Photic induction of mPer1 and mPer2 in cry-deficient mice lacking a biological clock

Circadian regulation of sleep in mammals: Role of the suprachiasmatic nucleus

Despite significant progress in elucidating the molecular basis for circadian oscillations, the neural mechanisms by which the circadian clock organizes daily rhythms of behavioral state in mammals remain poorly understood. The objective of this review is to critically evaluate a conceptual model that views sleep expression as the outcome of opponent processes-a circadian clock-dependent alerting process that opposes sleep during the daily wake period, and a homeostatic process by which sleep drive builds during waking and is dissipated during sleep after circadian alerting declines. This model is based primarily on the evidence that in a diurnal primate, the squirrel monkey (Saimiri sciureus), ablation of the master circadian clock (the suprachiasmatic nucleus; SCN) induces a significant expansion of total daily sleep duration and a reduction in sleep latency in the dark. According to this model, the circadian clock actively promotes wake but only passively gates sleep; thus, loss of circadian clock alerting by SCN ablation impairs the ability to sustain wakefulness and causes sleep to expand. For comparison, two additional conceptual models are described, one in which the circadian clock actively promotes sleep but not wake, and a third in which the circadian clock actively promotes both sleep and wake, at different circadian phases. Sleep in intact and SCN-damaged rodents and humans is first reviewed, to determine how well the data fit these conceptual models. Neuroanatomical and neurophysiological studies are then reviewed, to examine the evidence for direct and indirect interactions between the SCN circadian clock and sleep-wake circuits. Finally, sleep in SCN-ablated squirrel monkeys is re-examined, to consider its compatibility with alternative models of circadian regulation of sleep. In aggregate, the behavioral and neurobiological evidence suggests that in rodents and humans, the circadian clock actively promotes both wake and sleep, at different phases of the circadian cycle. The hypersomnia of SCN-ablated squirrel monkeys is unique in magnitude, but is not incompatible with a role for the SCN pacemaker in actively promoting sleep.

Non-image-forming ocular photoreception in vertebrates

It has been accepted for a hundred years or more that rods and cones are the only photoreceptive cells in the retina. The light signals generated in rods and cones, after processing by downstream retinal neurons (bipolar, horizontal, amacrine and ganglion cells), are transmitted to the brain via the axons of the ganglion cells for further analysis. In the past few years, however, convincing evidence has rapidly emerged indicating that a small subset of retinal ganglion cells in mammals is also intrinsically photosensitive. Melanopsin is the signaling photopigment in these cells. The main function of the inner-retina photoreceptors is to generate and transmit non-image-forming visual information, although some role in conventional vision (image detection) is also possible.

Circadian clock genes as modulators of sensitivity to genotoxic stress

A broad variety of organisms display circadian rhythms (i.e., oscillations with 24-hr periodicities) in many aspects of their behavior, physiology and metabolism. These rhythms are under genetic control and are generated endogenously at the cellular level. In mammals, the core molecular mechanism of the oscillator consists of two transcriptional activators, CLOCK and BMAL1, and their transcriptional targets, CRYPTOCHROMES (CRYS) and PERIODS (PERS). The CRY and PER proteins function as negative regulators of CLOCK/BMAL1 activity, thus forming the major circadian autoregulatory feedback loop. It is believed that the circadian clock system regulates daily variations in output physiology and metabolism through periodic activation/repression of the set of clock-controlled genes that are involved in various metabolic pathways. Importantly, circadian-controlled pathways include those that determine in vivo responses to genotoxic stress. By using circadian mutant mice deficient in different components of the molecular clock system, we have established genetic models that correlate with the two opposite extremes of circadian cycle as reflected by the activity of the CLOCK/BMAL1 transactivation complex. Comparison of the in vivo responses of these mutants to the chemotherapeutic drug, cyclophosphamide (CY), has established a direct correlation between drug toxicity and the functional status of the CLOCK/BMAL1 transcriptional complex. We have also demonstrated that CLOCK/BMAL1 modulates sensitivity to drug-induced toxicity by controlling B cell responses to active CY metabolites. These results suggest that the sensitivity of cells to genotoxic stress induced by anticancer therapy may be modulated by CLOCK/BMAL1 transcriptional activity. Further elucidation of the molecular mechanisms of circadian control as well as identification of specific pharmacological modulators of CLOCK/BMAL1 activity are likely to lead to the development of new anti-cancer treatment schedules with increased therapeutic index and reduced morbidity.

Environmental light and suprachiasmatic nucleus interact in the regulation of body temperature

The mammalian biological clock, located in the suprachiasmatic nucleus (SCN), is crucial for circadian rhythms in physiology and behavior. However, equivocal findings have been reported on its role in the circadian regulation of body temperature. The goal of the present studies was to investigate the interaction between the SCN and environmental light in the regulation of body temperature. All recordings were performed by telemetry in free moving male Wistar rats. Firstly, we demonstrated an endogenous circadian rhythm in body temperature independent of locomotor activity. This rhythm was abolished by stereotactic lesioning of the SCN. Secondly, we demonstrated a circadian phase-dependent suppressive effect of light ('negative masking') on body temperature. Light suppressed body temperature more at the end of the subjective night (circadian time [CT] 22) than in the middle (CT 6) and at the end (CT 10) of the subjective day. This circadian-phase dependent suppression was not demonstrated in SCN-lesioned animals. Surprisingly, after half a year of recovery from lesioning of the SCN, light regained its suppressing action on body temperature, resulting in a daily body temperature rhythm only under light-dark conditions. In contrast to body temperature, light could not substantially mimic a daytime inhibitory SCN-output in the regulation of heart rate and locomotor activity. The present results suggest that, after lesioning of the SCN as main relay station for the immediate body temperature-inhibition by light, secondary relay nuclei can fully take over this function of the SCN. These findings provide a possible explanation for the controversy in literature over the question whether the SCN is required for the diurnal rhythm in body temperature. Furthermore, they show that light may have an acute effect on behavior and physiology of the organism via the SCN, which extends beyond the generally acknowledged effect on melatonin secretion.

Reduced alpha-adrenoceptor responsiveness and enhanced baroreflex sensitivity in Cry-deficient mice lacking a biological clock

To reveal the role of clock genes in generating the circadian rhythm of baroreflexes, we continuously measured mean arterial pressure and baroreflex sensitivity in free-moving normal wild-type mice, and in Cry-deficient mice which lack a circadian rhythm, in constant darkness for 24 h. In wild-type mice the mean arterial pressure was higher at night than during the day, and was accompanied by a significantly enhanced baroreflex sensitivity of -13.6 +/- 0.8 at night compared with -9.7 +/- 0.7 beats min(-1) mmHg(-1) during the day (P < 0.001). On the other hand, diurnal changes in arterial pressure disappeared in Cry-deficient mice with remarkably enhanced baroreflex sensitivity compared with wild-type mice (P < 0.001): -21.9 +/- 1.6 at night and -23.1 +/- 2.1 beats min(-1) mmHg(-1) during the day. Moreover, the mean arterial pressure response to 10 microg kg(-1) of phenylephrine, an alpha1-adrenoceptor agonist, was severely suppressed in Cry-deficient mice regardless of time, while that for the wild-type mice was 10.1 +/- 1.9 mmHg in the night, significantly lower than 22.0 +/- 3.5 mmHg in the day (P < 0.01). These results suggest that CRY genes are involved in generating the circadian rhythm of baroreflex sensitivity, partially by regulating alpha(1)-adrenoceptor-mediated vasoconstriction in peripheral vessels.

Circadian clock genes directly regulate expression of the Na+/H+ exchanger NHE3 in the kidney

BACKGROUND

Daily rhythms in mammalian physiology are generated by a transcription/translation feedback loop orchestrated by a set of clock genes. However, little is known about the molecular cascade from the clock gene oscillators to cellular function.

METHODS

The mRNA expression profiles of NHE3 and clock genes were examined in mice and rat kidneys. First, luciferase assays followed by a site directed mutagenesis of an E-box sequence were used to assess the CLOCK:BMAL1-transactivated NHE3 promoter activity. A direct binding of CLOCK:BMAL1 heterodimers to an E-box sequences of NHE3 promoter was confirmed by electrophoretic mobility shift assay (EMSA).

RESULTS

We present evidence that renal tubular NHE3, the Na(+)/H(+) exchanger critical for systemic electrolyte and acid-base homeostasis, is a clock-controlled gene regulated directly by CLOCK:BMAL1 heterodimers in kidneys. NHE3 mRNA level in rat kidney displayed circadian kinetics, and this circadian expression was severely blunted in homozygous CRY1/2 double-deficient mice, suggesting that the transcriptional machinery of peripheral clocks in renal tubular cells directly regulates the circadian expression of NHE3. By analyzing the 5' upstream region of the NHE3 gene, we found an E box critical for the transcription of NHE3 via the CLOCK:BMAL1-driven circadian oscillator. The circadian expression of NHE3 mRNA was reflected by oscillating protein levels in the proximal tubules of the rat kidney.

CONCLUSION

NHE3 should represent an output gene of the peripheral oscillators in kidney, which is regulated directly by CLOCK:BMAL1 heterodimers.

Regulation of prokineticin 2 expression by light and the circadian clock

Background

The suprachiasmatic nucleus (SCN) contains the master circadian clock that regulates daily rhythms of many physiological and behavioural processes in mammals. Previously we have shown that prokineticin 2 (PK2) is a clock-controlled gene that may function as a critical SCN output molecule responsible for circadian locomotor rhythms. As light is the principal zeitgeber that entrains the circadian oscillator, and PK2 expression is responsive to nocturnal light pulses, we further investigated the effects of light on the molecular rhythm of PK2 in the SCN. In particular, we examined how PK2 responds to shifts of light/dark cycles and changes in photoperiod. We also investigated which photoreceptors are responsible for the light-induced PK2 expression in the SCN. To determine whether light requires an intact functional circadian pacemaker to regulate PK2, we examined PK2 expression in cryptochrome1,2-deficient (Cry1-/-Cry2-/-) mice that lack functional circadian clock under normal light/dark cycles and constant darkness.

Results

Upon abrupt shifts of the light/dark cycle, PK2 expression exhibits transients in response to phase advances but rapidly entrains to phase delays. Photoperiod studies indicate that PK2 responds differentially to changes in light period. Although the phase of PK2 expression expands as the light period increases, decreasing light period does not further condense the phase of PK2 expression. Genetic knockout studies revealed that functional melanopsin and rod-cone photoreceptive systems are required for the light-inducibility of PK2. In Cry1-/-Cry2-/- mice that lack a functional circadian clock, a low amplitude PK2 rhythm is detected under light/dark conditions, but not in constant darkness. This suggests that light can directly regulate PK2 expression in the SCN.

Conclusion

These data demonstrate that the molecular rhythm of PK2 in the SCN is regulated by both the circadian clock and light. PK2 is predominantly controlled by the endogenous circadian clock, while light plays a modulatory role. The Cry1-/-Cry2-/- mice studies reveal a light-driven PK2 rhythm, indicating that light can induce PK2 expression independent of the circadian oscillator. The light inducibility of PK2 suggests that in addition to its role in clock-driven rhythms of locomotor behaviour, PK2 may also participate in the photic entrainment of circadian locomotor rhythms.

Circadian sensitivity to the chemotherapeutic agent cyclophosphamide depends on the functional status of the CLOCK/BMAL1 transactivation complex

The circadian clock controls many aspects of mammalian physiology, including responses to cancer therapy. We find that wild-type and circadian mutant mice demonstrate striking differences in their response to the anticancer drug cyclophosphamide (CY). While the sensitivity of wild-type mice varies greatly, depending on the time of drug administration, Clock mutant and Bmal1 knockout mice are highly sensitive to treatment at all times tested. On the contrary, mice with loss-of-function mutations in Cryptochrome (Cry1-/-Cry2-/- double knockouts) were more resistant to CY compared with their wild-type littermates. Thus, both time-of-day and allelic-dependent variations in response to chemotherapy correlate with the functional status of the circadian CLOCK/BMAL1 transactivation complex. Pharmacokinetic analysis of plasma concentration of different CY metabolites shows that, in contrast to the traditional view, circadian variations in drug sensitivity cannot be attributed to the changes in the rates of CY metabolic activation and/or detoxification. At the same time, mice of different circadian genotypes demonstrate significant differences in B cell responses to toxic CY metabolites: B cell survival/recovery rate was directly correlated with the in vivo drug sensitivity. Based on these results, we propose that the CLOCK/BMAL1 transcriptional complex affects the lethality of chemotherapeutic agents by modulating the survival of the target cells necessary for the viability of the organism.

Clock genes in cell clocks: roles, actions, and mysteries

Cellular events must be organized in the time dimension as well as in the space dimension for many proteins to perform their cellular functions effectively. The intracellular molecular oscillating loops that compose the cell's circadian clock coordinate the timing of the expression of a variety of genes with basic or specific cellular functions. In mammals, the temporal pattern of clock gene expression generated in each SCN neuron is coupled to those of other cells and, amplified, spreads its signals through the brain and then, via feeding behavior, glucocorticoids, and sympathetic nerves, to peripheral organs. These peripheral organs have their own circadian clocks. In some tissues, such as liver, there is also a clock-regulating cell cycle, which interacts strongly with the components and temporal organization of the circadian clock. Some tissues, however, such as testis, express clock genes whose function, if any, remains unclear. Furthermore, circadian clock function may be suspended in differentiating tissue. Thus, the prominence of circadian organization may not apply equally to all tissues under all conditions.

A noncanonical E-box enhancer drives mouse Period2 circadian oscillations in vivo

The mouse Period2 (mPer2) locus is an essential negative-feedback element of the mammalian circadian-clock mechanism. Recent work has shown that mPer2 circadian gene expression persists in both central and peripheral tissues. Here, we analyze the mouse mPer2 promoter and identify a circadian enhancer (E2) with a noncanonical 5'-CACGTT-3' E-box located 20 bp upstream of the mPer2 transcription start site. The E2 enhancer accounts for most circadian transcriptional drive of the mPer2 locus by CLOCK:BMAL1, is a major site of DNaseI hypersensitivity in this region, and is constitutively bound by a transcriptional complex containing the CLOCK protein. Importantly, the E2 enhancer is sufficient to drive self-sustained circadian rhythms of luciferase activity in central and peripheral tissues from mPer2-E2::Luciferase transgenic mice with tissue-specific phase and period characteristics. Last, genetic analysis with mutations in Clock and Bmal1 shows that the E2 enhancer is a target of CLOCK and BMAL1 in vivo.

Oligodeoxynucleotide methods for analyzing the circadian clock in the suprachiasmatic nucleus

The recent identification of specific genes responsible for the generation of endogenous circadian rhythmicity in the suprachiasmatic nucleus presents a new level of investigation into endogenous rhythmicity and mechanisms of synchronization of this circadian clock with the environmental light?dark cycle. This article describes techniques that employ antisense and decoy oligodeoxynucleotides (ODN) to determine the roles of specific molecular substrates both in endogenous rhythmicity and in regulating the effects of light on the mammalian circadian clock. Application of antisense ODN technology has revealed a role for timeless (Tim) in the core clock mechanism and established that induction of period1 (Per1) is required for light responsiveness. Likewise, a decoy ODN designed to sequester activated CREB protein definitively demonstrated a requirement for CRE-mediated transcription in light signaling. Experiments designed with these molecular tools offer new insights on the interaction of cellular processes and signaling with the molecular clockworks.

Mammalian circadian biology: elucidating genome-wide levels of temporal organization

During the past decade, the molecular mechanisms underlying the mammalian circadian clock have been defined. A core set of circadian clock genes common to most cells throughout the body code for proteins that feed back to regulate not only their own expression, but also that of clock output genes and pathways throughout the genome. The circadian system represents a complex multioscillatory temporal network in which an ensemble of coupled neurons comprising the principal circadian pacemaker in the suprachiasmatic nucleus of the hypothalamus is entrained to the daily light/dark cycle and subsequently transmits synchronizing signals to local circadian oscillators in peripheral tissues. Only recently has the importance of this system to the regulation of such fundamental biological processes as the cell cycle and metabolism become apparent. A convergence of data from microarray studies, quantitative trait locus analysis, and mutagenesis screens demonstrates the pervasiveness of circadian regulation in biological systems. The importance of maintaining the internal temporal homeostasis conferred by the circadian system is revealed by animal models in which mutations in genes coding for core components of the clock result in disease, including cancer and disturbances to the sleep/wake cycle.

Melanopsin containing retinal ganglion cells are light responsive from birth

Photoentrainment of the biological clock located in the suprachiasmatic nucleus (SCN) begins shortly after birth. Here we show using c-FOS immunoreactivity as a marker for neuronal activity that the melanopsin/PACAP containing retinal ganglion cells (RGCs) which project to the SCN as the retinohypothalamic tract (RHT) are responsive to light from birth. After postnatal day 12 where the classical photoreceptors become functional other RGCs and cells of the inner nuclear cell layer also respond to light. Light also induces c-FOS immunoreactivity in the retinorecipient SCN from the first postnatal day and accordingly PACAP immunoreactive fibres are visible in the SCN. The results indicate that the retina is light responsive before functional rods and cones and that the RHT is functional from birth supporting that photoentrainment of the biological clock begins shortly after birth.

Model based conjectures on mammalian clock controversies

We explore some predictions of a previously developed detailed model of molecular timekeeping in mammals (Forger and Peskin, PNAS, 100:14806) in areas where our understanding of clock mechanisms are incomplete. It is conjectured that: (1) the clock's 24-h period depends on mRNA stability. (2) REV-ERBalpha suppresses and/or entrains rhythms in peripheral tissues by regulating CRY1 transcription. (3) CLK:BMAL1 oscillations are suppressed in the suprachiasmatic nuclei to enhance oscillations in other proteins. (4) PER2 is ineffective at causing phase advances because it is not induced by light during the late night. (5) The clock is a limit cycle oscillator that shows characteristics of the evening and morning oscillator model.

Circadian genes in a blind subterranean mammal III: molecular cloning and circadian regulation of cryptochrome genes in the blind subterranean mole rat, Spalax ehrenbergi superspecies

The blind subterranean mole rat superspecies Spalax ehrenbergi is an extreme example of mammalian adaptation to life underground. Though this rodent is totally visually blind, harboring a drastically degenerated subcutaneous rudimentary eye, its daily activity rhythm is entrainable to LD cycles. This indicates that it confers light information to the clock, as has been previously shown by the authors in behavioral studies as well as by molecular analyses of its Clock/MOP3 and its three Per genes. The Cryptochrome (Cry) genes found in animals and plants act both as photoreceptors and as essential components of the negative feedback mechanism of the biological clock. To further understand the circadian system of this unique mammal, the authors cloned and characterized the open reading frame of Spalax Cry1 and Cry2. The Spalax CRY1 protein is significantly closer to the human homolog than to the mice one, in contrast to the evolutionary expectations. They have found two isoforms of Cry2 in Spalax, which differ in their 5' end of the open reading frame and defined their expression in Spalax populations. They found a large and significant excess of heterozygotes of sCry2 (sCry2L/S genotype). Both sCry1 and sCry2 mRNAs were found in the SCN, the eye, the harderian gland, as well as in a wide range of peripheral tissues. Their expression pattern under different LD conditions has also been analyzed. As was already shown for other circadian genes, despite being blind and living in darkness, the Cry genes of Spalax behave in a similar, though not identical, pattern as in sighted animals. Once again, the results indicate that the uniquely hypertrophied harderian gland of Spalax plays a key role in its circadian system.

Regulation of photoreceptor Per1 and Per2 by light, dopamine and a circadian clock

In the Xenopus laevis retina, a principal model for retinal circadian organization, photoreceptors have all the properties of circadian oscillators. However, rhythmic oscillations of Per1 gene expression in the inner retina (but not photoreceptors) have been reported in mice with the suggestion that mice and frogs have a different retinal circadian organization. Although it is known that two period genes (xPer1 and xPer2) exhibit different temporal patterns of expression in the Xenopus retina, and that one (xPer2) is directly responsive to light and dopamine, it is not known whether this reflects the properties of period genes within photoreceptor oscillators or among distinct retinal cell populations. We addressed this by determining the cellular site of light and dopamine regulated xPer2 expression, and the diurnal expression of both xPer1 and xPer2 using in situ hybridization. Our data show that both xPer1 and xPer2 are expressed in most cell types in the retina, including inner nuclear neurons and ganglion cells. However, light and quinpirole, a dopamine agonist, increase xPer2 levels specifically in photoreceptors, and the effect of quinpirole, but not light, is blocked by pCPT-cAMP. Furthermore, antiphasic diurnal expression of xPer1 and xPer2 also occurs in photoreceptors. Our analysis does not provide insight into the near constitutive expression of period genes in the inner retina, but supports a model in which light- and dopamine regulated-xPer2 and rhythmic xPer1 play critical roles in entrainment and circadian oscillations within photoreceptors.

Modeling feedback loops of the Mammalian circadian oscillator

The suprachiasmatic nucleus governs daily variations of physiology and behavior in mammals. Within single neurons, interlocked transcriptional/translational feedback loops generate circadian rhythms on the molecular level. We present a mathematical model that reflects the essential features of the mammalian circadian oscillator to characterize the differential roles of negative and positive feedback loops. The oscillations that are obtained have a 24-h period and are robust toward parameter variations even when the positive feedback is replaced by a constantly expressed activator. This demonstrates the crucial role of the negative feedback for rhythm generation. Moreover, it explains the rhythmic phenotype of Rev-erbalpha-/- mutant mice, where a positive feedback is missing. The interplay of negative and positive feedback reveals a complex dynamics. In particular, the model explains the unexpected rescue of circadian oscillations in Per2Brdm1/Cry2-/- double-mutant mice (Per2Brdm1 single-mutant mice are arrhythmic). Here, a decrease of positive feedback strength associated with mutating the Per2 gene is compensated by the Cry2-/- mutation that simultaneously decreases the negative feedback strength. Finally, this model leads us to a testable prediction of a molecular and behavioral phenotype: circadian oscillations should be rescued when arrhythmic Per2Brdm1 mutant mice are crossed with Rev- erbalpha -/- mutant mice.

Circadian and light-induced transcription of clock gene Per1 depends on histone acetylation and deacetylation

Circadian clock genes are regulated through a transcriptional-translational feedback loop. Alterations of the chromatin structure by histone acetyltransferases and histone deacetylases (HDACs) are commonly implicated in the regulation of gene transcription. However, little is known about the transcriptional regulation of mammalian clock genes by chromatin modification. Here, we show that the state of acetylated histones fluctuated in parallel with the rhythm of mouse Per1 (mPer1) or mPer2 expression in fibroblast cells and liver. Mouse CRY1 (mCRY1) repressed transcription with HDACs and mSin3B, which was relieved by the HDAC inhibitor trichostatin A (TSA). In turn, TSA induced endogenous mPer1 expression as well as the acetylation of histones H3 and H4, which interacted with the mPer1 promoter region in fibroblast cells. Moreover, a light pulse stimulated rapid histone acetylation associated with the promoters of mPer1 or mPer2 in the suprachiasmatic nucleus (SCN) and the binding of phospho-CREB in the CRE of mPer1. We also showed that TSA administration into the lateral ventricle induced mPer1 and mPer2 expression in the SCN. Taken together, these data indicate that the rhythmic transcription and light induction of clock genes are regulated by histone acetylation and deacetylation.

Real-time imaging reveals spatiotemporal dynamics of cellular circadian clocks

Circadian clocks have been localized to discrete sites within the nervous systems of several organisms, and in mammals to the suprachiasmatic nucleus (SCN) in the anterior hypothalamus. The clock in the SCN is composed of multiple autonomous single-cell oscillators, and new studies now allow an unprecedented look at their oscillatory activities over repeated cycles in tissue slices in vitro. Initial data reinforce the importance of intercellular membrane events for constructing a functional and reliable tissue pacemaker.

A BMAL1 mutant with arginine 91 substituted with alanine acts as a dominant negative inhibitor

Basic-Helix-Loop-Helix-Per-Arnt-Sim (bHLH-PAS) transcription factor, Brain-Muscle-Arnt-Like-protein 1 (BMAL1), forms a heterodimer with the CLOCK protein. The BMAL1/CLOCK complex binds to a specific DNA sequence and plays an essential role in the generation of the circadian rhythm. The basic region of BMAL1 contains an E-R-X-R motif that is highly conserved among basic-helix-loop-helix (bHLH) transcription factors that bind to the E-box transcription element, and is thus thought to constitute a structure required for recognition of this DNA sequence. To understand the role of arginine 91 (E-K-R-R) within the basic region of BMAL1, we examined the effect of mutation of this residue on BMAL1 function. Co-immunoprecipitation and electrophoretic mobility shift assays (EMSA) showed that a BMAL1 R91A mutant forms a heterodimer with CLOCK, but is unable to support DNA binding in vitro. Consistent with these observations, plasmids encoding the R91A and R91H mutants of BMAL1 were unable to stimulate transcription from an E-box containing reporter construct in transient co-transfection analyses in NIH 3T3 cells. More importantly, these mutants suppressed the activation of transcription from the reporter construct mediated by wild-type BMAL1, indicating that they possess dominant negative activity in this assay. These results document further the importance of the basic region of BMAL1 in binding to DNA and suggest that this new mutant might be a useful tool for further analysis of BMAL1 function.

N-ethyl-N-nitrosourea mouse mutants in the dissection of behavioural and psychiatric disorders

Twin and adoption studies have consistently implicated genetics in the aetiology of psychiatric and behavioural disorders. The identification of the genes and molecular pathways that are associated with these traits using linkage studies has been difficult because psychiatric disorders are almost always non-mendelian, heterogeneous, involve multiple genetic loci and are influenced significantly by environmental factors. Mouse models that are based on intermediate signatures of psychiatric disease and pharmacological responsiveness hold promise as a complementary approach to dissecting the molecular basis of neurobehavioural disorders. This has been made possible by the development and refinement of gene targeting technologies and the use of super-efficient chemical mutagens. N-ethyl-N-nitrosourea (ENU) mutagenesis in the mouse, when coupled to a battery of sensitive behavioural screens, is an effective way of creating and identifying novel mouse behavioural mutants. Here, the concept of screening for ENU mutants is introduced while progress with two behavioural screens, an "anxiety" screen and a circadian screen, are presented. It is hoped that the study of mouse mutants that have arisen from these screens will provide new insights into the genetic basis of abnormal behaviour and that they might lead to the development of novel therapeutic compounds for human psychiatric disease.

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.

Indication of circadian oscillations in the rat pancreas

The central circadian oscillator of the suprachiasmatic nucleus controls diurnal rhythmicity of the body with light as its dominant zeitgeber. Recently, peripheral oscillators have been detected in liver and heart, which follow as yet unidentified cues. In this study real-time reverse transcription-polymerase chain reaction (RT-PCR) was used in analysis of the expression of the major clock genes Per1, Per2, Bmal1, Cry1, Tim (timeless) and Clock, as well as of the output genes Dbp and Rev-erbalpha in the pancreatic tissue of rats. The results presented here indicate a robust circadian expression of clock genes (e.g. Per1 and Bmal1) and the probable existence of a peripheral oscillator in the pancreas. Whether this oscillator regulates the diverse functions of the islets of Langerhans remains to be elucidated.

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.

Cyanobacterial circadian clocks — timing is everything

For more than three billion years, the organisms on this planet have known, like Little Orphan Annie, that "The sun'll come out tomorrow", and many have honed their biochemistry to exploit this knowledge. The cyanobacteria have had ample time to fashion a suitable timepiece, as they are among the oldest inhabitants of the earth. For these organisms, light is food, and it is a nutrient that shows up at the same time every day. Not surprisingly, cyanobacteria have learned to arrange their days around dinnertime.

A detailed predictive model of the mammalian circadian clock

Experimental data on the circadian (approximately 24-h) clock in mammalian cells are vast, diverse, and detailed. Mathematical models are therefore needed to piece these data together and to study overall clock behavior. Previous models have focused on Neurospora or Drosophila or can be converted to a Drosophila model simply by renaming variables. Those models used Hill-type terms for transcription regulation and Michaelis-Menten type or delay terms for posttranslation regulation. Recent mammalian experimental data call into question some of the assumptions in these approaches. Moreover, gene duplication has led to more proteins in the mammalian system than in lower organisms. Here we develop a detailed distinctly mammalian model by using mass action kinetics. Parameters for our model are found from experimental data by using a coordinate search method. The model accurately predicts the phase of entrainment, amplitude of oscillation, and shape of time profiles of clock mRNAs and proteins and is also robust to parameter changes and mutations.

Synchronization of cellular clocks in the suprachiasmatic nucleus

Individual cellular clocks in the suprachiasmatic nucleus (SCN), the circadian center, are integrated into a stable and robust pacemaker with a period length of about 24 hours. We used real-time analysis of gene expression to show synchronized rhythms of clock gene transcription across hundreds of neurons within the mammalian SCN in organotypic slice culture. Differentially phased neuronal clocks are topographically arranged across the SCN. A protein synthesis inhibitor set all cell clocks to the same initial phase and, after withdrawal, intrinsic interactions among cell clocks reestablished the stable program of gene expression across the assemblage. Na+-dependent action potentials contributed to establishing cellular synchrony and maintaining spontaneous oscillation across the SCN.

Chronotherapy for cancer

Cancer chronotherapy is attracting attention as a novel and logical therapy in which anti-cancer drugs are administered with optimal timing according to circadian rhythms of anti-cancer action and those of adverse effects on normal cells. Advances in chronobiology have identified the suprachiasmatic nucleus (SCN) as the center of biological rhythms and the area in which clock genes such as PER1, PER2, PER3, CLOCK, BMAL1, TIM, CRY1, CRY2, tau act to generate and coordinate biological rhythms. These findings have led to the development of chronotherapy. Clinically, patients with advanced gastrointestinal cancer have been treated by chronomodulated chemotherapy with good response. For colorectal cancer patients with unresectable liver metastases, chronotherapy with l-OHP + 5-FU + FA (folinic acid) has been reported to allow complete surgical resection of liver metastases, resulting in 39-50% 5-year survival. Many believe that chronotherapy will become accepted as a refined and advantageous therapeutic option for not only cancer but also for other diseases, due to its universally applicable principles.

An abrupt shift in the day/night cycle causes desynchrony in the mammalian circadian center

The suprachiasmatic nucleus (SCN) is the neuroanatomical locus of the mammalian circadian pacemaker. Here we demonstrate that an abrupt shift in the light/dark (LD) cycle disrupts the synchronous oscillation of circadian components in the rat SCN. The phases of the RNA cycles of the period genes Per1 and Per2 and the cryptochrome gene Cry1 shifted rapidly in the ventrolateral, photoreceptive region of the SCN, but were relatively slow to shift in the dorsomedial region. During the period of desynchrony, the animals displayed increased nighttime rest, the timing of which was inversely correlated with the expression of Per1 mRNA in the dorsomedial SCN. Molecular resynchrony required approximately 6 d after a 10 hr delay and 9 approximately 13 d after a 6 hr advance of the LD cycle and was accompanied by the reemergence of normal rest-activity patterns. This dissociation and slow resynchronization of endogenous oscillators within the SCN after an LD cycle shift suggests a mechanism for the physiological symptoms that constitute jet lag.

Loss of circadian rhythmicity in aging mPer1-/-mCry2-/- mutant mice

The mPer1, mPer2, mCry1, and mCry2 genes play a central role in the molecular mechanism driving the central pacemaker of the mammalian circadian clock, located in the suprachiasmatic nuclei (SCN) of the hypothalamus. In vitro studies suggest a close interaction of all mPER and mCRY proteins. We investigated mPER and mCRY interactions in vivo by generating different combinations of mPer/mCry double-mutant mice. We previously showed that mCry2 acts as a nonallelic suppressor of mPer2 in the core clock mechanism. Here, we focus on the circadian phenotypes of mPer1/mCry double-mutant animals and find a decay of the clock with age in mPer1-/- mCry2-/- mice at the behavioral and the molecular levels. Our findings indicate that complexes consisting of different combinations of mPER and mCRY proteins are not redundant in vivo and have different potentials in transcriptional regulation in the system of autoregulatory feedback loops driving the circadian clock.

Melanopsin, ganglion-cell photoreceptors, and mammalian photoentrainment

An understanding of the retinal mechanisms in mammalian photoentrainment will greatly facilitate optimization of the wavelength, intensity, and duration of phototherapeutic treatments designed to phase shift endogenous biological rhythms. A small population of widely dispersed retinal ganglion cells projecting to the suprachiasmatic nucleus in the hypothalamus is the source of the critical photic input. Recent evidence has shown that many of these ganglion cells are directly photosensitive and serve as photoreceptors. Melanopsin, a presumptive photopigment, is an essential component in the phototransduction cascade within these intrinsically photosensitive ganglion cells and plays an important role in the retinal photoentrainment pathway. This review summarizes recent findings related to melanopsin and melanopsin ganglion cells and lists other retinal proteins that might serve as photopigments in the mammalian photoentrainment input pathway.

The circadian E-box: when perfect is not good enough

Life on earth has evolved on a photic carousel, spinning through alternating periods of light and darkness. This playful image belies the fact that only those organisms that learned how to benefit from the recurring features in their environment were allowed to ride on. This selection process has engendered many daily rhythms in our biosphere, most of which rely on the anticipatory power of an endogenously generated marker of phase: the biological clock. The basic mechanisms driving this remarkable device have been really tough to decode but are finally beginning to unravel as chronobiologists probe deeper and wider in and around the recently discovered gears of the clock. Like its chemical predecessors, biological circadian oscillators are characterized by interlaced positive and negative feedback loops, but with constants and variables carefully balanced to achieve an approximately 24h period. The loops at the heart of these biological oscillators are sustained by specific patterns of gene expression and precisely tuned posttranscriptional modifications. It follows that a molecular understanding of the biological clock hinges, in no small measure, on a better understanding of the cis-acting elements that bestow a given gene with its circadian properties. The present review summarizes what is known about these elements and what remains to be elucidated.

Synchronization of the molecular clockwork by light- and food-related cues in mammals

The molecular clockwork in mammals involves various clock genes with specific temporal expression patterns. Synchronization of the master circadian clock located in the suprachiasmatic nucleus (SCN) is accomplished mainly via daily resetting of the phase of the clock by light stimuli. Phase shifting responses to light are correlated with induction of Per1, Per2 and Dec1 expression and a possible reduction of Cry2 expression within SCN cells. The timing of peripheral oscillators is controlled by the SCN when food is available ad libitum. Time of feeding, as modulated by temporal restricted feeding, is a potent 'Zeitgeber' (synchronizer) for peripheral oscillators with only weak synchronizing influence on the SCN clockwork. When restricted feeding is coupled with caloric restriction, however, timing of clock gene expression is altered within the SCN, indicating that the SCN function is sensitive to metabolic cues. The components of the circadian timing system can be differentially synchronized according to distinct, sometimes conflicting, temporal (time of light exposure and feeding) and homeostatic (metabolic) cues.

Suprachiasmatic nucleus grafts restore circadian behavioral rhythms of genetically arrhythmic mice

The mammalian master clock driving circadian rhythmicity in physiology and behavior resides within the suprachiasmatic nuclei (SCN) of the hypothalamus. SCN neurons contain a molecular oscillator composed of a set of clock genes that acts in intertwined negative and positive feedback loops [1]. In addition, all peripheral tissues analyzed thus far have been shown to contain circadian oscillators [2]. This raises the question of whether the central circadian pacemaker in the SCN is sufficient to evoke behavioral rhythms or whether peripheral circadian clockworks are also required. Mice with a mutated CLOCK protein (a transcriptional activator of E box-containing clock and clock output genes) or lacking both CRYPTOCHROMES, mCRY1 and mCRY2 proteins (inhibitors of E box-mediated transcription), lack circadian rhythmicity in behavior [3,4]. Here, we show that transplantation of mouse fetal SCN tissue into the hypothalamus restores free-running circadian behavioral rhythmicity in Clock mutant or mCry1/mCry2 double knockout mice. The periodicity of the emerged rhythms is determined by the genetic constitution (i.e., wild-type or mCry2 knockout) of the grafted SCN. Since transplanted mCry1/mCry2-deficient mice do not have functional circadian oscillators [5] other than those present in the grafted hypothalamus region, these findings suggest that the SCN can generate circadian behavioral rhythms in the absence of distant peripheral oscillators in the brain or elsewhere.

The network of time: understanding the molecular circadian system

The circadian clock provides a temporal structure that modulates biological functions from the level of gene expression to performance and behaviour. Pioneering work on the fruitfly Drosophila has provided a basis for understanding how the temporal sequence of daily events is controlled in mammals. New insights have come from work on mammals, specifically from studying the daily activity profiles of clock mutant mice; from more detailed recordings of clock gene expression under different experimental conditions and in different tissues; and from the discovery and analysis of a growing number of additional clock genes. These new results are moving the model paradigm away from a simple negative feedback loop to a molecular network. Understanding the coupling and interactions of this network will help us to understand the evolution of the circadian system, advance medical diagnosis and treatment, improve the health of shift workers and frequent travellers, and will generally enable the treatment of clock-related pathologies.

Mouse dexamethasone-induced RAS protein 1 gene is expressed in a circadian rhythmic manner in the suprachiasmatic nucleus

We identified the Dexamethasone-induced RAS protein 1 (Dexras1) gene as a cycling gene in the suprachiasmatic nucleus (SCN). Investigation of the whole brain using in situ hybridization demonstrated the localization of the expression of the gene in the SCN, thalamus, piriform cortex and hippocampus. However, rhythmic expression of the gene was observed only in the SCN. The rhythmic change in gene expression during 1 day was approximately five-fold, and the maximum expression was observed during subjective night. Real-time PCR using the SCN, paraventricular nucleus and cortex confirmed these results. Next, we analyzed the expression of the Dexras1 gene in the SCN of cryptochrome (Cry) 1 and 2 double knockout mice. We found that the rhythmic expression disappeared. The results indicate that Dexras1 rhythmicity and levels are dependent upon CRYs. This is the first time that the G protein, which may be involved in the input pathway, has been isolated as a cycling gene in the SCN.

Contrary to other non-photic cues, acute melatonin injection does not induce immediate changes of clock gene mRNA expression in the rat suprachiasmatic nuclei

The suprachiasmatic nuclei (SCN) contain the main clock of the mammalian circadian system. The endogenous oscillation machinery involves interactive positive and negative transcriptional and posttranslational feedback loops involving the clock genes Per1, Per2, Per3, Clock, Bmal1, Cry1 and Cry2. The SCN endogenous oscillation is entrained to 24 h by the light/dark cycle. Light induced regulation of Per1 and Per2 mRNA expression have been suggested to take part in the clock resetting. However, other factors have chronobiotic and synchronizing effects on SCN activity. Especially, the nocturnal pineal gland hormone, melatonin, which is involved in the regulation of both circadian and seasonal rhythms, is known to feedback on the SCN. Melatonin applied on SCN slices immediately phase-shifts their neuronal electrical activity, while daily injections of melatonin to free running rodents resynchronize their locomotor activity to 24 h. To determine whether melatonin feedback control on SCN activity implicates transcriptional regulation of the clock genes, we monitored the expression pattern of Per 1, 2, 3, Bmal1, Cry1 and AVP mRNAs after a single melatonin injection at the end of the subjective day. Results showed that melatonin injection affected none of the mRNA expression pattern during the first circadian night. Per1, Per3, Bmal1 and AVP expression patterns were, however, significantly but differentially affected, during the second subjective night after the melatonin injection. The present results strongly suggest that the immediate phase shifting effect of melatonin on the SCN molecular loop implicates rather post-translational than transcriptional mechanisms.

Roles for WHITE COLLAR-1 in circadian and general photoperception in Neurospora crassa

The transcription factors WHITE COLLAR-1 (WC-1) and WHITE COLLAR-2 (WC-2) interact to form a heterodimeric complex (WCC) that is essential for most of the light-mediated processes in Neurospora crassa. WCC also plays a distinct non-light-related role as the transcriptional activator in the FREQUENCY (FRQ)/WCC feedback loop that is central to the N. crassa circadian system. Although an activator role was expected for WC-1, unanticipated phenotypes resulting from some wc-1 alleles prompted a closer examination of an allelic series for WC-1 that has uncovered roles for this central regulator in constant darkness and in response to light. We analyzed the phenotypes of five different wc-1 mutants for expression of FRQ and WC-1 in constant darkness and following light induction. While confirming the absolute requirement of WC-1 for light responses, the data suggest multiple levels of control for light-regulated genes.

Circadian profile and photic regulation of clock genes in the suprachiasmatic nucleus of a diurnal mammal Arvicanthis ansorgei

The molecular mechanisms of the mammalian circadian clock located in the suprachiasmatic nucleus have been essentially studied in nocturnal species. Currently, it is not clear if the clockwork and the synchronizing mechanisms are similar between diurnal and nocturnal species. Here we investigated in a day-active rodent Arvicanthis ansorgei, some of the molecular mechanisms that participate in the generation of circadian rhythmicity and processing of photic signals. In situ hybridization was used to characterize circadian profiles of expression of Per1, Per2, Cry2 and Bmal1 in the suprachiasmatic nucleus of A. ansorgei housed in constant dim red light. All the clock genes studied showed a circadian expression. Per1 and Per2 mRNA increased during the subjective day and decreased during the subjective night. Also, Bmal1 exhibited a circadian expression, but in anti-phase to that of Per1. The expression of Cry2 displayed a circadian pattern, increasing during the late subjective day and decreasing during the late subjective night. We also obtained the phase responses to light for wheel-running rhythm and clock gene expression. At a behavioral level, light was able to induce phase shifts only during the subjective night, like in other diurnal and nocturnal species. At a molecular level, light pulse exposure during the night led to an up-regulation of Per1 and Per2 concomitant with a down-regulation of Cry2 in the suprachiasmatic nucleus of A. ansorgei. In contrast, Bmal1 expression was not affected by light pulses at the circadian times investigated. This study demonstrates that light exposure during the subjective night has opposite effects on the expression of the clock genes Per1 and Per2 compared with that of Cry2. These differential effects can participate in photic resetting of the circadian clock. Our data also indicate that the molecular mechanisms underlying circadian rhythmicity and photic synchronization share clear similarities between diurnal and nocturnal mammals.

Perchance to dream: solving the mystery of sleep through genetic analysis

Sleep has been identified in all mammals and nonmammalian vertebrates that have been critically evaluated. In addition, sleep-like states have also been identified and described in several invertebrates. Despite this prevalence throughout the animal kingdom, the function of sleep remains a mystery. The completion of several genome sequencing projects has led to the expectation that fundamental aspects of sleep can be elucidated through genetic dissection. Indeed, studies in both the mouse and fly have begun to reveal tantalizing suggestions about the underlying principles that regulate sleep homeostasis. In this article we will review recent studies that have used genetic techniques to evaluate sleep in the fruit fly and the mouse.

Photoperiod differentially regulates clock genes' expression in the suprachiasmatic nucleus of Syrian hamster

The suprachiasmatic nuclei (SCN) contain the master circadian pacemaker in mammals. Generation and maintenance of circadian oscillations involve clock genes which interact to form transcriptional/translational loops and constitute the molecular basis of the clock. There is some evidence that the SCN clock can integrate variations in day length, i.e. photoperiod. However, the effects of photoperiod on clock-gene expression remain largely unknown. We here report the expression pattern of Period (Per) 1, Per2, Per3, Cryptochrome (Cry) 1, Cry2, Bmal1 and Clock genes in the SCN of Syrian hamsters when kept under long (LP) and short (SP) photoperiods. Our data show that photoperiod differentially affects the expression of all clock genes studied. Among the components of the negative limb of the feedback loop, Per1, Per2, Per3, Cry2 but not Cry1 genes show a shortened duration of their peak expression under SP compared with LP. Moreover, mRNA expression of Per1, Per3 and Cry1 are phase advanced in SP compared with LP. Per3 shows an mRNA peak of higher amplitude under SP conditions whereas Per1 and Per2 peak amplitudes are unaffected by photoperiod changes. Bmal1 expression is phase advanced without a change of duration in SP compared with LP. Furthermore, the expression of Clock is rhythmic under SP whereas no rhythm is observed under LP. These results, which provide further evidence that the core clock mechanisms of the SCN integrate photoperiod, are discussed in the context of the existing molecular model.

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.

A role for cryptochromes in sleep regulation

BACKGROUND

The cryptochrome 1 and 2 genes (cry1 and cry2) are necessary for the generation of circadian rhythms, as mice lacking both of these genes (cry1,2-/-) lack circadian rhythms. We studied sleep in cry1,2-/- mice under baseline conditions as well as under conditions of constant darkness and enforced wakefulness to determine whether cryptochromes influence sleep regulatory processes.

RESULTS

Under all three conditions, cry1,2-/- mice exhibit the hallmarks of high non-REM sleep (NREMS) drive (i.e., increases in NREMS time, NREMS consolidation, and EEG delta power during NREMS). This unexpected phenotype was associated with elevated brain mRNA levels of period 1 and 2 (per1,2), and albumin d-binding protein (dbp), which are known to be transcriptionally inhibited by CRY1,2. To further examine the relationship between circadian genes and sleep homeostasis, we examined wild type mice and rats following sleep deprivation and found increased levels of per1,2 mRNA and decreased levels of dbp mRNA specifically in the cerebral cortex; these changes subsided with recovery sleep. The expression of per3, cry1,2, clock, npas2, bmal1, and casein-kinase-1epsilon did not change with sleep deprivation.

CONCLUSIONS

These results indicate that mice lacking cryptochromes are not simply a genetic model of circadian arrhythmicity in rodents and functionally implicate cryptochromes in the homeostatic regulation of sleep.

Dual regulation of cryptochrome 1 mRNA expression in chicken retina by light and circadian oscillators

The localization and regulation of chicken cryptochrome 1 (cCry1) mRNA expression in retina was investigated by laser capture microdissection and quantitative real-time RT-PCR. Laser capture microdissection (LCM) of retinal cell layers showed the highest level of cCry1 expression in the ganglion cell and photoreceptor layers. In both layers, expression was high during the daytime and low at night in subjects exposed to a 12:12 h light:dark cycle. Robust circadian oscillations of cCry1 mRNA levels were observed in constant (24 h day) light, but not in constant darkness, with the highest expression during daytime at zeitgeber time (ZT) 8. Unlike cCry1, circadian rhythms of the melatonin-synthesizing enzyme, arylalkylamine N-acetyltransferase, persisted in constant darkness, suggesting that rhythmic cCry1 expression is not essential for circadian clock function or output. On the second day of constant darkness, when cCry1 expression is arrhythmic, light exposure for 2 h significantly increased retinal cCry1 mRNA levels at ZT 4 and 8, times that cCry1 expression is induced in LD and LL. Similar light exposure ending at ZT 20 had no significant effect. Thus, expression of cCry1 mRNA is regulated dually by light and circadian 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.

[Chrono-endocrinology--quo vadis?]

The present review deals with important new chronobiological results especially in the field of chronoendocrinology, shedding new light on the circadian organisation of mammals including man. In vitro studies have shown that the concept of the existence of a single circadian oscillator located in the suprachiasmatic nucleus has to be extended. Circadian oscillators have also been found to exist in the retina, islets of Langerhans, liver, lung, and fibroblasts. Another major result is the detection of a new photopigment, melanopsin, present in a subpopulation of retinal ganglion cells which are lightsensitive and project to the suprachiasmatic nucleus, acting as zeitgeber for the photic entrainment of the circadian rhythm. We are only beginning to understand how the circadian oscillator transmits the circadian message to the endocrine system. The generation of circadian and seasonal rhythms of hormone synthesis is best understood in the pineal gland and its hormone melatonin. Seasonal changes of melatonin synthesis are transduced in the pars tuberalis of the adenohypophysis which is now entering the limelight of chronoendocrinological research. Currently, the elucidation of the genetic basis and the molecular organisation of the circadian oscillator within individual cells is a major thrust in chronobiological research.

Zebrafish CRY represses transcription mediated by CLOCK-BMAL heterodimer without inhibiting its binding to DNA

BACKGROUND

CLOCK and BMAL1 proteins, members of the basic helix-loop-helix PAS (PER-ARNT-SIM) superfamily of transcription factors which bind to the E-box DNA motif, are required for the high-level expression of the circadian clock genes period (per) and cryptochrome (cry). CRY inhibits transcriptional activity of the CLOCK-BMAL1 heterodimer, generating a negative-feedback loop that is the core element of the circadian oscillator.

RESULTS

We show that zebrafish CRY (zCRY1a) neither disrupts the association between zfCLOCK and zfBMAL nor inhibits binding of the zfCLOCK-zfBMAL heterodimer to an E-box-bearing DNA fragment. Instead it binds to the heterodimer to form a stable zCRY1a-zfCLOCK-zfBMAL-E-box complex. Another zebrafish CRY protein, zCRY4, does not have transcriptional inhibitor activity, whereas zCRY1a has strong activity. zCRY4 does not associate with zfCLOCK and zfBMAL. We also show that the presence of a chemical reductant in the reaction mixture is crucial for efficient binding of the CLOCK-BMAL heterodimer to E-box bearing DNA, which is indicative of the reduction/oxidation (redox)-sensitive character of the heterodimer.

CONCLUSIONS

Our findings suggest that CRY represses CLOCK-BMAL-mediated transcription by interacting directly with the zfCLOCK-zfBMAL-E-box complex.

Differential resynchronisation of circadian clock gene expression within the suprachiasmatic nuclei of mice subjected to experimental jet lag

Disruption of the circadian timing system arising from travel between time zones ("jet lag") and rotational shift work impairs mental and physical performance and severely compromises long-term health. Circadian disruption is more severe during adaptation to advances in local time, because the circadian clock takes much longer to phase advance than delay. The recent identification of mammalian circadian clock genes now makes it possible to examine time zone adjustments from the perspective of molecular events within the suprachiasmatic nucleus (SCN), the principal circadian oscillator. Current models of the clockwork posit interlocked transcriptional/post-translational feedback loops based on the light-sensitive Period (Per) genes and the Cryptochrome (Cry) genes, which are indirectly regulated by light. We show that circadian cycles of mPer expression in the mouse SCN react rapidly to an advance in the lighting schedule, whereas rhythmic mCry1 expression advances more slowly, in parallel to the gradual resetting of the activity-rest cycle. In contrast, during a delay in local time the mPer and mCry cycles react rapidly, completing the 6 hr shift together by the second cycle, in parallel with the activity-rest cycle. These results reveal the potential for dissociation of mPer and mCry expression within the central oscillator during circadian resetting and a differential molecular response of the clock during advance and delay resetting. They highlight the indirect photic regulation of mCry1 as a potentially rate-limiting factor in behavioral adjustment to time zone transitions.