Cloning and characterization of rat casein kinase 1epsilon

The tight junction protein TJP1 regulates the feeding-modulated hepatic circadian clock

Circadian clocks in the suprachiasmatic nucleus and peripheral tissues orchestrate behavioral and physiological activities of mammals in response to environmental cues. In the liver, the circadian clock is also modulated by feeding. However, the molecular mechanisms involved are unclear. Here, we show that TJP1 (tight junction protein 1) functions as a mediator of mTOR (mechanistic target of rapamycin) to modulate the hepatic circadian clock. TJP1 interacts with PER1 (period circadian regulator 1) and prevents its nuclear translocation. During feeding, mTOR phosphorylates TJP1 and attenuates its association with PER1, thereby enhancing nuclear shuttling of PER1 to dampen circadian oscillation. Therefore, our results provide a previously uncharacterized mechanistic insight into how feeding modulates the hepatic circadian clock.

The circadian clock regulates rhythms of physiology and metabolism in response to environmental cues such as food intake. Here, the authors show that tight junction protein 1 (TJP1) interacts with period 1 and modulates its nuclear translocation in a mTOR-dependent manner.

Non-transcriptional processes in circadian rhythm generation

'Biological clocks' orchestrate mammalian biology to a daily rhythm. Whilst 'clock gene' transcriptional circuits impart rhythmic regulation to myriad cellular systems, our picture of the biochemical mechanisms that determine their circadian (∼24 hour) period is incomplete. Here we consider the evidence supporting different models for circadian rhythm generation in mammalian cells in light of evolutionary factors. We find it plausible that the circadian timekeeping mechanism in mammalian cells is primarily protein-based, signalling biological timing information to the nucleus by the post-translational regulation of transcription factor activity, with transcriptional feedback imparting robustness to the oscillation via hysteresis. We conclude by suggesting experiments that might distinguish this model from competing paradigms.

Modelling the dual role of Per phosphorylation and its effect on the period and phase of the mammalian circadian clock

Circadian clocks are regulated at the post-translational level by a variety of processes among which protein phosphorylation plays a prominent, although complex, role. Thus, the phosphorylation of different sites on the clock protein PER by casein kinase I (CKI) can lead to opposite effects on the stability of the protein and on the period of circadian oscillations. Here the authors extend a computational model previously proposed for the mammalian circadian clock by incorporating two distinct phosphorylations of PER by CKI. On the basis of experimental observations the authors consider that phosphorylation at one site (denoted here PER-P1) enhances the rate of degradation of the protein and decreases the period, while phosphorylation at another site (PER-P2) stabilises the protein, enhances the transcription of the Per gene, and increases the period. The model also incorporates an additional phosphorylation of PER by the Glycogen Synthase Kinase 3 (GSK3). The authors show that the extended model incorporating the antagonistic effects of PER phosphorylations by CKI can account for observations pertaining to (i) the decrease in period in the Tau mutant, because of an increase in phosphorylation by CKI leading to PER-P1, and (ii) the familial advanced sleep phase syndrome (FASPS) in which the period is shortened and the phase of the oscillations is advanced when the rate of phosphorylation leading to PER-P2 is decreased. The model further accounts for the increase in period observed in the presence of CKI inhibitors that decrease the rate of phosphorylation leading to both PER-P1 and PER-P2. A similar increase in period results from inhibition of GSK3. [Includes supplementary material].

Endogenous casein kinase-1 modulates NMDA receptor activity of hypothalamic presympathetic neurons and sympathetic outflow in hypertension

KEY POINTS

Increased NMDA receptor activity and excitability of presympathetic neurons in the hypothalamus can increase sympathetic nerve discharges leading to hypertension. In this study, we determined how protein kinases and phosphatases are involved in regulating NMDA receptor activity and firing activity of presympathetic neurons in the hypothalamus in normotensive and hypertensive rats. We show that casein kinase-1 inhibition increases NMDA receptor activity and excitability of presympathetic neurons in the hypothalamus and augments sympathetic nerve discharges in normotensive, but not in hypertensive, rats. Our data indicate that casein kinase-1 tonically regulates NMDA receptor activity by interacting with casein kinase-2 and protein phosphatases in the hypothalamus and that imbalance of NMDA receptor phosphorylation can augment the excitability of hypothalamic presympathetic neurons and sympathetic nerve discharges in hypertension. These findings help us understand the neuronal mechanism of hypertension, and reducing the NMDA receptor phosphorylation level may be effective for treating neurogenic hypertension.

ABSTRACT

Increased N-methyl-d-aspartate receptor (NMDAR) activity in the paraventricular nucleus (PVN) of the hypothalamus is involved in elevated sympathetic outflow in hypertension. However, the molecular mechanisms underlying augmented NMDAR activity in hypertension remain unclear. In this study, we determined the role of casein kinase-1 (CK1) in regulating NMDAR activity in the PVN. NMDAR-mediated excitatory postsynaptic currents (EPSCs) and puff NMDA-elicited currents were recorded in spinally projecting PVN neurons in spontaneously hypertensive rats (SHRs) and Wistar-Kyoto (WKY) rats. The basal amplitudes of evoked NMDAR-EPSCs and puff NMDA currents were significantly higher in SHRs than in WKY rats. The CK1 inhibitor PF4800567 or PF670462 significantly increased the amplitude of NMDAR-EPSCs and puff NMDA currents in PVN neurons in WKY rats but not in SHRs. PF4800567 caused an NMDAR-dependent increase in the excitability of PVN neurons only in WKY rats. Also, the CK1ε protein level in the PVN was significantly lower in SHRs than in WKY rats. Furthermore, intracerebroventricular infusion of PF4800567 increased blood pressure and lumbar sympathetic nerve activity in WKY rats, and this effect was eliminated by microinjection of the NMDAR antagonist into the PVN. In addition, PF4800567 failed to increase NMDAR activity in brain slices of WKY rats pretreated with the protein phosphatase 1/2A, calcineurin, or casein kinase-2 inhibitor. Our findings suggest that CK1 tonically suppresses NMDAR activity in the PVN by reducing the NMDAR phosphorylation level. Diminished CK1 activity may contribute to potentiated glutamatergic synaptic input to PVN presympathetic neurons and elevated sympathetic vasomotor tone in neurogenic hypertension.

A design principle for a posttranslational biochemical oscillator

Multisite phosphorylation plays an important role in biological oscillators such as the circadian clock. Its general role, however, has been elusive. In this theoretical study, we show that a simple substrate with two modification sites acted upon by two opposing enzymes (e.g., a kinase and a phosphatase) can show oscillations in its modification state. An unbiased computational analysis of this oscillator reveals two common characteristics: a unidirectional modification cycle and sequestering of an enzyme by a specific modification state. These two motifs cause a substrate to act as a coupled system in which a unidirectional cycle generates single-molecule oscillators, whereas sequestration synchronizes the population by limiting the available enzyme under conditions in which substrate is in excess. We also demonstrate the conditions under which the oscillation period is temperature compensated, an important feature of the circadian clock. This theoretical model will provide a framework for analyzing and synthesizing posttranslational oscillators.

Involvement of stress kinase mitogen-activated protein kinase kinase 7 in regulation of mammalian circadian clock

The stress kinase mitogen-activated protein kinase kinase 7 (MKK7) is a specific activator of c-Jun N-terminal kinase (JNK), which controls various physiological processes, such as cell proliferation, apoptosis, differentiation, and migration. Here we show that genetic inactivation of MKK7 resulted in an extended period of oscillation in circadian gene expression in mouse embryonic fibroblasts. Exogenous expression in cultured mammalian cells of an MKK7-JNK fusion protein that functions as a constitutively active form of JNK induced phosphorylation of PER2, an essential circadian component. Furthermore, JNK interacted with PER2 at both the exogenous and endogenous levels, and MKK7-mediated JNK activation increased the half-life of PER2 protein by inhibiting its ubiquitination. Notably, the PER2 protein stabilization induced by MKK7-JNK fusion protein reduced the degradation of PER2 induced by casein kinase 1ε. Taken together, our results support a novel function for the stress kinase MKK7 as a regulator of the circadian clock in mammalian cells at steady state.

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

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

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

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

Roles of CLOCK phosphorylation in suppression of E-box-dependent transcription

In mammalian circadian clockwork, the CLOCK-BMAL1 heterodimer activates E-box-dependent transcription, while its activity is suppressed by circadian binding with negative regulators, such as CRYs. Here, we found that the CLOCK protein is kept mostly in the phosphorylated form throughout the day and is partly hyperphosphorylated in the suppression phase of E-box-dependent transcription in the mouse liver and NIH 3T3 cells. Coexpression of CRY2 in NIH 3T3 cells inhibited the phosphorylation of CLOCK, whereas CIPC coexpression markedly stimulated phosphorylation, indicating that CLOCK phosphorylation is regulated by a combination of the negative regulators in the suppression phase. CLOCK-BMAL1 purified from the mouse liver was subjected to tandem mass spectrometry analysis, which identified Ser38, Ser42, and Ser427 as in vivo phosphorylation sites of CLOCK. Ser38Asp and Ser42Asp mutations of CLOCK additively and markedly weakened the transactivation activity of CLOCK-BMAL1, with downregulation of the nuclear amount of CLOCK and the DNA-binding activity. On the other hand, CLOCK Delta 19, lacking the CIPC-binding domain, was far less phosphorylated and much more stabilized than wild-type CLOCK in vivo. Calyculin A treatment of cultured NIH 3T3 cells promoted CLOCK phosphorylation and facilitated its proteasomal degradation. Together, these results show that CLOCK phosphorylation contributes to the suppression of CLOCK-BMAL1-mediated transactivation through dual regulation: inhibition of CLOCK activity and promotion of its degradation.

Expression and role of TTF-1 in the rat suprachiasmatic nucleus

We have previously reported that TTF-1, a homeodomain-containing transcription factor, regulates circadian rhythm of pituitary adenylate cyclase-activating polypeptide gene expression in the rat hypothalamus. In this study we found that TTF-1 mRNA was specifically expressed in the rat suprachiasmatic nucleus (SCN) and colocalized with Period 2 (Per2), a circadian feedback loop controller. Interaction between TTF-1 and Per1 and Per2 was demonstrated by immunoprecipitation and immunoblot assays. Moreover, TTF-1 and Per proteins additively stimulated a transcriptional activity of angiotensinogen (AoGen) gene. TTF-1 also activated in vitro rhythm of AoGen transcription determined by secretary alkaline phosphatase (SEAP) reporter system in the NIH3T3 cells. These results suggest that TTF-1 plays a role in the circadian rhythm regulation of the AoGen gene expression via interacting with Per proteins in the rat SCN.

Interactions between Casein kinase Iepsilon (CKIepsilon) and two substrates from disparate signaling pathways reveal mechanisms for substrate-kinase specificity

BACKGROUND

Members of the Casein Kinase I (CKI) family of serine/threonine kinases regulate diverse biological pathways. The seven mammalian CKI isoforms contain a highly conserved kinase domain and divergent amino- and carboxy-termini. Although they share a preferred target recognition sequence and have overlapping expression patterns, individual isoforms often have specific substrates. In an effort to determine how substrates recognize differences between CKI isoforms, we have examined the interaction between CKIepsilon and two substrates from different signaling pathways.

METHODOLOGY/PRINCIPAL FINDINGS

CKIepsilon, but not CKIalpha, binds to and phosphorylates two proteins: Period, a transcriptional regulator of the circadian rhythms pathway, and Disheveled, an activator of the planar cell polarity pathway. We use GST-pull-down assays data to show that two key residues in CKIalpha's kinase domain prevent Disheveled and Period from binding. We also show that the unique C-terminus of CKIepsilon does not determine Dishevelled's and Period's preference for CKIepsilon nor is it essential for binding, but instead plays an auxillary role in stabilizing the interactions of CKIepsilon with its substrates. We demonstrate that autophosphorylation of CKIepsilon's C-terminal tail prevents substrate binding, and use mass spectrometry and chemical crosslinking to reveal how a phosphorylation-dependent interaction between the C-terminal tail and the kinase domain prevents substrate phosphorylation and binding.

CONCLUSIONS/SIGNIFICANCE

The biochemical interactions between CKIepsilon and Disheveled, Period, and its own C-terminus lead to models that explain CKIepsilon's specificity and regulation.

Drosophila DBT lacking protein kinase activity produces long-period and arrhythmic circadian behavioral and molecular rhythms

A mutation (K38R) which specifically eliminates kinase activity was created in the Drosophila melanogaster ckI gene (doubletime [dbt]). In vitro, DBT protein carrying the K38R mutation (DBT(K/R)) interacted with Period protein (PER) but lacked kinase activity. In cell culture and in flies, DBT(K/R) antagonized the phosphorylation and degradation of PER, and it damped the oscillation of PER in vivo. Overexpression of short-period, long-period, or wild-type DBT in flies produced the same circadian periods produced by the corresponding alleles of the endogenous gene. These mutations therefore dictate an altered "set point" for period length that is not altered by overexpression. Overexpression of the DBT(K/R) produced effects proportional to the titration of endogenous DBT, with long circadian periods at lower expression levels and arrhythmicity at higher levels. This first analysis of adult flies with a virtual lack of DBT activity demonstrates that DBT's kinase activity is necessary for normal circadian rhythms and that a general reduction of DBT kinase activity does not produce short periods.

Nocturnal expression of phosphorylated-ERK1/2 in gastrin-releasing peptide neurons of the rat suprachiasmatic nucleus

Extracellular regulated kinase (ERK) signalling is believed to play roles in various aspects of circadian clock mechanisms. In this study, we show in rat that the nuclear versus cytoplasmic intracellular distribution of the phosphorylated forms of ERK1/2 (P-ERK1/2) in the central clock, namely the suprachiasmatic nucleus (SCN), is proportionally constant across the light/dark cycle while the spatial distribution and neurochemical phenotype of cells expressing these activated forms are time-regulated according to a daily rhythm and light-regulated. P-ERK1/2 was exclusively found in neuronal elements. At daytime, it was detected throughout the dorsoventral extent of the SCN, partly within neurons synthesizing either arginine-vasopressin or vasoactive intestinal peptide (VIP). At night time, it was segregated in the ventrolateral aspect of the nucleus, within a cluster of cells 45% of which were gastrin-releasing peptide (GRP) neurons with or without co-localization with VIP. After a light pulse at night, expression of P-ERK1/2 increased in GRP neurons but also appeared in a population of neurons that stained for VIP only. These data show that the GRP neurons are closely associated with ERK1/2 activation at night and point to the importance of ERK1/2 signalling not only in intra-SCN transmission of photic information but also in maintenance of neuronal rhythms in the SCN.

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.

Post-translational modifications regulate the ticking of the circadian clock

Getting a good night's sleep is on everyone's to-do list. So is, no doubt, staying awake during late afternoon seminars. Our internal clocks control these and many more workings of the body, and disruptions of the circadian clocks predispose individuals to depression, obesity and cancer. Mutations in kinases and phosphatases in hamsters, flies, fungi and humans highlight how our timepieces are regulated and provide clues as to how we might be able to manipulate them.

Role of phosphorylation in the mammalian circadian clock

Circadian clocks regulate a wide variety of processes ranging from gene expression to behavior. At the molecular level, circadian rhythms are thought to be produced by a set of clock genes and proteins interconnected to form transcriptional-translational feedback loops. Rhythmic gene expression was formerly regarded as the major drive for rhythms in clock protein abundance, but recent findings underline the crucial importance of posttranslational mechanisms for both the generation and dynamics of circadian rhythms. In particular, the reversible phosphorylation of PER proteins-essential components within the negative feedback loop in Drosophila and mammals-seems to have a key role for the correct timing of nuclear repression. To understand how PER protein phosphorylation regulates the dynamics of the circadian oscillator, we have mapped endogenous phosphorylation sites in mPER2. Detailed investigation of the functional role of one particular phosphorylation site (Ser-659, which is mutated in the familial advanced sleep phase syndrome [FASPS]) led us propose a model of functionally different phosphorylation sites in PER2. This concept explains not only the FASPS phenotype, but also the effect of the tau mutation in hamster.

Serine 714 might be implicated in the regulation of the phosphorylation in other areas of mPer1 protein

The phosphorylation of mPer proteins may play important roles in the mechanism of the circadian clock via changes in subcellular localization and degradation. However, the mechanism has remained unclear. Previously, we identified three putative casein kinase (CK)1epsilon phosphorylation motif clusters in mPer1. In this work, we examined the role of the phosphorylation of serine residue, Ser(S)714, in mPer1. mPer1 S[714-726]A mutant, in which potential phosphorylation serine residues replaced by alanine residues, is rapidly phosphorylated compared with wild-type mPer1 by CK1epsilon. Coexpression with S[714]G mutant of mPer1 advanced phase of circadian expression of mPer2-luc expression, which was monitored by in vitro bioluminescence system. This result showed that the mPER1 S[714]G mutation affects circadian core oscillator. Considering these, it seems that Ser 714 might be involved in the regulation of the phosphorylation of other sites in mPer1 by CK1epsilon.

Physiological role for casein kinase 1 in glutamatergic synaptic transmission

Casein kinase 1 (CK1) is a highly conserved serine/threonine kinase, present in virtually all cell types, in which it phosphorylates a wide variety of substrates. So far, no role has been found for this ubiquitous protein kinase in the physiology of nerve cells. In the present study, we show that CK1 regulates fast synaptic transmission mediated by glutamate, the major excitatory neurotransmitter in the brain. Through the use of CK1 inhibitors, we present evidence that activation of CK1 decreases NMDA receptor activity in the striatum via a mechanism that involves activation by this kinase of protein phosphatase 1 and/or 2A and resultant increased dephosphorylation of NMDA receptors. Indeed, inhibition of CK1 increases NMDA-mediated EPSCs in medium spiny striatal neurons. This effect is associated with an increased phosphorylation of the NR1 and NR2B subunits of the NMDA receptor and is occluded by the phosphatase inhibitor okadaic acid. The mGluR1, but not mGluR5, subclass of metabotropic glutamate receptors uses CK1 to inhibit NMDA-mediated synaptic currents. These results provide the first evidence for a role of CK1 in the regulation of synaptic transmission in the brain.

Acute physical stress elevates mouse period1 mRNA expression in mouse peripheral tissues via a glucocorticoid-responsive element

In mammals, the circadian and stress systems (both centers of which are located in the hypothalamus) are involved in adaptation to predictable and unpredictable environmental stimuli, respectively. Although the interaction and relationship between these two systems are intriguing and have been studied in different ways since the "pre-clock gene" era, the molecular interaction between them remains largely unknown. Here, we show by systematic molecular biological analysis that acute physical stress elevated only Period1 (Per1) mRNA expression in mouse peripheral organs. Although behavioral rhythms in vivo and peripheral molecular clocks are rather stable against acute restraint stress, the results of a series of promoter analyses, including chromatin immunoprecipitation assays, indicate that a glucocorticoid-responsive element in the Per1 promoter is indispensable for induction of this mRNA both in vitro and in vivo. These results suggest that Per1 can be a potential stress marker and that a third pathway of Per1 transcriptional control may exist in addition to the clock-regulated CLOCK-BMAL1/E-box and light-responsive cAMP-responsive element-binding protein/cAMP-responsive element pathways.

A role for glycogen synthase kinase-3beta in the mammalian circadian clock

The Drosophila shaggy gene product is a mammalian glycogen synthase kinase-3beta (GSK-3beta) homologue that contributes to the circadian clock of the Drosophila through TIMELESS phosphorylation, and it regulates nuclear translocation of the PERIOD/TIMELESS heterodimer. We found that mammalian GSK-3beta is expressed in the suprachiasmatic nucleus and liver of mice and that GSK-3beta phosphorylation exhibits robust circadian oscillation. Rhythmic GSK-3beta phosphorylation is also observed in serum-shocked NIH3T3 cells. Exposing serum-shocked NIH3T3 cells to lithium chloride, a specific inhibitor of GSK-3beta, increases GSK-3beta phosphorylation and delays the phase of rhythmic clock gene expression. On the other hand, GSK-3beta overexpression advances the phase of clock gene expression. We also found that GSK-3beta interacts with PERIOD2 (PER2) in vitro and in vivo. Recombinant GSK-3beta can phosphorylate PER2 in vitro. GSK-3beta promotes the nuclear translocation of PER2 in COS1 cells. The present data suggest that GSK-3beta plays important roles in mammalian circadian clock.

Nuclear import of mPER3 in Xenopus oocytes and HeLa cells requires complex formation with mPER1

Several transcription factors with the function of setting the biological clock in vertebrates have been described. A detailed understanding of their nucleocytolasmic transport properties may uncover novel aspects of the regulation of the circadian rhythm. This assumption led us to perform a systematic analysis of the nuclear import characteristics of the different murine PER and CRY proteins, using Xenopus oocytes and HeLa cells as experimental systems. Our major finding is that nuclear import of mPER3 requires complex formation with mPER1. We further show that the nuclear localization signal (NLS) function of mPER1 and not activation of a masked NLS in mPER3 is critical for the import of the mPER1-mPER3 complex. Finally, and as previously described in other cell systems, nuclear import of mPER proteins in Xenopus oocytes correlates positively with their phosphorylation.

The circadian clock-containing photoreceptor cells in Xenopus laevis express several isoforms of casein kinase I

The frog (Xenopus laevis) retina has been an important model for the analysis of retinal circadian rhythms. In this paper, several isoforms of X. laevis casein kinase I (CKI) were analyzed to address whether they are involved in the phosphorylation and degradation of period protein (PER), as they are in the circadian oscillators of other species. cDNAs encoding two splice variants of CKI(delta) (a full-length form and deletion isoform, which is missing an exon that encodes a putative nuclear localization signal and two evolutionarily conserved protein kinase domains) were isolated and analyzed, together with a previously isolated CKI(epsilon) isoform. Both CKI(delta) and CKI(epsilon) were shown to be constitutively expressed in the photoreceptors of the retina, where a circadian clock has been localized. Both the full-length CKI(delta) and CKI(epsilon) were shown to have kinase activity in vitro, and the full-length CKI(delta) phosphorylated and degraded Drosophila PER when expressed in Drosophila S2 cells. The expression and biochemical characteristics of these CKIs are consistent with an evolutionarily conserved role for CKI in the Xenopus retinal clock. The CKI(delta) deletion isoform did not exhibit kinase activity and did not trigger degradation of PER. Subcellular localization of both CKI(delta) isoforms was cytoplasmic in several cell culture lines, but the full-length CKI(delta) , and not the deletion CKI(delta) isoform, was localized to both the nucleus and the cytoplasm in Drosophila S2 cells. These results indicate that the sequences missing in the deletion CKI(delta) isoform are important for the nuclear localization and kinase activity of the full-length isoform and that one or both of these features are necessary for degradation of Drosophila PER.

Posttranscriptional and posttranslational regulation of clock genes

Circadian rhythms have been observed in diverse organisms, including plants, animals, bacteria, and fungi. In such organisms, the circadian clock is primarily composed of a cell-autonomous transcriptional feedback loop. In addition to transcriptional regulation, the modification of core clock transcripts and proteins can dramatically affect the circadian clock. In this review, the authors discuss some of the posttranscriptional and posttranslational modifications and their effects on the circadian clock. The combined outcome of these modifications is to adjust the timing of the clock to produce a circadian oscillator that takes approximately 24 h.

A missense variation in human casein kinase I epsilon gene that induces functional alteration and shows an inverse association with circadian rhythm sleep disorders

Recent studies have shown that functional variations in clock genes, which generate circadian rhythms through interactive positive/negative feedback loops, contribute to the development of circadian rhythm sleep disorders in humans. Another potential candidate for rhythm disorder susceptibility is casein kinase I epsilon (CKIepsilon), which phosphorylates clock proteins and plays a pivotal role in the circadian clock. To determine whether variations in CKIepsilon induce vulnerability to human circadian rhythm sleep disorders, such as delayed sleep phase syndrome (DSPS) and non-24-h sleep-wake syndrome (N-24), we analyzed all of the coding exons of the human CKIepsilon gene. One of the variants identified encoded an amino-acid substitution S408N, eliminating one of the putative autophosphorylation sites in the carboxyl-terminal extension of CKIepsilon. The N408 allele was less common in both DSPS (p = 0.028) and N-24 patients (p = 0.035) compared to controls. When DSPS and N-24 subjects were combined, based on an a priori prediction of a common mechanism underlying both DSPS and N-24, the inverse association between the N408 allele and rhythm disorders was highly significant (p = 0.0067, odds ratio = 0.42, 95% confidence interval: 0.22-0.79). In vitro kinase assay revealed that CKIepsilon with the S408N variation was approximately 1.8-fold more active than wild-type CKIepsilon. These results indicate that the N408 allele in CKIepsilon plays a protective role in the development of DSPS and N-24 through alteration of the enzyme activity.

Serine phosphorylation of mCRY1 and mCRY2 by mitogen-activated protein kinase

The circadian oscillator is composed of a transcription/translation-based autoregulatory feedback loop in which Cryptochromes and Periods function as negative regulators for their own gene expression. Although post-translational modifications such as phosphorylation of these regulators appear crucial for circadian time-keeping mechanism, less is known about responsible protein kinases and their contribution to the function of the regulators. We found that mitogen-activated protein kinase (MAPK) associates with and phosphorylates mouse Cryptochromes (mCRY1 and mCRY2). Mass spectrometry analysis identified Ser265 and Ser557 of mCRY2 to be in vitro phospho-acceptor residues. Mutations of both the Ser residues to Ala completely abolished MAPK-mediated mCRY2 phosphorylation, suggesting that the two residues are the principal phosphorylation sites in mCRY2. Similarly, MAPK phosphorylates mCRY1 at Ser247, a site corresponding to Ser265 of mCRY2. An effect of the Ser phosphorylation was investigated by mutating Ser247 of mCRY1 and Ser265 of mCRY2 to Asp, which resulted in attenuation of each mCRYs' ability to inhibit BMAL1: CLOCK-mediated transcription, whereas a similar mutation at Ser557 of mCRY2 induced no measurable change in its activity. These results illustrate a model of MAPK-mediated negative regulation of mCRY function by phosphorylation at the specific Ser residue.

Analysis of the expression, localization and activity of rat casein kinase 1ε-3

Casein kinase 1epsilon (CK1epsilon) is a serine/threonine protein kinase that has been suggested to participate in the regulation of various signaling pathways. In this report, we examined the tissue distributions of three putative alternatively spliced forms of rCk1epsilon by RT-PCR. This analysis confirmed that all three isoforms are expressed in rat tissues with different tissue-specific expression patterns. Immunohistochemical analysis showed that the intracellular distribution of rCK1epsilon-3 in neurons was broader than that of rCK1epsilon-1. Moreover, the kinase activity of the rCK1epsilon-3 protein differed from that of rCK1epsilon-1. These data suggest that rCK1epsilon-1 and rCK1epsilon-3 may play different functional roles.

Negative Control of Circadian Clock Regulator E4BP4 by Casein Kinase Iϵ-Mediated Phosphorylation

Light-dependent transcriptional regulation of clock genes is a crucial step in the entrainment of the circadian clock. E4bp4 is a light-inducible gene in the chick pineal gland, and it encodes a bZIP protein that represses transcription of cPer2, a chick pineal clock gene. Here, we demonstrate that prolonged light period-dependent accumulation of E4BP4 protein is temporally coordinated with a delay of the rising phase of cPer2 in the morning. E4BP4 was phosphorylated progressively and then disappeared in parallel with induced cPer2 expression. Characterization of E4BP4 revealed Ser182, a phosphoacceptor site located at the amino-terminal border of the Ser/Thr cluster, which forms the phosphorylation motifs for casein kinase 1epsilon (CK1epsilon). CK1epsilon physically associated with E4BP4 and phosphorylated it. CK1epsilon-catalyzed phosphorylation of E4BP4 resulted in proteasomal proteolysis-dependent decrease of E4BP4 levels, while E4BP4 nuclear accumulation was attenuated by CK1epsilon in a kinase activity-independent manner. CK1epsilon-mediated posttranslational regulation was accompanied by reduction of the transcriptional repression executed by E4BP4. These results not only demonstrate a phosphorylation-dependent regulatory mechanism for E4BP4 function but also highlight the role of CK1epsilon as a negative regulator for E4BP4-mediated repression of cPer2.

Identification of mPer1 phosphorylation sites responsible for the nuclear entry

Casein kinase 1 epsilon (CK1 epsilon) is an essential component of the circadian clock in mammals and Drosophila. The phosphorylation of Period (Per) proteins by CK1 epsilon is believed to be implicated in their subcellular localization and degradation, but the precise mechanism by which CK1 epsilon affects Per proteins has not been determined. In this study, three putative CK1 epsilon phosphorylation motif clusters in mouse Per1 (mPer1) were identified, and the phosphorylation status of serine and threonine residues in these clusters was examined. Phosphorylation of residues within a region defined by amino acids 653-663 and in particular of Ser-661 and Ser-663, was identified as responsible for the nuclear translocation of mPer1. Furthermore, phosphorylation of these residues may influence the nuclear translocation of a clock protein complex containing mPer1. These findings indicate that mPer1 phosphorylation is a critical aspect of the circadian clock mechanism.

Phosphorylation of clock protein PER1 regulates its circadian degradation in normal human fibroblasts

Recent advances suggest that the molecular components of the circadian clock generate a self-sustaining transcriptional-translational feedback loop with a period of approx. 24 h. The precise expression profiles of human clock genes and their products have not been elucidated. We cloned human clock genes, including per1, per2, per3, cry2 and clock, and evaluated their circadian mRNA expression profiles in WI-38 fibroblasts stimulated with serum. Transcripts of hPer1, hPer2, hPer3, hBMAL1 and hCry2 (where h is human) underwent circadian oscillation. Serum-stimulation also caused daily oscillations of hPER1 protein and the apparent molecular mass of hPER1 changed. Inhibitor studies indicated that the CKI (casein kinase I) family, including CKIepsilon and CKIdelta, phosphorylated hPER1 and increased the apparent molecular mass of hPER1. The inhibition of hPER1 phosphorylation by CKI-7 [ N -(2-aminoethyl)-5-chloro-isoquinoline-8-sulphonamide], a CKI inhibitor, disturbed hPER1 degradation, delayed the nuclear entry of hPER1 and allowed it to persist for longer in the nucleus. Furthermore, proteasome inhibitors specifically blocked hPER1 degradation. However leptomycin B, an inhibitor of nuclear export, did not alter the degradation state of hPER1 protein. These findings indicate that circadian hPER1 degradation through a proteasomal pathway can be regulated through phosphorylation by CKI, but not by subcellular localization.

Resetting Mechanism of Central and Peripheral Circadian Clocks in Mammals

Almost all organisms on earth exhibit diurnal rhythms in physiology and behavior under the control of autonomous time-measuring system called circadian clock. The circadian clock is generally reset by environmental time cues, such as light, in order to synchronize with the external 24-h cycles. In mammals, the core oscillator of the circadian clock is composed of transcription/translation-based negative feedback loops regulating the cyclic expression of a limited number of clock genes (such as Per, Cry, Bmal1, etc.) and hundreds of output genes in a well-concerted manner. The central clock controlling the behavioral rhythm is localized in the hypothalamic suprachiasmatic nucleus (SCN), and peripheral clocks are present in other various tissues. The phase of the central clock is amenable to ambient light signal captured by the visual rod-cone photoreceptors and non-visual melanopsin in the retina. These light signals are transmitted to the SCN through the retinohypothalamic tract, and transduced therein by mitogen-activated protein kinase and other signaling molecules to induce Per gene expression, which eventually elicits phase-dependent phase shifts of the clock. The central clock controls peripheral clocks directly and indirectly by virtue of neural, humoral, and other signals in a coordinated manner. The change in feeding time resets the peripheral clocks in a SCN-independent manner, possibly by food metabolites and body temperature rhythms. In this article, we will provide an overview of recent molecular and genetic studies on the resetting mechanism of the central and peripheral circadian clocks in mammals.

Direct association between mouse PERIOD and CKIepsilon is critical for a functioning circadian clock

The mPER1 and mPER2 proteins have important roles in the circadian clock mechanism, whereas mPER3 is expendable. Here we examine the posttranslational regulation of mPER3 in vivo in mouse liver and compare it to the other mPER proteins to define the salient features required for clock function. Like mPER1 and mPER2, mPER3 is phosphorylated, changes cellular location, and interacts with other clock proteins in a time-dependent manner. Consistent with behavioral data from mPer2/3 and mPer1/3 double-mutant mice, either mPER1 or mPER2 alone can sustain rhythmic posttranslational events. However, mPER3 is unable to sustain molecular rhythmicity in mPer1/2 double-mutant mice. Indeed, mPER3 is always cytoplasmic and is not phosphorylated in the livers of mPer1-deficient mice, suggesting that mPER3 is regulated by mPER1 at a posttranslational level. In vitro studies with chimeric proteins suggest that the inability of mPER3 to support circadian clock function results in part from lack of direct and stable interaction with casein kinase Iepsilon (CKIepsilon). We thus propose that the CKIepsilon-binding domain is critical not only for mPER phosphorylation but also for a functioning circadian clock.

Drosophila doubletime mutations which either shorten or lengthen the period of circadian rhythms decrease the protein kinase activity of casein kinase I

In both mammals and fruit flies, casein kinase I has been shown to regulate the circadian phosphorylation of the period protein (PER). This phosphorylation regulates the timing of PER's nuclear accumulation and decline, and it is necessary for the generation of circadian rhythms. In Drosophila melanogaster, mutations affecting a casein kinase I (CKI) ortholog called doubletime (dbt) can produce short or long periods. The effects of both a short-period (dbt(S)) and long-period (dbt(L)) mutation on DBT expression and biochemistry were analyzed. Immunoblot analysis of DBT in fly heads showed that both the dbt(S) and dbt(L) mutants express DBT at constant levels throughout the day. Glutathione S-transferase pull-down assays and coimmunoprecipitation of DBT and PER showed that wild-type DBT, DBT(S), and DBT(L) proteins can bind to PER equivalently and that these interactions are mediated by the evolutionarily conserved N-terminal part of DBT. However, both the dbt(S) and dbt(L) mutations reduced the CKI-7-sensitive kinase activity of an orthologous Xenopus laevis CKIdelta expressed in Escherichia coli. Moreover, expression of DBT in Drosophila S2 cells produced a CKI-7-sensitive kinase activity which was reduced by both the dbt(S) and dbt(L) mutations. Thus, lowered enzyme activity is associated with both short-period and long-period phenotypes.

Nucleocytoplasmic shuttling and phosphorylation of BMAL1 are regulated by circadian clock in cultured fibroblasts

BACKGROUND

Recent discoveries of clock proteins have unveiled an important part of the mammalian circadian clock mechanism. However, the molecular clockwork that cause these fundamental feedback loops to stably oscillate with a approximately 24 h-periodicity remain unclear.

RESULTS

Serum-shocked fibroblasts were used as a cellular clock model. Circadian changes in the subcellular localization and phosphorylation of BMAL1 protein in these cells were assessed by immunocytochemistry and immunoblotting. A significant time lag between Bmal1 transcription and the cytoplasmic/nuclear accumulation of BMAL1 was observed. After its nuclear accumulation, BMAL1 accumulated in the cytoplasm again, mainly by nucleoexport, before the increase of Bmal1 transcripts. Nuclear accumulation of BMAL1 matched nuclear accumulation of CLOCK and the peak of Per1 transcription. Nuclear BMAL1 was gradually phosphorylated and then dephosphorylated in a temporally regulated manner, although cytoplasmic BMAL1 was not. In serum-shocked mCry1/mCry2 (CRY)-deficient fibroblasts, which lack a functional clock, both the cytoplasmic and nuclear BMAL1 were only present as hyperphosphorylated forms and their circadian nucleocytoplasmic shuttling was absent.

CONCLUSIONS

We propose that the nucleocytoplasmic shuttling and phosphorylation states of BMAL1 are regulated by circadian clock, and that this temporally regulated and time-delayed nuclear entry of BMAL1 is important in the maintenance of a stably oscillating clock.

Temperature compensation and temperature resetting of circadian rhythms in mammalian cultured fibroblasts

BACKGROUND

Circadian rhythms control many physiological processes. One of characteristic properties of circadian rhythms is insensitivity to temperature, called temperature compensation. Although this temperature-insensitive property has repeatedly been observed mainly in circadian output rhythms, temperature effect on autoregulatory feedback loops of clock gene expression, the rhythm-generating mechanisms, has not been fully investigated.

RESULTS

We show first that the circadian oscillation of clock gene expression in NIH3T3 fibroblasts, which is induced by TPA (12-O-tetradecanoylphorbol-13-acetate) treatment, is strongly temperature-compensated over the temperature range of 33-42 degrees C. We then show that heat treatment at 42 degrees C is able to trigger circadian oscillation of clock gene expression in NIH3T3 cells. This 42 degrees C heat treatment, unlike serum shock or TPA treatment, did not induce immediate expression of mPer1 mRNA, suggesting the existence of several different resetting mechanisms.

CONCLUSIONS

This is the first demonstration of temperature compensation of the rhythm-generating core feedback loops of clock gene expression in mammalian cultured cells. It is possible that cells in the periphery could sense the change of ambient temperature as a resetting cue and that the whole organism thus could be entrained rapidly at dawn, in cooperation with the resetting mechanism by light.

p38 mitogen-activated protein kinase regulates oscillation of chick pineal circadian clock

Extracellular signal-regulated kinase (ERK) and p38 are members of the mitogen-activated protein kinase (MAPK) family, and in some cases these kinases serve for closely related cellular functions within a cell. In a wide range of animal clock structures, ERK plays an important role in the circadian time-keeping mechanism. Here we found that immunoreactivity to p38 protein was uniformly distributed among cells in the chick pineal gland. On the other hand, a constant level of activated p38 was detected over the day, predominantly in the follicular and parafollicular pinealocytes that are potential circadian clock-containing cells. Chronic application of SB203580, a selective and reversible inhibitor of p38, to the cultured chick pineal cells markedly lengthened the period of the circadian rhythm of the melatonin release (up to 28.7 h). Noticeably, despite no significant temporal change of activated p38 level, a 4-h pulse treatment with SB203580 delayed the phase of the rhythm only when delivered during the subjective day. These results indicate a time-of-day-specific role of continuously activated p38 in the period length regulation of the chick pineal clock and suggest temporally separated regulation of the clock by two MAPKs, nighttime-activated ERK and daytime-working p38.

The circadian clock: pacemaker and tumour suppressor

The circadian rhythms are daily oscillations in various biological processes that are regulated by an endogenous clock. Disruption of these rhythms has been associated with cancer in humans. One of the cellular processes that is regulated by circadian rhythm is cell proliferation, which often shows asynchrony between normal and malignant tissues. This asynchrony highlights the importance of the circadian clock in tumour suppression in vivo and is one of the theoretical foundations for cancer chronotherapy. Investigation of the mechanisms by which the circadian clock controls cell proliferation and other cellular functions might lead to new therapeutic targets.

Genetic analysis of the circadian system in Drosophila melanogaster and mammals

The fruit fly, Drosophila melanogaster, has been a grateful object for circadian rhythm researchers over several decades. Behavioral, genetic, and molecular studies helped to reveal the genetic bases of circadian time keeping and rhythmic behaviors. Contrary, mammalian rhythm research until recently was mainly restricted to descriptive and physiologic approaches. As in many other areas of research, the surprising similarity of basic biologic principles between the little fly and our own species, boosted the progress of unraveling the genetic foundation of mammalian clock mechanisms. Once more, not only the basic mechanisms, but also the molecules involved in establishing our circadian system are taken or adapted from the fly. This review will try to give a comparative overview about the two systems, highlighting similarities as well as specifics of both insect and murine clocks.

Time after time: inputs to and outputs from the mammalian circadian oscillators

Oscillating levels of clock gene transcripts in the suprachiasmatic nucleus (SCN) are essential components of the mammalian circadian pacemaker. Their synchronization with daily light cycles involves neural connections from light-sensitive photoreceptor-containing retinal ganglion cells. This clock orchestrates rhythmic expression for approximately 10% of the SCN gene transcripts, of which only 10% are also rhythmically expressed in other tissues. Many of the transcripts expressed rhythmically only in the SCN are involved in neurosecretion, and their secreted products could mediate SCN control over physiological rhythms by coordinating rhythmicity in other nuclei within the brain. The coordination of clock gene transcript oscillations in peripheral tissues could be controlled directly by specific signals or indirectly by rhythmic behavior such as feeding.

Mechanism of regulation of casein kinase I activity by group I metabotropic glutamate receptors

Previously, we reported that (S)-3,5-dihydroxypenylglycine (DHPG), an agonist for group I metabotropic glutamate receptors (mGluRs), stimulates CK1 and Cdk5 kinase activities in neostriatal neurons, leading to enhanced phosphorylation, respectively, of Ser-137 and Thr-75 of DARPP-32 (dopamine and cAMP-regulated phosphoprotein, 32 kDa). We have now investigated the signaling pathway that leads from mGluRs to casein kinase 1 (CK1) activation. In mouse neostriatal slices, the effect of DHPG on phosphorylation of Ser-137 or Thr-75 of DARPP-32 was blocked by the phospholipase Cbeta inhibitor, the Ca(2+) chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA/AM), and the calcineurin inhibitor cyclosporin A. In neuroblastoma N2a cells, the effect of DHPG on the activity of transfected HA-tagged CK1(epsilon) was blocked by BAPTA/AM and cyclosporin A. In neostriatal slices, the effect of DHPG on Cdk5 activity was also abolished by BAPTA/AM and cyclosporin A, presumably through blocking activation of CK1. Metabolic labeling studies and phosphopeptide mapping revealed that a set of C-terminal sites in HA-CK1epsilon were transiently dephosphorylated in N2a cells upon treatment with DHPG, and this was blocked by cyclosporin A. A mutant CK1epsilon with a nonphosphorylatable C-terminal domain was not activated by DHPG. Together, these studies suggest that DHPG activates CK1(epsilon) via Ca(2+)-dependent stimulation of calcineurin and subsequent dephosphorylation of inhibitory C-terminal autophosphorylation sites.

Central and peripheral circadian oscillator mechanisms in flies and mammals

Circadian oscillators are cell-autonomous time-keeping mechanisms that reside in diverse tissues in many organisms. In flies and mice, the core molecular components that sustain these oscillators are highly conserved, but the functions of some of these components appear to have diverged significantly. One possible reason for these differences is that previous comparisons have focused primarily on the central oscillator of the mouse and peripheral oscillators in flies. Recent research on mouse and Drosophila peripheral oscillators shows that the function of the core components between these organisms may be more highly conserved than was first believed, indicating the following: (1) that central and peripheral oscillators in flies do not necessarily have the same molecular mechanisms; (2) that mammalian central oscillators are regulated differently from peripheral oscillators; and (3) that different peripheral oscillators within and across species show striking similarities. The core feedback loop in peripheral oscillators might therefore be functionally well conserved, and central oscillators could be specialized versions of a basic oscillator design.

The circadian regulatory proteins BMAL1 and cryptochromes are substrates of casein kinase Iepsilon

The serine/threonine protein kinase casein kinase I epsilon (CKIepsilon) is a key regulator of metazoan circadian rhythm. Genetic and biochemical data suggest that CKIepsilon binds to and phosphorylates the PERIOD proteins. However, the PERIOD proteins interact with a variety of circadian regulators, suggesting the possibility that CKIepsilon may interact with and phosphorylate additional clock components as well. We find that CRY1 and BMAL1 are phosphoproteins in cultured cells. Mammalian PERIOD proteins act as a scaffold with distinct domains that simultaneously bind CKIepsilon and mCRY1 and mCRY2 (mCRY). mCRY is phosphorylated by CKIepsilon only when both proteins are bound to mammalian PERIOD proteins. BMAL1 is also a substrate for CKIepsilon in vitro, and CKIepsilon kinase activity positively regulates BMAL1-dependent transcription from circadian promoters in reporter assays. We conclude that CKIepsilon phosphorylates multiple circadian substrates and may exert its effects on circadian rhythm in part by a direct effect on BMAL1-dependent transcription.

Control of intracellular dynamics of mammalian period proteins by casein kinase I epsilon (CKIepsilon) and CKIdelta in cultured cells

Recent studies have shown that casein kinase I epsilon (CKIepsilon) is an essential regulator of the mammalian circadian clock. However, the detailed mechanisms by which CKIepsilon regulates each component of the circadian negative-feedback loop have not been fully defined. We show here that mPer proteins, negative limbs of the autoregulatory loop, are specific substrates for CKIepsilon and CKIdelta. The CKI phosphorylation of mPer1 and mPer3 proteins results in their rapid degradation, which is dependent on the ubiquitin-proteasome pathway. Moreover, CKIepsilon and CKIdelta are able to induce nuclear translocation of mPer3, which requires its nuclear localization signal. The mutation in potential phosphorylation sites on mPer3 decreased the extent of both nuclear translocation and degradation of mPer3 that are stimulated by CKIepsilon. CKIepsilon and CKIdelta affected the inhibitory effect of mPer proteins on the transcriptional activity of BMAL1-CLOCK, but the inhibitory effect of mCry proteins on the activity of BMAL1-CLOCK was unaffected. These results suggest that CKIepsilon and CKIdelta regulate the mammalian circadian autoregulatory loop by controlling both protein turnover and subcellular localization of mPer proteins.

Nuclear export of mammalian PERIOD proteins

The timing of mammalian circadian rhythm is determined by interlocking negative and positive transcriptional feedback loops that govern the cyclic expression of both clock regulators and output genes. In mammals, nuclear localization of the circadian regulators PER1-3 is controlled by multiple mechanisms, including multimerization with PER and CRY proteins. In addition, nuclear entry of mammalian PER1 (mPER1) can be regulated by a phosphorylation-dependent masking of its nuclear localization signal. Here we present evidence suggesting that nuclear localization of PER proteins is a dynamic process determined by both nuclear import and previously unrecognized nuclear export pathways. Examination of the subcellular localization of a series of truncated mPER1 proteins demonstrated that cytoplasmic localization is mediated by an 11-amino acid region with homology to leucine-rich nuclear export signals (NESs). Similar sequences were identified in mPER2 and mPER3 as well as in several insect PER proteins. The putative NESs from mPER1 and mPER2 were able to direct cytoplasmic accumulation when fused to a heterologous protein. Mutations in conserved NES residues and the nuclear export inhibitor leptomycin B each blocked the function of the NES. Full-length mPER1 was also exported from microinjected Xenopus laevis oocyte nuclei in an NES-dependent manner. The presence of a functional NES in mPER1 and mPER2 as well as related sequences in a variety of other PER proteins suggests that nuclear export may be a conserved and important feature of circadian regulators.

A novel protein interacts with a clock-related protein, rPer1

Mammalian Per proteins are thought to be important in the mechanism of circadian rhythm. We identified a novel protein PIPS (Per1 interacting protein of the suprachiasmatic nucleus) with the yeast two-hybrid system using PAS domain of rat Per1 (rPer1) as a bait. PIPS is about a 180-kDa protein and expressed mainly in the brain, especially in the hypothalamus including the suprachiasmatic nuclei (SCN). PIPS interacts with mouse Per1 (mPer1) in vitro and in cultured cells transfected with both molecules. Furthermore, it was found that mPer1 translocated PIPS into the nuclei in the cultured cells. Thus, these findings suggest a possibility that PIPS is involved in the feedback loop or output mechanism of circadian rhythm through interacting with Per1 in the SCN.

Nuclear entry mechanism of rat PER2 (rPER2): role of rPER2 in nuclear localization of CRY protein

Mammalian PERIOD2 protein (PER2) is the product of a clock gene that controls circadian rhythms, because PER2-deficient mice have an arrhythmic phenotype. The nuclear entry regulation of clock gene products is a key step in proper circadian rhythm formation in both Drosophila and mammals, because the periodic transcription of clock genes is controlled by an intracellular, oscillating, negative feedback loop. The present study used deletion mutants of rat PER2 (rPER2) to identify the functional nuclear localization signal (NLS) in rPER2. The elimination of putative NLS (residues 778 to 794) from the rPER2 fragment resulted in the loss of nuclear entry activity. Adding the NLS to the cytosolic protein (bacterial alkaline phosphatase) translocates the fusion protein to the nuclei. The data indicate the presence of a functional NLS in rPER2. Furthermore, intact rPER2 was preferentially translocated from the cytoplasm to the nucleus when coexpressed with human CRY1 (hCRY1). However, rPER2 mutants lacking a carboxyl-terminal domain could not enter the nucleus even in the presence of hCRY1. In addition, coexpression of the nuclear localization domain (residues 512 to 794) lacking rPER2 and CRY1 changed the subcellular localization of CRY1 from the nucleus to the cytoplasm. In vitro protein interaction studies demonstrated that the carboxyl-terminal domain of rPER2 is essential for binding to CRY1. The data suggested that both the rPER2 NLS and carboxyl-terminal CRY binding domain are essential for nuclear entry of the rPER2-CRY1 complex.

Time zones: a comparative genetics of circadian clocks

The circadian clock is a widespread cellular mechanism that underlies diverse rhythmic functions in organisms from bacteria and fungi, to plants and animals. Intense genetic analysis during recent years has uncovered many of the components and molecular mechanisms comprising these clocks. Although autoregulatory genetic networks are a consistent feature in the design of all clocks, the weight of evidence favours their independent evolutionary origins in different kingdoms.

Molecular analysis of mammalian circadian rhythms

In mammals, a master circadian "clock" resides in the suprachiasmatic nuclei (SCN) of the anterior hypothalamus. The SCN clock is composed of multiple, single-cell circadian oscillators, which, when synchronized, generate coordinated circadian outputs that regulate overt rhythms. Eight clock genes have been cloned that are involved in interacting transcriptional-/translational-feedback loops that compose the molecular clockwork. The daily light-dark cycle ultimately impinges on the control of two clock genes that reset the core clock mechanism in the SCN. Clock-controlled genes are also generated by the central clock mechanism, but their protein products transduce downstream effects. Peripheral oscillators are controlled by the SCN and provide local control of overt rhythm expression. Greater understanding of the cellular and molecular mechanisms of the SCN clockwork provides opportunities for pharmacological manipulation of circadian timing.

Casein kinase I: another cog in the circadian clockworks

Multiple components of the circadian central clock are phosphoproteins, and it has become increasingly clear that posttranslational modification is an important regulator of circadian rhythm in diverse organisms, from dinoflagellates to humans. Genetic studies in Drosophila have identified double-time (dbt), a serine/threonine protein kinase that is highly homologous to human casein kinase I epsilon (CKIepsilon), as the first kinase linked to behavioral rhythms. Identification of a missense mutation in CKIepsilon as the tau mutation in the Syrian hamster places CKIepsilon within the core clock machinery in mammals. Most recently, identification of a phosphorylation site mutant of hPER2 in a family with an inherited circadian rhythm abnormality strongly suggests that PER2 is a physiologically relevant substrate of CKI. Phosphorylation may regulate multiple properties of clock proteins, including stability and intracellular localization.