The mammalian circadian timing system: from gene expression to physiology

Sunspot dynamics are reflected in human physiology and pathophysiology

Periodic episodes of increased sunspot activity (solar electromagnetic storms) occur with 10-11 and 5-6 year periodicities and may be associated with measurable biological events. We investigated whether this sunspot periodicity characterized the incidence of Pap smear-determined cervical epithelial histopathologies and human physiologic functions. From January 1983 through December 2003, monthly averages were obtained for solar flux and sunspot numbers; six infectious, premalignant and malignant changes in the cervical epithelium from 1,182,421 consecutive, serially independent, screening Pap smears (59°9″N, 4°29″E); and six human physiologic functions of a healthy man (oral temperature, pulse, systolic and diastolic blood pressure, respiration, and peak expiratory flow), which were measured ∼5 times daily during ∼34,500 self-measurement sessions (44°56″N, 93°8″W). After determining that sunspot numbers and solar flux, which were not annually rhythmic, occurred with a prominent 10-year and a less-prominent 5.75-year periodicity during this 21-year study span, each biological data set was analyzed with the same curve-fitting procedures. All six annually rhythmic Pap smear-detected infectious, premalignant and malignant cervical epithelial pathologies showed strong 10-year and weaker 5.75-year cycles, as did all six self-measured, annually rhythmic, physiologic functions. The phases (maxima) for the six histopathologic findings and five of six physiologic measurements were very near, or within, the first two quarters following the 10-year solar maxima. These findings add to the growing evidence that solar magnetic storm periodicities are mirrored by cyclic phase-locked rhythms of similar period length or lengths in human physiology and pathophysiology.

Genomics and systems approaches in the mammalian circadian clock

The circadian clock is an endogenous oscillator that regulates daily rhythms in behavior and physiology. In recent years, systems biology and genomics approaches re-shaped our view of the clock. Our understanding of outputs that regulate behavior and physiology has been enhanced through gene expression profiling and proteomic analyses. Systems approaches uncovered underlying principles of transcriptional regulation and robustness of the oscillator through perturbation analysis and synthetic methods. Finally, new clock components and modifiers were identified through cell-based screening efforts and proteomics.

Spotlight on post-transcriptional control in the circadian system

An endogenous timing mechanism, the circadian clock, causes rhythmic expression of a considerable fraction of the genome of most organisms to optimally align physiology and behavior with their environment. Circadian clocks are self-sustained oscillators primarily based on transcriptional feedback loops and post-translational modification of clock proteins. It is increasingly becoming clear that regulation at the RNA level strongly impacts the cellular circadian transcriptome and proteome as well as the oscillator mechanism itself. This review focuses on posttranscriptional events, discussing RNA-binding proteins that, by influencing the timing of pre-mRNA splicing, polyadenylation and RNA decay, shape rhythmic expression profiles. Furthermore, recent findings on the contribution of microRNAs to orchestrating circadian rhythms are summarized.

High-throughput chemical screen identifies a novel potent modulator of cellular circadian rhythms and reveals CKIα as a clock regulatory kinase

A novel compound “longdaysin” was found to dramatically slow down the speed of the circadian clock through simultaneous inhibition of protein kinases CKIδ, CKIα, and ERK2.

The circadian clock underlies daily rhythms of diverse physiological processes, and alterations in clock function have been linked to numerous pathologies. To apply chemical biology methods to modulate and dissect the clock mechanism with new chemical probes, we performed a circadian screen of ∼120,000 uncharacterized compounds on human cells containing a circadian reporter. The analysis identified a small molecule that potently lengthens the circadian period in a dose-dependent manner. Subsequent analysis showed that the compound also lengthened the period in a variety of cells from different tissues including the mouse suprachiasmatic nucleus, the central clock controlling behavioral rhythms. Based on the prominent period lengthening effect, we named the compound longdaysin. Longdaysin was amenable for chemical modification to perform affinity chromatography coupled with mass spectrometry analysis to identify target proteins. Combined with siRNA-mediated gene knockdown, we identified the protein kinases CKIδ, CKIα, and ERK2 as targets of longdaysin responsible for the observed effect on circadian period. Although individual knockdown of CKIδ, CKIα, and ERK2 had small period effects, their combinatorial knockdown dramatically lengthened the period similar to longdaysin treatment. We characterized the role of CKIα in the clock mechanism and found that CKIα-mediated phosphorylation stimulated degradation of a clock protein PER1, similar to the function of CKIδ. Longdaysin treatment inhibited PER1 degradation, providing insight into the mechanism of longdaysin-dependent period lengthening. Using larval zebrafish, we further demonstrated that longdaysin drastically lengthened circadian period in vivo. Taken together, the chemical biology approach not only revealed CKIα as a clock regulatory kinase but also identified a multiple kinase network conferring robustness to the clock. Longdaysin provides novel possibilities in manipulating clock function due to its ability to simultaneously inhibit several key components of this conserved network across species.

Most organisms show daily rhythms in physiology, behavior, and metabolism, which may be advantageous because they anticipate environmental changes thus optimize energy metabolism. These rhythms are controlled by the circadian clock, which produces cyclic expression of thousands of output genes. More than a dozen components of the circadian clock are called clock genes, and the proteins they encode form a transcription factor network that generates rhythmic gene expression. In this study, we set out to control the function of the circadian clock and to identify new clock proteins by means of chemical tools. We tested the effects on the clock in human cells of around 120,000 uncharacterized compounds. Here we describe identification of a novel compound “longdaysin” that markedly slows the circadian clock both in cultured mammalian cells and in living zebrafish. By using longdaysin as a chemical probe, we found new proteins that modulate clock function. Because defects of clock function have been linked to numerous diseases, longdaysin may form the basis for therapeutic strategies directed towards circadian rhythm-related disorders, shift-work fatigue, and jet lag.

Deficiency of circadian protein CLOCK reduces lifespan and increases age-related cataract development in mice

Circadian clock is implicated in the regulation of aging. The transcription factor CLOCK, a core component of the circadian system, operates in complex with another circadian clock protein BMAL1. Recently it was demonstrated that BMAL1 deficiency results in premature aging in mice. Here we investigate the aging of mice deficient for CLOCK protein. Deficiency of the CLOCK protein significantly affects longevity: the average lifespan of Clock-/- mice is reduced by 15% compared with wild type mice, while maximum lifespan is reduced by more than 20%. CLOCK deficiency also results in the development of two age-specific pathologies in these mice, cataracts and dermatitis, at a much higher rate than in wild type mice. In contrast to BMAL1 deficient animals, Clock-/- mice do not develop a premature aging phenotype and do not develop the multiple age-associated pathologies characteristic of BMAL1 deficiency. Thus, although CLOCK and BMAL1 form a transcriptional complex, the physiological result of their deficiency is different. Our results suggest that CLOCK plays an important role in aging, specifically; CLOCK activity is critical for the regulation of normal physiology and aging of the lens and skin.

Molecular basis for the dosing time-dependency of anti-allodynic effects of gabapentin in a mouse model of neuropathic pain

BACKGROUND

Neuropathic pain is characterized by hypersensitivity to innocuous stimuli (tactile allodynia) that is nearly always resistant to NSAIDs or even opioids. Gabapentin, a GABA analogue, was originally developed to treat epilepsy. Accumulating clinical evidence supports the effectiveness of this drug for diverse neuropathic pain. In this study, we showed that the anti-allodynic effect of gabapentin was changed by the circadian oscillation in the expression of its target molecule, the calcium channel α2δ-1 subunit.

RESULTS

Mice were underwent partial sciatic nerve ligation (PSL) to create a model of neuropathic pain. The paw withdrawal threshold (PWT) in PSL mice significantly decreased and fluctuated with a period length about 24 h. The PWT in PSL mice was dose-dependently increased by intraperitoneal injection of gabapentin, but the anti-allodynic effects varied according to its dosing time. The protein levels of α2δ-1 subunit were up-regulated in the DRG of PSL mice, but the protein levels oscillated in a circadian time-dependent manner. The time-dependent oscillation of α2δ-1 subunit protein correlated with fluctuations in the maximal binding capacity of gabapentin. The anti-allodynic effect of gabapentin was attenuated at the times of the day when α2δ-1 subunit protein was abundant.

CONCLUSIONS

These findings suggest that the dosing time-dependent difference in the anti-allodynic effects of gabapentin is attributable to the circadian oscillation of α2δ-1 subunit expression in the DRG and indicate that the optimizing its dosing schedule helps to achieve rational pharmacotherapy for neuropathic pain.

Circadian clocks and metabolism: the nutrient-sensing AKT and TOR pathways make the link

How do organisms sense metabolic and nutrient status to set their biological clocks? The significance of recent work showing that nutrient-sensing pathways influence central circadian clocks in Drosophila is discussed.

Systemic and cellular reflections on ageing and the circadian oscillator: a mini-review

From circulation to digestion to excretion, a circadian clock synchronizes most aspects of mammalian physiology with the solar day. During normal ageing, this daily coordination gradually erodes, and during pathological ageing such erosion is exacerbated. Recent experiments suggest that therapies aimed at sustaining circadian function increase quality of life in elderly patients. Hence, a better understanding of the interactions between the circadian clock and ageing - at both cellular and systemic levels - could lead to direct benefits for aged individuals.

The association of the Clock 3111 T/C SNP with lipids and lipoproteins including small dense low-density lipoprotein: results from the Mima study

BACKGROUND

The clock molecule plays major roles in circadian rhythmicity and regulating lipid and glucose metabolism in peripheral organs. Disruption of the circadian rhythm can lead to cardiometabolic disorders. The existence of small dense low-density lipoprotein (sdLDL) in the circulation, an abnormality of lipid metabolism, in part associated with lifestyle, is also one of risk parameters for cardiometabolic disorders. The 3111 T/C single nucleotide polymorphism (SNP) of the Clock gene has been reported to be associated with lifestyle including morning/evening preference. We investigated whether the Clock 3111 T/C SNP may affect lipids and lipoproteins including sdLDL.

METHODS

In 365 community-dwelling subjects (170 men and 195 women, mean age 63 ± 14 years), the 3111 T/C SNP was genotyped using a fluorescent allele-specific DNA primer assay system. The levels of sdLDL were measured with the electrophoretic separation of lipoproteins employing the Lipoprint system.

RESULTS

The frequency of the Clock 3111 C allele was 0.14. The area of sdLDL did not differ between the subjects with obesity and those without. In carriers of T/T homozygotes, the area of sdLDL was significantly higher compared with carriers of the C allele (T/C or C/C) (1.7 ± 3.4 vs. 0.8 ± 1.9%; p < 0.05). A multiple regression analysis showed that the area of sdLDL was significantly and negatively correlated with the Clock 3111 T/C SNP (β = -0.114, p < 0.05), independently of age, sex, body mass index, and exercise habits.

CONCLUSION

Our findings indicated that the Clock 3111 T/C SNP might be associated with the existence of sdLDL.

The physiological period length of the human circadian clock in vivo is directly proportional to period in human fibroblasts

Background

Diurnal behavior in humans is governed by the period length of a circadian clock in the suprachiasmatic nuclei of the brain hypothalamus. Nevertheless, the cell-intrinsic mechanism of this clock is present in most cells of the body. We have shown previously that for individuals of extreme chronotype (“larks” and “owls”), clock properties measured in human fibroblasts correlated with extreme diurnal behavior.

Methodology/Principal Findings

In this study, we have measured circadian period in human primary fibroblasts taken from normal individuals and, for the first time, compared it directly with physiological period measured in vivo in the same subjects. Human physiological period length was estimated via the secretion pattern of the hormone melatonin in two different groups of sighted subjects and one group of totally blind subjects, each using different methods. Fibroblast period length was measured via cyclical expression of a lentivirally delivered circadian reporter. Within each group, a positive linear correlation was observed between circadian period length in physiology and in fibroblast gene expression. Interestingly, although blind individuals showed on average the same fibroblast clock properties as sighted ones, their physiological periods were significantly longer.

Conclusions/Significance

We conclude that the period of human circadian behaviour is mostly driven by cellular clock properties in normal individuals and can be approximated by measurement in peripheral cells such as fibroblasts. Based upon differences among sighted and blind subjects, we also speculate that period can be modified by prolonged unusual conditions such as the total light deprivation of blindness.

Cardiac hypertrophy, low blood pressure, and low aldosterone levels in mice devoid of the three circadian PAR bZip transcription factors DBP, HLF, and TEF

The cardiovascular system is under the control of the circadian clock, and disturbed circadian rhythms can induce cardiovascular pathologies. This cyclic regulation is probably brought about by the circadian expression of genes encoding enzymes and regulators involved in cardiovascular functions. We have previously shown that the rhythmic transcription of output genes is, in part, regulated by the clock-controlled PAR bZip transcription factors DBP (albumin D-site binding protein), HLF (hepatic leukemia factor), and TEF (thyrotroph embryonic factor). The simultaneous deletion of all three PAR bZip transcription factors leads to increased morbidity and shortened life span. In the present study, we demonstrate that Dbp/ Tef/ Hlf triple knockout mice develop cardiac hypertrophy and left ventricular dysfunction associated with a low blood pressure. These dysfunctions are exacerbated by an abnormal response to this low blood pressure characterized by low aldosterone levels. The phenotype of PAR bZip knockout mice highlights the importance of circadian regulators in the modulation of cardiovascular functions.

Circadian regulation of cell cycle: Molecular connections between aging and the circadian clock

The circadian clock generates oscillations in physiology and behavior, known as circadian rhythms. Links between the circadian clock genes Periods, Bmal1, and Cryptochromes and aging and cancer are emerging. Circadian clock gene expression is changed in human pathologies, and transgenic mice with mutations in clock genes develop cancer and premature aging. Control of genome integrity and cell proliferation play key roles in the development of age-associated pathologies and carcinogenesis. Here, we review recent data on the connection between the circadian clock and control of the cell cycle. The circadian clock regulates the activity and expression of several critical cell cycle and cell cycle check-point-related proteins, and in turn cell cycle-associated proteins regulate circadian clock proteins. DNA damage can reset the circadian clock, which provides a molecular mechanism for reciprocal regulation between the circadian clock and the cell cycle. This circadian clock-dependent control of cell proliferation, together with other known physiological functions of the circadian clock such as the control of metabolism, oxidative and genotoxic stress response, and DNA repair, opens new horizons for understanding the mechanisms behind aging and carcinogenesis.

The adjustment and manipulation of biological rhythms by light, nutrition, and abused drugs

Daily restricted feeding entrains the circadian rhythm of mouse clock gene expression in the central nervous system, excluding the suprachiasmatic nucleus (SCN), as well as in the peripheral tissues such as the liver, lung, and heart. In addition to entrainment of the clock genes, daily restricted feeding induces a locomotor activity increase 2-3h before the restricted feeding time initiates. The increase in activity is called the food-anticipatory activity (FAA). In addition to FAA, daily restricted feeding can also entrain peripheral circadian clocks in other organs such as liver, lung, and heart. This type of oscillator is called the food-entrainable peripheral oscillator (FEPO). At present, the mechanisms for restricted feeding-induced entrainment of locomotor activity (FAA) and/or peripheral clock (FEPO) are still unknown. In this review, we describe the role of the central nervous system and peripheral tissues in FAA performance and also in the entrainment of clock gene expression. In addition, the mechanism for entrainment of circadian oscillators by the abuse of drugs, such as methamphetamine, is discussed.

Episodes of prolactin gene expression in GH3 cells are dependent on selective promoter binding of multiple circadian elements

Prolactin (PRL) gene expression in mammotropes occurs in pulses, but the mechanism(s) underlying this dynamic process remains obscure. Recent findings from our laboratory of an E-box in the rat PRL promoter (E-box133) that can interact with the circadian factors, circadian locomoter output cycles kaput (CLOCK) and brain and muscle aryl hydrocarbon receptor nuclear translocator-like protein (BMAL)-1, and was necessary for pulse activity raised the intriguing possibility that the circadian system may be central to this oscillatory process. In this study, we used serum-shocked GH(3) cells, established previously to synchronize PRL pulses between cells in culture, to reveal that pulses of PRL mRNA are linked temporally to the expression of bmal1, cry1, per1, and per3 mRNA in these cells. Moreover, we found that each of these circadian factors binds to the rat PRL promoter by chromatin immunoprecipitation analysis. Using EMSA analysis, we observed that two sites present in the proximal promoter region, E-box133 and E-box10, bind circadian factors differentially (E-box133 interacted with BMAL1, cryptochrome-1, period (PER)-1, and PER3 but not PER2 and E-box10 bound BMAL1, cryptochrome-1, PER2, PER3 but not PER1). More importantly, down-regulation of any factor binding E-box133 significantly reduced PRL mRNA levels during pulse periods. Our results demonstrate clearly that certain circadian elements binding to the E-box133 site are required for episodes of PRL mRNA expression in serum-shocked GH(3) cultures. Moreover, our findings of binding-related differences between functionally distinct E-boxes demonstrate not only that E-boxes can bind different components but suggest that the number and type of circadian elements that bind to an E-box is central in dictating its function.

Nuclear receptor Rev-erbalpha: a heme receptor that coordinates circadian rhythm and metabolism

Nuclear receptor Rev-erbα (NR1D1), previously considered to be an orphan nuclear receptor, is a receptor for heme, which promotes transcriptional repression via recruitment of the NCoR-HDAC3 corepressor complex. Rev-erbα gene regulation is circadian, and Rev-erbα comprises a critical negative limb of the core circadian clock by directly repressing the expression of the positive clock component, Bmal1. Rev-erbα also regulates the metabolic gene pathway, thus serving as a heme sensor for coordination of circadian and metabolic pathways.

The Mammalian Circadian Timing System: Organization and Coordination of Central and Peripheral Clocks

Most physiology and behavior of mammalian organisms follow daily oscillations. These rhythmic processes are governed by environmental cues (e.g., fluctuations in light intensity and temperature), an internal circadian timing system, and the interaction between this timekeeping system and environmental signals. In mammals, the circadian timekeeping system has a complex architecture, composed of a central pacemaker in the brain's suprachiasmatic nuclei (SCN) and subsidiary clocks in nearly every body cell. The central clock is synchronized to geophysical time mainly via photic cues perceived by the retina and transmitted by electrical signals to SCN neurons. In turn, the SCN influences circadian physiology and behavior via neuronal and humoral cues and via the synchronization of local oscillators that are operative in the cells of most organs and tissues. Thus, some of the SCN output pathways serve as input pathways for peripheral tissues. Here we discuss knowledge acquired during the past few years on the complex structure and function of the mammalian circadian timing system.

Development of the circadian oscillator during differentiation of mouse embryonic stem cells in vitro

The molecular oscillations underlying the generation of circadian rhythmicity in mammals develop gradually during ontogenesis. However, the developmental process of mammalian cellular circadian oscillator formation remains unknown. In differentiated somatic cells, the transcriptional-translational feedback loops (TTFL) consisting of clock genes elicit the molecular circadian oscillation. Using a bioluminescence imaging system to monitor clock gene expression, we show here that the circadian bioluminescence rhythm is not detected in the mouse embryonic stem (ES) cells, and that the ES cells likely lack TTFL regulation for clock gene expression. The circadian clock oscillation was induced during the differentiation culture of mouse ES cells without maternal factors. In addition, reprogramming of the differentiated cells by expression of Sox2, Klf4, Oct3/4, and c-Myc genes, which were factors to generate induced pluripotent stem (iPS) cells, resulted in the re-disappearance of circadian oscillation. These results demonstrate that an intrinsic program controls the formation of the circadian oscillator during the differentiation process of ES cells in vitro. The cellular differentiation and reprogramming system using cultured ES cells allows us to observe the circadian clock formation process and may help design new strategies to understand the key mechanisms responsible for the organization of the molecular oscillator in mammals.

Suprachiasmatic nucleus: cell autonomy and network properties

The suprachiasmatic nucleus (SCN) is the primary circadian pacemaker in mammals. Individual SCN neurons in dispersed culture can generate independent circadian oscillations of clock gene expression and neuronal firing. However, SCN rhythmicity depends on sufficient membrane depolarization and levels of intracellular calcium and cAMP. In the intact SCN, cellular oscillations are synchronized and reinforced by rhythmic synaptic input from other cells, resulting in a reproducible topographic pattern of distinct phases and amplitudes specified by SCN circuit organization. The SCN network synchronizes its component cellular oscillators, reinforces their oscillations, responds to light input by altering their phase distribution, increases their robustness to genetic perturbations, and enhances their precision. Thus, even though individual SCN neurons can be cell-autonomous circadian oscillators, neuronal network properties are integral to normal function of the SCN.

Antioxidant N-acetyl-L-cysteine ameliorates symptoms of premature aging associated with the deficiency of the circadian protein BMAL1

Deficiency of the circadian clock protein BMAL1 leads to premature aging and increased levels of reactivate oxygen species in several tissues of mice. In order to investigate the role of oxidative stress in accelerated aging and development of age-related pathologies, we continuously administered the antioxidant N-acetyl-L-cysteine toBmal1-deficient mice through their entire lifespan by supplementing drinking water. We found that the life long treatment with antioxidant significantly increased average and maximal lifespan and reduced the rate of age-dependent weight loss and development of cataracts. At the same time, it had no effect on time of onset and severity of other age-related pathologies characteristic of Bmal1-/- mice, such as joint ossification, reduced hair regrowth and sarcopenia. We conclude that chronic oxidative stress affects longevity and contributes to the development of at least some age-associated pathology, although ROS-independent mechanisms may also play a role. Our bioinformatics analysis identified the presence of a conservative E box element in the promoter regions of several genes encoding major antioxidant enzymes. We speculate that BMAL1 controls antioxidant defense by regulating the expression of major antioxidant enzymes.

Effect of hyperlipidemia on the expression of circadian genes in apolipoprotein E knock-out atherosclerotic mice

Background

Circadian patterns of cardiovascular vulnerability were well characterized, with a peak incidence of acute myocardial infarction and stroke secondary to atherosclerosis in the morning, which showed the circadian clock may take part in the pathological process of atherosclerosis induced by hyperlipidemia. Hence, the effect of hyperlipidemia on the expression of circadian genes was investigated in atherosclerotic mouse model.

Results

In apoE-/-mice on regular chow or high-fat diet, an atherosclerotic mouse model induced by heperlipidemia, we found that the peak concentration of serum lipids was showed four or eight hours later in apoE-/- mice, compared to C57BL/6J mice. During the artificial light period, a reduce in circulating level of serum lipids corresponded with the observed increase of the expression levels of some the transcription factors involved in lipid metabolism, such as PPARα and RXRα. Meanwhile, the expression of circadian genes was changed following with amplitude reduced or the peak mRNA level delayed.

Conclusions

Our studies indicated that heperlipidemia altered both the rhythmicity and expression of circadian genes. Diet-induced circadian disruption may affect the process of atherosclerosis and some acute cardiovascular disease.

Circadian Disruption, Fatigue, and Anorexia Clustering in Advanced Cancer Patients: Implications for Innovative Therapeutic Approaches

A disruption of the circadian timing system, as identified by monitoring of marker biorhythms, is common in cancer patients. The recording of the rest—activity rhythm with a wrist actigraph has been commonly used. This noninvasive monitoring allows a robust estimation of circadian disruption. The authors have previously found that altered patterns of circadian rest—activity rhythms are significantly and independently associated with the severity of fatigue and anorexia in patients with metastatic colorectal cancer. Elevated proinflammatory cytokines could partly account for this circadian disruption and its associated constitutional symptoms. Here, the authors present and discuss the data supporting the hypothesis that circadian disruption is often associated with fatigue and anorexia, which in turn further alter and dampen circadian synchronization, thus, creating a vicious cycle. This body of evidence paves the path for innovative therapeutic approaches targeting the circadian timing system in an effort to diminish constitutional symptoms induced by cancer and some anticancer treatments.

Self-sustained circadian rhythm in cultured human mononuclear cells isolated from peripheral blood

Disturbed circadian rhythmicity is associated with human diseases such as sleep and mood disorders. However, study of human endogenous circadian rhythm is laborious and time-consuming, which hampers the elucidation of diseases. It has been reported that peripheral tissues exhibit circadian rhythmicity as the suprachiasmatic nucleus-the center of the biological clock. We tried to study human circadian rhythm using cultured peripheral blood mononuclear cells (PBMCs) obtained from a single collection of venous blood. Activated human PBMCs showed self-sustained circadian rhythm of clock gene expression, which indicates that they are useful for investigating human endogenous circadian rhythm.

REV-ERBalpha participates in circadian SREBP signaling and bile acid homeostasis

The nuclear receptor REV-ERBα shapes the daily activity profile of Sterol Response Element Binding Protein (SREBP) and thereby participates in the circadian control of cholesterol and bile acid synthesis in the liver.

In mammals, many aspects of behavior and physiology, and in particular cellular metabolism, are coordinated by the circadian timing system. Molecular clocks are thought to rely on negative feedback loops in clock gene expression that engender oscillations in the accumulation of transcriptional regulatory proteins, such as the orphan receptor REV-ERBα. Circadian transcription factors then drive daily rhythms in the expression of clock-controlled output genes, for example genes encoding enzymes and regulators of cellular metabolism. To gain insight into clock output functions of REV-ERBα, we carried out genome-wide transcriptome profiling experiments with liver RNA from wild-type mice, Rev-erbα knock-out mice, or REV-ERBα overexpressing mice. On the basis of these genetic loss- and gain-of-function experiments, we concluded that REV-ERBα participates in the circadian modulation of sterol regulatory element-binding protein (SREBP) activity, and thereby in the daily expression of SREBP target genes involved in cholesterol and lipid metabolism. This control is exerted via the cyclic transcription of Insig2, encoding a trans-membrane protein that sequesters SREBP proteins to the endoplasmic reticulum membranes and thereby interferes with the proteolytic activation of SREBPs in Golgi membranes. REV-ERBα also participates in the cyclic expression of cholesterol-7α-hydroxylase (CYP7A1), the rate-limiting enzyme in converting cholesterol to bile acids. Our findings suggest that this control acts via the stimulation of LXR nuclear receptors by cyclically produced oxysterols. In conclusion, our study suggests that rhythmic cholesterol and bile acid metabolism is not just driven by alternating feeding–fasting cycles, but also by REV-ERBα, a component of the circadian clockwork circuitry.

The mammalian circadian timing system has a hierarchical architecture: a central pacemaker in the brain's suprachiasmatic nucleus (SCN) synchronizes subsidiary oscillators present in most peripheral cell types. In both SCN neurons and peripheral cells, circadian oscillators are thought to rely on two negative feedback loops. A major feedback loop involves the two cryptochromes CRY1 and CRY2 and the two period proteins PER1 and PER2, which serve as transcriptional repressors for their own genes. An accessory feedback loop couples the expression and activity of the transcriptional activators CLOCK and BMAL1 to the expression of cryptochrome and period proteins. The orphan nuclear receptor REV-ERBα is a key player in this accessory feedback loop, in that it periodically represses Bmal1 transcription. In liver, molecular clocks mediate the temporal gating of metabolic processes. Here we demonstrate that hepatocyte clocks participate in the control of cholesterol and bile acid homeostasis. According to this scenario, REV-ERBα shapes the circadian expression pattern of insulin-induced gene 2 (INSIG2), a resident protein of the endoplasmic reticulum that interferes with the proteolytic activation of sterol response element binding proteins (SREBPs). In turn SREBPs govern the rhythmic expression of enzymes with key functions in sterol and fatty acid synthesis. The circadian production of sterols (in particular oxysterols) may engender the cyclic activation of LXR nuclear receptors, which serve as critical activators of Cyp7a1 transcription. CYP7A1, also known as cholesterol 7α-hydroxylase, catalyzes the rate-limiting step in bile acid synthesis.

Circadian dysregulation disrupts bile acid homeostasis

Background

Bile acids are potentially toxic compounds and their levels of hepatic production, uptake and export are tightly regulated by many inputs, including circadian rhythm. We tested the impact of disrupting the peripheral circadian clock on integral steps of bile acid homeostasis.

Methodology/Principal Findings

Both restricted feeding, which phase shifts peripheral clocks, and genetic ablation in Per1^−/−/Per2^−/− (PERDKO) mice disrupted normal bile acid control and resulted in hepatic cholestasis. Restricted feeding caused a dramatic, transient elevation in hepatic bile acid levels that was associated with activation of the xenobiotic receptors CAR and PXR and elevated serum aspartate aminotransferase (AST), indicative of liver damage. In the PERDKO mice, serum bile acid levels were elevated and the circadian expression of key bile acid synthesis and transport genes, including Cyp7A1 and NTCP, was lost. This was associated with blunted expression of a primary clock output, the transcription factor DBP, which transactivates the promoters of both genes.

Conclusions/Significance

We conclude that disruption of the circadian clock results in dysregulation of bile acid homeostasis that mimics cholestatic disease.

How temperature affects the circadian clock ofNeurospora crassa

Light and temperature are major environmental cues that influence circadian clocks. The molecular effects of these zeitgebers on the circadian clock of Neurospora crassa have been studied intensively during the last decade. While signal transduction of light into the circadian clock is quite well characterized, we have only recently begun to understand the molecular mechanisms that underlie temperature sensing. Here we summarize briefly the current knowledge about the effects of temperature on the circadian clock of Neurospora crassa.

Melatonin in the multi‐oscillatory mammalian circadian world

In mammals, the complex interaction of neural, hormonal, and behavioral outputs from the suprachiasmatic nucleus (SCN) drives circadian expression of events, either directly or through coordination of the timing of peripheral oscillators. Melatonin, one of the endocrine output signals of the clock, provides the organism with circadian information and can be considered as an endogenous synchronizer, able to stabilize and reinforce circadian rhythms and to maintain their mutual phase-relationship at the different levels of the circadian network. Moreover, exogenous melatonin, through an action on the circadian clock, affects all levels of the circadian network. The molecular mechanisms underlying this chronobiotic effect have also been investigated in rats. REV-ERB alpha seems to be the initial molecular target.

Mini screening of kinase inhibitors affecting period-length of mammalian cellular circadian clock

In mammalian circadian rhythms, the transcriptional-translational feedback loop (TTFL) consisting of a set of clock genes is believed to elicit the circadian clock oscillation. The TTFL model explains that the accumulation and degradation of mPER and mCRY proteins control the period-length (tau) of the circadian clock. Although recent studies revealed that the Casein Kinase Iεδ (CKIεδ) regurates the phosphorylation of mPER proteins and the circadian period-length, other kinases are also likely to contribute the phosphorylation of mPER. Here, we performed small scale screening using 84 chemical compounds known as kinase inhibitors to identify candidates possibly affecting the circadian period-length in mammalian cells. Screening by this high-throughput real-time bioluminescence monitoring system revealed that the several chemical compounds apparently lengthened the cellular circadian clock oscillation. These compounds are known as inhibitors against kinases such as Casein Kinase II (CKII), PI3-kinase (PI3K) and c-Jun N-terminal Kinase (JNK) in addition to CKIεδ. Although these kinase inhibitors may have some non-specific effects on other factors, our mini screening identified new candidates contributing to period-length control in mammalian cells.

Integration of microRNA miR-122 in hepatic circadian gene expression

In liver, most metabolic pathways are under circadian control, and hundreds of protein-encoding genes are thus transcribed in a cyclic fashion. Here we show that rhythmic transcription extends to the locus specifying miR-122, a highly abundant, hepatocyte-specific microRNA. Genetic loss-of-function and gain-of-function experiments have identified the orphan nuclear receptor REV-ERBalpha as the major circadian regulator of mir-122 transcription. Although due to its long half-life mature miR-122 accumulates at nearly constant rates throughout the day, this miRNA is tightly associated with control mechanisms governing circadian gene expression. Thus, the knockdown of miR-122 expression via an antisense oligonucleotide (ASO) strategy resulted in the up- and down-regulation of hundreds of mRNAs, of which a disproportionately high fraction accumulates in a circadian fashion. miR-122 has previously been linked to the regulation of cholesterol and lipid metabolism. The transcripts associated with these pathways indeed show the strongest time point-specific changes upon miR-122 depletion. The identification of Pparbeta/delta and the peroxisome proliferator-activated receptor alpha (PPARalpha) coactivator Smarcd1/Baf60a as novel miR-122 targets suggests an involvement of the circadian metabolic regulators of the PPAR family in miR-122-mediated metabolic control.

Vasoactive intestinal polypeptide entrains circadian rhythms in astrocytes

Many mammalian cell types show daily rhythms in gene expression driven by a circadian pacemaker. For example, cultured astrocytes display circadian rhythms in Period1 and Period2 expression. It is not known, however, how or which intercellular factors synchronize and sustain rhythmicity in astrocytes. Because astrocytes are highly sensitive to vasoactive intestinal polypeptide (VIP), a neuropeptide released by neurons and important for the coordination of daily cycling, the authors hypothesized that VIP entrains circadian rhythms in astrocytes. They used astrocyte cultures derived from knock-in mice containing a bioluminescent reporter of PERIOD2 (PER2) protein, to assess the effects of VIP on the rhythmic properties of astrocytes. VIP induced a dose-dependent increase in the peak-to-trough amplitude of the ensemble rhythms of PER2 expression with maximal effects near 100 nM VIP and threshold values between 0.1 and 1 nM. VIP also induced dose- and phase-dependent shifts in PER2 rhythms and daily VIP administration entrained bioluminescence rhythms of astrocytes to a predicted phase angle. This is the first demonstration that a neuropeptide can entrain glial cells to a phase predicted by a phase-response curve. The authors conclude that VIP potently entrains astrocytes in vitro and is a candidate for coordinating daily rhythms among glia in the brain.

Retinoid-related orphan receptors (RORs): critical roles in development, immunity, circadian rhythm, and cellular metabolism

The last few years have witnessed a rapid increase in our knowledge of the retinoid-related orphan receptors RORα, -β, and -γ (NR1F1-3), their mechanism of action, physiological functions, and their potential role in several pathologies. The characterization of ROR-deficient mice and gene expression profiling in particular have provided great insights into the critical functions of RORs in the regulation of a variety of physiological processes. These studies revealed that RORα plays a critical role in the development of the cerebellum, that both RORα and RORβ are required for the maturation of photoreceptors in the retina, and that RORγ is essential for the development of several secondary lymphoid tissues, including lymph nodes. RORs have been further implicated in the regulation of various metabolic pathways, energy homeostasis, and thymopoiesis. Recent studies identified a critical role for RORγ in lineage specification of uncommitted CD4⁺ T helper cells into Th17 cells. In addition, RORs regulate the expression of several components of the circadian clock and may play a role in integrating the circadian clock and the rhythmic pattern of expression of downstream (metabolic) genes. Study of ROR target genes has provided insights into the mechanisms by which RORs control these processes. Moreover, several reports have presented evidence for a potential role of RORs in several pathologies, including osteoporosis, several autoimmune diseases, asthma, cancer, and obesity, and raised the possibility that RORs may serve as potential targets for chemotherapeutic intervention. This prospect was strengthened by recent evidence showing that RORs can function as ligand-dependent transcription factors.

Introduction of tau mutation into cultured Rat1-R12 cells by gene targeting, using recombinant adeno-associated virus vector

We aim to develop a cultured cell model, to serve as a system with which the altered circadian phenotypes produced by the clock gene variations could be studied in vitro. Tau mutation, which shortens the circadian period of hamsters and mice, was introduced into the CK1epsilon locus of cultured Rat1-R12 cells by gene targeting mediated by a recombinant adeno-associated virus (rAAV) vector. After transduction of Rat1-R12 cells with rAAV, about 0.14% of the drug-resistant cells underwent gene targeting at CK1epsilon locus. Of the three clones isolated, only one carried the targeted allele of tau mutation and two carried the targeted wild-type allele. The clone with the targeted tau mutant allele exhibited a significantly shorter circadian period compared to the clone with targeted wild-type allele. rAAV-mediated gene targeting in cultured somatic cells is a convenient and powerful tool for analyzing the phenotypic outcome of clock gene variations, and for elucidating the pathogenesis of the disorders associated with abnormal circadian rhythmicity.

Circadian clocks can take a few transcriptional knocks

A thought-provoking study in this issue of The EMBO Journal shows that the circadian clock in mouse fibroblasts is surprisingly insensitive to the inhibition of total cellular mRNA production. The authors go on to show intriguing parallels between compensation of period to changes in temperature and global transcription rate.

The "fourth dimension" of gene transcription

The three dimensions of space provide our relationship to position on the earth, but the fourth dimension of time has an equally profound influence on our lives. Everything from light and sound to weather and biology operate on the principle of measurable temporal periodicity. Consequently, a wide variety of time clocks affect all aspects of our existence. The annual (and biannual) cycles of activity, metabolism, and mating, the monthly physiological clocks of women and men, and the 24-h diurnal rhythms of humans are prime examples. Should it be surprising to us that the fourth dimension also impinges upon gene expression and that the genome itself is regulated by the fastest running of all biological clocks? Recent evidence substantiates the existence of such a ubiquitin-dependent transcriptional clock that is based upon the activation and destruction of transcriptional coactivators.

The 2008 Pittendrigh/Aschoff Lecture: Peripheral Phase Coordination in the Mammalian Circadian Timing System

The mammalian timing system is composed of a bodywide web of cell-autonomous and self-sustained oscillators. A master clock in the SCN synchronizes peripheral clocks through yet poorly understood molecular signaling pathways. In this lecture I shall present some of the experimental approaches we are employing to elucidate signaling routes through which the SCN may phase entrain peripheral clocks. These attempts unveiled several candidate pathways worth pursuing in future studies, including signaling through nuclear receptors, cytoskeleton components, Ca2+, fibroblast growth factors, ubiquitin ligases, Sirtuin 1 (a redox-sensing histone deacetylase), RNA binding proteins, and body temperature.

Nuclear receptor corepressor and histone deacetylase 3 govern circadian metabolic physiology

Rhythmic changes in histone acetylation at circadian clock genes suggest that temporal modulation of gene expression is regulated by chromatin modifications. Furthermore, recent studies demonstrate a critical relationship between circadian and metabolic physiology. The nuclear receptor corepressor 1 (Ncor1) functions as an activating subunit for the chromatin modifying enzyme histone deacetylase 3 (Hdac3). Lack of Ncor1 is incompatible with life, and hence it is unknown whether Ncor1, and particularly its regulation of Hdac3, is critical for adult mammalian physiology. Here we show that specific, genetic disruption of the Ncor1-Hdac3 interaction in mice causes aberrant regulation of clock genes and results in abnormal circadian behaviour. These mice are also leaner and more insulin-sensitive owing to increased energy expenditure. Unexpectedly, loss of a functional Ncor1-Hdac3 complex in vivo does not lead to sustained increases in known catabolic genes, but instead significantly alters the oscillatory patterns of several metabolic genes, demonstrating that circadian regulation of metabolism is critical for normal energy balance. These findings indicate that activation of Hdac3 by Ncor1 is a nodal point in the epigenetic regulation of circadian and metabolic physiology.

Remodeling the clock: coactivators and signal transduction in the circadian clockworks

Most organisms on earth such as cyanobacteria, fungi, plants, insects, animals, and humans synchronize their physiological and behavioral activities with the environmental cycles of day and night. Significant progress has been made in unraveling the genetic components that constitute a molecular circadian clock, which facilitates the temporal control of physiology and behavior. Clock genes assemble interlocked transcriptional/translational feedback loops that underlie the circadian oscillations. Recent investigations revealed that posttranslational regulation of clock proteins is crucial for functioning of the molecular oscillator and for precise temporal control of circadian transcription. This review provides an overview of the homologous clockworks in Drosophila and mammals, with a special focus on recent insights in the posttranslational regulation of clock proteins as well as the role of coactivators, repressors, and signal transduction for circadian controlled genome-wide transcription. The emerging mechanisms of clock gene regulation provide an understanding of the temporal control of transcription in general and the circadian orchestration of physiology and behavior in particular.

Establishment of human cell lines showing circadian rhythms of bioluminescence

We have established human retinal pigment epithelial cell lines stably expressing the luciferase gene, driven by the human Bmal1 promoter, to obtain human-derived cells that show circadian rhythms of bioluminescence after dexamethasone treatment. The average circadian period of bioluminescence for the obtained clones was 24.07+/-0.48 h. Lithium (10 mM) in the medium significantly lengthened the circadian period of bioluminescence, which is consistent with previous reports, while 2 mM or 5 mM lithium had no effect. This is the first report on the establishment of human-derived cell lines that proliferate infinitely and show circadian rhythms of bioluminescence, and also the first to investigate the effects of low-dose lithium on the circadian rhythms of human-derived cells in vitro. The established cells will be useful for various in vitro studies of human circadian rhythms and for the development of new therapies for human disorders related to circadian rhythm disturbances.

Phospholipase C beta 4 in mouse hepatocytes: rhythmic expression and cellular distribution

BACKGROUND

Circadian regulated physiological processes have been well documented in the mammalian liver. Phospholipases are important mediators of both cytoplasmic and nuclear signaling mechanisms in hepatocytes, and despite a potentially critical role for these enzymes in regulating the temporal aspect of hepatic physiology, their involvement in the circadian liver clock has not been the subject of much investigation. The phospholipase C beta4 (PLCbeta4) enzyme is of particular interest as it has been linked to circadian clock function. In general, there is no knowledge of the role of the PLCbeta4 isozyme in mammalian hepatocytes as this is the first report of its expression in the mammalian liver.

RESULTS

We found that in the liver of mice housed on a light:dark cycle, PLCbeta4 protein underwent a significant circadian rhythm with a peak occurring during the early night. In constant darkness, the protein rhythm was more robust and peaked around dusk. We also observed a significant oscillation in plcbeta4 gene expression in the livers of mice housed in both photoperiodic and constant dark conditions. The cellular distribution of the protein in hepatocytes varied over the course of the circadian day with PLCbeta4 primarily cytoplasmic around dusk and nuclear at dawn.

CONCLUSION

Our results indicate that PLCbeta4 gene and protein expression is regulated by a circadian clock in the mouse liver and is not dependent on the external photoperiod. A light-independent daily translocation of PLCbeta4 implies that it may play a key role in nuclear signaling in hepatocytes and serve as a daily temporal cue for physiological processes in the liver.

Circadian rhythm disturbances in depression

OBJECTIVE

The aim of this article is to review progress in understanding the mechanisms that underlie circadian and sleep rhythms, and their role in the pathogenesis and treatment of depression.

METHODS

Literature was selected principally by Medline searches, and additional reports were identified based on ongoing research activities in the authors' laboratory.

RESULTS

Many physiological processes show circadian rhythms of activity. Sleep and waking are the most obvious circadian rhythms in mammals. There is considerable evidence that circadian and sleep disturbances are important in the pathophysiology of mood disorders. Depressed patients often show altered circadian rhythms, sleep disturbances, and diurnal mood variation. Chronotherapies, including bright light exposure, sleep deprivation, and social rhythm therapies, may be useful adjuncts in non-seasonal and seasonal depression. Antidepressant drugs have marked effects on circadian processes and sleep.

CONCLUSIONS

Recent progress in understanding chronobiological and sleep regulation mechanisms may provide novel insights and avenues into the development of new pharmacological and behavioral treatment strategies for mood disorders.

Circadian photoreception: ageing and the eye's important role in systemic health

AIM

To analyse how age-related losses in crystalline lens transmittance and pupillary area affect circadian photoreception and compare the circadian performance of phakic and pseudophakic individuals of the same age.

METHODS

The spectral sensitivity of circadian photoreception peaks in the blue part of the spectrum at approximately 460 nm. Photosensitive retinal ganglion cells send unconscious information about environmental illumination to non-visual brain centres including the human body's master biological clock in the suprachiasmatic nuclei. This information permits human physiology to be optimised and aligned with geophysical day-night cycles using neural and hormonal messengers including melatonin. Age-related transmittance spectra of crystalline lenses and photopic pupil diameter are used with the spectral sensitivity of melatonin suppression and the transmittance spectra of intraocular lenses (IOLs) to analyse how ageing and IOL chromophores affect circadian photoreception.

RESULTS

Ageing increases crystalline lens light absorption and decreases pupil area resulting in progressive loss of circadian photoreception. A 10-year-old child has circadian photoreception 10-fold greater than a 95-year-old phakic adult. A 45-year-old adult retains only half the circadian photoreception of early youth. Pseudophakia improves circadian photoreception at all ages, particularly with UV-only blocking IOLs which transmit blue wavelengths optimal for non-visual photoreception.

CONCLUSIONS

Non-visual retinal ganglion photoreceptor responses to bright, properly timed light exposures help assure effective circadian photoentrainment and optimal diurnal physiological processes. Circadian photoreception can persist in visually blind individuals if retinal ganglion cell photoreceptors and their suprachiasmatic connections are intact. Retinal illumination decreases with ageing due to pupillary miosis and reduced crystalline lens light transmission especially of short wavelengths. Inadequate environmental light and/or ganglion photoreception can cause circadian disruption, increasing the risk of insomnia, depression, numerous systemic disorders and possibly early mortality. Artificial lighting is dimmer and less blue-weighted than natural daylight, contributing to age-related losses in unconscious circadian photoreception. Optimal intraocular lens design should consider the spectral requirements of both conscious and unconscious retinal photoreception.

Biological Rhythms Workshop IA: molecular basis of rhythms generation

Current circadian models are based on genetic, biochemical, and structural data that, when combined, provide a comprehensive picture of the molecular basis for rhythms generation. These models describe three basic elements-input pathways, oscillator, and output pathways-to which each molecular component is assigned. The lines between these elements are often blurred because some proteins function in more than one element of the circadian system. The end result of these molecular oscillations is the same in each system (near 24-hour timing), yet the proteins involved, the interactions among those proteins, and the regulatory feedback loops differ. Here, the currentmodels for the molecular basis for rhythms generation are described for the prokaryotic cyanobacterium Synechococcus elongatus as well as the eukaryotic systems Neurospora crassa, Drosophila melanogaster, Arabidopsis thaliana, and mammals (particularly rodents).

The nuclear receptors Rev-erbs and RORs integrate circadian rhythms and metabolism

Circadian rhythms are normal variations in physiological processes that occur over the period of a day. These rhythms are essential for the organism since they allow anticipatory metabolic regulations to prepare for the up-coming feeding or rest period. Disturbances of the biological clock predispose to metabolic disorders such as dyslipidaemia, insulin resistance and obesity. Moreover, certain pathological events, such as cardiovascular accidents (myocardial infarction, stroke) occur more frequently at specific times of the day. The nuclear receptors Rev-erbalpha and RORalpha are clock components involved in the regulation of the core clock circuitry. They are also important regulators of lipid and lipoprotein metabolism, adipogenesis and vascular inflammation. Moreover, they cross-talk with several other nuclear receptors controlling energy homeostasis. Therefore, Rev-erbalpha and RORalpha may play a central role in the coordination of metabolic processes and circadian outputs.

Evidence for an overlapping role of CLOCK and NPAS2 transcription factors in liver circadian oscillators

The mechanisms underlying the circadian control of gene expression in peripheral tissues and influencing many biological pathways are poorly defined. Factor VII (FVII), the protease triggering blood coagulation, represents a valuable model to address this issue in liver since its plasma levels oscillate in a circadian manner and its promoter contains E-boxes, which are putative DNA-binding sites for CLOCK-BMAL1 and NPAS2-BMAL1 heterodimers and hallmarks of circadian regulation. The peaks of FVII mRNA levels in livers of wild-type mice preceded those in plasma, indicating a transcriptional regulation, and were abolished in Clock(-/-); Npas2(-/-) mice, thus demonstrating a role for CLOCK and NPAS2 circadian transcription factors. The investigation of Npas2(-/-) and Clock(Delta19/Delta19) mice, which express functionally defective heterodimers, revealed robust rhythms of FVII expression in both animal models, suggesting a redundant role for NPAS2 and CLOCK. The molecular bases of these observations were established through reporter gene assays. FVII transactivation activities of the NPAS2-BMAL1 and CLOCK-BMAL1 heterodimers were (i) comparable (a fourfold increase), (ii) dampened by the negative circadian regulators PER2 and CRY1, and (iii) abolished upon E-box mutagenesis. Our data provide the first evidence in peripheral oscillators for an overlapping role of CLOCK and NPAS2 in the regulation of circadianly controlled genes.

Detection of a circadian enhancer in the mDbp promoter using prokaryotic transposon vector-based strategy

In mammals, the expression of 5-10% of genes occurs with circadian fluctuation in various organs and tissues. This cyclic transcription is thought to be directly or indirectly regulated through circadian transcriptional/translational feedback loops consisting of a set of clock genes. Among the clock genes in mammals, expression of the Dbp mRNA robustly oscillates both in vivo and in culture cells. Here, we present circadian enhancer detection strategy using prokaryotic transposon system. The mDbp promoter drives reporter gene expression in robust circadian cycles in rat-1 fibroblasts. To identify the circadian enhancer generating this robust rhythm, we developed a prokaryotic transposon-based enhancer detecting vector for in vitro transposition. Using this system, we identified a strong circadian enhancer region containing the CATGTG sequence in the 5' flanking region of the mDbp gene; this enhancer region is critical for the ability of the mDbp promoter to drive robust oscillation in living cells. This enhancer is classified as a CANNTG type non-canonical E-box. These findings strongly suggest that CANNTG-type non-canonical E-boxes may contribute, at least in part, to the regulation of robust circadian gene expression. Furthermore, these data may help explain the wider effects of the CLOCK/BMAL1 complex in control of clock output genes.

Molecular insights into human daily behavior

Human beings exhibit wide variation in their timing of daily behavior. We and others have suggested previously that such differences might arise because of alterations in the period length of the endogenous human circadian oscillator. Using dermal fibroblast cells from skin biopsies of 28 subjects of early and late chronotype (11 "larks" and 17 "owls"), we have studied the circadian period lengths of these two groups, as well as their ability to phase-shift and entrain to environmental and chemical signals. We find not only period length differences between the two classes, but also significant changes in the amplitude and phase-shifting properties of the circadian oscillator among individuals with identical "normal" period lengths. Mathematical modeling shows that these alterations could also account for the extreme behavioral phenotypes of these subjects. We conclude that human chronotype may be influenced not only by the period length of the circadian oscillator, but also by cellular components that affect its amplitude and phase. In many instances, these changes can be studied at the molecular level in primary dermal cells.

Expression levels of estrogen receptor beta are modulated by components of the molecular clock

Circadian regulation of gene expression plays a major role in health and disease. The precise role of the circadian system remains to be clarified, but it is known that circadian proteins generate physiological rhythms in organisms by regulating clock-controlled target genes. The estrogen receptor beta (ERbeta) is, together with ERalpha, a member of the nuclear receptor superfamily and a key mediator of estrogen action. Interestingly, recent studies show that disturbed circadian rhythmicity in humans can increase the risk of reproductive malfunctions, suggesting a link between the circadian system and ER-mediated transcription pathways. Here, we identify a novel level of regulation of estrogen signaling where ERbeta, but not ERalpha, is controlled by circadian clock proteins. We show that ERbeta mRNA levels fluctuate in different peripheral tissues following a robust circadian pattern, with a peak at the light-dark transition, which is maintained under free-running conditions. Interestingly, this oscillation is abolished in clock-deficient BMAL1 knockout mice. Circadian control of ERbeta expression is exerted through a conserved E-box element in the ERbeta promoter region that recruits circadian regulatory factors. Furthermore, using small interfering RNA-mediated knockdown assays, we show that the expression levels of the circadian regulatory factors directly influence estrogen signaling by regulating the intracellular levels of endogenous ERbeta.