Rhythmic leptin is required for weight gain from circadian desynchronized feeding in the mouse

The specificities, influencing factors, and medical implications of bone circadian rhythms

Physiological processes within the human body are regulated in approximately 24-h cycles known as circadian rhythms, serving to adapt to environmental changes. Bone rhythms play pivotal roles in bone development, metabolism, mineralization, and remodeling processes. Bone rhythms exhibit cell specificity, and different cells in bone display various expressions of clock genes. Multiple environmental factors, including light, feeding, exercise, and temperature, affect bone diurnal rhythms through the sympathetic nervous system and various hormones. Disruptions in bone diurnal rhythms contribute to the onset of skeletal disorders such as osteoporosis, osteoarthritis and skeletal hypoplasia. Conversely, these bone diseases can be effectively treated when aimed at the circadian clock in bone cells, including the rhythmic expressions of clock genes and drug targets. In this review, we describe the unique circadian rhythms in physiological activities of various bone cells. Then we summarize the factors synchronizing the diurnal rhythms of bone with the underlying mechanisms. Based on the review, we aim to build an overall understanding of the diurnal rhythms in bone and summarize the new preventive and therapeutic strategies for bone disorders.

Precision medicine in endocrinology: Unraveling metabolic health through time-restricted eating

As a promising avenue in nutrition, intermittent fasting, particularly time-restricted eating like the 8/16 protocol, requires careful individualization. This approach involves voluntary food restriction interspersed with normal eating, aiming to align with inner circadian rhythms for potential benefits in metabolism and weight management. Endocrinologists, responding to patient interest and backed by evidence-based medicine, can now delve into the intricacies of time-restricted eating. They consider each patient's unique medical history and expectations, integrating this approach into tailored treatment plans in a personalized medicine approach. Ongoing research is essential to deepen our comprehension of how time-restricted eating influences metabolic health, enabling the development of precise recommendations suitable for diverse populations and various clinical conditions. While time-restricted eating is a relevant metabolic approach, endocrinologists should exercise caution to prevent the promotion of eating disorders due to its restrictive nature.

Hypothalamic astrocytic-BMAL1 regulates energy homeostasis in a sex-dependent manner

Here, we demonstrate that hypothalamic astrocytic BMAL1 computes cyclic metabolic information to optimize energetic resources in a sexually dimorphic manner. Knockdown of BMAL1 in female astrocytes leads to negative energy balance and alters basal metabolic cycles without affecting circadian locomotor activity. Thus, astrocytic BMAL1 contributes to the control of energy balance through the modulation of the metabolic rate, hepatic and white adipose tissue lipogenesis, and the activity of brown adipose tissue. Importantly, most of these alterations are specific to hypothalamic astrocytic BMAL1. Moreover, female mice with BMAL1 knockdown in astrocytes exhibited a "male-like" metabolic obese phenotype when fed a high-fat diet. Overall, our results suggest a sexually dimorphic effect of astrocytic BMAL1 on the regulation of energy homeostasis, which may be of interest in the physiopathology of obesity and related comorbidities.

Leptin receptor neurons in the dorsomedial hypothalamus input to the circadian feeding network

Salient cues, such as the rising sun or availability of food, entrain biological clocks for behavioral adaptation. The mechanisms underlying entrainment to food availability remain elusive. Using single-nucleus RNA sequencing during scheduled feeding, we identified a dorsomedial hypothalamus leptin receptor-expressing (DMHLepR) neuron population that up-regulates circadian entrainment genes and exhibits calcium activity before an anticipated meal. Exogenous leptin, silencing, or chemogenetic stimulation of DMHLepR neurons disrupts the development of molecular and behavioral food entrainment. Repetitive DMHLepR neuron activation leads to the partitioning of a secondary bout of circadian locomotor activity that is in phase with the stimulation and dependent on an intact suprachiasmatic nucleus (SCN). Last, we found a DMHLepR neuron subpopulation that projects to the SCN with the capacity to influence the phase of the circadian clock. This direct DMHLepR-SCN connection is well situated to integrate the metabolic and circadian systems, facilitating mealtime anticipation.

A leptin-responsive hypothalamic circuit inputs to the circadian feeding network

Salient cues, such as the rising sun or the availability of food, play a crucial role in entraining biological clocks, allowing for effective behavioral adaptation and ultimately, survival. While the light-dependent entrainment of the central circadian pacemaker (suprachiasmatic nucleus, SCN) is relatively well defined, the molecular and neural mechanisms underlying entrainment associated with food availability remains elusive. Using single nucleus RNA sequencing during scheduled feeding (SF), we identified a leptin receptor (LepR) expressing neuron population in the dorsomedial hypothalamus (DMH) that upregulates circadian entrainment genes and exhibits rhythmic calcium activity prior to an anticipated meal. We found that disrupting DMHLepR neuron activity had a profound impact on both molecular and behavioral food entrainment. Specifically, silencing DMHLepR neurons, mis-timed exogenous leptin administration, or mis-timed chemogenetic stimulation of these neurons all interfered with the development of food entrainment. In a state of energy abundance, repetitive activation of DMHLepR neurons led to the partitioning of a secondary bout of circadian locomotor activity that was in phase with the stimulation and dependent on an intact SCN. Lastly, we discovered that a subpopulation of DMHLepR neurons project to the SCN with the capacity to influence the phase of the circadian clock. This leptin regulated circuit serves as a point of integration between the metabolic and circadian systems, facilitating the anticipation of meal times.

Leptin as an Antitorpor Hormone: An Explanation for the Increased Metabolic Efficiency and Cold Sensitivity of ob/ob Mice?

AbstractLeptin is recognized as an anorexigenic hormone. In its absence (e.g., in ob/ob mutant mice), mice become obese, primarily as a result of hyperphagia. A recurrent question is whether, additionally, leptin is thermogenic and thus also an antiobesity hormone in this way. We have earlier reviewed available data and have concluded that most articles implying a thermogenic effect of leptin have based this on a misconstrued division by body weight. Here, we have collected evidence that the remaining observations that imply that leptin is a thermogenic hormone are better understood as implying that leptin is an antitorpor hormone. Leptin levels increase in proportion to the body's energy reserves (i.e., stored lipids in the adipose tissue), and leptin thus serves as an indicator of energy availability. In the absence of leptin, ob/ob mice are exceedingly prone to enter daily torpor, since the absence of leptin causes them to perceive a lack of body energy reserves that, in combination with restricted or no food, induces them to enter the torpid state to save energy. This antitorpor effect of leptin probably explains the following earlier observations. First, ob/ob mice have the ability to gain weight even when pair fed with leptin-treated ob/ob mice. This is understood as follows: In the leptin-treated ob/ob mice, food intake is reduced. Untreated pair-fed mice enter daily torpor, and this markedly lowers total daily energy expenditure; the resulting surplus food energy is then accumulated as fat in these mice. However, ob/ob mice fed ad lib. do not enter torpor, so under normal conditions this mechanism does not contribute to the obesity found in the ob/ob mice. Second, neonatal ob/ob mice have the ability to become obese despite eating the same amount as wild-type mice: this is understood as these mice similarly entering daily torpor. Third, ob/ob mice on the C57BL/6J background have a lower metabolic rate: these mice were examined in the absence of food, and torpor was thus probably induced. Fourth, ob/ob mice have apparent high cold sensitivity: these mice experienced cold in the absence of food and would immediately enter deep torpor. It is suggested that this novel explanation of how the antitorpor effects of leptin affect mouse energy metabolism can open new avenues for leptin research.

Effects of mastication on antibody production under fasting conditions in mice

Chewing is beneficial not only for digestion and absorption of food, but also for various physiological functions, such as cognition and immunity. In this study, the effect of chewing on hormonal changes and the immune response was investigated under fasting conditions in mice. We investigated leptin and corticosterone levels, which are hormones with well-known associations with immune response and large changes during fasting. To study of effects of chewing under fasting conditions, one group of mice was provided with wooden sticks to stimulate chewing, one group was supplemented with 30% glucose solution, and one group received both treatments. We examined changes in serum leptin and corticosterone levels after 1 and 2 d of fasting. Antibody production was measured 2 weeks after subcutaneous immunization with bovine serum albumin on the last day of fasting. Under fasting conditions, serum leptin levels decreased and serum corticosterone levels increased. Supplementation with 30% glucose solution during fasting increased leptin levels above normal, but had little effect on corticosterone levels. In contrast, chewing stimulation inhibited the increase in corticosterone production, but did not affect the decrease in leptin levels. Antibody production significantly increased under separate and combined treatments. Taken together, our results showed that chewing stimulation during fasting inhibited the increase in corticosterone production and improved antibody production after immunization.

Circadian signatures of anterior hypothalamus in time-restricted feeding

Background: Meal timing resets circadian clocks in peripheral tissues, such as the liver, in seven days without affecting the phase of the central clock located in the suprachiasmatic nucleus (SCN) of the hypothalamus. Anterior hypothalamus plays an essential role in energy metabolism, circadian rhythm, and stress response. However, it remains to be elucidated whether and how anterior hypothalamus adapts its circadian rhythms to meal timing. Methods: Here, we applied transcriptomics to profile rhythmic transcripts in the anterior hypothalamus of nocturnal female mice subjected to day- (DRF) or night (NRF)-time restricted feeding for seven days. Results: This global profiling identified 128 and 3,518 rhythmic transcripts in DRF and NRF, respectively. NRF entrained diurnal rhythms among 990 biological processes, including 'Electron transport chain' and 'Hippo signaling' that reached peak time in the late sleep and late active phase, respectively. By contrast, DRF entrained only 20 rhythmic pathways, including 'Cellular amino acid catabolic process', all of which were restricted to the late active phase. The rhythmic transcripts found in both DRF and NRF tissues were largely resistant to phase entrainment by meal timing, which were matched to the action of the circadian clock. Remarkably, DRF for 36 days partially reversed the circadian clock compared to NRF. Conclusions: Collectively, our work generates a useful dataset to explore anterior hypothalamic circadian biology and sheds light on potential rhythmic processes influenced by meal timing in the brain (www.circametdb.org.cn).

Proanthocyanidins Restore the Metabolic Diurnal Rhythm of Subcutaneous White Adipose Tissue According to Time-Of-Day Consumption

Consumption of grape seed proanthocyanidin extract (GSPE) has beneficial effects on the functionality of white adipose tissue (WAT). However, although WAT metabolism shows a clear diurnal rhythm, whether GSPE consumption could affect WAT rhythmicity in a time-dependent manner has not been studied. Ninety-six male Fischer rats were fed standard (STD, two groups) or cafeteria (CAF, four groups) diet for 9 weeks (n = 16 each group). From week 6 on, CAF diet animals were supplemented with vehicle or 25 mg GSPE/kg of body weight either at the beginning of the light/rest phase (ZT0) or at the beginning of the dark/active phase (ZT12). The two STD groups were also supplemented with vehicle at ZT0 or ZT12. In week 9, animals were sacrificed at 6 h intervals (n = 4) to analyze the diurnal rhythms of subcutaneous WAT metabolites by nuclear magnetic resonance spectrometry. A total of 45 metabolites were detected, 19 of which presented diurnal rhythms in the STD groups. Although most metabolites became arrhythmic under CAF diet, GSPE consumption at ZT12, but not at ZT0, restored the rhythmicity of 12 metabolites including compounds involved in alanine, aspartate, and glutamate metabolism. These results demonstrate that timed GSPE supplementation may restore, at least partially, the functional dynamics of WAT when it is consumed at the beginning of the active phase. This study opens an innovative strategy for time-dependent polyphenol treatment in obesity and metabolic diseases.

Circadian Rhythm Disruption Influenced Hepatic Lipid Metabolism, Gut Microbiota and Promoted Cholesterol Gallstone Formation in Mice

BACKGROUND

Hepatic lipid metabolism regulates biliary composition and influences the formation of cholesterol gallstones. The genes Hmgcr and Cyp7a1, which encode key liver enzymes, are regulated by circadian rhythm-related transcription factors. We aimed to investigate the effect of circadian rhythm disruption on hepatic cholesterol and bile acid metabolism and the incidence of cholesterol stone formation.

METHODS

Adult male C57BL/6J mice were fed either a lithogenic diet (LD) only during the sleep phase (time-restricted lithogenic diet feeding, TRF) or an LD ad libitum (non-time-restricted lithogenic diet feeding, nTRF) for 4 weeks. Food consumption, body mass gain, and the incidence of gallstones were assessed. Circulating metabolic parameters, lipid accumulation in the liver, the circadian expression of hepatic clock and metabolic genes, and the gut microbiota were analyzed.

RESULTS

TRF caused a dysregulation of the circadian rhythm in the mice, characterized by significant differences in the circadian expression patterns of clock-related genes. In TRF mice, the circadian rhythms in the expression of genes involved in bile acid and cholesterol metabolism were disrupted, as was the circadian rhythm of the gut microbiota. These changes were associated with high biliary cholesterol content, which promoted gallstone formation in the TRF mice.

CONCLUSION

Disordered circadian rhythm is associated with abnormal hepatic bile acid and cholesterol metabolism in mice, which promotes gallstone formation.

Dietary Pterostilbene and Resveratrol Modulate the Gut Microbiota Influenced by Circadian Rhythm Dysregulation

SCOPE

A causal relationship between circadian misalignment and microbiota dysbiosis has been discussed recently, due to their association to pathogenesis. Herein, the possible impact of pterostilbene (PSB) and resveratrol (RES) on the gut microbiota brought by chronic jet-lag in mice is investigated.

METHODS AND RESULTS

Dietary supplement of RES and PSB (0.25%) are given to 16 week-jetlagged mice to examine the effects on microbiota and physiological functions. Jetlag significantly induces weight gained that could be effectively prevented by PSB. Both supplements also retain oscillation patterns that found to be lost in jetlag induced (JLG) group, including serum biochemical parameters and gut microbiota. The results of beta diversity suggest the supplementations efficiently lead to distinct gut microbial composition as compared to JLG group. Besides, the supplementation forestalls some microbial elevation, such as Eubacterium ventriosum and Acetitomaculum. Growth of health beneficial bacteria like Blautia and Lachnospiraceae UCG-001 is facilitated and abundance of these bacteria could be correlated to oscillation of biochemical parameters. Result of KEGG indicates distinct effect brought by microbial re-shaping.

CONCLUSION

The result suggests that supplementation of RES and PSB could potentially dampen some adverse effects of gut microbiota dysbiosis, and at the same time, re-composite and facilitate the growth of health beneficial microbiota.

Neuroimmune Interactions and Rhythmic Regulation of Innate Lymphoid Cells

The Earth’s rotation around its axis, is one of the parameters that never changed since life emerged. Therefore, most of the organisms from the cyanobacteria to humans have conserved natural oscillations to regulate their physiology. These daily oscillations define the circadian rhythms that set the biological clock for almost all physiological processes of an organism. They allow the organisms to anticipate and respond behaviorally and physiologically to changes imposed by the day/night cycle. As other physiological systems, the immune system is also regulated by circadian rhythms and while diurnal variation in host immune responses to lethal infection have been observed for many decades, the underlying mechanisms that affect immune function and health have only just started to emerge. These oscillations are generated by the central clock in our brain, but neuroendocrine signals allow the synchronization of the clocks in peripheral tissues. In this review, we discuss how the neuroimmune interactions create a rhythmic activity of the innate lymphoid cells. We highlight how the disruption of these rhythmic regulations of immune cells can disturb homeostasis and lead to the development of chronic inflammation in murine models.

Scrutinizing Mechanisms of the 'Obesity Paradox in Sepsis': Obesity Is Accompanied by Diminished Formation of Neutrophil Extracellular Traps (NETs) Due to Restricted Neutrophil-Platelet Interactions

Systemic inflammation is a detrimental condition associated with high mortality. However, obese individuals seem to have higher chances of surviving sepsis. To elucidate what immunological differences exist between obese and lean individuals we studied the course of endotoxemia in mice fed high-fat diet (HFD) and ob/ob animals. Intravital microscopy revealed that neutrophil extracellular trap (NET) formation in liver vasculature is negligible in obese mice in sharp contrast to their lean counterparts (ND). Unlike in lean individuals, neutrophil influx is not driven by leptin or interleukin 33 (IL-33), nor occurs via a chemokine receptor CXCR2. In obese mice less platelets interact with neutrophils forming less aggregates. Platelets transfer from ND to HFD mice partially restores NET formation, and even further so upon P-selectin blockage on them. The study reveals that in obesity the overexaggerated inflammation and NET formation are limited during sepsis due to dysfunctional platelets suggesting their targeting as a therapeutic tool in systemic inflammation.

The timing of fasting leads to different levels of food consumption and PYY3-36 in nocturnal mice

PURPOSE

The daily circadian cycle is known to modulate both feeding behavior and metabolism. As such, the timing of food consumption can play a role in regulating overall health. The purpose of this study is to determine whether fasting at different times of the day alters subsequent food consumption and levels of PYY_3-36, a hormone secreted after a meal which inhibits appetite.

METHODS

Separate groups of mice were fasted at different times of the day: (1) start of the day, (2) middle of the day, (3) start of the night, and (4) middle of the night, and either injected with vehicle or PYY_3-36 to assess their subsequent food consumption patterns, PYY_3-36 levels, and glucose and insulin levels. We also investigated whether light exposure during the night would alter food consumption and PYY_3-36 levels after fasting.

RESULTS

Mice fasted during the start of the daytime exhibited increased food consumption post-fast compared to mice fasted during the night. Injections of PYY_3-36 during the night were more effective in reducing food consumption compared to PYY_3-36 administration during the day. Constant light exposure suppressed food consumption after fasting and increased fasting PYY_3-36 levels.

CONCLUSIONS

These results indicate that mice exhibit distinct food consumption patterns after being presented with a fast at different times of the day. Light exposure also modulates both food consumption after a fast and levels of PYY_3-36.

Timing of Food Intake Drives the Circadian Rhythm of Blood Pressure

Timing of food intake has become a critical factor in determining overall cardiometabolic health. We hypothesized that timing of food intake entrains circadian rhythms of blood pressure (BP) and renal excretion in mice. Male C57BL/6J mice were fed ad libitum or reverse feeding (RF) where food was available at all times of day or only available during the 12-h lights-on period, respectively. Mice eating ad libitum had a significantly higher mean arterial pressure (MAP) during lights-off compared to lights-on (113 ± 2 mmHg vs 100 ± 2 mmHg, respectively; P < 0.0001); however, RF for 6 days inverted the diurnal rhythm of MAP (99 ± 3 vs 110 ± 3 mmHg, respectively; P < 0.0001). In contrast to MAP, diurnal rhythms of urine volume and sodium excretion remained intact after RF. Male Bmal1 knockout mice (Bmal1KO) underwent the same feeding protocol. As previously reported, Bmal1KO mice did not exhibit a diurnal MAP rhythm during ad libitum feeding (95 ± 1 mmHg vs 92 ± 3 mmHg, lights-off vs lights-on; P > 0.05); however, RF induced a diurnal rhythm of MAP (79 ± 3 mmHg vs 95 ± 2 mmHg, lights-off vs lights-on phase; P < 0.01). Transgenic PERIOD2::LUCIFERASE knock-in mice were used to assess the rhythm of the clock protein PERIOD2 in ex vivo tissue cultures. The timing of the PER2::LUC rhythm in the renal cortex and suprachiasmatic nucleus was not affected by RF; however, RF induced significant phase shifts in the liver, renal inner medulla, and adrenal gland. In conclusion, the timing of food intake controls BP rhythms in mice independent of Bmal1, urine volume, or sodium excretion.

Interplay between diet, exercise and the molecular circadian clock in orchestrating metabolic adaptations of adipose tissue

Disruption of circadian rhythmicity induced by prolonged light exposure, altered sleep patterns and shift work is associated with the development of obesity and related metabolic disorders, including type 2 diabetes and cardiovascular diseases. White and brown adipose tissue activity shows circadian rhythmicity, with daily variations in the regulation of metabolic processes such as lipolysis, glucose and lipid uptake, and adipokine secretion. The role of the circadian clock in the regulation of energy homeostasis has raised interest in clock-related strategies to mitigate metabolic disturbances associated with type 2 diabetes, including 'resynchronizing' metabolism through diet or targeting a particular time of a day to potentiate the effect of a pharmacological or physiological treatment. Exercise is an effective intervention to prevent insulin resistance and type 2 diabetes. Beyond its effect on skeletal muscle, exercise training also has a profound effect on adipose tissue. Adipose tissue partly mediates the beneficial effect of exercise on glucose and energy homeostasis, via its metabolic and endocrine function. The interaction between zeitgeber time and diet or exercise is likely to influence the metabolic response of adipose tissue and therefore impact the whole-body phenotype. Understanding the impact of circadian clock systems on human physiology and how this is regulated by exercise in a tissue-specific manner will yield new insights for the management of metabolic disorders.

Sharing Pathological Mechanisms of Insomnia and Osteoporosis, and a New Perspective on Safe Drug Choice

Lack of adequate sleep has become increasingly common in our 24/7 modern society. Reduced sleep has significant health consequences including metabolic and cardiovascular disorders, and mental problems including depression. In addition, although the increase in life expectancy has provided a dream of longevity to humans, the occurrence of osteoporosis is a big obstacle to this dream for both male and female. It is known that insomnia and bone health problems, which are very critical conditions in human life, interestingly, share a lot of pathogenesis in recent decades. Nevertheless, due to another side effects of the synthetic drugs being taken for the treatment of insomnia and osteoporosis, patients have substantial anxiety for the safety of drugs with therapeutic expectation. This review examines the pathogenesis shared by sleep and osteoporosis together and herbal medicine, which has recently been shown to be safe and efficacious in the treatment of both diseases other than synthetic drugs. We suggestions for how to treat osteoporosis. These efforts will be the first step toward enabling patients to have comfortable and safe prescriptions through a wide selection of therapeutic agents in the future.

Zebrafish Mutants Carrying Leptin a (lepa) Gene Deficiency Display Obesity, Anxiety, Less Aggression and Fear, and Circadian Rhythm and Color Preference Dysregulation

Leptin, a hormone secreted by peripheral adipose tissues, regulates the appetite in animals. Recently, evidence has shown that leptin also plays roles in behavioral response in addition to controlling appetite. In this study, we examined the potential function of leptin on non-appetite behaviors in zebrafish model. By using genome editing tool of Transcription activator-like effector nuclease (TALEN), we successfully knocked out leptin a (lepa) gene by deleting 4 bp within coding region to create a premature-translation stop. Morphological and appetite analysis showed the lepa KO fish display a phenotype with obese, good appetite and elevation of Agouti-related peptide (AgRP) and Ghrelin hormones, consistent with the canonical function of leptin in controlling food intake. By multiple behavior endpoint analyses, including novel tank, mirror biting, predator avoidance, social interaction, shoaling, circadian rhythm, and color preference assay, we found the lepa KO fish display an anxiogenic phenotype showing hyperactivity with rapid swimming, less freezing time, less fear to predator, loose shoaling area forming, and circadian rhythm and color preference dysregulations. Using biochemical assays, melatonin, norepinephrine, acetylcholine and serotonin levels in the brain were found to be significantly reduced in lepa KO fish, while the levels of dopamine, glycine and cortisol in the brain were significantly elevated. In addition, the brain ROS level was elevated, and the anti-oxidative enzyme catalase level was reduced. Taken together, by performing loss-of-function multiple behavior endpoint testing and biochemical analysis, we provide strong evidence for a critical role of lepa gene in modulating anxiety, aggression, fear, and circadian rhythm behaviors in zebrafish for the first time.

Short-term time-restricted feeding during the resting phase is sufficient to induce leptin resistance that contributes to development of obesity and metabolic disorders in mice

Feeding at unusual times of the day is thought to be associated with obesity and metabolic disorders in both experimental animals and humans. We previously reported that time-imposed feeding during the sleep phase (daytime feeding, DF) induces obesity and metabolic disorders compared with mice fed only during the active phase (nighttime feeding, NF). The present study aimed to determine whether leptin resistance is caused by DF, and whether it is involved in the underlying mechanisms of DF-induced obesity in mice, since leptin plays an essential role in regulating energy expenditure and adiposity in addition to food intake. We compared leptin sensitivity by evaluating the effects of exogenous injected leptin on food intake and body weight in wild-type C57BL/6J mice under NF and DF. The mice were fed with a high-fat high-sucrose diet throughout the study. To determine whether leptin resistance is a cause or a result of DF-induced obesity with metabolic disorders, we restricted the feeding times of leptin resistant db/db mice. We also examined leptin sensitivity in leptin deficient ob/ob mice under NF and DF to elucidate the underlying mechanisms of DF-induced leptin resistance. C57BL/6J mice under DF gained more weight and adiposity compared with mice under NF, and developed hyperleptinemia and hypothermia. We found that six days of DF abolished exogenous leptin-induced hypophagia and reduction in body weight in mice. We also found that the leptin injection significantly suppressed the mRNA expression of lipogenic genes in the liver of NF, but not in DF mice, suggesting that short-term DF was sufficient to induce metabolic leptin resistance. The DF-induced increases in body weight gain, food efficiency, adipose tissue mass, lipogenic gene expression in metabolic tissues, and hepatic lipid accumulation were abolished in db/db mice, suggesting that the leptin resistance is a cause of DF-induced metabolic disorders. DF resulted in deep hypothermia in db/db, as well as in wild-type mice, suggesting that a decrease in energy expenditure was not the main cause of DF-induced obesity. Exogenous leptin reduced the body weight of ob/ob mice under both NF and DF, and the effect was significantly higher in DF- than in NF-ob/ob mice. Therefore, the development of DF-induced leptin resistance requires endogenous leptin, and central leptin sensitivity fluctuates in a circadian manner. The present findings suggest that leptin resistance is responsible for DF-induced obesity and metabolic disorders, and that the circadian fluctuation of central leptin sensitivity might be involved in leptin resistance induced by DF, although further studies are needed to elucidate the mechanisms of metabolic disorders that depend on the time of feeding. Abbreviations: AMPK, adenosine monophosphate-activated protein kinase; ANOVA, analysis of variance; DF, daytime feeding; FFA, free fatty acid; HOMA-IR, homeostasis model assessment of insulin resistance; NEAT, non-exercise activity thermogenesis; NF, nighttime feeding; PI3, phosphatidylinositol 3; RF, restricted feeding; RW, running-wheel; SCN, suprachiasmatic nucleus; SEM, standard error of the mean; STAT3, signal transducer and activator of transcription 3; T-Cho, total cholesterol; TG, triglyceride; WAT, white adipose tissues.

The Mysterious Food-Entrainable Oscillator: Insights from Mutant and Engineered Mouse Models

The food-entrainable oscillator (FEO) is a mysterious circadian clock because its anatomical location(s) and molecular timekeeping mechanism are unknown. Food anticipatory activity (FAA), which is defined as the output of the FEO, emerges during temporally restricted feeding. FAA disappears immediately during ad libitum feeding and reappears during subsequent fasting. A free-running FAA rhythm has been observed only in rare circumstances when food was provided with a period outside the range of entrainment. Therefore, it is difficult to study the circadian properties of the FEO. Numerous studies have attempted to identify the critical molecular components of the FEO using mutant and genetically engineered mouse models. Herein we critically review the experimental protocols and findings of these studies in mouse models. Several themes emerge from these studies. First, there is little consistency in restricted feeding protocols between studies. Moreover, the protocols were sometimes not optimal, resulting in erroneous conclusions that FAA was absent in some mouse models. Second, circadian genes are not necessary for FEO timekeeping. Thus, another noncanonical timekeeping mechanism must exist in the FEO. Third, studies of mouse models have shown that signaling pathways involved in circadian timekeeping, reward (dopaminergic), and feeding and energy homeostasis can modulate, but are not necessary for, the expression of FAA. In sum, the approaches to date have been largely unsuccessful in discovering the timekeeping mechanism of the FEO. Moving forward, we propose the use of standardized and optimized experimental protocols that focus on identifying genes that alter the period of FAA in mutant and engineered mouse models. This approach is likely to permit discovery of molecular components of the FEO timekeeping mechanism.

The Circadian Clock in White and Brown Adipose Tissue: Mechanistic, Endocrine, and Clinical Aspects

Obesity is a major risk factor for the development of illnesses, such as insulin resistance and hypertension, and has become a serious public health problem. Mammals have developed a circadian clock located in the hypothalamic suprachiasmatic nuclei (SCN) that responds to the environmental light-dark cycle. Clocks similar to the one located in the SCN are found in peripheral tissues, such as the kidney, liver, and adipose tissue. The circadian clock regulates metabolism and energy homeostasis in peripheral tissues by mediating activity and/or expression of key metabolic enzymes and transport systems. Knockouts or mutations in clock genes that lead to disruption of cellular rhythmicity have provided evidence to the tight link between the circadian clock and metabolism. In addition, key proteins play a dual role in regulating the core clock mechanism, as well as adipose tissue metabolism, and link circadian rhythms with lipogenesis and lipolysis. Adipose tissues are distinguished as white, brown, and beige (or brite), each with unique metabolic characteristics. Recently, the role of the circadian clock in regulating the differentiation into the different adipose tissues has been investigated. In this review, the role of clock proteins and the downstream signaling pathways in white, brown, and brite adipose tissue function and differentiation will be reviewed. In addition, chronodisruption and metabolic disorders and clinical aspects of circadian adiposity will be addressed.

Circadian aspects of adipokine regulation in rodents

Most hormones display daily fluctuations of secretion during the 24-h cycle. This is also the case for adipokines, in particular the anorexigenic hormone, leptin. The temporal organization of the endocrine system is principally controlled by a network of circadian clocks. The circadian network comprises a master circadian clock, located in the suprachiasmatic nucleus of the hypothalamus, synchronized to the ambient light, and secondary circadian clocks found in various peripheral organs, such as the adipose tissues. Besides circadian clocks, other factors such as meals and metabolic status impact daily profiles of hormonal levels. In turn, the precise daily pattern of hormonal release provides temporal signaling information. This review will describe the reciprocal links between the circadian clocks and rhythmic secretion of leptin, and discuss the metabolic impact of circadian desynchronization and altered rhythmic leptin.

Role of estrogen receptor signaling in skeletal response to leptin in female ob/ob mice

Leptin, critical in regulation of energy metabolism, is also important for normal bone growth, maturation and turnover. Compared to wild type (WT) mice, bone mass is lower in leptin-deficient ob/ob mice. Osteopenia in growing ob/ob mice is due to decreased bone accrual, and is associated with reduced longitudinal bone growth, impaired cancellous bone maturation and increased marrow adipose tissue (MAT). However, leptin deficiency also results in gonadal dysfunction, disrupting production of gonadal hormones which regulate bone growth and turnover. The present study evaluated the role of increased estrogen in mediating the effects of leptin on bone in ob/ob mice. Three-month-old female ob/ob mice were randomized into one of the 3 groups: (1) ob/ob + vehicle (veh), (2) ob/ob + leptin (leptin) or (3) ob/ob + leptin and the potent estrogen receptor antagonist ICI 182,780 (leptin + ICI). Age-matched WT mice received vehicle. Leptin (40 µg/mouse, daily) and ICI (10 µg/mouse, 2×/week) were administered by subcutaneous injection for 1 month and bone analyzed by X-ray absorptiometry, microcomputed tomography and static and dynamic histomorphometry. Uterine weight did not differ between ob/ob mice and ob/ob mice receiving leptin + ICI, indicating that ICI successfully blocked the uterine response to leptin-induced increases in estrogen levels. Compared to leptin-treated ob/ob mice, ob/ob mice receiving leptin + ICI had lower uterine weight; did not differ in weight loss, MAT or bone formation rate; and had higher longitudinal bone growth rate and cancellous bone volume fraction. We conclude that increased estrogen signaling following leptin treatment is dispensable for the positive actions of leptin on bone and may attenuate leptin-induced bone growth.

Keeping fat on time: Circadian control of adipose tissue

Circadian clocks harmonize processes ranging from intracellular biochemistry to whole-body physiology in accordance with the Earth's 24h rotation. These intrinsic oscillators are based on an interlocked transcriptional-translational feedback loop comprised from a set of core clock factors. In addition to maintaining rhythmicity in nearly every cell of the body, these clock factors also mediate tissue specific metabolic functions. In this review, we will explore how the molecular clock shapes the unique features of different adipose depots.

Circadian Rhythms in Adipose Tissue Physiology

The different types of adipose tissues fulfill a wide range of biological functions-from energy storage to hormone secretion and thermogenesis-many of which show pronounced variations over the course of the day. Such 24-h rhythms in physiology and behavior are coordinated by endogenous circadian clocks found in all tissues and cells, including adipocytes. At the molecular level, these clocks are based on interlocked transcriptional-translational feedback loops comprised of a set of clock genes/proteins. Tissue-specific clock-controlled transcriptional programs translate time-of-day information into physiologically relevant signals. In adipose tissues, clock gene control has been documented for adipocyte proliferation and differentiation, lipid metabolism as well as endocrine function and other adipose oscillations are under control of systemic signals tied to endocrine, neuronal, or behavioral rhythms. Circadian rhythm disruption, for example, by night shift work or through genetic alterations, is associated with changes in adipocyte metabolism and hormone secretion. At the same time, adipose metabolic state feeds back to central and peripheral clocks, adjusting behavioral and physiological rhythms. In this overview article, we summarize our current knowledge about the crosstalk between circadian clocks and energy metabolism with a focus on adipose physiology. © 2017 American Physiological Society. Compr Physiol 7:383-427, 2017.

Modulation of circadian clocks by nutrients and food factors

Daily activity rhythms that are dominated by internal clocks are called circadian rhythms. A central clock is located in the suprachiasmatic nucleus of the hypothalamus, and peripheral clocks are located in most mammalian peripheral cells. The central clock is entrained by light/dark cycles, whereas peripheral clocks are entrained by feeding cycles. The effects of nutrients on the central and peripheral clocks have been investigated during the past decade and much interaction between them has come to light. For example, a high-fat diet prolongs the period of circadian behavior, a ketogenic diet advances the onset of locomotor activity rhythms, and a high-salt diet advances the phase of peripheral molecular clocks. Moreover, some food factors such as caffeine, nobiletin, and resveratrol, alter molecular and/or behavioral circadian rhythms. Here, we review nutrients and food factors that modulate mammalian circadian clocks from the cellular to the behavioral level.

Endocrine and cardiovascular rhythms differentially adapt to chronic phase-delay shifts in rats

Disturbances in regular circadian oscillations can have negative effects on cardiovascular function, but epidemiological data are inconclusive and new data from animal experiments elucidating critical biological mechanisms are needed. To evaluate the consequences of chronic phase shifts of the light/dark (LD) cycle on hormonal and cardiovascular rhythms, two experiments were performed. In Experiment 1, male rats were exposed to either a regular 12:12 LD cycle (CONT) or rotating 8-h phase-delay shifts of LD every second day (SHIFT) for 10 weeks. During this period, blood pressure (BP) was monitored weekly, and daily rhythms of melatonin, corticosterone, leptin and testosterone were evaluated at the end of the experiment. In Experiment 2, female rats were exposed to the identical shifted LD schedule for 12 weeks, and daily rhythms of BP, heart rate (HR) and locomotor activity were recorded using telemetry. Preserved melatonin rhythms were found in the pineal gland, plasma, heart and kidney of SHIFT rats with damped amplitude in the plasma and heart, suggesting that the central oscillator can adapt to chronic phase-delay shifts. In contrast, daily rhythms of corticosterone, testosterone and leptin were eliminated in SHIFT rats. Exposure to phase shifts did not lead to increased body weight and elevated BP. However, a shifted LD schedule substantially decreased the amplitude and suppressed the circadian power of the daily rhythms of BP and HR, implying weakened circadian control of physiological and behavioural processes. The results demonstrate that endocrine and cardiovascular rhythms can differentially adapt to chronic phase-delay shifts, promoting internal desynchronization between central and peripheral oscillators, which in combination with other negative environmental stimuli may result in negative health effects.

The acute effects of time-of-day-dependent high fat feeding on whole body metabolic flexibility in mice

Background:

Both circadian disruption and timing of feeding have important roles in the development of metabolic disease. Despite growing acceptance that the timing of food consumption has long-term impact on metabolic homeostasis, little is known regarding the immediate influence on whole body metabolism, or the mechanisms involved. We aimed to examine the acute effects of time-of-day-dependent high fat feeding on whole body substrate metabolism and metabolic plasticity, and to determine the potential contribution of the adipocyte circadian clock.

Methods:

Mice were fed a regimen of 4-h meal at the beginning and end of the dark (waking) cycle, separated by 4 h of fasting. Daily experimental conditions consisted of either an early very high fat or high fat (EVHF or EHF, 60 or 45% kcals from fat, respectively) or late (LVHF or LHF) meal, paired with a low fat (LF, 10% kcals from fat) meal. Metabolic parameters, glucose tolerance, body fat composition and weight were assessed. To determine the role of the adipocyte circadian clock, an aP2-CLOCK mutant (ACM) mouse model was used.

Results:

Mice in the EVHF or EHF groups showed a 13.2 or 8.84 higher percentage of caloric intake from fat and had a 0.013 or 0.026 lower daily average respiratory exchange ratio, respectively, compared with mice eating the opposite feeding regime. Changes in glucose tolerance, body fat composition and weight were not significant at the end of the 9-day restricted feeding period. ACM mice did not exhibit different metabolic responses to the feeding regimes compared with wild-type littermates. Circadian clock disruption did not influence the short-term response to timed feeding.

Conclusions:

Both the total fat composition of diet and the timing of fat intake may differentially mediate the effect of timed feeding on substrate metabolism, but may not induce acute changes in metabolic flexibility.

Short-term feeding at the wrong time is sufficient to desynchronize peripheral clocks and induce obesity with hyperphagia, physical inactivity and metabolic disorders in mice

BACKGROUND

The circadian clock regulates various physiological and behavioral rhythms such as feeding and locomotor activity. Feeding at unusual times of the day (inactive phase) is thought to be associated with obesity and metabolic disorders in experimental animals and in humans.

OBJECTIVE

The present study aimed to determine the underlying mechanisms through which time-of-day-dependent feeding influences metabolic homeostasis.

METHODS

We compared food consumption, wheel-running activity, core body temperature, hormonal and metabolic variables in blood, lipid accumulation in the liver, circadian expression of clock and metabolic genes in peripheral tissues, and body weight gain between mice fed only during the sleep phase (DF, daytime feeding) and those fed only during the active phase (NF, nighttime feeding). All mice were fed with the same high-fat high-sucrose diet throughout the experiment. To the best of our knowledge, this is the first study to examine the metabolic effects of time-imposed restricted feeding (RF) in mice with free access to a running wheel.

RESULTS

After one week of RF, DF mice gained more weight and developed hyperphagia, higher feed efficiency and more adiposity than NF mice. The daily amount of running on the wheel was rapidly and obviously reduced by DF, which might have been the result of time-of-day-dependent hypothermia. The amount of daily food consumption and hypothalamic mRNA expression of orexigenic neuropeptide Y and agouti-related protein were significantly higher in DF, than in NF mice, although levels of plasma leptin that fluctuate in an RF-dependent circadian manner, were significantly higher in DF mice. These findings suggested that the DF induced leptin resistance. The circadian phases of plasma insulin and ghrelin were synchronized to RF, although the corticosterone phase was unaffected. Peak levels of plasma insulin were remarkably higher in DF mice, although HOMA-IR was identical between the two groups. Significantly more free fatty acids, triglycerides and cholesterol accumulated in the livers of DF, than NF mice, which resulted from the increased expression of lipogenic genes such as Scd1, Acaca, and Fasn. Temporal expression of circadian clock genes became synchronized to RF in the liver but not in skeletal muscle, suggesting that uncoupling metabolic rhythms between the liver and skeletal muscle also contribute to DF-induced adiposity.

CONCLUSION

Feeding at an unusual time of day (inactive phase) desynchronizes peripheral clocks and causes obesity and metabolic disorders by inducing leptin resistance, hyperphagia, physical inactivity, hepatic fat accumulation and adiposity.

Impact of Sleep and Circadian Disruption on Energy Balance and Diabetes: A Summary of Workshop Discussions

A workshop was held at the National Institute for Diabetes and Digestive and Kidney Diseases with a focus on the impact of sleep and circadian disruption on energy balance and diabetes. The workshop identified a number of key principles for research in this area and a number of specific opportunities. Studies in this area would be facilitated by active collaboration between investigators in sleep/circadian research and investigators in metabolism/diabetes. There is a need to translate the elegant findings from basic research into improving the metabolic health of the American public. There is also a need for investigators studying the impact of sleep/circadian disruption in humans to move beyond measurements of insulin and glucose and conduct more in-depth phenotyping. There is also a need for the assessments of sleep and circadian rhythms as well as assessments for sleep-disordered breathing to be incorporated into all ongoing cohort studies related to diabetes risk. Studies in humans need to complement the elegant short-term laboratory-based human studies of simulated short sleep and shift work etc. with studies in subjects in the general population with these disorders. It is conceivable that chronic adaptations occur, and if so, the mechanisms by which they occur needs to be identified and understood. Particular areas of opportunity that are ready for translation are studies to address whether CPAP treatment of patients with pre-diabetes and obstructive sleep apnea (OSA) prevents or delays the onset of diabetes and whether temporal restricted feeding has the same impact on obesity rates in humans as it does in mice.

Circadian Dysfunction and Obesity: Is Leptin the Missing Link?

While accumulating evidence suggests that circadian desynchrony is linked to obesity and metabolic syndrome, the underlining mechanism is still poorly understood. In this issue, Kettner et al. (2015) demonstrate that leptin resistance, induced by circadian clock deficiency or chronic jet lag, may represent this missing link.

Time-fixed feeding prevents obesity induced by chronic advances of light/dark cycles in mouse models of jet-lag/shift work

Recent findings have uncovered intimate relationships between circadian clocks and energy metabolism. Epidemiological studies have shown that the frequency of obesity and metabolic disorders increases among shift-workers. Here we found that a chronic shift in light/dark (LD) cycles comprising an advance of six hours twice weekly, induced obesity in mice. Under such conditions that imitate jet lag/shift work, body weight and glucose intolerance increased, more fat accumulated in white adipose tissues and the expression profiles of metabolic genes changed in the liver compared with normal LD conditions. Mice fed at a fixed 12 h under the LD shift notably did not develop symptoms of obesity despite isocaloric intake. These results suggest that jet lag/shift work induces obesity as a result of fluctuating feeding times and it can be prevented by fixing meal times. This rodent model of obesity might serve as a useful tool for understanding why shift work induces metabolic disorders.

Modulation of sweet taste sensitivities by endogenous leptin and endocannabinoids in mice

KEY POINTS

Potential roles of endogenous leptin and endocannabinoids in sweet taste were examined by using pharmacological antagonists and mouse models including leptin receptor deficient (db/db) and diet-induced obese (DIO) mice. Chorda tympani (CT) nerve responses of lean mice to sweet compounds were increased after administration of leptin antagonist (LA) but not affected by administration of cannabinoid receptor antagonist (AM251). db/db mice showed clear suppression of CT responses to sweet compounds after AM251, increased endocannabinoid levels in the taste organ, and enhanced expression of a biosynthesizing enzyme of endocannabinoids in taste cells. The effect of LA was gradually decreased and that of AM251 was increased during the course of obesity in DIO mice. These findings suggest that circulating leptin, but not local endocannabinoids, is a dominant modulator for sweet taste in lean mice and endocannabinoids become more effective modulators of sweet taste under conditions of deficient leptin signalling.

ABSTRACT

Leptin is an anorexigenic mediator that reduces food intake by acting on hypothalamic receptor Ob-Rb. In contrast, endocannabinoids are orexigenic mediators that act via cannabinoid CB1 receptors in hypothalamus, limbic forebrain, and brainstem. In the peripheral taste system, leptin administration selectively inhibits behavioural, taste nerve and taste cell responses to sweet compounds. Opposing the action of leptin, endocannabinoids enhance sweet taste responses. However, potential roles of endogenous leptin and endocannabinoids in sweet taste remain unclear. Here, we used pharmacological antagonists (Ob-Rb: L39A/D40A/F41A (LA), CB1 : AM251) and examined the effects of their blocking activation of endogenous leptin and endocannabinoid signalling on taste responses in lean control, leptin receptor deficient db/db, and diet-induced obese (DIO) mice. Lean mice exhibited significant increases in chorda tympani (CT) nerve responses to sweet compounds after LA administration, while they showed no significant changes in CT responses after AM251. In contrast, db/db mice showed clear suppression of CT responses to sweet compounds after AM251, increased endocannabinoid (2-arachidonoyl-sn-glycerol (2-AG)) levels in the taste organ, and enhanced expression of a biosynthesizing enzyme (diacylglycerol lipase α (DAGLα)) of 2-AG in taste cells. In DIO mice, the LA effect was gradually decreased and the AM251 effect was increased during the course of obesity. Taken together, our results suggest that circulating leptin, but not local endocannabinoids, may be a dominant modulator for sweet taste in lean mice; however, endocannabinoids may become more effective modulators of sweet taste under conditions of deficient leptin signalling, possibly due to increased production of endocannabinoids in taste tissue.

Leptin normalizes photic synchronization in male ob/ob mice, via indirect effects on the suprachiasmatic nucleus

Mounting evidence indicates a strong link between metabolic diseases and circadian dysfunctions. The metabolic hormone leptin, substantially increased in dietary obesity, displays chronobiotic properties. Here we investigated whether leptin is involved in the alteration of timing associated with obesity, via direct or indirect effects on the suprachiasmatic nucleus (SCN), the site of the master clock. Photic synchronization was studied in obese ob/ob mice (deficient in leptin), either injected or not with high doses of recombinant murine leptin (5 mg/kg). This was performed first at a behavioral level, by shifting the light-dark cycle and inducing phase shifts by 30-minute light pulses and then at molecular levels (c-FOS and P-ERK1/2). Moreover, to characterize the targets mediating the chronomodulatory effects of leptin, we studied the induction of phosphorylated signal transducer and activator of transcription 3 (P-STAT3) in the SCN and in different structures projecting to the SCN, including the medial hypothalamus. Ob/ob mice showed altered photic synchronization, including augmented light-induced phase delays. Acute leptin treatment normalized the photic responses of the SCN at both the behavioral and molecular levels (decrease of light-induced c-FOS). Leptin-induced P-STAT3 was modulated by light in the arcuate nucleus and both the ventromedial and dorsomedial hypothalamic nuclei, whereas its expression was independent of the presence of leptin in the SCN. These results suggest an indirect action of leptin on the SCN, possibly mediated by the medial hypothalamus. Taken together, these results highlight a central role of leptin in the relationship between metabolic disturbances and circadian disruptions.

Circadian rhythms in liver metabolism and disease

Mounting research evidence demonstrates a significant negative impact of circadian disruption on human health. Shift work, chronic jet lag and sleep disturbances are associated with increased incidence of metabolic syndrome, and consequently result in obesity, type 2 diabetes and dyslipidemia. Here, these associations are reviewed with respect to liver metabolism and disease.

Manipulating the circadian and sleep cycles to protect against metabolic disease

Modernization of human society parallels an epidemic of metabolic disorders including obesity. Apart from excess caloric intake, a 24/7 lifestyle poses another important challenge to our metabolic health. Recent research under both laboratory and epidemiological settings has indicated that abnormal temporal organization of sleep and wakeful activities including food intake is a significant risk factor for metabolic disease. The circadian clock system is our intrinsic biological timer that regulates internal rhythms such as the sleep/wake cycle and also responses to external stimuli including light and food. Initially thought to be mainly involved in the timing of sleep, the clock, and/or clock genes may also play a role in sleep architecture and homeostasis. Importantly, an extensive body of evidence has firmly established a master regulatory role of the clock in energy balance. Together, a close relationship between well-timed circadian/sleep cycles and metabolic health is emerging. Exploiting this functional connection, an important holistic strategy toward curbing the epidemic of metabolic disorders (e.g., obesity) involves corrective measures on the circadian clock and sleep. In addition to behavioral and environmental interventions including meal timing and light control, pharmacological agents targeting sleep and circadian clocks promise convenient and effective applications. Recent studies, for example, have reported small molecules targeting specific clock components and displaying robust beneficial effects on sleep and metabolism. Furthermore, a group of clock-amplitude-enhancing small molecules (CEMs) identified via high-throughput chemical screens are of particular interest for future in vivo studies of their metabolic and sleep efficacies. Elucidating the functional relationship between clock, sleep, and metabolism will also have far-reaching implications for various chronic human diseases and aging.

The effects of light at night on circadian clocks and metabolism

Most organisms display endogenously produced ∼ 24-hour fluctuations in physiology and behavior, termed circadian rhythms. Circadian rhythms are driven by a transcriptional-translational feedback loop that is hierarchically expressed throughout the brain and body, with the suprachiasmatic nucleus of the hypothalamus serving as the master circadian oscillator at the top of the hierarchy. Appropriate circadian regulation is important for many homeostatic functions including energy regulation. Multiple genes involved in nutrient metabolism display rhythmic oscillations, and metabolically related hormones such as glucagon, insulin, ghrelin, leptin, and corticosterone are released in a circadian fashion. Mice harboring mutations in circadian clock genes alter feeding behavior, endocrine signaling, and dietary fat absorption. Moreover, misalignment between behavioral and molecular circadian clocks can result in obesity in both rodents and humans. Importantly, circadian rhythms are most potently synchronized to the external environment by light information and exposure to light at night potentially disrupts circadian system function. Since the advent of electric lights around the turn of the 20th century, exposure to artificial and irregular light schedules has become commonplace. The increase in exposure to light at night parallels the global increase in the prevalence of obesity and metabolic disorders. In this review, we propose that exposure to light at night alters metabolic function through disruption of the circadian system. We first provide an introduction to the circadian system, with a specific emphasis on the effects of light on circadian rhythms. Next we address interactions between the circadian system and metabolism. Finally, we review current experimental and epidemiological work directly associating exposure to light at night and metabolism.

Leptin expression is rhythmic in brain and liver of goldfish (Carassius auratus). Role of feeding time

Daily rhythms of feeding regulators are currently arousing research interest due to the relevance of the temporal harmony of endocrine regulators for growth and welfare in vertebrates. However, it is unknown the leptin circadian pattern in fish. The aim of this study is to investigate if leptin (gLep-aI and gLep-aII) expression is rhythmic in goldfish (Carassius auratus) liver and brain, and if such rhythms are driven by feeding time through a food entrainable oscillator. Fish maintained under 12-h light:12-h dark photoperiod and a scheduled feeding time showed 24-h locomotor activity and glycaemia rhythms. Moreover, hepatic gLep-aI and brain gLep-aI and gLep-aII expression were rhythmic with different daily profiles, showing a postprandial increase of leptin expression in the liver but not in the brain. Under constant light and different feeding regimes (scheduled fed at 10:00, 22:00 or randomly fed), feeding time synchronized daily rhythms in locomotor activity, glycaemia and clock gene expression (gPer1a, gPer3 and gCry3), but the rhythmic expression of hepatic gLep-aI and brain gLep-aII only remained in fed fish at 10:00. In summary, daily rhythms of leptin expression in goldfish are differently regulated at central and peripheral level, and they are not directly driven by clock genes. The role of food entrained oscillators on leptin expression rhythms in fish remains to be demonstrated.

Loss of NSCL-2 in gonadotropin releasing hormone neurons leads to reduction of pro-opiomelanocortin neurons in specific hypothalamic nuclei and causes visceral obesity

Regulation of sexual reproduction and energy homeostasis are closely interconnected, but only few efforts were made to explore the impact of gonadotropic neurons on metabolic processes. We have used Nscl-2 mutant mice suffering from adult onset of obesity and hypogonadotropic hypogonadism to study effects of gonadotropin releasing hormone (GnRH) neurons on neuronal circuits controlling energy balance. Inactivation of Nscl-2 in GnRH neurons but not in pro-opiomelanocortin (POMC) neurons reduced POMC neurons and increased visceral fat mass, suggesting a critical role of GnRH cells in the regulation of POMC neurons. In contrast, absence of POMC processing in the majority of Nscl-2-deficient POMC neurons had no effect on energy homeostasis. Finally, we investigated the cellular basis of the reduction of GnRH neurons in NSCL-2 mutants using a lineage tracing approach. We found that loss of Nscl-2 results in aberrant migration of GnRH neurons in Nscl-2 mutant mice causing a lineage switch of ectopically located GnRH neurons.

Differential regulation of adipokines may influence migratory behavior in the white-throated sparrow (Zonotrichia albicollis)

White-throated sparrows increase fat deposits during pre-migratory periods and rely on these fat stores to fuel migration. Adipose tissue produces hormones and signaling factors in a rhythmic fashion and may be controlled by a clock in adipose tissue or driven by a master clock in the brain. The master clock may convey photoperiodic information from the environment to adipose tissue to facilitate pre-migratory fattening, and adipose tissue may, in turn, release adipokines to indicate the extent of fat energy stores. Here, we present evidence that a change in signal from the adipokines adiponectin and visfatin may act to indicate body condition, thereby influencing an individual's decision to commence migratory flight, or to delay until adequate fat stores are acquired. We quantified plasma adiponectin and visfatin levels across the day in captive birds held under constant photoperiod. The circadian profiles of plasma adiponectin in non-migrating birds were approximately inverse the profiles from migrating birds. Adiponectin levels were positively correlated to body fat, and body fat was inversely related to the appearance of nocturnal migratory restlessness. Visfatin levels were constant across the day and did not correlate with fat deposits; however, a reduction in plasma visfatin concentration occurred during the migratory period. The data suggest that a significant change in the biological control of adipokine expression exists between the two migratory conditions and we propose a role for adiponectin, visfatin and adipose clocks in the regulation of migratory behaviors.

Disrupted circadian rhythmicity of the intestinal glucose transporter SGLT1 in Zucker diabetic fatty rats

BACKGROUND

Intestinal absorptive capacity shows a circadian rhythm synchronized with eating patterns. Disrupting these coordinated rhythms, e.g., with shift work, may contribute to metabolic disease. Circadian expression of nutrient transporters has not been studied in metabolic disease. We studied the circadian rhythm of intestinal transporter sodium glucose co-transporter type 1 (SGLT1) in an obese diabetic rat.

METHODS

We compared obese Zucker diabetic fatty (ZDF) rats to lean ZDF littermates. Temporal feeding patterns were assessed, then rats were harvested at Zeitgeber (ZT, ZT0 = 7:00 a.m.) 3, 9, or 15 to measure insulin resistance, SGLT1 expression and intestinal glucose absorption capacity. Regulators of SGLT1 (sweet taste receptor T1R2/3; clock genes) were measured to elucidate underlying mechanisms.

RESULTS

Both groups exhibited altered circadian food intake. Obese ZDF rats lost circadian rhythmicity of SGLT1 mRNA expression and functional activity. Lean ZDF rats maintained rhythmicity of SGLT1 mRNA expression but that of functional glucose absorption was blunted. Circadian rhythms of intestinal clock genes were maintained in both groups. Neither group had discernible rhythms of intestinal GLUT2 (glucose transporter) or T1R2 (sweet taste receptor component) mRNA expression. In summary, lean and obese ZDF rats exhibited similar disruptions in circadian feeding. Glucose intolerance was evident in lean rats, but only obese rats further developed diabetes and exhibited disrupted circadian rhythmicity of both SGLT1 mRNA expression and function.

CONCLUSIONS

Our findings suggest that disrupted circadian feeding rhythms contribute to glucose intolerance, but additional factors (genetics, changes in nutrient sensing/transport) are needed to lead to full diabetes.

Metabolic impact of sex chromosomes

Obesity and associated metabolic diseases are sexually dimorphic. To provide better diagnosis and treatment for both sexes, it is of interest to identify the factors that underlie male/female differences in obesity. Traditionally, sexual dimorphism has been attributed to effects of gonadal hormones, which influence numerous metabolic processes. However, the XX/XY sex chromosome complement is an additional factor that may play a role. Recent data using the four core genotypes mouse model have revealed that sex chromosome complement—independently from gonadal sex—plays a role in adiposity, feeding behavior, fatty liver and glucose homeostasis. Potential mechanisms for the effects of sex chromosome complement include differential gene dosage from X chromosome genes that escape inactivation, and distinct genomic imprints on X chromosomes inherited from maternal or paternal parents. Here we review recent data in mice and humans concerning the potential impact of sex chromosome complement on obesity and metabolic disease.