Tibetan PHD2, an allele with loss-of-function properties.

Tibetans have adapted to the chronic hypoxia of high altitude and display a distinctive suite of physiologic adaptations, including augmented hypoxic ventilatory response and resistance to pulmonary hypertension. Genome-wide studies have consistently identified compelling genetic signatures of natural selection in two genes of the Hypoxia Inducible Factor pathway, PHD2 and HIF2A The product of the former induces the degradation of the product of the latter. Key issues regarding Tibetan PHD2 are whether it is a gain-of-function or loss-of-function allele, and how it might contribute to high-altitude adaptation. Tibetan PHD2 possesses two amino acid changes, D4E and C127S. We previously showed that in vitro, Tibetan PHD2 is defective in its interaction with p23, a cochaperone of the HSP90 pathway, and we proposed that Tibetan PHD2 is a loss-of-function allele. Here, we report that additional PHD2 mutations at or near Asp-4 or Cys-127 impair interaction with p23 in vitro. We find that mice with the Tibetan Phd2 allele display augmented hypoxic ventilatory response, supporting this loss-of-function proposal. This is phenocopied by mice with a mutation in p23 that abrogates the PHD2:p23 interaction. Hif2a haploinsufficiency, but not the Tibetan Phd2 allele, ameliorates hypoxia-induced increases in right ventricular systolic pressure. The Tibetan Phd2 allele is not associated with hemoglobin levels in mice. We propose that Tibetans possess genetic alterations that both activate and inhibit selective outputs of the HIF pathway to facilitate successful adaptation to the chronic hypoxia of high altitude.

Mitochondrial ultrastructural adaptations in fast muscles of mice lacking IL15RA.

The pro-inflammatory cytokine interleukin-15 (IL15) and its receptor α (IL15RA) participate in the regulation of musculoskeletal function and metabolism. Deletion of the Il15ra gene in mice increases spontaneous activity, improves fatigue resistance in the glycolytic extensor digitorum longus (EDL) and protects from diet-induced obesity. In humans, IL15RA single-nucleotide polymorphisms (SNPs) have been linked to muscle strength, metabolism and performance in elite endurance athletes. Taken together, these features suggest a possible role for IL15RA in muscle mitochondrial structure and function. Here, we have investigated the consequences of loss of IL15RA on skeletal muscle fiber-type properties and mitochondrial ultrastructure. Immunostaining of the EDL for myosin heavy chain (MyHC) isoforms revealed no significant changes in fiber type. Electron microscopy (EM) analysis of the EDL indicated an overall higher mitochondria content, and increased cristae density in subsarcolemmal and A-band mitochondrial subpopulations. The higher cristae density in Il15ra-/- mitochondria was associated with higher OPA1 and cardiolipin levels. Overall, these data extend our understanding of the role of IL15RA signaling in muscle oxidative metabolism and adaptation to exercise.

Elongated mitochondrial constrictions and fission in muscle fatigue.

Mitochondria respond to stress and undergo fusion and fission at variable rates, depending on cell status. To understand mitochondrial behavior during muscle fatigue, we investigated mitochondrial ultrastructure and expression levels of a fission- and stress-related protein in fast-twitch muscle fibers of mice subjected to fatigue testing. Mice were subjected to running at increasing speed until exhaustion at 45 min-1 h. In further experiments, high-intensity muscle stimulation through the sciatic nerve simulated the forced treadmill exercise. We detected a rare phenotype characterized by elongated mitochondrial constrictions (EMCs) connecting two separate segments of the original organelles. EMCs are rare in resting muscles and their frequency increases, albeit still at low levels, in stimulated muscles. The constrictions are accompanied by elevated phosphorylation of Drp1 (Dnm1l) at Ser 616, indicating an increased translocation of Drp1 to the mitochondrial membrane. This is indicative of a mitochondrial stress response, perhaps leading to or facilitating a long-lasting fission event. A close apposition of sarcoplasmic reticulum (SR) to the constricted areas, detected using both transmission and scanning electron microscopy, is highly suggestive of SR involvement in inducing mitochondrial constrictions.

The nuclear receptor Rev-erbα controls circadian thermogenic plasticity.

Circadian oscillation of body temperature is a basic, evolutionarily conserved feature of mammalian biology. In addition, homeostatic pathways allow organisms to protect their core temperatures in response to cold exposure. However, the mechanism responsible for coordinating daily body temperature rhythm and adaptability to environmental challenges is unknown. Here we show that the nuclear receptor Rev-erbα (also known as Nr1d1), a powerful transcriptional repressor, links circadian and thermogenic networks through the regulation of brown adipose tissue (BAT) function. Mice exposed to cold fare considerably better at 05:00 (Zeitgeber time 22) when Rev-erbα is barely expressed than at 17:00 (Zeitgeber time 10) when Rev-erbα is abundant. Deletion of Rev-erbα markedly improves cold tolerance at 17:00, indicating that overcoming Rev-erbα-dependent repression is a fundamental feature of the thermogenic response to cold. Physiological induction of uncoupling protein 1 (Ucp1) by cold temperatures is preceded by rapid downregulation of Rev-erbα in BAT. Rev-erbα represses Ucp1 in a brown-adipose-cell-autonomous manner and BAT Ucp1 levels are high in Rev-erbα-null mice, even at thermoneutrality. Genetic loss of Rev-erbα also abolishes normal rhythms of body temperature and BAT activity. Thus, Rev-erbα acts as a thermogenic focal point required for establishing and maintaining body temperature rhythm in a manner that is adaptable to environmental demands.

IL-15Rα is a determinant of muscle fuel utilization, and its loss protects against obesity.

IL-15Rα is the widely expressed primary binding partner for IL-15. Because of the wide distribution in nonlymphoid tissues like skeletal muscle, adipose, or liver, IL-15/IL-15Rα take part in physiological and metabolic processes not directly related to immunity. In fast muscle, lack of IL-15Rα promotes an oxidative switch, with increased mitochondrial biogenesis and fatigue resistance. These effects are predicted to reproduce some of the benefits of exercise and, therefore, improve energy homeostasis. However, the direct effects of IL-15Rα on metabolism and obesity are currently unknown. We report that mice lacking IL-15Rα (IL-15Rα(-/-)) are resistant to diet-induced obesity (DIO). High-fat diet-fed IL-15Rα(-/-) mice have less body and liver fat accumulation than controls. The leaner phenotype is associated with increased energy expenditure and enhanced fatty acid oxidation by muscle mitochondria. Despite being protected against DIO, IL-15Rα(-/-) are hyperglycemic and insulin-resistant. These findings identify novel roles for IL-15Rα in metabolism and obesity.

AKT controls protein synthesis and oxidative metabolism via combined mTORC1 and FOXO1 signalling to govern muscle physiology.

BACKGROUND: Skeletomuscular diseases result in significant muscle loss and decreased performance, paralleled by a loss in mitochondrial and oxidative capacity. Insulin and insulin-like growth factor-1 (IGF-1) are two potent anabolic hormones that activate a host of signalling intermediates including the serine/threonine kinase AKT to influence skeletal muscle physiology. Defective AKT signalling is associated with muscle pathology, including cachexia, sarcopenia, and disuse; however, the mechanistic underpinnings remain unresolved. METHODS: To elucidate the role of AKT signalling in muscle mass and physiology, we generated both congenital and inducible mouse models of skeletal muscle-specific AKT deficiency. To understand the downstream mechanisms mediating AKT's effects on muscle biology, we generated mice lacking AKT1/2 and FOXO1 (M-AKTFOXO1TKO and M-indAKTFOXO1TKO) to inhibit downstream FOXO1 signalling, AKT1/2 and TSC1 (M-AKTTSCTKO and M-indAKTTSCTKO) to activate mTORC1, and AKT1/2, FOXO1, and TSC1 (M-QKO and M-indQKO) to simultaneously activate mTORC1 and inhibit FOXO1 in AKT-deficient skeletal muscle. Muscle proteostasis and physiology were assessed using multiple assays including metabolic labelling, mitochondrial function, fibre typing, ex vivo physiology, and exercise performance. RESULTS: Here, we show that genetic ablation of skeletal muscle AKT signalling resulted in decreased muscle mass and a loss of oxidative metabolism and muscle performance. Specifically, deletion of muscle AKT activity during development or in adult mice resulted in a significant reduction in muscle growth by 30-40% (P  < 0.0001; n = 12-20) and 15% (P < 0.01 and P < 0.0001; n = 20-30), respectively. Interestingly, this reduction in muscle mass was primarily due to an ~40% reduction in protein synthesis in both M-AKTDKO and M-indAKTDKO muscles (P < 0.05 and P < 0.01; n = 12-20) without significant changes in proteolysis or autophagy. Moreover, a significant reduction in oxidative capacity was observed in both M-AKTDKO (P < 0.05, P < 0.01 and P < 0.001; n = 5-12) and M-indAKTDKO (P < 0.05 and P < 0.01; n = 4). Mechanistically, activation and inhibition of mTORC1/FOXO1, respectively, but neither alone, were sufficient to restore protein synthesis, muscle oxidative capacity, and muscle function in the absence of AKT in vivo. In a mouse model of disuse-induced muscle loss, simultaneous activation of mTORC1 and inhibition of FOXO1 preserved muscle mass following immobilization (~5-10% reduction in casted M-indFOXO1TSCDKO muscles vs. ~30-40% casted M-indControl muscles, P < 0.05 and P < 0.0001; n = 8-16). CONCLUSIONS: Collectively, this study provides novel insights into the AKT-dependent mechanisms that underlie muscle protein homeostasis, function, and metabolism in both normal physiology and disuse-induced muscle wasting.

Inhibition of mitochondrial fission favours mutant over wild-type mitochondrial DNA.

Biased segregation of mitochondrial DNA variants has been widely documented, but little was known about its molecular basis. We set out to test the hypothesis that altering the balance between mitochondrial fusion and fission could influence the segregation of mutant and wild-type mtDNA variants, because it would modify the number of organelles per cell. Therefore human cells heteroplasmic for the pathological A3243G mitochondrial DNA mutation were transfected with constructs designed to silence Drp1 or hFis1, whose gene products are required for mitochondrial fission. Drp1 and hFis1 gene silencing were both associated with increased levels of mutant mitochondrial DNA. Thus, the extent of the mitochondrial reticular network appears to be an important factor in determining mutant load. The fact that the level of mutant and wild-type mitochondrial DNA can be manipulated by altering the expression of nuclear encoded factors involved in mitochondrial fission suggests new interventions for mitochondrial DNA disorders.

Examining the role of cyclin D1 in breast cancer.

Cyclin D1 overexpression is found in more than 50% of human breast cancers and causes mammary cancer in transgenic mice. Dysregulation of cyclin D1 gene expression or function contributes to the loss of normal cell cycle control during tumorigenesis. Recent studies have demonstrated that cyclin D1 conducts additional specific functions to regulate gene expression in the context of local chromatin, promote cellular migration and inhibit mitochondrial metabolism. It is anticipated that these additional functions contribute to the pathology associated with dysregulated cyclin D1 abundance. This article discusses evidence that examines the significance of cyclin D1 in breast cancer with emphasis on its role in breast cancer stem cell expansion.

Persistent NF-κB activation in muscle stem cells induces proliferation-independent telomere shortening.

During the repeated cycles of damage and repair in many muscle disorders, including Duchenne muscular dystrophy (DMD), the muscle stem cell (MuSC) pool becomes less efficient at responding to and repairing damage. The underlying mechanism of such stem cell dysfunction is not fully known. Here, we demonstrate that the distinct early telomere shortening of diseased MuSCs in both mice and young DMD patients is associated with aberrant NF-κB activation. We find that prolonged NF-κB activation in MuSCs in chronic injuries leads to shortened telomeres and Ku80 dysregulation and results in severe skeletal muscle defects. Our studies provide evidence of a role for NF-κB in regulating stem-cell-specific telomere length, independently of cell replication, and could be a congruent mechanism that is applicable to additional tissues and/or diseases characterized by systemic chronic inflammation.

A novel deletion in the GTPase domain of OPA1 causes defects in mitochondrial morphology and distribution, but not in function.

Autosomal dominant optic atrophy (ADOA), the commonest cause of inherited optic atrophy, is caused by mutations in the ubiquitously expressed gene optic atrophy 1 (OPA1), involved in fusion and biogenesis of the inner membrane of mitochondria. Bioenergetic failure, mitochondrial network abnormalities and increased apoptosis have all been proposed as possible causal factors. However, their relative contribution to pathogenesis as well as the prominent susceptibility of the retinal ganglion cell (RGC) in this disease remains uncertain. Here we identify a novel deletion of OPA1 gene in the GTPase domain in three patients affected by ADOA. Muscle biopsy of the patients showed neurogenic atrophy and abnormal morphology and distribution of mitochondria. Confocal microscopy revealed increased mitochondrial fragmentation in fibroblasts as well as in myotubes, where mitochondria were also unevenly distributed, with clustered organelles alternating with areas where mitochondria were sparse. These abnormalities were not associated with altered bioenergetics or increased susceptibility to pro-apoptotic stimuli. Therefore, changes in mitochondrial shape and distribution can be independent of other reported effects of OPA1 mutations, and therefore may be the primary cause of the disease. The arrangement of mitochondria in RGCs, which degenerate in ADOA, may be exquisitely sensitive to disturbance, and this may lead to bioenergetic crisis and/or induction of apoptosis. Our results highlight the importance of mitochondrial dynamics in the disease per se, and point to the loss of the fine positioning of mitochondria in the axons of RGCs as a possible explanation for their predominant degeneration in ADOA.

Overexpression of microRNA-206 in the skeletal muscle from myotonic dystrophy type 1 patients.

BACKGROUND: MicroRNAs are highly conserved, noncoding RNAs involved in post-transcriptional gene silencing. They have been shown to participate in a wide range of biological processes, including myogenesis and muscle regeneration. The goal of this study is to test the hypothesis that myo-miRs (myo = muscle + miR = miRNA) expression is altered in muscle from patients affected by myotonic dystrophy type 1 (DM1), the most frequently inherited neuromuscular disease in adults. In order to gain better insights about the role of miRNAs in the DM1 pathogenesis, we have also analyzed the muscular expression of miR-103 and miR-107, which have been identified in silico as attractive candidates for binding to the DMPK mRNA. METHODS: To this aim, we have profiled the expression of miR-133 (miR-133a, miR-133b), miR-1, miR-181 (miR-181a, miR-181b, miR-181c) and miR-206, that are specifically induced during myogenesis in cardiac and skeletal muscle tissues. miR-103 and miR-107, highly expressed in brain, heart and muscle have also been included in this study. QRT-PCR experiments have been performed on RNA from vastus lateralis biopsies of DM1 patients (n = 7) and control subjects (n = 4). Results of miRNAs expression have been confirmed by Northern blot, whereas in situ hybridization technique have been performed to localize misexpressed miRNAs on muscle sections from DM1 and control individuals. RESULTS: Only miR-206 showed an over-expression in 5 of 7 DM1 patients (threshold = 2, fold change between 1.20 and 13.22, average = 5.37) compared to the control group. This result has been further confirmed by Northern blot analysis (3.37-fold overexpression, R2 = 0.89). In situ hybridization localized miR-206 to nuclear site both in normal and DM1 tissues. Cellular distribution in DM1 tissues includes also the nuclear regions of centralized nuclei, with a strong signal corresponding to nuclear clumps. CONCLUSIONS: This work provides, for the first time, evidences about miRNAs misexpression in DM1 muscle tissues, adding a new element in the pathogenesis of this complex genetic disease.

PMO-based let-7c site blocking oligonucleotide (SBO) mediated utrophin upregulation in mdx mice, a therapeutic approach for Duchenne muscular dystrophy (DMD).

Upregulation of utrophin, a dystrophin related protein, is considered a promising therapeutic approach for Duchenne muscular dystrophy (DMD). Utrophin expression is repressed at the post-transcriptional level by a set of miRNAs, among which let-7c is evolutionarily highly conserved. We designed PMO-based SBOs complementary to the let-7c binding site in UTRN 3'UTR, with the goal of inhibiting let-7c interaction with UTRN mRNA and thus upregulating utrophin. We used the C2C12UTRN5'luc3' reporter cell line in which the 5'- and 3'-UTRs of human UTRN sequences flank luciferase, for reporter assays and the C2C12 cell line for utrophin western blots, to independently evaluate the site blocking efficiency of a series of let-7c PMOs in vitro. Treatment of one-month old mdx mice with the most effective let-7c PMO (i.e. S56) resulted in ca. two-fold higher utrophin protein expression in skeletal muscles and the improvement in dystrophic pathophysiology in mdx mice, in vivo. In summary, we show that PMO-based let-7c SBO has potential applicability for upregulating utrophin expression as a therapeutic approach for DMD.

Functional effects of muscle PGC-1alpha in aged animals.

PGC-1 (peroxisome-proliferator-activated receptor-γ coactivator-1) alpha is a potent transcriptional coactivator that coordinates the activation of numerous metabolic processes. Exercise strongly induces PGC-1alpha expression in muscle, and overexpression of PGC-1alpha in skeletal muscle activates mitochondrial oxidative metabolism and neovascularization, leading to markedly increased endurance. In light of these findings, PGC-1alpha has been proposed to protect from age-associated sarcopenia, bone loss, and whole-body metabolic dysfunction, although these findings have been controversial. We therefore comprehensively evaluated muscle and whole-body function and metabolism in 24-month-old transgenic mice that over-express PGC-1alpha in skeletal muscle. We find that the powerful effects of PGC-1alpha on promoting muscle oxidative capacity and protection from muscle fatigability persist in aged animals, although at the expense of muscle strength. However, skeletal muscle PGC-1alpha does not prevent bone loss and in fact accentuates it, nor does it have long-term benefit on whole-body metabolic composition or insulin sensitivity. Protection from sarcopenia is seen in male animals with overexpression of PGC-1alpha in skeletal muscle but not in female animals. In summary, muscle-specific expression of PGC-1alpha into old age has beneficial effects on muscle fatigability and may protect from sarcopenia in males, but does not improve whole-body metabolism and appears to worsen age-related trabecular bone loss.

Genome Editing-Mediated Utrophin Upregulation in Duchenne Muscular Dystrophy Stem Cells.

Utrophin upregulation is considered a promising therapeutic strategy for Duchenne muscular dystrophy (DMD). A number of microRNAs (miRNAs) post-transcriptionally regulate utrophin expression by binding their cognate sites in the 3' UTR. Previously we have shown that miRNA: UTRN repression can be alleviated using miRNA let-7c site blocking oligonucleotides (SBOs) to achieve utrophin upregulation and functional improvement in mdx mice. Here, we used CRISPR/Cas9-mediated genome editing to delete five miRNA binding sites (miR-150, miR-296-5p, miR-133b, let-7c, miR-196b) clustered in a 500 bp inhibitory miRNA target region (IMTR) within the UTRN 3' UTR, for achieving higher expression of endogenous utrophin. Deleting the UTRN IMTR in DMD patient-derived human induced pluripotent stem cells (DMD-hiPSCs) resulted in ca. 2-fold higher levels of utrophin protein. Differentiation of the UTRN edited DMD-hiPSCs (UTRNΔIMTR) by MyoD overexpression resulted in increased sarcolemmal α-sarcoglycan staining consistent with improved dystrophin glycoprotein complex (DGC) restoration. These results demonstrate that CRISPR/Cas9-based UTRN genome editing offers a novel utrophin upregulation therapeutic strategy applicable to all DMD patients, irrespective of the dystrophin mutation status.

Author Correction: High-throughput identification of post-transcriptional utrophin up-regulators for Duchenne muscle dystrophy (DMD) therapy.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

High-throughput identification of post-transcriptional utrophin up-regulators for Duchenne muscle dystrophy (DMD) therapy.

Upregulation of endogenous utrophin offers great promise for treating DMD, as it can functionally compensate for the lack of dystrophin caused by DMD gene mutations, without the immunogenic concerns associated with delivering dystrophin. However, post-transcriptional repression mechanisms targeting the 5' and 3' untranslated regions (UTRs) of utrophin mRNA significantly limit the magnitude of utrophin upregulation achievable by promoter activation. Using a utrophin 5'3'UTR reporter assay, we performed a high-throughput screen (HTS) for small molecules capable of relieving utrophin post-transcriptional repression. We identified 27 hits that were ranked using a using an algorithm that we designed for hit prioritization that we call Hit to Lead Prioritization Score (H2LPS). The top 10 hits were validated using an orthogonal assay for endogenous utrophin expression. Evaluation of the top scoring hit, Trichostatin A (TSA), demonstrated utrophin upregulation and functional improvement in the mdx mouse model of DMD. TSA and the other small molecules identified here represent potential starting points for DMD drug discovery efforts.

Development and characterization of polyspecific anti-mitochondrion antibodies for proteomics studies on in toto tissue homogenates.

We describe the characterization of polyclonal antibodies directed against the whole mitochondrial subproteome, as obtained by hyperimmunization of rabbits with an organelle fraction purified from human skeletal muscle and lysed by sonication. After 2-DE separations with either blue native electrophoresis or IPG as first dimension and blotting, the polyspecific antibodies detect 113 proteins in human muscle mitochondria, representative of all major biochemical pathways and oxidative phosphorylation (OXPHOS) complexes, and cross-react with 28 proteins in rat heart mitochondria. Using as sample cryosections of human muscle biopsies lysed in urea/thiourea/CHAPS, the mitochondrial subproteome can be detected against the background of contractile proteins. When comparing with controls samples from mitochondrial encephalomyopathy with lactic acidosis and stroke-like episodes patients, immunoblotting shows in the latter a drastic reduction for the subunits of OXPHOS complex I as well as an increase of several enzymes, including ATP synthase. This finding is the first evidence at the proteomic level of massive up-regulation in a number of metabolic pathways by which the affected tissues try to compensate for the deficit in the OXPHOS machinery.

New Insights into the Lactate Shuttle: Role of MCT4 in the Modulation of the Exercise Capacity.

Lactate produced by muscle during high-intensity activity is an important end product of glycolysis that supports whole body metabolism. The lactate shuttle model suggested that lactate produced by glycolytic muscle fibers is utilized by oxidative fibers. MCT4 is a proton coupled monocarboxylate transporter preferentially expressed in glycolytic muscle fibers and facilitates the lactate efflux. Here we investigated the exercise capacity of mice with disrupted lactate shuttle due to global deletion of MCT4 (MCT4-/-) or muscle-specific deletion of the accessory protein Basigin (iMSBsg-/-). Although MCT4-/- and iMSBsg-/- mice have normal muscle morphology and contractility, only MCT4-/- mice exhibit an exercise intolerant phenotype. In vivo measurements of compound muscle action potentials showed a decrement in the evoked response in the MCT4-/- mice. This was accompanied by a significant structural degeneration of the neuromuscular junctions (NMJs). We propose that disruption of the lactate shuttle impacts motor function and destabilizes the motor unit.

Effect of Interleukin-15 Receptor Alpha Ablation on the Metabolic Responses to Moderate Exercise Simulated by in vivo Isometric Muscle Contractions.

Lack of interleukin 15 receptor alpha (IL15RA) increases spontaneous activity, exercise capacity and protects from diet-induced obesity by enhancing muscle energy metabolism, suggesting a role as exercise mimetic for IL15RA antagonists. Using controlled in vivo muscle stimulation mimicking moderate exercise in normal and Il15ra-/- mice, we mapped and contrasted the metabolic pathways activated upon stimulation or deletion of IL15RA. Stimulation caused the differential regulation of 123 out of the 321 detected metabolites (FDR ≤ 0.05 and fold change ≥ ±1.5). The main energy pathways activated were fatty acid oxidation, nucleotide metabolism, and anaplerotic reactions. Notably, resting Il15ra-/- muscles were primed in a semi-exercised state, characterized by higher pool sizes of fatty acids oxidized to support muscle activity. These studies identify the role of IL15RA in the system-wide metabolic response to exercise and should enable translational studies to harness the potential of IL15RA blockade as a novel exercise mimetic strategy.

The HDAC3 enzymatic activity regulates skeletal muscle fuel metabolism.

Histone deacetylase 3 (HDAC3) is a major HDAC, whose enzymatic activity is targeted by small molecule inhibitors for treating a variety of conditions. However, its enzymatic activity is largely dispensable for its function in embryonic development and hepatic lipid metabolism. HDAC3 plays a pivotal role in regulating muscle fuel metabolism and contractile function. Here, we address whether these muscular functions of HDAC3 require its enzymatic activity. By mutating the NCoR/SMRT corepressors in a knock-in mouse model named NS-DADm, we ablated the enzymatic activity of HDAC3 without affecting its protein levels. Compared to the control mice, skeletal muscles from NS-DADm mice showed lower force generation, enhanced fatigue resistance, enhanced fatty acid oxidation, reduced glucose uptake during exercise, upregulated expression of metabolic genes involved in branched-chain amino acids catabolism, and reduced muscle mass during aging, without changes in the muscle fiber-type composition or mitochondrial protein content. These muscular phenotypes are similar to those observed in the HDAC3-depleted skeletal muscles, which demonstrates that, unlike that in the liver or embryonic development, the metabolic function of HDAC3 in skeletal muscles requires its enzymatic activity. These results suggest that drugs specifically targeting HDAC3 enzyme activity could be developed and tested to modulate muscle energy metabolism and exercise performance.