OCT4 is expressed in extraembryonic endoderm stem (XEN) cell progenitors during somatic cell reprogramming

During development, progenitors of embryonic stem (ES) and extraembryonic endoderm stem (XEN) cells are concomitantly specified within the inner cell mass (ICM) of the mouse blastocyst. Similarly, XEN cells are induced (iXEN cells) alongside induced pluripotent stem (iPS) cells following overexpression of Oct4, Sox2, Klf4 and Myc (OSKM) during somatic cell reprogramming. It is unclear how or why this cocktail produces both stem cell types, but OCT4 has been associated with non-pluripotent outcomes. In this report, we show that, during OSKM reprogramming, many individual Oct4-GFP-expressing cells are fated to become iXEN cells. Interestingly, SKM alone was also sufficient to induce iXEN cell formation, likely via activation of endogenous Oct4. These observations indicate that iXEN cell formation is not strictly an artifact of Oct4 overexpression. Moreover, our results suggest that a pathway to XEN may be an integral feature of establishing pluripotency during reprogramming, as in early embryo development.

Defined Conditions Control the Morphological Dualism of Rat Primitive Extraembryonic Endoderm Stem Cells

Rat primitive extraembryonic endoderm (pXEN) stem cell lines indefinitely preserve the characteristic features of the early extraembryonic endoderm (ExEn) in vitro, but require unknown serum factors and exhibit a hybrid (mesenchymal-epithelial) phenotype. We report two chemically defined conditions that differ by the addition of the cytokine leukemia inhibitory factor (Lif) and the β-catenin-stabilizing drug Chir99021, and enable permanent self-renewal as mesenchymal and epithelial morphotypes, respectively. The morphotypes are interconvertible and equipotent, as shown by the formation of well-differentiated organoids. Surprisingly, the proliferation of both morphotypes requires Lif-type Gp130/Stat3 signaling (autocrine in the absence of added Lif) and noncanonical Wnt signaling (autocrine). In addition, the epithelial version requires β-catenin for proliferation and morphology. Interestingly, the mesenchymal cells also express key epithelial markers, but those are improperly structured and/or not functional, indicating a primed state. These results provide an improved platform for studying the proliferation and plasticity of the early ExEn, which occurs in mesenchymal and epithelial forms in vivo.

Fluorescent Reporters Distinguish Stem Cell Colony Subtypes During Somatic Cell Reprogramming

Somatic cell reprogramming was first developed to create induced pluripotent stem (iPS) cells. Since that time, the highly dynamic and heterogeneous nature of the reprogramming process has come to be appreciated. Remarkably, a distinct type of stem cell, called induced extraembryonic endoderm (iXEN) stem cell, is also formed during reprogramming of mouse somatic cells by ectopic expression of the transcription factors, OCT4, SOX2, KLF4, and MYC (OSKM). The mechanisms leading somatic cells to adopt differing stem cell fates are challenging to resolve given that formation of either stem cell type is slow, stochastic, and rare. For these reasons, fluorescent gene expression reporters have provided an invaluable tool for revealing the path from the somatic state to pluripotency. However, no such reporters have been established for comparable studies of iXEN cell formation. In this study, we examined the expression of multiple fluorescent reporters, including Nanog, Oct4, and the endodermal genes, Gata4 and Gata6-alone and in combination, during reprogramming. We show that only simultaneous evaluation of Nanog and Gata4 reliably distinguishes iPS and iXEN cell colonies during reprogramming.

Cooperation between HDAC3 and DAX1 mediates lineage restriction of embryonic stem cells

Mouse embryonic stem cells (mESCs) are biased toward producing embryonic rather than extraembryonic endoderm fates. Here, we identify the mechanism of this barrier and report that the histone deacetylase Hdac3 and the transcriptional corepressor Dax1 cooperatively limit the lineage repertoire of mESCs by silencing an enhancer of the extraembryonic endoderm-specifying transcription factor Gata6. This restriction is opposed by the pluripotency transcription factors Nr5a2 and Esrrb, which promote cell type conversion. Perturbation of the barrier extends mESC potency and allows formation of 3D spheroids that mimic the spatial segregation of embryonic epiblast and extraembryonic endoderm in early embryos. Overall, this study shows that transcriptional repressors stabilize pluripotency by biasing the equilibrium between embryonic and extraembryonic lineages that is hardwired into the mESC transcriptional network.

Defined Stem Cell Culture Conditions to Model Mouse Blastocyst Development

The complex program of mouse development entails specification of the embryonic epiblast (Epi) as well as the extra-embryonic trophectoderm (TE) and primitive endoderm (PrE). These three lineages of mouse blastocyst can be modeled in vitro using stem cells derived from primary tissues. In these cultures, cells self-renew while retaining their developmental potential if put back into a developing embryo. Indeed, embryonic stem cells (ESC), when injected into a blastocyst, readily contribute to all embryonic lineages. Similarly, trophoblast stem cells (TSCs) will give rise to all TE-derived trophoblast lineages, and extraembryonic endoderm cells (XEN) will contribute to the PrE-derived yolk sack. These model systems are a powerful tool to study early development, lineage specification, and placenta formation. Only recently reproducible and chemically defined culture systems of these cells have been described. This overview discusses such novel methods for culturing ESC/TSC/XEN, as well as their molecular signatures and developmental potential. Recent strides in expanding the developmental potential of stem cells as well as achieving models more reminiscent of their in vivo counterparts are discussed. Finally, such in vitro stem cells can self-assemble into structures resembling embryos when used in novel 3D-culture systems. This article discusses the strengths and limitations of such "synthetic embryos" in studying developmental processes. © 2020 by John Wiley & Sons, Inc.

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.

Frizzled gene expression and negative regulation of canonical WNT-β-catenin signaling in mouse F9 teratocarcinoma cells

Mouse F9 cells differentiate into primitive endoderm (PrE) following the activation of the canonical WNT-β-catenin pathway. The upregulation of Wnt6 and activation of β-catenin-TCF-LEF-dependent transcription is known to accompany differentiation, but the Frizzled (FZD) receptor responsible for transducing the WNT6 signal is not known. Eight of the 10 Fzd genes were found to be expressed in F9 cells, with Fzd7 being the most highly expressed, and chosen for further analysis. To alter steady-state Fzd7 levels and test the effect this has on differentiation, siRNA and overexpression approaches were used to knock-down and ectopically express the Fzd7 message, respectively. siRNA knock-down of Fzd7 resulted in reduced DAB2 levels, and the overexpression activated a TCF-LEF reporter, but neither approach affected differentiation. Our focus turned to how canonical WNT6 signaling was attenuated to allow PrE cells to form parietal endoderm (PE). Dkk1, encoding a WNT antagonist, was examined and results showed that its expression increased in F9 cells treated with retinoic acid (RA) or overexpressing Wnt6. F9 cells overexpressing human DKK1 or treated with DKK1-conditioned medium and then treated with RA failed to differentiate, indicating that a negative feedback loop involving WNT6 and DKK1 attenuates canonical WNT-β-catenin signaling, thereby allowing PE cells to differentiate.

Gata6 potently initiates reprograming of pluripotent and differentiated cells to extraembryonic endoderm stem cells

Wamaitha et al. demonstrate that the transcription factor Gata6 can initiate reprograming of multiple cell types to induced extraembryonic endoderm cells. Profiling transcriptional changes following Gata6 induction in mES cells reveals step-wise pluripotency factor disengagement, with initial repression of Nanog and Esrrb, then Sox2, and finally Oct4, alongside step-wise activation of extraembryonic endoderm genes.

Transcription factor-mediated reprograming is a powerful method to study cell fate changes. In this study, we demonstrate that the transcription factor Gata6 can initiate reprograming of multiple cell types to induced extraembryonic endoderm stem (iXEN) cells. Intriguingly, Gata6 is sufficient to drive iXEN cells from mouse pluripotent cells and differentiated neural cells. Furthermore, GATA6 induction in human embryonic stem (hES) cells also down-regulates pluripotency gene expression and up-regulates extraembryonic endoderm (ExEn) genes, revealing a conserved function in mediating this cell fate switch. Profiling transcriptional changes following Gata6 induction in mES cells reveals step-wise pluripotency factor disengagement, with initial repression of Nanog and Esrrb, then Sox2, and finally Oct4, alongside step-wise activation of ExEn genes. Chromatin immunoprecipitation and subsequent high-throughput sequencing analysis shows Gata6 enrichment near pluripotency and endoderm genes, suggesting that Gata6 functions as both a direct repressor and activator. Together, this demonstrates that Gata6 is a versatile and potent reprograming factor that can act alone to drive a cell fate switch from diverse cell types.

Differentiation of embryonic stem cells is induced by GATA factors

Extraembryonic endoderm (ExE) is differentiated from the inner cell mass of the late blastocyst-stage embryo to form visceral and parietal endoderm, both of which have an important role in early embryogenesis. The essential roles of Gata-6 and Gata-4 on differentiation of visceral endoderm have been identified by analyses of knockout mice. Here we report that forced expression of either Gata-6 or Gata-4 in embryonic stem (ES) cells is sufficient to induce the proper differentiation program towards ExE. We believe that this is the first report of a physiological differentiation event induced by the ectopic expression of a transcription factor in ES cells.