Smad3 induces atrogin-1, inhibits mTOR and protein synthesis, and promotes muscle atrophy in vivo

Targeting Crosstalk of Signaling Pathways among Muscles-Bone-Adipose Tissue: A Promising Therapeutic Approach for Sarcopenia

The aging process is associated with the development of a wide range of degenerative disorders in mammals. These diseases are characterized by a progressive decline in function at multiple levels, including the molecular, cellular, tissue, and organismal. Furthermore, it is responsible for various healthcare costs in developing and developed countries. Sarcopenia is the deterioration in the quality and functionality of muscles, which is extremely concerning as it manages many functions in the human body. This article reviews the molecular crosstalk involved in sarcopenia and the specific roles of many mediator molecules in establishing cross-talk between muscles, bone, and fatty tissues, eventually leading to sarcopenia. Besides, the involvement of various etiological factors, such as neurology, endocrinology, lifestyle, etc., makes it exceedingly difficult for clinicians to develop a coherent hypothesis that may lead to the well-organized management system required to battle this debilitating disease. The several hallmarks contributing to the progression of the disease is a vital question that needs to be addressed to ensure an efficient treatment for sarcopenia patients. Also, the intricate molecular mechanism involved in developing this disease requires more studies. The direct relationship of cellular senescence with aging is one of the pivotal issues contributing to disease pathophysiology. Some patented treatment strategies have been discussed, including drugs undergoing clinical trials and emerging options like miRNA and protein-enclosed extracellular vesicles. A clear understanding of the secretome, including the signaling pathways involved between muscles, bone, and fatty tissues, is extremely beneficial for developing novel therapeutics for curing sarcopenia.

Lost in translation: challenges of current pharmacotherapy for sarcopenia

A healthy lifespan relies on independent living, in which active skeletal muscle is a critical element. The cost of not recognizing and acting earlier on unhealthy or aging muscle could be detrimental, since muscular weakness is inversely associated with all-cause mortality. Sarcopenia is characterized by a decline in skeletal muscle mass and strength and is associated with aging. Exercise is the only effective therapy to delay sarcopenia development and improve muscle health in older adults. Although numerous interventions have been proposed to reduce sarcopenia, none has yet succeeded in clinical trials. This review evaluates the biological gap between recent clinical trials targeting sarcopenia and the preclinical studies on which they are based, and suggests an alternative approach to bridge the discrepancy.

Skeletal muscle myostatin mRNA expression is upregulated in aged human adults with excess adiposity but is not associated with insulin resistance and ageing

Myostatin negatively regulates skeletal muscle growth and appears upregulated in human obesity and associated with insulin resistance. However, observations are confounded by ageing, and the mechanisms responsible are unknown. The aim of this study was to delineate between the effects of excess adiposity, insulin resistance and ageing on myostatin mRNA expression in human skeletal muscle and to investigate causative factors using in vitro models. An in vivo cross-sectional analysis of human skeletal muscle was undertaken to isolate effects of excess adiposity and ageing per se on myostatin expression. In vitro studies employed human primary myotubes to investigate the potential involvement of cross-talk between subcutaneous adipose tissue (SAT) and skeletal muscle, and lipid-induced insulin resistance. Skeletal muscle myostatin mRNA expression was greater in aged adults with excess adiposity than age-matched adults with normal adiposity (2.0-fold higher; P < 0.05) and occurred concurrently with altered expression of genes involved in the maintenance of muscle mass but did not differ between younger and aged adults with normal adiposity. Neither chronic exposure to obese SAT secretome nor acute elevation of fatty acid availability (which induced insulin resistance) replicated the obesity-mediated upregulation of myostatin mRNA expression in vitro. In conclusion, skeletal muscle myostatin mRNA expression is uniquely upregulated in aged adults with excess adiposity and insulin resistance but not by ageing alone. This does not appear to be mediated by the SAT secretome or by lipid-induced insulin resistance. Thus, factors intrinsic to skeletal muscle may be responsible for the obesity-mediated upregulation of myostatin, and future work to establish causality is required.

YTHDF2 governs muscle size through a targeted modulation of proteostasis

The regulation of proteostasis is fundamental for maintenance of muscle mass and function. Activation of the TGF-β pathway drives wasting and premature aging by favoring the proteasomal degradation of structural muscle proteins. Yet, how this critical post-translational mechanism is kept in check to preserve muscle health remains unclear. Here, we reveal the molecular link between the post-transcriptional regulation of m6A-modified mRNA and the modulation of SMAD-dependent TGF-β signaling. We show that the m6A-binding protein YTHDF2 is essential to determining postnatal muscle size. Indeed, muscle-specific genetic deletion of YTHDF2 impairs skeletal muscle growth and abrogates the response to hypertrophic stimuli. We report that YTHDF2 controls the mRNA stability of the ubiquitin ligase ASB2 with consequences on anti-growth gene program activation through SMAD3. Our study identifies a post-transcriptional to post-translational mechanism for the coordination of gene expression in muscle.

Sarcopenia is linked to higher levels of B-type natriuretic peptide and its N-terminal fragment in heart failure: a systematic review and meta-analysis

AIMS

Sarcopenia is linked to impaired physical function and exercise tolerance. The aim of this systematic review and meta-analysis was to examine the association of sarcopenia and low appendicular skeletal muscle (ASM) with biomarkers of cardiac function, B-type natriuretic peptide (BNP) and its N-terminal fragment (NT-proBNP), in patients with heart failure (HF).

METHODS AND RESULTS

From inception until May 2023, a systematic literature search of observational studies was undertaken utilizing the PubMed, Web of Science, Scopus, and Cochrane Library databases. A meta-analysis employing a random-effects model was used to compute the pooled effects (CRD42023418465). Overall, 16 studies were included in this systematic review and meta-analysis. Our main analysis showed that sarcopenia in HF was linked to significantly higher levels of BNP (MD: 87.76, 95% CI 20.74-154.78, I2 = 61%, P = 0.01) and NT-proBNP (MD: 947.45, 95% CI 98.97-1795.93, I2 = 35%, P = 0.03). Similarly, low ASM was associated with significantly higher levels of BNP (MD: 118.95, 95% CI 46.91-191.00, I2 = 93%, P < 0.01) and NT-proBNP (MD: 672.01, 95% CI 383.72-960.30, I2 = 2%, P < 0.01). The quality of the included cohort studies was considered moderate, using the binary AXIS checklist and the Cochrane Tool to Assess the Risk of Bias in Cohort Studies.

CONCLUSIONS

In patients with HF, sarcopenia and reduced ASM are associated with considerably higher plasma levels of BNP and NT-proBNP. Future research is required to investigate whether sarcopenia may express dysregulated biomarkers of cardiac function.

Sex-specific increases in myostatin and SMAD3 contribute to obesity-related insulin resistance in human skeletal muscle and primary human myotubes

The purpose of the present study was to determine the effects of obesity and biological sex on myostatin expression in humans and to examine the direct effects of myostatin, SMAD2, and SMAD3 on insulin signaling in primary human skeletal muscle cells (HSkMCs). For cohort 1, 15 lean [body mass index (BMI): 22.1 ± 0.5 kg/m2; n = 8 males; n = 7 females] and 14 obese (BMI: 40.6 ± 1.4 kg/m2; n = 7 males; n = 7 females) individuals underwent skeletal muscle biopsies and an oral glucose tolerance test. For cohort 2, 14 young lean (BMI: 22.4 ± 1.9 kg/m2; n = 6 males; n = 8 females) and 14 obese (BMI: 39.3 ± 7.9 kg/m2; n = 6 males; n = 8 females) individuals underwent muscle biopsies for primary HSkMC experiments. Plasma mature myostatin (P = 0.041), skeletal muscle precursor myostatin (P = 0.048), and skeletal muscle SMAD3 (P = 0.029) were elevated in obese females compared to lean females, and plasma mature myostatin (r = 0.58, P = 0.029) and skeletal muscle SMAD3 (r = 0.56, P = 0.037) were associated with insulin resistance in females but not males. Twenty-four hours of myostatin treatment impaired insulin signaling in primary HSkMCs derived from females (P < 0.024) but not males. Overexpression of SMAD3, but not SMAD2, impaired insulin-stimulated AS160 phosphorylation in HSkMCs derived from lean females (-27%, P = 0.040), whereas silencing SMAD3 improved insulin-stimulated AS160 phosphorylation and insulin-stimulated glucose uptake (25%, P < 0.014) in HSkMCs derived from obese females. These results suggest for the first time that myostatin-induced impairments in skeletal muscle insulin signaling are sex specific and that increased body fat in females is associated with detrimental elevations in myostatin and SMAD3, which contribute to obesity-related insulin resistance.NEW & NOTEWORTHY Obesity is considered a main risk factor for the development of insulin resistance and type 2 diabetes. The present study utilizes in vivo and in vitro experiments in human skeletal muscle to demonstrate for the first time that females are inherently more susceptible to myostatin-induced insulin resistance, which is further enhanced with obesity due to increased myostatin and SMAD3 expression.

Molecular and Structural Alterations of Skeletal Muscle Tissue Nuclei during Aging

Aging is accompanied by a progressive loss of skeletal muscle mass and strength. The mechanisms underlying this phenomenon are certainly multifactorial and still remain to be fully elucidated. Changes in the cell nucleus structure and function have been considered among the possible contributing causes. This review offers an overview of the current knowledge on skeletal muscle nuclei in aging, focusing on the impairment of nuclear pathways potentially involved in age-related muscle decline. In skeletal muscle two types of cells are present: fiber cells, constituting the contractile muscle mass and containing hundreds of myonuclei, and the satellite cells, i.e., the myogenic mononuclear stem cells occurring at the periphery of the fibers and responsible for muscle growth and repair. Research conducted on different experimental models and with different methodological approaches demonstrated that both the myonuclei and satellite cell nuclei of aged skeletal muscles undergo several structural and molecular alterations, affecting chromatin organization, gene expression, and transcriptional and post-transcriptional activities. These alterations play a key role in the impairment of muscle fiber homeostasis and regeneration, thus contributing to the age-related decrease in skeletal muscle mass and function.

Addressing cancer anorexia-cachexia in older patients: Potential therapeutic strategies and molecular pathways

Cancer cachexia (CC) syndrome, a feature of cancer-associated muscle wasting, is particularly pronounced in older patients, and is characterised by decreased energy intake and upregulated skeletal muscle catabolic pathways. To address CC, appetite stimulants, anabolic drugs, cytokine mediators, essential amino acid supplementation, nutritional counselling, cognitive behavioural therapy, and enteral nutrition have been utilised. However, pharmacological treatments that have also shown promising results, such as megestrol acetate, anamorelin, thalidomide, and delta-9-tetrahydrocannabinol, have been associated with gastrointestinal and cardiovascular complications. Emerging evidence on the efficacy of probiotics in modulating gut microbiota also presents a promising adjunct to traditional therapies, potentially enhancing nutritional absorption and systemic inflammation control. Additionally, low-dose olanzapine has demonstrated improved appetite and weight management in older patients undergoing chemotherapy, offering a potential refinement to current therapeutic approaches. This review aims to elucidate the molecular mechanisms underpinning CC, with a particular focus on the role of anorexia in exacerbating muscle wasting, and to propose pharmacological and non-pharmacological strategies to mitigate this syndrome, particularly emphasising the needs of an older demographic. Future research targeting CC should focus on refining appetite-stimulating drugs with fewer side-effects, specifically catering to the needs of older patients, and investigating nutritional factors that can either enhance appetite or minimise suppression of appetite in individuals with CC, especially within this vulnerable group.

Targeting myostatin using quercetin as a media supplement to improve myogenesis for cultured meat production: An in silico and in vitro study

Cultured meat (CM) is an alternative protein food and is produced by cultivating muscle satellite (stem) cells (MSCs) derived from livestock animals (bovine, chickens, and porcine) through myogenesis leading to generate muscle mass. Myostatin (MSTN) is well well-known negative regulator of myogenesis, and in the present study, in silico screening of natural compounds was performed to identify MSTN inhibitors. Interestingly, quercetin was found to inhibit MSTN (binding energy -7.40 kcal/mol), and this was further validated by a 100 ns molecular dynamics simulation. Quercetin was added to culture media to boost myogenesis, and its potent antioxidant property helped maintain media pH. Furthermore, quercetin increased the myotube thickness and length, increased MSC differentiation, and upregulated the gene and protein expressions of myoblast determination protein 1 (MYOD), Myogenin (MYOG), and Myosin heavy chains (MYH) in vitro. In addition, quercetin inhibited the activities of MSTN, activin receptor type-2B (ACVR2B), and SMAD2 and 3, and thus significantly enhanced MSC differentiation and myotube formation. Overall, this study shows that quercetin might be useful for enhancing large-scale CM production. It is hoped that this study provides a starting point for research in the CM area aimed to enhancing product quality, nutritional values, and the efficacy of large-scale production.

Adipokines in Rheumatoid Arthritis: Emerging Biomarkers and Therapeutic Targets

Rheumatoid arthritis (RA) is a chronic inflammatory disease manifested by joint involvement, extra-articular manifestations, and general symptoms. Adipose tissue, previously perceived as an inert energy storage organ, has been recognised as a significant contributor to RA pathophysiology. Adipokines modulate immune responses, inflammation, and metabolic pathways in RA. Although most adipokines have a pro-inflammatory and aggravating effect on RA, some could counteract this pathological process. The coexistence of RA and sarcopenic obesity (SO) has gained attention due to its impact on disease severity and outcomes. Sarcopenic obesity further contributes to the inflammatory milieu and metabolic disturbances. Recent research has highlighted the intricate crosstalk between adipose tissue and skeletal muscle, suggesting potential interactions between these tissues in RA. This review summarizes the roles of adipokines in RA, particularly in inflammation, immune modulation, and joint destruction. In addition, it explores the emerging role of adipomyokines, specifically irisin and myostatin, in the pathogenesis of RA and their potential as therapeutic targets. We discuss the therapeutic implications of targeting adipokines and adipomyokines in RA management and highlight the challenges and future directions for research in this field.

Single-Nuclei RNA-Sequencing of the Gastrocnemius Muscle in Peripheral Artery Disease

BACKGROUND

Lower extremity peripheral artery disease (PAD) is a growing epidemic with limited effective treatment options. Here, we provide a single-nuclei atlas of PAD limb muscle to facilitate a better understanding of the composition of cells and transcriptional differences that comprise the diseased limb muscle.

METHODS

We obtained gastrocnemius muscle specimens from 20 patients with PAD and 12 non-PAD controls. Nuclei were isolated and single-nuclei RNA-sequencing was performed. The composition of nuclei was characterized by iterative clustering via principal component analysis, differential expression analysis, and the use of known marker genes. Bioinformatics analysis was performed to determine differences in gene expression between PAD and non-PAD nuclei, as well as subsequent analysis of intercellular signaling networks. Additional histological analyses of muscle specimens accompany the single-nuclei RNA-sequencing atlas.

RESULTS

Single-nuclei RNA-sequencing analysis indicated a fiber type shift with patients with PAD having fewer type I (slow/oxidative) and more type II (fast/glycolytic) myonuclei compared with non-PAD, which was confirmed using immunostaining of muscle specimens. Myonuclei from PAD displayed global upregulation of genes involved in stress response, autophagy, hypoxia, and atrophy. Subclustering of myonuclei also identified populations that were unique to PAD muscle characterized by metabolic dysregulation. PAD muscles also displayed unique transcriptional profiles and increased diversity of transcriptomes in muscle stem cells, regenerating myonuclei, and fibro-adipogenic progenitor cells. Analysis of intercellular communication networks revealed fibro-adipogenic progenitors as a major signaling hub in PAD muscle, as well as deficiencies in angiogenic and bone morphogenetic protein signaling which may contribute to poor limb function in PAD.

CONCLUSIONS

This reference single-nuclei RNA-sequencing atlas provides a comprehensive analysis of the cell composition, transcriptional signature, and intercellular communication pathways that are altered in the PAD condition.

Acute In Vitro and In Vivo Effects of Dexamethasone in the Vocal Folds: a Pilot Study

OBJECTIVES/HYPOTHESIS

Glucocorticoids (GC)s are commonly employed to treat vocal fold (VF) pathologies. However, VF atrophy has been associated with intracordal GC injections. Dexamethasone-induced skeletal muscle atrophy is well-documented in other tissues and believed to be mediated by increased muscle proteolysis via upregulation of Muscle Ring Finger (MuRF)-1 and Atrogin-1. Mechanisms of dexamethasone-mediated VF atrophy have not been described. This pilot study employed in vitro and in vivo models to investigate the effects of dexamethasone on VF epithelium, thyroarytenoid (TA) muscle, and TA-derived myoblasts. We hypothesized that dexamethasone will increase atrophy-associated gene expression in TA muscle and myoblasts and decrease TA muscle fiber size and epithelial thickness.

STUDY DESIGN

In vitro, pre-clinical.

METHODS

TA myoblasts were isolated from a female Sprague-Dawley rat and treated with 1 μM dexamethasone for 24-h. In vivo, 15 New Zealand white rabbits were randomly assigned to three treatment groups: (1) bilateral intracordal injection of 40 μL dexamethasone (10 mg/ml; n = 5), (2) volume-matched saline (n = 5), and (3) untreated controls (n = 5). Larynges were harvested 7-days post-injection. Across in vivo and in vitro experimentation, MuRF-1 and Atrogin-1 mRNA expression were measured via RT-qPCR. TA muscle fiber cross-sectional area (CSA) and epithelial thickness were also quantified in vivo.

RESULTS

Dexamethasone increased MuRF-1 gene expression in TA myoblasts. Dexamethasone injection, however, did not alter atrophy-associated gene expression, TA CSA, or epithelial thickness in vivo.

CONCLUSION

Dexamethasone increased atrogene expression in TA myoblasts, providing foundational insight into GC induced atrophic gene transcription. Repeated dexamethasone injections may be required to elicit atrophy in vivo.

LEVEL OF EVIDENCE

NA Laryngoscope, 133:2264-2270, 2023.

Cancer cachexia: molecular mechanisms and treatment strategies

Muscle wasting is a consequence of physiological changes or a pathology characterized by increased catabolic activity that leads to progressive loss of skeletal muscle mass and strength. Numerous diseases, including cancer, organ failure, infection, and aging-associated diseases, are associated with muscle wasting. Cancer cachexia is a multifactorial syndrome characterized by loss of skeletal muscle mass, with or without the loss of fat mass, resulting in functional impairment and reduced quality of life. It is caused by the upregulation of systemic inflammation and catabolic stimuli, leading to inhibition of protein synthesis and enhancement of muscle catabolism. Here, we summarize the complex molecular networks that regulate muscle mass and function. Moreover, we describe complex multi-organ roles in cancer cachexia. Although cachexia is one of the main causes of cancer-related deaths, there are still no approved drugs for cancer cachexia. Thus, we compiled recent ongoing pre-clinical and clinical trials and further discussed potential therapeutic approaches for cancer cachexia.

Activation of β-catenin in mesenchymal progenitors leads to muscle mass loss

Loss of muscle mass is a common manifestation of chronic disease. We find the canonical Wnt pathway to be activated in mesenchymal progenitors (MPs) from cancer-induced cachectic mouse muscle. Next, we induce β-catenin transcriptional activity in murine MPs. As a result, we observe expansion of MPs in the absence of tissue damage, as well as rapid loss of muscle mass. Because MPs are present throughout the organism, we use spatially restricted CRE activation and show that the induction of tissue-resident MP activation is sufficient to induce muscle atrophy. We further identify increased expression of stromal NOGGIN and ACTIVIN-A as key drivers of atrophic processes in myofibers, and we verify their expression by MPs in cachectic muscle. Finally, we show that blocking ACTIVIN-A rescues the mass loss phenotype triggered by β-catenin activation in MPs, confirming its key functional role and strengthening the rationale for targeting this pathway in chronic disease.

Endoplasmic reticulum stress promotes sepsis-induced muscle atrophy via activation of STAT3 and Smad3

Endoplasmic reticulum (ER) stress is involved in skeletal muscle atrophy in various conditions, but the role of ER stress in sepsis-induced muscle atrophy is not well understood. In this study, we conducted experiments in wild-type (WT) mice and C/EBP homologous protein knockout (CHOP KO) mice to explore the role and mechanism of ER stress in sepsis-induced muscle atrophy. Cecal ligation and puncture (CLP) was used to establish a mouse model of sepsis. In WT mice, the body weight, muscle mass, and cross-sectional area of muscle fibers in CLP group both decreased significantly compared with sham group, which revealed that sepsis-induced dramatic muscle atrophy. Additionally, sepsis activated the ubiquitin-proteasome system (UPS), accompanied by the activation of ER stress. In vitro, inhibition of ER stress suppressed the activity of E3 ubiquitin ligases and alleviated the myotube atrophy. In vivo, CHOP KO also reduced the expression of E3 ubiquitin ligases and UPS-mediated protein degradation, and significantly attenuated sepsis-induced muscle atrophy. Deletion of CHOP also decreased the phosphorylation of signal transducer and activator of transcription 3 (STAT3) and Smad3, and inhibition of STAT3 and Smad3 partly reduced proteolysis caused by ER stress in vitro. These findings confirm that ER stress activates UPS-mediated proteolysis and promotes sepsis-induced muscle atrophy, which is partly achieved by activating STAT3 and Smad3.

Organ Crosstalk Contributes to Muscle Wasting in Chronic Kidney Disease

Muscle wasting (ie, atrophy) is a serious consequence of chronic kidney disease (CKD) that reduces muscle strength and function. It reduces the quality of life for CKD patients and increases the risks of comorbidities and mortality. Current treatment strategies to prevent or reverse skeletal muscle loss are limited owing to the broad and systemic nature of the initiating signals and the multifaceted catabolic mechanisms that accelerate muscle protein degradation and impair protein synthesis and repair pathways. Recent evidence has shown how organs such as muscle, adipose, and kidney communicate with each other through interorgan exchange of proteins and RNAs during CKD. This crosstalk changes cell functions in the recipient organs and represents an added dimension in the complex processes that are responsible for muscle atrophy in CKD. This complexity creates challenges for the development of effective therapies to ameliorate muscle wasting and weakness in patients with CKD.

The Preventive Effect of Specific Collagen Peptides against Dexamethasone-Induced Muscle Atrophy in Mice

Muscle atrophy, also known as muscle wasting, is the thinning of muscle mass due to muscle disuse, aging, or diseases such as cancer or neurological problems. Muscle atrophy is closely related to the quality of life and has high morbidity and mortality. However, therapeutic options for muscle atrophy are limited, so studies to develop therapeutic agents for muscle loss are always required. For this study, we investigated how orally administered specific collagen peptides (CP) affect muscle atrophy and elucidated its molecular mechanism using an in vivo model. We treated mice with dexamethasone (DEX) to induce a muscular atrophy phenotype and then administered CP (0.25 and 0.5 g/kg) for four weeks. In a microcomputed tomography analysis, CP (0.5 g/kg) intake significantly increased the volume of calf muscles in mice with DEX-induced muscle atrophy. In addition, the administration of CP (0.25 and 0.5 g/kg) restored the weight of the gluteus maximus and the fiber cross-sectional area (CSA) of the pectoralis major and calf muscles, which were reduced by DEX. CP significantly inhibited the mRNA expression of myostatin and the phosphorylation of Smad2, but it did not affect TGF-β, BDNF, or FNDC5 gene expression. In addition, AKT/mTOR, a central pathway for muscle protein synthesis and related to myostatin signaling, was enhanced in the groups that were administered CP. Finally, CP decreased serum albumin levels and increased TNF-α gene expression. Collectively, our in vivo results demonstrate that CP can alleviate muscle wasting through a multitude of mechanisms. Therefore, we propose CP as a supplement or treatment to prevent muscle atrophy.

Emerging Mechanisms of Skeletal Muscle Homeostasis and Cachexia: The SUMO Perspective

Mobility is an intrinsic feature of the animal kingdom that stimulates evolutionary processes and determines the biological success of animals. Skeletal muscle is the primary driver of voluntary movements. Besides, skeletal muscles have an immense impact on regulating glucose, amino acid, and lipid homeostasis. Muscle atrophy/wasting conditions are accompanied by a drastic effect on muscle function and disrupt steady-state muscle physiology. Cachexia is a complex multifactorial muscle wasting syndrome characterized by extreme loss of skeletal muscle mass, resulting in a dramatic decrease in life quality and reported mortality in more than 30% of patients with advanced cancers. The lack of directed treatments to prevent or relieve muscle loss indicates our inadequate knowledge of molecular mechanisms involved in muscle cell organization and the molecular etiology of cancer-induced cachexia (CIC). This review highlights the latest knowledge of regulatory mechanisms involved in maintaining muscle function and their deregulation in wasting syndromes, particularly in cachexia. Recently, protein posttranslational modification by the small ubiquitin-like modifier (SUMO) has emerged as a key regulatory mechanism of protein function with implications for different aspects of cell physiology and diseases. We also review an atypical association of SUMO-mediated pathways in this context and deliberate on potential treatment strategies to alleviate muscle atrophy.

Impact of cancer cachexia on respiratory muscle function and the therapeutic potential of exercise

Cancer cachexia is defined as a multi-factorial syndrome characterised by an ongoing loss of skeletal muscle mass and progressive functional impairment, estimated to affect 50-80% of patients and responsible for 20% of cancer deaths. Elevations in the morbidity and mortality rates of cachectic cancer patients has been linked to respiratory failure due to atrophy and dysfunction of the ventilatory muscles. Despite this, there is a distinct scarcity of research investigating the structural and functional condition of the respiratory musculature in cancer, with the majority of studies exclusively focusing on limb muscle. Treatment strategies are largely ineffective in mitigating the cachectic state. It is now widely accepted that an efficacious intervention will likely combine elements of pharmacology, nutrition and exercise. However, of these approaches, exercise has received comparatively little attention. Therefore, it is unlikely to be implemented optimally, whether in isolation or combination. In consideration of these limitations, the current review describes the mechanistic basis of cancer cachexia and subsequently explores the available respiratory- and exercise-focused literature within this context. The molecular basis of cachexia is thoroughly reviewed. The pivotal role of inflammatory mediators is described. Unravelling the mechanisms of exercise-induced support of muscle via antioxidant and anti-inflammatory effects in addition to promoting efficient energy metabolism via increased mitochondrial biogenesis, mitochondrial function and muscle glucose uptake provide avenues for interventional studies. Currently available pre-clinical mouse models including novel transgenic animals provide a platform for the development of multi-modal therapeutic strategies to protect respiratory muscles in people with cancer.

Pharmacological inhibition of HDAC6 improves muscle phenotypes in dystrophin-deficient mice by downregulating TGF-β via Smad3 acetylation

The absence of dystrophin in Duchenne muscular dystrophy disrupts the dystrophin-associated glycoprotein complex resulting in skeletal muscle fiber fragility and atrophy, associated with fibrosis as well as microtubule and neuromuscular junction disorganization. The specific, non-conventional cytoplasmic histone deacetylase 6 (HDAC6) was recently shown to regulate acetylcholine receptor distribution and muscle atrophy. Here, we report that administration of the HDAC6 selective inhibitor tubastatin A to the Duchenne muscular dystrophy, mdx mouse model increases muscle strength, improves microtubule, neuromuscular junction, and dystrophin-associated glycoprotein complex organization, and reduces muscle atrophy and fibrosis. Interestingly, we found that the beneficial effects of HDAC6 inhibition involve the downregulation of transforming growth factor beta signaling. By increasing Smad3 acetylation in the cytoplasm, HDAC6 inhibition reduces Smad2/3 phosphorylation, nuclear translocation, and transcriptional activity. These findings provide in vivo evidence that Smad3 is a new target of HDAC6 and implicate HDAC6 as a potential therapeutic target in Duchenne muscular dystrophy.

Harnessing conserved signaling and metabolic pathways to enhance the maturation of functional engineered tissues

The development of induced-pluripotent stem cell (iPSC)-derived cell types offers promise for basic science, drug testing, disease modeling, personalized medicine, and translatable cell therapies across many tissue types. However, in practice many iPSC-derived cells have presented as immature in physiological function, and despite efforts to recapitulate adult maturity, most have yet to meet the necessary benchmarks for the intended tissues. Here, we summarize the available state of knowledge surrounding the physiological mechanisms underlying cell maturation in several key tissues. Common signaling consolidators, as well as potential synergies between critical signaling pathways are explored. Finally, current practices in physiologically relevant tissue engineering and experimental design are critically examined, with the goal of integrating greater decision paradigms and frameworks towards achieving efficient maturation strategies, which in turn may produce higher-valued iPSC-derived tissues.

Myostatin and its Regulation: A Comprehensive Review of Myostatin Inhibiting Strategies

Myostatin (MSTN) is a well-reported negative regulator of muscle growth and a member of the transforming growth factor (TGF) family. MSTN has important functions in skeletal muscle (SM), and its crucial involvement in several disorders has made it an important therapeutic target. Several strategies based on the use of natural compounds to inhibitory peptides are being used to inhibit the activity of MSTN. This review delivers an overview of the current state of knowledge about SM and myogenesis with particular emphasis on the structural characteristics and regulatory functions of MSTN during myogenesis and its involvements in various muscle related disorders. In addition, we review the diverse approaches used to inhibit the activity of MSTN, especially in silico approaches to the screening of natural compounds and the design of novel short peptides derived from proteins that typically interact with MSTN.

CORP: Gene delivery into murine skeletal muscle using in vivo electroporation

The strategy of gene delivery into skeletal muscles has provided exciting avenues in identifying new potential therapeutics towards muscular disorders and addressing basic research questions in muscle physiology through overexpression and knockdown studies. In vivo electroporation methodology offers a simple, rapidly effective technique for the delivery of plasmid DNA into post-mitotic skeletal muscle fibers and the ability to easily explore the molecular mechanisms of skeletal muscle plasticity. The purpose of this review is to describe how to robustly electroporate plasmid DNA into different hindlimb muscles of rodent models. Further, key parameters (e.g., voltage, hyaluronidase, plasmid concentration) which contribute to the successful introduction of plasmid DNA into skeletal muscle fibers will be discussed. In addition, details on processing tissue for immunohistochemistry and fiber cross-sectional area (CSA) analysis will be outlined. The overall goal of this review is to provide the basic and necessary information needed for successful implementation of in vivo electroporation of plasmid DNA and thus open new avenues of discovery research in skeletal muscle physiology.

Lack of Tgfbr1 and Acvr1b synergistically stimulates myofibre hypertrophy and accelerates muscle regeneration

In skeletal muscle, transforming growth factor-β (TGF-β) family growth factors, TGF-β1 and myostatin, are involved in atrophy and muscle wasting disorders. Simultaneous interference with their signalling pathways may improve muscle function; however, little is known about their individual and combined receptor signalling. Here, we show that inhibition of TGF-β signalling by simultaneous muscle-specific knockout of TGF-β type I receptors Tgfbr1 and Acvr1b in mice, induces substantial hypertrophy, while such effect does not occur by single receptor knockout. Hypertrophy is induced by increased phosphorylation of Akt and p70S6K and reduced E3 ligases expression, while myonuclear number remains unaltered. Combined knockout of both TGF-β type I receptors increases the number of satellite cells, macrophages and improves regeneration post cardiotoxin-induced injury by stimulating myogenic differentiation. Extra cellular matrix gene expression is exclusively elevated in muscle with combined receptor knockout. Tgfbr1 and Acvr1b are synergistically involved in regulation of myofibre size, regeneration, and collagen deposition.

Myostatin/Activin Receptor Ligands in Muscle and the Development Status of Attenuating Drugs

Muscle wasting disease indications are among the most debilitating and often deadly noncommunicable disease states. As a comorbidity, muscle wasting is associated with different neuromuscular diseases and myopathies, cancer, heart failure, chronic pulmonary and renal diseases, peripheral neuropathies, inflammatory disorders, and, of course, musculoskeletal injuries. Current treatment strategies are relatively ineffective and can at best only limit the rate of muscle degeneration. This includes nutritional supplementation and appetite stimulants as well as immunosuppressants capable of exacerbating muscle loss. Arguably, the most promising treatments in development attempt to disrupt myostatin and activin receptor signaling because these circulating factors are potent inhibitors of muscle growth and regulators of muscle progenitor cell differentiation. Indeed, several studies demonstrated the clinical potential of "inhibiting the inhibitors," increasing muscle cell protein synthesis, decreasing degradation, enhancing mitochondrial biogenesis, and preserving muscle function. Such changes can prevent muscle wasting in various disease animal models yet many drugs targeting this pathway failed during clinical trials, some from serious treatment-related adverse events and off-target interactions. More often, however, failures resulted from the inability to improve muscle function despite preserving muscle mass. Drugs still in development include antibodies and gene therapeutics, all with different targets and thus, safety, efficacy, and proposed use profiles. Each is unique in design and, if successful, could revolutionize the treatment of both acute and chronic muscle wasting. They could also be used in combination with other developing therapeutics for related muscle pathologies or even metabolic diseases.

Pathophysiological mechanisms leading to muscle loss in chronic kidney disease

Loss of muscle proteins is a deleterious consequence of chronic kidney disease (CKD) that causes a decrease in muscle strength and function, and can lead to a reduction in quality of life and increased risk of morbidity and mortality. The effectiveness of current treatment strategies in preventing or reversing muscle protein losses is limited. The limitations largely stem from the systemic nature of diseases such as CKD, which stimulate skeletal muscle protein degradation pathways while simultaneously activating mechanisms that impair muscle protein synthesis and repair. Stimuli that initiate muscle protein loss include metabolic acidosis, insulin and IGF1 resistance, changes in hormones, cytokines, inflammatory processes and decreased appetite. A growing body of evidence suggests that signalling molecules secreted from muscle can enter the circulation and subsequently interact with recipient organs, including the kidneys, while conversely, pathological events in the kidney can adversely influence protein metabolism in skeletal muscle, demonstrating the existence of crosstalk between kidney and muscle. Together, these signals, whether direct or indirect, induce changes in the levels of regulatory and effector proteins via alterations in mRNAs, microRNAs and chromatin epigenetic responses. Advances in our understanding of the signals and processes that mediate muscle loss in CKD and other muscle wasting conditions will support the future development of therapeutic strategies to reduce muscle loss.

Role of MicroRNAs and Long Non-Coding RNAs in Sarcopenia

Sarcopenia is an age-related pathological process characterized by loss of muscle mass and function, which consequently affects the quality of life of the elderly. There is growing evidence that non-coding RNAs, including microRNAs (miRNAs) and long non-coding RNAs (lncRNAs), play a key role in skeletal muscle physiology. Alterations in the expression levels of miRNAs and lncRNAs contribute to muscle atrophy and sarcopenia by regulating various signaling pathways. This review summarizes the recent findings regarding non-coding RNAs associated with sarcopenia and provides an overview of sarcopenia pathogenesis promoted by multiple non-coding RNA-mediated signaling pathways. In addition, we discuss the impact of exercise on the expression patterns of non-coding RNAs involved in sarcopenia. Identifying non-coding RNAs associated with sarcopenia and understanding the molecular mechanisms that regulate skeletal muscle dysfunction during aging will provide new insights to develop potential treatment strategies.

Multifactorial Mechanism of Sarcopenia and Sarcopenic Obesity. Role of Physical Exercise, Microbiota and Myokines

Obesity and ageing place a tremendous strain on the global healthcare system. Age-related sarcopenia is characterized by decreased muscular strength, decreased muscle quantity, quality, and decreased functional performance. Sarcopenic obesity (SO) is a condition that combines sarcopenia and obesity and has a substantial influence on the older adults’ health. Because of the complicated pathophysiology, there are disagreements and challenges in identifying and diagnosing SO. Recently, it has become clear that dysbiosis may play a role in the onset and progression of sarcopenia and SO. Skeletal muscle secretes myokines during contraction, which play an important role in controlling muscle growth, function, and metabolic balance. Myokine dysfunction can cause and aggravate obesity, sarcopenia, and SO. The only ways to prevent and slow the progression of sarcopenia, particularly sarcopenic obesity, are physical activity and correct nutritional support. While exercise cannot completely prevent sarcopenia and age-related loss in muscular function, it can certainly delay development and slow down the rate of sarcopenia. The purpose of this review was to discuss potential pathways to muscle deterioration in obese individuals. We also want to present the current understanding of the role of various factors, including microbiota and myokines, in the process of sarcopenia and SO.

Sex-specific role of myostatin signaling in neonatal muscle growth, denervation atrophy, and neuromuscular contractures

Neonatal brachial plexus injury (NBPI) causes disabling and incurable muscle contractures that result from impaired longitudinal growth of denervated muscles. This deficit in muscle growth is driven by increased proteasome-mediated protein degradation, suggesting a dysregulation of muscle proteostasis. The myostatin (MSTN) pathway, a prominent muscle-specific regulator of proteostasis, is a putative signaling mechanism by which neonatal denervation could impair longitudinal muscle growth, and thus a potential target to prevent NBPI-induced contractures. Through a mouse model of NBPI, our present study revealed that pharmacologic inhibition of MSTN signaling induces hypertrophy, restores longitudinal growth, and prevents contractures in denervated muscles of female but not male mice, despite inducing hypertrophy of normally innervated muscles in both sexes. Additionally, the MSTN-dependent impairment of longitudinal muscle growth after NBPI in female mice is associated with perturbation of 20S proteasome activity, but not through alterations in canonical MSTN signaling pathways. These findings reveal a sex dimorphism in the regulation of neonatal longitudinal muscle growth and contractures, thereby providing insights into contracture pathophysiology, identifying a potential muscle-specific therapeutic target for contracture prevention, and underscoring the importance of sex as a biological variable in the pathophysiology of neuromuscular disorders.

Myostatin: Basic biology to clinical application

Myostatin is a member of the transforming growth factor (TGF)-β superfamily. It is expressed by animal and human skeletal muscle cells where it limits muscle growth and promotes protein breakdown. Its effects are influenced by complex mechanisms including transcriptional and epigenetic regulation and modulation by extracellular binding proteins. Due to its actions in promoting muscle atrophy and cachexia, myostatin has been investigated as a promising therapeutic target to counteract muscle mass loss in experimental models and patients affected by different muscle-wasting conditions. Moreover, growing evidence indicates that myostatin, beyond to regulate skeletal muscle growth, may have a role in many physiologic and pathologic processes, such as obesity, insulin resistance, cardiovascular and chronic kidney disease. In this chapter, we review myostatin biology, including intracellular and extracellular regulatory pathways, and the role of myostatin in modulating physiologic processes, such as muscle growth and aging. Moreover, we discuss the most relevant experimental and clinical evidence supporting the extra-muscle effects of myostatin. Finally, we consider the main strategies developed and tested to inhibit myostatin in clinical trials and discuss the limits and future perspectives of the research on myostatin.

MicroRNAs associated with signaling pathways and exercise adaptation in sarcopenia

Considering the expansion of human life-span over the past few decades; sarcopenia, a physiological consequence of aging process characterized with a diminution in mass and strength of skeletal muscle, has become more frequent. Thus, there is a growing need for expanding our knowledge on the molecular mechanisms of muscle atrophy in sarcopenia which are complex and involve many signaling pathways associated with protein degradation and synthesis. MicroRNAs (miRNAs) as evolutionary conserved small RNAs, could complementarily bind to their target mRNAs and post-transcriptionally inhibit their translation. Aberrant expression of miRNAs contributes to the development of sarcopenia by regulating the expression of critical genes involved in age-related skeletal muscle mass loss. Here we have a review on the signaling pathways along with the miRNAs controlling their components expression and subsequently we provide a brief overview on the effects of exercise on expression pattern of miRNAs in sarcopenia.

Myostatin/Activin-A Signaling in the Vessel Wall and Vascular Calcification

A current hypothesis is that transforming growth factor-β signaling ligands, such as activin-A and myostatin, play a role in vascular damage in atherosclerosis and chronic kidney disease (CKD). Myostatin and activin-A bind with different affinity the activin receptors (type I or II), activating distinct intracellular signaling pathways and finally leading to modulation of gene expression. Myostatin and activin-A are expressed by different cell types and tissues, including muscle, kidney, reproductive system, immune cells, heart, and vessels, where they exert pleiotropic effects. In arterial vessels, experimental evidence indicates that myostatin may mostly promote vascular inflammation and premature aging, while activin-A is involved in the pathogenesis of vascular calcification and CKD-related mineral bone disorders. In this review, we discuss novel insights into the biology and physiology of the role played by myostatin and activin in the vascular wall, focusing on the experimental and clinical data, which suggest the involvement of these molecules in vascular remodeling and calcification processes. Moreover, we describe the strategies that have been used to modulate the activin downward signal. Understanding the role of myostatin/activin signaling in vascular disease and bone metabolism may provide novel therapeutic opportunities to improve the treatment of conditions still associated with high morbidity and mortality.

Perturbed BMP signaling and denervation promote muscle wasting in cancer cachexia

Most patients with advanced solid cancers exhibit features of cachexia, a debilitating syndrome characterized by progressive loss of skeletal muscle mass and strength. Because the underlying mechanisms of this multifactorial syndrome are incompletely defined, effective therapeutics have yet to be developed. Here, we show that diminished bone morphogenetic protein (BMP) signaling is observed early in the onset of skeletal muscle wasting associated with cancer cachexia in mouse models and in patients with cancer. Cancer-mediated factors including Activin A and IL-6 trigger the expression of the BMP inhibitor Noggin in muscle, which blocks the actions of BMPs on muscle fibers and motor nerves, subsequently causing disruption of the neuromuscular junction (NMJ), denervation, and muscle wasting. Increasing BMP signaling in the muscles of tumor-bearing mice by gene delivery or pharmacological means can prevent muscle wasting and preserve measures of NMJ function. The data identify perturbed BMP signaling and denervation of muscle fibers as important pathogenic mechanisms of muscle wasting associated with tumor growth. Collectively, these findings present interventions that promote BMP-mediated signaling as an attractive strategy to counteract the loss of functional musculature in patients with cancer.

Concurrent BMP Signaling Maintenance and TGF-β Signaling Inhibition Is a Hallmark of Natural Resistance to Muscle Atrophy in the Hibernating Bear

Muscle atrophy arises from a multiplicity of physio-pathological situations and has very detrimental consequences for the whole body. Although knowledge of muscle atrophy mechanisms keeps growing, there is still no proven treatment to date. This study aimed at identifying new drivers for muscle atrophy resistance. We selected an innovative approach that compares muscle transcriptome between an original model of natural resistance to muscle atrophy, the hibernating brown bear, and a classical model of induced atrophy, the unloaded mouse. Using RNA sequencing, we identified 4415 differentially expressed genes, including 1746 up- and 2369 down-regulated genes, in bear muscles between the active versus hibernating period. We focused on the Transforming Growth Factor (TGF)-β and the Bone Morphogenetic Protein (BMP) pathways, respectively, involved in muscle mass loss and maintenance. TGF-β- and BMP-related genes were overall down- and up-regulated in the non-atrophied muscles of the hibernating bear, respectively, and the opposite occurred for the atrophied muscles of the unloaded mouse. This was further substantiated at the protein level. Our data suggest TGF-β/BMP balance is crucial for muscle mass maintenance during long-term physical inactivity in the hibernating bear. Thus, concurrent activation of the BMP pathway may potentiate TGF-β inhibiting therapies already targeted to prevent muscle atrophy.

The Role of Autophagy in Skeletal Muscle Diseases

Skeletal muscle is the most abundant type of tissue in human body, being involved in diverse activities and maintaining a finely tuned metabolic balance. Autophagy, characterized by the autophagosome–lysosome system with the involvement of evolutionarily conserved autophagy-related genes, is an important catabolic process and plays an essential role in energy generation and consumption, as well as substance turnover processes in skeletal muscles. Autophagy in skeletal muscles is finely tuned under the tight regulation of diverse signaling pathways, and the autophagy pathway has cross-talk with other pathways to form feedback loops under physiological conditions and metabolic stress. Altered autophagy activity characterized by either increased formation of autophagosomes or inhibition of lysosome-autophagosome fusion can lead to pathological cascades, and mutations in autophagy genes and deregulation of autophagy pathways have been identified as one of the major causes for a variety of skeleton muscle disorders. The advancement of multi-omics techniques enables further understanding of the molecular and biochemical mechanisms underlying the role of autophagy in skeletal muscle disorders, which may yield novel therapeutic targets for these disorders.

Resistance exercise training ameliorates chronic kidney disease outcomes in a 5/6 nephrectomy model

Chronic kidney disease (CKD) is defined by decreased glomerular filtration rate (GFR) or increased albumin excretion leading to renal injury. However, exercise training is an important non-pharmacological intervention that ameliorates and protects against Diabetes Mellitus, cardiovascular disease, and CKD.

AIM

Our aim was to evaluate the capability of resistance exercise training (RET) to improve CKD outcomes and the contribution of the renal and muscular Akt/mTOR signaling pathway for RET beneficial effects on a CKD model.

MAIN METHODS

Male Wistar rats were subjected to RET, followed for 10 weeks, and randomly divided into 5 groups: Sham: Sham-operated; sedentary and nephrectomy (5/6Nx) (SNS); exercising post-5/6Nx (SNE); exercising pre-5/6Nx (ENS); exercising pre- and post-5/6Nx (ENE). The systolic blood pressure (BP) was measured. Creatinine, proteinuria, and blood urea nitrogen (BUN) were evaluated. After euthanasia Renal and muscular Akt/mTOR signaling pathways were analyzed.

KEY FINDING

Our study showed that the SNS presented renal injury, hypertension, weight and muscular mass loss and a higher mortality rate. SNS group also decreased renal IL-10 and increased TNF-alfa and TGF-Beta. Renal AKT, mTOR, and rpS6 pathway were increased, PTEN was decreased on SNS. And muscular Akt and mTOR were decreased on SNS.

SIGNIFICANCE

The RET before and after the 5/6Nx ameliorates all these parameters mentioned above, suggesting that RET is a good non-pharmacological approach to diminish complications frequently found in CKD. We also suggest that the AKT-m-TOR pathway can play an important role in these beneficial outcomes of RET on the CKD animal model.

Effects of marathon race on selected myokines and sclerostin in middle-aged male amateur runners

In recent years, there has been increasing interest in the homeostatic response to extreme exercises, especially in the integrated function of muscle and bone. The aim of this study was to evaluate the effects of a marathon race on selected myokines and sclerostin in 10 male recreational runners (mean age 41 ± 7.7 years). Body composition, bone mineral density (BMD), and the serum concentration of myostatin, irisin, sclerostin, osteoprotegerin (OPG), 25-hydroxyvitamin D (25(OH)D), parathyroid hormone (PTH), high-sensitivity interleukin-6 (hsIL-6), tumor necrosis factor α (TNFα), high-sensitivity C-reactive protein (hsCRP) and myoglobin, were determined 24 h before and 24 h and 72 h after a marathon race. Post-marathon increases were observed in the levels of myostatin (1.2-fold), OPG (1.5-fold), and PTH (1.3-fold), hsIL-6 (1.9-fold), myoglobin (4.1-fold), hsCRP (fivefold), TNFα (2.6-fold), after 24 h; and in myostatin (1.2-fold), irisin (1.1-fold), sclerostin (1.3-fold), OPG (1.3-fold), and PTH (1.4-fold), hsIL-6 (1.4-fold), TNFα (1.9-fold), after 72 h compared to the baseline level. The results show that in response to the marathon run, a complex network of endocrine interactions is initiated. Further research is needed to fully elucidate the long-term impact of prolonged high intensity exercise on the human body.

Dynamic Changes to the Skeletal Muscle Proteome and Ubiquitinome Induced by the E3 Ligase, ASB2β

Ubiquitination is a posttranslational protein modification that has been shown to have a range of effects, including regulation of protein function, interaction, localization, and degradation. We have previously shown that the muscle-specific ubiquitin E3 ligase, ASB2β, is downregulated in models of muscle growth and that overexpression ASB2β is sufficient to induce muscle atrophy. To gain insight into the effects of increased ASB2β expression on skeletal muscle mass and function, we used liquid chromatography coupled to tandem mass spectrometry to investigate ASB2β-mediated changes to the skeletal muscle proteome and ubiquitinome, via a parallel analysis of remnant diGly-modified peptides. The results show that viral vector-mediated ASB2β overexpression in murine muscles causes progressive muscle atrophy and impairment of force-producing capacity, while ASB2β knockdown induces mild muscle hypertrophy. ASB2β-induced muscle atrophy and dysfunction were associated with the early downregulation of mitochondrial and contractile protein abundance and the upregulation of proteins involved in proteasome-mediated protein degradation (including other E3 ligases), protein synthesis, and the cytoskeleton/sarcomere. The overexpression ASB2β also resulted in marked changes in protein ubiquitination; however, there was no simple relationship between changes in ubiquitination status and protein abundance. To investigate proteins that interact with ASB2β and, therefore, potential ASB2β targets, Flag-tagged wild-type ASB2β, and a mutant ASB2β lacking the C-terminal SOCS box domain (dSOCS) were immunoprecipitated from C2C12 myotubes and subjected to label-free proteomic analysis to determine the ASB2β interactome. ASB2β was found to interact with a range of cytoskeletal and nuclear proteins. When combined with the in vivo ubiquitinomic data, our studies have identified novel putative ASB2β target substrates that warrant further investigation. These findings provide novel insight into the complexity of proteome and ubiquitinome changes that occur during E3 ligase-mediated skeletal muscle atrophy and dysfunction.

Drosophila Activin signaling promotes muscle growth through InR/TORC1-dependent and -independent processes

The Myostatin/Activin branch of the TGF-β superfamily acts as a negative regulator of vertebrate skeletal muscle size, in part, through downregulation of insulin/insulin-like growth factor 1 (IGF-1) signaling. Surprisingly, recent studies in Drosophila indicate that motoneuron-derived Activin signaling acts as a positive regulator of muscle size. Here we demonstrate that Drosophila Activin signaling promotes the growth of muscle cells along all three axes: width, thickness and length. Activin signaling positively regulates the insulin receptor (InR)/TORC1 pathway and the level of Myosin heavy chain (Mhc), an essential sarcomeric protein, via increased Pdk1 and Akt1 expression. Enhancing InR/TORC1 signaling in the muscle of Activin pathway mutants restores Mhc levels close to those of the wild type, but only increases muscle width. In contrast, hyperactivation of the Activin pathway in muscles increases overall larval body and muscle fiber length, even when Mhc levels are lowered by suppression of TORC1. Together, these results indicate that the Drosophila Activin pathway regulates larval muscle geometry and body size via promoting InR/TORC1-dependent Mhc production and the differential assembly of sarcomeric components into either pre-existing or new sarcomeric units depending on the balance of InR/TORC1 and Activin signals.

Single-nucleus RNA-seq identifies transcriptional heterogeneity in multinucleated skeletal myofibers

While the majority of cells contain a single nucleus, cell types such as trophoblasts, osteoclasts, and skeletal myofibers require multinucleation. One advantage of multinucleation can be the assignment of distinct functions to different nuclei, but comprehensive interrogation of transcriptional heterogeneity within multinucleated tissues has been challenging due to the presence of a shared cytoplasm. Here, we utilized single-nucleus RNA-sequencing (snRNA-seq) to determine the extent of transcriptional diversity within multinucleated skeletal myofibers. Nuclei from mouse skeletal muscle were profiled across the lifespan, which revealed the presence of distinct myonuclear populations emerging in postnatal development as well as aging muscle. Our datasets also provided a platform for discovery of genes associated with rare specialized regions of the muscle cell, including markers of the myotendinous junction and functionally validated factors expressed at the neuromuscular junction. These findings reveal that myonuclei within syncytial muscle fibers possess distinct transcriptional profiles that regulate muscle biology.

TMEPAI/PMEPA1 Is a Positive Regulator of Skeletal Muscle Mass

Inhibition of myostatin- and activin-mediated SMAD2/3 signaling using ligand traps, such as soluble receptors, ligand-targeting propeptides and antibodies, or follistatin can increase skeletal muscle mass in healthy mice and ameliorate wasting in models of cancer cachexia and muscular dystrophy. However, clinical translation of these extracellular approaches targeting myostatin and activin has been hindered by the challenges of achieving efficacy without potential effects in other tissues. Toward the goal of developing tissue-specific myostatin/activin interventions, we explored the ability of transmembrane prostate androgen-induced (TMEPAI), an inhibitor of transforming growth factor-β (TGF-β1)-mediated SMAD2/3 signaling, to promote growth, and counter atrophy, in skeletal muscle. In this study, we show that TMEPAI can block activin A, activin B, myostatin and GDF-11 activity in vitro. To determine the physiological significance of TMEPAI, we employed Adeno-associated viral vector (AAV) delivery of a TMEPAI expression cassette to the muscles of healthy mice, which increased mass by as much as 30%, due to hypertrophy of muscle fibers. To demonstrate that TMEPAI mediates its effects via inhibition of the SMAD2/3 pathway, tibialis anterior (TA) muscles of mice were co-injected with AAV vectors expressing activin A and TMEPAI. In this setting, TMEPAI blocked skeletal muscle wasting driven by activin-induced phosphorylation of SMAD3. In a model of cancer cachexia associated with elevated circulating activin A, delivery of AAV:TMEPAI into TA muscles of mice bearing C26 colon tumors ameliorated the muscle atrophy normally associated with cancer progression. Collectively, the findings indicate that muscle-directed TMEPAI gene delivery can inactivate the activin/myostatin-SMAD3 pathway to positively regulate muscle mass in healthy settings and models of disease.

MuRF1/TRIM63, Master Regulator of Muscle Mass

The E3 ubiquitin ligase MuRF1/TRIM63 was identified 20 years ago and suspected to play important roles during skeletal muscle atrophy. Since then, numerous studies have been conducted to decipher the roles, molecular mechanisms and regulation of this enzyme. This revealed that MuRF1 is an important player in the skeletal muscle atrophy process occurring during catabolic states, making MuRF1 a prime candidate for pharmacological treatments against muscle wasting. Indeed, muscle wasting is an associated event of several diseases (e.g., cancer, sepsis, diabetes, renal failure, etc.) and negatively impacts the prognosis of patients, which has stimulated the search for MuRF1 inhibitory molecules. However, studies on MuRF1 cardiac functions revealed that MuRF1 is also cardioprotective, revealing a yin and yang role of MuRF1, being detrimental in skeletal muscle and beneficial in the heart. This review discusses data obtained on MuRF1, both in skeletal and cardiac muscles, over the past 20 years, regarding the structure, the regulation, the location and the different functions identified, and the first inhibitors reported, and aim to draw the picture of what is known about MuRF1. The review also discusses important MuRF1 characteristics to consider for the design of future drugs to maintain skeletal muscle mass in patients with different pathologies.

Mechanisms of IGF-1-Mediated Regulation of Skeletal Muscle Hypertrophy and Atrophy

Insulin-like growth factor-1 (IGF-1) is a key growth factor that regulates both anabolic and catabolic pathways in skeletal muscle. IGF-1 increases skeletal muscle protein synthesis via PI3K/Akt/mTOR and PI3K/Akt/GSK3β pathways. PI3K/Akt can also inhibit FoxOs and suppress transcription of E3 ubiquitin ligases that regulate ubiquitin proteasome system (UPS)-mediated protein degradation. Autophagy is likely inhibited by IGF-1 via mTOR and FoxO signaling, although the contribution of autophagy regulation in IGF-1-mediated inhibition of skeletal muscle atrophy remains to be determined. Evidence has suggested that IGF-1/Akt can inhibit muscle atrophy-inducing cytokine and myostatin signaling via inhibition of the NF-κΒ and Smad pathways, respectively. Several miRNAs have been found to regulate IGF-1 signaling in skeletal muscle, and these miRs are likely regulated in different pathological conditions and contribute to the development of muscle atrophy. IGF-1 also potentiates skeletal muscle regeneration via activation of skeletal muscle stem (satellite) cells, which may contribute to muscle hypertrophy and/or inhibit atrophy. Importantly, IGF-1 levels and IGF-1R downstream signaling are suppressed in many chronic disease conditions and likely result in muscle atrophy via the combined effects of altered protein synthesis, UPS activity, autophagy, and muscle regeneration.

Myokines and adipokines in sarcopenia: understanding cross-talk between skeletal muscle and adipose tissue and the role of exercise

Detrimental age-associated changes in skeletal muscle and adipose tissue increase the risk of sarcopenia. Age-related changes in myokines, such as myostatin and irisin, as well as adipokines, such as leptin and adiponectin, contribute to cross-talk between muscle and adipose tissue. These age-related changes in myokines and adipokines have important implications for sarcopenia, however, recent literature highlights discrepancies in these relationships. Exercise may alter serum profiles and muscle receptor expression of these factors, but future work is needed to determine whether these changes in myokines and adipokines relate to improvements in muscle mass and function. Here, we describe myokine-mediated and adipokine-mediated interactions between muscle and adipose tissue, and discuss the fundamental importance of these cytokines to understanding the development of sarcopenia.

Preclinical insights into the gut-skeletal muscle axis in chronic gastrointestinal diseases

Muscle wasting represents a constant pathological feature of common chronic gastrointestinal diseases, including liver cirrhosis (LC), inflammatory bowel diseases (IBD), chronic pancreatitis (CP) and pancreatic cancer (PC), and is associated with increased morbidity and mortality. Recent clinical and experimental studies point to the existence of a gut‐skeletal muscle axis that is constituted by specific gut‐derived mediators which activate pro‐ and anti‐sarcopenic signalling pathways in skeletal muscle cells. A pathophysiological link between both organs is also provided by low‐grade systemic inflammation. Animal models of LC, IBD, CP and PC represent an important resource for mechanistic and preclinical studies on disease‐associated muscle wasting. They are also required to test and validate specific anti‐sarcopenic therapies prior to clinical application. In this article, we review frequently used rodent models of muscle wasting in the context of chronic gastrointestinal diseases, survey their specific advantages and limitations and discuss possibilities for further research activities in the field. We conclude that animal models of LC‐, IBD‐ and PC‐associated sarcopenia are an essential supplement to clinical studies because they may provide additional mechanistic insights and help to identify molecular targets for therapeutic interventions in humans.

Signaling Pathways That Control Muscle Mass

The loss of skeletal muscle mass under a wide range of acute and chronic maladies is associated with poor prognosis, reduced quality of life, and increased mortality. Decades of research indicate the importance of skeletal muscle for whole body metabolism, glucose homeostasis, as well as overall health and wellbeing. This tissue’s remarkable ability to rapidly and effectively adapt to changing environmental cues is a double-edged sword. Physiological adaptations that are beneficial throughout life become maladaptive during atrophic conditions. The atrophic program can be activated by mechanical, oxidative, and energetic distress, and is influenced by the availability of nutrients, growth factors, and cytokines. Largely governed by a transcription-dependent mechanism, this program impinges on multiple protein networks including various organelles as well as biosynthetic and quality control systems. Although modulating muscle function to prevent and treat disease is an enticing concept that has intrigued research teams for decades, a lack of thorough understanding of the molecular mechanisms and signaling pathways that control muscle mass, in addition to poor transferability of findings from rodents to humans, has obstructed efforts to develop effective treatments. Here, we review the progress made in unraveling the molecular mechanisms responsible for the regulation of muscle mass, as this continues to be an intensive area of research.

Differentiation of Murine C2C12 Myoblasts Strongly Reduces the Effects of Myostatin on Intracellular Signaling

Alongside in vivo models, a simpler and more mechanistic approach is required to study the effects of myostatin on skeletal muscle because myostatin is an important negative regulator of muscle size. In this study, myostatin was administered to murine (C2C12) and human (CHQ) myoblasts and myotubes. Canonical and noncanonical signaling downstream to myostatin, related ligands, and their receptor were analyzed. The effects of tumorkines were analyzed after coculture of C2C12 and colon cancer-C26 cells. The effects of myostatin on canonical and noncanonical signaling were strongly reduced in C2C12 cells after differentiation. This may be explained by increased follistatin, an endogenous blocker of myostatin and altered expression of activin receptor ligands. In contrast, CHQ cells were equally responsive to myostatin, and follistatin remained unaltered. Both myostatin administration and the coculture stimulated pathways associated with inflammation, especially in C2C12 cells. In conclusion, the effects of myostatin on intracellular signaling may be cell line- or organism-specific, and C2C12 myotubes seem to be a nonoptimal in vitro model for investigating the effects of myostatin on canonical and noncanonical signaling in skeletal muscle. This may be due to altered expression of activin receptor ligands and their regulators during muscle cell differentiation.

Is Mitochondrial Oxidative Stress the Key Contributor to Diaphragm Atrophy and Dysfunction in Critically Ill Patients?

Diaphragm dysfunction is prevalent in the progress of respiratory dysfunction in various critical illnesses. Respiratory muscle weakness may result in insufficient ventilation, coughing reflection suppression, pulmonary infection, and difficulty in weaning off respirators. All of these further induce respiratory dysfunction and even threaten the patients' survival. The potential mechanisms of diaphragm atrophy and dysfunction include impairment of myofiber protein anabolism, enhancement of myofiber protein degradation, release of inflammatory mediators, imbalance of metabolic hormones, myonuclear apoptosis, autophagy, and oxidative stress. Among these contributors, mitochondrial oxidative stress is strongly implicated to play a key role in the process as it modulates diaphragm protein synthesis and degradation, induces protein oxidation and functional alteration, enhances apoptosis and autophagy, reduces mitochondrial energy supply, and is regulated by inflammatory cytokines via related signaling molecules. This review aims to provide a concise overview of pathological mechanisms of diaphragmatic dysfunction in critically ill patients, with special emphasis on the role and modulating mechanisms of mitochondrial oxidative stress.

Exogenous miR-29a Attenuates Muscle Atrophy and Kidney Fibrosis in Unilateral Ureteral Obstruction Mice

Renal fibrosis leads to end-stage renal disease, but antifibrotic drugs are difficult to develop. Chronic kidney disease often results in muscle wasting, and thereby increases morbidity and mortality. In this work, adeno-associated virus (AAV)-mediated overexpressing miR-29a was hypothesized to counteract renal fibrosis and muscle wasting through muscle-kidney crosstalk in unilateral ureteral obstruction (UUO) mice. miR-29a level was downregulated in the kidney and skeletal muscle of UUO mice. The secretion of exosome-encapsulated miR-29a increased in cultured skeletal muscle satellite cells and HEK293 renal cells after stimulation with serum from UUO mice. This result was confirmed by qPCR and microRNA deep sequencing in the serum exosomes of mice with obstructed ureters. A recombinant AAV-miR-29a was generated to overexpress miR-29a and injected into the tibialis anterior muscle of the mice 2 weeks before UUO surgery. AAV-miR-29a abrogated the UUO-induced upregulation of YY1 and myostatin in skeletal muscles. Renal fibrosis was also partially improved in the UUO mice with intramuscular AAV-miR-29a transduction. AAV-miR-29a overexpression reversed the increase in transforming growth factor β, fibronectin, alpha-smooth muscle actin, and collagen 1A1 and 4A1 levels in the kidney of UUO mice. AAV-green fluorescent protein was applied to trace the AAV route in vivo, and fluorescence was significantly visible in the injected/uninjected muscles and in the kidneys. In conclusion, intramuscular AAV-miR-29a injection attenuates muscle wasting and ameliorates renal fibrosis by downregulating several fibrotic-related proteins in UUO mice.