Epigenetic regulation of skeletal muscle metabolism

Molecular characterization and induced changes of histone acetyltransferases in the tick Haemaphysalis longicornis in response to cold stress

BACKGROUND

Epigenetic modifications of histones play important roles in the response of eukaryotic organisms to environmental stress. However, many histone acetyltransferases (HATs), which are responsible for histone acetylation, and their roles in mediating the tick response to cold stress have yet to be identified. In the present study, HATs were molecularly characterized and their associations with the cold response of the tick Haemaphysalis longicornis explored.

METHODS

HATs were characterized by using polymerase chain reaction (PCR) based on published genome sequences, followed by multiple bioinformatic analyses. The differential expression of genes in H. longicornis under different cold treatment conditions was evaluated using reverse transcription quantitative PCR (RT-qPCR). RNA interference was used to explore the association of HATs with the cold response of H. longicornis.

RESULTS

Two HAT genes were identified in H. longicornis (Hl), a GCN5-related N-acetyltransferase (henceforth HlGNAT) and a type B histone acetyltransferase (henceforth HlHAT-B), which are respectively 960 base pairs (bp) and 1239 bp in length. Bioinformatics analysis revealed that HlGNAT and HlHAT-B are unstable hydrophilic proteins characterized by the presence of the acetyltransferase 16 domain and Hat1_N domain, respectively. RT-qPCR revealed that the expression of HlGNAT and HlHAT-B decreased after 3 days of cold treatment, but gradually increased with a longer period of cold treatment. The mortality rate following knockdown of HlGNAT or HlHAT-B by RNA interference, which was confirmed by RT-qPCR, significantly increased (P < 0.05) when H. longicornis was treated at the lowest lethal temperature (- 14 °C) for 2 h.

CONCLUSIONS

The findings demonstrate that HATs may play a crucial role in the cold response of H. longicornis. Thus further research is warranted to explore the mechanisms underlying the epigenetic regulation of the cold response in ticks.

A Circular RNA Generated from Nebulin (NEB) Gene Splicing Promotes Skeletal Muscle Myogenesis in Cattle as Detected by a Multi-Omics Approach

Cattle and the draught force provided by its skeletal muscle have been integral to agro-ecosystems of agricultural civilization for millennia. However, relatively little is known about the cattle muscle functional genomics (including protein coding genes, non-coding RNA, etc.). Circular RNAs (circRNAs), as a new class of non-coding RNAs, can be effectively translated into detectable peptides, which enlightened us on the importance of circRNAs in cattle muscle physiology function. Here, RNA-seq, Ribosome profiling (Ribo-seq), and peptidome data are integrated from cattle skeletal muscle, and detected five encoded peptides from circRNAs. It is further identified and functionally characterize a 907-amino acids muscle-specific peptide that is named circNEB-peptide because derived by the splicing of Nebulin (NEB) gene. This peptide localizes to the nucleus and cytoplasm and directly interacts with SKP1 and TPM1, key factors regulating physiological activities of myoblasts, via ubiquitination and myoblast fusion, respectively. The circNEB-peptide is found to promote myoblasts proliferation and differentiation in vitro, and induce muscle regeneration in vivo. These findings suggest circNEB-peptide is an important regulator of skeletal muscle regeneration and underscore the possibility that more encoding polypeptides derived by RNAs currently annotated as non-coding exist.

The epigenetic regulatory effect of histone acetylation and deacetylation on skeletal muscle metabolism-a review

Skeletal muscles, the largest organ responsible for energy metabolism in most mammals, play a vital role in maintaining the body's homeostasis. Epigenetic modification, specifically histone acetylation, serves as a crucial regulatory mechanism influencing the physiological processes and metabolic patterns within skeletal muscle metabolism. The intricate process of histone acetylation modification involves coordinated control of histone acetyltransferase and deacetylase levels, dynamically modulating histone acetylation levels, and precisely regulating the expression of genes associated with skeletal muscle metabolism. Consequently, this comprehensive review aims to elucidate the epigenetic regulatory impact of histone acetylation modification on skeletal muscle metabolism, providing invaluable insights into the intricate molecular mechanisms governing epigenetic modifications in skeletal muscle metabolism.

Methylome and proteome integration in human skeletal muscle uncover group and individual responses to high-intensity interval training

Exercise is a major beneficial contributor to muscle metabolism, and health benefits acquired by exercise are a result of molecular shifts occurring across multiple molecular layers (i.e., epigenome, transcriptome, and proteome). Identifying robust, across-molecular level targets associated with exercise response, at both group and individual levels, is paramount to develop health guidelines and targeted health interventions. Sixteen, apparently healthy, moderately trained (VO2 max = 51.0 ± 10.6 mL min-1  kg-1 ) males (age range = 18-45 years) from the Gene SMART (Skeletal Muscle Adaptive Responses to Training) study completed a longitudinal study composed of 12-week high-intensity interval training (HIIT) intervention. Vastus lateralis muscle biopsies were collected at baseline and after 4, 8, and 12 weeks of HIIT. DNA methylation (~850 CpG sites) and proteomic (~3000 proteins) analyses were conducted at all time points. Mixed models were applied to estimate group and individual changes, and methylome and proteome integration was conducted using a holistic multilevel approach with the mixOmics package. A total of 461 proteins significantly changed over time (at 4, 8, and 12 weeks), whilst methylome overall shifted with training only one differentially methylated position (DMP) was significant (adj.p-value < .05). K-means analysis revealed cumulative protein changes by clusters of proteins that presented similar changes over time. Individual responses to training were observed in 101 proteins. Seven proteins had large effect-sizes >0.5, among them are two novel exercise-related proteins, LYRM7 and EPN1. Integration analysis showed bidirectional relationships between the methylome and proteome. We showed a significant influence of HIIT on the epigenome and more so on the proteome in human muscle, and uncovered groups of proteins clustering according to similar patterns across the exercise intervention. Individual responses to exercise were observed in the proteome with novel mitochondrial and metabolic proteins consistently changed across individuals. Future work is required to elucidate the role of these proteins in response to exercise.

The Landscape of Accessible Chromatin and Developmental Transcriptome Maps Reveal a Genetic Mechanism of Skeletal Muscle Development in Pigs

The epigenetic regulation mechanism of porcine skeletal muscle development relies on the openness of chromatin and is also precisely regulated by transcriptional machinery. However, fewer studies have exploited the temporal changes in gene expression and the landscape of accessible chromatin to reveal the underlying molecular mechanisms controlling muscle development. To address this, skeletal muscle biopsy samples were taken from Landrace pigs at days 0 (D0), 60 (D60), 120 (D120), and 180 (D180) after birth and were then analyzed using RNA-seq and ATAC-seq. The RNA-seq analysis identified 8554 effective differential genes, among which ACBD7, TMEM220, and ATP1A2 were identified as key genes related to the development of porcine skeletal muscle. Some potential cis-regulatory elements identified by ATAC-seq analysis contain binding sites for many transcription factors, including SP1 and EGR1, which are also the predicted transcription factors regulating the expression of ACBD7 genes. Moreover, the omics analyses revealed regulatory regions that become ectopically active after birth during porcine skeletal muscle development after birth and identified 151,245, 53,435, 30,494, and 40,911 peaks. The enriched functional elements are related to the cell cycle, muscle development, and lipid metabolism. In summary, comprehensive high-resolution gene expression maps were developed for the transcriptome and accessible chromatin during postnatal skeletal muscle development in pigs.

5-Aza-2'-Deoxycytidine Regulates White Adipocyte Browning by Modulating miRNA-133a/Prdm16

The conversion of white adipocytes into brown adipocytes improves their thermogenesis and promotes energy consumption. Epigenetic modifications affect related genes and interfere with energy metabolism, and these are the basis of new ideas for obesity treatment. Neonatal mice show high levels of DNA hypermethylation in white adipose tissue early in life and low levels in brown adipose tissue. Thus, we considered that the regulation of DNA methylation may play a role in the conversion of white adipose to brown. We observed growth indicators, lipid droplets of adipocytes, brown fat specific protein, and miRNA-133a after treatment with 5-Aza-2'-deoxycytidine. The expression of Prdm16 and Ucp-1 in adipocytes was detected after inhibiting miRNA-133a. The results showed a decrease in total lipid droplet formation and an increased expression of the brown fat specific proteins Prdm16 and Ucp-1. This study indicated that 5-Aza-2'-deoxycytidine promotes white adipocyte browning following DNA demethylation, possibly via the modulation of miR-133a and Prdm16.

Histone Deacetylases as Modulators of the Crosstalk Between Skeletal Muscle and Other Organs

Skeletal muscle plays a major role in controlling body mass and metabolism: it is the most abundant tissue of the body and a major source of humoral factors; in addition, it is primarily responsible for glucose uptake and storage, as well as for protein metabolism. Muscle acts as a metabolic hub, in a crosstalk with other organs and tissues, such as the liver, the brain, and fat tissue. Cytokines, adipokines, and myokines are pivotal mediators of such crosstalk. Many of these circulating factors modulate histone deacetylase (HDAC) expression and/or activity. HDACs form a numerous family of enzymes, divided into four classes based on their homology to their orthologs in yeast. Eleven family members are considered classic HDACs, with a highly conserved deacetylase domain, and fall into Classes I, II, and IV, while class III members are named Sirtuins and are structurally and mechanistically distinct from the members of the other classes. HDACs are key regulators of skeletal muscle metabolism, both in physiological conditions and following metabolic stress, participating in the highly dynamic adaptative responses of the muscle to external stimuli. In turn, HDAC expression and activity are closely regulated by the metabolic demands of the skeletal muscle. For instance, NAD+ levels link Class III (Sirtuin) enzymatic activity to the energy status of the cell, and starvation or exercise affect Class II HDAC stability and intracellular localization. SUMOylation or phosphorylation of Class II HDACs are modulated by circulating factors, thus establishing a bidirectional link between HDAC activity and endocrine, paracrine, and autocrine factors. Indeed, besides being targets of adipo-myokines, HDACs affect the synthesis of myokines by skeletal muscle, altering the composition of the humoral milieu and ultimately contributing to the muscle functioning as an endocrine organ. In this review, we discuss recent findings on the interplay between HDACs and circulating factors, in relation to skeletal muscle metabolism and its adaptative response to energy demand. We believe that enhancing knowledge on the specific functions of HDACs may have clinical implications leading to the use of improved HDAC inhibitors for the treatment of metabolic syndromes or aging.

Issues on Trainability

Trainability is an adaptive response to given exercise loads and must be localized to the targeted physiological function since exercise-induced acute and chronic adaptations are systemic. Lack of adaptation or moderate level of adaptation in one organ or one physiological function would not mean that other organs or functions would not benefit from exercise training. The most beneficial training load could easily be different for skeletal muscle, brain, the gastro-intestinal track, or the immune systems. Hence, the term of non-responders should be used with caution and just referred to a given organ, cell type, molecular signaling, or function. The present paper aims to highlight some, certainly not all, issues on trainability especially related to muscle and cardiovascular system. The specificity of trainability and the systemic nature of exercise-induced adaptation are discussed, and the paper aims to provide suggestions on how to improve performance when faced with non-responders.

Zebrafish Model for Studying Dexamethasone-Induced Muscle Atrophy and Preventive Effect of Maca (Lepidium meyenii)

Loss of myofibers during muscle atrophy affects functional capacity and quality of life. Dexamethasone, an inducer of rapid atrophy of skeletal myofibers, has been studied as a glucocorticoid receptor in muscle atrophy or motor neurodegeneration. In this study, we examined dexamethasone-induced muscle atrophy using zebrafish (Danio rerio), a vertebrate model, and assessed whether administration of Lepidium meyenii (maca) as a dietary supplement can prevent muscle atrophy. Changes in skeletal myofibers in zebrafish were evaluated after exposure to dexamethasone for different periods and at different concentrations. Under optimized conditions, zebrafish pre-fed with maca for 3 days were exposed to 0.01% dexamethasone for 1 h/day for 7 days. Thereafter, myofiber loss, damaged muscle contractile proteins, and abnormal exploratory behavior due to the structural and functional impairment of skeletal muscle associated with muscle atrophy were investigated using hematoxylin-eosin, immunofluorescence staining, and behavioral analyses. Our findings suggest that dexamethasone induces muscle atrophy in zebrafish, inhibiting exploratory behavior by inducing myofiber loss, inhibiting muscle contraction, and causing changes in endurance and velocity. Thus, the zebrafish model can be used to screen pharmaceutical agents and to study muscle atrophy. Furthermore, maca is a potential dietary supplement to prevent muscle atrophy, as it protects muscle fibers.

Molecular Profiling of DNA Methylation and Alternative Splicing of Genes in Skeletal Muscle of Obese Rabbits

DNA methylation and the alternative splicing of precursor messenger RNAs (pre-mRNAs) are two important genetic modification mechanisms. However, both are currently uncharacterized in the muscle metabolism of rabbits. Thus, we constructed the Tianfu black rabbit obesity model (obese rabbits fed with a 10% high-fat diet and control rabbits from 35 days to 70 days) and collected the skeletal muscle samples from the two groups for Genome methylation sequencing and RNA sequencing. DNA methylation data showed that the promoter regions of 599 genes and gene body region of 2522 genes had significantly differential methylation rates between the two groups, of which 288 genes had differential methylation rates in promoter and gene body regions. Analysis of alternative splicing showed 555 genes involved in exon skipping (ES) patterns, and 15 genes existed in differential methylation regions. Network analysis showed that 20 hub genes were associated with ubiquitinated protein degradation, muscle development pathways, and skeletal muscle energy metabolism. Our findings suggest that the two types of genetic modification have potential regulatory effects on skeletal muscle development and provide a basis for further mechanistic studies in the rabbit.

Epigenetic underpinnings of freeze avoidance in the goldenrod gall moth, Epiblema scudderiana

The goldenrod gall moth (Epiblema scudderiana) is a cold hardy insect that survives subzero temperatures during the winter by supercooling bodily fluids to approximately -40 °C, allowing the insect to remain unfrozen despite the freezing temperatures. This is characterized by a drastic increase of cryoprotectant glycerol along with widespread downregulation of non-essential genes and processes to conserve cellular energy. This study examined the role of epigenetic enzymes in regulating this freeze-avoidant process across a range of freezing temperatures experienced in nature. Cold and subzero temperature exposure in E. scudderiana resulted in upregulation of select DNA methyltransferase (DNMT) enzymes with concurrent decreases in DNMT activity and no change in activity of the Ten-Eleven Translocation (TET) demethylation enzyme activities. Levels of histone acetyltransferase (HAT) and histone deacetylase (HDAC) activity decreased during cold exposures. The increase in DNMT expression and concurrent decrease in HAT activity suggests a role for DNA methylation to assist with transcriptional suppression. These findings propose that epigenetic regulation of genes and histones underpin the winter survival strategies of this insect.

The Influence of Epigenetic Modifications on Metabolic Changes in White Adipose Tissue and Liver and Their Potential Impact in Exercise

Background: Epigenetic marks are responsive to a wide variety of environmental stimuli and serve as important mediators for gene transcription. A number of chromatin modifying enzymes orchestrate epigenetic responses to environmental stimuli, with a growing body of research examining how changes in metabolic substrates or co-factors alter epigenetic modifications.

Scope of Review: Here, we provide a systematic review of existing evidence of metabolism-related epigenetic changes in white adipose tissue (WAT) and the liver and generate secondary hypotheses on how exercise may impact metabolism-related epigenetic marks in these tissues.

Major Conclusions: Epigenetic changes contribute to the complex transcriptional responses associated with WAT lipolysis, hepatic de novo lipogenesis, and hepatic gluconeogenesis. While these metabolic responses may hypothetically be altered with acute and chronic exercise, direct testing is needed.

Potential Physiological and Cellular Mechanisms of Exercise That Decrease the Risk of Severe Complications and Mortality Following SARS-CoV-2 Infection

The coronavirus disease 2019 (COVID-19) pandemic caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has unmasked mankind's vulnerability to biological threats. Although higher age is a major risk factor for disease severity in COVID-19, several predisposing risk factors for mortality are related to low cardiorespiratory and metabolic fitness, including obesity, cardiovascular disease, diabetes, and hypertension. Reaching physical activity (PA) guideline goals contribute to protect against numerous immune and inflammatory disorders, in addition to multi-morbidities and mortality. Elevated levels of cardiorespiratory fitness, being non-obese, and regular PA improves immunological function, mitigating sustained low-grade systemic inflammation and age-related deterioration of the immune system, or immunosenescence. Regular PA and being non-obese also improve the antibody response to vaccination. In this review, we highlight potential physiological, cellular, and molecular mechanisms that are affected by regular PA, increase the host antiviral defense, and may determine the course and outcome of COVID-19. Not only are the immune system and regular PA in relation to COVID-19 discussed, but also the cardiovascular, respiratory, renal, and hormonal systems, as well as skeletal muscle, epigenetics, and mitochondrial function.

Trichostatin A regulates fibro/adipogenic progenitor adipogenesis epigenetically and reduces rotator cuff muscle fatty infiltration

Rotator cuff (RC) muscle fatty infiltration (FI) is an important factor that determines the clinical outcome of patients with RC repair. There is no effective treatment for RC muscle FI at this time. The goal of this study is to define the role Trichostatin A (TSA), a histone deacetylase (HDAC) inhibitor in regulating muscle fibro/adipogenic progenitors (FAPs) adipogenesis and treating muscle fatty degeneration after massive RC tears in a mouse model. We hypothesize that TSA reduces muscle FI after massive RC tears. HDAC activity was measured in FAPs in RC muscle after tendon/nerve transection or sham surgery. FAPs were treated with TSA for 2 weeks and FAP adipogenesis was evaluated with perilipin and Oil Red O staining, as well as reverse transcript-polymerase chain reaction for adipogenesis-related genes. About 0.5 mg/kg TSA or dimethyl sulfoxide was administered to C57B/L6 mice with massive rotator cuff tears through daily intraperitoneal injection for 6 weeks. Supraspinatus muscles were harvested for biochemical and histology analysis. We found that FAPs showed significantly higher HDAC activity after RC tendon/nerve transection. TSA treatment significantly reduced HDAC activity and inhibited adipogenesis of FAPs. TSA also abolished the role of bone morphogenetic protein-7 in inducing FAP adipogenesis and promoted FAP brown/beige adipose tissue (BAT) differentiation. TSA injection significantly increased histone H3 acetylation and reduced FI of rotator cuff muscles after massive tendon tears. Results from this study showed that TSA can regulate FAP adipogenesis and promote FAP BAT differentiation epigenetically. HDAC inhibition may be a new treatment strategy to reduce muscle FI after RC tears and repair.

Epigenetic regulation by DNA methyltransferases during torpor in the thirteen-lined ground squirrel Ictidomys tridecemlineatus

The thirteen-lined ground squirrel, Ictidomys tridecemlineatus, is a mammal capable of lowering its T_b to almost 0 °C while undergoing deep torpor bouts over the winter. To decrease its metabolic rate to such a drastic extent, the squirrel must undergo multiple physiological, biological, and molecular alterations including downregulation of almost all nonessential processes. Epigenetic regulation allows for a dynamic range of transient phenotypes, allowing the squirrel to downregulate energy-expensive and nonessential pathways during torpor. DNA methylation is a prominent form of epigenetic regulation; therefore, the DNA methyltransferase (DNMT) family of enzymes were studied by measuring expression and activity levels of the five major proteins during torpor bouts. Additionally, specific cytosine marks on genomic DNA were quantified to further elucidate DNA methylation during hibernation. A tissue-specific response was observed that highlighted variant degrees of DNA methylation and DNMT expression/activity, demonstrating that DNA methylation is a highly complex form of epigenetic regulation and likely one of many regulatory mechanisms that enables metabolic rate depression in response to torpor.

Tissue-specific regulatory mechanism of LncRNAs and methylation in sheep adipose and muscle induced by Allium mongolicum Regel extracts

Allium mongolicum Regel (A. mongolicum) is a perennial and xerophytic Liliaceous allium plant in high altitude desert steppe and desert areas. Feeding A. mongolicum greatly reduced unpleasant mutton flavor and improves meat quality of sheep. We analyzed epigenetic regulatory mechanisms of water extracts of A. mongolicum (WEA) on sheep muscle and adipose using RNA-Seq and whole-genome Bisulfite sequencing. Feeding WEA reduced differentially expressed genes and long non-coding RNAs (lncRNAs) between two tissues but increased differentially methylation regions (DMRs). LncRNA and DMR targets were both involved in ATP binding, ubiquitin, protein kinase binding, regulation of cell proliferation, and related signaling pathways, but not unsaturated fatty acids metabolism. Besides, tissue specific targets were involved in distinct functional annotations, e.g., Golgi membrane and endoplasmic reticulum for muscle lncRNA, oxidative phosphorylation metabolism for adipose lncRNA, dsRNA binding for muscle DMRs. Epigenetic regulatory networks were also discovered to discovered essential co-regulated modules, e.g., co-regulated insulin secretion module (PDPK1, ATP1A2, CACNA1S and CAMK2D) in adipose. The results indicated that WEA induced distinct epigenetic regulation on muscle and adipose to diminish transcriptome differences between tissues, which highlights biological functions of A. mongolicum, tissue similarity and specificity, as well as regulatory mechanism of mutton odor.

Exercise adaptations: molecular mechanisms and potential targets for therapeutic benefit

Exercise is fundamental for good health, whereas physical inactivity underpins many chronic diseases of modern society. It is well appreciated that regular exercise improves metabolism and the metabolic phenotype in a number of tissues. The phenotypic alterations observed in skeletal muscle are partly mediated by transcriptional responses that occur following each individual bout of exercise. This adaptive response increases oxidative capacity and influences the function of myokines and extracellular vesicles that signal to other tissues. Our understanding of the epigenetic and transcriptional mechanisms that mediate the skeletal muscle gene expression response to exercise as well as of their upstream signalling pathways has advanced substantially in the past 10 years. With this knowledge also comes the opportunity to design new therapeutic strategies based on the biology of exercise for a variety of chronic conditions where regular exercise might be a challenge. This Review provides an overview of the beneficial adaptive responses to exercise and details the molecular mechanisms involved. The possibility of designing therapeutic interventions based on these molecular mechanisms is addressed, using relevant examples that have exploited this approach.

Inactivity and Skeletal Muscle Metabolism: A Vicious Cycle in Old Age

Aging is an inevitable and gradually progressive process affecting all organs and systems. The musculoskeletal system makes no exception, elderly exhibit an increased risk of sarcopenia (low muscle mass),dynapenia (declining muscle strength), and subsequent disability. Whereas in recent years the subject of skeletal muscle metabolic decline in the elderly has been gathering interest amongst researchers, as well as medical professionals, there are many challenges yet to be solved in order to counteract the effects of aging on muscle function efficiently. Noteworthy, it has been shown that aging individuals exhibit a decline in skeletal muscle metabolism, a phenomenon which may be linked to a number of predisposing (risk) factors such as telomere attrition, epigenetic changes, mitochondrial dysfunction, sedentary behavior (leading to body composition alterations), age-related low-grade systemic inflammation (inflammaging), hormonal imbalance, as well as a hypoproteic diet (unable to counterbalance the repercussions of the age-related increase in skeletal muscle catabolism). The present review aims to discuss the relationship between old age and muscle wasting in an effort to highlight the modifications in skeletal muscle metabolism associated with aging and physical activity.

Omics and the molecular exercise physiology

Exercise is a well-known non-pharmacologic agent used to prevent and treat a wide range of pathologic conditions such as metabolic and cardiovascular disease. In this sense, the classic field of exercise physiology has determined the main theoretical and practical bases of physiologic adaptations in response to exercise. However, the last decades were marked by significant advances in analytical laboratory techniques, where the field of biochemistry, genetics and molecular biology promoted exercise science to enter a new era. Regardless of its application, whether in the field of disease prevention or performance, the association of molecular biology with exercise physiology has been fundamental for unveiling knowledge of the molecular mechanisms related to the adaptation to exercise. This chapter will address the natural evolution of exercise physiology toward genetics and molecular biology, emphasizing the collection of integrated analytical approaches that composes the OMICS and their contribution to the field of molecular exercise physiology.

Genetic and epigenetic sex-specific adaptations to endurance exercise

In recent years, the interest in personalised interventions such as medicine, nutrition, and exercise is rapidly rising to maximize health outcomes and ensure the most appropriate treatments. Exercising regularly is recommended for both healthy and diseased populations to improve health. However, there are sex-specific adaptations to exercise that often are not taken into consideration. While endurance exercise training alters the human skeletal muscle epigenome and subsequent gene expression, it is still unknown whether it does so differently in men and women, potentially leading to sex-specific physiological adaptations. Elucidating sex differences in genetics, epigenetics, gene regulation and expression in response to exercise will have great health implications, as it may enable gene targets in future clinical interventions and may better individualised interventions. This review will cover this topic and highlight the recent findings of sex-specific genetic, epigenetic, and gene expression studies, address the gaps in the field, and offer recommendations for future research.

Diabetes induces tri-methylation at lysine 9 of histone 3 at Slc2a4 gene in skeletal muscle: A new target to improve glycemic control

Expression of the glucose transporter GLUT4, encoded by Slc2a4 gene, is reduced in both type 1 and type 2 diabetes (T1D and T2D), contributing to glycemic impairment. The present study investigated epigenetic regulations at the Slc2a4 promoter in skeletal muscle of T1D- and T2D-like experimental models. Slc2a4/GLUT4 repression was observed in T1D and T2D and that was reversed by insulin and resveratrol treatments, respectively. In both T1D-like and T2D-like animals, tri-methylation at lysine 9 of histone 3 (H3K9me3) increased in the Slc2a4 enhancer segment, whereas MEF2A/D binding into this segment was reduced; all effects were reversed by respective treatments. This study reveals that increased H3K9me3 in the Slc2a4 promoter enhancer segment contributes to reduce GLUT4 expression in skeletal muscle and to worse glycemic control in diabetes, pointing to the H3K9me3 of Slc2a4 promoter as a potential target for development of new approaches for treating diabetes.

MicroRNA and Long Non-coding RNA Regulation in Skeletal Muscle From Growth to Old Age Shows Striking Dysregulation of the Callipyge Locus

MicroRNAs (miRNAs) undergo high levels of regulation in skeletal muscle development and control skeletal muscle mass, function and metabolism over the lifespan. More recently, the role of long non-coding RNAs (lncRNAs) in skeletal muscle regulation has started to emerge. Following up on our recent study describing the expression pattern and putative roles of 768 miRNAs in the quadriceps muscle of mice at early life stages, we used a high-throughput miRNA qPCR-based array to assess the expression of the same miRNAs in 28-month old male mouse quadriceps muscle. In addition, we report the expression patterns of lncRNAs playing a putative role in muscle development and adaptation from growth to old age. Twelve miRNAs were significantly downregulated in 28-month old muscle when compared with 12-week old muscle. Ten of them clustered at the Dlk1-Dio3 locus, known as ‘Callipyge,’ which is associated with muscle development and hypertrophy. This collective downregulation was paralleled by decreases in the expression levels of the maternally expressed imprinted LncRNA coding genes Meg3 and Rian stemming from the same chromosomal region. In contrast, the paternally expressed imprinted Dlk1-Dio3 locus members Rtl1, Dio3, and Dlk1 and the muscle related lncRNAs lncMyoD1, Neat_v1, Neat_v2, and Malat1 underwent significant changes during growth, but their expression levels were not altered past the age of 12 weeks, suggesting roles limited to hyperplasia and early hypertrophy. In conclusion, collective muscle miRNA expression gradually decreases over the lifespan and a cluster of miRNAs and maternally expressed lncRNAs stemming from the Callipyge locus is significantly dysregulated in aging muscle. The Dlk1-Dio3 locus therefore represents a potential new mechanism for age-related muscle decline.

Decreased RNF41 expression leads to insulin resistance in skeletal muscle of obese women

CONTEXT

Toll-like receptor 4 (TLR4) activation contributes to obesity-associated insulin resistance in skeletal muscles (SM). TLR4 signaling involves two pathways: the myeloid differentiation primary response gene 88 (MyD88) leading to inflammatory cytokines production and the toll/interleukin-1 receptor domain-containing adapter-inducing interferon (IFN) I (TRIF)-dependent pathways leading to type 1 interferon (IFNI) and interferon stimulated genes (ISG) expression. The E3 ubiquitin ligase RNF41 allows the preferential activation of the TRIF-IFNI pathway; however, its role in insulin response has not been reported.

METHODS

We measured RNF41 level and IFNI pathway activation (ISG expression) in SM biopsies of obese insulin sensitive (OIS) and obese insulin resistant (OIR) women. Then we isolated and differentiated in myotubes, primary human SM cell progenitors from OIS and OIR SM biopsies. We modulated RNF41 and ISG expression in these myotubes and investigated their effects on insulin response.

RESULTS

RNF41 expression is down-regulated in vivo in OIR SM and myotubes compared to OIS SM and myotubes. TLR4 activation with palmitate induces TRIF-IFNI pathway and ISG in OIS myotubes but not in OIR myotubes. Inhibition of RNF41 expression with siRNF41 in OIS myotubes treated with palmitate attenuates insulin response, IFNI pathway activation and ISG induction, mimicking OIR phenotype. Further, overexpression of RNF41 in OIR myotubes increases insulin response and ISG expression. Exposure to IFNI or to its inducer polyinosinic-polycytidylic acid, restores ISG expression and insulin sensitivity in OIR myotubes and OIS myotubes transfected with siRNF41.

CONCLUSION

Our results identify RNF41 as essential to IFNI pathway activation in order to maintain muscle insulin sensitivity during human obesity.

Omics and Exercise: Global Approaches for Mapping Exercise Biological Networks

The application of global "-omics" technologies to exercise has introduced new opportunities to map the complexity and interconnectedness of biological networks underlying the tissue-specific responses and systemic health benefits of exercise. This review will introduce major research tracks and recent advancements in this emerging field, as well as critical gaps in understanding the orchestration of molecular exercise dynamics that will benefit from unbiased omics investigations. Furthermore, significant research hurdles that need to be overcome to effectively fill these gaps related to data collection, computation, interpretation, and integration across omics applications will be discussed. Collectively, a cross-disciplinary physiological and omics-based systems approach will lead to discovery of a wealth of novel exercise-regulated targets for future mechanistic validation. This frontier in exercise biology will aid the development of personalized therapeutic strategies to improve athletic performance and human health through precision exercise medicine.