What's So Special about FGF19-Unique Effects Reported on Skeletal Muscle Mass and Function. Glass David J Cell metabolism In a recent study published in Nature Medicine, Benoit et al. (2017) reported unique effects of FGF19 on mouse skeletal muscle: FGF19 induced skeletal muscle hypertrophy and blocked muscle atrophy, acting via FGF receptors and ßKlotho, while a related FGF21 hormone was ineffective. 10.1016/j.cmet.2017.07.017
    JNK regulates muscle remodeling via myostatin/SMAD inhibition. Lessard Sarah J,MacDonald Tara L,Pathak Prerana,Han Myoung Sook,Coffey Vernon G,Edge Johann,Rivas Donato A,Hirshman Michael F,Davis Roger J,Goodyear Laurie J Nature communications Skeletal muscle has a remarkable plasticity to adapt and remodel in response to environmental cues, such as physical exercise. Endurance exercise stimulates improvements in muscle oxidative capacity, while resistance exercise induces muscle growth. Here we show that the c-Jun N-terminal kinase (JNK) is a molecular switch that when active, stimulates muscle fibers to grow, resulting in increased muscle mass. Conversely, when muscle JNK activation is suppressed, an alternative remodeling program is initiated, resulting in smaller, more oxidative muscle fibers, and enhanced aerobic fitness. When muscle is exposed to mechanical stress, JNK initiates muscle growth via phosphorylation of the transcription factor, SMAD2, at specific linker region residues leading to inhibition of the growth suppressor, myostatin. In human skeletal muscle, this JNK/SMAD signaling axis is activated by resistance exercise, but not endurance exercise. We conclude that JNK acts as a key mediator of muscle remodeling during exercise via regulation of myostatin/SMAD signaling. 10.1038/s41467-018-05439-3
    The gut microbiota influences skeletal muscle mass and function in mice. Lahiri Shawon,Kim Hyejin,Garcia-Perez Isabel,Reza Musarrat Maisha,Martin Katherine A,Kundu Parag,Cox Laura M,Selkrig Joel,Posma Joram M,Zhang Hongbo,Padmanabhan Parasuraman,Moret Catherine,Gulyás Balázs,Blaser Martin J,Auwerx Johan,Holmes Elaine,Nicholson Jeremy,Wahli Walter,Pettersson Sven Science translational medicine The functional interactions between the gut microbiota and the host are important for host physiology, homeostasis, and sustained health. We compared the skeletal muscle of germ-free mice that lacked a gut microbiota to the skeletal muscle of pathogen-free mice that had a gut microbiota. Compared to pathogen-free mouse skeletal muscle, germ-free mouse skeletal muscle showed atrophy, decreased expression of insulin-like growth factor 1, and reduced transcription of genes associated with skeletal muscle growth and mitochondrial function. Nuclear magnetic resonance spectrometry analysis of skeletal muscle, liver, and serum from germ-free mice revealed multiple changes in the amounts of amino acids, including glycine and alanine, compared to pathogen-free mice. Germ-free mice also showed reduced serum choline, the precursor of acetylcholine, the key neurotransmitter that signals between muscle and nerve at neuromuscular junctions. Reduced expression of genes encoding Rapsyn and Lrp4, two proteins important for neuromuscular junction assembly and function, was also observed in skeletal muscle from germ-free mice compared to pathogen-free mice. Transplanting the gut microbiota from pathogen-free mice into germ-free mice resulted in an increase in skeletal muscle mass, a reduction in muscle atrophy markers, improved oxidative metabolic capacity of the muscle, and elevated expression of the neuromuscular junction assembly genes and Treating germ-free mice with short-chain fatty acids (microbial metabolites) partly reversed skeletal muscle impairments. Our results suggest a role for the gut microbiota in regulating skeletal muscle mass and function in mice. 10.1126/scitranslmed.aan5662
    Stochastic and genetic factors influence tissue-specific decline in ageing C. elegans. Herndon Laura A,Schmeissner Peter J,Dudaronek Justyna M,Brown Paula A,Listner Kristin M,Sakano Yuko,Paupard Marie C,Hall David H,Driscoll Monica Nature The nematode Caenorhabditis elegans is an important model for studying the genetics of ageing, with over 50 life-extension mutations known so far. However, little is known about the pathobiology of ageing in this species, limiting attempts to connect genotype with senescent phenotype. Using ultrastructural analysis and visualization of specific cell types with green fluorescent protein, we examined cell integrity in different tissues as the animal ages. We report remarkable preservation of the nervous system, even in advanced old age, in contrast to a gradual, progressive deterioration of muscle, resembling human sarcopenia. The age-1(hx546) mutation, which extends lifespan by 60-100%, delayed some, but not all, cellular biomarkers of ageing. Strikingly, we found strong evidence that stochastic as well as genetic factors are significant in C. elegans ageing, with extensive variability both among same-age animals and between cells of the same type within individuals. 10.1038/nature01135
    Role of beta-adrenoceptor signaling in skeletal muscle: implications for muscle wasting and disease. Lynch Gordon S,Ryall James G Physiological reviews The importance of beta-adrenergic signaling in the heart has been well documented, but it is only more recently that we have begun to understand the importance of this signaling pathway in skeletal muscle. There is considerable evidence regarding the stimulation of the beta-adrenergic system with beta-adrenoceptor agonists (beta-agonists). Although traditionally used for treating bronchospasm, it became apparent that some beta-agonists could increase skeletal muscle mass and decrease body fat. These so-called "repartitioning effects" proved desirable for the livestock industry trying to improve feed efficiency and meat quality. Studying beta-agonist effects on skeletal muscle has identified potential therapeutic applications for muscle wasting conditions such as sarcopenia, cancer cachexia, denervation, and neuromuscular diseases, aiming to attenuate (or potentially reverse) the muscle wasting and associated muscle weakness, and to enhance muscle growth and repair after injury. Some undesirable cardiovascular side effects of beta-agonists have so far limited their therapeutic potential. This review describes the physiological significance of beta-adrenergic signaling in skeletal muscle and examines the effects of beta-agonists on skeletal muscle structure and function. In addition, we examine the proposed beneficial effects of beta-agonist administration on skeletal muscle along with some of the less desirable cardiovascular effects. Understanding beta-adrenergic signaling in skeletal muscle is important for identifying new therapeutic targets and identifying novel approaches to attenuate the muscle wasting concomitant with many diseases. 10.1152/physrev.00028.2007
    Mitochondrial biogenesis in exercise and in ageing. Viña Jose,Gomez-Cabrera Mari Carmen,Borras Consuelo,Froio Teresa,Sanchis-Gomar Fabian,Martinez-Bello Vladimir E,Pallardo Federico V Advanced drug delivery reviews Mitochondrial biogenesis is critical for the normal function of cells. It is well known that mitochondria are produced and eventually after normal functioning they are degraded. Thus, the actual level of mitochondria in cells is dependent both on the synthesis and the degradation. Ever since the proposal of the mitochondrial theory of ageing by Jaime Miquel in the 70's, it was appreciated that mitochondria, which are both a target and a source of radicals in cells, are most important organelles to understand ageing. Thus, a common feature between cell physiology of ageing and exercise is that in both situations mitochondria are critical for normal cell functioning. Mitochondrial synthesis is stimulated by the PGC-1alpha-NRF1-TFAM pathway. PGC-1alpha is the first stimulator of mitochondrial biogenesis. NRF1 is an intermediate transcription factor which stimulates the synthesis of TFAM which is a final effector activating the duplication of mitochondrial DNA molecules. This pathway is impaired in ageing. On the contrary, exercise, particularly aerobic exercise, activates mitochondriogenesis in the young animal but its effects on mitochondrial biogenesis in the old animal are doubtful. In this chapter we consider the interrelationship between mitochondrial biogenesis stimulated by exercise and the possible impairment of this pathway in ageing leading to mitochondrial deficiency and eventually muscle sarcopenia. 10.1016/j.addr.2009.06.006
    Incorporating biomarkers into cancer and aging research. Hubbard Joleen M,Cohen Harvey J,Muss Hyman B Journal of clinical oncology : official journal of the American Society of Clinical Oncology The challenge in treating the older adult with cancer is accurately accounting for and adapting management to the heterogeneity in health status of the individual patient. Many oncologists recognize that chronological age alone should not be the determinant when deciding on a treatment regimen. Easily measurable markers that provide an assessment of functional age would be ideal to assess frailty, which may predispose the patient to complications from cancer treatment, including increased toxicity, functional decline, decreased quality of life, and poorer survival. Several categories of potential markers, including chronic inflammatory markers, markers of cellular senescence, and imaging to assess muscle mass to detect sarcopenia, may provide insight into the likelihood of treatment-related complications. This article discusses candidate markers and strategies to evaluate these markers in cancer treatment trials, with the aim of developing a method to assess risk of oncologic outcomes and guide management decisions for both the physician and patient. 10.1200/JCO.2014.55.4261
    MRF4 negatively regulates adult skeletal muscle growth by repressing MEF2 activity. Moretti Irene,Ciciliot Stefano,Dyar Kenneth A,Abraham Reimar,Murgia Marta,Agatea Lisa,Akimoto Takayuki,Bicciato Silvio,Forcato Mattia,Pierre Philippe,Uhlenhaut N Henriette,Rigby Peter W J,Carvajal Jaime J,Blaauw Bert,Calabria Elisa,Schiaffino Stefano Nature communications The myogenic regulatory factor MRF4 is highly expressed in adult skeletal muscle but its function is unknown. Here we show that Mrf4 knockdown in adult muscle induces hypertrophy and prevents denervation-induced atrophy. This effect is accompanied by increased protein synthesis and widespread activation of muscle-specific genes, many of which are targets of MEF2 transcription factors. MEF2-dependent genes represent the top-ranking gene set enriched after Mrf4 RNAi and a MEF2 reporter is inhibited by co-transfected MRF4 and activated by Mrf4 RNAi. The Mrf4 RNAi-dependent increase in fibre size is prevented by dominant negative MEF2, while constitutively active MEF2 is able to induce myofibre hypertrophy. The nuclear localization of the MEF2 corepressor HDAC4 is impaired by Mrf4 knockdown, suggesting that MRF4 acts by stabilizing a repressor complex that controls MEF2 activity. These findings open new perspectives in the search for therapeutic targets to prevent muscle wasting, in particular sarcopenia and cachexia. 10.1038/ncomms12397
    DRP1-mediated mitochondrial shape controls calcium homeostasis and muscle mass. Favaro Giulia,Romanello Vanina,Varanita Tatiana,Andrea Desbats Maria,Morbidoni Valeria,Tezze Caterina,Albiero Mattia,Canato Marta,Gherardi Gaia,De Stefani Diego,Mammucari Cristina,Blaauw Bert,Boncompagni Simona,Protasi Feliciano,Reggiani Carlo,Scorrano Luca,Salviati Leonardo,Sandri Marco Nature communications Mitochondrial quality control is essential in highly structured cells such as neurons and muscles. In skeletal muscle the mitochondrial fission proteins are reduced in different physiopathological conditions including ageing sarcopenia, cancer cachexia and chemotherapy-induced muscle wasting. However, whether mitochondrial fission is essential for muscle homeostasis is still unclear. Here we show that muscle-specific loss of the pro-fission dynamin related protein (DRP) 1 induces muscle wasting and weakness. Constitutive Drp1 ablation in muscles reduces growth and causes animal death while inducible deletion results in atrophy and degeneration. Drp1 deficient mitochondria are morphologically bigger and functionally abnormal. The dysfunctional mitochondria signals to the nucleus to induce the ubiquitin-proteasome system and an Unfolded Protein Response while the change of mitochondrial volume results in an increase of mitochondrial Ca uptake and myofiber death. Our findings reveal that morphology of mitochondrial network is critical for several biological processes that control nuclear programs and Ca handling. 10.1038/s41467-019-10226-9
    Muscle wasting in disease: molecular mechanisms and promising therapies. Cohen Shenhav,Nathan James A,Goldberg Alfred L Nature reviews. Drug discovery Atrophy occurs in specific muscles with inactivity (for example, during plaster cast immobilization) or denervation (for example, in patients with spinal cord injuries). Muscle wasting occurs systemically in older people (a condition known as sarcopenia); as a physiological response to fasting or malnutrition; and in many diseases, including chronic obstructive pulmonary disorder, cancer-associated cachexia, diabetes, renal failure, cardiac failure, Cushing syndrome, sepsis, burns and trauma. The rapid loss of muscle mass and strength primarily results from excessive protein breakdown, which is often accompanied by reduced protein synthesis. This loss of muscle function can lead to reduced quality of life, increased morbidity and mortality. Exercise is the only accepted approach to prevent or slow atrophy. However, several promising therapeutic agents are in development, and major advances in our understanding of the cellular mechanisms that regulate the protein balance in muscle include the identification of several cytokines, particularly myostatin, and a common transcriptional programme that promotes muscle wasting. Here, we discuss these new insights and the rationally designed therapies that are emerging to combat muscle wasting. 10.1038/nrd4467
    Antagonistic control of myofiber size and muscle protein quality control by the ubiquitin ligase UBR4 during aging. Hunt Liam C,Schadeberg Bronwen,Stover Jared,Haugen Benard,Pagala Vishwajeeth,Wang Yong-Dong,Puglise Jason,Barton Elisabeth R,Peng Junmin,Demontis Fabio Nature communications Sarcopenia is a degenerative condition that consists in age-induced atrophy and functional decline of skeletal muscle cells (myofibers). A common hypothesis is that inducing myofiber hypertrophy should also reinstate myofiber contractile function but such model has not been extensively tested. Here, we find that the levels of the ubiquitin ligase UBR4 increase in skeletal muscle with aging, and that UBR4 increases the proteolytic activity of the proteasome. Importantly, muscle-specific UBR4 loss rescues age-associated myofiber atrophy in mice. However, UBR4 loss reduces the muscle specific force and accelerates the decline in muscle protein quality that occurs with aging in mice. Similarly, hypertrophic signaling induced via muscle-specific loss of UBR4/poe and of ESCRT members (HGS/Hrs, STAM, USP8) that degrade ubiquitinated membrane proteins compromises muscle function and shortens lifespan in Drosophila by reducing protein quality control. Altogether, these findings indicate that these ubiquitin ligases antithetically regulate myofiber size and muscle protein quality control. 10.1038/s41467-021-21738-8
    ATP citrate lyase improves mitochondrial function in skeletal muscle. Das Suman,Morvan Frederic,Jourde Benjamin,Meier Viktor,Kahle Peter,Brebbia Pascale,Toussaint Gauthier,Glass David J,Fornaro Mara Cell metabolism Mitochondrial dysfunction is associated with skeletal muscle pathology, including cachexia, sarcopenia, and the muscular dystrophies. ATP citrate lyase (ACL) is a cytosolic enzyme that catalyzes mitochondria-derived citrate into oxaloacetate and acetyl-CoA. Here we report that activation of ACL in skeletal muscle results in improved mitochondrial function. IGF1 induces activation of ACL in an AKT-dependent fashion. This results in an increase in cardiolipin, thus increasing critical mitochondrial complexes and supercomplex activity, and a resultant increase in oxygen consumption and cellular ATP levels. Conversely, knockdown of ACL in myotubes not only reduces mitochondrial complex I, IV, and V activity but also blocks IGF1-induced increases in oxygen consumption. In vivo, ACL activity is associated with increased ATP. Activation of this IGF1/ACL/cardiolipin pathway combines anabolic signaling with induction of mechanisms needed to provide required ATP. 10.1016/j.cmet.2015.05.006
    GDF11 Increases with Age and Inhibits Skeletal Muscle Regeneration. Egerman Marc A,Cadena Samuel M,Gilbert Jason A,Meyer Angelika,Nelson Hallie N,Swalley Susanne E,Mallozzi Carolyn,Jacobi Carsten,Jennings Lori L,Clay Ieuan,Laurent Gaëlle,Ma Shenglin,Brachat Sophie,Lach-Trifilieff Estelle,Shavlakadze Tea,Trendelenburg Anne-Ulrike,Brack Andrew S,Glass David J Cell metabolism Age-related frailty may be due to decreased skeletal muscle regeneration. The role of TGF-β molecules myostatin and GDF11 in regeneration is unclear. Recent studies showed an age-related decrease in GDF11 and that GDF11 treatment improves muscle regeneration, which were contrary to prior studies. We now show that these recent claims are not reproducible and the reagents previously used to detect GDF11 are not GDF11 specific. We develop a GDF11-specific immunoassay and show a trend toward increased GDF11 levels in sera of aged rats and humans. GDF11 mRNA increases in rat muscle with age. Mechanistically, GDF11 and myostatin both induce SMAD2/3 phosphorylation, inhibit myoblast differentiation, and regulate identical downstream signaling. GDF11 significantly inhibited muscle regeneration and decreased satellite cell expansion in mice. Given early data in humans showing a trend for an age-related increase, GDF11 could be a target for pharmacologic blockade to treat age-related sarcopenia. 10.1016/j.cmet.2015.05.010
    p38 MAPK signaling underlies a cell-autonomous loss of stem cell self-renewal in skeletal muscle of aged mice. Bernet Jennifer D,Doles Jason D,Hall John K,Kelly Tanaka Kathleen,Carter Thomas A,Olwin Bradley B Nature medicine Skeletal muscle aging results in a gradual loss of skeletal muscle mass, skeletal muscle function and regenerative capacity, which can lead to sarcopenia and increased mortality. Although the mechanisms underlying sarcopenia remain unclear, the skeletal muscle stem cell, or satellite cell, is required for muscle regeneration. Therefore, identification of signaling pathways affecting satellite cell function during aging may provide insights into therapeutic targets for combating sarcopenia. Here, we show that a cell-autonomous loss in self-renewal occurs via alterations in fibroblast growth factor receptor-1, p38α and p38β mitogen-activated protein kinase signaling in satellite cells from aged mice. We further demonstrate that pharmacological manipulation of these pathways can ameliorate age-associated self-renewal defects. Thus, our data highlight an age-associated deregulation of a satellite cell homeostatic network and reveal potential therapeutic opportunities for the treatment of progressive muscle wasting. 10.1038/nm.3465
    Detection of muscle wasting in patients with chronic heart failure using C-terminal agrin fragment: results from the Studies Investigating Co-morbidities Aggravating Heart Failure (SICA-HF). Steinbeck Lisa,Ebner Nicole,Valentova Miroslava,Bekfani Tarek,Elsner Sebastian,Dahinden Pius,Hettwer Stefan,Scherbakov Nadja,Schefold Jörg C,Sandek Anja,Springer Jochen,Doehner Wolfram,Anker Stefan D,von Haehling Stephan European journal of heart failure AIMS:Skeletal muscle wasting affects 20% of patients with chronic heart failure and has serious implications for their activities of daily living. Assessment of muscle wasting is technically challenging. C-terminal agrin-fragment (CAF), a breakdown product of the synaptically located protein agrin, has shown early promise as biomarker of muscle wasting. We sought to investigate the diagnostic properties of CAF in muscle wasting among patients with heart failure. METHODS AND RESULTS:We assessed serum CAF levels in 196 patients who participated in the Studies Investigating Co-morbidities Aggravating Heart Failure (SICA-HF). Muscle wasting was identified using dual-energy X-ray absorptiometry (DEXA) in 38 patients (19.4%). Patients with muscle wasting demonstrated higher CAF values than those without (125.1 ± 59.5 pmol/L vs. 103.8 ± 42.9 pmol/L, P = 0.01). Using receiver operating characteristics (ROC), we calculated the optimal CAF value to identify patients with muscle wasting as >87.5 pmol/L, which had a sensitivity of 78.9% and a specificity of 43.7%. The area under the ROC curve was 0.63 (95% confidence interval 0.56-0.70). Using simple regression, we found that serum CAF was associated with handgrip (R = - 0.17, P = 0.03) and quadriceps strength (R = - 0.31, P < 0.0001), peak oxygen consumption (R = - 0.5, P < 0.0001), 6-min walk distance (R = - 0.32, P < 0.0001), and gait speed (R = - 0.2, P = 0.001), as well as with parameters of kidney and liver function, iron metabolism and storage. CONCLUSION:CAF shows good sensitivity for the detection of skeletal muscle wasting in patients with heart failure. Its assessment may be useful to identify patients who should undergo additional testing, such as detailed body composition analysis. As no other biomarker is currently available, further investigation is warranted. 10.1002/ejhf.400
    BubR1 allelic effects drive phenotypic heterogeneity in mosaic-variegated aneuploidy progeria syndrome. Sieben Cynthia J,Jeganathan Karthik B,Nelson Grace G,Sturmlechner Ines,Zhang Cheng,van Deursen Willemijn H,Bakker Bjorn,Foijer Floris,Li Hu,Baker Darren J,van Deursen Jan M The Journal of clinical investigation Mosaic-variegated aneuploidy (MVA) syndrome is a rare childhood disorder characterized by biallelic BUBR1, CEP57, or TRIP13 aberrations; increased chromosome missegregation; and a broad spectrum of clinical features, including various cancers, congenital defects, and progeroid pathologies. To investigate the mechanisms underlying this disorder and its phenotypic heterogeneity, we mimicked the BUBR1L1012P mutation in mice (BubR1L1002P) and combined it with 2 other MVA variants, BUBR1X753 and BUBR1H, generating a truncated protein and low amounts of wild-type protein, respectively. Whereas BubR1X753/L1002P and BubR1H/X753 mice died prematurely, BubR1H/L1002P mice were viable and exhibited many MVA features, including cancer predisposition and various progeroid phenotypes, such as short lifespan, dwarfism, lipodystrophy, sarcopenia, and low cardiac stress tolerance. Strikingly, although these mice had a reduction in total BUBR1 and spectrum of MVA phenotypes similar to that of BubR1H/H mice, several progeroid pathologies were attenuated in severity, which in skeletal muscle coincided with reduced senescence-associated secretory phenotype complexity. Additionally, mice carrying monoallelic BubR1 mutations were prone to select MVA-related pathologies later in life, with predisposition to sarcopenia correlating with mTORC1 hyperactivity. Together, these data demonstrate that BUBR1 allelic effects beyond protein level and aneuploidy contribute to disease heterogeneity in both MVA patients and heterozygous carriers of MVA mutations. 10.1172/JCI126863
    Neuromuscular junctions are stable in patients with cancer cachexia. Boehm Ines,Miller Janice,Wishart Thomas M,Wigmore Stephen J,Skipworth Richard Je,Jones Ross A,Gillingwater Thomas H The Journal of clinical investigation Cancer cachexia is a major cause of patient morbidity and mortality, with no efficacious treatment or management strategy. Despite cachexia sharing pathophysiological features with a number of neuromuscular wasting conditions, including age-related sarcopenia, the mechanisms underlying cachexia remain poorly understood. Studies of related conditions suggest that pathological targeting of the neuromuscular junction (NMJ) may play a key role in cachexia, but this has yet to be investigated in human patients. Here, high-resolution morphological analyses were undertaken on NMJs of rectus abdominis obtained from patients undergoing upper GI cancer surgery compared with controls (N = 30; n = 1,165 NMJs). Cancer patients included those with cachexia and weight-stable disease. Despite the low skeletal muscle index and significant muscle fiber atrophy (P < 0.0001) in patients with cachexia, NMJ morphology was fully conserved. No significant differences were observed in any of the pre- and postsynaptic variables measured. We conclude that NMJs remain structurally intact in rectus abdominis in both cancer and cachexia, suggesting that denervation of skeletal muscle is not a major driver of pathogenesis. The absence of NMJ pathology is in stark contrast to what is found in related conditions, such as age-related sarcopenia, and supports the hypothesis that intrinsic changes within skeletal muscle, independent of any changes in motor neurons, represent the primary locus of neuromuscular pathology in cancer cachexia. 10.1172/JCI128411
    Systemic Nutrient and Stress Signaling via Myokines and Myometabolites. Rai Mamta,Demontis Fabio Annual review of physiology Homeostatic systems mount adaptive responses to meet the energy demands of the cell and to compensate for dysfunction in cellular compartments. Such surveillance systems are also active at the organismal level: Nutrient and stress sensing in one tissue can lead to changes in other tissues. Here, we review the emerging understanding of the role of skeletal muscle in regulating physiological homeostasis and disease progression in other tissues. Muscle-specific genetic interventions can induce systemic effects indirectly, via changes in the mass and metabolic demand of muscle, and directly, via the release of muscle-derived cytokines (myokines) and metabolites (myometabolites) in response to nutrients and stress. In turn, myokines and myometabolites signal to various target tissues in an autocrine, paracrine, and endocrine manner, thereby determining organismal resilience to aging, disease, and environmental challenges. We propose that tailoring muscle systemic signaling by modulating myokine and myometabolite levels may combat many degenerative diseases and delay aging. 10.1146/annurev-physiol-021115-105305
    Skeletal muscle mitochondrial remodeling in exercise and diseases. Gan Zhenji,Fu Tingting,Kelly Daniel P,Vega Rick B Cell research Skeletal muscle fitness and plasticity is an important determinant of human health and disease. Mitochondria are essential for maintaining skeletal muscle energy homeostasis by adaptive re-programming to meet the demands imposed by a myriad of physiologic or pathophysiological stresses. Skeletal muscle mitochondrial dysfunction has been implicated in the pathogenesis of many diseases, including muscular dystrophy, atrophy, type 2 diabetes, and aging-related sarcopenia. Notably, exercise counteracts the effects of many chronic diseases on skeletal muscle mitochondrial function. Recent studies have revealed a finely tuned regulatory network that orchestrates skeletal muscle mitochondrial biogenesis and function in response to exercise and in disease states. In addition, increasing evidence suggests that mitochondria also serve to "communicate" with the nucleus and mediate adaptive genomic re-programming. Here we review the current state of knowledge relevant to the dynamic remodeling of skeletal muscle mitochondria in response to exercise and in disease states. 10.1038/s41422-018-0078-7
    Age-Associated Loss of OPA1 in Muscle Impacts Muscle Mass, Metabolic Homeostasis, Systemic Inflammation, and Epithelial Senescence. Tezze Caterina,Romanello Vanina,Desbats Maria Andrea,Fadini Gian Paolo,Albiero Mattia,Favaro Giulia,Ciciliot Stefano,Soriano Maria Eugenia,Morbidoni Valeria,Cerqua Cristina,Loefler Stefan,Kern Helmut,Franceschi Claudio,Salvioli Stefano,Conte Maria,Blaauw Bert,Zampieri Sandra,Salviati Leonardo,Scorrano Luca,Sandri Marco Cell metabolism Mitochondrial dysfunction occurs during aging, but its impact on tissue senescence is unknown. Here, we find that sedentary but not active humans display an age-related decline in the mitochondrial protein, optic atrophy 1 (OPA1), that is associated with muscle loss. In adult mice, acute, muscle-specific deletion of Opa1 induces a precocious senescence phenotype and premature death. Conditional and inducible Opa1 deletion alters mitochondrial morphology and function but not DNA content. Mechanistically, the ablation of Opa1 leads to ER stress, which signals via the unfolded protein response (UPR) and FoxOs, inducing a catabolic program of muscle loss and systemic aging. Pharmacological inhibition of ER stress or muscle-specific deletion of FGF21 compensates for the loss of Opa1, restoring a normal metabolic state and preventing muscle atrophy and premature death. Thus, mitochondrial dysfunction in the muscle can trigger a cascade of signaling initiated at the ER that systemically affects general metabolism and aging. 10.1016/j.cmet.2017.04.021
    Muscle protein anabolism in advanced cancer patients: response to protein and amino acids support, and to physical activity. Antoun S,Raynard B Annals of oncology : official journal of the European Society for Medical Oncology In the field of oncology, it is well recognized that a decrease in mass, density, strength, or function of skeletal muscle is associated to increased treatment toxicities and postoperative complications, as well as poor progression-free survival and overall survival. The ability of amino acids to stimulate protein synthesis in cancer patients is reduced. Considering nutritional intervention, this anabolic resistance could be in a part counteracted by increasing protein or by giving specific amino acids. In particular, Leucine might counteract this anabolic resistance not only by increasing substrate availability, but also by directly modulating the anabolic signal pathway. Few studies showed the possibility of increasing muscle protein synthesis by specific nutriments and/or by increasing amino acids or protein administration. In addition, whereas many studies provide evidence of a benefit of adapted physical activity in advanced cancer patients, it is difficult to specify the most appropriate type of exercise, and the optimum rhythm and intensity. Moreover, the benefits of physical activities and of protein support seem greater when it is started at the precachexia stage rather than at the cachexia stage, and their benefits are limited or nonexistent at the stage of refractory cachexia. Future approaches should integrate the combination of several complementary treatments in order to prevent (or improve) cachexia and/or sarcopenia in cancer patients. 10.1093/annonc/mdx809
    An embryonic CaVβ1 isoform promotes muscle mass maintenance via GDF5 signaling in adult mouse. Traoré Massiré,Gentil Christel,Benedetto Chiara,Hogrel Jean-Yves,De la Grange Pierre,Cadot Bruno,Benkhelifa-Ziyyat Sofia,Julien Laura,Lemaitre Mégane,Ferry Arnaud,Piétri-Rouxel France,Falcone Sestina Science translational medicine Deciphering the mechanisms that govern skeletal muscle plasticity is essential to understand its pathophysiological processes, including age-related sarcopenia. The voltage-gated calcium channel CaV1.1 has a central role in excitation-contraction coupling (ECC), raising the possibility that it may also initiate the adaptive response to changes during muscle activity. Here, we revealed the existence of a gene transcription switch of the CaV1.1 β subunit (CaVβ1) that is dependent on the innervation state of the muscle in mice. In a mouse model of sciatic denervation, we showed increased expression of an embryonic isoform of the subunit that we called CaVβ1E. CaVβ1E boosts downstream growth differentiation factor 5 (GDF5) signaling to counteract muscle loss after denervation in mice. We further reported that aged mouse muscle expressed lower quantity of CaVβ1E compared with young muscle, displaying an altered GDF5-dependent response to denervation. Conversely, CaVβ1E overexpression improved mass wasting in aging muscle in mice by increasing GDF5 expression. We also identified the human CaVβ1E analogous and show a correlation between CaVβ1E expression in human muscles and age-related muscle mass decline. These results suggest that strategies targeting CaVβ1E or GDF5 might be effective in reducing muscle mass loss in aging. 10.1126/scitranslmed.aaw1131
    Lrp4 is a retrograde signal for presynaptic differentiation at neuromuscular synapses. Yumoto Norihiro,Kim Natalie,Burden Steven J Nature Motor axons receive retrograde signals from skeletal muscle that are essential for the differentiation and stabilization of motor nerve terminals. Identification of these retrograde signals has proved elusive, but their production by muscle depends on the receptor tyrosine kinase, MuSK (muscle, skeletal receptor tyrosine-protein kinase), and Lrp4 (low-density lipoprotein receptor (LDLR)-related protein 4), an LDLR family member that forms a complex with MuSK, binds neural agrin and stimulates MuSK kinase activity. Here we show that Lrp4 also functions as a direct muscle-derived retrograde signal for early steps in presynaptic differentiation. We demonstrate that Lrp4 is necessary, independent of MuSK activation, for presynaptic differentiation in vivo, and we show that Lrp4 binds to motor axons and induces clustering of synaptic-vesicle and active-zone proteins. Thus, Lrp4 acts bidirectionally and coordinates synapse formation by binding agrin, activating MuSK and stimulating postsynaptic differentiation, and functioning in turn as a muscle-derived retrograde signal that is necessary and sufficient for presynaptic differentiation. 10.1038/nature11348
    Mitofusin 2 Regulates Axonal Transport of Calpastatin to Prevent Neuromuscular Synaptic Elimination in Skeletal Muscles. Wang Luwen,Gao Ju,Liu Jingyi,Siedlak Sandra L,Torres Sandy,Fujioka Hisashi,Huntley Mikayla L,Jiang Yinfei,Ji Haiyan,Yan Tingxiang,Harland Micah,Termsarasab Pichet,Zeng Sophia,Jiang Zhen,Liang Jingjing,Perry George,Hoppel Charles,Zhang Cheng,Li Hu,Wang Xinglong Cell metabolism Skeletal muscles undergo atrophy in response to diseases and aging. Here we report that mitofusin 2 (Mfn2) acts as a dominant suppressor of neuromuscular synaptic loss to preserve skeletal muscles. Mfn2 is reduced in spinal cords of transgenic SOD1 and aged mice. Through preserving neuromuscular synapses, increasing neuronal Mfn2 prevents skeletal muscle wasting in both SOD1 and aged mice, whereas deletion of neuronal Mfn2 produces neuromuscular synaptic dysfunction and skeletal muscle atrophy. Neuromuscular synaptic loss after sciatic nerve transection can also be alleviated by Mfn2. Mfn2 coexists with calpastatin largely in mitochondria-associated membranes (MAMs) to regulate its axonal transport. Genetic inactivation of calpastatin abolishes Mfn2-mediated protection of neuromuscular synapses. Our results suggest that, as a potential key component of a novel and heretofore unrecognized mechanism of cytoplasmic protein transport, Mfn2 may play a general role in preserving neuromuscular synapses and serve as a common therapeutic target for skeletal muscle atrophy. 10.1016/j.cmet.2018.06.011
    Testosterone therapy increases muscle mass in men with cirrhosis and low testosterone: A randomised controlled trial. Sinclair Marie,Grossmann Mathis,Hoermann Rudolf,Angus Peter W,Gow Paul J Journal of hepatology BACKGROUND & AIMS:Low testosterone and sarcopenia are common in men with cirrhosis and both are associated with increased mortality. Whether testosterone therapy in cirrhosis improves muscle mass and other outcomes is unknown. METHODS:We conducted a 12-month, double-blinded, placebo-controlled trial of intramuscular testosterone undecanoate in 101 men with established cirrhosis and low serum testosterone (total testosterone <12nmol/L or free testosterone <230pmol/L) in a single tertiary centre. Body composition was assessed using dual-energy X-ray absorptiometry at baseline, 6 and 12months. RESULTS:At study completion, appendicular lean mass was significant higher in testosterone-treated subjects, with a mean adjusted difference (MAD) of +1.69kg, (CI +0.40; +2.97kg, p=0.021). Secondary outcomes included a substantially higher total lean mass in the active group (MAD +4.74kg, CI +1.75; +7.74kg, p=0.008), matched by reduced fat mass (MAD -4.34kg, CI -6.65; -2.04, p<0.001). Total bone mass increased (MAD +0.08kg, CI +0.01; +0.15kg, p=0.009) as did bone mineral density at the femoral neck (MAD +0.287points, CI +0.140; +0.434, p<0.001). Haemoglobin was higher with testosterone therapy (MAD +10.2g/L, CI +1.50; +18.9g/L, p=0.041) and percentage glycosylated haemoglobin (HbA1c) lower (MAD -0.35%, CI -0.05; -0.54, p=0.028). Mortality was non-significantly lower in testosterone-treated patients (16% vs. 25.5%, p=0.352). There was no increase in adverse events in testosterone-treated subjects. CONCLUSION:Testosterone therapy in men with cirrhosis and low serum testosterone safely increases muscle mass, bone mass and haemoglobin, and reduces fat mass and HbA1c. This is the first evidence-based therapy for sarcopenia in cirrhosis and thus requires larger-scale investigation into its potential impact on mortality. LAY SUMMARY:Both low testosterone and muscle wasting are associated with increased risk of death in men with severe liver disease. Administering testosterone to men with liver disease who have low testosterone levels significantly increases their muscle mass. In addition, testosterone has non-muscle beneficial effects which may be able to increase survival in this population. CLINICAL TRIAL NUMBER:Australian New Zealand Clinical Trials Registry trial number ACTRN 12614000526673. 10.1016/j.jhep.2016.06.007
    Muscle Wasting Diseases: Novel Targets and Treatments. Furrer Regula,Handschin Christoph Annual review of pharmacology and toxicology Adequate skeletal muscle plasticity is an essential element for our well-being, and compromised muscle function can drastically affect quality of life, morbidity, and mortality. Surprisingly, however, skeletal muscle remains one of the most under-medicated organs. Interventions in muscle diseases are scarce, not only in neuromuscular dystrophies, but also in highly prevalent secondary wasting pathologies such as sarcopenia and cachexia. Even in other diseases that exhibit a well-established risk correlation of muscle dysfunction due to a sedentary lifestyle, such as type 2 diabetes or cardiovascular pathologies, current treatments are mostly targeted on non-muscle tissues. In recent years, a renewed focus on skeletal muscle has led to the discovery of various novel drug targets and the design of new pharmacological approaches. This review provides an overview of the current knowledge of the key mechanisms involved in muscle wasting conditions and novel pharmacological avenues that could ameliorate muscle diseases. 10.1146/annurev-pharmtox-010818-021041
    NAD metabolism and its roles in cellular processes during ageing. Covarrubias Anthony J,Perrone Rosalba,Grozio Alessia,Verdin Eric Nature reviews. Molecular cell biology Nicotinamide adenine dinucleotide (NAD) is a coenzyme for redox reactions, making it central to energy metabolism. NAD is also an essential cofactor for non-redox NAD-dependent enzymes, including sirtuins, CD38 and poly(ADP-ribose) polymerases. NAD can directly and indirectly influence many key cellular functions, including metabolic pathways, DNA repair, chromatin remodelling, cellular senescence and immune cell function. These cellular processes and functions are critical for maintaining tissue and metabolic homeostasis and for healthy ageing. Remarkably, ageing is accompanied by a gradual decline in tissue and cellular NAD levels in multiple model organisms, including rodents and humans. This decline in NAD levels is linked causally to numerous ageing-associated diseases, including cognitive decline, cancer, metabolic disease, sarcopenia and frailty. Many of these ageing-associated diseases can be slowed down and even reversed by restoring NAD levels. Therefore, targeting NAD metabolism has emerged as a potential therapeutic approach to ameliorate ageing-related disease, and extend the human healthspan and lifespan. However, much remains to be learnt about how NAD influences human health and ageing biology. This includes a deeper understanding of the molecular mechanisms that regulate NAD levels, how to effectively restore NAD levels during ageing, whether doing so is safe and whether NAD repletion will have beneficial effects in ageing humans. 10.1038/s41580-020-00313-x
    Adenosine/A2B Receptor Signaling Ameliorates the Effects of Aging and Counteracts Obesity. Gnad Thorsten,Navarro Gemma,Lahesmaa Minna,Reverte-Salisa Laia,Copperi Francesca,Cordomi Arnau,Naumann Jennifer,Hochhäuser Aileen,Haufs-Brusberg Saskia,Wenzel Daniela,Suhr Frank,Jespersen Naja Zenius,Scheele Camilla,Tsvilovskyy Volodymyr,Brinkmann Christian,Rittweger Joern,Dani Christian,Kranz Mathias,Deuther-Conrad Winnie,Eltzschig Holger K,Niemi Tarja,Taittonen Markku,Brust Peter,Nuutila Pirjo,Pardo Leonardo,Fleischmann Bernd K,Blüher Matthias,Franco Rafael,Bloch Wilhelm,Virtanen Kirsi A,Pfeifer Alexander Cell metabolism The combination of aging populations with the obesity pandemic results in an alarming rise in non-communicable diseases. Here, we show that the enigmatic adenosine A2B receptor (A2B) is abundantly expressed in skeletal muscle (SKM) as well as brown adipose tissue (BAT) and might be targeted to counteract age-related muscle atrophy (sarcopenia) as well as obesity. Mice with SKM-specific deletion of A2B exhibited sarcopenia, diminished muscle strength, and reduced energy expenditure (EE), whereas pharmacological A2B activation counteracted these processes. Adipose tissue-specific ablation of A2B exacerbated age-related processes and reduced BAT EE, whereas A2B stimulation ameliorated obesity. In humans, A2B expression correlated with EE in SKM, BAT activity, and abundance of thermogenic adipocytes in white fat. Moreover, A2B agonist treatment increased EE from human adipocytes, myocytes, and muscle explants. Mechanistically, A2B forms heterodimers required for adenosine signaling. Overall, adenosine/A2B signaling links muscle and BAT and has both anti-aging and anti-obesity potential. 10.1016/j.cmet.2020.06.006
    In Vivo Amelioration of Age-Associated Hallmarks by Partial Reprogramming. Ocampo Alejandro,Reddy Pradeep,Martinez-Redondo Paloma,Platero-Luengo Aida,Hatanaka Fumiyuki,Hishida Tomoaki,Li Mo,Lam David,Kurita Masakazu,Beyret Ergin,Araoka Toshikazu,Vazquez-Ferrer Eric,Donoso David,Roman Jose Luis,Xu Jinna,Rodriguez Esteban Concepcion,Nuñez Gabriel,Nuñez Delicado Estrella,Campistol Josep M,Guillen Isabel,Guillen Pedro,Izpisua Belmonte Juan Carlos Cell Aging is the major risk factor for many human diseases. In vitro studies have demonstrated that cellular reprogramming to pluripotency reverses cellular age, but alteration of the aging process through reprogramming has not been directly demonstrated in vivo. Here, we report that partial reprogramming by short-term cyclic expression of Oct4, Sox2, Klf4, and c-Myc (OSKM) ameliorates cellular and physiological hallmarks of aging and prolongs lifespan in a mouse model of premature aging. Similarly, expression of OSKM in vivo improves recovery from metabolic disease and muscle injury in older wild-type mice. The amelioration of age-associated phenotypes by epigenetic remodeling during cellular reprogramming highlights the role of epigenetic dysregulation as a driver of mammalian aging. Establishing in vivo platforms to modulate age-associated epigenetic marks may provide further insights into the biology of aging. 10.1016/j.cell.2016.11.052
    GSK-3α is a central regulator of age-related pathologies in mice. Zhou Jibin,Freeman Theresa A,Ahmad Firdos,Shang Xiying,Mangano Emily,Gao Erhe,Farber John,Wang Yajing,Ma Xin-Liang,Woodgett James,Vagnozzi Ronald J,Lal Hind,Force Thomas The Journal of clinical investigation Aging is regulated by conserved signaling pathways. The glycogen synthase kinase-3 (GSK-3) family of serine/threonine kinases regulates several of these pathways, but the role of GSK-3 in aging is unknown. Herein, we demonstrate premature death and acceleration of age-related pathologies in the Gsk3a global KO mouse. KO mice developed cardiac hypertrophy and contractile dysfunction as well as sarcomere disruption and striking sarcopenia in cardiac and skeletal muscle, a classical finding in aging. We also observed severe vacuolar degeneration of myofibers and large tubular aggregates in skeletal muscle, consistent with impaired clearance of insoluble cellular debris. Other organ systems, including gut, liver, and the skeletal system, also demonstrated age-related pathologies. Mechanistically, we found marked activation of mTORC1 and associated suppression of autophagy markers in KO mice. Loss of GSK-3α, either by pharmacologic inhibition or Gsk3a gene deletion, suppressed autophagy in fibroblasts. mTOR inhibition rescued this effect and reversed the established pathologies in the striated muscle of the KO mouse. Thus, GSK-3α is a critical regulator of mTORC1, autophagy, and aging. In its absence, aging/senescence is accelerated in multiple tissues. Strategies to maintain GSK-3α activity and/or inhibit mTOR in the elderly could retard the appearance of age-related pathologies. 10.1172/JCI64398
    Targeting β1-integrin signaling enhances regeneration in aged and dystrophic muscle in mice. Rozo Michelle,Li Liangji,Fan Chen-Ming Nature medicine Interactions between stem cells and their microenvironment, or niche, are essential for stem cell maintenance and function. Our knowledge of the niche for the skeletal muscle stem cell, i.e., the satellite cell (SC), is incomplete. Here we show that β1-integrin is an essential niche molecule that maintains SC homeostasis, and sustains the expansion and self-renewal of this stem cell pool during regeneration. We further show that β1-integrin cooperates with fibroblast growth factor 2 (Fgf2), a potent growth factor for SCs, to synergistically activate their common downstream effectors, the mitogen-activated protein (MAP) kinase Erk and protein kinase B (Akt). Notably, SCs in aged mice show altered β1-integrin activity and insensitivity to Fgf2. Augmenting β1-integrin activity with a monoclonal antibody restores Fgf2 sensitivity and improves regeneration after experimentally induced muscle injury. The same treatment also enhances regeneration and function of dystrophic muscles in mdx mice, a model for Duchenne muscular dystrophy. Therefore, β1-integrin senses the SC niche to maintain responsiveness to Fgf2, and this integrin represents a potential therapeutic target for pathological conditions of the muscle in which the stem cell niche is compromised. 10.1038/nm.4116
    Satellite cell senescence underlies myopathy in a mouse model of limb-girdle muscular dystrophy 2H. Kudryashova Elena,Kramerova Irina,Spencer Melissa J The Journal of clinical investigation Mutations in the E3 ubiquitin ligase tripartite motif-containing 32 (TRIM32) are responsible for the disease limb-girdle muscular dystrophy 2H (LGMD2H). Previously, we generated Trim32 knockout mice (Trim32-/- mice) and showed that they display a myopathic phenotype accompanied by neurogenic features. Here, we used these mice to investigate the muscle-specific defects arising from the absence of TRIM32, which underlie the myopathic phenotype. Using 2 models of induced atrophy, we showed that TRIM32 is dispensable for muscle atrophy. Conversely, TRIM32 was necessary for muscle regrowth after atrophy. Furthermore, TRIM32-deficient primary myoblasts underwent premature senescence and impaired myogenesis due to accumulation of PIAS4, an E3 SUMO ligase and TRIM32 substrate that was previously shown to be associated with senescence. Premature senescence of myoblasts was also observed in vivo in an atrophy/regrowth model. Trim32-/- muscles had substantially fewer activated satellite cells, increased PIAS4 levels, and growth failure compared with wild-type muscles. Moreover, Trim32-/- muscles exhibited features of premature sarcopenia, such as selective type II fast fiber atrophy. These results imply that premature senescence of muscle satellite cells is an underlying pathogenic feature of LGMD2H and reveal what we believe to be a new mechanism of muscular dystrophy associated with reductions in available satellite cells and premature sarcopenia. 10.1172/JCI59581
    Muscle Atrophy and the Sestrins. Martyn J A Jeevendra,Kaneki Masao The New England journal of medicine 10.1056/NEJMcibr2003528
    Oxytocin is an age-specific circulating hormone that is necessary for muscle maintenance and regeneration. Elabd Christian,Cousin Wendy,Upadhyayula Pavan,Chen Robert Y,Chooljian Marc S,Li Ju,Kung Sunny,Jiang Kevin P,Conboy Irina M Nature communications The regenerative capacity of skeletal muscle declines with age. Previous studies suggest that this process can be reversed by exposure to young circulation; however, systemic age-specific factors responsible for this phenomenon are largely unknown. Here we report that oxytocin--a hormone best known for its role in lactation, parturition and social behaviours--is required for proper muscle tissue regeneration and homeostasis, and that plasma levels of oxytocin decline with age. Inhibition of oxytocin signalling in young animals reduces muscle regeneration, whereas systemic administration of oxytocin rapidly improves muscle regeneration by enhancing aged muscle stem cell activation/proliferation through activation of the MAPK/ERK signalling pathway. We further show that the genetic lack of oxytocin does not cause a developmental defect in muscle but instead leads to premature sarcopenia. Considering that oxytocin is an FDA-approved drug, this work reveals a potential novel and safe way to combat or prevent skeletal muscle ageing. 10.1038/ncomms5082
    Ryanodine receptor oxidation causes intracellular calcium leak and muscle weakness in aging. Andersson Daniel C,Betzenhauser Matthew J,Reiken Steven,Meli Albano C,Umanskaya Alisa,Xie Wenjun,Shiomi Takayuki,Zalk Ran,Lacampagne Alain,Marks Andrew R Cell metabolism Age-related loss of muscle mass and force (sarcopenia) contributes to disability and increased mortality. Ryanodine receptor 1 (RyR1) is the skeletal muscle sarcoplasmic reticulum calcium release channel required for muscle contraction. RyR1 from aged (24 months) rodents was oxidized, cysteine-nitrosylated, and depleted of the channel-stabilizing subunit calstabin1, compared to RyR1 from younger (3-6 months) adults. This RyR1 channel complex remodeling resulted in "leaky" channels with increased open probability, leading to intracellular calcium leak in skeletal muscle. Similarly, 6-month-old mice harboring leaky RyR1-S2844D mutant channels exhibited skeletal muscle defects comparable to 24-month-old wild-type mice. Treating aged mice with S107 stabilized binding of calstabin1 to RyR1, reduced intracellular calcium leak, decreased reactive oxygen species (ROS), and enhanced tetanic Ca(2+) release, muscle-specific force, and exercise capacity. Taken together, these data indicate that leaky RyR1 contributes to age-related loss of muscle function. 10.1016/j.cmet.2011.05.014
    Muscle stem cells contribute to myofibres in sedentary adult mice. Keefe Alexandra C,Lawson Jennifer A,Flygare Steven D,Fox Zachary D,Colasanto Mary P,Mathew Sam J,Yandell Mark,Kardon Gabrielle Nature communications Skeletal muscle is essential for mobility, stability and whole body metabolism, and muscle loss, for instance, during sarcopenia, has profound consequences. Satellite cells (muscle stem cells) have been hypothesized, but not yet demonstrated, to contribute to muscle homeostasis and a decline in their contribution to myofibre homeostasis to play a part in sarcopenia. To test their role in muscle maintenance, we genetically labelled and ablated satellite cells in adult sedentary mice. We demonstrate via genetic lineage experiments that, even in the absence of injury, satellite cells contribute to myofibres in all adult muscles, although the extent and timing differs. However, genetic ablation experiments showed that satellite cells are not globally required to maintain myofibre cross-sectional area of uninjured adult muscle. 10.1038/ncomms8087
    Myokines: A promising therapeutic target for hepatic encephalopathy. Yang Chun,Luo Ai-Lin Journal of hepatology 10.1016/j.jhep.2016.10.040
    Lkb1 suppresses amino acid-driven gluconeogenesis in the liver. Just Pierre-Alexandre,Charawi Sara,Denis Raphaël G P,Savall Mathilde,Traore Massiré,Foretz Marc,Bastu Sultan,Magassa Salimata,Senni Nadia,Sohier Pierre,Wursmer Maud,Vasseur-Cognet Mireille,Schmitt Alain,Le Gall Morgane,Leduc Marjorie,Guillonneau François,De Bandt Jean-Pascal,Mayeux Patrick,Romagnolo Béatrice,Luquet Serge,Bossard Pascale,Perret Christine Nature communications Excessive glucose production by the liver is a key factor in the hyperglycemia observed in type 2 diabetes mellitus (T2DM). Here, we highlight a novel role of liver kinase B1 (Lkb1) in this regulation. We show that mice with a hepatocyte-specific deletion of Lkb1 have higher levels of hepatic amino acid catabolism, driving gluconeogenesis. This effect is observed during both fasting and the postprandial period, identifying Lkb1 as a critical suppressor of postprandial hepatic gluconeogenesis. Hepatic Lkb1 deletion is associated with major changes in whole-body metabolism, leading to a lower lean body mass and, in the longer term, sarcopenia and cachexia, as a consequence of the diversion of amino acids to liver metabolism at the expense of muscle. Using genetic, proteomic and pharmacological approaches, we identify the aminotransferases and specifically Agxt as effectors of the suppressor function of Lkb1 in amino acid-driven gluconeogenesis. 10.1038/s41467-020-19490-6
    Fibroblast growth factor 19 regulates skeletal muscle mass and ameliorates muscle wasting in mice. Benoit Bérengère,Meugnier Emmanuelle,Castelli Martina,Chanon Stéphanie,Vieille-Marchiset Aurélie,Durand Christine,Bendridi Nadia,Pesenti Sandra,Monternier Pierre-Axel,Durieux Anne-Cécile,Freyssenet Damien,Rieusset Jennifer,Lefai Etienne,Vidal Hubert,Ruzzin Jérôme Nature medicine The endocrine-derived hormone fibroblast growth factor (FGF) 19 has recently emerged as a potential target for treating metabolic disease. Given that skeletal muscle is a key metabolic organ, we explored the role of FGF19 in that tissue. Here we report a novel function of FGF19 in regulating skeletal muscle mass through enlargement of muscle fiber size, and in protecting muscle from atrophy. Treatment with FGF19 causes skeletal muscle hypertrophy in mice, while physiological and pharmacological doses of FGF19 substantially increase the size of human myotubes in vitro. These effects were not elicited by FGF21, a closely related endocrine FGF member. Both in vitro and in vivo, FGF19 stimulates the phosphorylation of the extracellular-signal-regulated protein kinase 1/2 (ERK1/2) and the ribosomal protein S6 kinase (S6K1), an mTOR-dependent master regulator of muscle cell growth. Moreover, mice with a skeletal-muscle-specific genetic deficiency of β-Klotho (KLB), an obligate co-receptor for FGF15/19 (refs. 2,3), were unresponsive to the hypertrophic effect of FGF19. Finally, in mice, FGF19 ameliorates skeletal muscle atrophy induced by glucocorticoid treatment or obesity, as well as sarcopenia. Taken together, these findings provide evidence that the enterokine FGF19 is a novel factor in the regulation of skeletal muscle mass, and that it has therapeutic potential for the treatment of muscle wasting. 10.1038/nm.4363
    Metabolic and molecular responses to leucine-enriched branched chain amino acid supplementation in the skeletal muscle of alcoholic cirrhosis. Tsien Cynthia,Davuluri Gangarao,Singh Dharmvir,Allawy Allawy,Ten Have Gabriella A M,Thapaliya Samjhana,Schulze John M,Barnes David,McCullough Arthur J,Engelen Marielle P K J,Deutz Nicolaas E P,Dasarathy Srinivasan Hepatology (Baltimore, Md.) UNLABELLED:Skeletal muscle loss (sarcopenia) is a major clinical complication in alcoholic cirrhosis with no effective therapy. Skeletal muscle autophagic proteolysis and myostatin expression (inhibitor of protein synthesis) are increased in cirrhosis and believed to contribute to anabolic resistance. A prospective study was performed to determine the mechanisms of sarcopenia in alcoholic cirrhosis and potential reversal by leucine. In six well-compensated, stable, alcoholic patients with cirrhosis and eight controls, serial vastus lateralis muscle biopsies were obtained before and 7 hours after a single oral branched chain amino acid mixture enriched with leucine (BCAA/LEU). Primed-constant infusion of l-[ring-(2) H5 ]-phenylalanine was used to quantify whole-body protein breakdown and muscle protein fractional synthesis rate using liquid chromatography/mass spectrometry. Muscle expression of myostatin, mammalian target of rapamycin (mTOR) targets, autophagy markers, protein ubiquitination, and the intracellular amino acid deficiency sensor general control of nutrition 2 were quantified by immunoblots and the leucine exchanger (SLC7A5) and glutamine transporter (SLC38A2), by real-time polymerase chain reaction. Following oral administration, plasma BCAA concentrations showed a similar increase in patients with cirrhosis and controls. Skeletal muscle fractional synthesis rate was 9.63 ± 0.36%/hour in controls and 9.05 ± 0.68%/hour in patients with cirrhosis (P = 0.54). Elevated whole-body protein breakdown in patients with cirrhosis was reduced with BCAA/LEU (P = 0.01). Fasting skeletal muscle molecular markers showed increased myostatin expression, impaired mTOR signaling, and increased autophagy in patients with cirrhosis compared to controls (P < 0.01). The BCAA/LEU supplement did not alter myostatin expression, but mTOR signaling, autophagy measures, and general control of nutrition 2 activation were consistently reversed in cirrhotic muscle (P < 0.01). Expression of SLC7A5 was higher in the basal state in patients with cirrhosis than controls (P < 0.05) but increased with BCAA/LEU only in controls (P < 0.001). CONCLUSIONS:Impaired mTOR1 signaling and increased autophagy in skeletal muscle of patients with alcoholic cirrhosis is acutely reversed by BCAA/LEU. 10.1002/hep.27717
    Dkk3 dependent transcriptional regulation controls age related skeletal muscle atrophy. Yin Jie,Yang Lele,Xie Yangli,Liu Yan,Li Sheng,Yang Wenjun,Xu Bo,Ji Hongbin,Ding Lianghua,Wang Kun,Li Gang,Chen Lin,Hu Ping Nature communications Age-related muscle atrophy (sarcopenia) is the leading cause for disability in aged population, but the underlying molecular mechanisms are poorly understood. Here we identify a novel role for the secreted glycoprotein Dickkopf 3 (Dkk3) in sarcopenia. Forced expression of Dkk3 in muscles in young mice leads to muscle atrophy. Conversely, reducing its expression in old muscles restores both muscle size and function. Dkk3 induces nuclear import of β-catenin and enhances its interaction with FoxO3, which in turn activates the transcription of E3 ubiquitin ligase Fbxo32 and Trim63, driving muscle atrophy. These findings suggest that Dkk3 may be used as diagnostic marker and as therapeutic target for age-related muscle atrophy, and reveal a distinct transcriptional control of Fbxo32 and Trim63. 10.1038/s41467-018-04038-6
    FABP3-mediated membrane lipid saturation alters fluidity and induces ER stress in skeletal muscle with aging. Lee Seung-Min,Lee Seol Hee,Jung Youngae,Lee Younglang,Yoon Jong Hyun,Choi Jeong Yi,Hwang Chae Young,Son Young Hoon,Park Sung Sup,Hwang Geum-Sook,Lee Kwang-Pyo,Kwon Ki-Sun Nature communications Sarcopenia is characterized by decreased skeletal muscle mass and function with age. Aged muscles have altered lipid compositions; however, the role and regulation of lipids are unknown. Here we report that FABP3 is upregulated in aged skeletal muscles, disrupting homeostasis via lipid remodeling. Lipidomic analyses reveal that FABP3 overexpression in young muscles alters the membrane lipid composition to that of aged muscle by decreasing polyunsaturated phospholipid acyl chains, while increasing sphingomyelin and lysophosphatidylcholine. FABP3-dependent membrane lipid remodeling causes ER stress via the PERK-eIF2α pathway and inhibits protein synthesis, limiting muscle recovery after immobilization. FABP3 knockdown induces a young-like lipid composition in aged muscles, reduces ER stress, and improves protein synthesis and muscle recovery. Further, FABP3 reduces membrane fluidity and knockdown increases fluidity in vitro, potentially causing ER stress. Therefore, FABP3 drives membrane lipid composition-mediated ER stress to regulate muscle homeostasis during aging and is a valuable target for sarcopenia. 10.1038/s41467-020-19501-6
    Metabolic adaptation of skeletal muscle to hyperammonemia drives the beneficial effects of l-leucine in cirrhosis. Davuluri Gangarao,Krokowski Dawid,Guan Bo-Jhih,Kumar Avinash,Thapaliya Samjhana,Singh Dharmvir,Hatzoglou Maria,Dasarathy Srinivasan Journal of hepatology BACKGROUND & AIMS:Increased skeletal muscle ammonia uptake with loss of muscle mass adversely affects clinical outcomes in cirrhosis. Hyperammonemia causes reduced protein synthesis and sarcopenia but the cellular responses to impaired proteostasis and molecular mechanism of l-leucine induced adaptation to ammonia induced stress were determined. METHODS:Response to activation of amino acid deficiency sensor, GCN2, in the skeletal muscle from cirrhotic patients and the portacaval anastomosis (PCA) rat were quantified. During hyperammonemia and l-leucine supplementation, protein synthesis, phosphorylation of eIF2α, mTORC1 signaling, l-leucine transport and response to l-leucine supplementation were quantified. Adaptation to cellular stress via ATF4 and its target GADD34 were also determined. RESULTS:Activation of the eIF2α kinase GCN2 and impaired mTORC1 signaling were observed in skeletal muscle from cirrhotic patients and PCA rats. Ammonia activated GCN2 mediated eIF2α phosphorylation (eIF2α-P) and impaired mTORC1 signaling that inhibit protein synthesis in myotubes and MEFs. Adaptation to ammonia induced stress did not involve translational reprogramming by activation transcription factor 4 (ATF4) dependent induction of the eIF2α-P phosphatase subunit GADD34. Instead, ammonia increased expression of the leucine/glutamine exchanger SLC7A5, l-leucine uptake and intracellular l-leucine levels, the latter not being sufficient to rescue the inhibition of protein synthesis, due to potentially enhanced mitochondrial sequestration of l-leucine. l-leucine supplementation rescued protein synthesis inhibition caused by hyperammonemia. CONCLUSIONS:Response to hyperammonemia is reminiscent of the cellular response to amino acid starvation, but lacks the adaptive ATF4 dependent integrated stress response (ISR). Instead, hyperammonemia-induced l-leucine uptake was an adaptive response to the GCN2-mediated decreased protein synthesis. LAY SUMMARY:Sarcopenia or skeletal muscle loss is the most frequent complication in cirrhosis but there are no treatments because the cause(s) of muscle loss in liver disease are not known. Results from laboratory experiments in animals and muscle cells were validated in human patients with cirrhosis to show that ammonia plays a key role in causing muscle loss in patients with cirrhosis. We identified a novel stress response to ammonia in the muscle that decreases muscle protein content that can be reversed by supplementation with the amino acid l-leucine. 10.1016/j.jhep.2016.06.004
    Age-related muscle loss - novel target identified. Morris Alan Nature reviews. Endocrinology 10.1038/s41574-020-0388-y
    Inhibition of prostaglandin-degrading enzyme 15-PGDH rejuvenates aged muscle mass and strength. Palla A R,Ravichandran M,Wang Y X,Alexandrova L,Yang A V,Kraft P,Holbrook C A,Schürch C M,Ho A T V,Blau H M Science (New York, N.Y.) 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 E (PGE)-degrading enzyme, as a hallmark of aged tissues, including skeletal muscle. The consequent reduction in PGE 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 PGE concentrations, which augmented mitochondrial function and autophagy and decreased transforming growth factor-β signaling and activity of ubiquitin-proteasome pathways. Thus, PGE signaling ameliorates muscle atrophy and rejuvenates muscle function, and 15-PGDH may be a suitable therapeutic target for countering sarcopenia. 10.1126/science.abc8059
    Genome-wide meta-analysis of muscle weakness identifies 15 susceptibility loci in older men and women. Jones Garan,Trajanoska Katerina,Santanasto Adam J,Stringa Najada,Kuo Chia-Ling,Atkins Janice L,Lewis Joshua R,Duong ThuyVy,Hong Shengjun,Biggs Mary L,Luan Jian'an,Sarnowski Chloe,Lunetta Kathryn L,Tanaka Toshiko,Wojczynski Mary K,Cvejkus Ryan,Nethander Maria,Ghasemi Sahar,Yang Jingyun,Zillikens M Carola,Walter Stefan,Sicinski Kamil,Kague Erika,Ackert-Bicknell Cheryl L,Arking Dan E,Windham B Gwen,Boerwinkle Eric,Grove Megan L,Graff Misa,Spira Dominik,Demuth Ilja,van der Velde Nathalie,de Groot Lisette C P G M,Psaty Bruce M,Odden Michelle C,Fohner Alison E,Langenberg Claudia,Wareham Nicholas J,Bandinelli Stefania,van Schoor Natasja M,Huisman Martijn,Tan Qihua,Zmuda Joseph,Mellström Dan,Karlsson Magnus,Bennett David A,Buchman Aron S,De Jager Philip L,Uitterlinden Andre G,Völker Uwe,Kocher Thomas,Teumer Alexander,Rodriguéz-Mañas Leocadio,García Francisco J,Carnicero José A,Herd Pamela,Bertram Lars,Ohlsson Claes,Murabito Joanne M,Melzer David,Kuchel George A,Ferrucci Luigi,Karasik David,Rivadeneira Fernando,Kiel Douglas P,Pilling Luke C Nature communications Low muscle strength is an important heritable indicator of poor health linked to morbidity and mortality in older people. In a genome-wide association study meta-analysis of 256,523 Europeans aged 60 years and over from 22 cohorts we identify 15 loci associated with muscle weakness (European Working Group on Sarcopenia in Older People definition: n = 48,596 cases, 18.9% of total), including 12 loci not implicated in previous analyses of continuous measures of grip strength. Loci include genes reportedly involved in autoimmune disease (HLA-DQA1 p = 4 × 10), arthritis (GDF5 p = 4 × 10), cell cycle control and cancer protection, regulation of transcription, and others involved in the development and maintenance of the musculoskeletal system. Using Mendelian randomization we report possible overlapping causal pathways, including diabetes susceptibility, haematological parameters, and the immune system. We conclude that muscle weakness in older adults has distinct mechanisms from continuous strength, including several pathways considered to be hallmarks of ageing. 10.1038/s41467-021-20918-w
    Sestrin prevents atrophy of disused and aging muscles by integrating anabolic and catabolic signals. Segalés Jessica,Perdiguero Eusebio,Serrano Antonio L,Sousa-Victor Pedro,Ortet Laura,Jardí Mercè,Budanov Andrei V,Garcia-Prat Laura,Sandri Marco,Thomson David M,Karin Michael,Hee Lee Jun,Muñoz-Cánoves Pura Nature communications A unique property of skeletal muscle is its ability to adapt its mass to changes in activity. Inactivity, as in disuse or aging, causes atrophy, the loss of muscle mass and strength, leading to physical incapacity and poor quality of life. Here, through a combination of transcriptomics and transgenesis, we identify sestrins, a family of stress-inducible metabolic regulators, as protective factors against muscle wasting. Sestrin expression decreases during inactivity and its genetic deficiency exacerbates muscle wasting; conversely, sestrin overexpression suffices to prevent atrophy. This protection occurs through mTORC1 inhibition, which upregulates autophagy, and AKT activation, which in turn inhibits FoxO-regulated ubiquitin-proteasome-mediated proteolysis. This study reveals sestrin as a central integrator of anabolic and degradative pathways preventing muscle wasting. Since sestrin also protected muscles against aging-induced atrophy, our findings have implications for sarcopenia. 10.1038/s41467-019-13832-9
    Mechanisms Regulating Neuromuscular Junction Development and Function and Causes of Muscle Wasting. Tintignac Lionel A,Brenner Hans-Rudolf,Rüegg Markus A Physiological reviews The neuromuscular junction is the chemical synapse between motor neurons and skeletal muscle fibers. It is designed to reliably convert the action potential from the presynaptic motor neuron into the contraction of the postsynaptic muscle fiber. Diseases that affect the neuromuscular junction may cause failure of this conversion and result in loss of ambulation and respiration. The loss of motor input also causes muscle wasting as muscle mass is constantly adapted to contractile needs by the balancing of protein synthesis and protein degradation. Finally, neuromuscular activity and muscle mass have a major impact on metabolic properties of the organisms. This review discusses the mechanisms involved in the development and maintenance of the neuromuscular junction, the consequences of and the mechanisms involved in its dysfunction, and its role in maintaining muscle mass during aging. As life expectancy is increasing, loss of muscle mass during aging, called sarcopenia, has emerged as a field of high medical need. Interestingly, aging is also accompanied by structural changes at the neuromuscular junction, suggesting that the mechanisms involved in neuromuscular junction maintenance might be disturbed during aging. In addition, there is now evidence that behavioral paradigms and signaling pathways that are involved in longevity also affect neuromuscular junction stability and sarcopenia. 10.1152/physrev.00033.2014
    RANKL inhibition improves muscle strength and insulin sensitivity and restores bone mass. Bonnet Nicolas,Bourgoin Lucie,Biver Emmanuel,Douni Eleni,Ferrari Serge The Journal of clinical investigation Receptor activator of Nfkb ligand (RANKL) activates, while osteoprotegerin (OPG) inhibits, osteoclastogenesis. In turn a neutralizing Ab against RANKL, denosumab improves bone strength in osteoporosis. OPG also improves muscle strength in mouse models of Duchenne's muscular dystrophy (mdx) and denervation-induce atrophy, but its role and mechanisms of action on muscle weakness in other conditions remains to be investigated. We investigated the effects of RANKL inhibitors on muscle in osteoporotic women and mice that either overexpress RANKL (HuRANKL-Tg+), or lack Pparb and concomitantly develop sarcopenia (Pparb-/-). In women, denosumab over 3 years improved appendicular lean mass and handgrip strength compared to no treatment, whereas bisphosphonate did not. HuRANKL-Tg+ mice displayed lower limb force and maximal speed, while their leg muscle mass was diminished, with a lower number of type I and II fibers. Both OPG and denosumab increased limb force proportionally to the increase in muscle mass. They markedly improved muscle insulin sensitivity and glucose uptake, and decrease anti-myogenic and inflammatory gene expression in muscle, such as myostatin and protein tyrosine phosphatase receptor-γ. Similarly, in Pparb-/-, OPG increased muscle volume and force, while also normalizing their insulin signaling and higher expression of inflammatory genes in skeletal muscle. In conclusions, RANKL deteriorates, while its inhibitor improves, muscle strength and insulin sensitivity in osteoporotic mice and humans. Hence denosumab could represent a novel therapeutic approach for sarcopenia. 10.1172/JCI125915
    Activated protein phosphatase 2A disrupts nutrient sensing balance between mTORC1 and AMPK causing sarcopenia in alcoholic liver disease. Davuluri Gangarao,Welch Nicole,Sekar Jinendiran,Gangadhariah Mahesha,Alsabbagh Alchirazi Khaled,Mohan Maradumane L,Kumar Avinash,Kant Sashi,Thapaliya Samjhana,Stine McKenzie,McMullen Megan R,McCullough Rebecca L,Stark George R,Nagy Laura E,Naga Prasad Sathyamangla V,Dasarathy Srinivasan Hepatology (Baltimore, Md.) BACKGROUND AND AIMS:Despite the high clinical significance of sarcopenia in alcoholic cirrhosis, there are currently no effective therapies because the underlying mechanisms are poorly understood. We determined the mechanisms of ethanol-induced impaired phosphorylation of mechanistic target of rapamycin complex 1 (mTORC1) and AMP kinase (AMPK) with consequent dysregulated skeletal muscle protein homeostasis (balance between protein synthesis and breakdown). APPROACH AND RESULTS:Differentiated murine myotubes, gastrocnemius muscle from mice with loss and gain of function of regulatory genes following ethanol treatment, and skeletal muscle from alcoholic cirrhotics were used. Ethanol increases skeletal muscle autophagy by dephosphorylating mTORC1, circumventing the classical kinase regulation by protein kinase B (Akt). Concurrently and paradoxically, ethanol exposure results in dephosphorylation and inhibition of AMPK, an activator of autophagy and inhibitor of mTORC1 signaling. However, AMPK remains inactive with ethanol exposure despite lower cellular and tissue ATP indicating a "pseudofed" state. We identified protein phosphatase 2A (PP2A) as a key mediator of ethanol-induced signaling and functional perturbations using loss and gain of function studies. Ethanol impairs binding of endogenous inhibitor of PP2A (I2-PP2A) to PP2A resulting in methylation and targeting of PP2A to cause dephosphorylation of mTORC1 and AMPK. Activity of phosphoinositide 3-kinase-γ (PI3Kγ), a negative regulator of PP2A, was decreased in response to ethanol. Ethanol-induced molecular and phenotypic perturbations in wild type mice were observed in PI3Kγ mice even at baseline. Importantly, overexpressing kinase-active PI3Kγ but not the kinase-dead mutant reversed ethanol-induced molecular perturbations. CONCLUSIONS:Our study describes the mechanistic underpinnings for previously unrecognized ethanol-mediated dysregulation of protein homeostasis by PP2A that leads to sarcopenia with a potential for therapeutic approaches by targeting the PI3Kγ-PP2A axis. 10.1002/hep.31524
    Mesenchymal Bmp3b expression maintains skeletal muscle integrity and decreases in age-related sarcopenia. Uezumi Akiyoshi,Ikemoto-Uezumi Madoka,Zhou Heying,Kurosawa Tamaki,Yoshimoto Yuki,Nakatani Masashi,Hitachi Keisuke,Yamaguchi Hisateru,Wakatsuki Shuji,Araki Toshiyuki,Morita Mitsuhiro,Yamada Harumoto,Toyoda Masashi,Kanazawa Nobuo,Nakazawa Tatsu,Hino Jun,Fukada So-Ichiro,Tsuchida Kunihiro The Journal of clinical investigation Age-related sarcopenia constitutes an important health problem associated with adverse outcomes. Sarcopenia is closely associated with fat infiltration in muscle, which is attributable to interstitial mesenchymal progenitors. Mesenchymal progenitors are nonmyogenic in nature but are required for homeostatic muscle maintenance. However, the underlying mechanism of mesenchymal progenitor-dependent muscle maintenance is not clear, nor is the precise role of mesenchymal progenitors in sarcopenia. Here, we show that mice genetically engineered to specifically deplete mesenchymal progenitors exhibited phenotypes markedly similar to sarcopenia, including muscle weakness, myofiber atrophy, alterations of fiber types, and denervation at neuromuscular junctions. Through searching for genes responsible for mesenchymal progenitor-dependent muscle maintenance, we found that Bmp3b is specifically expressed in mesenchymal progenitors, whereas its expression level is significantly decreased during aging or adipogenic differentiation. The functional importance of BMP3B in maintaining myofiber mass as well as muscle-nerve interaction was demonstrated using knockout mice and cultured cells treated with BMP3B. Furthermore, the administration of recombinant BMP3B in aged mice reversed their sarcopenic phenotypes. These results reveal previously unrecognized mechanisms by which the mesenchymal progenitors ensure muscle integrity and suggest that age-related changes in mesenchymal progenitors have a considerable impact on the development of sarcopenia. 10.1172/JCI139617
    Losartan restores skeletal muscle remodeling and protects against disuse atrophy in sarcopenia. Burks Tyesha N,Andres-Mateos Eva,Marx Ruth,Mejias Rebeca,Van Erp Christel,Simmers Jessica L,Walston Jeremy D,Ward Christopher W,Cohn Ronald D Science translational medicine Sarcopenia, a critical loss of muscle mass and function because of the physiological process of aging, contributes to disability and mortality in older adults. It increases the incidence of pathologic fractures, causing prolonged periods of hospitalization and rehabilitation. The molecular mechanisms underlying sarcopenia are poorly understood, but recent evidence suggests that increased transforming growth factor-β (TGF-β) signaling contributes to impaired satellite cell function and muscle repair in aged skeletal muscle. We therefore evaluated whether antagonism of TGF-β signaling via losartan, an angiotensin II receptor antagonist commonly used to treat high blood pressure, had a beneficial impact on the muscle remodeling process of sarcopenic mice. We demonstrated that mice treated with losartan developed significantly less fibrosis and exhibited improved in vivo muscle function after cardiotoxin-induced injury. We found that losartan not only blunted the canonical TGF-β signaling cascade but also modulated the noncanonical TGF-β mitogen-activated protein kinase pathway. We next assessed whether losartan was able to combat disuse atrophy in aged mice that were subjected to hindlimb immobilization. We showed that immobilized mice treated with losartan were protected against loss of muscle mass. Unexpectedly, this protective mechanism was not mediated by TGF-β signaling but was due to an increased activation of the insulin-like growth factor 1 (IGF-1)/Akt/mammalian target of rapamycin (mTOR) pathway. Thus, blockade of the AT1 (angiotensin II type I) receptor improved muscle remodeling and protected against disuse atrophy by differentially regulating the TGF-β and IGF-1/Akt/mTOR signaling cascades, two pathways critical for skeletal muscle homeostasis. Thus, losartan, a Food and Drug Administration-approved drug, may prove to have clinical benefits to combat injury-related muscle remodeling and provide protection against disuse atrophy in humans with sarcopenia. 10.1126/scitranslmed.3002227
    Sarcopenia associated with portosystemic shunting is reversed by follistatin. Dasarathy Srinivasan,McCullough Arthur J,Muc Sean,Schneyer Alan,Bennett Carole D,Dodig Milan,Kalhan Satish C Journal of hepatology BACKGROUND & AIMS:The distinct role of portosystemic shunting (PSS) in the pathogenesis of sarcopenia (skeletal muscle loss) that occurs commonly in cirrhosis is unclear. We have previously shown increased expression of myostatin (inhibitor of skeletal muscle mass) in the portacaval anastamosis (PCA) rat model of sarcopenia of PSS. The present study was performed to examine the mechanisms of sarcopenia following PCA. METHODS:In PCA and sham operated pair fed control rats, the phenylalanine flooding dose method was used to quantify the fractional and absolute protein synthesis rates in the skeletal muscle over time and in response to follistatin, a myostatin antagonist. The expression of myostatin and markers of satellite cell (myocyte precursors) proliferation and differentiation were quantified by real-time PCR and Western blot analyses. RESULTS:The absolute synthesis rate (ASR) was lower at 2, 4, and 6 weeks (p<0.05) and the fractional synthesis rate (FSR) of skeletal muscle protein was significantly lower (p<0.05) at week 2 in the PCA rats compared to control rats. Expression of myostatin was elevated while markers of satellite cell proliferation and differentiation were lower at 4 and 6 weeks after PCA. Follistatin increased skeletal muscle mass, muscle FSR and ASR, decreased expression of myostatin protein, and increased expression of markers of satellite cell function. CONCLUSIONS:Sarcopenia associated with PSS is caused by impaired protein synthesis and reduced satellite cell function due to increased myostatin expression. Confirming these alterations in human patients with cirrhosis will provide novel therapeutic targets for sarcopenia of liver disease. 10.1016/j.jhep.2010.08.032
    Myostatin inhibition for treatment of sarcopenia. Parise Gianni,Snijders Tim The lancet. Diabetes & endocrinology 10.1016/S2213-8587(15)00324-1
    Inducible depletion of satellite cells in adult, sedentary mice impairs muscle regenerative capacity without affecting sarcopenia. Fry Christopher S,Lee Jonah D,Mula Jyothi,Kirby Tyler J,Jackson Janna R,Liu Fujun,Yang Lin,Mendias Christopher L,Dupont-Versteegden Esther E,McCarthy John J,Peterson Charlotte A Nature medicine A key determinant of geriatric frailty is sarcopenia, the age-associated loss of skeletal muscle mass and strength. Although the etiology of sarcopenia is unknown, the correlation during aging between the loss of activity of satellite cells, which are endogenous muscle stem cells, and impaired muscle regenerative capacity has led to the hypothesis that the loss of satellite cell activity is also a cause of sarcopenia. We tested this hypothesis in male sedentary mice by experimentally depleting satellite cells in young adult animals to a degree sufficient to impair regeneration throughout the rest of their lives. A detailed analysis of multiple muscles harvested at various time points during aging in different cohorts of these mice showed that the muscles were of normal size, despite low regenerative capacity, but did have increased fibrosis. These results suggest that lifelong reduction of satellite cells neither accelerated nor exacerbated sarcopenia and that satellite cells did not contribute to the maintenance of muscle size or fiber type composition during aging, but that their loss may contribute to age-related muscle fibrosis. 10.1038/nm.3710
    Ammonia lowering reverses sarcopenia of cirrhosis by restoring skeletal muscle proteostasis. Kumar Avinash,Davuluri Gangarao,Silva Rafaella Nascimento E,Engelen Marielle P K J,Ten Have Gabrie A M,Prayson Richard,Deutz Nicolaas E P,Dasarathy Srinivasan Hepatology (Baltimore, Md.) Sarcopenia or skeletal muscle loss is a frequent, potentially reversible complication in cirrhosis that adversely affects clinical outcomes. Hyperammonemia is a consistent abnormality in cirrhosis that results in impaired skeletal muscle protein synthesis and breakdown (proteostasis). Despite the availability of effective ammonia-lowering therapies, whether lowering ammonia restores proteostasis and increases muscle mass is unknown. Myotube diameter, protein synthesis, and molecular responses in C2C12 murine myotubes to withdrawal of ammonium acetate following 24-hour exposure to 10 mM ammonium acetate were complemented by in vivo studies in the hyperammonemic portacaval anastomosis rat and sham-operated, pair-fed Sprague-Dawley rats treated with ammonia-lowering therapy by l-ornithine l-aspartate and rifaximin orally for 4 weeks. We observed reduced myotube diameter, impaired protein synthesis, and increased autophagy flux in response to hyperammonemia, which were partially reversed following 24-hour and 48-hour withdrawal of ammonium acetate. Consistently, 4 weeks of ammonia-lowering therapy resulted in significant lowering of blood and skeletal muscle ammonia, increase in lean body mass, improved grip strength, higher skeletal muscle mass and diameter, and an increase in type 2 fibers in treated compared to untreated portacaval anastomosis rats. The increased skeletal muscle myostatin expression, reduced mammalian target of rapamycin complex 1 function, and hyperammonemic stress response including autophagy markers normally found in portacaval anastomosis rats were reversed by treatment with ammonia-lowering therapy. Despite significant improvement, molecular and functional readouts were not completely reversed by ammonia-lowering measures. CONCLUSION:Ammonia-lowering therapy results in improvement in skeletal muscle phenotype and function and molecular perturbations of hyperammonemia; these preclinical studies complement previous studies on ammonia-induced skeletal muscle loss and lay the foundation for prolonged ammonia-lowering therapy to reverse sarcopenia of cirrhosis. (Hepatology 2017;65:2045-2058). 10.1002/hep.29107
    The exerkine apelin reverses age-associated sarcopenia. Vinel Claire,Lukjanenko Laura,Batut Aurelie,Deleruyelle Simon,Pradère Jean-Philippe,Le Gonidec Sophie,Dortignac Alizée,Geoffre Nancy,Pereira Ophelie,Karaz Sonia,Lee Umji,Camus Mylène,Chaoui Karima,Mouisel Etienne,Bigot Anne,Mouly Vincent,Vigneau Mathieu,Pagano Allan F,Chopard Angèle,Pillard Fabien,Guyonnet Sophie,Cesari Matteo,Burlet-Schiltz Odile,Pahor Marco,Feige Jerome N,Vellas Bruno,Valet Philippe,Dray Cedric Nature medicine Sarcopenia, the degenerative loss of skeletal muscle mass, quality and strength, lacks early diagnostic tools and new therapeutic strategies to prevent the frailty-to-disability transition often responsible for the medical institutionalization of elderly individuals. Herein we report that production of the endogenous peptide apelin, induced by muscle contraction, is reduced in an age-dependent manner in humans and rodents and is positively associated with the beneficial effects of exercise in older persons. Mice deficient in either apelin or its receptor (APLNR) presented dramatic alterations in muscle function with increasing age. Various strategies that restored apelin signaling during aging further demonstrated that this peptide considerably enhanced muscle function by triggering mitochondriogenesis, autophagy and anti-inflammatory pathways in myofibers as well as enhancing the regenerative capacity by targeting muscle stem cells. Taken together, these findings revealed positive regulatory feedback between physical activity, apelin and muscle function and identified apelin both as a tool for diagnosis of early sarcopenia and as the target of an innovative pharmacological strategy to prevent age-associated muscle weakness and restore physical autonomy. 10.1038/s41591-018-0131-6
    Sarcopenia from mechanism to diagnosis and treatment in liver disease. Dasarathy Srinivasan,Merli Manuela Journal of hepatology Sarcopenia or loss of skeletal muscle mass is the major component of malnutrition and is a frequent complication in cirrhosis that adversely affects clinical outcomes. These include survival, quality of life, development of other complications and post liver transplantation survival. Radiological image analysis is currently utilized to diagnose sarcopenia in cirrhosis. Nutrient supplementation and physical activity are used to counter sarcopenia but have not been consistently effective because the underlying molecular and metabolic abnormalities persist or are not influenced by these treatments. Even though alterations in food intake, hypermetabolism, alterations in amino acid profiles, endotoxemia, accelerated starvation and decreased mobility may all contribute to sarcopenia in cirrhosis, hyperammonemia has recently gained attention as a possible mediator of the liver-muscle axis. Increased muscle ammonia causes: cataplerosis of α-ketoglutarate, increased transport of leucine in exchange for glutamine, impaired signaling by leucine, increased expression of myostatin (a transforming growth factor beta superfamily member) and an increased phosphorylation of eukaryotic initiation factor 2α. In addition, mitochondrial dysfunction, increased reactive oxygen species that decrease protein synthesis and increased autophagy mediated proteolysis, also play a role. These molecular and metabolic alterations may contribute to the anabolic resistance and inadequate response to nutrient supplementation in cirrhosis. Central and skeletal muscle fatigue contributes to impaired exercise capacity and responses. Use of proteins with low ammoniagenic potential, leucine enriched amino acid supplementation, long-term ammonia lowering strategies and a combination of resistance and endurance exercise to increase muscle mass and function may target the molecular abnormalities in the muscle. Strategies targeting endotoxemia and the gut microbiome need further evaluation. 10.1016/j.jhep.2016.07.040
    Sarcopenia: Aging-Related Loss of Muscle Mass and Function. Larsson Lars,Degens Hans,Li Meishan,Salviati Leonardo,Lee Young Il,Thompson Wesley,Kirkland James L,Sandri Marco Physiological reviews Sarcopenia is a loss of muscle mass and function in the elderly that reduces mobility, diminishes quality of life, and can lead to fall-related injuries, which require costly hospitalization and extended rehabilitation. This review focuses on the aging-related structural changes and mechanisms at cellular and subcellular levels underlying changes in the individual motor unit: specifically, the perikaryon of the α-motoneuron, its neuromuscular junction(s), and the muscle fibers that it innervates. Loss of muscle mass with aging, which is largely due to the progressive loss of motoneurons, is associated with reduced muscle fiber number and size. Muscle function progressively declines because motoneuron loss is not adequately compensated by reinnervation of muscle fibers by the remaining motoneurons. At the intracellular level, key factors are qualitative changes in posttranslational modifications of muscle proteins and the loss of coordinated control between contractile, mitochondrial, and sarcoplasmic reticulum protein expression. Quantitative and qualitative changes in skeletal muscle during the process of aging also have been implicated in the pathogenesis of acquired and hereditary neuromuscular disorders. In experimental models, specific intervention strategies have shown encouraging results on limiting deterioration of motor unit structure and function under conditions of impaired innervation. Translated to the clinic, if these or similar interventions, by saving muscle and improving mobility, could help alleviate sarcopenia in the elderly, there would be both great humanitarian benefits and large cost savings for health care systems. 10.1152/physrev.00061.2017
    Epidemiology of sarcopenia and insight into possible therapeutic targets. Dennison Elaine M,Sayer Avan A,Cooper Cyrus Nature reviews. Rheumatology Musculoskeletal ageing is a major public health concern owing to demographic shifts in the population. Sarcopenia, generally defined as the age-related loss of muscle mass and function, is associated with considerable risk of falls, loss of independence in older adults and hospitalization with poorer health outcomes. This condition is therefore associated with increased morbidity and health care costs. As with bone mass, muscle mass and strength increase during late adolescence and early adulthood, but begin to decline substantially from ∼50 years of age. Sarcopenia is characterized by many features, which include loss of muscle mass, altered muscle composition, infiltration with fat and fibrous tissue and alterations in innervation. A better understanding of these factors might help us to develop strategies that target these effects. To date, however, methodological challenges and controversies regarding how best to define the condition, in addition to uncertainty about what outcome measures to consider, have delayed research into possible therapeutic options. Most pharmacological agents investigated to date are hormonal, although new developments have seen the emergence of agents that target myostatin signalling to increase muscle mass. In this review we consider the current approaching for defining sarcopenia and discuss its epidemiology, pathogenesis, and potential therapeutic opportunities. 10.1038/nrrheum.2017.60
    Mitochondrial oxidative capacity and NAD biosynthesis are reduced in human sarcopenia across ethnicities. Migliavacca Eugenia,Tay Stacey K H,Patel Harnish P,Sonntag Tanja,Civiletto Gabriele,McFarlane Craig,Forrester Terence,Barton Sheila J,Leow Melvin K,Antoun Elie,Charpagne Aline,Seng Chong Yap,Descombes Patrick,Feng Lei,Francis-Emmanuel Patrice,Garratt Emma S,Giner Maria Pilar,Green Curtis O,Karaz Sonia,Kothandaraman Narasimhan,Marquis Julien,Metairon Sylviane,Moco Sofia,Nelson Gail,Ngo Sherry,Pleasants Tony,Raymond Frederic,Sayer Avan A,Ming Sim Chu,Slater-Jefferies Jo,Syddall Holly E,Fang Tan Pei,Titcombe Philip,Vaz Candida,Westbury Leo D,Wong Gerard,Yonghui Wu,Cooper Cyrus,Sheppard Allan,Godfrey Keith M,Lillycrop Karen A,Karnani Neerja,Feige Jerome N Nature communications The causes of impaired skeletal muscle mass and strength during aging are well-studied in healthy populations. Less is known on pathological age-related muscle wasting and weakness termed sarcopenia, which directly impacts physical autonomy and survival. Here, we compare genome-wide transcriptional changes of sarcopenia versus age-matched controls in muscle biopsies from 119 older men from Singapore, Hertfordshire UK and Jamaica. Individuals with sarcopenia reproducibly demonstrate a prominent transcriptional signature of mitochondrial bioenergetic dysfunction in skeletal muscle, with low PGC-1α/ERRα signalling, and downregulation of oxidative phosphorylation and mitochondrial proteostasis genes. These changes translate functionally into fewer mitochondria, reduced mitochondrial respiratory complex expression and activity, and low NAD levels through perturbed NAD biosynthesis and salvage in sarcopenic muscle. We provide an integrated molecular profile of human sarcopenia across ethnicities, demonstrating a fundamental role of altered mitochondrial metabolism in the pathological loss of skeletal muscle mass and function in older people. 10.1038/s41467-019-13694-1
    The neuromuscular junction is a focal point of mTORC1 signaling in sarcopenia. Ham Daniel J,Börsch Anastasiya,Lin Shuo,Thürkauf Marco,Weihrauch Martin,Reinhard Judith R,Delezie Julien,Battilana Fabienne,Wang Xueyong,Kaiser Marco S,Guridi Maitea,Sinnreich Michael,Rich Mark M,Mittal Nitish,Tintignac Lionel A,Handschin Christoph,Zavolan Mihaela,Rüegg Markus A Nature communications With human median lifespan extending into the 80s in many developed countries, the societal burden of age-related muscle loss (sarcopenia) is increasing. mTORC1 promotes skeletal muscle hypertrophy, but also drives organismal aging. Here, we address the question of whether mTORC1 activation or suppression is beneficial for skeletal muscle aging. We demonstrate that chronic mTORC1 inhibition with rapamycin is overwhelmingly, but not entirely, positive for aging mouse skeletal muscle, while genetic, muscle fiber-specific activation of mTORC1 is sufficient to induce molecular signatures of sarcopenia. Through integration of comprehensive physiological and extensive gene expression profiling in young and old mice, and following genetic activation or pharmacological inhibition of mTORC1, we establish the phenotypically-backed, mTORC1-focused, multi-muscle gene expression atlas, SarcoAtlas (https://sarcoatlas.scicore.unibas.ch/), as a user-friendly gene discovery tool. We uncover inter-muscle divergence in the primary drivers of sarcopenia and identify the neuromuscular junction as a focal point of mTORC1-driven muscle aging. 10.1038/s41467-020-18140-1