Rapid disuse and denervation atrophy involve transcriptional changes similar to those of muscle wasting during systemic diseases

Integrated genomic and proteomic analyses identify stimulus-dependent molecular changes associated with distinct modes of skeletal muscle atrophy

Skeletal muscle atrophy is a debilitating condition that occurs with aging and disease, but the underlying mechanisms are incompletely understood. Previous work determined that common transcriptional changes occur in muscle during atrophy induced by different stimuli. However, whether this holds true at the proteome level remains largely unexplored. Here, we find that, contrary to this earlier model, distinct atrophic stimuli (corticosteroids, cancer cachexia, and aging) induce largely different mRNA and protein changes during muscle atrophy in mice. Moreover, there is widespread transcriptome-proteome disconnect. Consequently, atrophy markers (atrogenes) identified in earlier microarray-based studies do not emerge from proteomics as generally induced by atrophy. Rather, we identify proteins that are distinctly modulated by different types of atrophy (herein defined as “atroproteins”) such as the myokine CCN1/Cyr61, which regulates myofiber type switching during sarcopenia. Altogether, these integrated analyses indicate that different catabolic stimuli induce muscle atrophy via largely distinct mechanisms.

Skeletal muscle wasting is caused by many catabolic stimuli, which were thought to act via shared mechanisms. Hunt et al. now show that distinct catabolic stimuli induce muscle wasting via largely different molecular changes. The authors identify atrophy-associated proteins (“atroproteins”) that may represent diagnostic biomarkers and/or therapeutic targets.

Dicer-mediated miRNA processing is not involved in controlling muscle mass during muscle atrophy

Muscle atrophy occurs in a variety of physiological and pathological conditions. Specific molecular networks that govern protein synthesis and degradation play important roles in controlling muscle mass under diverse catabolic states. MicroRNAs (miRNAs) were previously found to be regulators of protein synthesis and degradation, and their expressions in skeletal muscle were altered in muscle wasting conditions. However, functional roles of miRNAs in muscle atrophy are poorly understood. In this study, we generated tamoxifen-inducible Dicer knockout (iDicer KO) mice and subjected them to 2 weeks of single hindlimb denervation. The expression of Dicer mRNA was significantly reduced in muscle of the iDicer KO mice compared to that of WT mice. The loss of Dicer moderately reduced levels of muscle-enriched miRNAs, miR-1, miR-133a and miR-206 in both innervated and denervated muscles of the iDicer KO mice. We also found that the extent of denervation-induced muscle atrophy as well as changes of signaling molecules related to protein synthesis/degradation pathways in the iDicer KO mice were comparable to these in WT mice. Taken together, Dicer knockout in adult skeletal muscle did not affect denervation-induced muscle atrophy.

Maslinic Acid Attenuates Denervation-Induced Loss of Skeletal Muscle Mass and Strength

Maslinic acid (MA) is a pentacyclic triterpene abundant in olive peels. MA reportedly increases skeletal muscle mass and strength in older adults; however, the underlying mechanism is unknown. This study aimed to investigate the effects of MA on denervated muscle atrophy and strength and to explore the underlying molecular mechanism. Mice were fed either a control diet or a 0.27% MA diet. One week after intervention, the sciatic nerves of both legs were cut to induce muscle atrophy. Mice were examined 14 days after denervation. MA prevented the denervation-induced reduction in gastrocnemius muscle mass and skeletal muscle strength. Microarray gene expression profiling in gastrocnemius muscle demonstrated several potential mechanisms for muscle maintenance. Gene set enrichment analysis (GSEA) revealed different enriched biological processes, such as myogenesis, PI3/AKT/mTOR signaling, TNFα signaling via NF-κB, and TGF-β signaling in MA-treated mice. In addition, qPCR data showed that MA induced Igf1 expression and suppressed the expressions of Atrogin-1, Murf1 and Tgfb. Altogether, our results suggest the potential of MA as a new therapeutic and preventive dietary ingredient for muscular atrophy and strength.

MERG1A Protein Abundance Increases in the Atrophied Skeletal Muscle of Denervated Mice, But Does Not Affect NFκB Activity

Skeletal muscle atrophy may occur with disease, injury, decreased muscle use, starvation, and normal aging. No reliably effective treatments for atrophy are available, thus research into the mechanisms contributing to muscle loss is essential. The ERG1A K+ channel contributes to muscle loss by increasing ubiquitin proteasome proteolysis (UPP) in the skeletal muscle of both unweighted and cachectic mice. Because the mechanisms which produce atrophy vary based upon the initiating factor, here we investigate atrophy produced by denervation. Using immunohistochemistry and immunoblots, we demonstrate that ERG1A protein abundance increases significantly in the Gastrocnemius muscle of rodents 7 days after both sciatic nerve transection and hind limb unweighting. Further, we reveal that ectopic expression of a Merg1a encoded plasmid in normal mouse Gastrocnemius muscle has no effect on activity of the NFκB transcription factor family, a group of proteins which contribute to muscle atrophy by modulation of the UPP. Further, although NFκB activity increases significantly after denervation, we show that expression of a plasmid encoding a dominant negative Merg1a mutant in Gastrocnemius muscle prior to denervation, has no effect on NFκB activity. Thus, although the ERG1A K+ channel increases UPP, it does not do so through modulation of NFκB transcription factors.

Microarray profiling of gene expression in C2C12 myotubes trained by electric pulse stimulation

Electric pulse-stimulated C2C12 myotubes are gaining interest in the field of muscle physiology and biotechnology because electric pulse stimulation (EPS) enhances sarcomere structure development and active tension generation capability. Recently, we found that termination of EPS results in the rapid loss of active tension generation accompanied by disassembly of the sarcomere structure, which may represent an in vitro muscle atrophy model. To elucidate the molecular mechanism underlying this rapid loss of active tension generation and sarcomere structure disassembly after termination of EPS, we performed transcriptomic analysis using microarray. After termination of EPS, 74 genes were upregulated and 120 genes were downregulated after 30 min; however, atrophy-related genes were not found among these genes. To further assess the effect of EPS on gene expression, we re-applied EPS after its termination for 8 h and searched for genes whose expression was reversed. Four genes were upregulated by termination of EPS and downregulated by the re-application of EPS, whereas two genes were downregulated by termination of EPS and upregulated by the re-application of EPS. Although none of these genes were atrophy- or hypertrophy-related, the results presented in this study will contribute to the understanding of gene expression changes that mediate rapid loss of active tension generation and sarcomere structure disassembly following termination of EPS in C2C12 myotubes.

Prioritizing movement to address the frailty phenotype in heart failure

Frailty is a highly prevalent multisystem syndrome in older adults with heart failure (HF) and is associated with poor clinical prognosis and increased complexity of care. While frailty is neither disease nor age specific, it is a clinical manifestation of aging-related processes that reflects a reduced physiological ability to tolerate and recover from stress associated with aging, disease, or therapy. Within this context, physical frailty, which is distinctly oriented to physical functional domains (e.g., muscle weakness, slowness, and low activity), has been recognized as a critical vital sign in older persons with HF. Identification and routine assessment of physical frailty, using objective physical performance measures, may guide the course of patient-centered treatment plans that maximize the likelihood of improving clinical outcomes in older HF patients. Exercise-based rehabilitation is a primary therapy to improve cardiovascular health in patients with HF; however, the limited evidence supporting the effectiveness of exercise tailored to older and frail HF patients underscores the current gaps in management of their care. Interdisciplinary exercise interventions designed with consideration of physical frailty as a therapeutic target may be an important strategy to counteract functional deficits characteristic of frailty and HF, and to improve patient-centered outcomes in this population. The purpose of this current review is to provide a better understanding of physical frailty and its relation to management of care in older patients with HF. Implications of movement-based interventions, including exercise and physical rehabilitation, to prevent or reverse physical frailty and improve clinical outcomes will further be discussed.

Identification of potential microRNAs and KEGG pathways in denervation muscle atrophy based on meta-analysis

The molecular mechanism of muscle atrophy has been studied a lot, but there is no comprehensive analysis focusing on the denervated muscle atrophy. The gene network that controls the development of denervated muscle atrophy needs further elucidation. We examined differentially expressed genes (DEGs) from five denervated muscle atrophy microarray datasets and predicted microRNAs that target these DEGs. We also included the differentially expressed microRNAs datasets of denervated muscle atrophy in previous studies as background information to identify potential key microRNAs. Finally, we compared denervated muscle atrophy with disuse muscle atrophy caused by other reasons, and obtained the Den-genes which only differentially expressed in denervated muscle atrophy. In this meta-analysis, we obtained 429 up-regulated genes, 525 down-regulated genes and a batch of key microRNAs in denervated muscle atrophy. We found eight important microRNA-mRNA interactions (miR-1/Jun, miR-1/Vegfa, miR-497/Vegfa, miR-23a/Vegfa, miR-206/Vegfa, miR-497/Suclg1, miR-27a/Suclg1, miR-27a/Mapk14). The top five KEGG pathways enriched by Den-genes are Insulin signaling pathway, T cell receptor signaling pathway, MAPK signaling pathway, Toll-like receptor signaling pathway and B cell receptor signaling pathway. Our research has delineated the RNA regulatory network of denervated muscle atrophy, and uncovered the specific genes and terms in denervated muscle atrophy.

Nuclear factor E2-related factor 2 (NRF2) deficiency accelerates fast fibre type transition in soleus muscle during space flight

Microgravity induces skeletal muscle atrophy, particularly in the soleus muscle, which is predominantly composed of slow-twitch myofibre (type I) and is sensitive to disuse. Muscle atrophy is commonly known to be associated with increased production of reactive oxygen species. However, the role of NRF2, a master regulator of antioxidative response, in skeletal muscle plasticity during microgravity-induced atrophy, is not known. To investigate the role of NRF2 in skeletal muscle within a microgravity environment, wild-type and Nrf2-knockout (KO) mice were housed in the International Space Station for 31 days. Gene expression and histological analyses demonstrated that, under microgravity conditions, the transition of type I (oxidative) muscle fibres to type IIa (glycolytic) was accelerated in Nrf2-KO mice without affecting skeletal muscle mass. Therefore, our results suggest that NRF2 affects myofibre type transition during space flight.

Identification of the MuRF1 Skeletal Muscle Ubiquitylome Through Quantitative Proteomics

MuRF1 (TRIM63) is a muscle-specific E3 ubiquitin ligase and component of the ubiquitin proteasome system. MuRF1 is transcriptionally upregulated under conditions that cause muscle loss, in both rodents and humans, and is a recognized marker of muscle atrophy. In this study, we used in vivo electroporation to determine whether MuRF1 overexpression alone can cause muscle atrophy and, in combination with ubiquitin proteomics, identify the endogenous MuRF1 substrates in skeletal muscle. Overexpression of MuRF1 in adult mice increases ubiquitination of myofibrillar and sarcoplasmic proteins, increases expression of genes associated with neuromuscular junction instability, and causes muscle atrophy. A total of 169 ubiquitination sites on 56 proteins were found to be regulated by MuRF1. MuRF1-mediated ubiquitination targeted both thick and thin filament contractile proteins, as well as, glycolytic enzymes, deubiquitinases, p62, and VCP. These data reveal a potential role for MuRF1 in not only the breakdown of the sarcomere but also the regulation of metabolism and other proteolytic pathways in skeletal muscle.

Mitochondrial Bioenergetics and Turnover during Chronic Muscle Disuse

Periods of muscle disuse promote marked mitochondrial alterations that contribute to the impaired metabolic health and degree of atrophy in the muscle. Thus, understanding the molecular underpinnings of muscle mitochondrial decline with prolonged inactivity is of considerable interest. There are translational applications to patients subjected to limb immobilization following injury, illness-induced bed rest, neuropathies, and even microgravity. Studies in these patients, as well as on various pre-clinical rodent models have elucidated the pathways involved in mitochondrial quality control, such as mitochondrial biogenesis, mitophagy, fission and fusion, and the corresponding mitochondrial derangements that underlie the muscle atrophy that ensues from inactivity. Defective organelles display altered respiratory function concurrent with increased accumulation of reactive oxygen species, which exacerbate myofiber atrophy via degradative pathways. The preservation of muscle quality and function is critical for maintaining mobility throughout the lifespan, and for the prevention of inactivity-related diseases. Exercise training is effective in preserving muscle mass by promoting favourable mitochondrial adaptations that offset the mitochondrial dysfunction, which contributes to the declines in muscle and whole-body metabolic health. This highlights the need for further investigation of the mechanisms in which mitochondria contribute to disuse-induced atrophy, as well as the specific molecular targets that can be exploited therapeutically.

Influence of Age on Skeletal Muscle Hypertrophy and Atrophy Signaling: Established Paradigms and Unexpected Links

Skeletal muscle atrophy in an inevitable occurrence with advancing age, and a consequence of disease including cancer. Muscle atrophy in the elderly is managed by a regimen of resistance exercise and increased protein intake. Understanding the signaling that regulates muscle mass may identify potential therapeutic targets for the prevention and reversal of muscle atrophy in metabolic and neuromuscular diseases. This review covers the major anabolic and catabolic pathways that regulate skeletal muscle mass, with a focus on recent progress and potential new players.

Parvalbumin affects skeletal muscle trophism through modulation of mitochondrial calcium uptake

Parvalbumin (PV) is a cytosolic Ca²⁺-binding protein highly expressed in fast skeletal muscle, contributing to an increased relaxation rate. Moreover, PV is an “atrogene” downregulated in most muscle atrophy conditions. Here, we exploit mice lacking PV to explore the link between the two PV functions. Surprisingly, PV ablation partially counteracts muscle loss after denervation. Furthermore, acute PV downregulation is accompanied by hypertrophy and upregulation by atrophy. PV ablation has a minor impact on sarcoplasmic reticulum but is associated with increased mitochondrial Ca²⁺ uptake, mitochondrial size and number, and contacts with Ca²⁺ release sites. Mitochondrial calcium uniporter (MCU) silencing abolishes the hypertrophic effect of PV ablation, suggesting that mitochondrial Ca²⁺ uptake is required for hypertrophy. In turn, an increase of mitochondrial Ca²⁺ is required to enhance expression of the pro-hypertrophy gene PGC-1α4, whose silencing blocks hypertrophy due to PV ablation. These results reveal how PV links cytosolic Ca²⁺ control to mitochondrial adaptations, leading to muscle mass regulation.

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PV is downregulated during skeletal muscle atrophy, and its levels affect trophism

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Skeletal muscle mitochondria undergo remodeling in PV knockout mice

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Mitochondria increase cytosolic Ca²⁺ buffer capacity in PV knockout skeletal muscles

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Increased mitochondrial Ca²⁺ triggers the PGC-1α4 pathway, inducing muscle growth

Transcriptome analysis of gravitational effects on mouse skeletal muscles under microgravity and artificial 1 g onboard environment

Spaceflight causes a decrease in skeletal muscle mass and strength. We set two murine experimental groups in orbit for 35 days aboard the International Space Station, under artificial earth-gravity (artificial 1 g; AG) and microgravity (μg; MG), to investigate whether artificial 1 g exposure prevents muscle atrophy at the molecular level. Our main findings indicated that AG onboard environment prevented changes under microgravity in soleus muscle not only in muscle mass and fiber type composition but also in the alteration of gene expression profiles. In particular, transcriptome analysis suggested that AG condition could prevent the alterations of some atrophy-related genes. We further screened novel candidate genes to reveal the muscle atrophy mechanism from these gene expression profiles. We suggest the potential role of Cacng1 in the atrophy of myotubes using in vitro and in vivo gene transductions. This critical project may accelerate the elucidation of muscle atrophy mechanisms.

Targeting the Activin Receptor Signaling to Counteract the Multi-Systemic Complications of Cancer and Its Treatments

Muscle wasting, i.e., cachexia, frequently occurs in cancer and associates with poor prognosis and increased morbidity and mortality. Anticancer treatments have also been shown to contribute to sustainment or exacerbation of cachexia, thus affecting quality of life and overall survival in cancer patients. Pre-clinical studies have shown that blocking activin receptor type 2 (ACVR2) or its ligands and their downstream signaling can preserve muscle mass in rodents bearing experimental cancers, as well as in chemotherapy-treated animals. In tumor-bearing mice, the prevention of skeletal and respiratory muscle wasting was also associated with improved survival. However, the definitive proof that improved survival directly results from muscle preservation following blockade of ACVR2 signaling is still lacking, especially considering that concurrent beneficial effects in organs other than skeletal muscle have also been described in the presence of cancer or following chemotherapy treatments paired with counteraction of ACVR2 signaling. Hence, here, we aim to provide an up-to-date literature review on the multifaceted anti-cachectic effects of ACVR2 blockade in preclinical models of cancer, as well as in combination with anticancer treatments.

Functional Non-coding RNA During Embryonic Myogenesis and Postnatal Muscle Development and Disease

Skeletal muscle is a highly heterogeneous tissue that plays a crucial role in mammalian metabolism and motion maintenance. Myogenesis is a complex biological process that includes embryonic and postnatal development, which is regulated by specific signaling pathways and transcription factors. Various non-coding RNAs (ncRNAs) account for the majority of total RNA in cells and have an important regulatory role in myogenesis. In this review, we introduced the research progress in miRNAs, circRNAs, and lncRNAs related to embryonic and postnatal muscle development. We mainly focused on ncRNAs that regulate myoblast proliferation, differentiation, and postnatal muscle development through multiple mechanisms. Finally, challenges and future perspectives related to the identification and verification of functional ncRNAs are discussed. The identification and elucidation of ncRNAs related to myogenesis will enrich the myogenic regulatory network, and the effective application of ncRNAs will enhance the function of skeletal muscle.

Chrysanthemum zawadskil Herbich attenuates dexamethasone-induced muscle atrophy through the regulation of proteostasis and mitochondrial function

Chrysanthemum zawadskii Herbich (CZH) is used in traditional medicine to treat inflammatory diseases and diabetes. However, the effects of CZH on muscle wasting remains to be studied. Here, we investigated the effect of CZH on dexamethasone (DEX), a synthetic glucocorticoid, induced muscle atrophy. To examine the effect of CZH on muscle atrophy, C2C12 myotubes were co-treated with DEX and CZH for 24 h. The treatment with CZH prevented DEX-induced myotube atrophy in a dose-dependent manner. CZH inhibited the DEX-induced decrease of the MHC isoforms and the upregulation of atrogin-1 and MuRF1 in C2C12 differentiated cells. C57BL/6 mice were supplemented with 0.1 % CZH for 8 weeks, with DEX-induced muscle atrophy stimulated in the last 3 weeks. In the mice, CZH supplementation effectively reversed DEX-induced skeletal muscle atrophy and increased the exercise capacity of the mice through the inhibition of glucocorticoid receptor translocation. Additionally, we observed that DEX-evoked impaired proteostasis was ameliorated via the Akt/mTOR pathway. CZH also prevented the DEX-induced decrease in the mitochondrial respiration. HPLC analysis demonstrated the highest concentration of acacetin-7-O-β-d-rutinoside (AR) among 4 compounds. Moreover, AR, a functional compound of CZH, prevented DEX-evoked muscle atrophy. Thus, we suggest that CZH could be a potential therapeutic candidate against muscle atrophy and AR is the main functional compound of CZH.

Breakdown of Filamentous Myofibrils by the UPS-Step by Step

Protein degradation maintains cellular integrity by regulating virtually all biological processes, whereas impaired proteolysis perturbs protein quality control, and often leads to human disease. Two major proteolytic systems are responsible for protein breakdown in all cells: autophagy, which facilitates the loss of organelles, protein aggregates, and cell surface proteins; and the ubiquitin-proteasome system (UPS), which promotes degradation of mainly soluble proteins. Recent findings indicate that more complex protein structures, such as filamentous assemblies, which are not accessible to the catalytic core of the proteasome in vitro, can be efficiently degraded by this proteolytic machinery in systemic catabolic states in vivo. Mechanisms that loosen the filamentous structure seem to be activated first, hence increasing the accessibility of protein constituents to the UPS. In this review, we will discuss the mechanisms underlying the disassembly and loss of the intricate insoluble filamentous myofibrils, which are responsible for muscle contraction, and whose degradation by the UPS causes weakness and disability in aging and disease. Several lines of evidence indicate that myofibril breakdown occurs in a strictly ordered and controlled manner, and the function of AAA-ATPases is crucial for their disassembly and loss.

Master Regulators of Muscle Atrophy: Role of Costamere Components

The loss of muscle mass and force characterizes muscle atrophy in several different conditions, which share the expression of atrogenes and the activation of their transcriptional regulators. However, attempts to antagonize muscle atrophy development in different experimental contexts by targeting contributors to the atrogene pathway showed partial effects in most cases. Other master regulators might independently contribute to muscle atrophy, as suggested by our recent evidence about the co-requirement of the muscle-specific chaperone protein melusin to inhibit unloading muscle atrophy development. Furthermore, melusin and other muscle mass regulators, such as nNOS, belong to costameres, the macromolecular complexes that connect sarcolemma to myofibrils and to the extracellular matrix, in correspondence with specific sarcomeric sites. Costameres sense a mechanical load and transduce it both as lateral force and biochemical signals. Recent evidence further broadens this classic view, by revealing the crucial participation of costameres in a sarcolemmal “signaling hub” integrating mechanical and humoral stimuli, where mechanical signals are coupled with insulin and/or insulin-like growth factor stimulation to regulate muscle mass. Therefore, this review aims to enucleate available evidence concerning the early involvement of costamere components and additional putative master regulators in the development of major types of muscle atrophy.

Metabolic signatures of muscle mass loss in an elderly Taiwanese population

To identify the association between metabolites and muscle mass in 305 elderly Taiwanese subjects, we conducted a multivariate analysis of 153 plasma samples. Based on appendicular skeletal muscle mass index (ASMI) quartiles, female and male participants were divided into four groups. Quartile 4 (Men: 5.67±0.35, Women: 4.70±0.32 Kg/m²) and quartile 1 (Men: 7.60±0.29, Women: 6.56±0.53 Kg/m²) represented low muscle mass and control groups, respectively. After multivariable adjustment, except for physical function, we found that blood urea nitrogen, creatinine, and age were associated with ASMI in men. However, only triglyceride level was related to ASMI in women. The multiple logistic regression models were used to analyze in each baseline characteristic and metabolite concentration. After the adjustment, we identify amino acid-related metabolites and show that glutamate levels in women and alpha-aminoadipate, Dopa, and citrulline/ornithine levels in men are gender-specific metabolic signatures of muscle mass loss.

Inhibition of prostaglandin-degrading enzyme 15-PGDH rejuvenates aged muscle mass and strength

Treatments are lacking for sarcopenia, a debilitating age-related skeletal muscle wasting syndrome. We identifed increased amounts of 15-hydroxyprostaglandin dehydrogenase (15-PGDH), the prostaglandin E₂ (PGE₂)-degrading enzyme, as a hallmark of aged tissues, including skeletal muscle. The consequent reduction in PGE₂ signaling contributed to muscle atrophy in aged mice and results from 15-PGDH-expressing myofibers and interstitial cells, such as macrophages, within muscle. Overexpression of 15-PGDH in young muscles induced atrophy. Inhibition of 15-PGDH, by targeted genetic depletion or a small-molecule inhibitor, increased aged muscle mass, strength, and exercise performance. These benefits arise from a physiological increase in PGE₂ concentrations, which augmented mitochondrial function and autophagy and decreased transforming growth factor-β signaling and activity of ubiquitin-proteasome pathways. Thus, PGE₂ signaling ameliorates muscle atrophy and rejuvenates muscle function, and 15-PGDH may be a suitable therapeutic target for countering sarcopenia.

Cell autonomous requirement of neurofibromin (Nf1) for postnatal muscle hypertrophic growth and metabolic homeostasis

BACKGROUND

Neurofibromatosis type 1 (NF1) is a multi-organ disease caused by mutations in neurofibromin 1 (NF1). Amongst other features, NF1 patients frequently show reduced muscle mass and strength, impairing patients' mobility and increasing the risk of fall. The role of Nf1 in muscle and the cause for the NF1-associated myopathy are mostly unknown.

METHODS

To dissect the function of Nf1 in muscle, we created muscle-specific knockout mouse models for NF1, inactivating Nf1 in the prenatal myogenic lineage either under the Lbx1 promoter or under the Myf5 promoter. Mice were analysed during prenatal and postnatal myogenesis and muscle growth.

RESULTS

Nf1^Lbx1 and Nf1^Myf5 animals showed only mild defects in prenatal myogenesis. Nf1^Lbx1 animals were perinatally lethal, while Nf1^Myf5 animals survived only up to approximately 25 weeks. A comprehensive phenotypic characterization of Nf1^Myf5 animals showed decreased postnatal growth, reduced muscle size, and fast fibre atrophy. Proteome and transcriptome analyses of muscle tissue indicated decreased protein synthesis and increased proteasomal degradation, and decreased glycolytic and increased oxidative activity in muscle tissue. High-resolution respirometry confirmed enhanced oxidative metabolism in Nf1^Myf5 muscles, which was concomitant to a fibre type shift from type 2B to type 2A and type 1. Moreover, Nf1^Myf5 muscles showed hallmarks of decreased activation of mTORC1 and increased expression of atrogenes. Remarkably, loss of Nf1 promoted a robust activation of AMPK with a gene expression profile indicative of increased fatty acid catabolism. Additionally, we observed a strong induction of genes encoding catabolic cytokines in muscle Nf1^Myf5 animals, in line with a drastic reduction of white, but not brown adipose tissue.

CONCLUSIONS

Our results demonstrate a cell autonomous role for Nf1 in myogenic cells during postnatal muscle growth required for metabolic and proteostatic homeostasis. Furthermore, Nf1 deficiency in muscle drives cross-tissue communication and mobilization of lipid reserves.

(-)-Epicatechin reduces muscle waste after complete spinal cord transection in a murine model: role of ubiquitin-proteasome system

The skeletal muscle mass reduces 30-60% after spinal cord injury, this is mostly due to protein degradation through ubiquitin-proteasome system. In this work, we propose that the flavanol (-)-epicatechin, due its widespread biological effects on muscle health, can prevent muscle mass decrease after spinal cord injury. Thirty-six female Long Evans rats were randomized into 5 groups: (1) Spinal cord injury 7 days, (2) Spinal cord injury + (-)-epicatechin 7 days, (3) Spinal cord injury 30 days, (4) Spinal cord injury + (-)-epicatechin 30 days and (5) Sham (Only laminectomy). Hind limb perimeter, muscle cross section area, fiber cross section area and ubiquitin-proteasome system protein expression together with total protein ubiquitination were assessed. At 30 days Spinal cord injury group lost 49.52 ± 2.023% of muscle cross section area (-)-epicatechin treated group lost only 24.28 ± 15.45% being a significant difference. Ubiquitin-proteasome markers showed significant changes. FOXO1a increased in spinal cord injury group vs Sham (-)-epicatechin reduced this increase. In spinal cord injury group MAFbx increased significantly vs Sham but decrease in (-)-epicatechin treatment group at 30 days. At 7 and 30 days MuRF1 increased in the spinal cord injury and decreased in the (-)-epicatechin group. The global protein ubiquitination increases after spinal cord injury, epicatechin treatment induce a significant decrease in protein ubiquitination. These results suggest that (-)-epicatechin reduces the muscle waste after spinal cord injury through down regulation of the ubiquitin-proteasome system.

Skeletal Muscle Anti-Atrophic Effects of Leucine Involve Myostatin Inhibition

Lack of mechanical load leads to skeletal muscle atrophy, and one major underlying mechanism involves the myostatin pathway that negatively regulates protein synthesis and also activates Atrogin-1/MAFbx and MuRF1 genes. In hindlimb immobilization, leucine was observed to attenuate the upregulation of the referred atrogenes, thereby shortening the impact on fiber cross-sectional area, nonetheless, the possible connection with myostatin is still elusive. This study sought to verify the impact of leucine supplementation on myostatin expression. Male Wistar rats were supplemented with leucine and hindlimb immobilized for 3 and 7 days, after which soleus muscles were removed for morphometric measurements and analyzed for gene and protein expression by real-time PCR and Western blotting, respectively. Muscle wasting was prominent 7 days after immobilization, as expected, leucine feeding mitigated this effect. Atrogin-1/MAFbx gene expression was upregulated only after 3 days of immobilization, and this effect was attenuated by leucine supplementation. Atrogin-1/MAFbx protein levels were elevated after 7 days of immobilization, which leucine supplementation was not able to lessen. On the other hand, myostatin gene expression was upregulated in immobilization for 3 and 7 days, which returned to normal levels after leucine supplementation. Myostatin protein levels followed gene expression at a 3-day time point only. Follistatin gene expression was upregulated during immobilization and accentuated by leucine after 3 days of supplementation. Concerning protein expression, follistatin was not altered neither by immobilization nor in immobilized animals treated with leucine. In conclusion, leucine protects against skeletal muscle mass loss during disuse, and the underlying molecular mechanisms appear to involve myostatin inhibition and Atrogin-1 normalization independently of follistatin signaling.

Characteristics of Early Internal Laryngeal Muscle Atrophy After Recurrent Laryngeal Nerve Injuries in Rats

OBJECTIVES/HYPOTHESIS

The present study investigated the characteristics of early internal laryngeal muscle atrophy in recurrent laryngeal nerve injury (RLNI) rats.

STUDY DESIGN

To observe the characteristics of early internal laryngeal muscle atrophy post RLNI.

METHODS

Rats were divided into three groups: sham-operated control group (n = 20), recurrent laryngeal nerve transverse injury group (RLNTI, n = 50), and recurrent laryngeal nerve blunt contusion group (RLNBC, n = 50). Five weeks after RLNI, certain rats were sacrificed weekly, and their laryngeal tissues were harvested. The atrophic features of internal laryngeal muscles were detected using hematoxylin and eosin. NF-κB and MuRF-1 levels were tested using IHC.

RESULTS

The atrophic degree and fibrosis of thyroarytenoid, posterior cricoarytenoid, and lateral cricoarytenoid muscles were related to the type of RLNI. The average myofiber cross-sectional areas increased before an obvious decrease in the RLNTI and RLNBC groups. Muscle recovery occurred in the RLNBC group starting 4 weeks after RLNI, but only a weak trend was observed in the RLNTI group in the 5th week. During the muscle atrophy process, MuRF-1 and NF-κB were upregulated early and were maintained at a high level, which showed a trend similar to muscle atrophy. However, NF-κB expression was opposite to MuRF-1 expression and muscle atrophy when the muscles recovered.

CONCLUSION

The atrophy degree of internal laryngeal muscles was associated with the type of RLNI. The NF-κB/MuRF-1 signaling pathway was involved in internal laryngeal muscle atrophy after RLNI, which is different from skeletal muscle after denervation.

LEVEL OF EVIDENCE

NA Laryngoscope, 131:E1256-E1264, 2021.

The connection between the dynamic remodeling of the mitochondrial network and the regulation of muscle mass

The dynamic coordination of processes controlling the quality of the mitochondrial network is crucial to maintain the function of mitochondria in skeletal muscle. Changes of mitochondrial proteolytic system, dynamics (fusion/fission), and mitophagy induce pathways that affect muscle mass and performance. When muscle mass is lost, the risk of disease onset and premature death is dramatically increased. For instance, poor quality of muscles correlates with the onset progression of several age-related disorders such as diabetes, obesity, cancer, and aging sarcopenia. To date, there are no drug therapies to reverse muscle loss, and exercise remains the best approach to improve mitochondrial health and to slow atrophy in several diseases. This review will describe the principal mechanisms that control mitochondrial quality and the pathways that link mitochondrial dysfunction to muscle mass regulation.

Hyperbaric Oxygen Treatment Following Mid-Cervical Spinal Cord Injury Preserves Diaphragm Muscle Function

Oxidative damage to the diaphragm as a result of cervical spinal cord injury (SCI) promotes muscle atrophy and weakness. Respiratory insufficiency is the leading cause of morbidity and mortality in cervical spinal cord injury (SCI) patients, emphasizing the need for strategies to maintain diaphragm function. Hyperbaric oxygen (HBO) increases the amount of oxygen dissolved into the blood, elevating the delivery of oxygen to skeletal muscle and reactive oxygen species (ROS) generation. It is proposed that enhanced ROS production due to HBO treatment stimulates adaptations to diaphragm oxidative capacity, resulting in overall reductions in oxidative stress and inflammation. Therefore, we tested the hypothesis that exposure to HBO therapy acutely following SCI would reduce oxidative damage to the diaphragm muscle, preserving muscle fiber size and contractility. Our results demonstrated that lateral contusion injury at C3/4 results in a significant reduction in diaphragm muscle-specific force production and fiber cross-sectional area, which was associated with augmented mitochondrial hydrogen peroxide emission and a reduced mitochondrial respiratory control ratio. In contrast, rats that underwent SCI followed by HBO exposure consisting of 1 h of 100% oxygen at 3 atmospheres absolute (ATA) delivered for 10 consecutive days demonstrated an improvement in diaphragm-specific force production, and an attenuation of fiber atrophy, mitochondrial dysfunction and ROS production. These beneficial adaptations in the diaphragm were related to HBO-induced increases in antioxidant capacity and a reduction in atrogene expression. These findings suggest that HBO therapy may be an effective adjunctive therapy to promote respiratory health following cervical SCI.

Altering aspects of mitochondrial quality to improve musculoskeletal outcomes in disuse atrophy

Muscle atrophy is a significant moderator for disease prognosis; as such, interventions to mitigate disuse-induced muscle loss are imperative to improve clinical interventions. Mitochondrial deteriorations may underlie disuse-induced myopathies; therefore, improving mitochondrial quality may be an enticing therapeutic intervention. However, different mitochondria-based treatments may have divergent impacts on the prognosis of disuse atrophy. Therefore, the purpose of this study was to investigate different mitochondria-centered interventions during disuse atrophy in hindlimb unloaded male and female mice. Male and female mice overexpressing peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC-1α) or mitochondrially targeted catalase (MCAT) and their respective wild-type (WT) littermate controls were hindlimb unloaded for 7 days to induce disuse atrophy or allowed normal ambulatory activity (cage control; CON). After designated interventions, animals were euthanized, and tissues were collected for measures of mitochondrial quality control and protein turnover. Although PGC-1α overexpression mitigated ubiquitin-proteasome activation (MuRF1 and Atrogin mRNA content), this did not correspond to phenotypic protections from disuse-induced atrophy. Rather, PGC-1α mice appeared to have a greater reliance on autophagic protein breakdown compared with WT mice. In MCAT mice, females exhibited a mitigated response to disuse atrophy; however, this effect was not noted in males. Despite these phenotypic differences, there were no clear cellular signaling differences between MCAT hindlimb unloaded females and MCAT fully loaded females. PGC-1α overexpression does not protect against phenotypic alterations during disuse atrophy but appears to shift catabolic pathways moderating atrophy. However, increased mitochondrially targeted catalase activity appears to blunt disuse atrophy within highly oxidative muscles specifically in female mice.NEW & NOTEWORTHY We present data suggesting that mitochondria-based interventions may mitigate disuse atrophy. However, the efficacy of mitochondria-based interventions may vary depending on the specific target of the intervention and the sex of the organism. Females appear to be more responsive to increased mitochondrial catalase as a potential therapeutic for mitigating disuse atrophy.

A Review of the Pharmacological Properties of Psoralen

Psoralen is the principal bioactive component in the dried fruits of Cullen corylifolium (L.) Medik (syn. Psoralea corylifolia L), termed "Buguzhi" in traditional Chinese medicine (TCM). Recent studies have demonstrated that psoralen displays multiple bioactive properties, beneficial for the treatment of osteoporosis, tumors, viruses, bacteria, and inflammation. The present review focuses on the research evidence relating to the properties of psoralen gathered over recent years. Firstly, multiple studies have demonstrated that psoralen exerts strong anti-osteoporotic effects via regulation of osteoblast/osteoclast/chondrocyte differentiation or activation due to the participation in multiple molecular mechanisms of the wnt/β-catenin, bone morphogenetic protein (BMP), inositol-requiring enzyme 1 (IRE1)/apoptosis signaling kinase 1 (ASK1)/c-jun N-terminal kinase (JNK) and the Protein Kinase B(AKT)/activator protein-1 (AP-1) axis, and the expression of miR-488, peroxisome proliferators-activated receptor-gamma (PPARγ), and matrix metalloproteinases (MMPs). In addition, the antitumor properties of psoralen are associated with the induction of ER stress-related cell death via enhancement of PERK: Pancreatic Endoplasmic Reticulum Kinase (PERK)/activating transcription factor (ATF), 78kD glucose-regulated protein (GRP78)/C/EBP homologous protein (CHOP), and 94kD glucose-regulated protein (GRP94)/CHOP signaling, and inhibition of P-glycoprotein (P-gp) or ATPase that overcomes multidrug resistance. Furthermore, multiple articles have shown that the antibacterial, anti-inflammatory and neuroprotective effects of psoralen are a result of its interaction with viral polymerase (Pol), destroying the formation of biofilm, and regulating the activation of tumor necrosis factor alpha (TNF-α), transforming growth factor beta (TGF-β), interleukin 4/5/6/8/12/13 (IL-4/5/6/8/12/13), GATA-3, acetylcholinesterase (AChE), and the hypothalamic-pituitary-adrenal (HPA) axis. Finally, the toxic effects and mechanisms of action of psoralen have also been reviewed.

Docosahexaenoic Acid, a Potential Treatment for Sarcopenia, Modulates the Ubiquitin-Proteasome and the Autophagy-Lysosome Systems

One of the characteristic features of aging is the progressive loss of muscle mass, a nosological syndrome called sarcopenia. It is also a pathologic risk factor for many clinically adverse outcomes in older adults. Therefore, delaying the loss of muscle mass, through either boosting muscle protein synthesis or slowing down muscle protein degradation using nutritional supplements could be a compelling strategy to address the needs of the world’s aging population. Here, we review the recently identified properties of docosahexaenoic acid (DHA). It was shown to delay muscle wasting by stimulating intermediate oxidative stress and inhibiting proteasomal degradation of muscle proteins. Both the ubiquitin–proteasome and the autophagy–lysosome systems are modulated by DHA. Collectively, growing evidence indicates that DHA is a potent pharmacological agent that could improve muscle homeostasis. Better understanding of cellular proteolytic systems associated with sarcopenia will allow us to identify novel therapeutic interventions, such as omega-3 polyunsaturated fatty acids, to treat this disease.

Modified expression of vitamin D receptor and CYP27B1 in denervation-induced muscle atrophy

The vitamin D pathway is related to the mass and function of skeletal muscles. Several studies have demonstrated the role of vitamin D receptor (VDR) and CYP27B1 in skeletal muscles, suggesting that these proteins may regulate skeletal muscles and their function. However, it remains unclear whether the expression of VDR and CYP27B1 is modified in skeletal muscle atrophy. We investigated whether denervation-induced muscle atrophy is associated with altered expression of VDR and CYP27B1 in murine skeletal muscles. Skeletal muscles were excised from C57BL/6J mice, 3 and 7 days after the mice underwent denervation surgery. Denervation induced muscle atrophy and enhanced the expression of MuRF1 and Atrogin-1 in the gastrocnemius and soleus. The protein expression of VDR was increased in the denervated gastrocnemius; in contrast, denervation decreased the protein expression of CYP27B1 in the gastrocnemius and soleus. These results suggest that denervation-induced muscle atrophy is associated with changes in the expression of vitamin D-related proteins in murine skeletal muscles.

Supplementary Oral Anamorelin Mitigates Anorexia and Skeletal Muscle Atrophy Induced by Gemcitabine Plus Cisplatin Systemic Chemotherapy in a Mouse Model

Chemotherapy-induced adverse effects can reduce the relative dose intensity and quality of life. In this study, we investigated the potential benefit of supplementary anamorelin and 5-aminolevulinic acid (5-ALA) as preventive interventions against a gemcitabine and cisplatin (GC) combination chemotherapy-induced adverse effects in a mouse model. Non-cancer-bearing C3H mice were randomly allocated as follows and treated for 2 weeks—(1) non-treated control, (2) oral anamorelin alone, (3) oral 5-ALA alone, (4) gemcitabine and cisplatin (GC) chemotherapy, (5) GC plus anamorelin, and (6) GC plus 5-ALA. GC chemotherapy significantly decreased body weight, food intake, skeletal muscle mass and induced severe gastric mucositis, which resulted in decreased ghrelin production and blood ghrelin level. The supplementation of oral anamorelin to GC chemotherapy successfully mitigated decrease of food intake during the treatment period and body weight loss at day 8. In addition, analysis of the resected muscles and stomach revealed that anamorelin suppressed chemotherapy-induced skeletal muscle atrophy by mediating the downregulation of forkhead box protein O-1 (FOXO1)/atrogin-1 signaling and gastric damage. Our findings suggest the preventive effect of anamorelin against GC combination chemotherapy, which was selected for patients with some types of advanced malignancies in clinical practice.

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.

MEF2 impairment underlies skeletal muscle atrophy in polyglutamine disease

Polyglutamine (polyQ) tract expansion leads to proteotoxic misfolding and drives a family of nine diseases. We study spinal and bulbar muscular atrophy (SBMA), a progressive degenerative disorder of the neuromuscular system caused by the polyQ androgen receptor (AR). Using a knock-in mouse model of SBMA, AR113Q mice, we show that E3 ubiquitin ligases which are a hallmark of the canonical muscle atrophy machinery are not induced in AR113Q muscle. Similarly, we find no evidence to suggest dysfunction of signaling pathways that trigger muscle hypertrophy or impairment of the muscle stem cell niche. Instead, we find that skeletal muscle atrophy is characterized by diminished function of the transcriptional regulator Myocyte Enhancer Factor 2 (MEF2), a regulator of myofiber homeostasis. Decreased expression of MEF2 target genes is age- and glutamine tract length-dependent, occurs due to polyQ AR proteotoxicity, and is associated with sequestration of MEF2 into intranuclear inclusions in muscle. Skeletal muscle from R6/2 mice, a model of Huntington disease which develops progressive atrophy, also sequesters MEF2 into inclusions and displays age-dependent loss of MEF2 target genes. Similarly, SBMA patient muscle shows loss of MEF2 target gene expression, and restoring MEF2 activity in AR113Q muscle rescues fiber size and MEF2-regulated gene expression. This work establishes MEF2 impairment as a novel mechanism of skeletal muscle atrophy downstream of toxic polyglutamine proteins and as a therapeutic target for muscle atrophy in these disorders.

4-Hydroxyderricin and xanthoangelol isolated from Angelica keiskei prevent dexamethasone-induced muscle loss

Since a decrease in muscle mass leads to an increased risk of mortality, the prevention of muscle wasting contributes to maintaining the quality of life. Recently, we reported that glabridin, a prenylated flavonoid in licorice, prevents dexamethasone-induced muscle loss. In this study, we focused on the other prenylated chalcones 4-hydroxyderricin and xanthoangelol in Ashitaba (Angelica keiskei) and investigated their prevention effect on dexamethasone-induced muscle loss. It was found that 4-hydroxyderricin and xanthoangelol significantly prevented dexamethasone-induced protein degradation in C2C12 myotubes by suppressing the expression of ubiquitin ligases, Cbl-b and MuRF-1. These prenylated chalcones acted as the antagonists of the glucocorticoid receptor and inhibited the binding of dexamethasone to this receptor and its subsequent nuclear translocation. In addition, the chalcones suppressed the phosphorylation of p38 and FoxO3a as the upstream factors for ubiquitin ligases. Dexamethasone-induced protein degradation and upregulation of Cbl-b were attenuated by the knockdown of the glucocorticoid receptor but not by the knockdown of p38. In male C57BL/6J mice, the Ashitaba extract, containing 4-hydroxyderricin and xanthoangelol, suppressed dexamethasone-induced muscle mass wasting accompanied by a decrease in the expression of ubiquitin ligases by inhibiting the nuclear translocation of the glucocorticoid receptor and phosphorylation of FoxO3a. In conclusion, 4-hydroxyderricin and xanthoangelol are effective compounds to inhibit steroid-induced muscle loss.

Transcriptional changes in muscle of hibernating arctic ground squirrels (Urocitellus parryii): implications for attenuation of disuse muscle atrophy

Physical inactivity generates muscle atrophy in most mammalian species. In contrast, hibernating mammals demonstrate limited muscle loss over the prolonged intervals of immobility during winter, which suggests that they have adaptive mechanisms to reduce disuse muscle atrophy. To identify transcriptional programs that underlie molecular mechanisms attenuating muscle loss, we conducted a large-scale gene expression profiling in quadriceps muscle of arctic ground squirrels, comparing hibernating (late in a torpor and during torpor re-entry after arousal) and summer active animals using next generation sequencing of the transcriptome. Gene set enrichment analysis showed a coordinated up-regulation of genes involved in all stages of protein biosynthesis and ribosome biogenesis during both stages of hibernation that suggests induction of translation during interbout arousals. Elevated proportion of down-regulated genes involved in apoptosis, NFKB signaling as well as significant under expression of atrogenes, upstream regulators (FOXO1, FOXO3, NFKB1A), key components of the ubiquitin proteasome pathway (FBXO32, TRIM63, CBLB), and overexpression of PPARGC1B inhibiting proteolysis imply suppression of protein degradation in muscle during arousals. The induction of protein biosynthesis and decrease in protein catabolism likely contribute to the attenuation of disuse muscle atrophy through prolonged periods of immobility of hibernation.

Mice carrying an analogous heterozygous dynamin 2 K562E mutation that causes neuropathy in humans develop predominant characteristics of a primary myopathy

Some mutations affecting dynamin 2 (DNM2) can cause dominantly inherited Charcot-Marie-Tooth (CMT) neuropathy. Here, we describe the analysis of mice carrying the DNM2 K562E mutation which has been associated with dominant-intermediate CMT type B (CMTDIB). Contrary to our expectations, heterozygous DNM2 K562E mutant mice did not develop definitive signs of an axonal or demyelinating neuropathy. Rather, we found a primary myopathy-like phenotype in these mice. A likely interpretation of these results is that the lack of a neuropathy in this mouse model has allowed the unmasking of a primary myopathy due to the DNM2 K562E mutation which might be overshadowed by the neuropathy in humans. Consequently, we hypothesize that a primary myopathy may also contribute to the disease mechanism in some CMTDIB patients. We propose that these findings should be considered in the evaluation of patients, the determination of the underlying disease processes and the development of tailored potential treatment strategies.

CRISPR/Cas9-Mediated miR-29b Editing as a Treatment of Different Types of Muscle Atrophy in Mice

Muscle atrophy is the loss of skeletal muscle mass and strength in response to diverse catabolic stimuli. At present, no effective treatments except exercise have been shown to reduce muscle atrophy clinically. Here, we report that CRISPR/Cas9-mediated genome editing through local injection into gastrocnemius muscles or tibialis anterior muscle efficiently targets the biogenesis processing sites in pre-miR-29b. In vivo, this CRISPR-based treatment prevented the muscle atrophy induced by angiotensin II (AngII), immobilization, and denervation via activation of the AKT-FOXO3A-mTOR signaling pathway and protected against AngII-induced myocyte apoptosis in mice, leading to significantly increased exercise capacity. Our work establishes CRISPR/Cas9-based gene targeting on miRNA as a potential durable therapy for the treatment of muscle atrophy and expands the strategies available interrogating miRNA function in vivo.

Loss of high-temperature requirement protein A2 protease activity induces mitonuclear imbalance via differential regulation of mitochondrial biogenesis in sarcopenia

Cellular homeostasis requires tight coordination between nucleus and mitochondria, organelles that each possesses their own genomes. Disrupted mitonuclear communication has been found to be implicated in many aging processes. However, little is known about mitonuclear signaling regulator in sarcopenia which is a major contributor to the risk of poor health-related quality of life, disability, and premature death in older people. High-temperature requirement protein A2 (HtrA2/Omi) is a mitochondrial protease and plays an important role in mitochondrial proteostasis. HtrA2^mnd2(-/-) mice harboring protease-deficient HtrA2/Omi Ser276Cys missense mutants exhibit premature aging phenotype. Additionally, HtrA2/Omi has been established as a signaling regulator in nervous system and tumors. We therefore asked whether HtrA2/Omi participates in mitonuclear signaling regulation in muscle degeneration. Using motor functional, histological, and molecular biological methods, we characterized the phenotype of HtrA2^mnd2(-/-) muscle. Furthermore, we isolated the gastrocnemius muscle of HtrA2^mnd2(-/-) mice and determined expression of genes in mitochondrial unfolded protein response (UPR^mt ), mitohormesis, electron transport chain (ETC), and mitochondrial biogenesis. Here, we showed that HtrA2/Omi protease deficiency induced denervation-independent skeletal muscle degeneration with sarcopenia phenotypes. Despite mitochondrial hypofunction, upregulation of UPR^mt and mitohormesis-related genes and elevated total reactive oxygen species (ROS) production were not observed in HtrA2^mnd2(-/-) mice, contrary to previous assumptions that loss of protease activity of HtrA2/Omi would lead to mitochondrial dysfunction as a result of proteostasis disturbance and ROS burst. Instead, we showed that HtrA2/Omi protease deficiency results in different changes between the expression of nuclear DNA- and mitochondrial DNA-encoded ETC subunits, which is in consistent with their transcription factors, nuclear respiratory factors 1 and 2, and coactivator peroxisome proliferator-activated receptor γ coactivator 1α. These results reveal that loss of HtrA2/Omi protease activity induces mitonuclear imbalance via differential regulation of mitochondrial biogenesis in sarcopenia. The novel mechanistic insights may be of importance in developing new therapeutic strategies for sarcopenia.

Muscle Disuse Atrophy Caused by Discord of Intracellular Signaling

Significance: Regular contractile activity plays a critical role in maintaining skeletal muscle morphological integrity and physiological function. If the muscle is forced to stop contraction, such as during limb immobilization (IM), the IGF/Akt/mTOR signaling pathway that normally stimulates protein synthesis and inhibits proteolysis will be suppressed, whereas the FoxO-controlled catabolic pathways such as ubiquitin-proteolysis and autophagy/mitophagy will be activated and dominate, resulting in muscle fiber atrophy. Recent Advances: Mitochondria occupy a central position in the regulation of both protein synthesis and degradation through several redox-sensitive pathways, including peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC-1α), mitochondrial fusion and fission proteins, mitophagy, and sirtuins. Prolonged IM downregulates PGC-1α due to AMPK (5'-AMP-activated protein kinase) and FoxO activation, thus decreasing mitochondrial biogenesis and causing oxidative damage. Decrease of mitochondrial inner membrane potential and increase of mitochondrial fission can trigger cascades of mitophagy leading to loss of mitochondrial homeostasis (mitostasis), inflammation, and apoptosis. The phenotypic outcomes of these disorders are compromised muscle function and fiber atrophy. Critical Issues: Given the molecular mechanism of the pathogenesis, it is imperative that the integrity of intracellular signaling be restored to prevent the deterioration. So far, overexpression of PGC-1α via transgene and in vivo DNA transfection has been found to be effective in ameliorating mitostasis and reduces IM-induced muscle atrophy. Nutritional supplementation of select amino acids and phytochemicals also provides mechanistic and practical insights into the prevention of muscle disuse atrophy. Future Directions: In light of the importance of mitochondria in regulating the various critical signaling pathways, future work should focus on exploring new epigenetic strategies to restore mitostasis and redox balance.

Cancer Cachexia Induces Preferential Skeletal Muscle Myosin Loss When Combined With Denervation

Patients with cancer cachexia (CCX) suffer from muscle wasting, which is often but not always accompanied by selective loss of myosin. Here we examined the effects of CCX on muscle mass and myosin heavy chain (MyHC) expression in denervated (DEN) muscles, especially focusing on the protein synthesis and degradation pathways. Male CD2F1 mice were randomly divided into control (CNT) and CCX groups and their left sciatic nerve was transected. CCX was induced by an intraperitoneal injection of colon 26 cells. After 14 days, the serum concentration of IL-6 and corticosteroid was higher in CCX mice than in CNT mice. The combination of CCX with DEN (CCX + DEN) resulted in a marked reduction of the gastrocnemius muscle weight (−69%) that was significantly lower than DEN (−53%) or CCX (−36%) alone. CCX had no effect on MyHC content, but it elicited a preferential MyHC loss when combined with DEN. The expression levels of autophagy markers cathepsin D and LC3BII/I ratio were markedly higher in the CCX + DEN group than in the CNT + DEN and the CCX groups. Paradoxically, there was an increase in protein synthesis rate and phosphorylation levels of p70S6K and rpS6, markers of mTORC1 signaling, in the CNT + DEN group, and these molecular alterations were inhibited in the CCX + DEN group. Our data indicate that CCX aggravates muscle atrophy in DEN muscles by inducing seletive loss of myosin, which involves inactivity dependent mechanisms that is likely to be a consequence of increased autophagy-mediated protein breakdown coupled with impaired protein synthesis.

Detecting the Effects of the Glucocorticoid Dexamethasone on Primary Human Skeletal Muscle Cells-Differences to the Murine Cell Line

Skeletal muscle atrophy is characterized by a decrease in muscle fiber size as a result of a decreased protein synthesis, which leads to degradation of contractile muscle fibers. It can occur after denervation and immobilization, and glucocorticoids (GCs) may also increase protein breakdown contributing to the loss of muscle mass and myofibrillar proteins. GCs are already used in vitro to induce atrophic conditions, but until now no studies with primary human skeletal muscle existed. Therefore, this study deals with the effects of the GC dexamethasone (dex) on primary human myoblasts and myotubes. After incubation with 1, 10, and 100 µM dex for 48 and 72 h, gene and protein expression analyses were performed by qPCR and Western blot. Foxo, MuRF-1, and MAFbx were significantly upregulated by dex, and there was increased gene expression of myogenic markers. However, prolonged incubation periods demonstrated no Myosin protein degradation, but an increase of MuRF-1 expression. In conclusion, applying dex did not only differently affect primary human myoblasts and myotubes, as differences were also observed when compared to murine cells. Based on our findings, studies using cell lines or animal cells should be interpreted with caution as signaling transduction and functional behavior might differ in diverse species.

Leucine Improved Growth Performance, Muscle Growth, and Muscle Protein Deposition Through AKT/TOR and AKT/FOXO3a Signaling Pathways in Hybrid Catfish Pelteobagrus vachelli × Leiocassis longirostris

(1) Background: l-leucine (Leu) plays a positive role in regulating protein turnover in skeletal muscle in mammal. However, the molecular mechanism for the effects of Leu on muscle growth and protein deposition is not clearly demonstrated in fish. This study investigated the effects of dietary Leu on growth performance and muscle growth, protein synthesis, and degradation-related signaling pathways of hybrid catfish (Pelteobagrus vachelli♀ × Leiocassis longirostris♂). (2) Methods: A total of 630 hybrid catfish (23.19 ± 0.20 g) were fed 6 different experimental diets containing graded levels of Leu at 10.0 (control), 15.0, 20.0, 25.0, 30.0, 35.0, and 40.0 g Leu kg^-1 for 8 weeks. (3) Results: Results showed that dietary Leu increased percent weight gain (PWG), specific growth rate (SGR), FI (feed intake), feed efficiency (FE), protein efficiency ratio (PER), muscle fibers diameter, and muscle fibers density; up-regulated insulin-like growth factor I (IGF-I), insulin-like growth factor I receptor (IGF-IR), proliferating cell nuclear antigen (PCNA), myogenic regulation factors (MyoD, Myf5, MyoG, and Mrf4), and MyHC mRNA levels; increased muscle protein synthesis via regulating the AKT/TOR signaling pathway; and attenuated protein degradation via regulating the AKT/FOXO3a signaling pathway. (4) Conclusions: These results suggest that Leu has potential role to improve muscle growth and protein deposition in fish, which might be due to the regulation of IGF mRNA expression, muscle growth related gene, and protein synthesis and degradation-related signaling pathways. Based on the broken-line model, the Leu requirement of hybrid catfish (23.19-54.55 g) for PWG was estimated to be 28.10 g kg^-1 of the diet (73.04 g kg^-1 of dietary protein). These results will improve our understanding of the mechanisms responsible for muscle growth and protein deposition effects of Leu in fish.

Skeletal Muscle Atrophy: Discovery of Mechanisms and Potential Therapies

Skeletal muscle atrophy proceeds through a complex molecular signaling network that is just beginning to be understood. Here, we discuss examples of recently identified molecular mechanisms of muscle atrophy and how they highlight an immense need and opportunity for focused biochemical investigations and further unbiased discovery work.

Nicotinamide protects against skeletal muscle atrophy in streptozotocin-induced diabetic mice

Context: Skeletal muscle atrophy is a complication of diabetes, partially induced by nicotinamide adenine dinucleotide (NAD+) deficiency. Objective: This study investigates the potential of nicotinamide (NAM) supplementation, a precursor of NAD+, against muscle atrophy. Methods: Mice were separated into normal control group, normal control with NAM administration group, diabetic group, and diabetic mice with NAM administration group. Basic characteristics, muscle weight, maximal grip strength, and myofibers cross-sectional area were analysed. Markers reflecting muscle atrophy and hypertrophy, and transforming growth factor β1/Smad2 (TGF-β1/Smad2) pathway were examined. Results: NAM did not influence body weight and blood glucose. In diabetic mice, NAM increased NAD+ level, rescued muscle weight and strength loss, and increased myofibers cross-sectional area. NAM inhibited MuRF1 and Atrogin1, while elevated phosphorylation of Akt. Overactivation of TGF-β1/Smad2 pathway was repressed by NAM. Conclusion: NAM ameliorated diabetic muscle atrophy by rebalancing protein anabolism and catabolism, probably through de-activation of TGF-β1/Smad2 signaling.

Ectopic TWEAKR expression in type I fiber of stroke-prone spontaneously hypertensive rats is related to slow muscle-specific hypotrophy

AIMS

Stroke-prone spontaneously hypertensive rats (SHRSP) show significantly lower body weight than normotensive Wistar-Kyoto rats (WKY). Our hypotheses are as follows: weight loss of the skeletal muscle is related to hypertension-related diseases, and muscle hypotrophy is useful as a therapeutic target for hypertension and hypertension-related diseases. In this study, we aimed to investigate the pathophysiological characteristics of muscle hypotrophy in SHRSP to determine the therapeutic target molecule(s).

MAIN METHODS

The difference in skeletal muscles in the lower leg between WKY and SHRSP was evaluated mainly through weight/tibial length, histological, gene expression, and protein expression analyses.

KEY FINDINGS

SHRSP had a significantly lower weight/tibial length in soleus and gastrocnemius, but not in plantaris and tibialis anterior, indicating that muscles consisting of a relatively high amount of slow muscle fiber were affected. This result was confirmed by the histological analysis of soleus, showing that type I fiber mainly decreased the fiber size. Microarray and protein expression analyses showed that the muscle-specific ubiquitin ligase, muscle RING finger 1 (MuRF1), but not atrogin-1, was highly expressed in soleus, but not in plantaris, in SHRSP. TNF-like weak inducer of apoptosis receptor (TWEAKR) was predicted as a MuRF1 up-regulator by Ingenuity Pathway Analysis and immunostained only in type II fiber in WKY but in both type I and II fibers in SHRSP.

SIGNIFICANCE

TWEAKR is a type II-specific receptor in the skeletal muscle. Ectopic TWEAKR expression in type I fiber of SHRSP is most likely involved in slow muscle-specific hypotrophy through MuRF1 overexpression.

Microarray Analysis of Gene Expression Provides New Insights Into Denervation-Induced Skeletal Muscle Atrophy

Denervation induces skeletal muscle atrophy, accompanied by complex biochemical and physiological changes, with potentially devastating outcomes even an increased mortality. Currently, however, there remains a paucity of effective strategies to treat skeletal muscle atrophy. Therefore, it is required to understand the molecular mechanisms of skeletal muscle atrophy and formulate new treatment strategies. In this study, we investigated the transcriptional profile of denervated skeletal muscle after peripheral nerve injury in rats. The cDNA microarray analysis showed that a huge number of genes in tibialis anterior (TA) muscles were differentially expressed at different times after sciatic nerve transection. Notably, the 24 h of denervation might be a critical time point for triggering TA muscle atrophy. According to the data from self-organizing map (SOM), Pearson correlation heatmap, principal component analysis (PCA), and hierarchical clustering analysis, three nodal transitions in gene expression profile of the denervated TA muscle might partition the period of 0.25 h–28 days post nerve injury into four distinct transcriptional phases. Moreover, the four transcriptional phases were designated as “oxidative stress stage”, “inflammation stage”, “atrophy stage” and “atrophic fibrosis stage”, respectively, which was concluded from Kyoto Encyclopedia of Genes and Genomes (KEGG) and Gene ontology (GO) analyses at each transcriptional phase. Importantly, the differentially expressed genes at 24 h post sciatic nerve transection seemed to be mainly involved in inflammation, which might be a critical process in denervation-induced muscle atrophy. Overall, our study would contribute to the understanding of molecular aspects of denervation-induced muscle atrophy, and may also provide a new insight into the time window for targeted therapy.

One week, but not 12 hours, of cast immobilization alters promotor DNA methylation patterns in the nNOS gene in mouse skeletal muscle

KEY POINTS

DNA methylation may play an important role in regulating gene expression in skeletal muscle to adapt to physical activity and inactivity. Neuronal nitric oxide synthase (nNOS) in skeletal muscle is a key regulator of skeletal muscle mass; however, it is unclear whether nNOS expression is regulated by DNA methylation. We found that 1 week of cast immobilization increased nNOS DNA methylation levels and downregulated nNOS gene expression in atrophic slow-twitch soleus muscle from the mouse leg. These changes were not detected in non-atrophic fast-twitch extensor digitorum longus muscle. Twelve hours of cast immobilization decreased nNOS gene expression, whereas nNOS DNA methylation levels were unchanged, suggesting that downregulation of nNOS gene expression by short-term muscle inactivity is independent of the DNA methylation pattern. These findings contribute to a better understanding of the maintenance of skeletal muscle mass and prevention of muscle atrophy by epigenetic mechanisms via the nNOS/NO pathway.

ABSTRACT

DNA methylation is a mechanism that controls gene expression in skeletal muscle under various environmental stimuli, such as physical activity and inactivity. Neuronal nitric oxide synthase (nNOS) regulates muscle atrophy in skeletal muscle. However, the mechanisms regulating nNOS expression in atrophic muscle remain unclear. We hypothesized that nNOS expression in atrophic muscle is regulated by DNA methylation of the nNOS promotor in soleus (Sol; slow-twitch fibre dominant) and extensor digitorum longus (EDL; fast-twitch fibre dominant) muscles. One week of cast immobilization induced significant muscle atrophy in Sol but not in EDL. We showed that 1 week of cast immobilization increased nNOS DNA methylation levels in Sol, although only a minor change was detected in EDL. Consistent with the increased DNA methylation levels in atrophic Sol, the gene expression levels of total nNOS and nNOSµ (i.e. the major splicing variant of nNOS in skeletal muscle) decreased. The abundance of the nNOS protein and cell membrane (especially type IIa fibre) immunoreactivity also decreased in atrophic Sol. These changes were not observed in EDL after 1 week of cast immobilization. Furthermore, despite the lack of significant atrophy, 12 h of cast immobilization decreased gene expression levels of total nNOS and nNOSµ in Sol. However, no association was detected between nNOS DNA methylation and gene expression. The expression of the nNOSβ gene, another splicing variant of nNOS, in EDL was unchanged by cast immobilization, whereas its expression was not detected in Sol. We concluded that chronic adaptation of nNOS gene expression in cast immobilized muscle may involve nNOS DNA methylation.

Longitudinal RNA-Seq analysis of acute and chronic neurogenic skeletal muscle atrophy

Skeletal muscle is a highly adaptable tissue capable of changes in size, contractility, and metabolism according to functional demands. Atrophy is a decline in mass and strength caused by pathologic loss of myofibrillar proteins, and can result from disuse, aging, or denervation caused by injury or peripheral nerve disorders. We provide a high-quality longitudinal RNA-Seq dataset of skeletal muscle from a cohort of adult C57BL/6J male mice subjected to tibial nerve denervation for 0 (baseline), 1, 3, 7, 14, 30, or 90 days. Using an unbiased genomics approach to identify gene expression changes across the entire longitudinal course of muscle atrophy affords the opportunity to (1) establish acute responses to denervation, (2) detect pathways that mediate rapid loss of muscle mass within the first week after denervation, and (3) capture the molecular phenotype of chronically atrophied muscle at a stage when it is largely resistant to recovery.