Differential regulation of the muscle-specific GLUT4 enhancer in regenerating and adult skeletal muscle

Regulation of injury-induced skeletal myofiber regeneration by glucose transporter 4 (GLUT4)

BACKGROUND

Insulin resistance and type 2 diabetes impair cellular regeneration in multiple tissues including skeletal muscle. The molecular basis for this impairment is largely unknown. Glucose uptake via glucose transporter GLUT4 is impaired in insulin resistance. In healthy muscle, acute injury stimulates glucose uptake. Whether decreased glucose uptake via GLUT4 impairs muscle regeneration is presently unknown. The goal of this study was to determine whether GLUT4 regulates muscle glucose uptake and/or regeneration following acute injury.

METHODS

Tibialis anterior and extensor digitorum longus muscles from wild-type, control, or muscle-specific GLUT4 knockout (mG4KO) mice were injected with the myotoxin barium chloride to induce muscle injury. After 3, 5, 7, 10, 14, or 21 days (in wild-type mice), or after 7 or 14 days (in control & mG4KO) mice, muscles were isolated to examine [3H]-2-deoxyglucose uptake, GLUT4 levels, extracellular fluid space, fibrosis, myofiber cross-sectional area, and myofiber centralized nuclei.

RESULTS

In wild-type mice, muscle glucose uptake was increased 3, 5, 7, and 10 days post-injury. There was a rapid decrease in GLUT4 protein levels that were restored to baseline at 5-7 days post-injury, followed by a super-compensation at 10-21 days. In mG4KO mice, there were no differences in muscle glucose uptake, extracellular fluid space, muscle fibrosis, myofiber cross-sectional areas, or percentage of centrally nucleated myofibers at 7 days post-injury. In contrast, at 14 days injured muscles from mG4KO mice exhibited decreased glucose uptake, muscle weight, myofiber cross sectional areas, and centrally nucleated myofibers, with no change in extracellular fluid space or fibrosis.

CONCLUSIONS

Collectively, these findings demonstrate that glucose uptake via GLUT4 regulates skeletal myofiber regeneration following acute injury.

Chicken GLUT4 function via enhancing mitochondrial oxidative phosphorylation and inhibiting ribosome pathway in skeletal muscle satellite cells

Glucose Transporter 4 (GLUT4) is a crucial protein facilitating glucose uptake and metabolism across cell membranes in mammals. However, information on GLUT4 in birds has historically been limited. In this study, we investigated the dynamic expression profile of chicken GLUT4 using real-time quantitative PCR (RT-qPCR) and examined its potential effects and mechanisms via GLUT4 overexpression and RNA sequencing (RNA-seq) in chicken primary skeletal muscle satellite cells (CP-SMSCs). Our results demonstrated that chicken GLUT4 is differentially expressed across tissues, with predominant expression in skeletal muscles, and across developmental stages of CP-SMSCs, with notable upregulation during the phases of cell proliferation and early differentiation. Notably, 0.1 μM insulin for 60 min significantly elevated the expression of GLUT4 in CP-SMSCs (P < 0.05). GLUT4 overexpression in CP-SMSCs promoted cell proliferation, as evidenced by Cell Counting Kit-8 (CCK-8) (P < 0.05) and 5-Ethynyl-2'-Deoxyuridine (EDU) assays (P < 0.05), and enhanced glucose consumption following 0.1 μM insulin treatment (P < 0.05). However, it inhibited glucose consumption 12 h after the addition of 5 g/L glucose (P < 0.05). After overexpressing GLUT4, we identified 302 differentially expressed genes (DEGs) in CP-SMSCs, with 134 upregulated and 168 downregulated. These DEGs are primarily enriched in pathways such as oxidative phosphorylation, ribosome, cardiac muscle contraction, ATP metabolic processes, and mitochondrial protein complexes. Specifically, in the enriched oxidative phosphorylation pathway, the upregulated DEGs (12) encode mitochondrial proteins, while the downregulated DEGs (6) are nuclear genome-derived. The ribosomal pathway is predominantly inhibited, accompanying with the downregulation of the translocase of outer mitochondrial membrane 7 (TOMM7)/translocase of inner mitochondrial membrane 8 (TIMM8A) complex responsible for mitochondrial protein transport, and a reduction in 28S (LOC121106978) and 18S (LOC112533601) ribosomal rRNAs. In conclusion, chicken GLUT4 is dynamically modulated during development and acts as an insulin responder that significantly regulates cellular glucose uptake and cell proliferation. This regulation occurs mainly through enhancing the mitochondrial oxidative phosphorylation and inhibiting ribosomal pathway.

Diabetes: discovery of insulin, genetic, epigenetic and viral infection mediated regulation

Diabetes mellitus, commonly referred to as diabetes, is a combination of many metabolic diseases. Insulin deficiency in our body is the main cause of diabetes. Insulin is one of the most well studied proteins, yet the genesis of its discovery was not getting much attention so far. Nevertheless, the history of the discovery of insulin is an exemplary of solving observational and scientific riddles, drudgery, patience and even professional turmoil. It is an inspiration for all medical personnel and scientists who are practising in the field of molecular medicine. Additionally, the genetic and epigenetic regulation of different types of diabetes needs to be addressed because of the widespread nature of the disease. Diabetes not only involves genetic predisposition but environmental factors, lifestyle etc. can be the major contributor for its inception. Nonetheless, viral infections at an early age are also found to trigger the onset of type I diabetes. In this review article, the history of the discovery of insulin is detailed along with the justification for the genetic and epigenetic regulatory mechanisms of diabetes and explained how viral infections can also trigger the onset of diabetes.

Epigenetic modification and therapeutic targets of diabetes mellitus

The prevalence of diabetes and its related complications are increasing significantly globally. Collected evidence suggested that several genetic and environmental factors contribute to diabetes mellitus. Associated complications such as retinopathy, neuropathy, nephropathy and other cardiovascular complications are a direct result of diabetes. Epigenetic factors include deoxyribonucleic acid (DNA) methylation and histone post-translational modifications. These factors are directly related with pathological factors such as oxidative stress, generation of inflammatory mediators and hyperglycemia. These result in altered gene expression and targets cells in the pathology of diabetes mellitus without specific changes in a DNA sequence. Environmental factors and malnutrition are equally responsible for epigenetic states. Accumulated evidence suggested that environmental stimuli alter the gene expression that result in epigenetic changes in chromatin. Recent studies proposed that epigenetics may include the occurrence of 'metabolic memory' found in animal studies. Further study into epigenetic mechanism might give us new vision into the pathogenesis of diabetes mellitus and related complication thus leading to the discovery of new therapeutic targets. In this review, we discuss the possible epigenetic changes and mechanism that happen in diabetes mellitus type 1 and type 2 separately. We highlight the important epigenetic and non-epigenetic therapeutic targets involved in the management of diabetes and associated complications.

The Impact of Skeletal Muscle ERα on Mitochondrial Function and Metabolic Health

The incidence of chronic disease is elevated in women after menopause. Increased expression of ESR1 (the gene that encodes the estrogen receptor alpha, ERα) in muscle is highly associated with metabolic health and insulin sensitivity. Moreover, reduced muscle expression levels of ESR1 are observed in women, men, and animals presenting clinical features of the metabolic syndrome (MetSyn). Considering that metabolic dysfunction elevates chronic disease risk, including type 2 diabetes, heart disease, and certain cancers, treatment strategies to combat metabolic dysfunction and associated pathologies are desperately needed. This review will provide published work supporting a critical and protective role for skeletal muscle ERα in the regulation of mitochondrial function, metabolic homeostasis, and insulin action. We will provide evidence that muscle-selective targeting of ERα may be effective for the preservation of mitochondrial and metabolic health. Collectively published findings support a compelling role for ERα in the control of muscle metabolism via its regulation of mitochondrial function and quality control. Studies identifying ERα-regulated pathways essential for disease prevention will lay the important foundation for the design of novel therapeutics to improve metabolic health of women while limiting secondary complications that have historically plagued traditional hormone replacement interventions.

The impact of ERα action on muscle metabolism and insulin sensitivity - Strong enough for a man, made for a woman

Background

The incidence of chronic disease is elevated in women after menopause. Natural variation in muscle expression of the estrogen receptor (ER)α is inversely associated with plasma insulin and adiposity. Moreover, reduced muscle ERα expression levels are observed in women and animals presenting clinical features of the metabolic syndrome (MetSyn). Considering that metabolic dysfunction impacts nearly a quarter of the U.S. adult population and elevates chronic disease risk including type 2 diabetes, heart disease, and certain cancers, treatment strategies to combat metabolic dysfunction and associated pathologies are desperately needed.

Scope of the review

This review will provide evidence supporting a critical and protective role for skeletal muscle ERα in the regulation of metabolic homeostasis and insulin sensitivity, and propose novel ERα targets involved in the maintenance of metabolic health.

Major conclusions

Studies identifying ERα-regulated pathways essential for disease prevention will lay the important foundation for the rational design of novel therapeutics to improve the metabolic health of women while limiting secondary complications that have plagued traditional hormone replacement interventions.

Effects of Class II-Selective Histone Deacetylase Inhibitor on Neuromuscular Function and Disease Progression in SOD1-ALS Mice

Emerging evidence indicates that transcriptome alterations due to epigenetic deregulation concur to ALS pathogenesis. Accordingly, pan-histone deacetylase (HDAC) inhibitors delay ALS development in mice, but these compounds failed when tested in ALS patients. Possibly, lack of selectivity toward specific classes of HDACs weakens the therapeutic effects of pan-HDAC inhibitors. Here, we tested the effects of the HDAC Class II selective inhibitor MC1568 on disease evolution, motor neuron survival as well as skeletal muscle function in SOD1G93A mice. We report that HDACs did not undergo expression changes during disease evolution in isolated motor neurons of adult mice. Conversely, increase in specific Class II HDACs (-4, -5 and -6) occurs in skeletal muscle of mice with severe neuromuscular impairment. Importantly, treatment with MC1568 causes early improvement of motor performances that vanishes at later stages of disease. Notably, motor improvement is not paralleled by reduced motor neuron degeneration but by increased skeletal muscle electrical potentials, reduced activation of mir206/FGFBP1-dependent muscle reinnervation signaling, and increased muscle expression of myogenic genes.

The Role of Skeletal Muscle Estrogen Receptors in Metabolic Homeostasis and Insulin Sensitivity

Women in the modern era are challenged with facing menopausal symptoms as well as heightened disease risk associated with increasing adiposity and metabolic dysfunction for up to three decades of life. Treatment strategies to combat metabolic dysfunction and associated pathologies have been hampered by our lack of understanding regarding the biological causes of these clinical conditions and our incomplete understanding regarding the effects of estrogens and the tissue-specific functions and molecular actions of its receptors. In this chapter we provide evidence supporting a critical and protective role for skeletal muscle estrogen receptor α in the maintenance of metabolic homeostasis and insulin sensitivity. Studies identifying the critical ER-regulated pathways essential for disease prevention will lay the important foundation for the rational design of novel therapeutic strategies to improve the health of women while limiting secondary complications that have plagued traditional hormone replacement interventions.

Signaling pathways involved in cardiac energy metabolism

Various signaling pathways coordinate energy metabolism and contractile function in the heart. Myocardial uptake of long-chain fatty acids largely occurs by facilitated diffusion, involving the membrane-associated protein, CD36. Glucose uptake, the rate-limiting step in glucose utilization, is mediated predominantly by the glucose transporter protein, GLUT4. Insulin and contraction-mediated AMPK signaling each are implicated in tightly regulating these myocardial 'gate-keepers' of energy balance, that is, CD36 and GLUT4. The insulin and AMPK signaling cascades are complex and their cross-talk is only beginning to be understood. Moreover, transcriptional regulation of the CD36 and GLUT4 is significantly understudied. This review focuses on recent advances on the role of these signaling pathways and transcription factors involved in the regulation of CD36 and GLUT4.

Glucose Transporters in Cardiac Metabolism and Hypertrophy

The heart is adapted to utilize all classes of substrates to meet the high-energy demand, and it tightly regulates its substrate utilization in response to environmental changes. Although fatty acids are known as the predominant fuel for the adult heart at resting stage, the heart switches its substrate preference toward glucose during stress conditions such as ischemia and pathological hypertrophy. Notably, increasing evidence suggests that the loss of metabolic flexibility associated with increased reliance on glucose utilization contribute to the development of cardiac dysfunction. The changes in glucose metabolism in hypertrophied hearts include altered glucose transport and increased glycolysis. Despite the role of glucose as an energy source, changes in other nonenergy producing pathways related to glucose metabolism, such as hexosamine biosynthetic pathway and pentose phosphate pathway, are also observed in the diseased hearts. This article summarizes the current knowledge regarding the regulation of glucose transporter expression and translocation in the heart during physiological and pathological conditions. It also discusses the signaling mechanisms governing glucose uptake in cardiomyocytes, as well as the changes of cardiac glucose metabolism under disease conditions.

Impaired estrogen receptor action in the pathogenesis of the metabolic syndrome

Considering the current trends in life expectancy, women in the modern era are challenged with facing menopausal symptoms as well as heightened disease risk associated with increasing adiposity and metabolic dysfunction for up to three decades of life. Treatment strategies to combat metabolic dysfunction and associated pathologies have been hampered by our lack of understanding regarding the biological underpinnings of these clinical conditions and our incomplete understanding of the effects of estrogens and the tissue-specific functions and molecular actions of its receptors. In this review we provide evidence supporting a critical and protective role for the estrogen receptor α specific form in the maintenance of metabolic homeostasis and insulin sensitivity. Studies identifying the ER-regulated pathways required for disease prevention will lay the important foundation for the rational design of targeted therapeutics to improve women's health while limiting complications that have plagued traditional hormone replacement interventions.

JAZF1 promotes proliferation of C2C12 cells, but retards their myogenic differentiation through transcriptional repression of MEF2C and MRF4-Implications for the role of Jazf1 variants in oncogenesis and type 2 diabetes

Single-nucleotide polymorphisms associated with type 2 diabetes (T2D) have been identified in Jazf1, which is also involved in the oncogenesis of endometrial stromal tumors. To understand how Jazf1 variants confer a risk of tumorigenesis and T2D, we explored the functional roles of JAZF1 and searched for JAZF1 target genes in myogenic C2C12 cells. Consistent with an increase of Jazf1 transcripts during myoblast proliferation and their decrease during myogenic differentiation in regenerating skeletal muscle, JAZF1 overexpression promoted cell proliferation, whereas it retarded myogenic differentiation. Examination of myogenic genes revealed that JAZF1 overexpression transcriptionally repressed MEF2C and MRF4 and their downstream genes. AMP deaminase1 (AMPD1) was identified as a candidate for JAZF1 target by gene array analysis. However, promoter assays of Ampd1 demonstrated that mutation of the putative binding site for the TR4/JAZF1 complex did not alleviate the repressive effects of JAZF1 on promoter activity. Instead, JAZF1-mediated repression of Ampd1 occurred through the MEF2-binding site and E-box within the Ampd1 proximal regulatory elements. Consistently, MEF2C and MRF4 expression enhanced Ampd1 promoter activity. AMPD1 overexpression and JAZF1 downregulation impaired AMPK phosphorylation, while JAZF1 overexpression also reduced it. Collectively, these results suggest that aberrant JAZF1 expression contributes to the oncogenesis and T2D pathogenesis.

The CHC22 clathrin-GLUT4 transport pathway contributes to skeletal muscle regeneration

Mobilization of the GLUT4 glucose transporter from intracellular storage vesicles provides a mechanism for insulin-responsive glucose import into skeletal muscle. In humans, clathrin isoform CHC22 participates in formation of the GLUT4 storage compartment in skeletal muscle and fat. CHC22 function is limited to retrograde endosomal sorting and is restricted in its tissue expression and species distribution compared to the conserved CHC17 isoform that mediates endocytosis and several other membrane traffic pathways. Previously, we noted that CHC22 was expressed at elevated levels in regenerating rat muscle. Here we investigate whether the GLUT4 pathway in which CHC22 participates could play a role in muscle regeneration in humans and we test this possibility using CHC22-transgenic mice, which do not normally express CHC22. We observed that GLUT4 expression is elevated in parallel with that of CHC22 in regenerating skeletal muscle fibers from patients with inflammatory and other myopathies. Regenerating human myofibers displayed concurrent increases in expression of VAMP2, another regulator of GLUT4 transport. Regenerating fibers from wild-type mouse skeletal muscle injected with cardiotoxin also showed increased levels of GLUT4 and VAMP2. We previously demonstrated that transgenic mice expressing CHC22 in their muscle over-sequester GLUT4 and VAMP2 and have defective GLUT4 trafficking leading to diabetic symptoms. In this study, we find that muscle regeneration rates in CHC22 mice were delayed compared to wild-type mice, and myoblasts isolated from these mice did not proliferate in response to glucose. Additionally, CHC22-expressing mouse muscle displayed a fiber type switch from oxidative to glycolytic, similar to that observed in type 2 diabetic patients. These observations implicate the pathway for GLUT4 transport in regeneration of both human and mouse skeletal muscle, and demonstrate a role for this pathway in maintenance of muscle fiber type. Extrapolating these findings, CHC22 and GLUT4 can be considered markers of muscle regeneration in humans.

The role of estrogens in control of energy balance and glucose homeostasis

Estrogens play a fundamental role in the physiology of the reproductive, cardiovascular, skeletal, and central nervous systems. In this report, we review the literature in both rodents and humans on the role of estrogens and their receptors in the control of energy homeostasis and glucose metabolism in health and metabolic diseases. Estrogen actions in hypothalamic nuclei differentially control food intake, energy expenditure, and white adipose tissue distribution. Estrogen actions in skeletal muscle, liver, adipose tissue, and immune cells are involved in insulin sensitivity as well as prevention of lipid accumulation and inflammation. Estrogen actions in pancreatic islet β-cells also regulate insulin secretion, nutrient homeostasis, and survival. Estrogen deficiency promotes metabolic dysfunction predisposing to obesity, the metabolic syndrome, and type 2 diabetes. We also discuss the effect of selective estrogen receptor modulators on metabolic disorders.

The role of CaMKII in regulating GLUT4 expression in skeletal muscle

Contractile activity during physical exercise induces an increase in GLUT4 expression in skeletal muscle, helping to improve glucose transport capacity and insulin sensitivity. An important mechanism by which exercise upregulates GLUT4 is through the activation of Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) in response to elevated levels of cytosolic Ca(2+) during muscle contraction. This review discusses the mechanism by which Ca(2+) activates CaMKII, explains research techniques currently used to alter CaMK activity in cells, and highlights various exercise models and pharmacological agents that have been used to provide evidence that CaMKII plays an important role in regulating GLUT4 expression. With regard to transcriptional mechanisms, the key research studies that identified myocyte enhancer factor 2 (MEF2) and GLUT4 enhancer factor as the major transcription factors regulating glut4 gene expression, together with their binding domains, are underlined. Experimental evidence showing that CaMK activation induces hyperacetylation of histones in the vicinity of the MEF2 domain and increases MEF2 binding to its cis element to influence MEF2-dependent Glut4 gene expression are also given along with data suggesting that p300 might be involved in acetylating histones on the Glut4 gene. Finally, an appraisal of the roles of other calcium- and non-calcium-dependent mechanisms, including the major HDAC kinases in GLUT4 expression, is also given.

The deiodinases and the control of intracellular thyroid hormone signaling during cellular differentiation

Background

Thyroid hormone influences gene expression in virtually all vertebrates. Its action is initiated by the activation of T4 to T3, an outer ring deiodination reaction that is catalyzed by the type 1 or the type 2 iodothyronine selenodeiodinases (D1 or D2). Inactivation of T4 and T3 occurs via inner ring deiodination catalyzed by the type 3 iodothyronine selenodeiodinases (D3). The T4 concentration is generally quite stable in human plasma, with T3 levels also remaining constant. Deiodinase actions are tightly regulated in both pre- and post-natal life when they are required to make local adjustments of intracellular T3 concentrations in a precise spatio- and temporal manner. Although all the signals governing the dynamic expression of deiodinases in specific cell types are not known, many important regulatory factors have been deciphered.

Scope of review

This review provides striking examples from the recent literature illustrating how the expression of D2 and D3 is finely tuned during maturation of different organs, and how their action play a critical role in different settings to control intracellular T3 availability.

Major conclusions

Emerging evidence indicates that in various cell contexts, D2 and D3 are expressed in a dynamic balance, in which the expression of one enzyme is coordinately regulated with that of the other to tightly control intracellular T3 levels commensurate with cell requirements at that time.

General significance

Deiodinases control TH action in a precise spatio-temporal fashion thereby providing a novel mechanism for the local paracrine and autocrine regulation of TH action. This remarkable tissue-specific regulation of intracellular thyroid status remains hidden due to the maintenance of constant circulating TH concentrations by the hypothalamic–pituitary–thyroid axis. This article is part of a Special Issue entitled Thyroid hormone signalling.

Thyroid hormone receptors are down-regulated in skeletal muscle of patients with non-thyroidal illness syndrome secondary to non-septic shock

AIM

Non-thyroidal illness syndrome (NTIS) is related to changes in thyroid hormone (TH) physiology. Skeletal muscle (SM) plays a major role in metabolism, and TH regulates SM phenotype and metabolism. We aimed to characterize the SM of non-septic shock NTIS patients in terms of: i) expression of genes and proteins involved in TH metabolism and actions; and ii) NFKB's pathway activation, a responsible factor for some of the phenotypic changes in NTIS. We also investigated whether the patient's serum can induce in vitro the effects observed in vivo.

METHODS

Serum samples and SM biopsies from 14 patients with non-septic shock NTIS and 11 controls. Gene and protein expression and NFKB1 activation were analyzed by quantitative PCR and immunoblotting. Human SM cell (HSkMC) cultures to investigate the effects of patient's serum on TH action mediators.

RESULTS

Patients with non-septic shock NTIS showed higher levels of pro-inflammatory cytokines than controls. Expression of TRβ (THRB), TRα1 (THRA), and retinoid X receptor γ (RXRG) was decreased in NTIS patients. RXRA gene expression was higher, but its protein was lower in NTIS than controls, suggesting the existence of a post-transcriptional mechanism that down-regulates protein levels. NFKB1 pathway activation was not different between NTIS and control patients. HSkMC incubated with patient's serum increased TH receptor and RXRG gene expression after 48  h.

CONCLUSIONS

Patients with non-septic shock NTIS showed decreased expression of TH receptors and RXRs, which were not related to increased activation of the NFKB1 pathway. These findings could not be replicated in cultures of HSkMCs incubated in the patient's serum.

Epigenetic mechanisms in the development of type 2 diabetes

Type 2 diabetes (T2D) is a disorder of complex genetics influenced by interactions between susceptible genetic loci and environmental perturbations. Intrauterine growth retardation is one such environmental perturbation linked to the development of T2D in adulthood. An abnormal metabolic intrauterine milieu affects fetal development by permanently modifying expression of key genes regulating beta-cell development (Pdx1) and glucose transport (Glut4) in muscle. Epigenetic regulation of gene expression is one mechanism by which genetic susceptibility and environmental insults can lead to T2D. Therefore, therapeutic agents targeting epigenetic gene regulation can ultimately be used to treat T2D; however, there is much to be learned about genome-wide epigenetic programming of health and disease before these therapies can be used in patient care.

Multiple signalling pathways redundantly control glucose transporter GLUT4 gene transcription in skeletal muscle

Increased glucose transporter GLUT4 expression in skeletal muscle is an important benefit of regular exercise, resulting in improved insulin sensitivity and glucose tolerance. The Ca(2+)-calmodulin-dependent kinase II (CaMKII), calcineurin and AMPK pathways have been implicated in GLUT4 gene regulation based on pharmacological evidence. Here, we have used a more specific genetic approach to establish the relative role of the three pathways in fast and slow muscles. Plasmids coding for protein inhibitors of CaMKII or calcineurin were co-transfected in vivo with a GLUT4 enhancer-reporter construct either in normal mice or in mice expressing a kinase dead (KD) AMPK mutant. GLUT4 reporter activity was not inhibited in the slow soleus muscle by blocking either CaMKII or calcineurin alone, but was inhibited by blocking both pathways. GLUT4 reporter activity was likewise unchanged in the soleus of KD-AMPK mice, but was significantly reduced by incapacitation of either CaMKII or calcineurin in these mice. On the other hand, in the fast tibialis anterior (TA) muscle, calcineurin appears to exert a prominent role in the control of GLUT4 reporter activity, independent of CaMKII and AMPK. The results point to a muscle type-specific and redundant regulation of GLUT4 enhancer based on the interplay of multiple signalling pathways, all of which are known to affect myocyte enhancing factor 2 (MEF2) transcriptional activity, a point of convergence of different pathways on muscle gene regulation.

Developmental origins of adult disease

Intrauterine growth retardation (IUGR) has been linked to development of type 2 diabetes in adulthood. Using a rat model, we tested the hypothesis that uteroplacental insufficiency disrupts the function of the electron transport chain in the fetal beta-cell and leads to a debilitating cascade of events. The net result is progressive loss of beta-cell function and eventual development of type 2 diabetes in the adult. Studies in the IUGR rat demonstrate that an abnormal intrauterine environment induces epigenetic modifications of key genes regulating beta-cell development; experiments directly link chromatin remodeling with suppression of transcription. Future research will be directed at elucidating the mechanisms underlying epigenetic modifications in offspring.

Contractile activity per se induces transcriptional activation of SLC2A4 gene in soleus muscle: involvement of MEF2D, HIF-1a, and TRalpha transcriptional factors

Skeletal muscle is a target tissue for approaches that can improve insulin sensitivity in insulin-resistant states. In muscles, glucose uptake is performed by the GLUT-4 protein, which is encoded by the SLC2A4 gene. SLC2A4 gene expression increases in response to conditions that improve insulin sensitivity, including chronic exercise. However, since chronic exercise improves insulin sensitivity, the increased SLC2A4 gene expression could not be clearly attributed to the muscle contractile activity per se and/or to the improved insulin sensitivity. The present study was designed to investigate the role of contractile activity per se in the regulation of SLC2A4 gene expression as well as in the participation of the transcriptional factors myocyte enhancer factor 2D (MEF2D), hypoxia inducible factor 1a (HIF-1a), and thyroid hormone receptor-alpha (TRalpha). The performed in vitro protocol excluded the interference of metabolic, hormonal, and neural effects. The results showed that, in response to 10 min of electrically induced contraction of soleus muscle, an early 40% increase in GLUT-4 mRNA (30 min) occurred, with a subsequent 65% increase (120 min) in GLUT-4 protein content. EMSA and supershift assays revealed that the stimulus rapidly increased the binding activity of MEF2D, HIF-1a, and TRalpha into the SLC2A4 gene promoter. Furthermore, chromatin immunoprecipitation assay confirmed, in native nucleosome, that contraction induced an approximate fourfold (P < 0.01) increase in MEF2D and HIF-1a-binding activity. In conclusion, muscle contraction per se enhances SLC2A4 gene expression and that involves MEF2D, HIF-1a, and TRalpha transcription factor activation. This finding reinforces the importance of physical activity to improve glycemic homeostasis independently of other additional insulin sensitizer approaches.

The effect of passive movement on denervated soleus highlights a differential nerve control on SERCA and MyHC isoforms

The sarco-endoplasmic reticulum Ca2+ ATP-ase (SERCA) and myosin heavy chain (MyHC) levels were measured in hindlimb-denervated and selectively denervated rat soleus muscles. Selective denervation allowed passive movement of the soleus, whereas hindlimb denervation rendered it to passivity. To minimize chronic effects, we followed the changes only for 2 weeks. Selective denervation resulted in less muscle atrophy, a faster slow-to-fast transition of MyHC isoforms, and less coordinated expressions of the slow vs fast isoforms of MyHC and SERCA. Generally, expression of the slow-twitch type SERCA2a was found to be less dependent, whereas the slow-twitch type MyHC1 was the most dependent on innervation. Our study shows that passive movement is able to ameliorate denervation-induced atrophy of the soleus and that it also accentuates the dyscoordination in the expression of the corresponding slow and fast isoforms of MyHC and SERCA.

Histone code modifications repress glucose transporter 4 expression in the intrauterine growth-restricted offspring

We examined transcriptional and epigenetic mechanism(s) behind diminished skeletal muscle GLUT4 mRNA in intrauterine growth-restricted (IUGR) female rat offspring. An increase in MEF2D (inhibitor) with a decline in MEF2A (activator) and MyoD (co-activator) binding to the glut4 promoter in IUGR versus control was observed. The functional role of MEF2/MyoD-binding sites and neighboring three CpG clusters in glut4 gene transcription was confirmed in C2C12 muscle cells. No differential methylation of these three and other CpG clusters in the glut4 promoter occurred. DNA methyltransferase 1 (DNMT1) in postnatal, DNMT3a, and DNMT3b in adult was differentially recruited with increased MeCP2 (methyl CpG-binding protein) concentrations to bind the IUGR glut4 gene. Covalent modifications of the histone (H) code consisted of H3.K14 de-acetylation by recruitment of histone deacetylase (HDAC) 1 and enhanced association of HDAC4 enzymes. This set the stage for Suv39H1 methylase-mediated di-methylation of H3.K9 and increased recruitment of heterochromatin protein 1alpha, which partially inactivates postnatal and adult IUGR glut4 gene transcription. Further increased interactions in the adult IUGR between DNMT3a/DNMT3b and HDAC1 and MEF2D and HDAC1/HDAC4 and decreased association between MyoD and MEF2A existed. We conclude that epigenetic mechanisms consisting of histone code modifications repress skeletal muscle glut4 transcription in the postnatal period and persist in the adult female IUGR offspring.

Identification of a novel modulator of thyroid hormone receptor-mediated action

Background

Diabetes is characterized by reduced thyroid function and altered myogenesis after muscle injury. Here we identify a novel component of thyroid hormone action that is repressed in diabetic rat muscle.

Methodology/Principal Findings

We have identified a gene, named DOR, abundantly expressed in insulin-sensitive tissues such as skeletal muscle and heart, whose expression is highly repressed in muscle from obese diabetic rats. DOR expression is up-regulated during muscle differentiation and its loss-of-function has a negative impact on gene expression programmes linked to myogenesis or driven by thyroid hormones. In agreement with this, DOR enhances the transcriptional activity of the thyroid hormone receptor TR_α1. This function is driven by the N-terminal part of the protein. Moreover, DOR physically interacts with TR _α1 and to T₃-responsive promoters, as shown by ChIP assays. T₃ stimulation also promotes the mobilization of DOR from its localization in nuclear PML bodies, thereby indicating that its nuclear localization and cellular function may be related.

Conclusions/Significance

Our data indicate that DOR modulates thyroid hormone function and controls myogenesis. DOR expression is down-regulated in skeletal muscle in diabetes. This finding may be of relevance for the alterations in muscle function associated with this disease.

Thyroid-State Influence on Protein-Expression Profile of Rat Skeletal Muscle

We analyzed the whole-cell protein content of gastrocnemius muscles from rats in different thyroid states. Twenty differentially expressed proteins were unambiguously identified. They were involved in substrates and energy metabolism, stress response, cell structure, and gene expression. This study represents the first systematic identification of thyroid state-induced changes in the skeletal muscle protein-expression profile and reveals new cellular pathways as targets for thyroid hormone action.

The GLUT4 glucose transporter

Few physiological parameters are more tightly and acutely regulated in humans than blood glucose concentration. The major cellular mechanism that diminishes blood glucose when carbohydrates are ingested is insulin-stimulated glucose transport into skeletal muscle. Skeletal muscle both stores glucose as glycogen and oxidizes it to produce energy following the transport step. The principal glucose transporter protein that mediates this uptake is GLUT4, which plays a key role in regulating whole body glucose homeostasis. This review focuses on recent advances on the biology of GLUT4.

Evidence for a mitochondrial regulatory pathway defined by peroxisome proliferator-activated receptor-gamma coactivator-1 alpha, estrogen-related receptor-alpha, and mitofusin 2

Mitofusin 2 (Mfn2) is a mitochondrial membrane protein that participates in mitochondrial fusion and regulates mitochondrial metabolism in mammalian cells. Here, we show that Mfn2 gene expression is induced in skeletal muscle and brown adipose tissue by conditions associated with enhanced energy expenditure, such as cold exposure or beta(3)-adrenergic agonist treatment. In keeping with the role of peroxisome proliferator-activated receptor-gamma coactivator (PGC)-1 alpha on energy expenditure, we demonstrate a stimulatory effect of PGC-1 alpha on Mfn2 mRNA and protein expression in muscle cells. PGC-1 alpha also stimulated the activity of the Mfn2 promoter, which required the integrity of estrogen-related receptor-alpha (ERR alpha)-binding elements located at -413/-398. ERR alpha also activated the transcriptional activity of the Mfn2 promoter, and the effects were synergic with those of PGC-1 alpha. Mfn2 loss of function reduced the stimulatory effect of PGC-1 alpha on mitochondrial membrane potential. Exposure to cold substantially increased Mfn2 gene expression in skeletal muscle from heterozygous Mfn2 knock-out mice, which occurred in the presence of higher levels of PGC-1 alpha mRNA compared with control mice. Our results indicate the existence of a regulatory pathway involving PGC-1 alpha, ERR alpha, and Mfn2. Alterations in this regulatory pathway may participate in the pathophysiology of insulin-resistant conditions and type 2 diabetes.

Exercise increases MEF2- and GEF DNA-binding activity in human skeletal muscle

Overexpression of GLUT4 exclusively in skeletal muscle enhances insulin action and improves glucose homeostasis. Transgenic studies have discovered two regions on the GLUT4 promoter conserved across several species that are required for normal GLUT4 expression in skeletal muscle. These regions contain binding motifs for the myocyte enhancer factor 2 (MEF2) family and GLUT4 enhancer factor (GEF). A single bout of exercise increases both GLUT4 transcription and mRNA abundance; however, the molecular mechanisms mediating this response remain largely unexplored. Thus, the aim of this study was to determine whether a single, acute bout of exercise increased the DNA-binding activities of MEF2 and GEF in human skeletal muscle. Seven subjects performed 60 min of cycling at approximately 70% of VO2peak. After exercise, the DNA-binding activities of both the MEF2A/D heterodimer and GEF were increased (P<0.05). There was no change in nuclear MEF2D or GEF abundance after exercise, but nuclear MEF2A abundance was increased (P<0.05). These data demonstrate that exercise increases MEF2 and GEF DNA binding and imply that these transcription factors could be potential targets for modulating GLUT4 expression in human skeletal muscle.

Skeletal muscle and nuclear hormone receptors: implications for cardiovascular and metabolic disease

Skeletal muscle is a major mass peripheral tissue that accounts for approximately 40% of the total body mass and a major player in energy balance. It accounts for >30% of energy expenditure, is the primary tissue of insulin stimulated glucose uptake, disposal, and storage. Furthermore, it influences metabolism via modulation of circulating and stored lipid (and cholesterol) flux. Lipid catabolism supplies up to 70% of the energy requirements for resting muscle. However, initial aerobic exercise utilizes stored muscle glycogen but as exercise continues, glucose and stored muscle triglycerides become important energy substrates. Endurance exercise increasingly depends on fatty acid oxidation (and lipid mobilization from other tissues). This underscores the importance of lipid and glucose utilization as an energy source in muscle. Consequently skeletal muscle has a significant role in insulin sensitivity, the blood lipid profile, and obesity. Moreover, caloric excess, obesity and physical inactivity lead to skeletal muscle insulin resistance, a risk factor for the development of type II diabetes. In this context skeletal muscle is an important therapeutic target in the battle against cardiovascular disease, the worlds most serious public health threat. Major risk factors for cardiovascular disease include dyslipidemia, hypertension, obesity, sedentary lifestyle, and diabetes. These risk factors are directly influenced by diet, metabolism and physical activity. Metabolism is largely regulated by nuclear hormone receptors which function as hormone regulated transcription factors that bind DNA and mediate the patho-physiological regulation of gene expression. Metabolism and activity, which directly influence cardiovascular disease risk factors, are primarily driven by skeletal muscle. Recently, many nuclear receptors expressed in skeletal muscle have been shown to improve glucose tolerance, insulin resistance, and dyslipidemia. Skeletal muscle and nuclear receptors are rapidly emerging as critical targets in the battle against cardiovascular disease risk factors. Understanding the function of nuclear receptors in skeletal muscle has enormous pharmacological utility for the treatment of cardiovascular disease. This review focuses on the molecular regulation of metabolism by nuclear receptors in skeletal muscle in the context of dyslipidemia and cardiovascular disease.

Role of histone and transcription factor acetylation in diabetes pathogenesis

Globally, diabetes (and, in particular, type 2 diabetes) represents a major challenge to world health. Currently in the United States, the costs of treating diabetes and its associated complications exceed 100 billion US dollars annually, and this figure is expected to soar in the near future. Despite decades of intense research efforts, the genetic basis of the events involved in the pathogenesis of diabetes is still poorly understood. Diabetes is a complex multigenic syndrome primarily due to beta-cell dysfunction associated with a variable degree of insulin resistance. Recent advances have led to exciting new developments with regard to our understanding of the mechanisms that regulate insulin transcription. These include data that implicate chromatin as a critical regulator of this event. The 'Histone Code' is a widely accepted hypothesis, whereby sequential modifications to the histones in chromatin lead to regulated transcription of genes. One of the modifications used in the histone code is acetylation. This is probably the best characterized modification of histones, which is carried out under the control of histone acetyltransferases (HATs) and histone deacetylases (HDACs). These enzymes also regulate the activity of a number of transcription factors through acetylation. Increasing evidence links possible dysregulation of these mechanisms in the pathogenesis of diabetes, with important therapeutic implications.

Mechanisms regulating GLUT4 glucose transporter expression and glucose transport in skeletal muscle

Skeletal muscle is a major glucose-utilizing tissue in the absorptive state and the major glucose transporter expressed in muscle in adulthood is GLUT4. GLUT4 expression is exquisitely regulated in muscle and this seems important in the regulation of insulin-stimulated glucose uptake by this tissues. Thus, muscle GLUT4 overexpression in transgenic animals ameliorates insulin resistance associated with obesity or diabetes. Recent information indicates that glut4 gene transcription is regulated by a number of factors in skeletal muscle that include MEF2, MyoD myogenic proteins, thyroid hormone receptors, Kruppel-like factor KLF15, NF1, Olf-1/Early B cell factor and GEF/HDBP1. In addition, studies in vivo indicate that under normal conditions the activity of the muscle-specific GLUT4 enhancer is low in adult skeletal muscle compared with the maximal potential activity that it can attain at high levels of the MRF transcription factors, MEF2, and TRalpha1. This finding indicates that glut4 transcription may be greatly up-regulated via activation of this enhancer through an increase in the levels of expression or activity of these transcription factors. Understanding the molecular basis of the expression of glut4 will be useful for the appropriate therapeutic design of treatments for insulin-resistant states. The nature of the intracellular signals that mediate the stimulation of glucose transport in response to insulin or exercise is also reviewed.

Control of muscle bioenergetic gene expression: implications for allometric scaling relationships of glycolytic and oxidative enzymes

Muscle metabolic properties vary with body size, with larger animals relying relatively less on oxidative metabolism as a result of lower specific activities of mitochondrial enzymes and greater specific activities of glycolytic enzymes. While many have argued reasons why such relationships might be grounded in physical relationships, an explanation for the regulatory basis of the differences in enzyme levels remains unexplored. Focusing on skeletal muscle, we review potential cellular and genetic explanations for the relationship between bioenergetic enzymes and body mass. Differences in myonuclear domain (the ratio of fiber volume to nuclei number) in conjunction with constitutive expression may explain part of the variation in mitochondrial content among fiber types and species. Superimposed on such constitutive determinants are (1) extrinsic signalling pathways that control the muscle contractile and metabolic phenotype and (2) intrinsic signalling pathways that translate changes in cellular milieu (ions, metabolites, oxygen,redox) arising through the contractile phenotype into changes in enzyme synthesis. These signalling pathways work through transcriptional regulation,as well as post-transcriptional, translational and post-translational regulation, acting via synthesis and degradation.