Properties, entrainment, and physiological functions of mammalian peripheral oscillators

The effects of combining serotonin reuptake inhibition and 5-HT7 receptor blockade on circadian rhythm regulation in rodents

Disruption of circadian rhythms may lead to mood disorders. The present study investigated the potential therapeutic utility of combining a 5-HT7 antagonist with a selective serotonin (5-HT) reuptake inhibitor (SSRI), the standard of care in depression, on circadian rhythm regulation. In tissue explants of the suprachiasmatic nucleus (SCN) from PER2::LUC mice genetically modified to report changes in the expression of a key clock protein, the period length of PER2 bioluminescence was shortened in the presence of AS19, a 5-HT7 partial agonist. This reduction was blocked by SB269970, a selective 5-HT7 antagonist. The SSRI, escitalopram, had no effect alone on period length, but a combination with SB269970, yielded significant increases. Dosed in vivo, escitalopram had little impact on the occurrence of activity onsets in rats given access to running wheels, whether the drug was given acutely or sub-chronically. However, preceding the escitalopram treatment with a single acute dose of SB269970 produced robust phase delays, in keeping with the in vitro explant data. Taken together, these findings suggest that the combination of an SSRI and a 5-HT7 receptor antagonist has a greater impact on circadian rhythms than that observed with either agent alone, and that such a multimodal approach may be of therapeutic value in treating patients with poor clock function.

Dipper and non-dipper blood pressure 24-hour patterns: circadian rhythm-dependent physiologic and pathophysiologic mechanisms

Neuroendocrine mechanisms are major determinants of the normal 24-h blood pressure (BP) pattern. At the central level, integration of the major driving factors of this temporal variability is mediated by circadian rhythms of monoaminergic systems in conjunction with those of the hypothalamic-pituitary-adrenal, hypothalamic-pituitary-thyroid, opioid, renin-angiotensin-aldosterone, plus endothelial systems and specific vasoactive peptides. Humoral secretions are typically episodic, coupled either to sleep and/or the circadian endogenous (suprachiasmatic nucleus) central pacemaker clock, but exhibiting also weekly, monthly, seasonal, and annual periodicities. Sleep induction and arousal are influenced also by many hormones and chemical substances that exhibit 24-h variation, e.g., arginine vasopressin, vasoactive intestinal peptide, melatonin, somatotropin, insulin, steroids, serotonin, corticotropin-releasing factor, adrenocorticotropic hormone, thyrotropin-releasing hormone, endogenous opioids, and prostaglandin E2, all with established effects on the cardiovascular system. As a consequence, physical, mental, and pathologic stimuli that activate or inhibit neuroendocrine effectors of biological rhythmicity may also interfere with, or modify, the temporal BP structure. Moreover, immediate adjustment to exogenous components/environment demands by BP rhythms is modulated by the circadian-time-dependent responsiveness of biological oscillators and their neuroendocrine effectors. This knowledge contributes to a better understanding of the pathophysiology of abnormalities of the 24-h BP pattern and level and their correction through circadian rhythm-based chronotherapeutic strategies.

Disruption of gene expression rhythms in mice lacking secretory vesicle proteins IA-2 and IA-2β

Insulinoma-associated protein (IA)-2 and IA-2β are transmembrane proteins involved in neurotransmitter secretion. Mice with targeted disruption of both IA-2 and IA-2β (double-knockout, or DKO mice) have numerous endocrine and physiological disruptions, including disruption of circadian and diurnal rhythms. In the present study, we have assessed the impact of disruption of IA-2 and IA-2β on molecular rhythms in the brain and peripheral oscillators. We used in situ hybridization to assess molecular rhythms in the hypothalamic suprachiasmatic nuclei (SCN) of wild-type (WT) and DKO mice. The results indicate significant disruption of molecular rhythmicity in the SCN, which serves as the central pacemaker regulating circadian behavior. We also used quantitative PCR to assess gene expression rhythms in peripheral tissues of DKO, single-knockout, and WT mice. The results indicate significant attenuation of gene expression rhythms in several peripheral tissues of DKO mice but not in either single knockout. To distinguish whether this reduction in rhythmicity reflects defective oscillatory function in peripheral tissues or lack of entrainment of peripheral tissues, animals were injected with dexamethasone daily for 15 days, and then molecular rhythms were assessed throughout the day after discontinuation of injections. Dexamethasone injections improved gene expression rhythms in liver and heart of DKO mice. These results are consistent with the hypothesis that peripheral tissues of DKO mice have a functioning circadian clockwork, but rhythmicity is greatly reduced in the absence of robust, rhythmic physiological signals originating from the SCN. Thus, IA-2 and IA-2β play an important role in the regulation of circadian rhythms, likely through their participation in neurochemical communication among SCN neurons.

Cold-inducible RNA-binding protein modulates circadian gene expression posttranscriptionally

In mammalian tissues, circadian gene expression can be driven by local oscillators or systemic signals controlled by the master pacemaker in the suprachiasmatic nucleus. We show that simulated body temperature cycles, but not peripheral oscillators, controlled the rhythmic expression of cold-inducible RNA-binding protein (CIRP) in cultured fibroblasts. In turn, loss-of-function experiments indicated that CIRP was required for high-amplitude circadian gene expression. The transcriptome-wide identification of CIRP-bound RNAs by a biotin-streptavidin-based cross-linking and immunoprecipitation (CLIP) procedure revealed several transcripts encoding circadian oscillator proteins, including CLOCK. Moreover, CLOCK accumulation was strongly reduced in CIRP-depleted fibroblasts. Because ectopic expression of CLOCK improved circadian gene expression in these cells, we surmise that CIRP confers robustness to circadian oscillators through regulation of CLOCK expression.

Brain-specific rescue of Clock reveals system-driven transcriptional rhythms in peripheral tissue

The circadian regulatory network is organized in a hierarchical fashion, with a central oscillator in the suprachiasmatic nuclei (SCN) orchestrating circadian oscillations in peripheral tissues. The nature of the relationship between central and peripheral oscillators, however, is poorly understood. We used the tetOFF expression system to specifically restore Clock function in the brains of Clock(Δ19) mice, which have compromised circadian clocks. Rescued mice showed normal locomotor rhythms in constant darkness, with activity period lengths approximating wildtype controls. We used microarray analysis to assess whether brain-specific rescue of circadian rhythmicity was sufficient to restore circadian transcriptional output in the liver. Compared to Clock mutants, Clock-rescue mice showed significantly larger numbers of cycling transcripts with appropriate phase and period lengths, including many components of the core circadian oscillator. This indicates that the SCN oscillator overcomes local circadian defects and signals directly to the molecular clock. Interestingly, the vast majority of core clock genes in liver were responsive to Clock expression in the SCN, suggesting that core clock genes in peripheral tissues are intrinsically sensitive to SCN cues. Nevertheless, most circadian output in the liver was absent or severely low-amplitude in Clock-rescue animals, demonstrating that the majority of peripheral transcriptional rhythms depend on a fully functional local circadian oscillator. We identified several new system-driven rhythmic genes in the liver, including Alas1 and Mfsd2. Finally, we show that 12-hour transcriptional rhythms (i.e., circadian "harmonics") are disrupted by Clock loss-of-function. Brain-specific rescue of Clock converted 12-hour rhythms into 24-hour rhythms, suggesting that signaling via the central circadian oscillator is required to generate one of the two daily peaks of expression. Based on these data, we conclude that 12-hour rhythms are driven by interactions between central and peripheral circadian oscillators.

Assessment of circadian rhythms in humans: comparison of real-time fibroblast reporter imaging with plasma melatonin

We compared the period of the rhythm of plasma melatonin, driven by the hypothalamic circadian pacemaker, to in vitro periodicity in cultured peripheral fibroblasts to assess the effects on these rhythms of a polymorphism of PER3 (rs57875989), which is associated with sleep timing. In vitro circadian period was determined using luminometry of cultured fibroblasts, in which the expression of firefly luciferase was driven by the promoter of the circadian gene Arntl (Bmal1). The period of the melatonin rhythm was assessed in a 9-d forced desynchrony protocol, minimizing confounding effects of sleep-wake and light-dark cycles on circadian rhythmicity. In vitro periods (32 participants, 24.61±0.33 h, mean±SD) were longer than in vivo periods (31 participants, 24.16±0.17 h; P<0.0001) but did not differ between PER3 genotypes (P>0.4). Analyses of replicate in vitro assessments demonstrated that circadian period was reproducible within individuals (intraclass correlation=0.62), but in vivo and in vitro period assessments did not correlate (P>0.9). In accordance with circadian entrainment theory, in vivo period correlated with the timing of melatonin (P<0.05) at baseline and with diurnal preference (P<0.05). Individual circadian rhythms can be reliably assessed in fibroblasts but may not correlate with physiological rhythms driven by the central circadian pacemaker.

Epigenetic Control of Circadian Clock Operation during Development

The molecular players of circadian clock oscillation have been identified and extensively characterized. The epigenetic mechanisms behind the circadian gene expression control has also been recently studied, although there are still details to be illucidated. In this review, we briefly summarize the current understanding of the mammalian clock. We also provide evidence for the lack of circadian oscillation in particular cell types. As the circadian clock has intimate interaction with the various cellular functions in different type of cells, it must have plasticity and specicity in its operation within different epigenetic environments. The lack of circadian oscillation in certain cells provide an unique opportunity to study the required epigenetic environment in the cell that permit circadian oscillation and to idenfify key influencing factors for proper clock function. How epigenetic mechansims, including DNA methylaiton and chromatin modifications, participate in control of clock oscillation still awaits future studies at the genomic scale.

Entrainment of peripheral clock genes by cortisol

Circadian rhythmicity in mammals is primarily driven by the suprachiasmatic nucleus (SCN), often called the central pacemaker, which converts the photic information of light and dark cycles into neuronal and hormonal signals in the periphery of the body. Cells of peripheral tissues respond to these centrally mediated cues by adjusting their molecular function to optimize organism performance. Numerous systemic cues orchestrate peripheral rhythmicity, such as feeding, body temperature, the autonomic nervous system, and hormones. We propose a semimechanistic model for the entrainment of peripheral clock genes by cortisol as a representative entrainer of peripheral cells. This model demonstrates the importance of entrainer's characteristics in terms of the synchronization and entrainment of peripheral clock genes, and predicts the loss of intercellular synchrony when cortisol moves out of its homeostatic amplitude and frequency range, as has been observed clinically in chronic stress and cancer. The model also predicts a dynamic regime of entrainment, when cortisol has a slightly decreased amplitude rhythm, where individual clock genes remain relatively synchronized among themselves but are phase shifted in relation to the entrainer. The model illustrates how the loss of communication between the SCN and peripheral tissues could result in desynchronization of peripheral clocks.

Daytime restricted feeding modifies 24 h rhythmicity and subcellular distribution of liver glucocorticoid receptor and the urea cycle in rat liver

The timing system in mammals is formed by a set of peripheral biological clocks coordinated by a light-entrainable pacemaker located in the suprachiasmatic nucleus. Daytime restricted feeding (DRF) modifies the circadian control and uncouples the light-dependent physiological rhythmicity, food access becoming the principal external time cue. In these conditions, an alternative biological clock is expressed, the food-entrainable oscillator (FEO). Glucocorticoid hormones are an important part of the humoral mechanisms in the daily synchronisation of the metabolic response of peripheral oscillators by the timing system. A peak of circulating corticosterone has been reported before food access in DRF protocols. In the present study we explored in the liver the 24 h variations of: (1) the subcellular distribution of glucocorticoid receptor (GCR), (2) the activities of the corticosterone-forming and NADPH-generating enzymes (11β-hydroxysteroid dehydrogenase type 1 (11β-HSD-1) and hexose-6-phosphate dehydrogenase (H6PDH)), and, (3) parameters related with the urea cycle (circulating urea and activities of carbamoyl phosphate synthetase and ornithine transcarbamylase) elicited by DRF. The results showed that DRF promoted an increase of more than two times of the hepatic GCR, but exclusively in the cytosolic compartment, since the GCR in the nuclear fraction showed a reduction. No changes were observed in the activities of 11β-HSD-1 and H6PDH, but the rhythmicity of all of the urea cycle-related parameters was modified. It is concluded that liver glucocorticoid signalling and the urea cycle are responsive to feeding-restricted schedules and could be part of the FEO.

The long-term abnormalities in circadian expression of Period 1 and Period 2 genes in response to stress is normalized by agomelatine administered immediately after exposure

In mammals, the circadian and stress systems are involved in adaptation to predictable and unpredictable stimuli, respectively. A series of experiments examined the relationship between stress-induced posttraumatic stress (PTSD)-like behavioral response patterns in rats and brain levels of genes related to circadian rhythms. The effects of agomelatine, administered immediately after exposure, on stress-related behavior and on local expression of Per1 and Per2 were assessed. Animals were exposed to predator scent stress. The outcome measures included behavior in an elevated plus-maze (EPM) and acoustic startle response (ASR) 7days after the exposure. Pre-set cut-off behavioral criteria classified exposed animals according to behavioral responses in EPM and ASR paradigms as those with 'extreme behavioral response' (EBR), 'minimal behavioral response (MBR),' or 'partial behavioral response' (PBR). Per1 and Per2 expression in hippocampal subregions, frontal cortex and suprachiasmatic nucleus (SCN) 8days after exposure were evaluated using immunohistochemical and RT-PCR techniques at zeitgeber-times 19 and 13. The effects of agomelatine, on behavioral tests were evaluated on Day 8. Local brain expression of Per1 and Per2 mRNA was subsequently assessed. Data were analyzed in relation to individual behavior patterns. Animals with extreme behavioral response (EBR) displayed a distinct pattern of Per1 and Per2 expression in the SCN, which was the opposite of that observed in the control and MBR animals. In the DG, no variation in Per2 expression was observed in the EBR and PBR animals. Immediate post-exposure treatment with agomelatine significantly reduced percentage of extreme-responders and normalized the expression of Per1 and Per2 as compared to controls. Stress-induced alterations in Per genes in the EBR animals may represent an imbalance between normally precisely orchestrated physiological and behavioral processes and psychopathological processes. These findings indicate that these circadian-related genes play a role in the neurobiological response to predator scent stress and provide supportive evidence that the use of agomelatine immediately after traumatic experience may be protective against the subsequent development of PTSD.

Simulated body temperature rhythms reveal the phase-shifting behavior and plasticity of mammalian circadian oscillators

The circadian pacemaker in the suprachiasmatic nuclei (SCN) of the hypothalamus maintains phase coherence in peripheral cells through metabolic, neuronal, and humoral signaling pathways. Here, we investigated the role of daily body temperature fluctuations as possible systemic cues in the resetting of peripheral oscillators. Using precise temperature devices in conjunction with real-time monitoring of the bioluminescence produced by circadian luciferase reporter genes, we showed that simulated body temperature cycles of mice and even humans, with daily temperature differences of only 3°C and 1°C, respectively, could gradually synchronize circadian gene expression in cultured fibroblasts. The time required for establishing the new steady-state phase depended on the reporter gene, but after a few days, the expression of each gene oscillated with a precise phase relative to that of the temperature cycles. Smooth temperature oscillations with a very small amplitude could synchronize fibroblast clocks over a wide temperature range, and such temperature rhythms were also capable of entraining gene expression cycles to periods significantly longer or shorter than 24 h. As revealed by genetic loss-of-function experiments, heat-shock factor 1 (HSF1), but not HSF2, was required for the efficient synchronization of fibroblast oscillators to simulated body temperature cycles.

Peripheral circadian rhythms and their regulatory mechanism in insects and some other arthropods: a review

Many physiological functions of insects show a rhythmic change to adapt to daily environmental cycles. These rhythms are controlled by a multi-clock system. A principal clock located in the brain usually organizes the overall behavioral rhythms, so that it is called the "central clock". However, the rhythms observed in a variety of peripheral tissues are often driven by clocks that reside in those tissues. Such autonomous rhythms can be found in sensory organs, digestive and reproductive systems. Using Drosophila melanogaster as a model organism, researchers have revealed that the peripheral clocks are self-sustained oscillators with a molecular machinery slightly different from that of the central clock. However, individual clocks normally run in harmony with each other to keep a coordinated temporal structure within an animal. How can this be achieved? What is the molecular mechanism underlying the oscillation? Also how are the peripheral clocks entrained by light-dark cycles? There are still many questions remaining in this research field. In the last several years, molecular techniques have become available in non-model insects so that the molecular oscillatory mechanisms are comparatively investigated among different insects, which give us more hints to understand the essential regulatory mechanism of the multi-oscillatory system across insects and other arthropods. Here we review current knowledge on arthropod's peripheral clocks and discuss their physiological roles and molecular mechanisms.

[Aging of the circadian system]

Ageing affects all organic structures and processes, including the circadian system and its principal sign, the biological rhythms. The circadian system temporarily organizes the living organisms. It is made up of structures receiving information from the external environment (that synchronize the circadian clock), the central circadian pacemaker and the peripheral clocks that depends on it, and several outputs that are the overt rhythms. Ageing produces losses in function of all these three components: receptors (the eye in its capacity to transmit the more active light information to the circadian system), the central pacemaker (due to alterations in neuronal function) and the outputs. This leads to the alteration of overt rhythms, with losses in the phase relationship between them, a reduction in amplitude, an increase in fragmentation and an advancement of its phase.

Daily rhythms in olfactory discrimination depend on clock genes but not the suprachiasmatic nucleus

The suprachiasmatic nucleus (SCN) regulates a wide range of daily behaviors and has been described as the master circadian pacemaker. The role of daily rhythmicity in other tissues, however, is unknown. We hypothesized that circadian changes in olfactory discrimination depend on a genetic circadian oscillator outside the SCN. We developed an automated assay to monitor olfactory discrimination in individual mice throughout the day. We found olfactory sensitivity increased approximately 6-fold from a minimum during the day to a peak in the early night. This circadian rhythm was maintained in SCN-lesioned mice and mice deficient for the Npas2 gene but was lost in mice lacking Bmal1 or both Per1 and Per2 genes. We conclude that daily rhythms in olfactory sensitivity depend on the expression of canonical clock genes. Olfaction is, thus, the first circadian behavior that is not based on locomotor activity and does not require the SCN.

The adrenal peripheral clock: glucocorticoid and the circadian timing system

The mammalian circadian timing system is organized in a hierarchy, with the master clock residing in the suprachiasmatic nucleus (SCN) of the hypothalamus and subsidiary peripheral clocks in other brain regions as well as peripheral tissues. Since the local oscillators in most cells contain a similar molecular makeup to that in the central pacemaker, determining the role of the peripheral clocks in the regulation of rhythmic physiology and behavior is an important issue. Glucocorticoids (GCs) are a class of multi-functional adrenal steroid hormones, which exhibit a robust circadian rhythm, with a peak linked with the onset of the daily activity phase. It has long been believed that the production and secretion of GC is primarily governed through the hypothalamus-pituitary-adrenal (HPA) neuroendocrine axis in mammals. Growing evidence, however, strongly supports the notion that the periodicity of GC involves the integrated activity of multiple regulatory mechanisms related to circadian timing system along with the classical HPA neuroendocrine regulation. The adrenal-intrinsic oscillator as well as the central pacemaker plays a pivotal role in its rhythmicity. GC influences numerous biological processes, such as metabolic, cardiovascular, immune and even higher brain functions, and also acts as a resetting signal for the ubiquitous peripheral clocks, suggesting its importance in harmonizing circadian physiology and behavior. In this review, we will therefore focus on the recent advances in our understanding of the circadian regulation of adrenal GC and its functional relevance.

Crosstalk between environmental light and internal time in humans

Daily exposure to environmental light is the most important zeitgeber in humans, and all studied characteristics of light pattern (timing, intensity, rate of change, duration, and spectrum) influence the circadian system. However, and due to lack of current studies on environmental light exposure and its influence on the circadian system, the aim of this work is to determine the characteristics of a naturalistic regimen of light exposure and its relationship with the functioning of the human circadian system. Eighty-eight undergraduate students (18-23 yrs) were recruited in Murcia, Spain (latitude 38°01'N) to record wrist temperature (WT), light exposure, and sleep for 1 wk under free-living conditions. Light-exposure timing, rate of change, regularity, intensity, and contrast were calculated, and their effects on the sleep pattern and WT rhythm were then analyzed. In general, higher values for interdaily stability, relative amplitude, mean morning light, and light quality index (LQI) correlated with higher interdaily stability and relative amplitude, and phase advance in sleep plus greater stability in WT and phase advance of the WT circadian rhythm. On the other hand, a higher fragmentation of the light-exposure rhythm was associated with more fragmented sleep. Naturalistic studies using 24-h ambulatory light monitoring provide essential information about the main circadian system input, necessary for maintaining healthy circadian tuning. Correcting light-exposure patterns accordingly may help prevent or even reverse health problems associated with circadian disruption.

Sucrose modifies c-fos mRNA expression in the brain of rats maintained on feeding schedules

Food intake is regulated according to circadian activity, metabolic needs and the hedonic value of food. Rodents placed on a fixed feeding schedule show behavioral and physiological anticipation of mealtime referred to as food-anticipatory activity (FAA). FAA is driven by the food-entrainable oscillator (FEO), whose anatomical substrate is not yet known. Recent data have shown that restricted feeding schedules for regular chow and daily limited access to palatable food in free-feeding rats activate distinct brain regions during FAA. The combination of a deprivation regimen and scheduled access to palatable food may give rise to a more global anticipatory mechanism because the temporal cycles of energy balance would be strongly modulated by the incentive properties of palatable food; however, the neuronal response to this combined treatment is not yet known. The present study investigated how adding palatable sucrose to feeding schedules affects the pattern of brain c-fos mRNA expression during FAA (0-3 h) and 1 h following feeding. The rats maintained on scheduled chow access increased their daily chow intake, while the rats maintained on scheduled sucrose and chow mainly increased their daily sucrose intake. Adding sucrose to scheduled feeding displaced c-fos mRNA expression from the dorsomedial and paraventricular hypothalamic nuclei and posterior lateral hypothalamus (LH) to the prefrontal cortex, lateral septum, nucleus accumbens and anterior LH. During refeeding, the rats on scheduled sucrose demonstrated higher activation of the nucleus of the solitary tract. The present results suggest that palatable sucrose combined with restricted feeding schedules activate a distinct neuronal network compared to neuronal activation produced by scheduled access to regular chow. These data provide evidence that the brain may contain different food-oscillatory systems and that food palatability may shift the neuronal activity from the medial hypothalamus to the limbic and reward-related areas even at the negative metabolic state.

Experience-independent development of the hamster circadian visual system

Experience-dependent functional plasticity is a hallmark of the primary visual system, but it is not known if analogous mechanisms govern development of the circadian visual system. Here we investigated molecular, anatomical, and behavioral consequences of complete monocular light deprivation during extended intervals of postnatal development in Syrian hamsters. Hamsters were raised in constant darkness and opaque contact lenses were applied shortly after eye opening and prior to the introduction of a light-dark cycle. In adulthood, previously-occluded eyes were challenged with visual stimuli. Whereas image-formation and motion-detection were markedly impaired by monocular occlusion, neither entrainment to a light-dark cycle, nor phase-resetting responses to shifts in the light-dark cycle were affected by prior monocular deprivation. Cholera toxin-b subunit fluorescent tract-tracing revealed that in monocularly-deprived hamsters the density of fibers projecting from the retina to the suprachiasmatic nucleus (SCN) was comparable regardless of whether such fibers originated from occluded or exposed eyes. In addition, long-term monocular deprivation did not attenuate light-induced c-Fos expression in the SCN. Thus, in contrast to the thalamocortical projections of the primary visual system, retinohypothalamic projections terminating in the SCN develop into normal adult patterns and mediate circadian responses to light largely independent of light experience during development. The data identify a categorical difference in the requirement for light input during postnatal development between circadian and non-circadian visual systems.

A noradrenergic sensitive endogenous clock is present in the rat pineal gland

The aim of this study was to examine the occurrence of endogenous oscillations of Per1, Per2, Bmal1 and Rev-erbα genes in rat pineal explants and to investigate their regulation by adrenergic ligands. Our results show a significant and sustained rhythm of Per2,Bmal1 and Rev-erbα gene expression for up to 48 h in cultured pineal gland with a pattern similar to that observed in vivo. By contrast, the rhythms of Per1 and Aa-nat, the rate-limiting enzyme for melatonin synthesis, were strongly attenuated after 24 h in culture. Addition of the exogenous adrenergic agonist isoproterenol on cultured pineal glands induced a short-term increase in mRNA levels of Per1 and Aa-nat, but not those of Per2,Bmal1 and Rev-erbα. This study demonstrates that the rat pineal gland hosts a circadian oscillator as evidenced by the sustained, noradrenergic-independent, endogenous oscillations of Per2, Bmal1 and Rev-erbα mRNA levels in cultured tissues. Only expression of Per1 was stimulated by adrenergic ligands suggesting that, in vivo, the adrenergic input could synchronize the pineal clock by acting selectively on Per1.

Skin surface temperature rhythms as potential circadian biomarkers for personalized chronotherapeutics in cancer patients

Chronotherapeutics involve the administration of treatments according to circadian rhythms. Circadian timing of anti-cancer medications has been shown to improve treatment tolerability up to fivefold and double efficacy in experimental and clinical studies. However, the physiological and the molecular components of the circadian timing system (CTS), as well as gender, critically affect the success of a standardized chronotherapeutic schedule. In addition, a wrongly timed therapy or an excessive drug dose disrupts the CTS. Therefore, a non-invasive approach to accurately detect and monitor circadian rhythms is needed for a dynamic assessment of the CTS in order to personalize chronomodulated drug delivery schedule in cancer patients. Since core body temperature is a robust circadian biomarker, we recorded temperature at multiple locations on the skin of the upper chest and back of controls and cancer patients continuously. Variability in the circadian phase existed among patch locations in individual subjects over the course of 2-6 days, demonstrating the need to monitor multiple skin temperature locations to determine the precise circadian phase. Additionally, we observed that locations identified by infrared imaging as relatively cool had the largest 24 h temperature variations. Disruptions in skin temperature rhythms during treatment were found, pointing to the need to continually assess circadian timing and personalize chronotherapeutic schedules.

Temperature as a universal resetting cue for mammalian circadian oscillators

Environmental temperature cycles are a universal entraining cue for all circadian systems at the organismal level with the exception of homeothermic vertebrates. We report here that resistance to temperature entrainment is a property of the suprachiasmatic nucleus (SCN) network and is not a cell-autonomous property of mammalian clocks. This differential sensitivity to temperature allows the SCN to drive circadian rhythms in body temperature, which can then act as a universal cue for the entrainment of cell-autonomous oscillators throughout the body. Pharmacological experiments show that network interactions in the SCN are required for temperature resistance and that the heat shock pathway is integral to temperature resetting and temperature compensation in mammalian cells. These results suggest that the evolutionarily ancient temperature resetting response can be used in homeothermic animals to enhance internal circadian synchronization.

CLOCK and BMAL1 regulate MyoD and are necessary for maintenance of skeletal muscle phenotype and function

MyoD, a master regulator of myogenesis, exhibits a circadian rhythm in its mRNA and protein levels, suggesting a possible role in the daily maintenance of muscle phenotype and function. We report that MyoD is a direct target of the circadian transcriptional activators CLOCK and BMAL1, which bind in a rhythmic manner to the core enhancer of the MyoD promoter. Skeletal muscle of Clock(Δ19) and Bmal1(-/-) mutant mice exhibited ∼30% reductions in normalized maximal force. A similar reduction in force was observed at the single-fiber level. Electron microscopy (EM) showed that the myofilament architecture was disrupted in skeletal muscle of Clock(Δ19), Bmal1(-/-), and MyoD(-/-) mice. The alteration in myofilament organization was associated with decreased expression of actin, myosins, titin, and several MyoD target genes. EM analysis also demonstrated that muscle from both Clock(Δ19) and Bmal1(-/-) mice had a 40% reduction in mitochondrial volume. The remaining mitochondria in these mutant mice displayed aberrant morphology and increased uncoupling of respiration. This mitochondrial pathology was not seen in muscle of MyoD(-/-) mice. We suggest that altered expression of both Pgc-1α and Pgc-1β in Clock(Δ19) and Bmal1(-/-) mice may underlie this pathology. Taken together, our results demonstrate that disruption of CLOCK or BMAL1 leads to structural and functional alterations at the cellular level in skeletal muscle. The identification of MyoD as a clock-controlled gene provides a mechanism by which the circadian clock may generate a muscle-specific circadian transcriptome in an adaptive role for the daily maintenance of adult skeletal muscle.

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.

Chronobiological aspects of nutrition, metabolic syndrome and obesity

The present review starts from the classical physiological and nutritional studies related with food intake control, digestion, transport and absorption of nutrients. It continues with studies related with the metabolism of adipose tissue, and finish with modern experiments in genetics and molecular biology - all from a fresh, chronobiological point of view. Obesity will be explained as a fault in the circadian system, as pathology associated with "chronodisruption". The main gaps in chronobiological research related to obesity will be also identified and chronobiological-based therapies will be proposed in order to allow the resetting of the circadian rhythm among obese subjects.

Chronic treatment with a selective inhibitor of casein kinase I delta/epsilon yields cumulative phase delays in circadian rhythms

INTRODUCTION

Casein kinase I epsilon/delta phosphorylates certain clock-related proteins as part of a complex arrangement of transcriptional/translational feedback loops that comprise the circadian oscillator in mammals. Pharmacologic inhibition leads to a delay of the oscillations with the magnitude of this effect dependent upon the timing of drug administration.

OBJECTIVE

Earlier studies by our lab described the actions of a selective CKI epsilon/delta inhibitor, PF-670462, on circadian behavior following acute dosing; the present work extended these studies to chronic once-daily treatment.

METHODS

Gross motor activity was used to estimate the circadian rhythms of rats maintained under a 12 L:12 D cycle. PF-670462, 10 or 30 mg/kg/day s.c., was administered once daily for 20 days either at ZT6 or ZT11 (i.e., 6 or 11 h after light onset).

RESULTS

Chronic administration of PF-670462, performed at a fixed time of day, produced delays in the activity onsets of rats that cumulated with the duration of dosing. Dosing at ZT11 yielded more robust delays than dosing at ZT6 in keeping with earlier phase-response analyses with this agent.

CONCLUSIONS

The magnitude of the shifts in activity onsets achieved with chronic dosing of PF-670462 appears to be a function of the dose and the previously established phase relationship. Its cumulative effect further suggests that the pharmacodynamic t (1/2) of the drug greatly exceeds its pharmacokinetic one. Most importantly, these changes in circadian behavior occurred in the presence of a fixed L:D cycle, confirming the drug to be a robust modulator of circadian phase in the presence of the natural zeitgeber.

Lateralization of the central circadian pacemaker output: a test of neural control of peripheral oscillator phase

To evaluate the contribution of neural pathways to the determination of the circadian oscillator phase in peripheral organs, we assessed lateralization of clock gene expression in Syrian hamsters induced to split rhythms of locomotor activity by exposure to constant light. We measured the ratio of haPer1, haPer2, and haBmal1 mRNA on the high vs. low (H/L) side at 3-h intervals prior to the predicted activity onset (pAO). We also calculated expression on the sides ipsilateral vs. contralateral (I/C) to the side of the suprachiasmatic nucleus (SCN) expressing higher haPer1. The extent of asymmetry in split hamsters varied between specific genes, phases, and organs. Although the magnitude of asymmetry in peripheral organs was never as great as that in the SCN, we observed significantly greater lateralization of clock gene expression in the adrenal medulla and cortex, lung, and skeletal muscle, but not in liver or kidney, of split hamsters than of unsplit controls. We observed fivefold lateralization of expression of the clock-controlled gene, albumin site D-element binding protein (Dbp), in skeletal muscle (H/L: 10.7 +/- 3.7 at 3 h vs. 2.2 +/- 0.3 at 0 h pAO; P = 0.03). Furthermore, tyrosine hydroxylase expression was asymmetrical in the adrenal medulla of split (H/L: 1.9 +/- 0.5 at 0 h) vs. unsplit hamsters (1.2 +/- 0.04; P < 0.05). Consistent with a model of neurally controlled gene expression, we found significant correlations between the phase angle between morning and evening components (psi(me)) and the level of asymmetry (H/L or I/C). Our results indicate that neural pathways contribute to, but cannot completely account for, SCN regulation of the phase of peripheral oscillators.

The down-regulation of the human MYC gene by the nuclear hormone 1alpha,25-dihydroxyvitamin D3 is associated with cycling of corepressors and histone deacetylases

MYC is a pleiotropic transcription factor that coordinates the expression of diverse programs that are together necessary for the growth and expansion of somatic cells. The nuclear hormone 1alpha,25-dihydroxyvitamin D(3) down-regulates MYC expression, but the exact mechanism is still elusive. We found in RWPE-1 normal human prostate cells that 1alpha,25-dihydroxyvitamin D(3) down-regulates MYC mRNA with a periodicity of 30-90 min. In silico screening of the MYC gene locus identified six putative binding sites [vitamin D response elements (VDREs)] for the vitamin D receptor (VDR). Two of these VDREs efficiently bound VDR-retinoid X receptor heterodimers in vitro, and their genomic regions associated with VDR in RWPE-1 cells. Gene-specific small inhibitory RNA silencing indicated that basal MYC mRNA expression, as well as its down-regulation, depended on the exchange factor TBL1X (transducer beta-like 1, X-linked), the corepressor silencing mediator for retinoid and thyroid hormone receptor, and histone deacetylases (HDACs) 2, 6, and 11. Assaying the association of these five proteins with the VDRE-containing genomic regions of the MYC gene locus showed characteristic ligand-dependent profiles of TBL1X, silencing mediator for retinoid and thyroid hormone receptor, HDAC6, and HDAC11, in particular on an evolutionarily conserved VDRE. In conclusion, our data suggest that dynamically composed protein complexes that dock via VDR to the two VDREs may explain the repression of the MYC gene.

Circadian clocks in mood-related behaviors

The circadian clock organizes biochemical and physiological processes of an organism in a temporal fashion. This temporal organization is crucial to avoid interference of processes that have adverse effects on each other. Thus, disruption of temporal organization can lead to health problems and behavioral disorders related to mood alterations. To alleviate the consequences of a disrupted temporal organization in the body, it is of importance to understand the processes involved in the synchronization of all body clocks and their phase relationship to the environmental day/night cycle at the mechanistic level. This review will focus on internal and external factors affecting synchronization and function of the circadian system and highlight connections to mood-related behavior.

Genetic suppression of the circadian Clock mutation by the melatonin biosynthesis pathway

Most laboratory mouse strains including C57BL/6J do not produce detectable levels of pineal melatonin owing to deficits in enzymatic activity of arylalkylamine N-acetyltransferase (AANAT) and N-acetylserotonin O-methyl transferase (ASMT), two enzymes necessary for melatonin biosynthesis. Here we report that alleles segregating at these two loci in C3H/HeJ mice, an inbred strain producing melatonin, suppress the circadian period-lengthening effect of the Clock mutation. Through a functional mapping approach, we localize mouse Asmt to chromosome X and show that it, and the Aanat locus on chromosome 11, are significantly associated with pineal melatonin levels. Treatment of suprachiasmatic nucleus (SCN) explant cultures from Period2(Luciferase) (Per2(Luc)) Clock/+ reporter mice with melatonin, or the melatonin agonist, ramelteon, phenocopies the genetic suppression of the Clock mutant phenotype observed in living animals. These results demonstrate that melatonin suppresses the Clock/+ mutant phenotype and interacts with Clock to affect the mammalian circadian system.

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.

Temporal dynamics of mouse hippocampal clock gene expression support memory processing

Hippocampal plasticity and mnemonic processing exhibit a striking time-of-day dependence and likely implicate a temporally structured replay of memory traces. Molecular mechanisms fulfilling the requirements of sensing time and capturing time-related information are coded in dynamics of so-called clock genes and their protein products, first discovered and described in the hypothalamic suprachiasmatic nucleus. Using real-time PCR and immunohistochemical analyses, we show that in wildtype mice core clock components (mPer1/PER1, mPer2/PER2, mCry1/CRY1, mCry2/CRY2, mClock/CLOCK, mBmal1/BMAL1) are expressed in neurons of all subregions of the hippocampus in a time-locked fashion over a 24-h (diurnal) day/night cycle. Temporal profiling of these transcriptional regulators reveals distinct and parallel peaks, at times when memory traces are usually formed and/or consolidated. The coordinated rhythmic expression of hippocampal clock gene expression is greatly disordered in mice deficient for the clock gene mPer1, a key player implicated in both, maintenance and adaptative plasticity of circadian clocks. Moreover, Per1-knockout animals are severely handicapped in a hippocampus-dependent long-term spatial learning paradigm. We propose that the dynamics of hippocampal clock gene expression imprint a temporal structure on memory processing and shape at the same time the efficacy of behavioral learning.

Melatonin, circadian rhythms, and the clock genes in bipolar disorder

Sleep disturbances and dysregulation of circadian rhythms are core elements of bipolar disorder that might be involved in its pathogenesis. It has been proposed that patients with bipolar disorder have an abnormally shifted or arrhythmic circadian system and that the disturbance of circadian rhythms may be caused by an alteration in the circadian clock machinery. Chronotherapeutic strategies based on controlled exposures to environmental stimuli that act on biological rhythms have shown good efficacy in the treatment of illness episodes, thus confirming ex juvantibus the clinical relevance of internal timing in this illness.

Molecular clock is involved in predictive circadian adjustment of renal function

Renal excretion of water and major electrolytes exhibits a significant circadian rhythm. This functional periodicity is believed to result, at least in part, from circadian changes in secretion/reabsorption capacities of the distal nephron and collecting ducts. Here, we studied the molecular mechanisms underlying circadian rhythms in the distal nephron segments, i.e., distal convoluted tubule (DCT) and connecting tubule (CNT) and the cortical collecting duct (CCD). Temporal expression analysis performed on microdissected mouse DCT/CNT or CCD revealed a marked circadian rhythmicity in the expression of a large number of genes crucially involved in various homeostatic functions of the kidney. This analysis also revealed that both DCT/CNT and CCD possess an intrinsic circadian timing system characterized by robust oscillations in the expression of circadian core clock genes (clock, bma11, npas2, per, cry, nr1d1) and clock-controlled Par bZip transcriptional factors dbp, hlf, and tef. The clock knockout mice or mice devoid of dbp/hlf/tef (triple knockout) exhibit significant changes in renal expression of several key regulators of water or sodium balance (vasopressin V2 receptor, aquaporin-2, aquaporin-4, alphaENaC). Functionally, the loss of clock leads to a complex phenotype characterized by partial diabetes insipidus, dysregulation of sodium excretion rhythms, and a significant decrease in blood pressure. Collectively, this study uncovers a major role of molecular clock in renal function.

Time's arrow flies like a bird: two paradoxes for avian circadian biology

Biological timekeeping in birds is a fundamental feature of avian physiology, behavior and ecology. The physiological basis for avian circadian rhythmicity has pointed to a multi-oscillator system of mutually coupled pacemakers in the pineal gland, eyes and hypothalamic suprachiasmatic nuclei (SCN). In passerines, the role of the pineal gland and its hormone melatonin is particularly important. More recent molecular biological studies have pointed to a highly conserved mechanism involving rhythmic transcription and translation of "clock genes". However, studies attempting to reconcile the physiological role of pineal melatonin with molecular studies have largely failed. Recent work in our laboratory has suggested that melatonin-sensitive physiological processes are only loosely coupled to transcriptional oscillations. Similarly, although the pineal gland has been shown to be critical for overt circadian behaviors, its role in annual cycles of reproductive function appears to be minimal. Recent work on the seasonal control of birdsong, however, suggests that, although the pineal gland does not directly affect gonadal cycles, it is important for seasonal changes in song. Experimental analyses that address these paradoxes will shed light on the roles the biological clock play in birds and in vertebrates in general.

A time to fast, a time to feast: the crosstalk between metabolism and the circadian clock

The cyclic environmental conditions brought about by the 24 h rotation of the earth have allowed the evolution of endogenous circadian clocks that control the temporal alignment of behaviour and physiology, including the uptake and processing of nutrients. Both metabolic and circadian regulatory systems are built upon a complex feedback network connecting centres of the central nervous system and different peripheral tissues. Emerging evidence suggests that circadian clock function is closely linked to metabolic homeostasis and that rhythm disruption can contribute to the development of metabolic disease. At the same time, metabolic processes feed back into the circadian clock, affecting clock gene expression and timing of behaviour. In this review, we summarize the experimental evidence for this bimodal interaction, with a focus on the molecular mechanisms mediating this exchange, and outline the implications for clock-based and metabolic diseases.

Negative feedback maintenance of heme homeostasis by its receptor, Rev-erbalpha

Intracellular heme levels must be tightly regulated to maintain proper mitochondrial respiration while minimizing toxicity, but the homeostatic mechanisms are not well understood. Here we report a novel negative feedback mechanism whereby the nuclear heme receptor Rev-erbalpha tightly controls the level of its own ligand. Heme binding to Rev-erbalpha recruits the NCoR/histone deacetylase 3 (HDAC3) corepressor complex to repress the transcription of the coactivator PGC-1alpha, a potent inducer of heme synthesis. Depletion of Rev-erbalpha derepresses PGC-1alpha, resulting in increased heme levels. Conversely, increased Rev-erbalpha reduces intracellular heme, and impairs mitochondrial respiration in a heme-dependent manner. Consistent with this bioenergetic impairment, overexpression of Rev-erbalpha dramatically inhibits cell growth due to a cell cycle arrest. Thus, Rev-erbalpha modulates the synthesis of its own ligand in a negative feedback pathway that maintains heme levels and regulates cellular energy metabolism.

Inhibition of casein kinase I epsilon/delta produces phase shifts in the circadian rhythms of Cynomolgus monkeys

INTRODUCTION

Circadian rhythms in mammals depend upon the cyclic oscillations of transcriptional/translational feedback loops in pacemaker cells of the suprachiasmatic nucleus. The rise and fall of clock-related proteins is a function of synthesis and degradation, the latter involving phosphorylation by casein kinase Iepsilon and delta.

OBJECTIVE

Earlier studies by our lab described the actions of a selective CKIepsilon/delta inhibitor, PF-670462, on circadian behavior in rats; the present work extended these studies to a diurnal species, Cynomolgus monkeys.

MATERIALS AND METHODS

General cage activity was used to estimate the circadian rhythms of eight telemeterized monkeys under baseline conditions and following s.c. doses of PF-670462.

RESULTS AND DISCUSSION

Consolidated bouts of activity were noted during periods of light with a repeating period length of roughly 24 h based on their onset. Reassessment in constant dim light (42 vs. 450 lx) again yielded period lengths of 24 h, in this instance revealing the animals' endogenous rhythm. PF-670462 (10-100 mg/kg s.c.) produced a dose-dependent phase delay in much the same manner as that observed previously in rats. Dosing occurred 1.5 h prior to lights-off, roughly coincident with peaking levels of PER protein, a primary substrate of CKIepsilon/delta.

CONCLUSIONS

These findings suggest that the time of dosing, when held constant in both the monkey and rat studies, produced nearly identical effects despite the subjects' diurnal or nocturnal preference. Importantly, these changes in rhythm occurred in the presence of light, revealing the drug as a powerful zeitgeber in a non-human primate and, by extension, in man.

Diurnal physiology: core principles with application to the pathogenesis, diagnosis, prevention, and treatment of myocardial hypertrophy and failure

The circadian system has been shown to be fundamentally important in human health and disease. Recently, there have been major advances in our understanding of daily rhythmicity, and its relevance to human physiology, and to the pathogenesis and treatment of cardiac hypertrophy and heart failure. Cardiovascular tissues, such as heart and blood vessels, show remarkable daily variation in gene expression, metabolism, growth, and remodeling. Moreover, synchrony of daily molecular and physiological rhythms is integral to healthy organ growth and renewal. Disruption of these rhythms adversely affects normal growth, also the remodeling mechanisms in disease, leading to gross abnormalities in heart and vessels. These observations provide new insights into the pathogenesis, diagnosis, treatment, and prevention of heart disease. In this review, we focus on the recent advances in circadian biology and cardiovascular function, with particular emphasis on how this applies to human myocardial hypertrophy and heart failure, and the implications and importance for translational medicine.

Expression of the circadian clock gene Period2 in the hippocampus: possible implications for synaptic plasticity and learned behaviour

Genes responsible for generating circadian oscillations are expressed in a variety of brain regions not typically associated with circadian timing. The functions of this clock gene expression are largely unknown, and in the present study we sought to explore the role of the Per2 (Period 2) gene in hippocampal physiology and learned behaviour. We found that PER2 protein is highly expressed in hippocampal pyramidal cell layers and that the expression of both protein and mRNA varies with a circadian rhythm. The peaks of these rhythms occur in the late night or early morning and are almost 180° out-of-phase with the expression rhythms measured from the suprachiasmatic nucleus of the same animals. The rhythms in Per2 expression are autonomous as they are present in isolated hippocampal slices maintained in culture. Physiologically, Per2-mutant mice exhibit abnormal long-term potentiation. The underlying mechanism is suggested by the finding that levels of phosphorylated cAMP-response-element-binding protein, but not phosphorylated extracellular-signal-regulated kinase, are reduced in hippocampal tissue from mutant mice. Finally, Per2-mutant mice exhibit deficits in the recall of trace, but not cued, fear conditioning. Taken together, these results provide evidence that hippocampal cells contain an autonomous circadian clock. Furthermore, the clock gene Per2 may play a role in the regulation of long-term potentiation and in the recall of some forms of learned behaviour.

NPAS2 and PER2 are linked to risk factors of the metabolic syndrome

Background

Mammalian circadian clocks control multiple physiological events. The principal circadian clock generates seasonal variations in behavior as well. Seasonality elevates the risk for metabolic syndrome, and evidence suggests that disruption of the clockwork can lead to alterations in metabolism. Our aim was to analyze whether circadian clock polymorphisms contribute to seasonal variations in behavior and to the metabolic syndrome.

Methods

We genotyped 39 single-nucleotide polymorphisms (SNP) from 19 genes which were either canonical circadian clock genes or genes related to the circadian clockwork from 517 individuals drawn from a nationwide population-based sample. Associations between these SNPs and seasonality, metabolic syndrome and its risk factors were analyzed using regression analysis. The p-values were corrected for multiple testing.

Results

Our findings link circadian gene variants to the risk factors of the metabolic syndrome, since Npas2 was associated with hypertension (P-value corrected for multiple testing = 0.0024) and Per2 was associated with high fasting blood glucose (P-value corrected for multiple testing = 0.049).

Conclusion

Our findings support the view that relevant relationships between circadian clocks and the metabolic syndrome in humans exist.

Circadian rhythms and memory: not so simple as cogs and gears

The influence of circadian rhythms on memory has long been studied; however, the molecular prerequisites for their interaction remain elusive. The hippocampus, which is a region of the brain important for long-term memory formation and temporary maintenance, shows circadian rhythmicity in pathways central to the memory-consolidation process. As neuronal plasticity is the translation of numerous inputs, illuminating the direct molecular links between circadian rhythms and memory consolidation remains a daunting task. However, the elucidation of how clock genes contribute to synaptic plasticity could provide such a link. Furthermore, the idea that memory training could actually function as a zeitgeber for hippocampal neurons is worth consideration, based on our knowledge of the entrainment of the circadian clock system. The integration of many inputs in the hippocampus affects memory consolidation at both the cellular and the systems level, leaving the molecular connections between circadian rhythmicity and memory relatively obscure but ripe for investigation.

Casein kinase 1 delta regulates the pace of the mammalian circadian clock

Both casein kinase 1 delta (CK1delta) and epsilon (CK1epsilon) phosphorylate core clock proteins of the mammalian circadian oscillator. To assess the roles of CK1delta and CK1epsilon in the circadian clock mechanism, we generated mice in which the genes encoding these proteins (Csnk1d and Csnk1e, respectively) could be disrupted using the Cre-loxP system. Cre-mediated excision of the floxed exon 2 from Csnk1d led to in-frame splicing and production of a deletion mutant protein (CK1delta(Delta2)). This product is nonfunctional. Mice homozygous for the allele lacking exon 2 die in the perinatal period, so we generated mice with liver-specific disruption of CK1delta. In livers from these mice, daytime levels of nuclear PER proteins, and PER-CRY-CLOCK complexes were elevated. In vitro, the half-life of PER2 was increased by approximately 20%, and the period of PER2::luciferase bioluminescence rhythms was 2 h longer than in controls. Fibroblast cultures from CK1delta-deficient embryos also had long-period rhythms. In contrast, disruption of the gene encoding CK1epsilon did not alter these circadian endpoints. These results reveal important functional differences between CK1delta and CK1epsilon: CK1delta plays an unexpectedly important role in maintaining the 24-h circadian cycle length.

Endogenous rhythmicity of Bmal1 and Rev-erb alpha in the hamster pineal gland is not driven by norepinephrine

Pineal melatonin is synthesized with daily and seasonal rhythms following the hypothalamic clock-driven release of norepinephrine (NE). The pineal gland of rats and mice, like the biological clock, expresses a number of clock genes. However, the role of pineal clock elements in pineal physiology is still unknown. We examined the expression and regulation of several clock genes (Per1, Cry2, Bmal1 and Rev-erb alpha) under different lighting conditions or following adrenergic treatments in the Syrian hamster, a seasonal rodent. We found that Per1 and Cry2 genes were similarly regulated by the nocturnal release of NE: levels of Per1 and Cry2 mRNA displayed a nocturnal increase that was maintained after 2 days in constant darkness (DD) but abolished after 2 days under constant light (LL), a condition that suppresses endogenous NE release, or after an early night administration of the adrenergic antagonist propranolol. In contrast, Bmal1 and Rev-erb alpha exhibited a different pattern of expression and regulation. mRNA levels of both clock genes displayed a marked daily variation, maintained in DD, with higher values at midday for Bmal1 and at day/night transition for Rev-erb alpha. Remarkably, the daily variation of both Bmal1 and Rev-erb alpha mRNA was maintained in LL conditions and was not affected by propranolol. This study confirms the daily regulation of Per1 and Cry2 gene expression by NE in the pineal gland of rodents and shows for the first time that a second set of clock genes, Bmal1 and Rev-erb alpha are expressed with a circadian rhythm independent of the hypothalamic clock-driven noradrenergic signal.