The role of satellite cells in muscle hypertrophy

Abstract

Issue Title: European Muscle Conference 2013 - "Muscle physiology: move to translation", a special issue handled by Coen A.C. Ottenheijm, Richard T. Jaspers, Rob C.I. Wüst and Jolanda van der Velden

The role of satellite cells in muscle hypertrophy has long been a debated issue. In the late 1980s it was shown that proteins remain close to the myonucleus responsible for its synthesis, giving rise to the idea of a nuclear domain. This, together with the observation that during various models of muscle hypertrophy there is an activation of the muscle stem cells, i.e. satellite cells, lead to the idea that satellite cell activation is required for muscle hypertrophy. Thus, satellite cells are not only responsible for muscle repair and regeneration, but also for hypertrophic growth. Further support for this line of thinking was obtained after studies showing that irradiation of skeletal muscle, and therefore elimination of all satellite cells, completely prevented overload-induced hypertrophy. Recently however, using different transgenic approaches, it has become clear that muscle hypertrophy can occur without a contribution of satellite cells, even though in most situations of muscle hypertrophy satellite cells are activated. In this review we will discuss the contribution of satellite cells, and other muscle-resident stem cells, to muscle hypertrophy both in mice as well as in humans.[PUBLICATION ABSTRACT]

J Muscle Res Cell Motil (2014) 35:310 DOI 10.1007/s10974-014-9376-y

REVIEW

The role of satellite cells in muscle hypertrophy

Bert Blaauw Carlo Reggiani

Received: 16 December 2013 / Accepted: 25 January 2014 / Published online: 7 February 2014 Springer International Publishing Switzerland 2014

Abstract The role of satellite cells in muscle hypertrophy has long been a debated issue. In the late 1980s it was shown that proteins remain close to the myonucleus responsible for its synthesis, giving rise to the idea of a nuclear domain. This, together with the observation that during various models of muscle hypertrophy there is an activation of the muscle stem cells, i.e. satellite cells, lead to the idea that satellite cell activation is required for muscle hypertrophy. Thus, satellite cells are not only responsible for muscle repair and regeneration, but also for hypertrophic growth. Further support for this line of thinking was obtained after studies showing that irradiation of skeletal muscle, and therefore elimination of all satellite cells, completely prevented overload-induced hypertrophy. Recently however, using different transgenic approaches, it has become clear that muscle hypertrophy can occur without a contribution of satellite cells, even though in most situations of muscle hypertrophy satellite cells are activated. In this review we will discuss the contribution of satellite cells, and other muscle-resident stem cells, to muscle hypertrophy both in mice as well as in humans.

Keywords Hypertrophy Satellite cells Exercise

Skeletal muscle

Introduction

A remarkable feature of skeletal muscles is their ability to adapt to new functional demands. This adaptation is the mechanism exploited by all training protocols, leading either to hypertrophy and increased strength or to improved endurance.

Muscle mass is a major determinant of muscle strength and is not stable; there is a continuous process of production and subsequent degradation of muscle proteins and the balance between protein synthesis and degradation determines whether a net gain (hypertrophy) or a loss (atrophy) of muscle mass occurs. The modulation of protein synthesis and degradation is the focus of intense research, and many important signals, pathways and cellular adaptations are currently being elucidated, see for a recent review (Blaauw et al. 2013).

The increase in muscle proteins during muscle hyper-trophy can be achieved either by increasing RNA and protein synthesis from the existing nuclei or maintaining the same level of RNA and protein synthesis from each nucleus and adding new nuclei to the bers. Since the adult muscle ber nuclei (myonuclei) are unable to divide, the new nuclei, which are incorporated by the ber, originate from outside the ber. Satellite cells are the major donors of new nuclei, being myogenic precursor cells, important for muscle development, for muscle regeneration and possibly also for muscle hypertrophy in response to exercise, training and hormonal stimulation (Montarras et al. 2013). The nuclei of the satellite cells are situated beneath the basal lamina, but in contrast to proper myonuclei they

B. Blaauw (&)

Venetian Institute of Molecular Medicine (VIMM), Via Orus 2, 35129 Padua, Italye-mail: [email protected]; [email protected]

B. Blaauw C. Reggiani (&)

Department of Biomedical Sciences, University of Padova, Via Marzolo 3, 35129 Padua, Italye-mail: [email protected]

C. ReggianiCNR Institute of Neuroscience, Padua, Italy

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are located outside the sarcolemma. Based on their peculiar location between the sarcolemma and the basal membrane, satellite cells can be identied either with electron microscopy (Mauro 1961) or with immunostaining of marker proteins of the sarcolemma and basal membrane. In addition several molecular markers specic to satellite cells are used to identify them in immunohistochemistry: transcription factors, such as Pax7, or surface membrane proteins, such as N-CAM, M-cadherin and CD34A1 are among the most popular and a complete updated list is reported by Yin et al. (2013).

Satellite cells contribute to muscle ber growth during development by providing new myonuclei. In neonatal rodent muscles, satellite cells represents up to 3035 % of the total number of nuclei along muscle bers and this proportion decreases while the number of myonuclei increases during the rst weeks of postnatal development (Schultz 1996; White et al. 2010). Accordingly, during this developmental phase, the nuclear domain size, i.e. the amount of cytoplasm that each myonucleus can control, increases less than muscle ber volume due to the effect of new myonuclei addition. In adult life, during development of muscle hypertrophy, the growing myobers may well need additional myonuclei, since the myonuclear domain size is regarded as virtually constant (Allen et al. 1999). There are clear demonstrations that activation of satellite cells occurs during overload-induced muscle hypertrophy (Schiafno et al. 1972, 1976) and, actually, precede the hypertrophic growth (Bruusgaard et al. 2010). Resistance training in humans has been proven to be very effective in activating satellite cells and increasing their density (Mackey et al. 2009). Hypertrophic growth induced by humoral/hormonal stimuli, such as IGF-1 and androgens, also induces satellite cell activation [see for a discussion (Yin et al. 2013)]. In contrast to this body of evidence, it has been shown that hypertrophy in some cases can occur without myonuclei addition and satellite cell activation (Amthor et al. 2009; Blaauw et al. 2009; McCarthy and Esser 2007; OConnor and Pavlath 2007). This issue, which was the subject of a Point/Counterpoint discussion on the Journal of Applied Physiology in 2007 (McCarthy and Esser 2007; OConnor and Pavlath 2007), is still debated and many studies have been recently published, aiming to dene whether hypertrophy requires the contribution of satellite cells and which are the signals that trigger satellite cell activation. In addition, another important and related question is still open, namely whether other stem cell populations, together with satellite cells, are activated during the hypertrophic response to overload or hormonal stimuli. The present review aims to summarize and discuss some of the most recent results on these topics (Figs. 1, 2).

mTORC1

Mechanical stress BMP

SC

?

SC

Myostatin

Musclehypertrophy Akt

SRF SC

IL4, IL6

IL4, IL6 Macrophages

Mesangioblasts

IGF-1

Fig. 1 Signaling pathways involved in the activation of satellite cells during muscle hypertrophy. Several extracellular signals like mechanical stress, cytokines or hormones are potentially involved in the activation of satellite cells in various models of hypertrophy and during exercise

Fig. 2 Effects of a single exercise session (a) and training (b) on satellite cells in human muscles. a Eight individuals performed a single heavy session of resistance training with one leg, while contralateral leg served as a control. Biopsy samples were taken from both legs (vastus lateralis) before and 2, 4, 8 days after exercise: means and SE, asterisk statistical signicance. Redrawn from (Crameri et al. 2004). b Eight individuals performed a 16-week resistance training for lower limb muscles, while seven other untrained individuals served as a control. Biopsy samples were collected from vastus lateralis before and after 4, 8, 16 weeks. The biopsy at 8 weeks was skipped in control untrained subjects: means and SE, Asterisk statistical signicance. Redrawn from (Olsen et al. 2006)

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Satellite cells, muscle homeostasis and hypertrophy

The structural integrity of muscle bers is required for the maintenance of the quiescent state of the satellite cell. Indeed, injection of Marcaine, leading to a rapid degradation of the muscle bers without degrading the basal lamina, immediately activates satellite cells. It is possible that in a similar manner, the presence of satellite cells in their niche is required for the maintenance of muscle ber homeostasis. Ablation of all satellite cells using diphtheria toxin leads to an immediate reduction in muscle size (Sambasivan et al. 2011) or even death (Lepper et al. 2011) as Pax7 is also required in central nervous system. This differential response between the two studies can potentially be explained by different levels of Cre-expression as reviewed very nicely by Relaix and Zammit (2012). In another example it was shown that conditional ablation of the miRNA processing enzyme Dicer in the satellite cells (Cheung et al. 2012) leads to a small but signicant reduction of muscle size over longer periods, suggesting that the presence of satellite cells is required to maintain adult muscle mass. These results can be explained either by the fact that satellite cells occasionally need to fuse to the muscle bers in order for muscles to maintain their mass or that satellite cells release factors which impinge positively on the protein synthesis rate. So far, however, no obvious candidates have emerged which could play this paracrine role in the maintenance of the mass of muscle bers, following secretion by the satellite cell.

In the last few years numerous reports have discussed the role of satellite cells in adult muscle, in relation with muscle hypertrophy. Initial reports in the late 90s showed that overload hypertrophy was severely blunted after gamma-irradiation (Rosenblatt et al. 1994) and this reduced hyper-trophic response was suggested to be due to the complete ablation of satellite cells. This, together with the fact that the protein synthesized under the control of a single myonucleus are conned in a restricted area of the ber, i.e. the myonu-clear domain (Pavlath et al. 1989), and that the amount of cytoplasm surrounding each myonucleus in adult muscle bers is relatively constant (Allen et al. 1999), generated the theory that satellite cells are required for muscle hypertrophy in order to keep the myonuclear domain constant. Nicely correlating with the contribution of satellite cells to the hypertrophic response is the fact that satellite cells proliferate and are incorporated into growing muscle bers during muscle hypertrophy (Bruusgaard et al. 2010; Schiafno et al. 1976). The role of satellite cells as donors of myonuclei had been rstly shown in a pioneer study by Schiafno et al. (1972), in which compensatory hypertrophy was induced in the rat EDL by agonist removal. The radioactive labeling determined by an injection of 3H-thymidine at different times after the interventions was detectable rstly both in satellite cell nuclei and true myonuclei and later only in

myonuclei. This indicated that newly formed satellite cells during the initial phase of hypertrophy were later incorporated into muscle bers. Bruusgaard et al. (2010) designed an elegant approach in which the number of myonuclei in a given ber was counted at different moments during progression of overload hypertrophy, and observed that the incorporation of new myonuclei is an event which precedes the subsequent ber growth.

The signals triggering satellite cell activation in relation to mechanical overload are still unclear. Minimal ber damage represents one possibility, but a mechanical sensitivity of the satellite cells to load inside their niche is an interesting alternative. Actually, a recent paper has shown that satellite cells express collagen VI, at variance of adult myobers and myoblasts (Urciuolo et al. 2013), and collagen VI contributes to establish a three-dimensional support with specic mechanical properties important for satellite cell activation, possibly mediated by integrins (Cosgrove et al. 2009). Actually, genetic ablation of collagen VI in knock out mice strongly impairs the ability of satellite cells to respond to stimuli suitable for their activation (Urciuolo et al. 2013). In line with the idea that hormonal/humoral signals are important for the activation of satellite cells, several studies have shown a relation between myonuclei accumulation and hypertrophic growth induced by anabolic compounds such as androgens (Chen et al. 2005; Egner et al. 2013; Kadi et al. 1999) and IGF-1 (Barton-Davis et al. 1999; Musaro et al. 2001; Musaro et al. 1999). Also in these models of hypertrophy, satellite cells are activated and appear as the most likely donors of myonuclei.

More recently, however, the view that myonuclear domain is constant as previously assumed (Allen et al. 1999) has been challenged. Firstly, during postnatal growth the myonuclear domain increases signicantly starting from p21, as the growth of the bers continues at the same rate as before p21 yet no increase in the number of myonuclei occurs (White et al. 2010). Secondly, in contrast to the results obtained by Allen et al., a lack of a quantitative link between myonuclei number and ber size was found in adult murine muscle bers (Bruusgaard et al. 2006). Finally, several transgenic models of muscle hypertrophy show a signicant increase in muscle mass, which is not accompanied by an increase in myonuclei (Amthor et al. 2009; Blaauw et al. 2009; Lee et al. 2012; Raffaello et al. 2010).

Satellite cell activation and muscle function

While it can be generally accepted that in many models of muscle hypertrophy satellite cell activation and incorporation occurs, no nal conclusion has been reached yet on whether or not this process is absolutely required. Interestingly, some animal models of muscle hypertrophy

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suggest that if increases in ber size are not accompanied by an increase in myonuclei, functional properties are compromised. Over-expression of the oncogene c-ski has shown to lead to a signicant hypertrophy without incorporation of new myonuclei and with a parallel decrease in specic force (Bruusgaard et al. 2005). Similarly, mice lacking myostatin show a major muscle hypertrophy and hyperplasia which is not accompanied by incorporation of satellite cells (Amthor et al. 2009; Sartori et al. 2009). Interestingly, also in this case muscle force did not increase in the enlarged hypertrophic muscles leading to a signi-cant reduction of specic muscle force (Amthor et al. 2007). Despite these earlier results linking the myonuclear domain to force production, more recent evidence suggests that the myonuclear domain does not need to remain constant for increases in force production. Using transgenic animals in which 90 % of all satellite cells are ablated, overload-induced hypertrophy was not compromised and there was no drop in specic tension measured in skinned bers (McCarthy et al. 2011). The hyperplasia which normally occurs during overload after synergist ablation was strongly reduced, suggesting that satellite cells are required for the formation of new bers (McCarthy et al. 2011). Surprisingly, despite the fact that almost all satellite cells were ablated, there was still proliferation as 6 % of the hypertrophic ber still showed BrdU-positive myonuclei. Considering the presence of other stem cells in skeletal muscle, it is tempting to assume that other muscle-resident stem cells contribute to muscle hypertrophy. Indeed, it has been shown that if stem cells are removed from their niche, but the niche remains intact, other stem cells can populate them and be reprogrammed to perform some of the tasks normally performed by the resident stem cell (Losick et al. 2011). Giving further support to the hypothesis that satellite cell activation is not required for a functional hypertrophy are the results found in a transgenic mouse model in which Akt is activated in skeletal muscle (Blaauw et al. 2009). In this model, activation of Akt leads to a rapid, functional hypertrophy, but without incorporation of BrdU-positive nuclei into the growing bers. These results clearly show the capacity of adult muscle bers to support elevated levels of protein synthesis, with ber size increasing by at least 50 %, without the need of new myonuclei. Both of these studies however evaluated muscle function at only relatively short time periods after the induction of hypertrophy, therefore making it hard to speculate about the effects over longer periods.

Myostatin, hypertrophy and satellite cells

As mentioned in the previous paragraph, one of the major regulators of skeletal muscle mass is the negative regulator

myostatin. Despite the fact that mice lacking myostatin show no increased activation of satellite cells, there are various reports linking myostatin signaling to the activation of satellite cells. Considering this dual role in regulating both muscle ber growth as well as directly determining the activation of satellite cells, we will discuss the role of myostatin in more detail and potentially linking it to exercise.

The myostatin-smad2/3 pathway is a major negative regulator of adult skeletal muscle mass. It is well established that mice lacking myostatin or treatment with animals with the soluble activin receptor, and showing therefore a reduced myostatin-dependent signaling, show a rapid and signicant muscle hypertrophy. Furthermore, exercise is known to reduce myostatin signaling (Raue et al. 2006) potentially playing a role in the growth of muscle bers after exercise. Besides this direct effect on ber growth, various reports have shown that myostatin is also a strong repressor of satellite cell activation and myoblast fusion (Langley et al. 2002; Thomas et al. 2000). Indeed, animals lacking myostatin show an increase in muscle mass, which is mainly due to a very pronounced hyperplasia (McPherron et al. 1997). This increased number of bers is very likely an event which occurs during early stages of development, since treatment of adult mice with antibodies for myostatin does not affect ber number, but only increases ber size (Bogdanovich et al. 2002). When examining the effects of myostatin inhibition on satellite cell activation in the adult there are some contrasting reports. Considering the inhibitory effect of myostatin on satellite cell activation one would expect an increase in the number of satellite cells in knock out animals. This, however, does not seem to be completely clear, as some studies demonstrate an increase in the number of satellite cells (McCroskery et al. 2003), some dont nd any difference (Wang and McPherron 2012), and others nd a decrease (Amthor et al. 2009). Treatment of adult wild type animals with the soluble activin receptor 2B (sActRIIB) leads to a rapid hypertrophy which is followed by a mild incorporation of BrdU-positive myonuclei in a dose-dependent manner. The number of nuclei incorporated is 510 fold lower than that seen after overload hypertrophy or during postnatal growth (Blaauw et al. 2009; Wang and McPherron 2012), making it plausible that this minimal contribution of new myonuclei is not a prerequisite for myostatin-related muscle growth. It is however possible that this minor addition of new myonuclei is one of the mechanisms involved in exercise-related satellite cell activation. Indeed, physiological stimuli leading to muscle growth and satellite cell proliferation, like resistance exercise, are accompanied by a reduction in myostatin signaling (MacKenzie et al. 2013).

Recently it has been shown that, in adult mice, the muscle growth seen after myostatin inhibition is due to an

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increase in BMP-signaling by recruitment of the transcription factor smad4 which is also required for myostatin-dependent signaling (Sartori et al. 2013). Similar mechanisms seem to be active in satellite cells, as BMP signaling stimulates proliferation of satellite cells and slows down the fusion into adult bers, which is only stimulated once levels of Noggin increase, leading to an increase in differentiation (Ono et al. 2011).

Hypertrophy and other cell types

It has been clearly shown that satellite cells are required for a proper regeneration of adult skeletal muscle, showing an absolute requirement for satellite cells, and not other stem cells, in the regeneration process (Ciciliot and Schiafno 2010). Hypertrophy on the other hand is not prevented, as a virtually complete removal of satellite cells does not compromise the level of hypertrophy induced by synergist ablation. A contribution of other circulating cells, to regeneration, and potentially also to hypertrophic growth of adult muscle has been suggested, even though clear demonstrations are still missing. Different interstitial stem cells, like PW1-positive interstitial cells, mesangioblasts, or pericytes, have shown myogenic potential and could therefore contribute to muscle growth. Indeed, overload-induced hypertrophy is completely abrogated in mice lacking the urokinase-type plasminogen activator or after treatment with clodronate liposomes, both reducing inltration of macrophages (DiPasquale et al. 2007). The authors of this paper also nd a signicant reduction of BrdU-incorporation, but do not identify which cells are involved. Very likely part of the reduction in BrdU-positive cells is due to a lack of proliferation of satellite cells as macrophages secrete IL-6 and IL-4 which are known to stimulate satellite cell proliferation and fusion respectively. Indeed, overload hypertrophy is compromised in muscles from mice lacking the transcription factor SRF (Serum Response Factor) which is specically expressed in the muscle bers and contributes to control transcription of many muscle specic genes. The negative impact of SRF knock out on muscle hypertrophy is attributed to a reduction in Il-6, IL-4 and Cox2 produced by the myobers (Guerci et al. 2012). Overexpression of either IL-4 or Cox2, and therefore stimulation of fusion of satellite cells to growing muscle bers, restored the decit in overload-induced hypertrophy observed in SRF knock out mice. In addition, muscle hypertrophy is blunted in mice lacking IL-6, and this reduced growth was linked to a compromised activation and fusion of satellite cells (Serrano et al. 2008). Giving further strength to the role of interleukins in the regulation of muscle mass, is the fact that overexpression of IL-6 leads to a loss of muscle mass (Tsujinaka et al.

1995). Interestingly, this loss of muscle mass seen during cancer cachexia is linked to a strong upregulation of Pax7 in satellite cells which lead to a compromised muscle repair (He et al. 2013). Furthermore, reduction of Pax7 levels during cancer cachexia is sufcient to prevent muscle wasting, underling the role of satellite cells in preserving muscle mass. While it is tempting to link the positive effect of inammation on muscle hypertrophy to satellite cells, also other potential muscle resident stem cells could mediate this response. Mesangioblasts, which are known to have a strong myogenic potential (Morosetti et al. 2006), respond to the cytokine HMGB1, a signal released by inammatory cells, leading to the migration across the vessel wall and proliferation (Palumbo et al. 2004). Indeed, overexpression of HMGB1 is sufcient to induce a rapid swelling after 24 h, something also observed in the early phases of overload hypertrophy, followed by an accumulation of mesangioblasts close to the bers. Considering the angiogenic potential of these cells (Sachdev et al. 2012) it is also tempting to assume that these cells play an important role in stimulating angiogenesis, leading to an improved muscle perfusion. Similar pro-angiogenic effects were observed in the functional hypertrophy induced by overexpression of Akt within muscle bers, as a signicant increase in proliferating NG2/CD31-positive interstitial cells was found (Blaauw et al. 2009). Taken together, these results show that several types of interstitial stem cells can play an important role in the adaptations of muscle during muscle hypertrophy, however clear demonstrations of this are still missing.

Myonuclei and satellite cells in muscle response to exercise in humans

Several studies on resistance training and on comparison between athletes and sedentary subjects have provided strong evidence that exercise can stimulate satellite cells in human muscles to re-enter the cell cycle and proliferate. In untrained unaccustomed subjects a period of 816 weeks of resistance training is sufcient to signicantly increase (3050 %) the density of satellite cells as revealed by immuno-histochemistry or electron microscopy (Kadi et al. 2004; Kadi and Thornell 2000; Mackey et al. 2007; Roth et al. 2001). Interestingly, the comparison between young and elderly exposed to similar training protocol revealed a less pronounced response in the latter group (Petrella et al. 2008). Subsequent studies, however, have shown that this difference is partly explained by the difference in ber type distribution between young and elderly subjects. Actually, in sedentary elderly compared to young subjects the content in satellite cells is lower in fast bers, with only minor difference in slow bers (Verdijk et al. 2007; Verney et al.

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2008). A resistance training protocol of 1216 weeks is sufcient to increase the density of satellite cells in fast bers to values similar to those of young subjects.

There is also evidence that a single bout of exercise is sufcient to increase the number of satellite cells, even in well-trained athletes. A signicant increase, and in some case even a doubling of the satellite cell density has been observed at time variable between 1 day and 8 days after a single bout of heavy eccentric exercise (Crameri et al. 2004, 2007 Dreyer et al. 2006; Mikkelsen et al. 2009; OReilly et al. 2008; Paulsen et al. 2010). It is worth to underline a possibly relevant difference between the effects of training and those of a single bout of heavy exercise. While at the end of a training protocol lasting several weeks the accumulation of new satellite cells can be more safely associated with the adaptive response of muscle, i.e. hypertrophy, in the period immediately subsequent an exceptional eccentric performance the activation of satellite cells to proliferate is more likely the expression of the regenerative process which follows the damage induced by the eccentric contractions. In this respect, a transversal analysis which compares well trained elite power lifters to sedentary controls gives an even more reliable representation of the effects of physical activity and exercise: two studies (Eriksson et al. 2005; Kadi et al. 1999) have found a higher (?30 %) density of satellite cells in the trained athletes.

Although less investigated than resistance training, also endurance training is able to activate satellite cells: not only a training of 14 weeks (Chari et al. 2003) but also a single bout of exercise (36 km run in moderately trained men (Mackey et al. 2007)) is able to increase (approximately 30 %) satellite cell density. In partial contrast with the documented effects of training, a very recent comparison between leg muscles of healthy old trained men who had been running more than 40 km a week for approximately 30 years of their life and age-matched untrained controls has not shown any difference in satellite cell density. This result, however, has been considered as a proof that the repeated cycles of damage-repair involving satellite cells activation over a period of 30 years did not impair the satellite cell population (Mackey et al. 2013).

Although caution must be taken in view of the small size of the biopsy sample analyzed and of the possible misinterpretation of the immuno-histochemical methods used for satellite cells identication, the available data strongly support that satellite cells are activated by exercise in human muscles. In the rst hours and days after heavy exercise sessions such activation is likely directed to repair and regeneration, but long lasting increase in satellite cells populations at the end of a training period appear as a part of the adaptive remodeling of the muscle architecture.

Conclusions

The discordant conclusions drawn on the contribution of satellite cells during muscle hypertrophy may well depend on the stimulus responsible for the induction of muscle growth. Stimuli, like overload or exercise, leading to signicant mechanical stress placed on the muscle bers, are very effective in inducing the activation of satellite cells. Indeed, it was recently shown that mechanical signals/ stiffness of the satellite cell niche is important in the activation of satellite cells (Urciuolo et al. 2013). The same can be true when hypertrophy is induced by hormonal factors, such as androgens or IGF-1. However, while in many situations of muscle growth satellite cell activation occurs, various studies using transgenic mice to either induce hypertrophy or to ablate satellite cells, have shown that satellite cells are not required for the induction of a functional hypertrophy (Blaauw et al. 2009; McCarthy et al. 2011). The difference might be related to the fact that in many models of hypertrophy, even a minimal amount of damage leads to an inammatory response which attracts both circulating stem cells with myogenic potential and activates satellite cells, in addition to exposing the muscle to a mechanical stress. Considering that in virtually all known non-transgenic models of hypertrophy satellite cell activation does occur, it leads to an interesting question, namely if the activation and incorporation of satellite cells is sufcient to induce muscle hypertrophy by itself. To conclude, while it seems likely that satellite cells are not required for muscle hypertrophy to be established, the fact that satellite cell activation occurs in virtually all hyper-trophy models underlines the need of further studies to understand which signals trigger activation and which role the fusion of satellite cells play during muscle hypertrophy.

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