Effects of endurance training on satellite cell frequency in skeletal muscle of old men

Impaired skeletal muscle regeneration in diabetes: From cellular and molecular mechanisms to novel treatments

Diabetes represents a major public health concern with a considerable impact on human life and healthcare expenditures. It is now well established that diabetes is characterized by a severe skeletal muscle pathology that limits functional capacity and quality of life. Increasing evidence indicates that diabetes is also one of the most prevalent disorders characterized by impaired skeletal muscle regeneration, yet underlying mechanisms and therapeutic treatments remain poorly established. In this review, we describe the cellular and molecular alterations currently known to occur during skeletal muscle regeneration in people with diabetes and animal models of diabetes, including its associated comorbidities, e.g., obesity, hyperinsulinemia, and insulin resistance. We describe the role of myogenic and non-myogenic cell types on muscle regeneration in conditions with or without diabetes. Therapies for skeletal muscle regeneration and gaps in our knowledge are also discussed, while proposing future directions for the field.

Muscle fibre size and myonuclear positioning in trained and aged humans

Changes in myonuclear architecture and positioning are associated with exercise adaptations and ageing. However, data on the positioning and number of myonuclei following exercise are inconsistent. Additionally, whether myonuclear domains (MNDs; i.e., the theoretical volume of cytoplasm within which a myonucleus is responsible for transcribing DNA) and myonuclear positioning are altered with age remains unclear. The aim of this investigation was to investigate relationships between age and activity status and myonuclear domains and positioning. Vastus lateralis muscle biopsies from younger endurance-trained (YT) and older endurance-trained (OT) individuals were compared with age-matched untrained counterparts (YU and OU; OU samples were acquired during surgical operation). Serial, optical z-slices were acquired throughout isolated muscle fibres and analysed to give three-dimensional coordinates for myonuclei and muscle fibre dimensions. The mean cross-sectional area (CSA) of muscle fibres from OU individuals was 33%-53% smaller compared with the other groups. The number of nuclei relative to fibre CSA was 90% greater in OU compared with YU muscle fibres. Additionally, scaling of MND volume with fibre size was altered in older untrained individuals. The myonuclear arrangement, in contrast, was similar across groups. Fibre CSA and most myonuclear parameters were significantly associated with age in untrained individuals, but not in trained individuals. These data indicate that regular endurance exercise throughout the lifespan might better preserve the size of muscle fibres in older age and maintain the relationship between fibre size and MND volumes. Inactivity, however, might result in reduced muscle fibre size and altered myonuclear parameters.

Effect of non-pharmacological interventions on the prevention of sarcopenia in menopausal women: a systematic review and meta-analysis of randomized controlled trials

BACKGROUND

Sarcopenia is a chronic disease marked by gradual muscle system and functional decline. Prior research indicates its prevalence in those under 60 varies from 8 to 36%. There is limited evidence on the effectiveness of non-pharmacological interventions for sarcopenia prevention in menopausal women aged 40-60. This study examines the influence of such interventions for sarcopenia prevention on these women.

METHODS

PubMed, EMBASE, Medline, Cochrane Library, CINAHL, PEDro, and Airiti Library were searched from inception until May 5, 2023. Randomized controlled trials that examined exercise, vitamin D and protein supplementation effects on muscle mass, strength, and physical function. Quality assessment used the Cochrane risk of bias tool, and analysis employed Comprehensive Meta-Analysis version 2.0.

RESULTS

A total of 27 randomized controlled trials, involving 1,989 participants were identified. Meta-analysis results showed exercise improved lean body mass (SMD = 0.232, 95% CI: 0.097, 0.366), handgrip strength (SMD = 0.901, 95% CI: 0.362, 1.441), knee extension strength (SMD = 0.698, 95% CI: 0.384, 1.013). Resistance training had a small effect on lean body mass, longer exercise duration (> 12 weeks) and higher frequency (60-90 min, 3 sessions/week) showed small to moderate effects on lean body mass. Vitamin D supplementation improved handgrip strength (SMD = 0.303, 95% CI: 0.130, 0.476), but not knee extension strength. There was insufficient data to assess the impact of protein supplementation on muscle strength.

CONCLUSIONS

Exercise effectively improves muscle mass, and strength in menopausal women. Resistance training with 3 sessions per week, lasting 20-90 min for at least 6 weeks, is most effective. Vitamin D supplementation enhances small muscle group strength. Further trials are needed to assess the effects of vitamin D and protein supplementation on sarcopenia prevention.

REGISTRATION NUMBER

This review was registered on PROSPERO CRD42022329273.

DEHP exposure impairs human skeletal muscle cell proliferation in primary culture conditions: preliminary study

The plasticizer di (2-ethylhexyl) phthalate (DEHP) inhibits differentiation, impairs glucose metabolism, and decreases mitochondrial function in murine muscle satellite cells; however, if these effects are translated to human cells is unknown. The goal of this study was to evaluate changes in morphology and proliferation of primary human skeletal muscle cells exposed to DEHP. Rectus abdominis muscle samples were obtained from healthy women undergoing programed cesarean surgery. Skeletal muscle cells were isolated and grown under standard primary culture conditions, generating two independent sample groups of 25 subcultures each. Cells from the first group were exposed to 1 mM DEHP for 13 days and monitored for changes in cell morphology, satellite cell frequency and total cell abundance, while the second group remained untreated (control). Differences between treated and untreated groups were compared using generalized linear mixed models (GLMM). Cell membrane and nuclear envelope boundary alterations, loss of cell volume and presence of stress bodies were observed in DEHP-treated cultures. DEHP-treated cultures also showed a significant reduction in satellite cell frequency compared to controls. Exposure to DEHP reduced human skeletal muscle cell abundance. Statistical differences were found between the GLMM slopes, suggesting that exposure to DEHP reduced growth rate. These results suggest that exposure to DEHP inhibits human skeletal muscle cell proliferation, as evidenced by reduced cell abundance, potentially compromising long-term culture viability. Therefore, DEHP induces human skeletal muscle cell deterioration potentially inducing an inhibitory effect of myogenesis by depleting satellite cells.

Graphical abstract

Skeletal Muscle Satellite Cells in Sickle Cell Disease Patients and Their Responses to a Moderate-intensity Endurance Exercise Training Program

We previously demonstrated that 8 weeks of moderate-intensity endurance training is safe and improves muscle function and characteristics of sickle cell disease (SCD) patients. Here, we investigated skeletal muscle satellite cells (SCs) in SCD patients and their responses to a training program. Fifteen patients followed the training program while 18 control patients maintained a normal lifestyle. Biopsies of the vastus lateralis muscle were performed before and after training. After training, the cross-sectional area and myonuclear content in type I fibers were slightly increased in the training patients compared to non-training patients. The SC pool was unchanged in type I fibers while it was slightly decreased in type II fibers in the training patients compared to non-training patients. No necrotic fibers were detected in patients before or after training. Therefore, the slight myonuclear accretion in type I fibers in trained SCD patients may highlight the contribution of SCs to training-induced slight type I fiber hypertrophy without expansion of the SC pool. The low training intensity and the short duration of training sessions could explain the low SC response to the training program. However, the lack of necrotic fibers suggests that the training program seemed to be safe for patients' muscle tissue.

Myogenic Precursor Cells Show Faster Activation and Enhanced Differentiation in a Male Mouse Model Selected for Advanced Endurance Exercise Performance

Satellite cells (SATC), the most abundant skeletal muscle stem cells, play a main role in muscle plasticity, including the adaptive response following physical activity. Thus, we investigated how long-term phenotype selection of male mice for high running performance (Dummerstorf high Treadmill Performance; DUhTP) affects abundance, creatine kinase activity, myogenic marker expression (Pax7, MyoD), and functionality (growth kinetics, differentiation) of SATC and their progeny. SATC were isolated from sedentary male DUhTP and control (Dummerstorf Control; DUC) mice at days 12, 43, and 73 of life and after voluntary wheel running for three weeks (day 73). Marked line differences occur at days 43 and 73 (after activity). At both ages, analysis of SATC growth via xCELLigence system revealed faster activation accompanied by a higher proliferation rate and lower proportion of Pax7+ cells in DUhTP mice, indicating reduced reserve cell formation and faster transition into differentiation. Cultures from sedentary DUhTP mice contain an elevated proportion of actively proliferating Pax7+/MyoD+ cells and have a higher fusion index leading to the formation of more large and very large myotubes at day 43. This robust hypertrophic response occurs without any functional load in the donor mice. Thus, our selection model seems to recruit myogenic precursor cells/SATC with a lower activation threshold that respond more rapidly to external stimuli and are more primed for differentiation at the expense of more primitive cells.

Angiotensin II inhibition: a potential treatment to slow the progression of sarcopenia

Sarcopenia is defined as the progressive and generalized loss of skeletal muscle mass and strength, which is associated with increased likelihood of adverse outcomes including falls, fractures, physical disability, and mortality. The etiology of sarcopenia has been postulated to be multifactorial with genetics, aging, immobility, nutritional deficiencies, inflammation, stress, and endocrine factors all contributing to the imbalance of muscle anabolism and catabolism. The prevalence of sarcopenia is estimated to range from 13 to 24% in adults over 60 years of age and up to 50% in persons aged 80 and older. As the population continues to age, the prevalence of sarcopenia continues to increase and is expected to affect 500 million people by the year 2050. Sarcopenia impacts the overall health of patients through limitations in functional status, increase in hospital readmissions, poorer hospital outcomes, and increase in overall mortality. Thus, there exists a need to prevent or reduce the occurrence of sarcopenia. Here, we explore the potential mechanisms and current studies regarding angiotensin receptor blockers (ARBs) and angiotensin-converting enzyme (ACE) inhibitors on reducing the development of sarcopenia through the associated changes in cardiovascular function, renal function, muscle fiber composition, inflammation, endothelial dysfunction, metabolic efficiency, and mitochondrial function.

Fusion and beyond: Satellite cell contributions to loading-induced skeletal muscle adaptation

Satellite cells support adult skeletal muscle fiber adaptations to loading in numerous ways. The fusion of satellite cells, driven by cell-autonomous and/or extrinsic factors, contributes new myonuclei to muscle fibers, associates with load-induced hypertrophy, and may support focal membrane damage repair and long-term myonuclear transcriptional output. Recent studies have also revealed that satellite cells communicate within their niche to mediate muscle remodeling in response to resistance exercise, regulating the activity of numerous cell types through various mechanisms such as secretory signaling and cell-cell contact. Muscular adaptation to resistance and endurance activity can be initiated and sustained for a period of time in the absence of satellite cells, but satellite cell participation is ultimately required to achieve full adaptive potential, be it growth, function, or proprioceptive coordination. While significant progress has been made in understanding the roles of satellite cells in adult muscle over the last few decades, many conclusions have been extrapolated from regeneration studies. This review highlights our current understanding of satellite cell behavior and contributions to adaptation outside of regeneration in adult muscle, as well as the roles of satellite cells beyond fusion and myonuclear accretion, which are gaining broader recognition.

Myofibre Hypertrophy in the Absence of Changes to Satellite Cell Content Following Concurrent Exercise Training in Young Healthy Men

Concurrent exercise training has been suggested to create an ‘interference effect,’ attenuating resistance training-based skeletal muscle adaptations, including myofibre hypertrophy. Satellite cells support myofibre hypertrophy and are influenced by exercise mode. To determine whether satellite cells contribute to the ‘interference effect’ changes in satellite cell and myonuclear content were assessed following a period of training in 32 recreationally active males (age: 25 ± 5 year; body mass index: 24 ± 3 kg⋅m^–2; mean ± SD) who undertook 12-week of either isolated (3 d⋅w^–1) resistance (RES; n = 10), endurance (END; n = 10), or alternate day (6 d⋅w^–1) concurrent (CET, n = 12) training. Skeletal muscle biopsies were obtained pre-intervention and after 2, 8, and 12 weeks of training to determine fibre type-specific cross-sectional area (CSA), satellite cell content (Pax7⁺DAPI⁺), and myonuclei (DAPI⁺) using immunofluorescence microscopy. After 12 weeks, myofibre CSA increased in all training conditions in type II (P = 0.0149) and mixed fibres (P = 0.0102), with no difference between conditions. Satellite cell content remained unchanged after training in both type I and type II fibres. Significant correlations were observed between increases in fibre type-specific myonuclear content and CSA of Type I (r = 0.63, P < 0.0001), Type II (r = 0.69, P < 0.0001), and mixed fibres (r = 0.72, P < 0.0001). Resistance, endurance, and concurrent training induce similar myofibre hypertrophy in the absence of satellite cell and myonuclear pool expansion. These findings suggest that myonuclear accretion via satellite cell fusion is positively correlated with hypertrophy after 12 weeks of concurrent training, and that individuals with more myonuclear content displayed greater myofibre hypertrophy.

Role of endocrine factors and stem cells in skeletal muscle regeneration

The process of reconstructing damaged skeletal muscles involves degeneration, inflammatory and immune responses, regeneration and reorganization, which are regulated by a number of immune-endocrine factors affecting muscle cells and satellite cells (SCs). One of these molecules is testosterone (T), which binds to the androgen receptor (AR) to initiate the expression of the muscle isoform of insulin-like growth factor 1 (IGF-1Ec). The interaction between T and IGF-1Ec stimulates the growth and regeneration of skeletal muscles by inhibiting apoptosis, enhancement of SCs proliferation and myoblasts differentiation. As a result of sarcopenia, muscle dystrophy or wasting diseases, the SCs population is significantly reduced. Regular physical exercise attenuates a decrease in SCs count, and thus elevates the regenerative potential of muscles in both young and elderly people. One of the challenges of modern medicine is the application of SCs and extracellular matrix scaffolds in regenerative and molecular medicine, especially in the treatment of degenerative diseases and post-traumatic muscle reconstruction. The aim of the study is to present current information on the molecular and cellular mechanisms of skeletal muscle regenera,tion, the role of testosterone and growth factors in the activation of SCs and the possibility of their therapeutic use in stimulating the reconstruction of damaged muscle fibers.

Both Traditional and Stair Climbing-based HIIT Cardiac Rehabilitation Induce Beneficial Muscle Adaptations

PURPOSE

There is a lack of knowledge as to how different exercise-based cardiac rehabilitation programming affects skeletal muscle adaptations in coronary artery disease (CAD) patients. We first characterized the skeletal muscle from adults with CAD compared with a group of age- and sex-matched healthy adults. We then determined the effects of a traditional moderate-intensity continuous exercise program (TRAD) or a stair climbing-based high-intensity interval training program (STAIR) on skeletal muscle metabolism in CAD.

METHODS

Sixteen adults (n = 16, 61 ± 7 yr), who had undergone recent treatment for CAD, were randomized to perform (3 d·wk-1) either TRAD (n = 7, 30 min at 60%-80% of peak heart rate) or STAIR (n = 9, 3 × 6 flights) for 12 wk. Muscle biopsies were collected at baseline in both CAD and healthy controls (n = 9), and at 4 and 12 wk after exercise training in CAD patients undertaking TRAD or STAIR.

RESULTS

We found that CAD had a lower capillary-to-fiber ratio (C/Fi, 35% ± 25%, P = 0.06) and capillary-to-fiber perimeter exchange (CFPE) index (23% ± 29%, P = 0.034) in Type II fibers compared with healthy controls. However, 12 wk of cardiac rehabilitation with either TRAD or STAIR increased C/Fi (Type II, 23% ± 14%, P < 0.001) and CFPE (Type I, 10% ± 23%, P < 0.01; Type II, 18% ± 22%, P = 0.002).

CONCLUSION

Cardiac rehabilitation via TRAD or STAIR exercise training improved the compromised skeletal muscle microvascular phenotype observed in CAD patients.

Can High-Intensity Interval Training Promote Skeletal Muscle Anabolism?

Exercise training in combination with optimal nutritional support is an effective strategy to maintain or increase skeletal muscle mass. A single bout of resistance exercise undertaken with adequate protein availability increases rates of muscle protein synthesis and, when repeated over weeks and months, leads to increased muscle fiber size. While resistance-based training is considered the 'gold standard' for promoting muscle hypertrophy, other modes of exercise may be able to promote gains in muscle mass. High-intensity interval training (HIIT) comprises short bouts of exercise at or above the power output/speed that elicits individual maximal aerobic capacity, placing high tensile stress on skeletal muscle, and somewhat resembling the demands of resistance exercise. While HIIT induces rapid increases in skeletal muscle oxidative capacity, the anabolic potential of HIIT for promoting concurrent gains in muscle mass and cardiorespiratory fitness has received less scientific inquiry. In this review, we discuss studies that have determined muscle growth responses after HIIT, with a focus on molecular responses, that provide a rationale for HIIT to be implemented among populations who are susceptible to muscle loss (e.g. middle-aged or older adults) and/or in clinical settings (e.g. pre- or post-surgery).

Sarcopenia: Molecular Pathways and Potential Targets for Intervention

Aging is associated with sarcopenia. The loss of strength results in decreased muscle mass and motor function. This process accelerates the progressive muscle deterioration observed in older adults, favoring the presence of debilitating pathologies. In addition, sarcopenia leads to a decrease in quality of life, significantly affecting self-sufficiency. Altogether, these results in an increase in economic resources from the National Health Systems devoted to mitigating this problem in the elderly, particularly in developed countries. Different etiological determinants are involved in the progression of the disease, including: neurological factors, endocrine alterations, as well as nutritional and lifestyle changes related to the adoption of more sedentary habits. Molecular and cellular mechanisms have not been clearly characterized, resulting in the absence of an effective treatment for sarcopenia. Nevertheless, physical activity seems to be the sole strategy to delay sarcopenia and its symptoms. The present review intends to bring together the data explaining how physical activity modulates at a molecular and cellular level all factors that predispose or favor the progression of this deteriorating pathology.

Effects of physical exercise on the prevention of stem cells senescence

Regular physical activity is essential for maintaining wellbeing; physical inactivity, on the contrary, is considered by the World Health Organization (WHO) as one of the most important risk factors for global mortality. During physical exercise different growth factors, cytokines and hormones are released, which affect positively the functions of heart, bone, brain and skeletal muscle. It has been reported that physical activity is able to stimulate tissue remodeling. Therefore, in this scenario, it is important to deepen the topic of physical activity-induced effects on stem cells.

High Levels of Physical Activity in Later Life Are Associated With Enhanced Markers of Mitochondrial Metabolism

The age-associated reduction in muscle mass is well characterized; however, less is known regarding the mechanisms responsible for the decline in oxidative capacity also observed with advancing age. The purpose of the current study was therefore to compare mitochondrial gene expression and protein content between young and old recreationally active, and older highly active individuals. Muscle biopsies were obtained from the vastus lateralis of young males (YG: 22 ± 3 years) and older (OG: 67 ± 2 years) males not previously engaged in formal exercise and older male master cyclists (OT: 65 ± 5 years) who had undertaken cycling exercise for 32 ± 17 years. Comparison of gene expression between YG, OG, and OT groups revealed greater expression of mitochondrial-related genes, namely, electron transport chain (ETC) complexes II, III, and IV (p < .05) in OT compared with YG and OG. Gene expression of mitofusion (MFN)-1/2, mitochondrial fusion genes, was greater in OT compared with OG (p < .05). Similarly, protein content of ETC complexes I, II, and IV was significantly greater in OT compared with both YG and OG (p < .001). Protein content of peroxisome proliferator-activated receptor gamma, coactivator 1 α (PGC-1α), was greater in OT compared with YG and OG (p < .001). Our results suggest that the aging process per se is not associated with a decline in gene expression and protein content of ETC complexes. Mitochondrial-related gene expression and protein content are substantially greater in OT, suggesting that exercise-mediated increases in mitochondrial content can be maintained into later life.

The effect of endurance training on levels of LINC complex proteins in skeletal muscle fibers of STZ-induced diabetic rats

The changes of the linker of nucleoskeleton and cytoskeleton (LINC) complex have been studied in many muscular abnormality conditions; however, the effects of diabetes and physical activities on it have still remained to be defined. Therefore, the purpose of the this study was to evaluate the impacts of a six-week endurance training on the levels of SUN1 and Nesprin-1 proteins in Soleus and EDL muscles from diabetic wistar rats. A total number of 48 male Wistar rats (10 weeks, 200-250 gr) were randomly divided into healthy control (HC, N = 12), healthy trained (HT, N = 12), diabetic control (DC, N = 12), and diabetic trained (DT, N = 12) groups. Diabetes was also induced by a single intraperitoneally injection of streptozocin (45 mg/kg). The training groups ran a treadmill for five consecutive days within six weeks. The levels of the SUN1 and the Nesprin-1 proteins were further determined via ELISA method. The induction of diabetes had significantly decreased the levels of Nesprin-1 protein in the soleus and EDL muscles but it had no effects on the SUN1 in these muscles. As well, the findings revealed that six weeks of endurance training had significantly increased the levels of Nesprin-1 in DT and HT groups in the soleus as well as the EDL muscles; however, it had no impacts on the SUN1 in these muscles. The muscle fiber cross-sectional area (CSA) and myonuclei also decreased in diabetic control rats in both studied muscles. The training further augmented these parameters in both studied muscles in HT and DT groups. The present study provides new evidence that diabetes changes Nesprin-1 protein levels in skeletal muscle and endurance exercise training can modify it.

Resistance training preserves high-intensity interval training induced improvements in skeletal muscle capillarization of healthy old men: a randomized controlled trial

Skeletal muscle capillarization is a determining factor in gas and metabolite exchange, while its impairments may contribute to the development of sarcopenia. Studies on the potential of resistance training (RT) to induce angiogenesis in older muscles have been inconclusive, and effects of sequential endurance training (ET) and RT on capillarization are unknown. Healthy older men (66.5 ± 3.8 years) were engaged in either 12 weeks of habitual course observation (HC) followed by 12 weeks of RT (n = 8), or 12 weeks of high-intensity interval training (HIIT) followed by 12 weeks of RT (n = 9). At baseline, following 12 and 24 weeks, m. vastus lateralis biopsies were obtained. (Immuno-)histochemistry was used to assess indices of muscle fiber capillarization, muscle fiber morphology and succinate dehydrogenase (SDH) activity. Single periods of RT and HIIT resulted in similar improvements in capillarization and SDH activity. During RT following HIIT, improved capillarization and SDH activity, as well as muscle fiber morphology remained unchanged. The applied RT and HIIT protocols were thus similarly effective in enhancing capillarization and oxidative enzyme activity and RT effectively preserved HIIT-induced adaptations of these parameters. Hence, both, RT and HIIT, are valid training modalities for older men to improve skeletal muscle vascularization.

Restoring the regenerative balance in neuromuscular disorders: satellite cell activation as therapeutic target in Pompe disease

Skeletal muscle is capable of efficiently regenerating after damage in a process mediated by tissue-resident stem cells called satellite cells. This regenerative potential is often compromised under muscle-degenerative conditions. Consequently, the damage produced during degeneration is not efficiently repaired and the balance between repair and damage is lost. Here we review recent progress on the role of satellite cell-mediated repair in neuromuscular disorders with a focus on Pompe disease, an inherited metabolic myopathy caused by deficiency of the lysosomal enzyme acid alpha glucosidase (GAA). Studies performed in patient biopsies as well as in Pompe disease mouse models demonstrate that muscle regeneration activity is compromised despite progressing muscle damage. We describe disease-specific mechanisms of satellite cell dysfunction to highlight the differences between Pompe disease and muscle dystrophies. The mechanisms involved provide possible targets for therapy, such as modulation of autophagy, muscle exercise, and pharmacological modulation of satellite cell activation. Most of these approaches are still experimental, although promising in animal models, still warrant caution with respect to their safety and efficiency profile.

Human skeletal muscle macrophages increase following cycle training and are associated with adaptations that may facilitate growth

Skeletal muscle macrophages participate in repair and regeneration following injury. However, their role in physiological adaptations to exercise is unexplored. We determined whether endurance exercise training (EET) alters macrophage content and characteristics in response to resistance exercise (RE), and whether macrophages are associated with other exercise adaptations. Subjects provided vastus lateralis biopsies before and after one bout of RE, after 12 weeks of EET (cycling), and after a final bout of RE. M2 macrophages (CD11b+/CD206+) did not increase with RE, but increased in response to EET (P < 0.01). Increases in M2 macrophages were positively correlated with fiber hypertrophy (r = 0.49) and satellite cells (r = 0.47). M2c macrophages (CD206+/CD163+) also increased following EET (P < 0.001), and were associated with fiber hypertrophy (r = 0.64). Gene expression was quantified using NanoString. Following EET, the change in M2 macrophages was positively associated with changes in HGF, IGF1, and extracellular matrix genes. EET decreased expression of IL6 (P < 0.05), C/EBPβ (P < 0.01), and MuRF (P < 0.05), and increased expression of IL-4 (P < 0.01), TNFα (P < 0.01) and the TWEAK receptor FN14 (P < 0.05). The change in FN14 gene expression was inversely associated with changes in C/EBPβ (r = -0.58) and MuRF (r = -0.46) following EET. In cultured human myotubes, siRNA inhibition of FN14 increased expression of C/EBPβ (P < 0.05) and MuRF (P < 0.05). Our data suggest that macrophages contribute to the muscle response to EET, potentially including modulation of TWEAK-FN14 signaling.

Satellite cell response to concurrent resistance exercise and high-intensity interval training in sedentary, overweight/obese, middle-aged individuals

Purpose

Sarcopenia can begin from the 4–5th decade of life and is exacerbated by obesity and inactivity. A combination of resistance exercise (RE) and endurance exercise is recommended to combat rising obesity and inactivity levels. However, work continues to elucidate whether interference in adaptive outcomes occur when RE and endurance exercise are performed concurrently. This study examined whether a single bout of concurrent RE and high-intensity interval training (HIIT) alters the satellite cell response following exercise compared to RE alone.

Methods

Eight sedentary, overweight/obese, middle-aged individuals performed RE only (8 × 8 leg extensions at 70% 1RM), or RE + HIIT (10 × 1 min at 90% HR_max on a cycle ergometer). Muscle biopsies were collected from the vastus lateralis before and 96 h after the RE component to determine muscle fiber type-specific total (Pax7⁺ cells) and active (MyoD⁺ cells) satellite cell number using immunofluorescence microscopy.

Results

Type-I-specific Pax7⁺ (P = 0.001) cell number increased after both exercise trials. Type-I-specific MyoD⁺ (P = 0.001) cell number increased after RE only. However, an elevated baseline value in RE + HIIT compared to RE (P = 0.046) was observed, with no differences between exercise trials at 96 h (P = 0.21). Type-II-specific Pax7⁺ and MyoD⁺ cell number remained unchanged after both exercise trials (all P ≥ 0.13).

Conclusion

Combining a HIIT session after a single bout of RE does not interfere with the increase in type-I-specific total, and possibly active, satellite cell number, compared to RE only. Concurrent RE + HIIT may offer a time-efficient way to maximise the physiological benefits from a single bout of exercise in sedentary, overweight/obese, middle-aged individuals.

Geriatric syndromes: new frontiers in HIV and sarcopenia

: HIV infection, in many circumstances, can now be managed as a chronic disease due to the marked increase in life expectancy since the introduction of combination antiretroviral therapy (ART). As the patients who first had access to combination ART age into their 50s and 60s, the effects of chronic HIV infection on health have become an important research focus in HIV infection. People living with HIV appear to exhibit an earlier occurrence of some aging-related conditions compared to people without HIV, in part due to higher rates of comorbidities, high-risk behaviors (e.g. smoking, substance use), chronic immune activation, inflammation, and ART-specific factors. Some studies have even suggested an earlier-than-expected appearance of the 'geriatric syndromes,' which are complex medical syndromes of older adults that are associated with morbidity and mortality. The geriatric syndromes include a wide variety of disease processes ranging from incontinence and dementia to impairments in physical function. This review will focus on one geriatric syndrome, sarcopenia, in older HIV-infected populations, and its relation to other aging syndromes, including frailty and falls. The contribution of HIV itself, ART exposure, and specific comorbidities, and the importance of early recognition and prevention of these aging syndromes will be highlighted.

Satellite cell activation induced by aerobic muscle adaptation in response to endurance exercise in humans and rodents

Although the requirement of satellite cells activation and expansion following injury, mechanical load or growth stimulus provoked by resistance exercise has been well established, their function in response to aerobic exercise adaptation remains unclear. A clear relationship between satellite cell expansion in fiber-type specific myosin heavy chain and aerobic performance has been related, independent of myonuclear accretion or muscle growth. However, the trigger for this activation process is not fully understood yet and it seems to be a multi-faceted and well-orchestrated process. Emerging in vitro studies suggest a role for metabolic pathways and oxygen availability for satellite cell activation, modulating the self-renewal potential and cell fate control. The goal of this review is to describe and discuss the current knowledge about the satellite cell activation and expansion in response to aerobic exercise adaptation in human and rodent models. Additionally, findings about the in vitro metabolic control, which seems be involved in the satellite cell activation and cell fate control, are presented and discussed.

Fiber Type-Specific Satellite Cell Content in Cyclists Following Heavy Training with Carbohydrate and Carbohydrate-Protein Supplementation

The central purpose of this study was to evaluate the fiber type-specific satellite cell and myonuclear responses of endurance-trained cyclists to a block of intensified training, when supplementing with carbohydrate (CHO) vs. carbohydrate-protein (PRO). In a crossover design, endurance-trained cyclists (n = 8) performed two consecutive training periods, once supplementing with CHO (de facto “control” condition) and the other with PRO. Each training period consisted of 10 days of intensified cycle training (ICT–120% increase in average training duration) followed by 10 days of recovery (RVT–reduced volume training; 33% volume reduction vs. normal training). Skeletal muscle biopsies were obtained from the vastus lateralis before and after ICT and again following RVT. Immunofluorescent microscopy was used to quantify SCs (Pax7+), myonuclei (DAPI+), and myosin heavy chain I (MyHC I). Data are expressed as percent change ± 90% confidence limits. The 10-day block of ICT_CHO increased MyHC I SC content (35 ± 28%) and myonuclear density (16 ± 6%), which remained elevated following RVT_CHO (SC = 69 ± 50% vs. PRE; Nuclei = 17 ± 15% vs. PRE). MyHC II SC and myonuclei were not different following ICT_CHO, but were higher following RVT_CHO (SC = +33 ± 31% vs. PRE; Nuclei = 15 ± 14% vs. PRE), indicating a delayed response compared to MyHC I fibers. The MyHC I SC pool increased following ICT_PRO (37 ± 37%), but without a concomitant increase in myonuclei. There were no changes in MyHC II SC or myonuclei following ICT_PRO. Collectively, these trained endurance cyclists possessed a relatively large pool of SCs that facilitated rapid (MyHC I) and delayed (MyHC II) satellite cell proliferation and myonuclear accretion under carbohydrate conditions. The current findings strengthen the growing body of evidence demonstrating alterations in satellite cell number in the absence of hypertrophy. Satellite cell pool expansion is typically viewed as an advantageous response to exercise. However, when coupled with our previous report that PRO possibly enhanced whole muscle recovery and increased MyHC I and II fiber size, the limited satellite cell/myonuclear response observed with carbohydrate-protein seem to indicate that protein supplementation may have minimized the necessity for satellite cell involvement, thereby suggesting that protein may benefit skeletal muscle during periods of heavy training.

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.

Safety and efficacy of a 6-month home-based exercise program in patients with facioscapulohumeral muscular dystrophy: A randomized controlled trial

BACKGROUND

Previous randomized controlled trials investigating exercise training programs in facioscapulohumeral muscular dystrophy (FSHD) patients are scarce and of short duration only. This study assessed the safety and efficacy of a 6-month home-based exercise training program on fitness, muscle, and motor function in FSHD patients.

METHODS

Sixteen FSHD patients were randomly assigned to training (TG) and control (CG) groups (both n = 8) in a home-based exercise intervention. Training consisted of cycling 3 times weekly for 35 minutes (combination of strength, high-intensity interval, and low-intensity aerobic) at home for 24 weeks. Patients in CG also performed an identical training program (CTG) after 24 weeks. The primary outcome was change in peak oxygen uptake (VO2 peak) measured every 6 weeks. The principal secondary outcomes were maximal quadriceps strength (MVC) and local quadriceps endurance every 12 weeks. Other outcome measures included maximal aerobic power (MAP) and experienced fatigue every 6 weeks, 6-minute walking distance every 12 weeks, and muscle characteristics from vastus lateralis biopsies taken pre- and postintervention.

RESULTS

The compliance rate was 91% in TG. Significant improvements with training were observed in the VO2 peak (+19%, P = 0.002) and MAP by week 6 and further to week 24. Muscle endurance, MVC, and 6-minute walking distance increased and experienced fatigue decreased. Muscle fiber cross-sectional area and citrate synthase activity increased by 34% (P = 0.008) and 46% (P = 0.003), respectively. Dystrophic pathophysiologic patterns were not exacerbated. Similar improvements were experienced by TG and CTG.

CONCLUSIONS

A combined strength and interval cycling exercise-training program compatible with patients' daily professional and social activities leads to significant functional benefits without compromising muscle tissue.

Satellite cell response to erythropoietin treatment and endurance training in healthy young men

KEY POINT

Erythropoietin (Epo) treatment may induce myogenic differentiation factor (MyoD) expression and prevent apoptosis in satellite cells (SCs) in murine and in vitro models. Endurance training stimulates SC proliferation in vivo in murine and human skeletal muscle. In the present study, we show, in human skeletal muscle, that treatment with an Epo-stimulating agent (darbepoetin-α) in vivo increases the content of MyoD(+) SCs in healthy young men. Moreover, we report that Epo receptor mRNA is expressed in adult human SCs, suggesting that Epo may directly target SCs through ligand-receptor interaction. Moreover, endurance training, but not Epo treatment, increases the SC content in type II myofibres, as well as the content of MyoD(+) SCs. Collectively, our results suggest that Epo treatment can regulate human SCs in vivo, supported by Epo receptor mRNA expression in human SCs. In effect, long-term Epo treatment during disease conditions involving anaemia may impact SCs and warrants further investigation. Satellite cell (SC) proliferation is observed following erythropoitin treatment in vitro in murine myoblasts and endurance training in vivo in human skeletal muscle. The present study aimed to investigate the effects of prolonged erythropoiesis-stimulating agent (ESA; darbepoetin-α) treatment and endurance training, separately and combined, on SC quantity and commitment in human skeletal muscle. Thirty-five healthy, untrained men were randomized into four groups: sedentary-placebo (SP, n = 9), sedentary-ESA (SE, n = 9), training-placebo (TP, n = 9) or training-ESA (TE, n = 8). ESA/placebo was injected once weekly and training consisted of ergometer cycling three times a week for 10 weeks. Prior to and following the intervention period, blood samples and muscle biopsies were obtained and maximal oxygen uptake (V̇O2, max) was measured. Immunohistochemical analyses were used to quantify fibre type specific SCs (Pax7(+)), myonuclei and active SCs (Pax7(+)/MyoD(+)). ESA treatment led to elevated haematocrit, whereas endurance training increased V̇O2, max. Endurance training led to an increase in SCs associated with type II fibres (P < 0.05), whereas type I fibres showed no changes. Both ESA treatment and endurance training increased Pax7(+)/MyoD(+) cells, whereas only ESA treatment increased the total content of MyoD(+) cells. Epo-R mRNA presence in adult SC was tested with real-time RT-PCR using fluorescence-activated cell sorting (CD56(+)/CD45(-)/CD31(-)) to isolate cells from a human rectus abdominis muscle and was found to be considerably higher than in whole muscle. In conclusion, endurance training and ESA treatment may separately stimulate SC commitment to the myogenic program. Furthermore, ESA-treatment may alter SC activity by direct interaction with the Epo-R expressed on SCs.

Skeletal muscle morphology in sarcopenia defined using the EWGSOP criteria: findings from the Hertfordshire Sarcopenia Study (HSS)

Background

Sarcopenia is defined as the loss of muscle mass and function with age and is associated with decline in mobility, frailty, falls and mortality. There is considerable interest in understanding the underlying mechanisms. Our aim was to characterise muscle morphology changes associated with sarcopenia among community dwelling older men.

Methods

One hundred and five men aged 68–76 years were recruited to the Hertfordshire Sarcopenia Study (HSS) for detailed characterisation of muscle including measures of muscle mass, strength and function. Muscle tissue was obtained from a biopsy of the vastus lateralis for 99 men and was processed for immunohistochemical studies to determine myofibre distribution and area, capillarisation and satellite cell (SC) density.

Results

Six (6 %) men had sarcopenia as defined by the European Working Group on Sarcopenia in Older People (EWGSOP) criteria. These men had lower SC density (1.7 cells/mm² vs 3.8 cells/mm², p = 0.06) and lower SC/fibre ratio (0.02 vs 0.06, p = 0.06) than men without sarcopenia. Although men with sarcopenia tended to have smaller myofibres and lower capillary to fibre ratio, these relationships were not statistically significant.

Conclusion

We have shown that there may be altered muscle morphology parameters in older men with sarcopenia. These results have the potential to help identify cell and molecular targets for therapeutic intervention. This work now requires extension to larger studies which also include women.

The effect of exercise mode on the acute response of satellite cells in old men

AIM

A dysregulation of satellite cells may contribute to the progressive loss of muscle mass that occurs with age; however, older adults retain the ability to activate and expand their satellite cell pool in response to exercise. The modality of exercise capable of inducing the greatest acute response is unknown. We sought to characterize the acute satellite cell response following different modes of exercise in older adults.

METHODS

Sedentary older men (n = 22; 67 ± 4 years; 27 ± 2.6 kg*m(-2) ) were randomly assigned to complete an acute bout of either resistance exercise, high-intensity interval exercise on a cycle ergometer or moderate-intensity aerobic exercise. Muscle biopsies were obtained before, 24 and 48 h following each exercise bout. The satellite cell response was analysed using immunofluorescent microscopy of muscle cross sections.

RESULTS

Satellite cell expansion associated with type I fibres was observed 24 and 48 h following resistance exercise only (P ˂ 0.05), while no expansion of type II-associated satellite cells was observed in any group. There was a greater number of activated satellite cells 24 h following resistance exercise (pre: 1.3 ± 0.1, 24 h: 4.8 ± 0.5 Pax7 + /MyoD+cells/100 fibres) and high-intensity interval exercise (pre: 0.7 ± 0.3, 24 h: 3.1 ± 0.3 Pax7 + /MyoD+cells/100 fibres) (P ˂ 0.05). The percentage of type I-associated SC co-expressing MSTN was reduced only in the RE group 24 h following exercise (pre: 87 ± 4, 24 h: 57 ± 5%MSTN+ type I SC) (P < 0.001).

CONCLUSION

Although resistance exercise is the most potent exercise type to induce satellite cell pool expansion, high-intensity interval exercise was also more potent than moderate-intensity aerobic exercise in inducing satellite cell activity.

Health benefits of cycle ergometer training for older adults over 70: a review

As the number of older adults continues to increase worldwide, more attention is being paid to geriatric health care needs, and successful ageing is becoming an important topic in the medical literature. A preventive approach to the care of older adults is thus a priority in our aging societies. The purpose of this study was to update evidence for the health benefits of cycle ergometer training for older adults over 70. We searched online electronic databases up to September 2014 for original observational and intervention studies on the relationship between cycle ergometer training and health among older patients over 70. Twenty-five studies examined interventions aimed specifically at promoting cycling for older adults over 70. These studies reported a positive effect on the prevention of cardiovascular disease, and a significant improvement in metabolic responses. Improving functional status, muscle strength and cognitive performance are also well established. Overall, this review demonstrates a positive effect of cycle ergometer training with functional benefits and positive health outcomes for older adults over 70. Based on this evidence, clinicians can now encourage older adults to profit from the health benefits of cycle ergometer training to be able to pursue their daily activities independently.

Satellite cells in human skeletal muscle plasticity

Skeletal muscle satellite cells are considered to play a crucial role in muscle fiber maintenance, repair and remodeling. Our knowledge of the role of satellite cells in muscle fiber adaptation has traditionally relied on in vitro cell and in vivo animal models. Over the past decade, a genuine effort has been made to translate these results to humans under physiological conditions. Findings from in vivo human studies suggest that satellite cells play a key role in skeletal muscle fiber repair/remodeling in response to exercise. Mounting evidence indicates that aging has a profound impact on the regulation of satellite cells in human skeletal muscle. Yet, the precise role of satellite cells in the development of muscle fiber atrophy with age remains unresolved. This review seeks to integrate recent results from in vivo human studies on satellite cell function in muscle fiber repair/remodeling in the wider context of satellite cell biology whose literature is largely based on animal and cell models.

Satellite cell activity, without expansion, after nonhypertrophic stimuli

The purpose of the present studies was to determine the effect of various nonhypertrophic exercise stimuli on satellite cell (SC) pool activity in human skeletal muscle. Previously untrained men and women (men: 29 ± 9 yr and women: 29 ± 2 yr, n = 7 each) completed 6 wk of very low-volume high-intensity sprint interval training. In a separate study, recreationally active men (n = 16) and women (n = 3) completed 6 wk of either traditional moderate-intensity continuous exercise (n = 9, 21 ± 4 yr) or low-volume sprint interval training (n = 10, 21 ± 2 yr). Muscle biopsies were obtained from the vastus lateralis before and after training. The fiber type-specific SC response to training was determined, as was the activity of the SC pool using immunofluorescent microscopy of muscle cross sections. Training did not induce hypertrophy, as assessed by muscle cross-sectional area, nor did the SC pool expand in any group. However, there was an increase in the number of active SCs after each intervention. Specifically, the number of activated (Pax7(+)/MyoD(+), P ≤ 0.05) and differentiating (Pax7(-)/MyoD(+), P ≤ 0.05) SCs increased after each training intervention. Here, we report evidence of activated and cycling SCs that may or may not contribute to exercise-induced adaptations while the SC pool remains constant after three nonhypertrophic exercise training protocols.

Exercise and Stem Cells

Stem cells are traditionally studied in the context of embryonic development, yet studies confirm that a fraction remains in the adult organism for the purpose of daily remodeling and rejuvenation of multiple tissues following injury. Adult stem cells (ASCs) are found in close proximity to vessels and respond to tissue-specific cues in the microenvironment that dictate their fate and function. Exercise can dramatically alter strain sensing, extracellular matrix composition, and inflammation, and such changes in the niche likely alter ASC quantity and function postexercise. The field of stem cell biology is still in its infancy and identification and terminology of ASCs continues to evolve; thus, current information regarding exercise and stem cells is lacking. This chapter summarizes the literature that reports on the ASC response to acute exercise and exercise training, with particular emphasis on hematopoietic stem cells, endothelial progenitor cells, and mesenchymal stem cells.

Interactions between muscle stem cells, mesenchymal-derived cells and immune cells in muscle homeostasis, regeneration and disease

Recent evidence has revealed the importance of reciprocal functional interactions between different types of mononuclear cells in coordinating the repair of injured muscles. In particular, signals released from the inflammatory infiltrate and from mesenchymal interstitial cells (also known as fibro-adipogenic progenitors (FAPs)) appear to instruct muscle stem cells (satellite cells) to break quiescence, proliferate and differentiate. Interestingly, conditions that compromise the functional integrity of this network can bias muscle repair toward pathological outcomes that are typically observed in chronic muscular disorders, that is, fibrotic and fatty muscle degeneration as well as myofiber atrophy. In this review, we will summarize the current knowledge on the regulation of this network in physiological and pathological conditions, and anticipate the potential contribution of its cellular components to relatively unexplored conditions, such as aging and physical exercise.

Long-Term Endurance Exercise in Humans Stimulates Cell Fusion of Myoblasts along with Fusogenic Endogenous Retroviral Genes In Vivo

Myogenesis is defined as growth, differentiation and repair of muscles where cell fusion of myoblasts to multinucleated myofibers is one major characteristic. Other cell fusion events in humans are found with bone resorbing osteoclasts and placental syncytiotrophoblasts. No unifying gene regulation for natural cell fusions has been found. We analyzed skeletal muscle biopsies of competitive cyclists for muscle-specific attributes and expression of human endogenous retrovirus (ERV) envelope genes due to their involvement in cell fusion of osteoclasts and syncytiotrophoblasts. Comparing muscle biopsies from post- with the pre-competitive seasons a significant 2.25-fold increase of myonuclei/mm fiber, a 2.38-fold decrease of fiber area/nucleus and a 3.1-fold decrease of satellite cells (SCs) occurred. We propose that during the pre-competitive season SC proliferation occurred following with increased cell fusion during the competitive season. Expression of twenty-two envelope genes of muscle biopsies demonstrated a significant increase of putative muscle-cell fusogenic genes Syncytin-1 and Syncytin-3, but also for the non-fusogenic erv3. Immunohistochemistry analyses showed that Syncytin-1 mainly localized to the sarcolemma of myofibers positive for myosin heavy-chain isotypes. Cellular receptors SLC1A4 and SLC1A5 of Syncytin-1 showed significant decrease of expression in post-competitive muscles compared with the pre-competitive season, but only SLC1A4 protein expression localized throughout the myofiber. Erv3 protein was strongly expressed throughout the myofiber, whereas envK1-7 localized to SC nuclei and myonuclei. Syncytin-1 transcription factors, PPARγ and RXRα, showed no protein expression in the myofiber, whereas the pCREB-Ser133 activator of Syncytin-1 was enriched to SC nuclei and myonuclei. Syncytin-1, Syncytin-3, SLC1A4 and PAX7 gene regulations along with MyoD1 and myogenin were verified during proliferating or actively-fusing human primary myoblast cell cultures, resembling muscle biopsies of cyclists. Myoblast treatment with anti-Synycytin-1 abrogated cell fusion in vitro. Our findings support functional roles for ERV envelope proteins, especially Syncytin-1, contributing to cell fusion of myotubes.

Reduced satellite cell number in situ in muscular contractures from children with cerebral palsy

Satellite cells (SC) are quiescent adult muscle stem cells critical for postnatal development. Children with cerebral palsy have impaired muscular growth and develop contractures. While flow cytometry previously demonstrated a reduced SC population, extracellular matrix abnormalities may influence the cell isolation methods used, systematically isolating fewer cells from CP muscle and creating a biased result. Consequently, the purpose of this study was to use immunohistochemistry on serial muscle sections to quantify SC in situ. Serial cross-sections from human gracilis muscle biopsies (n = 11) were labeled with fluorescent antibodies for Pax7 (SC transcriptional marker), laminin (basal lamina), and 4',6-diamidino-2-phenylindole (nuclei). Fluorescence microscopy under high magnification was used to identify SC based on labeling and location. Mean SC/100 myofibers was reduced by ∼70% (p < 0.001) in children with CP (2.89 ± 0.39) compared to TD children (8.77 ± 0.79). Furthermore, SC distribution across fields was different (p < 0.05) with increased percentage of SC in fields being solitary cells (p < 0.01) in children with CP. Quantification of SC number in situ, without any other tissue manipulation confirms children with spastic CP have a reduced number. This stem cell loss may, in part, explain impaired muscle growth and apparent decreased responsiveness of CP muscle to exercise.

Mixed lactate and caffeine compound increases satellite cell activity and anabolic signals for muscle hypertrophy

We examined whether a mixed lactate and caffeine compound (LC) could effectively elicit proliferation and differentiation of satellite cells or activate anabolic signals in skeletal muscles. We cultured C2C12 cells with either lactate or LC for 6 h. We found that lactate significantly increased myogenin and follistatin protein levels and phosphorylation of P70S6K while decreasing the levels of myostatin relative to the control. LC significantly increased protein levels of Pax7, MyoD, and Ki67 in addition to myogenin, relative to control. LC also significantly increased follistatin expression relative to control and stimulated phosphorylation of mTOR and P70S6K. In an in vivo study, male F344/DuCrlCrlj rats were assigned to control (Sed, n = 10), exercise (Ex, n = 12), and LC supplementation (LCEx, n = 13) groups. LC was orally administered daily. The LCEx and Ex groups were exercised on a treadmill, running for 30 min at low intensity every other day for 4 wk. The LCEx group experienced a significant increase in the mass of the gastrocnemius (GA) and tibialis anterior (TA) relative to both the Sed and Ex groups. Furthermore, the LCEx group showed a significant increase in the total DNA content of TA compared with the Sed group. The LCEx group experienced a significant increase in myogenin and follistatin expression of GA relative to the Ex group. These results suggest that administration of LC can effectively increase muscle mass concomitant with elevated numbers of myonuclei, even with low-intensity exercise training, via activated satellite cells and anabolic signals.

Interaction between bone and muscle in older persons with mobility limitations

Aging is associated with a progressive loss of bone-muscle mass and strength. When the decline in mass and strength reaches critical thresholds associated with adverse health outcomes, they are operationally considered geriatric conditions and named, respectively, osteoporosis and sarcopenia. Osteoporosis and sarcopenia share many of the same risk factors and both directly or indirectly cause higher risk of mobility limitations, falls, fractures and disability in activities of daily living. This is not surprising since bones adapt their morphology and strength to the long-term loads exerted by muscle during anti-gravitational and physical activities. Non-mechanical systemic and local factors also modulate the mechanostat effect of muscle on bone by affecting the bidirectional osteocyte-muscle crosstalk, but the specific pathways that regulate these homeostatic mechanisms are not fully understood. More research is required to reach a consensus on cut points in bone and muscle parameters that identify individuals at high risk for adverse health outcomes, including falls, fractures and disability. A better understanding of the muscle-bone physiological interaction may help to develop preventive strategies that reduce the burden of musculoskeletal diseases, the consequent disability in older persons and to limit the financial burden associated with such conditions. In this review, we summarize age-related bone-muscle changes focusing on the biomechanical and homeostatic mechanisms that explain bone-muscle interaction and we speculate about possible pathological events that occur when these mechanisms become impaired. We also report some recent definitions of osteoporosis and sarcopenia that have emerged in the literature and their implications in clinical practice. Finally, we outline the current evidence for the efficacy of available anti-osteoporotic and proposed antisarcopenic interventions in older persons.

Skeletal muscle hypertrophy after aerobic exercise training

Current dogma suggests that aerobic exercise training has minimal effects on skeletal muscle size. We and others have demonstrated that aerobic exercise acutely and chronically alters protein metabolism and induces skeletal muscle hypertrophy. These findings promote an antithesis to the status quo by providing novel perspective on skeletal muscle mass regulation and insight into exercise countermeasures for populations prone to muscle loss.

Skeletal muscle as a regulator of the longevity protein, Klotho

Klotho is a powerful longevity protein that has been linked to the prevention of muscle atrophy, osteopenia, and cardiovascular disease. Similar anti-aging effects have also been ascribed to exercise and physical activity. While an association between muscle function and Klotho expression has been previously suggested from longitudinal cohort studies, a direct relationship between circulating Klotho and skeletal muscle has not been investigated. In this paper, we present a review of the literature and preliminary evidence that, together, suggests Klotho expression may be modulated by skeletal muscle activity. Our pilot clinical findings performed in young and aged individuals suggest that circulating Klotho levels are upregulated in response to an acute exercise bout, but that the response may be dependent on fitness level. A similar upregulation of circulating Klotho is also observed in response to an acute exercise in young and old mice, suggesting that this may be a good model for mechanistically probing the role of physical activity on Klotho expression. Finally, we highlight overlapping signaling pathways that are modulated by both Klotho and skeletal muscle and propose potential mechanisms for cross-talk between the two. It is hoped that this review will stimulate further consideration of the relationship between skeletal muscle activity and Klotho expression, potentially leading to important insights into the well-documented systemic anti-aging effects of exercise.

Lactate regulates myogenesis in C2C12 myoblasts in vitro

Satellite cells (SCs) are the resident stem cells of skeletal muscle tissue which play a major role in muscle adaptation, e.g. as a response to physical training. The aim of this study was to examine the effects of an intermittent lactate (La) treatment on the proliferation and differentiation of C2C12 myoblasts, simulating a microcycle of high intensity endurance training. Furthermore, the involvement of reactive oxygen species (ROS) in this context was examined. C2C12 myoblasts were therefore repeatedly incubated for 2 h each day with 10 mM or 20 mM La differentiation medium (DM) and in some cases 20 mM La DM plus different antioxidative substances for up to 5 days. La free (0 mM) DM served as a control. Immunocytochemical staining, Western blot analysis and colorimetric assays were used to assess oxidative stress, proliferation, and differentiation. Results show that La induces oxidative stress, enhances cell-cycle withdrawal, and initiates early differentiation but delays late differentiation in a timely and dose-dependent manner. These effects can be reversed by the addition of antioxidants to the La DM. We therefore conclude that La has a regulatory role in C2C12 myogenesis via a ROS-sensitive mechanism which elicits implications for reassessing some aspects of training and the use of nutritional supplements.

Fibre type-specific satellite cell response to aerobic training in sedentary adults

In the present study, we sought to determine the effect of a traditional, 12 week aerobic training protocol on skeletal muscle fibre type distribution and satellite cell content in sedentary subjects. Muscle biopsies were obtained from the vastus lateralis [n = 23 subjects (six male and 17 female); body mass index 30.7 ± 1.2 kg m(-2)] before and after 12 weeks of aerobic training performed on a cycle ergometer. Immunohistochemical analyses were used to quantify myosin heavy chain (MyHC) isoform expression, cross-sectional area and satellite cell and myonuclear content. Following training, a decrease in MyHC hybrid type IIa/IIx fibre frequency occurred, with a concomitant increase in pure MyHC type IIa fibres. Pretraining fibre type correlated with body mass index, and the change in fibre type following training was associated with improvements in maximal oxygen consumption. Twelve weeks of aerobic training also induced increases in mean cross-sectional area in both MyHC type I and type IIa fibres. Satellite cell content was also increased following training, specifically in MyHC type I fibres, with no change in the number of satellite cells associated with MyHC type II fibres. With the increased satellite cell content following training, an increase in myonuclear number per fibre also occurred in MyHC type I fibres. Hypertrophy of MyHC type II fibres occurred without detectable myonuclear addition, suggesting that the mechanisms underlying growth in fast and slow fibres differ. These data provide intriguing evidence for a fibre type-specific role of satellite cells in muscle adaptation following aerobic training.