Generation of genome-edited mouse epiblast stem cells via a detour through ES cell-chimeras

L-Proline Supplementation Drives Self-Renewing Mouse Embryonic Stem Cells to a Partially Primed Pluripotent State: The Early Primitive Ectoderm-Like Cell

Mouse embryonic stem cells (mESCs) can be grown under a variety of culture conditions as discrete cell states along the pluripotency continuum, ranging from the least mature "ground state" to being "primed" to differentiate. Cells along this continuum are demarcated by differences in gene expression, X chromosome inactivation, ability to form chimeras and epigenetic marks. We have developed a protocol to differentiate "naïve" mESCs to a "partially primed" state by adding the amino acid L-proline to self-renewal medium. These cells express the primitive ectoderm markers Dnmt3b and Fgf5, and are thus called early primitive ectoderm-like (EPL) cells. In addition to changes in gene expression, these cells undergo a morphological change to flattened, dispersed colonies, have an increased proliferation rate, and a predisposition to neural fate. EPL cells can be used to study the cell states along the pluripotency continuum, peri-implantation embryogenesis, and as a starting point for efficient neuronal differentiation.

Differential impact of TGFβ/SMAD signaling activity elicited by Activin A and Nodal on endoderm differentiation of epiblast stem cells

Allocation of cells to an endodermal fate in the gastrulating embryo is driven by Nodal signaling and consequent activation of TGFβ pathway. In vitro methodologies striving to recapitulate the process of endoderm differentiation, however, use TGFβ family member Activin in place of Nodal. This is despite Activin not known to have an in vivo role in endoderm differentiation. In this study, five epiblast stem cell lines were subjected to directed differentiation using both Activin A and Nodal to induce endodermal fate. A reporter line harboring endoderm markers FoxA2 and Sox17 was further analyzed for TGFβ pathway activation and WNT response. We demonstrated that Activin A-treated cells remain more primitive streak-like when compared to Nodal-treated cells that have a molecular profile suggestive of more advanced differentiation. Activin A elicited a robust TGFβ/SMAD activity, enhanced WNT signaling activity and promoted the generation of DE precursors. Nodal treatment resulted in lower TGFβ/SMAD activity, and a weaker, sustained WNT response, and ultimately failed to upregulate endoderm markers. This is despite signaling response resembling more closely the activity seen in vivo. These findings emphasize the importance of understanding the downstream activities of Activin A and Nodal signaling in directing in vitro endoderm differentiation of primed-state epiblast stem cells.

Small Interfering RNA (siRNA) Transfection in Epiblast Stem Cells

Lipid-based transfection of siRNA is a technique routinely used to investigate gene function in experiments using mammalian cells cultured in vitro. Due to innate differences in cellular characteristics, the efficiency of lipid-based transfection is variable across cell types. Pluripotent cells which exist in a "primed" state such as human embryonic stem cells (hESCs) and mouse epiblast stem cells (mEpiSCs) are notorious for being refractory to lipid-based transfection systems. Herein we describe a forward transfection protocol which we routinely use to achieve upwards of 70% transfection efficiency rates in mEpiSCs. Our protocol also includes a suggested transfection timeline and details pertaining to the techniques we use to validate transfection success.

Establishment of Mouse Epiblast Stem Cells

Epiblast stem cells are made from the epiblast of mouse post-implantation embryo. They have been critical in the understanding of mammalian pluripotent stem cells as they share similar properties, such as their incapability to contribute to the formation of an embryo after injection into blastocyst. The epiblast stem cells (EpiSC) have delineated a novel status of pluripotency called "primed." How to establish EpiSC from mouse embryo is described in detail in this chapter.

Embryo-derived and induced pluripotent stem cells: Towards naive pluripotency and chimeric competency in rabbits

Both embryo-derived (ESC) and induced pluripotent stem cell (iPSC) lines have been established in rabbit. They exhibit the essential characteristics of primed pluripotency. In this review, we described their characteristic features at both molecular and functional levels. We also described the attempts to reprogram rabbit pluripotent stem cells (rbPSCs) toward the naive state of pluripotency using methods established previously to capture this state in rodents and primates. In the last section, we described and discussed our current knowledge of rabbit embryo development pertaining to the mechanisms of early lineage segregation. We argued that the molecular signature of naive-state pluripotency differs between mice and rabbits. We finally discussed some of the key issues to be addressed for capturing the naive state in rbPSCs, including the generation of embryo/PSC chimeras.

Regulation of Cyclin E by transcription factors of the naïve pluripotency network in mouse embryonic stem cells

Continuous, non-cell cycle-dependent expression of cyclin E is a characteristic feature of mouse embryonic stem cells (mESCs). We studied the 5′ regulatory region of Cyclin E, also known as Ccne1, and identified binding sites for transcription factors of the naïve pluripotency network, including Esrrb, Klf4, and Tfcp2l1 within 1 kilobase upstream of the transcription start site. Luciferase assay and chromatin immunoprecipitation-quantitative polymerase chain reaction (ChiP–qPCR) study highlighted one binding site for Esrrb that is essential to transcriptional activity of the promoter region, and three binding sites for Klf4 and Tfcp2l1. Knockdown of Esrrb, Klf4, and Tfcp2l1 reduced Cyclin E expression whereas overexpression of Esrrb and Klf4 increased it, indicating a strong correlation between the expression level of these factors and that of cyclin E. We observed that cyclin E overexpression delays differentiation induced by Esrrb depletion, suggesting that cyclin E is an important target of Esrrb for differentiation blockade. We observed that mESCs express a low level of miR-15a and that transfection of a miR-15a mimic decreases Cyclin E mRNA level. These results lead to the conclusion that the high expression level of Cyclin E in mESCs can be attributed to transcriptional activation by Esrrb as well as to the absence of its negative regulator, miR-15a.

Dynamics of Wnt activity on the acquisition of ectoderm potency in epiblast stem cells

During embryogenesis, the stringent regulation of Wnt activity is crucial for the morphogenesis of the head and brain. The loss of function of the Wnt inhibitor Dkk1 results in elevated Wnt activity, loss of ectoderm lineage attributes from the anterior epiblast, and the posteriorisation of anterior germ layer tissue towards the mesendoderm. The modulation of Wnt signalling may therefore be crucial for the allocation of epiblast cells to ectoderm progenitors during gastrulation. To test this hypothesis, we examined the lineage characteristics of epiblast stem cells (EpiSCs) that were derived and maintained under different signalling conditions. We showed that suppression of Wnt activity enhanced the ectoderm propensity of the EpiSCs. Neuroectoderm differentiation of these EpiSCs was further empowered by the robust re-activation of Wnt activity. Therefore, during gastrulation, the tuning of the signalling activities that mediate mesendoderm differentiation is instrumental for the acquisition of ectoderm potency in the epiblast.

Gene Editing of Mouse Embryonic and Epiblast Stem Cells

Efficient and reliable methods for gene editing are critical for the generation of loss-of-gene function stem cells and genetically modified mice. Here, we outline the application of CRISPR-Cas9 technology for gene editing in mouse embryonic stem cells (mESCs) to generate knockout ESC chimeras for the fast-tracked analysis of gene function. Furthermore, we describe the application of gene editing directly to mouse epiblast stem cells (mEpiSCs) for modelling germ layer differentiation in vitro.

[Fifty shades of pluripotency]

Since the derivation of the first pluripotent embryonic stem cell lines in mice in the early 1980s, a plethora of lines has been obtained from various mammalian species including rodents, lagomorphs and primates. These lines are distinguished by their molecular and functional characteristics and correspond to the different pluripotency states observed in the developing embryo between the "blastocyst" and "gastrula" stages. These cell lines are positioned along a gradient, or continuum of pluripotency, the ends of which are epitomized by the naïve and primed states, respectively. Conventional human pluripotent stem cells self-renew in the primed state of pluripotency (ie, at the bottom of the gradient), a position that is undoubtedly the cause of their natural instability. Recent studies aim to generate naive human pluripotent stem cells (at the top of the gradient). The importance of this research in the perspective of medical applications will be discussed.