SCL/TAL1 regulates hematopoietic specification from human embryonic stem cells

Dynamics of Endothelial Cell Diversity and Plasticity in Health and Disease

Endothelial cells (ECs) are vital structural units of the cardiovascular system possessing two principal distinctive properties: heterogeneity and plasticity. Endothelial heterogeneity is defined by differences in tissue-specific endothelial phenotypes and their high predisposition to modification along the length of the vascular bed. This aspect of heterogeneity is closely associated with plasticity, the ability of ECs to adapt to environmental cues through the mobilization of genetic, molecular, and structural alterations. The specific endothelial cytoarchitectonics facilitate a quick structural cell reorganization and, furthermore, easy adaptation to the extrinsic and intrinsic environmental stimuli, known as the epigenetic landscape. ECs, as universally distributed and ubiquitous cells of the human body, play a role that extends far beyond their structural function in the cardiovascular system. They play a crucial role in terms of barrier function, cell-to-cell communication, and a myriad of physiological and pathologic processes. These include development, ontogenesis, disease initiation, and progression, as well as growth, regeneration, and repair. Despite substantial progress in the understanding of endothelial cell biology, the role of ECs in healthy conditions and pathologies remains a fascinating area of exploration. This review aims to summarize knowledge and concepts in endothelial biology. It focuses on the development and functional characteristics of endothelial cells in health and pathological conditions, with a particular emphasis on endothelial phenotypic and functional heterogeneity.

The genesis of human hematopoietic stem cells

Developmental hematopoiesis consists of multiple, partially overlapping hematopoietic waves that generate the differentiated blood cells required for embryonic development while establishing a pool of undifferentiated hematopoietic stem cells (HSCs) for postnatal life. This multilayered design in which active hematopoiesis migrates through diverse extra and intraembryonic tissues has made it difficult to define a roadmap for generating HSCs vs non-self-renewing progenitors, especially in humans. Recent single-cell studies have helped in identifying the rare human HSCs at stages when functional assays are unsuitable for distinguishing them from progenitors. This approach has made it possible to track the origin of human HSCs to the unique type of arterial endothelium in the aorta-gonad-mesonephros region and document novel benchmarks for HSC migration and maturation in the conceptus. These studies have delivered new insights into the intricate process of HSC generation and provided tools to inform the in vitro efforts to replicate the physiological developmental journey from pluripotent stem cells via distinct mesodermal and endothelial intermediates to HSCs.

The aberrant epigenome of DNMT3B-mutated ICF1 patient iPSCs is amenable to correction, with the exception of a subset of regions with H3K4me3- and/or CTCF-based epigenetic memory

Bi-allelic hypomorphic mutations in DNMT3B disrupt DNA methyltransferase activity and lead to immunodeficiency, centromeric instability, facial anomalies syndrome, type 1 (ICF1). Although several ICF1 phenotypes have been linked to abnormally hypomethylated repetitive regions, the unique genomic regions responsible for the remaining disease phenotypes remain largely uncharacterized. Here we explored two ICF1 patient-derived induced pluripotent stem cells (iPSCs) and their CRISPR-Cas9-corrected clones to determine whether DNMT3B correction can globally overcome DNA methylation defects and related changes in the epigenome. Hypomethylated regions throughout the genome are highly comparable between ICF1 iPSCs carrying different DNMT3B variants, and significantly overlap with those in ICF1 patient peripheral blood and lymphoblastoid cell lines. These regions include large CpG island domains, as well as promoters and enhancers of several lineage-specific genes, in particular immune-related, suggesting that they are premarked during early development. CRISPR-corrected ICF1 iPSCs reveal that the majority of phenotype-related hypomethylated regions reacquire normal DNA methylation levels following editing. However, at the most severely hypomethylated regions in ICF1 iPSCs, which also display the highest increases in H3K4me3 levels and/or abnormal CTCF binding, the epigenetic memory persists, and hypomethylation remains uncorrected. Overall, we demonstrate that restoring the catalytic activity of DNMT3B can reverse the majority of the aberrant ICF1 epigenome. However, a small fraction of the genome is resilient to this rescue, highlighting the challenge of reverting disease states that are due to genome-wide epigenetic perturbations. Uncovering the basis for the persistent epigenetic memory will promote the development of strategies to overcome this obstacle.

De Novo Generation of Human Hematopoietic Stem Cells from Pluripotent Stem Cells for Cellular Therapy

The ability to manufacture human hematopoietic stem cells (HSCs) in the laboratory holds enormous promise for cellular therapy of human blood diseases. Several differentiation protocols have been developed to facilitate the emergence of HSCs from human pluripotent stem cells (PSCs). Most approaches employ a stepwise addition of cytokines and morphogens to recapitulate the natural developmental process. However, these protocols globally lack clinical relevance and uniformly induce PSCs to produce hematopoietic progenitors with embryonic features and limited engraftment and differentiation capabilities. This review examines how key intrinsic cues and extrinsic environmental inputs have been integrated within human PSC differentiation protocols to enhance the emergence of definitive hematopoiesis and how advances in genomics set the stage for imminent breakthroughs in this field.

DNA binding protein CgIkaros-like regulates the proliferation of agranulocytes and granulocytes in oyster (Crassostrea gigas)

DNA-binding protein Ikaros is a major determinant of haematopoietic lineage, especially in the development, differentiation and proliferation of lymphocytes. In the present study, a Ikaros homologue (designed as CgIkaros-like) was identified and characterized as a vital determinant in the proliferation of haemocytes during haematopoiesis of Pacific oyster Crassostrea gigas. The complete coding sequence of CgIkaros-like was of 1329 bp encoding a predicted polypeptide of 442 amino acids with four ZnF regions, locating at the C-terminus and N-terminus respectively. The highest expression level of CgIkaros-like mRNA was found in gills, followed by haemocytes and gonad. The mRNA transcripts of CgIkaros-like could be detected in all the haemocytes with higher abundance in semi-granulocytes and agranulocytes. CgIkaros-like protein was localized in both of cytoplasm and nucleus with higher abundance in nucleus of oyster haemocytes. The mRNA and protein expression levels of agranulocyte marker CgCD9, granulocyte marker CgAATase, cell cycle related gene CgCDK2, Notch receptor CgNotch and Notch target gene CgHes1 all increased significantly (p < 0.05) after CgIkaros-like was interfered by siRNAs, which were about 27.33-, 2.63-, 24.34-, 4.45- and 6.08-fold of that in the siRNA-NC control group, respectively. While the transcripts of CgGATA3 and CgRunx did not change significantly after CgIkaros-like was interfered. These results demonstrated that CgIkaros-like functioned as a transcription factor combined with Notch pathway to mediate CgCDK2 and regulate the proliferation of oyster haemocytes.

A Benchmark Side-by-Side Comparison of Two Well-Established Protocols for in vitro Hematopoietic Differentiation From Human Pluripotent Stem Cells

The generation of transplantable hematopoietic stem cells (HSCs) from human pluripotent stem cells (hPSCs) remains challenging. Current differentiation protocols from hPSCs generate mostly hematopoietic progenitors of the primitive HSC-independent program, and it remains unclear what is the best combination of cytokines and hematopoietic growth factors (HGFs) for obtaining functional hematopoietic cells in vitro. Here, we have used the AND1 and H9 hESC lines and the H9:dual-reporter RUNX1C-GFP-SOX17-Cherry to compare the hematopoietic differentiation in vitro based on the treatment of embryoid bodies (EBs) with the ventral mesoderm inducer BMP4 plus HGFs in the absence (protocol 1) or presence (protocol 2) of stage-specific activation of Wnt/β-catenin and inhibition of Activin/Nodal. Despite a slight trend in favor of protocol 1, no statistically significant differences were observed between protocols at any time point analyzed throughout EB development regarding the frequency of hemogenic endothelial (HE) precursors; CD43+ CD45−, CD45+, and CD45 + CD34 + hematopoietic derivatives; or the output of clonogenic progenitors. Similarly, the kinetics of emergence throughout EB development of both SOX17 + HE and RUNX1C + definitive hematopoiesis was very similar for both protocols. The expression of the early master mesendodermal transcription factors Brachyury, MIXL1, and KDR revealed similar gene expression kinetics prior to the emergence of RUNX1C + definitive hematopoiesis for both protocols. Collectively, the simpler protocol 1 is, at least, as efficient as protocol 2, suggesting that supplementation with additional morphogens/HGFs and modulation of Activin/Nodal and Wnt/β-catenin pathways seem dispensable for in vitro hematopoietic differentiation of hPSCs.

-Omic Analysis of the Sepia officinalis White Body: New Insights into Multifunctionality and Haematopoiesis Regulation

Cephalopods, like other protostomes, lack an adaptive immune system and only rely on an innate immune system. The main immune cells are haemocytes (Hcts), which are able to respond to pathogens and external attacks. First reports based on morphological observations revealed that the white body (WB) located in the optic sinuses of cuttlefish was the origin of Hcts. Combining transcriptomic and proteomic analyses, we identified several factors known to be involved in haematopoiesis in vertebrate species in cuttlefish WB. Among these factors, members of the JAK-STAT signaling pathway were identified, some of them for the first time in a molluscan transcriptome and proteome. Immune factors, such as members of the Toll/NF-κB signaling pathway, pattern recognition proteins and receptors, and members of the oxidative stress responses, were also identified, and support an immune role of the WB. Both transcriptome and proteome analyses revealed that the WB harbors an intense metabolism concurrent with the haematopoietic function. Finally, a comparative analysis of the WB and Hct proteomes revealed many proteins in common, confirming previous morphological studies on the origin of Hcts in cuttlefish. This molecular work demonstrates that the WB is multifunctional and provides bases for haematopoiesis regulation in cuttlefish.

A non-linear reverse-engineering method for inferring genetic regulatory networks

Hematopoiesis is a highly complex developmental process that produces various types of blood cells. This process is regulated by different genetic networks that control the proliferation, differentiation, and maturation of hematopoietic stem cells (HSCs). Although substantial progress has been made for understanding hematopoiesis, the detailed regulatory mechanisms for the fate determination of HSCs are still unraveled. In this study, we propose a novel approach to infer the detailed regulatory mechanisms. This work is designed to develop a mathematical framework that is able to realize nonlinear gene expression dynamics accurately. In particular, we intended to investigate the effect of possible protein heterodimers and/or synergistic effect in genetic regulation. This approach includes the Extended Forward Search Algorithm to infer network structure (top-down approach) and a non-linear mathematical model to infer dynamical property (bottom-up approach). Based on the published experimental data, we study two regulatory networks of 11 genes for regulating the erythrocyte differentiation pathway and the neutrophil differentiation pathway. The proposed algorithm is first applied to predict the network topologies among 11 genes and 55 non-linear terms which may be for heterodimers and/or synergistic effect. Then, the unknown model parameters are estimated by fitting simulations to the expression data of two different differentiation pathways. In addition, the edge deletion test is conducted to remove possible insignificant regulations from the inferred networks. Furthermore, the robustness property of the mathematical model is employed as an additional criterion to choose better network reconstruction results. Our simulation results successfully realized experimental data for two different differentiation pathways, which suggests that the proposed approach is an effective method to infer the topological structure and dynamic property of genetic regulations.

MSX2 suppression through inhibition of TGFβ signaling enhances hematopoietic differentiation of human embryonic stem cells

Background

Strategies of generating functional blood cells from human pluripotent stem cells (hPSCs) remain largely unsuccessful due to the lack of a comprehensive understanding of hematopoietic development. Endothelial-to-hematopoietic transition (EHT) serves as the pivotal mechanism for the onset of hematopoiesis and is negatively regulated by TGF-β signaling. However, little is known about the underlying details of TGF-β signaling during EHT.

Methods

In this study, by applying genome-wide gene profiling, we identified muscle segment homeobox2 (MSX2) as a potential mediator of TGF-β signaling during EHT. We generated MSX2-deleted human embryonic stem cell (hESC) lines using the CRISPR/Cas9 technology and induced them to undergo hematopoietic differentiation. The role of MSX2 in hematopoiesis and functional regulation of TGFβ signaling in EHT was studied.

Results

We identified MSX2 as a novel regulator of human hematopoiesis. MSX2 deletion promotes the production of hematopoietic cells from hESCs. Functional and bioinformatics studies further demonstrated that MSX2 deletion augments hematopoietic differentiation of hESCs by facilitating EHT. Mechanistically, MSX2 acts as a downstream target of TGFβ signaling to mediate its function during EHT.

Conclusions

Our results not only improve the understanding of EHT, but may also provide novel insight into the efficient production of functional blood cells from hPSCs for regenerative medicine.

Gene therapy of hematological disorders: current challenges

Recent advances in genetic engineering technology and stem cell biology have spurred great interest in developing gene therapies for hereditary, as well as acquired hematological disorders. Currently, hematopoietic stem cell transplantation is used to cure disorders such as hemoglobinopathies and primary immunodeficiencies; however, this method is limited by the availability of immune-matched donors. Using autologous cells coupled with genome editing bypasses this limitation and therefore became the focus of many research groups aiming to develop efficient and safe genomic modification. Hence, gene therapy research has witnessed a noticeable growth in recent years with numerous successful achievements; however, several challenges have to be overcome before gene therapy becomes widely available for patients. In this review, I discuss tools used in gene therapy for hematological disorders, choices of target cells, and delivery vehicles with emphasis on current hurdles and attempts to solve them, and present examples of successful clinical trials to give a glimpse of current progress.

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.

Hematopoietic Differentiation of Human Pluripotent Stem Cells: HOX and GATA Transcription Factors as Master Regulators

Numerous human disorders of the blood system would directly or indirectly benefit from therapeutic approaches that reconstitute the hematopoietic system. Hematopoietic stem cells (HSCs), either from matched donors or ex vivo manipulated autologous tissues, are the most used cellular source of cell therapy for a wide range of disorders. Due to the scarcity of matched donors and the difficulty of ex vivo expansion of HSCs, there is a growing interest in harnessing the potential of pluripotent stem cells (PSCs) as a de novo source of HSCs. PSCs make an ideal source of cells for regenerative medicine in general and for treating blood disorders in particular because they could expand indefinitely in culture and differentiate to any cell type in the body. However, advancement in deriving functional HSCs from PSCs has been slow. This is partly due to an incomplete understanding of the molecular mechanisms underlying normal hematopoiesis. In this review, we discuss the latest efforts to generate human PSC (hPSC)-derived HSCs capable of long-term engraftment. We review the regulation of the key transcription factors (TFs) in hematopoiesis and hematopoietic differentiation, the Homeobox (HOX) and GATA genes, and the interplay between them and microRNAs. We also propose that precise control of these master regulators during the course of hematopoietic differentiation is key to achieving functional hPSC-derived HSCs.

MEIS2 regulates endothelial to hematopoietic transition of human embryonic stem cells by targeting TAL1

Background

Despite considerable progress in the development of methods for hematopoietic differentiation, efficient generation of transplantable hematopoietic stem cells (HSCs) and other genuine functional blood cells from human embryonic stem cells (hESCs) is still unsuccessful. Therefore, a better understanding of the molecular mechanism underlying hematopoietic differentiation of hESCs is highly demanded.

Methods

In this study, by using whole-genome gene profiling, we identified Myeloid Ectopic Viral Integration Site 2 homolog (MEIS2) as a potential regulator of hESC early hematopoietic differentiation. We deleted MEIS2 gene in hESCs using the CRISPR/CAS9 technology and induced them to hematopoietic differentiation, megakaryocytic differentiation.

Results

In this study, we found that MEIS2 deletion impairs early hematopoietic differentiation from hESCs. Furthermore, MEIS2 deletion suppresses hemogenic endothelial specification and endothelial to hematopoietic transition (EHT), leading to the impairment of hematopoietic differentiation. Mechanistically, TAL1 acts as a downstream gene mediating the function of MEIS2 during early hematopoiesis. Interestingly, unlike MEIS1, MEIS2 deletion exerts minimal effects on megakaryocytic differentiation and platelet generation from hESCs.

Conclusions

Our findings advance the understanding of human hematopoietic development and may provide new insights for large-scale generation of functional blood cells for clinical applications.

Electronic supplementary material

The online version of this article (10.1186/s13287-018-1074-z) contains supplementary material, which is available to authorized users.

Early Human Hemogenic Endothelium Generates Primitive and Definitive Hematopoiesis In Vitro

The differentiation of human embryonic stem cells (hESCs) to hematopoietic lineages initiates with the specification of hemogenic endothelium, a transient specialized endothelial precursor of all blood cells. This in vitro system provides an invaluable model to dissect the emergence of hematopoiesis in humans. However, the study of hematopoiesis specification is hampered by a lack of consensus in the timing of hemogenic endothelium analysis and the full hematopoietic potential of this population. Here, our data reveal a sharp decline in the hemogenic potential of endothelium populations isolated over the course of hESC differentiation. Furthermore, by tracking the dynamic expression of CD31 and CD235a at the onset of hematopoiesis, we identified three populations of hematopoietic progenitors, representing primitive and definitive subsets that all emerge from the earliest specified hemogenic endothelium. Our data establish that hemogenic endothelium populations endowed with primitive and definitive hematopoietic potential are specified simultaneously from the mesoderm in differentiating hESCs.

Transcriptome sequencing reveals the involvement of reactive oxygen species in the hematopoiesis from Chinese mitten crab Eriocheir sinensis

Reactive oxygen species (ROS) produced in vivo during various electron transfer reactions are generally kept at a certain level since they are harmful to cells. However, it can sensitize hematopoietic progenitors to differentiation, and plays a signaling role in the regulation of hematopoietic cell fate. In the present study, the transcriptomes of crab HPT and hemocytes were sequenced using the Ion Torrent Proton sequencing platform. A total of 51,229,690 single end reads were obtained from six single-end libraries, which were assembled into 31346 unireads as reference. After mapping and transcript assembling, 362 differently expressed genes were identified and 301 of them were deemed to be more abundant in HPT. GO annotation revealed that they were mostly implicated in DNA, RNA and protein synthesis, cell division, mitochondria activities and energy metabolism. The expression level of mitochondrial complexes I (mitochondrial NADH-ubiquinone oxidoreductase) which was the main natural producers of mitochondrial ROS was found to be 8.6-fold (p < 0.01) higher in HPT than that in hemocytes. In hemocytes, the proteinase genes associated with proPO activation from the 61 up-regulated genes in hemocytes were the main up-regulated genes which might be the potential markers for mature hemocytes. ROS level in HPT cells was relatively higher which was confirmed with the high expression level of mitochondria related genes identified by transcriptome sequencing. After the ROS level was depressed by N-acetyl-l-cysteine (NAC), the production of hemocytes from HPT was inhibited, and the recovery of the total hemocytes counts was delayed. These results collectively indicated that the genes in redox system were more active in HPT, and ROS could function as an important modulator in the hematopoiesis of crab and promote the production of hemocytes from HPT.

Identification and Phylogenetic Analysis of Basic Helix-Loop-Helix Genes in the Diamondback Moth

Basic helix-loop-helix (bHLH) transcription factors play essential roles in regulating eukaryotic developmental and physiological processes such as neuron generation, myocyte formation, intestinal tissue development, and response to environmental stress. In this study, the diamondback moth, Plutella xylostella (L.) (Lepidoptera: Plutellidae), genome was found to encode 52 bHLH genes. All 52 P. xylostella bHLH (PxbHLH) genes were classified into correspondent bHLH families according to their orthology with bHLHs from fruit fly and other insect species. Among these 52 PxbHLH genes, 19 have been annotated consistently with our classification in GenBank database. The remaining 33 PxbHLH genes are either annotated as general bHLH genes or as hypothetical genes. Therefore, our data provide useful information for updating annotations to PxbHLH genes. P. xylostella has four stem cell leukemia (SCL) genes (one of them has three copies), two Dys genes, two copies of MyoR, Mitf, and Sima genes, and three copies of Sage genes. Further studies may be conducted to elucidate functions of these specific bHLH genes in regulating P. xylostella growth and development.

Downregulation of LGR5 Expression Inhibits Cardiomyocyte Differentiation and Potentiates Endothelial Differentiation from Human Pluripotent Stem Cells

Understanding molecules involved in differentiation of human pluripotent stem cells (hPSCs) into cardiomyocytes and endothelial cells is important in advancing hPSCs for cell therapy and drug testing. Here, we report that LGR5, a leucine-rich repeat-containing G-protein-coupled receptor, plays a critical role in hPSC differentiation into cardiomyocytes and endothelial cells. LGR5 expression was transiently upregulated during the early stage of cardiomyocyte differentiation, and knockdown of LGR5 resulted in reduced expression of cardiomyocyte-associated markers and poor cardiac differentiation. In contrast, knockdown of LGR5 promoted differentiation of endothelial-like cells with increased expression of endothelial cell markers and appropriate functional characteristics, including the ability to form tube-like structures and to take up acetylated low-density lipoproteins. Furthermore, knockdown of LGR5 significantly reduced the proliferation of differentiated cells and increased the nuclear translocation of β-catenin and expression of Wnt signaling-related genes. Therefore, regulation of LGR5 may facilitate efficient generation of cardiomyocytes or endothelial cells from hPSCs.

•

LGR5 expression is upregulated in the early stage of cardiomyocyte differentiation

•

Knockdown of LGR5 inhibits differentiation of cardiomyocytes

•

Knockdown of LGR5 increases differentiation of endothelial cells

•

Knockdown of LGR5 decreases the expression of Wnt signaling-related genes

A novel GATA-like zinc finger transcription factor involving in hematopoiesis of Eriocheir sinensis

GATA transcription factor is a family of DNA-binding proteins that can recognize and bind to sequence of (A/T) GATA (A/G). In the present study, a GATA-like protein (named as EsGLP) was characterized from Eriocheir sinensis, including an 834 bp full length open reading frame of EsGLP, encoding a polypeptide of 277 amino acids. The deduced amino acid sequence of EsGLP contained one conserved GATA-type zinc finger of the form Cys-X2-Cys-X17-Cys-X2-Cys, with four cysteine sites. The EsGLP mRNA transcripts were mainly detected in the hematopoietic tissue, hepatopancreas and gonad. The recombinant EsGLP protein was prepared for the antibody production. The EsGLP protein was mainly distributed in the edge of lobules in the HPT and the cytoplasm of hemocytes. The mRNA transcripts of EsGLP in hemocytes were significantly decreased at 24 h (0.39-fold and 0.27-fold, p < .05) and 48 h (0.35-fold and 0.16-fold, p < .05) after LPS and Aeromonas hydrophila stimulation, respectively. However, one peak of EsGLP mRNA transcripts were recorded at 24 h (8.71-fold, p < .05) in HPT after A. hydrophila stimulation. The expression level of EsGLP mRNA in HPT was significantly up-regulated at 2 h, 2.5 h and 9 h (41.74-fold, 45.38-fold and 26.07-fold, p < .05) after exsanguination stimulation. When EsGLP gene expression was inhibited by the injection of double-stranded RNA, both the total hemocytes counts and the rate of EdU-positive hemocytes were significantly decreased (0.32-fold and 0.56-fold compared to that in control group, p < .05). All these results suggested that EsGLP was an important regulatory factor in E. sinensis which involved in the hemocytes generation and the immune response against invading pathogens.

MEIS1 Regulates Hemogenic Endothelial Generation, Megakaryopoiesis, and Thrombopoiesis in Human Pluripotent Stem Cells by Targeting TAL1 and FLI1

Human pluripotent stem cells (hPSCs) provide an unlimited source for generating various kinds of functional blood cells. However, efficient strategies for generating large-scale functional blood cells from hPSCs are still lacking, and the mechanism underlying human hematopoiesis remains largely unknown. In this study, we identified myeloid ectopic viral integration site 1 homolog (MEIS1) as a crucial regulator of hPSC early hematopoietic differentiation. MEIS1 is vital for specification of APLNR⁺ mesoderm progenitors to functional hemogenic endothelial progenitors (HEPs), thereby controlling formation of hematopoietic progenitor cells (HPCs). TAL1 mediates the function of MEIS1 in HEP specification. In addition, MEIS1 is vital for megakaryopoiesis and thrombopoiesis from hPSCs. Mechanistically, FLI1 acts as a downstream gene necessary for the function of MEIS1 during megakaryopoiesis. Thus, MEIS1 controls human hematopoiesis in a stage-specific manner and can be potentially manipulated for large-scale generation of HPCs or platelets from hPSCs for therapeutic applications in regenerative medicine.

•

MEIS1 knockout impairs hematopoiesis of hPSCs by suppressing HEP specification

•

MEIS1^−/− megakaryocytes fail to undergo polyploidization and thrombopoiesis

•

TAL1 mediates the function of MEIS1 in HEP specification

•

FLI1 acts as a downstream target of MEIS1 during megakaryopoiesis and thrombopoiesis

The Role of TAL1 in Hematopoiesis and Leukemogenesis

TAL1 (SCL/TAL1, T-cell acute leukemia protein 1) is a transcription factor that is involved in the process of hematopoiesis and leukemogenesis. It participates in blood cell formation, forms mesoderm in early embryogenesis, and regulates hematopoiesis in adult organisms. TAL1 is essential in maintaining the multipotency of hematopoietic stem cells (HSC) and keeping them in quiescence (stage G0). TAL1 forms complexes with various transcription factors, regulating hematopoiesis (E2A/HEB, GATA1-3, LMO1-2, Ldb1, ETO₂, RUNX1, ERG, FLI1). In these complexes, TAL1 regulates normal myeloid differentiation, controls the proliferation of erythroid progenitors, and determines the choice of the direction of HSC differentiation. The transcription factors TAL1, E2A, GATA1 (or GATA2), LMO2, and Ldb1 are the major components of the SCL complex. In addition to normal hematopoiesis, this complex may also be involved in the process of blood cell malignant transformation. Upregulation of C-KIT expression is one of the main roles played by the SCL complex. Today, TAL1 and its partners are considered promising therapeutic targets in the treatment of T-cell acute lymphoblastic leukemia.

RUNX1c Regulates Hematopoietic Differentiation of Human Pluripotent Stem Cells Possibly in Cooperation with Proinflammatory Signaling

Runt-related transcription factor 1 (Runx1) is a master hematopoietic transcription factor essential for hematopoietic stem cell (HSC) emergence. Runx1-deficient mice die during early embryogenesis due to the inability to establish definitive hematopoiesis. Here, we have used human pluripotent stem cells (hPSCs) as model to study the role of RUNX1 in human embryonic hematopoiesis. Although the three RUNX1 isoforms a, b, and c were induced in CD45+ hematopoietic cells, RUNX1c was the only isoform induced in hematoendothelial progenitors (HEPs)/hemogenic endothelium. Constitutive expression of RUNX1c in human embryonic stem cells enhanced the appearance of HEPs, including hemogenic (CD43+) HEPs and promoted subsequent differentiation into blood cells. Conversely, specific deletion of RUNX1c dramatically reduced the generation of hematopoietic cells from HEPs, indicating that RUNX1c is a master regulator of human hematopoietic development. Gene expression profiling of HEPs revealed a RUNX1c-induced proinflammatory molecular signature, supporting previous studies demonstrating proinflammatory signaling as a regulator of HSC emergence. Collectively, RUNX1c orchestrates hematopoietic specification of hPSCs, possibly in cooperation with proinflammatory signaling. Stem Cells 2017;35:2253-2266.

Activation of KLF1 Enhances the Differentiation and Maturation of Red Blood Cells from Human Pluripotent Stem Cells

Blood transfusion is widely used in the clinic but the source of red blood cells (RBCs) is dependent on donors, procedures are susceptible to transfusion-transmitted infections and complications can arise from immunological incompatibility. Clinically-compatible and scalable protocols that allow the production of RBCs from human embryonic stem cells (hESCs) and induced pluripotent stem cells (iPSCs) have been described but progress to translation has been hampered by poor maturation and fragility of the resultant cells. Genetic programming using transcription factors has been used to drive lineage determination and differentiation so we used this approach to assess whether exogenous expression of the Erythroid Krüppel-like factor 1 (EKLF/KLF1) could augment the differentiation and stability of iPSC-derived RBCs. To activate KLF1 at defined time points during later stages of the differentiation process and to avoid transgene silencing that is commonly observed in differentiating pluripotent stem cells, we targeted a tamoxifen-inducible KLF1-ERT2 expression cassette into the AAVS1 locus. Activation of KLF1 at day 10 of the differentiation process when hematopoietic progenitor cells were present, enhanced erythroid commitment and differentiation. Continued culture resulted the appearance of more enucleated cells when KLF1 was activated which is possibly due to their more robust morphology. Globin profiling indicated that these conditions produced embryonic-like erythroid cells. This study demonstrates the successful use of an inducible genetic programing strategy that could be applied to the production of many other cell lineages from human induced pluripotent stem cells with the integration of programming factors into the AAVS1 locus providing a safer and more reproducible route to the clinic. Stem Cells 2017;35:886-897.

Intragenic CpG islands play important roles in bivalent chromatin assembly of developmental genes

CpG, 5'-C-phosphate-G-3', islands (CGIs) have long been known for their association with enhancers, silencers, and promoters, and for their epigenetic signatures. They are maintained in embryonic stem cells (ESCs) in a poised but inactive state via the formation of bivalent chromatin containing both active and repressive marks. CGIs also occur within coding sequences, where their functional role has remained obscure. Intragenic CGIs (iCGIs) are largely absent from housekeeping genes, but they are found in all genes associated with organ development and cell lineage control. In this paper, we investigated the epigenetic status of iCGIs and found that they too reside in bivalent chromatin in ESCs. Cell type-specific DNA methylation of iCGIs in differentiated cells was linked to the loss of both the H3K4me3 and H3K27me3 marks, and disruption of physical interaction with promoter regions, resulting in transcriptional activation of key regulators of differentiation such as PAXs, HOXs, and WNTs. The differential epigenetic modification of iCGIs appears to be mediated by cell type-specific transcription factors distinct from those bound by promoter, and these transcription factors may be involved in the hypermethylation of iCGIs upon cell differentiation. iCGIs thus play a key role in the cell type-specific regulation of transcription.

A synthetic three-dimensional niche system facilitates generation of functional hematopoietic cells from human-induced pluripotent stem cells

Background

The efficient generation of hematopoietic stem cells (HSCs) from human-induced pluripotent stem cells (iPSCs) holds great promise in personalized transplantation therapies. However, the derivation of functional and transplantable HSCs from iPSCs has had very limited success thus far.

Methods

We developed a synthetic 3D hematopoietic niche system comprising nanofibers seeded with bone marrow (BM)-derived stromal cells and growth factors to induce functional hematopoietic cells from human iPSCs in vitro.

Results

Approximately 70 % of human CD34⁺ hematopoietic cells accompanied with CD43⁺ progenitor cells could be derived from this 3D induction system. Colony-forming-unit (CFU) assay showed that iPSC-derived CD34⁺ cells formed all types of hematopoietic colonies including CFU-GEMM. TAL-1 and MIXL1, critical transcription factors associated with hematopoietic development, were expressed during the differentiation process. Furthermore, iPSC-derived hematopoietic cells gave rise to both lymphoid and myeloid lineages in the recipient NOD/SCID mice after transplantation.

Conclusions

Our study underscores the importance of a synthetic 3D niche system for the derivation of transplantable hematopoietic cells from human iPSCs in vitro thereby establishing a foundation towards utilization of human iPSC-derived HSCs for transplantation therapies in the clinic.

Electronic supplementary material

The online version of this article (doi:10.1186/s13045-016-0326-6) contains supplementary material, which is available to authorized users.

Development Refractoriness of MLL-Rearranged Human B Cell Acute Leukemias to Reprogramming into Pluripotency

Induced pluripotent stem cells (iPSCs) are a powerful tool for disease modeling. They are routinely generated from healthy donors and patients from multiple cell types at different developmental stages. However, reprogramming leukemias is an extremely inefficient process. Few studies generated iPSCs from primary chronic myeloid leukemias, but iPSC generation from acute myeloid or lymphoid leukemias (ALL) has not been achieved. We attempted to generate iPSCs from different subtypes of B-ALL to address the developmental impact of leukemic fusion genes. OKSM(L)-expressing mono/polycistronic-, retroviral/lentiviral/episomal-, and Sendai virus vector-based reprogramming strategies failed to render iPSCs in vitro and in vivo. Addition of transcriptomic-epigenetic reprogramming “boosters” also failed to generate iPSCs from B cell blasts and B-ALL lines, and when iPSCs emerged they lacked leukemic fusion genes, demonstrating non-leukemic myeloid origin. Conversely, MLL-AF4-overexpressing hematopoietic stem cells/B progenitors were successfully reprogrammed, indicating that B cell origin and leukemic fusion gene were not reprogramming barriers. Global transcriptome/DNA methylome profiling suggested a developmental/differentiation refractoriness of MLL-rearranged B-ALL to reprogramming into pluripotency.

•

Neither primary B-ALL blasts nor leukemic B cell lines can be reprogrammed to iPSCs

•

Global transcriptome and DNA methylome suggest a developmental refractoriness

Large-scale production of megakaryocytes from human pluripotent stem cells by chemically defined forward programming

The production of megakaryocytes (MKs)--the precursors of blood platelets--from human pluripotent stem cells (hPSCs) offers exciting clinical opportunities for transfusion medicine. Here we describe an original approach for the large-scale generation of MKs in chemically defined conditions using a forward programming strategy relying on the concurrent exogenous expression of three transcription factors: GATA1, FLI1 and TAL1. The forward programmed MKs proliferate and differentiate in culture for several months with MK purity over 90% reaching up to 2 × 10(5) mature MKs per input hPSC. Functional platelets are generated throughout the culture allowing the prospective collection of several transfusion units from as few as 1 million starting hPSCs. The high cell purity and yield achieved by MK forward programming, combined with efficient cryopreservation and good manufacturing practice (GMP)-compatible culture, make this approach eminently suitable to both in vitro production of platelets for transfusion and basic research in MK and platelet biology.

Concise review: programming human pluripotent stem cells into blood

Blood disorders are treated with cell therapies including haematopoietic stem cell (HSC) transplantation as well as platelet and red blood cell transfusions. However the source of cells is entirely dependent on donors, procedures are susceptible to transfusion-transmitted infections and serious complications can arise in recipients due to immunological incompatibility. These problems could be alleviated if it was possible to produce haematopoietic cells in vitro from an autologous and renewable cell source. The production of haematopoietic cells in the laboratory from human induced pluripotent stem cells (iPSCs) may provide a route to realize this goal but it has proven challenging to generate long-term reconstituting HSCs. To date, the optimization of differentiation protocols has mostly relied on the manipulation of extrinsic signals to mimic the in vivo environment. We review studies that have taken an alternative approach to modulate intrinsic signals by enforced expression of transcription factors. Single and combinations of multiple transcription factors have been used in a variety of contexts to enhance the production of haematopoietic cells from human pluripotent stem cells. This programming approach, together with the recent advances in the production and use of synthetic transcription factors, holds great promise for the production of fully functional HSCs in the future.

A Role for the Long Noncoding RNA SENCR in Commitment and Function of Endothelial Cells

Despite the increasing importance of long noncoding RNA in physiology and disease, their role in endothelial biology remains poorly understood. Growing evidence has highlighted them to be essential regulators of human embryonic stem cell differentiation. SENCR, a vascular-enriched long noncoding RNA, overlaps the Friend Leukemia Integration virus 1 (FLI1) gene, a regulator of endothelial development. Therefore, we wanted to test the hypothesis that SENCR may contribute to mesodermal and endothelial commitment as well as in endothelial function. We thus developed new differentiation protocols allowing generation of endothelial cells from human embryonic stem cells using both directed and hemogenic routes. The expression of SENCR was markedly regulated during endothelial commitment using both protocols. SENCR did not control the pluripotency of pluripotent cells; however its overexpression significantly potentiated early mesodermal and endothelial commitment. In human umbilical endothelial cell (HUVEC), SENCR induced proliferation, migration, and angiogenesis. SENCR expression was altered in vascular tissue and cells derived from patients with critical limb ischemia and premature coronary artery disease compared to controls. Here, we showed that SENCR contributes to the regulation of endothelial differentiation from pluripotent cells and controls the angiogenic capacity of HUVEC. These data give novel insight into the regulatory processes involved in endothelial development and function.

SCL/TAL1 in Hematopoiesis and Cellular Reprogramming

SCL, a transcription factor of the basic helix-loop-helix family, is a master regulator of hematopoiesis. Scl specifies lateral plate mesoderm to a hematopoietic fate and establishes boundaries by inhibiting the cardiac lineage. A combinatorial interaction between Scl and Vegfa/Flk1 sets in motion the first wave of primitive hematopoiesis. Subsequently, definitive hematopoietic stem cells (HSCs) emerge from the embryo proper via an endothelial-to-hematopoietic transition controlled by Runx1, acting with Scl and Gata2. Past this stage, Scl in steady state HSCs is redundant with Lyl1, a highly homologous factor. However, Scl is haploinsufficient in stress response, when a rare subpopulation of HSCs with very long term repopulating capacity is called into action. SCL activates transcription by recruiting a core complex on DNA that necessarily includes E2A/HEB, GATA1-3, LIM-only proteins LMO1/2, LDB1, and an extended complex comprising ETO2, RUNX1, ERG, or FLI1. These interactions confer multifunctionality to a complex that can control cell proliferation in erythroid progenitors or commitment to terminal differentiation through variations in single component. Ectopic SCL and LMO1/2 expression in immature thymocytes activates of a stem cell gene network and reprogram cells with a finite lifespan into self-renewing preleukemic stem cells (pre-LSCs), an initiating event in T-cell acute lymphoblastic leukemias. Interestingly, fate conversion of fibroblasts to hematoendothelial cells requires not only Scl and Lmo2 but also Gata2, Runx1, and Erg, indicating a necessary collaboration between these transcription factors for hematopoietic reprogramming. Nonetheless, full reprogramming into self-renewing multipotent HSCs may require additional factors and most likely, a permissive microenvironment.

Activated KRAS Cooperates with MLL-AF4 to Promote Extramedullary Engraftment and Migration of Cord Blood CD34+ HSPC But Is Insufficient to Initiate Leukemia

The MLL-AF4 (MA4) fusion gene is the genetic hallmark of an aggressive infant pro-B-acute lymphoblastic leukemia (B-ALL). Our understanding of MA4-mediated transformation is very limited. Whole-genome sequencing studies revealed a silent mutational landscape, which contradicts the aggressive clinical outcome of this hematologic malignancy. Only RAS mutations were recurrently detected in patients and found to be associated with poorer outcome. The absence of MA4-driven B-ALL models further questions whether MA4 acts as a single oncogenic driver or requires cooperating mutations to manifest a malignant phenotype. We explored whether KRAS activation cooperates with MA4 to initiate leukemia in cord blood-derived CD34(+) hematopoietic stem/progenitor cells (HSPC). Clonogenic and differentiation/proliferation assays demonstrated that KRAS activation does not cooperate with MA4 to immortalize CD34(+) HSPCs. Intrabone marrow transplantation into immunodeficient mice further showed that MA4 and KRAS(G12V) alone or in combination enhanced hematopoietic repopulation without impairing myeloid-lymphoid differentiation, and that mutated KRAS did not cooperate with MA4 to initiate leukemia. However, KRAS activation enhanced extramedullary hematopoiesis of MA4-expressing cell lines and CD34(+) HSPCs that was associated with leukocytosis and central nervous system infiltration, both hallmarks of infant t(4;11)(+) B-ALL. Transcriptional profiling of MA4-expressing patients supported a cell migration gene signature underlying the mutant KRAS-mediated phenotype. Collectively, our findings demonstrate that KRAS affects the homeostasis of MA4-expressing HSPCs, suggesting that KRAS activation in MA4(+) B-ALL is important for tumor maintenance rather than initiation. Cancer Res; 76(8); 2478-89. ©2016 AACR.

Hematopoietic stem cell expansion and generation: the ways to make a breakthrough

Hematopoietic stem cell transplantation (HSCT) is the first field where human stem cell therapy was successful. Flooding interest on human stem cell therapy to cure previously incurable diseases is largely indebted to HSCT success. Allogeneic HSCT has been an important modality to cure various diseases including hematologic malignancies, various non-malignant hematologic diseases, primary immunodeficiency diseases, and inborn errors of metabolism, while autologous HSCT is generally performed to rescue bone marrow aplasia following high-dose chemotherapy for solid tumors or multiple myeloma. Recently, HSCs are also spotlighted in the field of regenerative medicine for the amelioration of symptoms caused by neurodegenerative diseases, heart diseases, and others. Although the demand for HSCs has been growing, their supply often fails to meet the demand of the patients needing transplant due to a lack of histocompatible donors or a limited cell number. This review focuses on the generation and large-scale expansion of HSCs, which might overcome current limitations in the application of HSCs for clinical use. Furthermore, current proof of concept to replenish hematological homeostasis from non-hematological origin will be covered.

Making a Hematopoietic Stem Cell

Previous attempts to either generate or expand hematopoietic stem cells (HSCs) in vitro have involved either ex vivo expansion of pre-existing patient or donor HSCs or de novo generation from pluripotent stem cells (PSCs), comprising both embryonic stem cells (ESCs) and induced pluripotent stem cells (iPSCs). iPSCs alleviated ESC ethical issues but attempts to generate functional mature hematopoietic stem and progenitor cells (HSPCs) have been largely unsuccessful. New efforts focus on directly reprogramming somatic cells into definitive HSCs and HSPCs. To meet clinical needs and to advance drug discovery and stem cell therapy, alternative approaches are necessary. In this review, we synthesize the strategies used and the key findings made in recent years by those trying to make an HSC.

Reprogramming human B cells into induced pluripotent stem cells and its enhancement by C/EBPα

B cells have been shown to be refractory to reprogramming and B-cell-derived induced pluripotent stem cells (iPSC) have only been generated from murine B cells engineered to carry doxycycline-inducible Oct4, Sox2, Klf4 and Myc (OSKM) cassette in every tissue and from EBV/SV40LT-immortalized lymphoblastoid cell lines. Here, we show for the first time that freshly isolated non-cultured human cord blood (CB)- and peripheral blood (PB)-derived CD19+CD20+ B cells can be reprogrammed to iPSCs carrying complete VDJH immunoglobulin (Ig) gene monoclonal rearrangements using non-integrative tetracistronic, but not monocistronic, OSKM-expressing Sendai Virus. Co-expression of C/EBPα with OSKM facilitates iPSC generation from both CB- and PB-derived B cells. We also demonstrate that myeloid cells are much easier to reprogram than B and T lymphocytes. Differentiation potential back into the cell type of their origin of B-cell-, T-cell-, myeloid- and fibroblast-iPSCs is not skewed, suggesting that their differentiation does not seem influenced by 'epigenetic memory'. Our data reflect the actual cell-autonomous reprogramming capacity of human primary B cells because biased reprogramming was avoided by using freshly isolated primary cells, not exposed to cytokine cocktails favoring proliferation, differentiation or survival. The ability to reprogram CB/PB-derived primary human B cells offers an unprecedented opportunity for studying developmental B lymphopoiesis and modeling B-cell malignancies.

The Notch ligand DLL4 specifically marks human hematoendothelial progenitors and regulates their hematopoietic fate

Notch signaling is essential for definitive hematopoiesis, but its role in human embryonic hematopoiesis is largely unknown. We show that in hESCs the expression of the Notch ligand DLL4 is induced during hematopoietic differentiation. We found that DLL4 is only expressed in a sub-population of bipotent hematoendothelial progenitors (HEPs) and segregates their hematopoietic versus endothelial potential. We demonstrate at the clonal level and through transcriptome analyses that DLL4(high) HEPs are enriched in endothelial potential, whereas DLL4(low/-) HEPs are committed to the hematopoietic lineage, albeit both populations still contain bipotent cells. Moreover, DLL4 stimulation enhances hematopoietic differentiation of HEPs and increases the amount of clonogenic hematopoietic progenitors. Confocal microscopy analysis of whole differentiating embryoid bodies revealed that DLL4(high) HEPs are located close to DLL4(low/-) HEPs, and at the base of clusters of CD45+ cells, resembling intra-aortic hematopoietic clusters found in mouse embryos. We propose a model for human embryonic hematopoiesis in which DLL4(low/-) cells within hemogenic endothelium receive Notch-activating signals from DLL4(high) cells, resulting in an endothelial-to-hematopoietic transition and their differentiation into CD45+ hematopoietic cells.

V-myc immortalizes human neural stem cells in the absence of pluripotency-associated traits

A better understanding of the molecular mechanisms governing stem cell self-renewal will foster the use of different types of stem cells in disease modeling and cell therapy strategies. Immortalization, understood as the capacity for indefinite expansion, is needed for the generation of any cell line. In the case of v-myc immortalized multipotent human Neural Stem Cells (hNSCs), we hypothesized that v-myc immortalization could induce a more de-differentiated state in v-myc hNSC lines. To test this, we investigated the expression of surface, biochemical and genetic markers of stemness and pluripotency in v-myc immortalized and control hNSCs (primary precursors, that is, neurospheres) and compared these two cell types to human Embryonic Stem Cells (hESCs) and fibroblasts. Using a Hierarchical Clustering method and a Principal Component Analysis (PCA), the v-myc hNSCs associated with their counterparts hNSCs (in the absence of v-myc) and displayed a differential expression pattern when compared to hESCs. Moreover, the expression analysis of pluripotency markers suggested no evidence supporting a reprogramming-like process despite the increment in telomerase expression. In conclusion, v-myc expression in hNSC lines ensures self-renewal through the activation of some genes involved in the maintenance of stem cell properties in multipotent cells but does not alter the expression of key pluripotency-associated genes.

De novo generation of HSCs from somatic and pluripotent stem cell sources

Generating human hematopoietic stem cells (HSCs) from autologous tissues, when coupled with genome editing technologies, is a promising approach for cellular transplantation therapy and for in vitro disease modeling, drug discovery, and toxicology studies. Human pluripotent stem cells (hPSCs) represent a potentially inexhaustible supply of autologous tissue; however, to date, directed differentiation from hPSCs has yielded hematopoietic cells that lack robust and sustained multilineage potential. Cellular reprogramming technologies represent an alternative platform for the de novo generation of HSCs via direct conversion from heterologous cell types. In this review, we discuss the latest advancements in HSC generation by directed differentiation from hPSCs or direct conversion from somatic cells, and highlight their applications in research and prospects for therapy.

The polycomb group protein L3MBTL1 represses a SMAD5-mediated hematopoietic transcriptional program in human pluripotent stem cells

Epigenetic regulation of key transcriptional programs is a critical mechanism that controls hematopoietic development, and, thus, aberrant expression patterns or mutations in epigenetic regulators occur frequently in hematologic malignancies. We demonstrate that the Polycomb protein L3MBTL1, which is monoallelically deleted in 20q- myeloid malignancies, represses the ability of stem cells to drive hematopoietic-specific transcriptional programs by regulating the expression of SMAD5 and impairing its recruitment to target regulatory regions. Indeed, knockdown of L3MBTL1 promotes the development of hematopoiesis and impairs neural cell fate in human pluripotent stem cells. We also found a role for L3MBTL1 in regulating SMAD5 target gene expression in mature hematopoietic cell populations, thereby affecting erythroid differentiation. Taken together, we have identified epigenetic priming of hematopoietic-specific transcriptional networks, which may assist in the development of therapeutic approaches for patients with anemia.

•

L3MBTL1 is a chromatin-binding protein that represses SMAD5 expression

•

Lack of L3MBTL1 primes the hematopoietic development of pluripotent stem cells

•

L3MBTL1 regulates erythroid differentiation

Identification of Cdca7 as a novel Notch transcriptional target involved in hematopoietic stem cell emergence

Guiu et al. use ChIP-on-chip analysis for the Notch partner RBPj, using embryonic tissue from the aorta-gonad-mesonephros region to identify potential novel Notch target genes involved in HSC emergence. They show that c-MYC–responsive gene Cdca7 is expressed in different HSC and progenitor subpopulations and that CDCA7 is important for maintaining the undifferentiated phenotype. Cdca7 acts downstream of Notch in HSCs in zebrafish, mouse, and human, indicating a highly conserved Notch/RBPj/Cdca7 axis in hematopoietic development.

Hematopoietic stem cell (HSC) specification occurs in the embryonic aorta and requires Notch activation; however, most of the Notch-regulated elements controlling de novo HSC generation are still unknown. Here, we identify putative direct Notch targets in the aorta-gonad-mesonephros (AGM) embryonic tissue by chromatin precipitation using antibodies against the Notch partner RBPj. By ChIP-on-chip analysis of the precipitated DNA, we identified 701 promoter regions that were candidates to be regulated by Notch in the AGM. One of the most enriched regions corresponded to the Cdca7 gene, which was subsequently confirmed to recruit the RBPj factor but also Notch1 in AGM cells. We found that during embryonic hematopoietic development, expression of Cdca7 is restricted to the hematopoietic clusters of the aorta, and it is strongly up-regulated in the hemogenic population during human embryonic stem cell hematopoietic differentiation in a Notch-dependent manner. Down-regulation of Cdca7 mRNA in cultured AGM cells significantly induces hematopoietic differentiation and loss of the progenitor population. Finally, using loss-of-function experiments in zebrafish, we demonstrate that CDCA7 contributes to HSC emergence in vivo during embryonic development. Thus, our study identifies Cdca7 as an evolutionary conserved Notch target involved in HSC emergence.

SCL/TAL1-mediated transcriptional network enhances megakaryocytic specification of human embryonic stem cells

Human embryonic stem cells (hESCs) are a unique in vitro model for studying human developmental biology and represent a potential source for cell replacement strategies. Platelets can be generated from cord blood progenitors and hESCs; however, the molecular mechanisms and determinants controlling the in vitro megakaryocytic specification of hESCs remain elusive. We have recently shown that stem cell leukemia (SCL) overexpression accelerates the emergence of hemato-endothelial progenitors from hESCs and promotes their subsequent differentiation into blood cells with higher clonogenic potential. Given that SCL participates in megakaryocytic commitment, we hypothesized that it may potentiate megakaryopoiesis from hESCs. We show that ectopic SCL expression enhances the emergence of megakaryocytic precursors, mature megakaryocytes (MKs), and platelets in vitro. SCL-overexpressing MKs and platelets respond to different activating stimuli similar to their control counterparts. Gene expression profiling of megakaryocytic precursors shows that SCL overexpression renders a megakaryopoietic molecular signature. Connectivity Map analysis reveals that trichostatin A (TSA) and suberoylanilide hydroxamic acid (SAHA), both histone deacetylase (HDAC) inhibitors, functionally mimic SCL-induced effects. Finally, we confirm that both TSA and SAHA treatment promote the emergence of CD34(+) progenitors, whereas valproic acid, another HDAC inhibitor, potentiates MK and platelet production. We demonstrate that SCL and HDAC inhibitors are megakaryopoiesis regulators in hESCs.

Bottlenecks in deriving definitive hematopoietic stem cells from human pluripotent stem cells: a CIRM mini-symposium and workshop report

On August 29, 2013, the California Institute for Regenerative Medicine (CIRM) convened a small group of investigators in San Francisco, CA, to discuss a longstanding challenge in the stem cell field: the inability to derive fully functional, definitive hematopoietic stem cells (HSCs) from pluripotent stem cells (PSCs). To date, PSC-derived HSCs have been deficient in their developmental potential and their ability to self-renew and engraft upon transplantation. Tasked with identifying key challenges to overcoming this "HSC bottleneck", workshop participants identified critical knowledge gaps in two key areas: (a) understanding the ontogeny of human HSCs, and (b) understanding of the intrinsic and extrinsic factors that govern HSC behavior and function. They agreed that development of new methods and tools is critical for addressing these knowledge gaps. These include molecular profiling of key HSC properties, development of new model systems/assays for predicting and assessing HSC function, and novel technological advancements for manipulating cell culture conditions and genetic programs. The workshop produced tangible advances, including providing a current definition of the nature and challenge of the HSC bottleneck and identifying key mechanistic studies of HSC biology that should be prioritized for future funding initiatives (e.g., including higher risk approaches that have potential for high gain).

HOXA9 promotes hematopoietic commitment of human embryonic stem cells

The molecular determinants regulating the specification of human embryonic stem cells (hESCs) into hematopoietic cells remain elusive. HOXA9 plays a relevant role in leukemogenesis and hematopoiesis. It is highly expressed in hematopoietic stem and progenitor cells (HSPCs) and is downregulated upon differentiation. Hoxa9-deficient mice display impaired hematopoietic development, and deregulation of HOXA9 expression is frequently associated with acute leukemia. Analysis of the genes differentially expressed in cord blood HSPCs vs hESC-derived HSPCs identified HOXA9 as the most downregulated gene in hESC-derived HSPCs, suggesting that expression levels of HOXA9 may be crucial for hematopoietic differentiation of hESC. Here we show that during hematopoietic differentiation of hESCs, HOXA9 expression parallels hematopoietic development, but is restricted to the hemogenic precursors (HEP) (CD31(+)CD34(+)CD45(-)), and diminishes as HEPs differentiate into blood cells (CD45(+)). Different gain-of-function and loss-of-function studies reveal that HOXA9 enhances hematopoietic differentiation of hESCs by specifically promoting the commitment of HEPs into primitive and total CD45(+) blood cells. Gene expression analysis suggests that nuclear factor-κB signaling could be collaborating with HOXA9 to increase hematopoietic commitment. However, HOXA9 on its own is not sufficient to confer in vivo long-term engraftment potential to hESC-hematopoietic derivatives, reinforcing the idea that additional molecular regulators are needed for the generation of definitive in vivo functional HSPCs from hESC.

The essential roles of core binding factors CfRunt and CfCBFβ in hemocyte production of scallop Chlamys farreri

Core binding factor (CBF) is a family of heterodimeric transcription factors composed of a DNA-binding CBFα subunit and a non-DNA-binding CBFβ subunit, which plays critical roles in regulating hematopoiesis, osteogenesis and neurogenesis. In the present study, two genes encoding Runt (designed as CfRunt) and CBFβ (designed as CfCBFβ) were cloned and characterized from scallop Chlamys farreri. The full-length cDNA of CfRunt and CfCBFβ consists of 2128 bp and 1729 bp encoding a predicted polypeptide of 530 and 183 amino acids with a conserved Runt domain and CBFβ domain, respectively. Electrophoretic mobility shift assay demonstrated that the recombinant CfRunt protein (rCfRunt) exhibited solid ability to bind specific DNA, whereas rCfCBFβ could remarkably increase the DNA-binding affinity of rCfRunt. The mRNA transcripts of CfRunt and CfCBFβ could be detected in all tested tissues, especially in hemocytes, heart, hepatopancreas or muscle. After bacterial challenge, the circulating total hemocyte count (THC) of scallop reduced to the lowest level at 6h (P<0.05), and then it recovered gradually to the control level at 48-96 h, while the mRNA expressions of CfRunt and CfCBFβ were significant up-regulated between 6 and 48 h (P<0.05). After CfRunt gene was silenced by RNA interference, the hemocyte renewal rate and circulating THC both decreased significantly (P<0.05). However, following the RNA interference of CfRunt, the mRNA expression of CfRunt was significantly induced (P<0.05) and the attenuated hemocyte renewal rate and circulating THC could be repaired partially by LPS stimulation in the CfRunt-silenced scallops. The results collectively indicated that CfRunt and CfCBFβ, as conserved transcription factors, played essential roles in regulating hemocyte production of scallop.

The genome-wide molecular signature of transcription factors in leukemia

Transcription factors control expression of genes essential for the normal functioning of the hematopoietic system and regulate development of distinct blood cell types. During leukemogenesis, aberrant regulation of transcription factors such as RUNX1, CBFβ, MLL, C/EBPα, SPI1, GATA, and TAL1 is central to the disease. Here, we will discuss the mechanisms of transcription factor deregulation in leukemia and how in recent years next-generation sequencing approaches have helped to elucidate the molecular role of many of these aberrantly expressed transcription factors. We will focus on the complexes in which these factors reside, the role of posttranslational modification of these factors, their involvement in setting up higher order chromatin structures, and their influence on the local epigenetic environment. We suggest that only comprehensive knowledge on all these aspects will increase our understanding of aberrant gene expression in leukemia as well as open new entry points for therapeutic intervention.

Early aberrant DNA methylation events in a mouse model of acute myeloid leukemia

Background

Aberrant DNA methylation is frequently found in human malignancies including acute myeloid leukemia (AML). While most studies focus on later disease stages, the onset of aberrant DNA methylation events and their dynamics during leukemic progression are largely unknown.

Methods

We screened genome-wide for aberrant CpG island methylation in three disease stages of a murine AML model that is driven by hypomorphic expression of the hematopoietic transcription factor PU.1. DNA methylation levels of selected genes were correlated with methylation levels of CD34+ cells and lineage negative, CD127-, c-Kit+, Sca-1+ cells; common myeloid progenitors; granulocyte-macrophage progenitors; and megakaryocyte-erythroid progenitors.

Results

We identified 1,184 hypermethylated array probes covering 762 associated genes in the preleukemic stage. During disease progression, the number of hypermethylated genes increased to 5,465 in the late leukemic disease stage. Using publicly available data, we found a significant enrichment of PU.1 binding sites in the preleukemic hypermethylated genes, suggesting that shortage of PU.1 makes PU.1 binding sites in the DNA accessible for aberrant methylation. Many known AML associated genes such as RUNX1 and HIC1 were found among the preleukemic hypermethylated genes. Nine novel hypermethylated genes, FZD5, FZD8, PRDM16, ROBO3, CXCL14, BCOR, ITPKA, HES6 and TAL1, the latter four being potential PU.1 targets, were confirmed to be hypermethylated in human normal karyotype AML patients, underscoring the relevance of the mouse model for human AML.

Conclusions

Our study identified early aberrantly methylated genes as potential contributors to onset and progression of AML.

Role of BRD4 in hematopoietic differentiation of embryonic stem cells

The bromodomain and extra terminal (BET) protein family member BRD4 is a transcriptional regulator, critical for cell cycle progression and cellular viability. Here, we show that BRD4 plays an important role in embryonic stem cell (ESC) regulation. During differentiation of ESCs, BRD4 expression is upregulated and its gene promoter becomes demethylated. Disruption of BRD4 expression in ESCs did not induce spontaneous differentiation but severely diminished hematoendothelial potential. Although BRD4 regulates c-Myc expression, our data show that the role of BRD4 in hematopoietic commitment is not exclusively mediated by c-Myc. Our results indicate that BRD4 is epigenetically regulated during hematopoietic differentiation ESCs in the context of a still unknown signaling pathway.

Ligand-independent FLT3 activation does not cooperate with MLL-AF4 to immortalize/transform cord blood CD34+ cells

MLL-AF4 fusion is hallmark in high-risk infant pro-B-acute lymphoblastic leukemia (pro-B-ALL). Our limited understanding of MLL-AF4-mediated transformation reflects the absence of human models reproducing this leukemia. Hematopoietic stem/progenitor cells (HSPCs) constitute likely targets for transformation. We previously reported that MLL-AF4 enhanced hematopoietic engraftment and clonogenic potential in cord blood (CB)-derived CD34+ HSPCs but was not sufficient for leukemogenesis, suggesting that additional oncogenic lesions are required for MLL-AF4-mediated transformation. MLL-AF4+ pro-B-ALL display enormous levels of FLT3, and occasionally FLT3-activating mutations, thus representing a candidate cooperating event in MLL-AF4+ pro-B-ALL. We have explored whether FLT3.TKD (tyrosine kinase domain) mutation or increased expression of FLT3.WT (wild type) cooperates with MLL-AF4 to immortalize/transform CB-CD34+ HSPCs. In vivo, FLT3.TKD/FLT3.WT alone, or in combination with MLL-AF4, enhances hematopoietic repopulating function of CB-CD34+ HSPCs without impairing migration or hematopoietic differentiation. None of the animals transplanted with MLL-AF4+FLT3.TKD/WT-CD34+ HSPCs showed any sign of disease after 16 weeks. In vitro, enforced expression of FLT3.TKD/FLT3.WT conveys a transient overexpansion of MLL-AF4-expressing CD34+ HSPCs associated to higher proportion of cycling cells coupled to lower apoptotic levels, but does not augment clonogenic potential nor confer stable replating. Together, FLT3 activation does not suffice to immortalize/transform MLL-AF4-expressing CB-CD34+ HSPCs, suggesting the need of alternative (epi)-genetic cooperating oncogenic lesions.