Healthy skeletal muscle aging: The role of satellite cells, somatic mutations and exercise

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Healthy skeletal muscle aging: The role of satellite cells, somatic mutations and exercise Irene Francoa,*, Rodrigo Fernandez-Gonzalob, Peter Vrta
cnika, Tommy R. Lundbergb, Maria Erikssona, Thomas Gustafssonb a

Department of Biosciences and Nutrition, Center for Innovative Medicine, Karolinska Institutet, Huddinge, Sweden Division of Clinical Physiology, Department of Laboratory Medicine, Karolinska Institutet, and Unit of Clinical Physiology, Karolinska University Hospital, Stockholm, Sweden *Corresponding author: e-mail address: [email protected] b

Contents 1. Introduction 2. Characterization and function of the satellite cells 2.1 Properties and heterogeneity 2.2 Satellite cells during homeostatic turnover 2.3 The role of satellite cells in muscle injury and regeneration 2.4 The response of satellite cells to different exercise modes 2.5 The response of satellite cells to unloading 3. Effects: How the age-related decline in the satellite cell pool affects the skeletal muscle 3.1 Hampered regeneration after muscle damage or injury 3.2 Disuse and reload 3.3 Attenuated response to exercise 4. Causes: Why satellite cell number and function decline with increasing age 4.1 Intrinsic and extrinsic factors 4.2 Changes in the genome, epigenome, transcriptome 4.3 Circulating factors 5. Exercise: An intervention to modulate satellite cell deterioration with aging 6. Conclusions Acknowledgments References

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Abstract Satellite cells (SCs) form the resident stem cell population in the skeletal muscle tissue. While their function in mediating tissue regeneration after injury is well described, their role in the undamaged-, aging-, and exercising muscle is only starting to be unraveled. Although direct evidence linking the loss of SC function to the onset of age-related loss

International Review of Cell and Molecular Biology ISSN 1937-6448 https://doi.org/10.1016/bs.ircmb.2019.03.003

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of muscle mass and function (i.e., sarcopenia) is currently lacking, satellite cells are increasingly seen as an important component for the decline of tissue function seen with aging. This is evident from the pertinent role of SCs in maintaining homeostasis, and in mediating remodeling- and repair-responses, in the skeletal muscle. This narrative review focuses on human studies, but includes cellular and animal models, to describe the role of SCs in different physiological scenarios relevant for human aging. The intrinsic and extrinsic mechanisms underlying age-induced alterations in the SC pool are discussed, with particular emphasis on the genomic modifications that accumulate in human SCs during a lifetime (i.e., somatic mutation-burden). Finally, the role of exercise as a potential countermeasure to age-induced SC alterations is explored in the different scenarios covered.

1. Introduction Skeletal muscle comprises 40–50% of our total body mass and is necessary for movement, balance, oxygen consumption, and as a metabolic reserve. Skeletal muscle deconditioning, illustrated by a loss of both muscle mass and function, occurs systematically, yet at a low rate in older people (primary sarcopenia), or happens at an accelerated rate as a physiological response to many life events, including disease and injury; i.e., secondary sarcopenia (Cruz-Jentoft et al., 2010). Regardless the triggering factors, this muscle deconditioning can lead to frailty, reduced quality of life, and increased morbidity and mortality (Cohen et al., 2015), which together with the estimated future prevalence of sarcopenia calls for immediate actions (Ethgen et al., 2017). Advances in our understanding of the cellular events that regulate skeletal muscle integrity have occurred during the last decade (Wall et al., 2013). In general, the loss of skeletal muscle mass results from excessive protein breakdown together with a reduced protein synthetic response to anabolic stimuli and feeding (i.e., “anabolic resistance”) (Burd et al., 2013). Such changes in skeletal muscle protein turnover observed with age are influenced by an interaction of cellular aging processes and age-associated changes in external factors. The physiological processes include, e.g., DNA damage and mutations, insulin resistance, low-grade inflammation, and loss of motor neurons, as others and we have shown (Franco et al., 2018; Lopez et al., 2013; Piasecki et al., 2018). In addition, several environmental factors have been associated with loss of muscle function during aging, including physical inactivity, poor nutrition, and co-morbidity from other diseases (Wall et al., 2013).

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In the last decades, it has become apparent that a particular cell type within the skeletal muscle may have an important role in aging-induced alterations, i.e., the satellite cell (SC). The SCs constitute a pluripotent cell population with stem cells features in the adult skeletal muscle tissue. Similar to other stem cell populations, SCs become quiescent once body growth has been completed and the individual reaches adulthood. Yet, they have the capacity to be activated, proliferate and/or differentiate to support muscle turnover. Given their role in muscle plasticity and repair, the SC pool becomes a target of interest to explain the declined regenerative and remodeling capacity observed with aging (Sadeh, 1988). Several groups have shown reduced number of SCs in aged human muscles, preferentially in type II fibers (Suetta et al., 2013; Verdijk et al., 2007), as well as an impairment in SC proliferation and differentiation in aged individuals (Snijders et al., 2015). These age-related changes have been attributed to deterioration of intracellular pathways and components in the SCs (Bigot et al., 2015; Franco et al., 2018; Sousa-Victor et al., 2014). Moreover, the aged environment contributes to the decline of the SC compartment and age-induced immune alterations (Saini et al., 2016), hormonal changes (La Colla et al., 2015) and extracellular matrix adaptations (Boers et al., 2018) have a direct impact on SC number and function. To date, the best strategy to combat the age-induced decline in skeletal muscle mass and function is exercise, and in particular resistance-type exercise (Phu et al., 2015). The efficacy of exercise to counteract muscle deconditioning may partly rely on adaptations in the SC pool, as indicated by an increased number of SCs after an acute exercise bout (Snijders et al., 2015). Yet, the specific role of SCs to support muscle remodeling induced by exercise training is currently a matter of debate among scientists. Nevertheless, the mechanisms by which exercise exerts beneficial effects on the aging-muscle process, and the potential role of SCs in mediating these adaptations, are now starting to be unraveled. This narrative review, including cellular, animal, and human studies, will describe the role of SCs in different physiological scenarios, such as homeostatic conditions, injury, and mechanically induced activation (i.e., exercise and unloading). Then, the effects of aging on SCs will be discussed, with a special focus on the loss of regenerative capacity to injury or immobilization, and the potentially hampered remodeling response to exercise. Moreover, the mechanisms behind age-induced alterations in the SC pool will be scrutinized, and, finally, the role of exercise as a potential countermeasure to age-induced SC alterations will be explored.

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2. Characterization and function of the satellite cells 2.1 Properties and heterogeneity The SCs are located tightly squeezed between the plasma membrane of the muscle fiber and the basement membrane. They are intimately connected with the skeletal muscle fiber protruding inward by pushing the myofibrils aside. SCs contain a single nucleus with high nucleus-to-cytoplasm ratio that dictates their shape (Mauro, 1961). They are normally quiescent in the adult skeletal muscle but become activated upon exposure to stimuli, like exercise or injury (Crameri et al., 2004; McKay et al., 2012; Murphy et al., 2011). Early radioactive labeling experiments demonstrated that SCs were capable of adding nuclei into the mature fibers (Moss and Leblond, 1970, 1971; Reznik, 1969; Schiaffino et al., 1976) and that the mitotically quiescent SCs entered the cell cycle in the muscle of injured rats (Snow, 1977). These studies, corroborated by earlier in vitro evidence showing that myoblasts from various animal models were able to fuse and create multinucleated myotubes (Konigsberg et al., 1975; Yaffe, 1969), led to the hypothesis that SCs are the cells responsible for muscle regeneration. More recent mouse experiments based on the transplantation of single intact myofibers (Collins et al., 2005) or single SCs (Sacco et al., 2008) into the muscle of irradiated mice have shown that SCs possess all the properties that commonly define a stem cell. SCs transplanted into an injured tissue show remarkable ability to proliferate and significantly expand their numbers, and maintain this ability even after serial rounds of transplantations (Sacco et al., 2008). SC expansion produces committed progenitors that fuse with existing muscle fibers or form new ones, successfully regenerating the muscle tissue, but also maintains a stabile and functional population of uncommitted mononucleated cells. This important property is defined as stem cell selfrenewal (Collins et al., 2005; Sacco et al., 2008). Further characterization of the SC population was achieved through the use of transgenic animal models. Genetic engineering was used to drive the expression of the green fluorescence protein (GFP) downstream of the promoter of the SC-specific marker Pax7 in reporter mice (Bosnakovski et al., 2008; Tichy et al., 2018). This strategy enabled efficient labeling of SCs in vivo and their prospective FACS-assisted isolation for downstream in vitro characterization (Bosnakovski et al., 2008). In addition, tamoxifen-controled Cre-mediated loxP recombination has been used to conditionally inactivate target gene expression specifically in SCs (Lepper et al., 2009).

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A distinctive feature of the SCs compared to other stem cell populations is their heterogeneity. Results mainly obtained from rodent models showed that, at the level of the single cell, SCs are different in terms of proliferative capacities, symmetric/asymmetric divisions, and degree of commitment toward the differentiated lineage (Tierney and Sacco, 2016). The paired box transcription factor Pax7 is recognized as a broad SC marker in mouse and human muscles, defining SC identity during both embryogenesis and adult life (Lepper et al., 2009; Seale et al., 2000). In the adult skeletal muscle, Pax7 is expressed in both quiescent and activated SCs (Zammit et al., 2004). The interplay between Pax7 and the myogenic regulatory factors (MRFs) Myf5, MyoD, Mrf4 and myogenin regulates the SC progression through the myogenic program (Seale et al., 2000). Upon SC activation, Pax7 and MyoD functionally counteract each other, with MyoD pushing toward proliferation and initiation of terminal differentiation and Pax7 pushing toward quiescence (Olguin et al., 2007; Zammit et al., 2004). High levels of Pax7 expression at this stage are sufficient to restore quiescence (Day et al., 2007; Zammit et al., 2004). Instead, if the myogenic program is started, Pax7 levels decrease until complete repression of the gene, alongside an increased expression of myogenin (Olguin et al., 2007). For example, a concomitant expression of Pax7 and MyoD exists in the proliferative phase that follows SC activation and precedes SC differentiation (Olguin et al., 2007, Zammit et al., 2004). Thus, the classical cascade of events leading to SC differentiation consists of concomitant downregulation of Pax7 and upregulation of Myf5 and MyoD upon SC activation, followed by Mrf4 and myogenin to initiate terminal differentiation (Seale et al., 2000). Hence, Pax7 and MRFs expression levels can be used to define different subpopulations of SCs and different degrees of commitment toward the myogenic lineage, as well as a wide range of proliferative and self-renewing capacities (Tierney and Sacco, 2016).

2.2 Satellite cells during homeostatic turnover The requirement of SCs in the maintenance of a healthy muscle in the absence of acute perturbation events is debated. Depletion of the Pax7 + compartment does not accelerate the onset of muscle wasting seen with aging (sarcopenia), nor does it worsen the outcome in sedentary mice. Mice that lived from early adult life to old age with skeletal muscles depleted of SCs showed equal fiber size and numbers of myonuclei per fiber as controls (Fry et al., 2015; Keefe et al., 2015). The main difference observed was an

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increase of fibrosis. Thus, SCs seem to prevent deposition of excessive extracellular matrix and consequent increased stiffness in the old muscle, but they do not directly counteract the loss of mass and nuclei seen at old age (Fry et al., 2015, 2017). This conclusion needs to be balanced with evidence showing that SCs constantly contribute with new nuclei to differentiated fibers in non-injured skeletal muscles of adult sedentary animals. The data rely on mouse models and in vivo cell tracking techniques from different labs (Fry et al., 2015; Keefe et al., 2015; Pawlikowski et al., 2015; Tierney et al., 2018; Yamamoto et al., 2018). Labeling of Pax7 + cells in adult muscles and tracking of their labeled progeny at middle (12 months) and old (24 months) age showed that the labeled SCs contributed to myonuclear accretion in all muscles analyzed, but the degree of contribution differed between and within muscles, and by fiber type (Keefe et al., 2015). A higher amount of labeled fibers in old vs. middle-aged animals indicated active integration of SC nuclei to fibers even in the latest stages of life (Keefe et al., 2015). Similar analyses, but in a shorter time frame, provided conflicting quantification of the extent of nuclei addition in adult and old animals. Results range from 0.1% (Fry et al., 2015) to 20% (Pawlikowski et al., 2015) of fibers acquiring a new nucleus in a period of 2 weeks. Inconsistencies might be due to different assay specificity. For example, the low rate of tissue turnover was determined tracking cells that duplicated and incorporated BrdU (Fry et al., 2015). Yet, Pax7+ SCs might differentiate and fuse with fibers without undergoing cell duplication, as reported in a mouse model of skeletal muscle overloading (Egner et al., 2016). A brainbow-2.1 reporter was used in mice to label Pax7 cells with four different colors in order to appreciate the simultaneous fusion of multiple SCs to the same fiber in long periods of time (Tierney et al., 2018). In this case the labeling was done in growing animals (4 weeks) and tracked for either 2–3 (young) or 24–26 months (old). The majority of fibers from the old animals showed labeling with one or more colors, meaning that new nuclei are provided during adult life to almost every fiber. This experiment also allowed the imaging of the SC clones associated with muscle fibers, and originated by the proliferation of a single Pax7+ cell after the labeling. The clone size was generally smaller in skeletal muscles from old animals, demonstrating that aged SCs are characterized by reduced proliferation (Tierney et al., 2018). However, an equal number of clones and a similar distribution of the different colors were observed in young and old animals, meaning a similar clonal complexity. The clonal complexity is a measure of the dynamics within the population of SCs. Despite the

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heterogeneity at the level of the single cells, the SC population shows a welldefined capacity to regenerate adult fibers. This is evidenced by the balance between symmetric/asymmetric divisions, numbers of highly/lowly proliferating cells and numbers of cells in the different levels of commitment toward the myogenic lineage (Tierney and Sacco, 2016). The fact that SCs from the old animals show the same in vivo dynamics as the SCs from the young animals provides support that, when the muscles remain in un-challenged/ physiological conditions, the SC pool is functional even at old age. Conversely, the clonal complexity was severely disrupted when the young muscles were challenged by serial rounds of injury (Tierney et al., 2018). Overall these results show that skeletal muscle fibers in rodents that naturally age in a cage are subjected to a basal cellular turnover. The pool of SCs is sufficient to sustain this phenomenon for the entire lifetime. However, external perturbations, like repeated injury and regeneration, seem to be able to disrupt this equilibrium, to exhaust the SC pool and favor the selection of more proliferating clones (Tierney et al., 2018). Our group obtained data on the basal proliferation of SCs in uninjured human skeletal muscles making use of genomic data (Franco et al., 2018). In fact, duplication of DNA during cell division produces mutations in the genome. These genetic marks accumulate following each cell division, from the early embryonic stages to the adult age. The result is that every genome in an adult tissue is unique (Zhang and Vijg, 2018). This genetic diversity, termed somatic variance, can be exploited with two purposes. One is estimating the number of cell divisions that have occurred in vivo in a period of time. In this case, we measured the average numbers of somatic mutations in single SCs of the vastus lateralis muscle of young (20-years-old) and old (63–78-years-old) individuals (Franco et al., 2018). We found increased numbers of mutations in aged SCs and calculated an accumulation of 13 somatic single nucleotide substitutions per year. These mutations correspond to 5 cell duplications per year or 250 in 50 years, challenging the concept of quiescent SCs in the uninjured human muscle. This technology can be exploited further and used to track cell lineages in adult tissues (Behjati et al., 2014; Lee-Six et al., 2018; Lodato et al., 2015). Specific mutations can be selected to unequivocally identify a specific SC. The same mutations can then be detected in the tissue to track the progeny of single SCs in human skeletal muscles, similar to the fluorescent labeling of Pax7 cells obtained by genetic manipulation in mouse. We provided the proof of concept by tracking six mutations back to the muscle biopsy from which the SCs were originally isolated. One of the mutation was found in 2.6% of

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genomes of the tissue portion, demonstrating that the mutated SC had been amplified by proliferation and its progeny contributed to the differentiated tissue during the individual’s life (Franco et al., 2018). More advanced analyses isolating SCs associated to single fibers and detecting the mutations in the same single fibers will provide valuable cell tracking data directly from human samples. This technology has the potential to finally clarify the rhythm of quiescence and proliferation of the SCs in human skeletal muscles in a number of conditions.

2.3 The role of satellite cells in muscle injury and regeneration The induction of skeletal muscle injury is a widely used approach to test the ability of SCs to mediate muscle regeneration in vivo in animal models. In addition, an injury/repair model has been recently established to directly study the process in humans, demonstrating a substantially overlapping cascade of events in human and mouse injured muscles (Mackey and Kjaer, 2017). In animals, injury can be introduced in several different ways, e.g., using physical procedures (freeze injury, irradiation, segmental crushing, denervation, transplantation), chemicals (barium chloride) or myotoxic agents (notexin, cardiotoxin) (Hardy et al., 2016). Depending on the injury model used, slight differences have been observed in the kinetics of the molecular, cellular and tissue responses (Hardy et al., 2016). In rodent models, several consecutive injuries separated by recovery intervals are often combined to study the SC function under severe pressure and over time (Sacco et al., 2008; Tierney et al., 2018). In human skeletal muscles, injury has been caused by a series of electrical stimulations of the neurojunctions that forced eccentric muscle actions, fiber lengthening and necrosis in the vastus lateralis (Mackey and Kjaer, 2017). Regardless of how the initial trauma is induced, general features of skeletal muscle regeneration are highly stereotypical in human and mouse, and are normally completed within 28–30 days (Hardy et al., 2016; Mackey and Kjaer, 2017). The regeneration process starts with the necrosis of the damaged myofibers. Neutrophil infiltration is characteristic for the very acute phase of the regeneration (18 h post-injury), while macrophage numbers reach their peak at around day 4 post-injury. Normally there are no B or T lymphocytes present (Hardy et al., 2016). Migration and proliferation of activated Pax7-positive SCs begins immediately after injury and the SC pool reaches its largest size by day 5 post-injury. Afterward, the cells either differentiate into myoblasts

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and myofibers downregulating Pax7 expression, or return to their quiescent state. By day 28 post-injury, the number of quiescent Pax7-positive SCs is reduced to normal levels (Murphy et al., 2011). A similar expansion of the SC pool following injury and fusion of SCs to damaged fibers characterizes the repair process also in the human muscle (Mackey and Kjaer, 2017). Importantly, SC dynamics seems to be significantly different during the regeneration of the injured muscle compared to the normal homeostatic turnover. In vivo microscopy in zebrafish showed that regenerated muscle fibers are usually reconstituted by a single SC clone, which is the one located in closer proximity to the damaged fiber (Gurevich et al., 2016). Conversely, fibers formed during growth and maintained through homeostasis exhibit a far greater degree of heterogeneity, consistent with a larger number of contributing SCs (Gurevich et al., 2016). Similar results were obtained by using mouse models (Tierney et al., 2018) (see Section 2.2). Through genetic manipulation in mice, it has been shown that Pax7-expressing SCs represent the only effective source of muscle stem cells and that they are essential for muscle regeneration following injury (Lepper et al., 2011; Murphy et al., 2011; Sambasivan et al., 2011). These models showed that in the absence of SCs there is no regenerated skeletal muscle tissue, which gets replaced with fibrotic connective tissue and infiltrating adipocytes (Lepper et al., 2011, Murphy et al., 2011, Sambasivan et al., 2011). Despite the primary role of SCs, there are other cell types that exert important functions in the repair process. In parallel to the SC expansion, the muscle connective tissue, comprising Tcf-positive fibroblasts secreting extracellular matrix (ECM), expands upon injury. Both fibroblast numbers and ECM abundance show a peak at day 3 post-injury and gradually return to baseline levels by day 28 post-injury (Murphy et al., 2011). The ECM has the capability to control SC activity and renewal (Calve et al., 2010; Gilbert et al., 2010), and fibroblasts exert an important regulatory effect on early SC dynamics in mouse (Murphy et al., 2011). In support, significantly reduced fibroblast numbers impair early SC expansion by causing their premature activation and differentiation. This leads to slightly reduced regeneration efficiency, extending the time for the regeneration process to reach completion (Murphy et al., 2011). Other important players in the repairing tissue are endothelial and inflammatory cells, not only for their function in the regulation of the connective tissue expansion (Schaefer, 2018), but also for a direct interaction with the SCs.

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In fact, endothelial cells and macrophages secrete mitogenic factors that modulate SC activation and proliferation in the human muscle (Schiaffino et al., 2017; Verma et al., 2018).

2.4 The response of satellite cells to different exercise modes The injury/repair model is mainly used in animal studies, while the most widely used stimulus to assess the role of SCs in the human skeletal muscle is exercise and, in particular, single bouts of unaccustomed eccentric (i.e., lengthening) exercise that produce muscle damage (Crameri et al., 2004; Dreyer et al., 2006; McKay et al., 2008, 2009; Mikkelsen et al., 2009; O’reilly et al., 2008; Toth et al., 2011). Using this type of exercise, marked increases in SC counts are observed in the first few days after exercise (reviewed in Snijders et al., 2015). The overall temporal pattern of SC expansion after resistance-type exercise appears to be comparable to the response seen with damaging exercise insults (Snijders et al., 2015). The SC response is apparent at 24-h post-exercise, and peaks at about 72-h after the bout. Similar SC response has also been observed after endurance exercise (Mackey et al., 2007; Nederveen et al., 2015), blood-flow restricted exercise (Wernbom et al., 2013), high-intensity interval training (Nederveen et al., 2015) and combined endurance/resistance exercise (Pugh et al., 2018; Snijders et al., 2012). While it is difficult to cross-sectionally compare these studies since they are performed in different populations, Nederveen et al. assigned sedentary older men to complete an acute bout of either resistance exercise, high-intensity interval exercise, or moderate-intensity aerobic exercise (Nederveen et al., 2015). The results showed that resistance exercise was the most potent to induce expansion of the SC pool, yet high-intensity interval exercise was more effective than moderate-intensity aerobic exercise. Thus, it appears that there is greater SC activation with higher-compared with lower-intensity exercise bouts, with the greatest effect seen with resistance-type exercise. Given that the temporal response of the increase in SC content post-exercise is similar to what has been noted after damaging lengthening muscle actions, it seems that muscle damage is not an obligatory event for post-exercise SC expansion. However, a correlation between the extent of skeletal muscle damage and expansion of the SC pool has been reported ( Joanisse et al., 2017; Nederveen et al., 2015). Moreover, SC density is greater after damaging vs. non-damaging muscle actions in training-unaccustomed men (Crameri et al., 2007; Hyldahl et al., 2014). Notwithstanding, it seems that most exercise regimes are associated with an increased number of satellite cells (Mackey et al., 2007).

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2.5 The response of satellite cells to unloading Although there are sparse data on the effects of unloading on the human SC niche, Arentson-Lantz et al. reported a 40% reduction in SC content after 14 days of bed rest in middle-aged adults. The change in SC correlated with the change in mean fiber cross-sectional area and was paralleled by a small decline in myonuclear content (Arentson-Lantz et al., 2016). Also, chronic muscle disuse as seen in spinal-cord injury results in fiber atrophy with a concomitant reduction in SC content (Verdijk et al., 2012), and Suetta et al. reported lower numbers of Pax7 positive cells in old compared with young individuals, specifically in type II fibers following 2 weeks immobilization (Suetta et al., 2013). In contrast, SC content did not change during 28 days of bed rest in middle-aged men receiving essential amino acid supplementation during the bed rest period (Brooks et al., 2010), or after a 2-week period of lower-limb immobilization (Snijders et al., 2014b). In mouse and rat studies, reduced SC content and proliferation with hind limb unloading has been reported (Darr and Schultz, 1989; Mitchell and Meck, 2004). Yet the specific role of SCs during unloading in these animal models might depend on the age of the mice, since growing mice seem more affected by, e.g., SC depletion than adult mice ( Jackson et al., 2012; Mitchell and Pavlath, 2001; Murach et al., 2017). Thus, differences in duration of unloading, subject age and disuse model may contribute to the discrepancies observed across human studies.

3. Effects: How the age-related decline in the satellite cell pool affects the skeletal muscle It has been estimated that the number of SCs in young healthy men amounts to 4% of total myonuclei (Snijders et al., 2015). This frequency, however, varies with muscle fiber type and age. While some studies reported no differences in SC counts in skeletal muscle biopsies collected from young and old men (Dreyer et al., 2006), others showed a significant decline in the numbers of SCs specifically associated with type II fibers in the elderly (McKay et al., 2012; Verdijk et al., 2007). The effect of aging on SC numbers has also been shown in animal models, i.e., a clear age-related decline in SC frequency is observed in mouse hind limb muscles with Pax7-positive cells accounting for almost 4% of the mononuclear cells in neonatal mice compared to less than 1% in adult and middle-aged mice (Bosnakovski et al., 2008). The rate of SC loss seems to differ among skeletal muscle

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groups; for example, in the extensor digitorum longus muscle a major decline in SC numbers and increase in the frequency of myofibers lacking or having just one SC occurred already in the middle-aged mice (1 year of age), while in the soleus muscle a similar extent of loss was observed in old mice (2.5 years of age) (Shefer et al., 2006). This difference probably reflects the difference in fiber composition, since the extensor digitorum longus is richer in type II fibers, which are known to be more susceptible to SC loss with aging (McKay et al., 2012, Verdijk et al., 2007). Finally, a more prominent reduction in SC numbers is also observed in old female mice compared to males (Day et al., 2010). Besides a decrease in numbers, SCs experience a functional decline with aging (Bengal et al., 2017). This loss of function is traditionally measured by inducing an acute stimulus and registering the ability of SCs to respond (Tierney et al., 2018). Injury repair and muscle remodeling after reload or exercise are the most widely used models. Even though these are only experimental conditions, they are valuable to understand the biology of muscle aging at large (Bengal et al., 2017). Muscle loss during aging is worsened by acute and chronic diseases and co-morbidities, as well as micro-traumas such as accidents, hospitalization and fall-related injuries. Such events may lead to reduced physical activity and periods of bed rest, but also increased generation of pro-inflammatory responses and exacerbated proteolysis, which ultimately contribute to the development of sarcopenia in older persons (Larsson et al., 2019; Santilli et al., 2014). Similar events may also progressively deteriorate the SC pool, which becomes non-responsive to subsequent traumas. In such a scenario, the loss of SC function might constitute a complicating factor that exacerbates skeletal muscle loss in humans. An additional complicating factor is inactivity in itself (Ingram, 2000). While moderate or elevated skeletal muscle activity preserves muscle mass in the older population and reduces the prevalence of sarcopenia (McPhee et al., 2016), sedentary aged individuals are characterized by very low levels of muscle activity (Ingram, 2000). This leads to a decrease in protein synthesis rate followed by an accelerated rate of protein degradation, which results in more pronounced weakness and muscle atrophy than what is observed in more active elderly individuals (Larsson et al., 2019; McPhee et al., 2016). In this respect, the sedentary life of a mouse does not represent a good model for the sedentary human life. Even if restricted to a cage, mice spend most of their time moving, in a condition that can be viewed as moderate exercise (Bjursell et al., 2008). In addition, the life-style of laboratory animals is easy to control and lacks accidents and events that are common in the human life

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and can stress the SC pool (e.g., fractures, diseases, bed rest periods, rounds of intensive training). Because of all these lifestyle differences, results from mouse models indicating a minor contribution of the SCs to skeletal muscle maintenance (Fry et al., 2015; Keefe et al., 2015), and a preserved function of the SC pool during physiological aging (Tierney et al., 2018), might not be directly translatable to the human muscle. Unfortunately, evidences concerning the importance of SCs for lifelong muscle maintenance in humans, regardless whether they are inactive or not, are still lacking. In the next sections, however, we will discuss the response of SCs to acute stressors like injury, disuse/reload and exercise.

3.1 Hampered regeneration after muscle damage or injury Aging is accompanied by a decline in the regenerative potential of most tissues, including the skeletal muscle (Bengal et al., 2017). When older mice (24–26 months) are subjected to a model of muscle injury, the repair process is not as efficient as in young mice. In particular, the cross-sectional area of regenerated fibers is reduced (Sadeh, 1988; Sinha et al., 2014; Sousa-Victor et al., 2014; Tierney et al., 2018) and an increased fibrosis is observed at the site of injury (Conboy et al., 2005). Injury-repair studies in the human muscle are complicated (see previous section on “the role of SCs on muscle injury and regeneration.”) However, tissue damage caused by, e.g., leg immobilization followed by acute exercise has been studied in the leg muscle vastus lateralis. Analogous to the results obtained with injury models in rodents, histological inspection of the regenerating tissues showed reduced fiber area and increased fibrosis in 70-year-old individuals compared to young ones (Carlson et al., 2009). In both humans and rodents, a primary cause for the age-related loss of regenerative capacities is the attenuated SC response, which originated from combined effects of cell-intrinsic and extrinsic changes (Dumont et al., 2015; Zhou et al., 2017) (see next section on the causes of SC aging). Results from animal models showed that the aged SC compartment can count on a smaller percentage of cells fully able to regenerate the tissue (Cosgrove et al., 2014), and that SCs that show proliferative capacities give rise to smaller clones, compared to young ones (Tierney et al., 2018). Overall, these data indicate that impaired function of aged SCs is mainly due to activation and proliferation defects, while there seem to be no reduction in the rate of early myogenic differentiation or propensity to fuse and form myotubes with age (Conboy et al., 2003; Tierney et al., 2018).

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3.2 Disuse and reload Elderly individuals generally require a longer recovery phase in order to return to initial muscle mass levels following short-term immobilization compared with young individuals (Hvid et al., 2010, 2014; Suetta et al., 2009, 2013). However, relatively few studies have investigated the specific role of SCs in the disuse-induced atrophy. An immobilization/reload model is useful to study both the molecular mechanisms entailing the early atrophy process (Brooks and Myburgh, 2014), and the following recovery of muscle mass after reloading. Two studies showed that 2 weeks of immobilization did not affect SC numbers regardless of the age of the individual (Carlson et al., 2009; Snijders et al., 2014b), and these results are supported by animal studies in which SC-depleted muscles subjected to 2 weeks of hind limb-suspension could recover both fiber area and strength ( Jackson et al., 2012). As mentioned earlier, however, Suetta et al. reported lower numbers of Pax7 positive cells in old compared with young individuals, specifically in type II fibers following 2 weeks immobilization (Suetta et al., 2013). Thus, the potential role of SCs during atrophy remains controversial, and there is a lack of experiments that have attempted to mechanistically assess the role of SCs during muscle atrophy related to aging. In the analysis of the subsequent reloading phase, however, age-related defects have been revealed that potentially could be attributed to a declined SC function (Carlson et al., 2009; Suetta et al., 2013). More specifically, the blunted SC activation registered in the elderly occurred in parallel with diminished activation of Notch-signaling (Carlson et al., 2009), whereas activation of MAPK/Notch restored myogenic responses and made them more similar to SCs of young adults (Suetta et al., 2013). Further evaluation of the causal relationship between the functional decline of SCs in the elderly and impaired recovery of muscle fiber size and function following short disuse periods is an important challenge in future studies.

3.3 Attenuated response to exercise It is often put forth that the myogenic capacity is diminished with aging, and that a reduction in SC activity in response to mechanical stimuli might contribute to this phenomenon. Mackey et al. investigated the effect of lifelong endurance running on the SC pool of the vastus lateralis of 65-years-old individuals (Mackey et al., 2014). The effect of training on SC numbers was very small and mainly observed as a specific protection of type II fibers from the age-induced decline in SC numbers. Interestingly, old-trained, similar to

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young trained or untrained individuals, showed a linear correlation between fiber area and SC counts per fiber, while this correlation was lost in old untrained individuals. This result is an indication that endurance running promotes SC-mediated remodeling of muscle fibers even at old age. In line with these findings, some studies showed no changes in myogenic capacity with aging (Roth et al., 2001; Walker et al., 2012), or reported an increase in SC content in older men and women after chronic resistance-type training (Mackey et al., 2007; Verdijk et al., 2009; Verney et al., 2008). Yet, when SC content was analyzed 24-h after maximal eccentric muscle actions, the increase was blunted in older compared with younger men (Dreyer et al., 2006). This is supported by several other studies reporting attenuated or delayed SC responses in old compared with young individuals after either acute or chronic resistance exercise (McKay et al., 2012; Petrella et al., 2006; Snijders et al., 2014a). Thus, taken collectively, these studies show that the SCs response in older muscle to resistance-type exercise is not as robust as that of young. This deregulation in SC activation may impair the skeletal muscle remodeling capacity to exercise. A recent report suggested that lower rates of muscle hypertrophy in older individuals might be related to age-related decline in muscle fiber capillarization, which in turn was associated with a reduced increase in SC content after chronic resistance training in older adults (Snijders et al., 2017). This, together with the established positive correlation between increases in SC content and fiber size following chronic resistance training in both young and old individuals (Bellamy et al., 2014; Mackey et al., 2011, 2014; Snijders et al., 2015), suggests that hampered SC function with age could contribute to reduced fiber growth with aging. It has been proposed that one of the important roles of the SCs during muscle hypertrophy is to fuse with existing fibers, thereby protecting the relationship between the numbers of nuclei per fiber area. This is often referred to as the myonuclear domain theory (Allen et al., 1999). In support for the hypothesis that diminished SC function in the elderly might translate into reduced capacity for myonuclear addition, a comparison between young and old muscle showed that myonuclei addition was more effectively accomplished in the young (Petrella et al., 2006). Indeed, this has been put forth as an explanation for the sometimes reported attenuated hypertrophic response in older compared with younger individuals (Kosek et al., 2006; Petrella et al., 2006; Phillips et al., 2017). While the collective body of evidence indicates an association between SC activation, myonuclei addition and fiber hypertrophy, myonuclei addition may still not be a strict

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prerequisite for muscle hypertrophy (Kadi et al., 2004; Murach et al., 2018a). In fact, the myonuclear domain appears to be relatively flexible during fiber hypertrophy in humans (Murach et al., 2018a). Few studies have explored the effect of age on myonuclear addition (Petrella et al., 2006), but a recent meta-analysis concluded that age had no significant impact on neither myonuclei addition nor fiber-size increases in response to chronic resistance training, suggesting that the potential detrimental effect of aging on myonuclei addition and fiber hypertrophy is rather small (Conceicao et al., 2018). In mice, fiber hypertrophy was effectively accomplished during 2 weeks of overload in mouse muscle, despite the SC pool being depleted (McCarthy et al., 2011). These findings were later challenged by another laboratory showing that SC depletion prevented fiber hypertrophy (Egner et al., 2016). Translating findings from mouse to human muscle is, however, complicated since the importance of SCs for overload-induced hypertrophy appears to be dependent on the age of the animal. In particular, the young growing muscles are dependent on SCs for intact hypertrophy whereas some degree of myonuclear domain expansion seems to be tolerated in adult muscles lacking SCs (Murach et al., 2018b). Altogether, existing data indicate that SCs have a role in skeletal muscle growth and in the regulation of myonuclear density. Yet, more research is needed to define the functional consequences of the diminished SC activation following exercise in skeletal muscles in older compared with younger individuals..

4. Causes: Why satellite cell number and function decline with increasing age Aging induces a diminished function of cellular components and pathways and overall impacts on the cell ability to respond to stimuli. In addition, multiple types of cellular damage induce senescence, a cell status characterized by a permanent exit from the cell cycle and a specific metabolic and secretory phenotype (Lopez-Otin et al., 2013). In the skeletal muscle, expression of the senescence marker p16 has been detected in SCs from very old mice (28–32 months of age) and humans (around 75 years of age) (Sousa-Victor et al., 2014). In particular, SCs from older animals have been shown to switch to a senescent, non-proliferative state in response to activation stimuli (Sousa-Victor et al., 2014). This mechanism contributes to the reduced SC activity seen with aging, but the main triggers of SC senescence, and SC functional decline in general, are still unclear. Both cell-intrinsic and extrinsic factors have been observed to influence SC

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function during aging. The topic is vast and excellently addressed by recent reviews (Almada and Wagers, 2016; Blau et al., 2015; Brack and MunozCanoves, 2016; Dumont et al., 2015). This section will provide a brief overview of the intrinsic and extrinsic factors, followed by a more extensive discussion of the changes at the level of the genome, epigenome and transcriptome and a final summary of the circulating factors that modulate the SC function in the aging human muscle.

4.1 Intrinsic and extrinsic factors Discerning the causes and mechanisms controlling the decline of SC number and function with age could lead to new strategies to offset the aging effect in the SC pool, and hence maintain the regenerative capacity of the muscle (Blau et al., 2015). Specific in vitro and in vivo assays have been ideated to address the question whether the defects observed in aged SCs can be reverted by their exposure to young factors or environments (Bengal et al., 2017). These experiments, mainly conducted in animal models, allowed defining intrinsic and extrinsic factors contributing to SC aging (Dumont et al., 2015). Intrinsic factors are cell-autonomous processes that occur in the SCs with aging, independent from the environment where they reside. These factors include the deterioration of cellular components and altered activation of intracellular signaling (Bigot et al., 2015; Franco et al., 2018; Sousa-Victor et al., 2014). Conversely, extrinsic factors correspond to signals that the SCs receive from the outside. Within their niche, SCs are influenced by cell-cell and cell-matrix interactions and by auto- and paracrine signaling (Dumont et al., 2015). The paracrine signaling influencing SC activity is mainly connected to the capillarization, inflammatory and hormonal changes that characterize the aged muscle (see next section). In addition, the interaction of the SCs with the ECM mediates both anchoring to the basal lamina and intracellular transduction of external mechanical loading (Mayer, 2003). With age, the ECM expands within the skeletal muscle increasing the stiffness of the tissue (Parker et al., 2017), which may have implications for the SCs (Boers et al., 2018). Importantly, in real life, intrinsic and extrinsic factors extensively influence each other and work in symphony to determine the outcome of muscle aging and the SC niche (Almada and Wagers, 2016; Dumont et al., 2015). The loss of self-renewal and consequent excessive commitment toward differentiation of the SC progeny is regarded as a major factor compromising SC function in the aged muscle (Cheung and Rando, 2013; Giordani et al., 2018).

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Thanks to extensive studies in animal models, it has been established that the loss of self-renewal and consequent exhaustion of the uncommitted SC pool is determined by both intrinsic and extrinsic factors (Baghdadi et al., 2018; Chakkalakal et al., 2012; Rozo et al., 2016). The main intracellular pathways discovered to regulate self-renewal are the Notch (Bjornson et al., 2012; Chakkalakal et al., 2012; Wen et al., 2012), the p38/MAPK (mitogen activated protein kinase) (Bernet et al., 2014; Cosgrove et al., 2014; Rozo et al., 2016) and the IL6-STAT3 cascades (Price et al., 2014; Tierney et al., 2014). Engagement of these pathways has been confirmed in cultured human myoblasts upon activation (Bigot et al., 2015; Charville et al., 2015), and strategies aimed at fine-tuning these cascades might be of therapeutic importance. Efforts have been made to produce an artificial niche that provides soluble factors and mechanical support to optimally maintain selfrenewal of human SCs in culture (Quarta et al., 2016). Beneficial effects have also been observed upon in vitro treatment of human myoblasts with the antioxidant vitamin D treatment. This compound has been found to potentiate the expression of factors promoting self-renewal, such as FOXO3 and the components of the Notch pathway, and to inhibit SC proliferation and differentiation (Olsson et al., 2016).

4.2 Changes in the genome, epigenome, transcriptome The genome (Zhang and Vijg, 2018), epigenome (Sen et al., 2016) and transcriptome (Rodriguez et al., 2016) are known to encounter progressive deterioration with aging in any cell type, and supporting evidences are starting to accumulate for human SCs (Fig. 1). The transcriptome of young and old human SCs presents remarkable differences, especially at the level of genes involved in the regulation of self-renewal (Bigot et al., 2015). Chromatin alterations and epigenetic changes, such as a general increase of DNA methylation, were observed in cultured human SCs from 70-year-old individuals compared to younger ones (Bigot et al., 2015). These epigenetic alterations repressed the transcription of loci relevant to the regulation of self-renewal, like the SPRTY1 locus. In agreement, old myoblasts were characterized by stronger expression of differentiation markers and failed to maintain the Pax7+ pool both in vitro and when transplanted in mouse muscles (Bigot et al., 2015). These results are in line with the epigenetic profile of SCs from young and old mice (Liu et al., 2013). Liu et al. analyzed the whole genome of quiescent cells and found that specific promoters marked by active histones actively maintain quiescence.

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Fig. 1 Graphic representation of the changes occurring in the nucleus of aged satellite cells (SCs) and their effects on SC activation upon stimulation and the cascade of events leading to tissue remodeling. Mechanisms active in functional SCs and healthy tissues are written in black, while the red color represents age-related alterations. SPRY1: Sprouty, RTK signaling antagonist 1; Hoxa9: Homeobox protein Hox A9; SASP: senescence-associated secretory phenotype.

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These sites started to be repressed after SC activation. Interestingly, SCs from old animals presented a widespread increase of repressive methylation of the histones (H3K27me3) both at promoters and in non-coding regions during quiescence. The resulting epigenetic landscape was more similar to activated SCs and suggests a link to the loss of self-renewal with aging. Moreover, the spread of repressive marks may underline a general loss of transcriptional potential with age (Liu et al., 2013). Also the locus encoding for the master regulator of development Hoxa9 was differentially methylated in SCs from young and old mice and this led to the re-expression of Hoxa9 in activated aged SCs (Schworer et al., 2016). Hoxa9, in turn, is a transcription factor involved in the regulation of intracellular pathways known to limit SC activation and self-renewal (e.g., MAPK, TGFβ, Wnt and JAK/STAT), and promote senescence (Schworer et al., 2016). In summary, the regulation of the chromatin status varies with aging and is a crucial determinant of SC function. Besides the epigenomic modifications, changes to the genomic sequence itself can affect the transcriptional landscape. Studies exploring the whole genome of single adult human cells have demonstrated a linear increase of somatic mutations with aging in multiple healthy tissues (Alexandrov et al., 2015; Blokzijl et al., 2016; Franco et al., 2018; Lo Sardo et al., 2017; Lodato et al., 2018). However, the functional impact of these mutations is still unclear. On the one hand, a very small portion of somatic mutations is expected to affect protein sequence and function. The majority of mutations are predicted to have no effect, and the cells carrying a mutation affecting cell viability are usually cleared by natural selection (Martincorena et al., 2018). A creative strategy allowed the parallel acquisition of somatic mutation and gene expression data from single cell RNA sequencing (Enge et al., 2017). A concomitant increase of genomic somatic variability and transcriptional variability accompanied chronological aging. However, the genetic and transcriptional variability were not interdependent, indicating a negligible effect of genomic somatic alterations on transcription (Enge et al., 2017). On the other hand, a study from our group specifically addressed the changes occurring in the genome of human SCs and found that somatic mutations accumulated with aging and were correlated with impaired proliferative capacities at the single cell level (Franco et al., 2018). In particular, the number of somatic mutations was registered in single SC genomes. In parallel, the ability of the single SCs to clonally expand in vitro was assessed. These assay showed that the clonal progeny of the SCs carrying more than 1250 somatic mutations presented slower

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in vitro proliferation and increased senescent morphology (Franco et al., 2018). This correlation could not be explained by mutations mapped to specific genes, promoters and enhancers normally expressed in SCs. Conversely, it seemed that the whole mutation burden concurred to establish the slowly proliferating phenotype (Franco et al., 2018). Thus, SCs might be sensitive to genomic mutations and activate a response, for example, senescence, which prevents the expansion of highly mutated clones. In mouse SCs, senescence has been shown to occur in response to DNA double strand break (DSB) accumulation (Didier et al., 2012). In particular, heterozygous loss of the Ku80 subunit of the Ku heterodimer complex that is active in the repair of DNA DSBs induces p53 activation, senescence and accelerated muscle aging phenotypes (Didier et al., 2012). However, no major signs of DSB accumulation were detected in physiologically aged SCs (Cousin et al., 2013), in agreement with SCs showing a higher intrinsic ability to repair DSBs compared to more differentiated muscle cells (Vahidi Ferdousi et al., 2014). Yet, genetic induction of a broader range of DNA errors, via inactivation of the nucleotide excision repair complex, impaired in vitro proliferation and differentiation of SCs at a similar degree as physiological aging (Lavasani et al., 2012). It is still unclear whether single nucleotide variants and small insertions/deletions identified in aged human SCs (Franco et al., 2018) are able to induce a response that interferes with cell cycle entry. While the meaning of the threshold of 1250 mutations per genome found to affect proliferation is also unknown (Franco et al., 2018), the increase of somatic mutations with aging implies that the higher the age of the skeletal muscle, the higher the number of SCs that carry a mutation burden able to impair expansion. Mouse experiments showed that the Pax7 positive cells able to expand after transplant in an injured skeletal muscle are reduced to two-thirds in old animals (24 months of age) compared with young animals (Cosgrove et al., 2014). The hypothesis of a link between this reduction and an increased mutation burden is attractive, but still needs to be verified. An increase in the number of senescent cells as a consequence of high mutation burden might also explain the association between average number of mutations and loss of differentiation observed in SCs from aged donors (Franco et al., 2018). In agreement, the DNAdamage response that is responsible for SC senescence has been found to also prevent mouse myoblast differentiation downstream of MyoD activation (Latella et al., 2017). Age-related proliferation and differentiation defects observed with clonal assays (Franco et al., 2018) were not detected when the SCs were analyzed

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as a population (bulk culture) (Bigot et al., 2015). In particular, bulk cultures of SCs from 70-year-old subjects showed normal replicative capacities, negligible signs of senescence and an increased tendency to differentiate (Bigot et al., 2015). An explanation for these discrepancies might rely on differences in the assay specificities. In particular, the bulk culture shows an intact potential of the SC pool even at old ages, while the clonal assay shows the heterogeneity of the SC pool and age-related differences at the single cell level. In addition, the properties of the single SC clones were tested after multiple in vitro replication cycles, which are expected to stress the cells and enhance age-related defects (Franco et al., 2018). In agreement, an attenuated activation of the myogenic program has been observed in human myoblasts that entered replicative senescence after extensive culture (Bigot et al., 2008). The single cell analysis of human SCs also provided information on specific genes mutated in the muscle of aged individuals (Franco et al., 2018). Mutations that accumulate randomly in the genome, like ageinduced somatic mutations, have a certain probability to hit and inactivate a gene that is important or necessary for the cell function. Our data show that, in the leg muscles of 70-years-old individuals, 1 out of 200 SCs carry a mutation in a gene that is differentially expressed in response to exercise training (Franco et al., 2018). When this mutation is propagated to differentiated fibers, a defective gene will be expressed and the cumulative effect of similar somatic mutations across all fibers has the potential to progressively impair remodeling and repair capacities in aged individuals (Franco et al., 2018). A great part of the studies searching for a link between genomic changes and aging has focused on telomeres (Blasco, 2007). A known feature of aging is the progressive shortening of telomeres, the chromosome extremities that need to be elongated by a specific enzyme in order to be maintained. This enzyme is called telomerase (TERT) and generally stops to be expressed in adult cells. The lack of telomerase activity causes a progressive loss of the terminal bases of the chromosomes, and the net result is that the telomeres become shorter at every cycle of DNA synthesis (Blasco, 2007). When telomeres become too short, a DNA-damage response is started and cells become senescent (d’Adda di Fagagna et al., 2003). In the skeletal muscle, the mechanism of telomere attrition has been identified as a leading pathogenic mechanism for the Duchenne muscular dystrophy (Sacco et al., 2010). This study showed that chronic muscle damage triggered continuous rounds of SC proliferation. In such scenario, the SC telomeres became too short and induced cell cycle arrest, ultimately impairing the SC ability to

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regenerate the damaged tissue (Sacco et al., 2010). Thus, in teenager individuals affected by the disease the majority of SCs were not functional. An attractive possibility is that the same mechanism occurs in sarcopenic and extremely old muscles. A new tool to quantify telomere length in single SCs using Fluorescence-In Situ-Hybridization (FISH) and accurate microscopy measurements has been developed (Tichy et al., 2017). This technology could verify the reduction in telomere length in SCs from mice and adolescent individuals with Duchenne dystrophy. Yet, no difference has been found between young and aged healthy mice (Tichy et al., 2017). While measurements from SCs from young and older humans skeletal muscle using this technology are still lacking, earlier qPCR measurements did not find any difference (Bigot et al., 2015). Nonetheless, telomere shortening was observed in the lower limb muscles of aged human subjects (Venturelli et al., 2014). In summary, while the impact of aging on SC telomere length is still debated, age-related changes occurring in the nucleus at the level of the DNA (genetic and epigenetic mutations) and consequent transcription are envisaged as a major intrinsic factor in the attenuated response to stimuli observed in SCs from aged skeletal muscle (Fig. 1).

4.3 Circulating factors Since SCs are located in close proximity to skeletal muscle capillaries, they are a perfect candidate to respond to any change in the concentration of circulating factors in the blood (Christov et al., 2007). The age-induced decline in sex hormones concentration (Morley and Malmstrom, 2013) is closely related to the loss of muscle mass and function in elderly individuals (Velders and Diel, 2013). Nonetheless, the role of androgens (e.g., testosterone) in regulating the SC niche is still unclear. Although, there is evidence indicating that testosterone has the ability to increase the SC number in human skeletal muscle, at least during short-term administration (Kadi, 2000; Kadi et al., 2008; Sinha-Hikim et al., 2003). This increment in SC number was also noted in elderly individuals, alongside with an increase in fiber cross-sectional area (Sinha-Hikim et al., 2006). Interestingly, this study demonstrated that graded doses of testosterone administered to elderly men induced a dose-dependent increase in the SC number (Serra et al., 2013). From a mechanistic perspective, it appears that testosterone supplementation may offset the aging-induced alterations by targeting molecular pathways associated to SC proliferation and differentiation, such as phospho-Akt, myostatin, and Notch signaling (Kovacheva et al., 2010).

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As the most important female sex hormone, estrogens are in charge of the development of the reproductive system in females, as well as promoting secondary sex characteristics. Currently, the link between estrogens and the muscle regenerative capacity is based on anti-catabolic and antiinflammatory effects of these hormones. This assumption comes from studies using female participants with and without hormonal replacement therapy (HRT). Williamson et al. demonstrated a higher basal gene expression level of myostatin, MuRF-1, and FOXO3a in old compared to young women after a period of resistance training (Williamson et al., 2010). Also, women on HRT suffered from less muscle damage after an acute eccentric exercise bout than women not taking HRT, indicated by a lower increase in several pro-inflammatory markers (e.g., IL-6, IL-8, IL-15, and TNF-α) in the skeletal muscle (Dieli-Conwright et al., 2009). In a similar follow-up study by the same authors, women on HRT demonstrated a greater downregulation of myostatin than their counterparts not taking HRT after eccentric exercise (Dieli-Conwright et al., 2012). Such protective role of estrogens on the inflammatory and metabolic environment in the skeletal muscle could have implications for SC number and activity (Enns and Tiidus, 2008; La Colla et al., 2015; Milanesi et al., 2008; Pronsato et al., 2013; Thomas et al., 2010; Vasconsuelo et al., 2013; Velders et al., 2012). Insulin-like growth factor-1 (IGF-1) is a circulating factor that can be locally produced by the muscle, but whose principal source is the liver. IGF-1 has been related to the activation and differentiation of SCs (Hill and Goldspink, 2003) and there seems to be a temporal association between the myogenic program and the different IGF-1 splice variants (McKay et al., 2008; Philippou et al., 2009). Importantly, aging induced a reduction of IGF-1 activity that was associated with the loss of muscle mass and function in elderly population (Sharples et al., 2015). Myostatin, one of the most potent and well-described negative regulators of muscle mass, has also been shown to impact some aspects of the SC pool. Myostatin is a member of the transforming growth factor-β family. It can hinder SC self-renewal and proliferation (McCroskery et al., 2003), as well as downregulate MyoD, which has negative consequences for the SC differentiation process (Langley et al., 2002). However, the results obtained in in vitro experiments have not always been successfully replicated in in vivo models (Snijders et al., 2015). Despite this controversy, myostatin protein concentration has been reported to be twofold higher in the skeletal muscle of old men when compared with young subjects (McKay et al., 2012), and a phase 2 clinical trial, testing the effects of a myostatin inhibitor in elderly and frail subjects,

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showed positive results in terms of muscle mass and function, partly attributed to a more active SC pool (Becker et al., 2015). It is also worth noting that a key factor regulating angiogenesis, i.e., vascular endothelial growth factor (VEGF), appears capable to stimulate the SCs through autocrine stimulation, as well as through paracrine effects from endothelial cells or adult muscle fibers (Verma et al., 2018). This provides an additional link between the SC pool and the vascular niche. Indeed, it has been argued that age-related changes in capillarization may impact the capacity to activate the SC pool (Nederveen et al., 2018). Given that aged individuals present lower capillaraization and reduced VEGF levels in the skeletal muscle, as compared with young subjects (Frontera et al., 2000; Ryan et al., 2006), these alterations offer further support to an age-induced alteration in the dynamics between the SC and the vascular niche, which contributes to the reduced remodeling capacity and muscle loss in the aged population ( Joanisse et al., 2017). Cytokines and other inflammatory factors produced either locally in the muscle or coming from the circulation influence SC function (Saini et al., 2016). Among these factors, IL-6 has been suggested to play a key role by taking part in the initial inflammatory response shortly after a muscle damage event, favoring infiltration of monocytes and macrophages into the muscle (Saini et al., 2016). Particularly important for the SCs is the IL-6/STAT3 signaling, since IL-6-activated STAT3 controls SC performance through regulation of Myod1 (Tierney et al., 2014). McKay et al. performed a study investigating the role of IL-6 on age-related SC dysfunction by comparing the response of old (70 years) vs. young (20 years) men to an acute resistance exercise bout, with multiple muscle biopsy time-points (McKay et al., 2013). This investigation demonstrated that the age-induced blunted response of SCs to exercise was at least partially mediated by an elevation in suppressors of cytokine signaling (SOCS) proteins, together with a delayed increase of IL-6 and pSTAT3. Interestingly, the authors noted that the differences in basal values of most of these factors in old vs. young men appeared to be important for the SC response, offering additional data supporting the negative effects of the age-induced pro-inflammatory state on the SC pool (McKay et al., 2013).

5. Exercise: An intervention to modulate satellite cell deterioration with aging Despite the fact that the remodeling capacity decreases with age in an individual-specific fashion, and that the exercise response also differs between young and old, beneficial effects of exercise are still evident for

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most individuals even at very old age (Fiatarone et al., 1990; Suetta et al., 2013). Moreover, old individuals that experienced lifelong endurance training show signs of active SC-mediated remodeling (Bellamy et al., 2014; Mackey et al., 2011, 2014; Snijders et al., 2015). Even though physical activity and exercise have an impact on practically all the organ systems of the body (Hawley et al., 2014), one of the main target is the skeletal muscle, and in turn the SCs. Indeed, exercise training has been shown to profoundly change the epigenome (Lindholm et al., 2014; Seaborne et al., 2018; Sharples et al., 2016), transcriptome (Damas et al., 2018; Lindholm et al., 2016), proteome (Robinson et al., 2017) and mitochondrial-related enzymatic activity (Hood et al., 2019; Kim et al., 2017) in the human muscle. For example, exercise-induced adaptations on mitochondrial activity and cell metabolism are opposite to those caused by aging (Hood et al., 2019; Seo et al., 2016), and counteract the muscle loss in the elderly (Bruseghini et al., 2015) (Fig. 2). However, it is important to note that age-related muscle deconditioning is not “just the opposite” of exercise-induced muscle adaptations, and the major effects of exercise appears not to counteract key pathways in the “general” aging process, i.e., muscle aging is not simply inactivity (Phillips et al., 2013). Although, it is known that exercise has the capacity to optimize environmental factors and enhance other non-aging related processes that help to counteract the phenotypic and/or functional consequences of the aging processes (McPhee et al., 2016). Little is known about the effect of exercise on the genome. Exercise could contribute to create an environment with low levels of mutagenic factors (e.g., reducing the concentration of reactive oxygen species (ROS) within the muscle, Thirupathi and Pinho, 2018), and thereby slow down the mutation accumulation process. Alternatively, an enhanced SC proliferation triggered by exercise could increase the amount of genetic changes (e.g., somatic mutations) and act synergistically with aging in promoting the degeneration of the genome. To date, the effect of exercise on SC proliferation as well as the connection between SC somatic mutations and functional decline have started to be addressed, but the overall impact on muscle physiology is not clear (Crameri et al., 2007; Franco et al., 2018; Hyldahl et al., 2014; Joanisse et al., 2015; Nederveen et al., 2015). Conversely, it must be acknowledged that there are overwhelming data supporting the beneficial effects of exercise on muscle mass and function in the elderly (McPhee et al., 2016). Thus, the consequences of a potential exercise-driven increase in genetic alterations are most likely exceeded by the overall favorable effects of physical activity and exercise training.

Intrinsic factors

Impaired self-renewal Deterioration of cellular components Changes in the genome, epigenome & transcriptome

Positive metabolic adaptations Reduced concentration of free radicals Somatic mutations accumulation (?)

Muscle adaptations

Tailored exercise

Satellite cell function

Aging

Changes in endocrine functins (♀;?)

Extrinsic factors

Altered immune system Changes in the hormonal balance Extracellular matrix alterations

↓ pro-inflammation Changes in endocrine functions (♂) ECM remodeling Effects on IGF-1 and myostatin

Systemic adaptations

Fig. 2 Schematic representation of the counteracting forces shaping the satellite cell (SC) function. Aging modifies both intrinsic and extrinsic factors and interferes with normal SC activation and function. Conversely, the effect of exercise is expected to be favorable for SC maintenance and activity and counteract the age-related decline in muscle function.

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Somatic mutations accumulation (?)

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Telomere length has been quite extensively studied in relation to exercise, but only in whole muscle samples, and the results are still not consistent among different cohorts (Arsenis et al., 2017). This is probably because different training routines cause different effects in the muscle and influence not only SC proliferation, but also many aspects of the muscle environment. For example, physical inactivity is detrimental for telomere maintenance (Venturelli et al., 2014), and in fact, physically inactive subjects showed shorter telomeres compared to age-matched active individuals, and the level of physical activity influenced telomere length to a greater degree than age itself (Venturelli et al., 2014). An explanation might be that moderate and regular exercise may confer positive adaptations providing a protective environment for telomeres. However, the positive effect of exercise is reduced when the training is too intense or prolonged (Collins et al., 2003; Kadi et al., 2008; Rae et al., 2010). It is conceivable that high-intensity bouts of both aerobic and resistance exercise cause telomere shortening by inducing muscle damage (Del Coso et al., 2017; Fernandez-Gonzalo et al., 2014) and increased cell proliferation and tissue turnover. In addition, the inflammatory response (Fernandez-Gonzalo et al., 2012) and ROS (Thirupathi and Pinho, 2018) generated by high-intensity training could damage the DNA and have a further negative impact on telomere length. It is pertinent to further explore the overall effect of exercise and training on the genome (somatic mutation accumulation, telomere shortening and epigenetic changes) as findings from these studies will be the cornerstone in the future design of training routines aimed at counteracting the aging-induced changes in the SCs. Numerous potential effects of exercise on extrinsic factors affecting SC function exist. We and others have shown that different forms of exercise have a positive effect on the pro-inflammatory profile present in elderly people. Thus, reductions in circulating pro-inflammatory markers such as TNF-α or IL-6 occur after a period of training in older subjects (Monteiro-Junior et al., 2018; Nicklas and Brinkley, 2009; RodriguezMiguelez et al., 2014, 2015). It is then reasonable to assume that such reduction will have a positive impact on SC number and function (Fig. 2). In addition, exercise may restore some of the age-induced endocrine alterations ( Janssen, 2016) and affect hormones with a known effect on SCs. For example, 6 weeks of high-intensity training increased the concentration of free testosterone in old men (Herbert et al., 2017). Also, chronic training tends to decrease estrogen levels in elderly women, which is associated with reduced body fat content (McTiernan et al., 2004). Thus, the potential effects of exercise in elderly women vs. men may have opposite

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consequences on the SC pool, e.g., reduced estrogen levels in women which may negatively alter the SC microenvironment vs. increased testosterone levels which tend to increase SC number (Fig. 2). The effects of exercise on IGF-1 in the elderly may be dependent on the particular fitness level of each individual. Despite a general decline in IGF-1 with age, fit elderly subjects still show increased circulating IGF-1 after exercise. This response is not seen in less-fit, older persons (Amir et al., 2007; Janssen, 2016). Thus, high fitness may protect from the deleterious effects of age on IGF-1-related alterations in the SC pool. In young subjects, myostatin decreases in muscle after an acute bout of exercise (Fernandez-Gonzalo et al., 2013), although basal values tend to be maintained after a period of training (Lundberg et al., 2013). While the acute response is still present in old individuals, the magnitude of such response seems to be reduced, together with a smaller decline in myostatin-positive SCs in elderly vs. young men (Snijders et al., 2014a). Therefore, although exercise still affects myostatin levels and activity in elderly subjects, the response appears to be somewhat compromised (Snijders et al., 2014a). One of the first factors demonstrated to activate SCs was the hepatocyte growth factor (HGF). HGF levels increased in parallel with SC number in young subjects (O’reilly et al., 2008). However, whether similar effects occur in elderly subjects is still unknown. It should be noted that, in studies using arrays or RNAseq in skeletal muscle following exercise, there are numerous factors with described effects on cell cycle and growth (Phillips et al., 2013). Such explorative analyses provide multiple exercise-induced candidate factors with the potential to influence the SC pool. For example, in both animals and humans, it is well documented that increased physical activity stimulate capillary growth, and that cessation of exercise training induce a rapid regression in capillarity (Andersen and Henriksson, 1977; Klausen et al., 1981), with VEGF playing a central role in such adaptations (Gustafsson et al., 1999, 2007). Despite the somewhat blunted response of VEGF to exercise with increased age (Ryan et al., 2006), and given the connection between the microvasculature and the SC niches, these data support the positive impact of exercise on the SC pool in the aged population. Also, modulation of genes involved in ECM processes has been described after aerobic exercise training in humans (Timmons et al., 2005). Yet, it appears that resistance exercise is a more powerful stimulus to induce ECM remodeling (Hughes et al., 2015; Hyldahl et al., 2015) and improve its mechanical properties (Garg and Boppart, 2016). Hence, altohugh more evidence is needed, these adaptations may counteract the age-induced alterations in the ECM with a known negative impact of SC function (Fig. 2).

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6. Conclusions SCs form the resident stem cell population in the skeletal muscle tissue and are a key component of the skeletal muscle regeneration and remodeling machinery. Using different in vitro and in vivo approaches, it has been shown that skeletal muscle SC numbers and function are affected by aging in both human and animal models. Cell-intrinsic and extrinsic causes have been associated with this decline. Among the intrinsic factors, somatic mutations have been found to accummulate in the SC genomes with increasing age and associate with proliferation and differentiation defects. Further changes in the epigenome and transcriptome, as well as the progressive deterioration of cellular components and signaling pathways regulating self-renewal in the pool of uncommitted SC progenitors, progressively compromise SC function in the elderly. In parallel, extrinsic factors such as immune, hormonal and extracellular matrix alterations further decrease the capacity of SCs to be activated, differentiate and proliferate. Under the concomitant pressure of intrinsic and extrinsic factors, SC tends to enter an irreversible state of senescence and the numbers of functional SCs progressively decrease in the muscle. As a consequence, the aged skeletal muscle presents a reduced capacity to regain the original muscle mass after negative events such as injury, immobilization, or inactivity. Nonetheless, at least a part of the SC pool in elderly individuals maintains its ability to respond to stimuli such as exercise and undergo muscle remodeling. This portion of functional SCs can be exploited for therapeutic intervention. Given the beneficial effects of exercise at both the systemic and muscular level, exercise routines should be further developed (e.g., exercise type, intensity, volume) to prevent and/or counteract the intrinsic and extrinsic negative events occurring in SCs with aging.

Acknowledgments Authors wish to thank Valentino M. Davio for graphic contribution to figures. This study was supported by grants to I.F. from the Hagelen, Jeanssons and Osterman Foundations, Svenska L€akares€allskapet and Riksbankens Jubileumsfond; to M.E. from the Swedish Research Council and the Center for Innovative Medicine; to P.V. from the Osterman and Stohnes Foundations; to T.G. from the Swedish Medical Research Council (2013-09305) Marcus and Marianne Wallenberg foundation and CIMED samverkansbidrag.

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References Alexandrov, L.B., Jones, P.H., Wedge, D.C., Sale, J.E., Campbell, P.J., Nik-Zainal, S., Stratton, M.R., 2015. Clock-like mutational processes in human somatic cells. Nat. Genet. 47, 1402–1407. Allen, D.L., Roy, R.R., Edgerton, V.R., 1999. Myonuclear domains in muscle adaptation and disease. Muscle Nerve 22, 1350–1360. Almada, A.E., Wagers, A.J., 2016. Molecular circuitry of stem cell fate in skeletal muscle regeneration, ageing and disease. Nat. Rev. Mol. Cell Biol. 17, 267–279. Amir, R., Ben-Sira, D., Sagiv, M., 2007. Igf-I and fgf-2 responses to Wingate anaerobic test in older men. J. Sports Sci. Med. 6, 227–232. Andersen, P., Henriksson, J., 1977. Capillary supply of the quadriceps femoris muscle of man: adaptive response to exercise. J. Physiol. 270, 677–690. Arentson-Lantz, E.J., English, K.L., Paddon-Jones, D., Fry, C.S., 2016. Fourteen days of bed rest induces a decline in satellite cell content and robust atrophy of skeletal muscle fibers in middle-aged adults. J. Appl. Physiol. (1985) 120, 965–975. Arsenis, N.C., You, T., Ogawa, E.F., Tinsley, G.M., Zuo, L., 2017. Physical activity and telomere length: impact of aging and potential mechanisms of action. Oncotarget 8, 45008–45019. Baghdadi, M.B., Castel, D., Machado, L., Fukada, S.I., Birk, D.E., Relaix, F., Tajbakhsh, S., Mourikis, P., 2018. Reciprocal signalling by notch-collagen V-CALCR retains muscle stem cells in their niche. Nature 557, 714–718. Becker, C., Lord, S.R., Studenski, S.A., Warden, S.J., Fielding, R.A., Recknor, C.P., Hochberg, M.C., Ferrari, S.L., Blain, H., Binder, E.F., Rolland, Y., Poiraudeau, S., Benson, C.T., Myers, S.L., Hu, L., Ahmad, Q.I., Pacuch, K.R., Gomez, E.V., Benichou, O., STEADY Group, 2015. Myostatin antibody (LY2495655) in older weak fallers: a proof-of-concept, randomised, phase 2 trial. Lancet Diabetes Endocrinol. 3, 948–957. Behjati, S., Huch, M., Van Boxtel, R., Karthaus, W., Wedge, D.C., Tamuri, A.U., Martincorena, I., Petljak, M., Alexandrov, L.B., Gundem, G., Tarpey, P.S., Roerink, S., Blokker, J., Maddison, M., Mudie, L., Robinson, B., Nik-Zainal, S., Campbell, P., Goldman, N., Van De Wetering, M., Cuppen, E., Clevers, H., Stratton, M.R., 2014. Genome sequencing of normal cells reveals developmental lineages and mutational processes. Nature 513, 422–425. Bellamy, L.M., Joanisse, S., Grubb, A., Mitchell, C.J., McKay, B.R., Phillips, S.M., Baker, S., Parise, G., 2014. The acute satellite cell response and skeletal muscle hypertrophy following resistance training. PLoS One 9, e109739. Bengal, E., Perdiguero, E., Serrano, A.L., Munoz-Canoves, P., 2017. Rejuvenating stem cells to restore muscle regeneration in aging. F1000Res 6, 76. Bernet, J.D., Doles, J.D., Hall, J.K., Kelly Tanaka, K., Carter, T.A., Olwin, B.B., 2014. p38 MAPK signaling underlies a cell-autonomous loss of stem cell self-renewal in skeletal muscle of aged mice. Nat. Med. 20, 265–271. Bigot, A., Jacquemin, V., Debacq-Chainiaux, F., Butler-Browne, G.S., Toussaint, O., Furling, D., Mouly, V., 2008. Replicative aging down-regulates the myogenic regulatory factors in human myoblasts. Biol. Cell 100, 189–199. Bigot, A., Duddy, W.J., Ouandaogo, Z.G., Negroni, E., Mariot, V., Ghimbovschi, S., Harmon, B., Wielgosik, A., Loiseau, C., Devaney, J., Dumonceaux, J., Butler-Browne, G., Mouly, V., Duguez, S., 2015. Age-associated methylation suppresses SPRY1, leading to a failure of re-quiescence and loss of the reserve stem cell Pool in elderly muscle. Cell Rep. 13, 1172–1182.

ARTICLE IN PRESS 32

Irene Franco et al.

Bjornson, C.R., Cheung, T.H., Liu, L., Tripathi, P.V., Steeper, K.M., Rando, T.A., 2012. Notch signaling is necessary to maintain quiescence in adult muscle stem cells. Stem Cells 30, 232–242. Bjursell, M., Gerdin, A.K., Lelliott, C.J., Egecioglu, E., Elmgren, A., Tornell, J., Oscarsson, J., Bohlooly, Y.M., 2008. Acutely reduced locomotor activity is a major contributor to Western diet-induced obesity in mice. Am. J. Physiol. Endocrinol. Metab. 294, E251–E260. Blasco, M.A., 2007. Telomere length, stem cells and aging. Nat. Chem. Biol. 3, 640–649. Blau, H.M., Cosgrove, B.D., Ho, A.T., 2015. The central role of muscle stem cells in regenerative failure with aging. Nat. Med. 21, 854–862. Blokzijl, F., De Ligt, J., Jager, M., Sasselli, V., Roerink, S., Sasaki, N., Huch, M., Boymans, S., Kuijk, E., Prins, P., Nijman, I.J., Martincorena, I., Mokry, M., Wiegerinck, C.L., Middendorp, S., Sato, T., Schwank, G., Nieuwenhuis, E.E., Verstegen, M.M., Van Der Laan, L.J., De Jonge, J., Jn, I.J., Vries, R.G., Van De Wetering, M., Stratton, M.R., Clevers, H., Cuppen, E., Van Boxtel, R., 2016. Tissue-specific mutation accumulation in human adult stem cells during life. Nature 538, 260–264. Boers, H.E., Haroon, M., Le Grand, F., Bakker, A.D., Klein-Nulend, J., Jaspers, R.T., 2018. Mechanosensitivity of aged muscle stem cells. J. Orthop. Res. 36, 632–641. Bosnakovski, D., Xu, Z., Li, W., Thet, S., Cleaver, O., Perlingeiro, R.C., Kyba, M., 2008. Prospective isolation of skeletal muscle stem cells with a Pax7 reporter. Stem Cells 26, 3194–3204. Brack, A.S., Munoz-Canoves, P., 2016. The ins and outs of muscle stem cell aging. Skelet. Muscle 6, 1. Brooks, N.E., Myburgh, K.H., 2014. Skeletal muscle wasting with disuse atrophy is multi-dimensional: the response and interaction of myonuclei, satellite cells and signaling pathways. Front. Physiol. 5, 99. Brooks, N.E., Cadena, S.M., Vannier, E., Cloutier, G., Carambula, S., Myburgh, K.H., Roubenoff, R., Castaneda-Sceppa, C., 2010. Effects of resistance exercise combined with essential amino acid supplementation and energy deficit on markers of skeletal muscle atrophy and regeneration during bed rest and active recovery. Muscle Nerve 42, 927–935. Bruseghini, P., Calabria, E., Tam, E., Milanese, C., Oliboni, E., Pezzato, A., Pogliaghi, S., Salvagno, G.L., Schena, F., Mucelli, R.P., Capelli, C., 2015. Effects of eight weeks of aerobic interval training and of isoinertial resistance training on risk factors of cardiometabolic diseases and exercise capacity in healthy elderly subjects. Oncotarget 6, 16998–17015. Burd, N.A., Gorissen, S.H., Van Loon, L.J., 2013. Anabolic resistance of muscle protein synthesis with aging. Exerc. Sport Sci. Rev. 41, 169–173. Calve, S., Odelberg, S.J., Simon, H.G., 2010. A transitional extracellular matrix instructs cell behavior during muscle regeneration. Dev. Biol. 344, 259–271. Carlson, M.E., Suetta, C., Conboy, M.J., Aagaard, P., Mackey, A., Kjaer, M., Conboy, I., 2009. Molecular aging and rejuvenation of human muscle stem cells. EMBO Mol. Med. 1, 381–391. Chakkalakal, J.V., Jones, K.M., Basson, M.A., Brack, A.S., 2012. The aged niche disrupts muscle stem cell quiescence. Nature 490, 355–360. Charville, G.W., Cheung, T.H., Yoo, B., Santos, P.J., Lee, G.K., Shrager, J.B., Rando, T.A., 2015. Ex vivo expansion and in vivo self-renewal of human muscle stem cells. Stem Cell Rep. 5, 621–632. Cheung, T.H., Rando, T.A., 2013. Molecular regulation of stem cell quiescence. Nat. Rev. Mol. Cell Biol. 14, 329–340.

ARTICLE IN PRESS Satellite cells in skeletal muscle aging

33

Christov, C., Chretien, F., Abou-Khalil, R., Bassez, G., Vallet, G., Authier, F.J., Bassaglia, Y., Shinin, V., Tajbakhsh, S., Chazaud, B., Gherardi, R.K., 2007. Muscle satellite cells and endothelial cells: close neighbors and privileged partners. Mol. Biol. Cell 18, 1397–1409. Cohen, S., Nathan, J.A., Goldberg, A.L., 2015. Muscle wasting in disease: molecular mechanisms and promising therapies. Nat. Rev. Drug Discov. 14, 58–74. Collins, M., Renault, V., Grobler, L.A., St Clair Gibson, A., Lambert, M.I., Wayne Derman, E., Butler-Browne, G.S., Noakes, T.D., Mouly, V., 2003. Athletes with exerciseassociated fatigue have abnormally short muscle DNA telomeres. Med. Sci. Sports Exerc. 35, 1524–1528. Collins, C.A., Olsen, I., Zammit, P.S., Heslop, L., Petrie, A., Partridge, T.A., Morgan, J.E., 2005. Stem cell function, self-renewal, and behavioral heterogeneity of cells from the adult muscle satellite cell niche. Cell 122, 289–301. Conboy, I.M., Conboy, M.J., Smythe, G.M., Rando, T.A., 2003. Notch-mediated restoration of regenerative potential to aged muscle. Science 302, 1575–1577. Conboy, I.M., Conboy, M.J., Wagers, A.J., Girma, E.R., Weissman, I.L., Rando, T.A., 2005. Rejuvenation of aged progenitor cells by exposure to a young systemic environment. Nature 433, 760–764. Conceicao, M.S., Vechin, F.C., Lixandrao, M., Damas, F., Libardi, C.A., Tricoli, V., Roschel, H., Camera, D., Ugrinowitsch, C., 2018. Muscle Fiber hypertrophy and myonuclei addition: a systematic review and meta-analysis. Med. Sci. Sports Exerc. 50, 1385–1393. Cosgrove, B.D., Gilbert, P.M., Porpiglia, E., Mourkioti, F., Lee, S.P., Corbel, S.Y., Llewellyn, M.E., Delp, S.L., Blau, H.M., 2014. Rejuvenation of the muscle stem cell population restores strength to injured aged muscles. Nat. Med. 20, 255–264. Cousin, W., Ho, M.L., Desai, R., Tham, A., Chen, R.Y., Kung, S., Elabd, C., Conboy, I.M., 2013. Regenerative capacity of old muscle stem cells declines without significant accumulation of DNA damage. PLoS One 8, e63528. Crameri, R.M., Langberg, H., Magnusson, P., Jensen, C.H., SchrØder, H.D., Olesen, J.L., Suetta, C., Teisner, B., Kjaer, M., 2004. Changes in satellite cells in human skeletal muscle after a single bout of high intensity exercise. J. Physiol. 558, 333–340. Crameri, R.M., Aagaard, P., Qvortrup, K., Langberg, H., Olesen, J., Kjaer, M., 2007. Myofibre damage in human skeletal muscle: effects of electrical stimulation versus voluntary contraction. J. Physiol. 583, 365–380. Cruz-Jentoft, A.J., Baeyens, J.P., Bauer, J.M., Boirie, Y., Cederholm, T., Landi, F., Martin, F.C., Michel, J.P., Rolland, Y., Schneider, S.M., Topinkova, E., Vandewoude, M., Zamboni, M., European Working Group on Sarcopenia in Older People, 2010. Sarcopenia: European consensus on definition and diagnosis: report of the European working group on sarcopenia in older people. Age Ageing 39, 412–423. d’Adda di Fagagna, F., Reaper, P.M., Clay-Farrace, L., Fiegler, H., Carr, P., Von Zglinicki, T., Saretzki, G., Carter, N.P., Jackson, S.P., 2003. A DNA damage checkpoint response in telomere-initiated senescence. Nature 426, 194–198. Damas, F., Ugrinowitsch, C., Libardi, C.A., Jannig, P.R., Hector, A.J., Mcglory, C., Lixandrao, M.E., Vechin, F.C., Montenegro, H., Tricoli, V., Roschel, H., Phillips, S.M., 2018. Resistance training in young men induces muscle transcriptome-wide changes associated with muscle structure and metabolism refining the response to exercise-induced stress. Eur. J. Appl. Physiol. 118, 2607–2616. Darr, K.C., Schultz, E., 1989. Hindlimb suspension suppresses muscle growth and satellite cell proliferation. J. Appl. Physiol. 67, 1827–1834. Day, K., Shefer, G., Richardson, J.B., Enikolopov, G., Yablonka-Reuveni, Z., 2007. Nestin-GFP reporter expression defines the quiescent state of skeletal muscle satellite cells. Dev. Biol. 304, 246–259.

ARTICLE IN PRESS 34

Irene Franco et al.

Day, K., Shefer, G., Shearer, A., Yablonka-Reuveni, Z., 2010. The depletion of skeletal muscle satellite cells with age is concomitant with reduced capacity of single progenitors to produce reserve progeny. Dev. Biol. 340, 330–343. Del Coso, J., Salinero, J.J., Lara, B., Abian-Vicen, J., Gallo-Salazar, C., Areces, F., 2017. A comparison of the physiological demands imposed by competing in a half-marathon vs. a marathon. J. Sports Med. Phys. Fitness 57, 1399–1406. Didier, N., Hourde, C., Amthor, H., Marazzi, G., Sassoon, D., 2012. Loss of a single allele for Ku80 leads to progenitor dysfunction and accelerated aging in skeletal muscle. EMBO Mol. Med. 4, 910–923. Dieli-Conwright, C.M., Spektor, T.M., Rice, J.C., Sattler, F.R., Schroeder, E.T., 2009. Hormone therapy attenuates exercise-induced skeletal muscle damage in postmenopausal women. J. Appl. Physiol. (1985) 107, 853–858. Dieli-Conwright, C.M., Spektor, T.M., Rice, J.C., Sattler, F.R., Schroeder, E.T., 2012. Hormone therapy and maximal eccentric exercise alters myostatin-related gene expression in postmenopausal women. J. Strength Cond. Res. 26, 1374–1382. Dreyer, H.C., Blanco, C.E., Sattler, F.R., Schroeder, E.T., Wiswell, R.A., 2006. Satellite cell numbers in young and older men 24 hours after eccentric exercise. Muscle Nerve 33, 242–253. Dumont, N.A., Wang, Y.X., Rudnicki, M.A., 2015. Intrinsic and extrinsic mechanisms regulating satellite cell function. Development 142, 1572–1581. Egner, I.M., Bruusgaard, J.C., Gundersen, K., 2016. Satellite cell depletion prevents fiber hypertrophy in skeletal muscle. Development 143, 2898–2906. Enge, M., Arda, H.E., Mignardi, M., Beausang, J., Bottino, R., Kim, S.K., Quake, S.R., 2017. Single-cell analysis of human pancreas reveals transcriptional signatures of aging and somatic mutation patterns. Cell 171 (321–330), e14. Enns, D.L., Tiidus, P.M., 2008. Estrogen influences satellite cell activation and proliferation following downhill running in rats. J. Appl. Physiol. (1985) 104, 347–353. Ethgen, O., Beaudart, C., Buckinx, F., Bruyere, O., Reginster, J.Y., 2017. The future prevalence of sarcopenia in Europe: a claim for public health action. Calcif. Tissue Int. 100, 229–234. Fernandez-Gonzalo, R., De Paz, J.A., Rodriguez-Miguelez, P., Cuevas, M.J., GonzalezGallego, J., 2012. Effects of eccentric exercise on toll-like receptor 4 signaling pathway in peripheral blood mononuclear cells. J. Appl. Physiol. (1985) 112, 2011–2018. Fernandez-Gonzalo, R., Lundberg, T.R., Tesch, P.A., 2013. Acute molecular responses in untrained and trained muscle subjected to aerobic and resistance exercise training versus resistance training alone. Acta Physiol. 209, 283–294. Fernandez-Gonzalo, R., Lundberg, T.R., Alvarez-Alvarez, L., De Paz, J.A., 2014. Muscle damage responses and adaptations to eccentric-overload resistance exercise in men and women. Eur. J. Appl. Physiol. 114, 1075–1084. Fiatarone, M.A., Marks, E.C., Ryan, N.D., Meredith, C.N., Lipsitz, L.A., Evans, W.J., 1990. High-intensity strength training in nonagenarians. Effects on skeletal muscle. JAMA 263, 3029–3034. Franco, I., Johansson, A., Olsson, K., Vrtacnik, P., Lundin, P., Helgadottir, H.T., Larsson, M., Revechon, G., Bosia, C., Pagnani, A., Provero, P., Gustafsson, T., Fischer, H., Eriksson, M., 2018. Somatic mutagenesis in satellite cells associates with human skeletal muscle aging. Nat. Commun. 9, 800. Frontera, W.R., Hughes, V.A., Fielding, R.A., Fiatarone, M.A., Evans, W.J., Roubenoff, R., 2000. Aging of skeletal muscle: a 12-yr longitudinal study. J. Appl. Physiol. (1985) 88, 1321–1326. Fry, C.S., Lee, J.D., Mula, J., Kirby, T.J., Jackson, J.R., Liu, F., Yang, L., Mendias, C.L., DuPont-Versteegden, E.E., McCarthy, J.J., Peterson, C.A., 2015. Inducible depletion of satellite cells in adult, sedentary mice impairs muscle regenerative capacity without affecting sarcopenia. Nat. Med. 21, 76–80.

ARTICLE IN PRESS Satellite cells in skeletal muscle aging

35

Fry, C.S., Kirby, T.J., Kosmac, K., McCarthy, J.J., Peterson, C.A., 2017. Myogenic progenitor cells control extracellular matrix production by fibroblasts during skeletal muscle hypertrophy. Cell Stem Cell 20, 56–69. Garg, K., Boppart, M.D., 2016. Influence of exercise and aging on extracellular matrix composition in the skeletal muscle stem cell niche. J. Appl. Physiol. (1985) 121, 1053–1058. Gilbert, P.M., Havenstrite, K.L., Magnusson, K.E., Sacco, A., Leonardi, N.A., Kraft, P., Nguyen, N.K., Thrun, S., Lutolf, M.P., Blau, H.M., 2010. Substrate elasticity regulates skeletal muscle stem cell self-renewal in culture. Science 329, 1078–1081. Giordani, L., Parisi, A., Le Grand, F., 2018. Satellite cell self-renewal. Curr. Top. Dev. Biol. 126, 177–203. Gurevich, D.B., Nguyen, P.D., Siegel, A.L., Ehrlich, O.V., Sonntag, C., Phan, J.M., Berger, S., Ratnayake, D., Hersey, L., Berger, J., Verkade, H., Hall, T.E., Currie, P.D., 2016. Asymmetric division of clonal muscle stem cells coordinates muscle regeneration in vivo. Science 353, aad9969. Gustafsson, T., Puntschart, A., Kaijser, L., Jansson, E., Sundberg, C.J., 1999. Exercise-induced expression of angiogenesis-related transcription and growth factors in human skeletal muscle. Am. J. Physiol. 276, H679–H685. Gustafsson, T., Rundqvist, H., Norrbom, J., Rullman, E., Jansson, E., Sundberg, C.J., 2007. The influence of physical training on the angiopoietin and VEGF-A systems in human skeletal muscle. J. Appl. Physiol. (1985) 103, 1012–1020. Hardy, D., Besnard, A., Latil, M., Jouvion, G., Briand, D., Thepenier, C., Pascal, Q., Guguin, A., Gayraud-Morel, B., Cavaillon, J.M., Tajbakhsh, S., Rocheteau, P., Chretien, F., 2016. Comparative study of injury models for studying muscle regeneration in mice. PLoS One 11, e0147198. Hawley, J.A., Hargreaves, M., Joyner, M.J., Zierath, J.R., 2014. Integrative biology of exercise. Cell 159, 738–749. Herbert, P., Hayes, L.D., Sculthorpe, N.F., Grace, F.M., 2017. HIIT produces increases in muscle power and free testosterone in male masters athletes. Endocr. Connect. 6, 430–436. Hill, M., Goldspink, G., 2003. Expression and splicing of the insulin-like growth factor gene in rodent muscle is associated with muscle satellite (stem) cell activation following local tissue damage. J. Physiol. 549, 409–418. Hood, D.A., Memme, J.M., Oliveira, A.N., Triolo, M., 2019. Maintenance of skeletal muscle mitochondria in health, exercise, and aging. Annu. Rev. Physiol. 81, 19–41. Hughes, D.C., Wallace, M.A., Baar, K., 2015. Effects of aging, exercise, and disease on force transfer in skeletal muscle. Am. J. Physiol. Endocrinol. Metab. 309, E1–e10. Hvid, L., Aagaard, P., Justesen, L., Bayer, M.L., Andersen, J.L., Ortenblad, N., Kjaer, M., Suetta, C., 2010. Effects of aging on muscle mechanical function and muscle fiber morphology during short-term immobilization and subsequent retraining. J. Appl. Physiol. (1985) 109, 1628–1634. Hvid, L.G., Suetta, C., Nielsen, J.H., Jensen, M.M., Frandsen, U., Ortenblad, N., Kjaer, M., Aagaard, P., 2014. Aging impairs the recovery in mechanical muscle function following 4 days of disuse. Exp. Gerontol. 52, 1–8. Hyldahl, R.D., Olson, T., Welling, T., Groscost, L., Parcell, A.C., 2014. Satellite cell activity is differentially affected by contraction mode in human muscle following a workmatched bout of exercise. Front. Physiol. 5, 485. Hyldahl, R.D., Nelson, B., Xin, L., Welling, T., Groscost, L., Hubal, M.J., Chipkin, S., Clarkson, P.M., Parcell, A.C., 2015. Extracellular matrix remodeling and its contribution to protective adaptation following lengthening contractions in human muscle. FASEB J. 29, 2894–2904. Ingram, D.K., 2000. Age-related decline in physical activity: generalization to nonhumans. Med. Sci. Sports Exerc. 32, 1623–1629.

ARTICLE IN PRESS 36

Irene Franco et al.

Jackson, J.R., Mula, J., Kirby, T.J., Fry, C.S., Lee, J.D., Ubele, M.F., Campbell, K.S., McCarthy, J.J., Peterson, C.A., DuPont-Versteegden, E.E., 2012. Satellite cell depletion does not inhibit adult skeletal muscle regrowth following unloading-induced atrophy. Am. J. Physiol. Cell Physiol. 303, C854–C861. Janssen, J.A., 2016. Impact of physical exercise on endocrine aging. Front. Horm. Res. 47, 68–81. Joanisse, S., McKay, B.R., Nederveen, J.P., Scribbans, T.D., Gurd, B.J., Gillen, J.B., Gibala, M.J., Tarnopolsky, M., Parise, G., 2015. Satellite cell activity, without expansion, after nonhypertrophic stimuli. Am. J. Physiol. Regul. Integr. Comp. Physiol. 309, R1101–R1111. Joanisse, S., Nederveen, J.P., Snijders, T., McKay, B.R., Parise, G., 2017. Skeletal muscle regeneration, repair and Remodelling in aging: the importance of muscle stem cells and vascularization. Gerontology 63, 91–100. Kadi, F., 2000. Adaptation of human skeletal muscle to training and anabolic steroids. Acta Physiol. Scand. Suppl. 646, 1–52. Kadi, F., Schjerling, P., Andersen, L.L., Charifi, N., Madsen, J.L., Christensen, L.R., Andersen, J.L., 2004. The effects of heavy resistance training and detraining on satellite cells in human skeletal muscles. J. Physiol. 558, 1005–1012. Kadi, F., Ponsot, E., Piehl-Aulin, K., Mackey, A., Kjaer, M., Oskarsson, E., Holm, L., 2008. The effects of regular strength training on telomere length in human skeletal muscle. Med. Sci. Sports Exerc. 40, 82–87. Keefe, A.C., Lawson, J.A., Flygare, S.D., Fox, Z.D., Colasanto, M.P., Mathew, S.J., Yandell, M., Kardon, G., 2015. Muscle stem cells contribute to myofibres in sedentary adult mice. Nat. Commun. 6, 7087. Kim, Y., Triolo, M., Hood, D.A., 2017. Impact of aging and exercise on mitochondrial quality control in skeletal muscle. Oxid. Med. Cell. Longev. 2017, 3165396. Klausen, K., Andersen, L.B., Pelle, I., 1981. Adaptive changes in work capacity, skeletal muscle capillarization and enzyme levels during training and detraining. Acta Physiol. Scand. 113, 9–16. Konigsberg, U.R., Lipton, B.H., Konigsberg, I.R., 1975. The regenerative response of single mature muscle fibers isolated in vitro. Dev. Biol. 45, 260–275. Kosek, D.J., Kim, J.S., Petrella, J.K., Cross, J.M., Bamman, M.M., 2006. Efficacy of 3 days/wk resistance training on myofiber hypertrophy and myogenic mechanisms in young vs. older adults. J. Appl. Physiol. (1985) 101, 531–544. Kovacheva, E.L., Hikim, A.P., Shen, R., Sinha, I., Sinha-Hikim, I., 2010. Testosterone supplementation reverses sarcopenia in aging through regulation of myostatin, c-Jun NH2-terminal kinase, Notch, and Akt signaling pathways. Endocrinology 151, 628–638. La Colla, A., Pronsato, L., Milanesi, L., Vasconsuelo, A., 2015. 17beta-Estradiol and testosterone in sarcopenia: role of satellite cells. Ageing Res. Rev. 24, 166–177. Langley, B., Thomas, M., Bishop, A., Sharma, M., Gilmour, S., Kambadur, R., 2002. Myostatin inhibits myoblast differentiation by down-regulating MyoD expression. J. Biol. Chem. 277, 49831–49840. Larsson, L., Degens, H., Li, M., Salviati, L., Lee, Y.I., Thompson, W., Kirkland, J.L., Sandri, M., 2019. Sarcopenia: aging-related loss of muscle mass and function. Physiol. Rev. 99, 427–511. Latella, L., Dall’agnese, A., Boscolo, F.S., Nardoni, C., Cosentino, M., Lahm, A., Sacco, A., Puri, P.L., 2017. DNA damage signaling mediates the functional antagonism between replicative senescence and terminal muscle differentiation. Genes Dev. 31, 648–659. Lavasani, M., Robinson, A.R., Lu, A., Song, M., Feduska, J.M., Ahani, B., Tilstra, J.S., Feldman, C.H., Robbins, P.D., Niedernhofer, L.J., Huard, J., 2012. Muscle-derived stem/progenitor cell dysfunction limits healthspan and lifespan in a murine progeria model. Nat. Commun. 3, 608.

ARTICLE IN PRESS Satellite cells in skeletal muscle aging

37

Lee-Six, H., Obro, N.F., Shepherd, M.S., Grossmann, S., Dawson, K., Belmonte, M., Osborne, R.J., Huntly, B.J.P., Martincorena, I., Anderson, E., O’neill, L., Stratton, M.R., Laurenti, E., Green, A.R., Kent, D.G., Campbell, P.J., 2018. Population dynamics of normal human blood inferred from somatic mutations. Nature 561, 473–478. Lepper, C., Conway, S.J., Fan, C.M., 2009. Adult satellite cells and embryonic muscle progenitors have distinct genetic requirements. Nature 460, 627–631. Lepper, C., Partridge, T.A., Fan, C.M., 2011. An absolute requirement for Pax7-positive satellite cells in acute injury-induced skeletal muscle regeneration. Development 138, 3639–3646. Lindholm, M.E., Marabita, F., Gomez-Cabrero, D., Rundqvist, H., Ekstrom, T.J., Tegner, J., Sundberg, C.J., 2014. An integrative analysis reveals coordinated reprogramming of the epigenome and the transcriptome in human skeletal muscle after training. Epigenetics 9, 1557–1569. Lindholm, M.E., Giacomello, S., Werne Solnestam, B., Fischer, H., Huss, M., Kjellqvist, S., Sundberg, C.J., 2016. The impact of endurance training on human skeletal muscle memory, global isoform expression and novel transcripts. PLoS Genet. 12, e1006294. Liu, L., Cheung, T.H., Charville, G.W., Hurgo, B.M., Leavitt, T., Shih, J., Brunet, A., Rando, T.A., 2013. Chromatin modifications as determinants of muscle stem cell quiescence and chronological aging. Cell Rep. 4, 189–204. Lo Sardo, V., Ferguson, W., Erikson, G.A., Topol, E.J., Baldwin, K.K., Torkamani, A., 2017. Influence of donor age on induced pluripotent stem cells. Nat. Biotechnol. 35, 69–74. Lodato, M.A., Woodworth, M.B., Lee, S., Evrony, G.D., Mehta, B.K., Karger, A., Lee, S., Chittenden, T.W., D’gama, A.M., Cai, X., Luquette, L.J., Lee, E., Park, P.J., Walsh, C.A., 2015. Somatic mutation in single human neurons tracks developmental and transcriptional history. Science 350, 94–98. Lodato, M.A., Rodin, R.E., Bohrson, C.L., Coulter, M.E., Barton, A.R., Kwon, M., Sherman, M.A., Vitzthum, C.M., Luquette, L.J., Yandava, C.N., Yang, P., Chittenden, T.W., Hatem, N.E., Ryu, S.C., Woodworth, M.B., Park, P.J., Walsh, C.A., 2018. Aging and neurodegeneration are associated with increased mutations in single human neurons. Science 359, 555–559. Lopez, D.H., Fiol-Deroque, M.A., Noguera-Salva, M.A., Teres, S., Campana, F., Piotto, S., Castro, J.A., Mohaibes, R.J., Escriba, P.V., Busquets, X., 2013. 2-hydroxy arachidonic acid: a new non-steroidal anti-inflammatory drug. PLoS One 8, e72052. Lopez-Otin, C., Blasco, M.A., Partridge, L., Serrano, M., Kroemer, G., 2013. The hallmarks of aging. Cell 153, 1194–1217. Lundberg, T.R., Fernandez-Gonzalo, R., Gustafsson, T., Tesch, P.A., 2013. Aerobic exercise does not compromise muscle hypertrophy response to short-term resistance training. J. Appl. Physiol. (1985) 114, 81–89. Mackey, A.L., Kjaer, M., 2017. The breaking and making of healthy adult human skeletal muscle in vivo. Skelet. Muscle 7, 24. Mackey, A.L., Esmarck, B., Kadi, F., Koskinen, S.O., Kongsgaard, M., Sylvestersen, A., Hansen, J.J., Larsen, G., Kjaer, M., 2007. Enhanced satellite cell proliferation with resistance training in elderly men and women. Scand. J. Med. Sci. Sports 17, 34–42. Mackey, A.L., Andersen, L.L., Frandsen, U., Sjogaard, G., 2011. Strength training increases the size of the satellite cell pool in type I and II fibres of chronically painful trapezius muscle in females. J. Physiol. 589, 5503–5515. Mackey, A.L., Karlsen, A., Couppe, C., Mikkelsen, U.R., Nielsen, R.H., Magnusson, S.P., Kjaer, M., 2014. Differential satellite cell density of type I and II fibres with lifelong endurance running in old men. Acta Physiol (Oxf.) 210, 612–627.

ARTICLE IN PRESS 38

Irene Franco et al.

Martincorena, I., Raine, K.M., Gerstung, M., Dawson, K.J., Haase, K., Van Loo, P., Davies, H., Stratton, M.R., Campbell, P.J., 2018. Universal patterns of selection in Cancer and somatic tissues. Cell 173, 1823. Mauro, A., 1961. Satellite cell of skeletal muscle fibers. J. Biophys. Biochem. Cytol. 9, 493–495. Mayer, U., 2003. Integrins: redundant or important players in skeletal muscle? J. Biol. Chem. 278, 14587–14590. McCarthy, J.J., Mula, J., Miyazaki, M., Erfani, R., Garrison, K., Farooqui, A.B., Srikuea, R., Lawson, B.A., Grimes, B., Keller, C., Van Zant, G., Campbell, K.S., Esser, K.A., DuPont-Versteegden, E.E., Peterson, C.A., 2011. Effective fiber hypertrophy in satellite cell-depleted skeletal muscle. Development 138, 3657–3666. McCroskery, S., Thomas, M., Maxwell, L., Sharma, M., Kambadur, R., 2003. Myostatin negatively regulates satellite cell activation and self-renewal. J. Cell Biol. 162, 1135–1147. McKay, B.R., O’reilly, C.E., Phillips, S.M., Tarnopolsky, M.A., Parise, G., 2008. Co-expression of IGF-1 family members with myogenic regulatory factors following acute damaging muscle-lengthening contractions in humans. J. Physiol. 586, 5549–5560. McKay, B.R., De Lisio, M., Johnston, A.P., O’reilly, C.E., Phillips, S.M., Tarnopolsky, M.A., Parise, G., 2009. Association of interleukin-6 signalling with the muscle stem cell response following muscle-lengthening contractions in humans. PLoS One 4, e6027. McKay, B.R., Ogborn, D.I., Bellamy, L.M., Tarnopolsky, M.A., Parise, G., 2012. Myostatin is associated with age-related human muscle stem cell dysfunction. FASEB J. 26, 2509–2521. McKay, B.R., Ogborn, D.I., Baker, J.M., Toth, K.G., Tarnopolsky, M.A., Parise, G., 2013. Elevated SOCS3 and altered IL-6 signaling is associated with age-related human muscle stem cell dysfunction. Am. J. Physiol. Cell Physiol. 304, C717–C728. McPhee, J.S., French, D.P., Jackson, D., Nazroo, J., Pendleton, N., Degens, H., 2016. Physical activity in older age: perspectives for healthy ageing and frailty. Biogerontology 17, 567–580. McTiernan, A., Tworoger, S.S., Rajan, K.B., Yasui, Y., Sorenson, B., Ulrich, C.M., Chubak, J., Stanczyk, F.Z., Bowen, D., Irwin, M.L., Rudolph, R.E., Potter, J.D., Schwartz, R.S., 2004. Effect of exercise on serum androgens in postmenopausal women: a 12-month randomized clinical trial. Cancer Epidemiol. Biomarkers Prev. 13, 1099–1105. Mikkelsen, U.R., Langberg, H., Helmark, I.C., Skovgaard, D., Andersen, L.L., Kjaer, M., Mackey, A.L., 2009. Local NSAID infusion inhibits satellite cell proliferation in human skeletal muscle after eccentric exercise. J. Appl. Physiol. (1985) 107, 1600–1611. Milanesi, L., Russo De Boland, A., Boland, R., 2008. Expression and localization of estrogen receptor alpha in the C2C12 murine skeletal muscle cell line. J. Cell. Biochem. 104, 1254–1273. Mitchell, B.M., Meck, J.V., 2004. Short-duration spaceflight does not prolong QTc intervals in male astronauts. Am. J. Cardiol. 93, 1051–1052. Mitchell, P.O., Pavlath, G.K., 2001. A muscle precursor cell-dependent pathway contributes to muscle growth after atrophy. Am. J. Physiol. Cell Physiol. 281, C1706–C1715. Monteiro-Junior, R.S., De Tarso Maciel-Pinheiro, P., Da Matta Mello Portugal, E., Da Silva Figueiredo, L.F., Terra, R., Carneiro, L.S.F., Rodrigues, V.D., Nascimento, O.J.M., Deslandes, A.C., Laks, J., 2018. Effect of exercise on inflammatory profile of older persons: systematic review and meta-analyses. J. Phys. Act. Health 15, 64–71. Morley, J.E., Malmstrom, T.K., 2013. Frailty, sarcopenia, and hormones. Endocrinol. Metab. Clin. North Am. 42, 391–405.

ARTICLE IN PRESS Satellite cells in skeletal muscle aging

39

Moss, F.P., Leblond, C.P., 1970. Nature of dividing nuclei in skeletal muscle of growing rats. J. Cell Biol. 44, 459–462. Moss, F.P., Leblond, C.P., 1971. Satellite cells as the source of nuclei in muscles of growing rats. Anat. Rec. 170, 421–435. Murach, K.A., White, S.H., Wen, Y., Ho, A., DuPont-Versteegden, E.E., McCarthy, J.J., Peterson, C.A., 2017. Differential requirement for satellite cells during overload-induced muscle hypertrophy in growing versus mature mice. Skelet. Muscle 7, 14. Murach, K.A., Englund, D.A., DuPont-Versteegden, E.E., McCarthy, J.J., Peterson, C.A., 2018a. Myonuclear domain flexibility challenges rigid assumptions on satellite cell contribution to skeletal muscle Fiber hypertrophy. Front. Physiol. 9, 635. Murach, K.A., Fry, C.S., Kirby, T.J., Jackson, J.R., Lee, J.D., White, S.H., DuPontVersteegden, E.E., McCarthy, J.J., Peterson, C.A., 2018b. Starring or supporting role? Satellite cells and skeletal muscle fiber size regulation. Physiology (Bethesda) 33, 26–38. Murphy, M.M., Lawson, J.A., Mathew, S.J., Hutcheson, D.A., Kardon, G., 2011. Satellite cells, connective tissue fibroblasts and their interactions are crucial for muscle regeneration. Development 138, 3625–3637. Nederveen, J.P., Joanisse, S., Seguin, C.M., Bell, K.E., Baker, S.K., Phillips, S.M., Parise, G., 2015. The effect of exercise mode on the acute response of satellite cells in old men. Acta Physiol (Oxf.) 215, 177–190. Nicklas, B.J., Brinkley, T.E., 2009. Exercise training as a treatment for chronic inflammation in the elderly. Exerc. Sport Sci. Rev. 37, 165–170. Nederveen, J.P., Joanisse, S., Snijders, T., Thomas, A.C.Q., Kumbhare, D., Parise, G., 2018. The influence of capillarization on satellite cell pool expansion and activation following exercise-induced muscle damage in healthy young men. J. Physiol. 596 (6), 1063–1078. Olguin, H.C., Yang, Z., Tapscott, S.J., Olwin, B.B., 2007. Reciprocal inhibition between Pax7 and muscle regulatory factors modulates myogenic cell fate determination. J. Cell Biol. 177, 769–779. Olsson, K., Saini, A., Stromberg, A., Alam, S., Lilja, M., Rullman, E., Gustafsson, T., 2016. Evidence for vitamin D receptor expression and direct effects of 1alpha,25(OH)2D3 in human skeletal muscle precursor cells. Endocrinology 157, 98–111. O’reilly, C., McKay, B., Phillips, S., Tarnopolsky, M., Parise, G., 2008. Hepatocyte growth factor (HGF) and the satellite cell response following muscle lengthening contractions in humans. Muscle Nerve 38, 1434–1442. Parker, L., Caldow, M.K., Watts, R., Levinger, P., Cameron-Smith, D., Levinger, I., 2017. Age and sex differences in human skeletal muscle fibrosis markers and transforming growth factor-beta signaling. Eur. J. Appl. Physiol. 117, 1463–1472. Pawlikowski, B., Pulliam, C., Betta, N.D., Kardon, G., Olwin, B.B., 2015. Pervasive satellite cell contribution to uninjured adult muscle fibers. Skelet. Muscle 5, 42. Petrella, J.K., Kim, J.S., Cross, J.M., Kosek, D.J., Bamman, M.M., 2006. Efficacy of myonuclear addition may explain differential myofiber growth among resistance-trained young and older men and women. Am. J. Physiol. Endocrinol. Metab. 291, E937–E946. Philippou, A., Papageorgiou, E., Bogdanis, G., Halapas, A., Sourla, A., Maridaki, M., Pissimissis, N., Koutsilieris, M., 2009. Expression of IGF-1 isoforms after exercise-induced muscle damage in humans: characterization of the MGF E peptide actions in vitro. In Vivo 23, 567–575. Phillips, B.E., Williams, J.P., Gustafsson, T., Bouchard, C., Rankinen, T., Knudsen, S., Smith, K., Timmons, J.A., Atherton, P.J., 2013. Molecular networks of human muscle adaptation to exercise and age. PLoS Genet. 9, e1003389. Phillips, B.E., Williams, J.P., Greenhaff, P.L., Smith, K., Atherton, P.J., 2017. Physiological adaptations to resistance exercise as a function of age. JCI Insight 2, 1–16. Phu, S., Boersma, D., Duque, G., 2015. Exercise and sarcopenia. J. Clin. Densitom. 18, 488–492.

ARTICLE IN PRESS 40

Irene Franco et al.

Piasecki, M., Ireland, A., Piasecki, J., Stashuk, D.W., Swiecicka, A., Rutter, M.K., Jones, D.A., McPhee, J.S., 2018. Failure to expand the motor unit size to compensate for declining motor unit numbers distinguishes sarcopenic from non-sarcopenic older men. J. Physiol. 596, 1627–1637. Price, F.D., Von Maltzahn, J., Bentzinger, C.F., Dumont, N.A., Yin, H., Chang, N.C., Wilson, D.H., Frenette, J., Rudnicki, M.A., 2014. Inhibition of JAK-STAT signaling stimulates adult satellite cell function. Nat. Med. 20, 1174–1181. Pronsato, L., Boland, R., Milanesi, L., 2013. Non-classical localization of androgen receptor in the C2C12 skeletal muscle cell line. Arch. Biochem. Biophys. 530, 13–22. Pugh, J.K., Faulkner, S.H., Turner, M.C., Nimmo, M.A., 2018. Satellite cell response to concurrent resistance exercise and high-intensity interval training in sedentary, overweight/obese, middle-aged individuals. Eur. J. Appl. Physiol. 118, 225–238. Quarta, M., Brett, J.O., Dimarco, R., De Morree, A., Boutet, S.C., Chacon, R., Gibbons, M.C., Garcia, V.A., Su, J., Shrager, J.B., Heilshorn, S., Rando, T.A., 2016. An artificial niche preserves the quiescence of muscle stem cells and enhances their therapeutic efficacy. Nat. Biotechnol. 34, 752–759. Rae, D.E., Vignaud, A., Butler-Browne, G.S., Thornell, L.E., Sinclair-Smith, C., Derman, E.W., Lambert, M.I., Collins, M., 2010. Skeletal muscle telomere length in healthy, experienced, endurance runners. Eur. J. Appl. Physiol. 109, 323–330. Reznik, M., 1969. Thymidine-3H uptake by satellite cells of regenerating skeletal muscle. J. Cell Biol. 40, 568–571. Robinson, M.M., Dasari, S., Konopka, A.R., Johnson, M.L., Manjunatha, S., Esponda, R.R., Carter, R.E., Lanza, I.R., Nair, K.S., 2017. Enhanced protein translation underlies improved metabolic and physical adaptations to different exercise training modes in young and old humans. Cell Metab. 25, 581–592. Rodriguez, S.A., Grochova, D., McKenna, T., Borate, B., Trivedi, N.S., Erdos, M.R., Eriksson, M., 2016. Global genome splicing analysis reveals an increased number of alternatively spliced genes with aging. Aging Cell 15, 267–278. Rodriguez-Miguelez, P., Fernandez-Gonzalo, R., Almar, M., Mejias, Y., Rivas, A., De Paz, J.A., Cuevas, M.J., Gonzalez-Gallego, J., 2014. Role of Toll-like receptor 2 and 4 signaling pathways on the inflammatory response to resistance training in elderly subjects. Age 36, 9734. Rodriguez-Miguelez, P., Fernandez-Gonzalo, R., Collado, P.S., Almar, M., MartinezFlorez, S., De Paz, J.A., Gonzalez-Gallego, J., Cuevas, M.J., 2015. Whole-body vibration improves the anti-inflammatory status in elderly subjects through toll-like receptor 2 and 4 signaling pathways. Mech. Ageing Dev. 150, 12–19. Roth, S.M., Martel, G.F., Ivey, F.M., Lemmer, J.T., Tracy, B.L., Metter, E.J., Hurley, B.F., Rogers, M.A., 2001. Skeletal muscle satellite cell characteristics in young and older men and women after heavy resistance strength training. J. Gerontol. A Biol. Sci. Med. Sci. 56, B240–B247. Rozo, M., Li, L., Fan, C.M., 2016. Targeting beta1-integrin signaling enhances regeneration in aged and dystrophic muscle in mice. Nat. Med. 22, 889–896. Ryan, N.A., Zwetsloot, K.A., Westerkamp, L.M., Hickner, R.C., Pofahl, W.E., Gavin, T.P., 2006. Lower skeletal muscle capillarization and VEGF expression in aged vs. young men. J. Appl. Physiol. (1985) 100, 178–185. Sacco, A., Doyonnas, R., Kraft, P., Vitorovic, S., Blau, H.M., 2008. Self-renewal and expansion of single transplanted muscle stem cells. Nature 456, 502–506. Sacco, A., Mourkioti, F., Tran, R., Choi, J., Llewellyn, M., Kraft, P., Shkreli, M., Delp, S., Pomerantz, J.H., Artandi, S.E., Blau, H.M., 2010. Short telomeres and stem cell exhaustion model duchenne muscular dystrophy in mdx/mTR mice. Cell 143, 1059–1071. Sadeh, M., 1988. Effects of aging on skeletal muscle regeneration. J. Neurol. Sci. 87, 67–74.

ARTICLE IN PRESS Satellite cells in skeletal muscle aging

41

Saini, J., McPhee, J.S., Al-Dabbagh, S., Stewart, C.E., Al-Shanti, N., 2016. Regenerative function of immune system: modulation of muscle stem cells. Ageing Res. Rev. 27, 67–76. Sambasivan, R., Yao, R., Kissenpfennig, A., Van Wittenberghe, L., Paldi, A., GayraudMorel, B., Guenou, H., Malissen, B., Tajbakhsh, S., Galy, A., 2011. Pax7-expressing satellite cells are indispensable for adult skeletal muscle regeneration. Development 138, 3647–3656. Santilli, V., Bernetti, A., Mangone, M., Paoloni, M., 2014. Clinical definition of sarcopenia. Clin. Cases Miner. Bone Metab. 11, 177–180. Schaefer, L., 2018. Decoding fibrosis: mechanisms and translational aspects. Matrix Biol. 68-69, 1–7. Schiaffino, S., Bormioli, S.P., Aloisi, M., 1976. The fate of newly formed satellite cells during compensatory muscle hypertrophy. Virchows Arch. B Cell Pathol. 21, 113–118. Schiaffino, S., Pereira, M.G., Ciciliot, S., Rovere-Querini, P., 2017. Regulatory T cells and skeletal muscle regeneration. FEBS J. 284, 517–524. Schworer, S., Becker, F., Feller, C., Baig, A.H., Kober, U., Henze, H., Kraus, J.M., Xin, B., Lechel, A., Lipka, D.B., Varghese, C.S., Schmidt, M., Rohs, R., Aebersold, R., Medina, K.L., Kestler, H.A., Neri, F., Von Maltzahn, J., Tumpel, S., Rudolph, K.L., 2016. Epigenetic stress responses induce muscle stem-cell ageing by Hoxa9 developmental signals. Nature 540, 428–432. Seaborne, R.A., Strauss, J., Cocks, M., Shepherd, S., O’brien, T.D., Van Someren, K.A., Bell, P.G., Murgatroyd, C., Morton, J.P., Stewart, C.E., Sharples, A.P., 2018. Human skeletal muscle possesses an epigenetic memory of hypertrophy. Sci. Rep. 8, 1898. Seale, P., Sabourin, L.A., Girgis-Gabardo, A., Mansouri, A., Gruss, P., Rudnicki, M.A., 2000. Pax7 is required for the specification of myogenic satellite cells. Cell 102, 777–786. Sen, P., Shah, P.P., Nativio, R., Berger, S.L., 2016. Epigenetic mechanisms of longevity and aging. Cell 166, 822–839. Seo, D.Y., Lee, S.R., Kim, N., Ko, K.S., Rhee, B.D., Han, J., 2016. Age-related changes in skeletal muscle mitochondria: the role of exercise. Integr. Med. Res. 5, 182–186. Serra, C., Tangherlini, F., Rudy, S., Lee, D., Toraldo, G., Sandor, N.L., Zhang, A., Jasuja, R., Bhasin, S., 2013. Testosterone improves the regeneration of old and young mouse skeletal muscle. J. Gerontol. A Biol. Sci. Med. Sci. 68, 17–26. Sharples, A.P., Hughes, D.C., Deane, C.S., Saini, A., Selman, C., Stewart, C.E., 2015. Longevity and skeletal muscle mass: the role of IGF signalling, the sirtuins, dietary restriction and protein intake. Aging Cell 14, 511–523. Sharples, A.P., Stewart, C.E., Seaborne, R.A., 2016. Does skeletal muscle have an ’epi’-memory? The role of epigenetics in nutritional programming, metabolic disease, aging and exercise. Aging Cell 15, 603–616. Shefer, G., Van De Mark, D.P., Richardson, J.B., Yablonka-Reuveni, Z., 2006. Satellite-cell pool size does matter: defining the myogenic potency of aging skeletal muscle. Dev. Biol. 294, 50–66. Sinha, M., Jang, Y.C., Oh, J., Khong, D., Wu, E.Y., Manohar, R., Miller, C., Regalado, S.G., Loffredo, F.S., Pancoast, J.R., Hirshman, M.F., Lebowitz, J., Shadrach, J.L., Cerletti, M., Kim, M.J., Serwold, T., Goodyear, L.J., Rosner, B., Lee, R.T., Wagers, A.J., 2014. Restoring systemic GDF11 levels reverses age-related dysfunction in mouse skeletal muscle. Science 344, 649–652. Sinha-Hikim, I., Roth, S.M., Lee, M.I., Bhasin, S., 2003. Testosterone-induced muscle hypertrophy is associated with an increase in satellite cell number in healthy, young men. Am. J. Physiol. Endocrinol. Metab. 285, E197–E205. Sinha-Hikim, I., Cornford, M., Gaytan, H., Lee, M.L., Bhasin, S., 2006. Effects of testosterone supplementation on skeletal muscle fiber hypertrophy and satellite cells in community-dwelling older men. J. Clin. Endocrinol. Metab. 91, 3024–3033.

ARTICLE IN PRESS 42

Irene Franco et al.

Snijders, T., Verdijk, L.B., Beelen, M., McKay, B.R., Parise, G., Kadi, F., Van Loon, L.J., 2012. A single bout of exercise activates skeletal muscle satellite cells during subsequent overnight recovery. Exp. Physiol. 97, 762–773. Snijders, T., Verdijk, L.B., Smeets, J.S., McKay, B.R., Senden, J.M., Hartgens, F., Parise, G., Greenhaff, P., Van Loon, L.J., 2014a. The skeletal muscle satellite cell response to a single bout of resistance-type exercise is delayed with aging in men. Age (Dordr.) 36, 9699. Snijders, T., Wall, B.T., Dirks, M.L., Senden, J.M., Hartgens, F., Dolmans, J., Losen, M., Verdijk, L.B., Van Loon, L.J., 2014b. Muscle disuse atrophy is not accompanied by changes in skeletal muscle satellite cell content. Clin. Sci. (Lond.) 126, 557–566. Snijders, T., Nederveen, J.P., McKay, B.R., Joanisse, S., Verdijk, L.B., Van Loon, L.J., Parise, G., 2015. Satellite cells in human skeletal muscle plasticity. Front. Physiol. 6, 283. Snijders, T., Nederveen, J.P., Joanisse, S., Leenders, M., Verdijk, L.B., Van Loon, L.J., Parise, G., 2017. Muscle fibre capillarization is a critical factor in muscle fibre hypertrophy during resistance exercise training in older men. J. Cachexia. Sarcopenia Muscle 8, 267–276. Snow, M.H., 1977. Myogenic cell formation in regenerating rat skeletal muscle injured by mincing. II. An autoradiographic study. Anat. Rec. 188, 201–217. Sousa-Victor, P., Gutarra, S., Garcia-Prat, L., Rodriguez-Ubreva, J., Ortet, L., RuizBonilla, V., Jardi, M., Ballestar, E., Gonzalez, S., Serrano, A.L., Perdiguero, E., Munoz-Canoves, P., 2014. Geriatric muscle stem cells switch reversible quiescence into senescence. Nature 506, 316–321. Suetta, C., Hvid, L.G., Justesen, L., Christensen, U., Neergaard, K., Simonsen, L., Ortenblad, N., Magnusson, S.P., Kjaer, M., Aagaard, P., 2009. Effects of aging on human skeletal muscle after immobilization and retraining. J. Appl. Physiol. (1985) 107, 1172–1180. Suetta, C., Frandsen, U., Mackey, A.L., Jensen, L., Hvid, L.G., Bayer, M.L., Petersson, S.J., Schroder, H.D., Andersen, J.L., Aagaard, P., Schjerling, P., Kjaer, M., 2013. Ageing is associated with diminished muscle re-growth and myogenic precursor cell expansion early after immobility-induced atrophy in human skeletal muscle. J. Physiol. 591, 3789–3804. Thirupathi, A., Pinho, R.A., 2018. Effects of reactive oxygen species and interplay of antioxidants during physical exercise in skeletal muscles. J. Physiol. Biochem. 74, 359–367. Thomas, A., Bunyan, K., Tiidus, P.M., 2010. Oestrogen receptor-alpha activation augments post-exercise myoblast proliferation. Acta Physiol (Oxf.) 198, 81–89. Tichy, E.D., Sidibe, D.K., Tierney, M.T., Stec, M.J., Sharifi-Sanjani, M., Hosalkar, H., Mubarak, S., Johnson, F.B., Sacco, A., Mourkioti, F., 2017. Single stem cell imaging and analysis reveals telomere length differences in diseased human and mouse skeletal muscles. Stem Cell Rep. 9, 1328–1341. Tichy, E.D., Sidibe, D.K., Greer, C.D., Oyster, N.M., Rompolas, P., Rosenthal, N.A., Blau, H.M., Mourkioti, F., 2018. A robust Pax7egfp mouse that enables the visualization of dynamic behaviors of muscle stem cells. Skelet. Muscle 8, 27. Tierney, M.T., Sacco, A., 2016. Satellite cell heterogeneity in skeletal muscle homeostasis. Trends Cell Biol. 26, 434–444. Tierney, M.T., Aydogdu, T., Sala, D., Malecova, B., Gatto, S., Puri, P.L., Latella, L., Sacco, A., 2014. STAT3 signaling controls satellite cell expansion and skeletal muscle repair. Nat. Med. 20, 1182–1186. Tierney, M.T., Stec, M.J., Rulands, S., Simons, B.D., Sacco, A., 2018. Muscle stem cells exhibit distinct clonal dynamics in response to tissue repair and homeostatic aging. Cell Stem Cell 22. 119-127.e3. Timmons, J.A., Jansson, E., Fischer, H., Gustafsson, T., Greenhaff, P.L., Ridden, J., Rachman, J., Sundberg, C.J., 2005. Modulation of extracellular matrix genes reflects the magnitude of physiological adaptation to aerobic exercise training in humans. BMC Biol. 3, 19.

ARTICLE IN PRESS Satellite cells in skeletal muscle aging

43

Toth, K.G., McKay, B.R., De Lisio, M., Little, J.P., Tarnopolsky, M.A., Parise, G., 2011. IL-6 induced STAT3 signalling is associated with the proliferation of human muscle satellite cells following acute muscle damage. PLoS One 6, e17392. Vahidi Ferdousi, L., Rocheteau, P., Chayot, R., Montagne, B., Chaker, Z., Flamant, P., Tajbakhsh, S., Ricchetti, M., 2014. More efficient repair of DNA double-strand breaks in skeletal muscle stem cells compared to their committed progeny. Stem Cell Res. 13, 492–507. Vasconsuelo, A., Milanesi, L., Boland, R., 2013. Actions of 17beta-estradiol and testosterone in the mitochondria and their implications in aging. Ageing Res. Rev. 12, 907–917. Velders, M., Diel, P., 2013. How sex hormones promote skeletal muscle regeneration. Sports Med. 43, 1089–1100. Velders, M., Schleipen, B., Fritzemeier, K.H., Zierau, O., Diel, P., 2012. Selective estrogen receptor-beta activation stimulates skeletal muscle growth and regeneration. FASEB J. 26, 1909–1920. Venturelli, M., Morgan, G.R., Donato, A.J., Reese, V., Bottura, R., Tarperi, C., Milanese, C., Schena, F., Reggiani, C., Naro, F., Cawthon, R.M., Richardson, R.S., 2014. Cellular aging of skeletal muscle: telomeric and free radical evidence that physical inactivity is responsible and not age. Clin. Sci. (Lond.) 127, 415–421. Verdijk, L.B., Koopman, R., Schaart, G., Meijer, K., Savelberg, H.H., Van Loon, L.J., 2007. Satellite cell content is specifically reduced in type II skeletal muscle fibers in the elderly. Am. J. Physiol. Endocrinol. Metab. 292, E151–E157. Verdijk, L.B., Gleeson, B.G., Jonkers, R.A., Meijer, K., Savelberg, H.H., Dendale, P., Van Loon, L.J., 2009. Skeletal muscle hypertrophy following resistance training is accompanied by a fiber type-specific increase in satellite cell content in elderly men. J. Gerontol. A Biol. Sci. Med. Sci. 64, 332–339. Verdijk, L.B., Dirks, M.L., Snijders, T., Prompers, J.J., Beelen, M., Jonkers, R.A., Thijssen, D.H., Hopman, M.T., Van Loon, L.J., 2012. Reduced satellite cell numbers with spinal cord injury and aging in humans. Med. Sci. Sports Exerc. 44, 2322–2330. Verma, M., Asakura, Y., Murakonda, B.S.R., Pengo, T., Latroche, C., Chazaud, B., Mcloon, L.K., Asakura, A., 2018. Muscle satellite cell cross-talk with a vascular niche maintains quiescence via VEGF and notch signaling. Cell Stem Cell 23. 530-543.e9. Verney, J., Kadi, F., Charifi, N., Feasson, L., Saafi, M.A., Castells, J., Piehl-Aulin, K., Denis, C., 2008. Effects of combined lower body endurance and upper body resistance training on the satellite cell pool in elderly subjects. Muscle Nerve 38, 1147–1154. Walker, D.K., Fry, C.S., Drummond, M.J., Dickinson, J.M., Timmerman, K.L., Gundermann, D.M., Jennings, K., Volpi, E., Rasmussen, B.B., 2012. PAX7 + satellite cells in young and older adults following resistance exercise. Muscle Nerve 46, 51–59. Wall, B.T., Dirks, M.L., Van Loon, L.J., 2013. Skeletal muscle atrophy during short-term disuse: implications for age-related sarcopenia. Ageing Res. Rev. 12, 898–906. Wen, Y., Bi, P., Liu, W., Asakura, A., Keller, C., Kuang, S., 2012. Constitutive notch activation upregulates Pax7 and promotes the self-renewal of skeletal muscle satellite cells. Mol. Cell. Biol. 32, 2300–2311. Wernbom, M., Apro, W., Paulsen, G., Nilsen, T.S., Blomstrand, E., Raastad, T., 2013. Acute low-load resistance exercise with and without blood flow restriction increased protein signalling and number of satellite cells in human skeletal muscle. Eur. J. Appl. Physiol. 113, 2953–2965. Williamson, D.L., Raue, U., Slivka, D.R., Trappe, S., 2010. Resistance exercise, skeletal muscle FOXO3A, and 85-year-old women. J. Gerontol. A Biol. Sci. Med. Sci. 65, 335–343. Yaffe, D., 1969. Cellular aspects of muscle differentiation in vitro. Curr. Top. Dev. Biol. 4, 37–77.

ARTICLE IN PRESS 44

Irene Franco et al.

Yamamoto, M., Legendre, N.P., Biswas, A.A., Lawton, A., Yamamoto, S., Tajbakhsh, S., Kardon, G., Goldhamer, D.J., 2018. Loss of MyoD and Myf5 in skeletal muscle stem cells results in altered myogenic programming and failed regeneration. Stem Cell Rep. 10, 956–969. Zammit, P.S., Golding, J.P., Nagata, Y., Hudon, V., Partridge, T.A., Beauchamp, J.R., 2004. Muscle satellite cells adopt divergent fates: a mechanism for self-renewal? J. Cell Biol. 166, 347–357. Zhang, L., Vijg, J., 2018. Somatic mutagenesis in mammals and its implications for human disease and aging. Annu. Rev. Genet. 52, 397–419. Zhou, Y., Lovell, D., Bethea, M., Yoseph, B., Poteracki, J., Soker, S., Criswell, T., 2017. (*) The impact of age on skeletal muscle progenitor cell survival and fate after injury. Tissue Eng. Part C Methods 23, 1012–1021.