A simple and robust method for establishing homogeneous mouse epiblast stem cell lines by wnt inhibition

Wnt Inhibition Safeguards Porcine Embryonic Stem Cells From the Acquisition of Extraembryonic Endoderm Cell Fates

Porcine embryonic stem cells (ESCs) are excellent models for exploring embryogenesis, producing genetically enhanced farm animals, and improving breeding. Various chemicals have been applied to generate porcine ESCs from embryos, which differ from mouse and human ESC derivation. Wnt inhibitors XAV939 or IWR1 are required to isolate and maintain porcine ESCs. How Wnt inhibitors specify porcine ESC fate decisions remains poorly understood. Additionally, whether porcine ESCs can be converted to extraembryonic endoderm (XEN) cells without genetic interventions has not been reported. Here, it is reported that Wnt inhibitors (i.e., XAV939 and IWR1) safeguard porcine ESCs from acquiring the XEN lineage. Porcine ESCs rely on Wnt inhibitors to maintain pluripotency. Without them, porcine ESCs exit from pluripotency and convert to XEN cells. An efficient strategy and culture conditions are further developed to directly derive porcine XEN cells from ESCs without gene editing. The resulting XEN cells from ESCs exhibit similar transcriptome and chromatin accessibility features to XEN cells from embryos and contribute to mouse extraembryonic tissues. This study will deepen the understanding of porcine pluripotency, lay the foundation for deriving high-quality porcine ESCs with germline chimerism and transmission, and provide valuable materials to study extraembryonic development and lineage segregation in livestock.

Epigenetic control of cell identities from epiblast to gastrulation

Epigenetic modifications of chromatin are essential for the establishment of cell identities during embryogenesis. Between embryonic days 3.5-7.5 of murine development, major cell lineage decisions are made that discriminate extraembryonic and embryonic tissues, and the embryonic primary germ layers are formed, thereby laying down the basic body plan. In this review, we cover the contribution of dynamic chromatin modifications by DNA methylation, changes of chromatin accessibility, and histone modifications, that in combination with transcription factors control gene expression programs of different cell types. We highlight the differences in regulation of enhancer and promoter marks and discuss their requirement in cell lineage specification. Importantly, in many cases, lineage-specific targeting of epigenetic modifiers is carried out by pioneer or master transcription factors, that in sum mediate the chromatin landscape and thereby control the transcription of cell-type-specific gene programs and thus, cell identities.

Metabolic activities are selective modulators for individual segmentation clock processes

Numerous cellular and molecular processes during embryonic development prompt the fundamental question of how their tempos are coordinated and whether a common global modulator exists. While the segmentation clock tempo scales with the kinetics of gene expression and degradation processes of the core clock gene Hes7 across mammals, the coordination of these processes remains unclear. This study examines whether metabolic activities serve as a global modulator for the segmentation clock, finding them to be selective instead. Several metabolic inhibitions extend the clock period but affect key processes differently: glycolysis inhibition slows Hes7 protein degradation and production delay without altering intron delay, while electron transport chain inhibition extends intron delay without influencing the other processes. Combinations of distinct metabolic inhibitions exhibit synergistic effects. We propose that the scaled kinetics of segmentation clock processes across species may result from combined selective modulators shaped by evolutionary constraints, rather than a single global modulator.

Relationship of PSC to embryos: Extending and refining capture of PSC lines from mammalian embryos

Pluripotent stem cell lines derived from preimplantation mouse embryos have opened opportunities for the study of early mammalian development and generation of genetically uncompromised material for differentiation into specific cell types. Murine embryonic stem cells are highly versatile and can be engineered and introduced into host embryos, transferred to recipient females, and gestated to investigate gene function at multiple levels as well as developmental mechanisms, including lineage segregation and cell competition. In this review, we summarize the biomedical motivation driving the incremental modification to culture regimes and analyses that have advanced stem cell research to its current state. Ongoing investigation into divergent mechanisms of early developmental processes adopted by other species, such as agriculturally beneficial mammals and birds, will continue to enrich knowledge and inform strategies for future in vitro models.

A PLURIPOTENT STEM CELL PLATFORM FOR IN VITRO SYSTEMS GENETICS STUDIES OF MOUSE DEVELOPMENT

The directed differentiation of pluripotent stem cells (PSCs) from panels of genetically diverse individuals is emerging as a powerful experimental system for characterizing the impact of natural genetic variation on developing cell types and tissues. Here, we establish new PSC lines and experimental approaches for modeling embryonic development in a genetically diverse, outbred mouse stock (Diversity Outbred mice). We show that a range of inbred and outbred PSC lines can be stably maintained in the primed pluripotent state (epiblast stem cells -- EpiSCs) and establish the contribution of genetic variation to phenotypic differences in gene regulation and directed differentiation. Using pooled in vitro fertilization, we generate and characterize a genetic reference panel of Diversity Outbred PSCs (n = 230). Finally, we demonstrate the feasibility of pooled culture of Diversity Outbred EpiSCs as "cell villages", which can facilitate the differentiation of large numbers of EpiSC lines for forward genetic screens. These data can complement and inform similar efforts within the stem cell biology and human genetics communities to model the impact of natural genetic variation on phenotypic variation and disease-risk.

Development of a chemically disclosed serum-free medium for mouse pluripotent stem cells

Mouse embryonic stem cells (mESCs) have been widely used as a model system to study the basic biology of pluripotency and to develop cell-based therapies. Traditionally, mESCs have been cultured in a medium supplemented with fetal bovine serum (FBS). However, serum with its inconsistent chemical composition has been problematic for reproducibility and for studying the role of specific components. While some serum-free media have been reported, these media contain commercial additives whose detailed components have not been disclosed. Recently, we developed a serum-free medium, DA-X medium, which can maintain a wide variety of adherent cancer lines. In this study, we modified the DA-X medium and established a novel serum-free condition for both naïve mESCs in which all components are chemically defined and disclosed (DA-X-modified medium for robust growth of pluripotent stem cells: DARP medium). The DARP medium fully supports the normal transcriptome and differentiation potential in teratoma and the establishment of mESCs from blastocysts that retain the developmental potential in all three germ layers, including germ cells in chimeric embryos. Utility of chemically defined DA-X medium for primed mouse epiblast stem cells (mEpiSCs) revealed that an optimal amount of cholesterol is required for the robust growth of naïve-state mESCs, but is dispensable for the maintenance of primed-state mEpiSCs. Thus, this study provides reliable and reproducible culture methods to investigate the role of specific components regulating self-renewal and pluripotency in a wide range of pluripotent states.

Rat epiblast-derived stem cells recapitulate the attributes of pre-gastrulation epiblast

Iwatsuki and colleagues have generated self-renewing pluripotent stem cells from the pre-gastrulation epiblast of the rat embryo and from other cellular sources: rat embryonic stem cells (rESCs) and epiblast-like cells derived from the rESCs. These rat epiblast-derived stem cells (rEpiSCs) display germ-line competence that is characteristic of mouse formative stem cells and early signature of specification of germ layer lineages typical of primed state mouse epiblast stem cells.

Rat post-implantation epiblast-derived pluripotent stem cells produce functional germ cells

In mammals, pluripotent cells transit through a continuum of distinct molecular and functional states en route to initiating lineage specification. Capturing pluripotent stem cells (PSCs) mirroring in vivo pluripotent states provides accessible in vitro models to study the pluripotency program and mechanisms underlying lineage restriction. Here, we develop optimal culture conditions to derive and propagate post-implantation epiblast-derived PSCs (EpiSCs) in rats, a valuable model for biomedical research. We show that rat EpiSCs (rEpiSCs) can be reset toward the naive pluripotent state with exogenous Klf4, albeit not with the other five candidate genes (Nanog, Klf2, Esrrb, Tfcp2l1, and Tbx3) effective in mice. Finally, we demonstrate that rat EpiSCs retain competency to produce authentic primordial germ cell-like cells that undergo functional gametogenesis leading to the birth of viable offspring. Our findings in the rat model uncover principles underpinning pluripotency and germline competency across species.

Replication dynamics identifies the folding principles of the inactive X chromosome

Chromosome-wide late replication is an enigmatic hallmark of the inactive X chromosome (Xi). How it is established and what it represents remains obscure. By single-cell DNA replication sequencing, here we show that the entire Xi is reorganized to replicate rapidly and uniformly in late S-phase during X-chromosome inactivation (XCI), reflecting its relatively uniform structure revealed by 4C-seq. Despite this uniformity, only a subset of the Xi became earlier replicating in SmcHD1-mutant cells. In the mutant, these domains protruded out of the Xi core, contacted each other and became transcriptionally reactivated. 4C-seq suggested that they constituted the outermost layer of the Xi even before XCI and were rich in escape genes. We propose that this default positioning forms the basis for their inherent heterochromatin instability in cells lacking the Xi-binding protein SmcHD1 or exhibiting XCI escape. These observations underscore the importance of 3D genome organization for heterochromatin stability and gene regulation.

A stem cell zoo uncovers intracellular scaling of developmental tempo across mammals

Differential speeds in biochemical reactions have been proposed to be responsible for the differences in developmental tempo between mice and humans. However, the underlying mechanism controlling the species-specific kinetics remains to be determined. Using in vitro differentiation of pluripotent stem cells, we recapitulated the segmentation clocks of diverse mammalian species varying in body weight and taxa: marmoset, rabbit, cattle, and rhinoceros. Together with mouse and human, the segmentation clock periods of the six species did not scale with the animal body weight, but with the embryogenesis length. The biochemical kinetics of the core clock gene HES7 displayed clear scaling with the species-specific segmentation clock period. However, the cellular metabolic rates did not show an evident correlation. Instead, genes involving biochemical reactions showed an expression pattern that scales with the segmentation clock period. Altogether, our stem cell zoo uncovered general scaling laws governing species-specific developmental tempo.

Reduced Cell-ECM Interactions in the EpiSC Colony Center Cause Heterogeneous Differentiation

Mechanoregulation of cell-extracellular matrix (ECM) interactions are crucial for dictating pluripotent stem cell differentiation. However, not all pluripotent cells respond homogeneously which results in heterogeneous cell populations. When cells, such as mouse epiblast stem cells (EpiSCs), are cultured in clusters, the heterogeneity effect during differentiation is even more pronounced. While past studies implicated variations in signaling pathways to be the root cause of heterogeneity, the biophysical aspects of differentiation have not been thoroughly considered. Here, we demonstrate that the heterogeneity of EpiSC differentiation arises from differences in the colony size and varying degrees of interactions between cells within the colonies and the ECM. Confocal imaging demonstrates that cells in the colony periphery established good contact with the surface while the cells in the colony center were separated by an average of 1-2 µm from the surface. Traction force measurements of the cells within the EpiSC colonies show that peripheral cells generate large tractions while the colony center cells do not. A finite element modeling of EpiSC colonies shows that tractions generated by the cells at the colony periphery lift off the colony center preventing the colony center from undergoing differentiation. Together, our results demonstrate a biophysical regulation of heterogeneous EpiSC colony differentiation.

Progesterone Receptor Modulates Extraembryonic Mesoderm and Cardiac Progenitor Specification during Mouse Gastrulation

Progesterone treatment is commonly employed to promote and support pregnancy. While maternal tissues are the main progesterone targets in humans and mice, its receptor (PGR) is expressed in the murine embryo, questioning its function during embryonic development. Progesterone has been previously associated with murine blastocyst development. Whether it contributes to lineage specification is largely unknown. Gastrulation initiates lineage specification and generation of the progenitors contributing to all organs. Cells passing through the primitive streak (PS) will give rise to the mesoderm and endoderm. Cells emerging posteriorly will form the extraembryonic mesodermal tissues supporting embryonic growth. Cells arising anteriorly will contribute to the embryonic heart in two sets of distinct progenitors, first (FHF) and second heart field (SHF). We found that PGR is expressed in a posterior–anterior gradient in the PS of gastrulating embryos. We established in vitro differentiation systems inducing posterior (extraembryonic) and anterior (cardiac) mesoderm to unravel PGR function. We discovered that PGR specifically modulates extraembryonic and cardiac mesoderm. Overexpression experiments revealed that PGR safeguards cardiac differentiation, blocking premature SHF progenitor specification and sustaining the FHF progenitor pool. This role of PGR in heart development indicates that progesterone administration should be closely monitored in potential early-pregnancy patients undergoing infertility treatment.

Stepwise pluripotency transitions in mouse stem cells

Pluripotent cells in mouse embryos, which first emerge in the inner cell mass of the blastocyst, undergo gradual transition marked by changes in gene expression, developmental potential, polarity, and morphology as they develop from the pre-implantation until post-implantation gastrula stage. Recent studies of cultured mouse pluripotent stem cells (PSCs) have clarified the presence of intermediate pluripotent stages between the naïve pluripotent state represented by embryonic stem cells (ESCs-equivalent to the pre-implantation epiblast) and the primed pluripotent state represented by epiblast stem cells (EpiSCs-equivalent to the late post-implantation gastrula epiblast). In this review, we discuss these recent findings in light of our knowledge on peri-implantation mouse development and consider the implications of these new PSCs to understand their temporal sequence and the feasibility of using them as model system for pluripotency.

SmcHD1 underlies the formation of H3K9me3 blocks on the inactive X chromosome in mice

Stable silencing of the inactive X chromosome (Xi) in female mammals is crucial for the development of embryos and their postnatal health. SmcHD1 is essential for stable silencing of the Xi, and its functional deficiency results in derepression of many X-inactivated genes. Although SmcHD1 has been suggested to play an important role in the formation of higher-order chromatin structure of the Xi, the underlying mechanism is largely unknown. Here, we explore the epigenetic state of the Xi in SmcHD1-deficient epiblast stem cells and mouse embryonic fibroblasts in comparison with their wild-type counterparts. The results suggest that SmcHD1 underlies the formation of H3K9me3-enriched blocks on the Xi, which, although the importance of H3K9me3 has been largely overlooked in mice, play a crucial role in the establishment of the stably silenced state. We propose that the H3K9me3 blocks formed on the Xi facilitate robust heterochromatin formation in combination with H3K27me3, and that the substantial loss of H3K9me3 caused by SmcHD1 deficiency leads to aberrant distribution of H3K27me3 on the Xi and derepression of X-inactivated genes.

Timely stimulation of early embryo promotes the acquisition of pluripotency

Mouse embryonic stem cells (ESCs) and epiblast stem cells (EpiSCs) are both pluripotent stem cells from early embryos. Another type of pluripotent stem cells, which are similar with EpiSCs and derive from pre-implantation embryos in feeder-free and chemically defined medium containing Activin A and basic fibroblast growth factors (bFGF), is termed as AFSCs. The pluripotency and self-renewal maintenance of ESCs rely on Leukemia inhibitory factor (LIF)/STAT/BMP4/SMAD signaling, while the pluripotency and self-renewal maintenance of EpiSCs and AFSCs rely on bFGF and Activin/Nodal signaling. However, the establishment efficiency of AFSCs lines is low. In this study, we stimulated early embryos by 2i/LIF (CHIR99021 + PD0325901 + LIF) and Activin A + bFGF respectively, to change the cell fate in inner cell mass (ICM). The "fate changed embryos" by 2i/LIF can efficiently produce AFSCs in feeder-free and chemically defined medium, but the efficiency of embryos treated with Activin A + bFGF were poor. The AFSCs from fate-changed embryos share similar molecular characteristics with conventional AFSCs and EpiSCs. Our results suggest that the advanced stimulation of 2i/LIF and the premature stimulation of Activin A + bFGF contribute to capturing the pluripotent stem cells in early embryos, and the FGF/MAPK signaling dominate early embryo development. Our study provides a new approach to capturing pluripotency from pre-implantation embryos.

β-catenin perturbations control differentiation programs in mouse embryonic stem cells

The Wnt/β-catenin pathway is involved in development, cancer, and embryonic stem cell (ESC) maintenance; its dual role in stem cell self-renewal and differentiation is still controversial. Here, by applying an in vitro system enabling inducible gene expression control, we report that moderate induction of transcriptionally active exogenous β-catenin in β-catenin null mouse ESCs promotes epiblast-like cell (EpiLC) derivation in vitro. Instead, in wild-type cells, moderate chemical pre-activation of the Wnt/β-catenin pathway promotes EpiLC in vitro derivation. Finally, we suggest that moderate β-catenin levels in β-catenin null mouse ESCs favor early stem cell commitment toward mesoderm if the exogenous protein is induced only in the “ground state” of pluripotency condition, or endoderm if the induction is maintained during the differentiation. Overall, our results confirm previous findings about the role of β-catenin in pluripotency and differentiation, while indicating a role for its doses in promoting specific differentiation programs.

•

Moderate β-catenin levels promote EpiLCs derivation in vitro

•

Chemical pre-activation of the Wnt pathway enhances ESC-EpiLC transition

•

β-catenin overexpression tips the balance between mesoderm and endoderm

•

Cell fate is influenced by the extent of β-catenin induction

Definitive Endoderm from EpiSC Aggregates in Matrigel

This chapter describes the protocol to derive definitive endoderm cells from epiblast stem cells (EpiSCs) via a process analogous to gastrulation in embryos. The basis of this protocol mimicking the in vivo gastrulation process makes a contrast with those using sequential administration of pharmacological molecules and recombinant signaling proteins even at nonphysiological levels. In the experimental setup, EpiSCs are first freed from the dish-adherent condition to form free-floating aggregates, where endoderm precursor pools are produced. Embedding the EpiSC aggregates in the Matrigel allows the endoderm precursors to interact with the Matrigel mimicking the laminin-rich basement membrane underlying the egg cylinder epiblast in embryos, and let the precursors migrate into the Matrigel-filled external zone and develop into endodermal epithelial tissues.

Differentiation of EpiLCs on Micropatterned Substrates Generated by Micro-Contact Printing

During the last decades, signaling pathways responsible for the initiation of gastrulation in mammalian embryos have been identified. However, the physical rules governing the tissue spatial patterning and the extensive morphogenetic movements occurring during that process are still elusive. Progress on these issues is slowed by the difficulty to record or perturb the patterning events in real time, especially in mammalian embryos that develop in utero. Because they permit easy observation and manipulation, in vitro model systems offer an exciting opportunity to dissect the rules governing the organization of the mammalian gastrula. For instance, it is sufficient to cultivate human embryonic stem cells on micropatterned substrates to reveal their self-organization potential. We present here a method to obtain micropatterned mouse Epiblast Like Cells colonies, providing a convenient way to compare spatial organization of mouse and human pluripotent stem cells and to complement the characterization of mutant embryos in a controlled environment.

Generation of Sheep Induced Pluripotent Stem Cells With Defined DOX-Inducible Transcription Factors via piggyBac Transposition

Pluripotent stem cells (PSCs) have the potential to differentiate to all cell types of an adult individual and are useful for studying mammalian development. Establishing induced pluripotent stem cells (iPSCs) capable of expressing pluripotent genes and differentiating to three germ layers will not only help to explain the mechanisms underlying somatic reprogramming but also lay the foundation for the establishment of sheep embryonic stem cells (ESCs) in vitro. In this study, sheep somatic cells were reprogrammed in vitro into sheep iPSCs with stable morphology, pluripotent marker expression, and differentiation ability, delivered by piggyBac transposon system with eight doxycycline (DOX)-inducible exogenous reprogramming factors: bovine OCT4, SOX2, KLF4, cMYC, porcine NANOG, human LIN28, SV40 large T antigen, and human TERT. Sheep iPSCs exhibited a chimeric contribution to the early blastocysts of sheep and mice and E6.5 mouse embryos in vitro. A transcriptome analysis revealed the pluripotent characteristics of somatic reprogramming and insights into sheep iPSCs. This study provides an ideal experimental material for further study of the construction of totipotent ESCs in sheep.

Stepwise conversion methods between ground states pluripotency from naïve to primed

Pluripotent stem cells (PSCs) are in vitro adaptations of in vivo pluripotency continuum and can be broadly classified into naïve state characteristic of pre-implantation epiblast and primed state resembling peri-gastrulation epiblasts. Naïve and primed PSCs differ in their cellular and molecular characteristics, e.g., molecular mechanisms for maintaining undifferentiated state. Naïve-to-primed PSC transition provides a tractable in vitro model to study pluripotency development in vivo. We previously developed a protocol that enabled high-efficient (100%) and homogenous derivation of ground state of primed epiblast stem cells (rsEpiSCs) by culturing the isolated post-implantation mouse epiblast under the culture condition containing FGF2 and a Wnt signaling inhibitor (IWR1) (F/R1 condition). Based on F/R1 condition, in this study, we developed three naïve-to-primed conversion methods for generating rsEpiSCs from naïve ground state of mouse ESCs (2i/LIF condition). We found that stepwise methods, but not directly, were effective for bona fide rsEpiSCs conversion from mouse ESCs. In sum, we established a robust and efficient ground states of naïve-to-primed PSC conversion strategy that will facilitate the study of genetic, epigenetic and metabolic processes involved in pluripotency progression in vivo.

AMPK activation reverts mouse epiblast stem cells to naive state

Despite increasing knowledge on primed and naive pluripotency, the cell signaling that regulates the pluripotency type in stem cells remains not fully understood. Here we show that AMP kinase (AMPK) activators can induce the reversion of primed mouse epiblast stem cells (mEpiSCs) to the naive pluripotent state. The addition of AMPK activators alone or together with leukemia inhibitory factor to primed mEpiSCs induced the appearance of naive-like cells. After passaging in naive culture conditions, the colony morphology, protein expression, and global gene expression profiles indicated the naive state, as did germline transmission ability. Loss-of-function and gain-of-function studies suggested that p38 is a critical downstream target in AMPK activation. Finally, single-cell RNA sequencing analysis revealed that the reversion process through AMPK signaling passes an intermediate naive-like population. In conclusion, the AMPK pathway is a critical driving force in the reversion of primed to naive pluripotency.

All models are wrong, but some are useful: Establishing standards for stem cell-based embryo models

Detailed studies of the embryo allow an increasingly mechanistic understanding of development, which has proved of profound relevance to human disease. The last decade has seen in vitro cultured stem cell-based models of embryo development flourish, which provide an alternative to the embryo for accessible experimentation. However, the usefulness of any stem cell-based embryo model will be determined by how accurately it reflects in vivo embryonic development, and/or the extent to which it facilitates new discoveries. Stringent benchmarking of embryo models is thus an important consideration for this growing field. Here we provide an overview of means to evaluate both the properties of stem cells, the building blocks of most embryo models, as well as the usefulness of current and future in vitro embryo models.

Importance of WNT-dependent signaling for derivation and maintenance of primed pluripotent bovine embryonic stem cells†

The WNT signaling system plays an important but paradoxical role in the regulation of pluripotency. In the cow, IWR-1, which inhibits canonical WNT activation and has WNT-independent actions, promotes the derivation of primed pluripotent embryonic stem cells from the blastocyst. Here, we describe a series of experiments to determine whether derivation of embryonic stem cells could be generated by replacing IWR-1 with other inhibitors of WNT signaling. Results confirm the importance of inhibition of canonical WNT signaling for the establishment of pluripotent embryonic stem cells in cattle and indicate that the actions of IWR-1 can be mimicked by the WNT secretion inhibitor IWP2 but not by the tankyrase inhibitor XAV939 or WNT inhibitory protein dickkopf 1. The role of Janus kinase-mediated signaling pathways for the maintenance of pluripotency of embryonic stem cells was also evaluated. Maintenance of pluripotency of embryonic stem cells lines was blocked by a broad inhibitor of Janus kinase, even though the cells did not express phosphorylated signal transducer and activator of transcription 3 (pSTAT3). Further studies with blastocysts indicated that IWR-1 blocks the activation of pSTAT3. A likely explanation is that IWR-1 blocks differentiation of embryonic stem cells into a pSTAT3+ lineage. In conclusion, results presented here indicate the importance of inhibition of WNT signaling for the derivation of pluripotent bovine embryonic stem cells, the role of Janus kinase signaling for maintenance of pluripotency, and the participation of IWR-1 in the inhibition of activation of STAT3.

Gastruloids generated without exogenous Wnt activation develop anterior neural tissues

When stimulated with a pulse from an exogenous WNT pathway activator, small aggregates of mouse embryonic stem cells (ESCs) can undergo embryo-like axial morphogenesis and patterning along the three major body axes. However, these structures, called gastruloids, currently lack the anterior embryonic regions, such as those belonging to the brain. Here, we describe an approach to generate gastruloids that have a more complete antero-posterior development. We used hydrogel microwell arrays to promote the robust derivation of mouse ESCs into post-implantation epiblast-like (EPI) aggregates in a reproducible and scalable manner. These EPI aggregates break symmetry and axially elongate without external chemical stimulation. Inhibition of WNT signaling in early stages of development leads to the formation of gastruloids with anterior neural tissues. Thus, we provide a new tool to study the development of the mouse after implantation in vitro, especially the formation of anterior neural regions.

•

Mouse embryonic stem cells can be aggregated to form epiblast-like (EPI) aggregates

•

EPI aggregates undergo axial morphogenesis in the absence of exogenous WNT activation

•

Initial aggregate size is a major determinant for axial morphogenesis

•

Early WNT inhibition is essential for the emergence of anterior neural progenitors

Distinct dynamics and functions of H2AK119ub1 and H3K27me3 in mouse preimplantation embryos

Polycomb repressive complexes 1 and 2 (PRC1/2) maintain transcriptional silencing of developmental genes largely by catalyzing the formation of mono-ubiquitinated histone H2A at lysine 119 (H2AK119ub1) and trimethylated histone H3 at lysine 27 (H3K27me3), respectively. How Polycomb domains are reprogrammed during mammalian preimplantation development remains largely unclear. Here we show that, although H2AK119ub1 and H3K27me3 are highly colocalized in gametes, they undergo differential reprogramming dynamics following fertilization. H3K27me3 maintains thousands of maternally biased domains until the blastocyst stage, whereas maternally biased H2AK119ub1 distribution in zygotes is largely equalized at the two-cell stage. Notably, while maternal PRC2 depletion has a limited effect on global H2AK119ub1 in early embryos, it disrupts allelic H2AK119ub1 at H3K27me3 imprinting loci including Xist. By contrast, acute H2AK119ub1 depletion in zygotes does not affect H3K27me3 imprinting maintenance, at least by the four-cell stage. Importantly, loss of H2AK119ub1, but not H3K27me3, causes premature activation of developmental genes during zygotic genome activation (ZGA) and subsequent embryonic arrest. Thus, our study reveals distinct dynamics and functions of H3K27me3 and H2AK119ub1 in mouse preimplantation embryos.

Human ES Cell Culture Conditions Fail to Preserve the Mouse Epiblast State

Mouse embryonic stem cells (mESCs) and mouse epiblast stem cells (mEpiSCs) are the pluripotent stem cells (PSCs), derived from the inner cell mass (ICM) of preimplantation embryos at embryonic day 3.5 (E3.5) and postimplantation embryos at E5.5-E7.5, respectively. Depending on their environment, PSCs can exist in the so-called naïve (ESCs) or primed (EpiSCs) states. Exposure to EpiSC or human ESC (hESC) culture condition can convert mESCs towards an EpiSC-like state. Here, we show that the undifferentiated epiblast state is however not stabilized in a sustained manner when exposing mESCs to hESC or EpiSC culture condition. Rather, prolonged exposure to EpiSC condition promotes a transition to a primitive streak- (PS-) like state via an unbiased epiblast-like intermediate. We show that the Brachyury-positive PS-like state is likely promoted by endogenous WNT signaling, highlighting a possible species difference between mouse epiblast-like stem cells and human Embryonic Stem Cells.

Dynamics of transcription-mediated conversion from euchromatin to facultative heterochromatin at the Xist promoter by Tsix

The fine-scale dynamics from euchromatin (EC) to facultative heterochromatin (fHC) has remained largely unclear. Here, we focus on Xist and its silencing initiator Tsix as a paradigm of transcription-mediated conversion from EC to fHC. In mouse epiblast stem cells, induction of Tsix recapitulates the conversion at the Xist promoter. Investigating the dynamics reveals that the conversion proceeds in a stepwise manner. Initially, a transient opened chromatin structure is observed. In the second step, gene silencing is initiated and dependent on Tsix, which is reversible and accompanied by simultaneous changes in multiple histone modifications. At the last step, maintenance of silencing becomes independent of Tsix and irreversible, which correlates with occupation of the -1 position of the transcription start site by a nucleosome and initiation of DNA methylation introduction. This study highlights the hierarchy of multiple chromatin events upon stepwise gene silencing establishment.

High-throughput screening in postimplantation haploid epiblast stem cells reveals Hs3st3b1 as a modulator for reprogramming

Epiblast stem cells (EpiSCs) derived from postimplantation epiblast are pluripotent stem cells, epigenetically distinct from embryonic stem cells (ESCs), which are widely used in reprogramming studies. Recent achieved haploid cell lines in mammalian species open a new era for high-throughput genetic screening, due to their homozygous phenotypes. Here, we report the generation of mouse haploid EpiSCs (haEpiSCs) from postimplantation chimeric embryos at embryonic day 6.5 (E6.5). These cells maintain one set of chromosomes, express EpiSC-specific genes, and have potentials to differentiate into three germ layers. We also develop a massive mutagenesis protocol with haEpiSCs, and subsequently perform reprogramming selection using this genome-wide mutation library. Multiple modules related to various pathways are implicated. The validation experiments prove that knockout of Hst3st3b1 (one of the candidates) can promote reprogramming of EpiSCs to the ground state efficiently. Our results open the feasibility of utilizing haEpiSCs to elucidate fundamental biological processes including cell fate alternations.

Derivation of stable embryonic stem cell-like, but transcriptionally heterogenous, induced pluripotent stem cells from non-permissive mouse strains

Genetic background is known to play a role in the ability to derive pluripotent, embryonic stem cells (ESC), a trait referred to as permissiveness. Previously we demonstrated that induced pluripotent stem cells (iPSC) can be readily derived from non-permissive mouse strains by addition of serum-based media supplemented with GSK3B and MEK inhibitors, termed 2iS media, 3 days into reprogramming. Here, we describe the derivation of second type of iPSC colony from non-permissive mouse strains that can be stably maintained independently of 2iS media. The resulting cells display transcriptional heterogeneity similar to that observed in ESC from permissive genetic backgrounds derived in conventional serum containing media supplemented with leukemia inhibitor factor. However, unlike previous studies that report exclusive subpopulations, we observe both exclusive and simultaneous expression of naive and primed cell surface markers. Herein, we explore shifts in pluripotency in the presence of 2iS and characterize heterogenous subpopulations to determine their pluripotent state and role in heterogenous iPSCs derived from the non-permissive NOD/ShiLtJ strain. We conclude that heterogeneity is a naturally occurring, necessary quality of stem cells that allows for the maintenance of pluripotency. This study further demonstrates the efficacy of the 2iS reprogramming technique. It is also the first study to derive stable ESC-like stem cells from the non-permissive NOD/ShiLtJ and WSB/EiJ strains, enabling easier and broader research possibilities into pluripotency for these and similar non-permissive mouse strains and species.

Wnt pathway modulation generates blastomere-derived mouse embryonic stem cells with different pluripotency features

PURPOSE

This study aimed to determine the role of Wnt pathway in mouse embryonic stem cell (mESC) derivation from single blastomeres isolated from eight-cell embryos and in the pluripotency features of the mESC established.

METHODS

Wnt activator CHIR99021, Wnt inhibitor IWR-1-endo, and MEK inhibitor PD0325901 were used alone or in combination during ESC derivation and maintenance from single blastomeres biopsied from eight-cell embryos. Alkaline phosphatase activity, FGF5 levels, expression of key pluripotency genes, and chimera formation were assessed to determine the pluripotency state of the mESC lines.

RESULTS

Derivation efficiencies were highest when combining pairs of inhibitors (15-24.7%) than when using single inhibitors or none (1.4-10.1%). Full naïve pluripotency was only achieved in CHIR- and 2i-treated mESC lines, whereas IWR and PD treatments or the absence of treatment resulted in co-existence of naïve-like and primed-like pluripotency features. IWR + CHIR- and IWR + PD-treated mESC displayed features of primed pluripotency, but IWR + CHIR-treated lines were able to generate germline-competent chimeric mice, resembling the predicted properties of formative pluripotency.

CONCLUSION

Wnt and MAPK pathways have a key role in the successful derivation and pluripotency features of mESC from single precompaction blastomeres. Modulation of these pathways results in mESC lines with various degrees of naïve-like and primed-like pluripotency features.

Naive Pluripotent Stem Cells Exhibit Phenotypic Variability that Is Driven by Genetic Variation

Variability among pluripotent stem cell (PSC) lines is a prevailing issue that hampers not only experimental reproducibility but also large-scale applications and personalized cell-based therapy. This variability could result from epigenetic and genetic factors that influence stem cell behavior. Naive culture conditions minimize epigenetic fluctuation, potentially overcoming differences in PSC line differentiation potential. Here we derived PSCs from distinct mouse strains under naive conditions and show that lines from distinct genetic backgrounds have divergent differentiation capacity, confirming a major role for genetics in PSC phenotypic variability. This is explained in part through inconsistent activity of extra-cellular signaling, including the Wnt pathway, which is modulated by specific genetic variants. Overall, this study shows that genetic background plays a dominant role in driving phenotypic variability of PSCs.

Livestock pluripotency is finally captured in vitro

Pluripotent stem cells (PSCs) have demonstrated great utility in improving our understanding of mammalian development and continue to revolutionise regenerative medicine. Thanks to the improved understanding of pluripotency in mice and humans, it has recently become feasible to generate stable livestock PSCs. Although it is unlikely that livestock PSCs will be used for similar applications as their murine and human counterparts, new exciting applications that could greatly advance animal agriculture are being developed, including the use of PSCs for complex genome editing, cellular agriculture, gamete generation and invitro breeding schemes.

Pleiotropic roles of tankyrase/PARP proteins in the establishment and maintenance of human naïve pluripotency

Tankyrase 1 (TNKS1; PARP-5a) and Tankyrase 2 (TNKS2; PARP-5b) are poly-ADP-ribosyl-polymerase (PARP)-domain-containing proteins that regulate the activities of a wide repertoire of target proteins via post-translational addition of poly-ADP-ribose polymers (PARylation). Although tankyrases were first identified as regulators of human telomere elongation, important and expansive roles of tankyrase activity have recently emerged in the development and maintenance of stem cell states. Herein, we summarize the current state of knowledge of the various tankyrase-mediated activities that may promote human naïve and 'extended' pluripotency'. We review the putative role of tankyrase and PARP inhibition in trophectoderm specification, telomere elongation, DNA repair and chromosomal segregation, metabolism, and PTEN-mediated apoptosis. Importantly, tankyrases possess PARP-independent activities that include regulation of MDC1-associated DNA repair by homologous recombination (HR) and autophagy/pexophagy, which is an essential mechanism of protein synthesis in the preimplantation embryo. Additionally, tankyrases auto-regulate themselves via auto-PARylation which augments their cellular protein levels and potentiates their non-PARP tankyrase functions. We propose that these non-PARP-related activities of tankyrase proteins may further independently affect both naïve and extended pluripotency via mechanisms that remain undetermined. We broadly outline a hypothetical framework for how inclusion of a tankyrase/PARP inhibitor in small molecule cocktails may stabilize and potentiate naïve and extended pluripotency via pleiotropic routes and mechanisms.

Induction and maintenance of specific multipotent progenitor stem cells synergistically mediated by Activin A and BMP4 signaling

We recently reported that epiblast stem cells (EpiSCs)-like cells could be derived from preimplantation embryos (named as AFSCs). Here, we established AFSCs from pre-implantation embryos of multiple mouse strains and showed that unlike EpiSCs, the derivation efficiency of AFSCs was affected by the genetic background. We then used AFSCs lines to dissect the roles of Activin A (Act A) and basic fibroblast growth factor and reported that Act A alone was capable of maintaining self-renewal but not developmental potential in vivo. Finally, we established a novel experimental system, in which AFSCs were efficiently converted to multipotent progenitor stem cells using Act A and bone morphogenetic protein 4 (named as ABSCs). Importantly, these ABSCs contributed to neural mesodermal progenitors and lateral plate mesoderm in postimplantation chimeras. Taken together, our study established a robust experimental system for the generation of specific multipotent progenitor stem cells that was self-renewable and capable of contributing to embryonic and extra-embryonic tissues.

Modeling early stages of endoderm development in epiblast stem cell aggregates with supply of extracellular matrices

Endoderm precursors expressing FoxA2 and Sox17 develop from the epiblast through the gastrulation process. In this study, we developed an experimental system to model the endoderm-generating gastrulation process using epiblast stem cells (EpiSCs). To this end, we established an EpiSC line i22, in which enhanced green fluorescent protein is coexpressed with Foxa2. Culturing i22 EpiSCs as aggregates for a few days was sufficient to initiate Foxa2 expression, and further culturing of the aggregates in Matrigel promoted the sequential activation of transcription factor genes involved in endoderm precursor development, e.g., Eomes, Gsc, and Sox17. In aggregation culture of i22 cells for 3 days, all cells expressed POU5F1, SOX2, and E-cadherin, a signature of the epiblast, whereas expression of GATA4 and SOX17 was also activated moderately in dispersed cells, suggesting priming of these cells to endodermal development. Embedding the aggregates in Matrigel for further 3 days elicited migration of the cells into the lumen of laminin-rich matrices covering the aggregates, in which FOXA2 and SOX17 were expressed at a high level with the concomitant loss of E-cadherin, indicating the migratory phase of endodermal precursors. Prolonged culturing of the aggregates generated three segregating cell populations found in post-gastrulation stage embryos: (1) definitive endoderm co-expressing high SOX17, GATA4, and E-cadherin, (2) mesodermal cells expressing a low level of GATA4 and lacking E-cadherin, and (3) primed epiblast cells expressing POU5F1, SOX2 without E-cadherin. Thus, aggregation of EpiSCs followed by embedding of aggregates in the laminin-rich matrix models the gastrulation-dependent endoderm precursor development.

Modulation of Wnt and Activin/Nodal supports efficient derivation, cloning and suspension expansion of human pluripotent stem cells

Various culture systems have been used to derive and maintain human pluripotent stem cells (hPSCs), but they are inefficient in sustaining cloning and suspension expansion of hPSCs. Through systematically modulating Wnt and Activin/Nodal signaling, we developed a defined medium (termed AIC), which enables efficient cloning and long-term expansion of hPSCs (AIC-hPSCs) through single-cell passage on feeders, matrix or in suspension (25-fold expansion in 4 days) and maintains genomic stability of hPSCs over extensive expansion. Moreover, the AIC medium supports efficient derivation of hPSCs from blastocysts or somatic cells under feeder-free conditions. Compared to conventional hPSCs, AIC-hPSCs have similar gene expression profiles but down-regulated differentiation genes and display higher metabolic activity. Additionally, the AIC medium shows a good compatibility for different hPSC lines under various culture conditions. Our study provides a robust culture system for derivation, cloning and suspension expansion of high-quality hPSCs that benefits GMP production and processing of therapeutic hPSC products.

Recapitulating the human segmentation clock with pluripotent stem cells

Pluripotent stem cells are increasingly used to model different aspects of embryogenesis and organ formation¹. Despite recent advances in in vitro induction of major mesodermal lineages and cell types^2,3, experimental model systems that can recapitulate more complex features of human mesoderm development and patterning are largely missing. Here we used induced pluripotent stem cells for the stepwise in vitro induction of presomitic mesoderm and its derivatives to model distinct aspects of human somitogenesis. We focused initially on modelling the human segmentation clock, a major biological concept believed to underlie the rhythmic and controlled emergence of somites, which give rise to the segmental pattern of the vertebrate axial skeleton. We observed oscillatory expression of core segmentation clock genes, including HES7 and DKK1, determined the period of the human segmentation clock to be around five hours, and demonstrated the presence of dynamic travelling-wave-like gene expression in in vitro-induced human presomitic mesoderm. Furthermore, we identified and compared oscillatory genes in human and mouse presomitic mesoderm derived from pluripotent stem cells, which revealed species-specific and shared molecular components and pathways associated with the putative mouse and human segmentation clocks. Using CRISPR-Cas9-based genome editing technology, we then targeted genes for which mutations in patients with segmentation defects of the vertebrae, such as spondylocostal dysostosis, have been reported (HES7, LFNG, DLL3 and MESP2). Subsequent analysis of patient-like and patient-derived induced pluripotent stem cells revealed gene-specific alterations in oscillation, synchronization or differentiation properties. Our findings provide insights into the human segmentation clock as well as diseases associated with human axial skeletogenesis.

Heterogeneity in Epiblast Stem Cells

Epiblast stem cells (EpiSCs) are pluripotent cells that are derived from mouse embryos at gastrulation stages. They represent the primed state of pluripotency, in which cells are on the verge of differentiation and already express markers of the three primary lineages (mesoderm, endoderm, neurectoderm). EpiSCs display some heterogeneity intra- and inter-cell lines in the expression of some of these lineage markers. We relate this heterogeneity to signalling pathways that are active in EpiSCs, either due to addition of growth factors (FGF2 and activin) in the culture medium, or endogenously active (FGF, Nodal, and Wnt). By modulating Wnt or activin/nodal pathways, cell lines close to EpiSCs but with different properties can be obtained. These signalling pathways are all at work in vivo to pattern the pluripotent epiblast and specify cellular fates.

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.

WNT Inhibition and Increased FGF Signaling Promotes Derivation of Less Heterogeneous Primed Human Embryonic Stem Cells, Compatible with Differentiation

Human embryonic stem cells (hESCs) hold great value for future clinical applications. However, standard culture conditions maintain hESCs in a primed state, which bears heterogeneity in pluripotency and a tendency for spontaneous differentiation. To counter these drawbacks, primed hESCs have been converted to a naive state, but this has restricted the efficiency of existing directed differentiation protocols. In mouse, WNT inhibition by inhibitor of WNT production-2, together with a higher dose of fibroblast growth factor 2 (12 ng/mL) in DMEM/F12 basal medium (DhiFI), markedly improved derivation and maintenance of primed mouse epiblast stem cells. In this study, we show that DhiFI conditions similarly improved primed hESC traits, such as conferring a primed transcriptional signature with high levels of pluripotency markers and reduced levels of differentiation markers. When triggered to differentiate to neuronal and cardiac lineages, DhiFI hESCs and isogenic primed hESCs progressed similarly. Moreover, DhiFI conditions supported the derivation of hESC lines from a post-inner cell mass intermediate (PICMI). DhiFI-derived hESCs showed less spontaneous differentiation and expressed significantly lower levels of lineage-specific markers, compared to primed-derived lines from the same PICMI. Overall, DhiFI hESCs retained advantages of both primed and naive pluripotency and may ultimately represent a more favorable starting point for differentiation toward clinically desired cell types.

CXCL4/PF4 is a predictive biomarker of cardiac differentiation potential of human induced pluripotent stem cells

Selection of human induced pluripotent stem cell (hiPSC) lines with high cardiac differentiation potential is important for regenerative therapy and drug screening. We aimed to identify biomarkers for predicting cardiac differentiation potential of hiPSC lines by comparing the gene expression profiles of six undifferentiated hiPSC lines with different cardiac differentiation capabilities. We used three platforms of gene expression analysis, namely, cap analysis of gene expression (CAGE), mRNA array, and microRNA array to efficiently screen biomarkers related to cardiac differentiation of hiPSCs. Statistical analysis revealed candidate biomarker genes with significant correlation between the gene expression levels in the undifferentiated hiPSCs and their cardiac differentiation potential. Of the candidate genes, PF4 was validated as a biomarker expressed in undifferentiated hiPSCs with high potential for cardiac differentiation in 13 additional hiPSC lines. Our observations suggest that PF4 may be a useful biomarker for selecting hiPSC lines appropriate for the generation of cardiomyocytes.

Maternal Eed knockout causes loss of H3K27me3 imprinting and random X inactivation in the extraembryonic cells

In this study, Inoue et al. investigated the regulatory mechanisms and functions of the maternal H3K27me3 mechanism. They found that maternal Eed, an essential component of the Polycomb group complex 2 (PRC2), is required for establishing H3K27me3 imprinting, and their results also reveal unique XCI dynamics in the absence of Xist imprinting.

Genomic imprinting is essential for mammalian development. Recent studies have revealed that maternal histone H3 Lys27 trimethylation (H3K27me3) can mediate DNA methylation-independent genomic imprinting. However, the regulatory mechanisms and functions of this new imprinting mechanism are largely unknown. Here we demonstrate that maternal Eed, an essential component of the Polycomb group complex 2 (PRC2), is required for establishing H3K27me3 imprinting. We found that all H3K27me3-imprinted genes, including Xist, lose their imprinted expression in Eed maternal knockout (matKO) embryos, resulting in male-biased lethality. Surprisingly, although maternal X-chromosome inactivation (XmCI) occurs in Eed matKO embryos at preimplantation due to loss of Xist imprinting, it is resolved at peri-implantation. Ultimately, both X chromosomes are reactivated in the embryonic cell lineage prior to random XCI, and only a single X chromosome undergoes random XCI in the extraembryonic cell lineage. Thus, our study not only demonstrates an essential role of Eed in H3K27me3 imprinting establishment but also reveals a unique XCI dynamic in the absence of Xist imprinting.

A Simplified and Efficient Protocol for Derivation and Maintenance of High-Quality Mouse Primed Pluripotent Stem Cells Using Wnt Inhibition

Epiblast stem cells (EpiSCs) are primed pluripotent stem cells (PSCs) derived from mouse postimplantation embryos. Interestingly, EpiSCs share many characteristics with human PSCs such as human embryonic stem cells (hESCs) and human induced PSCs (hiPSC). Thus, EpiSCs can serve as a model for studying primed states of pluripotency. This article describes a simple yet highly efficient protocol for EpiSC derivation and maintenance of homogenous EpiSCs using an inhibitor of WNT secretion. Using this method, EpiSCs can be readily derived from mouse strains with different genetic background including C57BL/6N. The EpiSCs derived by this protocol maintain a homogenous, undifferentiated status, yet retain high differentiation potential. Unlike EpiSCs established by the original protocol, the new EpiSC lines require the continued presence of WNT inhibitor, suggesting intrinsic differences from EpiSCs made by the original method. This new version of EpiSCs will provide clues to understand the nature of primed states of mammalian pluripotent cells and may facilitate establishment of a better protocol for directed differentiation from the primed state. © 2018 by John Wiley & Sons, Inc.

Overlapping functions of Krüppel-like factor family members: targeting multiple transcription factors to maintain the naïve pluripotency of mouse embryonic stem cells

Krüppel-like factors (Klfs) have a pivotal role in maintaining self-renewal of mouse embryonic stem cells (mESCs). The functions of three Klf family members (Klf2, Klf4 and Klf5) have been identified, and are suggested to largely overlap. For further dissection of their functions, we applied an inducible knockout system for these Klf family members and assessed the effects of combinatorial loss of function. As a result, we confirmed that any one of Klf2, Klf4 and Klf5 was sufficient to support self-renewal, whereas the removal of all three compromised it. The activity of any single transcription factor, except for a Klf family member, was not sufficient to restore self-renewal of triple-knockout mESCs. However, some particular combinations of transcription factors were capable of the restoration. The triple-knockout mESCs were successfully captured at primed state. These data indicate that the pivotal function of a Klf family member is transduced into the activation of multiple transcription factors in a naïve-state-specific manner.

Comparative analysis of naive, primed and ground state pluripotency in mouse embryonic stem cells originating from the same genetic background

Mouse embryonic stem cells (mESCs) exist in a naive, primed and ground state of pluripotency. While comparative analyses of these pluripotency states have been reported, the mESCs utilized originated from various genetic backgrounds and were derived in different laboratories. mESC derivation in conventional LIF + serum culture conditions is strain dependent, with different genetic backgrounds potentially affecting subsequent stem cell characteristics. In the present study, we performed a comprehensive characterization of naive, primed and ground state mESCs originating from the same genetic background within our laboratory, by comparing their transcriptional profiles. We showed unique transcriptional profiles for naive, primed and ground state mESCs. While naive and ground state mESCs have more similar but not identical profiles, primed state mESCs show a very distinct profile. We further demonstrate that the differentiation propensity of mESCs to specific germ layers is highly dependent on their respective state of pluripotency.

Efficient derivation of stable primed pluripotent embryonic stem cells from bovine blastocysts

Embryonic stem cells (ESCs) are derived from the inner cell mass of preimplantation blastocysts. From agricultural and biomedical perspectives, the derivation of stable ESCs from domestic ungulates is important for genomic testing and selection, genome engineering, and modeling human diseases. Cattle are one of the most important domestic ungulates that are commonly used for food and bioreactors. To date, however, it remains a challenge to produce stable pluripotent bovine ESC lines. Employing a culture system containing fibroblast growth factor 2 and an inhibitor of the canonical Wnt-signaling pathway, we derived pluripotent bovine ESCs (bESCs) with stable morphology, transcriptome, karyotype, population-doubling time, pluripotency marker gene expression, and epigenetic features. Under this condition bESC lines were efficiently derived (100% in optimal conditions), were established quickly (3-4 wk), and were simple to propagate (by trypsin treatment). When used as donors for nuclear transfer, bESCs produced normal blastocyst rates, thereby opening the possibility for genomic selection, genome editing, and production of cattle with high genetic value.

The Art of Capturing Pluripotency: Creating the Right Culture

Embryonic stem cells (ESCs) are a unique tool for genetic perturbation of mammalian cellular and organismal processes additionally in humans offer unprecedented opportunities for disease modeling and cell therapy. Furthermore, ESCs are a powerful system for exploring the fundamental biology of pluripotency. Indeed understanding the control of self-renewal and differentiation is key to realizing the potential of ESCs. Building on previous observations, we found that mouse ESCs can be derived and maintained with high efficiency through insulation from differentiation cues combined with consolidation of an innate cell proliferation program. This finding of a pluripotent ground state has led to conceptual and practical advances, including the establishment of germline-competent ESCs from recalcitrant mouse strains and for the first time from the rat. Here, we summarize historical and recent progress in defining the signaling environment that supports self-renewal. We compare the contrasting requirements of two types of pluripotent stem cell, naive ESCs and primed post-implantation epiblast stem cells (EpiSCs), and consider the outstanding challenge of generating naive pluripotent stem cells from different mammals.

Pluripotency Deconstructed

Pluripotency denotes the flexible capacity of single cells to give rise to all somatic lineages and typically also the germline. Mouse ES cells and post-implantation epiblast-derived stem cells (EpiSC) are widely used pluripotent cell culture systems. These two in vitro stem cell types have divergent characteristics. They are considered as representative of distinct developmental stages, distinguished by using the terms "naïve" and "primed". A binary description is an over-simplification, however. Here, we discuss an intermediate stage of pluripotency that we term "formative". Formative pluripotency features a gene regulatory network switch from the naïve state and comprises capacitation of enhancers, signaling pathways and epigenetic machinery in order to install competence for lineage specification.