Circadian Clocks, Stress, and Immunity

In mammals, molecular circadian clocks are present in most cells of the body, and this circadian network plays an important role in synchronizing physiological processes and behaviors to the appropriate time of day. The hypothalamic-pituitary-adrenal endocrine axis regulates the response to acute and chronic stress, acting through its final effectors - glucocorticoids - released from the adrenal cortex. Glucocorticoid secretion, characterized by its circadian rhythm, has an important role in synchronizing peripheral clocks and rhythms downstream of the master circadian pacemaker in the suprachiasmatic nucleus. Finally, glucocorticoids are powerfully anti-inflammatory, and recent work has implicated the circadian clock in various aspects and cells of the immune system, suggesting a tight interplay of stress and circadian systems in the regulation of immunity. This mini-review summarizes our current understanding of the role of the circadian clock network in both the HPA axis and the immune system, and discusses their interactions.

Adrenal Clocks and the Role of Adrenal Hormones in the Regulation of Circadian Physiology

The mammalian circadian timing system consists of a master pacemaker in the suprachiasmatic nucleus (SCN) and subordinate clocks that disseminate time information to various central and peripheral tissues. While the function of the SCN in circadian rhythm regulation has been extensively studied, we still have limited understanding of how peripheral tissue clock function contributes to the regulation of physiological processes. The adrenal gland plays a special role in this context as adrenal hormones show strong circadian secretion rhythms affecting downstream physiological processes. At the same time, they have been shown to affect clock gene expression in various other tissues, thus mediating systemic entrainment to external zeitgebers and promoting internal circadian alignment. In this review, we discuss the function of circadian clocks in the adrenal gland, how they are reset by the SCN and may further relay time-of-day information to other tissues. Focusing on glucocorticoids, we conclude by outlining the impact of adrenal rhythm disruption on neuropsychiatric, metabolic, immune, and malignant disorders.

Dissociation of Molecular and Endocrine Circadian Rhythms in Male Mice Lacking Bmal1 in the Adrenal Cortex

The circadian rhythm of glucocorticoids affects diverse physiological systems, including stress responses and the coordination of rhythmic functions in peripheral and central tissues. Circadian clocks are considered to be important coordinators of glucocorticoid release and loss of the core clock component Brain and muscle Arnt-like protein-1 leads to ablation of behavioral and physiological rhythms, hypocortisolism, impaired ACTH, and behavioral stress responses. Transplantation and conditional clock gene knock-down studies in mice suggest an important role of local adrenocortical clock function in this context. Here, we present a Cre-loxP-mediated conditional knockout of Bmal1 in the steroidogenic cells of the adrenal cortex in mice. Mutant animals show a loss of molecular clock gene activity rhythms in this tissue with subsequent disruption of rhythmic steroidogenic gene expression. However, despite this loss of normal clock rhythmicity in the adrenal cortex, behavioral and physiological rhythms and acute stress responses persist in mutant mice. These findings reveal a dissociation of transcriptional and endocrine rhythm regulation in the adrenal cortex, arguing for a less pivotal function of the local clock machinery in the regulation of circadian and acute glucocorticoid outputs.

Circadian Clocks and the Interaction between Stress Axis and Adipose Function

Many physiological processes and most endocrine functions show fluctuations over the course of the day. These so-called circadian rhythms are governed by an endogenous network of cellular clocks and serve as an adaptation to daily and, thus, predictable changes in the organism's environment. Circadian clocks have been described in several tissues of the stress axis and in adipose cells where they regulate the rhythmic and stimulated release of stress hormones, such as glucocorticoids, and various adipokine factors. Recent work suggests that both adipose and stress axis clock systems reciprocally influence each other and adrenal-adipose rhythms may be key players in the development and therapy of metabolic disorders. In this review, we summarize our current understanding of adrenal and adipose tissue rhythms and clocks and how they might interact to regulate energy homoeostasis and stress responses under physiological conditions. Potential chronotherapeutic strategies for the treatment of metabolic and stress disorders are discussed.

Linking the FTO obesity rs1421085 variant circuitry to cellular, metabolic, and organismal phenotypes in vivo

Variants in FTO have the strongest association with obesity; however, it is still unclear how those noncoding variants mechanistically affect whole-body physiology. We engineered a deletion of the rs1421085 conserved cis-regulatory module (CRM) in mice and confirmed in vivo that the CRM modulates Irx3 and Irx5 gene expression and mitochondrial function in adipocytes. The CRM affects molecular and cellular phenotypes in an adipose depot-dependent manner and affects organismal phenotypes that are relevant for obesity, including decreased high-fat diet-induced weight gain, decreased whole-body fat mass, and decreased skin fat thickness. Last, we connected the CRM to a genetically determined effect on steroid patterns in males that was dependent on nutritional challenge and conserved across mice and humans. Together, our data establish cross-species conservation of the rs1421085 regulatory circuitry at the molecular, cellular, metabolic, and organismal level, revealing previously unknown contextual dependence of the variant's action.

Effect of exercise on photoperiod-regulated hypothalamic gene expression and peripheral hormones in the seasonal Dwarf Hamster Phodopus sungorus

The Siberian hamster (Phodopus sungorus) is a seasonal mammal responding to the annual cycle in photoperiod with anticipatory physiological adaptations. This includes a reduction in food intake and body weight during the autumn in anticipation of seasonally reduced food availability. In the laboratory, short-day induction of body weight loss can be reversed or prevented by voluntary exercise undertaken when a running wheel is introduced into the home cage. The mechanism by which exercise prevents or reverses body weight reduction is unknown, but one hypothesis is a reversal of short-day photoperiod induced gene expression changes in the hypothalamus that underpin body weight regulation. Alternatively, we postulate an exercise-related anabolic effect involving the growth hormone axis. To test these hypotheses we established photoperiod-running wheel experiments of 8 to 16 weeks duration assessing body weight, food intake, organ mass, lean and fat mass by magnetic resonance, circulating hormones FGF21 and insulin and hypothalamic gene expression. In response to running wheel activity, short-day housed hamsters increased body weight. Compared to short-day housed sedentary hamsters the body weight increase was accompanied by higher food intake, maintenance of tissue mass of key organs such as the liver, maintenance of lean and fat mass and hormonal profiles indicative of long day housed hamsters but there was no overall reversal of hypothalamic gene expression regulated by photoperiod. Therefore the mechanism by which activity induces body weight gain is likely to act largely independently of photoperiod regulated gene expression in the hypothalamus.

The Chemerin-CMKLR1 Axis is Functionally important for Central Regulation of Energy Homeostasis

Chemerin is an adipokine involved in inflammation, adipogenesis, angiogenesis and energy metabolism, and has been hypothesized as a link between obesity and type II diabetes. In humans affected by obesity, chemerin gene expression in peripheral tissues and circulating levels are elevated. In mice, plasma levels of chemerin are upregulated by high-fat feeding and gain and loss of function studies show an association of chemerin with body weight, food intake and glucose homeostasis. Therefore, chemerin is an important blood-borne mediator that, amongst its other functions, controls appetite and body weight. Almost all studies of chemerin to date have focused on its release from adipose tissue and its effects on peripheral tissues with the central effects largely overlooked. To demonstrate a central role of chemerin, we manipulated chemerin signaling in the hypothalamus, a brain region associated with appetite regulation, using pharmacological and genetic manipulation approaches. Firstly, the selective chemerin receptor CMKLR1 antagonist α-NETA was administered i.c.v. to rats to test for an acute physiological effect. Secondly, we designed a short-hairpin-RNA (shRNA) lentivirus construct targeting expression of CMKLR1. This shRNA construct, or a control construct was injected bilaterally into the arcuate nucleus of male Sprague Dawley rats on high-fat diet (45%). After surgery, rats were maintained on high-fat diet for 2 weeks and then switched to chow diet for a further 2 weeks. We found a significant weight loss acutely and inhibition of weight gain chronically. This difference became apparent after diet switch in arcuate nucleus-CMKLR1 knockdown rats. This was not accompanied by a difference in blood glucose levels. Interestingly, appetite-regulating neuropeptides remained unaltered, however, we found a significant reduction of the inflammatory marker TNF-α suggesting reduced expression of CMKLR1 protects from high-fat diet induced neuroinflammation. In white and brown adipose tissue, mRNA expression of chemerin, its receptors and markers of adipogenesis, lipogenesis and brown adipocyte activation remained unchanged confirming that the effects are driven by the brain. Our behavioral analyses suggest that knockdown of CMKLR1 had an impact on object recognition. Our data demonstrate that CMKLR1 is functionally important for the central effects of chemerin on body weight regulation and neuroinflammation.

Disruption of the homeodomain transcription factor orthopedia homeobox (Otp) is associated with obesity and anxiety

Objective

Genetic studies in obese rodents and humans can provide novel insights into the mechanisms involved in energy homeostasis.

Methods

In this study, we genetically mapped the chromosomal region underlying the development of severe obesity in a mouse line identified as part of a dominant N-ethyl-N-nitrosourea (ENU) mutagenesis screen. We characterized the metabolic and behavioral phenotype of obese mutant mice and examined changes in hypothalamic gene expression. In humans, we examined genetic data from people with severe early onset obesity.

Results

We identified an obese mouse heterozygous for a missense mutation (pR108W) in orthopedia homeobox (Otp), a homeodomain containing transcription factor required for the development of neuroendocrine cell lineages in the hypothalamus, a region of the brain important in the regulation of energy homeostasis. Otp^R108W/+ mice exhibit increased food intake, weight gain, and anxiety when in novel environments or singly housed, phenotypes that may be partially explained by reduced hypothalamic expression of oxytocin and arginine vasopressin. R108W affects the highly conserved homeodomain, impairs DNA binding, and alters transcriptional activity in cells. We sequenced OTP in 2548 people with severe early-onset obesity and found a rare heterozygous loss of function variant in the homeodomain (Q153R) in a patient who also had features of attention deficit disorder.

Conclusions

OTP is involved in mammalian energy homeostasis and behavior and appears to be necessary for the development of hypothalamic neural circuits. Further studies will be needed to investigate the contribution of rare variants in OTP to human energy homeostasis.

•

A mouse Otp mutation alters hypothalamic neuropeptide expression leading to increased food intake, obesity and anxiety.

•

In severe early onset obesity, we found a heterozygous LOF variant in a patient with attention deficit disorder features.

•

These studies show for the first time that mutations in the Otp/OTP gene cause obesity.

Effects of inhibition of gonadotropin releasing hormone secretion on the response to novel objects in young male and female sheep

This study investigated the actions of blocking the GnRH receptor using a specific agonist on the response of male and female sheep to a novel object placed in their pen. The study is part of a series performed on 46 same sex twin animals. One of the pair received a subcutaneous implant of the GnRH agonist Goserelin acetate every four weeks while the other remained untreated. Implantation began immediately prior to puberty; at 8 weeks in the males and 28 weeks in the females (as timing of puberty is sex specific). To determine the effects of agonist treatment on the reproductive axis blood samples were collected for measurement of testosterone in the males and progesterone in the females. In addition the volume of the scrotum was determined. The present study aimed to determine whether there are sexually differentiated behavioural responses to a novel object at different stages of brain development (8, 28 and 48 weeks of age) and whether these responses are altered by GnRHa treatment. Approach behaviour towards and interactions with the novel object were monitored as was the number of vocalisations per unit time during the test period. GnRHa treatment suppressed testosterone concentrations and testicular growth in the males and progesterone release in the females. Sheep vocalised significantly more prior to weaning (8 weeks of age) than post weaning (28 and 48 weeks of age) suggesting stress on separation from their dams. Our current study shows that males are more likely to leave their conspecifics to approach a novel object than females. As this behaviour was not altered by suppression of the reproductive axis we suggest that, although sex differences are more obviously expressed in the phenotype after puberty, these may be developed during adolescence but not primarily altered during puberty by sex hormones.

A Wars2 mutant mouse shows a sex and diet specific change in fat distribution, reduced food intake and depot-specific upregulation of WAT browning

Background: Increased waist-to-hip ratio (WHR) is associated with increased mortality and risk of type 2 diabetes and cardiovascular disease. The TBX15-WARS2 locus has consistently been associated with increased WHR. Previous study of the hypomorphic Wars2 V117L/V117L mouse model found phenotypes including severely reduced fat mass, and white adipose tissue (WAT) browning, suggesting Wars2 could be a potential modulator of fat distribution and WAT browning. Methods: To test for differences in browning induction across different adipose depots of Wars2 V117L/V117L mice, we measured multiple browning markers of a 4-month old chow-fed cohort in subcutaneous and visceral WAT and brown adipose tissue (BAT). To explain previously observed fat mass loss, we also tested for the upregulation of plasma mitokines FGF21 and GDF15 and for differences in food intake in the same cohort. Finally, to test for diet-associated differences in fat distribution, we placed Wars2 V117L/V117L mice on low-fat or high-fat diet (LFD, HFD) and assessed their body composition by Echo-MRI and compared terminal adipose depot weights at 6 months of age. Results: The chow-fed Wars2 V117L/V117L mice showed more changes in WAT browning marker gene expression in the subcutaneous inguinal WAT depot (iWAT) than in the visceral gonadal WAT depot (gWAT). These mice also demonstrated reduced food intake and elevated plasma FGF21 and GDF15, and mRNA from heart and BAT. When exposed to HFD, the Wars2 V117L/V117L mice showed resistance to diet-induced obesity and a male and HFD-specific reduction of gWAT: iWAT ratio. Conclusion: Severe reduction of Wars2 gene function causes a systemic phenotype which leads to upregulation of FGF21 and GDF15, resulting in reduced food intake and depot-specific changes in browning and fat mass.

Modelling the genetic aetiology of complex disease: human-mouse conservation of noncoding features and disease-associated loci

Understanding the genetic aetiology of loci associated with a disease is crucial for developing preventative measures and effective treatments. Mouse models are used extensively to understand human pathobiology and mechanistic functions of disease-associated loci. However, the utility of mouse models is limited in part by evolutionary divergence in transcription regulation for pathways of interest. Here, we summarize the alignment of genomic (exonic and multi-cell regulatory) annotations alongside Mendelian and complex disease-associated variant sites between humans and mice. Our results highlight the importance of understanding evolutionary divergence in transcription regulation when interpreting functional studies using mice as models for human disease variants.

Somatostatin Agonist Pasireotide Inhibits Exercise-Stimulated Growth in the Male Siberian Hamster (Phodopus sungorus)

The Siberian hamster (Phodopus sungorus) is a seasonal mammal, exhibiting a suite of physiologically and behaviourally distinct traits dependent on the time of year and governed by changes in perceived day length (photoperiod). These attributes include significant weight loss, reduced food intake, gonadal atrophy and pelage change with short-day photoperiod as in winter. The central mechanisms driving seasonal phenotype change during winter are mediated by a reduced availability of hypothalamic triiodothyronine (T3), although the downstream mechanisms responsible for physiological and behavioural changes are yet to be fully clarified. With access to a running wheel (RW) in short photoperiod, Siberian hamsters that have undergone photoperiod-mediated weight loss over-ride photoperiod-drive for reduced body weight and regain weight similar to a hamster held in long days. These changes occur despite retaining the majority of hypothalamic gene expression profiles appropriate for short-day hamsters. Utilising the somatostatin agonist pasireotide, we recently provided evidence for an involvement of the growth hormone (GH) axis in the seasonal regulation of bodyweight. In the present study, we employed pasireotide to test for the possible involvement of the GH axis in RW-induced body weight regulation. Pasireotide successfully inhibited exercise-stimulated growth in short-day hamsters and this was accompanied by altered hypothalamic gene expression of key GH axis components. Our data provide support for an involvement of the GH axis in the RW response in Siberian hamsters.

An appetite for growth: The role of the hypothalamic - pituitary - growth hormone axis in energy balance

Links between the regulation of growth and energy balance are clear; to fuel growth, there must be consumption of energy. Therefore, it is perhaps intuitive that interactions between the hypothalamic - pituitary - growth hormone axis (growth axis) and pathways that drive metabolic processes exist. Overproduction of growth hormone has been associated with diabetes and metabolic disease for decades and the opposing effects of growth hormone and insulin have been studied since early experiments almost a century ago. The relationship between neuroendocrine axes can be complex and the growth axis is no exception, interacting with energy balance in several organ systems, both in the periphery and centrally in hypothalamic nuclei. Much is known about peripheral interactions between growth axis hormones and processes such as glucose homeostasis and adipogenesis. More is still being learned about the molecular actions of growth axis hormones in adipose and other metabolically active tissues, and recent findings are discussed in this perspective. However, less is known about interactions with central energy balance pathways in the hypothalamus. This perspective aims to summarise what is known about these interactions, taking lessons from human studies and animal genetic and seasonal models, and discusses what this may mean in an evolving landscape of personalised medicine.

A lead candidate functional single nucleotide polymorphism within the WARS2 gene associated with waist-hip-ratio does not alter RNA stability

We have prioritised a single nucleotide polymorphism (SNP) rs2645294 as one candidate functional SNP in the TBX15-WARS2 waist-hip-ratio locus using posterior probability analysis. This SNP is located in the 3' untranslated region of the WARS2 (tryptophanyl tRNA synthetase 2, mitochondrial) gene with which it has an expression quantitative trait in subcutaneous white adipose tissue. We show that transcripts of the WARS2 gene in a human white adipose cell line, heterozygous for the rs2645294 SNP, showed allelic imbalance. We tested whether the rs2645294 SNP altered WARS2 RNA stability using three different methods: actinomycin-D inhibition and RNA decay, mature and nascent RNA analysis and luciferase reporter assays. We found no evidence of a difference in RNA stability between the rs2645294 alleles indicating that the allelic expression imbalance was likely due to transcriptional regulation.

A missense mutation in zinc finger homeobox-3 (ZFHX3) impedes growth and alters metabolism and hypothalamic gene expression in mice

A protein altering variant in the gene encoding zinc finger homeobox-3 (ZFHX3) has recently been associated with lower BMI in a human genome-wide association study. We investigated metabolic parameters in mice harboring a missense mutation in Zfhx3 (Zfhx3Sci/+ ) and looked for altered in situ expression of transcripts that are associated with energy balance in the hypothalamus to understand how ZFHX3 may influence growth and metabolic effects. One-year-old male and female Zfhx3Sci/+ mice weighed less, had shorter body length, lower fat mass, smaller mesenteric fat depots, and lower circulating insulin, leptin, and insulin-like growth factor-1 (IGF1) concentrations than Zfhx3+/+ littermates. In a second cohort of 9-20-week-old males and females, Zfhx3Sci/+ mice ate less than wildtype controls, in proportion to body weight. In a third cohort of female-only Zfhx3Sci/+ and Zfhx3+/+ mice that underwent metabolic phenotyping from 6 to 14 weeks old, Zfhx3Sci/+ mice weighed less and had lower lean mass and energy expenditure, but fat mass did not differ. We detected increased expression of somatostatin and decreased expression of growth hormone-releasing hormone and growth hormone-receptor mRNAs in the arcuate nucleus (ARC). Similarly, ARC expression of orexigenic neuropeptide Y was decreased and ventricular ependymal expression of orphan G protein-coupled receptor Gpr50 was decreased. We demonstrate for the first time an energy balance effect of the Zfhx3Sci mutation, likely by altering expression of key ARC neuropeptides to alter growth, food intake, and energy expenditure.