Notch and hippo converge on Cdx2 to specify the trophectoderm lineage in the mouse blastocyst

Emerging Principles in the Transcriptional Control by YAP and TAZ

Simple Summary

YAP and TAZ are transcriptional cofactors that integrate several upstream signals to generate context-dependent transcriptional responses. This requires extensive integration with epigenetic regulators and other transcription factors. The molecular and genomic characterization of YAP and TAZ nuclear function has broad implications both in physiological and pathological settings.

Abstract

Yes-associated protein (YAP) and TAZ are transcriptional cofactors that sit at the crossroad of several signaling pathways involved in cell growth and differentiation. As such, they play essential functions during embryonic development, regeneration, and, once deregulated, in cancer progression. In this review, we will revise the current literature and provide an overview of how YAP/TAZ control transcription. We will focus on data concerning the modulation of the basal transcriptional machinery, their ability to epigenetically remodel the enhancer–promoter landscape, and the mechanisms used to integrate transcriptional cues from multiple pathways. This reveals how YAP/TAZ activation in cancer cells leads to extensive transcriptional control that spans several hallmarks of cancer. The definition of the molecular mechanism of transcriptional control and the identification of the pathways regulated by YAP/TAZ may provide therapeutic opportunities for the effective treatment of YAP/TAZ-driven tumors.

Human Embryogenesis: A Comparative Perspective

Understanding human embryology has historically relied on comparative approaches using mammalian model organisms. With the advent of low-input methods to investigate genetic and epigenetic mechanisms and efficient techniques to assess gene function, we can now study the human embryo directly. These advances have transformed the investigation of early embryogenesis in nonrodent species, thereby providing a broader understanding of conserved and divergent mechanisms. Here, we present an overview of the major events in human preimplantation development and place them in the context of mammalian evolution by comparing these events in other eutherian and metatherian species. We describe the advances of studies on postimplantation development and discuss stem cell models that mimic postimplantation embryos. A comparative perspective highlights the importance of analyzing different organisms with molecular characterization and functional studies to reveal the principles of early development. This growing field has a fundamental impact in regenerative medicine and raises important ethical considerations.

Molecular fingerprint of female bovine embryos produced in vitro with high competence to establish and maintain pregnancy†

The objective was to identify the transcriptomic profile of in vitro-derived embryos with high competence to establish and maintain gestation. Embryos produced with X-sorted sperm were cultured from day 5 to day 7 in serum-free medium containing 10 ng/ml recombinant bovine colony-stimulating factor 2 (CSF2) or vehicle. The CSF2 was administered because this molecule can increase blastocyst competence for survival after embryo transfer. Blastocysts were harvested on day 7 of culture and manually bisected. One demi-embryo from a single blastocyst was transferred into a synchronized recipient and the other half was used for RNA-seq analysis. Using P < 0.01 and a fold change >2-fold or <0.5 fold as cutoffs, there were 617 differentially expressed genes (DEG) between embryos that survived to day 30 of gestation vs those that did not, 470 DEG between embryos that survived to day 60 and those that did not, 432 DEG between embryos that maintained pregnancy from day 30 to day 60 vs those where pregnancy failed after day 30, and 635 DEG regulated by CSF2. Pathways and ontologies in which DEG were overrepresented included many related to cellular responses to stress and cell survival. It was concluded that gene expression in the blastocyst is different between embryos that are competent to establish and maintain pregnancy vs those that are not. The relationship between expression of genes related to cell stress and subsequent embryonic survival probably reflects cellular perturbations caused by embryonic development taking place in the artificial environment associated with cell culture.

Nested information processing in the living world

Living organisms create, copy, and make use of information, the content depending on the level of organization. In cells, a network of signal chain proteins regulates gene expression and other cell functions. Incoming information is encoded through signal reception, processed by the network, and decoded by the synthesis of new gene products and other biological functions. Signaling proteins represent nodes, and signal transmission proceeds via allosteric binding, chemical and structural modifications, synthesis, sequestering, and degradation. The induction of the gene caudal type homeobox 2 (CDX2) in the mammalian preimplantation embryo is outlined as a demonstration of this concept. CDX2 is involved in the decision of cells to enter the trophoblast lineage. Two signal chains are coordinated into an information processing model with the help of logic gates. The model introduces a formal structure that incorporates experimental and morphological data. Above the cell level, information flow relates to tissue formation and functioning, and whole cells play the role of network nodes. This is described for the anatomical patterning of bone with implications for bone formation and homeostasis. The information usage in cells and tissues is set into a context of the nervous system and the interaction of human individuals in societies, both established scenes of information processing.

Expression levels and activation status of Yap splicing isoforms determine self-renewal and differentiation potential of embryonic stem cells

Yap is the key effector of Hippo signaling; however, its role in embryonic stem cells (ESCs) remains controversial. Here, we identify two Yap splicing isoforms (Yap472 and Yap488), which show equal expression levels but heterogeneous distribution in ESCs. Knockout (KO) of both isoforms reduces ESC self-renewal, accelerates pluripotency exit, but arrests terminal differentiation, while overexpression of each isoform leads to the reverse phenotype. The effect of both Yap isoforms on self-renewal is Teads-dependent and mediated by c-Myc. Nonetheless, different isoforms are found to affect overlapping yet distinct genes, and confer different developmental potential to Yap-KO cells, with Yap472 exerting a more pronounced biological effect and being more essential for neuroectoderm differentiation. Constitutive activation of Yaps, particularly Yap472, dramatically upregulates p53 and Cdx2, inducing trophectoderm trans-differentiation even under self-renewal conditions. These findings reveal the combined roles of different Yap splicing isoforms and mechanisms in regulating self-renewal efficiency and differentiation potential of ESCs.

In Silico Logical Modelling to Uncover Cooperative Interactions in Cancer

The multistep development of cancer involves the cooperation between multiple molecular lesions, as well as complex interactions between cancer cells and the surrounding tumour microenvironment. The search for these synergistic interactions using experimental models made tremendous contributions to our understanding of oncogenesis. Yet, these approaches remain labour-intensive and challenging. To tackle such a hurdle, an integrative, multidisciplinary effort is required. In this article, we highlight the use of logical computational models, combined with experimental validations, as an effective approach to identify cooperative mechanisms and therapeutic strategies in the context of cancer biology. In silico models overcome limitations of reductionist approaches by capturing tumour complexity and by generating powerful testable hypotheses. We review representative examples of logical models reported in the literature and their validation. We then provide further analyses of our logical model of Epithelium to Mesenchymal Transition (EMT), searching for additional cooperative interactions involving inputs from the tumour microenvironment and gain of function mutations in NOTCH.

Roles of cell differentiation factors in preimplantation development of domestic animals

In mammalian embryos, the first visible differentiation event is the segregation of the inner cell mass (ICM) and trophectoderm (TE) during the transition from the morula to the blastocyst stage. The ICM, which is attached to the inside of the TE, develop into the fetus and extraembryonic tissues, while the TE, which is a single layer surrounding the fluid-filled cavity called the blastocoel, will provide extraembryonic structures such as the placenta. ICM/TE differentiation is regulated by the interaction between various transcriptional factors. However, little information is available on the segregation of the ICM and TE lineages in preimplantation embryos of domestic animals, such as cattle and pigs. This review focuses on the roles of cell differentiation factors that regulate the ICM/TE segregation of preimplantation bovine and porcine embryos. Understanding the mechanism of cell differentiation in early embryos is necessary to improve the in vitro production systems for bovine and porcine embryos.

NOTCH signaling pathway is required for bovine early embryonic development†

The NOTCH signaling pathway plays an important role in regulating various biological processes, including lineage specification and apoptosis. Multiple components of the NOTCH pathway have been identified in mammalian preimplantation embryos. However, the precise role of the NOTCH pathway in early embryonic development is poorly understood, especially in large animals. Here, we show that the expression of genes encoding key transcripts of the NOTCH pathway is dynamic throughout early embryonic development. We also confirm the presence of active NOTCH1 and RBPJ. By using pharmacological and RNA interference tools, we demonstrate that the NOTCH pathway is required for the proper development of bovine early embryos. This functional consequence could be partly attributed to the major transcriptional mediator, Recombination Signal Binding Protein For Immunoglobulin Kappa J Region (RBPJ), whose deficiency also compromised the embryo quality. Indeed, both NOTCH1 and RBPJ knockdown cause a significant increase of histone H3 serine 10 phosphorylation (pH3S10, a mitosis marker) positive blastomeres, suggesting a cell cycle arrest at mitosis. Importantly, RNA sequencing analyses reveal that either NOTCH1 or RBPJ depletion triggers a reduction in H1FOO that encodes the oocyte-specific linker histone H1 variant. Interestingly, depleting H1FOO results in detrimental effects on the developmental competence of early embryos, similar with NOTCH1 inhibition. Overall, our results reveal a crucial role for NOTCH pathway in regulating bovine preimplantation development, likely by controlling cell proliferation and maintaining H1FOO expression.

Single-cell multiomics sequencing reveals the functional regulatory landscape of early embryos

Extensive epigenetic reprogramming occurs during preimplantation embryo development. However, it remains largely unclear how the drastic epigenetic reprogramming contributes to transcriptional regulatory network during this period. Here, we develop a single-cell multiomics sequencing technology (scNOMeRe-seq) that enables profiling of genome-wide chromatin accessibility, DNA methylation and RNA expression in the same individual cell. We apply this method to depict a single-cell multiomics map of mouse preimplantation development. We find that genome-wide DNA methylation remodeling facilitates the reconstruction of genetic lineages in early embryos. Further, we construct a zygotic genome activation (ZGA)-associated regulatory network and reveal coordination among multiple epigenetic layers, transcription factors and repeat elements that instruct proper ZGA. Cell fates associated cis-regulatory elements are activated stepwise in post-ZGA stages. Trophectoderm (TE)-specific transcription factors play dual roles in promoting the TE program while repressing the inner cell mass (ICM) program during the ICM/TE separation.

A Comparative Analysis of Hippo Signaling Pathway Components during Murine and Bovine Early Mammalian Embryogenesis

The time required for successful blastocyst formation varies among multiple species. The formation of a blastocyst is governed by numerous molecular cell signaling pathways, such as the Hippo signaling pathway. The Hippo signaling pathway is initiated by increased cell-cell contact and via apical polarity proteins (AMOT, PARD6, and NF2) during the period of preimplantation embryogenesis. Cell-cell contact and cell polarity activate (phosphorylates) the core cascade components of the pathway (mammalian sterile twenty like 1 and 2 (MST1/2) and large tumor suppressor 1 and 2 (LATS1/2)), which in turn phosphorylate the downstream effectors of the pathway (YAP1/TAZ). The Hippo pathway remains inactive with YAP1 (Yes Associated protein 1) present inside the nucleus in the trophectoderm (TE) cells (polar blastomeres) of the mouse blastocyst. In the inner cell mass (ICM) cells (apolar blastomeres), the pathway is activated with p-YAP1 present in the cytoplasm. On the contrary, during bovine embryogenesis, p-YAP1 is exclusively present in the nucleus in both TE and ICM cells. Contrary to mouse embryos, transcription co activator with PDZ-binding motif (TAZ) (also known as WWTR1) is also predominantly present in the cytoplasm in all the blastomeres during bovine embryogenesis. This review outlines the major differences in the localization and function of Hippo signaling pathway components of murine and bovine preimplantation embryos, suggesting significant differences in the regulation of this pathway in between the two species. The variance observed in the Hippo signaling pathway between murine and bovine embryos confirms that both of these early embryonic models are quite distinct. Moreover, based on the similarity of the Hippo signaling pathway between bovine and human early embryo development, bovine embryos could be an alternate model for understanding the regulation of the Hippo signaling pathway in human embryos.

Context-dependent roles of YAP/TAZ in stem cell fates and cancer

Hippo effectors YAP and TAZ control cell fate and survival through various mechanisms, including transcriptional regulation of key genes. However, much of this research has been marked by conflicting results, as well as controversy over whether YAP and TAZ are redundant. A substantial portion of the discordance stems from their contradictory roles in stem cell self-renewal vs. differentiation and cancer cell survival vs. apoptosis. In this review, we present an overview of the multiple context-dependent functions of YAP and TAZ in regulating cell fate decisions in stem cells and organoids, as well as their mechanisms of controlling programmed cell death pathways in cancer.

OCT4 induces embryonic pluripotency via STAT3 signaling and metabolic mechanisms

OCT4 is a fundamental component of the molecular circuitry governing pluripotency in vivo and in vitro. To determine how OCT4 establishes and protects the pluripotent lineage in the embryo, we used comparative single-cell transcriptomics and quantitative immunofluorescence on control and OCT4 null blastocyst inner cell masses at two developmental stages. Surprisingly, activation of most pluripotency-associated transcription factors in the early mouse embryo occurs independently of OCT4, with the exception of the JAK/STAT signaling machinery. Concurrently, OCT4 null inner cell masses ectopically activate a subset of trophectoderm-associated genes. Inspection of metabolic pathways implicates the regulation of rate-limiting glycolytic enzymes by OCT4, consistent with a role in sustaining glycolysis. Furthermore, up-regulation of the lysosomal pathway was specifically detected in OCT4 null embryos. This finding implicates a requirement for OCT4 in the production of normal trophectoderm. Collectively, our findings uncover regulation of cellular metabolism and biophysical properties as mechanisms by which OCT4 instructs pluripotency.

Characterisation of the Hippo signalling pathway during bovine preimplantation embryo development

Blastocyst formation is an important milestone during preimplantation embryo development. During murine preimplantation embryogenesis, the Hippo signalling pathway is known to play a significant role in lineage segregation and henceforth the formation of blastocysts. However, the role of this cell signalling pathway during bovine embryogenesis remains unknown. Thus, the aim of the present study was to characterise the Hippo signalling pathway during bovine preimplantation embryo development. mRNA transcripts of Hippo signalling pathway constituents (i.e. crumbs cell polarity complex component 3 (CRB3), mammalian sterile 20-like 1 (MST1), mammalian sterile 20-like 2 (MST2), Yes associated protein 1 (YAP1), transcriptional coactivator with PDZ-binding motif (TAZ)) were observed during all stages of bovine preimplantation embryo development. To evaluate the localisation of Hippo pathway components, bovine embryos at timed stages of development were stained using specific antibodies and observed under a laser confocal microscope. Although MST1/2 proteins were in the cytoplasm during various stages of bovine embryonic development, TAZ and phosphorylated (p-) YAP were detected in the nucleus during the blastocyst stages. Localisation of TAZ and p-YAP proteins was distinct in the bovine compared with mouse model, suggesting that the Hippo signalling pathway is regulated differently in early bovine embryos.

ZC3H4-a novel Cys-Cys-Cys-His-type zinc finger protein-is essential for early embryogenesis in mice†

Zinc finger domains of the Cys-Cys-Cys-His (CCCH) class are evolutionarily conserved proteins that bind nucleic acids and are involved in various biological processes. Nearly 60 CCCH-type zinc finger proteins have been identified in humans and mice, most have not been functionally characterized. Here, we provide the first in vivo functional characterization of ZC3H4-a novel CCCH-type zinc finger protein. Our results show that although Zc3h4 mutant embryos exhibit normal morphology at E3.5 blastocyst stage, they cannot be recovered at E7.5 early post-gastrulation stage, suggesting implantation failure. Outgrowth assays reveal that mutant blastocysts either fail to hatch from the zona pellucida, or can hatch but do not form a typical inner cell mass colony, the source of embryonic stem cells (ESCs). Although there is no change in levels of reactive oxygen species, Zc3h4 mutants display severe DNA breaks and reduced cell proliferation. Analysis of lineage specification reveals that both epiblast and primitive endoderm lineages are compromised with severe reductions in cell number and/or specification in the mutant blastocysts. In summary, these findings demonstrate the essential role of ZC3H4 during early mammalian embryogenesis.

Induction of Human Trophoblast Stem Cells from Somatic Cells and Pluripotent Stem Cells

Human trophoblast stem cells (hTSCs) derived from blastocysts and first-trimester cytotrophoblasts offer an unprecedented opportunity to study the placenta. However, access to human embryos and first-trimester placentas is limited, thus preventing the establishment of hTSCs from diverse genetic backgrounds associated with placental disorders. Here, we show that hTSCs can be generated from numerous genetic backgrounds using post-natal cells via two alternative methods: (1) somatic cell reprogramming of adult fibroblasts with OCT4, SOX2, KLF4, MYC (OSKM) and (2) cell fate conversion of naive and extended pluripotent stem cells. The resulting induced/converted hTSCs recapitulated hallmarks of hTSCs including long-term self-renewal, expression of specific transcription factors, transcriptomic signature, and the potential to differentiate into syncytiotrophoblast and extravillous trophoblast cells. We also clarified the developmental stage of hTSCs and show that these cells resemble day 8 cytotrophoblasts. Altogether, hTSC lines of diverse genetic origins open the possibility to model both placental development and diseases in a dish.

MCRS1 is essential for epiblast development during early mouse embryogenesis

Microspherule protein 1 (MCRS1, also known as MSP58) is an evolutionarily conserved protein that has been implicated in various biological processes. Although a variety of functions have been attributed to MCRS1 in vitro, mammalian MCRS1 has not been studied in vivo. Here we report that MCRS1 is essential during early murine development. Mcrs1 mutant embryos exhibit normal morphology at the blastocyst stage but cannot be recovered at gastrulation, suggesting an implantation failure. Outgrowth (OG) assays reveal that mutant blastocysts do not form a typical inner cell mass (ICM) colony, the source of embryonic stem cells (ESCs). Surprisingly, cell death and histone H4 acetylation analysis reveal that apoptosis and global H4 acetylation are normal in mutant blastocysts. However, analysis of lineage specification reveals that while the trophoblast and primitive endoderm are properly specified, the epiblast lineage is compromised and exhibits a severe reduction in cell number. In summary, our study demonstrates the indispensable role of MCRS1 in epiblast development during early mammalian embryogenesis.

The chromatin-binding protein PHF6 functions as an E3 ubiquitin ligase of H2BK120 via H2BK12Ac recognition for activation of trophectodermal genes

Epigenetic regulation is important for establishing lineage-specific gene expression during early development. Although signaling pathways have been well-studied for regulation of trophectoderm reprogramming, epigenetic regulation of trophectodermal genes with histone modification dynamics have been poorly understood. Here, we identify that plant homeodomain finger protein 6 (PHF6) is a key epigenetic regulator for activation of trophectodermal genes using RNA-sequencing and ChIP assays. PHF6 acts as an E3 ubiquitin ligase for ubiquitination of H2BK120 (H2BK120ub) via its extended plant homeodomain 1 (PHD1), while the extended PHD2 of PHF6 recognizes acetylation of H2BK12 (H2BK12Ac). Intriguingly, the recognition of H2BK12Ac by PHF6 is important for exerting its E3 ubiquitin ligase activity for H2BK120ub. Together, our data provide evidence that PHF6 is crucial for epigenetic regulation of trophectodermal gene expression by linking H2BK12Ac to H2BK120ub modification.

Living a Sweet Life: Glucose Instructs Cell Fate in the Mouse Embryo

It has long been known that glucose is required for the development of pre-implantation mouse embryos, but the mechanism accounting for such a requirement has remained unknown. In this issue of Developmental Cell, Chi et al. dissect the molecular pathways that respond to the state of glucose metabolism to drive the morula to blastocyst transition.

Coordination between patterning and morphogenesis ensures robustness during mouse development

The mammalian preimplantation embryo is a highly tractable, self-organizing developmental system in which three cell types are consistently specified without the need for maternal factors or external signals. Studies in the mouse over the past decades have greatly improved our understanding of the cues that trigger symmetry breaking in the embryo, the transcription factors that control lineage specification and commitment, and the mechanical forces that drive morphogenesis and inform cell fate decisions. These studies have also uncovered how these multiple inputs are integrated to allocate the right number of cells to each lineage despite inherent biological noise, and as a response to perturbations. In this review, we summarize our current understanding of how these processes are coordinated to ensure a robust and precise developmental outcome during early mouse development. This article is part of a discussion meeting issue 'Contemporary morphogenesis'.

Spatial protein analysis in developing tissues: a sampling-based image processing approach

Advances in fluorescence microscopy approaches have made it relatively easy to generate multi-dimensional image volumes and have highlighted the need for flexible image analysis tools for the extraction of quantitative information from such data. Here we demonstrate that by focusing on simplified feature-based nuclear segmentation and probabilistic cytoplasmic detection we can create a tool that is able to extract geometry-based information from diverse mammalian tissue images. Our open-source image analysis platform, called 'SilentMark', can cope with three-dimensional noisy images and with crowded fields of cells to quantify signal intensity in different cellular compartments. Additionally, it provides tissue geometry related information, which allows one to quantify protein distribution with respect to marked regions of interest. The lightweight SilentMark algorithms have the advantage of not requiring multiple processors, graphics cards or training datasets and can be run even with just several hundred megabytes of memory. This makes it possible to use the method as a Web application, effectively eliminating setup hurdles and compatibility issues with operating systems. We test this platform on mouse pre-implantation embryos, embryonic stem cell-derived embryoid bodies and mouse embryonic heart, and relate protein localization to tissue geometry. This article is part of a discussion meeting issue 'Contemporary morphogenesis'.

Loss of POLR1D results in embryonic lethality prior to blastocyst formation in mice

In eukaryotic cells, RNA polymerase (Pol) I and Pol III are dedicated to the synthesis of ribosomal RNA precursors and a variety of small RNAs, respectively. Although RNA Pol I and Pol III complexes are crucial for the regulation of cell growth and cell cycle in all cell types, many of the components of the Pol I and Pol III complexes have not been functionally characterized in mammals. Here, we provide the first in vivo functional characterization of POLR1D, a subunit shared by RNA Pol I and Pol III, during early mammalian embryo development. Our results show that Polr1d mutant embryos cannot be recovered at E7.5 early post-gastrulation stage, suggesting failed implantation. Although Polr1d mutants can be recovered at E3.5, they exhibit delayed/stalled development with morula morphology rather than differentiation into blastocysts. Even with extended time in culture, mutant embryos fail to form blastocysts and eventually die. Analysis of E3.0 embryos revealed severe DNA damage in Polr1d mutants. Additionally, lineage assessment reveals that trophectoderm specification is compromised in the absence of Polr1d. In summary, these findings demonstrate the essential role of POLR1D during early mammalian embryogenesis and highlight cell-lethal phenotype without Polr1d function.

The unknown human trophectoderm: implication for biopsy at the blastocyst stage

Trophectoderm biopsy is increasingly performed for pre-implantation genetic testing of aneuploidies and considered a safe procedure on short-term clinical outcome, without strong assessment of long-term consequences. Poor biological information on human trophectoderm is available due to ethical restrictions. Therefore, most studies have been conducted in vitro (choriocarcinoma cell lines, embryonic and pluripotent stem cells) and on murine models that nevertheless poorly reflect the human counterpart. Polarization, compaction, and blastomere differentiation (e.g., the basis to ascertain trophectoderm origin) are poorly known in humans. In addition, the trophectoderm function is poorly known from a biological point of view, although a panoply of questionable and controversial microarray studies suggest that important genes overexpressed in trophectoderm are involved in pluripotency, metabolism, cell cycle, endocrine function, and implantation. The intercellular communication system between the trophectoderm cells and the inner cell mass, modulated by cell junctions and filopodia in the murine model, is obscure in humans. For the purpose of this paper, data mainly on primary cells from human and murine embryos has been reviewed. This review suggests that the trophectoderm origin and functions have been insufficiently ascertained in humans so far. Therefore, trophectoderm biopsy should be considered an experimental procedure to be undertaken only under approved rigorous experimental protocols in academic contexts.

Follistatin treatment modifies DNA methylation of the CDX2 gene in bovine preimplantation embryos

CDX2 plays a crucial role in the formation and maintenance of the trophectoderm epithelium in preimplantation embryos. Follistatin supplementation during the first 72 hr of in vitro culture triggers a significant increase in blastocyst rates, CDX2 expression, and trophectoderm cell numbers. However, the underlying epigenetic mechanisms by which follistatin upregulates CDX2 expression are not known. Here, we investigated whether stimulatory effects of follistatin are linked to alterations in DNA methylation within key regulatory regions of the CDX2 gene. In vitro-fertilized (IVF) zygotes were cultured with or without 10 ng/ml of recombinant human follistatin for 72 hr, then cultured without follistatin until Day 7. The bisulfite-sequencing analysis revealed differential methylation (DM) at specific CpG sites within the CDX2 promoter and intron 1 following follistatin treatment. These DM CpG sites include five hypomethylated sites at positions -1384, -1283, -297, -163, and -23, and four hypermethylated sites at positions -1501, -250, -243, and +20 in the promoter region. There were five hypomethylated sites at positions +3060, +3105, +3219, +3270, and +3545 in intron 1. Analysis of transcription factor binding sites using MatInspector combined with a literature search revealed a potential association between differentially methylated CpG sites and putative binding sites for key transcription factors involved in regulating CDX2 expression. The hypomethylated sites are putative binding sites for FXR, STAF, OCT1, KLF, AP2 family, and P53 protein, whereas the hypermethylated sites are putative binding sites for NRSF. Collectively, our results suggest that follistatin may increase CDX2 expression in early bovine embryos, at least in part, by modulating DNA methylation at key regulatory regions.

From pluripotency to totipotency: an experimentalist's guide to cellular potency

Embryonic stem cells (ESCs) are derived from the pre-implantation mammalian blastocyst. At this point in time, the newly formed embryo is concerned with the generation and expansion of both the embryonic lineages required to build the embryo and the extra-embryonic lineages that support development. When used in grafting experiments, embryonic cells from early developmental stages can contribute to both embryonic and extra-embryonic lineages, but it is generally accepted that ESCs can give rise to only embryonic lineages. As a result, they are referred to as pluripotent, rather than totipotent. Here, we consider the experimental potential of various ESC populations and a number of recently identified in vitro culture systems producing states beyond pluripotency and reminiscent of those observed during pre-implantation development. We also consider the nature of totipotency and the extent to which cell populations in these culture systems exhibit this property.

MED20 is essential for early embryogenesis and regulates NANOG expression

Mediator is an evolutionarily conserved multi-subunit complex, bridging transcriptional activators and repressors to the general RNA polymerase II (Pol II) initiation machinery. Though the Mediator complex is crucial for the transcription of almost all Pol II promoters in eukaryotic organisms, the phenotypes of individual Mediator subunit mutants are each distinct. Here, we report for the first time, the essential role of subunit MED20 in early mammalian embryo development. Although Med20 mutant mouse embryos exhibit normal morphology at E3.5 blastocyst stage, they cannot be recovered at early post-gastrulation stages. Outgrowth assays show that mutant blastocysts cannot hatch from the zona pellucida, indicating impaired blastocyst function. Assessments of cell death and cell lineage specification reveal that apoptosis, inner cell mass, trophectoderm and primitive endoderm markers are normal in mutant blastocysts. However, the epiblast marker NANOG is ectopically expressed in the trophectoderm of Med20 mutants, indicative of defects in trophoblast specification. These results suggest that MED20 specifically, and the Mediator complex in general, are essential for the earliest steps of mammalian development and cell lineage specification.

YAP/TAZ affects the development of pulmonary fibrosis by regulating multiple signaling pathways

YAP and TAZ are important co-activators of various biological processes in human body. YAP/TAZ plays a vital role in the development of pulmonary fibrosis. Dysregulation of the YAP/TAZ signaling pathway is one of the most important causes of pulmonary fibrosis. Therefore, considering its crucial role, summary of the signal mechanism of YAP/TAZ is of certain guiding significance for the research of YAP/TAZ as a therapeutic target. The present review provided a detailed introduction to various YAP/TAZ-related signaling pathways and clarified the specific role of YAP/TAZ in these pathways. In the meantime, we summarized and evaluated possible applications of YAP/TAZ in the treatment of pulmonary fibrosis. Overall, our study is of guiding significance for future research on the functional mechanism of YAP/TAZ underlying lung diseases as well as for identification of novel therapeutic targets specific to pulmonary fibrosis.

Common principles of early mammalian embryo self-organisation

Pre-implantation mammalian development unites extreme plasticity with a robust outcome: the formation of a blastocyst, an organised multi-layered structure ready for implantation. The process of blastocyst formation is one of the best-known examples of self-organisation. The first three cell lineages in mammalian development specify and arrange themselves during the morphogenic process based on cell-cell interactions. Despite decades of research, the unifying principles driving early mammalian development are still not fully defined. Here, we discuss the role of physical forces, and molecular and cellular mechanisms, in driving self-organisation and lineage formation that are shared between eutherian mammals.

Mechanisms of human embryo development: from cell fate to tissue shape and back

Gene regulatory networks and tissue morphogenetic events drive the emergence of shape and function: the pillars of embryo development. Although model systems offer a window into the molecular biology of cell fate and tissue shape, mechanistic studies of our own development have so far been technically and ethically challenging. However, recent technical developments provide the tools to describe, manipulate and mimic human embryos in a dish, thus opening a new avenue to exploring human development. Here, I discuss the evidence that supports a role for the crosstalk between cell fate and tissue shape during early human embryogenesis. This is a critical developmental period, when the body plan is laid out and many pregnancies fail. Dissecting the basic mechanisms that coordinate cell fate and tissue shape will generate an integrated understanding of early embryogenesis and new strategies for therapeutic intervention in early pregnancy loss.

Trophoblast lineage specification in the mammalian preimplantation embryo

BACKGROUND

The establishment of the trophectoderm (TE) and the inner cell mass (ICM) is the first cell lineage segregation that occurs in mammalian preimplantation development. TE will contribute to the placenta while ICM cells give rise to the epiblast (EPI) and primitive endoderm (PrE). There are two historical models for TE/ICM segregation: the positional (inside-outside) model and the polarity model, but both models alone cannot explain the mechanism of TE/ICM segregation.

METHODS

This article discusses a current possible model based on recent studies including the finding through live-cell imaging of the expression patterns of caudal type homeobox 2 (Cdx2), a key transcription factor of TE differentiation in the mouse embryo.

RESULTS

It was observed that a part of outer Cdx2-expressing blastomeres was internalized at the around 20- to 30-cell stage, downregulates Cdx2, ceases TE differentiation, and participates in ICM lineages.

CONCLUSION

The early blastomere, which starts differentiation toward the TE cell fate, still has plasticity and can change its fate. Differentiation potency of all blastomeres until approximately the 32-cell stage is presumably not irreversibly restricted even if they show heterogeneity in their epigenetic modifications or gene expression patterns.

The role of RHOA signaling in trophectoderm cell-fate decision in cattle

Mammalian blastocysts are composed of two distinct cell lineages, namely the inner cell mass (ICM) and trophectoderm (TE). TE cells that give rise to the embryonic placenta are marked by an exclusive expression of the key determinant transcription factor, CDX2. Although Hippo signaling pathway is known to be responsible for this TE-specific expression of CDX2, the upstream regulator of this pathway in mammalian embryos is still controversial. In the present study, the involvement of the small molecular G protein, RHOA, in TE cell-fate decision in cattle was investigated. Inhibition of RHOA by the specific inhibitor, C3 transferase (C3), severely impaired the blastocyst formation. Further, C3 treatment significantly decreased the number of blastomeres with nuclearized YAP1, the prominent effector of Hippo pathway. An artificial isolation of ICM cells from blastocysts followed by the continuing culture to regenerate TE cells was conducted and showed that TE re-emergence from the isolated ICM is governed by Hippo pathway and suppressed by C3 treatment like that observed in developing embryos. Finally, the long-term exposure to C3 suggests the presence of alternative regulators of CDX2 expression other than RHOA signaling because there were still CDX2-positive cells after C3 treatment. These results demonstrated that RHOA signaling plays a significant role in TE cell-fate decision by regulating Hippo pathway in cattle.

Notch signaling in mouse blastocyst development and hatching

Background

Mammalian early embryo development requires a well-orchestrated interplay of cell signaling pathways. Notch is a major regulatory pathway involved in cell-fate determination in embryonic and adult scenarios. However, the role of Notch in embryonic pre-implantation development is controversial. In particular, Notch role on blastocyst development and hatching remains elusive, and a complete picture of the transcription and expression patterns of Notch components during this time-period is not available.

Results

This study provided a comprehensive view on the dynamics of individual embryo gene transcription and protein expression patterns of Notch components (receptors Notch1–4; ligands Dll1 and Dll4, Jagged1–2; and effectors Hes1–2), and their relationship with transcription of gene markers of pluripotency and differentiation (Sox2, Oct4, Klf4, Cdx2) during mouse blastocyst development and hatching. Transcription of Notch1–2, Jagged1–2 and Hes1 was highly prevalent and dynamic along stages of development, whereas transcription of Notch3–4, Dll4 and Hes2 had a low prevalence among embryos. Transcription levels of Notch1, Notch2, Jagged2 and Hes1 correlated with each other and with those of pluripotency and differentiation genes. Gene transcription was associated to protein expression, except for Jagged2, where high transcription levels in all embryos were not translated into protein. Presence of Notch signaling activity was confirmed through nuclear NICD and Hes1 detection, and downregulation of Hes1 transcription following canonical signaling blockade with DAPT. In vitro embryo culture supplementation with Jagged1 had no effect on embryo developmental kinetics. In contrast, supplementation with Jagged2 abolished Jagged1 transcription, downregulated Cdx2 transcription and inhibited blastocyst hatching. Notch signaling blockade by DAPT downregulated transcription of Sox2, and retarded embryo hatching.

Conclusion

Transcription of Notch genes showed a dynamic pattern along blastocyst development and hatching. Data confirmed Notch signaling activity, and lead to the suggestion that Notch canonical signaling may be operating through Notch1, Notch3, Jagged1 and Hes1. Embryo culture supplementation with Jagged1 and Jagged2 unveiled a possible regulatory effect between Jagged1, Cdx2 and blastocyst hatching. Overall, results indicate that a deregulation in Notch signaling, either by its over or under-activation, affects blastocyst development and hatching.

Generation of Blastocyst-like Structures from Mouse Embryonic and Adult Cell Cultures

A single mouse blastomere from an embryo until the 8-cell stage can generate an entire blastocyst. Whether laboratory-cultured cells retain a similar generative capacity remains unknown. Starting from a single stem cell type, extended pluripotent stem (EPS) cells, we established a 3D differentiation system that enabled the generation of blastocyst-like structures (EPS-blastoids) through lineage segregation and self-organization. EPS-blastoids resembled blastocysts in morphology and cell-lineage allocation and recapitulated key morphogenetic events during preimplantation and early postimplantation development in vitro. Upon transfer, some EPS-blastoids underwent implantation, induced decidualization, and generated live, albeit disorganized, tissues in utero. Single-cell and bulk RNA-sequencing analysis revealed that EPS-blastoids contained all three blastocyst cell lineages and shared transcriptional similarity with natural blastocysts. We also provide proof of concept that EPS-blastoids can be generated from adult cells via cellular reprogramming. EPS-blastoids provide a unique platform for studying early embryogenesis and pave the way to creating viable synthetic embryos by using cultured cells.

RHPCG: a database of the Regulation of the Hippo Pathway in Cancer Genome

The Hippo signaling pathway is a highly conserved pathway controlling organ size, cell proliferation, apoptosis and other biological functions. Recent studies have shown that Hippo signaling pathway also plays important roles in cancer initiation and progression. However, a database offering multi-omics analyses and visualization of Hippo pathway genes in cancer, as well as comprehensive Hippo regulatory relationships is still lacking. To fill this gap, we constructed the Regulation of the Hippo Pathway in Cancer Genome (RHPCG) database. Currently, RHPCG focuses on analyzing the 21 core Hippo-protein-encoding genes in over 10 000 patients across 33 TCGA (The Cancer Genome Atlas) cancer types at the levels of genomic, epigenomic and transcriptomic landscape. Concurrently, RHPCG provides in its motif section 11 regulatory motif types associated with 21 core Hippo pathway genes containing 180 miRNAs, 6182 lncRNAs, 728 circRNAs and 335 protein coding genes. Thus, RHPCG is a powerful tool that could help researchers understand gene alterations and regulatory mechanisms in the Hippo signaling pathway in cancer.

The transition from local to global patterns governs the differentiation of mouse blastocysts

During mammalian blastocyst development, inner cell mass (ICM) cells differentiate into epiblast (Epi) or primitive endoderm (PrE). These two fates are characterized by the expression of the transcription factors NANOG and GATA6, respectively. Here, we investigate the spatio-temporal distribution of NANOG and GATA6 expressing cells in the ICM of the mouse blastocysts with quantitative three-dimensional single cell-based neighbourhood analyses. We define the cell neighbourhood by local features, which include the expression levels of both fate markers expressed in each cell and its neighbours, and the number of neighbouring cells. We further include the position of a cell relative to the centre of the ICM as a global positional feature. Our analyses reveal a local three-dimensional pattern that is already present in early blastocysts: 1) Cells expressing the highest NANOG levels are surrounded by approximately nine neighbours, while 2) cells expressing GATA6 cluster according to their GATA6 levels. This local pattern evolves into a global pattern in the ICM that starts to emerge in mid blastocysts. We show that FGF/MAPK signalling is involved in the three-dimensional distribution of the cells and, using a mutant background, we further show that the GATA6 neighbourhood is regulated by NANOG. Our quantitative study suggests that the three-dimensional cell neighbourhood plays a role in Epi and PrE precursor specification. Our results highlight the importance of analysing the three-dimensional cell neighbourhood while investigating cell fate decisions during early mouse embryonic development.

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.

Recent Advances of the Hippo/YAP Signaling Pathway in Brain Development and Glioma

The Hippo signaling pathway is highly conserved from Drosophila melanogaster to mammals and plays a crucial role in organ size control, tissue regeneration, and tumor suppression. The Yes-associated protein (YAP) is an important transcriptional co-activator that is negatively regulated by the Hippo signaling pathway. The Hippo signaling pathway is also regulated by various upstream regulators, such as cell polarity, adhesion proteins, and other signaling pathways (the Wnt/β-catenin, Notch, and MAPK pathways). Recently, accumulated evidence suggests that the Hippo/YAP signaling pathway plays important roles in central nervous system development and brain tumor, including glioma. In this review, we summarize the results of recent studies on the physiological effect of the Hippo/YAP signaling pathway in neural stem cells, neural progenitor cells, and glial cells. In particular, we also focus on the expression of MST1/2, LATS1/2, and the downstream effector YAP, in glioma, and offer a review of the latest research of the Hippo/YAP signaling pathway in glioma pathogenesis. Finally, we also present future research directions and potential therapeutic strategies for targeting the Hippo/YAP signaling in glioma.

Notch signaling: Its essential roles in bone and craniofacial development

Notch is a cell–cell signaling pathway that is involved in a host of activities including development, oncogenesis, skeletal homeostasis, and much more. More specifically, recent research has demonstrated the importance of Notch signaling in osteogenic differentiation, bone healing, and in the development of the skeleton. The craniofacial skeleton is complex and understanding its development has remained an important focus in biology. In this review we briefly summarize what recent research has revealed about Notch signaling and the current understanding of how the skeleton, skull, and face develop. We then discuss the crucial role that Notch plays in both craniofacial development and the skeletal system, and what importance it may play in the future.

Specification of the First Mammalian Cell Lineages In Vivo and In Vitro

Our understanding of how the first mammalian cell lineages arise has been shaped largely by studies of the preimplantation mouse embryo. Painstaking work over many decades has begun to reveal how a single totipotent cell is transformed into a multilayered structure representing the foundations of the body plan. Here, we review how the first lineage decision is initiated by epigenetic regulation but consolidated by the integration of morphological features and transcription factor activity. The establishment of pluripotent and multipotent stem cell lines has enabled deeper analysis of molecular and epigenetic regulation of cell fate decisions. The capability to assemble these stem cells into artificial embryos is an exciting new avenue of research that offers a long-awaited window into cell fate specification in the human embryo. Together, these approaches are poised to profoundly increase our understanding of how the first lineage decisions are made during mammalian embryonic development.

Glycolysis-Independent Glucose Metabolism Distinguishes TE from ICM Fate during Mammalian Embryogenesis

The mouse embryo undergoes compaction at the 8-cell stage, and its transition to 16 cells generates polarity such that the outer apical cells are trophectoderm (TE) precursors and the inner cell mass (ICM) gives rise to the embryo. Here, we report that this first cell fate specification event is controlled by glucose. Glucose does not fuel mitochondrial ATP generation, and glycolysis is dispensable for blastocyst formation. Furthermore, glucose does not help synthesize amino acids, fatty acids, and nucleobases. Instead, glucose metabolized by the hexosamine biosynthetic pathway (HBP) allows nuclear localization of YAP1. In addition, glucose-dependent nucleotide synthesis by the pentose phosphate pathway (PPP), along with sphingolipid (S1P) signaling, activates mTOR and allows translation of Tfap2c. YAP1, TEAD4, and TFAP2C interact to form a complex that controls TE-specific gene transcription. Glucose signaling has no role in ICM specification, and this process of developmental metabolism specifically controls TE cell fate.

Effect of Downregulating the Hippo Pathway Members YAP1 and LATS2 Transcripts on Early Development and Gene Expression Involved in Differentiation in Porcine Embryos

In mouse development, differentiation of the inner cell mass (ICM) and trophectoderm (TE) during the transition from the morula to blastocyst stage is regulated by the Hippo pathway; however, the functions of the Hippo pathway in porcine embryogenesis have not been investigated. In the present study, we examined the gene expression patterns of the Hippo pathway members yes-associated protein 1 (YAP1) and large tumor suppressor 2 (LATS2) and the functions of these genes during porcine preimplantation development using RNA interference. Both YAP1 and LATS2 mRNA levels were shown high in the in vitro matured oocytes and 1-cell stage embryos and fell progressively with development. YAP1 nuclear localization was detected at the morula and blastocyst stages. Downregulation of either YAP1 or LATS2 inhibited porcine preimplantation development and affected the expression levels of POU class 5 homeobox 1 (OCT-4) and SRY-related HMG-box gene 2 (SOX2), transcription factors necessary for the ICM/TE differentiation. Taken together, YAP1 and LATS2 are essential for porcine preimplantation development, and it is possible that the Hippo pathway has important roles in porcine ICM/TE segregation.

ROCK and RHO Playlist for Preimplantation Development: Streaming to HIPPO Pathway and Apicobasal Polarity in the First Cell Differentiation

In placental mammalian development, the first cell differentiation produces two distinct lineages that emerge according to their position within the embryo: the trophectoderm (TE, placenta precursor) differentiates in the surface, while the inner cell mass (ICM, fetal body precursor) forms inside. Here, we discuss how such position-dependent lineage specifications are regulated by the RHOA subfamily of small GTPases and RHO-associated coiled-coil kinases (ROCK). Recent studies in mouse show that activities of RHO/ROCK are required to promote TE differentiation and to concomitantly suppress ICM formation. RHO/ROCK operate through the HIPPO signaling pathway, whose cell position-specific modulation is central to establishing unique gene expression profiles that confer cell fate. In particular, activities of RHO/ROCK are essential in outside cells to promote nuclear localization of transcriptional co-activators YAP/TAZ, the downstream effectors of HIPPO signaling. Nuclear localization of YAP/TAZ depends on the formation of apicobasal polarity in outside cells, which requires activities of RHO/ROCK. We propose models of how RHO/ROCK regulate lineage specification and lay out challenges for future investigations to deepen our understanding of the roles of RHO/ROCK in preimplantation development. Finally, as RHO/ROCK may be inhibited by certain pharmacological agents, we discuss their potential impact on human preimplantation development in relation to fertility preservation in women.

Transcriptional and posttranscriptional regulation of the SMC-selective blood pressure-associated gene, ARHGAP42

We previously showed that ARHGAP42 is a smooth muscle cell (SMC)-selective, RhoA-specific GTPase activating protein that regulates blood pressure and that a minor allele single nucleotide variation within a DNAse hypersensitive regulatory element in intron1 (Int1DHS) increased ARHGAP42 expression by promoting serum response factor binding. The goal of the current study was to identify additional transcriptional and posttranscriptional mechanisms that control ARHGAP42 expression. Using deletion/mutation, gel shift, and chromatin immunoprecipitation experiments, we showed that recombination signal binding protein for immunoglobulin κ-J region (RBPJ) and TEA domain family member 1 (TEAD1) binding to a conserved core region was required for full IntDHS transcriptional activity. Importantly, overexpression of the notch intracellular domain (NICD) or plating SMCs on recombinant jagged-1 increased IntDHS activity and endogenous ARHGAP42 expression while siRNA-mediated knockdown of TEAD1 inhibited ARHGAP42 mRNA levels. Re-chromatin immunoprecipitation experiments indicated that RBPJ and TEAD1 were bound to the Int1DHS enhancer at the same time, and coimmunoprecipitation assays indicated that these factors interacted physically. Our results also suggest TEAD1 and RBPJ bound cooperatively to the Int1DHS and that the presence of TEAD1 promoted the recruitment of NICD by RBPJ. Finally, we showed that ARHGAP42 expression was inhibited by micro-RNA 505 (miR505) which interacted with the ARHGAP42 3'-untranslated region (UTR) to facilitate its degradation and by AK124326, a long noncoding RNA that overlaps with the ARHGAP42 transcription start site on the opposite DNA strand. Since siRNA-mediated depletion of AK124326 was associated with increased H3K9 acetylation and RNA Pol-II binding at the ARHGAP42 gene, it is likely that AK124326 inhibits ARHGAP42 transcription.NEW & NOTEWORTHY First, RBPJ and TEAD1 converge at an intronic enhancer to regulate ARHGAP42 expression in SMCs. Second, TEAD1 and RBPJ interact physically and bind cooperatively to the ARHGAP42 enhancer. Third, miR505 interacts with the ARHGAP42 3'-UTR to facilitate its degradation. Finally, LncRNA, AK124326, inhibits ARHGAP42 transcription.

Hippo-Yap/Taz signaling: Complex network interactions and impact in epithelial cell behavior

The Hippo pathway has emerged as a crucial integrator of signals in biological events from development to adulthood and in diseases. Although extensively studied in Drosophila and in cell cultures, major gaps of knowledge still remain on how this pathway functions in mammalian systems. The pathway consists of a growing number of components, including core kinases and adaptor proteins, which control the subcellular localization of the transcriptional co-activators Yap and Taz through phosphorylation of serines at key sites. When localized to the nucleus, Yap/Taz interact with TEAD transcription factors to induce transcriptional programs of proliferation, stemness, and growth. In the cytoplasm, Yap/Taz interact with multiple pathways to regulate a variety of cellular functions or are targeted for degradation. The Hippo pathway receives cues from diverse intracellular and extracellular inputs, including growth factor and integrin signaling, polarity complexes, and cell-cell junctions. This review highlights the mechanisms of regulation of Yap/Taz nucleocytoplasmic shuttling and their implications for epithelial cell behavior using the lung as an intriguing example of this paradigm. This article is categorized under: Gene Expression and Transcriptional Hierarchies > Regulatory Mechanisms Signaling Pathways > Cell Fate Signaling Establishment of Spatial and Temporal Patterns > Cytoplasmic Localization.

Genome-wide histone modification profiling of inner cell mass and trophectoderm of bovine blastocysts by RAT-ChIP

Chromatin immunoprecipitation coupled with next-generation sequencing (ChIP-seq) has revolutionized our understanding of chromatin-related biological processes. The method, however, requires thousands of cells and has therefore limited applications in situations where cell numbers are limited. Here we describe a novel method called Restriction Assisted Tagmentation Chromatin Immunoprecipitation (RAT-ChIP) that enables global histone modification profiling from as few as 100 cells. The method is simple, cost-effective and takes a single day to complete. We demonstrate the sensitivity of the method by deriving the first genome-wide maps of histone H3K4me3 and H3K27me3 modifications of inner cell mass and trophectoderm of bovine blastocyst stage embryos.

The first cell fate decision in pre-implantation mouse embryos

Fertilization happens when sperm and oocytes meet, which is a complicated process involving many important types of biological activation. Beginning in the 2-cell stage, an important event referred to as zygotic genome activation (ZGA) occurs, which governs the subsequent development of the embryo. In ZGA, multiple epigenetic modifications are involved and critical for pre-implantation development. These changes occur after ZGA, resulting in blastomeres segregate into two different lineages. Some blastomeres develop into the inner cell mass (ICM), and others develop into the trophectoderm (TE), which is considered the first cell fate decision. How this process is initiated and the exact molecular mechanisms involved are fascinating questions that remain to be answered. In this review, we introduce some possible developmental models of the first cell fate decision and discuss the signalling pathways and transcriptional networks regulating this process.

Maternal factors regulating preimplantation development in mice

Mammalian embryogenesis depends on maternal factors accumulated in eggs prior to fertilization and on placental transfers later in gestation. In this review, we focus on initial events when the organism has insufficient newly synthesized embryonic factors to sustain development. These maternal factors regulate preimplantation embryogenesis both uniquely in pronuclear formation, genome reprogramming and cell fate determination and more universally in regulating cell division, transcription and RNA metabolism. Depletion, disruption or inappropriate persistence of maternal factors can result in developmental defects in early embryos. To better understand the origins of these maternal effects, we include oocyte maturation processes that are responsible for their production. We focus on recent publications and reference comprehensive reviews that include earlier scientific literature of early mouse development.

A framework for TRIM21-mediated protein depletion in early mouse embryos: recapitulation of Tead4 null phenotype over three days

Background

While DNA and RNA methods are routine to disrupt the expression of specific genes, complete understanding of developmental processes requires also protein methods, because: oocytes and early embryos accumulate proteins and these are not directly affected by DNA and RNA methods. When proteins in the oocyte encounter a specific antibody and the TRIpartite Motiv-containing 21 (TRIM21) ubiquitin-protein ligase, they can be committed to degradation in the proteasome, producing a transient functional knock-out that reveals the role of the protein. However, there are doubts about whether this targeted proteolysis could be successfully used to study mammalian development, because duration of the transient effect is unknown, and also because amounts of reagents delivered must be adequate in relation to the amount of target protein, which is unknown, too.

Results

We show that the mouse egg contains up to 1E-02 picomoles/protein, as estimated by mass spectrometry using the intensity-based absolute quantification (iBAQ) algorithm. However, the egg can only accommodate ≈1E-04 picomoles of antibody or TRIM21 without incurring toxic effects. Within this framework, we demonstrate that TRIM21-mediated protein depletion efficiently disrupts the embryonic process of trophectoderm formation, which critically depends on the TEA domain family member 4 (Tead4) gene. TEAD4 depletion starting at the 1-cell stage lasts for 3 days prior to a return of gene and protein expression to baseline. This time period is long enough to result in a phenotype entirely consistent with that of the published null mutation and RNA interference studies: significant underexpression of trophectodermal genes Cdx2 and Gata3 and strongly impaired ability of embryos to cavitate and implant in the uterus. Omics data are available via ProteomeXchange (PXD012613) and GEO (GSE124844).

Conclusions

TRIM21-mediated protein depletion can be an effective means to disrupt gene function in mouse development, provided the target gene is chosen carefully and the method is tuned accurately. The knowledge gathered in this study provides the basic know-how (prerequisites, requirements, limitations) to expedite the protein depletion of other genes besides Tead4.

TEAD4, YAP1 and WWTR1 prevent the premature onset of pluripotency prior to the 16-cell stage

In mice, pluripotent cells are thought to derive from cells buried inside the embryo around the 16-cell stage. Sox2 is the only pluripotency gene known to be expressed specifically within inside cells at this stage. To understand how pluripotency is established, we therefore investigated the mechanisms regulating the initial activation of Sox2 expression. Surprisingly, Sox2 expression initiated normally in the absence of both Nanog and Oct4 (Pou5f1), highlighting differences between embryo and stem cell models of pluripotency. However, we observed precocious ectopic expression of Sox2 prior to the 16-cell stage in the absence of Yap1, Wwtr1 and Tead4. Interestingly, the repression of premature Sox2 expression was sensitive to LATS kinase activity, even though LATS proteins normally do not limit activity of TEAD4, YAP1 and WWTR1 during these early stages. Finally, we present evidence for direct transcriptional repression of Sox2 by YAP1, WWTR1 and TEAD4. Taken together, our observations reveal that, while embryos are initially competent to express Sox2 as early as the four-cell stage, transcriptional repression prevents the premature expression of Sox2, thereby restricting the pluripotency program to the stage when inside cells are first created.

Highlighted Article: The pluripotency marker SOX2 is not initially regulated by OCT4 and NANOG, but by HIPPO pathway members during the first 2 days of mouse embryogenesis.

Morphogenesis of extra-embryonic tissues directs the remodelling of the mouse embryo at implantation

Mammalian embryos change shape dramatically upon implantation. The cellular and molecular mechanism underlying this transition are largely unknown. Here, we show that this transition is directed by cross talk between the embryonic epiblast and the first extra-embryonic tissue, the trophectoderm. Specifically, we show via visualisation of a Cdx2-GFP reporter line and pharmacologically mediated loss and gain of function experiments that the epiblast provides FGF signal that results in differential fate acquisition in the multipotent trophectoderm leading to the formation of a tissue boundary within this tissue. The trophectoderm boundary becomes essential for expansion of the tissue into a multi-layered epithelium. Folding of this multi-layered trophectoderm induces spreading of the second extra-embryonic tissue, the primitive endoderm. Together, these events remodel the pre-implantation embryo into its post-implantation cylindrical shape. Our findings uncover how communication between embryonic and extra-embryonic tissues provides positional cues to drive shape changes in mammalian development during implantation.

Generation of Cdx2-mCherry knock-in murine ES cell line to model trophectoderm and intestinal lineage differentiation

Caudal-type homeobox 2 (.Cdx2) transcription factor is an essential regulator of differentiation to the intestinal epithelium, somatic mesoderm and trophectoderm function in the mouse. However, the regulation of Cdx2 in these processes is poorly understood. Separation of viable Cdx2 expressing cells during differentiation for downstream experiments is not possible due to its nuclear localization, limiting experimental possibilities and studying Cdx2 regulation. Here, we report generation of a Cdx2-mCherry knock-in reporter mouse embryonic stem cell line (TCMC), for modeling and studying in vitro differentiation of mESCs to intestinal epithelia, somatic mesoderm, and trophectoderm.