Environmental Optimization Enables Maintenance of Quiescent Hematopoietic Stem Cells Ex Vivo

Treating genetic blood disorders in the era of CRISPR-mediated genome editing

In the setting of monogenic disease, advances made in genome editing technologies can, in principle, be deployed as a therapeutic strategy to precisely correct a specific gene mutation in an affected cell type and restore functionality. Using the β-hemoglobinopathies and hemophilia as exemplars, we review recent experimental breakthroughs using CRISPR-derived genome editing technology that have translated to significant improvements in the management of inherited hematologic disorders. Yet there are also challenges facing the use of CRISPR-mediated genome editing in these patients; we discuss possible ways to obviate those issues for furtherance of clinical benefit.

Fatty Acid β-Oxidation May Be Associated with the Erythropoietin Resistance Index in Stable Patients Undergoing Haemodialysis

BACKGROUND/OBJECTIVES

Lipid metabolism and adiponectin modulate erythropoiesis in vitro and in general population studies and may also affect responsiveness to erythropoietin in patients undergoing haemodialysis (HD). However, little is known about the impact of lipid-associated biomarkers on reticulocyte production and erythropoietin resistance index (ERI) in patients undergoing HD. Therefore, we aimed to investigate their impacts in 167 stable patients undergoing HD.

METHODS

Pre-dialysis blood samples were collected and analysed for reticulocyte counts and serum lipid profiles by routine analyses and serum carnitine profiles (C0-C18) by LC-MS/MS. ERI was calculated as erythropoietin dose/kg/week normalized for haemoglobin levels.

RESULTS

The independent positive determinants of reticulocyte count were log [Triglyceride (TG)] and logC18:1. A large proportion of longer-chain acylcarnitines was positively correlated with reticulocyte counts, possibly resulting from the accumulation of acylcarnitines in mitochondria undergoing fateful exocytosis from reticulocytes. These results indicate a possible association between reticulocyte formation and reduced β-oxidation, which occurs during the peripheral phase of erythroblast enucleation. Total cholesterol (TC) and log [C2/(C16 + C18:1)] as a putative marker of β-oxidation efficiency were negative independent determinants of ERI. Moreover, acyl chain length had a significantly positive impact on the correlation coefficients of individual acylcarnitines with ERI, suggesting that enhanced β-oxidation may be associated with reduced ERI. Finally, adiponectin had no independent association with reticulocyte counts or ERI despite its negative association with HDL-C levels.

CONCLUSIONS

Enhanced fatty acid β-oxidation and higher TC levels may be associated with lower ERI, whereas higher TG levels and longer acylcarnitines may be related to the latest production of reticulocytes in stable patients undergoing HD.

SDHAF1 confers metabolic resilience to aging hematopoietic stem cells by promoting mitochondrial ATP production

Aging generally predisposes stem cells to functional decline, impairing tissue homeostasis. Here, we report that hematopoietic stem cells (HSCs) acquire metabolic resilience that promotes cell survival. High-resolution real-time ATP analysis with glucose tracing and metabolic flux analysis revealed that old HSCs reprogram their metabolism to activate the pentose phosphate pathway (PPP), becoming more resistant to oxidative stress and less dependent on glycolytic ATP production at steady state. As a result, old HSCs can survive without glycolysis, adapting to the physiological cytokine environment in bone marrow. Mechanistically, old HSCs enhance mitochondrial complex II metabolism during stress to promote ATP production. Furthermore, increased succinate dehydrogenase assembly factor 1 (SDHAF1) in old HSCs, induced by physiological low-concentration thrombopoietin (TPO) exposure, enables rapid mitochondrial ATP production upon metabolic stress, thereby improving survival. This study provides insight into the acquisition of resilience through metabolic reprogramming in old HSCs and its molecular basis to ameliorate age-related hematopoietic abnormalities.

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.

PIK3CA inhibition in models of proliferative glomerulonephritis and lupus nephritis

Proliferative glomerulonephritis is a severe condition that often leads to kidney failure. There is a significant lack of effective treatment for these disorders. Here, following the identification of a somatic PIK3CA gain-of-function mutation in podocytes of a patient, we demonstrate using multiple genetically engineered mouse models, single-cell RNA sequencing, and spatial transcriptomics the crucial role played by this pathway for proliferative glomerulonephritis development by promoting podocyte proliferation, dedifferentiation, and inflammation. Additionally, we show that alpelisib, a PI3Kα inhibitor, improves glomerular lesions and kidney function in different mouse models of proliferative glomerulonephritis and lupus nephritis by targeting podocytes. Surprisingly, we determined that pharmacological inhibition of PI3Kα affects B and T lymphocyte populations in lupus nephritis mouse models, with a decrease in the production of proinflammatory cytokines, autoantibodies, and glomerular complement deposition, which are all characteristic features of PI3Kδ inhibition, the primary PI3K isoform expressed in lymphocytes. Importantly, PI3Kα inhibition does not impact lymphocyte function under normal conditions. These findings were then confirmed in human lymphocytes isolated from patients with active lupus nephritis. In conclusion, we demonstrate the major role played by PI3Kα in proliferative glomerulonephritis and show that in this condition, alpelisib acts on both podocytes and the immune system.

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.

A mitochondrial NADPH-cholesterol axis regulates extracellular vesicle biogenesis to support hematopoietic stem cell fate

Mitochondrial fatty acid oxidation (FAO) is essential for hematopoietic stem cell (HSC) self-renewal; however, the mechanism by which mitochondrial metabolism controls HSC fate remains unknown. Here, we show that within the hematopoietic lineage, HSCs have the largest mitochondrial NADPH pools, which are required for proper HSC cell fate and homeostasis. Bioinformatic analysis of the HSC transcriptome, biochemical assays, and genetic inactivation of FAO all indicate that FAO-generated NADPH fuels cholesterol synthesis in HSCs. Interference with FAO disturbs the segregation of mitochondrial NADPH toward corresponding daughter cells upon single HSC division. Importantly, we have found that the FAO-NADPH-cholesterol axis drives extracellular vesicle (EV) biogenesis and release in HSCs, while inhibition of EV signaling impairs HSC self-renewal. These data reveal the existence of a mitochondrial NADPH-cholesterol axis for EV biogenesis that is required for hematopoietic homeostasis and highlight the non-stochastic nature of HSC fate determination.

Ex vivo hematopoietic stem cell expansion technologies: recent progress, applications, and open questions

Hematopoietic stem cells (HSCs) are a rare but potent cell type that support life-long hematopoiesis and stably regenerate the entire blood and immune system following transplantation. HSC transplantation represents a mainstay treatment for various diseases of the blood and immune systems. The ex vivo expansion and manipulation of HSCs therefore represents an important approach to ask biological questions in experimental hematology and to help improve clinical HSC transplantation therapies. However, it has remained challenging to expand transplantable HSCs ex vivo. This review summarizes recent progress in ex vivo HSC expansion technologies and their applications to biological and clinical problems and discusses current questions in the field.

Evaluating the function of murine quiescent hematopoietic stem cells following non-homologous end joining-based genome editing

Preculture is indispensable for achieving highly efficient non-homologous end joining (NHEJ)-based genome editing. Here, we present a protocol for optimizing genome editing conditions for murine hematopoietic stem cells (HSCs) and evaluating their function following NHEJ-based genome editing. We describe steps for sgRNA preparation, cell sorting, preculture, and electroporation. We then detail post-editing culture and transplanting of bone marrow. This protocol can be used to study genes related to HSC quiescence. For complete details on the use and execution of this protocol, please refer to Shiroshita et al.¹.

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.

Mitochondrial Transfer to Host Cells from Ex Vivo Expanded Donor Hematopoietic Stem Cells

Mitochondrial dysfunction is observed in various conditions, from metabolic syndromes to mitochondrial diseases. Moreover, mitochondrial DNA (mtDNA) transfer is an emerging mechanism that enables the restoration of mitochondrial function in damaged cells. Hence, developing a technology that facilitates the transfer of mtDNA can be a promising strategy for the treatment of these conditions. Here, we utilized an ex vivo culture of mouse hematopoietic stem cells (HSCs) and succeeded in expanding the HSCs efficiently. Upon transplantation, sufficient donor HSC engraftment was attained in-host. To assess the mitochondrial transfer via donor HSCs, we used mitochondrial-nuclear exchange (MNX) mice with nuclei from C57BL/6J and mitochondria from the C3H/HeN strain. Cells from MNX mice have C57BL/6J immunophenotype and C3H/HeN mtDNA, which is known to confer a higher stress resistance to mitochondria. Ex vivo expanded MNX HSCs were transplanted into irradiated C57BL/6J mice and the analyses were performed at six weeks post transplantation. We observed high engraftment of the donor cells in the bone marrow. We also found that HSCs from the MNX mice could transfer mtDNA to the host cells. This work highlights the utility of ex vivo expanded HSC to achieve the mitochondrial transfer from donor to host in the transplant setting.

Regulation of adult stem cell quiescence and its functions in the maintenance of tissue integrity

Adult stem cells are important for mammalian tissues, where they act as a cell reserve that supports normal tissue turnover and can mount a regenerative response following acute injuries. Quiescent stem cells are well established in certain tissues, such as skeletal muscle, brain, and bone marrow. The quiescent state is actively controlled and is essential for long-term maintenance of stem cell pools. In this Review, we discuss the importance of maintaining a functional pool of quiescent adult stem cells, including haematopoietic stem cells, skeletal muscle stem cells, neural stem cells, hair follicle stem cells, and mesenchymal stem cells such as fibro-adipogenic progenitors, to ensure tissue maintenance and repair. We discuss the molecular mechanisms that regulate the entry into, maintenance of, and exit from the quiescent state in mice. Recent studies revealed that quiescent stem cells have a discordance between RNA and protein levels, indicating the importance of post-transcriptional mechanisms, such as alternative polyadenylation, alternative splicing, and translation repression, in the control of stem cell quiescence. Understanding how these mechanisms guide stem cell function during homeostasis and regeneration has important implications for regenerative medicine.

Quantitative metaproteomics reveals composition and metabolism characteristics of microbial communities in Chinese liquor fermentation starters

Introduction

Daqu, the Chinese liquor fermentation starter, contains complex microbial communities that are important for the yield, quality, and unique flavor of produced liquor. However, the composition and metabolism of microbial communities in the different types of high-temperature Daqu (i.e., white, yellow, and black Daqu) have not been well understood.

Methods

Herein, we used quantitative metaproteomics based on data-independent acquisition (DIA) mass spectrometry to analyze a total of 90 samples of white, yellow, and black Daqu collected in spring, summer, and autumn, revealing the taxonomic and metabolic profiles of different types of Daqu across seasons.

Results

Taxonomic composition differences were explored across types of Daqu and seasons, where the under-fermented white Daqu showed the higher microbial diversity and seasonal stability. It was demonstrated that yellow Daqu had higher abundance of saccharifying enzymes for raw material degradation. In addition, considerable seasonal variation of microbial protein abundance was discovered in the over-fermented black Daqu, suggesting elevated carbohydrate and amino acid metabolism in autumn black Daqu.

Discussion

We expect that this study will facilitate the understanding of the key microbes and their metabolism in the traditional fermentation process of Chinese liquor production.

A culture platform to study quiescent hematopoietic stem cells following genome editing

Other than genetically engineered mice, few reliable platforms are available for the study of hematopoietic stem cell (HSC) quiescence. Here we present a platform to analyze HSC cell cycle quiescence by combining culture conditions that maintain quiescence with a CRISPR-Cas9 genome editing system optimized for HSCs. We demonstrate that preculture of HSCs enhances editing efficiency by facilitating nuclear transport of ribonucleoprotein complexes. For post-editing culture, mouse and human HSCs edited based on non-homologous end joining and cultured under low-cytokine, low-oxygen, and high-albumin conditions retain their phenotypes and quiescence better than those cultured under the proliferative conditions. Using this approach, HSCs regain quiescence even after editing by homology-directed repair. Our results show that low-cytokine culture conditions for gene-edited HSCs are a useful approach for investigating HSC quiescence ex vivo.

Biomedical polymer scaffolds mimicking bone marrow niches to advance in vitro expansion of hematopoietic stem cells

Hematopoietic stem cell (HSC) transplantation provides an effective platform for the treatment of hematological disorders. However, the donor shortage of HSCs and immune responses severely restrict the clinical applications of HSCs. Compared to allogeneic transplantation, autogenous transplantation poses less risk to the immune system, but the problem associated with insufficient HSCs remains a substantial challenge. A significant strategy for obtaining sufficient HSCs is to promote the expansion of HSCs. In vivo, a bone marrow microenvironment supports the survival and hematopoiesis of HSCs. Therefore, it is crucial to establish a platform that mimics the features of a bone marrow microenvironment for the in vitro expansion of HSCs. Three-dimensional (3D) scaffolds have emerged as the most powerful tools to mimic cellular microenvironments for the growth and proliferation of stem cells. Biomedical polymers have been widely utilized as cell scaffolds due to their advantageous features including favorable biocompatibility, biodegradability, as well as adjustable physical and chemical properties. This review focuses on recent advances in the study of biomedical polymer scaffolds that mimic bone marrow microenvironments for the in vitro expansion of HSCs. Bone marrow transplantation and microenvironments are first introduced. Then, biomedical polymer scaffolds for the expansion of HSCs and future prospects are summarized and discussed.

Ex Vivo Expansion of Phenotypic and Transcriptomic Chronic Myeloid Leukemia Stem Cells

Despite decades of research, standard therapies remain ineffective for most leukemias, pushing toward an essential unmet need for targeted drug screens. Moreover, preclinical drug testing is an important consideration for success of clinical trials without affecting non-transformed stem cells. Using the transgenic chronic myeloid leukemia (CML) mouse model, we determine that leukemic stem cells (LSCs) are transcriptionally heterogenous with a preexistent drug-insensitive signature. To test targeting of potentially important pathways, we establish ex vivo expanded LSCs that have long-term engraftment and give rise to multilineage hematopoiesis. Expanded LSCs share transcriptomic signatures with primary LSCs including enrichment in Wnt, JAK-STAT, MAPK, mTOR and transforming growth factor β signaling pathways. Drug testing on expanded LSCs show that transforming growth factor β and Wnt inhibitors had significant effects on the viability of LSCs, but not leukemia-exposed healthy HSCs. This platform allows testing of multiple drugs at the same time to identify vulnerabilities of LSCs.

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.

Meteorin links the bone marrow hypoxic state to hematopoietic stem/progenitor cell mobilization

Hematopoietic stem/progenitor cells (HSPCs) are supported and regulated by niche cells in the bone marrow with an important characterization of physiological hypoxia. However, how hypoxia regulates HSPCs is still unclear. Here, we find that meteorin (Metrn) from hypoxic macrophages restrains HSPC mobilization. Hypoxia-induced factor 1α and Yin Yang 1 induce the high expression of Metrn in macrophages, and macrophage-specific Metrn knockout increases HSPC mobilization through modulating HSPC proliferation and migration. Mechanistically, Metrn interacts with its receptor 5-hydroxytryptamine receptor 2b (Htr2b) to regulate the reactive oxygen species levels in HSPCs through targeting phospholipase C signaling. The reactive oxygen species levels are reduced in HSPCs of macrophage-specific Metrn knockout mice with activated phospholipase C signaling. Targeting the Metrn/Htr2b axis could therefore be a potential strategy to improve HSPC mobilization for stem cell-based therapy.

Development and clinical translation considerations for the next wave of gene modified hematopoietic stem and progenitor cells therapies

INTRODUCTION

Consistent and reliable manufacture of gene modified hematopoietic stem and progenitor cell (HPSC) therapies will be of the utmost importance as they become more mainstream and address larger populations. Robust development campaigns will be needed to ensure that these products will be delivered to patients with the highest quality standards.

AREAS COVERED

Through publicly available manuscripts, press releases, and news articles - this review touches on aspects related to HSPC therapy, development, and manufacturing.

EXPERT OPINION

Recent advances in genome modification technology coupled with the longstanding clinical success of HSPCs warrants great optimism for the next generation of engineered HSPC-based therapies. Treatments for some diseases that have thus far been intractable now appear within reach. Reproducible manufacturing will be of critical importance in delivering these therapies but will be challenging due to the need for bespoke materials and methods in combination with the lack of off-the-shelf solutions. Continued progress in the field will manifest in the form of industrialization which currently requires attention and resources directed toward the custom reagents, a focus on closed and automated processes, and safer and more precise genome modification technologies that will enable broader, faster, and safer access to these life-changing therapies.

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.

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.

Inhibition of canonical Wnt signaling promotes ex vivo maintenance and proliferation of hematopoietic stem cells in zebrafish

The maintenance and proliferation of hematopoietic stem cells (HSCs) are tightly regulated by their niches in the bone marrow. The analysis of niche cells or stromal cell lines that can support HSCs has facilitated the finding of novel supporting factors for HSCs. Despite large efforts in the murine bone marrow, however, HSC expansion is still difficult ex vivo, highlighting the need for new approaches to elucidate the molecular elements that regulate HSCs. The zebrafish provides a unique model to study hematopoietic niches as HSCs are maintained in the kidney, allowing for a parallel view of hematopoietic niches over evolution. Here, using a stromal cell line from the zebrafish kidney, zebrafish kidney stromal (ZKS), we uncover that an inhibitor of canonical Wnt signaling, IWR-1-endo, is a potent regulator of HSCs. Co-culture assays revealed that ZKS cells were in part supportive of maintenance, but not expansion, of gata2a:GFP+runx1:mCherry+ (gata2a+runx1+) HSCs. Transcriptome analysis revealed that, compared to candidate niche cells in the kidney, ZKS cells weakly expressed HSC maintenance factor genes, thpo and cxcl12, but highly expressed canonical Wnt ligand genes, wnt1, 7bb, and 9a. Thpo supplementation in ZKS culture slightly increased, but inhibition of canonical Wnt signaling by IWR-1-endo treatment largely increased the number of gata2a+runx1+ cells (> 2-fold). Moreover, we found that gata2a+runx1+ cells can be maintained by supplementing both IWR-1-endo and Thpo without stromal cells. Collectively, our data provide evidence that IWR-1-endo can be used as a novel supporting factor for HSCs.

Isolation, Maintenance and Expansion of Adult Hematopoietic Stem/Progenitor Cells and Leukemic Stem Cells

Hematopoietic stem cells (HSCs) are rare, self-renewing cells that perch on top of the hematopoietic tree. The HSCs ensure the constant supply of mature blood cells in a tightly regulated process producing peripheral blood cells. Intense efforts are ongoing to optimize HSC engraftment as therapeutic strategy to treat patients suffering from hematopoietic diseases. Preclinical research paves the way by developing methods to maintain, manipulate and expand HSCs ex vivo to understand their regulation and molecular make-up. The generation of a sufficient number of transplantable HSCs is the Holy Grail for clinical therapy. Leukemia stem cells (LSCs) are characterized by their acquired stem cell characteristics and are responsible for disease initiation, progression, and relapse. We summarize efforts, that have been undertaken to increase the number of long-term (LT)-HSCs and to prevent differentiation towards committed progenitors in ex vivo culture. We provide an overview and compare methods currently available to isolate, maintain and enrich HSC subsets, progenitors and LSCs and discuss their individual advantages and drawbacks.

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.

CD244 expression represents functional decline of murine hematopoietic stem cells after in vitro culture

Isolation of long-term hematopoietic stem cell (HSC) is possible by utilizing flow cytometry with multiple cell surface markers. However, those cell surface phenotypes do not represent functional HSCs after in vitro culture. Here we show that cultured HSCs express mast cell-related genes including Cd244. After in vitro culture, phenotypic HSCs were divided into CD244^- and CD244⁺ subpopulations, and only CD244^- cells that have low mast cell gene expression and maintain HSC-related genes sustain reconstitution potential. The result was same when HSCs were cultured in an efficient expansion medium containing polyvinyl alcohol. Chemically induced endoplasmic reticulum (ER) stress signal increased the CD244⁺ subpopulation, whereas ER stress suppression using a molecular chaperone, TUDCA, decreased CD244⁺ population, which was correlated to improved reconstitution output. These data suggest CD244 is a potent marker to exclude non-functional HSCs after in vitro culture thereby useful to elucidate mechanism of functional decline of HSCs during ex vivo treatment.

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Murine HSCs up-regulate mast cell-related genes including Cd244 during in vitro culture

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Long-term HSCs after in vitro culture are enriched in CD244⁻CD48⁻KSL population

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Induction of unfolded protein response is involved in the increase of CD244⁺HSC

Three-Dimensional Avian Hematopoietic Stem Cell Cultures as a Model for Studying Disease Pathogenesis

Three-dimensional (3D) cell culture is attracting increasing attention today because it can mimic tissue environments and provide more realistic results than do conventional cell cultures. On the other hand, very little attention has been given to using 3D cell cultures in the field of avian cell biology. Although mimicking the bone marrow niche is a classic challenge of mammalian stem cell research, experiments have never been conducted in poultry on preparing in vitro the bone marrow niche. It is well known, however, that all diseases cause immunosuppression and target immune cells and their development. Hematopoietic stem cells (HSC) reside in the bone marrow and constitute a source for immune cells of lymphoid and myeloid origins. Disease prevention and control in poultry are facing new challenges, such as greater use of alternative breeding systems and expanding production of eggs and chicken meat in developing countries. Moreover, the COVID-19 pandemic will draw greater attention to the importance of disease management in poultry because poultry constitutes a rich source of zoonotic diseases. For these reasons, and because they will lead to a better understanding of disease pathogenesis, in vivo HSC niches for studying disease pathogenesis can be valuable tools for developing more effective disease prevention, diagnosis, and control. The main goal of this review is to summarize knowledge about avian hematopoietic cells, HSC niches, avian immunosuppressive diseases, and isolation of HSC, and the main part of the review is dedicated to using 3D cell cultures and their possible use for studying disease pathogenesis with practical examples. Therefore, this review can serve as a practical guide to support further preparation of 3D avian HSC niches to study the pathogenesis of avian diseases.

Engineering strategies to achieve efficient in vitro expansion of haematopoietic stem cells: development and improvement

Haematopoietic stem cells are the basis for building and maintaining lifelong haematopoietic mechanisms and important resources for the treatment of blood disorders. Haematopoietic niches are microenvironment in the body where...

Molecular characterization of hematopoietic stem cells after in vitro amplification on biomimetic 3D PDMS cell culture scaffolds

Hematopoietic stem cell (HSC) transplantation is successfully applied since the late 1950s. However, its efficacy can be impaired by insufficient numbers of donor HSCs. A promising strategy to overcome this hurdle is the use of an advanced ex vivo culture system that supports the proliferation and, at the same time, maintains the pluripotency of HSCs. Therefore, we have developed artificial 3D bone marrow-like scaffolds made of polydimethylsiloxane (PDMS) that model the natural HSC niche in vitro. These 3D PDMS scaffolds in combination with an optimized HSC culture medium allow the amplification of high numbers of undifferentiated HSCs. After 14 days in vitro cell culture, we performed transcriptome and proteome analysis. Ingenuity pathway analysis indicated that the 3D PDMS cell culture scaffolds altered PI3K/AKT/mTOR pathways and activated SREBP, HIF1α and FOXO signaling, leading to metabolic adaptations, as judged by ELISA, Western blot and metabolic flux analysis. These molecular signaling pathways can promote the expansion of HSCs and are involved in the maintenance of their pluripotency. Thus, we have shown that the 3D PDMS scaffolds activate key molecular signaling pathways to amplify the numbers of undifferentiated HSCs ex vivo effectively.

Reconstructing Boolean network ensembles from single-cell data for unraveling dynamics in the aging of human hematopoietic stem cells

Regulatory dependencies in molecular networks are the basis of dynamic behaviors affecting the phenotypical landscape. With the advance of high throughput technologies, the detail of omics data has arrived at the single-cell level. Nevertheless, new strategies are required to reconstruct regulatory networks based on populations of single-cell data. Here, we present a new approach to generate populations of gene regulatory networks from single-cell RNA-sequencing (scRNA-seq) data. Our approach exploits the heterogeneity of single-cell populations to generate pseudo-timepoints. This allows for the first time to uncouple network reconstruction from a direct dependency on time series measurements. The generated time series are then fed to a combined reconstruction algorithm. The latter allows a fast and efficient reconstruction of ensembles of gene regulatory networks. Since this approach does not require knowledge on time-related trajectories, it allows us to model heterogeneous processes such as aging. Applying the approach to the aging-associated NF-κB signaling pathway-based scRNA-seq data of human hematopoietic stem cells (HSCs), we were able to reconstruct eight ensembles, and evaluate their dynamic behavior. Moreover, we propose a strategy to evaluate the resulting attractor patterns. Interaction graph-based features and dynamic investigations of our model ensembles provide a new perspective on the heterogeneity and mechanisms related to human HSCs aging.

Systems analysis of hematopoiesis using single-cell lineage tracing

PURPOSE OF REVIEW

In the last few decades, revolutionary advances in next-generation sequencing have led to single-cell lineage tracing technologies that now enable researchers to identify and quantify hematopoietic cell behavior with unprecedented detail. Combined readouts of cell lineage and cell state from the same cell mitigate the need to prospectively isolate populations of interest, and allow a system-level understanding of dynamic developmental processes. We will discuss the advantages and shortcomings of these technologies, the intriguing discoveries that stemmed from lineage tracing hematopoiesis at the single-cell level and the directions toward which the field is moving.

RECENT FINDINGS

Single-cell lineage tracing studies unveiled extensive functional heterogeneity within discrete immunophenotypic populations. Recently, several groups merged lineage tracing with single-cell RNA sequencing to visualize clonal relationships directly on transcriptional landscapes without the requirement for prospective isolation of cell types by FACS. To study the cell dynamics of hematopoiesis, without perturbation in their native niche, researchers have developed mouse models with endogenous single-cell lineage tracing systems, which can simultaneously trace thousands of hematopoietic progenitor cells in a single mouse, without transplantation. The emerging picture is that multiple hematopoietic hierarchies coexist within a single individual, each with distinct regulatory features. These hierarchies are imprinted during development much earlier than previously predicted, persisting well into adulthood and even after injury and transplantation.

SUMMARY

Clone-tracking experiments allow stem-cell researchers to characterize lineage hierarchies during blood development and regeneration. Combined with single-cell genomics analyses, these studies are allowing system-level description of hematopoiesis in mice and humans. Early exploratory studies have unveiled features with important implications for human biology and disease. VIDEO ABSTRACT.

The Impact of Oxygen Availability and Multilineage Communication on Organoid Maturation

Significance: An optimal supply with oxygen is of high importance during embryogenesis and a prerequisite for proper organ development. Different tissues require varying amounts of oxygen, and even within single organs, different phases of development go alongside with either physiological hypoxia or the need for sufficient oxygen supply. Recent Advances: Human induced pluripotent stem cell-derived organoid models are state of the art cell culture platforms for the investigation of developmental processes, disease modeling, and drug testing. Organoids modeling the development of multiple tissues were developed within the past years. Critical Issues: Until now, optimization of oxygen supply and its role during organoid growth, differentiation, and maturation have only rarely been addressed. Recent publications indicate that hypoxia-induced processes play an important role in three-dimensional tissue cultures, triggering multilineage communication between mesenchymal cells, the endothelium, as well as organotypic cells. Later in culture, a sufficient supply with oxygen is of high importance to allow larger organoid sizes. Moreover, cellular stress is reduced and tissue maturation is improved. Therefore, a functional blood vessel network is required. Future Directions: In this review, we will briefly summarize aspects of the role of oxygen during embryonic development and organogenesis, present an update on novel organoid models with a special focus on organoid vascularization, and discuss the importance of complex organoids involving parenchymal cells, mesenchymal cells, inflammatory cells, and functional blood vessels for the generation of mature and fully functional tissues in vitro. Antioxid. Redox Signal. 35, 217-233.

GPX4 and vitamin E cooperatively protect hematopoietic stem and progenitor cells from lipid peroxidation and ferroptosis

Ferroptosis, a newly defined mode of regulated cell death caused by unbalanced lipid redox metabolism, is implicated in various tissue injuries and tumorigenesis. However, the role of ferroptosis in stem cells has not yet been investigated. Glutathione peroxidase 4 (GPX4) is a critical suppressor of lipid peroxidation and ferroptosis. Here, we study the function of GPX4 and ferroptosis in hematopoietic stem and progenitor cells (HSPCs) in mice with Gpx4 deficiency in the hematopoietic system. We find that Gpx4 deletion solely in the hematopoietic system has no significant effect on the number and function of HSPCs in mice. Notably, hematopoietic stem cells (HSCs) and hematopoietic progenitor cells lacking Gpx4 accumulated lipid peroxidation and underwent ferroptosis in vitro. α-Tocopherol, the main component of vitamin E, was shown to rescue the Gpx4-deficient HSPCs from ferroptosis in vitro. When Gpx4 knockout mice were fed a vitamin E-depleted diet, a reduced number of HSPCs and impaired function of HSCs were found. Furthermore, increased levels of lipid peroxidation and cell death indicated that HSPCs undergo ferroptosis. Collectively, we demonstrate that GPX4 and vitamin E cooperatively maintain lipid redox balance and prevent ferroptosis in HSPCs.

Differentiated Cells Derived from Hematopoietic Stem Cells and Their Applications in Translational Medicine

Hematopoietic stem cells (HSCs) and their development are one of the most widely studied model systems in mammals. In adults, HSCs are predominantly found in the bone marrow, from where they maintain homeostasis. Besides bone marrow and mobilized peripheral blood, cord blood is also being used as an alternate allogenic source of transplantable HSCs. HSCs from both autologous and allogenic sources are being applied for the treatment of various conditions like blood cancers, anemia, etc. HSCs can further differentiate to mature blood cells. Differentiation process of HSCs is being extensively studied so as to obtain a large number of pure populations of various differentiated cells in vitro so that they can be taken up for clinical trials. The ability to generate sufficient quantity of clinical-grade specialized blood cells in vitro would take the field of hematology a step ahead in translational medicine.

Hematopoietic stem cells retain functional potential and molecular identity in hibernation cultures

Advances in the isolation and gene expression profiling of single hematopoietic stem cells (HSCs) have permitted in-depth resolution of their molecular program. However, long-term HSCs can only be isolated to near purity from adult mouse bone marrow, thereby precluding studies of their molecular program in different physiological states. Here, we describe a powerful 7-day HSC hibernation culture system that maintains HSCs as single cells in the absence of a physical niche. Single hibernating HSCs retain full functional potential compared with freshly isolated HSCs with respect to colony-forming capacity and transplantation into primary and secondary recipients. Comparison of hibernating HSC molecular profiles to their freshly isolated counterparts showed a striking degree of molecular similarity, further resolving the core molecular machinery of HSC self-renewal while also identifying key factors that are potentially dispensable for HSC function, including members of the AP1 complex (Jun, Fos, and Ncor2), Sult1a1 and Cish. Finally, we provide evidence that hibernating mouse HSCs can be transduced without compromising their self-renewal activity and demonstrate the applicability of hibernation cultures to human HSCs.

Mobilization efficiency is critically regulated by fat via marrow PPARδ

The mobilization efficiency of hematopoietic stem/progenitor cells from bone marrow (BM) to circulation by granulocyte colony-stimulating factor (G-CSF) is dramatically dispersed in humans and mice with no mechanistic lead for poor mobilizers. The regulatory mechanism for mobilization efficiency by dietary fat was assessed in mice. Fat-free diet (FFD) for 2 weeks greatly increased mobilization compared to normal diet (ND). The BM mRNA level of peroxisome proliferator-activated receptor δ (PPARδ), a receptor for lipid mediators, was markedly up-regulated by G-CSF in mice fed with ND and displayed strong positive correlation with widely scattered mobilization efficiency. It was hypothesized that BM fat ligand for PPARδ might inhibit mobilization. The PPARδ agonist inhibited mobilization in mice fed with ND and enhanced mobilization by FFD. Treatment with the PPARδ antagonist and chimeric mice with PPARδ+/- BM showed enhanced mobilization. Immunohistochemical staining and flow cytometry revealed that BM PPARδ expression was enhanced by G-CSF mainly in mature/immature neutrophils. BM lipid mediator analysis revealed that G-CSF treatment and FFD resulted in the exhaustion of ω3-polyunsaturated fatty acids such as eicosapentaenoic acid (EPA). EPA induced the up-regulation of genes downstream of PPARδ, such as carnitine palmitoyltransferase-1α and angiopoietin-like protein 4 (Angptl4), in mature/immature neutrophils in vitro and inhibited enhanced mobilization in mice fed with FFD in vivo. Treatment of wild-type mice with the anti-Angptl4 antibody enhanced mobilization together with BM vascular permeability. Collectively, PPARδ signaling in BM mature/immature neutrophils induced by dietary fatty acids negatively regulates mobilization, at least partially, via Angptl4 production.

Granulocyte colony-stimulating factor directly acts on mouse lymphoid-biased but not myeloid-biased hematopoietic stem cells

Granulocyte colony-stimulating factor (G-CSF) is widely used in clinical settings to mobilize hematopoietic stem cells (HSC) into the circulation for HSC harvesting and transplantation. However, whether G-CSF directly stimulates HSC to change their cell cycle state and fate is controversial. HSC are a heterogeneous population consisting of different types of HSC, such as myeloid-biased HSC and lymphoid-biased HSC. We hypothesized that G-CSF has different effects on different types of HSC. To verify this, we performed serum-free single-cell culture and competitive repopulation with cultured cells. Single highly purified HSC and hematopoietic progenitor cells (HPC) were cultured with stem cell factor (SCF), SCF + G-CSF, SCF + granulocyte/macrophage (GM)-CSF, or SCF + thrombopoietin (TPO) for 7 days. Compared with SCF alone, SCF + G-CSF increased the number of divisions of cells from the lymphoid-biased HSCenriched population but not that of cells from the My-bi HSC-enriched population. SCF + G-CSF enhanced the level of reconstitution of lymphoidbiased HSC but not that of myeloid-biased HSC. Clonal transplantation assay also showed that SCF + G-CSF did not increase the frequency of myeloid-biased HSC. These data showed that G-CSF directly acted on lymphoid- biased HSC but not myeloid-biased HSC. Our study also revised the cytokine network at early stages of hematopoiesis: SCF directly acted on myeloid-biased HSC; TPO directly acted on myeloid-biased HSC and lymphoid- biased HSC; and GM-CSF acted only on HPC. Early hematopoiesis is controlled differentially and sequentially by a number of cytokines.

Lipid metabolism in focus: how the build-up and breakdown of lipids affects stem cells

Cellular metabolism has recently emerged as a key regulator of stem cell behavior. Various studies have suggested that metabolic regulatory mechanisms are conserved in different stem cell niches, suggesting a common level of stem cell regulation across tissues. Although the balance between glycolysis and oxidative phosphorylation has been shown to be distinct in stem cells and their differentiated progeny, much less is known about lipid metabolism in stem cell regulation. In this Review, we focus on how stem cells are affected by two major lipid metabolic pathways: the build-up of lipids, called de novo lipogenesis, and the breakdown of lipids, called fatty acid beta-oxidation. We cover the recent literature on hematopoietic stem cells, intestinal stem cells, neural stem/progenitor cells and cancer stem cells, where these two lipid pathways have been studied in more depth.

Mitochondria Turnover and Lysosomal Function in Hematopoietic Stem Cell Metabolism

Hematopoietic stem cells (HSCs) reside in a hypoxic microenvironment that enables glycolysis-fueled metabolism and reduces oxidative stress. Nonetheless, metabolic regulation in organelles such as the mitochondria and lysosomes as well as autophagic processes have been implicated as essential for the determination of HSC cell fate. This review encompasses the current understanding of anaerobic metabolism in HSCs as well as the emerging roles of mitochondrial metabolism and lysosomal regulation for hematopoietic homeostasis.

p38α plays differential roles in hematopoietic stem cell activity dependent on aging contexts

Hematopoietic stem cells (HSCs) and their progeny sustain lifetime hematopoiesis. Aging alters HSC function, number, and composition and increases risk of hematological malignancies, but how these changes occur in HSCs remains unclear. Signaling via p38 mitogen-activated kinase (p38MAPK) has been proposed as a candidate mechanism underlying induction of HSC aging. Here, using genetic models of both chronological and premature aging, we describe a multimodal role for p38α, the major p38MAPK isozyme in hematopoiesis, in HSC aging. We report that p38α regulates differentiation bias and sustains transplantation capacity of HSCs in the early phase of chronological aging. However, p38α decreased HSC transplantation capacity in the late progression phase of chronological aging. Furthermore, codeletion of p38α in mice deficient in ataxia–telangiectasia mutated, a model of premature aging, exacerbated aging-related HSC phenotypes seen in ataxia–telangiectasia mutated single-mutant mice. Overall, these studies provide new insight into multiple functions of p38MAPK, which both promotes and suppresses HSC aging context dependently.

Hypoxia promotes erythroid differentiation through the development of progenitors and proerythroblasts

Oxygen is a critical noncellular component of the bone marrow microenvironment that plays an important role in the development of hematopoietic cell lineages. In this study, we investigated the impact of low oxygen (hypoxia) on ex vivo myeloerythroid differentiation of human cord blood-derived CD34⁺ hematopoietic stem and progenitor cells. We characterized the culture conditions to demonstrate that low oxygen inhibits cell proliferation and causes a metabolic shift in the stem and progenitor populations. We found that hypoxia promotes erythroid differentiation by supporting the development of progenitor populations. Hypoxia also increases the megakaryoerythroid potential of the common myeloid progenitors and the erythroid potential of megakaryoerythroid progenitors and significantly accelerates maturation of erythroid cells. Specifically, we determined that hypoxia promotes the loss of CD71 and the appearance of the erythroid markers CD235a and CD239. Further, evaluation of erythroid populations revealed a hypoxia-induced increase in proerythroblasts and in enucleation of CD235a⁺ cells. These results reveal the extensive role of hypoxia at multiple steps during erythroid development. Overall, our work establishes a valuable model for further investigations into the relationship between erythroid progenitors and/or erythroblast populations and their hypoxic microenvironment.

Mito-managing ROS & redox to reboot the immune system: Tapping mitochondria & redox management to extend the reach of hematopoietic stem cell transplantation

Hematopoietic stem cells (HSCs) are responsible for life-long production of blood and immune cells. HSC transplantation (HSCT) is the original cell therapy which can cure hematological disorders but also has the potential to treat other diseases if technical and safety barriers are overcome. To maintain homeostatic hematopoiesis or to restore hematopoiesis during transplantation HSCs must perform both self-renewal, replication of themselves, and differentiation, generation of mature blood and immune cells. These are just two of the cell fate choices HSCs have; the transitional phases where HSCs undergo these cell fate decisions are regulated by reduction-oxidation (redox) signaling, mitochondrial activity, and cellular metabolism. Recent studies revealed that mitochondria, a key source of redox signaling components, are central to HSC cell fate decisions. Here we highlight how mitochondria serve as hubs in HSCs to manage redox signaling and metabolism and thus guide HSC fate choices. We focus on how mitochondrial activity is modulated by their clearance, biogenesis, dynamics, distribution, and quality control in HSCs. We also note how modulating mitochondria in HSCs can help overcome technical barriers limiting further use of HSCT.

3D Scaffolds to Model the Hematopoietic Stem Cell Niche: Applications and Perspectives

Hematopoietic stem cells (HSC) are responsible for the production of blood and immune cells during life. HSC fate decisions are dependent on signals from specialized microenvironments in the bone marrow, termed niches. The HSC niche is a tridimensional environment that comprises cellular, chemical, and physical elements. Introductorily, we will revise the current knowledge of some relevant elements of the niche. Despite the importance of the niche in HSC function, most experimental approaches to study human HSCs use bidimensional models. Probably, this contributes to the failure in translating many in vitro findings into a clinical setting. Recreating the complexity of the bone marrow microenvironment in vitro would provide a powerful tool to achieve in vitro production of HSCs for transplantation, develop more effective therapies for hematologic malignancies and provide deeper insight into the HSC niche. We previously demonstrated that an optimized decellularization method can preserve with striking detail the ECM architecture of the bone marrow niche and support HSC culture. We will discuss the potential of this decellularized scaffold as HSC niche model. Besides decellularized scaffolds, several other methods have been reported to mimic some characteristics of the HSC niche. In this review, we will examine these models and their applications, advantages, and limitations.

Hematopoietic Stem Cell Metabolism during Development and Aging

Cellular metabolism in hematopoietic stem cells (HSCs) is an area of intense research interest, but the metabolic requirements of HSCs and their adaptations to their niches during development have remained largely unaddressed. Distinctive from other tissue stem cells, HSCs transition through multiple hematopoietic sites during development. This transition requires drastic metabolic shifts, insinuating the capacity of HSCs to meet the physiological demand of hematopoiesis. In this review, we highlight how mitochondrial metabolism determines HSC fate, and especially focus on the links between mitochondria, endoplasmic reticulum (ER), and lysosomes in HSC metabolism.

Hematopoietic Stem Cells Culture, Expansion and Differentiation: An Insight into Variable and Available Media

Owing to differentiation and self-renewal capacity, hematopoietic stem cells clasp potentiality to engender all blood cell types, leading to their immense competence to play a diverse role in therapeutic applications. Although these stem cells are the most investigated and exploited until now, further research is still essential to comprehend their nature, fate, and potential. Enhanced usage of hematopoietic stem cells in research and therapeutics intensified the requirement of expansion and differentiation of hematopoietic stem cells under in vitro conditions. Since these cells remain in senescence for a prolonged period before isolation, selection of appropriate growth medium along with supplements and culture conditions are crucial to initiate their cell division and to designate their destiny. The precise equilibrium between self-renewal and differentiation of stem cells sustained by exclusive medium along with special growth or differentiation factors is accountable for generating diverse cell lineages. Maintenance of hematopoietic stem and progenitor cell lines along with the advancement of research work generate an inexorable demand for production and commercialization of specialized stem cell culture media, with or without serum along with specific growth factors and supplements. Media commercialization for precise stem cell types, culturing and differentiation is a cost-effective developing field. Here in this review, we are assembling various types of hematopoietic stem cell self-renewal, expansion and differentiation media along with supplements and culture conditions, either developed and used by various scientists or are available commercially.

Protocol for the Maintenance of Quiescent Murine Hematopoietic Stem Cells

Mammalian hematopoietic stem cells (HSCs) maintain life-long hematopoiesis in the bone marrow. HSCs remain quiescent in vivo, unlike more differentiated progenitors, and enter the cell cycle rapidly after bone marrow injury or in vitro culture. We have recently demonstrated the ability to maintain HSC quiescence in vitro by mimicking the bone marrow microenvironment. Here, we provide a detailed protocol for keeping functional HSCs in the quiescent state in vitro. For complete details on the use and execution of this protocol, please refer to Kobayashi et al. (2019).

Towards Mimicking the Fetal Liver Niche: The Influence of Elasticity and Oxygen Tension on Hematopoietic Stem/Progenitor Cells Cultured in 3D Fibrin Hydrogels

Hematopoietic stem/progenitor cells (HSPCs) are responsible for the generation of blood cells throughout life. It is believed that, in addition to soluble cytokines and niche cells, biophysical cues like elasticity and oxygen tension are responsible for the orchestration of stem cell fate. Although several studies have examined the effects of bone marrow (BM) niche elasticity on HSPC behavior, no study has yet investigated the effects of the elasticity of other niche sites like the fetal liver (FL), where HSPCs expand more extensively. In this study, we evaluated the effect of matrix stiffness values similar to those of the FL on BM-derived HSPC expansion. We first characterized the elastic modulus of murine FL tissue at embryonic day E14.5. Fibrin hydrogels with similar stiffness values as the FL (soft hydrogels) were compared with stiffer fibrin hydrogels (hard hydrogels) and with suspension culture. We evaluated the expansion of total nucleated cells (TNCs), Lin⁻/cKit⁺ cells, HSPCs (Lin⁻/Sca⁺/cKit⁺ (LSK) cells), and hematopoietic stem cells (HSCs: LSK- Signaling Lymphocyte Activated Molecule (LSK-SLAM) cells) when cultured in 5% O₂ (hypoxia) or in normoxia. After 10 days, there was a significant expansion of TNCs and LSK cells in all culture conditions at both levels of oxygen tension. LSK cells expanded more in suspension culture than in both fibrin hydrogels, whereas TNCs expanded more in suspension culture and in soft hydrogels than in hard hydrogels, particularly in normoxia. The number of LSK-SLAM cells was maintained in suspension culture and in the soft hydrogels but not in the hard hydrogels. Our results indicate that both suspension culture and fibrin hydrogels allow for the expansion of HSPCs and more differentiated progeny whereas stiff environments may compromise LSK-SLAM cell expansion. This suggests that further research using softer hydrogels with stiffness values closer to the FL niche is warranted.

Kinetics of leukemic cells in 3D culture with stromal cells and with arginine deprivation stress

Previously, we established a three-dimensional (3D) bone marrow culture system that maintains normal hematopoiesis, including prolongation of hematopoietic stem cell proliferation and differentiation. To analyze the role of bone marrow stromal cells that compose the microenvironment, the growth of a leukemic cell line (K562) in the 3D condition and with arginine deprivation stress was compared with two-dimensional stromal cell monolayers (2D) and suspension cultures without stromal cells (stroma (-)). Arginine is essential for the proliferation and differentiation of erythrocytes. The proliferation and differentiation of K562 cells cultured in the 3D system were stabilized compared with cells in 2D or stroma (-). Furthermore, the number of K562 cells in the G₀/G₁ phase in 3D was increased significantly compared with cells grown in 2D or stroma (-). Interestingly, the mRNA expression of various hematopoietic growth factors of stromal cells in 3D was not different from 2D, even though supportive activity on K562 cell growth was observed in the arginine deprivation condition. Thus, the hematopoietic microenvironment involves multi-dimensional and complex systems including biochemical and physiochemical factors that regulate quiescence, proliferation, activation, and differentiation of normal hematopoietic cells and cloned leukemic cells. Our 3D culture system may be a valuable new tool for investigating leukemic cell-stromal cell interactions in vitro.

Awakening the HSC: Dynamic Modeling of HSC Maintenance Unravels Regulation of the TP53 Pathway and Quiescence

Hematopoietic stem cells (HSCs) provide all types of blood cells during the entire life of the organism. HSCs are mainly quiescent and can eventually enter the cell cycle to differentiate. HSCs are maintained and tightly regulated in a particular environment. The stem cell niche regulates dormancy and awakening. Deregulations of this interplay can lead to hematopoietic failure and diseases. In this paper, we present a Boolean network model that recapitulates HSC regulation in virtue of external signals coming from the niche. This Boolean network integrates and summarizes the current knowledge of HSC regulation and is based on extensive literature research. Furthermore, dynamic simulations suggest a novel systemic regulation of TP53 in homeostasis. Thereby, our model indicates that TP53 activity is balanced depending on external stimulations, engaging a regulatory mechanism involving ROS regulators and RAS activated transcription factors. Finally, we investigated different mouse models and compared them to in silico knockout simulations. Here, the model could recapitulate in vivo observed behaviors and thus sustains our results.

Alpha lipoic acid promotes development of hematopoietic progenitors derived from human embryonic stem cells by antagonizing ROS signals

Antagonism of ROS signaling can inhibit cell apoptosis and autophagy, thus favoring the maintenance and expansion of hematopoietic stem cells. Alpha lipoic acid (ALA), a small antioxidant molecule, affects cell apoptosis by lowering the ROS level. In this study, we show that ALA promoted production of human pluripotent stem cells (hPSCs) derived hemogenic endothelial cells and hematopoietic stem/progenitor cells in vitro. Transcriptome analysis of hPSCs derived hemogenic endothelial cells showed that ALA promoted endothelial‐to‐hematopoietic transition by up‐regulating RUNX1, GFI1, GFI1B, MEIS2, and HIF1A and down‐regulating SOX17, TGFB1, TGFB2, TGFB3, TGFBR1, and TGFBR2. ALA also up‐regulated sensor genes of ROS signals, including HIF1A, FOXO1, FOXO3, ATM, PETEN, SIRT1, and SIRT3, during the process of hPSCs derived hemogenic endothelial cells generation. However, in more mature hPSC‐derived hematopoietic stem/progenitor cells, ALA reduced ROS levels and inhibited apoptosis. In particular, ALA enhanced development of hPSCs derived hematopoietic stem/progenitor cells by up‐regulating HIF1A in response to a hypoxic environment. Furthermore, addition of ALA in ex vivo culture greatly improved the maintenance of functional cord blood HSCs by in vivo transplantation assay. Our findings support the conjecture that ALA plays an important role in efficient regeneration of hematopoietic stem/progenitor cells from hPSCs and maintenance of functional HSCs, providing insight into understanding of regeneration of early hematopoiesis for engineering clinically useful hPSCs derived hematopoietic stem/progenitor cells transplantation. Thus, ALA can be used in the study of hPSCs derived HSCs.