Evidence for involvement of activin A and bone morphogenetic protein 4 in mammalian mesoderm and hematopoietic development

Ontogeny of the hematopoietic system

Blood cells are constantly produced in the bone marrow (BM) of adult mammals. This constant turnover ultimately depends on a rare population of progenitors that displays self-renewal and multilineage differentiation potential, the hematopoietic stem cells (HSCs). It is generally accepted that HSCs are generated during embryonic development and sequentially colonize the fetal liver, the spleen, and finally the BM. Here we discuss the experimental evidence that argues for the extrinsic origin of HSCs and the potential locations where HSC generation might occur. The identification of the cellular components playing a role in the generation process, in these precise locations, will be important in understanding the molecular mechanisms involved in HSC production from undifferentiated mesoderm.

Differentiation of human embryonic stem cells in serum-free medium reveals distinct roles for bone morphogenetic protein 4, vascular endothelial growth factor, stem cell factor, and fibroblast growth factor 2 in hematopoiesis

We have utilized a serum- and stromal cell-free "spin embryoid body (EB)" differentiation system to investigate the roles of four growth factors, bone morphogenetic protein 4 (BMP4), vascular endothelial growth factor (VEGF), stem cell factor (SCF), and basic fibroblast growth factor (FGF2), singly and in combination, on the generation of hematopoietic cells from human embryonic stem cells (HESCs). Of the four factors, only BMP4 induced expression of genes that signaled the emergence of the primitive streak-like population required for the subsequent development of hematopoietic mesoderm. In addition, BMP4 initiated the expression of genes marking hematopoietic mesoderm and supported the generation of hematopoietic progenitor cells at a low frequency. However, the appearance of robust numbers of hematopoietic colony forming cells and their mature progeny required the inclusion of VEGF. Finally, the combination of BMP4, VEGF, SCF, and FGF2 further enhanced the total yield of hematopoietic cells. These data demonstrate the utility of the serum-free spin EB system in dissecting the roles of specific growth factors required for the directed differentiation of HESCs toward the hematopoietic lineage.

Germ layer induction in ESC--following the vertebrate roadmap

Controlled differentiation of pluripotential cells takes place routinely and with great success in developing vertebrate embryos. It therefore makes sense to take note of how this is achieved and use this knowledge to control the differentiation of embryonic stem cells (ESCs). An added advantage is that the differentiated cells resulting from this process in embryos have proven functionality and longevity. This unit reviews what is known about the embryonic signals that drive differentiation in one of the most informative of the vertebrate animal models of development, the amphibian Xenopus laevis. It summarizes their identities and the extent to which their activities are dose-dependent. The unit details what is known about the transcription factor responses to these signals, describing the networks of interactions that they generate. It then discusses the target genes of these transcription factors, the effectors of the differentiated state. Finally, how these same developmental programs operate during germ layer formation in the context of ESC differentiation is summarized.

In vitro differentiation of murine embryonic stem cells toward a renal lineage

Embryonic stem (ES) cells have the capacity to differentiate into all cells of the developing embryo and may provide a renewable resource for future cell replacement therapies. The addition of bone morphogenetic protein 4 (BMP4) to serum-free ES cell culture has previously been shown to induce transcription factors, signaling molecules, and cell adhesion proteins expressed during mesoderm specification of the embryo. Here, we show the dynamics of primitive streak mesoderm differentiation in ES cells is comparable between serum and serum-free embryoid body (EB) cultures, supplemented with BMP4. Furthermore, we show a delayed wave of expression of a cohort of genes (Pax2, WT1, podocalyxin, pod-1, and nephrin), which play important roles during embryonic kidney development. The paired box transcription factor, Pax2, is one of the earliest genes expressed during kidney organogenesis and is required for normal urogenital development. ES cell lines containing either a modified Pax2 promoter-lacZ or bacterial artificial chromosome-green fluorescent protein (GFP) transgene were generated, which enabled the quantitative analysis of kidney rather than neuronal Pax2 expression within EBs. Both beta-galactosidase activity and GFP expression were detected by immunohistochemical and flow cytometric analysis following 16 days of EB culture, which correlated with an increase in Pax2 transcript levels. Together, these results suggest a spontaneous kidney gene expression program develops in mature EBs grown in both serum and serum-free conditions, when supplemented with BMP4. Further, the recombinant growth factors BMP2, BMP4, and BMP7 strongly influence gene expression within mesoderm induced EBs. BMP4 promotes ventral (blood) and intermediate (kidney) mesoderm gene expression, whereas BMP2 and BMP7 promote kidney outcomes at the expense of hematopoietic commitment. This induction assay and these unique ES cell lines will be useful for the generation of mesoderm-derived cell populations with implications for future cell therapeutic/integration assays.

Differential requirements for hematopoietic commitment between human and rhesus embryonic stem cells

Progress toward clinical application of ESC-derived hematopoietic cellular transplantation will require rigorous evaluation in a large animal allogeneic model. However, in contrast to human ESCs (hESCs), efforts to induce conclusive hematopoietic differentiation from rhesus macaque ESCs (rESCs) have been unsuccessful. Characterizing these poorly understood functional differences will facilitate progress in this area and likely clarify the critical steps involved in the hematopoietic differentiation of ESCs. To accomplish this goal, we compared the hematopoietic differentiation of hESCs with that of rESCs in both EB culture and stroma coculture. Initially, undifferentiated rESCs and hESCs were adapted to growth on Matrigel without a change in their phenotype or karyotype. Subsequent differentiation of rESCs in OP9 stroma led to the development of CD34(+)CD45(-) cells that gave rise to endothelial cell networks in methylcellulose culture. In the same conditions, hESCs exhibited convincing hematopoietic differentiation. In cytokine-supplemented EB culture, rESCs demonstrated improved hematopoietic differentiation with higher levels of CD34(+) and detectable levels of CD45(+) cells. However, these levels remained dramatically lower than those for hESCs in identical culture conditions. Subsequent plating of cytokine-supplemented rhesus EBs in methylcellulose culture led to the formation of mixed colonies of erythroid, myeloid, and endothelial cells, confirming the existence of bipotential hematoendothelial progenitors in the cytokine-supplemented EB cultures. Evaluation of four different rESC lines confirmed the validity of these disparities. Although rESCs have the potential for hematopoietic differentiation, they exhibit a pause at the hemangioblast stage of hematopoietic development in culture conditions developed for hESCs.

A scaleable and defined system for generating neural stem cells from human embryonic stem cells

The ability to differentiate human ESCs (hESCs) to defined lineages in a totally controlled manner is fundamental to developing cell-based therapies and studying human developmental mechanisms. We report a novel, scaleable, and widely applicable system for deriving and propagating neural stem cells from hESCs without the use of animal products, proprietary formulations, or genetic manipulation. This system provides a definitive platform for studying human neural development and has potential therapeutic implications.

Comparative proteomic analysis reveals differential expression of Hsp25 following the directed differentiation of mouse embryonic stem cells

Murine embryonic stem (ES) cells can be committed to neural differentiation with high efficiency in culture through the use of feeder- and serum-free media. This system is proving to be an excellent model to study processes involved in ES cell commitment to neural cell fate. We used this approach to generate neurogenic embryoid bodies (NEBs) in a serum-free culture system to perform proteomic analysis of soluble fractions and identify early changes in protein expression as ES cells differentiate. Ten candidate proteins were altered significantly in expression levels. One of the most significant alterations was for the small heat shock protein Hsp25. Three species of Hsp25 are detected in ES cells, and this expression pattern changes during the first 24 h of differentiation until expression is decreased to levels that are barely detectable at 4 days following differentiation. We used immunofluorescence studies to confirm that following ES cell differentiation, expression of Hsp25 becomes excluded from neural precursors as well as other differentiating cells, making it a potentially useful marker of early ES cell differentiation.

Embryonic stem cells remain highly pluripotent following long term expansion as aggregates in suspension bioreactors

Increasing attention has been drawn towards pluripotent embryonic stem cells (ESCs) and their potential use as the primary material in various tissue engineering applications. Successful clinical implementation of this technology would require a quality controlled reproducible culture system for the expansion of the cells to be used in the generation of functional tissues. Recently, we showed that suspension bioreactors could be used in the regulated large-scale expansion of highly pluripotent murine ESCs. The current study illustrates that these bioreactor protocols can be adapted for long term culture and that murine ESC cultures remain highly undifferentiated, when serially passaged in suspension bioreactors for extended periods. Flow cytometry analysis and gene expression profiles of several pluripotency markers, in addition to colony and embryoid body (EB) formation tests were conducted at the start and end of the experiment and all showed that the ESC cultures remained highly undifferentiated over extended culture time in suspension. In vivo teratoma formation and in vitro differentiation into neural, cardiomyocyte, osteoblast and chondrocyte lineages, performed at the end of the long term culture, further supported the presence of functional and undifferentiated ESCs in the expanded population. Overall, this system enables the controlled expansion of highly pluripotent murine ESC populations.

Human embryonic stem cells: an in vitro model to study mechanisms controlling pluripotency in early mammalian development

The property of pluripotency confers the capacity for differentiation into a large number of cell types including extra-embryonic, somatic and germinal cells. During normal development, pluripotency is acquired by the cells of the early embryo, which shortly thereafter undergo differentiation, whereas embryonic stem cells (ESCs) uniquely maintain pluripotency while undergoing extensive in vitro proliferation. Studies using ESCs have begun to unravel the network of cytokines and transcription factors responsible for their maintenance of pluripotency. Surprisingly, mouse and human ESCs display significant differences in such mechanisms despite their similar embryonic origins. In this review, we compare the properties of pluripotent embryonic cells with those of ESCs to establish a general model for the mechanisms maintaining pluripotency. We first consider whether mouse and human ESCs represent comparable stages of early embryonic development. We then describe how human embryoid body (EB) differentiation could be used as a model of embryonic development. Finally, to concretely illustrate the discussion, we discuss our recent results concerning Nodal function in controlling cell fate at early stages of human EB development. With the new perspective of these findings, we suggest a previously unrecognized role of TGF-beta pathway signaling in maintaining pluripotency at early stages of mammalian embryonic development.

Cross-species transcriptional profiles establish a functional portrait of embryonic stem cells

An understanding of the regulatory mechanisms responsible for pluripotency in embryonic stem cells (ESCs) is critical for realizing their potential in medicine and science. Significant similarities exist among ESCs harvested from different species, yet major differences have also been observed. Here, by cross-species analysis of a large set of functional categories and all transcription factors and growth factors, we reveal conserved and divergent functional landscapes underlining fundamental and species-specific mechanisms that regulate ESC development. Global transcriptional trends derived from all expressed genes, instead of differentially expressed genes alone, were examined, allowing for a higher discriminating power in the functional portrait. We demonstrate that cross-species correlation of transcriptional changes that occur upon ESC differentiation is a powerful predictor of ESC-important biological pathways and functional cores within a pathway. Hundreds of functional modules, as defined by Gene Ontology, were associated with conserved expression patterns but bear no overt relationship to ESC development, suggestive of new mechanisms critical to ESC pluripotency. Yet other functional modules were not conserved; instead, they were significantly up-regulated in ESCs of either species, suggestive of species-specific regulation. The comparisons of ESCs across species and between human ESCs and embryonal carcinoma stem cells suggest that while pluripotency as an essential function in multicellular organisms is conserved throughout evolution, mechanisms primed for differentiation are less conserved and contribute substantially to the differences among stem cells derived from different tissues or species. Our findings establish a basis for defining the "stemness" properties of ESCs from the perspective of functional conservation and variation. The data and analyses resulting from this study provide a framework for new hypotheses and research directions and a public resource for functional genomics of ESCs.

N-cadherin is a useful marker for the progenitor of cardiomyocytes differentiated from mouse ES cells in serum-free condition

Cardiomyocytes are known to differentiate spontaneously from embryonic stem (ES) cells when they formed aggregates, so called "embryoid bodies", in the presence of serum. In this study, we explored the induction of cardiomyocytes from mouse ES cells in chemically defined serum-free medium by using a mesoderm-inducing factor, BMP4. Comparing the different inductive methods, we found a candidate cell surface marker, N-cadherin, for cardiomyocyte progenitors from ES cells. N-cadherin-positive cells highly expressed cardiogenic markers, Nkx2.5, Tbx5, and Isl1, and showed a high differentiation rate into cardiomyocyte lineage. These results indicate that N-cadherin can be a useful cell surface marker for the progenitors of cardiomyocyte differentiated from ES cells in the serum-free culture.

Expansion of Undifferentiated Murine Embryonic Stem Cells as Aggregates in Suspension Culture Bioreactors

Pluripotent embryonic stem cells (ESCs) have recently been considered as a primary material for regenerating tissues lost to injuries and degenerative diseases. For clinical implementation of this technology, a quality controlled, reproducible culture system is necessary for the expansion and differentiation of the cells. Used in many bioprocess applications, suspension bioreactors have gained considerable attention for the regulated large-scale expansion of cells. The current study presents a bioreactor process for the large-scale expansion of undifferentiated murine ESCs as aggregates. In this system, the level of ESC aggregation and differentiation was effectively controlled by adjusting shear forces and inoculation density, achieving a 31-fold expansion in 5 days. Pluripotency markers Oct-4, Nanog, SSEA-1, ALP, and rex-1 were assessed using flow cytometry analysis and gene expression profiles and showed that the undifferentiated nature of the cells within the ESC aggregates was maintained. Colony-forming efficiencies and embryoid body formation tests of the expanded cultures demonstrated that characteristic functional attributes of undifferentiated cells were not lost. Overcoming a major impediment in the area of ESC expansion, this study describes a successful process for the controlled and reproducible largescale expansion of ESCs using suspension culture bioreactors.

Smad1 expands the hemangioblast population within a limited developmental window

Bone morphogenetic protein (BMP) signaling is an important regulator of hematovascular development. However, the progenitor population that responds to BMP signaling is undefined, and the relative role of downstream mediators including Smad1 is unclear. We find that Smad1 shows a distinctive expression profile as embryonic stem (ES) cells undergo differentiation in the embryoid body (EB) system, with peak levels in cell populations enriched for the hemangioblast. To test the functional relevance of this observation, we generated an ES cell line that allows temporal control of ectopic Smad1 expression. Continuous expression of Smad1 from day 2 of EB culture does not disturb hematopoiesis, according to colony assays. In contrast, a pulse of Smad1 expression exclusively between day 2 and day 2.25 expands the population of progenitors for primitive erythroblasts and other hematopoietic lineages. This effect correlates with increased levels of transcripts encoding markers for the hemangioblast, including Runx1, Scl, and Gata2. Indeed, the pulse of Smad1 induction also expands the blast colony-forming cell (BL-CFC) population at a level that is fully sufficient to explain subsequent increases in hematopoiesis. Our data demonstrate that Smad1 expression is sufficient to expand the number of cells that commit to hemangioblast fate.

Tal1/Scl Gene Transduction Using a Lentiviral Vector Stimulates Highly Efficient Hematopoietic Cell Differentiation from Common Marmoset (Callithrix jacchus) Embryonic Stem Cells

The development of embryonic stem cell (ESC) therapies requires the establishment of efficient methods to differentiate ESCs into specific cell lineages. Here, we report the in vitro differentiation of common marmoset (CM) (Callithrix jacchus) ESCs into hematopoietic cells after exogenous gene transfer using vesicular stomatitis virus-glycoprotein-pseudotyped lentiviral vectors. We transduced hematopoietic genes, including tal1/scl, gata1, gata2, hoxB4, and lhx2, into CM ESCs. By immunochemical and morphological analyses, we demonstrated that overexpression of tal1/scl, but not the remaining genes, dramatically increased hematopoiesis of CM ESCs, resulting in multiple blood-cell lineages. Furthermore, flow cytometric analysis demonstrated that CD34, a hematopoietic stem/progenitor cell marker, was highly expressed in tal1/scl-overexpressing embryoid body cells. Similar results were obtained from three independent CM ESC lines. These results suggest that transduction of exogenous tal1/scl cDNA into ESCs is a promising method to induce the efficient differentiation of CM ESCs into hematopoietic stem/progenitor cells.

Canonical Wnt signaling is required for development of embryonic stem cell-derived mesoderm

Formation of mesoderm from the pluripotent epiblast depends upon canonical Wnt/beta-catenin signaling, although a precise molecular basis for this requirement has not been established. To develop a robust model of this developmental transition, we examined the role of Wnt signaling during the analogous stage of embryonic stem cell differentiation. We show that the kinetics of Wnt ligand expression and pathway activity in vitro mirror those found in vivo. Furthermore, inhibition of this endogenous Wnt signaling abrogates the functional competence of differentiating ES cells, reflected by their failure to generate Flk1(+) mesodermal precursors and subsequent mature mesodermal lineages. Microarray analysis at various times during early differentiation reveal that mesoderm- and endoderm-associated genes fail to be induced in the absence of Wnt signaling, indicating a lack of germ layer induction that normally occurs during gastrulation in vivo. The earliest genes displaying Wnt-dependent expression, however, were those expressed in vivo in the primitive streak. Using an inducible form of stabilized beta-catenin, we find that Wnt activity, although required, does not autonomously promote primitive streak-associated gene expression in vitro. Our results suggest that Wnt signaling functions in this model system to regulate the thresholds or stability of responses to other effector pathways and demonstrate that differentiating ES cells represent a useful model system for defining complex regulatory interactions underlying primary germ layer induction.

Developmental relationship between hematopoietic and endothelial cells

Blood (hematopoietic cells) and blood vessels (endothelial cells) develop from mesoderm via a transitional progenitor known as the hemangioblast. Flk-1, a receptor tyrosine kinase, and Scl, a basic helix-loop-helix transcription factor, are two critical molecules functioning in this process. Recent studies have shown that Flk-1 expressing mesoderm contributes to the circulatory system, including hematopoietic, endothelial, smooth muscle, skeletal muscle, and cardiac muscle cells. Our studies suggest that hemangioblast specification within Flk-1 expressing mesoderm is regulated by Scl expression. Herein, we review studies that have utilized transgenic mouse models as well as an in vitro model of embryonic stem cell differentiation, both of which have greatly contributed to the current understanding of the cellular and molecular pathways regulating hemangioblast development and differentiation.

Bone morphogenetic protein receptor 1A signaling is dispensable for hematopoietic development but essential for vessel and atrioventricular endocardial cushion formation

Bone morphogenetic protein 4 (BMP4) is crucial for the formation of FLK1-expressing (FLK1(+)) mesodermal cells. To further define the requirement for BMP signaling in the differentiation of blood, endothelial and smooth muscle cells from FLK1(+) mesoderm, we inactivated Alk3 (Bmpr1a) in FLK1(+) cells by crossing Alk3(floxed/floxed) and Flk1(+/Cre)Alk3(+/floxed) mice. Alk3 conditional knockout (CKO) mice died between E10.5 and E11.5. Unexpectedly, Alk3 CKO embryos did not show any hematopoietic defects. However, Alk3 CKO embryos displayed multiple abnormalities in vascular development, including vessel remodeling and maturation, which contributed to severe abdominal hemorrhage. Alk3 CKO embryos also displayed defects in atrioventricular canal (AVC) endocardial cushion formation in the heart. Collectively, our studies indicate a crucial role for ALK3 in vessel remodeling, vessel integrity and endocardial cushion formation during the development of the circulation system.

Embryonic stem cell-derived neural progenitors display temporal restriction to neural patterning

Neural stem cells have considerable therapeutic potential because of their ability to generate defined neuronal cell types for use in drug screening studies or cell-based therapies for neurodegenerative diseases. In this study, we differentiate mouse embryonic stem cells to neural progenitors with an initial forebrain identity in a defined system that enables systematic manipulation to generate more caudal fates, including motoneurons. We demonstrate that the ability to pattern embryonic stem cell-derived neural progenitors is temporally restricted and show that the loss of responsiveness to morphogenetic cues correlates with constitutive expression of the basic helix-loop-helix transcription factors Olig2 and Mash1, epidermal growth factor receptor, and vimentin and parallels the onset of gliogenesis. We provide evidence for two temporal classes of embryonic stem cell-derived putative radial glia that coincide with a transition from neurogenesis to gliogenesis and a concomitant loss of regional identity.

Nanog binds to Smad1 and blocks bone morphogenetic protein-induced differentiation of embryonic stem cells

ES cells represent a valuable model for investigating early embryo development and hold promise for future regenerative medicine strategies. The self-renewal of pluripotent mouse ES cells has been shown to require extrinsic stimulation by the bone morphogenetic protein (BMP) and leukemia inhibitory factor signaling pathways and the expression of the transcription factors Oct4 and Nanog. However, the network of interactions among extrinsic and intrinsic determinants of ES cell pluripotency is currently poorly understood. Here, we show that Nanog expression is up-regulated in mouse ES cells by the binding of T (Brachyury) and STAT3 to an enhancer element in the mouse Nanog gene. We further show that Nanog blocks BMP-induced mesoderm differentiation of ES cells by physically interacting with Smad1 and interfering with the recruitment of coactivators to the active Smad transcriptional complexes. Taken together, our findings illustrate the existence of ES cell-specific regulatory networks that underlie the maintenance of ES cell pluripotency and provide mechanistic insights into the role of Nanog in this process.

Neural conversion of ES cells by an inductive activity on human amniotic membrane matrix

Here we report a human-derived material with potent inductive activity that selectively converts ES cells into neural tissues. Both mouse and human ES cells efficiently differentiate into neural precursors when cultured on the matrix components of the human amniotic membrane in serum-free medium [amniotic membrane matrix-based ES cell differentiation (AMED)]. AMED-induced neural tissues have regional characteristics (brainstem) similar to those induced by coculture with mouse PA6 stromal cells [a common method called stromal cell-derived inducing activity (SDIA) culture]. Like the SDIA culture, the AMED system is applicable to the in vitro generation of various CNS tissues, including dopaminergic neurons, motor neurons, and retinal pigment epithelium. In contrast to the SDIA method, which uses animal cells, the AMED culture uses a noncellular inductive material derived from an easily available human tissue; therefore, AMED should provide a more suitable and versatile system for generating a variety of neural tissues for clinical applications.

Mechanisms of self-renewal in human embryonic stem cells

Embryonic stem cells (ESCs) are the pluripotent cell population derived from the inner cell mass of pre-implantation embryos and are characterised by prolonged self-renewal and the potential to differentiate into cells representing all three germ layers both in vitro and in vivo. Preservation of the undifferentiated status of the ESC population requires the maintenance of self-renewal whilst inhibiting differentiation and regulating senescence and apoptosis. In this review, we discuss the intrinsic and extrinsic factors associated with self-renewal process, together with possible signalling pathway interactions and mechanisms of regulation.

Human embryonic stem cells: the battle between self-renewal and differentiation

Human embryonic stem cells are pluripotent cells derived from the inner cell mass of blastocyst-stage embryos. These cells possess two unique properties: an indefinite self-renewal capacity and pluripotency, the ability to differentiate to cells from the three germ layers. The pathways governing self-renewal and pluripotency are currently under intensive research. Much effort is devoted to the establishment of feeder-free cultures by elucidation of the cytokines and growth factors required for cell propagation. These seem thus far, to be distinct from those required by mouse embryonic stem cells. In addition, transcriptional regulators unique to embryonic stem cells seem to govern the pluripotent state. These transcriptional regulators determine cell fate, and decide whether the cell will remain pluripotent or differentiate. Together, the understanding of the exogenous and endogenous factors determining cell fate will facilitate the use of these cells in cell-based therapies and will allow understanding of early developmental processes.

Embryonic stem cells and cardiomyocyte differentiation: phenotypic and molecular analyses

Embryonic stem (ES) cell lines, derived from the inner cell mass (ICM) of blastocyst-stage embryos, are pluripotent and have a virtually unlimited capacity for self-renewal and differentiation into all cell types of an embryoproper. Both human and mouse ES cell lines are the subject of intensive investigation for potential applications in developmental biology and medicine. ES cells from both sources differentiate in vitro into cells of ecto-, endoand meso-dermal lineages, and robust cardiomyogenic differentiation is readily observed in spontaneously differentiating ES cells when cultured under appropriate conditions. Molecular, cellular and physiologic analyses demonstrate that ES cell-derived cardiomyocytes are functionally viable and that these cell derivatives exhibit characteristics typical of heart cells in early stages of cardiac development. Because terminal heart failure is characterized by a significant loss of cardiomyocytes, the use of human ES cell-derived progeny represents one possible source for cell transplantation therapies. With these issues in mind, this review will focus on the differentiation of pluripotent embryonic stem cells into cardiomyocytes as a developmental model, and the possible use of ES cell-derived cardiomyocytes as source of donor cells.

Activin A maintains pluripotency of human embryonic stem cells in the absence of feeder layers

To date, all human embryonic stem cells (hESCs) available for research require unidentified soluble factors secreted from feeder layers to maintain the undifferentiated state and pluripotency. Activation of STAT3 by leukemia inhibitory factor is required to maintain "stemness" in mouse embryonic stem cells, but not in hESCs, suggesting the existence of alternate signaling pathways for self-renewal and pluripotency in human cells. Here we show that activin A is secreted by mouse embryonic feeder layers (mEFs) and that culture medium enriched with activin A is capable of maintaining hESCs in the undifferentiated state for >20 passages without the need for feeder layers, conditioned medium from mEFs, or STAT3 activation. hESCs retained both normal karyotype and markers of undifferentiated cells, including Oct-4, nanog, and TRA-1-60 and remained pluripotent, as shown by the in vivo formation of teratomas.

Characterization and Evaluation of Human Embryonic Stem Cells

Human embryonic stem cells (hESCs) provide great opportunities for regenerative medicine, pharmacological and toxicological investigation, and the study of human embryonic development. These applications require proper derivation, maintenance, and extensive characterization of undifferentiated cells before being used for differentiation into cells of interest. Undifferentiated hESCs possess several unique features, including their extensive proliferation capacity in the undifferentiated state, ability to maintain a normal karyotype after long-term culture, expression of markers characteristic of stem cells, high constitutive telomerase activity, and capacity to differentiate into essentially all somatic cell types. This chapter will summarize the current development in culture conditions and provide technical details for the evaluation and characterization of hESCs.

Hematopoietic Cells from Primate Embryonic Stem Cells

Embryonic stem (ES) cells, derived from early stage embryos, are pluripotent precursors of all of the tissues and organs of the body. ES cells from the mouse have been shown to undergo differentiation in vitro to form a variety of different cell types, including the differentiated progeny of hematopoietic precursors. These hematopoietic cells, however, exhibit numerous differences from those of human cells, and it has become increasingly clear that mouse ES cell differentiation has significant limitations as a model of human developmental biology. The more recent isolation and characterization of nonhuman primate ES cell lines have made available an experimental model with characteristics considerably more close to human biology. We have developed experimental conditions that promote efficient differentiation of these cells to produce progeny cells with considerable similarity to hematopoietic precursors harvested from bone marrow of adult animals.

A road map toward defining the role of Smad signaling in hematopoietic stem cells

The transforming growth factor-beta (TGF-beta) superfamily encompasses the ligands and receptors for TGF-beta, bone morphogenic proteins (BMPs), and Activins. Cellular response to ligand is context-dependent and may be controlled by specificity and/or redundancy of expression of these superfamily members. Several pathways within this family have been implicated in the proliferation, differentiation, and renewal of hematopoietic stem cells (HSCs); however, their roles and redundancies at the molecular level are poorly understood in the rare HSC. Here we have characterized the expression of TGF-beta superfamily ligands, receptors, and Smads in murine HSCs and in the Lhx2-hematopoietic progenitor cell (Lhx2-HPC) line. We demonstrate a remarkable likeness between these two cell types with regard to expression of the majority of receptors and Smads necessary for the transduction of signals from TGF-beta, BMP, and Activin. We have also evaluated the response of these two cell types to various ligands in proliferation assays. In this regard, primary cells and the Lhx2-HPC line behave similarly, revealing a suppressive effect of Activin-A that is similar to that of TGF-beta in bulk cultures and no effect of BMP-4 on proliferation. Signaling studies that verify the phosphorylation of Smad2 (Activin and TGF-beta) and Smad1/5 (BMP) confirm cytosolic responses to these ligands. In addition to providing a thorough characterization of TGF-beta superfamily expression in HSCs, our results define the Lhx2-HPC line as an appropriate model for molecular characterization of Smad signaling.

Maintenance of olfactory neurogenesis requires HSF1, a major heat shock transcription factor in mice

Heat shock transcription factors (HSFs) play roles not only in heat shock response but also in development of the reproductive organs, brain, and lens. Here, we analyzed sensory organs and found abnormalities of the olfactory epithelium in adult HSF1-null mice, which is developmentally related to the lens. The olfactory epithelium was normal until postnatal 3 weeks but was not maintained later than 4 weeks in HSF1-null mice. The olfactory epithelium was atrophied with increased cell death of olfactory sensory neurons. Analysis of the epithelium revealed that induction of HSP expression and reduction of LIF expression are lacking in adult HSF1-null mice. We found that DNA binding activity of HSF1 is induced in the olfactory epithelium later than 4 weeks and that HSF1 binds directly to Lif gene and inhibits its expression. HSF4 has opposing effects on LIF expression and olfactory neurogenesis. These data indicate that HSF1 is required for the precise expression of Hsp and cytokine genes that is obligatory for maintenance of olfactory neurogenesis in adult mice and suggest that stress-related processes are involved in its maintenance.

Generation of Chromosome-Specific Monoclonal Antibodies Using In Vitro-Differentiated Transchromosomic Mouse Embryonic Stem Cells

Monoclonal antibodies (MoAbs) recognizing lineage- and stage-specific human cell-surface antigens are valuable reagents for the characterization and isolation of various specialized cell populations derived from human embryonic stem cells (hESCs). In this report, we examined the use of in vitro differentiated transchromosomic mouse embryonic stem cells (TC-ESCs) as immunogens to obtain MoAbs against human cell-surface antigens. Immunization of a neural-cell population derived from differentiating human chromosome 4 and 11 TC-ESCs resulted in two chromosome-specific MoAbs, h4-neural1 and h11-neural1, respectively. The staining profiles of differentiated TC-ESCs and human embryonic carcinoma cells with these MoAbs were similar to the expression profile of nestin, a well-characterized intracellular marker for neural progenitor cells. We also described the successful purification and identification of the gene for h4-neural1 antigen (CD133, 4p15.32) with immunoaffinity chromatography. This procedure may have significant utility in generating MoAbs useful for understanding the mechanism that regulates the in vitro differentiation of hESCs.

Functions of the superfamily in human embryonic stem cells

The establishment of human embryonic stem (ES) cells has opened possibilities for cell replacement therapy to treat diseases such as diabetes, Parkinson's disease and cardiac myopathies. Self-renewal is one of the essential defining characteristics of stem cells. If stem cells are to have widespread therapeutic applications, it is essential to identify the extrinsic and intrinsic factors maintaining self-renewal, particularly in culture. Insight into the regulation of known self-renewal transcription factors and cross-talk between their upstream signalling pathways is important for a better understanding of how stem cell self-renewal and differentiation are related to downstream target genes. This may lead to the establishment of protocols for obtaining a large supply of ES cells. Here, we review the role that TGFbeta superfamily members are thought to play in self-renewal and differentiation of human and mouse ES cells. We focus on the prototype TGFbeta, TGFbeta1, activin A, nodal and bone morphogenetic proteins and their expression, activity and function in embryonic stem cells.

Transcriptional profiling of the developmentally important signalling pathways in human embryonic stem cells

BACKGROUND

Embryonic stem cells (ESC) maintain their 'stemness' by self-renewal. However, the molecular mechanisms underlying self-renewal of human embryonic stem cells (hESC) remain to be elucidated. In this study, expression profiles of the molecules of developmentally important signalling pathways were investigated to better understand the relationships of the signalling pathways for self-renewal in hESC.

METHODS

Two human ESC lines were cultured on mouse embryonic fibroblast (MEF) feeder cells. Gene expression was analysed by RT-PCR, real-time RT-PCR and Western blotting.

RESULTS

In the bone morphogenetic protein (BMP4), transforming growth factor (TGF-beta) and fibroblast growth factor (FGF4) signalling pathways, ligands and antagonists were highly expressed in hESC compared with human embryoid body (hEB). Human ESC showed abundant transcripts of intracellular molecules in the Wnt, Hh and Notch signalling pathways. No difference was detected in the expression level of the JAK/STAT signalling molecules between hESC and hEB. Western blot analysis showed that the transcriptional levels of the signalling molecules in hESC were consistent with translational levels. From the real-time PCR analysis, expression levels of some genes, such as Oct3/4, Nodal and beta-catenin, were different between two hESC lines.

CONCLUSION

The self-renewal of hESC is probably maintained by coordinated regulation of signalling-specific molecules and in a signalling-specific manner.

The molecular basis and prospects in pancreatic development

Studies on the signaling mechanism that control the specification of endoderm-derived organs have only recently begun. While many studies revealed genes involved in the differentiation, growth and morphogenesis of the pancreas through studies of mutant mice, still little is known about how endoderm give rise to specific domains. Although many genes are known to have a role in pancreatic differentiation, growth and morphogenesis, few genes are known to take part in the specification of the pancreas so far. Hallmarks as well as gene markers for early development of the pancreas, which are however still very limited, will be useful for dissecting early events in pancreatic specification. Here, I give a summary on the origin of the dorsal and ventral pancreatic progenitors, signals for inductions, and genes so far known to function in pancreatic differentiation. I also give a future prospect in the use of ES cells and other experimental models, towards a comprehensive understanding of gene networks in the progenitor cells or intermediate cell types which arise during various stages of differentiation.

Ultrastructural analysis of mouse embryonic stem cell-derived chondrocytes

Pluripotent embryonic stem (ES) cells cultivated as cellular aggregates, so called embryoid bodies (EBs), differentiate spontaneously into different cell types of all three germ layers in vitro resembling processes of cellular differentiation during embryonic development. Regarding chondrogenic differentiation, murine ES cells differentiate into progenitor cells, which form pre-cartilaginous condensations in the EB-outgrowths and express marker molecules characteristic for mesenchymal cell types such as Sox5 and Sox6. Later, mature chondrocytes appear which express collagen type II, and the collagen fibers show a typical morphology as demonstrated by electron-microscopical analysis. These mature chondrogenic cells are organized in cartilage nodules and produce large amounts of extracellular proteoglycans as revealed by staining with cupromeronic blue. Finally, cells organized in nodules express collagen type X, indicating the hypertrophic stage. In conclusion, differentiation of murine ES cells into chondrocytes proceeds from the undifferentiated stem cell via progenitor cells up to mature chondrogenic cells, which then undergo hypertrophy. Furthermore, because the ES-cell-derived chondrocytes did not express elastin, a marker for elastic cartilage tissue, we suggest the cartilage nodules to resemble hyaline cartilage tissue.

Ascorbic acid promotes osteoclastogenesis from embryonic stem cells

Ascorbic acid (AA) is known to regulate cell differentiation; however, the effects of AA on osteoclastogenesis, especially on its early stages, remain unclear. To examine the effects of AA throughout the process of osteoclast development, we established a culture system in which tartrate-resistant acid phosphate (TRAP)-positive osteoclasts were induced from embryonic stem cells without stromal cell lines. In this culture system, the number of TRAP-positive cells was strongly increased by the addition of AA during the development of osteoclast precursors, and reducing agents, 2-mercaptoethanol, monothioglycerol, and dithiothreitol, failed to substitute for AA. The effect of AA was stronger when it was added during the initial 4 days during the development of mesodermal cells than when it was added during the last 4 days. On day 4 of the culture period, AA increased the total cell recovery and frequency of osteoclast precursors. Magnetic cell sorting using anti-Flk-1 antibody enriched osteoclast precursors on day 4, and the proportion of Flk-1-positive cells but not that of platelet-derived growth factor receptor alpha-positive cells was increased by the addition of AA. These results suggest that AA might promote osteoclastogenesis of ES cells through increasing Flk-1-positive cells, which then give rise to osteoclast precursors.

Activin/Nodal and FGF pathways cooperate to maintain pluripotency of human embryonic stem cells

Maintenance of pluripotency is crucial to the mammalian embryo's ability to generate the extra-embryonic and embryonic tissues that are needed for intrauterine survival and foetal development. The recent establishment of embryonic stem cells from human blastocysts (hESCs) provides an opportunity to identify the factors supporting pluripotency at early stages of human development. Using this in vitro model, we have recently shown that Nodal can block neuronal differentiation, suggesting that TGFbeta family members are involved in cell fate decisions of hESCs, including preservation of their pluripotency. Here, we report that Activin/Nodal signalling through Smad2/3 activation is necessary to maintain the pluripotent status of hESCs. Inhibition of Activin/Nodal signalling by follistatin and by overexpression of Lefty or Cerberus-Short, or by the Activin receptor inhibitor SB431542, precipitates hESC differentiation. Nevertheless, neither Nodal nor Activin is sufficient to sustain long-term hESC growth in a chemically defined medium without serum. Recent studies have shown that FGF2 can also maintain long-term expression of pluripotency markers, and we find that inhibition of the FGF signalling pathway by the tyrosine kinase inhibitor SU5402 causes hESC differentiation. However, this effect of FGF on hESC pluripotency depends on Activin/Nodal signalling, because it is blocked by SB431542. Finally, long-term maintenance of in-vitro pluripotency can be achieved with a combination of Activin or Nodal plus FGF2 in the absence of feeder-cell layers, conditioned medium or Serum Replacer. These findings suggest that the Activin/Nodal pathway maintains pluripotency through mechanism(s) in which FGF acts as a competence factor and therefore provide further evidence of distinct mechanisms for preservation of pluripotency in mouse and human ESCs.

Neural differentiation of mouse embryonic stem cells in chemically defined medium

Directed differentiation of embryonic stem (ES) cells has enormous potential to derive a wide variety of defined cell populations of therapeutic value. To achieve this, it is necessary to use protocols that promote cell differentiation under defined culture conditions. Furthermore, understanding the mechanisms of cell differentiation in vitro will allow the development of rationale approaches to systematically manipulate cell fates. Here we have analysed the differentiation of mouse ES cells to the neural lineage under serum and feeder cell-free conditions, using a previously described chemically defined medium (CDM). In CDM, ES cell differentiation is highly neurogenic. Cell differentiation was monitored by analysis of a gene expression array (Clontech-Atlas) and by semi-quantitative RT-PCR for a panel of genes involved in cell lineage specification and patterning of the epiblast. In addition to expression of neural markers, data identified a transient expression of several genes associated with the organising activities of the embryonic node and visceral endoderm, including regulators of WNT, BMP, Hedgehog and FGF signaling pathways. Neural differentiation in CDM does not occur by a simple default mechanism, but was dependent on endogenous FGF signaling, and could be blocked by adding BMP4, and LiCl to simulate WNT activation. Neural differentiation was also inhibited by antagonising endogenous hedgehog activity. Taken together the profile of gene expression changes seen in CDM cultures recapitulates those seen in the early embryo, and is suggestive of common developmental mechanisms.

Stepwise commitment from embryonic stem to hematopoietic and endothelial cells

There is great excitement in generating different types of somatic cells from in vitro differentiated embryonic stem (ES) cells, because they can potentially be utilized for therapies for human diseases for which there are currently no effective treatments. Successful generation and application of ES-derived somatic cells requires better understanding of molecular mechanisms that regulate self-renewal and lineage commitment. Accordingly, many studies are aimed toward understanding mechanisms for maintaining the stem cell state and pathways leading to lineage specification. In this chapter we discuss recent studies that examine molecules that are critical for ES cell self-renewal, as well as hematopoietic and endothelial cell lineage differentiation from ES cells.

Germ layer induction from embryonic stem cells

Embryonic stem (ES) cells have the potential to develop into all cell types of the adult body. This capability provides the basis for considering the ES cell system as a novel and unlimited source of cells for replacement therapies for the treatment of a wide range of diseases. Before the cell-based therapy potential of ES cells can be realized, a better understanding of the pathways regulating lineage-specific differentiation is required. Current studies suggest that the bone morphogenic protein, transforming growth factor-beta, Wnt, and fibroblast growth factor pathways that are required for gastrulation and germ layer induction in the embryo are also essential for differentiation of ES cells in culture. The current understanding of how these factors influence germ layer induction in both the embryo and in the ES cell differentiation system is addressed in this review.

Patterning definitive hematopoietic stem cells from embryonic stem cells

Derived from the inner cell mass of blastocysts, embryonic stem cells (ESCs) retain the pluripotent features of early embryonic epiblast cells. In vitro, ESCs undergo spontaneous differentiation into a multitude of tissues, and thus are a powerful tool for the study of early developmental processes and a promising resource for cell-based therapies. We have pursued the derivation of functional, multipotent and engraftable hematopoietic stem cells (HSCs) from ESCs in order to investigate the genetic pathways specifying blood formation, as well as to lay the foundation for hematopoietic cell replacement therapies based on engineered ESCs. Theoretically, the generation of HSCs from patient-specific ESCs derived by nuclear transfer could provide for autologous hematopoietic therapies for the treatment of malignant and genetic bone marrow disorders. Although significant progress has been made in achieving hematopoietic differentiation from both murine and human ESCs, we have only a primitive understanding of the underlying mechanisms that specify hematopoietic cell fate, and a very limited capacity to direct the differentiation of the definitive HSC that would be suitable for clinical engraftment studies. Here we will review the progress to date and the significant problems that remain, and outline a strategy to achieve the directed differentiation of HSCs under conditions that might be appropriate for clinical scale-up and disease applications.

The role of Smad signaling in hematopoiesis

The TGF-beta family of ligands, including TGF-beta, bone morphogenetic protein (BMP) and activin, signal through Smad pathways to regulate the fate of hematopoietic progenitor and stem cells during development and postnatally. BMP regulates hematopoietic stem cell (HSC) specification during development, while TGF-beta1, 2 and 3 are not essential for the generation of HSCs. BMP4 can increase proliferation of human hematopoietic progenitors, while TGF-beta acts as a negative regulator of hematopoietic progenitor and stem cells in vitro. In contrast, TGF-beta signaling deficiency in vivo does not affect proliferation of HSCs and does not affect lineage choice either. Therefore, the outcome of Smad signaling is very context dependent in hematopoiesis and regulation of hematopoietic stem and progenitor cells is more complicated in the bone marrow microenvironment in vivo than is seen in liquid cultures ex vivo. Smad signaling regulates hematopoiesis by crosstalk with other regulatory signals and future research will define in more detail how the various pathways interact and how the knowledge obtained can be used to develop advanced cell therapies.

Overexpression of Nodal promotes differentiation of mouse embryonic stem cells into mesoderm and endoderm at the expense of neuroectoderm formation

Understanding how to direct the fate of embryonic stem (ES) cells upon differentiation is critical to their eventual use in therapeutic applications. Clues for controlling ES cell differentiation may be found in the early embryo because mouse ES cells form derivatives of all three embryonic germ layers upon injection into blastocysts. One promising candidate for influencing the differentiation of ES cells into the embryonic germ layers is the transforming growth factor-beta (TGF-beta) growth factor, Nodal. Nodal null mouse mutants lack mesoderm, and injection of Nodal mRNA into nonmammalian embryos induces mesodermal and endodermal tissues. We find that overexpression of Nodal in mouse ES cells leads not only to up-regulation of mesodermal and endodermal cell markers but also to downregulation of neuroectodermal markers. These findings demonstrate the importance of Nodal's influence on the differentiation of pluripotent cells to all three of the primary germ layers. Accordingly, altering expression of factors responsible for cell differentiation in the intact embryo provides an approach for directing ES cell fates in vitro toward therapeutically useful cell types.

RXR agonist enhances the differentiation of cardiomyocytes derived from embryonic stem cells in serum-free conditions

Signaling from the retinoic acid receptors (RARs) and retinoid X receptors (RXRs) is essential for cardiovascular morphogenesis in vivo. RAR and/or RXR signaling can also enhance the in vitro induction of cardiomyocytes from murine embryonic stem (ES) cells in the presence of serum. The present study examined the effect of RXR agonist that was specifically bound to RXRs on the differentiation of mouse ES cells into cardiomyocytes in vitro in the absence of serum. The number of beating embryoid body-like spheres (EBSs) derived from the ES cells increased significantly following treatment with PA024, an RXR agonist. In contrast, when EBSs were treated with PA452, which was specifically bound to RXR and worked as an antagonist, the number of beating EBSs was decreased in a dose-dependent manner. These results suggest that RXR signaling regulates cardiomyocyte numbers during the differentiation of ES cells in vitro and probably in normal development.

Forced aggregation of defined numbers of human embryonic stem cells into embryoid bodies fosters robust, reproducible hematopoietic differentiation

To realize the therapeutic potential of human embryonic stem cells (hESCs), it is necessary to regulate their differentiation in a uniform and reproducible manner. We have developed a method in which known numbers of hESCs in serum-free medium were aggregated by centrifugation to foster the formation of embryoid bodies (EBs) of uniform size (spin EBs). These spin EBs differentiated efficiently and synchronously, as evidenced by the sequential expression of molecular markers representing stem cells, primitive streak, and mesoderm. In the presence of hematopoietic growth factors, reproducible differentiation was achieved with blood cells formed in more than 90% of EBs. Using chimeric EBs generated from mixtures of green fluorescence protein-positive (GFP(+)) and GFP(-) hESCs in a clonogenic assay, hematopoietic precursor frequency was estimated to be approximately 1:500 input cells. This method of EB formation provides a generally applicable means for modulating and objectively monitoring the directed differentiation of hESCs.

Embryonic stem cells: prospects for developmental biology and cell therapy

Stem cells represent natural units of embryonic development and tissue regeneration. Embryonic stem (ES) cells, in particular, possess a nearly unlimited self-renewal capacity and developmental potential to differentiate into virtually any cell type of an organism. Mouse ES cells, which are established as permanent cell lines from early embryos, can be regarded as a versatile biological system that has led to major advances in cell and developmental biology. Human ES cell lines, which have recently been derived, may additionally serve as an unlimited source of cells for regenerative medicine. Before therapeutic applications can be realized, important problems must be resolved. Ethical issues surround the derivation of human ES cells from in vitro fertilized blastocysts. Current techniques for directed differentiation into somatic cell populations remain inefficient and yield heterogeneous cell populations. Transplanted ES cell progeny may not function normally in organs, might retain tumorigenic potential, and could be rejected immunologically. The number of human ES cell lines available for research may also be insufficient to adequately determine their therapeutic potential. Recent molecular and cellular advances with mouse ES cells, however, portend the successful use of these cells in therapeutics. This review therefore focuses both on mouse and human ES cells with respect to in vitro propagation and differentiation as well as their use in basic cell and developmental biology and toxicology and presents prospects for human ES cells in tissue regeneration and transplantation.

Nuclei of embryonic stem cells reprogram somatic cells

The restricted potential of a differentiated cell can be reverted back to a pluripotent state by cell fusion; totipotency can even be regained after somatic cell nuclear transfer. To identify factors involved in resetting the genetic program of a differentiated cell, we fused embryonic stem cells (ESCs) with neurosphere cells (NSCs). The fusion activated Oct4, a gene essential for pluripotency, in NSCs. To further identify whether cytoplasmic or nuclear factors are responsible for its reactivation, we fused either karyoplasts or cytoplasts of ESCs with NSCs. Our results show that ESC karyoplasts could induce Oct4 expression in the somatic genome, but cytoplasts lacked this ability. In addition, mitomycin C-treated ESCs, although incapable of DNA replication and cell division, could reprogram 5-azacytidine-treated NSCs. We therefore conclude that the Oct4 reprogramming capacity resides in the ESC karyoplast and that gene reactivation is independent of DNA replication and cell division.

Enforced Bcl-2 expression overrides serum and feeder cell requirements for mouse embryonic stem cell self-renewal

Leukemia inhibitory factor (LIF) is required, but not sufficient, for pluripotent mouse embryonic stem (ES) cell expansion in vitro in the absence of serum or a feeder cell layer, suggesting that additional signals are provided by serum or feeders that are necessary to support self-renewal. Here we show that transgenic ES cell lines expressing Bcl-2, an antiapoptotic protein, continue to self-renew in serum- and feeder-free conditions when supplemented with LIF; even in the absence of bone morphogenic proteins. Bcl-2-expressing clones sustain the characteristics of undifferentiated, pluripotent ES cells during long-term culture, and maintain their potential to differentiate into mature cell types. These results suggest that LIF and Bcl-2 overexpression are sufficient to expand these mouse pluripotent stem cells in vitro.

Pluripotent differentiation in vitro of murine ES-D3 embryonic stem cells

Although the ES-D3 murine embryonic stem cell line was one of the first derived, little information exists on the in vitro differentiation potential of these cells. We have used immunocytochemical and flow cytometric methods to monitor ES-D3 embryoid body differentiation in vitro during a 21-d period. Spontaneous differentiation of embryoid body cells was induced by leukemia inhibitory factor withdrawal in the absence of feeder cells. The pluripotent stem cell markers Oct-3/4, SSEA-1, and EMA-1 were found to persist for at least 7 d, whereas the primitive endoderm marker cytokeratin endo-A was expressed at increasing levels from day 6. The localization of these antigens within the embryoid bodies suggested that embryonic ectoderm- and primitive endoderm-derived tissues were segregated. Localized expression of class III beta-tubulin and sarcomeric myosin also was detected, indicating that representatives of all three embryonic germ layers were present after induction of differentiation in vitro.

A regulatory role of Wnt signaling pathway in the hematopoietic differentiation of murine embryonic stem cells

One of the most important issues in stem cell research is to understand the regulatory mechanisms responsible for their differentiation. An extensive understanding of mechanism underlying the process of differentiation is crucial in order to prompt stem cells to perform a particular function after differentiation. To elucidate the molecular mechanisms responsible for the hematopoietic differentiation of embryonic stem cells (ESCs), we investigated murine ES cells for the presence of hematopoietic lineage markers as well as Wnt signaling pathway during treatments with different cytokines alone or in combination with another. Here we report that Wnt/beta-catenin signaling is down-regulated in hematopoietic differentiation of murine ES cells. We also found that differentiation induced by the interleukin-3, interleukin-6, and erythropoietin combinations resulted in high expression of CD3e, CD11b, CD45R/B220, Ly-6G, and TER-119 in differentiated ES cells. A high expression of beta-catenin was observed in two undifferentiated ES cell lines. Gene and protein expression analysis revealed that the members downstream of Wnt in this signaling pathway including beta-catenin, GSK-3beta, Axin, and TCF4 were significantly down-regulated as ES cells differentiated into hematopoietic progenitors. Our results show that the Wnt/beta-catenin signaling pathway plays a role in the hematopoietic differentiation of murine ESCs and also may support beta-catenin as a crucial factor in the maintenance of ES cells in their undifferentiated state.