian clockwork is comprised of interlocked transcriptional-translational feedback loops (TTLs). Three Period ( Per1-3) and 2 Dec ( Dec1/2) genes interact in regulating the activity of the transcriptional activators CLOCK/NPAS2 and BMAL1. While deletion of Per1 and Per2 in mice results in behavioral arrhythmicity, Dec deletion has less dramatic effects on activity rhythms, affecting primarily phase of entrainment and free-running period. In intact animals, clock gene mutant phenotypes are often masked due to intercellular coupling mechanisms that stabilize cellular rhythms. Therefore, to study Per/Dec genetic interaction at the cellular level, we isolated fibroblasts from different tissues of Per1, Per2, and Dec2 single and double mutant mice. We show that in the cellular TTL, Pers and Dec2 act in a principally synergistic way, but tissue-specific differences in this interaction are seen. A rescue of rhythmicity in Per2 mutant cells after additional deletion of Dec2 was observed, indicating that in the absence of Per2, DEC2 destabilizes TTL function. Rhythm power in Per1/Dec2 and Per2/Dec2 double mutants was strongly reduced, suggesting that interaction of Dec2 with both Per genes is important for stabilizing clock period. Contrary to what was observed for behavior, nonsynergistic effects of Dec2 and Per1/2 mutations were observed on cellular clock phase regulation that do not correlate with period effects. Our data reveal cell type-specific interactions of Per1/2 and Dec2 in the regulation of period, phase, and rhythm sustainment, emphasizing the differential organization of the mammalian clock machinery in different tissues.

Biological clock dysfunction exacerbates contact hypersensitivity in mice

BACKGROUND

Immediate-type skin allergic reactions, such as passive cutaneous anaphylactic reaction, are associated with circadian rhythm, but the role of circadian mechanisms on delayed-type skin allergic reactions, such as contact hypersensitivity (CHS), remains uncertain. In mice, CHS, a T-cell-mediated immune response, is a classic model of human allergic contact dermatitis.

OBJECTIVES

  We investigated whether biological clock dysfunction affects CHS pathogenesis in CLOCK mutant mice compared with wild-type (WT) mice.

METHODS

 Mice were treated with 2,4,6-trinitro-1-chlorobenzene (TNCB) on the abdominal skin on day 0 (sensitization) and then treated with TNCB on the ears on day 5 (challenge).

RESULTS

We found that biological clock dysfunction resulted in severe inflammation. Ear swelling, serum immunoglobulin E level and mast cell number were significantly increased in CLOCK mutant mice compared with WT mice. These results provide evidence that CLOCK mutation promotes the T-helper type 2 immune response and exacerbates CHS. Corticosterone has a protective effect on CHS. The serum corticosterone level lost rhythmicity and showed a decreased daily level in CLOCK mutant mice compared with WT mice, supporting the exacerbating effect of CLOCK mutation on CHS. Adrenalectomy markedly worsened TNCB-induced CHS in WT mice but not in CLOCK mutant mice. In addition, dramatic dexamethasone-induced protection of CHS was observed in CLOCK mutant mice compared with WT mice.

CONCLUSIONS

 The present results suggest that circadian rhythm might be an important factor in the regulation of CHS via corticosterone rhythmicity and/or level.

Does the circadian system regulate lactation?

Environmental variables such as photoperiod, heat, stress, nutrition and other external factors have profound effects on quality and quantity of a dairy cow's milk. The way in which the environment interacts with genotype to impact milk production is unknown; however, evidence from our laboratory suggests that circadian clocks play a role. Daily and seasonal endocrine rhythms are coordinated in mammals by the master circadian clock in the hypothalamus. Peripheral clocks are distributed in every organ and coordinated by signals from the master clock. We and others have shown that there is a circadian clock in the mammary gland. Approximately 7% of the genes expressed during lactation had circadian patterns including core clock and metabolic genes. Amplitude changes occurred in the core mammary clock genes during the transition from pregnancy to lactation and were coordinated with changes in molecular clocks among multiple tissues. In vitro studies using a bovine mammary cell line showed that external stimulation synchronized mammary clocks, and expression of the core clock gene, BMAL1, was induced by lactogens. Female clock/clock mutant mice, which have disrupted circadian rhythms, have impaired mammary development and their offspring failed to thrive suggesting that the dam's milk production was not adequate enough to nourish their young. We envision that, in mammals, during the transition from pregnancy to lactation the master clock is modified by environmental and physiological cues that it receives, including photoperiod length. In turn, the master clock coordinates changes in endocrine milieu that signals peripheral tissues. In dairy cows, it is clear that changes in photoperiod during the dry period and/or during lactation influences milk production. We believe that the photoperiod effect on milk production is mediated, in part by the 'setting' of the master clock with light, which modifies peripheral circadian clocks including the mammary core clock and subsequently impacts milk yield and may impact milk composition.

Lactation Biology Symposium: circadian clocks as mediators of the homeorhetic response to lactation

The transition from pregnancy to lactation is the most stressful period in the life of a cow. During this transition, homeorhetic adaptations are coordinated across almost every organ and are marked by changes in hormones and metabolism to accommodate the increased energetic demands of lactation. Recent data from our laboratory showed that changes in circadian clocks occur in multiple tissues during the transition period in rats and indicate that the circadian system coordinates changes in the physiology of the dam needed to support lactation. Circadian rhythms coordinate the timing of physiological processes and synchronize these processes with the environment of the animal. Circadian rhythms are generated by molecular circadian clocks located in the hypothalamus (the master clock) and peripherally in every organ of the body. The master clock receives environmental and physiological cues and, in turn, synchronizes internal physiology by coordinating endocrine rhythms and metabolism through peripheral clocks. The effect of the circadian clock on lactation may be inferred by the photoperiod effect on milk production, which is accompanied by coordinated changes in the endocrine system and metabolic capacity of the dam to respond to changes in day length. We have shown that bovine mammary epithelial cells possess a functional clock that can be synchronized by external stimuli, and the expression of the aryl hydrocarbon receptor nuclear translocator-like gene, a positive limb of the core clock, is responsive to prolactin in bovine mammary explants. Others showed that 7% of genes expressed in breasts of lactating women had circadian patterns of expression, and we report that the diurnal variation of composition of bovine milk is associated with changes in expression of mammary core clock genes. Together these studies indicate that the circadian system coordinates the metabolic and hormonal changes needed to initiate and sustain lactation, and we believe that the capacity of the dam to produce milk and cope with metabolic stresses in early lactation is related to her ability to set circadian rhythms during the transition period.

Circadian rhythms and fertility

Circadian rhythms impact on a wide range of physiological systems and this impact extends to fertility, such that disruptions to timing systems can impact upon reproductive capacity. This is highlighted most obviously in mutant mouse models whereby deletion or mutation of single genes results not only in disrupted circadian rhythmicity, but also compromised male and female reproductive function. In this review, we discuss the presence of circadian clocks in female and male reproductive tissues and the role these clocks play in the generation of oestrus cycles, ovulation, sperm generation, implantation and the maintenance of pregnancy. Given the increased incidence of shiftwork and international travel which disrupt circadian rhythmicity, and the increasing prevalence of reproductive technologies whereby early embryo development occurs without external time cues, it is important for us to consider the role of circadian rhythms in fertility.

Circadian regulation of pineal gland rhythmicity

The pineal gland is a neuroendocrine organ of the brain. Its main task is to synthesize and secrete melatonin, a nocturnal hormone with diverse physiological functions. This review will focus on the central and pineal mechanisms in generation of mammalian pineal rhythmicity including melatonin production. In particular, this review covers the following topics: (1) local control of serotonin and melatonin rhythms; (2) neurotransmitters involved in central control of melatonin; (3) plasticity of the neural circuit controlling melatonin production; (4) role of clock genes in melatonin formation; (5) phase control of pineal rhythmicity; (6) impact of light at night on pineal rhythms; and (7) physiological function of the pineal rhythmicity.

Adipokines and adipocyte function in Clock mutant mice that retain melatonin rhythmicity

Clock(δ19)+MEL mutant mice, which retain melatonin rhythmicity, but lack peripheral tissue rhythmicity have impaired glucose tolerance, but reduced plasma free fatty acids, increased plasma adiponectin, and improved insulin sensitivity. Here, we report their response to a high-fat diet and adipocyte rhythmicity and function. The diet increased epigonadal fat weight similarly (twofold) in both wild-type and Clock(δ19)+MEL mice. The Clock(δ19) mutation abolished rhythmicity of Per2, Rev erbα and peroxisome proliferator-activated receptor-γ (Pparγ ) mRNA in epigonadal fat, but not Bmal1 mRNA, and reduced Rev erbα mRNA by 59 and 70% compared to the wild-type mice on the control and high-fat diets, respectively. The mutants had increased Adipoq mRNA expression in epigonadal fat (22%; P < 0.05) on a control diet, but showed no further change on a high-fat diet, and no change in Lep, Nampt or Retn mRNA on either diet. The Clock(δ19) mutation abolished rhythmicity of genes in epigonadal fat that contribute to plasma free fatty acids for mice on both diets, and increased Lipe mRNA expression in those on the high-fat diet. The persistent melatonin rhythm and reduced plasma free fatty acids in Clock(δ19)+MEL mutants may contribute to their enhanced insulin sensitivity, ameliorate the extent of impaired glucose homeostasis, and protect against the adverse effects of a high-fat diet.

Photic resetting and entrainment in CLOCK-deficient mice

Mice lacking the CLOCK protein have a relatively subtle circadian phenotype, including a slightly shorter period in constant darkness, differences in phase resetting after 4-hour light pulses in the early and late night, and a variably advanced phase angle of entrainment in a light-dark (LD) cycle. The present series of experiments was conducted to more fully characterize the circadian phenotype of Clock(-/-) mice under various lighting conditions. A phase-response curve (PRC) to 4-hour light pulses in free-running mice was conducted; the results confirm that Clock(-/-) mice exhibit very large phase advances after 4-hour light pulses in the late subjective night but have relatively normal responses to light at other phases. The abnormal shape of the PRC to light may explain the tendency of CLOCK-deficient mice to begin activity before lights-out when housed in a 12-hour light:12-hour dark lighting schedule. To assess this relationship further, Clock(-/-) and wild-type control mice were entrained to skeleton lighting cycles (1L:23D and 1L:10D:1L:12D). Comparing entrainment under the 2 types of skeleton photoperiods revealed that exposure to 1-hour light in the morning leads to a phase advance of activity onset (expressed the following afternoon) in Clock(-/-) mice but not in the controls. Constant light typically causes an intensity-dependent increase in circadian period in mice, but this did not occur in CLOCK-deficient mice. The failure of Clock(-/-) mice to respond to the period-lengthening effect of constant light likely results from the increased functional impact of light falling in the phase advance zone of the PRC. Collectively, these experiments reveal that alterations in the response of CLOCK-deficient mice to light in several paradigms are likely due to an imbalance in the shape of the PRC to light.

Circadian variation of the response of T cells to antigen

Circadian clocks regulate many important aspects of physiology, and their disturbance leads to various medical conditions. Circadian variations have been found in immune system variables, including daily rhythms in circulating WBC numbers and serum concentration of cytokines. However, control of immune functional responses by the circadian clock has remained relatively unexplored. In this study, we show that mouse lymph nodes exhibit rhythmic clock gene expression. T cells from lymph nodes collected over 24 h show a circadian variation in proliferation after stimulation via the TCR, which is blunted in Clock gene mutant mice. The tyrosine kinase ZAP70, which is just downstream of the TCR in the T cell activation pathway and crucial for T cell function, exhibits rhythmic protein expression. Lastly, mice immunized with OVA peptide-loaded dendritic cells in the day show a stronger specific T cell response than mice immunized at night. These data reveal circadian control of the Ag-specific immune response and a novel regulatory mode of T cell proliferation, and may provide clues for more efficient vaccination strategies.

Sex and dosing-time dependencies in irinotecan-induced circadian disruption

Circadian disruption accelerates malignant growth; thus, it should be avoided in anticancer therapy. The circadian disruptive effects of irinotecan, a topoisomerase I inhibitor, was investigated according to dosing time and sex. In previous work, irinotecan achieved best tolerability following dosing at zeitgeber time (ZT) 11 in male and ZT15 in female mice, whereas worst toxicity corresponded to treatment at ZT23 and ZT3 in male and female mice, respectively. Here, irinotecan (50 mg/kg intravenous [i.v.]) was delivered at the sex-specific optimal or worst circadian timing in male and female B6D2F1 mice. Circadian disruption was assessed with rest-activity, body temperature, plasma corticosterone, and liver mRNA expressions of clock genes Rev-erbα, Per2, and Bmal1. Baseline circadian rhythms in rest-activity, body temperature, and plasma corticosterone were more prominent in females as compared to males. Severe circadian disruption was documented for all physiology and molecular clock endpoints in female mice treated at the ZT of worst tolerability. Conversely, irinotecan administration at the ZT of best tolerability induced slight alteration of circadian physiology and clock-gene expression patterns in female mice. In male mice, irinotecan produced moderate alterations of circadian physiology and clock-gene expression patterns, irrespective of treatment ZT. However, the average expression of Rev-erbα, Per2, and Bmal1 were down-regulated 2- to 10-fold with irinotecan at the worst ZT, while being minimally or unaffected at the best ZT, irrespective of sex. Corticosterone secretion increased acutely within 2 h with a sex-specific response pattern, resulting in a ZT-dependent phase-advance or -delay in both sex. The mRNA expressions of irinotecan clock-controlled metabolism genes Ce2, Ugt1a1, and Top1 were unchanged or down-regulated according to irinotecan timing and sex. This study shows that the circadian timing system represents an important toxicity target of irinotecan in female mice, where circadian disruption persists after wrongly timed treatment. As a result, the mechanisms underling cancer chronotherapeutics are expectedly more susceptible to disruption in females as compared to males. Thus, the optimal circadian timing of chemotherapy requires precise determination according to sex, and should involve the noninvasive monitoring of circadian biomarkers.

Disrupting the circadian clock: gene-specific effects on aging, cancer, and other phenotypes

The circadian clock imparts 24-hour rhythmicity on gene expression and cellular physiology in virtually all cells. Disruption of the genes necessary for the circadian clock to function has diverse effects, including aging-related phenotypes. Some circadian clock genes have been described as tumor suppressors, while other genes have less clear functions in aging and cancer. In this Review, we highlight a recent study [Dubrovsky et al., Aging 2: 936-944, 2010] and discuss the much larger field examining the relationship between circadian clock genes, circadian rhythmicity, aging-related phenotypes, and cancer.

Expression pattern of clock under acute phase-delay of the light/dark cycle in the chicken pineal model

Shift workers have a higher risk of metabolic syndrome, a condition that also develops in mice carrying mutation in their circadian clock gene clock. To collect more data on the transcriptional changes of clock under phase-shifted light/dark LD conditions, we examined the 24h patterns of clock mRNA expression in vivo and in vitro in chickens exposed acutely to a reversed LD (DL) cycle. Under controlled LD conditions (lights on at 6:00, lights off at 20:00), clock mRNA expression peaked in vivo at 2:00 (Zeitgeber Time 20, ZT20) and in vitro at 22:00 (ZT16). Even higher mRNA contents were measured in the first cycle of in vivo DL conditions between 22:00 and 6:00 (lights at night), but in the second cycle by 2:00, lower mRNA contents were detected than the control peak values seen at this time point. Furthermore, no alterations were found in vitro in clock mRNA content during the first 12h of DL conditions (lights at night). The differences seen between the first and the second DL cycles in vivo and between the in vivo and in vitro data for the first DL cycle support the idea that neurohumoral signals perturbed by a phase-delayed light-dark cycle may also play a role in the in vivo rapid transcriptional resetting of the circadian clock in the chicken pineal model.

Clock genes at the heart of depression

The rhythms of life are ever pervasive, touching almost every aspect of our lives. We are finely tuned to the cycle of light and dark, so that we normally sleep during the night and are active during the day. Physiological rhythms are, however, not just slaves to the solar day, but are actually generated endogenously within the suprachiasmatic nuclei in the hypothalamus and are entrained via the retina. The circadian timing system is organized hierarchically with the suprachiasmatic nuclei providing neural and/or hormonal cues to the various organ systems, allowing them to express their own rhythmic physiological output. There is now a substantial body of evidence emerging that disruption of rhythmicity through altered sleep/wake patterns and exposure to light, or through endogenous disruption of key determinants of endogenous rhythms, can be detrimental to health. Circadian rhythm disturbances have long been associated with mood disorders, especially delayed sleep onset, and evidence is accumulating that alterations to the cellular timing system may underpin some aspects of the disorders. For example, mice carrying mutations in either Clock or per2 spend less time immobile in swim tests, which has been suggested as mimicking mania. In humans, single nucleotide polymorphisms in Clock and other clock genes have been associated with depression. With this increasing knowledge we may predict that new antidepressant drugs will emerge that, as a primary or secondary mechanism of action, target and correct abnormalities in the circadian timing system.

Reproductive biology of female Bmal1 null mice

The light/dark cycle and suprachiasmatic nucleus rhythmicity are known to have important influences on reproductive function of rodents. We studied reproductive function in female heterozygous and homozygous brain and muscle ARNT-like protein 1 (Bmal1, also known as Arntl) null mice, which lack central and peripheral cellular rhythms. Heterozygous Bmal1 mice developed normally and were fertile, with apparent normal pregnancy progression and litter size, although postnatal mortality up to weaning was high (1.1-1.3/litter). The genotype distribution was skewed with both heterozygous and null genotypes underrepresented (1.0:1.7:0.7; P<0.05), suggesting loss of a single Bmal1 allele may impact on postnatal survival. Homozygous Bmal1 null mice were 30% lighter at weaning, and while they grew at a similar rate to the wild-type mice, they never achieved a comparable body weight. They had delayed vaginal opening (4 days), disrupted estrus cyclicity, and reduced ovarian weight (30%). Bmal1 null mice had a 40% reduction in ductal length and a 43% reduction in ductal branches in the mammary gland. Surprisingly, the Bmal1 mice ovulated, but progesterone synthesis was reduced in conjunction with altered corpora lutea formation. Pregnancy failed prior to implantation presumably due to poor embryo development. While Bmal1 null ovaries responded to pregnant mare serum gonadotropin/human chorionic gonadotropin stimulation, ovulation rate was reduced, and the fertilized oocytes progressed poorly to blastocysts and failed to implant. The loss of Bmal1 gene expression resulted in a loss of rhythmicity of many genes in the ovary and downregulation of Star. In conclusion, it is clear that the profound infertility of Bmal1 null mice is multifactorial.

Measurement of internal body time by blood metabolomics

Detection of internal body time (BT) via a few-time-point assay has been a longstanding challenge in medicine, because BT information can be exploited to maximize potency and minimize toxicity during drug administration and thus will enable highly optimized medication. To address this challenge, we previously developed the concept, "molecular-timetable method," which was originally inspired by Linné's flower clock. In Linné's flower clock, one can estimate the time of the day by watching the opening and closing pattern of various flowers. Similarly, in the molecular-timetable method, one can measure the BT of the day by profiling the up and down patterns of substances in the molecular timetable. To make this method clinically feasible, we now performed blood metabolome analysis and here report the successful quantification of hundreds of clock-controlled metabolites in mouse plasma. Based on circadian blood metabolomics, we can detect individual BT under various conditions, demonstrating its robustness against genetic background, sex, age, and feeding differences. The power of this method is also demonstrated by the sensitive and accurate detection of circadian rhythm disorder in jet-lagged mice. These results suggest the potential for metabolomics-based detection of BT ("metabolite-timetable method"), which will lead to the realization of chronotherapy and personalized medicine.

Fat circadian biology

While adipose tissue has long been recognized for its major role in metabolism, it is now appreciated as an endocrine organ. A growing body of literature has emerged that identifies circadian mechanisms as a critical regulator of adipose tissue differentiation, metabolism, and adipokine secretory function in both health and disease. This concise review focuses on recent data from murine and human models that highlights the interplay between the core circadian regulatory proteins and adipose tissue in the context of energy, fat, and glucose metabolism. It will be important to integrate circadian mechanisms and networks into future descriptions of adipose tissue physiology.

Oscillating perceptions: the ups and downs of the CLOCK protein in the mouse circadian system

A functional mouse CLOCK protein has long been thought to be essential for mammalian circadian clockwork function, based mainly on studies of mice bearing a dominant negative, antimorphic mutation in the Clock gene. However, new discoveries using recently developed Clock-null mutant mice have shaken up this view. In this review, I discuss how this recent work impacts and alters the previous view of the role of CLOCK in the mouse circadian clockwork.

Impaired steroidogenesis and implantation failure in Bmal1-/- mice

Evidence in humans and rodents suggests that normal circadian rhythmicity is important for supporting reproductive function. A molecular clock underlies circadian rhythmicity. Impaired fertility is observed in some genetically altered mice with deficiencies in genes of the molecular clock, suggesting a critical role for these genes in reproduction. Here we systematically characterize the reproductive phenotype of females deficient in the clock gene Bmal1. Bmal1(-/-) females are infertile. They exhibit progression through the estrous cycle, although these cycles are prolonged. Normal follicular development occurs in Bmal1(-/-) females, and healthy embryos of the expected developmental stage are found in the reproductive tract of Bmal1(-/-) females 3.5 d after mating to wild-type males. However, serum progesterone levels are significantly lower in Bmal1(-/-) vs. Bmal1(+/+/-) females on d 3.5 of gestation. Low progesterone levels in Bmal1(-/-) females are accompanied by decreased expression of steroidogenic acute regulatory protein in corpora lutea of Bmal1(-/-) vs. Bmal1(+/+/-) females. Whereas implantation of embryos is not observed in untreated or vehicle-treated Bmal1(-/-) females, exogenous administration of progesterone to Bmal1(-/-) females is able to reinstitute implantation. These data suggest that implantation failure due to impaired steroidogenesis causes infertility of Bmal1(-/-) females.

Glucocorticoids and the circadian clock

Glucocorticoids, hormones produced by the adrenal gland cortex, perform numerous functions in body homeostasis and the response of the organism to external stressors. One striking feature of their regulation is a diurnal release pattern, with peak levels linked to the start of the activity phase. This release is under control of the circadian clock, an endogenous biological timekeeper that acts to prepare the organism for daily changes in its environment. Circadian control of glucocorticoid production and secretion involves a central pacemaker in the hypothalamus, the suprachiasmatic nucleus, as well as a circadian clock in the adrenal gland itself. Central circadian regulation is mediated via the hypothalamic-pituitary-adrenal axis and the autonomic nervous system, while the adrenal gland clock appears to control sensitivity of the gland to the adrenocorticopic hormone (ACTH). The rhythmically released glucocorticoids in turn might contribute to synchronisation of the cell-autonomous clocks in the body and interact with them to time physiological dynamics in their target tissues around the day.

Probing the relative importance of molecular oscillations in the circadian clock

Circadian ( approximately 24 hr) rhythms of behavior and physiology are driven by molecular clocks that are endogenous to most organisms. The mechanisms underlying these clocks are remarkably conserved across evolution and typically consist of auto-regulatory loops in which specific proteins (clock proteins) rhythmically repress expression of their own genes. Such regulation maintains 24-hr cycles of RNA and protein expression. Despite the conservation of these mechanisms, however, questions are now being raised about the relevance of different molecular oscillations. Indeed, several studies have demonstrated that oscillations of some critical clock genes can be eliminated without loss of basic clock function. Here, we describe the multiple levels at which clock gene/protein expression and function can be rhythmically regulated-transcription, protein expression, post-translational modification, and localization-and speculate as to which aspect of this regulation is most critical. While the review is focused on Drosophila, we include some discussion of mammalian clocks to indicate the extent to which the questions concerning clock mechanisms are similar, regardless of the organism under study.

Metabolic homeostasis in mice with disruptedClockgene expression in peripheral tissues

The role of peripheral vs. central circadian rhythms and Clock in the maintenance of metabolic homeostasis and with aging was examined by using ClockΔ19+MEL mice. These have preserved suprachiasmatic nucleus and pineal gland rhythmicity but arrhythmic Clock gene expression in the liver and skeletal muscle. ClockΔ19+MEL mice showed fasting hypoglycemia in young-adult males, fasting hyperglycemia in older females, and substantially impaired glucose tolerance overall. ClockΔ19+MEL mice had substantially reduced plasma insulin and plasma insulin/glucose nocturnally in males and during a glucose tolerance test in females, suggesting impaired insulin secretion. ClockΔ19+MEL mice had reduced hepatic expression and loss of rhythmicity of gck, pfkfb3, and pepck mRNA, which is likely to impair glycolysis and gluconeogenesis. ClockΔ19+MEL mice also had reduced glut4 mRNA in skeletal muscle, and this may contribute to poor glucose tolerance. Whole body insulin tolerance was enhanced in ClockΔ19+MEL mice, however, suggesting enhanced insulin sensitivity. These responses occurred although the ClockΔ19mutation did not cause obesity and reduced plasma free fatty acids while increasing plasma adiponectin. These studies on clock-gene disruption in peripheral tissues and metabolic homeostasis provide compelling evidence of a relationship between circadian rhythms and the glucose/insulin and adipoinsular axes. It is, however, premature to declare that clock-gene disruption causes the full metabolic syndrome.

Challenging the omnipotence of the suprachiasmatic timekeeper: are circadian oscillators present throughout the mammalian brain?

The suprachiasmatic nucleus of the hypothalamus (SCN) is the master circadian pacemaker or clock in the mammalian brain. Canonical theory holds that the output from this single, dominant clock is responsible for driving most daily rhythms in physiology and behaviour. However, important recent findings challenge this uniclock model and reveal clock-like activities in many neural and non-neural tissues. Thus, in addition to the SCN, a number of areas of the mammalian brain including the olfactory bulb, amygdala, lateral habenula and a variety of nuclei in the hypothalamus, express circadian rhythms in core clock gene expression, hormone output and electrical activity. This review examines the evidence for extra-SCN circadian oscillators in the mammalian brain and highlights some of the essential properties and key differences between brain oscillators. The demonstration of neural pacemakers outside the SCN has wide-ranging implications for models of the circadian system at a whole-organism level.

Molecular components of the mammalian circadian clock

Circadian rhythms are approximately 24-h oscillations in behavior and physiology, which are internally generated and function to anticipate the environmental changes associated with the solar day. A conserved transcriptional-translational autoregulatory loop generates molecular oscillations of 'clock genes' at the cellular level. In mammals, the circadian system is organized in a hierarchical manner, in which a master pacemaker in the suprachiasmatic nucleus (SCN) regulates downstream oscillators in peripheral tissues. Recent findings have revealed that the clock is cell-autonomous and self-sustained not only in a central pacemaker, the SCN, but also in peripheral tissues and in dissociated cultured cells. It is becoming evident that specific contribution of each clock component and interactions among the components vary in a tissue-specific manner. Here, we review the general mechanisms of the circadian clockwork, describe recent findings that elucidate tissue-specific expression patterns of the clock genes and address the importance of circadian regulation in peripheral tissues for an organism's overall well-being.

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.