A calcium flux is required for circadian rhythm generation in mammalian pacemaker neurons

Understanding the intricacies of cellular mechanisms in remyelination: The role of circadian rhythm

The term "circadian rhythm" refers to the 24-h oscillations found in various physiological processes in organisms, responsible for maintaining bodily homeostasis. Many neurological diseases mainly involve the process of demyelination, and remyelination is crucial for the treatment of neurological diseases. Current research mainly focuses on the key role of circadian clocks in the pathophysiological mechanisms of multiple sclerosis. Various studies have shown that the circadian rhythm regulates various cellular molecular mechanisms and signaling pathways involved in remyelination. The process of remyelination is primarily mediated by oligodendrocyte precursor cells (OPCs), oligodendrocytes, microglia, and astrocytes. OPCs are activated, proliferate, migrate, and ultimately differentiate into oligodendrocytes after demyelination, involving many key signaling pathway and regulatory factors. Activated microglia secretes important cytokines and chemokines, promoting OPC proliferation and differentiation, and phagocytoses myelin debris that inhibits remyelination. Astrocytes play a crucial role in supporting remyelination by secreting signals that promote remyelination or facilitate the phagocytosis of myelin debris by microglia. Additionally, cell-to-cell communication via gap junctions allows for intimate contact between astrocytes and oligodendrocytes, providing metabolic support for oligodendrocytes. Therefore, gaining a deeper understanding of the mechanisms and molecular pathways of the circadian rhythm at various stages of remyelination can help elucidate the fundamental characteristics of remyelination and provide insights into treating demyelinating disorders.

Circadian rhythm disruption and endocrine-related tumors

This review delved into the intricate relationship between circadian clocks and physiological processes, emphasizing their critical role in maintaining homeostasis. Orchestrated by interlocked clock genes, the circadian timekeeping system regulates fundamental processes like the sleep-wake cycle, energy metabolism, immune function, and cell proliferation. The central oscillator in the hypothalamic suprachiasmatic nucleus synchronizes with light-dark cycles, while peripheral tissue clocks are influenced by cues such as feeding times. Circadian disruption, linked to modern lifestyle factors like night shift work, correlates with adverse health outcomes, including metabolic syndrome, cardiovascular diseases, infections, and cancer. We explored the molecular mechanisms of circadian clock genes and their impact on metabolic disorders and cancer pathogenesis. Specific associations between circadian disruption and endocrine tumors, spanning breast, ovarian, testicular, prostate, thyroid, pituitary, and adrenal gland cancers, are highlighted. Shift work is associated with increased breast cancer risk, with PER genes influencing tumor progression and drug resistance. CLOCK gene expression correlates with cisplatin resistance in ovarian cancer, while factors like aging and intermittent fasting affect prostate cancer. Our review underscored the intricate interplay between circadian rhythms and cancer, involving the regulation of the cell cycle, DNA repair, metabolism, immune function, and the tumor microenvironment. We advocated for integrating biological timing into clinical considerations for personalized healthcare, proposing that understanding these connections could lead to novel therapeutic approaches. Evidence supports circadian rhythm-focused therapies, particularly chronotherapy, for treating endocrine tumors. Our review called for further research to uncover detailed connections between circadian clocks and cancer, providing essential insights for targeted treatments. We emphasized the importance of public health interventions to mitigate lifestyle-related circadian disruptions and underscored the critical role of circadian rhythms in disease mechanisms and therapeutic interventions.

The interplay between the PI3K/AKT pathway and circadian clock in physiologic and cancer-related pathologic conditions

The circadian clock is responsible for the regulation of different cellular processes, and its disturbance has been linked to the development of different diseases, such as cancer. The main molecular mechanism for this issue has been linked to the crosstalk between core clock regulators and intracellular pathways responsible for cell survival. The PI3K/AKT signalling pathway is one of the most known intracellular pathways in the case of cancer initiation and progression. This pathway regulates different aspects of cell survival including proliferation, apoptosis, metabolism, and response to environmental stimuli. Accumulating evidence indicates that there is a link between the PI3K/AKT pathway activity and circadian rhythm in physiologic and cancer-related pathogenesis. Different classes of PI3Ks and AKT isoforms are involved in regulating circadian clock components in a transcriptional and functional manner. Reversely, core clock components induce a rhythmic fashion in PI3K and AKT activity in physiologic and pathogenic conditions. The aim of this review is to re-examine the interplay between this pathway and circadian clock components in normal condition and cancer pathogenesis, which provides a better understanding of how circadian rhythms may be involved in cancer progression.

Combined use of multiparametric high-content-screening and in vitro circadian reporter assays in neurotoxicity evaluation

Accumulating evidence indicates that chronic circadian rhythm disruption is associated with the development of neurodegenerative diseases induced by exposure to neurotoxic chemicals. Herein, we examined the relationship between cellular circadian rhythm disruption and cytotoxicity in neural cells. Moreover, we evaluated the potential application of an in vitro cellular circadian rhythm assay in determining circadian rhythm disruption as a sensitive and early marker of neurotoxicant-induced adverse effects. To explore these objectives, we established an in vitro cellular circadian rhythm assay using human glioblastoma (U87 MG) cells stably transfected with a circadian reporter vector (PER2-dLuc) and determined the lowest-observed-adverse-effect levels (LOAELs) of several common neurotoxicants. Additionally, we determined the LOAEL of each compound on multiple cytotoxicity endpoints (nuclear size [NC], mitochondrial membrane potential [MMP], calcium ions, or lipid peroxidation) using a multiparametric high-content screening (HCS) assay using transfected U87 MG cells treated with the same neurotoxicants for 24 and 72 h. Based on our findings, the LOAEL for cellular circadian rhythm disruption for most chemicals was slightly higher than that for most cytotoxicity indicators detected using HCS, and the LOAEL for MMP in the first 24 h was the closest to that for cellular circadian rhythm disruption. Dietary antioxidants (methylselenocysteine and N-acetyl-l-cysteine) prevented or restored neurotoxicant-induced cellular circadian rhythm disruption. Our results suggest that cellular circadian rhythm disruption is as sensitive as cytotoxicity indicators and occurs early as much as cytotoxic events during disease development. Moreover, the in vitro cellular circadian rhythm assay warrants further evaluation as an early screening tool for neurotoxicants.

Circadian Rhythm Regulation by Pacemaker Neuron Chloride Oscillation in Flies

Circadian rhythms in physiology and behavior sync organisms to external environmental cycles. Here, circadian oscillation in intracellular chloride in central pacemaker neurons of the fly, Drosophila melanogaster, is reviewed. Intracellular chloride links SLC12 cation-coupled chloride transporter function with kinase signaling and the regulation of inwardly rectifying potassium channels.

The Suprachiasmatic Nucleus at 50: Looking Back, Then Looking Forward

It has been 50 years since the suprachiasmatic nucleus (SCN) was first identified as the central circadian clock and 25 years since the last overview of developments in the field was published in the Journal of Biological Rhythms. Here, we explore new mechanisms and concepts that have emerged in the subsequent 25 years. Since 1997, methodological developments, such as luminescent and fluorescent reporter techniques, have revealed intricate relationships between cellular and network-level mechanisms. In particular, specific neuropeptides such as arginine vasopressin, vasoactive intestinal peptide, and gastrin-releasing peptide have been identified as key players in the synchronization of cellular circadian rhythms within the SCN. The discovery of multiple oscillators governing behavioral and physiological rhythms has significantly advanced our understanding of the circadian clock. The interaction between neurons and glial cells has been found to play a crucial role in regulating these circadian rhythms within the SCN. Furthermore, the properties of the SCN network vary across ontogenetic stages. The application of cell type-specific genetic manipulations has revealed components of the functional input-output system of the SCN and their correlation with physiological functions. This review concludes with the high-risk effort of identifying open questions and challenges that lie ahead.

A PDZ scaffolding/CaM-mediated pathway in Cryptochrome signaling

Cryptochromes are cardinal constituents of the circadian clock, which orchestrates daily physiological rhythms in living organisms. A growing body of evidence points to their participation in pathways that have not traditionally been associated with circadian clock regulation, implying that cryptochromes may be subject to modulation by multiple signaling mechanisms. In this study, we demonstrate that human CRY2 (hCRY2) forms a complex with the large, modular scaffolding protein known as Multi-PDZ Domain Protein 1 (MUPP1). This interaction is facilitated by the calcium-binding protein Calmodulin (CaM) in a calcium-dependent manner. Our findings suggest a novel cooperative mechanism for the regulation of mammalian cryptochromes, mediated by calcium ions (Ca2+ ) and CaM. We propose that this Ca2+ /CaM-mediated signaling pathway may be an evolutionarily conserved mechanism that has been maintained from Drosophila to mammals, most likely in relation to its potential role in the broader context of cryptochrome function and regulation. Further, the understanding of cryptochrome interactions with other proteins and signaling pathways could lead to a better definition of its role within the intricate network of molecular interactions that govern circadian rhythms.

Contribution of membrane-associated oscillators to biological timing at different timescales

Environmental rhythms such as the daily light-dark cycle selected for endogenous clocks. These clocks predict regular environmental changes and provide the basis for well-timed adaptive homeostasis in physiology and behavior of organisms. Endogenous clocks are oscillators that are based on positive feedforward and negative feedback loops. They generate stable rhythms even under constant conditions. Since even weak interactions between oscillators allow for autonomous synchronization, coupling/synchronization of oscillators provides the basis of self-organized physiological timing. Amongst the most thoroughly researched clocks are the endogenous circadian clock neurons in mammals and insects. They comprise nuclear clockworks of transcriptional/translational feedback loops (TTFL) that generate ∼24 h rhythms in clock gene expression entrained to the environmental day-night cycle. It is generally assumed that this TTFL clockwork drives all circadian oscillations within and between clock cells, being the basis of any circadian rhythm in physiology and behavior of organisms. Instead of the current gene-based hierarchical clock model we provide here a systems view of timing. We suggest that a coupled system of autonomous TTFL and posttranslational feedback loop (PTFL) oscillators/clocks that run at multiple timescales governs adaptive, dynamic homeostasis of physiology and behavior. We focus on mammalian and insect neurons as endogenous oscillators at multiple timescales. We suggest that neuronal plasma membrane-associated signalosomes constitute specific autonomous PTFL clocks that generate localized but interlinked oscillations of membrane potential and intracellular messengers with specific endogenous frequencies. In each clock neuron multiscale interactions of TTFL and PTFL oscillators/clocks form a temporally structured oscillatory network with a common complex frequency-band comprising superimposed multiscale oscillations. Coupling between oscillator/clock neurons provides the next level of complexity of an oscillatory network. This systemic dynamic network of molecular and cellular oscillators/clocks is suggested to form the basis of any physiological homeostasis that cycles through dynamic homeostatic setpoints with a characteristic frequency-band as hallmark. We propose that mechanisms of homeostatic plasticity maintain the stability of these dynamic setpoints, whereas Hebbian plasticity enables switching between setpoints via coupling factors, like biogenic amines and/or neuropeptides. They reprogram the network to a new common frequency, a new dynamic setpoint. Our novel hypothesis is up for experimental challenge.

Misalignment of Circadian Rhythms in Diet-Induced Obesity

The biological clocks of the circadian timing system coordinate cellular and physiological processes and synchronize them with daily cycles. While the central clock in the suprachiasmatic nucleus (SCN) is mainly synchronized by the light/dark cycles, the peripheral clocks react to other stimuli, including the feeding/fasting state, nutrients, sleep-wake cycles, and physical activity. During the disruption of circadian rhythms due to genetic mutations or social and occupational obligations, incorrect arrangement between the internal clock system and environmental rhythms leads to the development of obesity. Desynchronization between the central and peripheral clocks by altered timing of food intake and diet composition leads to uncoupling of the peripheral clocks from the central pacemaker and to the development of metabolic disorders. The strong coupling of the SCN to the light-dark cycle creates a situation of misalignment when food is ingested during the "wrong" time of day. Food-anticipatory activity is mediated by a self-sustained circadian timing, and its principal component is a food-entrainable oscillator. Modifying the time of feeding alone greatly affects body weight, whereas ketogenic diet (KD) influences circadian biology, through the modulation of clock gene expression. Night-eating behavior is one of the causes of circadian disruption, and night eaters have compulsive and uncontrolled eating with severe obesity. By contrast, time-restricted eating (TRE) restores circadian rhythms through maintaining an appropriate daily rhythm of the eating-fasting cycle. The hypothalamus has a crucial role in the regulation of energy balance rather than food intake. While circadian locomotor output cycles kaput (CLOCK) expression levels increase with high-fat diet-induced obesity, peroxisome proliferator-activated receptor-alpha (PPARα) increases the transcriptional level of brain and muscle aryl hydrocarbon receptor nuclear translocator (ARNT)-like 1 (BMAL1) in obese subjects. In this context, effective timing of chronotherapies aiming to correct SCN-driven rhythms depends on an accurate assessment of the SCN phase. In fact, in a multi-oscillator system, local rhythmicity and its disruption reflects the disruption of either local clocks or central clocks, thus imposing rhythmicity on those local tissues, whereas misalignment of peripheral oscillators is due to exosome-based intercellular communication.Consequently, disruption of clock genes results in dyslipidemia, insulin resistance, and obesity, while light exposure during the daytime, food intake during the daytime, and sleeping during the biological night promote circadian alignment between the central and peripheral clocks. Thus, shift work is associated with an increased risk of obesity, diabetes, and cardiovascular diseases because of unusual eating times as well as unusual light exposure and disruption of the circadian rhythm.

In-phasic cytosolic-nuclear Ca2+ rhythms in suprachiasmatic nucleus neurons

The suprachiasmatic nucleus (SCN) of the hypothalamus is the master circadian clock in mammals. SCN neurons exhibit circadian Ca2+ rhythms in the cytosol, which is thought to act as a messenger linking the transcriptional/translational feedback loop (TTFL) and physiological activities. Transcriptional regulation occurs in the nucleus in the TTFL model, and Ca2+-dependent kinase regulates the clock gene transcription. However, the Ca2+ regulatory mechanisms between cytosol and nucleus as well as the ionic origin of Ca2+ rhythms remain unclear. In the present study, we monitored circadian-timescale Ca2+ dynamics in the nucleus and cytosol of SCN neurons at the single-cell and network levels. We observed robust nuclear Ca2+ rhythm in the same phase as the cytosolic rhythm in single SCN neurons and entire regions. Neuronal firing inhibition reduced the amplitude of both nuclear and cytosolic Ca2+ rhythms, whereas blocking of Ca2+ release from the endoplasmic reticulum (ER) via ryanodine and inositol 1,4,5-trisphosphate (IP3) receptors had a minor effect on either Ca2+ rhythms. We conclude that the in-phasic circadian Ca2+ rhythms in the cytosol and nucleus are mainly driven by Ca2+ influx from the extracellular space, likely through the nuclear pore. It also raises the possibility that nuclear Ca2+ rhythms directly regulate transcription in situ.

Immediate responses to ambient light in vivo reveal distinct subpopulations of suprachiasmatic VIP neurons

The circadian rhythm pacemaker, the suprachiasmatic nucleus (SCN), mediates light entrainment via vasoactive intestinal peptide (VIP) neurons (SCNVIP). Yet, how these neurons uniquely respond and connect to intrinsically photosensitive retinal ganglion cells (ipRGCs) expressing melanopsin (Opn4) has not been determined functionally in freely behaving animals. To address this, we first used monosynaptic tracing from SCNVIP neurons in mice and identified two SCNVIP subpopulations. Second, we recorded calcium changes in response to ambient light, at both bulk and single-cell levels, and found two unique activity patterns in response to high- and low-intensity blue light. The activity patterns of both subpopulations could be manipulated by application of an Opn4 antagonist. These results suggest that the two SCNVIP subpopulations connect to two types of Opn4-expressing ipRGCs, likely M1 and M2, but only one is responsive to red light. These findings have important implications for our basic understanding of non-image-forming circadian light processing.

In vivo recording of the circadian calcium rhythm in Prokineticin 2 neurons of the suprachiasmatic nucleus

Prokineticin 2 (Prok2) is a small protein expressed in a subpopulation of neurons in the suprachiasmatic nucleus (SCN), the primary circadian pacemaker in mammals. Prok2 has been implicated as a candidate output molecule from the SCN to control multiple circadian rhythms. Genetic manipulation specific to Prok2-producing neurons would be a powerful approach to understanding their function. Here, we report the generation of Prok2-tTA knock-in mice expressing the tetracycline transactivator (tTA) specifically in Prok2 neurons and an application of these mice to in vivo recording of Ca2+ rhythms in these neurons. First, the specific and efficient expression of tTA in Prok2 neurons was verified by crossing the mice with EGFP reporter mice. Prok2-tTA mice were then used to express a fluorescent Ca2+ sensor protein to record the circadian Ca2+ rhythm in SCN Prok2 neurons in vivo. Ca2+ in these cells showed clear circadian rhythms in both light-dark and constant dark conditions, with their peaks around midday. Notably, the hours of high Ca2+ nearly coincided with the rest period of the behavioral rhythm. These observations fit well with the predicted function of Prok2 neurons as a candidate output pathway of the SCN by suppressing locomotor activity during both daytime and subjective daytime.

Aging affects GABAergic function and calcium homeostasis in the mammalian central clock

Introduction

Aging impairs the function of the central circadian clock in mammals, the suprachiasmatic nucleus (SCN), leading to a reduction in the output signal. The weaker timing signal from the SCN results in a decline in rhythm strength in many physiological functions, including sleep-wake patterns. Accumulating evidence suggests that the reduced amplitude of the SCN signal is caused by a decreased synchrony among the SCN neurons. The present study was aimed to investigate the hypothesis that the excitation/inhibition (E/I) balance plays a role in synchronization within the network.

Methods

Using calcium (Ca²⁺) imaging, the polarity of Ca²⁺ transients in response to GABA stimulation in SCN slices of old mice (20-24 months) and young controls was studied.

Results

We found that the amount of GABAergic excitation was increased, and that concordantly the E/I balance was higher in SCN slices of old mice when compared to young controls. Moreover, we showed an effect of aging on the baseline intracellular Ca²⁺ concentration, with higher Ca²⁺ levels in SCN neurons of old mice, indicating an alteration in Ca²⁺ homeostasis in the aged SCN. We conclude that the change in GABAergic function, and possibly the Ca²⁺ homeostasis, in SCN neurons may contribute to the altered synchrony within the aged SCN network.

Ion dynamics and the regulation of circadian cellular physiology

Circadian rhythms in physiology and behavior allow organisms to anticipate the daily environmental changes imposed by the rotation of our planet around its axis. Although these rhythms eventually manifest at the organismal level, a cellular basis for circadian rhythms has been demonstrated. Significant contributors to these cell-autonomous rhythms are daily cycles in gene expression and protein translation. However, recent data revealed cellular rhythms in other biological processes, including ionic currents, ion transport, and cytosolic ion abundance. Circadian rhythms in ion currents sustain circadian variation in action potential firing rate, which coordinates neuronal behavior and activity. Circadian regulation of metal ions abundance and dynamics is implicated in distinct cellular processes, from protein translation to membrane activity and osmotic homeostasis. In turn, studies showed that manipulating ion abundance affects the expression of core clock genes and proteins, suggestive of a close interplay. However, the relationship between gene expression cycles, ion dynamics, and cellular function is still poorly characterized. In this review, I will discuss the mechanisms that generate ion rhythms, the cellular functions they govern, and how they feed back to regulate the core clock machinery.

Using bioluminescence to image gene expression and spontaneous behavior in freely moving mice

Bioluminescence imaging (BLI) of gene expression in live animals is a powerful method for monitoring development, tumor growth, infections, healing, and other progressive, long-term biological processes. BLI remains an effective approach for reducing the number of animals needed to monitor dynamic changes in gene activity because images can be captured repeatedly from the same animals. When examining these ongoing changes, it is sometimes necessary to remove rhythmic effects on the bioluminescence signal caused by the circadian clock's daily modulation of gene expression. Furthermore, BLI using freely moving animals remains limited because the standard procedures can alter normal behaviors. Another obstacle with conventional BLI of animals is that luciferin, the firefly luciferase substrate, is usually injected into mice that are then imaged while anesthetized. Unfortunately, the luciferase signal declines rapidly during imaging as luciferin is cleared from the body. Alternatively, mice are imaged after they are surgically implanted with a pump or connected to a tether to deliver luciferin, but stressors such as this surgery and anesthesia can alter physiology, behavior, and the actual gene expression being imaged. Consequently, we developed a strategy that minimizes animal exposure to stressors before and during sustained BLI of freely moving unanesthetized mice. This technique was effective when monitoring expression of the Per1 gene that serves in the circadian clock timing mechanism and was previously shown to produce circadian bioluminescence rhythms in live mice. We used hairless albino mice expressing luciferase that were allowed to drink luciferin and engage in normal behaviors during imaging with cooled electron-multiplying-CCD cameras. Computer-aided image selection was developed to measure signal intensity of individual mice each time they were in the same posture, thereby providing comparable measurements over long intervals. This imaging procedure, performed primarily during the animal's night, is compatible with entrainment of the mouse circadian timing system to the light cycle while allowing sampling at multi-day intervals to monitor long-term changes. When the circadian expression of a gene is known, this approach provides an effective alternative to imaging immobile anesthetized animals and can removing noise caused by circadian oscillations and body movements that can degrade data collected during long-term imaging studies.

Multiple calcium signaling genes play a role in the circadian period of Neurospora crassa

The Ca2+ signaling genes cpe-1, plc-1, ncs-1, splA2, camk-1, camk-2, camk-3, camk-4, cmd, and cnb-1 are necessary for a normal circadian period length in Neurospora crassa. In addition, the Q10 values ranged between 0.8 and 1.2 for the single mutants lacking cpe-1, splA2, camk-1, camk-2, camk-3, camk-4, and cnb-1, suggesting that the circadian clock exhibits standard temperature compensation. However, the Q10 value for the ∆plc-1 mutant was 1.41 at 25 and 30 °C, 1.53 and 1.40 for the ∆ncs-1 mutant at 20 and 25 °C, and at 20 and 30 °C, respectively, suggesting a partial loss of temperature compensation in these two mutants. Moreover, expression of frq, a regulator of the circadian period, and the blue light receptor wc-1, were increased >2-fold in the Δplc-1, ∆plc-1; ∆cpe-1, and the ∆plc-1; ∆splA2 mutants at 20 °C. The frq mRNA level was increased >2-fold in the Δncs-1 mutant compared to the ras-1bd strain at 20 °C. Therefore, multiple Ca2+ signaling genes regulate the circadian period, by influencing expression of the frq and wc-1 genes that are critical for maintaining the normal circadian period length in N. crassa.

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.

Neural function of Bmal1: an overview

Bmal1 (Brain and muscle arnt-like, or Arntl) is a bHLH/PAS domain transcription factor central to the transcription/translation feedback loop of the biologic clock. Although Bmal1 is well-established as a major regulator of circadian rhythm, a growing number of studies in recent years have shown that dysfunction of Bmal1 underlies a variety of psychiatric, neurodegenerative-like, and endocrine metabolism-related disorders, as well as potential oncogenic roles. In this review, we systematically summarized Bmal1 expression in different brain regions, its neurological functions related or not to circadian rhythm and biological clock, and pathological phenotypes arising from Bmal1 knockout. This review also discusses oscillation and rhythmicity, especially in the suprachiasmatic nucleus, and provides perspective on future progress in Bmal1 research.

An extra-clock ultradian brain oscillator sustains circadian timekeeping

The master circadian clock generates 24-hour rhythms to orchestrate daily behavior, even running freely under constant conditions. Traditionally, the master clock is considered self-sufficient in sustaining free-running timekeeping via its cell-autonomous molecular clocks and interneuronal communications within the circadian neural network. Here, we find a set of bona fide ultradian oscillators in the Drosophila brain that support free-running timekeeping, despite being located outside the master clock circuit and lacking clock gene expression. These extra-clock electrical oscillators (xCEOs) generate cell-autonomous ultradian bursts, pacing widespread burst firing and promoting rhythmic resting membrane potentials in clock neurons via parallel monosynaptic connections. Silencing xCEOs disrupts daily electrical rhythms in clock neurons and impairs cycling of neuropeptide pigment dispersing factor, leading to the loss of free-running locomotor rhythms. Together, we conclude that the master clock is not self-sufficient to sustain free-running behavior rhythms but requires additional endogenous inputs to the clock from the extra-clock ultradian brain oscillators.

ISX-9 potentiates CaMKIIδ-mediated BMAL1 activation to enhance circadian amplitude

Circadian dysregulation associates with numerous diseases including metabolic dysfunction, sleep disorder, depression and aging. Given that declined circadian amplitude is a trait commonly found with compromised health, interventions that design in precluding circadian amplitude from dampening will aid to mitigate complex, circadian-related diseases. Here we identify a neurogenic small molecule ISX-9 that is able to support persistent and higher amplitude of circadian oscillations. ISX-9 improves diurnal metabolic rhythms in middle-aged mice. Moreover, the ISX-9-treated mice show better sleep homeostasis with increased delta power during the day time and higher locomotive activity in the dark period. ISX-9 augments CaMKIIδ expression and increases BMAL1 activity via eliciting CaMKIIδ-mediated phosphorylation on BMAL1 residues S513/S515/S516, accordingly composes a positive feedback effect on enhancing circadian amplitude. CaMKIIδ-targeting, and the use of ISX-9 may serve as decent choices for treating circadian-related disorders.

The Circadian Molecular Machinery in CNS Cells: A Fine Tuner of Neuronal and Glial Activity With Space/Time Resolution

The circadian molecular machinery is a fine timekeeper with the capacity to harmonize physiological and behavioral processes with the external environment. This tight-knit regulation is coordinated by multiple cellular clocks across the body. In this review, we focus our attention on the molecular mechanisms regulated by the clock in different brain areas and within different cells of the central nervous system. Further, we discuss evidence regarding the role of circadian rhythms in the regulation of neuronal activity and neurotransmitter systems. Not only neurons, but also astrocytes and microglia actively participate in the maintenance of timekeeping within the brain, and the diffusion of circadian information among these cells is fine-tuned by neurotransmitters (e.g., dopamine, serotonin, and γ-aminobutyric acid), thus impacting on the core clock machinery. The bidirectional interplay between neurotransmitters and the circadian clockwork is fundamental in maintaining accuracy and precision in daily timekeeping throughout different brain areas. Deepening the knowledge of these correlations allows us to define the basis of drug interventions to restore circadian rhythms, as well as to predict the onset of drug treatment/side effects that might promote daily desynchronization. Furthermore, it may lead to a deeper understanding of the potential impacts of modulations in rhythmic activities on the pace of aging and provide an insight in to the pathogenesis of psychiatric diseases and neurodegenerative disorders.

Circadian pacemaker neurons display cophasic rhythms in basal calcium level and in fast calcium fluctuations

Circadian pacemaker neurons in the Drosophila brain display daily rhythms in the levels of intracellular calcium. These calcium rhythms are driven by molecular clocks and are required for normal circadian behavior. To study their biological basis, we employed genetic manipulations in conjunction with improved methods of in vivo light-sheet microscopy to measure calcium dynamics in individual pacemaker neurons over complete 24-h durations at sampling frequencies as high as 5 Hz. This technological advance unexpectedly revealed cophasic daily rhythms in basal calcium levels and in high-frequency calcium fluctuations. Further, we found that the rhythms of basal calcium levels and of fast calcium fluctuations reflect the activities of two proteins that mediate distinct forms of calcium fluxes. One is the inositol trisphosphate receptor (ITPR), a channel that mediates calcium fluxes from internal endoplasmic reticulum calcium stores, and the other is a T-type voltage-gated calcium channel, which mediates extracellular calcium influx. These results suggest that Drosophila molecular clocks regulate ITPR and T-type channels to generate two distinct but coupled rhythms in basal calcium and in fast calcium fluctuations. We propose that both internal and external calcium fluxes are essential for circadian pacemaker neurons to provide rhythmic outputs and thereby, regulate the activities of downstream brain centers.

Molecular regulations of circadian rhythm and implications for physiology and diseases

The term “circadian rhythms” describes endogenous oscillations with ca. 24-h period associated with the earth’s daily rotation and light/dark cycle. Such rhythms reflect the existence of an intrinsic circadian clock that temporally orchestrates physiological processes to adapt the internal environment with the external cues. At the molecular level, the circadian clock consists of multiple sets of transcription factors resulting in autoregulatory transcription-translation feedback loops. Notably, in addition to their primary role as generator of circadian rhythm, the biological clock plays a key role in controlling physiological functions of almost all tissues and organs. It regulates several intracellular signaling pathways, ranging from cell proliferation, DNA damage repair and response, angiogenesis, metabolic and redox homeostasis, to inflammatory and immune response. In this review, we summarize findings showing the crosstalk between the circadian molecular clock and some key intracellular pathways, describing a scenario wherein their reciprocal regulation impinges upon several aspects of mammalian physiology. Moreover, based on evidence indicating that circadian rhythms can be challenged by environmental factors, social behaviors, as well as pre-existing pathological conditions, we discuss implications of circadian misalignment in human pathologies, such as cancer and inflammatory diseases. Accordingly, disruption of circadian rhythm has been reported to affect several physiological processes that are relevant to human diseases. Expanding our understanding of this field represents an intriguing and transversal medicine challenge in order to establish a circadian precision medicine.

The light response in chickens divergently selected for feather pecking behavior reveals mechanistic insights towards psychiatric disorders

Background

Feather pecking is a serious behavioral disorder in chickens that has a considerable impact on animal welfare and poses an economic burden for poultry farming. To study the underlying genetics of feather pecking animals were divergently selected for feather pecking over 15 generations based on estimated breeding values for the behavior.

Methods and results

By characterizing the transcriptomes of whole brains isolated from high and low feather pecking chickens in response to light stimulation we discovered a putative dysregulation of micro RNA processing caused by a lack of Dicer1. This results in a prominent downregulation of the GABRB2 gene and other GABA receptor transcripts, which might cause a constant high level of excitation in the brains of high feather pecking chickens. Moreover, our results point towards an increase in immune system-related transcripts that may be caused by higher interferon concentrations due to Dicer1 downregulation.

Conclusion

Based on our results, we conclude that feather pecking in chickens and schizophrenia in humans have numerous common features. For instance, a Dicer1 dependent disruption of miRNA biogenesis and the lack of GABRB2 expression have been linked to schizophrenia pathogenesis. Furthermore, disturbed circadian rhythms and dysregulation of genes involved in the immune system are common features of both conditions.

Supplementary Information

The online version contains supplementary material available at 10.1007/s11033-021-07111-4.

Multi-Modal Regulation of Circadian Physiology by Interactive Features of Biological Clocks

The circadian clock is a fundamental biological timing mechanism that generates nearly 24 h rhythms of physiology and behaviors, including sleep/wake cycles, hormone secretion, and metabolism. Evolutionarily, the endogenous clock is thought to confer living organisms, including humans, with survival benefits by adapting internal rhythms to the day and night cycles of the local environment. Mirroring the evolutionary fitness bestowed by the circadian clock, daily mismatches between the internal body clock and environmental cycles, such as irregular work (e.g., night shift work) and life schedules (e.g., jet lag, mistimed eating), have been recognized to increase the risk of cardiac, metabolic, and neurological diseases. Moreover, increasing numbers of studies with cellular and animal models have detected the presence of functional circadian oscillators at multiple levels, ranging from individual neurons and fibroblasts to brain and peripheral organs. These oscillators are tightly coupled to timely modulate cellular and bodily responses to physiological and metabolic cues. In this review, we will discuss the roles of central and peripheral clocks in physiology and diseases, highlighting the dynamic regulatory interactions between circadian timing systems and multiple metabolic factors.

Cut the noise or couple up: Coordinating circadian and synthetic clocks

Circadian clocks are important to much of life on Earth and are of inherent interest to humanity, implicated in fields ranging from agriculture and ecology to developmental biology and medicine. New techniques show that it is not simply the presence of clocks, but coordination between them that is critical for complex physiological processes across the kingdoms of life. Recent years have also seen impressive advances in synthetic biology to the point where parallels can be drawn between synthetic biological and circadian oscillators. This review will emphasize theoretical and experimental studies that have revealed a fascinating dichotomy of coupling and heterogeneity among circadian clocks. We will also consolidate the fields of chronobiology and synthetic biology, discussing key design principles of their respective oscillators.

Comparative Ca2+ channel contributions to intracellular Ca2+ levels in the circadian clock

Circadian rhythms in mammals are coordinated by the central clock in the brain, located in the suprachiasmatic nucleus (SCN). Multiple molecular and cellular signals display a circadian variation within SCN neurons, including intracellular Ca2+, but the mechanisms are not definitively established. SCN cytosolic Ca2+ levels exhibit a peak during the day, when both action potential firing and Ca2+ channel activity are increased, and are decreased at night, correlating with a reduction in firing rate. In this study, we employ a single-color fluorescence anisotropy reporter (FLARE), Venus FLARE-Cameleon, and polarization inverted selective-plane illumination microscopy to measure rhythmic changes in cytosolic Ca2+ in SCN neurons. Using this technique, the Ca2+ channel subtypes contributing to intracellular Ca2+ at the peak and trough of the circadian cycle were assessed using a pharmacological approach with Ca2+ channel inhibitors. Peak (218 ± 16 nM) and trough (172 ± 13 nM) Ca2+ levels were quantified, indicating a 1.3-fold circadian variance in Ca2+ concentration. Inhibition of ryanodine-receptor-mediated Ca2+ release produced a larger relative decrease in cytosolic Ca2+ at both time points compared to voltage-gated Ca2+channels. These results support the hypothesis that circadian Ca2+ rhythms in SCN neurons are predominantly driven by intracellular Ca2+ channels, although not exclusively so. The study provides a foundation for future experiments to probe Ca2+ signaling in a dynamic biological context using FLAREs.

The Drosophila ortholog of the schizophrenia-associated CACNA1A and CACNA1B voltage-gated calcium channels regulate memory, sleep and circadian rhythms

Schizophrenia exhibits up to 80% heritability. A number of genome wide association studies (GWAS) have repeatedly shown common variants in voltage-gated calcium (Ca_v) channel genes CACNA1C, CACNA1I and CACNA1G have a major contribution to the risk of the disease. More recently, studies using whole exome sequencing have also found that CACNA1B (Ca_v2.2 N-type) deletions and rare disruptive variants in CACNA1A (Ca_v2.1 P/Q-type) are associated with schizophrenia. The negative symptoms of schizophrenia include behavioural defects such as impaired memory, sleep and circadian rhythms. It is not known how variants in schizophrenia-associated genes contribute to cognitive and behavioural symptoms, thus hampering the development of treatment for schizophrenia symptoms. In order to address this knowledge gap, we studied behavioural phenotypes in a number of loss of function mutants for the Drosophila ortholog of the Ca_v2 gene family called cacophony (cac). cac mutants showed several behavioural features including decreased night-time sleep and hyperactivity similar to those reported in human patients. The change in timing of sleep-wake cycles suggested disrupted circadian rhythms, with the loss of night-time sleep being caused by loss of cac just in the circadian clock neurons. These animals also showed a reduction in rhythmic circadian behaviour a phenotype that also could be mapped to the central clock. Furthermore, reduction of cac just in the clock resulted in a lengthening of the 24 h period. In order to understand how loss of Ca_v2 function may lead to cognitive deficits and underlying cellular pathophysiology we targeted loss of function of cac to the memory centre of the fly, called the mushroom bodies (MB). This manipulation was sufficient to cause reduction in both short- and intermediate-term associative memory. Memory impairment was accompanied by a decrease in Ca²⁺ transients in response to a depolarizing stimulus, imaged in the MB presynaptic terminals. This work shows loss of cac Ca_v2 channel function alone causes a number of cognitive and behavioural deficits and underlying reduced neuronal Ca²⁺ transients, establishing Drosophila as a high-throughput in vivo genetic model to study the Ca_v channel pathophysiology related to schizophrenia.

Reciprocal Relationship Between Calcium Signaling and Circadian Clocks: Implications for Calcium Homeostasis, Clock Function, and Therapeutics

In animals, circadian clocks impose a daily rhythmicity to many behaviors and physiological processes. At the molecular level, circadian rhythms are driven by intracellular transcriptional/translational feedback loops (TTFL). Interestingly, emerging evidence indicates that they can also be modulated by multiple signaling pathways. Among these, Ca²⁺ signaling plays a key role in regulating the molecular rhythms of clock genes and of the resulting circadian behavior. In addition, the application of in vivo imaging approaches has revealed that Ca²⁺ is fundamental to the synchronization of the neuronal networks that make up circadian pacemakers. Conversely, the activity of circadian clocks may influence Ca²⁺ signaling. For instance, several genes that encode Ca²⁺ channels and Ca²⁺-binding proteins display a rhythmic expression, and a disruption of this cycling affects circadian function, underscoring their reciprocal relationship. Here, we review recent advances in our understanding of how Ca²⁺ signaling both modulates and is modulated by circadian clocks, focusing on the regulatory mechanisms described in Drosophila and mice. In particular, we examine findings related to the oscillations in intracellular Ca²⁺ levels in circadian pacemakers and how they are regulated by canonical clock genes, neuropeptides, and light stimuli. In addition, we discuss how Ca²⁺ rhythms and their associated signaling pathways modulate clock gene expression at the transcriptional and post-translational levels. We also review evidence based on transcriptomic analyzes that suggests that mammalian Ca²⁺ channels and transporters (e.g., ryanodine receptor, ip3r, serca, L- and T-type Ca²⁺ channels) as well as Ca²⁺-binding proteins (e.g., camk, cask, and calcineurin) show rhythmic expression in the central brain clock and in peripheral tissues such as the heart and skeletal muscles. Finally, we discuss how the discovery that Ca²⁺ signaling is regulated by the circadian clock could influence the efficacy of pharmacotherapy and the outcomes of clinical interventions.

Na+/Ca2+ exchanger mediates cold Ca2+ signaling conserved for temperature-compensated circadian rhythms

Circadian rhythms are based on biochemical oscillations generated by clock genes/proteins, which independently evolved in animals, fungi, plants, and cyanobacteria. Temperature compensation of the oscillation speed is a common feature of the circadian clocks, but the evolutionary-conserved mechanism has been unclear. Here, we show that Na⁺/Ca²⁺ exchanger (NCX) mediates cold-responsive Ca²⁺ signaling important for the temperature-compensated oscillation in mammalian cells. In response to temperature decrease, NCX elevates intracellular Ca²⁺, which activates Ca²⁺/calmodulin-dependent protein kinase II and accelerates transcriptional oscillations of clock genes. The cold-responsive Ca²⁺ signaling is conserved among mice, Drosophila, and Arabidopsis The mammalian cellular rhythms and Drosophila behavioral rhythms were severely attenuated by NCX inhibition, indicating essential roles of NCX in both temperature compensation and autonomous oscillation. NCX also contributes to the temperature-compensated transcriptional rhythms in cyanobacterial clock. Our results suggest that NCX-mediated Ca²⁺ signaling is a common mechanism underlying temperature-compensated circadian rhythms both in eukaryotes and prokaryotes.

Cisplatin's dual-effect on the circadian clock triggers proliferation and apoptosis

The circadian clock, which generates the internal daily rhythm largely mediated through release of melatonin, can be disrupted in various ways. Multiple factors result in a disruption of the circadian cycle in the clinical context, of interest are anti-cancer drugs such as cisplatin. Cisplatin modulates the circadian clock through two mechanisms: 1) the circadian clock control of DNA excision repair and 2) the effect of circadian clock disruption on apoptosis. Cisplatin can stimulate multiple classified molecules, including DNA repair factors, DNA damage recognition factors and transcription factors in drug resistance and cisplatin-induced signal transduction. These factors interact with each other and can be transformed by DNA damage. Hence, these molecular interactions are intimately involved in cell proliferation and damage-induced apoptosis. Cisplatin has a dual-effect on circadian genes: upregulation of CLOCK expression causes an increase in proliferation but upregulation of BMAL1 expression causes an increase in apoptosis. Therefore, the interference of circadian genes by cisplatin can have multiple, opposing effects on apoptosis and cell proliferation, which may have unintended pro-cancer effects. Melatonin and intracellular Ca²⁺ also have a dual-effect on cell proliferation and apoptosis and can disrupt circadian rhythms.

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Cisplatin has a dual-effect on components of the circadian clock, increasing or decreasing cell proliferation and apoptosis.

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DNA excision repair and apoptosis are controlled by circadian rhythms.

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When cisplatin is combined with other agents, the effects are enhanced.

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These findings provide clinicians with the prospect to create effective chrono-cisplatin regimens for patients.

Contraction influences Per2 gene expression in skeletal muscle through a calcium-dependent pathway

KEY POINTS

Exercising at different times of day elicits different effects on exercise performance and metabolic health. However, the specific signals driving the observed time-of-day specific effects of exercise have not been fully identified. Exercise influences the skeletal muscle circadian clock, although the relative contribution of muscle contraction and extracellular signals is unknown. Here, we show that contraction acutely increases the expression of the core circadian clock gene Period Circadian Regulator 2 (Per2) and phase-shifts Per2 rhythmicity in muscle cells. This contraction effect on core clock genes is mediated through a calcium-dependant mechanism; The results obtained in the present study suggest that a proportion of the ability of exercise to entrain the skeletal muscle clock is driven directly by muscle contraction. Contraction interventions may be used to mimic some time-of-day specific effects of exercise on metabolism and muscle performance.

ABSTRACT

Exercise entrains the central and peripheral circadian clocks, although the mechanism by which exercise modulates expression of skeletal muscle clock genes is unclear. The present study aimed to determine whether skeletal muscle contraction alone could directly influence circadian rhythmicity and uncover the underlying mechanism by which contraction modulates clock gene expression. We investigated the expression of core clock genes in human skeletal muscle after acute exercise, as well as following in vitro contraction in mouse soleus muscle and cultured C2C12 skeletal muscle myotubes. Additionally, we interrogated the molecular pathways by which skeletal muscle contraction could influence clock gene expression. Contraction acutely increased the expression of the core circadian clock gene Period Circadian Regulator 2 (Per2) and phase-shifted Per2 rhythmicity in C2C12 myotubes in vitro. Further investigation revealed that pharmacologically increasing cytosolic calcium concentrations by ionomycin treatment mimicked the effect of contraction on Per2 expression. Similarly, treatment with a calcium channel blocker, nifedipine, blocked the effect of electric pulse stimulation-induced contraction on Per2 expression. Increased calcium influx from contraction lead to binding of the phosphorylated form of cAMP response element-binding protein (CREB) to the Per2 promoter, suggesting a role of CREB in contraction-induced Per2 transcription. Thus, by dissociating the effect of muscle contraction alone from the whole effect of exercise, our investigations indicate that a proportion of the ability of exercise to entrain the skeletal muscle clock is driven directly by contraction.

Eukaryotic cell biology is temporally coordinated to support the energetic demands of protein homeostasis

Yeast physiology is temporally regulated, this becomes apparent under nutrient-limited conditions and results in respiratory oscillations (YROs). YROs share features with circadian rhythms and interact with, but are independent of, the cell division cycle. Here, we show that YROs minimise energy expenditure by restricting protein synthesis until sufficient resources are stored, while maintaining osmotic homeostasis and protein quality control. Although nutrient supply is constant, cells sequester and store metabolic resources via increased transport, autophagy and biomolecular condensation. Replete stores trigger increased H⁺ export which stimulates TORC1 and liberates proteasomes, ribosomes, chaperones and metabolic enzymes from non-membrane bound compartments. This facilitates translational bursting, liquidation of storage carbohydrates, increased ATP turnover, and the export of osmolytes. We propose that dynamic regulation of ion transport and metabolic plasticity are required to maintain osmotic and protein homeostasis during remodelling of eukaryotic proteomes, and that bioenergetic constraints selected for temporal organisation that promotes oscillatory behaviour.

Calcium Sets the Clock in Ameloblasts

Background

Stromal interaction molecule 1 (STIM1) is one of the main components of the store operated Ca²⁺ entry (SOCE) signaling pathway. Individuals with mutated STIM1 present severely hypomineralized enamel characterized as amelogenesis imperfecta (AI) but the downstream molecular mechanisms involved remain unclear. Circadian clock signaling plays a key role in regulating the enamel thickness and mineralization, but the effects of STIM1-mediated AI on circadian clock are unknown.

Objectives

The aim of this study is to examine the potential links between SOCE and the circadian clock during amelogenesis.

Methods

We have generated mice with ameloblast-specific deletion of Stim1 (Stim1^fl/fl/Amelx-iCre^+/+, Stim1 cKO) and analyzed circadian gene expression profile in Stim1 cKO compared to control (Stim1^fl/fl/Amelx-iCre^–/–) using ameloblast micro-dissection and RNA micro-array of 84 circadian genes. Expression level changes were validated by qRT-PCR and immunohistochemistry.

Results

Stim1 deletion has resulted in significant upregulation of the core circadian activator gene Brain and Muscle Aryl Hydrocarbon Receptor Nuclear Translocation 1 (Bmal1) and downregulation of the circadian inhibitor Period 2 (Per2). Our analyses also revealed that SOCE disruption results in dysregulation of two additional circadian regulators; p38α mitogen-activated protein kinase (MAPK14) and transforming growth factor-beta1 (TGF-β1). Both MAPK14 and TGF-β1 pathways are known to play major roles in enamel secretion and their dysregulation has been previously implicated in the development of AI phenotype.

Conclusion

These data indicate that disruption of SOCE significantly affects the ameloblasts molecular circadian clock, suggesting that alteration of the circadian clock may be partly involved in the development of STIM1-mediated AI.

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.

Ion Channels Controlling Circadian Rhythms in Suprachiasmatic Nucleus Excitability

Animals synchronize to the environmental day-night cycle by means of an internal circadian clock in the brain. In mammals, this timekeeping mechanism is housed in the suprachiasmatic nucleus (SCN) of the hypothalamus and is entrained by light input from the retina. One output of the SCN is a neural code for circadian time, which arises from the collective activity of neurons within the SCN circuit and comprises two fundamental components: 1) periodic alterations in the spontaneous excitability of individual neurons that result in higher firing rates during the day and lower firing rates at night, and 2) synchronization of these cellular oscillations throughout the SCN. In this review, we summarize current evidence for the identity of ion channels in SCN neurons and the mechanisms by which they set the rhythmic parameters of the time code. During the day, voltage-dependent and independent Na⁺ and Ca²⁺ currents, as well as several K⁺ currents, contribute to increased membrane excitability and therefore higher firing frequency. At night, an increase in different K⁺ currents, including Ca²⁺-activated BK currents, contribute to membrane hyperpolarization and decreased firing. Layered on top of these intrinsically regulated changes in membrane excitability, more than a dozen neuromodulators influence action potential activity and rhythmicity in SCN neurons, facilitating both synchronization and plasticity of the neural code.

Aging Affects the Capacity of Photoperiodic Adaptation Downstream from the Central Molecular Clock

Aging impairs circadian clock function, leading to disrupted sleep-wake patterns and a reduced capability to adapt to changes in environmental light conditions. This makes shift work or the changing of time zones challenging for the elderly and, importantly, is associated with the development of age-related diseases. However, it is unclear what levels of the clock machinery are affected by aging, which is relevant for the development of targeted interventions. We found that naturally aged mice of >24 months had a reduced rhythm amplitude in behavior compared with young controls (3-6 months). Moreover, the old animals had a strongly reduced ability to adapt to short photoperiods. Recording PER2::LUC protein expression in the suprachiasmatic nucleus revealed no impairment of the rhythms in PER2 protein under the 3 different photoperiods tested (LD: 8:16, 12:12, and 16:8). Thus, we observed a discrepancy between the behavioral phenotype and the molecular clock, and we conclude that the aging-related deficits emerge downstream of the core molecular clock. Since it is known that aging affects several intracellular and membrane components of the central clock cells, it is likely that an impairment of the interaction between the molecular clock and these components is contributing to the deficits in photoperiod adaptation.

The NRON complex controls circadian clock function through regulated PER and CRY nuclear translocation

Post-translational regulation plays a central role in the circadian clock mechanism. However, nucleocytoplasmic translocation of core clock proteins, a key step in circadian timekeeping, is not fully understood. Earlier we found that the NRON scaffolding complex regulates nuclear translocation of NFAT and its signaling. Here, we show that components of the NRON complex also regulate the circadian clock. In peripheral cell clock models, genetic perturbation of the NRON complex affects PER and CRY protein nuclear translocation, dampens amplitude, and alters period length. Further, we show small molecules targeting the NFAT pathway alter nuclear translocation of PER and CRY proteins and impact circadian rhythms in peripheral cells and tissue explants of the master clock in the suprachiasmatic nucleus. Taken together, these studies highlight a key role for the NRON complex in regulating PER/CRY subcellular localization and circadian timekeeping.

GABA in the suprachiasmatic nucleus refines circadian output rhythms in mice

In mammals, the circadian rhythms are regulated by the central clock located in the hypothalamic suprachiasmatic nucleus (SCN), which is composed of heterogeneous neurons with various neurotransmitters. Among them an inhibitory neurotransmitter, γ-Amino-Butyric-Acid (GABA), is expressed in almost all SCN neurons, however, its role in the circadian physiology is still unclear. Here, we show that the SCN of fetal mice lacking vesicular GABA transporter (VGAT-/-) or GABA synthesizing enzyme, glutamate decarboxylase (GAD65-/-/67-/-), shows burst firings associated with large Ca2+ spikes throughout 24 hours, which spread over the entire SCN slice in synchrony. By contrast, circadian PER2 rhythms in VGAT-/- and GAD65-/-/67-/- SCN remain intact. SCN-specific VGAT deletion in adult mice dampens circadian behavior rhythm. These findings indicate that GABA in the fetal SCN is necessary for refinement of the circadian firing rhythm and, possibly, for stabilizing the output signals, but not for circadian integration of multiple cellular oscillations.

From clock to functional pacemaker

In mammals, the central pacemaker that coordinates 24‐hr rhythms is located in the suprachiasmatic nucleus (SCN). Individual neurons of the SCN have a molecular basis for rhythm generation and hence, they function as cell autonomous oscillators. Communication and synchronization among these neurons are crucial for obtaining a coherent rhythm at the population level, that can serve as a pace making signal for brain and body. Hence, the ability of single SCN neurons to produce circadian rhythms is equally important as the ability of these neurons to synchronize one another, to obtain a bona fide pacemaker at the SCN tissue level. In this chapter we will discuss the mechanisms underlying synchronization, and plasticity herein, which allows adaptation to changes in day length. Furthermore, we will discuss deterioration in synchronization among SCN neurons in aging, and gain in synchronization by voluntary physical activity or exercise.

The regulation of circadian clock by tumor necrosis factor alpha

All organisms display circadian rhythms which are under the control of the circadian clock located in the hypothalamus at the suprachiasmatic nucleus, (SCN). The circadian rhythms allow individuals to adjust their physiological activities and daily behavior for the diurnal changes in the living environment. To achieve these, all metabolic processes are aligned with the sleep/wake and fasting/feeding cycles. Subtle changes of daily behavior or food intake can result in misalignment of circadian rhythms. This can cause development of variety of metabolic diseases and even cancer. Although light plays a pivotal role for the activation of the master clock in SCN, the peripheral secondary clocks (or non-SCN), such as melatonin, growth hormone (GH), insulin, adiponectin and Ghrelin also are important in maintaining the circadian rhythms in the brain and peripheral organs. In recent years, growing body of evidence strongly suggest that CA²⁺ signaling, tumor necrosis factor alpha (TNFα) and transforming growth factor beta (TGFβ) also play very important roles in the regulation of circadian rhythms by regulating the transcription of the clock genes.

NMDA Receptor-mediated Ca2+ Influx in the Absence of Mg2+ Block Disrupts Rest: Activity Rhythms in Drosophila

Study Objectives

The correlated activation of pre- and postsynaptic neurons is essential for the NMDA receptor-mediated Ca2+ influx by removing Mg2+ from block site and NMDA receptors have been implicated in phase resetting of circadian clocks. So we assessed rest:activity rhythms in Mg2+ block defective animals.

Methods

Using Drosophila locomotor monitoring system, we checked circadian rest:activity rhythms of different mutants under constant darkness (DD) and light:dark (LD) conditions. We recorded NMDA receptor-mediated currents or Ca2+ increase in neurons using patch-clamp and Ca2+ imaging techniques.

Results

We found that Mg2+ block defective mutant flies were completely arrhythmic under DD. To further understand the role of Mg2+ block in daily circadian rest:activity, we observed the mutant files under LD cycles, and we found severely reduced morning anticipation and advanced evening peak compared to control flies. We also used tissue-specific expression of Mg2+ block defective NMDA receptors and demonstrated pigment-dispersing factor receptor (PDFR)-expressing circadian neurons were implicated in mediating the circadian rest:activity deficits. Endogenous functional NMDA receptors are expressed in most Drosophila neurons, including in a subgroup of dorsal neurons (DN1s). Subsequently, we determined that the uncorrelated extra Ca2+ influx may act in part through Ca2+/Calmodulin (CaM)-stimulated PDE1c pathway leading to morning behavior phenotypes.

Conclusions

These results demonstrate that Mg2+ block of NMDA receptors at resting potential is essential for the daily circadian rest:activity rhythms and we propose that Mg2+ block functions to suppress CaM-stimulated PDE1c activation at resting potential, thus regulating Ca2+ and cyclic AMP oscillations in circadian and sleep circuits.

The circadian clock: A tale of genetic-electrical interplay and synaptic integration

Pioneering work in Drosophila uncovered the building blocks of the molecular clock, consisting of transcription-translation feedback loops (TTFLs). Subsequent experimental work demonstrated that the mammalian TTFL is localized in cells and tissues throughout the brain and body. Further research established that neuronal activity forms an essential aspect of clock function. However, how the membrane electrical activity of clock neurons of the suprachiasmatic nucleus collaborate with the TTFL to drive circadian behaviors remains mostly unknown. Intercellular communication synchronizes the individual circadian oscillators to produce a precise and coherent circadian output. Here, we briefly review significant research that is increasing our understanding of the critical interactions between the TTFL and neuronal and glial activity in the generation of circadian timing signals.

Differential regulation of nimodipine-sensitive and -insensitive Ca2+ influx by the Na+/Ca2+ exchanger and mitochondria in the rat suprachiasmatic nucleus neurons

Background

Transmembrane Ca²⁺ influx is critical for molecular rhythmicity, metabolic activity, and neuropeptide release in the central clock of the suprachiasmatic nucleus (SCN). We previously reported that both the Na⁺/Ca²⁺ exchanger (NCX) and mitochondria play a role in regulating intracellular Ca²⁺ homeostasis in the rat SCN neurons. Here we present evidence to show differential regulation by NCX and mitochondria of nimodipine-sensitive and -insensitive Ca²⁺ influx.

Methods

Ratiometric Ca²⁺ imaging was used to measure change in [Ca²⁺]_i and patch clamp recordings to study spontaneous firing, membrane potential, and voltage-dependent Ca²⁺ channels in neurons from reduced SCN slice preparations. Immunofluorescent staining was used to determine the distribution pattern of CaV1.2 and CaV1.3 and their colocalization with NCX1.

Results

Ratiometric Ca²⁺ imaging indicates that nimodipine (2 μM) blocked most of 20 (mM) K⁺-induced, but less so of 50 K⁺-induced, Ca²⁺ rise. The nimodipine-sensitive 50 K⁺-induced Ca²⁺ transient rose more rapidly but decayed similarly with the nimodipine-insensitive component, suggesting both components were extruded by NCX. Immunofluorescent stains showed the expression of both CaV1.2 and CaV1.3 and their colocalization with NCX1, whereas functional studies suggest that CaV1.2 mediated most of the nimodipine-sensitive Ca²⁺ rise but had insignificant effect on spontaneous firing. After normalization relative to the Ca²⁺-free solution, nimodipine reduced ~ 65% of basal Ca²⁺ influx, and TTX lowered it by ~ 35%, leaving ~ 25% basal Ca²⁺ influx in the combined presence of TTX and nimodipine. With the mitochondrial uncoupler carbonyl cyanide-p-trifluoromethoxyphenylhydrazone (FCCP) to inhibit mitochondrial Ca²⁺ uptake, 20 K⁺-induced Ca²⁺ transients became larger and slower, both in the absence and presence of nimodipine. FCCP markedly enhanced nimodipine-insensitive, but not nimodipine-sensitive, Ca²⁺ transients, suggesting that mitochondria preferentially buffer nimodipine-insensitive Ca²⁺ influx. Results from using CaV2 channel blockers further indicate that FCCP enhanced Ca²⁺ transients mediated by N-, P/Q-, and the blocker cocktail-insensitive Ca²⁺ channels.

Conclusions

The differential regulation of transmembrane Ca²⁺ influx by NCX and mitochondria suggests that Ca²⁺ entry via different sources may be regulated differently to play different roles in SCN physiology.

Circadian redox rhythms in the regulation of neuronal excitability

Oxidation-reduction reactions are essential to life as the core mechanisms of energy transfer. A large body of evidence in recent years presents an extensive and complex network of interactions between the circadian and cellular redox systems. Recent advances show that cellular redox state undergoes a ~24-h (circadian) oscillation in most tissues and is conserved across the domains of life. In nucleated cells, the metabolic oscillation is dependent upon the circadian transcription-translation machinery and, vice versa, redox-active proteins and cofactors feed back into the molecular oscillator. In the suprachiasmatic nucleus (SCN), a hypothalamic region of the brain specialized for circadian timekeeping, redox oscillation was found to modulate neuronal membrane excitability. The SCN redox environment is relatively reduced in daytime when neuronal activity is highest and relatively oxidized in nighttime when activity is at its lowest. There is evidence that the redox environment directly modulates SCN K⁺ channels, tightly coupling metabolic rhythms to neuronal activity. Application of reducing or oxidizing agents produces rapid changes in membrane excitability in a time-of-day-dependent manner. We propose that this reciprocal interaction may not be unique to the SCN. In this review, we consider the evidence for circadian redox oscillation and its interdependencies with established circadian timekeeping mechanisms. Furthermore, we will investigate the effects of redox on ion-channel gating dynamics and membrane excitability. The susceptibility of many different ion channels to modulation by changes in the redox environment suggests that circadian redox rhythms may play a role in the regulation of all excitable cells.

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.

TNF-α induces expression of the circadian clock gene Bmal1 via dual calcium-dependent pathways in rheumatoid synovial cells

Tumor necrosis factor (TNF)-α is responsible for expressions of several clock genes and affects joint symptoms of rheumatoid arthritis (RA) with diurnal fluctuation. We tried to determine the mechanism involved in over-expression of Bmal1, induced by TNF-α, in primary cultured rheumatoid synovial cells. Cells were incubated with intra-cellular Ca2+ chelator BAPTA-AM, calcineurin inhibitor FK506 and p300/CBP (CREB binding protein) inhibitor C646, respectively, or transfected with p300 and CBP small interfering RNA (siRNA) before stimulation with TNF-α. Oscillation phase and amplitude of Bmal1, transcriptional activator Rorα, transcriptional repressor Rev-erbα, and histone acetyltransferases (p300 and Cbp) were evaluated by quantitative real-time PCR. As results, TNF-α did not influence the oscillation phase of Rev-erbα, while enhanced those of Rorα, resulting in over-expression of Bmal1. When Ca2+ influx was inhibited by BAPTA-AM, TNF-α-mediated up-regulation of Rorα was cancelled, however, that of Bmal1 was still apparent. When we further explored another pathway between TNF-α and Bmal1, TNF-α suppressed the expression of Rev-erbα in the absence of Ca2+ influx, as well as those of p300 and Cbp genes. Finally, actions of TNF-α, in increasing Bmal1/Rorα and decreasing Rev-erbα, were cancelled by C646 treatment or silencing of both p300 and Cbp. In conclusion, we determined a novel role of TNF-α in inducing Bmal1 via dual calcium dependent pathways; Rorα was up-regulated in the presence of Ca2+ influx and Rev-erbα was down-regulated in the absence of that. Results proposed that inhibition of p300/CBP could be new therapeutic targets for RA.

Encoding seasonal information in a two-oscillator model of the multi-oscillator circadian clock

The suprachiasmatic nucleus (SCN) is a collection of about 10 000 neurons, each of which functions as a circadian clock with slightly different periods and phases, that work in concert with form and maintain the master circadian clock for the organism. The diversity among neurons confers on the SCN the ability to robustly encode both the 24-h light pattern as well as the seasonal time. Cluster synchronization brings the different neurons into line and reduces the large population to essentially two oscillators, coordinated by a macroscopic network motif of asymmetric repulsive-attractive coupling. We recount the steps leading to this simplification and rigorously examine the two-oscillator case by seeking an analytical solution. Through these steps, we identify physiologically relevant parameters that shape the behaviour of the SCN network and delineate its ability to store past details of seasonal variation in photoperiod.