Multilayer omics analysis reveals a non-classical retinoic acid signaling axis that regulates hematopoietic stem cell identity

Metabolism in hematology: Technological advances open new perspectives on disease biology and treatment

The term metabolism refers to the multi-faceted biochemical reactions within a cell or an organism that occur to maintain energy homeostasis, cell growth, and oxidative balance. Cells possess a high metabolic plasticity, allowing them to adapt to the dynamic requirements of their functional state and environment. Deregulated cellular metabolism is a hallmark of many diseases, including benign and malignant hematological conditions. In the last decade, multiple technological innovations in the metabolism field have made in-depth metabolic analysis broadly applicable. Such studies are shedding new light on normal and malignant hematopoiesis and open avenues to a better understanding of the biology of hematological diseases. In this review, we will first give a brief overview of central metabolic processes. Furthermore, we discuss the most commonly used methods to study metabolism. We begin by elaborating on the use of next-generation sequencing to detect metabolism-related genomic mutations and study transcriptional signatures. Furthermore, we discuss methods for measuring protein expression, such as mass spectrometry (MS), flow cytometry, and cytometry time-of-flight. Next, we describe the use of nuclear magnetic resonance spectroscopy, MS, and flow cytometry for metabolite quantification. Finally, we highlight functional assays to probe metabolic pathways in real-time. We illustrate how these technologies and their combination have advanced our understanding of the role of metabolism. Our goal is to provide hematologists with a comprehensive guide to modern techniques in metabolism research, their benefits and disadvantages, and how they guide our understanding of disease and potentially future personalized therapy decisions.

Retinoic acid modulates peritoneal macrophage function and distribution to enhance antibacterial defense during inflammation

Background

Peritoneal macrophages, comprising large peritoneal macrophages (LPMs) and small peritoneal macrophages (SPMs), play a vital role in maintaining immune defenses during inflammation. However, the molecular mechanisms governing their responses, particularly the impact of retinoic acid (RA), remain poorly understood. This study aims to elucidate the role of RA in modulating macrophage function, distribution, and immune responses during bacterial infections.

Methods

A murine model of peritonitis was established using Escherichia coli expressing a tdTomato fluorescence marker. The effects of RA on macrophage phagocytic capacity, population dynamics, and transcriptomic profiles were assessed using immunofluorescence, flow cytometry, RNA sequencing, and quantitative PCR. Additionally, RA-loaded ZIF-8 nanoparticles were employed to investigate the sustained effects of RA delivery.

Results

RA significantly enhanced macrophage phagocytic activity, delayed functional decline, and promoted the recruitment of SPMs in the peritoneal cavity. Transcriptomic analysis revealed upregulation of leukocyte migration and cell adhesion pathways in RA-treated SPMs. RA treatment also induced distinct gene expression profiles in macrophage subpopulations, reflecting its role in immune modulation. Notably, RA-loaded ZIF-8 nanoparticles prolonged RA retention within macrophages, sustaining its effects.

Conclusion

RA enhances antibacterial defense by modulating macrophage activity, providing new insights into immune regulation. These findings underscore the therapeutic potential of RA and its nanoparticle formulations in managing bacterial infections and inflammation.

Modulation of bone marrow haematopoietic stem cell activity as a therapeutic strategy after myocardial infarction: a preclinical study

Myocardial infarction (MI) is a major global health concern. Although myeloid cells are crucial for tissue repair in emergency haematopoiesis after MI, excessive myelopoiesis can exacerbate scarring and impair cardiac function. Bone marrow (BM) haematopoietic stem cells (HSCs) have the unique capability to replenish the haematopoietic system, but their role in emergency haematopoiesis after MI has not yet been established. Here we collected human sternal BM samples from over 150 cardiac surgery patients, selecting 49 with preserved cardiac function. We show that MI causes detrimental transcriptional and functional changes in human BM HSCs. Lineage tracing experiments suggest that HSCs are contributors of pro-inflammatory myeloid cells infiltrating cardiac tissue after MI. Therapeutically, enforcing HSC quiescence with the vitamin A metabolite 4-oxo-retinoic acid dampens inflammatory myelopoiesis, thereby modulating tissue remodelling and preserving long-term cardiac function after MI.

All-trans retinoic acid beyond acute promyelocytic leukemia

All-trans retinoic acid (ATRA), a known regulator of hematopoiesis, is a key component of the established therapeutic regimen for treating acute promyelocytic leukemia (APL). In this issue of Cancer Cell, Mosialou et al. present a niche-based mechanism of ATRA targeting osteoblasts, repurposing ATRA treatment beyond APL.

DNA methylation biomarkers for predicting lymph node metastasis in colorectal cancer

Colorectal cancer is one of the most common cancers worldwide. Lymph node metastasis is an important marker of colorectal cancer progression and plays a key role in the evaluation of patient prognosis. Accurate preoperative assessment of lymph node metastasis is crucial for devising appropriate treatment plans. However, current clinical imaging methods have limitations in many aspects. Therefore, the discovery of a method for accurately predicting lymph node metastasis is crucial clinical decision-making. DNA methylation is a common epigenetic modification that can regulate gene expression, which also has an important impact on the development of colorectal cancer. It is considered to be a promising biomarker with good specificity and stability and has promising application in predicting lymph node metastasis in patients with colorectal cancer. This article reviews the characteristics and limitations of currently available methods for predicting lymph node metastasis in patients with colorectal cancer and discusses the role of DNA methylation as a biomarker.

Hematopoietic stem cell metabolism within the bone marrow niche - insights and opportunities

Hematopoiesis unfolds within the bone marrow niche where hematopoietic stem cells (HSCs) play a central role in continually replenishing blood cells. The hypoxic bone marrow environment imparts peculiar metabolic characteristics to hematopoietic processes. Here, we discuss the internal metabolism of HSCs and describe external influences exerted on HSC metabolism by the bone marrow niche environment. Importantly, we suggest that the metabolic environment and metabolic cues are intertwined with HSC cell fate, and are crucial for hematopoietic processes. Metabolic dysregulation within the bone marrow niche during acute stress, inflammation, and chronic inflammatory conditions can lead to reduced HSC vitality. Additionally, we raise questions regarding metabolic stresses imposed on HSCs during implementation of stem cell protocols such as allo-SCT and gene therapy, and the potential ramifications. Enhancing our comprehension of metabolic influences on HSCs will expand our understanding of pathophysiology in the bone marrow and improve the application of stem cell therapies.

From Complexity to Clarity: Expanding Metabolome Coverage With Innovative Analytical Strategies

Metabolomics, a powerful discipline within systems biology, aims at comprehensive profiling of small molecules in biological samples. The challenges of biological sample complexity are addressed through innovative sample preparation methods, including solid-phase extraction and microextraction techniques, enhancing the detection and quantification of low-abundance metabolites. Advances in chromatographic separation, particularly liquid chromatography (LC) and gas chromatography (GC), coupled with high-resolution (HR) mass spectrometry (MS), have significantly improved the sensitivity, selectivity, and throughput of metabolomic studies. Cutting-edge techniques, such as ion-mobility mass spectrometry (IM-MS) and tandem MS (MS/MS), further expand the capacity for comprehensive metabolite profiling. These advanced analytical platforms each offer unique advantages for metabolomics, with continued technological improvements driving deeper insights into metabolic pathways and biomarker discovery. By providing a detailed overview of current trends and techniques, this review aims to offer valuable insights into the future of metabolomics in human health research and its translational potential in clinical settings. Toward the end, this review also highlights the biomedical applications of metabolomics, emphasizing its role in biomarker discovery, disease diagnostics, personalized medicine, and drug development.

Ascorbate deficiency increases quiescence and self-renewal in hematopoietic stem cells and multipotent progenitors

ABSTRACT

Ascorbate (vitamin C) limits hematopoietic stem cell (HSC) function and suppresses leukemia development, partly by promoting the function of the Tet2 tumor suppressor. In humans, ascorbate is obtained from the diet, whereas in mice, it is synthesized in the liver. In this study, we show that deletion of the Slc23a2 ascorbate transporter from hematopoietic cells depleted ascorbate to undetectable levels in HSCs and multipotent hematopoietic progenitors (MPPs) without altering the plasma ascorbate levels. Slc23a2 deficiency increased HSC reconstituting potential and self-renewal potential upon transplantation into irradiated mice. Slc23a2 deficiency also increased the reconstituting and self-renewal potentials of MPPs, conferring the ability to reconstitute irradiated mice long term. Slc23a2-deficient HSCs and MPPs divided much less frequently than control HSCs and MPPs. Increased self-renewal and reconstituting potential were observed particularly in quiescent Slc23a2-deficient HSCs and MPPs. The effect of Slc23a2 deficiency on MPP self-renewal was not mediated by reduced Tet2 function. Ascorbate thus regulates quiescence and restricts self-renewal potential in HSCs and MPPs such that ascorbate deficiency confers MPPs with long-term self-renewal potential.

Optimization of pre-enrichment strategies for mouse hematopoietic stem cell isolation and metabolomic analysis

Blood cell production arises from the activity of hematopoietic stem cells (HSCs), defined by their self-renewal capacity and ability to give rise to all mature blood cell types. The mouse remains one of the most studied species in hematological research, and markers to define and isolate mouse HSCs are well-established. Given the very low frequency of HSCs in the bone marrow, stem cell pre-enrichment by red blood cell lysis and magnetic cell separation is often performed as part of the isolation process to reduce sorting times. Several pre-enrichment strategies are available, differing in their speed, degree of enrichment, final cell yield, and cost. In the current study, we performed a side-by-side comparison and provide a decision tree to help researchers select a pre-enrichment strategy for mouse HSC isolation depending on their downstream application. We then compared different pre-enrichment techniques in combination with metabolomics analysis of HSCs, where speed, yield and temperature during pre-enrichment are crucial factors, and found that the choice of pre-enrichment strategy significantly impacts the number of metabolites detected and levels of individual metabolites in HSCs.

CXCR4 signaling determines the fate of hematopoietic multipotent progenitors by stimulating mTOR activity and mitochondrial metabolism

Both cell-intrinsic and niche-derived, cell-extrinsic cues drive the specification of hematopoietic multipotent progenitors (MPPs) in the bone marrow, which comprise multipotent MPP1 cells and lineage-restricted MPP2, MPP3, and MPP4 subsets. Patients with WHIM syndrome, a rare congenital immunodeficiency caused by mutations that prevent desensitization of the chemokine receptor CXCR4, have an excess of myeloid cells in the bone marrow. Here, we investigated the effects of increased CXCR4 signaling on the localization and fate of MPPs. Knock-in mice bearing a WHIM syndrome-associated CXCR4 mutation (CXCR41013) phenocopied the myeloid skewing of bone marrow in patients. Whereas MPP4 cells in wild-type mice differentiated into lymphoid cells, MPP4s in CXCR41013 knock-in mice differentiated into myeloid cells. This myeloid rewiring of MPP4s in CXCR41013 knock-in mice was associated with enhanced signaling mediated by the kinase mTOR and increased oxidative phosphorylation (OXPHOS). MPP4s also localized further from arterioles in the bone marrow of knock-in mice compared with wild-type mice, suggesting that the loss of extrinsic cues from the perivascular niche may also contribute to their myeloid skewing. Chronic treatment with the CXCR4 antagonist AMD3100 or the mTOR inhibitor rapamycin restored the lymphoid potential of MPP4s in knock-in mice. Thus, CXCR4 desensitization drives the lymphoid potential of MPP4 cells by dampening the mTOR-dependent metabolic changes that promote myeloid differentiation.

Metabolic regulation in normal and leukemic stem cells

Hematopoietic stem cells (HSCs) and leukemic stem cells (LSCs) are crucial for ensuring hematopoietic homeostasis and driving leukemia progression, respectively. Recent research has revealed that metabolic adaptations significantly regulate the function and survival of these stem cells. In this review, we provide an overview of how metabolic pathways regulate oxidative and proteostatic stresses in HSCs during homeostasis and aging. Furthermore, we highlight targetable metabolic pathways and explore their interactions with epigenetics and the microenvironment in addressing the chemoresistance and immune evasion capacities of LSCs. The metabolic differences between HSCs and LSCs have profound implications for therapeutic strategies.

Retinoids Molecular Probes by Late-stage Azide Insertion - Functional Tools to Decrypt Retinoid Metabolism

Studying the complex and intricate retinoids metabolic pathways by chemical biology approaches requires design and synthesis of biologically functional molecular probes. Only few of such molecular retinoid probes could be found in literature, most of them bearing a molecular structure quite different from natural retinoids. To provide close-to-native retinoid probes, we have developed a versatile late-stage method for the insertion of azide function at the C4 position of several retinoids. This one-step process opens straightforward access to different retinoid and carotenoid probes from commercially available precursors. We have further demonstrated that the different molecular probes retain ability of the original compound to activate genes' transcription, despite azide insertion, highlighting biological activities that were further validated in zebrafish in vivo model. The present work paves the way to future studies on vitamin A's metabolism.

Metabolism and HSC fate: what NADPH is made for

Mitochondrial metabolism plays a central role in the regulation of hematopoietic stem cell (HSC) biology. Mitochondrial fatty acid oxidation (FAO) is pivotal in controlling HSC self-renewal and differentiation. Herein, we discuss recent evidence suggesting that NADPH generated in the mitochondria can influence the fate of HSCs. Although NADPH has multiple functions, HSCs show high levels of NADPH that are preferentially used for cholesterol biosynthesis. Endogenous cholesterol supports the biogenesis of extracellular vesicles (EVs), which are essential for maintaining HSC properties. We also highlight the significance of EVs in hematopoiesis through autocrine signaling. Elucidating the mitochondrial NADPH-cholesterol axis as part of the metabolic requirements of healthy HSCs will facilitate the development of new therapies for hematological disorders.

IκBα controls dormancy in hematopoietic stem cells via retinoic acid during embryonic development

Recent findings suggest that Hematopoietic Stem Cells (HSC) and progenitors arise simultaneously and independently of each other already in the embryonic aorta-gonad mesonephros region, but it is still unknown how their different features are established. Here, we uncover IκBα (Nfkbia, the inhibitor of NF-κB) as a critical regulator of HSC proliferation throughout development. IκBα balances retinoic acid signaling levels together with the epigenetic silencer, PRC2, specifically in HSCs. Loss of IκBα decreases proliferation of HSC and induces a dormancy related gene expression signature instead. Also, IκBα deficient HSCs respond with superior activation to in vitro culture and in serial transplantation. At the molecular level, chromatin regions harboring binding motifs for retinoic acid signaling are hypo-methylated for the PRC2 dependent H3K27me3 mark in IκBα deficient HSCs. Overall, we show that the proliferation index in the developing HSCs is regulated by a IκBα-PRC2 axis, which controls retinoic acid signaling.

EFSA Panel on Nutrition, Novel Foods and Food Allergens (NDA), Turck D, Bohn T, Castenmiller J, de Henauw S, Hirsch‐Ernst K, Knutsen HK, Maciuk A, Mangelsdorf I, McArdle HJ, Pentieva K, Siani A, Thies F, Tsabouri S, Vinceti M, Lietz G, Passeri G, Craciun I, Fabiani L, Horvath Z, Valtueña Martínez S, Naska A. Scientific opinion on the tolerable upper intake level for preformed vitamin A and β-carotene

Following two requests from the European Commission, the EFSA Panel on Nutrition, Novel Foods and Food Allergens (NDA) was asked to deliver a scientific opinion on the revision of the tolerable upper intake level (UL) for preformed vitamin A and β-carotene. Systematic reviews of the literature were conducted for priority adverse health effects of excess vitamin A intake, namely teratogenicity, hepatotoxicity and endpoints related to bone health. Available data did not allow to address whether β-carotene could potentiate preformed vitamin A toxicity. Teratogenicity was selected as the critical effect on which to base the UL for preformed vitamin A. The Panel proposes to retain the UL for preformed vitamin A of 3000 μg RE/day for adults. This UL applies to men and women, including women of child-bearing age, pregnant and lactating women and post-menopausal women. This value was scaled down to other population groups using allometric scaling (body weight0.75), leading to ULs between 600 μg RE/day (infants 4-11 months) and 2600 μg RE/day (adolescents 15-17 years). Based on available intake data, European populations are unlikely to exceed the UL for preformed vitamin A if consumption of liver, offal and products thereof is limited to once per month or less. Women who are planning to become pregnant or who are pregnant are advised not to consume liver products. Lung cancer risk was selected as the critical effect of excess supplemental β-carotene. The available data were not sufficient and suitable to characterise a dose-response relationship and identify a reference point; therefore, no UL could be established. There is no indication that β-carotene intake from the background diet is associated with adverse health effects. Smokers should avoid consuming food supplements containing β-carotene. The use of supplemental β-carotene by the general population should be limited to the purpose of meeting vitamin A requirements.

Nutrient metabolism in regulating intestinal stem cell homeostasis

Intestinal stem cells (ISCs) are known for their remarkable proliferative capacity, making them one of the most active cell populations in the body. However, a high turnover rate of intestinal epithelium raises the likelihood of dysregulated homeostasis, which is known to cause various diseases, including cancer. Maintaining precise control over the homeostasis of ISCs is crucial to preserve the intestinal epithelium's integrity during homeostasis or stressed conditions. Recent research has indicated that nutrients and metabolic pathways can extensively modulate the fate of ISCs. This review will explore recent findings concerning the influence of various nutrients, including lipids, carbohydrates, and vitamin D, on the delicate balance between ISC proliferation and differentiation.

Crosstalk between DNA Damage Repair and Metabolic Regulation in Hematopoietic Stem Cells

Self-renewal and differentiation are two characteristics of hematopoietic stem cells (HSCs). Under steady physiological conditions, most primitive HSCs remain quiescent in the bone marrow (BM). They respond to different stimuli to refresh the blood system. The transition from quiescence to activation is accompanied by major changes in metabolism, a fundamental cellular process in living organisms that produces or consumes energy. Cellular metabolism is now considered to be a key regulator of HSC maintenance. Interestingly, HSCs possess a distinct metabolic profile with a preference for glycolysis rather than oxidative phosphorylation (OXPHOS) for energy production. Byproducts from the cellular metabolism can also damage DNA. To counteract such insults, mammalian cells have evolved a complex and efficient DNA damage repair (DDR) system to eliminate various DNA lesions and guard genomic stability. Given the enormous regenerative potential coupled with the lifetime persistence of HSCs, tight control of HSC genome stability is essential. The intersection of DDR and the HSC metabolism has recently emerged as an area of intense research interest, unraveling the profound connections between genomic stability and cellular energetics. In this brief review, we delve into the interplay between DDR deficiency and the metabolic reprogramming of HSCs, shedding light on the dynamic relationship that governs the fate and functionality of these remarkable stem cells. Understanding the crosstalk between DDR and the cellular metabolism will open a new avenue of research designed to target these interacting pathways for improving HSC function and treating hematologic disorders.

Hematopoietic Stem Cells as an Integrative Hub Linking Lifestyle to Cardiovascular Health

Despite breakthroughs in modern medical care, the incidence of cardiovascular disease (CVD) is even more prevalent globally. Increasing epidemiologic evidence indicates that emerging cardiovascular risk factors arising from the modern lifestyle, including psychosocial stress, sleep problems, unhealthy diet patterns, physical inactivity/sedentary behavior, alcohol consumption, and tobacco smoking, contribute significantly to this worldwide epidemic, while its underpinning mechanisms are enigmatic. Hematological and immune systems were recently demonstrated to play integrative roles in linking lifestyle to cardiovascular health. In particular, alterations in hematopoietic stem cell (HSC) homeostasis, which is usually characterized by proliferation, expansion, mobilization, megakaryocyte/myeloid-biased differentiation, and/or the pro-inflammatory priming of HSCs, have been shown to be involved in the persistent overproduction of pro-inflammatory myeloid leukocytes and platelets, the cellular protagonists of cardiovascular inflammation and thrombosis, respectively. Furthermore, certain lifestyle factors, such as a healthy diet pattern and physical exercise, have been documented to exert cardiovascular protective effects through promoting quiescence, bone marrow retention, balanced differentiation, and/or the anti-inflammatory priming of HSCs. Here, we review the current understanding of and progression in research on the mechanistic interrelationships among lifestyle, HSC homeostasis, and cardiovascular health. Given that adhering to a healthy lifestyle has become a mainstream primary preventative approach to lowering the cardiovascular burden, unmasking the causal links between lifestyle and cardiovascular health from the perspective of hematopoiesis would open new opportunities to prevent and treat CVD in the present age.

Context-dependent modification of PFKFB3 in hematopoietic stem cells promotes anaerobic glycolysis and ensures stress hematopoiesis

Metabolic pathways are plastic and rapidly change in response to stress or perturbation. Current metabolic profiling techniques require lysis of many cells, complicating the tracking of metabolic changes over time after stress in rare cells such as hematopoietic stem cells (HSCs). Here, we aimed to identify the key metabolic enzymes that define differences in glycolytic metabolism between steady-state and stress conditions in murine HSCs and elucidate their regulatory mechanisms. Through quantitative 13C metabolic flux analysis of glucose metabolism using high-sensitivity glucose tracing and mathematical modeling, we found that HSCs activate the glycolytic rate-limiting enzyme phosphofructokinase (PFK) during proliferation and oxidative phosphorylation (OXPHOS) inhibition. Real-time measurement of ATP levels in single HSCs demonstrated that proliferative stress or OXPHOS inhibition led to accelerated glycolysis via increased activity of PFKFB3, the enzyme regulating an allosteric PFK activator, within seconds to meet ATP requirements. Furthermore, varying stresses differentially activated PFKFB3 via PRMT1-dependent methylation during proliferative stress and via AMPK-dependent phosphorylation during OXPHOS inhibition. Overexpression of Pfkfb3 induced HSC proliferation and promoted differentiated cell production, whereas inhibition or loss of Pfkfb3 suppressed them. This study reveals the flexible and multilayered regulation of HSC glycolytic metabolism to sustain hematopoiesis under stress and provides techniques to better understand the physiological metabolism of rare hematopoietic cells.

Ascorbate depletion increases quiescence and self-renewal potential in hematopoietic stem cells and multipotent progenitors

Ascorbate (vitamin C) limits hematopoietic stem cell (HSC) function and suppresses leukemia development by promoting the function of the Tet2 tumor suppressor. In humans, ascorbate is obtained from the diet while in mice it is synthesized in the liver. In this study, we show that deletion of the Slc23a2 ascorbate transporter severely depleted ascorbate from hematopoietic cells. Slc23a2 deficiency increased HSC reconstituting potential and self-renewal potential upon transplantation into irradiated mice. Slc23a2 deficiency also increased the reconstituting and self-renewal potential of multipotent hematopoietic progenitors (MPPs), conferring the ability to long-term reconstitute irradiated mice. Slc23a2-deficient HSCs and MPPs divided much less frequently than control HSCs and MPPs. Increased self-renewal and reconstituting potential were observed particularly in quiescent Slc23a2-deficient HSCs and MPPs. The effect of Slc23a2 deficiency on MPP self-renewal was not mediated by reduced Tet2 function. Ascorbate thus regulates quiescence and restricts self-renewal potential in HSCs and MPPs such that ascorbate depletion confers MPPs with long-term self-renewal potential.

Transgenerational inheritance of methylmercury and vitamin A-induced toxicological effects in a Wistar rats environmental-based model

Mercury (Hg) and vitamin A (VitA) are two environmental factors with potential health impacts, especially during pregnancy and early childhood. Fish and seafood may present elevated levels of methylmercury (MeHg), the major Hg derivative, and VitA. This study aimed to evaluate the transgenerational effects of exposure to MeHg and/or VitA on epigenetic and toxicological parameters in a Wistar rat model. Our findings revealed persistent toxicological effects in generations F1 and F2 following low/mild doses of MeHg and/or VitA exposure during dams' (F0) gestation and breastfeeding. Toxicological effects observed in F2 included chronic DNA damage, bone marrow toxicity, altered microglial content, reduced neuronal signal, and diminished male longevity. Sex-specific patterns were also observed. Co-exposure to MeHg and VitA showed both synergistic and antagonistic effects. Additionally, the study demonstrated that MeHg and VitA affected histone methylation and caused consistent effects in F2. While MeHg exposure has been associated with transgenerational inheritance effects in other organisms, this study provides the first evidence of transgenerational inheritance of MeHg and VitA-induced toxicological effects in rodents. Although the exact mechanism is not yet fully understood, these findings suggest that MeHg and VitA may perpetuate their impacts across generations. The study highlights the need for remedial policies and interventions to mitigate the potential health problems faced by future generations exposed to MeHg or VitA. Further research is warranted to investigate the transgenerational effects beyond F2 and determine the matrilineal or patrilineal inheritance patterns.

Retinoic acid signaling in development and differentiation commitment and its regulatory topology

Retinoic acid (RA), the derivative of vitamin A/retinol, is a signaling molecule with important implications in health and disease. It is a well-known developmental morphogen that functions mainly through the transcriptional activity of nuclear RA receptors (RARs) and, uncommonly, through other nuclear receptors, including peroxisome proliferator-activated receptors. Intracellular RA is under spatiotemporally fine-tuned regulation by synthesis and degradation processes catalyzed by retinaldehyde dehydrogenases and P450 family enzymes, respectively. In addition to dictating the transcription architecture, RA also impinges on cell functioning through non-genomic mechanisms independent of RAR transcriptional activity. Although RA-based differentiation therapy has achieved impressive success in the treatment of hematologic malignancies, RA also has pro-tumor activity. Here, we highlight the relevance of RA signaling in cell-fate determination, neurogenesis, visual function, inflammatory responses and gametogenesis commitment. Genetic and post-translational modifications of RAR are also discussed. A better understanding of RA signaling will foster the development of precision medicine to improve the defects caused by deregulated RA signaling.

Protocol for high-sensitivity western blot on murine hematopoietic stem cells

Hematopoietic stem cells (HSCs) sustain hematopoiesis during homeostasis and regeneration. However, their limited availability poses a challenge for protein analysis. Here, we present a protocol for performing high-sensitivity western blot on HSCs using two techniques that enhance HSC isolation from mice and boost sensitivity for low cell numbers. We describe steps for isolating murine bone marrow cells, antibody staining, and cell sorting and post-sort analysis. We then detail a western blot procedure suitable for low numbers of HSCs. For complete details on the use and execution of this protocol, please refer to Li et al (2022).1,2.

Bidirectional interplay between metabolism and epigenetics in hematopoietic stem cells and leukemia

During the last decades, remarkable progress has been made in further understanding the complex molecular regulatory networks that maintain hematopoietic stem cell (HSC) function. Cellular and organismal metabolisms have been shown to directly instruct epigenetic alterations, and thereby dictate stem cell fate, in the bone marrow. Epigenetic regulatory enzymes are dependent on the availability of metabolites to facilitate DNA- and histone-modifying reactions. The metabolic and epigenetic features of HSCs and their downstream progenitors can be significantly altered by environmental perturbations, dietary habits, and hematological diseases. Therefore, understanding metabolic and epigenetic mechanisms that regulate healthy HSCs can contribute to the discovery of novel metabolic therapeutic targets that specifically eliminate leukemia stem cells while sparing healthy HSCs. Here, we provide an in-depth review of the metabolic and epigenetic interplay regulating hematopoietic stem cell fate. We discuss the influence of metabolic stress stimuli, as well as alterations occurring during leukemic development. Additionally, we highlight recent therapeutic advancements toward eradicating acute myeloid leukemia cells by intervening in metabolic and epigenetic pathways.

How nutrition regulates hematopoietic stem cell features

Our dietary choices significantly impact all the cells in our body. Increasing evidence suggests that diet-derived metabolites influence hematopoietic stem cell (HSC) metabolism and function, thereby actively modulating blood homeostasis. This is of particular relevance because regulating the metabolic activity of HSCs is crucial for maintaining stem cell fitness and mitigating the risk of hematologic disorders. In this review, we examine the current scientific knowledge of the impact of diet on stemness features, and we specifically highlight the established mechanisms by which dietary components modulate metabolic and transcriptional programs in adult HSCs. Gaining a deeper understanding of how nutrition influences our HSC compartment may pave the way for targeted dietary interventions with the potential to decelerate aging and improve the effectiveness of transplantation and cancer therapies.

Neutral sphingomyelinase blockade enhances hematopoietic stem cell fitness through an integrated stress response

Hematopoietic stem and progenitor cell (HSPC) transplantation serves as a curative therapy for many benign and malignant hematopoietic disorders and as a platform for gene therapy. However, growing needs for ex vivo manipulation of HSPC-graft products are limited by barriers in maintaining critical self-renewal and quiescence properties. The role of sphingolipid metabolism in safeguarding these essential cellular properties has been recently recognized, but not yet widely explored. Here, we demonstrate that pharmacologic and genetic inhibition of neutral sphingomyelinase 2 (nSMase-2) leads to sustained improvements in long-term competitive transplantation efficiency after ex vivo culture. Mechanistically, nSMase-2 blockade activates a canonical integrated stress response (ISR) and promotes metabolic quiescence in human and murine HSPCs. These adaptations result in part from disruption in sphingolipid metabolism that impairs the release of nSMase-2-dependent extracellular vesicles (EVs). The aggregate findings link EV trafficking and the ISR as a regulatory dyad guarding HSPC homeostasis and long-term fitness. Translationally, transient nSMase-2 inhibition enables ex vivo graft manipulation with enhanced HSPC potency.

Hematopoietic stem cells through the ages: A lifetime of adaptation to organismal demands

Hematopoietic stem cells (HSCs), which govern the production of all blood lineages, transition through a series of functional states characterized by expansion during fetal development, functional quiescence in adulthood, and decline upon aging. We describe central features of HSC regulation during ontogeny to contextualize how adaptive responses over the life of the organism ultimately form the basis for HSC functional degradation with age. We particularly focus on the role of cell cycle regulation, inflammatory response pathways, epigenetic changes, and metabolic regulation. We then explore how the knowledge of age-related changes in HSC regulation can inform strategies for the rejuvenation of old HSCs.

A Bloody Feast-Nutritional Regulation of Hematopoiesis

Hematopoietic stem cells provide us with a lifelong supply of blood cells. Hence, their proper function is absolutely essential for life, and their dysfunction can lead to infectious and malignant diseases. These cells have specific metabolic requirements to enable their lifelong function and blood-producing capacity. With the words of the Roman poet Juvenal "a healthy mind in a healthy body" in mind, it is intriguing to understand the connection between our daily diet and the quality of our blood, with the hope that through specific dietary adjustments we can improve our hematopoietic stem cell function and prevent disease. Nowadays, dietary supplements are an expanding market filled with potential and promises for better health. However, the link between many of those supplements and human physiology is obscure. Several groups have begun to shed light on this by investigating the metabolic regulation of hematopoiesis by specific nutrients. Beyond the link to dietary supplementation, these studies have also significantly improved our understanding of basic hematopoietic stem cell biology. Herein we summarize recent knowledge on the effect of specific vitamins and amino acids, which might be considered as dietary supplements, on normal hematopoiesis and hematopoietic stem cell function. We propose that improving our understanding of the link between nutrition in general and blood physiology can ultimately lead to the optimization of health-care policies, protocols, and standards of care.

Nutrient regulation of development and cell fate decisions

Diet contributes to health at all stages of life, from embryonic development to old age. Nutrients, including vitamins, amino acids, lipids and sugars, have instructive roles in directing cell fate and function, maintaining stem cell populations, tissue homeostasis and alleviating the consequences of aging. This Review highlights recent findings that illuminate how common diets and specific nutrients impact cell fate decisions in healthy and disease contexts. We also draw attention to new models, technologies and resources that help to address outstanding questions in this emerging field and may lead to dietary approaches that promote healthy development and improve disease treatments.

Linking cell cycle to hematopoietic stem cell fate decisions

Hematopoietic stem cells (HSCs) have the properties to self-renew and/or differentiate into any blood cell lineages. In order to balance the maintenance of the stem cell pool with supporting mature blood cell production, the fate decisions to self-renew or to commit to differentiation must be tightly controlled, as dysregulation of this process can lead to bone marrow failure or leukemogenesis. The contribution of the cell cycle to cell fate decisions has been well established in numerous types of stem cells, including pluripotent stem cells. Cell cycle length is an integral component of hematopoietic stem cell fate. Hematopoietic stem cells must remain quiescent to prevent premature replicative exhaustion. Yet, hematopoietic stem cells must be activated into cycle in order to produce daughter cells that will either retain stem cell properties or commit to differentiation. How the cell cycle contributes to hematopoietic stem cell fate decisions is emerging from recent studies. Hematopoietic stem cell functions can be stratified based on cell cycle kinetics and divisional history, suggesting a link between Hematopoietic stem cells activity and cell cycle length. Hematopoietic stem cell fate decisions are also regulated by asymmetric cell divisions and recent studies have implicated metabolic and organelle activity in regulating hematopoietic stem cell fate. In this review, we discuss the current understanding of the mechanisms underlying hematopoietic stem cell fate decisions and how they are linked to the cell cycle.

How can early life adversity still exert an effect decades later? A question of timing, tissues and mechanisms

Exposure to any number of stressors during the first 1000 days from conception to age 2 years is important in shaping an individual's life trajectory of health and disease. Despite the expanding range of stressors as well as later-life phenotypes and outcomes, the underlying molecular mechanisms remain unclear. Our previous data strongly suggests that early-life exposure to a stressor reduces the capacity of the immune system to generate subsequent generations of naïve cells, while others have shown that, early life stress impairs the capacity of neuronal stem cells to proliferate as they age. This leads us to the "stem cell hypothesis" whereby exposure to adversity during a sensitive period acts through a common mechanism in all the cell types by programming the tissue resident progenitor cells. Furthermore, we review the mechanistic differences observed in fully differentiated cells and suggest that early life adversity (ELA) may alter mitochondria in stem cells. This may consequently alter the destiny of these cells, producing the lifelong "supply" of functionally altered fully differentiated cells.

DNA Methylation of Genes Participating in Hepatic Metabolisms and Function in Fetal Calf Liver Is Altered by Maternal Undernutrition during Gestation

This study aimed to elucidate the effects of maternal undernutrition (MUN) on epigenetic modification of hepatic genes in Japanese Black fetal calves during gestation. Using a previously established experimental design feeding the dams with 60% (LN) or 120% (HN) of their global nutritional requirements during the 8.5-month gestational period, DNA methylation in the fetal liver was analyzed with reduced representation bisulfite sequencing (RRBS). The promoters and gene bodies in the LN fetuses were hypomethylated compared to HN fetuses. Pathway analysis showed that the genes with DMR in the exon/intron in the LN group were associated with pathways involved in Cushing syndrome, gastric acid secretion, and aldosterone synthesis and secretion. Promoter hypomethylation in the LN group was frequently observed in genes participating in various signaling pathways (thyroid hormone, Ras/Rap1, PIK3-Akt, cAMP), fatty acid metabolism, and cholesterol metabolism. The promoter hypomethylated genes ALPL and GNAS were upregulated in the LN group, whereas the promoter hypermethylated genes GRB10 and POR were downregulated. The intron/exon hypomethylated genes IGF2, IGF2R, ACAD8, TAT, RARB, PINK1, and SOAT2 were downregulated, whereas the hypermethylated genes IGF2BP2, NOS3, and NR2F1 were upregulated. Collectively, MUN alters the promoter and gene body methylation of genes associated with hepatic metabolisms (energy, cholesterol, mitochondria) and function, suggesting an impact of altered gene methylation on the dysregulation of gene expression in the fetal liver.

VarID2 quantifies gene expression noise dynamics and unveils functional heterogeneity of ageing hematopoietic stem cells

Variability of gene expression due to stochasticity of transcription or variation of extrinsic signals, termed biological noise, is a potential driving force of cellular differentiation. Utilizing single-cell RNA-sequencing, we develop VarID2 for the quantification of biological noise at single-cell resolution. VarID2 reveals enhanced nuclear versus cytoplasmic noise, and distinct regulatory modes stratified by correlation between noise, expression, and chromatin accessibility. Noise levels are minimal in murine hematopoietic stem cells (HSCs) and increase during differentiation and ageing. Differential noise identifies myeloid-biased Dlk1+ long-term HSCs in aged mice with enhanced quiescence and self-renewal capacity. VarID2 reveals noise dynamics invisible to conventional single-cell transcriptome analysis.

Differentiation latency and dormancy signatures define fetal liver HSCs at single cell resolution

Decoding the gene regulatory mechanisms mediating self-renewal of hematopoietic stem cells (HSCs) during their amplification in the fetal liver (FL) is relevant for advancing therapeutic applications aiming to expand transplantable HSCs, a long-standing challenge. Here, to explore intrinsic and extrinsic regulation of self-renewal in FL-HSCs at the single cell level, we engineered a culture platform designed to recapitulate the FL endothelial niche, which supports the amplification of serially engraftable HSCs ex vivo. Leveraging this platform in combination with single cell index flow cytometry, serial transplantation assays, and single cell RNA-sequencing, we elucidated previously unrecognized heterogeneity in immunophenotypically defined FL-HSCs and demonstrated that differentiation latency and transcriptional signatures of biosynthetic dormancy are distinguishing properties of self-renewing FL-HSCs with capacity for serial, long-term multilineage hematopoietic reconstitution. Altogether, our findings provide key insights into HSC expansion and generate a novel resource for future exploration of the intrinsic and niche-derived signaling pathways that support FL-HSC self-renewal.

Harnessing endogenous transcription factors directly by small molecules for chemically induced pluripotency inception

Chemistry-alone approach has recently been applied for incepting pluripotency in somatic cells, representing a breakthrough in biology. However, chemical reprogramming is hampered by low efficiency, and the underlying molecular mechanisms remain unclear. Particularly, chemical compounds do not have specific DNA-recognition domains or transcription regulatory domains, and then how do small molecules work as a driving force for reinstating pluripotency in somatic cells? Furthermore, how to efficiently clear materials and structures of an old cell to prepare the rebuilding of a new one? Here, we show that small molecule CD3254 activates endogenous existing transcription factor RXRα to significantly promote mouse chemical reprogramming. Mechanistically, CD3254-RXRα axis can directly activate all the 11 RNA exosome component genes (Exosc1-10 and Dis3) at transcriptional level. Unexpectedly, rather than degrading mRNAs as its substrates, RNA exosome mainly modulates the degradation of transposable element (TE)-associated RNAs, particularly MMVL30, which is identified as a new barrier for cell-fate determination. In turn, MMVL30-mediated inflammation (IFN-γ and TNF-α pathways) is reduced, contributing to the promotion of successful reprogramming. Collectively, our study provides conceptual advances for translating environmental cues into pluripotency inception, particularly, identifies that CD3254-RXRα-RNA exosome axis can promote chemical reprogramming, and suggests modulation of TE-mediated inflammation via CD3254-inducible RNA exosome as important opportunities for controlling cell fates and regenerative medicine.

Retinoic acid receptor regulation of decision-making for cell differentiation

All-trans retinoic acid (ATRA) activation of retinoic acid receptors (RARs) is crucial to an organism's proper development as established by findings for mouse foetuses from dams fed a vitamin A-deficient diet. ATRA influences decision-making by embryonic stem (ES) cells for differentiation including lineage fate. From studies of knockout mice, RARα and RARγ regulate haematopoiesis whereby active RARα modulates the frequency of decision-making for myeloid differentiation, but is not essential for myelopoiesis, and active RARγ supports stem cell self-renewal and maintenance. From studies of zebrafish embryo development, active RARγ plays a negative role in stem cell decision-making for differentiation whereby, in the absence of exogenous ATRA, selective agonism of RARγ disrupted stem cell decision-making for differentiation patterning for development. From transactivation studies, 0.24 nM ATRA transactivated RARγ and 19.3 nM (80-fold more) was needed to transactivate RARα. Therefore, the dose of ATRA that cells are exposed to in vivo, from gradients created by cells that synthesize and metabolize, is important to RARγ versus RARα and RARγ activation and balancing of the involvements in modulating stem cell maintenance versus decision-making for differentiation. RARγ activation favours stemness whereas concomitant or temporal activation of RARγ and RARα favours differentiation. Crosstalk with signalling events that are provoked by membrane receptors is also important.

Small molecule metabolites: discovery of biomarkers and therapeutic targets

Metabolic abnormalities lead to the dysfunction of metabolic pathways and metabolite accumulation or deficiency which is well-recognized hallmarks of diseases. Metabolite signatures that have close proximity to subject's phenotypic informative dimension, are useful for predicting diagnosis and prognosis of diseases as well as monitoring treatments. The lack of early biomarkers could lead to poor diagnosis and serious outcomes. Therefore, noninvasive diagnosis and monitoring methods with high specificity and selectivity are desperately needed. Small molecule metabolites-based metabolomics has become a specialized tool for metabolic biomarker and pathway analysis, for revealing possible mechanisms of human various diseases and deciphering therapeutic potentials. It could help identify functional biomarkers related to phenotypic variation and delineate biochemical pathways changes as early indicators of pathological dysfunction and damage prior to disease development. Recently, scientists have established a large number of metabolic profiles to reveal the underlying mechanisms and metabolic networks for therapeutic target exploration in biomedicine. This review summarized the metabolic analysis on the potential value of small-molecule candidate metabolites as biomarkers with clinical events, which may lead to better diagnosis, prognosis, drug screening and treatment. We also discuss challenges that need to be addressed to fuel the next wave of breakthroughs.

LC-MS-Based Targeted Metabolomics for FACS-Purified Rare Cells

Metabolism plays a fundamental role in regulating cellular functions and fate decisions. Liquid chromatography-mass spectrometry (LC-MS)-based targeted metabolomic approaches provide high-resolution insights into the metabolic state of a cell. However, the typical sample size is in the order of 10⁵-10⁷ cells and thus not compatible with rare cell populations, especially in the case of a prior flow cytometry-based purification step. Here, we present a comprehensively optimized protocol for targeted metabolomics on rare cell types, such as hematopoietic stem cells and mast cells. Only 5000 cells per sample are required to detect up to 80 metabolites above background. The use of regular-flow liquid chromatography allows for robust data acquisition, and the omission of drying or chemical derivatization avoids potential sources of error. Cell-type-specific differences are preserved while the addition of internal standards, generation of relevant background control samples, and targeted metabolite with quantifiers and qualifiers ensure high data quality. This protocol could help numerous studies to gain thorough insights into cellular metabolic profiles and simultaneously reduce the number of laboratory animals and the time-consuming and costly experiments associated with rare cell-type purification.

Retinoid X receptor promotes hematopoietic stem cell fitness and quiescence and preserves hematopoietic homeostasis

Hematopoietic stem cells (HSCs) balance self-renewal and differentiation to maintain hematopoietic fitness throughout life. In steady-state conditions, HSC exhaustion is prevented by the maintenance of most HSCs in a quiescent state, with cells entering the cell cycle only occasionally. HSC quiescence is regulated by retinoid and fatty-acid ligands of transcriptional factors of the nuclear retinoid X receptor (RXR) family. Herein, we show that dual deficiency for hematopoietic RXRα and RXRβ induces HSC exhaustion, myeloid cell/megakaryocyte differentiation, and myeloproliferative-like disease. RXRα and RXRβ maintain HSC quiescence, survival, and chromatin compaction; moreover, transcriptome changes in RXRα;RXRβ-deficient HSCs include premature acquisition of an aging-like HSC signature, MYC pathway upregulation, and RNA intron retention. Fitness loss and associated RNA transcriptome and splicing alterations in RXRα;RXRβ-deficient HSCs are prevented by Myc haploinsufficiency. Our study reveals the critical importance of RXRs for the maintenance of HSC fitness and their protection from premature aging.

The influence of high-order chromatin state in the regulation of stem cell fate

In eukaryotic cells, genomic DNA is hierarchically compacted by histones into chromatin, which is initially assembled by the nucleosome and further folded into orderly and flexible structures that include chromatin fiber, chromatin looping, topologically associated domains (TADs), chromosome compartments, and chromosome territories. These distinct structures and motifs build the three-dimensional (3D) genome architecture, which precisely controls spatial and temporal gene expression in the nucleus. Given that each type of cell is characterized by its own unique gene expression profile, the state of high-order chromatin plays an essential role in the cell fate decision. Accumulating evidence suggests that the plasticity of high-order chromatin is closely associated with stem cell fate. In this review, we summarize the biological roles of the state of high-order chromatin in embryogenesis, stem cell differentiation, the maintenance of stem cell identity, and somatic cell reprogramming. In addition, we highlight the roles of epigenetic factors and pioneer transcription factors (TFs) involved in regulating the state of high-order chromatin during the determination of stem cell fate and discuss how H3K9me3-heterochromatin restricts stem cell fate. In summary, we review the most recent progress in research on the regulatory functions of high-order chromatin dynamics in the determination and maintenance of stem cell fate.

Retinoid metabolism: new insights

Vitamin A (retinol) is a critical micronutrient required for the control of stem cell functions, cell differentiation, and cell metabolism in many different cell types, both during embryogenesis and in the adult organism. However, we must obtain vitamin A from food sources. Thus, the uptake and metabolism of vitamin A by intestinal epithelial cells, the storage of vitamin A in the liver, and the metabolism of vitamin A in target cells to more biologically active metabolites, such as retinoic acid (RA) and 4-oxo-RA, must be precisely regulated. Here, I will discuss the enzymes that metabolize vitamin A to RA and the cytochrome P450 Cyp26 family of enzymes that further oxidize RA. Because much progress has been made in understanding the regulation of ALDH1a2 (RALDH2) actions in the intestine, one focus of this review is on the metabolism of vitamin A in intestinal epithelial cells and dendritic cells. Another focus is on recent data that 4-oxo-RA is a ligand required for the maintenance of hematopoietic stem cell dormancy and the important role of RARβ (RARB) in these stem cells. Despite this progress, many questions remain in this research area, which links vitamin A metabolism to nutrition, immune functions, developmental biology, and nuclear receptor pharmacology.

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.

Lysosomes and Their Role in Regulating the Metabolism of Hematopoietic Stem Cells

Hematopoietic stem cells (HSCs) have the capacity to renew blood cells at all stages of life and are largely quiescent at a steady state. It is essential to understand the processes that govern quiescence in HSCs to enhance bone marrow transplantation. It is hypothesized that in their quiescent state, HSCs primarily use glycolysis for energy production rather than mitochondrial oxidative phosphorylation (OXPHOS). In addition, the HSC switch from quiescence to activation occurs along a continuous developmental path that is driven by metabolism. Specifying the metabolic regulation pathway of HSC quiescence will provide insights into HSC homeostasis for therapeutic application. Therefore, understanding the metabolic demands of HSCs at a steady state is key to developing innovative hematological therapeutics. Lysosomes are the major degradative organelle in eukaryotic cells. Catabolic, anabolic, and lysosomal function abnormalities are connected to an expanding list of diseases. In recent years, lysosomes have emerged as control centers of cellular metabolism, particularly in HSC quiescence, and essential regulators of cell signaling have been found on the lysosomal membrane. In addition to autophagic processes, lysosomal activities have been shown to be crucial in sustaining quiescence by restricting HSCs access to a nutritional reserve essential for their activation into the cell cycle. Lysosomal activity may preserve HSC quiescence by altering glycolysis-mitochondrial biogenesis. The understanding of HSC metabolism has significantly expanded over the decade, revealing previously unknown requirements of HSCs in both their dividing (active) and quiescent states. Therefore, understanding the role of lysosomes in HSCs will allow for the development of innovative treatment methods based on HSCs to fight clonal hematopoiesis and HSC aging.

Lipids and the cancer stemness regulatory system in acute myeloid leukemia

Acute myeloid leukemia (AML) is a heterogeneous disease of impaired myeloid differentiation and a caricature of normal hematopoiesis. Leukemic stem cells (LSCs) are responsible for long-term clonal propagation in AML just as hematopoietic stem cells (HSCs) sustain lifelong hematopoiesis. LSCs are often resistant to standard chemotherapy and are responsible for clinical relapse. Although AML is highly heterogeneous, determinants of stemness are prognostic for AML patient survival and can predict AML drug sensitivity. Therefore, one way to overcome challenges preventing efficacious treatment outcomes is to target LSC stemness. Metabolomic and lipidomic studies of serum and cells from AML patients are emerging to complement genomic, transcriptomic, epigenetic, and proteomic data sets to characterize and stratify AML. Recent studies have shown the value of fractionating LSCs versus blasts when characterizing metabolic pathways and implicate the importance of lipid balance to LSCs function. As more extensive metabolic studies coupled to functional in vivo assays are conducted on highly purified HSCs, bulk AML, and LSCs, the similarities and differences in lipid homeostasis in stem-like versus more mature AML subtypes as well as from normal HSCs are emerging. Here, we discuss the latest findings from studies of lipid function in LSCs, with a focus on sphingolipids (SLs) as stemness/lineage fate mediators in AML, and the balance of fatty acid anabolism and catabolism fueling metabolic flexibility and drug resistance in AML. We also discuss how designing successful strategies to target lipid vulnerabilities and improve AML patient survival should take into consideration the hierarchical nature 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.

Stem cells "aclymatise" to regenerate the blood system

How blood stem cells balance fate decisions between quiescence maintenance and differentiation during recovery from cancer treatment remains poorly understood. A recent study by Umemoto et al (2022) uncovers an unexpected linkage between metabolic and epigenetic regulation of haematopoiesis, suggesting new targets in haematopoietic regeneration, with possible implications in leukaemogenesis and therapy resistance.

Retinoids in the Pathogenesis and Treatment of Liver Diseases

Vitamin A (VA), all-trans-retinol (ROL), and its analogs are collectively called retinoids. Acting through the retinoic acid receptors RARα, RARβ, and RARγ, all-trans-retinoic acid, an active metabolite of VA, is a potent regulator of numerous biological pathways, including embryonic and somatic cellular differentiation, immune functions, and energy metabolism. The liver is the primary organ for retinoid storage and metabolism in humans. For reasons that remain incompletely understood, a body of evidence shows that reductions in liver retinoids, aberrant retinoid metabolism, and reductions in RAR signaling are implicated in numerous diseases of the liver, including hepatocellular carcinoma, non-alcohol-associated fatty liver diseases, and alcohol-associated liver diseases. Conversely, restoration of retinoid signaling, pharmacological treatments with natural and synthetic retinoids, and newer agonists for specific RARs show promising benefits for treatment of a number of these liver diseases. Here we provide a comprehensive review of the literature demonstrating a role for retinoids in limiting the pathogenesis of these diseases and in the treatment of liver diseases.