Regulation of glycolysis by Pdk functions as a metabolic checkpoint for cell cycle quiescence in hematopoietic stem cells

Cell-intrinsic factors governing quiescence vis-à-vis activation of adult hematopoietic stem cells

Hematopoiesis is a highly complex process, regulated by both intrinsic and extrinsic factors. Often, these two regulatory arms work in tandem to maintain the steady-state condition of hematopoiesis. However, at times, certain intrinsic attributes of hematopoietic stem cells (HSCs) override the external stimuli and dominate the outcome. These could be genetic events like mutations or environmentally induced epigenetic or transcriptomic changes. Since leukemic stem cells (LSCs) share molecular pathways that also regulate normal HSCs, identifying specific, dominantly acting intrinsic factors could help in the development of novel therapeutic approaches. Here we have reviewed such dominantly acting intrinsic factors governing quiescence vis-à-vis activation of the HSCs in the face of external forces acting on them. For brevity, we have restricted our review to the articles dealing with adult HSCs of human and mouse origin that have been published in the last 10 years. Hematopoietic stem cells (HSCs) are closely associated with various stromal cells in their microenvironment and, thus, constantly receive signaling cues from them. The illustration depicts some dominantly acting intrinsic or cell-autonomous factors operative in the HSCs. These fall into various categories, such as epigenetic regulators, transcription factors, cell cycle regulators, tumor suppressor genes, signaling pathways, and metabolic regulators, which counteract the outcome of extrinsic signaling exerted by the HSC niche.

Loss of sphingosine kinase 2 promotes the expansion of hematopoietic stem cells by improving their metabolic fitness

Hematopoietic stem cells (HSCs) have reduced capacities to properly maintain and replenish the hematopoietic system during myelosuppressive injury or aging. Expanding and rejuvenating HSCs for therapeutic purposes has been a long-sought goal with limited progress. Here, we show that the enzyme Sphk2 (sphingosine kinase 2), which generates the lipid metabolite sphingosine-1-phosphate, is highly expressed in HSCs. The deletion of Sphk2 markedly promotes self-renewal and increases the regenerative potential of HSCs. More importantly, Sphk2 deletion globally preserves the young HSC gene expression pattern, improves the function, and sustains the multilineage potential of HSCs during aging. Mechanistically, Sphk2 interacts with prolyl hydroxylase 2 and the Von Hippel-Lindau protein to facilitate HIF1α ubiquitination in the nucleus independent of the Sphk2 catalytic activity. Deletion of Sphk2 increases hypoxic responses by stabilizing the HIF1α protein to upregulate PDK3, a glycolysis checkpoint protein for HSC quiescence, which subsequently enhances the function of HSCs by improving their metabolic fitness; specifically, it enhances anaerobic glycolysis but suppresses mitochondrial oxidative phosphorylation and generation of reactive oxygen species. Overall, targeting Sphk2 to enhance the metabolic fitness of HSCs is a promising strategy to expand and rejuvenate functional HSCs.

Central gene transcriptional regulatory networks shaping monocyte development in bone marrow

The development of monocytes in bone marrow is a complex process with multiple steps. We used RNA-seq data to analyze the transcriptome profiles in developing stages of monocytes, including hematopoietic stem cells (HSCs), common myeloid progenitors (CMPs), granulocyte-monocyte progenitors (GMPs), and monocytes. We found that genes related to potassium and other cation transmembrane activities and ion binding were upregulated during the differentiation of HSCs into CMPs. Protein transport and membrane surface functional molecules were significantly upregulated in the GMP stage. The CD42RAC and proteasome pathways are significantly upregulated during the development of HSCs into monocytes. Transcription factors Ank1, Runx2, Hmga2, Klf1, Nfia, and Bmyc were upregulated during the differentiation of HSCs into CMPs; Gfi1 and Hmgn2 were highly expressed during the differentiation of CMPs into GMPs; Seventeen transcription factors including Foxo1, Cdkn2d, Foxo3, Ep300, Pias1, Nfkb1, Creb1, Bcl6, Ppp3cb, Stat5b, Nfatc4, Mef2a, Stat6, Ifnar2, Irf7, Irf5, and Cebpb were identified as potentially involved in the development of GMPs into monocytes in mice and humans. In metabolism pathway regulation, HSCs have high glucose, lipid, and nucleic acid metabolism activities; CMPs mainly up regulate the TCA cycle related genes; and GMPs have extremely active metabolisms, with significantly elevated pentose phosphate pathway, TCA cycle, histidine metabolism, and purine metabolism. In the monocyte phase, the tricarboxylic acid (TCA) cycle is reduced, and the anaerobic glycolysis process becomes dominated. Overall, our studies offer the kinetics and maps of gene transcriptional expressions and cell metabolisms during monocyte development in bone marrow.

Approaches towards Elucidating the Metabolic Program of Hematopoietic Stem/Progenitor Cells

Hematopoietic stem cells (HSCs) in bone marrow continuously supply a large number of blood cells throughout life in collaboration with hematopoietic progenitor cells (HPCs). HSCs and HPCs are thought to regulate and utilize intracellular metabolic programs to obtain metabolites, such as adenosine triphosphate (ATP), which is necessary for various cellular functions. Metabolites not only provide stem/progenitor cells with nutrients for ATP and building block generation but are also utilized for protein modification and epigenetic regulation to maintain cellular characteristics. In recent years, the metabolic programs of tissue stem/progenitor cells and their underlying molecular mechanisms have been elucidated using a variety of metabolic analysis methods. In this review, we first present the advantages and disadvantages of the current approaches applicable to the metabolic analysis of tissue stem/progenitor cells, including HSCs and HPCs. In the second half, we discuss the characteristics and regulatory mechanisms of HSC metabolism, including the decoupling of ATP production by glycolysis and mitochondria. These technologies and findings have the potential to advance stem cell biology and engineering from a metabolic perspective and to establish therapeutic approaches.

Mitochondrial Deep Dive into Hematopoietic Stem Cell Dormancy: Not Much Glycolysis but Plenty of Sluggish Lysosomes

The discovery of hematopoietic stem cells (HSCs) heterogeneity has had major implications for investigations of hematopoietic stem cell disorders, clonal hematopoiesis, and HSC aging. More recent studies of the heterogeneity of HSCs' organelles have begun to provide additional insights into HSCs' behavior with far-reaching ramifications for the mechanistic understanding of aging of HSCs and stem cell-derived diseases. Mitochondrial heterogeneity has been explored to expose HSC subsets with distinct properties and functions. Here we review some of the recent advances in these lines of studies that challenged the classic view of glycolysis in HSCs and led to the identification of lysosomes as dynamic pivotal switches in controlling HSC quiescence versus activation beyond their function in autophagy.

Stem cells: Mitochondrial biogenesis links growth and EGFR signaling

Epidermal growth factor receptor signaling is central to cell proliferation, growth, and survival and is often deregulated in cancers. A new study links downstream effectors of this receptor to stem cell growth via mitochondrial biogenesis and metabolic reprogramming.

Identification of the Differentiation Stages of Living Cells from the Six Most Immature Murine Hematopoietic Cell Populations by Multivariate Analysis of Single-Cell Raman Spectra

Efforts to expand hematopoietic stem and progenitor cells (HSPCs) in vitro are motivated by their use in the treatment of leukemias and other blood and immune system diseases. The combinations of extrinsic cues within the hematopoietic stem cell (HSC) niche that lead to HSC fate decisions remain unknown. New noninvasive and location-specific techniques are needed to enable identification of the differentiation stages of individual hematopoietic cells on biomaterial microarray screening platforms that minimize the usage of rare HSCs. Here, we show that a combination of Raman microspectroscopy and partial least-squares discriminant analysis (PLS-DA) enables the location-specific identification of individual living cells from the six most immature hematopoietic cell populations, HSC, multipotent progenitor (MPP)-1, MPP-2, MPP-3, common myeloid progenitor, and common lymphoid progenitor. Better than 90% accuracy was achieved. We show that the accuracy of this differentiation stage identification was based on spectral features associated with cell biochemistries. This work establishes that PLS-DA can capture the subtle spectral variations between as many as six closely related cell populations in the presence of potentially significant within-population spectral variation. This noninvasive approach can be used to screen HSC fate decisions elicited by extrinsic cues within biomaterial microarray screening platforms.

Crosstalk between AML and stromal cells triggers acetate secretion through the metabolic rewiring of stromal cells

Acute myeloid leukaemia (AML) cells interact and modulate components of their surrounding microenvironment into their own benefit. Stromal cells have been shown to support AML survival and progression through various mechanisms. Nonetheless, whether AML cells could establish beneficial metabolic interactions with stromal cells is underexplored. By using a combination of human AML cell lines and AML patient samples together with mouse stromal cells and a MLL-AF9 mouse model, here we identify a novel metabolic crosstalk between AML and stromal cells where AML cells prompt stromal cells to secrete acetate for their own consumption to feed the tricarboxylic acid cycle (TCA) and lipid biosynthesis. By performing transcriptome analysis and tracer-based metabolic NMR analysis, we observe that stromal cells present a higher rate of glycolysis when co-cultured with AML cells. We also find that acetate in stromal cells is derived from pyruvate via chemical conversion under the influence of reactive oxygen species (ROS) following ROS transfer from AML to stromal cells via gap junctions. Overall, we present a unique metabolic communication between AML and stromal cells and propose two different molecular targets, ACSS2 and gap junctions, that could potentially be exploited for adjuvant therapy.

Mechanisms involved in hematopoietic stem cell aging

Hematopoietic stem cells (HSCs) undergo progressive functional decline over time due to both internal and external stressors, leading to aging of the hematopoietic system. A comprehensive understanding of the molecular mechanisms underlying HSC aging will be valuable in developing novel therapies for HSC rejuvenation and to prevent the onset of several age-associated diseases and hematological malignancies. This review considers the general causes of HSC aging that range from cell-intrinsic factors to cell-extrinsic factors. In particular, epigenetics and inflammation have been implicated in the linkage of HSC aging, clonality, and oncogenesis. The challenges in clarifying mechanisms of HSC aging have accelerated the development of therapeutic interventions to rejuvenate HSCs, the major goal of aging research; these details are also discussed in this review.

Hematopoietic Stem Cells and Regeneration

Hematopoietic stem cell (HSC) regeneration is the remarkable process by which extremely rare, normally inactive cells of the bone marrow can replace an entire organ if called to do so by injury or harnessed by transplantation. HSC research is arguably the first quantitative single-cell science and the foundation of adult stem cell biology. Bone marrow transplant is the oldest and most refined technique of regenerative medicine. Here we review the intertwined history of the discovery of HSCs and bone marrow transplant, the molecular and cellular mechanisms of HSC self-renewal, and the use of HSCs and their derivatives for cell therapy.

The role of cellular quiescence in cancer - beyond a quiet passenger

Quiescence, the ability to temporarily halt proliferation, is a conserved process that initially allowed survival of unicellular organisms during inhospitable times and later contributed to the rise of multicellular organisms, becoming key for cell differentiation, size control and tissue homeostasis. In this Review, we explore the concept of cancer as a disease that involves abnormal regulation of cellular quiescence at every step, from malignant transformation to metastatic outgrowth. Indeed, disrupted quiescence regulation can be linked to each of the so-called 'hallmarks of cancer'. As we argue here, quiescence induction contributes to immune evasion and resistance against cell death. In contrast, loss of quiescence underlies sustained proliferative signalling, evasion of growth suppressors, pro-tumorigenic inflammation, angiogenesis and genomic instability. Finally, both acquisition and loss of quiescence are involved in replicative immortality, metastasis and deregulated cellular energetics. We believe that a viewpoint that considers quiescence abnormalities that occur during oncogenesis might change the way we ask fundamental questions and the experimental approaches we take, potentially contributing to novel discoveries that might help to alter the course of cancer therapy.

Amino acid catabolism regulates hematopoietic stem cell proteostasis via a GCN2-eIF2α axis

Hematopoietic stem cells (HSCs) adapt their metabolism to maintenance and proliferation; however, the mechanism remains incompletely understood. Here, we demonstrated that homeostatic HSCs exhibited high amino acid (AA) catabolism to reduce cellular AA levels, which activated the GCN2-eIF2α axis, a protein synthesis inhibitory checkpoint to restrain protein synthesis for maintenance. Furthermore, upon proliferation conditions, HSCs enhanced mitochondrial oxidative phosphorylation (OXPHOS) for higher energy production but decreased AA catabolism to accumulate cellular AAs, which inactivated the GCN2-eIF2α axis to increase protein synthesis and coupled with proteotoxic stress. Importantly, GCN2 deletion impaired HSC function in repopulation and regeneration. Mechanistically, GCN2 maintained proteostasis and inhibited Src-mediated AKT activation to repress mitochondrial OXPHOS in HSCs. Moreover, the glycolytic metabolite, NAD⁺ precursor nicotinamide riboside (NR), accelerated AA catabolism to activate GCN2 and sustain the long-term function of HSCs. Overall, our study uncovered direct links between metabolic alterations and translation control in HSCs during homeostasis and proliferation.

Energy Metabolism on Mitochondrial Maturation and Its Effects on Cardiomyocyte Cell Fate

Alterations in energy metabolism play a major role in the lineage of cardiomyocytes, such as the dramatic changes that occur in the transition from neonate to newborn. As cardiomyocytes mature, they shift from a primarily glycolytic state to a mitochondrial oxidative metabolic state. Metabolic intermediates and metabolites may have epigenetic and transcriptional roles in controlling cell fate by increasing mitochondrial biogenesis. In the maturing cardiomyocyte, such as in the postnatal heart, fatty acid oxidation increases in conjunction with increased mitochondrial biogenesis driven by the transcriptional coregulator PGC1-α. PGC1-α is necessary for mitochondrial biogenesis in the heart at birth, with deficiencies leading to postnatal cardiomyopathy. While stem cell therapy as a treatment for heart failure requires further investigation, studies suggest that adult stem cells may secrete cardioprotective factors which may regulate cardiomyocyte differentiation and survival. This review will discuss how metabolism influences mitochondrial biogenesis and how mitochondrial biogenesis influences cell fate, particularly in the context of the developing cardiomyocyte. The implications of energy metabolism on stem cell differentiation into cardiomyocytes and how this may be utilized as a therapy against heart failure and cardiovascular disease will also be discussed.

Drug Resistance Mechanisms of Acute Myeloid Leukemia Stem Cells

Acute myeloid leukemia (AML) is a polyclonal and heterogeneous hematological malignancy. Relapse and refractory after induction chemotherapy are still challenges for curing AML. Leukemia stem cells (LSCs), accepted to originate from hematopoietic stem/precursor cells, are the main root of leukemogenesis and drug resistance. LSCs are dynamic derivations and possess various elusive resistance mechanisms. In this review, we summarized different primary resistance and remolding mechanisms of LSCs after chemotherapy, as well as the indispensable role of the bone marrow microenvironment on LSCs resistance. Through a detailed and comprehensive review of the spectacle of LSCs resistance, it can provide better strategies for future researches on eradicating LSCs and clinical treatment of AML.

Metabolic Regulation of Hematopoietic Stem Cells

Cellular metabolism is a key regulator of hematopoietic stem cell (HSC) maintenance. HSCs rely on anaerobic glycolysis for energy production to minimize the production of reactive oxygen species and shift toward mitochondrial oxidative phosphorylation upon differentiation. However, increasing evidence has shown that HSCs still maintain a certain level of mitochondrial activity in quiescence, and exhibit high mitochondrial membrane potential, which both support proper HSC function. Since glycolysis and the tricarboxylic acid (TCA) cycle are not directly connected in HSCs, other nutrient pathways, such as amino acid and fatty acid metabolism, generate acetyl-CoA and provide it to the TCA cycle. In this review, we discuss recent insights into the regulatory roles of cellular metabolism in HSCs. Understanding the metabolic requirements of healthy HSCs is of critical importance to the development of new therapies for hematological disorders.

The new metabolic needs of hematopoietic stem cells

PURPOSE OF REVIEW

Hematopoietic stem cells (HSCs) are endowed with high regenerative potential to supply mature blood cells throughout life, under steady state or stress conditions. HSCs are thought to rely on glycolysis when in a quiescent state and to switch to oxidative phosphorylation to meet their metabolic needs during activation. Recently, a series of important studies reveals a higher degree of complexity that goes well beyond the dichotomy between glycolysis and oxidative phosphorylation. The purpose of this review is to summarize the recent findings highlighting the multifaceted metabolic requirements of HSC homeostasis.

RECENT FINDINGS

Emerging evidence points to the importance of lysosomal catabolic activity and noncanonical retinoic acid pathway in maintaining HSC quiescence and stemness. HSC activation into cycle seems to be accompanied by a switch to glycolysis-mitochondrial coupling and to anabolic pathways, including Myc, aspartate-mediated purine synthesis.

SUMMARY

Knowledge of metabolism of HSCs has dramatically increased in the past 2 years and reveals unexpected needs of HSCs during both their quiescent and activated state. Understanding how HSCs use metabolism for their functions will offer new opportunity for HSC-based therapies.

ID2 and HIF-1α collaborate to protect quiescent hematopoietic stem cells from activation, differentiation, and exhaustion

Defining mechanism(s) that maintain tissue stem quiescence is important for improving tissue regeneration, cell therapies, aging, and cancer. We report here that genetic ablation of Id2 in adult hematopoietic stem cells (HSCs) promotes increased HSC activation and differentiation, which results in HSC exhaustion and bone marrow failure over time. Id2^Δ/Δ HSCs showed increased cycling, ROS production, mitochondrial activation, ATP production, and DNA damage compared with Id2^+/+ HSCs, supporting the conclusion that Id2^Δ/Δ HSCs are less quiescent. Mechanistically, HIF-1α expression was decreased in Id2^Δ/Δ HSCs, and stabilization of HIF-1α in Id2^Δ/Δ HSCs restored HSC quiescence and rescued HSC exhaustion. Inhibitor of DNA binding 2 (ID2) promoted HIF-1α expression by binding to the von Hippel-Lindau (VHL) protein and interfering with proteasomal degradation of HIF-1α. HIF-1α promoted Id2 expression and enforced a positive feedback loop between ID2 and HIF-1α to maintain HSC quiescence. Thus, sustained ID2 expression could protect HSCs during stress and improve HSC expansion for gene editing and cell therapies.

Emerging Evidence of the Significance of Thioredoxin-1 in Hematopoietic Stem Cell Aging

The United States is undergoing a demographic shift towards an older population with profound economic, social, and healthcare implications. The number of Americans aged 65 and older will reach 80 million by 2040. The shift will be even more dramatic in the extremes of age, with a projected 400% increase in the population over 85 years old in the next two decades. Understanding the molecular and cellular mechanisms of ageing is crucial to reduce ageing-associated disease and to improve the quality of life for the elderly. In this review, we summarized the changes associated with the ageing of hematopoietic stem cells (HSCs) and what is known about some of the key underlying cellular and molecular pathways. We focus here on the effects of reactive oxygen species and the thioredoxin redox homeostasis system on ageing biology in HSCs and the HSC microenvironment. We present additional data from our lab demonstrating the key role of thioredoxin-1 in regulating HSC ageing.

Deciphering Metabolic Adaptability of Leukemic Stem Cells

Therapeutic targeting of leukemic stem cells is widely studied to control leukemia. An emerging approach gaining popularity is altering metabolism as a potential therapeutic opportunity. Studies have been carried out on hematopoietic and leukemic stem cells to identify vulnerable pathways without impacting the non-transformed, healthy counterparts. While many metabolic studies have been conducted using stem cells, most have been carried out in vitro or on a larger population of progenitor cells due to challenges imposed by the low frequency of stem cells found in vivo. This creates artifacts in the studies carried out, making it difficult to interpret and correlate the findings to stem cells directly. This review discusses the metabolic difference seen between hematopoietic stem cells and leukemic stem cells across different leukemic models. Moreover, we also shed light on the advancements of metabolic techniques and current limitations and areas for additional research of the field to study stem cell metabolism.

Microenvironmental Features Driving Immune Evasion in Myelodysplastic Syndromes and Acute Myeloid Leukemia

Bone marrow, besides the known functions of hematopoiesis, is an active organ of the immune system, functioning as a sanctuary for several mature immune cells. Moreover, evidence suggests that hematopoietic stem cells (the bone marrow’s functional unit) are capable of directly sensing and responding to an array of exogenous stimuli. This chronic immune stimulation is harmful to normal hematopoietic stem cells, while essential for the propagation of myeloid diseases, which show a dysregulated immune microenvironment. The bone marrow microenvironment in myelodysplastic syndromes (MDS) is characterized by chronic inflammatory activity and immune dysfunction, that drive excessive cellular death and through immune evasion assist in cancer cell expansion. Acute myeloid leukemia (AML) is another example of immune response failure, with features that augment immune evasion and suppression. In this review, we will outline some of the functions of the bone marrow with immunological significance and describe the alterations in the immune landscape of MDS and AML that drive disease progression.

Quiescence regulation by normal haematopoietic stem cells and leukaemia stem cells

The haematopoietic system is maintained by rare haematopoietic stem cells (HSCs), which are quiescent most of the time and only divide occasionally to self-renew and/or to undergo commitment to clonal expansion via the generation of highly proliferative progenitor cells. The latter is responsible for the generation of all mature cells of the system through subsequent lineage commitment and terminal differentiation. Cells with similar properties also exist in leukaemias and are known as leukaemia stem cells (LSCs). Quiescence provides essential protection for both HSC and LSC from cytotoxic stress and DNA damage and, in the case of LSCs, likely causes therapy resistance and disease relapse in leukaemia patients. Specific inhibition of LSC quiescence has been considered a promising strategy for eliminating LSCs and curing leukaemias. Although the understanding of mechanisms responsible for quiescence maintenance in these cells remains limited, particularly for LSCs, recent studies have suggested potential differences in their dependency on certain pathways and their levels of stress and DNA damage caused by increased cycling. Such differences likely stem from oncogenic mutations in LSCs and could be specifically exploited for the elimination of LSCs while sparing normal HSCs in the future.

Toward Elucidating Epigenetic and Metabolic Regulation of Stem Cell Lineage Plasticity in Skin Aging

Skin is the largest organ in human body, harboring a plethora of cell types and serving as the organismal barrier. Skin aging such as wrinkling and hair graying is graphically pronounced, and the molecular mechanisms behind these phenotypic manifestations are beginning to unfold. As in many other organs and tissues, epigenetic and metabolic deregulations have emerged as key aging drivers. Particularly in the context of the skin epithelium, the epigenome and metabolome coordinately shape lineage plasticity and orchestrate stem cell function during aging. Our review discusses recent studies that proposed molecular mechanisms that drive the degeneration of hair follicles, a major appendage of the skin. By focusing on skin while comparing it to model organisms and adult stem cells of other tissues, we summarize literature on genotoxic stress, nutritional sensing, metabolic rewiring, mitochondrial activity, and epigenetic regulations of stem cell plasticity. Finally, we speculate about the rejuvenation potential of rate-limiting upstream signals during aging and the dominant role of the tissue microenvironment in dictating aged epithelial stem cell function.

An efficient simplified method for the generation of corneal epithelial cells from human pluripotent stem cells

Corneal epithelial cells derived from human pluripotent stem cells (hPSCs) are an important cell source for preclinical models to test ophthalmic drugs. However, current differentiation protocols lack instructions regarding optimal culturing conditions, which hinders the quality of cells and limits scale-up. Here, we introduce a simplified small molecule-based corneal induction method (SSM-CI) to generate corneal epithelial cells from hPSCs. SSM-CI provides the advantage of minimizing cell-culturing time using two defined culturing media containing TGF-β, and Wnt/β-catenin pathway inhibitors, and bFGF growth factor over 25 days. Compared to the conventional human corneal epithelial cell line (HCE-T) and human primary corneal epithelial cells (hPCEpCs), corneal epithelial cells generated by SSM-CI are well differentiated and express relevant maturation markers, including PAX6 and CK12. RNA-seq analysis indicated the faithful differentiation of hPSCs into corneal epithelia, with significant upregulation of corneal progenitor and adult corneal epithelial phenotypes. Furthermore, despite the initial inhibition of TGF-β and Wnt/β-catenin, upregulation of these pathway-related transcripts was observed in the later stages, indicating their necessity in the generation of mature corneal epithelial cells. Moreover, we observed a shift in gene signatures associated with the metabolic characteristics of mature corneal epithelial cells, involving a decrease in glycolysis and an increase in fatty acid oxidation. This was also attributed to the overexpression of metabolic enzymes and transporter-related transcripts responsible for fatty acid metabolism. Thus, SSM-CI provides a comprehensive method for the generation of functional corneal epithelial cells for use in preclinical models.

Bone Marrow Niches of Hematopoietic Stem and Progenitor Cells

The mammalian hematopoietic system is remarkably efficient in meeting an organism’s vital needs, yet is highly sensitive and exquisitely regulated. Much of the organismal control over hematopoiesis comes from the regulation of hematopoietic stem cells (HSCs) by specific microenvironments called niches in bone marrow (BM), where HSCs reside. The experimental studies of the last two decades using the most sophisticated and advanced techniques have provided important data on the identity of the niche cells controlling HSCs functions and some mechanisms underlying niche-HSC interactions. In this review we discuss various aspects of organization and functioning of the HSC cell niche in bone marrow. In particular, we review the anatomy of BM niches, various cell types composing the niche, niches for more differentiated cells, metabolism of HSCs in relation to the niche, niche aging, leukemic transformation of the niche, and the current state of HSC niche modeling in vitro.

Blood progenitor redox homeostasis through olfaction-derived systemic GABA in hematopoietic growth control in Drosophila

The role of reactive oxygen species (ROS) in myeloid development is well established. However, its aberrant generation alters hematopoiesis. Thus, a comprehensive understanding of events controlling ROS homeostasis forms the central focus of this study. We show that, in homeostasis, myeloid-like blood progenitor cells of the Drosophila larvae, which reside in a specialized hematopoietic organ termed the lymph gland, use TCA to generate ROS. However, excessive ROS production leads to lymph gland growth retardation. Therefore, to moderate blood progenitor ROS, Drosophila larvae rely on olfaction and its downstream systemic GABA. GABA internalization and its breakdown into succinate by progenitor cells activates pyruvate dehydrogenase kinase (PDK), which controls inhibitory phosphorylation of pyruvate dehydrogenase (PDH). PDH is the rate-limiting enzyme that connects pyruvate to the TCA cycle and to oxidative phosphorylation. Thus, GABA metabolism via PDK activation maintains TCA activity and blood progenitor ROS homeostasis, and supports normal lymph gland growth. Consequently, animals that fail to smell also fail to sustain TCA activity and ROS homeostasis, which leads to lymph gland growth retardation. Overall, this study describes the requirement of animal odor-sensing and GABA in myeloid ROS regulation and hematopoietic growth control.

Summary: Ablation of olfactory receptor neurons reveals that odor-sensing and GABA are involved in myeloid reactive oxygen species regulation and hematopoietic growth control.

Mitochondria bridge HIF signaling and ferroptosis blockage in acute kidney injury

Ferroptosis, a form of regulated cell death, plays an important role in acute kidney injury (AKI). Previous studies have shown that prolyl hydroxylase domain protein (PHD) inhibitors that activate HIF signaling provide strong protection against AKI, which is characterized by marked cell death. However, the relationship between PHD inhibition/HIF signaling and ferroptosis in AKI has not been elucidated. Here, we review recent studies to explore the issue. First, we will review the literature concerning the functions of HIF in promoting mitophagy, suppressing mitochondrial respiration and modulating redox homeostasis. Second, we will describe the current understanding of ferroptosis and its role in AKI, particularly from the perspective of mitochondrial dysfunction. Finally, we will discuss the possibility that mitochondria link PHD inhibition/HIF signaling and ferroptosis in AKI. In conclusion, we propose that HIF may protect renal cells against ferroptosis in AKI by reducing mitochondrial oxidative stress and damage.

Molecular regulation of hematopoietic stem cell quiescence

Hematopoietic stem cells (HSCs) are primarily dormant in a cell-cycle quiescence state to preserve their self-renewal capacity and long-term maintenance, which is essential for the homeostasis of hematopoietic system. Dysregulation of quiescence causes HSC dysfunction and may result in aberrant hematopoiesis (e.g., myelodysplastic syndrome and bone marrow failure syndromes) and leukemia transformation. Accumulating evidence indicates that both intrinsic molecular networks and extrinsic signals regulate HSC quiescence, including cell-cycle regulators, transcription factors, epigenetic factors, and niche factors. Further, the transition between quiescence and activation of HSCs is a continuous developmental path driven by cell metabolism (e.g., protein synthesis, glycolysis, oxidative phosphorylation, and autophagy). Elucidating the complex regulatory networks of HSC quiescence will expand the knowledge of HSC hemostasis and benefit for clinical HSC use. Here, we review the current understanding and progression on the molecular and metabolic regulation of HSC quiescence, providing a more complete picture regarding the mechanisms of HSC quiescence maintenance.

Increasing Complexity of Molecular Landscapes in Human Hematopoietic Stem and Progenitor Cells during Development and Aging

The past five decades have seen significant progress in our understanding of human hematopoiesis. This has in part been due to the unprecedented development of advanced technologies, which have allowed the identification and characterization of rare subsets of human hematopoietic stem and progenitor cells and their lineage trajectories from embryonic through to adult life. Additionally, surrogate in vitro and in vivo models, although not fully recapitulating human hematopoiesis, have spurred on these scientific advances. These approaches have heightened our knowledge of hematological disorders and diseases and have led to their improved diagnosis and therapies. Here, we review human hematopoiesis at each end of the age spectrum, during embryonic and fetal development and on aging, providing exemplars of recent progress in deciphering the increasingly complex cellular and molecular hematopoietic landscapes in health and disease. This review concludes by highlighting links between chronic inflammation and metabolic and epigenetic changes associated with aging and in the development of clonal hematopoiesis.

A non-dividing cell population with high pyruvate dehydrogenase kinase activity regulates metabolic heterogeneity and tumorigenesis in the intestine

Although reprogramming of cellular metabolism is a hallmark of cancer, little is known about how metabolic reprogramming contributes to early stages of transformation. Here, we show that the histone deacetylase SIRT6 regulates tumor initiation during intestinal cancer by controlling glucose metabolism. Loss of SIRT6 results in an increase in the number of intestinal stem cells (ISCs), which translates into enhanced tumor initiating potential in APC^min mice. By tracking down the connection between glucose metabolism and tumor initiation, we find a metabolic compartmentalization within the intestinal epithelium and adenomas, where a rare population of cells exhibit features of Warburg-like metabolism characterized by high pyruvate dehydrogenase kinase (PDK) activity. Our results show that these cells are quiescent cells expressing +4 ISCs and enteroendocrine markers. Active glycolysis in these cells suppresses ROS accumulation and enhances their stem cell and tumorigenic potential. Our studies reveal that aerobic glycolysis represents a heterogeneous feature of cancer, and indicate that this metabolic adaptation can occur in non-dividing cells, suggesting a role for the Warburg effect beyond biomass production in tumors.

Metabolic reprogramming upon SIRT6 loss induces tumour formation in the intestine but the mechanism is unclear. Here, the authors show that loss of SIRT6 leads to the expansion of epithelial cells with high pyruvate dehydrogenase kinase activity resulting in enhanced stem cell activity and tumour-initiating potential

CD36-Mediated Fatty Acid Oxidation in Hematopoietic Stem Cells Is a Novel Mechanism of Emergency Hematopoiesis in Response to Infection

Hematopoietic homeostasis depends on the close regulation of hematopoietic stem cell (HSC) activity in the bone marrow. Quiescence and activation in response to stress, among other changes in state, are mediated by shifts in HSC metabolic activity. Although HSC steady-state metabolism is well established, the mechanisms driving HSC activation, proliferation, and differentiation in response to stress remain poorly understood. Here we discuss a study by Mistry et al. that describes a novel metabolic mechanism that fuels HSC activation and expansion. The authors show that to meet their metabolic needs in response to infection, hematopoietic stem and progenitor cells uptake free fatty acids from their microenvironment via CD36 to fuel fatty acid oxidation. These exciting findings suggest that in the context of infection, HSCs undergo a metabolic shift toward fatty acid metabolism that drives emergency hematopoiesis and raise questions about the role of the microenvironment in this process.

Neutrophil Migratory Patterns: Implications for Cardiovascular Disease

The body's inflammatory response involves a series of processes that are necessary for the immune system to mitigate threats from invading pathogens. Leukocyte migration is a crucial process in both homeostatic and inflammatory states. The mechanisms involved in immune cell recruitment to the site of inflammation are numerous and require several cascades and cues of activation. Immune cells have multiple origins and can be recruited from primary and secondary lymphoid, as well as reservoir organs within the body to generate an immune response to certain stimuli. However, no matter the origin, an important aspect of any inflammatory response is the web of networks that facilitates immune cell trafficking. The vasculature is an important organ for this trafficking, especially during an inflammatory response, mainly because it allows cells to migrate towards the source of insult/injury and serves as a reservoir for leukocytes and granulocytes under steady state conditions. One of the most active and vital leukocytes in the immune system's arsenal are neutrophils. Neutrophils exist under two forms in the vasculature: a marginated pool that is attached to the vessel walls, and a demarginated pool that freely circulates within the blood stream. In this review, we seek to present the current consensus on the mechanisms involved in leukocyte margination and demargination, with a focus on the role of neutrophil migration patterns during physio-pathological conditions, in particular diabetes and cardiovascular disease.

The Glycolytic Gatekeeper PDK1 defines different metabolic states between genetically distinct subtypes of human acute myeloid leukemia

Acute myeloid leukemia remains difficult to treat due to strong genetic heterogeneity between and within individual patients. Here, we show that Pyruvate dehydrogenase kinase 1 (PDK1) acts as a targetable determinant of different metabolic states in acute myeloid leukemia (AML). PDK1^low AMLs are OXPHOS-driven, are enriched for leukemic granulocyte-monocyte progenitor (L-GMP) signatures, and are associated with FLT3-ITD and NPM1cyt mutations. PDK1^high AMLs however are OXPHOS^low, wild type for FLT3 and NPM1, and are enriched for stemness signatures. Metabolic states can even differ between genetically distinct subclones within individual patients. Loss of PDK1 activity releases glycolytic cells into an OXPHOS state associated with increased ROS levels resulting in enhanced apoptosis in leukemic but not in healthy stem/progenitor cells. This coincides with an enhanced dependency on glutamine uptake and reduced proliferation in vitro and in vivo in humanized xenograft mouse models. We show that human leukemias display distinct metabolic states and adaptation mechanisms that can serve as targets for treatment.

Acute myeloid leukemia (AML) is genetically a very heterogeneous disease. Here, Erdem et al. uncover heterogeneity in the metabolic landscape of AML and identify Pyruvate dehydrogenase kinase 1 (PDK1) as a targetable determinant of different metabolic states in distinct subtypes of AML.

Exploring the Metabolic Landscape of AML: From Haematopoietic Stem Cells to Myeloblasts and Leukaemic Stem Cells

Despite intensive chemotherapy regimens, up to 60% of adults with acute myeloid leukaemia (AML) will relapse and eventually succumb to their disease. Recent studies suggest that leukaemic stem cells (LSCs) drive AML relapse by residing in the bone marrow niche and adapting their metabolic profile. Metabolic adaptation and LSC plasticity are novel hallmarks of leukemogenesis that provide important biological processes required for tumour initiation, progression and therapeutic responses. These findings highlight the importance of targeting metabolic pathways in leukaemia biology which might serve as the Achilles' heel for the treatment of AML relapse. In this review, we highlight the metabolic differences between normal haematopoietic cells, bulk AML cells and LSCs. Specifically, we focus on four major metabolic pathways dysregulated in AML; (i) glycolysis; (ii) mitochondrial metabolism; (iii) amino acid metabolism; and (iv) lipid metabolism. We then outline established and emerging drug interventions that exploit metabolic dependencies of leukaemic cells in the treatment of AML. The metabolic signature of AML cells alters during different biological conditions such as chemotherapy and quiescence. Therefore, targeting the metabolic vulnerabilities of these cells might selectively eradicate them and improve the overall survival of patients with AML.

Metabolic regulation of somatic stem cells in vivo

Metabolism has been studied mainly in cultured cells or at the level of whole tissues or whole organisms in vivo. Consequently, our understanding of metabolic heterogeneity among cells within tissues is limited, particularly when it comes to rare cells with biologically distinct properties, such as stem cells. Stem cell function, tissue regeneration and cancer suppression are all metabolically regulated, although it is not yet clear whether there are metabolic mechanisms unique to stem cells that regulate their activity and function. Recent work has, however, provided evidence that stem cells do have a metabolic signature that is distinct from that of restricted progenitors and that metabolic changes influence tissue homeostasis and regeneration. Stem cell maintenance throughout life in many tissues depends upon minimizing anabolic pathway activation and cell division. Consequently, stem cell activation by tissue injury is associated with changes in mitochondrial function, lysosome activity and lipid metabolism, potentially at the cost of eroding self-renewal potential. Stem cell metabolism is also regulated by the environment: stem cells metabolically interact with other cells in their niches and are able to sense and adapt to dietary changes. The accelerating understanding of stem cell metabolism is revealing new aspects of tissue homeostasis with the potential to promote tissue regeneration and cancer suppression.

Di(2-ethylhexyl)phthalate impairs erythropoiesis via inducing Klotho expression and not via bioenergetic reprogramming

Di(2-ethylhexyl)phthalate (DEHP) is the most widely used phthalate to manufacture various plastic products. However, the potential effects of DEHP on erythropoiesis have not been investigated comprehensively. Here, we aimed to investigate whether DEHP modulated the function of hematopoietic stem and progenitor cells (HSPCs) to influence erythropoiesis, and to explore the associated mechanisms. In the present study, human cell lines with a capacity to differentiate into erythroid cells and murine bone marrow cells were treated with DEHP. DEHP not only impaired HSPC function, but also suppressed erythroid differentiation in a dose-dependent manner. In addition, DEHP removal restored HSPC activity. To explore how DEHP interfered with erythroid differentiation, we focused on energy metabolism and Klotho expression. DEHP suppressed erythroid differentiation via upregulating Klotho expression, while it did not via modulating cellular bioenergetics. Therefore, our results provided a novel insight into the pathophysiological link between phthalates and dysregulated erythroid differentiation.

Aging and Clonal Behavior of Hematopoietic Stem Cells

Hematopoietic stem cells (HSCs) are the only cell population that possesses both a self-renewing capacity and multipotency, and can give rise to all lineages of blood cells throughout an organism's life. However, the self-renewal capacity of HSCs is not infinite, and cumulative evidence suggests that HSCs alter their function and become less active during organismal aging, leading ultimately to the disruption of hematopoietic homeostasis, such as anemia, perturbed immunity and increased propensity to hematological malignancies. Thus, understanding how HSCs alter their function during aging is a matter of critical importance to prevent or overcome these age-related changes in the blood system. Recent advances in clonal analysis have revealed the functional heterogeneity of murine HSC pools that is established upon development and skewed toward the clonal expansion of functionally poised HSCs during aging. In humans, next-generation sequencing has revealed age-related clonal hematopoiesis that originates from HSC subsets with acquired somatic mutations, and has highlighted it as a significant risk factor for hematological malignancies and cardiovascular diseases. In this review, we summarize the current fate-mapping strategies that are used to track and visualize HSC clonal behavior during development or after stress. We then review the age-related changes in HSCs that can be inherited by daughter cells and act as a cellular memory to form functionally distinct clones. Altogether, we link aging of the hematopoietic system to HSC clonal evolution and discuss how HSC clones with myeloid skewing and low regenerative potential can be expanded during aging.

Integrated Transcriptomics and Metabolomics Analyses of Stress-Induced Murine Hair Follicle Growth Inhibition

Psychological stress plays an important role in hair loss, but the underlying mechanisms are not well-understood, and the effective therapies available to regrow hair are rare. In this study, we established a chronic restraint stress (CRS)-induced hair growth inhibition mouse model and performed a comprehensive analysis of metabolomics and transcriptomics. Metabolomics data analysis showed that the primary and secondary metabolic pathways, such as carbohydrate metabolism, amino acid metabolism, and lipid metabolism were significantly altered in skin tissue of CRS group. Transcriptomics analysis also showed significant changes of genes expression profiles involved in regulation of metabolic processes including arachidonic acid metabolism, glutathione metabolism, glycolysis gluconeogenesis, nicotinate and nicotinamide metabolism, purine metabolism, retinol metabolism and cholesterol metabolism. Furthermore, RNA-Seq analyses also found that numerous genes associated with metabolism were significantly changed, such as Hk-1, in CRS-induced hair growth inhibition. Overall, our study supplied new insights into the hair growth inhibition induced by CRS from the perspective of integrated metabolomics and transcriptomics analyses.

Mitochondrial calcium homeostasis in hematopoietic stem cell: Molecular regulation of quiescence, function, and differentiation

Hematopoiesis is based on the existence of hematopoietic stem cells (HSC) with the capacity to self-proliferate and self-renew or to differentiate into specialized cells. The hematopoietic niche is the essential microenvironment where stem cells reside and integrate various stimuli to determine their fate. Recent studies have identified niche containing high level of calcium (Ca²⁺) suggesting that HSCs are sensitive to Ca²⁺. This is a highly versatile and ubiquitous second messenger that regulates a wide variety of cellular functions. Advanced methods for measuring its concentrations, genetic experiments, cell fate tracing data, single-cell imaging, and transcriptomics studies provide information into its specific roles to integrate signaling into an array of mechanisms that determine HSC identity, lineage potential, maintenance, and self-renewal. Accumulating and contrasting evidence, are revealing Ca²⁺ as a previously unacknowledged feature of HSC, involved in functional maintenance, by regulating multiple actors including transcription and epigenetic factors, Ca²⁺-dependent kinases and mitochondrial physiology. Mitochondria are significant participants in HSC functions and their responsiveness to cellular demands is controlled to a significant extent via Ca²⁺ signals. Recent reports indicate that mitochondrial Ca²⁺ uptake also controls HSC fate. These observations reveal a physiological feature of hematopoietic stem cells that can be harnessed to improve HSC-related disease. In this review, we discuss the current knowledge Ca²⁺ in hematopoietic stem cell focusing on its potential involvement in proliferation, self-renewal and maintenance of HSC and discuss future research directions.

A Long Journey before Cycling: Regulation of Quiescence Exit in Adult Muscle Satellite Cells

Skeletal muscle harbors a pool of stem cells called muscle satellite cells (MuSCs) that are mainly responsible for its robust regenerative capacities. Adult satellite cells are mitotically quiescent in uninjured muscles under homeostasis, but they exit quiescence upon injury to re-enter the cell cycle to proliferate. While most of the expanded satellites cells differentiate and fuse to form new myofibers, some undergo self-renewal to replenish the stem cell pool. Specifically, quiescence exit describes the initial transition of MuSCs from quiescence to the first cell cycle, which takes much longer than the time required for subsequent cell cycles and involves drastic changes in cell size, epigenetic and transcriptomic profiles, and metabolic status. It is, therefore, an essential period indispensable for the success of muscle regeneration. Diverse mechanisms exist in MuSCs to regulate quiescence exit. In this review, we summarize key events that occur during quiescence exit in MuSCs and discuss the molecular regulation of this process with an emphasis on multiple levels of intrinsic regulatory mechanisms. A comprehensive understanding of how quiescence exit is regulated will facilitate satellite cell-based muscle regenerative therapies and advance their applications in various disease and aging conditions.

Pyruvate metabolism guides definitive lineage specification during hematopoietic emergence

During embryonic development, hematopoiesis occurs through primitive and definitive waves, giving rise to distinct blood lineages. Hematopoietic stem cells (HSCs) emerge from hemogenic endothelial (HE) cells, through endothelial-to-hematopoietic transition (EHT). In the adult, HSC quiescence, maintenance, and differentiation are closely linked to changes in metabolism. However, metabolic processes underlying the emergence of HSCs from HE cells remain unclear. Here, we show that the emergence of blood is regulated by multiple metabolic pathways that induce or modulate the differentiation toward specific hematopoietic lineages during human EHT. In both in vitro and in vivo settings, steering pyruvate use toward glycolysis or OXPHOS differentially skews the hematopoietic output of HE cells toward either an erythroid fate with primitive phenotype, or a definitive lymphoid fate, respectively. We demonstrate that glycolysis-mediated differentiation of HE toward primitive erythroid hematopoiesis is dependent on the epigenetic regulator LSD1. In contrast, OXPHOS-mediated differentiation of HE toward definitive hematopoiesis is dependent on cholesterol metabolism. Our findings reveal that during EHT, metabolism is a major regulator of primitive versus definitive hematopoietic differentiation.

Increased RNA Transcription of Energy Source Transporters in Circulating White Blood Cells of Aged Mice

Circulating white blood cells (WBC) contribute toward maintenance of cerebral metabolism and brain function. Recently, we showed that during aging, transcription of metabolism related genes, including energy source transports, in the brain significantly decreased at the hippocampus resulting in impaired neurological functions. In this article, we investigated the changes in RNA transcription of metabolism related genes (glucose transporter 1 [Glut1], Glut3, monocarboxylate transporter 4 [MCT4], hypoxia inducible factor 1-α [Hif1-α], prolyl hydroxylase 3 [PHD3] and pyruvate dehydrogenase kinase 1 [PDK1]) in circulating WBC and correlated these with brain function in mice. Contrary to our expectations, most of these metabolism related genes in circulating WBC significantly increased in aged mice, and correlation between their increased RNA transcription and impaired neurological functions was observed. Bone marrow mononuclear transplantation into aged mice decreased metabolism related genes in WBC with accelerated neurogenesis in the hippocampus. In vitro analysis revealed that cell-cell interaction between WBC and endothelial cells via gap junction is impaired with aging, and blockade of the interaction increased their transcription in WBC. Our findings indicate that gross analysis of RNA transcription of metabolism related genes in circulating WBC has the potential to provide significant information relating to impaired cell-cell interaction between WBC and endothelial cells of aged mice. Additionally, this can serve as a tool to evaluate the change of the cell-cell interaction caused by various treatments or diseases.

Srebf1c preserves hematopoietic stem cell function and survival as a switch of mitochondrial metabolism

Mitochondria are fundamental but complex determinants for hematopoietic stem cell (HSC) maintenance. However, the factors involved in the regulation of mitochondrial metabolism in HSCs and the underlying mechanisms have not been fully elucidated. Here, we identify sterol regulatory element binding factor-1c (Srebf1c) as a key factor in maintaining HSC biology under both steady-state and stress conditions. Srebf1c knockout (Srebf1c^-/-) mice display increased phenotypic HSCs and less HSC quiescence. In addition, Srebf1c deletion compromises the function and survival of HSCs in competitive transplantation or following chemotherapy and irradiation. Mechanistically, SREBF1c restrains the excessive activation of mammalian target of rapamycin (mTOR) signaling and mitochondrial metabolism in HSCs by regulating the expression of tuberous sclerosis complex 1 (Tsc1). Our study demonstrates that Srebf1c plays an important role in regulating HSC fate via the TSC1-mTOR-mitochondria axis.

Chemical augmentation of mitochondrial electron transport chains tunes T cell activation threshold in tumors

BACKGROUND

Cancer immunotherapy shows insufficient efficacy for low immunogenic tumors. Furthermore, tumors often downregulate antigen and major histocompatibility complex expression to escape recognition by T cells, resulting in insufficient T cell receptor (TCR) stimulation in the tumor microenvironment. Thus, augmenting TCR-mediated recognition of tumor antigens is a useful strategy to improve the efficacy of cancer immunotherapy.

METHODS

We screened 310 small molecules from our library and identified PQDN, a small molecule that activates CD8 T cells after TCR engagement, even when antigen stimulation is too weak for their activation. We used inhibitors of mitochondrial functions and Seahorse Flux Analyzer to investigate the mechanism underlying the effect of PQDN on T cells. Effect of PQDN on tumor-infiltrating CD8 T cells was examined using flow cytometry and TCR repertoire analysis.

RESULTS

PQDN increased mitochondrial reciprocal capacity through enhancement of electron transport chains (ETCs) and facilitated glycolysis via mTOR/AKT signaling, resulting in augmented CD8 T cell activation, even when antigen stimulation is extremely weak. Intratumoral administration of this compound into tumor-bearing mice tunes inactivated T cell with tumor antigen recognition potent and expanded functional T cell receptor diversity of tumor-infiltrating T cells, augmenting antitumor immune responses and retarding tumor growth. Furthermore, PQDN has a synergistic potent with T cell dependent immunotherapy, such as checkpoint inhibitory therapy or adoptive cell therapy, even in a low immunogenic tumor. We also demonstrated that this compound enhances the activation of human CD8 T cells.

CONCLUSIONS

These data suggest that tuning the T cell activation threshold by chemical activation of mitochondrial ETC is a new strategy for improving therapeutic efficacy through the activation of low-avidity tumor-specific T cells.

Cadmium exposure reprograms energy metabolism of hematopoietic stem cells to promote myelopoiesis at the expense of lymphopoiesis in mice

Cadmium (Cd) is a highly toxic heavy metal in our living environment. Hematopoietic stem cells (HSC) are ancestors for all blood cells. Therefore understanding the impact of Cd on HSC is significant for public health. The aim of this study was to investigate the impact of Cd²⁺ on energy metabolism of HSC and its involvement in hematopoiesis. Wild-type C57BL/6 mice were treated with 10 ppm of Cd²⁺ via drinking water for 3 months, and thereafter glycolysis and mitochondrial (MT) oxidative phosphorylation (OXPHOS) of HSC in the bone marrow (BM) and their impact on hematopoiesis were evaluated. After Cd²⁺ treatment, HSC had reduced lactate dehydrogenase (LDH) activity and lactate production while having increased pyruvate dehydrogenase (PDH) activity, MT membrane potential, ATP production, oxygen (O₂) consumption and reactive oxygen species (ROS), indicating that Cd²⁺ switched the pattern of energy metabolism from glycolysis to OXPHOS in HSC. Moreover, Cd²⁺ switch of HSC energy metabolism was critically dependent on Wnt5a/Cdc42/calcium (Ca²⁺) signaling triggered by a direct action of Cd²⁺ on HSC. To test the biological significance of Cd²⁺ impact on HSC energy metabolism, HSC were intervened for Ca²⁺, OXPHOS, or ROS in vitro, and thereafter the HSC were transplanted into lethally irradiated recipients to reconstitute the immune system; the transplantation assay indicated that Ca²⁺-dependent MT OXPHOS dominated the skewed myelopoiesis of HSC by Cd²⁺ exposure. Collectively, we revealed that Cd²⁺ exposure activated Wnt5a/Cdc42/Ca²⁺ signaling to reprogram the energy metabolism of HSC to drive myelopoiesis at the expense of lymphopoiesis.

Cholinergic signals preserve haematopoietic stem cell quiescence during regenerative haematopoiesis

The sympathetic nervous system has been evolutionary selected to respond to stress and activates haematopoietic stem cells via noradrenergic signals. However, the pathways preserving haematopoietic stem cell quiescence and maintenance under proliferative stress remain largely unknown. Here we found that cholinergic signals preserve haematopoietic stem cell quiescence in bone-associated (endosteal) bone marrow niches. Bone marrow cholinergic neural signals increase during stress haematopoiesis and are amplified through cholinergic osteoprogenitors. Lack of cholinergic innervation impairs balanced responses to chemotherapy or irradiation and reduces haematopoietic stem cell quiescence and self-renewal. Cholinergic signals activate α7 nicotinic receptor in bone marrow mesenchymal stromal cells leading to increased CXCL12 expression and haematopoietic stem cell quiescence. Consequently, nicotine exposure increases endosteal haematopoietic stem cell quiescence in vivo and impairs hematopoietic regeneration after haematopoietic stem cell transplantation in mice. In humans, smoking history is associated with delayed normalisation of platelet counts after allogeneic haematopoietic stem cell transplantation. These results suggest that cholinergic signals preserve stem cell quiescence under proliferative stress.

The STAT3-MYC axis promotes survival of leukemia stem cells by regulating SLC1A5 and oxidative phosphorylation

Acute myeloid leukemia (AML) is characterized by the presence of leukemia stem cells (LSCs), and failure to fully eradicate this population contributes to disease persistence/relapse. Prior studies have characterized metabolic vulnerabilities of LSCs, which demonstrate preferential reliance on oxidative phosphorylation (OXPHOS) for energy metabolism and survival. In the present study, using both genetic and pharmacologic strategies in primary human AML specimens, we show that signal transducer and activator of transcription 3 (STAT3) mediates OXPHOS in LSCs. STAT3 regulates AML-specific expression of MYC, which in turn controls transcription of the neutral amino acid transporter gene SLC1A5. We show that genetic inhibition of MYC or SLC1A5 acts to phenocopy the impairment of OXPHOS observed with STAT3 inhibition, thereby establishing this axis as a regulatory mechanism linking STAT3 to energy metabolism. Inhibition of SLC1A5 reduces intracellular levels of glutamine, glutathione, and multiple tricarboxylic acid (TCA) cycle metabolites, leading to reduced TCA cycle activity and inhibition of OXPHOS. Based on these findings, we used a novel small molecule STAT3 inhibitor, which binds STAT3 and disrupts STAT3-DNA, to evaluate the biological role of STAT3. We show that STAT3 inhibition selectively leads to cell death in AML stem and progenitor cells derived from newly diagnosed patients and patients who have experienced relapse while sparing normal hematopoietic cells. Together, these findings establish a STAT3-mediated mechanism that controls energy metabolism and survival in primitive AML cells.