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

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.

Differential Phase Arrangement of Cellular Clocks along the Tonotopic Axis of the Mouse Cochlea Ex Vivo

Topological distributions of individual cellular clocks have not been demonstrated in peripheral organs. The cochlea displays circadian patterns of core clock gene expression [1, 2]. PER2 protein is expressed in the hair cells and spiral ganglion neurons of the cochlea in the spiral ganglion neurons [1]. To investigate the topological organization of cellular oscillators in the cochlea, we recorded circadian rhythms from mouse cochlear explants using highly sensitive real-time tracking of PER2::LUC bioluminescence. Here, we show cell-autonomous and self-sustained oscillations originating from hair cells and spiral ganglion neurons. Multi-phased cellular clocks were arranged along the length of the cochlea with oscillations initiating at the apex (low-frequency region) and traveling toward the base (high-frequency region). Phase differences of 3 hr were found between cellular oscillators in the apical and middle regions and from isolated individual cochlear regions, indicating that cellular networks organize the rhythms along the tonotopic axis. This is the first demonstration of a spatiotemporal arrangement of circadian clocks at the cellular level in a peripheral organ. Cochlear rhythms were disrupted in the presence of either voltage-gated potassium channel blocker (TEA) or extracellular calcium chelator (BAPTA), demonstrating that multiple types of ion channels contribute to the maintenance of coherent rhythms. In contrast, preventing action potentials with tetrodotoxin (TTX) or interfering with cell-to-cell communication the broad-spectrum gap junction blocker (CBX [carbenoxolone]) had no influence on cochlear rhythms. These findings highlight a dynamic regulation and longitudinal distribution of cellular clocks in the cochlea.

Calcium Circadian Rhythmicity in the Suprachiasmatic Nucleus: Cell Autonomy and Network Modulation

Circadian rhythms of mammalian physiology and behavior are coordinated by the suprachiasmatic nucleus (SCN) in the hypothalamus. Within SCN neurons, various aspects of cell physiology exhibit circadian oscillations, including circadian clock gene expression, levels of intracellular Ca²⁺ ([Ca²⁺]_i), and neuronal firing rate. [Ca²⁺]_i oscillates in SCN neurons even in the absence of neuronal firing. To determine the causal relationship between circadian clock gene expression and [Ca²⁺]_i rhythms in the SCN, as well as the SCN neuronal network dependence of [Ca²⁺]_i rhythms, we introduced GCaMP3, a genetically encoded fluorescent Ca²⁺ indicator, into SCN neurons from PER2::LUC knock-in reporter mice. Then, PER2 and [Ca²⁺]_i were imaged in SCN dispersed and organotypic slice cultures. In dispersed cells, PER2 and [Ca²⁺]_i both exhibited cell autonomous circadian rhythms, but [Ca²⁺]_i rhythms were typically weaker than PER2 rhythms. This result matches the predictions of a detailed mathematical model in which clock gene rhythms drive [Ca²⁺]_i rhythms. As predicted by the model, PER2 and [Ca²⁺]_i rhythms were both stronger in SCN slices than in dispersed cells and were weakened by blocking neuronal firing in slices but not in dispersed cells. The phase relationship between [Ca²⁺]_i and PER2 rhythms was more variable in cells within slices than in dispersed cells. Both PER2 and [Ca²⁺]_i rhythms were abolished in SCN cells deficient in the essential clock gene Bmal1. These results suggest that the circadian rhythm of [Ca²⁺]_i in SCN neurons is cell autonomous and dependent on clock gene rhythms, but reinforced and modulated by a synchronized SCN neuronal network.

Acute Knockdown of Kv4.1 Regulates Repetitive Firing Rates and Clock Gene Expression in the Suprachiasmatic Nucleus and Daily Rhythms in Locomotor Behavior

Rapidly activating and inactivating A-type K⁺ currents (I_A) encoded by Kv4.2 and Kv4.3 pore-forming (α) subunits of the Kv4 subfamily are key regulators of neuronal excitability. Previous studies have suggested a role for Kv4.1 α-subunits in regulating the firing properties of mouse suprachiasmatic nucleus (SCN) neurons. To test this, we utilized an RNA-interference strategy to knockdown Kv4.1, acutely and selectively, in the SCN. Current-clamp recordings revealed that the in vivo knockdown of Kv4.1 significantly (p < 0.0001) increased mean ± SEM repetitive firing rates in SCN neurons during the day (6.4 ± 0.5 Hz) and at night (4.3 ± 0.6 Hz), compared with nontargeted shRNA-expressing SCN neurons (day: 3.1 ± 0.5 Hz; night: 1.6 ± 0.3 Hz). I_A was also significantly (p < 0.05) reduced in Kv4.1-targeted shRNA-expressing SCN neurons (day: 80.3 ± 11.8 pA/pF; night: 55.3 ± 7.7 pA/pF), compared with nontargeted shRNA-expressing (day: 121.7 ± 10.2 pA/pF; night: 120.6 ± 16.5 pA/pF) SCN neurons. The magnitude of the effect of Kv4.1-targeted shRNA expression on firing rates and I_A was larger at night. In addition, Kv4.1-targeted shRNA expression significantly (p < 0.001) increased mean ± SEM nighttime input resistance (R_in; 2256 ± 166 MΩ), compared to nontargeted shRNA-expressing SCN neurons (1143 ± 93 MΩ). Additional experiments revealed that acute knockdown of Kv4.1 significantly (p < 0.01) shortened, by ∼0.5 h, the circadian period of spontaneous electrical activity, clock gene expression and locomotor activity demonstrating a physiological role for Kv4.1-encoded I_A channels in regulating circadian rhythms in neuronal excitability and behavior.

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.

Synchronous circadian voltage rhythms with asynchronous calcium rhythms in the suprachiasmatic nucleus

The suprachiasmatic nucleus (SCN), the master circadian clock, contains a network composed of multiple types of neurons which are thought to form a hierarchical and multioscillator system. The molecular clock machinery in SCN neurons drives membrane excitability and sends time cue signals to various brain regions and peripheral organs. However, how and at what time of the day these neurons transmit output signals remain largely unknown. Here, we successfully visualized circadian voltage rhythms optically for many days using a genetically encoded voltage sensor, ArcLightD. Unexpectedly, the voltage rhythms are synchronized across the entire SCN network of cultured slices, whereas simultaneously recorded Ca²⁺ rhythms are topologically specific to the dorsal and ventral regions. We further found that the temporal order of these two rhythms is cell-type specific: The Ca²⁺ rhythms phase-lead the voltage rhythms in AVP neurons but Ca²⁺ and voltage rhythms are nearly in phase in VIP neurons. We confirmed that circadian firing rhythms are also synchronous and are coupled with the voltage rhythms. These results indicate that SCN networks with asynchronous Ca²⁺ rhythms produce coherent voltage rhythms.

Dual origins of the intracellular circadian calcium rhythm in the suprachiasmatic nucleus

In mammals, the master circadian clock is located in the suprachiasmatic nucleus (SCN), where most neurons show circadian rhythms of intracellular Ca²⁺ levels. However, the origin of these Ca²⁺ rhythms remains largely unknown. In this study, we successfully monitored the intracellular circadian Ca²⁺ rhythms together with the circadian PER2 and firing rhythms in a single SCN slice ex vivo, which enabled us to explore the origins. The phase relation between the circadian PER2 and Ca²⁺ rhythms, but not between the circadian PER2 and firing rhythms, was significantly altered in Cry1/Cry2 double knockout mice, which display a loss of intercellular synchronization in the SCN. In addition, in Cry1/Cry2 double knockout mice, circadian Ca²⁺ rhythms were abolished in the dorsolateral SCN, but were maintained in the majority of the ventromedial SCN. These findings indicate that intracellular circadian Ca²⁺ rhythms are composed of an exogenous and endogenous component involving PER2 expression.

Temporal variations in presynaptic release probability in the lateral habenula

Rhythmicity plays an important role in a number of biological systems. The habenular complex is reported to contain an intrinsic molecular clock and to show rhythmic expression of circadian clock genes and proteins including per2/PER2. In this study, we observed that there is a temporal rhythmicity in the presynaptic efficacy of the lateral habenula (LHb) neurons. We collected a substantial number of recordings at different time points of the day during the light phase. The frequency and amplitude of spontaneous excitatory transmission were increased in the afternoon compared to recordings performed in the morning. In addition, the paired-pulse ratio and the success rate of minimal stimulation were also significantly different depending on the time of the recording. We did not see any significant differences in recordings obtained from pyramidal neurons of the hippocampus in the same brain slices. Taken together, our data indicates that the LHb exhibits intrinsic temporal oscillation in basal neurotransmission and in presynaptic release probability. Given the rapidly growing interest on the function of the LHb, more careful examination of synaptic transmission in the LHb is thus required.

Regulating the Suprachiasmatic Nucleus (SCN) Circadian Clockwork: Interplay between Cell-Autonomous and Circuit-Level Mechanisms

The suprachiasmatic nucleus (SCN) is the principal circadian clock of the brain, directing daily cycles of behavior and physiology. SCN neurons contain a cell-autonomous transcription-based clockwork but, in turn, circuit-level interactions synchronize the 20,000 or so SCN neurons into a robust and coherent daily timer. Synchronization requires neuropeptide signaling, regulated by a reciprocal interdependence between the molecular clockwork and rhythmic electrical activity, which in turn depends on a daytime Na⁺ drive and nighttime K⁺ drag. Recent studies exploiting intersectional genetics have started to identify the pacemaking roles of particular neuronal groups in the SCN. They support the idea that timekeeping involves nonlinear and hierarchical computations that create and incorporate timing information through the interactions between key groups of neurons within the SCN circuit. The field is now poised to elucidate these computations, their underlying cellular mechanisms, and how the SCN clock interacts with subordinate circadian clocks across the brain to determine the timing and efficiency of the sleep-wake cycle, and how perturbations of this coherence contribute to neurological and psychiatric illness.

Circadian Rhythms in Diet-Induced Obesity

The biological clocks of the circadian timing system coordinate cellular and physiological processes and synchronizes these with daily cycles, feeding patterns also regulates circadian clocks. The clock genes and adipocytokines show circadian rhythmicity. Dysfunction of these genes are involved in the alteration of these adipokines during the development of obesity. Food availability promotes the stimuli associated with food intake which is a circadian oscillator outside of the suprachiasmatic nucleus (SCN). Its circadian rhythm is arranged with the predictable daily mealtimes. Food anticipatory activity is mediated by a self-sustained circadian timing and its principal component is food entrained oscillator. However, the hypothalamus has a crucial role in the regulation of energy balance rather than food intake. Fatty acids or their metabolites can modulate neuronal activity by brain nutrient-sensing neurons involved in the regulation of energy and glucose homeostasis. The timing of three-meal schedules indicates close association with the plasma levels of insulin and preceding food availability. Desynchronization between the central and peripheral clocks by altered timing of food intake and diet composition can lead to uncoupling of peripheral clocks from the central pacemaker and to the development of metabolic disorders. Metabolic dysfunction is associated with circadian disturbances at both central and peripheral levels and, eventual disruption of circadian clock functioning can lead to obesity. While CLOCK expression levels are increased with high fat diet-induced obesity, peroxisome proliferator-activated receptor (PPAR) alpha increases the transcriptional level of brain and muscle ARNT-like 1 (BMAL1) in obese subjects. Consequently, disruption of clock genes results in dyslipidemia, insulin resistance and obesity. Modifying the time of feeding alone can greatly affect body weight. Changes in the circadian clock are associated with temporal alterations in feeding behavior and increased weight gain. Thus, shift work is associated with increased risk for obesity, diabetes and cardio-vascular diseases as a result of unusual eating time and disruption of circadian rhythm.

Ryanodine-sensitive intracellular Ca2+ channels are involved in the output from the SCN circadian clock

The suprachiasmatic nuclei (SCN) contain the major circadian clock responsible for generation of circadian rhythms in mammals. The time measured by the molecular circadian clock must eventually be translated into a neuronal firing rate pattern to transmit a meaningful signal to other tissues and organs in the animal. Previous observations suggest that circadian modulation of ryanodine receptors (RyR) is a key element of the output pathway from the molecular circadian clock. To directly test this hypothesis, we studied the effects of RyR activation and inhibition on real time expression of PERIOD2::LUCIFERASE, intracellular calcium levels and spontaneous firing frequency in mouse SCN neurons. Furthermore, we determined whether the RyR-2 mRNA is expressed with a daily variation in SCN neurons. We provide evidence that pharmacological manipulation of RyR in mice SCN neurons alters the free [Ca²⁺ ]_i in the cytoplasm and the spontaneous firing without affecting the molecular clock mechanism. Our data also show a daily variation in RyR-2 mRNA from single mouse SCN neurons with highest levels during the day. Together, these results confirm the hypothesis that RyR-2 is a key element of the circadian clock output from SCN neurons.

Peptidergic circadian clock circuits in the Madeira cockroach

Circadian clocks control physiology and behavior of organisms in synchrony with external light dark cycles in changing photoperiods. The Madeira cockroach Rhyparobia maderae was the first model organism in which an endogenous circadian clock in the brain was identified. About 240 neurons constitute the cockroach circadian pacemaker network in the accessory medulla. The expression of high concentrations of neuropeptides, among them the most prominent circadian coupling factor pigment-dispersing factor, as well as their ability to generate endogenous ultradian and circadian rhythms in electrical activity and clock gene expression distinguish these pacemaker neurons. We assume that entrainment to light-dark cycles and the control of 24h rest-activity rhythms is achieved via peptidergic circuits forming autoreceptive labeled lines.

The core clock gene Per1 phases molecular and electrical circadian rhythms in SCN neurons

The brain’s biological clock, the suprachiasmatic nucleus (SCN), exhibits endogenous 24-hour rhythms in gene expression and spontaneous firing rate; however, the functional relationship between these neuronal rhythms is not fully understood. Here, we used a Per1::GFP transgenic mouse line that allows for the simultaneous quantification of molecular clock state and firing rate in SCN neurons to examine the relationship between these key components of the circadian clock. We find that there is a stable, phased relationship between E-box-driven clock gene expression and spontaneous firing rate in SCN neurons and that these relationships are independent of light input onto the system or of GABA_A receptor-mediated synaptic activity. Importantly, the concordant phasing of gene and neural rhythms is disrupted in the absence of the homologous clock gene Per1, but persists in the absence of the core clock gene Per2. These results suggest that Per1 plays a unique, non-redundant role in phasing gene expression and firing rate rhythms in SCN neurons to increase the robustness of cellular timekeeping.

Encoding the Ins and Outs of Circadian Pacemaking

The SCN of the mammalian hypothalamus comprises a self-sustained, biological clock that generates endogenous ca. 24-h (circadian) rhythms. Circadian rhythmicity in the SCN originates from the interaction of a defined set of “clock genes” that participate in transcription/translation feedback loops. In order for the SCN to serve as an internal clock that times an internal day corresponding to the external solar day, the intracellular molecular oscillations must be output as physiological signals and be reset by appropriate environmental inputs. Here, the authors consider the mechanisms by which the SCN circadian pacemaker encodes rhythmic output and light input. In particular, they focus on the ionic mechanisms by which SCN neurons encode clock gene output as circa-dian rhythms in spike frequency, as well as cellular and molecular mechanisms by which SCN neurons encode circadian light input through phase heterogeneity in the SCN network. The authors propose that there are 2 distinct classes of ionic mechanisms supporting spike frequency rhythms output—modulation of basal membrane potential and conductance versus modulation of spike production—whereas light input is transformed by cellular communication within the SCN network and encoded by the relative phase relationships among SCN neurons.

The Methamphetamine-Sensitive Circadian Oscillator (MASCO) in Mice

The suprachiasmatic nucleus (SCN) orchestrates synchrony among many peripheral oscillators and is required for circadian rhythms of locomotor activity and many physiological processes. However, the unique effects of methamphetamine (MAP) on circadian behavior suggest the presence of an SCN-independent, methamphetamine-sensitive circadian oscillator (MASCO). Substantial data collected using rat models show that chronic methamphetamine dramatically lengthens circadian period of locomotor activity rhythms and induces rhythms in animals lacking an SCN. However, the anatomical substrate and the molecular components of the MASCO are unknown. The response to MAP is less well studied in mice, a model that would provide the genetic tools to probe the molecular components of this extra-SCN oscillator. The authors tested the effects of chronic MAP on 2 strains of intact and SCN-lesioned mice in constant dark and constant light. Furthermore, they applied various MAP availability schedules to SCN-lesioned mice to confirm the circadian nature of the underlying oscillator. The results indicate that this oscillator has circadian properties. In intact mice, the MASCO interacts with the SCN in a manner that is strain, sex, and dose dependent. In SCN-lesioned mice, it induces robust free-running locomotor rhythmicity, which persists for up to 14 cycles after methamphetamine is withdrawn. In the future, localization of the MASCO and characterization of its underlying molecular mechanism, as well as its interactions with other oscillators in the body, will be essential to a complete understanding of the organization of the mammalian circadian system.

Lithium effects on circadian rhythms in fibroblasts and suprachiasmatic nucleus slices from Cry knockout mice

Lithium is widely used as a treatment of bipolar disorder, a neuropsychiatric disorder associated with disrupted circadian rhythms. Lithium is known to lengthen period and increase amplitude of circadian rhythms. One possible pathway for these effects involves inhibition of glycogen synthase kinase-3β (GSK-3β), which regulates degradation of CRY2, a canonical clock protein determining circadian period. CRY1 is also known to play important roles in regulating circadian period and phase, although there is no evidence that it is similarly phosphorylated by GSK-3β. In this paper, we tested the hypothesis that lithium affects circadian rhythms through CRYs. We cultured fibroblasts and slices of the suprachiasmatic nucleus (SCN), the master circadian pacemaker of the brain, from Cry1-/-, Cry2-/-, or wild-type (WT) mice bearing the PER2:LUC circadian reporter. Lithium was applied in the culture medium, and circadian rhythms of PER2 expression were measured. In WT and Cry2-/- fibroblasts, 10mM lithium increased PER2 expression and rhythm amplitude but not period, and 1mM lithium did not affect either period or amplitude. In non-rhythmic Cry1-/- fibroblasts, 10mM lithium increased PER2 expression. In SCN slices, 1mM lithium lengthened period ∼1h in all genotypes, but did not affect amplitude except in Cry2-/- SCN. Thus, the amplitude-enhancing effect of lithium in WT fibroblasts was unaffected by Cry2 knockout and occurred in the absence of period-lengthening, whereas the period-lengthening effect of lithium in WT SCN was unaffected by Cry1 or Cry2 knockout and occurred in the absence of rhythm amplification, suggesting that these two effects of lithium on circadian rhythms are independent of CRYs and of each other.

The Circadian Clock Gene Period1 Connects the Molecular Clock to Neural Activity in the Suprachiasmatic Nucleus

The neural activity patterns of suprachiasmatic nucleus (SCN) neurons are dynamically regulated throughout the circadian cycle with highest levels of spontaneous action potentials during the day. These rhythms in electrical activity are critical for the function of the circadian timing system and yet the mechanisms by which the molecular clockwork drives changes in the membrane are not well understood. In this study, we sought to examine how the clock gene Period1 (Per1) regulates the electrical activity in the mouse SCN by transiently and selectively decreasing levels of PER1 through use of an antisense oligodeoxynucleotide. We found that this treatment effectively reduced SCN neural activity. Direct current injection to restore the normal membrane potential partially, but not completely, returned firing rate to normal levels. The antisense treatment also reduced baseline [Ca²⁺]i levels as measured by Fura2 imaging technique. Whole cell patch clamp recording techniques were used to examine which specific potassium currents were altered by the treatment. These recordings revealed that the large conductance [Ca²⁺]i-activated potassium currents were reduced in antisense-treated neurons and that blocking this current mimicked the effects of the anti-sense on SCN firing rate. These results indicate that the circadian clock gene Per1 alters firing rate in SCN neurons and raise the possibility that the large conductance [Ca²⁺]i-activated channel is one of the targets.

Circadian Clock Genes Are Essential for Normal Adult Neurogenesis, Differentiation, and Fate Determination

Adult neurogenesis creates new neurons and glia from stem cells in the human brain throughout life. It is best understood in the dentate gyrus (DG) of the hippocampus and the subventricular zone (SVZ). Circadian rhythms have been identified in the hippocampus, but the role of any endogenous circadian oscillator cells in hippocampal neurogenesis and their importance in learning or memory remains unclear. Any study of stem cell regulation by intrinsic circadian timing within the DG is complicated by modulation from circadian clocks elsewhere in the brain. To examine circadian oscillators in greater isolation, neurosphere cultures were prepared from the DG of two knockout mouse lines that lack a functional circadian clock and from mPer1::luc mice to identify circadian oscillations in gene expression. Circadian mPer1 gene activity rhythms were recorded in neurospheres maintained in a culture medium that induces neurogenesis but not in one that maintains the stem cell state. Although the differentiating neural stem progenitor cells of spheres were rhythmic, evidence of any mature neurons was extremely sparse. The circadian timing signal originated in undifferentiated cells within the neurosphere. This conclusion was supported by immunocytochemistry for mPER1 protein that was localized to the inner, more stem cell-like neurosphere core. To test for effects of the circadian clock on neurogenesis, media conditions were altered to induce neurospheres from BMAL1 knockout mice to differentiate. These cultures displayed unusually high differentiation into glia rather than neurons according to GFAP and NeuN expression, respectively, and very few BetaIII tubulin-positive, immature neurons were observed. The knockout neurospheres also displayed areas visibly devoid of cells and had overall higher cell death. Neurospheres from arrhythmic mice lacking two other core clock genes, Cry1 and Cry2, showed significantly reduced growth and increased astrocyte proliferation during differentiation, but they generated normal percentages of neuronal cells. Neuronal fate commitment therefore appears to be controlled through a non-clock function of BMAL1. This study provides insight into how cell autonomous circadian clocks and clock genes regulate adult neural stem cells with implications for treating neurodegenerative disorders and impaired brain functions by manipulating neurogenesis.

KPNB1 mediates PER/CRY nuclear translocation and circadian clock function

Regulated nuclear translocation of the PER/CRY repressor complex is critical for negative feedback regulation of the circadian clock of mammals. However, the precise molecular mechanism is not fully understood. Here, we report that KPNB1, an importin β component of the ncRNA repressor of nuclear factor of activated T cells (NRON) ribonucleoprotein complex, mediates nuclear translocation and repressor function of the PER/CRY complex. RNAi depletion of KPNB1 traps the PER/CRY complex in the cytoplasm by blocking nuclear entry of PER proteins in human cells. KPNB1 interacts mainly with PER proteins and directs PER/CRY nuclear transport in a circadian fashion. Interestingly, KPNB1 regulates the PER/CRY nuclear entry and repressor function, independently of importin α, its classical partner. Moreover, inducible inhibition of the conserved Drosophila importin β in lateral neurons abolishes behavioral rhythms in flies. Collectively, these data show that KPNB1 is required for timely nuclear import of PER/CRY in the negative feedback regulation of the circadian clock.

DOI:http://dx.doi.org/10.7554/eLife.08647.001

Most organisms have an internal clock—known as the circadian clock—that regulates many aspects of their biology and behavior in roughly 24-hr long cycles. In animals, the core of the circadian clock is made of two ‘activator’ proteins and two ‘repressor’ proteins that inhibit the activators so that the levels of all four proteins in cells fluctuate over the cycle.

The activator proteins switch on the genes that encode the repressor proteins. This increases the production of the repressor proteins in an area of the cell called the cytoplasm. The repressor proteins then bind to each other and then move into the nucleus of the cell to inactivate the activator proteins. However, it was not clear how the repressor proteins move into the nucleus.

Lee et al. used a technique called ‘RNA interference’ to study the circadian clock in human cells and fruit flies. The experiments show that a protein called importin β enables the repressor proteins to move into the nucleus. Importin β directly interacted with only one of the repressor proteins (called PER). Previous studies have shown that importin β is able to interact with another protein called importin α, but Lee et al.'s results show that this interaction is not important for importin β's role in the movement of the repressor proteins.

Blocking importin β activity resulted in the loss of circadian rhythms in both human cells and fruit flies, which suggests that importin β performs the same role in many different animals. The circadian clock is disrupted in many cancers, so Lee et al.'s findings may also help to lead us to new treatments to fight these diseases.

DOI:http://dx.doi.org/10.7554/eLife.08647.002

IA Channels Encoded by Kv1.4 and Kv4.2 Regulate Circadian Period of PER2 Expression in the Suprachiasmatic Nucleus

Neurons in the suprachiasmatic nucleus (SCN), the master circadian pacemaker in mammals, display daily rhythms in electrical activity with more depolarized resting potentials and higher firing rates during the day than at night. Although these daily variations in the electrical properties of SCN neurons are required for circadian rhythms in physiology and behavior, the mechanisms linking changes in neuronal excitability to the molecular clock are not known. Recently, we reported that mice deficient for either Kcna4 (Kv1.4(-/-)) or Kcnd2 (Kv4.2(-/-); but not Kcnd3, Kv4.3(-/-)), voltage-gated K(+) (Kv) channel pore-forming subunits that encode subthreshold, rapidly activating, and inactivating K(+) currents (IA), have shortened (0.5 h) circadian periods in SCN firing and in locomotor activity compared with wild-type (WT) mice. In the experiments here, we used a mouse (Per2(Luc)) line engineered with a bioluminescent reporter construct, PERIOD2::LUCIFERASE (PER2::LUC), replacing the endogenous Per2 locus, to test the hypothesis that the loss of Kv1.4- or Kv4.2-encoded IA channels also modifies circadian rhythms in the expression of the clock protein PERIOD2 (PER2). We found that SCN explants from Kv1.4(-/-)Per2(Luc) and Kv4.2(-/-) Per2(Luc), but not Kv4.3(-/-)Per2(Luc), mice have significantly shorter (by approximately 0.5 h) circadian periods in PER2 rhythms, compared with explants from Per2(Luc) mice, revealing that the membrane properties of SCN neurons feedback to regulate clock (PER2) expression. The combined loss of both Kv1.4- and Kv4.2-encoded IA channels in Kv1.4(-/-)/Kv4.2(-/-)Per2(Luc) SCN explants did not result in any further alterations in PER2 rhythms. Interestingly, however, mice lacking both Kv1.4 and Kv4.2 show a striking (approximately 1.8 h) advance in their daily activity onset in a light cycle compared with WT mice, suggesting additional roles for Kv1.4- and Kv4.2-encoded IA channels in controlling the light-dependent responses of neurons within and/or outside of the SCN to regulate circadian phase of daily activity.

Circadian Tick-Talking Across the Neuroendocrine System and Suprachiasmatic Nuclei Circuits: The Enigmatic Communication Between the Molecular and Electrical Membrane Clocks

As with many processes in nature, appropriate timing in biological systems is of paramount importance. In the neuroendocrine system, the efficacy of hormonal influence on major bodily functions, such as reproduction, metabolism and growth, relies on timely communication within and across many of the brain's homeostatic systems. The activity of these circuits is tightly orchestrated with the animal's internal physiological demands and external solar cycle by a master circadian clock. In mammals, this master clock is located in the hypothalamic suprachiasmatic nucleus (SCN), where the ensemble activity of thousands of clock neurones generates and communicates circadian time cues to the rest of the brain and body. Many regions of the brain, including areas with neuroendocrine function, also contain local daily clocks that can provide feedback signals to the SCN. Although much is known about the molecular processes underpinning endogenous circadian rhythm generation in SCN neurones and, to a lesser extent, extra-SCN cells, the electrical membrane clock that acts in partnership with the molecular clockwork to communicate circadian timing across the brain is poorly understood. The present review focuses on some circadian aspects of reproductive neuroendocrinology and processes involved in circadian rhythm communication in the SCN, aiming to identify key gaps in our knowledge of cross-talk between our daily master clock and neuroendocrine function. The intention is to highlight our surprisingly limited understanding of their interaction in the hope that this will stimulate future work in these areas.

A specific role for the REV-ERBα-controlled L-Type Voltage-Gated Calcium Channel CaV1.2 in resetting the circadian clock in the late night

Within the suprachiasmatic nucleus (SCN) of the hypothalamus, circadian timekeeping and resetting have been shown to be largely dependent on both membrane depolarization and intracellular second-messenger signaling. In both of these processes, voltage-gated calcium channels (VGCCs) mediate voltage-dependent calcium influx, which propagates neural impulses by stimulating vesicle fusion and instigates intracellular pathways resulting in clock gene expression. Through the cumulative actions of these processes, the phase of the internal clock is modified to match the light cycle of the external environment. To parse out the distinct roles of the L-type VGCCs, we analyzed mice deficient in Cav1.2 (Cacna1c) in brain tissue. We found that mice deficient in the Cav1.2 channel exhibited a significant reduction in their ability to phase-advance circadian behavior when subjected to a light pulse in the late night. Furthermore, the study revealed that the expression of Cav1.2 mRNA was rhythmic (peaking during the late night) and was regulated by the circadian clock component REV-ERBα. Finally, the induction of clock genes in both the early and late subjective night was affected by the loss of Cav1.2, with reductions in Per2 and Per1 in the early and late night, respectively. In sum, these results reveal a role of the L-type VGCC Cav1.2 in mediating both clock gene expression and phase advances in response to a light pulse in the late night.

Development of circadian oscillators in neurosphere cultures during adult neurogenesis

Circadian rhythms are common in many cell types but are reported to be lacking in embryonic stem cells. Recent studies have described possible interactions between the molecular mechanism of circadian clocks and the signaling pathways that regulate stem cell differentiation. Circadian rhythms have not been examined well in neural stem cells and progenitor cells that produce new neurons and glial cells during adult neurogenesis. To evaluate circadian timing abilities of cells undergoing neural differentiation, neurospheres were prepared from the mouse subventricular zone (SVZ), a rich source of adult neural stem cells. Circadian rhythms in mPer1 gene expression were recorded in individual spheres, and cell types were characterized by confocal immunofluorescence microscopy at early and late developmental stages in vitro. Circadian rhythms were observed in neurospheres induced to differentiate into neurons or glia, and rhythms emerged within 3–4 days as differentiation proceeded, suggesting that the neural stem cell state suppresses the functioning of the circadian clock. Evidence was also provided that neural stem progenitor cells derived from the SVZ of adult mice are self-sufficient clock cells capable of producing a circadian rhythm without input from known circadian pacemakers of the organism. Expression of mPer1 occurred in high frequency oscillations before circadian rhythms were detected, which may represent a role for this circadian clock gene in the fast cycling of gene expression responsible for early cell differentiation.

Network-mediated encoding of circadian time: the suprachiasmatic nucleus (SCN) from genes to neurons to circuits, and back

The transcriptional architecture of intracellular circadian clocks is similar across phyla, but in mammals interneuronal mechanisms confer a higher level of circadian integration. The suprachiasmatic nucleus (SCN) is a unique model to study these mechanisms, as it operates as a ∼24 h clock not only in the living animal, but also when isolated in culture. This "clock in a dish" can be used to address fundamental questions, such as how intraneuronal mechanisms are translated by SCN neurons into circuit-level emergent properties and how the circuit decodes, and responds to, light input. This review addresses recent developments in understanding the relationship between electrical activity, [Ca(2+)]i, and intracellular clocks. Furthermore, optogenetic and chemogenetic approaches to investigate the distinct roles of neurons and glial cells in circuit encoding of circadian time will be discussed, as well as the epigenetic and circuit-level mechanisms that enable the SCN to translate light input into coherent daily rhythms.

Role of Na⁺/Ca²⁺ exchanger in Ca²⁺ homeostasis in rat suprachiasmatic nucleus neurons

Intracellular Ca(2+) is critical to the central clock of the suprachiasmatic nucleus (SCN). However, the role of Na(+)/Ca(2+) exchanger (NCX) in intracellular Ca(2+) concentration ([Ca(2+)]i) homeostasis in the SCN is unknown. Here we show that NCX is an important mechanism for somatic Ca(2+) clearance in SCN neurons. In control conditions Na(+)-free solution lowered [Ca(2+)]i by inhibiting TTX-sensitive as well as nimodipine-sensitive Ca(2+) influx. With use of the Na(+) ionophore monensin to raise intracellular Na(+) concentration ([Na(+)]i), Na(+)-free solution provoked rapid Ca(2+) uptake via reverse NCX. The peak amplitude of 0 Na(+)-induced [Ca(2+)]i increase was larger during the day than at night, with no difference between dorsal and ventral SCN neurons. Ca(2+) extrusion via forward NCX was studied by determining the effect of Na(+) removal on Ca(2+) clearance after high-K(+)-induced Ca(2+) loads. The clearance of Ca(2+) proceeded with two exponential decay phases, with the fast decay having total signal amplitude of ∼85% and a time constant of ∼7 s. Na(+)-free solution slowed the fast decay rate threefold, whereas mitochondrial protonophore prolonged mostly the slow decay. In contrast, blockade of plasmalemmal and sarco(endo)plasmic reticulum Ca(2+) pumps had little effect on the kinetics of Ca(2+) clearance. RT-PCR indicated the expression of NCX1 and NCX2 mRNAs. Immunohistochemical staining showed the presence of NCX1 immunoreactivity in the whole SCN but restricted distribution of NCX2 immunoreactivity in the ventrolateral SCN. Together our results demonstrate an important role of NCX, most likely NCX1, as well as mitochondrial Ca(2+) uptake in clearing somatic Ca(2+) after depolarization-induced Ca(2+) influx in SCN neurons.

Effects of caffeine on circadian phase, amplitude and period evaluated in cells in vitro and peripheral organs in vivo in PER2::LUCIFERASE mice

BACKGROUND AND PURPOSE

Caffeine is one of the most commonly used psychoactive substances. Circadian rhythms consist of the main suprachiasmatic nucleus (SCN) clocks and peripheral clocks. Although caffeine lengthens circadian rhythms and modifies phase changes in SCN-operated rhythms, the effects on caffeine on the phase, period and amplitude of peripheral organ clocks are not known. In addition, the role of cAMP/Ca(2+) signalling in effects of caffeine on rhythm has not been fully elucidated.

EXPERIMENTAL APPROACH

We examined whether chronic or transient application of caffeine affects circadian period/amplitude and phase by evaluating bioluminescence rhythm in PER2::LUCIFERASE knock-in mice. Circadian rhythms were monitored in vitro using fibroblasts and ex vivo and in vivo for monitoring of peripheral clocks.

KEY RESULTS

Chronic application of caffeine (0.1-10 mM) increased period and amplitude in vitro. Transient application of caffeine (10 mM) near the bottom of the decreasing phase of bioluminescence rhythm caused phase advance in vitro. Caffeine (0.1%) intake caused a phase delay under light-dark or constant dark conditions, suggesting a period-lengthening effect in vivo. Caffeine (20 mg·kg(-1) ) at daytime or at late night-time caused phase advance or delay in bioluminescence rhythm in the liver and kidney respectively. The complicated roles of cAMP/Ca(2+) signalling may be involved in the caffeine-induced increase of period and amplitude in vitro.

CONCLUSIONS AND IMPLICATIONS

Caffeine affects circadian rhythm in mice by lengthening the period and causing a phase shift of peripheral clocks. These results suggest that caffeine intake with food/drink may help with food-induced resetting of peripheral circadian clocks.

Regulation of circadian and acute activity levels by the murine suprachiasmatic nuclei

The suprachiasmatic nuclei (SCN) coordinate the daily sleep-wake cycle by generating a circadian rhythm in electrical impulse frequency. While period and phase of the SCN rhythm have been considered as major output parameters, we propose that the waveform of the rhythm of the SCN also has significance. Using implanted micro-electrodes, we recorded SCN impulse frequency in freely moving mice and manipulated its circadian waveform by exposing mice to light-dark (LD) cycle durations ranging from 22 hours (LD 11∶11) to 26 hours (LD 13∶13). Adaptation to long T-cycles (>24 h) resulted in a trough in electrical activity at the beginning of the night while in short T-cycles (<24 h), SCN activity reached a trough at the end of night. In all T-cycle durations, the intensity of behavioral activity was maximal during the trough of SCN electrical activity and correlated negatively with increasing levels of SCN activity. Interestingly, small changes in T-cycle duration could induce large changes in waveform and in the time of trough (about 3.5 h), and accordingly in the timing of behavioral activity. At a smaller timescale (minutes to hours), we observed a negative correlation between SCN activity and behavioral activity, and acute silencing of SCN neurons by tetrodotoxin (TTX) during the inactive phase of the animal triggered behavioral activity. Thus, the SCN electrical activity levels appear crucially involved in determining the temporal profile of behavioral activity and controls behavior beyond the circadian time domain.

Circadian clock and stress interactions in the molecular biology of psychiatric disorders

Many psychiatric disorders are characterized by circadian rhythm abnormalities, including disturbed sleep/wake cycles, changes in locomotor activity, and abnormal endocrine function. Animal models with mutations in circadian "clock genes" commonly show disturbances in reward processing, locomotor activity and novelty seeking behaviors, further supporting the idea of a connection between the circadian clock and psychiatric disorders. However, if circadian clock dysfunction is a common risk factor for multiple psychiatric disorders, it is unknown if and how these putative clock abnormalities could be expressed differently, and contribute to multiple, distinct phenotypes. One possible explanation is that the circadian clock modulates the biological responses to stressful environmental factors that vary with an individual's experience. It is known that the circadian clock and the stress response systems are closely related: Circadian clock genes regulate the physiological sensitivity to and rhythmic release of glucocorticoids (GC). In turn, GCs have reciprocal effects on the clock. Since stressful life events or increased vulnerability to stress are risk factors for multiple psychiatric disorders, including post-traumatic stress disorder (PTSD), attention deficit hyperactivity disorder (ADHD), bipolar disorder (BD), major depressive disorder (MDD), alcohol use disorder (AUD) and schizophrenia (SCZ), we propose that modulation of the stress response is a common mechanism by which circadian clock genes affect these illnesses. Presently, we review how molecular components of the circadian clock may contribute to these six psychiatric disorders, and present the hypothesis that modulation of the stress response may constitute a common mechanism by which the circadian clock affects multiple psychiatric disorders.

Circadian gating of neuronal functionality: a basis for iterative metaplasticity

Brain plasticity, the ability of the nervous system to encode experience, is a modulatory process leading to long-lasting structural and functional changes. Salient experiences induce plastic changes in neurons of the hippocampus, the basis of memory formation and recall. In the suprachiasmatic nucleus (SCN), the central circadian (~24-h) clock, experience with light at night induces changes in neuronal state, leading to circadian plasticity. The SCN's endogenous ~24-h time-generator comprises a dynamic series of functional states, which gate plastic responses. This restricts light-induced alteration in SCN state-dynamics and outputs to the nighttime. Endogenously generated circadian oscillators coordinate the cyclic states of excitability and intracellular signaling molecules that prime SCN receptivity to plasticity signals, generating nightly windows of susceptibility. We propose that this constitutes a paradigm of ~24-h iterative metaplasticity, the repeated, patterned occurrence of susceptibility to induction of neuronal plasticity. We detail effectors permissive for the cyclic susceptibility to plasticity. We consider similarities of intracellular and membrane mechanisms underlying plasticity in SCN circadian plasticity and in hippocampal long-term potentiation (LTP). The emerging prominence of the hippocampal circadian clock points to iterative metaplasticity in that tissue as well. Exploring these links holds great promise for understanding circadian shaping of synaptic plasticity, learning, and memory.

CaMKII is essential for the cellular clock and coupling between morning and evening behavioral rhythms

Daily behavioral rhythms in mammals are governed by the central circadian clock located in the suprachiasmatic nucleus (SCN), and calcium is thought to play a role in the oscillation of the SCN. Kon et al. now find that Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) activity promotes dimerization of CLOCK and BMAL1 and is essential for the cellular oscillation and synchronization among oscillators in the SCN. Kinase-dead CaMKIIα weakened the behavioral rhythmicity and elicited decoupling between the morning and evening activity rhythms.

Daily behavioral rhythms in mammals are governed by the central circadian clock, located in the suprachiasmatic nucleus (SCN). The behavioral rhythms persist even in constant darkness, with a stable activity time due to coupling between two oscillators that determine the morning and evening activities. Accumulating evidence supports a prerequisite role for Ca²⁺ in the robust oscillation of the SCN, yet the underlying molecular mechanism remains elusive. Here, we show that Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) activity is essential for not only the cellular oscillation but also synchronization among oscillators in the SCN. A kinase-dead mutation in mouse CaMKIIα weakened the behavioral rhythmicity and elicited decoupling between the morning and evening activity rhythms, sometimes causing arrhythmicity. In the mutant SCN, the right and left nuclei showed uncoupled oscillations. Cellular and biochemical analyses revealed that Ca²⁺–calmodulin–CaMKII signaling contributes to activation of E-box-dependent gene expression through promoting dimerization of circadian locomotor output cycles kaput (CLOCK) and brain and muscle Arnt-like protein 1 (BMAL1). These results demonstrate a dual role of CaMKII as a component of cell-autonomous clockwork and as a synchronizer integrating circadian behavioral activities.

Brain circadian oscillators and redox regulation in mammals

SIGNIFICANCE

Functional states of organisms vary rhythmically with a period of about a day (i.e., circadian). This endogenous dynamic is shaped by day-night alternations in light and energy. Mammalian circadian rhythms are orchestrated by the hypothalamic suprachiasmatic nucleus (SCN), a brain region specialized for timekeeping. These autonomous ~24-h oscillations are cell-based, requiring transcription-translation-based regulation. SCN circadian oscillations include the maintenance of intrinsic rhythms, sensitivities to input signals, and generation of output signals. These change predictably as time proceeds from dawn to day, dusk, and through the night. SCN neuronal excitability, a highly energy-demanding process, also oscillates over ~24 h. The nature of the relationship of cellular metabolism and excitability had been unknown.

RECENT ADVANCES

Global SCN redox state was found to undergo an autonomous circadian rhythm. Redox state is relatively reduced in daytime, when neuronal activity is high, and oxidized during nighttime, when neurons are relatively inactive. Redox modulates neuronal excitability via tight coupling: imposed reducing or oxidizing shifts immediately alter membrane excitability. Whereas an intact transcription-translation oscillator is necessary for the redox oscillation, metabolic modulation of excitability is too rapid to be under clockwork control.

CRITICAL ISSUES

Our observations lead to the hypothesis that redox state and neuronal activity are coupled nontranscriptional circadian oscillators in SCN neurons. Critical issues include discovering molecular and cellular substrates and functional consequences of this redox oscillator.

FUTURE DIRECTIONS

Understanding interdependencies between cellular energy metabolism, neuronal activity, and circadian rhythms is critical to developing therapeutic strategies for treating neurodegenerative diseases and brain metabolic syndromes.

Molecular mechanisms of the circadian clockwork in mammals

Circadian rhythms enable organisms to co-ordinate biological processes with the predictable 24 h cycle of day and night. Given that molecular clocks that coordinate such biological timing have evolved in almost all organisms, it is clear that being synchronous with the external environment confers competitive advantage. Conversely, it is apparent that being out of phase is detrimental, resulting in a number of clinical conditions, many of which are linked to metabolic dysfunction. The canonical clockwork involves a core set of genes that negatively regulate themselves through a so-called transcription translation feedback loop. However, recent studies describing evolutionarily conserved oscillations in redox reactions link circadian rhythms to metabolic processes, and in particular, redox pathways. In this review we describe the evidence for the interaction between transcriptional loops, redox and metabolism in mammals and suggest the clock may be potential target for the treatment of disease.

Translational profiling of clock cells reveals circadianly synchronized protein synthesis

This study describes, for the first time, the rhythmic translational program within circadian clock cells. The results indicate that most clock cell mRNAs are translated at low-energy times of either mid-day or mid-night, and also that related cellular functions are coordinately regulated by the synchronized translation of relevant mRNAs at the same time of day.

Genome-wide studies of circadian transcription or mRNA translation have been hindered by the presence of heterogeneous cell populations in complex tissues such as the nervous system. We describe here the use of a Drosophila cell-specific translational profiling approach to document the rhythmic “translatome” of neural clock cells for the first time in any organism. Unexpectedly, translation of most clock-regulated transcripts—as assayed by mRNA ribosome association—occurs at one of two predominant circadian phases, midday or mid-night, times of behavioral quiescence; mRNAs encoding similar cellular functions are translated at the same time of day. Our analysis also indicates that fundamental cellular processes—metabolism, energy production, redox state (e.g., the thioredoxin system), cell growth, signaling and others—are rhythmically modulated within clock cells via synchronized protein synthesis. Our approach is validated by the identification of mRNAs known to exhibit circadian changes in abundance and the discovery of hundreds of novel mRNAs that show translational rhythms. This includes Tdc2, encoding a neurotransmitter synthetic enzyme, which we demonstrate is required within clock neurons for normal circadian locomotor activity.

The circadian clock controls daily rhythms in physiology and behavior via mechanisms that regulate gene expression. While numerous studies have examined the clock regulation of gene transcription and documented rhythms in mRNA abundance, less is known about how circadian changes in protein synthesis contribute to the orchestration of physiological and behavioral programs. Here we have monitored mRNA ribosomal association (as a proxy for translation) to globally examine the circadian timing of protein synthesis specifically within clock cells of Drosophila. The results reveal, for the first time in any organism, the complete circadian program of protein synthesis (the “circadian translatome”) within these cells. A novel finding is that most mRNAs within clock cells are translated at one of two predominant circadian phases—midday or mid-night—times of low energy expenditure. Our work also finds that many clock cell processes, including metabolism, redox state, signaling, neurotransmission, and even protein synthesis itself, are coordinately regulated such that mRNAs required for similar cellular functions are translated in synchrony at the same time of day.

Causes and consequences of hyperexcitation in central clock neurons

Hyperexcited states, including depolarization block and depolarized low amplitude membrane oscillations (DLAMOs), have been observed in neurons of the suprachiasmatic nuclei (SCN), the site of the central mammalian circadian (~24-hour) clock. The causes and consequences of this hyperexcitation have not yet been determined. Here, we explore how individual ionic currents contribute to these hyperexcited states, and how hyperexcitation can then influence molecular circadian timekeeping within SCN neurons. We developed a mathematical model of the electrical activity of SCN neurons, and experimentally verified its prediction that DLAMOs depend on post-synaptic L-type calcium current. The model predicts that hyperexcited states cause high intracellular calcium concentrations, which could trigger transcription of clock genes. The model also predicts that circadian control of certain ionic currents can induce hyperexcited states. Putting it all together into an integrative model, we show how membrane potential and calcium concentration provide a fast feedback that can enhance rhythmicity of the intracellular circadian clock. This work puts forward a novel role for electrical activity in circadian timekeeping, and suggests that hyperexcited states provide a general mechanism for linking membrane electrical dynamics to transcription activation in the nucleus.

Diurnal and nutritional adjustments of intracellular Ca2+ release channels and Ca2+ ATPases associated with restricted feeding schedules in the rat liver

Background

Intracellular calcium is a biochemical messenger that regulates part of the metabolic adaptations in the daily fed-fast cycle. The aim of this study was to characterize the 24-h variations of the liver ryanodine and IP₃ receptors (RyR and IP₃R) as well as of the endoplasmic-reticulum and plasma-membrane Ca²⁺-ATPases (SERCA and PMCA) in daytime restricted feeding protocol.

Methods

A biochemical and immunohistochemical approach was implemented in this study: specific ligand-binding for RyR and IP₃R, enzymatic activity (SERCA and PMCA), and protein levels and zonational hepatic-distribution were determined by immunoblot and immunohistochemistry respectively under conditions of fasting, feeding, and temporal food-restriction.

Results

Binding assays and immunoblots for IP₃R1 and 2 showed a peak at the light/dark transition in the ad-libitum (AL) group, whereas in the restricted-feeding (RF) group the peak shifted towards the food-access time. In the case of RyR binding experiments, both AL and RF groups showed a modest elevation during the dark period, with the RF rats exhibiting increased binding in response to feeding. The AL group showed 24-h rhythmicity in SERCA level; in contrast, RF group showed a pronounced amplitude elevation and a peak phase-shift during the light-period in SERCA level and activity. The activity of PMCA was constant along day in both groups; PMCA1 levels showed a 24-h rhythmicity in the RF rats (with a peak in the light period), meanwhile PMCA4 protein levels showed rhythmicity in both groups. The fasted condition promoted an increase in IP₃R binding and protein level; re-feeding increased the amount of RyR; neither the activity nor expression of SERCA and PMCA protein was affected by fasting–re-feeding conditions. Histochemical experiments showed that the distribution of the Ca²⁺-handling proteins, between periportal and pericentral zones of the liver, varied with the time of day and the feeding protocol.

Conclusions

Our findings show that RF influences mainly the phase and amplitude of hepatic IP₃R and SERCA rhythms as well as discrete zonational distribution for RyR, IP₃Rs, SERCA, and PMCA within the liver acinus, suggesting that intracellular calcium dynamics could be part of the rheostatic adaptation of the liver due to diurnal meal entrainment/food entrained oscillator expression.

Fibroblast PER2 circadian rhythmicity depends on cell density

Like neurons in the suprachiasmatic nucleus (SCN), the master circadian pacemaker in the brain, single fibroblasts can function as independent oscillators. In the SCN, synaptic and paracrine signaling among cells creates a robust, synchronized circadian oscillation, whereas there is no evidence for such integration in fibroblast cultures. However, interactions among single-cell fibroblast oscillators cannot be completely excluded, because fibroblasts were not isolated in previous work. In this study, we tested the autonomy of fibroblasts as single-cell circadian oscillators in high- and low-density culture, by single-cell imaging of cells from PER2::LUC circadian reporter mice. We found greatly reduced PER2::LUC rhythmicity in low-density cultures, which could result from lack of either constitutive or rhythmic paracrine signals from neighboring fibroblasts. To discriminate between these 2 possibilities, we mixed PER2::LUC wild-type (WT) cells with nonluminescent, nonrhythmic Bmal1-/- cells, so that density of rhythmic cells was low but overall cell density remained high. In this condition, WT cells showed clear rhythmicity similar to high-density cultures. We also mixed PER2::LUC WT cells with nonluminescent, long period Cry2-/- cells. In this condition, WT cells showed a period no different from cells cultured with rhythmic WT cells or nonrhythmic Bmal1-/- cells. In previous work, we found that low K⁺ suppresses fibroblast rhythmicity, and we and others have found that either low K⁺ or low Ca²⁺ suppresses SCN rhythmicity. Therefore, we attempted to rescue rhythmicity of low-density fibroblasts with high K⁺ (21 mM), high Ca²⁺ (3.6 mM), or conditioned medium. Conditioned medium from high-density fibroblast cultures rescued rhythmicity of low-density cultures, whereas high K⁺ or Ca²⁺ medium did not consistently rescue rhythmicity. These data suggest that fibroblasts require paracrine signals from adjacent cells for normal expression of rhythmicity, but that these signals do not have to be rhythmic, and that rhythmic signals from other cells do not affect the intrinsic periods of fibroblasts.

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

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

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

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

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Selective Gq-mediated reprogramming mediated by intrinsic VIPergic signaling

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Internal structure and unanticipated plasticity of the SCN circadian network unveiled

A mechanism for circadian control of pacemaker neuron excitability

Although the intracellular molecular clocks that regulate circadian (~24 h) behavioral rhythms are well understood, it remains unclear how molecular clock information is transduced into rhythmic neuronal activity that in turn drives behavioral rhythms. To identify potential clock outputs, the authors generated expression profiles from a homogeneous population of purified pacemaker neurons (LN(v)s) from wild-type and clock mutant Drosophila. They identified a group of genes with enriched expression in LN(v)s and a second group of genes rhythmically expressed in LN(v)s in a clock-dependent manner. Only 10 genes fell into both groups: 4 core clock genes, including period (per) and timeless (tim), and 6 genes previously unstudied in circadian rhythms. The authors focused on one of these 6 genes, Ir, which encodes an inward rectifier K(+) channel likely to regulate resting membrane potential, whose expression peaks around dusk. Reducing Ir expression in LN(v)s increased larval light avoidance and lengthened the period of adult locomotor rhythms, consistent with increased LN(v) excitability. In contrast, increased Ir expression made many adult flies arrhythmic and dampened PER protein oscillations. The authors propose that rhythmic Ir expression contributes to daily rhythms in LN(v) neuronal activity, which in turn feed back to regulate molecular clock oscillations.

Cellular mechanisms of circadian pacemaking: beyond transcriptional loops

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

Topological specificity and hierarchical network of the circadian calcium rhythm in the suprachiasmatic nucleus

The circadian pacemaker in the hypothalamic suprachiasmatic nucleus (SCN) is a hierarchical multioscillator system in which neuronal networks play crucial roles in expressing coherent rhythms in physiology and behavior. However, our understanding of the neuronal network is still incomplete. Intracellular calcium mediates the input signals, such as phase-resetting stimuli, to the core molecular loop involving clock genes for circadian rhythm generation and the output signals from the loop to various cellular functions, including changes in neurotransmitter release. Using a unique large-scale calcium imaging method with genetically encoded calcium sensors, we visualized intracellular calcium from the entire surface of SCN slice in culture including the regions where autonomous clock gene expression was undetectable. We found circadian calcium rhythms at a single-cell level in the SCN, which were topologically specific with a larger amplitude and more delayed phase in the ventral region than the dorsal. The robustness of the rhythm was reduced but persisted even after blocking the neuronal firing with tetrodotoxin (TTX). Notably, TTX dissociated the circadian calcium rhythms between the dorsal and ventral SCN. In contrast, a blocker of gap junctions, carbenoxolone, had only a minor effect on the calcium rhythms at both the single-cell and network levels. These results reveal the topological specificity of the circadian calcium rhythm in the SCN and the presence of coupled regional pacemakers in the dorsal and ventral regions. Neuronal firings are not necessary for the persistence of the calcium rhythms but indispensable for the hierarchical organization of rhythmicity in the SCN.

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

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

Electrical activity can impose time of day on the circadian transcriptome of pacemaker neurons

BACKGROUND

Circadian (∼24 hr) rhythms offer one of the best examples of how gene expression is tied to behavior. Circadian pacemaker neurons contain molecular clocks that control 24 hr rhythms in gene expression that in turn regulate electrical activity rhythms to control behavior.

RESULTS

Here we demonstrate the inverse relationship: there are broad transcriptional changes in Drosophila clock neurons (LN(v)s) in response to altered electrical activity, including a large set of circadian genes. Hyperexciting LN(v)s creates a morning-like expression profile for many circadian genes while hyperpolarization leads to an evening-like transcriptional state. The electrical effects robustly persist in per(0) mutant LN(v)s but not in cyc(0) mutant LN(v)s, suggesting that neuronal activity interacts with the transcriptional activators of the core circadian clock. Bioinformatic and immunocytochemical analyses suggest that CREB family transcription factors link LN(v) electrical state to circadian gene expression.

CONCLUSIONS

The electrical state of a clock neuron can impose time of day to its transcriptional program. We propose that this acts as an internal zeitgeber to add robustness and precision to circadian behavioral rhythms.

The effects of glycogen synthase kinase-3beta in serotonin neurons

Glycogen synthase kinase-3 (GSK3) is a constitutively active protein kinase in brain. Increasing evidence has shown that GSK3 acts as a modulator in the serotonin neurotransmission system, including direct interaction with serotonin 1B (5-HT1B) receptors in a highly selective manner and prominent modulating effect on 5-HT1B receptor activity. In this study, we utilized the serotonin neuron-selective GSK3β knockout (snGSK3β-KO) mice to test if GSK3β in serotonin neurons selectively modulates 5-HT1B autoreceptor activity and function. The snGSK3β-KO mice were generated by crossbreeding GSK3β-floxed mice and ePet1-Cre mice. These mice had normal growth and physiological characteristics, similar numbers of tryptophan hydroxylase-2 (TpH2)-expressing serotonin neurons, and the same brain serotonin content as in littermate wild type mice. However, the expression of GSK3β in snGSK3β-KO mice was diminished in TpH2-expressing serotonin neurons. Compared to littermate wild type mice, snGSK3β-KO mice had a reduced response to the 5-HT1B receptor agonist anpirtoline in the regulation of serotonergic neuron firing, cAMP production, and serotonin release, whereas these animals displayed a normal response to the 5-HT1A receptor agonist 8-OH-DPAT. The effect of anpirtoline on the horizontal, center, and vertical activities in the open field test was differentially affected by GSK3β depletion in serotonin neurons, wherein vertical activity, but not horizontal activity, was significantly altered in snGSK3β-KO mice. In addition, there was an enhanced anti-immobility response to anpirtoline in the tail suspension test in snGSK3β-KO mice. Therefore, results of this study demonstrated a serotonin neuron-targeting function of GSK3β by regulating 5-HT1B autoreceptors, which impacts serotonergic neuron firing, serotonin release, and serotonin-regulated behaviors.

Circadian rhythm of redox state regulates excitability in suprachiasmatic nucleus neurons

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

Ca²⁺-dependent ion channels underlying spontaneous activity in insect circadian pacemaker neurons

Electrical activity in the gamma frequency range is instrumental for temporal encoding on the millisecond scale in attentive vertebrate brains. Surprisingly, also circadian pacemaker neurons in the cockroach Rhyparobia maderae (Leucophaea maderae) employ fast spontaneous rhythmic activity in the gamma band frequency range (20-70  Hz) together with slow rhythmic activity. The ionic conductances controlling this fast spontaneous activity are still unknown. Here, Ca(2+) imaging combined with pharmacology was employed to analyse ion channels underlying spontaneous activity in dispersed circadian pacemakers of the adult accessory medulla, which controls circadian locomotor activity rhythms. Fast spontaneous Ca(2+) transients in circadian pacemakers accompany tetrodotoxin (TTX)-blockable spontaneous action potentials. In contrast to vertebrate pacemakers, the spontaneous depolarisations from rest appear to be rarely initiated via TTX-sensitive sustained Na(+) channels. Instead, they are predominantly driven by mibefradil-sensitive, low-voltage-activated Ca(2+) channels and DK-AH269-sensitive hyperpolarisation-activated, cyclic nucleotide-gated cation channels. Rhythmic depolarisations activate voltage-gated Na(+) channels and nifedipine-sensitive high-voltage-activated Ca(2+) channels. Together with Ca(2+) rises, the depolarisations open repolarising small-conductance but not large-conductance Ca(2+) -dependent K(+) channels. In contrast, we hypothesise that P/Q-type Ca(2+) channels coupled to large-conductance Ca(2+) -dependent K(+) channels are involved in input-dependent activity.

Neuropeptide Y-induced phase shifts of PER2::LUC rhythms are mediated by long-term suppression of neuronal excitability in a phase-specific manner

Endogenous circadian rhythms are entrained to the 24-h light/dark cycle by both light and nonphotic stimuli. During the day, nonphotic stimuli, such as novel wheel-induced exercise, produce large phase advances. Neuropeptide Y (NPY) release from the thalamus onto suprachiasmatic nucleus (SCN) neurons at least partially mediates this nonphotic signal. The authors examined the hypothesis that NPY-induced phase advances are accompanied by suppression of PER2 and are mediated by long-term depression of neuronal excitability in a phase-specific manner. First, it was found that NPY-induced phase advances in PER2::LUC SCN cultures are largest when NPY (2.35 µM) is given in the early part of the day (circadian time [CT] 0-6). In addition, PER2::LUC levels in NPY-treated (compared to vehicle-treated) samples were suppressed beginning 6-7 h after treatment. Similar NPY application to organotypic Per1::GFP SCN cultures resulted in long-term suppression of spike rate of green fluorescent protein-positive (GFP+) cells when slices were treated with NPY during the early or middle of the day (zeitgeber time [ZT] 2 or 6), but not during the late day (ZT 10). Furthermore, 1-h bath application of NPY to acute SCN brain slices decreased general neuronal activity measured through extracellular recordings. Finally, NPY-induced phase advances of PER2::LUC rhythms were blocked by latent depolarization with 34.5 mM K(+) 3 h after NPY application. These results suggest that NPY-induced phase advances may be mediated by long-term depression of neuronal excitability. This model is consistent with findings in other brain regions that NPY-induced persistent hyperpolarization underlies mechanisms of energy homeostasis, anxiety-related behavior, and thalamocortical synchronous firing.

Diurnal rhythms in neurexins transcripts and inhibitory/excitatory synapse scaffold proteins in the biological clock

The neurexin genes (NRXN1/2/3) encode two families (α and β) of highly polymorphic presynaptic proteins that are involved in excitatory/inhibitory synaptic balance. Recent studies indicate that neuronal activation and memory formation affect NRXN1/2/3α expression and alternative splicing at splice sites 3 and 4 (SS#3/SS#4). Neurons in the biological clock residing in the suprachiasmatic nuclei of the hypothalamus (SCN) act as self-sustained oscillators, generating rhythms in gene expression and electrical activity, to entrain circadian bodily rhythms to the 24 hours day/night cycles. Cell autonomous oscillations in NRXN1/2/3α expression and SS#3/SS#4 exons splicing and their links to rhythms in excitatory/inhibitory synaptic balance in the circadian clock were explored. NRXN1/2/3α expression and SS#3/SS#4 splicing, levels of neurexin-2α and the synaptic scaffolding proteins PSD-95 and gephyrin (representing excitatory and inhibitory synapses, respectively) were studied in mRNA and protein extracts obtained from SCN of C3H/J mice at different times of the 24 hours day/night cycle. Further studies explored the circadian oscillations in these components and causality relationships in immortalized rat SCN2.2 cells. Diurnal rhythms in mNRXN1α and mNRXN2α transcription, SS#3/SS#4 exon-inclusion and PSD-95 gephyrin and neurexin-2α levels were found in the SCN in vivo. No such rhythms were found with mNRXN3α. SCN2.2 cells also exhibited autonomous circadian rhythms in rNRXN1/2 expression SS#3/SS#4 exon inclusion and PSD-95, gephyrin and neurexin-2α levels. rNRXN3α and rNRXN1/2β were not expressed. Causal relationships were demonstrated, by use of specific siRNAs, between rNRXN2α SS#3 exon included transcripts and gephyrin levels in the SCN2.2 cells. These results show for the first time dynamic, cell autonomous, diurnal rhythms in expression and splicing of NRXN1/2 and subsequent effects on the expression of neurexin-2α and postsynaptic scaffolding proteins in SCN across the 24-h cycle. NRXNs gene transcripts may have a role in coupling the circadian clock to diurnal rhythms in excitatory/inhibitory synaptic balance.

Endurance exercise modifies the circadian clock in zebrafish (Danio rerio) temperature independently

AIM

Several rodent and human studies revealed that physical exercise acts as a non-photic zeitgeber for the circadian clock. The intrinsic entraining mechanism is still unknown, although it was assumed that the exercise-mediated increase in core temperature could be the underlying zeitgeber. As the homoeostatic control of mammalian core temperature interferes strongly with the investigation of this hypothesis, the present study used the poikilotherm zebrafish to answer this question.

METHODS

Gene transcription levels of the two circadian core clock genes period1 and clock1 were quantified using real-time qPCR of whole animal zebrafish larvae.

RESULTS

Long-term endurance exercise of zebrafish larvae aged 9-15 days post-fertilization (dpf) or 21-32 dpf at a constant water temperature of 25 °C caused significantly altered transcription levels of the circadian genes period1 and clock1. Cosinor analysis of diurnal transcription profiles obtained after 3 days of swim training revealed significant differences regarding acrophase, mesor and amplitude of period1, resulting in a phase delay of the gene oscillation. After termination of the exercise bout, at 15 dpf, oscillation amplitudes of both circadian genes were significantly reduced.

CONCLUSION

The results showed that physical exercise is able to affect the transcription of circadian genes in developing zebrafish larvae. Considering the poikilothermy of zebrafish, an exercise-mediated change in body core temperature could be excluded as the underlying intrinsic zeitgeber. However, the day-active zebrafish arises as a useful model to address the synchronizing effect of exercise on the circadian clock.