Cooperative roles of the suprachiasmatic nucleus central clock and the adrenal clock in controlling circadian glucocorticoid rhythm

The effects of ambient temperature and feeding regimens on cecum bacteria composition and circadian rhythm in growing rabbits

Seasonal environmental shifts and improper eating habits are the important causes of diarrhea in children and growing animals. Whether adjusting feeding time at varying temperatures can modify cecal bacterial structure and improve diarrhea remains unknown. Three batches growing rabbits with two groups per batch were raised under different feeding regimens (fed at daytime vs. nighttime) in spring, summer and winter separately, and contents were collected at six time points in 1 day and used 16S rRNA sequencing to investigate the effects of feeding regimens and season on the composition and circadian rhythms of cecum bacteria. Randomized forest regression screened 12 genera that were significantly associated with seasonal ambient temperature changes. Nighttime feeding reduced the abundance of the conditionally pathogenic bacteria Desulfovibrio and Alistipes in summer and Campylobacter in winter. And also increases the circadian rhythmic Amplicon Sequence Variants in the cecum, enhancing the rhythm of bacterial metabolic activity. This rhythmic metabolic profile of cecum bacteria may be conducive to the digestion and absorption of nutrients in the host cecum. In addition, this study has identified 9 genera that were affected by the combination of seasons and feeding time. In general, we found that seasons and feeding time and their combinations affect cecum composition and circadian rhythms, and that daytime feeding during summer and winter disrupts the balance of cecum bacteria of growing rabbits, which may adversely affect cecum health and induce diarrhea risk.

Hidden variables in stress neurobiology research

Among the central goals of stress neurobiology research is to understand the mechanisms by which stressors change neural circuit function to precipitate or exacerbate psychiatric symptoms. Yet despite decades of effort, psychiatric medications that target the biological substrates of the stress response are largely lacking. We propose that the clinical advancement of stress response-based therapeutics for psychiatric disorders may be hindered by 'hidden variables' in stress research, including considerations of behavioral study design (stressors and outcome measures), individual variability, sex differences, and the interaction of the body's stress hormone system with endogenous circadian and ultradian rhythms. We highlight key issues and suggest ways forward in stress neurobiology research that may improve the ability to assess stress mechanisms and translate preclinical findings.

Mutual Shaping of Circadian Body-Wide Synchronization by the Suprachiasmatic Nucleus and Circulating Steroids

Background

Steroids are lipid hormones that reach bodily tissues through the systemic circulation, and play a major role in reproduction, metabolism, and homeostasis. All of these functions and steroids themselves are under the regulation of the circadian timing system (CTS) and its cellular/molecular underpinnings. In health, cells throughout the body coordinate their daily activities to optimize responses to signals from the CTS and steroids. Misalignment of responses to these signals produces dysfunction and underlies many pathologies.

Questions Addressed

To explore relationships between the CTS and circulating steroids, we examine the brain clock located in the suprachiasmatic nucleus (SCN), the daily fluctuations in plasma steroids, the mechanisms producing regularly recurring fluctuations, and the actions of steroids on their receptors within the SCN. The goal is to understand the relationship between temporal control of steroid secretion and how rhythmic changes in steroids impact the SCN, which in turn modulate behavior and physiology.

Evidence Surveyed

The CTS is a multi-level organization producing recurrent feedback loops that operate on several time scales. We review the evidence showing that the CTS modulates the timing of secretions from the level of the hypothalamus to the steroidogenic gonadal and adrenal glands, and at specific sites within steroidogenic pathways. The SCN determines the timing of steroid hormones that then act on their cognate receptors within the brain clock. In addition, some compartments of the body-wide CTS are impacted by signals derived from food, stress, exercise etc. These in turn act on steroidogenesis to either align or misalign CTS oscillators. Finally this review provides a comprehensive exploration of the broad contribution of steroid receptors in the SCN and how these receptors in turn impact peripheral responses.

Conclusion

The hypothesis emerging from the recognition of steroid receptors in the SCN is that mutual shaping of responses occurs between the brain clock and fluctuating plasma steroid levels.

Induction of internal circadian desynchrony by misaligning zeitgebers

24-h rhythms in physiology and behaviour are orchestrated by an endogenous circadian clock system. In mammals, these clocks are hierarchically organized with a master pacemaker residing in the hypothalamic suprachiasmatic nucleus (SCN). External time signals-so-called zeitgebers-align internal with geophysical time. During shift work, zeitgeber input conflicting with internal time induces circadian desynchrony which, in turn, promotes metabolic and psychiatric disorders. However, little is known about how internal desynchrony is expressed at the molecular level under chronodisruptive environmental conditions. We here investigated the effects of zeitgeber misalignment on circadian molecular organisation by combining 28-h light-dark (LD-28) cycles with either 24-h (FF-24) or 28-h feeding-fasting (FF-28) regimes in mice. We found that FF cycles showed strong effects on peripheral clocks, while having little effect on centrally coordinated activity rhythms. Systemic, i.e., across-tissue internal circadian desynchrony was profoundly induced within four days in LD-28/FF-24, while phase coherence between tissue clocks was maintained to a higher degree under LD-28/FF-28 conditions. In contrast, temporal coordination of clock gene activity across tissues was reduced under LD-28/FF-28 conditions compared to LD-28/FF-24. These results indicate that timed food intake may improve internal synchrony under disruptive zeitgeber conditions but may, at the same time, weaken clock function at the tissue level.

Psychotropic Drug Effects on Steroid Stress Hormone Release and Possible Mechanisms Involved

There is no doubt that chronic stress accompanied by adrenocortical stress hormone release affects the development and treatment outcome of several mental disorders. Less attention has been paid to the effects of psychotropic drugs on adrenocortical steroids, particularly in clinical studies. This review focuses on the knowledge related to the possible modulation of cortisol and aldosterone secretion under non-stress and stress conditions by antipsychotic drugs, which are being used in the treatment of several psychotic and affective disorders. The molecular mechanisms by which antipsychotic drugs may influence steroid stress hormones include the modulation of central and/or adrenocortical dopamine and serotonin receptors, modulation of inflammatory cytokines, influence on regulatory mechanisms in the central part of the hypothalamic-pituitary axis, inhibition of corticotropin-releasing hormone gene promoters, influencing glucocorticoid receptor-mediated gene transcription, indirect effects via prolactin release, alteration of signaling pathways of glucocorticoid and mineralocorticoid actions. Clinical studies performed in healthy subjects, patients with psychosis, and patients with bipolar disorder suggest that single and repeated antipsychotic treatments either reduce cortisol concentrations or do not affect its secretion. A single and potentially long-term treatment with dopamine receptor antagonists, including antipsychotics, has a stimulatory action on aldosterone release.

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

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

Glucocorticoid-Mediated Developmental Programming of Vertebrate Stress Responsivity

Glucocorticoids, vertebrate steroid hormones produced by cells of the adrenal cortex or interrenal tissue, function dynamically to maintain homeostasis under constantly changing and occasionally stressful environmental conditions. They do so by binding and thereby activating nuclear receptor transcription factors, the Glucocorticoid and Mineralocorticoid Receptors (MR and GR, respectively). The GR, by virtue of its lower affinity for endogenous glucocorticoids (cortisol or corticosterone), is primarily responsible for transducing the dynamic signals conveyed by circadian and ultradian glucocorticoid oscillations as well as transient pulses produced in response to acute stress. These dynamics are important determinants of stress responsivity, and at the systemic level are produced by feedforward and feedback signaling along the hypothalamus-pituitary-adrenal/interrenal axis. Within receiving cells, GR signaling dynamics are controlled by the GR target gene and negative feedback regulator fkpb5. Chronic stress can alter signaling dynamics via imperfect physiological adaptation that changes systemic and/or cellular set points, resulting in chronically elevated cortisol levels and increased allostatic load, which undermines health and promotes development of disease. When this occurs during early development it can "program" the responsivity of the stress system, with persistent effects on allostatic load and disease susceptibility. An important question concerns the glucocorticoid-responsive gene regulatory network that contributes to such programming. Recent studies show that klf9, a ubiquitously expressed GR target gene that encodes a Krüppel-like transcription factor important for metabolic plasticity and neuronal differentiation, is a feedforward regulator of GR signaling impacting cellular glucocorticoid responsivity, suggesting that it may be a critical node in that regulatory network.

Time-Restricted Feeding in Mice Prevents the Disruption of the Peripheral Circadian Clocks and Its Metabolic Impact during Chronic Jetlag

We used time-restricted feeding (TRF) to investigate whether microbial metabolites and the hunger hormone ghrelin can become the dominant entraining factor during chronic jetlag to prevent disruption of the master and peripheral clocks, in order to promote health. Therefore, hypothalamic clock gene and Agrp/Npy mRNA expression were measured in mice that were either chronically jetlagged and fed ad libitum, jetlagged and fed a TRF diet, or not jetlagged and fed a TRF diet. Fecal short-chain fatty acid (SCFA) concentrations, plasma ghrelin and corticosterone levels, and colonic clock gene mRNA expression were measured. Preventing the disruption of the food intake pattern during chronic jetlag using TRF restored the rhythmicity in hypothalamic clock gene mRNA expression of Reverbα but not of Arntl. TRF countered the changes in plasma ghrelin levels and in hypothalamic Npy mRNA expression induced by chronic jetlag, thereby reestablishing the food intake pattern. Increase in body mass induced by chronic jetlag was prevented. Alterations in diurnal fluctuations in fecal SCFAs during chronic jetlag were prevented thereby re-entraining the rhythmic expression of peripheral clock genes. In conclusion, TRF during chronodisruption re-entrains the rhythms in clock gene expression and signals from the gut that regulate food intake to normalize body homeostasis.

Intermittent exposure to green and white light-at-night activates hepatic glycogenolytic and gluconeogenetic activities in male Wistar rats

OBJECTIVES

Exposure to light-at-night (LAN) has been reported to impair blood glucose regulation. The liver modulates blood glucose through mechanisms influenced by several factors that include peroxisome proliferator-activated receptor gamma coactivator-1alpha (PGC-1α) and glucose-6-phosphatase (G6Pase). This study investigated the effect of intermittent exposure to green and white LAN on some hepatic glucose regulatory factors in male Wistar rats.

METHODS

Animals were divided into three equal groups. Group I (control) was exposed to normal housing conditions. Groups II and III were each daily exposed to either green or white LAN for 2 h (7-9 pm) for 14 days. Body weight and blood glucose was monitored on days 0, 7, and 14. Thereafter, retro-orbital sinus blood was obtained after light thiopental anaesthesia and serum insulin was determined. Liver samples were also obtained and evaluated for glycogen, PGC-1α, and G6Pase activity. Insulin resistance was estimated using the HOMA-IR equation.

RESULTS

Body weight and blood glucose on days 7 and 14 increased in groups II and III compared to control. Hepatic PGC-1α and G6Pase increased in group II (2.33 ± 0.31; 2.07 ± 0.22) and III (2.31 ± 0.20; 0.98 ± 0.23) compared to control (1.73 ± 0.21; 0.47 ± 0.11). Hepatic glycogen was 71.8 and 82.4% reduced in groups II and III compared to control. Insulin in group II increased (63.6%) whiles group III values reduced (27.3%) compared to control. Insulin resistance increased in group II (0.29 ± 0.09) compared to control (0.12 ± 0.03) and group III (0.11 ± 0.03), respectively.

CONCLUSIONS

Exposure to 2 h green and white LAN in the early dark phase increases hepatic glycogenolysis and gluconeogenetic activities resulting in increased blood glucose. In male Wistar rats, exposure to green but not white LAN may predispose to insulin resistance.

Effects of antipsychotics on circadian rhythms in humans: a systematic review and meta-analysis

Antipsychotics are widely used to treat psychiatric illness and insomnia. However, the etiology of insomnia is multifactorial, including disrupted circadian rhythms. Several studies show that antipsychotics might modulate even healthy circadian rhythms. The purpose of this systematic review is to integrate current knowledge about the effects of antipsychotics on the circadian rhythms in humans, and to conduct a meta- analysis with the available data. Nine electronic databases were searched. We followed the PRISMA guidelines and included randomized controlled trials (RCTs), non-RCTs, case-control studies, case series, and case reports. Of 7,217 articles, 70 were included. The available data was mainly from healthy individuals, or patients having schizophrenia, but the findings showed a transdiagnostic impact on circadian parameters. This was consistently seen as decreased amplitude of cortisol, melatonin, and body temperature. Particularly, a meta-analysis of 16 RCTs measuring cortisol rhythm showed that antipsychotics, especially atypical antipsychotics, decreased the cortisol area under the curve and morning cortisol level, compared to placebo. The data with melatonin or actigraphy was limited. Overall, this evidence about the circadian effect of antipsychotics showed a need for longitudinal, real-time monitoring of specific circadian markers to differentiate a change in amplitude from a shift in phasing, and for knowledge about optimal timing of administration of antipsychotics, according to individual baseline circadian parameters. Standardizing selection criteria and outcome methods could facilitate good quality intervention studies and evidence-based treatment guidelines. This is relevant considering the accumulating evidence of the high prevalence and unfavorable impact of disrupted circadian rhythms in psychiatric disorders.

Involvement of clock gene expression, bone morphogenetic protein and activin in adrenocortical steroidogenesis by human H295R cells

Functional interactions between the levels of clock gene expression and adrenal steroidogenesis were studied in human adrenocortical H295R cells. Fluctuations of Bmal1, Clock, Per2 and Cry1 mRNA levels were found in H295R cells treated with forskolin (FSK) in a serum-free condition. The changes of clock gene expression levels were diverged, with Clock mRNA level being significantly higher than Cry1 and Per2 mRNA levels after 12-h stimulation with FSK. After FSK induction, mRNA levels of StAR and CYP11B2 were highest at 12 hours and CYP17 mRNA level reached a peak at 6 hours, but HSD3B1 mRNA level was transiently decreased at 3 hours. The expression levels of Clock mRNA showed a significant positive correlation with StAR among the interrelationships between mRNA levels of key steroidogenic factors and clock genes. Knockdown of Clock gene by siRNA led to a significant reduction of FSK-induced expression of StAR and CYP17 after 12-h treatment with FSK. BMP-6 and activin, which modulate adrenal steroidogenesis, had inhibitory effects on Clock mRNA expression, whereas treatment with follistatin, a binding protein of activin, increased Clock mRNA levels in the presence of FSK, suggesting an endogenous function of activin in regulation of Clock mRNA expression. Collectively, the results indicated that changes of Clock mRNA expression, being upregulated by FSK and suppressed by BMP-6 and activin, were tightly linked to StAR expression by human adrenocortical cells.

Circadian rhythms and the HPA axis: A systems view

The circadian timing system comprises a network of time-keeping clocks distributed across a living host whose responsibility is to allocate resources and distribute functions temporally to optimize fitness. The molecular structures generating these rhythms have evolved to accommodate the rotation of the earth in an attempt to primarily match the light/dark periods during the 24-hr day. To maintain synchrony of timing across and within tissues, information from the central clock, located in the suprachiasmatic nucleus, is conveyed using systemic signals. Leading among those signals are endocrine hormones, and while the hypothalamic-pituitary-adrenal axis through the release of glucocorticoids is a major pacesetter. Interestingly, the fundamental units at the molecular and physiological scales that generate local and systemic signals share critical structural properties. These properties enable time-keeping systems to generate rhythmic signals and allow them to adopt specific properties as they interact with each other and the external environment. The purpose of this review is to provide a broad overview of these structures, discuss their functional characteristics, and describe some of their fundamental properties as these related to health and disease. This article is categorized under: Immune System Diseases > Computational Models Immune System Diseases > Biomedical Engineering.

Colonization with the commensal fungus Candida albicans perturbs the gut-brain axis through dysregulation of endocannabinoid signaling

Anxiety disorders are the most prevalent mental health disorder worldwide, with a lifetime prevalence of 5-7 % of the human population. Although the etiology of anxiety disorders is incompletely understood, one aspect of host health that affects anxiety disorders is the gut-brain axis. Adolescence is a key developmental window in which stress and anxiety disorders are a major health concern. We used adolescent female mice in a gastrointestinal (GI) colonization model to demonstrate that the commensal fungus Candida albicans affects host health via the gut-brain axis. In mice, bacterial members of the gut microbiota can influence the host gut-brain axis, affecting anxiety-like behavior and the hypothalamus-pituitary-adrenal (HPA) axis which produces the stress hormone corticosterone (CORT). Here we showed that mice colonized with C. albicans demonstrated increased anxiety-like behavior and increased basal production of CORT as well as dysregulation of CORT production following acute stress. The HPA axis and anxiety-like behavior are negatively regulated by the endocannabinoid N-arachidonoylethanolamide (AEA). We demonstrated that C. albicans-colonized mice exhibited changes in the endocannabinoidome. Further, increasing AEA levels using the well-characterized fatty acid amide hydrolase (FAAH) inhibitor URB597 was sufficient to reverse both neuroendocrine phenotypes in C. albicans-colonized mice. Thus, a commensal fungus that is a common colonizer of humans had widespread effects on the physiology of its host. To our knowledge, this is the first report of microbial manipulation of the endocannabinoid (eCB) system that resulted in neuroendocrine changes contributing to anxiety-like behavior.

Molecular Signatures of Sinus Node Dysfunction Induce Structural Remodeling in the Right Atrial Tissue

The sinus node (SN) is located at the apex of the cardiac conduction system, and SN dysfunction (SND)-characterized by electrical remodeling-is generally attributed to idiopathic fibrosis or ischemic injuries in the SN. SND is associated with increased risk of cardiovascular disorders, including syncope, heart failure, and atrial arrhythmias, particularly atrial fibrillation. One of the histological SND hallmarks is degenerative atrial remodeling that is associated with conduction abnormalities and increased right atrial refractoriness. Although SND is frequently accompanied by increased fibrosis in the right atrium (RA), its molecular basis still remains elusive. Therefore, we investigated whether SND can induce significant molecular changes that account for the structural remodeling of RA. Towards this, we employed a rabbit model of experimental SND, and then compared the genome-wide RNA expression profiles in RA between SND-induced rabbits and sham-operated controls to identify the differentially expressed transcripts. The accompanying gene enrichment analysis revealed extensive pro-fibrotic changes within 7 days after the SN ablation, including activation of transforming growth factor-β (TGF-β) signaling and alterations in the levels of extracellular matrix components and their regulators. Importantly, our findings suggest that periostin, a matricellular factor that regulates the development of cardiac tissue, might play a key role in mediating TGF-β-signaling-induced aberrant atrial remodeling. In conclusion, the present study provides valuable information regarding the molecular signatures underlying SND-induced atrial remodeling, and indicates that periostin can be potentially used in the diagnosis of fibroproliferative cardiac dysfunctions.

Programming effects of maternal stress on the circadian system of adult offspring

Maternal stress has long-lasting influences on the brain functions of offspring, and several brain regions have been proposed to mediate such programming. Although perinatal programming of crosstalk between the circadian and stress systems has been proposed, the functional consequences of prenatal stress on the circadian system and the underlying mechanisms remain largely unknown. Therefore, we investigated whether exposing pregnant mice to chronic restraint stress had prolonged effects on the suprachiasmatic nucleus (SCN), which bears the central pacemaker for mammalian circadian rhythms, of offspring. SCN explants from maternally stressed mice exhibited altered cyclic expression patterns of a luciferase reporter under control of the mouse Per1 promoter (mPer1::LUC), which manifested as a decreased amplitude and impaired stability of the rhythm. Bioluminescence imaging at the single-cell level subsequently revealed that impaired synchrony among individual cells was responsible for the impaired rhythmicity. These intrinsic defects appeared to persist during adulthood. Adult male offspring from stressed mothers showed advanced-phase behavioral rhythms with impaired stability as well as altered clock gene expression in the SCN. In addition to affecting the central rhythm, maternal stress also had prolonged influences on the circadian characteristics of the adrenal gland and liver, as determined by circulating corticosterone levels and hepatic glycogen content, and on canonical clock gene mRNA expression in those tissues. Taken together, our findings suggest that the SCN is a key target of the programming effects of maternal stress. The widespread effects of circadian disruptions caused by a misprogrammed clock may have further impacts on metabolic and mental health in later life.

When pregnant mothers are stressed, the fetus’s circadian rhythms are reprogrammed, increasing the risk of health complications later in life. Stress during pregnancy was known to negatively affect the fetus, but how it affected circadian rhythms (day/night patterns of alertness) was poorly understood. Kyungjin Kim (Daegu Gyeongbuk Institute of Science and Technology) and Sooyoung Chung (Ewha Womans University, Seoul), both in South Korea, and co-workers stressed pregnant mice by confining them in small tubes, then measured the effects on their offspring. Pups of stressed mothers showed disturbed circadian rhythms, and the effects persisted into adulthood. Further analysis showed that the rhythms were disrupted because individual cells in the key brain region regulating circadian rhythms were poorly synchronized. These results suggest potential treatments to counteract the negative effects of prenatal stress on circadian rhythms.

Adrenal and Glucocorticoid Effects on the Circadian Rhythm of Murine Intraocular Pressure

Purpose

Intraocular pressure (IOP) fluctuates with a robust circadian rhythm, which is synchronized to organismal rhythmicity through the master circadian clock located in the suprachiasmatic nuclei. The mechanisms maintaining circadian synchrony between the suprachiasmatic nuclei and IOP rhythms are presently unknown. The purpose of this study was to evaluate the necessity and sufficiency of adrenal and glucocorticoid function for the entrainment of iris-ciliary body (iris-CB) and IOP circadian rhythms in mice.

Methods

Iris-CB complexes were dissected from C57Bl/6J mice that were kept in 12-hour light/dark cycles at 3-hour intervals, and their core clock gene (Per1, Per2, and Bmal1) and glucocorticoid receptor mRNA expression were quantified. Iris-CB complexes from period2::luciferase mice were dissected and cultured to measure in vitro rhythmicity. To determine the phase-shifting effect of glucocorticoids on the iris-CB, dexamethasone or vehicle was added to the cultured tissues at defined circadian phases. The diurnal IOP rhythms of adrenalectomized or sham-operated mice under the 12-hour light/dark cycles were also measured.

Results

In iris-CB complexes, glucocorticoid receptor mRNA expression remained stable throughout the day, whereas the mRNA of core clock genes showed a robust circadian rhythmicity. Dexamethasone significantly induced phase-delays when administered between circadian time 8 (CT8) to CT12 and phase-advance when given between CT16 to CT20. Adrenalectomy abolished circadian IOP rhythmicity, particularly diminishing nocturnal IOP elevation compared with sham-operated mice.

Conclusions

Glucocorticoids are sufficient for phase shifting the circadian clock in iris-CB. Intact adrenal function is required for manifest circadian rhythms of IOP in mice. Taken together, these data are consistent with the hypothesis that glucocorticoids mediate circadian entrainment of IOP to the master circadian oscillator.

Coupling the Circadian Clock to Homeostasis: The Role of Period in Timing Physiology

A plethora of physiological processes show stable and synchronized daily oscillations that are either driven or modulated by biological clocks. A circadian pacemaker located in the suprachiasmatic nucleus of the ventral hypothalamus coordinates 24-hour oscillations of central and peripheral physiology with the environment. The circadian clockwork involved in driving rhythmic physiology is composed of various clock genes that are interlocked via a complex feedback loop to generate precise yet plastic oscillations of ∼24 hours. This review focuses on the specific role of the core clockwork gene Period1 and its paralogs on intra-oscillator and extra-oscillator functions, including, but not limited to, hippocampus-dependent processes, cardiovascular function, appetite control, as well as glucose and lipid homeostasis. Alterations in Period gene function have been implicated in a wide range of physical and mental disorders. At the same time, a variety of conditions including metabolic disorders also impact clock gene expression, resulting in circadian disruptions, which in turn often exacerbates the disease state.

Insights into the Role of Circadian Rhythms in Bone Metabolism: A Promising Intervention Target?

Numerous physiological processes of mammals, including bone metabolism, are regulated by the circadian clock system, which consists of a central regulator, the suprachiasmatic nucleus (SCN), and the peripheral oscillators of the BMAL1/CLOCK-PERs/CRYs system. Various bone turnover markers and bone metabolism-regulating hormones such as melatonin and parathyroid hormone (PTH) display diurnal rhythmicity. According to previous research, disruption of the circadian clock due to shift work, sleep restriction, or clock gene knockout is associated with osteoporosis or other abnormal bone metabolism, showing the importance of the circadian clock system for maintaining homeostasis of bone metabolism. Moreover, common causes of osteoporosis, including postmenopausal status and aging, are associated with changes in the circadian clock. In our previous research, we found that agonism of the circadian regulators REV-ERBs inhibits osteoclast differentiation and ameliorates ovariectomy-induced bone loss in mice, suggesting that clock genes may be promising intervention targets for abnormal bone metabolism. Moreover, osteoporosis interventions at different time points can provide varying degrees of bone protection, showing the importance of accounting for circadian rhythms for optimal curative effects in clinical treatment of osteoporosis. In this review, we summarize current knowledge about circadian rhythms and bone metabolism.

MECHANISMS IN ENDOCRINOLOGY: A sense of time of the glucocorticoid circadian clock: from the ontogeny to the diagnosis of Cushing's syndrome

The circadian rhythm of glucocorticoids has long been recognised within the last 75 years. Since the beginning, researchers have sought to identify basic mechanisms underlying the origin and emergence of the corticosteroid circadian rhythmicity among mammals. Accordingly, Young, Hall and Rosbash, laureates of the 2017 Nobel Prize in Physiology or Medicine, as well as Takahashi's group among others, have characterised the molecular cogwheels of the circadian system, describing interlocking transcription/translation feedback loops essential for normal circadian rhythms. Plasma glucocorticoid circadian variation depends on the expression of intrinsic clock genes within the anatomic components of the hypothalamic-pituitary-adrenal axis, which are organised in a hierarchical manner. This review presents a general overview of the glucocorticoid circadian clock mechanisms, highlighting the ontogeny of the pituitary-adrenal axis diurnal rhythmicity as well as the involvement of circadian rhythm abnormalities in the physiopathology and diagnosis of Cushing's disease.

Multimodal Regulation of Circadian Glucocorticoid Rhythm by Central and Adrenal Clocks

Adrenal glucocorticoids (GCs) control a wide range of physiological processes, including metabolism, cardiovascular and pulmonary activities, immune and inflammatory responses, and various brain functions. During stress responses, GCs are secreted through activation of the hypothalamic-pituitary-adrenal axis, whereas circulating GC levels in unstressed states follow a robust circadian oscillation with a peak around the onset of the active period of a day. A recent advance in chronobiological research has revealed that multiple regulatory mechanisms, along with classical neuroendocrine regulation, underlie this GC circadian rhythm. The hierarchically organized circadian system, with a central pacemaker in the suprachiasmatic nucleus of the hypothalamus and local oscillators in peripheral tissues, including the adrenal gland, mediates periodicities in physiological processes in mammals. In this review, we primarily focus on our understanding of the circadian regulation of adrenal GC rhythm, with particular attention to the cooperative actions of the suprachiasmatic nucleus central and adrenal local clocks, and the clinical implications of this rhythm in human diseases.

Chronic stress alters adrenal clock function in a sexually dimorphic manner

Glucocorticoid production is gated at the molecular level by the circadian clock in the adrenal gland. Stress influences daily rhythms in behavior and physiology, but it remains unclear how stress affects the function of the adrenal clock itself. Here, we examine the influence of stress on adrenal clock function by tracking PERIOD2::LUCIFERASE (PER2::LUC) rhythms in vitro Relative to non-stressed controls, adrenals from stressed mice displayed marked changes in PER2::LUC rhythms. Interestingly, the effect of stress on adrenal rhythms varied by sex and the type of stress experienced in vivo To investigate the basis of sex differences in the adrenal response to stress, we next stimulated male and female adrenals in vitro with adrenocorticotropic hormone (ACTH). ACTH shifted phase and increased amplitude of adrenal PER2::LUC rhythms. Both phase and amplitude responses were larger in female adrenals than in male adrenals, an observation consistent with previously described sex differences in the physiological response to stress. Lastly, we reversed the sex difference in adrenal clock function using stress and sex hormone manipulations to test its role in driving adrenal responses to ACTH. We find that adrenal responsiveness to ACTH is inversely proportional to the amplitude of adrenal PER2::LUC rhythms. This suggests that larger ACTH responses from female adrenals may be driven by their lower amplitude molecular rhythms. Collectively, these results indicate a reciprocal relationship between stress and the adrenal clock, with stress influencing adrenal clock function and the state of the adrenal clock gating the response to stress in a sexually dimorphic manner.