cAMP-dependent signaling as a core component of the mammalian circadian pacemaker

Socially synchronized circadian oscillators

Daily rhythms of physiology and behaviour are governed by an endogenous timekeeping mechanism (a circadian 'clock'). The alternation of environmental light and darkness synchronizes (entrains) these rhythms to the natural day-night cycle, and underlying mechanisms have been investigated using singly housed animals in the laboratory. But, most species ordinarily would not live out their lives in such seclusion; in their natural habitats, they interact with other individuals, and some live in colonies with highly developed social structures requiring temporal synchronization. Social cues may thus be critical to the adaptive function of the circadian system, but elucidating their role and the responsible mechanisms has proven elusive. Here, we highlight three model systems that are now being applied to understanding the biology of socially synchronized circadian oscillators: the fruitfly, with its powerful array of molecular genetic tools; the honeybee, with its complex natural society and clear division of labour; and, at a different level of biological organization, the rodent suprachiasmatic nucleus, site of the brain's circadian clock, with its network of mutually coupled single-cell oscillators. Analyses at the 'group' level of circadian organization will likely generate a more complex, but ultimately more comprehensive, view of clocks and rhythms and their contribution to fitness in nature.

Clock and light regulation of the CREB coactivator CRTC1 in the suprachiasmatic circadian clock

The CREB/CRE transcriptional pathway has been implicated in circadian clock timing and light-evoked clock resetting. To date, much of the work on CREB in circadian physiology has focused on how changes in the phosphorylation state of CREB regulate the timing processes. However, beyond changes in phosphorylation, CREB-dependent transcription can also be regulated by the CREB coactivator CRTC (CREB-regulated transcription coactivator), also known as TORC (transducer of regulated CREB). Here we profiled both the rhythmic and light-evoked regulation of CRTC1 and CRTC2 in the murine suprachiasmatic nucleus (SCN), the locus of the master mammalian clock. Immunohistochemical analysis revealed rhythmic expression of CRTC1 in the SCN. CRTC1 expression was detected throughout the dorsoventral extent of the SCN in the middle of the subjective day, with limited expression during early night, and late night expression levels intermediate between mid-day and early night levels. In contrast to CRTC1, robust expression of CRTC2 was detected during both the subjective day and night. During early and late subjective night, a brief light pulse induced strong nuclear accumulation of CRTC1 in the SCN. In contrast with CRTC1, photic stimulation did not affect the subcellular localization of CRTC2 in the SCN. Additionally, reporter gene profiling and chromatin immunoprecipitation analysis indicated that CRTC1 was associated with CREB in the 5' regulatory region of the period1 gene, and that overexpression of CRTC1 leads to a marked upregulation in period1 transcription. Together, these data raise the prospect that CRTC1 plays a role in fundamental aspects of SCN clock timing and entrainment.

CRY2 genetic variants associate with dysthymia

People with mood disorders often have disruptions in their circadian rhythms. Recent molecular genetics has linked circadian clock genes to mood disorders. Our objective was to study two core circadian clock genes, CRY1 and CRY2 as well as TTC1 that interacts with CRY2, in relation to depressive and anxiety disorders. Of these three genes, 48 single-nucleotide polymorphisms (SNPs) whose selection was based on the linkage disequilibrium and potential functionality were genotyped in 5910 individuals from a nationwide population-based sample. The diagnoses of major depressive disorder, dysthymia and anxiety disorders were assessed with a structured interview (M-CIDI). In addition, the participants filled in self-report questionnaires on depressive and anxiety symptoms. Logistic and linear regression models were used to analyze the associations of the SNPs with the phenotypes. Four CRY2 genetic variants (rs10838524, rs7121611, rs7945565, rs1401419) associated significantly with dysthymia (false discovery rate q<0.05). This finding together with earlier CRY2 associations with winter depression and with bipolar type 1 disorder supports the view that CRY2 gene has a role in mood disorders.

Pharmacological modulators of the circadian clock as potential therapeutic drugs: focus on genotoxic/anticancer therapy

The circadian clock is an evolutionary conserved intrinsic timekeeping mechanism that controls daily variations in multiple biological processes. One important process that is modulated by the circadian clock is an organism's response to genotoxic stress, such as that induced by anticancer drug and radiation treatments. Numerous observations made in animal models have convincingly demonstrated that drug-induced toxicity displays prominent daily variations; therefore, undesirable side effects could be significantly reduced by administration of drugs at specific times when they are better tolerated. In some cases, these critical times of the day coincide with increased sensitivity of tumor cells allowing for a greater therapeutic index. Despite encouraging results of chronomodulated therapies, our knowledge of molecular mechanisms underlying these observations remains sketchy. Here we review recent progress in deciphering mechanistic links between circadian and stress response pathways with a focus on how these findings could be applied to anticancer clinical practice. We discuss the potential for using high-throughput screens to identify small molecules that can modulate basic parameters of the entire circadian machinery as well as functional activity of its individual components. We also describe the discovery of several small molecules that can pharmacologically modulate clock and that have a potential to be developed into therapeutic drugs. We believe that translational applications of clock-targeting pharmaceuticals are twofold: they may be developed into drugs to treat circadian-related disorders or used in combination with existing therapeutic strategies to improve therapeutic index of a given genotoxic treatment via the intrinsic clock mechanism.

Regional circadian period difference in the suprachiasmatic nucleus of the mammalian circadian center

The suprachiasmatic nucleus (SCN) is the mammalian circadian rhythm center. Individual oscillating neurons have different endogenous circadian periods, but they are usually synchronized by an intercellular coupling mechanism. The differences in the period of each oscillating neuron have been extensively studied; however, the clustering of oscillators with similar periods has not been reported. In the present study, we artificially disrupted the intercellular coupling among oscillating neurons in the SCN and observed regional differences in the periods of the oscillating small-latticed regions of the SCN using a transgenic rat carrying a luciferase reporter gene driven by regulatory elements from a per2 clock gene (Per2::dluc rat). The analysis divided the SCN into two regions--aregion with periods shorter than 24 h (short-period region, SPR) and another with periods longer than 24 h (long-period region, LPR). The SPR was located in the smaller medial region of the dorsal SCN, whereas the LPR occupied the remaining larger region. We also found that slices containing the medial region of the SCN generated shorter circadian periods than slices that contained the lateral region of the SCN. Interestingly, the SPR corresponded well with the region where the SCN phase wave is generated. We numerically simulated the relationship between the SPR and a large LPR. A mathematical model of the SCN based on our findings faithfully reproduced the kinetics of the oscillators in the SCN in synchronized conditions, assuming the existence of clustered short-period oscillators.

Role of miR-142-3p in the post-transcriptional regulation of the clock gene Bmal1 in the mouse SCN

MicroRNAs (miRNAs) are small non-coding RNAs that function as post-transcriptional modulators by regulating stability or translation of target mRNAs. Recent studies have implicated miRNAs in the regulation of mammalian circadian rhythms. To explore the role of miRNAs in the post-transcriptional modulation of core clock genes in the master circadian pacemaker, we examined miR-142-3p for evidence of circadian expression in the suprachiasmatic nuclei (SCN), regulation of its putative clock gene target Bmal1 via specific binding sites in the 3' UTR and overexpression-induced changes in the circadian rhythm of BMAL1 protein levels in SCN cells. In mice exposed to constant darkness (DD), miR-142-3p levels in the SCN were characterized by circadian rhythmicity with peak expression during early subjective day at CT 3. Mutagenesis studies indicate that two independent miRNA recognition elements located at nucleotides 1-7 and 335-357 contribute equally to miR-142-3p-induced repression of luciferase-reported Bmal1 3' UTR activity. Importantly, overexpression of miR-142-3p in immortalized SCN cells abolished circadian variation in endogenous BMAL1 protein levels in vitro. Collectively, our results suggest that miR-142-3p may play a role in the post-transcriptional modulation of Bmal1 and its oscillatory regulation in molecular feedback loops mediating SCN circadian function.

Vasoactive intestinal peptide produces long-lasting changes in neural activity in the suprachiasmatic nucleus

The neuropeptide vasoactive intestinal peptide (VIP) is expressed at high levels in the neurons of the suprachiasmatic nucleus (SCN). While VIP is known to be important to the input and output pathways from the SCN, the physiological effects of VIP on electrical activity of SCN neurons are not well known. Here the impact of VIP on firing rate of SCN neurons was investigated in mouse slice cultures recorded during the night. The application of VIP produced an increase in electrical activity in SCN slices that lasted several hours after treatment. This is a novel mechanism by which this peptide can produce long-term changes in central nervous system physiology. The increase in action potential frequency was blocked by a VIP receptor antagonist and lost in a VIP receptor knockout mouse. In addition, inhibitors of both the Epac family of cAMP binding proteins and cAMP-dependent protein kinase (PKA) blocked the induction by VIP. The persistent increase in spike rate following VIP application was not seen in SCN neurons from mice deficient in Kv3 channel proteins and was dependent on the clock protein PER1. These findings suggest that VIP regulates the long-term firing rate of SCN neurons through a VIPR2-mediated increase in the cAMP pathway and implicate the fast delayed rectifier (FDR) potassium currents as one of the targets of this regulation.

Diurnal variation in vascular and metabolic function in diet-induced obesity: divergence of insulin resistance and loss of clock rhythm

Circadian rhythms are integral to the normal functioning of numerous physiological processes. Evidence from human and mouse studies suggests that loss of rhythm occurs in obesity and cardiovascular disease and may be a neglected contributor to pathophysiology. Obesity has been shown to impair the circadian clock mechanism in liver and adipose tissue but its effect on cardiovascular tissues is unknown. We investigated the effect of diet-induced obesity in C57BL6J mice upon rhythmic transcription of clock genes and diurnal variation in vascular and metabolic systems. In obesity, clock gene function and physiological rhythms were preserved in the vasculature but clock gene transcription in metabolic tissues and rhythms of glucose tolerance and insulin sensitivity were blunted. The most pronounced attenuation of clock rhythm occurred in adipose tissue, where there was also impairment of clock-controlled master metabolic genes and both AMPK mRNA and protein. Across tissues, clock gene disruption was associated with local inflammation but diverged from impairment of insulin signaling. We conclude that vascular tissues are less sensitive to pathological disruption of diurnal rhythms during obesity than metabolic tissues and suggest that cellular disruption of clock gene rhythmicity may occur by mechanisms shared with inflammation but distinct from those leading to insulin resistance.

Analysis of core circadian feedback loop in suprachiasmatic nucleus of mCry1-luc transgenic reporter mouse

The suprachiasmatic nucleus (SCN) coordinates circadian rhythms that adapt the individual to solar time. SCN pacemaking revolves around feedback loops in which expression of Period (Per) and Cryptochrome (Cry) genes is periodically suppressed by their protein products. Specifically, PER/CRY complexes act at E-box sequences in Per and Cry to inhibit their transactivation by CLOCK/BMAL1 heterodimers. To function effectively, these closed intracellular loops need to be synchronized between SCN cells and to the light/dark cycle. For Per expression, this is mediated by neuropeptidergic and glutamatergic extracellular cues acting via cAMP/calcium-responsive elements (CREs) in Per genes. Cry genes, however, carry no CREs, and how CRY-dependent SCN pacemaking is synchronized remains unclear. Furthermore, whereas reporter lines are available to explore Per circadian expression in real time, no Cry equivalent exists. We therefore created a mouse, B6.Cg-Tg(Cry1-luc)01Ld, carrying a transgene (mCry1-luc) consisting of mCry1 elements containing an E-box and E'-box driving firefly luciferase. mCry1-luc organotypic SCN slices exhibited stable circadian bioluminescence rhythms with appropriate phase, period, profile, and spatial organization. In SCN lacking vasoactive intestinal peptide or its receptor, mCry1 expression was damped and desynchronized between cells. Despite the absence of CREs, mCry1-luc expression was nevertheless (indirectly) sensitive to manipulation of cAMP-dependent signaling. In mPer1/2-null SCN, mCry1-luc bioluminescence was arrhythmic and no longer suppressed by elevation of cAMP. Finally, an SCN graft procedure showed that PER-independent as well as PER-dependent mechanisms could sustain circadian expression of mCry1. The mCry1-luc mouse therefore reports circadian mCry1 expression and its interactions with vasoactive intestinal peptide, cAMP, and PER at the heart of the SCN pacemaker.

A Gq-Ca2+ axis controls circuit-level encoding of circadian time in the suprachiasmatic nucleus

The role of intracellular transcriptional/post-translational feedback loops (TTFL) within the circadian pacemaker of the suprachiasmatic nucleus (SCN) is well established. In contrast, contributions from G-coupled pathways and cytosolic rhythms to the intercellular control of SCN pacemaking are poorly understood. We therefore combined viral transduction of SCN slices with fluorescence/bioluminescence imaging to visualize GCaMP3-reported circadian oscillations of intracellular calcium [Ca²⁺]_i alongside activation of Ca²⁺/cAMP-responsive elements. We phase-mapped them to the TTFL, in time and SCN space, and demonstrated their dependence upon G-coupled vasoactive intestinal peptide (VIP) signaling. Pharmacogenetic manipulation revealed the individual contributions of Gq, Gs, and Gi to cytosolic and TTFL circadian rhythms. Importantly, activation of Gq-dependent (but not Gs or Gi) pathways in a minority of neurons reprogrammed [Ca²⁺]_i and TTFL rhythms across the entire SCN. This reprogramming was mediated by intrinsic VIPergic signaling, thus revealing a Gq/[Ca²⁺]_i-VIP leitmotif and unanticipated plasticity within network encoding of SCN circadian time.

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SCN [Ca²⁺]i and TTFL circadian landscape phase-mapped by real-time imaging

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SCN network reprogrammed by recruitment of Gq-[Ca²⁺]i axis in a minority of neurons

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Selective Gq-mediated reprogramming mediated by intrinsic VIPergic signaling

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Internal structure and unanticipated plasticity of the SCN circadian network unveiled

Exchange protein directly activated by cAMP (epac): a multidomain cAMP mediator in the regulation of diverse biological functions

Since the discovery nearly 60 years ago, cAMP is envisioned as one of the most universal and versatile second messengers. The tremendous feature of cAMP to tightly control highly diverse physiologic processes, including calcium homeostasis, metabolism, secretion, muscle contraction, cell fate, and gene transcription, is reflected by the award of five Nobel prizes. The discovery of Epac (exchange protein directly activated by cAMP) has ignited a new surge of cAMP-related research and has depicted novel cAMP properties independent of protein kinase A and cyclic nucleotide-gated channels. The multidomain architecture of Epac determines its activity state and allows cell-type specific protein-protein and protein-lipid interactions that control fine-tuning of pivotal biologic responses through the "old" second messenger cAMP. Compartmentalization of cAMP in space and time, maintained by A-kinase anchoring proteins, phosphodiesterases, and β-arrestins, contributes to the Epac signalosome of small GTPases, phospholipases, mitogen- and lipid-activated kinases, and transcription factors. These novel cAMP sensors seem to implement certain unexpected signaling properties of cAMP and thereby to permit delicate adaptations of biologic responses. Agonists and antagonists selective for Epac are developed and will support further studies on the biologic net outcome of the activation of Epac. This will increase our current knowledge on the pathophysiology of devastating diseases, such as diabetes, cognitive impairment, renal and heart failure, (pulmonary) hypertension, asthma, and chronic obstructive pulmonary disease. Further insights into the cAMP dynamics executed by the Epac signalosome will help to optimize the pharmacological treatment of these diseases.

Attenuating myocardial ischemia by targeting A2B adenosine receptors

Myocardial ischemia is associated with profound tissue hypoxia due to an imbalance in oxygen supply and demand, and studies of hypoxia-elicited adaptive responses during myocardial ischemia revealed a cardioprotective role for the signaling molecule adenosine. In ischemic human hearts, the A2B adenosine receptor (ADORA2B) is selectively induced. Functional studies in genetic models show that ADORA2B signaling attenuates myocardial infarction by adapting metabolism towards more oxygen efficient utilization of carbohydrates. This adenosine-mediated cardio-adaptive response involves the transcription factor hypoxia-inducible factor HIF1α and the circadian rhythm protein PER2. In this article, we discuss advances in the understanding of adenosine-elicited cardioprotection with particular emphasis on ADORA2B, its downstream targets, and the implications for novel strategies to prevent or treat myocardial ischemia.

Metabolism and the circadian clock converge

Circadian rhythms occur in almost all species and control vital aspects of our physiology, from sleeping and waking to neurotransmitter secretion and cellular metabolism. Epidemiological studies from recent decades have supported a unique role for circadian rhythm in metabolism. As evidenced by individuals working night or rotating shifts, but also by rodent models of circadian arrhythmia, disruption of the circadian cycle is strongly associated with metabolic imbalance. Some genetically engineered mouse models of circadian rhythmicity are obese and show hallmark signs of the metabolic syndrome. Whether these phenotypes are due to the loss of distinct circadian clock genes within a specific tissue versus the disruption of rhythmic physiological activities (such as eating and sleeping) remains a cynosure within the fields of chronobiology and metabolism. Becoming more apparent is that from metabolites to transcription factors, the circadian clock interfaces with metabolism in numerous ways that are essential for maintaining metabolic homeostasis.

The circadian clock circuitry and the AHR signaling pathway in physiology and pathology

Life forms populating the Earth must face environmental challenges to assure individual and species survival. The strategies predisposed to maintain organismal homeostasis and grant selective advantage rely on anticipatory phenomena facing periodic modifications, and compensatory phenomena facing unpredictable changes. Biological processes bringing about these responses are respectively driven by the circadian timing system, a complex of biological oscillators entrained to the environmental light/dark cycle, and by regulatory and metabolic networks that precisely direct the body's adjustments to variations of external conditions and internal milieu. A critical role in organismal homeostatic functions is played by the aryl hydrocarbon receptor (AHR) complex, which senses environmental and endogenous compounds, influences metabolic responses controlling phase I/II gene expression, and modulates vital phenomena such as development, inflammation and adaptive immunity. A physiological cross-talk between circadian and AHR signaling pathways has been evidenced. The alteration of AHR signaling pathway deriving from genetic damage with polymorphisms or mutations, or produced by exogenous or endogenous AHR activation, and chronodisruption caused by mismatch between the body's internal clock and geophysical time/social schedules, are capable of triggering pathological mechanisms involved in metabolic, immune-related and neoplastic diseases. On the other hand, the molecular components of the circadian clock circuitry and AHR signaling pathway may represent useful tools for preventive interventions and valuable targets of therapeutic approaches.

Fast synchronous oscillations of firing rate in cultured rat suprachiasmatic nucleus neurons: possible role in circadian synchronization in the intact nucleus

The coherent circadian rhythm of the brain's master circadian pacemaker, the suprachiasmatic nucleus (SCN), is a result of synchronization of electrical activity of many SCN neurons possessing their own circadian oscillators. However, how the activity of these neurons is synchronized is not precisely known. By plotting the electrical firing rates of dispersed rat SCN neurons in multi-electrode array dishes with 20-s averaging of action-potential activity, we have investigated a novel phenomenon: fast (relative to the circadian cycle) oscillations of firing rate (FOFR) with duration of bursts ∼10min and interburst interval varying in a range from 20 to 60min in different cells, remaining nevertheless rather regular in individual cells. In many cases, separate neurons in distant parts of the 1mm recording area of an array exhibited correlated FOFR. FOFR of individual cells were positively or negatively correlated with those of other cells in a functioning neural network. Intriguingly, in occasional neuron pairs, transformation of their irregular firing to circadian peaks was accompanied by appearance of FOFR and an increase in the magnitude of firing correlation. We hypothesize that this FOFR observed in cultured SCN neurons contribute to synchronization of the circadian rhythm in the intact SCN.

Molecular components of the Mammalian circadian clock

Mammals synchronize their circadian activity primarily to the cycles of light and darkness in the environment. This is achieved by ocular photoreception relaying signals to the suprachiasmatic nucleus (SCN) in the hypothalamus. Signals from the SCN cause the synchronization of independent circadian clocks throughout the body to appropriate phases. Signals that can entrain these peripheral clocks include humoral signals, metabolic factors, and body temperature. At the level of individual tissues, thousands of genes are brought to unique phases through the actions of a local transcription/translation-based feedback oscillator and systemic cues. In this molecular clock, the proteins CLOCK and BMAL1 cause the transcription of genes which ultimately feedback and inhibit CLOCK and BMAL1 transcriptional activity. Finally, there are also other molecular circadian oscillators which can act independently of the transcription-based clock in all species which have been tested.

Cellular mechanisms of circadian pacemaking: beyond transcriptional loops

Circadian clocks drive the daily rhythms in our physiology and behaviour that adapt us to the 24-h solar and social worlds. Because they impinge upon every facet of metabolism, their acute or chronic disruption compromises performance (both physical and mental) and systemic health, respectively. Equally, the presence of such rhythms has significant implications for pharmacological dynamics and efficacy, because the fate of a drug and the state of its therapeutic target will vary as a function of time of day. Improved understanding of the cellular and molecular biology of circadian clocks therefore offers novel approaches for therapeutic development, for both clock-related and other conditions. At the cellular level, circadian clocks are pivoted around a transcriptional/post-translational delayed feedback loop (TTFL) in which the activation of Period and Cryptochrome genes is negatively regulated by their cognate protein products. Synchrony between these, literally countless, cellular clocks across the organism is maintained by the principal circadian pacemaker, the suprachiasmatic nucleus (SCN) of the hypothalamus. Notwithstanding the success of the TTFL model, a diverse range of experimental studies has shown that it is insufficient to account for all properties of cellular pacemaking. Most strikingly, circadian cycles of metabolic status can continue in human red blood cells, devoid of nuclei and thus incompetent to sustain a TTFL. Recent interest has therefore focused on the role of oscillatory cytosolic mechanisms as partners to the TTFL. In particular, cAMP- and Ca²⁺-dependent signalling are important components of the clock, whilst timekeeping activity is also sensitive to a series of highly conserved kinases and phosphatases. This has led to the view that the 'proto-clock' may have been a cytosolic, metabolic oscillation onto which evolution has bolted TTFLs to provide robustness and amplify circadian outputs in the form of rhythmic gene expression. This evolutionary ascent of the clock has culminated in the SCN, a true pacemaker to the innumerable clock cells distributed across the body. On the basis of findings from our own and other laboratories, we propose a model of the SCN pacemaker that synthesises the themes of TTFLs, intracellular signalling, metabolic flux and interneuronal coupling that can account for its unique circadian properties and pre-eminence.

Circadian clock disruptions and the risk of cancer

Disrupted circadian rhythms may lead to failures in the control of the cell division cycle and the subsequent malignant cell growth. In order to understand the pathogenesis of cancer more in detail, it is crucial to identify those mechanisms of action which contribute to the loss of control of the cell division cycle. This mini-review focuses on the recent findings concerning the links between the human circadian clock and cancer. Clinical implications concern not only feasible methods for the assessment of the circadian time of an individual or for the determination of the best time for administration of a drug of treatment, but also in the future genetic tests for screening and for planning treatment.

Small molecule modifiers of circadian clocks

Circadian clocks orchestrate 24-h oscillations of essential physiological and behavioral processes in response to daily environmental changes. These clocks are remarkably precise under constant conditions yet highly responsive to resetting signals. With the molecular composition of the core oscillator largely established, recent research has increasingly focused on clock-modifying mechanisms/molecules. In particular, small molecule modifiers, intrinsic or extrinsic, are emerging as powerful tools for understanding basic clock biology as well as developing putative therapeutic agents for clock-associated diseases. In this review, we will focus on synthetic compounds capable of modifying the period, phase, or amplitude of circadian clocks, with particular emphasis on the mammalian clock. We will discuss the potential of exploiting these small molecule modifiers in both basic and translational research.

Fibroblast circadian rhythms of PER2 expression depend on membrane potential and intracellular calcium

The suprachiasmatic nucleus (SCN) of the hypothalamus synchronizes circadian rhythms of cells and tissues throughout the body. In SCN neurons, rhythms of clock gene expression are suppressed by manipulations that hyperpolarize the plasma membrane or lower intracellular Ca(2+). However, whether clocks in other cells also depend on membrane potential and calcium is unknown. In this study, the authors investigate the effects of membrane potential and intracellular calcium on circadian rhythms in mouse primary fibroblasts. Rhythms of clock gene expression were monitored using a PER2::LUC knockin reporter. Rhythms were lost or delayed at lower (hyperpolarizing) K(+) concentrations. Bioluminescence imaging revealed that this loss of rhythmicity in cultures was due to loss of rhythmicity of single cells rather than loss of synchrony among cells. In lower Ca(2+) concentrations, rhythms were advanced or had shorter periods. Buffering intracellular Ca(2+) by the calcium chelator 1,2-Bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid tetrakis acetoxymethyl ester (BAPTA-AM) or manipulation of inositol triphosphate (IP(3))-sensitive intracellular calcium stores by thapsigargin delayed rhythms. These results suggest that the circadian clock in fibroblasts, as in SCN neurons, is regulated by membrane potential and Ca(2+). Changes in intracellular Ca(2+) may mediate the effects of membrane potential observed in this study.

Hypothesis: Cryptochromes and Brown Fat are Essential for Adaptation and Affect Mood and Mood-Related Behaviors

Solar radiation and ambient temperature have acted as selective physical forces among populations and thereby guided species distributions in the globe. Circadian clocks are universal and evolve when subjected to selection, and their properties contribute to variations in fitness within specific environments. Concerning humans, as compared to the remaining, the "evening owls" have a greater deviation from the 24 h cycle, are under a greater pressure to circadian desynchrony and more prone to a cluster of health hazards with the increased mortality. Because of their position in the hierarchy and repressive actions, cryptochromes are the key components of the feedback loops on which circadian clocks are built. Based on the evidence a new hypothesis is formulated in which brown adipocytes with their cryptochromes are responsive to a broad range of physical stimuli from the habitat and through their activity ensure adaptation of the individual. The over-activated brown adipose tissue with deficient cryptochromes might induce disrupted thermoregulation and circadian desynchrony, and thereby contribute to lowered mood and pronounced depressive behaviors.

The clock shop: coupled circadian oscillators

Daily rhythms in neural activity underlie circadian rhythms in sleep-wake and other daily behaviors. The cells within the mammalian suprachiasmatic nucleus (SCN) are intrinsically capable of 24-h timekeeping. These cells synchronize with each other and with local environmental cycles to drive coherent rhythms in daily behaviors. Recent studies have identified a small number of neuropeptides critical for this ability to synchronize and sustain coordinated daily rhythms. This review highlights the roles of specific intracellular and intercellular signals within the SCN that underlie circadian synchrony.

In vivo circadian oscillation of dCREB2 and NF-κB activity in the Drosophila nervous system

cAMP response element-binding protein (CREB) and nuclear factor kappa-B (NF-κB) are two ubiquitous transcription factors involved in a wide number of cellular processes, including the circadian system. Many previous studies on these factors use cellular assays that provide limited information on circadian activity or anatomical specificity. The ability to study transcription factors in defined tissue within intact animals will help to bridge the gap between cellular and in vivo data. We have used the GAL4-UAS and FLP-FRT systems to gain spatial control over reporter gene expression. Using a luciferase-based reporter, we show in vivo that Drosophila dCREB2- and NF-κB-mediated transcription oscillates in neuronal cells, glia, and in the mushroom body, a higher-order brain center in flies. This oscillation is under circadian control, cycling with a 24-hour rhythm, under both light-dark and dark-dark conditions. In light-light conditions, dCREB2 and NF-κB reporter flies exhibit a suppression of rhythmic activity. Furthermore, neuronal cycling of dCREB2 and NF-κB activity are modulated in period mutant flies, indicating these oscillations are controlled through the central clock. This study shows for the first time region-specific circadian oscillation of dCREB2/NF-κB activity in the Drosophila nervous system.

Structure-based discovery of highly selective phosphodiesterase-9A inhibitors and implications for inhibitor design

A new series of phosphodiesterase-9 (PDE9) inhibitors that contain a scaffold of 6-amino-pyrazolopyrimidinone have been discovered by a combination of structure-based design and computational docking. This procedure significantly saved the load of chemical synthesis and is an effective method for the discovery of inhibitors. The best compound 28 has an IC(50) of 21 nM and 3.3 μM, respectively, for PDE9 and PDE5 and about 3 orders of magnitude of selectivity against other PDE families. The crystal structure of the PDE9 catalytic domain in complex with 28 has been determined and shows a hydrogen bond between 28 and Tyr424. This hydrogen bond may account for the 860-fold selectivity of 28 against PDE1B, in comparison with about 30-fold selectivity of BAY73-6691. Thus, our studies suggest that Tyr424, a unique residue of PDE8 and PDE9, is a potential target for improvement of selectivity of PDE9 inhibitors.

Monitoring cell-autonomous circadian clock rhythms of gene expression using luciferase bioluminescence reporters

In mammals, many aspects of behavior and physiology such as sleep-wake cycles and liver metabolism are regulated by endogenous circadian clocks (reviewed). The circadian time-keeping system is a hierarchical multi-oscillator network, with the central clock located in the suprachiasmatic nucleus (SCN) synchronizing and coordinating extra-SCN and peripheral clocks elsewhere. Individual cells are the functional units for generation and maintenance of circadian rhythms, and these oscillators of different tissue types in the organism share a remarkably similar biochemical negative feedback mechanism. However, due to interactions at the neuronal network level in the SCN and through rhythmic, systemic cues at the organismal level, circadian rhythms at the organismal level are not necessarily cell-autonomous. Compared to traditional studies of locomotor activity in vivo and SCN explants ex vivo, cell-based in vitro assays allow for discovery of cell-autonomous circadian defects. Strategically, cell-based models are more experimentally tractable for phenotypic characterization and rapid discovery of basic clock mechanisms. Because circadian rhythms are dynamic, longitudinal measurements with high temporal resolution are needed to assess clock function. In recent years, real-time bioluminescence recording using firefly luciferase as a reporter has become a common technique for studying circadian rhythms in mammals, as it allows for examination of the persistence and dynamics of molecular rhythms. To monitor cell-autonomous circadian rhythms of gene expression, luciferase reporters can be introduced into cells via transient transfection or stable transduction. Here we describe a stable transduction protocol using lentivirus-mediated gene delivery. The lentiviral vector system is superior to traditional methods such as transient transfection and germline transmission because of its efficiency and versatility: it permits efficient delivery and stable integration into the host genome of both dividing and non-dividing cells. Once a reporter cell line is established, the dynamics of clock function can be examined through bioluminescence recording. We first describe the generation of P(Per2)-dLuc reporter lines, and then present data from this and other circadian reporters. In these assays, 3T3 mouse fibroblasts and U2OS human osteosarcoma cells are used as cellular models. We also discuss various ways of using these clock models in circadian studies. Methods described here can be applied to a great variety of cell types to study the cellular and molecular basis of circadian clocks, and may prove useful in tackling problems in other biological systems.

Circadian rhythm of redox state regulates excitability in suprachiasmatic nucleus neurons

Daily rhythms of mammalian physiology, metabolism, and behavior parallel the day-night cycle. They are orchestrated by a central circadian clock in the brain, the suprachiasmatic nucleus (SCN). Transcription of clock genes is sensitive to metabolic changes in reduction and oxidation (redox); however, circadian cycles in protein oxidation have been reported in anucleate cells, where no transcription occurs. We investigated whether the SCN also expresses redox cycles and how such metabolic oscillations might affect neuronal physiology. We detected self-sustained circadian rhythms of SCN redox state that required the molecular clockwork. The redox oscillation could determine the excitability of SCN neurons through nontranscriptional modulation of multiple potassium (K(+)) channels. Thus, dynamic regulation of SCN excitability appears to be closely tied to metabolism that engages the clockwork machinery.

Role of activating transcription factor-4 in 24-hour rhythm of serotonin transporter expression in the mouse midbrain

Serotonin (5-HT) transporter (5-HTT) plays a key role in the control of 5-HT neuronal activity by reuptaking extracellular 5-HT from the synapse cleft. We have previously demonstrated that 5-HTT mRNA expression levels and its uptake activity in the mouse midbrain are significantly higher in the dark phase than those in the light phase. However, the molecular mechanisms of time-dependent expression of 5-HTT have not been clarified. In this study, expression of 5-HTT mRNA in the mouse midbrain showed a significant 24-h rhythm and was higher in the dark phase. Although such an oscillation was eliminated by a Clock gene mutation, CLOCK and BMAL1 did not activate 5-HTT transcription in the luciferase reporter assay. Activating transcription factor-4 (ATF4), a member of the ATF/cAMP response element (CRE)-binding protein family, is a component responsible for sustaining circadian oscillations of CRE-mediated gene expression. ATF4 significantly activated 5-HTT transcription in vitro and time dependently bound to the CRE site in the 5-HTT promoter in the mouse midbrain. In addition, mutation of the Clock gene disrupted temporal binding of ATF4 to the CRE site in the 5-HTT promoter. These results indicated that the circuit of circadian-basis molecular regulation between the clockwork system and mouse 5-HTT gene was connected by the ATF4 signaling pathway.

Circadian clock protein cryptochrome regulates the expression of proinflammatory cytokines

Chronic sleep deprivation perturbs the circadian clock and increases susceptibility to diseases such as diabetes, obesity, and cancer. Increased inflammation is one of the common underlying mechanisms of these diseases, thus raising a hypothesis that circadian-oscillator components may regulate immune response. Here we show that absence of the core clock component protein cryptochrome (CRY) leads to constitutive elevation of proinflammatory cytokines in a cell-autonomous manner. We observed a constitutive NF-κB and protein kinase A (PKA) signaling activation in Cry1(-/-);Cry2(-/-) cells. We further demonstrate that increased phosphorylation of p65 at S276 residue in Cry1(-/-);Cry2(-/-) cells is due to increased PKA signaling activity, likely induced by a significantly high basal level of cAMP, which we detected in these cells. In addition, we report that CRY1 binds to adenylyl cyclase and limits cAMP production. Based on these data, we propose that absence of CRY protein(s) might release its (their) inhibition on cAMP production, resulting in elevated cAMP and increased PKA activation, subsequently leading to NF-κB activation through phosphorylation of p65 at S276. These results offer a mechanistic framework for understanding the link between circadian rhythm disruption and increased susceptibility to chronic inflammatory diseases.

The circadian neuropeptide PDF signals preferentially through a specific adenylate cyclase isoform AC3 in M pacemakers of Drosophila

To synchronize a network of pacemakers in the Drosophila brain, a neuropeptide receptor specifically associates with adenylate cyclase 3 to create a “circadian signalosome.”

The neuropeptide Pigment Dispersing Factor (PDF) is essential for normal circadian function in Drosophila. It synchronizes the phases of M pacemakers, while in E pacemakers it decelerates their cycling and supports their amplitude. The PDF receptor (PDF-R) is present in both M and subsets of E cells. Activation of PDF-R stimulates cAMP increases in vitro and in M cells in vivo. The present study asks: What is the identity of downstream signaling components that are associated with PDF receptor in specific circadian pacemaker neurons? Using live imaging of intact fly brains and transgenic RNAi, we show that adenylate cyclase AC3 underlies PDF signaling in M cells. Genetic disruptions of AC3 specifically disrupt PDF responses: they do not affect other Gs-coupled GPCR signaling in M cells, they can be rescued, and they do not represent developmental alterations. Knockdown of the Drosophila AKAP-like scaffolding protein Nervy also reduces PDF responses. Flies with AC3 alterations show behavioral syndromes consistent with known roles of M pacemakers as mediated by PDF. Surprisingly, disruption of AC3 does not alter PDF responses in E cells—the PDF-R(+) LNd. Within M pacemakers, PDF-R couples preferentially to a single AC, but PDF-R association with a different AC(s) is needed to explain PDF signaling in the E pacemakers. Thus critical pathways of circadian synchronization are mediated by highly specific second messenger components. These findings support a hypothesis that PDF signaling components within target cells are sequestered into “circadian signalosomes,” whose compositions differ between E and M pacemaker cell types.

In the fruit fly Drosophila melanogaster, the neuropeptide Pigment Dispersing Factor (PDF) supports circadian function by synchronizing two types of pacemaker cells, M cells and E cells. The PDF receptor (PDF-R) is a G protein coupled receptor (GPCR) whose activation stimulates adenylate cyclase (AC), thereby elevating levels of the second messenger cAMP in many different pacemakers including M cells. Drosophila contains at least 12 genes that encode potential ACs. In this study, we identify the AC downstream of the PDF receptor specifically in M cells and show that PDF signals preferentially through AC3. However, other GPCRs in the very same cells do not rely on AC3. A different scaffolding protein also influences PDF responses in M cells, suggesting that signaling components are spatially grouped to allow for coupling of specific receptors with downstream components. Remarkably, in E pacemakers, AC3 disruptions have no effect. These findings suggest that distinct PDF circadian signals exist in M versus in E pacemakers, and more generally, we propose a mechanism to differentiate signaling pathways that use common components.

Role of Epac in brain and heart

Epacs (exchange proteins directly activated by cAMP) are guanine-nucleotide-exchange factors for the Ras-like small GTPases Rap1 and Rap2. Epacs were discovered in 1998 as new sensors for the second messenger cAMP acting in parallel to PKA (protein kinase A). As cAMP regulates many important physiological functions in brain and heart, the existence of Epacs raises many questions regarding their role in these tissues. The present review focuses on the biological roles and signalling pathways of Epacs in neurons and cardiac myocytes. We discuss the potential involvement of Epacs in the manifestation of cardiac and central diseases such as cardiac hypertrophy and memory disorders.

(Re)inventing the circadian feedback loop

For 20 years, researchers have thought that circadian clocks are defined by feedback loops of transcription and translation. The rediscovery of posttranslational circadian oscillators in diverse organisms forces us to rethink this paradigm. Meanwhile, the original "basic" feedback loops of canonical circadian clocks have swelled to include dozens of additional proteins acting in interlocked loops. We review several self-sustained clock mechanisms and propose that minimum requirements for diurnal timekeeping might be simpler than those of actual free-running circadian oscillators. Thus, complex mechanisms of circadian timekeeping might have evolved from random connections between unrelated feedback loops with independent but limited time-telling capability.

Circadian Oscillations within the Hippocampus Support Memory Formation and Persistence

The ability to sustain memories over long periods of time, sometimes even a lifetime, is one of the most remarkable properties of the brain. Much knowledge has been gained over the past few decades regarding the molecular correlates of memory formation. Once a memory is forged, however, the molecular events that provide permanence are as of yet unclear. Studies in multiple organisms have revealed that circadian rhythmicity is important for the formation, stability, and recall of memories (Gerstner et al., 2009).The neuronal events that provide this link need to be explored further. This article will discuss the findings related to the circadian regulation of memory-dependent processes in the hippocampus. Specifically, the circadian-controlled mitogen-activated protein kinase (MAPK) and cAMP signal transduction pathway plays critical roles in the consolidation of hippocampus-dependent memory. A series of studies have revealed the circadian oscillation of this pathway within the hippocampus, an activity that is absent in memory-deficient, transgenic mice lacking Ca(2+)-stimulated adenylyl cyclases. Interference with these oscillations proceeding the cellular memory consolidation period impairs the persistence of hippocampus-dependent memory. These data suggest that the persistence of long-term memories may depend upon reactivation of this signal transduction pathway in the hippocampus during the circadian cycle. New data reveals the dependence of hippocampal oscillation in MAPK activity on the suprachiasmatic nucleus, again underscoring the importance of this region in maintaining the circadian physiology of memory. Finally, the downstream ramification of these oscillations in terms of gene expression and epigenetics should be considered, as emerging evidence is pointing strongly to a circadian link between epigenetics and long-term synaptic plasticity.

Adora2b-elicited Per2 stabilization promotes a HIF-dependent metabolic switch crucial for myocardial adaptation to ischemia

Adenosine signaling has been implicated in cardiac adaptation to limited oxygen availability. In a wide search for adenosine receptor A2b (Adora2b)-elicited cardioadaptive responses, we identified the circadian rhythm protein period 2 (Per2) as an Adora2b target. Adora2b signaling led to Per2 stabilization during myocardial ischemia, and in this setting, Per2(-/-) mice had larger infarct sizes compared to wild-type mice and loss of the cardioprotection conferred by ischemic preconditioning. Metabolic studies uncovered a limited ability of ischemic hearts in Per2(-/-) mice to use carbohydrates for oxygen-efficient glycolysis. This impairment was caused by a failure to stabilize hypoxia-inducible factor-1α (Hif-1α). Moreover, stabilization of Per2 in the heart by exposing mice to intense light resulted in the transcriptional induction of glycolytic enzymes and Per2-dependent cardioprotection from ischemia. Together, these studies identify adenosine-elicited stabilization of Per2 in the control of HIF-dependent cardiac metabolism and ischemia tolerance and implicate Per2 stabilization as a potential new strategy for treating myocardial ischemia.

Adora2b-elicited Per2 stabilization promotes a HIF-dependent metabolic switch crucial for myocardial adaptation to ischemia

Adenosine signaling has been implicated in cardiac adaptation to limited oxygen availability. In a wide search for adenosine receptor A2b (Adora2b)-elicited cardioadaptive responses, we identified the circadian rhythm protein period 2 (Per2) as an Adora2b target. Adora2b signaling led to Per2 stabilization during myocardial ischemia, and in this setting, Per2(-/-) mice had larger infarct sizes compared to wild-type mice and loss of the cardioprotection conferred by ischemic preconditioning. Metabolic studies uncovered a limited ability of ischemic hearts in Per2(-/-) mice to use carbohydrates for oxygen-efficient glycolysis. This impairment was caused by a failure to stabilize hypoxia-inducible factor-1α (Hif-1α). Moreover, stabilization of Per2 in the heart by exposing mice to intense light resulted in the transcriptional induction of glycolytic enzymes and Per2-dependent cardioprotection from ischemia. Together, these studies identify adenosine-elicited stabilization of Per2 in the control of HIF-dependent cardiac metabolism and ischemia tolerance and implicate Per2 stabilization as a potential new strategy for treating myocardial ischemia.

Coordination of the transcriptome and metabolome by the circadian clock

The circadian clock governs a large array of physiological functions through the transcriptional control of a significant fraction of the genome. Disruption of the clock leads to metabolic disorders, including obesity and diabetes. As food is a potent zeitgeber (ZT) for peripheral clocks, metabolites are implicated as cellular transducers of circadian time for tissues such as the liver. From a comprehensive dataset of over 500 metabolites identified by mass spectrometry, we reveal the coordinate clock-controlled oscillation of many metabolites, including those within the amino acid and carbohydrate metabolic pathways as well as the lipid, nucleotide, and xenobiotic metabolic pathways. Using computational modeling, we present evidence of synergistic nodes between the circadian transcriptome and specific metabolic pathways. Validation of these nodes reveals that diverse metabolic pathways, including the uracil salvage pathway, oscillate in a circadian fashion and in a CLOCK-dependent manner. This integrated map illustrates the coherence within the circadian metabolome, transcriptome, and proteome and how these are connected through specific nodes that operate in concert to achieve metabolic homeostasis.

On the evolution of memory: a time for clocks

Evolutionarily, what was the earliest engram? Biology has evolved to encode representations of past events, and in neuroscience, we are attempting to link experience-dependent changes in molecular signaling with cellular processes that ultimately lead to behavioral output. The theory of evolution has guided biological research for decades, and since phylogenetically conserved mechanisms drive circadian rhythms, these processes may serve as common predecessors underlying more complex behavioral phenotypes. For example, the cAMP/MAPK/CREB cascade is interwoven with the clock to trigger circadian output, and is also known to affect memory formation. Time-of-day dependent changes have been observed in long-term potentiation (LTP) within the suprachiasmatic nucleus and hippocampus, along with light-induced circadian phase resetting and fear conditioning behaviors. Together this suggests during evolution, similar processes underlying metaplasticity in more simple circuits may have been redeployed in higher-order brain regions. Therefore, this notion predicts a model that LTP and metaplasticity may exist in neural circuits of other species, through phylogenetically conserved pathways, leading to several testable hypotheses.

Novel insights into the downstream pathways and targets controlled by transcription factors CREM in the testis

The essential role of the Crem gene in normal sperm development is widely accepted and is confirmed by azoospermia in male mice lacking the Crem gene. The exact number of genes affected by Crem absence is not known, however a large difference has been observed recently between the estimated number of differentially expressed genes found in Crem knock-out (KO) mice compared to the number of gene loci bound by CREM. We therefore re-examined global gene expression in male mice lacking the Crem gene using whole genome transcriptome analysis with Affymetrix microarrays and compared the lists of differentially expressed genes from Crem-/- mice to a dataset of genes where binding of CREM was determined by Chip-seq. We determined the global effect of CREM on spermatogenesis as well as distinguished between primary and secondary effects of the CREM absence. We demonstrated that the absence of Crem deregulates over 4700 genes in KO testis. Among them are 101 genes associated with spermatogenesis 41 of which are bound by CREM and are deregulated in Crem KO testis. Absence of several of these genes in mouse models has proven their importance for normal spermatogenesis and male fertility. Our study showed that the absence of Crem plays a more important role on different aspects of spermatogenesis as estimated previously, with its impact ranging from apoptosis induction to deregulation of major circadian clock genes, steroidogenesis and the cell-cell junction dynamics. Several new genes important for normal spermatogenesis and fertility are down-regulated in KO testis and are therefore possible novel targets of CREM.

The essential role of cAMP/Ca2+ signalling in mammalian circadian timekeeping

Approximately daily, or circadian, rhythms are ubiquitous across eukaryotes. They are manifest in the temporal co-ordination of metabolism, physiology and behaviour, thereby allowing organisms to anticipate and synchronize with daily environmental cycles. Although cellular rhythms are self-sustained and cell-intrinsic, in mammals, the master regulator of timekeeping is localized within the hypothalamic SCN (suprachiasmatic nucleus). Molecular models for mammalian circadian rhythms have focused largely on transcriptional-translational feedback loops, but recent data have revealed essential contributions by intracellular signalling mechanisms. cAMP and Ca2+ signalling are not only regulated by the cellular clock, but also contribute directly to the timekeeping mechanism, in that appropriate manipulations determine the canonical pacemaker properties of amplitude, phase and period. It is proposed that daily auto-amplification of second messenger activity, through paracrine neuropeptidergic coupling, is necessary and sufficient to account for the increased amplitude, accuracy and robustness of SCN timekeeping.

Reciprocal cholinergic and GABAergic modulation of the small ventrolateral pacemaker neurons of Drosophila's circadian clock neuron network

The relatively simple clock neuron network of Drosophila is a valuable model system for the neuronal basis of circadian timekeeping. Unfortunately, many key neuronal classes of this network are inaccessible to electrophysiological analysis. We have therefore adopted the use of genetically encoded sensors to address the physiology of the fly's circadian clock network. Using genetically encoded Ca(2+) and cAMP sensors, we have investigated the physiological responses of two specific classes of clock neuron, the large and small ventrolateral neurons (l- and s-LN(v)s), to two neurotransmitters implicated in their modulation: acetylcholine (ACh) and γ-aminobutyric acid (GABA). Live imaging of l-LN(v) cAMP and Ca(2+) dynamics in response to cholinergic agonist and GABA application were well aligned with published electrophysiological data, indicating that our sensors were capable of faithfully reporting acute physiological responses to these transmitters within single adult clock neuron soma. We extended these live imaging methods to s-LN(v)s, critical neuronal pacemakers whose physiological properties in the adult brain are largely unknown. Our s-LN(v) experiments revealed the predicted excitatory responses to bath-applied cholinergic agonists and the predicted inhibitory effects of GABA and established that the antagonism of ACh and GABA extends to their effects on cAMP signaling. These data support recently published but physiologically untested models of s-LN(v) modulation and lead to the prediction that cholinergic and GABAergic inputs to s-LN(v)s will have opposing effects on the phase and/or period of the molecular clock within these critical pacemaker neurons.

Receptor signaling and the cell biology of synaptic transmission

This volume describes a series of psychiatric and neuropsychiatric disorders, connects some aspects of somatic and psychiatric medicine, and describes various current and emerging therapies. The purpose of this chapter is to set the stage for the volume by developing the theoretical basis of synaptic transmission and introducing the various neurotransmitters and their receptors involved in the process. The intent is to provide not only a historical context through which to understand neurotransmitters, but a current contextual basis for understanding neuronal signal transduction and applying this knowledge to facilitate treatment of maladies of the brain and mind.

Regulation of metabolism: the circadian clock dictates the time

Circadian rhythms occur with a periodicity of approximately 24h and regulate a wide array of metabolic and physiologic functions. Accumulating epidemiological and genetic evidence indicates that disruption of circadian rhythms can be directly linked to many pathological conditions, including sleep disorders, depression, metabolic syndrome and cancer. Intriguingly, several molecular gears constituting the clock machinery have been found to establish functional interplays with regulators of cellular metabolism. Although the circadian clock regulates multiple metabolic pathways, metabolite availability and feeding behavior can in turn regulate the circadian clock. An in-depth understanding of this reciprocal regulation of circadian rhythms and cellular metabolism may provide insights into the development of therapeutic intervention against specific metabolic disorders.

Circadian system from conception till adulthood

In mammals, the circadian system is composed of the central clock in the hypothalamic suprachiasmatic nuclei and of peripheral clocks that are located in other neural structures and in cells of the peripheral tissues and organs. In adults, the system is hierarchically organized so that the central clock provides the other clocks in the body with information about the time of day. This information is needed for the adaptation of their functions to cyclically changing external conditions. During ontogenesis, the system undergoes substantial development and its sensitivity to external signals changes. Perinatally, maternal cues are responsible for setting the phase of the developing clock, while later postnatally, the LD cycle is dominant. The central clock attains its functional properties during a gradual and programmed process. Peripheral clocks begin to exhibit rhythmicity independent of each other at various developmental stages. During the early developmental stages, the peripheral clocks are set or driven by maternal feeding, but later the central clock becomes fully functional and begins to entrain the periphery. During the perinatal period, the central and peripheral clocks seem to be vulnerable to disturbances in external conditions. Further studies are needed to understand the processes of how the circadian system develops and what degree of plasticity and resilience it possesses during ontogenesis. These data may lead to an assessment of the contribution of disturbances of the circadian system during early ontogenesis to the occurrence of circadian diseases in adulthood.

Identification of diverse modulators of central and peripheral circadian clocks by high-throughput chemical screening

The circadian clock coordinates daily oscillations of essential physiological and behavioral processes. Conversely, aberrant clocks with damped amplitude and/or abnormal period have been associated with chronic diseases and aging. To search for small molecules that perturb or enhance circadian rhythms, we conducted a high-throughput screen of approximately 200,000 synthetic compounds using Per2lucSV reporter fibroblast cells and validated 11 independent classes of molecules with Bmal1:luciferase reporter cells as well as with suprachiasmatic nucleus and peripheral tissue explants. Four compounds were found to lengthen the period in both central and peripheral clocks, including three compounds that inhibited casein kinase Iε in vitro and a unique benzodiazepine derivative acting through a non-GABA(A) receptor target. In addition, two compounds acutely induced Per2lucSV reporter bioluminescence, delayed the rhythm, and increased intracellular cAMP levels, but caused rhythm damping. Importantly, five compounds shortened the period of peripheral clocks; among them, four compounds also enhanced the amplitude of central and/or peripheral reporter rhythms. Taken together, these studies highlight diverse activities of drug-like small molecules in manipulating the central and peripheral clocks. These small molecules constitute a toolbox for probing clock regulatory mechanisms and may provide putative lead compounds for treatment of clock-associated diseases.

In vitro circadian rhythms: imaging and electrophysiology

In vitro assays have localized circadian pacemakers to individual cells, revealed genetic determinants of rhythm generation, identified molecular players in cell-cell synchronization and determined physiological events regulated by circadian clocks. Although they allow strict control of experimental conditions and reduce the number of variables compared with in vivo studies, they also lack many of the conditions in which cellular circadian oscillators normally function. The present review highlights methods to study circadian timing in cultured mammalian cells and how they have shaped the hypothesis that all cells are capable of circadian rhythmicity.