A role for thrombopoietin in hemangioblast development

Ferric citrate and apo-transferrin enable erythroblast maturation with β-globin from hemogenic endothelium

Red blood cell (RBC) generation from human pluripotent stem cells (PSCs) offers potential for innovative cell therapy in regenerative medicine as well as developmental studies. Ex vivo erythropoiesis from PSCs is currently limited by the low efficiency of functional RBCs with β-globin expression in culture systems. During induction of β-globin expression, the absence of a physiological microenvironment, such as a bone marrow niche, may impair cell maturation and lineage specification. Here, we describe a simple and reproducible culture system that can be used to generate erythroblasts with β-globin expression. We prepared a two-dimensional defined culture with ferric citrate treatment based on definitive hemogenic endothelium (HE). Floating erythroblasts derived from HE cells were primarily CD45+CD71+CD235a+ cells, and their number increased remarkably upon Fe treatment. Upon maturation, the erythroblasts cultured in the presence of ferric citrate showed high transcriptional levels of β-globin and enrichment of genes associated with heme synthesis and cell cycle regulation, indicating functionality. The rapid maturation of these erythroblasts into RBCs was observed when injected in vivo, suggesting the development of RBCs that were ready to grow. Hence, induction of β-globin expression may be explained by the effects of ferric citrate that promote cell maturation by binding with soluble transferrin and entering the cells.Taken together, upon treatment with Fe, erythroblasts showed advanced maturity with a high transcription of β-globin. These findings can help devise a stable protocol for the generation of clinically applicable RBCs.

CD34dim cells identified as pluripotent stem cell-derived definitive hemogenic endothelium purified using bone morphogenetic protein 4

Hemogenic endothelium (HE) plays a pivotal and inevitable role in haematopoiesis and can generate all blood and endothelial lineage cells in the aorta-gonad-mesonephros of mouse embryos. Whether definitive HE can prospectively isolate pure HE from human pluripotent stem cells that can spontaneously differentiate into heterogeneous cells remains unknown. Here, we identified and validated a CD34^dim subpopulation with hemogenic potential. We also purified CD34 cells with a CXCR4^- CD73^- phenotype as a definitive HE population that generated haematopoietic stem cells and lymphocytes. The frequency of CXCR4^- CD73^- CD34^dim was evidently increased by bone morphogenetic protein 4, and purified HE cells differentiated into haematopoietic cells with myeloid and T lymphoid lineages including Vδ2+ subset of γ/δ T cells. We developed a simple method to purify HE cells that were enriched in CD34^dim cells. We uncovered an initial step in differentiating haematopoietic lineage cells that could be applied to basic and translational investigations into regenerative medicine.

Thrombopoietin receptor agonist for treatment of immune thrombocytopenia in pregnancy: a narrative review

The treatment of immune thrombocytopenia (ITP) in adults has evolved rapidly over the past decade. The second-generation thrombopoietin receptor agonists (TPO-RAs), romiplostim, eltrombopag, and avatrombopag are approved for the treatment of chronic ITP in adults. However, their use in pregnancy is labeled as category C by the United States Food and Drug Administration (FDA) due to the lack of clinical data on human subjects. ITP is a common cause of thrombocytopenia in the first and second trimester of pregnancy, which not only affects the mother but can also lead to thrombocytopenia in the neonatal thrombocytopenia secondary to maternal immune thrombocytopenia (NMITP). Corticosteroids, intravenous immunoglobulins (IVIGs) are commonly used for treating acute ITP in pregnant patients. Drugs such as rituximab, anti-D, and azathioprine that are used to treat ITP in adults, are labeled category C and seldom used in pregnant patients. Cytotoxic chemotherapy (vincristine, cyclophosphamide), danazol, and mycophenolate are contraindicated in pregnant women. In such a scenario, TPO-RAs present an attractive option to treat ITP in pregnant patients. Current evidence on the use of TPO-RAs in pregnant women with ITP is limited. In this narrative review, we will examine the preclinical and the clinical literature regarding the use of TPO-RAs in the management of ITP in pregnancy and their effect on neonates with NMITP.

High-Efficiency Serum-Free Feeder-Free Erythroid Differentiation of Human Pluripotent Stem Cells Using Small Molecules

This article describes a highly efficient, fully feeder-free, serum-free method for erythroid differentiation of induced pluripotent stem cells and human embryonic stem cells, including a clinical-grade line, that is amenable to scale-up and as such will be of significant value for basic and translational studies of hematopoiesis and erythropoiesis.

This article describes a good manufacturing practice (GMP)-compatible, feeder-free and serum-free method to produce large numbers of erythroid cells from human pluripotent stem cells (hPSCs), either embryonic or induced. This multistep protocol combines cytokines and small molecules to mimic and surpass the early stages of development. It produces, without any selection or sorting step, a population of cells in which 91.8% ± 5.4% express CD34 at day 7, 98.6% ± 1.3% express CD43 at day 10, and 99.1% ± 0.95% of cells are CD235a positive by day 31 of the differentiation process. Moreover, this differentiation protocol supports extensive expansion, with a single hPSC producing up to 150 hematopoietic progenitor cells by day 10 and 50,000–200,000 erythroid cells by day 31. The erythroid cells produced exhibit a definitive fetal hematopoietic type, with 90%–95% fetal globin and variable proportion of embryonic and adult globin at the protein level. The presence of small molecules during the differentiation protocol has quantitative and qualitative effects; it increases the proportion of adult globin and decreases the proportion of embryonic globin. Given its level of definition, this system provides a powerful tool for investigation of the mechanisms governing early hematopoiesis and erythropoiesis, including globin switching and enucleation. The early stages of the differentiation protocol could also serve as a starting point for the production of endothelial cells and other hematopoietic cells, or to investigate the production of long-term reconstituting hematopoietic stem cells from hPSCs.

Significance

This differentiation protocol allows the production of a large amount of erythroid cells from pluripotent stem cells. Its efficiency is compatible with that of in vitro red blood cell production, and it can be a considerable asset for studying developmental erythropoiesis and red blood cell enucleation, thereby aiding both basic and translational research. In addition to red cells, the early stages of the protocol could also be used as a starting point for the large-scale production of other hematopoietic cell types, including the ultimate goal of generating long-term reconstituting hematopoietic stem cells.

Modeling murine yolk sac hematopoiesis with embryonic stem cell culture systems

The onset of hematopoiesis in mammals is defined by generation of primitive erythrocytes and macrophage progenitors in embryonic yolk sac. Laboratories have met the challenge of transient and swiftly changing specification events from ventral mesoderm through multipotent progenitors and maturing lineage-restricted hematopoietic subtypes, by developing powerful in vitro experimental models to interrogate hematopoietic ontogeny. Most importantly, studies of differentiating embryonic stem cell derivatives in embryoid body and stromal coculture systems have identified crucial roles for transcription factor networks (e.g. Gata1, Runx1, Scl) and signaling pathways (e.g. BMP, VEGF, WNT) in controlling stem and progenitor cell output. These and other relevant pathways have pleiotropic biological effects, and are often associated with early embryonic lethality in knockout mice. Further refinement in subsequent studies has allowed conditional expression of key regulatory genes, and isolation of progenitors via cell surface markers (e.g. FLK1) and reporter-tagged constructs, with the purpose of measuring their primitive and definitive hematopoietic potential. These observations continue to inform attempts to direct the differentiation, and augment the expansion, of progenitors in human cell culture systems that may prove useful in cell replacement therapies for hematopoietic deficiencies. The purpose of this review is to survey the extant literature on the use of differentiating murine embryonic stem cells in culture to model the developmental process of yolk sac hematopoiesis.

WT1 regulates murine hematopoiesis via maintenance of VEGF isoform ratio

Mutations in the Wilms tumor suppressor 1 (WT1) gene are as frequent in acute myeloid leukemia (AML) as in nephroblastma and predict poor prognosis. However, the role of WT1 in hematopoiesis remains unclear. We show that Wt1-deficient mouse embryonic stem cells exhibit reduced hematopoietic potential caused by vascular endothelial growth factor A (Vegf-a)-dependent apoptosis of hematopoietic progenitor cells associated with overproduction of the Vegf-a120 isoform. We demonstrate that Wt1 promotes exon inclusion using a Vegf-a minigene-based splicing assay. These data identify a critical role for Wt1 in hematopoiesis and Vegf-a as a cellular RNA whose splicing is potentially regulated by Wt1. The correction of Wt1 deficiency by treatment with exogenous Vegf-a protein indicates that the Wt1/Vegf-a axis is a molecular pathway that could be exploited for the management/treatment of poor prognosis AMLs.

Insights into the pathogenesis of systemic sclerosis based on the gene expression profile of progenitor-derived endothelial cells

OBJECTIVE

To determine the gene expression profile of endothelial cells derived from the endothelial progenitor cells (EPCs) of patients with systemic sclerosis (SSc).

METHODS

Microarray experiments were performed on Affymetrix GeneChip Human Exon 1.0 ST Arrays in unstimulated and hypoxia-stimulated EPC-derived cells from patients with SSc and control subjects. Followup of the raised hypotheses was performed ex vivo by immunohistochemical analysis of skin tissue.

RESULTS

Signals from 92 probe sets and 188 probe sets were different in unstimulated and hypoxia-stimulated cells, respectively, from patients with SSc compared with controls. Within the largest groups of genes related to cell-cell interaction and vascular remodeling, down-regulation of tumor necrosis factor ligand superfamily member 10 (TNFSF10) and homeobox A9 (HOX-A9) was confirmed by real-time polymerase chain reaction and Western blots in EPC-derived cells and by immunohistochemistry in SSc skin tissue. Signals from 221 and 307 probe sets were different in unstimulated and hypoxia-stimulated cells, respectively, from patients with diffuse cutaneous SSc compared with patients with limited cutaneous SSc. Within the largest group of genes related to the inflammatory response, differential expression of TNFα-induced protein 3 and prostaglandin-endoperoxide synthase 2 was observed in EPC-derived cells and skin tissue from patients with SSc.

CONCLUSION

Our data revealed important gene expression changes in EPC-derived endothelial cells from patients with SSc, characterized by a proadhesive, proinflammatory, and activated phenotype. Differential expression in lesional SSc skin tissue of new targets, such as TNF family members and HOX-A9, may contribute to the pathogenesis of SSc and deserves more in-depth exploration.

Itga2b regulation at the onset of definitive hematopoiesis and commitment to differentiation

Product of the Itga2b gene, CD41 contributes to hematopoietic stem cell (HSC) and megakaryocyte/platelet functions. CD41 expression marks the onset of definitive hematopoiesis in the embryo where it participates in regulating the numbers of multipotential progenitors. Key to platelet aggregation, CD41 expression also characterises their precursor, the megakaryocyte, and is specifically up regulated during megakaryopoiesis. Though phenotypically unique, megakaryocytes and HSC share numerous features, including key transcription factors, which could indicate common sub-regulatory networks. In these respects, Itga2b can serve as a paradigm to study features of both developmental-stage and HSC- versus megakaryocyte-specific regulations. By comparing different cellular contexts, we highlight a mechanism by which internal promoters participate in Itga2b regulation. A developmental process connects epigenetic regulation and promoter switching leading to CD41 expression in HSC. Interestingly, a similar process can be observed at the Mpl locus, which codes for another receptor that defines both HSC and megakaryocyte identities. Our study shows that Itga2b expression is controlled by lineage-specific networks and associates with H4K8ac in megakaryocyte or H3K27me3 in the multipotential hematopoietic cell line HPC7. Correlating with the decrease in H3K27me3 at the Itga2b Iocus, we find that following commitment to megakaryocyte differentiation, the H3K27 demethylase Jmjd3 up-regulation influences both Itga2b and Mpl expression.

Effect of endoglin overexpression during embryoid body development

Increasing evidence points to endoglin (Eng), an accessory receptor for the transforming growth factor-β superfamily commonly associated with the endothelial lineage, as an important regulator of the hematopoietic lineage. We have shown that lack of Eng results in reduced numbers of primitive erythroid colonies as well as downregulation of key hematopoietic genes. To determine the effect of Eng overexpression in hematopoietic development, we generated a doxycycline-inducible embryonic stem cell line. Our results demonstrate that induction of Eng during embryoid body differentiation leads to a significant increase in the frequency of hematopoietic progenitors, in particular, the erythroid lineage, which correlated with upregulation of Scl, Gata1, Runx1, and embryonic globin. Interestingly, activation of the hematopoietic program happened at the expense of endothelial and cardiac cells, as differentiation into these mesoderm lineages was compromised. Eng-induced enhanced erythroid activity was accompanied by high levels of Smad1 phosphorylation. This effect was attenuated by addition of a bone morphogenetic protein (BMP) signaling inhibitor to these cultures. Among the BMPs, BMP4 is well known for its role in hematopoietic specification from mesoderm by promoting expression of several hematopoietic genes, including Scl. Because Scl is considered the master regulator of the hematopoietic program, we investigated whether Scl would be capable of rescuing the defective hematopoietic phenotype observed in Eng(-/-) embryonic stem cells. Scl expression in Eng-deficient embryonic stem cells resulted in increased erythroid colony-forming activity and upregulation of Gata1 and Gata2, positioning Eng upstream of Scl. Taken together, these findings support the premise that Eng modulates the hematopoietic transcriptional network, most likely through regulation of BMP4 signaling.

Modulation of TGF-β signaling by endoglin in murine hemangioblast development and primitive hematopoiesis

Endoglin (Eng), an accessory receptor for the transforming growth factor β (TGF-β) superfamily, is required for proper hemangioblast and primitive hematopoietic development. However the mechanism by which endoglin functions at this early developmental stage is currently unknown. Transcriptional analyses of differentiating eng(-/-) and eng(+/+) ES cells revealed that lack of endoglin leads to profound reductions in the levels of key hematopoietic regulators, including Scl, Lmo2, and Gata2. We also detected lower levels of phosphorylated Smad1 (pSmad1), a downstream target signaling molecule associated with the TGF-β pathway. Using doxycycline-inducible ES cell lines, we interrogated the TGF-β signaling pathway by expressing activated forms of ALK-1 and ALK-5, type I receptors for TGF-β. Our results indicate that ALK-1 signaling promotes hemangioblast development and hematopoiesis, as evidenced by colony assays, gene expression and FACS analyses, whereas signaling by ALK-5 leads to the opposite effect, inhibition of hemangioblast and hematopoietic development. In Eng(-/-) ES cells, ALK-1 rescued both the defective hemangioblast development, and primitive erythropoiesis, indicating that ALK-1 signaling can compensate for the absence of endoglin. We propose that endoglin regulates primitive hematopoiesis by modulating the activity of the Smad1/5 signaling pathway in early stages of development.

Challenges and strategies for generating therapeutic patient-specific hemangioblasts and hematopoietic stem cells from human pluripotent stem cells

Recent characterization of hemangioblasts differentiated from human embryonic stem cells (hESC) has further confirmed evidence from murine, zebrafish and avian experimental systems that hematopoietic and endothelial lineages arise from a common progenitor. Such progenitors may provide a valuable resource for delineating the initial developmental steps of human hemato-endotheliogenesis, which is a process normally difficult to study due to the very limited accessibility of early human embryonic/fetal tissues. Moreover, efficient hemangioblast and hematopoietic stem cell (HSC) generation from patient-specific pluripotent stem cells has enormous potential for regenerative medicine, since it could lead to strategies for treating a multitude of hematologic and vascular disorders. However, significant scientific challenges remain in achieving these goals, and the generation of transplantable hemangioblasts and HSC derived from hESC currently remains elusive. Our previous work has suggested that the failure to derive engraftable HSC from hESC is due to the fact that current methodologies for differentiating hESC produce hematopoietic progenitors developmentally similar to those found in the human yolk sac, and are therefore too immature to provide adult-type hematopoietic reconstitution. Herein, we outline the nature of this challenge and propose targeted strategies for generating engraftable human pluripotent stem cell-derived HSC from primitive hemangioblasts using a developmental approach. We also focus on methods by which reprogrammed somatic cells could be used to derive autologous pluripotent stem cells, which in turn could provide unlimited sources of patient-specific hemangioblasts and HSC.

SIRT1 deficiency compromises mouse embryonic stem cell hematopoietic differentiation, and embryonic and adult hematopoiesis in the mouse

SIRT1 is a founding member of a sirtuin family of 7 proteins and histone deacetylases. It is involved in cellular resistance to stress, metabolism, differentiation, aging, and tumor suppression. SIRT1(-/-) mice demonstrate embryonic and postnatal development defects. We examined hematopoietic and endothelial cell differentiation of SIRT1(-/-) mouse embryonic stem cells (ESCs) in vitro, and hematopoietic progenitors in SIRT1(+/+)(+/-), and (-/-) mice. SIRT1(-/-) ESCs formed fewer mature blast cell colonies. Replated SIRT1(-/-) blast colony-forming cells demonstrated defective hematopoietic potential. Endothelial cell production was unaltered, but there were defects in formation of a primitive vascular network from SIRT1(-/-)-derived embryoid bodies. Development of primitive and definitive progenitors derived from SIRT1(-/-) ESCs were also delayed and/or defective. Differentiation delay/defects were associated with delayed capacity to switch off Oct4, Nanog and Fgf5 expression, decreased β-H1 globin, β-major globin, and Scl gene expression, and reduced activation of Erk1/2. Ectopic expression of SIRT1 rescued SIRT1(-/-) ESC differentiation deficiencies. SIRT1(-/-) yolk sacs manifested fewer primitive erythroid precursors. SIRT1(-/-) and SIRT1(+/-) adult marrow had decreased numbers and cycling of hematopoietic progenitors, effects more apparent at 5%, than at 20%, oxygen tension, and these progenitors survived less well in vitro under conditions of delayed growth factor addition. This suggests a role for SIRT1 in ESC differentiation and mouse hematopoiesis.

Common features of megakaryocytes and hematopoietic stem cells: what's the connection?

Megakaryocytes (Mks) are rare polyploid bone marrow cells whose function is to produce blood platelets. Since the purification and cloning of the major Mk cytokine, thrombopoietin, in 1994, considerable progress has been made in understanding the biology of Mk development. Remarkably, these advances have revealed a number of key features of Mks that are shared with hematopoietic stem cells (HSCs), such as common surface receptors, lineage-specific transcription factors, and specialized signaling pathways. Why there should be such a close connection between these two cell types remains unclear. In this Prospect article, we summarize the data supporting these shared features and speculate on possible teleological bases. In particular, we focus on common links involving developmental hierarchy, endothelial cells, and bone marrow niche interactions. This discussion highlights new data showing close ontologic relationship between HSCs and specialized "hemogenic" endothelial cells during development, and functional overlap between Mks/platelets and endothelial cells. Overall, these findings may be of relevance in the development of techniques for HSC ex vivo culture and/or possible generation of HSCs via somatic cell reprogramming.

Association of hereditary thrombocythemia and distal limb defects with a thrombopoietin gene mutation

Hereditary thrombocythemia is a rare autosomal dominant disorder caused by mutations in either the thrombopoietin gene (TPO) or its receptor c-MPL. TPO mutations described so far lead to thrombopoietin overproduction through increased translation of m-RNA. Unilateral transverse reduction limb defects are usually sporadic and generally thought to be caused by vascular disruptions. Reports of inherited unilateral limb defects are extremely rare. In the present study, we describe a family with segregation of G185T TPO mutation in the 5′ UTR region in 4 subjects with thrombocythemia. Three of these patients also present congenital transverse limb defects. Association of these events gives a strong hint of the in vivo involvement of thrombopoietin in vasculogenesis, confirming the role of TPO in human development of the hemangioblast, the embryonic progenitor of the hematopoietic and endothelial lineages. This is the first report showing that vascular disruptions could be secondary to specific gene derangements.

BMP and Wnt specify hematopoietic fate by activation of the Cdx-Hox pathway

The formation of blood in the embryo is dependent on bone morphogenetic protein (BMP), but how BMP signaling intersects with other regulators of hematopoietic development is unclear. Using embryonic stem (ES) cells, we show that BMP4 first induces ventral-posterior (V-P) mesoderm and subsequently directs mesodermal cells toward blood fate by activating Wnt3a and upregulating Cdx and Hox genes. When BMP signaling is blocked during this latter phase, enforced expression of either Cdx1 or Cdx4 rescues hematopoietic development, thereby placing BMP4 signaling upstream of the Cdx-Hox pathway. Wnt signaling cooperates in BMP-induced hemogenesis, and the Wnt effector LEF1 mediates BMP4 activation of Cdx genes. Our data suggest that BMP signaling plays two distinct and sequential roles during blood formation, initially as an inducer of mesoderm, and later to specify blood via activation of Wnt signaling and the Cdx-Hox pathway.

Hematopoiesis from Human Embryonic Stem Cells: Overcoming the Immune Barrier in Stem Cell Therapies

The multipotency and proliferative capacity of human embryonic stem cells (hESCs) make them a promising source of stem cells for transplant therapies and of vital importance given the shortage in organ donation. Recent studies suggest some immune privilege associated with hESC-derived tissues. However, the adaptability of the immune system makes it unlikely that fully differentiated tissues will permanently evade immune rejection. One promising solution is to induce a state of immune tolerance to a hESC line using tolerogenic hematopoietic cells derived from it. This could provide acceptance of other differentiated tissues from the same line. However, this approach will require efficient multilineage hematopoiesis from hESCs. This review proposes that more efficient differentiation of hESCs to the tolerogenic cell types required is most likely to occur through applying knowledge gained of the ontogeny of complex regulatory signals used by the embryo for definitive hematopoietic development in vivo. Stepwise formation of mesoderm, induction of definitive hematopoietic stem cells, and the application of factors key to their self-renewal may improve in vitro production both quantitatively and qualitatively.

Analysis of the temporal and concentration-dependent effects of BMP-4, VEGF, and TPO on development of embryonic stem cell-derived mesoderm and blood progenitors in a defined, serum-free media

OBJECTIVE

To develop a robust serum-free (SF) system for generation of hemogenic mesoderm and blood progenitors from pluripotent cells.

MATERIALS AND METHODS

Embryonic stem cells (ESCs) maintained in N2B27 supplemented with leukemia inhibitory factor (LIF) and bone morphogenetic protein (BMP)-4 were induced to differentiate into Brachyury/T-expressing cells (measured using a green fluorescent protein reporter) and myeloid-erythroid colony-forming cells (ME-CFCs), by removing LIF, changing the base media formulation, and via the time- and concentration-dependent addition of other factors.

RESULTS

Presence of 10 ng/mL BMP-4 permitted the emergence of cells expressing T and the vascular endothelial growth factor receptor (VEGFR)-2, however, <5% of the cells were double-positive on day 4. Adjusting the SF media formulation allowed only 5 ng/mL BMP-4 to yield 24% +/- 4% Brachyury-green fluorescent protein VEGFR-2(+) cells by day 4. These cells could develop into ME-CFC, producing 4.4 +/- 0.8 CFC per 1000 cells at day 8. We also examined the timing and concentration sensitivity of BMP-4, VEGF, and thrombopoietin (TPO) during differentiation. BMP-4 with 50 ng/mL TPO generated 232 +/- 48 CFC per 5 x 10(4) cells, similar to the serum-control, and this response could be enhanced to 292 +/- 42 CFC per 5 x 10(4) cells by early (between day 0-5), but not late (after day 5) VEGF treatment.

CONCLUSION

Moving to SF systems facilitates directed differentiation by eliminating confounding signals. This article describes modifications to the N2B27 media that amplify mesoderm induction and extends earlier work defining blood progenitor cell induction from ESC with BMP-4, VEGF, and TPO.

Dual role of Mpl receptor during the establishment of definitive hematopoiesis

Cytokine signaling pathways are important in promoting hematopoietic stem cell (HSC) self-renewal, proliferation and differentiation. Mpl receptor and its ligand, TPO, have been shown to play an essential role in the early steps of adult hematopoiesis. We previously demonstrated that the cytoplasmic domain of Mpl promotes hematopoietic commitment of embryonic stem cells in vitro, and postulated that Mpl could be important in the establishment of definitive hematopoiesis. To answer this question, we investigated the temporal expression of Mpl during mouse development by in situ hybridization. We found Mpl expression in the HSCs clusters emerging in the AGM region, and in the fetal liver (FL) as early as E10.5. Using Mpl(-/-) mice, the functional relevance of Mpl expression was tested by comparing the hematopoietic progenitor (HP) content, long-term hematopoietic reconstitution (LTR) abilities and HSC content of control and Mpl(-/-) embryos at different times of development. In the AGM, we observed delayed production of HSCs endowed with normal LTR but presenting a self-renewal defect. During FL development, we detected a decrease in HP and HSC potential associated with a defect in amplification and self-renewal/survival of the lin(-) AA4.1(+) Sca1(+) population of HSCs. These results underline the dual role of Mpl in the generation and expansion of HSCs during establishment of definitive hematopoiesis.

Endoglin is required for hemangioblast and early hematopoietic development

Endoglin (ENG), an ancillary receptor for several members of the transforming growth factor (TGF)-beta superfamily, has a well-studied role in endothelial function. Here, we report that endoglin also plays an important role early in development at the level of the hemangioblast, an embryonic progenitor of the hematopoietic and endothelial lineages. Eng(-/-), Eng(+/-) and Eng(+/+) mouse embryonic stem (ES) cells were differentiated as embryoid bodies (EBs) and assayed for blast colony-forming cells (BL-CFCs). Our results showed a profound reduction in hemangioblast frequency in the absence of endoglin. Furthermore, cell-sorting experiments revealed that endoglin marks the hemangioblast on day 3 of EB differentiation. When analyzed for hematopoietic and endothelial activity, replated Eng(-/-) BL-CFCs presented limited hematopoietic potential, whereas endothelial differentiation was unaltered. Analysis of hematopoietic colony formation of EBs, at different time points, further supports a function for endoglin in early hematopoiesis. Taken together, these findings point to a role for endoglin in both hemangioblast specification and hematopoietic commitment.

The Cdx-Hox Pathway in Hematopoietic Stem Cell Formation from Embryonic Stem Cells

Embryonic stem cells (ESCs) differentiated in vitro will yield a multitude of hematopoietic derivatives, yet progenitors displaying true stem cell activity remain difficult to obtain. Possible causes are a biased differentiation to primitive yolk sac-type hematopoiesis, and a variety of developmental or functional deficiencies. Recent studies in the zebrafish have identified the caudal homeobox transcription factors (cdx1/4) and posterior hox genes (hoxa9a, hoxb7a) as key regulators for blood formation during embryonic development. Activation of Cdx and Hox genes during the in vitro differentiation of mouse ESCs followed by co-culture on supportive stromal cells generates ESC-derived hematopoietic stem cells (HSCs) capable of multilineage repopulation of lethally irradiated adult mice. We show here that brief pulses of ectopic Cdx4 or HoxB4 expression are sufficient to enhance hematopoiesis during ESC differentiation, presumably by acting as developmental switches to activate posterior Hox genes. Insights into the role of the Cdx-Hox gene pathway during embryonic hematopoietic development in the zebrafish have allowed us to improve the derivation of repopulating HSCs from murine ESCs.

Identification of high proliferative potential precursors with hemangioblastic activity in the mouse aorta-gonad- mesonephros region

Hemangioblast, a precursor possessing hematopoietic and endothelial potential, is identified as the blast colony-forming cell in the murine gastrulating embryos (E7.0-E7.5). Whether hemangioblast exists in the somite-stage embryos is unknown, even though hemogenic endothelium is regarded as the precursor of definitive hematopoiesis in the aorta-gonad-mesonephros (AGM) region. To address the issue, we developed a unique three-step assay of high proliferative potential (HPP) precursors. The AGM region contained a kind of HPP precursor that displayed hematopoietic self-renewal capacity and was able to differentiate into functional endothelial cells in vitro (i.e., incorporating DiI-acetylated low-density lipoprotein, expressing von Willebrand factors, and forming network structures in Matrigel). The clonal nature was verified by cell mixing assay. However, the bilineage precursor with high proliferative potential-the HPP-hemangioblast (HA)-was not readily detected in the yolk sac (E8.25-E12.5), embryonic circulation (E10.5), placenta (E10.5-E11.5), fetal liver (E11.5-E12.5), and even umbilical artery (E11.5), reflective of its strictly spatial-regulated ontogeny. Expression of CD45, a panhematopoietic marker, distinguished hematopoietic-restricted HPP-colony-forming cell from the bipotential HPP-HA. Finally, we revealed that basic fibroblast growth factor, other than vascular endothelial growth factor or transforming growth factor-beta1, was a positive modulator of the HPP-HA proliferation. Taken together, the HPP-HA represents a novel model for definitive hemangioblast in the mouse AGM region and will shed light on molecular mechanisms underlying the hemangioblast development. Disclosure of potential conflicts of interest is found at the end of this article.

The megakaryocyte lineage originates from hemangioblast precursors and is an integral component both of primitive and of definitive hematopoiesis

In the adult, platelets are derived from unipotential megakaryocyte colony-forming cells (Meg-CFCs) that arise from bipotential megakaryocyte/erythroid progenitors (MEPs). To better define the developmental origin of the megakaryocyte lineage, several aspects of megakaryopoiesis, including progenitors, maturing megakaryocytes, and circulating platelets, were examined in the murine embryo. We found that a majority of hemangioblast precursors during early gastrulation contains megakaryocyte potential. Combining progenitor assays with immunohistochemical analysis, we identified 2 waves of MEPs in the yolk sac associated with the primitive and definitive erythroid lineages. Primitive MEPs emerge at E7.25 along with megakaryocyte and primitive erythroid progenitors, indicating that primitive hematopoiesis is bilineage in nature. Subsequently, definitive MEPs expand in the yolk sac with Meg-CFCs and definitive erythroid progenitors. The first GP1bbeta-positive cells in the conceptus were identified in the yolk sac at E9.5, while large, highly reticulated platelets were detected in the embryonic bloodstream beginning at E10.5. At this time, the number of megakaryocyte progenitors begins to decline in the yolk sac and expand in the fetal liver. We conclude that the megakaryocyte lineage initially originates from hemangioblast precursors during early gastrulation and is closely associated both with primitive and with definitive erythroid lineages in the yolk sac prior to the transition of hematopoiesis to intraembryonic sites.

Prenatal transplantation of cytokine-stimulated marrow improves early chimerism in a resistant strain combination but results in poor long-term engraftment

OBJECTIVE

In the absence of immunodeficiency, only microchimerism (<0.1%) has been achieved in human fetal recipients or nonhuman primates following in utero hematopoietic cell transplantation (IUHCT). We hypothesized that enhanced long-term engraftment might be more reliably achieved in microchimeric systems if higher levels of chimerism existed during development of adaptive immunity. To evaluate this hypothesis, we stimulated the donor cells with vascular endothelial growth factor (VEGF) and stem cell factor (SCF) prior to IUHCT in a chimerism-resistant murine strain combination.

METHODS

Donor Balb/c marrow was cultured in media with or without VEGF and SCF supplementation for 12 hours prior to IUHCT into B6 fetuses at 14 days postcoitum (dpc). Donor cell phenotype, homing, and chimerism were assessed at short and long-term time points and transplanted animals received skin allografts at 8 weeks.

RESULTS

In pretreated allogeneic recipients, early chimerism rates were more than double that of controls (71% vs 33%, p = 0.01). These differences were associated with higher numbers of pretransplant donor cell colony-forming cells without change in donor cell homing. Despite prolonged skin allograft survival for pretreated recipients compared with controls (mean survival = 20.8 vs 8.2 days, p < 0.001), long-term engraftment was unchanged.

CONCLUSIONS

These findings demonstrate that higher levels of early chimerism in recipients of cytokine-stimulated marrow result in improved short-term chimerism and tolerance. Future studies are needed to confirm the existence of a "threshold" level of chimerism necessary to sustain long-term engraftment.

Ape1 regulates hematopoietic differentiation of embryonic stem cells through its redox functional domain

Ape1 is a molecule with dual functions in DNA repair and redox regulation of transcription factors. In Ape1-deficient mice, embryos do not survive beyond embryonic day 9, indicating that this molecule is required for normal embryo development. Currently, direct evidence of the role of Ape1 in regulating hematopoiesis is lacking. We used the embryonic stem (ES) cell differentiation system and an siRNA approach to knockdown Ape1 gene expression to test the role of Ape1 in hematopoiesis. Hemangioblast development from ES cells was reduced 2- to 3-fold when Ape1 gene expression was knocked down by Ape1-specific siRNA, as was primitive and definitive hematopoiesis. Impaired hematopoiesis was not associated with increased apoptosis in siRNA-treated cells. To begin to explore the mechanism whereby Ape1 regulates hematopoiesis, we found that inhibition of the redox activity of Ape1 with E3330, a specific Ape1 redox inhibitor, but not Ape1 DNA repair activity, which was blocked using the small molecule methoxyamine, affected cytokine-mediated hemangioblast development in vitro. In summary, these data indicate Ape1 is required in normal embryonic hematopoiesis and that the redox function, but not the repair endonuclease activity, of Ape1 is critical in normal embryonic hematopoietic development.

The Mpl receptor expressed on endothelial cells does not contribute significantly to the regulation of circulating thrombopoietin levels

OBJECTIVE

Blood levels of thrombopoietin (TPO) are regulated in part by cellular degradation following its binding to the cell surface receptor c-mpl. Previous reports have demonstrated that in addition to hematopoietic cells, c-mpl is expressed on and functions in several types of endothelial cells (ECs). We hypothesized that the c-mpl expressed on ECs would contribute to the regulation of circulating TPO levels.

METHODS

To test this hypothesis we transplanted c-mpl-null and wild-type (WT) control mice with WT marrow stem cells, resulting in two groups of posttransplant chimeric animals, one expressing c-mpl on megakaryocytes and platelets only and one in which the receptor is expressed on both hematopoietic and ECs. Should EC c-mpl take up TPO and degrade it, we predicted that c-mpl-null mice reconstituted with WT cells would display increased TPO levels and an increased steady state platelet count compared to the WT recipients.

RESULTS

Contrary to our prediction, for up to 6 months posttransplantation both platelet counts and TPO levels in both groups of transplanted mice were virtually identical.

CONCLUSIONS

Our results indicate that the EC c-mpl receptor does not contribute significantly to the regulation of TPO levels or to steady-state platelet counts. These results also imply that patients with congenital amegakaryocytic thrombocytopenia, lacking the c-mpl receptor, who have successfully been engrafted with normal hematopoietic stem cells should have normal (not elevated) TPO levels and that gene replacement strategies designed to restore c-mpl in these patients do not need to target ECs to establish the normal regulation of TPO.

The thrombopoietin receptor, c-Mpl, is a selective surface marker for human hematopoietic stem cells

Background

Thrombopoietin (TPO), the primary cytokine regulating megakaryocyte proliferation and differentiation, exerts significant influence on other hematopoietic lineages as well, including erythroid, granulocytic and lymphoid lineages. We previously demonstrated that the receptor for TPO, c-mpl, is expressed by a subset of human adult bone marrow hematopoietic stem/progenitor cells (HSC/PC) that are enriched for long-term multilineage repopulating ability in the SCID-hu Bone in vivo model of human hematopoiesis.

Methods

Here, we employ flow cytometry and an anti-c-mpl monoclonal antibody to comprehensively define the surface expression pattern of c-mpl in four differentiation stages of human CD34⁺ HSC/PC (I: CD34⁺38^--, II: CD34⁺38^dim, III: CD34⁺38⁺, IV: CD34^dim38⁺) for the major sources of human HSC: fetal liver (FL), umbilical cord blood (UCB), adult bone marrow (ABM), and cytokine-mobilized peripheral blood stem cells (mPBSC). We use a surrogate in vivo model of human thymopoiesis, SCID-hu Thy/Liv, to compare the capacity of c-mpl⁺ vs. c-mpl^-- CD34⁺38^--/dim HSC/PC for thymocyte reconstitution.

Results

For all tissue sources, the percentage of c-mpl⁺ cells was significantly highest in stage I HSC/PC (FL 72 ± 10%, UCB 67 ± 19%, ABM 82 ± 16%, mPBSC 71 ± 15%), and decreased significantly through stages II, III, and IV ((FL 3 ± 3%, UCB 8 ± 13%, ABM 0.6 ± 0.6%, mPBSC 0.2 ± 0.1%) [ANOVA: P < 0.0001]. The relative median fluorescence intensity of c-mpl expression was similarly highest in stage I, decreasing through stage IV [ANOVA: P < 0.0001]. No significant differences between tissue sources were observed for either % c-mpl⁺ cells [P = 0.89] or intensity of c-mpl expression [P = 0.21]. Primary Thy/Liv grafts injected with CD34⁺38^--/dimc-mpl⁺ cells showed slightly higher levels of donor HLA⁺ thymocyte reconstitution vs. CD34⁺38^--/dimc-mpl^---injected grafts and non-injected controls (c-mpl⁺ vs. c-mpl^--: CD2⁺ 6.8 ± 4.5% vs. 2.8 ± 3.3%, CD4⁺8^-- 54 ± 35% vs. 31 ± 29%, CD4^--8⁺ 29 ± 19% vs. 18 ± 14%).

Conclusion

These findings support the hypothesis that the TPO receptor, c-mpl, participates in the regulation of primitive human HSC from mid-fetal through adult life. This study extends our previous work documenting human B-lineage, myeloid and CD34⁺ cell repopulation by c-mpl⁺ progenitors to show that c-mpl⁺ HSC/PC are also capable of significant T-lineage reconstitution in vivo. These results suggest that c-mpl merits consideration as a selective surface marker for the identification and isolation of human HSC in both basic research and clinical settings.

Embryonic stem cell-derived hematopoietic stem cells

Despite two decades of studies documenting the in vitro blood-forming potential of murine embryonic stem cells (ESCs), achieving stable long-term blood engraftment of ESC-derived hematopoietic stem cells in irradiated mice has proven difficult. We have exploited the Cdx-Hox pathway, a genetic program important for blood development, to enhance the differentiation of ESCs along the hematopoietic lineage. Using an embryonic stem cell line engineered with tetracycline-inducible Cdx4, we demonstrate that ectopic Cdx4 expression promotes hematopoietic mesoderm specification, increases hematopoietic progenitor formation, and, together with HoxB4, enhances multilineage hematopoietic engraftment of lethally irradiated adult mice. Clonal analysis of retroviral integration sites confirms a common stem cell origin of lymphoid and myeloid populations in engrafted primary and secondary mice. These data document the cardinal stem cell features of self-renewal and multilineage differentiation of ESC-derived hematopoietic stem cells.

Thrombopoietin gene transfer-mediated enhancement of angiogenic responses to acute ischemia

The development of new blood vessels is a complex process, likely requiring the synergy of multiple angiogenic mediators. This study focuses on the proximal angiogenic response using the platelet as a complex carrier of critical mediators of angiogenesis. Platelet levels are controlled by circulating levels of thrombopoietin (TPO) functioning to activate megakaryocyte differentiation and platelet release through the c-mpl receptor. We hypothesized that TPO gene transfer should enhance correction of experimental ischemia by providing increased levels of platelets and hence platelet-derived mediators of angiogenesis. To evaluate this hypothesis, we dissected the role of the TPO-c-mpl-megakaryocyte-platelet pathway in the angiogenic response using a model of acute hindlimb ischemia of wild-type, TPO(-/-), and c-mpl(-/-) mice. The data demonstrate that infusion of platelets will enhance the angiogenic response in wild-type mice and that the endogenous angiogenic response is blunted in TPO(-/-) and c-mpl(-/-) mice. Consistent with this observation, adenovirus (Ad)-mediated transfer of TPO (AdTPO) enhanced the correction of ischemia in wild-type and TPO(-/-), but not c-mpl(-/-), mice. Local versus systemic administration of AdTPO showed that the effect of TPO gene transfer was systemic, not local, and it could be replaced by gene transfer of VEGF, one of the many mediators of angiogenesis carried by the platelets, even in the absence of components in the TPO-c-mpl-megakaryocyte-platelet pathway.

Soluble factors elaborated by human brain endothelial cells induce the concomitant expansion of purified human BM CD34+CD38- cells and SCID-repopulating cells

The CD34(+)CD38- phenotype identifies a population in the bone marrow that is enriched in the steady state for hematopoietic stem cells (HSCs). Following ex vivo culture of CD34(+) cells, HSC content is difficult to measure since committed CD34(+)CD38+ progenitors down-regulate CD38 surface expression during culture. In this study, we sought to define the phenotype of human HSCs following ex vivo culture under conditions that support the expansion of human cells capable of repopulating non-obese diabetic/severe combined immunodeficiency (SCID)-repopulating cells (SRCs). Contact coculture of fluorescence-activated cell sorter (FACS)-sorted bone marrow (BM) CD34(+)CD38- cells with human brain endothelial cells (HUBECs) supported a 4.4-fold increase in CD34(+)CD38- cells with a concordant 3.6-fold increase in SRCs over 7 days. Noncontact HUBEC cultures and the addition of thrombopoietin, stem cell factor (SCF), and macrophage colony stimulating factor I receptor (Fms)-like tyrosine kinase 3 (Flt-3) ligand supported further increases in CD34(+)CD38- cells (6.4-fold and 13.1-fold), which correlated with significant increases in SRC activity. Moreover, cell-sorting studies performed on HUBEC-cultured populations demonstrated that SRCs were significantly enriched within the CD34(+)CD38- subset compared with the CD34(-)CD38- population after culture. These results indicate that human HSCs can be identified and characterized by phenotype following expansion culture. These studies also demonstrate that HUBEC-elaborated soluble factors mediate a unique and potent expansion of human HSCs.

Commentary: the role of cell migration in the ontogeny of the lymphoid system

The foundations of experimental hematology were laid by histologists, and while their contributions were enormous, they were limited in their interpretation of very dynamic processes by the static nature of the methodology. The middle of the twentieth century saw the introduction of techniques for hematopoietic cell marking and development of in vitro and in vivo assays for primitive hematopoietic cells, allowing dynamic studies of hematopoiesis. Paralleling this was an understanding of cellular immunology with the discovery of the role of the thymus and the identification of T and B lymphocyte lineages. In the 1960s a series of ontogenetic studies in birds and subsequently in mice revealed that hematopoietic and lymphoid development involved migration streams of primitive cells that colonized developing primary lymphoid organs as well as spleen, marrow, and liver. The yolk sac was proposed as the ultimate origin of these lympho-hematopoietic precursors. Subsequent studies identified a region associated with the dorsal aorta as the primary site of "definitive" stem cells. These opposing views are currently achieving a compromise that recognizes that both sites contribute stem cells involved in seeding the developing tissues. The clear distinction between the local origin of the inducing microenvironment provided by the endoderm or by stroma derived from mesenchymal stem cells of mesodermal origin, and the immigrant origin of the hematopoietic stem cells and progenitors, raises intriguing questions in the current climate of stem cell plasticity, cell fusion, and discovery of stem cells in adult marrow with the capacity to generate hematopoiesis as well as other mesodermal, ectodermal, and endodermal lineages.

Origins of mammalian hematopoiesis: in vivo paradigms and in vitro models

Though a topic of medical interest for centuries, our understanding of vertebrate hematopoietic or "blood-forming" tissue development has improved greatly only in recent years and given a series of scientific and technical milestones. Key among these observations was the description of procedures that allowed the transplantation of blood-forming activity. Beyond this, other advances include the creation of a variety of knock-out animals (mice and more recently zebrafish), microdissection of embryonic and fetal blood-forming tissues, hematopoietic stem (HSC) and progenitor cell (HPC) colony-forming assays, the discovery of cytokines with defined hematopoietic activities, gene transfer technologies, and the description of lineage-specific surface antigens for the identification and purification of pluripotent and differentiated blood cells. The availability of both murine and human embryonic stem cells (ESC) and the delineation of in vitro systems to direct their differentiation have now been added to this analytical arsenal. Such tools have allowed researchers to interrogate the complex developmental processes behind both primitive (yolk sac or extraembryonic) and definitive (intraembryonic) hematopoietic tissue formation. Using ES cells, we hope to not only gain additional basic insights into hematopoietic development but also to develop platforms for therapeutic use in patients suffering from hematological disease. In this review, we will focus on points of convergence and divergence between murine and human hematopoiesis in vivo and in vitro, and use these observations to evaluate the literature regarding attempts to create hematopoietic tissue from embryonic stem cells, the pitfalls encountered therein, and what challenges remain.