The mPer2 gene encodes a functional component of the mammalian circadian clock

Differential daily expression of Per1 and Per2 mRNA in the suprachiasmatic nucleus of fetal and early postnatal mice

It is well known that there are circadian rhythms of 2-deoxyglucose uptake and neuronal firing in the rat suprachiasmatic nucleus (SCN) during fetal and early postnatal periods. A core clock mechanism in the mouse SCN appears to involve a transcriptional feedback loop in which CLOCK and BMAL1 function as positive regulators and three mPeriod (mPer) genes play a role in negative feedback. Per genes expression occurs not only in the adult SCN but also in the fetal SCN. However, the developmental change in these genes remains unclear. In this experiment, we examined the day--night pattern of expression of Per1 and Per2 mRNA in the mouse SCN and cerebral cortex on embryonic day 17, postnatal day 3, and in young adult mice under a light-dark cycle. Daily rhythms of mRNA content were observed in mPer1 but not mPer2 in the fetal SCN. Interestingly, the expression of mPer2 in the SCN was high throughout the entire day, and a significant daily rhythm of this gene was observed on postnatal day 6. The expression pattern of SCN mPer1 in constant darkness was similar to that seen in the light-dark cycle. The present results suggest that the daily oscillation of mPer1 but not of mPer2 in the SCN in fetal and early postnatal mice may be associated with the daily rhythms of 2-deoxyglucose uptake and neuronal firing.

A mouse genetic locus with death clock and life clock features

A senility syndrome, with weight loss and priapism, occurs in CBAT6/T6 mice, an exceptionally long-lived strain. Instead of dying at the expected time, these mice get senile weight loss and priapism and go on living. We have postulated that a mutant death clock kills the wrong neurons. Crosses with the NZW and C57BL/6 strains show causation by a single genetic locus (Priap1), with a pronounced gene dosage effect on timing. We report here that various cancers were the cause of death in 31 of 32 NZW mice, compared to only five of 22 CBAT6/T6 mice, a highly significant difference (P<0.001). The longevity of (CBAT6/T6xNZW)F1 hybrids, and the segregation of longevity with priapism and senile weight loss in (CBAT6/T6xNZW) F2 hybrids, indicates that Priap1, or a linked gene, inhibits the cancers that usually shorten the lives of NZW mice. If a timer gene is involved, the cancer resistance action could be because the locus impedes the normal mid-life regression of anti-cancer defence. The priapism suggests loss of the medullary reticular formation neurons which normally inhibit male spinal sexual reflexes. In this region of the medulla there are also the respiratory and cardiac control centres, where apoptotic neuron destruction by the wild-type locus could govern maximal life-span. The CBAT6/T6 locus may be a mutant life-stage control clock. Its discovery could be the revelation of a new, major class of aetiology of disease.

Mop3 is an essential component of the master circadian pacemaker in mammals

Circadian oscillations in mammalian physiology and behavior are regulated by an endogenous biological clock. Here we show that loss of the PAS protein MOP3 (also known as BMAL1) in mice results in immediate and complete loss of circadian rhythmicity in constant darkness. Additionally, locomotor activity in light-dark (LD) cycles is impaired and activity levels are reduced in Mop3-/- mice. Analysis of Period gene expression in the suprachiasmatic nucleus (SCN) indicates that these behavioral phenotypes arise from loss of circadian function at the molecular level. These results provide genetic evidence that MOP3 is the bona fide heterodimeric partner of mCLOCK. Furthermore, these data demonstrate that MOP3 is a nonredundant and essential component of the circadian pacemaker in mammals.

Functional redundancy of cryptochromes and classical photoreceptors for nonvisual ocular photoreception in mice

The daily light-dark (LD) cycle exerts a powerful influence on the temporal organization of behavior and physiology. Much of this influence is preserved in behaviorally blind retinally degenerate mice; the photoreceptors underlying this nonvisual phototransduction are unknown. The mammalian eye contains at least two classes of photoactive pigments, the vitamin A-based opsins and the vitamin B(2)-based cryptochromes. To genetically define the roles of these pigments in light modulation of behavior, we generated rd/rd;mCry1(-)/mCry1(-);mCry2(-)/mCry2(-) mutant mice lacking rods and most cones as well as both cryptochrome proteins. The response of the mutant mouse to photic input was analyzed at both behavioral and molecular levels. Behaviorally, mice lacking either classical photoreceptors or cryptochromes exhibited strongly rhythmic locomotor responses to 10 and 100 lux daily LD 12 h/12-h cycles; however, triple mutant mice carrying both cryptochrome and retinal degenerate mutations were nearly arrhythmic under both LD cycles and in constant darkness. At the molecular level, the light induction of c-fos transcription in the suprachiasmatic nucleus was markedly reduced in the triple mutant mouse compared with either rd/rd or cryptochrome mutant mice. These data indicate that classical opsins and cryptochromes serve functionally redundant roles in the transduction of light information to behavioral modulation and suggest a pleomorphic role for cryptochromes in both photoreception and central clock mechanism.

Phase-dependent responses of Per1 and Per2 genes to a light-stimulus in the suprachiasmatic nucleus of the rat

Single brief and discrete light treatments are sufficient to reset the overt mammalian rhythms of nocturnal rodents. In the present study, we examined the phase-dependent response of the mammalian clock genes, Per1 and Per2, to a brief strong light-stimulus (1000 lux) in the circadian oscillator center, the suprachiasmatic nucleus (SCN) of rats. Light-induced elevation of Per1 mRNA was observed through the subjective night (CT16, CT20 and CT0 (=CT24)) with a marked peak at the subjective dawn (CT0). However, the light influence was very limited for the induction of Per2; only weak elevation of Per2 mRNA was detected at CT16. The effect of light-stimulus on the Per1 gene was transient, and the effect was restricted to ventrolateral SCN neurons in both CT0 and CT16 after light exposure. Since it is known that these rats show a light-induced behavioral phase-shift throughout the subjective night with being strongest at subjective dawn, the present results suggest that the transient induction of Per1 in ventrolateral SCN neurons is a critical step in the resetting of the biological clock to environmental light-dark schedule.

Antiphase oscillation of the left and right suprachiasmatic nuclei

An unusual property of the circadian timekeeping systems of animals is rhythm "splitting," in which a single daily period of physical activity (usually measured as wheel running) dissociates into two stably coupled components about 12 hours apart; this behavior has been ascribed to a clock composed of two circadian oscillators cycling in antiphase. We analyzed gene expression in the hypothalamic circadian clock, the suprachiasmatic nucleus (SCN), of behaviorally "split" hamsters housed in constant light. The results show that the two oscillators underlying the split condition correspond to the left and right sides of the bilaterally paired SCN.

Conditional disruption of the aryl hydrocarbon receptor nuclear translocator (Arnt) gene leads to loss of target gene induction by the aryl hydrocarbon receptor and hypoxia-inducible factor 1alpha

To determine the function of the aryl hydrocarbon receptor nuclear translocator (ARNT), a conditional gene knockout mouse was made using the Cre-loxP system. Exon 6, encoding the conserved basic-helix-loop-helix domain of the protein, was flanked by loxP sites and introduced into the Arnt gene by standard gene disruption techniques using embryonic stem cells. Mice homozygous for the floxed allele were viable and had no readily observable phenotype. The Mx1-Cre transgene, in which Cre is under control of the interferon-gamma promoter, was introduced into the Arnt-floxed mouse line. Treatment with polyinosinic-polycytidylic acid to induce expression of Cre resulted in complete disruption of the Arnt gene and loss of ARNT messenger RNA (mRNA) expression in liver. To determine the role of ARNT in gene control in the intact animal mouse liver, expression of target genes under control of an ARNT dimerization partner, the aryl hydrocarbon receptor (AHR), was monitored. Induction of CYP1A1, CYP1A2, and UGT1*06 mRNAs by the AHR ligand 2,3,7,8-tetrachlorodibenzo-p-dioxin was absent in livers of Arnt-floxed/Mx1-Cre mice treated with polyinosinic-polycytidylic. These data demonstrate that ARNT is required for AHR function in the intact animal. Partial deletion of the Arnt allele was found in kidney, heart, intestine, and lung. Despite more than 80% loss of the ARNT expression in lung, maximal induction of CYP1A1 was found, indicating that the expression level of ARNT is not limiting to AHR signaling. Cobalt chloride induction of the glucose transporter-1 and heme oxygenase-1 mRNAs was also markedly abrogated in mice lacking ARNT expression, suggesting an inhibition of HIF-1alpha activity. These studies establish a critical role for ARNT in AHR and HIF-1alpha signal transduction in the intact mouse.

Molecular genetics of circadian rhythms in mammals

Recent gene discovery approaches have led to a new era in our understanding of the molecular basis of circadian oscillators in animals. A conserved set of genes in Drosophila and mammals (Clock, Bmal1, Period, and Timeless) provide a molecular framework for the circadian mechanism. These genes define a transcription-translation-based negative autoregulatory feedback loop that comprises the core elements generating circadian rhythmicity. This circadian core provides a focal point for understanding how circadian rhythms arise, how environmental inputs entrain the oscillatory system, and how the circadian system regulates its outputs. The addition of molecular genetic approaches to the existing physiological understanding of the mammalian circadian system provides new opportunities for understanding this basic life process.

Circadian rhythm genetics: from flies to mice to humans

A successful genetic dissection of the circadian regulation of behaviour has been achieved through phenotype-driven mutagenesis screens in flies and mice. Cloning and biochemical analysis of these evolutionarily conserved proteins has led to detailed molecular insight into the clock mechanism. Few behaviours enjoy the degree of understanding that exists for circadian rhythms at the genetic, cellular and anatomical levels. The circadian clock has so eagerly spilled her secrets that we may soon know the unbroken chain of events from gene to behaviour. It will likely be fruitful to wield this uncommon degree of knowledge to attack one of the most challenging problems in genetics: the basis of complex human behavioural disorders. We review here the genetic screens that provided the entreé into the heart of the circadian clock, the model of the clock mechanism that has resulted, and the prospects for using the homologues as candidate genes in studies of human circadian dysrhythmias.

Targeted disruption of the mPer3 gene: subtle effects on circadian clock function

Neurons in the mammalian suprachiasmatic nucleus (SCN) contain a cell-autonomous circadian clock that is based on a transcriptional-translational feedback loop. The basic helix-loop-helix-PAS proteins CLOCK and BMAL1 are positive regulators and drive the expression of the negative regulators CRY1 and CRY2, as well as PER1, PER2, and PER3. To assess the role of mouse PER3 (mPER3) in the circadian timing system, we generated mice with a targeted disruption of the mPer3 gene. Western blot analysis confirmed the absence of mPER3-immunoreactive proteins in mice homozygous for the targeted allele. mPer1, mPer2, mCry1, and Bmal1 RNA rhythms in the SCN did not differ between mPER3-deficient and wild-type mice. Rhythmic expression of mPer1 and mPer2 RNAs in skeletal muscle also did not differ between mPER3-deficient and wild-type mice. mPer3 transcripts were rhythmically expressed in the SCN and skeletal muscle of mice homozygous for the targeted allele, but the level of expression of the mutant transcript was lower than that in wild-type controls. Locomotor activity rhythms in mPER3-deficient mice were grossly normal, but the circadian cycle length was significantly (0.5 h) shorter than that in controls. The results demonstrate that mPer3 is not necessary for circadian rhythms in mice.

Cloning and characterization of rat casein kinase 1epsilon

Genes differentially expressed in the subjective day and night in the rat suprachiasmatic nucleus (SCN) were surveyed by differential display. A gene homologous to human casein kinase 1epsilon (CK1epsilon) was isolated, which initially appeared to be expressed in the suprachiasmatic nucleus (SCN) in a circadian manner. We here describe the cDNA cloning of the rat CK1epsilon and characterization of the protein products. The rCK1epsilon is predominantly expressed in the brain including the SCN, binds and phosphorylates mPer1, mPer2, and mPer3 in vitro, and translocates mPer1 and mPer3, but not mPer2, to the cell nucleus depending on its kinase activity when coexpressed with these Per proteins in COS-7 cells.

An inherited functional circadian clock in zebrafish embryos

Circadian clocks are time-keeping systems found in most organisms. In zebrafish, expression of the clock gene Period3 (Per3) oscillates throughout embryogenesis in the central nervous system and the retina. Per3 rhythmic expression was free-running and was reset by light but not by the developmental delays caused by low temperature. The time of fertilization had no effect on Per3 expression. Per3 messenger RNA accumulates rhythmically in oocytes and persists in embryos. Our results establish that the circadian clock functions during early embryogenesis in zebrafish. Inheritance of maternal clock gene products suggests a mechanism of phase inheritance through ovogenesis.

Nuclear entry of the circadian regulator mPER1 is controlled by mammalian casein kinase I epsilon

The molecular oscillator that keeps circadian time is generated by a negative feedback loop. Nuclear entry of circadian regulatory proteins that inhibit transcription from E-box-containing promoters appears to be a critical component of this loop in both Drosophila and mammals. The Drosophila double-time gene product, a casein kinase I epsilon (CKIepsilon) homolog, has been reported to interact with dPER and regulate circadian cycle length. We find that mammalian CKIepsilon binds to and phosphorylates the murine circadian regulator mPER1. Unlike both dPER and mPER2, mPER1 expressed alone in HEK 293 cells is predominantly a nuclear protein. Two distinct mechanisms appear to retard mPER1 nuclear entry. First, coexpression of mPER2 leads to mPER1-mPER2 heterodimer formation and cytoplasmic colocalization. Second, coexpression of CKIepsilon leads to masking of the mPER1 nuclear localization signal and phosphorylation-dependent cytoplasmic retention of both proteins. CKIepsilon may regulate mammalian circadian rhythm by controlling the rate at which mPER1 enters the nucleus.

Involvement of the MAP kinase cascade in resetting of the mammalian circadian clock

Although the suprachiasmatic nucleus (SCN) is the major pacemaker in mammals, the peripheral cells or immortalized cells also contain a circadian clock. The SCN and the periphery may use different entraining signals—light and some humoral factors, respectively. We show that induction of the circadian oscillation of gene expression is triggered by TPA treatment of NIH-3T3 fibroblasts, which is inhibited by a MEK inhibitor, and that prolonged activation of the MAPK cascade is sufficient to trigger circadian gene expression. Therefore, such prolonged activation of MAPK by entraining cues may be involved in the resetting of the circadian clock.

CLOCK, an essential pacemaker component, controls expression of the circadian transcription factor DBP

DBP, the founding member of the PAR leucine zipper transcription factor family, is expressed according to a robust daily rhythm in the suprachiasmatic nucleus and several peripheral tissues. Previous studies with mice deleted for the Dbp gene have established that DBP participates in the regulation of several clock outputs, including locomotor activity, sleep distribution, and liver gene expression. Here we present evidence that circadian Dbptranscription requires the basic helix–loop–helix–PAS protein CLOCK, an essential component of the negative-feedback circuitry generating circadian oscillations in mammals and fruit flies. Genetic and biochemical experiments suggest that CLOCK regulates Dbpexpression by binding to E-box motifs within putative enhancer regions located in the first and second introns. Similar E-box motifs have been found previously in the promoter sequence of the murine clock genemPeriod1. Hence, the same molecular mechanisms generating circadian oscillations in the expression of clock genes may directly control the rhythmic transcription of clock output regulators such asDbp.

The transcription factor DBP affects circadian sleep consolidation and rhythmic EEG activity

Albumin D-binding protein (DBP) is a PAR leucine zipper transcription factor that is expressed according to a robust circadian rhythm in the suprachiasmatic nuclei, harboring the circadian master clock, and in most peripheral tissues. Mice lacking DBP display a shorter circadian period in locomotor activity and are less active. Thus, although DBP is not essential for circadian rhythm generation, it does modulate important clock outputs. We studied the role of DBP in the circadian and homeostatic aspects of sleep regulation by comparing DBP deficient mice (dbp-/-) with their isogenic controls (dbp+/+) under light-dark (LD) and constant-dark (DD) baseline conditions, as well as after sleep loss. Whereas total sleep duration was similar in both genotypes, the amplitude of the circadian modulation of sleep time, as well as the consolidation of sleep episodes, was reduced in dbp-/- under both LD and DD conditions. Quantitative EEG analysis demonstrated a marked reduction in the amplitude of the sleep-wake-dependent changes in slow-wave sleep delta power and an increase in hippocampal theta peak frequency in dbp-/- mice. The sleep deprivation-induced compensatory rebound of EEG delta power was similar in both genotypes. In contrast, the rebound in paradoxical sleep was significant in dbp+/+ mice only. It is concluded that the transcriptional regulatory protein DBP modulates circadian and homeostatic aspects of sleep regulation.

Circadian clocks in the mammalian brain

Daily cycles in physiology and behaviour are probably a universal feature of multicellular organisms. These rhythms are predominantly driven by endogenous clocks with a periodicity approximating to one day, i.e. circadian. In mammals, the circadian clock governing activity/ rest, neuroendocrine and autonomic rhythms lies in the hypothalamus, in the suprachiasmatic nuclei (SCN). Intrinsic circadian oscillators are also present in the retina. The SCN "clockwork" is based on a cell autonomous, genetically determined mechanism. Mammalian homologues of a number of Drosophila genes which encode elements of the fly circadian mechanism have recently been identified. In Drosophila, the protein products of these genes interact in a negative feedback loop, establishing a circadian cycle in gene expression. Characterisation of the roles played by putative mammalian clock genes in the SCN, and how the emergent cellular signal imposes order over the entire neuraxis, will provide a fundamental contribution to our understanding of the molecular basis of behaviour. BioEssays 22:23-31, 2000.

Rapid down-regulation of mammalian period genes during behavioral resetting of the circadian clock

The pervasive role of circadian clocks in regulating physiology and behavior is widely recognized. Their adaptive value is their ability to be entrained by environmental cues such that the internal circadian phase is a reliable predictor of solar time. In mammals, both light and nonphotic behavioral cues can entrain the principal oscillator of the hypothalamic suprachiasmatic nuclei (SCN). However, although light can advance or delay the clock during circadian night, behavioral events trigger phase advances during the subjective day, when the clock is insensitive to light. The recent identification of Period (Per) genes in mammals, homologues of dperiod, which encodes a core element of the circadian clockwork in Drosophila, now provides the opportunity to explain circadian timing and entrainment at a molecular level. In mice, expression of mPer1 and mPer2 in the SCN is rhythmic and acutely up-regulated by light. Moreover, the temporal relations between mRNA and protein cycles are consistent with a clock based on a transcriptional/translational feedback loop. Here we describe circadian oscillations of Per1 and Per2 in the SCN of the Syrian hamster, showing that PER1 protein and mRNA cycles again behave in a manner consistent with a negative-feedback oscillator. Furthermore, we demonstrate that nonphotic resetting has the opposite effect to light: acutely down-regulating these genes. Their sensitivity to nonphotic resetting cues supports their proposed role as core elements of the circadian oscillator. Moreover, this study provides an explanation at the molecular level for the contrasting but convergent effects of photic and nonphotic cues on the clock.

Light-independent role of CRY1 and CRY2 in the mammalian circadian clock

Cryptochrome (CRY), a photoreceptor for the circadian clock in Drosophila, binds to the clock component TIM in a light-dependent fashion and blocks its function. In mammals, genetic evidence suggests a role for CRYs within the clock, distinct from hypothetical photoreceptor functions. Mammalian CRY1 and CRY2 are here shown to act as light-independent inhibitors of CLOCK-BMAL1, the activator driving Per1 transcription. CRY1 or CRY2 (or both) showed light-independent interactions with CLOCK and BMAL1, as well as with PER1, PER2, and TIM. Thus, mammalian CRYs act as light-independent components of the circadian clock and probably regulate Per1 transcriptional cycling by contacting both the activator and its feedback inhibitors.

Differential regulation of mammalian period genes and circadian rhythmicity by cryptochromes 1 and 2

Cryptochromes regulate the circadian clock in animals and plants. Humans and mice have two cryptochrome (Cry) genes. A previous study showed that mice lacking the Cry2 gene had reduced sensitivity to acute light induction of the circadian gene mPer1 in the suprachiasmatic nucleus (SCN) and had an intrinsic period 1 hr longer than normal. In this study, Cry1(-/-) and Cry1(-/-)Cry2(-/-) mice were generated and their circadian clocks were analyzed at behavioral and molecular levels. Behaviorally, the Cry1(-/-) mice had a circadian period 1 hr shorter than wild type and the Cry1(-/-)Cry2(-/-) mice were arrhythmic in constant darkness (DD). Biochemically, acute light induction of mPer1 mRNA in the SCN was blunted in Cry1(-/-) and abolished in Cry1(-/-)Cry2(-/-) mice. In contrast, the acute light induction of mPer2 in the SCN was intact in Cry1(-/-) and Cry1(-/-)Cry2(-/-) animals. Importantly, in double mutants, mPer1 expression was constitutively elevated and no rhythmicity was detected in either 12-hr light/12-hr dark or DD, whereas mPer2 expression appeared rhythmic in 12-hr light/12-hr dark, but nonrhythmic in DD with intermediate levels. These results demonstrate that Cry1 and Cry2 are required for the normal expression of circadian behavioral rhythms, as well as circadian rhythms of mPer1 and mPer2 in the SCN. The differential regulation of mPer1 and mPer2 by light in Cry double mutants reveals a surprising complexity in the role of cryptochromes in mammals.