An expandable, inducible hemangioblast state regulated by fibroblast growth factor

Current status of producing autologous hematopoietic stem cells

Hematopoietic stem cells (HSCs) transplantation is an established therapy for many diseases of the hematopoietic system, for example aplastic anemia, acute myeloid leukemia and acute lymphoblastic leukemia. With the development of the HSCs research, HSCs provide an attractive method for treating hereditary blood disorders and immunotherapy of cancer by introducing gene modification. Compared with allogenic HSCs transplantation, using autologous HSCs or HSCs from induced pluripotent stem cells (iPSCs) would eliminate the probability of alloimmunization and transfusion-transmitted infectious diseases. The methods for obtaining autologous HSCs include amplifying patients' HSCs or inducing patients' somatic cells to HSCs (graph abstract). However, the biggest problem is inducing HSCs to proliferate in vitro and maintaining their stemness at the same time. Although many tests have been made to transform iPSCs to HSCs, the artificially generated HSCs still have substantial disparity compared with physiological HSCs. This review summarized the application status and obstacles to implantation of autologous HSCs and iPSC-derived HSCs. Meanwhile, we summarized the latest research progress in HSCs amplification and iPSCs reprogramming methods, which will help to solve the problems mentioned above.

LMO2 is essential to maintain the ability of progenitors to differentiate into T-cell lineage in mice

Notch signaling primarily determines T-cell fate. However, the molecular mechanisms underlying the maintenance of T-lineage potential in pre-thymic progenitors remain unclear. Here, we established two murine Ebf1-deficient pro-B cell lines, with and without T-lineage potential. The latter expressed lower levels of Lmo2; their potential was restored via ectopic expression of Lmo2. Conversely, the CRISPR/Cas9-mediated deletion of Lmo2 resulted in the loss of the T-lineage potential. Introduction of Bcl2 rescued massive cell death of Notch-stimulated pro-B cells without efficient LMO2-driven Bcl11a expression but was not sufficient to retain their T-lineage potential. Pro-B cells without T-lineage potential failed to activate Tcf7 due to DNA methylation; Tcf7 transduction restored this capacity. Moreover, direct binding of LMO2 to the Bcl11a and Tcf7 loci was observed. Altogether, our results highlight LMO2 as a crucial player in the survival and maintenance of T-lineage potential in T-cell progenitors via the regulation of the expression of Bcl11a and Tcf7.

FGF primes angioblast formation by inducing ETV2 and LMO2 via FGFR1/BRAF/MEK/ERK

It is critical to specify a signal that directly drives the transition that occurs between cell states. However, such inferences are often confounded by indirect intercellular communications or secondary transcriptomic changes due to primary transcription factors. Although FGF is known for its importance during mesoderm-to-endothelium differentiation, its specific role and signaling mechanisms are still unclear due to the confounding factors referenced above. Here, we attempted to minimize the secondary artifacts by manipulating FGF and its downstream mediators with a short incubation time before sampling and protein-synthesis blockage in a low-density angioblastic/endothelial differentiation system. In less than 8 h, FGF started the conversion of KDRlow/PDGFRAlow nascent mesoderm into KDRhigh/PDGFRAlow angioblasts, and the priming by FGF was necessary to endow endothelial formation 72 h later. Further, the angioblastic conversion was mediated by the FGFR1/BRAF/MEK/ERK pathway in mesodermal cells. Finally, two transcription factors, ETV2 and LMO2, were the early direct functional responders downstream of the FGF pathway, and ETV2 alone was enough to complement the absence of FGF. FGF's selective role in mediating the first-step, angioblastic conversion from mesoderm-to-endothelium thus allows for refined control over acquiring and manipulating angioblasts. The noise-minimized differentiation/analysis platform presented here is well-suited for studies on the signaling switches of other mesodermal-lineage fates as well.

The hemogenic endothelium: a critical source for the generation of PSC-derived hematopoietic stem and progenitor cells

In vitro generation of hematopoietic cells and especially hematopoietic stem cells (HSCs) from human pluripotent stem cells (PSCs) are subject to intensive research in recent decades, as these cells hold great potential for regenerative medicine and autologous cell replacement therapies. Despite many attempts, in vitro, de novo generation of bona fide HSCs remains challenging, and we are still far away from their clinical use, due to insufficient functionality and quantity of the produced HSCs. The challenges of generating PSC-derived HSCs are already apparent in early stages of hemato-endothelial specification with the limitation of recapitulating complex, dynamic processes of embryonic hematopoietic ontogeny in vitro. Further, these current shortcomings imply the incompleteness of our understanding of human ontogenetic processes from embryonic mesoderm over an intermediate, specialized hemogenic endothelium (HE) to their immediate progeny, the HSCs. In this review, we examine the recent investigations of hemato-endothelial ontogeny and recently reported progress for the conversion of PSCs and other promising somatic cell types towards HSCs with the focus on the crucial and inevitable role of the HE to achieve the long-standing goal—to generate therapeutically applicable PSC-derived HSCs in vitro.

Transcriptional control of blood cell emergence

The haematopoietic system is established during embryonic life through a series of developmental steps that culminates with the generation of haematopoietic stem cells. Characterisation of the transcriptional network that regulates blood cell emergence has led to the identification of transcription factors essential for this process. Among the many factors wired within this complex regulatory network, ETV2, SCL and RUNX1 are the central components. All three factors are absolutely required for blood cell generation, each one controlling a precise step of specification from the mesoderm germ layer to fully functional blood progenitors. Insight into the transcriptional control of blood cell emergence has been used for devising protocols to generate blood cells de novo, either through reprogramming of somatic cells or through forward programming of pluripotent stem cells. Interestingly, the physiological process of blood cell generation and its laboratory-engineered counterpart have very little in common.

Haematopoietic stem cell reprogramming and the hope for a universal blood product

Haematopoietic stem cells (HSCs) are the only adult stem cells with a demonstrated clinical use, even though a tractable method to maintain and expand human HSCs in vitro has not yet been found. Owing to the introduction of transplantation strategies for the treatment of haematological malignancies and, more recently, the promise of gene therapy, the need to improve the generation, manipulation and scalability of autologous or allogeneic HSCs has risen steeply over the past decade. In that context, reprogramming strategies based on the expression of exogenous transcription factors have emerged as a means to produce functional HSCs in vitro. These approaches largely stem from the assumption that key master transcription factors direct the expression of downstream target genes thereby triggering haematopoiesis. Both somatic and pluripotent cells have been used to this end, yielding variable results in terms of haematopoietic phenotype and functionality. Here, we present an overview of the haematopoietic reprogramming methods reported to date, provide the appropriate historical context and offer some critical insight about where the field stands at present.

Induction of human hemogenesis in adult fibroblasts by defined factors and hematopoietic coculture

Transcription factor (TF)-based reprogramming of somatic tissues holds great promise for regenerative medicine. Previously, we demonstrated that the TFs GATA2, GFI1B, and FOS convert mouse and human fibroblasts to hemogenic endothelial-like precursors that generate hematopoietic stem progenitor (HSPC)-like cells over time. This conversion is lacking in robustness both in yield and biological function. Herein, we show that inclusion of GFI1 to the reprogramming cocktail significantly expands the HSPC-like population. AFT024 coculture imparts functional potential to these cells and allows quantification of stem cell frequency. Altogether, we demonstrate an improved human hemogenic induction protocol that could provide a valuable human in vitro model of hematopoiesis for disease modeling and a platform for cell-based therapeutics. DATABASE: Gene expression data are available in the Gene Expression Omnibus (GEO) database under the accession number GSE130361.

Expandable Arterial Endothelial Precursors from Human CD34+ Cells Differ in Their Proclivity to Undergo an Endothelial-to-Mesenchymal Transition

Arterial diseases continue to pose a major health concern but in vitro studies are limited because explanted cells can exhibit poor proliferative capacity and a loss of specificity. Here, we find that two transcription factors, MYCN and SOX17, induce and indefinitely expand in culture precursors of human arterial endothelial cells (expandable arterial endothelial precursors [eAEPs]). The eAEPs are derived from CD34⁺ cells found in umbilical cord blood or adult bone marrow. Independent eAEP lines differ in their proclivity to undergo an endothelial-to-mesenchymal transition (EndoMT), a hallmark event in a broad array of vascular diseases and disorders. Some cell lines spontaneously become mesenchymal over time in culture, an effect exacerbated by inhibition of the fibroblast growth factor receptor, while others do not readily convert. These distinctions were exploited to identify genes that correlate with resistance to an EndoMT and to elucidate transcriptional changes that underpin the transition.

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MYCN and SOX17 induce and expand human arterial endothelial precursors (eAEPs)

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The eAEPs arise from CD34⁺ cells from umbilical cord blood or adult bone marrow

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The eAEPs preferentially arise from PROM1^low/−THY1⁻CD45^highCD45RA⁻ cells

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Some eAEPs readily transition to mesenchymal state and others do not

Human haematopoietic stem cell development: from the embryo to the dish

Haematopoietic stem cells (HSCs) emerge during embryogenesis and give rise to the adult haematopoietic system. Understanding how early haematopoietic development occurs is of fundamental importance for basic biology and medical sciences, but our knowledge is still limited compared with what we know of adult HSCs and their microenvironment. This is particularly true for human haematopoiesis, and is reflected in our current inability to recapitulate the development of HSCs from pluripotent stem cells in vitro In this Review, we discuss what is known of human haematopoietic development: the anatomical sites at which it occurs, the different temporal waves of haematopoiesis, the emergence of the first HSCs and the signalling landscape of the haematopoietic niche. We also discuss the extent to which in vitro differentiation of human pluripotent stem cells recapitulates bona fide human developmental haematopoiesis, and outline some future directions in the field.

Reprogramming Mouse Embryonic Fibroblasts with Transcription Factors to Induce a Hemogenic Program

This protocol details the induction of a hemogenic program in mouse embryonic fibroblasts (MEFs) via overexpression of transcription factors (TFs). We first designed a reporter screen using MEFs from human CD34-tTA/TetO-H2BGFP (34/H2BGFP) double transgenic mice. CD34⁺ cells from these mice label H2B histones with GFP, and cease labeling upon addition of doxycycline (DOX). MEFS were transduced with candidate TFs and then observed for the emergence of GFP⁺ cells that would indicate the acquisition of a hematopoietic or endothelial cell fate. Starting with 18 candidate TFs, and through a process of combinatorial elimination, we obtained a minimal set of factors that would induce the highest percentage of GFP⁺ cells. We found that Gata2, Gfi1b, and cFos were necessary and the addition of Etv6 provided the optimal induction. A series of gene expression analyses done at different time points during the reprogramming process revealed that these cells appeared to go through a precursor cell that underwent an endothelial to hematopoietic transition (EHT). As such, this reprogramming process mimics developmental hematopoiesis "in a dish," allowing study of hematopoiesis in vitro and a platform to identify the mechanisms that underlie this specification. This methodology also provides a framework for translation of this work to the human system in the hopes of generating an alternative source of patient-specific hematopoietic stem cells (HSCs) for a number of applications in the treatment and study of hematologic diseases.

Emerging concepts for the in vitro derivation of murine haematopoietic stem and progenitor cells

Well into the second decade of the 21st century, the field of regenerative medicine is bursting with hopes and promises to heal young and old. The bespoken generation of cells is thought to offer unprecedented cures for a vast range of diseases. Haematological disorders have already benefited tremendously from stem cell therapy in the form of bone marrow transplantation. However, lack of compatible donors often means that patients remain on transplantation waiting lists for too long. The in vitro derivation of haematopoietic stem cells offers the possibility to generate tailor-made cells for the treatment of these patients. Promising approaches to generate in vitro-derived blood progenitors include the directed differentiation of pluripotent stem cells and the reprogramming of somatic cells.

Concise Review: Recent Advances in the In Vitro Derivation of Blood Cell Populations

: Hematopoietic cell-based therapies are currently available treatment options for many hematological and nonhematological disorders. However, the scarcity of allogeneic donor-derived cells is a major hurdle in treating these disorders. Embryonic stem cell-based directed differentiation and direct reprogramming of somatic cells provide excellent tools for the potential generation of hematopoietic stem cells usable in the clinic for cellular therapies. In addition to blood stem cell transplantation, mature blood cells such as red blood cells, platelets, and engineered T cells have also been increasingly used to treat several diseases. Besides cellular therapies, induced blood progenitor cells generated from autologous sources (either induced pluripotent stem cells or somatic cells) can be useful for disease modeling of bone marrow failures and acquired blood disorders. However, although great progress has been made toward these goals, we are still far from the use of in vitro-derived blood products in the clinic. We review the current state of knowledge on the directed differentiation of embryonic stem cells and the reprogramming of somatic cells toward the generation of blood stem cells and derivatives.

SIGNIFICANCE

Hematopoietic cell-based therapies are currently available treatment options for many hematological and nonhematological disorders. However, the scarcity of allogeneic donor-derived cells is a major hurdle in treating these disorders. The current state of knowledge on the directed differentiation of embryonic stem cells and the reprogramming of somatic cells toward the generation of blood stem cells and derivatives is reviewed.

Single-cell RNA-seq reveals novel regulators of human embryonic stem cell differentiation to definitive endoderm

BACKGROUND

Human pluripotent stem cells offer the best available model to study the underlying cellular and molecular mechanisms of human embryonic lineage specification. However, it is not fully understood how individual stem cells exit the pluripotent state and transition towards their respective progenitor states.

RESULTS

Here, we analyze the transcriptomes of human embryonic stem cell-derived lineage-specific progenitors by single-cell RNA-sequencing (scRNA-seq). We identify a definitive endoderm (DE) transcriptomic signature that leads us to pinpoint a critical time window when DE differentiation is enhanced by hypoxia. The molecular mechanisms governing the emergence of DE are further examined by time course scRNA-seq experiments, employing two new statistical tools to identify stage-specific genes over time (SCPattern) and to reconstruct the differentiation trajectory from the pluripotent state through mesendoderm to DE (Wave-Crest). Importantly, presumptive DE cells can be detected during the transitory phase from Brachyury (T) (+) mesendoderm toward a CXCR4 (+) DE state. Novel regulators are identified within this time window and are functionally validated on a screening platform with a T-2A-EGFP knock-in reporter engineered by CRISPR/Cas9. Through loss-of-function and gain-of-function experiments, we demonstrate that KLF8 plays a pivotal role modulating mesendoderm to DE differentiation.

CONCLUSIONS

We report the analysis of 1776 cells by scRNA-seq covering distinct human embryonic stem cell-derived progenitor states. By reconstructing a differentiation trajectory at single-cell resolution, novel regulators of the mesendoderm transition to DE are elucidated and validated. Our strategy of combining single-cell analysis and genetic approaches can be applied to uncover novel regulators governing cell fate decisions in a variety of systems.

Lost in translation: pluripotent stem cell-derived hematopoiesis

Pluripotent stem cells (PSCs) such as embryonic stem cells or induced pluripotent stem cells represent a promising cell type to gain novel insights into human biology. Understanding the differentiation process of PSCs in vitro may allow for the identification of cell extrinsic/intrinsic factors, driving the specification process toward all cell types of the three germ layers, which may be similar to the human in vivo scenario. This would not only lay the ground for an improved understanding of human embryonic development but would also contribute toward the generation of novel cell types used in cell replacement therapies. In this line, especially the developmental process of mesodermal cells toward the hematopoietic lineage is of great interest. Therefore, this review highlights recent progress in the field of hematopoietic specification of pluripotent stem cell sources. In addition, we would like to shed light on emerging factors controlling primitive and definitive hematopoietic development and to highlight recent approaches to improve the differentiation potential of PSC sources toward hematopoietic stem/progenitor cells. While the generation of fully defined hematopoietic stem cells from PSCs remains challenging in vitro, we here underline the instructive role of cell extrinsic factors such as cytokines for the generation of PSC-derived mature hematopoietic cells. Thus, we have comprehensively examined the role of cytokines for the derivation of mature hematopoietic cell types such as macrophages, granulocytes, megakaryocytes, erythrocytes, dendritic cells, and cells of the B- and T-cell lineage.

LMO2 at 25 years: a paradigm of chromosomal translocation proteins

LMO2 was first discovered through proximity to frequently occurring chromosomal translocations in T cell acute lymphoblastic leukaemia (T-ALL). Subsequent studies on its role in tumours and in normal settings have highlighted LMO2 as an archetypical chromosomal translocation oncogene, activated by association with antigen receptor gene loci and a paradigm for translocation gene activation in T-ALL. The normal function of LMO2 in haematopoietic cell fate and angiogenesis suggests it is a master gene regulator exerting a dysfunctional control on differentiation following chromosomal translocations. Its importance in T cell neoplasia has been further emphasized by the recurrent findings of interstitial deletions of chromosome 11 near LMO2 and of LMO2 as a target of retroviral insertion gene activation during gene therapy trials for X chromosome-linked severe combined immuno-deficiency syndrome, both types of event leading to similar T cell leukaemia. The discovery of LMO2 in some B cell neoplasias and in some epithelial cancers suggests a more ubiquitous function as an oncogenic protein, and that the current development of novel inhibitors will be of great value in future cancer treatment. Further, the role of LMO2 in angiogenesis and in haematopoietic stem cells (HSCs) bodes well for targeting LMO2 in angiogenic disorders and in generating autologous induced HSCs for application in various clinical indications.

Differentiation of pluripotent stem cells into endothelial cells

PURPOSE OF REVIEW

Methods to isolate endothelial cells from murine and human pluripotent stem cells continue to evolve and increasingly diverse endothelial cell populations have been generated. This review provides an update of key articles published within the past year that report on some of those advances.

RECENT FINDINGS

Cooperative interactions among microRNA (miRNA), transcription factors and some downstream interacting proteins have been reported to enhance endothelial specification from embryonic stem cells (ESCs) and induced pluripotent stem cells (iPSCs). Endothelial cell differentiation can also be modulated by various growth factor additions, Notch pathway activation or inhibition, and modulation of the microenvironment of the differentiating ESC and iPSC. Functionality of the derived endothelium has been demonstrated by a variety of in-vitro and in-vivo assays. Finally, two recent reports have identified endothelial progenitor populations with robust proliferative potential.

SUMMARY

Progress in differentiating endothelial cells from ESC and iPSC has been made. The recent report of formation of endothelial colony forming cells from human ESC and iPSC provides a protocol that can generate clinically relevant numbers of cells for human cell therapy.

Converting cell fates: generating hematopoietic stem cells de novo via transcription factor reprogramming

Even though all paradigms of stem cell therapy and regenerative medicine emerged from the study of hematopoietic stem cells (HSCs), the inability to generate these cells de novo or expand them in vitro persists. Initial efforts to obtain these cells began with the use of embryonic stem cell (ESC) and induced pluripotent stem cell (iPSC) technologies, but these strategies have yet to yield fully functional cells. Subsequently, more recent approaches involve transcription factor (TF) overexpression to reprogram PSCs and various somatic cells. The induction of pluripotency with just four TFs by Yamanaka informs our ability to convert cell fates and demonstrates the feasibility of utilizing terminally differentiated cells to generate cells with multilineage potential. In this review, we discuss the recent efforts undertaken using TF-based reprogramming strategies to convert several cell types into HSCs.

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.

Defining minimum essential factors to derive highly pure human endothelial cells from iPS/ES cells in an animal substance-free system

It is desirable to obtain unlimited supplies of endothelial cells for research and therapeutics. However, current methods of deriving endothelial cells from humans suffer from issues, such as limited supplies, contamination from animal substances, and lengthy/complicated procedures. In this article we developed a way to differentiate human iPS and ES cells to highly pure endothelial cells in 5 days. The chemically defined system is robust, easy to perform, and free of animal substances. Using the system, we verified that combined TGFβ and canonical Wnt agonists are essential and sufficient for iPS/ES cell-to-mesoderm transition. Besides, VEGF-KDR signaling alone is required for endothelial formation at high density while supplementation with FGF allows for colonial endothelial differentiation. Finally, anti-adsorptive agents could enrich the endothelial output by allowing selective attachment of the endothelial precursors. The system was validated to work on multiple iPS/ES cells lines to produce endothelial cells capable of forming capillary-like structures in vitro and integrating into host vasculature in vivo. In sum, the simple yet robust differentiation system permits the unlimited supply of human endothelial cells. The defined and animal substance-free nature of the system is compatible with clinical applications and characterization of endothelial differentiation in an unbiased manner.