Satellite cell content is specifically reduced in type II skeletal muscle fibers in the elderly

Myotendinous junction plasticity in aged ovariectomized rats submitted to aquatic training

The study aims to describe the tissue plasticity of MTJ through the morphological analysis of MTJ soleus in ovariectomized aged female Wistar rats submitted to aquatic training. Forty aged Wistar rats, 1 year and 2 months of age, were divided into four groups: sedentary (S), trained (T), ovariectomized (O), and trained/ovariectomized (OT). Employing the transmission electron microscopy, the ultrastructural and morphometric elements were revealed. In the S group, changes in morphological characteristics as a consequence of the aging process were seen, demonstrated by the conical shape of the muscle cell extremity, a large area with collagen deposit, and misalignment of sarcomeres in series. The T group presented ample adjustments when revealed the organization of MTJ, through the increase of the contact area and greater lengths of sarcoplasmatic invaginations and evaginations. The O group revealed extensive tissue disorganization with muscle atrophy, reduction of MTJ contact area, and consequently, changes in sarcoplasmatic invaginations and evaginations. The OT group demonstrated extensive remodeling with restructuring MTJ through the increase of tissue contact area, extensive organization, parallel arrangement, and increased length of sarcoplasmatic invaginations and evaginations. The distal sarcomeres presented higher lengths compared to the proximal sarcomeres in both the groups. We conclude that aquatic training was effective in the organization and structural remodeling of the myotendinous interface of ovariectomized aged rats. There was a greater area of contact, and consequently, greater resistance in the myotendinous interface promoting a lower predisposition to injuries.

The role of DNA-PK in aging and energy metabolism

DNA-dependent protein kinase (DNA-PK) is a very large holoenzyme comprised of the p470 kDa DNA-PK catalytic subunit (DNA-PK_cs ) and the Ku heterodimer consisting of the p86 (Ku 80) and p70 (Ku 70) subunits. It is best known for its nonhomologous end joining (NHEJ) activity, which repairs double-strand DNA (dsDNA) breaks (DSBs). As expected, the absence of DNA-PK activity results in sensitivity to ionizing radiation, which generates DSBs and defect in lymphocyte development, which requires NHEJ of the V(D)J region in the immunoglobulin and T-cell receptor loci. DNA-PK also has been reported to have functions seemingly unrelated to NHEJ. For example, DNA-PK responds to insulin signaling to facilitate the conversion of carbohydrates to fatty acids in the liver. More recent evidence indicates that DNA-PK activity increases with age in skeletal muscle, promoting mitochondrial loss and weight gain. These discoveries suggest that our understanding of DNA-PK is far from complete. As many excellent reviews have already been written about the role of DNA-PK in NHEJ, here we will review the non-NHEJ role of DNA-PK with a focus on its role in aging and energy metabolism.

Potential Roles of n-3 PUFAs during Skeletal Muscle Growth and Regeneration

Omega-3 polyunsaturated fatty acids (n-3 PUFAs), which are commonly found in fish oil supplements, are known to possess anti-inflammatory properties and more recently alter skeletal muscle function. In this review, we discuss novel findings related to how n-3 PUFAs modulate molecular signaling responsible for growth and hypertrophy as well as the activity of muscle stem cells. Muscle stem cells commonly known as satellite cells, are primarily responsible for driving the skeletal muscle repair process to potentially damaging stimuli, such as mechanical stress elicited by exercise contraction. To date, there is a paucity of human investigations related to the effects of n-3 PUFAs on satellite cell content and activity. Based on current in vitro investigations, this review focuses on novel mechanisms linking n-3 PUFA’s to satellite cell activity and how they may improve muscle repair. Understanding the role of n-3 PUFAs during muscle growth and regeneration in association with exercise could lead to the development of novel supplementation strategies that increase muscle mass and strength, therefore possibly reducing the burden of muscle wasting with age.

Effects of Exercise and Aging on Skeletal Muscle

A substantial loss of muscle mass and strength (sarcopenia), a decreased regenerative capacity, and a compromised physical performance are hallmarks of aging skeletal muscle. These changes are typically accompanied by impaired muscle metabolism, including mitochondrial dysfunction and insulin resistance. A challenge in the field of muscle aging is to dissociate the effects of chronological aging per se on muscle characteristics from the secondary influence of lifestyle and disease processes. Remarkably, physical activity and exercise are well-established countermeasures against muscle aging, and have been shown to attenuate age-related decreases in muscle mass, strength, and regenerative capacity, and slow or prevent impairments in muscle metabolism. We posit that exercise and physical activity can influence many of the changes in muscle during aging, and thus should be emphasized as part of a lifestyle essential to healthy aging.

Sarcopenic Dysphagia. A Narrative Review

Sarcopenic dysphagia is described as difficulty swallowing due to generalized sarcopenia of skeletal muscles and thus swallowing muscles. It is a recently recognized condition. It draws attention due to its important complications. The risk factors of dysphagia include age, history of clinical disease, and physical frailty, including reduced activities of daily living. It is a common syndrome among the elderly and demands multidisciplinary therapeutic interventions, including nutritional support and rehabilitation programs, which are non-invasive but effective methods, mandatory for the best outcome. The prevention, assessment, and intervention methods for sarcopenic dysphagia are very important. Recent studies demonstrate that new concepts in rehabilitation and nutritional support render promising results.

Differing Effects of Younger and Older Human Plasma on C2C12 Myocytes in Vitro

Ageing is associated with a general reduction of physiological function and a reduction of muscle mass and strength. Endocrine factors such as myostatin, activin A, growth and differentiation factor 11 (GDF-11) and their inhibitory peptides influence muscle mass in health and disease. We hypothesised that myocytes cultured in plasma from older and younger individuals would show an ageing effect, with reduced proliferation and differentiation in older environments. C2C12 myoblasts were grown as standard and stimulated with media conditioned with 5% plasma from healthy male participants that were either younger (n = 6, 18–35 years of age) or older (n = 6, >57 years of age). Concentration of plasma myostatin (total and free), follistatin-like binding protein (FLRG), GDF-11 and activin A were quantified by ELISA. Both FLRG and activin A were elevated in older individuals (109.6 and 35.1% increase, respectively), whilst myostatin (free and total) and GDF-11 were not. Results indicated that plasma activin A and FLRG were increased in older vs. younger participants, GDF11 and myostatin did not differ. Myoblasts in vitro showed no difference in proliferation rate between ages, however scratch closure was greater in younger vs. older plasma stimulated myoblasts (78.2 vs. 87.2% of baseline scratch diameter, respectively). Myotube diameters were larger in cells stimulated with younger plasma than with older at 24 and 48 h, but not at 2 h. A significant negative correlation was noted between in vivo plasma FLRG concentration and in vitro myotube diameter 48 h following plasma stimulation (r² = 0.392, p = 0.030). Here we show that myoblasts and myotubes cultured in media conditioned with plasma from younger or older individuals show an ageing effect, and further this effect moderately correlates with circulating FLRG concentration in vivo. The effect of ageing on muscle function may not be innate to the tissue, but involve a general cellular environment change. Further work is needed to examine the effect of increased FLRG concentration on muscle function in ageing populations.

Skeletal muscle performance and ageing

The world population is ageing rapidly. As society ages, the incidence of physical limitations is dramatically increasing, which reduces the quality of life and increases healthcare expenditures. In western society, ~30% of the population over 55 years is confronted with moderate or severe physical limitations. These physical limitations increase the risk of falls, institutionalization, co-morbidity, and premature death. An important cause of physical limitations is the age-related loss of skeletal muscle mass, also referred to as sarcopenia. Emerging evidence, however, clearly shows that the decline in skeletal muscle mass is not the sole contributor to the decline in physical performance. For instance, the loss of muscle strength is also a strong contributor to reduced physical performance in the elderly. In addition, there is ample data to suggest that motor coordination, excitation-contraction coupling, skeletal integrity, and other factors related to the nervous, muscular, and skeletal systems are critically important for physical performance in the elderly. To better understand the loss of skeletal muscle performance with ageing, we aim to provide a broad overview on the underlying mechanisms associated with elderly skeletal muscle performance. We start with a system level discussion and continue with a discussion on the influence of lifestyle, biological, and psychosocial factors on elderly skeletal muscle performance. Developing a broad understanding of the many factors affecting elderly skeletal muscle performance has major implications for scientists, clinicians, and health professionals who are developing therapeutic interventions aiming to enhance muscle function and/or prevent mobility and physical limitations and, as such, support healthy ageing.

The Influence of Clinically Diagnosed Neuropathy on Respiratory Muscle Strength in Type 2 Diabetes Mellitus

OBJECTIVES

This cross-sectional study investigated the influence of clinically diagnosed neuropathy (cdNP) on respiratory muscle strength in patients with type 2 diabetes mellitus (T2DM).

METHODS

110 T2DM patients and 35 nondiabetic healthy controls (≥60 years) were allocated to one of three groups depending on the presence of cdNP: T2DM without cdNP (D-; n = 28), T2DM with cdNP (D+; n = 82), and controls without cdNP (C; n = 35). Clinical neurological diagnostic examination consisted of Vibration Perception Threshold and Diabetic Neuropathy Symptom score. Respiratory muscle strength was registered by maximal Inspiratory and Expiratory Pressures (PI_max and PE_max), and respiratory function by Peak Expiratory Flow (PEF). Isometric Handgrip Strength and Short Physical Performance Battery were used to evaluate peripheral skeletal muscle strength and physical performance. Univariate analysis of covariance was used with age, level of physical activity, and body mass index as covariates.

RESULTS

PI_max, PE_max, and PEF were higher in C compared to D- and D+. Exploring more in detail, PI_max, PE_max, and PEF were significantly lower in D+ compared to C. PE_max and PEF were also significantly lower in D- versus C. Measures of peripheral muscle strength and physical performance showed less associations with cdNP and T2DM.

CONCLUSIONS

The presence of cdNP affects respiratory muscle strength in T2DM patients compared to healthy controls. Both cdNP and diabetes in themselves showed a distinctive impact on respiratory muscle strength and function; however, an accumulating effect could not be ascertained in this study. As commonly used measures of peripheral muscle strength and physical performance seemed to be less affected at the given time, the integration of PI_max, PE_max, and PEF measurements in the assessment of respiratory muscle weakness could be of added value in the (early) screening for neuropathy in patients with T2DM.

The role of declining adaptive homeostasis in ageing

Adaptive homeostasis is "the transient expansion or contraction of the homeostatic range for any given physiological parameter in response to exposure to sub-toxic, non-damaging, signalling molecules or events, or the removal or cessation of such molecules or events" (Davies, 2016). Adaptive homeostasis enables biological systems to make continuous short-term adjustments for optimal functioning despite ever-changing internal and external environments. Initiation of adaptation in response to an appropriate signal allows organisms to successfully cope with much greater, normally toxic, stresses. These short-term responses are initiated following effective signals, including hypoxia, cold shock, heat shock, oxidative stress, exercise-induced adaptation, caloric restriction, osmotic stress, mechanical stress, immune response, and even emotional stress. There is now substantial literature detailing a decline in adaptive homeostasis that, unfortunately, appears to manifest with ageing, especially in the last third of the lifespan. In this review, we present the hypothesis that one hallmark of the ageing process is a significant decline in adaptive homeostasis capacity. We discuss the mechanistic importance of diminished capacity for short-term (reversible) adaptive responses (both biochemical and signal transduction/gene expression-based) to changing internal and external conditions, for short-term survival and for lifespan and healthspan. Studies of cultured mammalian cells, worms, flies, rodents, simians, apes, and even humans, all indicate declining adaptive homeostasis as a potential contributor to age-dependent senescence, increased risk of disease, and even mortality. Emerging work points to Nrf2-Keap1 signal transduction pathway inhibitors, including Bach1 and c-Myc, both of whose tissue concentrations increase with age, as possible major causes for age-dependent loss of adaptive homeostasis.

The Muscle Stem Cell Niche in Health and Disease

The regulation of stem cells that maintain and regenerate postnatal tissues depends on extrinsic signals originating from their microenvironment, commonly referred to as the stem cell niche. Complex higher-order regulatory interrelationships with the tissue and factors in the systemic circulation are integrated and propagated to the stem cells through the niche. The stem cell niche in skeletal muscle tissue is both a paradigm for a structurally and functionally relatively static niche that maintains stem cell quiescence during tissue homeostasis, and a highly dynamic regenerative niche that is subject to extensive structural remodeling and a flux of different support cell populations. Conditions ranging from aging to chronically degenerative skeletal muscle diseases affect the composition of the niche and thereby impair the regenerative potential of muscle stem cells. A holistic and integrative understanding of the extrinsic mechanisms regulating muscle stem cells in health and disease in a broad systemic context will be imperative for the identification of regulatory hubs in the niche interactome that can be targeted to maintain, restore, or enhance the regenerative capacity of muscle tissue. Here, we review the microenvironmental regulation of muscle stem cells, summarize how niche dysfunction can contribute to disease, and discuss emerging therapeutic implications.

The response of muscle progenitor cells to cutaneous thermal injury

BACKGROUND

Severe burn results in a systemic response that leads to significant muscle wasting. It is believed that this rapid loss in muscle mass occurs due to increased protein degradation combined with reduced protein synthesis. Alterations in the microenvironment of muscle progenitor cells may partially account for this pathology. The aim of this study was to ascertain the response of muscle progenitor cells following thermal injury in mice and to enlighten the cellular cascades that contribute to the muscle wasting.

METHODS

C57BL/6 mice received a 20% total body surface area (TBSA) thermal injury. Gastrocnemius muscle was harvested at days 2, 7, and 14 following injury for protein and histological analysis.

RESULTS

We observed a decrease in myofiber cross-sectional area at 2 days post-burn. This muscle atrophy was compensated for by an increase in myofiber cross-sectional area at 7 and 14 days post-burn. Myeloperoxidase (MPO)-positive cells (neutrophils) increased significantly at 2 days. Moreover, through Western blot analysis of two key mediators of the proteolytic pathway, we show there is an increase in Murf1 and NF-κB 2 days post-burn. MPO-positive cells were also positive for NF-κB, suggesting that neutrophils attain NF-κB activity in the muscle. Unlike inflammatory and proteolytic pathways, the number of Pax7-positive muscle progenitor cells decreased significantly 2 days post-burn. This was followed by a recovery in the number of Pax7-positive cells at 7 and 14 days, suggesting proliferation of muscle progenitors that accompanied regrowth.

CONCLUSION

Our data show a biphasic response in the muscles of mice exposed to burn injury, with phenotypic characteristics of muscle atrophy at 2 days while compensation was observed later with a change in Pax7-positive muscle progenitor cells. Targeting muscle progenitors may be of therapeutic benefit in muscle wasting observed after burn injury.

Vitamin D and muscle function

Muscle weakness is a hallmark of severe vitamin D deficiency, but the effect of milder vitamin D deficiency or insufficiency on muscle mass and performance and risk of falling is uncertain. In this presentation, I review the evidence that vitamin D influences muscle mass and performance, balance, and risk of falling in older adults. Special consideration is given to the impact of both the starting 25-hydroxyvitamin D [25(OH)D] level and the dose administered on the clinical response to supplemental vitamin D in older men and women. Based on available evidence, older adults with serum 25(OH)D levels <40nmol/L appear most likely to improve their muscle performance with supplementation. The vitamin D dose range of 800-1000 IU per day has been effective in many studies; lower doses have generally been ineffective and several doses above this range have increased the risk of falls. In conclusion, older adults with serum 25(OH)D levels <40nmol/L are likely to have fewer falls if supplemented with 800-1000 IU per day of vitamin D.

The Combination of Physical Exercise with Muscle-Directed Antioxidants to Counteract Sarcopenia: A Biomedical Rationale for Pleiotropic Treatment with Creatine and Coenzyme Q10

Sarcopenia represents an increasing public health risk due to the rapid aging of the world's population. It is characterized by both low muscle mass and function and is associated with mobility disorders, increased risk of falls and fractures, loss of independence, disabilities, and increased risk of death. Despite the urgency of the problem, the development of treatments for sarcopenia has lagged. Increased reactive oxygen species (ROS) production and decreased antioxidant (AO) defences seem to be important factors contributing to muscle impairment. Studies have been conducted to verify whether physical exercise and/or AOs could prevent and/or delay sarcopenia through a normalization of the etiologically relevant ROS imbalance. Despite the strong rationale, the results obtained were contradictory, particularly with regard to the effects of the tested AOs. A possible explanation might be that not all the agents included in the general heading of "AOs" could fulfill the requisites to counteract the complex series of events causing/accelerating sarcopenia: the combination of the muscle-directed antioxidants creatine and coenzyme Q10 with physical exercise as a biomedical rationale for pleiotropic prevention and/or treatment of sarcopenia is discussed.

Divergent effects of resistance training and anabolic steroid on the postsynaptic region of different skeletal muscles of aged rats

This study aimed to analyze the effects of resistance training associated with testosterone administration in the neuromuscular junction (NMJ) postsynaptic region of different skeletal muscle types of aged rats. Wistar rats were divided into: SEI - 20-months-old control, SEF - 24-months-old control, T - 20-months-old with testosterone, S - 20-months-old resistance trained and ST - 20-months-old with resistance training associated with testosterone propionate. All groups were submitted to familiarization and maximum load carrying testing (MLCT). The MLCT was applied before and after the resistance training (RT) period. RT (6-8×/session with progressive loads of 50 to 100%, 3×/week and 120s interval) was performed in ladder climbing for 15weeks. The administration of testosterone propionate was performed 2×/week (10mg/kg/body weight). After euthanize, soleus and plantaris muscles were removed and prepared for histochemistry and cytofluorescence. T, S and ST significantly increased their maximum carrying load capacity compared to SEI and SEF (p<0.05). For soleus postsynaptic region, ST had lower total and stained area than SEF (p<0.05). For plantaris, the postsynaptic component of T was statistically larger than SEI (p<0.05). For soleus histochemistry, T, S and ST groups showed the same magnitude of type I myofibers hypertrophy, thus statistically different from SEI and SEF (p<0.05). The cross-sectional area of the type IIa myofibers of the ST was larger than SEF (p<0.05). The volume density of type I myofibers show to be lower in ST than SEI (p<0.05). As for type IIa myofibers, ST increased Vv [type IIa] compared to SEI and SEF (p<0.05). For plantaris, T significantly hypertrophied type I myofibers compared to SEI and SEF (p<0.05). S and ST demonstrated significant increases of type I myofibers compared to SEI and SEF (p<0.05). As for type IIx myofibers, both S and ST showed myofibers larger than SEI (p<0.05). However, only the ST had significant difference compared to SEF (p<0.05). In conclusion, both therapies, alone or combined, have little effect on the morphology of the NMJ postsynaptic region of distinct muscles. Moreover, the three therapies are potentially stimulating for strength gains and muscle hypertrophy.

Muscle Stem Cell and Physical Activity: What Point is the Debate at?

In the last 15 years, it emerged that the practice of regular physical activity reduces the risks of many diseases (cardiovascular diseases, diabetes, etc.) and it is fundamental in weight control and energy consuming to contrast obesity. Different groups proposed many molecular mechanisms as responsible for the positive effects of physical activity in healthy life. However, many points remain to be clarified. In this mini-review we reported the latest observations on the effects of physical exercise on healthy skeletal and cardiac muscle focusing on muscle stem cells. The last ones represent the fundamental elements for muscle regeneration post injury, but also for healthy muscle homeostasis.

Interestingly, in both muscle tissues the morphological consequence of physical activity is a physiological hypertrophy that depends on different phenomena both in differentiated cells and stem cells. The signaling pathways for physical exercise effects present common elements in skeletal and cardiac muscle, like activation of specific transcription factors, proliferative pathways, and cytokines. More recently, post translational (miRNAs) or epigenetic (DNA methylation) modifications have been demonstrated. However, several points remain unresolved thus requiring new research on the effect of exercise on muscle stem cells.

Aging in Rats Differentially Affects Markers of Transcriptional and Translational Capacity in Soleus and Plantaris Muscle

Alterations in transcriptional and translational mechanisms occur during skeletal muscle aging and such changes may contribute to age-related atrophy. Herein, we examined markers related to global transcriptional output (i.e., myonuclear number, total mRNA and RNA pol II levels), translational efficiency [i.e., eukaryotic initiation and elongation factor levels and muscle protein synthesis (MPS) levels] and translational capacity (ribosome density) in the slow-twitch soleus and fast-twitch plantaris muscles of male Fischer 344 rats aged 3, 6, 12, 18, and 24 months (n = 9-10 per group). We also examined alterations in markers of proteolysis and oxidative stress in these muscles (i.e., 20S proteasome activity, poly-ubiquinated protein levels and 4-HNE levels). Notable plantaris muscle observations included: (a) fiber cross sectional area (CSA) was 59% (p < 0.05) and 48% (p < 0.05) greater in 12 month vs. 3 month and 24 month rats, respectively, suggesting a peak lifetime value near 12 months and age-related atrophy by 24 months, (b) MPS levels were greatest in 18 month rats (p < 0.05) despite the onset of atrophy, (c) while regulators of ribosome biogenesis [c-Myc and upstream binding factor (UBF) protein levels] generally increased with age, ribosome density linearly decreased from 3 months of age and RNA polymerase (Pol) I protein levels were lowest in 24 month rats, and d) 20S proteasome activity was robustly up-regulated in 6 and 24 month rats (p < 0.05). Notable soleus muscle observations included: (a) fiber CSA was greatest in 6 month rats and was maintained in older age groups, and (b) 20S proteasome activity was modestly but significantly greater in 24 month vs. 3/12/18 month rats (p < 0.05), and (c) total mRNA levels (suggestive of transcriptional output) trended downward in older rats despite non-significant between-group differences in myonuclear number and/or RNA Pol II protein levels. Collectively, these findings suggest that plantaris, not soleus, atrophy occurs following 12 months of age in male Fisher rats and this may be due to translational deficits (i.e., changes in MPS and ribosome density) and/or increases in proteolysis rather than increased oxidative stress and/or alterations in global transcriptional mechanisms.

Preparation and Culture of Myogenic Precursor Cells/Primary Myoblasts from Skeletal Muscle of Adult and Aged Humans

Skeletal muscle homeostasis depends on muscle growth (hypertrophy), atrophy and regeneration. During ageing and in several diseases, muscle wasting occurs. Loss of muscle mass and function is associated with muscle fiber type atrophy, fiber type switching, defective muscle regeneration associated with dysfunction of satellite cells, muscle stem cells, and other pathophysiological processes. These changes are associated with changes in intracellular as well as local and systemic niches. In addition to most commonly used rodent models of muscle ageing, there is a need to study muscle homeostasis and wasting using human models, which due to ethical implications, consist predominantly of in vitro cultures. Despite the wide use of human Myogenic Progenitor Cells (MPCs) and primary myoblasts in myogenesis, there is limited data on using human primary myoblast and myotube cultures to study molecular mechanisms regulating different aspects of age-associated muscle wasting, aiding in the validation of mechanisms of ageing proposed in rodent muscle. The use of human MPCs, primary myoblasts and myotubes isolated from adult and aged people, provides a physiologically relevant model of molecular mechanisms of processes associated with muscle growth, atrophy and regeneration. Here we describe in detail a robust, inexpensive, reproducible and efficient protocol for the isolation and maintenance of human MPCs and their progeny — myoblasts and myotubes from human muscle samples using enzymatic digestion. Furthermore, we have determined the passage number at which primary myoblasts from adult and aged people undergo senescence in an in vitro culture. Finally, we show the ability to transfect these myoblasts and the ability to characterize their proliferative and differentiation capacity and propose their suitability for performing functional studies of molecular mechanisms of myogenesis and muscle wasting in vitro.

Investigating human skeletal muscle physiology with unilateral exercise models: when one limb is more powerful than two

Direct sampling of human skeletal muscle using the needle biopsy technique can facilitate insight into the biochemical and histological responses resulting from changes in exercise or feeding. However, the muscle biopsy procedure is invasive, and analyses are often expensive, which places pragmatic restraints on sample sizes. The unilateral exercise model can serve to increase statistical power and reduce the time and cost of a study. With this approach, 2 limbs of a participant are randomized to 1 of 2 treatments that can be applied almost concurrently or sequentially depending on the nature of the intervention. Similar to a typical repeated measures design, comparisons are made within participants, which increases statistical power by reducing the amount of between-person variability. A washout period is often unnecessary, reducing the time needed to complete the experiment and the influence of potential confounding variables such as habitual diet, activity, and sleep. Variations of the unilateral exercise model have been employed to investigate the influence of exercise, diet, and the interaction between the 2, on a wide range of variables including mitochondrial content, capillary density, and skeletal muscle hypertrophy. Like any model, unilateral exercise has some limitations: it cannot be used to study variables that potentially transfer across limbs, and it is generally limited to exercises that can be performed in pairs of treatments. Where appropriate, however, the unilateral exercise model can yield robust, well-controlled investigations of skeletal muscle responses to a wide range of interventions and conditions including exercise, dietary manipulation, and disuse or immobilization.

Skeletal muscle satellite cells are located at a closer proximity to capillaries in healthy young compared with older men

BACKGROUND

Skeletal muscle satellite cells (SC) are instrumental in maintenance of muscle fibres, the adaptive responses to exercise, and there is an age-related decline in SC. A spatial relationship exists between SC and muscle fibre capillaries. In the present study, we aimed to investigate whether chronologic age has an impact on the spatial relationship between SC and muscle fibre capillaries. Secondly, we determined whether this spatial relationship changes in response to a single session of resistance exercise.

METHODS

Muscle biopsies were obtained from the vastus lateralis of previously untrained young men (YM, 24 ± 3 years; n = 23) and older men (OM, 67 ± 4 years; n = 22) at rest. A subset of YM (n = 9) performed a single bout of resistance exercise, where additional muscle biopsies taken at 24 and 72 h post-exercise recovery. Skeletal muscle fibre capillarization, SC content, and activation status were assessed using immunofluorescent microscopy of muscle cross sections.

RESULTS

Type II muscle fibre SC and capillary content was significantly lower in the YM compared with OM (P < 0.05). Furthermore, type II muscle fibre SC were located at a greater distance from the nearest capillary in OM compared with YM (21.6 ± 1.3 vs. 17.0 ± 0.8 µm, respectively; P < 0.05). In response to a single bout of exercise, we observed a significant increase in SC number and activation status (P < 0.05). In addition, activated vs. quiescent SC were situated closer (P < 0.05) to capillaries.

CONCLUSIONS

We demonstrate that there is a greater distance between capillaries and type II fibre-associated SC in OM as compared with YM. Furthermore, quiescent SC are located significantly further away from capillaries than active SC after single bout of exercise. Our data have implications for how muscle adapts to exercise and how aging may affect such adaptations.

Altered muscle satellite cell activation following 16 wk of resistance training in young men

Skeletal muscle satellite cells (SC) play an important role in muscle adaptation. In untrained individuals, SC content and activation status have been observed to increase in response to a single bout of exercise. Muscle fiber characteristics change considerably when resistance exercise is performed chronically, but whether training status affects the activity of SC in response to a single bout of exercise remains unknown. We examined the changes in SC content and activation status following a single bout of resistance exercise, before and following a 16-wk progressive resistance training (RT) program in 14 young (25 ± 3 yr) men. Before and after RT, percutaneous biopsies from the vastus lateralis muscle were taken before a single bout of resistance exercise and after 24 and 72 h of postexercise recovery. Muscle fiber size, capillarization, and SC response were determined by immunohistochemistry. Following RT, there was a greater activation of SC after 24 h in response to a single bout of resistance exercise (Pre, 1.4 ± 0.3; 24 h, 3.1 ± 0.3 Pax7⁺/MyoD⁺ cells per 100 fibers) compared with before RT (Pre, 1.4 ± 0.3; 24 h, 2.2 ± 0.3 Pax7⁺/MyoD⁺ cells per 100 fibers, P < 0.05); no difference was observed 72 h postexercise. Following 16 wk of RT, MyoD mRNA expression increased from basal to 24 h after the single bout of exercise (P < 0.05); this change was not observed before training. Individual capillary-to-fiber ratio (C/Fi) increased in both type I (1.8 ± 0.3 to 2.0 ± 0.3 C/Fi, P < 0.05) and type II (1.7 ± 0.3 to 2.2 ± 0.3 C/Fi, P < 0.05) fibers in response to RT. After RT, enhanced activation of SC in response to resistance exercise is accompanied by increases in muscle fiber capillarization.

Does the muscle protein synthetic response to exercise and amino acid-based nutrition diminish with advancing age? A systematic review

The precise role of age-related muscle anabolic resistance in the progression of sarcopenia and functional decline in older individuals is unclear. The present aim was to assess whether the muscle protein synthesis (MPS) response to acute exercise (endurance or resistance) and/or amino acid-based nutrition is attenuated in older compared with young individuals. A systematic review was conducted on studies that directly examined the influence of age on the MPS response to exercise and/or amino acid-based nutrition. Each study arm was synthesized and reported as providing sufficient or insufficient "evidence of age-related muscle anabolic resistance". Subsequently, three models were established to compare age-related differences in the MPS response to 1) exercise alone, 2) amino acid-based nutrition alone, or 3) the combination of exercise and amino acid-based nutrition. Following exercise alone, 8 of the 17 study arms provided sufficient evidence of age-related muscle anabolic resistance, while in response to amino acid-based nutrition alone, 8 of the 21 study arms provided sufficient evidence of age-related muscle anabolic resistance. When exercise and amino acid-based nutrition were combined, only 2 of the 10 study arms provided sufficient evidence of age-related muscle anabolic resistance. Our results highlight that optimization of exercise and amino acid-based nutrition is sufficient to induce a comparable MPS response between young and older individuals. However, the exercise volume completed and/or the amino acid/protein dose and leucine content must exceed a certain threshold to stimulate equivalent MPS rates in young and older adults, below which age-related muscle anabolic resistance may become apparent.

The aging neuromuscular system and motor performance

Age-related changes in the basic functional unit of the neuromuscular system, the motor unit, and its neural inputs have a profound effect on motor function, especially among the expanding number of old (older than ∼60 yr) and very old (older than ∼80 yr) adults. This review presents evidence that age-related changes in motor unit morphology and properties lead to impaired motor performance that includes 1) reduced maximal strength and power, slower contractile velocity, and increased fatigability; and 2) increased variability during and between motor tasks, including decreased force steadiness and increased variability of contraction velocity and torque over repeat contractions. The age-related increase in variability of motor performance with aging appears to involve reduced and more variable synaptic inputs that drive motor neuron activation, fewer and larger motor units, less stable neuromuscular junctions, lower and more variable motor unit action potential discharge rates, and smaller and slower skeletal muscle fibers that coexpress different myosin heavy chain isoforms in the muscle of older adults. Physical activity may modify motor unit properties and function in old men and women, although the effects on variability of motor performance are largely unknown. Many studies are of cross-sectional design, so there is a tremendous opportunity to perform high-impact and longitudinal studies along the continuum of aging that determine 1) the influence and cause of the increased variability with aging on functional performance tasks, and 2) whether lifestyle factors such as physical exercise can minimize this age-related variability in motor performance in the rapidly expanding numbers of very old adults.

One Week of Bed Rest Leads to Substantial Muscle Atrophy and Induces Whole-Body Insulin Resistance in the Absence of Skeletal Muscle Lipid Accumulation

Short (<10 days) periods of muscle disuse, often necessary for recovery from illness or injury, lead to various negative health consequences. The current study investigated mechanisms underlying disuse-induced insulin resistance, taking into account muscle atrophy. Ten healthy, young males (age: 23 ± 1 years; BMI: 23.0 ± 0.9 kg · m(-2)) were subjected to 1 week of strict bed rest. Prior to and after bed rest, lean body mass (dual-energy X-ray absorptiometry) and quadriceps cross-sectional area (CSA; computed tomography) were assessed, and peak oxygen uptake (VO2peak) and leg strength were determined. Whole-body insulin sensitivity was measured using a hyperinsulinemic-euglycemic clamp. Additionally, muscle biopsies were collected to assess muscle lipid (fraction) content and various markers of mitochondrial and vascular content. Bed rest resulted in 1.4 ± 0.2 kg lean tissue loss and a 3.2 ± 0.9% decline in quadriceps CSA (both P < 0.01). VO2peak and one-repetition maximum declined by 6.4 ± 2.3 (P < 0.05) and 6.9 ± 1.4% (P < 0.01), respectively. Bed rest induced a 29 ± 5% decrease in whole-body insulin sensitivity (P < 0.01). This was accompanied by a decline in muscle oxidative capacity, without alterations in skeletal muscle lipid content or saturation level, markers of oxidative stress, or capillary density. In conclusion, 1 week of bed rest substantially reduces skeletal muscle mass and lowers whole-body insulin sensitivity, without affecting mechanisms implicated in high-fat diet-induced insulin resistance.

Satellite Cells Contribution to Exercise Mediated Muscle Hypertrophy and Repair

Satellite cells (SCs) are the most abundant skeletal muscle stem cells. They are widely recognized for their contributions to maintenance of muscle mass, regeneration and hypertrophy during the human life span. These cells are good candidates for cell therapy due to their self-renewal capabilities and presence in an undifferentiated form. Presently, a significant gap exists between our knowledge of SCs behavior and their application as a means for human skeletal muscle tissue repair and regeneration. Both physiological and pathological stimuli potentially affect SCs activation, proliferation, and terminal differentiation the former category being the focus of this article. Activation of SCs occurs following exercise, post-training micro-injuries, and electrical stimulation. Exercise, as a potent and natural stimulus, is at the center of numerous studies on SC activation and relevant fields. According to research, different exercise modalities end with various effects. This review article attempts to picture the state of the art of the SCs life span and their engagement in muscle regeneration and hypertrophy in exercise.

Identification of morphological markers of sarcopenia at early stage of aging in skeletal muscle of mice

The gastrocnemius muscle (GM) of young (3months) and aged (12months) female wild-type C57/BL6 mice was examined by light and electron microscopy, looking for the presence of structural changes at early stage of the aging process. Morphometrical parameters including body and gastrocnemius weights, number and type of muscle fibers, cross section area (CSA), perimeter, and Feret's diameter of single muscle fiber, were measured. Moreover, lengths of the sarcomere, A-band, I-band, H-zone, and number and CSA of intermyofibrillar mitochondria (IFM), were also determined. The results provide evidence that 12month-old mice had significant changes on skeletal muscle structure, beginning with the reduction of gastrocnemius weight to body weight ratio, compatible with an early loss of skeletal muscle function and strength. Moreover, light microscopy revealed increased muscle fibers size, with a significant increase on their CSA, perimeter, and diameter of both type I and type II muscle fibers, and a reduction in the percentage of muscle area occupied by type II fibers. Enhanced connective tissue infiltrations, and the presence of centrally nucleated muscle fibers, were also found in aged mice. These changes may underlie an attempt to compensate the loss of muscle mass and muscle fibers number. Furthermore, electron microscopy discovered a significant age-dependent increase in the length of sarcomeres, I and H bands, and reduction on the overlapped actin/myosin length, supporting contractile force loss with age. Electron microscopy also showed an increased number and CSA of IFM with age, which may reveal more endurance at 12months of age. Together, mice at early stage of aging already show significant changes in gastrocnemius muscle morphology and ultrastructure that are suggestive of the onset of sarcopenia.

Composition of Muscle Fiber Types in Rat Rotator Cuff Muscles

The rat is a suitable model to study human rotator cuff pathology owing to the similarities in morphological anatomy structure. However, few studies have reported the composition muscle fiber types of rotator cuff muscles in the rat. In this study, the myosin heavy chain (MyHC) isoforms were stained by immunofluorescence to show the muscle fiber types composition and distribution in rotator cuff muscles of the rat. It was found that rotator cuff muscles in the rat were of mixed fiber type composition. The majority of rotator cuff fibers labeled positively for MyHCII. Moreover, the rat rotator cuff muscles contained hybrid fibers. So, compared with human rotator cuff muscles composed partly of slow-twitch fibers, the majority of fast-twitch fibers in rat rotator cuff muscles should be considered when the rat model study focus on the pathological process of rotator cuff muscles after injury. Gaining greater insight into muscle fiber types in rotator cuff muscles of the rat may contribute to elucidate the mechanism of pathological change in rotator cuff muscles-related diseases. Anat Rec, 299:1397-1401, 2016. © 2016 Wiley Periodicals, Inc.

Frailty and sarcopenia as the basis for the phenotypic manifestation of chronic diseases in older adults

Frailty is a functional status that precedes disability and is characterized by decreased functional reserve and increased vulnerability. In addition to disability, the frailty phenotype predicts falls, institutionalization, hospitalization and mortality. Frailty is the consequence of the interaction between the aging process and some chronic diseases and conditions that compromise functional systems and finally produce sarcopenia. Many of the clinical manifestations of frailty are explained by sarcopenia which is closely related to poor physical performance. Reduced regenerative capacity, malperfusion, oxidative stress, mitochondrial dysfunction and inflammation compose the sarcopenic skeletal muscle alterations associated to the frailty phenotype. Inflammation appears as a common determinant for chronic diseases, sarcopenia and frailty. The strategies to prevent the frailty phenotype include an adequate amount of physical activity and exercise as well as pharmacological interventions such as myostatin inhibitors and specific androgen receptor modulators. Cell response to stress pathways such as Nrf2, sirtuins and klotho could be considered as future therapeutic interventions for the management of frailty phenotype and aging-related chronic diseases.

Ageing in relation to skeletal muscle dysfunction: redox homoeostasis to regulation of gene expression

Ageing is associated with a progressive loss of skeletal muscle mass, quality and function—sarcopenia, associated with reduced independence and quality of life in older generations. A better understanding of the mechanisms, both genetic and epigenetic, underlying this process would help develop therapeutic interventions to prevent, slow down or reverse muscle wasting associated with ageing. Currently, exercise is the only known effective intervention to delay the progression of sarcopenia. The cellular responses that occur in muscle fibres following exercise provide valuable clues to the molecular mechanisms regulating muscle homoeostasis and potentially the progression of sarcopenia. Redox signalling, as a result of endogenous generation of ROS/RNS in response to muscle contractions, has been identified as a crucial regulator for the adaptive responses to exercise, highlighting the redox environment as a potentially core therapeutic approach to maintain muscle homoeostasis during ageing. Further novel and attractive candidates include the manipulation of microRNA expression. MicroRNAs are potent gene regulators involved in the control of healthy and disease-associated biological processes and their therapeutic potential has been researched in the context of various disorders, including ageing-associated muscle wasting. Finally, we discuss the impact of the circadian clock on the regulation of gene expression in skeletal muscle and whether disruption of the peripheral muscle clock affects sarcopenia and altered responses to exercise. Interventions that include modifying altered redox signalling with age and incorporating genetic mechanisms such as circadian- and microRNA-based gene regulation, may offer potential effective treatments against age-associated sarcopenia.

Alterations in Skeletal Muscle Oxidative Phenotype in Mice Exposed to 3 Weeks of Normobaric Hypoxia

Skeletal muscle of patients with chronic respiratory failure is prone to loss of muscle mass and oxidative phenotype. Tissue hypoxia has been associated with cachexia and emphysema in humans. Experimental research on the role of hypoxia in loss of muscle oxidative phenotype, however, has yielded inconsistent results. Animal studies are frequently performed in young animals, which may hinder translation to generally older aged patients. Therefore, in this study, we tested the hypothesis that hypoxia induces loss of skeletal muscle oxidative phenotype in a model of aged (52 weeks) mice exposed to 3 weeks of hypoxia. Additional groups of young (4 weeks) and adult (12 weeks) mice were included to examine age effects. To verify hypoxia-induced cachexia, fat pad and muscle weights as well as muscle fiber cross-sectional areas were determined. Muscle oxidative phenotype was assessed by expression and activity of markers of mitochondrial metabolism and fiber-type distribution. A profound loss of muscle and fat was indeed accompanied by a slightly lower expression of markers of muscle oxidative capacity in the aged hypoxic mice. In contrast, hypoxia-associated changes of fiber-type composition were more prominent in the young mice. The differential response of the muscle of young, adult, and aged mice to hypoxia suggests that age matters and that the aged mouse is a better model for translation of findings to elderly patients with chronic respiratory disease. Furthermore, the findings warrant further mechanistic research into putative accelerating effects of hypoxia-induced loss of oxidative phenotype on the cachexia process in chronic respiratory disease.

Muscle wasting and aging: Experimental models, fatty infiltrations, and prevention

Identification of cost-effective interventions to maintain muscle mass, muscle strength, and physical performance during muscle wasting and aging is an important public health challenge. It requires understanding of the cellular and molecular mechanisms involved. Muscle-deconditioning processes have been deciphered by means of several experimental models, bringing together the opportunities to devise comprehensive analysis of muscle wasting. Studies have increasingly recognized the importance of fatty infiltrations or intermuscular adipose tissue for the age-mediated loss of skeletal-muscle function and emphasized that this new important factor is closely linked to inactivity. The present review aims to address three main points. We first mainly focus on available experimental models involving cell, animal, or human experiments on muscle wasting. We next point out the role of intermuscular adipose tissue in muscle wasting and aging and try to highlight new findings concerning aging and muscle-resident mesenchymal stem cells called fibro/adipogenic progenitors by linking some cellular players implicated in both FAP fate modulation and advancing age. In the last part, we review the main data on the efficiency and molecular and cellular mechanisms by which exercise, replacement hormone therapies, and β-hydroxy-β-methylbutyrate prevent muscle wasting and sarcopenia. Finally, we will discuss a potential therapeutic target of sarcopenia: glucose 6-phosphate dehydrogenase.

Age-related changes in miR-143-3p:Igfbp5 interactions affect muscle regeneration

A common characteristic of aging is defective regeneration of skeletal muscle. The molecular pathways underlying age-related decline in muscle regenerative potential remain elusive. microRNAs are novel gene regulators controlling development and homeostasis and the regeneration of most tissues, including skeletal muscle. Here, we use satellite cells and primary myoblasts from mice and humans and an in vitro regeneration model, to show that disrupted expression of microRNA-143-3p and its target gene, Igfbp5, plays an important role in muscle regeneration in vitro. We identified miR-143 as a regulator of the insulin growth factor-binding protein 5 (Igfbp5) in primary myoblasts and show that the expression of miR-143 and its target gene is disrupted in satellite cells from old mice. Moreover, we show that downregulation of miR-143 during aging may act as a compensatory mechanism aiming at improving myogenesis efficiency; however, concomitant upregulation of miR-143 target gene, Igfbp5, is associated with increased cell senescence, thus affecting myogenesis. Our data demonstrate that dysregulation of miR-143-3p:Igfbp5 interactions in satellite cells with age may be responsible for age-related changes in satellite cell function.

Pharmacologic Options for the Treatment of Sarcopenia

Sarcopenia is now clinically defined as a loss of muscle mass coupled with functional deterioration (either walking speed or distance or grip strength). Based on the FRAX studies suggesting that the questions without bone mineral density can be used to screen for osteoporosis, there is now a valid simple questionnaire to screen for sarcopenia, i.e., the SARC-F. Numerous factors have been implicated in the pathophysiology of sarcopenia. These include genetic factors, mitochondrial defects, decreased anabolic hormones (e.g., testosterone, vitamin D, growth hormone and insulin growth hormone-1), inflammatory cytokine excess, insulin resistance, decreased protein intake and activity, poor blood flow to muscle and deficiency of growth derived factor-11. Over the last decade, there has been a remarkable increase in our understanding of the molecular biology of muscle, resulting in a marked increase in potential future targets for the treatment of sarcopenia. At present, resistance exercise, protein supplementation, and vitamin D have been established as the basic treatment of sarcopenia. High-dose testosterone increases muscle power and function, but has a number of potentially limiting side effects. Other drugs in clinical development include selective androgen receptor molecules, ghrelin agonists, myostatin antibodies, activin IIR antagonists, angiotensin converting enzyme inhibitors, beta antagonists, and fast skeletal muscle troponin activators. As sarcopenia is a major predictor of frailty, hip fracture, disability, and mortality in older persons, the development of drugs to treat it is eagerly awaited.

Cellular and Physiological Effects of Dietary Supplementation with β-Hydroxy-β-Methylbutyrate (HMB) and β-Alanine in Late Middle-Aged Mice

There is growing evidence that severe decline of skeletal muscle mass and function with age may be mitigated by exercise and dietary supplementation with protein and amino acid ingredient technologies. The purposes of this study were to examine the effects of the leucine catabolite, beta-hydroxy-beta-methylbutyrate (HMB), in C₂C₁₂ myoblasts and myotubes, and to investigate the effects of dietary supplementation with HMB, the amino acid β-alanine and the combination thereof, on muscle contractility in a preclinical model of pre-sarcopenia. In C₂C₁₂ myotubes, HMB enhanced sarcoplasmic reticulum (SR) calcium release beyond vehicle control in the presence of all SR agonists tested (KCl, P<0.01; caffeine, P = 0.03; ionomycin, P = 0.03). HMB also improved C₂C₁₂ myoblast viability (25 μM HMB, P = 0.03) and increased proliferation (25 μM HMB, P = 0.04; 125 μM HMB, P<0.01). Furthermore, an ex vivo muscle contractility study was performed on EDL and soleus muscle from 19 month old, male C57BL/6nTac mice. For 8 weeks, mice were fed control AIN-93M diet, diet with HMB, diet with β-alanine, or diet with HMB and β-alanine. In β-alanine fed mice, EDL muscle showed a 7% increase in maximum absolute force compared to the control diet (202 ± 3vs. 188± 5 mN, P = 0.02). At submaximal frequency of stimulation (20 Hz), EDL from mice fed HMB plus β-alanine showed an 11% increase in absolute force (88.6 ± 2.2 vs. 79.8 ± 2.4 mN, P = 0.025) and a 13% increase in specific force (12.2 ± 0.4 vs. 10.8 ± 0.4 N/cm², P = 0.021). Also in EDL muscle, β-alanine increased the rate of force development at all frequencies tested (P<0.025), while HMB reduced the time to reach peak contractile force (TTP), with a significant effect at 80 Hz (P = 0.0156). In soleus muscle, all experimental diets were associated with a decrease in TTP, compared to control diet. Our findings highlight beneficial effects of HMB and β-alanine supplementation on skeletal muscle function in aging mice.

Short-term ursolic acid promotes skeletal muscle rejuvenation through enhancing of SIRT1 expression and satellite cells proliferation

Ursolic acid (UA) is a triterpenoid compound, which exerts its influences on the skeletal muscles. However, the mechanisms underlying these effects are still unclear. In this study, muscle satellite cells were isolated and purified by high-throughput pre-plating method (∼>60%) from 10 days old mice skeletal muscles. Evaluation of paired-box 7 (Pax7) expressions then confirmed the purification. Treatment of the cells with UA showed that UA up-regulated SIRT1 (∼35 folds) and overexpressed PGC-1α (∼175 folds) gene significantly. Moreover, the number of muscle satellite cells, which accompanied by initiation of neomyogenesis in the animal skeletal muscles, was increased (∼3.4 times). We also evaluated UA-mediated changes in the cellular energy status in the skeletal muscles. The results revealed that in the UA-treated mice, ATP and ADP contents in the various skeletal muscle tissue types, including: Gastrocnemius (Gas), Tibialis Anterior (Tib) and Gluteus Maximus (Glu) have been significantly decreased (P≤0.001); 2.2, 3.2, 2 times for ATP, and 9.6, 35.7, 11.6 times for ADP, respectively; however to compensate this process mitochondrial biogenesis occurred (12.33%±1.5 times). Furthermore, a rise in ATP/ADP ratio was observed 2.5, 4.5, 2.05 times for Gas, Tib and Glu muscles, respectively (P≤0.001). Alternatively, UA enhanced the expression of myoglobin (∼2 folds) in concert with remodeling of glycolytic muscle fibers to mainly fast IIA (∼30%) and slow-twitch (∼4%) types as well. Finally, our study indicated that UA indirectly mimicked beneficial effects of short-term calorie restriction and exercise (fast-oxidative) by directing the skeletal muscle composition toward oxidative metabolism.

Neural Activation During Submaximal Contractions Seems More Reflective of Neuromuscular Ageing than Maximal Voluntary Activation

This study aimed at testing the hypothesis that differences in neural activation strategy during submaximal but not maximal plantarflexions exist between young and older men. Eleven young men (YM, 26 ± 4 years) and thirteen old men (OM, 76 ± 3 years) volunteered for the investigation. Maximal voluntary torque (MVT) was 38.2%, lower (p < 0.001) in OM than in YM, while voluntary activation was equivalent (~97%). The relationship between the interpolated twitch-torque and the voluntary torque (IT-VT relationship) was composite (curvilinear + exponential) for both age-groups. However, the OM showed accentuated concavity, as attested by the occurrence of the deviation from linearity at a lower contraction intensity (OM: 54.9 vs. YM: 71.9% MVT). In conclusion, ageing does not affect the capacity to fully activate the plantar flexors during maximal performances, but it alters the activation pattern for submaximal levels of effort. The greater age-related concavity of the IT-VT relationship suggests that, during submaximal contractions, OM need to reach a level of activation higher than YM to develop an equivalent relative torque.

Age modifies respiratory complex I and protein homeostasis in a muscle type-specific manner

Changes in mitochondrial function with age vary between different muscle types, and mechanisms underlying this variation remain poorly defined. We examined whether the rate of mitochondrial protein turnover contributes to this variation. Using heavy label proteomics, we measured mitochondrial protein turnover and abundance in slow‐twitch soleus (SOL) and fast‐twitch extensor digitorum longus (EDL) from young and aged mice. We found that mitochondrial proteins were longer lived in EDL than SOL at both ages. Proteomic analyses revealed that age‐induced changes in protein abundance differed between EDL and SOL with the largest change being increased mitochondrial respiratory protein content in EDL. To determine how altered mitochondrial proteomics affect function, we measured respiratory capacity in permeabilized SOL and EDL. The increased mitochondrial protein content in aged EDL resulted in reduced complex I respiratory efficiency in addition to increased complex I‐derived H₂O₂ production. In contrast, SOL maintained mitochondrial quality, but demonstrated reduced respiratory capacity with age. Thus, the decline in mitochondrial quality with age in EDL was associated with slower protein turnover throughout life that may contribute to the greater decline in mitochondrial dysfunction in this muscle. Furthermore, mitochondrial‐targeted catalase protected respiratory function with age suggesting a causal role of oxidative stress. Our data clearly indicate divergent effects of age between different skeletal muscles on mitochondrial protein homeostasis and function with the greatest differences related to complex I. These results show the importance of tissue‐specific changes in the interaction between dysregulation of respiratory protein expression, oxidative stress, and mitochondrial function with age.

Fourteen days of bed rest induces a decline in satellite cell content and robust atrophy of skeletal muscle fibers in middle-aged adults

Bed rest, a ground-based spaceflight analog, induces robust atrophy of skeletal muscle, an effect that is exacerbated with increasing age. We examined the effect of 14 days of bed rest on skeletal muscle satellite cell content and fiber type atrophy in middle-aged adults, an understudied age demographic with few overt signs of muscle aging that is representative of astronauts who perform long-duration spaceflight. Muscle biopsies were obtained from the vastus lateralis of healthy middle-aged adults [n= 7 (4 male, 3 female); age: 51 ± 1 yr] before (Pre-BR) and after (Post-BR) 14 days of bed rest. Immunohistochemical analyses were used to quantify myosin heavy chain (MyHC) isoform expression, cross-sectional area (CSA), satellite cell and myonuclear content, and capillary density. Peak oxygen consumption, knee extensor strength, and body composition were also measured Pre-BR and Post-BR. Post-BR MyHC type 2a fiber percentage was reduced, and mean CSA decreased in all fiber types (-24 ± 5%;P< 0.05). Satellite cell content was also reduced Post-BR (-39 ± 9%;P< 0.05), and the change in satellite cell content was significantly correlated with the change in mean fiber CSA (r(2)= 0.60;P< 0.05). A decline in capillary density was observed Post-BR (-23 ± 6%;P< 0.05), and Post-BR capillary content was significantly associated with Post-BR peak aerobic capacity (r(2)= 0.59;P< 0.05). A subtle decline in myonuclear content occurred during bed rest (-5 ± 1%;P< 0.05). The rapid maladaptation of skeletal muscle to 14 days of mechanical unloading in middle-aged adults emphasizes the need for robust countermeasures to preserve muscle function in astronauts.

Exercise attenuates the major hallmarks of aging

Regular exercise has multi-system anti-aging effects. Here we summarize how exercise impacts the major hallmarks of aging. We propose that, besides searching for novel pharmaceutical targets of the aging process, more research efforts should be devoted to gaining insights into the molecular mediators of the benefits of exercise and to implement effective exercise interventions for elderly people.

Proliferation of Human Primary Myoblasts Is Associated with Altered Energy Metabolism in Dependence on Ageing In Vivo and In Vitro

Background. Ageing is associated with suppressed regenerative potential of muscle precursor cells due to decrease of satellite cells and suppressive intramuscular milieu on their activation, associated with ageing-related low-grade inflammation. The aim of the study was to characterize the function of oxidative phosphorylation (OXPHOS), glycolysis, adenylate kinase (AK), and creatine kinase (CK) mediated systems in young and older individuals. Materials and Methods. Myoblasts were cultivated from biopsies taken by transcutaneous conchotomy from vastus lateralis muscle in young (20–29 yrs, n = 7) and older (70–79 yrs, n = 7) subjects. Energy metabolism was assessed in passages 2 to 6 by oxygraphy and enzyme analysis. Results. In myoblasts of young and older subjects the rate of OXPHOS decreased during proliferation from passages 2 to 6. The total activities of CK and AK decreased. Myoblasts of passage 2 cultivated from young muscle showed higher rate of OXPHOS and activities of CK and AK compared to myoblasts from older subjects while hexokinase and pyruvate kinase were not affected by ageing. Conclusions. Proliferation of myoblasts in vitro is associated with downregulation of OXPHOS and energy storage and transfer systems. Ageing in vivo exerts an impact on satellite cells which results in altered metabolic profile in favour of the prevalence of glycolytic pathways over mitochondrial OXPHOS of myoblasts.