NODAL Secures Pluripotency upon Embryonic Stem Cell Progression from the Ground State

Control of cell state transitions by post-transcriptional regulation

Cell state transitions are prevalent in biology, playing a fundamental role in development, homeostasis and repair. Dysregulation of cell state transitions can lead to or occur in a wide range of diseases. In this letter, I explore and highlight the role of post-transcriptional regulatory mechanisms in determining the dynamics of cell state transitions. I propose that regulation of protein levels after transcription provides an under-appreciated regulatory route to obtain fast and sharp transitions between distinct cell states. This article is part of a discussion meeting issue 'Causes and consequences of stochastic processes in development and disease'.

Hallmarks of totipotent and pluripotent stem cell states

Though totipotency and pluripotency are transient during early embryogenesis, they establish the foundation for the development of all mammals. Studying these in vivo has been challenging due to limited access and ethical constraints, particularly in humans. Recent progress has led to diverse culture adaptations of epiblast cells in vitro in the form of totipotent and pluripotent stem cells, which not only deepen our understanding of embryonic development but also serve as invaluable resources for animal reproduction and regenerative medicine. This review delves into the hallmarks of totipotent and pluripotent stem cells, shedding light on their key molecular and functional features.

Pluripotency state transition of embryonic stem cells requires the turnover of histone chaperone FACT on chromatin

The differentiation of embryonic stem cells (ESCs) begins with the transition from the naive to the primed state. The formative state was recently established as a critical intermediate between the two states. Here, we demonstrate the role of the histone chaperone FACT in regulating the naive-to-formative transition. We found that the Q265K mutation in the FACT subunit SSRP1 increased the binding of FACT to histone H3-H4, impaired nucleosome disassembly in vitro, and reduced the turnover of FACT on chromatin in vivo. Strikingly, mouse ESCs harboring this mutation showed elevated naive-to-formative transition. Mechanistically, the SSRP1-Q265K mutation enriched FACT at the enhancers of formative-specific genes to increase targeted gene expression. Together, these findings suggest that the turnover of FACT on chromatin is crucial for regulating the enhancers of formative-specific genes, thereby mediating the naive-to-formative transition. This study highlights the significance of FACT in fine-tuning cell fate transition during early development.

Derivation of human primordial germ cell-like cells in an embryonic-like culture

Primordial germ cells (PGCs) are the embryonic precursors of sperm and eggs. They transmit genetic and epigenetic information across generations. Given the prominent role of germline defects in diseases such as infertility, detailed understanding of human PGC (hPGC) development has important implications in reproductive medicine and studying human evolution. Yet, hPGC specification remains an elusive process. Here, we report the induction of hPGC-like cells (hPGCLCs) in a bioengineered human pluripotent stem cell (hPSC) culture that mimics peri-implantation human development. In this culture, amniotic ectoderm-like cells (AMLCs), derived from hPSCs, induce hPGCLC specification from hPSCs through paracrine signaling downstream of ISL1. Our data further show functional roles of NODAL, WNT, and BMP signaling in hPGCLC induction. hPGCLCs are successfully derived from eight non-obstructive azoospermia (NOA) participant-derived hPSC lines using this biomimetic platform, demonstrating its promise for screening applications.

The many dimensions of germline competence

Primordial germ cell (PGC) specification is the first step in the development of the germline. Recent work has elucidated human-mouse differences in PGC differentiation and identified cell states with enhanced competency for PGC-like cell (PGCLC) differentiation in vitro in both species. However, it remains a subject of debate how different PGC competent states in vitro relate to each other, to embryonic development, and to the origin of PGCs in vivo. Here we review recent literature on human PGCLC differentiation in the context of mouse and non-human primate models. In contrast to what was previously thought, recent work suggests human pluripotent stem cells (hPSCs) are highly germline competent. We argue that paradoxical observations regarding the origin and signaling requirements of hPGCLCs may be due to local cell interactions. These confound assays of competence by generating endogenous signaling gradients and spatially modulating the ability to receive exogenous inductive signals. Furthermore, combinatorial signaling suggests that there is no unique germline competent state: rather than a one-dimensional spectrum of developmental progression, competence should be considered in a higher dimensional landscape of cell states.

TOBF1 modulates mouse embryonic stem cell fate through regulating alternative splicing of pluripotency genes

Embryonic stem cells (ESCs) can undergo lineage-specific differentiation, giving rise to different cell types that constitute an organism. Although roles of transcription factors and chromatin modifiers in these cells have been described, how the alternative splicing (AS) machinery regulates their expression has not been sufficiently explored. Here, we show that the long non-coding RNA (lncRNA)-associated protein TOBF1 modulates the AS of transcripts necessary for maintaining stem cell identity in mouse ESCs. Among the genes affected is serine/arginine splicing factor 1 (SRSF1), whose AS leads to global changes in splicing and expression of a large number of downstream genes involved in the maintenance of ESC pluripotency. By overlaying information derived from TOBF1 chromatin occupancy, the distribution of its pluripotency-associated OCT-SOX binding motifs, and transcripts undergoing differential expression and AS upon its knockout, we describe local nuclear territories where these distinct events converge. Collectively, these contribute to the maintenance of mouse ESC identity.

Epigenetic dynamics during capacitation of naïve human pluripotent stem cells

Human pluripotent stem cells (hPSCs) are of fundamental relevance in regenerative medicine. Naïve hPSCs hold promise to overcome some of the limitations of conventional (primed) hPSCs, including recurrent epigenetic anomalies. Naïve-to-primed transition (capacitation) follows transcriptional dynamics of human embryonic epiblast and is necessary for somatic differentiation from naïve hPSCs. We found that capacitated hPSCs are transcriptionally closer to postimplantation epiblast than conventional hPSCs. This prompted us to comprehensively study epigenetic and related transcriptional changes during capacitation. Our results show that CpG islands, gene regulatory elements, and retrotransposons are hotspots of epigenetic dynamics during capacitation and indicate possible distinct roles of specific epigenetic modifications in gene expression control between naïve and primed hPSCs. Unexpectedly, PRC2 activity appeared to be dispensable for the capacitation. We find that capacitated hPSCs acquire an epigenetic state similar to conventional hPSCs. Significantly, however, the X chromosome erosion frequently observed in conventional female hPSCs is reversed by resetting and subsequent capacitation.

Esrrb guides naive pluripotent cells through the formative transcriptional programme

During embryonic development, naive pluripotent epiblast cells transit to a formative state. The formative epiblast cells form a polarized epithelium, exhibit distinct transcriptional and epigenetic profiles and acquire competence to differentiate into all somatic and germline lineages. However, we have limited understanding of how the transition to a formative state is molecularly controlled. Here we used murine embryonic stem cell models to show that ESRRB is both required and sufficient to activate formative genes. Genetic inactivation of Esrrb leads to illegitimate expression of mesendoderm and extra-embryonic markers, impaired formative expression and failure to self-organize in 3D. Functionally, this results in impaired ability to generate formative stem cells and primordial germ cells in the absence of Esrrb. Computational modelling and genomic analyses revealed that ESRRB occupies key formative genes in naive cells and throughout the formative state. In so doing, ESRRB kickstarts the formative transition, leading to timely and unbiased capacity for multi-lineage differentiation.

Cell state transitions: catch them if you can

The Company of Biologists' 2022 workshop on 'Cell State Transitions: Approaches, Experimental Systems and Models' brought together an international and interdisciplinary team of investigators spanning the fields of cell and developmental biology, stem cell biology, physics, mathematics and engineering to tackle the question of how cells precisely navigate between distinct identities and do so in a dynamic manner. This second edition of the workshop was organized after a successful virtual workshop on the same topic that took place in 2021.

The transforming growth factor beta ligand TIG-2 modulates the function of neuromuscular junction and muscle energy metabolism in Caenorhabditis elegans

Deciphering the physiological function of TGF-β (the transforming growth factor beta) family ligands is import for understanding the role of TGF-β in animals' development and aging. Here, we investigate the function of TIG-2, one of the ligands in Caenorhabditis elegans TGF-β family, in animals' behavioral modulation. Our results show that a loss-of-function mutation in tig-2 gene result in slower locomotion speed in the early adulthood and an increased density of cholinergic synapses, but a decreased neurotransmitter release at neuromuscular junctions (NMJs). Further tissue-specific rescue results reveal that neuronal and intestinal TIG-2 are essential for the formation of cholinergic synapses at NMJs. Interestingly, tig-2(ok3416) mutant is characterized with reduced muscle mitochondria content and adenosine triphosphate (ATP) production, although the function of muscle acetylcholine receptors and the morphology muscle fibers in the mutant are comparable to that in wild-type animals. Our result suggests that TIG-2 from different neuron and intestine regulates worm locomotion by modulating synaptogenesis and neurotransmission at NMJs, as well as energy metabolism in postsynaptic muscle cells.

Rapid and robust directed differentiation of mouse epiblast stem cells into definitive endoderm and forebrain organoids

Directed differentiation of pluripotent stem cells (PSCs) is a powerful model system for deconstructing embryonic development. Although mice are the most advanced mammalian model system for genetic studies of embryonic development, state-of-the-art protocols for directed differentiation of mouse PSCs into defined lineages require additional steps and generates target cell types with lower purity than analogous protocols for human PSCs, limiting their application as models for mechanistic studies of development. Here, we examine the potential of mouse epiblast stem cells cultured in media containing Wnt pathway inhibitors as a starting point for directed differentiation. As a proof of concept, we focused our efforts on two specific cell/tissue types that have proven difficult to generate efficiently and reproducibly from mouse embryonic stem cells: definitive endoderm and neural organoids. We present new protocols for rapid generation of nearly pure definitive endoderm and forebrain-patterned neural organoids that model the development of prethalamic and hippocampal neurons. These differentiation models present new possibilities for combining mouse genetic tools with in vitro differentiation to characterize molecular and cellular mechanisms of embryonic development.

The RNA helicase DDX6 controls early mouse embryogenesis by repressing aberrant inhibition of BMP signaling through miRNA-mediated gene silencing

The evolutionarily conserved RNA helicase DDX6 is a central player in post-transcriptional regulation, but its role during embryogenesis remains elusive. We here show that DDX6 enables proper cell lineage specification from pluripotent cells by analyzing Ddx6 knockout (KO) mouse embryos and employing an in vitro epiblast-like cell (EpiLC) induction system. Our study unveils that DDX6 is an important BMP signaling regulator. Deletion of Ddx6 causes the aberrant upregulation of the negative regulators of BMP signaling, which is accompanied by enhanced expression of Nodal and related genes. Ddx6 KO pluripotent cells acquire higher pluripotency with a strong inclination toward neural lineage commitment. During gastrulation, abnormally expanded Nodal and Eomes expression in the primitive streak likely promotes endoderm cell fate specification while inhibiting mesoderm differentiation. We also genetically dissected major DDX6 pathways by generating Dgcr8, Dcp2, and Eif4enif1 KO models in addition to Ddx6 KO. We found that the miRNA pathway mutant Dgcr8 KO phenocopies Ddx6 KO, indicating that DDX6 mostly works along with the miRNA pathway during early development, whereas its P-body-related functions are dispensable. Therefore, we conclude that DDX6 prevents aberrant upregulation of BMP signaling inhibitors by participating in miRNA-mediated gene silencing processes. Overall, this study delineates how DDX6 affects the development of the three primary germ layers during early mouse embryogenesis and the underlying mechanism of DDX6 function.

Arp2/3 complex activity is necessary for mouse ESC differentiation, times formative pluripotency, and enables lineage specification

Mouse embryonic stem cells (mESCs), a model for differentiation into primed epiblast-like cells (EpiLCs), have revealed transcriptional and epigenetic control of early embryonic development. The control and significance of morphological changes, however, remain less defined. We show marked changes in morphology and actin architectures during differentiation that depend on Arp2/3 complex but not formin activity. Inhibiting Arp2/3 complex activity pharmacologically or genetically does not block exit from naive pluripotency, but attenuates increases in EpiLC markers. We find that inhibiting Arp2/3 complex activity delays formative pluripotency and causes globally defective lineage specification as indicated by RNA sequencing, with significant effects on TBX3-depedendent transcriptional programs. We also identify two previously unreported indicators of mESC differentiation, namely, MRTF and FHL2, which have inverse Arp2/3 complex-dependent nuclear translocation. Our findings on Arp2/3 complex activity in differentiation and the established role of formins in EMT indicate that these two actin nucleators regulate distinct modes of epithelial plasticity.

Dissecting signaling hierarchies in the patterning of the mouse primitive streak using micropatterned EpiLC colonies

Embryo studies have established that the patterning of the mouse gastrula depends on a regulatory network in which the WNT, BMP, and NODAL signaling pathways cooperate, but aspects of their respective contributions remain unclear. Studying their impact on the spatial organization and developmental trajectories of micropatterned epiblast-like cell (EpiLC) colonies, we show that NODAL is required prior to BMP action to establish the mesoderm and endoderm lineages. The presence of BMP then forces NODAL and WNT to support the formation of posterior primitive streak (PS) derivatives, while its absence allows them to promote that of anterior PS derivatives. Also, a Nodal mutation elicits more severe patterning defects in vitro than in the embryo, suggesting that ligands of extra-embryonic origin can rescue them. These results support the implication of a combinatorial process in PS patterning and illustrate how the study of micropatterned EpiLC colonies can complement that of embryos.

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BMP or WNT cannot rescue the impact a Nodal KO has on primitive streak formation

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BMP exposure results in Nodal promoting posterior rather than anterior PS formation

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The maintenance of posterior mesodermal identities is dependent on Nodal expression

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Low Nodal expression does not prevent the emergence of anterior PS derivatives

Efficient differentiation of human primordial germ cells through geometric control reveals a key role for Nodal signaling

Human primordial germ cells (hPGCs) form around the time of implantation and are the precursors of eggs and sperm. Many aspects of hPGC specification remain poorly understood because of the inaccessibility of the early postimplantation human embryo for study. Here, we show that micropatterned human pluripotent stem cells (hPSCs) treated with BMP4 give rise to hPGC-like cells (hPGCLC) and use these as a quantitatively reproducible and simple in vitro model to interrogate this important developmental event. We characterize micropatterned hPSCs up to 96 hr and show that hPGCLC populations are stable and continue to mature. By perturbing signaling during hPGCLC differentiation, we identify a previously unappreciated role for Nodal signaling and find that the relative timing and duration of BMP and Nodal signaling are critical parameters controlling the number of hPGCLCs. We formulate a mathematical model for a network of cross-repressive fates driven by Nodal and BMP signaling, which predicts the measured fate patterns after signaling perturbations. Finally, we show that hPSC colony size dictates the efficiency of hPGCLC specification, which led us to dramatically improve the efficiency of hPGCLC differentiation.

In humans and other animals, eggs and sperm are unique cells that pass on genetic material to the next generation. They originate from a small group of cells called primordial germ cells that form early in life in the developing embryo. Several different signal molecules including ones known as BMP4, Wnt, and Nodal, instruct certain cells in the embryo to become primordial germ cells.

The process by which primordial germ cells are made in humans is very different to how primordial germ cells are made in mice and other so-called model animals that are commonly used in research. This has made it more challenging to uncover the details of the process in humans. Fortunately, new methods have recently been created that mimic aspects of how human embryos develop using human stem cells in a laboratory dish, providing an opportunity to gain a deeper understanding of how human germ cells form.

Jo et al. used a technique called micropatterning to control the shape and size of groups of human stem cells growing in a laboratory dish. Treating these cells with a signal known as BMP4 gave rise to cells that resembled primordial germ cells. The team then used this system as a model to study how primordial germ cells form in humans. The experiments found that reducing Wnt signals in stem cells stopped primordial germ cells from forming in response to BMP4, confirming that Wnt signals made by the cells in response to BMP4 are essential. However, this block was overcome by providing the stem cells with another signal called Nodal. This suggests that the role of Wnt signaling in primordial germ cell formation is in part indirect by switching on Nodal in stem cells.

Defects in eggs and sperm may lead to infertility, therefore, the findings of Jo et al. have the potential to help researchers develop new fertility treatments that use eggs or sperm grown in a laboratory from the patients’ own stem cells. Such research would benefit from first developing a better understanding of how to make primordial germ cells.

NMD is required for timely cell fate transitions by fine-tuning gene expression and regulating translation

Here, Huth et al. investigated the role of components of the nonsense-mediated mRNA decay (NMD) pathway in regulating embryonic stem cell (ESC) differentiation, and show that NMD controls expression levels of the translation initiation factor Eif4a2 and its premature termination codon-encoding isoform (Eif4a2^PTC). Their findings expose an intricate link between mRNA homeostasis and mTORC1 activity that must be maintained for normal dynamics of cell state transitions.

Cell fate transitions depend on balanced rewiring of transcription and translation programs to mediate ordered developmental progression. Components of the nonsense-mediated mRNA decay (NMD) pathway have been implicated in regulating embryonic stem cell (ESC) differentiation, but the exact mechanism is unclear. Here we show that NMD controls expression levels of the translation initiation factor Eif4a2 and its premature termination codon-encoding isoform (Eif4a2^PTC). NMD deficiency leads to translation of the truncated eIF4A2^PTC protein. eIF4A2^PTC elicits increased mTORC1 activity and translation rates and causes differentiation delays. This establishes a previously unknown feedback loop between NMD and translation initiation. Furthermore, our results show a clear hierarchy in the severity of target deregulation and differentiation phenotypes between NMD effector KOs (Smg5 KO > Smg6 KO > Smg7 KO), which highlights heterodimer-independent functions for SMG5 and SMG7. Together, our findings expose an intricate link between mRNA homeostasis and mTORC1 activity that must be maintained for normal dynamics of cell state transitions.

RNA polymerase II pausing in development: orchestrating transcription

The coordinated regulation of transcriptional networks underpins cellular identity and developmental progression. RNA polymerase II promoter-proximal pausing (Pol II pausing) is a prevalent mechanism by which cells can control and synchronize transcription. Pol II pausing regulates the productive elongation step of transcription at key genes downstream of a variety of signalling pathways, such as FGF and Nodal. Recent advances in our understanding of the Pol II pausing machinery and its role in transcription call for an assessment of these findings within the context of development. In this review, we discuss our current understanding of the molecular basis of Pol II pausing and its function during organismal development. By critically assessing the tools used to study this process we conclude that combining recently developed genomics approaches with refined perturbation systems has the potential to expand our understanding of Pol II pausing mechanistically and functionally in the context of development and beyond.

A miniature dialysis-culture device allows high-density human-induced pluripotent stem cells expansion from growth factor accumulation

Three-dimensional aggregate-suspension culture is a potential biomanufacturing method to produce a large number of human induced pluripotent stem cells (hiPSCs); however, the use of expensive growth factors and method-induced mechanical stress potentially result in inefficient production costs and difficulties in preserving pluripotency, respectively. Here, we developed a simple, miniaturized, dual-compartment dialysis-culture device based on a conventional membrane-culture insert with deep well plates. The device improved cell expansion up to approximately ~3.2 to 4×10⁷ cells/mL. The high-density expansion was supported by reduction of excessive shear stress and agglomeration mediated by the addition of the functional polymer FP003. The results revealed accumulation of several growth factors, including fibroblast growth factor 2 and insulin, along with endogenous Nodal, which acts as a substitute for depleted transforming growth factor-β1 in maintaining pluripotency. Because we used the same growth-factor formulation per volume in the upper culture compartment, the cost reduced in inverse proportional manner with the cell density. We showed that growth-factor-accumulation dynamics in a low-shear-stress environment successfully improved hiPSC proliferation, pluripotency, and differentiation potential. This miniaturised dialysis-culture system demonstrated the feasibility of cost-effective mass production of hiPSCs in high-density culture.

The exploration of pluripotency space: Charting cell state transitions in peri-implantation development

Pluripotent cells in the mammalian embryo undergo state transitions marked by changes in patterns of gene expression and developmental potential as they progress from pre-implantation through post-implantation stages of development. Recent studies of cultured mouse and human pluripotent stem cells (hPSCs) have identified cells representative of an intermediate stage (referred to as the formative state) between naive pluripotency (equivalent to pre-implantation epiblast) and primed pluripotency (equivalent to late post-implantation epiblast). We examine these recent findings in light of our knowledge of peri-implantation mouse and human development, and we consider the implications of this work for deriving human embryo models from pluripotent cells.

StemBond hydrogels control the mechanical microenvironment for pluripotent stem cells

Studies of mechanical signalling are typically performed by comparing cells cultured on soft and stiff hydrogel-based substrates. However, it is challenging to independently and robustly control both substrate stiffness and extracellular matrix tethering to substrates, making matrix tethering a potentially confounding variable in mechanical signalling investigations. Moreover, unstable matrix tethering can lead to poor cell attachment and weak engagement of cell adhesions. To address this, we developed StemBond hydrogels, a hydrogel in which matrix tethering is robust and can be varied independently of stiffness. We validate StemBond hydrogels by showing that they provide an optimal system for culturing mouse and human pluripotent stem cells. We further show how soft StemBond hydrogels modulate stem cell function, partly through stiffness-sensitive ERK signalling. Our findings underline how substrate mechanics impact mechanosensitive signalling pathways regulating self-renewal and differentiation, indicating that optimising the complete mechanical microenvironment will offer greater control over stem cell fate specification.

Cell state transitions: definitions and challenges

A fundamental challenge when studying biological systems is the description of cell state dynamics. During transitions between cell states, a multitude of parameters may change - from the promoters that are active, to the RNAs and proteins that are expressed and modified. Cells can also adopt different shapes, alter their motility and change their reliance on cell-cell junctions or adhesion. These parameters are integral to how a cell behaves and collectively define the state a cell is in. Yet, technical challenges prevent us from measuring all of these parameters simultaneously and dynamically. How, then, can we comprehend cell state transitions using finite descriptions? The recent virtual workshop organised by The Company of Biologists entitled 'Cell State Transitions: Approaches, Experimental Systems and Models' attempted to address this question. Here, we summarise some of the main points that emerged during the workshop's themed discussions. We also present examples of cell state transitions and describe models and systems that are pushing forward our understanding of how cells rewire their state.

Enhancer-associated H3K4 methylation safeguards in vitro germline competence

Germline specification in mammals occurs through an inductive process whereby competent cells in the post-implantation epiblast differentiate into primordial germ cells (PGC). The intrinsic factors that endow epiblast cells with the competence to respond to germline inductive signals remain unknown. Single-cell RNA sequencing across multiple stages of an in vitro PGC-like cells (PGCLC) differentiation system shows that PGCLC genes initially expressed in the naïve pluripotent stage become homogeneously dismantled in germline competent epiblast like-cells (EpiLC). In contrast, the decommissioning of enhancers associated with these germline genes is incomplete. Namely, a subset of these enhancers partly retain H3K4me1, accumulate less heterochromatic marks and remain accessible and responsive to transcriptional activators. Subsequently, as in vitro germline competence is lost, these enhancers get further decommissioned and lose their responsiveness to transcriptional activators. Importantly, using H3K4me1-deficient cells, we show that the loss of this histone modification reduces the germline competence of EpiLC and decreases PGCLC differentiation efficiency. Our work suggests that, although H3K4me1 might not be essential for enhancer function, it can facilitate the (re)activation of enhancers and the establishment of gene expression programs during specific developmental transitions.

Establishment of Mouse Primed Stem Cells by Combination of Activin and LIF Signaling

In mice, embryonic stem cells (ESCs) and epiblast stem cells (EpiSCs) are established from pre- and post-implantation embryos and represent the naive and primed state, respectively. Herein we used mouse leukemia inhibitory factor (LIF), which supports ESCs self-renewal and Activin A (Act A), which is the main factor in maintaining EpiSCs in post-implantation epiblast cultures, to derive a primed stem cell line named ALSCs. Like EpiSCs, ALSCs express key pluripotent genes Oct4, Sox2, and Nanog; one X chromosome was inactivated; and the cells failed to contribute to chimera formation in vivo. Notably, compared to EpiSCs, ALSCs efficiently reversed to ESCs (rESCs) on activation of Wnt signaling. Moreover, we also discovered that culturing EpiSCs in AL medium for several passages favored Wnt signaling-driven naive pluripotency. Our results show that ALSCs is a primed state stem cell and represents a simple model to study the control of pluripotency fate and conversion from the primed to the naive state.

Building Pluripotency Identity in the Early Embryo and Derived Stem Cells

The fusion of two highly differentiated cells, an oocyte with a spermatozoon, gives rise to the zygote, a single totipotent cell, which has the capability to develop into a complete, fully functional organism. Then, as development proceeds, a series of programmed cell divisions occur whereby the arising cells progressively acquire their own cellular and molecular identity, and totipotency narrows until when pluripotency is achieved. The path towards pluripotency involves transcriptome modulation, remodeling of the chromatin epigenetic landscape to which external modulators contribute. Both human and mouse embryos are a source of different types of pluripotent stem cells whose characteristics can be captured and maintained in vitro. The main aim of this review is to address the cellular properties and the molecular signature of the emerging cells during mouse and human early development, highlighting similarities and differences between the two species and between the embryos and their cognate stem cells.

MicroRNAs and Stem-like Properties: The Complex Regulation Underlying Stemness Maintenance and Cancer Development

Embryonic stem cells (ESCs) have the extraordinary properties to indefinitely proliferate and self-renew in culture to produce different cell progeny through differentiation. This latter process recapitulates embryonic development and requires rounds of the epithelial-mesenchymal transition (EMT). EMT is characterized by the loss of the epithelial features and the acquisition of the typical phenotype of the mesenchymal cells. In pathological conditions, EMT can confer stemness or stem-like phenotypes, playing a role in the tumorigenic process. Cancer stem cells (CSCs) represent a subpopulation, found in the tumor tissues, with stem-like properties such as uncontrolled proliferation, self-renewal, and ability to differentiate into different cell types. ESCs and CSCs share numerous features (pluripotency, self-renewal, expression of stemness genes, and acquisition of epithelial-mesenchymal features), and most of them are under the control of microRNAs (miRNAs). These small molecules have relevant roles during both embryogenesis and cancer development. The aim of this review was to recapitulate molecular mechanisms shared by ESCs and CSCs, with a special focus on the recently identified classes of microRNAs (noncanonical miRNAs, mirtrons, isomiRs, and competitive endogenous miRNAs) and their complex functions during embryogenesis and cancer development.

A case for revisiting Nodal signaling in human pluripotent stem cells

Nodal is a transforming growth factor-β (TGF-β) superfamily member that plays a number of critical roles in mammalian embryonic development. Nodal is essential for the support of the peri-implantation epiblast in the mouse embryo and subsequently acts to specify mesendodermal fate at the time of gastrulation and, later, left-right asymmetry. Maintenance of human pluripotent stem cells (hPSCs) in vitro is dependent on Nodal signaling. Because it has proven difficult to prepare a biologically active form of recombinant Nodal protein, Activin or TGFB1 are widely used as surrogates for NODAL in hPSC culture. Nonetheless, the expression of the components of an endogenous Nodal signaling pathway in hPSC provides a potential autocrine pathway for the regulation of self-renewal in this system. Here we review recent studies that have clarified the role of Nodal signaling in pluripotent stem cell populations, highlighted spatial restrictions on Nodal signaling, and shown that Nodal functions in vivo as a heterodimer with GDF3, another TGF-β superfamily member expressed by hPSC. We discuss the role of this pathway in the maintenance of the epiblast and hPSC in light of these new advances.

Cooperative genetic networks drive embryonic stem cell transition from naïve to formative pluripotency

In the mammalian embryo, epiblast cells must exit the naïve state and acquire formative pluripotency. This cell state transition is recapitulated by mouse embryonic stem cells (ESCs), which undergo pluripotency progression in defined conditions in vitro. However, our understanding of the molecular cascades and gene networks involved in the exit from naïve pluripotency remains fragmentary. Here, we employed a combination of genetic screens in haploid ESCs, CRISPR/Cas9 gene disruption, large‐scale transcriptomics and computational systems biology to delineate the regulatory circuits governing naïve state exit. Transcriptome profiles for 73 ESC lines deficient for regulators of the exit from naïve pluripotency predominantly manifest delays on the trajectory from naïve to formative epiblast. We find that gene networks operative in ESCs are also active during transition from pre‐ to post‐implantation epiblast in utero. We identified 496 naïve state‐associated genes tightly connected to the in vivo epiblast state transition and largely conserved in primate embryos. Integrated analysis of mutant transcriptomes revealed funnelling of multiple gene activities into discrete regulatory modules. Finally, we delineate how intersections with signalling pathways direct this pivotal mammalian cell state transition.

Capture of Mouse and Human Stem Cells with Features of Formative Pluripotency

Pluripotent cells emerge as a naive founder population in the blastocyst, acquire capacity for germline and soma formation, and then undergo lineage priming. Mouse embryonic stem cells (ESCs) and epiblast-derived stem cells (EpiSCs) represent the initial naive and final primed phases of pluripotency, respectively. Here, we investigate the intermediate formative stage. Using minimal exposure to specification cues, we derive stem cells from formative mouse epiblast. Unlike ESCs or EpiSCs, formative stem (FS) cells respond directly to germ cell induction. They colonize somatic tissues and germline in chimeras. Whole-transcriptome analyses show similarity to pre-gastrulation formative epiblast. Signal responsiveness and chromatin accessibility features reflect lineage capacitation. Furthermore, FS cells show distinct transcription factor dependencies, relying critically on Otx2. Finally, FS cell culture conditions applied to human naive cells or embryos support expansion of similar stem cells, consistent with a conserved staging post on the trajectory of mammalian pluripotency.

Abscission Couples Cell Division to Embryonic Stem Cell Fate

Cell fate transitions are key to development and homeostasis. It is thus essential to understand the cellular mechanisms controlling fate transitions. Cell division has been implicated in fate decisions in many stem cell types, including neuronal and epithelial progenitors. In other stem cells, such as embryonic stem (ES) cells, the role of division remains unclear. Here, we show that exit from naive pluripotency in mouse ES cells generally occurs after a division. We further show that exit timing is strongly correlated between sister cells, which remain connected by cytoplasmic bridges long after division, and that bridge abscission progressively accelerates as cells exit naive pluripotency. Finally, interfering with abscission impairs naive pluripotency exit, and artificially inducing abscission accelerates it. Altogether, our data indicate that a switch in the division machinery leading to faster abscission regulates pluripotency exit. Our study identifies abscission as a key cellular process coupling cell division to fate transitions.

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Mouse embryonic stem cells exit naive pluripotency after mitosis

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Naive embryonic stem cells display slow abscission and remain connected by bridges

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Cells exiting naive pluripotency display faster abscission

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Accelerating abscission facilitates exit from naive pluripotency

Microfluidic platform for 3D cell culture with live imaging and clone retrieval

Combining live imaging with the ability to retrieve individual cells of interest remains a technical challenge. Combining imaging with precise cell retrieval is of particular interest when studying highly dynamic or transient, asynchronous, or heterogeneous cell biological and developmental processes. Here, we present a method to encapsulate live cells in a 3D hydrogel matrix, via hydrogel bead compartmentalisation. Using a small-scale screen, we optimised matrix conditions for the culture and multilineage differentiation of mouse embryonic stem cells. Moreover, we designed a custom microfluidic platform that is compatible with live imaging. With this platform we can long-term culture and subsequently extract individual cells-in-beads by media flow only, obviating the need for enzymatic cell removal from the platform. Specific beads may be extracted from the platform in isolation, without disrupting the adjacent beads. We show that we can differentiate mouse embryonic stem cells, monitor reporter expression by live imaging, and retrieve individual beads for functional assays, correlating reporter expression with functional response. Overall, we present a highly flexible 3D cell encapsulation and microfluidic platform that enables both monitoring of cellular dynamics and retrieval for molecular and functional assays.

Vascular mimicry: changing the therapeutic paradigms in cancer

Cancer is a major problem in the health system, and despite many efforts to effectively treat it, none has yet been fully successful. Angiogenesis and metastasis are considered as major challenges in the treatment of various cancers. Researchers have struggled to succeed with anti-angiogenesis drugs for the effective treatment of cancer, although new challenges have emerged in the treatment with the emergence of resistance to anti-angiogenesis and anti-metastatic drugs. Numerous studies have shown that different cancers can resist anti-angiogenesis drugs in a new process called vascular mimicry (VM). The studies have revealed that cells resistant to anti-angiogenesis cancer therapies are more capable of forming VMs in the in vivo and in vitro environment, although there is a link between the presence of VM and poor clinical outcomes. Given the importance of the VM in the challenges facing cancer treatment, researchers are trying to identify factors that prevent the formation of these structures. In this review article, it is attempted to provide a comprehensive overview of the molecules and main signaling pathways involved in VM phenomena, as well as the agents currently being identified as anti-VM and the role of VM in response to treatment and prognosis of cancer patients.

Signal regulators of human naïve pluripotency

Pluripotent cells transiently develop during peri-implantation embryogenesis and have the capacity to convert into three embryonic lineages. Two typical states of pluripotency, naïve and primed, can be experimentally induced in vitro. The in vitro naïve state can be stabilized in response to environmental inductive cues via a unique transcriptional regulatory program. However, interference with various signaling pathways creates a spectrum of alternative pluripotent cells that display different functions and molecular expression patterns. Similarly, human naïve pluripotent cells can be placed into two main levels - intermediate and bona fide. Here, we discuss several culture conditions that have been used to establish naïve-associated gene regulatory networks in human pluripotent cells. We also describe different transcriptional patterns in various culture systems that are associated with these two levels of human naïve pluripotency.

Combinatorial Smad2/3 Activities Downstream of Nodal Signaling Maintain Embryonic/Extra-Embryonic Cell Identities during Lineage Priming

Epiblast cells in the early post-implantation stage mammalian embryo undergo a transition described as lineage priming before cell fate allocation, but signaling pathways acting upstream remain ill defined. Genetic studies demonstrate that Smad2/3 double-mutant mouse embryos die shortly after implantation. To learn more about the molecular disturbances underlying this abrupt failure, here we characterized Smad2/3-deficient embryonic stem cells (ESCs). We found that Smad2/3 double-knockout ESCs induced to form epiblast-like cells (EpiLCs) display changes in naive and primed pluripotency marker gene expression, associated with the disruption of Oct4-bound distal regulatory elements. In the absence of Smad2/3, we observed enhanced Bmp target gene expression and de-repression of extra-embryonic gene expression. Cell fate allocation into all three embryonic germ layers is disrupted. Collectively, these experiments demonstrate that combinatorial Smad2/3 functional activities are required to maintain distinct embryonic and/or extra-embryonic cell identity during lineage priming in the epiblast before gastrulation.

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Smad2/3 alters the transcriptome and activity of distal regulatory elements in EpiLCs

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Smad2 prevents expression of extra-embryonic genes during priming and differentiation

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Smad2/3 is essential for mesoderm and definitive endoderm cell fate allocation

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Smad2/3 signaling balances Bmp signaling during neural precursor differentiation

Signaling regulation during gastrulation: Insights from mouse embryos and in vitro systems

Gastrulation is the process whereby cells exit pluripotency and concomitantly acquire and pattern distinct cell fates. This is driven by the convergence of WNT, BMP, Nodal and FGF signals, which are tightly spatially and temporally controlled, resulting in regional and stage-specific signaling environments. The combination, level and duration of signals that a cell is exposed to, according its position within the embryo and the developmental time window, dictates the fate it will adopt. The key pathways driving gastrulation exhibit complex interactions, which are difficult to disentangle in vivo due to the complexity of manipulating multiple signals in parallel with high spatiotemporal resolution. Thus, our current understanding of the signaling dynamics regulating gastrulation is limited. In vitro stem cell models have been established, which undergo organized cellular differentiation and patterning. These provide amenable, simplified, deconstructed and scalable models of gastrulation. While the foundation of our understanding of gastrulation stems from experiments in embryos, in vitro systems are now beginning to reveal the intricate details of signaling regulation. Here we discuss the current state of knowledge of the role, regulation and dynamic interaction of signaling pathways that drive mouse gastrulation.

Zfp281 orchestrates interconversion of pluripotent states by engaging Ehmt1 and Zic2

Developmental cell fate specification is a unidirectional process that can be reverted in response to injury or experimental reprogramming. Whether differentiation and de-differentiation trajectories intersect mechanistically is unclear. Here, we performed comparative screening in lineage-related mouse naïve embryonic stem cells (ESCs) and primed epiblast stem cells (EpiSCs), and identified the constitutively expressed zinc finger transcription factor (TF) Zfp281 as a bidirectional regulator of cell state interconversion. We showed that subtle chromatin binding changes in differentiated cells translate into activation of the histone H3 lysine 9 (H3K9) methyltransferase Ehmt1 and stabilization of the zinc finger TF Zic2 at enhancers and promoters. Genetic gain-of-function and loss-of-function experiments confirmed a critical role of Ehmt1 and Zic2 downstream of Zfp281 both in driving exit from the ESC state and in restricting reprogramming of EpiSCs. Our study reveals that cell type-invariant chromatin association of Zfp281 provides an interaction platform for remodeling the cis-regulatory network underlying cellular plasticity.

Parsing the pluripotency continuum in humans and non-human primates for interspecies chimera generation

Pluripotency refers to the potential of single cells to form all cells and tissues of an organism. The observation that pluripotent stem cells can chimerize the embryos of evolutionarily distant species, albeit at very low efficiencies, could with further modifications, facilitate the production of human-animal interspecies chimeras. The generation of human-animal interspecies chimeras, if achieved, will enable practitioners to recapitulate pathologic human tissue formation in vivo and produce patient-specific organs inside livestock species. However, little is known about the nature of chimera-competent cellular states in primates. Here, I discuss recent advances in our understanding of the pluripotency continuum in humans and non-human primates (NHPs). Although undefined differences between humans and NHPs still justify the utility of studying human cells, the complementary use of NHP PS cells could also allow one to conduct pilot studies testing interspecies chimera generation strategies with reduced ethical concerns associated with human interspecies neurological chimerism. However, the availability of standardized, high-quality and validated NHP PS cell lines covering the spectrum of primate pluripotent states is lacking. Therefore, a clearer understanding of the primate pluripotency continuum will facilitate the complementary use of both human and NHP PS cells for testing interspecies organogenesis strategies, with the hope of one day enabling human organ generation inside livestock species.

Differential regulation of OCT4 targets facilitates reacquisition of pluripotency

Ectopic transcription factor expression enables reprogramming of somatic cells to pluripotency, albeit with generally low efficiency. Despite steady progress in the field, the exact molecular mechanisms that coordinate this remarkable transition still remain largely elusive. To better characterize the final steps of pluripotency induction, we optimized an experimental system where pluripotent stem cells are differentiated for set intervals before being reintroduced to pluripotency-supporting conditions. Using this approach, we identify a transient period of high-efficiency reprogramming where ectopic transcription factors, but not serum/LIF alone, rapidly revert cells to pluripotency with near 100% efficiency. After this period, cells reprogram with somatic-like kinetics and efficiencies. We identify a set of OCT4 bound cis-regulatory elements that are dynamically regulated during this transient phase and appear central to facilitating reprogramming. Interestingly, these regions remain hypomethylated during in vitro and in vivo differentiation, which may allow them to act as primary targets of ectopically induced factors during somatic cell reprogramming.

Targeting Nodal and Cripto-1: Perspectives Inside Dual Potential Theranostic Cancer Biomarkers

BACKGROUND

Elucidating the mechanisms of recurrence of embryonic signaling pathways in tumorigenesis has led to the discovery of onco-fetal players which have physiological roles during normal development but result aberrantly re-activated in tumors. In this context, Nodal and Cripto-1 are recognized as onco-developmental factors, which are absent in normal tissues but are overexpressed in several solid tumors where they can serve as theranostic agents.

OBJECTIVE

To collect, review and discuss the most relevant papers related to the involvement of Nodal and Cripto-1 in the development, progression, recurrence and metastasis of several tumors where they are over-expressed, with a particular attention to their occurrence on the surface of the corresponding sub-populations of cancer stem cells (CSC).

RESULTS

We have gathered, rationalized and discussed the most interesting findings extracted from some 370 papers related to the involvement of Cripto-1 and Nodal in all tumor types where they have been detected. Data demonstrate the clear connection between Nodal and Cripto-1 presence and their multiple oncogenic activities across different tumors. We have also reviewed and highlighted the potential of targeting Nodal, Cripto-1 and the complexes that they form on the surface of tumor cells, especially of CSC, as an innovative approach to detect and suppress tumors with molecules that block one or more mechanisms that they regulate.

CONCLUSION

Overall, Nodal and Cripto-1 represent two innovative and effective biomarkers for developing potential theranostic anti-tumor agents that target normal as well as CSC subpopulations and overcome both pharmacological resistance and tumor relapse.

Complementary Activity of ETV5, RBPJ, and TCF3 Drives Formative Transition from Naive Pluripotency

The gene regulatory network (GRN) of naive mouse embryonic stem cells (ESCs) must be reconfigured to enable lineage commitment. TCF3 sanctions rewiring by suppressing components of the ESC transcription factor circuitry. However, TCF3 depletion only delays and does not prevent transition to formative pluripotency. Here, we delineate additional contributions of the ETS-family transcription factor ETV5 and the repressor RBPJ. In response to ERK signaling, ETV5 switches activity from supporting self-renewal and undergoes genome relocation linked to commissioning of enhancers activated in formative epiblast. Independent upregulation of RBPJ prevents re-expression of potent naive factors, TBX3 and NANOG, to secure exit from the naive state. Triple deletion of Etv5, Rbpj, and Tcf3 disables ESCs, such that they remain largely undifferentiated and locked in self-renewal, even in the presence of differentiation stimuli. Thus, genetic elimination of three complementary drivers of network transition stalls developmental progression, emulating environmental insulation by small-molecule inhibitors.

Capacitation of human naïve pluripotent stem cells for multi-lineage differentiation

Human naïve pluripotent stem cells (PSCs) share features with the pre-implantation epiblast. They therefore provide an unmatched opportunity for characterising the developmental programme of pluripotency in Homo sapiens Here, we confirm that naïve PSCs do not respond directly to germ layer induction, but must first acquire competence. Capacitation for multi-lineage differentiation occurs without exogenous growth factor stimulation and is facilitated by inhibition of Wnt signalling. Whole-transcriptome profiling during this formative transition highlights dynamic changes in gene expression, which affect many cellular properties including metabolism and epithelial features. Notably, naïve pluripotency factors are exchanged for postimplantation factors, but competent cells remain devoid of lineage-specific transcription. The gradual pace of transition for human naïve PSCs is consistent with the timespan of primate development from blastocyst to gastrulation. Transcriptome trajectory during in vitro capacitation of human naïve cells tracks the progression of the epiblast during embryogenesis in Macaca fascicularis, but shows greater divergence from mouse development. Thus, the formative transition of naïve PSCs in a simple culture system may recapitulate essential and specific features of pluripotency dynamics during an inaccessible period of human embryogenesis.

Defined conditions for propagation and manipulation of mouse embryonic stem cells

The power of mouse embryonic stem (ES) cells to colonise the developing embryo has revolutionised mammalian developmental genetics and stem cell research. This power is vulnerable, however, to the cell culture environment, deficiencies in which can lead to cellular heterogeneity, adaptive phenotypes, epigenetic aberrations and genetic abnormalities. Here, we provide detailed methodologies for derivation, propagation, genetic modification and primary differentiation of ES cells in 2i or 2i+LIF media without serum or undefined serum substitutes. Implemented diligently, these procedures minimise variability and deviation, thereby improving the efficiency, reproducibility and biological validity of ES cell experimentation.

Genetic dissection of Nodal and Bmp signalling requirements during primordial germ cell development in mouse

The essential roles played by Nodal and Bmp signalling during early mouse development have been extensively documented. Here we use conditional deletion strategies to investigate functional contributions made by Nodal, Bmp and Smad downstream effectors during primordial germ cell (PGC) development. We demonstrate that Nodal and its target gene Eomes provide early instructions during formation of the PGC lineage. We discover that Smad2 inactivation in the visceral endoderm results in increased numbers of PGCs due to an expansion of the PGC niche. Smad1 is required for specification, whereas in contrast Smad4 controls the maintenance and migration of PGCs. Additionally we find that beside Blimp1, down-regulated phospho-Smad159 levels also distinguishes PGCs from their somatic neighbours so that emerging PGCs become refractory to Bmp signalling that otherwise promotes mesodermal development in the posterior epiblast. Thus balanced Nodal/Bmp signalling cues regulate germ cell versus somatic cell fate decisions in the early posterior epiblast.

Physiological Microenvironmental Conditions in Different Scalable Culture Systems for Pluripotent Stem Cell Expansion and Differentiation

Human Pluripotent Stem Cells (PSCs) are a valuable cell type that has a wide range of biomedical applications because they can differentiate into many types of adult somatic cell. Numerous studies have examined the clinical applications of PSCs. However, several factors such as bioreactor design, mechanical stress, and the physiological environment have not been optimized. These factors can significantly alter the pluripotency and proliferation properties of the cells, which are important for the mass production of PSCs. Nutritional mass transfer and oxygen transfer must be effectively maintained to obtain a high yield. Various culture systems are currently available for optimum cell propagation by maintaining the physiological conditions necessary for cell cultivation. Each type of culture system using a different configuration with various advantages and disadvantages affecting the mechanical conditions in the bioreactor, such as shear stress. These factors make it difficult to preserve the cellular viability and pluripotency of PSCs. Additional limitations of the culture system for PSCs must also be identified and overcome to maintain the culture conditions and enable large-scale expansion and differentiation of PSCs. This review describes the different physiological conditions in the various culture systems and recent developments in culture technology for PSC expansion and differentiation.

Long-Term Perfusion Culture of Monoclonal Embryonic Stem Cells in 3D Hydrogel Beads for Continuous Optical Analysis of Differentiation

Developmental cell biology requires technologies in which the fate of single cells is followed over extended time periods, to monitor and understand the processes of self-renewal, differentiation, and reprogramming. A workflow is presented, in which single cells are encapsulated into droplets (Ø: 80 µm, volume: ≈270 pL) and the droplet compartment is later converted to a hydrogel bead. After on-chip de-emulsification by electrocoalescence, these 3D scaffolds are subsequently arrayed on a chip for long-term perfusion culture to facilitate continuous cell imaging over 68 h. Here, the response of murine embryonic stem cells to different growth media, 2i and N2B27, is studied, showing that the exit from pluripotency can be monitored by fluorescence time-lapse microscopy, by immunostaining and by reverse-transcription and quantitative PCR (RT-qPCR). The defined 3D environment emulates the natural context of cell growth (e.g., in tissue) and enables the study of cell development in various matrices. The large scale of cell cultivation (in 2000 beads in parallel) may reveal infrequent events that remain undetected in lower throughput or ensemble studies. This platform will help to gain qualitative and quantitative mechanistic insight into the role of external factors on cell behavior.

Frontiers of Pluripotency

Humans develop from a unique group of pluripotent cells in early embryos that can produce all cells of the human body. While pluripotency is only transiently manifest in the embryo, scientists have identified conditions that sustain pluripotency indefinitely in the laboratory. Pluripotency is not a monolithic entity, however, but rather comprises a spectrum of different cellular states. Questions regarding the scientific value of examining the continuum of pluripotent stem (PS) cell states have gained increased significance in light of attempts to generate interspecies chimeras between humans and animals. In this chapter, I review our ever-evolving understanding of the continuum of pluripotency. Historically, the discovery of two different PS cell states in mice fostered a general conception of pluripotency comprised of two distinct attractor states: naïve and primed. Naïve pluripotency has been defined by competence to form germline chimeras and governance by unique KLF-based transcription factor (TF) circuitry, whereas primed state is distinguished by an inability to generate chimeras and alternative TF regulation. However, the discovery of many alternative PS cell states challenges the concept of pluripotency as a binary property. Moreover, it remains unclear whether the current molecular criteria used to classify human naïve-like pluripotency also identify human chimera-competent PS cells. Therefore, I examine the pluripotency continuum more closely in light of recent advances in PS cell research and human interspecies chimera research.

FGFR1 regulates trophectoderm development and facilitates blastocyst implantation

FGF signaling plays important roles in many aspects of mammalian development. Fgfr1^-/- and Fgfr1^-/-Fgfr2^-/- mouse embryos on a 129S4 co-isogenic background fail to survive past the peri-implantation stage, whereas Fgfr2^-/- embryos die at midgestation and show defects in limb and placental development. To investigate the basis for the Fgfr1^-/- and Fgfr1^-/-Fgfr2^-/- peri-implantation lethality, we examined the role of FGFR1 and FGFR2 in trophectoderm (TE) development. In vivo, Fgfr1^-/- TE cells failed to downregulate CDX2 in the mural compartment and exhibited abnormal apicobasal E-Cadherin polarity. In vitro, we were able to derive mutant trophoblast stem cells (TSCs) from Fgfr1^-/- or Fgfr2^-/- single mutant, but not from Fgfr1^-/-Fgfr2^-/- double mutant blastocysts. Fgfr1^-/- TSCs however failed to efficiently upregulate TE differentiation markers upon differentiation. These results suggest that while the TE is specified in Fgfr1^-/- mutants, its differentiation abilities are compromised leading to defects at implantation.

Negative feedback via RSK modulates Erk-dependent progression from naïve pluripotency

Mitogen-activated protein kinase (MAPK)/extracellular signal-regulated kinase (ERK) signalling is implicated in initiation of embryonic stem (ES) cell differentiation. The pathway is subject to complex feedback regulation. Here, we examined the ERK-responsive phosphoproteome in ES cells and identified the negative regulator RSK1 as a prominent target. We used CRISPR/Cas9 to create combinatorial mutations in RSK family genes. Genotypes that included homozygous null mutations in Rps6ka1, encoding RSK1, resulted in elevated ERK phosphorylation. These RSK-depleted ES cells exhibit altered kinetics of transition into differentiation, with accelerated downregulation of naïve pluripotency factors, precocious expression of transitional epiblast markers and early onset of lineage specification. We further show that chemical inhibition of RSK increases ERK phosphorylation and expedites ES cell transition without compromising multilineage potential. These findings demonstrate that the ERK activation profile influences the dynamics of pluripotency progression and highlight the role of signalling feedback in temporal control of cell state transitions.

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.

TGF-β signaling pathway in early mouse development and embryonic stem cells

TGF-β superfamily signaling pathways essentially contribute to the broad spectrum of early developmental events including embryonic patterning, cell fate determination and dynamic movements. In this review, we first introduced some key developmental processes that require TGF-β signaling to show the fundamental importance of these pathways. Then we discuss how their activities are regulated, and new findings about how the TGF-β superfamily ligands bind to the chromatin to regulate transcription during embryo development.