System-driven and oscillator-dependent circadian transcription in mice with a conditionally active liver clock

From starvation to time-restricted eating: a review of fasting physiology

We have long known that subjects with obesity who fast for several weeks survive. Calculations that assume the brain can only use glucose indicated that all carbohydrate and protein sources would be consumed by the brain within several weeks yet subjects with obesity who fasted for several weeks survived. This anomaly led to the determination of the metabolic role of ketone bodies. Subsequent studies transformed our understanding of ketone bodies and illustrated the value of challenging the norm and adapting theory to evidence. Although prolonged fasting is no longer a treatment for obesity, the early studies of starvation provided valuable insights about macronutrient metabolism and ketone body adaptations that fasting elicits. Intermittent fasting and its variants such as time-restricted eating are fasting models that are far less regimented than starvation and severe calorie restriction; yet they produce metabolic benefits. The mechanisms that produce the metabolic changes that intermittent fasting elicits are relatively unknown. In this article, we review the physiology of starvation, starvation adaptation diets, diet-induced ketosis, and intermittent fasting. Understanding the premise and physiology that these regimens induce is necessary to draw parallels and provoke thoughts on the mechanisms underlying the metabolic benefits of intermittent fasting and its variants.

Grape Seed Proanthocyanidin Extract Attenuates Cafeteria-Diet-Induced Liver Metabolic Disturbances in Rats: Influence of Photoperiod

This study investigated the influence of photoperiod (day length) on the efficacy of grape seed proanthocyanidin extract (GSPE) in mitigating metabolic disorders in obese rats fed a cafeteria diet. Rats were exposed to standard (L12), long (L18), or short (L6) photoperiods and treated with GSPE or vehicle. In the standard photoperiod, GSPE reduced body weight gain (50.5%), total cholesterol (37%), and triglycerides (34.8%), while increasing the expression of hepatic metabolic genes. In the long photoperiod, GSPE tended to decrease body weight gain, increased testosterone levels (68.3%), decreased liver weight (12.4%), and decreased reverse serum amino acids. In the short photoperiod, GSPE reduced glycemia (~10%) and lowered triglyceride levels (38.5%), with effects modified by diet. The standard photoperiod showed the greatest efficacy against metabolic syndrome-associated diseases. The study showed how day length affects GSPE's benefits and underscores considering biological rhythms in metabolic disease therapies.

Steven A. Brown and the synchronization of circadian rhythms by body temperature cycles

The mammalian circadian timing system has a hierarchical architecture, with a central pacemaker located in the brain's suprachiasmatic nucleus orchestrating rhythms in behaviour and physiology. In cooperation with environmental cycles, it synchronizes the phases of peripheral oscillators operating in most cells of the body. Even cells kept in tissue culture harbour self-sustained and cell-autonomous circadian clocks that keep ticking throughout their lifespan. The master pacemaker in the suprachiasmatic nucleus is synchronized primarily by light-dark cycles, whereas peripheral oscillators are phase entrained by a multitude of systemic signalling pathways. These include pathways depending on feeding-fasting cycles, cellular actin polymerization dynamics, endocrine rhythms and, surprisingly, body temperature oscillations. Using tissue culture and murine models, Steve Brown was the first one to demonstrate that shallow rhythms of mammalian body temperature are timing cues (zeitgebers) for peripheral circadian clocks.

Alleviative effects of the parthenolide derivative ACT001 on insulin resistance induced by sodium propionate combined with a high-fat diet and its potential mechanisms

The increasing side effects of traditional medications used to treat type II diabetes have made research into the development of safer and more effective natural medications necessary. ACT001, a derivative of parthenolide, has been shown to have good anti-inflammatory and antitumor effects; however, its role in diabetes is unclear. The short-chain fatty acid propionate is a common food preservative that has been found to cause disturbances in glucose metabolism in mice and humans. This study aimed to investigate whether sodium propionate could aggravate insulin resistance in obese mice and cause diabetes and to study the alleviative effects and potential mechanisms of action of ACT001 on insulin resistance in diabetic mice. Type II diabetic mice were adminietered sodium propionate combined with a high-fat diet (HFD + propionate) by gavage daily for four weeks. Biochemical analysis showed that ACT001 significantly affected blood glucose concentration in diabetic mice, mainly by downregulating the expression of phosphoenolpyruvate carboxykinase 2 and glucose-6-phosphatase. Meanwhile, the level of fatty acid-binding protein 4 in the liver was significantly decreased. ACT001 has a protective effect on the liver and adipose tissue of mice. In addition, the results of the running wheel experiment indicated that ACT001 alleviated the circadian rhythm disorder caused by insulin resistance to a certain extent. This study revealed the potential mechanism by which ACT001 alleviates insulin resistance and provides ideas for developing natural antidiabetic drugs.

Telomerase: a nexus between cancer nanotherapy and circadian rhythm

Cancer represents a complex disease category defined by the unregulated proliferation and dissemination of anomalous cells within the human body. According to the GLOBOCAN 2020 report, the year 2020 witnessed the diagnosis of approximately 19.3 million new cases of cancer and 10.0 million individuals succumbed to the disease. A typical cell eventually becomes cancerous because of a long-term buildup of genetic instability and replicative immortality. Telomerase is a crucial regulator of cancer progression as it induces replicative immortality. In cancer cells, telomerase inhibits apoptosis by elongating the length of the telomeric region, which usually protects the genome from shortening. Many nanoparticles are documented as being available for detecting the presence of telomerase, and many were used as delivery systems to transport drugs. Furthermore, telomere homeostasis is regulated by the circadian time-keeping machinery, leading to 24-hour rhythms in telomerase activity and TERT mRNA expression in mammals. This review provides a comprehensive discussion of various kinds of nanoparticles used in telomerase detection, inhibition, and multiple drug-related pathways, as well as enlightens an imperative association between circadian rhythm and telomerase activity from the perspective of nanoparticle-based anticancer therapeutics.

miR-2940-1 is involved in the circadian regulation of oviposition in Aedes albopictus

The vast majority of all global species have circadian rhythm cycles that allow them to adapt to natural environments. These regular rhythms are regulated by core clock genes and recent studies have also implicated roles for microRNAs in this regulation. Oviposition is an important circadian behavior in the reproductive cycle of insect vectors of diseases, and little is known about the rhythm or its regulation in mosquitoes. Aedes albopictus is a diurnal mosquito that transmits arboviruses and is the major cause of outbreaks of dengue fever in China. We analyzed the oviposition rhythm patterns of A. albopictus under different light/dark conditions and show that the mosquitoes have an oviposition peak between zeitgeber time 9 (ZT 9) and ZT 12. Furthermore, the antagomir-mediated knockdown of expression of the microRNA miR-2940-1 affected the oviposition rhythm of A. albopictus. These data support the conclusion that miR-2940-1 is involved in the regulation of oviposition rhythm in A. albopictus and provide a foundation for using oviposition rhythms as a new target for vector mosquito control.

Behavior and physiology in female Cricetulus barabensis are associated with the expression of circadian genes

The circadian clock regulates the behavior, physiology, and metabolism of mammals, and these characteristics, such as sleep-wake cycles, exercise capacity, and hormone levels, exhibit circadian rhythms. Light signaling is the main stimulator of the mammalian circadian system. The photoperiod regulates the reproductive cycle of seasonal breeding animals, and the circadian clock plays a pivotal role in this process. However, the role of the clock in coordinating animal behavior and physiology in response to photoperiodic changes needs further investigation. The present study investigated the changes and correlation of behavioral activities, physiological indicators, and gene expression in female striped hamsters (Cricetulus barabensis) within 24 h under a 12L:12D photoperiod. We found that the daily rhythms of sleep-wake and open field were significant in hamsters. The expression of clock genes, melatonin receptor genes, and genes involved in general metabolism oscillated significantly in central and peripheral tissues (brain, hypothalamus, liver, ovary, and thymus) and was significantly associated with behavior and physiology. Our results revealed that the neuroendocrine system regulated the rhythmicity of behavior and physiology, and central and peripheral clock genes (Bmal1, Clock, Per1, Per2, Cry1, and Cry2), melatonin receptor genes (MT1, MT2, and GPR50), and metabolizing genes (SIRT1, FGF21, and PPARα) played important roles. Our results suggest that central and peripheral circadian clocks, melatonin receptors, and genes involved in general metabolism may play key roles in maintaining circadian behavior and metabolic homeostasis in striped hamsters. Our results may have important implication for rodent pest control.

Circadian Clock and Body Temperature

The circadian fluctuation of body temperature is one of the most prominent and stable outputs of the circadian clock and plays an important role in maintaining optimal day-night energy homeostasis. The body temperature of homothermic animals is not strictly constant, but it shows daily oscillation within a range of 1-3 °C, which is sufficient to synchronize the clocks of peripheral tissues throughout the body. The thermal entrainment mechanisms of the clock are partly mediated by the action of the heat shock transcription factor and cold-inducible RNA-binding protein-both have the ability to affect clock gene expression. Body temperature in the poikilotherms is not completely passive to the ambient temperature change; they can travel to the place of preferred temperature in a manner depending on the time of their endogenous clock. Based on this behavior-level thermoregulation, flies exhibit a clear body temperature cycle. Noticeably, flies and mice share the same molecular circuit for the controlled body temperature; in both species, the calcitonin receptors participate in the formation of body temperature rhythms during the active phase and exhibit rather specific expression in subsets of clock neurons in the brain. We summarize knowledge on mutual relationships between body temperature regulation and the circadian clock.

Lysine-independent ubiquitination and degradation of REV-ERBα involves a bi-functional degradation control sequence at its N-terminus

REV-ERBα and REV-ERBβ proteins play crucial roles in linking the circadian system to overt daily rhythms in mammalian physiology and behavior. In most tissues, REV-ERBα protein robustly cycles such that it is detected only within a tight interval of 4-6 hours each day, suggesting both its synthesis and degradation are tightly controlled. Several ubiquitin ligases are known to drive REV-ERBα degradation, but how they interact with REV-ERBα and which lysine residues they ubiquitinate to promote degradation are unknown. In this study, we attempted to identify both ubiquitin-ligase-binding and ubiquitination sites within REV-ERBα required for its degradation. Surprisingly, mutating all lysine residues, the common sites for ubiquitin conjugation, in REV-ERBα to arginines (K20R), did very little to impair its degradation in cells. K20R were degraded much faster by co-expression of two E3 ligases, SIAH2 or SPSB4, suggesting possible N-terminal ubiquitination. To explore this, we examined if small deletions at the N-terminus of REV-ERBα would alter its degradation. Interestingly, deletion of amino acid (AA) residues 2 to 9 (delAA2-9) clearly resulted in a less stable REV-ERBα. We found that it was the length (i.e. 8 AA), and not the specific sequence, that confers stability in this region. Simultaneously, we also mapped the interaction site of the E3 ligase SPSB4 to this same region, specifically requiring AA4-9 of REV-ERBα. Thus, the first 9 AA of REV-ERBα has two opposing roles in regulating REV-ERBα turnover. Further, deleting eight additional AAs (delAA2-17) from the N-terminus strongly prevents REV-ERBα degradation. Combined, these results suggest that complex interactions within the first 25AAs potentially act as an endogenous 'switch' that allows REV-ERBα to exist in a stabilized conformation in order to accumulate at one time of day, but then rapidly shifts to a destabilized form, to enhance its removal at the end of its daily cycle.

Special Issue: Circadian Rhythms and Age Related Disorder: How Does Aging Impact Mammalian Circadian Organization?

Aging significantly impacts circadian timing in mammals. The amplitude and precision of behavioral, endocrine, and metabolic rhythms decline with age. This is accompanied with an age-related decline in the amplitude of central pacemaker output, although the molecular clock in the suprachiasmatic nucleus exhibit robust oscillation. Peripheral clocks also exhibit robust oscillation during aging, when extensive reprogramming of other genes' expression rhythms occurs in peripheral tissues. The age-related dissociation between the molecular clock and downstream rhythms in both central and peripheral tissues indicates that mechanisms other than the molecular clock are involved in mediating the impact the aging on circadian organization. In this article, findings are reviewed on the impact of aging on circadian timing functions, and the potential role of increased inflammatory response in age-related changes in circadian organization is highlighted.

George M, Tan A, Chrisler EA, Izumo M, Takahashi JS, Rao MC, Leone VA, Chang EB. Gut microbes and the liver circadian clock partition glucose and lipid metabolism

Circadian rhythms govern glucose homeostasis, and their dysregulation leads to complex metabolic diseases. Gut microbes exhibit diurnal rhythms that influence host circadian networks and metabolic processes, yet underlying mechanisms remain elusive. Here, we showed hierarchical, bidirectional communication among the liver circadian clock, gut microbes, and glucose homeostasis in mice. To assess this relationship, we utilized mice with liver-specific deletion of the core circadian clock gene Bmal1 via Albumin-cre maintained in either conventional or germ-free housing conditions. The liver clock, but not the forebrain clock, required gut microbes to drive glucose clearance and gluconeogenesis. Liver clock dysfunctionality expanded proportions and abundances of oscillating microbial features by 2-fold relative to that in controls. The liver clock was the primary driver of differential and rhythmic hepatic expression of glucose and fatty acid metabolic pathways. Absent the liver clock, gut microbes provided secondary cues that dampened these rhythms, resulting in reduced lipid fuel utilization relative to carbohydrates. All together, the liver clock transduced signals from gut microbes that were necessary for regulating glucose and lipid metabolism and meeting energy demands over 24 hours.

Daily rhythms of REV-ERBα and its role as transcriptional repressor of clock genes in fish hepatic oscillator

The REV-ERBα nuclear receptor is a key component of the molecular machinery of circadian oscillators in mammals. While the rhythmic expression of this receptor has been described in teleosts, several critical aspects of its regulation remain unknown, such as which synchronizers entrain its rhythm, and whether it can modulate the expression of other clock genes. The objective of this study was to gain deeper understanding of the role of REV-ERBα in the fish circadian system. To this end, we first investigated the cues that entrain the rhythm of rev-erbα expression in the goldfish (Carassius auratus) liver and hypothalamus. A 12-h shift in feeding time induced a parallel shift in the hepatic rhythm of rev-erbα expression, confirming that this gene is food-entrainable in the goldfish liver. In contrast, light seems the main driver of rev-erbα rhythmic expression in the hypothalamus. Next, we examined the effects of REV-ERBα activation on locomotor activity and hepatic expression of clock genes. Subchronic treatment with the REV-ERBα agonist SR9009 slightly decreased locomotor activity anticipating light onset and food arrival, and downregulated hepatic bmal1a, clock1a, cry1a, per1a and pparα expression. This generalized repressing action of REV-ERBα on the expression of hepatic clock genes was confirmed in vitro by using agonists (SR9009 and GSK4112) and antagonist (SR8278) of this receptor. Overall, the present work reveals that REV-ERBα modulates the daily expression of the main genes of the teleostean liver clock, reinforcing its role in the liver temporal homeostasis, which seems highly conserved in both fish and mammals.

Diurnal rhythms of wrist temperature are associated with future disease risk in the UK Biobank

Many chronic disease symptomatologies involve desynchronized sleep-wake cycles, indicative of disrupted biorhythms. This can be interrogated using body temperature rhythms, which have circadian as well as sleep-wake behavior/environmental evoked components. Here, we investigated the association of wrist temperature amplitudes with a future onset of disease in the UK Biobank one year after actigraphy. Among 425 disease conditions (range n = 200-6728) compared to controls (range n = 62,107-91,134), a total of 73 (17%) disease phenotypes were significantly associated with decreased amplitudes of wrist temperature (Benjamini-Hochberg FDR q < 0.05) and 26 (6.1%) PheCODEs passed a more stringent significance level (Bonferroni-correction α < 0.05). A two-standard deviation (1.8° Celsius) lower wrist temperature amplitude corresponded to hazard ratios of 1.91 (1.58-2.31 95% CI) for NAFLD, 1.69 (1.53-1.88) for type 2 diabetes, 1.25 (1.14-1.37) for renal failure, 1.23 (1.17-1.3) for hypertension, and 1.22 (1.11-1.33) for pneumonia (phenome-wide atlas available at http://bioinf.itmat.upenn.edu/biorhythm_atlas/ ). This work suggests peripheral thermoregulation as a digital biomarker.

Regulation of Pol II Pausing during Daily Gene Transcription in Mouse Liver

Cell autonomous circadian oscillation is present in central and various peripheral tissues. The intrinsic tissue clock and various extrinsic cues drive gene expression rhythms. Transcription regulation is thought to be the main driving force for gene rhythms. However, how transcription rhythms arise remains to be fully characterized due to the fact that transcription is regulated at multiple steps. In particular, Pol II recruitment, pause release, and premature transcription termination are critical regulatory steps that determine the status of Pol II pausing and transcription output near the transcription start site (TSS) of the promoter. Recently, we showed that Pol II pausing exhibits genome-wide changes during daily transcription in mouse liver. In this article, we review historical as well as recent findings on the regulation of transcription rhythms by the circadian clock and other transcription factors, and the potential limitations of those results in explaining rhythmic transcription at the TSS. We then discuss our results on the genome-wide characteristics of daily changes in Pol II pausing, the possible regulatory mechanisms involved, and their relevance to future research on circadian transcription regulation.

Circadian control of Nocturnin and its regulatory role in health and disease

Circadian rhythms are generated by intrinsic 24-h oscillations that anticipate the extrinsic changes associated with solar day. A conserved transcriptional-translational feedback loop generates these molecular oscillations of clock genes at the organismal and the cellular levels. One of the recently discovered outputs of circadian clock is Nocturnin (Noct) or Ccrn4l. In mice, Noct mRNA is broadly expressed in cells throughout the body, with a particularly high-amplitude rhythm in liver. NOCT belongs to the EEP family of proteins with the closest similarity to the CCR4 family of deadenylases. Multiple studies have investigated the role of Nocturnin in development, adipogenesis, lipid metabolism, inflammation, osteogenesis, and obesity. Further, mice lacking Noct (Noct KO or Noct-/-) are protected from high-fat diet-induced obesity and hepatic steatosis. Recent studies had provided new insights by investigating various aspects of Nocturnin, ranging from its sub-cellular localization to identification of its target transcripts. However, a profound understanding of its molecular function remains elusive. This review article seeks to integrate the available literature into our current understanding of the functions of Nocturnin, their regulatory roles in key tissues and to throw light on the existing scientific lacunae.

Liver and muscle circadian clocks cooperate to support glucose tolerance in mice

Physiology is regulated by interconnected cell and tissue circadian clocks. Disruption of the rhythms generated by the concerted activity of these clocks is associated with metabolic disease. Here we tested the interactions between clocks in two critical components of organismal metabolism, liver and skeletal muscle, by rescuing clock function either in each organ separately or in both organs simultaneously in otherwise clock-less mice. Experiments showed that individual clocks are partially sufficient for tissue glucose metabolism, yet the connections between both tissue clocks coupled to daily feeding rhythms support systemic glucose tolerance. This synergy relies in part on local transcriptional control of the glucose machinery, feeding-responsive signals such as insulin, and metabolic cycles that connect the muscle and liver. We posit that spatiotemporal mechanisms of muscle and liver play an essential role in the maintenance of systemic glucose homeostasis and that disrupting this diurnal coordination can contribute to metabolic disease.

Circadian mechanisms in adipose tissue bioenergetics and plasticity

The circadian clock plays an essential role in coordinating feeding and metabolic rhythms with the light/dark cycle. Disruption of clocks is associated with increased adiposity and metabolic disorders, whereas aligning feeding time with cell-autonomous rhythms in metabolism improves health. Here, we provide a comprehensive overview of recent literature in adipose tissue biology as well as our understanding of molecular mechanisms underlying the circadian regulation of transcription, metabolism, and inflammation in adipose tissue. We highlight recent efforts to uncover the mechanistic links between clocks and adipocyte metabolism, as well as its application to dietary and behavioral interventions to improve health and mitigate obesity.

The circadian neutrophil, inside-out

The circadian clock has sway on a myriad of physiological targets, among which the immune and inflammatory systems are particularly prominent. In this review, we discuss how neutrophils, the wildcard of the immune system, are regulated by circadian oscillations. We describe cell-intrinsic and extrinsic diurnal mechanisms governing the general physiology and function of these cells, from purely immune to homeostatic. Repurposing the concepts discovered in other cell types, we then speculate on various uncharted avenues of neutrophil-circadian relationships, such as topology, metabolism, and the regulation of tissue clocks, with the hope of identifying exciting new avenues of work in the context of circadian immunity.

The REV-ERB Nuclear Receptors: Timekeepers for the Core Clock Period and Metabolism

REV-ERB nuclear receptors are potent transcriptional repressors that play an important role in the core mammalian molecular clock and metabolism. Deletion of both REV-ERBα and its largely redundant isoform REV-ERBβ in a murine tissue-specific manner have shed light on their specific functions in clock mechanisms and circadian metabolism. This review highlights recent findings that establish REV-ERBs as crucial circadian timekeepers in a variety of tissues, regulating overlapping and distinct processes that maintain normal physiology and protect from metabolic dysfunction.

Heat shock factor 1 induces a short burst of transcription of the clock gene Per2 during interbout arousal in mammalian hibernation

During winter hibernation, a diverse range of small mammals can enter prolonged torpor. They spend the non-hibernation season as a homeotherm, but the hibernation season as a heterotherm. In the hibernation season, chipmunks (Tamias asiaticus) cycle regularly between 5-6 day long deep torpor with a body temperature (Tb) of 5-7 ºC and interbout arousal of ∼ 20 h, during which, their Tb returns to the normothermic level. Here, we investigated Per2 expression in the liver to elucidate the regulation of the peripheral circadian clock in a mammalian hibernator. In the non-hibernation season, as in mice, heat shock factor 1 (HSF1), activated by elevated Tb during the wake period, activated Per2 transcription in the liver, which contributed to synchronizing the peripheral circadian clock to the Tb rhythm. In the hibernation season, we determined that the Per2 mRNA was at low levels during deep torpor, but Per2 transcription was transiently activated by HSF1, which was activated by elevated Tb during interbout arousal. Nevertheless, we found that the mRNA from the core clock gene Bmal1 exhibited arrhythmic expression during interbout arousal. Since circadian rhythmicity is dependent on negative feedback loops involving the clock genes, these results suggest that the peripheral circadian clock in the liver is nonfunctional in the hibernation season.

The cold-sensing ion channel TRPM8 regulates central and peripheral clockwork and the circadian oscillations of body temperature

AIM

Physiological functions in mammals show circadian oscillations, synchronized by daily cycles of light and temperature. Central and peripheral clocks participate in this regulation. Since the ion channel TRPM8 is a critical cold sensor, we investigated its role in circadian function.

METHODS

We used TRPM8 reporter mouse lines and TRPM8-deficient mice. mRNA levels were determined by in situ hybridization or RT-qPCR and protein levels by immunofluorescence. A telemetry system was used to measure core body temperature (Tc).

RESULTS

TRPM8 is expressed in the retina, specifically in cholinergic amacrine interneurons and in a subset of melanopsin-positive ganglion cells which project to the central pacemaker, the suprachiasmatic nucleus (SCN) of the hypothalamus. TRPM8-positive fibres were also found innervating choroid and ciliary body vasculature, with a putative function in intraocular temperature, as shown in TRPM8-deficient mice. Interestingly, Trpm8^-/- animals displayed increased expression of the clock gene Per2 and vasopressin (AVP) in the SCN, suggesting a regulatory role of TRPM8 on the central oscillator. Since SCN AVP neurons control body temperature, we studied Tc in driven and free-running conditions. TRPM8-deficiency increased the amplitude of Tc oscillations and, under dim constant light, induced a greater phase delay and instability of Tc rhythmicity. Finally, TRPM8-positive fibres innervate peripheral organs, like liver and white adipose tissue. Notably, Trpm8^-/- mice displayed a dysregulated expression of Per2 mRNA in these metabolic tissues.

CONCLUSION

Our findings support a function of TRPM8 as a temperature sensor involved in the regulation of central and peripheral clocks and the circadian control of Tc.

Minimal upstream open reading frame of Per2 mediates phase fitness of the circadian clock to day/night physiological body temperature rhythm

Body temperature in homeothermic animals does not remain constant but displays a regular circadian fluctuation within a physiological range (e.g., 35°C-38.5°C in mice), constituting a fundamental systemic signal to harmonize circadian clock-regulated physiology. Here, we find the minimal upstream open reading frame (uORF) encoded by the 5' UTR of the mammalian core clock gene Per2 and reveal its role as a regulatory module for temperature-dependent circadian clock entrainment. A temperature shift within the physiological range does not affect transcription but instead increases translation of Per2 through its minimal uORF. Genetic ablation of the Per2 minimal uORF and inhibition of phosphoinositide-3-kinase, lying upstream of temperature-dependent Per2 protein synthesis, perturb the entrainment of cells to simulated body temperature cycles. At the organismal level, Per2 minimal uORF mutant skin shows delayed wound healing, indicating that uORF-mediated Per2 modulation is crucial for optimal tissue homeostasis. Combined with transcriptional regulation, Per2 minimal uORF-mediated translation may enhance the fitness of circadian physiology.

Diurnal rhythmicity of wearable device-measured wrist temperature predicts future disease incidence in the UK Biobank

Many chronic disease symptomatologies involve desynchronized sleep-wake cycles, indicative of disrupted biorhythms. This can be interrogated using body temperature rhythms, which are well-established biomarkers for circadian clock function. Here, we investigated the association of wrist temperature amplitudes with a future onset of disease in the UK Biobank one year after actigraphy. Among 425 disease conditions (range n =  200-6,728) compared to controls (range n =  62,107 - 91,134), a total of 73 (36.5%) disease phenotypes were significantly associated with decreased amplitudes of wrist temperature (Benjamini-Hochberg FDR q < 0.05) and 26 (13%) PheCODEs passed a more stringent significance level (Bonferroni-correction α < 0.05). Here, for example, a two-standard deviation (1.8° Celsius) lower wrist temperature amplitude corresponded to hazard ratios of 1.91 (1.58-2.31 95% CI) for NAFLD, 1.69 (1.53-1.88) for type 2 diabetes, 1.25 (1.14-1.37) for renal failure, 1.23 (1.17-1.3) for hypertension, and 1.22 (1.11-1.33) for pneumonia. A comprehensive phenome-wide atlas of the identified mappings has been made available at http://bioinf.itmat.upenn.edu/biorhythm_atlas/. These findings strongly suggest peripheral thermoregulation as a digital biomarker.

Spatiotemporal Metabolic Liver Zonation and Consequences on Pathophysiology

Hepatocytes are the main workers in the hepatic factory, managing metabolism of nutrients and xenobiotics, production and recycling of proteins, and glucose and lipid homeostasis. Division of labor between hepatocytes is critical to coordinate complex complementary or opposing multistep processes, similar to distributed tasks at an assembly line. This so-called metabolic zonation has both spatial and temporal components. Spatial distribution of metabolic function in hepatocytes of different lobular zones is necessary to perform complex sequential multistep metabolic processes and to assign metabolic tasks to the right environment. Moreover, temporal control of metabolic processes is critical to align required metabolic processes to the feeding and fasting cycles. Disruption of this complex spatiotemporal hepatic organization impairs key metabolic processes with both local and systemic consequences. Many metabolic diseases, such as nonalcoholic steatohepatitis and diabetes, are associated with impaired metabolic liver zonation. Recent technological advances shed new light on the spatiotemporal gene expression networks controlling liver function and how their deregulation may be involved in a large variety of diseases. We summarize the current knowledge about spatiotemporal metabolic liver zonation and consequences on liver pathobiology.

A conditional Smg6 mutant mouse model reveals circadian clock regulation through the nonsense-mediated mRNA decay pathway

Nonsense-mediated messenger RNA (mRNA) decay (NMD) has been intensively studied as a surveillance pathway that degrades erroneous transcripts arising from mutations or RNA processing errors. While additional roles in physiological control of mRNA stability have emerged, possible functions in mammalian physiology in vivo remain unclear. Here, we created a conditional mouse allele that allows converting the NMD effector nuclease SMG6 from wild-type to nuclease domain-mutant protein. We find that NMD down-regulation affects the function of the circadian clock, a system known to require rapid mRNA turnover. Specifically, we uncover strong lengthening of free-running circadian periods for liver and fibroblast clocks and direct NMD regulation of Cry2 mRNA, encoding a key transcriptional repressor within the rhythm-generating feedback loop. Transcriptome-wide changes in daily mRNA accumulation patterns in the entrained liver, as well as an altered response to food entrainment, expand the known scope of NMD regulation in mammalian gene expression and physiology.

Circadian rhythms in the blood-brain barrier: impact on neurological disorders and stress responses

Circadian disruption has become more prevalent in society due to the increase in shift work, sleep disruption, blue light exposure, and travel via different time zones. The circadian rhythm is a timed transcription-translation feedback loop with positive regulators, BMAL1 and CLOCK, that interact with negative regulators, CRY and PER, to regulate both the central and peripheral clocks. This review highlights the functions of the circadian rhythm, specifically in the blood-brain barrier (BBB), during both healthy and pathological states. The BBB is a highly selective dynamic interface composed of CNS endothelial cells, astrocytes, pericytes, neurons, and microglia that form the neurovascular unit (NVU). Circadian rhythms modulate BBB integrity through regulating oscillations of tight junction proteins, assisting in functions of the NVU, and modulating transporter functions. Circadian disruptions within the BBB have been observed in stress responses and several neurological disorders, including brain metastasis, epilepsy, Alzheimer's disease, and Parkinson's disease. Further understanding of these interactions may facilitate the development of improved treatment options and preventative measures.

Consumption of a high-fat diet alters transcriptional rhythmicity in liver from pubertal mice

Introduction

Childhood obesity is associated with adult obesity, which is a risk factor for chronic diseases. Obesity, as an environmental cue, alters circadian rhythms. The hypothesis of this study was that consumption of a high-fat diet alters metabolic rhythms in pubertal mice.

Methods

Weanling female C57BL/6NHsd mice were fed a standard AIN93G diet or a high-fat diet (HFD) for 3 weeks. Livers were collected from six-week-old mice every 4 h over a period of 48 h for transcriptome analysis.

Results and discussion

The HFD altered rhythmicity of differentially rhythmic transcripts in liver. Specifically, the HFD elevated expression of circadian genes Clock, Per1, and Cry1 and genes encoding lipid metabolism Fads1 and Fads2, while decreased expression of circadian genes Bmal1 and Per2 and lipid metabolism genes Acaca, Fasn, and Scd1. Hierarchical clustering analysis of differential expression genes showed that the HFD-mediated metabolic disturbance was most active in the dark phase, ranging from Zeitgeber time 16 to 20. The Kyoto Encyclopedia of Genes and Genomes enrichment analysis of differentially expressed genes showed that the HFD up-regulated signaling pathways related to fatty acid and lipid metabolism, steroid and steroid hormone biosynthesis, amino acid metabolism and protein processing in the endoplasmic reticulum, glutathione metabolism, and ascorbate and aldarate metabolism in the dark phase. Down-regulations included MAPK pathway, lipolysis in adipocytes, Ras and Rap1 pathways, and pathways related to focal adhesion, cell adhesion molecules, and extracellular matrix-receptor interaction. In summary, the HFD altered metabolic rhythms in pubertal mice with the greatest alterations in the dark phase. These alterations may disrupt metabolic homeostasis in puberty and lead to metabolic disorders.

Fatty acid metabolism in liver and muscle is strongly modulated by photoperiod in Fischer 344 rats

Circadian and seasonal variations produce variations in physiological processes throughout the day and the year, respectively. In this sense, both the light and the moment of feeding are strong modulators of the central and peripheral clocks. However, little is known about its influence on certain metabolic parameters and on the composition of liver and muscle fatty acids (FA). In the present study, 24 Fischer 344 rats were exposed for 11 weeks to different photoperiods, L6, L12 and L18, with 6, 12 and 18 h of light/day, respectively. They were fed a standard diet. Serum metabolic parameters, gene expression of liver enzymes and gastrocnemius muscle involved in the synthesis, elongation, desaturation and β-oxidation of FA were analyzed. We have found that exposure to different hours of light has a clear effect on FA composition and gene expression in the liver. Mainly, the biosynthesis of unsaturated FA was altered in the L18 animals with respect to those exposed to L12, while the L6 did not show significant changes. At the muscle level, differences were observed in the concentration of mono and polyunsaturated FA. A multivariate analysis confirmed the differences between L12 and L18 in a significant way. We conclude that exposure to long days produces changes in the composition of liver and muscle FA, as well as changes in the gene expression of oxidative enzymes compared to exposure to L12, which could be a consequence of different seasonal eating patterns.

Chrononutrition-When We Eat Is of the Essence in Tackling Obesity

Obesity is a chronic and relapsing public health problem with an extensive list of associated comorbidities. The worldwide prevalence of obesity has nearly tripled over the last five decades and continues to pose a serious threat to wider society and the wellbeing of future generations. The pathogenesis of obesity is complex but diet plays a key role in the onset and progression of the disease. The human diet has changed drastically across the globe, with an estimate that approximately 72% of the calories consumed today come from foods that were not part of our ancestral diets and are not compatible with our metabolism. Additionally, multiple nutrient-independent factors, e.g., cost, accessibility, behaviours, culture, education, work commitments, knowledge and societal set-up, influence our food choices and eating patterns. Much research has been focused on 'what to eat' or 'how much to eat' to reduce the obesity burden, but increasingly evidence indicates that 'when to eat' is fundamental to human metabolism. Aligning feeding patterns to the 24-h circadian clock that regulates a wide range of physiological and behavioural processes has multiple health-promoting effects with anti-obesity being a major part. This article explores the current understanding of the interactions between the body clocks, bioactive dietary components and the less appreciated role of meal timings in energy homeostasis and obesity.

Potential Interactions Between Cerebellar Dysfunction and Sleep Disturbances in Dystonia

Dystonia is the third most common movement disorder. It causes debilitating twisting postures that are accompanied by repetitive and sometimes intermittent co- or over-contractions of agonist and antagonist muscles. Historically diagnosed as a basal ganglia disorder, dystonia is increasingly considered a network disorder involving various brain regions including the cerebellum. In certain etiologies of dystonia, aberrant motor activity is generated in the cerebellum and the abnormal signals then propagate through a “dystonia circuit” that includes the thalamus, basal ganglia, and cerebral cortex. Importantly, it has been reported that non-motor defects can accompany the motor symptoms; while their severity is not always correlated, it is hypothesized that common pathways may nevertheless be disrupted. In particular, circadian dysfunction and disordered sleep are common non-motor patient complaints in dystonia. Given recent evidence suggesting that the cerebellum contains a circadian oscillator, displays sleep-stage-specific neuronal activity, and sends robust long-range projections to several subcortical regions involved in circadian rhythm regulation, disordered sleep in dystonia may result from cerebellum-mediated dysfunction of the dystonia circuit. Here, we review the evidence linking dystonia, cerebellar network dysfunction, and cerebellar involvement in sleep. Together, these ideas may form the basis for the development of improved pharmacological and surgical interventions that could take advantage of cerebellar circuitry to restore normal motor function as well as non-motor (sleep) behaviors in dystonia.

Circadian disruption enhances HSF1 signaling and tumorigenesis in Kras-driven lung cancer

Disrupted circadian rhythmicity is a prominent feature of modern society and has been designated as a probable carcinogen by the World Health Organization. However, the biological mechanisms that connect circadian disruption and cancer risk remain largely undefined. We demonstrate that exposure to chronic circadian disruption [chronic jetlag (CJL)] increases tumor burden in a mouse model of KRAS-driven lung cancer. Molecular characterization of tumors and tumor-bearing lung tissues revealed that CJL enhances the expression of heat shock factor 1 (HSF1) target genes. Consistently, exposure to CJL disrupted the highly rhythmic nuclear trafficking of HSF1 in the lung, resulting in an enhanced accumulation of HSF1 in the nucleus. HSF1 has been shown to promote tumorigenesis in other systems, and we find that pharmacological or genetic inhibition of HSF1 reduces the growth of KRAS-mutant human lung cancer cells. These findings implicate HSF1 as a molecular link between circadian disruption and enhanced tumorigenesis.

Circadian mechanism disruption is associated with dysregulation of inflammatory and immune responses: a systematic review

The circadian rhythms are regulated by the circadian clock which is under the control of suprachiasmatic nucleus of hypothalamus. The central and peripheral clocks on different tissue together synchronize to form circadian system. Factors disrupt the circadian rhythm, such as irregular eating patterns, sleep/wake time, night shift work and temperature. Due to the misalignment of central clock components, it has been recognized as the pathophysiology of lifestyle-related diseases mediated by the inflammation such as diabetes, obesity, neurological disorder and hormonal imbalance. Also we discuss the therapeutic effect of time-restricted feeding over diabetes and obesity caused by miscommunication between central and peripheral clock. The genetic and epigenetic changes involve due to the deregulation of circadian system. The aim of the present review is to discuss the circadian mechanisms that are involved in the complex interaction between host and external factors and its disruption is associated with deregulation of inflammatory and immune responses. Hence, we need to understand the mechanism of functioning of our biological clocks so that it helps us treat health-related problems such as jet lags, sleep disorders due to night-time shift work, obesity and mental disturbances. We hope minimal cost behavioural and lifestyle changes can improve circadian rhythms and presumably provide a better health.

The Circadian Axis and Cardiometabolic Syndrome

Circadian rhythm refers to the daily physiologically fluctuating patterns of systemic processes that occur within a circa 24-hour timeframe, independently of external factors. There is evidence that in time, external and internal cycle misalignment leads to severe health consequences, resulting in the development of cardiometabolic disturbances. Desynchronized hormonal fluctuations along with daily specific macronutrient utilization patterns are also discussed, which by consequence, are all predictors of metabolic syndrome. The aim of this paper is to provide insight on the circadian clock’s organization throughout the human body and to explain the underlying genetic background. By understanding these well-established molecular mechanisms and processes, we believe this paper will provide accuracy regarding the importance of the circadian clock’s integrity and will highlight its role in the etiopathology of cardiometabolic syndrome.

The central clock suffices to drive the majority of circulatory metabolic rhythms

Life on Earth anticipates recurring 24-hour environmental cycles via genetically encoded molecular clocks active in all mammalian organs. Communication between these clocks controls circadian homeostasis. Intertissue communication is mediated, in part, by temporal coordination of metabolism. Here, we characterize the extent to which clocks in different organs control systemic metabolic rhythms, an area that remains largely unexplored. We analyzed the metabolome of serum from mice with tissue-specific expression of the clock gene Bmal1. Having functional hepatic and muscle clocks can only drive a minority (13%) of systemic metabolic rhythms. Conversely, limiting Bmal1 expression to the central pacemaker in the brain restores rhythms to 57% of circulatory metabolites. Rhythmic feeding imposed on clockless mice resulted in a similar rescue, indicating that the central clock mainly regulates metabolic rhythms via behavior. These findings explicate the circadian communication between tissues and highlight the importance of the central clock in governing those signals.

Time-restricted Eating for the Prevention and Management of Metabolic Diseases

Time-restricted feeding (TRF, animal-based studies) and time-restricted eating (TRE, humans) are an emerging behavioral intervention approach based on the understanding of the role of circadian rhythms in physiology and metabolism. In this approach, all calorie intake is restricted within a consistent interval of less than 12 hours without overtly attempting to reduce calories. This article will summarize the origin of TRF/TRE starting with concept of circadian rhythms and the role of chronic circadian rhythm disruption in increasing the risk for chronic metabolic diseases. Circadian rhythms are usually perceived as the sleep-wake cycle and dependent rhythms arising from the central nervous system. However, the recent discovery of circadian rhythms in peripheral organs and the plasticity of these rhythms in response to changes in nutrition availability raised the possibility that adopting a consistent daily short window of feeding can sustain robust circadian rhythm. Preclinical animal studies have demonstrated proof of concept and identified potential mechanisms driving TRF-related benefits. Pilot human intervention studies have reported promising results in reducing the risk for obesity, diabetes, and cardiovascular diseases. Epidemiological studies have indicated that maintaining a consistent long overnight fast, which is similar to TRE, can significantly reduce risks for chronic diseases. Despite these early successes, more clinical and mechanistic studies are needed to implement TRE alone or as adjuvant lifestyle intervention for the prevention and management of chronic metabolic diseases.

Casein kinase 1 and disordered clock proteins form functionally equivalent, phospho-based circadian modules in fungi and mammals

Circadian clocks rely on negative feedback loops. The core circadian inhibitors, FRQ in Neurospora and PERs in animals, are progressively hyperphosphorylated, inactivated, and degraded. CK1 is essential for these clocks. Despite our knowledge of the role of CK1, it is not known how many other kinases are required and how multisite phosphorylation might contribute to circadian timekeeping. We show here that CK1 alone is sufficient to slowly phosphorylate low-affinity sites in FRQ or PER2. The reaction is nearly temperature compensated, and the phosphorylation state of FRQ or PER2 corresponds to the time elapsed since the start of the reaction. Thus, CK1 and FRQ or PER2 form equivalent modules that are in principle capable of measuring time on a circadian scale.

Circadian clocks are timing systems that rhythmically adjust physiology and metabolism to the 24-h day–night cycle. Eukaryotic circadian clocks are based on transcriptional–translational feedback loops (TTFLs). Yet TTFL-core components such as Frequency (FRQ) in Neurospora and Periods (PERs) in animals are not conserved, leaving unclear how a 24-h period is measured on the molecular level. Here, we show that CK1 is sufficient to promote FRQ and mouse PER2 (mPER2) hyperphosphorylation on a circadian timescale by targeting a large number of low-affinity phosphorylation sites. Slow phosphorylation kinetics rely on site-specific recruitment of Casein Kinase 1 (CK1) and access of intrinsically disordered segments of FRQ or mPER2 to bound CK1 and on CK1 autoinhibition. Compromising CK1 activity and substrate binding affects the circadian clock in Neurospora and mammalian cells, respectively. We propose that CK1 and the clock proteins FRQ and PERs form functionally equivalent, phospho-based timing modules in the core of the circadian clocks of fungi and animals.

Roles of peripheral clocks: lessons from the fly

To adapt to and anticipate rhythmic changes in the environment such as daily light-dark and temperature cycles, internal timekeeping mechanisms called biological clocks evolved in a diverse set of organisms, from unicellular bacteria to humans. These biological clocks play critical roles in organisms' fitness and survival by temporally aligning physiological and behavioral processes to the external cues. The central clock is located in a small subset of neurons in the brain and drives daily activity rhythms, whereas most peripheral tissues harbor their own clock systems, which generate metabolic and physiological rhythms. Since the discovery of Drosophila melanogaster clock mutants in the early 1970s, the fruit fly has become an extensively studied model organism to investigate the mechanism and functions of circadian clocks. In this review, we primarily focus on D. melanogaster to survey key discoveries and progresses made over the past two decades in our understanding of peripheral clocks. We discuss physiological roles and molecular mechanisms of peripheral clocks in several different peripheral tissues of the fly.

Circadian effects on UV-induced damage and mutations

Skin cancer is the most diagnosed type of cancer in the United States, and while most of these malignancies are highly treatable, treatment costs still exceed $8 billion annually. Over the last 50 years, the annual incidence of skin cancer has steadily grown; therefore, understanding the environmental factors driving these types of cancer is a prominent research-focus. A causality between ultraviolet radiation (UVR) exposure and skin cancer is well-established, but exposure to UVR alone is not necessarily sufficient to induce carcinogenesis. The emerging field of circadian biology intersects strongly with the physiological systems of the mammalian body and introduces a unique opportunity for analyzing mechanisms of homeostatic disruption. The circadian clock refers to the approximate 24-hour cycle, in which protein levels of specific clock-controlled genes (CCGs) fluctuate based on the time of day. Though these CCGs are tissue specific, the skin has been observed to have a robust circadian clock that plays a role in its response to UVR exposure. This in-depth review will detail the mechanisms of the circadian clock and its role in cellular homeostasis. Next, the skin's response to UVR exposure and its induction of DNA damage and mutations will be covered - with an additional focus placed on how the circadian clock influences this response through nucleotide excision repair. Lastly, this review will discuss current models for studying UVR-induced skin lesions and perturbations of the circadian clock, as well as the impact of these factors on human health.

The sleep-wake distribution contributes to the peripheral rhythms in PERIOD-2

In the mouse, Period-2 (Per2) expression in tissues peripheral to the suprachiasmatic nuclei (SCN) increases during sleep deprivation and at times of the day when animals are predominantly awake spontaneously, suggesting that the circadian sleep-wake distribution directly contributes to the daily rhythms in Per2. We found support for this hypothesis by recording sleep-wake state alongside PER2 bioluminescence in freely behaving mice, demonstrating that PER2 bioluminescence increases during spontaneous waking and decreases during sleep. The temporary reinstatement of PER2-bioluminescence rhythmicity in behaviorally arrhythmic SCN-lesioned mice submitted to daily recurring sleep deprivations substantiates our hypothesis. Mathematical modeling revealed that PER2 dynamics can be described by a damped harmonic oscillator driven by two forces: a sleep-wake-dependent force and an SCN-independent circadian force. Our work underscores the notion that in peripheral tissues the clock gene circuitry integrates sleep-wake information and could thereby contribute to behavioral adaptability to respond to homeostatic requirements.

Circadian rhythms are daily cycles in behavior and physiology which repeat approximately every 24 hours. The master regulator of these rhythms is located in a small part of the brain called the supra-chiasmatic nucleus. This brain structure regulates the timing of sleep and wakefulness and is also thought to control the daily rhythms of cells throughout the body on a molecular level. It does this by synchronizing the activity of a set of genes called clock genes.

Under normal conditions, the levels of proteins coded for by clock genes change throughout the day following a rhythm that matches sleep-wake patterns. However, keeping animals and humans awake at their preferred sleeping times affects the protein levels of clock genes in many tissues of the body. This suggests that, in addition to the supra-chiasmatic nucleus, sleep-wake cycles may also influence clock-gene rhythms throughout the body.

To test this theory, Hoekstra, Jan et al. measured the levels of PERIOD-2, a protein coded for by the clock gene Period-2, while tracking sleep-wake states in mice. They did this by imaging a bioluminescent version of the PERIOD-2 protein in the brain and the kidneys, at the same time as they recorded the brain activity, movement and muscle response of animals. Results showed that PERIOD-2 increased on waking and decreased when mice fell asleep. Additionally, in mice lacking a circadian rhythm in sleep-wake behavior – whose changes in PERIOD-2 levels with respect to time were greatly reduced – imposing a regular sleep-wake cycle restored normal PERIOD-2 rhythmicity.

Next, Hoekstra, Jan et al. developed a mathematical model to understand how sleep-wake cycles together with circadian rhythms affect clock-gene activity in the brain and kidneys. Computer simulations suggested that sleep-wake cycles and circadian factors act as forces of comparable strength driving clock-gene dynamics. Both need to act in concert to keep clock-genes rhythmic. The model also predicted the large and immediate effects of sleep deprivation on PERIOD-2 levels, giving further credence to the idea that waking accelerated clock-gene rhythms while sleeping slowed them down. Modelling also suggested that having regular clock-gene rhythms protects against sleep disturbances.

In summary, this work shows how sleep patterns contribute to the daily rhythms in clock genes in the brain and body. The findings support the idea that well-timed sleep-wake schedules could help people to adjust to new time zones. It might also be useful to inform other strategies to reduce the health impacts of shift work.

Dietary Conjugated Linoleic Acid Modulates the Hepatic Circadian Clock Program via PPARα/REV-ERBα-Mediated Chromatin Modification in Mice

Scope: Disruptions of circadian rhythm cause metabolic disorders and are closely related to dietary factors. In this study, we investigated the interplays between the dietary conjugated linoleic acid (CLA)-induced hepatic steatosis and the circadian clock regulation, in association with lipid homeostasis. Methods and Results: Exposure of mice to 1.5% dietary CLA for 28 days caused insulin resistance, enlarged livers, caused hepatic steatosis, and increased triglyceride levels. Transcriptional profiling showed that hepatic circadian clock genes were significantly downregulated with increased expression of the negative transcription factor, REV-ERBα. We uncovered that the nuclear receptor (NR) PPARα, as a major target of dietary CLA, drives REV-ERBα expression via its binding to key genes of the circadian clock, including Cry1 and Clock, and the recruitment of histone marks and cofactors. The PPARα or REV-ERBα inhibition blocked the physical connection of this NR pair, reduced the cobinding of PPARα and REV-ERBα to the genomic DNA response element, and abolished histone modifications in the CLA-hepatocytes. In addition, we demonstrated that CLA promotes PPARα driving REV-ERBα transcriptional activity by directly binding to the PPAR response element (PPRE) at the Nr1d1 gene. Conclusions: Our results add a layer to the understanding of the peripheral clock feedback loop, which involves the PPARα-REV-ERBα, and provide guidance for nutrients optimization in circadian physiology.

Circadian Synchrony: Sleep, Nutrition, and Physical Activity

The circadian clock in mammals regulates the sleep/wake cycle and many associated behavioral and physiological processes. The cellular clock mechanism involves a transcriptional negative feedback loop that gives rise to circadian rhythms in gene expression with an approximately 24-h periodicity. To maintain system robustness, clocks throughout the body must be synchronized and their functions coordinated. In mammals, the master clock is located in the suprachiasmatic nucleus (SCN) of the hypothalamus. The SCN is entrained to the light/dark cycle through photic signal transduction and subsequent induction of core clock gene expression. The SCN in turn relays the time-of-day information to clocks in peripheral tissues. While the SCN is highly responsive to photic cues, peripheral clocks are more sensitive to non-photic resetting cues such as nutrients, body temperature, and neuroendocrine hormones. For example, feeding/fasting and physical activity can entrain peripheral clocks through signaling pathways and subsequent regulation of core clock genes and proteins. As such, timing of food intake and physical activity matters. In an ideal world, the sleep/wake and feeding/fasting cycles are synchronized to the light/dark cycle. However, asynchronous environmental cues, such as those experienced by shift workers and frequent travelers, often lead to misalignment between the master and peripheral clocks. Emerging evidence suggests that the resulting circadian disruption is associated with various diseases and chronic conditions that cause further circadian desynchrony and accelerate disease progression. In this review, we discuss how sleep, nutrition, and physical activity synchronize circadian clocks and how chronomedicine may offer novel strategies for disease intervention.

Circadian Synchrony: Sleep, Nutrition, and Physical Activity

The circadian clock in mammals regulates the sleep/wake cycle and many associated behavioral and physiological processes. The cellular clock mechanism involves a transcriptional negative feedback loop that gives rise to circadian rhythms in gene expression with an approximately 24-h periodicity. To maintain system robustness, clocks throughout the body must be synchronized and their functions coordinated. In mammals, the master clock is located in the suprachiasmatic nucleus (SCN) of the hypothalamus. The SCN is entrained to the light/dark cycle through photic signal transduction and subsequent induction of core clock gene expression. The SCN in turn relays the time-of-day information to clocks in peripheral tissues. While the SCN is highly responsive to photic cues, peripheral clocks are more sensitive to non-photic resetting cues such as nutrients, body temperature, and neuroendocrine hormones. For example, feeding/fasting and physical activity can entrain peripheral clocks through signaling pathways and subsequent regulation of core clock genes and proteins. As such, timing of food intake and physical activity matters. In an ideal world, the sleep/wake and feeding/fasting cycles are synchronized to the light/dark cycle. However, asynchronous environmental cues, such as those experienced by shift workers and frequent travelers, often lead to misalignment between the master and peripheral clocks. Emerging evidence suggests that the resulting circadian disruption is associated with various diseases and chronic conditions that cause further circadian desynchrony and accelerate disease progression. In this review, we discuss how sleep, nutrition, and physical activity synchronize circadian clocks and how chronomedicine may offer novel strategies for disease intervention.

Integration of feeding behavior by the liver circadian clock reveals network dependency of metabolic rhythms

The mammalian circadian clock, expressed throughout the brain and body, controls daily metabolic homeostasis. Clock function in peripheral tissues is required, but not sufficient, for this task. Because of the lack of specialized animal models, it is unclear how tissue clocks interact with extrinsic signals to drive molecular oscillations. Here, we isolated the interaction between feeding and the liver clock by reconstituting Bmal1 exclusively in hepatocytes (Liver-RE), in otherwise clock-less mice, and controlling timing of food intake. We found that the cooperative action of BMAL1 and the transcription factor CEBPB regulates daily liver metabolic transcriptional programs. Functionally, the liver clock and feeding rhythm are sufficient to drive temporal carbohydrate homeostasis. By contrast, liver rhythms tied to redox and lipid metabolism required communication with the skeletal muscle clock, demonstrating peripheral clock cross-talk. Our results highlight how the inner workings of the clock system rely on communicating signals to maintain daily metabolism.

Central and Peripheral Clock Control of Circadian Feeding Rhythms

Many behaviors exhibit ~24-h oscillations under control of an endogenous circadian timing system that tracks time of day via a molecular circadian clock. In the fruit fly, Drosophila melanogaster, most circadian research has focused on the generation of locomotor activity rhythms, but a fundamental question is how the circadian clock orchestrates multiple distinct behavioral outputs. Here, we have investigated the cells and circuits mediating circadian control of feeding behavior. Using an array of genetic tools, we show that, as is the case for locomotor activity rhythms, the presence of feeding rhythms requires molecular clock function in the ventrolateral clock neurons of the central brain. We further demonstrate that the speed of molecular clock oscillations in these neurons dictates the free-running period length of feeding rhythms. In contrast to the effects observed with central clock cell manipulations, we show that genetic abrogation of the molecular clock in the fat body, a peripheral metabolic tissue, is without effect on feeding behavior. Interestingly, we find that molecular clocks in the brain and fat body of control flies gradually grow out of phase with one another under free-running conditions, likely due to a long endogenous period of the fat body clock. Under these conditions, the period of feeding rhythms tracks with molecular oscillations in central brain clock cells, consistent with a primary role of the brain clock in dictating the timing of feeding behavior. Finally, despite a lack of effect of fat body selective manipulations, we find that flies with simultaneous disruption of molecular clocks in multiple peripheral tissues (but with intact central clocks) exhibit decreased feeding rhythm strength and reduced overall food intake. We conclude that both central and peripheral clocks contribute to the regulation of feeding rhythms, with a particularly dominant, pacemaker role for specific populations of central brain clock cells.

Diet-altered body temperature rhythms are associated with altered rhythms of clock gene expression in peripheral tissues in vivo

Temperature rhythms can act as potent signals for the modulation of the amplitude and phase of clock gene expression in peripheral organs in vitro, but the relevance of the circadian rhythm of core body temperature (T_c) as a modulating signal in vivo has not yet been investigated. Using calorie restriction and cafeteria feeding, we induced a larger and a dampened T_c amplitude, respectively, in male Wistar rats, and investigated the circadian expression profile of the core clock genes Bmal1, Per2, Cry1, and Rev-erbα, the heat-responsive genes heat shock protein 90 (Hsp90) and cold-inducible RNA binding protein (Cirbp), and Pgc1α, Pparα/γ/δ, Glut1/4, and Chop10 in the liver, skeletal muscle, white adipose tissue (WAT), and adrenal glands. Diet-altered T_c rhythms differentially affected the profiles of clock genes, Hsp90, and Cirbp expression in peripheral tissues. Greater T_c amplitudes elicited by calorie restriction were associated with large amplitudes of Hsp90 and Cirbp expression in the liver and WAT, in which larger amplitudes of clock gene expression were also observed. The amplitudes of metabolic gene expression were greater in the WAT, but not in the liver, in calorie-restricted rats. Conversely, diet-altered T_c rhythms were not translated to distinct changes in the amplitude of Hsp90, Cirbp, or clock or metabolic genes in the skeletal muscle or adrenal glands. While it was not possible to disentangle the effects of diet and temperature in this model, taken together with previous in vitro studies, our study presents novel data consistent with the notion that the circadian T_c rhythm can modulate the amplitude of circadian gene expression in vivo. The different responses of Hsp90 and Cirbp in peripheral tissues may be linked to the tissue-specific responses of peripheral clocks to diet and/or body temperature rhythms, but the association with the amplitude of metabolic gene expression is limited to the WAT.

Adipocyte NR1D1 dictates adipose tissue expansion during obesity

The circadian clock component NR1D1 (REVERBα) is considered a dominant regulator of lipid metabolism, with global Nr1d1 deletion driving dysregulation of white adipose tissue (WAT) lipogenesis and obesity. However, a similar phenotype is not observed under adipocyte-selective deletion (Nr1d1Flox2-6:AdipoqCre), and transcriptional profiling demonstrates that, under basal conditions, direct targets of NR1D1 regulation are limited, and include the circadian clock and collagen dynamics. Under high-fat diet (HFD) feeding, Nr1d1Flox2-6:AdipoqCre mice do manifest profound obesity, yet without the accompanying WAT inflammation and fibrosis exhibited by controls. Integration of the WAT NR1D1 cistrome with differential gene expression reveals broad control of metabolic processes by NR1D1 which is unmasked in the obese state. Adipocyte NR1D1 does not drive an anticipatory daily rhythm in WAT lipogenesis, but rather modulates WAT activity in response to alterations in metabolic state. Importantly, NR1D1 action in adipocytes is critical to the development of obesity-related WAT pathology and insulin resistance.

Chronic jetlag-induced alterations in pancreatic diurnal gene expression

Cell-autonomous circadian clocks exist in nearly every organ and function to maintain homeostasis through a complex series of transcriptional-translational feedback loops. The response of these peripheral clocks to external perturbations, such as chronic jetlag and shift work, has been extensively investigated. However, an evaluation of the effects of chronic jetlag on the mouse pancreatic transcriptome is still lacking. Herein, we report an evaluation of the diurnal variations encountered in the pancreatic transcriptome following exposure to an established chronic jetlag protocol. We found approximately 5.4% of the pancreatic transcriptome was rhythmic. Following chronic jetlag, we found the number of rhythmic transcripts decreased to approximately 3.6% of the transcriptome. Analysis of the core clock genes, which orchestrate circadian physiology, revealed that nearly all exhibited a shift in the timing of peak gene expression-known as a phase shift. Similarly, over 95% of the rhythmically expressed genes in the pancreatic transcriptome exhibited a phase shift, many of which were found to be important for metabolism. Evaluation of the genes involved in pancreatic exocrine secretion and insulin signaling revealed many pancreas-specific genes were also rhythmically expressed and several displayed a concomitant phase shift with chronic jetlag. Phase differences were found 9 days after normalization, indicating a persistent failure to reentrain to the new light-dark cycle. This study is the first to evaluate the endogenous pancreatic clock and rhythmic gene expression in whole pancreas over 48 h, and how the external perturbation of chronic jetlag affects the rhythmic expression of genes in the pancreatic transcriptome.

Melatonin Adjusts the Phase of Mouse Circadian Clocks in the Cornea Both Ex Vivo and In Vivo

The presence of an endogenous circadian clock within most mammalian cells is associated with the amazing observation that within a given tissue, these clocks are largely in synchrony with each other. Different tissues use a variety of systemic or environmental cues to precisely coordinate the phase of these clocks. The cornea is a unique tissue in that it is largely isolated from the direct blood supply that most tissues experience, it is transparent to visible light, and it is exposed directly to environmental light and temperature. Melatonin is a hormone that has been implicated in regulation of the cornea's circadian clocks. Here, we analyze the ability of rhythmic melatonin to entrain corneas ex vivo, and analyze the phase of corneal circadian clocks in vivo both in light: dark cycles and in constant darkness. We find that the presence of a retina from a melatonin-proficient mouse strain, C3Sn, can photoentrain the circadian clocks of a co-cultured mouse cornea, but a retina from a melatonin-deficient strain, C57Bl/6, cannot. Furthermore, pharmacologic blockade of melatonin or use of a retina with advanced retinal degeneration, Pde6b^rd1, blocks the photoentraining effect. Corneal circadian clocks in vivo adopt an advanced phase in C3Sn mice compared with C57Bl/6, but the circadian clocks in the liver are unaffected. This observation is not attributable to a shorter endogenous period of the cornea or behavior between the strains. Some transcripts of circadian genes in the corneas of C3Sn mice also show an advanced phase of expression in a light: dark cycle, while the transcript of Per2 exhibits a light-dependent transient induction at the onset of darkness. We conclude that melatonin acts as a phase modifying factor in a rhythmic manner for the circadian clocks of the cornea.

Long-Term Normothermic Machine Preservation of Partial Livers: First Experience With 21 Human Hemi-Livers

OBJECTIVE

The aim of this study was to maintain long-term full function and viability of partial livers perfused ex situ for sufficient duration to enable ex situ treatment, repair, and regeneration.

BACKGROUND

Organ shortage remains the single most important factor limiting the success of transplantation. Autotransplantation in patients with nonresectable liver tumors is rarely feasible due to insufficient tumor-free remnant tissue. This limitation could be solved by the availability of long-term preservation of partial livers that enables functional regeneration and subsequent transplantation.

METHODS

Partial swine livers were perfused with autologous blood after being procured from healthy pigs following 70% in-vivo resection, leaving only the right lateral lobe. Partial human livers were recovered from patients undergoing anatomic right or left hepatectomies and perfused with a blood based perfusate together with various medical additives. Assessment of physiologic function during perfusion was based on markers of hepatocyte, cholangiocyte, vascular and immune compartments,, as well as histology.

RESULTS

Following the development phase with partial swine livers, 21 partial human livers (14 right and 7 left hemi-livers) were perfused, eventually reaching the targeted perfusion duration of 1 week with the final protocol. These partial livers disclosed a stable perfusion with normal hepatic function including bile production (5-10 mL/h), lactate clearance, and maintenance of energy exhibited by normal of adenosine triphosphate (ATP) and glycogen levels, and preserved liver architecture for up to 1 week.

CONCLUSION

This pioneering research presents the inaugural evidence for long-term machine perfusion of partial livers and provides a pathway for innovative and relevant clinical applications to increase the availability of organs and provide novel approaches in hepatic oncology.