Role of satellite cells in muscle growth and maintenance of muscle mass

Changes in muscle mass may result from changes in protein turnover, reflecting the balance between protein synthesis and protein degradation, and changes in cell turnover, reflecting the balance between myonuclear accretion and myonuclear loss. Myonuclear accretion, i.e. increase in the number of myonuclei within the muscle fibers, takes place via proliferation and fusion of satellite cells, myogenic stem cells associated to skeletal muscle fibers and involved in muscle regeneration. In developing muscle, satellite cells undergo extensive proliferation and most of them fuse with myofibers, thus contributing to the increase in myonuclei during early postnatal stages. A similar process is induced in adult skeletal muscle by functional overload and exercise. In contrast, satellite cells and myonuclei may undergo apoptosis during muscle atrophy, although it is debated whether myonuclear loss occurs in atrophying muscle. An increase in myofiber size can also occur by changes in protein turnover without satellite cell activation, e.g. in late phases of postnatal development or in some models of muscle hypertrophy. The relative role of protein turnover and cell turnover in muscle adaptation and in the establishment of functional muscle hypertrophy remains to be established. The identification of the signaling pathways mediating satellite cell activation may provide therapeutic targets for combating muscle wasting in a variety of pathological conditions, including cancer cachexia, renal and cardiac failure, neuromuscular diseases, as well as aging sarcopenia.     

Androgen receptor expression during C2C12 skeletal muscle cell line differentiation

The Androgen Receptor (AR) pathway is involved in the development of skeletal muscle but the molecular basis of androgen-related myogenic enhancement is still unclear. We have investigated AR expression and localization during myoblasts-myotubes differentiation in skeletal muscle cell line C2C12. AR expression increases during proliferation and commitment phase and its levels remain elevated in myotubes. In proliferating and committed cells in the absence of testosterone, AR protein localizes in the nuclei whereas it is almost totally localized in the cytoplasm in myotubes. Low testosterone doses shift the receptor in the nuclei without increasing the amount of total protein. High doses of T induce a significant increase of AR expression during proliferation and differentiation. Little information is available on AR targets that drive the myogenic process. In our study, testosterone induces myogenin, myosin heavy chains (MyHC) and GRIP-1 expression, suggesting that AR and its coregulatory proteins are pivotal factors in skeletal muscle differentiation.     

小鼠骨骼肌干细胞蛋白标志JAK的发现

目的通过骨骼肌卫星细胞的体外培养,探讨骨骼肌干细胞蛋白表达标志Pax7,发现新的与骨骼肌干细胞的生长、分化密切相关的表达蛋白。方法采用两种小鼠骨骼肌干细胞分离、纯化及培养方法,一种是采取成年小鼠的趾短屈肌(FDB)分离切割后,体外培养出单个骨骼肌干细胞;另一种是取成年小鼠FDB,分离出单个肌纤维丝(fiber)并体外培养。应用免疫荧光染色法对骨骼肌干细胞进行鉴定。运用RNAi技术将JAK1蛋白敲除。结果发现Pax7表达阳性的细胞质内有JAK1蛋白表达,并运用RNAi技术将JAK1蛋白敲除,结果显示骨骼肌干细胞迅速分化。结论 FDB肌纤维的体外培养是一个良好的体内肌肉状态的模型,同时发现了一种新的与骨骼肌增殖分化有关的骨骼肌干细胞蛋白标志JAK1。     

Stem cells in adult skeletal muscle

Muscle satellite cells are a self-renewing pool of stem cells that give rise to daughter myogenic precursor cells in adult skeletal muscle, where they function in postnatal tissue growth and regeneration. Adult skeletal muscle also contains a novel stem cell population purified as a side population (SP), which actively excludes Hoechst 33342 dye. Muscle SP cells that express the hematopoietic stem cell marker Sca-1 possess the ability to differentiate into hematopoietic cells, skeletal muscle, and satellite cells following transplantation. The muscle SP fraction also contains cells expressing the hematopoietic marker CD45 that are capable of differentiation into hematopoietic cells and muscle cells. Thus, these novel muscle stem cells appear to have characteristics similar to those of hematopoietic stem cells, and can participate in muscle regeneration. This review outlines recent findings regarding different stem cell populations in skeletal muscle, and discusses their involvement in muscle regeneration.     

Aging increases the susceptibility of skeletal muscle derived satellite cells to apoptosis

The mechanisms causing the impaired regenerative response to injury observed in skeletal muscle of old animals are unknown. Satellite cells, stem cell descendants within adult skeletal muscle, are the primary source of regenerating muscle fibers. Apoptosis may be a mechanism responsible for the depletion of satellite cells in old animals. This work tested the hypothesis that aging increases the susceptibility of satellite cells to apoptosis. Satellite cells were cultured from the extensor digitorum longus muscles of young (3-month-old), adult (9-month-old), and old (31-month-old) Brown Norway rats. Satellite cells were treated for 24h with the pro-apoptotic agents TNF-alpha (20 ng/mL) and Actinomycin D (250 ng/mL). Immunostaining for activated caspases and terminal deoxynucleotydil transferase-mediated dutp nick-end labeling (TUNEL) was performed to identify apoptotic satellite cells. Quantity of the anti-apoptotic protein bcl-2 was determined by Western blot analysis. Satellite cells from old animals demonstrated significantly higher percentages of cells with activated caspases and TUNEL-positive cells, and significantly lower amounts of bcl-2 compared to young and adult animals. These data support the hypothesis that aging increases satellite cell susceptibility to apoptosis. In old muscle, apoptosis may play a causative role in the depletion of satellite cells, impairing the regenerative response to injury.     

Effects of testosterone supplementation on skeletal muscle fiber hypertrophy and satellite cells in community-dwelling older men

OBJECTIVE: In this study, we determined the effects of graded doses of testosterone on muscle fiber cross-sectional area (CSA) and satellite cell number and replication in older men. PARTICIPANTS: Healthy men, 60-75 yr old, received a long-acting GnRH agonist to suppress endogenous testosterone production and 25, 50, 125, 300, or 600 mg testosterone enanthate im weekly for 20 wk. METHODS: Immunohistochemistry, light and confocal microscopy, and electron microscopy were used to perform fiber typing and quantitate myonuclear and satellite cell number in vastus lateralis biopsies, obtained before and after 20 wk of treatment. RESULTS: Testosterone administration in older men was associated with dose-dependent increases in CSA of both types I and II fibers. Satellite cell number increased dose dependently at the three highest doses (3% at baseline vs. 6.2, 9.2, and 13.0% at 125, 300, and 600 mg doses, P < 0.05). Testosterone administration was associated with an increase in the number of proliferating cell nuclear antigen+ satellite cells (1.8% at baseline vs. 3.9, 7.5, and 13% at 125, 300, and 600 mg doses, P < 0.005). The expression of activated Notch, examined only in the 300-mg group (baseline, 2.3 vs. 9.0% after treatment, P < 0.005), increased in satellite cells after testosterone treatment. The expression of myogenin (baseline, 6.2 vs. 20.7% after treatment, P < 0.005), examined only in the 300-mg group, increased significantly in muscle fiber nuclei after testosterone treatment, but Numb expression did not change. CONCLUSIONS: Older men respond to graded doses of testosterone with a dose-dependent increase in muscle fiber CSA and satellite cell number. Testosterone-induced skeletal muscle hypertrophy in older men is associated with increased satellite cell replication and activation.     

Effect of muscle origin and phenotype on satellite cell muscle-specific gene expression

Satellite cells from adult mouse tongue, diaphragm, vastus lateralis, rectus femoris, tibialis anterior, soleus, and extensor digitorum longus muscles were isolated, expanded, and differentiated under identical culture conditions. Proliferating myoblasts and differentiated myofiber cultures were analyzed via SDS-PAGE, immunochemical, and PCR methods for expression of myosin heavy chains (MyHC) and muscle creatine kinase (MCK) as indices of muscle fiber type. Contralateral muscles were harvested for simultaneous, parallel analysis utilizing these assays. The MyHC profile of differentiated primary satellite cells was equivalent across all cultures with MyHC(2A) and MyHC(1/slow) co-expressed in all myotube and myofiber structures. Trace amounts of MyHC(2B) and MyHC(neo) were detected in a few myofibers. MCK was expressed at a uniform, similar level among these cultures. In contrast, contralateral muscles expressed each muscle-specific indicator at levels correlated with the fiber-type distribution within each muscle. MM14 and C2C12 cells, mouse satellite cell lines, were expanded and compared to primary cell cultures. MM14 cells had a high differentiation index (>95%) and co-expressed MyHC(2A) and MyHC(1/slow) along with trace amounts of MyHC(neo) throughout myotube cultures. Myofibers obtained from C2C12 cells exhibited less differentiation (~75%) with MyHC(2A) as the dominant isoform. These data indicate that primary satellite cells from adult muscle form a uniform differentiated cell type regardless of the fiber-type, anatomic location, and embryonic origin of the donor muscles. MM14 cells expressed an adult MyHC isoform profile similar to primary satellite cells. The results suggest that satellite cells provide a uniform cell source for use in autologous transplantation studies and do not acquire a heritable fiber-type-specific phenotype from their host muscle.     

bFGF基因修饰骨骼肌卫星细胞移植对心肌梗死兔心功能的影响

目的:观察bFGF基因修饰骨骼肌卫星细胞在心肌梗死区存活情况以及对心功能的影响。方法:分离培养兔骨骼肌卫星细胞,构建bFGF基因修饰骨骼肌卫星细胞。建立急性心肌梗死兔模型,随机分为骨骼肌卫星细胞组、bFGF基因修饰骨骼肌卫星细胞组和对照组,每组6只,在各组动物梗死心肌内分别注射骨骼肌卫星细胞、bFGF基因修饰骨骼肌卫星细胞及等量的细胞培养液。造模前及细胞移植4周后,心脏超声测定兔左室舒张末期内径(LVEDD)、左室收缩末期内径(LVESD)、短轴缩短率(FS)和射血分数(EF),观察移植4周后心脏病理切片中心肌梗死边缘区骨骼肌卫星细胞存活情况、bFGF表达情况及新生血管形成情况。结果:细胞移植4周后病理学检查提示骨骼肌卫星细胞在心肌梗死边缘区存活,bFGF基因修饰骨骼肌卫星细胞组可见大量EGFPbFGF融合蛋白表达。与对照组相比,骨骼肌卫星细胞组和bFGF基因修饰骨骼肌卫星细胞组心肌梗死边缘区微血管密度均有增加(78.3±5.2和98.5±8.6对25.2±4.6,P均0.05),且bFGF基因修饰骨骼肌卫星细胞组微血管密度较骨骼肌卫星细胞组明显增加(P0.05)。与造模前相比,移植4周后对照组和骨骼肌卫星细胞组均出现LVESD和LVEDD增大,FS和EF降低(P均0.05),bFGF基因修饰骨骼肌卫星细胞组各指标与造模前相比差异无统计学意义。移植4周后,骨骼肌卫星细胞组和bFGF基因修饰骨骼肌卫星细胞组LVESD及LVEDD均小于对照组,FS和EF均高于对照组(P均0.05);与骨骼肌卫星细胞组相比,bFGF基因修饰骨骼肌卫星细胞组LVESD及LVEDD减小,FS和EF升高(P均0.05)。结论:bFGF基因修饰骨骼肌卫星细胞自体移植可增加急性心肌梗死兔心肌梗死边缘区新生血管形成,改善心功能。     

Delta-4-androstene-3,17-dione binds androgen receptor, promotes myogenesis in vitro, and increases serum testosterone levels, fat-free mass, and muscle strength in hypogonadal men

Previous studies of Delta 4-androstene-3,17-dione (4-androstenedione) administration in men have not demonstrated sustained increments in testosterone levels, fat-free mass (FFM), and muscle strength, and failure to demonstrate androstenedione's androgenic/anabolic effects has stifled efforts to regulate its sales. To determine whether 4-androstenedione has androgenic/anabolic properties, we evaluated its association with androgen receptor (AR) and its effects on myogenesis in vitro. Additionally, we studied the effects of a high dose of 4-androstenedione on testosterone levels, FFM, and muscle strength in hypogonadal men. We determined the dissociation constant (K(d)) for 4-androstenedione using fluorescence anisotropy measurement of competitive displacement of fluorescent androgen from AR ligand-binding domain. AR nuclear translocation and myogenic activity of androstenedione were evaluated in mesenchymal, pluripotent C3H10T1/2 cells, in which androgens stimulate myogenesis through an AR pathway. We determined effects of a high dose of androstenedione (500 mg thrice daily) given for 12 wk on FFM, muscle strength, and hormone levels in nine healthy, hypogonadal men. 4-Androstenedione competitively displaced fluorescent androgen from AR ligand-binding domain with a lower affinity than dihydrotestosterone (K(d), 648 +/- 21 and 10 +/- 0.4 nm, respectively). In C3H10T1/2 cells, 4-androstenedione caused nuclear translocation of AR and stimulated myogenesis, as indicated by a dose-dependent increase in myosin heavy chain II+ myotube area and up-regulation of MyoD protein. Stimulatory effects of 4-androstenedione on myosin heavy chain II+ myotubes and myogenic determination factor expression were attenuated by bicalutamide, an AR antagonist. Administration of 1500 mg 4-androstenedione daily to hypogonadal men significantly increased serum androstenedione, total and free testosterone, estradiol, and estrone levels and suppressed SHBG and high-density lipoprotein cholesterol levels. 4-androstenedione administration was associated with significant gains in FFM (+1.7 +/- 0.5 kg; P = 0.012) and muscle strength in bench press (+4.3 +/- 3.1 kg; P = 0.006) and leg press exercises (+18.8 +/- 17.3 kg; P = 0.045). 4-androstenedione is an androgen that binds AR, induces AR nuclear translocation, and promotes myogenesis in vitro, with substantially lower potency than dihydrotestosterone. 4-androstenedione administration in high doses to hypogonadal men increases testosterone levels, FFM, and muscle strength, although at the dose tested, the anabolic effects in hypogonadal men are likely because of its conversion to testosterone.     

Regulation of myogenic differentiation by androgens: cross talk between androgen receptor/ beta-catenin and follistatin/transforming growth factor-beta signaling pathways

Androgens are important regulators of body composition and promote myogenic differentiation and inhibit adipogenesis of mesenchymal, multipotent cells. Here, we investigated the mechanisms by which androgens induce myogenic differentiation of mesenchymal multipotent cells. Incubation of mesenchymal multipotent C3H 10T1/2 cells with testosterone and dihydrotestosterone promoted nuclear translocation of androgen receptor (AR)/beta-catenin complex and physical interaction of AR, beta-catenin, and T-cell factor-4 (TCF-4). Inhibition of beta-catenin by small inhibitory RNAs significantly decreased testosterone-induced stimulation of myogenic differentiation. Overexpression of TCF-4, a molecule downstream of beta-catenin in Wnt signaling cascade, in C3H 10T1/2 cells significantly up-regulated expression of myoD and myosin heavy chain II proteins and of follistatin (Fst), which binds and antagonizes native ligands of the TGF-beta/Smad pathway. Gene array analysis of C3H 10T1/2 cells treated with testosterone revealed that testosterone up-regulated the expression of Fst and modified the expression of several signaling molecules involved in the TGF-beta/Smad pathway, including Smad7. Lowering of testosterone levels in mice by orchidectomy led to a significant decrease in Fst and Smad7 expression; conversely, testosterone supplementation in castrated mice up-regulated Fst and Smad7 mRNA expression in androgen-responsive levator ani muscle. Testosterone-induced up-regulation of MyoD and myosin heavy chain II proteins in C3H 10T1/2 cells was abolished in cells simultaneously treated with anti-Fst antibody, suggesting an essential role of Fst during testosterone regulation of myogenic differentiation. In conclusion, our data suggest the involvement of AR, beta-catenin, and TCF-4 pathway during androgen action to activate a number of Wnt target genes, including Fst, and cross communication with the Smad signaling pathway.     

Steroid-sensitive gene-1 is an androgen-regulated gene expressed in prostatic smooth muscle cells in vivo

Steroid-sensitive gene-1 (SSG1) is a novel gene we cloned, found regulated by 17beta-estradiol in the rat uterus and mammary gland, and over-expressed in 7,12-dimethylbenz(a)anthracene-induced rat mammary tumors. We show here that SSG1 mRNA and protein expression are regulated by androgens in the rat ventral prostate. Increases in SSG1 mRNA levels were detected by Northern blotting after 24 h and reached a 27-fold peak 96 h following castration, relative to SSG1 mRNA expression in sham-operated rats. Dihydrotestosterone or testosterone supplementation of castrated rats prevented this rise in SSG1 mRNA. In contrast with SSG1 mRNA expression, SSG1 protein was decreased 16-fold 2 weeks following castration but was at control levels in the prostates of castrated rats receiving dihydrotestosterone or testosterone. Although SSG1 is regulated by androgens in vivo, treatment of LnCap cells with dihydrotestosterone, cyproterone acetate or flutamide did not result in the regulation of SSG1 protein levels in vitro. Immunofluorescence studies show that SSG1 is mainly expressed in prostatic smooth muscle cells. These results indicate that SSG1 is an androgen-regulated gene that is expressed in the smooth muscle component of the rat ventral prostate in vivo.     

Androgens stimulate myogenic differentiation and inhibit adipogenesis in C3H 10T1/2 pluripotent cells through an androgen receptor-mediated pathway

Testosterone supplementation increases skeletal muscle mass and decreases fat mass; however, the underlying mechanisms are unknown. We hypothesized that testosterone regulates body composition by promoting the commitment of mesenchymal pluripotent cells into myogenic lineage and inhibiting their differentiation into adipogenic lineage. Mouse C3H 10T1/2 pluripotent cells were treated with testosterone (0-300 nM) or dihydrotestosterone (DHT, 0-30 nM) for 0-14 d, and myogenic conversion was evaluated by immunocytochemical staining for early (MyoD) and late (myosin heavy chain II; MHC) myogenic markers and by measurements of MyoD and MHC mRNA and protein. Adipogenic differentiation was assessed by adipocyte counting and by measurements of peroxisomal proliferator-activated receptor gamma 2 (PPAR gamma 2) mRNA and PPAR gamma 2 protein and CCAAT/enhancer binding protein alpha. The number of MyoD+ myogenic cells and MHC+ myotubes and MyoD and MHC mRNA and protein levels increased dose dependently in response to testosterone and DHT treatment. Both testosterone and DHT decreased the number of adipocytes and down-regulated the expression of PPAR gamma 2 mRNA and PPAR gamma 2 protein and CCAAT/enhancer binding protein alpha. Androgen receptor mRNA and protein levels were low at baseline but increased after testosterone or DHT treatment. The effects of testosterone and DHT on myogenesis and adipogenesis were blocked by bicalutamide. Therefore, testosterone and DHT regulate lineage determination in mesenchymal pluripotent cells by promoting their commitment to the myogenic lineage and inhibiting their differentiation into the adipogenic lineage through an androgen receptor-mediated pathway. The observation that differentiation of pluripotent cells is androgen dependent provides a unifying explanation for the reciprocal effects of androgens on muscle and fat mass in men.     

骨骼肌干细胞移植对犬缺血心肌结构的影响

目的　观察自体骨骼肌干细胞移植于缺血心肌后对心肌结构变化的影响。方法　取 12只成年犬臀大肌 ,分离卫星细胞、培养、传代、用 4’ ,6 二乙酰基 2 苯基吲哚 (DAPI)标记卫星细胞 ;在已建立的急性心肌梗死动物模型基础上 ,将DAPI标记的卫星细胞 ,自左冠状动脉前降支灌入缺血心肌中。分别于 2、4、8周后取出心脏 ,对缺血心肌的纤维化程度及植入的卫星细胞进行观察。结果　在 2 4例标本中卫星细胞分化成为带有横纹德肌纤维 ;卫星细胞在缺血心肌中可分化成心肌细胞样细胞 ;在卫星细胞移植区域 ,原有心肌细胞由于得到保护而不发生玻璃样变性 ,且排列有序 ;对照组缺血心肌发生玻璃样变性 ,心肌细胞基本结构紊乱。结论　自体骨骼肌卫星细胞在缺血心肌中可分化成心肌细胞样细胞 ,并可抑制缺血心肌的纤维化 ,心肌的基本结构得到保护 ;干细胞移植有望为心肌损伤提供一条新的治疗途径     

Muscle hypertrophy driven by myostatin blockade does not require stem/precursor-cell activity

Myostatin, a member of the TGF-β family, has been identified as a powerful inhibitor of muscle growth. Absence or blockade of myostatin induces massive skeletal muscle hypertrophy that is widely attributed to proliferation of the population of muscle fiber-associated satellite cells that have been identified as the principle source of new muscle tissue during growth and regeneration. Postnatal blockade of myostatin has been proposed as a basis for therapeutic strategies to combat muscle loss in genetic and acquired myopathies. But this approach, according to the accepted mechanism, would raise the threat of premature exhaustion of the pool of satellite cells and eventual failure of muscle regeneration. Here, we show that hypertrophy in the absence of myostatin involves little or no input from satellite cells. Hypertrophic fibers contain no more myonuclei or satellite cells and myostatin had no significant effect on satellite cell proliferation in vitro, while expression of myostatin receptors dropped to the limits of detectability in postnatal satellite cells. Moreover, hypertrophy of dystrophic muscle arising from myostatin blockade was achieved without any apparent enhancement of contribution of myonuclei from satellite cells. These findings contradict the accepted model of myostatin-based control of size of postnatal muscle and reorient fundamental investigations away from the mechanisms that control satellite cell proliferation and toward those that increase myonuclear domain, by modulating synthesis and turnover of structural muscle fiber proteins. It predicts too that any benefits of myostatin blockade in chronic myopathies are unlikely to impose any extra stress on the satellite cells.     

Adult skeletal muscle stem cell migration is mediated by a blebbing/amoeboid mechanism

Adult skeletal muscle possesses a resident stem cell population called satellite cells, which are responsible for tissue repair following damage. Satellite cell migration is crucial in promoting rapid tissue regeneration, but it is a poorly understood process. Furthermore, the mechanisms facilitating satellite cell movement have yet to be elucidated. This study investigates the process of satellite cell migration, revealing that they undergo two distinct phases of movement, first under the basal lamina and then rapidly increasing their velocity when on the myofiber surface. Most significantly, we show that satellite cells move using a highly dynamic blebbing or amoeboid-based mechanism and not via lamellipodia-mediated propulsion. We show that nitric oxide and noncanonical Wnt signaling pathways are necessary for regulating the formation of blebs and the migration of satellite cells. In summary, we propose that the formation of blebs and their necessity for satellite cell migration has significant implications in the future development of therapeutic regimes aimed at promoting skeletal muscle regeneration.     

Vascular smooth muscle differentiation of murine stroma: a sequential model

Previous studies by our group showed that stromal cells from human long-term marrow cultures were mesenchymal cells following a vascular smooth muscle pathway. The present study using 58 immortalized stromal lines from different hematopoietic sites was conducted to verify whether this hypothesis also held true for murine stroma. Principal components analysis performed using cytoskeletal and extracellular matrix proteins allowed the segregation of five factors explaining more than 70% of the variance. Factor I, including osteopontin and vimentin, and factor II, laminins and fibronectins, were representative of the mesenchyme. The remaining three factors were representative of vascular smooth muscle: factor III, including alphaSM actin, SM alpha actinin, SM22alpha, EDa+ fibronectin, and thrombospondin-1; factor IV, metavinculin and h-caldesmon; and factor V, smooth muscle myosin SM1 and desmin. All lines expressed factors I and II; 53 lines expressed factor III, 35 lines expressed factor IV; and 11 lines expressed factor V. A second principal components analysis including membrane antigens indicated the cosegregration of vascular cell adhesion molecule-1 with osteopontin and that of Ly6A/E with vimentin, whereas CD34 and Thy-1 appeared to be independent factors. The heterogeneity of vascular smooth muscle markers expression suggests that harmonious maintenance of hematopoiesis depends on the cooperation between different stromal cell clones.     

Role of satellite cells versus myofibers in muscle hypertrophy induced by inhibition of the myostatin/activin signaling pathway

Myostatin and activin A are structurally related secreted proteins that act to limit skeletal muscle growth. The cellular targets for myostatin and activin A in muscle and the role of satellite cells in mediating muscle hypertrophy induced by inhibition of this signaling pathway have not been fully elucidated. Here we show that myostatin/activin A inhibition can cause muscle hypertrophy in mice lacking either syndecan4 or Pax7, both of which are important for satellite cell function and development. Moreover, we show that muscle hypertrophy after pharmacological blockade of this pathway occurs without significant satellite cell proliferation and fusion to myofibers and without an increase in the number of myonuclei per myofiber. Finally, we show that genetic ablation of Acvr2b, which encodes a high-affinity receptor for myostatin and activin A specifically in myofibers is sufficient to induce muscle hypertrophy. All of these findings are consistent with satellite cells playing little or no role in myostatin/activin A signaling in vivo and render support that inhibition of this signaling pathway can be an effective therapeutic approach for increasing muscle growth even in disease settings characterized by satellite cell dysfunction.