INAUGURAL ARTICLE by a Recently Elected Academy Member:Functional redundancy of type I and type II receptors in the regulation of skeletal muscle growth by myostatin and activin A

Myostatin (MSTN) is a transforming growth factor-β (TGF-β) family member that normally acts to limit muscle growth. The function of MSTN is partially redundant with that of another TGF-β family member, activin A. MSTN and activin A are capable of signaling through a complex of type II and type I receptors. Here, we investigated the roles of two type II receptors (ACVR2 and ACVR2B) and two type I receptors (ALK4 and ALK5) in the regulation of muscle mass by these ligands by genetically targeting these receptors either alone or in combination specifically in myofibers in mice. We show that targeting signaling in myofibers is sufficient to cause significant increases in muscle mass, showing that myofibers are the direct target for signaling by these ligands in the regulation of muscle growth. Moreover, we show that there is functional redundancy between the two type II receptors as well as between the two type I receptors and that all four type II/type I receptor combinations are utilized in vivo. Targeting signaling specifically in myofibers also led to reductions in overall body fat content and improved glucose metabolism in mice fed either regular chow or a high-fat diet, demonstrating that these metabolic effects are the result of enhanced muscling. We observed no effect, however, on either bone density or muscle regeneration in mice in which signaling was targeted in myofibers. The latter finding implies that MSTN likely signals to other cells, such as satellite cells, in addition to myofibers to regulate muscle homeostasis.

Myostatin (MSTN) is a secreted signaling molecule that normally acts to limit skeletal muscle growth (for review, see ref. 1). Mice lacking MSTN exhibit dramatic increases in muscle mass throughout the body, with individual muscles growing to about twice the normal size (2). MSTN appears to play two distinct roles in regulating muscle size, one to regulate the number of muscle fibers that are formed during development and a second to regulate the growth of those fibers postnatally. The sequence of MSTN has been highly conserved through evolution, with the mature MSTN peptide being identical in species as divergent as humans and turkeys (3). The function of MSTN has also been conserved, and targeted or naturally occurring mutations in MSTN have been shown to cause increased muscling in numerous species, including cattle (3–5), sheep (6), dogs (7), rabbits (8), rats (9), swine (10), goats (11), and humans (12). Numerous pharmaceutical and biotechnology companies have developed biologic agents capable of blocking MSTN activity, and these have been tested in clinical trials for a wide range of indications, including Duchenne and facioscapulohumeral muscular dystrophy, inclusion body myositis, muscle atrophy following falls and hip fracture surgery, age-related sarcopenia, Charcot–Marie–Tooth disease, and cachexia due to chronic obstructive pulmonary disease, end-stage kidney disease, and cancer.The finding that certain inhibitors of MSTN signaling can increase muscle mass even in Mstn^−/− mice revealed that the function of MSTN as a negative regulator of muscle mass is partially redundant with at least one other TGF-β family member (13, 14), and subsequent studies have identified activin A as one of these cooperating ligands (15, 16). MSTN and activin A share many key regulatory and signaling components. For example, the activities of both MSTN and activin A can be modulated extracellularly by naturally occurring inhibitory binding proteins, including follistatin (17, 18) and the follistatin-related protein, FSTL-3 or FLRG (19, 20). Moreover, MSTN and activin A also appear to share receptor components. Based on in vitro studies, MSTN is capable of binding initially to the activin type II receptors, ACVR2 and ACVR2B (also called ActRIIA and ActRIIB) (18) followed by engagement of the type I receptors, ALK4 and ALK5 (21). In previous studies, we presented genetic evidence supporting a role for both ACVR2 and ACVR2B in mediating MSTN signaling and regulating muscle mass in vivo. Specifically, we showed that mice expressing a truncated, dominant-negative form of ACVR2B in skeletal muscle (18) or carrying deletion mutations in Acvr2 and/or Acvr2b (13) have significantly increased muscle mass. One limitation of the latter study, however, was that we could not examine the consequence of complete loss of both receptors using the deletion alleles, as double homozygous mutants die early during embryogenesis (22). Moreover, the roles that the two type I receptors, ALK4 and ALK5, play in regulating MSTN and activin A signaling in muscle in vivo have not yet been documented using genetic approaches. Here, we present the results of studies in which we used floxed alleles for each of the type II and type I receptor genes in order to target these receptors alone and in combination in muscle fibers. We show that these receptors are functionally redundant and that signaling through each of these receptors contributes to the overall control of muscle mass.     

“骨肉不相亲”理论指导下肌抑素在肌少-骨质疏松症中的机制探讨

随着人口老龄化的日益加剧,"肌少-骨质疏松症"(sarco-osteopenia)这类肌肉及骨协同退变的疾病日益增多,给社会带来巨大负担。中医很早就对肌骨协同退变疾病进行了记录,其中"骨肉不相亲"理论是中医学对肌骨协同退变病理机制的高度概括。近年研究发现,肌肉生长抑制素(Myostatin)由骨骼肌分泌,通过多条途径参与了骨骼肌和骨组织生长代谢的调节过程,可能成为肌骨之间调节的重要因子。笔者在"骨肉不相亲"理论的指导下,通过探究肌肉生长抑制素调控骨骼肌与骨代谢的机制,以期揭示肌少-骨质疏松症的病理机制,探索中医理论的现代医学实质,为该类疾病的防治提供新的思路。     

Myostatin在小鼠腓肠肌失神经支配萎缩过程中的表达

目的:分析myostatin在腓肠肌失神经支配萎缩过程中的表达变化及其在肌萎缩过程中的作用。方法:采用坐骨神经横断术制备小鼠腓肠肌失神经支配模型,实时荧光定量PCR和Western印迹法分别检测失神经支配不同时段腓肠肌myostatin mRNA和蛋白表达水平,并对失神经前后肌肉湿重比、肌纤维横截面积进行比较。结果:失神经支配1 d时,腓肠肌myostatin mRNA迅速上升,3 d达到高峰,随后逐渐下降,而相应蛋白水平逐渐增高,7 d达到高峰继而逐渐下降,至56 d时mRNA和蛋白水平仍略高于正常水平。结论:腓肠肌失神经支配萎缩过程中myostatin的表达变化是一重要的分子事件。     

老年慢性心力衰竭病人血清suPAR、MSTN水平与心功能和预后的关系

目的探讨老年慢性心力衰竭(CHF)病人血清可溶性尿激酶型纤溶酶原激活物受体(suPAR)、肌肉生长抑制素(MSTN)水平及其与心功能和预后的关系。方法选取2017年1月至2019年6月我科收治的CHF病人120例为观察组,同期选取我院体检健康志愿者100例为对照组。采用酶联免疫吸附法检测2组血清suPAR、MSTN水平。出院后随访1年,根据CHF病人是否发生心血管不良事件分为预后不良组(n=32)及预后良好组(n=88)。绘制ROC曲线,分析血清suPAR、MSTN对CHF病人不良预后的预测价值。结果观察组血清suPAR、MSTN水平较对照组明显升高,差异有统计学意义(P<0.05)。不同心功能分级病人血清suPAR、MSTN、氨基末端脑钠肽原(NT-proBNP)、LVEF水平比较,差异有统计学意义(P<0.05)。Pearson相关性分析示血清suPAR、MSTN与NT-proBNP呈正相关,与LVEF呈负相关(均P<0.05)。预后不良组血清suPAR、MSTN、NT-proBNP、LVEF水平与预后良好组相比,差异有统计学意义(P<0.05)。ROC曲线示,血清suPAR、MSTN及两项联合检测预测病人不良预后的灵敏度分别为0.785、0.754、0.908,特异度分别为0.743、0.716、0.779,AUC分别为0.867、0.815、0.912。结论老年CHF病人血清suPAR、MSTN水平明显升高,其与病人的心功能分级及临床预后密切相关,联合检测血清suPAR、MSTN对老年CHF病人的预后判断有较高临床价值,可作为病情的监测指标。     

Myokines in skeletal muscle physiology and metabolism: Recent advances and future perspectives

Myokines are molecules produced and secreted by skeletal muscle to act in an auto‐, para‐ and endocrine manner to alter physiological function of target tissues. The growing number of effects of myokines on metabolism of distant tissues provides a compelling case for crosstalk between skeletal muscle and other tissues and organs to regulate metabolic homoeostasis. In this review, we summarize and discuss the current knowledge regarding the impact on metabolism of several canonical and recently identified myokines. We focus specifically on myostatin, β‐aminoisobutyric acid, interleukin‐15, meteorin‐like and myonectin, and discuss how these myokines are induced and regulated as well as their overall function. We also review how these myokines may serve as potential prognostic biomarkers that reflect whole‐body metabolism and how they may be attractive therapeutic targets for treating muscle and metabolic diseases.     

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)-β 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.     

Single nucleotide polymorphisms in the myostatin (MSTN) and muscle creatine kinase (CKM) genes are not associated with elite endurance performance

Maximal oxygen uptake (VO_2max) is one of the most important determinants of elite endurance performance. VO_2max is determined by a whole range of genetic and environmental factors. Single nucleotide polymorphisms (SNPs) in muscle myostatin (MSTN) and creatine kinase (CKM) genes are candidates for VO_2max and skeletal muscle performance phenotypes. Common MSTN (rs3791783, rs11681628 and rs7570532) and CKM (rs344816, rs10410448, rs432979, rs1133190, rs7260359, rs7260463 and rs4884) SNPs, selected from HapMap CEU data in order to tag the genetic variability of the proteins, were genotyped in 316 male Caucasian elite endurance athletes and 304 sedentary controls from the Genathlete study. Association with elite endurance performance was determined by logistic regression analysis. The P‐value for statistical significance was set at <0.01. None of the SNPs or haplotypes showed a significant association with elite endurance status. We conclude that common variants of MSTN and CKM genes do not play a role in attaining high‐level endurance performance in Caucasian populations.     

The effects of hypermuscularity on shoulder morphology in myostatin-deficient mice

Mechanical loads, particularly those generated by skeletal muscle, play a significant role in determining long-bone shape and strength, but it is less clear how these loads influence the morphology of flat bones like the scapula. While scapular morphology has been shown to vary with locomotor mode in mammals, this study seeks to better understand whether genetically modified muscle size can influence scapular shape in the absence of significant locomotor differences. The soft- and hard-tissue morphological characteristics were examined in 11 hypermuscular, mutant (myostatin-deficient), 20 heterozygote, and 15 wild-type mouse shoulders. Body mass did not significantly differ among the genotype groups, but homozygous mutant and heterozygote mice had significantly larger shoulder muscles than wild-type mice. Mutant mice also differed significantly from the wild-type controls in several aspects of scapular size and shape, including glenohumeral joint orientation, total scapular length, superior border length, and supraspinous and infraspinous fossa length. Conversely, several traits describing superoinferior scapular breadth measures (e.g. total breadth and dorsal scapular fossa breadth) did not significantly differ between mutant and wild-type mice. Since the intrinsic musculature of the scapula is oriented in a mediolateral fashion, it follows that mediolaterally configured hard-tissue features like scapular length were most distinct among genotype groups. As had been noted previously with long bones, this study demonstrates that genetically enhanced muscle size has marked effects on the morphological characteristics of the shoulder.     

Myostatin blockade improves function but not histopathology in a murine model of limb-girdle muscular dystrophy 2C

Myostatin is a negative regulator of skeletal muscle growth. Myostatin mutations and pharmacological strategies increase muscle mass in vivo, suggesting that myostatin blockade may prove useful in diseases characterized by muscle wasting, such as the muscular dystrophies. We subjected the gamma-sarcoglycan-deficient (Sgcg(-/-)) mouse model of limb-girdle muscular dystrophy (LGMD) 2C to antibody-mediated myostatin blockade in vivo. Myostatin inhibition led to increased fiber size, muscle mass, and absolute force. However, no clear improvement in muscle histopathology was evident, demonstrating discordance between physiological and histological improvement. These results and previous studies on the dyw/dyw mouse model of congenital muscular dystrophy and in the late-stage delta-sarcoglycan-deficient (Sgcd(-/-)) mouse model of LGMD2F document disease-specific limitations to therapeutic strategies based on myostatin blockade in the more severe mouse models of different muscular dystrophies.