Nodal inhibits differentiation of human embryonic stem cells along the neuroectodermal default pathway

Guided differentiation of embryonic stem cells into Pdx1-expressing regional-specific definitive endoderm

The generation of specific lineages of the definitive endoderm from embryonic stem (ES) cells is an important issue in developmental biology, as well as in regenerative medicine. This study demonstrates that ES cells are induced sequentially into regional-specific gut endoderm lineages, such as pancreatic, hepatic, and other cell lineages, when they are cultured directly on a monolayer of mesoderm-derived supporting cells. A detailed chronological analysis revealed that Activin, fibroblast growth factor, or bone morphogenetic protein signals are critical at various steps and that additional short-range signals are required for differentiation into Pdx1-expressing cells. Under selective culture conditions, definitive endoderm (47%) or Pdx1-positive pancreatic progenitors (30%) are yielded at a high efficiency. When transplanted under the kidney capsule, the Pdx1-positive cells further differentiated into all three pancreatic lineages, namely endocrine, exocrine, and duct cells.

TGFbeta/Activin/Nodal pathway in inhibition of human embryonic stem cell differentiation by mechanical strain

Cyclic biaxial mechanical strain has been reported to inhibit human embryonic stem cell differentiation without selecting against survival of differentiated or undifferentiated cells. We show that TGFbeta/Activin/Nodal signaling plays a crucial role in repression of human embryonic stem cell (hESC) differentiation under mechanical strain. Strain-induced transcription of TGFbeta1, Activin A, and Nodal, and upregulated Similar to Mothers Against Decapentaplegic homolog (Smad)2/3 phosphorylation in undifferentiated hESC. TGFbeta/Activin/Nodal receptor inhibitor SB431542 stimulated differentiation of hESCs cultured under biaxial strain. Exogenous addition of TGFbeta1, Activin A, or Nodal alone was insufficient to stimulate hESC self-renewal to replicate behavior of hESCs in presence of strain. However, exogenous TGFbeta1 and Activin A in combination partially replicated the self-renewing phenotype induced by strain but when combined with strain did not further stimulate self-renewal. In presence of mechanical strain, addition of a neutralizing antibody to TGFbeta1 promoted hESC differentiation whereas inhibition of Activin A by Follistatin promoted hESC differentiation to a lesser extent. Together, these findings show that TGFbeta superfamily activation of Smad2/3 is required for repression of spontaneous differentiation under strain and suggest that strain may induce autocrine or paracrine signaling through TGFbeta superfamily ligands.

Neural induction requires continued suppression of both Smad1 and Smad2 signals during gastrulation

Vertebrate neural induction requires inhibition of bone morphogenetic protein (BMP) signaling in the ectoderm. However, whether inhibition of BMP signaling is sufficient to induce neural tissues in vivo remains controversial. Here we have addressed why inhibition of BMP/Smad1 signaling does not induce neural markers efficiently in Xenopus ventral ectoderm, and show that suppression of both Smad1 and Smad2 signals is sufficient to induce neural markers. Manipulations that inhibit both Smad1 and Smad2 pathways, including a truncated type IIB activin receptor, Smad7 and Ski, induce early neural markers and inhibit epidermal genes in ventral ectoderm; and co-expression of BMP inhibitors with a truncated activin/nodal-specific type IB activin receptor leads to efficient neural induction. Conversely, stimulation of Smad2 signaling in the neural plate at gastrula stages results in inhibition of neural markers, disruption of the neural tube and reduction of head structures, with conversion of neural to neural crest and mesodermal fates. The ability of activated Smad2 to block neural induction declines by the end of gastrulation. Our results indicate that prospective neural cells are poised to respond to Smad2 and Smad1 signals to adopt mesodermal and non-neural ectodermal fates even at gastrula stages, after the conventionally assigned end of mesodermal competence, so that continued suppression of both mesoderm- and epidermis-inducing Smad signals leads to efficient neural induction.

Albumin-associated lipids regulate human embryonic stem cell self-renewal

BACKGROUND

Although human embryonic stem cells (hESCs) hold great promise as a source of differentiated cells to treat several human diseases, many obstacles still need to be surmounted before this can become a reality. First among these, a robust chemically-defined system to expand hESCs in culture is still unavailable despite recent advances in the understanding of factors controlling hESC self-renewal.

METHODOLOGY/PRINCIPAL FINDINGS

In this study, we attempted to find new molecules that stimulate long term hESC self-renewal. In order to do this, we started from the observation that a commercially available serum replacement product has a strong positive effect on the expansion of undifferentiated hESCs when added to a previously reported chemically-defined medium. Subsequent experiments demonstrated that the active ingredient within the serum replacement is lipid-rich albumin. Furthermore, we show that this activity is trypsin-resistant, strongly suggesting that lipids and not albumin are responsible for the effect. Consistent with this, lipid-poor albumin shows no detectable activity. Finally, we identified the major lipids bound to the lipid-rich albumin and tested several lipid candidates for the effect.

CONCLUSIONS/SIGNIFICANCE

Our discovery of the role played by albumin-associated lipids in stimulating hESC self-renewal constitutes a significant advance in the knowledge of how hESC pluripotency is maintained by extracellular factors and has important applications in the development of increasingly chemically defined hESC culture systems.

Sox3 expression is maintained by FGF signaling and restricted to the neural plate by Vent proteins in the Xenopus embryo

The formation of the nervous system is initiated when ectodermal cells adopt the neural fate. Studies in Xenopus demonstrate that inhibition of BMP results in the formation of neural tissue. However, the molecular mechanism driving the expression of early neural genes in response to this inhibition is unknown. Moreover, controversy remains regarding the sufficiency of BMP inhibition for neural induction. To address these questions, we performed a detailed analysis of the regulation of the soxB1 gene, sox3, one of the earliest genes expressed in the neuroectoderm. Using ectodermal explant assays, we analyzed the role of BMP, Wnt and FGF signaling in the regulation of sox3 and the closely related soxB1 gene, sox2. Our results demonstrate that both sox3 and sox2 are induced in response to BMP antagonism, but by distinct mechanisms and that the activation of both genes is independent of FGF signaling. However, both require FGF for the maintenance of their expression. Finally, sox3 genomic elements were identified and characterized and an element required for BMP-mediated repression via Vent proteins was identified through the use of transgenesis and computational analysis. Interestingly, none of the elements required for sox3 expression were identified in the sox2 locus. Together our data indicate that two closely related genes have unique mechanisms of gene regulation at the onset of neural development.

Inhibition of Activin/Nodal signaling promotes specification of human embryonic stem cells into neuroectoderm

Nodal, a member of the TGF-beta family of signaling molecules, has been implicated in pluripotency in human embryonic stem cells (hESCs) [Vallier, L., Reynolds, D., Pedersen, R.A., 2004a. Nodal inhibits differentiation of human embryonic stem cells along the neuroectodermal default pathway. Dev. Biol. 275, 403-421], a finding that seems paradoxical given Nodal's central role in mesoderm/endoderm specification during gastrulation. In this study, we sought to clarify the role of Nodal signaling during hESC differentiation by constitutive overexpression of the endogenous Nodal inhibitors Lefty2 (Lefty) and truncated Cerberus (Cerb-S) and by pharmacological interference using the Nodal receptor antagonist SB431542. Compared to wildtype (WT) controls, embryoid bodies (EBs) derived from either Lefty or Cerb-S overexpressing hESCs showed increased expression of neuroectoderm markers Sox1, Sox3, and Nestin. Conversely, they were negative for a definitive endoderm marker (Sox17) and did not generate beating cardiomyocyte structures in conditions that allowed mesendoderm differentiation from WT hESCs. EBs derived from either Lefty or Cerb-S expressing hESCs also contained a greater abundance of neural rosette structures as compared to controls. Differentiating EBs derived from Lefty expressing hESCs generated a dense network of beta-tubulin III positive neurites, and when Lefty expressing hESCs were grown as a monolayer and allowed to differentiate, they generated significantly higher numbers of beta-tubulin positive neurons as compared to wildtype hESCs. SB431542 treatments reproduced the neuralising effects of Lefty overexpression in hESCs. These results show that inhibition of Nodal signaling promotes neuronal specification, indicating a role for this pathway in controlling early neural development of pluripotent cells.

GATA factors induce mouse embryonic stem cell differentiation toward extraembryonic endoderm

The GATA family of transcription factors are implicated in early embryonic development. There are six factors in this family in vertebrates. GATA4 and GATA6 have been demonstrated to induce mouse embryonic stem (mES) cells differentiation toward extraembryonic endoderm (ExE). We investigated the effect of GATA3 on the differentiation of mES cells both in the ES cell and in the embryoid body (EB) states. The results demonstrate that GATA3 overexpression can initiate the ES cell differentiation program toward ExE. Furthermore, overexpression of GATA1 and GATA2 in ES cells and EBs resulted in similar effects. We believe this finding can augment our understanding of mouse ES cell differentiation.

Retinoic acid and bone morphogenetic protein signaling synergize to efficiently direct epithelial differentiation of human embryonic stem cells

Human embryonic stem cells (hESCs) can differentiate to various somatic lineages, including stratified squamous epithelia, although the molecular mechanisms of epithelial specification from hESCs currently remain undefined. Here, we demonstrate a novel, stage-specific effect of retinoic acid (RA) on epithelial differentiation of hESCs. RA strongly upregulated expression of keratin 18 and the transcription factor p63, which is involved in epidermal morphogenesis and ectodermal specification, while repressing early neural marker transcription. RA-induced hESCs efficiently differentiated to keratin 14-expressing epithelial cells, although this effect was dependent upon on the context of bone morphogenetic protein signaling. Furthermore, these hESC-derived keratinocytes could be subcultured to obtain relatively pure keratinocyte populations that retained the capacity to terminally differentiate. These findings suggest that RA plays an important role in epithelial differentiation of hESCs.

Activin B mediated induction of Pdx1 in human embryonic stem cell derived embryoid bodies

Human embryonic stem cells (hESCs) have the potential to provide alternative sources for pancreatic islet grafts. In the present study we have investigated the influence of Activin A and Activin B on the expression of the pancreas marker gene Pdx1 in hESCs differentiated as embryoid bodies (EBs). We report here that Activin B in a dose depend manner markedly up-regulates Pdx1 expression as compared to Activin A and untreated cultures. Pdx1(+) cells co-express FOXA2 but lacks, however, co-expression with nkx6.1, a marker combination that in the present study is shown precisely to identify embryonic and fetal pancreas anlage in humans. Pdx1(+) cells are found in cell clusters also expressing Serpina1 and FABP1, suggesting activation of intestinal/liver developmental programs. Moreover, Activin B up-regulates Sonic Hedgehog (Shh) and its target Gli1, which during normal development is suppressed in the pancreatic anlage. In conclusion, Activin B is a potent inducer of Pdx1 as well as Shh in differentiating hESCs. The data suggest that additional suppression of Shh signaling may be required to allow for proper specification of pancreatic cell lineages in hESCs.

IGF and FGF cooperatively establish the regulatory stem cell niche of pluripotent human cells in vitro

Distinctive properties of stem cells are not autonomously achieved, and recent evidence points to a level of external control from the microenvironment. Here, we demonstrate that self-renewal and pluripotent properties of human embryonic stem (ES) cells depend on a dynamic interplay between human ES cells and autologously derived human ES cell fibroblast-like cells (hdFs). Human ES cells and hdFs are uniquely defined by insulin-like growth factor (IGF)- and fibroblast growth factor (FGF)-dependence. IGF 1 receptor (IGF1R) expression was exclusive to the human ES cells, whereas FGF receptor 1 (FGFR1) expression was restricted to surrounding hdFs. Blocking the IGF-II/IGF1R pathway reduced survival and clonogenicity of human ES cells, whereas inhibition of the FGF pathway indirectly caused differentiation. IGF-II is expressed by hdFs in response to FGF, and alone was sufficient in maintaining human ES cell cultures. Our study demonstrates a direct role of the IGF-II/IGF1R axis on human ES cell physiology and establishes that hdFs produced by human ES cells themselves define the stem cell niche of pluripotent human stem cells.

The Sox axis, Nodal signaling, and germ layer specification

Asymmetries in the egg, established during oogenesis, set the stage for a cascade of intercellular signaling events leading to differential gene expression and subsequent tissue and organ formation. Maternally supplied Sox-type transcription factors have recently emerged as key components in the patterning of the early embryo and the regulation of embryonic stem cell differentiation. In deuterostomes, B1-type Soxs are asymmetrically localized to the future animal/ectodermal region where they act to suppress mesendodermal, and favor neuroectodermal differentiation, while vegetally localized F-type Soxs are involved in mesendodermal differentiation. Here, we review past observations and present new data from studies on the clawed frog Xenopus laevis. Animally localized Sox3 acts to inhibit Nodal (Xnr5 and Xnr6) expression, and induces the expression of genes (Ectodermin, Xema, and Coco) whose products repress Nodal signaling. Vegetally localized Sox7 positively regulates Nodal (Xnr4, Xnr5, and Xnr6) expression, as well as the expression of genes involved in mesodermal (Xmenf, Slug, and Snail) and endodermal (Endodermin and Sox17beta) differentiation. Given the evolutionary strategy of using common regulatory networks, it seems likely that a homologous Sox-Axis is active during embryonic development in many metazoans.

The commonality of plasticity underlying multipotent tumor cells and embryonic stem cells

Aggressive cancer cells and pluripotent stem cells converge in their capacity for self-renewal, proliferation and plasticity. Recent studies have capitalized on these similarities by demonstrating that tumors arise from specific cancer stem cell populations that, in a manner reminiscent of normal stem cells, are able to both self-renew and give rise to a heterogeneous tumor population. This stem cell like function of aggressive cancer cells is likely attributable to the ectopic expression of embryonic factors such as Nodal and Cancer Testis Specific Antigens (CTAs), which maintain a functional plasticity by promoting pluripotency and immortality. During development, the expression of these embryonic factors is tightly regulated by a dynamic array of mediators, including the spatial and temporal expression of inhibitors such as Lefty, and the epigenetic modulation of the genome. In aggressive cancer cells, particularly melanoma, this balance of regulatory mediators is disrupted, leading to the aberrant expression of pluripotency-associated genes. By exposing aggressive cancer cells to embryonic microenvironments, this balance of regulatory mediators is restored, thereby reprogramming tumor cells to a more benign phenotype. These stem cell-derived mediators, as well as the genes they regulate, provide therapeutic targets designed to specifically differentiate and eradicate aggressive cancers.

New cell lines from mouse epiblast share defining features with human embryonic stem cells

The application of human embryonic stem (ES) cells in medicine and biology has an inherent reliance on understanding the starting cell population. Human ES cells differ from mouse ES cells and the specific embryonic origin of both cell types is unclear. Previous work suggested that mouse ES cells could only be obtained from the embryo before implantation in the uterus. Here we show that cell lines can be derived from the epiblast, a tissue of the post-implantation embryo that generates the embryo proper. These cells, which we refer to as EpiSCs (post-implantation epiblast-derived stem cells), express transcription factors known to regulate pluripotency, maintain their genomic integrity, and robustly differentiate into the major somatic cell types as well as primordial germ cells. The EpiSC lines are distinct from mouse ES cells in their epigenetic state and the signals controlling their differentiation. Furthermore, EpiSC and human ES cells share patterns of gene expression and signalling responses that normally function in the epiblast. These results show that epiblast cells can be maintained as stable cell lines and interrogated to understand how pluripotent cells generate distinct fates during early development.

Exploiting the Convergence of Embryonic and Tumorigenic Signaling Pathways to Develop New Therapeutic Targets

As our understanding of embryonic stem cell biology becomes more sophisticated, the similarities between multipotent cancer cells and these totipotent precursors are increasingly striking. Both multipotent cancer cells and embryonic stem cells possess the ability to self-renew, epigenetically alter their neighboring cellular architecture, and populate a tissue mass with a phenotypically heterogeneous composition of cells. While the molecular signature of these cell types continues to be elucidated, new insights are emerging related to the convergence of embryonic and tumorigenic signaling pathways. Understanding the molecular underpinnings of these two stem cell phenotypes may lead to new therapeutic targets for the elusive cancer cell. While still in its infancy, the potential of adapting embryonic stem cells, and more specifically the factors they produce, is enormous for clinical application. Here we outline evidence that demonstrates the inductive influence of embryonic stem cells and their microenvironment to reprogram cancer cells to exhibit a more benign phenotype, with profound implications for differentiation therapy.

Regulated Nodal signaling promotes differentiation of the definitive endoderm and mesoderm from ES cells

Nodal signaling induces the formation of the endoderm and mesoderm during gastrulation. Nodal expression persists until the definitive endoderm progenitor has completely formed, and disappears thereafter. A tightly regulated Nodal expression system is essential for the differentiation of embryonic stem (ES) cells into distinct tissue lineages. On this basis, we established an ES cell differentiation system with the tetracycline-regulated expression of Nodal. The upregulated Nodal signaling pathway and its downstream transcriptional targets induced the specification of ES cells into definitive endoderm and mesoderm derivatives, and the subsequent downregulation of Nodal signaling promoted further maturation of the gut tube both in vitro and in vivo. Sustained expression of the Nodal gene inhibited the maturation of the definitive endoderm owing to persistent Oct3 and/or Oct4 expression and teratoma formation. Furthermore, quantitative single cell analysis by flow cytometry using CXCR4, VEGFR2 and PDGFR-α indicated that this protocol for definitive endoderm and mesoderm differentiation is superior to any other available protocol. Our findings also indicated that the Nodal or Nodal-related molecules secreted from Nodal-expressing ES cells could cause genetically unmanipulated ES cells to induce the expression of the Nodal signaling pathway and its downstream targets, which consequently leads to the differentiation of the ES cells into definitive endoderm and mesoderm. Our differentiation system, using tightly regulated Nodal expression, enabled us to investigate the mechanism of ES cell differentiation into definitive endoderm or mesoderm derivatives. Our findings also demonstrate that Nodal-expressing ES cells might be a source of highly active proteins that could be used for developing endoderm or mesoderm tissues in regenerative medicine.

Directed differentiation of human embryonic stem cells towards a pancreatic cell fate

AIMS/HYPOTHESIS

The relative lack of successful pancreatic differentiation of human embryonic stem cells (hESCs) may suggest that directed differentiation of hESCs into definitive endoderm and subsequent commitment towards a pancreatic fate are not readily achieved. The aim of this study was to investigate whether sequential exposure of hESCs to epigenetic signals that mimic in vivo pancreatic development can efficiently generate pancreatic endodermal cells, and whether these cells can be further matured and reverse hyperglycaemia upon transplantation.

MATERIALS AND METHODS

The hESCs were sequentially treated with serum, activin and retinoic acid (RA) during embryoid body formation. The patterns of gene expression and protein production associated with embryonic germ layers and pancreatic endoderm were analysed by RT-PCR and immunostaining. The developmental competence and function of hESC-derived PDX1-positive cells were evaluated after in vivo transplantation.

RESULTS

Sequential treatment with serum, activin and RA highly upregulated the expression of the genes encoding forkhead box protein A2 (FOXA2), SRY-box containing gene 17 (SOX17), pancreatic and duodenal homeobox 1 (PDX1) and homeobox HB9 (HLXB9). The population of pancreatic endodermal cells that produced PDX1 was significantly increased at the expense of ectodermal differentiation, and a subset of the PDX1-positive cells also produced FOXA2, caudal-type homeobox transcription factor 2 (CDX2), and nestin (NES). After transplantation, the PDX1-positive cells further differentiated into mature cell types producing insulin and glucagon, resulting in amelioration of hyperglycaemia and weight loss in streptozotocin-treated diabetic mice.

CONCLUSIONS/INTERPRETATION

Our strategy allows the progressive differentiation of hESCs into pancreatic endoderm capable of generating mature pancreatic cell types that function in vivo. These findings may establish the basis of further investigations for the purification of transplantable islet progenitors derived from hESCs.

Conditional gene expression in human embryonic stem cells

Human embryonic stem cells (hESCs) possess unique properties for studying mechanisms controlling cell fate commitment during early mammalian development. Gain of function is a common strategy to study the function of specific genes involved in these mechanisms. However, transgene toxicity can be a major limitation, especially with factors influencing proliferation or differentiation. Here, we describe an efficient method based on the inducible recombinase Cre-ERT2 for conditional gene expression in hESCs and their differentiated derivatives. Using this approach, we have established several hESC sublines inducible for the expression of the enhanced green fluorescent protein and the transforming growth factor beta family member Nodal. Together, these results demonstrate that Cre-ERT2 can be used to control gene expression in undifferentiated and differentiated cells, thereby providing the first conditional transgene expression system that works effectively in hESCs. Disclosure of potential conflicts of interest is found at the end of this article.

Conversion of embryonic stem cells into pancreatic beta-cell surrogates guided by ontogeny

Cellular therapies to treat Type 1 diabetes are being devised and the use of human embryonic stem cells (hESCs) offers a solution to the issue of supply, because hESCs can be maintained in a pluripotent state indefinitely. Furthermore, hESCs have advantages in terms of their plasticity and reduced immunogenicity. Several strategies that have so far been investigated indicate that hESCs are capable of differentiating into insulin producing beta-cell surrogates. However the efficiency of the differentiation procedures used is generally quite low and the cell populations derived are often highly heterogenous. A strategy that appears to have long term potential is to design differentiation procedures based on the ontogeny of the beta-cell. The focus of this strategy is to replicate signaling processes that are known to be involved in the maturation of a beta-cell. The earliest pancreatic progenitors found in the developing vertebrate fetus are produced via a process known as gastrulation and form part of the definitive endoderm germ layer. hESCs have recently been differentiated into definitive endoderm with high efficiency using a differentiation procedure that mimics the signaling that occurs during gastrulation and the formation of the definitive endoderm. Subsequent events during pancreas development involve a section of the definitive endoderm forming into pancreatic epithelium, which then branches into the pancreatic mesenchyme to form islet clusters of endocrine cells. A proportion of the endocrine precursor cells within islets develop into insulin producing beta-cells. The challenge currently is to design hESC differentiation procedures that mimic the combined events of these stages of beta-cell development.

Stage-specific role of endogenous Smad2 activation in cardiomyogenesis of embryonic stem cells

The role of Smads and their specific ligands during cardiomyogenesis in ES cells was examined. Smad2 was activated bimodally in the early and late phases of cardiac differentiation, whereas Smad1 was activated after the middle phase. Nodal and Cripto were expressed in the early stage and then downregulated, whereas transforming growth factor-beta and activin were expressed only in the late phase. Suppression of early Smad2 activation by SB-431542 produced complete inhibition of endodermal and mesodermal induction but augmented neuroectodermal differentiation, followed by poor cardiomyogenesis, whereas inhibition during the late phase alone promoted cardiomyogenesis. Inhibitory effect of Smad2 on cardiomyogenesis in the late phase was mainly mediated by transforming growth factor-beta, and inhibition of transforming growth factor-beta-mediated Smad2 activation resulted in a greater replicative potential in differentiated cardiac myocytes and enhanced differentiation of nonmyocytes into cardiac myocytes. Thus, endogenous Smad2 activation is indispensable for endodermal and mesodermal induction in the early phase. In the late phase, endogenous transforming growth factor-beta negatively regulates cardiomyogenesis through Smad2 activation by modulating proliferation and differentiation of cardiac myocytes.

Neural induction and neural stem cell development

Embryonic stem (ES) cells are a pluripotent and renewable cellular resource with tremendous potential for broad applications in regenerative medicine. Arguably the most important consideration for stem cell-based therapies is the ability to precisely direct the differentiation of stem cells along a preferred cellular lineage. During development, lineage commitment is a multistep process requiring the activation and repression of sets of genes at various stages, from an ES cell identity to a tissue-specific stem cell identity and beyond. Thus, the challenge is to ensure that the pattern of genomic regulation is recapitulated during the in vitro differentiation of ES cells into stem/progenitor cells of the appropriate tissue in a robust, predictable and stable manner. To address this issue, we must understand the ontogeny of tissue-specific stem cells during normal embryogenesis and compare the ontogeny of tissue-specific stem cells in ES cell models. Here, we discuss the issue of directed differentiation of pluripotent ES cells into neural stem cells, which is fundamentally linked to two early events in the development of the mammalian nervous system: the 'decision' of the ectoderm to acquire a neural identity (neural determination) and the origin of neural stem cells within this neural-committed population of cells. A clearer understanding of the molecular and cellular mechanisms that govern mammalian neural cell fate determination will lead to improved ES technology applications in neural regeneration.

Silencing of core transcription factors in human EC cells highlights the importance of autocrine FGF signaling for self-renewal

Background

Despite their distinct origins, human embryonic stem (hES) and embryonic carcinoma (hEC) cells share a number of similarities such as surface antigen expression, growth characteristics, the ability to either self-renew or differentiate, and control of the undifferentiated state by the same core transcription factors. To obtain further insights into the regulation of self-renewal, we have silenced hES/hEC cell-specific genes in NCCIT hEC cells and analysed the downstream effects by means of microarrays.

Results

RNAi-mediated silencing of OCT4 and SOX2 induced differentiation with mesodermal characteristics. Markers of trophoblast induction were only transiently up-regulated in the OCT4 knock-down. Independent knock-downs of NANOG produced a proliferation rather than a differentiation phenotype, which may be due to high NANOG expression levels in the cell line used. Published ChIP-chip data from hES cells were used to identify putative direct targets. RNAi-mediated differentiation was accompanied by direct down-regulation of known hES/hEC cell markers. This included all three core transcription factors in the case of the OCT4 and SOX2 knock-downs, confirming previous findings of reciprocal activation in ES cells. Furthermore, large numbers of histone genes as well as epigenetic regulators were differentially expressed, pointing at chromatin remodeling as an additional regulatory level in the differentiation process. Moreover, loss of self-renewal was accompanied by the down-regulation of genes involved in FGF signaling. FGF receptor inhibition for short and prolonged periods of time revealed that the ERK/MAPK cascade is activated by endogenously expressed fibroblast growth factors and that FGF signaling is cruicial for maintaining the undifferentiated state of hEC cells, like in hES cells.

Conclusion

Control of self-renewal appears to be very similar in hEC and hES cells. This is supported by large numbers of common transcription factor targets and the requirement for autocrine FGF signaling.

Whole Genome Analysis of Human Neural Stem Cells Derived from Embryonic Stem Cells and Stem and Progenitor Cells Isolated from Fetal Tissue

Multipotent neural stem cells (NSC) have been derived from human embryonic stem cells (hESC) as well as isolated from fetal tissues. However, there have been few exclusive markers of NSC identified to date, and the differences between NSC from various sources are poorly understood. Although cells isolated from these two sources share many important characteristics, it is not clear how closely they are related in terms of gene expression. Here, we compare the gene expression profiles of 11 lines of NSC derived from hESC (ES_NSC), four lines of NSC isolated from fetus (F_NSC), and two lines of restricted progenitors in order to characterize these cell populations and identify differences between NSC derived from these two sources. We showed that ES_NSC were clustered together with high transcriptional similarities but were distinguished from F_NSC, oligodendrocyte precursor cells, and astrocyte precursor cells. There were 17 genes expressed in both ES_NSC and F_NSC whose expression was not identified in restricted neural progenitors. Furthermore, the major differences between ES_NSC and F_NSC were mostly observed in genes related to the key neural differentiation pathways. Here, we show that comparison of global gene expression profiles of ES_NSC, F_NSC, and restricted neural progenitor cells makes it possible to identify some of the common characteristics of NSC and differences between similar stem cell populations derived from hESCs or isolated from fetal tissue. Disclosure of potential conflicts of interest is found at the end of this article.

Targeting Nodal in malignant melanoma cells

Metastatic melanoma continues to be a significantly deadly cancer with a cure rate of < 20% and a median survival of 6 - 9 months. The aggressiveness associated with metastatic melanoma is largely attributable to its inherent plasticity, a property that is mediated by the secretion of Nodal, a stem-cell associated protein belonging to the transforming growth factor-beta superfamily. This is supported by the observations that Nodal expression is limited to invasive vertical growth phase and metastatic melanoma lesions, and that inhibition of Nodal signaling promotes the reversion of metastatic melanoma cells toward a more differentiated, less invasive non-tumorigenic phenotype. Hence, due to its restricted expression pattern and function as a melanoma-tumor-promoter, Nodal (and its signaling partners) present unique targets for both immunologic and pharmacologic therapies.

Activin-Nodal signaling is involved in propagation of mouse embryonic stem cells

Embryonic stem (ES) cells are self-renewing cells that maintain pluripotency to differentiate into all types of cells. Because of their potential to provide a variety of tissues for use in regenerative medicine, there is great interest in the identification of growth factors that govern these unique properties of ES cells. However, the signaling pathways controlling ES cell proliferation remain largely unknown. Since transforming growth factor beta (TGFbeta) superfamily members have been implicated in the processes of early embryogenesis, we investigated their roles in ES cell self-renewal. Inhibition of activin-Nodal-TGFbeta signaling by Smad7 or SB-431542 dramatically decreased ES cell proliferation without decreasing ES pluripotency. By contrast, inhibition of bone morphogenetic protein (BMP) signaling by Smad6 did not exhibit such effects, suggesting that activin-Nodal-TGFbeta signaling, but not BMP signaling, is indispensable for ES cell propagation. In serum-free culture, supplementation of recombinant activin or Nodal, but not TGFbeta or BMP, significantly enhanced ES cell propagation without affecting pluripotency. We also found that activin-Nodal signaling was constitutively activated in an autocrine fashion in serum-free cultured ES cells, and that inhibition of such endogenous signaling by SB-431542 decreased ES cell propagation in serum-free conditions. These findings suggest that endogenously activated autocrine loops of activin-Nodal signaling promote ES cell self-renewal.

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.

Nodal signaling: developmental roles and regulation

Nodal-related ligands of the transforming growth factor-beta (TGFbeta) superfamily play central roles in patterning the early embryo during the induction of mesoderm and endoderm and the specification of left-right asymmetry. Additional roles for this pathway in the maintenance of embryonic stem cell pluripotency and in carcinogenesis have been uncovered more recently. Consistent with its crucial developmental functions, Nodal signaling is tightly regulated by diverse mechanisms including the control of ligand processing, utilization of co-receptors, expression of soluble antagonists, as well as positive- and negative-feedback activities.

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.

Directing the differentiation of embryonic stem cells to neural stem cells

Embryonic stem cells (ESCs) are a potential source of neural derivatives that can be used in stem cell-based therapies designed to treat neurological disorders. The derivation of specific neuronal or glial cell types from ESCs invariably includes the production of neural stem cells (NSCs). We describe the basic mechanisms of neural induction during vertebrate embryogenesis and how this information helped formulate several protocols used to generate NSCs from ESCs. We highlight the advantages and disadvantages of each approach and review what has been learned about the intermediate stages in the transition from ESC to NSC. Recent data describing how specific growth factors and signaling molecules regulate production of NSCs are described and a model synthesizing this information is presented.

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.

FoxD3 regulation of Nodal in the Spemann organizer is essential for Xenopus dorsal mesoderm development

Induction and patterning of the mesodermal germ layer is a key early step of vertebrate embryogenesis. We report that FoxD3 function in the Xenopus gastrula is essential for dorsal mesodermal development and for Nodal expression in the Spemann organizer. In embryos and explants, FoxD3 induced mesodermal genes, convergent extension movements and differentiation of axial tissues. Engrailed-FoxD3, but not VP16-FoxD3, was identical to native FoxD3 in mesoderm-inducing activity, indicating that FoxD3 functions as a transcriptional repressor to induce mesoderm. Antagonism of FoxD3 with VP16-FoxD3 or morpholino-knockdown of FoxD3 protein resulted in a complete block to axis formation, a loss of mesodermal gene expression, and an absence of axial mesoderm, indicating that transcriptional repression by FoxD3 is required for mesodermal development. FoxD3 induced mesoderm in a non-cell-autonomous manner, indicating a role for secreted inducing factors in the response to FoxD3. Consistent with this mechanism, FoxD3 was necessary and sufficient for the expression of multiple Nodal-related genes, and inhibitors of Nodal signaling blocked mesoderm induction by FoxD3. Therefore, FoxD3 is required for Nodal expression in the Spemann organizer and this function is essential for dorsal mesoderm formation.

Constitutive gene expression predisposes morphogen-mediated cell fate responses of NT2/D1 and 27X-1 human embryonal carcinoma cells

Human embryonal carcinoma (EC) cell lines exhibit considerable heterogeneity in their levels of pluripotency. Thus, NT2/D1 cells differentiate into neural lineages upon exposure to all-trans retinoic acid (ATRA) and non-neural epithelial lineages upon exposure to bone morphogenetic protein-2 (BMP-2). In contrast, 27X-1 cells differentiate into extra-embryonic endodermal (ExE) cells upon treatment with either morphogen. To understand the molecular basis for the differential responses of the two cell lines, we performed gene expression profiling at the undifferentiated EC cell line state to identify constitutive differences in gene expression. NT2/D1 cells preferentially expressed transcripts associated with neurectodermal development, whereas 27X-1 cells expressed high levels of transcripts associated with mesendodermal characteristics. We then determined temporal expression profiles of 27X-1 cells during ExE differentiation upon treatment with ATRA and BMP-2 and compared the data with changes in gene expression observed during BMP-2- and ATRA-induced differentiation of NT2/D1 cells. ATRA and BMP-2 induced distinct sets of transcription factors and phenotypic markers in the two EC cell lines, underlying distinct lineage choices. Although 27X-1 differentiation yielded comprehensive gene expression profiles of parietal endodermal lineages, we were able to use the combined analysis of 27X-1 data with data derived from yolk sac tumors for the identification of transcripts associated with visceral endoderm formation. Our results demonstrate constitutive differences in the levels of pluripotency between NT2/D1 and 27X-1 cells that correlate with lineage potential. This study also demonstrates that EC cells can serve as robust models to investigate early lineage choices during both embryonic and extra-embryonic human development.

The SHB Adapter Protein Is Required for Normal Maturation of Mesoderm during in Vitro Differentiation of Embryonic Stem Cells

Definitive mesoderm arises from a bipotent mesendodermal population, and to study processes controlling its development at this stage, embryonic stem (ES) cells can be employed. SHB (Src homology 2 protein in beta-cells) is an adapter protein previously found to be involved in ES cell differentiation to mesoderm. To further study the role of SHB in this context, we have established ES cell lines deficient for one (SHB+/-) or both SHB alleles (SHB-/-). Differentiating embryoid bodies (EBs) derived from these ES cell lines were used for gene expression analysis. Alternatively, EBs were stained for the blood vessel marker CD31. For hematopoietic differentiation, EBs were differentiated in methylcellulose. SHB-/- EBs exhibited delayed down-regulation of the early mesodermal marker Brachyury. Later mesodermal markers relatively specific for the hematopoietic, vascular, and cardiac lineages were expressed at lower levels on day 6 or 8 of differentiation in EBs lacking SHB. The expression of vascular endothelial growth factor receptor-2 and fibroblast growth factor receptor-1 was also reduced in SHB-/- EBs. SHB-/- EBs demonstrated impaired blood vessel formation after vascular endothelial growth factor stimulation. In addition, the SHB-/- ES cells formed fewer blood cell colonies than SHB+/+ ES cells. It is concluded that SHB is required for appropriate hematopoietic and vascular differentiation and that delayed down-regulation of Brachyury expression may play a role in this context.

Fibroblast growth factor 2 modulates transforming growth factor beta signaling in mouse embryonic fibroblasts and human ESCs (hESCs) to support hESC self-renewal

Fibroblast growth factor 2 (FGF2) is known to promote self-renewal of human embryonic stem cells (hESCs). In addition, it has been shown that transforming growth factor beta (TGFbeta) signaling is crucial in that the TGFbeta/Activin/Nodal branch of the pathway needs to be activated and the bone morphogenic protein (BMP)/GDF branch repressed to prevent differentiation. This holds particularly true for Serum Replacement-based medium containing BMP-like activity. We have reinvestigated a widely used protocol for conditioning hESC medium with mouse embryonic fibroblasts (MEFs). We show that FGF2 acts on MEFs to release supportive factors and reduce differentiation-inducing activity. FGF2 stimulation experiments with supportive and nonsupportive MEFs followed by genome-wide expression profiling revealed that FGF2 regulates the expression of key members of the TGFbeta pathway, with Inhba, Tgfb1, Grem1, and Bmp4 being the most likely candidates orchestrating the above activities. In addition, restimulation experiments in hESCs combined with global expression analysis revealed downstream targets of FGF2 signaling in these cells. Among these were the same factors previously identified in MEFs, thus suggesting that FGF2, at least in part, promotes self-renewal of hESCs by modulating the expression of TGFbeta ligands, which, in turn, act on hESCs in a concerted and autocrine manner.

The Regulation of Self-Renewal in Human Embryonic Stem Cells

Human embryonic stem (hES) cells have the ability to self-renew while maintaining their pluripotency. The ability of stem cells to self-renew expansively is essential in both development and maintenance of adult tissues. ES cell lines were first generated from mouse blastocysts, these lines provided much needed information regarding ES cell propagation, growth factor dependence, and marker expression. However, the application potential of murine models is restricted in value because many differences between mouse and human ES cells have since been identified. The process of hES cells self-renewal appears to be regulated by many different pathways; however, the molecular mechanisms enabling this process are not fully characterized. Further defining these mechanisms will enable growth of hES cells under defined conditions and aid controlled differentiation of cells into specified lineages, in turn providing cells suitable for therapeutic applications. This review provides a summary of the mechanisms known to control self-renewal and pluripotency in hES cells.

Pluripotent stem cells and their niches

The ability of stem cells to self-renew and to replace mature cells is fundamental to ontogeny and tissue regeneration. Stem cells of the adult organism can be categorized as mono-, bi-, or multipotent, based on the number of mature cell types to which they can give rise. In contrast, pluripotent stem cells of the early embryo have the ability to form every cell type of the adult body. Permanent lines of pluripotent stem cells have been derived from preimplantation embryos (embryonic stem cells), fetal primordial germ cells (embryonic germ cells), and malignant teratocarcinomas (embryonal carcinoma cells). Cultured pluripotent stem cells can easily be manipulated genetically, and they can be matured into adult-type stem cells and terminally differentiated cell types in vitro, thereby, providing powerful model systems for the study of mammalian embryogenesis and disease processes. In addition, human embryonic stem cell lines hold great promise for the development of novel regenerative therapies. To fully utilize the potential of these cells, we must first understand the mechanisms that control pluripotent stem cell fate and function. In recent decades, the microenvironment or niche has emerged as particularly critical for stem cell regulation. In this article, we review how pluripotent stem cell signal transduction mechanisms and transcription factor circuitries integrate information provided by the microenvironment. In addition, we consider the potential existence and location of adult pluripotent stem cell niches, based on the notion that a revealing feature indicating the presence of stem cells in a given tissue is the occurrence of tumors whose characteristics reflect the normal developmental potential of the cognate stem cells.

Guiding embryonic stem cells towards differentiation: lessons from molecular embryology

Embryonic stem cells are uniquely endowed with the capacity of self-renewal and the potential to give rise to all possible cell types, including germ cells. These qualities have made mouse embryonic stem cells a valuable resource for genetic manipulation of the mouse genome. In addition, they present a powerful system for the in vitro dissection of mammalian embryonic development. The recent isolation of human embryonic stem cells has raised a lot of interest for the potential of transposing our knowledge of lineage-specific differentiation of embryonic stem cells to cell-based therapy of human disease. Recent reports have provided insights into the specific differentiation of embryonic stem cells to different cell types of the embryo. However, progress in this direction seems to depend on the knowledge of the mechanisms controlling lineage decisions during embryogenesis.

Embryonic and tumorigenic pathways converge via Nodal signaling: role in melanoma aggressiveness

Bidirectional cellular communication is integral to both cancer progression and embryological development. In addition, aggressive tumor cells are phenotypically plastic, sharing many properties with embryonic cells. Owing to the similarities between these two types of cells, the developing zebrafish can be used as a biosensor for tumor-derived signals. Using this system, we show that aggressive melanoma cells secrete Nodal (a potent embryonic morphogen) and consequently can induce ectopic formation of the embryonic axis. We further show that Nodal is present in human metastatic tumors, but not in normal skin, and thus may be involved in melanoma pathogenesis. Inhibition of Nodal signaling reduces melanoma cell invasiveness, colony formation and tumorigenicity. Nodal inhibition also promotes the reversion of melanoma cells toward a melanocytic phenotype. These data suggest that Nodal signaling has a key role in melanoma cell plasticity and tumorigenicity, thereby providing a previously unknown molecular target for regulating tumor progression.

Three-dimensional culture for expansion and differentiation of mouse embryonic stem cells

Differentiation of embryonic stem (ES) cells typically requires cell-cell aggregation in the form of embryoid bodies (EBs). This process is not very well controlled and final cell numbers can be limited by EB agglomeration and the inability to drive differentiation towards a desired cell type. This study compares three-dimensional (3D) fibrin culture to conventional two-dimensional (2D) suspension culture and to culture in a semisolid methylcellulose medium solution. Two types of fibrin culture were evaluated, including a PEGylated fibrin gel. PEGylation with a difunctional PEG derivative retarded fibrinogen migration during through sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) as a result of crosslinking, similarly, degradation was slowed in the PEGylated gel. ES cell proliferation was higher in both the fibrin and PEGylated fibrin gels versus 2D and methylcellulose controls. FACS analysis and real-time-PCR revealed differences in patterns of differentiation for the various culture systems. Culture in PEGylated fibrin or methylcellulose culture demonstrated features characteristic of less extensive differentiation relative to fibrin and 2D culture as evidenced by the transcription factor Oct-4. Fibrin gels showed gene and protein expression similar to that in 2D culture. Both fibrin and 2D cultures demonstrated statistically greater cell numbers positive for the vascular mesoderm marker, VE-cadherin.

Absence of Nodal signaling promotes precocious neural differentiation in the mouse embryo

After implantation, mouse embryos deficient for the activity of the transforming growth factor-beta member Nodal fail to form both the mesoderm and the definitive endoderm. They also fail to specify the anterior visceral endoderm, a specialized signaling center which has been shown to be required for the establishment of anterior identity in the epiblast. Our study reveals that Nodal-/- epiblast cells nevertheless express prematurely and ectopically molecular markers specific of anterior fate. Our analysis shows that neural specification occurs and regional identities characteristic of the forebrain are established precociously in the Nodal-/- mutant with a sequential progression equivalent to that of wild-type embryo. When explanted and cultured in vitro, Nodal-/- epiblast cells readily differentiate into neurons. Genes normally transcribed in organizer-derived tissues, such as Gsc and Foxa2, are also expressed in Nodal-/- epiblast. The analysis of Nodal-/-;Gsc-/- compound mutant embryos shows that Gsc activity plays no critical role in the acquisition of forebrain characters by Nodal-deficient cells. This study suggests that the initial steps of neural specification and forebrain development may take place well before gastrulation in the mouse and highlights a possible role for Nodal, at pregastrula stages, in the inhibition of anterior and neural fate determination.

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.

A novel chemical-defined medium with bFGF and N2B27 supplements supports undifferentiated growth in human embryonic stem cells

Traditionally, undifferentiated human embryonic stem cells (hESCs) are maintained on mouse embryonic fibroblast (MEF) cells or on matrigel with an MEF-conditioned medium (CM), which hampers the clinical applications of hESCs due to the contamination by animal pathogens. Here we report a novel chemical-defined medium using DMEM/F12 supplemented with N2, B27, and basic fibroblast growth factor (bFGF) [termed NBF]. This medium can support prolonged self-renewal of hESCs. hESCs cultured in NBF maintain an undifferentiated state and normal karyotype, are able to form embryoid bodies in vitro, and differentiate into three germ layers and extraembryonic cells. Furthermore, we find that hESCs cultured in NBF possess a low apoptosis rate and a high proliferation rate compared with those cultured in MEF-CM. Our findings provide a novel, simplified chemical-defined culture medium suitable for further therapeutic applications and developmental studies of hESCs.

Derivation of distal lung epithelial progenitors from murine embryonic stem cells using a novel three-step differentiation protocol

Embryonic stem cells (ESCs) are a potential source for the cell-based therapy of a wide variety of lung diseases for which the only current treatment is transplantation. However, distal lung epithelium, like many other endodermally derived somatic cell lineages, is proving difficult to obtain from both murine and human ESCs. We have previously obtained alveolar epithelium from ESCs, although final cell yield remained extremely low. Here, we present an optimized three-step protocol for the derivation of distal lung epithelial cells from murine ESCs. This protocol incorporates (a) treatment of early differentiating embryoid bodies with activin A to enhance the specification of the endodermal germ layer, followed by (b) adherent culture in serum-free medium and (c) the final application of a commercial, lung-specific medium. As well as enhancing the specification of distal lung epithelium, this protocol was found to yield cells with a phenotype most closely resembling that of lung-committed progenitor cells present in the foregut endoderm and the early lung buds during embryonic development. This is in contrast to our previous differentiation method, which drives differentiation through to mature type II alveolar epithelial cells. The derivation of a committed lung progenitor cell type from ESCs is particularly significant for regenerative medicine because the therapeutic implantation of progenitor cells has several clear advantages over the transplantation of mature, terminally differentiated somatic cells.

HOXB4 overexpression promotes hematopoietic development by human embryonic stem cells

Human embryonic stem cells (hESCs) are a potential source of hematopoietic cells for therapeutic transplantation and can provide a model for human hematopoiesis. Culture of hESCs on murine stromal layers or in stromal-free conditions as embryoid bodies results in low levels of hematopoietic cells. Here we demonstrate that overexpression of the transcription factor HOXB4 considerably augments hematopoietic development of hESCs. Stable HOXB4-expressing hESC clones were generated by lipofection and could be maintained in the undifferentiated state for prolonged passages. Moreover, differentiation of hESCs as embryoid bodies in serum-containing medium without the use of additional cytokines led to sequential expansion of first erythroid and then myeloid and monocytic progenitors from day 10 of culture. These cells retained the capacity to develop into formed blood elements during in vitro culture. Consistent with the development of committed hematopoietic cells, we observed the expression of transcription factors known to be critical for hematopoietic development. We thus demonstrate successful use of enforced gene expression to promote the differentiation of hESCs into a terminally differentiated tissue, thereby revealing an important role for HOXB4 in supporting their in vitro development along the hematopoietic pathway.

Long-term self-renewal and directed differentiation of human embryonic stem cells in chemically defined conditions

Chemically defined medium (CDM) conditions for controlling human embryonic stem cell (hESC) fate will not only facilitate the practical application of hESCs in research and therapy but also provide an excellent system for studying the molecular mechanisms underlying self-renewal and differentiation, without the multiple unknown and variable factors associated with feeder cells and serum. Here we report a simple CDM that supports efficient self-renewal of hESCs grown on a Matrigel-coated surface over multiple passages. Expanded hESCs under such conditions maintain expression of multiple hESC-specific markers, retain the characteristic hESC morphology, possess a normal karyotype in vitro, as well as develop teratomas in vivo. Additionally, several growth factors were found to selectively induce monolayer differentiation of hESC cultures toward neural, definitive endoderm/pancreatic and early cardiac muscle cells, respectively, in our CDM conditions. Therefore, this CDM condition provides a basic platform for further characterization of hESC self-renewal and directed differentiation, as well as the development of novel therapies.

Notch signaling is inactive but inducible in human embryonic stem cells

The NOTCH signaling pathway performs a wide range of critical functions in a number of different cell types during development and differentiation. The role of NOTCH signals in human embryonic stem cells (hESCs) has not been tested. We measured the activity of canonical NOTCH signaling in undifferentiated embryonic stem (ES) cells and tested the requirement for NOTCH activity in hESC self-renewal or differentiation by growing hESCs in the presence of gamma-secretase inhibitors. Our results suggest that NOTCH signaling is not required for the propagation of undifferentiated human ES cells but instead is required for the maintenance of the differentiating cell types that accumulate in human ES cell cultures. Our studies suggest that NOTCH signaling is not required in human embryonic differentiation until the formation of extraembryonic, germ layer, or tissue-specific stem cells and progenitors.

The Vg1-related protein Gdf3 acts in a Nodal signaling pathway in the pre-gastrulation mouse embryo

The formation of the anterior visceral endoderm (AVE) in the pre-gastrulation mouse embryo represents a crucial event in patterning of the anterior-posterior axis. Here, we show that the transforming growth factor beta (Tgfbeta) family member Gdf3 (growth-differentiation factor 3), a close relative of Xenopus Vg1, resembles the Tgfbeta ligand Nodal in both its signaling activity and its role in AVE formation in vivo. Thus, in cell culture, Gdf3 signaling requires the EGF-CFC co-receptor Cripto and can be inhibited by Lefty antagonists. In Xenopus embryos, Gdf3 misexpression results in secondary axis formation, and induces morphogenetic elongation and mesendoderm formation in animal caps. In mouse embryos, Gdf3 is expressed in the inner cell mass and epiblast, and null mutants frequently exhibit abnormal formation or positioning of the AVE. This phenotype correlates with defects in mesoderm and definitive endoderm formation, as well as abnormal Nodal expression levels. Our findings indicate that Gdf3 acts in a Nodal-like signaling pathway in pre-gastrulation development, and provide evidence for the functional conservation of Vg1 activity in mice.

Activin A Maintains Self-Renewal and Regulates Fibroblast Growth Factor, Wnt, and Bone Morphogenic Protein Pathways in Human Embryonic Stem Cells

Human embryonic stem cells (hESCs) self-renew indefinitely while maintaining pluripotency. The molecular mechanism underlying hESCs self-renewal and pluripotency is poorly understood. To identify the signaling pathway molecules that maintain the proliferation of hESCs, we performed a microarray analysis comparing an aneuploid H1 hESC line (named H1T) versus euploid H1 hESC line because the H1T hESC line demonstrates a self-renewal advantage while maintaining pluripotency. We find differential gene expression for the Nodal/Activin, fibroblast growth factor (FGF), Wnt, and Hedgehog (Hh) signaling pathways in the H1T line, which implicates each of these molecules in maintaining the undifferentiated state, whereas the bone morphogenic protein (BMP) and Notch pathways could promote hESCs differentiation. Experimentally, we find that Activin A is necessary and sufficient for the maintenance of self-renewal and pluripotency of hESCs and supports long-term feeder and serum-free growth of hESCs. We show that Activin A induces the expression of Oct4, Nanog, Nodal, Wnt3, basic FGF, and FGF8 and suppresses the BMP signal. Our data indicates Activin A as a key regulator in maintenance of the stemness in hESCs. This finding will help elucidate the complex signaling network that maintains the hESC phenotype and function.

Automated mechanical passaging: a novel and efficient method for human embryonic stem cell expansion

There is a need for more standardized methods of maintenance and propagation of human embryonic stem cell (hESC) cultures. Enzymatic passaging currently represents the most widely used method for expansion of hESCs. Although rapid and straightforward, this technique results in variable-sized cell clusters and significant cellular trauma, which may apply selective pressure in long-term culture. Mechanical passaging has the potential advantages of defined colony fragment sizes, reduced cellular trauma, and the possibility of selecting undifferentiated colonies for transfer. However, manual dissection of individual colonies is a prohibitively time-consuming process unsuitable for maintaining large numbers of hESCs without the use of additional chemical means. In this study we report an efficient automated method for mechanically passaging hESCs. We have used this method exclusively to maintain hESCs in long-term undifferentiated culture without the use of enzymatic digestion for longer than 100 days. This automated technique can thus be used routinely to culture hESCs in the laboratory.