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Obesity and type 2 diabetes are metabolic diseases, often associated with sarcopenia and muscle dysfunction. MOTS-c, a mitochondrial-derived peptide, acts as a systemic hormone and has been implicated in metabolic homeostasis. Although MOTS-c improves insulin sensitivity in skeletal muscle, whether MOTS-c impacts muscle atrophy is not known. Myostatin is a negative regulator of skeletal muscle mass and also one of the possible mediators of insulin resistance-induced skeletal muscle wasting. Interestingly, we found that plasma MOTS-c levels are inversely correlated with myostatin levels in human subjects. We further demonstrated that MOTS-c prevents palmitic acid-induced atrophy in differentiated C2C12 myotubes, whereas MOTS-c administration decreased myostatin levels in plasma in diet-induced obese mice. By elevating AKT phosphorylation, MOTS-c inhibits the activity of an upstream transcription factor for myostatin and other muscle wasting genes, FOXO1. MOTS-c increases mTORC2 and inhibits PTEN activity, which modulates AKT phosphorylation. Further upstream, MOTS-c increases CK2 activity, which leads to PTEN inhibition. These results suggest that through inhibition of myostatin, MOTS-c could be a potential therapy for insulin resistance-induced skeletal muscle atrophy as well as other muscle wasting phenotypes including sarcopenia. NEW & NOTEWORTHY MOTS-c, a mitochondrial-derived peptide reduces high-fat-diet-induced muscle atrophy signaling by reducing myostatin expression. The CK2-PTEN-mTORC2-AKT-FOXO1 pathways play key roles in MOTS-c action on myostatin expression.

Insulin-like growth factor-1 (IGF1) is crucial for regulating post-natal growth and, along with myostatin (MSTN), regulates muscle size. Here, we sought to clarify the roles of these two genes in regulating sexually dimorphic growth of body and muscle mass. In the first study, we established that Igf1 mRNA was increased to a greater extent and Igf1 receptor mRNA increased earlier in male, than in female, gastrocnemius muscles during the rapid phase of growth (from 2 to 6 weeks) were unchanged, thereafter, to 32 weeks of age in WT mice (P < 0.001). In the second study, we sought to determine if supplemental IGF1 could overcome the sexual dimorphism of muscle and body mass, when myostatin is absent. We crossed myostatin null (Mstn-/-) mice with mice over-expressing Igf1 in skeletal muscle (Igf1+) to generate six genotypes; control (Mstn+/+), Mstn+/-, Mstn-/-, Mstn+/+:Igf1+, Mstn+/-:Igf1+ and Mstn-/-:Igf1+ (n = 8 per genotype and sex). In both sexes, body mass at 12 weeks was increased by at least 1.6-fold and muscle mass by at least 3-fold in Mstn-/-:Igf1+ compared with Mstn+/+ mice (P < 0.001). The abundance of AKT was increased in muscles of mice transgenic for Mstn, while phosphorylation of AKTS473 was increased in both male and female mice transgenic for Igf1+. The ratio of phosphorylated to total AKT was 1.9-fold greater in male mice (P < 0.001). Thus, despite increased growth of skeletal muscle and body size when myostatin was absent and IGF1 was in excess, sexual dimorphism persisted, an effect consistent with greater IGF1-induced activation of AKT in skeletal muscles of males.

Context: Distinct from the muscle atrophy that develops from inactivity or disuse, atrophy that occurs after traumatic joint injury continues despite the patient being actively engaged in exercise. Recognizing the multitude of factors and cascade of events that are present and negatively influence the regulation of muscle mass after traumatic joint injury will likely enable clinicians to design more effective treatment strategies. To provide sports medicine practitioners with the best strategies to optimize muscle mass, the purpose of this clinical review is to discuss the predominant mechanisms that control muscle atrophy for disuse and posttraumatic scenarios, and to highlight how they differ., Evidence Acquisition: Articles that reported on disuse atrophy and muscle atrophy after traumatic joint injury were collected from peer-reviewed sources available on PubMed (2000 through December 2019). Search terms included the following: disuse muscle atrophy OR disuse muscle mass OR anterior cruciate ligament OR ACL AND mechanism OR muscle loss OR atrophy OR neurological disruption OR rehabilitation OR exercise ., Study Design: Clinical review., Level of Evidence: Level 5., Results: We highlight that (1) muscle atrophy after traumatic joint injury is due to a broad range of atrophy-inducing factors that are resistant to standard resistance exercises and need to be effectively targeted with treatments and (2) neurological disruptions after traumatic joint injury uncouple the nervous system from muscle tissue, contributing to a more complex manifestation of muscle loss as well as degraded tissue quality., Conclusion: Atrophy occurring after traumatic joint injury is distinctly different from the muscle atrophy that develops from disuse and is likely due to the broad range of atrophy-inducing factors that are present after injury. Clinicians must challenge the standard prescriptive approach to combating muscle atrophy from simply prescribing physical activity to targeting the neurophysiological origins of muscle atrophy after traumatic joint injury.

The point mutation in myostatin (MSTN) can produce the Texel sheep double muscle phenotype. However, whether other species have the same mode of action as MSTN and whether breeding materials can be obtained through cross-species genetic editing remain unclear. The mutation in the mouse MSTN 3'UTR could create a target site for mmu-miR-1/206, as verified by the dual luciferase reporter system. A C2C12 cell model with the mutation in MSTN 3'UTR was constructed using CRISPR/Cas9 gene editing. Then, the mRNA and protein expression of MSTN was analyzed in the mutant C2C12 cell model. Results revealed that the mutation blocked the translational level of MSTN. By inhibiting mmu-mir-206, low expression of MSTN protein in mutant C2C12 cell can be rescued. Furthermore, the proliferation and differentiation abilities of the mutant C2C12 cell model were tested by RT-PCR, CCK8 analysis, EDU (5-ethynyl-2'-deoxyuridine) proliferation analysis, immunofluorescence analysis, Western blot, and myotube fusion statistics. This study may serve as a reference for elucidating the function and molecular mechanism of MSTN and as a foundation for accurate breeding improvement., Competing Interests: Declaration of competing interest The authors declare no competing interest., (Copyright © 2020. Published by Elsevier B.V.)

Skeletal muscle and adipose tissue play an important role in maintaining metabolic homeostasis and thermogenesis. We aimed to investigate the effects of single and repeated exposure to whole-body cryotherapy in volunteers with different physical fitness levels on 25-hydroxyvitamin D (25(OH)D) and myokines. The study included 22 healthy male volunteers (mean age: 21 ± 1.17 years), who underwent 10 consecutive sessions in a cryogenic chamber once daily (3 minutes, -110 °C). Blood samples were collected before and 30 minutes and 24 hours after the first and last cryotherapy sessions. Prior to treatment, body composition and physical fitness levels were measured. After 10 cryotherapy treatments, significant changes were found in myostatin concentrations in the low physical fitness level (LPhL) group. The 25(OH)D levels were increased in the high physical fitness level (HPhL) group and decreased in the LPhL group. The HPhL group had significant changes in the level of high-sensitivity interleukin-6 after the first treatment. The LPhL group had significant changes in 25(OH)D, irisin, and myostatin levels after the tenth treatment. Our data demonstrated that in healthy young men, cryotherapy affects 25(OH)D levels, but they were small and transient. The body's response to a series of 10 cryotherapy treatments is modified by physical fitness level.

Many studies have been conducted to improve economically important livestock traits such as feed efficiency and muscle growth. Genome editing technologies represent a major advancement for both basic research and agronomic biotechnology development. The clustered regularly interspaced short palindromic repeats (CRISPR)/Cas9 technical platform is a powerful tool used to engineer specific targeted loci. However, the potential occurrence of off-target effects, including the cleavage of unintended targets, limits the practical applications of Cas9-mediated genome editing. In this study, to minimize the off-target effects of this technology, we utilized D10A-Cas9 nickase to generate myostatin-knockout (MSTN KO) chickens via primordial germ cells. D10A-Cas9 nickase (Cas9n)-mediated MSTN KO chickens exhibited significantly larger skeletal muscles in the breast and leg. Degrees of skeletal muscle hypertrophy and hyperplasia induced by myostatin deletion differed by sex and muscle type. The abdominal fat deposition was dramatically lower in MSTN KO chickens than in wild-type chickens. Our results demonstrate that the D10A-Cas9 technical platform can facilitate precise and efficient targeted genome engineering and may broaden the range of applications for genome-edited chickens in practical industrialization and as animal models of human diseases., (© 2020 The Authors. The FASEB Journal published by Wiley Periodicals, Inc. on behalf of Federation of American Societies for Experimental Biology.)

Myostatin is an inhibitor of skeletal muscle growth and might be involved in adaptations to caloric restriction (CR). We compared responses to 12-week 30% CR in male mice of Berlin high strain with myostatin dysfunction (BEH) and wild-type myostatin (BEH+/+). BEH mice were heavier than BEH+/+ mice (58.8 ± 2.0 versus 53.1 ± 2.7 g, p < 0.001), had 1.8-fold greater hind limb muscle mass and were less (p < 0.05) physically active when fed ad libitum. After CR, BEH and BEH+/+ strains experienced similar weight loss (24.7 ± 5.7 versus 20.6 ± 6.5%, p > 0.05, respectively) and decreases (p < 0.001) in plasma IGF-1 and total cholesterol, but loss of hind limb muscle mass was greater (p < 0.001) in BEH mice than BEH+/+ mice. BEH mice had better (p < 0.001) glucose tolerance and showed smaller (p < 0.05) improvements of it than BEH+/+ mice after CR (1038.2 ± 174.7 versus 744.4 ± 95.8 glucose mM× 120 min, p < 0.01 for BEH; 1365.8 ± 218.5 versus 831.5 ± 134.4 glucose mM ×120 min, p < 0.001, for BEH+/+, respectively). In summary, myostatin dysfunction is associated with muscle hypertrophy and high glucose tolerance, but greater muscle wasting and smaller improvements in glucose tolerance in response to CR., (Copyright © 2019 Elsevier Inc. All rights reserved.)

Spinal muscular atrophy (SMA) is a neuromuscular disease characterized by loss of α-motor neurons, leading to profound skeletal muscle atrophy. Patients also suffer from decreased bone mineral density and increased fracture risk. The majority of treatments for SMA, approved or in clinic trials, focus on addressing the underlying cause of disease, insufficient production of full-length SMN protein. While restoration of SMN has resulted in improvements in functional measures, significant deficits remain in both mice and SMA patients following treatment. Motor function in SMA patients may be additionally improved by targeting skeletal muscle to reduce atrophy and improve muscle strength. Inhibition of myostatin, a negative regulator of muscle mass, offers a promising approach to increase muscle function in SMA patients. Here we demonstrate that muSRK-015P, a monoclonal antibody which specifically inhibits myostatin activation, effectively increases muscle mass and function in two variants of the pharmacological mouse model of SMA in which pharmacologic restoration of SMN has taken place either 1 or 24 days after birth to reflect early or later therapeutic intervention. Additionally, muSRK-015P treatment improves the cortical and trabecular bone phenotypes in these mice. These data indicate that preventing myostatin activation has therapeutic potential in addressing muscle and bone deficiencies in SMA patients. An optimized variant of SRK-015P, SRK-015, is currently in clinical development for treatment of SMA., (© The Author(s) 2018. Published by Oxford University Press.)

Expression of myostatin (MSTN, also known as growth differentiation factor 8, GDF8) was recently detected in cumulus-oocytes complexes (COCs), however little is known about its role in in vitro maturation (IVM) and fertilization (IVF) in large animals. Therefore, this study was designed to investigate the effect of MSTN inhibition on IVM of buffalo oocytes through investigation of IVM efficiency and expression of some specific genes in COCs from IVM till subsequent developmental stages following IVF. To reach this goal, we prepared a construct of adeno-associated virus (AAV) carrying MSTN pro-peptides (AAV-MSTNP) to inhibit MSTN. Over-expression of MSTNP was verified by upregulated expression of MSTNP and downregulated expression of the TGFβ receptor ActRIIb, the TGFβ signal transducer SMAD2 in COCs using qPCR. Microinjection of AAV-MSTNP to oocytes before IVM yielded a significant decrease in maturation rate as revealed by less cumulus cells expansion, fewer oocytes reaching metaphase II, and downregulation of cumulus expansion-related genes pentraxin 3 (Ptx3) and prostaglandin-endoperoxide synthase 2 (Ptgs2) as compared to the control and vehicle groups. These changes were also accompanied by elevated intracellular reactive oxygen species (ROS), upregulated expression of the apoptotic Bax gene, reduced antioxidant enzymes (SOD, CAT, GPX) activities, and downregulated expression of the antioxidant gene nuclear factor erythroid 2 like 2 (Nrf2), and the anti-apoptotic gene Bcl2 in COCs after IVM. Overexpression of MSTN inhibitor, MSTNP, also inhibited GDF9 and BMP15 genes expression in COCs. Additionally, both the fertilization efficiency and cleavage and blastocyst rates were significantly lower in MSTNP group than in the control and vehicle groups. The obtained data suggest an important role for MSTN during IVM and the subsequent developmental stages probably through, at least in part, inhibition of ROS production and apoptosis and modulation of IVM-related gene expression in COCs., (Copyright © 2018 Elsevier Inc. All rights reserved.)

Background: Burn injury leads to a hypercatabolic response and ultimately muscle wasting with drastic implications for recovery of bodily functions, patient's quality of life (QoL), and long-term survival. Several treatment options target the body's initial stress response, but pharmacological approaches to specifically address muscle protein metabolism have only been poorly investigated., Objective: The aim of this study was to assess the role of myostatin and follistatin in burn injury and its possible implications in muscle wasting syndrome., Methods: We harvested serum from male patients within 48 h and again 9-12 months after severe burn injury (>20% of total body surface area). By means of myoblast cultures, immunohistochemistry, immunoblotting, and scratch assay, the role of myostatin and its implications in post-burn muscle metabolism and myoblast proliferation and differentiation was analyzed., Results: We were able to show increased proliferative and myogenic capacity, decreased myostatin, decreased SMAD 2/3, and elevated follistatin concentrations in human skeletal myoblast cultures with serum conditioned medium of patients in the acute phase of burn injury and conversely a reversed situation in patients in the chronic phase of burn injury. Thus, there is a biphasic response to burn trauma, initiated by an anabolic state and followed by long-term hypercatabolism., Conclusion: We conclude that the myostatin signaling pathway plays an important regulative role in burn-associated muscle wasting and that blockade of myostatin could prove to be a valuable treatment approach improving the rehabilitation process, QoL, and long-term survival after severe burn injury., (© 2019 S. Karger AG, Basel.)

Variants of the Myostatin gene have been shown to have an influence on muscle hypertrophy phenotypes in a wide range of mammalian species. Recently, a Thoroughbred horse with a C-Allele at the g.66493737C/T single-nucleotide polymorphism (SNP) has been reported to be suited to short-distance racing. In this study, we examined the effect of the Myostatin SNP on muscle fiber properties in young Thoroughbred horses during a training period. To investigate the effect of the Myostatin SNP on muscle fiber before training, several mRNA expressions were relatively quantified in biopsy samples from the middle gluteal muscle of 27 untrained male Thoroughbred horses (1.5 years old) using real-time RT-PCR analysis. The remaining muscle samples were used for immunohistochemical analysis to determine the population and area of each fiber type. All measurements were revaluated in biopsy samples of the same horses after a 5-month period of conventional training. Although the expressions of Myostatin mRNA decreased in all SNP genotypes, a significant decrease was found in only the C/C genotype after training. While, expression of VEGFa, PGC1α, and SDHa mRNAs, which relate to the biogenesis of mitochondria and capillaries, was significantly higher (54-82%) in the T/T than the C/C genotypes after training. It is suggested that hypertrophy of muscle fiber is directly associated with a decrease in Myostatin mRNA expression in the C/C genotype, and that increased expressions of VEGFa, PGC1α, and SDHa in the T/T genotype might be indirectly caused by the Myostatin SNP.

Exercise is generally considered to have health benefits for the body, although its beneficial mechanisms have not been fully elucidated. Recent progressive research suggests that myokines, bioactive substances secreted from skeletal muscle, play an important role in mediating the benefits of exercise. There are three types of myokines in terms of the muscular secretion mechanism: those in which the secretion is promoted by stimulation, such as irisin, interleukin (IL)-6, and IL-15; those whose secretion is constitutive, such as thioredoxin, glutaredoxin, and peroxiredoxin; and those whose secretion is suppressed by stimulation, such as by a macrophage migration inhibitory factor. Although dozens of myokines have been reported, their physiological roles are not well understood. Therefore, there currently exists no advanced drug discovery research specifically targeting myokines, with the exception of Myostatin. Myostatin was discovered as a negative regulator of muscle growth. Myostatin is secreted from muscle cells as a myokine; it signals via an activin type IIB receptor in an autocrine manner, and regulates gene expressions involved in myogenesis. Given the studies to date that have been conducted on the utilization of myostatin inhibitors for the treatment of muscle weakness, including cachexia and sarcopenia, other myokines may also be new potential drug targets.

Myostatin (Mstn) is an inhibitor of myogenesis, regulating the number and size of skeletal myocytes. In addition to its myogenic regulatory function, Mstn plays important roles in the development of adipose tissues and in metabolism. In the present study, an Mstn knockout rat model was generated using the zinc finger nuclease (ZFN) technique in order to further investigate the function and mechanism of Mstn in metabolism. The knockout possesses a frame shift mutation resulting in an early termination codon and a truncated peptide of 109 amino acids rather than the full 376 amino acids. The absence of detectable mRNA confirmed successful knockout of Mstn. Relative to wild-type (WT) littermates, Knockout (KO) rats exhibited significantly greater body weight, body circumference, and muscle mass. However, no significant differences in grip force was observed, indicating that Mstn deletion results in greater muscle mass but not greater muscle fiber strength. Additionally, KO rats were found to possess less body fat relative to WT littermates, which is consistent with previous studies in mice and cattle. The aforementioned results indicate that Mstn knockout increases muscle mass while decreasing fat content, leading to observed increases in body weight and body circumference. The Mstn knockout rat model provides a novel means to study the role of Mstn in metabolism and Mstn-related muscle hypertrophy., (Copyright © 2016 The Authors. Published by Elsevier Inc. All rights reserved.)

The present study investigated the influence of a diet largely comprising rice whole-crop silage (rWCS) on growth performance, carcass and meat characteristics, and expression of genes involved in muscle growth of Japanese Black steers. Steers were randomly separated into rWCS-fed (rWCS ad libitum and restricted feeding of concentrate) and concentrate-fed groups. Total digestible nutrient intake and daily gain (DG) decreased in rWCS-fed steers in comparison with concentrate-fed steers, whereas dressed carcass weight and final body weight did not significantly differ between the groups. Decreases in drip loss in the muscle of rWCS-fed steers may be caused by α-tocopherol and β-carotene in muscle. Feeding large amounts of rWCS to steers may maintain quantitative productivity of beef steers equally to a concentrate-based diet, and improve the qualitative productivity. Results of gene expression suggest that activation of skeletal muscle growth in rWCS-fed steers may occur at the late fattening period owing to a decrease in myostatin and increase in myosin heavy chain gene expression. Preadipocyte factor-1 and myostatin genes may be strongly involved in the control of lipid accumulation. This rearing system would allow beef production to switch to rWCS-based diets from concentrate-based diets., (© 2015 Japanese Society of Animal Science.)

Growth differentiation factor 11 (GDF11) and myostatin (or GDF8) are closely related members of the transforming growth factor β superfamily and are often perceived to serve similar or overlapping roles. Yet, despite commonalities in protein sequence, receptor utilization and signaling, accumulating evidence suggests that these 2 ligands can have distinct functions in many situations. GDF11 is essential for mammalian development and has been suggested to regulate aging of multiple tissues, whereas myostatin is a well-described negative regulator of postnatal skeletal and cardiac muscle mass and modulates metabolic processes. In this review, we discuss the biochemical regulation of GDF11 and myostatin and their functions in the heart, skeletal muscle, and brain. We also highlight recent clinical findings with respect to a potential role for GDF11 and/or myostatin in humans with heart disease. Finally, we address key outstanding questions related to GDF11 and myostatin dynamics and signaling during development, growth, and aging., (© 2016 American Heart Association, Inc.)

This "Controversies in Cardiovascular Research" article evaluates the evidence for and against the hypothesis that the circulating blood level of growth differentiation factor 11 (GDF11) decreases in old age and that restoring normal GDF11 levels in old animals rejuvenates their skeletal muscle and reverses pathological cardiac hypertrophy and cardiac dysfunction. Studies supporting the original GDF11 hypothesis in skeletal and cardiac muscle have not been validated by several independent groups. These new studies have either found no effects of restoring normal GDF11 levels on cardiac structure and function or have shown that increasing GDF11 or its closely related family member growth differentiation factor 8 actually impairs skeletal muscle repair in old animals. One possible explanation for what seems to be mutually exclusive findings is that the original reagent used to measure GDF11 levels also detected many other molecules so that age-dependent changes in GDF11 are still not well known. The more important issue is whether increasing blood [GDF11] repairs old skeletal muscle and reverses age-related cardiac pathologies. There are substantial new and existing data showing that GDF8/11 can exacerbate rather than rejuvenate skeletal muscle injury in old animals. There is also new evidence disputing the idea that there is pathological hypertrophy in old C57bl6 mice and that GDF11 therapy can reverse cardiac pathologies. Finally, high [GDF11] causes reductions in body and heart weight in both young and old animals, suggestive of a cachexia effect. Our conclusion is that elevating blood levels of GDF11 in the aged might cause more harm than good., (© 2016 American Heart Association, Inc.)

Sarcopenia, or skeletal muscle atrophy, is a debilitating comorbidity of many physiological and pathophysiological processes, including normal aging. There are no approved therapies for sarcopenia, but the antihypertrophic myokine myostatin is a potential therapeutic target. Here, we show that treatment of young and old mice with an anti-myostatin antibody (ATA 842) for 4 wk increased muscle mass and muscle strength in both groups. Furthermore, ATA 842 treatment also increased insulin-stimulated whole body glucose metabolism in old mice, which could be attributed to increased insulin-stimulated skeletal muscle glucose uptake as measured by a hyperinsulinemic-euglycemic clamp. Taken together, these studies provide support for pharmacological inhibition of myostatin as a potential therapeutic approach for age-related sarcopenia and metabolic disease.

Unlabelled: Mice with osteogenesis imperfecta (+/oim), a disorder of bone fragility, were bred to mice with muscle over growth to test whether increasing muscle mass genetically would improve bone quality and strength. The results demonstrate that femora from mice carrying both mutations have greater mechanical integrity than their +/oim littermates., Introduction: Osteogenesis imperfecta is a heritable connective tissue disorder due primarily to mutations in the type I collagen genes resulting in skeletal deformity and fragility. Currently, there is no cure, and therapeutic strategies encompass the use of antiresorptive pharmaceuticals and surgical bracing, with limited success and significant potential for adverse effects. Bone, a mechanosensing organ, can respond to high mechanical loads by increasing new bone formation and altering bone geometry to withstand increased forces. Skeletal muscle is a major source of physiological loading on bone, and bone strength is proportional to muscle mass., Methods: To test the hypothesis that congenic increases in muscle mass in the osteogenesis imperfecta murine model mouse (oim) will improve their compromised bone quality and strength, heterozygous (+/oim) mice were bred to mice deficient in myostatin (+/mstn), a negative regulator of muscle growth. The resulting adult offspring were evaluated for hindlimb muscle mass, and bone microarchitecture, physiochemistry, and biomechanical integrity., Results: +/oim mice deficient in myostatin (+/mstn +/oim) were generated and demonstrated that myostatin deficiency increased body weight, muscle mass, and biomechanical strength in +/mstn +/oim mice as compared to +/oim mice. Additionally, myostatin deficiency altered the physiochemical properties of the +/oim bone but did not alter bone remodeling., Conclusions: Myostatin deficiency partially improved the reduced femoral bone biomechanical strength of adult +/oim mice by increasing muscle mass with concomitant improvements in bone microarchitecture and physiochemical properties.

Myostatin (also known as growth and differentiation factor 8) is a secreted member of the transforming growth factor-β (TGF-β) family that is mainly expressed in skeletal muscle, which is also its primary target tissue. Deletion of the myostatin gene (Mstn) in mice leads to muscle hypertrophy, and animal studies support the concept that myostatin is a negative regulator of muscle growth and regeneration. However, myostatin deficiency also increases bone formation, mainly through loading-associated effects on bone. Here we report a previously unknown direct role for myostatin in osteoclastogenesis and in the progressive loss of articular bone in rheumatoid arthritis (RA). We demonstrate that myostatin is highly expressed in the synovial tissues of RA subjects and of human tumor necrosis factor (TNF)-α transgenic (hTNFtg) mice, a model for human RA. Myostatin strongly accelerates receptor activator of nuclear factor κB ligand (RANKL)-mediated osteoclast formation in vitro through transcription factor SMAD2-dependent regulation of nuclear factor of activated T-cells (NFATC1). Myostatin deficiency or antibody-mediated inhibition leads to an amelioration of arthritis severity in hTNFtg mice, chiefly reflected by less bone destruction. Consistent with these effects in hTNFtg mice, the lack of myostatin leads to increased grip strength and less bone erosion in the K/BxN serum-induced arthritis model in mice. The results strongly suggest that myostatin is a potent therapeutic target for interfering with osteoclast formation and joint destruction in RA.

Chronic kidney disease (CKD) is associated with a decline in muscle mass, strength, and function, collectively called "sarcopenia." Sarcopenia is associated with hospitalizations and mortality in CKD and is therefore important to understand and characterize. While the focus of skeletal health in CKD has traditionally focused on bone and mineral aberrations, it is now recognized that sarcopenia must also play a role in poor musculoskeletal health in this population. In this paper, we present an overview of skeletal muscle changes in CKD, including defects in skeletal muscle catabolism and anabolism in uremic tissue. There are many gaps in knowledge in this field that should be the focus for future research to unravel pathogenesis and therapies for musculoskeletal health in CKD.

Myostatin is a critical negative regulator of skeletal muscle development, and has been reported to be involved in the progression of obesity and diabetes. In the present study, we explored the effects of myostatin on the proliferation and differentiation of 3T3-L1 preadipocytes by using 3-[4,5-dimethylthiazol-2-yl] 2,5-diphenyl tetrazolium bromide spectrophotometry, intracellular triglyceride (TG) assays, and real-time quantitative RT-PCR methods. The results indicated that recombinant myostatin significantly promoted the proliferation of 3T3-L1 preadipocytes and the expression of proliferation-related genes, including Cyclin B2, Cyclin D1, Cyclin E1, Pcna, and c-Myc, and IGF1 levels in the medium of 3T3-L1 were notably upregulated by 35.2, 30.5, 20.5, 33.4, 51.2, and 179% respectively (all P<0.01) in myostatin-treated 3T3-L1 cells. Meanwhile, the intracellular lipid content of myostatin-treated cells was notably reduced as compared with the non-treated cells. Additionally, the mRNA levels of Pparγ, Cebpα, Gpdh, Dgat, Acs1, Atgl, and Hsl were significantly downregulated by 22-76% in fully differentiated myostatin-treated adipocytes. Finally, myostatin regulated the mRNA levels and secretion of adipokines, including Adiponectin, Resistin, Visfatin, and plasminogen activator inhibitor-1 (PAI-1) in 3T3-L1 adipocytes (all P<0.001). Above all, myostatin promoted 3T3-L1 proliferation by increasing the expression of cell-proliferation-related genes and by stimulating IGF1 secretion. Myostatin inhibited 3T3-L1 adipocyte differentiation by suppressing Pparγ and Cebpα expression, which consequently deceased lipid accumulation in 3T3-L1 cells by inhibiting the expression of critical lipogenic enzymes and by promoting the expression of lipolytic enzymes. Finally, myostatin modulated the expression and secretion of adipokines in fully differentiated 3T3-L1 adipocytes., (© 2015 Society for Endocrinology.)

The use of genetic markers to aid in selection decisions to improve carcass and growth characteristics is of great interest to the beef industry. However, it is important to examine potential antagonistic interactions with fertility in cows before widespread application of marker-assisted selection. The objective of the current experiment was to examine the influence of 2 commercially available markers currently in use for improving carcass traits, the myostatin (MSTN) F94L and μ-calpain (CAPN1) 316 and 4751 polymorphisms, on heifer development and reproductive performance. In Exp. 1, beef heifers (n = 146) were evaluated for growth and reproductive traits over a 3-yr period to determine if these polymorphisms influenced reproductive performance. In Exp. 2, heifers representing the 2 homozygous genotypes for the MSTN F94L polymorphism were slaughtered on d 4 of the estrous cycle and reproductive tracts were collected for morphological examination. In Exp. 1, there was a tendency (P = 0.06) for birth BW to be affected by MSTN with the Leu allele increasing birth BW in an additive fashion. Additionally, MSTN significantly affected the proportion of pubertal heifers by the start of the breeding season (P < 0.05) with the Leu allele additively decreasing the proportion pubertal; however, this did not result in a delay in conception or a decrease in pregnancy rates during the first breeding season (P > 0.15). The GT haplotype of CAPN1, which was previously associated with decreased meat tenderness, was associated with an additive decrease in birth BW of the first calf born to these heifers (P < 0.05). In Exp. 2, there were no differences between the MSTN genotypes for gross or histological morphology of the anterior pituitary, uterus, or ovaries (P > 0.05). From these results, we concluded that the MSTN F94L and CAPN1 polymorphisms can be used to improve carcass traits without compromising fertility in beef heifers. The influence of these markers on cow performance and herd life remains to be determined. While the delay in puberty associated with the MSTN F94L polymorphism did not negatively impact reproductive performance in heifers, caution should be used when combining this marker with other markers for growth or carcass traits until the potential interactions are more clearly understood.

Improving the traits that underlie meat quality is a major challenge in the beef industry. The objective of this paper was to detect QTL linked to sensory meat quality traits in 3 French beef cattle breeds. We genotyped 1,059, 1,219, and 947 young bulls and their sires belonging to the Charolais, Limousin, and Blonde d'Aquitaine breeds, respectively, using the Illumina BovineSNP50 BeadChip (Illumina Inc., San Diego, CA). After estimating relevant genetic parameters using VCE software, we performed a linkage disequilibrium and linkage analysis on 4 meat traits: intramuscular fat content, muscle lightness, shear force, and tenderness score. Heritability coefficients largely ranged between 0.10 and 0.24; however, they reached a maximum of 0.44 and 0.50 for intramuscular fat content and tenderness score, respectively, in the Charolais breed. The 2 meat texture traits, shear force and tenderness score, were strongly genetically correlated (-0.91 in the Charolais and Limousin breed and -0.86 in the Blonde d'Aquitaine breed), indicating that they are 2 different measures of approximately the same trait. The genetic correlation between tenderness and intramuscular fat content differed across breeds. Using a significance threshold of 5 × 10(-4) for QTL detection, we found more than 200 significant positions across the 29 autosomal chromosomes for the 4 traits in the Charolais and Blonde d'Aquitaine breeds; in contrast, there were only 78 significant positions in the Limousin breed. Few QTL were common across breeds. We detected QTL for intramuscular fat content located near the myostatin gene in the Charolais and Blonde d'Aquitaine breeds. No mutation in this gene has been reported for the Blonde d'Aquitaine breed; therefore, it suggests that an unknown mutation could be segregating in this breed. We confirmed that, in certain breeds, markers in the calpastatin and calpain 1 gene regions affect tenderness. We also found new QTL as several QTL on chromosome 3 that are significantly associated with meat tenderness in the Blonde d'Aquitaine breed. Overall, these results greatly contribute to the goal of building a panel of markers that can be used to select animals of high meat quality.

Mammalian aging is accompanied by a progressive loss of skeletal muscle, a process called sarcopenia. Myostatin, a secreted member of the transforming growth factor-β family of signaling molecules, has been shown to be a potent inhibitor of muscle growth. Here, we examined whether muscle growth could be promoted in aged animals by antagonizing the activity of myostatin through the neutralizing activity of the myostatin propeptide. We show that a single injection of an AAV8 virus expressing the myostatin propeptide induced an increase in whole body weights and all muscles examined within 7 weeks of treatment. Our cellular studies demonstrate that muscle enlargement was due to selective fiber type hypertrophy, which was accompanied by a shift toward a glycolytic phenotype. Our molecular investigations elucidate the mechanism underpinning muscle hypertrophy by showing a decrease in the expression of key genes that control ubiquitin-mediated protein breakdown. Most importantly, we show that the hypertrophic muscle that develops as a consequence of myostatin propeptide in aged mice has normal contractile properties. We suggest that attenuating myostatin signaling could be a very attractive strategy to halt and possibly reverse age-related muscle loss., (© The Author 2014. Published by Oxford University Press on behalf of The Gerontological Society of America. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com.)

Rationale: Myostatin is a major negative regulator of skeletal muscle mass and initiates multiple metabolic changes, including enhanced insulin sensitivity. However, the function of myostatin in the heart is barely understood, although it is upregulated in the myocardium under several pathological conditions., Objective: Here, we aimed to decipher the role of myostatin and myostatin-dependent signaling pathways for cardiac function and cardiac metabolism in adult mice. To avoid potential counterregulatory mechanisms occurring in constitutive and germ-line-based myostatin mutants, we generated a mouse model that allows myostatin inactivation in adult cardiomyocytes., Methods and Results: Cardiac MRI revealed that genetic inactivation of myostatin signaling in the adult murine heart caused cardiac hypertrophy and heart failure, partially recapitulating effects of the age-dependent decline of the myostatin paralog growth and differentiation factor 11. We found that myostatin represses AMP-activated kinase activation in the heart via transforming growth factor-β-activated kinase 1, thereby preventing a metabolic switch toward glycolysis and glycogen accumulation. Furthermore, myostatin stimulated expression of regulator of G-protein signaling 2, a GTPase-activating protein that restricts Gaq and Gas signaling and thereby protects against cardiac failure. Inhibition of AMP-activated kinase in vivo rescued cardiac hypertrophy and prevented enhanced glycolytic flow and glycogen accumulation after inactivation of myostatin in cardiomyocytes., Conclusions: Our results uncover an important role of myostatin in the heart for maintaining cardiac energy homeostasis and preventing cardiac hypertrophy., (© 2014 American Heart Association, Inc.)

Background: A sedentary lifestyle is an independent risk factor for cardiovascular disease and exercise has been shown to ameliorate this risk. Inactivity is associated with a loss of muscle mass, which is also reversed with isometric exercise training. The relationship between muscle mass and vascular function is poorly defined. The aims of the current study were to determine whether increasing muscle mass by genetic deletion of myostatin, a negative regulator of muscle growth, can influence vascular function in mesenteric arteries from obese db/db mice., Methods and Results: Myostatin expression was elevated in skeletal muscle of obese mice and associated with reduced muscle mass (30% to 50%). Myostatin deletion increased muscle mass in lean (40% to 60%) and obese (80% to 115%) mice through increased muscle fiber size (P<0.05). Myostatin deletion decreased adipose tissue in lean mice, but not obese mice. Markers of insulin resistance and glucose tolerance were improved in obese myostatin knockout mice. Obese mice demonstrated an impaired endothelial vasodilation, compared to lean mice. This impairment was improved by superoxide dismutase mimic Tempol. Deletion of myostatin improved endothelial vasodilation in mesenteric arteries in obese, but not in lean, mice. This improvement was blunted by nitric oxide (NO) synthase inhibitor l-NG-nitroarginine methyl ester (l-NAME). Prostacyclin (PGI2)- and endothelium-derived hyperpolarizing factor (EDHF)-mediated vasodilation were preserved in obese mice and unaffected by myostatin deletion. Reactive oxygen species) was elevated in the mesenteric endothelium of obese mice and down-regulated by deletion of myostatin in obese mice. Impaired vasodilation in obese mice was improved by NADPH oxidase inhibitor (GKT136901). Treatment with sepiapterin, which increases levels of tetrahydrobiopterin, improved vasodilation in obese mice, an improvement blocked by l-NAME., Conclusions: Increasing muscle mass by genetic deletion of myostatin improves NO-, but not PGI2- or EDHF-mediated vasodilation in obese mice; this vasodilation improvement is mediated by down-regulation of superoxide., (© 2014 The Authors. Published on behalf of the American Heart Association, Inc., by Wiley Blackwell.)

The growth of muscle fibers can be negatively regulated by bovine myostatin. The first two exons of myostatin gene code for the N-propeptide and its third exon codes for the C-polypeptide. Myostatin is secreted as a latent configuration formed by dimerization of two matured C peptides non-covalently linked with the N terminal pro-peptide. Pro-peptide has two distinct functions in guiding protein folding and regulating biological activity of myostatin. When the structure of the leader peptide is altered via mutations resulting in more tight binding with the mature peptide, myostatin function is inhibited, resulting in the changes of P21 and CDK2 expression levels which are related to the regulation of cell cycle. In the present study, the coding region of bMSTN (bovine myostatin) gene was amplified and mutated (A224C and G938A) through fusion PCR, and the mutated bMSTN gene (bMSTN-mut) was inserted in frame into the pEF1a-IRES-DsRed-Express2 vector and transfected into bovine fibroblast cells. The expression levels of bMSTN-mut, P21 and CDK2 (cyclin dependent kinase 2) were examined with qPCR and Western-blotting. Changes in cell cycle after transfection were also analyzed with flow cytometry. The results indicated that leader peptide mutation resulted in down-regulation of P21 expression levels and up-regulation of CDK2 expression levels. The flow cytometry results showed that the proportion of cells in the G0/G1-phase was lower and that of cells in the S-phase was higher in bMSTN-mut transfected group than that in the control group. The proliferation rate of bMSTN-mut transfected cells was also significantly higher than that of the control cells. In conclusion, the studies have shown that the pEF1a-IRES-DsRed-Express2-bMSTN-mut recombinant plasmid could effectively promote the proliferation of bovine fibroblast cells. The site-directed mutagenesis of bMSTN gene leader peptide and in vitro expression in bovine fibroblast cells could be helpful to further the studies of bMSTN in regulating bovine muscle cell growth and development., (Copyright © 2014 Elsevier B.V. All rights reserved.)

Myostatin, a member of the transforming growth factor β family, is a potent negative regulator of skeletal muscle growth, as myostatin-deficient mice show a great increase in muscle mass. Yet the physical performance of these animals is reduced. As an explanation for this, alterations in the steps in excitation-contraction coupling were hypothesized and tested for in mice with the 12 bp deletion in the propeptide region of the myostatin precursor (Mstn(Cmpt-dl1Abc) or Cmpt). In voluntary wheel running, control C57BL/6 mice performed better than the mutant animals in both maximal speed and total distance covered. Despite the previously described lower specific force of Cmpt animals, the pCa-force relationship, determined on chemically permeabilized fibre segments, did not show any significant difference between the two mouse strains. While resting intracellular Ca(2+) concentration ([Ca(2+)]i) measured on single intact flexor digitorum brevis (FDB) muscle fibres using Fura-2 AM was similar to control (72.0 ± 1.7 vs. 78.1 ± 2.9 nM, n = 38 and 45), the amplitude of KCl-evoked calcium transients was smaller (360 ± 49 vs. 222 ± 45 nM, n = 22) in the mutant strain. Similar results were obtained using tetanic stimulation and Rhod-2 AM, which gave calcium transients that were smaller (2.42 ± 0.11 vs. 2.06 ± 0.10 ΔF/F0, n = 14 and 13, respectively) on Cmpt mice. Sarcoplasmic reticulum (SR) calcium release flux calculated from these transients showed a reduced peak (23.7 ± 3.0 vs. 15.8 ± 2.1 mM s(-1)) and steady level (5.7 ± 0.7 vs. 3.7 ± 0.5 mM s(-1)) with no change in the peak-to-steady ratio. The amplitude and spatial spread of calcium release events detected on permeabilized FDB fibres were also significantly smaller in mutant mice. These results suggest that reduced SR calcium release underlies the reduced muscle force in Cmpt animals.

Smad (Sma and Mad-related protein) 2/3 are downstream signaling molecules for TGF-β and myostatin (Mstn). Recently, Mstn was shown to induce reactive oxygen species (ROS) in skeletal muscle via canonical Smad3, nuclear factor-κB, and TNF-α pathway. However, mice lacking Smad3 display skeletal muscle atrophy due to increased Mstn levels. Hence, our aims were first to investigate whether Mstn induced muscle atrophy in Smad3(-/-) mice by increasing ROS and second to delineate Smad3-independent signaling mechanism for Mstn-induced ROS. Herein we show that Smad3(-/-) mice have increased ROS levels in skeletal muscle, and inactivation of Mstn in these mice partially ablates the oxidative stress. Furthermore, ROS induction by Mstn in Smad3(-/-) muscle was not via nuclear factor-κB (p65) signaling but due to activated p38, ERK MAPK signaling and enhanced IL-6 levels. Consequently, TNF-α, nicotinamide adenine dinucleotide phosphate oxidase, and xanthine oxidase levels were up-regulated, which led to an increase in ROS production in Smad3(-/-) skeletal muscle. The exaggerated ROS in the Smad3(-/-) muscle potentiated binding of C/EBP homology protein transcription factor to MuRF1 promoter, resulting in enhanced MuRF1 levels leading to muscle atrophy.

The immortal C2C12 cell line originates from dystrophic mouse thigh muscle and has been used to study the endocrine control of muscle cell growth, development, and function, including those actions regulated by myostatin. Previous studies suggest that high concentrations of recombinant myostatin generated in bacteria inhibit C2C12 proliferation and differentiation. Recombinant myostatin generated in eukaryotic systems similarly inhibits the proliferation of primary myosatellite cells, but consequently initiates, rather than inhibits, their differentiation and is bioactive at far lower concentrations. Our studies indicate that 2 different sources of recombinant myostatin made in eukaryotes stimulate, not inhibit, C2C12 proliferation. This effect occurred at different cell densities and serum concentrations and in the presence of IGF-I, a potent myoblast mitogen. This stimulatory effect was comparable to that obtained with TGFβ1, a related factor that also inhibits primary myosatellite cell proliferation. Attenuating the myostatin/activin (ie, Acvr2b) and TGFβ1 receptor signaling pathways with the Alk4/5 and Alk5 inhibitors, SB431542 and SB505142, respectively, similarly attenuated proliferation induced by serum, myostatin or TGFβ1 and in a dose-dependent manner. In serum-free medium, both myostatin and TGFβ1 stimulated Smad2 phosphorylation, but not that of Smad3, and a Smad3 inhibitor (SIS3) only inhibited proliferation in cells cultured in high serum. Thus, myostatin and TGFβ1 stimulate C2C12 proliferation primarily via Smad2. These results together question the physiological relevance of the C2C12 model and previous studies using recombinant myostatin generated in bacteria. They also support the alternative use of primary myosatellite cells and recombinant myostatin generated in eukaryotes.

The progressive loss of skeletal muscle mass, strength and/or function with advancing age, termed sarcopenia, poses a major threat to independence and quality of life. Therefore, there is significant merit in better understanding the biology of sarcopenia and developing therapeutic interventions to prevent, slow or reverse its progression. Since the discovery of myostatin, a potent negative regulator of growth that is highly enriched in skeletal muscle, there has been great interest in it as a potential mediator of sarcopenia as well as a therapeutic target. The complex biology of myostatin, the promise of myostatin inhibition as an effective means to counter sarcopenia, and the challenges facing its clinical translation are reviewed herein., (© 2014 S. Karger AG, Basel.)

Myostatin, a member of the transforming growth factor beta (TGF-β) superfamily, was first described in 1997. Since then, myostatin has gained growing attention because of the discovery that myostatin inhibition leads to muscle mass accrual. Myostatin not only plays a key role in muscle homeostasis, but also affects fat and bone. This review will focus on the impact of myostatin and its inhibition on muscle mass/function, adipose tissue and bone density/geometry in humans. Although existing data are sparse, myostatin inhibition leads to increased lean mass and 1 study found a decrease in fat mass and increase in bone formation. In addition, myostatin levels are increased in sarcopenia, cachexia and bed rest whereas they are increased after resistance training, suggesting physiological regulatory of myostatin. Increased myostatin levels have also been found in obesity and levels decrease after weight loss from caloric restriction. Knowledge on the relationship of myostatin with bone is largely based on animal data where elevated myostatin levels lead to decreased BMD and myostatin inhibition improved BMD. In summary, myostatin appears to be a key factor in the integrated physiology of muscle, fat, and bone. It is unclear whether myostatin directly affects fat and bone, or indirectly via muscle. Whether via direct or indirect effects, myostatin inhibition appears to increase muscle and bone mass and decrease fat tissue-a combination that truly appears to be a holy grail. However, at this time, human data for both efficacy and safety are extremely limited. Moreover, whether increased muscle mass also leads to improved function remains to be determined. Ultimately potential beneficial effects of myostatin inhibition will need to be determined based on hard outcomes such as falls and fractures.

Myostatin is a growth and differentiation factor and acts as a negative regulator of skeletal muscle mass. Although the mechanism whereby myostatin controls muscle cell growth is mostly clarified, the regulation of myostatin activity after its secretion into the extracellular matrix (ECM) is still unclear. In the present study, we investigated the interaction between laminin and myostatin and the effect of laminin on myostatin signaling in vitro. The surface plasmon resonance assay showed that laminin bound to mature myostatin and activin receptor type IIB (ActRIIB), but did not bind to latency-associated protein, which remains non-covalently linked to mature myostatin. Furthermore, kinetic analysis demonstrated that the affinity of mature myostatin for laminin was similar to that for ActRIIB. Next, we examined the action of laminin on the myostatin signaling pathway using a conventional reporter assay. The luciferase activity of myostatin-treated cells was repressed significantly (P < 0.05) by coincubation of laminin. These results suggest that laminin has a potential to regulate myostatin activity through binding to mature myostatin and/or its receptor ActRIIB., (© 2013 Japanese Society of Animal Science.)

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.

Recent works suggest that Activin A (ActA) and Myostatin (Mstn), two members of the TGFβ superfamily, could contribute to skeletal muscle atrophy observed in some cancers. It is known that several human tumoral cell lines synthesize and secrete ActA and Mstn. In addition, systemic treatment with ActA and Mstn in mice induce muscle atrophy. Likewise, Inhibin-α knock-out mice, which are characterized by elevated circulating levels of ActA, exhibit muscle atrophy and die of cachexia. Finally, administration of ActA and Mstn antagonists prevents muscular atrophy and mortality induced by some animal tumors. Collectively, these findings suggest that ActA or Mstn production by several cancers could contribute to cachexia and thus to mortality associated with some cancers in human. This hypothesis is very interesting since new molecules that are able to inhibit ActA and Mstn, in particularly the sActRIIB, are under development., (Copyright © 2013 Elsevier Masson SAS. All rights reserved.)

Myostatin (Mstn) is predominantly expressed in skeletal muscles and plays important roles in regulating muscle growth and development, as well as fat deposition. Mstn-knockout (Mstn(-/-)) mice exhibit increased muscle mass due to both hypertrophy and hyperplasia, and leaner body composition due to reduced fat mass. Here, we show that white adipose tissue (WAT) of Mstn(-/-) develops characteristics of brown adipose tissue (BAT) with dramatically increased expression of BAT signature genes, including Ucp1 and Pgc1α, and beige adipocyte markers Tmem26 and CD137. Strikingly, the observed browning phenotype is non-cell autonomous and is instead driven by the newly defined myokine irisin (Fndc5) secreted from Mstn(-/-) skeletal muscle. Within the muscle, Mstn(-/-) leads to increased expression of AMPK and its phosphorylation, which subsequently activates PGC1α and Fndc5. Together, our study defines a paradigm of muscle-fat crosstalk mediated by Fndc5, which is up-regulated and secreted from muscle to induce beige cell markers and the browning of WAT in Mstn(-/-) mice. These results suggest that targeting muscle Mstn and its downstream signaling represents a therapeutic approach to treat obesity and type 2 diabetes.

Catabolic conditions including chronic kidney disease (CKD), cancer, and diabetes cause muscle atrophy. The loss of muscle mass worsens the burden of disease because it is associated with increased morbidity and mortality. To avoid these problems or to develop treatment strategies, the mechanisms leading to muscle wasting must be identified. Specific mechanisms uncovered in CKD generally occur in other catabolic conditions. These include stimulation of protein degradation in muscle arising from activation of caspase-3 and the ubiquitin-proteasome system (UPS). These proteases act in a coordinated fashion with caspase-3 initially cleaving the complex structure of proteins in muscle, yielding fragments that are substrates that are degraded by the UPS. Fortunately, the UPS exhibits remarkable specificity for proteins to be degraded because it is the major intracellular proteolytic system. Without a high level of specificity cellular functions would be disrupted. The specificity is accomplished by complex reactions that depend on recognition of a protein substrate by specific E3 ubiquitin ligases. In muscle, the specific ligases are Atrogin-1 and MuRF-1, and their expression has characteristics of a biomarker of accelerated muscle proteolysis. Specific complications of CKD (metabolic acidosis, insulin resistance, inflammation, and angiotensin II) activate caspase-3 and the UPS through mechanisms that include glucocorticoids and impaired insulin or IGF-1 signaling. Mediators activate myostatin, which functions as a negative growth factor in muscle. In models of cancer or CKD, strategies that block myostatin prevent muscle wasting, suggesting that therapies that block myostatin could prevent muscle wasting in catabolic conditions.

Myostatin, also known as growth differentiation factor (GDF-8), regulates proliferation of muscle satellite cells, and suppresses differentiation of myoblasts into myotubes via down-regulation of key myogenic differentiation factors including MyoD. Recent advances in stem cell biology have enabled generation of myoblasts from pluripotent stem cells, but it remains to be clarified whether myostatin is also involved in regulation of artificial differentiation of myoblasts from pluripotent stem cells. Here we show that the human induced pluripotent stem (iPS) cell-derived cells that were induced to differentiate into myoblasts expressed myostatin and its receptor during the differentiation. An addition of recombinant human myostatin (rhMyostatin) suppressed induction of MyoD and Myo5a, resulting in significant suppression of myoblast differentiation. The rhMyostatin treatment also inhibited proliferation of the cells at a later phase of differentiation. RNAi-mediated silencing of myostatin promoted differentiation of human iPS-derived embryoid body (EB) cells into myoblasts. These results strongly suggest that myostatin plays an important role in regulation of myoblast differentiation from iPS cells of human origin. The present findings also have significant implications for potential regenerative medicine for muscular diseases., (Copyright © 2013 Elsevier Inc. All rights reserved.)

Myostatin, a member of the transforming growth factor-β family, acts as a negative regulator of skeletal muscle mass. In this study, myostatin-targeted caprine fibroblasts were obtained and subjected to SCNT to determine whether myostatin-knockout goats could be created. Fibroblasts from a 2-mo-old goat were transfected with a myostatin-targeted vector to prepare transgenic donor cells for nuclear transfer. After serum-starvation (for synchronization of the cell cycle), the percentage of transgenic fibroblasts in the G(0)/G(1) phase increased (66.2% vs. 82.9%; P < 0.05) compared with that in the control group, whereas the apoptosis rate and mitochondrial membrane potential were unaffected (P > 0.05). There were no significant differences between in vivo- and in vitro-matured oocytes as recipient cytoplasts for rates of fusion (86.5% vs. 78.4%), pregnancy (21.6% vs. 16.7%), or kidding (2.7% vs. 0%). One female kid from an in vivo-matured oocyte was born, but died a few hours later. Microsatellite analysis and polymerase chain reaction identification confirmed that this kid was genetically identical to the donor cells. Based on Western blot analysis, myostatin of the cloned kid was not expressed compared with that of nontransgenic kids. In conclusion, SCNT using myostatin-targeted 2-mo-old goat fibroblasts as donors has potential as a method for producing myostatin-targeted goats., (Copyright © 2013 Elsevier Inc. All rights reserved.)