Muscle insulin sensitivity and glucose metabolism are controlled by the intrinsic muscle clock

A comprehensive view of muscle glucose uptake: regulation by insulin, contractile activity, and exercise

Skeletal muscle is the main site of glucose deposition in the body during meals and the major glucose utilizer during physical activity. Although in both instances the supply of glucose from the circulation to the muscle is of paramount importance, in most conditions the rate-limiting step in glucose uptake, storage, and utilization is the transport of glucose across the muscle cell membrane. This step is dependent upon the translocation of the insulin- and contraction-responsive glucose transporter GLUT4 from intracellular storage sites to the sarcolemma and T tubules. Here, we first analyze how glucose can traverse the capillary wall into the muscle interstitial space. We then review the molecular processes that regulate GLUT4 translocation in response to insulin and muscle contractions and the methodologies utilized to unravel them. We further discuss how physical activity and inactivity, respectively, lead to increased and decreased insulin action in muscle and touch upon sex differences in glucose metabolism. Although many key processes regulating glucose uptake in muscle are known, the advent of newer and bioinformatics tools has revealed further molecular signaling processes reaching a staggering level of complexity. Much of this molecular mapping has emerged from cellular and animal studies and more recently from application of a variety of -omics in human tissues. In the future, it will be imperative to validate the translatability of results drawn from experimental systems to human physiology.

Muscle peripheral circadian clock drives nocturnal protein degradation via raised Ror/Rev-erb balance and prevents premature sarcopenia

How central and peripheral circadian clocks regulate protein metabolism and affect tissue mass homeostasis has been unclear. Circadian shifts in the balance between anabolism and catabolism control muscle growth rate in young zebrafish independent of behavioral cycles. Here, we show that the ubiquitin-proteasome system (UPS) and autophagy, which mediate muscle protein degradation, are each upregulated at night under the control of the muscle peripheral clock. Perturbation of the muscle transcriptional molecular clock disrupts nocturnal proteolysis, increases muscle growth measured over 12 h, and compromises muscle function. Mechanistically, the shifting circadian balance of Ror and Rev-erb regulates nocturnal UPS, autophagy, and muscle growth through altered TORC1 activity. Although environmental zeitgebers initially mitigate defects, lifelong muscle clock inhibition reduces muscle size and growth rate, accelerating aging-related loss of muscle mass and function. Circadian misalignment such as shift work, sleep deprivation, or dementia may thus unsettle muscle proteostasis, contributing to muscle wasting and sarcopenia.

Cancer-induced FOXP1 disrupts and reprograms skeletal-muscle circadian transcription in cachexia

Cancer cachexia is a debilitating metabolic disorder characterized by involuntary loss of body and muscle mass, leading to increased morbidity and mortality. We previously found that forkhead box P1 (FoxP1) upregulation in skeletal muscle causes muscle wasting and is required for muscle wasting in response to cancer. However, transcriptional networks targeted by FoxP1 in skeletal muscles undergoing cancer-induced wasting remain largely unknown. Here, we identify FoxP1 as a key disruptor of the skeletal-muscle clock in response to cancer that reprograms circadian patterns of gene expression at cachexia onset. Specifically, we show that cancer-induced FoxP1 rewires the skeletal-muscle circadian transcriptome toward pathways associated with muscle wasting and disrupts the temporal patterning of pathways governing glucose, lipid, and oxidative metabolism. These findings thus implicate cancer/disease-specific functions of FOXP1 in the disruption and reprograming of the skeletal-muscle circadian transcriptome, which may contribute to muscle wasting and the development of cachexia.

Rhythmic TDP-43 affects RNA splicing of USP13, resulting in alteration of BMAL1 ubiquitination

Circadian rhythm disorders are common characteristics of neurodegenerative diseases. The pathological aggregation of transactive response DNA-binding protein 43 (TDP-43) is associated with multiple neurodegenerative diseases, such as amyotrophic lateral sclerosis. However, the relationship between TDP-43 and circadian rhythm remains unknown. Here, we found that TDP-43 is rhythmically expressed both in vivo and in vitro. TDP-43 knockdown affected the expression of circadian genes, including BMAL1, CLOCK, CRY1, and PER2, and impaired autonomous circadian wheel behavior, cognitive functions, and balance abilities in mice. Furthermore, TDP-43 knockdown induced aberrant splicing of ubiquitin-specific peptidase 13 (USP13) and blocked USP13 rhythmic expression, enhancing the ubiquitination of BMAL1. Meanwhile, TDP-43 knockdown altered the rhythmic expression of phospho-AMPKα (Thr172) and platelet-type phosphofructokinase (PFKP), which may change cellular glucose uptake and ATP production. Our findings further the understanding of the role of TDP-43 dysfunction in circadian rhythm disruption in neurodegenerative diseases and provide new mechanistic evidence supporting the interaction between circadian rhythm disruption and neurodegeneration.

Enhanced metabolic adaptations following late dark phase wheel running in high-fat diet-fed mice

Exercise interventions represent an effective strategy to prevent and treat metabolic diseases and the time-of-day-dependent effects of exercise on metabolic outcomes are becoming increasingly apparent. We aimed to study the influence of time-restricted wheel running on whole-body energy and glucose homeostasis. Male, 8-week-old, C57BL/6NTac mice were fed either a 60% high-fat diet (HFD) or a 10% low-fat diet (LFD) for 4 weeks. Following this, mice were given access to a running wheel between zeitgeber time (ZT) 12-16 (early dark phase) or ZT 20-0 (late dark phase). Sedentary mice had access to a permanently locked wheel. Mice were housed under these conditions in metabolic chambers for 4 weeks in which LFD and HFD conditions were maintained. Following the exercise intervention, body composition and glucose tolerance were assessed. Wheel running during either the early or late dark phase resulted in metabolic improvements such as attenuation in body weight gain, enhanced glucose tolerance and reduced ectopic lipid deposition. However, late dark phase exercise resulted in a greater reduction in body weight gain, as well as enhanced metabolic flexibility and insulin sensitivity. Our data suggest that late dark phase versus early dark phase exercise confers greater metabolic adaptations in HFD-fed mice.

Circadian clock controlled glycolipid metabolism and its relevance to disease management

The circadian clock is a critical regulator of physiological rhythms, orchestrating metabolic processes to adapt to daily environmental changes. This review focuses on the intricate relationship between circadian regulation and glycolipid metabolism, with implications for metabolic diseases. Central and peripheral clocks coordinate the rhythmic expression of key enzymes and transporters, ensuring glycolipid homeostasis. Disruptions to these rhythms can result in metabolic disorders characterized by altered glucose utilization, insulin sensitivity, and lipid storage. The molecular mechanisms underlying these processes include transcriptional-translational feedback loops involving clock factors that regulate glycolipid metabolism. Emerging therapeutic strategies, such as pharmacological and dietary interventions, highlight the translational potential of circadian biology. This review underscores the importance of circadian rhythm maintenance for glycolipid metabolism and its role in preventing metabolic disorders. Further elucidation of the molecular mechanisms linking circadian regulation to glycolipid metabolism could pave the way for precision medicine approaches tailored to individual circadian profiles.

Frequent Shifts During Chronic Jet Lag Uncouple Liver Rhythms From the Light Cycle in Male Mice

Circadian disruption is pervasive in modern society and associated with increased risk of disease. Chronic jet lag paradigms are popular experimental tools aiming to emulate human circadian disruption experienced during rotating and night shift work. Chronic jet lag induces metabolic phenotypes tied to liver and systemic functions, yet lack of a clear definition for how rhythmic physiology is impaired under these conditions hinders the ability to identify the underlying molecular mechanisms. Here, we compared 2 common chronic jet lag paradigms and found that neither induced arrythmicity of the liver and each had distinct effects on rhythmicity. Instead, more frequent 8-h forward shifts of the light schedule induced more severe misalignment and non-fasted hyperglycemia. Every other day shifts eventually uncoupled behavioral and hepatic rhythms from the light cycle, reminiscent of free-running conditions. These results point to misalignment, not arrhythmicity, as the initial disturbance tied to metabolic dysfunction in environmental circadian disruption and highlight considerations for the interpretation and design of chronic jet lag studies.

Rhythm is essential: Unraveling the relation between the circadian clock and cancer

Physiological processes such as the sleep-wake cycle, metabolism, hormone secretion, neurotransmitter release, sensory capabilities, and a variety of behaviors, including sleep, are controlled by a circadian rhythm adapted to 24-hour day-night periodicity. Disruption of circadian rhythm may lead to the risks of numerous diseases, including cancers. Several epidemiological and clinical data reveal a connection between the disruption of circadian rhythms and cancer. On the contrary, oncogenic processes may suppress the homeostatic balance imposed by the circadian clock. The integration of circadian biology into cancer research offers new options for making cancer treatment more effective, and the pharmacological modulation of core clock genes is a new approach in cancer therapy. This review highlights the role of the circadian clock in tumorigenesis, how clock disruption alters the tumor microenvironment, and discusses how pharmacological modulation of circadian clock genes can lead to new therapeutic options.

Scheduled Exercise Partially Offsets Alcohol-Induced Clock Dysfunction in Skeletal Muscle and Liver of Female Mice

Binge and chronic alcohol intake impair skeletal muscle and liver circadian clocks. Scheduled exercise is suggested to protect against circadian misalignment, like that induced by alcohol. It was tested whether scheduled, voluntary daily wheel running would protect the gastrocnemius and liver clocks against alcohol-induced perturbations. Female C57BL6/Hsd mice were assigned to 1 of 4 groups: control-sedentary (CON SED, n = 26), control-exercise (CON EX, n = 28), alcohol-sedentary (ETOH SED, n = 27), or alcohol-exercise (ETOH EX, n = 25). Exercise mice were granted access to running wheels for 2 h/day (ZT13-15) while ETOH mice consumed alcohol-containing liquid diet for 6 weeks. Tissues were collected every 4 h starting at ZT12 from 4-5 mice/group and were used for RNA/cDNA/RT-PCR (gastrocnemius and liver) and Western blotting (gastrocnemius). A second cohort of mice were weaned off alcohol, given regular chow, and continued daily exercise (2 h/day) for ~2 weeks. Then, all mice (EX and SED) were given 24-h wheel access for 1 week to assess cyclic running behaviors during abstinence. While alcohol differentially disrupted muscle and liver clocks in sedentary mice, differences between exercised groups were minimized. BMAL1 protein expression increased in the nuclear-enriched fraction in the gastrocnemius of both exercise groups compared to both sedentary groups. In the second cohort, wheel running was increased in ETOH EX compared to ETOH SED in the dark cycle. In the light cycle, ETOH mice ran less than CON mice, and EX mice ran less than SED mice despite all mice receiving chow diet and no EtOH. Overall, scheduled wheel running partially offset the alcohol-induced perturbations in the muscle and liver clock while ETOH and EX both influenced the timing of subsequent activity after the dietary intervention ended.

Melatonin and Exercise Restore Myogenesis and Mitochondrial Dynamics Deficits Associated With Sarcopenia in iMS-Bmal1-/- Mice

Sarcopenia, a condition associated with aging, involves progressive loss of muscle mass, strength, and function, leading to impaired mobility, health, and increased mortality. The underlying mechanisms remain unclear, which limits the development of effective therapeutic interventions. Emerging evidence implicates chronodisruption as a key contributor to sarcopenia, emphasizing the role of Bmal1, a circadian clock gene critical for muscle integrity and mitochondrial function. In a skeletal muscle-specific and inducible Bmal1 knockout model (iMS-Bmal1-/-), we observed hallmark features of sarcopenia, including disrupted rhythms, impaired muscle function, and mitochondrial dysfunction. Exercise and melatonin treatment reversed these deficits independently of Bmal1. Building on these findings, the present study elucidates several mechanisms underlying these changes and the pathways by which melatonin and exercise exert their beneficial effects. Our findings indicate that iMS-Bmal1-/- mice exhibit reduced expression of satellite cell and muscle regulatory factors, indicating impaired muscle regeneration. While mitochondrial respiration remained unchanged, notable alterations in mitochondrial dynamics disrupted mitochondria in skeletal muscle. In addition, these mice showed alterations in muscle energy metabolism, compromised antioxidant defense, and inflammatory response. Remarkably, exercise and/or melatonin successfully mitigated these deficits, restoring muscle health in Bmal1-deficient mice. These findings position exercise and melatonin as promising therapeutic candidates for combating sarcopenia and emphasize the need to elucidate the molecular pathways underlying their protective effects.

Control of circadian muscle glucose metabolism through the BMAL1-HIF axis in obesity

Disruptions of circadian rhythms are widespread in modern society and lead to accelerated and worsened symptoms of metabolic syndrome. In healthy mice, the circadian clock factor BMAL1 is required for skeletal muscle function and metabolism. However, the importance of muscle BMAL1 in the development of metabolic diseases, such as diet-induced obesity (DIO), remains unclear. Here, we demonstrate that skeletal muscle-specific BMAL1-deficient mice exhibit worsened glucose tolerance upon high-fat diet feeding, despite no evidence of increased weight gain. Metabolite profiling from Bmal1-deficient muscles revealed impaired glucose utilization specifically at early steps in glycolysis that dictate the switch between anabolic and catabolic glucose fate. We provide evidence that this is due to abnormal control of the nutrient stress-responsive hypoxia-inducible factor (HIF) pathway. Genetic HIF1α stabilization in muscle Bmal1-deficient mice restores glucose tolerance and expression of 217/736 dysregulated genes during DIO, including glycolytic enzymes. Together, these data indicate that during DIO, skeletal muscle BMAL1 is an important regulator of HIF-driven glycolysis and metabolic flexibility, which influences the development of high-fat-diet-induced glucose intolerance.

The metabolic significance of peripheral tissue clocks

The circadian clock is a transcriptional-translational feedback loop which oscillates in virtually all nucleated cells of the body. In the decades since its discovery, it has become evident that the molecular clockwork is inextricably linked to energy metabolism. Given the frequency with which metabolic dysfunction and clock disruption co-occur, understanding why and how clock and metabolic processes are reciprocally coupled will have important implications for supporting human health and wellbeing. Here, we discuss the relevance of molecular clock function in metabolic tissues and explore its role not only as a driver of day-night variation in gene expression, but as a key mechanism for maintaining metabolic homeostasis in the face of fluctuating energy supply and demand.

Characterizing 24-Hour Skeletal Muscle Gene Expression Alongside Metabolic and Endocrine Responses Under Diurnal Conditions

CONTEXT

Skeletal muscle plays a central role in the storage, synthesis, and breakdown of nutrients, yet little research has explored temporal responses of this human tissue, especially with concurrent measures of systemic biomarkers of metabolism.

OBJECTIVE

To characterize temporal profiles in skeletal muscle expression of genes involved in carbohydrate metabolism, lipid metabolism, circadian clocks, and autophagy and descriptively relate them to systemic metabolites and hormones during a controlled laboratory protocol.

METHODS

Ten healthy adults (9M/1F, [mean ± SD] age 30 ± 10 years; BMI 24.1 ± 2.7 kg·m-2) rested in the laboratory for 37 hours with all data collected during the final 24 hours (08:00-08:00 hours). Participants ingested hourly isocaloric liquid meal replacements alongside appetite assessments during waking before a sleep opportunity from 22:00 to 07:00 hours. Blood samples were collected hourly for endocrine and metabolite analyses, with muscle biopsies occurring every 4 hours from 12:00 to 08:00 hours the following day to quantify gene expression.

RESULTS

Plasma insulin displayed diurnal rhythmicity peaking at 18:04 hours. Expression of skeletal muscle genes involved in carbohydrate metabolism (Name, Acrophase [hours]: GLUT4, 14:40; PPARGC1A, 16:13; HK2, 18:24) and lipid metabolism (FABP3, 12:37; PDK4, 05:30; CPT1B, 12:58) displayed 24-hour rhythmicity that reflected the temporal rhythm of insulin. Equally, circulating glucose (00:19 hours), nonesterified fatty acids (04:56), glycerol (04:32), triglyceride (23:14), urea (00:46), C-terminal telopeptide (05:07), and cortisol (22:50) concentrations also all displayed diurnal rhythmicity.

CONCLUSION

Diurnal rhythms were present in human skeletal muscle gene expression as well systemic metabolites and hormones under controlled diurnal conditions. The temporal patterns of genes relating to carbohydrate and lipid metabolism alongside circulating insulin are consistent with diurnal rhythms being driven in part by the diurnal influence of cyclic feeding and fasting.

The circadian rhythm: A new target of natural products that can protect against diseases of the metabolic system, cardiovascular system, and nervous system

BACKGROUND

The treatment of metabolic system, cardiovascular system, and nervous system diseases remains to be explored. In the internal environment of organisms, the metabolism of substances such as carbohydrates, lipids and proteins (including biohormones and enzymes) exhibit a certain circadian rhythm to maintain the energy supply and material cycle needed for the normal activities of organisms. As a key factor for the health of organisms, the circadian rhythm can be disrupted by pathological conditions, and this disruption accelerates the progression of diseases and results in a vicious cycle. The current treatments targeting the circadian rhythm for the treatment of metabolic system, cardiovascular system, and nervous system diseases have certain limitations, and the identification of safer and more effective circadian rhythm regulators is needed.

AIM OF THE REVIEW

To systematically assess the possibility of using the biological clock as a natural product target for disease intervention, this work reviews a range of evidence on the potential effectiveness of natural products targeting the circadian rhythm to protect against diseases of the metabolic system, cardiovascular system, and nervous system. This manuscript focuses on how natural products restore normal function by affecting the amplitude of the expression of circadian factors, sleep/wake cycles and the structure of the gut microbiota.

KEY SCIENTIFIC CONCEPTS OF THE REVIEW

This work proposes that the circadian rhythm, which is regulated by the amplitude of the expression of circadian rhythm-related factors and the sleep/wake cycle, is crucial for diseases of the metabolic system, cardiovascular system and nervous system and is a new target for slowing the progression of diseases through the use of natural products. This manuscript provides a reference for the molecular modeling of natural products that target the circadian rhythm and provides a new perspective for the time-targeted action of drugs.

Morning sprint interval training produces greater physical performance adaptations than evening training in soccer players

While the literature provides evidence supporting the effectiveness of evening sprint interval training (SIT), there remains a limited amount of research investigating the effects of morning SIT among soccer athletes. This study examined the effects of morning versus evening SIT during the preparatory phase on physical and physiological performance in collegiate soccer players. Thirty male players volunteered to participate in the study and were randomly divided into morning, evening, or control groups, each group consisting of 10 subjects. The evaluation of physical (countermovement vertical jump [CMVJ], 20-m sprint, Illinois change of direction, and Yo-Yo intermittent recovery test level 1 [Yo-Yo IR1]) and physiological (graded exercise test and Wingate anaerobic power test) performance took place prior to and following the 7-week SIT, both in the morning and evening testing sessions. Both training groups showed significant improvements in physical and physiological performance from pre- to post-training, regardless of testing session timing (p < 0.05). The morning SIT group exhibited greater adaptive changes (p < 0.05) compared to the evening SIT group in CMVJ, 20-m sprint, Yo-Yo IR1, peak and mean power outputs at both the morning and evening testing sessions. These findings suggest that male soccer players can benefit from both morning and evening SIT sessions; however, conducting SIT in the morning could result in greater adaptive changes than evening training. To optimize physical performance adaptations, coaches should schedule SIT sessions in the morning. It is recommended to schedule conditioning workouts in the morning and team practices in the afternoon for optimal adaptations.

Aerobic exercise timing affects mitochondrial dynamics and insulin resistance by regulating the circadian clock protein expression and NAD+-SIRT1-PPARα-MFN2 pathway in the skeletal muscle of high-fat-diet-induced diabetes mice

The circadian clock regulates mitochondrial function and affects time-dependent metabolic responses to exercise. The present study aimed to determine the effects of aerobic exercise timing at the light-dark phase on the proteins expression of the circadian clock, mitochondrial dynamics, and, NAD+-SIRT1-PPARα axis in skeletal muscle of high-fat diet-induced diabetic mice. In this experimental study, thirty male mice were randomly assigned into two groups based on time: the early light phase, ZT3, and the early dark phase, ZT15, and three groups at each time: (1) Healthy Control (HC), (2) Diabetic Control (DC), and (3) Diabetic + Exercise (DE). Diabetes was induced by 5 weeks of feeding with a high-fat diet and Streptozotocin injection. Following confirmation of diabetes, animals underwent treadmill running at ZT3 and ZT15 for eight-weeks (5 days, 60-80 min, 50-60%Vmax). The expression of proteins of muscle aryl-hydrocarbon receptor nuclear translocator-like-1 (BMAL1), period-2 (PER2), mitofusin-2 (MFN2), dynamin-related proteins-1 (DRP-1), glucose transporter (GLUT4), sirtuin-1 (SIRT1), peroxisome proliferator-activated receptor-alpha (PPARα), and nicotinamide adenine dinucleotide (NAD+) level were analyzed in gastrocnemius muscle at both exercise times. The results showed that aerobic exercise at both times reversed the dysregulation of the diabetes-induced skeletal muscle clock by increasing the BMAL1 and PER2 protein levels. Aerobic exercise, especially at ZT15 compared to ZT3, increased GLUT4-mediated glucose uptake, and improved the diabetes-induced imbalance of mitochondrial fusion-fission by a significant increase in MFN2 protein level. Moreover, time-dependent aerobic exercise only at ZT15 increased the SIRT1 and PPARα protein levels and reduced diabetes-induced hyperglycemia. However, the aerobic exercise timing could not restore the attenuation of diabetes-induced NAD+ levels and DRP-1 protein. Our findings demonstrated that the synchronization of aerobic exercise with the circadian rhythm of NAD+-SIRT1 may boost MFN2-mediated mitochondrial fusion by activating the BMAL1-PER2-SIRT1-PPARα axis in the skeletal muscle of diabetic mice and be more effective in facilitating glycemic control and insulin resistance.

Unraveling the role of muscle mass and strength in predicting type 2 diabetes risk: a systematic review

INTRODUCTION

Skeletal muscle is the largest insulin-sensitive tissue in the human body, alteration in muscle mass and strength substantially impact glucose metabolism. This systematic review aims to investigate further the relationship between muscle mass and strength towards type 2 diabetes mellitus (T2DM) incidence.

METHODS

This systematic review included cohort studies that examinedthe relationship between muscle mass and/or muscle strength on T2DM incidence. A comprehensive search was conducted across PubMed, EBSCO, ProQuest, and Google scholar employing specific Medical Subject Headings (MeSH) and relevant keywords related to or synonymous with "muscle mass", "muscle strength", and "Type 2 Diabetes Mellitus incidence".

RESULTS

Twenty-five cohort studies were included, 11 studies on muscle mass and 16 studies on muscle strength. Participants included were 278,475 for muscle mass and 400,181 for muscle strength. Skeletal muscle mass normalized to body weight (SMM/BW), appendicular skeletal musce mass normalized to body weight (ASM/BW), and handgrip strength normalized to body mass index (HGS/BMI) consistently demonstrate significant inverse association with T2DM even after sex and/or BMI stratification. Handgrip strength normalized to body weight (HGS/BW) demonstrates a strong inverse association with T2DM incidence, however, adiposity should be considered.

CONCLUSION

Muscle mass and strength demonstrate strong association with T2DM incidence. Adiposity, a key T2DM risk factor, should also be assessed through a simple BMI or a sophisticated technique with BIA or CT-scan. The combination of muscle variables and adiposity could further enhance T2DM risk assessment. However, T2DM risks are multifactorial, with various contributing factors, further large-scale studies are needed to validate these findings.

Timing of resistance exercise and cardiometabolic outcomes in adults with prediabetes: a secondary analysis

The objective of this study was to explore whether the time of day (AM vs. PM) resistance exercise is performed influences glucose and insulin concentrations, body composition, and muscular strength in adults with prediabetes. A secondary data analysis was conducted using data from the "Resist Diabetes" study, a phase II exercise intervention. Participants (age: 59.9 ± 5.4 yr; BMI: 33 ± 3.7 kg/m2) with prediabetes and overweight or obesity were categorized into AM (n = 73) or PM (n = 80) exercisers based on when they completed all of their supervised exercise sessions during a 12-wk, 2×/wk resistance exercise intervention. Blood glucose and insulin derived from oral glucose tolerance tests, body composition, and muscular strength were assessed pre- and post resistance exercise training. Inverse propensity score weighting approach was used to estimate the efficacy of AM versus PM exercise on the change of clinical responses. Paired samples t test was used to compare pre-/post-outcomes within AM and PM groups. No differences between AM and PM exercisers were detected in the change in glucose or insulin area under the curve (AUC), body composition, or muscular strength. When exploring within-group changes, PM exercisers reduced glucose AUC (change: -800.6 mg/dL·120 min; P = 0.01), whereas no significant change was detected for AM exercisers (change: -426.9 mg/dL·120 min; P = 0.26). Only AM exercisers increased fat-free mass (change: 0.6 kg; P = 0.001). The time of day resistance exercise is performed may have some impact on glucose concentrations and body composition response. Future randomized clinical trials are needed to understand how exercise timing influences cardiometabolic outcomes in at-risk adults.NEW & NOTEWORTHY In this secondary analysis, there was no difference between AM and PM exercisers in blood glucose, insulin, body composition, or muscular strength following 12 wk of supervised exercise. However, examining within-group changes, glucose area under the curve (AUC) was significantly reduced in PM exercisers, but not in AM exercisers.

Ablation of satellite cell-specific clock gene, Bmal1, alters force production, muscle damage, and repair following contractile-induced injury

Following injury, skeletal muscle undergoes repair via satellite cell (SC)-mediated myogenic progression. In SCs, the circadian molecular clock gene, Bmal1, is necessary for appropriate myogenic progression and repair with evidence that muscle molecular clocks can also affect force production. Utilizing a mouse model allowing for inducible depletion of Bmal1 within SCs, we determined contractile function, SC myogenic progression and muscle damage and repair following eccentric contractile-induced injury. At baseline, SC-Bmal1iKO animals exhibited a ~20-25% reduction in normalized force production (ex vivo and in vivo) versus control SC-Bmal1Cntrl and SC-Bmal1iKO untreated littermates (p < .05). Following contractile injury, SC-Bmal1iKO animals displayed reduced muscle damage and subsequent repair post-injury (Dystrophinnegative fibers 24 h: SC-Bmal1Cntrl 199 ± 41; SC-Bmal1iKO 36 ± 13, p < .05) (eMHC+ fibers 7 day: SC-Bmal1Cntrl 217.8 ± 115.5; SC-Bmal1iKO 27.8 ± 17.3; Centralized nuclei 7 day: SC-Bmal1Cntrl 160.7 ± 70.5; SC-Bmal1iKO 46.2 ± 15.7). SC-Bmal1iKO animals also showed reduced neutrophil infiltration, consistent with less injury (Neutrophil content 24 h: SC-Bmal1Cntrl 2.4 ± 0.4; SC-Bmal1iKO 0.4 ± 0.2, % area fraction, p < .05). SC-Bmal1iKO animals had greater SC activation/proliferation at an earlier timepoint (p < .05) and an unexplained increase in activation 7 days post injury. Collectively, these data suggest SC-Bmal1 plays a regulatory role in force production, influencing the magnitude of muscle damage/repair, with an altered SC myogenic progression following contractile-induced muscle injury.

Markers of mitochondrial function and oxidative metabolism in female skeletal muscle do not display intrinsic circadian regulation

Mitochondria are key regulators of metabolism and ATP supply in skeletal muscle, while circadian rhythms influence many physiological processes. However, whether mitochondrial function is intrinsically regulated in a circadian manner in mouse skeletal muscle is inadequately understood. Accordingly, we measured post-absorptive transcript abundance of markers of mitochondrial biogenesis, dynamics, and metabolism (extensor digitorum longus [EDL], soleus, gastrocnemius), protein abundance of electron transport chain complexes (EDL and soleus), enzymatic activity of SDH (tibialis anterior and plantaris), and maximum uncoupled respiration (tibialis anterior) in different skeletal muscles from female C57BL/6NJ mice at four zeitgeber times (ZT), ZT 1, 7, 13, and 19. Our findings demonstrate that markers of mitochondrial function and oxidative metabolism do not display intrinsic time-of-day regulation at the gene, protein, enzymatic, or functional level. The core-clock genes Bmal1 and Dbp exhibited intrinsic circadian rhythmicity in skeletal muscle (i.e., EDL, soleus, gastrocnemius) and circadian amplitude varied by muscle type. These findings demonstrate that female mouse skeletal muscle does not display circadian regulation of markers of mitochondrial function or oxidative metabolism over 24 hours.

Effects of time-of-day on the noradrenaline, adrenaline, cortisol and blood lipidome response to an ice bath

While the effect of time-of-day (morning versus evening) on hormones, lipids and lipolysis has been studied in relation to meals and exercise, there are no studies that have investigated the effects of time-of-day on ice bath induced hormone and lipidome responses. In this crossover-designed study, a group of six women and six men, 26 ± 5 years old, 176 ± 7 cm tall, weighing 75 ± 10 kg, and a BMI of 23 ± 2 kg/m2 had an ice bath (8-12 °C for 5 min) both in the morning and evening on separate days. Absence from intense physical exercise, nutrient intake and meal order was standardized in the 24 h prior the ice baths to account for confounders such as diet or exercise. We collected venous blood samples before and after (5 min and 30 min) the ice baths to measure hormones (noradrenaline, adrenaline, and cortisol) and lipid levels in plasma via liquid chromatography mass spectrometry shotgun lipidomics. We found that ice baths in the morning increase plasma fatty acids more than in the evening. Overall plasma lipid composition significantly differed in-between the morning and evening, and only in the morning ice bathing is accompanied by significantly increased plasma fatty acids from 5.1 ± 2.2% to 6.0 ± 2.4% (P = 0.029) 5 min after and to 6.3 ± 3.1% (P = 0.008) 30 min after. Noradrenaline was not affected by time-of-day and increased significantly immediately after the ice baths in the morning by 127 ± 2% (pre: 395 ± 158 pg/ml, post 5 min: 896 ± 562 pg/ml, P = 0.025) and in the evening by 144 ± 2% (pre: 385 ± 146 pg/ml, post 5 min: 937 ± 547 pg/ml, P = 0.015). Cortisol was generally higher in the morning than in the evening (pre: 179 ± 108 pg/ml versus 91 ± 59 pg/ml, P = 0.013; post 5 min: 222 ± 96 pg/ml versus 101 ± 52 pg/ml, P = 0.001; post 30 min: 190 ± 96 pg/ml versus 98 ± 54 pg/ml, P = 0.009). There was no difference in the hormonal and lipidome response to an ice bath between women and men. The main finding of the study was that noradrenaline, adrenaline, cortisol and plasma lipidome responses are similar after an ice bath in the morning and evening. However, ice baths in the morning increase plasma fatty acids more than in the evening.

Clockwork conditioning: Aligning the skeletal muscle clock with time-of-day exercise for cardiometabolic health

Circadian rhythms have evolved to synchronize gene expression, physiology, and behavior with time-of-day changes in the external environment. In every mammalian cell exists a core clock mechanism that consists of a transcriptional-translational feedback loop that drives rhythmic gene expression. Circadian disruption, as observed in shift workers and genetic mouse models, contributes to the onset and progression of cardiometabolic disorders. The central clock, located in the hypothalamus, is uniquely sensitive to external light cues, while the peripheral clocks are responsive to non-photic stimuli such as feeding and activity in addition to signals from the central clock. Recent research has illustrated the sensitivity of the skeletal muscle circadian clock to exercise timing, offering a promising avenue for therapeutic intervention in cardiometabolic health. Here we provide an in-depth examination of the molecular mechanisms underlying skeletal muscle clock function and its impact on cardiometabolic pathways, including glucose and lipid metabolism, as well as inflammation. To highlight the role of exercise as a time-cue for the skeletal muscle clock, we discuss evidence of exercise-induced shifts in the skeletal muscle clock and the differential response to exercise performed at different times of the day. Furthermore, we present data in support of time-of-day exercise as a potential therapeutic strategy for mitigating cardiometabolic disease burden. By exploring the relationship between the skeletal muscle clock, exercise timing, and cardiometabolic health, we identify new areas for future research and offer valuable insights into novel therapeutic approaches aimed at improving cardiometabolic disease outcomes.

Sarcopenia and cachexia: molecular mechanisms and therapeutic interventions

Sarcopenia is defined as a muscle-wasting syndrome that occurs with accelerated aging, while cachexia is a severe wasting syndrome associated with conditions such as cancer and immunodeficiency disorders, which cannot be fully addressed through conventional nutritional supplementation. Sarcopenia can be considered a component of cachexia, with the bidirectional interplay between adipose tissue and skeletal muscle potentially serving as a molecular mechanism for both conditions. However, the underlying mechanisms differ. Recognizing the interplay and distinctions between these disorders is essential for advancing both basic and translational research in this area, enhancing diagnostic accuracy and ultimately achieving effective therapeutic solutions for affected patients. This review discusses the muscle microenvironment's changes contributing to these conditions, recent therapeutic approaches like lifestyle modifications, small molecules, and nutritional interventions, and emerging strategies such as gene editing, stem cell therapy, and gut microbiome modulation. We also address the challenges and opportunities of multimodal interventions, aiming to provide insights into the pathogenesis and molecular mechanisms of sarcopenia and cachexia, ultimately aiding in innovative strategy development and improved treatments.

BMAL1 and CLOCK proteins exhibit differential association with mitochondrial dynamics, protein synthesis pathways and muscle strength in human muscle

Murine models lacking CLOCK/BMAL1 proteins in skeletal muscle (SkM) present muscle deterioration and mitochondria abnormalities. It is unclear whether humans with lower levels of these proteins in the SkM have similar alterations. Here we evaluated the association between BMAL1 and CLOCK protein mass with mitochondrial dynamics parameters and molecular and functional SkM quality markers in males. SkM biopsies were taken from the vastus lateralis of 16 male (non-athletes, non-obese and non-diabetic) subjects (8-9 a.m.). The morphology of mitochondria and their interaction with the sarcoplasmic reticulum (mitochondria-SR) were determined using transmission electron microscopy images. Additionally, protein abundance of the OXPHOS complex, mitochondria fusion/fission regulators, mitophagy and signalling proteins related to muscle protein synthesis were measured. To evaluate the quality of SkM, the cross-sectional area and maximal SkM strength were also measured. The results showed that BMAL1 protein mass was positively associated with mitochondria-SR distance, mitochondria size, mitochondria cristae density and mTOR protein mass. On the other hand, CLOCK protein mass was negatively associated with mitochondria-SR interaction, but positively associated with mitochondria complex III, OPA1 and DRP1 protein mass. Furthermore, CLOCK protein mass was positively associated with the protein synthesis signalling pathway (total mTOR, AKT and P70S6K protein mass) and SkM strength. These findings suggest that the BMAL1 and CLOCK proteins play different roles in regulating mitochondrial dynamics and SkM function in males, and that modulation of these proteins could be a potential therapeutic target for treating muscle diseases. KEY POINTS: In murine models, reductions in BMAL1 and CLOCK proteins lead to changes in mitochondria biology and a decline in muscle function. However, this association has not been explored in humans. We found that in human skeletal muscle, a decrease in BMAL1 protein mass is linked to smaller intermyofibrillar mitochondria, lower mitochondria cristae density, higher interaction between mitochondria and sarcoplasmic reticulum, and reduced mTOR protein mass. Additionally, we found that a decrease in CLOCK protein mass is associated with a higher interaction between mitochondria and sarcoplasmic reticulum, lower protein mass of OPA1 and DRP1, which regulates mitochondria fusion and fission, lower protein synthesis signalling pathway (mTOR, AKT and P70S6K protein mass), and decreased skeletal muscle strength. According to our findings in humans, which are supported by previous studies in animals, the mitochondrial dynamics and skeletal muscle function could be regulated differently by BMAL1 and CLOCK proteins. As a result, targeting the modulation of these proteins could be a potential therapeutic approach for treating muscle diseases and metabolic disorders related to muscle.

More than the clock: distinct regulation of muscle function and metabolism by PER2 and RORα

Circadian rhythms, governed by the dominant central clock, in addition to various peripheral clocks, regulate almost all biological processes, including sleep-wake cycles, hormone secretion and metabolism. In certain contexts, the regulation and function of the peripheral oscillations can be decoupled from the central clock. However, the specific mechanisms underlying muscle-intrinsic clock-dependent modulation of muscle function and metabolism remain unclear. We investigated the outcome of perturbations of the primary and secondary feedback loops of the molecular clock in skeletal muscle by specific gene ablation of Period circadian regulator 2 (Per2) and RAR-related orphan receptor alpha (Rorα), respectively. In both models, a dampening of core clock gene oscillation was observed, while the phase was preserved. Moreover, both loops seem to be involved in the homeostasis of amine groups. Highly divergent outcomes were seen for overall muscle gene expression, primarily affecting circadian rhythmicity in the PER2 knockouts and non-oscillating genes in the RORα knockouts, leading to distinct outcomes in terms of metabolome and phenotype. These results highlight the entanglement of the molecular clock and muscle plasticity and allude to specific functions of different clock components, i.e. the primary and secondary feedback loops, in this context. The reciprocal interaction between muscle contractility and circadian clocks might therefore be instrumental to determining a finely tuned adaptation of muscle tissue to perturbations in health and disease. KEY POINTS: Specific perturbations of the primary and secondary feedback loop of the molecular clock result in specific outcomes on muscle metabolism and function. Ablation of Per2 (primary loop) or Rorα (secondary loop) blunts the amplitude of core clock genes, in absence of a shift in phase. Perturbation of the primary feedback loop by deletion of PER2 primarily affects muscle gene oscillation. Knockout of RORα and the ensuing modulation of the secondary loop results in the aberrant expression of a large number of non-clock genes and proteins. The deletion of PER2 and RORα affects muscle metabolism and contractile function in a circadian manner, highlighting the central role of the molecular clock in modulating muscle plasticity.

PRC2-EZH1 contributes to circadian gene expression by orchestrating chromatin states and RNA polymerase II complex stability

Circadian rhythmicity of gene expression is a conserved feature of cell physiology. This involves fine-tuning between transcriptional and post-transcriptional mechanisms and strongly depends on the metabolic state of the cell. Together these processes guarantee an adaptive plasticity of tissue-specific genetic programs. However, it is unclear how the epigenome and RNA Pol II rhythmicity are integrated. Here we show that the PcG protein EZH1 has a gateway bridging function in postmitotic skeletal muscle cells. On the one hand, the circadian clock master regulator BMAL1 directly controls oscillatory behavior and periodic assembly of core components of the PRC2-EZH1 complex. On the other hand, EZH1 is essential for circadian gene expression at alternate Zeitgeber times, through stabilization of RNA Polymerase II preinitiation complexes, thereby controlling nascent transcription. Collectively, our data show that PRC2-EZH1 regulates circadian transcription both negatively and positively by modulating chromatin states and basal transcription complex stability.

Inflammation o'clock: interactions of circadian rhythms with inflammation-induced skeletal muscle atrophy

Circadian rhythms are ∼24 h cycles evident in behaviour, physiology and metabolism. The molecular mechanism directing circadian rhythms is the circadian clock, which is composed of an interactive network of transcription-translation feedback loops. The core clock genes include Bmal1, Clock, Rev-erbα/β, Per and Cry. In addition to keeping time, the core clock regulates a daily programme of gene expression that is important for overall cell homeostasis. The circadian clock mechanism is present in all cells, including skeletal muscle fibres, and disruption of the muscle clock is associated with changes in muscle phenotype and function. Skeletal muscle atrophy is largely associated with a lower quality of life, frailty and reduced lifespan. Physiological and genetic modification of the core clock mechanism yields immune dysfunction, alters inflammatory factor expression and secretion and is associated with skeletal muscle atrophy in multiple conditions, such as ageing and cancer cachexia. Here, we summarize the possible interplay between the circadian clock modulation of immune cells, systemic inflammatory status and skeletal muscle atrophy in chronic inflammatory conditions. Although there is a clear disruption of circadian clocks in various models of atrophy, the mechanism behind such alterations remains unknown. Understanding the modulatory potential of muscle and immune circadian clocks in inflammation and skeletal muscle health is essential for the development of therapeutic strategies to protect skeletal muscle mass and function of patients with chronic inflammation.

Hierarchy or Heterarchy of Mammalian Circadian Timekeepers?

Mammalian circadian biologists commonly characterize the relation between circadian clocks as hierarchical, with the clock in the suprachiasmatic nucleus at the top of the hierarchy. The lineage of research since the suprachiasmatic nucleus (SCN) was first identified as the clock in mammals has challenged this perspective, revealing clocks in peripheral tissues, showing that they respond to their own zeitgebers, coordinate oscillations among themselves, and in some cases modify the behavior of the SCN. Increasingly circadian timekeepers appear to constitute a heterarchical network, with control distributed and operating along multiple pathways. One reason for the continued invocation of hierarchy in mammalian circadian biology is that it is difficult to understand how a heterarchical system could operate effectively so as to maintain the organism. Evolved mechanisms, however, need not respect hierarchy and those that have survived have demonstrated the ability of heterarchical organizaton to maintain organisms.

Transcriptional analysis of cancer cachexia: conserved and unique features across preclinical models and biological sex

Studies suggest heterogeneity in cancer cachexia (CC) among models and biological sexes, yet examinations comparing models and sexes are scarce. We compared the transcriptional landscape of skeletal muscle across murine CC models and biological sexes during early and late CC. Global gene expression analyses were performed on gastrocnemius [Lewis lung carcinoma (LLC)], quadriceps (KPC-pancreatic), and tibialis anterior [Colon-26 (C26)-colorectal and ApcMin/+] muscles across biological sexes. Differentially expressed genes (DEGs) were identified using an adj-P value of <0.05, followed by pathway and computational cistrome analyses. Integrating all controls, early and late stages of all models and sexes revealed up to 68% of DEGs and pathways were enriched at early and late CC, indicating a conserved transcriptional profile during CC development. Comparing DEGs and pathways within sexes and across models, in early CC, the transcriptional response was highly heterogeneous. At late stage, 11.5% of upregulated and 10% of downregulated genes were shared between models in males, whereas 18.9% of upregulated and 7% of downregulated DEGs were shared in females. Shared DEGs were enriched in proteasome and mitophagy/autophagy pathways (upregulated), and downregulation of energy metabolism pathways in males only. Between sexes, though the proportion of shared DEGs was low (<16%), similar pathway enrichment was observed, including proteasome and mitophagy at late-stage CC. In early CC, oncostatin M receptor (Osmr) upregulation was the only commonality across all models and sexes, whereas CLOCK and ARNTL/BMAL1 were predicted transcriptional factors associated with dysregulations in all three male models. This study highlights sex and model differences in CC progression and suggests conserved transcriptional changes as potential therapeutic targets.NEW & NOTEWORTHY This study is among the first to integrate and compare the skeletal muscle transcriptional landscape across multiple preclinical models and biological sexes. We highlight that 1) early CC transcriptional changes are two-thirds conserved at late stages, 2) DEGs are largely model and sex specific, and 3) transcriptional factors including CLOCK and ARNTL/BMAL1, which influence early CC gene expression, might represent a global therapeutic target with a chance of efficacy across various cancer types.

Exercise training modifies skeletal muscle clock gene expression but not 24-hour rhythmicity in substrate metabolism of men with insulin resistance

Twenty-four hour rhythmicity in whole-body substrate metabolism, skeletal muscle clock gene expression and mitochondrial respiration is compromised upon insulin resistance. With exercise training known to ameliorate insulin resistance, our objective was to test if exercise training can reinforce diurnal variation in whole-body and skeletal muscle metabolism in men with insulin resistance. In a single-arm longitudinal design, 10 overweight and obese men with insulin resistance performed 12 weeks of high-intensity interval training recurrently in the afternoon (between 14.00 and 18.00 h) and were tested pre- and post-exercise training, while staying in a metabolic research unit for 2 days under free-living conditions with regular meals. On the second days, indirect calorimetry was performed at 08.00, 13.00, 18.00, 23.00 and 04.00 h, muscle biopsies were taken from the vastus lateralis at 08.30, 13.30 and 23.30 h, and blood was drawn at least bi-hourly over 24 h. Participants did not lose body weight over 12 weeks, but improved body composition and exercise capacity. Exercise training resulted in reduced 24-h plasma glucose levels, but did not modify free fatty acid and triacylglycerol levels. Diurnal variation of muscle clock gene expression was modified by exercise training with period genes showing an interaction (time × exercise) effect and reduced mRNA levels at 13.00 h. Exercise training increased mitochondrial respiration without inducing diurnal variation. Twenty-four-hour substrate metabolism and energy expenditure remained unchanged. Future studies should investigate alternative exercise strategies or types of interventions (e.g. diet or drugs aiming at improving insulin sensitivity) for their capacity to reinforce diurnal variation in substrate metabolism and mitochondrial respiration. KEY POINTS: Insulin resistance is associated with blunted 24-h flexibility in whole-body substrate metabolism and skeletal muscle mitochondrial respiration, and disruptions in the skeletal muscle molecular circadian clock. We hypothesized that exercise training modifies 24-h rhythmicity in whole-body substrate metabolism and diurnal variation in skeletal muscle molecular clock and mitochondrial respiration in men with insulin resistance. We found that metabolic inflexibility over 24 h persisted after exercise training, whereas mitochondrial respiration increased independent of time of day. Gene expression of Per1-3 and Rorα in skeletal muscle changed particularly close to the time of day at which exercise training was performed. These results provide the rationale to further investigate the differential metabolic impact of differently timed exercise to treat metabolic defects of insulin resistance that manifest at a particular time of day.

Interaction Between Early Meals (Big-Breakfast Diet), Clock Gene mRNA Expression, and Gut Microbiome to Regulate Weight Loss and Glucose Metabolism in Obesity and Type 2 Diabetes

The circadian clock gene system plays a pivotal role in coordinating the daily rhythms of most metabolic processes. It is synchronized with the light-dark cycle and the eating-fasting schedule. Notably, the interaction between meal timing and circadian clock genes (CGs) allows for optimizing metabolic processes at specific times of the day. Breakfast has a powerful resetting effect on the CG network. A misaligned meal pattern, such as skipping breakfast, can lead to a discordance between meal timing and the endogenous CGs, and is associated with obesity and T2D. Conversely, concentrating most calories and carbohydrates (CH) in the early hours of the day upregulates metabolic CG expression, thus promoting improved weight loss and glycemic control. Recently, it was revealed that microorganisms in the gastrointestinal tract, known as the gut microbiome (GM), and its derived metabolites display daily oscillation, and play a critical role in energy and glucose metabolism. The timing of meal intake coordinates the oscillation of GM and GM-derived metabolites, which in turn influences CG expression, playing a crucial role in the metabolic response to food intake. An imbalance in the gut microbiota (dysbiosis) can also reciprocally disrupt CG rhythms. Evidence suggests that misaligned meal timing may cause such disruptions and can lead to obesity and hyperglycemia. This manuscript focuses on the reciprocal interaction between meal timing, GM oscillation, and circadian CG rhythms. It will also review studies demonstrating how aligning meal timing with the circadian clock can reset and synchronize CG rhythms and GM oscillations. This synchronization can facilitate weight loss and improve glycemic control in obesity and those with T2D.

Circadian rhythms in muscle health and diseases

All major life forms from bacteria to humans have internal clocks that regulate essential biological processes in a roughly 24-h cycle. In mammals, the central clock in the suprachiasmatic nucleus (SCN) is historically considered the top of a hierarchical organisation that dominates subordinate clocks in peripheral tissues and dictates the circadian behaviours of an organism. Recent studies, however, underscore the importance of the local circadian oscillators, such as the skeletal muscle clock, in regulating local metabolism and physiology. Studies in animal models show that the muscle peripheral clock per se is required for the expression of genes involved in glucose, lipid, and amino acid metabolism. Disruption of the muscle clock leads to glucose intolerance, insulin resistance, and alterations in muscle size and force. This highlights the vital role of the muscle clock in controlling muscle physiology and metabolism. In humans, a perturbation in the muscle circadian rhythms is seen in metabolic disorders such as type 2 diabetes, and muscle diseases such as dystrophies. Disruption of muscle metabolism is also seen when the internal rhythms are misaligned with the external rhythms (circadian misalignments) as in shift work. Understanding the mechanisms by which the muscle clock regulates circadian functions may help the development of new strategies, such as chronotherapy, to potentially prevent or treat muscle pathologies and maintain muscle health.

Disparity in the effect of partial gravity simulated using a new apparatus on different rat hindlimb muscles

The days of returning to the Moon and landing on Mars are approaching. These long-duration missions present significant challenges, such as changes in gravity, which pose serious threats to human health. Maintaining muscle function and health is essential for successful spaceflight and exploration of the Moon and Mars. This study aimed to observe the adaptation of rat hindlimb muscles to partial gravity conditions by simulating the gravity of space (microgravity (µG)), Moon (1/6G), and Mars (3/8G) using our recently invented ground-based apparatus. A total of 25 rats were included in this study. The rats were divided into five groups: control (1G), sham (1G), simulated Mars (3/8G), simulated Moon (1/6G), and simulated Space (µG). Muscle mass, fiber proportion, and fiber cross-sectional area (CSA) of four types of hindlimb muscles were measured: gastrocnemius (GA), tibialis anterior (TA), extensor digitorum longus (EDL), soleus (Sol). Sol and GA exhibited the most significant alterations in response to the changes in gravity after 10 days of the experiment. A notable decline in muscle mass was observed in the simulated µG, Moon, and Mars groups, with the µG group exhibiting the most noticeable decline. In Sol, a noteworthy decline in the proportion of slow-twitch type I fibers, CSA of slow-twitch type I fibers, and average CSA of the whole muscle fibers was observed in the simulated groups. The GA red, mixed, and white portions were examined, and the GA mixed portion showed significant differences in fiber proportion and CSA. A notable increase in the proportion of slow-twitch type I fibers was observed in the simulated groups, with a significant decrease in CSA of type IIb. In EDL or TA, no discernible changes in muscle mass, fiber proportion, or fiber CSA were observed in any of the five groups. These findings indicate that weight-bearing muscles, such as Sol and GA, are more sensitive to changes in partial gravity. Furthermore, partial gravity is insufficient to preserve the normal physiological and functional properties of the hindlimb muscles. Therefore, targeted muscle interventions are required to ensure astronauts' health and mission success. Furthermore, these findings demonstrate the viability and durability of our ground-based apparatus for partial gravity simulation.

The deleterious effects and potential therapeutic strategy of fluorene-9-bisphenol on circadian activity and liver diseases in zebrafish and mice

Increasing evidence indicates that disturbance of the clock genes, which leads to systemic endocrine perturbation, plays a crucial role in the pathogenesis of metabolic and liver diseases. Fluorene-9-bisphenol (BHPF) is utilized in the manufacturing of plastic materials but its biological effects on liver homeostasis remain unknown. The impacts and involved mechanisms of BHPF on the liver diseases, metabolism, and circadian clock were comprehensively studied by zebrafish and mouse models. The therapeutic effect of melatonin (MT) was also verified. Zebrafish and mouse models with either acute exposure (0.5 and 1 µmol/L, 1-4 days post-fertilization) or chronic oral exposure (0.5 and 50 mg/(kg·2 days), 30 days) were established with various BHPF concentrations. Herein, we identified a crucial role for estrogenic regulation in liver development and circadian locomotor rhythms damaged by BHPF in a zebrafish model. BHPF mice showed chaos in circadian activity through the imbalance of circadian clock component Brain and Muscle Aryl hydrocarbon receptor nuclear translocator-like 1 in the liver and brain. The liver sexual dimorphic alteration along with reduced growth hormone and estrogens played a critical role in damaged glucose metabolism, hepatic inflammation, and fibrosis induced by BHPF. Besides, sleep improvement by exogenous MT alleviated BHPF-related glucose metabolism and liver injury in mice. We proposed the pathogenesis of metabolic and liver disease resulting from BHPF and promising targeted therapy for liver metabolism disorders associated with endocrine perturbation chemicals. These results might play a warning role in the administration of endocrine-disrupting chemicals in everyday life and various industry applications.

Activity and phosphatidylcholine transfer protein interactions of skeletal muscle thioesterase Them2 enable hepatic steatosis and insulin resistance

Thioesterase superfamily member 2 (Them2), a long-chain fatty acyl-CoA thioesterase that is highly expressed in oxidative tissues, interacts with phosphatidylcholine transfer protein (PC-TP) to regulate hepatic lipid and glucose metabolism and to suppress insulin signaling. High-fat diet-fed mice lacking Them2 globally or specifically in skeletal muscle, but not liver, exhibit reduced hepatic steatosis and insulin resistance. Here, we report that the capacity of Them2 in skeletal muscle to promote hepatic steatosis and insulin resistance depends on both its catalytic activity and interaction with PC-TP. Two residues of Them2 catalytic site were mutated (N50A/D65A) to produce the inactive enzyme while maintaining its homotetrameric structure and interaction with PC-TP. Restoration of skeletal muscle expression in Them2-/- mice using recombinant adeno-associated virus revealed that WT, but not N50A/D65A Them2, promoted high-fat diet-induced weight gain and hepatic steatosis. This was accompanied by greater impairment of insulin sensitivity in WT than N50A/D65A Them2. Pharmacological inhibition or genetic ablation of PC-TP attenuated these effects. In reductionist experiments, conditioned medium collected from WT primary cultured myotubes promoted excess lipid accumulation in oleic acid-treated primary cultured hepatocytes relative to Them2-/- myotubes, which was attributable to secreted extracellular vesicles. Reconstitution of Them2 expression in Them2-/- myotubes affirmed the requirements for catalytic activity and PC-TP interactions for extracellular vesicles to promote lipid accumulation in hepatocytes. These studies provide valuable mechanistic insights, whereby Them2 in skeletal muscle promotes hepatic steatosis and establish both Them2 and PC-TP as attractive targets for managing metabolic dysfunction-associated steatotic liver disease.

Skeletal muscle p53-depletion uncovers a mechanism of fuel usage suppression that enables efficient energy conservation

BACKGROUND

The ability of skeletal muscle to respond adequately to changes in nutrient availability, known as metabolic flexibility, is essential for the maintenance of metabolic health and loss of flexibility contributes to the development of diabetes and obesity. The tumour suppressor protein, p53, has been linked to the control of energy metabolism. We assessed its role in the acute control of nutrient allocation in skeletal muscle in the context of limited nutrient availability.

METHODS

A mouse model with inducible deletion of the p53-encoding gene, Trp53, in skeletal muscle was generated using the Cre-loxP-system. A detailed analysis of nutrient metabolism in mice with control and knockout genotypes was performed under ad libitum fed and fasting conditions and in exercised mice.

RESULTS

Acute deletion of p53 in myofibres of mice activated catabolic nutrient usage pathways even under ad libitum fed conditions, resulting in significantly increased overall energy expenditure (+10.6%; P = 0.0385) and a severe nutrient deficit in muscle characterized by depleted intramuscular glucose and glycogen levels (-62,0%; P < 0.0001 and -52.7%; P < 0.0001, respectively). This was accompanied by changes in marker gene expression patterns of circadian rhythmicity and hyperactivity (+57.4%; P = 0.0068). These metabolic changes occurred acutely, within 2-3 days after deletion of Trp53 was initiated, suggesting a rapid adaptive response to loss of p53, which resulted in a transient increase in lactate release to the circulation (+46.6%; P = 0.0115) from non-exercised muscle as a result of elevated carbohydrate mobilization. Conversely, an impairment of proteostasis and amino acid metabolism was observed in knockout mice during fasting. During endurance exercise testing, mice with acute, muscle-specific Trp53 inactivation displayed an early exhaustion phenotype with a premature shift in fuel usage and reductions in multiple performance parameters, including a significantly reduced running time and distance (-13.8%; P = 0.049 and -22.2%; P = 0.0384, respectively).

CONCLUSIONS

These findings suggest that efficient nutrient conservation is a key element of normal metabolic homeostasis that is sustained by p53. The homeostatic state in metabolic tissues is actively maintained to coordinate efficient energy conservation and metabolic flexibility towards nutrient stress. The acute deletion of Trp53 unlocks mechanisms that suppress the activity of nutrient catabolic pathways, causing substantial loss of intramuscular energy stores, which contributes to a fasting-like state in muscle tissue. Altogether, these findings uncover a novel function of p53 in the short-term regulation of nutrient metabolism in skeletal muscle and show that p53 serves to maintain metabolic homeostasis and efficient energy conservation.

Targeted Bmal1 restoration in muscle prolongs lifespan with systemic health effects in aging model

Disruption of the circadian clock in skeletal muscle worsens local and systemic health, leading to decreased muscle strength, metabolic dysfunction, and aging-like phenotypes. Whole-body knockout mice that lack Bmal1, a key component of the molecular clock, display premature aging. Here, by using adeno-associated viruses, we rescued Bmal1 expression specifically in the skeletal muscle fibers of Bmal1-KO mice and found that this engaged the circadian clock and clock output gene expression, contributing to extended lifespan. Time course phenotypic analyses found that muscle strength, mobility, and glucose tolerance were improved with no effects on muscle mass or fiber size or type. A multiomics approach at 2 ages further determined that restored muscle Bmal1 improved glucose handling pathways while concomitantly reducing lipid and protein metabolic pathways. The improved glucose tolerance and metabolic flexibility resulted in the systemic reduction of inflammatory signatures across peripheral tissues, including liver, lung, and white adipose fat. Together, these findings highlight the critical role of muscle Bmal1 and downstream target genes for skeletal muscle homeostasis with considerable implications for systemic health.

Light-phase time-restricted feeding disrupts the muscle clock and insulin sensitivity yet potentially induces muscle fiber remodeling in mice

Skeletal muscle plays a critical role in regulating systemic metabolic homeostasis. It has been demonstrated that time-restricted feeding (TRF) during the rest phase can desynchronize the suprachiasmatic nucleus (SCN) and peripheral clocks, thereby increasing the risk of metabolic diseases. However, the impact of dietary timing on the muscle clock and health remains poorly understood. Here, through the analysis of cycling genes and differentially expressed genes in the skeletal muscle transcriptome, we identified disruptions in muscle diurnal rhythms by 2 weeks of light-phase TRF. Furthermore, compared with ad libitum (AL) feeding mice, 2 weeks of light-phase TRF was found to induce insulin resistance, muscle fiber type remodeling, and changes in the expression of muscle growth-related genes, while both light-phase and dark-phase TRF having a limited impact on bone quality relative to AL mice. In summary, our research reveals that the disruption of the skeletal muscle clock may contribute to the abnormal metabolic phenotype resulting from feeding restricted to the inactive period. Additionally, our study provides a comprehensive omics atlas of the diurnal rhythms in skeletal muscle regulated by dietary timing.

Unresolved questions in the regulation of skeletal muscle insulin action by reactive oxygen species

Reactive oxygen species (ROS) are well-established signaling molecules implicated in a wide range of cellular processes, including both oxidative stress and intracellular redox signaling. In the context of insulin action within its target tissues, ROS have been reported to exert both positive and negative regulatory effects. However, the precise molecular mechanisms underlying this duality remain unclear. This Review examines the complex role of ROS in insulin action, with a particular focus on skeletal muscle. We aim to address three critical aspects: (a) the proposed intracellular pro-oxidative redox shift elicited by insulin, (b) the evidence supporting that redox-sensitive cysteine modifications impact insulin signaling and action, and (c) cellular mechanisms underlying how ROS can paradoxically act as both enhancers and inhibitors of insulin action. This Review underscores the urgent need for more systematic research to identify specific reactive species, redox targets, and the physiological significance of redox signaling in maintaining insulin action and metabolic health, with a particular emphasis on human skeletal muscle.

BMAL1/PGC1α4-FNDC5/irisin axis impacts distinct outcomes of time-of-day resistance exercise

BACKGROUND

Resistance exercise leads to improved muscle function and metabolic homeostasis. Yet how circadian rhythm impacts exercise outcomes and its molecular transduction remains elusive.

METHODS

Human volunteers were subjected to 4 weeks of resistance training protocols at different times of day to assess training outcomes and their associations with myokine irisin. Based on rhythmicity of Fibronectin type III domain containing 5 (FNDC5/irisin), we trained wild type and FNDC5 knockout mice at late active phase (high FNDC5/irisin level) or late rest phase (low FNDC5/irisin level) to analyze exercise benefits on muscle function and metabolic homeostasis. Molecular analysis was performed to understand the regulatory mechanisms of FNDC5 rhythmicity and downstream signaling transduction in skeletal muscle.

RESULTS

In this study, we showed that regular resistance exercises performed at different times of day resulted in distinct training outcomes in humans, including exercise benefits and altered plasma metabolomics. We found that muscle FNDC5/irisin levels exhibit rhythmicity. Consistent with human data, compared to late rest phase (low irisin level), mice trained chronically at late active phase (high irisin level) gained more muscle capacity along with improved metabolic fitness and metabolomics/lipidomics profiles under a high-fat diet, whereas these differences were lost in FNDC5 knockout mice. Mechanistically, Basic helix-loop-helix ARNT like 1 (BMAL1) and Peroxisome proliferative activated receptor, gamma, coactivator 1 alpha 4 (PGC1α4) induce FNDC5/irisin transcription and rhythmicity, and the signaling is transduced via αV integrin in muscle.

CONCLUSION

Together, our results offered novel insights that exercise performed at distinct times of day determines training outcomes and metabolic benefits through the rhythmic regulation of the BMAL1/PGC1α4-FNDC5/irisin axis.

Skeletal muscle BMAL1 is necessary for transcriptional adaptation of local and peripheral tissues in response to endurance exercise training

OBJECTIVE

In this investigation, we addressed the contribution of the core circadian clock factor, BMAL1, in skeletal muscle to both acute transcriptional responses to exercise and transcriptional remodeling in response to exercise training. Additionally, we adopted a systems biology approach to investigate how loss of skeletal muscle BMAL1 altered peripheral tissue homeostasis as well as exercise training adaptations in iWAT, liver, heart, and lung of male mice.

METHODS

Combining inducible skeletal muscle specific BMAL1 knockout mice, physiological testing and standardized exercise protocols, we performed a multi-omic analysis (transcriptomics, chromatin accessibility and metabolomics) to explore loss of muscle BMAL1 on muscle and peripheral tissue responses to exercise.

RESULTS

Muscle-specific BMAL1 knockout mice demonstrated a blunted transcriptional response to acute exercise, characterized by the lack of upregulation of well-established exercise responsive transcription factors including Nr4a3 and Ppargc1a. Six weeks of exercise training in muscle-specific BMAL1 knockout mice induced significantly greater and divergent transcriptomic and metabolomic changes in muscle. Surprisingly, liver, lung, inguinal white adipose and heart showed divergent exercise training transcriptomes with less than 5% of 'exercise-training' responsive genes shared for each tissue between genotypes.

CONCLUSIONS

Our investigation has uncovered the critical role that BMAL1 plays in skeletal muscle as a key regulator of gene expression programs for both acute exercise and training adaptations. In addition, our work has uncovered the significant impact that altered exercise response in muscle and its likely impact on the system plays in the peripheral tissue adaptations to exercise training. Our work also demonstrates that if the muscle adaptations diverge to a more maladaptive state this is linked to increased gene expression signatures of inflammation across many tissues. Understanding the molecular targets and pathways contributing to health vs. maladaptive exercise adaptations will be critical for the next stage of therapeutic design for exercise mimetics.

Lactate transported by MCT1 plays an active role in promoting mitochondrial biogenesis and enhancing TCA flux in skeletal muscle

Once considered as a "metabolic waste," lactate is now recognized as a major fuel for tricarboxylic acid (TCA) cycle. Our metabolic flux analysis reveals that skeletal muscle mainly uses lactate to fuel TCA cycle. Lactate is transported through the cell membrane via monocarboxylate transporters (MCTs) in which MCT1 is highly expressed in the muscle. We analyzed how MCT1 affects muscle functions using mice with specific deletion of MCT1 in skeletal muscle. MCT1 deletion enhances running performance, increases oxidative fibers while decreasing glycolytic fibers, and enhances flux of glucose to TCA cycle. MCT1 deficiency increases the expression of mitochondrial proteins, augments cell respiration rate, and elevates mitochondrial activity in the muscle. Mechanistically, the protein level of PGC-1α, a master regulator of mitochondrial biogenesis, is elevated upon loss of MCT1 via increases in cellular NAD+ level and SIRT1 activity. Collectively, these results demonstrate that MCT1-mediated lactate shuttle plays a key role in regulating muscle functions by modulating mitochondrial biogenesis and TCA flux.

Liver as a nexus of daily metabolic cross talk

Over the course of a day, the circadian clock promotes a homeostatic balance between energy intake and energy expenditure by aligning metabolism with nutrient availability. In mammals, this process is driven by central clocks in the brain that control feeding behavior, the peripheral nervous system, and humoral outputs, as well as by peripheral clocks in non-brain tissues that regulate gene expression locally. Circadian organization of metabolism is critical, as circadian disruption is associated with increased risk of metabolic disease. Emerging evidence shows that circadian metabolism hinges upon inter-organ cross talk involving the liver, a metabolic hub that integrates many facets of systemic energy homeostasis. Here, we review spatiotemporal interactions, mainly metabolite exchange, signaling factors, and hormonal control, between the liver and skeletal muscle, pancreas, gut, microbiome, and adipose tissue. Modern society presents the challenge of circadian disturbances from rotating shift work to social jet lag and 24/7 food availability. Thus, it is important to better understand the mechanisms by which the clock system controls metabolic homeostasis and work toward targeted therapies.

Metabolic plasticity and obesity-associated changes in diurnal postexercise metabolism in mice

BACKGROUND

Circadian disruption is widespread and increases the risk of obesity. Timing of therapeutic interventions may promote coherent and efficient gating of metabolic processes and restore energy homeostasis.

AIM

To characterize the diurnal postexercise metabolic state in mice and to identify the influence of diet-induced obesity on identified outcomes.

METHODS

C57BL6/NTac male mice (6 wks of age) were fed a standard chow or high-fat diet for 5 weeks. At week 5, mice were subjected to a 60-min (16 m/min, 5 % incline) running bout (or sham) during the early rest (day) or early active (night) phase. Tissue and serum samples were collected immediately post-exercise (n = 6/group). In vivo glucose oxidation was measured after oral administration of 13C-glucose via 13CO2 exhalation analysis in metabolic cages. Basal and isoproterenol-stimulated adipose tissue lipolysis was assessed ex vivo for 1 h following exercise.

RESULTS

Lean mice displayed exercise-timing-specific plasticity in metabolic outcomes, including phase-specificity in systemic glucose metabolism and adipose-tissue-autonomous lipolytic activity depending on time of day. Conversely, obesity impaired temporal postexercise differences in whole-body glucose oxidation, as well as the phase- and exercise-mediated induction of lipolysis in isolated adipose tissue. This obesity-induced alteration in diurnal metabolism, as well as the indistinct response to exercise, was observed concomitant with disruption of core clock gene expression in peripheral tissues.

CONCLUSIONS

Overall, high-fat fed obese mice exhibit metabolic inflexibility, which is also evident in the diurnal exercise response. Our study provides physiological insight into exercise timing-dependent aspects in the dynamic regulation of metabolism and the influence of obesity on this biology.

Molecular Pathways Regulating Circadian Rhythm and Associated Diseases

Circadian rhythms, the natural cycles of physical, mental, and behavioral changes that follow a roughly 24-hour cycle, are known to have a profound effect on the human body. Light plays an important role in the regulation of circadian rhythm in human body. When light from the outside enters the eyes, cones, rods, and specialized retinal ganglion cells receive the light signal and transmit it to the suprachiasmatic nucleus of the hypothalamus. The central rhythm oscillator of the suprachiasmatic nucleus regulates the rhythm oscillator of tissues all over the body. Circadian rhythms, the natural cycles of physical, mental, and behavioral changes that follow a roughly 24-hour cycle, are known to have a profound effect on the human body. As the largest organ in the human body, skin plays an important role in the peripheral circadian rhythm regulation system. Like photoreceptor cells in the retina, melanocytes express opsins. Studies show that melanocytes in the skin are also sensitive to light, allowing the skin to "see" light even without the eyes. Upon receiving light signals, melanocytes in the skin release hormones that maintain homeostasis. This process is called "photoneuroendocrinology", which supports the health effects of light exposure. However, inappropriate light exposure, such as prolonged work in dark environments or exposure to artificial light at night, can disrupt circadian rhythms. Such disruptions are linked to a variety of health issues, emphasizing the need for proper light management in daily life. Conversely, harnessing light's beneficial effects through phototherapy is gaining attention as an adjunctive treatment modality. Despite these advancements, the field of circadian rhythm research still faces several unresolved issues and emerging challenges. One of the most exciting prospects is the use of the skin's photosensitivity to treat diseases. This approach could revolutionize how we think about and manage various health conditions, leveraging the skin's unique ability to respond to light for therapeutic purposes. As research continues to unravel the complexities of circadian rhythms and their impact on health, the potential for innovative treatments and improved wellbeing is immense.

Brain-muscle communication prevents muscle aging by maintaining daily physiology

A molecular clock network is crucial for daily physiology and maintaining organismal health. We examined the interactions and importance of intratissue clock networks in muscle tissue maintenance. In arrhythmic mice showing premature aging, we created a basic clock module involving a central and a peripheral (muscle) clock. Reconstituting the brain-muscle clock network is sufficient to preserve fundamental daily homeostatic functions and prevent premature muscle aging. However, achieving whole muscle physiology requires contributions from other peripheral clocks. Mechanistically, the muscle peripheral clock acts as a gatekeeper, selectively suppressing detrimental signals from the central clock while integrating important muscle homeostatic functions. Our research reveals the interplay between the central and peripheral clocks in daily muscle function and underscores the impact of eating patterns on these interactions.

The impact of aerobic exercise timing on BMAL1 protein expression and antioxidant responses in skeletal muscle of mice

It is well known that the adaptations of muscular antioxidant system to aerobic exercise depend on the frequency, intensity, duration, type of the exercise. Nonetheless, the timing of aerobic exercise, related to circadian rhythms or biological clock, may also affect the antioxidant defense system, but its impact remains uncertain. Bain and muscle ARNT-like 1 (BMAL1) is the core orchestrator of molecular clock, which can maintain cellular redox homeostasis by directly controlling the transcriptional activity of nuclear factor erythroid 2-related factor 2 (NRF2). So, our research objective was to evaluate the impacts of aerobic exercise training at various time points of the day on BMAL1 and NRF2-mediated antioxidant system in skeletal muscle. C57BL/6J mice were assigned to the control group, the group exercising at Zeitgeber Time 12 (ZT12), and the group exercising at ZT24. Control mice were not intervened, while ZT12 and ZT24 mice were trained for four weeks at the early and late time point of their active phase, respectively. We observed that the skeletal muscle of ZT12 mice exhibited higher total antioxidant capacity and lower reactive oxygen species compared to ZT24 mice. Furthermore, ZT12 mice improved the colocalization of BMAL1 with nucleus, the protein expression of BMAL1, NRF2, NAD(P)H quinone oxidoreductase 1, heme oxygenase 1, glutamate-cysteine ligase modifier subunit and glutathione reductase in comparison to those of ZT24 mice. In conclusion, the 4-week aerobic training performed at ZT12 is more effective for enhancing NRF2-mediated antioxidant responses of skeletal muscle, which may be attributed to the specific activation of BMAL1.

Anti-obesity effect of butyrate links to modulation of gut microbiome and epigenetic regulation of muscular circadian clock

The role of the muscle circadian clock in regulating oxidative metabolism exerts a significant influence on whole-body energy metabolism; however, research on the connection between the muscle circadian clock and obesity is limited. Moreover, there is a lack of studies demonstrating the regulatory effects of dietary butyrate on muscle circadian clock and the resulting antiobesity effects. This study aimed to investigate the impacts of dietary butyrate on metabolic and microbiome alterations and muscle circadian clock in a diet-induced obesity model. Male Sprague-Dawley rats were fed a high-fat diet with or without butyrate. Gut microbiota and serum metabolome were analyzed, and molecular changes were examined using tissues and a cell line. Further correlation analysis was performed on butyrate-induced results. Butyrate supplementation reduced weight gain, even with increased food intake. Gut microbiome analysis revealed an increased abundance of Firmicutes in butyrate group. Serum metabolite profile in butyrate group exhibited reduced amino acid and increased fatty acid content. Muscle circadian clock genes were upregulated, resulting in increased transcription of fatty acid oxidation-related genes. In myoblast cells, butyrate also enhanced pan-histone acetylation via histone deacetylase inhibition, particularly modulating acetylation at the promoter of circadian clock genes. Correlation analysis revealed potential links between Firmicutes phylum, including certain genera within it, and butyrate-induced molecular changes in muscle as well as phenotypic alterations. The butyrate-driven effects on diet-induced obesity were associated with alterations in gut microbiota and a muscle-specific increase in histone acetylation, leading to the transcriptional activation of circadian clock genes and their controlled genes.