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

Aging-like circadian disturbances in folate-deficient mice

The elderly population shows various circadian disturbances, including dampened amplitude of rhythmicity and decreased responsiveness to light. The common poor folate status in the elderly might account for these aging-related circadian disturbances. To test this hypothesis, we investigated whether folate deficiency in mice affects circadian oscillations of the master clock in the suprachiasmatic nuclei, and the shifting responses to light. Mice fed a diet without folate for 6 weeks displayed markedly reduced (4.5-fold) erythrocyte folate concentration and increased (2.3-fold) homocysteinemia compared with control mice. Folate deficiency decreased the circadian amplitude of vasopressin and the clock protein PERIOD 2 (PER2) in the master clock, slowed the rate of re-entrainment of behavioral rhythms after delayed light-dark cycle and reduced light-induced phase-delays, without detectable morphologic changes in the retina, such as the number of melanopsinergic ganglion cells, that might have impaired photodetection. In conclusion, folate deficiency and consecutive hyperhomocysteinemia led to dampened PER2 and vasopressin oscillations in the master clock and reduced responsiveness to photic resetting, which constitute hallmarks of aging effects on circadian rhythmicity.

Mitogen- and stress-activated protein kinase 1 modulates photic entrainment of the suprachiasmatic circadian clock

The master circadian clock in mammals, the suprachiasmatic nucleus (SCN), is under the entraining influence of the external light cycle. At a mechanistic level, intracellular signaling via the p42/44 mitogen-activated protein kinase pathway appears to play a central role in light-evoked clock entrainment; however, the precise downstream mechanisms by which this pathway influences clock timing are not known. Within this context, we have previously reported that light stimulates activation of the mitogen-activated protein kinase effector mitogen-stress-activated kinase 1 (MSK1) in the SCN. In this study, we utilised MSK1(-/-) mice to further investigate the potential role of MSK1 in circadian clock timing and entrainment. Locomotor activity analysis revealed that MSK1 null mice entrained to a 12 h light/dark cycle and exhibited circadian free-running rhythms in constant darkness. Interestingly, the free-running period in MSK1 null mice was significantly longer than in wild-type control animals, and MSK1 null mice exhibited a significantly greater variance in activity onset. Further, MSK1 null mice exhibited a significant reduction in the phase-delaying response to an early night light pulse (100 lux, 15 min), and, using an 8 h phase-advancing 'jet-lag' experimental paradigm, MSK1 knockout animals exhibited a significantly delayed rate of re-entrainment. At the molecular level, early night light-evoked cAMP response element-binding protein (CREB) phosphorylation, histone phosphorylation and Period1 gene expression were markedly attenuated in MSK1(-/-) animals relative to wild-type mice. Together, these data provide key new insights into the molecular mechanisms by which MSK1 affects the SCN clock.

Circadian pacemaker in the suprachiasmatic nuclei of teleost fish revealed by rhythmic period2 expression

In mammals, the role of the suprachiasmatic nucleus (SCN) as the primary circadian clock that coordinates the biological rhythms of peripheral oscillators is well known. However, in teleosts, it remains unclear whether the SCN also functions as a circadian pacemaker. We used in situ hybridization (ISH) techniques to demonstrate that the molecular clock gene, per2, is expressed in the SCN of flounder (Paralichthys olivaceus) larvae during the day and down-regulated at night, demonstrating that a circadian pacemaker exists in the SCN of this teleost. The finding that per2 expression in the SCN was also observed in the amberjack (Seriola dumerili), but not in medaka (Oryzias latipes), implies that interspecific variation exists in the extent to which the SCN controls the circadian rhythms of fish species, presumably reflecting their lifestyle. Rhythmic per2 expression was also detected in the pineal gland and pituitary, and aperiodic per2 expression was observed in the habenula, which is known to exhibit circadian rhythms in rodents. Since the ontogeny of per2 expression in the brain of early flounder larvae can be monitored by whole mount ISH, it is possible to investigate the effects of drugs and environmental conditions on the functional development of circadian clocks in the brain of fish larvae. In addition, flounder would be a good model for understanding the rhythmicity of marine fish. Our findings open a new frontier for investigating the role of the SCN in teleost circadian rhythms.

Disrupted circadian rhythm in rats with nephrectomy-induced chronic kidney disease

AIMS

Our study investigated the role of circadian rhythm in the pathogenesis of sleep disturbance in patients with chronic kidney disease (CKD) based on an animal model.

MAIN METHODS

Sixteen Sprague-Dawley (SD) rats (eight from 5/6 nephrectomized CKD group and eight from control group) were used for electroencephalography (EEG) and electromyography (EMG) recording. Eight rats (four from CKD and four from control group) were sacrificed at six Zeitgeber time (ZT) points and determined the mRNA expression of clock genes, rPer1, rPer2 and rBMAL1b in the hypothalamus.

KEY FINDINGS

Our results demonstrated that both slow wave sleep (SWS) and rapid eye movement (REM) sleep were significantly increased in the ZT22-24 Zeitgeber time point of the dark period in the CKD rats when compared with those sleep architectures obtained from the control rats. The CKD-induced sleep disruptions were associated with significant upregulations of rPer1 (in ZT2, ZT6 and ZT14) and rPer2 mRNA expression (in ZT2 and ZT14) in the hypothalamus.

SIGNIFICANCE

Our study elucidated that the increases of SWS and REM sleep during ZT22-24 of the dark period in the CKD rats might be due to the enhancement of rPer1 and rPer2 clock genes in the hypothalamus, suggesting that disrupted circadian rhythm plays a role in the pathogenesis of sleep disturbance in patients with CKD.

Control of sleep and wakefulness

This review summarizes the brain mechanisms controlling sleep and wakefulness. Wakefulness promoting systems cause low-voltage, fast activity in the electroencephalogram (EEG). Multiple interacting neurotransmitter systems in the brain stem, hypothalamus, and basal forebrain converge onto common effector systems in the thalamus and cortex. Sleep results from the inhibition of wake-promoting systems by homeostatic sleep factors such as adenosine and nitric oxide and GABAergic neurons in the preoptic area of the hypothalamus, resulting in large-amplitude, slow EEG oscillations. Local, activity-dependent factors modulate the amplitude and frequency of cortical slow oscillations. Non-rapid-eye-movement (NREM) sleep results in conservation of brain energy and facilitates memory consolidation through the modulation of synaptic weights. Rapid-eye-movement (REM) sleep results from the interaction of brain stem cholinergic, aminergic, and GABAergic neurons which control the activity of glutamatergic reticular formation neurons leading to REM sleep phenomena such as muscle atonia, REMs, dreaming, and cortical activation. Strong activation of limbic regions during REM sleep suggests a role in regulation of emotion. Genetic studies suggest that brain mechanisms controlling waking and NREM sleep are strongly conserved throughout evolution, underscoring their enormous importance for brain function. Sleep disruption interferes with the normal restorative functions of NREM and REM sleep, resulting in disruptions of breathing and cardiovascular function, changes in emotional reactivity, and cognitive impairments in attention, memory, and decision making.

Setting the main circadian clock of a diurnal mammal by hypocaloric feeding

Caloric restriction attenuates the onset of a number of pathologies related to ageing. In mammals, circadian rhythms, controlled by the hypothalamic suprachiasmatic (SCN) clock, are altered with ageing. Although light is the main synchronizer for the clock, a daily hypocaloric feeding (HF) may also modulate the SCN activity in nocturnal rodents. Here we report that a HF also affects behavioural, physiological and molecular circadian rhythms of the diurnal rodent Arvicanthis ansorgei. Under constant darkness HF, but not normocaloric feeding (NF), entrains circadian behaviour. Under a light–dark cycle, HF at midnight led to phase delays of the rhythms of locomotor activity and plasma corticosterone. Furthermore, Per2 and vasopressin gene oscillations in the SCN were phase delayed in HF Arvicanthis compared with animals fed ad libitum. Moreover, light-induced expression of Per genes in the SCN was modified in HF Arvicanthis, despite a non-significant effect on light-induced behavioural phase delays. Together, our data show that HF affects the circadian system of the diurnal rodent Arvicanthis ansorgei differentially from nocturnal rodents. The Arvicanthis model has relevance for the potential use of HF to manipulate circadian rhythms in diurnal species including humans.

The daily melatonin pattern in Djungarian hamsters depends on the circadian phenotype

Djungarian hamsters (Phodopus sungorus) bred at the Institute of Halle reveal three different circadian phenotypes. The wild type (WT) shows normal locomotor activity patterns, whereas in hamsters of the DAO (delayed activity onset) type, the activity onset is continuously delayed. Since the activity offset in those hamsters remains coupled to "light-on," the activity time becomes compressed. Hamsters of the AR (arrhythmic) type are episodically active throughout the 24 h. Previous studies showed that a disturbed interaction of the circadian system with the light-dark (LD) cycle contributes to the phenomenon observed in DAO hamsters. To gain better insight into the underlying mechanisms, the authors investigated the daily melatonin rhythm, as it is a reliable marker of the circadian clock. Hamsters were kept individually under standardized laboratory conditions (LD 14:10, T=22°C±2°C, food and water ad libitum). WT, DAO (with exactly 5 h delay of activity onset), and AR hamsters were used for pineal melatonin and urinary 6-sulfatoxymelatonin (aMT6s) measurement. Pineal melatonin content was determined at 3 time points: 4 h after "light-off" [D+4], 1 h before "light-on" [L-1], and 1h after "light-on" [L+1]). The 24-h profile of melatonin secretion was investigated by transferring the animals to metabolic cages for 27?h to collect urine at 3-h intervals for aMT6s analysis. WT hamsters showed high pineal melatonin content during the dark time (D+4, L-1), which significantly decreased at the beginning of the light period (L+1). In contrast, DAO hamsters displayed low melatonin levels during the part of the dark period when animals were still resting (D+4). At the end of the dark period (L-1), melatonin content increased significantly and declined again when light was switched on (L+1). AR hamsters showed low melatonin levels, comparable to daytime values, at all 3 time points. The results were confirmed by aMT6s data. WT hamsters showed a marked circadian pattern of aMT6s excretion. The concentration started to increase 3?h after "light-off" and reached daytime values 5 h after "light-on." In DAO hamsters, in contrast, aMT6s excretion started about 6?h later and reached significantly lower levels compared to WT hamsters. In AR animals, aMT6s excretion was low at all times. The results clearly indicate the rhythm of melatonin secretion in DAO hamsters is delayed in accord with their delayed activity onset, whereas AR hamsters display no melatonin rhythm at all. Since the regulatory pathways for the rhythms of locomotor activity and melatonin synthesis (which are downstream from the suprachiasmatic nucleus [SCN]) are different but obviously convey the same signal, we conclude that the origin of the phenomenon observed in DAO hamsters must be located upstream of the SCN, or in the SCN itself.

Contribution of FSH and triiodothyronine to the development of circadian clocks during granulosa cell maturation

The involvement of FSH and triiodothyronine (T(3)) in circadian clocks was investigated using immature granulosa cells of ovaries during the progress of cell maturation. Granulosa cells were prepared from preantral follicles of mouse Period2 (Per2)-dLuc reporter gene transgenic rats injected subcutaneously with the synthetic nonsteroidal estrogen diethylstilbestrol. Analysis of the cellular clock of the immature granulosa cells was performed partly using a serum-free culture system. Several bioluminescence oscillations of Per2-dLuc promoter activity were generated in the presence of FSH + fetal bovine serum, but not in the presence of either FSH or serum. As revealed by bioluminescence recording and analysis of clock gene expression, the granulosa cells lack the functional cellular clock at the immature stage, although Lhr was greatly expressed during the period of cell maturation. The granulosa cells gained a strong circadian rhythm of bioluminescence during stimulation with FSH, whereas LH reset the cellular clock of matured granulosa cells. During strong circadian rhythms of clock genes, the Star gene showed significant expression in matured granulosa cells. In contrast, T(3) showed an inhibitory effect on the development of the functional cellular clock during the period of cell maturation. These results indicate that FSH provides a cue for the development of the functional cellular clock of the immature granulosa cells, and T(3) blocks the development of the cellular clock.

miR-212/132 expression and functions: within and beyond the neuronal compartment

During the last two decades, microRNAs (miRNAs) emerged as critical regulators of gene expression. By modulating the expression of numerous target mRNAs mainly at the post-transcriptional level, these small non-coding RNAs have been involved in most, if not all, biological processes as well as in the pathogenesis of a number of diseases. miR-132 and miR-212 are tandem miRNAs whose expression is necessary for the proper development, maturation and function of neurons and whose deregulation is associated with several neurological disorders, such as Alzheimer's disease and tauopathies (neurodegenerative diseases resulting from the pathological aggregation of tau protein in the human brain). Although their involvement in neuronal functions is the most described, evidences point towards a role of these miRNAs in many other biological processes, including inflammation and immune functions. Incidentally, miR-132 was recently classified as a ‘neurimmiR’, a class of miRNAs operating within and between the neural and immune compartments. In this review, we propose an outline of the current knowledge about miR-132 and miR-212 functions in neurons and immune cells, by describing the signalling pathways and transcription factors regulating their expression as well as their putative or demonstrated roles and validated mRNA targets.

PML regulates PER2 nuclear localization and circadian function

Studies have suggested that the clock regulator PER2 is a tumour suppressor. A cancer network involving PER2 raises the possibility that some tumour suppressors are directly involved in the mammalian clock. Here, we show that the tumour suppressor promyelocytic leukaemia (PML) protein is a circadian clock regulator and can physically interact with PER2. In the suprachiasmatic nucleus (SCN), PML expression and PML-PER2 interaction are under clock control. Loss of PML disrupts and dampens the expression of clock regulators Per2, Per1, Cry1, Bmal1 and Npas2. In the presence of PML and PER2, BMAL1/CLOCK-mediated transcription is enhanced. In Pml(-/-) SCN and mouse embryo fibroblast cells, the cellular distribution of PER2 is primarily perinuclear/cytoplasmic. PML is acetylated at K487 and its deacetylation by SIRT1 promotes PML control of PER2 nuclear localization. The circadian period of Pml(-/-) mice displays reduced precision and stability consistent with PML having a role in the mammalian clock mechanism.

Tissue-specific function of Period3 in circadian rhythmicity

The mammalian circadian system is composed of multiple central and peripheral clocks that are temporally coordinated to synchronize physiology and behavior with environmental cycles. Mammals have three homologs of the circadian Period gene (Per1, 2, 3). While numerous studies have demonstrated that Per1 and Per2 are necessary for molecular timekeeping and light responsiveness in the master circadian clock in the suprachiasmatic nuclei (SCN), the function of Per3 has been elusive. In the current study, we investigated the role of Per3 in circadian timekeeping in central and peripheral oscillators by analyzing PER2::LUCIFERASE expression in tissues explanted from C57BL/6J wild-type and Per3⁻/⁻ mice. We observed shortening of the periods in some tissues from Per3⁻/⁻ mice compared to wild-types. Importantly, the periods were not altered in other tissues, including the SCN, in Per3⁻/⁻ mice. We also found that Per3-dependent shortening of endogenous periods resulted in advanced phases of those tissues, demonstrating that the in vitro phenotype is also present in vivo. Our data demonstrate that Per3 is important for endogenous timekeeping in specific tissues and those tissue-specific changes in endogenous periods result in internal misalignment of circadian clocks in Per3⁻/⁻ mice. Taken together, our studies demonstrate that Per3 is a key player in the mammalian circadian system.

Rhythmic interaction between Period1 mRNA and hnRNP Q leads to circadian time-dependent translation

The mouse PERIOD1 (mPER1) protein, along with other clock proteins, plays a crucial role in the maintenance of circadian rhythms. mPER1 also provides an important link between the circadian system and the cell cycle system. Here we show that the circadian expression of mPER1 is regulated by rhythmic translational control of mPer1 mRNA together with transcriptional modulation. This time-dependent translation was controlled by an internal ribosomal entry site (IRES) element in the 5' untranslated region (5'-UTR) of mPer1 mRNA along with the trans-acting factor mouse heterogeneous nuclear ribonucleoprotein Q (mhnRNP Q). Knockdown of mhnRNP Q caused a decrease in mPER1 levels and a slight delay in mPER1 expression without changing mRNA levels. The rate of IRES-mediated translation exhibits phase-dependent characteristics through rhythmic interactions between mPer1 mRNA and mhnRNP Q. Here, we demonstrate 5'-UTR-mediated rhythmic mPer1 translation and provide evidence for posttranscriptional regulation of the circadian rhythmicity of core clock genes.

The circadian clock, reward, and memory

During our daily activities, we experience variations in our cognitive performance, which is often accompanied by cravings for small rewards, such as consuming coffee or chocolate. This indicates that the time of day, cognitive performance, and reward may be related to one another. This review will summarize data that describe the influence of the circadian clock on addiction and mood-related behavior and put the data into perspective in relation to memory processes.

Distinct patterns of Period gene expression in the suprachiasmatic nucleus underlie circadian clock photoentrainment by advances or delays

The circadian clock in the mammalian hypothalamic suprachiasmatic nucleus (SCN) is entrained by the ambient light/dark cycle, which differentially acts to cause the clock to advance or delay. Light-induced changes in the rhythmic expression of SCN clock genes are believed to be a critical step in this process, but how the two entrainment modalities--advances vs. delays--engage the molecular clockwork remains incompletely understood. We investigated molecular substrates of photic entrainment of the clock in the SCN by stably entraining hamsters to T cycles (non-24-h light/dark cycles) consisting of a single 1-h light pulse repeated as either a short (23.33-h) or a long (24.67-h) cycle; under these conditions, the light pulse of the short cycle acts as "dawn," whereas that of the long cycle acts as "dusk." Analyses of the expression of the photoinducible and rhythmic clock genes Period 1 and 2 (Per1 and Per2) in the SCN revealed fundamental differences under these two entrainment modes. Light at dawn advanced the clock, advancing the onset of the Per1 mRNA rhythm and acutely increasing mRNA transcription, whereas light at dusk delayed the clock, delaying the offset of the Per2 mRNA rhythm and tonically increasing mRNA stability. The results suggest that the underlying molecular mechanisms of circadian entrainment differ with morning (advancing) or evening (delaying) light exposure, and such differences may reflect how entrainment takes place in nocturnal animals under natural conditions.

Masking responses to light in period mutant mice

Masking is an acute effect of an external signal on an overt rhythm and is distinct from the process of entrainment. In the current study, we investigated the phase dependence and molecular mechanisms regulating masking effects of light pulses on spontaneous locomotor activity in mice. The circadian genes, Period1 (Per1) and Per2, are necessary components of the timekeeping machinery and entrainment by light appears to involve the induction of the expression of Per1 and Per2 mRNAs in the suprachiasmatic nuclei (SCN). We assessed the roles of the Per genes in regulating masking by assessing the effects of light pulses on nocturnal locomotor activity in C57BL/6J Per mutant mice. We found that Per1(-/-) and Per2(-/-) mice had robust negative masking responses to light. In addition, the locomotor activity of Per1(-/-)/Per2(-/-) mice appeared to be rhythmic in the light-dark (LD) cycle, and the phase of activity onset was advanced (but varied among individual mice) relative to lights off. This rhythm persisted for 1 to 2 days in constant darkness in some Per1(-/-)/Per2(-/-) mice. Furthermore, Per1(-/-)/Per2(-/-) mice exhibited robust negative masking responses to light. Negative masking was phase dependent in wild-type mice such that maximal suppression was induced by light pulses at zeitgeber time 14 (ZT14) and gradually weaker suppression occurred during light pulses at ZT16 and ZT18. By measuring the phase shifts induced by the masking protocol (light pulses were administered to mice maintained in the LD cycle), we found that the phase responsiveness of Per mutant mice was altered compared to wild-types. Together, our data suggest that negative masking responses to light are robust in Per mutant mice and that the Per1(-/-)/Per2(-/-) SCN may be a light-driven, weak/damping oscillator.

Effects of different light intensities in the morning on dim light melatonin onset

The present study evaluated the effects of exposure to light intensity in the morning on dim light melatonin onset (DLMO). The tested light intensities were 750 lux, 150 lux, 3000 lux, 6000 lux and 12,000 lux (horizontal illuminance at cornea), using commercial 5000 K fluorescent lamps. Eleven healthy males aged 21-31 participated in 2-day experiments for each light condition. On the first experimental day (day 1), subjects were exposed to dim light (<30 lux) for 3 h in the morning (09:00-12:00). On the same day, saliva samples were taken in dim light (<30 lux) every 30 min from 21:00 to 01:00 to determine the DLMO phase. The subjects were allowed to sleep from 01:00 to 08:00. On the second experimental day (day 2), the subjects were exposed to experimental light conditions for 3 h in the morning. The experimental schedule after light exposure was the same as on day 1. On comparing day 2 with day 1, significant phase advances of DLMO were obtained at 3000 lux, 6000 lux and 12,000 lux. These findings indicate that exposure to a necessary intensity from an ordinary light source, such as a fluorescent lamp, in the morning within one day affects melatonin secretion.

Uncovering the proteome response of the master circadian clock to light using an AutoProteome system

In mammals, the suprachiasmatic nucleus (SCN) is the central circadian pacemaker that governs rhythmic fluctuations in behavior and physiology in a 24-hr cycle and synchronizes them to the external environment by daily resetting in response to light. The bilateral SCN is comprised of a mere ~20,000 neurons serving as cellular oscillators, a fact that has, until now, hindered the systematic study of the SCN on a global proteome level. Here we developed a fully automated and integrated proteomics platform, termed AutoProteome system, for an in-depth analysis of the light-responsive proteome of the murine SCN. All requisite steps for a large-scale proteomic study, including preconcentration, buffer exchanging, reduction, alkylation, digestion and online two-dimensional liquid chromatography-tandem MS analysis, are performed automatically on a standard liquid chromatography-MS system. As low as 2 ng of model protein bovine serum albumin and up to 20 μg and 200 μg of SCN proteins can be readily processed and analyzed by this system. From the SCN tissue of a single mouse, we were able to confidently identify 2131 proteins, of which 387 were light-regulated based on a spectral counts quantification approach. Bioinformatics analysis of the light-inducible proteins reveals their diverse distribution in different canonical pathways and their heavy connection in 19 protein interaction networks. The AutoProteome system identified vasopressin-neurophysin 2-copeptin and casein kinase 1 delta, both of which had been previously implicated in clock timing processes, as light-inducible proteins in the SCN. Ras-specific guanine nucleotide-releasing factor 1, ubiquitin protein ligase E3A, and X-linked ubiquitin specific protease 9, none of which had previously been implicated in SCN clock timing processes, were also identified in this study as light-inducible proteins. The AutoProteome system opens a new avenue to systematically explore the proteome-wide events that occur in the SCN, either in response to light or other stimuli, or as a consequence of its intrinsic pacemaker capacity.

Lab mice in the field: unorthodox daily activity and effects of a dysfunctional circadian clock allele

Daily patterns of animal behavior are potentially of vast functional importance. Fitness benefits have been identified in nature by the association between individual timing and survival or by the fate of individuals after experimental deletion of their circadian pacemaker. The recent advances in unraveling the molecular basis of circadian timing enable new approaches to natural selection on timing. The investigators report on the effect and fate of the mutant Per2(Brdm1) allele in 4 replicate populations of house mice in a seminatural outside environment over 2 years. This allele is known to compromise circadian organization and entrainment and to cause multiple physiological disturbances. Mice (N=250) bred from Per2(Brdm1) heterozygotes were implanted subcutaneously with transponders and released in approximately Mendelian ratios in four 400 m(2) pens. An electronic system stored the times of all visits to feeders of each individual. The study first demonstrates that mice are not explicitly nocturnal in this natural environment. Feeding activity was predominantly and sometimes exclusively diurnal and spread nearly equally over day and night under the protective snow cover in winter. The effect of Per2(Brdm1) on activity timing is negligible compared to seasonal changes in all genotypes. Second, the Per2(Brdm1) allele did not have persistent negative effects on fitness. In the first year, the allele gradually became less frequent by reducing survival. New cohorts captured had the same Per2(Brdm1) frequency as the survivors from previous cohorts, consistent with an absence of an effect on reproduction. In the second year, the allele recovered to about its initial frequency (0.54). These changes in selective advantage were primarily due to female mice, as females lived longer and the sex ratio dropped to about 25% males in the population. While it is unknown which selective advantage led to the recovery, the results caution against inferences from laboratory experiments on fitness consequences in the natural environment. It also demonstrates that the activity of mice, while strictly nocturnal in the laboratory, may be partially or completely diurnal in the field. The new method allows assessment of natural selection on specific alleles on a day-today basis.

The circadian clock and mood-related behavior

Over many years evidence accumulated that circadian rhythms are related to psychiatric disorders.1-3 However, a mechanistic relationship between the circadian clock and mood related behaviors remained enigmatic. Now, we have reported that monoamine oxidase A (MAOA), a mitochondrial enzyme degrading catecholamines including dopamine, is regulated by components of the circadian clock.4 Interestingly, this regulation is variable depending on cell type, indicating the presence of cell type specific factors modulating BMAL1/NPAS2 or BMAL1/CLOCK dependent transcription. In the mesolimbic dopaminergic reward circuit, including the ventral tegmental area (VTA) and the ventral striatum/nucleus accumbens (NAc) we found a positive influence of PERIOD 2 (PER2) on transcriptional activation of Maoa using mice mutant in the Per2 gene. These animals show less Maoa mRNA expression and MAO activity compared to wild type littermates. This is probably the reason for the observed increase in dopamine levels in the striatum of Per2 mutant mice what leads to alteration in despair-based behavioral tests. These results suggest that clock components can influence dopamine metabolism and mood-related behaviors.

Protein phosphatase 1 (PP1) is a post-translational regulator of the mammalian circadian clock

Circadian clocks coordinate the timing of important biological processes. Interconnected transcriptional and post-translational feedback loops based on a set of clock genes generate and maintain these rhythms with a period of about 24 hours. Many clock proteins undergo circadian cycles of post-translational modifications. Among these modifications, protein phosphorylation plays an important role in regulating activity, stability and intracellular localization of clock components. Several protein kinases were characterized as regulators of the circadian clock. However, the function of protein phosphatases, which balance phosphorylation events, in the mammalian clock mechanism is less well understood. Here, we identify protein phosphatase 1 (PP1) as regulator of period and light-induced resetting of the mammalian circadian clock. Down-regulation of PP1 activity in cells by RNA interference and in vivo by expression of a specific inhibitor in the brain of mice tended to lengthen circadian period. Moreover, reduction of PP1 activity in the brain altered light-mediated clock resetting behavior in mice, enhancing the phase shifts in either direction. At the molecular level, diminished PP1 activity increased nuclear accumulation of the clock component PER2 in neurons. Hence, PP1, may reduce PER2 phosphorylation thereby influencing nuclear localization of this protein. This may at least partially influence period and phase shifting properties of the mammalian circadian clock.

Circadian rhythms, sleep, and substance abuse

Substance abuse is linked to numerous mental and physical health problems, including disturbed sleep. The association between substance use and sleep appears to be bidirectional, in that substance use may directly cause sleep disturbances, and difficulty sleeping may be a risk factor for relapse to substance use. Growing evidence similarly links substance use to disturbances in circadian rhythms, although many gaps in knowledge persist, particularly regarding whether circadian disturbance leads to substance abuse or dependence. Given the integral role circadian rhythms play in regulating sleep, circadian mechanisms may account in part for sleep-substance abuse interactions. Furthermore, a burgeoning research base supports a role for the circadian system in regulating reward processing, indicating that circadian mechanisms may be directly linked to substance abuse independently of sleep pathways. More work in this area is needed, particularly in elucidating how sleep and circadian disturbance may contribute to initiation of, and/or relapse to, substance use. Sleep and circadian-based interventions could play a critical role in the prevention and treatment of substance use disorders.

Chronic unpredictable stress induces a reversible change of PER2 rhythm in the suprachiasmatic nucleus

Many clinical studies have shown that circadian rhythm abnormalities are strongly associated with major depression. The master clock of the circadian system in mammals is located in the suprachiasmatic nucleus (SCN) within the anterior hypothalamus, where Per1 and Per2 are essential core components of circadian rhythm oscillation. Chronic unpredictable stress (CUS) is a reliable animal model of depression with good face, predictive, and constructive validity. In the present study, we investigated the effects of CUS on the circadian expression of PER1 and PER2 in the SCN. We found that CUS led to depressive-like behavior and reduced the amplitude of PER2 oscillation in the SCN, which were blocked by 3 weeks of desipramine (DMI) treatment. 2 weeks after termination of CUS, the decreased peak of PER2 expression returned to control levels, whereas depressive-like behavior remained unchanged. Our findings suggest that the dampened amplitude of PER2 expression in the SCN may participate in the development of depressive-like behavior induced by CUS but is unlikely involved in the long-lasting effects of CUS on depressive-like behavior.

Daily morphine injection and withdrawal disrupt 24-h wheel running and PERIOD2 expression patterns in the rat limbic forebrain

Symptoms of opiate withdrawal include disturbances in circadian rhythms. We examined in male Wistar rats (n=48) the effects of a daily, mid-morning morphine injection (5-40 mg/kg, i.p.) and its withdrawal on 24-h wheel-running activity and on the expression of the clock protein, PERIOD2 (PER2), in the suprachiasmatic nucleus (SCN), oval nucleus of the bed nucleus of the stria terminalis (BNSTov), central amygdala (CEA), and dorsal striatum. Rats were killed over 2 days at 10, 22, 46, and 58 h after the last daily morphine injection at zeitgeber times (ZT) 1 or ZT13. Daily morphine injections and their withdrawal suppressed nighttime wheel running, but did not entrain any increase in activity in advance of the injection. Neither morphine injection nor its withdrawal affected PER2 expression in the SCN, whereas the normal daily peaks of PER2 in the BNSTov, CEA, and dorsal striatum were blunted both during morphine administration and its withdrawal. Treatment with a dopaminergic agonist (the D2/3 agonist, quinpirole, 1.0 mg/kg) or a noradrenergic agonist (alpha2 agonist, clonidine, 0.1 mg/kg) in morphine withdrawal did not restore normal PER2 patterns in each affected region; however, both quinpirole and clonidine themselves altered normal daily PER2 expression patterns in morphine-naive rats. These findings confirm and extend previous observations that opiates disrupt daily patterns of clock gene expression in the limbic forebrain. Furthermore, catecholaminergic drugs, which have been previously found to alleviate symptoms of opiate withdrawal, do not alleviate the effects of morphine withdrawal on PER2, but do modulate daily patterns of PER2 expression in saline controls.

Positive autoregulation delays the expression phase of mammalian clock gene Per2

In mammals, cellular circadian rhythms are generated by a transcriptional-translational autoregulatory network that consists of clock genes that encode transcriptional regulators. Of these clock genes, Period1 (Per1) and Period2 (Per2) are essential for sustainable circadian rhythmicity and photic entrainment. Intriguingly, Per1 and Per2 mRNAs exhibit circadian oscillations with a 4-hour phase difference, but they are similarly transactivated by CLOCK-BMAL1. In this study, we investigated the mechanism underlying the phase difference between Per1 and Per2 through a combination of mathematical simulations and molecular experiments. Mathematical analyses of a model for the mammalian circadian oscillator demonstrated that the slow synthesis and fast degradation of mRNA tend to advance the oscillation phase of mRNA expression. However, the phase difference between Per1 and Per2 was not reproduced by the model, which implemented a 1.1-fold difference in degradation rates and a 3-fold difference in CLOCK-BMAL1 mediated inductions of Per1 and Per2 as estimated in cultured mammalian cells. Thus, we hypothesized the existence of a novel transcriptional activation of Per2 by PER1/2 such that the Per2 oscillation phase was delayed. Indeed, only the Per2 promoter, but not Per1, was strongly induced by both PER1 and PER2 in the presence of CLOCK-BMAL1 in a luciferase reporter assay. Moreover, a 3-hour advance was observed in the transcriptional oscillation of the delta-Per2 reporter gene lacking cis-elements required for the induction by PER1/2. These results indicate that the Per2 positive feedback regulation is a significant factor responsible for generating the phase difference between Per1 and Per2 gene expression.

It's time to swim! Zebrafish and the circadian clock

The zebrafish represents a fascinating model for studying key aspects of the vertebrate circadian timing system. Easy access to early embryonic development has made this species ideal for investigating how the clock is first established during embryogenesis. In particular, the molecular basis for the functional development of the zebrafish pineal gland has received much attention. In addition to this dedicated clock and photoreceptor organ, and unlike the situation in mammals, the clocks in zebrafish peripheral tissues and even cell lines are entrainable by direct exposure to light thus providing unique insight into the function and evolution of the light input pathway. Finally, the small size, low maintenance costs and high fecundity of this fish together with the availability of genetic tools make this an attractive model for forward genetic analysis of the circadian clock. Here, we review the work that has established the zebrafish as a valuable clock model organism and highlight the key questions that will shape the future direction of research.

Deficiency of antinociception and excessive grooming induced by acute immobilization stress in Per1 mutant mice

Acute stressors induce changes in numerous behavioral parameters through activation of the hypothalamic-pituitary-adrenal (HPA) axis. Several important hormones in paraventricular nucleus of the hypothalamus (PVN) play the roles in these stress-induced reactions. Corticotropin-releasing hormone (CRH), arginine-vasopressin (AVP) and corticosterone are considered as molecular markers for stress-induced grooming behavior. Oxytocin in PVN is an essential modulator for stress-induced antinociception. The clock gene, Per1, has been identified as an effecter response to the acute stresses, but its function in neuroendocrine stress systems remains unclear. In the present study we observed the alterations in grooming and nociceptive behaviors induced by acute immobilization stress in Per1 mutant mice and other genotypes (wild types and Per2 mutant). The results displayed that stress elicited a more robust effect on grooming behavior in Per1 mutant mice than in other genotypes. Subsequently, the obvious stress-induced antinociception was observed in the wild-type and Per2 mutant mice, however, in Per1 mutant, this antinociceptive effects were partially-reversed (mechanical sensitivity), or over-reversed to hyperalgesia (thermal sensitivity). The real-time qPCR results showed that in PVN, there were stress-induced up-regulations of Crh, Avp and c-fos in all of genotypes; moreover, the expression change of Crh in Per1 mutant mice was much larger than in others. Another hormonal gene, Oxt, was up-regulated induced by stress in wild-type and Per2 mutant but not in Per1 mutant. In addition, the stress significantly elevated the serum corticosterone levels without genotype-dependent differences, and accordingly the glucocorticoid receptor gene, Nr3c1, expressed with a similar pattern in PVN of all strains. Taken together, the present study indicated that in acute stress treated Per1 mutant mice, there are abnormal hormonal responses in PVN, correlating with the aberrant performance of stress-induced behaviors. Therefore, our findings suggest a novel functional role of Per1 in neuroendocrine stress system, which further participates in analgesic regulation.

Interactions of polymorphisms in different clock genes associated with circadian phenotypes in humans

Several studies have shown that mutations and polymorphisms in clock genes are associated with abnormal circadian parameters in humans and also with more subtle non-pathological phenotypes like chronotypes. However, there have been conflicting results, and none of these studies analyzed the combined effects of more than one clock gene. Up to date, association studies in humans have focused on the analysis of only one clock gene per study. Since these genes encode proteins that physically interact with each other, combinations of polymorphisms in different clock genes could have a synergistic or an inhibitory effect upon circadian phenotypes. In the present study, we analyzed the combined effects of four polymorphisms in four clock genes (Per2, Per3, Clock and Bmal1) in people with extreme diurnal preferences (morning or evening). We found that a specific combination of polymorphisms in these genes is more frequent in people who have a morning preference for activity and there is a different combination in individuals with an evening preference for activity. Taken together, these results show that it is possible to detect clock gene interactions associated with human circadian phenotypes and bring an innovative idea of building a clock gene variation map that may be applied to human circadian biology.

A common origin: signaling similarities in the regulation of the circadian clock and DNA damage responses

Circadian clocks are intrinsic, time-tracking systems that endow organisms with a survival advantage. Studies of animal models and human tumor samples have revealed that the disruption of circadian rhythms is an important endogenous factor that can contribute to mammalian cancer development. The core of the circadian clock mechanism is a cell-autonomous and self-sustained oscillator system mediated by a transcription/translation-based negative feedback loop that relies on positive and negative elements. Recent studies have implicated these core circadian components in the regulation of both the cell cycle and DNA damage responses (DDR). Indeed, the circadian feedback loop controls the timing of cell proliferation by regulating the expression of key cell cycle genes. Conversely, several intracellular signaling cascades and post-translational modifications that play important roles in the cell cycle and DDR are also essential for circadian clock regulation. Importantly, alteration of a cell's reduction-oxidation (redox) state triggers the transduction of photic signals that regulate circadian clock gene transcription, suggesting that cellular responses to photo-oxidative stress may have been the evolutionary origin of the circadian clock. This review describes selected regulatory aspects of circadian machinery that are evidence of a molecular link between the circadian clock and DDR, focusing particularly on the signaling cascades involved in the light entrainment of the zebrafish circadian clock.

Photic entrainment of period mutant mice is predicted from their phase response curves

A fundamental property of circadian clocks is that they entrain to environmental cues. The circadian genes, Period1 and Period2, are involved in entrainment of the mammalian circadian system. To investigate the roles of the Period genes in photic entrainment, we constructed phase response curves (PRC) to light pulses for C57BL/6J wild-type, Per1(-/-), Per2(-/-), and Per3(-/-) mice and tested whether the PRCs accurately predict entrainment to non-24 light-dark cycles (T-cycles) and constant light (LL). The PRCs of wild-type and Per3(-/-) mice are similar in shape and amplitude and have relatively large delay zones and small advance zones, resulting in successful entrainment to 26 h T-cycles (T26), but not T21, with similar phase angles. Per1(-/-) mice have a high-amplitude PRC, resulting in entrainment to a broad range of T-cycles. Per2(-/-) mice also entrain to a wide range of T-cycles because the advance portion of their PRC is larger than wild types. Period aftereffects following entrainment to T-cycles were similar among all genotypes. We found that the ratio of the advance portion to the delay portion of the PRC accurately predicts the lengthening of the period of the activity rhythm in LL. Wild-type, Per1(-/-), and Per3(-/-) mice had larger delay zones than advance zones and lengthened (>24 h) periods in LL, whereas Per2(-/-) mice had delay and advance zones that were equal in size and no period lengthening in LL. Together, these results demonstrate that PRCs are powerful tools for predicting and understanding photic entrainment of circadian mutant mice.

High amplitude phase resetting in rev-erbalpha/per1 double mutant mice

Over time, organisms developed various strategies to adapt to their environment. Circadian clocks are thought to have evolved to adjust to the predictable rhythms of the light-dark cycle caused by the rotation of the Earth around its own axis. The rhythms these clocks generate persist even in the absence of environmental cues with a period of about 24 hours. To tick in time, they continuously synchronize themselves to the prevailing photoperiod by appropriate phase shifts. In this study, we disrupted two molecular components of the mammalian circadian oscillator, Rev-Erbalpha and Period1 (Per1). We found that mice lacking these genes displayed robust circadian rhythms with significantly shorter periods under constant darkness conditions. Strikingly, they showed high amplitude resetting in response to a brief light pulse at the end of their subjective night phase, which is rare in mammals. Surprisingly, Cry1, a clock component not inducible by light in mammals, became slightly inducible in these mice. Taken together, Rev-Erbalpha and Per1 may be part of a mechanism preventing drastic phase shifts in mammals.

Physiology of circadian entrainment

Mammalian circadian rhythms are controlled by endogenous biological oscillators, including a master clock located in the hypothalamic suprachiasmatic nuclei (SCN). Since the period of this oscillation is of approximately 24 h, to keep synchrony with the environment, circadian rhythms need to be entrained daily by means of Zeitgeber ("time giver") signals, such as the light-dark cycle. Recent advances in the neurophysiology and molecular biology of circadian rhythmicity allow a better understanding of synchronization. In this review we cover several aspects of the mechanisms for photic entrainment of mammalian circadian rhythms, including retinal sensitivity to light by means of novel photopigments as well as circadian variations in the retina that contribute to the regulation of retinal physiology. Downstream from the retina, we examine retinohypothalamic communication through neurotransmitter (glutamate, aspartate, pituitary adenylate cyclase-activating polypeptide) interaction with SCN receptors and the resulting signal transduction pathways in suprachiasmatic neurons, as well as putative neuron-glia interactions. Finally, we describe and analyze clock gene expression and its importance in entrainment mechanisms, as well as circadian disorders or retinal diseases related to entrainment deficits, including experimental and clinical treatments.

Nighttime dim light exposure alters the responses of the circadian system

The daily light dark cycle is the most salient entraining factor for the circadian system. However, in modern society, darkness at night is vanishing as light pollution steadily increases. The impact of brighter nights on wild life ecology and human physiology is just now being recognized. In the present study, we tested the possible detrimental effects of dim light exposure on the regulation of circadian rhythms, using CD1 mice housed in light/dim light (LdimL, 300 lux:20 lux) or light/dark (LD, 300 lux:1 lux) conditions. We first examined the expression of clock genes in the suprachiasmatic nucleus (SCN), the locus of the principal brain clock, in the animals of the LD and LdimL groups. Under the entrained condition, there was no difference in PER1 peak expression between the two groups, but at the trough of the PER 1 rhythm, there was an increase in PER1 in the LdimL group, indicating a decrease in the amplitude of the PER1 rhythm. After a brief light exposure (30 min, 300 lux) at night, the light-induced expression of mPer1 and mPer2 genes was attenuated in the SCN of LdimL group. Next, we examined the behavioral rhythms by monitoring wheel-running activity to determine whether the altered responses in the SCN of LdimL group have behavioral consequence. Compared to the LD controls, the LdimL group showed increased daytime activity. After being released into constant darkness, the LdimL group displayed shorter free-running periods. Furthermore, following the light exposure, the phase shifting responses were smaller in the LdimL group. The results indicate that nighttime dim light exposure can cause functional changes of the circadian system, and suggest that altered circadian function could be one of the mechanisms underlying the adverse effects of light pollution on wild life ecology and human physiology.

Environmental modulation of alcohol intake in hamsters: effects of wheel running and constant light exposure

BACKGROUND

Alcohol abuse leads to marked disruptions of circadian rhythms, and these disturbances in themselves can increase the drive to drink. Circadian clock timing is regulated by light, as well as by nonphotic influences such as food, social interactions, and wheel running. We previously reported that alcohol markedly disrupts photic and nonphotic modes of circadian rhythm regulation in Syrian hamsters. As an extension of this work, we characterize the hedonic interrelationship between wheel running and ethanol (EtOH) intake and the effects of environmental circadian disruption (long-term exposure to constant light [LL]) on the drive to drink.

METHODS

First, we tested the effect of wheel running on chronic free-choice consumption of a 20% (v/v) EtOH solution and water. Second, the effect of this alcohol drinking on wheel running in alcohol-naive animals was investigated. Third, we assessed the influence of LL, known to suppress locomotor activity and cause circadian rhythm disruption, on EtOH consumption and wheel-running behavior.

RESULTS

Inhibitory effects of wheel running on EtOH intake and vice versa were observed. Exposure to LL, while not affecting EtOH intake, induced rhythm splitting in 75% of the animals. Notably, the splitting phenotype was associated with lower levels of EtOH consumption and preference prior to, and throughout, the period of LL exposure.

CONCLUSIONS

These results are evidence that exercise may offer an efficacious clinical approach to reducing EtOH intake. Also, predisposition for light-induced (or other) forms of circadian disruption may modulate the drive to drink.

Mammalian target of rapamycin signaling modulates photic entrainment of the suprachiasmatic circadian clock

Inducible gene expression appears to be an essential event that couples light to entrainment of the master mammalian circadian clock located in the suprachiasmatic nucleus (SCN) of the hypothalamus. Recently, we reported that light triggers phase-dependent activation of the mammalian target of rapamycin (mTOR) signaling pathway, a major regulator of protein synthesis, in the SCN, thus raising the possibility that mTOR-evoked mRNA translation contributes to clock entrainment. Here, we used a combination of cellular, molecular, and behavioral assays to address this question. To this end, we show that the in vivo infusion of the mTOR inhibitor rapamycin led to a significant attenuation of the phase-delaying effect of early-night light. Conversely, disruption of mTOR during the late night augmented the phase-advancing effect of light. To assess the role of mTOR signaling within the context of molecular entrainment, the effects of rapamycin on light-induced expression of PERIOD1 and PERIOD2 were examined. At both the early- and late-night time points, abrogation of mTOR signaling led to a significant attenuation of light-evoked PERIOD protein expression. Our results also reveal that light-induced mTOR activation leads to the translation of mRNAs with a 5'-terminal oligopyrimidine tract such as eukaryotic elongation factor 1A and the immediate early gene JunB. Together, these data indicate that the mTOR pathway functions as potent and selective regulator of light-evoked protein translation and SCN clock entrainment.

Lack of calbindin-D28k alters response of the murine circadian clock to light

A strong stimulus adjusting the circadian clock to the prevailing light-dark cycle is light. However, the circadian clock is reset by light only at specific times of the day. The mechanisms mediating such gating of light input to the CNS are not well understood. There is evidence that Ca(2+) ions play an important role in intracellular signaling mechanisms, including signaling cascades stimulated by light. Therefore, Ca(2+) is hypothesized to play a role in the light-mediated resetting of the circadian clock. Calbindin-D28k (CB; gene symbol: Calb1) is a Ca(2+) binding protein implicated in Ca(2+) homeostasis and sensing. The absence of this protein influences Ca(2+) buffering capacity of a cell, alters spatio-temporal aspects of intracellular Ca(2+) signaling, and hence might alter transmission of light information to the circadian clock in neurons of the suprachiasmatic nuclei (SCN). We tested mice lacking a functional Calb1 gene (Calb1(-/-)) and found an increased phase-delay response to light applied at circadian time (CT) 14 in these animals. This is accompanied by elevated induction of Per2 gene expression in the SCN. Period length and circadian rhythmicity were comparable between Calb1(-/-) and wild-type animals. Our findings indicate an involvement of CB in the signaling pathway that modulates the behavioral and molecular response to light.

Molecular cloning, tissue distribution, and daily rhythms of expression of per1 gene in European sea bass (Dicentrarchus labrax)

Circadian rhythms are controlled by interlocked autoregulatory feedback loops consisting of interactions of a group of circadian clock genes and their proteins. The Period family is a group of genes that are essential components of the molecular clock. In the present study, we cloned a period gene (per1) of the European sea bass, a marine teleost of chronobiological interest. The cloned sequence encoded a protein consisting of 1436 amino acids that homology and phylogenic analyses showed to be related with fish PER1 proteins possessing very high identity with Oryzias latipes (Medaka) per1. Polymerase chain reaction screening of per1 expression showed that this gene is expressed in all the tissues analyzed (brain, heart, liver, gill, muscle, digestive tract, adipose tissue, spleen, and retina). In addition, a daily expression rhythm, with an acrophase (peak time) approximately ZT0 (lights-on), was found in the two tissue types investigated: neural (brain) and peripheral (liver and heart). In conclusion, identification and characterization of the gene encoding sea bass per1 provide valuable information for understanding the circadian mechanism at the molecular level in this species, although further research is needed to clarify the exact role that per1 plays in the circadian oscillator and the dual behavior of European sea bass.

Light-dependent behavioral phenotypes in PER3-deficient mice

A functional knockout of Period3 in mice (mPer3(-/-)) results in a mildly altered circadian phenotype, and mPer3 shows a redundant role within the circadian clock. In this study, the authors reevaluated the Per3(-/ -) behavioral phenotype on a C57Bl/6J background and report altered responses to light. In constant light, free-running activity period was shorter than that of wild-type, whereas in constant darkness, no difference was observed between genotypes. The effect of light was parametric, and the difference in free-running period between genotypes increased under constant light with increasing light intensity. An attenuated response to light in Per3(-/-) mice was also demonstrated through reduced negative masking in activity in an ultradian protocol and a slower reentrainment to a shifted light-dark cycle when activity falls in the light period of the new light-dark cycle. Behavioral phase-shifts in response to a single delaying or advancing light pulse in the Per3(-/-) mouse were not compromised. This demonstrates that the mPer3(- /-) phenotype is characterized predominantly by altered sensitivity to light and not by the ability of the circadian system to respond to light. In addition to its redundant role within the molecular clock, these data suggest a new role for Per3 outside of the circadian clock and contributing to light input pathways.

The Mammalian Circadian Timing System: Organization and Coordination of Central and Peripheral Clocks

Most physiology and behavior of mammalian organisms follow daily oscillations. These rhythmic processes are governed by environmental cues (e.g., fluctuations in light intensity and temperature), an internal circadian timing system, and the interaction between this timekeeping system and environmental signals. In mammals, the circadian timekeeping system has a complex architecture, composed of a central pacemaker in the brain's suprachiasmatic nuclei (SCN) and subsidiary clocks in nearly every body cell. The central clock is synchronized to geophysical time mainly via photic cues perceived by the retina and transmitted by electrical signals to SCN neurons. In turn, the SCN influences circadian physiology and behavior via neuronal and humoral cues and via the synchronization of local oscillators that are operative in the cells of most organs and tissues. Thus, some of the SCN output pathways serve as input pathways for peripheral tissues. Here we discuss knowledge acquired during the past few years on the complex structure and function of the mammalian circadian timing system.

The circadian clock gene Per1 suppresses cancer cell proliferation and tumor growth at specific times of day

Cell cycle progression is tightly regulated. The expressions of cell cycle regulators, the products of which either promote or inhibit cell proliferation, oscillate during each cell cycle. Cellular proliferation and the expression of cell cycle regulators are also controlled by the circadian clock. Disruption of the circadian clock may thereby lead to deregulated cell proliferation. Mammalian Per2 is a core clock gene, the product of which suppresses cancer cell proliferation and tumor growth in vivo and in vitro. Because Per1, another key clock gene, is mutated in human breast cancers, and because its clock functions are similar and complementary to those of Per2, we have studied its role in modulating breast cancer cell proliferation and tumor growth. We find that breast cancer growth rate is gated by the circadian clock with two daily peaks and troughs, and that they are coupled to the daily expression patterns of clock-controlled genes that regulate cell proliferation. Down-regulation of the expression of tumor Per1 increases cancer cell growth in vitro and tumor growth in vivo by enhancing the circadian amplitude of the two daily tumor growth peaks. The data of the study suggest Per1 has tumor-suppressor function that diminishes cancer proliferation and tumor growth, but only at specific times of day.

Circadian temperature variation and ageing

In the present paper, an attempt is made to summarize current knowledge concerning the daily body temperature rhythm and its age-dependent alterations. Homeostatic and circadian control mechanisms are considered. Special attention is paid to the circadian system, as the mechanisms of autonomic control are the topic of another contribution to this special issue. Also, the interactions of the core body temperature rhythm with other circadian functions are discussed in detail as they constitute an essential part of the internal temporal order of living systems and thus guarantee their optimal functioning. In the second part of the paper, age-dependent changes in the circadian body temperature rhythm and their putative causes, considering circadian and homeostatic components, are described. Consequences for health and fitness and some possibilities to prevent adverse effect are mentioned in the final section.

Expression of clock genes in the suprachiasmatic nucleus: effect of environmental lighting conditions

The suprachiasmatic nucleus (SCN) is the anatomical substrate for the principal circadian clock coordinating daily rhythms in a vast array of behavioral and physiological responses. Individual SCN neurons are cellular oscillators and are organized into a multi-oscillator network following unique spatiotemporal patterns. The rhythms generated in the SCN are generally entrained to the environmental light dark cycle, which is the most salient cue influencing the network organization of the SCN. The neural network in the SCN is a heterogeneous structure, containing two major compartments identified by applying physiological and functional criteria, namely the retinorecipient core region and the highly rhythmic shell region. Changes in the environmental lighting condition are first detected and processed by the core region, and then conveyed to the rest of the SCN, leading to adaptive responses of the entire network. This review will focus on the studies that explore the responses of the SCN network by examining the expression of clock genes, under various lighting paradigms, such as acute light exposure, lighting schedules or exposure to different light durations. The results will be discussed under the framework of functionally distinct SCN sub regions and oscillator groups. The evidence presented here suggests that the environmental lighting conditions alter the spatiotemporal organization of the cellular oscillators within the SCN, which consequently affect the overt rhythms in behavior and physiology. Thus, information on how the SCN network elements respond to environmental cues is key to understanding the human health problems that stem from circadian rhythm disruption.

Circadian disruption, Per3, and human cytokine secretion

Circadian disruption has been linked with inflammation, an established cancer risk factor. Per3 clock gene polymorphisms have also been associated with circadian disruption and with increased cancer risk. Patients completed a questionnaire and provided a blood sample prior to undergoing a colonoscopy (n = 70). Adjusted mean serum cytokine concentrations (IL-6, TNF-alpha, gamma-INF, IL-1ra, IL-1-beta, VEGF) were compared among patients with high and low scores for fatigue (Multidimensional Fatigue Inventory), depressive symptoms (Beck Depression Inventory II), or sleep disruption (Pittsburgh Sleep Quality Index), or among patients with different Per3 clock gene variants. Poor sleep was associated with elevated VEGF, and fatigue-related reduced activity was associated with elevated TNF-alpha concentrations. Participants with the 4/5 or 5/5 Per3 variable tandem repeat sequence had elevated IL-6 concentrations compared to those with the 4/4 genotype. Biological processes linking circadian disruption with cancer remain to be elucidated. Increased inflammatory cytokine secretion may play a role.