Isolation of Oct4-expressing extraembryonic endoderm precursor cell lines

Dissecting embryonic and extraembryonic lineage crosstalk with stem cell co-culture

Embryogenesis necessitates harmonious coordination between embryonic and extraembryonic tissues. Although stem cells of both embryonic and extraembryonic origins have been generated, they are grown in different culture conditions. In this study, utilizing a unified culture condition that activates the FGF, TGF-β, and WNT pathways, we have successfully derived embryonic stem cells (FTW-ESCs), extraembryonic endoderm stem cells (FTW-XENs), and trophoblast stem cells (FTW-TSCs) from the three foundational tissues of mouse and cynomolgus monkey (Macaca fascicularis) blastocysts. This approach facilitates the co-culture of embryonic and extraembryonic stem cells, revealing a growth inhibition effect exerted by extraembryonic endoderm cells on pluripotent cells, partially through extracellular matrix signaling. Additionally, our cross-species analysis identified both shared and unique transcription factors and pathways regulating FTW-XENs. The embryonic and extraembryonic stem cell co-culture strategy offers promising avenues for developing more faithful embryo models and devising more developmentally pertinent differentiation protocols.

Generation of Mouse Primitive Endoderm Stem Cells

The blastocysts consist of dozens of cells of three distinct lineages: epiblast (Epi), trophoblast (TB), and primitive endoderm (PrE). All embryonic and extraembryonic tissues are derived from Epi, TB, and PrE. Stem cell lines representing preimplantation Epi and TB have been established and are known as embryonic stem cells (ESCs) and trophoblast stem cells (TSCs). Extraembryonic endoderm cells (XENCs) constitute a cell line that has been established from PrE. Although in vivo, PrE gives rise to visceral endoderm (VE), parietal endoderm (PE), and marginal zone endoderm (MZE); XENCs, on blastocyst injection into chimeras, primarily contribute to the distal region of PE. Here, we provide a comprehensive protocol for the establishment of fully potent primitive endoderm stem cell (PrESC) lines. PrESCs are established and maintained on mouse embryonic fibroblast (MEF) feeder cells in a serum-free medium supplemented with fibroblast growth factor 4 (FGF4), heparin, CHIR99021, and platelet-derived growth factor-AA (PDGF-AA). PrESCs co-express markers indicative of pluripotency and endoderm lineage commitment, exhibiting characteristics akin to those of PrE. On transplantation of PrESCs into blastocysts, they demonstrate a high efficiency in contributing to VE, PE, and MZE. PrESCs serve as a valuable model for studying PrE, sharing similarities in gene expression profiles and differentiation potential. PrESCs constitute a pivotal cornerstone for in vitro analysis of early developmental mechanisms and for studies of embryo reconstitution in vitro, particularly in conjunction with ESCs and TSCs. Key features • Establishment and maintenance of primitive endoderm stem cell (PrESCs) capable of recapitulating the developmental prowess inherent to PrE. • Offering a source of PrE lineage for embryo-like organoid reconstitution studies.

Klf4 methylated by Prmt1 restrains the commitment of primitive endoderm

The second cell fate decision in the early stage of mammalian embryonic development is pivotal; however, the underlying molecular mechanism is largely unexplored. Here, we report that Prmt1 acts as an important regulator in primitive endoderm (PrE) formation. First, Prmt1 depletion promotes PrE gene expression in mouse embryonic stem cells (ESCs). Single-cell RNA sequencing and flow cytometry assays demonstrated that Prmt1 depletion in mESCs contributes to an emerging cluster, where PrE genes are upregulated significantly. Furthermore, the efficiency of extraembryonic endoderm stem cell induction increased in Prmt1-depleted ESCs. Second, the pluripotency factor Klf4 methylated at Arg396 by Prmt1 is required for recruitment of the repressive mSin3a/HDAC complex to silence PrE genes. Most importantly, an embryonic chimeric assay showed that Prmt1 inhibition and mutated Klf4 at Arg 396 induce the integration of mouse ESCs into the PrE lineage. Therefore, we reveal a regulatory mechanism for cell fate decisions centered on Prmt1-mediated Klf4 methylation.

Extraembryonic Endoderm (XEN) Cells Capable of Contributing to Embryonic Chimeras Established from Pig Embryos

Most of our current knowledge regarding early lineage specification and embryo-derived stem cells comes from studies in rodent models. However, key gaps remain in our understanding of these developmental processes from nonrodent species. Here, we report the detailed characterization of pig extraembryonic endoderm (pXEN) cells, which can be reliably and reproducibly generated from primitive endoderm (PrE) of blastocyst. Highly expandable pXEN cells express canonical PrE markers and transcriptionally resemble rodent XENs. The pXEN cells contribute both to extraembryonic tissues including visceral yolk sac as well as embryonic gut when injected into host blastocysts, and generate live offspring when used as a nuclear donor in somatic cell nuclear transfer (SCNT). The pXEN cell lines provide a novel model for studying lineage segregation, as well as a source for genome editing in livestock.

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Primitive endoderm (PrE) is the predominant lineage emerging from pig blastocyst outgrowths

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pXEN cells exhibit key features of PrE-progenitors and resemble rodent XEN cells

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pXEN cells contribute to extraembryonic and embryonic (gut) endoderm in vivo

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pXEN cells can support full-term development via somatic cell nuclear transfer

Characterisation of extraembryonic endoderm-like cells from mouse embryonic fibroblasts induced using chemicals alone

Background

The development of somatic reprogramming, especially purely chemical reprogramming, has significantly advanced biological research. And chemical-induced extraembryonic endoderm-like (ciXEN) cells have been confirmed to be an indispensable intermediate stage of chemical reprogramming. They resemble extraembryonic endoderm (XEN) cells in terms of transcriptome, reprogramming potential, and developmental ability in vivo. However, the other characteristics of ciXEN cells and the effects of chemicals and bFGF on the in vitro culture of ciXEN cells have not been systematically reported.

Methods

Chemicals and bFGF in combination with Matrigel were used to induce the generation of ciXEN cells derived from mouse embryonic fibroblasts (MEFs). RNA sequencing was utilised to examine the transcriptome of ciXEN cells, and PCR/qPCR assays were performed to evaluate the mRNA levels of the genes involved in this study. Hepatic functions were investigated by periodic acid-Schiff staining and indocyanine green assay. Lactate production, ATP detection, and extracellular metabolic flux analysis were used to analyse the energy metabolism of ciXEN cells.

Results

ciXEN cells expressed XEN-related genes, exhibited high proliferative capacity, had the ability to differentiate into visceral endoderm in vitro, and possessed the plasticity allowing for their differentiation into induced hepatocytes (iHeps). Additionally, the upregulated biological processes of ciXEN cells compared to those in MEFs focused on metabolism, but their energy production was independent of glycolysis. Furthermore, without the cocktail of chemicals and bFGF, which are indispensable for the generation of ciXEN cells, induced XEN (iXEN) cells remained the expression of XEN markers, the high proliferative capacity, and the plasticity to differentiate into iHeps in vitro.

Conclusions

ciXEN cells had high plasticity, and energy metabolism was reconstructed during chemical reprogramming, but it did not change from aerobic oxidation to glycolysis. And the cocktail of chemicals and bFGF were non-essential for the in vitro culture of ciXEN cells.

Basal-type lumenogenesis in extraembryonic endoderm stem cells models the early visceral endoderm

Cultured rat primitive extraembryonic endoderm (pXEN) cells easily form free-floating multicellular vesicles de novo, exemplifying a poorly studied type of morphogenesis. Here, we reveal the underlying mechanism and the identity of the vesicles. We resolve the morphogenesis into vacuolization, vesiculation and maturation, and define the molecular characteristics and requirements of each step. Vacuolization is fueled by macropinocytosis and occurs by default if not blocked by high cell density or matrix proteins. Fine-tuned cell-cell contact then forms nascent three-cell vesicles with vacuole-derived lumina. In maturation, the vesicles complete epithelialization, expand via mitosis and continued fluid uptake, and differentiate further. The mature vesicles consist of a simple squamous epithelium with an apical-outside/basal-inside polarity that we trace back to the single cell stage. The polarity and gene expression pattern of the vesicles are similar to those of the early visceral endoderm. pXEN cells provide a useful in vitro model for study of matrix-independent, basal-type lumenogenesis and the physiology of the visceral endoderm.This article has an associated First Person interview with the first author of the paper.

Transcriptome analysis shows ambiguous phenotypes of murine primitive endoderm-related stem cell lines

Primitive endoderm (PrE)-related cell lines (XEN, pXEN and nEnd cells) show key features of the PrE. By transcriptome analysis, we show: (a) Compared to embryonic stem cells, PrE-related cell lines are less in vivo like, although early nEnd cells are most similar to the PrE. (b) These cell lines show post-PrE features of parietal (XEN and pXEN cells) or visceral (nEnd cells) endoderm, likely driven by Tgf-β and Wnt/Activin signaling, respectively. (c) pXEN and nEnd cell lines additionally show pre-PrE features. Hence, neither pXEN nor nEnd cell cultures represent a distinct in vivo entity. Rather, their properties are compatible with mixed and hybrid phenotypes. Our findings indicate that pre-PrE, PrE and early post-PrE phenotypes result from different niches, which need to be better understood to derive cell lines that distinctly represent the early stages of the extraembryonic endoderm.

Isolation of primitive mouse extraembryonic endoderm (pXEN) stem cell lines

Mouse blastocysts contain the committed precursors of the extraembryonic endoderm (ExEn), which express the key transcription factor Oct4, depend on LIF/LIF-like factor-driven Jak/Stat signaling, and initially exhibit lineage plasticity. Previously described rat blastocyst-derived ExEn precursor-like cell lines (XENP cells/HypoSCs) also show these features, but equivalent mouse blastocyst-derived cell lines are lacking. We now present mouse blastocyst-derived cell lines, named primitive XEN (pXEN) cells, which share these and additional characteristics with the XENP cells/HypoSCs, but not with previously known mouse blastocyst-derived XEN cell lines. Otherwise, pXEN cells are highly similar to XEN cells by morphology, lineage-intrinsic differentiation potential, and multi-gene expression profile, although the pXEN cell profile correlates better with the blastocyst stage. Finally, we show that pXEN cells easily convert into XEN-like cells but not vice versa. The findings indicate that (i) pXEN cells are more representative than XEN cells of the blastocyst stage; (ii) mouse pXEN, rather than XEN, cells are homologs of rat XENP cells/HypoSCs, which we propose to call rat pXEN cells.

From engineering to editing the rat genome

Since its domestication over 100 years ago, the laboratory rat has been the preferred experimental animal in many areas of biomedical research (Lindsey and Baker The laboratory rat. Academic, New York, pp 1-52, 2006). Its physiology, size, genetics, reproductive cycle, cognitive and behavioural characteristics have made it a particularly useful animal model for studying many human disorders and diseases. Indeed, through selective breeding programmes numerous strains have been derived that are now the mainstay of research on hypertension, obesity and neurobiology (Okamoto and Aoki Jpn Circ J 27:282-293, 1963; Zucker and Zucker J Hered 52(6):275-278, 1961). Despite this wealth of genetic and phenotypic diversity, the ability to manipulate and interrogate the genetic basis of existing phenotypes in rat strains and the methodology to generate new rat models has lagged significantly behind the advances made with its close cousin, the laboratory mouse. However, recent technical developments in stem cell biology and genetic engineering have again brought the rat to the forefront of biomedical studies and enabled researchers to exploit the increasingly accessible wealth of genome sequence information. In this review, we will describe how a breakthrough in understanding the molecular basis of self-renewal of the pluripotent founder cells of the mammalian embryo, embryonic stem (ES) cells, enabled the derivation of rat ES cells and their application in transgenesis. We will also describe the remarkable progress that has been made in the development of gene editing enzymes that enable the generation of transgenic rats directly through targeted genetic modifications in the genomes of zygotes. The simplicity, efficiency and cost-effectiveness of the CRISPR/Cas gene editing system, in particular, mean that the ability to engineer the rat genome is no longer a limiting factor. The selection of suitable targets and gene modifications will now become a priority: a challenge where ES culture and gene editing technologies can play complementary roles in generating accurate bespoke rat models for studying biological processes and modelling human disease.

Generation of Canine Induced Extraembryonic Endoderm-Like Cell Line That Forms Both Extraembryonic and Embryonic Endoderm Derivatives

Extraembryonic endoderm (XEN) cells are stem cell lines derived from primitive endoderm cells of inner cell mass in blastocysts. These cells have self-renewal properties and differentiate into visceral endoderm (VE) and parietal endoderm (PE) of the yolk sac. Recently, it has been reported that XEN cells can contribute to fetal embryonic endoderm, and their unique potency has been evaluated. In this study, we have described the induction and characterization of new canine stem cell lines that closely resemble to XEN cells. These cells, which we designated canine induced XEN (ciXEN)-like cells, were induced from canine embryonic fibroblasts by introducing four transgenes. ciXEN-like cells expressed XEN markers, which could be maintained over 50 passages in N2B27 medium supplemented with inhibitors of mitogen-activated protein kinase p38 and transforming growth factor-beta 1. Our ciXEN-like cells were maintained without transgene expression and exhibited upregulated expression of VE and PE markers in feeder-free conditions. The cells differentiated from ciXEN-like cells using a coculture system showed multiple nuclei and expressed albumin protein, similar to characteristics of hepatocytes. Furthermore, these cells expressed the adult hepatocyte marker, CYP3A4. Interestingly, these cells also formed a net structure expressing the bile epithelium capillary marker, multidrug resistance-associated protein 2. Thus, we have demonstrated the induction of a new canine stem cell line, ciXEN-like cells, which could form an embryonic endodermal cell layer. Our ciXEN-like cells may be a helpful tool to study the canine embryo development and represent a promising cell source for proceeding human and canine regenerative medicine.

Capturing Identity and Fate Ex Vivo: Stem Cells from the Mouse Blastocyst

During mouse preimplantation development, three molecularly, morphologically, and spatially distinct lineages are formed, the embryonic epiblast, the extraembryonic primitive endoderm, and the trophectoderm. Stem cell lines representing each of these lineages have now been derived and can be indefinitely maintained and expanded in culture, providing an unlimited source of material to study the interplay of tissue-specific transcription factors and signaling pathways involved in these fundamental cell fate decisions. Here we outline our current understanding of the derivation, maintenance, and properties of these in vitro stem cell models representing the preimplantation embryonic lineages.

Osteogenic Potential of Multipotent Adult Progenitor Cells for Calvaria Bone Regeneration

Osteogenic cells derived from rat multipotent adult progenitor cells (rMAPCs) were investigated for their potential use in bone regeneration. rMAPCs are adult stem cells derived from bone marrow that have a high proliferation capacity and the differentiation potential to multiple lineages. They may also offer immunomodulatory properties favorable for applications for regenerative medicine. rMAPCs were cultivated as single cells or as 3D aggregates in osteogenic media for up to 38 days, and their differentiation to bone lineage was then assessed by immunostaining of osteocalcin and collagen type I and by mineralization assays. The capability of rMAPCs in facilitating bone regeneration was evaluated in vivo by the direct implantation of multipotent adult progenitor cell (MAPC) aggregates in rat calvarial defects. Bone regeneration was examined radiographically, histologically, and histomorphometrically. Results showed that rMAPCs successfully differentiated into osteogenic lineage by demonstrating mineralized extracellular matrix formation in vitro and induced new bone formation by the effect of rMAPC aggregates in vivo. These outcomes confirm that rMAPCs have a good osteogenic potential and provide insights into rMAPCs as a novel adult stem cell source for bone regeneration.

Bidirectional developmental potential in reprogrammed cells with acquired pluripotency

We recently discovered an unexpected phenomenon of somatic cell reprogramming into pluripotent cells by exposure to sublethal stimuli, which we call stimulus-triggered acquisition of pluripotency (STAP). This reprogramming does not require nuclear transfer or genetic manipulation. Here we report that reprogrammed STAP cells, unlike embryonic stem (ES) cells, can contribute to both embryonic and placental tissues, as seen in a blastocyst injection assay. Mouse STAP cells lose the ability to contribute to the placenta as well as trophoblast marker expression on converting into ES-like stem cells by treatment with adrenocorticotropic hormone (ACTH) and leukaemia inhibitory factor (LIF). In contrast, when cultured with Fgf4, STAP cells give rise to proliferative stem cells with enhanced trophoblastic characteristics. Notably, unlike conventional trophoblast stem cells, the Fgf4-induced stem cells from STAP cells contribute to both embryonic and placental tissues in vivo and transform into ES-like cells when cultured with LIF-containing medium. Taken together, the developmental potential of STAP cells, shown by chimaera formation and in vitro cell conversion, indicates that they represent a unique state of pluripotency.

Concise review: Bone marrow meets blastocyst: lessons from an unlikely encounter

This article discusses the implications of the recent discovery that rat bone marrow-derived multipotent adult progenitor cells (rMAPCs), a cell type with broad somatic differentiation potential but of uncertain lineage identity, are similar to rat blastocyst-derived extraembryonic endoderm precursor (rXENP) cells, which appear to represent the committed extraembryonic endoderm precursor of the blastocyst. It was found that under rMAPC culture conditions, rXENP cells can be homogeneously cultured and similar cells, named rat hypoblast stem cells (rHypoSCs), can be derived from rat blastocysts more rapidly and directly. The detailed comparison of rHypoSCs, rXENP cells, and rMAPCs revealed highly similar gene expression profiles and developmental potentials. The significance of these findings for embryology, stem cell biology, and medicine is discussed. Specifically, the results assign a lineage identity to rMAPCs, indicate that rMAPCs originated by environmental reprogramming, and imply that HypoSCs, XENP cells, and MAPCs possess lineage plasticity. The MAPC-HypoSC relation also strengthens the consistency of rat and mouse embryology and consequently the idea that HypoSCs represent the committed extraembryonic endoderm precursor of the blastocyst. On this basis, it is argued that the direct comparison of HypoSCs (now available in pure form) with embryonic stem cells will be highly useful for the understanding of pluripotency and plasticity. Finally, the new findings suggest an explanation for an obscure observation on stem cell-induced transplantation tolerance. Thus, the HypoSC/XENP/MAPC phenotype provides a unique but broadly instructive model with which to study stem cell plasticity, reprogramming, and transplantation tolerance, all central themes in regenerative medicine.

Culture of mouse amniotic fluid-derived cells on irradiated STO feeders results in the generation of primitive endoderm cell lines capable of self-renewal in vitro

The cells present in amniotic fluid (AF) are currently used for prenatal diagnosis of fetal anomalies but are also a potential source of cells for cell therapy. To better characterize putative progenitor cell populations present in AF, we used culture conditions that support self-renewal to determine if these promoted the generation of stable cell lines from AF-derived cells (AFC). Cells isolated from E11.5 mouse were cultured on irradiated STO fibroblast feeder layers in human embryonic germ cell derivation conditions. The cultures grew multicellular epithelial colonies that could be repropagated from single cells. Reverse transcription semiquantitative polymerase chain reaction of established cell lines revealed that they belonged to the extraembryonic endoderm (ExEn) expressing high levels of Gata6, Gata4, Sox17, Foxa2 and Sox7 mRNA. Hierarchical clustering based on the whole transcriptome expression profile of the AFC lines (AFCL) shows significant correlation between transcription profiles of AFCL and blastocyst-derived XEN, an ExEn cell line. In vitro differentiation of AFCL results in the generation of cells expressing albumin and α-fetoprotein (AFP), while intramuscular injection of AFCL into immunodeficient mice produced AFP-positive tumors with primitive endodermal appearance. Hence, E11.5 mouse AF contains cells that efficiently produce XEN lines. These AF-derived XEN lines do not spontaneously differentiate into embryonic-type cells but are phenotypically stable and have the capacity for extensive expansion. The lack of requirement for reprogramming factors to turn AF-derived progenitor cells into stable cell lines capable of massive expansion together with the known ability of ExEn to contribute to embryonic tissue suggests that this cell type may be a candidate for banking for cell therapies.

Immunological characteristics of human mesenchymal stem cells and multipotent adult progenitor cells

Somatic, also termed adult, stem cells are highly attractive biomedical cell candidates because of their extensive replication potential and functional multilineage differentiation capacity. They can be used for drug and toxicity screenings in preclinical studies, as in vitro model to study differentiation or for regenerative medicine to aid in the repair of tissues or replace tissues that are lost upon disease, injury or ageing. Multipotent adult progenitor cells (MAPCs) and mesenchymal stem cells (MSCs) are two types of adult stem cells derived from bone marrow that are currently being used clinically for tissue regeneration and for their immunomodulatory and trophic effects. This review will give an overview of the phenotypic and functional differences between human MAPCs and MSCs, with a strong emphasis on their immunological characteristics. Finally, we will discuss the clinical studies in which MSCs and MAPCs are already used.

Reversal of hyperglycemia by insulin-secreting rat bone marrow- and blastocyst-derived hypoblast stem cell-like cells

β-cell replacement may efficiently cure type 1 diabetic (T1D) patients whose insulin-secreting β-cells have been selectively destroyed by autoantigen-reactive T cells. To generate insulin-secreting cells we used two cell sources: rat multipotent adult progenitor cells (rMAPC) and the highly similar rat extra-embryonic endoderm precursor (rXEN-P) cells isolated under rMAPC conditions from blastocysts (rHypoSC). rMAPC/rHypoSC were sequentially committed to definitive endoderm, pancreatic endoderm, and β-cell like cells. On day 21, 20% of rMAPC/rHypoSC progeny expressed Pdx1 and C-peptide. rMAPCr/HypoSC progeny secreted C-peptide under the stimulus of insulin agonist carbachol, and was inhibited by the L-type voltage-dependent calcium channel blocker nifedipine. When rMAPC or rHypoSC differentiated d21 progeny were grafted under the kidney capsule of streptozotocin-induced diabetic nude mice, hyperglycemia reversed after 4 weeks in 6/10 rMAPC- and 5/10 rHypoSC-transplanted mice. Hyperglycemia recurred within 24 hours of graft removal and the histological analysis of the retrieved grafts revealed presence of Pdx1-, Nkx6.1- and C-peptide-positive cells. The ability of both rMAPC and HypoSC to differentiate to functional β-cell like cells may serve to gain insight into signals that govern β-cell differentiation and aid in developing culture systems to commit other (pluripotent) stem cells to clinically useful β-cells for cell therapy of T1D.

Oct4 switches partnering from Sox2 to Sox17 to reinterpret the enhancer code and specify endoderm

How regulatory information is encoded in the genome is poorly understood and poses a challenge when studying biological processes. We demonstrate here that genomic redistribution of Oct4 by alternative partnering with Sox2 and Sox17 is a fundamental regulatory event of endodermal specification. We show that Sox17 partners with Oct4 and binds to a unique 'compressed' Sox/Oct motif that earmarks endodermal genes. This is in contrast to the pluripotent state where Oct4 selectively partners with Sox2 at 'canonical' binding sites. The distinct selection of binding sites by alternative Sox/Oct partnering is underscored by our demonstration that rationally point-mutated Sox17 partners with Oct4 on pluripotency genes earmarked by the canonical Sox/Oct motif. In an endodermal differentiation assay, we demonstrate that the compressed motif is required for proper expression of endodermal genes. Evidently, Oct4 drives alternative developmental programs by switching Sox partners that affects enhancer selection, leading to either an endodermal or pluripotent cell fate. This work provides insights in understanding cell fate transcriptional regulation by highlighting the direct link between the DNA sequence of an enhancer and a developmental outcome.

Extraembryonic endoderm cells as a model of endoderm development

In recent years the multipotent extraembryonic endoderm (XEN) stem cells have been the center of much attention. In vivo, XEN cells contribute to the formation of the extraembryonic endoderm, visceral and parietal endoderm and later on, the yolk sac. Recent data have shown that the distinction between embryonic and extraembryonic endoderm is not as strict as previously thought due to the integration, and not the displacement, of the visceral endoderm into the definitive embryonic endoderm. Therefore, cells from the extraembryonic endoderm also contribute to definitive endoderm. Many research groups focused on unraveling the potential and ability of XEN cells to both support differentiation and/or differentiate into endoderm-like tissues as an alternative to embryonic stem (ES) cells. Moreover, the conversion of ES to XEN cells, shown recently without genetic manipulations, uncovers significant and novel molecular mechanisms involved in extraembryonic endoderm and definitive endoderm development. XEN cell lines provide a unique model for an early mammalian lineage that complements the established ES and trophoblast stem cell lines. Through the study of essential genes and signaling requirements for XEN cells in vitro, insights will be gained about the developmental program of the extraembryonic and embryonic endodermal lineage in vivo. This review will provide an overview on the current literature focusing on XEN cells as a model for primitive endoderm and possibly definitive endoderm as well as the potential of using these cells for therapeutic applications.

Epiblast cell number and primary embryonic stem cell colony generation are increased by culture of cleavage stage embryos in insulin

Human embryos for hESC derivation are often donated at the cleavage stage and of reduced quality. Poor quality embryos have lower efficiency for hESC derivation. However, cleavage stage mouse embryos develop into higher quality expanded blastocysts if they are cultured with insulin, suggesting that this approach could be used to improve hESC derivation from poor quality cleavage stage embryos. The present study used a mouse model to examine this approach. In particular we examined the effect of insulin on the number of epiblast cells in blastocysts on days 4, 5 and 6 using Oct4 and Nanog co-expression. Second we examined the effect of insulin on the frequency with which outgrowths can be derived from these. Finally, we tested whether prior culture in the presence of insulin results in blastocysts with increased capacity to generate ESC colonies. Culture of cleavage stage embryos with insulin increased the number of Oct4 and Nanog positive cells in blastocysts at all time points examined. Prior culture with insulin had no effect on outgrowths generated from blastocysts plated on days 4 or 5. However, insulin treatment of blastocysts plated on day 6 resulted in increased numbers of outgrowths with larger epiblasts compared with controls. 13% of insulin treated day 6 blastocysts produced primary ESC colonies compared with 6% of controls. In conclusion, treatment with insulin can improve epiblast cell number in mice leading to an increase with which primary ESC colonies can be generated and may improve hESC isolation from reduced quality embryos donated at the cleavage stage.

Monoallelic gene targeting in hypoblast stem cells reveals X chromosome inactivation

We recently isolated hypoblast stem cells (HypoSC), which are related to embryonic stem (ES) cells. ES cells efficiently perform homologous recombination (HR) and lack X chromosome inactivation (Xi), but it is unknown whether the same applies to HypoSC. Using the X-linked hypoxanthine phosphoribosyl transferase (HPRT) gene, we find that HypoSC perform HR with similar frequency as ES cells. Monoallelic targeting in female HypoSC eliminated HPRT gene expression, implying epigenetic inactivation of the other allele. Although density-induced differentiation complicated selection, the targeted clones maintained their original properties. These results will facilitate targeted genetic manipulation of HypoSC and the study of Xi.

Activation of the PTHRP/adenylate cyclase pathway promotes differentiation of rat XEN cells into parietal endoderm, whereas Wnt/β-catenin signaling promotes differentiation into visceral endoderm

During early mammalian development, primitive endoderm (PrE) is specified and segregated away from the pluripotent epiblast. At a later developmental stage, PrE forms motile parietal endoderm (PE) lying proximal to the trophectoderm, and visceral endoderm (VE) that contacts the developing epiblast and extraembryonic ectoderm. Mouse extraembryonic endoderm (XEN) cells were isolated and became widely used to study signals governing lineage specification. Rat XEN cell lines have also been derived, but were distinguished from mouse by expression of SSEA1 and Oct4. We showed here that rat XEN cells grown in the presence of a GSK3 inhibitor or overexpressing β-catenin exhibited enhanced formation of cell contacts and decreased motility. Rat XEN cells treated with BMP4 revealed similar morphological changes. Furthermore, we observed that rat XEN cells cultured with GSK3 inhibitor formed adhesion and tight junctions, and acquired bottom-top polarity, indicating the formation of VE cells. In contrast, forskolin, an activator of the cAMP pathway, induced the disruption of cell contacts in rat XEN cells. Treatment with forskolin induced PE formation and epithelial-mesenchymal transition (EMT) in rat XEN cells. Using microarray and real-time PCR assays, we found that VE versus PE formation of rat XEN cells was correlated with change in expression levels of VE or PE marker genes. Similar to forskolin, EMT was prompted upon treatment of rat XEN cells with recombinant parathyroid hormone related peptide (PTHRP), an activator of the cAMP pathway in vivo. Taken together, our data suggest that rat XEN cells are PrE-like cells. The activation of Wnt or BMP4 pathways in rat XEN cells leads to the acquisition of VE characteristics, whereas the activation of the PTHRP/cAMP pathway leads to EMT and the formation of PE.

A focused microarray for screening rat embryonic stem cell lines

Here, we describe a focused microarray for screening rat embryonic stem cells (ESCs) and provide validation data that this array can distinguish undifferentiated rat ESCs from rat trophoblast stem (TS) cells, rat extraembryonic endoderm cells, mouse embryonic fibroblast feeder cells, and differentiated rat ESCs. Using this tool, genuine rat ESC lines, which have been expanded in a conventional rat ESC medium containing two inhibitors (2i), for example, glycogen synthase kinase 3 (GSK3) and mitogen-activated protein kinase (MEK) inhibitors, and leukemia inhibitory factor, and genuine rat ESCs, which have been expanded in rat ESC medium containing four inhibitors (4i), for example, GSK3, MEK, Alk5, and Rho-associated kinase inhibitors were compared; as were genuine rat ESCs from 4 different strains of rats. Expression of Cdx2, a gene associated with trophoblast determination, was observed in genuine, undifferentiated rat ESCs from 4 strains and from both 2i and 4i ESC derivation medium. This finding is in contrast to undifferentiated mouse ESCs that do not express Cdx2. The rat ESC focused microarray described in this report has utility for rapid screening of rat ESCs. This tool will enable optimization of culture conditions in the future.

Troika of the mouse blastocyst: lineage segregation and stem cells

The initial period of mammalian embryonic development is primarily devoted to cell commitment to the pluripotent lineage, as well as to the formation of extraembryonic tissues essential for embryo survival in utero. This phase of development is also characterized by extensive morphological transitions. Cells within the preimplantation embryo exhibit extraordinary cell plasticity and adaptation in response to experimental manipulation, highlighting the use of a regulative developmental strategy rather than a predetermined one resulting from the non-uniform distribution of maternal information in the cytoplasm. Consequently, early mammalian development represents a useful model to study how the three primary cell lineages; the epiblast, primitive endoderm (also referred to as the hypoblast) and trophoblast, emerge from a totipotent single cell, the zygote. In this review, we will discuss how the isolation and genetic manipulation of murine stem cells representing each of these three lineages has contributed to our understanding of the molecular basis of early developmental events.

The endogenous retrovirus ENS-1 provides active binding sites for transcription factors in embryonic stem cells that specify extra embryonic tissue

Background

Long terminal repeats (LTR) from endogenous retroviruses (ERV) are source of binding sites for transcription factors which affect the host regulatory networks in different cell types, including pluripotent cells. The embryonic epiblast is made of pluripotent cells that are subjected to opposite transcriptional regulatory networks to give rise to distinct embryonic and extraembryonic lineages. To assess the transcriptional contribution of ERV to early developmental processes, we have characterized in vitro and in vivo the regulation of ENS-1, a host adopted and developmentally regulated ERV that is expressed in chick embryonic stem cells.

Results

We show that Ens-1 LTR activity is controlled by two transcriptional pathways that drive pluripotent cells to alternative developmental fates. Indeed, both Nanog that maintains pluripotency and Gata4 that induces differentiation toward extraembryonic endoderm independently activate the LTR. Ets coactivators are required to support Gata factors' activity thus preventing inappropriate activation before epigenetic silencing occurs during differentiation. Consistent with their expression patterns during chick embryonic development, Gata4, Nanog and Ets1 are recruited on the LTR in embryonic stem cells; in the epiblast the complementary expression of Nanog and Gata/Ets correlates with the Ens-1 gene expression pattern; and Ens-1 transcripts are also detected in the hypoblast, an extraembryonic tissue expressing Gata4 and Ets2, but not Nanog. Accordingly, over expression of Gata4 in embryos induces an ectopic expression of Ens-1.

Conclusion

Our results show that Ens-1 LTR have co-opted conditions required for the emergence of extraembryonic tissues from pluripotent epiblasts cells. By providing pluripotent cells with intact binding sites for Gata, Nanog, or both, Ens-1 LTR may promote distinct transcriptional networks in embryonic stem cells subpopulations and prime the separation between embryonic and extraembryonic fates.

Derivation and characterization of embryonic stem cells lines derived from transgenic Fischer 344 and Dark Agouti rats

Rat embryonic stem cell (ESC) lines are not widely available, and there are only 2 lines available for distribution. Here, ESC lines were derived and characterized from Fischer 344 (F344) rats that express marker transgenes either β-galactosidase or human placental alkaline phosphatase (AP), nontransgenic F344 rats, and from Dark Agouti (DA) rats. The ESC lines were maintained in an undifferentiated state as characterized by colony morphology, expression of Oct4, Nanog, Sox-2, Cdx2, and Stella, staining for AP, and stage-specific embryonic antigen-1. Pluripotency was demonstrated in vitro by differentiation to embryoid bodies, followed by embryonic monsters. The Cdx2 expression by ESCs was unexpected and was confirmed via reverse transcriptase-polymerase chain reaction, immunocytochemistry. Pluripotency of ESCs was demonstrated in vivo by production of teratoma after an injection into F344 nontransgenic rats, and by an injection of male DA ESCs into F344 or Sprague-Dawley rat blastocysts and the generation of chimeric rats and germline contribution. ESCs from both F344 and DA contributed to chimeric rats, and one DA ESC line was proved to be germline competent. ESC sublines were created by transfection with a plasmid expressing enhanced green fluorescent protein (eGFP) under the control of a beta actin promoter and cytomegalovirus enhancer (pCX-eGFP) or by transfection with a plasmid expressing GFP under the control of a 3.1 kb portion of the rat Oct4 promoter (pN1-Oct4-GFP). In pN1-Oct4-GFP sublines, GFP gene expression and fluorescence were shown to be correlated with endogenous Oct4 gene expression. Therefore, these new ESC lines may be useful for tissue engineering and transplantation studies or for optimizing culture conditions required for self-renewal and differentiation of rat ESCs. While they made chimeric rats, further work is needed to confirm whether the transgenic F344 rat ESCs described here are germline-competent ESCs.

Regulation of extra-embryonic endoderm stem cell differentiation by Nodal and Cripto signaling

The signaling pathway for Nodal, a ligand of the TGFβ superfamily, plays a central role in regulating the differentiation and/or maintenance of stem cell types that can be derived from the peri-implantation mouse embryo. Extra-embryonic endoderm stem (XEN) cells resemble the primitive endoderm of the blastocyst, which normally gives rise to the parietal and the visceral endoderm in vivo, but XEN cells do not contribute efficiently to the visceral endoderm in chimeric embryos. We have found that XEN cells treated with Nodal or Cripto (Tdgf1), an EGF-CFC co-receptor for Nodal, display upregulation of markers for visceral endoderm as well as anterior visceral endoderm (AVE), and can contribute to visceral endoderm and AVE in chimeric embryos. In culture, XEN cells do not express Cripto, but do express the related EGF-CFC co-receptor Cryptic (Cfc1), and require Cryptic for Nodal signaling. Notably, the response to Nodal is inhibited by the Alk4/Alk5/Alk7 inhibitor SB431542, but the response to Cripto is unaffected, suggesting that the activity of Cripto is at least partially independent of type I receptor kinase activity. Gene set enrichment analysis of genome-wide expression signatures generated from XEN cells under these treatment conditions confirmed the differing responses of Nodal- and Cripto-treated XEN cells to SB431542. Our findings define distinct pathways for Nodal and Cripto in the differentiation of visceral endoderm and AVE from XEN cells and provide new insights into the specification of these cell types in vivo.

Concise review: culture mediated changes in fate and/or potency of stem cells

Although Gurdon demonstrated already in 1958 that the nucleus of intestinal epithelial cells could be reprogrammed to give rise to adult frogs, the field of cellular reprogramming has only recently come of age with the description by Takahashi and Yamanaka in 2006, which defined transcription factors can reprogram fibroblasts to an embryonic stem cell-like fate. With the mounting interest in the use of human pluripotent stem cells and culture-expanded somatic stem/progenitor cells, such as mesenchymal stem cells, increasing attention has been given to the effect of changes in the in vitro microenvironment on the fate of stem cells. These studies have demonstrated that changes in culture conditions may change the potency of pluripotent stem cells or reprogram adult stem/progenitor cells to endow them with a broader differentiation potential. The mechanisms underlying these fate and potency changes by ex vivo culture should be further investigated and considered when designing clinical therapies with stem/progenitor cells.

Medaka cleavage embryos are capable of generating ES-like cell cultures

Mammalian embryos at the blastocyst stage have three major lineages, which in culture can give rise to embryonic stem (ES) cells from the inner cell mass or epiblast, trophoblast stem cells from the trophectoderm, and primitive endoderm stem cells. None of these stem cells is totipotent, because they show gene expression profiles characteristic of their sources and usually contribute only to the lineages of their origins in chimeric embryos. It is unknown whether embryos prior to the blastocyst stage can be cultivated towards totipotent stem cell cultures. Medaka is an excellent model for stem cell research. This laboratory fish has generated diploid and even haploid ES cells from the midblastula embryo with ~2000 cells. Here we report in medaka that dispersed cells from earlier embryos can survive, proliferate and attach in culture. We show that even 32-cells embryos can be dissociated into individual cells capable of producing continuously growing ES-like cultures. Our data point to the possibility to derive stable cell culture from cleavage embryos in this organism.

Fish stem cell cultures

Stem cells have the potential for self-renewal and differentiation. First stem cell cultures were derived 30 years ago from early developing mouse embryos. These are pluripotent embryonic stem (ES) cells. Efforts towards ES cell derivation have been attempted in other mammalian and non-mammalian species. Work with stem cell culture in fish started 20 years ago. Laboratory fish species, in particular zebrafish and medaka, have been the focus of research towards stem cell cultures. Medaka is the second organism that generated ES cells and the first that gave rise to a spermatogonial stem cell line capable of test-tube sperm production. Most recently, the first haploid stem cells capable of producing whole animals have also been generated from medaka. ES-like cells have been reported also in zebrafish and several marine species. Attempts for germline transmission of ES cell cultures and gene targeting have been reported in zebrafish. Recent years have witnessed the progress in markers and procedures for ES cell characterization. These include the identification of fish homologs/paralogs of mammalian pluripotency genes and parameters for optimal chimera formation. In addition, fish germ cell cultures and transplantation have attracted considerable interest for germline transmission and surrogate production. Haploid ES cell nuclear transfer has proven in medaka the feasibility of semi-cloning as a novel assisted reproductive technology. In this special issue on "Fish Stem Cells and Nuclear Transfer", we will focus our review on medaka to illustrate the current status and perspective of fish stem cells in research and application. We will also mention semi-cloning as a new development to conventional nuclear transfer.

Sequence-specific regulator Prdm14 safeguards mouse ESCs from entering extraembryonic endoderm fates

Prdm14 is a PR-domain and zinc-finger protein whose expression is restricted to the pluripotent cells of the early embryo, embryonic stem cells (ESCs), and germ cells. Here, we show that Prdm14 safeguards mouse ESC (mESC) maintenance by preventing induction of extraembryonic endoderm (ExEn) fates. Conversely, Prdm14 overexpression impairs ExEn differentiation during embryoid body formation. Prdm14 occupies and represses genomic loci encoding ExEn differentiation factors, while also binding to and promoting expression of genes associated with mESC self-renewal. Prdm14-associated genomic regions substantially overlap those occupied by Nanog and Oct4, are enriched in a chromatin signature associated with distal regulatory elements and contain a unique DNA-sequence motif recognized by Prdm14 in vitro. Our work identifies a new member of the mESC transcriptional network, Prdm14, which plays a dual role as a context-dependent transcriptional repressor or activator.

Human embryonic and rat adult stem cells with primitive endoderm-like phenotype can be fated to definitive endoderm, and finally hepatocyte-like cells

Stem cell-derived hepatocytes may be an alternative cell source to treat liver diseases or to be used for pharmacological purposes. We developed a protocol that mimics mammalian liver development, to differentiate cells with pluripotent characteristics to hepatocyte-like cells. The protocol supports the stepwise differentiation of human embryonic stem cells (ESC) to cells with characteristics of primitive streak (PS)/mesendoderm (ME)/definitive endoderm (DE), hepatoblasts, and finally cells with phenotypic and functional characteristics of hepatocytes. Remarkably, the same protocol can also differentiate rat multipotent adult progenitor cells (rMAPCs) to hepatocyte-like cells, even though rMAPC are isolated clonally from cultured rat bone marrow (BM) and have characteristics of primitive endoderm cells. A fraction of rMAPCs can be fated to cells expressing genes consistent with a PS/ME/DE phenotype, preceding the acquisition of phenotypic and functional characteristics of hepatocytes. Although the hepatocyte-like progeny derived from both cell types is mixed, between 10-20% of cells are developmentally consistent with late fetal hepatocytes that have attained synthetic, storage and detoxifying functions near those of adult hepatocytes. This differentiation protocol will be useful for generating hepatocyte-like cells from rodent and human stem cells, and to gain insight into the early stages of liver development.

Characterization of trophoblast and extraembryonic endoderm cell lineages derived from rat preimplantation embryos

BACKGROUND

Previous attempts to isolate pluripotent cell lines from rat preimplantation embryo in mouse embryonic stem (ES) cell culture conditions (serum and LIF) were unsuccessful, however the resulting cells exhibited the expression of such traditional pluripotency markers as SSEA-1 and alkaline phosphatase. We addressed the question, which kind of cell lineages are produced from rat preimplantation embryo under "classical" mouse ES conditions.

RESULTS

We characterized two cell lines (C5 and B10) which were obtained from rat blastocysts in medium with serum and LIF. In the B10 cell line we found the expression of genes known to be expressed in trophoblast, Cdx-2, cytokeratin-7, and Hand-1. Also, B10 cells invaded the trophectodermal layer upon injection into rat blastocysts. In contrast to mouse Trophoblast Stem (TS) cells proliferation of B10 cells occurred independently of FGF4. Cells of the C5 line expressed traditional markers of extraembryonic-endoderm (XEN) cells, in particular, GATA-4, but also the pluripotency markers SSEA-1 and Oct-4. C5 cell proliferation exhibited dependence on LIF, which is not known to be required by mouse XEN cells.

CONCLUSIONS

Our results confirm and extend previous findings about differences between blastocyst-derived cell lines of rat and mice. Our data show, that the B10 cell line represents a population of FGF4-independent rat TS-like cells. C5 cells show features that have recently become known as characteristic of rat XEN cells. Early passages of C5 and B10 cells contained both, TS and XEN cells. We speculate, that mechanisms maintaining self-renewal of cell lineages in rat preimplantation embryo and their in vitro counterparts, including ES, TS and XEN cells are different than in respective mouse lineages.