Current evidence that exercise can increase the number of adult stem cells

The impact of acute and chronic aerobic and resistance exercise on stem cell mobilization: A review of effects in healthy and diseased individuals across different age groups

Stem cells (SCs) play a crucial role in tissue repair, regeneration, and maintaining physiological homeostasis. Exercise mobilizes and enhances the function of SCs. This review examines the effects of acute and chronic aerobic and resistance exercise on the population of SCs in healthy and diseased individuals across different age groups. Both acute intense exercise and moderate regular training increase circulating precursor cells CD34+ and, in particular, the subset of angiogenic progenitor cells (APCs) CD34+/KDR+. Conversely, chronic exercise training has conflicting effects on circulating CD34+ cells and their function, which are likely influenced by exercise dosage, the health status of the participants, and the methodologies employed. While acute activity promotes transient mobilization, regular exercise often leads to an increased number of progenitors and more sustainable functionality. Short interventions lasting 10-21 days mobilize CD34+/KDR + APCs in sedentary elderly individuals, indicating the inherent capacity of the body to rapidly activate tissue-reparative SCs during activity. However, further investigation is needed to determine the optimal exercise regimens for enhancing SC mobilization, elucidating the underlying mechanisms, and establishing functional benefits for health and disease prevention. Current evidence supports the integration of intense exercise with chronic training in exercise protocols aimed at activating the inherent regenerative potential through SC mobilization. The physical activity promotes endogenous repair processes, and research on exercise protocols that effectively mobilize SCs can provide innovative guidelines designed for lifelong tissue regeneration. An artificial neural network (ANN) was developed to estimate the effects of modifying elderly individuals and implementing chronic resistance exercise on stem cell mobilization and its impact on individuals and exercise. The network's predictions were validated using linear regression and found to be acceptable compared to experimental results.

A bibliometric analysis from 2013 to 2023 reveals research hotspots and trends in the connection between sport and regenerative medicine

The field of regenerative medicine for sports injuries has grown significantly in the 21st century. This study attempted to provide an overview of the current state of research and key findings regarding the relationship between sport and regenerative medicine in general, identifying trends and hotspots in research topics. We gathered the literature from the Web of Science (WOS) database covering the last 10 years (2013-2023) pertaining to regenerative medicine for sporter and applied Citespace to assess the knowledge mapping. The findings demonstrated that there were 572, with a faster increase after 2018. The country, institution, and author with the most publications are the USA, Harvard University, and Maffulli Nicola. In addition, the most co-cited reference is J Acad Nutr Diet (2016) (199). Adipose tissue, high tibial osteotomy, and bone marrow are the hot spots in this field in the next few years.

Slow or fast: Implications of myofibre type and associated differences for manifestation of neuromuscular disorders

Many neuromuscular disorders can have a differential impact on a specific myofibre type, forming the central premise of this review. The many different skeletal muscles in mammals contain a spectrum of slow- to fast-twitch myofibres with varying levels of protein isoforms that determine their distinctive contractile, metabolic, and other properties. The variations in functional properties across the range of classic 'slow' to 'fast' myofibres are outlined, combined with exemplars of the predominantly slow-twitch soleus and fast-twitch extensor digitorum longus muscles, species comparisons, and techniques used to study these properties. Other intrinsic and extrinsic differences are discussed in the context of slow and fast myofibres. These include inherent susceptibility to damage, myonecrosis, and regeneration, plus extrinsic nerves, extracellular matrix, and vasculature, examined in the context of growth, ageing, metabolic syndrome, and sexual dimorphism. These many differences emphasise the importance of carefully considering the influence of myofibre-type composition on manifestation of various neuromuscular disorders across the lifespan for both sexes. Equally, understanding the different responses of slow and fast myofibres due to intrinsic and extrinsic factors can provide deep insight into the precise molecular mechanisms that initiate and exacerbate various neuromuscular disorders. This focus on the influence of different myofibre types is of fundamental importance to enhance translation for clinical management and therapies for many skeletal muscle disorders.

Effect of resistance training on satellite cells in old mice - a transcriptome study : implications for sarcopenia

Aims

The decrease in the number of satellite cells (SCs), contributing to myofibre formation and reconstitution, and their proliferative capacity, leads to muscle loss, a condition known as sarcopenia. Resistance training can prevent muscle loss; however, the underlying mechanisms of resistance training effects on SCs are not well understood. We therefore conducted a comprehensive transcriptome analysis of SCs in a mouse model.

Methods

We compared the differentially expressed genes of SCs in young mice (eight weeks old), middle-aged (48-week-old) mice with resistance training intervention (MID+ T), and mice without exercise (MID) using next-generation sequencing and bioinformatics.

Results

After the bioinformatic analysis, the PI3K-Akt signalling pathway and the regulation of actin cytoskeleton in particular were highlighted among the top ten pathways with the most differentially expressed genes involved in the young/MID and MID+ T/MID groups. The expression of Gng5, Atf2, and Rtor in the PI3K-Akt signalling pathway was higher in the young and MID+ T groups compared with the MID group. Similarly, Limk1, Arhgef12, and Araf in the regulation of the actin cytoskeleton pathway had a similar bias. Moreover, the protein expression profiles of Atf2, Rptor, and Ccnd3 in each group were paralleled with the results of NGS.

Conclusion

Our results revealed that age-induced muscle loss might result from age-influenced genes that contribute to muscle development in SCs. After resistance training, age-impaired genes were reactivated, and age-induced genes were depressed. The change fold in these genes in the young/MID mice resembled those in the MID + T/MID group, suggesting that resistance training can rejuvenate the self-renewing ability of SCs by recovering age-influenced genes to prevent sarcopenia.

Cite this article: Bone Joint Res 2022;11(2):121–133.

Stem cells and regenerative medicine in sport science

The estimated cost of acute injuries in college-level sport in the USA is ∼1.5 billion dollars per year, without taking into account the cost of follow up rehabilitation. In addition to this huge financial burden, without appropriate diagnosis and relevant interventions, sport injuries may be career-ending for some athletes. With a growing number of females participating in contact based and pivoting sports, middle aged individuals returning to sport and natural injuries of ageing all increasing, such costs and negative implications for quality of life will expand. For those injuries, which cannot be predicted and prevented, there is a real need, to optimise repair, recovery and function, post-injury in the sporting and clinical worlds. The 21st century has seen a rapid growth in the arena of regenerative medicine for sporting injuries, in a bid to progress recovery and to facilitate return to sport. Such interventions harness knowledge relating to stem cells as a potential for injury repair. While the field is rapidly growing, consideration beyond the stem cells, to the factors they secrete, should be considered in the development of effective, affordable treatments.

Role of Regular Physical Activity in Neuroprotection against Acute Ischemia

One of the major obstacles that prevents an effective therapeutic intervention against ischemic stroke is the lack of neuroprotective agents able to reduce neuronal damage; this results in frequent evolution towards a long-term disability with limited alternatives available to aid in recovery. Nevertheless, various treatment options have shown clinical efficacy. Neurotrophins such as brain-derived neurotrophic factor (BDNF), widely produced throughout the brain, but also in distant tissues such as the muscle, have demonstrated regenerative properties with the potential to restore damaged neural tissue. Neurotrophins play a significant role in both protection and recovery of function following neurological diseases such as ischemic stroke or traumatic brain injury. Unfortunately, the efficacy of exogenous administration of these neurotrophins is limited by rapid degradation with subsequent poor half-life and a lack of blood-brain-barrier permeability. Regular exercise seems to be a therapeutic approach able to induce the activation of several pathways related to the neurotrophins release. Exercise, furthermore, reduces the infarct volume in the ischemic brain and ameliorates motor function in animal models increasing astrocyte proliferation, inducing angiogenesis and reducing neuronal apoptosis and oxidative stress. One of the most critical issues is to identify the relationship between neurotrophins and myokines, newly discovered skeletal muscle-derived factors released during and after exercise able to exert several biological functions. Various myokines (e.g., Insulin-Like Growth Factor 1, Irisin) have recently shown their ability to protects against neuronal injury in cerebral ischemia models, suggesting that these substances may influence the degree of neuronal damage in part via inhibiting inflammatory signaling pathways. The aim of this narrative review is to examine the main experimental data available to date on the neuroprotective and anti-ischemic role of regular exercise, analyzing also the possible role played by neurotrophins and myokines.

The redox-dependent regulation of satellite cells following aseptic muscle trauma (SpEED): study protocol for a randomized controlled trial

Background

Muscle satellite cells (SCs) are crucial for muscle regeneration following muscle trauma. Acute skeletal muscle damage results in inflammation and the production of reactive oxygen species (ROS) which may be implicated in SCs activation. Protection of these cells from oxidative damage is essential to ensure sufficient muscle regeneration. The aim of this study is to determine whether SCs activity under conditions of aseptic skeletal muscle trauma induced by exercise is redox-dependent.

Methods/design

Based on the SCs content in their vastus lateralis skeletal muscle, participants will be classified as either high or low respondents. In a randomized, double-blind, crossover, repeated-measures design, participants will then receive either placebo or N-acetylcysteine (alters redox potential in muscle) during a preliminary 7-day loading phase, and for eight consecutive days following a single bout of intense muscle-damaging exercise. In both trials, blood samples and muscle biopsies will be collected, and muscle performance and soreness will be measured at baseline, pre-exercise, 2 and 8 days post exercise. Biological samples will be analyzed for redox status and SCs activity. Between trials, a 4-week washout period will be implemented.

Discussion

This study is designed to investigate the impact of redox status on SCs mobilization and thus skeletal muscle potential for regeneration under conditions of aseptic inflammation induced by exercise. Findings of this trial should provide insight into (1) molecular pathways involved in SCs recruitment and muscle healing under conditions of aseptic skeletal muscle trauma present in numerous catabolic conditions and (2) whether skeletal muscle’s potential for regeneration depends on its basal SCs content.

Trial registration

ClinicalTrials.gov, ID: NCT03711838. Registered on 19 Oct 2018.

Electronic supplementary material

The online version of this article (10.1186/s13063-019-3557-3) contains supplementary material, which is available to authorized users.

Highly Porous Microcarriers for Minimally Invasive In Situ Skeletal Muscle Cell Delivery

Microscale cell carriers have recently garnered enormous interest in repairing tissue defects by avoiding substantial open surgeries using implants for tissue regeneration. In this study, the highly open porous microspheres (HOPMs) are fabricated using a microfluidic technique for harboring proliferating skeletal myoblasts and evaluating their feasibility toward cell delivery application in situ. These biocompatible HOPMs with particle sizes of 280-370 µm possess open pores of 10-80 µm and interconnected paths. Such structure of the HOPMs conveniently provide a favorable microenvironment, where the cells are closely arranged in elongated shapes with the deposited extracellular matrix, facilitating cell adhesion and proliferation, as well as augmented myogenic differentiation. Furthermore, in vivo results in mice confirm improved cell retention and vascularization, as well as partial myoblast differentiation. These modular cell-laden microcarriers potentially allow for in situ tissue construction after minimally invasive delivery providing a convenient means for regeneration medicine.

Hsp60 in Skeletal Muscle Fiber Biogenesis and Homeostasis: From Physical Exercise to Skeletal Muscle Pathology

Hsp60 is a molecular chaperone classically described as a mitochondrial protein with multiple roles in health and disease, participating to the maintenance of protein homeostasis. It is well known that skeletal muscle is a complex tissue, rich in proteins, that is, subjected to continuous rearrangements, and this homeostasis is affected by many different types of stimuli and stresses. The regular exercise induces specific histological and biochemical adaptations in skeletal muscle fibers, such as hypertrophy and an increase of mitochondria activity and oxidative capacity. The current literature is lacking in information regarding Hsp60 involvement in skeletal muscle fiber biogenesis and regeneration during exercise, and in disease conditions. Here, we briefly discuss the functions of Hsp60 in skeletal muscle fibers during exercise, inflammation, and ageing. Moreover, the potential usage of Hsp60 as a marker for disease and the evaluation of novel treatment options is also discussed. However, some questions remain open, and further studies are needed to better understand Hsp60 involvement in skeletal muscle homeostasis during exercise and in pathological condition.

Investigation of Circulating Extracellular Vesicle MicroRNA Following Two Consecutive Bouts of Muscle-Damaging Exercise

Background: Extracellular vesicles (EVs) are nano-sized vesicles that are known to be powerful mediators of intercellular communication via their microRNA (miR) content. A paucity of information on EV-mediated communication arising from skeletal muscle (SkM) in response to exercise-induced muscle damage is present in the published literature. Lack of such information inhibits our understanding of muscle injury and repair processes. Aims: To assess circulating EV levels and selected miR content within them, in response to two consecutive bouts of muscle-damaging exercise. Methods: Serum creatine kinase activity (CK) and EVs were analyzed from the blood of 9 healthy, untrained males at baseline, and at 2 and 24 h post-exercise. The exercise regimen consisted of a combination of plyometric jumping and downhill running. Perceived muscle pain (PMP) was assessed on a scale from 1 to 10. Plasma EVs were isolated using size exclusion columns and visualized with transmission electron microscopy (TEM). EV size and number were quantified using nanoparticle tracking analysis (NTA). miR expression was quantified using qPCR, with normalization to an exogenous control (cel-miR-39). Results: PMP and CK were significantly elevated post-exercise compared to baseline levels, providing indirect evidence for muscle damage. EV visualization using TEM revealed an abundant and heterogeneously sized pool of intact particles within the exosome size range (30-150 nm). No significant change in mean EV size or number was seen over time. The SkM-specific miR-206 in EVs was found to be variable among participants and no significant change occurred in SkM-important miRs; 1, 133a, 133b, 486, and 499a. However, EV miR-31 decreased from baseline to 24 h post-exercise (p = 0.027). Conclusion: Mild to moderate exercise-induced muscle damage altered the miR-31 profile of circulating EVs within the first 24 h post-exercise, but not that of myomiRs in EVs. These data demonstrate that EVs carry selectively packaged cargo which can be affected by exercise. Future research into the total miR content of EVs in response to exercise-induced muscle damage may reveal other miRs responsive to this relatively mild perturbation. More time points post-muscle-damaging exercise would provide a better understanding of the temporal EV myomiR response.

Physical exercise promotes proliferation and differentiation of endogenous neural stem cells via ERK in rats with cerebral infarction

Physical exercise is beneficial for the functional recovery of neurons after stroke. It has been suggested that exercise regulates proliferation and differentiation of endogenous neural stem cells (NSCs); however, the underlying molecular mechanisms are still largely unknown. In the present study, the aim was to investigate whether physical exercise activates the extracellular signal-regulated kinase (ERK) signaling pathway to promote proliferation and differentiation of NSCs in rats with cerebral infarction, thereby improving neurological function. Following middle cerebral artery occlusion, rats underwent physical exercise and neurological behavior was analyzed at various time points. Immunofluorescence staining was performed to detect proliferation and differentiation of NSCs, and western blotting was used to analyze cyclin-dependent kinase 4 (CDK4), Cyclin D1, retinoblastoma protein (p-Rb), P-16, phosphorylated (p)-ERK1/2 and c-Fos expression. The results indicated that physical exercise promoted proliferation and differentiation of NSCs, and led to improved neural function. In addition, the expression levels of CDK4, Cyclin D1, p-Rb, p-ERK1/2 and c-Fos were upregulated, whereas the expression of P-16 was downregulated following exercise. U0126, an inhibitor of ERK signaling, reversed the beneficial effects of exercise. Therefore, it may be hypothesized that physical exercise enhances proliferation and differentiation of endogenous NSCs in the hippocampus of rats with cerebral infarction via the ERK signaling pathway.

European Week of Sport: innovative initiative of European Commission that inspires children to be active

BACKGROUND

Estimates indicate that more than one third of European adults are inactive, despite the known benefits of physical activity. In 2015 the European Commission launched the European Week of Sport (EWoS), to encourage people to engage in sport and physical activity. The aim of this study was to evaluate if participation in the EWoS could motivate children to participate in physical activity in future.

METHODS

A total of 10,892 children (aged 6-14), from 6 EU cities (Palermo, Italy; Ankara, Turkey; Lousada, Portugal; Gardabaer, Iceland; Rijeka, Croatia; Albacete, Spain), were enrolled in sport activities (running sport event, extra hours of physical activity, seminars on physical activity, and a family sport festival during the weekend) during the EWoS 2016. A questionnaire was set up and distributed amongst participants to identify the physical activity habits of schoolchildren and whether the activities conducted during the project were able to establish the desire to participate in physical activity.

RESULTS

Data has shown that 15% of the individuals (respondents from the 6 countries) did not practice sport, although large variability among participating countries exists. The majority (15%) of these children showed an interest in practicing sport in ensuing months following EWoS.

CONCLUSIONS

The results suggest that the participation in sport activities during the EWoS encouraged inactive European children to practice physical activity in the months that followed. Future researchers should however investigate whether the motivation to participate in sport observed in this study became reality.

Exercise restores muscle stem cell mobilization, regenerative capacity and muscle metabolic alterations via adiponectin/AdipoR1 activation in SAMP10 mice

BACKGROUND

Exercise train (ET) stimulates muscle response in pathological conditions, including aging. The molecular mechanisms by which exercise improves impaired adiponectin/adiponectin receptor 1 (AdipoR1)-related muscle actions associated with aging are poorly understood. Here we observed that in a senescence-accelerated mouse prone 10 (SAMP10) model, long-term ET modulated muscle-regenerative actions.

METHODS

25-week-old male SAMP10 mice were randomly assigned to the control and the ET (45 min/time, 3/week) groups for 4 months. Mice that were maintained in a sedentary condition served controls.

RESULTS

ET ameliorated aging-related muscle changes in microstructure, mitochondria, and performance. The amounts of proteins or mRNAs for p-AMPKα, p-Akt, p-ERK1/2, p-mTOR, Bcl-XL, p-FoxO3, peroxisome proliferators-activated receptor-γ coactivator, adiponectin receptor1 (adpoR1), and cytochrome c oxidase-IV, and the numbers of CD34⁺ /integrin-α₇⁺ muscle stem cells (MuSCs) and proliferating cells in the muscles and bone-marrow were enhanced by ET, whereas the levels of p-GSK-3α and gp91phox proteins and apoptotic cells were reduced by ET. The ET also resulted in increased levels of plasma adiponectin and the numbers of bone-marrow (BM)-derived circulating CD34⁺ /integrin-α₇⁺ MuSCs and their functions. Integrin-α₇⁺ MuSCs of exercised mice had improved changes of those beneficial molecules. These ET-mediated aged muscle benefits were diminished by adiponectin and AdipoR1 blocking as well as AMPK inhibition. Finally, recombinant mouse adiponectin enhanced AMPK and mTOR phosphorylations in BM-derived integrin-α₇⁺ cells.

CONCLUSIONS

These findings suggest that ET can improve aging-related impairments of BM-derived MuSC regenerative capacity and muscle metabolic alterations via an AMPK-dependent mechanism that is mediated by an adiponectin/AdipoR1 axis in SAMP10 mice.

Reduced cancer mortality at high altitude: The role of glucose, lipids, iron and physical activity

Residency at high altitude (HA) demands adaptation to challenging environmental conditions with hypobaric hypoxia being the most important one. Epidemiological and experimental data suggest that chronic exposure to HA reduces cancer mortality and lowers prevalence of metabolic disorders like diabetes and obesity implying that adaption to HA modifies a broad spectrum of physiological, metabolic and cellular programs with a generally beneficial outcome for humans. However, the complexity of multiple, potentially tumor-suppressive pathways at HA impedes the understanding of mechanisms leading to reduced cancer mortality. Many adaptive processes at HA are tightly interconnected and thus it cannot be ruled out that the entirety or at least some of the HA-related alterations act in concert to reduce cancer mortality. In this review we discuss tumor formation as a concept of competition between healthy and cancer cells with improved fitness - and therefore higher competitiveness - of healthy cells at high altitude. We discuss HA-related changes in glucose, lipid and iron metabolism that may have an impact on tumorigenesis. Additionally, we discuss two parameters with a strong impact on tumorigenesis, namely drug metabolism and physical activity, to underpin their potential contribution to HA-dependent reduced cancer mortality. Future studies are needed to unravel why cancer mortality is reduced at HA and how this knowledge might be used to prevent and to treat cancer patients.

Effect of Low-Magnitude, High-Frequency Vibration Treatment on Retardation of Sarcopenia: Senescence-Accelerated Mouse-P8 Model

Sarcopenia-related falls and fall-related injuries in community-dwelling elderly people garnered more and more interest in recent years. Low-magnitude high-frequency vibration (LMHFV) was proven beneficial to musculoskeletal system and recommended for sarcopenia treatment. This study aimed to evaluate the effects of LMHFV on the sarcopenic animals and explore the mechanism of the stimulatory effects. Senescence-accelerated mouse P8 (SAMP8) mice at month 6 were randomized into control (Ctrl) and vibration (Vib) groups and the mice in the Vib group were given LMHFV (0.3 g, 20 min/day, 5 days/week) treatment. At months 0, 1, 2, 3, and 4 post-treatment, muscle mass, structure, and function were assessed. The potential proliferation capacity of the muscle was also evaluated by investigating satellite cells (SCs) pool and serum myostatin expression. At late stage, the mice in the Vib group showed higher muscle strength (month 4, p = 0.028). Generally, contractibility was significantly improved by LMHFV (contraction time [CT], p = 0.000; half-relaxation time [RT50], p = 0.000). Enlarged cross-sectional area of fiber type IIA was observed in the Vib group when compared with Ctrl group (p = 0.000). No significant difference of muscle mass was observed. The promotive effect of LMHFV on myoregeneration was reflected by suppressed SC pool reduction (month 3, p = 0.000; month 4, p = 0.000) and low myostatin expression (p = 0.052). LMHFV significantly improved the structural and functional outcomes of the skeletal muscle, hence retarding the progress of sarcopenia in SAMP8. It would be a good recommendation for prevention of the diseases related to skeletal muscle atrophy.

Endurance Exercise Mobilizes Developmentally Early Stem Cells into Peripheral Blood and Increases Their Number in Bone Marrow: Implications for Tissue Regeneration

Endurance exercise has been reported to increase the number of circulating hematopoietic stem/progenitor cells (HSPCs) in peripheral blood (PB) as well as in bone marrow (BM). We therefore became interested in whether endurance exercise has the same effect on very small embryonic-like stem cells (VSELs), which have been described as a population of developmentally early stem cells residing in BM. Mice were run daily for 1 hour on a treadmill for periods of 5 days or 5 weeks. Human volunteers had trained in long-distance running for one year, six times per week. FACS-based analyses and RT-PCR of murine and human VSELs and HSPCs from collected bone marrow and peripheral blood were performed. We observed that endurance exercise increased the number of VSELs circulating in PB and residing in BM. In parallel, we observed an increase in the number of HSPCs. These observations were subsequently confirmed in young athletes, who showed an increase in circulating VSELs and HSPCs after intensive running exercise. We provide for the first time evidence that endurance exercise may have beneficial effects on the expansion of developmentally early stem cells. We hypothesize that these circulating stem cells are involved in repairing minor exercise-related tissue and organ injuries.

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.

Skeletal muscle: a brief review of structure and function

Skeletal muscle is one of the most dynamic and plastic tissues of the human body. In humans, skeletal muscle comprises approximately 40% of total body weight and contains 50-75% of all body proteins. In general, muscle mass depends on the balance between protein synthesis and degradation and both processes are sensitive to factors such as nutritional status, hormonal balance, physical activity/exercise, and injury or disease, among others. In this review, we discuss the various domains of muscle structure and function including its cytoskeletal architecture, excitation-contraction coupling, energy metabolism, and force and power generation. We will limit the discussion to human skeletal muscle and emphasize recent scientific literature on single muscle fibers.

The need to more precisely define aspects of skeletal muscle regeneration

A more precise definition of the term 'skeletal muscle regeneration' is required to reduce confusion and misconceptions. In this paper the term is used only for events that follow myofibre necrosis, to result in myogenesis and new muscle formation: other key events include early inflammation and revascularisation, and later fibrosis and re-innervation. The term 'muscle regeneration' is sometimes used casually for situations that do not involve myonecrosis; such as restoration of muscle mass by hypertrophy after atrophy, and other forms of damage to muscle tissue components. These situations are excluded from the definition in this paper which is focussed on mammalian muscles with the long-term aim of clinical translation to enhance new muscle formation after acute or chronic injury or during surgery to replace whole muscles. The paper briefly outlines the cellular events involved in myogenesis during development and post-natal muscle growth, discusses the role of satellite cells in mature normal muscles, and the likely incidence of myofibre necrosis/regeneration in healthy ageing mammals (even when subjected to exercise). The importance of the various components of regeneration is outlined to emphasise that problems in each of these aspects can influence overall new muscle formation; thus care is needed for correct interpretation of altered kinetics. Various markers used to identify regenerating myofibres are critically discussed and, since these can all occur in other conditions, caution is required for accurate interpretation of these cellular events. Finally, clinical situations are outlined where there is a need to enhance skeletal muscle regeneration: these include acute and chronic injuries or transplantation with bioengineering to form new muscles, therapeutic approaches to muscular dystrophies, and comment on proposed stem cell therapies to reduce age-related loss of muscle mass and function. This article is part of a directed issue entitled: Regenerative Medicine: the challenge of translation.

Exercise mitigates the adverse effects of hyperhomocysteinemia on macrophages, MMP-9, skeletal muscle, and white adipocytes

Regular exercise is a great medicine with its benefits encompassing everything from prevention of cardiovascular risk to alleviation of different muscular myopathies. Interestingly, elevated levels of homocysteine (Hcy), also known as hyperhomocysteinemia (HHcy), antagonizes beta-2 adrenergic receptors (β2AR), gamma amino butyric acid (GABA), and peroxisome proliferator-activated receptor-gamma (PPARγ) receptors. HHcy also stimulates an elevation of the M1/M2 macrophage ratio, resulting in a more inflammatory profile. In this review we discuss several potential targets altered by HHcy that result in myopathy and excessive fat accumulation. Several of these HHcy mediated changes can be countered by exercise and culminate into mitigation of HHcy induced myopathy and metabolic syndrome. We suggest that exercise directly impacts levels of Hcy, matrix metalloproteinase 9 (MMP-9), macrophages, and G-protein coupled receptors (GPCRs, especially Gs). While HHcy promotes the M1 macrophage phenotype, it appears that exercise may diminish the M1/M2 ratio, resulting in a less inflammatory phenotype. HHcy through its influence on GPCRs, specifically β₂AR, PPARγ and GABA receptors, promotes accumulation of white fat, whereas exercise enhances the browning of white fat and counters HHcy-mediated effects on GPCRs. Alleviation of HHcy-associated pathologies with exercise also includes reversal of excessive MMP-9 activation. Moreover, exercise, by reducing plasma Hcy levels, may prevent skeletal muscle myopathy, improve exercise capacity and rescue the obese phenotype. The purpose of this review is to summarize the pathological conditions surrounding HHcy and to clarify the importance of regular exercise as a method of disease prevention.

Exercise is the real polypill

The concept of a "polypill" is receiving growing attention to prevent cardiovascular disease. Yet similar if not overall higher benefits are achievable with regular exercise, a drug-free intervention for which our genome has been haped over evolution. Compared with drugs, exercise is available at low cost and relatively free of adverse effects. We summarize epidemiological evidence on the preventive/therapeutic benefits of exercise and on the main biological mediators involved.

Acute environmental hypoxia induces LC3 lipidation in a genotype-dependent manner

Hypoxia-induced muscle wasting is a phenomenon often described with prolonged stays at high altitude, which has been attributed to altered protein metabolism. We hypothesized that acute normobaric hypoxia would induce a negative net protein balance by repressing anabolic and activating proteolytic signaling pathways at rest and postexercise and that those changes could be partially genetically determined. Eleven monozygotic twins participated in an experimental trial in normoxia and hypoxia (10.7% O2). Muscle biopsy samples were obtained before and after a 20-min moderate cycling exercise. In hypoxia at rest, autophagic flux was increased, as indicated by an increased microtubule-associated protein 1 light chain 3 type II/I (LC3-II/I) ratio (+25%) and LC3-II expression (+60%) and decreased p62/SQSTM1 expression (-25%; P<0.05), whereas exercise reversed those changes to a level similar to that with normoxia except for p62/SQSTM1, which was further decreased (P<0.05). Hypoxia also increased Bnip3 (+34%) and MAFbx (+18%) mRNA levels as well as REDD1 expression (+439%) and AMP-activated protein kinase phosphorylation (+22%; P<0.05). Among the molecular responses to hypoxia and/or exercise, high monozygotic similarity was found for REDD1, LC3-II, and LC3-II/I (P<0.05). Our results indicate that environmental hypoxia modulates protein metabolism at rest and after moderate exercise by primarily increasing markers of protein breakdown and, more specifically, markers of the autophagy-lysosomal system, with a modest genetic contribution.

Endurance exercise and conjugated linoleic acid (CLA) supplementation up-regulate CYP17A1 and stimulate testosterone biosynthesis

A new role for fat supplements, in particular conjugated linoleic acid (CLA), has been delineated in steroidogenesis, although the underlying molecular mechanisms have not yet been elucidated. The aims of the present study were to identify the pathway stimulated by CLA supplementation using a cell culture model and to determine whether this same pathway is also stimulated in vivo by CLA supplementation associated with exercise. In vitro, Leydig tumour rat cells (R2C) supplemented with different concentrations of CLA exhibited increasing testosterone biosynthesis accompanied by increasing levels of CYP17A1 mRNA and protein. In vivo, trained mice showed an increase in free plasma testosterone and an up-regulation of CYP17A1 mRNA and protein. The effect of training on CYP17A1 expression and testosterone biosynthesis was significantly higher in the trained mice supplemented with CLA compared to the placebo. The results of the present study demonstrated that CLA stimulates testosterone biosynthesis via CYP17A1, and endurance training led to the synthesis of testosterone in vivo by inducing the overexpression of CYP17A1 mRNA and protein in the Leydig cells of the testis. This effect was enhanced by CLA supplementation. Therefore, CLA-associated physical activity may be used for its steroidogenic property in different fields, such as alimentary industry, human reproductive medicine, sport science, and anti-muscle wasting.

Cellular Transplantation Alters the Disease Progression in Becker's Muscular Dystrophy

Becker's Muscular Dystrophy (BMD) is a dystrophinopathy manifested as progressive muscle degeneration. Autologous Bone Marrow Mononuclear Cells (BMMNCs) have shown some myogenic potential. The paracrine effects of the BMMNCs reduce the inflammation and are thought to reduce muscle degeneration. We treated a 39 year old dental surgeon suffering from BMD. Muscle strength was reduced when measured using modified Medical Research Council's Manual Muscle Testing (mMRC-MMT). Static sitting balance was poor. He was wheelchair dependent for ambulation and moderately independent in Activities of Daily Living (ADL). Functional Independence Measure (FIM) score was 93. Musculoskeletal Magnetic Resonance Imaging (MRI-MSK) showed moderate fatty infiltration in the muscles. Three cellular transplantations were carried out. Clinical assessment and the investigations were repeated. Progressive increase in the muscle strength was noted. Ambulation was independent using push-knee splints and minimal assistance when weary. Static and dynamic balance in sitting and standing improved. FIM score increased from 93 to 105. There was no increase in the degree of fatty infiltration, as seen on the MRI-MSK. The case study provides evidence for the putative benefits of cellular therapy in altering the disease progression in BMD. It also suggests augmented clinical benefits of combination of cellular therapy and rehabilitation.

Stem cell populations in the heart and the role of Isl1 positive cells

Cardiac progenitor cells are multipotent stem cells isolated from both embryonic and adult hearts in several species and are able to differentiate at least into smooth muscle cells, endothelial cells and cardiomyocytes. The embryonic origin of these cells has not yet been demonstrated, but it has been suggested that these cells may derive from the first and secondary heart fields and from the neural crest. In the last decade, two diffe-rent populations of cardiac progenitor or stem cells have been identified and isolated, i.e., the Islet1 positive (Isl1+) and c-Kit positive (c-Kit+)/Stem Cell Antigen-1 positive (Sca-1+) cells. Until 2012, these two populations have been considered two separate entities with different roles and a different origin, but new evidence now suggests a con-nection between the two populations and that the two populations may represent two subpopulations of a unique pool of cardiac stem cells, derived from a common immature primitive cell. To find a common consensus on this concept is very important in furthe-ring the application of stem cells to cardiac tissue engineering.

Satellite cell pool expansion is affected by skeletal muscle characteristics

INTRODUCTION

We investigated changes in satellite cell (SC) pool size after an acute bout of strenuous exercise and evaluated the influence of baseline SC count and fiber type.

METHODS

Participants completed a downhill running (DHR) intervention (5 × 8 min, 2-min rest; 80% VO2max ; -10% gradient). Muscle biopsies were taken 7 days before VO₂max and 7-9 days after the DHR intervention. Delayed-onset muscle soreness (DOMS) and creatine kinase activity (CK) were measured on days 1, 2, 7, and 9 post-DHR. SCs were identified by Pax7 and laminin staining. Relative distribution of MHC isoforms was determined by electrophoresis.

RESULTS

DOMS and CK peaked on day 1 post-DHR (P < 0.01). The SC pool increased (26%) after DHR (P = 0.005). SCs/total myonuclei after recovery correlated with baseline SCs (r = 0.979, P = 0.003) and VO₂max (r = 0.956, P = 0.011), whereas change in SC pool (Pax7(+) cells/total myonuclei: recovery minus baseline) tended to correlate with percent MHC II (r = 0.848; P = 0.06).

CONCLUSION

Interindividual physiological characteristics affect SC pool expansion after a single bout of DHR and are influenced by VO₂max .

Do fat supplements increase physical performance?

Fish oil and conjugated linoleic acid (CLA) belong to a popular class of food supplements known as "fat supplements", which are claimed to reduce muscle glycogen breakdown, reduce body mass, as well as reduce muscle damage and inflammatory responses. Sport athletes consume fish oil and CLA mainly to increase lean body mass and reduce body fat. Recent evidence indicates that this kind of supplementation may have other side-effects and a new role has been identified in steroidogenensis. Preliminary findings demonstrate that fish oil and CLA may induce a physiological increase in testosterone synthesis. The aim of this review is to describe the effects of fish oil and CLA on physical performance (endurance and resistance exercise), and highlight the new results on the effects on testosterone biosynthesis. In view of these new data, we can hypothesize that fat supplements may improve the anabolic effect of exercise.