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Key Points Stem cells perpetuate themselves through self-renewal and they replenish mature cells to maintain tissue homeostasis throughout the lifespan of an organism. Stem cell function is precisely regulated by various intrinsic mechanisms, in coordination with extrinsic stimuli; recent studies have revealed critical roles for stem cell metabolism in maintaining stem cell self-renewal. Stem cells reside within specialized niches (for example, a hypoxic niche), where specific local conditions have a role in maintaining a quiescent state that is essential for preserving the self-renewal capacity of stem cells. Many types of stem cells heavily rely on anaerobic glycolysis, rather than mitochondrial oxidative phosphorylation, to produce the low levels of intracellular reactive oxygen species, which inhibit stem cell ageing. The balance between stem cell quiescence and proliferation is regulated by the nutrient-sensitive PI3K–AKT–mTOR and AMPK pathways, and Gln metabolism. Recent studies have uncovered a crucial role for fatty acid metabolism in the self-renewal of haematopoietic stem cells (HSCs) through the control it exerts over stem cell fate decisions. Studies of mouse models and advances in metabolomic analysis, particularly of haematopoietic stem cells, have revealed how metabolic cues from anaerobic glycolysis, bioenergetic signalling, the AKT–mTOR pathway, and Gln and fatty acid metabolism, affect the balance between stem cell self-renewal and differentiation. Understanding how metabolic pathways regulate fate decisions may be beneficial therapeutically. A distinctive feature of stem cells is their capacity to self-renew to maintain pluripotency. Studies of genetically-engineered mouse models and recent advances in metabolomic analysis, particularly in haematopoietic stem cells, have deepened our understanding of the contribution made by metabolic cues to the regulation of stem cell self-renewal. Many types of stem cells heavily rely on anaerobic glycolysis, and stem cell function is also regulated by bioenergetic signalling, the AKT–mTOR pathway, Gln metabolism and fatty acid metabolism. As maintenance of a stem cell pool requires a finely-tuned balance between self-renewal and differentiation, investigations into the molecular mechanisms and metabolic pathways underlying these decisions hold great therapeutic promise.

Mitochondrial dysfunction and stem cell exhaustion are two hallmarks of aging. In the hematopoietic system, aging is linked to imbalanced immune response and reduced regenerative capacity in hematopoietic stem cells (HSCs), as well as an increased predisposition to a spectrum of diseases, including myelodysplastic syndrome and acute myeloid leukemia. Myeloid-biased differentiation and loss of polarity are distinct features of aged HSCs, which generally exhibit enhanced mitochondrial oxidative phosphorylation and increased production of reactive oxygen species (ROS), suggesting a direct role for mitochondria in the degenerative process. Here, we provide an overview of current knowledge of the mitochondrial mechanisms that contribute to age-related phenotypes in HSCs. These include mitochondrial ROS production, alteration/activation of mitochondrial metabolism, the quality control pathway of mitochondria, and inflammation. Greater understanding of the key machineries of HSC aging will allow us to identify new therapeutic targets for preventing, delaying, or even reversing aspects of this process.

HSCs have a fate choice when they divide; they can self-renew, producing new HSCs, or produce daughter cells that will mature to become committed cells. Technical challenges, however, have long obscured the mechanics of these choices. Advances in flow-sorting have made possible the purification of HSC populations, but available HSC-enriched fractions still include substantial heterogeneity, and single HSCs have proven extremely difficult to track and observe. Advances in single-cell approaches, however, have led to the identification of a highly purified population of hematopoietic stem cells (HSCs) that make a critical contribution to hematopoietic homeostasis through a preference for self-renewing division. Metabolic cues are key regulators of this cell fate choice, and the importance of controlling the population and quality of mitochondria has recently been highlighted to maintain the equilibrium of HSC populations. Leukemic cells also demand tightly regulated metabolism, and shifting the division balance of leukemic cells toward commitment has been considered as a promising therapeutic strategy. A deeper understanding of precisely how specific modes of metabolism control HSC fate is, therefore, of great biological interest, and more importantly will be critical to the development of new therapeutic strategies that target HSC division balance for the treatment of hematological disease.

Leukemia stem cells (LSCs, also known as leukemia-initiating cells) not only drive leukemia initiation and progression, but also contribute to drug resistance and/or disease relapse. Therefore, eradication of every last LSC is critical for a patient’s long-term cure. Chronic myeloid leukemia (CML) is a myeloproliferative disorder that arises from multipotent hematopoietic stem and progenitor cells. Tyrosine kinase inhibitors (TKIs) have dramatically improved long-term outcomes and quality of life for patients with CML in the chronic phase. Point mutations of the kinase domain of BCR-ABL1 lead to TKI resistance through a reduction in drug binding, and as a result, several new generations of TKIs have been introduced to the clinic. Some patients develop TKI resistance without known mutations, however, and the presence of LSCs is believed to be at least partially associated with resistance development and CML relapse. We previously proposed targeting quiescent LSCs as a therapeutic approach to CML, and a number of potential strategies for targeting insensitive LSCs have been presented over the last decade. The identification of specific markers distinguishing CML-LSCs from healthy HSCs, and the potential contributions of the bone marrow microenvironment to CML pathogenesis, have also been explored. Nonetheless, 25% of CML patients are still expected to switch TKIs at least once, and various TKI discontinuation studies have shown a wide range in the incidence of molecular relapse (from 30% to 60%). In this review, we revisit the current knowledge regarding the role(s) of LSCs in CML leukemogenesis and response to pharmacological treatment and explore how durable treatment-free remission may be achieved and maintained after discontinuing TKI treatment.

Land plants produce diverse flavonoids for growth, survival, and reproduction. Chalcone synthase is the first committed enzyme of the flavonoid biosynthetic pathway and catalyzes the production of 2′,4,4′,6′-tetrahydroxychalcone (THC). However, it also produces other polyketides, including p -coumaroyltriacetic acid lactone (CTAL), because of the derailment of the chalcone-producing pathway. This promiscuity of CHS catalysis adversely affects the efficiency of flavonoid biosynthesis, although it is also believed to have led to the evolution of stilbene synthase and p -coumaroyltriacetic acid synthase. In this study, we establish that chalcone isomerase-like proteins (CHILs), which are encoded by genes that are ubiquitous in land plant genomes, bind to CHS to enhance THC production and decrease CTAL formation, thereby rectifying the promiscuous CHS catalysis. This CHIL function has been confirmed in diverse land plant species, and represents a conserved strategy facilitating the efficient influx of substrates from the phenylpropanoid pathway to the flavonoid pathway. Chalcone synthase is the first committed enzyme in the plant flavonoid biosynthesis pathway, yet shows low product specificity in vitro. Here Waki et al. show that chalcone isomerase-like proteins bind to and reduce the catalytic promiscuity of chalcone synthase, ensuring efficient flavonoid production in planta.

Cell cycle quiescence is a critical feature contributing to haematopoietic stem cell (HSC) maintenance. Although various candidate stromal cells have been identified as potential HSC niches, the spatial localization of quiescent HSCs in the bone marrow remains unclear. Here, using a novel approach that combines whole-mount confocal immunofluorescence imaging techniques and computational modelling to analyse significant three-dimensional associations in the mouse bone marrow among vascular structures, stromal cells and HSCs, we show that quiescent HSCs associate specifically with small arterioles that are preferentially found in endosteal bone marrow. These arterioles are ensheathed exclusively by rare NG2 (also known as CSPG4) + pericytes, distinct from sinusoid-associated leptin receptor (LEPR) + cells. Pharmacological or genetic activation of the HSC cell cycle alters the distribution of HSCs from NG2 + periarteriolar niches to LEPR + perisinusoidal niches. Conditional depletion of NG2 + cells induces HSC cycling and reduces functional long-term repopulating HSCs in the bone marrow. These results thus indicate that arteriolar niches are indispensable for maintaining HSC quiescence. Immunofluorescence imaging and computational modelling are used to study the spatial distribution of different cell types within the haematopoietic stem cell (HSC) niche; findings show that quiescent HSCs associate specifically with small arterioles that are preferentially found in the endosteal bone marrow and are essential in maintaining this quiescence. Haematopoietic stem cell niche characterized Paul Frenette and colleagues used whole-mount confocal immunofluorescence imaging and computational modelling to study the spatial distribution of different cell types within the hematopoietic stem cell (HSC) niche. They found that quiescent HSCs associate specifically with small arterioles that are preferentially found in endosteal bone marrow and that these arterioles are essential in maintaining HSC quiescence. These results thus suggest that distinct HSC niches, quiescent or proliferative, are conferred by distinct blood vessel types.

Humans have approximately 400 different olfactory receptors (hORs) and recognize odorants through the repertoire of hOR responses. Although the cell surface expression of hORs is critical to evaluate their response, hORs are poorly expressed on the surface of heterologous cells. To address this problem, previous studies have focused on hOR transportation to the membrane. Nevertheless, the response pattern of hORs to odorants has yet to be successfully linked, and the response sensitivity still remains to be improved. In this study, we demonstrate that increasing the transcriptional level can result in a significant increase in cell surface and functional expression of hORs. We used the TAR-Tat system, which increases the transcription efficiency through positive feedback, and found that OR1A1, OR6N2, and OR51M1 exhibited robust expression. Moreover, this system induces enhanced hOR responses to odorants, thus defining four hORs as novel n-hexanal receptors and n-hexanal is an inverse agonist to one of them. Our results suggested that using the TAR-Tat system and increasing the transcriptional level of hORs can help understanding the relationship between hORs and odorants that were previously undetectable. This finding could facilitate the understanding of the sense of smell by decoding the repertoire of hOR responses.

RNA modifications are emerging as key determinants of gene expression. However, compelling genetic demonstrations of their relevance to human disease are lacking. Here, we link ribosomal RNA 2′-O-methylation (2′-O-Me) to the etiology of dyskeratosis congenita. We identify nucleophosmin (NPM1) as an essential regulator of 2′-O-Me on rRNA by directly binding C/D box small nucleolar RNAs, thereby modulating translation. We demonstrate the importance of 2′-O-Me-regulated translation for cellular growth, differentiation and hematopoietic stem cell maintenance, and show that Npm1 inactivation in adult hematopoietic stem cells results in bone marrow failure. We identify NPM1 germline mutations in patients with dyskeratosis congenita presenting with bone marrow failure and demonstrate that they are deficient in small nucleolar RNA binding. Mice harboring a dyskeratosis congenita germline Npm1 mutation recapitulate both hematological and nonhematological features of dyskeratosis congenita. Thus, our findings indicate that impaired 2′-O-Me can be etiological to human disease. NPM1 regulates ribosomal RNA 2′-O-methylation by binding to small nucleolar RNAs, thereby modulating translation. NPM1 mutations lead to altered 2′-O-methylation and impaired ribosomal function, resulting in bone marrow failure and leukemia susceptibility.

The promyelocytic leukemia (PML) tumor suppressor is a pleiotropic modulator of apoptosis. However, the molecular basis for such a diverse proapoptotic role is currently unknown. We show that extranuclear Pml was specifically enriched at the endoplasmic reticulum (ER) and at the mitochondria-associated membranes, signaling domains involved in ER-to-mitochondria calcium ion (Ca²⁺) transport and in induction of apoptosis. We found Pml in complexes of large molecular size with the inositol 1,4,5-trisphosphate receptor (IP₃R), protein kinase Akt, and protein phosphatase 2a (PP2a). Pml was essential for Akt- and PP2a-dependent modulation of IP₃R phosphorylation and in turn for IP₃R-mediated Ca²⁺ release from ER. Our findings provide a mechanistic explanation for the pleiotropic role of Pml in apoptosis and identify a pharmacological target for the modulation of Ca²⁺ signals.

Hematopoietic stem cells (HSCs) undergo self-renewing cell divisions and maintain blood production for their lifetime 1 . Appropriate control of HSC self-renewal is crucial for the maintenance of hematopoietic homeostasis. Here we show that activation of p38 MAPK in response to increasing levels of reactive oxygen species (ROS) limits the lifespan of HSCs in vivo . In Atm −/− mice, elevation of ROS levels induces HSC-specific phosphorylation of p38 MAPK accompanied by a defect in the maintenance of HSC quiescence. Inhibition of p38 MAPK rescued ROS-induced defects in HSC repopulating capacity and in the maintenance of HSC quiescence, indicating that the ROS–p38 MAPK pathway contributes to exhaustion of the stem cell population. Furthermore, prolonged treatment with an antioxidant or an inhibitor of p38 MAPK extended the lifespan of HSCs from wild-type mice in serial transplantation experiments. These data show that inactivation of p38 MAPK protects HSCs against loss of self-renewal capacity. Our characterization of molecular mechanisms that limit HSC lifespan may lead to beneficial therapies for human disease.