Hematopoietic stem cell development requires transient Wnt/β-catenin activity

Dynamic WNT signaling controls differentiation of hematopoietic progenitor cells from human pluripotent stem cells

Human pluripotent stem cells (hPSCs) can in theory give rise to any hematopoietic lineages, thereby offering opportunities for disease modeling, drug screening and cell therapies. However, gaps in our knowledge of the signaling requirements for the specification of human hematopoietic stem/progenitor cells (HSPCs), which lie at the apex of all hematopoietic lineages, greatly limit the potential of hPSC in hematological research and application. Transcriptomic analysis reveals aberrant regulation of WNT signaling during maturation of hPSC-derived hematopoietic progenitor cells (hPSC-HPCs), which results in higher mitochondria activity, misregulation of HOX genes, loss of self-renewal and precocious differentiation. These defects are partly due to the activation of the WNT target gene CDX2. Late-stage WNT inhibition improves the yield, self-renewal, multilineage differentiation, and transcriptional and metabolic profiles of hPSC-HPCs. Genome-wide mapping of transcription factor (TF) accessible chromatin reveals a significant overrepresentation of myeloid TF binding motifs in hPSC-HPCs, which could underlie their myeloid-biased lineage potential. Together our findings uncover a previously unappreciated dynamic requirement of the WNT signaling pathway during the specification of human HSPCs. Modulating the WNT pathway with small molecules normalizes the molecular differences between hPSC-HPCs and endogenous hematopoietic stem cells (HSCs), thereby representing a promising approach to improve the differentiation and function of hPSC-HPCs.

Secreted Frizzled-Related Protein 1a regulates hematopoietic development in a dose-dependent manner

Hematopoietic stem and progenitor cells (HSPCs) arise only during embryonic development, and their identity specification, emergence from the floor of the dorsal aorta, and proliferation are all tightly regulated by molecular mechanisms such as signaling cues. Among these, Wnt signaling plays an important role in HSPC specification, differentiation, and self-renewal, requiring precise modulation for proper development and homeostasis. Wnt signaling is initiated when a Wnt ligand binds to cell surface receptors such as those encoded by the frizzled gene family, activating intracellular signaling pathways that regulate gene expression. Secreted frizzled-related proteins (Sfrps) are known modulators of Wnt signaling, acting as both agonists and antagonists of this pathway. Yet, in vivo functions of Sfrps in HSPC development remain incompletely understood. Here, we demonstrate that Sfrp1a regulates zebrafish HSPC development and differentiation in a dose-dependent manner. In Sfrp1a loss of function animals, we observe an increase in HSPCs, an upregulation of canonical Wnt signaling, and a decrease in differentiation into both lymphoid and myeloid lineages. Conversely, at low-dose sfrp1a overexpression, there is a decrease in HSPCs and an increase in lymphoid differentiation. High-dose sfrp1a overexpression phenocopies the loss of function animals, with an increase in HSPCs, increased canonical Wnt signaling, and decreased lymphoid and myeloid differentiation. These findings highlight the importance of dose-dependent modulation of Sfrps, paralleling what is observed in hematopoietic cancers where SFRP1 loss-of-function and gain-of-function variants can drive tumorigenesis.

The role of the haematopoietic stem cell niche in development and ageing

Blood production depends on rare haematopoietic stem cells (HSCs) and haematopoietic stem and progenitor cells (HSPCs) that ultimately take up residence in the bone marrow during development. HSPCs and HSCs are subject to extrinsic regulation by the bone marrow microenvironment, or niche. Studying the interactions between HSCs and their niche is critical for improving ex vivo culturing conditions and genetic manipulation of HSCs, which is pivotal for improving autologous HSC therapies and transplantations. Additionally, understanding how the complex molecular network in the bone marrow is altered during ageing is paramount for developing novel therapeutics for ageing-related haematopoietic disorders. HSCs are unique amongst stem and progenitor cell pools in that they engage with multiple physically distinct niches during their ontogeny. HSCs are specified from haemogenic endothelium in the aorta, migrate to the fetal liver and, ultimately, colonize their final niche in the bone marrow. Recent studies employing single-cell transcriptomics and microscopy have identified novel cellular interactions that govern HSC specification and engagement with their niches throughout ontogeny. New lineage-tracing models and microscopy tools have raised questions about the numbers of HSCs specified, as well as the functional consequences of HSCs interacting with each developmental niche. Advances have also been made in understanding how these niches are modified and perturbed during ageing, and the role of these altered interactions in haematopoietic diseases. In this Review, we discuss these new findings and highlight the questions that remain to be explored.

Endothelial heterogeneity in bone marrow: insights across development, adult life and leukemia

The central role of the endothelial microenvironment in orchestrating bone marrow (BM) homeostasis and hematopoietic support has been confirmed at various developmental stages and in adult life. The BM vasculature is crucial in mediating communication between BM parenchyma and circulating blood, displaying remarkable heterogeneity in structure and function. While vascular cell diversity in other tissues has long been recognized, the molecular basis of this phenomenon in BM is just now emerging. Over the past decade, single-cell approaches and microscopic observations have expanded our understanding of BM vasculature. While solely characterized for their paracrine properties in the past, recent advances have revolutionized our perception of endothelial function, revealing distinct anatomical locations associated with diverse endothelial cell states. The identification of phenotypic differences between normal and pathological conditions has therefore deepened our understanding of vascular dynamics and their impact on hematopoiesis in health and disease. In this review, we highlight key milestones and recent advances in understanding vascular heterogeneity within BM microenvironment during development, adulthood and aging. We also explore how leukemia affects this heterogeneity and how we can take this knowledge forward to improve clinical practices. By synthesizing existing literature, we aim to address unresolved questions and outline future research directions.

Deciphering the evolving niche interactome of human hematopoietic stem cells from ontogeny to aging

Hematopoietic stem cells (HSC) reside within specialized microenvironments that undergo dynamic changes throughout development and aging to support HSC function. However, the evolving cell-cell communication networks within these niches remain largely unexplored. This study integrates single-cell RNA sequencing datasets to systematically characterize the HSC niche interactome from ontogeny to aging. We reconstructed single-cell atlases of HSC niches at different developmental stages, revealing stage-specific cellular compositions and interactions targeting HSC. During HSC maturation, our analysis identified distinct patterns of ligand-receptor interactions and signaling pathways that govern HSC emergence, expansion, and maintenance. HSC aging was accompanied by a decrease in supportive niche interactions, followed by an adaptive increase in interaction strength in old adult bone marrow. This complex aging process involved the emergence of interactions associated with inflammation, altered stem cell function, and a decline in the efficacy of key signaling pathways. Our findings provide a comprehensive understanding of the dynamic remodeling of the HSC niche interactome throughout life, paving the way for targeted interventions to maintain HSC function and promote healthy aging. This study offers valuable insights into the intricate cell-cell communication networks that govern HSC behavior and fate, with implications for hematological disorders and regenerative medicine.

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.

Ecological insights into hematopoiesis regulation: unraveling the influence of gut microbiota

The gut microbiota constitutes a vast ecological system within the human body, forming a mutually interdependent entity with the host. In recent years, advancements in molecular biology technologies have provided a clearer understanding of the role of the gut microbiota. They not only influence the local immune status and metabolic functions of the host's intestinal tract but also impact the functional transformation of hematopoietic stem cells (HSCs) through the gut-blood axis. In this review, we will discuss the role of the gut microbiota in influencing hematopoiesis. We analyze the interactions between HSCs and other cellular components, with a particular emphasis on the direct functional regulation of HSCs by the gut microbiota and their indirect influence through cellular components in the bone marrow microenvironment. Additionally, we propose potential control targets for signaling pathways triggered by the gut microbiota to regulate hematopoietic function, filling crucial knowledge gaps in the development of this research field.

The RNA-binding protein CSDE1 promotes hematopoietic stem and progenitor cell generation via translational control of Wnt signaling

In vertebrates, the earliest hematopoietic stem and progenitor cells (HSPCs) are derived from a subset of specialized endothelial cells, hemogenic endothelial cells, in the aorta-gonad-mesonephros region through endothelial-to-hematopoietic transition. HSPC generation is efficiently and accurately regulated by a variety of factors and signals; however, the precise control of these signals remains incompletely understood. Post-transcriptional regulation is crucial for gene expression, as the transcripts are usually bound by RNA-binding proteins (RBPs) to regulate RNA metabolism. Here, we report that the RBP protein Csde1-mediated translational control is essential for HSPC generation during zebrafish early development. Genetic mutants and morphants demonstrated that depletion of csde1 impaired HSPC production in zebrafish embryos. Mechanistically, Csde1 regulates HSPC generation through modulating Wnt/β-catenin signaling activity. We demonstrate that Csde1 binds to ctnnb1 mRNAs (encoding β-catenin, an effector of Wnt signaling) and regulates translation but not stability of ctnnb1 mRNA, which further enhances β-catenin protein level and Wnt signal transduction activities. Together, we identify Csde1 as an important post-transcriptional regulator and provide new insights into how Wnt/β-catenin signaling is precisely regulated at the post-transcriptional level.

Wnt regulation of hematopoietic stem cell development and disease

Hematopoietic stem cells (HSCs) are multipotent stem cells that give rise to all cells of the blood and most immune cells. Due to their capacity for unlimited self-renewal, long-term HSCs replenish the blood and immune cells of an organism throughout its life. HSC development, maintenance, and differentiation are all tightly regulated by cell signaling pathways, including the Wnt pathway. Wnt signaling is initiated extracellularly by secreted ligands which bind to cell surface receptors and give rise to several different downstream signaling cascades. These are classically categorized either β-catenin dependent (BCD) or β-catenin independent (BCI) signaling, depending on their reliance on the β-catenin transcriptional activator. HSC development, homeostasis, and differentiation is influenced by both BCD and BCI, with a high degree of sensitivity to the timing and dosage of Wnt signaling. Importantly, dysregulated Wnt signals can result in hematological malignancies such as leukemia, lymphoma, and myeloma. Here, we review how Wnt signaling impacts HSCs during development and in disease.

Ex Vivo Expansion and Homing of Human Cord Blood Hematopoietic Stem Cells

Cord blood (CB) has been proven to be an alternative source of haematopoietic stem cells (HSCs) for clinical transplantation and has multiple advantages, including but not limited to greater HLA compatibility, lower incidence of graft-versus-host disease (GvHD), higher survival rates and lower relapse rates among patients with minimal residual disease. However, the limited number of HSCs in a single CB unit limits the wider use of CB in clinical treatment. Many efforts have been made to enhance the efficacy of CB HSC transplantation, particularly by ex vivo expansion or enhancing the homing efficiency of HSCs. In this chapter, we will document the major advances regarding human HSC ex vivo expansion and homing and will also discuss the possibility of clinical translation of such laboratory work.

Using Pluripotent Stem Cells to Understand Normal and Leukemic Hematopoietic Development

Several decades have passed since the generation of the first embryonic stem cell (ESC) lines both in mice and in humans. Since then, stem cell biologists have tried to understand their potential biological and clinical uses for their implementation in regenerative medicine. The hematopoietic field was a pioneer in establishing the potential use for the development of blood cell products and clinical applications; however, early expectations have been truncated by the difficulty in generating bonafide hematopoietic stem cells (HSCs). Despite some progress in understanding the origin of HSCs during embryonic development, the reproduction of this process in vitro is still not possible, but the knowledge acquired in the embryo is slowly being implemented for mouse and human pluripotent stem cells (PSCs). In contrast, ESC-derived hematopoietic cells may recapitulate some leukemic transformation processes when exposed to oncogenic drivers. This would be especially useful to model prenatal leukemia development or other leukemia-predisposing syndromes, which are difficult to study. In this review, we will review the state of the art of the use of PSCs as a model for hematopoietic and leukemia development.

Synthetic gene circuits for cell state detection and protein tuning in human pluripotent stem cells

During development, cell state transitions are coordinated through changes in the identity of molecular regulators in a cell type‐ and dose‐specific manner. The ability to rationally engineer such transitions in human pluripotent stem cells (hPSC) will enable numerous applications in regenerative medicine. Herein, we report the generation of synthetic gene circuits that can detect a desired cell state using AND‐like logic integration of endogenous miRNAs (classifiers) and, upon detection, produce fine‐tuned levels of output proteins using an miRNA‐mediated output fine‐tuning technology (miSFITs). Specifically, we created an “hPSC ON” circuit using a model‐guided miRNA selection and circuit optimization approach. The circuit demonstrates robust PSC‐specific detection and graded output protein production. Next, we used an empirical approach to create an “hPSC‐Off” circuit. This circuit was applied to regulate the secretion of endogenous BMP4 in a state‐specific and fine‐tuned manner to control the composition of differentiating hPSCs. Our work provides a platform for customized cell state‐specific control of desired physiological factors in hPSC, laying the foundation for programming cell compositions in hPSC‐derived tissues and beyond.

PDGFRβ+ cells play a dual role as hematopoietic precursors and niche cells during mouse ontogeny

Hematopoietic stem cell (HSC) generation in the aorta-gonad-mesonephros region requires HSC specification signals from the surrounding microenvironment. In zebrafish, PDGF-B/PDGFRβ signaling controls hematopoietic stem/progenitor cell (HSPC) generation and is required in the HSC specification niche. Little is known about murine HSPC specification in vivo and whether PDGF-B/PDGFRβ is involved. Here, we show that PDGFRβ is expressed in distinct perivascular stromal cell layers surrounding the mid-gestation dorsal aorta, and its deletion impairs hematopoiesis. We demonstrate that PDGFRβ⁺ cells play a dual role in murine hematopoiesis. They act in the aortic niche to support HSPCs, and in addition, PDGFRβ⁺ embryonic precursors give rise to a subset of HSPCs that persist into adulthood. These findings provide crucial information for the controlled production of HSPCs in vitro.

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PDGFRβ deletion affects hematopoietic development in the AGM in vivo

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The transcriptome and hematopoietic support of the PDGFRβ-KO niche are altered

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The osteogenic gene profile and differentiation of KO AGM MSCs are affected

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PDGFRβ⁺ early embryonic precursors contribute to EC and HSPC lineages in vivo

The Wnt Pathway: From Signaling Mechanisms to Synthetic Modulators

The Wnt pathway is central to a host of developmental and disease-related processes. The remarkable conservation of this intercellular signaling cascade throughout metazoan lineages indicates that it coevolved with multicellularity to regulate the generation and spatial arrangement of distinct cell types. By regulating cell fate specification, mitotic activity, and cell polarity, Wnt signaling orchestrates development and tissue homeostasis, and its dysregulation is implicated in developmental defects, cancer, and degenerative disorders. We review advances in our understanding of this key pathway, from Wnt protein production and secretion to relay of the signal in the cytoplasm of the receiving cell. We discuss the evolutionary history of this pathway as well as endogenous and synthetic modulators of its activity. Finally, we highlight remaining gaps in our knowledge of Wnt signal transduction and avenues for future research. Expected final online publication date for the Annual Review of Biochemistry, Volume 91 is June 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Ezh2 is essential for the generation of functional yolk sac derived erythro-myeloid progenitors

Yolk sac (YS) hematopoiesis is critical for the survival of the embryo and a major source of tissue-resident macrophages that persist into adulthood. Yet, the transcriptional and epigenetic regulation of YS hematopoiesis remains poorly characterized. Here we report that the epigenetic regulator Ezh2 is essential for YS hematopoiesis but dispensable for subsequent aorta-gonad-mesonephros (AGM) blood development. Loss of EZH2 activity in hemogenic endothelium (HE) leads to the generation of phenotypically intact but functionally deficient erythro-myeloid progenitors (EMPs), while the generation of primitive erythroid cells is not affected. EZH2 activity is critical for the generation of functional EMPs at the onset of the endothelial-to-hematopoietic transition but subsequently dispensable. We identify a lack of Wnt signaling downregulation as the primary reason for the production of non-functional EMPs. Together, our findings demonstrate a critical and stage-specific role of Ezh2 in modulating Wnt signaling during the generation of EMPs from YS HE.

Nxhl Controls Angiogenesis by Targeting VE-PTP Through Interaction With Nucleolin

Precise regulation of angiogenesis is required for organ development, wound repair, and tumor progression. Here, we identified a novel gene, nxhl (New XingHuo light), that is conserved in vertebrates and that plays a crucial role in vascular integrity and angiogenesis. Bioinformatic analysis uncovered its essential roles in development based on co-expression with several key developmental genes. Knockdown of nxhl in zebrafish causes global and pericardial edema, loss of blood circulation, and vascular defects characterized by both reduced vascularization in intersegmental vessels and decreased sprouting in the caudal vein plexus. The nxhl gene also affects human endothelial cell behavior in vitro. We found that nxhl functions in part by targeting VE-PTP through interaction with NCL (nucleolin). Loss of ptprb (a VE-PTP ortholo) in zebrafish resulted in defects similar to nxhl knockdown. Moreover, nxhl deficiency attenuates tumor invasion and proteins (including VE-PTP and NCL) associated with angiogenesis and EMT. These findings illustrate that nxhl can regulate angiogenesis via a novel nxhl-NCL-VE-PTP axis, providing a new therapeutic target for modulating vascular formation and function, especially for cancer treatment.

Canonical Wnt: a safeguard and threat for erythropoiesis

Myeloid dysplastic syndrome (MDS) reflects a preleukemic bone marrow (BM) disorder with limited treatment options and poor disease survival. As only a minority of MDS patients are eligible for curative hematopoietic stem cell transplantation, there is an urgent need to develop alternative treatment options. Chronic activation of Wnt/β-catenin has been implicated to underlie MDS formation and recently assigned to drive MDS transformation to acute myeloid leukemia. Wnt/β-catenin signaling therefore may harbor a pharmaceutical target to treat MDS and/or prevent leukemia formation. However, targeting the Wnt/β-catenin pathway will also affect healthy hematopoiesis in MDS patients. The control of Wnt/β-catenin in healthy hematopoiesis is poorly understood. Whereas Wnt/β-catenin is dispensable for steady-state erythropoiesis, its activity is essential for stress erythropoiesis in response to BM injury and anemia. Manipulation of Wnt/β-catenin signaling in MDS may therefore deregulate stress erythropoiesis and even increase anemia severity. Here, we provide a comprehensive overview of the most recent and established insights in the field to acquire more insight into the control of Wnt/β-catenin signaling in healthy and inefficient erythropoiesis as seen in MDS.

First blood: the endothelial origins of hematopoietic progenitors

Hematopoiesis in vertebrate embryos occurs in temporally and spatially overlapping waves in close proximity to blood vascular endothelial cells. Initially, yolk sac hematopoiesis produces primitive erythrocytes, megakaryocytes, and macrophages. Thereafter, sequential waves of definitive hematopoiesis arise from yolk sac and intraembryonic hemogenic endothelia through an endothelial-to-hematopoietic transition (EHT). During EHT, the endothelial and hematopoietic transcriptional programs are tightly co-regulated to orchestrate a shift in cell identity. In the yolk sac, EHT generates erythro-myeloid progenitors, which upon migration to the liver differentiate into fetal blood cells, including erythrocytes and tissue-resident macrophages. In the dorsal aorta, EHT produces hematopoietic stem cells, which engraft the fetal liver and then the bone marrow to sustain adult hematopoiesis. Recent studies have defined the relationship between the developing vascular and hematopoietic systems in animal models, including molecular mechanisms that drive the hemato-endothelial transcription program for EHT. Moreover, human pluripotent stem cells have enabled modeling of fetal human hematopoiesis and have begun to generate cell types of clinical interest for regenerative medicine.

Multispecies RNA tomography reveals regulators of hematopoietic stem cell birth in the embryonic aorta

The defined location of a stem cell within a niche regulates its fate, behavior, and molecular identity via a complex extrinsic regulation that is far from being fully elucidated. To explore the molecular characteristics and key components of the aortic microenvironment, where the first hematopoietic stem cells are generated during development, we performed genome-wide RNA tomography sequencing on zebrafish, chicken, mouse, and human embryos. The resulting anterior-posterior and dorsal-ventral transcriptional maps provided a powerful resource for exploring genes and regulatory pathways active in the aortic microenvironment. By performing interspecies comparative RNA sequencing analyses and functional assays, we explored the complexity of the aortic microenvironment landscape and the fine-tuning of various factors interacting to control hematopoietic stem cell generation, both in time and space in vivo, including the ligand-receptor couple ADM-RAMP2 and SVEP1. Understanding the regulatory function of the local environment will pave the way for improved stem cell production in vitro and clinical cell therapy.

A drug targeting 5-lipoxygenase enhances the activity of a JAK2 inhibitor in CD34+ bone marrow cells from patients with JAK2V617F-positive polycythemia vera in vitro

Janus kinase 2 (JAK2) inhibitors, the first targeted treatments for myeloproliferative neoplasms (MPNs), provide substantial benefits, including a marked reduction in splenomegaly and MPN-associated symptoms. However, these drugs rarely induce molecular remission in patients with MPNs. Zileuton, a 5-lipoxygenase (5-LO) inhibitor, has been demonstrated to selectively deplete hematopoietic stem cells (HSCs) expressing a JAK2 point mutation (JAK2V617F) in mouse models of JAK2V617F-induced polycythemia vera (PV). To determine the potential activity of 5-LO inhibitors in combination with JAK inhibitors against human PV HSCs, the present study first analyzed 5-LO expression in CD34⁺ bone marrow cells from patients with JAK2V617F-positive PV using western blotting and reverse transcription-quantitative PCR, and then examined the effect of zileuton combined with ruxolitinib on colony formation using a colony formation assay. Furthermore, cell cycle and apoptosis in CD34⁺ cells from patients with PV and healthy volunteers were determined by flow cytometry. In the present study, 5-LO expression was upregulated in CD34⁺ cells from patients with PV compared with in CD34⁺ cells from healthy volunteers. Higher levels of leukotriene B4, a product of the 5-LO signaling pathway, were detected in patients with PV compared with in healthy volunteers. Zileuton treatment suppressed the colony formation of CD34⁺ cells from patients with PV in a dose-dependent manner. Furthermore, zileuton and ruxolitinib exerted their anticancer effects by suppressing hematopoietic colony formation, inducing apoptosis and arresting the cell cycle of human CD34⁺ cells from patients with PV. The combination of these two drugs exerted a more beneficial effect than either agent alone. Based on these data, zileuton enhanced the antitumor activity of low-dose ruxolitinib in hematopoietic progenitor cells from patients with PV, providing conceptual validation for further clinical applications of combination treatment with ruxolitinib and zileuton for patients with PV.

JEM career launchpad

JEM has been a launching pad for scientific careers since its inception. Here is a collection of testimonials attesting to the diversity of the scientific community it serves.

Sry-related High Mobility Group Box 17 Functions as a Tumor Suppressor by Antagonizing the Wingless-related Integration Site Pathway

A transcription factor Sry-related high mobility group box (Sox) 17 is involved in developmental processes including spermatogenesis, cardiovascular system, endoderm formation, and so on. In this article, we firstly review the studies on the relation between the Sox17 expression and tumor malignancy. Although Sox17 positively promotes various tissue development, most of the cancers associated with Sox17 show decreased expression levels of Sox17, and an inverse correlation between Sox17 expression and malignancy is revealed. We briefly discuss the mechanism of such Sox17 down-regulation by focusing on DNA methylation of CpG sites located in the Sox17 gene promoter. Next, we overview the function of Sox17 in the fetal hematopoiesis, particularly in the dorsal aorta in midgestation mouse embryos. The Sox17 expression in hematopoietic stem cell (HSC)-containing intra-aortic hematopoietic cell cluster (IAHCs) is important for the cluster formation with the hematopoietic ability. The sustained expression of Sox17 in adult bone marrow HSCs and the cells in IAHCs of the dorsal aorta indicate abnormalities that are low lymphocyte chimerism and the aberrant proliferation of common myeloid progenitors in transplantation experiments. We then summarize the perspectives of Sox17 research in cancer control.

Molecular Modulation of Fetal Liver Hematopoietic Stem Cell Mobilization into Fetal Bone Marrow in Mice

Development of hematopoietic stem cells is a complex process, which has been extensively investigated. Hematopoietic stem cells (HSCs) in mouse fetal liver are highly expanded to prepare for mobilization of HSCs into the fetal bone marrow. It is not completely known how the fetal liver niche regulates HSC expansion without loss of self-renewal ability. We reviewed current progress about the effects of fetal liver niche, chemokine, cytokine, and signaling pathways on HSC self-renewal, proliferation, and expansion. We discussed the molecular regulations of fetal HSC expansion in mouse and zebrafish. It is also unknown how HSCs from the fetal liver mobilize, circulate, and reside into the fetal bone marrow niche. We reviewed how extrinsic and intrinsic factors regulate mobilization of fetal liver HSCs into the fetal bone marrow, which provides tools to improve HSC engraftment efficiency during HSC transplantation. Understanding the regulation of fetal liver HSC mobilization into the fetal bone marrow will help us to design proper clinical therapeutic protocol for disease treatment like leukemia during pregnancy. We prospect that fetal cells, including hepatocytes and endothelial and hematopoietic cells, might regulate fetal liver HSC expansion. Components from vascular endothelial cells and bones might also modulate the lodging of fetal liver HSCs into the bone marrow. The current review holds great potential to deeply understand the molecular regulations of HSCs in the fetal liver and bone marrow in mammals, which will be helpful to efficiently expand HSCs in vitro.

Haematopoietic stem cell-dependent Notch transcription is mediated by p53 through the Histone chaperone Supt16h

Haematopoietic stem and progenitor cells (HSPCs) have been the focus of developmental and regenerative studies, yet our understanding of the signalling events regulating their specification remains incomplete. We demonstrate that supt16h, a component of the Facilitates chromatin transcription (FACT) complex, is required for HSPC formation. Zebrafish supt16h mutants express reduced levels of Notch-signalling components, genes essential for HSPC development, due to abrogated transcription. Whereas global chromatin accessibility in supt16h mutants is not substantially altered, we observe a specific increase in p53 accessibility, causing an accumulation of p53. We further demonstrate that p53 influences expression of the Polycomb-group protein PHC1, which functions as a transcriptional repressor of Notch genes. Suppression of phc1 or its upstream regulator, p53, rescues the loss of both Notch and HSPC phenotypes in supt16h mutants. Our results highlight a relationship between supt16h, p53 and phc1 to specify HSPCs via modulation of Notch signalling.

Location, Location, Location: How Vascular Specialization Influences Hematopoietic Fates During Development

During embryonic development, sequential waves of hematopoiesis give rise to blood-forming cells with diverse lineage potentials and self-renewal properties. This process must accomplish two important yet divergent goals: the rapid generation of differentiated blood cells to meet the needs of the developing embryo and the production of a reservoir of hematopoietic stem cells to provide for life-long hematopoiesis in the adult. Vascular beds in distinct anatomical sites of extraembryonic tissues and the embryo proper provide the necessary conditions to support these divergent objectives, suggesting a critical role for specialized vascular niche cells in regulating disparate blood cell fates during development. In this review, we will examine the current understanding of how organ- and stage-specific vascular niche specialization contributes to the development of the hematopoietic system.

Combination of SB431542, Chir9901, and Bpv as a novel supplement in the culture of umbilical cord blood hematopoietic stem cells

BACKGROUND

Small molecule compounds have been well recognized for their promising power in the generation, expansion, and maintenance of embryonic or adult stem cells. The aim of this study was to identify a novel combination of small molecules in order to optimize the ex vivo expansion of umbilical cord blood-derived CD34+ cells.

METHODS

Considering the most important signaling pathways involved in the self-renewal of hematopoietic stem cells, CB-CD34+ cells were expanded with cytokines in the presence of seven small molecules including SB, PD, Chir, Bpv, Pur, Pμ, and NAM. The eliminativism approach was used to find the best combination of selected small molecules for effective ex vivo expansion of CD34+ cell. In each step, proliferation, self-renewal, and clonogenic potential of the expanded cells as well as expression of some hematopoietic stem cell-related genes were studied. Finally, the engraftment potential of expanded cells was also examined by the mouse intra-uterine transplantation model.

RESULTS

Our data shows that the simultaneous use of SB431542 (TGF-β inhibitor), Chir9901 (GSK3 inhibitor), and Bpv (PTEN inhibitor) resulted in a 50-fold increase in the number of CD34+CD38- cells. This was further reflected in approximately 3 times the increase in the clonogenic potential of the small molecule cocktail-expanded cells. These cells, also, showed a 1.5-fold higher engraftment potential in the peripheral blood of the NMRI model of in utero transplantation. These results are in total conformity with the upregulation of HOXB4, GATA2, and CD34 marker gene as well as the CXCR4 homing gene.

CONCLUSION

Taken together, our findings introduce a novel combination of small molecules to improve the yield of existing protocols used in the expansion of hematopoietic stem cells.

Regulation of Hemogenic Endothelial Cell Development and Function

Embryonic definitive hematopoiesis generates hematopoietic stem and progenitor cells (HSPCs) essential for establishment and maintenance of the adult blood system. This process requires the specification of a subset of vascular endothelial cells to become blood-forming, or hemogenic, and the subsequent endothelial-to-hematopoietic transition to generate HSPCs therefrom. The mechanisms that regulate these processes are under intensive investigation, as their recapitulation in vitro from human pluripotent stem cells has the potential to generate autologous HSPCs for clinical applications. In this review, we provide an overview of hemogenic endothelial cell development and highlight the molecular events that govern hemogenic specification of vascular endothelial cells and the generation of multilineage HSPCs from hemogenic endothelium. We also discuss the impact of hemogenic endothelial cell development on adult hematopoiesis.

β-catenin and γ-catenin are dispensable for T lymphocytes and AML leukemic stem cells

The β-catenin transcriptional coregulator is involved in various biological and pathological processes; however, its requirements in hematopoietic cells remain controversial. We re-targeted the Ctnnb1 gene locus to generate a true β-catenin-null mutant mouse strain. Ablation of β-catenin alone, or in combination with its homologue γ-catenin, did not affect thymocyte maturation, survival or proliferation. Deficiency in β/γ-catenin did not detectably affect differentiation of CD4⁺T follicular helper cells or that of effector and memory CD8⁺ cytotoxic cells in response to acute viral infection. In an MLL-AF9 AML mouse model, genetic deletion of β-catenin, or even all four Tcf/Lef family transcription factors that interact with β-catenin, did not affect AML onset in primary recipients, or the ability of leukemic stem cells (LSCs) in propagating AML in secondary recipients. Our data thus clarify on a long-standing controversy and indicate that β-catenin is dispensable for T cells and AML LSCs.

Ginsenoside Rg1 Improves Differentiation by Inhibiting Senescence of Human Bone Marrow Mesenchymal Stem Cell via GSK-3β and β-Catenin

Objectives

To demonstrate the effect of Ginsenoside Rg1 on the differentiation of human bone marrow-derived mesenchymal stem cells (hBM-MSCs). Subsequently, a rational mechanism for the detection of Rg1 which affects mesenchymal stem cell differentiation was explored.

Methods

Flow cytometry is used for cell identification. The differentiation ability of hBM-MSCs was studied by differentiation culture. SA-β-gal staining is used to detect cell senescence levels. Western blot and immunofluorescence were used to determine protein expression levels. RT-qPCR is used to detect mRNA expression levels.

Results

Rg1 regulates the differentiation of hBM-MSCs. Differentiation culture analysis showed that Rg1 promoted cells to osteogenesis and chondrogenesis. Western blot results showed that Rg1 regulated the overactivation of the β-catenin signaling pathway and significantly adjusted the phosphorylation of GSK-3β. GSK-3β inhibitor (Licl) significantly increased Rg1-induced phosphorylation of GSK-3β, which in turn reduced Rg1-induced differentiation of hBM-MSCs.

Conclusion

Ginsenoside Rg1 can reduce the excessive activation of the Wnt pathway in senescent cells by inhibiting the phosphorylation of GSK-3β and regulate the mesenchymal stem cell differentiation ability.

Benzene and its effects on cell signaling pathways related to hematopoiesis and leukemia

Benzene is an environmental toxicant found in many consumer products. It is an established human carcinogen and is known to cause acute myeloid leukemia in adults. Epidemiological evidence has since shown that benzene can cross the placenta and affect the fetal liver. Animal studies have shown that in utero exposure to benzene can increase tumor incidence in offspring. Although there have been risk factors established for acute myeloid leukemia, they still do not account for many of the cases. Clearly then, current efforts to elucidate the mechanism by which benzene exerts its carcinogenic properties have been superficial. Owing to the critical role of cell signaling pathways in the development of an organism and its various organ systems, it seems plausible to suspect that these pathways may have a role in leukemogenesis. This review article assesses current evidence of the effects of benzene on critical hematopoietic signaling pathways. Pathways discussed included Hedgehog, Notch/Delta, Wingless/Integrated, nuclear factor-kappaB and others. Following a review of the literature, it seems that current evidence about the effects of benzene on these critical signaling pathways remains limited. Given the important role of these pathways in hematopoiesis, more attention should be given to them.

Network Pharmacology-Based Investigation of the System-Level Molecular Mechanisms of the Hematopoietic Activity of Samul-Tang, a Traditional Korean Herbal Formula

Hematopoiesis is a dynamic process of the continuous production of diverse blood cell types to meet the body's physiological demands and involves complex regulation of multiple cellular mechanisms in hematopoietic stem cells, including proliferation, self-renewal, differentiation, and apoptosis. Disruption of the hematopoietic system is known to cause various hematological disorders such as myelosuppression. There is growing evidence on the beneficial effects of herbal medicines on hematopoiesis; however, their mechanism of action remains unclear. In this study, we conducted a network pharmacological-based investigation of the system-level mechanisms underlying the hematopoietic activity of Samul-tang, which is an herbal formula consisting of four herbal medicines, including Angelicae Gigantis Radix, Rehmanniae Radix Preparata, Paeoniae Radix Alba, and Cnidii Rhizoma. In silico analysis of the absorption-distribution-metabolism-excretion model identified 16 active phytochemical compounds contained in Samul-tang that may target 158 genes/proteins associated with myelosuppression to exert pharmacological effects. Functional enrichment analysis suggested that the targets of Samul-tang were significantly enriched in multiple pathways closely related to the hematopoiesis and myelosuppression development, including the PI3K-Akt, MAPK, IL-17, TNF, FoxO, HIF-1, NF-kappa B, and p53 signaling pathways. Our study provides novel evidence regarding the system-level mechanisms underlying the hematopoiesis-promoting effect of herbal medicines for hematological disorder treatment.

Revisiting β-Catenin Signaling in T-Cell Development and T-Cell Acute Lymphoblastic Leukemia

β-Catenin/CTNNB1 is critical for leukemia initiation or the stem cell capacity of several hematological malignancies. This review focuses on a general evaluation of β-catenin function in normal T-cell development and T-cell acute lymphoblastic leukemia (T-ALL). The integration of the existing literature offers a state-of-the-art dissection of the complexity of β-catenin function in leukemia initiation and maintenance in both Notch-dependent and independent contexts. In addition, β-catenin mutations are screened for in T-ALL primary samples, and it is found that they are rare and with little clinical relevance. Transcriptional analysis of Wnt family members (Ctnnb1, Axin2, Tcf7, and Lef1) and Myc in different publicly available T-ALL cohorts indicates that the expression of these genes may correlate with T-ALL subtypes and/or therapy outcomes.

In Vitro Expansion of Hematopoietic Stem Cells by Inhibition of Both GSK3 and p38 Signaling

Hematopoietic stem cell (HSC) transplantation therapy is one of the most effective treatments for life-threatening hematopoietic diseases. Bone marrow (BM) and mobilized peripheral blood are the major sources of HSCs, but these resources are limited by a paucity of human leukocyte antigen (HLA)-matched donors. Umbilical cord blood (UCB) is the most promising alternative to obtain HSCs for transplantation therapy. However, UCB transplantation therapy is limited by low numbers of HSCs per unit of UCB. In vitro HSC expansion is believed to be the most effective and applicable strategy to address this issue. Here we report that a moderate concentration of GSK3 inhibitor promotes HSC expansion by inducing moderate levels of β-catenin activity in HSCs. However, such a concentration of GSK3 inhibitor also stimulates myeloid cells to produce inflammatory cytokines, which attenuate HSC expansion by inducing p38 activation. Thus, when unpurified HSCs were used in culture, inhibition of p38-induced inflammatory cytokine signaling was required to ensure HSC expansion induced by the low concentration of GSK3 inhibitor. Our study suggests that the combination of a moderate concentration of p38 inhibitor plus a GSK3 inhibitor synergistically promotes the expansion of both murine BM HSCs and human UCB HSCs.

Blood Development: Hematopoietic Stem Cell Dependence and Independence

Evidence of the diversity and multi-layered organization of the hematopoietic system is leading to new insights that may inform ex vivo production of blood cells. Interestingly, not all long-lived hematopoietic cells derive from hematopoietic stem cells (HSCs). Here we review the current knowledge on HSC-dependent cell lineages and HSC-independent tissue-resident hematopoietic cells and how they arise during embryonic development. Classical embryological and genetic experiments, cell fate tracing data, single-cell imaging, and transcriptomics studies provide information on the molecular/cell trajectories that form the complete hematopoietic system. We also discuss the current developmentally informed efforts toward generating engraftable and multilineage blood cells.

Understanding the Journey of Human Hematopoietic Stem Cell Development

Hematopoietic stem cells (HSCs) surface during embryogenesis leading to the genesis of the hematopoietic system, which is vital for immune function, homeostasis balance, and inflammatory responses in the human body. Hematopoiesis is the process of blood cell formation, which initiates from hematopoietic stem/progenitor cells (HSPCs) and is responsible for the generation of all adult blood cells. With their self-renewing and pluripotent properties, human pluripotent stem cells (hPSCs) provide an unprecedented opportunity to create in vitro models of differentiation that will revolutionize our understanding of human development, especially of the human blood system. The utilization of hPSCs provides newfound approaches for studying the origins of human blood cell diseases and generating progenitor populations for cell-based treatments. Current shortages in our knowledge of adult HSCs and the molecular mechanisms that control hematopoietic development in physiological and pathological conditions can be resolved with better understanding of the regulatory networks involved in hematopoiesis, their impact on gene expression, and further enhance our ability to develop novel strategies of clinical importance. In this review, we delve into the recent advances in the understanding of the various cellular and molecular pathways that lead to blood development from hPSCs and examine the current knowledge of human hematopoietic development. We also review how in vitro differentiation of hPSCs can undergo hematopoietic transition and specification, including major subtypes, and consider techniques and protocols that facilitate the generation of hematopoietic stem cells.

Lrp5 and Lrp6 are required for maintaining self-renewal and differentiation of hematopoietic stem cells

Hematopoietic stem cells (HSCs) have the capacity for self-renewal to maintain the HSCs' pool and the ability for multilineage differentiation, which are responsible for sustained production of multiple blood lineages. The regulation of HSC development is controlled precisely by complex signal networks and hematopoietic microenvironment, which has been termed the HSCs' niche. The Wnt signaling pathway is one of a variety of signaling pathways that have been involved in HSC self-renewal and maintenance. Previous studies are indeterminant on the regulation of adult HSCs upon canonical Wnt signaling pathways because of the different experimental systems and models used. In this study, we generated the conditional knockout Wnt coreceptor low-density lipoprotein receptor-related protein 5 (Lrp5) and low-density lipoprotein receptor-related protein 6 (Lrp6) mice in adult hematopoiesis via Vav-Cre Loxp system. Inactivation of Lrp5 and -6 in a hematopoietic system diminished the pool of HSCs, but there were no obvious defects in mature immune cells. Lrp5 and -6 double deficiency HSCs showed intrinsic defects in self-renewal and differentiation due to reduced proliferation and increased quiescence of the cell cycle. Analysis of HSC gene expression suggested that the quiescence regulators were significantly up-regulated, such as Egr1, Cdkn1a, Nr4a1, Gata2, Junb and Btg2, and the positive cell cycle regulators were correspondingly down-regulated, such as Ccna2 and Ranbp1. Taken together, we investigated the roles of Lrp5 and -6 in HSCs by functional and bioinformatic assays, and we demonstrated that Lrp5 and -6 are required for the self-renewal and differentiation of adult HSCs. The canonical Wnt pathway may contribute to maintaining the HSC pool and regulate the differentiation of adult HSCs by controlling cell cycle gene regulatory module.-Liu, J., Cui, Z., Wang, F., Yao, Y., Yu, G., Liu, J., Cao, D., Niu, S., You, M., Sun, Z., Lian, D., Zhao, T., Kang, Y., Zhao, Y., Xue, H.-H., Yu, S. Lrp5 and Lrp6 are required for maintaining self-renewal and differentiation of hematopoietic stem cells.

VE-Cadherin and ACE Co-Expression Marks Highly Proliferative Hematopoietic Stem Cells in Human Embryonic Liver

Despite advances to engineer transplantable hematopoietic stem and progenitor cells (HSPCs) for research and therapy, an in-depth characterization of the developing human hematopoietic system is still lacking. The human embryonic liver is at the crossroad of several hematopoietic sites and harbors a complex hematopoietic hierarchy, including the first actively dividing HSPCs that will further seed the definitive hematopoietic organs. However, few are known about the phenotypic and functional HSPC organization operating at these stages of development. In this study, using a combination of four endothelial and hematopoietic surface markers, that is, the endothelial-specific marker vascular endothelial-cadherin (Cdh5, CD144), the pan-leukocyte antigen CD45, the hemato-endothelial marker CD34, and the angiotensin-converting enzyme (ACE, CD143), we identified distinct HSPC subsets, and among them, a population co-expressing the four markers that uniquely harbored an outstanding proliferation potential both ex vivo and in vivo. Moreover, we traced back this population to the yolk sac (YS) and aorta-gonad-mesonephros (AGM) sites of hematopoietic emergence. Taken together, our data will help to identify human HSPC self-renewal and amplification mechanisms for future cell therapies.

Distinct Bone Marrow Sources of Pleiotrophin Control Hematopoietic Stem Cell Maintenance and Regeneration

Bone marrow (BM) perivascular stromal cells and vascular endothelial cells (ECs) are essential for hematopoietic stem cell (HSC) maintenance, but the roles of distinct niche compartments during HSC regeneration are less understood. Here we show that Leptin receptor-expressing (LepR+) BM stromal cells and ECs dichotomously regulate HSC maintenance and regeneration via secretion of pleiotrophin (PTN). BM stromal cells are the key source of PTN during steady-state hematopoiesis because its deletion from stromal cells, but not hematopoietic cells, osteoblasts, or ECs, depletes the HSC pool. Following myelosuppressive irradiation, PTN expression is increased in bone marrow endothelial cells (BMECs), and PTN⁺ ECs are more frequent in the niche. Moreover, deleting Ptn from ECs impairs HSC regeneration whereas Ptn deletion from BM stromal cells does not. These findings reveal dichotomous and complementary regulation of HSC maintenance and regeneration by BM stromal cells and ECs.

Chromatin and Transcriptional Analysis of Mesoderm Progenitor Cells Identifies HOPX as a Regulator of Primitive Hematopoiesis

We analyzed chromatin dynamics and transcriptional activity of human embryonic stem cell (hESC)-derived cardiac progenitor cells (CPCs) and KDR⁺/CD34⁺ endothelial cells generated from different mesodermal origins. Using an unbiased algorithm to hierarchically rank genes modulated at the level of chromatin and transcription, we identified candidate regulators of mesodermal lineage determination. HOPX, a non-DNA-binding homeodomain protein, was identified as a candidate regulator of blood-forming endothelial cells. Using HOPX reporter and knockout hESCs, we show that HOPX regulates blood formation. Loss of HOPX does not impact endothelial fate specification but markedly reduces primitive hematopoiesis, acting at least in part through failure to suppress Wnt/β-catenin signaling. Thus, chromatin state analysis permits identification of regulators of mesodermal specification, including a conserved role for HOPX in governing primitive hematopoiesis.

WNT9A Is a Conserved Regulator of Hematopoietic Stem and Progenitor Cell Development

Hematopoietic stem cells (HSCs) differentiate into all cell types of the blood and can be used therapeutically to treat hematopoietic cancers and disorders. Despite decades of research, it is not yet possible to derive therapy-grade HSCs from pluripotent precursors. Analysis of HSC development in model organisms has identified some of the molecular cues that are necessary to instruct hematopoiesis in vivo, including Wnt9A, which is required during an early time window in zebrafish development. Although bona fide HSCs cannot be derived in vitro, it is possible to model human hematopoietic progenitor development by differentiating human pluripotent stem cells to hematopoietic cells. Herein, we modulate WNT9A expression during the in vitro differentiation of human embryonic stem cells to hematopoietic progenitor cells and demonstrate that WNT9A also regulates human hematopoietic progenitor cell development in vitro. Overexpression of WNT9A only impacts differentiation to CD34⁺/CD45⁺ cells during early time windows and does so in a dose-dependent manner. The cells that receive the Wnt signal—not the cells that secrete WNT9A—differentiate most efficiently to hematopoietic progenitors; this mimics the paracrine action of Wnt9a during in vivo hematopoiesis. Taken together, these data indicate that WNT9A is a conserved regulator of zebrafish and human hematopoietic development.

Wnt Signaling in Hematological Malignancies

Leukemia and lymphoma are a wide encompassing term for a diverse set of blood malignancies that affect people of all ages and result in approximately 23,000 deaths in the United States per year (Siegel RL, Miller KD, Jemal A. Cancer statistics, 2016. CA Cancer J Clin. 2016;66(1):7-30.). Hematopoietic stem cells (HSCs) are tissue-specific stem cells at the apex of the hierarchy that gives rise to all of the terminally differentiated blood cells, through progressively restricted progenitor populations, a process that is known to be Wnt-responsive. In particular, the progenitor populations are subject to uncontrolled expansion during oncogenic processes, namely the common myeloid progenitor and common lymphoid progenitor, as well as the myeloblast and lymphoblast. Unregulated growth of these cell-types leads to mainly three types of blood cancers (i.e., leukemia, lymphoma, and myeloma), which frequently exhibit deregulation of the Wnt signaling pathway. Generally, leukemia is caused by the expansion of myeloid progenitors, leading to an overproduction of white blood cells; as such, patients are unable to make sufficient numbers of red blood cells and platelets. Likewise, an overproduction of lymphocytes leads to clogging of the lymph system and impairment of the immune system in lymphomas. Finally, cancer of the plasma cells in the blood is called myeloma, which also leads to immune system failure. Within each of these three types of blood cancers, there are multiple subtypes, usually characterized by their timeline of onset and their cell type of origin. Of these, 85% of leukemias are encompassed by the four most common diseases, that is, acute myeloid leukemia (AML), chronic myeloid leukemia (CML), acute lymphoblastic leukemia (ALL), and chronic lymphocytic leukemia (CLL); AML accounts for the majority of leukemia-related deaths (Siegel RL, Miller KD, Jemal A. Cancer statistics, 2016. CA Cancer J Clin. 2016;66(1):7-30.). Through understanding how HSCs are normally developed and maintained, we can understand how the normal functions of these pathways are disrupted during blood cancer progression; the Wnt pathway is important in regulation of both normal and malignant hematopoiesis. In this chapter, we will discuss the role of Wnt signaling in normal and aberrant hematopoiesis. Our understanding the relationship between Wnt and HSCs will provide novel insights into therapeutic targets.

Wnt9a Is Required for the Aortic Amplification of Nascent Hematopoietic Stem Cells

All mature blood cell types in the adult animal arise from hematopoietic stem and progenitor cells (HSPCs). However, the developmental cues regulating HSPC ontogeny are incompletely understood. In particular, the details surrounding a requirement for Wnt/β-catenin signaling in the development of mature HSPCs are controversial and difficult to consolidate. Using zebrafish, we demonstrate that Wnt signaling is required to direct an amplification of HSPCs in the aorta. Wnt9a is specifically required for this process and cannot be replaced by Wnt9b or Wnt3a. This proliferative event occurs independently of initial HSPC fate specification, and the Wnt9a input is required prior to aorta formation. HSPC arterial amplification occurs prior to seeding of secondary hematopoietic tissues and proceeds, in part, through the cell cycle regulator myca (c-myc). Our results support a general paradigm, in which early signaling events, including Wnt, direct later HSPC developmental processes.

Hematopoietic stem and progenitor cells (HSPCs) give rise to all of the blood cells of the adult organism; however, how these cells are derived in vivo is still incompletely understood. Using zebrafish, Grainger et al. find that Wnt9a mediates amplification of HSPCs prior to their migration to secondary hematopoietic sites.

A molecular roadmap of the AGM region reveals BMPER as a novel regulator of HSC maturation

Through transcriptional profiling of the mouse AGM region, McGarvey et al. identify potential niche regulators of HSC development. They show a new function of BMPER in regulating HSC maturation, likely via its modulation of BMP signalling.

In the developing embryo, hematopoietic stem cells (HSCs) emerge from the aorta-gonad-mesonephros (AGM) region, but the molecular regulation of this process is poorly understood. Recently, the progression from E9.5 to E10.5 and polarity along the dorso-ventral axis have been identified as clear demarcations of the supportive HSC niche. To identify novel secreted regulators of HSC maturation, we performed RNA sequencing over these spatiotemporal transitions in the AGM region and supportive OP9 cell line. Screening several proteins through an ex vivo reaggregate culture system, we identify BMPER as a novel positive regulator of HSC development. We demonstrate that BMPER is associated with BMP signaling inhibition, but is transcriptionally induced by BMP4, suggesting that BMPER contributes to the precise control of BMP activity within the AGM region, enabling the maturation of HSCs within a BMP-negative environment. These findings and the availability of our transcriptional data through an accessible interface should provide insight into the maintenance and potential derivation of HSCs in culture.

Transcription factor MafB may play an important role in secondary hyperparathyroidism

The transcription factor MafB is essential for development of the parathyroid glands, the expression of which persists after morphogenesis and in adult parathyroid glands. However, the function of MafB in adult parathyroid tissue is unclear. To investigate this, we induced chronic kidney disease (CKD) in wild-type and MafB heterozygote (MafB+/-) mice by feeding them an adenine-supplemented diet, leading to secondary hyperparathyroidism. The elevated serum creatinine and blood urea nitrogen levels in heterozygous and wild-type mice fed the adenine-supplemented diet were similar. Interestingly, secondary hyperparathyroidism, characterized by serum parathyroid hormone elevation and enlargement of parathyroid glands, was suppressed in MafB+/- mice fed the adenine-supplemented diet compared to similarly fed wild-type littermates. Quantitative RT-PCR and immunohistochemical analyses showed that the increased expression of parathyroid hormone and cyclin D2 in mice with CKD was suppressed in the parathyroid glands of heterozygous CKD mice. A reporter assay indicated that MafB directly regulated parathyroid hormone and cyclin D2 expression. To exclude an effect of a developmental anomaly in MafB+/- mice, we analyzed MafB tamoxifen-induced global knockout mice. Hypocalcemia-stimulated parathyroid hormone secretion was significantly impaired in MafB knockout mice. RNA-sequencing analysis indicated PTH, Gata3 and Gcm2 depletion in the parathyroid glands of MafB knockout mice. Thus, MafB appears to play an important role in secondary hyperparathyroidism by regulation of parathyroid hormone and cyclin D2 expression. Hence, MafB may represent a new therapeutic target in secondary hyperparathyroidism.

The molecular basis of endothelial cell plasticity

The endothelium is capable of remarkable plasticity. In the embryo, primitive endothelial cells differentiate to acquire arterial, venous or lymphatic fates. Certain endothelial cells also undergo hematopoietic transition giving rise to multi-lineage hematopoietic stem and progenitors while others acquire mesenchymal properties necessary for heart development. In the adult, maintenance of differentiated endothelial state is an active process requiring constant signalling input. The failure to do so leads to the development of endothelial-to-mesenchymal transition that plays an important role in pathogenesis of a number of diseases. A better understanding of these phenotypic changes may lead to development of new therapeutic interventions.

Vascular endothelium possesses remarkable plasticity in response to cues from its surroundings, leading to great heterogeneity of endothelial cells in different vascular beds. Here the authors explain the molecular basis of endothelial plasticity during embryogenesis and in various diseases.

R-spondin 1 is required for specification of hematopoietic stem cells through Wnt16 and Vegfa signaling pathways

Hematopoietic stem cells (HSCs) are the therapeutic component of bone marrow transplants, but finding immune-compatible donors limits treatment availability and efficacy. Recapitulation of endogenous specification during development is a promising approach to directing HSC specification in vitro, but current protocols are not capable of generating authentic HSCs with high efficiency. Across phyla, HSCs arise from hemogenic endothelium in the ventral floor of the dorsal aorta concurrent with arteriovenous specification and intersegmental vessel (ISV) sprouting, processes regulated by Notch and Wnt. We hypothesized that coordination of HSC specification with vessel patterning might involve modulatory regulatory factors such as R-spondin 1 (Rspo1), an extracellular protein that enhances β-catenin-dependent Wnt signaling and has previously been shown to regulate ISV patterning. We find that Rspo1 is required for HSC specification through control of parallel signaling pathways controlling HSC specification: Wnt16/DeltaC/DeltaD and Vegfa/Tgfβ1. Our results define Rspo1 as a key upstream regulator of two crucial pathways necessary for HSC specification.

Enhanced targeting of CML stem and progenitor cells by inhibition of porcupine acyltransferase in combination with TKI

Tyrosine kinase inhibitor (TKI) treatment of chronic myeloid leukemia (CML) has limited efficacy against leukemia stem cells (LSC) responsible for disease propagation, and most CML patients require continued TKI treatment to maintain remission. LSC maintenance is related, at least in part, to signals from the bone marrow microenvironment (BMM). Our previous studies have shown that Wnt signaling from the BMM contributes to preservation of CML LSC following TKI treatment. Secretion of Wnt ligands requires their modification by the O-acyl transferase Porcupine (PORCN). Here we investigated the activity of a potent and selective PORCN inhibitor, WNT974, against CML stem and progenitor cells. WNT974 efficiently antagonized Wnt signaling in human CML CD34⁺ cells, and in combination with the TKI nilotinib (NIL) significantly enhanced inhibition of proliferation and colony-forming potential of CML stem and progenitor cells and reduced their growth in immunodeficient mice in vivo, in comparison with NIL alone. Treatment of transgenic CML mice in vivo with NIL in combination with WNT974 significantly reduced leukemic stem and progenitor cell numbers, reduced regeneration of leukemic long-term hematopoietic stem cells in secondary transplant recipients, and enhanced survival of mice after discontinuation of treatment, in comparison with NIL alone. CML progenitors demonstrated enhanced sensitivity to Wnt stimulation, associated with increased expression of the FZD4 receptor. FZD4 knockdown inhibited CML progenitor growth. These results support further investigation of PORCN targeting to inhibit Wnt secretion and signaling and enhance targeting of CML stem cells while sparing their normal counterparts.