Embryonic origin of the adult hematopoietic system: advances and questions

Embryonic macrophages orchestrate niche cell homeostasis for the establishment of the definitive hematopoietic stem cell pool

Embryonic macrophages emerge before the onset of definitive hematopoiesis, seed into discrete tissues and contribute to specialized resident macrophages throughout life. Presence of embryonic macrophages in the bone marrow and functional impact on hematopoietic stem cells (HSC) or the niche remains unclear. Here we show that bone marrow macrophages consist of two ontogenetically distinct cell populations from embryonic and adult origin. Newborn mice lacking embryonic macrophages have decreased HSC numbers in the bone marrow suggesting an important function for embryo-derived macrophages in orchestrating HSC trafficking around birth. The establishment of a normal cellular niche space in the bone marrow critically depends on embryonic macrophages that are important for the development of mesenchymal stromal cells, but not other non-hematopoietic niche cells, providing evidence for a specific role for embryo-derived macrophages in the establishment of the niche environment pivotal for the establishment of a normally sized HSC pool.

Advantages and challenges of ex vivo generation and expansion of human hematopoietic stem cells from pluripotent stem cells

Hematopoietic stem cell transplantation (HSCT) is an essential and increasing therapeutic approach for treating conditions such as leukemia, lymphoma, and other blood cancers. However, its widespread use faces significant challenges, including limited donor availability, pathogens, and the risk of immune rejection. The emergence of pluripotent stem cells (PSCs) offers a potential solution to these challenges. By enabling the generation of hematopoietic stem cells (HSCs) and blood cells in vitro, PSCs open pathways to address the limitations of traditional HSC sources. Self-induced or gene-edited PSCs from patients may provide an accessible and personalized option for clinical applications. In this review, we examine the current protocols for differentiating PSCs into HSCs and blood cells, highlighting their benefits and shortcomings. Despite advancements in this field, two primary challenges persist: low differentiation efficiency and difficulties in isolating and enriching functional HSCs. These problems make it difficult to obtain HSCs for long-term survival. Thus, we propose innovative strategies and potential improvements including induction scheme optimization, reprogramming, and cell fate tracking. Future research should prioritize the development of efficient and reliable differentiation protocols for PSCs to obtain more functional HSCs. Additionally, establishing effective methods for enriching functional HSCs and blood cells will be critical for optimizing their use in clinical applications. These efforts hold the promise of overcoming current limitations and advancing the therapeutic potential of PSC-derived blood cells.

[Extramedullary hematopoiesis, a rare complication of sickle cell disease: A six-case series and literature review]

INTRODUCTION

Extramedullary hematopoiesis (EMH) is very rarely described during sickle cell disease (SCD). Our aim was to describe six cases of EMH occurring in adult SCD patients and to conduct a literature review.

METHODS

Retrospective, descriptive, and monocentric study, identifying all cases of EMH recorded in our cohort of adult SCD patients, up to April 2024. A literature review via PubMed included thirty-five articles (44 patients).

RESULTS

Six patients (4 men, 83.3% with SS genotype [n=5], 1 SC), with a median age of 22 (range 12-64) years at the time of EMH diagnosis were included. Four patients (66.7%) had an aseptic osteonecrosis of the hip. The localization of EMH varied: paravertebral (n=3), peri-articular in the hip (n=1), adrenal (n=1), hepatic (n=1), splenic (n=1) and was similar to the localizations reported in the literature. EMH was symptomatic at diagnosis in half of the cases. The diagnosis was established by histology (n=3/3) and/or typic magnetic resonance imaging (MRI) (n=4/4). The median baseline hemoglobin was 9.1 (extremes 5.8-10.9) g/dL. A watch-and-wait approach was primarily observed.

CONCLUSION

EMH in SCD patients appears to be rare, with varied localizations. Its diagnosis is made with MRI and/or biopsy, and its treatment is not consensual.

Wdr5-mediated H3K4 methylation facilitates HSPC development via maintenance of genomic stability in zebrafish

During fetal stage, hematopoietic stem and progenitor cells (HSPCs) undergo rapid proliferation with a tight control of genomic stability. Although histone H3 lysine 4 (H3K4) methylation has been reported to stabilize the genome in proliferating cells, its specific role in HSPC development remains elusive. In this study, we demonstrated that tryptophan-aspartic acid (WD) repeat protein 5 (Wdr5)-mediated H3K4 methylation is crucial for maintaining genomic stability of proliferating HSPCs in zebrafish embryos. Loss of wdr5 led to a severe reduction of HSPC pool in the caudal hematopoietic tissue, accompanied with attenuated H3K4 methylation level and evident p53-dependent apoptosis in the HSPCs. Mechanistically, Wdr5-mediated H3K4 methylation maintains genomic stability by inhibiting the formation of abnormal R-loops in the HSPCs, whereas accumulation of R-loops exacerbates DNA damage. Moreover, the absence of H3K4 trimethylation leads to an inactivated DNA damage response (DDR) pathway, which is deleterious to DNA damage repair and genomic stability. Subsequently, we found that DDR-associated genes, mutL homolog 1 and breast and ovarian cancer interacting helicase 1, are important to ensure HSPC survival, likely by stabilizing their genome. In summary, these findings reveal that Wdr5-mediated H3K4 methylation is essential for HSPC development through tight control of R-loop accumulation and DDR-associated program to ensure genomic stability and survival of proliferating HSPCs.

Placenta-derived factors contribute to human iPSC-liver organoid growth

Organoids derived from human induced pluripotent stem cells (hiPSC) are potentially applicable for regenerative medicine. However, the applications have been hampered by limited organoid size and function as a consequence of a lack of progenitor expansion. Here, we report the recapitulation of progenitor expansion in hiPSC-liver organoids based on the analysis of mouse development. Visualization of blood perfusion and oxygen levels in mouse embryos reveals a transient hypoxic environment during hepatoblast expansion, despite active blood flow. During this specific stage, the placenta expresses various growth factors. Human and mouse placenta-liver interaction analysis identifies various placenta-derived factors. Among them, IL1α efficiently induces the growth in hiPSC-liver organoids as well as mouse fetal livers following progenitor expansion under hypoxia. Furthermore, subsequent oxygenation demonstrates that progenitors expanded by IL1α contribute to hiPSC-liver organoid size and function. Taken together, we demonstrate that treatment with the placenta-derived factor under hypoxia is a crucial human organoid culture technique that efficiently induces progenitor expansion.

Modeling GATA2 deficiency in mice: the R396Q mutation disrupts normal hematopoiesis

GATA2 deficiency is an autosomal dominant germline disorder of immune dysfunction and bone marrow failure with a high propensity for leukemic transformation. While sequencing studies have identified several secondary mutations thought to contribute to malignancy, the mechanisms of disease progression have been difficult to identify due to a lack of disease-specific experimental models. Here, we describe a murine model of one of the most common GATA2 mutations associated with leukemic progression in GATA2 deficiency, Gata2R396Q/+. While mutant mice exhibit mild defects in peripheral blood, they display significant hematopoietic abnormalities in the bone marrow, including a reduction in hematopoietic stem cell (HSC) function and intrinsic biases toward specific stem cell subsets that differ from previous models of GATA2 loss. Supporting this observation, single-cell RNA sequencing of hematopoietic progenitors revealed a loss of stemness, myeloid-bias, and indications of accelerated aging. Importantly, we show that Gata2R396Q/+ exerts effects early in hematopoietic development, as mutant mice generate fewer HSCs in the aorta gonad mesonephros, and fetal liver HSCs have reduced function. This reduced and altered pool of HSCs could be potential contributors to leukemic transformation in patients, and our model provides a useful tool to study the mechanisms of malignant transformation in GATA2 deficiency.

Beclin 1 prevents ISG15-mediated cytokine storms to secure fetal hematopoiesis and survival

Proper control of inflammatory responses is essential for embryonic development, but the underlying mechanism is poorly understood. Here, we show that under physiological conditions, inactivation of ISG15, an inflammation amplifier, is associated with the interaction of Beclin 1 (Becn1), via its evolutionarily conserved domain, with STAT3 in the major fetal hematopoietic organ of mice. Conditional loss of Becn1 caused sequential dysfunction and exhaustion of fetal liver hematopoietic stem cells, leading to lethal inflammatory cell-biased hematopoiesis in the fetus. Molecularly, the absence of Becn1 resulted in the release of STAT3 from Becn1 tethering and subsequent phosphorylation and translocation to the nucleus, which in turn directly activated the transcription of ISG15 in fetal liver hematopoietic cells, coupled with increased ISGylation and production of inflammatory cytokines, whereas inactivating STAT3 reduced ISG15 transcription and inflammation but improved hematopoiesis potential, and further silencing ISG15 mitigated the above collapse in the Becn1-null hematopoietic lineage. The Becn1/STAT3/ISG15 axis remains functional in autophagy-disrupted fetal hematopoietic organs. These results suggest that Becn1, in an autophagy-independent manner, secures hematopoiesis and survival of the fetus by directly inhibiting STAT3/ISG15 activation to prevent cytokine storms. Our findings highlight a previously undocumented role of Becn1 in governing ISG15 to safeguard the fetus.

The placenta as a cradle, but not source, of blood?

An important question is whether the placenta is a source of, or merely a niche for, blood-forming hematopoietic stem cells. A recent PLOS Biology study suggests that the placenta does not directly give rise to hematopoietic stem cells.

Potential effects of liver dysfunction at the time of diagnosis in patients with acute myeloid leukemia

Whilst severe liver dysfunction is rarely encountered at the time of diagnosis for patients with acute myeloid leukemia (AML), mild elevations aminotransferase (<5 times the upper limit of normal) may be more frequently seen. Liver dysfunction at the time of diagnosis of AML is a parameter that requires investigation and can assist the clinicians in predicting prognosis. The aim of the present study was to investigate liver dysfunction at the time of diagnosis using the assoicated parameters in patients with AML. The present retrospective study included 90 patients diagnosed with AML who were hospitalised in the Hematology Clinic of Dışkapı Yıldırım Beyazıt Training and Research Hospital (Ankara, Turkey). The demographic characteristics of the patients were recorded together with hemogram results, anemia parameters, measurable residual disease positivity (MRD) and risk category, the presence of hepatosplenomegaly, infection, neutrophil recovery time (NRT), platelet recovery time (PRT) and liver dysfunction. The patients were analyzed in two groups following sorting into the liver dysfunction (n=45) and normal liver function test group (n=45). In the analysis of independent quantitative data (age, white blood cell count, hemoglobin, platelet, international normalized ratio, albumin, B12 vitamin, NRT, PRT) the Mann Whitney U-test was used. Independent qualitative data (sex, hepatomegaly, splenomegaly, MRD, risk category, infection) were analyzed using the χ2 test or the Fischer test. The effect level was investigated using univariate and multivariate logistic regression. A receiver operating characteristic curve was applied to determine the effect level and cut-off values. In the group with liver dysfunction, NRT, PRT, MRD positivity, risk category and the presence of infection were found to be statistically significantly higher. These findings suggest that during the first evaluation of patients diagnosed with AML, liver function tests are simple, rapid and necessary. The results obtained in the present study showed that liver dysfunction at diagnosis can be associated with the high-risk group, in addition to more frequent infection, poorer prognosis and mortality.

NR4A1 and NR4A2 orphan nuclear receptors regulate endothelial-to-hematopoietic transition in mouse hematopoietic stem cell specification

Hematopoietic stem cells (HSCs) sustain life-long hematopoiesis and emerge during mid-gestation from hemogenic endothelial progenitors via an endothelial-to-hematopoietic transition (EHT). The full scope of molecular mechanisms governing this process remains unclear. The NR4A subfamily of orphan nuclear receptors act as tumor suppressors in myeloid leukemogenesis and have never been implicated in HSC specification. Here, we report that Nr4a1 and Nr4a2 expression is upregulated in hemogenic endothelium during EHT. Progressive genetic ablation of Nr4a gene dosage results in a gradual decrease in numbers of nascent c-Kit+ hematopoietic progenitors in developing embryos, c-Kit+ cell cluster size in the dorsal aorta, and a block in HSC maturation, revealed by an accumulation of pro-HSCs and pre-HSC-type I cells and decreased numbers of pre-HSC-type II cells. Consistent with these observations, cells isolated from embryonic day 11.5 Nr4a1-/-; Nr4a2-/- aorta-gonads-mesonephros are devoid of in vivo long-term hematopoietic repopulating potential. Molecularly, employing spatial transcriptomic analysis we determined that the genetic ablation of Nr4a1 and Nr4a2 prevents Notch signaling from being downregulated in intra-aortic clusters and thus for pro-HSCs to mature into HSCs. Interestingly, this defect is partially rescued by ex vivo culture of dissected aorta-gonads-mesonephros with SCF, IL3 and FLT3L, which may bypass Notch-dependent regulation. Overall, our data reveal a role for the NR4A family of orphan nuclear receptors in EHT.

Sox17 and Other SoxF-Family Proteins Play Key Roles in the Hematopoiesis of Mouse Embryos

During mouse development, hematopoietic cells first form in the extraembryonic tissue yolk sac. Hematopoietic stem cells (HSCs), which retain their ability to differentiate into hematopoietic cells for a long time, form intra-aortic hematopoietic cell clusters (IAHCs) in the dorsal aorta at midgestation. These IAHCs emerge from the hemogenic endothelium, which is the common progenitor of hematopoietic cells and endothelial cells. HSCs expand in the fetal liver, and finally migrate to the bone marrow (BM) during the peripartum period. IAHCs are absent in the dorsal aorta in mice deficient in transcription factors such as Runx-1, GATA2, and c-Myb that are essential for definitive hematopoiesis. In this review, we focus on the transcription factor Sry-related high mobility group (HMG)-box (Sox) F family of proteins that is known to regulate hematopoiesis in the hemogenic endothelium and IAHCs. The SoxF family is composed of Sox7, Sox17, and Sox18, and they all have the HMG box, which has a DNA-binding ability, and a transcriptional activation domain. Here, we describe the functional and phenotypic properties of SoxF family members, with a particular emphasis on Sox17, which is the most involved in hematopoiesis in the fetal stages considering that enhanced expression of Sox17 in hemogenic endothelial cells and IAHCs leads to the production and maintenance of HSCs. We also discuss SoxF-inducing signaling pathways.

The evolving hematopoietic niche during development

Mammalian hematopoietic stem cells (HSCs) emerge from the hemogenic endothelium in the major embryonic arteries. HSCs undergo a complex journey first migrating to the fetal liver (FL) and from there to the fetal bone marrow (FBM), where they mostly remain during adult life. In this process, a pool of adult HSCs is produced, which sustains lifelong hematopoiesis. Multiple cellular components support HSC maturation and expansion and modulate their response to environmental and developmental cues. While the adult HSC niche has been extensively studied over the last two decades, the niches present in the major embryonic arteries, FL, FBM and perinatal bone marrow (BM) are poorly described. Recent investigations highlight important differences among FL, FBM and adult BM niches and emphasize the important role that inflammation, microbiota and hormonal factors play regulating HSCs and their niches. We provide a review on our current understanding of these important cellular microenvironments across ontogeny. We mainly focused on mice, as the most widely used research model, and, when possible, include relevant insights from other vertebrates including birds, zebrafish, and human. Developing a comprehensive picture on these processes is critical to understand the earliest origins of childhood leukemia and to achieve multiple goals in regenerative medicine, such as mimicking HSC development in vitro to produce HSCs for broad transplantation purposes in leukemia, following chemotherapy, bone marrow failure, and in HSC-based gene therapy.

Protocol for in vitro generating innate lymphoid cells from mouse α4β7+ lymphoid progenitors

Building a simple and efficient in vitro differentiation system is crucial for studying the regulatory mechanisms during the development of innate lymphoid cells (ILCs). Here, we present a protocol for generating ILC subsets from α4β7+ lymphoid progenitors (αLPs). We describe steps for murine cell isolation from fetal liver and adult bone marrow, flow cytometry sorting for αLPs, and cell culture. We then detail procedures for flow cytometry analysis of ILCs. This protocol significantly simplifies the differentiation process through ILC differentiation in vitro. For complete details on the use and execution of this protocol, please refer to Wu et al.1.

Variant-to-function mapping of late-onset Alzheimer's disease GWAS signals in human microglial cell models implicates RTFDC1 at the CASS4 locus

Late-onset Alzheimer's disease (LOAD) research has principally focused on neurons over the years due to their known role in the production of amyloid beta plaques and neurofibrillary tangles. In contrast, recent genomic studies of LOAD have implicated microglia as culprits of the prolonged inflammation exacerbating the neurodegeneration observed in patient brains. Indeed, recent LOAD genome-wide association studies (GWAS) have reported multiple loci near genes related to microglial function, including TREM2, ABI3, and CR1. However, GWAS alone cannot pinpoint underlying causal variants or effector genes at such loci, as most signals reside in non-coding regions of the genome and could presumably confer their influence frequently via long-range regulatory interactions. We elected to carry out a combination of ATAC-seq and high-resolution promoter-focused Capture-C in two human microglial cell models (iPSC-derived microglia and HMC3) in order to physically map interactions between LOAD GWAS-implicated candidate causal variants and their corresponding putative effector genes. Notably, we observed consistent evidence that rs6024870 at the GWAS CASS4 locus contacted the promoter of nearby gene, RTFDC1. We subsequently observed a directionallly consistent decrease in RTFDC1 expression with the the protective minor A allele of rs6024870 via both luciferase assays in HMC3 cells and expression studies in primary human microglia. Through CRISPR-Cas9-mediated deletion of the putative regulatory region harboring rs6024870 in HMC3 cells, we observed increased pro-inflammatory cytokine secretion and decreased DNA double strand break repair related, at least in part, to RTFDC1 expression levels. Our variant-to-function approach therefore reveals that the rs6024870-harboring regulatory element at the LOAD 'CASS4' GWAS locus influences both microglial inflammatory capacity and DNA damage resolution, along with cumulative evidence implicating RTFDC1 as a novel candidate effector gene.

Discovery of an embryonically derived bipotent population of endothelial-macrophage progenitor cells in postnatal aorta

Converging evidence indicates that extra-embryonic yolk sac is the source of both macrophages and endothelial cells in adult mouse tissues. Prevailing views are that these embryonically derived cells are maintained after birth by proliferative self-renewal in their differentiated states. Here we identify clonogenic endothelial-macrophage (EndoMac) progenitor cells in the adventitia of embryonic and postnatal mouse aorta, that are independent of Flt3-mediated bone marrow hematopoiesis and derive from an early embryonic CX3CR1+ and CSF1R+ source. These bipotent progenitors are proliferative and vasculogenic, contributing to adventitial neovascularization and formation of perfused blood vessels after transfer into ischemic tissue. We establish a regulatory role for angiotensin II, which enhances their clonogenic and differentiation properties and rapidly stimulates their proliferative expansion in vivo. Our findings demonstrate that embryonically derived EndoMac progenitors participate in local vasculogenic responses in the aortic wall by contributing to the expansion of endothelial cells and macrophages postnatally.

Lysosomal membrane protein TMEM106B modulates hematopoietic stem and progenitor cell proliferation and differentiation by regulating LAMP2A stability

Hematopoietic stem and progenitor cells (HSPCs) are successfully employed for hematological transplantations, and impaired HSPC function causes hematological diseases and aging. HSPCs maintain the lifelong homeostasis of blood and immune cells through continuous self-renewal and maintenance of the multilineage differentiation potential. TMEM106B is a transmembrane protein localized on lysosomal membranes and associated with neurodegenerative and cardiovascular diseases; however, its roles in HSPCs and hematopoiesis are unknown. Here, we established tmem106bb-/- knockout (KO) zebrafish and showed that tmem106bb KO reduced the proliferation of HSPCs during definitive hematopoiesis. The differentiation potential of HSPCs to lymphoid lineage was reduced, whereas the myeloid and erythroid differentiation potentials of HPSCs were increased in tmem106bb-/- zebrafish. Similar results were obtained with morpholino knockdown of tmem106bb. Mechanistically, TMEM106B interacted with LAMP2A, the lysosomal associated membrane protein 2A, impaired LAMP2A-Cathepsin A interaction, and enhanced LAMP2A stability; tmem106bb KO or TMEM106B knockdown caused LAMP2A degradation and impairment of chaperone-mediated autophagy (CMA). Knockdown of lamp2a caused similar phenotypes to that in tmem106bb-/- zebrafish, and overexpression of lamp2a rescued the impaired phenotypes of HSPCs in tmem106bb-/- embryos. These results uncover a novel molecular mechanism for the maintenance of HSPC proliferation and differentiation through stabilizing LAMP2A via TMEM106B-LAMP2A interaction.

Clonal analysis of fetal hematopoietic stem/progenitor cells reveals how post-transplantation capabilities are distributed

It has been proposed that adult hematopoiesis is sustained by multipotent progenitors (MPPs) specified during embryogenesis. Adult-like hematopoietic stem cell (HSC) and MPP immunophenotypes are present in the fetus, but knowledge of their functional capacity is incomplete. We found that fetal MPP populations were functionally similar to adult cells, albeit with some differences in lymphoid output. Clonal assessment revealed that lineage biases arose from differences in patterns of single-/bi-lineage differentiation. Long-term (LT)- and short-term (ST)-HSC populations were distinguished from MPPs according to capacity for clonal multilineage differentiation. We discovered that a large cohort of long-term repopulating units (LT-RUs) resides within the ST-HSC population; a significant portion of these were labeled using Flt3-cre. This finding has two implications: (1) use of the CD150+ LT-HSC immunophenotype alone will significantly underestimate the size and diversity of the LT-RU pool and (2) LT-RUs in the ST-HSC population have the attributes required to persist into adulthood.

Neurobeachin regulates hematopoietic progenitor differentiation and survival by modulating Notch activity

ABSTRACT

Hematopoietic stem cells (HSCs) can generate all blood cells. This ability is exploited in HSC transplantation (HSCT) to treat hematologic disease. A clear understanding of the molecular mechanisms that regulate HSCT is necessary to continue improving transplant protocols. We identified the Beige and Chediak-Higashi domain-containing protein (BDCP), Neurobeachin (NBEA), as a putative regulator of HSCT. Here, we demonstrated that NBEA and related BDCPs, including LPS Responsive Beige-Like Anchor Protein (LRBA), Neurobeachin Like 1 (NBEAL1) and Lysosomal Trafficking Regulator (LYST), are required during HSCT to efficiently reconstitute the hematopoietic system of lethally irradiated mice. Nbea knockdown in mouse HSCs induced apoptosis and a differentiation block after transplantation. Nbea deficiency in hematopoietic progenitor cells perturbed the expression of genes implicated in vesicle trafficking and led to changes in NOTCH receptor localization. This resulted in perturbation of the NOTCH transcriptional program, which is required for efficient HSC engraftment. In summary, our findings reveal a novel role for NBEA in the control of NOTCH receptor turnover in hematopoietic cells and supports a model in which BDCP-regulated vesicle trafficking is required for efficient HSCT.

Mechanistic insights into the developmental origin of pediatric hematologic disorders

Embryonic and fetal hematopoietic stem and progenitor cells differ in some key properties from cells that are part of the adult hematopoietic system. These include higher proliferation and self-renewal capacity, different globin gene usage, and differing lineage biases. Although these evolved to cover specific requirements of embryonic development, they can have serious consequences for the pathogenesis of hematologic malignancies that initiate prebirth in fetal blood cells and may result in a particularly aggressive disease that does not respond well to treatments that have been designed for adult leukemias. This indicates that these infant/pediatric leukemias should be considered developmental diseases, where a thorough understanding of their unique biology is essential for designing more effective therapies. In this review, we will highlight some of these unique fetal properties and detail the underlying molecular drivers of these phenotypes. We specifically focus on those that are pertinent to disease pathogenesis and that may therefore reveal disease vulnerabilities. We have also included an extensive description of the origins, phenotypes, and key molecular drivers of the main infant and pediatric leukemias that have a known prenatal origin. Importantly, successes in recent years in generating faithful models of these malignancies in which cellular origins, key drivers, and potential vulnerabilities can be investigated have resulted in uncovering potential, new therapeutic avenues.

The molecular and cellular hematopoietic stem cell specification niche

Hematopoietic stem cells (HSCs) are a population of tissue-specific stem cells that reside in the bone marrow of adult mammals, where they self-renew and continuously regenerate the adult hematopoietic lineages over the life of the individual. Prominence as a stem cell model and clinical usefulness have driven interest in understanding the physiologic processes that lead to the specification of HSCs during embryonic development. High-efficiency directed differentiation of HSCs by the instruction of defined progenitor cells using sequentially defined instructive molecules and conditions remains impossible, indicating that comprehensive knowledge of the complete set of precursor intermediate identities and required inductive inputs remains incompletely understood. Recently, interest in the molecular and cellular microenvironment where HSCs are specified from endothelial precursors-the "specification niche"-has increased. Here we review recent progress in understanding these niche spaces across vertebrate phyla, as well as how a better characterization of the origin and molecular phenotypes of the niche cell populations has helped inform and complicate previous understanding of signaling required for HSC emergence and maturation.

Lineage-tracing hematopoietic stem cell origins in vivo to efficiently make human HLF+ HOXA+ hematopoietic progenitors from pluripotent stem cells

The developmental origin of blood-forming hematopoietic stem cells (HSCs) is a longstanding question. Here, our non-invasive genetic lineage tracing in mouse embryos pinpoints that artery endothelial cells generate HSCs. Arteries are transiently competent to generate HSCs for 2.5 days (∼E8.5-E11) but subsequently cease, delimiting a narrow time frame for HSC formation in vivo. Guided by the arterial origins of blood, we efficiently and rapidly differentiate human pluripotent stem cells (hPSCs) into posterior primitive streak, lateral mesoderm, artery endothelium, hemogenic endothelium, and >90% pure hematopoietic progenitors within 10 days. hPSC-derived hematopoietic progenitors generate T, B, NK, erythroid, and myeloid cells in vitro and, critically, express hallmark HSC transcription factors HLF and HOXA5-HOXA10, which were previously challenging to upregulate. We differentiated hPSCs into highly enriched HLF+ HOXA+ hematopoietic progenitors with near-stoichiometric efficiency by blocking formation of unwanted lineages at each differentiation step. hPSC-derived HLF+ HOXA+ hematopoietic progenitors could avail both basic research and cellular therapies.

Cis inhibition of NOTCH1 through JAGGED1 sustains embryonic hematopoietic stem cell fate

Hematopoietic stem cells (HSCs) develop from the hemogenic endothelium (HE) in the aorta- gonads-and mesonephros (AGM) region and reside within Intra-aortic hematopoietic clusters (IAHC) along with hematopoietic progenitors (HPC). The signalling mechanisms that distinguish HSCs from HPCs are unknown. Notch signaling is essential for arterial specification, IAHC formation and HSC activity, but current studies on how Notch segregates these different fates are inconsistent. We now demonstrate that Notch activity is highest in a subset of, GFI1 + , HSC-primed HE cells, and is gradually lost with HSC maturation. We uncover that the HSC phenotype is maintained due to increasing levels of NOTCH1 and JAG1 interactions on the surface of the same cell (cis) that renders the NOTCH1 receptor from being activated. Forced activation of the NOTCH1 receptor in IAHC activates a hematopoietic differentiation program. Our results indicate that NOTCH1-JAG1 cis-inhibition preserves the HSC phenotype in the hematopoietic clusters of the embryonic aorta.

Haematopoietic development and HSC formation in vitro: promise and limitations of gastruloid models

Haematopoietic stem cells (HSCs) are the most extensively studied adult stem cells. Yet, six decades after their first description, reproducible and translatable generation of HSC in vitro remains an unmet challenge. HSC production in vitro is confounded by the multi-stage nature of blood production during development. Specification of HSC is a late event in embryonic blood production and depends on physical and chemical cues which remain incompletely characterised. The precise molecular composition of the HSC themselves is incompletely understood, limiting approaches to track their origin in situ in the appropriate cellular, chemical and mechanical context. Embryonic material at the point of HSC emergence is limiting, highlighting the need for an in vitro model of embryonic haematopoietic development in which current knowledge gaps can be addressed and exploited to enable HSC production. Gastruloids are pluripotent stem cell-derived 3-dimensional (3D) cellular aggregates which recapitulate developmental events in gastrulation and early organogenesis with spatial and temporal precision. Gastruloids self-organise multi-tissue structures upon minimal and controlled external cues, and are amenable to live imaging, screening, scaling and physicochemical manipulation to understand and translate tissue formation. In this review, we consider the haematopoietic potential of gastruloids and review early strategies to enhance blood progenitor and HSC production. We highlight possible strategies to achieve HSC production from gastruloids, and discuss the potential of gastruloid systems in illuminating current knowledge gaps in HSC specification.

CK2β Regulates Hematopoietic Stem Cell Biology and Erythropoiesis

The Ser-Thr kinase CK2 plays important roles in sustaining cell survival and resistance to stress and these functions are exploited by different types of blood tumors. Yet, the physiological involvement of CK2 in normal blood cell development is poorly known. Here, we discovered that the β regulatory subunit of CK2 is critical for normal hematopoiesis in the mouse. Fetal livers of conditional CK2β knockout embryos showed increased numbers of hematopoietic stem cells associated to a higher proliferation rate compared to control animals. Both hematopoietic stem and progenitor cells (HSPCs) displayed alterations in the expression of transcription factors involved in cell quiescence, self-renewal, and lineage commitment. HSPCs lacking CK2β were functionally impaired in supporting both in vitro and in vivo hematopoiesis as demonstrated by transplantation assays. Furthermore, KO mice developed anemia due to a reduced number of mature erythroid cells. This compartment was characterized by dysplasia, proliferative defects at early precursor stage, and apoptosis at late-stage erythroblasts. Erythroid cells exhibited a marked compromise of signaling cascades downstream of the cKit and erythropoietin receptor, with a defective activation of ERK/JNK, JAK/STAT5, and PI3K/AKT pathways and perturbations of several transcriptional programs as demonstrated by RNA-Seq analysis. Moreover, we unraveled an unforeseen molecular mechanism whereby CK2 sustains GATA1 stability and transcriptional proficiency. Thus, our work demonstrates new and crucial functions of CK2 in HSPC biology and in erythropoiesis.

Generating hematopoietic cells from human pluripotent stem cells: approaches, progress and challenges

Human pluripotent stem cells (hPSCs) have been suggested as a potential source for the production of blood cells for clinical application. In two decades, almost all types of blood cells can be successfully generated from hPSCs through various differentiated strategies. Meanwhile, with a deeper understanding of hematopoiesis, higher efficiency of generating progenitors and precursors of blood cells from hPSCs is achieved. However, how to generate large-scale mature functional cells from hPSCs for clinical use is still difficult. In this review, we summarized recent approaches that generated both hematopoietic stem cells and mature lineage cells from hPSCs, and remarked their efficiency and mechanisms in producing mature functional cells. We also discussed the major challenges in hPSC-derived products of blood cells and provided some potential solutions. Our review summarized efficient, simple, and defined methodologies for developing good manufacturing practice standards for hPSC-derived blood cells, which will facilitate the translation of these products into the clinic.

Ontogenetically distinct neutrophils differ in function and transcriptional profile in zebrafish

The current view of hematopoiesis considers leukocytes on a continuum with distinct developmental origins, and which exert non-overlapping functions. However, there is less known about the function and phenotype of ontogenetically distinct neutrophil populations. In this work, using a photoconvertible transgenic zebrafish line; Tg(mpx:Dendra2), we selectively label rostral blood island-derived and caudal hematopoietic tissue-derived neutrophils in vivo during steady state or upon injury. By comparing the migratory properties and single-cell expression profiles of both neutrophil populations at steady state we show that rostral neutrophils show higher csf3b expression and migration capacity than caudal neutrophils. Upon injury, both populations share a core transcriptional profile as well as subset-specific transcriptional signatures. Accordingly, both rostral and caudal neutrophils are recruited to the wound independently of their distance to the injury. While rostral neutrophils respond uniformly, caudal neutrophils respond heterogeneously. Collectively, our results reveal that co-existing neutrophils populations with ontogenically distinct origin display functional differences.

GATA2 mitotic bookmarking is required for definitive haematopoiesis

In mitosis, most transcription factors detach from chromatin, but some are retained and bookmark genomic sites. Mitotic bookmarking has been implicated in lineage inheritance, pluripotency and reprogramming. However, the biological significance of this mechanism in vivo remains unclear. Here, we address mitotic retention of the hemogenic factors GATA2, GFI1B and FOS during haematopoietic specification. We show that GATA2 remains bound to chromatin throughout mitosis, in contrast to GFI1B and FOS, via C-terminal zinc finger-mediated DNA binding. GATA2 bookmarks a subset of its interphase targets that are co-enriched for RUNX1 and other regulators of definitive haematopoiesis. Remarkably, homozygous mice harbouring the cyclin B1 mitosis degradation domain upstream Gata2 partially phenocopy knockout mice. Degradation of GATA2 at mitotic exit abolishes definitive haematopoiesis at aorta-gonad-mesonephros, placenta and foetal liver, but does not impair yolk sac haematopoiesis. Our findings implicate GATA2-mediated mitotic bookmarking as critical for definitive haematopoiesis and highlight a dependency on bookmarkers for lineage commitment.

The Zinc Transporter SLC39A10 Plays an Essential Role in Embryonic Hematopoiesis

The role of zinc in hematopoiesis is currently unclear. Here, SLC39A10 (ZIP10) is identified as a key zinc transporter in hematopoiesis. The results show that in zebrafish, Slc39a10 is a key regulator of the response to zinc deficiency. Surprisingly, both slc39a10 mutant zebrafish and hematopoietic Slc39a10-deficient mice develop a more severe form of impaired hematopoiesis than animals lacking transferrin receptor 1, a well-characterized iron gatekeeper, indicating that zinc plays a larger role than iron in hematopoiesis, at least in early hematopoietic stem cells (HSCs). Furthermore, it is shown that loss of Slc39a10 causes zinc deficiency in fetal HSCs, which in turn leads to DNA damage, apoptosis, and G₁ cell cycle arrest. Notably, zinc supplementation largely restores colony formation in HSCs derived from hematopoietic Slc39a10-deficient mice. In addition, inhibiting necroptosis partially restores hematopoiesis in mouse HSCs, providing mechanistic insights into the requirement for zinc in mediating hematopoiesis. Together, these findings indicate that SLC39A10 safeguards hematopoiesis by protecting against zinc deficiency-induced necroptosis, thus providing compelling evidence that SLC39A10 and zinc homeostasis promote the development of fetal HSCs. Moreover, these results suggest that SLC39A10 may serve as a novel therapeutic target for treating anemia and zinc deficiency-related disorders.

SOX18-enforced expression diverts hemogenic endothelium-derived progenitors from T towards NK lymphoid pathways

Hemogenic endothelium (HE) is the main source of blood cells in the embryo. To improve blood manufacturing from human pluripotent stem cells (hPSCs), it is essential to define the molecular determinants that enhance HE specification and promote development of the desired blood lineage from HE. Here, using SOX18-inducible hPSCs, we revealed that SOX18 forced expression at the mesodermal stage, in contrast to its homolog SOX17, has minimal effects on arterial specification of HE, expression of HOXA genes and lymphoid differentiation. However, forced expression of SOX18 in HE during endothelial-to-hematopoietic transition (EHT) greatly increases NK versus T cell lineage commitment of hematopoietic progenitors (HPs) arising from HE predominantly expanding CD34⁺CD43⁺CD235a/CD41a^-CD45^- multipotent HPs and altering the expression of genes related to T cell and Toll-like receptor signaling. These studies improve our understanding of lymphoid cell specification during EHT and provide a new tool for enhancing NK cell production from hPSCs for immunotherapies.

Linking Benzene, in Utero Carcinogenicity and Fetal Hematopoietic Stem Cell Niches: A Mechanistic Review

Previous research reported that prolonged benzene exposure during in utero fetal development causes greater fetal abnormalities than in adult-stage exposure. This phenomenon increases the risk for disease development at the fetal stage, particularly carcinogenesis, which is mainly associated with hematological malignancies. Benzene has been reported to potentially act via multiple modes of action that target the hematopoietic stem cell (HSCs) niche, a complex microenvironment in which HSCs and multilineage hematopoietic stem and progenitor cells (HSPCs) reside. Oxidative stress, chromosomal aberration and epigenetic modification are among the known mechanisms mediating benzene-induced genetic and epigenetic modification in fetal stem cells leading to in utero carcinogenesis. Hence, it is crucial to monitor exposure to carcinogenic benzene via environmental, occupational or lifestyle factors among pregnant women. Benzene is a well-known cause of adult leukemia. However, proof of benzene involvement with childhood leukemia remains scarce despite previously reported research linking incidences of hematological disorders and maternal benzene exposure. Furthermore, accumulating evidence has shown that maternal benzene exposure is able to alter the developmental and functional properties of HSPCs, leading to hematological disorders in fetus and children. Since HSPCs are parental blood cells that regulate hematopoiesis during the fetal and adult stages, benzene exposure that targets HSPCs may induce damage to the population and trigger the development of hematological diseases. Therefore, the mechanism of in utero carcinogenicity by benzene in targeting fetal HSPCs is the primary focus of this review.

Hematopoietic Stem Cells and the Immune System in Development and Aging

Hematopoietic stem cells (HSCs) support haematopoiesis throughout life and give rise to the whole variety of cells of the immune system. Developing in the early embryo, passing through the precursor stage, and maturing into the first HSCs, they undergo a fairly large number of divisions while maintaining a high regenerative potential due to high repair activity. This potential is greatly reduced in adult HSCs. They go into a state of dormancy and anaerobic metabolism to maintain their stemness throughout life. However, with age, changes occur in the pool of HSCs that negatively affect haematopoiesis and the effectiveness of immunity. Niche aging and accumulation of mutations with age reduces the ability of HSCs to self-renew and changes their differentiation potential. This is accompanied by a decrease in clonal diversity and a disturbance of lymphopoiesis (decrease in the formation of naive T- and B-cells) and the predominance of myeloid haematopoiesis. Aging also affects mature cells, regardless of HSC, therefore, phagocytic activity and the intensity of the oxidative burst decrease, and the efficiency of processing and presentation of antigens by myeloid cells is impaired. Aging cells of innate and adaptive immunity produce factors that form a chronic inflammatory background. All these processes have a serious negative impact on the protective properties of the immune system, increasing inflammation, the risk of developing autoimmune, oncological, and cardiovascular diseases with age. Understanding the mechanisms of reducing the regenerative potential in a comparative analysis of embryonic and aging HSCs, the features of inflammatory aging will allow us to get closer to deciphering the programs for the development, aging, regeneration and rejuvenation of HSCs and the immune system.

Wnt signaling in stem cells during development and cell lineage specification

During embryo development, cell proliferation, cell fate specification and tissue patterning are coordinated and tightly regulated by a handful of evolutionarily conserved signaling pathways activated by secreted growth factor families including fibroblast growth factor (FGF), Nodal/bone morphogenetic protein (BMP), Hedgehog and Wnt. The spatial and temporal activation of these signaling pathways elicit context-specific cellular responses that ultimately shape the different tissues of the embryo. Extensive efforts have been dedicated to identifying the molecular mechanisms underlying these signaling pathways during embryo development, adult tissue homeostasis and regeneration. In this review, we first describe the role of the Wnt/β-catenin signaling pathway during early embryo development, axis specification and cell differentiation as a prelude to highlight how this knowledge is being leveraged to manipulate Wnt/β-catenin signaling activity with small molecules and biologics for the directed differentiation of pluripotent stem cells into various cell lineages that are physiologically relevant for stem cell therapy and regenerative medicine.

The role of microenvironment in the initiation and evolution of B-cell precursor acute lymphoblastic leukemia

B cell precursor acute lymphoblastic leukemia (BCP-ALL) is a malignant disorder of immature B lineage immune progenitors and is the commonest cancer in children. Despite treatment advances it remains a leading cause of death in childhood and response rates in adults remain poor. A preleukemic state predisposing children to BCP-ALL frequently arises in utero, with an incidence far higher than that of transformed leukemia, offering the potential for early intervention to prevent disease. Understanding the natural history of this disease requires an appreciation of how cell-extrinsic pressures, including microenvironment, immune surveillance and chemotherapy direct cell-intrinsic genetic and epigenetic evolution. In this review, we outline how microenvironmental factors interact with BCP-ALL at different stages of tumorigenesis and highlight emerging therapeutic avenues.

Immuno-localization of definitive hematopoietic stem cells in the vascular niche of mouse fetal liver

Understanding the murine fetal liver (FL) hematopoietic microenvironment, which promotes HSC proliferation, warrants identifying innate relationships between stem cells and the niche. An inclusive study of these cell associations remains elusive. Here, we optimized a protocol to immunolabel HSCs alongside the FL vasculature, a promising niche component. We provide a comprehensive plan from tissue processing, immunohistochemistry, and confocal microscopy, to three-dimensional distance analyses between HSCs and vasculature. This technique can be adapted for achieving congruous outcomes for other cell types. For complete details on the use and execution of this protocol, please refer to Biswas et al. (2020).

Murine foetal liver supports limited detectable expansion of life-long haematopoietic progenitors

Current dogma asserts that the foetal liver (FL) is an expansion niche for recently specified haematopoietic stem cells (HSCs) during ontogeny. Indeed, between embryonic day of development (E)12.5 and E14.5, the number of transplantable HSCs in the murine FL expands from 50 to about 1,000. Here we used a non-invasive, multi-colour lineage tracing strategy to interrogate the embryonic expansion of murine haematopoietic progenitors destined to contribute to the adult HSC pool. Our data show that this pool of fated progenitors expands only two-fold during FL ontogeny. Although Histone2B-GFP retention in vivo experiments confirmed substantial proliferation of phenotypic FL-HSC between E12.5 and E14.5, paired-daughter cell assays revealed that many mid-gestation phenotypic FL-HSCs are biased to differentiate, rather than self-renew, relative to phenotypic neonatal and adult bone marrow HSCs. In total, these data support a model in which the FL-HSC pool fated to contribute to adult blood expands only modestly during ontogeny.

Development of the immune system in the human embryo

The fetal immune system is highly specialized which is to generate both tolerogenic and protective immune responses to tolerate both self- and maternal-antigens. Fetal T cells with pro-inflammatory potential are born in a tolerogenic environment and are tightly controlled by both cell-intrinsic and -extrinsic mechanisms. Fetal B-1 and B-2 B cells involved in innate and adaptive immune responses, respectively, arise in staggered waves of development from distinct progenitors. Innate immune responses are the key to the protection against infection and adaptive immunity creates memory after an initial response to a specific pathogen. This review aims to discuss the recent advances in understanding the development of immune system in fetus. IMPACT: During gestation, essential developmental changes occur to survive the neonates. At early stage, developmental signals and changes may be influenced due to immune deficiencies.

Murine fetal bone marrow does not support functional hematopoietic stem and progenitor cells until birth

While adult bone marrow (BM) hematopoietic stem and progenitor cells (HSPCs) and their extrinsic regulation is well studied, little is known about the composition, function, and extrinsic regulation of the first HSPCs to enter the BM during development. Here, we functionally interrogate murine BM HSPCs from E15.5 through P0. Our work reveals that fetal BM HSPCs are present by E15.5, but distinct from the HSPC pool seen in fetal liver, both phenotypically and functionally, until near birth. We also generate a transcriptional atlas of perinatal BM HSPCs and the BM niche in mice across ontogeny, revealing that fetal BM lacks HSPCs with robust intrinsic stem cell programs, as well as niche cells supportive of HSPCs. In contrast, stem cell programs are preserved in neonatal BM HSPCs, which reside in a niche expressing HSC supportive factors distinct from those seen in adults. Collectively, our results provide important insights into the factors shaping hematopoiesis during this understudied window of hematopoietic development.

Relatively little is known about the first hematopoietic stem and progenitor cells to arrive in the fetal bone marrow. Here they characterize the frequency, function, and molecular identity of fetal BM HSPCs and their bone marrow niche, and show that most BM HSPCs have little hematopoietic function until birth.

Characterization of CD41+ cells in the lymph node

Lymph nodes (LNs) are the critical sites of immunity, and the stromal cells of LNs are crucial to their function. Our understanding of the stromal compartment of the LN has deepened recently with the characterization of nontraditional stromal cells. CD41 (integrin αIIb) is known to be expressed by platelets and hematolymphoid cells. We identified two distinct populations of CD41⁺Lyve1⁺ and CD41⁺Lyve1^- cells in the LNs. CD41⁺Lyve1^- cells appear in the LN mostly at the later stages of the lives of mice. We identified CD41⁺ cells in human LNs as well. We demonstrated that murine CD41⁺ cells express mesodermal markers, such as Sca-1, CD105 and CD29, but lack platelet markers. We did not observe the presence of platelets around the HEVs or within proximity to fibroblastic reticular cells of the LN. Examination of thoracic duct lymph fluid showed the presence of CD41⁺Lyve1^- cells, suggesting that these cells recirculate throughout the body. FTY720 reduced their trafficking to lymph fluid, suggesting that their egress is controlled by the S1P1 pathway. CD41⁺Lyve1^- cells of the LNs were sensitive to radiation, suggestive of their replicative nature. Single cell RNA sequencing data showed that the CD41⁺ cell population in naïve mouse LNs expressed largely stromal cell markers. Further studies are required to examine more deeply the role of CD41⁺ cells in the function of LNs.

Gastruloids as in vitro models of embryonic blood development with spatial and temporal resolution

Gastruloids are three-dimensional embryonic organoids that reproduce key features of early mammalian development in vitro with unique scalability, accessibility, and spatiotemporal similarity to real embryos. Recently, we adapted the gastruloid culture conditions to promote cardiovascular development. In this work, we extended these conditions to capture features of embryonic blood development through a combination of immunophenotyping, detailed transcriptomics analysis, and identification of blood stem/progenitor cell potency. We uncovered the emergence of blood progenitor and erythroid-like cell populations in late gastruloids and showed the multipotent clonogenic capacity of these cells, both in vitro and after transplantation into irradiated mice. We also identified the spatial localization near a vessel-like plexus in the anterior portion of gastruloids with similarities to the emergence of blood stem cells in the mouse embryo. These results highlight the potential and applicability of gastruloids to the in vitro study of complex processes in embryonic blood development with spatiotemporal fidelity.

Single-cell transcriptome analysis of embryonic and adult endothelial cells allows to rank the hemogenic potential of post-natal endothelium

Hematopoietic stem cells (HSCs) are crucial for the continuous production of blood cells during life. The transplantation of these cells is one of the most common treatments to cure patient suffering of blood diseases. However, the lack of suitable donors is a major limitation. One option to get HSCs matching perfectly a patient is cellular reprogramming. HSCs emerge from endothelial cells in blood vessels during embryogenesis through the endothelial to hematopoietic transition. Here, we used single-cell transcriptomics analysis to compare embryonic and post-natal endothelial cells to investigate the potential of adult vasculature to be reprogrammed in hematopoietic stem cells. Although transcriptional similarities have been found between embryonic and adult endothelial cells, we found some key differences in term of transcription factors expression. There is a deficit of expression of Runx1, Tal1, Lyl1 and Cbfb in adult endothelial cells compared to their embryonic counterparts. Using a combination of gene expression profiling and gene regulatory network analysis, we found that endothelial cells from the pancreas, brain, kidney and liver appear to be the most suitable targets for cellular reprogramming into HSCs. Overall, our work provides an important resource for the rational design of a reprogramming strategy for the generation of HSCs.

Lifelong multilineage contribution by embryonic-born blood progenitors

Haematopoietic stem cells (HSCs) arise in the embryo from the arterial endothelium through a process known as the endothelial-to-haematopoietic transition (EHT)1-4. This process generates hundreds of blood progenitors, of which a fraction go on to become definitive HSCs. It is generally thought that most adult blood is derived from those HSCs, but to what extent other progenitors contribute to adult haematopoiesis is not known. Here we use in situ barcoding and classical fate mapping to assess the developmental and clonal origins of adult blood in mice. Our analysis uncovers an early wave of progenitor specification-independent of traditional HSCs-that begins soon after EHT. These embryonic multipotent progenitors (eMPPs) predominantly drive haematopoiesis in the young adult, have a decreasing yet lifelong contribution over time and are the predominant source of lymphoid output. Putative eMPPs are specified within intra-arterial haematopoietic clusters and represent one fate of the earliest haematopoietic progenitors. Altogether, our results reveal functional heterogeneity during the definitive wave that leads to distinct sources of adult blood.

GFI1 regulates chromatin state essential in human endothelial-to-haematopoietic transition

Objectives

During embryonic haematopoiesis, haematopoietic stem/progenitor cells (HSPCs) develop from hemogenic endothelial cells (HECs) though endothelial to haematopoietic transition (EHT). However, little is known about how EHT is regulated in human. Here, we report that GFI1 plays an essential role in enabling normal EHT during haematopoietic differentiation of human embryonic stem cells (hESCs).

Results

GFI1 deletion in hESCs leads to a complete EHT defect due to a closed chromatin state of hematopoietic genes in HECs. Mechanically, directly regulates important signaling pathways essential for the EHT such as PI3K signaling.etc.

Conclutions

Together, our findings reveal an essential role of GFI1 mediated epigenetic mechanism underlying human EHT during hematopoiesis.

In the spotlight: the role of TGFβ signalling in haematopoietic stem and progenitor cell emergence

Haematopoietic stem and progenitor cells (HSPCs) sustain haematopoiesis by generating precise numbers of mature blood cells throughout the lifetime of an individual. In vertebrates, HSPCs arise during embryonic development from a specialised endothelial cell population, the haemogenic endothelium (HE). Signalling by the Transforming Growth Factor β (TGFβ) pathway is key to regulate haematopoiesis in the adult bone marrow, but evidence for a role in the formation of HSPCs has only recently started to emerge. In this review, we examine recent work in various model systems that demonstrate a key role for TGFβ signalling in HSPC emergence from the HE. The current evidence underpins two seemingly contradictory views of TGFβ function: as a negative regulator of HSPCs by limiting haematopoietic output from HE, and as a positive regulator, by programming the HE towards the haematopoietic fate. Understanding how to modulate the requirement for TGFβ signalling in HSC emergence may have critical implications for the generation of these cells in vitro for therapeutic use.

Notch Signaling in HSC Emergence: When, Why and How

The hematopoietic stem cell (HSC) sustains blood homeostasis throughout life in vertebrates. During embryonic development, HSCs emerge from the aorta-gonads and mesonephros (AGM) region along with hematopoietic progenitors within hematopoietic clusters which are found in the dorsal aorta, the main arterial vessel. Notch signaling, which is essential for arterial specification of the aorta, is also crucial in hematopoietic development and HSC activity. In this review, we will present and discuss the evidence that we have for Notch activity in hematopoietic cell fate specification and the crosstalk with the endothelial and arterial lineage. The core hematopoietic program is conserved across vertebrates and here we review studies conducted using different models of vertebrate hematopoiesis, including zebrafish, mouse and in vitro differentiated Embryonic stem cells. To fulfill the goal of engineering HSCs in vitro, we need to understand the molecular processes that modulate Notch signaling during HSC emergence in a temporal and spatial context. Here, we review relevant contributions from different model systems that are required to specify precursors of HSC and HSC activity through Notch interactions at different stages of development.

Articulating the "stem cell niche" paradigm through the lens of non-model aquatic invertebrates

Stem cells (SCs) in vertebrates typically reside in "stem cell niches" (SCNs), morphologically restricted tissue microenvironments that are important for SC survival and proliferation. SCNs are broadly defined by properties including physical location, but in contrast to vertebrates and other "model" organisms, aquatic invertebrate SCs do not have clearly documented niche outlines or properties. Life strategies such as regeneration or asexual reproduction may have conditioned the niche architectural variability in aquatic or marine animal groups. By both establishing the invertebrates SCNs as independent types, yet allowing inclusiveness among them, the comparative analysis will allow the future functional characterization of SCNs.

Cdx regulates gene expression through PRC2-mediated epigenetic mechanisms

The extra-embryonic yolk sac contains adjacent layers of mesoderm and visceral endoderm. The mesodermal layer serves as the first site of embryonic hematopoiesis, while the visceral endoderm provides a means of exchanging nutrients and waste until the development of the chorioallantoic placenta. While defects in chorioallantoic fusion and yolk sac hematopoiesis have been described in Cdx mutant mouse models, little is known about the gene targets and molecular mechanisms through which Cdx members regulate these processes. To this end, we used RNA-seq to examine Cdx-dependent gene expression changes in the yolk sac. We find that loss of Cdx function impacts the expression of genes involved in yolk sac hematopoiesis, as previously described, as well as novel Cdx2 target genes. In addition, we observed Cdx-dependent changes in PRC2 subunit expression accompanied by altered H3K27me3 deposition at a subset of Cdx target genes as early as E7.5 in the embryo proper. This study identifies additional Cdx target genes and provides further evidence for Cdx-dependent epigenetic regulation of gene expression in the early embryo, and that this regulation is required to maintain gene expression programs in the extra-embryonic yolk sac at later developmental stages.

Guidelines for the use of flow cytometry and cell sorting in immunological studies (third edition)

The third edition of Flow Cytometry Guidelines provides the key aspects to consider when performing flow cytometry experiments and includes comprehensive sections describing phenotypes and functional assays of all major human and murine immune cell subsets. Notably, the Guidelines contain helpful tables highlighting phenotypes and key differences between human and murine cells. Another useful feature of this edition is the flow cytometry analysis of clinical samples with examples of flow cytometry applications in the context of autoimmune diseases, cancers as well as acute and chronic infectious diseases. Furthermore, there are sections detailing tips, tricks and pitfalls to avoid. All sections are written and peer-reviewed by leading flow cytometry experts and immunologists, making this edition an essential and state-of-the-art handbook for basic and clinical researchers.

Extramedullary hematopoiesis in myeloproliferative neoplasms: Pathophysiology and treatment strategies

Extramedullary hematopoiesis (EMH) is often a physiologic response to ineffective marrow production of hematologic cells. While this can be found incidentally in various physiologic and pathophysiologic states, the myeloproliferative neoplasms (MPNs) are some of the most common underlying conditions found in patients with EMH. Although this process can assist with hematologic production in defective states, the burden of EMH can lead to symptomatic discomfort and mechanical obstructive complications, most commonly in the spleen and liver. Here we describe the pathophysiology of EMH, treatment options, including medical, surgical and radiation-based approaches.

Mek1 and Mek2 Functional Redundancy in Erythropoiesis

Several studies have established the crucial role of the extracellular signal–regulated kinase (ERK)/mitogen-activated protein kinase pathway in hematopoietic cell proliferation and differentiation. MEK1 and MEK2 phosphorylate and activate ERK1 and ERK2. However, whether MEK1 and MEK2 differentially regulate these processes is unknown. To define the function of Mek genes in the activation of the ERK pathway during hematopoiesis, we generated a mutant mouse line carrying a hematopoietic-specific deletion of the Mek1 gene function in a Mek2 null background. Inactivation of both Mek1 and Mek2 genes resulted in death shortly after birth with a severe anemia revealing the essential role of the ERK pathway in erythropoiesis. Mek1 and Mek2 functional ablation also affected lymphopoiesis and myelopoiesis. In contrast, mice that retained one functional Mek1 (1Mek1) or Mek2 (1Mek2) allele in hematopoietic cells were viable and fertile. 1Mek1 and 1Mek2 mutants showed mild signs of anemia and splenomegaly, but the half-life of their red blood cells and the response to erythropoietic stress were not altered, suggesting a certain level of Mek redundancy for sustaining functional erythropoiesis. However, subtle differences in multipotent progenitor distribution in the bone marrow were observed in 1Mek1 mice, suggesting that the two Mek genes might differentially regulate early hematopoiesis.

Hematopoietic stem cell stretches and moves in its bone marrow niche

Hematopoietic stem cells are the most illustrious inhabitants of the bone marrow. Direct visualization of endogenous hematopoietic stem cells in this niche is essential to study their functions. Until recently this was not possible in live animals. Recent studies, using state-of-the-art technologies, including sophisticated in vivo inducible genetic approaches in combination with two-photon laser scanning microscopy, allow the follow-up of endogenous hematopoietic stem cells' behavior in their habitat. Strikingly, the new findings reveal that quiescent hematopoietic stem cells are more mobile than previously thought, and link their retained steady state within the niche to a mobile behavior. The arising knowledge from this research will be critical for the therapy of several hematological diseases. Here, we review recent progress in our understanding of hematopoietic stem cell biology in their niches.