Hematoendothelial differentiation of human embryonic stem cells

Proteomics of novel induced pluripotent stem cell-derived vascular endothelial cells reveal extensive similarity with an immortalized human endothelial cell line

The vascular endothelium constitutes the inner lining of the blood vessel, and malfunction and injuries of the endothelium can cause cardiovascular diseases as well as other diseases including stroke, tumor growth, and chronic kidney failure. Generation of effective sources to replace injured endothelial cells (ECs) could have significant clinical impact, and somatic cell sources like peripheral or cord blood cannot credibly supply enough endothelial cell progenitors for multitude of treatments. Pluripotent stem cells are a promising source for a reliable EC supply, which have the potential to restore tissue function and treat vascular diseases. We have developed methods to differentiate induced pluripotent stem cells (iPSCs) efficiently and robustly across multiple iPSC lines into nontissue-specific pan vascular ECs (iECs) with high purity. These iECs present with canonical endothelial cell markers and exhibit measures of endothelial cell functionality with the uptake of Dil fluorescent dye-labeled acetylated low-density lipoprotein (Dil-Ac-LDL) and tube formation. Using proteomic analysis, we revealed that the iECs are more proteomically similar to established human umbilical vein ECs (HUVECs) than to iPSCs. Posttranslational modifications (PTMs) were most shared between HUVECs and iECs, and potential targets for increasing the proteomic similarity of iECs to HUVECs were identified. Here we demonstrate an efficient robust method to differentiate iPSCs into functional ECs, and for the first time provide a comprehensive protein expression profile of iECs, which indicates their similarities with a widely used immortalized HUVECs, allowing for further mechanistic studies of EC development, signaling, and metabolism for future regenerative applications.NEW & NOTEWORTHY We have developed methods to differentiate induced pluripotent stem cells (iPSCs) across multiple iPSC lines into nontissue-specific pan vascular ECs (iECs) and demonstrated the proteomic similarity of these cells to a widely used endothelial cell line (HUVECs). We also identified posttranslational modifications and targets for increasing the proteomic similarity of iECs to HUVECs. In the future, iECs can be used to study EC development, signaling, and metabolism for future regenerative applications.

Erythroid Differentiation of Induced Pluripotent Stem Cells Co-cultured with OP9 Cells for Diagnostic Purposes

Background

Reagent red blood cells (RBCs) are prepared from donated whole blood, resulting in various combinations of blood group antigens. This inconsistency can be resolved by producing RBCs with uniform antigen expression. Induced pluripotent stem cells (iPSCs) generated directly from mature cells constitute an unlimited source for RBC production. We aimed to produce erythroid cells from iPSCs for diagnostic purposes. We hypothesized that cultured erythroid cells express surface antigens that can be recognized by blood group antibodies.

Methods

iPSCs were co-cultured with OP9 stromal cells to stimulate differentiation into the erythroid lineage. Cell differentiation was examined using microscopy and flow cytometry. Hemoglobin electrophoresis and oxygen-binding capacity testing were performed to verify that the cultured erythroid cells functioned normally. The agglutination reactions of the cultured erythroid cells to antibodies were investigated to confirm that the cells expressed blood group antigens.

Results

The generated iPSCs showed stemness characteristics and could differentiate into the erythroid lineage. As differentiation progressed, the proportion of nucleated RBCs increased. Hemoglobin electrophoresis revealed a sharp peak in the hemoglobin F region. The oxygen-binding capacity test results were similar between normal RBCs and cultured nucleated RBCs. ABO and Rh-Hr blood grouping confirmed similar antigen expression between the donor RBCs and cultured nucleated RBCs.

Conclusions

We generated blood group antigen-expressing nucleated RBCs from iPSCs co-cultured with OP9 cells that can be used for diagnostic purposes. iPSCs from rare blood group donors could serve as an unlimited source for reagent production.

Wiskott-Aldrich syndrome protein forms nuclear condensates and regulates alternative splicing

The diverse functions of WASP, the deficiency of which causes Wiskott-Aldrich syndrome (WAS), remain poorly defined. We generated three isogenic WAS models using patient induced pluripotent stem cells and genome editing. These models recapitulated WAS phenotypes and revealed that WASP deficiency causes an upregulation of numerous RNA splicing factors and widespread altered splicing. Loss of WASP binding to splicing factor gene promoters frequently leads to aberrant epigenetic activation. WASP interacts with dozens of nuclear speckle constituents and constrains SRSF2 mobility. Using an optogenetic system, we showed that WASP forms phase-separated condensates that encompasses SRSF2, nascent RNA and active Pol II. The role of WASP in gene body condensates is corroborated by ChIPseq and RIPseq. Together our data reveal that WASP is a nexus regulator of RNA splicing that controls the transcription of splicing factors epigenetically and the dynamics of the splicing machinery through liquid-liquid phase separation.

Role of TGFβ1 and WNT6 in FGF2 and BMP4-driven endothelial differentiation of murine embryonic stem cells

Embryonic stem cells (ES) are a valuable source of endothelial cells. By co-culturing ES cells with the stromal PA6 cells, the endothelial commitment can be achieved by adding exogenous FGF2 or BMP4. In this work, the molecular pathways that direct the differentiation of ES cells toward endothelium in response to FGF2 are evaluated and compared to those activated by BMP4. To this purpose the genes expression profiles of both ES/PA6 co-cultures and of pure cultures of PA6 cells were obtained by microarray technique at different time points. The bioinformatics processing of the data indicated TGFβ1 as the most represented upstream regulator in FGF2-induced endothelial commitment while WNT pathway as the most represented in BMP4-activated endothelial differentiation. Loss of function experiments were performed to validate the importance of TGFβ1 and WNT6 respectively in FGF2 and BMP4-induced endothelial differentiation. The loss of TGFβ1 expression significantly impaired the accomplishment of the endothelial commitment unless exogenous recombinant TGFβ1 was added to the culture medium. Similarly, silencing WNT6 expression partially affected the endothelial differentiation of the ES cells upon BMP4 stimulation. Such dysfunction was recovered by the addition of recombinant WNT6 to the culture medium. The ES/PA6 co-culture system recreates an in vitro complete microenvironment in which endothelial commitment is accomplished in response to alternative signals through different mechanisms. Given the importance of WNT and TGFβ1 in mediating the crosstalk between tumor and stromal cells this work adds new insights in the mechanism of tumor angiogenesis and of its possible inhibition.

Down syndrome iPSC model: endothelial perspective on tumor development

Trisomy 21 (T21), known as Down syndrome (DS), is a widely studied chromosomal abnormality. Previous studies have shown that DS individuals have a unique cancer profile. While exhibiting low solid tumor prevalence, DS patients are at risk for hematologic cancers, such as acute megakaryocytic leukemia and acute lymphoblastic leukemia. We speculated that endothelial cells are active players in this clinical background. To this end, we hypothesized that impaired DS endothelial development and functionality, impacted by genome-wide T21 alterations, potentially results in a suboptimal endothelial microenvironment with the capability to prevent solid tumor growth.

To test this hypothesis, we assessed molecular and phenotypic differences of endothelial cells differentiated from Down syndrome and euploid iPS cells. Microarray, RNA-Seq, and bioinformatic analyses revealed that most significantly expressed genes belong to angiogenic, cytoskeletal rearrangement, extracellular matrix remodeling, and inflammatory pathways. Interestingly, the majority of these genes are not located on Chromosome 21. To substantiate these findings, we carried out functional assays. The obtained phenotypic results correlated with the molecular data and showed that Down syndrome endothelial cells exhibit decreased proliferation, reduced migration, and a weak TNF-α inflammatory response. Based on this data, we provide a set of genes potentially associated with Down syndrome’s elevated leukemic incidence and its unfavorable solid tumor microenvironment—highlighting the potential use of these genes as therapeutic targets in translational cancer research.

Generation of T cells from Human and Nonhuman Primate Pluripotent Stem Cells

Pluripotent stem cells (PSCs) have the potential to provide homogeneous cell populations of T cells that can be grown at a clinical scale and genetically engineered to meet specific clinical needs. OP9-DLL4, a stromal line ectopically expressing the Notch ligand Delta-like 4 (DLL4) is used to support differentiation of PSCs to T-lymphocytes. This article outlines several protocols related to generation of T cells from human and non-human primate (NHP) PSCs, including initial hematopoietic differentiation of PSC on OP9 feeders or defined conditions, followed by coculture of the OP9-DLL4 cells with the PSC-derived hematopoietic progenitors (HPs), leading to efficient differentiation to T lymphocytes. In addition, we describe a protocol for robust T cell generation from hPSCs conditionally expressing ETS1. The presented protocols provide a platform for T cell production for disease modeling and evaluating their use for immunotherapy in large animal models.

NOX4 is the main NADPH oxidase involved in the early stages of hematopoietic differentiation from human induced pluripotent stem cells

Reactive oxygen species (ROS) produced in hematopoietic stem cells (HSCs) are involved in the balance between quiescence, self-renewal, proliferation and differentiation processes. However the role of NOX enzymes on the early stages of hematopoietic differentiation is poorly investigated. For that, we used induced pluripotent stem cells (iPSCs) derived from X-linked Chronic Granulomatous Disease (X⁰CGD) patients with deficiency in NOX2, and AR22⁰CGD patients with deficiency in p22^phox subunit which decreases NOX1, NOX2, NOX3 and NOX4 activities. CD34⁺ hematopoietic progenitors were obtained after 7, 10 and 13 days of iPS/OP9 co-culture differentiation system. Neither NOX expression nor activity was found in Wild-type (WT), X⁰CGD and AR22⁰CGD iPSCs. Although NOX2 and NOX4 mRNA were found in WT, X⁰CGD and AR22⁰CGD iPSC-derived CD34⁺ cells at day 10 and 13 of differentiation, NOX4 protein was the only NOX enzyme expressed in these cells. A NADPH oxidase activity was measured in WT and X⁰CGD iPSC-derived CD34⁺ cells but not in AR22⁰CGD iPSC-derived CD34⁺ cells because of the absence of p22^phox, which is essential for the NOX4 activity. The absence of NOX4 activity and the poor NOX-independent ROS production in AR22⁰CGD iPSC-derived CD34⁺ cells favored the CD34⁺ cells production but lowered their hematopoietic potential compared to WT and X⁰CGD iPSC-derived CD34⁺ cells. In addition we found a large production of primitive AR22⁰CGD iPSC-derived progenitors at day 7 compared to the WT and X⁰CGD cell types. In conclusion NOX4 is the major NOX enzyme involved in the early stages of hematopoietic differentiation from iPSCs and its activity can modulate the production, the hematopoietic potential and the phenotype of iPSC-derived CD34⁺.

Strategies for derivation of endothelial lineages from human stem cells

Accumulating evidence demonstrates that pre-vascularization of tissue-engineered constructs can significantly enhance their survival and engraftment upon transplantation. Endothelial cells (ECs), the basic component of vasculatures, are indispensable to the entire process of pre-vascularization. However, the source of ECs still poses an issue. Recent studies confirmed that diverse approaches are available in the derivation of ECs for tissue engineering, such as direct isolation of autologous ECs, reprogramming of somatic cells, and induced differentiation of stem cells in typology. Herein, we discussed a variety of human stem cells (i.e., totipotent, pluripotent, multipotent, oligopotent, and unipotent stem cells), which can be induced to differentiate into ECs and reviewed the multifarious approaches for EC generation, such as 3D EB formation for embryonic stem cells (ESCs), stem cell-somatic cell co-culture, and directed endothelial differentiation with growth factors in conventional 2D culture.

Enhanced hemato-endothelial specification during human embryonic differentiation through developmental cooperation between AF4-MLL and MLL-AF4 fusions

The t(4;11)(q21;q23) translocation is associated with high-risk infant pro-B-cell acute lymphoblastic leukemia and arises prenatally during embryonic/fetal hematopoiesis. The developmental/pathogenic contribution of the t(4;11)-resulting MLL-AF4 (MA4) and AF4-MLL (A4M) fusions remains unclear; MA4 is always expressed in patients with t(4;11)+ B-cell acute lymphoblastic leukemia, but the reciprocal fusion A4M is expressed in only half of the patients. Because prenatal leukemogenesis manifests as impaired early hematopoietic differentiation, we took advantage of well-established human embryonic stem cell-based hematopoietic differentiation models to study whether the A4M fusion cooperates with MA4 during early human hematopoietic development. Co-expression of A4M and MA4 strongly promoted the emergence of hemato-endothelial precursors, both endothelial- and hemogenic-primed. Double fusion-expressing hemato-endothelial precursors specified into significantly higher numbers of both hematopoietic and endothelial-committed cells, irrespective of the differentiation protocol used and without hijacking survival/proliferation. Functional analysis of differentially expressed genes and differentially enriched H3K79me3 genomic regions by RNA-sequencing and H3K79me3 chromatin immunoprecipitation-sequencing, respectively, confirmed a hematopoietic/endothelial cell differentiation signature in double fusion-expressing hemato-endothelial precursors. Importantly, chromatin immunoprecipitation-sequencing analysis revealed a significant enrichment of H3K79 methylated regions specifically associated with HOX-A cluster genes in double fusion-expressing differentiating hematopoietic cells. Overall, these results establish a functional and molecular cooperation between MA4 and A4M fusions during human hematopoietic development.

Generation of Endothelial Cells From Human Pluripotent Stem Cells

Endothelial cells (ECs) are critical for several aspects of cardiovascular disease therapy, including vascular regeneration, personalized drug development, and tissue engineering. Human pluripotent stem cells (hPSCs) afford us with an unprecedented opportunity to produce virtually unlimited quantities of human ECs. In this review, we highlight key developments and outstanding challenges in our ability to derive ECs de novo from hPSCs. Furthermore, we consider strategies for recapitulating the vessel- and tissue-specific functional heterogeneity of ECs in vitro. Finally, we discuss ongoing attempts to utilize hPSC-derived ECs and their progenitors for various therapeutic applications. Continued progress in generating hPSC-derived ECs will profoundly enhance our ability to discover novel drug targets, revascularize ischemic tissues, and engineer clinically relevant tissue constructs. Visual Overview- An online visual overview is available for this article.

NOTCH Activation at the Hematovascular Mesoderm Stage Facilitates Efficient Generation of T Cells with High Proliferation Potential from Human Pluripotent Stem Cells

Human pluripotent stem cells (hPSCs) offer the potential to serve as a versatile and scalable source of T cells for immunotherapies, which could be coupled with genetic engineering technologies to meet specific clinical needs. To improve T cell production from hPSCs, it is essential to identify cell subsets that are highly enriched in T cell progenitors and those stages of development at which NOTCH activation induces the most potent T cells. In this study, we evaluated the efficacy of T cell production from cell populations isolated at different stages of hematopoietic differentiation, including mesoderm, hemogenic endothelium (HE), and multipotent hematopoietic progenitors. We demonstrate that KDR^hiCD31^- hematovascular mesodermal progenitors (HVMPs) with definitive hematopoietic potential produce the highest numbers of T cells when cultured on OP9-DLL4 as compared with downstream progenitors, including HE and multipotent hematopoietic progenitors. In addition, we found that T cells generated from HVMPs have the capacity to expand for 6-7 wk in vitro, in comparison with T cells generated from HE and hematopoietic progenitors, which could only be expanded for 4-5 wk. Demonstrating the critical need of NOTCH activation at the HVMP stage of hematopoietic development to establish robust T cell production from hPSCs may aid in establishing protocols for the efficient off-the-shelf production and expansion of T cells for treating hematologic malignancies.

It Takes Two: Endothelial-Perivascular Cell Cross-Talk in Vascular Development and Disease

The formation of new blood vessels is a crucial step in the development of any new tissue both during embryogenesis and in vitro models as without sufficient perfusion the tissue will be unable to grow beyond the size where nutrition and oxygenation can be managed by diffusion alone. Endothelial cells are the primary building block of blood vessels and are capable of forming tube like structures independently however they are unable to independently form functional vasculature which is capable of conducting blood flow. This requires support from other structures including supporting perivascular cells and the extracellular matrix. The crosstalk between endothelial cells and perivascular cells is vital in regulating vasculogenesis and angiogenesis and the consequences when this is disrupted can be seen in a variety of congenital and acquired disease states. This review details the mechanisms of vasculogenesis in vivo during embryogenesis and compares this to currently employed in vitro techniques. It also highlights clinical consequences of defects in the endothelial cell-pericyte cross-talk and highlights therapies which are being developed to target this pathway. Improving the understanding of the intricacies of endothelial-pericyte signaling will inform pathophysiology of multiple vascular diseases and allow the development of effective in vitro models to guide drug development and assist with approaches in tissue engineering to develop functional vasculature for regenerative medicine applications.

Functional Heterogeneity of Endothelial Cells Derived from Human Pluripotent Stem Cells

Specification of endothelial cells (ECs) into arterial, venous, and lymphatic cells is a crucial process of vascular development, and expanding our knowledge about EC specification from human pluripotent stem cells (hPSCs) will aid the design of optimal strategies for producing desired types of ECs for therapies. In our prior studies, we revealed that hPSC-derived VE-cadherin(V)⁺CD31⁺CD34⁺ ECs are heterogeneous and include at least three major subsets with distinct hemogenic properties: V⁺CD43/235a^-CD73^- hemogenic endothelial progenitors (HEPs), V⁺CD43^loCD235a⁺73^- angiogenic hematopoietic progenitors (AHPs), and V⁺CD43/235a^-73⁺ non-HEPs. In this study, using angiogenesis assays, we demonstrated that ECs within these subsets have distinct endothelial colony- and tube-forming properties, proliferative and migratory properties, and endothelial nitric oxide synthase and inflammatory cytokine production potentials. Culture of isolated subsets in arterial, venous, and lymphatic conditions revealed that AHPs are skewed toward lymphatic, HEPs toward arterial, and non-HEPs toward venous differentiation in vitro. These findings suggest that selection and enhancement of production of a particular EC subset may aid in generating desirable EC populations with arterial, venous, or lymphatic properties from hPSCs.

Specification and Diversification of Pericytes and Smooth Muscle Cells from Mesenchymoangioblasts

Elucidating the pathways that lead to vasculogenic cells, and being able to identify their progenitors and lineage-restricted cells, is critical to the establishment of human pluripotent stem cell (hPSC) models for vascular diseases and development of vascular therapies. Here, we find that mesoderm-derived pericytes (PCs) and smooth muscle cells (SMCs) originate from a clonal mesenchymal progenitor mesenchymoangioblast (MB). In clonogenic cultures, MBs differentiate into primitive PDGFRβ⁺ CD271⁺CD73⁻ mesenchymal progenitors, which give rise to proliferative PCs, SMCs, and mesenchymal stem/stromal cells. MB-derived PCs can be further specified to CD274⁺ capillary and DLK1⁺ arteriolar PCs with a proinflammatory and contractile phenotype, respectively. SMC maturation was induced using a MEK inhibitor. Establishing the vasculogenic lineage tree, along with identification of stage- and lineage-specific markers, provides a platform for interrogating the molecular mechanisms that regulate vasculogenic cell specification and diversification and manufacturing well-defined mural cell populations for vascular engineering and cellular therapies from hPSCs.

Kumar et al. find that mesodermal pericytes and smooth muscle cells in human pluripotent stem cell cultures originate from a common endothelial and mesenchymal cell precursor, the mesenchymoangioblast. They show how different lineages of mural cells are specified from mesenchymoangioblasts and define stage- and lineage-specific markers for vasculogenic cells.

StemRegenin 1 selectively promotes expansion of Multipotent Hematopoietic Progenitors derived from Human Embryonic Stem Cells

Human embryonic stem cell (hESC)-derived hematopoietic stem/progenitor cells hold tremendous potential as alternative cell sources for the treatment of various hematological diseases, drug discovery and toxicological screening. However, limited number of hematopoietic stem/progenitor cells generated from the differentiation of hESCs hinders their downstream applications. Here, we show that aryl hydrocarbon receptor antagonist StemRegenin 1 (SR1) selectively promotes expansion of hESC-derived lin^-CD34⁺ hematopoietic progenitors in a concentration-dependent manner. The colony-forming cell (CFC) activity was found to be enriched in the CD34⁺ cells that were expanded with SR1; however, these cells have less colony-forming activity as compared to unexpanded cells (1,338 vs. 7 of CD34⁺ cells to form 1 colony, respectively). Interestingly, SR1 showed a bipotential effect on the proliferation of CD34 negative population, that is low dose of SR1 (1 µM) enhanced cell proliferation, whereas it was repressed at higher doses (>5 µM). In summary, our results suggest that SR1 has the potential to facilitate expansion of hESC-derived lin^-CD34⁺ hematopoietic progenitors, which further retain the potential to form multilineage hematopoietic colonies.

Engineered LINE-1 retrotransposition in nondividing human neurons

Half the human genome is made of transposable elements (TEs), whose ongoing activity continues to impact our genome. LINE-1 (or L1) is an autonomous non-LTR retrotransposon in the human genome, comprising 17% of its genomic mass and containing an average of 80-100 active L1s per average genome that provide a source of inter-individual variation. New LINE-1 insertions are thought to accumulate mostly during human embryogenesis. Surprisingly, the activity of L1s can further impact the somatic human brain genome. However, it is currently unknown whether L1 can retrotranspose in other somatic healthy tissues or if L1 mobilization is restricted to neuronal precursor cells (NPCs) in the human brain. Here, we took advantage of an engineered L1 retrotransposition assay to analyze L1 mobilization rates in human mesenchymal (MSCs) and hematopoietic (HSCs) somatic stem cells. Notably, we have observed that L1 expression and engineered retrotransposition is much lower in both MSCs and HSCs when compared to NPCs. Remarkably, we have further demonstrated for the first time that engineered L1s can retrotranspose efficiently in mature nondividing neuronal cells. Thus, these findings suggest that the degree of somatic mosaicism and the impact of L1 retrotransposition in the human brain is likely much higher than previously thought.

GSK3β Inhibition Promotes Efficient Myeloid and Lymphoid Hematopoiesis from Non-human Primate-Induced Pluripotent Stem Cells

Advances in the scalable production of blood cells from induced pluripotent stem cells (iPSCs) open prospects for the clinical translation of de novo generated blood products, and evoke the need for preclinical evaluation of their efficacy, safety, and immunogenicity in large animal models. Due to substantial similarities with humans, the outcomes of cellular therapies in non-human primate (NHP) models can be readily extrapolated to a clinical setting. However, the use of this model is hampered by relatively low efficiency of blood generation and lack of lymphoid potential in NHP-iPSC differentiation cultures. Here, we generated transgene-free iPSCs from different NHP species and showed the efficient induction of mesoderm, myeloid, and lymphoid cells from these iPSCs using a GSK3β inhibitor. Overall, our studies enable scalable production of hematopoietic progenitors from NHP-iPSCs, and lay the foundation for preclinical testing of iPSC-based therapies for blood and immune system diseases in an NHP model.

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Generation of transgene-free iPSCs from various non-human primate (NHP) species

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GSK3β inhibition is essential for induction of mesoderm and blood from NHP-iPSCs

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Efficient generation of CD34⁺CD45⁺CD90⁺CD38⁻CD45RA⁻ progenitors from NHP-iPSCs

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Efficient T and NK cell production from NHP-iPSCs

CCR5 Disruption in Induced Pluripotent Stem Cells Using CRISPR/Cas9 Provides Selective Resistance of Immune Cells to CCR5-tropic HIV-1 Virus

The chemokine (C-C motif) receptor 5 (CCR5) serves as an HIV-1 co-receptor and is essential for cell infection with CCR5-tropic viruses. Loss of functional receptor protects against HIV infection. Here, we report the successful targeting of CCR5 in GFP-marked human induced pluripotent stem cells (iPSCs) using CRISPR/Cas9 with single and dual guide RNAs (gRNAs). Following CRISPER/Cas9-mediated gene editing using a single gRNA, 12.5% of cell colonies demonstrated CCR5 editing, of which 22.2% showed biallelic editing as determined by a Surveyor nuclease assay and direct sequencing. The use of dual gRNAs significantly increased the efficacy of CCR5 editing to 27% with a biallelic gene alteration frequency of 41%. To ensure the homogeneity of gene editing within cells, we used single cell sorting to establish clonal iPSC lines. Single cell-derived iPSC lines with homozygous CCR5 mutations displayed the typical characteristics of pluripotent stem cells and differentiated efficiently into hematopoietic cells, including macrophages. Although macrophages from both wild-type and CCR5-edited iPSCs supported CXCR4-tropic virus replication, macrophages from CCR5-edited iPSCs were uniquely resistant to CCR5-tropic virus challenge. This study demonstrates the feasibility of applying iPSC technology for the study of the role of CCR5 in HIV infection in vitro, and generation of HIV-resistant cells for potential therapeutic applications.

Direct Induction of Hemogenic Endothelium and Blood by Overexpression of Transcription Factors in Human Pluripotent Stem Cells

During development, hematopoietic cells arise from a specialized subset of endothelial cells, hemogenic endothelium (HE). Modeling HE development in vitro is essential for mechanistic studies of the endothelial-hematopoietic transition and hematopoietic specification. Here, we describe a method for the efficient induction of HE from human pluripotent stem cells (hPSCs) by way of overexpression of different sets of transcription factors. The combination of ETV2 and GATA1 or GATA2 TFs is used to induce HE with pan-myeloid potential, while a combination of GATA2 and TAL1 transcription factors allows for the production of HE with erythroid and megakaryocytic potential. The addition of LMO2 to GATA2 and TAL1 combination substantially accelerates differentiation and increases erythroid and megakaryocytic cells production. This method provides an efficient and rapid means of HE induction from hPSCs and allows for the observation of the endothelial-hematopoietic transition in a culture dish. The protocol includes hPSCs transduction procedures and post-transduction analysis of HE and blood progenitors.

OP9 Stromal Cells Proteins Involved in Hematoendothelial Differentiation from Human Embryonic Stem Cells

Hematopoietic cells (HCs) and endothelial cells (ECs) can be produced in vitro from human embryonic stem cells (hESCs), but the differentiation systems used are still inefficient. To overcome this obstacle, it is necessary to understand the differentiation process. One of the methods used to obtain HCs and ECs from hESCs is their co-culture with stromal cells. The soluble factors secreted by these cells and cell-cell contact have a great impact on the differentiation process. Here, we performed comparative proteomic analyses of proteins obtained from the total extract of OP9 stromal cells and secreted by these cells before and during in vitro generation of HCs and ECs (hematoendothelial) from hESCs. We identified a total of 83 secreted and 759 intracellular proteins during differentiation. Twenty-five secreted and 181 proteins from the total extract were more abundant. Some secreted proteins are involved in cell-matrix interactions and HC and/or EC development. Moreover, 13 proteins of the total extract from OP9 cells that were exclusive/or more abundant during differentiation are involved in the Nrf2/Nfe2l2 gene pathway, that is, they are described to have a key role in oxidative stress and in hematopoietic development and maturation. Our proteomic profiles provide valuable insight about the proteins involved in in vitro hematoendothelial cell generation and in the future they might be used to optimize the differentiation process and produce both cell types in vitro.

Genetic heterogeneity of induced pluripotent stem cells: results from 24 clones derived from a single C57BL/6 mouse

Induced pluripotent stem cells (iPSCs) have tremendous potential as a tool for disease modeling, drug testing, and other applications. Since the generation of iPSCs "captures" the genetic history of the individual cell that was reprogrammed, iPSC clones (even those derived from the same individual) would be expected to demonstrate genetic heterogeneity. To assess the degree of genetic heterogeneity, and to determine whether some cells are more genetically "fit" for reprogramming, we performed exome sequencing on 24 mouse iPSC clones derived from skin fibroblasts obtained from two different sites of the same 8-week-old C57BL/6J male mouse. While no differences in the coding regions were detected in the two parental fibroblast pools, each clone had a unique genetic signature with a wide range of heterogeneity observed among the individual clones: a total of 383 iPSC variants were validated for the 24 clones (mean 16.0/clone, range 0-45). Since these variants were all present in the vast majority of the cells in each clone (variant allele frequencies of 40-60% for heterozygous variants), they most likely preexisted in the individual cells that were reprogrammed, rather than being acquired during reprogramming or cell passaging. We then tested whether this genetic heterogeneity had functional consequences for hematopoietic development by generating hematopoietic progenitors in vitro and enumerating colony forming units (CFUs). While there was a range of hematopoietic potentials among the 24 clones, only one clone failed to differentiate into hematopoietic cells; however, it was able to form a teratoma, proving its pluripotent nature. Further, no specific association was found between the mutational spectrum and the hematopoietic potential of each iPSC clone. These data clearly highlight the genetic heterogeneity present within individual fibroblasts that is captured by iPSC generation, and suggest that most of the changes are random, and functionally benign.

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.

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.

Tenascin C promotes hematoendothelial development and T lymphoid commitment from human pluripotent stem cells in chemically defined conditions

The recent identification of hemogenic endothelium (HE) in human pluripotent stem cell (hPSC) cultures presents opportunities to investigate signaling pathways that are essential for blood development from endothelium and provides an exploratory platform for de novo generation of hematopoietic stem cells (HSCs). However, the use of poorly defined human or animal components limits the utility of the current differentiation systems for studying specific growth factors required for HE induction and manufacturing clinical-grade therapeutic blood cells. Here, we identified chemically defined conditions required to produce HE from hPSCs growing in Essential 8 (E8) medium and showed that Tenascin C (TenC), an extracellular matrix protein associated with HSC niches, strongly promotes HE and definitive hematopoiesis in this system. hPSCs differentiated in chemically defined conditions undergo stages of development similar to those previously described in hPSCs cocultured on OP9 feeders, including the formation of VE-Cadherin⁺CD73⁻CD235a/CD43⁻ HE and hematopoietic progenitors with myeloid and T lymphoid potential.

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Hemogenic endothelium is generated in a completely defined xenogen-free system

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The system reproduces all stages of hematopoietic development

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Tenascin C enhances hematoendothelial development from pluripotent stem cells

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Tenesacin C uniquely supports T cell specification

Modelling Fanconi anemia pathogenesis and therapeutics using integration-free patient-derived iPSCs

Fanconi anaemia (FA) is a recessive disorder characterized by genomic instability, congenital abnormalities, cancer predisposition and bone marrow (BM) failure. However, the pathogenesis of FA is not fully understood partly due to the limitations of current disease models. Here, we derive integration free-induced pluripotent stem cells (iPSCs) from an FA patient without genetic complementation and report in situ gene correction in FA-iPSCs as well as the generation of isogenic FANCA-deficient human embryonic stem cell (ESC) lines. FA cellular phenotypes are recapitulated in iPSCs/ESCs and their adult stem/progenitor cell derivatives. By using isogenic pathogenic mutation-free controls as well as cellular and genomic tools, our model serves to facilitate the discovery of novel disease features. We validate our model as a drug-screening platform by identifying several compounds that improve hematopoietic differentiation of FA-iPSCs. These compounds are also able to rescue the hematopoietic phenotype of FA patient BM cells.

Identification of multipotent progenitors that emerge prior to hematopoietic stem cells in embryonic development

Hematopoiesis in the embryo proceeds in a series of waves, with primitive erythroid-biased waves succeeded by definitive waves, within which the properties of hematopoietic stem cells (multilineage potential, self-renewal, and engraftability) gradually arise. Whereas self-renewal and engraftability have previously been examined in the embryo, multipotency has not been thoroughly addressed, especially at the single-cell level or within well-defined populations. To identify when and where clonal multilineage potential arises during embryogenesis, we developed a single-cell multipotency assay. We find that, during the initiation of definitive hematopoiesis in the embryo, a defined population of multipotent, engraftable progenitors emerges that is much more abundant within the yolk sac (YS) than the aorta-gonad-mesonephros (AGM) or fetal liver. These experiments indicate that multipotent cells appear in concert within both the YS and AGM and strongly implicate YS-derived progenitors as contributors to definitive hematopoiesis.

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A multipotency assay reveals eight hematopoietic lineages from a single cell

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The first embryonic multipotent cells are defined as CD11A⁻ KIT⁺ SCA-1⁺ (CD11A⁻ KLS)

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Transplanted CD11A− KLS cells give rise to robust hematopoiesis in newborn mice

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The early yolk sac is implicated as the major source of CD11A⁻ KLS cells

Regeneration of human tumor antigen-specific T cells from iPSCs derived from mature CD8(+) T cells

Antigen-specific T cells represent a potential therapeutic avenue for a variety of conditions, but current approaches for generating such cells for therapeutic purposes are limited. In this study, we established iPSCs from mature cytotoxic T cells specific for the melanoma epitope MART-1. When cocultured with OP9/DLL1 cells, these iPSCs efficiently generated TCRβ(+)CD4(+)CD8(+) double positive (DP) cells expressing a T cell receptor (TCR) specific for the MART-1 epitope. Stimulation of these DP cells with anti-CD3 antibody generated a large number of CD8(+) T cells, and more than 90% of the resulting cells were specific for the original MART-1 epitope. Stimulation of the CD8(+) T cells with MART-1 antigen-presenting cells led to the secretion of IFNγ, demonstrating their specific reactivity. The present study therefore illustrates an approach for cloning and expanding functional antigen-specific CD8(+) T cells that might be applicable in cell-based therapy of cancer.

Vascular tissue engineering: biodegradable scaffold platforms to promote angiogenesis

The ability to understand and regulate human vasculature development and differentiation has the potential to benefit patients suffering from a variety of ailments, including cardiovascular disease, peripheral vascular disease, ischemia, and burn wounds. Current clinical treatments for vascular-related diseases commonly use the grafting from patients of autologous vessels, which are limited and often damaged due to disease. Considerable progress is being made through a tissue engineering strategy in the vascular field. Tissue engineering takes a multidisciplinary approach seeking to repair, improve, or replace biological tissue function in a controlled and predictable manner. To address the clinical need to perfuse and repair damaged, ischemic tissue, one approach of vascular engineering aims to understand and promote the growth and differentiation of vascular networks. Vascular tissue engineered constructs enable the close study of vascular network assembly and vessel interactions with the surrounding microenvironment. Scaffold platforms provide a method to control network development through the biophysical regulation of different scaffold properties, such as composition, mechanics, dimensionality, and so forth. Following a short description of vascular physiology and blood vessel biomechanics, the key principles in vascular tissue engineering are discussed. This review focuses on various biodegradable scaffold platforms and demonstrates how they are being used to regulate, promote, and understand angiogenesis and vascular network formation.

Endothelial cells derived from nuclear reprogramming

The endothelium plays a pivotal role in vascular homeostasis, regulating the tone of the vascular wall, and its interaction with circulating blood elements. Alterations in endothelial functions facilitate the infiltration of inflammatory cells and permit vascular smooth muscle proliferation and platelet aggregation. Therefore, endothelial dysfunction is an early event in disease processes including atherosclerosis, and because of its critical role in vascular health, the endothelium is worthy of the intense focus it has received. However, there are limitations to studying human endothelial function in vivo, or human vascular segments ex vivo. Thus, methods for endothelial cell (EC) culture have been developed and refined. Recently, methods to derive ECs from pluripotent cells have extended the scientific range of human EC studies. Pluripotent stem cells may be generated, expanded, and then differentiated into ECs for in vitro studies. Constructs for molecular imaging can also be employed to facilitate tracking these cells in vivo. Furthermore, one can generate patient-specific ECs to study the effects of genetic or epigenetic alterations on endothelial behavior. Finally, there is the opportunity to apply these cells for vascular therapy. This review focuses on the generation of ECs from stem cells; their characterization by genetic, histological, and functional studies; and their translational applications.

Concise review: production of cultured red blood cells from stem cells

In the Western world, the volunteer-based collection system covers most transfusion needs, but transient shortages regularly develop and blood supplies are vulnerable to potentially major disruptions. The production of cultured red blood cells from stem cells is slowly emerging as a potential alternative. The various cell sources, the niche applications most likely to reach the clinic first, and some of the remaining technical issues are reviewed here.

Identification of the hemogenic endothelial progenitor and its direct precursor in human pluripotent stem cell differentiation cultures

Hemogenic endothelium (HE) has been recognized as a source of hematopoietic stem cells (HSCs) in the embryo. Access to human HE progenitors (HEPs) is essential for enabling the investigation of the molecular determinants of HSC specification. Here, we show that HEPs capable of generating definitive hematopoietic cells can be obtained from human pluripotent stem cells (hPSCs) and identified precisely by a VE-cadherin(+)CD73(-)CD235a/CD43(-) phenotype. This phenotype discriminates true HEPs from VE-cadherin(+)CD73(+) non-HEPs and VE-cadherin(+)CD235a(+)CD41a(-) early hematopoietic cells with endothelial and FGF2-dependent hematopoietic colony-forming potential. We found that HEPs arise at the post-primitive-streak stage of differentiation directly from VE-cadherin-negative KDR(bright)APLNR(+)PDGFRα(low/-) hematovascular mesodermal precursors (HVMPs). In contrast, hemangioblasts, which are capable of forming endothelium and primitive blood cells, originate from more immature APLNR(+)PDGFRα(+) mesoderm. The demarcation of HEPs and HVMPs provides a platform for modeling blood development from endothelium with a goal of facilitating the generation of HSCs from hPSCs.

Dynamic expression of specific miRNAs during erythroid differentiation of human embryonic stem cells

MicroRNAs (miRNAs) are small non-coding RNAs that regulate gene expression at post-transcriptional levels through mRNA degradation or translation inhibition. Little is known regarding miRNA participation in regulating hematopoietic, or more specifically erythroid differentiation. This study was aimed at identifying erythroid lineage-specific miRNAs expressed during in vitro erythropoiesis using human embryonic stem cells (hESCs) and human umbilical cord blood (CB) CD34+ cells. CD34+ hematopoietic cells were produced from hESCs in vitro and subsequently induced to differentiate into erythroid cells by culture in sequence on OP9 feeder cells and then with mesenchymal stromal cells (MSC) in the presence of cytokines. Expression profiles of erythroid lineage-specific miRNAs were analyzed by quantitative PCR during in vitro differentiation. Expression levels of miR-142-3p, miR-142-5p, miR-146a and miR-451 were dynamically changed during differentiation of hESCs to CD34+ hematopoietic cells, and in subsequent differentiation of the CD34+ cells into the erythroid lineage. This suggests that these four miRNAs might be involved in regulating erythropoiesis.

Differentiation efficiency of induced pluripotent stem cells depends on the number of reprogramming factors

Reprogramming of somatic cells into induced pluripotent stem cells (iPSCs) by retroviral overexpression of the transcription factors Oct4, Sox2, Klf4, and c-Myc holds great promise for the development of personalized cell replacement therapies. In an attempt to minimize the risk for chromosomal disruption and to simplify reprogramming, several studies demonstrated that a reduced set of reprogramming factors is sufficient to generate iPSC, albeit at lower efficiency. To elucidate the influence of factor reduction on subsequent differentiation, we compared the efficiency of neuronal differentiation in iPSC generated from postnatal murine neural stem cells with either one (Oct4; iPSC(1F-NSC) ), two (Oct4, Klf4; iPSC(2F-NSC) ), or all four factors (iPSC(4F-NSC) ) with those of embryonic stem cells (ESCs) and iPSC produced from fibroblasts with all four factors (iPSC(4F-MEF) ). After 2 weeks of coculture with PA6 stromal cells, neuronal differentiation of iPSC(1F-NSC) and iPSC(2F-NSC) was less efficient compared with iPSC(4F-NSC) and ESC, yielding lower proportions of colonies that stained positive for early and late neuronal markers. Electrophysiological analyses after 4 weeks of differentiation identified functional maturity in neurons differentiated from ESC, iPSC(2F-NSC) , iPSC(4F-NSC) , and iPSC(4F-MEF) but not in those from iPSC(1F-NSC) . Similar results were obtained after hematoendothelial differentiation on OP9 bone marrow stromal cells, where factor-reduced iPSC generated lower proportions of colonies with hematoendothelial progenitors than colonies of ESC, iPSC(4F-NSC) , and iPSC(4F-MEF) . We conclude that a reduction of reprogramming factors does not only reduce reprogramming efficiency but may also worsen subsequent differentiation and hinder future application of iPSC in cell replacement therapies.

SCL/TAL1 regulates hematopoietic specification from human embryonic stem cells

Determining the molecular regulators/pathways responsible for the specification of human embryonic stem cells (hESCs) into hematopoietic precursors has far-reaching implications for potential cell therapies and disease modeling. Mouse models lacking SCL/TAL1 (stem cell leukemia/T-cell acute lymphocytic leukemia 1) do not survive beyond early embryogenesis because of complete absence of hematopoiesis, indicating that SCL is a master early hematopoietic regulator. SCL is commonly found rearranged in human leukemias. However, there is barely information on the role of SCL on human embryonic hematopoietic development. Differentiation and sorting assays show that endogenous SCL expression parallels hematopoietic specification of hESCs and that SCL is specifically expressed in hematoendothelial progenitors (CD45(-)CD31(+)CD34(+)) and, to a lesser extent, on CD45(+) hematopoietic cells. Enforced expression of SCL in hESCs accelerates the emergence of hematoendothelial progenitors and robustly promotes subsequent differentiation into primitive (CD34(+)CD45(+)) and total (CD45(+)) blood cells with higher clonogenic potential. Short-hairpin RNA-based silencing of endogenous SCL abrogates hematopoietic specification of hESCs, confirming the early hematopoiesis-promoting effect of SCL. Unfortunately, SCL expression on its own is not sufficient to confer in vivo engraftment to hESC-derived hematopoietic cells, suggesting that additional yet undefined master regulators are required to orchestrate the stepwise hematopoietic developmental process leading to the generation of definitive in vivo functional hematopoiesis from hESCs.

Genetic correction of β-thalassemia patient-specific iPS cells and its use in improving hemoglobin production in irradiated SCID mice

The generation of induced pluripotent stem cells (iPSCs) from differentiated somatic cells by over-expression of several transcription factors has the potential to cure many genetic and degenerative diseases currently recalcitrant to traditional clinical approaches. One such genetic disease is β-thalassemia major (Cooley's anemia). This disease is caused by either a point mutation or the deletion of several nucleotides in the β-globin gene, and it threatens the lives of millions of people in China. In the present study, we successfully generated iPSCs from fibroblasts collected from a 2-year-old patient who was diagnosed with a homozygous 41/42 deletion in his β-globin gene. More importantly, we successfully corrected this genetic mutation in the β-thalassemia iPSCs by homologous recombination. Furthermore, transplantation of the genetically corrected iPSCs-derived hematopoietic progenitors into sub-lethally irradiated immune deficient SCID mice showed improved hemoglobin production compared with the uncorrected iPSCs. Moreover, the generation of human β-globin could be detected in the mice transplanted with corrected iPSCs-derived hematopietic progenitors. Our study provides strong evidence that iPSCs generated from a patient with a genetic disease can be corrected by homologous recombination and that the corrected iPSCs have potential clinical uses.

An antibody against SSEA-5 glycan on human pluripotent stem cells enables removal of teratoma-forming cells

An important risk in the clinical application of human pluripotent stem cells (hPSCs), including human embryonic and induced pluripotent stem cells (hESCs and hiPSCs), is teratoma formation by residual undifferentiated cells. We raised a monoclonal antibody against hESCs, designated anti-stage-specific embryonic antigen (SSEA)-5, which binds a previously unidentified antigen highly and specifically expressed on hPSCs--the H type-1 glycan. Separation based on SSEA-5 expression through fluorescence-activated cell sorting (FACS) greatly reduced teratoma-formation potential of heterogeneously differentiated cultures. To ensure complete removal of teratoma-forming cells, we identified additional pluripotency surface markers (PSMs) exhibiting a large dynamic expression range during differentiation: CD9, CD30, CD50, CD90 and CD200. Immunohistochemistry studies of human fetal tissues and bioinformatics analysis of a microarray database revealed that concurrent expression of these markers is both common and specific to hPSCs. Immunodepletion with antibodies against SSEA-5 and two additional PSMs completely removed teratoma-formation potential from incompletely differentiated hESC cultures.

Generation of red blood cells from human induced pluripotent stem cells

Differentiation of human induced pluripotent stem cells (hiPSCs) and embryonic stem cells (hESCs) into the erythroid lineage of cells offers a novel opportunity to study erythroid development, regulation of globin switching, drug testing, and modeling of red blood cell (RBC) diseases in vitro. Here we describe an approach for the efficient generation of RBCs from hiPSC/hESCs using an OP9 coculture system to induce hematopoietic differentiation followed by selective expansion of erythroid cells in serum-free media with erythropoiesis-supporting cytokines. We showed that fibroblast-derived transgenic hiPSCs generated using lentivirus-based vectors and transgene-free hiPSCs generated using episomal vectors can be differentiated into RBCs with an efficiency similar to that of H1 hESCs. Erythroid cultures established with this approach consisted of an essentially pure population of CD235a(+)CD45(-) leukocyte-free RBCs with robust expansion potential and long life span (up to 90 days). Similar to hESCs, hiPSC-derived RBCs expressed predominately fetal γ and embryonic ɛ globins, indicating complete reprogramming of β-globin locus following transition of fibroblasts to the pluripotent state. Although β-globin expression was detected in hiPSC/hESC-derived erythroid cells, its expression was substantially lower than the embryonic and fetal globins. Overall, these results demonstrate the feasibility of large-scale production of erythroid cells from fibroblast-derived hiPSCs, as has been described for hESCs. Since RBCs generated from transgene-free hiPSCs lack genomic integration and background expression of reprogramming genes, they would be a preferable cell source for modeling of diseases and for gene function studies.

L3MBTL1 deficiency directs the differentiation of human embryonic stem cells toward trophectoderm

Human embryonic stem cells (hESCs) can be used to study the early events in human development and, hopefully, to understand how to differentiate human pluripotent cells for clinical use. To define how L3MBTL1, a chromatin-associated polycomb group protein with transcriptional repressive activities, regulates early events in embryonic cell differentiation, we created hESC lines that constitutively express shRNAs directed against L3MBTL1. The L3MBTL1 knockdown (KD) hESCs maintained normal morphology, proliferation, cell cycle kinetics, cell surface markers, and karyotype after 40 passages. However, under conditions that promote spontaneous differentiation, the L3MBTL1 KD cells differentiated into a relatively homogeneous population of large, flat trophoblast-like cells, unlike the multilineage differentiation seen with the control cells. The differentiated L3MBTL1 KD cells expressed numerous trophoblast markers and secreted placental hormones. Although the L3MBTL1 KD cells could be induced to differentiate into various embryonic lineages, they adopted an exclusive trophoblast fate during spontaneous differentiation. Our data demonstrate that depletion of L3MBTL1 does not affect hESC self-renewal, rather it enhances differentiation toward extra-embryonic trophoblast tissues.

A mesoderm-derived precursor for mesenchymal stem and endothelial cells

Among the three embryonic germ layers, the mesoderm is a major source of the mesenchymal precursors giving rise to skeletal and connective tissues, but these precursors have not previously been identified and characterized. Using human embryonic stem cells directed toward mesendodermal differentiation, we show that mesenchymal stem/stromal cells (MSCs) originate from a population of mesodermal cells identified by expression of apelin receptor. In semisolid medium, these precursors form FGF2-dependent compact spheroid colonies containing mesenchymal cells with a transcriptional profile representative of mesoderm-derived embryonic mesenchyme. When transferred to adherent cultures, individual colonies give rise to MSC lines with chondro-, osteo-, and adipogenic differentiation potentials. Although the MSC lines lacked endothelial potential, endothelial cells could be derived from the mesenchymal colonies, suggesting that, similar to hematopoietic cells, MSCs arise from precursors with angiogenic potential. Together, these studies identified a common precursor of mesenchymal and endothelial cells, mesenchymoangioblast, as the source of mesoderm-derived MSCs.

Hematopoietic differentiation and production of mature myeloid cells from human pluripotent stem cells

In this paper, we describe a protocol for hematopoietic differentiation of human pluripotent stem cells (hPSCs) and generation of mature myeloid cells from hPSCs through expansion and differentiation of hPSC-derived lin(-)CD34(+)CD43(+)CD45(+) multipotent progenitors. The protocol comprises three major steps: (i) induction of hematopoietic differentiation by coculture of hPSCs with OP9 bone marrow stromal cells; (ii) short-term expansion of multipotent myeloid progenitors with a high dose of granulocyte-macrophage colony-stimulating factor; and (iii) directed differentiation of myeloid progenitors into neutrophils, eosinophils, dendritic cells, Langerhans cells, macrophages and osteoclasts. The generation of multipotent hematopoietic progenitors from hPSCs requires 9 d of culture and an additional 2 d to expand myeloid progenitors. Differentiation of myeloid progenitors into mature myeloid cells requires an additional 5-19 d of culture with cytokines, depending on the cell type.

Efficient generation of transgene-free induced pluripotent stem cells from normal and neoplastic bone marrow and cord blood mononuclear cells

Reprogramming blood cells to induced pluripotent stem cells (iPSCs) provides a novel tool for modeling blood diseases in vitro. However, the well-known limitations of current reprogramming technologies include low efficiency, slow kinetics, and transgene integration and residual expression. In the present study, we have demonstrated that iPSCs free of transgene and vector sequences could be generated from human BM and CB mononuclear cells using non-integrating episomal vectors. The reprogramming described here is up to 100 times more efficient, occurs 1-3 weeks faster compared with the reprogramming of fibroblasts, and does not require isolation of progenitors or multiple rounds of transfection. Blood-derived iPSC lines lacked rearrangements of IGH and TCR, indicating that their origin is non-B- or non-T-lymphoid cells. When cocultured on OP9, blood-derived iPSCs could be differentiated back to the blood cells, albeit with lower efficiency compared to fibroblast-derived iPSCs. We also generated transgene-free iPSCs from the BM of a patient with chronic myeloid leukemia (CML). CML iPSCs showed a unique complex chromosomal translocation identified in marrow sample while displaying typical embryonic stem cell phenotype and pluripotent differentiation potential. This approach provides an opportunity to explore banked normal and diseased CB and BM samples without the limitations associated with virus-based methods.

In vivo evaluation of putative hematopoietic stem cells derived from human pluripotent stem cells

Efficient derivation and isolation of hematopoietic stem cells (HSCs) from human pluripotent stem cell (hPSC) populations remains a major goal in the field of developmental hematopoiesis. These enticing pluripotent stem cells (comprising both human embryonic stem cells and induced pluripotent stem cells) have been successfully used to generate a wide array of hematopoietic cells in vitro, from primitive hematoendothelial precursors to mature myeloid, erythroid, and lymphoid lineage cells. However, to date, PSC-derived cells have demonstrated only limited potential for long-term multilineage hematopoietic engraftment in vivo - the test by which putative HSCs are defined. Successful generation and characterization of HSCs from hPSCs not only requires an efficient in vitro differentiation system that provides insight into the developmental fate of hPSC-derived cells, but also necessitates an in vivo engraftment model that allows identification of specific mechanisms that hinder or promote hematopoietic engraftment. In this chapter, we will describe a method that utilizes firefly luciferase-expressing hPSCs and bioluminescent imaging to noninvasively track the survival, proliferation, and migration of transplanted hPSC-derived cells. Combined with lineage and functional analyses of engrafted cells, this system is a useful tool to gain insight into the in vivo potential of hematopoietic cells generated from hPSCs.

Role of the microenvironment in the specification of endothelial progenitors derived from embryonic stem cells

Embryonic stem (ES) cells are pluripotent cells capable of differentiating in all the cell types present in a living organism. They derive from the inner cell mass of blastocysts of different species including humans. Given their unlimited potential, ES cells represent an invaluable resource of different cell types for transplantation and tissue engineering applications. However, in order to accomplish these therapeutic purposes, efficient and controlled in vitro systems of directing ES cell differentiation are mandatory. ES cell differentiation is strongly influenced by physical, chemical and cellular signals provided by the local microenvironment. Understanding the relationships occurring between differentiating cells and surrounding environment is pivotal for a successful ES cells-based therapy. This review describes three different methods of in vitro differentiation of ES cells by outlining the environmental elements required for endothelial fate specification. For each system, the efficiency of endothelial differentiation, the accessibility and the advantages are discussed. The main conclusion that arises from this analysis is that the knowledge of the role played by microenvironment in cell fate determination is essential to control and take advantage of ES cells potential.

In vitro and in vivo differentiation of human embryonic stem cells into retina-like organs and comparison with that from mouse pluripotent epiblast stem cells

Correctly inducing the differentiation of pluripotent hESCs to a specific lineage with high purity is highly desirable for regenerative cell therapy. Our first effort to perform in vitro differentiation of hESCs resulted in a limited recapitulation of the ocular tissue structures. When undifferentiated hESCs were placed in vivo into the ocular tissue, in this case into the vitreous cavity, 3-dimensional retina-like structures reminiscent of the invagination of the optic vesicle were generated. Immunohistochemical analysis confirmed the presence of both a neural retina-like cell layer and a retinal pigmented epithelium-like cell layer, possibly equivalent to the developing E12.5 mouse retina. Furthermore, mouse epiblast-derived stem cells, which are reported to share some characteristics with hESCs, but not with mouse ESCs, also generated retinal anlage-like structures in vivo. hESC-derived retina-like structures present a novel therapeutic possibility for retinal diseases and also provide a novel experimental system to study early human eye development.

Hematopoietic and endothelial differentiation of human induced pluripotent stem cells

Induced pluripotent stem cells (iPSCs) provide an unprecedented opportunity for modeling of human diseases in vitro, as well as for developing novel approaches for regenerative therapy based on immunologically compatible cells. In this study, we employed an OP9 differentiation system to characterize the hematopoietic and endothelial differentiation potential of seven human iPSC lines obtained from human fetal, neonatal, and adult fibroblasts through reprogramming with POU5F1, SOX2, NANOG, and LIN28 and compared it with the differentiation potential of five human embryonic stem cell lines (hESC, H1, H7, H9, H13, and H14). Similar to hESCs, all iPSCs generated CD34(+)CD43(+) hematopoietic progenitors and CD31(+)CD43(-) endothelial cells in coculture with OP9. When cultured in semisolid media in the presence of hematopoietic growth factors, iPSC-derived primitive blood cells formed all types of hematopoietic colonies, including GEMM colony-forming cells. Human induced pluripotent cells (hiPSCs)-derived CD43(+) cells could be separated into the following phenotypically defined subsets of primitive hematopoietic cells: CD43(+)CD235a(+)CD41a(+/-) (erythro-megakaryopoietic), lin(-)CD34(+)CD43(+)CD45(-) (multipotent), and lin(-)CD34(+)CD43(+)CD45(+) (myeloid-skewed) cells. Although we observed some variations in the efficiency of hematopoietic differentiation between different hiPSCs, the pattern of differentiation was very similar in all seven tested lines obtained through reprogramming of human fetal, neonatal, or adult fibroblasts with three or four genes. Although several issues remain to be resolved before iPSC-derived blood cells can be administered to humans for therapeutic purposes, patient-specific iPSCs can already be used for characterization of mechanisms of blood diseases and for identification of molecules that can correct affected genetic networks.

Generation of mature human myelomonocytic cells through expansion and differentiation of pluripotent stem cell-derived lin-CD34+CD43+CD45+ progenitors

Basic research into human mature myelomonocytic cell function, myeloid lineage diversification and leukemic transformation, and assessment of myelotoxicity in preclinical drug development requires a constant supply of donor blood or bone marrow samples and laborious purification of mature myeloid cells or progenitors, which are present in very small quantities. To overcome these limitations, we have developed a protocol for efficient generation of neutrophils, eosinophils, macrophages, osteoclasts, DCs, and Langerhans cells from human embryonic stem cells (hESCs). As a first step, we generated lin-CD34+CD43+CD45+ hematopoietic cells highly enriched in myeloid progenitors through coculture of hESCs with OP9 feeder cells. After expansion in the presence of GM-CSF, these cells were directly differentiated with specific cytokine combinations toward mature cells of particular types. Morphologic, phenotypic, molecular, and functional analyses revealed that hESC-derived myelomonocytic cells were comparable to their corresponding somatic counterparts. In addition, we demonstrated that a similar protocol could be used to generate myelomonocytic cells from induced pluripotent stem cells (iPSCs). This technology offers an opportunity to generate large numbers of patient-specific myelomonocytic cells for in vitro studies of human disease mechanisms as well as for drug screening.