Stem Cell Reports Review Interplay between Metabolites and the Epigenome in Regulating Embryonic and Adult Stem Cell Potency and Maintenance Alexandra Harvey,1,4 Giuseppina Caretti,2,4 Viviana Moresi,3,4,* Alessandra Renzini,3 and Sergio Adamo3 1School of BioSciences, University of Melbourne, Parkville, VIC 2010, Australia 2Department of Biosciences, Universita` degli Studi di Milano, Milan, Italy 3Department of Anatomy, Histology, Forensic Medicine & Orthopedics, Histology & Medical Embryology Section, Sapienza University of Rome and Interuniversity Institute of Myology, Rome, Italy 4Co-first author *Correspondence: [email protected] https://doi.org/10.1016/j.stemcr.2019.09.003 The environment surrounding stem cells has the ability to elicit pluripotency is a continuum of cell states that give rise to profound, heritable epigenetic changes orchestrated by multiple the three germ lineages. epigenetic mechanisms, which can be modulated by the level of In contrast to the high proliferation rates of ESCs, ASCs specific metabolites. In this review, we highlight the significance generally exist in a quiescent state where transient cell-cy- of metabolism in regulating stem cell homeostasis, cell state, and cle inhibition prevents exhaustion of the stem cell pool (Ito differentiation capacity, using metabolic regulation of embryonic and Suda, 2014). However, in contrast to pluripotent ESCs, and adult muscle stem cells as examples, and cast light on the inter- most ASCs are lineage restricted, therefore multipotent, action between cellular metabolism and epigenetics. These new regulatory networks, based on the dynamic interplay between maintaining tissue homeostasis, responding to damage metabolism and epigenetics in stem cell biology, are important, and/or stress. Depending on the tissues, certain ASCs not only for understanding tissue homeostasis, but to determine display extensive plasticity and can give rise to different in vitro culture conditions which accurately support normal cell specialized cell types in different organs (Raff, 2003), physiology. whereas other ASCs exhibit more restricted plasticity (Wa- gers and Weissman, 2004). ASCs reside within specialized Basic Features of Embryonic and Adult Stem Cells niches which provide specific cues, including stromal cells, As a result of their ability to self-renew and to differentiate extracellular matrix (ECM), vascularization, and innerva- into different cell types, stem cells play critical roles in tion that support their capacity for self-renewal (Jones development and tissue homeostasis. Both embryonic and Wagers, 2008). ASCs can divide either symmetrically, stem cells (ESCs) and adult stem cells (ASCs) are affected thereby producing two identical cells that replicate and by intrinsic and extrinsic signals, which regulate their expand in number, or asymmetrically, thereby producing self-renewal capacity and the balance between stem cell po- one identical and one committed stem cell, depending tency and differentiation (Jaenisch and Young, 2008). on developmental and environmental signals. Upon The totipotent cleavage-stage preimplantation embryo recruitment, or in certain pathological conditions, ASCs gives rise to the blastocyst, within which the inner cell exit from their quiescent state, re-enter the cell cycle, pro- mass (ICM) is representative of a transient, highly prolifer- liferate, and commit to and differentiate into specific tissue ative pluripotent cell population from which ESCs are lineages. Most ASCs have the ability to switch between derived. In response to local cues, the pluripotent ICM un- asymmetric and symmetric division, and an imbalance be- dergoes lineage specification toward each of the three germ tween the two modalities is often associated with disease layers, progressively differentiating into more specialized states. cell types. A number of pluripotent states have been Muscle stem cells (MuSCs), or satellite cells, are ASCs described in vivo and in vitro (reviewed by Davidson et al., located between the basal lamina and the sarcolemma of 2015), underpinned by differing growth factor and muscle fibers and are crucial for skeletal muscle growth signaling pathway requirements. Self-renewal and mainte- and regeneration (Comai and Tajbakhsh, 2014). Whereas nance of pluripotency of mouse ESCs relies on leukemia MuSC activation and proliferation rely on Notch activity inhibitory factor (LIF) (Brook and Gardner, 1997), whereas (Conboy et al., 2003), the commitment and onset of differ- human ESCs, derived from a later ‘‘primed’’ pluripotent cell entiation is due to a transition from Notch to Wnt population likely representative of post-implantation signaling, the latter being an important regulator of termi- epiblast, rely on activin/Nodal and fibroblast growth factor nal differentiation (Brack et al., 2008). Several growth fac- (FGF) signaling (Vallier et al., 2005). Alternatively, a more tors in the satellite cell niche affect MuSCs, in part by influ- naive pluripotent state can be achieved in culture through encing the temporal transition from Notch to Wnt the inhibition of signaling pathways that regulate differen- signaling. FGF, hepatocyte growth factor, and platelet- tiation (Ying et al., 2008; Zimmerlin et al., 2017). Thus, derived growth factor promote activation and proliferation Stem Cell Reports j Vol. 13 j 573–589 j October 8, 2019 j ª 2019 The Authors. 573 This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Stem Cell Reports Review of MuSCs but delay terminal differentiation. Conversely, however, it maintains a high level of oxygen consumption. MuSC differentiation is primarily promoted by the insu- Indeed, the ICM is predominantly glycolytic, while the lin-like growth factor 1 but severely inhibited by transform- trophectoderm is more oxidative (Hewitson and Leese, ing growth factor b family members (Kuang et al., 2008). 1993). Significantly, these metabolic changes coincide However, while growth factors, cytokines, and the ECM with migration of the embryo from the oviduct to the have traditionally been considered as the signals that uterus, accompanied by changes in the nutrient composi- regulate cell decisions through pathway activation, it is tion of reproductive tract fluid (Gardner et al., 1996), now becoming increasingly apparent that metabolites including a reduction in oxygen availability from 7% to can also act as signaling molecules, interacting with their 2% (Fischer and Bavister, 1993). own receptors and regulating a vast array of cellular func- The glycolytic metabolic state of the pluripotent ICM is tions. Increasing evidence supports a role for metabolism replicated in vitro, whereby ESCs rely on aerobic glycolysis in regulating the complexity of early development and to support biosynthesis (Harvey et al., 2016a), even in the lineage specification. presence of sufficient oxygen, termed the Warburg effect (Harvey et al., 2013). Furthermore, the transition between Metabolic Control of Stem Cells naive and primed pluripotency is accompanied by changes Metabolism underpins cell function, with coordinated in metabolism, whereby naive mouse ESCs exhibit lower nutrient utilization necessary to maintain homeostasis, rates of glycolysis compared with primed ESCs, accompa- including cellular energy (ATP) production and biosyn- nied by upregulation of oxidative metabolism (Zhou thesis to support proliferation (Metallo and Vander Heiden, et al., 2012). In contrast, cultured naive human ESCs 2013). Cell function and the surrounding nutrient micro- exhibit both upregulation of OXPHOS and increased environment determine cellular metabolic requirements, glycolysis relative to their primed counterparts (Gu et al., which are supported by the activity of core metabolic path- 2016), suggesting that differing metabolic profiles likely ways, including glycolysis, the pentose phosphate reflect different cell states within pluripotency. However, pathway, the tricarboxylic acid (TCA) cycle and oxidative these divergent metabolic states may also reflect differences phosphorylation (OXPHOS), which enable adaptation to in medium composition (and hence relative metabolite nutrient availability. This flexibility promotes cell and or- levels) and culture conditions (Zhang et al., 2016a). Indeed, ganism survival, and supports dynamic, stage-specific en- the availability of metabolites has been shown to impact ergy demands through development. However, long-term the balance between pluripotency and differentiation and adaptation contributes to altered cell health, as demon- is important for the maintenance of cell state. strated by numerous diseases characterized by perturba- Glucose and glutamine availability regulate self-renewal tions in metabolism (Cai et al., 2012; Perl, 2017; Wallace, in cultured mouse and human ESCs, through the provision 2012). Consequently, once considered mere by-products of acetyl-CoA and alpha-ketoglutarate (aKG) (Carey et al., of energy production, metabolites are increasingly recog- 2015; Moussaieff et al., 2015). Unlike primed mouse nized for their diverse roles in mediating cell signaling, ESCs, naive mouse ESCs are not dependent on glutamine with emerging evidence from stem cells implicating metab- to maintain proliferation; however, they utilize both olites in the regulation of self-renewal, differentiation, and glucose and glutamine to maintain self-renewal by main- cell state. taining high levels of intracellular aKG (Carey et al., Changing Metabolic Demands