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The neuromuscular junction is a focal point of mTORC1 signaling in sarcopenia [Aging-Rapamycin data set].

ABSTRACT: Purpose: Despite demonstrating that the overall effect of long-term rapamycin-treatment is overwhelmingly positive in aging skeletal muscle, we observed muscle-specificity in the responsiveness to rapamycin, leading us to hypothesize that the primary drivers of age-related muscle loss and therefore effective intervention strategies may differ between muscles. To address this question and dissect the key signaling nodes associated with mTORC1-driven muscle aging, we created a comprehensive multi-muscle gene expression atlas in adult, sarcopenic and rapamycin-treated mice using RNA-seq. Methods: To examine the impact of long-term rapamycin treatment, male C57BL/6 mice were fed encapsulated rapamycin incorporated into a standardized AIN93M diet at 42 ppm (i.e. mg per kg of food), corresponding to a dose of ~4 mg·kg-1·day-1, from 15- to 30-months of age. This dose of rapamycin has been shown to extend lifespan maximally in male C57BL/6 mice. We performed RNA-seq on gastrocnemius (GAS), tibialis anterior (TA), triceps brachii (TRI) and soleus (SOL) muscles from each of six mice for 10m, 30m and 30mRM groups. The four muscle types were chosen to encompass fore- and hindlimb locations (e.g. TRI and GAS); slow and fast contraction properties (e.g. SOL and GAS); anterior and posterior positioning (e.g. TA and GAS) and the extent of protection by rapamycin (TA and TRI: protected; SOL: partiallly protected; GAS: not protected). Results: Age-related gene expression changes were remarkably consistent across the four different muscles, varying only in magnitude. Despite having the smallest age-related muscle loss of the four muscles, TA had the strongest age-related gene expression response which was associated with an increased reinnervation response. Despite the strong between-muscle commonality of age-related changes, gene expression responses to prolonged rapamycin treatment differed substantially between muscles. Rapamycin partially reversed many age-related changes in mRNA expression in the TA and TRI, but not in SOL or GAS muscle. Principle component analysis (PCA) showed that rapamycin had common 'anti-aging' effects on all muscles, while also exerting muscle-specific pro-aging effects on muscles not protected by rapamycin. Conclusions: Sustained, muscle fiber-specific mTORC1 activity drives sarcopenia-like gene expression programs, and the hyperactive mTORC1 seen in sarcopenic muscle may therefore contribute to sarcopenia.

ORGANISM(S): Mus musculus

PROVIDER: GSE139204 | GEO | 2020/06/19

REPOSITORIES: GEO

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Project description:Mitochondrial dysfunction caused by mitochondrial (mtDNA) deletions have been associated with skeletal muscle atrophy and myofiber loss. However, whether such defects occurring in myofibers cause sarcopenia is unclear. Also, the contribution of mtDNA alterations in muscle stem cells (MuSCs) to sarcopenia remain to be investigated. We expressed a dominant-negative variant of the mitochondrial helicase, which induces mtDNA alterations, specifically in differentiated myofibers (K320Eskm mice) and MuSCs (K320Emsc mice), respectively, and investigated their impact on muscle structure and function by immunohistochemistry, analysis of mtDNA and respiratory chain content, muscle transcriptome and functional tests. K320Eskm mice at 24 months of age had higher levels of mtDNA deletions compared to controls in soleus (SOL, 0.07673 % vs 0.00015 %, p=0.0167), extensor digitorium longi (EDL, 0.0649 vs 0.000925, p= 0.0015) and gastrocnemius (GAS, 0.09353 vs 0.000425, p=0.0004). K320Eskm mice revealed a progressive increase in the proportion of cytochrome c oxidase deficient fibers (COX-) in cross sections, reaching a maximum of 3.03%, 4.36%, 13.58% and 17.08% in EDL, SOL, tibialis anterior (TA) and GAS, respectively. However, mice did not show accelerated loss of muscle mass, muscle strength or physical performance. Histological analyses revealed ragged red fibers but also stimulated regeneration, indicating activation of MuSCs. RNAseq demonstrated enhanced expression of genes associated with protein synthesis, but also degradation, as well as muscle fiber differentiation and cell proliferation. In contrast, 7 days after destruction by cardiotoxin, regenerating TA of K320Emsc mice showed 30% of COX- fibers. Notably, regenerated muscle showed dystrophic changes, increased fibrosis (2.5% vs 1.6%, p=0.0003), increased abundance of fat cells (2.76% vs 0.23%, p=0.0144) and reduced muscle mass (regenerated TA: 40.0mg vs 60.2mg, p=0.0171). In contrast to muscles from K320Eskm mice, freshly isolated MuSCs from aged K320Emsc mice were completely devoid of mtDNA alterations. However, after passaging, mtDNA copy number as well as respiratory chain subunits and p62 levels gradually decreased.Taken together, accumulation of large-scale mtDNA alterations in myofibers alone is not sufficient to cause sarcopenia. Expression of K320E is tolerated in quiescent MuSCs, but progressively leads to mtDNA and respiratory chain depletion upon activation, in vivo and in vitro, possibly caused by an increased mitochondrial removal. Altogether, our results suggest that the accumulation of mtDNA alterations in myofibers activates regeneration during aging, which leads to sarcopenia if such alterations have expanded in MuSCs as well.

Project description:Belt electrode-skeletal muscle electrical stimulation (B-SES) involves the use of belt-shaped electrodes to simultaneously contract multiple muscle groups. Twitch contractions have been demonstrated to protect against denervation-induced muscle atrophy in rats, possibly via mitochondrial biosynthesis. In this study, we examined whether inducing tetanus contractions with B-SES suppresses muscle atrophy and identified the underlying molecular mechanisms. We evaluated the effects of acute (60 Hz, 5 min) and chronic (60 Hz, 5 min, every alternate day for 1 week) B-SES on the tibialis anterior (TA) and gastrocnemius (GAS) muscles in Sprague Dawley rats using belt electrodes attached to both ankle joints. In acute stimulation, a significant decrease in glycogen content in the left and right TA and GAS was observed, suggesting that B-SES causes simultaneous contractions in multiple muscle groups. B-SES also enhanced p70S6K phosphorylation, an indicator of the mechanistic target of rapamycin complex 1 activity. During chronic stimulations, rats were divided into control (CONT), denervation-induced atrophy (DEN), and DEN+electrically stimulated with B-SES (DEN＋ES) groups. After 7 days of treatment, muscle wet weight (n = 8-11 for each group) and muscle fiber cross-sectional area (CSA, n = 6 for each group) of the TA and GAS muscles were reduced in the DEN and DEN+ES groups compared to those in the CON group. The DEN+ES group showed significantly higher muscle weight and CSA than the DEN group. Although RNA-seq and pathway analysis suggested that mitochondrial and ribosome biogenesis are key events in this phenomenon, mitochondrial content showed no difference. In contrast, ribosomal RNA 28S and 45S (n = 6) levels in the DEN+ES group were higher than those in the DEN group. The mRNA levels of the muscle proteolytic molecules Atrogin-1 and MuRF1 were significantly higher in DEN than in CONT but were suppressed in DEN+ES. In conclusion, tetanic electrical stimulation of both ankles using belt electrodes was effective in preventing denervation-induced atrophy in multiple muscle groups. Unlike twitch contractions, ribosomal biosynthesis plays a key role in tetanic contractions to prevent muscle atrophy.

Project description:Maintaining skeletal muscle mass is of high importance as muscle atrophy like during sarcopenia or cachexia lead to a decrease in independence and a higher risk of morbidity and mortality. A leading compound in the treatment against ageing and cancer is rapamycin, an inhibitor of mechanistic target of rapamycin complex 1 (mTORC1). Whether the treatment with mTORC1 inhibitors would work at a cost of losing muscle mass is unclear, as most studies have been focusing on the role of mTORC1 specifically during hypertrophy. In order to answer this question we developed an inducible muscle specific knockout mouse model in which raptor can be ablated during adulthood to eliminate mTORC1 activity. We analysed the muscles after different time points and found that after 3 months the mice showed a fiber shift towards slower fiber types, a loss in oxidative capacity but only very few myopathic features. After 5 months the myopathic features became more apparent, however it did not largely affect the ex vivo muscle force. Surprisingly despite the myopathy we did not see a significant loss of muscle mass even after 5 months, that we hypothesised based on mTORC1s central role in protein synthesis. We assume that the myopathy after long-term mTORC1 inactivation is mostly a result of secondary effects through the loss of mitochondria, alterations in metabolism and in cytoskeletal components. In conclusion, during skeletal muscle maintenance mTORC1 is more essential for metabolic processes than it is for maintaining basal muscle mass.Maintaining skeletal muscle mass is of high importance as muscle atrophy like during sarcopenia or cachexia lead to a decrease in independence and a higher risk of morbidity and mortality. A leading compound in the treatment against ageing and cancer is rapamycin, an inhibitor of mechanistic target of rapamycin complex 1 (mTORC1). Whether the treatment with mTORC1 inhibitors would work at a cost of losing muscle mass is unclear, as most studies have been focusing on the role of mTORC1 specifically during hypertrophy. In order to answer this question we developed an inducible muscle specific knockout mouse model in which raptor can be ablated during adulthood to eliminate mTORC1 activity. We analysed the muscles after different time points and found that after 3 months the mice showed a fiber shift towards slower fiber types, a loss in oxidative capacity but only very few myopathic features. After 5 months the myopathic features became more apparent, however it did not largely affect the ex vivo muscle force. Surprisingly despite the myopathy we did not see a significant loss of muscle mass even after 5 months, that we hypothesised based on mTORC1s central role in protein synthesis. We assume that the myopathy after long-term mTORC1 inactivation is mostly a result of secondary effects through the loss of mitochondria, alterations in metabolism and in cytoskeletal components. In conclusion, during skeletal muscle maintenance mTORC1 is more essential for metabolic processes than it is for maintaining basal muscle mass.

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The neuromuscular junction is a focal point of mTORC1 signaling in sarcopenia [Aging-Rapamycin data set].

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