Download - Wiley Online Library

REVIEW ARTICLE
Metabolic reprogramming as a novel regulator of skeletal
muscle development and regeneration
James G. Ryall
University of Melbourne, Victoria, Australia
Keywords
cell fate; glycolysis; metabolism; satellite
cells; stem cells
Correspondence
J. G. Ryall, Department of Physiology,
University of Melbourne, Victoria, 3010,
Australia
Fax: +613 83445818
Tel: +613 83443672
E-mail: [email protected]
Website: www.about.me/JamesRyall
(Received 17 December 2012, revised 6
February 2013, accepted 8 February 2013)
doi:10.1111/febs.12189
Adult skeletal muscle contains a resident population of stem cells, termed
satellite cells, that exist in a quiescent state. In response to an activating
signal (such as physical trauma), satellite cells enter the cell cycle and
undergo multiple rounds of proliferation, followed by differentiation,
fusion, and maturation. Over the last 10–15 years, our understanding of
the transcriptional regulation of this stem cell population has greatly
expanded, but there remains a dearth of knowledge with regard to the initiating signal leading to these changes in transcription. The recent renewed
interest in the metabolic regulation of both cancer and stem cells, combined with previous findings indicating that satellite cells preferentially colocalize with blood vessels, suggests that satellite cell function may be
regulated by changes in cellular metabolism. This review aims to describe
what is currently known about satellite cell metabolism during changes in
cell fate, as well as to describe some of the exciting findings in other cell
types and how these might relate to satellite cells.
Introduction
Skeletal muscle shows a high potential for regeneration
following an insult or injury; this regenerative capacity
is the direct result of a population of skeletal muscle
stem cells, termed satellite cells (SCs). These were first
described (separately) by Alexander Mauro and Bernard Katz over 50 years ago [1], and our understanding of the regulation of SCs has dramatically increased
since then. Lying between the sarcolemma and the
basal lamina (the so-called SC ‘niche’), the SC is perfectly positioned to receive local signals from the muscle fiber, fibroblasts, and endothelial cells, in addition
to systemic signals from blood vessels, with which SCs
have been found to preferentially colocalize (Fig. 1)
[2,3]. Therefore, it seems likely that, in addition to cell
–cell signaling, changes in local and systemic nutrient
quantity and quality, and oxygen availability, might
inform the SC and alter the cell state. The purpose of
this review is not to provide a comprehensive description of SC biology, for which a number of excellent
reviews already exist [4–6], but rather to examine how
metabolic reprogramming of SCs may play a role in
determining SC fate.
The skeletal muscle stem cell
Adult SCs exist in a quiescent cell state (G0), with little
to no basal turnover [7]. During this quiescent phase,
SCs express (among others) the paired homeobox transcription factor Pax7 and the myogenic regulatory factor Myf5. In one of the first studies to directly
demonstrate that quiescent SCs exist as a heterogeneous
population, Kuang et al. identified a subpopulation of
Abbreviations
AMPK, AMP-activated kinase; CR, caloric restriction; ESC, embryonic stem cell; FFA, free fatty acid; hnRNP, heterogeneous RNP; LRC,
label-retaining cell; OXPHOS, oxidative phosphorylation; PGC1a, peroxisome proliferator-activated receptor-c coactivator 1a; PKM, pyruvate
kinase M; PPP, pentose phosphate pathway; SC, satellite cell; TCA, tricarboxylic cycle.
4004
FEBS Journal 280 (2013) 4004–4013 ª 2013 The Author Journal compilation ª 2013 FEBS
Satellite cell metabolism
J. G. Ryall
Myonuclei
Capillary
Satellite cell
Pax7 CD31 DAPI
Fig. 1. Immunofluorescence/differential
interference
contrast
image highlighting the colocalization of SCs [marked by Pax7
(green) staining] with blood vessels [marked by CD31 (red)
staining] in skeletal muscle [2]. Nuclei are stained with 4′,6diamidino-2-phenylindole (DAPI) (blue), and myonuclei can be
identified as the nuclei located within the myofiber.
quiescent SCs that do not express Myf5. These authors
proposed that this Pax7+Myf5 population of SCs
(~ 10% of all SCs) represents the population of ‘true’
muscle stem cells, capable of repopulating the quiescent
SC pool, whereas the Pax7+Myf5+ population represents a pool of committed muscle progenitor cells [8].
Furthermore, following activation, Pax7+Myf5 SCs
were found to divide both symmetrically and asymmetrically, the latter resulting in one Pax7+Myf5 SC and
one Pax7+Myf5+ SC.
In addition to differential protein segregation during
asymmetric SC division, DNA has also been found to
selectively segregate during cell division. In this model,
it has been observed that, during cell division, the template DNA strand is retained by the daughter cell,
which will return to the niche (the muscle stem cell),
and the replicant strand is segregated into the daughter
cell, which will undergo further rounds of symmetric
division and eventually differentiate [9]. In this manner, the true stem-like SC population has been proposed to limit mutations in the DNA introduced
during replication, a process that has been termed the
‘immortal DNA strand’ hypothesis [10].
The creative use of transgenic mouse models has shed
further light on the heterogeneity of the endogenous SC
pool. Chakkalakal et al. used a transgenic mouse line
harboring an inducible histone H2B–green fluorescent
protein mutation to model SC turnover via transient
green fluorescent protein induction followed by a
20-month chase [11]. In this study, the authors demonstrated two clear populations of SCs, label-retaining
cells (LRCs) and non-LRCs, with the LRC population
expressing higher levels of Pax7 and markers of quiescence, and lower levels of Myf5. Finally, in transplantation experiments, the LRC population of SCs showed a
greater propensity for self-renewal, providing support
for the LRC population containing the SC stem population [11].
Since the identification of heterogeneity among the
SC population, numerous studies have identified similar small subpopulations of ‘stem-like’ cells, which
have been referred to as side-population cells, reserve
cells, LRCs, and/or muscle-derived stem cells. The system is further complicated by the discovery that nonmyogenic cells can also contribute to the myogenic
lineage, such as PW1+/Pax7 interstitial cells and mesangioblasts [12,13].
In response to an activating signal, such as physical
or chemically induced trauma, SCs leave the quiescent
cell state (G0) and enter the cell cycle (G1; Fig. 2). During this phase, expression of the master myogenic regulator MyoD1 is induced, and SCs undergo multiple
rounds of proliferation. Cycles of proliferation are followed by cellular differentiation [indicated via upregulation of myogenin (MyoG)], fusion, and growth and
maturation to form mature myofibers. A small population of proliferating SCs (probably the SC stem cell
population) exit the cell cycle early and return to the
quiescent state; in this way, SCs are able to repopulate
after a bout of activation and proliferation [5,14].
Over the last few years, a number of researchers
have focused on the active regulation of the quiescent
SC state, with investigations identifying numerous factors that maintain and/or restore quiescence to a previously activated SC. In one such study, Shea et al.
demonstrated the importance of the receptor tyrosine
kinase inhibitor Sprouty1 (Spry1) for both the maintenance of quiescence and promotion of the self-renewal
capacity of SCs [15]. A follow-up study by this group
went on to demonstrate that Spry1 expression was elevated in the LRC population of SCs [11], providing
further support for a role for Spry1 in the process of
SC self-renewal. Furthermore, Spry1 was found to be
negatively regulated via fibroblast growth factor 2
released from the muscle fiber and its receptor present
on the SC, such that an increase in fibroblast growth
factor 2 in the fiber inhibited SC Spry1 and led to SC
activation. These exciting results demonstrate the crosstalk that exists between the skeletal muscle fiber and
the SC pool [16].
The space that surrounds the SC between the basal
lamina and the sarcolemma has been termed the SC
niche. The majority of adult stem cells have been
found to localize to a specialized niche [17], and a
FEBS Journal 280 (2013) 4004–4013 ª 2013 The Author Journal compilation ª 2013 FEBS
4005
Satellite cell metabolism
J. G. Ryall
Quiescence
Pax7+/Myf5+/–
G0
Activation/
SpecificaƟon
Self-renewal
Pax7+/Myf5+
Pax7+/Myf5–
G1
Commitment
Pax7+/Myf5+/MyoD+
M
Proliferation
Pax7+/Myf5+/MyoD+
S
Oxidative versus glycolytic
metabolism
G2
Differentiation
Pax7–/Myf5–/MyoD+/MyoG+
Fusion/MaturaƟon
Fig. 2. Proposed model of the SC life cycle. SCs exist as a
heterogeneous population, as determined via Myf5 positivity
(among other markers), in the quiescent (G0) state. Upon receiving
an activating signal, SCs enter the cell cycle (G1), upregulate
MyoD, and undergo multiple rounds of proliferation and expansion.
This expansion process is then followed by the eventual terminal
differentiation of SCs (marked via the expression of MyoG), fusion,
and maturation. A subpopulation of SCs (proposed to be Myf5negative) re-enter the quiescent state, allowing for the self-renewal
of the SC population and prevention of SC exhaustion.
number of exciting studies have proposed that stem
cell function can be regulated via changes to the niche
environment [11,18]. It is interesting to speculate that
the metabolic milieu of the SC niche may be different
from that of the muscle fiber and/or the extracellular
space. Thus, damage or trauma to the muscle would
be expected to destroy the niche and expose the SC to
an altered metabolic environment, leading to rapid
changes in both nutrient uptake and intracellular
metabolism – a result likely to lead to changes in
metabolite levels. Similarly, any inflammatory response
that was initiated in response to injury or trauma
would be expected to alter the availability of both
nutrients and oxygen, owing to the infiltration of
energy-demanding immune cells (for review see [19]).
This infiltration of immune cells would likely lead to a
hypoxic environment low in nutrients during the early
stages of damage to skeletal muscle.
4006
Whereas the last few decades have seen a great
expansion in our understanding of the transcriptional
regulation of the processes of activation, specification,
proliferation and differentiation of SCs, it is likely that
the next decade will focus on the initiating signals that
lead to the activation of these complex transcriptional
networks. This review will aim to discuss our current
understanding of how metabolic cell status can inform
cell fate decisions and cellular proliferation in other
stem cell populations, as well as to look at what has
previously been described in tumor cells and how this
might relate to SC function. The major metabolic
pathways will first be briefly described.
SCs, like all other cells, require energy to carry out the
reactions necessary for life. However, it is clear that
the energetic demands placed on quiescent, proliferating and differentiating cells are likely to be very
different. Furthermore, both proliferating and differentiating cells must rapidly generate new biomass in the
form of nucleotides, proteins, and phospholipids, to
support rapid cell division and growth.
The breakdown of ATP into ADP or AMP and
inorganic phosphate is the predominant source of cellular energy used to drive enzymatic reactions, and is
required to maintain cellular homeostasis. The generation of ATP predominantly occurs in the mitochondria
via oxidative phosphorylation (OXPHOS), or in the
cytoplasm via glycolysis, and a brief overview of these
major pathways is given in Fig. 3, and in more detail
in a number of excellent reviews [20,21]. Substrate
(fats, carbohydrates, and proteins) and oxygen availability, as well as energy demand, can dictate the pathway used for ATP generation. At the cellular level,
both free fatty acids (FFAs) and glucose are broken
down into CoA-SAc in the presence of oxygen, via
b-oxidation (FFAs) or glycolysis (glucose). CoA-SAc,
in turn, enters the tricarboxylic cycle (TCA) (also
referred to as the citric acid or Krebs cycle), resulting
in the reduction of NAD+ to NADH, which is used
to drive complex I (the NADH dehydrogenase), and
the production of succinate, which is used to drive
complex II (the succinate dehydrogenase) of the mitochondrial electron transport chain to generate ATP.
In the absence of oxygen, glucose is shunted away
from the OXPHOS pathway, and ATP is instead generated via anaerobic glycolysis, a process leading to
the conversion of pyruvate into lactate, instead of
CoA-SAc. Glycolysis is an inefficient method of generating ATP, as each molecule of glucose generates a net
FEBS Journal 280 (2013) 4004–4013 ª 2013 The Author Journal compilation ª 2013 FEBS
Satellite cell metabolism
J. G. Ryall
Glucose
FA
Extracellular
Intracellular
Glucose 6phosphate
Ribose 5phosphate
Fructose 6phosphate
NAD+
FFA
Glyceraldehyde
3-phosphate
NAD+
NADH
NADH
3-Phosphoglycerate
NAD+
NADH
Nucleotide
biosynthesis
Lipid
biosynthesis
Amino acid
biosynthesis
Phosphoenolpyruvate
NAD+
Lactate
NADH
Pkm2
Pyruvate
Pkm1
CoA-SAc
Acyl-CoA
TCA cycle
NADH
NAD+
ETC
NADH
NAD+
Mitochondria
ATP
Fig. 3. Metabolic pathways leading to ATP generation. Fatty acids (FAs) and glucose serve as the two major energy substrates to generate
ATP (glutamine is a third substrate but is not described here). Whereas both FAs and glucose can be used to generate ATP, only glucose
can generate the glycolytic intermediates required for macromolecular synthesis. ETC, electron transport chain.
gain of two ATP molecules; in the OXPHOS pathway,
32–36 molecules of ATP are generated. Although it is
relatively inefficient, glycolysis provides a number of
important advantages for cells, including the ability to
rapidly generate ATP in response to acute changes in
energy demand [22], as well as generating the necessary
glycolytic intermediates for the biosynthesis of new
macromolecules via the pentose phosphate pathway
(PPP) (Fig. 3).
SC metabolism during quiescence,
specification, and proliferation
The metabolic regulation of skeletal muscle at both
the whole muscle and single-fiber levels has received
significant attention over the last century [23–25]; how-
ever, very little is known about the energy demands
and metabolic status of SCs, or how the energy
demands may change as SCs shift from quiescence to
an actively proliferating state, and then again to differentiation. As compared with actively proliferating SCs
(and myofibers), quiescent SCs are small in size, and
are composed primarily of the nucleus surrounded by
a small layer of cytoplasm. Like many stem cell populations [26], SCs contain a paucity of mitochondria,
which appear to be tightly packed around the nucleus
[27]. Following differentiation and fusion into myotubes/myofibers, there is a dramatic increase in both the
density of mitochondria, and the complexity and organization.
It is interesting to note that SC numbers can be
influenced by the fiber that they are attached to, with
FEBS Journal 280 (2013) 4004–4013 ª 2013 The Author Journal compilation ª 2013 FEBS
4007
Satellite cell metabolism
J. G. Ryall
more SCs being associated with fibers that are predominantly oxidative (slow, type I fibers) than with
fibers that rely primarily on glycolysis (fast, type II
fibers) [2,28]. However, whether this is attributable to
direct signaling from the fiber to the SC population
and what role the metabolic status of the fiber may
play in SC biology have yet to be investigated. An
interesting corollary to the observation that oxidative
fibers contain more SCs is the finding that aging is
associated with an increase in the proportion of oxidative fibers [29,30], but the number of SCs decreases
[11]. These disparate findings between the metabolic
status of the fiber and the SC number during aging is
worthy of further investigation, as is the underlying
question of whether the metabolic status of the fiber
influences SC number.
With the advent of large-scale microarray technology and, more recently, whole transcriptome sequencing (RNA sequencing), our ability to investigate the
gene profiles of different SC states has been greatly
enhanced [31,32]. Fukada et al. used a combination of
fluorescence-activated cell sorting and Affymetrix
microarrays to probe the gene expression of both quiescent SCs and cultured proliferating SCs [31]. In addition to identifying a number of expected changes in
genes that regulate cell–cell adhesion in quiescent SCs,
these authors identified an enrichment of genes that
regulate lipid transport. A similar observation has
been made in hematopoietic stem cells, which have
been found to consume high levels of fatty acids
[33,34]. Lipid biosynthesis and/or liberation of fatty
acids from lipid stores is essential for integration into
new plasma membranes, but fatty acids can also act as
second messenger signaling molecules, as is the case
with arachidonic acid, prostaglandins, and diacylyglycerol. Whether these signaling molecules are present in
quiescent SCs and what role (if any) they play in
maintaining the quiescent state or cell identity remain
to be seen [35].
In one of the first studies to examine metabolic
activity in quiescent SCs, Rocheteau et al. identified
two distinct populations of quiescent SCs based on
Pax7 expression levels: a Pax7Hi population and a
Pax7Lo population [9]. The Pax7Hi population of SCs
showed reduced mitochondrial staining and levels of
ATP production, suggesting that they were metabolically quieter than the Pax7Lo SC population. Furthermore, the expression levels of both mitochondrial
transcription factor A, mitochondrial-specific polymerase c (PolG) and polymerase c2 were reduced in the
Pax7Hi population of SCs. Interestingly, the Pax7Hi
population took longer to undergo first division,
expressed higher levels of the Cxcr4 and CD34 genes
4008
(markers of ‘stemness’), underwent template DNA
strand selection, and was identified as the population
responsible for SC self-renewal.
In another study, the viability of SCs isolated from
cadavers was examined [27]. Although the number of
viable SCs progressively declined, a population of
functional and viable SCs could still be isolated at
17 days post-mortem. Like the population of Pax7Hi
cells, the viable SCs from cadavers were found to exist
at a lower metabolic state, as demonstrated by a
reduced level of oxygen consumption and ATP production. These results suggest that differences in
metabolism exist in the heterogeneous population of
SCs, and that these differences are associated with
both cell fate decisions leading to self-renewal and SC
survival.
Further evidence of a link between metabolic status
and stem cell function is provided by a number of
studies that have found that caloric restriction (CR)
helps to maintain stem cell function with aging [36–
38]. Cerletti et al. found that the mitochondrial abundance and oxygen consumption of SCs was increased
in CR mice, and this was associated with an increase
in SC transplant efficiency [36]. These findings are
interesting, as CR has been found to induce the activity of both AMP-activated kinase (AMPK), and the
class III histone deacetylase SIRT1, two proteins that
act as energy and redox sensors, respectively (discussed
in more detail below). Although very little is known
about the role of these proteins in SC quiescence,
increased AMPK activity has been proposed to inhibit
both cell proliferation and differentiation [39–41], and
SIRT1 has been found to inhibit cell differentiation
[39,42–44]. Cerletti et al. found an increase in the level
of SIRT1 protein in SCs isolated from CR mice; however, AMPK was not investigated. Further studies will
be required to help identify the link between these two
important regulators, the regulation of SC quiescence,
activation, and proliferation, and the maintenance of
cell identity.
It is worth noting that Rocheteau et al. identified
SCs showing the lowest level of mitochondrial activity
as the population most capable of replenishing the SC
pool [9]. In contrast, Cerletti et al. proposed the elevated SC mitochondrial activity as the likely mechanism for the observed increase in SC transplant
efficiency [36]. Although not investigated by these
authors, it would be interesting to examine whether
donor SCs from CR animals contribute to the quiescent SC pool of the host animal, or whether the donor
SCs are limited to contributing to differentiated myofibers. Similarly, a measure of the frequency of symmetric versus asymmetric divisions in the CR population
FEBS Journal 280 (2013) 4004–4013 ª 2013 The Author Journal compilation ª 2013 FEBS
Satellite cell metabolism
J. G. Ryall
of SCs may provide insights into the mechanism of
these interesting findings.
Metabolic reprogramming during SC
differentiation and skeletal muscle
growth
In contrast to the early stages of the SC cycle (quiescence, specification/activation, and proliferation), the
importance of nutrient availability and metabolism in
the process of differentiation has been well described.
Utilizing the C2C12 myogenic cell line, a number of
early studies demonstrated a clear reliance upon glycolysis in proliferating myoblasts, with an increase in
both mitochondrial density and OXPHOS activity following differentiation [45–47]. In addition, gene
expression analyses have revealed dramatic changes in
the expression levels of genes that regulate both
nucleic acid and protein metabolism, with an overall
reduction in both of these processes following differentiation [48].
In their 2008 study, Fulco et al. demonstrated that
glucose restriction significantly impaired the differentiation of C2C12 cells, and this defect in differentiation
induced by glucose depletion could not be overcome
by incubation with FFAs, suggesting that glycolytic
byproducts may be required for successful differentiation and growth [39]. These authors also went on to
show that the inhibition of differentiation of C2C12
cells by glucose restriction was dependent on both
AMPK and SIRT1.
As discussed, both AMPK and SIRT1 are able to
respond to changes in the local metabolic environment
to initiate changes in gene transcription; AMPK is
activated in response to a rise in the AMP/ATP ratio
(as occurs during a decrease in nutrient availability, or
a rise in energy expenditure [49]), whereas SIRT1
requires NAD+ to induce substrate deacetylation
[50,51]. Interestingly, the levels of intracellular NAD+
are believed to be directly regulated via cellular metabolism, with glycolysis leading to a decrease in NAD+
levels and an increase in OXPHOS activity (Fig. 3)
[44]. Unexpectedly, the levels of NAD+ have been
found to decrease with C2C12 differentiation, despite
an increase in OXPHOS activity [39], so there are
clearly other factors regulating the level of cellular
NAD+ during changes in cell state.
Both AMPK and SIRT1 have been found to regulate the transcriptional activity of the peroxisome proliferator-activated receptor-c coactivator 1a (PGC1a),
which is believed to be a master regulator of mitochondrial biogenesis. Interestingly, AMPK has been
found to regulate PGC1a activity via SIRT1-mediated
deacetylation, leading to increased PGC1a transcriptional activity [52,53], and increased PGC1a activity
has been shown to inhibit C2C12 differentiation [40].
Together, these results indicate that, for differentiation
to proceed successfully, the activities of both SIRT1
and AMPK need to be downregulated.
Lessons from development, cancer,
and other stem cells – pyruvate kinase
Through research in developmental, cancer and stem
cell biology, it has become apparent that cellular
metabolism plays a large role in the determination of
cell fate. Studies investigating the metabolic changes
occurring in the developing embryo have identified a
shift from OXPHOS in the single-cell embryo to glycolysis in the morula and blastocyst stages [54]. This
switch to glycolysis is believed to allow the developing
embryo to strike a balance between the catabolic processes needed to generate ATP, and the production of
nucleotides, proteins and phospholipids needed for the
anabolic processes required for cell growth [55].
As development proceeds, stem cell populations
become specified to different lineages, a process in
which alterations in energy supply and demand have
been proposed to play an important role [55]. A study
by Yanes et al. used MS-based metabolomics to identify a metabolic signature for pluripotent embryonic
stem cells (ESCs) and differentiated cardiomyoblasts
and neuronal cells [56]. As part of the identified signature, ESCs were found to contain a higher proportion
of unsaturated metabolites, whereas differentiated cells
contained higher proportions of saturated FFAs and
acyl-carnitines. Unsaturated metabolites are rapidly
released from the cell membrane in response to stress,
and are highly prone to oxidation. The authors proposed that the high levels of these metabolites in ESCs
confer metabolic plasticity, priming these cells for differentiation [56]. Given the colocalization of SCs and
blood vessels, it would be interesting to identify a similar metabolomic signature for both quiescent and
active SCs.
Prior to birth, the mammalian fetus receives continual nutrients high in carbohydrates and amino acids
from the mother; in contrast, the immediate postnatal
period is marked by a shift towards an intermittent
nutrient supply that is high in fats (milk) [57]. This dramatic shift in nutrient composition and availability
occurs during a period marked by changes in both SC
number and SC activity. In the embryo, rapid cellular
proliferation, migration and differentiation are required
to generate the embryonic and fetal muscles. The early
postnatal phase is marked by further SC proliferation,
FEBS Journal 280 (2013) 4004–4013 ª 2013 The Author Journal compilation ª 2013 FEBS
4009
Satellite cell metabolism
J. G. Ryall
as well as entry into the SC niche and quiescence for
the first time. Whether the change in whole body
metabolism during this early postnatal phase is linked
to the change in SC function is worthy of further investigation, particularly in light of the previous discussion
regarding lipid transporters in quiescent SCs.
Differential splicing of the muscle isoform of pyruvate kinase M (PKM) at exons 9 and 10 has been
found to be an important regulator of the decision to
shunt glucose breakdown products to either form
CoA-SAc for entry into the mitochondria and the
TCA cycle, or to instead enter the PPP to produce nucleotides, proteins and phospholipids for cell growth
[23,58]. Inclusion of exon 9 produces PKM1, which
catalyzes the dephosphorylation of phosphoenolpyruvate, and promotes the entry of pyruvate into the
mitochondria for conversion to CoA-SAc. In contrast,
exon 10 inclusion produces the PKM2 splice isoform,
which has a reduced affinity for phosphoenolpyruvate
and leads to the build-up of glycolytic intermediates
that are available for entry into the PPP, even in the
presence of oxygen [23]. In tumorigenic cells, this process has been referred to as aerobic glycolysis, or the
Warburg effect [59]. Interestingly, highly proliferative
cells such as ESCs and tumor cells show elevated levels
of PKM2, whereas fully differentiated cells have high
levels of PKM1 [60,61]. The increased expression of
PKM2 in highly proliferative cells has been proposed
to be essential to allow the cells to generate sufficient
intermediates for the generation of new macromolecules through the PPP, including nucleotides and
amino acids.
In 1995, Harada et al. found that PKM2 predominated in proliferating C2C12 myoblasts, but, following
induction of differentiation, the level of PKM1 rapidly
increased, so that it became the predominant splice
isoform [62]. A more recent study confirmed these
results in C2C12 cells at the protein level, and
extended this finding to identify three heterogeneous
nuclear RNPs (hnRNPs) (hnRNPA1, hnRNPA2, and
polypyrimidine tract binding protein 1) as regulators
of the differential splicing of PKM. The hnRNPs were
found to repressively bind to the flanking regions of
exon 9 of the PKM gene, resulting in exon 10 inclusion (PKM2 isoform) in proliferating C2C12 cells,
whereas downregulation of the hnRNPs and exon 9
inclusion (PKM1) was seen in differentiating C2C12
myotubes [63].
PKM2 clearly plays an important role in regulating
glucose metabolism and entry into the PPP, but a series of recent studies have also suggested that it may
play an important role in the regulation of transcription in rapidly proliferating cancer cells. Yang et al.
4010
first demonstrated that PKM2 (and not PKM1) translocated into the nuclei of cancer cells following epidermal growth factor receptor activation [64]. In a followup study, these authors went on to show that this epidermal growth factor receptor-dependent translocation
of PKM2 led to the phosphorylation of histone H3 on
Thr11, which subsequently led to acetylation on the
adjacent Lys9 and transcription of the cell cycle regulators cyclin D1 and C-myc [65]. These exciting findings, combined with those previously indicating
preferential expression of PKM2 in proliferating
C2C12 cells, may indicate a novel role for PKM2 in
regulating the proliferation of SCs.
Conclusions
Whereas a wealth of information exists regarding the
transcriptional regulation of SCs, very little is known
about the initiating signal(s) leading to the activation
of these pathways. Recent work in both cancer and
stem cells has identified changes in cellular metabolism
as an important regulator of cell fate, and studies linking cellular metabolism to SC function are beginning
to appear. With the ongoing advances in our ability to
combine molecular and cellular biology with cellular
physiology, it will be interesting to follow future developments in the re-emerging field of cellular metabolism
and stem cell fate.
Acknowledgements
J. G. Ryall is supported by an Overseas Biomedical
Research Fellowship from the National Health and
Medical Research Council of Australia (NH&MRC).
This work was supported in part by the Intramural
Research Program of the National Institute of Arthritis, and Musculoskeletal and Skin diseases (NIAMS)
at the National Institutes of Health (NIH, Bethesda,
MD, USA), and the University of Melbourne
(Melbourne, Victoria, Australia).
References
1 Scharner J & Zammit PS (2011) The muscle satellite cell
at 50: the formative years. Skelet Muscle 1, 28.
2 Christov C, Chretien F, Abou-Khalil R, Bassez G,
Vallet G, Authier FJ, Bassaglia Y, Shinin V, Tajbakhsh
S, Chazaud B et al. (2007) Muscle satellite cells and
endothelial cells: close neighbors and privileged
partners. Mol Biol Cell 18, 397–409.
3 Mounier R, Chretien F & Chazaud B (2001) Blood
vessels and the satellite cell niche. Curr Top Dev Biol
96, 121–138.
FEBS Journal 280 (2013) 4004–4013 ª 2013 The Author Journal compilation ª 2013 FEBS
Satellite cell metabolism
J. G. Ryall
4 Yin H, Price F & Rudnicki MA (2013) Satellite cells
and the muscle stem cell niche. Physiol Rev 93, 23–67.
5 Brack AS & Rando TA (2012) Tissue-specific stem cells:
lessons from the skeletal muscle satellite cell. Cell Stem
Cell 10, 504–514.
6 Tajbakhsh S (2009) Skeletal muscle stem cells in
developmental versus regenerative myogenesis. J Intern
Med 266, 372–389.
7 Decary S, Mouly V, Hamida CB, Sautet A, Barbet JP
& Butler-Browne GS (1997) Replicative potential and
telomere length in human skeletal muscle: implications
for satellite cell-mediated gene therapy. Hum Gene Ther
8, 1429–1438.
8 Kuang S, Kuroda K, Le Grand F & Rudnicki MA
(2007) Asymmetric self-renewal and commitment of
satellite stem cells in muscle. Cell 129, 999–1010.
9 Rocheteau P, Gayraud-Morel B, Siegl-Cachedenier I,
Blasco MA & Tajbakhsh S (2012) A subpopulation of
adult skeletal muscle stem cells retains all template
DNA strands after cell division. Cell 148, 112–125.
10 Tajbakhsh S & Gonzalez C (2009) Biased segregation
of DNA and centrosomes: moving together or drifting
apart? Nat Rev Mol Cell Biol 10, 804–810.
11 Chakkalakal JV, Jones KM, Basson MA & Brack AS
(2012) The aged niche disrupts muscle stem cell
quiescence. Nature 490, 355–360.
12 Mitchell KJ, Pannerec A, Cadot B, Parlakian A,
Besson V, Gomes ER, Marazzi G & Sassoon DA
(2010) Identification and characterization of a nonsatellite cell muscle resident progenitor during postnatal
development. Nat Cell Biol 12, 257–266.
13 Sampaolesi M, Torrente Y, Innocenzi A, Tonlorenzi R,
D’Antona G, Pellegrino MA, Barresi R, Bresolin N, De
Angelis MG, Campbell KP et al. (2003) Cell therapy of
alpha-sarcoglycan null dystrophic mice through intraarterial delivery of mesoangioblasts. Science 301, 487–
492.
14 Relaix F & Zammit PS (2012) Satellite cells are
essential for skeletal muscle regeneration: the cell on the
edge returns centre stage. Development 139,
2845–2856.
15 Shea KL, Xiang W, LaPorta VS, Licht JD, Keller C,
Basson MA & Brack AS (2010) Sprouty1 regulates
reversible quiescence of a self-renewing adult muscle
stem cell pool during regeneration. Cell Stem Cell 6,
117–129.
16 Rodgers JT & Rando TA (2012) Sprouting: a new take
on stem cell aging. EMBO J 31, 4103–4105.
17 Lander AD, Kimble J, Clevers H, Fuchs E, Montarras
D, Buckingham M, Calof AL, Trumpp A & Oskarsson
T (2012) What does the concept of the stem cell niche
really mean today? BMC Biol 10, 19.
18 Gilbert PM, Havenstrite KL, Magnusson KE, Sacco A,
Leonardi NA, Kraft P, Nguyen NK, Thrun S, Lutolf
MP & Blau HM (2010) Substrate elasticity regulates
19
20
21
22
23
24
25
26
27
28
29
30
31
32
skeletal muscle stem cell self-renewal in culture. Science
329, 1078–1081.
Kominsky DJ, Campbell EL & Colgan SP (2010)
Metabolic shifts in immunity and inflammation. J
Immunol 184, 4062–4068.
Lunt SY & Vander Heiden MG (2011) Aerobic
glycolysis: meeting the metabolic requirements of cell
proliferation. Annu Rev Cell Dev Biol 27, 441–464.
Smeitink J, van den Heuvel L & DiMauro S (2001) The
genetics and pathology of oxidative phosphorylation.
Nat Rev Genet 2, 342–352.
Pfeiffer T, Schuster S & Bonhoeffer S (2001)
Cooperation and competition in the evolution of ATPproducing pathways. Science 292, 504–507.
Gupta RK, Rosen ED & Spiegelman BM (2011)
Identifying novel transcriptional components
controlling energy metabolism. Cell Metab 14, 739–745.
McGee SL & Hargreaves M (2010) Histone
modifications and skeletal muscle metabolic gene
expression. Clin Exp Pharmacol Physiol 37, 392–396.
Schiaffino S & Reggiani C (2011) Fiber types in
mammalian skeletal muscles. Physiol Rev 91, 1447–
1531.
Facucho-Oliveira JM & St John JC (2009) The
relationship between pluripotency and mitochondrial
DNA proliferation during early embryo development
and embryonic stem cell differentiation. Stem Cell Rev
5, 140–158.
Latil M, Rocheteau P, Ch^atre L, Sanulli S, Memet S,
Ricchetti M, Tajbakhsh S & Chretien F (2012) Skeletal
muscle stem cells adopt a dormant cell state post
mortem and retain regenerative capacity. Nat Commun
3, 903.
Manzano R, Toivonen JM, Calvo AC, Miana-Mena
FJ, Zaragoza P, Mu~
noz MJ, Montarras D & Osta R
(2011) Sex, fiber-type, and age dependent in vitro
proliferation of mouse muscle satellite cells. J Cell
Biochem 112, 2825–2836.
Ryall JG, Schertzer JD & Lynch GS (2007)
Attenuation of age-related muscle wasting and
weakness in rats after formoterol treatment: therapeutic
implications for sarcopenia. J Gerontol A Biol Sci Med
Sci 62, 813–823.
Ryall JG, Schertzer JD & Lynch GS (2008) Cellular
and molecular mechanisms underlying age-related
skeletal muscle wasting and weakness. Biogerontology 9,
213–228.
Fukada S, Uezumi A, Ikemoto M, Masuda S, Segawa
M, Tanimura N, Yamamoto H, Miyagoe-Suzuki Y &
Takeda S (2007) Molecular signature of quiescent
satellite cells in adult skeletal muscle. Stem Cells 25,
2448–2459.
Pallafacchina G, Francßois S, Regnault B, Czarny B,
Dive V, Cumano A, Montarras D & Buckingham M
(2010) An adult tissue-specific stem cell in its niche:
FEBS Journal 280 (2013) 4004–4013 ª 2013 The Author Journal compilation ª 2013 FEBS
4011
Satellite cell metabolism
33
34
35
36
37
38
39
40
41
42
43
44
45
J. G. Ryall
a gene profiling analysis of in vivo quiescent and
activated muscle satellite cells. Stem Cell Res 4, 77–91.
Ito K, Carracedo A, Weiss D, Arai F, Ala U, Avigan
DE, Schafer ZT, Evans RM, Suda T, Lee CH et al.
(2012) A PML–PPAR-d pathway for fatty acid
oxidation regulates hematopoietic stem cell
maintenance. Nat Med 18, 1350–1358.
Suda T, Takubo K & Semenza GL (2011) Metabolic
regulation of hematopoietic stem cells in the hypoxic
niche. Cell Stem Cell 9, 298–310.
Zhang J, Nuebel E, Daley GQ, Koehler CM & Teitell
MA (2012) Metabolic regulation in pluripotent stem
cells during reprogramming and self-senewal. Cell Stem
Cell 11, 589–595.
Cerletti M, Jang YC, Finley LW, Haigis MC & Wagers
AJ (2012) Short-term calorie restriction enhances
skeletal muscle stem cell function. Cell Stem Cell 10,
515–519.
Chen J, Astle CM & Harrison DE (2003)
Hematopoietic senescence is postponed and
hematopoietic stem cell function is enhanced by dietary
restriction. Exp Hematol 31, 1097–1103.
€
Yilmaz OH,
Katajisto P, Lamming DW, G€
ultekin Y,
Bauer-Rowe KE, Sengupta S, Birsoy K, Dursun A,
Yilmaz VO, Selig M et al. (2012) mTORC1 in the
Paneth cell niche couples intestinal stem-cell function to
calorie intake. Nature 486, 490–495.
Fulco M, Cen Y, Zhao P, Hoffman EP, McBurney
MW, Sauve AA & Sartorelli V (2008) Glucose
restriction inhibits skeletal myoblast differentiation by
activating SIRT1 through AMPK-mediated regulation
of Nampt. Dev Cell 14, 661–673.
Williamson DL, Butler DC & Alway SE (2009) AMPK
inhibits myoblast differentiation through a PGC1alpha-dependent mechanism. Am J Physiol Endocrinol
Metab 297, E304–E314.
Jones RG, Plas DR, Kubek S, Buzzai M, Mu J, Xu Y,
Birnbaum MJ & Thompson CB (2005) AMP-activated
protein kinase induces a p53-dependent metabolic
checkpoint. Mol Cell 18, 283–293.
Fulco M, Schiltz RL, Iezzi S, King MT, Zhao P,
Kashiwaya Y, Hoffman E, Veech RL & Sartorelli V
(2003) Sir2 regulates skeletal muscle differentiation as a
potential sensor of the redox state. Mol Cell 12,
51–62.
Rathbone CR, Booth FW & Lees SJ (2009) Sirt1
increases skeletal muscle precursor cell proliferation.
Eur J Cell Biol 88, 35–44.
Ryall JG (2012) The role of sirtuins in the regulation of
metabolic homeostasis in skeletal muscle. Curr Opin
Clin Nutr Metab Care 15, 561–566.
Leary SC, Battersby BJ, Hansford RG & Moyes CD
(1998) Interactions between bioenergetics and
mitochondrial biogenesis. Biochim Biophys Acta 1365,
522–530.
4012
46 Lyons CN, Leary SC & Moyes CD (2004) Bioenergetic
remodeling during cellular differentiation: changes in
cytochrome c oxidase regulation do not affect the
metabolic phenotype. Biochem Cell Biol 82, 391–399.
47 Kraft CS, LeMoine CM, Lyons CN, Michaud D,
Mueller CR & Moyes CD (2006) Control of
mitochondrial biogenesis during myogenesis. Am J
Physiol Cell Physiol 290, C1119–C1127.
48 Tomczak KK, Marinescu VD, Ramoni MF, Sanoudou
D, Montanaro F, Han M, Kunkel LM, Kohane IS &
Beggs AH (2004) Expression profiling and identification
of novel genes involved in myogenic differentiation.
FASEB J 18, 403–405.
49 Steinberg GR & Kemp BE (2009) AMPK in health and
disease. Physiol Rev 89, 1025–1078.
50 Vaziri H, Dessain SK, Ng Eaton E, Imai SI, Frye RA,
Pandita TK, Guarente L & Weinberg RA (2001) hSIR2
(SIRT1) functions as an NAD-dependent p53
deacetylase. Cell 107, 149–159.
51 Sauve AA, Celic I, Avalos J, Deng H, Boeke JD &
Schramm VL (2001) Chemistry of gene silencing: the
mechanism of NAD+-dependent deacetylation
reactions. Biochemistry 40, 15456–15463.
52 Cant
o C, Gerhart-Hines Z, Feige JN, Lagouge M,
Noriega L, Milne JC, Elliott PJ, Puigserver P &
Auwerx J (2009) AMPK regulates energy expenditure
by modulating NAD+ metabolism and SIRT1 activity.
Nature 458, 1056–1060.
53 Cant
o C, Jiang LQ, Deshmukh AS, Mataki C, Coste
A, Lagouge M, Zierath JR & Auwerx J (2010)
Interdependence of AMPK and SIRT1 for metabolic
adaptation to fasting and exercise in skeletal muscle.
Cell Metab 11, 213–219.
54 Johnson MT, Mahmood S & Patel MS (2003)
Intermediary metabolism and energetics during murine
early embryogenesis. J Biol Chem 278, 31457–31460.
55 Folmes CD, Dzeja PP, Nelson TJ & Terzic A (2012)
Metabolic plasticity in stem cell homeostasis and
differentiation. Cell Stem Cell 11, 596–606.
56 Yanes O, Clark J, Wong DM, Patti GJ, Sanchez-Ruiz
A, Benton HP, Trauger SA, Desponts C, Ding S &
Siuzdak G (2010) Metabolic oxidation regulates
embryonic stem cell differentiation. Nat Chem Biol 6,
411–417.
57 Schiaffino S, Mammucari C & Sandri M (2008) The
role of autophagy in neonatal tissues: just a response
to amino acid starvation? Autophagy 4, 727–730.
58 Macintyre AN & Rathmell JC (2011) PKM2 and
the tricky balance of growth and energy in cancer.
Mol Cell 42, 713–714.
59 Warburg O (1956) On respiratory impairment in cancer
cells. Science 124, 269–270.
60 Ye J, Mancuso A, Tong X, Ward PS, Fan J,
Rabinowitz JD & Thompson CB (2012) Pyruvate
kinase M2 promotes de novo serine synthesis to sustain
FEBS Journal 280 (2013) 4004–4013 ª 2013 The Author Journal compilation ª 2013 FEBS
Satellite cell metabolism
J. G. Ryall
mTORC1 activity and cell proliferation. Proc Natl
Acad Sci USA 109, 6904–6909.
61 Lv L, Li D, Zhao D, Lin R, Chu Y, Zhang H, Zha Z,
Liu Y, Li Z, Xu Y et al. (2011) Acetylation targets the
M2 isoform of pyruvate kinase for degradation through
chaperone-mediated autophagy and promotes tumor
growth. Mol Cell 42, 719–730.
62 Harada Y, Nakamura M & Asano A (1995)
Temporally distinctive changes of alternative splicing
patterns during myogenic differentiation of C2C12 cells.
J Biochem 118, 780–790.
63 David CJ, Chen M, Assanah M, Canoll P & Manley
JL (2010) HnRNP proteins controlled by c-Myc
deregulate pyruvate kinase mRNA splicing in cancer.
Nature 463, 364–368.
64 Yang W, Xia Y, Ji H, Zheng Y, Liang J, Huang W,
Gao X, Aldape K & Lu Z (2011) Nuclear PKM2
regulates b-catenin transactivation upon EGFR
activation. Nature 480, 118–122.
65 Yang W, Xia Y, Hawke D, Li X, Liang J, Xing D,
Aldape K, Hunter T, Alfred Yung WK & Lu Z (2012)
PKM2 phosphorylates histone H3 and promotes gene
transcription and tumorigenesis. Cell 150, 685–696.
FEBS Journal 280 (2013) 4004–4013 ª 2013 The Author Journal compilation ª 2013 FEBS
4013