MPer1 and mper2 are essential for normal resetting of the circadian clock

Light directs zebrafish period2 expression via conserved D and E boxes

For most species, light represents the principal environmental signal for entraining the endogenous circadian clock. The zebrafish is a fascinating vertebrate model for studying this process since unlike mammals, direct exposure of most of its tissues to light leads to local clock entrainment. Importantly, light induces the expression of a set of genes including certain clock genes in most zebrafish cell types in vivo and in vitro. However, the mechanism linking light to gene expression remains poorly understood. To elucidate this key mechanism, here we focus on how light regulates transcription of the zebrafish period2 (per2) gene. Using transgenic fish and stably transfected cell line-based assays, we define a Light Responsive Module (LRM) within the per2 promoter. The LRM lies proximal to the transcription start site and is both necessary and sufficient for light-driven gene expression and also for a light-dependent circadian clock regulation. Curiously, the LRM sequence is strongly conserved in other vertebrate per2 genes, even in species lacking directly light-sensitive peripheral clocks. Furthermore, we reveal that the human LRM can substitute for the zebrafish LRM to confer light-regulated transcription in zebrafish cells. The LRM contains E- and D-box elements that are critical for its function. While the E-box directs circadian clock regulation by mediating BMAL/CLOCK activity, the D-box confers light-driven expression. The zebrafish homolog of the thyrotroph embryonic factor binds efficiently to the LRM D-box and transactivates expression. We demonstrate that tef mRNA levels are light inducible and that knock-down of tef expression attenuates light-driven transcription from the per2 promoter in vivo. Together, our results support a model where a light-dependent crosstalk between E- and D-box binding factors is a central determinant of per2 expression. These findings extend the general understanding of the mechanism whereby the clock is entrained by light and how the regulation of clock gene expression by light has evolved in vertebrates.

Neurogenetics of food anticipation

Circadian clocks enable the organisms to anticipate predictable cycling events in the environment. The mechanisms of the main circadian clock, localized in the suprachiasmatic nuclei of the hypothalamus, involve intracellular autoregulatory transcriptional loops of specific genes, called clock genes. In the suprachiasmatic clock, circadian oscillations of clock genes are primarily reset by light, thus allowing the organisms to be in phase with the light-dark cycle. Another circadian timing system is dedicated to preparing the organisms for the ongoing meal or food availability: the so-called food-entrainable system, characterized by food-anticipatory processes depending on a circadian clock whose location in the brain is not yet identified with certainty. Here we review the current knowledge on food anticipation in mice lacking clock genes or feeding-related genes. The food-entrainable clockwork in the brain is currently thought to be made of transcriptional loops partly divergent from those described in the light-entrainable suprachiasmatic nuclei. Possible confounding effects associated with behavioral screening of meal anticipation in mutant mice are also discussed.

How nuclear receptors tell time

Most organisms adapt their behavior and physiology to the daily changes in their environment through internal ( approximately 24 h) circadian clocks. In mammals, this time-keeping system is organized hierarchically, with a master clock located in the suprachiasmatic nuclei of the hypothalamus that is reset by light, and that, in turn, coordinates the oscillation of local clocks found in all cells. Central and peripheral clocks control, in a highly tissue-specific manner, hundreds of target genes, resulting in the circadian regulation of most physiological processes. A great deal of knowledge has accumulated during the last decade regarding the molecular basis of mammalian circadian clocks. These studies have collectively demonstrated how a set of clock genes and their protein products interact together in complex feedback transcriptional/translational loops to generate 24-h oscillations at the molecular, cellular, and organism levels. In recent years, a number of nuclear receptors (NRs) have been implicated as important regulators of the mammalian clock mechanism. REV-ERB and retinoid-related orphan receptor NRs regulate directly the core feedback loop and increase its robustness. The glucocorticoid receptor mediates the synchronizing effect of glucocorticoid hormones on peripheral clocks. Other NR family members, including the orphan NR EAR2, peroxisome proliferator activated receptors-alpha/gamma, estrogen receptor-alpha, and retinoic acid receptors, are also linked to the clockwork mechanism. These findings together establish nuclear hormone receptor signaling as an integral part of the circadian timing system.

Period2Expression Pattern and its Role in the Development of the Pineal Circadian Clock in Zebrafish

In zebrafish, pineal arylalkylamine-N-acetyltransferase (zfaanat2) mRNA expression begins at 22 h post-fertilization (hpf), and the clock-controlled rhythm of its transcript begins on the third day of development. Here we describe the role of light and of the clock gene, period2 (zper2) in the development of this rhythm. In 1-day-old zebrafish embryos, zper2 expression is transiently up-regulated by light in the pineal gland and, to a lesser extent, in other areas of the brain. Expression of zper2 that was not affected by light occurred in the olfactory placode and lactotroph cells of the pituitary primordium. Circadian analysis of pineal zfaanat2 mRNA expression indicated that light exposure is required for proper development of the circadian clock-controlled rhythmic expression of this gene. Knockdown of zPER2 using antisense technology abolished the effect of light on development of the zfaanat2 rhythm in the pineal gland, corroborating the role of zper2 in light entrainment of the circadian oscillator in zebrafish. Further analysis of zper2 expression at earlier stages of development revealed that light exposure at the blastula to mid-segmentation stages also caused a transient increase in zper2 expression. At mid-segmentation, before pineal differentiation, light-induced zper2 expression was enhanced in pineal progenitor cells. Thus, a possible role for early photoreception and light-induced zper2 expression in the development of clock-controlled rhythms remains to be investigated.

rPer1 and rPer2 induction during phases of the circadian cycle critical for light resetting of the circadian clock

Photic resetting of a biological clock is one of the fundamental characteristics of circadian systems and allows living organisms to adjust to a particular environment. Nocturnal light induces the Per1 and Per2 genes, which leads to a resetting of the circadian clock in the suprachiasmatic nucleus (SCN), the mammalian circadian center. In our present study, we investigated whether light differentially induces the rat Per1 (rPer1) and Per2 (rPer2) genes to enable resetting of their circadian clocks. In a 24-hour LD cycle (12 h light:12 h dark), which is shorter than the normal free-running period for rats, Per1 alone showed strong induction in the ventrolateral region of the SCN (VLSCN) during the early day. In contrast, in a 25 hour LD cycle (12.5 h light:12.5 h dark), which is longer than the free running period for these animals, rPer2 alone was strongly induced in the VLSCN, at the end of the light phase and during the early dark periods. Our current findings therefore suggest that Per1 and Per2 are differentially regulated for daily entrainment to the LD cycle.

Compartmentalized expression of light-induced clock genes in the suprachiasmatic nucleus of the diurnal grass rat (Arvicanthis niloticus)

Photic responses of the circadian system are mediated through light-induced clock gene expression in the suprachiasmatic nucleus (SCN). In nocturnal rodents, depending on the timing of light exposure, Per1 and Per2 gene expression shows distinct compartmentalized patterns that correspond to the behavioral responses. Whether the gene- and region-specific induction patterns are unique to nocturnal animals, or are also present in diurnal species is unknown. We explored this question by examining the light-induced Per1 and Per2 gene expression in functionally distinct SCN subregions, using diurnal grass rats Arvicanthis niloticus. Light exposure during nighttime induced Per1 and Per2 expression in the SCN, showing unique spatiotemporal profiles depending on the phase of the light exposure. After a phase delaying light pulse (LP) in the early night, strong Per1 induction was observed in the retinorecipient core region of the SCN, while strong Per2 induction was observed throughout the entire SCN. After a phase advancing LP in the late night, Per1 was first induced in the core and then extended into the whole SCN, accompanied by a weak Per2 induction. This compartmentalized expression pattern is very similar to that observed in nocturnal rodents, suggesting that the same molecular and intercellular pathways underlying acute photic responses are present in both diurnal and nocturnal species. However, after an LP in early subjective day, which induces phase advances in diurnal grass rats, but not in nocturnal rodents, we did not observe any Per1 or Per2 induction in the SCN. This result suggests that in spite of remarkable similarities in the SCN of diurnal and nocturnal rodents, unique mechanisms are involved in mediating the phase shifts of diurnal animals during the subjective day.

Brain Clocks: From the Suprachiasmatic Nuclei to a Cerebral Network

Circadian timing affects almost all life’s processes. It not only dictates when we sleep, but also keeps every cell and tissue working under a tight temporal regimen. The daily variations of physiology and behavior are controlled by a highly complex system comprising of a master circadian clock in the suprachiasmatic nuclei (SCN) of the hypothalamus, extra-SCN cerebral clocks, and peripheral oscillators. Here are presented similarities and differences in the molecular mechanisms of the clock machinery between the primary SCN clock and extra-SCN brain clocks. Diversity of secondary clocks in the brain, their specific sensitivities to time-giving cues, as their differential coupling to the master SCN clock, may allow more plasticity in the ability of the circadian timing system to integrate a wide range of temporal information. Furthermore, it raises the possibility that pathophysiological alterations of internal timing that are deleterious for health may result from internal desynchronization within the network of cerebral clocks.

Strong resetting of the mammalian clock by constant light followed by constant darkness

The mammalian molecular circadian clock in the suprachiasmatic nuclei (SCN) regulates locomotor activity rhythms as well as clocks in peripheral tissues (Reppert and Weaver, 2002; Ko and Takahashi, 2006). Constant light (LL) can induce behavioral and physiological arrhythmicity by desynchronizing clock cells in the SCN (Ohta et al., 2005). We examined how the disordered clock cells resynchronize by probing the molecular clock and measuring behavior in mice transferred from LL to constant darkness (DD). The circadian locomotor activity rhythms disrupted in LL become robustly rhythmic again from the beginning of DD, and the starting phase of the rhythm in DD is specific, not random, suggesting that the desynchronized clock cells are quickly reset in an unconventional manner by the L/D transition. By measuring mPERIOD protein rhythms, we showed that the SCN and peripheral tissue clocks quickly become rhythmic again in phase with the behavioral rhythms. We propose that this resetting mechanism may be different from conventional phase shifting, which involves light induction of Period genes (Albrecht et al., 1997; Shearman et al., 1997; Shigeyoshi et al., 1997). Using our functional insights, we could shift the circadian phase of locomotor activity rhythms by 12 h using a 15 h LL treatment: essentially producing phase reversal by a single light pulse, a feat that has not been reported previously in wild-type mice and that has potential clinical utility.

Circadian polymorphisms associated with affective disorders

BACKGROUND

Clinical symptoms of affective disorders, their response to light treatment, and sensitivity to other circadian interventions indicate that the circadian system has a role in mood disorders. Possibly the mechanisms involve circadian seasonal and photoperiodic mechanisms. Since genetic susceptibilities contribute a strong component to affective disorders, we explored whether circadian gene polymorphisms were associated with affective disorders in four complementary studies.

METHODS

Four groups of subjects were recruited from several sources: 1) bipolar proband-parent trios or sib-pair-parent nuclear families, 2) unrelated bipolar participants who had completed the BALM morningness-eveningness questionnaire, 3) sib pairs from the GenRed Project having at least one sib with early-onset recurrent unipolar depression, and 4) a sleep clinic patient group who frequently suffered from depression. Working mainly with the SNPlex assay system, from 2 to 198 polymorphisms in genes related to circadian function were genotyped in the participant groups. Associations with affective disorders were examined with TDT statistics for within-family comparisons. Quantitative trait associations were examined within the unrelated samples.

RESULTS

In NR1D1, rs2314339 was associated with bipolar disorder (P = 0.0005). Among the unrelated bipolar participants, 3 SNPs in PER3 and CSNK1E were associated with the BALM score. A PPARGC1B coding SNP, rs7732671, was associated with affective disorder with nominal significance in bipolar family groups and independently in unipolar sib pairs. In TEF, rs738499 was associated with unipolar depression; in a replication study, rs738499 was also associated with the QIDS-SR depression scale in the sleep clinic patient sample.

CONCLUSION

Along with anti-manic effects of lithium and the antidepressant effects of bright light, these findings suggest that perturbations of the circadian gene network at several levels may influence mood disorders, perhaps ultimately through regulation of MAOA and its modulation of dopamine transmission. Twenty-three associations of circadian polymorphisms with affective symptoms met nominal significance criteria (P < 0.05), whereas 15 would be expected by chance, indicating that many represented false discoveries (Type II errors). Some evidence of replication has been gathered, but more studies are needed to ascertain if circadian gene polymorphisms contribute to susceptibility to affective disorders.

Beta-catenin induces beta-TrCP-mediated PER2 degradation altering circadian clock gene expression in intestinal mucosa of ApcMin/+ mice

Proliferation of intestinal epithelial cells is rhythmic throughout the day. This temporal organization occurs through the interaction between the endogenous peripheral circadian clock and pathways controlling cell cycle progression. Per2, a core clock gene with tumour suppresser function, is critical to clock function and to the regulation of cellular proliferation. Circadian disruption, which increases colon cancer incidence, may do so by deregulating clock controlled epithelial cell proliferation. Increased expression of beta-catenin is a contributing cause of most familial and spontaneous human colon cancer and the cause of multiple intestinal neoplasia of the Apc(Min/+) mouse. Here we report that increased beta-catenin destabilizes PER2 clock protein by inducing beta-TrCP, an F-box protein of SCF ubiquitin E3 ligase. In the intestinal mucosa of the Apc(Min/)(+) mouse, the decrease in PER2 protein levels is associated with altered circadian rhythms of clock genes, Per1 and Per2, and clock controlled genes, Dbp and Wee1. These findings suggest that disruption of the peripheral intestinal circadian clock may be intimately involved in beta-catenin induced intestinal epithelial neoplastic transformation in both mouse and man.

Period gene expression in the diurnal degu (Octodon degus) differs from the nocturnal laboratory rat (Rattus norvegicus)

Recent data suggest that both nocturnal and diurnal mammals generate circadian rhythms using similarly phased feedback loops involving Period genes in the suprachiasmatic nuclei (SCN) of the hypothalamus. These molecular oscillations also exist in the brain outside of the SCN, but the relationship between SCN and extra-SCN oscillations is unclear. We hypothesized that a comparison of "diurnal" and "nocturnal" central nervous system Per rhythms would uncover differences in the underlying circadian mechanisms between these two chronotypes. Therefore, this study compared the 24-h oscillatory patterns of Per1 and Per2 mRNA in the SCN and putative striatum and cortex of Octodon degus (degu), a diurnal hystricognath rodent, with those of the nocturnal laboratory rat, Rattus norvegicus. The brains of adult male degus and rats were collected at 2-h intervals across 24 h in entrained light-dark and constant darkness conditions, and sections were analyzed via in situ hybridization. In the SCN, degu Per1 and Per2 hybridization signal exhibited 24-h oscillatory patterns similar in phasing to those seen in other rodents, with peaks occurring during the light period and troughs during the dark period. However, Per1 remained elevated for five fewer hours in the degu than in the rat, and Per2 remained elevated for two fewer hours in the degu. In brain areas outside of the SCN, the phase of Per2 hybridization signal rhythms in the degu were 180 degrees out of phase with those found in the rat, and Per1 hybridization signal lacked significant rhythmicity. These results suggest that, while certain basic components of the transcriptional-translational feedback loop generating circadian rhythms are similar in diurnal and nocturnal mammals, there are variations that may reflect adaptations to circadian niche.

A "bottom-counting" video system for measuring cocaine-induced behaviors in Drosophila

Cocaine exposure elicits a set of stereotypic behaviors in Drosophila that are strikingly similar to the cocaine-induced behaviors observed in vertebrates. This provides a valuable model for the study of cocaine abuse and has led to the discovery of a connection between the cocaine response pathway and the circadian system. This article describes a simplified assessment of cocaine-induced behavior combined with an image acquisition system, which allows the assay to be semiautomated. With this new system, cocaine response can be evaluated in a fraction of the time required by the previous assay, and subjectivity in scoring is reduced dramatically.

Mouse period 2 mRNA circadian oscillation is modulated by PTB-mediated rhythmic mRNA degradation

Circadian mRNA oscillations are the main feature of core clock genes. Among them, period 2 is a key component in negative-feedback regulation, showing robust diurnal oscillations. Moreover, period 2 has been found to have a physiological role in the cell cycle or the tumor suppression. The present study reports that 3′-untranslated region (UTR)-dependent mRNA decay is involved in the regulation of circadian oscillation of period 2 mRNA. Within the mper2 3′UTR, both the CU-rich region and polypyrimidine tract-binding protein (PTB) are more responsible for mRNA stability and degradation kinetics than are other factors. Depletion of PTB with RNAi results in mper2 mRNA stabilization. During the circadian oscillations of mper2, cytoplasmic PTB showed a reciprocal expression profile compared with mper2 mRNA and its peak amplitude was increased when PTB was depleted. This report on the regulation of mper2 proposes that post-transcriptional mRNA decay mediated by PTB is a fine-tuned regulatory mechanism that includes dampening-down effects during circadian mRNA oscillations.

Low reproductive success in Per1 and Per2 mutant mouse females due to accelerated ageing?

Recent studies on mice with mutations in the Clock gene have shown that this mutation disrupts oestrus cyclicity and interferes with successful pregnancy. In order to determine whether two other molecular components of the main clock, namely the period genes, Per1 and Per2, have an effect on the length of the oestrous cycle and the reproductive success, we used Per1- and Per2-deficient females. We show that although fecundity of young adult Per mutant females does not differ from that of wild-type females, middle-aged Per mutant mice are characterised by lower reproductive success than the control group. This may be a consequence of irregularity and acyclicity of the oestrous cycle of the middle-aged mutants. Besides, we demonstrate that Per mutant females have significantly more embryonal implantations in the uterus than successfully delivered offspring. The reproductive deficits of the middle-aged Per mutant females are comparable with those seen in aged wild-type mice. This suggests that Per1 and Per2 mutations cause an advanced ageing.

The multiple facets of Per2

The Period 2 (Per2) gene is an important component of the circadian system. It appears to be not only part of the core oscillator mechanism, but also part of the input and output pathways of the clock. Because of its involvement at multiple levels of the circadian system, Per2 needs to meet a variety of different demands. We discuss how Per2 might be able to fulfill multiple functions by reviewing known facts and combine this with speculations based on these facts. This might provide new views about Per2 function and help to better understand diseases that are rooted in the circadian system.

The nuclear hormone receptor family round the clock

Daily rhythms in behavior and physiology are observed in most organisms. These rhythms are controlled by internal self-sustained circadian ( approximately 24 h) clocks, which are present in virtually all cells. The 24-h oscillations are generated by a molecular mechanism entrained by external or internal time cues and which, in turn, regulate rhythmic outputs. In mammals, the circadian system comprises a master clock located in the hypothalamus that is directly entrained by the light-dark cycle and which coordinates the phases of local clocks in the periphery in order to ensure optimal timing of the physiology. Nuclear receptors (NRs) form a large family of transcription factors that include both ligand-inducible and orphan receptors. These NRs are key regulators of major biological processes such as reproduction, development, cell growth and death, inflammation, immunity, and metabolic homeostasis. Recent observations indicate that several NR signaling pathways play a critical role in central and peripheral circadian clocks. The REV-ERB/retinoid-related orphan receptor orphan NR subfamily regulates the expression of core clock genes and contributes to the robustness of the clock mechanism. Glucocorticoid and retinoic acid receptors are involved in the resetting of peripheral clocks. Several other NRs such as peroxisome proliferator-activated receptor-alpha, short heterodimer partner, and constitutive androstane receptor act as molecular links between clock genes and specific rhythmic metabolic outputs. The expanding functional links between NRs and circadian clocks open novel perspectives for understanding the hormonal regulation of the mammalian circadian system as well as for exploring the role of circadian clocks in the pathogenesis of NR-related diseases such as cancer and metabolic syndrome.

Photic regulation of the mTOR signaling pathway in the suprachiasmatic circadian clock

Here we analyzed the light-responsiveness of the mammalian target of rapamycin (mTOR) cascade, a key regulator of inducible translation, in the suprachiasmatic nuclei (SCN), the locus of the master circadian clock. Brief light exposure during the subjective night, but not during the subjective day, triggered rapid phosphorylation (a marker of catalytic activity) of the mTOR translation effectors p70 S6K, ribosomal S6 protein (S6) and 4E-BP1. In the absence of photic stimulation, marked S6 and 4E-BP1 phosphorylation was detected, indicating tonic mTOR activity in the SCN. Light stimulated the colocalized activation of p70 S6K and extracellular signal-regulated protein kinase (ERK), and pharmacological disruption of ERK signaling abolished light-induced mTOR activity, revealing that the MAPK cascade is an essential intermediate that couples light to mTOR. Together these data identify a light-responsive mTOR cascade in the SCN, and thus, raise the possibility that inducible translation contributes to the clock entrainment process.

SCN-AVP release of mPer1/mPer2 double-mutant mice in vitro

Background

Circadian organisation of behavioural and physiological rhythms in mammals is largely driven by the clock in the suprachiasmatic nuclei (SCN) of the hypothalamus. In this clock, a molecular transcriptional repression and activation mechanism generates near 24 hour rhythms. One of the outputs of the molecular clock in specific SCN neurons is arginine-vasopressin (AVP), which is responsive to transcriptional activation by clock gene products. As negative regulators, the protein products of the period genes are thought to repress transcriptional activity of the positive limb after heterodimerisation with CRYPTOCHROME. When both the Per1 and Per2 genes are dysfunctional by targeted deletion of the PAS heterodimer binding domain, mice lose circadian organization of behaviour upon release into constant environmental conditions. To which degree the period genes are involved in the control of AVP output is unknown.

Methods

Using an in vitro slice culture setup, SCN-AVP release of cultures made of 10 wildtype and 9 Per1/2 double-mutant mice was assayed. Mice were sacrificed in either the early light phase of the light-dark cycle, or in the early subjective day on the first day of constant dark.

Results

Here we report that in arrhythmic homozygous Per1/2 double-mutant mice there is still a diurnal peak in in vitro AVP release from the SCN similar to that of wildtypes but distinctively different from the release pattern from the paraventricular nucleus. Such a modulation of AVP release is unexpected in mice where the circadian clockwork is thought to be disrupted.

Conclusion

Our results suggest that the circadian clock in these animals, although deficient in (most) behavioural and molecular rhythms, may still be (partially) functional, possibly as an hourglass mechanism. The level of perturbation of the clock in Per1/2 double mutants may therefore be less than was originally thought.

Interactions of GABA A receptor activation and light on period mRNA expression in the suprachiasmatic nucleus

Activation of gamma-aminobutyric acid (GABA) A receptors in the suprachiasmatic nucleus (SCN) resets the circadian clock during the day and inhibits the ability of light to reset the clock at night. Light in turn acts during the day to inhibit the phase-resetting effects of GABA. Some evidence suggests that Period mRNA changes in the SCN are responsible for these interactions between light and GABA. Here, the hypothesis that light and the GABA A receptor interact by altering the expression of Period 1 and/or Period 2 mRNA in the SCN is tested. The GABA A agonist muscimol was injected near the SCN just prior to a light pulse, during the mid-subjective day and the early and late subjective night. Changes in Period 1 and Period 2 mRNA were measured in the SCN by in situ hybridization. Light-induced Period 1 mRNA was inhibited by GABA A receptor activation in the early and late subjective night, while Period 2 mRNA was only inhibited during the late night. During the subjective day, light had no effect on the ability of muscimol to suppress Period 1 mRNA hybridization signal. Thus, light and GABA A receptor activation inhibit each other's ability to induce behavioral phase shifts throughout the subjective day and night. However, only in the late night are these behavioral effects correlated with changes in Period gene expression. Together, our data support the hypothesis that the interacting effects of light and GABA are the result of the opposing actions of these stimuli on Period mRNA, but only during the subjective night.

Diurnal rhythmicity of the canonical clock genes Per1, Per2 and Bmal1 in the rat adrenal gland is unaltered after hypophysectomy

Circadian rhythms are generated by endogenous clocks in the central brain oscillator, the suprachiasmatic nucleus (SCN), and peripheral tissues. The molecular basis for the circadian clock consists of a number of genes and proteins that form transcriptional/translational feedback loops. Rhythmic expression of clock genes in the adrenal glands has previously been reported. Since the central clock in the SCN communicates with the adrenal glands via circadian release of adrenocorticotrophic hormone, we quantified the mRNAs for the canonical clock genes, Per1, Per2 and Bmal1 in the adrenal glands by real-time reverse transcription-polymerase chain reaction during a 24-h-cycle in normal and hypophysectomised rats. The mRNAs for all the three clock genes disclosed rhythmic oscillations with a period of 24 h and the phase did not differ between the hypophysectomised and intact rats. The expression pattern of Per1 and Bmal1 was in antiphase in both groups of animals. In situ hybridisation histochemistry using antisense RNA probes demonstrated that, at times of peak expression, mRNAs for all the three clock genes were expressed in the adrenal cortex with a particularly strong labelling in the zona reticularis. In accordance with the mRNA localisation, immunostaining for PER1 protein was visualised in cells of the adrenal cortex, being most intense in the inner zone. The immunostaining also demonstrated a translocation of PER1 protein from the cytoplasm to the nucleus during the daily cycle, supporting the existence of a core oscillator in the individual adrenal gland cells. Our findings support the existence of a circadian core oscillator in cells of the rat adrenal cortex and indicate that the activity of the oscillator is independent of SCN signalling via the pituitary gland. The adrenal cortical clock could be involved in rhythmic transcriptional activation of genes associated with hormonal biosynthesis, involved in gating of the response of the adrenal cortex to external cues or involved in apoptosis of adrenal cortical cells.

The role of circadian clock genes in mental disorders

The study of molecular clock mechanisms in psychiatric disorders is gaining significant interest due to data suggesting that a misalignment between the endogenous circadian system and the sleep-wake cycle might contribute to the clinical status of patients suffering from a variety of psychiatric disorders. Sleep disturbances in major depressive disorder (MDD) are characterized by increased sleep latency, poorer sleep efficiency, reduced latency to the first rapid eye movement (REM) sleep episode, and early-morning awakening, but there is little data to indicate a role of circadian clock genes in MDD. There is also relatively little information regarding the role of clock genes in anxiety. In contrast, a significant amount of evidence gathered in bipolar disorder (BPD) patients suggests a circadian rhythm disorder, namely an advanced circadian rhythm and state-dependent alterations of REM sleep latency. Most research on the role of clock genes in BPD has focused on polymorphisms of CLOCK, but the lithium target GSK3 may also play a significant role. A circadian phase shift is also theorized to contribute to the pathophysiology of winter seasonal affective disorder (SAD). Certain allelic combinations of NPAS2, PER3, and BMAL1 appear to contribute to the risk of SAD. In chronic shizophrenia, disturbances of sleep including insomnia and reduced sleep efficiency have been observed. Genetic studies have found associations with CLOCK, PER1, PER3, and TIMELESS. Sleep and circadian changes associated with dementia due to Alzheimer's disease suggest a functional change in the circadian master clock, which is supported by postmortem studies of clock gene expression in the brain.

The daily timing of gene expression and physiology in mammals

Mammalian behavior and physiology undergo daily rhythms that are coordinated by an endogenous circadian timing system. This system has a hierarchical structure, in that a master pacemaker, residing in the suprachiasmatic nucleus of the ventral hypothalamus, synchronizes peripheral oscillators in virtually all body cells. While the basic molecular mechanisms generating the daily rhythms are similar in all cells, most clock outputs are cell-specific. This conclusion is based on genome-wide transcriptome profiling studies in several tissues that have revealed hundreds of rhythmically expressed genes. Cyclic gene expression in the various organs governs overt rhythms in behavior and physiology, encompassing sleep-wake cycles, metabolism, xenobiotic detoxification, and cellular proliferation. As a consequence, chronic perturbation of this temporal organization may lead to increased morbidity and reduced lifespan.

Minireview: Entrainment of the suprachiasmatic clockwork in diurnal and nocturnal mammals

Daily rhythmicity, including timing of wakefulness and hormone secretion, is mainly controlled by a master clock located in the suprachiasmatic nucleus (SCN) of the hypothalamus. The SCN clockwork involves various clock genes, with specific temporal patterns of expression that are similar in nocturnal and diurnal species (e.g. the clock gene Per1 in the SCN peaks at midday in both categories). Timing of sensitivity to light is roughly similar, during nighttime, in diurnal and nocturnal species. Molecular mechanisms of photic resetting are also comparable in both species categories. By contrast, in animals housed in constant light, exposure to darkness can reset the SCN clock, mostly during the resting period, i.e. at opposite circadian times between diurnal and nocturnal species. Nonphotic stimuli, such as scheduled voluntary exercise, food shortage, exogenous melatonin, or serotonergic receptor activation, are also capable of shifting the master clock and/or modulating photic synchronization. Comparison between day- and night-active species allows classifications of nonphotic cues in two, arousal-independent and arousal-dependent, families of factors. Arousal-independent factors, such as melatonin (always secreted during nighttime, independently of daily activity pattern) or gamma-aminobutyric acid (GABA), have shifting effects at the same circadian times in both nocturnal and diurnal rodents. By contrast, arousal-dependent factors, such as serotonin (its cerebral levels follow activity pattern), induce phase shifts only during resting and have opposite modulating effects on photic resetting between diurnal and nocturnal species. Contrary to light and arousal-independent nonphotic cues, arousal-dependent nonphotic stimuli provide synchronizing feedback signals to the SCN clock in circadian antiphase between nocturnal and diurnal animals.

Forebrain oscillators ticking with different clock hands

Clock proteins like PER1 and PER2 are expressed in the brain, but little is known about their functionality outside the main suprachiasmatic clock. Here we show that PER1 and PER2 were neither uniformly present nor identically phased in forebrain structures of mice fed ad libitum. Altered expression of the clock gene Cry1 was observed in respective Per1 or Per2 mutants. In response to hypocaloric feeding, PERs timing was not markedly affected in few forebrain structures (hippocampus). In most other forebrain oscillators, including those expressing only PER1 (e.g., dorsomedial hypothalamus), PER2 (e.g., paraventricular hypothalamus) or both (e.g., paraventricular thalamus), PER1 was up-regulated and PER2 largely phase-advanced. Cry1 expression was selectively modified in the forebrain of Per mutants challenged with hypocaloric feeding. Our results suggest that there is not one single cerebral clock, but a system of multiple brain oscillators ticking with different clock hands and differentially sensitive to nutritional cues.

Circadian rhythms and reproduction

There is a growing recognition that the circadian timing system, in particular recently discovered clock genes, plays a major role in a wide range of physiological systems. Microarray studies, for example, have shown that the expression of hundreds of genes changes many fold in the suprachiasmatic nucleus, liver heart and kidney. In this review, we discuss the role of circadian rhythmicity in the control of reproductive function in animals and humans. Circadian rhythms and clock genes appear to be involved in optimal reproductive performance, but there are sufficient redundancies in their function that many of the knockout mice produced do not show overt reproductive failure. Furthermore, important strain differences have emerged from the studies especially between the various Clock (Circadian Locomotor Output Cycle Kaput) mutant strains. Nevertheless, there is emerging evidence that the primary clock genes, Clock and Bmal1 (Brain and Muscle ARNT-like protein 1, also known as Mop3), strongly influence reproductive competency. The extent to which the circadian timing system affects human reproductive performance is not known, in part, because many of the appropriate studies have not been done. With the role of Clock and Bmal1 in fertility becoming clearer, it may be time to pursue the effect of polymorphisms in these genes in relation to the various types of infertility in humans.

Orchestration of gene expression and physiology by the circadian clock

In mammals, the master circadian clock that drives many biochemical, physiological and behavioral rhythms is located in the suprachiasmatic nuclei (SCN) of the hypothalamus. Generation and maintenance of circadian rhythms rely on complex interlaced feedback loops based on transcriptional and posttranscriptional events involving clock genes and kinases. This clock serves the purpose to organize an organism's biochemistry on a 24 h time scale thereby avoiding interference between biochemical pathways and optimizing performance. Synchronization to environmental 24 h oscillations tunes physiological processes optimally with nature. In this review, I briefly describe the principle of the clock mechanism, its synchronization to the environment and consequences on health when the circadian clock is disrupted.

"Feeding time" for the brain: a matter of clocks

Circadian clocks are autonomous time-keeping mechanisms that allow living organisms to predict and adapt to environmental rhythms of light, temperature and food availability. At the molecular level, circadian clocks use clock and clock-controlled genes to generate rhythmicity and distribute temporal signals. In mammals, synchronization of the master circadian clock located in the suprachiasmatic nuclei of the hypothalamus is accomplished mainly by light stimuli. Meal time, that can be experimentally modulated by temporal restricted feeding, is a potent synchronizer for peripheral oscillators with no clear synchronizing influence on the suprachiasmatic clock. Furthermore, food-restricted animals are able to predict meal time, as revealed by anticipatory bouts of locomotor activity, body temperature and plasma corticosterone. These food anticipatory rhythms have long been thought to be under the control of a food-entrainable clock (FEC). Analysis of clock mutant mice has highlighted the relevance of some, but not all of the clock genes for food-entrainable clockwork. Mutations of Clock or Per1 do not impair expression of food anticipatory components, suggesting that these clock genes are not essential for food-entrainable oscillations. By contrast, mice mutant for Npas2 or deficient for Cry1 and Cry2 show more or less altered responses to restricted feeding conditions. Moreover, a lack of food anticipation is specifically associated with a mutation of Per2, demonstrating the critical involvement of this gene in the anticipation of meal time. The actual location of the FEC is not yet clearly defined. Nevertheless, current knowledge of the putative brain regions involved in food-entrainable oscillations is discussed. We also describe several neurochemical pathways, including orexinergic and noradrenergic, likely to participate in conveying inputs to and outputs from the FEC to control anticipatory processes.

microRNA modulation of circadian-clock period and entrainment

microRNAs (miRNAs) are a class of small, noncoding RNAs that regulate the stability or translation of mRNA transcripts. Although recent work has implicated miRNAs in development and in disease, the expression and function of miRNAs in the adult mammalian nervous system have not been extensively characterized. Here, we examine the role of two brain-specific miRNAs, miR-219 and miR-132, in modulating the circadian clock located in the suprachiasmatic nucleus. miR-219 is a target of the CLOCK and BMAL1 complex, exhibits robust circadian rhythms of expression, and the in vivo knockdown of miR-219 lengthens the circadian period. miR-132 is induced by photic entrainment cues via a MAPK/CREB-dependent mechanism, modulates clock-gene expression, and attenuates the entraining effects of light. Collectively, these data reveal miRNAs as clock- and light-regulated genes and provide a mechanistic examination of their roles as effectors of pacemaker activity and entrainment.

Light Entrainment of the Mammalian Circadian Clock by a PRKCA-Dependent Posttranslational Mechanism

Light is the most potent stimulus for synchronizing endogenous circadian rhythms with external time. Photic clock resetting in mammals involves cAMP-responsive element binding protein (CREB)-mediated transcriptional activation of Period clock genes in the suprachiasmatic nuclei (SCN). Here we provide evidence for an additional photic input pathway to the mammalian circadian clock based on Protein Kinase C alpha (PRKCA). We found that Prkca-deficient mice show an impairment of light-mediated clock resetting. In the SCN of wild-type mice, light exposure evokes a transient interaction between PRKCA and PERIOD 2 (PER2) proteins that affects PER2 stability and nucleocytoplasmic distribution. These posttranslational events, together with CREB-mediated transcriptional regulation, are key factors in the molecular mechanism of photic clock resetting.

Circadian rhythms: mechanisms and therapeutic implications

The mammalian circadian system is organized in a hierarchical manner in that a central pacemaker in the suprachiasmatic nucleus (SCN) of the brain's hypothalamus synchronizes cellular circadian oscillators in most peripheral body cells. Fasting-feeding cycles accompanying rest-activity rhythms are the major timing cues in the synchronization of many, if not most, peripheral clocks, suggesting that the temporal coordination of metabolism and proliferation is a major task of the mammalian timing system. The inactivation of noxious food components by hepatic, intestinal, and renal detoxification systems is among the metabolic processes regulated in a circadian manner, with the understanding of the involved clock output pathways emerging. The rhythmic control of xenobiotic detoxification provides the molecular basis for the dosing time-dependence of drug toxicities and efficacy. This knowledge can in turn be used in improving or designing chronotherapeutics for the patients who suffer from many of the major human diseases.

Mutation of the circadian clock gene Per2 alters vascular endothelial function

BACKGROUND

The circadian clock regulates biological processes including cardiovascular function and metabolism. In the present study, we investigated the role of the circadian clock gene Period2 (Per2) in endothelial function in a mouse model.

METHODS AND RESULTS

Compared with the wild-type littermates, mice with Per2 mutation exhibited impaired endothelium-dependent relaxations to acetylcholine in aortic rings suspended in organ chambers. During transition from the inactive to active phase, this response was further increased in the wild-type mice but further decreased in the Per2 mutants. The endothelial dysfunction in the Per2 mutants was also observed with ionomycin, which was improved by the cyclooxygenase inhibitor indomethacin. No changes in the expression of endothelial acetylcholine-M3 receptor or endothelial nitric oxide synthase protein but increased cyclooxygenase-1 (not cyclooxygenase-2) protein levels were observed in the aortas of the Per2 mutants. Compared with Per2 mutants, a greater endothelium-dependent relaxation to ATP was observed in the wild-type mice, which was reduced by indomethacin. In quiescent aortic rings, ATP caused greater endothelium-dependent contractions in the Per2 mutants than in the wild-type mice, contractions that were abolished by indomethacin. The endothelial dysfunction in the Per2 mutant mice is not associated with hypertension or dyslipidemia.

CONCLUSIONS

Mutation in the Per2 gene in mice is associated with aortic endothelial dysfunction involving decreased production of NO and vasodilatory prostaglandin(s) and increased release of cyclooxygenase-1-derived vasoconstrictor(s). The results suggest an important role of the Per2 gene in maintenance of normal cardiovascular functions.

Possible evidence for morning and evening oscillators in Drosophila melanogaster populations selected for early and late adult emergence

In this paper, we report the results of our study aimed at a systematic analysis of the circadian phenotypes of fruit flies Drosophila melanogaster selected for early and late adult emergence, in light of the "morning and evening oscillator" (M and E) model for circadian clocks. We monitored adult emergence and activity/rest rhythms in these flies under light/dark (LD) cycles with short (8:16 h), normal (12:12 h) and long (16:8 h) photoperiods, as well as under constant darkness (DD). Across all the three LD cycles, the early populations displayed a morning phenotype with peak of emergence and activity occurring earlier than the controls and greater anticipation to "lights-on" and weak anticipation to "lights-off", while the late populations showed an evening phenotype with peak of emergence and activity occurring later than the controls and greater anticipation to lights-off and weak anticipation to lights-on. The gate of adult emergence and duration of activity in the early populations was narrower than the controls, while those of the late populations were wider than the controls. In addition, the circadian periodicities of adult emergence and activity/rest rhythms of the early flies were significantly shorter than the controls, while those of the late flies were significantly longer than the controls. In summary, the circadian phenotypes indicate that the early populations have evolved a dominant M oscillator, while the late populations have evolved a dominant E oscillator, thus providing an empirical support for the M and E model in Drosophila.

LARK activates posttranscriptional expression of an essential mammalian clock protein, PERIOD1

The mammalian molecular clock is composed of feedback loops to keep circadian 24-h rhythms. Although much focus has been on transcriptional regulation, it is clear that posttranscriptional controls also play important roles in molecular circadian clocks. In this study, we found that mouse LARK (mLARK), an RNA binding protein, activates the posttranscriptional expression of the mouse Period1 (mPer1) mRNA. A strong circadian cycling of the mLARK protein is observed in the suprachiasmatic nuclei with a phase similar to that of mPER1, although the level of the Lark transcripts are not rhythmic. We demonstrate that LARK causes increased mPER1 protein levels, most likely through translational regulation and that the LARK1 protein binds directly to a cis element in the 3' UTR of the mPer1 mRNA. Alterations of mLark expression in cycling cells caused significant changes in circadian period, with mLark knockdown by siRNA resulting in a shorter circadian period, and the overexpression of mLARK1 resulting in a lengthened period. These data indicate that mLARKs are novel posttranscriptional regulators of mammalian circadian clocks.

Clock gene expression in the retina of melatonin-proficient (C3H) and melatonin-deficient (C57BL) mice

In several mammalian species, the retina contains an autonomous circadian clock and is capable of synthesizing melatonin. The function of circadian clocks depends on interlocking transcriptional/translational feedback loops involving several clock genes. Here we investigated the expression of two clock genes (Per1, Cry2) and the level of phosphorylated (p) cyclic AMP response element binding protein (CREB) in retinae of melatonin-deficient (C57BL) with an intact retina and melatonin-proficient (C3H) mice with degenerated outer nuclear layer. RNase protection assay and in situ hybridization revealed in both strains a rhythm in transcript levels for Per1 with a peak at zeitgeber time (ZT) 08, but not for Cry2. Immunoreactions for PER1, CRY2 and pCREB were localized to the nuclei of cells in the inner nuclear layer (INL) and ganglion cell layer (GC) of both strains and to the outer nuclear layer of C57BL. In C3H, protein levels of PER1 and CRY2 followed a clear day/night rhythm in the INL and the GC with a peak at the end of the day (ZT14). pCREB levels peaked at the beginning of the day. Noteably, in melatonin-deficient C57BL mice, protein levels of PER1, CRY2 and pCREB did not show significant changes over a 16L/8D cycle. These data suggest that melatonin influences PER1 and CRY2 protein levels via post-transcriptional mechanisms and also plays a role in rhythmic regulation of pCREB levels in the mammalian retina.

Quantitative analyses of circadian gene expression in mammalian cell cultures

The central circadian pacemaker is located in the hypothalamus of mammals, but essentially the same oscillating system operates in peripheral tissues and even in immortalized cell lines. Using luciferase reporters that allow automated monitoring of circadian gene expression in mammalian fibroblasts, we report the collection and analysis of precise rhythmic data from these cells. We use these methods to analyze signaling pathways of peripheral tissues by studying the responses of Rat-1 fibroblasts to ten different compounds. To quantify these rhythms, which show significant variation and large non-stationarities (damping and baseline drifting), we developed a new fast Fourier transform–nonlinear least squares analysis procedure that specifically optimizes the quantification of amplitude for circadian rhythm data. This enhanced analysis method successfully distinguishes among the ten signaling compounds for their rhythm-inducing properties. We pursued detailed analyses of the responses to two of these compounds that induced the highest amplitude rhythms in fibroblasts, forskolin (an activator of adenylyl cyclase), and dexamethasone (an agonist of glucocorticoid receptors). Our quantitative analyses clearly indicate that the synchronization mechanisms by the cAMP and glucocorticoid pathways are different, implying that actions of different genes stimulated by these pathways lead to distinctive programs of circadian synchronization.

The circadian biological clock controls the adaptation of animals and plants to the daily environmental cycle of light and darkness. As such, this clock is responsible for jet lag and has consequences for mental health (e.g., depression), physical health (e.g., athletic performance and the timing of heart attacks), and social issues (e.g., shift work). The central circadian pacemaker is located in the hypothalamus of the mammalian brain, but essentially the same oscillating system operates in nonneural tissues. Using luciferase, an enzyme that emits light, the authors could monitor circadian gene expression in mammalian fibroblasts via luminescence emission that is controlled by the biological clock. Using this method, they report the collection and analysis of precise rhythmic data from these cells. These methods were used to analyze signaling pathways by studying the responses of fibroblasts to a variety of different treatments, including drugs, growth factors, and serum. The authors developed a new analysis procedure that specifically optimizes the quantification of amplitude for cyclic data to analyze these rhythms. This enhanced analysis method successfully distinguishes among the various signaling treatments for their rhythm inducing properties. The quantitative analyses clearly indicate that the synchronization mechanisms by the cyclic AMP and glucocorticoid pathways are different. Therefore, these pathways lead to distinctive programs of circadian synchronization.

Lack of food anticipation in Per2 mutant mice

Predicting time of food availability is key for survival in most animals. Under restricted feeding conditions, this prediction is manifested in anticipatory bouts of locomotor activity and body temperature. This process seems to be driven by a food-entrainable oscillator independent of the main, light-entrainable clock located in the suprachiasmatic nucleus (SCN) of the hypothalamus . Although the SCN clockwork involves self-sustaining transcriptional and translational feedback loops based on rhythmic expression of mRNA and proteins of clock genes , the molecular mechanisms responsible for food anticipation are not well understood. Period genes Per1 and Per2 are crucial for the SCN's resetting to light . Here, we investigated the role of these genes in circadian anticipatory behavior by studying rest-activity and body-temperature rhythms of Per1 and Per2 mutant mice under restricted feeding conditions. We also monitored expression of clock genes in the SCN and peripheral tissues. Whereas wild-type and Per1 mutant mice expressed regular food-anticipatory activity, Per2 mutant mice did not show food anticipation. In peripheral tissues, however, phase shifts of clock-gene expression in response to timed food restriction were comparable in all genotypes. In conclusion, a mutation in Per2 abolishes anticipation of mealtime, without interfering with peripheral synchronization by feeding cycles.

Light and GABAAreceptor activation alterPeriodmRNA levels in the SCN of diurnal Nile grass rats

We examined Period (Per) mRNA rhythms in the suprachiasmatic nucleus (SCN) of a diurnal rodent and assessed how phase-shifting stimuli acutely affect SCN Per mRNA using semiquantitative in situ hybridization. First, Per1 and Per2 varied rhythmically in the SCN over the course of one circadian cycle in constant darkness: Per1 mRNA was highest in the early to mid-subjective day, while Per2 mRNA levels peaked in the late subjective day. Second, acute light exposure in the early subjective night significantly increased both Per1 and Per2 mRNA. Third, Per2 but not Per1 levels decreased 1 and 2 h after injection of the gamma-aminobutyric acid (GABA)(A) receptor agonist muscimol into the SCN during the subjective day. Fourth, muscimol also reduced the light-induced Per2 in the early subjective night, but Per1 induction by light was not significantly affected. Consistent with previous studies, these data demonstrate that diurnal and nocturnal animals show very similar daily patterns of Per mRNA and light-induced Per increases in the SCN. As with light, muscimol alters circadian phase, and daytime phase alterations induced by muscimol are associated with significant decreases in Per2 mRNA. In diurnal animals, muscimol-induced decreases in Per are associated with phase delays rather than advances. The direction of the daytime phase shift may be determined by the relative suppression of Per1 vs. Per2 in SCN cells. As in nocturnal animals, changes in Per1 and Per2 mRNA by photic and non-photic stimuli appear to be associated with circadian phase alteration.

DARPP-32 involvement in the photic pathway of the circadian system

The multifunctional regulator of protein kinases and phosphatases dopamine- and cAMP-regulated phosphoprotein of 32 kDa (DARPP-32) is an important molecular target of the dopamine signaling pathway. In the present study, we investigated the possible involvement of DARPP-32 regulation in the circadian system using DARPP-32 knock-out (KO) mice. These mice showed normal entrainment to a 12 h light/dark cycle and free run in constant darkness with a period similar to that of wild-type controls. After light exposure, however, the behavioral phase-delay response and the expression of light-induced clock gene mPer2 were attenuated in the DARPP-32 KO mice. Attenuated phase delays were also seen in animals bearing a point mutation in DARPP-32 at the PKA (Thr34) but not at the casein kinase I (Ser130) phosphorylation site. We next examined DARPP-32 expression in the retina and intergeniculate leaflet (IGL), both of which convey light information to the suprachiasmatic nucleus (SCN), the locus of a master circadian clock, and in the SCN itself. DARPP-32 was expressed in the retina but not in the IGL or the SCN. The results indicate that DARPP-32 is involved in the retinal pathway transmitting photic information that resets the circadian clock.

Evidence supporting a circadian control of natural killer cell function

Natural killer (NK) cells participate in the immune response against infection and cancer. An emerging body of epidemiological data supports that circadian homeostasis may constitute a factor risk for cancer development. Physiological rhythms under circadian control persist in the absence of light entrainment and ultimately rely on a molecular clock. We have previously shown that NK cell cytolytic activity follows a daily rhythm and that NK cells enriched from light-entrained rats present 24-h oscillations of clock genes, cytolytic factors, and cytokines. To investigate whether these oscillations are under a genuine circadian control, we assessed the daily expression of clock genes (Per1, Per2, Clock, and Bmal1), a clock-controlled gene (Dbp), cytolytic factors (granzyme B and perforin), and cytokines (IFN-gamma and TNF-alpha) in NK cells enriched from rats maintained in constant darkness (DD). In addition, we investigated whether the disruption of the NK cell clock by RNA interference (RNAi) affects the expression of cytolytic factors and cytokines. Persistent 24-h oscillations were found in the expression levels of clock genes, cytolytic factors, and cytokines in NK cells enriched from DD rats. In addition, RNAi-mediated Per2 knockdown caused a significant decrease of granzyme B and perforin levels in the rat derived NK cell line RNK16. Taken together, these results provide evidence supporting that NK cell function is under circadian regulation.

Design principles underlying circadian clocks

A fundamental problem for regulatory networks is to understand the relation between form and function: to uncover the underlying design principles of the network. Circadian clocks present a particularly interesting instance, as recent work has shown that they have complex structures involving multiple interconnected feedback loops with both positive and negative feedback. While several authors have speculated on the reasons for this, a convincing explanation is still lacking. We analyse both the flexibility of clock networks and the relationships between various desirable properties such as robust entrainment, temperature compensation, and stability to environmental variations and parameter fluctuations. We use this to argue that the complexity provides the flexibility necessary to simultaneously attain multiple key properties of circadian clocks. As part of our analysis we show how to quantify the key evolutionary aims using infinitesimal response curves, a tool that we believe will be of general utility in the analysis of regulatory networks. Our results suggest that regulatory and signalling networks might be much less flexible and of lower dimension than their apparent complexity would suggest.

Diurnal rhythmicity of the clock genes Per1 and Per2 in the rat ovary

Circadian rhythms are generated by endogenous clocks in the central brain oscillator, the suprachiasmatic nucleus, and peripheral tissues. The molecular basis for the circadian clock consists of a number of genes and proteins that form transcriptional/translational feedback loops. In the mammalian gonads, clock genes have been reported in the testes, but the expression pattern is developmental rather than circadian. Here we investigated the daily expression of the two core clock genes, Per1 and Per2, in the rat ovary using real-time RT-PCR, in situ hybridization histochemistry, and immunohistochemistry. Both Per1 and Per2 mRNA displayed a statistically significant rhythmic oscillation in the ovary with a period of 24 h in: 1) a group of rats during proestrus and estrus under 12-h light,12-h dark cycles; 2) a second group of rats representing a mixture of all 4 d of the estrous cycle under 12-h light,12-h dark conditions; and 3) a third group of rats representing a mixture of all 4 d of estrous cycle during continuous darkness. Per1 mRNA was low at Zeitgeber time 0-2 and peaked at Zeitgeber time 12-14, whereas Per2 mRNA was delayed by approximately 4 h relative to Per1. By in situ hybridization histochemistry, Per mRNAs were localized to steroidogenic cells in preantral, antral, and preovulatory follicles; corpora lutea; and interstitial glandular tissue. With newly developed antisera, we substantiated the expression of Per1 and Per2 in these cells by single/double immunohistochemistry. Furthermore, we visualized the temporal intracellular movements of PER1 and PER2 proteins. These findings suggest the existence of an ovarian circadian clock, which may play a role both locally and in the hypothalamo-pituitary-ovarian axis.

Essential Role of 3′-Untranslated Region-mediated mRNA Decay in Circadian Oscillations of Mouse Period3 mRNA

Daily oscillations in mRNA levels are a general feature of most clock genes. Although mRNA oscillations largely depend on transcriptional regulation, it has been suggested that post-transcriptional controls also contribute to mRNA oscillations in Drosophila. Currently, however, there is no direct evidence for post-transcriptional regulation of mammalian clock genes. To investigate the roles of post-transcriptional regulations, we focused on the 3'-untranslated region (3'-UTR) of mouse Period3 (mPer3) mRNA, one of the clock genes. Insertion of the entire mPer3 3'-UTR downstream of a reporter gene resulted in a dramatic decrease in mRNA stability. Deletion and point mutation analyses led to the identification of critical sequences responsible for mRNA decay. To explore the effects of the mPer3 3'-UTR-mediated mRNA decay on circadian oscillations, we established NIH3T3 stable cell lines that express luciferase mRNA with wild-type or mutant mPer3 3'-UTR. Interestingly, a stabilizing mutation of 3'-UTR induced a significant alteration in the oscillation profile of luciferase mRNA. Above all, the peak time, during which the mRNAs reached their highest levels, was significantly delayed (for 12 h). In addition, the luciferase mRNA level with mutant 3'-UTR began to increase earlier than that in the presence of wild-type 3'-UTR. Consequently, luciferase mRNA with mutant 3'-UTR displayed oscillation patterns with a prolonged rising phase. Our results indicate that mPer3 3'-UTR-mediated mRNA decay plays an essential role in mRNA cycling and provide direct evidence for post-transcriptional control of circadian mRNA oscillations.

The mouse Clock mutation reduces circadian pacemaker amplitude and enhances efficacy of resetting stimuli and phase-response curve amplitude

The mouse Clock gene encodes a basic helix-loop-helix-PAS transcription factor, CLOCK, that acts in concert with BMAL1 to form the positive elements of the circadian clock mechanism in mammals. The original Clock mutant allele is a dominant negative (antimorphic) mutation that deletes exon 19 and causes an internal deletion of 51 aa in the C-terminal activation domain of the CLOCK protein. Here we report that heterozygous Clock/+ mice exhibit high-amplitude phase-resetting responses to 6-h light pulses (Type 0 resetting) as compared with wild-type mice that have low amplitude (Type 1) phase resetting. The magnitude and time course of acute light induction in the suprachiasmatic nuclei of the only known light-induced core clock genes, Per1 and Per2, are not affected by the Clock/+ mutation. However, the amplitude of the circadian rhythms of Per gene expression are significantly reduced in Clock homozygous and heterozygous mutants. Rhythms of PER2::LUCIFERASE expression in suprachiasmatic nuclei explant cultures also are reduced in amplitude in Clock heterozygotes. The phase-response curves to changes in culture medium are Type 0 in Clock heterozygotes, but Type 1 in wild types, similar to that seen for light in vivo. The increased efficacy of resetting stimuli and decreased PER expression amplitude can be explained in a unified manner by a model in which the Clock mutation reduces circadian pacemaker amplitude in the suprachiasmatic nuclei.

Differential sorting of the vesicular glutamate transporter 1 into a defined vesicular pool is regulated by light signaling involving the clock gene Period2

Synaptic strength depends on the amount of neurotransmitter stored in synaptic vesicles. The vesicular transmitter content has recently been shown to be directly dependent on the expression levels of vesicular neurotransmitter transporters indicating that the transport capacity of synaptic vesicles is a critical determinant for synaptic efficacy. Using synaptic vesicles prepared from whole brain at different times of the day we now show that the amount of vesicular glutamate transporter (VGLUT) 1 undergoes strong diurnal cycling. VGLUT1 protein levels are high before the start of the light period, decline at noon, increase again before start of the dark period, and decline again at midnight. Mice kept in complete darkness showed within a 24-h period only a single peak of VGLUT1 expression in the middle of the rest phase. In contrast, mice lacking the period gene Period 2, a core component of the circadian clock, did not show any light-cycle-dependent changes of VGLUT1 levels. No other of several synaptic vesicle proteins examined underwent circadian cycling. Circadian cycling of VGLUT1 was not seen when analyzing homogenate or synaptosomes, the starting fraction for vesicle preparation. Circadian cycling of VGLUT1 was also not reflected at the mRNA level. We conclude that nerve terminals are endowed with mechanisms that regulate quantal size by changing the copy number of transporters in synaptic vesicles. A reduced amount of VGLUT1 per vesicle is probably achieved by means of selective sorting controlled by clock genes.

Mouse Period1 (mPER1) acts as a circadian adaptor to entrain the oscillator to environmental light/dark cycles by regulating mPER2 protein

Mouse period1 (mPer1) and mPer2 are mammalian homologs of the Drosophila clock gene period that show robust oscillation in the suprachiasmatic nucleus, the mammalian master clock, and have been implicated as essential components of the core clock mechanism. Gene-targeting studies have demonstrated that mPer2 plays a dominant function in behavioral rhythm generation, although the role of mPer1 has not been fully clarified. Here, we report that prolongation of the lighting period (4-16 h) induces a larger-delay phase shift of the behavioral rhythm in mPer1-deficient (mPer1-/-) mice. During the light-elongation task, mPER2 protein decay in mPer1-/- mice is slower (approximately 4 h) than in wild-type mice, which thereby causes larger behavioral phase delay. mPer1-/- mice could not adapt to environmental light/dark cycles in long complete photoperiods with dim light or in long skeleton photoperiods. These photoperiodic conditions mimic natural environmental changes present at high latitudes, indicating that mPer1 could operate in the adaptation of the circadian clock of nocturnal mice to large seasonal changes of environmental light/dark cycles.

Circadian time-keeping during early stages of development

The zebrafish pineal gland is a photoreceptive organ containing an intrinsic central circadian oscillator, which drives daily rhythms of gene expression and the melatonin hormonal signal. Here we investigated the effect of light, given at early developmental stages before pineal gland formation, on the pineal circadian oscillator. Embryos that were exposed to light at 0-6, 10-13, or 10-16 h after fertilization exhibited clock-controlled rhythms of arylalkylamine-N-acetyltransferase (zfaanat2) mRNA in the pineal gland during the third and fourth day of development. This rhythm was absent in embryos that were placed in continuous dark within 2 h after fertilization (before blastula stage). Differences in the phases of these rhythms indicate that they are determined by the time of illumination. Light treatments at these stages also caused a transient increase in period2 mRNA levels, and the development of zfaanat2 mRNA rhythm was abolished by PERIOD2 knock-down. These results indicate that light exposure at early developmental stages, and light-induced expression of period2, are both required for setting the phase of the circadian clock. The 24-h rhythm is then maintained throughout rapid proliferation and, remarkably, differentiation.

Light stimulates MSK1 activation in the suprachiasmatic nucleus via a PACAP-ERK/MAP kinase-dependent mechanism

Signaling via the p42/44 mitogen-activated protein kinase (MAPK) pathway has been shown to be a key intracellular signaling event that couples light to entrainment of the mammalian circadian clock located in the suprachiasmatic nucleus (SCN). Because many of the physiological effects of the MAPK pathway are mediated by extracellular signal-regulated kinase (ERK)-regulated kinases, it was of interest to identify kinase targets of ERK in the SCN. In this study, we examined whether mitogen- and stress-activated protein kinase 1 (MSK1) is a downstream target of ERK in the SCN and whether it couples to clock gene expression. Here we show that photic stimulation during the subjective night stimulates MSK1 phosphorylation at serine 360, an event required for robust kinase activation. Activated ERK and MSK1 were colocalized in SCN cell nuclei after photic stimulation. The in vivo administration of the MAP kinase kinase 1/2 inhibitor U0126 [1,4-diamino-2,3-dicyano-1,4-bis(o-aminophenylmercapto) butadiene] attenuated MSK1 phosphorylation. MSK1 phosphorylation was more responsive to late-night than early-night photic stimulation, indicating that MSK1 may differentially contribute to light-induced phase advancing and phase delaying of the clock. The potential connection between pituitary adenylate cyclase-activating polypeptide (PACAP) (a regulator of clock entrainment) and MSK1 phosphorylation was examined. PACAP infusion stimulated MSK1 phosphorylation, whereas PACAP receptor antagonist infusion attenuated light-induced MSK1 phosphorylation in the SCN. In reporter gene assays, MSK1 was shown to couple to mPeriod1 via a cAMP response element-binding protein-dependent mechanism. Together, these data identify MSK1 as both a downstream target of the MAPK cascade within the SCN and a regulator of clock gene expression.

Constitutive expression of the Period1 gene impairs behavioral and molecular circadian rhythms

Three mammalian Period (Per) genes, termed Per1, Per2, and Per3, have been identified as structural homologues of the Drosophila circadian clock gene, period (per). The three Per genes are rhythmically expressed in the suprachiasmatic nucleus (SCN), the central circadian pacemaker in mammals. The phases of peak mRNA levels for the three Per genes in the SCN are slightly different. Light sequentially induces the transcripts of Per1 and Per2 but not of Per3 in mice. These data and others suggest that each Per gene has a different but partially redundant function in mammals. To elucidate the function of Per1 in the circadian system in vivo, we generated two transgenic rat lines in which the mouse Per1 (mPer1) transcript was constitutively expressed under the control of either the human elongation factor-1alpha (EF-1alpha) or the rat neuron-specific enolase (NSE) promoter. The transgenic rats exhibited an approximately 0.6-1.0-h longer circadian period than their wild-type siblings in both activity and body temperature rhythms. Entrainment in response to light cycles was dramatically impaired in the transgenic rats. Molecular analysis revealed that the amplitudes of oscillation in the rat Per1 (rPer1) and rat Per2 (rPer2) mRNAs were significantly attenuated in the SCN and eyes of the transgenic rats. These results indicate that either the level of Per1, which is raised by overexpression, or its rhythmic expression, which is damped or abolished in over expressing animals, is critical for normal entrainment of behavior and molecular oscillation of other clock genes.