mTOR signaling regulates central and peripheral circadian clock function

Clinical and functional studies of MTOR variants in Smith-Kingsmore syndrome reveal deficits of circadian rhythm and sleep-wake behavior

Heterozygous de novo or inherited gain-of-function mutations in the MTOR gene cause Smith-Kingsmore syndrome (SKS). SKS is a rare autosomal dominant condition, and individuals with SKS display macrocephaly/megalencephaly, developmental delay, intellectual disability, and seizures. A few dozen individuals are reported in the literature. Here, we report a cohort of 28 individuals with SKS that represent nine MTOR pathogenic variants. We conducted a detailed natural history study and found pathophysiological deficits among individuals with SKS in addition to the common neurodevelopmental symptoms. These symptoms include sleep-wake disturbance, hyperphagia, and hyperactivity, indicative of homeostatic imbalance. To characterize these variants, we developed cell models and characterized their functional consequences. We showed that these SKS variants display a range of mechanistic target of rapamycin (mTOR) activities and respond to the mTOR inhibitor, rapamycin, differently. For example, the R1480_C1483del variant we identified here and the previously known C1483F are more active than wild-type controls and less responsive to rapamycin. Further, we showed that SKS mutations dampened circadian rhythms and low-dose rapamycin improved the rhythm amplitude, suggesting that optimal mTOR activity is required for normal circadian function. As SKS is caused by gain-of-function mutations in MTOR, rapamycin was used to treat several patients. While higher doses of rapamycin caused delayed sleep-wake phase disorder in a subset of patients, optimized lower doses improved sleep. Our study expands the clinical and molecular spectrum of SKS and supports further studies for mechanism-guided treatment options to improve sleep-wake behavior and overall health.

Circadian regulation of macromolecular complex turnover and proteome renewal

Although costly to maintain, protein homeostasis is indispensable for normal cellular function and long-term health. In mammalian cells and tissues, daily variation in global protein synthesis has been observed, but its utility and consequences for proteome integrity are not fully understood. Using several different pulse-labelling strategies, here we gain direct insight into the relationship between protein synthesis and abundance proteome-wide. We show that protein degradation varies in-phase with protein synthesis, facilitating rhythms in turnover rather than abundance. This results in daily consolidation of proteome renewal whilst minimising changes in composition. Coupled rhythms in synthesis and turnover are especially salient to the assembly of macromolecular protein complexes, particularly the ribosome, the most abundant species of complex in the cell. Daily turnover and proteasomal degradation rhythms render cells and mice more sensitive to proteotoxic stress at specific times of day, potentially contributing to daily rhythms in the efficacy of proteasomal inhibitors against cancer. Our findings suggest that circadian rhythms function to minimise the bioenergetic cost of protein homeostasis through temporal consolidation of protein turnover.

Melatonin in energy control: Circadian time-giver and homeostatic monitor

Melatonin is a neurohormone synthesized from dietary tryptophan in various organs, including the pineal gland and the retina. In the pineal gland, melatonin is produced at night under the control of the master clock located in the suprachiasmatic nuclei of the hypothalamus. Under physiological conditions, the pineal gland seems to constitute the unique source of circulating melatonin. Melatonin is involved in cellular metabolism in different ways. First, the circadian rhythm of melatonin helps the maintenance of proper internal timing, the disruption of which has deleterious effects on metabolic health. Second, melatonin modulates lipid metabolism, notably through diminished lipogenesis, and it has an antidiabetic effect, at least in several animal models. Third, pharmacological doses of melatonin have antioxidative, free radical-scavenging, and anti-inflammatory properties in various in vitro cellular models. As a result, melatonin can be considered both a circadian time-giver and a homeostatic monitor of cellular metabolism, via multiple mechanisms of action that are not all fully characterized. Aging, circadian disruption, and artificial light at night are conditions combining increased metabolic risks with diminished circulating levels of melatonin. Accordingly, melatonin supplementation could be of potential therapeutic value in the treatment or prevention of metabolic disorders. More clinical trials in controlled conditions are needed, notably taking greater account of circadian rhythmicity.

Circadian rhythm regulates the function of immune cells and participates in the development of tumors

Circadian rhythms are present in almost all cells and play a crucial role in regulating various biological processes. Maintaining a stable circadian rhythm is essential for overall health. Disruption of this rhythm can alter the expression of clock genes and cancer-related genes, and affect many metabolic pathways and factors, thereby affecting the function of the immune system and contributing to the occurrence and progression of tumors. This paper aims to elucidate the regulatory effects of BMAL1, clock and other clock genes on immune cells, and reveal the molecular mechanism of circadian rhythm's involvement in tumor and its microenvironment regulation. A deeper understanding of circadian rhythms has the potential to provide new strategies for the treatment of cancer and other immune-related diseases.

Time-Restricted Feeding Attenuates Metabolic Dysfunction-Associated Steatohepatitis and Hepatocellular Carcinoma in Obese Male Mice

Metabolic dysfunction-associated steatotic liver disease (MASLD) has surpassed the hepatitis B virus and hepatitis C virus as the leading cause of chronic liver disease in most parts of the Western world. MASLD (formerly known as NAFLD) encompasses both simple steatosis and more aggressive metabolic dysfunction-associated steatohepatitis (MASH), which is accompanied by inflammation, fibrosis, and cirrhosis, and ultimately can lead to hepatocellular carcinoma (HCC). There are currently very few approved therapies for MASH. Weight loss strategies such as caloric restriction can ameliorate the harmful metabolic effect of MASH and inhibit HCC; however, it is difficult to implement and maintain in daily life, especially in individuals diagnosed with HCC. In this study, we tested a time-restricted feeding (TRF) nutritional intervention in mouse models of MASH and HCC. We show that TRF abrogated metabolic dysregulation induced by a Western diet without any calorie restriction or weight loss. TRF improved insulin sensitivity and reduced hyperinsulinemia, liver steatosis, inflammation, and fibrosis. Importantly, TRF inhibited liver tumors in two mouse models of obesity-driven HCC. Our data suggest that TRF is likely to be effective in abrogating MASH and HCC and warrant further studies of time-restricted eating in humans with MASH who are at higher risk of developing HCC.

Interorgan rhythmicity as a feature of healthful metabolism

The finding that animals with circadian gene mutations exhibit diet-induced obesity and metabolic syndrome with hypoinsulinemia revealed a distinct role for the clock in the brain and peripheral tissues. Obesogenic diets disrupt rhythmic sleep/wake patterns, feeding behavior, and transcriptional networks, showing that metabolic signals reciprocally control the clock. Providing access to high-fat diet only during the sleep phase (light period) in mice accelerates weight gain, whereas isocaloric time-restricted feeding during the active period enhances energy expenditure due to circadian induction of adipose thermogenesis. This perspective focuses on advances and unanswered questions in understanding the interorgan circadian control of healthful metabolism.

Stroke in the Time of Circadian Medicine

Time-of-day significantly influences the severity and incidence of stroke. Evidence has emerged not only for circadian governance over stroke risk factors, but also for important determinants of clinical outcome. In this review, we provide a comprehensive overview of the interplay between chronobiology and cerebrovascular disease. We discuss circadian regulation of pathophysiological mechanisms underlying stroke onset or tolerance as well as in vascular dementia. This includes cell death mechanisms, metabolism, mitochondrial function, and inflammation/immunity. Furthermore, we present clinical evidence supporting the link between disrupted circadian rhythms and increased susceptibility to stroke and dementia. We propose that circadian regulation of biochemical and physiological pathways in the brain increase susceptibility to damage after stroke in sleep and attenuate treatment effectiveness during the active phase. This review underscores the importance of considering circadian biology for understanding the pathology and treatment choice for stroke and vascular dementia and speculates that considering a patient's chronotype may be an important factor in developing precision treatment following stroke.

Lifelong Chronic Sleep Disruption in a Mouse Model of Traumatic Brain Injury

Chronic sleep/wake disturbances (SWDs) are strongly associated with traumatic brain injury (TBI) in patients and are being increasingly recognized. However, the underlying mechanisms are largely understudied and there is an urgent need for animal models of lifelong SWDs. The objective of this study was to develop a chronic TBI rodent model and investigate the lifelong chronic effect of TBI on sleep/wake behavior. We performed repetitive midline fluid percussion injury (rmFPI) in 4-month-old mice and monitored their sleep/wake behavior using the non-invasive PiezoSleep system. Sleep/wake states were recorded before injury (baseline) and then monthly thereafter. We found that TBI mice displayed a significant decrease in sleep duration in both the light and dark phases, beginning at 3 months post-TBI and continuing throughout the study. Consistent with the sleep phenotype, these TBI mice showed circadian locomotor activity phenotypes and exhibited reduced anxiety-like behavior. TBI mice also gained less weight, and had less lean mass and total body water content, compared to sham controls. Further, TBI mice showed extensive brain tissue loss and increased glial fibrillary acidic protein and ionized calcium-binding adaptor molecule 1 levels in the hypothalamus and vicinity of the injury, indicative of chronic neuropathology. In summary, our study identified a critical time window of TBI pathology and associated circadian and sleep/wake phenotypes. Future studies should leverage this mouse model to investigate the molecular mechanisms underlying the chronic sleep/wake phenotypes post-TBI early in life.

The Antihypertensive Guanabenz Exacerbates Integrated Stress Response and Disrupts the Brain Circadian Clock

The circadian clock regulates a variety of biological processes that are normally synchronized with the solar day. Disruption of circadian rhythms is associated with health problems. Understanding the signaling mechanisms that couple cell physiology and metabolism to circadian timekeeping will help to develop novel therapeutic strategies. The integrated stress response (ISR) is activated by the cellular stressors to maintain physiological homeostasis by orchestrating mRNA translation. Aberrant ISR has been found in a number of neurological diseases that exhibit disrupted circadian rhythms and sleep. Recent work has started to uncover a critical role for the ISR in regulating the physiology of the circadian clock. Guanabenz (2,6-dichlorobenzylidene aminoguanidine acetate) is an orally bioavailable α2-adrenergic receptor agonist that has been used as an antihypertensive for decades. Recent studies demonstrated that guanabenz can regulate the ISR. Here, we assessed the effects of guanabenz on cellular and behavioral circadian rhythms using a multidisciplinary approach. We found that guanabenz can induce the ISR by increasing eIF2α phosphorylation in cultured fibroblasts as well as in the mouse brain. The hyperphosphorylation of eIF2α by guanabenz is associated with the shortened circadian period in cells and animals and the disruption of behavioral circadian rhythms in mice. Guanabenz administration disrupted circadian oscillations of the clock protein Per1 and Per2 in the mouse suprachiasmatic nucleus, the master pacemaker. These results uncover a significant yet previously unidentified role of guanabenz in regulating circadian rhythms and indicate that exacerbated ISR activation can impair the functions of the brain's circadian clock by disrupting clock gene expression.

Circadian Regulation of the Neuroimmune Environment Across the Lifespan: From Brain Development to Aging

Circadian clocks confer 24-h periodicity to biological systems, to ultimately maximize energy efficiency and promote survival in a world with regular environmental light cycles. In mammals, circadian rhythms regulate myriad physiological functions, including the immune, endocrine, and central nervous systems. Within the central nervous system, specialized glial cells such as astrocytes and microglia survey and maintain the neuroimmune environment. The contributions of these neuroimmune cells to both homeostatic and pathogenic demands vary greatly across the day. Moreover, the function of these cells changes across the lifespan. In this review, we discuss circadian regulation of the neuroimmune environment across the lifespan, with a focus on microglia and astrocytes. Circadian rhythms emerge in early life concurrent with neuroimmune sculpting of brain circuits and wane late in life alongside increasing immunosenescence and neurodegeneration. Importantly, circadian dysregulation can alter immune function, which may contribute to susceptibility to neurodevelopmental and neurodegenerative diseases. In this review, we highlight circadian neuroimmune interactions across the lifespan and share evidence that circadian dysregulation within the neuroimmune system may be a critical component in human neurodevelopmental and neurodegenerative diseases.

Time-restricted eating for chronodisruption-related chronic diseases

The circadian timing system enables organisms to adapt their physiology and behavior to the cyclic environmental changes including light-dark cycle or food availability. Misalignment between the endogenous circadian rhythms and external cues is known as chronodisruption and is closely associated with the development of metabolic and gastrointestinal disorders, cardiovascular diseases, and cancer. Time-restricted eating (TRE, in human) is an emerging dietary approach for weight management. Recent studies have shown that TRE or time-restricted feeding (TRF, when referring to animals) has several beneficial health effects, which, however, are not limited to weight management. This review summarizes the effects of TRE/TRF on regulating energy metabolism, gut microbiota and homeostasis, development of cardiovascular diseases and cancer. Furthermore, we will address the role of circadian clocks in TRE/TRF and propose ways to optimize TRE as a dietary strategy to obtain maximal health benefits.

Autistic-like behavior and cerebellar dysfunction in Bmal1 mutant mice ameliorated by mTORC1 inhibition

Although circadian and sleep disorders are frequently associated with autism spectrum disorders (ASD), it remains elusive whether clock gene disruption can lead to autistic-like phenotypes in animals. The essential clock gene Bmal1 has been associated with human sociability and its missense mutations are identified in ASD. Here we report that global Bmal1 deletion led to significant social impairments, excessive stereotyped and repetitive behaviors, as well as motor learning disabilities in mice, all of which resemble core behavioral deficits in ASD. Furthermore, aberrant cell density and immature morphology of dendritic spines were identified in the cerebellar Purkinje cells (PCs) of Bmal1 knockout (KO) mice. Electrophysiological recordings uncovered enhanced excitatory and inhibitory synaptic transmission and reduced firing rates in the PCs of Bmal1 KO mice. Differential expression of ASD- and ataxia-associated genes (Ntng2, Mfrp, Nr4a2, Thbs1, Atxn1, and Atxn3) and dysregulated pathways of translational control, including hyperactivated mammalian target of rapamycin complex 1 (mTORC1) signaling, were identified in the cerebellum of Bmal1 KO mice. Interestingly, the antidiabetic drug metformin reversed mTORC1 hyperactivation and alleviated major behavioral and PC deficits in Bmal1 KO mice. Importantly, conditional Bmal1 deletion only in cerebellar PCs was sufficient to recapitulate autistic-like behavioral and cellular changes akin to those identified in Bmal1 KO mice. Together, these results unveil a previously unidentified role for Bmal1 disruption in cerebellar dysfunction and autistic-like behaviors. Our findings provide experimental evidence supporting a putative role for dysregulation of circadian clock gene expression in the pathogenesis of ASD.

The Case for the Target of Rapamycin Pathway as a Candidate Circadian Oscillator

The molecular mechanisms that drive circadian (24 h) rhythmicity have been investigated for many decades, but we still do not have a complete picture of eukaryotic circadian systems. Although the transcription/translation feedback loop (TTFL) model has been the primary focus of research, there are many examples of circadian rhythms that persist when TTFLs are not functioning, and we lack any good candidates for the non-TTFL oscillators driving these rhythms. In this hypothesis-driven review, the author brings together several lines of evidence pointing towards the Target of Rapamycin (TOR) signalling pathway as a good candidate for a non-TTFL oscillator. TOR is a ubiquitous regulator of metabolism in eukaryotes and recent focus in circadian research on connections between metabolism and rhythms makes TOR an attractive candidate oscillator. In this paper, the evidence for a role for TOR in regulating rhythmicity is reviewed, and the advantages of TOR as a potential oscillator are discussed. Evidence for extensive feedback regulation of TOR provides potential mechanisms for a TOR-driven oscillator. Comparison with ultradian yeast metabolic cycles provides an example of a potential TOR-driven self-sustained oscillation. Unanswered questions and problems to be addressed by future research are discussed.

Targeted screening and identification of chlorhexidine as a pro-myogenic circadian clock activator

BACKGROUND

The circadian clock is an evolutionarily conserved mechanism that exerts pervasive temporal control in stem cell behavior. This time-keeping machinery is required for orchestrating myogenic progenitor properties in regenerative myogenesis that ameliorates muscular dystrophy. Here we report a screening platform to discover circadian clock modulators that promote myogenesis and identify chlorhexidine (CHX) as a clock-activating molecule with pro-myogenic activities.

METHODS

A high-throughput molecular docking pipeline was applied to identify compounds with a structural fit for a hydrophobic pocket within the key circadian transcription factor protein, Circadian Locomotor Output Cycles Kaput (CLOCK). These identified molecules were further screened for clock-modulatory activities and functional validations for pro-myogenic properties.

RESULTS

CHX was identified as a clock activator that promotes distinct aspects of myogenesis. CHX activated circadian clock that reduced cycling period length and augmented amplitude. This action was mediated by the targeted CLOCK structure via augmented interaction with heterodimer partner Bmal1, leading to enhanced CLOCK/Bmal1-controlled transcription with upregulation of core clock genes. Consistent with its clock-activating function, CHX displayed robust effects on stimulating myogenic differentiation in a clock-dependent manner. In addition, CHX augmented the proliferative and migratory activities of myoblasts.

CONCLUSION

Our findings demonstrate the feasibility of a screening platform to discover clock modulators with myogenic regulatory activities. Discovery of CHX as a pro-myogenic molecule could be applicable to promote regenerative capacities in ameliorating dystrophic or degenerative muscle diseases.

Epilepsy, gut microbiota, and circadian rhythm

In recent years, relevant studies have found changes in gut microbiota (GM) in patients with epilepsy. In addition, impaired sleep and circadian patterns are common symptoms of epilepsy. Moreover, the types of seizures have a circadian rhythm. Numerous reports have indicated that the GM and its metabolites have circadian rhythms. This review will describe changes in the GM in clinical and animal studies under epilepsy and circadian rhythm disorder, respectively. The aim is to determine the commonalities and specificities of alterations in GM and their impact on disease occurrence in the context of epilepsy and circadian disruption. Although clinical studies are influenced by many factors, the results suggest that there are some commonalities in the changes of GM. Finally, we discuss the links among epilepsy, gut microbiome, and circadian rhythms, as well as future research that needs to be conducted.

d-Glucosamine induces circadian phase delay by promoting BMAL1 degradation through AMPK/mTOR pathway

Circadian rhythms are closely linked to the metabolic network through circadian feedback regulation. The hexosamine biosynthetic pathway (HBP) is a branch of glucose metabolism that affects circadian rhythms through the O-linked N-acetylglucosamine modification (O-GlcNAcylation) of clock proteins. Here, we found out that, among the downstream metabolites regulated by d-glucosamine (GlcN) in HBP salvage pathway, only GlcN is able to induce circadian phase delay both in vitro and in vivo. Mechanistic studies indicated that the phase-shift induced by GlcN is independent of O-GlcNAcylation. Instead, GlcN selectively up-regulates p-AMPK activity, leading to the inhibition of mTOR signaling pathway, and thus down-regulation of p-BMAL1 both in human cell line and mouse tissues. Moreover, GlcN promoted BMAL1 degradation via proteasome pathway. These findings reveal a novel molecular mechanism of GlcN in regulating clock phase and suggest the therapeutic potential of GlcN as new use for an old drug in the future treatment of shift work and circadian misalignment.

The shared molecular mechanisms underlying aging of the brain, major depressive disorder, and Alzheimer's disease: The role of circadian rhythm disturbances

An association with circadian clock function and pathophysiology of aging, major depressive disorder (MDD), and Alzheimer's disease (AD) is well established and has been proposed as a factor in the development of these diseases. Depression and changes in circadian rhythm have been increasingly suggested as the two primary overlapping and interpenetrating changes that occur with aging. The relationship between AD and depression in late life is not completely understood and probably is complex. Patients with major depression or AD suffer from disturbed sleep/wake cycles and altered rhythms in daily activities. Although classical monoaminergic hypotheses are traditionally proposed to explain the pathophysiology of MDD, several clinical and preclinical studies have reported a strong association between circadian rhythm and mood regulation. In addition, a large body of evidence supports an association between disruption of circadian rhythm and AD. Some clock genes are dysregulated in rodent models of depression. If aging, AD, and MDD share a common biological basis in pathophysiology, common therapeutic tools could be investigated for their prevention and treatment. Nitro-oxidative stress (NOS), for example, plays a fundamental role in aging, as well as in the pathogenesis of AD and MDD and is associated with circadian clock disturbances. Thus, development of therapeutic possibilities with these NOS-related conditions is advisable. This review describes recent findings that link disrupted circadian clocks to aging, MDD, and AD and summarizes the experimental evidence that supports connections between the circadian clock and molecular pathologic factors as shared common pathophysiological mechanisms underlying aging, AD, and MDD.

Aryl hydrocarbon receptor (AhR) impairs circadian regulation: impact on the aging process

Circadian clocks control the internal sleep-wake rhythmicity of 24hours which is synchronized by the solar cycle. Circadian regulation of metabolism evolved about 2.5 billion years ago, i.e., the rhythmicity has been conserved from cyanobacteria and Archaea through to mammals although the mechanisms utilized have developed with evolution. While the aryl hydrocarbon receptor (AhR) is an evolutionarily conserved defence mechanism against environmental threats, it has gained many novel functions during evolution, such as the regulation of cell cycle, proteostasis, and many immune functions. There is robust evidence that AhR signaling impairs circadian rhythmicity, e.g., by interacting with the core BMAL1/CLOCK complex and disturbing the epigenetic regulation of clock genes. The maintenance of circadian rhythms is impaired with aging, disturbing metabolism and many important functions in aged organisms. Interestingly, it is known that AhR signaling promotes an age-related tissue degeneration, e.g., it is able to inhibit autophagy, enhance cellular senescence, and disrupt extracellular matrix. These alterations are rather similar to those induced by a long-term impairment of circadian rhythms. However, it is not known whether AhR signaling enhances the aging process by impairing circadian homeostasis. I will examine the experimental evidence indicating that AhR signaling is able to promote the age-related degeneration via a disruption of circadian rhythmicity.

Reciprocal Interactions between Circadian Clocks, Food Intake, and Energy Metabolism

Like other biological functions, food intake and energy metabolism display daily rhythms controlled by the circadian timing system that comprises a main circadian clock and numerous secondary clocks in the brain and peripheral tissues. Each secondary circadian clock delivers local temporal cues based on intracellular transcriptional and translational feedback loops that are tightly interconnected to intracellular nutrient-sensing pathways. Genetic impairment of molecular clocks and alteration in the rhythmic synchronizing cues, such as ambient light at night or mistimed meals, lead to circadian disruption that, in turn, negatively impacts metabolic health. Not all circadian clocks are sensitive to the same synchronizing signals. The master clock in the suprachiasmatic nuclei of the hypothalamus is mostly synchronized by ambient light and, to a lesser extent, by behavioral cues coupled to arousal and exercise. Secondary clocks are generally phase-shifted by timed metabolic cues associated with feeding, exercise, and changes in temperature. Furthermore, both the master and secondary clocks are modulated by calorie restriction and high-fat feeding. Taking into account the regularity of daily meals, the duration of eating periods, chronotype, and sex, chrononutritional strategies may be useful for improving the robustness of daily rhythmicity and maintaining or even restoring the appropriate energy balance.

Mealtime alcohol consumption suppresses skeletal muscle mTORC1 signaling in female mice

OBJECTIVE

To determine whether alcohol consumed within the meal influences the feeding induced increase in mTORC1 signaling.

METHODS

Alcohol provided in the liquid diet was consumed by alcohol naïve, fasted, C57BL/6Hsd female mice and gastrocnemius was collected 1hr after the refeeding. Subsequent experiments determined the extent to which changes in mTORC1 signaling persisted across the day.

RESULTS

Compared with control mice, protein synthesis, mTORC1 (Ser2448), 4EBP1 (Ser65), S6K1 (Thr389), rpS6 (Ser240/244), Akt (Thr308), and ULK1 (Ser757) were lower in EtOH. Similar suppressive patterns were observed in the hours following consumption of alcohol containing food throughout the dark cycle. Higher peak blood alcohol concentrations induced by intraperitoneal injection of alcohol extended the time and magnitude of mTORC1 pathway suppression.

CONCLUSION

Alcohol administered as part of the meal results in lower skeletal muscle mTORC1 signaling while subsequent models show that alcohol may influence this pathway across the day.

Is obstructive sleep apnea a circadian rhythm disorder?

Obstructive sleep apnea is the most common sleep-related breathing disorder worldwide and remains underdiagnosed. Its multiple associated comorbidities contribute to a decreased quality of life and work performance as well as an increased risk of death. Standard treatment seems to have limited effects on cardiovascular and metabolic aspects of the disease, emphasising the need for early diagnosis and additional therapeutic approaches. Recent evidence suggests that the dysregulation of circadian rhythms, processes with endogenous rhythmicity that are adjusted to the environment through various cues, is involved in the pathogenesis of comorbidities. In patients with obstructive sleep apnea, altered circadian gene expression patterns have been demonstrated. Obstructive respiratory events may promote circadian dysregulation through the effects of sleep disturbance and intermittent hypoxia, with subsequent inflammation and disruption of neural and hormonal homeostasis. In this review, current knowledge on obstructive sleep apnea, circadian rhythm regulation, and circadian rhythm sleep disorders is summarised. Studies that connect obstructive sleep apnea to circadian rhythm abnormalities are critically evaluated. Furthermore, pathogenetic mechanisms that may underlie this association, most notably hypoxia signalling, are presented. A bidirectional relationship between obstructive sleep apnea and circadian rhythm dysregulation is proposed. Approaching obstructive sleep apnea as a circadian rhythm disorder may prove beneficial for the development of new, personalised diagnostic, therapeutic and prognostic tools. However, further studies are needed before the clinical approach to obstructive sleep apnea includes targeting the circadian system.

Ion dynamics and the regulation of circadian cellular physiology

Circadian rhythms in physiology and behavior allow organisms to anticipate the daily environmental changes imposed by the rotation of our planet around its axis. Although these rhythms eventually manifest at the organismal level, a cellular basis for circadian rhythms has been demonstrated. Significant contributors to these cell-autonomous rhythms are daily cycles in gene expression and protein translation. However, recent data revealed cellular rhythms in other biological processes, including ionic currents, ion transport, and cytosolic ion abundance. Circadian rhythms in ion currents sustain circadian variation in action potential firing rate, which coordinates neuronal behavior and activity. Circadian regulation of metal ions abundance and dynamics is implicated in distinct cellular processes, from protein translation to membrane activity and osmotic homeostasis. In turn, studies showed that manipulating ion abundance affects the expression of core clock genes and proteins, suggestive of a close interplay. However, the relationship between gene expression cycles, ion dynamics, and cellular function is still poorly characterized. In this review, I will discuss the mechanisms that generate ion rhythms, the cellular functions they govern, and how they feed back to regulate the core clock machinery.

Circadian disruption: from mouse models to molecular mechanisms and cancer therapeutic targets

The circadian clock is a timekeeping system for numerous biological rhythms that contribute to the regulation of numerous homeostatic processes in humans. Disruption of circadian rhythms influences physiology and behavior and is associated with adverse health outcomes, especially cancer. However, the underlying molecular mechanisms of circadian disruption-associated cancer initiation and development remain unclear. It is essential to construct good circadian disruption models to uncover and validate the detailed molecular clock framework of circadian disruption in cancer development and progression. Mouse models are the most widely used in circadian studies due to their relatively small size, fast reproduction cycle, easy genome manipulation, and economic practicality. Here, we reviewed the current mouse models of circadian disruption, including suprachiasmatic nuclei destruction, genetic engineering, light disruption, sleep deprivation, and other lifestyle factors in our understanding of the crosstalk between circadian rhythms and oncogenic signaling, as well as the molecular mechanisms of circadian disruption that promotes cancer growth. We focused on the discoveries made with the nocturnal mouse, diurnal human being, and cell culture and provided several circadian rhythm-based cancer therapeutic strategies.

Clocking Epilepsies: A Chronomodulated Strategy-Based Therapy for Rhythmic Seizures

Epilepsy is a neurological disorder characterized by hypersynchronous recurrent neuronal activities and seizures, as well as loss of muscular control and sometimes awareness. Clinically, seizures have been reported to display daily variations. Conversely, circadian misalignment and circadian clock gene variants contribute to epileptic pathogenesis. Elucidation of the genetic bases of epilepsy is of great importance because the genetic variability of the patients affects the efficacies of antiepileptic drugs (AEDs). For this narrative review, we compiled 661 epilepsy-related genes from the PHGKB and OMIM databases and classified them into 3 groups: driver genes, passenger genes, and undetermined genes. We discuss the potential roles of some epilepsy driver genes based on GO and KEGG analyses, the circadian rhythmicity of human and animal epilepsies, and the mutual effects between epilepsy and sleep. We review the advantages and challenges of rodents and zebrafish as animal models for epileptic studies. Finally, we posit chronomodulated strategy-based chronotherapy for rhythmic epilepsies, integrating several lines of investigation for unraveling circadian mechanisms underpinning epileptogenesis, chronopharmacokinetic and chronopharmacodynamic examinations of AEDs, as well as mathematical/computational modeling to help develop time-of-day-specific AED dosing schedules for rhythmic epilepsy patients.

Major roles of the circadian clock in cancer

Circadian rhythms are natural rhythms that widely exist in all creatures, and regulate the processes and physiological functions of various biochemical reactions. The circadian clock is critical for cancer occurrence and progression. Its function is regulated by metabolic activities, and the expression and transcription of various genes. This review summarizes the composition of the circadian clock; the biological basis for its function; its relationship with, and mechanisms in, cancer; its various functions in different cancers; the effects of anti-tumor treatment; and potential therapeutic targets. Research in this area is expected to advance understanding of circadian locomotor output cycles kaput (CLOCK) and brain and muscle ARNT-like protein 1 (BMAL1) in tumor diseases, and contribute to the development of new anti-tumor treatment strategies.

Nutritional compensation of the circadian clock is a conserved process influenced by gene expression regulation and mRNA stability

Compensation is a defining principle of a true circadian clock, where its approximately 24-hour period length is relatively unchanged across environmental conditions. Known compensation effectors directly regulate core clock factors to buffer the oscillator's period length from variables in the environment. Temperature Compensation mechanisms have been experimentally addressed across circadian model systems, but much less is known about the related process of Nutritional Compensation, where circadian period length is maintained across physiologically relevant nutrient levels. Using the filamentous fungus Neurospora crassa, we performed a genetic screen under glucose and amino acid starvation conditions to identify new regulators of Nutritional Compensation. Our screen uncovered 16 novel mutants, and together with 4 mutants characterized in prior work, a model emerges where Nutritional Compensation of the fungal clock is achieved at the levels of transcription, chromatin regulation, and mRNA stability. However, eukaryotic circadian Nutritional Compensation is completely unstudied outside of Neurospora. To test for conservation in cultured human cells, we selected top hits from our fungal genetic screen, performed siRNA knockdown experiments of the mammalian orthologs, and characterized the cell lines with respect to compensation. We find that the wild-type mammalian clock is also compensated across a large range of external glucose concentrations, as observed in Neurospora, and that knocking down the mammalian orthologs of the Neurospora compensation-associated genes CPSF6 or SETD2 in human cells also results in nutrient-dependent period length changes. We conclude that, like Temperature Compensation, Nutritional Compensation is a conserved circadian process in fungal and mammalian clocks and that it may share common molecular determinants.

Circadian medicine for aging attenuation and sleep disorders: Prospects and challenges

Aging causes progressive deterioration of daily rhythms in behavioral and metabolic processes and disruption in the regular sleep-wake cycle. Circadian disruption is directly related to diverse age-induced health abnormalities. Rising evidence from various organisms shows that core clock gene mutations cause premature aging, reduced lifespan, and sleeping irregularities. Improving the clock functions and correcting its disruption by pharmacological interventions or time-regulated feeding patterns could be a novel avenue for effective clinical management of aging and sleep disorders. To this end, many drugs for sleep disorders and anti-aging compounds interact with the core clock machinery and alter the circadian output. Evaluation of dosing time-dependency and circadian regulation of drug metabolism for therapeutic improvement of the existing drugs is another fundamental facet of chronomedicine. Multiple studies have demonstrated dose-dependent manipulation of the circadian period and phase-shifting by pharmacologically active compounds. The chronobiology research field is gradually moving towards the development of novel therapeutic strategies based on targeting the molecular clock or dosing time-oriented medications. However, such translational research ventures would require more experimental evidence from studies on humans. This review discusses the impact of circadian rhythms on aging and sleep, emphasizing the potentiality of circadian medicine in aging attenuation and sleep disorders.

Step in Time: Conservation of Circadian Clock Genes in Animal Evolution

Over the past few decades, the molecular mechanisms responsible for circadian phenotypes of animals have been studied in increasing detail in mammals, some insects, and other invertebrates. Particular circadian proteins and their interactions are shared across evolutionary distant animals, resulting in a hypothesis for the canonical circadian clock of animals. As the number of species for which the circadian clockwork has been described increases, the circadian clock in animals driving cyclical phenotypes becomes less similar. Our focus in this review is to develop and synthesize the current literature to better understand the antiquity and evolution of the animal circadian clockwork. Here, we provide an updated understanding of circadian clock evolution in animals, largely through the lens of conserved genes characterized in the circadian clock identified in bilaterian species. These comparisons reveal extensive variation within the likely composition of the core clock mechanism, including losses of many genes, and that the ancestral clock of animals does not equate to the bilaterian clock. Despite the loss of these core genes, these species retain circadian behaviors and physiology, suggesting novel clocks have evolved repeatedly. Additionally, we highlight highly conserved cellular processes (e.g., cell division, nutrition) that intersect with the circadian clock of some animals. The conservation of these processes throughout the animal tree remains essentially unknown, but understanding their role in the evolution and maintenance of the circadian clock will provide important areas for future study.

Development of Cell Therapies for Renal Disease and Regenerative Medicine

The incidence of renal disease is gradually increasing worldwide, and this condition has become a major public health problem because it is a trigger for many other chronic diseases. Cell therapies using multipotent mesenchymal stromal cells, hematopoietic stem cells, macrophages, and other cell types have been used to induce regeneration and provide a cure for acute and chronic kidney disease in experimental models. This review describes the advances in cell therapy protocols applied to acute and chronic kidney injuries and the attempts to apply these treatments in a clinical setting.

TOR kinase, a GPS in the complex nutrient and hormonal signaling networks to guide plant growth and development

To survive and sustain growth, sessile plants have developed sophisticated internal signalling networks that respond to various external and internal cues. Despite the central roles of nutrient and hormone signaling in plant growth and development, how hormone-driven processes coordinate with metabolic status remains largely enigmatic. Target of rapamycin (TOR) kinase is an evolutionarily conserved master regulator that integrates energy, nutrients, growth factors, hormones, and stress signals to promote growth in all eukaryotes. Inspired by recent comprehensive systems, chemical, genetic, and genomic studies on TOR in plants, this review discusses a potential role of TOR as a 'global positioning system' that directs plant growth and developmental programs both temporally and spatially by integrating dynamic information in the complex nutrient and hormonal signaling networks. We further evaluate and depict the possible functional and mechanistic models for how a single protein kinase, TOR, is able to recognize, integrate, and even distinguish a plethora of positive and negative input signals to execute appropriate and distinct downstream biological processes via multiple partners and effectors.

Hepatic mTORC2 Signaling Facilitates Acute Glucagon Receptor Enhancement of Insulin-Stimulated Glucose Homeostasis in Mice

Long-term glucagon receptor (GCGR) agonism is associated with hyperglycemia and glucose intolerance, while acute GCGR agonism enhances whole-body insulin sensitivity and hepatic AKTSer473 phosphorylation. These divergent effects establish a critical gap in knowledge surrounding GCGR action. mTOR complex 2 (mTORC2) is composed of seven proteins, including RICTOR, which dictates substrate binding and allows for targeting of AKTSer473. We used a liver-specific Rictor knockout mouse (RictorΔLiver) to investigate whether mTORC2 is necessary for insulin receptor (INSR) and GCGR cross talk. RictorΔLiver mice were characterized by impaired AKT signaling and glucose intolerance. Intriguingly, RictorΔLiver mice were also resistant to GCGR-stimulated hyperglycemia. Consistent with our prior report, GCGR agonism increased glucose infusion rate and suppressed hepatic glucose production during hyperinsulinemic-euglycemic clamp of control animals. However, these benefits to insulin sensitivity were ablated in RictorΔLiver mice. We observed diminished AKTSer473 and GSK3α/βSer21/9 phosphorylation in RictorΔLiver mice, whereas phosphorylation of AKTThr308 was unaltered in livers from clamped mice. These signaling effects were replicated in primary hepatocytes isolated from RictorΔLiver and littermate control mice, confirming cell-autonomous cross talk between GCGR and INSR pathways. In summary, our study reveals the necessity of RICTOR, and thus mTORC2, in GCGR-mediated enhancement of liver and whole-body insulin action.

The actin cytoskeleton-MRTF/SRF cascade transduces cellular physical niche cues to entrain the circadian clock

The circadian clock is entrained to daily environmental cues. Integrin-linked signaling via actin cytoskeleton dynamics transduces physical niche cues from the extracellular matrix to myocardin-related transcription factor (MRTF)/serum response factor (SRF)-mediated transcription. The actin cytoskeleton organization and SRF-MRTF activity display diurnal oscillations. By interrogating disparate upstream events in the actin cytoskeleton-MRTF-A/SRF signaling cascade, we show that this pathway transduces extracellular niche cues to modulate circadian clock function. Pharmacological inhibition of MRTF-A/SRF by disrupting actin polymerization or blocking the ROCK kinase induced period lengthening with augmented clock amplitude, and genetic loss of function of Srf or Mrtfa mimicked the effects of treatment with actin-depolymerizing agents. In contrast, actin polymerization shortened circadian clock period and attenuated clock amplitude. Moreover, interfering with the cell-matrix interaction through blockade of integrin, inhibition of focal adhesion kinase (FAK, encoded by Ptk2) or attenuating matrix rigidity reduced the period length while enhancing amplitude. Mechanistically, we identified that the core clock repressors Per2, Nr1d1 and Nfil3 are direct transcriptional targets of MRTF-A/SRF in mediating actin dynamics-induced clock response. Collectively, our findings defined an integrin-actin cytoskeleton-MRTF/SRF pathway in linking clock entrainment with extracellular cues that might facilitate cellular adaptation to the physical niche environment.

mTOR-AKT Signaling in Cellular Clock Resetting Triggered by Osmotic Stress

Aims: The circadian clock oscillates in a cell-autonomous manner with a period of ∼24 h, and the phase is regulated by various time cues such as light and temperature through multiple clock input pathways. We previously found that osmotic and oxidative stress strongly affected the circadian period and phase of cellular rhythms, and triple knockout of apoptosis signal-regulating kinase (ASK) family members, Ask1, Ask2, and Ask3, abolished the phase shift (clock resetting) induced by hyperosmotic pulse treatment. We aimed at exploring a key molecule(s) and signaling events in the clock input pathway dependent on ASK kinases. Results: The phase shift of the cellular clock induced by the hyperosmotic pulse treatment was significantly reduced by combined deficiencies of the clock(-related) genes, Dec1, Dec2, and E4 promoter-binding protein 4 (also known as Nfil3) (E4bp4). In addition, liquid chromatography mass/mass spectrometry (LC-MS/MS)-based proteomic analysis identified hyperosmotic pulse-induced phosphorylation of circadian locomotor output cycles caput (CLOCK) Ser845 in an AKT-dependent manner. We found that AKT kinase was phosphorylated at Ser473 (i.e., activated) in response to the hyperosmotic pulse experiments. Inhibition of mechanistic target of rapamycin (mTOR) kinase by Torin 1 treatment completely abolished the AKT activation, suppressed the phosphorylation of CLOCK Ser845, and blocked the clock resetting induced by the hyperosmotic pulse treatment. Innovation and Conclusions: We conclude that mTOR-AKT signaling is indispensable for the CLOCK Ser845 phosphorylation, which correlates with the clock resetting induced by the hyperosmotic pulse treatment. Immediate early induction of the clock(-related) genes and CLOCK carboxyl-terminal (C-terminal) region containing Ser845 also play important roles in the clock input pathway through redox-sensitive ASK kinases. Antioxid. Redox Signal. 37, 631-646.

The interaction between circadian rhythm and epilepsy

Evidence about the interaction between circadian rhythms (CR) and epilepsy has been expanded with the application of advanced detection technology. An adequate understanding of how circadian system and epilepsy interact with each other could contribute to more accurate seizure prediction as well as rapid development of potential treatment timed to specific phases of CR. In this review, we present the reciprocal relationship between CR and epileptic activities from aspects of sleep effect, genetic modulation and brain biochemistry. It has been found that sleep-wake patterns, circadian timing systems and multidien rhythms have essential roles in seizure activities and interictal epileptiform discharge (IED). For instance, specific distribution patterns of seizures and IED have been reported, i.e., lighter non-rapid eye movement (NREM) sleep stage (stage 2) induces seizures while deeper NREM sleep stage (stage 3) activates IEDs. Furthermore, the epilepsy type, seizure type and seizure onset zone can significantly affect the rhythms of seizure occurrence. Apart from the common seizure types, several specific epilepsy syndromes also have a close correlation with sleep-wakefulness patterns. Sleep influences the epilepsy rhythm, and conversely, epilepsy alters the sleep rhythm through multiple pathways. Clock genes accompanied by two feedback loops of regulation have an important role in cortical excitability and seizure occurrence, which may be involved in the mTORopathy. The suprachiasmatic nuclei (SCN) has a rhythm of melatonin and cortisol secretion under the circadian pattern, and then these hormones can feed back into a central oscillator to affect the SCN-dependent rhythms, leading to variable but prominent influence on epilepsy. Furthermore, we discuss the precise predictive algorithms and chronotherapy strategies based on different temporal patterns of seizure occurrence for patients with epilepsy, which may offer a valuable indication for non-invasive closed-loop treatment system. Optimization of the time and dose of antiseizure medications, and resynchronization of disturbed CR (by hormone therapy, light exposure, ketogenic diet, novel small molecules) would be beneficial for epileptic patients in the future. Before formal clinical practice, future large-scale studies are urgently needed to assist prediction and treatment of circadian seizure activities and address unsolved restrictions.

The trilateral interactions between mammalian target of rapamycin (mTOR) signaling, the circadian clock, and psychiatric disorders: an emerging model

Circadian (~24 h) rhythms in physiology and behavior are evolutionarily conserved and found in almost all living organisms. The rhythms are endogenously driven by daily oscillatory activities of so-called "clock genes/proteins", which are widely distributed throughout the mammalian brain. Mammalian (mechanistic) target of rapamycin (mTOR) signaling is a fundamental intracellular signal transduction cascade that controls important neuronal processes including neurodevelopment, synaptic plasticity, metabolism, and aging. Dysregulation of the mTOR pathway is associated with psychiatric disorders including autism spectrum disorders (ASD) and mood disorders (MD), in which patients often exhibit disrupted daily physiological rhythms and abnormal circadian gene expression in the brain. Recent work has found that the activities of mTOR signaling are temporally controlled by the circadian clock and exhibit robust circadian oscillations in multiple systems. In the meantime, mTOR signaling regulates fundamental properties of the central and peripheral circadian clocks, including period length, entrainment, and synchronization. Whereas the underlying mechanisms remain to be fully elucidated, increasing clinical and preclinical evidence support significant crosstalk between mTOR signaling, the circadian clock, and psychiatric disorders. Here, we review recent progress in understanding the trilateral interactions and propose an "interaction triangle" model between mTOR signaling, the circadian clock, and psychiatric disorders (focusing on ASD and MD).

An Overview of the Circadian Clock in the Frame of Chronotherapy: From Bench to Bedside

Most living organisms in both the plant and animal kingdoms have evolved processes to stay in tune with the alternation of day and night, and to optimize their physiology as a function of light supply. In mammals, a circadian clock relying on feedback loops between key transcription factors will thus control the temporally regulated pattern of expression of most genes. Modern ways of life have highly altered the synchronization of human activities with their circadian clocks. This review discusses the links between an altered circadian clock and the rise of pathologies. We then sum up the proofs of concept advocating for the integration of circadian clock considerations in chronotherapy for health care, medicine, and pharmacotherapy. Finally, we discuss the current challenges that circadian biology must face and the tools to address them.

Mammalian PERIOD2 regulates H2A.Z incorporation in chromatin to orchestrate circadian negative feedback

Mammalian circadian oscillators are built on a feedback loop in which the activity of the transcription factor CLOCK-BMAL1 is repressed by the PER-CRY complex. Here, we show that murine Per^-/- fibroblasts display aberrant nucleosome occupancy around transcription start sites (TSSs) and at promoter-proximal and distal CTCF sites due to impaired histone H2A.Z deposition. Knocking out H2A.Z mimicked the Per null chromatin state and disrupted cellular rhythms. We found that endogenous mPER2 complexes retained CTCF as well as the specific H2A.Z-deposition chaperone YL1-a component of the ATP-dependent remodeler SRCAP and p400-TIP60 complex. While depleting YL1 or mutating chaperone-binding sites on H2A.Z lengthened the circadian period, H2A.Z deletion abrogated BMAL1 chromatin recruitment and promoted its proteasomal degradation. We propose that a PER2-mediated H2A.Z deposition pathway (1) compacts CLOCK-BMAL1 binding sites to establish negative feedback, (2) organizes circadian chromatin landscapes using CTCF and (3) bookmarks genomic loci for BMAL1 binding to impinge on the positive arm of the subsequent cycle.

Nutrition, metabolism, and epigenetics: pathways of circadian reprogramming

Food intake profoundly affects systemic physiology. A large body of evidence has indicated a link between food intake and circadian rhythms, and ~24-h cycles are deemed essential for adapting internal homeostasis to the external environment. Circadian rhythms are controlled by the biological clock, a molecular system remarkably conserved throughout evolution. The circadian clock controls the cyclic expression of numerous genes, a regulatory program common to all mammalian cells, which may lead to various metabolic and physiological disturbances if hindered. Although the circadian clock regulates multiple metabolic pathways, metabolic states also provide feedback on the molecular clock. Therefore, a remarkable feature is reprogramming by nutritional challenges, such as a high-fat diet, fasting, ketogenic diet, and caloric restriction. In addition, various factors such as energy balance, histone modifications, and nuclear receptor activity are involved in the remodeling of the clock. Herein, we review the interaction of dietary components with the circadian system and illustrate the relationships linking the molecular clock to metabolism and critical roles in the remodeling process.

Stress hormone signaling inhibits Th1 polarization in a CD4 T-cell-intrinsic manner via mTORC1 and the circadian gene PER1

Stress hormones are believed to skew the CD4 T‐cell differentiation towards a Th2 response via a T‐cell‐extrinsic mechanism. Using isolated primary human naïve and memory CD4 T cells, here we show that both adrenergic‐ and glucocorticoid‐mediated stress signalling pathways play a CD4 naïve T‐cell‐intrinsic role in regulating the Th1/Th2 differentiation balance. Both stress hormones reduced the Th1 programme and cytokine production by inhibiting mTORC1 signalling via two parallel mechanisms. Stress hormone signalling inhibited mTORC1 in naïve CD4 T cells (1) by affecting the PI3K/AKT pathway and (2) by regulating the expression of the circadian rhythm gene, period circadian regulator 1 (PER1). Both stress hormones induced the expression of PER1, which inhibited mTORC1 signalling, thus reducing Th1 differentiation. This previously unrecognized cell‐autonomous mechanism connects stress hormone signalling with CD4 T‐cell differentiation via mTORC1 and a specific circadian clock gene, namely PER1.

A wrinkle in time: circadian biology in pulmonary vascular health and disease

An often overlooked element of pulmonary vascular disease is time. Cellular responses to time, which are regulated directly by the core circadian clock, have only recently been elucidated. Despite an extensive collection of data regarding the role of rhythmic contribution to disease pathogenesis (such as systemic hypertension, coronary artery, and renal disease), the roles of key circadian transcription factors in pulmonary hypertension remain understudied. This is despite a large degree of overlap in the pulmonary hypertension and circadian rhythm fields, not only including shared signaling pathways, but also cell-specific effects of the core clock that are known to result in both protective and adverse lung vessel changes. Therefore, the goal of this review is to summarize the current dialogue regarding common pathways in circadian biology, with a specific emphasis on its implications in the progression of pulmonary hypertension. In this work, we emphasize specific proteins involved in the regulation of the core molecular clock while noting the circadian cell-specific changes relevant to vascular remodeling. Finally, we apply this knowledge to the optimization of medical therapy, with a focus on sleep hygiene and the role of chronopharmacology in patients with this disease. In dissecting the unique relationship between time and cellular biology, we aim to provide valuable insight into the practical implications of considering time as a therapeutic variable. Armed with this information, physicians will be positioned to more efficiently use the full four dimensions of patient care, resulting in improved morbidity and mortality of pulmonary hypertension patients.

The Circadian Clock and Obesity

The modern way of life has dramatically affected our biological rhythms. Circadian rhythms, which are generated by an endogenous circadian clock, are observed in a large number of physiological functions including metabolism. Proper peripheral clock synchronization by different signals including appropriate feeding/fasting cycles is essential to coordinate and temporally gate metabolic processes. In this chapter, we emphasize the importance of nutrient sensing by peripheral clocks and highlight the major role of peripheral and central clock communication to locally regulate metabolic processes and ensure optimal energy storage and expenditure. As a consequence, changes in eating behavior and/or bedtime, as occurs upon shift work and jet lag, have direct consequences on metabolism and participate in the increasing prevalence of obesity and associated metabolic diseases such as type 2 diabetes and non-alcoholic fatty liver disease. In this setting, time-restricted feeding has been suggested as an efficient approach to ameliorate metabolic parameters and control body weight.

Ultradian rhythms of AKT phosphorylation and gene expression emerge in the absence of the circadian clock components Per1 and Per2

Rhythmicity of biological processes can be elicited either in response to environmental cycles or driven by endogenous oscillators. In mammals, the circadian clock drives about 24-hour rhythms of multitude metabolic and physiological processes in anticipation to environmental daily oscillations. Also at the intersection of environment and metabolism is the protein kinase—AKT. It conveys extracellular signals, primarily feeding-related signals, to regulate various key cellular functions. Previous studies in mice identified rhythmicity in AKT activation (pAKT) with elevated levels in the fed state. However, it is still unknown whether rhythmic AKT activation can be driven through intrinsic mechanisms. Here, we inspected temporal changes in pAKT levels both in cultured cells and animal models. In cultured cells, pAKT levels showed circadian oscillations similar to those observed in livers of wild-type mice under free-running conditions. Unexpectedly, in livers of Per1,2^−/− but not of Bmal1^−/− mice we detected ultradian (about 16 hours) oscillations of pAKT levels. Importantly, the liver transcriptome of Per1,2^−/− mice also showed ultradian rhythms, corresponding to pAKT rhythmicity and consisting of AKT-related genes and regulators. Overall, our findings reveal ultradian rhythms in liver gene expression and AKT phosphorylation that emerge in the absence of environmental rhythms and Per1,2^−/− genes.

This study reveals ultradian (16-hour) rhythms in the activation of the protein kinase AKT in the livers of mice, accompanied by corresponding downstream changes in gene expression. Intriguingly, these oscillations emerge in the absence of rhythmic environmental cues and in mice lacking the circadian clock proteins Per1 and Per2.

Circadian Rhythm Effects on the Molecular Regulation of Physiological Systems

Nearly every system within the body contains an intrinsic cellular circadian clock. The circadian clock contributes to the regulation of a variety of homeostatic processes in mammals through the regulation of gene expression. Circadian disruption of physiological systems is associated with pathophysiological disorders. Here, we review the current understanding of the molecular mechanisms contributing to the known circadian rhythms in physiological function. This article focuses on what is known in humans, along with discoveries made with cell and rodent models. In particular, the impact of circadian clock components in metabolic, cardiovascular, endocrine, musculoskeletal, immune, and central nervous systems are discussed. © 2021 American Physiological Society. Compr Physiol 11:1-30, 2021.

Disturbances of the Gut Microbiota, Sleep Architecture, and mTOR Signaling Pathway in Patients with Severe Obstructive Sleep Apnea-Associated Hypertension

Intermittent hypoxia and sleep fragmentation are pathophysiological processes involved in obstructive sleep apnea (OSA) which affect gut microbiota, sleep architecture, and mTOR signaling pathway. However, the involvement of these elements in the pathogenesis mechanism of OSA-associated hypertension remains unclear. Therefore, this study investigated whether the OSA-associated hypertension mechanism is regulated by the gut microbiota and mTOR signaling pathway. Patients were diagnosed by polysomnography; their fecal samples were obtained and analyzed for their microbiome composition by 16S ribosomal RNA pyrosequencing and bioinformatics analysis. Transcript genes on fasting peripheral blood mononuclear cells (PBMCs) were examined using Illumina RNA-sequencing analysis. Totally, we enrolled 60 patients with severe OSA [without hypertension (n = 27) and with hypertension (n = 33)] and 12 controls (neither OSA nor hypertension). Results revealed that severe-OSA patients with hypertension had an altered gut microbiome, decreased short-chain fatty acid-producing bacteria (P < 0.05), and reduced arginine and proline metabolism pathways (P=0.001), compared with controls; also, they had increased stage N1 sleep and reduced stages N2 and N3 sleep accompanied by repeated arousals (P < 0.05). Analysis of PBMCs using the Kyoto Encyclopedia of Genes and Genomes database showed that the mTOR signaling pathway (P=0.006) was the most important differential gene-enriched pathway in severe-OSA patients with hypertension. Our findings extend prior work and suggest a possibility that the regulation of the mTOR signaling pathway is involved in developing OSA-associated hypertension through its interaction with the disturbance of the gut microbiome and sleep architecture.

Understanding circadian regulation of mammalian cell function, protein homeostasis, and metabolism

Circadian rhythms are ∼24 h cycles of organismal and cellular activity ubiquitous to mammalian physiology. A prevailing paradigm suggests that timing information flows linearly from rhythmic transcription via protein abundance changes to drive circadian regulation of cellular function. Challenging this view, recent evidence indicates daily variation in many cellular functions arises through rhythmic post-translational regulation of protein activity. We suggest cellular circadian timing primarily functions to maintain proteome homeostasis rather than perturb it. Indeed, although relevant to timekeeping mechanism, daily rhythms of clock protein abundance may be the exception, not the rule. Informed by insights from yeast and mammalian models, we propose that optimal bioenergetic efficiency results from coupled rhythms in mammalian target of rapamycin complex activity, protein synthesis/turnover, ion transport and protein sequestration, which drive facilitatory rhythms in metabolic flux and substrate utilisation. Such daily consolidation of proteome renewal would account for many aspects of circadian cell biology whilst maintaining osmotic homeostasis.

Dysfunction of the circadian clock in the kidney tubule leads to enhanced kidney gluconeogenesis and exacerbated hyperglycemia in diabetes

The circadian clock is a ubiquitous molecular time-keeping mechanism which synchronizes cellular, tissue, and systemic biological functions with 24-hour environmental cycles. Local circadian clocks drive cell type- and tissue-specific rhythms and their dysregulation has been implicated in pathogenesis and/or progression of a broad spectrum of diseases. However, the pathophysiological role of intrinsic circadian clocks in the kidney of diabetics remains unknown. To address this question, we induced type I diabetes with streptozotocin in mice devoid of the circadian transcriptional regulator BMAL1 in podocytes (cKOp mice) or in the kidney tubule (cKOt mice). There was no association between dysfunction of the circadian clock and the development of diabetic nephropathy in cKOp and cKOt mice with diabetes. However, cKOt mice with diabetes exhibited exacerbated hyperglycemia, increased fractional excretion of glucose in the urine, enhanced polyuria, and a more pronounced kidney hypertrophy compared to streptozotocin-treated control mice. mRNA and protein expression analyses revealed substantial enhancement of the gluconeogenic pathway in kidneys of cKOt mice with diabetes as compared to diabetic control mice. Transcriptomic analysis along with functional analysis of cKOt mice with diabetes identified changes in multiple mechanisms directly or indirectly affecting the gluconeogenic pathway. Thus, we demonstrate that dysfunction of the intrinsic kidney tubule circadian clock can aggravate diabetic hyperglycemia via enhancement of gluconeogenesis in the kidney proximal tubule and further highlight the importance of circadian behavior in patients with diabetes.

NF-κB modifies the mammalian circadian clock through interaction with the core clock protein BMAL1

In mammals, the circadian clock coordinates cell physiological processes including inflammation. Recent studies suggested a crosstalk between these two pathways. However, the mechanism of how inflammation affects the clock is not well understood. Here, we investigated the role of the proinflammatory transcription factor NF-κB in regulating clock function. Using a combination of genetic and pharmacological approaches, we show that perturbation of the canonical NF-κB subunit RELA in the human U2OS cellular model altered core clock gene expression. While RELA activation shortened period length and dampened amplitude, its inhibition lengthened period length and caused amplitude phenotypes. NF-κB perturbation also altered circadian rhythms in the master suprachiasmatic nucleus (SCN) clock and locomotor activity behavior under different light/dark conditions. We show that RELA, like the clock repressor CRY1, repressed the transcriptional activity of BMAL1/CLOCK at the circadian E-box cis-element. Biochemical and biophysical analysis showed that RELA binds to the transactivation domain of BMAL1. These data support a model in which NF-kB competes with CRY1 and coactivator CBP/p300 for BMAL1 binding to affect circadian transcription. This is further supported by chromatin immunoprecipitation analysis showing that binding of RELA, BMAL1 and CLOCK converges on the E-boxes of clock genes. Taken together, these data support a significant role for NF-κB in directly regulating the circadian clock and highlight mutual regulation between the circadian and inflammatory pathways.

Cellular Senescence and its Impact on the Circadian Clock

Aging is one of the greatest risk factors for chronic non-communicable diseases, and cellular senescence is one of the major causes of aging and age-related diseases. The persistent presence of senescent cells in late life seems to cause disarray in a tissue-specific manner. Aging disrupts the circadian clock system, which results in the development of many age-related diseases such as metabolic syndrome, cancer, cardiac diseases, and sleep disorders and an increased susceptibility to infections. In this review, we first discuss cellular senescence and some of its basic characteristics and detrimental roles. Then, we discuss a relatively unexplored topic on the link between cellular senescence and the circadian clock and attempt to determine whether cellular senescence could be the underlying factor for circadian clock disruption.