Spatial and temporal regulation of mitogen-activated protein kinase phosphorylation in the mouse suprachiasmatic nucleus

Circadian Regulation of Retinal Pigment Epithelium Function

The retinal pigment epithelium (RPE) is a single layer of cells located between the choriocapillaris vessels and the light-sensitive photoreceptors in the outer retina. The RPE performs physiological processes necessary for the maintenance and support of photoreceptors and visual function. Among the many functions performed by the RPE, the timing of the peak in phagocytic activity by the RPE of the photoreceptor outer segments that occurs 1-2 h. after the onset of light has captured the interest of many investigators and has thus been intensively studied. Several studies have shown that this burst in phagocytic activity by the RPE is under circadian control and is present in nocturnal and diurnal species and rod and cone photoreceptors. Previous investigations have demonstrated that a functional circadian clock exists within multiple retinal cell types and RPE cells. However, the anatomical location of the circadian controlling this activity is not clear. Experimental evidence indicates that the circadian clock, melatonin, dopamine, and integrin signaling play a key role in controlling this rhythm. A series of very recent studies report that the circadian clock in the RPE controls the daily peak in phagocytic activity. However, the loss of the burst in phagocytic activity after light onset does not result in photoreceptor or RPE deterioration during aging. In the current review, we summarized the current knowledge on the mechanism controlling this phenomenon and the physiological role of this peak.

Circadian rhythm and sleep-wake systems share the dynamic extracellular synaptic milieu

The mammalian circadian and sleep-wake systems are closely aligned through their coordinated regulation of daily activity patterns. Although they differ in their anatomical organization and physiological processes, they utilize overlapping regulatory mechanisms that include an assortment of proteins and molecules interacting within the extracellular space. These extracellular factors include proteases that interact with soluble proteins, membrane-attached receptors and the extracellular matrix; and cell adhesion molecules that can form complex scaffolds connecting adjacent neurons, astrocytes and their respective intracellular cytoskeletal elements. Astrocytes also participate in the dynamic regulation of both systems through modulating neuronal appositions, the extracellular space and/or through release of gliotransmitters that can further contribute to the extracellular signaling processes. Together, these extracellular elements create a system that integrates rapid neurotransmitter signaling across longer time scales and thereby adjust neuronal signaling to reflect the daily fluctuations fundamental to both systems. Here we review what is known about these extracellular processes, focusing specifically on areas of overlap between the two systems. We also highlight questions that still need to be addressed. Although we know many of the extracellular players, far more research is needed to understand the mechanisms through which they modulate the circadian and sleep-wake systems.

Glycogen synthase kinase 3 regulates photic signaling in the suprachiasmatic nucleus

Glycogen synthase kinase 3 (GSK3) is a serine-threonine kinase that regulates mammalian circadian rhythms at the behavioral, molecular and neurophysiological levels. In the central circadian pacemaker, the suprachiasmatic nucleus (SCN), inhibitory phosphorylation of GSK3 exhibits a rhythm across the 24 h day. We have recently shown that GSK3 is capable of influencing both the molecular clock and SCN neuronal activity rhythms. However, it is not known whether GSK3 regulates the response to environmental cues such as light. The goal of this study was to test the hypothesis that GSK3 activation mediates light-induced SCN excitability and photic entrainment. Immunofluorescence staining in the SCN of mice showed that late-night light exposure significantly increased GSK3 activity (decreased pGSK3β levels) 30-60 min after the light-pulse. In addition, pharmacological inhibition of GSK3 blocked the expected light-induced excitability in SCN neurons; however, this effect was not associated with changes in resting membrane potential or input resistance. Behaviorally, mice with constitutively active GSK3 (GSK3-KI) re-entrained to a 6-h phase advance in the light-dark cycle in significantly fewer days than WT control animals. Furthermore, the behavioral and SCN neuronal activity of GSK3-KI mice was phase-advanced compared to WT, in both normal and light-exposed conditions. Finally, GSK3-KI mice exhibited normal negative-masking behavior and electroretinographic responses to light, suggesting that the enhanced photic entrainment is not due to an overall increased sensitivity to light in these animals. Taken together, these results provide strong evidence that GSK3 activation contributes to light-induced phase-resetting at both the neurophysiological and behavioral levels.

Acute morphine affects the rat circadian clock via rhythms of phosphorylated ERK1/2 and GSK3β kinases and Per1 expression in the rat suprachiasmatic nucleus

BACKGROUND AND PURPOSE

Opioids affect the circadian clock and may change the timing of many physiological processes. This study was undertaken to investigate the daily changes in sensitivity of the circadian pacemaker to an analgesic dose of morphine, and to uncover a possible interplay between circadian and opioid signalling.

EXPERIMENTAL APPROACH

A time-dependent effect of morphine (1 mg·kg(-1) , i.p.) applied either during the day or during the early night was followed, and the levels of phosphorylated ERK1/2, GSK3β, c-Fos and Per genes were assessed by immunohistochemistry and in situ hybridization. The effect of morphine pretreatment on light-induced pERK and c-Fos was examined, and day/night difference in activity of opioid receptors was evaluated by [(35) S]-GTPγS binding assay.

KEY RESULTS

Morphine stimulated a rise in pERK1/2 and pGSK3β levels in the suprachiasmatic nucleus (SCN) when applied during the day but significantly reduced both kinases when applied during the night. Morphine at night transiently induced Period1 but not Period2 in the SCN and did not attenuate the light-induced level of pERK1/2 and c-Fos in the SCN. The activity of all three principal opioid receptors was high during the day but decreased significantly at night, except for the δ receptor. Finally, we demonstrated daily profiles of pERK1/2 and pGSK3β levels in the rat ventrolateral and dorsomedial SCN.

CONCLUSIONS AND IMPLICATIONS

Our data suggest that the phase-shifting effect of opioids may be mediated via post-translational modification of clock proteins by means of activated ERK1/2 and GSK3β.

Delayed Effect of the Light Pulse on Phosphorylated ERK1/2 and GSK3β Kinases in the Ventrolateral Suprachiasmatic Nucleus of Rat

The intrinsic period of circadian clock in the suprachiasmatic nucleus is entrained to a 24-h cycle by external cues, mainly light. Previous studies have shown that light applied at night induces robust phosphorylation of extracellular-signal-regulated kinase that is necessary to process the light pulse into the phase shift of the clock phase. In this study, we show the persistent downregulation of phosphorylated extracellular-signal-regulated kinase and transient downregulation of phosphorylated glycogen synthase kinase-3beta in the ventrolateral part of the suprachiasmatic nucleus to photic stimuli starting at 2 h after the beginning of the light pulse. As both kinases are involved in regulation of circadian clockwork, we hypothesize that these changes may contribute to the phase-shifting effect of light at night.

The MAP kinase p38 is part of Drosophila melanogaster's circadian clock

All organisms have to adapt to acute as well as to regularly occurring changes in the environment. To deal with these major challenges organisms evolved two fundamental mechanisms: the p38 mitogen-activated protein kinase (MAPK) pathway, a major stress pathway for signaling stressful events, and circadian clocks to prepare for the daily environmental changes. Both systems respond sensitively to light. Recent studies in vertebrates and fungi indicate that p38 is involved in light-signaling to the circadian clock providing an interesting link between stress-induced and regularly rhythmic adaptations of animals to the environment, but the molecular and cellular mechanisms remained largely unknown. Here, we demonstrate by immunocytochemical means that p38 is expressed in Drosophila melanogaster's clock neurons and that it is activated in a clock-dependent manner. Surprisingly, we found that p38 is most active under darkness and, besides its circadian activation, additionally gets inactivated by light. Moreover, locomotor activity recordings revealed that p38 is essential for a wild-type timing of evening activity and for maintaining ∼ 24 h behavioral rhythms under constant darkness: flies with reduced p38 activity in clock neurons, delayed evening activity and lengthened the period of their free-running rhythms. Furthermore, nuclear translocation of the clock protein Period was significantly delayed on the expression of a dominant-negative form of p38b in Drosophila's most important clock neurons. Western Blots revealed that p38 affects the phosphorylation degree of Period, what is likely the reason for its effects on nuclear entry of Period. In vitro kinase assays confirmed our Western Blot results and point to p38 as a potential "clock kinase" phosphorylating Period. Taken together, our findings indicate that the p38 MAP Kinase is an integral component of the core circadian clock of Drosophila in addition to playing a role in stress-input pathways.

Diverse roles for MAPK signaling in circadian clocks

The mitogen-activated protein kinase (MAPK) family of genes aids cells in sensing both extracellular and intracellular stimuli, and emerging data indicate that MAPKs have fundamental, yet diverse, roles in the circadian biological clock. In the mammalian suprachiasmatic nucleus (SCN), MAPK pathways can function as inputs allowing the endogenous clock to entrain to 24h environmental cycles. MAPKs can also interact physically and/or genetically with components of the molecular circadian oscillator, implying that MAPKs can affect the cycling of the clock. Finally, circadian rhythms in MAPK pathway activation exist in many different tissue types and in model organisms, providing a mechanism to coordinately control the expression tissue-specific target genes at the proper time of day. As such, it should probably not come as a surprise that MAPK signaling pathways and circadian clocks affect similar biological processes and defects in either pathway lead to many of the same types of human diseases, highlighting the need to better define the mechanisms that link these two fundamental pathways together.

Direct transcriptional control of a p38 MAPK pathway by the circadian clock in Neurospora crassa

MAPK signal transduction pathways are important regulators of stress responses, cellular growth, and differentiation. In Neurospora, the circadian clock controls rhythms in phosphorylation of the p38-like MAPK (OS-2); however, the mechanism for this regulation is not known. We show that the WCC, a transcription factor and clock component, binds to the os-4 MAPKKK promoter in response to light and rhythmically in constant darkness, peaking in the subjective morning. Deletion of the WCC binding sites in the os-4 promoter disrupts both os-4 mRNA and OS-2 phosphorylation rhythms. The clock also indirectly regulates rhythmic expression of the histidyl-phosphotransferase gene, hpt-1, which peaks in the evening. Anti-phase expression of positive (OS-4) and negative (HPT-1) MAPK pathway regulators likely coordinate to enhance rhythmic MAPK activation to prepare cells to respond to osmotic stress during the day in the natural environment. Consistent with this idea, we show that wild type cells have a clock-dependent morning kinetic advantage in glycerol accumulation after salt stress as compared to evening treatment. Thus, circadian transcriptional control of MAPK pathway components leads to striking time-of-day-specific effects on the signaling status and physiological response of the pathway.

Circadian regulation of intracellular G-protein signalling mediates intercellular synchrony and rhythmicity in the suprachiasmatic nucleus

Synchronous oscillations of thousands of cellular clocks in the suprachiasmatic nucleus (SCN), the circadian centre, are coordinated by precisely timed cell–cell communication, the principle of which is largely unknown. Here we show that the amount of RGS16 (regulator of G protein signalling 16), a protein known to inactivate Gαi, increases at a selective circadian time to allow time-dependent activation of intracellular cyclic AMP signalling in the SCN. Gene ablation of Rgs16 leads to the loss of circadian production of cAMP and as a result lengthens circadian period of behavioural rhythm. The temporally precise regulation of the cAMP signal by clock-controlled RGS16 is needed for the dorsomedial SCN to maintain a normal phase-relationship to the ventrolateral SCN. Thus, RGS16-dependent temporal regulation of intracellular G protein signalling coordinates the intercellular synchrony of SCN pacemaker neurons and thereby defines the 24 h rhythm in behaviour.

Circadian rhythm is controlled by the suprachiasmatic nucleus and the mechanisms that control the rhythm are largely undiscovered. In this study, a G protein regulator, RGS16, is shown to be involved in the production of cyclic AMP that is required for the suprachiasmatic nucleus to maintain rhythm

Physiological responses of the circadian clock to acute light exposure at night

Circadian rhythms in physiological, endocrine and metabolic functioning are controlled by a neural clock located in the suprachiasmatic nucleus (SCN). This structure is endogenously rhythmic and the phase of this rhythm can be reset by light information from the eye. A key feature of the SCN is that while it is a small structure containing on the order of about 20,000 cells, it is amazingly heterogeneous. It is likely that anatomical heterogeneity reflects an underlying functional heterogeneity. In this review, we examine the physiological responses of cells in the SCN to light stimuli that reset the phase of the circadian clock, highlighting where possible the spatial pattern of such responses. Increases in intracellular calcium are an important signal in response to light, and this increase triggers many biochemical cascades that mediate responses to light. Furthermore, only some cells in the SCN are actually endogenously rhythmic, and these cells likely do not receive strong direct input from the retina. Therefore, this review also considers how light information is conveyed from the retinorecipient cells to the endogenously rhythmic cells that track circadian phase. A number of neuropeptides, including vasoactive intestinal polypeptide, gastrin-releasing peptide and substance P, may be particularly important in relaying such signals, but other neurochemicals such as GABA and nitric oxide may participate as well. A thorough understanding of the intracellular and intercellular responses to light, as well as the spatial arrangements of such responses may help identify important pharmacological targets for therapeutic interventions to treat sleep and circadian disorders.

A connection between MAPK pathways and circadian clocks

Circadian clocks and mitogen-activated protein kinase (MAPK) signaling pathways are fundamental features of eukaryotic cells. Both pathways provide mechanisms for cells to respond to environmental stimuli, and links between them are known. We recently reported that the circadian clock in Neurospora crassa regulates daily rhythms in accumulation of phosphorylated, and thus active, OS-2 MAPK, a relative of mammalian p38 MAPK, when cells are grown in constant conditions. In the absence of acute stress, rhythmically activated MAPK then signals to downstream effector molecules to regulate rhythmic expression of target genes of the pathway. Clock regulation of MAPK signaling pathways provides a mechanism to coordinately control major groups of genes such that they peak at the appropriate times of day to provide a growth and survival advantage to the organism by anticipating stresses. MAPK pathways are well known for their role in cell proliferation and tumor suppression. New evidence reveals that some mammalian clock components also function as tumor suppressors and rhythms in phospho-MAPK have been observed in higher eukaryotes. Thus, the role of the clock in regulation of the activity of MAPK pathways provides important clues into the function of the circadian clock as a tumor suppressor.

Circadian rhythmicity mediated by temporal regulation of the activity of p38 MAPK

Circadian clocks are composed of central oscillators, input pathways that transduce external information to the oscillators, and output pathways that allow the oscillators to temporally regulate cellular processes. Little is known about the output pathways. In this study, we show that the Neurospora crassa osmosensing MAPK pathway, essential for osmotic stress responses, is a circadian output pathway that regulates daily rhythms in the expression of downstream genes. Rhythmic activation of the highly conserved stress-activated p38-type MAPK [Osmotically Sensitive-2 (OS-2)] by the N. crassa circadian clock allows anticipation and preparation for hyperosmotic stress and desiccation that begin at sunrise. These results suggest a conserved role for MAPK pathways in circadian rhythmicity.

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.

Constant light housing during nursing causes human DSPS (delayed sleep phase syndrome) behaviour in Clock-mutant mice

Delayed sleep phase syndrome (DSPS) is very often seen among patients with sleep-wake rhythm disorders. Humans with the 3111C allele of the human Clock gene tend to demonstrate a higher evening preference on the morningness-eveningness (ME) preference test. DSPS is thought to be an extreme form of this evening preference. Clock-mutant mice have been proposed as an animal model of evening preference. In this study, we looked at whether constant light (LL) housing of Clock-mutant mice during lactation would result in evening preference and/or DSPS. Housed under light-dark (LD) or constant dark (DD) conditions during the lactation period, both wild-type and Clock-mutant mice did not show a phase-delay in the locomotor activity measured under light-dark conditions, whereas constant light housing during lactation significantly caused a delayed onset. The magnitude of the delay during the light-dark cycle was positively associated with free-running period measured during constant darkness. Among wild, heterozygote, and homozygote pups born from heterozygous dams, only homozygote pups showed a delayed onset. Constant light-housed Clock-mutant mice exhibited a lower number and delayed peak of phospho-MAPK-immunoreactive cells in core regions of the suprachiasmatic nucleus (SCN) compared to light-dark housed wild-type or Clock-mutant mice. Activity onset returned to normal with daily melatonin injection at the lights-off time for 5 days. The present results demonstrate that Clock-mutant mice exposed to constant light during lactation can function as an animal model of DSPS and can be used to gain an understanding of the ethological aspects of DSPS as well as to find medication for its treatment.

Light-inducible and clock-controlled expression of MAP kinase phosphatase 1 in mouse central pacemaker neurons

MAP kinase phosphatase 1 (MKP1) is a negative regulator for the mitogen-activated protein kinase (MAPK)-mediated signal transduction, a key pathway that leads to the regulated expression of circadian clock genes. Here the authors analyzed mkp1 expression by in situ hybridization and found that mkp1 is a light-inducible and clock-controlled gene expressed in the central pacemaker neurons of the hypothalamic SCN. Interestingly, mkp1 presents a marked similarity to the clock core gene per1 in terms of the gene expression profiles as well as the gene promoter organization. Both mkp1 and per1 are subject to bimodal regulation in the SCN: the external light-dependent acute up-regulation and the functional clock-dependent circadian oscillation. Consistent with this, the authors show that mkp1 gene has a per1-like promoter that contains 2 functionally distinct elements: cAMP-responsive element (CRE) and E-box. CRE sites present in the mkp1 promoter constitute the functional binding sites for the CRE binding protein (CREB), which serves as an important regulator that mediates the light-induced signaling cascades in the SCN neurons. Furthermore, the authors show that the E-box present in the mkp1 promoter is necessary and sufficient for transcriptional control exerted by circadian clock core regulators that include a positive complex CLOCK/BMAL1 and a negative factor CRY1. The authors' studies on mkp1 have identified for the first time a gene encoding a phosphatase that functions in light-dependent and time-of-day-dependent manners in the mammalian central clock structure SCN.

Diurnal regulation of the gastrin-releasing peptide receptor in the mouse circadian clock

In mammals, circadian rhythms are generated by the suprachiasmatic nuclei (SCN) of the hypothalamus. SCN neurons are heterogeneous and can be classified according to their function, anatomical connections, morphology and/or peptidergic identity. We focus here on gastrin-releasing peptide- (GRP) and on GRP receptor- (GRPr) expressing cells of the SCN. Pharmacological application of GRP in vivo or in vitro can shift the phase of circadian rhythms, and GRPr-deficient mice show blunted photic phase shifting. Given the in vivo and in vitro effects of GRP on circadian behavior and on SCN neuronal activity, we investigated whether the GRPr might be under circadian and/or diurnal control. Using in situ hybridization and autoradiographic receptor binding, we localized the GRPr in the mouse SCN and determined that GRP binding varies with time of day in animals housed in a light-dark cycle but not in conditions of constant darkness. The latter results were confirmed with Western blots of SCN tissue. Together, the present findings reveal that changes in GRPr are light driven and not endogenously organized. Diurnal variation in GRPr activity probably underlies intra-SCN signaling important for entrainment and phase shifting.

Cellular senescence impairs circadian expression of clock genes in vitro and in vivo

Circadian rhythms are regulated by a set of clock genes that form transcriptional feedback loops and generate circadian oscillation with a 24-hour cycle. Aging alters a broad spectrum of physiological, endocrine, and behavioral rhythms. Although recent evidence suggests that cellular aging contributes to various age-associated diseases, its effects on the circadian rhythms have not been examined. We report here that cellular senescence impairs circadian rhythmicity both in vitro and in vivo. Circadian expression of clock genes in serum-stimulated senescent cells was significantly weaker compared with that in young cells. Introduction of telomerase completely prevented this reduction of clock gene expression associated with senescence. Stimulation by serum activated the cAMP response element-binding protein, but the activation of this signaling pathway was significantly weaker in senescent cells. Treatment with activators of this pathway effectively restored the impaired clock gene expression of senescent cells. When young cells were implanted into young mice or old mice, the implanted cells were effectively entrained by the circadian rhythm of the recipients. In contrast, the entrainment of implanted senescent cells was markedly impaired. These results suggest that senescence decreases the ability of cells to transmit circadian signals to their clocks and that regulation of clock gene expression may be a novel strategy for the treatment of age-associated impairment of circadian rhythmicity.

A novel neuronal cell line derived from the ventrolateral region of the suprachiasmatic nucleus

The suprachiasmatic nucleus of the anterior hypothalamus is the center of an internal biological clock in mammals. Glutamate is the neurotransmitter of retino-hypothalamic tract responsible for mediating the circadian actions of light in rodents. N-methyl-d-aspartate receptors, particularly NR2B subunit are reported to be principally involved in photic resetting of the biological clock in vivo and in slice culture. But, the precise cellular mechanisms of the resetting are not elucidated, because no adequate neuronal cell lines derived from the suprachiasmatic nucleus have been established. We established a neuronal cell line, N14.5, derived from the suprachiasmatic nucleus of a transgenic rat harboring the temperature-sensitive simian virus 40 large T-antigen gene. When the cells were cultured at 39 degrees C, the morphological features were turned fibroblastic into neuronal round cell body with neurite extensions. These cells showed immunoreactivities for neuronal markers (betaIII-tubulin, microtubule-associated protein 2 and TAU2) and as well as for vasoactive intestinal peptide which is expressed in the ventrolateral region of the suprachiasmatic nucleus. The cells expressed N-methyl-d-aspartate receptors, particularly NR1 and NR2B subunits as revealed by quantitative PCR. N-methyl-d-aspartate activated phosphorylation of p44/42 mitogen-activated protein kinase and increased expression level of Per1 and Per2 mRNA. These results suggest that the N14.5 is a novel neuronal cell line derived from the ventrolateral region of the suprachiasmatic nucleus, and that N-methyl-d-aspartate receptors expressed in the cells are a functional receptor. The N14.5 cells may be a useful tool to elucidate numerous chronobiological processes, especially resetting mechanism induced by an external light signal.

The suprachiasmatic nucleus is a functionally heterogeneous timekeeping organ

Ever since the locus of the brain clock in the suprachiasmatic nucleus (SCN) was first described, methods available have both enabled and encumbered our understanding of its nature at the level of the cell, the tissue, and the animal. A combination of in vitro and in vivo approaches has shown that the SCN is a complex heterogeneous neuronal network. The nucleus is composed of cells that are retinorecipient and reset by photic input; those that are reset by nonphotic inputs; slave oscillators that are rhythmic only in the presence of the retinohypothalamic tract; endogenously rhythmic cells, with diverse period, phase, and amplitude responses; and cells that do not oscillate, at least on some measures. Network aspects of SCN organization are currently being revealed, but mapping these properties onto cellular characteristics of electrical responses and patterns of gene expression are in early stages. While previous mathematical models focused on properties of uniform coupled oscillators, newer models of the SCN as a brain clock now incorporate oscillator and gated, nonoscillator elements.

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.

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.

Effect of lithium on the circadian rhythms of locomotor activity and glycogen synthase kinase-3 protein expression in the mouse suprachiasmatic nuclei

A circadian clock located in the suprachiasmatic nucleus (SCN) regulates the period of physiological and behavioural rhythms to approximately 24 h. Lithium can lengthen the period of circadian rhythms in most organisms although little is known about the underlying mechanism. In the present study, we examined Drosophila shaggy ortholog glycogen synthase kinase-3 (GSK-3) protein expression in the SCN after lithium treatment. When locomotor activity was assessed, we found an association between the effect of lithium and the period of circadian oscillation as well as the level of GSK-3 protein expression. The decreased expression of GSK-3 and increased expression of phosphorylated GSK-3 (pGSK-3) resulted in an antiphasic circadian rhythm between the two in the SCN of lithium-treated mice housed under both light-dark and constant dark conditions. The enzyme activity of GSK-3 in the SCN was low when the level of pGSK-3 protein was high, as examined by immunoblotting analysis. Thus, GSK-3 enzyme activity has a correlation with the expression of GSK-3 protein in the SCN. Although both GSK-3 and pGSK-3 proteins are also expressed in the arcuate nucleus, lithium did not affect their expression. Based on the association that we found between lengthened circadian period and GSK-3 protein and GSK-3 activity in the SCN, we suggest that GSK-3 plays a role in regulating the period of the mammalian circadian pacemaker.

Temporal regulation of light-induced extracellular signal-regulated kinase activation in the suprachiasmatic nucleus

Signaling via the p42/p44 mitogen activated protein kinase (MAPK) pathway has been implicated as an intermediate event coupling light to entrainment of the mammalian circadian clock located in the suprachiasmatic nucleus (SCN). To examine how photic input dynamically regulates the activation state of the MAPK pathway, we monitored extracellular signal-regulated kinase (ERK) activation using different light stimulus paradigms. Compared with control animals not exposed to light, a 15 min light exposure during the early night triggered a marked increase in ERK activation and the translocation of ERK from the cytosol to the nucleus. ERK activation peaked 15 min after light onset, then returned to near basal levels within approximately 45 min. The MAPK pathway could be reactivated multiple times by light pulses spaced 45 min apart, indicating that the MAPK cascade rapidly resets and resolves individual light pulses into discrete signaling events. Under conditions of constant light (120 min), the time course for ERK activation, nuclear translocation, and inactivation was similar to the time course observed after a 15-min light treatment. The parallels between the ERK inactivation profiles elicited by a 15 and a 120 min light exposure suggest that SCN cells contain a MAPK pathway signal-termination mechanism that limits the duration of pathway activation. This concept was supported by the observation that the small G protein Ras, a regulator of the MAPK pathway, remained in the active, GTP-bound, state under conditions of constant light (120-min duration), indicating that photic information was relayed to the SCN and that SCN cells maintained their responsiveness for the duration of the light treatment. The SCN expressed both nuclear MAPK phosphatases (MKP-1 and MKP-2) and the cytosolic MAPK phosphatase Mkp-3, thus providing mechanisms by which light-induced ERK activation is terminated. Collectively, these observations provide important new information regarding the regulation of the MAPK cascade, a signaling intermediate that couples light to resetting of the SCN clock.