REVIEW

Intrinsic and extrinsic mechanisms regulating satellite cell functionNicolas A. Dumont1, Yu Xin Wang1,2 and Michael A. Rudnicki1,2,*

ABSTRACTMuscle stem cells, termed satellite cells, are crucial for skeletalmuscle growth and regeneration. In healthy adult muscle, satellitecells are quiescent but poised for activation. During muscleregeneration, activated satellite cells transiently re-enter the cellcycle to proliferate and subsequently exit the cell cycle to differentiateor self-renew. Recent studies have demonstrated that satellite cellsare heterogeneous and that subpopulations of satellite stem cells areable to perform asymmetric divisions to generate myogenicprogenitors or symmetric divisions to expand the satellite cell pool.Thus, a complex balance between extrinsic cues and intrinsicregulatory mechanisms is needed to tightly control satellite cellcycle progression and cell fate determination. Defects in satellite cellregulation or in their niche, as observed in degenerative conditionssuch as aging, can impair muscle regeneration. Here, we reviewrecent discoveries of the intrinsic and extrinsic factors that regulatesatellite cell behaviour in regenerating and degenerating muscles.

KEY WORDS: Satellite cell, Muscle stem cell, Myogenesis,Quiescence, Cell cycle regulation, Asymmetric division,Self-renewal, Skeletal muscle, Regeneration, Aging

IntroductionSkeletal muscles are made up of numerous multinucleatedmyofibers that possess the contractile machinery to generatemovement. Skeletal muscles also contain a population of smallmononucleated muscle stem cells, termed satellite cells, thatrepresent 2-10% of total myonuclei (2×105 to 1×106 cells/gmuscle) (Hawke and Garry, 2001; White et al., 2010). Satellitecells are located in a specialized niche between the myofibersarcolemma and the surrounding extracellular matrix (ECM), whichis termed the basal lamina (Fig. 1). In resting adult muscles, satellitecells are quiescent and are characterized by the expression of thepaired box protein Pax7. Following injury or growth stimulus,satellite cells become activated and express the myogenic regulatoryfactors Myf5 and/or MyoD (also known as Myod1). Activatedsatellite cells, which are called myoblasts, proliferate massively togenerate the myogenic progenitors needed for muscle regeneration(Bischoff, 1986a; Hurme and Kalimo, 1992; Zammit et al., 2002).Thereafter, myoblasts downregulate the expression of Pax7 andupregulate the expression of factors such as myogenin (Myog) andmyogenic regulatory factor 4 (MRF4; also known as Myf6) to exitthe cell cycle, differentiate and fuse to form newly regeneratingmyofibers (Fig. 2). Satellite cells are responsible for the tremendousregenerative capacity of skeletal muscles; many studies have

demonstrated a complete lack of regeneration in adult skeletalmuscles depleted of Pax7-expressing satellite cells (Günther et al.,2013; Lepper et al., 2011; Sambasivan et al., 2011; von Maltzahnet al., 2013).

Following muscle injury, activated satellite cells give rise to themyogenic progenitors needed to form new myofibers, but they alsoneed to self-renew tomaintain the satellite cell pool. Repeated injuryexperiments have shown that satellite cell numbers remain constanteven after multiple traumas (Shi and Garry, 2006). The ability ofsatellite cells to appropriately balance quiescence, self-renewal andcommitment is thus crucial to ensure the life-long maintenance ofskeletal muscle. Importantly, many studies now support the notionthat satellite cells are heterogeneous and comprise a subpopulationof committed satellite cells that are more prone to progress into themyogenic lineage and a subpopulation of satellite stem cells that arepredisposed to undergo self-renewal (Beauchamp et al., 2000;Collins et al., 2005; Kuang et al., 2007; Rocheteau et al., 2012;Sacco et al., 2008).

In recent years, many intrinsic and extrinsic factors governingsatellite cell functions have been discovered. Together, these studiesdemonstrate that the fine-tuned regulation of cell cycle repressorsand activators is crucial to coordinate the different states of satellitecells. In this Review, we analyze the intrinsic mechanisms thatcontrol satellite cell cycle progression throughout the differentstages of adult myogenesis. We also describe how intrinsicmechanisms and extrinsic signals from the satellite cell nichecooperate to influence satellite cell fate decisions, in particular self-renewal versus myogenic commitment. Finally, we discuss theintrinsic defects of aged satellite cells and describe how thesedeficits modulate the ability of satellite cells to appropriatelyregulate the cell cycle and cell fate decisions, which ultimatelyaffects their regenerative capacity.

The development and heterogeneity of satellite cellsDuring development, the myogenic progenitors that generateskeletal muscles arise from the dorsal portion of the somites,known as the dermomyotome (Bentzinger et al., 2012). Embryonicmyogenic progenitors are characterized by expression of the pairedbox proteins Pax3 and Pax7 (Bober et al., 1994; Relaix et al., 2004;Seale et al., 2000). Embryonic myogenesis in these muscleprecursor cells follows the hierarchical activation of Myf5 and/orMyoD, followed by Myog and MRF4, resulting in the formation ofembryonic muscle compartments (Hasty et al., 1993; Rawls et al.,1998; Rudnicki et al., 1993; Zhang et al., 1995). In parallel tomyofiber formation, a subpopulation of myogenic precursor cellsthat do not express the MRFs and maintain Pax3/Pax7 expressionis observed adjacent to the myofibers late during mouse fetaldevelopment, at around E16.5-18.5 (Kassar-Duchossoy et al.,2005). It is hypothesized that these cells give rise to the satellitecell population found in adult muscle. Although adult satellite cellsdo not express MyoD in resting conditions, the use of a MyoD-iCremouse strain with a lineage-tracing reporter allele suggests that theprogenitors of essentially all adult satellite cells transcribed MyoD

1Sprott Centre for Stem Cell Research, Ottawa Hospital Research Institute, Ottawa,Ontario, Canada K1H 8L6. 2Faculty of Medicine, Department of Cellular andMolecular Medicine, University of Ottawa, Ottawa, Ontario, Canada K1H 8M5.

*Author for correspondence (mrudnicki@ohri.ca)

This is an Open Access article distributed under the terms of the Creative Commons AttributionLicense (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,distribution and reproduction in any medium provided that the original work is properly attributed.

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prenatally (Kanisicak et al., 2009). Contrary to MyoD expression,distinct populations of Myf5-positive and Myf5-negative satellitecells are present in adult muscles, as observed in Myf5-nlacZreporter mice and by the direct detection of Myf5 protein levels(Beauchamp et al., 2000; Gayraud-Morel et al., 2012; Kuang et al.,2007). To determine whether the Myf5-negative satellite cellsrepresent a distinct population that has never expressed Myf5during development, Myf5-Cre/ROSA26-YFP mice, in which cellsexpressing Myf5 and their progeny are permanently labelled withyellow fluorescent protein (YFP), were used. These analysesrevealed that a subpopulation of ∼10% of total satellite cells neverexpresses Myf5 during development (Kuang et al., 2007).This heterogeneity in the developmental origins of satellite cells

raises the possibility that subsets of satellite cells have self-renewalcapacity and act as muscle stem cells. Accordingly, in Myf5-Cre/ROSA26-YFP mice, the YFP-negative satellite cells possesshigher self-renewal ability than YFP-positive cells, which aremore prone to commit into myogenic progenitors. Transplantationexperiments clearly highlight the differences between satellite stemcell (YFP−) and committed satellite cell (YFP+) subpopulations,with the former resulting in long-term engraftment into thetransplanted muscle while the latter leading to differentiation andfusion to the host myofibers (Kuang et al., 2007). Using Pax7-nGFP mice, it was shown that, under regenerating conditions,activated satellite cells expressing higher levels of Pax7 are lessprone to commitment than those expressing lower levels of Pax7(Rocheteau et al., 2012). Experiments on TetO-H2B-GFP mice,which are used to report proliferative history, showed that somesatellite cells retain the expression of H2B-GFP (termed label-retaining cells, or LRCs), whereas others lose the labelling overtime (non-LRCs) (Chakkalakal et al., 2014). LRCs represent apopulation of satellite cells that are able to self-renew,whereas non-LRCs are committed to differentiation. The findings regardingLRCs in the satellite cell pool agrees with previous experimentsthat defined satellite cell heterogeneity by cell cycle kinetics andwith other recent studies that suggest better self-renewal capacity inslow-dividing cells (Ono et al., 2012; Schultz, 1996).

Together, these studies demonstrate that satellite cells are in fact aheterogeneous population that can be divided into subpopulationsof committed satellite cells (i.e. cells that are predisposed to progressthrough the myogenic lineage once activated) as well as asubpopulation of satellite stem cells (i.e. cells that are able to self-renew and maintain the satellite cell pool). However, whether thesatellite stem cell populations identified with the various reportermouse models represent the same or different subsets of satellitestem cells remains to be determined.

Cell cycle regulation in satellite cellsMuscle regeneration is characterized by different myogenic stages,namely: activation, proliferation, differentiation, and self-renewal/return to quiescence. Careful regulation of the cell cycle is essentialto ensure appropriate progression through these various overlappingstates. The following sections describe the intrinsic mechanisms andextrinsic signals that regulate the satellite cell cycle.

Satellite cell quiescenceIn resting adult muscles, satellite cells exist in a dormant stateknown as quiescence or the reversible G0 state (Fig. 2). The abilityof satellite cells to maintain quiescence in the resting state isessential for the long-term conservation of the satellite cell pool(Bjornson et al., 2012; Mourikis et al., 2012). This quiescent state isdistinct from the cell cycle exit observed prior to differentiation, themost notable difference being its reversibility, which allows cells toreturn to a proliferative state in response to injury. The rapid cellcycle re-entry of satellite cells after injury suggests that the quiescentstate is highly regulated and represents a ‘ready’ state that is primedfor activation. Microarray analyses revealed that more than 500genes are highly upregulated in quiescent satellite cells comparedwith cycling myoblasts (Fukada et al., 2007; Liu et al., 2013).Within this quiescence signature are negative regulators of the cellcycle, including cyclin-dependent kinase inhibitors 1B (Cdkn1b;also known as p27 or p27Kip1) and 1C (Cdkn1c; also known as p57or p57Kip2), the retinoblastoma tumor suppressor protein (Rb; alsoknown as Rb1), regulator of G-protein signalling 2 and 5 (Rgs2,

A Quiescent satellite cells B Activated satellite cells

Myofiber

Basal lamina

Quiescent satellite cell

NuclearPax7emerinlamin A/Cp57

Surfaceα7-integrincaveolin 1CalrCD34Cxcr4

Quiescent satellite cell markers

N-CAMM-cadherinsyndecan 3/4V-CAM

NuclearPax7MyoDemerinlamin A/CKi67

Surfaceα7-integrincaveolin 1Cxcr4desmin

Activated satellite cell markers

M-cadherinsyndecan 3/4V-CAMN-CAM

Activated satellite cells

Fig. 1. The satellite cell and its niche. Satellite cells are locatedjuxtaposed to mature muscle fibers in a quiescent state. Uponactivation by extrinsic factors, satellite cells re-enter the cell cycleand proliferate to generate sufficient numbers of progeny to formnew myofibers. Micrographs show Pax7-expressing (red) quiescent(A) and activated (B) satellite cells on cultured single myofibers. Theschematics beneath represent the quiescent and activated satellitecells in their niche and enumerate the nuclear and surfacemolecularmarkers associated with each state.

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Rgs5), peripheral myelin protein 22 (Pmp22), and the negativeregulator of fibroblast growth factor (FGF) signalling sprouty 1(Spry1). These quiescence genes act in concert to prevent theprecocious activation of quiescent satellite cells, and it was shownthat the conditional knockout of p27Kip1 or Rb protein results inaberrant satellite cell activation and proliferation (Chakkalakal et al.,2014; Hosoyama et al., 2011). Importantly, impairments in theability of satellite cells to maintain quiescence reduce self-renewalcapacity and muscle regeneration.Given the highly regulated state of satellite cell quiescence, it is

important to consider the transcriptional regulation of quiescencegenes. In recent years, the Notch pathway has emerged as a masterregulator of satellite cell quiescence. Notch signalling activity ishigher in quiescent satellite cells than in activated myogenic cells(Mourikis et al., 2012). This increased activity could be mediated bythe interaction between the Notch ligand Delta1 (delta-like 1 inmouse), which is expressed by myofibers, and the Notch receptor(and its co-receptor syndecan 3), which is present on satellite cells(Conboy and Rando, 2002; Pisconti et al., 2010). In addition,expression of the Notch1 and Notch3 receptors is activated by theforkhead transcription factor Foxo3, which is also enriched inquiescent satellite cells (Gopinath et al., 2014). Upon binding ofNotch ligand to its receptor, the Notch intracellular domain (NICD)is released and translocates into the nucleus where it interacts withrecombining binding protein suppressor of hairless (Rbpj) andtriggers the transcription of various genes, including those in theHes and Hey families. Accordingly, the conditional depletion ofRbpj in satellite cells or the double knockout of Hey1 and HeyLresults in spontaneous activation of quiescent satellite cells,

impairment in self-renewal and depletion of the satellite cell pool(Bjornson et al., 2012; Fukada et al., 2011; Mourikis et al., 2012).Interestingly, genetic models of Notch inactivation also lead to thespontaneous differentiation of satellite cells, suggesting that Notchhas a dual role in maintaining the myogenic progenitor state. Duringregeneration, downregulation of Notch is mandatory to allowmyogenic cell lineage progression (Brack et al., 2008).

Satellite cell quiescence is also maintained by microRNAs(miRNAs). The overall requirement of miRNAs in satellite cellquiescence is exemplified by their precocious activation afterdeletion of Dicer, the enzyme involved in pre-miRNA hairpincleavage (Cheung et al., 2012). Although our overall understandingof miRNA regulation in satellite cells is limited, the functions ofspecific families of miRNAs were recently discovered. Muscle-specific miRNAs, such as miR-1/206 and miR-133, are thought tomaintain the myogenic program and facilitate the transition intodifferentiation (Ma et al., 2015; Townley-Tilson et al., 2010).Although miR-1 and miR-206 share identical seed sequences, theindependently regulated expression of the three genomic copyvariants (miR-1-1/miR-133a-2, miR-1-2/miR-133a-1, miR-206/miR-133b clusters) confounds the exact genetic requirement forany one particular locus (Besser et al., 2014; Boettger et al., 2014;Wystub et al., 2013). However, the lack of skeletal musclephenotypes in the knockout of any single miR-1/206/133 clustersuggests that they share overlapping functions or that alternativemechanisms could circumvent their function in these specificknockout models. Other miRNAs regulate the cell cycle and preventpremature differentiation. For instance, miR-489 is highly enrichedin quiescent satellite cells where it post-transcriptionally represses

Activated satellite cellsand myoblastsCycling state

Cell size DNA content

Cell number

Myogenic factors

Quiescent satellite cellsG0 state

Cell cycle genesTranscription Metabolism

p27Kip1 Spry1 Hes/Hey

‘Alert’ satellite cellsGAlert state

Cell cycle genes

TranscriptionMetabolism

G1 phase

S phase

M phase

G2 phase

Hgf

mTORC1 activation

Notchactivation

Spry1 Fgf2

p38α/β MAPKactivation

MyocytesDifferentiation

p21Cip1

Sarcomeric proteins

Cell cycle genesp19Arf p57

Cell cycle exit

Cell cyclere-entry

Cell cycle exit

Fig. 2. Regulation of the cell cycle in satellite cells. In resting conditions, intrinsic regulators of the cell cycle maintain satellite cells in a reversible andquiescent G0 state. Activated satellite cells then re-enter the cell cycle, either directly or via an intermediate state referred to asGAlert. After activation, satellite cellscan exit the cell cycle and return to quiescence by upregulating Spry1 or by increasing Notch signalling. Proliferating myoblasts also exit the cell cycle todifferentiate into myocytes and progress into the myogenic lineage.

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the oncogene Dek and prevents cell cycle entry (Cheung et al.,2012). Additionally, the ability of satellite cells to maintainquiescence but be poised for activation was shown to be at leastpartially due to the sequestration of miRNAs together with mRNAs.Indeed, the storage of miR-31 together with Myf5 in satellite cellmRNP granules prevents the translation of this myogenic regulatoryfactor and repress myogenic commitment (Crist et al., 2012).

Activation and cell cycle re-entryMitogenic factors liberated following injury drive quiescent satellitecells to re-enter the cell cycle (Bischoff, 1986b, 1990). Recentanalyses of systemic effects of muscle injury revealed anintermediate state between the G0 quiescent and activated cellstates, termed GAlert (Fig. 2). It was shown that, following muscleinjury by intramuscular BaCl2 injection, satellite cells from thecontralateral uninjured leg are phenotypically different fromclassical quiescent cells (Rodgers et al., 2014). Interestingly,satellite cells from contralateral legs are larger, have moreintracellular ATP and higher metabolic activity than quiescentsatellite cells. Intriguingly, these ‘alert’ satellite cells express anumber of cell cycle genes similar to those expressed in activatedsatellite cells, but they do not take up BrdU (a proliferation marker)or enter the cell cycle. This intermediate GAlert state allows satellitecells to perform their first division faster than satellite cells in theG0 state (Rodgers et al., 2014). This pre-activation state isparticularly important because the first cell cycle takes muchlonger to complete than subsequent cell cycles, which indicates thatexit from quiescence is a relatively slow process (Bischoff, 1986b;Siegel et al., 2011). Ultimately, satellite cells in the GAlert state havegreater regenerative potential. Although the contralateral leg issubject to overloading in these types of injury experiment, it wassuggested that the release of hepatocyte growth factor (Hgf) at thesite of injury could have a systemic effect and activate mTORsignalling in distant quiescent satellite cells, thereby driving theirentry into the GAlert state (Rodgers et al., 2014). This hypothesis isconsistent with a previous study showing that Hgf is present incrushed muscle extract and that direct injection of Hgf intouninjured muscle leads to satellite cell activation (Tatsumi et al.,1998). This implies the existence of a system-wide responsemechanism that primes satellite cells to become activated in aregenerative environment. Hence, it will be interesting to assesswhether this response is associated with exercise-induced trainingeffects or is possibly altered in pathological conditions.It is well characterized that the vast majority of satellite cells and

myogenic progenitors proliferate in a coordinated effort in responseto injury. A peak in myoblast proliferation is reached between thesecond and fifth days after cardiotoxin-induced muscle injury(Murphy et al., 2014; Ogawa et al., 2014; Rodgers et al., 2014).Damaged muscles release various growth factors that activatesignalling pathways involved in satellite cell cycle entry (Andersonet al., 1995). For instance, insulin-like growth factor (Igf1) wasshown to inactivate the transcription factor Foxo1, leading todownregulation of the cell cycle inhibitor p27Kip1 and ultimatelyresulting in cell cycle entry (Chakravarthy et al., 2000; Machidaet al., 2003). Fgf2 is also highly expressed in regenerating muscleand is known to activate different MAPK pathways in satellite cells(Yablonka-Reuveni et al., 1999). For example, studies of isolatedmyofibers have shown that p38α/βMAPK (Mapk14/11) is activatedand quickly translocates into the nucleus of activated satellite cellsafter myofiber isolation and, accordingly, the inhibition of p38α/βMAPK impairs the ability of satellite cells to enter the cell cycle(Jones et al., 2005). In addition, activation of the ERK1/2 (Mapk3/1)

pathway by Fgf2 was shown to be crucial for the G1 to S phasetransition in satellite cells (Jones et al., 2001). JNK, another MAPKsignalling pathway, was also shown to trigger activation of cyclinD1 and promote cell cycle progression (Perdiguero et al., 2007).Furthermore, microarray analyses confirmed that a number of cellcycle regulators, including cyclins A, B, D, E, F and G, are enrichedin cultured myoblasts compared with freshly sorted quiescentsatellite cells (Fukada et al., 2007).

Cell cycle exit and the return to quiescenceOnce activated, satellite cells are then instructed to undergodifferentiation (into myocytes) or to return to quiescence, twoprocesses that involve cell cycle exit (Fig. 2). This cell cycle exitrequires the upregulation of specific cyclin-dependent kinase inhibitors.Whereas the return to quiescence requires p27Kip1, differentiationrequires the concerted upregulation of p21Cip1 (Cdkn1a), p19Arf

(Cdkn2a) and p57 (Cao et al., 2003; Chakkalakal et al., 2014; Pajciniet al., 2010).

Recent evidence has partially unravelled the mechanisms thatallow satellite cells to return to quiescence. The tyrosine kinasesignalling inhibitor Spry1 is expressed at low levels in activatedsatellite cells, but is upregulated in satellite cells returning toquiescence (Shea et al., 2010). Spry1 promotes cell cycle exit byinhibiting the ERK signalling pathway. Accordingly, satellite cellsfrom Spry1 null mice show an impaired ability to return toquiescence, resulting in a decreased satellite cell pool after muscleinjury (Shea et al., 2010). Activation of Notch signalling is alsocrucial for satellite cells to return to quiescence. During theasymmetric division of activated satellite stem cells, the daughtercell that is committed to differentiate expresses high levels ofDelta1, whereas the daughter satellite stem cell expresses theNotch3 receptor, which results in the activation of Notch signallingin this cell and promotes its return to quiescence (Kuang et al.,2007). Notch activation also inhibits the expression of MyoD andpromotes the expression of Pax7, thus encouraging maintenance ofthe primitive satellite cell state (Bröhl et al., 2012; Wen et al., 2012).Moreover, the loss of Notch leads to a failure in the homing ofsatellite cells to their sublaminar position due to the inability ofthese cells to produce a variety of ECM proteins, such as collagenisoforms, integrin alpha 7, and cell adhesion molecules includingcomponents of the Dystroglycan complex (Bröhl et al., 2012). Thissuggests that, in order for satellite cells to return to quiescencein vivo, cell-cell adhesion and ECM deposition are necessarycheckpoints that are ultimately intrinsically regulated throughautocrine mechanisms.

The regulation of satellite cell fate decisionsIn addition to re-entering the cell cycle, activated satellite cells mustalso determine the cell fate of their daughter cells, in particularwhether they self-renew or generate myogenic progenitors. Theanalysis of MRF expression in proliferating satellite cells revealedasymmetric expression of determination factors, including Myf5,MyoD and Myog, in the daughter cells of subsets of satellite celldivisions (Kuang et al., 2007; Shinin et al., 2006; Troy et al., 2012).It is believed that stem cell subpopulations are able to undergo bothsymmetric and asymmetric modes of self-renewal to maintainsatellite cell numbers through repetitive rounds of regeneration.Tracking the activation of Myf5 with Myf5-Cre/ROSA26-YFPmice revealed that YFP-negative satellite stem cells are able toperform symmetric divisions, which give rise to two identicaldaughter cells that will self-renew the satellite stem cell pool (Kuanget al., 2007). Alternatively, satellite stem cells can perform

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asymmetric divisions, which generate one stem cell and onecommitted daughter cell that will progress through the myogeniclineage (Fig. 3) (Kuang et al., 2007). This section will describe howcell fate decisions are determined during stem cell divisions, andhow the niche and intrinsic processes, such as polarity andmetabolism, regulate this process.

Symmetric and asymmetric divisionsIn Myf5-Cre/ROSA26-YFP mice ∼30-40% of the first divisions ofYFP-negative cells are symmetric, while the remaining divisions areasymmetric (Le Grand et al., 2009). Similar percentages wereobserved in Pax7-nGFPHi satellite cells (Yennek et al., 2014).Symmetric division is promoted by activation of the planar cellpolarity (PCP) pathway, which leads to the symmetric distributionof polarity effectors such as Vangl2 in daughter cells (Le Grandet al., 2009). Knockdown of Vangl2 in isolated myofibers results inincreased numbers of Myog-positive progenitors but decreases totalsatellite cell numbers, indicating a role for symmetric division in theself-renewal and expansion of the satellite cell pool (Le Grand et al.,2009).Asymmetric division is characterized by the segregation of

different cell fate determinants into the daughter cells. During theasymmetric division of Myf5-negative satellite stem cells, theNotch3 receptor is enriched in the daughter satellite stem (Myf5-negative) cell, whereas the committed daughter (Myf5-positive) cellinherits the Notch ligand Delta1 (Kuang et al., 2007). Accordingly,the Notch antagonist Numbwas shown to be asymmetrically locatedin the committed Myog-positive daughter cell (Conboy and Rando,2002; Conboy et al., 2005). These results are consistent with the roleof Notch in promoting satellite cell return to quiescence and self-renewal (Gopinath et al., 2014; Pisconti et al., 2010; Wen et al.,2012). After asymmetric division, the ability of the two daughtercells to activate the myogenic program is also controlled by Pax7transcriptional activity. Pax7 was shown to recruit the histonemethyltransferase complex Wdr5-Ash2l-Mll2 (Kmt2) to the Myf5locus, resulting in permissive chromatin modifications that stimulatetranscriptional activation ofMyf5 (McKinnell et al., 2008).However,to recruit this histone methyltransferase complex, Pax7 must bemethylated by the arginine methyltransferase Carm1 (Kawabe et al.,2012). During asymmetric division, interaction between Carm1 andPax7 in the nucleus of the committed daughter cell activates thetranscriptional expression ofMyf5 (Kawabe et al., 2012).Other types of asymmetric divisions and asymmetric segregations

of factors have also been described inmyoblasts (Fig. 3) (Wang et al.,

2013). For example, MyoD was shown to be able to asymmetricallydistribute in the two daughter cells, giving rise to one Pax7+ MyoD−

reserve cell and one Pax7− MyoD+ committed myogenic cell (Liuet al., 2012). Similar results were obtained with Myog, withmyoblasts generating one Pax7+ Myog− and one Pax7− Myog+

daughter cell (Yennek et al., 2014). Asymmetric segregation oftemplate DNA has also been proposed as a model to measureasymmetric division in activated satellite cells. Indeed, usingtransgenic Pax7-nGFP it was shown that, whereas activatedsatellite cells expressing low levels of Pax7 (Pax7Lo) performrandom DNA segregation, those expressing higher levels of Pax7(Pax7Hi) mostly perform asymmetric DNA segregation during celldivision (Rocheteau et al., 2012). The daughter cell inheriting the oldtemplate DNA retains Pax7 expression, whereas that receiving thenew template DNA expresses Myog (Yennek et al., 2014).Asymmetric DNA segregation measured by BrdU incorporationshowed that it is also associated with the asymmetric inheritance ofcell fate determinants, with the daughter cell that inherits the oldtemplateDNAexpressing SCA-1 (Ly6a) and the cell that receives thenew template DNA being positive for desmin (Conboy et al., 2007).

The capacity of satellite cells to choose whether to performsymmetric or asymmetric division allows them to coordinate theiractivity with the needs of the regenerating muscle. For example, anincreased proportionof symmetric divisionswould promote expansionof the satellite stem cell pool, whereas increased asymmetric divisionswould favour the generationofmyogenic progenitors andmaintenanceof the stem cell pool (Fig. 3) (Wang et al., 2014). Thus, a dynamicbalance must exist between symmetric and asymmetric divisions thatfluctuates during the different stages of muscle regeneration.Conversely, one could hypothesize that a prolonged imbalance in thesymmetric:asymmetric division ratio will lead to impaired muscleregeneration. A better understanding of the mechanisms drivingsatellite cell fate decisions is needed to determine whether intrinsicdefects in satellite cells are involved in pathological conditions.

Establishing polarityAsymmetric divisions are controlled by two major interactingevents: (1) the unequal distribution of polarity proteins and cell fatedeterminants; and (2) mitotic spindle orientation. How theseprocesses are regulated in satellite cells remains largely elusive,although recent studies highlight a role for members of thePartitioning-defective protein (PAR) family, which are masterregulators of cell polarity establishment in many different stem celltypes (Suzuki and Ohno, 2006). In muscle stem cells, recent

Pax7 Pax7Myf5 and/or MyoD

Myogenin

Symmetric stem cell division

Asymmetric stem cell division

Progenitorproliferation

Stem cell expansion

Stem cellmaintenance

Stem cellcommitment

Asymmetricprogenitor divisions

Satellite stem cells

Commited myogenic progenitors

Differentiatingmyocytes

Fig. 3. Satellite cell fate decisions.Activated satellite stem cells(Pax7+, Myf5–, MyoD–) can undergo symmetric divisions toexpand the satellite stem cell population, or asymmetric divisionsto maintain the stem cell population and generate myogenicprogenitors. Satellite cells can also commit to the myogeniclineage and proliferate to give rise to committed myogenicprogenitors (Pax7+, Myf5+ and/or MyoD+). Myogenic progenitorsare able to asymmetrically divide or directly differentiate intomyocytes (Myog+), which fuse and form new myofibers.

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findings revealed that during asymmetric division the PAR3-PKCλ(Pard3-Prkcι) complex is asymmetrically distributed in thecommitted daughter cell (Troy et al., 2012). Phospho-p38α/βMAPK also colocalizes asymmetrically in the committed cell withthe PAR3-PKCλ complex, where it acts as a determination factorand triggers the activation of MyoD, leading to the commitment ofthe daughter cell into a myogenic progenitor (Troy et al., 2012). Invarious stem cell types, polarity establishment has been shown toregulate mitotic spindle orientation and the direction of cell division(Lu and Johnston, 2013). In regenerating muscle, satellite cellsymmetric divisions occur mostly in a planar orientation (parallel tothe myofiber), whereas asymmetric divisions occur in an apicobasalorientation (perpendicular to the myofiber), suggesting that celldivision orientation is a decisive factor in cell fate determination(Kuang et al., 2007).

The role of the satellite cell nicheThe analysis of different adult stem cell populations, such asepithelial stem cells, neural stem cells and haematopoietic stemcells, has revealed that in all these tissues the stem cell nicheprovides essential cues that influence cell fate decisions (Cheungand Rando, 2013). In skeletal muscle, satellite cells are entrappedbetween the myofiber and the ECM. In quiescent satellite cells,these two opposing microenvironments lead to the asymmetricdistribution of different proteins, includingM-cadherin (cadherin 15)and β-catenin that are located on the myofiber side, and ECM-interacting proteins, such as integrin alpha 7, that are located on thebasal membrane side. During muscle regeneration, the feedbacksignals sent by the microenvironment to the activated satellite cellsare important to establish cell division orientation, which is a keyfactor in determining daughter cell fate. Real-time imaging ofmyofibers showed that, for the first cell division, the proportions ofapicobasal versus planar divisions are 35% and 65%, respectively(Siegel et al., 2011).A number of proteins located in the satellite cell niche have been

shown to affect the orientation of division and, hence, cell fate choice.For example, collagen VI present in the ECM reduces myogeniccommitment and promotes the self-renewal of activated satellite cells(Urciuolo et al., 2013), and its deletion strongly impairs themaintenance of satellite cell number following multiple muscleinjuries. Fibronectin, which is transiently expressed by activatedsatellite cells during muscle regeneration, also promotes expansion ofthe satellite stem cell population. Accordingly, knockdown offibronectin severely reduces satellite cell engraftment and self-renewal potential (Bentzinger et al., 2013). Fibronectin functions bybinding to syndecan 4 (Sdc4), which is located on the satellite cellmembrane and forms a co-receptorwith the frizzled 7 (Fzd7) receptor.The wingless family member Wnt7a, a soluble factor released in thesatellite cell niche by regeneratingmyofibers, alsobinds toFzd7-Sdc4-fibronectin and, together, this complex stimulates the symmetricdivision and expansion of the satellite stem cell subpopulation (LeGrand et al., 2009). Intracellularly, the binding of Wnt7a to Fzd7activates the small Rho-GTPase Rac1, which is a member of the PCPpathway, leading to the symmetric polarization of different PCPeffectors, such as Vangl2, at opposite poles of the cell along themyofiber axis (Le Grand et al., 2009; von Maltzahn et al., 2012).Activationof thePCPpathway leads integrin alpha7 to colocalizewithVangl2 on both ends of the dividing cell, thereby allowing the twodaughter cells to bind to laminin (Le Grand et al., 2009). Together,these various components of the satellite cell niche promote thesymmetric planar division of satellite cells, allowing the two daughter

cells to receive similar feedbacks from their microenvironment byremaining in contact with both the myofiber and the ECM.

Conversely, asymmetric divisions take place in an apicobasalorientation, such that the satellite stem cell remains attached to thebasal membrane while the committed satellite cell is in contact withthe myofiber (Kuang et al., 2007). The daughter stem cell and thedaughter committed cell are therefore in physical contact withdistinct microenvironments that differently influence cell fatedecisions. During apicobasal divisions, the satellite stem cell(Myf5 negative) remains attached to the basal lamina, while thecommitted progenitor (Myf5 positive) is pushed towards themyofiber and retains no contact with the basal lamina. Notably,during muscle regeneration, satellite cells can traverse the basallamina and locate in interstitial areas (Ogawa et al., 2014). Theinfluence of ECM binding on cell fate decisions was also studied inmyoblasts cultured in vitro on symmetric or asymmetricmicropatterns coated with fibronectin and fibrinogen (Yenneket al., 2014). Myogenic cells submitted to ECM in asymmetricadherence motifs (such that one side of the cell has higher adherenceto ECM proteins than the other) are more prone to performasymmetric division (Yennek et al., 2014). Interestingly, thesubpopulation of Pax7Hi satellite cells that intrinsically tends toperform asymmetric divisions is not affected by symmetric orasymmetric micropatterns, indicating that intrinsic factors alsopredetermine satellite cell fate (Rocheteau et al., 2012; Yenneket al., 2014).

Overall, cell fate decisions in satellite cells involve a finely tunedbalance between intrinsic and extrinsic factors. The tight regulationof this balance is essential for optimal muscle regeneration, and anyperturbations might lead to diverse muscle pathologies. The satellitecell niche provides crucial feedback to establish cell polarity andorient cell division. Intrinsically, the symmetric or asymmetricdistribution of cell fate determinants also influences the status of thedaughter cells. However, the exact mechanisms by which polarity isestablished in satellite cells remain to be characterized.

Metabolic control of satellite cellsQuiescent satellite cells have a low metabolic rate, but theiractivation and entry into the cell cycle are characterized by majormetabolic changes. Isolated satellite cells cultured in vitroexperience a switch from oxidative to glycolytic metabolism(Ryall et al., 2015). Interestingly, emerging studies indicate a rolefor metabolic pathways in satellite cell fate decisions. The switch toglycolytic metabolism observed after satellite cell activation wasshown to decrease the levels of NAD+, which in turn decreases theactivity of the histone deacetylase sirtuin 1 (Sirt1) and increasesH4K16 acetylation, ultimately leading to the activation of musclegene transcription (Ryall et al., 2015). Pax7-specific Sirt1 knockoutmice show premature differentiation of activated satellite cells andimpaired muscle regeneration (Ryall et al., 2015). Moreover, Sirt1also regulates the autophagy process that provides the nutrients togenerate ATP and sustain high energy demands during satellite cellactivation (Tang and Rando, 2014). Consequently, Sirt1 knockoutor autophagy inhibition results in impaired satellite cell activation(Tang and Rando, 2014). Oxygen availability also regulates satellitecell fate decisions, and myoblasts cultured in hypoxic conditionsexhibit increased Notch signalling that, in turn, promotes high levelsof Pax7 expression (Liu et al., 2012). Therefore, myoblasts culturedin hypoxic conditions have decreased differentiation ability butenhanced self-renewal potential, leading to higher transplantationefficiency (Liu et al., 2012). Similarly, satellite cell transplantation

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efficiency is increased following calorie restriction in either thedonor or the recipient mice (Cerletti et al., 2012).Together, these studies indicate that the metabolic status of

satellite cells regulates key pathways involved in cell fate decisions.However, the metabolic modifications observed in satellite cells canaffect various molecular signalling pathways in the cell. Therefore,additional studies into the molecular events following metabolicchanges in satellite cells are needed and will likely lead to theidentification of novel cellular targets that regulate satellite cellbehaviour.

Changes in satellite cells during agingBoth skeletal muscle mass and strength gradually declinewith aging(Frontera et al., 2000). Reduced myofiber size is observed incombination with the accumulation of intramuscular fat, fibrosisand chronic inflammation, suggesting that aged muscles exhibitimpaired regeneration potential (Barani et al., 2003; Grounds,1998). Previous studies suggested that the decreased numbers ofsatellite cells observed in aged muscles could partially explain thisreduced regenerative potential (Shefer et al., 2006). However, theextent towhich the satellite cell population is reduced is still debatedand varies considerably between studies and animal models (Brackand Rando, 2007). Moreover, the density of satellite cells remainsrelatively stable between aged (20-24 months) and very old or‘geriatric’ (28-32 months) mice, whereas the numbers and size ofthe myofibers decline sharply during the same period. Therefore,accumulating evidence indicates that it is the intrinsic regenerativepotential of satellite cells, together with the satellite cell niche, thatis impaired in old muscles.

Impaired self-renewal of aged satellite cellsThe exact nature of the defects observed in aged satellite cells isbeginning to emerge. For example, in vitro experiments have shownthat activation takes longer in aged satellite cells, but that cell cycleprogression is not affected (Barani et al., 2003).Moreover, timecoursemeasurements of the expression of myogenic regulatory factors aftercardiotoxin injury indicate that MyoD and Myog levels remainelevated at later time points in aged as compared with young muscles(Marsh et al., 1997). These results suggest that the commitment of

satellite cells is unaffected in aged muscles but that self-renewal andreturn to quiescence could be impaired. Accordingly, the analysis ofmarkers of cell proliferation, such as BrdU incorporation and Ki67staining, confirmed that aged satellite cells have increased cyclingactivity in resting conditions (Chakkalakal et al., 2012). Althoughmost satellite cells show impaired self-renewal in aging, a smallsubpopulation of aged satellite cells seems to maintain its ability toself-renew, suggesting that a stem cell subpopulation could haveintrinsic resistance to aging (Collins et al., 2007).

The mechanisms driving this inability of old satellite cells tomaintain or return to quiescence and to self-renew have beenextensively studied in recent years. The expression of Fgf2, which isinvolved in satellite cell activation, was shown to be upregulated in theaged satellite cell microenvironment, whereas the FGF signallinginhibitor Spry1 is downregulated (Chakkalakal et al., 2012).Interestingly, inhibition of FGF signalling rescues the self-renewalcapacity in old satellite cells (Chakkalakal et al., 2012). p38α/βMAPK, which acts downstream of the FGF receptor, is alsooverstimulated in aged satellite cells (Bernet et al., 2014). Thisoveractivation of the p38α/β MAPK pathway induces satellite cellactivation and disturbs the asymmetric division of aged satellite cells,leading to the increased generation of committed progenitors and areduction in self-renewal (Fig. 4). Furthermore, the activation of p38MAPK in old satellite cells cannot be rescued by transplantation into ayoung myofiber, indicating that it is a deficit inherent to the agedsatellite cells that cannot be overcome by the satellite cell niche (Bernetet al., 2014; Cosgrove et al., 2014). However, pharmacologicalinhibition of p38α/β MAPK in aged satellite cells is able to restoreengraftment potential and improve their self-renewal ability.

Genome-wide expression analyses of freshly isolated satellite cellsfrom young or old muscles revealed that JAK/STAT is anothersignalling pathway upregulated in aged satellite cells (Price et al.,2014). In vitro experiments showed that Jak1-Stat1-Stat3 signallingpromotesmyoblast proliferation and represses premature differentiationby regulating the cell cycle inhibitors p21Cip1 and p27Kip1 (Sun et al.,2007).High levels of the inflammatory cytokine IL-6 in aged serumarepartially responsible for this increased JAK/STAT signalling (Fig. 4)(Tierney et al., 2014). Consequently, aged muscle satellite stem cellspreferentially undergo asymmetric divisions, and this predisposition

Young

Aged

‘Geriatric’

IL-6

p16INK4a

p38 MAPK

Stat3Fgf2

Fig. 4. Satellite cells in aging. In young muscle, acritical balance between lineage commitment andself-renewal is maintained during regeneration. Bycontrast, aged muscles show increased lineagecommitment (solid arrows) to myogenicprogenitors (orange cells) and a lack of self-renewal (dashed arrows), resulting in impairedregeneration and slow exhaustion of the satellitecell reserve (green cells). Satellite cells from veryold (‘geriatric’) muscles enter senescence and losetheir ability to re-enter the cell cycle.

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can be reversed by treatment with siRNA targeting Jak2 or Stat3 (Priceet al., 2014). Interestingly, this bias and the resulting lack of symmetricdivisions could partially explain the inability of aged satellite cells toself-renew. Accordingly, JAK/STAT inhibition in isolated myofibercultures increases the number of Pax7+ satellite cells in aged mice anddecreases the number of committed Pax7− MyoD+ myogenicprogenitors (Price et al., 2014; Tierney et al., 2014). Pharmacologicalinhibition of the JAK/STAT pathway also strongly improves theengraftment potential of young and aged satellite cells. Moreover, thedirect injection of JAK/STAT inhibitors into regenerating muscleimproves myofiber size, satellite cell numbers and the recovery ofmuscle force (Price et al., 2014).Altogether, these results indicate that the ability of satellite cells to

maintain/return to quiescence and self-renew is impaired in agedsatellite cells. More importantly, the discovery of dysregulatedintracellular pathways involved in these defects opens up newtherapeutic avenues. However, pharmacological inhibitors of thep38 MAPK or JAK/STAT pathways that promote satellite cellself-renewal must be used in a transient manner to replenish thesatellite cell population without affecting muscle regeneration. Forinstance, genetic deletion of Stat3 in satellite cells improves theirexpansion but impairs myogenic differentiation, whereas the transientdownregulation of Stat3 increases both satellite cell density andmuscle fiber size (Tierney et al., 2014). Nonetheless, pharmacologicalinhibitors provide a proof-of-concept that the regenerative potential ofaged satellite cells can be rejuvenated.

Modifications in the aged satellite cell nicheWhether the defects in the regenerative capacity of aged muscle arecaused by changes in the satellite cell niche or whether they areintrinsically inherent to aged satellite cells is still under debate.However, a number of studies have shown that modifications to thesatellite cell microenvironment, resulting from local or systemicchanges, are at least partially responsible for the impaired satellite cellregenerative potential of oldmuscles. For example, in vitro experimentsshowed that the addition of young serum to old myoblasts improvestheir myogenesis potential (Carlson and Conboy, 2007). Likewise, invivo experiments indicated that the exposure of satellite cells to a youngenvironment partially rescues their regenerative potential (Carlson andFaulkner, 1989; Conboy et al., 2005).The expression of a numberof different cytokines and growth factors

ismodulated in agedmuscles. TGFβ (Tgfβ1), for example, is one factorthat is overexpressed in old muscle and that induces phosphorylationand activation of Smad3 in aged satellite cells (Carlson et al., 2008).Smad3 activation, in turn, upregulates various cyclin-dependent kinaseinhibitors, including p15, p16, p21 and p27. Importantly, a reduction inTGFβ/Smad3 activation restores the regenerative potential of oldsatellite cells (Carlson et al., 2008). Interestingly, Smad3 activationdirectly antagonizes the Notch signalling pathway, which is known toregulate satellite cell quiescence and self-renewal (Bjornson et al., 2012;Carlson et al., 2008). Accordingly, decreased expression of the Notchligand Delta1 and impaired Notch signalling were observed in satellitecells from old regenerating muscles (Conboy et al., 2003). Forcedactivation of Notch with specific antibodies rescues the regenerativepotential of aged muscle (Conboy et al., 2003). Similar results wereobtained using heterochronic parabioses (models in which old andyoung mice share circulatory systems), with young serum restoringNotch signalling and the regenerative potential of old satellite cells(Conboy et al., 2005).Serum from aged mice also contains a higher concentration ofWnt

ligands (Brack et al., 2007). Canonical Wnt signalling directlyantagonizes Notch signalling in satellite cells (Brack et al., 2008), and

it was shown that the inhibition of Wnt signalling in old musclesrestores satellite cell regenerative potential (Brack et al., 2007).Moreover, high expression levels of Wnt in aged serum force theconversion of myogenic cells toward the fibrogenic lineage (Bracket al., 2007). Interestingly, manyECMcomponents physically interactwith satellite cells, and the excessive fibrosis observed in aged muscleis thus likely to perturb satellite cell behaviour. Altogether, theseresults indicate that changes in the aged satellite cell niche are partiallyresponsible for the impaired regenerative potential of aged muscle.

Satellite cell senescenceConsistent with the inability of aged satellite cells to maintainquiescence it was demonstrated in geriatric mice (30 months old)that a proportion of satellite cells displays a loss of reversiblequiescence (pre-senescence) (Sousa-Victor et al., 2014a). Underproliferative pressure induced by muscle injury, satellite cells fromgeriatric muscles thus fail to activate and proliferate and insteadundergo full senescence and irreversibly withdraw from the cellcycle (geroconversion) (Sousa-Victor et al., 2014b). The strongupregulation of the cyclin-dependent kinase inhibitor p16INK4a, analternative spliced variant of p19Arf (Fig. 4), is at least partiallyresponsible for this loss of reversible quiescence (Sousa-Victoret al., 2014a). This geroconversion further reduces the self-renewaland regenerative ability of old muscle satellite cells and cannot becompensated by a youthful environment.

Concluding remarks and future perspectivesSatellite cells are the protagonists of muscle regeneration. Toappropriately fulfil their functions, they must maintain a dynamicbalance between their different cell states, namely quiescence,commitment, differentiation and self-renewal. As we havehighlighted above, many intrinsic mechanisms are required toregulate the cell cycle and cell fate determination in satellite cells,and dysregulation of these mechanisms results in the loss ofregeneration in degenerative conditions, including aging. Inparticular, aged satellite cells lose their ability to maintainquiescence (through increased cycling or senescence) and, onceactivated, their cell fate is forced toward commitment to themyogenic lineage in lieu of self-renewal. Whether these changesare caused by modifications in the satellite cell niche or by intrinsicsatellite cell defects remains a matter of debate. Some dysfunctionsin aged satellite cells can be rescued by a young environment,whereas others cannot, suggesting that both intrinsic and extrinsicdeficiencies are present simultaneously.

Many recent studies have shown that, regardless of their origin,these deficits can be overcome by pharmacological inhibitors.These discoveries could represent a cornerstone in the treatment ofpathologies such as sarcopenia, an irreversible loss of muscle massinduced by aging that causes serious health issues and currently hasno therapy. It is likely that intrinsic perturbations in satellite cellregulatory mechanisms are present in other pathologies, includingmuscular dystrophies, and further studies are needed to explorethese possibilities and to aid our understanding of muscledevelopment, regeneration and degeneration.

Competing interestsThe authors declare no competing or financial interests.

FundingN.A.D. is supported by a Postdoctoral Fellowship from the Canadian Institutes ofHealth Research. Y.X.W. is supported by fellowships from QEII-GSST and theCanadian Institutes of Health Research. M.A.R. holds the Canada Research Chair inMolecular Genetics. These studies were carried out with support of grants to M.A.R.

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from the US National Institutes of Health [R01AR044031], the Canadian Institutesfor Health Research [MOP-12080, MOP-81288], E-Rare-2: Canadian Institutes ofHealth Research/Muscular Dystrophy Canada [ERA-132935], the Stem CellNetwork, and the Ontario Ministry of Economic Development and Innovation[RE05-084]. Deposited in PMC for release after 12 months.

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