The Journal of Physiological Sciences (2018) 68:207–219 https://doi.org/10.1007/s12576-018-0597-5

REVIEW

The mammalian circadian system: a hierarchical multi‑oscillator structure for generating

Sato Honma1

Received: 11 December 2017 / Accepted: 25 January 2018 / Published online: 19 February 2018 © The Physiological Society of Japan and Springer Japan KK, part of Springer Nature 2018

Abstract The circadian nature of physiology and behavior is regulated by a circadian clock that generates intrinsic rhythms with a periodicity of approximately 24 h. The mammalian circadian system is composed of a hierarchical multi-oscillator struc- ture, with the central clock located in the (SCN) of the regulating the peripheral clocks found throughout the body. In the past two decades, key clock have been discovered in mammals and shown to be interlocked in transcriptional and translational feedback loops. At the cellular level, each cell is governed by its own independent clock; and yet, these cellular circadian clocks in the SCN form regional oscillators that are further coupled to one another to generate a single rhythm for the tissue. The oscillatory coupling within and between the regional oscillators appears to be critical for the extraordinary stability and the wide range of adaptability of the circadian clock, the mechanism of which is now being elucidated with newly advanced molecular tools.

Keywords Circadian clock · Suprachiasmatic nucleus · Oscillatory coupling · Clock · Luciferase reporter

Introduction Characteristics of circadian rhythms

Robust circadian rhythms in physiological functions and For a biological process to be considered circadian, it must behaviors are conserved across all organisms, from cyano- exhibit the following three characteristics: an endogenous bacteria to humans. These rhythms are driven by an intrinsic free-running cycle, entrainability, and temperature com- machinery called the circadian clock, which generates self- pensation. Under constant environmental conditions, such sustaining rhythms with a periodicity of about (circa) 1 day as continuous darkness (DD) under constant temperature, (dies). In the last two decades, we have come to appreciate a circadian rhythm exhibits intrinsic, self-sustaining cycles the underlying mechanisms of the mammalian circadian with a close to but slightly deviated from 24 h [4]. clock at the molecular, cellular, network, and organismal This cycle is called the free-running rhythm, and distin- levels. Indeed, the 2017 Nobel Prize for Physiology and guishes the process from one that is merely a reaction to Medicine was awarded to three chronobiologists who frst external signals. Free-running rhythms are observed not only cloned the Drosophila clock gene, Period (Per) in 1984 [1, at the organism level, e.g., -wakefulness and behavior 2]. The frst mammalian clock gene, clock, was successfully (Fig. 1a), but also at the cellular level, such as in fuctuations cloned in 1997 [3] and, owing to the remarkable similarities of gene/protein expression and the release of hormones and of the circadian pacemaker system between mammals and (Fig. 1c) [4]. Drosophila, our understanding of the clock mechanism has Although circadian rhythms are intrinsic, living organ- advanced rapidly. isms are also under the infuence of cyclic changes of envi- ronments, such as day–night cycles and seasonal changes in photoperiods and temperature. The alignment of the cir- cadian rhythm to such environmental cues or “” * Sato Honma [email protected] to maintain periodicity is referred to as entrainment. For all organisms, light is the strongest . The circadian 1 Research and Education Center for Brain Science, Hokkaido clock entrains to a 24-h light–dark (LD) cycle resulting in University, North 15, West 7, Kita‑ku, Sapporo 060‑8638, diurnal, nocturnal, or crepuscular behavior. Japan

Vol.:(0123456789)1 3 208 The Journal of Physiological Sciences (2018) 68:207–219

ab2 [6]. For example, a single light pulse at the subjective early LocalTime (h) h)

s( night induces a phase-delay shift, while a pulse during the

6186ft 0 0 shi subjective late night to morning results in a phase-advance shift. No shift is observed when a pulse is given during the -2

10 Phase

LD middle of subjective day. Interestingly, the shape of the PRC 0 6 12 18 24 6 Circadian time (h) has been shown to be remarkably similar across all living

20 )

c in 6 organisms recorded thus far [7], which strongly suggests that 30 the circadian clock developed very early during evolution. s 3 counts/m 0 Experiments using mutants with diferent Day 40

(x1,000 -3 circadian periodicities have demonstrated the evolutionary 50 -6 advantage of having a circadian clock [8]. Without competi- RLU DD 2 4 6 8 10 Days in culture tors, cyanobacteria mutants with a short or prolonged perio-

60 d ) 12

(h dicity proliferated at the same speed as wild type cyanobac-

s 6 70 ft teria. However, when populations with diferent periodicities

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e were mixed, the colony with a circadian period most similar -6 to the environmental periodicity eventually outpaced their Phas -12 0 6 12 18 24 unsynchronized counterparts, highlighting the importance Circadian time (h) of matching environmental cues to life itself. Finally, a circadian rhythm requires the ability to com- Fig. 1 Phase–response curves (PRCs) of the behavior rhythm and pensate for changes in temperature to maintain periodicity. the peripheral clock of mice. a Behavioral activity of a C57BL/6 J Typically, a circadian oscillation would have a Q­ 10 coef- mouse monitored by an area sensor under light–dark (LD) and con- tinuous darkness (DD) cycles. Under DD, the mouse showed a stable cient (a measure of the rate of change in a biological system free-running rhythm that phase-dependently shifted by light pulses dependent upon temperature) of approximately 1.0, meaning (30 min, 300 lx) at circadian times (CT) 14 (on day 48) and CT22 that the periodicity is not afected by changes in increas- (on day 68). The onset phase of behavior activity is designated as ing temperatures that can afect the kinetics of cellular pro- CT12. White stars in the actogram indicate the timing of the light pulses. Regression lines ftted to the onset of consecutive activity cesses [9]. In homothermic animals like mammals and birds, before and after the light pulse demonstrate that the light induced a the signifcance of temperature compensation was initially phase-delay at CT14 and a phase-advance at CT22. b PRC of behav- overlooked as the central circadian clock is located within ioral rhythm after light pulses (mean and standard error) demon- the central nervous system where the temperature is held strates that light pulses (30 min, 300 lx) during early subjective night induce a phase-delay (negative number of the ordinate), while light constant. However, the discovery of peripheral clocks has pulses during the subjective late night to early morning induce phase- reignited the feld to research the importance of temperature advance shifts (positive number of the ordinate). No shift is observed compensation. While the mechanism of how the circadian when a light pulse is administered during subjective day. Shaded clock compensates for the change in temperature is not com- area, subjective night. c Cultured nasal mucosa of a mouse carrying a luciferase reporter for PER2 exhibits robust circadian rhythm in pletely understood, it has been shown that the structure of a PER2::LUCIFERASE (PER2::LUC) levels, which is reset by dexa- clock protein KaiC in cyanobacteria is critical for tempera- methasone (Dex). Data was collected using a photomultiplier tube ture compensation by slowing down the speed of oscillation and is expressed in relative light units (RLU). An arrow indicates the and reducing energy expenditure [10]. Furthermore, casein time of Dex or vehicle treatment. Original data was detrended by sub- tracting the 24-h moving average. Solid lines indicate cultures treated kinase 1 ε was demonstrated to play an important role in with Dex at diferent phases, and the dashed line indicates cultures temperature compensation in mammals [11]. treated with vehicle. d PRC of the nasal clock shows a large phase- The mammalian circadian system is composed of a cen- advance (approximately 10 h) and phase-delay shifts (approximately tral clock located in the suprachiasmatic nucleus (SCN) of − 12 h) (see Ref. [114]) the hypothalamus that regulates additional circadian rhythms found in non-SCN brain regions and in peripheral organs The circadian clock can be phase-shifted by any of these throughout the body [12–14]. The SCN is the only clock that time cues, and the extent and direction (advance or delay) of is reset by the light from the environment; the photic sig- the shifts depend on the phase of the circadian clock at which nals are transferred to the SCN via the retinohypothalamic the cues are introduced. The phase–response curve (PRC) tract (RHT). Once the SCN pacemaker is light-entrained, it summarizes the relationship between the time when the cue, synchronizes the peripheral clocks to the same 24-h cycle such as light (Fig. 1b) and dexamethasone (Fig. 1d), is intro- through mechanisms that remain largely unknown. duced and the induced phase shifts [5]. Using the PRC, we The circadian clock allows us to prepare and adapt our can predict how the clock will respond to the environment, physiology to cyclic changes in our environment. Notably, and we can calculate the time course of re-entrainment after ablation of the circadian clock does not directly terminate changes in the LD cycle, as well as the limit of entrainment the life of organisms, but without a fully functioning clock,

1 3 The Journal of Physiological Sciences (2018) 68:207–219 209 the organisms appear to become vulnerable when placed in a naturally or socially competitive environment. The circadian Cytosol stabilization clock thus plays a critical role in the survival of individuals destabilization FBXL21 CRY and of species as a whole. FBXL3 * CK1 Nucleus PER CRY CRY Core loop PER BMAL CLOCK PER CRY Molecular machinery that generates LRE E-box circadian rhythm at a cellular level Per1,Per2, Cry1,Cry2 Interlocked loop PER CRY degradation CLOCK BMAL Dec1,Dec2 CLOCK DEC LRE E-box Clock genes and molecular feedback BMAL Dec loop RORE Bmal1 The frst known mammalian clock gene, Clock, encoding Bmal loop a basic helix-loop-helix (bHLH) Period-Arnt-Sim (PAS)- RORα CLOCK BMAL E-box REV type factor, was cloned in 1997 [3], which was Reverb, Ror soon followed by the cloning of the clock genes Period 1 (Per1) [15] and Bmal1 [16, 17], encoding a PAS protein and a bHLH-PAS transcription factor, respectively. BMAL1 het- activation rhythmic expression inhibition photic signals erodimerizes with CLOCK and binds to an E-box enhancer (CANNTG) site upstream of the Per gene, inducing Per tran- Fig. 2 scription. The PER protein then translocates to the nucleus Molecular clock of a mammalian cell. Molecular machinery of the circadian clock is composed of interlocked molecular feed- with (CRY) 1 and 2 and binds to the CLOCK/ back loops. The core loop is composed of positive elements (CLOCK BMAL1 heterodimer to inhibit enhancer activity [18]. One and BMAL1) and negative elements (PERs and CRYs). The loop is cycle of this negative feedback loop takes about 24 h, thus interlocked antiphasic to the Bmal1 loop (positive element: ROR; generating circadian rhythmicity (Fig. 2). CRY was initially negative element: REB-ERBα) and in phase with the Dec loop (posi- tive elements: CLOCK and BMAL1; negative elements: DECs). identifed as a photoreceptor in fungi and plants [19, 20], Light-responsive elements such as cAMP response element (CRE), although in mammals, CRY1 and CRY2 have no such pho- to which phosphorylated CREB binds to induce transcription, exists toreceptor function [21]. In mammals, the Cry1 and 2 genes upstream of Per1, Per2, and Dec1 transcription start sites. Post-tran- are also regulated by an E-box enhancer. scriptional and post-translational modifcations such as ubiquitination and are important for regulating the circadian period Due to the similarities between the mammalian and Drosophila clock [1, 2], key members of transcriptional and translational feedbacks were quickly identifed in mammals. by an E-box enhancer located upstream of its transcription And in the case of Clock and Bmal1 (called Cycle in Dros- start site, resulting in an expression pattern that subsequently ophila), Drosophila orthologs were cloned after the clon- puts Bmal1 completely out of phase with Per1 and Per2 [28]. ing of mammalian genes [22, 23]. However, it is important Dec1 and Dec2 are two other elements of an interlocking to note that there are also a few diferences between mam- loop found to negatively afect circadian rhythms [29]. Both mals and Drosophila. For example, mammals have three Dec1 and Dec2 have E-box enhancers and are positively Per homologs, Per1, Per2, and Per3, each of them playing regulated by CLOCK and BMAL1, and negatively by PER1, a specifc role [24]. Furthermore, in Drosophila, TIME- PER2, CRY1, and CRY2. Furthermore, the transcription of LESS protein exhibits photosensitivity and plays a role in Dec1 and Dec2 is self-regulated by their encoded proteins. regulating the circadian rhythm. In contrast, the mammalian DECs also form a mutually interlocked molecular loop with TIMELESS has been shown to regulate and car- the Per loop. These interlocked multiple molecular loops are cinogenesis [25], and its role in the circadian rhythm has thought to be advantageous by providing stability and fne remained elusive. tuning to the circadian periodicity [30]. In addition to the core molecular feedback loop consist- ing of PER1, PER2, CRY1, CRY2, CLOCK, and BMAL1, Molecular mechanisms of light entrainment interlocking feedback loops have also been identifed to regulate circadian rhythms. One example is the Bmal1 loop, The circadian clock in the SCN is reset by photic signals that which interlocks with the core loop [26] and exhibits an are received by -expressing, intrinsically photosen- antiphasic pattern [17]. Bmal1 is regulated sitive retinal ganglion cells (ipRGCs) in the retina [31–33] and by a retinoic acid receptor-related (ROR) are transmitted via the RHT [34]. Among the fve subtypes enhancer site located upstream of the Bmal1 gene; ROR of ipRGCs (M1–M5), the M1 ipRGCs predominantly (and, binding activates gene expression, while REB-ERBα bind- to a lesser extent, the M2 ipRGCs) project photic signals to ing inhibits transcription [27]. REB-ERBα itself is regulated the SCN to entrain the circadian clock [35]. ipRGCs are also

1 3 210 The Journal of Physiological Sciences (2018) 68:207–219 involved in light-induced pupil constriction, masking behavior indicating a critical role of post-transcriptional regulation in in response to light, and even certain kinds of image-forming the molecular circadian clock. For example, an RNA edit- visual pathways [36–39]. ing enzyme, Adarb1, has been shown to have a circadian The molecular mechanisms of light entrainment have expression pattern. As a result, its downstream targets also been intensively studied. Photic signals are projected to the exhibited post-transcriptionally regulated oscillations [44]. SCN core where glutamate, the of the RHT, Post-translational modifcations also afect the circadian induces phosphorylation of Ca­ 2+-cAMP response element- rhythm, especially the period length. Tau was the frst mam- binding (CREB) protein, resulting in the transcription of some malian clock gene identifed in a mutant hamster that dis- key clock genes (Fig. 2). Photic induction of Per1 and Per2 played a short circadian period [45]. The cloning of tau was [40, 41], but not Per3 [24], is detected only during the photo- delayed compared to that of Clock [3] and Per [15] for many sensitive zone of PRC (Fig. 1). Dec1, but not Dec2, expression years because it was found in hamsters in which genome is also induced by phase-resetting light stimuli [29]. Photic sig- information was limited. Tau mutation is a missense muta- nals administered during the dead-zone of PRC do not induce tion in the phosphorylation site of casein kinase 1 ε (CK1ε) Per or Dec expression and do not lead to shifts in behavioral [46], a kinase which phosphorylates many clock proteins, rhythms. Interestingly, light induction of these gene expres- including PER and CRY. Soon after the fnding of the Tau sions is observed at the core of the SCN where the circadian mutation in hamsters, patients with advanced sleep-phase clock gene expression level is low, whereas circadian gene syndrome (ASPS) with short circadian periods, were shown expression is high in the shell region of the SCN [40]. Among to possess a missense mutation in the Per2 gene at the CK1ε these genes, Per1 and Dec1 expressions are induced by lights phosphorylation site [47]. Interestingly, further studies with the kinetics comparable to those of an immediately early revealed the Tau mutation to be a gain-of-function muta- gene. Furthermore, Per1 antisense injection into the SCN tion, which instead induced hyperphosphorylation in other blocked light-induced phase shifts, suggesting a critical role regions of the PER2 protein, resulting in shortening of the of Per1 in mediating light entrainment. Per and Dec genes molecular oscillation period [48]. Other kinases were also exhibit robust circadian expression rhythms in the SCN with found to afect the periodicity, such as glycogen synthase a peak located during the subjective day, i.e., the dead-zone of kinase 3 β [49]. PRC (Fig. 2). When light is given in subjective evening when In addition to phosphorylation, other post-translational Per expression is decreasing, light-induced gene expression modifcations such as ubiquitination [50, 51], acetylation may elongate this part of the cycle, resulting in a phase-delay [52], and SUMOylation [53] have been shown to play of the molecular clock [40]. In contrast, when light is given in important roles in the fne-tuning of the circadian period. the subjective morning hours when Per expression is increas- For example, FBXL3, an F-box-type E3 ligase, ubiquitinates ing, light-induced gene expression makes an early start of the CRY1 and CRY2 in the nucleus, which mediates their deg- rising phase of the cycle, resulting in the advancement of the radation and shortens the circadian period (Fig. 2). In con- molecular clock. Since the gene expression would already be trast, FBXL21 ubiquitinates CRYs in the cytosol and sta- high during the subjective day at the dead-zone of PRC, we bilizes them. Thus, FBXL21 in the cytosol counteracts the would not expect a light-induced shift of the molecular clock. period-shortening efects of FBXL3 in the nucleus [54, 55]. Although this mechanism explains light-induced phase In addition to the familiar ASPS due to Per2 mutation [47], shifts and light entrainment, Per1, Per2, or Dec1 knockout various mutations have already been identifed in diferent mice show LD-entrained behavioral rhythms similar to those types of sleep–wake rhythm disorders in humans [56–58]. of wild type mice, suggesting redundancy in gene functions Clock gene functions have also become research targets for or alternative mechanisms. Among the light-inducible clock mood [59–61] and neurodevelopmental [62–64] disorders genes, Per1 and Dec1 are expressed immediately following that result in disrupted circadian rhythms. light exposure, and their mRNAs have a relatively short half- The multiple interacting molecular loops, post-tran- life [29, 40]. Subsequently, Per2 expression is observed, and scriptional, and post-translational modifications of the its high mRNA level is maintained for a longer period than clock genes increase the stability as well as adaptability of for Per1 [41, 42]. Thus, it appears that Per1 and Per2 have the system. Further study is needed to fully understand the non-redundant roles in light entrainment. mechanisms that regulate the clock at the molecular and cellular levels. Post‑transcriptional and post‑translational regulation of molecular oscillation Technical advances in long‑term monitoring of physiological functions Recent studies have demonstrated that 70–80% of mRNA demonstrating circadian rhythms in their content did not Canonical clock genes such as Per1, Per2, and Bmal1 exhibit exhibit circadian rhythm in their de novo transcription [43], robust circadian rhythms in their transcriptional activity and

1 3 The Journal of Physiological Sciences (2018) 68:207–219 211 protein levels [15, 17, 41, 65]. To monitor their circadian a rhythms in living cells for prolonged periods of time, biolu- minescent and fuorescent reporters have been introduced. Among them, luciferase reporters have been widely used to analyze clock gene expression and circadian rhythms because detection of luminescence produced by the lucif- erin–luciferase reaction requires no excitation. This low U RL phototoxicity is advantageous for long-term, continuous recording of the reporters. To detect these signals, a sensi- b tive photon counting device called a photomultiplier tube (PMT) can be used; however, PMTs lack spatial resolution [14, 66]. To overcome this, a charge coupled device (CCD) (counts/h)

camera can be used to image bioluminescence to monitor U RL circadian rhythms at the cellular level. To detect very dim Dexsamethasone (100nM) luminescence, CCD and electron multiplying CCD cameras can be cooled to − 60 to −80 ℃ to reduce background noise Time (hours) [67, 68]. c Since 1972, it was thought that the SCN was the only site 200 ) of a circadian clock in mammals [34, 69]. However, recent studies using the luciferase reporter technique revealed that 100 30 (count/min every tissue of the body actually has its own circadian clock, 20 ior called the peripheral clocks (Fig. 3a) [14, 65]. Furthermore, 10 RLU (counts/min)

every single cell in culture, as well as the ones that were 0 0 Behav frozen and stored in the laboratory for many generations, 2 4 6 8 10 12 14 Time (days) maintain their cellular clock (Fig. 3b) [70]. Cellular rhythms are synchronized and amplifed by dexamethasone treatment, Fig. 3 suggesting the glucocorticoids as time cues for peripheral Circadian rhythm of clock gene expression monitored using luciferase reporters. a Bioluminescence rhythms of cultured explants clocks. Moreover, continuous in vivo bioluminescence (300-μm-thick slices of the SCN, lung, and liver) and an eye from monitoring (Fig. 3c) in freely moving mice is now available transgenic mice carrying a Bmal1 promoter-driven luciferase reporter [71], which allows us to examine the responses of the cir- (Bmal1-luc) (see Ref. [115]). Data was monitored by a PMT and cadian clock to various external stimuli [72, 73]. How such expressed in relative light units (RLU). Horizontal gray and black bars on the abscissa indicate the light–dark (LD) cycles to which ani- techniques are contributing to the research of mammalian mals were entrained. b Bmal1-luc rhythm of a single Rat-1 fbroblast circadian clocks, especially in the SCN, will be described cell before and during dexamethasone treatment. c In vivo recording in the following sections. of bioluminescence using fber optics situated above the SCN (gray line). Behavioral activity (black histogram) was monitored simultane- ously from a freely moving mouse carrying a Per1-luc reporter. Hor- izontal gray and black bars on the abscissa indicate subjective light The central circadian pacemaker, SCN and dark phases, respectively. Bioluminescence data was smoothed using the 4-h moving average method [72] Architecture of the SCN

The SCN is a pair of oval nuclei in the anterior hypothala- mus, located just above the optic chiasm, and its structure intergeniculate leafet. In contrast, the shell consists of pri- is conserved from rodents to primates. The SCN is cyto- marily arginine vasopressin (AVP) neurons, while a small chemically and functionally classifed into two regions, number of somatostatin neurons are also found. The SCN the core (also called the ventrolateral region) and the shell shell is considered to play an important role in rhythm (dorsomedial region) (Fig. 4a, b) [74–76]. The core pre- output. In addition, neuromedin S-containing neurons are dominantly consists of vasoactive intestinal polypeptide distributed both in the core and the shell [77]. Neuromedin (VIP) and gastrin-releasing peptide (GRP), and receives S is expressed almost exclusively in the SCN in the brain, major aferents. The RHT from the retina is the most although its physiological role in the SCN is still unknown important input and its neurotransmitters are glutamate [78]. Interestingly, almost all SCN neurons contain GABA and PCAP. The core also receives serotonergic inputs from [79, 80]; however, it is also not understood why the central the raphe nuclei and GABA/NPY inputs from the thalamic circadian pacemaker would be composed almost entirely of GABAergic neurons.

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culture are synchronized to one another, and the slice-to- a b slice variability is much smaller than that observed in dis- AVP/VIP dorsal V3 sociated cell culture [84]. Free-running periods of behavior V3 Shell (AVP/GABA) were distributed in an even narrower range than in slices NPY, GABA [85]. These fndings suggest that in an intact SCN, individual Core IGL neurons communicate to achieve synchronized periodicity (VIP/GRP/GABA) 5HT Raphe representative of the majority of constitutional cellular oscil- lators. The coupling of the oscillator network in the SCN is ventral OC OC Retina Glu. PACAP critical for generating a coherent output from the SCN and, subsequently, for the regulation of the peripheral clocks. cdLD18:6 LD6:18 Bioluminescence imaging of coronal SCN slices of V3 either transgenic mice expressing a Per1-promoter-driven post luciferase reporter (Per1-luc) or knock-in mice carrying a PER2::LUCIFERASE fusion reporter (PER2::LUC) dem- onstrated robust cellular circadian rhythms with synchro- nized periodicity. However, the phase distribution within the SCN was specifc to topology and changed dynamically 200 OC ant in response to a number of perturbing signals, in particular, photic signals [86, 87]. Abrupt phase shifts in an LD cycle 0012 0012 induced phase shifts in behavioral rhythm with a transient Local time (h) Local time (h) period of approximately 1 week. This transient period is ef regarded as the lag between the immediate phase shift of post. 180 210 the central clock and the gradual shifts of the peripheral ant. clocks involved in overt rhythm expression. Interestingly, 120 140 the rate of change in clock gene expression rhythms appears 60 70 to be region-dependent; a shift was immediately observed in the SCN core following the LD perturbation, while a more Bioluminescence 0 0 0 12 0 12 0 12 0 0 12 0 12 0 12 0 gradual change was recorded over several days in the shell Local time (h) Local time (h) [86]. This fnding supports the presence of regional oscilla- tors in the SCN, which are synchronized under steady-state Fig. 4 Photoperiodic clock in the SCN. a A coronal section of a conditions, but become distinct in the presence of external mouse SCN immunolabeled with anti-AVP (green) and anti-VIP stimuli. Previously, we reported the separation of the core (red). Scale bar: 200 μm; OC, optic chiasm; V3, third ventricle. b and shell rhythms by measuring AVP and VIP releases from Schematic drawing showing the cytoarchitecture of the SCN with an organotypic SCN slice culture [88]. We found that the major aferents and their neurotransmitters. Glu, glutamate; NPY, neuropeptide Y. c, d Phase maps for Per1-luc rhythms of horizon- circadian rhythms of the release of the two peptides were tal SCN slices from mice housed under light–dark (LD) cycles 18:6 initially synchronized; however, if they were treated with (L:3:00–21:00) (c) and 6:18 (L:9:00–15:00) (d). Pseudocolored antimitotic drugs at the beginning of the culture, the two Per1 luc bioluminescent images of peak phases of - rhythms (each systems became gradually desynchronized over time. This pixel = 3.7 × 3.7 μm). The peak phases are widely distributed in LD18:6 (from approximately 3:00 in the central SCN to 22:00 in the demonstrated that the synchrony was kept among the iden- anterior SCN), and are consolidated between 9:00–14:00 in LD6:18. tical peptidergic cells, but AVP and VIP neurons could be (see Ref. [116]). e, f, Bioluminescence from single pixels of horizon- separated, suggesting the presence of two types of cellular tal SCN slices indicate a large regional phase distribution from the networks within the SCN. posterior (post) to anterior (ant) SCN in LD18:6, but are synchro- nized across the SCN as a whole in LD18:6 (see Ref. [93]) Another type of regional oscillator was reported in ham- sters exhibiting split rhythms (two activity components that were 180 degrees out of phase) under constant light Regional oscillators and networks (LL). When rhythm splitting was observed in their behavior, gene expression rhythms of clock genes such as Per1, Per2, Dispersed SCN neurons exhibit circadian rhythms in their and Bmal1 were desynchronized and exhibited anti-phasic clock gene expression and spontaneous firing [81–83]. rhythms between the right and left SCN [89], even though Importantly, SCN neurons in wild type animals exhibit inde- the phase relation among these clock genes was intact uni- pendent circadian rhythms, with cell-specifc periods with laterally. Additionally, each SCN independently regulated a Gaussian distribution covering a wide range of 20–30 h. their output functions, such as the release of gonadotrophin- In contrast, the circadian periods of SCN neurons in a slice releasing hormone [90]. This result demonstrates that under

1 3 The Journal of Physiological Sciences (2018) 68:207–219 213 certain circumstances, the circadian clocks of the right and distinct localization of the E and M oscillator cells were not left SCN can also be separated. detected in the PER2::LUC mice. Per1 and Per2 were shown to have non-redundant roles in the photoperiodic response. Oscillators in the SCN that regulate seasonal changes in behavior Clock gene mutation revealed clock mechanisms at cellular and network levels Another set of regional oscillations were found to change their phase relations when animals were exposed to difer- Gene mutations of clock and clock-related genes can induce ent photoperiods. Nocturnal rodents are known to entrain rhythm disturbances at multiple levels in the organism. In to a wide range of photoperiods, from an LD cycle of light most mutant mice that exhibited altered circadian periods in 0.5 h and dark 23.5 h (LD 0.5:23.5) to an LD 20:4. In order behavior, similar deviations were also observed in their dis- to explain this wide range of adaptability, the two-oscillator persed SCN cells [94, 95]. However, behavior rhythms are model was proposed in 1976 [91]. In this model, the even- also afected by the mutation afecting rhythm coupling and ing (E) oscillator is reset by a light-of signal and regulates output. Despite no known role of VIP in the generation of the onset of activity, whereas the morning (M) oscillator is circadian rhythm in the SCN, mice lacking its receptor, VIP reset by a light-on signal and regulates the end of activity. Receptor 2 (VIPR2), often exhibit behavioral arrhythmicity In order to study the localization and the molecular mecha- in DD [96]. They also display a variety of abnormalities nisms of the E and M oscillators, we exposed the Per1-luc such as splitting and relative coordination [97]. VIPR2 is mice to either LD18:6 (long day) or LD 6:18 (short day) for widely expressed in neurons of both the SCN core and shell. 3 weeks. On the last day of the experiment, we collected two In cultured SCN slices, individual SCN cells exhibited sig- consecutive coronal SCN slices and found that the Per1-luc nifcant circadian rhythms, but their circadian periods were rhythm of the posterior SCN phase-led the anterior counter- distributed over a wider range of time compared to that of part without exception [92]. Notably, the phase diference wild type mice [98]. Collectively, these results demonstrate between the two SCN slices increased with the length of the the importance of coupling in vivo, and that the mechanism light phase, and in mice exposed to LD18:6, the Per1-luc that translates circadian rhythm in the SCN to behavior is far signal peaked twice a day in the anterior SCN slice, in the more complicated than we had previously expected. / / morning and in the evening. Furthermore, we found that Cry1/Cry2-double-deficient (Cry1− −/Cry2− −) mice the Per1-luc rhythm in the anterior SCN was phase-locked become behaviorally arrhythmic immediately after they to the onset of behavior activity, while that of the posterior are exposed to DD [99]. Since CRYs are critical mol- SCN coincided with the end of behavior activity, indicat- ecules for inhibiting CLOCK and BMAL1 heterodimer- ing the anterior and posterior SCN as sites of the E and M mediated activation of Per, the arrhythmic behavior in / / oscillators, respectively. We also observed a third oscillator, Cry1− −/Cry2− − mice has been regarded as a result of the which exhibited a peak in the early morning in the anterior termination of molecular oscillation. In addition, noctur- / / SCN only under LD18:6; the function of this was unknown. nal behavior of Cry1− −/Cry2− − mice under an LD cycle To identify the precise location of the three oscillators in has been regarded as the masking of behavior by light and the SCN, we made horizontal SCN slices that contained both darkness [99]. However, we found that the onset phase of / / the anterior and posterior SCNs in a single slice. A geomet- nocturnal behavior in Cry1− −/Cry2− − mice systematically rical transformation technique was applied to set the shape changed according to the period length of the LD cycle (T of the cultured SCNs to the same shape as the arbitrary- cycle) [100], which further indicated that a light-entrain- selected template SCN so that the bioluminescence image able circadian oscillator was present even in mice lack- data could be statistically evaluated [93]. We found that the ing CRYs. We predicted that if individual cellular clocks oscillator cells located in the posterior tip corresponded to with a wide range of periods were unable to synchronize, the M oscillator; cells of the open-ring shape covering the but still retained the ability to entrain to an LD cycle, anterior SCN corresponded to the E oscillator; and cells in then coherent rhythm outputs observed under LD would the center appeared to respond to light. The light-responsive be immediately lost due to desynchronization of consti- area may transfer the lights-on and -of signals to the M and tutional cellular clocks. As expected, adult SCN slices of / / E oscillators, respectively. Cry1− −/Cry2− − lacked circadian rhythm in a whole SCN We also analyzed the rhythms of PER2::LUC mice tissue; however, sloppy but statistically signifcant circadian to compare the roles of Per1 and Per2 in photoperiodic rhythms were observed in individual SCN cells. Surpris- / / responses. Intriguingly, the peak phases of Per1-luc rhythms ingly, Cry1− −/Cry2− − SCNs of neonatal mice exhibited changed almost proportionally to the changes in photoperi- robust and synchronous circadian rhythms in their clock ods (Fig. 4c–f), whereas those of PER2:LUC exhibited gene expression (Fig. 5a). The ability of the cellular oscil- minimal changes in the circadian phase within a slice. The lators to synchronize decreased over time, and the SCN

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/ / tissue became totally arrhythmic by postnatal week 3 [101]. of cellular rhythms in Cry1− −/Cry2− − mice resulted in Furthermore, when the SCN neurons from postnatal day 6 arrhythmicity of the SCN tissue and behavior. Interestingly, mice (when synchronized PER2::LUC rhythms were still their clock gene expression rhythms in the SCN tissue were / / observed in Cry1− −/Cry2− − SCN) were dissociated or when rescued upon co-culturing with wild type SCN [101], sug- those in a cultured SCN slice were treated with tetrodotoxin gesting that a difusible factor(s) was involved in the syn- (TTX), individual cellular rhythms were desynchronized chronization of cellular clocks. Furthermore, only neonatal and period lengths were distributed over an extremely wide wild type SCN were able to rescue the arrhythmic adult / / range, compared to the narrow distribution of periodic- Cry1− −/Cry2− − SCN, indicating that the release of this dif- ity observed in wild type SCN cells. Desynchronization fusible factor was age-dependent. Although the wild type SCN exhibits robust circadian rhythms with synchronized cellular oscillations throughout their life, these results sug- a Wild type Cry1-/-/Cry2-/- gest that the mechanism for rhythm synchronization changes 8 from a CRY-independent to a CRY-dependent coupling one during postnatal development. P7 4 Using triple knockout mice lacking VIPR2, CRY1, / / / and CRY2 (VIPR2− −/Cry1− −/Cry2− −), we demonstrated 0 that the CRY-independent coupling found during the neo- 8 natal period was dependent on VIP signaling, and that

counts/min) AVP and VIP had redundant roles in the coupling of cel- 4 P14 lular rhythms [98]. AVP expression was attenuated in the / / Cry1− −/Cry2− − SCN and VIP signaling naturally lost its 10,000 0 8 ability to regulate rhythm coupling over time, thus result- ing in the loss of synchronous cellular rhythms in adult / / RLU (x − − − − 4 P21 Cry1 /Cry2 SCN slices. Recovery of AVP signaling from the neonatal wild type SCN rescued circadian rhythms −/− −/− −/− 0 in both the adult VIPR2 /Cry1 /Cry2 (Fig. 5b) and / / 0 2 4 6 8 0 2 4 6 8 Cry1− −/Cry2− − SCNs. It appears that peptidergic signal- Days in culture ing is critical for generating coordinated rhythm expression b from the SCN, and may be advantageous over the fast clas- -/- -/- -/- ) Recipient: Vipr2 /Cry1 /Cry2 (adult PER2::LUC) sical neurotransmitter signaling, which exerts its efect in the 4 order of milliseconds. Graft: wild type (P7, no reporter) 3 Antagonists, vehicle Circadian rhythms in calcium levels and membrane counts/min 2 potential 4

10 1 Intracellular ­Ca2+ mediates input signals to the molecular 0 clock of the cell (e.g., phase-resetting stimuli to the SCN) RLU (x 0 4 8 12 16 and output signals from the molecular clock (e.g., neuro- Days in culture transmitter release). Taking advantage of the improvements in long-term and large-scale ­Ca2+ imaging, we were able to 2+ Fig. 5 Development of cellular networks in the suprachiasmatic monitor circadian Ca­ rhythms across all neurons of cul- nucleus (SCN). a PER2::LUC rhythms in SCN slices at postnatal tured SCN slices [102, 103]. In every section of the SCN day (P) 7, P14, and P21 were monitored using a photomultiplier tube Cry1−/−/Cry2−/− neuron examined so far — including the regions with low (PMT). The SCNs of mice exhibit a robust circa- 2+ dian rhythm at P7; the rhythm is gradually dampened and completely clock gene expression — a robust circadian ­Ca rhythm undetectable by P21. Wild type SCN exhibits robust rhythms in both with a topologically specifc phase distribution was observed neonatal and adult cultures. Data from three littermates are shown [104]. Circadian ­Ca2+ rhythms of the dorsal SCN neurons for each panel. Vertical lines in the graph indicate 0:00 local time. phase-led those of the ventral neurons, similarly to the Numbers on the right margin indicate the age of each SCN culture. −/− −/− −/− Per1 Per2 b PER2::LUC rhythm of an adult Vipr2 /Cry1 /Cry2 SCN is phase-distribution pattern observed for and [67]. rescued by co-culturing with neonatal SCN of wild type mice carry- The gap junction blocker carbenoxolone had minimal efects ing no luciferase reporter on the 5th day of culture (indicated by an on the ­Ca2+ rhythm at the single cell as well as at the net- arrow). The antagonists for V1a and V1b AVP receptors suppressed work levels, while the Na­ + channel blocker TTX uncoupled the rhythm (gray line), indicating that AVP from the wild type SCN 2+ is critical for the coupling of cellular rhythm. Black, vehicle treatment the ­Ca rhythms between the dorsal and ventral SCNs. 2+ (see Ref. [98]) Under TTX treatment, Ca­ rhythms continued although

1 3 The Journal of Physiological Sciences (2018) 68:207–219 215 the amplitude was reduced by approximately 30% [104]. a FEO, MAO These fndings suggest that neural fring may be necessary to establish the hierarchy among the multiple regional oscil- Oscillator networks lators in the SCN, even though the synchronization within Retina in the CNS the regional oscillator are independent of the cellular trans- Light SCN mission via ­Na+ channels or gap junctions. The 30% reduc- tion of amplitude under TTX treatment is thought to refect the ­Ca2+ increase due to input signals in the overall ­Ca2+ Oscillator networks Shell in the SCN rhythms. b The SCN outputs circadian signals to multiple brain areas Core and peripheral clocks to orchestrate the circadian rhythms (anterior posterior) throughout the body. In addition to the multielectrode array left right dish (MED) to record spontaneous fring from multiple neurons of the SCN, we found that the genetically encoded Neural signals Humoral signals voltage sensor ArchLightD is a powerful tool to monitor c circadian rhythm of the membrane potential from entire SCN slices. Unexpectedly, the circadian voltage rhythms were synchronous throughout the coronal SCN slice [105, 106]. Circadian rhythms of spontaneous fring, simultane- ously monitored by the MED system, were also synchronous between the dorsal and ventral SCN, despite the topologi- cally specifc phase distribution of Ca­ 2+ rhythms within the SCN. The similarity in phase distribution between the Per1- luc/PER2::LUC and the ­Ca2+ rhythms indicates that the syn- Outputs to behavior and peripheral clocks chronous rhythm of the neural outputs at the network level is independent of the molecular clock of individual neurons. Fig. 6 Hierarchical multi-oscillator circadian system. a In addition The diferences in regulation of the fring and clock gene to the central circadian clock in the SCN, two extra-SCN oscillators, food-entrainable oscillator (FEO) and methamphetamine-induced expression rhythms [86, 87] may allow for the precise phase oscillator (MAO), are known to regulate various physiological func- regulation of rhythm outputs. tions, including sleep–wake rhythms. These are under the control of the SCN, but can be dissociated when food is restricted at cer- tain hours in daytime (FEO) or methamphetamine is administered Perspectives (MAO). b The SCN is composed of regional oscillators such as those in the core and shell, right and left, and anterior and posterior, each of which is comprised of multiple cellular oscillators. Regional oscilla- In this review, I have highlighted the recent progress made tors are dissociable under specifc conditions such as LD shifts, con- in the understanding of the mammalian circadian clock, stant light, and photoperiod changes. c Peripheral clocks and behav- ioral rhythms are thought to be orchestrated by the SCN via neural with a special focus on cellular oscillation mechanisms signals (e.g., sympathetic nerves) and humoral signals (e.g., hor- and oscillator networks in the SCN. We now know that the mones and cytokines) circadian system is composed of the central clock in the SCN and peripheral clocks found throughout in the body (Fig. 6). The mechanisms remain to be elucidated as to how The circadian clock regulates neuronal, metabolic, and the SCN orchestrates these peripheral clocks and how these hormonal functions, and many clock gene mutants exhibit clocks cross their rhythm signals. In addition, mammals disturbances in sleep–wake rhythms and metabolisms have at least two extra-SCN circadian oscillators in the cen- [112, 113]. Further research is needed to reveal how the tral nervous system, namely, the food entrainable oscillator SCN orchestrates all of the peripheral clocks, how these [107, 108] and methamphetamine-induced oscillator [109]. clocks communicate their circadian rhythm, and how the These oscillators drive rhythms of circadian periodicity in hierarchical multi-oscillator circadian system is estab- various physiological functions including sleep-wakefulness, lished. Finally, the structure and functions of the extra- feeding, drinking, body temperature and autonomic nervous SCN oscillators would be fascinating research areas to functions [110, 111] even in SCN lesioned animals. The target. hierarchical multi-oscillator system composed of the SCN central clock, extra SCN brain oscillators and peripheral Compliance with ethical standards clocks throughout the body highlight the complicated nature of the circadian system. Conflict of interest The author declares no conficts of interest.

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