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Mitochondria Turnover and Lysosomal Function in Hematopoietic Stem Cell Metabolism

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Mitochondria Turnover and Lysosomal Function in Hematopoietic Stem Cell Metabolism International Journal of Molecular Sciences Review Mitochondria Turnover and Lysosomal Function in Hematopoietic Stem Cell Metabolism Makiko Mochizuki-Kashio 1, Hiroko Shiozaki 2, Toshio Suda 3,4 and Ayako Nakamura-Ishizu 1,* 1 Microanatomy and Developmental Biology, Tokyo Women’s Medical University, 8-1 Kawadacho, Shinjuku-ku, Tokyo 162-8666, Japan; [email protected] 2 Department of Hematology, Tokyo Women’s Medical University, 8-1 Kawadacho, Shinjuku-ku, Tokyo 162-8666, Japan; [email protected] 3 Cancer Science Institute, National University of Singapore, 14 Medical Drive, MD6, Singapore 117599, Singapore; [email protected] 4 International Research Center for Medical Sciences, Kumamoto University, 2-2-1 Honjo, Chuo-ku, Kumamoto City 860-0811, Japan * Correspondence: [email protected] Abstract: Hematopoietic stem cells (HSCs) reside in a hypoxic microenvironment that enables glycolysis-fueled metabolism and reduces oxidative stress. Nonetheless, metabolic regulation in organelles such as the mitochondria and lysosomes as well as autophagic processes have been implicated as essential for the determination of HSC cell fate. This review encompasses the current understanding of anaerobic metabolism in HSCs as well as the emerging roles of mitochondrial metabolism and lysosomal regulation for hematopoietic homeostasis. Keywords: hematopoietic stem cells; ROS; mitochondria; lysosome; autophagy; folliculin Citation: Mochizuki-Kashio, M.; Shiozaki, H.; Suda, T.; Nakamura-Ishizu, A. Mitochondria 1. Introduction Turnover and Lysosomal Function in Hematopoietic cells consist of a heterogenous group of cells originating from hematopoi- Hematopoietic Stem Cell Metabolism. etic stem cells (HSCs) [1]. HSCs differentiate into multi-potent progenitor cells (MPPs) Int. J. Mol. Sci. 2021, 22, 4627. https:// which further produce circulating and tissue-residing blood cells of specific lineage [2]. doi.org/10.3390/ijms22094627 The bone marrow (BM) is the main hematopoietic organ in an adult and houses millions of immature and mature hematopoietic cells. HSCs reside as a rare cell population in the BM Academic Editor: Verena Tretter where they are maintained in quiescence as a reserve pool for hematopoiesis [3]. When Received: 30 March 2021 hematopoiesis is compromised, HSCs self-renew, proliferate and differentiate to replenish Accepted: 26 April 2021 hematopoietic cells. Published: 28 April 2021 As with all cells in an organism, hematopoietic cells utilize adenosine triphosphate (ATP) as a common energy currency. While it is fundamentally known that ATP is pro- Publisher’s Note: MDPI stays neutral duced anaerobically through glycolysis and aerobically through mitochondrial oxidative with regard to jurisdictional claims in phosphorylation (OXPHOS), the contribution of these processes for energy production published maps and institutional affil- varies between hematopoietic cell type. While the BM microenvironment is predominantly iations. hypoxic [4], different hematopoietic cells utilize both glycolysis and OXPHOS at varying degrees for survival. Here we review the metabolic state and regulation of murine HSCs, particularly emphasizing non-oxygen- and oxygen-associated processes in HSC regulation. We will also highlight folliculin (FLCN)-Transcription Factor Binding To IGHM Enhancer Copyright: © 2021 by the authors. 3 (TFE3)/Transcription Factor EB (TFEB) pathway as a critical metabolic regulator of Licensee MDPI, Basel, Switzerland. mitochondria and lysosomes in HSCs. This article is an open access article distributed under the terms and 2. Glycolysis and Oxidative Phosphorylation (OXPHOS) during Hematopoietic Differentiation conditions of the Creative Commons Attribution (CC BY) license (https:// Stem cells rely on glycolysis for energy production in order to reduce oxidative stress, creativecommons.org/licenses/by/ yet upon differentiation, require a robust upregulation of mitochondrial OXPHOS [5]. 4.0/). Likewise, HSCs require a shift in anaerobic to aerobic metabolism to efficiently proliferate Int. J. Mol. Sci. 2021, 22, 4627. https://doi.org/10.3390/ijms22094627 https://www.mdpi.com/journal/ijms Int. J. Mol. Sci. 2021, 22, 4627 2 of 14 and produce mature hematopoietic cells (hypoxic regulation of HSCs has been reviewed extensively in [6]). HSCs reside in the BM niche that is hypoxic compared to other tis- sues [4,7]. The BM hypoxic niche influences HSCs to produce low amounts of energy via anaerobic glycolysis. HSCs express high levels of hypoxia inducible factor-1 alpha (HIF1α) and Hif1α deletion results in loss of HSC quiescence and stem cell potential [6,8]. The deletion of Meis Homeobox 1 (Meis1), which activates transcription of Hif1α, also results in a similar loss of function in HSCs [9,10]. Remaining in a state of metabolic quiescence, HSCs withstand premature exhaustion from cell proliferation. In association, hypoxic culture conditions maintain HSCs in quiescence ex vivo [11]. Specifically, hypoxic culture conditions (in 1% O2 conditions), low concentrations of stem cell factor (SCF) and thrombopoietin (Thpo) with the addition of 4% bovine serum albumin (BSA) as a source of fatty acids maintained quiescent murine HSCs for 1 month in culture [11]. While another recent protocol succeeded in optimizing ex vivo expansion of HSCs under normal oxygen pressure [12], hypoxic culture characteristically maintained HSCs in quiescence. Adult HSCs depend upon discrete genetic and proteomic control in order to utilize anaerobic glycolysis as their main energy source [8,13]. HSCs exhibit high expression of glycolytic enzyme, pyruvate dehydrogenase kinase (Pdk), which downregulates aerobic metabolism through inhibition of pyruvate dehydrogenase (PDH)-mediated conversion of pyruvate to acetyl-CoA [14]. Loss of Pdk2 and Pdk4 in HSCs results in loss of stem cell potential. Furthermore, BM cells from mice deficient of glycolytic enzyme, lactate dehydrogenase A (Ldha) or M2 pyruvate kinase isoform (Pkm2) exhibit significantly low repopulation potential upon BM transplantation [15]. Interestingly, mitochondria activity and reactive oxygen species (ROS) production increased in HSCs only with Ldha and not with Pkm2 deficiency suggesting a complex regulatory network between anaerobic and aerobic metabolism in HSCs. Mitochondria contain the major enzymes that oxidize carbohydrates, proteins and lipids to produce ATP [16]. In this process, appropriate substrates are catabolized to acetyl-CoA which enters the TCA cycle (tricarboxylic acid cycle, or Krebs cycle) in the mi- tochondrial matrix to form reduced cofactors, NADH (nicotinamide adenine dinucleotide) and FADH2 (flavin adenine dinucleotide). NADH and FADH2 are fueled tothe electron transport complex (ETC) on the inner mitochondria membrane (IMM) for aerobic ATP pro- duction [17]. Although HSCs predominantly rely on glycolysis, mitochondrial metabolism is not completely dispensable for their survival. Molecular components of mitochondria OXPHOS have been implicated for the maintenance of HSCs. Deletion of an IMM-specific phosphatase (PTEN-like mitochondrial phosphatase, PTPMT1) resulted in defective aerobic mitochondria metabolism and BM failure [18]. Interestingly, deletion of Ptpmt1 in mature hematopoietic lineage cells (Ptpmt1fl/fl/LysM-Cre+ (deletion in granulocyte-macrophage lineage), Ptpmt1fl/fl/Lck-Cre+ (deletion in T cell lineage) and Ptpmt1fl/fl/CD19-Cre+ (dele- tion in B cell lineage)) did not influence the number and function of these mature cells. Deletion of Rieske iron-sulfur protein (RISP) located in mitochondrial complex III of the ETC also impaired both fetal and adult hematopoiesis [19]. RISP-deficient fetal HSCs exhibit defective differentiation resulting in severe anemia. Induction of deletion in mice also results in a loss of quiescence and survival potential in adult HSCs. Importantly, deletion of RISP resulted in decreased NAD+/NADH ratio, which caused accumulation of 2-hydroxyglutrate (2-HG), an epigenetic moderator. Also, a small portion of DNA is located in the mitochondria (mtDNA) and its integrity influences HSC aging. HSCs with defective a proof-reading-deficient version of mitochondrial DNA polymerase gamma (Polg) exhibit premature aging phenotype [20]. Further investigation is necessary to characterize HSC mitochondria functions under hypoxic conditions and to identify specific metabolites of mitochondria which potentially serve to maintain HSCs. Glycolysis and mitochondrial OXPHOS in HSCs are interlinked with other metabolic pathways such as glutamine metabolism. Glutamine metabolism, or glutaminolysis, pro- vides energy for proliferative cells and involves the deamination of glutamine by glutam- inase (Gls1) in the mitochondria. HSCs exhibit low rates in both glucose and glutamine Int. J. Mol. Sci. 2021, 22, 4627 3 of 14 consumption [21]. It was shown that alternative polyadenylation (APA) regulated conver- sion of Glsisoforms which resulted in upregulation of glutamine metabolism to fuel energy for HSC self-renewal [22]. Glucose and glutamine metabolism have also been implicated in HSC differentiation to erythroid lineage [23]. HSCs commit to erythroid lineage differ- entiation through high expression of the glutamine transporter, ASCT2 (SLC1A5), which stimulated glutamine-driven de novo nucleotide biosynthesis. 3. Reactive Oxygen Species (ROS) Production in Hematopoietic Stem Cells (HSCs) ROS are a group of oxygen-containing molecules that easily interact with and cause oxidative damage to lipids, proteins, and nucleic acids [24]. The electron reduction
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