Effects of aging and caloric restriction on fiber type composition, mitochondrial morphology and dynamics in rat oxidative and glycolytic muscles

Aging is associated with a progressive decline in muscle mass and strength, a process known as sarcopenia. Evidence indicates that mitochondrial dysfunction plays a causal role in sarcopenia and suggests that alterations in mitochondrial dynamics/morphology may represent an underlying mechanism. Caloric restriction (CR) is among the most efficient nonpharmacological interventions to attenuate sarcopenia in rodents and is thought to exert its beneficial effects by improving mitochondrial function. However, CR effects on mitochondrial morphology and dynamics, especially in aging muscle, remain unknown. To address this issue, we investigated mitochondrial morphology and dynamics in the oxidative soleus (SOL) and glycolytic white gastrocnemius (WG) muscles of adult (9-month-old) ad libitum-fed (AL; A-AL), old (22-month-old) AL-fed (O-AL), and old CR (O-CR) rats. We show that CR attenuates the aging-related decline in the muscle-to-body-weight ratio, a sarcopenic index. CR also prevented the effects of aging on muscle fiber type composition in both muscles. With aging, the SOL displayed fragmented SubSarcolemmal (SS) and InterMyoFibrillar (IMF) mitochondria, an effect attenuated by CR. Aged WG displayed enlarged SS and more complex/branched IMF mitochondria. CR had marginal anti-aging effects on WG mitochondrial morphology. In the SOL, DRP1 (pro-fission protein) content was higher in O-AL vs YA-AL, and Mfn2 (pro-fusion) content was higher in O-CR vs A-AL. In the gastrocnemius, Mfn2, Drp1, and Fis1 (pro-fission) contents were higher in O-AL vs A-AL. CR reduced this aging-related increase in Mfn2 and Fis1 content. Overall, these results reveal for the first time that aging differentially impacts mitochondrial morphology and dynamics in different muscle fiber types, by increasing fission/fragmentation in oxidative fibers while enhancing mitochondrial size and branching in glycolytic fibers. Our results also indicate that although CR partially attenuates aging-related changes in mitochondrial dynamics in glycolytic fibers, its anti-aging effect on mitochondrial morphology is restricted to oxidative fibers

Lack of compensatory mitophagy in skeletal muscles during sepsis

Abstract Skeletal muscle dysfunction is a major problem in critically ill patients suffering from sepsis. This condition is associated with mitochondrial dysfunction and increased autophagy in skeletal muscles. Autophagy is a proteolytic mechanism involved in eliminating dysfunctional cellular components, including mitochondria. The latter process, referred to as mitophagy, is essential for maintaining mitochondrial quality and skeletal muscle health. Recently, a fluorescent reporter system called mito-QC (i.e. mitochondrial quality control) was developed to specifically quantify mitophagy levels. In the present study, we used mito-QC transgenic mice and confocal microscopy to morphologically monitor mitophagy levels during sepsis. To induce sepsis, Mito-QC mice received Escherichia coli lipopolysaccharide (10 mg kg–1 i.p.) or phosphate-buffered saline and skeletal muscles (hindlimb and diaphragm) were excised 48 h later. In control groups, there was a negative correlation between the basal mitophagy level and overall muscle mitochondrial content. Sepsis increased general autophagy in both limb muscles and diaphragm but had no effect on mitophagy levels. Sepsis was associated with a downregulation of certain mitophagy receptors (Fundc1, Bcl2L13, Fkbp8 and Phbb2). The present study suggests that general autophagy and mitophagy can be dissociated from one another, and that the characteristic accumulation of damaged mitochondria in skeletal muscles under the condition of sepsis may reflect a failure of adequate compensatory mitophagy. Key points - There was a negative correlation between the basal level of skeletal muscle mitophagy and the mitochondrial content of individual muscles. - Mitophagy levels in limb muscles and the diaphragm were unaffected by lipopolysaccharide (LPS)-induced sepsis. - With the exception of BNIP3 in sepsis, LPS administration induced either no change or a downregulation of mitophagy receptors in skeletal muscles

Parkin overexpression attenuates sepsis-induced muscle wasting

Sepsis elicits skeletal muscle weakness and fiber atrophy. The accumulation of injured mitochondria and depressed mitochondrial functions are considered as important triggers of sepsis-induced muscle atrophy. It is unclear whether mitochondrial dysfunctions in septic muscles are due to the inadequate activation of quality control processes. We hypothesized that overexpressing Parkin, a protein responsible for the recycling of dysfunctional mitochondria by the autophagy pathway (mitophagy), would confer protection against sepsis-induced muscle atrophy by improving mitochondrial quality and content. Parkin was overexpressed for four weeks in the limb muscles of four-week old mice using intramuscular injections of adeno-associated viruses (AAVs). The cecal ligation and perforation (CLP) procedure was used to induce sepsis. Sham operated animals were used as controls. All animals were studied for 48 h post CLP. Sepsis resulted in major body weight loss and myofiber atrophy. Parkin overexpression prevented myofiber atrophy in CLP mice. Quantitative two-dimensional transmission electron microscopy revealed that sepsis is associated with the accumulation of enlarged and complex mitochondria, an effect which was attenuated by Parkin overexpression. Parkin overexpression also prevented a sepsis-induced decrease in the content of mitochondrial subunits of NADH dehydrogenase and cytochrome C oxidase. We conclude that Parkin overexpression prevents sepsis-induced skeletal muscle atrophy, likely by improving mitochondrial quality and contents

MFN2 overexpression in skeletal muscles of young and old mice causes a mild hypertrophy without altering mitochondrial respiration and H2O2 emission

Abstract Aim Sarcopenia, the aging-related loss of muscle mass and function, is a debilitating process negatively impacting the quality of life of affected individuals. Although the mechanisms underlying sarcopenia are incompletely understood, impairments in mitochondrial dynamics, including mitochondrial fusion, have been proposed as a contributing factor. However, the potential of upregulating mitochondrial fusion proteins to alleviate the effects of aging on skeletal muscles remains unexplored. We therefore hypothesized that overexpressing Mitofusin 2 (MFN2) in skeletal muscle in vivo would mitigate the effects of aging on muscle mass and improve mitochondrial function. Methods MFN2 was overexpressed in young (7 mo) and old (24 mo) male mice for 4 months through intramuscular injections of an adeno-associated viruses. The impacts of MFN2 overexpression on muscle mass and fiber size (histology), mitochondrial respiration, and H2O2 emission (Oroboros fluororespirometry), and various signaling pathways (qPCR and western blotting) were investigated. Results MFN2 overexpression increased muscle mass and fiber size in both young and old mice. No sign of fibrosis, necrosis, or inflammation was found upon MFN2 overexpression, indicating that the hypertrophy triggered by MFN2 overexpression was not pathological. MFN2 overexpression even reduced the proportion of fibers with central nuclei in old muscles. Importantly, MFN2 overexpression had no impact on muscle mitochondrial respiration and H2O2 emission in both young and old mice. MFN2 overexpression attenuated the increase in markers of impaired autophagy in old muscles. Conclusion MFN2 overexpression may be a viable approach to mitigate aging-related muscle atrophy and may have applications for other muscle disorders

Mitochondrial morphology is altered in atrophied skeletal muscle of aged mice

Abstract Skeletal muscle aging is associated with a progressive decline in muscle mass and strength, a process termed sarcopenia. Evidence suggests that accumulation of mitochondrial dysfunction plays a causal role in sarcopenia, which could be triggered by impaired mitophagy. Mitochondrial function, mitophagy and mitochondrial morphology are interconnected aspects of mitochondrial biology, and may coordinately be altered with aging. However, mitochondrial morphology has remained challenging to characterize in muscle, and whether sarcopenia is associated with abnormal mitochondrial morphology remains unknown. Therefore, we assessed the morphology of SubSarcolemmal (SS) and InterMyoFibrillar (IMF) mitochondria in skeletal muscle of young (8-12wk-old) and old (88-96wk-old) mice using a quantitative 2-dimensional transmission electron microscopy approach. We show that sarcopenia is associated with larger and less circular SS mitochondria. Likewise, aged IMF mitochondria were longer and more branched, suggesting increased fusion and/or decreased fission. Accordingly, although no difference in the content of proteins regulating mitochondrial dynamics (Mfn1, Mfn2, Opa1 and Drp1) was observed, a mitochondrial fusion index (Mfn2-to-Drp1 ratio) was significantly increased in aged muscles. Our results reveal that sarcopenia is associated with complex changes in mitochondrial morphology that could interfere with mitochondrial function and mitophagy, and thus contribute to aging-related accumulation of mitochondrial dysfunction and sarcopenia

Resistance training in women with myotonic dystrophy type 1: A multisystemic therapeutic avenue

Abstract Myotonic dystrophy type 1 (DM1) is a hereditary disease characterized by muscular impairments. Fundamental and clinical positive effects of strength training have been reported in men with DM1, but its impact on women remains unknown. We evaluated the effects of a 12-week supervised strength training on physical and neuropsychiatric health. Women with DM1 performed a twice-weekly supervised resistance training program (3 series of 6–8 repetitions of squat, leg press, plantar flexion, knee extension, and hip abduction). Lower limb muscle strength, physical function, apathy, anxiety and depression, fatigue and excessive somnolence, pain, and patient-reported outcomes were assessed before and after the intervention, as well as three and six months after completion of the training program. Muscle biopsies of the vastus lateralis were also taken before and after the training program to assess muscle fiber growth. Eleven participants completed the program (attendance: 98.5 %). Maximal hip and knee extension strength (p < 0.006), all One-Repetition Maximum strength measures (p < 0.001), apathy (p = 0.0005), depression (p = 0.02), pain interference (p = 0.01) and perception of the lower limb function (p = 0.003) were significantly improved by training. Some of these gains were maintained up to six months after the training program. Strength training is a good therapeutic strategy for women with DM1

MYTHO is a novel regulator of skeletal muscle autophagy and integrity

Autophagy is a critical process in the regulation of muscle mass, function and integrity. The molecular mechanisms regulating autophagy are complex and still partly understood. Here, we identify and characterize a novel FoxO-dependent gene, d230025d16rik which we named Mytho (Macroautophagy and YouTH Optimizer), as a regulator of autophagy and skeletal muscle integrity in vivo. Mytho is significantly up-regulated in various mouse models of skeletal muscle atrophy. Short term depletion of MYTHO in mice attenuates muscle atrophy caused by fasting, denervation, cancer cachexia and sepsis. While MYTHO overexpression is sufficient to trigger muscle atrophy, MYTHO knockdown results in a progressive increase in muscle mass associated with a sustained activation of the mTORC1 signaling pathway. Prolonged MYTHO knockdown is associated with severe myopathic features, including impaired autophagy, muscle weakness, myofiber degeneration, and extensive ultrastructural defects, such as accumulation of autophagic vacuoles and tubular aggregates. Inhibition of the mTORC1 signaling pathway in mice using rapamycin treatment attenuates the myopathic phenotype triggered by MYTHO knockdown. Skeletal muscles from human patients diagnosed with myotonic dystrophy type 1 (DM1) display reduced Mytho expression, activation of the mTORC1 signaling pathway and impaired autophagy, raising the possibility that low Mytho expression might contribute to the progression of the disease. We conclude that MYTHO is a key regulator of muscle autophagy and integrity

The impact of ageing, physical activity, and pre-frailty on skeletal muscle phenotype, mitochondrial content, and intramyocellular lipids in men

Abtract Background: The exact impact of ageing on skeletal muscle phenotype and mitochondrial and lipid content remains controversial, probably because physical activity, which greatly influences muscle physiology, is rarely accounted for. The present study was therefore designed to investigate the effects of ageing, physical activity, and pre-frailty on skeletal muscle phenotype, and mitochondrial and intramyocellular lipid content in men. Methods: Recreationally active young adult (20–30 yo; YA); active (ACT) and sedentary (SED) middle-age (50–65 yo; MA-ACT and MA-SED); and older (65 + yo; 65 + ACT and 65 + SED) and pre-frail older (65 + PF) men were recruited. Muscle biopsies from the vastus lateralis were collected to assess, on muscle cross sections, muscle phenotype (using myosin heavy chain isoforms immunolabelling), the fibre type-specific content of mitochondria (by quantifying the succinate dehydrogenase stain intensity), and the fibre type-specific lipid content (by quantifying the Oil Red O stain intensity). Results: Only 65 + SED and 65 + PF displayed significantly lower overall and type IIa fibre sizes vs. YA. 65 + SED displayed a lower type IIa fibre proportion vs. YA. MA-SED and 65 + SED displayed a higher hybrid type IIa/IIx fibre proportion vs. YA. Sedentary and pre-frail, but not active, men displayed lower mitochondrial content irrespective of fibre type vs. YA. 65 + SED, but not 65 + ACT, displayed a higher lipid content in type I fibres vs. YA. Finally, mitochondrial content, but not lipid content, was positively correlated with indices of muscle function, functional capacity, and insulin sensitivity across all subjects. Conclusions: Taken altogether, our results indicate that ageing in sedentary men is associated with (i) complex changes in muscle phenotype preferentially affecting type IIa fibres; (ii) a decline in mitochondrial content affecting all fibre types; and (iii) an increase in lipid content in type I fibres. They also indicate that physical activity partially protects from the effects of ageing on muscle phenotype, mitochondrial content, and lipid accumulation. No skeletal specific muscle phenotype of pre-frailty was observed