The role of myostatin and bone morphogenetic proteins in muscular disorders

Skeletal muscle is the largest organ in the human body, and plays an important role in body movement and metabolism. Skeletal muscle mass is lost in genetic disorders such as muscular dystrophy, muscle wasting and ageing. Chemicals and proteins that restore muscle mass and function are potential drugs that can improve human health and could be used in the clinic. Myostatin is a muscle-specific member of the transforming growth factor (TGF)-beta superfamily that plays an essential role in the negative regulation of muscle growth. Inhibition of myostatin activity is a promising therapeutic method for restoring muscle mass and strength. Potential inhibitors of myostatin include follistatin domain-containing proteins, myostatin propeptide, myostatin antibodies and chemical compounds. These inhibitors could be beneficial for the development of clinical drugs for the treatment of muscular disorders. Bone morphogenetic protein (BMP) plays a significant role in the development of neuromuscular architecture and its proper functions. Modulation of BMP activity could be beneficial for muscle function in muscular disorders. This review will describe the current progress in therapy for muscular disorders, emphasising the importance of myostatin as a drug target.     

AAV-mediated delivery of a mutated myostatin propeptide ameliorates calpain 3 but not alpha-sarcoglycan deficiency

Myostatin is a negative regulator of muscle mass whose inhibition has been proposed as a therapeutic strategy for muscle-wasting conditions. Indeed, blocking myostatin action through different strategies has proved beneficial for the pathophysiology of the dystrophin-deficient mdx mouse. In this report, we tested the inhibition of myostatin by AAV-mediated expression of a mutated propeptide in animal models of two limb-girdle muscular dystrophies: LGMD2A caused by mutations in the calpain 3 (CAPN3) gene and LGMD2D caused by mutations in the alpha-sarcoglycan gene (SGCA). In the highly regenerative Sgca-null mice, survival of the alpha-sarcoglycan-deficient muscle fibers did not improve after transfer of the myostatin propeptide. In calpain 3-deficient mice, a boost in muscle mass and an increase in absolute force were obtained, suggesting that myostatin inhibition could constitute a therapeutic strategy in this predominantly atrophic disorder.     

肌肉生长抑制素基因多态性与肌肉生长敏感性的关联

背景:肌肉生长抑制素(growth differentiation factor 8,GDF-8)基因在人类中对于调节肌肉生长起重要的作用.但关于中国人群该基因的研究甚少.目的:实验从肌肉生长抑制素基因多性AluⅠ酶切位点限制性酶切位点入手,观察GDF-8基因多性与人体肌肉生长的关联.方法:选取天津体育学院无训练汉族学生92名,进行为期2个月的力量训练,用B超测定运动前后肱二头肌和股四头肌的肌肉厚度.测试训练前后形态学各指标包括身高、体质量、体脂肪含量、瘦体质量的变化、运动前后肱二头肌和股四头肌的肌肉厚度变化趋势以及在GDF-8 AluⅠ酶切位点限制性酶切位点上的差异.含AluⅠ位点片段由引物扩增后长度为135 bp,定义为A,经AluⅠ限制性酶切割后,含有AluⅠ酶切位点的等位基因出现80,55 bp的片段,定义为T,这样即出现3种基因型AA,AT和TT.结果与结论:从形态学指标上受试者群体在AluⅠ位点上A/T杂合子及T/T纯合子在身高,训练前后的体质量,瘦体质量,肌肉厚度和肱二头肌及股四头肌直径厚度上均高于A/A纯合子(P < 0.05或P < 0.01).结果证实,AluⅠ位点多态性与人体身高,体质量及瘦体质量关联,等位基因T为肌肉生长的先天敏感因子.     

IRE1α regulates skeletal muscle regeneration through myostatin mRNA decay

Skeletal muscle can undergo a regenerative process in response to injury or disease to preserve muscle mass and function, which are critically influenced by cellular stress responses. Inositol-requiring enzyme 1 (IRE1) is an ancient endoplasmic reticulum stress sensor and mediates a key branch of the unfolded protein response. In mammals, IRE1α is implicated in the homeostatic control of stress responses during tissue injury and regeneration. Here, we show that IRE1α serves as a myogenic regulator in skeletal muscle regeneration in response to injury and muscular dystrophy. We found in mice that IRE1α was activated during injury-induced muscle regeneration, and muscle-specific IRE1α ablation resulted in impaired regeneration upon cardiotoxin-induced injury. Gain- and loss-of-function studies in myocytes demonstrated that IRE1α acts to sustain both differentiation in myoblasts and hypertrophy in myotubes through regulated IRE1-dependent decay (RIDD) of mRNA encoding myostatin, a key negative regulator of muscle repair and growth. Furthermore, in the mouse model of Duchenne muscular dystrophy, loss of muscle IRE1α resulted in augmented myostatin signaling and exacerbated the dystrophic phenotypes. These results reveal a pivotal role for the RIDD output of IRE1α in muscle regeneration, offering insight into potential therapeutic strategies for muscle loss diseases.     

Myostatin propeptide gene delivery by adeno-associated virus serotype 8 vectors enhances muscle growth and ameliorates dystrophic phenotypes in mdx mice

Myostatin has been extensively documented as a negative regulator of muscle growth. Myostatin inhibition is therefore considered an attractive strategy for the treatment of muscle-wasting diseases such as muscular dystrophies. To investigate whether systemic gene delivery of myostatin propeptide (MRPO), a natural inhibitor of myostatin, could enhance body-wide skeletal muscle growth, we used adeno-associated virus serotype 8 (AAV8) vectors to deliver the MRPO gene into either normal mice or mdx mice, a murine model of Duchenne muscular dystrophy (DMD). In normal mice, a significant increase in skeletal muscle mass was observed after either an intraperitoneal injection of AAV-MPRO into neonates, or an intravenous injection of AAV-MPRO76AFc (a modified MPRO fused with IgG Fc) into adults. Enhanced muscle growth occurred because of myofiber hypertrophy, not hyperplasia. In mdx mice, a significant increase in skeletal muscle mass was also observed after AAV-MPRO76AFc injection. The treated mdx mice showed larger and more uniform myofibers, fewer infiltrating mononuclear cells, less fibrosis, and lower serum creatine kinase levels. In addition, a grip force test and an in vitro tetanic contractile force test showed improved muscle strength. A treadmill test, however, showed reduced endurance of the treated mdx mice compared with their untreated counterparts. Importantly, no cardiac hypertrophy was observed in either normal or mdx mice after myostatin inhibition by gene delivery. These results clearly demonstrate the efficacy of AAV8-mediated myostatin propeptide gene delivery in a rodent model of DMD, and warrant further investigation in large animal models and eventually in human patients.     

肌肉生长抑制素基因多态性与肌肉生长敏感性的关联

背景：肌肉生长抑制素（growth differentiation factor 8,GDF-8）基因在人类中对于调节肌肉生长起重要的作用。但关于中国人群该基因的研究甚少。目的：实验从肌肉生长抑制素基因多性AluⅠ酶切位点限制性酶切位点入手,观察GDF-8基因多性与人体肌肉生长的关联。方法：选取天津体育学院无训练汉族学生92名,进行为期2个月的力量训练,用B超测定运动前后肱二头肌和股四头肌的肌肉厚度。测试训练前后形态学各指标包括身高、体质量、体脂肪含量、瘦体质量的变化、运动前后肱二头肌和股四头肌的肌肉厚度变化趋势以及在GDF-8AluⅠ酶切位点限制性酶切位点上的差异。含AluⅠ位点片段由引物扩增后长度为135bp,定义为A,经AluⅠ限制性酶切割后,含有AluⅠ酶切位点的等位基因出现80,55bp的片段,定义为T,这样即出现3种基因型AA,AT和TT。结果与结论：从形态学指标上受试者群体在AluⅠ位点上A/T杂合子及T/T纯合子在身高,训练前后的体质量,瘦体质量,肌肉厚度和肱二头肌及股四头肌直径厚度上均高于A/A纯合子（P〈0.05或P〈0.01）。结果证实,AluⅠ位点多态性与人体身高,体质量及瘦体质量关联,等位基因T为肌肉生长的先天敏感因子。     

Long-term systemic myostatin inhibition via liver-targeted gene transfer in golden retriever muscular dystrophy

Duchenne muscular dystrophy (DMD) is a lethal, X-linked recessive disease affecting 1 in 3,500 newborn boys for which there is no effective treatment or cure. One novel strategy that has therapeutic potential for DMD is inhibition of myostatin, a negative regulator of skeletal muscle mass that may also promote fibrosis. Therefore, our goal in this study was to evaluate systemic myostatin inhibition in the golden retriever model of DMD (GRMD). GRMD canines underwent liver-directed gene transfer of a self-complementary adeno-associated virus type 8 vector designed to express a secreted dominant-negative myostatin peptide (n = 4) and were compared with age-matched, untreated GRMD controls (n = 3). Dogs were followed with serial magnetic resonance imaging (MRI) for 13 months to assess cross-sectional area and volume of skeletal muscle, then euthanized so that tissue could be harvested for morphological and histological analysis. We found that systemic myostatin inhibition resulted in increased muscle mass in GRMD dogs as assessed by MRI and confirmed at tissue harvest. We also found that hypertrophy of type IIA fibers was largely responsible for the increased muscle mass and that reductions in serum creatine kinase and muscle fibrosis were associated with long-term myostatin inhibition in GRMD. This is the first report describing the effects of long-term, systemic myostatin inhibition in a large-animal model of DMD, and we believe that the simple and effective nature of our liver-directed gene-transfer strategy makes it an ideal candidate for evaluation as a novel therapeutic approach for DMD patients.     

Hydrodynamic limb vein injection of adeno-associated virus serotype 8 vector carrying canine myostatin propeptide gene into normal dogs enhances muscle growth

Inhibition or blockade of myostatin, a negative growth factor of skeletal muscle, enhances muscle growth and therefore is considered a promising strategy for the treatment of muscle-wasting diseases such as the muscular dystrophies. Previously, we showed that myostatin blockade in both normal and dystrophin-deficient mdx mice by systemic delivery of the myostatin propeptide (MPRO) gene by an adeno-associated virus serotype 8 (AAV8) vector could enhance muscle growth and ameliorate dystrophic lesions. Here, we further investigate whether the muscle growth effect of myostatin blockade can be achieved in dogs by gene transfer. First, we cloned the canine MPRO gene, packaged it in the AAV8 vector, and showed robust muscle-enhancing effects after systemic delivery into neonatal mice. This vector was then further tested in two 3-month-old normal dogs (weighing 9.7 and 6.3 kg). The vector was delivered to one limb by hydrodynamic vein injection, and the contralateral limb served as a control. The delivery procedure was safe, without discernible adverse effects. AAV vector DNA and MPRO gene expression were detected by quantitative polymerase chain reaction, Western blotting, and immunofluorescence staining of muscle biopsies. Overexpression of MPRO resulted in enhanced muscle growth without a cytotoxic T lymphocytic immune response, as evidenced by larger myofibers in multiple muscles, increased muscle volume determined by magnetic resonance imaging, and the lack of CD4+ and CD8+ T cell infiltration in the vector-injected limbs. Our preliminary study thus supports further investigation of this therapeutic strategy in the dystrophin-deficient golden retriever muscular dystrophy dog model.     

Indomethacin and ibuprofen preserve gastrocnemius muscle mass in mice bearing the colon-26 adenocarcinoma

Skeletal muscle wasting is a prominent feature of cancer cachexia and involves decreased muscle protein synthesis and increased activity of the ubiquitin-proteasome pathway of protein degradation. We report that both indomethacin and ibuprofen improved body weight and weight of the gastrocnemius muscle in tumor-bearing mice. Ibuprofen increased the soluble protein content of the muscle without affecting muscle levels of phosphorylated p70 S6 kinase, a ribosomal kinase involved in protein synthesis. Paradoxically, indomethacin increased levels of ubiquitin-conjugated proteins. Further study is needed to understand the mechanism of action by which indomethacin and ibuprofen preserve body weight and muscle mass in the tumor-bearing mice. The data suggest that ibuprofen may have beneficial effects in the treatment of cancer cachexia.     

骨形态发生蛋白复合带血供肌瓣修复骨缺损成骨过程中骨骼肌的转归

背景局部软组织条件不好的难治性骨缺损修复是临床的重大难题和亟待解决的问题.目的探讨带血供肌瓣作为骨形态发生蛋白载体修复骨缺损成骨过程中骨骼肌的变化.设计观察对比实验.单位南方医科大学南方医院实验动物中心.对象重组人骨形态发生蛋白2;清洁级健康新西兰大白兔20只,1.5～2.5 kg,雌雄不拘.方法实验于2000-01/2002-08在南方医科大学南方医院实验动物中心完成.取10只兔,制作兔桡骨下段15 mm骨缺损,将指深屈肌肌瓣去神经后,转位至缺损区,将纤维蛋白与重组人骨形态发生蛋白2复合物植入肌瓣中.分别于1,2,4,6,8周麻醉状态下各处死4只兔,进行骨缺损区肌瓣大体观察、组织学、原位末端标记及超微结构观察.主要观察指标兔桡骨缺损区肌瓣大体观察、组织学检查、原位末端标记法染色结果及超微结构.结果20只兔均进入结果分析.①兔桡骨缺损区肌瓣大体观察结果骨骼肌逐渐发生萎缩,8周时肌纤维几乎完全消失,新生骨组织呈条索状桥接两断端.②兔桡骨缺损区肌瓣组织学检查结果组织学显示骨骼肌萎缩的同时,部分细胞核出现崩解,并有凋亡小体出现.③兔桡骨缺损区肌瓣细胞凋亡情况原位末端标记法染色萎缩的骨骼肌纤维内大量显色阳性的细胞核或核碎片.④兔桡骨缺损区肌瓣超微结构骨骼肌肌丝随时间延长逐渐消失,细胞核出现染色质边集、固缩、碎裂,呈典型的凋亡改变.结论带血供肌瓣复合骨形态发生蛋白修复骨缺损成骨过程中,肌纤维逐渐发生萎缩、凋亡,最终被骨组织所替代.     

Combination of Myostatin Pathway Interference and Dystrophin Rescue Enhances Tetanic and Specific Force in Dystrophic mdx Mice

Duchenne muscular dystrophy is characterized by muscular atrophy, fibrosis, and fat accumulation. Several groups have demonstrated that in the mdx mouse, the exon-skipping strategy can restore a quasi-dystrophin in almost 100% of the muscle fibers. On the other hand, inhibition of the myostatin pathway in adult mice has been described to enhance muscle growth and improve muscle force. Our aim was to combine these two strategies to evaluate a possible additive effect. We have chosen to inhibit the myostatin pathway using the technique of RNA interference directed against the myostatin receptor AcvRIIb mRNA (sh-AcvRIIb). The restoration of a quasi-dystrophin was mediated by the vectorized U7 exon-skipping technique (U7-DYS). Adeno-associated vectors carrying either the sh-AcvrIIb construct alone, the U7-DYS construct alone, or a combination of both constructs were injected in the tibialis anterior (TA) muscle of dystrophic mdx mice. We show that even if each separate approach has some effects on muscle physiology, the combination of the dystrophin rescue and the downregulation of the myostatin receptor is required to massively improve both the tetanic force and the specific force. This study provides a novel pharmacogenetic strategy for treatment of certain neuromuscular diseases associated with muscle wasting.     

Targeting the myostatin signaling pathway to treat muscle loss and metabolic dysfunction

Since the discovery of myostatin (MSTN; also known as GDF-8) as a critical regulator of skeletal muscle mass in 1997, there has been an extensive effort directed at understanding the cellular and physiological mechanisms underlying MSTN activity, with the long-term goal of developing strategies and agents capable of blocking MSTN signaling to treat patients with muscle loss. Considerable progress has been made in elucidating key components of this regulatory system, and in parallel with this effort has been the development of numerous biologics that have been tested in clinical trials for a wide range of indications, including muscular dystrophy, sporadic inclusion body myositis, spinal muscular atrophy, cachexia, muscle loss due to aging or following falls, obesity, and type 2 diabetes. Here, I review what is known about the MSTN regulatory system and the current state of efforts to target this pathway for clinical applications.     

Suppression of body fat accumulation in myostatin-deficient mice

Myostatin is a TGF-beta family member that acts as a negative regulator of muscle growth. Mice lacking the myostatin gene (Mstn) have a widespread increase in skeletal muscle mass resulting from a combination of muscle fiber hypertrophy and hyperplasia. Here we show that Mstn-null mice have a significant reduction in fat accumulation with increasing age compared with wild-type littermates, even in the setting of normal food intake (relative to body weight), normal body temperature, and a slightly decreased resting metabolic rate. To investigate whether myostatin might be an effective target for suppressing the development of obesity in settings of abnormal fat accumulation, we analyzed the effect of the Mstn mutation in two genetic models of obesity, agouti lethal yellow (A(y)) and obese (Lep(ob/ob)). In each case, loss of Mstn led to a partial suppression of fat accumulation and of abnormal glucose metabolism. Our findings raise the possibility that pharmacological agents that block myostatin function may be useful not only for enhancing muscle growth, but also for slowing or preventing the development of obesity and type 2 diabetes.     

Combination Antisense Treatment for Destructive Exon Skipping of Myostatin and Open Reading Frame Rescue of Dystrophin in Neonatal mdx Mice

The fatal X-linked Duchenne muscular dystrophy (DMD), characterized by progressive muscle wasting and muscle weakness, is caused by mutations within the DMD gene. The use of antisense oligonucleotides (AOs) modulating pre-mRNA splicing to restore the disrupted dystrophin reading frame, subsequently generating a shortened but functional protein has emerged as a potential strategy in DMD treatment. AO therapy has recently been applied to induce out-of-frame exon skipping of myostatin pre-mRNA, knocking-down expression of myostatin protein, and such an approach is suggested to enhance muscle hypertrophy/hyperplasia and to reduce muscle necrosis. Within this study, we investigated dual exon skipping of dystrophin and myostatin pre-mRNAs using phosphorodiamidate morpholino oligomers conjugated with an arginine-rich peptide (B-PMOs). Intraperitoneal administration of B-PMOs was performed in neonatal mdx males on the day of birth, and at weeks 3 and 6. At week 9, we observed in treated mice (as compared to age-matched, saline-injected controls) normalization of muscle mass, a recovery in dystrophin expression, and a decrease in muscle necrosis, particularly in the diaphragm. Our data provide a proof of concept for antisense therapy combining dystrophin restoration and myostatin inhibition for the treatment of DMD.     

Myostatin antisense RNA-mediated muscle growth in normal and cancer cachexia mice

Myostatin is a negative regulator of myogenesis, and inactivation of myostatin leads to muscle growth. Here we have used modified RNA oligonucleotides targeting the myostatin mRNA and examined the therapeutic potential in normal and cancer cachexia mouse models. We found that the RNA oligonucleotides could suppress the myostatin expression in vivo, leading to the increase in muscle growth both in normal and cachectic mice. We also established that the effect of myostatin inhibition caused by the RNA oligonucleotides may be through the MyoD pathway, as evidenced by a significant upregulation of MyoD expression. Taken together, these results demonstrate the feasibility using antisense strategy for the treatment of muscle wasting conditions.     

Genetic ablation of complement C3 attenuates muscle pathology in dysferlin-deficient mice

Mutations in the dysferlin gene underlie a group of autosomal recessive muscle-wasting disorders denoted as dysferlinopathies. Dysferlin has been shown to play roles in muscle membrane repair and muscle regeneration, both of which require vesicle-membrane fusion. However, the mechanism by which muscle becomes dystrophic in these disorders remains poorly understood. Although muscle inflammation is widely recognized in dysferlinopathy and dysferlin is expressed in immune cells, the contribution of the immune system to the pathology of dysferlinopathy remains to be fully explored. Here, we show that the complement system plays an important role in muscle pathology in dysferlinopathy. Dysferlin deficiency led to increased expression of complement factors in muscle, while muscle-specific transgenic expression of dysferlin normalized the expression of complement factors and eliminated the dystrophic phenotype present in dysferlin-null mice. Furthermore, genetic disruption of the central component (C3) of the complement system ameliorated muscle pathology in dysferlin-deficient mice but had no significant beneficial effect in a genetically distinct model of muscular dystrophy, mdx mice. These results demonstrate that complement-mediated muscle injury is central to the pathogenesis of dysferlinopathy and suggest that targeting the complement system might serve as a therapeutic approach for this disease.     

Myostatin expression in ventricular myocardium in a rat model of volume-overload heart failure

BACKGROUND: Mechanical stress increases myocardial myostatin expression. However, the expression of myostatin in chronic heart failure resulting from volume-overload and after treatment with beta-blockers is little known. The authors hypothesize that myostatin plays a role in the failing myocardium because of volume-overload. MATERIALS AND METHODS: Aorto-caval shunt was created over a 4-week period in adult Sprague-Dawley rats to induce volume-overload heart failure. RESULTS: Heart weight and body weight ratio significantly increased after shunting. The left ventricular end-diastolic dimension also significantly increased. Treatment with carvedilol in the shunt group reversed the increase in heart weight and ventricular dimension to the baseline values. Myocardial and skeletal myostatin proteins were up-regulated in the shunt group. The mRNA of myocardial myostatin also increased in the shunt group. Treatment with carvedilol reversed both protein and mRNA of myocardial myostatin to the baseline values. Treatment with N-acetylcysteine and doxazosin partially decreased myostatin mRNA and protein expression as compared with the shunt group. Carvedilol normalized the increased immunohistochemical labelling of myocardial myostatin in the shunt group. CONCLUSION: Myocardial myostatin mRNA and protein expression were up-regulated in the rat model of volume-overload heart failure. Treatment with carvedilol is associated with a limitation of increased myostatin expression in the failing ventricular myocardium.     

Antagonism of myostatin enhances muscle regeneration during sarcopenia.

A reduction in muscle mass and strength is often observed with aging, and this phenomenon is known as sarcopenia. This age-related atrophy frequently correlates with insufficient levels of muscle regeneration resulting from impairment of satellite cell involvement and myogenesis brought about by the aged environment. Using myostatin-null mice, we recently showed that negative regulators of muscle mass such as myostatin play an active role in the regulation of myogenesis during aging. The present study specifically tests the therapeutic value of a myostatin antagonist in sarcopenia. We report here that a short-term blockade of myostatin, through stage-specific administration of a myostatin antagonist, significantly enhanced muscle regeneration in aged mice after injury and during sarcopenia. Antagonism of myostatin led to satellite cell activation, increased Pax7 and MyoD protein levels, and greater myoblast and macrophage cell migration, resulting in enhanced muscle regeneration after notexin injury in aged mice. In addition, the antagonist demonstrated a high degree of efficacy, as only minimal doses during the critical period of regeneration after injury were sufficient to restore the myogenic and inflammatory responses in the aged environment. Thus, we propose that the antagonism of myostatin has significant therapeutic potential in the alleviation of sarcopenia.