Encyclopedia
Please note this is a comparison between Version 1 by Henrik Oster and Version 2 by Jason Zhu.
24-h rhythms in physiology and behaviour are organized by a body-wide network of endogenous circadian clocks. In mammals, a central pacemaker in the hypothalamic suprachiasmatic nucleus (SCN) integrates external light information to adapt cellular clocks in all tissues and organs to the external light-dark cycle.
- SCN
- Clock
- 24-h rhythms
- circadian clock
1. Introduction
Life on earth is subjected to recurrent changes in environmental conditions due to the 24-h rotation of the earth around its axis. The coordination of physiology and behaviour in a 24-h time-dependent manner depends on circadian clocks.
Early studies identified the hypothalamic suprachiasmatic nuclei (SCN) as the master central pacemaker, since bilateral lesions of the SCN in rodents result in a complete loss of rhythmic locomotor activity, drinking behaviour, food consumption, hormone release and body temperature [1][2][3,4]. The SCN receive photic input from the retina via the retinohypothalamic tract (RHT) but also non-photic input via the intergeniculate leaflet and geniculohypothalamic tract projections [3][4][5][5,6,7]. The neurons in the SCN are organized as a coupled network that conveys the temporal information to other clocks in the brain and peripheral tissues through neural projections and humoral signals finally regulating physiology. The idea of a strict hierarchical organization of the circadian system around the central clock in the SCN has been replaced by the concept of a more “federated” organization since other pacemakers (such as food- and drug-sensitive oscillators) are able to introduce rhythmic circadian output in the absence of the SCN [6][7][8,9]. Husse et al. showed that peripheral tissues remain synchronized with the external light-dark cycle even when the SCN clock is absent [8][9][10,11]. Exposure to light can immediately affect clock gene expression in non-SCN clocks like the pineal gland or liver, likely through autonomic innervation [10][12]. Moreover, light can directly activate clock gene expression in adrenal glands independent of photic responses at the level of the central clock [11][12][13,14].
Although light has been traditionally considered as the main zeitgeber for the circadian system, the time of food intake has a strong impact on liver, kidney, heart and pancreas clocks without affecting the central clock in the SCN [13][15]. Additionally, scheduled exercise can induce phase shifts in skeleton muscle and lung clocks, probably altering metabolic processes to cope with changing energy demands [14][16]. Thus, a more federated organization of tissue clocks is necessary to process, integrate and translate environmental signals and ensure the adaptation of rhythmic physiological processes to the environment (see in [15][17]).
At the molecular level, the circadian clock is based on interlocked transcriptional–translational feedback loops (TTLs) comprised of a set of core clock genes. The positive limb of the mammalian core TTL is formed by the two transcription factors brain and muscle aryl hydrocarbon receptor nuclear translocator-like protein 1 (BMAL1) [16][17][18][19][18,19,20,21] and circadian locomotor output cycles kaput (CLOCK) [20][22], which induce the expression of three period (Per1-3) [21][22][23,24] and two cryptochrome (Cry1/2) [23][25] genes. In turn, PER and CRY proteins dimerize, translocate to the nuclei and suppress their own transcription [24][25][26][26,27,28] defining the autoregulatory 24 h-loop of the core TTL. Furthermore, BMAL:CLOCK initiate the expression of other genes either directly, via binding to E-box motifs in the promotor of genes e.g., Reverse-erythroblastosis virus (RevErbα/β) [27][29] or albumin d-element-binding protein (Dbp) [28][30], or indirectly via the oscillation of output genes, so called clock controlled genes (ccgs). Through these mechanisms the TTLs drive the rhythmic expression of thousands of protein-coding and -noncoding genes. Recent studies show that more than 40% of protein-coding RNAs in mice and 80% in baboons exhibit a ~24-h rhythmic expression in at least one tissue [29][30][31,32].
The identity of clock-controlled genes is highly tissue specific as only a minor fraction of oscillating genes is shared across different tissues. In this way, the timing of expression of a particular gene is in phase with the timing of the particular pathway in which this gene is involved [31][33]. One of the most graphical examples of the former is the rhythmic regulation of liver functions. The liver plays a central role in regulating glucose and lipid metabolism, biosynthesis of serum proteins, cholesterol and xenobiotic biotransformation, among others. The circadian expression of glucose transporters, glucagon and rate-limiting enzymes of glucose utilization is synchronized to peak to prepare for activity [31][33].
2. Central Clock
2.1. Local Synchronization Routes
The ability of the SCN to synchronize peripheral clocks rely on a strong neuronal network that transmit time signals to other hypothalamic and extra-hypothalamic nuclei, and connections with the autonomic nervous and the endocrine system. Thus, the SCN entrain other tissue clocks by organizing the rhythms of hormone release and influencing, through the autonomic nervous system, the sensitivity of peripheral tissues to these hormones (reviewed in [32][35]).
Traditionally, the SCN have been divided into dorsomedial shell and ventrolateral core, based on the retinal innervation patterns and the neuropeptidergic repertoire of the neurons. This feature has been shown in hamsters, mice, rats and humans [33][34][35][36,37,38]. The light signal is transduced by photoreceptive cells in the retina and transmitted to the SCN core along the RHT by glutamate, substance P (SP) and pituitary adenylyl cyclase activating peptide (PACAP). Glutamate activates N-methyl-d-aspartate (NMDA) receptors increasing intracellular Ca2+ concentrations, which results in the phosphorylation and activation of cyclic adenosine monophosphate (cAMP)-response-element-binding protein (CREB) by protein kinase A (PKA). Activated CREB binds to cAMP response elements (CREs) in Per1 and Per2 promoters inducing their transcription. Thus, the rhythmicity of neurons from the SCN core is determined mainly by the photic input. Neurons in the SCN core communicate with the neurons in the SCN shell by several neurotransmitters such as vasoactive intestinal polypeptide (VIP), gastrin-releasing peptide (GRP) and SP [36][37][38][39,40,41]. Cells in the SCN shell exhibit a self-sustained rhythmicity [39][42] driven by the autoregulatory TTL of clock genes. This property of the SCN was observed when the rhythmic behaviour of dissociate neurons was compared with SCN organotypic cultures [40][43]. Dissociated SCN neurons exhibit circadian clock gene expression and spontaneous firing rhythms with periods ranging between 20 and 30 hrs. In contrast, SCN neurons in an organotypic culture are synchronized to each other and the period range is much smaller [39][42]. This suggests that the light signal received by the SCN core pulls the phases of individual neurons in the SCN shell closer together in order to generate a robust and rhythmic output. In turn, SCN shell neurons communicate with hypothalamic and extra-hypothalamic targets by releasing vasopressin (AVP), gamma-Aminobutyric acid (GABA) and diffusible signals (reviewed by [41][44]).
2.2. Central Output
The connections of the SCN with different target organs prepare both the body for upcoming changes in the circadian cycle, and single organs for receiving the hormonal signals associated with these changes. Tracing techniques have shown multiple neuronal projections connecting the SCN with other brain regions. Most of the SCN connections are within the medial hypothalamus where the key cell groups are involved in organizing hormone release and autonomic control [42][43][50,51]. These cell groups are located in the medial preoptic area (MPO), the sub-paraventricular area (sub-PVN) and the dorsomedial hypothalamus (DMH). In addition to the ventral and dorsal borders of the PVN, SCN fibres innervate the arcuate nucleus (ARC) and the lateral hypothalamus [44][52]. Optogenetic induction or suppression of firing in SCN neurons is sufficient to reset the phase and period of the molecular clockwork and alter SCN-dependent entrainment of behavioural rhythms [45][53].
At the same time, the SCN also receive input from hypothalamic and extra-hypothalamic regions, which allows the adjustment of SCN outputs. Reciprocal neuronal connections mediating this feedback have been described with the ARC [46][54], the nucleus of the solitary tract (NTS) [47][55] and the DMH [48][56]. In a recent paper, Buijs and colleagues tested the SCN–ARC reciprocity by specifically disrupting the connection between these without altering other projections from the SCN to subparaventricular zone-paraventricular nucleus (SPZ-PVN), PVN or DMH. Surprisingly, a specific elimination of SCN–ARC crosstalk results in a complete loss of rhythmicity of one of the main synchronizing hormones, corticosterone, without disrupting SCN clock gene expression [44][52]. A significant limitation of SCN lesion experiments is that they interrupt neuronal networks with low specificity and may interfere with optic signalling through the nearby RHT. Therefore, in order to dissect the role of the pacemaker clock itself, several models with a specific genetic deletion of the SCN clockwork were generated. These experiments demonstrate that the central pacemaker is important to keep the synchrony of the circadian network in absence of external zeitgebers or when conflicting zeitgeber signals are received [8][49][10,57].
2.3. Systemic Synchronization Routes
The influence of the SCN on hormonal secretion and, in turn, the action of these hormones in peripheral tissues is considered as one of the main systemic synchronization routes. However, the SCN also modulate the autonomic nervous system to adapt the sensitivity of peripheral organs to those hormones (reviewed in [32][35]).
The regulation of circadian glucocorticoid (GC) secretion is one of the best-studied examples of circadian coordination involving a cooperation between the SCN, the autonomic system and adrenocortical clocks [50][58]. The SCN, via the activation of corticoliberin (CRH) secretion from the paraventricular nucleus of the hypothalamus (PVN), controls the rhythmic release of adrenocorticotropic hormone (ACTH) from the pituitary. ACTH, in turn, stimulates GC production in zona fasciculata cells of the adrenal cortex. Via another route involving autonomic pathways, either directly to the adrenal cortex or through connections with the adjacent medulla, the SCN synchronizes adrenal clocks, thereby regulating the sensitivity of the steroidogenic machinery to ACTH stimulation [11][50][51][13,58,59]. Transplantation and knock-down studies suggest that this sensitivity is gated by the adrenal clock. The adrenal gland is more sensitive to ACTH just before the onset of the activity period—as a result, with the same ACTH stimulus the adrenal cortex releases more corticosterone at the beginning of the activity period than at the beginning of the sleep period [50][52][58,60]. GC effects are primarily exerted by the glucocorticoid receptor (GR), which is widely expressed throughout the body and within the brain (reviewed by [53][61]) with the noted exception of the SCN [54][62]. GCs also bind to and activate the mineralocorticoid receptor (MR) whose expression is restricted to certain tissues [55][63]. Due to the higher GC affinity to MR, this receptor is tonically activated while GRs are activated only during peaks of ultradian GC pulses or during acute stress responses [56][64]. GRs act as ligand-activated transcription factors. Upon GC binding, GRs translocate from the cytosol to the nucleus, bind to GRE (glucocorticoid responsive element) DNA motifs in regulatory regions of target genes and modulate transcriptional activity. The daily peak of GCs is synchronized with the need to mobilize energy from tissue stores anticipating the active phase, e.g., promoting gluconeogenesis and glucose release from the liver [57][65] and fatty acids release from adipose tissues [58][66]. GR signalling and the molecular clock machinery interact in multiple and reciprocal ways. Hormone-bound GR binds GREs in the promoter regions of several clock genes [59][67]. Moreover, several clock proteins regulate GR intracellular localization and activity. These multiple bi-directional interactions explain the role of GCs as a major entrainment signal and their role in gating the sensitivity of peripheral tissue to systemic signals across the day (reviewed by [53][61]).
Melatonin is considered as another key-synchronizing signal, since it exhibits a strong circadian rhythmicity with higher levels during the night (in phase and in anti-phase with GC rhythms in nocturnal and diurnal animals, respectively). In most mammals, rhythmic release of melatonin is regulated by light through sympathetic neuronal connections from the SCN to the pineal gland [1][3]. Melatonin signal is transduced by G protein-coupled receptors expressed in the SCN, the pituitary and several peripheral organs such as adrenal glands, lung, heart, liver, etc. (reviewed by [60][68]). For instance, melatonin signalling has been involved in transmitting seasonal day length information to the pars tuberalis, and the circadian regulation of insulin secretion and blood glucose levels [61][69].
Early studies showed that autonomic nerve activity changes after light exposure while this effect is absent in SCN-lesioned animals [62][70]. Later, tracing techniques demonstrated that the SCN is connected with several peripheral organs such as adipose tissue, adrenal, heart, liver, ovary, kidney, pancreas, etc. [63][64][65][66][67][68][69][71,72,73,74,75,76,77]. A combination of tracing techniques and selective denervation also revealed that the SCN is connected with these organs through sympathetic and parasympathetic pre-ganglionic neurons [70][78]. For instance, leptin, a hormone secreted by adipose tissue, displays a diurnal pattern that is controlled by the SCN through the sympathetic innervation [71][79]. Although leptin induces phase advances in SCN slices, it may not shift the activity rhythm in vivo but instead potentiate the phase-shifting effect of a light pulse in the late subjective night [72][73][80,81]. Electrophysiological experiments demonstrate that the connections of the SCN with neuroendocrine centres in the hypothalamus are physically separated from autonomic connections representing an independent communication route to the periphery [74][75][82,83].
