A clockwork web: circadian timing in brain and periphery, in health and disease

Effect of chronic ethanol feeding on 24-hour rhythms of mitogenic responses and lymphocyte subset populations in thymus and spleen of peripubertal male rats

This work analyzes the effect of chronic ethanol feeding on the 24-hour variation of mitogenic responses and lymphocyte subset populations in thymus and spleen. Animals were maintained under a 12:12-hour light/dark photoperiod and they received a liquid diet for 4 weeks, starting on day 35 of life. The ethanol-fed group received a similar diet to controls except that maltose was isocalorically replaced by ethanol. Ethanol replacement provided 36% of the total caloric content of the diet. Rats were killed at 6 time intervals around the clock, beginning at Zeitgeber time (ZT) 1 (ZT 0 = lights on). Under ethanol intake the splenic and thymic weight decreased. In addition, mean values of the thymic, but not of the splenic T cell number decreased, and mean values of the thymic and splenic CD8+ and CD4+CD8+ number increased. Consequently, the thymic T/B ratio and the thymic and splenic CD4+/CD8+ ratio decreased in ethanol-fed rats. At the same time there was a significant increase in the response of the thymic cells to LPS. The ethanol diet modified the 24-hour rhythmicity of thymic and splenic T, B and CD4+CD8+ cells, thymic CD4+ and splenic CD8+ cells, thymic and splenic T/B and CD4+/CD8+ ratios, as well as of mitogenic responses in both tissues. Chronic ethanol administration presumably affects the endogenous clock that modulates the circadian variation of immune responsiveness in growing rats.

Role of Neuronal Membrane Events in Circadian Rhythm Generation

Circadian clock systems are composed of an input or "entrainment" pathway by which synchronization to the external environment occurs, a pacemaker responsible for generating rhythmicity, and an output or "expression" pathway through which rhythmic signals act to modulate physiology and behavior. The circadian pacemaker contains molecular feedback loops of rhythmically expressed genes and their protein products, which, through interactions, generate a circa 24-h cycle of transcription and translation of clock and clock-controlled genes. Neuronal membrane events appear to play major roles in entrainment of circadian rhythms in mollusks and mammals. In mammals, the suprachiasmatic nuclei of the hypothalamus receive photic information via the retinohypothalamic tract. Retinal signals, mediated by glutamate, induce calcium release and activate a number of intracellular cascades involved in photic gating and phase shifting. Membrane events are also involved in rhythm expression. Calcium and potassium currents influence the electrical output of pacemaker neurons by altering shape and intervals of impulse prepotentials, afterhyperpolarization periods, and interspike intervals, as well as altering membrane potentials and thereby shaping the spontaneous rhythmic spiking patterns. Unlike the involvement of membrane events in circadian entrainment and expression, it is less clear whether electrical activity, postsynaptic events, and transmembrane ion fluxes also are essential elements in rhythm generation. Studies, however, suggest that neuronal membrane activity may indeed play a crucial role in circadian rhythm generation.

Effect of interferon-gamma treatment on 24-hour variations in plasma ACTH, growth hormone, prolactin, luteinizing hormone and follicle-stimulating hormone of male rats

OBJECTIVE

Interferon-gamma (IFN-gamma) is a cytokine produced by T helper cells on antigenic challenge that may affect the release of several pituitary hormones. However, in vitro or in vivo studies have yielded disparate results with stimulatory, inhibitory or absent effects of IFN on pituitary hormone release. One of the reasons for these discrepancies could be that hormone changes were commonly assessed at a single time point in the day-night cycle. In this study we measured the circadian pattern of plasma ACTH, growth hormone (GH), prolactin, luteinizing hormone (LH) and follicle-stimulating hormone (FSH) at 6 different time points within a 24-hour cycle in adult male Wistar rats.

METHODS

Groups of 6-8 rats kept under light from 08:00 to 20:00 h daily received 5 daily injections intraperitoneally of human IFN-gamma (10(5) IU/kg body weight) or saline at 08:30 h. Plasma ACTH, GH, prolactin, LH and FSH levels were measured by a homologous specific double antibody RIA.

RESULTS

A factorial ANOVA for main effects indicated a significant 43% increase of circulating prolactin in IFN-gamma-treated rats. Time of day changes were significant for the five hormones examined and these diurnal variations became altered by IFN-gamma administration, with a phase advance of ACTH peak, a suppression of the rest phase peak of GH, the appearance of a second peak of prolactin at an early phase of daily photoperiod, and the blunting of the 24-hour variations of plasma FSH.

CONCLUSION

The data point out an effect of IFN-gamma on the mechanisms responsible for the circadian organization of pituitary hormone release.

Analysis of circadian mechanisms in the suprachiasmatic nucleus by transgenesis and biolistic transfection

Analysis of the cellular and molecular mechanisms that underlie the circadian pacemaker of the suprachiasmatic nuclei (SCN) requires in vitro preparations amenable to genetic manipulation that can provide dynamic measures of circadian activity in real time over multiple circadian cycles. This article focuses on the value of the SCN organotypic slice for such studies. Specifically, it describes the use of tissues from genetically modified mice in which the circadian promoter of the mPer1 gene is used to drive the expression of either firefly luciferase or destabilized green fluorescent protein optical reporters. Furthermore, we describe a procedure for biolistic (particle-mediated) transfection of SCN organotypic slices with fluorescent reporters that can be used to explore the cis-acting elements and trans-acting factors that control circadian patterning, and also the interactions between subpopulations of neuronal oscillators within the SCN assemblage.

The clock gene Per2 influences the glutamatergic system and modulates alcohol consumption

Period (Per) genes are involved in regulation of the circadian clock and are thought to modulate several brain functions. We demonstrate that Per2(Brdm1) mutant mice, which have a deletion in the PAS domain of the Per2 protein, show alterations in the glutamatergic system. Lowered expression of the glutamate transporter Eaat1 is observed in these animals, leading to reduced uptake of glutamate by astrocytes. As a consequence, glutamate levels increase in the extracellular space of Per2(Brdm1) mutant mouse brains. This is accompanied by increased alcohol intake in these animals. In humans, variations of the PER2 gene are associated with regulation of alcohol consumption. Acamprosate, a drug used to prevent craving and relapse in alcoholic patients is thought to act by dampening a hyper-glutamatergic state. This drug reduced augmented glutamate levels and normalized increased alcohol consumption in Per2(Brdm1) mutant mice. Collectively, these data establish glutamate as a link between dysfunction of the circadian clock gene Per2 and enhanced alcohol intake.

Identification of putative rat ribonuclease III by differential display: a novel rat mRNA expressed in a circadian manner in the rat suprachiasmatic nucleus

The suprachiasmatic nucleus (SCN) of the hypothalamus constitutes the principal site responsible for the generation and entrainment of circadian rhythms in mammals. The mechanisms of the circadian clock involve periodic gene expression. Here we report the use of differential display reverse transcriptase polymerase chain reaction to identify a novel rat mRNA sequence which is highly homologous to human ribonuclease III. Analysis of its expression in the rat brain by in situ hybridization histochemistry showed this transcript to be expressed at differing intensities at various sites. Temporal variation in expression was observed in the SCN, with a peak at circadian time (CT) 2 and a nadir at CT14. No significant changes in its expression were detected across the cycle within the supraoptic nucleus, cingulate cortex or caudate putamen.

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.

Human molecular chronotyping in sight?

Recent research on mouse models has taken us closer to deciphering the molecular clock mechanism that defines an individual's 'body time'.

Recent research on mouse models has taken us closer to deciphering the molecular clock mechanism that defines an individual's 'body time'. How feasible will it be to create a molecular timetable that allows determination of individual body time from tissue harvested at a single time point?

BMAL1 and CLOCK, two essential components of the circadian clock, are involved in glucose homeostasis

Circadian timing is generated through a unique series of autoregulatory interactions termed the molecular clock. Behavioral rhythms subject to the molecular clock are well characterized. We demonstrate a role for Bmal1 and Clock in the regulation of glucose homeostasis. Inactivation of the known clock components Bmal1 (Mop3) and Clock suppress the diurnal variation in glucose and triglycerides. Gluconeogenesis is abolished by deletion of Bmal1 and is depressed in Clock mutants, but the counterregulatory response of corticosterone and glucagon to insulin-induced hypoglycaemia is retained. Furthermore, a high-fat diet modulates carbohydrate metabolism by amplifying circadian variation in glucose tolerance and insulin sensitivity, and mutation of Clock restores the chow-fed phenotype. Bmal1 and Clock, genes that function in the core molecular clock, exert profound control over recovery from insulin-induced hypoglycaemia. Furthermore, asynchronous dietary cues may modify glucose homeostasis via their interactions with peripheral molecular clocks.

Transcriptional oscillation of canonical clock genes in mouse peripheral tissues

Background

The circadian rhythm of about 24 hours is a fundamental physiological function observed in almost all organisms from prokaryotes to humans. Identification of clock genes has allowed us to study the molecular bases for circadian behaviors and temporal physiological processes such as hormonal secretion, and has prompted the idea that molecular clocks reside not only in a central pacemaker, the suprachiasmatic nuclei (SCN) of hypothalamus in mammals, but also in peripheral tissues, even in immortalized cells. Furthermore, previous molecular dissection revealed that the mechanism of circadian oscillation at a molecular level is based on transcriptional regulation of clock and clock-controlled genes.

Results

We systematically analyzed the mRNA expression of clock and clock-controlled genes in mouse peripheral tissues. Eight genes (mBmal1, mNpas2, mRev-erbα, mDbp, mRev-erbβ, mPer3, mPer1 and mPer2; given in the temporal order of the rhythm peak) showed robust circadian expressions of mRNAs in all tissues except testis, suggesting that these genes are core molecules of the molecular biological clock. The bioinformatics analysis revealed that these genes have one or a combination of 3 transcriptional elements (RORE, DBPE, and E-box), which are conserved among human, mouse, and rat genome sequences, and indicated that these 3 elements may be responsible for the biological timing of expression of canonical clock genes.

Conclusions

The observation of oscillatory profiles of canonical clock genes is not only useful for physiological and pathological examination of the circadian clock in various organs but also important for systematic understanding of transcriptional regulation on a genome-wide basis. Our finding of the oscillatory expression of canonical clock genes with a temporal order provides us an interesting hypothesis, that cyclic timing of all clock and clock-controlled genes may be dependent on several transcriptional elements including 3 known elements, E-box, RORE, and DBPE.

Effect of ethanol on 24-hour hormonal changes in peripubertal male rats

We analyzed the effect of chronic (4 weeks) ethanol feeding on 24-h variation of pituitary-testicular function in peripubertal male Wistar rats by measuring circulating concentrations of prolactin, follicle-stimulating hormone, luteinizing hormone, testosterone, and thyrotropin. Animals were maintained under a 12-h light: 12-h dark photoperiod and received a liquid diet for 4 weeks, starting on day 35 of life. The ethanol-fed group received a diet similar to that provided to control animals, except that maltose was replaced isocalorically with ethanol. Ethanol replacement provided 36% of the total caloric content of the diet. Rats were killed at one of six times around the clock, beginning at zeitgeber time (ZT) 1 (ZT 0 = lights on). In ethanol-fed rats globally, secretion of prolactin was augmented, whereas secretion of follicle-stimulating hormone, luteinizing hormone, testosterone, and thyrotropin was decreased. Significant changes in the 24-h secretory pattern of circulating hormones occurred in rats receiving ethanol, including the appearance of two peaks (at ZT 1 and ZT 9), rather than one peak, of follicle-stimulating hormone during the inactive phase of the daily cycle, suppression of the maximum plasma luteinizing hormone concentration during the first part of the inactive phase, and appearance of a second peak of testosterone and prolactin during the second part of the inactive phase (at ZT 5 and ZT 9, respectively) and of a second peak of plasma thyrotropin during the first part of the active phase (at ZT 13). The significant positive correlation between testosterone and individual luteinizing hormone and prolactin concentrations in control animals was no longer observed after ethanol administration. Chronic ethanol administration presumably affects the endogenous clock that modulates the circadian variation of the pituitary-gonadal axis and thyrotropin release in growing male rats.

24-hour pattern of circulating prolactin and growth hormone levels and submaxillary lymph node immune responses in growing male rats subjected to social isolation

To assess the effect of social isolation of growing rats on 24-h rhythmicity of circulating prolactin and growth hormone (GH) levels and submaxillary lymph node immune responses, male Wistar rats were either individually caged or kept in groups (4-5 animals per cage) for 30 d starting on d 35 of life. Plasma prolactin and GH levels, and submaxillary lymph node lymphocyte subset populations, interferon (IFN)-gamma release and mitogenic responses to concanavalin A (Con A) and lipopolysaccharide (LPS) were determined at six time intervals during the 24 h span. Social isolation brought about changes in mean values and 24-h pattern of plasma prolactin and GH levels and lymph node immune responses. After isolation, prolactin and GH mean values decreased, and lymph node T, B, non T-non B, CD8+, and CD4+-CD8+ cells augmented, whereas lymph node CD4+/CD8+ ratio, IFN-gamma release and mitogenic responses decreased. Social isolation resulted in disruption of 24 h rhythmicity of every immune parameter tested. CD4+/CD8+ ratio, IFN-gamma release and Concanavalin A (Con A) and lipopolysaccharide (LPS) responses correlated significantly with plasma prolactin or GH levels while T/B ratio correlated with plasma prolactin levels only. B, non T-non B, and CD4+-CD8+ cells correlated negatively with plasma prolactin. Modifications in mean value and 24-h rhythmicity of plasma prolactin and GH levels are presumably involved in the effect of social isolation on immune responsiveness.

Disruption of Ultradian and Circadian Rhythms of Blood Pressure in Nondipper Hypertensive Patients

Ultradian rhythms in blood pressure (BP) are known to exist, but their modification in hypertension is largely unknown. The present study was undertaken to assess the integrity of ultradian and 24-hour BP rhythms in dipper (n=100) and nondipper (n=20) hypertensive patients compared with 44 dipper normotensive individuals. Fourier analysis was used to fit ultradian (12, 8, and 6 hour) and 24-hour rhythms in BP and heart rate (HR). Mesor, amplitude, and acrophase were calculated for individual and overall rhythm curves. All subjects showed significant ultradian or 24-hour BP and HR rhythms. Systolic and diastolic BP mesor was higher in hypertensive patients compared with normotensive patients. The percentage of variability in ambulatory BP that could be explained by fitting ultradian and 24-hour rhythms was reduced in nondippers compared with normotensives or dippers. Amplitude of ultradian and 24-hour rhythms in BP increased in dippers and decreased in nondippers. Ultradian and 24-hour rhythms in HR did not differ among the 3 groups examined. Results indicate that in nondippers, blunted ultradian and 24-hour rhythm amplitude in BP was accompanied by a loss of rhythm integrity.

A functional genomics strategy reveals Rora as a component of the mammalian circadian clock

The mammalian circadian clock plays an integral role in timing rhythmic physiology and behavior, such as locomotor activity, with anticipated daily environmental changes. The master oscillator resides within the suprachiasmatic nucleus (SCN), which can maintain circadian rhythms in the absence of synchronizing light input. Here, we describe a genomics-based approach to identify circadian activators of Bmal1, itself a key transcriptional activator that is necessary for core oscillator function. Using cell-based functional assays, as well as behavioral and molecular analyses, we identified Rora as an activator of Bmal1 transcription within the SCN. Rora is required for normal Bmal1 expression and consolidation of daily locomotor activity and is regulated by the core clock in the SCN. These results suggest that opposing activities of the orphan nuclear receptors Rora and Rev-erb alpha, which represses Bmal1 expression, are important in the maintenance of circadian clock function.

Circadian dependency of nocturnal immediate-early protein induction in rat retina

Daily rhythms in the mammalian retina are regulated by an endogenous circadian clock. Previously it was found that neuronal elements of the rat retina respond to light:dark (L:D) transitions with cell-specific changes in expression of the c-fos gene. Using a pan-Fos antibody to probe Western blots of rat retina, we have now shown that darkness is associated with a 60-fold increase in c-Fos protein, whereas levels of FosB and Fos-related antigens are invariant. The induction of c-Fos exhibits circadian dependency; accumulation of c-Fos protein was significantly enhanced, by a factor of 2.5-fold, when darkness onset was coincident with the established L:D transition. c-Fos exhibited only a low amplitude circadian rhythm in the absence of L:D cycles. Similar results were obtained for another immediate early gene (IEG) protein, Egr-1. These findings show that IEG induction in the rodent retina exhibits circadian clock dependency.

Why Trypanosomes Cause Sleeping Sickness

African trypanosomiasis or sleeping sickness is hallmarked by sleep and wakefulness disturbances. In contrast to other infections, there is no hypersomnia, but the sleep pattern is fragmented. This overview discusses that the causative agents, the parasites Trypanosoma brucei, target circumventricular organs in the brain, causing inflammatory responses in hypothalamic structures that may lead to dysfunctions in the circadian-timing and sleep-regulatory systems.

Circadian clocks and natural antisense RNA

Eukaryotes regulate gene expression in a number of different ways. On a daily and seasonal timescale, the orchestration of gene expression is to a large extent governed by circadian clocks. These endogenous timekeepers enable organisms to prepare for predictable environmental conditions from one day to the next and thus allow adaptation to a given temporal niche. In general, circadian clocks have been shown to employ the classical transcriptional and posttranscriptional control mechanisms to generate rhythmicity. However, the discovery of antisense clock gene transcripts suggests that mechanisms of gene regulation operating through antisense RNA may also be integral to the circadian clockwork. Following a brief history of the impact of genetic and molecular techniques in aiding our understanding of circadian clocks, this review concentrates on the few examples of antisense clock gene transcripts so far investigated and their effect on circadian timing.

Persistent twenty-four hour changes in liver and bone marrow despite suprachiasmatic nuclei ablation in mice

Rest-activity or cortisol rhythms can be altered in cancer patients, a condition that may impair the benefits from a timed delivery of anticancer treatments. In rodents, the circadian pattern in rest-activity is suppressed by the destruction of the suprachiasmatic nuclei (SCN) in the hypothalamus. We sought whether such ablation would result in a similar alteration of cellular rhythms known to be relevant for anticancer drug chronopharmacology. The SCN of 77 B6D2F(1) mice synchronized with 12 h of light and 12 h of darkness were destroyed by electrocoagulation [SCN(-)], while 34 animals were sham operated. Activity and body temperature were recorded by telemetry. Blood and organs were sampled at one of six circadian times for determinations of serum corticosterone concentration, blood leukocyte count, reduced glutathione (GSH), and dihydropyrimidine dehydrogenase (DPD) mRNA expression in liver and cell cycle phase distribution of bone marrow cells. Sham-operated mice displayed significant 24-h rhythms in rest-activity and body temperature, whereas such rhythms were found in none and in 15% of the SCN(-) mice, respectively. SCN lesions markedly altered the rhythmic patterns in serum corticosterone and liver GSH, which became nonsinusoidal. Liver DPD mRNA expression and bone marrow cell cycle phase distribution displayed similar 24-h sinusoidal patterns in sham-operated and SCN(-) mice. These results support the existence of another light-dark entrainable pacemaker that can coordinate cellular functions in peripheral organs. They suggest that the delivery of anticancer treatments at an optimal time of day may still be beneficial, despite suppressed rest-activity or cortisol rhythms.

Transgenic growth hormone mice exposed to lifetime constant illumination: gender-specific effects

Photoperiod affects most of the features altered in transgenic growth hormone (TG) mice, and laboratory rats and mice retain some sensitivity to photoperiod. We examined growth, feeding, longevity, and reproduction of TG mice and normal control mice (Mus musculus L., 1758) in 12 h light : 12 h dark (LD) and 24 h light (LL) photoperiods. Sexual dichotomy in growth and hepatic gene expression are considered to require gender-specific patterns of growth hormone secretion that are absent in TG mice. Regardless, in the LD photoperiod mature TG females were 82.8% (46.8 g) of the mass of TG males (56.5 g, p < 0.05), whereas control mice showed no size dichotomy (≈33 g). Mature masses of TG males and of control mice of either gender were unaffected by the LL photoperiod. TG females, however, reached a mature mass 92% (50.9 g) of that of mature TG males in the LL photoperiod, attenuating the sexual size dichotomy expressed in the LD photoperiod. Growth of females was slower than that of males, even in the control group. TG females in the LL photoperiod expressed faster growth, higher reproduction, and greater mean longevity than TG females in the LD photoperiod. Differences in age-related feeding associated with gender and photoperiod reflected differential growth rates. Females grew more slowly and ate more than males of similar age because they were smaller (i.e., had lower growth efficiencies). The LL photoperiod improved the energy balance of TG females. Possible mechanisms mediating such gender-specific effects are explored.

A circadian rhythm in the expression of PERIOD2 protein reveals a novel SCN-controlled oscillator in the oval nucleus of the bed nucleus of the stria terminalis

Circadian rhythms in mammals are regulated not only globally by the master clock in the suprachiasmatic nucleus (SCN), but also locally by widely distributed populations of clock cells in the brain and periphery that control tissue-specific rhythmic outputs. Here we show that the oval nucleus of the bed nucleus of the stria terminalis (BNST-OV) exhibits a robust circadian rhythm in expression of the Period2 (PER2) clock protein. PER2 expression is rhythmic in the BNST-OV in rats housed under a light/dark cycle or in constant darkness, in blind rats, and in mice, and is in perfect synchrony with the PER2 rhythm of the SCN. Constant light or bilateral SCN lesions abolish the rhythm of PER2 in the BNST-OV. Large abrupt shifts in the light schedule transiently uncouple the BNST-OV rhythm from that of the SCN. Re-entrainment of the PER2 rhythm is faster in the SCN than in the BNST-OV, and it is faster after a delay than an advance shift. Bilateral adrenalectomy blunts the PER2 rhythm in the BNST-OV. Thus, the BNST-OV contains circadian clock cells that normally oscillate in synchrony with the SCN, but these cells appear to require both input from the SCN and circulating glucocorticoids to maintain their circadian oscillation. Taken together with what is known about the functional organization of the connections of the BNST-OV with systems of the brain involved in stress and motivational processes, these findings place BNST-OV oscillators in a position to influence specific physiological and behavioral rhythms downstream from the SCN clock.

Resetting Mechanism of Central and Peripheral Circadian Clocks in Mammals

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

Identification of human Clock gene variants by denaturing high-performance liquid chromatography

The human Clock gene ( hClock) encodes the CLOCK protein essential for the function of the circadian system. We have screened the entire coding region, including the 5' and 3' untranslated regions (UTRs) and the flanking intronic regions, of the hClock gene for sequence variations in 70 unrelated Chinese Singaporeans with denaturing high-performance liquid chromatography (dHPLC). A total of 15 sequence variations were detected, five of which were novel. All involved single base changes. There were 12 substitutions and three insertions/deletions. All except one were found in the introns or the UTRs. Frequencies of the minor allele for all 15 polymorphisms ranged from 0.7% to almost 50%. For the eight sites whose minor allele frequency was found to be at least 10%, pair-wise comparisons revealed that all except one were in almost complete linkage disequilibrium. Our identification of additional single nucleotide polymorphisms in the hClock gene would provide markers whose frequencies could be established in the selected population and used for further analysis of the phenotypic effects. Our results would also be useful for better planning in the selection of polymorphisms for future genetic association studies involving the hClock gene.

PERIOD2::LUCIFERASE real-time reporting of circadian dynamics reveals persistent circadian oscillations in mouse peripheral tissues

Mammalian circadian rhythms are regulated by the suprachiasmatic nucleus (SCN), and current dogma holds that the SCN is required for the expression of circadian rhythms in peripheral tissues. Using a PERIOD2::LUCIFERASE fusion protein as a real-time reporter of circadian dynamics in mice, we report that, contrary to previous work, peripheral tissues are capable of self-sustained circadian oscillations for >20 cycles in isolation. In addition, peripheral organs expressed tissue-specific differences in circadian period and phase. Surprisingly, lesions of the SCN in mPer2(Luciferase) knockin mice did not abolish circadian rhythms in peripheral tissues, but instead caused phase desynchrony among the tissues of individual animals and from animal to animal. These results demonstrate that peripheral tissues express self-sustained, rather than damped, circadian oscillations and suggest the existence of organ-specific synchronizers of circadian rhythms at the cell and tissue level.

Circadian clocks: self-assembling oscillators?

Circadian clocks are auto-regulatory loops in which 'clock' gene products feedback to regulate their own expression. This explains how they keep oscillating, but how do they first get going? It appears that the transcription factor dClock not only drives the oscillation within the fruit fly's clock, but also plays a pivotal role in pre-assembling the clockwork.