Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Gene regulatory network wikipedia , lookup
Signal transduction wikipedia , lookup
Evolution of metal ions in biological systems wikipedia , lookup
Paracrine signalling wikipedia , lookup
Polyclonal B cell response wikipedia , lookup
Vectors in gene therapy wikipedia , lookup
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