Circadian regulation of intracellular G-protein signalling mediates intercellular synchrony and rhythmicity in the suprachiasmatic nucleus

On the origin and evolution of the dual oscillator model underlying the photoperiodic clockwork in the suprachiasmatic nucleus

Decades have now passed since Colin Pittendrigh first proposed a model of a circadian clock composed of two coupled oscillators, individually responsive to the rising and setting sun, as a flexible solution to the challenge of behavioral and physiological adaptation to the changing seasons. The elegance and predictive power of this postulation has stimulated laboratories around the world in searches to identify and localize such hypothesized evening and morning oscillators, or sets of oscillators, in insects, rodents, and humans, with experimental designs and approaches keeping pace over the years with technological advances in biology and neuroscience. Here, we recount the conceptual origin and highlight the subsequent evolution of this dual oscillator model for the circadian clock in the mammalian suprachiasmatic nucleus; and how, despite our increasingly sophisticated view of this multicellular pacemaker, Pittendrigh's binary conception has remained influential in our clock models and metaphors.

Diverse genetic alteration dysregulates neuropeptide and intracellular signalling in the suprachiasmatic nuclei

In mammals, intrinsic 24 h or circadian rhythms are primarily generated by the suprachiasmatic nuclei (SCN). Rhythmic daily changes in the transcriptome and proteome of SCN cells are controlled by interlocking transcription-translation feedback loops (TTFLs) of core clock genes and their proteins. SCN cells function as autonomous circadian oscillators, which synchronize through intercellular neuropeptide signalling. Physiological and behavioural rhythms can be severely disrupted by genetic modification of a diverse range of genes and proteins in the SCN. With the advent of next generation sequencing, there is unprecedented information on the molecular profile of the SCN and how it is affected by genetically targeted alteration. However, whether the expression of some genes is more readily affected by genetic alteration of the SCN is unclear. Here, using publicly available datasets from recent RNA-seq assessments of the SCN from genetically altered and control mice, we evaluated whether there are commonalities in transcriptome dysregulation. This was completed for four different phases across the 24 h cycle and was augmented by Gene Ontology Molecular Function (GO:MF) and promoter analysis. Common differentially expressed genes (DEGs) and/or enriched GO:MF terms included signalling molecules, their receptors, and core clock components. Finally, examination of the JASPAR database indicated that E-box and CRE elements in the promoter regions of several commonly dysregulated genes. From this analysis, we identify differential expression of genes coding for molecules involved in SCN intra- and intercellular signalling as a potential cause of abnormal circadian rhythms.

A genome-wide association study identifies candidate genes for sleep disturbances in depressed individuals

OBJECTIVE

This study aimed to identify candidate loci and genes related to sleep disturbances in depressed individuals and clarify the co-occurrence of sleep disturbances and depression from the genetic perspective.

METHODS

The study subjects (including 58,256 self-reported depressed individuals and 6,576 participants with PHQ-9 score ≥ 10, respectively) were collected from the UK Biobank, which were determined based on the Patient Health Questionnaire (PHQ-9) and self-reported depression status, respectively. Sleep related traits included chronotype, insomnia, snoring and daytime dozing. Genome-wide association studies (GWASs) of sleep related traits in depressed individuals were conducted by PLINK 2.0 adjusting age, sex, Townsend deprivation index and 10 principal components as covariates. The CAUSALdb database was used to explore the mental traits associated with the candidate genes identified by the GWAS.

RESULTS

GWAS detected 15 loci significantly associated with chronotype in the subjects with self-reported depression, such as rs12736689 at RNASEL (P = 1.00 × 10- 09), rs509476 at RGS16 (P = 1.58 × 10- 09) and rs1006751 at RFX4 (P = 1.54 × 10- 08). 9 candidate loci were identified in the subjects with PHQ-9 ≥ 10, of which 2 loci were associated with insomnia such as rs115379847 at EVC2 (P = 3.50 × 10- 08), and 7 loci were associated with daytime dozing, such as rs140876133 at SMYD3 (P = 3.88 × 10- 08) and rs139156969 at ROBO2 (P = 3.58 × 10- 08). Multiple identified genes, such as RNASEL, RGS16, RFX4 and ROBO2 were reported to be associated with chronotype, depression or cognition in previous studies.

CONCLUSION

Our study identified several candidate genes related to sleep disturbances in depressed individuals, which provided new clues for understanding the biological mechanism underlying the co-occurrence of depression and sleep disorders.

Circadian rhythm and host genetics

This chapter aims to explore the usefulness of the latest advances in genetic studies in the field of the circadian system in the future development of individualised strategies for health improvement based on lifestyle intervention. Due to the multifactorial and complex nature of the circadian system, we focus on the highly prevalent phenotypes in the population that are key to understanding its biology from an evolutionary perspective and that can be modulated by lifestyle. Therefore, we leave in the background those phenotypes that constitute infrequent pathologies or in which the current level of scientific evidence does not favour the implementation of practical approaches of this type. Therefore, from an evolutionary paradigm, this chapter addresses phenotypes such as morning chronotypes, evening chronotypes, extreme chronotypes, and other key concepts such as circadian rhythm amplitude, resilience to changes in circadian rhythm, and their relationships with pathologies associated with circadian rhythm imbalances.

LimoRhyde2: Genomic analysis of biological rhythms based on effect sizes

Genome-scale data have revealed daily rhythms in various species and tissues. However, current methods to assess rhythmicity largely restrict their focus to quantifying statistical significance, which may not reflect biological relevance. To address this limitation, we developed a method called LimoRhyde2 (the successor to our method LimoRhyde), which focuses instead on rhythm-related effect sizes and their uncertainty. For each genomic feature, LimoRhyde2 fits a curve using a series of linear models based on periodic splines, moderates the fits using an Empirical Bayes approach called multivariate adaptive shrinkage (Mash), then uses the moderated fits to calculate rhythm statistics such as peak-to-trough amplitude. The periodic splines capture non-sinusoidal rhythmicity, while Mash uses patterns in the data to account for different fits having different levels of noise. To demonstrate LimoRhyde2's utility, we applied it to multiple circadian transcriptome datasets. Overall, LimoRhyde2 prioritized genes having high-amplitude rhythms in expression, whereas a prior method (BooteJTK) prioritized "statistically significant" genes whose amplitudes could be relatively small. Thus, quantifying effect sizes using approaches such as LimoRhyde2 has the potential to transform interpretation of genomic data related to biological rhythms.

Prolonged Light Exposure Induces Circadian Impairment in Aquaporin-4-Knockout Mice

Astrocytes are densely present in the suprachiasmatic nucleus (SCN), which is the master circadian oscillator in mammals, and are presumed to play a key role in circadian oscillation. However, specific astrocytic molecules that regulate the circadian clock are not yet well understood. In our study, we found that the water channel aquaporin-4 (AQP4) was abundantly expressed in SCN astrocytes, and we further examined its circadian role using AQP4-knockout mice. There was no prominent difference in circadian behavioral rhythms between Aqp4^-/- and Aqp4^+/+ mice subjected to light-dark cycles and constant dark conditions. However, exposure to constant light induced a greater decrease in the Aqp4^-/- mice rhythmicity. Although the damped rhythm in long-term constant light recovered after transfer to constant dark conditions in both genotypes, the period until the reappearance of original rhythmicity was severely prolonged in Aqp4^-/- mice. In conclusion, AQP4 absence exacerbates the prolonged light-induced impairment of circadian oscillations and delays their recovery to normal rhythmicity.

Niacin/β-hydroxybutyrate regulates milk fat and milk protein synthesis via the GPR109A/Gi/mTORC1 pathway

As a crucial receptor of BHBA and niacin, GPR109A is largely expressed in the mammary gland. However, the role of GPR109A in milk synthesis and its underlying mechanism is still largely unknown. In this study, we first investigated the effect of GPR109A agonists (niacin/BHBA) on milk fat and milk protein synthesis in a mouse mammary epithelial cell line (HC11) and PMECs (porcine mammary epithelial cells). The results showed that both niacin and BHBA promote milk fat and milk protein synthesis with the activation of mTORC1 signaling. Importantly, knockdown GPR109A attenuated the niacin-induced increase of milk fat and protein synthesis and the niacin-induced activation of mTORC1 signaling. Furthermore, we found that GPR109A downstream G protein-G_αi and -G_βγ participated in the regulation of milk synthesis and the activation of mTORC1 signaling. Consistent with the finding in vitro, dietary supplementation with niacin increases milk fat and protein synthesis in mice with the activation of GPR109A-mTORC1 signaling. Collectively, GPR109A agonists promote the synthesis of milk fat and milk protein through the GPR109A/G_i/mTORC1 signaling pathway.

Shedding light on myopia by studying complete congenital stationary night blindness

Myopia is the most common eye disorder, caused by heterogeneous genetic and environmental factors. Rare progressive and stationary inherited retinal disorders are often associated with high myopia. Genes implicated in myopia encode proteins involved in a variety of biological processes including eye morphogenesis, extracellular matrix organization, visual perception, circadian rhythms, and retinal signaling. Differentially expressed genes (DEGs) identified in animal models mimicking myopia are helpful in suggesting candidate genes implicated in human myopia. Complete congenital stationary night blindness (cCSNB) in humans and animal models represents an ON-bipolar cell signal transmission defect and is also associated with high myopia. Thus, it represents also an interesting model to identify myopia-related genes, as well as disease mechanisms. While the origin of night blindness is molecularly well established, further research is needed to elucidate the mechanisms of myopia development in subjects with cCSNB. Using whole transcriptome analysis on three different mouse models of cCSNB (in Gpr179^-/-, Lrit3^-/- and Grm6^-/-), we identified novel actors of the retinal signaling cascade, which are also novel candidate genes for myopia. Meta-analysis of our transcriptomic data with published transcriptomic databases and genome-wide association studies from myopia cases led us to propose new biological/cellular processes/mechanisms potentially at the origin of myopia in cCSNB subjects. The results provide a foundation to guide the development of pharmacological myopia therapies.

Circadian regulation of dentate gyrus excitability mediated by G-protein signaling

The central circadian regulator within the suprachiasmatic nucleus transmits time of day information by a diurnal spiking rhythm driven by molecular clock genes controlling membrane excitability. Most brain regions, including the hippocampus, harbor similar intrinsic circadian transcriptional machinery, but whether these molecular programs generate oscillations of membrane properties is unclear. Here, we show that intrinsic excitability of mouse dentate granule neurons exhibits a 24-h oscillation that controls spiking probability. Diurnal changes in excitability are mediated by antiphase G-protein regulation of potassium and sodium currents that reduce excitability during the Light phase. Disruption of the circadian transcriptional machinery by conditional deletion of Bmal1 enhances excitability selectively during the Light phase by removing G-protein regulation. These results reveal that circadian transcriptional machinery regulates intrinsic excitability by coordinated regulation of ion channels by G-protein signaling, highlighting a potential novel mechanism of cell-autonomous oscillations.

LimoRhyde2: genomic analysis of biological rhythms based on effect sizes

Genome-scale data have revealed daily rhythms in various species and tissues. However, current methods to assess rhythmicity largely restrict their focus to quantifying statistical significance, which may not reflect biological relevance. To address this limitation, we developed a method called LimoRhyde2 (the successor to our method LimoRhyde), which focuses instead on rhythm-related effect sizes and their uncertainty. For each genomic feature, LimoRhyde2 fits a curve using a series of linear models based on periodic splines, moderates the fits using an Empirical Bayes approach called multivariate adaptive shrinkage (Mash), then uses the moderated fits to calculate rhythm statistics such as peak-to-trough amplitude. The periodic splines capture non-sinusoidal rhythmicity, while Mash uses patterns in the data to account for different fits having different levels of noise. To demonstrate LimoRhyde2's utility, we applied it to multiple circadian transcriptome datasets. Overall, LimoRhyde2 prioritized genes having high-amplitude rhythms in expression, whereas a prior method (BooteJTK) prioritized "statistically significant" genes whose amplitudes could be relatively small. Thus, quantifying effect sizes using approaches such as LimoRhyde2 has the potential to transform interpretation of genomic data related to biological rhythms.

Timed exercise stabilizes behavioral rhythms but not molecular programs in the brain's suprachiasmatic clock

Timed daily access to a running-wheel (scheduled voluntary exercise; SVE) synchronizes rodent circadian rhythms and promotes stable, 24h rhythms in animals with genetically targeted impairment of neuropeptide signaling (Vipr2 ^-/- mice). Here we used RNA-seq and/or qRT-PCR to assess how this neuropeptide signaling impairment as well as SVE shapes molecular programs in the brain clock (suprachiasmatic nuclei; SCN) and peripheral tissues (liver and lung). Compared to Vipr2 ^+/+ animals, the SCN transcriptome of Vipr2 ^-/- mice showed extensive dysregulation which included core clock components, transcription factors, and neurochemicals. Furthermore, although SVE stabilized behavioral rhythms in these animals, the SCN transcriptome remained dysregulated. The molecular programs in the lung and liver of Vipr2 ^-/- mice were partially intact, although their response to SVE differed to that of these peripheral tissues in the Vipr2 ^+/+ mice. These findings highlight that SVE can correct behavioral abnormalities in circadian rhythms without causing large scale alterations to the SCN transcriptome.

Network-driven intracellular cAMP coordinates circadian rhythm in the suprachiasmatic nucleus

The mammalian central circadian clock, located in the suprachiasmatic nucleus (SCN), coordinates the timing of physiology and behavior to local time cues. In the SCN, second messengers, such as cAMP and Ca²⁺, are suggested to be involved in the input and/or output of the molecular circadian clock. However, the functional roles of second messengers and their dynamics in the SCN remain largely unclear. In the present study, we visualized the spatiotemporal patterns of circadian rhythms of second messengers and neurotransmitter release in the SCN. Here, we show that neuronal activity regulates the rhythmic release of vasoactive intestinal peptides from the SCN, which drives the circadian rhythms of intracellular cAMP in the SCN. Furthermore, optical manipulation of intracellular cAMP levels in the SCN shifts molecular and behavioral circadian rhythms. Together, our study demonstrates that intracellular cAMP is a key molecule in the organization of the SCN circadian neuronal network.

A light-induced small G-protein gem limits the circadian clock phase-shift magnitude by inhibiting voltage-dependent calcium channels

Calcium signaling is pivotal to the circadian clockwork in the suprachiasmatic nucleus (SCN), particularly in rhythm entrainment to environmental light-dark cycles. Here, we show that a small G-protein Gem, an endogenous inhibitor of high-voltage-activated voltage-dependent calcium channels (VDCCs), is rapidly induced by light in SCN neurons via the calcium (Ca²⁺)-mediated CREB/CRE transcriptional pathway. Gem attenuates light-induced calcium signaling through its interaction with VDCCs. The phase-shift magnitude of locomotor activity rhythms by light, at night, increases in Gem-deficient (Gem^-/-) mice. Similarly, in SCN slices from Gem^-/- mice, depolarizing stimuli induce larger phase shifts of clock gene transcription rhythms that are normalized by the application of an L-type VDCC blocker, nifedipine. Voltage-clamp recordings from SCN neurons reveal that Ca²⁺ currents through L-type channels increase in Gem^-/- mice. Our findings suggest that transcriptionally activated Gem feeds back to suppress excessive light-evoked L-type VDCC activation, adjusting the light-induced phase-shift magnitude to an appropriate level in mammals.

Gpr19 is a circadian clock-controlled orphan GPCR with a role in modulating free-running period and light resetting capacity of the circadian clock

Gpr19 encodes an evolutionarily conserved orphan G-protein-coupled receptor (GPCR) with currently no established physiological role in vivo. We characterized Gpr19 expression in the suprachiasmatic nucleus (SCN), the locus of the master circadian clock in the brain, and determined its role in the context of the circadian rhythm regulation. We found that Gpr19 is mainly expressed in the dorsal part of the SCN, with its expression fluctuating in a circadian fashion. A conserved cAMP-responsive element in the Gpr19 promoter was able to produce circadian transcription in the SCN. Gpr19^−/− mice exhibited a prolonged circadian period and a delayed initiation of daily locomotor activity. Gpr19 deficiency caused the downregulation of several genes that normally peak during the night, including Bmal1 and Gpr176. In response to light exposure at night, Gpr19^−/− mice had a reduced capacity for light-induced phase-delays, but not for phase-advances. This defect was accompanied by reduced response of c-Fos expression in the dorsal region of the SCN, while apparently normal in the ventral area of the SCN, in Gpr19^−/− mice. Thus, our data demonstrate that Gpr19 is an SCN-enriched orphan GPCR with a distinct role in circadian regulation and may provide a potential target option for modulating the circadian clock.

Single-cell transcriptomics of suprachiasmatic nuclei reveal a Prokineticin-driven circadian network

Circadian rhythms in mammals are governed by the hypothalamic suprachiasmatic nucleus (SCN), in which 20,000 clock cells are connected together into a powerful time‐keeping network. In the absence of network‐level cellular interactions, the SCN fails as a clock. The topology and specific roles of its distinct cell populations (nodes) that direct network functions are, however, not understood. To characterise its component cells and network structure, we conducted single‐cell sequencing of SCN organotypic slices and identified eleven distinct neuronal sub‐populations across circadian day and night. We defined neuropeptidergic signalling axes between these nodes, and built neuropeptide‐specific network topologies. This revealed their temporal plasticity, being up‐regulated in circadian day. Through intersectional genetics and real‐time imaging, we interrogated the contribution of the Prok2‐ProkR2 neuropeptidergic axis to network‐wide time‐keeping. We showed that Prok2‐ProkR2 signalling acts as a key regulator of SCN period and rhythmicity and contributes to defining the network‐level properties that underpin robust circadian co‐ordination. These results highlight the diverse and distinct contributions of neuropeptide‐modulated communication of temporal information across the SCN.

Comprehensive Multi-omics Analysis Reveals Mitochondrial Stress as a Central Biological Hub for Spaceflight Impact

Spaceflight is known to impose changes on human physiology with unknown molecular etiologies. To reveal these causes, we used a multi-omics, systems biology analytical approach using biomedical profiles from fifty-nine astronauts and data from NASA's GeneLab derived from hundreds of samples flown in space to determine transcriptomic, proteomic, metabolomic, and epigenetic responses to spaceflight. Overall pathway analyses on the multi-omics datasets showed significant enrichment for mitochondrial processes, as well as innate immunity, chronic inflammation, cell cycle, circadian rhythm, and olfactory functions. Importantly, NASA's Twin Study provided a platform to confirm several of our principal findings. Evidence of altered mitochondrial function and DNA damage was also found in the urine and blood metabolic data compiled from the astronaut cohort and NASA Twin Study data, indicating mitochondrial stress as a consistent phenotype of spaceflight.

A multi-level assessment of the bidirectional relationship between aging and the circadian clock

The daily temporal order of physiological processes and behavior contribute to the wellbeing of many organisms including humans. The central circadian clock, which coordinates the timing within our body, is located in the suprachiasmatic nucleus (SCN) of the hypothalamus. Like in other parts of the brain, aging impairs the SCN function, which in turn promotes the development and progression of aging-related diseases. We here review the impact of aging on the different levels of the circadian clock machinery-from molecules to organs-with a focus on the role of the SCN. We find that the molecular clock is less effected by aging compared to other cellular components of the clock. Proper rhythmic regulation of intracellular signaling, ion channels and neuronal excitability of SCN neurons are greatly disturbed in aging. This suggests a disconnection between the molecular clock and the electrophysiology of these cells. The neuronal network of the SCN is able to compensate for some of these cellular deficits. However, it still results in a clear reduction in the amplitude of the SCN electrical rhythm, suggesting a weakening of the output timing signal. Consequently, other brain areas and organs not only show aging-related deficits in their own local clocks, but also receive a weaker systemic timing signal. The negative spiral completes with the weakening of positive feedback from the periphery to the SCN. Consequently, chronotherapeutic interventions should aim at strengthening overall synchrony in the circadian system using life-style and/or pharmacological approaches.

REV-ERB in GABAergic neurons controls diurnal hepatic insulin sensitivity

Systemic insulin sensitivity shows a diurnal rhythm with a peak upon waking1,2. The molecular mechanism that underlies this temporal pattern is unclear. Here we show that the nuclear receptors REV-ERB-α and REV-ERB-β (referred to here as 'REV-ERB') in the GABAergic (γ-aminobutyric acid-producing) neurons in the suprachiasmatic nucleus (SCN) (SCNGABA neurons) control the diurnal rhythm of insulin-mediated suppression of hepatic glucose production in mice, without affecting diurnal eating or locomotor behaviours during regular light-dark cycles. REV-ERB regulates the rhythmic expression of genes that are involved in neurotransmission in the SCN, and modulates the oscillatory firing activity of SCNGABA neurons. Chemogenetic stimulation of SCNGABA neurons at waking leads to glucose intolerance, whereas restoration of the temporal pattern of either SCNGABA neuron firing or REV-ERB expression rescues the time-dependent glucose metabolic phenotype caused by REV-ERB depletion. In individuals with diabetes, an increased level of blood glucose after waking is a defining feature of the 'extended dawn phenomenon'3,4. Patients with type 2 diabetes with the extended dawn phenomenon exhibit a differential temporal pattern of expression of REV-ERB genes compared to patients with type 2 diabetes who do not have the extended dawn phenomenon. These findings provide mechanistic insights into how the central circadian clock regulates the diurnal rhythm of hepatic insulin sensitivity, with implications for our understanding of the extended dawn phenomenon in type 2 diabetes.

Role of α2δ3 in Cellular Synchronization of the Suprachiasmatic Nucleus Under Constant Light Conditions

By the effort to identify candidate signaling molecules important for the formation of robust circadian rhythms in the suprachiasmatic nucleus (SCN), the mammalian circadian center, here we characterize the role of α2δ proteins, synaptic molecules initially identified as an auxiliary subunit of the voltage dependent calcium channel, in circadian rhythm formation. In situ hybridization study demonstrated that type 3 α2δ gene (α2δ3) was strongly expressed in the SCN. Mice without this isoform (Cacna2d3^-/-) did not maintain proper circadian locomotor activity rhythms under a constant light (LL) condition, whereas under a constant dark (DD) condition, these mice showed a similar period length and similar light-responsiveness as compared to wild type mice. Reflecting this behavioral phenotype, Cacna2d3^-/- mice showed a severely impaired Per1 expression rhythm in the SCN under LL, but not under DD. Cultured SCN slices from Per1-luc transgenic Cacna2d3^-/- mice revealed reduced synchrony of Per1-luc gene expression rhythms among SCN neurons. These findings suggest that α2δ3 is essential for synchronized cellular oscillations in the SCN and thereby contributes to enhancing the sustainability of circadian rhythms in behavior.

Light Input to the Mammalian Circadian Clock

Circadian rhythms are 24-h cycles in physiology and behavior that occur in virtually all organisms. These processes are not simply driven by changes in the external environment as they persist under constant conditions, providing evidence for an internal biological clock. In mammals, this clock is located in the hypothalamic suprachiasmatic nuclei (SCN) and is based upon an intracellular mechanism composed of a transcriptional-translational feedback loop composed of a number of core clock genes. However, a clock is of no use unless it can be set to the correct time. The primary time cue for the molecular clock in the SCN is light detected by the eye. The photoreceptors involved in this process include the rods and cones that mediate vision, as well as the recently identified melanopsin-expressing photosensitive retinal ganglion cells (pRGCs). Light information is conveyed to the SCN via the retinohypothalamic tract, resulting in an intracellular signaling cascade which converges on cAMP-response elements in the promoters of several key clock genes. Over the last two decades a number of studies have investigated the transcriptional response of the SCN to light stimuli with the aim of further understanding these molecular signaling pathways. Here we provide an overview of these studies and provide protocols for studying the molecular responses to light in the SCN clock.

Genomic perspectives on the circadian clock hypothesis of psychiatric disorders

Circadian rhythm disturbances are frequently described in psychiatric disorders such as major depressive disorder, bipolar disorder, and schizophrenia. Growing evidence suggests a biological connection between mental health and circadian rhythmicity, including the circadian influence on brain function and mood and the requirement for circadian entrainment by external factors, which is often impaired in mental illness. Mental (as well as physical) health is also adversely affected by circadian misalignment. The marked interindividual differences in this combined susceptibility, in addition to the phenotypic spectrum in traits related both to circadian rhythms and mental health, suggested the possibility of a shared genetic background and that circadian clock genes may also be candidate genes for psychiatric disorders. This hypothesis was further strengthened by observations in animal models where clock genes had been knocked out or mutated. The introduction of genome-wide association studies (GWAS) enabled hypothesis-free testing. GWAS analysis of chronotype confirmed the prominent role of circadian genes in these phenotypes and their extensive polygenicity. However, in GWAS on psychiatric traits, only one clock gene, ARNTL (BMAL1) was identified as one of the few loci differentiating bipolar disorder from schizophrenia, and macaque monkeys where the ARNTL gene has been knocked out display symptoms similar to schizophrenia. Another lesson from genomic analyses is that chronotype has an important genetic correlation with several psychiatric disorders and that this effect is unidirectional. We conclude that the effect of circadian disturbances on psychiatric disorders probably relates to modulation of rhythm parameters and extend beyond the core clock genes themselves.

Dual-Color Single-Cell Imaging of the Suprachiasmatic Nucleus Reveals a Circadian Role in Network Synchrony

The suprachiasmatic nucleus (SCN) acts as a master pacemaker driving circadian behavior and physiology. Although the SCN is small, it is composed of many cell types, making it difficult to study the roles of particular cells. Here we develop bioluminescent circadian reporter mice that are Cre dependent, allowing the circadian properties of genetically defined populations of cells to be studied in real time. Using a Color-Switch PER2::LUCIFERASE reporter that switches from red PER2::LUCIFERASE to green PER2::LUCIFERASE upon Cre recombination, we assess circadian rhythms in two of the major classes of peptidergic neurons in the SCN: AVP (arginine vasopressin) and VIP (vasoactive intestinal polypeptide). Surprisingly, we find that circadian function in AVP neurons, not VIP neurons, is essential for autonomous network synchrony of the SCN and stability of circadian rhythmicity.

A Global Map of G Protein Signaling Regulation by RGS Proteins

The control over the extent and timing of G protein signaling is provided by the regulator of G protein signaling (RGS) proteins that deactivate G protein α subunits (Gα). Mammalian genomes encode 20 canonical RGS and 16 Gα genes with key roles in physiology and disease. To understand the principles governing the selectivity of Gα regulation by RGS, we examine the catalytic activity of all canonical human RGS proteins and their selectivity for a complete set of Gα substrates using real-time kinetic measurements in living cells. The data reveal rules governing RGS-Gα recognition, the structural basis of its selectivity, and provide principles for engineering RGS proteins with defined selectivity. The study also explores the evolution of RGS-Gα selectivity through ancestral reconstruction and demonstrates how naturally occurring non-synonymous variants in RGS alter signaling. These results provide a blueprint for decoding signaling selectivity and advance our understanding of molecular recognition principles.

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Systematic analysis reveals G protein selectivity of all canonical RGS proteins

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RGS proteins rely on selectivity bar codes for selective G protein recognition

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Transplantation of bar codes across RGS proteins switches their G protein preferences

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Natural variants, mutations, and evolution shape RGS selectivity

It's about time: clocks in the developing lung

The discovery of peripheral intracellular clocks revealed circadian oscillations of clock genes and their targets in all cell types, including those in the lung, sparking exploration of clocks in lung disease pathophysiology. While the focus has been on the role of these clocks in adult airway diseases, clock biology is also likely to be important in perinatal lung development, where it has received far less attention. Historically, fetal circadian rhythms have been considered irrelevant owing to lack of external light exposure, but more recent insights into peripheral clock biology raise questions of clock emergence, its concordance with tissue-specific structure/function, the interdependence of clock synchrony and functionality in perinatal lung development, and the possibility of lung clocks in priming the fetus for postnatal life. Understanding the perinatal molecular clock may unravel mechanistic targets for chronic airway disease across the lifespan. With current research providing more questions than answers, it is about time to investigate clocks in the developing lung.

Time-Restricted G-Protein Signaling Pathways via GPR176, Gz, and RGS16 Set the Pace of the Master Circadian Clock in the Suprachiasmatic Nucleus

G-protein-coupled receptors (GPCRs) are an important source of drug targets with diverse therapeutic applications. However, there are still more than one hundred orphan GPCRs, whose ligands and functions remain unidentified. The suprachiasmatic nucleus (SCN) is the central circadian clock of the brain, directing daily rhythms in activity–rest behavior and physiology. Malfunction of the circadian clock has been linked to a wide variety of diseases, including sleep–wake disorders, obesity, diabetes, cancer, and hypertension, making the circadian clock an intriguing target for drug development. The orphan receptor GPR176 is an SCN-enriched orphan GPCR that sets the pace of the circadian clock. GPR176 undergoes asparagine (N)-linked glycosylation, a post-translational modification required for its proper cell-surface expression. Although its ligand remains unknown, this orphan receptor shows agonist-independent basal activity. GPR176 couples to the unique G-protein subclass G_z (or G_x) and participates in reducing cAMP production during the night. The regulator of G-protein signaling 16 (RGS16) is equally important for the regulation of circadian cAMP synthesis in the SCN. Genome-wide association studies, employing questionnaire-based evaluations of individual chronotypes, revealed loci near clock genes and in the regions containing RGS16 and ALG10B, a gene encoding an enzyme involved in protein N-glycosylation. Therefore, increasing evidence suggests that N-glycosylation of GPR176 and its downstream G-protein signal regulation may be involved in pathways characterizing human chronotypes. This review argues for the potential impact of focusing on GPCR signaling in the SCN for the purpose of fine-tuning the entire body clock.

The VIP-VPAC2 neuropeptidergic axis is a cellular pacemaking hub of the suprachiasmatic nucleus circadian circuit

The hypothalamic suprachiasmatic nuclei (SCN) are the principal mammalian circadian timekeeper, co-ordinating organism-wide daily and seasonal rhythms. To achieve this, cell-autonomous circadian timing by the ~20,000 SCN cells is welded into a tight circuit-wide ensemble oscillation. This creates essential, network-level emergent properties of precise, high-amplitude oscillation with tightly defined ensemble period and phase. Although synchronised, regional cell groups exhibit differentially phased activity, creating stereotypical spatiotemporal circadian waves of cellular activation across the circuit. The cellular circuit pacemaking components that generate these critical emergent properties are unknown. Using intersectional genetics and real-time imaging, we show that SCN cells expressing vasoactive intestinal polypeptide (VIP) or its cognate receptor, VPAC2, are neurochemically and electrophysiologically distinct, but together they control de novo rhythmicity, setting ensemble period and phase with circuit-level spatiotemporal complexity. The VIP/VPAC2 cellular axis is therefore a neurochemically and topologically specific pacemaker hub that determines the emergent properties of the SCN timekeeper.

Circadian activity modulation in the suprachiasmatic nucleus (SCN) is a network-level emergent property that requires neuropeptide VIP signaling, yet the precise cellular mechanisms are unknown. Patton et al. show that cells expressing VIP or its receptor VPAC2 together determine these emergent properties of the SCN.

GABAergic mechanisms in the suprachiasmatic nucleus that influence circadian rhythm

The mammalian central circadian clock is located in the suprachiasmatic nucleus (SCN) of the hypothalamus. The SCN contains multiple circadian oscillators which synchronize with each other via several neurotransmitters. Importantly, an inhibitory neurotransmitter, γ-amino butyric acid (GABA), is expressed in almost all SCN neurons. In this review, we discuss how GABA influences circadian rhythms in the SCN. Excitatory and inhibitory effects of GABA may depend on intracellular Cl^- concentration, in which several factors such as day-length, time of day, development, and region in the SCN may be involved. GABA also mediates oscillatory coupling of the circadian rhythms in the SCN. Recent genetic approaches reveal that GABA refines circadian output rhythms, but not circadian oscillations in the SCN. Since several efferent projections of the SCN have been suggested, GABA might work downstream of neuronal pathways from the SCN which regulate the temporal order of physiology and behavior.

Timekeeping in the hindbrain: a multi-oscillatory circadian centre in the mouse dorsal vagal complex

Metabolic and cardiovascular processes controlled by the hindbrain exhibit 24 h rhythms, but the extent to which the hindbrain possesses endogenous circadian timekeeping is unresolved. Here we provide compelling evidence that genetic, neuronal, and vascular activities of the brainstem’s dorsal vagal complex are subject to intrinsic circadian control with a crucial role for the connection between its components in regulating their rhythmic properties. Robust 24 h variation in clock gene expression in vivo and neuronal firing ex vivo were observed in the area postrema (AP) and nucleus of the solitary tract (NTS), together with enhanced nocturnal responsiveness to metabolic cues. Unexpectedly, we also find functional and molecular evidence for increased penetration of blood borne molecules into the NTS at night. Our findings reveal that the hindbrain houses a local network complex of neuronal and non-neuronal autonomous circadian oscillators, with clear implications for understanding local temporal control of physiology in the brainstem.

Lukasz Chrobok, Rebecca Northeast et al. show circadian variation in clock gene expression and neuronal firing within the area postrema and the nucleus of the solitary tract in mice. These regions also exhibit variation in metabolic processes and blood-brain barrier permeability across the 24 hour cycle suggesting the presence of circadian oscillators within the dorsal vagal complex.

Identification and functional characterisation of N-linked glycosylation of the orphan G protein-coupled receptor Gpr176

G-protein-coupled receptors (GPCRs) are important drug targets with diverse therapeutic applications. However, there are still more than a hundred orphan GPCRs, whose protein functions and biochemical features remain unidentified. Gpr176 encodes a class-A orphan GPCR that has a role in circadian clock regulation in mouse hypothalamus and is also implicated in human breast cancer transcriptional response. Here we show that Gpr176 is N-glycosylated. Peptide-N-glycosidase treatment of mouse hypothalamus extracts revealed that endogenous Gpr176 undergoes N-glycosylation. Using a heterologous expression system, we show that N-glycosylation occurs at four conserved asparagine residues in the N-terminal region of Gpr176. Deficient N-glycosylation due to mutation of these residues reduced the protein expression of Gpr176. At the molecular function level, Gpr176 has constitutive, agonist-independent activity that leads to reduced cAMP synthesis. Although deficient N-glycosylation did not compromise this intrinsic activity, the resultant reduction in protein expression was accompanied by attenuation of cAMP-repressive activity in the cells. We also demonstrate that human GPR176 is N-glycosylated. Importantly, missense variations in the conserved N-glycosylation sites of human GPR176 (rs1473415441; rs761894953) affected N-glycosylation and thereby attenuated protein expression and cAMP-repressive activity in the cells. We show that N-glycosylation is a prerequisite for the efficient protein expression of functional Gpr176/GPR176.

Circadian rhythms of sorting nexin 25 in the mouse suprachiasmatic nucleus

Entrainment of mammalian circadian rhythms requires receptor-mediated signaling in the hypothalamic suprachiasmatic nucleus (SCN), the site of the master circadian pacemaker. Receptor-mediated signaling is regulated by endocytosis, indicating that endocytosis-related proteins contribute to SCN pacemaking. Sorting nexin 25 (SNX25) belongs to the sorting nexin superfamily, whose members are responsible for membrane attachment to organelles of the endocytic system. In this study, we showed that Snx25 mRNA and SNX25 protein are highly expressed and exhibit remarkable circadian rhythms in the SCN of adult mice. Expression was maximal at about zeitgeber time (ZT) 16 in the subjective night and minimal at ZT8 in the subjective day. Prominent SNX25 immunoreactivity was found in the arginine vasopressin-positive neurons of the SCN. These findings suggest that SNX25 is a new actor in endocytic signaling, perhaps contributing to the circadian pacemaking system.

The Concept of Coupling in the Mammalian Circadian Clock Network

The circadian clock network regulates daily rhythms in mammalian physiology and behavior to optimally adapt the organism to the 24-h day/night cycle. A central pacemaker, the hypothalamic suprachiasmatic nucleus (SCN), coordinates subordinate cellular oscillators in the brain, as well as in peripheral organs to align with each other and external time. Stability and coordination of this vast network of cellular oscillators is achieved through different levels of coupling. Although coupling at the molecular level and across the SCN is well established and believed to define its function as pacemaker structure, the notion of coupling in other tissues and across the whole system is less well understood. In this review, we describe the different levels of coupling in the mammalian circadian clock system - from molecules to the whole organism. We highlight recent advances in gaining knowledge of the complex organization and function of circadian network regulation and its significance for the generation of stable but plastic intrinsic 24-h rhythms.

Generation of circadian rhythms in the suprachiasmatic nucleus

The suprachiasmatic nucleus (SCN) of the hypothalamus is remarkable. Despite numbering only about 10,000 neurons on each side of the third ventricle, the SCN is our principal circadian clock, directing the daily cycles of behaviour and physiology that set the tempo of our lives. When this nucleus is isolated in organotypic culture, its autonomous timing mechanism can persist indefinitely, with precision and robustness. The discovery of the cell-autonomous transcriptional and post-translational feedback loops that drive circadian activity in the SCN provided a powerful exemplar of the genetic specification of complex mammalian behaviours. However, the analysis of circadian time-keeping is moving beyond single cells. Technical and conceptual advances, including intersectional genetics, multidimensional imaging and network theory, are beginning to uncover the circuit-level mechanisms and emergent properties that make the SCN a uniquely precise and robust clock. However, much remains unknown about the SCN, not least the intrinsic properties of SCN neurons, its circuit topology and the neuronal computations that these circuits support. Moreover, the convention that the SCN is a neuronal clock has been overturned by the discovery that astrocytes are an integral part of the timepiece. As a test bed for examining the relationships between genes, cells and circuits in sculpting complex behaviours, the SCN continues to offer powerful lessons and opportunities for contemporary neuroscience.

Non-coding cis-element of Period2 is essential for maintaining organismal circadian behaviour and body temperature rhythmicity

Non-coding cis-regulatory elements are essential determinants of development, but their exact impacts on behavior and physiology in adults remain elusive. Cis-element-based transcriptional regulation is believed to be crucial for generating circadian rhythms in behavior and physiology. However, genetic evidence supporting this model is based on mutations in the protein-coding sequences of clock genes. Here, we report generation of mutant mice carrying a mutation only at the E'-box cis-element in the promoter region of the core clock gene Per2. The Per2 E'-box mutation abolishes sustainable molecular clock oscillations and renders circadian locomotor activity and body temperature rhythms unstable. Without the E'-box, Per2 messenger RNA and protein expression remain at mid-to-high levels. Our work delineates the Per2 E'-box as a critical nodal element for keeping sustainable cell-autonomous circadian oscillation and reveals the extent of the impact of the non-coding cis-element in daily maintenance of animal locomotor activity and body temperature rhythmicity.

Genetic Analysis of Rare Human Variants of Regulators of G Protein Signaling Proteins and Their Role in Human Physiology and Disease

Regulators of G protein signaling (RGS) proteins modulate the physiologic actions of many neurotransmitters, hormones, and other signaling molecules. Human RGS proteins comprise a family of 20 canonical proteins that bind directly to G protein-coupled receptors/G protein complexes to limit the lifetime of their signaling events, which regulate all aspects of cell and organ physiology. Genetic variations account for diverse human traits and individual predispositions to disease. RGS proteins contribute to many complex polygenic human traits and pathologies such as hypertension, atherosclerosis, schizophrenia, depression, addiction, cancers, and many others. Recent analysis indicates that most human diseases are due to extremely rare genetic variants. In this study, we summarize physiologic roles for RGS proteins and links to human diseases/traits and report rare variants found within each human RGS protein exome sequence derived from global population studies. Each RGS sequence is analyzed using recently described bioinformatics and proteomic tools for measures of missense tolerance ratio paired with combined annotation-dependent depletion scores, and protein post-translational modification (PTM) alignment cluster analysis. We highlight selected variants within the well-studied RGS domain that likely disrupt RGS protein functions and provide comprehensive variant and PTM data for each RGS protein for future study. We propose that rare variants in functionally sensitive regions of RGS proteins confer profound change-of-function phenotypes that may contribute, in newly appreciated ways, to complex human diseases and/or traits. This information provides investigators with a valuable database to explore variation in RGS protein function, and for targeting RGS proteins as future therapeutic targets.

Melatonin as a Hormone: New Physiological and Clinical Insights

Melatonin is a ubiquitous molecule present in almost every live being from bacteria to humans. In vertebrates, besides being produced in peripheral tissues and acting as an autocrine and paracrine signal, melatonin is centrally synthetized by a neuroendocrine organ, the pineal gland. Independently of the considered species, pineal hormone melatonin is always produced during the night and its production and secretory episode duration are directly dependent on the length of the night. As its production is tightly linked to the light/dark cycle, melatonin main hormonal systemic integrative action is to coordinate behavioral and physiological adaptations to the environmental geophysical day and season. The circadian signal is dependent on its daily production regularity, on the contrast between day and night concentrations, and on specially developed ways of action. During its daily secretory episode, melatonin coordinates the night adaptive physiology through immediate effects and primes the day adaptive responses through prospective effects that will only appear at daytime, when melatonin is absent. Similarly, the annual history of the daily melatonin secretory episode duration primes the central nervous/endocrine system to the seasons to come. Remarkably, maternal melatonin programs the fetuses' behavior and physiology to cope with the environmental light/dark cycle and season after birth. These unique ways of action turn melatonin into a biological time-domain-acting molecule. The present review focuses on the above considerations, proposes a putative classification of clinical melatonin dysfunctions, and discusses general guidelines to the therapeutic use of melatonin.

The Phosphorylation of CREB at Serine 133 Is a Key Event for Circadian Clock Timing and Entrainment in the Suprachiasmatic Nucleus

Within the suprachiasmatic nucleus (SCN)-the locus of the master circadian clock- transcriptional regulation via the CREB/CRE pathway is implicated in the functioning of the molecular clock timing process, and is a key conduit through which photic input entrains the oscillator. One event driving CRE-mediated transcription is the phosphorylation of CREB at serine 133 (Ser¹³³). Indeed, numerous reporter gene assays have shown that an alanine point mutation in Ser¹³³ reduces CREB-mediated transcription. Here, we sought to examine the contribution of Ser¹³³ phosphorylation to the functional role of CREB in SCN clock physiology in vivo. To this end, we used a CREB knock-in mouse strain, in which Ser¹³³ was mutated to alanine (S/A CREB). Under a standard 12 h light-dark cycle, S/A CREB mice exhibited a marked alteration in clock-regulated wheel running activity. Relative to WT mice, S/A CREB mice had highly fragmented bouts of locomotor activity during the night phase, elevated daytime activity, and a delayed phase angle of entrainment. Further, under free-running conditions, S/A CREB mice had a significantly longer tau than WT mice and reduced activity amplitude. In S/A CREB mice, light-evoked clock entrainment, using both Aschoff type 1 and 6 h "jet lag" paradigms, was markedly reduced relative to WT mice. S/A CREB mice exhibited attenuated transcriptional drive, as assessed by examining both clock-gated and light-evoked gene expression. Finally, SCN slice culture imaging detected a marked disruption in cellular clock phase synchrony following a phase-resetting stimulus in S/A CREB mice. Together, these data indicate that signaling through CREB phosphorylation at Ser¹³³ is critical for the functional fidelity of both SCN timing and entrainment.

Gαi3 signaling is associated with sexual dimorphic expression of the clock-controlled output gene Dbp in murine liver

The albumin D-box binding protein (DBP) is a member of the PAR bZip (proline and acidic amino acid-rich basic leucine zipper) transcription factor family and functions as important regulator of circadian core and output gene expression. Gene expression of DBP itself is under the control of E-box-dependent binding by the Bmal1-Clock heterodimer and CRE-dependent binding by the cAMP responsive element binding protein (CREB). However, the signaling mechanism mediating CREB-dependent regulation of DBP expression in the peripheral clock remains elusive. In this study, we examined the role of the GPCR (G-protein-coupled receptor)/Gα_i3 (Galphai3) controlled cAMP-CREB signaling pathway in the regulation of hepatic expression of core clock and clock-regulated genes, including Dbp. Analysis of circadian gene expression revealed that rhythmicity of hepatic transcript levels of the majority of core clock (including Per1) and clock-regulated genes were not affected by Gα_i3 deficiency. Consistently, the period length of primary Gα_i3 deficient tail fibroblasts expressing a Bmal1-Luciferase reporter was not affected. Interestingly, however, Gα_i3 deficient female but not male mice showed a tendentiously increased activation of CREB (nuclear pSer133-CREB) accompanied by an advanced peak in Dbp gene expression and elevated mRNA levels of the cytochrome P₄₅₀ family member Cyp3a11, a target gene of DBP. Accordingly, selective inhibition of CREB led to a strongly decreased expression of DBP and CYP3A4 (human Cyp3a11 homologue) in HepG2 liver cells. In summary, our data suggest that the Gα_i3-pCREB signalling pathway functions as a regulator of sexual-dimorphic expression of DBP and its xenobiotic target enzymes Cyp3a11/CYP3A4.

Circadian locomotor output cycles kaput affects the proliferation and migration of breast cancer cells by regulating the expression of E-cadherin via IQ motif containing GTPase activating protein 1

The circadian rhythm regulates numerous physiological activities, including sleep and wakefulness, behavior, immunity and metabolism. Previous studies have demonstrated that circadian rhythm disorder is associated with the occurrence of tumors. Responsible for regulating a number of functions, the Circadian locomotor output cycles kaput (Clock) gene is one of the core regulatory genes of circadian rhythm. The Clock gene has also been implicated in the occurrence and development of tumors in previously studies. The present study evaluated the role of the Clock gene in the proliferation and migration of mouse breast cancer 4T1 cells, and investigated its possible regulatory pathways and mechanisms. It was reported that downregulation of Clock facilitated the proliferation and migration of breast cancer cells. Further investigation revealed the involvement of IQ motif containing GTPase activating protein 1 (IQGAP1) protein expression in the Clock regulatory pathway, further influencing the expression of E-cadherin, a known proprietor of tumor cell migration and invasion. To the best of our knowledge, the present study is the first to report that Clock, acting through the regulation of the scaffolding protein IQGAP1, regulates the downstream expression of E-cadherin, thereby affecting tumor cell structure and motility. These results confirmed the role of Clock in breast cancer tumor etiology and provide insight regarding the molecular avenues of its regulatory nature, which may translate beyond breast cancer into other known functions of the gene.

Improving Reproducibility and Candidate Selection in Transcriptomics Using Meta-analysis

Transcriptomic experiments are often used in neuroscience to identify candidate genes of interest for further study. However, the lists of genes identified from comparable transcriptomic studies often show limited overlap. One approach to addressing this issue of reproducibility is to combine data from multiple studies in the form of a meta-analysis. Here, we discuss recent work in the field of circadian biology, where transcriptomic meta-analyses have been used to improve candidate gene selection. With the increasing availability of microarray and RNA-Seq data due to deposition in public databases, combined with freely available tools and code, transcriptomic meta-analysis provides an ideal example of how open data can benefit neuroscience research.

Neuronal oscillations on an ultra-slow timescale: daily rhythms in electrical activity and gene expression in the mammalian master circadian clockwork

Neuronal oscillations of the brain, such as those observed in the cortices and hippocampi of behaving animals and humans, span across wide frequency bands, from slow delta waves (0.1 Hz) to ultra-fast ripples (600 Hz). Here, we focus on ultra-slow neuronal oscillators in the hypothalamic suprachiasmatic nuclei (SCN), the master daily clock that operates on interlocking transcription-translation feedback loops to produce circadian rhythms in clock gene expression with a period of near 24 h (< 0.001 Hz). This intracellular molecular clock interacts with the cell's membrane through poorly understood mechanisms to drive the daily pattern in the electrical excitability of SCN neurons, exhibiting an up-state during the day and a down-state at night. In turn, the membrane activity feeds back to regulate the oscillatory activity of clock gene programs. In this review, we emphasise the circadian processes that drive daily electrical oscillations in SCN neurons, and highlight how mathematical modelling contributes to our increasing understanding of circadian rhythm generation, synchronisation and communication within this hypothalamic region and across other brain circuits.

CLOCKΔ19 mutation modifies the manner of synchrony among oscillation neurons in the suprachiasmatic nucleus

In mammals, the principal circadian oscillator exists in the hypothalamic suprachiasmatic nucleus (SCN). In the SCN, CLOCK works as an essential component of molecular circadian oscillation, and ClockΔ19 mutant mice show unique characteristics of circadian rhythms such as extended free running periods, amplitude attenuation, and high-magnitude phase-resetting responses. Here we investigated what modifications occur in the spatiotemporal organization of clock gene expression in the SCN of ClockΔ19 mutants. The cultured SCN, sampled from neonatal homozygous ClockΔ19 mice on an ICR strain comprising PERIOD2::LUCIFERASE, demonstrated that the Clock gene mutation not only extends the circadian period, but also affects the spatial phase and period distribution of circadian oscillations in the SCN. In addition, disruption of the synchronization among neurons markedly attenuated the amplitude of the circadian rhythm of individual oscillating neurons in the mutant SCN. Further, with numerical simulations based on the present studies, the findings suggested that, in the SCN of the ClockΔ19 mutant mice, stable oscillation was preserved by the interaction among oscillating neurons, and that the orderly phase and period distribution that makes a phase wave are dependent on the functionality of CLOCK.

Circadian clock regulates hepatic polyploidy by modulating Mkp1-Erk1/2 signaling pathway

Liver metabolism undergoes robust circadian oscillations in gene expression and enzymatic activity essential for liver homeostasis, but whether the circadian clock controls homeostatic self-renewal of hepatocytes is unknown. Here we show that hepatocyte polyploidization is markedly accelerated around the central vein, the site of permanent cell self-renewal, in mice deficient in circadian Period genes. In these mice, a massive accumulation of hyperpolyploid mononuclear and binuclear hepatocytes occurs due to impaired mitogen-activated protein kinase phosphatase 1 (Mkp1)-mediated circadian modulation of the extracellular signal-regulated kinase (Erk1/2) activity. Time-lapse imaging of hepatocytes suggests that the reduced activity of Erk1/2 in the midbody during cytokinesis results in abscission failure, leading to polyploidization. Manipulation of Mkp1 phosphatase activity is sufficient to change the ploidy level of hepatocytes. These data provide clear evidence that the Period genes not only orchestrate dynamic changes in metabolic activity, but also regulate homeostatic self-renewal of hepatocytes through Mkp1-Erk1/2 signaling pathway.

Circadian clock regulates hepatic gene expression and functions. Here Chao et al. show that alteration of circadian clock genes by Period deletion induces polyploidy in hepatocytes due to impaired regulation of Erk signaling by mitogen-activated protein kinase phosphatase 1.

Circadian Profiling of Amino Acids in the SCN and Cerebral Cortex by Laser Capture Microdissection-Mass Spectrometry

The suprachiasmatic nucleus (SCN) is an extremely robust self-sustained oscillator, containing virtually the same molecular clock present in other tissues in the body but, in addition, endowed with tight intercellular coupling dependent on multiple neurotransmitter systems that allow the SCN to function as the "master clock." Several studies on the circadian SCN transcriptome have been published and compared with the transcriptome of other tissues, but the recent focus shift toward the circadian metabolome and the importance of small molecules for circadian timekeeping has so far been limited to macroscopic tissues such as the liver. Here, we report the successful use of laser capture microdissection coupled with liquid chromatography/tandem mass spectrometry for the circadian profiling of SCN amino acids. Among 18 amino acids detected, 10 (55.5%) showed significant variations, particularly marked for proline, lysine, and histidine, with higher levels during the subjective day. Moreover, we compared SCN and cortical amino acid levels between wild-type and Bmal1-deficient animals, either in the whole body or specifically in the liver. Interestingly, lack of Bmal1 in the whole body led to a significant increase in most amino acids in the SCN but not in the cerebral cortex. In contrast, deletion of Bmal1 in the liver mostly affected cortical amino acid levels during the subjective day. This study demonstrates that laser capture microdissection can be used for the isolation of microscopic brain structures for metabolomic purposes and reveals interactions between liver and SCN amino acid metabolism.

Meta-analysis of transcriptomic datasets identifies genes enriched in the mammalian circadian pacemaker

The master circadian pacemaker in mammals is located in the suprachiasmatic nuclei (SCN) which regulate physiology and behaviour, as well as coordinating peripheral clocks throughout the body. Investigating the function of the SCN has often focused on the identification of rhythmically expressed genes. However, not all genes critical for SCN function are rhythmically expressed. An alternative strategy is to characterize those genes that are selectively enriched in the SCN. Here, we examined the transcriptome of the SCN and whole brain (WB) of mice using meta-analysis of publicly deposited data across a range of microarray platforms and RNA-Seq data. A total of 79 microarrays were used (24 SCN and 55 WB samples, 4 different microarray platforms), alongside 17 RNA-Seq data files (7 SCN and 10 WB). 31 684 MGI gene symbols had data for at least one platform. Meta-analysis using a random effects model for weighting individual effect sizes (derived from differential expression between relevant SCN and WB samples) reliably detected known SCN markers. SCN-enriched transcripts identified in this study provide novel insights into SCN function, including identifying genes which may play key roles in SCN physiology or provide SCN-specific drivers.

Genome-Wide Screen for Genes Involved in Caenorhabditis elegans Developmentally Timed Sleep

In Caenorhabditis elegans, Notch signaling regulates developmentally timed sleep during the transition from L4 larval stage to adulthood (L4/A) . To identify core sleep pathways and to find genes acting downstream of Notch signaling, we undertook the first genome-wide, classical genetic screen focused on C. elegans developmentally timed sleep. To increase screen efficiency, we first looked for mutations that suppressed inappropriate anachronistic sleep in adult hsp::osm-11 animals overexpressing the Notch coligand OSM-11 after heat shock. We retained suppressor lines that also had defects in L4/A developmentally timed sleep, without heat shock overexpression of the Notch coligand. Sixteen suppressor lines with defects in developmentally timed sleep were identified. One line carried a new allele of goa-1; loss of GOA-1 Gαo decreased C. elegans sleep. Another line carried a new allele of gpb-2, encoding a Gβ5 protein; Gβ5 proteins have not been previously implicated in sleep. In other scenarios, Gβ5 GPB-2 acts with regulators of G protein signaling (RGS proteins) EAT-16 and EGL-10 to terminate either EGL-30 Gαq signaling or GOA-1 Gαo signaling, respectively. We found that loss of Gβ5 GPB-2 or RGS EAT-16 decreased L4/A sleep. By contrast, EGL-10 loss had no impact. Instead, loss of RGS-1 and RGS-2 increased sleep. Combined, our results suggest that, in the context of L4/A sleep, GPB-2 predominantly acts with EAT-16 RGS to inhibit EGL-30 Gαq signaling. These results confirm the importance of G protein signaling in sleep and demonstrate that these core sleep pathways function genetically downstream of the Notch signaling events promoting sleep.

G-protein-coupled receptor signaling through Gpr176, Gz, and RGS16 tunes time in the center of the circadian clock [Review]

G-protein-coupled receptors (GPCRs) constitute an immensely important class of drug targets with diverse clinical applications. There are still more than 120 orphan GPCRs whose cognate ligands and physiological functions are not known. A set of circadian pacemaker neurons that governs daily rhythms in behavior and physiology resides in the suprachiasmatic nucleus (SCN) in the brain. Malfunction of the circadian clock has been linked to a multitude of diseases, such as sleeping disorders, obesity, diabetes, cardiovascular diseases, and cancer, which makes the clock an attractive target for drug development. Here, we review a recently identified role of Gpr176 in the SCN. Gpr176 is an SCN-enriched orphan GPCR that sets the pace of the circadian clock in the SCN. Even without known ligand, this orphan receptor has an agonist-independent basal activity to reduce cAMP signaling. A unique cAMP-repressing G-protein subclass Gz is required for the activity of Gpr176. We also provide an overview on the circadian regulation of G-protein signaling, with an emphasis on a role for the regulator of G-protein signaling 16 (RGS16). RGS16 is indispensable for the circadian regulation of cAMP in the SCN. Developing drugs that target the SCN remains an unfulfilled opportunity for the circadian pharmacology. This review argues for the potential impact of focusing on GPCRs in the SCN for the purpose of tuning the body clock.

Membrane Currents, Gene Expression, and Circadian Clocks

Neuronal circadian oscillators in the mammalian and Drosophila brain express a circadian clock comprised of interlocking gene transcription feedback loops. The genetic clock regulates the membrane electrical activity by poorly understood signaling pathways to generate a circadian pattern of action potential firing. During the day, Na⁺ channels contribute an excitatory drive for the spontaneous activity of circadian clock neurons. Multiple types of K⁺ channels regulate the action potential firing pattern and the nightly reduction in neuronal activity. The membrane electrical activity possibly signaling by changes in intracellular Ca²⁺ and cyclic adenosine monophosphate (cAMP) regulates the activity of the gene clock. A decline in the signaling pathways that link the gene clock and neural activity during aging and disease may weaken the circadian output and generate significant impacts on human health.