Forskolin induces circadian gene expression of rPer1, rPer2 and dbp in mammalian rat-1 fibroblasts

Asthma: Chronopharmacotherapy and the molecular clock

Bronchial asthma is characterized by chronic airways inflammation and reversible airflow limitation. In patients with asthma, symptoms generally worsen during the early hours of the morning, and pulmonary function often deteriorates at the same time, suggesting a role for chronopharmacotherapy. Several drugs for asthma have been developed based on chronopharmacology. Most medications employed for the chronotherapy of asthma are administered once at night with the goal of preventing chronic airway inflammation or development of airflow limitation. In addition to bronchodilators, the inhaled glucocorticosteroid ciclesonide is now available with once-daily dosing, which also improves patients' compliance. Numerous investigations have demonstrated the usefulness of chronotherapy for asthma, especially for patients with nocturnal asthma. This review focuses on chronotherapy of asthma, and also provides a molecular biological explanation for the influence of asthma medications on the clock genes.

Mammalian circadian clock system: Molecular mechanisms for pharmaceutical and medical sciences

An internal circadian (from the Latin "circa" meaning "about" and "dien" meaning "day") clock has been found across kingdoms of life, a testimony that circadian rhythms are a basic feature of life on earth. Physiologically relevant circadian time is generated at the level of transcription-(post)translation feedback loop of clock genes, which machinery can be found in most cells throughout the body. Lesions of the hypothalamic suprachiasmatic nucleus (SCN) abolish clock oscillations in the body, indicating thereby that rhythm generation is a hierarchial system with the SCN at the top. Disrupting this exquisitely harmonious system causes abnormal expression of cell-type specific clock-controlled genes, as revealed by the etiology of life-style related diseases such as hypertension.

Circadian-independent cell mitosis in immortalized fibroblasts

Two prominent timekeeping systems, the cell cycle, which controls cell division, and the circadian system, which controls 24-h rhythms of physiology and behavior, are found in nearly all living organisms. A distinct feature of circadian rhythms is that they are temperature-compensated such that the period of the rhythm remains constant (approximately 24 h) at different ambient temperatures. Even though the speed of cell division, or growth rate, is highly temperature-dependent, the cell-mitosis rhythm is temperature-compensated. Twenty-four-hour fluctuations in cell division have also been observed in numerous species, suggesting that the circadian system is regulating the timing of cell division. We tested whether the cell-cycle rhythm was coupled to the circadian system in immortalized rat-1 fibroblasts by monitoring cell-cycle gene promoter-driven luciferase activity. We found that there was no consistent phase relationship between the circadian and cell cycles, and that the cell-cycle rhythm was not temperature-compensated in rat-1 fibroblasts. These data suggest that the circadian system does not regulate the cell-mitosis rhythm in rat-1 fibroblasts. These findings are inconsistent with numerous studies that suggest that cell mitosis is regulated by the circadian system in mammalian tissues in vivo. To account for this discrepancy, we propose two possibilities: (i) There is no direct coupling between the circadian rhythm and cell cycle but the timing of cell mitosis is synchronized with the rhythmic host environment, or (ii) coupling between the circadian rhythm and cell cycle exists in normal cells but it is disconnected in immortalized cells.

Systems biology of mammalian circadian clocks

Systems biology is a natural extension of molecular biology; it can be defined as biology after identification of key gene(s). Systems-biological research is a multistage process beginning with (a) the comprehensive identification and (b) quantitative analysis of individual system components and their networked interactions, which lead to the ability to (c) control existing systems toward the desired state and (d) design new ones based on an understanding of the underlying structure and dynamical principles. In this review, we use the mammalian circadian clock as a model system and describe the application of systems-biological approaches to fundamental problems in this model. This application has allowed the identification of transcriptional/posttranscriptional circuits, the discovery of a temperature-insensitive period-determining process, and the discovery of desynchronization of individual clock cells underlying the singularity behavior of mammalian clocks.

Development of the circadian oscillator during differentiation of mouse embryonic stem cells in vitro

The molecular oscillations underlying the generation of circadian rhythmicity in mammals develop gradually during ontogenesis. However, the developmental process of mammalian cellular circadian oscillator formation remains unknown. In differentiated somatic cells, the transcriptional-translational feedback loops (TTFL) consisting of clock genes elicit the molecular circadian oscillation. Using a bioluminescence imaging system to monitor clock gene expression, we show here that the circadian bioluminescence rhythm is not detected in the mouse embryonic stem (ES) cells, and that the ES cells likely lack TTFL regulation for clock gene expression. The circadian clock oscillation was induced during the differentiation culture of mouse ES cells without maternal factors. In addition, reprogramming of the differentiated cells by expression of Sox2, Klf4, Oct3/4, and c-Myc genes, which were factors to generate induced pluripotent stem (iPS) cells, resulted in the re-disappearance of circadian oscillation. These results demonstrate that an intrinsic program controls the formation of the circadian oscillator during the differentiation process of ES cells in vitro. The cellular differentiation and reprogramming system using cultured ES cells allows us to observe the circadian clock formation process and may help design new strategies to understand the key mechanisms responsible for the organization of the molecular oscillator in mammals.

Impact of the circadian clock on in vitro genotoxic risk assessment assays

Our society expects safety assessment for drugs, chemicals, cosmetics, and foods, which to date cannot be achieved without the use of laboratory animals. At the same time, society aims at refining, reducing, and (ultimately) replacing animal testing. As a consequence, much effort is taken to establish alternatives, such as toxicogenomics-based risk assessment assays on cultured cells and tissues. Evidently, the properties of cells in vitro will considerably differ from the in vivo situation. This review will discuss the impact of the circadian clock, an internal time keeping system that drives 24-h rhythms in metabolism, physiology and behavior, on in vitro genotoxic risk assessment. Our recent observation that DNA damaging agents can synchronize the circadian clock of individual cells in culture (and as a consequence the cyclic expression of clock-controlled genes, comprising up to 10% of the transcriptome) implies that the circadian clock should not be neglected when developing cell or tissue-based alternatives for chronic rodent toxicity assays.

Pharmacological modulators of the circadian clock as potential therapeutic drugs

Circadian clocks are molecular time-keeping systems that underlie daily fluctuations in multiple physiological and biochemical processes. It is well recognized now that dysfunctions of the circadian system (both genetically and environmentally induced) are associated with the development of various pathological conditions. Here we describe the application of high throughput screening approach designed to search for small molecules capable of pharmacological modulation of the molecular clock. We provide evidence for the feasibility and value of this approach for both scientific and therapeutic purposes.

Induction of circadian rhythm in cultured human mesenchymal stem cells by serum shock and cAMP analogs in vitro

Circadian clocks have been shown to operate developmentally in mouse and human hematopoietic stem and progenitor cells in vivo, but little is known about their possible oscillations in vitro. Here, we show that repeated circadian oscillations could be induced in both cultured bone marrow-derived mesenchymal- and adipose-derived stem cells (MSCs and ASCs, respectively) by serum shock. In particular, the novel finding of rhythmic clock gene expression induced by cAMP analogs showed similarities as well as differences to serum-induced oscillations. Rhythmic PER1 expression was found in serum-shocked MSCs, suggesting the phosphorylation status of PER1 is important for its activity in circadian rhythms. Furthermore, immunofluoresent staining showed that the localization of PER1 was dependent on the level of PER1 expression. These inducible self-sustained circadian clocks in primary cultures of human MSCs in vitro with rhythmic changes in expression levels, phosphorylation, and localization of clock protein, PER1, may be of importance for maintaining the induced oscillations in stem cells. Therefore, the established cell models described here appear to be valuable for studying the molecular mechanism driving and coordinating the circadian network between stem and stromal cells.

Assessment of circadian function in fibroblasts of patients with bipolar disorder

Previous studies have implicated the circadian system in the pathophysiology of bipolar disorder, but conclusive evidence for altered circadian clocks is lacking. Cultured fibroblasts harbor circadian clocks representative of those in the master clock resident in the suprachiasmatic nuclei, providing a new avenue to investigate the core clock machinery in patients with bipolar illness. We examined the rhythmic expression patterns of core clock genes (BMAL1, PER1, PER2, REV-ERBalpha, DEC2, DBP) in fibroblasts from 12 bipolar patients and 12 healthy controls. Although we did not detect differences in the circadian period between bipolar patients and controls, the amplitude of rhythmic expression for BMAL1, REV-ERBalpha and DBP, as well as the overall mRNA expression level for DEC2 and DBP was reduced in fibroblasts from bipolar patients. Bonferroni's correction for multiple comparisons still resulted in significantly reduced DBP expression level, and trends toward reduced overall expression level of DEC2 and circadian amplitude of BMAL1, in fibroblasts from bipolar patients. We next examined an expanded cohort of 18 bipolar patients and 35 healthy controls for mRNA expression levels of four kinases (CKIdelta, CKIepsilon, GSK3alpha and GSK3beta) and the protein and phosphorylation levels of two of them (GSK3alpha and GSK3beta). We did not detect differences in steady-state mRNA levels or protein levels of these kinases between bipolar patients and controls, but the level of GSK3beta phosphorylation was significantly reduced in bipolar patients within an Old Order Amish bipolar kindred. Our results suggest that the reduced amplitudes and overall expression levels of circadian genes, and the decreased phosphorylation level of GSK3beta may lead to dysregulation of downstream genes, which could explain some pathological features of bipolar disorder.

The melatonin receptor MT1 is required for the differential regulatory actions of melatonin on neuronal 'clock' gene expression in striatal neurons in vitro

Through inhibitory G protein-coupled melatonin receptors, melatonin regulates intracellular signaling systems and also the transcriptional activity of certain genes. Clock genes are proposed as regulatory factors in forming dopamine-related behaviors and mood and melatonin has the ability to regulate these processes. Melatonin-mediated changes in clock gene expression have been reported in brain regions, including the striatum, that are crucial for the development of dopaminergic behaviors and mood. However, it is not known whether melatonin receptors present in striatum mediate these effects. Therefore, we investigated the role of the melatonin/melatonin receptor system on clock gene expression using a model of primary neuronal cultures prepared from striatum. We found that melatonin at the receptor affinity range (i.e., nm) affects the expression of the clock genes mPer1, mClock, mBmal1 and mNPAS2 (neuronal PAS domain protein 2) differentially in a pertussis toxin-sensitive manner: a decrease in Per1 and Clock, an increase in NPAS2 and no change in Bmal1 expression. Furthermore, mutating MT1 melatonin receptor (i.e., MT1 knockouts, MT1(-/-)) reversed melatonin-induced changes, indicating the involvement of MT1 receptor in the regulatory action of melatonin on neuronal clock gene expression. Therefore, by controlling clock gene expression we propose melatonin receptors (i.e., MT1) as novel therapeutic targets for the pathobiologies of dopamine-related behaviors and mood.

The resetting of the circadian rhythm by Prostaglandin J2 is distinctly phase-dependent

The circadian rhythm can be reset by a variety of substances. Prostaglandin J(2) (PGJ(2)) is one such substance and resets the circadian rhythm in fibroblasts. In our current study, we examined the phase-dependent phase shift following PGJ(2) treatment using a real-time luciferase luminescence monitoring system. In the phase response curves, we observed 12h differences in the times of peaks in comparison with the same analysis for forskolin. Quantification of clock gene mRNAs following PGJ(2) administration additionally revealed a rapid decrease in the Per1, Rev-erbAalpha and Dbp levels. Our current findings thus suggest that PGJ(2) resets the peripheral circadian clock via a mechanism that is distinct from that used by forskolin (FK).

Dopamine receptor-mediated regulation of neuronal "clock" gene expression

Using a transgenic mice model (i.e. "clock" knockouts), clock transcription factors have been suggested as critical regulators of dopaminergic behaviors induced by drugs of abuse. Moreover, it has been shown that systemic administration of psychostimulants, such as cocaine and methamphetamine regulates the striatal expression of clock genes. However, it is not known whether dopamine receptors mediate these regulatory effects of psychostimulants at the cellular level. Primary striatal neurons in culture express dopamine receptors as well as clock genes and have been successfully used in studying dopamine receptor functioning. Therefore, we investigated the role of dopamine receptors on neuronal clock gene expression in this model using specific receptor agonists. We found an inhibitory effect on the expression of mClock and mPer1 genes with the D2-class (i.e. D2/D3) receptor agonist quinpirole. We also found a generalized stimulatory effect on the expression of clock genes mPer1, mClock, mNPAS2 (neuronal PAS domain protein 2), and mBmal1 with the D1-class (i.e. D1) receptor agonist SKF38393. Further, we tested whether systemic administration of dopamine receptor agonists causes similar changes in striatal clock gene expression in vivo. We found quinpirole-induced alterations in mPER1 protein levels in the mouse striatum (i.e. rhythm shift). Collectively, our results indicate that the dopamine receptor system may mediate psychostimulant-induced changes in clock gene expression. Using striatal neurons in culture as a model, further research is needed to better understand how dopamine signaling modulates the expression dynamics of clock genes (i.e. intracellular signaling pathways) and thereby influences neuronal gene expression, neuronal transmission, and brain functioning.

Systems biology of mammalian circadian clocks

Systems Biology is a natural extension of molecular biology and can be defined as biology after identification of key gene(s). Systems-biological research is hence seen as a multistage process, beginning with the comprehensive identification and quantitative analysis of individual system components and their networked interactions and leading to the ability to control existing systems toward the desired state and design new ones based on an understanding of structure and underlying dynamical principles. In this chapter, we take mammalian circadian clocks as a model system and describe systems-biological approaches, including the identification of clock-controlled genes, clock-controlled cis elements, and clock transcriptional circuits driven by functional genomics; the parameter change of clock components followed by quantitative measurement; and the dynamic and quantitative perturbation of the clock and its application to one of the fundamental but yet-unsolved questions: singularity behavior of clocks. As perspective for systems-biological investigations, we also introduce the system-level dynamical questions related to the core of clocks, including delay, nonlinearity, temperature-compensation and synchronization of mammalian circadian oscillator(s), and the system-level information problems related to clocks in the environment, including the internal representation of light change through perfect adaptation and internal representation of day length through photoperiodism in mammals.

DEC1 modulates the circadian phase of clock gene expression

DEC1 suppresses CLOCK/BMAL1-enhanced promoter activity, but its role in the circadian system of mammals remains unclear. Here we examined the effect of Dec1 overexpression or deficiency on circadian gene expression triggered with 50% serum. Overexpression of Dec1 delayed the phase of clock genes such as Dec1, Dec2, Per1, and Dbp that contain E boxes in their regulatory regions, whereas it had little effect on the circadian phase of Per2 and Cry1 carrying CACGTT E' boxes. In contrast, Dec1 deficiency advanced the phase of the E-box-containing clock genes but not that of the E'-box-containing clock genes. Accordingly, DEC1 showed strong binding and transrepression on the E box, but not on the E' box, in chromatin immunoprecipitation, electrophoretic mobility shift, and luciferase reporter assays. Dec1-/- mice showed behavioral rhythms with slightly but significantly longer circadian periods under conditions of constant darkness and faster reentrainment to a 6-h phase-advanced shift of a light-dark cycle. Knockdown of Dec2 with small interfering RNA advanced the phase of Dec1 and Dbp expression, and double knockdown of Dec1 and Dec2 had much stronger effects on the expression of the E-box-containing clock genes. These findings suggest that DEC1, along with DEC2, plays a role in the finer regulation and robustness of the molecular clock.

[Advances in interactions between glucocorticoid hormones and circadian gene expression]

Circadian rhythm, which is internally generated by cell autonomous biological clocks, has been greatly concerned in recent years. This circadian system in mammals is composed of a master pacemaker in the suprachiasmatic nucleus (SCN) of the hypothalamus and slave clocks in most peripheral cell types. The clock genes and their coding proteins compose the feed-back loops of the circadian system. Light and food are two major Zeitgebers to synchronize circadian clocks. Light can induce clock genes expression and glucocorticoids release in the adrenal gland, while glucocorticoids can slow down the food-induced phase-shifting of peripheral circadian oscillators, suggesting that a close relationships may exist between glucocorticoids and the circadian gene expression. This article briefly reviews the recent progress in the interactions between them and suggests the direction of future researches.

Are circadian rhythms the code of hypothalamic-immune communication? Insights from natural killer cells

Circadian rhythms in physiology and behavior are ultimately regulated at the hypothalamic level by the suprachiasmatic nuclei (SCN). This central oscillator transduces photic information to the cellular clocks in the periphery through the autonomic nervous system and the neuroendocrine system. The fact that these two systems have been shown to modulate leukocyte physiology supports the concept that the circadian component is an important aspect of hypothalamic-immune communication. Circadian disruption has been linked to immune dysregulation, and recent reports suggest that several circadian clock genes, in addition to their time-keeping role, are involved in the immune response. In this overview, we summarize the findings demonstrating that Natural Killer (NK) cell function is under circadian control.

Does a melatonin-dependent circadian oscillator in the pars tuberalis drive prolactin seasonal rhythmicity?

The pars tuberalis (PT) of the adenohypophysis expresses a high density of melatonin receptors and is thought to be a crucial relay for the actions of melatonin on seasonal rhythmicity of prolactin secretion by the pars distalis (PD). In common with the suprachiasmatic nucleus of the hypothalamus and most other peripheral tissues, the PT rhythmically expresses a range of 'clock genes'. Interestingly, this expression is highly dependent upon melatonin/photoperiod, with several aspects unique to the PT. These observations led to the establishment of a conceptual framework for the encoding of seasonal timing in this tissue. This review summarises current knowledge of the morphological, functional and molecular aspects of the PT and considers its role in seasonal timing. The strengths and weaknesses of current hypotheses that link melatonin action in the PT to its seasonal effect on lactotrophs of the PD are discussed and alternative working hypotheses are suggested.

Aldosterone induces circadian gene expression of clock genes in H9c2 cardiomyoblasts

We examined mRNA expression of the clock genes (Per1, Per2, and Bmal1) and PAI-1 (plasminogen activator inhibitor-1) after aldosterone treatment every 4 h up to 48 h in H9c2 cardiomyoblasts by reverse transcription-polymerase chain reaction. To block the MR (mineralocorticoid receptor), the MR antagonist, spironolactone, was added to the medium 1 h before aldosterone treatment. Aldosterone induced an initial increase and rhythmic expression of Per1, while spironolactone attenuated the acute increase in Per1 mRNA induced by aldosterone. On the other hand, aldosterone did not increase the Per2 mRNA in the acute phase, but thereafter induced a rhythmic expression of Per2. Aldosterone also induced rhythmic expression of Bmal1, a positive element of the clock genes. The rhythm of Bmal1 mRNA was anti-phase of that of Per2 mRNA. Aldosterone induced an acute increase in PAI-1 mRNA, but did not induce rhythmic expression of PAI-1. The present study demonstrated first that aldosterone regulates expression of the clock genes Per1, Per2, and Bmal1, and increases PAI-1 expression in H9c2 cardiomyoblasts. Second, an acute increase in Per1 mRNA after aldosterone treatment is mediated through MR. Third, clock genes are not related to PAI-1 expression in H9c2 cardiomyoblasts.

Gonadotropic regulation of circadian clockwork in rat granulosa cells

The circadian clock is responsible for the generation of circadian rhythms in hormonal secretion and metabolism. These peripheral clocks could be reset by various cues in order to adapt to environmental variations. The ovary can be characterized as having highly dynamic physiology regulated by gonadotropins. Here, we aimed to address the status of circadian clock in the ovary, and to explore how gonadotropins could regulate clockwork in granulosa cells (GCs). To this end, we mainly utilized the immunohistochemistry, RT-PCR, and real-time monitoring of gene expression methods. PER1 protein was constantly abundant across the daily cycle in the GCs of immature ovaries. In contrast, PER1 protein level was obviously cyclic through the circadian cycle in the luteal cells of pubertal ovaries. In addition, both FSH and LH induced Per1 expression in cultured immature and mature GCs, respectively. The promoter analysis revealed that the Per1 expression was mediated by the cAMP response element binding protein. In cultured transgenic GCs, both FSH and LH also induced the circadian oscillation of Per2. However, the Per2 oscillation promoted by FSH quickly dampened within only one cycle, whereas the Per2 oscillation promoted by LH was persistently maintained. Collectively, these findings strongly suggest that both FSH and LH play an important role in regulating circadian clock in the ovary; however, they might exert differential actions on the clockwork in vivo due to each specific role within ovarian physiology.

Circadian rhythms in the CNS and peripheral clock disorders: function of clock genes: influence of medication for bronchial asthma on circadian gene

Bronchial asthma is a chronic inflammatory disorder of the airways, in which inflammation causes bronchial hyper-responsiveness and flow limitation in the presence of various stimuli. Pulmonary function in asthmatic patients frequently deteriorates between midnight and early morning, which has suggested a role for chronotherapy. Although relationships between bronchial asthma and the function of clock genes remain unclear, some medications given for asthma such as glucocorticoids or beta(2)-adrenoceptor agonists may influence clock genes in vivo. In our studies of clock gene mRNA expressions in human bronchial epithelial cells in vitro and peripheral blood cells in vivo, we demonstrated that glucocorticoid or beta(2)-adrenoceptor agonist treatment strongly induced human Per1 mRNA expression both in vitro and in vivo. Human peripheral blood cells provide a useful indication of peripheral clock gene mRNA expression in vivo.

Regulation of circadian gene expression in liver by systemic signals and hepatocyte oscillators

The mammalian circadian timing system has a hierarchical structure, in that a master pacemaker located in the suprachiasmatic nuclei (SCN) coordinates slave oscillators present in virtually all body cells. In both the SCN and peripheral organs, the rhythm-generating oscillators are self-sustained and cell-autonomous, and it is likely that the molecular makeup of master and slave oscillators is nearly identical. However, due to variations in period length, the phase coherence between peripheral oscillators in intact animals must be established by daily signals emanating directly or indirectly from the SCN master clock. The synchronization of individual cellular clocks in peripheral organs is probably accomplished by immediate-early genes that interpret the cyclic systemic signals and convey this phase information to core clock components. This model predicts that circadian gene expression in peripheral organs can be influenced either by systemic signals emanating from the SCN master clock, local oscillators, or both. We developed a transgenic mouse strain in which hepatocyte clocks are only operative when the tetracycline analog doxycycline is added to the food or drinking water. The genome-wide mapping of genes whose cyclic expression in liver does not depend on functional hepatocyte oscillators unveiled putative signaling pathways that may participate in the phase entrainment of peripheral clocks.

Quantitative analyses of circadian gene expression in mammalian cell cultures

The central circadian pacemaker is located in the hypothalamus of mammals, but essentially the same oscillating system operates in peripheral tissues and even in immortalized cell lines. Using luciferase reporters that allow automated monitoring of circadian gene expression in mammalian fibroblasts, we report the collection and analysis of precise rhythmic data from these cells. We use these methods to analyze signaling pathways of peripheral tissues by studying the responses of Rat-1 fibroblasts to ten different compounds. To quantify these rhythms, which show significant variation and large non-stationarities (damping and baseline drifting), we developed a new fast Fourier transform–nonlinear least squares analysis procedure that specifically optimizes the quantification of amplitude for circadian rhythm data. This enhanced analysis method successfully distinguishes among the ten signaling compounds for their rhythm-inducing properties. We pursued detailed analyses of the responses to two of these compounds that induced the highest amplitude rhythms in fibroblasts, forskolin (an activator of adenylyl cyclase), and dexamethasone (an agonist of glucocorticoid receptors). Our quantitative analyses clearly indicate that the synchronization mechanisms by the cAMP and glucocorticoid pathways are different, implying that actions of different genes stimulated by these pathways lead to distinctive programs of circadian synchronization.

The circadian biological clock controls the adaptation of animals and plants to the daily environmental cycle of light and darkness. As such, this clock is responsible for jet lag and has consequences for mental health (e.g., depression), physical health (e.g., athletic performance and the timing of heart attacks), and social issues (e.g., shift work). The central circadian pacemaker is located in the hypothalamus of the mammalian brain, but essentially the same oscillating system operates in nonneural tissues. Using luciferase, an enzyme that emits light, the authors could monitor circadian gene expression in mammalian fibroblasts via luminescence emission that is controlled by the biological clock. Using this method, they report the collection and analysis of precise rhythmic data from these cells. These methods were used to analyze signaling pathways by studying the responses of fibroblasts to a variety of different treatments, including drugs, growth factors, and serum. The authors developed a new analysis procedure that specifically optimizes the quantification of amplitude for cyclic data to analyze these rhythms. This enhanced analysis method successfully distinguishes among the various signaling treatments for their rhythm inducing properties. The quantitative analyses clearly indicate that the synchronization mechanisms by the cyclic AMP and glucocorticoid pathways are different. Therefore, these pathways lead to distinctive programs of circadian synchronization.

Bidirectional CLOCK/BMAL1-dependent circadian gene regulation by retinoic acid in vitro

A central circadian clock located in the suprachiasmatic nucleus (SCN) of the mammalian hypothalamus entrains peripheral clocks through both neural and humoral factors. Although candidates for entrainment factors have been described, their details remain obscure. Here, we screened ligands for nuclear receptors that affect CLOCK/BMAL1-dependent transactivation of the mouse Period1 (mPer1) gene in NIH3T3 cells. We found that retinoic acids (RAs) significantly up-regulate mPer1 expression in an E-box-dependent manner. We also found that RAs up-regulate the expression of other E-box-dependent circadian genes such as mPer2, arginine vasopressin (mAVP), and peroxisome proliferator-activated receptor alpha (mPPARalpha). Surprisingly, the effect of RAs on CLOCK/BMAL1 (E-box)-dependent mRNA expression was bidirectional and depended on the presence of exogenous retinoic acid receptor alpha (RARalpha). These results suggest that RAs regulate the CLOCK/BMAL1-dependent transcription of circadian genes in a complex manner.

Circadian clocks regulate adenylyl cyclase activity rhythms in human RPE cells

Genes and components of the circadian clock may represent relevant drug targets for diseases involving circadian dysfunctions. By exploiting an established cell line derived from human retinal pigment epithelium (HRPE), the cell constituting the blood-retinal barrier that is essential to maintain the visual functions of the sensorineural retina, we showed serum-shock induction of rhythmic changes in forskolin-evoked adenylyl cyclase (AC) activity. In the presence of Ca2+ and protein kinase A, the forskolin-induced AC activity is significantly, but not completely inhibited, suggesting the involvement of both Ca2+-sensitive and Ca2+-insensitive AC isoforms in the regulation of circadian rhythmicity in these cells. Semi-quantitative RT-PCR showed circadian profile in the expression of three AC isoforms, the Ca2+-inhibitable AC5 and AC6 and the Ca2+-insensitive AC7, and the clock genes hPer1 and hPer2. Our results demonstrate for the first time circadian rhythmicity in a human cell line, identifying the isoforms involved in the circadian profile of AC activity and showing a rhythmicity of the clock gene mRNA expression in these cells. Therefore, the results reported here provide evidence for an intertwine between AC/[Ca2+]i signalling pathways and Per genes in the HRPE circadian clockwork.

Gq/11-induced intracellular calcium mobilization mediates Per2 acute induction in Rat-1 fibroblasts

Phase resetting is one of the essential properties of circadian clocks that is required for the adjustment to a particular environment and the induction of Per1 and Per2 clock genes is believed to be a primary molecular event during this process. Although the intracellular signal transduction pathway underlying Per1 gene activation has been well characterized, the mechanisms that control Per2 up-regulation have not yet been elucidated. In our present study, we demonstrate that Gq/11 coupled receptors mediate serum-induced immediate rat Per2 (rPer2) transactivation in Rat-1 fibroblasts via intracellular Ca2+ mobilization. Stimulation of these cells with a high concentration of serum was found to rapidly increase the intracellular Ca2+ levels and strongly up-regulated rPer2 gene. rPer2 induction by serum stimulation was abrogated by intracellular Ca2+ chelation and depletion of intracellular Ca2+ store, which suggests that the calcium mobilization is necessary for the up-regulation of rPer2 gene. In addition, suppression of Gq/11 function was observed to inhibit both Ca2+ mobilization and rPer2 induction. Further, we demonstrated that endothelin-induced acute rPer2 transactivation via Gq/11-coupled endothelin receptors is also suppressed by a Gq/11 specific inhibitor. These findings together suggest that serum and endothelin utilize a common Gq/11-PLC mediated pathway for the transactivation of rPer2, which involves the mobilization of calcium from the intracellular calcium store.

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

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

Modeling the VPAC2-activated cAMP/PKA signaling pathway: from receptor to circadian clock gene induction

Increasing evidence suggests an important role for VPAC2-activated signal transduction pathways in maintaining a synchronized biological clock in the suprachiasmatic nucleus (SCN). Activation of the VPAC2 signaling pathway induces per1 gene expression in the SCN and phase-shifts the circadian clock. Mice without the VPAC2 receptor lack an overt, coherent circadian rhythm in clock gene expression, SCN neuron firing rate, and locomotor behavior. Using a systems approach, we have developed a kinetic model integrating VPAC2 signaling mediated by the cyclic AMP (cAMP)/protein kinase A (PKA) pathway and leading to induced circadian clock gene expression. We fit the model to experimental data from the literature for cAMP accumulation, PKA activation, cAMP-response element binding protein phosphorylation, and per1 induction. By linking the VPAC2 model to a published circadian clock model, we also simulated clock phase shifts induced by vasoactive intestinal polypeptide (VIP) and matched experimental data for the VIP response. The simulated phase response curve resembled the hamster response to a related neuropeptide, GRP1-27, and light. Simulations using pulses of VIP revealed that the system response is extraordinarily robust to input signal duration, a result with physiologically relevant consequences. Lastly, simulations using varied receptor levels matched literature experimental data from animals overexpressing VPAC2 receptors.

Clock genes of mammalian cells: Practical implications in tissue culture

The clock genes family is expressed by all the somatic cells driving central and peripheral circadian rhythms through transcription/translation feedback loops. The circadian clock provides a local time for a cell and a way to integrate the normal environmental changes to smoothly adapt the cellular machinery to new conditions. The central circadian rhythm is retained in primary cultures by neurons of the suprachiasmatic nuclei. The peripheral circadian rhythms of the other somatic cells are progressively dampened down up to loss unless neuronal signals of the central clock are provided for re-entrainment. Under typical culture conditions (obscurity, 37 +/- 1 degrees C, 5-7% CO(2)), freshly explanted peripheral cells harbor chaotic expression of clock genes for 12-14 h and loose, coordinated oscillating patterns of clock components. Cells of normal or cancerous phenotypes established in culture harbor low levels of clock genes idling up to the re-occurrence of new synchronizer signals. Synchronizers are physicochemical cues (like thermic oscillations, short-term exposure to high concentrations of serum or single medium exchange) able to re-induce molecular oscillations of clock genes. The environmental synchronizers are integrated by response elements located in the promoter region of period genes that drive the central oscillator complex (CLOCK:BMAL1 and NPAS2:BMAL1 heterodimers). Only a few cell lines from different species and lineages have been tested for the existence or the functioning of a circadian clockwork. The best characterized cell lines are the immortalized SCN2.2 neurons of rat suprachiasmatic nuclei for the central clock and the Rat-1 fibroblasts or the NIH/3T3 cells for peripheral clocks. Isolation methods of fragile cell phenotypes may benefit from research on the biological clocks to design improved tissue culture media and new bioassays to diagnose pernicious consequences for health of circadian rhythm alterations.

Beta2-adrenoceptor agonists induce the mammalian clock gene, hPer1, mRNA in cultured human bronchial epithelium cells in vitro

The mammalian Per1 gene is one of the most important components of circadian clock function of the suprachiasmatic nucleus and peripheral tissues. We examined whether the beta2-adrenoceptor agonists, procaterol and fenoterol, induce human Per1 mRNA expression in human bronchial epithelium. The in vitro stimulation of beta2-adrenoceptor agonists in BEAS-2B cells led to a remarkable increase in the level of hPer1 mRNA. Moreover, fenoterol or procaterol induced the phosphorylation of CREB in BEAS-2B cells as verified by immunoblot analysis. beta2-adrenoceptor agonists induced human Per1 mRNA expression by the signaling pathways of cAMP-CREB in BEAS-2B cells.

mPER1-mediated nuclear export of mCRY1/2 is an important element in establishing circadian rhythm

Receptor-mediated nucleocytoplasmic transport of clock proteins is an important, conserved element of the core mechanism for circadian rhythmicity. A systematic analysis of the nuclear export characteristics for the different murine period (mPER) and cryptochrome (mCRY) proteins using Xenopus oocytes as an experimental system demonstrates that all three mPER proteins, but neither mCRY1 nor mCRY2, are exported if injected individually. However, nuclear injection of heterodimeric complexes that contain combinations of mPER and mCRY proteins shows that mPER1 serves as an export adaptor for mCRY1 and mCRY2. Functional analysis of dominant-negative mPER1 variants designed either to sequester mPER3 to the cytoplasm or to inhibit nuclear export of mCRY1/2 in synchronized, stably transfected fibroblasts suggests that mPER1-mediated export of mCRY1/2 defines an important new element of the core clock machinery in vertebrates.

Circadian clock genes as modulators of sensitivity to genotoxic stress

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

Circadian Oscillations of Clock Genes, Cytolytic Factors, and Cytokines in Rat NK Cells

A growing body of knowledge is revealing the critical role of circadian physiology in the development of specific pathological entities such as cancer. NK cell function participates in the immune response against infection and malignancy. We have reported previously the existence of a physiological circadian rhythm of NK cell cytolytic activity in rats, suggesting the existence of circadian mechanisms subjacent to NK cell function. At the cellular level, circadian rhythms are originated by the sustained transcriptional-translational oscillation of clock genes that form the cellular clock apparatus. Our aim in this study was to investigate the presence of molecular clock mechanisms in NK cells as well as the circadian expression of critical factors involved in NK cell function. For that purpose, we measured the circadian changes in the expression of clock genes (Per1, Per2, Bmal1, Clock), Dbp (a clock-controlled output gene), CREB (involved in clock signaling), cytolytic factors (granzyme B and perforin), and cytokines (IFN-gamma and TNF-alpha) in NK cells enriched from the rat spleen. The results obtained from this study demonstrate for the first time the existence of functional molecular clock mechanisms in NK cells. Moreover, the circadian expression of cytolytic factors and cytokines in NK cells reported in this study emphasizes the circadian nature of NK cell function.

Circadian clocks: neural and peripheral pacemakers that impact upon the cell division cycle

Circadian clocks are pervasive entities that allow organisms to maintain rhythms of approximately 24h, independently of external cues, thereby adapting them to the solar cycle. Recent studies have shown that molecular circadian clocks are important for the proper orchestration of the cell division cycle. For the first time, this provides a framework to understand the interactions between these two evolutionarily linked timers. Here we review the current model of the circadian clock and the molecular methods that can be used to investigate its function. We then map out links to the cell cycle at the cellular level. Furthermore, we review recent progress that has linked dysfunction of the clockwork with the pathogenesis of cancer. Disruption of circadian timing (as occurs in jet-lag, shift work and dementia) thus has far reaching consequences for normal regulation of cell division. The implications of this for the health of a "24-h society" are apparent.

Altered food-anticipatory activity rhythm in Cryptochrome-deficient mice

In nocturnal rodents, restricted feeding to daytime (RF) causes feeding-associated diurnal locomotor activity that persists for the next 1-2 days when food is withheld. Along with this anticipatory behavior, the expression pattern of clock genes such as mPer1 and mPer2 changes from a nocturnal to diurnal pattern in the liver and cerebral cortex but not in the suprachiasmatic nucleus (SCN). Whether the molecular clockwork, in which mCry1 and mCry2 genes are essential components, is involved in food-anticipatory circadian rhythms is unknown. In this study, we investigated the impact of the absence of mCRY products upon the locomotion pattern induced by RF. RF caused an increase in daytime activity that lasted even for 2 days after food was withheld, in wild-type and mCry1-/-mCry2-/- mice. However, RF-induced activity was less stable and appeared more gradually in mutant mice. Similar results were obtained with mice housed under constant darkness or with SCN-lesioned wild-type and mutant mice. Our data reveal that mCry proteins are basically dispensable for food-entrainable oscillation. However, it is also important to note that mCry deficiency affects the stability and development of RF-induced anticipatory locomotor activity.

Effect of Haloperidol on mPer1 Gene Expression in Mouse Suprachiasmatic Nuclei

The effect of a typical neuroleptic haloperidol (Hal) on mPer1 gene expression was investigated in mouse suprachiasmatic nuclei (SCN). Hal induced mPer1 mRNA levels both in vivo and in cultured SCN cells. For mechanisms underlying Hal-induced mPer1 expression, N-methyl-d-aspartate (NMDA) glutamate receptor subtype, the phosphorylation form of the transcription factor, and the Ser-133 phosphorylation form of cAMP-responsive element-binding protein (CREB) played an important role, because the induction of mPer1 mRNA significantly decreased after pretreatment with a non-competitive NMDA receptor antagonist, such as MK-801 or CREB antisense. These results suggest that Hal may increase CREB phosphorylation and mPer1 expression according to the activation of the NMDA receptor through the dopaminergic pathways. Although the injection of Hal during the light period increased the amplitude of mPer1 mRNA rhythmicity in a nondrug state, the injection of the drug during the dark period disturbed the rhythmic pattern of mPer1 mRNA. These results suggest that the rhythmicity of clock genes in SCN may be disturbed depending on the dosing time of Hal. On the other hand, because the induction of mPer1 mRNA by Hal seems to be at least partly caused by the NMDA receptor, showing a phase shift or resetting effect of the circadian clock, Hal may also cause such phase shift effects.

Circadian gene expression in individual fibroblasts: cell-autonomous and self-sustained oscillators pass time to daughter cells

The mammalian circadian timing system is composed of a central pacemaker in the suprachiasmatic nucleus (SCN) of the brain and subsidiary oscillators in most peripheral cell types. While oscillators in SCN neurons are known to function in a self-sustained fashion, peripheral oscillators have been thought to damp rapidly when disconnected from the control exerted by the SCN. Using two reporter systems, we monitored circadian gene expression in NIH3T3 mouse fibroblasts in real time and in individual cells. In conjunction with mathematical modeling and cell co-culture experiments, these data demonstrated that in vitro cultured fibroblasts harbor self-sustained and cell-autonomous circadian clocks similar to those operative in SCN neurons. Circadian gene expression in fibroblasts continues during cell division, and our experiments unveiled unexpected interactions between the circadian clock and the cell division clock. Specifically, the circadian oscillator gates cytokinesis to defined time windows, and mitosis elicits phase shifts in circadian cycles.

Circadian Gene Expression in Cultured Cells

In mammals, circadian oscillators not only exist in specialized neurons of the suprachiasmatic nucleus, but in almost all peripheral cell types. These oscillators are operative even in established fibroblast cell lines, such as Rat-1 cells or NIH3T3 cells, and in primary fibroblasts from mouse embryos or adult animals. This can be demonstrated by treating such cells for a short time period with high concentrations of serum or chemicals that activate a large number of known signaling pathways. The possibility of studying circadian rhythms in cultured cells should facilitate the biochemical and genetic dissection of the circadian clockwork and should promote the discovery of new clock components.

The role of circadian rhythmicity in reproduction

Circadian rhythmicity is evident in a wide range of physiological systems including the reproductive axis. The recent discoveries of rhythmic clock gene expression in peripheral tissues, including reproductive tissue, suggests that they may play an important role in optimizing fertility. The evidence for rhythmic control of reproduction from studies in laboratory animals is reviewed and where possible this includes evidence from human studies. Clock genes are highly conserved across species including humans and there is no reason to suggest that they are functionless in humans. The challenge issued here is for researchers to probe their function and the consequences of their disruption in both animal and human reproduction.

The mammalian circadian timing system: from gene expression to physiology

Many physiological processes in organisms from bacteria to man are rhythmic, and some of these are controlled by self-sustained oscillators that persist in the absence of external time cues. Circadian clocks are perhaps the best characterized biological oscillators and they exist in virtually all light-sensitive organisms. In mammals, they influence nearly all aspects of physiology and behavior, including sleep-wake cycles, cardiovascular activity, endocrinology, body temperature, renal activity, physiology of the gastro-intestinal tract, and hepatic metabolism. The master pacemaker is located in the suprachiasmatic nuclei, two small groups of neurons in the ventral part of the hypothalamus. However, most peripheral body cells contain self-sustained circadian oscillators with a molecular makeup similar to that of SCN (suprachiasmatic nucleus) neurons. This organization implies that the SCN must synchronize countless subsidiary oscillators in peripheral tissues, in order to coordinate cyclic physiology. In this review, we will discuss some recent studies on the structure and putative functions of the mammalian circadian timing system, but we will also point out some apparent inconsistencies in the currently publicized model for rhythm generation.

Toward construction of a self-sustained clock-like expression system based on the mammalian circadian clock

Despite recent advances in circadian biology, detailed understanding of how a biological pacemaker system is assembled, maintained, and regulated continues to be a significant challenge. We have assembled and characterized a first-generation, regulatable, self-sustained clock-like expression system based on key components of the mammalian circadian clock. The molecular setup of the clock-like oscillator was reduced to the core set of positive and negative elements common to all known circadian pacemakers. Sophisticated tetracycline-responsive multi-cistronic expression integrated with forefront lentiviral transduction tools enabled autoregulated reporter transgene expression in a human cell line. We characterized transgene expression kinetics of an artificial oscillator and showed that its expression profiles could be modulated by a serum shock and administration of regulating tetracycline antibiotics. Design of a generic mammalian clock-like expression system will offer novel opportunities to study circadian biology and may provide a unique tool for rhythmic expression of desired transgenes fostering advances in biopharmaceutical manufacturing, gene therapy, and tissue engineering.

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.

Genetic clock of biologic rhythms

Biologic rhythms are tightly regulated in all species, in order to adapt to circadian variations of light, temperature and other biological related living conditions. The intrinsic mechanisms of rhythm regulation from every cell to a whole organism is embedded into the genetic clockwork. The mammalian clock genes are reviewed with their interrelation. The per gene promoter is stimulated by clock/bmal1 heterodimer and trigger the per proteins synthesis. Initially considered as a simple negative feed-back, from to the phosphorylated per/cry heterodimer towards clock/bmal1 transcription stimulus, the clockweb of the cell became complex. General characteristics of cell clock genes are integrated in a model. In addition, the involvement of clock genes in peripheral tissues is discussed.

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.