In vitro generation of HSC-like cells from murine ESCs/iPSCs by enforced expression of LIM-homeobox transcription factor Lhx2

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

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

Stepwise dual-release microparticles of BMP-4 and SCF in induced pluripotent stem cell spheroids enhance differentiation into hematopoietic stem cells

Recently, the formation of three-dimensional (3D) cell aggregates known as embryoid bodies (EBs) grown in media supplemented with HSC-specific morphogens has been utilized for the directed differentiation of embryonic stem cells (ESCs) and induced pluripotent stem cells (iPSCs), into clinically relevant hematopoietic stem cells (HSCs). However, delivering growth factors and nutrients have become ineffective in inducing synchronous differentiation of cells due to their 3D conformation. Moreover, irregularly sized EBs often lead to the formation of necrotic cores in larger EBs, impairing differentiation. Here, we developed two gelatin microparticles (GelMPs) with different release patterns and two HSC-related growth factors conjugated to them. Slow and fast releasing GelMPs were conjugated with bone morphogenic factor-4 (BMP-4) and stem cell factor (SCF), respectively. The sequential presentation of BMP-4 and SCF in GelMPs resulted in efficient and effective hematopoietic differentiation, shown by the enhanced gene and protein expression of several mesoderm and HSC-related markers, and the increased concentration of released HSC-related cytokines. In the present study, we were able to generate CD34+, CD133+, and FLT3+ cells with similar cellular and molecular morphology as the naïve HSCs that can produce colony units of different blood cells, in vitro.

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

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

Induced pluripotent stem cell-derived hematopoietic stem and progenitor cells induce mixed chimerism and donor-specific allograft tolerance

In transplantation using allogeneic induced pluripotent stem cells (iPSCs), strategies focused on major histocompatibility complexes were adopted to avoid immune rejection. We showed that minor antigen mismatches are a risk factor for graft rejection, indicating that immune regulation remains one of the most important issues. In organ transplantation, it has been known that mixed chimerism using donor-derived hematopoietic stem/progenitor cells (HSPCs) can induce donor-specific tolerance. However, it is unclear whether iPSC-derived HSPCs (iHSPCs) can induce allograft tolerance. We showed that 2 hematopoietic transcription factors, Hoxb4 and Lhx2, can efficiently expand iHSPCs with a c-Kit+Sca-1+Lineage- phenotype, which possesses long-term hematopoietic repopulating potential. We also demonstrated that these iHSPCs can form hematopoietic chimeras in allogeneic recipients and induce allograft tolerance in murine skin and iPSC transplantation. With mechanistic analyses, both central and peripheral mechanisms were suggested. We demonstrated the basic concept of tolerance induction using iHSPCs in allogeneic iPSC-based transplantation.

An interferon-γ/FLT3 axis positively regulates hemopoietic progenitor cell expansion from human pluripotent stem cells

Since it became possible to differentiate human pluripotent stem cells (hPSCs) into hematopoietic cells in vitro, great efforts have been made to obtain highly potent hematopoietic stem/progenitor cells (HSPCs) from hPSCs. Immunophenotypical HSPCs can be obtained from hPSCs, but their repopulating potential in vivo is low. Here, we developed a novel hematopoietic differentiation method for human induced pluripotent stem cells (hiPSCs) to determine why the existing hPSC differentiation systems are inadequate. hiPSC-derived CD45 +CD34 + cells in our system were mostly CD38 - immunophenotypical HSPCs. The vast majority of human CD45 +CD34 + cells in umbilical cord blood, fetal liver, and bone marrow are CD38 + hematopoietic progenitor cells (HPCs); therefore, the poor production of CD38 + HPCs was indicative of a systematic problem. hiPSC-derived CD45 +CD34 + cells did not express FLT3, a receptor tyrosine kinase. Exogenous FLT3 activity significantly enhanced production of CD38 + HPCs from hiPSCs. Thus, poor production of CD38 + HPCs was due to a lack of FLT3 expression. Interferon-γ upregulated expression of FLT3 and increased the number of CD38 + HPCs among hiPSC-derived CD45 +CD34 + cells. These results suggest that poor production of CD38 + HPCs with hPSC differentiation systems is due to a lack of FLT3 expression, and that addition of interferon-γ can solve this problem.

Generation and clinical potential of functional T lymphocytes from gene-edited pluripotent stem cells

Engineered T cells have been shown to be highly effective in cancer immunotherapy, although T cell exhaustion presents a challenge for their long-term function. Additional T-cell sources must be exploited to broaden the application of engineered T cells for immune defense and reconstitution. Unlimited sources of pluripotent stem cells (PSCs) have provided a potential opportunity to generate precise-engineered therapeutic induced T (iT) cells. Single-cell transcriptome analysis of PSC-derived induced hematopoietic stem and progenitor cells (iHSPC)/iT identified the developmental pathways and possibilities of generating functional T cell from PSCs. To date, the PSC-to-iT platforms encounter several problems, including low efficiency of conventional T subset specification, limited functional potential, and restrictions on large-scale application, because of the absence of a thymus-like organized microenvironment. The updated PSC-to-iT platforms, such as the three-dimensional (3D) artificial thymic organoid (ATO) co-culture system and Runx1/Hoxa9-enforced iT lymphopoiesis, provide fresh perspectives for coordinating culture conditions and transcription factors, which may greatly improve the efficiency of T-cell generation greatly. In addition, the improved PSC-to-iT platform coordinating gene editing technologies will provide various functional engineered unconventional or conventional T cells. Furthermore, the clinical applications of PSC-derived immune cells are accelerating from bench to bedside.

In vivo generation of bone marrow from embryonic stem cells in interspecies chimeras

Generation of bone marrow (BM) from embryonic stem cells (ESCs) promises to accelerate the development of future cell therapies for life-threatening disorders. However, such approach is limited by technical challenges to produce a mixture of functional BM progenitor cells able to replace all hematopoietic cell lineages. Herein, we used blastocyst complementation to simultaneously produce BM cell lineages from mouse ESCs in a rat. Based on fluorescence-activated cell sorting analysis and single-cell RNA sequencing, mouse ESCs differentiated into multiple hematopoietic and stromal cell types that were indistinguishable from normal mouse BM cells based on gene expression signatures and cell surface markers. Receptor-ligand interactions identified Cxcl12-Cxcr4, Lama2-Itga6, App-Itga6, Comp-Cd47, Col1a1-Cd44, and App-Il18rap as major signaling pathways between hematopoietic progenitors and stromal cells. Multiple hematopoietic progenitors, including hematopoietic stem cells (HSCs) in mouse-rat chimeras derived more efficiently from mouse ESCs, whereas chondrocytes predominantly derived from rat cells. In the dorsal aorta and fetal liver of mouse-rat chimeras, mouse HSCs emerged and expanded faster compared to endogenous rat cells. Sequential BM transplantation of ESC-derived cells from mouse-rat chimeras rescued lethally irradiated syngeneic mice and demonstrated long-term reconstitution potential of donor HSCs. Altogether, a fully functional BM was generated from mouse ESCs using rat embryos as 'bioreactors'.

Cell and Gene Therapy for Anemia: Hematopoietic Stem Cells and Gene Editing

Hereditary anemia has various manifestations, such as sickle cell disease (SCD), Fanconi anemia, glucose-6-phosphate dehydrogenase deficiency (G6PDD), and thalassemia. The available management strategies for these disorders are still unsatisfactory and do not eliminate the main causes. As genetic aberrations are the main causes of all forms of hereditary anemia, the optimal approach involves repairing the defective gene, possibly through the transplantation of normal hematopoietic stem cells (HSCs) from a normal matching donor or through gene therapy approaches (either in vivo or ex vivo) to correct the patient’s HSCs. To clearly illustrate the importance of cell and gene therapy in hereditary anemia, this paper provides a review of the genetic aberration, epidemiology, clinical features, current management, and cell and gene therapy endeavors related to SCD, thalassemia, Fanconi anemia, and G6PDD. Moreover, we expound the future research direction of HSC derivation from induced pluripotent stem cells (iPSCs), strategies to edit HSCs, gene therapy risk mitigation, and their clinical perspectives. In conclusion, gene-corrected hematopoietic stem cell transplantation has promising outcomes for SCD, Fanconi anemia, and thalassemia, and it may overcome the limitation of the source of allogenic bone marrow transplantation.

A robust approach for the generation of functional hematopoietic progenitor cell lines to model leukemic transformation

Studies of molecular mechanisms of hematopoiesis and leukemogenesis are hampered by the unavailability of progenitor cell lines that accurately mimic the situation in vivo. We now report a robust method to generate and maintain LSK (Lin-, Sca-1+, c-Kit+) cells, which closely resemble MPP1 cells. HPCLSKs reconstitute hematopoiesis in lethally irradiated recipient mice over >8 months. Upon transformation with different oncogenes including BCR/ABL, FLT3-ITD, or MLL-AF9, their leukemic counterparts maintain stem cell properties in vitro and recapitulate leukemia formation in vivo. The method to generate HPCLSKs can be applied to transgenic mice, and we illustrate it for CDK6-deficient animals. Upon BCR/ABLp210 transformation, HPCLSKsCdk6-/- induce disease with a significantly enhanced latency and reduced incidence, showing the importance of CDK6 in leukemia formation. Studies of the CDK6 transcriptome in murine HPCLSK and human BCR/ABL+ cells have verified that certain pathways depend on CDK6 and have uncovered a novel CDK6-dependent signature, suggesting a role for CDK6 in leukemic progenitor cell homing. Loss of CDK6 may thus lead to a defect in homing. The HPCLSK system represents a unique tool for combined in vitro and in vivo studies and enables the production of large quantities of genetically modifiable hematopoietic or leukemic stem/progenitor cells.

Mechanisms of anti-GPIbα antibody-induced thrombocytopenia in mice

Immune thrombocytopenia (ITP) is an acquired bleeding disorder characterized by antibody-mediated platelet destruction. Different mechanisms have been suggested to explain accelerated platelet clearance and impaired thrombopoiesis, but the pathophysiology of ITP has yet to be fully delineated. In this study, we tested 2 mouse models of immune-mediated thrombocytopenia using the rat anti-mouse GPIbα monoclonal antibody 5A7, generated in our laboratory. After a single IV administration of high-dose (2 mg/kg) 5A7, opsonized platelets were rapidly cleared from the circulation into the spleen and liver; this was associated with rapid upregulation of thrombopoietin (TPO) messenger RNA. In contrast, subcutaneous administration of low-dose 5A7 (0.08-0.16 mg/kg) every 3 days gradually lowered the platelet count; in this case, opsonized platelets were observed only in the spleen, and TPO levels remained unaltered. Interestingly, in both models, the 5A7 antibody was found on the surface of, as well as internalized to, bone marrow megakaryocytes. Consequently, platelets generated in the chronic phase of repeated subcutaneous 5A7 administration model showed reduced GPIbα membrane expression on their surface. Our findings indicate that evaluation of platelet surface GPIbα relative to platelet size may be a useful marker to support the diagnosis of anti-GPIbα antibody-induced ITP.

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.

Platelet production using adipose-derived mesenchymal stem cells: Mechanistic studies and clinical application

Megakaryocytes (MKs) are platelet progenitor stem cells found in the bone marrow. Platelets obtained from blood draws can be used for therapeutic applications, especially platelet transfusion. The needs for platelet transfusions for clinical situation is increasing, due in part to the growing number of patients undergoing chemotherapy. Platelets obtained from donors, however, have the disadvantages of a limited storage lifespan and the risk of donor-related infection. Extensive effort has therefore been directed at manufacturing platelets ex vivo. Here, we review ex vivo technologies for MK development, focusing on human adipose tissue-derived mesenchymal stem/stromal cell line (ASCL)-based strategies and their potential clinical application. Bone marrow and adipose tissues contain mesenchymal stem/stromal cells that have an ability to differentiate into MKs, which release platelets. Taking advantage of this mechanism, we developed a donor-independent system for manufacturing platelets for clinical application using ASCL established from adipose-derived mesenchymal stem/stromal cells (ASCs). Culture of ASCs with endogenous thrombopoietin and its receptor c-MPL, and endogenous genes such as p45NF-E2 leads to MK differentiation and subsequent platelet production. ASCs compose heterogeneous cells, however, and are not suitable for clinical application. Thus, we established ASCLs, which expand into a more homogeneous population, and fulfill the criteria for mesenchymal stem cells set by the International Society for Cellular Therapy. Using our ASCL culture system with MK lineage induction medium without recombinant thrombopoietin led to peak production of platelets within 12 days, which may be sufficient for clinical application.

Generation of hematopoietic cells from mouse pluripotent stem cells in a 3D culture system of self-assembling peptide hydrogel

In vitro generation of hematopoietic stem cells from pluripotent stem cells (PSCs) can be regarded as novel therapeutic approaches for replacing bone marrow transplantation without immune rejection or graft versus host disease. To date, many different approaches have been evaluated in terms of directing PSCs toward different hematopoietic cell types, yet, low efficiency and no function restrict the further hematopoietic differentiation study, our research aims to develop a three dimension (3D) hematopoietic differentiation approach that serves as recapitulation of embryonic development in vitro to a degree of complexity not achievable in a two dimension culture system. We first found that mouse PSCs could be efficiently induced to hematopoietic differentiation with an expression of hematopoietic makers, such as c-kit, CD41, and CD45 within self-assembling peptide hydrogel. Colony-forming cells assay results suggested mouse PSCs (mPSCs) could be differentiated into multipotential progenitor cells and 3D induction system derived hematopoietic colonies owned potential of differentiating into lymphocyte cells. In addition, in vivo animal transplantation experiment showed that mPSCs (CD45.2) could be embedded into nonobese diabetic/severe combined immunodeficiency mice (CD45.1) with about 3% engraftment efficiency after 3 weeks transplantation. This study demonstrated that we developed the 3D induction approach that could efficiently promote the hematopoietic differentiation of mPSCs in vitro and obtained the multipotential progenitors that possessed the short-term engraftment potential.

In vitro differentiation of mouse T cell-derived hybrid cells obtained through cell fusion with embryonic stem cells

Nuclear reprogramming is an innovative advance in cell biology. An important research initiative in this field is cell fusion-mediated nuclear reprogramming, wherein the nuclei of somatic cells, such as thymocytes, are initialized through cell fusion with embryonic stem cells (ESCs). However, hybrid cells obtained through cell fusion between ESCs and thymocytes failed to contribute to the embryo proper when injected into blastocysts, which suggested that there are fundamental defects in such hybrid cells. Here, we performed side-by-side comparative analyses of the in vitro growth and differentiation capacities of ESCs and ESC-T hybrid cells. We found that the hybrid cells were larger and proliferated more slowly than the ESCs in 2i/LIF medium. Upon in vitro induction of differentiation, hybrid cells gave rise to cells of the three germ layers. Under culture conditions for hematopoietic differentiation, hybrid cells successively differentiated into lateral mesodermal cells, hemogenic endothelial cells, and various types of hematopoietic cells, including erythroid, myeloid, and lymphoid cells, although T cell maturation in the CD4/CD8 double-negative fraction was delayed. These results verified the multi-lineage differentiation capacity of ESC-T hybrid cells. The minimal contribution of hybrid cells to chimeric embryos may be due to their slow growth.

LIM homeobox 2 promotes interaction between human iPS-derived hepatic progenitors and iPS-derived hepatic stellate-like cells

Human induced pluripotent stem (iPS) cells can differentiate into hepatocyte lineages, although the phenotype of the differentiated cells is immature compared to adult hepatocytes. Improvement of cell-cell interactions between epithelium and mesenchyme is a potential approach to address this phenotype issue. In this study, we developed a model system for improving interactions between human iPS-derived hepatic progenitor cells (iPS-HPCs) and human iPS-derived hepatic stellate cell-like cells (iPS-HSCs). The phenotype of iPS-HSCs, including gene and protein expression profiles and vitamin A storage, resembled that of hepatic stellate cells. Direct co-culture of iPS-HSCs with iPS-HPCs significantly improved hepatocytic maturation in iPS-HPCs, such as their capacity for albumin production. Next, we generated iPS cell lines overexpressing LIM homeobox 2 (LHX2), which suppresses myofibroblastic changes in HSCs in mice. Hepatocytic maturation in iPS-HPCs was significantly increased in direct co-culture with iPS-HSCs overexpressing LHX2, but not in co-culture with a human hepatic stellate cell line (LX-2) overexpressing LHX2. LHX2 regulated the expression of extracellular matrices, such as laminin and collagen, in iPS-HSCs. In conclusion, this study provides an evidence that LHX2 upregulation in iPS-HSCs promotes hepatocytic maturation of iPS-HPCs, and indicates that genetically modified iPS-HSCs will be of value for research into cell-cell interactions.

Tyrosyl-tRNA synthetase stimulates thrombopoietin-independent hematopoiesis accelerating recovery from thrombocytopenia

Aminoacyl-tRNA synthetases (aaRSs) catalyze aminoacylation of tRNAs in the first step of protein synthesis in the cytoplasm. However, in higher eukaryotes, they acquired additional functions beyond translation. In the present study, we show that an activated form of tyrosyl-tRNA synthetase (YRS^ACT) functions to enhance megakaryopoiesis and platelet production in vitro and in vivo. These findings were confirmed with human megakaryocytes differentiated from peripheral blood CD34⁺ hematopoietic stem cells and with human induced pluripotent stem (iPS) cells. The activity of YRS^ACT is independent of thrombopoietin (TPO), as evidenced by expansion of the megakaryocytes from iPS cell-derived hematopoietic stem cells from a patient deficient in TPO signaling. These findings demonstrate a previously unrecognized function of an aaRS which may have implications for therapeutic interventions.

New mechanisms behind blood cell formation continue to be uncovered, with therapeutic approaches for hematological diseases being of great interest. Here we report an enzyme in protein synthesis, known for cell-based activities beyond translation, is a factor inducing megakaryocyte-biased hematopoiesis, most likely under stress conditions. We show an activated form of tyrosyl-tRNA synthetase (YRS^ACT), prepared either by rationally designed mutagenesis or alternative splicing, induces expansion of a previously unrecognized high-ploidy Sca-1⁺ megakaryocyte population capable of accelerating platelet replenishment after depletion. Moreover, YRS^ACT targets monocytic cells to induce secretion of transacting cytokines that enhance megakaryocyte expansion stimulating the Toll-like receptor/MyD88 pathway. Platelet replenishment by YRS^ACT is independent of thrombopoietin (TPO), as evidenced by expansion of the megakaryocytes from induced pluripotent stem cell-derived hematopoietic stem cells from a patient deficient in TPO signaling. We suggest megakaryocyte-biased hematopoiesis induced by YRS^ACT offers new approaches for treating thrombocytopenia, boosting yields from cell-culture production of platelet concentrates for transfusion, and bridging therapy for hematopoietic stem cell transplantation.

Domain-specific biological functions of the transcription factor Gata2 on hematopoietic differentiation of mouse embryonic stem cells

The generation of mouse hematopoietic stem cells from hemogenic endothelial cells (HECs) in the aorta/gonad/mesonephros region of developing embryos requires a zinc finger transcription factor Gata2. In the previous study, an enforced expression of Gata2 in vitro promoted the production of HECs from mesodermal cells differentiated from mouse embryonic stem cells (ESCs). Our research group has previously demonstrated that the enforced expression of Gata2 in ESC-derived HECs enhances erythroid and megakaryocyte differentiation and inhibits macrophage differentiation. However, the manner in which the multiple functions of Gata2 are regulated remains unclear. Mouse ESCs differentiate into various types of hematopoietic cells when cocultured with OP9 stromal cells (OP9 system). Using this system and the inducible gene cassette exchange system, which facilitates the establishment of ESCs carrying inducible transgenes under an identical gene expression regulatory unit, the domain-specific functions of Gata2 were systematically dissected in this study. We determined that the N-terminal (amino acid 1-110) region of Gata2 was an erythroid-inducing region, both the middle (amino acid 111-200) and C-terminal (amino acid 413-480) regions were megakaryocyte-inducing regions. Furthermore, the present data strongly suggest that intramolecular antagonistic interactions between each of these regions fine-tune the biological functions of Gata2.

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.

Identification of novel genes and networks governing hematopoietic stem cell development

Hematopoietic stem cells (HSCs) are capable of giving rise to all blood cell lineages throughout adulthood, and the generation of engraftable HSCs from human pluripotent stem cells is a major goal for regenerative medicine. Here, we describe a functional genome‐wide RNAi screen to identify genes required for the differentiation of embryonic stem cell (ESC) into hematopoietic stem/progenitor cells (HSPCs) in vitro. We report the discovery of novel genes important for the endothelial‐to‐hematopoietic transition and subsequently for HSPC specification. High‐throughput sequencing and bioinformatic analyses identified twelve groups of genes, including a set of 351 novel genes required for HSPC specification. As in vivo proof of concept, four of these genes, Ap2a1, Mettl22, Lrsam1, and Hal, are selected for validation, confirmed to be essential for HSPC development in zebrafish and for maintenance of human HSCs. Taken together, our results not only identify a number of novel regulatory genes and pathways essential for HSPC development but also serve as valuable resource for directed differentiation of therapy grade HSPCs using human pluripotent stem cells.

Zbtb1 Safeguards Genome Integrity and Prevents p53-Mediated Apoptosis in Proliferating Lymphoid Progenitors

Expression of the transcription factor Zbtb1 is required for normal lymphoid development. We report in the present study that Zbtb1 maintains genome integrity in immune progenitors, without which cells undergo increased DNA damage and p53-mediated apoptosis during replication and differentiation. Increased DNA damage in Zbtb1-mutant (ScanT) progenitors was due to increased sensitivity to replication stress, which was a consequence of inefficient activation of the S-phase checkpoint response. Increased p53-mediated apoptosis affected not only lymphoid but also myeloid development in competitive bone marrow chimeras, and prevention of apoptosis by transgenic Bcl2 expression and p53 deficiency rescued lymphoid as well as myeloid development from Zbtb1-mutant progenitors. Interestingly, however, protection from apoptosis rescued only the early stages of T cell development, and thymocytes remained arrested at the double-negative 3 developmental stage, indicating a strict requirement of Zbtb1 at later T cell developmental stages. Collectively, these results indicate that Zbtb1 prevents DNA damage in replicating immune progenitors, allowing the generation of B cells, T cells, and myeloid cells.

OP9-Lhx2 stromal cells facilitate derivation of hematopoietic progenitors both in vitro and in vivo

Generating engraftable hematopoietic stem cells (HSCs) from pluripotent stem cells (PSCs) is an ideal approach for obtaining induced HSCs for cell therapy. However, the path from PSCs to robustly induced HSCs (iHSCs) in vitro remains elusive. We hypothesize that the modification of hematopoietic niche cells by transcription factors facilitates the derivation of induced HSCs from PSCs. The Lhx2 transcription factor is expressed in fetal liver stromal cells but not in fetal blood cells. Knocking out Lhx2 leads to a fetal hematopoietic defect in a cell non-autonomous role. In this study, we demonstrate that the ectopic expression of Lhx2 in OP9 cells (OP9-Lhx2) accelerates the hematopoietic differentiation of PSCs. OP9-Lhx2 significantly increased the yields of hematopoietic progenitor cells via co-culture with PSCs in vitro. Interestingly, the co-injection of OP9-Lhx2 and PSCs into immune deficient mice also increased the proportion of hematopoietic progenitors via the formation of teratomas. The transplantation of phenotypic HSCs from OP9-Lhx2 teratomas but not from the OP9 control supported a transient repopulating capability. The upregulation of Apln gene by Lhx2 is correlated to the hematopoietic commitment property of OP9-Lhx2. Furthermore, the enforced expression of Apln in OP9 cells significantly increased the hematopoietic differentiation of PSCs. These results indicate that OP9-Lhx2 is a good cell line for regeneration of hematopoietic progenitors both in vitro and in vivo.

Efficient production of T cells from mouse pluripotent stem cells by controlled expression of Lhx2

LIM-homeobox transcription factor Lhx2 induces ex vivo amplification of adult hematopoietic stem cells (HSCs) in mice. We previously showed that engraftable HSC-like cells are generated from mouse embryonic stem cells (ESCs) and induced pluripotent stem cells by enforced expression of Lhx2. However, when these HSC-like cells were transplanted into irradiated congenic mice, donor-derived T cells were barely detectable, whereas other lineages of hematopoietic cells were continuously produced. Here we investigated T-cell differentiation potential of the Lhx2-induced HSC-like cells using ESCs carrying doxycycline (dox)-inducible Lhx2 expression cassette. Dox-mediated over-expression of Lhx2 conferred a self-renewing activity to ESC-derived c-Kit(+) CD41(+) embryonic hematopoietic progenitor cells (HPCs), thereby converting them to HSC-like cells. When these HSC-like cells were transplanted into irradiated immunodeficient mice and they were supplied with a dox-containing water, CD4/8 double negative T cells were detected in their thymi. Once the Lhx2 expression was terminated, differentiation of CD4/8 double positive and single positive T cells was initiated in the thymi of transplanted mice and mature T cells were released in the peripheral blood. These results showed that engraftable HSC-like cells with full hematopoietic potential can be obtained from ESCs by the conditional expression of Lhx2.

In vivo repopulating activity emerges at the onset of hematopoietic specification during embryonic stem cell differentiation

The generation of in vivo repopulating hematopoietic cells from in vitro differentiating embryonic stem cells has remained a long-standing challenge. To date, hematopoietic engraftment has mostly been achieved through the enforced expression of ectopic transcription factors. Here, we describe serum-free culture conditions that allow the generation of in vivo repopulating hematopoietic cells in the absence of ectopically expressed factors. We show that repopulating activity arises immediately upon the commitment of mesodermal precursors to the blood program, within the first wave of hematopoietic specification. We establish that the formation of these progenitors is extremely transient and exquisitely sensitive to the cytokine milieu. Our findings define the precise differentiating stage at which hematopoietic repopulating activity first appears in vitro, and suggest that during embryonic stem cell differentiation, all hematopoietic programs are unraveled simultaneously from the mesoderm in the absence of cues that restrict the coordinated emergence of each lineage as is normally observed during embryogenesis.

Cell therapies for treatment of human immunodeficiency virus infection

After the serendipitous discovery of HIV eradication in the "Berlin patient", interest has grown in curing HIV infection by replacing the patient's replication-competent blood cells with infection-resistant ones. At the same time, induced pluripotent stem cell technologies and genetic engineering have boosted cell therapy transfer into the clinic. Currently available cell therapy approaches to attempt to cure HIV infection include the following: (1) Transplantation of autologous or allogeneic cells spontaneously resistant or edited to resist HIV infection; (2) Transplantation of autologous T-lymphocytes spontaneously targeting or redirected against HIV; and (3) Transplantation of autologous cells engineered to work as anti-HIV antibody factories. We review here the preliminary results and potential for future applications of these approaches.

Progress and challenges in generating functional hematopoietic stem/progenitor cells from human pluripotent stem cells

The generation of hematopoietic stem cells (HSCs) from human pluripotent stem cells (hPSCs) in vitro holds great potential for providing alternative sources of donor cells for clinical HSC transplantation. However, the low efficiency of current protocols for generating blood lineages and the dysfunction identified in hPSC-derived hematopoietic cells limit their use for full hematopoietic reconstitution in clinics. This review outlines the current understanding of in vitro hematopoietic differentiation from hPSCs, emphasizes the intrinsic and extrinsic molecular mechanisms that are attributed to the aberrant phenotype and function in hPSC-derived hematopoietic cells, pinpoints the current challenges to develop the truly functional HSCs from hPSCs for clinical applications and explores their potential solutions.

LIM homeobox transcription factor Lhx2 inhibits skeletal muscle differentiation in part via transcriptional activation of Msx1 and Msx2

LIM homeobox transcription factor Lhx2 is known to be an important regulator of neuronal development, homeostasis of hair follicle stem cells, and self-renewal of hematopoietic stem cells; however, its function in skeletal muscle development is poorly understood. In this study, we found that overexpression of Lhx2 completely inhibits the myotube-forming capacity of C2C12 cells and primary myoblasts. The muscle dedifferentiation factors Msx1 and Msx2 were strongly induced by the Lhx2 overexpression. Short interfering RNA-mediated knockdown of Lhx2 in the developing limb buds of mouse embryos resulted in a reduction in Msx1 and Msx2 mRNA levels, suggesting that they are downstream target genes of Lhx2. We found two Lhx2 consensus-binding sites in the -2097 to -1189 genomic region of Msx1 and two additional sites in the -536 to +73 genomic region of Msx2. These sequences were shown by luciferase reporter assay to be essential for Lhx2-mediated transcriptional activation. Moreover, electrophoretic mobility shift assays and chromatin immunoprecipitation assays showed that Lhx2 is present in chromatin DNA complexes bound to the enhancer regions of the Msx1 and Msx2 genes. These data demonstrate that Msx1 and Msx2 are direct transcriptional targets of Lhx2. In addition, overexpression of Lhx2 significantly enhanced the mRNA levels of bone morphogenetic protein 4 and transforming growth factor beta family genes. We propose that Lhx2 is involved in the early stage of skeletal muscle development by inducing multiple differentiation inhibitory factors.

A novel system for simultaneous or sequential integration of multiple gene-loading vectors into a defined site of a human artificial chromosome

Human artificial chromosomes (HACs) are gene-delivery vectors suitable for introducing large DNA fragments into mammalian cells. Although a HAC theoretically incorporates multiple gene expression cassettes of unlimited DNA size, its application has been limited because the conventional gene-loading system accepts only one gene-loading vector (GLV) into a HAC. We report a novel method for the simultaneous or sequential integration of multiple GLVs into a HAC vector (designated as the SIM system) via combined usage of Cre, FLP, Bxb1, and φC31 recombinase/integrase. As a proof of principle, we first attempted simultaneous integration of three GLVs encoding EGFP, Venus, and TdTomato into a gene-loading site of a HAC in CHO cells. These cells successfully expressed all three fluorescent proteins. Furthermore, microcell-mediated transfer of HACs enabled the expression of those fluorescent proteins in recipient cells. We next demonstrated that GLVs could be introduced into a HAC one-by-one via reciprocal usage of recombinase/integrase. Lastly, we introduced a fourth GLV into a HAC after simultaneous integration of three GLVs by FLP-mediated DNA recombination. The SIM system expands the applicability of HAC vectors and is useful for various biomedical studies, including cell reprogramming.

Generation of iPSCs from genetically corrected Brca2 hypomorphic cells: implications in cell reprogramming and stem cell therapy

Fanconi anemia (FA) is a complex genetic disease associated with a defective DNA repair pathway known as the FA pathway. In contrast to many other FA proteins, BRCA2 participates downstream in this pathway and has a critical role in homology-directed recombination (HDR). In our current studies, we have observed an extremely low reprogramming efficiency in cells with a hypomorphic mutation in Brca2 (Brca2(Δ) (27/) (Δ27)), that was associated with increased apoptosis and defective generation of nuclear RAD51 foci during the reprogramming process. Gene complementation facilitated the generation of Brca2(Δ) (27/) (Δ27) induced pluripotent stem cells (iPSCs) with a disease-free FA phenotype. Karyotype analyses and comparative genome hybridization arrays of complemented Brca2(Δ) (27/) (Δ27) iPSCs showed, however, the presence of different genetic alterations in these cells, most of which were not evident in their parental Brca2(Δ) (27/) (Δ27) mouse embryonic fibroblasts. Gene-corrected Brca2(Δ) (27/) (Δ27) iPSCs could be differentiated in vitro toward the hematopoietic lineage, although with a more limited efficacy than WT iPSCs or mouse embryonic stem cells, and did not engraft in irradiated Brca2(Δ) (27/) (Δ27) recipients. Our results are consistent with previous studies proposing that HDR is critical for cell reprogramming and demonstrate that reprogramming defects characteristic of Brca2 mutant cells can be efficiently overcome by gene complementation. Finally, based on analysis of the phenotype, genetic stability, and hematopoietic differentiation potential of gene-corrected Brca2(Δ) (27/) (Δ) (27) iPSCs, achievements and limitations in the application of current reprogramming approaches in hematopoietic stem cell therapy are also discussed.

Ncor2 is required for hematopoietic stem cell emergence by inhibiting Fos signaling in zebrafish

Nuclear receptor corepressors (Ncors) are important for developmental and homeostatic processes in vertebrates, which exert transcriptional repression by coordinating with histone deacetylases. However, little is known about their roles in definitive hematopoiesis. In this study, we show that in zebrafish, ncor2 is required for hematopoietic stem cell (HSC) development by repressing fos-vegfd signaling. ncor2 is specifically expressed in the aorta-gonad-mesonephros (AGM) region in zebrafish embryos. ncor2 deficiency reduced the population of HSCs in both the AGM region and T cells in the thymus. Mechanistically, ncor2 knockdown upregulated fos transcription by modulating the acetylation level in the fos promoter region, which then enhanced Vegfd signaling. Consequently, the augmented Vegfd signaling induced Notch signaling to promote the arterial endothelial fate, therefore, possibly repressing the hemogenic endothelial specification, which is a prerequisite for HSC emergence. Thus, our findings identify a novel regulatory mechanism for Ncor2 through Fos-Vegfd-Notch signaling cascade during HSC development in zebrafish embryos.

Induced pluripotent stem cells in hematology: current and future applications

Reprogramming somatic cells into induced pluripotent stem (iPS) cells is nowadays approaching effectiveness and clinical grade. Potential uses of this technology include predictive toxicology, drug screening, pathogenetic studies and transplantation. Here, we review the basis of current iPS cell technology and potential applications in hematology, ranging from disease modeling of congenital and acquired hemopathies to hematopoietic stem and other blood cell transplantation.

Architectural niche organization by LHX2 is linked to hair follicle stem cell function

In adult skin, self-renewing, undifferentiated hair follicle stem cells (HF-SCs) reside within a specialized niche, where they spend prolonged times as a single layer of polarized, quiescent epithelial cells. When sufficient activating signals accumulate, HF-SCs become mobilized to fuel tissue regeneration and hair growth. Here, we show that architectural organization of the HF-SC niche by transcription factor LHX2 plays a critical role in HF-SC behavior. Using genome-wide chromatin and transcriptional profiling of HF-SCs in vivo, we show that LHX2 directly transactivates genes that orchestrate cytoskeletal dynamics and adhesion. Conditional ablation of LHX2 results in gross cellular disorganization and HF-SC polarization within the niche. LHX2 loss leads to a failure to maintain HF-SC quiescence and hair anchoring, as well as progressive transformation of the niche into a sebaceous gland. These findings suggest that niche organization underlies the requirement for LHX2 in hair follicle structure and function.

Lhx2 balances progenitor maintenance with neurogenic output and promotes competence state progression in the developing retina

The LIM-Homeodomain transcription factor Lhx2 is an essential organizer of early eye development and is subsequently expressed in retinal progenitor cells (RPCs). To determine its requirement in RPCs, we performed a temporal series of conditional inactivations in mice with the early RPC driver Pax6 α-Cre and the tamoxifen-inducible Hes1(CreERT2) driver. Deletion of Lhx2 caused a significant reduction of the progenitor population and a corresponding increase in neurogenesis. Precursor fate choice correlated with the time of inactivation; early and late inactivation led to the overproduction of retinal ganglion cells (RGCs) and rod photoreceptors, respectively. In each case, however, the overproduction was selective, occurring at the expense of other cell types and indicating a role for Lhx2 in generating cell type diversity. RPCs that persisted in the absence of Lhx2 continued to generate RGC precursors beyond their normal production window, suggesting that Lhx2 facilitates a transition in competence state. These results identify Lhx2 as a key regulator of RPC properties that contribute to the ordered production of multiple cell types during retinal tissue formation.

Closing the door to human immunodeficiency virus

The pandemic of human immunodeficiency virus type one (HIV-1), the major etiologic agent of acquired immunodeficiency disease (AIDS), has led to over 33 million people living with the virus, among which 18 million are women and children. Until now, there is neither an effective vaccine nor a therapeutic cure despite over 30 years of efforts. Although the Thai RV144 vaccine trial has demonstrated an efficacy of 31.2%, an effective vaccine will likely rely on a breakthrough discovery of immunogens to elicit broadly reactive neutralizing antibodies, which may take years to achieve. Therefore, there is an urgency of exploring other prophylactic strategies. Recently, antiretroviral treatment as prevention is an exciting area of progress in HIV-1 research. Although effective, the implementation of such strategy faces great financial, political and social challenges in heavily affected regions such as developing countries where drug resistant viruses have already been found with growing incidence. Activating latently infected cells for therapeutic cure is another area of challenge. Since it is greatly difficult to eradicate HIV-1 after the establishment of viral latency, it is necessary to investigate strategies that may close the door to HIV-1. Here, we review studies on non-vaccine strategies in targeting viral entry, which may have critical implications for HIV-1 prevention.

OP9 bone marrow stroma cells differentiate into megakaryocytes and platelets

Platelets are essential for hemostatic plug formation and thrombosis. The mechanisms of megakaryocyte (MK) differentiation and subsequent platelet production from stem cells remain only partially understood. The manufacture of megakaryocytes (MKs) and platelets from cell sources including hematopoietic stem cells and pluripotent stem cells have been highlighted for studying the platelet production mechanisms as well as for the development of new strategies for platelet transfusion. The mouse bone marrow stroma cell line OP9 has been widely used as feeder cells for the differentiation of stem cells into MK lineages. OP9 cells are reported to be pre-adipocytes. We previously reported that 3T3-L1 pre-adipocytes differentiated into MKs and platelets. In the present study, we examined whether OP9 cells differentiate into MKs and platelets using MK lineage induction (MKLI) medium previously established to generate MKs and platelets from hematopoietic stem cells, embryonic stem cells, and pre-adipocytes. OP9 cells cultured in MKLI medium had megakaryocytic features, i.e., positivity for surface markers CD41 and CD42b, polyploidy, and distinct morphology. The OP9-derived platelets had functional characteristics, providing the first evidence for the differentiation of OP9 cells into MKs and platelets. We then analyzed gene expressions of critical factors that regulate megakaryopoiesis and thrombopoiesis. The gene expressions of p45NF-E2, FOG, Fli1, GATA2, RUNX1, thrombopoietin, and c-mpl were observed during the MK differentiation. Among the observed transcription factors of MK lineages, p45NF-E2 expression was increased during differentiation. We further studied MK and platelet generation using p45NF-E2-overexpressing OP9 cells. OP9 cells transfected with p45NF-E2 had enhanced production of MKs and platelets. Our findings revealed that OP9 cells differentiated into MKs and platelets in vitro. OP9 cells have critical factors for megakaryopoiesis and thrombopoiesis, which might be involved in a mechanism of this differentiation. p45NF-E2 might also play important roles in the differentiation of OP9 cells into MK lineages cells.

Embryonic development of hematopoietic stem cells: implications for clinical use

Hematopoietic stem cell (HSC) transplantation is an important treatment modality for hematological malignancies or to correct congenital immunodeficiency disorders. Several stem cell sources are currently applied clinically, with a recent increased application of umbilical cord blood. The low number of HSCs available, particularly in umbilical cord blood, is a limiting factor, and different lines of research are ongoing to circumvent this issue. In this review, we will describe the research strategies developed to expand adult HSCs in vitro and to generate new HSCs from pluripotent stem cell lines. We will also discuss the importance of studying the embryonic microenvironment since it allows both generation and extensive expansion of HSCs. Understanding the mechanisms that underlie HSC production, self-renewal and differentiation is necessary for the establishment of optimal in vitro HSC cultures, where a limitless and manipulatable resource of HSCs would be available for both clinical and fundamental research.

Generation of red blood cells from human embryonic/induced pluripotent stem cells for blood transfusion

Red blood cell (RBC) transfusion is necessary for many patients with emergency or hematological disorders. However, to date the supply of RBCs remains labile and dependent on voluntary donations. In addition, the transmission of infectious disease via blood transfusion from unspecified donors remains a risk. Establishing a large quantity of safe RBCs would help to address this issue. Human embryonic stem (hES) cells and the recently established human induced pluripotent stem (hiPS) cells represent potentially unlimited sources of donor-free RBCs for blood transfusion, as they can proliferate indefinitely in vitro. Extensive research has been done to efficiently generate transfusable RBCs from hES/iPS cells. Nevertheless, a number of challenges must be overcome before the clinical usage of hES/iPS cell-derived RBCs can become a reality.

Modeling human hematopoietic cell development from pluripotent stem cells

Understanding the steps and cues that allow hematopoietic cells to be generated during development holds great clinical as well as biological interest. Analysis of these events in mice has provided many important insights into the processes involved, but features that might be unique to humans remain challenging to elucidate because they cannot be studied directly in vivo. Human embryonic stem or induced pluripotent stem cells offer attractive in vitro alternatives to analyze the process. Here we review recent efforts to develop defined and quantitative systems to address outstanding developmental questions against a background of what we know about the development of hematopoietic cells in the fetus and derived from mouse embryonic stem cells.