Gene expression profiling of skeletal muscles treated with a soluble activin type IIB receptor

YTHDF2 regulates MSS51 expression contributing to mitochondria dysfunction of granulosa cells in polycystic ovarian syndrome patients

RESEARCH QUESTION

Granulosa cells (GCs) dysfunction plays a crucial role in the pathogenesis of polycystic ovary syndrome (PCOS). It is reported that YTH domain-containing family protein 2 (YTHDF2) is upregulated in mural GCs of PCOS patients. What effect does the differential expression of YTHDF2 have in PCOS patients?

DESIGN

Mural GCs and cumulus GCs from 15 patients with PCOS and 15 ovulatory controls and 4 cases of pathological sections in each group were collected. Real-time PCR, Western Blot, immunohistochemistry, and immunofluorescence experiments were conducted to detect gene and protein expression. RNA immunoprecipitation assay was performed to evaluate the binding relationship between YTHDF2 and MSS51. Mitochondrial morphology, cellular ATP and ROS levels and glycolysis-related gene expression were detected after YTHDF2 overexpression or MSS51 inhibition.

RESULTS

In the present study, we found that YTHDF2 was upregulated in GCs of PCOS patients while MSS51 was downregulated. YTHDF2 protein can bind to MSS51 mRNA and affect MSS51 expression. The reduction of MSS51 expression or the increase in YTHDF2 expression can lead to mitochondrial damage, reduced ATP levels, increased ROS levels and reduced expression of LDHA, PFKP and PKM.

CONCLUSIONS

YTHDF2 may regulate the expression of MSS51, affecting the structure and function of mitochondria in GCs and interfering with cellular glycolysis, which may disturb the normal biological processes of GCs and follicle development in PCOS patients.

Mss51 protein inhibition serves as a novel target for type 2 diabetes: a molecular docking and simulation study

Myostatin is a widely recognized inhibitory factor of skeletal muscle growth and significantly influences muscle development and metabolism. In mice, myostatin inhibition improves insulin sensitivity, increases glucose uptake by skeletal muscle, and reduces body fat. Furthermore, Mss51 is downregulated in response to myostatin inhibition, and its deletion appears to improve the metabolic state of skeletal muscle and reduce adipose tissue, which makes Mss51 a potential target for the treatment of obesity and type 2 diabetes. Here, we report a computationally predicted and validated three-dimensional structure of Mss51. Computational screening was used to identify naturally occurring compounds from the Herbal and Specs chemical database that might inhibit Mss51, based on binding affinities and physiochemical and ADMET properties. ZINC00338371, ZINC95099599 and ZINC08214878 were found to bind to Mss51 with high binding affinity and specificity. In addition, 100 ns molecular dynamics simulations were conducted to assess the stabilities of the interactions between the three compounds and Mss51. MD simulation demonstrated that all three compounds bind to the active pocket site of Mss51 stably and cause conformation changes. ZINC00338371 was found to bind most stably with binding free energy -229.022 ± 13.776 kJ/mol to Mss51, suggesting that it has therapeutic potential as a treatment option for obesity and type 2 diabetes.Communicated by Ramaswamy H. Sarma.

Identification of Male-Specific Molecular Marker and Development of PCR-Based Genetic Sex Identification Technique in Spotted Knifejaw (Oplegnathus punctatus)

Spotted knifejaw (Oplegnathus punctatus) is a marine teleost species that is economically important for aquaculture and marine pasture proliferation and shows obvious bisexual growth dimorphism, but molecular sex markers are currently lacking. A 290 bp (base pair) insertion with two fragments (230 bp and 60 bp) was identified in male individuals of O. punctatus based on whole-genome sequencing scanning and structural variation analyses. The gene annotation results showed that the insertion event occurred in the Igfn1 gene of male O. punctatus. The results of amino acid analysis further showed that the insertion event resulted in the functional variation of Igfn1 in male O. punctatus, and recombination caused the inactivation of Igfn1. According to the male-specific insertion information, we designed a PCR-based genetic amplification technique for rapid sex identification in O. punctatus. The results of agarose gel electrophoresis showed that two DNA fragments of 635 bp and 925 bp were amplified in male O. punctatus, while only a single DNA fragment of 635 bp was amplified in female individuals. The sex of individuals identified by this method was consistent with their known phenotypic sex, which will improve sex identification efficiency. This method provides a new DNA marker for rapid sex identification in O. punctatus, which has great significance and application value in monosex breeding and provides new insights for the study of Igfn1 gene recombination and inactivation in male O. punctatus.

Pharmacogenomic Study of Statin-Associated Muscle Symptoms in the ODYSSEY OUTCOMES Trial

Statin-associated muscle symptoms (SAMS) are the most frequently reported adverse events for statin therapies. Previous studies have reported an association between the p.Val174Ala missense variant in SLCO1B1 and SAMS in simvastatin-treated subjects; however, evidence for genetic predictors of SAMS in atorvastatin- or rosuvastatin-treated subjects is currently lacking.

GCN5 maintains muscle integrity by acetylating YY1 to promote dystrophin expression

Protein lysine acetylation is a post-translational modification that regulates protein structure and function. It is targeted to proteins by lysine acetyltransferases (KATs) or removed by lysine deacetylases. This work identifies a role for the KAT enzyme general control of amino acid synthesis protein 5 (GCN5; KAT2A) in regulating muscle integrity by inhibiting DNA binding of the transcription factor/repressor Yin Yang 1 (YY1). Here we report that a muscle-specific mouse knockout of GCN5 (Gcn5skm-/-) reduces the expression of key structural muscle proteins, including dystrophin, resulting in myopathy. GCN5 was found to acetylate YY1 at two residues (K392 and K393), disrupting the interaction between the YY1 zinc finger region and DNA. These findings were supported by human data, including an observed negative correlation between YY1 gene expression and muscle fiber diameter. Collectively, GCN5 positively regulates muscle integrity through maintenance of structural protein expression via acetylation-dependent inhibition of YY1. This work implicates the role of protein acetylation in the regulation of muscle health and for consideration in the design of novel therapeutic strategies to support healthy muscle during myopathy or aging.

Mss51 deletion increases endurance and ameliorates histopathology in the mdx mouse model of Duchenne muscular dystrophy

Mitochondrial derangement is an important contributor to the pathophysiology of muscular dystrophies and may be among the earliest cellular deficits. We have previously shown that disruption of Mss51, a mammalian skeletal muscle protein that localizes to the mitochondria, results in enhanced muscle oxygen consumption rate, increased endurance capacity, and improved limb muscle strength in mice with wildtype background. Here, we investigate whether Mss51 deletion in the mdx murine model of Duchenne muscular dystrophy (mdx-Mss51 KO) counteracts the muscle pathology and mitochondrial irregularities observed in mdx mice. We found that mdx-Mss51 KO mice had increased myofiber oxygen consumption rates and an amelioration of muscle histopathology compared to mdx counterparts. This corresponded with greater treadmill endurance and less percent fatigue in muscle physiology, but no improvement in forelimb grip strength or limb muscle force production. These findings suggest that although Mss51 deletion ameliorates the skeletal muscle mitochondrial respiration defects in mdx and improves fatigue resistance in vivo, the lack of improvement in force production suggests that this target alone may be insufficient for a therapeutic effect.

p21-Activated Kinase 1 Is Permissive for the Skeletal Muscle Hypertrophy Induced by Myostatin Inhibition

Skeletal muscle, the most abundant tissue in the body, plays vital roles in locomotion and metabolism. Understanding the cellular processes that govern regulation of muscle mass and function represents an essential step in the development of therapeutic strategies for muscular disorders. Myostatin, a member of the TGF-β family, has been identified as a negative regulator of muscle development. Indeed, its inhibition induces an extensive skeletal muscle hypertrophy requiring the activation of Smad 1/5/8 and the Insulin/IGF-I signaling pathway, but whether other molecular mechanisms are involved in this process remains to be determined. Using transcriptomic data from various Myostatin inhibition models, we identified Pak1 as a potential mediator of Myostatin action on skeletal muscle mass. Our results show that muscle PAK1 levels are systematically increased in response to Myostatin inhibition, parallel to skeletal muscle mass, regardless of the Myostatin inhibition model. Using Pak1 knockout mice, we investigated the role of Pak1 in the skeletal muscle hypertrophy induced by different approaches of Myostatin inhibition. Our findings show that Pak1 deletion does not impede the skeletal muscle hypertrophy magnitude in response to Myostatin inhibition. Therefore, Pak1 is permissive for the skeletal muscle mass increase caused by Myostatin inhibition.

Proteomic resolution of IGFN1 complexes reveals a functional interaction with the actin nucleating protein COBL

The Igfn1 gene produces multiple proteins by alternative splicing predominantly expressed in skeletal muscle. Igfn1 deficient clones derived from C2C12 myoblasts show reduced fusion index and morphological differences compared to control myotubes. Here, we first show that G:F actin ratios are significantly higher in differentiating IGFN1-deficient C2C12 myoblasts, suggesting that fusion and differentiation defects are underpinned by deficient actin remodelling. We obtained pull-downs from skeletal muscle with IGFN1 fragments and applied a proteomics approach. The proteomic composition of IGFN1 complexes identified the cytoskeleton and an association with the proteasome as the main networks. The actin nucleating protein COBL was selected for further validation. COBL is expressed in C2C12 myoblasts from the first stages of myoblast fusion but not in proliferating cells. COBL is also expressed in adult muscle and, as IGFN1, localizes to the Z-disc. We show that IGFN1 interacts, stabilizes and colocalizes with COBL and prevents the ability of COBL to form actin ruffles in COS7 cells. COBL loss of function C2C12-derived clones are able to fuse, therefore indicating that COBL or the IGFN1/COBL interaction are not essential for myoblast fusion.

Muscle NAD+ depletion and Serpina3n as molecular determinants of murine cancer cachexia-the effects of blocking myostatin and activins

Objective

Cancer cachexia and muscle loss are associated with increased morbidity and mortality. In preclinical animal models, blocking activin receptor (ACVR) ligands has improved survival and prevented muscle wasting in cancer cachexia without an effect on tumour growth. However, the underlying mechanisms are poorly understood. This study aimed to identify cancer cachexia and soluble ACVR (sACVR) administration-evoked changes in muscle proteome.

Methods

Healthy and C26 tumour-bearing (TB) mice were treated with recombinant sACVR. The sACVR or PBS control were administered either prior to the tumour formation or by continued administration before and after tumour formation. Muscles were analysed by quantitative proteomics with further examination of mitochondria and nicotinamide adenine dinucleotide (NAD⁺) metabolism. To complement the first prophylactic experiment, sACVR (or PBS) was injected as a treatment after tumour cell inoculation.

Results

Muscle proteomics in TB cachectic mice revealed downregulated signatures for mitochondrial oxidative phosphorylation (OXPHOS) and increased acute phase response (APR). These were accompanied by muscle NAD⁺ deficiency, alterations in NAD⁺ biosynthesis including downregulation of nicotinamide riboside kinase 2 (Nrk2), and decreased muscle protein synthesis. The disturbances in NAD⁺ metabolism and protein synthesis were rescued by treatment with sACVR. Across the whole proteome and APR, in particular, Serpina3n represented the most upregulated protein and the strongest predictor of cachexia. However, the increase in Serpina3n expression was associated with increased inflammation rather than decreased muscle mass and/or protein synthesis.

Conclusions

We present evidence implicating disturbed muscle mitochondrial OXPHOS proteome and NAD⁺ homeostasis in experimental cancer cachexia. Treatment of TB mice with a blocker of activin receptor ligands restores depleted muscle NAD⁺ and Nrk2, as well as decreased muscle protein synthesis. These results indicate putative new treatment therapies for cachexia and that although acute phase protein Serpina3n may serve as a predictor of cachexia, it more likely reflects a condition of elevated inflammation.

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Cachectic muscle proteome shows decreased OXPHOS and increased acute phase response.

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Cancer cachexia is characterized by lowered muscle Nrk2 expression and NAD⁺ levels.

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Blocking activin receptor 2B ligands rescues muscle NAD⁺ homeostasis in cachexia.

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Blocking activin receptor 2B ligands prevents affected protein synthesis in cachexia.

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Serpina3n predicts cachexia and cancer-induced APR independently from muscle atrophy.

Duchenne muscular dystrophy hiPSC-derived myoblast drug screen identifies compounds that ameliorate disease in mdx mice

Duchenne muscular dystrophy (DMD) is the most common muscular dystrophy. In the present study, when human induced pluripotent stem cells (hiPSCs) were differentiated into myoblasts, the myoblasts derived from DMD patient hiPSCs (DMD hiPSC-derived myoblasts) exhibited an identifiable DMD-relevant phenotype: myogenic fusion deficiency. Based on this model, we developed a DMD hiPSC-derived myoblast screening platform employing a high-content imaging (BD Pathway 855) approach to generate parameters describing morphological as well as myogenic marker protein expression. Following treatment of the cells with 1524 compounds from the Johns Hopkins Clinical Compound Library, compounds that enhanced myogenic fusion of DMD hiPSC-derived myoblasts were identified. The final hits were ginsenoside Rd and fenofibrate. Transcriptional profiling revealed that ginsenoside Rd is functionally related to FLT3 signaling, while fenofibrate is linked to TGF-β signaling. Preclinical tests in mdx mice showed that treatment with these 2 hit compounds can significantly ameliorate some of the skeletal muscle phenotypes caused by dystrophin deficiency, supporting their therapeutic potential. Further study revealed that fenofibrate could inhibit mitochondrion-induced apoptosis in DMD hiPSC-derived cardiomyocytes. We have developed a platform based on DMD hiPSC-derived myoblasts for drug screening and identified 2 promising small molecules with in vivo efficacy.

Systemic blockade of ACVR2B ligands attenuates muscle wasting in ischemic heart failure without compromising cardiac function

Signaling through activin receptors regulates skeletal muscle mass and activin receptor 2B (ACVR2B) ligands are also suggested to participate in myocardial infarction (MI) pathology in the heart. In this study, we determined the effect of systemic blockade of ACVR2B ligands on cardiac function in experimental MI, and defined its efficacy to revert muscle wasting in ischemic heart failure (HF). Mice were treated with soluble ACVR2B decoy receptor (ACVR2B-Fc) to study its effect on post-MI cardiac remodeling and on later HF. Cardiac function was determined with echocardiography, and myocardium analyzed with histological and biochemical methods for hypertrophy and fibrosis. Pharmacological blockade of ACVR2B ligands did not rescue the heart from ischemic injury or alleviate post-MI remodeling and ischemic HF. Collectively, ACVR2B-Fc did not affect cardiomyocyte hypertrophy, fibrosis, angiogenesis, nor factors associated with cardiac regeneration except modification of certain genes involved in metabolism or cell growth/survival. ACVR2B-Fc, however, was able to reduce skeletal muscle wasting in chronic ischemic HF, accompanied by reduced LC3II as a marker of autophagy and increased mTOR signaling and Cited4 expression as markers of physiological hypertrophy in quadriceps muscle. Our results ascertain pharmacological blockade of ACVR2B ligands as a possible therapy for skeletal muscle wasting in ischemic HF. Pharmacological blockade of ACVR2B ligands preserved myofiber size in ischemic HF, but did not compromise cardiac function nor exacerbate cardiac remodeling after ischemic injury.

Mss51 deletion enhances muscle metabolism and glucose homeostasis in mice

Myostatin is a negative regulator of muscle growth and metabolism and its inhibition in mice improves insulin sensitivity, increases glucose uptake into skeletal muscle, and decreases total body fat. A recently described mammalian protein called MSS51 is significantly downregulated with myostatin inhibition. In vitro disruption of Mss51 results in increased levels of ATP, β-oxidation, glycolysis, and oxidative phosphorylation. To determine the in vivo biological function of Mss51 in mice, we disrupted the Mss51 gene by CRISPR/Cas9 and found that Mss51-KO mice have normal muscle weights and fiber-type distribution but reduced fat pads. Myofibers isolated from Mss51-KO mice showed an increased oxygen consumption rate compared with WT controls, indicating an accelerated rate of skeletal muscle metabolism. The expression of genes related to oxidative phosphorylation and fatty acid β-oxidation were enhanced in skeletal muscle of Mss51-KO mice compared with that of WT mice. We found that mice lacking Mss51 and challenged with a high-fat diet were resistant to diet-induced weight gain, had increased whole-body glucose turnover and glycolysis rate, and increased systemic insulin sensitivity and fatty acid β-oxidation. These findings demonstrate that MSS51 modulates skeletal muscle mitochondrial respiration and regulates whole-body glucose and fatty acid metabolism, making it a potential target for obesity and diabetes.

Activin type IIB receptor blockade does not limit adenosine triphosphate supply in mouse skeletal muscle in Vivo

INTRODUCTION

Postnatal activin/myostatin type IIB receptor (ActRIIB) blockade increases skeletal muscle mass and strength but also increases muscle fatigability and impairs oxidative metabolism. The objective of this study was to determine in vivo whether this increased fatigability is due to energy supply limitation.

METHODS

The impact of 8-week ActRIIB blockade with soluble receptor (sActRIIB-Fc) on muscle function and adenosine triphosphate (ATP) fluxes was investigated noninvasively by using multimodal magnetic resonance and indirect calorimetry measurements in wild-type mice.

RESULTS

Activin/myostatin type IIB receptor blockade reduced (-41%) the muscle apparent mitochondrial capacity and increased (+11%) the basal body energy expenditure. During a fatiguing exercise, ActRIIB blockade decreased both oxidative ATP production rate (-32%) and fatigue resistance (-36%), but these changes affected neither the total ATP production rate nor the contractile ATP cost.

DISCUSSION

These findings demonstrate that the increased fatigability after ActRIIB blockade is not due to limitation in energy supply and/or disturbance in contractile ATP cost. Muscle Nerve 58:834-842, 2018.

Zmynd17 controls muscle mitochondrial quality and whole-body metabolism

Muscle mitochondria are crucial for systemic metabolic function, yet their regulation remains unclear. The zinc finger MYND domain-containing protein 17 (Zmynd17) was recently identified as a muscle-specific gene in mammals. Here, we investigated the role of Zmynd17 in mice. We found Zmynd17 predominantly expressed in skeletal muscle, especially in fast glycolytic muscle. Genetic Zmynd17 inactivation led to morphologic and functional abnormalities in muscle mitochondria, resulting in decreased respiratory function. Metabolic stress induced by a high-fat diet upregulated Zmynd17 expression and further exacerbated muscle mitochondrial morphology in Zmynd17-deficient mice. Strikingly, Zmynd17 deficiency significantly aggravated metabolic stress-induced hepatic steatosis, glucose intolerance, and insulin resistance. Furthermore, middle-aged mice lacking Zmynd17 exhibited impaired aerobic exercise performance, glucose intolerance, and insulin resistance. Thus, our results indicate that Zmynd17 is a metabolic stress-inducible factor that maintains muscle mitochondrial integrity, with its deficiency profoundly affecting whole-body glucose metabolism.-Fujita, R., Yoshioka, K., Seko, D., Suematsu, T., Mitsuhashi, S., Senoo, N., Miura, S., Nishino, I., Ono, Y. Zmynd17 controls muscle mitochondrial quality and whole-body metabolism.

Prevention of chemotherapy-induced cachexia by ACVR2B ligand blocking has different effects on heart and skeletal muscle

BACKGROUND

Toxicity of chemotherapy on skeletal muscles and the heart may significantly contribute to cancer cachexia, mortality, and decreased quality of life. Doxorubicin (DOX) is an effective cytostatic agent, which unfortunately has toxic effects on many healthy tissues. Blocking of activin receptor type IIB (ACVR2B) ligands is an often used strategy to prevent skeletal muscle loss, but its effects on the heart are relatively unknown.

METHODS

The effects of DOX treatment with or without pre-treatment with soluble ACVR2B-Fc (sACVR2B-Fc) were investigated. The mice were randomly assigned into one of the three groups: (1) vehicle (PBS)-treated controls, (2) DOX-treated mice (DOX), and (3) DOX-treated mice administered with sACVR2B-Fc during the experiment (DOX + sACVR2B-Fc). DOX was administered with a cumulative dose of 24 mg/kg during 2 weeks to investigate cachexia outcome in the heart and skeletal muscle. To understand similarities and differences between skeletal and cardiac muscles in their responses to chemotherapy, the tissues were collected 20 h after a single DOX (15 mg/kg) injection and analysed with genome-wide transcriptomics and mRNA and protein analyses. The combination group was pre-treated with sACVR2B-Fc 48 h before DOX administration. Major findings were also studied in mice receiving only sACVR2B-Fc.

RESULTS

The DOX treatment induced similar (~10%) wasting in skeletal muscle and the heart. However, transcriptional changes in response to DOX were much greater in skeletal muscle. Pathway analysis and unbiased transcription factor analysis showed that p53-p21-REDD1 is the main common pathway activated by DOX in both skeletal and cardiac muscles. These changes were attenuated by blocking ACVR2B ligands especially in skeletal muscle. Tceal7 (3-fold to 5-fold increase), transferrin receptor (1.5-fold increase), and Ccl21 (0.6-fold to 0.9-fold decrease) were identified as novel genes responsive to blocking ACVR2B ligands. Overall, at the transcriptome level, ACVR2B ligand blocking had only minor influence in the heart while it had marked effects in skeletal muscle. The same was also true for the effects on tissue wasting. This may be explained in part by about 18-fold higher gene expression of myostatin in skeletal muscle compared with the heart.

CONCLUSIONS

Cardiac and skeletal muscles display similar atrophy after DOX treatment, but the mechanisms for this may differ between the tissues. The present results suggest that p53-p21-REDD1 signalling is the main common DOX-activated pathway in these tissues and that blocking activin receptor ligands attenuates this response, especially in skeletal muscle supporting the overall stronger effects of this treatment in skeletal muscles.

The Hippo Signaling Pathway in the Regulation of Skeletal Muscle Mass and Function

The Hippo signaling pathway regulates the activity of the proteins Yes-associated protein (Yap) and transcriptional co-activator with PDZ-binding motif (Taz) to control tissue growth in many different cell types. Previously, we demonstrated that Yap is a critical regulator of skeletal muscle mass. We hypothesize that alterations in Yap and Taz activity modulate the anabolic adaptations of skeletal muscle to resistance exercise.

Soluble activin receptor type IIB decoy receptor differentially impacts murine osteogenesis imperfecta muscle function

INTRODUCTION

Osteogenesis imperfecta (OI) is characterized by skeletal fragility and muscle weakness. In this study we investigated the effects of soluble activin type IIB receptor (sActRIIB-mFc) on muscle mass and function in 2 distinct mouse models of OI: osteogenesis imperfecta murine (oim) and +/G610C.

METHODS

Wild-type (WT), +/G610C, and oim/oim mice were treated from 2 to 4 months of age with Tris-buffered saline (vehicle) or sActRIIB-mFc and their hindlimb muscles evaluated for mass, morphology, and contractile function.

RESULTS

sActRIIB-mFc-treated WT, +/G610C, and oim/oim mice had increased hindlimb muscle weights and myofiber cross-sectional area compared with vehicle-treated counterparts. sActRIIB-mFc-treated oim/oim mice also exhibited increased contractile function relative to vehicle-treated counterparts.

DISCUSSION

Blocking endogenous ActRIIB was effective at increasing muscle size in mouse models of OI, and increasing contractile function in oim/oim mice. ActRIIB inhibitors may provide a potential mutation-specific therapeutic option for compromised muscle function in OI. Muscle Nerve 57: 294-304, 2018.

Comparative Proteomic and Transcriptomic Analysis of Follistatin-Induced Skeletal Muscle Hypertrophy

Skeletal muscle, the most abundant body tissue, plays vital roles in locomotion and metabolism. Myostatin is a negative regulator of skeletal muscle mass. In addition to increasing muscle mass, Myostatin inhibition impacts muscle contractility and energy metabolism. To decipher the mechanisms of action of the Myostatin inhibitors, we used proteomic and transcriptomic approaches to investigate the changes induced in skeletal muscles of transgenic mice overexpressing Follistatin, a physiological Myostatin inhibitor. Our proteomic workflow included a fractionation step to identify weakly expressed proteins and a comparison of fast versus slow muscles. Functional annotation of altered proteins supports the phenotypic changes induced by Myostatin inhibition, including modifications in energy metabolism, fiber type, insulin and calcium signaling, as well as membrane repair and regeneration. Less than 10% of the differentially expressed proteins were found to be also regulated at the mRNA level but the Biological Process annotation, and the KEGG pathways analysis of transcriptomic results shows a great concordance with the proteomic data. Thus this study describes the most extensive omics analysis of muscle overexpressing Follistatin, providing molecular-level insights to explain the observed muscle phenotypic changes.

Myostatin deficiency but not anti-myostatin blockade induces marked proteomic changes in mouse skeletal muscle

Pharmacologic blockade of the myostatin (Mstn)/activin receptor pathway is being pursued as a potential therapy for several muscle wasting disorders. The functional benefits of blocking this pathway are under investigation, in particular given the findings that greater muscle hypertrophy results from Mstn deficiency arising from genetic ablation compared to post-developmental Mstn blockade. Using high-resolution MS coupled with SILAC mouse technology, we quantitated the relative proteomic changes in gastrocnemius muscle from Mstn knockout (Mstn(-/-) ) and mice treated for 2-weeks with REGN1033, an anti-Mstn antibody. Relative to wild-type animals, Mstn(-/-) mice had a two-fold greater muscle mass and a >1.5-fold change in expression of 12.0% of 1137 quantified muscle proteins. In contrast, mice treated with REGN1033 had minimal changes in muscle proteome (0.7% of 1510 proteins >1.5-fold change, similar to biological difference 0.5% of 1310) even though the treatment induced significant 20% muscle mass increase. Functional annotation of the altered proteins in Mstn(-/-) mice corroborates the mutiple physiological changes including slow-to-fast fiber type switch. Thus, the proteome-wide protein expression differs between Mstn(-/-) mice and mice subjected to specific Mstn blockade post-developmentally, providing molecular-level insights to inform mechanistic hypotheses to explain the observed functional differences.

IGFN1_v1 is required for myoblast fusion and differentiation

Igfn1 is a complex locus that codes for multiple splicing variants of Immunoglobulin- and Fibronectin-like domain containing proteins predominantly expressed in skeletal muscle. To reveal possible roles for Igfn1, we applied non-selective knock-down by shRNAs as well as specific targeting of Igfn1 exon 13 by CRISPR/Cas9 mutagenesis in C2C12 cells. Decreased expression of Igfn1 variants via shRNAs against the common 3'-UTR region caused a total blunting of myoblast fusion, but did not prevent expression of differentiation markers. Targeting of N-terminal domains by elimination of exon 13 via CRISPR/Cas9 mediated homologous recombination, also resulted in fusion defects as well as large multinucleated cells. Expression of IGFN1_v1 partially rescued fusion and myotube morphology in the Igfn1 exon 13 knock-out cell line, indicating a role for this variant in myoblast fusion and differentiation. However, in vivo overexpression of IGFN1_v1 or the Igfn1 Exon 13 CRISPR/Cas9 targeting vector did not result in significant size changes in transfected fibres.

Metabolic adaptations in skeletal muscle after 84 days of bed rest with and without concurrent flywheel resistance exercise

As metabolic changes in human skeletal muscle after long-term (simulated) spaceflight are not well understood, this study examined the effects of long-term microgravity, with and without concurrent resistance exercise, on skeletal muscle oxidative and glycolytic capacity. Twenty-one men were subjected to 84 days head-down tilt bed rest with (BRE; n = 9) or without (BR; n = 12) concurrent flywheel resistance exercise. Activity and gene expression of glycogen synthase, glycogen phosphorylase (GPh), hexokinase, phosphofructokinase-1 (PFK-1), and citrate synthase (CS), as well as gene expression of succinate dehydrogenase (SDH), vascular endothelial growth factor (VEFG), peroxisome proliferator-activated receptor gamma coactivator-1 (PGC-1α), and myostatin, were analyzed in samples from m. vastus lateralis collected before and after bed rest. Activity and gene expression of enzymes controlling oxidative metabolism (CS, SDH) decreased in BR but were partially maintained in BRE. Activity of enzymes regulating anaerobic glycolysis (GPh, PFK-1) was unchanged in BR. Resistance exercise increased the activity of GPh. PGC-1α and VEGF expression decreased in both BR and BRE. Myostatin increased in BR but decreased in BRE after bed rest. The analyses of these unique samples indicate that long-term microgravity induces marked alterations in the oxidative, but not the glycolytic, energy system. The proposed flywheel resistance exercise was effective in counteracting some of the metabolic alterations triggered by 84-day bed rest. Given the disparity between gene expression vs. enzyme activity in several key metabolic markers, posttranscriptional mechanisms should be explored to fully evaluate metabolic adaptations to long-term microgravity with/without exercise countermeasures in human skeletal muscle.

New function of the myostatin/activin type I receptor (ALK4) as a mediator of muscle atrophy and muscle regeneration

Skeletal muscle fibrosis and impaired muscle regeneration are major contributors to muscle wasting in Duchenne muscular dystrophy (DMD). Muscle growth is negatively regulated by myostatin (MSTN) and activins. Blockage of these pathways may improve muscle quality and function in DMD. Antisense oligonucleotides (AONs) were designed specifically to block the function of ALK4, a key receptor for the MSTN/activin pathway in skeletal muscle. AON-induced exon skipping resulted in specific Alk4 down-regulation, inhibition of MSTN activity, and increased myoblast differentiation in vitro. Unexpectedly, a marked decrease in muscle mass (10%) was found after Alk4 AON treatment in mdx mice. In line with in vitro results, muscle regeneration was stimulated, and muscle fiber size decreased markedly. Notably, when Alk4 was down-regulated in adult wild-type mice, muscle mass decreased even more. RNAseq analysis revealed dysregulated metabolic functions and signs of muscle atrophy. We conclude that ALK4 inhibition increases myogenesis but also regulates the tight balance of protein synthesis and degradation. Therefore, caution must be used when developing therapies that interfere with MSTN/activin pathways.—Pasteuning-Vuhman, S., Boertje-van der Meulen, J. W., van Putten, M., Overzier, M., ten Dijke, P., Kiełbasa, S. M., Arindrarto, W., Wolterbeek, R., Lezhnina, K. V., Ozerov, I. V., Aliper, A. M., Hoogaars, W. M., Aartsma-Rus, A., Loomans, C. J. M. New function of the myostatin/activin type I receptor (ALK4) as a mediator of muscle atrophy and muscle regeneration.

Systemic blockade of ACVR2B ligands prevents chemotherapy-induced muscle wasting by restoring muscle protein synthesis without affecting oxidative capacity or atrogenes

Doxorubicin is a widely used and effective chemotherapy drug. However, cardiac and skeletal muscle toxicity of doxorubicin limits its use. Inhibiting myostatin/activin signalling can prevent muscle atrophy, but its effects in chemotherapy-induced muscle wasting are unknown. In the present study we investigated the effects of doxorubicin administration alone or combined with activin receptor ligand pathway blockade by soluble activin receptor IIB (sACVR2B-Fc). Doxorubicin administration decreased body mass, muscle size and bone mineral density/content in mice. However, these effects were prevented by sACVR2B-Fc administration. Unlike in many other wasting situations, doxorubicin induced muscle atrophy without markedly increasing typical atrogenes or protein degradation pathways. Instead, doxorubicin decreased muscle protein synthesis which was completely restored by sACVR2B-Fc. Doxorubicin administration also resulted in impaired running performance without effects on skeletal muscle mitochondrial capacity/function or capillary density. Running performance and mitochondrial function were unaltered by sACVR2B-Fc administration. Tumour experiment using Lewis lung carcinoma cells demonstrated that sACVR2B-Fc decreased the cachectic effects of chemotherapy without affecting tumour growth. These results demonstrate that blocking ACVR2B signalling may be a promising strategy to counteract chemotherapy-induced muscle wasting without damage to skeletal muscle oxidative capacity or cancer treatment.

ActRIIB blockade increases force-generating capacity and preserves energy supply in exercising mdx mouse muscle in vivo

Postnatal blockade of the activin type IIB receptor (ActRIIB) represents a promising therapeutic strategy for counteracting dystrophic muscle wasting. However, its impact on muscle function and bioenergetics remains poorly documented in physiologic conditions. We have investigated totally noninvasively the effect of 8-wk administration of either soluble ActRIIB signaling inhibitor (sActRIIB-Fc) or vehicle PBS (control) on gastrocnemius muscle force-generating capacity, energy metabolism, and anatomy in dystrophic mdx mice using magnetic resonance (MR) imaging and dynamic [³¹P]-MR spectroscopy ([³¹P]-MRS) in vivo ActRIIB inhibition increased muscle volume (+33%) without changing fiber-type distribution, and increased basal animal oxygen consumption (+22%) and energy expenditure (+23%). During an in vivo standardized fatiguing exercise, maximum and total absolute contractile forces were larger (+40 and 24%, respectively) in sActRIIB-Fc treated animals, whereas specific force-generating capacity and fatigue resistance remained unaffected. Furthermore, sActRIIB-Fc administration did not alter metabolic fluxes, ATP homeostasis, or contractile efficiency during the fatiguing bout of exercise, although it dramatically reduced the intrinsic mitochondrial capacity for producing ATP. Overall, sActRIIB-Fc treatment increased muscle mass and strength without altering the fundamental weakness characteristic of dystrophic mdx muscle. These data support the clinical interest of ActRIIB blockade for reversing dystrophic muscle wasting.-Béchir, N., Pecchi, E., Vilmen, C., Le Fur, Y., Amthor, H., Bernard, M., Bendahan, D., Giannesini, B. ActRIIB blockade increases force-generating capacity and preserves energy supply in exercising mdx mouse muscle in vivo.

Mitochondrial impairment induced by postnatal ActRIIB blockade does not alter function and energy status in exercising mouse glycolytic muscle in vivo

Because it leads to a rapid and massive muscle hypertrophy, postnatal blockade of the activin type IIB receptor (ActRIIB) is a promising therapeutic strategy for counteracting muscle wasting. However, the functional consequences remain very poorly documented in vivo. Here, we have investigated the impact of 8-wk ActRIIB blockade with soluble receptor (sActRIIB-Fc) on gastrocnemius muscle anatomy, energy metabolism, and force-generating capacity in wild-type mice, using totally noninvasive magnetic resonance imaging (MRI) and dynamic(31)P-MRS. Compared with vehicle (PBS) control, sActRIIB-Fc treatment resulted in a dramatic increase in body weight (+29%) and muscle volume (+58%) calculated from hindlimb MR imaging, but did not alter fiber type distribution determined via myosin heavy chain isoform analysis. In resting muscle, sActRIIB-Fc treatment induced acidosis and PCr depletion, thereby suggesting reduced tissue oxygenation. During an in vivo fatiguing exercise (6-min repeated maximal isometric contraction electrically induced at 1.7 Hz), maximal and total absolute forces were larger in sActRIIB-Fc treated animals (+26 and +12%, respectively), whereas specific force and fatigue resistance were lower (-30 and -37%, respectively). Treatment with sActRIIB-Fc further decreased the maximal rate of oxidative ATP synthesis (-42%) and the oxidative capacity (-34%), but did not alter the bioenergetics status in contracting muscle. Our findings demonstrate in vivo that sActRIIB-Fc treatment increases absolute force-generating capacity and reduces mitochondrial function in glycolytic gastrocnemius muscle, but this reduction does not compromise energy status during sustained activity. Overall, these data support the clinical interest of postnatal ActRIIB blockade.

Protective effect of myostatin gene deletion on aging-related muscle metabolic decline

While myostatin gene deletion is a promising therapy to fight muscle loss during aging, this approach induces also skeletal muscle metabolic changes such as mitochondrial deficits, redox alteration and increased fatigability. In the present study, we evaluated the effects of aging on these features in aged wild-type (WT) and mstn knockout (KO) mice. Moreover, to determine whether an enriched-antioxidant diet may be useful to prevent age-related disorders, we orally administered to the two genotypes a melon concentrate rich in superoxide dismutase for 12 weeks. We reported that mitochondrial functional abnormalities persisted (decreased state 3 and 4 of respiration; p<0.05) in skeletal muscle from aged KO mice; however, differences with WT mice were attenuated at old age in line with reduced difference on running endurance between the two genotypes. Interestingly, we showed an increase in glutathione levels, associated with lower lipid peroxidation levels in KO muscle. Enriched antioxidant diet reduced the aging-related negative effects on maximal aerobic velocity and running limit time (p<0.05) in both groups, with systemic adaptations on body weight. The redox status and the hypertrophic phenotype appeared to be beneficial to KO mice, mitigating the effect of aging on the skeletal muscle metabolic remodeling.

Mammalian Mss51 is a skeletal muscle-specific gene modulating cellular metabolism

BACKGROUND

The transforming growth factor β (TGF-β) signaling pathways modulate skeletal muscle growth, regeneration, and cellular metabolism. Several recent gene expression studies have shown that inhibition of myostatin and TGF-β1 signaling consistently leads to a significant reduction in expression of Mss51, also named Zmynd17. The function of mammalian Mss51 is unknown although a putative homolog in yeast is a mitochondrial translational activator.

OBJECTIVE

The objective of this work was to characterize mammalian Mss51.

METHODS

Quantitative RT-PCR and immunoblot of subcellular fractionation were used to determine expression patterns and localization of Mss51. The CRISPR/Cas9 system was used to reduce expression of Mss51 in C2C12 myoblasts and the function of Mss51 was evaluated in assays of proliferation, differentiation and cellular metabolism.

RESULTS

Mss51 was predominantly expressed in skeletal muscle and in those muscles dominated by fast-twitch fibers. In vitro, its expression was upregulated upon differentiation of C2C12 myoblasts into myotubes. Expression of Mss51 was modulated in response to altered TGF-β family signaling. In human muscle, Mss51 localized to the mitochondria. Its genetic disruption resulted in increased levels of cellular ATP, β-oxidation, glycolysis, and oxidative phosphorylation.

CONCLUSIONS

Mss51 is a novel, skeletal muscle-specific gene and a key target of myostatin and TGF-β1 signaling. Unlike myostatin, TGF-β1 and IGF-1, Mss51 does not regulate myoblast proliferation or differentiation. Rather, Mss51 appears to be one of the effectors of these growth factors on metabolic processes including fatty acid oxidation, glycolysis and oxidative phosphorylation.

Myostatin/activin blocking combined with exercise reconditions skeletal muscle expression profile of mdx mice

Duchenne muscular dystrophy is characterized by muscle wasting and decreased aerobic metabolism. Exercise and blocking of myostatin/activin signaling may independently or combined counteract muscle wasting and dystrophies. The effects of myostatin/activin blocking using soluble activin receptor-Fc (sActRIIB-Fc) administration and wheel running were tested alone or in combination for 7 weeks in dystrophic mdx mice. Expression microarray analysis revealed decreased aerobic metabolism in the gastrocnemius muscle of mdx mice compared to healthy mice. This was not due to reduced home-cage physical activity, and was further downregulated upon sActRIIB-Fc treatment in enlarged muscles. However, exercise activated pathways of aerobic metabolism and counteracted the negative effects of sActRIIB-Fc. Exercise and sActRIIB-Fc synergistically increased expression of major urinary protein, but exercise blocked sActRIIB-Fc induced phosphorylation of STAT5 in gastrocnemius muscle. In conclusion, exercise alone or in combination with myostatin/activin blocking corrects aerobic gene expression profiles of dystrophic muscle toward healthy wild type mice profiles.

Combined Strategies for Maintaining Skeletal Muscle Mass and Function in Aging: Myostatin Inactivation and AICAR-Associated Oxidative Metabolism Induction

Myostatin (mstn) blockade, resulting in muscle hypertrophy, is a promising therapy to counteract age-related muscle loss. However, oxidative and mitochondrial deficit observed in young mice with myostatin inhibition could be detrimental with aging. The aim of this study was (a) to bring original data on metabolic and mitochondrial consequences of mstn inhibition in old mice, and (b) to examine whether 4-weeks of AICAR treatment, a pharmacological compound known to upregulate oxidative metabolism, may be useful to improve exercise capacity and mitochondrial deficit of 20-months mstn KO versus wild-type (WT) mice. Our results show that despite the enlarged muscle mass, the oxidative and mitochondrial deficit associated with reduced endurance running capacity is maintained in old mstn KO mice but not worsened by aging. Importantly, AICAR treatment induced a significant beneficial effect on running limit time only in old mstn KO mice, with a marked increase in PGC-1α expression and slight beneficial effects on mitochondrial function. We showed that AICAR effects were autophagy-independent. This study underlines the relevance of aged muscle remodelling by complementary approaches that impact both muscle mass and function, and suggest that mstn inhibition and aerobic metabolism activators should be co-developed for delaying age-related deficits in skeletal muscle.

Myostatin is a key mediator between energy metabolism and endurance capacity of skeletal muscle

Myostatin (Mstn) participates in the regulation of skeletal muscle size and has emerged as a regulator of muscle metabolism. Here, we hypothesized that lack of myostatin profoundly depresses oxidative phosphorylation-dependent muscle function. Toward this end, we explored Mstn −/− mice as a model for the constitutive absence of myostatin and AAV-mediated overexpression of myostatin propeptide as a model of myostatin blockade in adult wild-type mice. We show that muscles from Mstn −/− mice, although larger and stronger, fatigue extremely rapidly. Myostatin deficiency shifts muscle from aerobic toward anaerobic energy metabolism, as evidenced by decreased mitochondrial respiration, reduced expression of PPAR transcriptional regulators, increased enolase activity, and exercise-induced lactic acidosis. As a consequence, constitutively reduced myostatin signaling diminishes exercise capacity, while the hypermuscular state of Mstn−/− mice increases oxygen consumption and the energy cost of running. We wondered whether these results are the mere consequence of the congenital fiber-type switch toward a glycolytic phenotype of constitutive Mstn −/− mice. Hence, we overexpressed myostatin propeptide in adult mice, which did not affect fiber-type distribution, while nonetheless causing increased muscle fatigability, diminished exercise capacity, and decreased Pparb/d and Pgc1a expression. In conclusion, our results suggest that myostatin endows skeletal muscle with high oxidative capacity and low fatigability, thus regulating the delicate balance between muscle mass, muscle force, energy metabolism, and endurance capacity.

Blockade of ActRIIB signaling triggers muscle fatigability and metabolic myopathy

Myostatin regulates skeletal muscle size via the activin receptor IIB (ActRIIB). However, its effect on muscle energy metabolism and energy-dependent muscle function remains largely unexplored. This question needs to be solved urgently since various therapies for neuromuscular diseases based on blockade of ActRIIB signaling are being developed. Here, we show in mice, that 4-month pharmacological abrogation of ActRIIB signaling by treatment with soluble ActRIIB-Fc triggers extreme muscle fatigability. This is associated with elevated serum lactate levels and a severe metabolic myopathy in the mdx mouse, an animal model of Duchenne muscular dystrophy. Blockade of ActRIIB signaling downregulates porin, a crucial ADP/ATP shuttle between cytosol and mitochondrial matrix leading to a consecutive deficiency of oxidative phosphorylation as measured by in vivo Phosphorus Magnetic Resonance Spectroscopy ((31)P-MRS). Further, ActRIIB blockade reduces muscle capillarization, which further compounds the metabolic stress. We show that ActRIIB regulates key determinants of muscle metabolism, such as Pparβ, Pgc1α, and Pdk4 thereby optimizing different components of muscle energy metabolism. In conclusion, ActRIIB signaling endows skeletal muscle with high oxidative capacity and low fatigability. The severe metabolic side effects following ActRIIB blockade caution against deploying this strategy, at least in isolation, for treatment of neuromuscular disorders.

Fam65b is important for formation of the HDAC6-dysferlin protein complex during myogenic cell differentiation

Previously, we identified family with sequence similarity 65, member B (Fam65b), as a protein transiently up-regulated during differentiation and fusion of human myogenic cells. Silencing of Fam65b expression results in severe reduction of myogenin expression and consequent lack of myoblast fusion. The molecular function of Fam65b and whether misregulation of its expression could be causative of muscle diseases are unknown. Protein pulldowns were used to identify Fam65b-interacting proteins in differentiating human muscle cells and regenerating muscle tissue. In vitro, human muscle cells were treated with histone-deacetylase (HDAC) inhibitors, and expression of Fam65b and interacting proteins was studied. Nontreated cells were used as controls. In vivo, expression of Fam65b was down-regulated in developing zebrafish to determine the effects on muscle development. Fam65b binds to HDAC6 and dysferlin, the protein mutated in limb girdle muscular dystrophy 2B. The tricomplex Fam65b-HDAC6-dysferlin is transient, and Fam65b expression is necessary for the complex to form. Treatment of myogenic cells with pan-HDAC or HDAC6-specific inhibitors alters Fam65b expression, while dysferlin expression does not change. Inhibition of Fam65b expression in developing zebrafish results in abnormal muscle, with low birefringence, tears at the myosepta, and increased embryo lethality. Fam65b is an essential component of the HDAC6-dysferlin complex. Down-regulation of Fam65b in developing muscle causes changes consistent with muscle disease.-Balasubramanian, A., Kawahara, G., Gupta, V. A., Rozkalne, A., Beauvais, A., Kunkel, L. M., Gussoni, E. Fam65b is important for formation of the HDAC6-dysferlin protein complex during myogenic cell differentiation.

Elevated expression of activins promotes muscle wasting and cachexia

In models of cancer cachexia, inhibiting type IIB activin receptors (ActRIIBs) reverse muscle wasting and prolongs survival, even with continued tumor growth. ActRIIB mediates signaling of numerous TGF-β proteins; of these, we demonstrate that activins are the most potent negative regulators of muscle mass. To determine whether activin signaling in the absence of tumor-derived factors induces cachexia, we used recombinant serotype 6 adeno-associated virus (rAAV6) vectors to increase circulating activin A levels in C57BL/6 mice. While mice injected with control vector gained ~10% of their starting body mass (3.8±0.4 g) over 10 wk, mice injected with increasing doses of rAAV6:activin A exhibited weight loss in a dose-dependent manner, to a maximum of -12.4% (-4.2±1.1 g). These reductions in body mass in rAAV6:activin-injected mice correlated inversely with elevated serum activin A levels (7- to 24-fold). Mechanistically, we show that activin A reduces muscle mass and function by stimulating the ActRIIB pathway, leading to deleterious consequences, including increased transcription of atrophy-related ubiquitin ligases, decreased Akt/mTOR-mediated protein synthesis, and a profibrotic response. Critically, we demonstrate that the muscle wasting and fibrosis that ensues in response to excessive activin levels is fully reversible. These findings highlight the therapeutic potential of targeting activins in cachexia.

Exercise restores decreased physical activity levels and increases markers of autophagy and oxidative capacity in myostatin/activin-blocked mdx mice

The importance of adequate levels of muscle size and function and physical activity is widely recognized. Myostatin/activin blocking increases skeletal muscle mass but may decrease muscle oxidative capacity and can thus be hypothesized to affect voluntary physical activity. Soluble activin receptor IIB (sActRIIB-Fc) was produced to block myostatin/activins. Modestly dystrophic mdx mice were injected with sActRIIB-Fc or PBS with or without voluntary wheel running exercise for 7 wk. Healthy mice served as controls. Running for 7 wk attenuated the sActRIIB-Fc-induced increase in body mass by decreasing fat mass. Running also enhanced/restored the markers of muscle oxidative capacity and autophagy in mdx mice to or above the levels of healthy mice. Voluntary running activity was decreased by sActRIIB-Fc during the first 3-4 wk correlating with increased body mass. Home cage physical activity of mice, quantified from the force plate signal, was decreased by sActRIIB-Fc the whole 7-wk treatment in sedentary mice. To understand what happens during the first weeks after sActRIIB-Fc administration, when mice are less active, healthy mice were injected with sActRIIB-Fc or PBS for 2 wk. During the sActRIIB-Fc-induced rapid 2-wk muscle growth period, oxidative capacity and autophagy were reduced, which may possibly explain the decreased running activity. These results show that increased muscle size and decreased markers of oxidative capacity and autophagy during the first weeks of myostatin/activin blocking are associated with decreased voluntary activity levels. Voluntary exercise in dystrophic mice enhances the markers of oxidative capacity and autophagy to or above the levels of healthy mice.

Increasing muscle mass to improve metabolism

Skeletal muscle insulin resistance is a predictor of the development of type 2 diabetes and maintenance of adequate muscle glucose disposal in muscle may help to prevent diabetes. Lipodystrophy is a type of diabetes caused by a reduction of white adipose tissue and the adipokine leptin. Lipidemia, insulin resistance and hyperphagia develop as a consequence. In a recent study, we showed that increasing skeletal muscle mass by inhibiting signaling of myostatin, a transforming growth factor β (TGFβ) family member that negatively regulates muscle growth, prevents the development of diabetes in a mouse model of lipodystrophy. Muscle-specific myostatin inhibition also prevented hyperphagia suggesting muscle may regulate food intake. Here we discuss these results in the context of strategies to increase muscle insulin sensitivity as well as new findings about the effects of myostatin and other TGFβ family members on similar metabolic processes.

Muscle protein synthesis, mTORC1/MAPK/Hippo signaling, and capillary density are altered by blocking of myostatin and activins

Loss of muscle mass and function occurs in various diseases. Myostatin blocking can attenuate muscle loss, but downstream signaling is not well known. Therefore, to elucidate associated signaling pathways, we used the soluble activin receptor IIb (sActRIIB-Fc) to block myostatin and activins in mice. Within 2 wk, the treatment rapidly increased muscle size as expected but decreased capillary density per area. sActRIIB-Fc increased muscle protein synthesis 1-2 days after the treatment correlating with enhanced mTORC1 signaling (phosphorylated rpS6 and S6K1, r = 0.8). Concurrently, increased REDD1 and eIF2Bε protein contents and phosphorylation of 4E-BP1 and AMPK was observed. In contrast, proangiogenic MAPK signaling and VEGF-A protein decreased. Hippo signaling has been characterized recently as a regulator of organ size and an important regulator of myogenesis in vitro. The phosphorylation of YAP (Yes-associated protein), a readout of activated Hippo signaling, increased after short- and longer-term myostatin and activin blocking and in exercised muscle. Moreover, dystrophic mdx mice had elevated phosphorylated and especially total YAP protein content. These results show that the blocking of myostatin and activins induce rapid skeletal muscle growth. This is associated with increased protein synthesis and mTORC1 signaling but decreased capillary density and proangiogenic signaling. It is also shown for the first time that Hippo signaling is activated in skeletal muscle after myostatin blocking and exercise and also in dystrophic muscle. This suggests that Hippo signaling may have a role in skeletal muscle in various circumstances.

Myostatin: genetic variants, therapy and gene doping

Since its discovery, myostatin (MSTN) has been at the forefront of muscle therapy research because intrinsic mutations or inhibition of this protein, by either pharmacological or genetic means, result in muscle hypertrophy and hyperplasia. In addition to muscle growth, MSTN inhibition potentially disturbs connective tissue, leads to strength modulation, facilitates myoblast transplantation, promotes tissue regeneration, induces adipose tissue thermogenesis and increases muscle oxidative phenotype. It is also known that current advances in gene therapy have an impact on sports because of the illicit use of such methods. However, the adverse effects of these methods, their impact on athletic performance in humans and the means of detecting gene doping are as yet unknown. The aim of the present review is to discuss biosynthesis, genetic variants, pharmacological/genetic manipulation, doping and athletic performance in relation to the MSTN pathway. As will be concluded from the manuscript, MSTN emerges as a promising molecule for combating muscle wasting diseases and for triggering wide-ranging discussion in view of its possible use in gene doping.

Muscle wasting and the temporal gene expression pattern in a novel rat intensive care unit model

BACKGROUND

Acute quadriplegic myopathy (AQM) or critical illness myopathy (CIM) is frequently observed in intensive care unit (ICU) patients. To elucidate duration-dependent effects of the ICU intervention on molecular and functional networks that control the muscle wasting and weakness associated with AQM, a gene expression profile was analyzed at time points varying from 6 hours to 14 days in a unique experimental rat model mimicking ICU conditions, i.e., post-synaptically paralyzed, mechanically ventilated and extensively monitored animals.

RESULTS

During the observation period, 1583 genes were significantly up- or down-regulated by factors of two or greater. A significant temporal gene expression pattern was constructed at short (6 h-4 days), intermediate (5-8 days) and long (9-14 days) durations. A striking early and maintained up-regulation (6 h-14d) of muscle atrogenes (muscle ring-finger 1/tripartite motif-containing 63 and F-box protein 32/atrogin-1) was observed, followed by an up-regulation of the proteolytic systems at intermediate and long durations (5-14d). Oxidative stress response genes and genes that take part in amino acid catabolism, cell cycle arrest, apoptosis, muscle development, and protein synthesis together with myogenic factors were significantly up-regulated from 5 to 14 days. At 9-14 d, genes involved in immune response and the caspase cascade were up-regulated. At 5-14d, genes related to contractile (myosin heavy chain and myosin binding protein C), regulatory (troponin, tropomyosin), developmental, caveolin-3, extracellular matrix, glycolysis/gluconeogenesis, cytoskeleton/sarcomere regulation and mitochondrial proteins were down-regulated. An activation of genes related to muscle growth and new muscle fiber formation (increase of myogenic factors and JunB and down-regulation of myostatin) and up-regulation of genes that code protein synthesis and translation factors were found from 5 to 14 days.

CONCLUSIONS

Novel temporal patterns of gene expression have been uncovered, suggesting a unique, coordinated and highly complex mechanism underlying the muscle wasting associated with AQM in ICU patients and providing new target genes and avenues for intervention studies.