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The human blastocyst contains the pluripotent epiblast from which human embryonic stem cells (hESCs) can be derived. ACTIVIN/NODAL signaling maintains expression of the transcription factor NANOG and in vitro propagation of hESCs. It is unknown whether this reflects a functional requirement for epiblast development in human embryos. Here, we characterized NODAL signaling activity during pre-implantation human development. We showed that NANOG is an early molecular marker restricted to the nascent human pluripotent epiblast and was initiated prior to the onset of NODAL signaling. We further demonstrated that expression of pluripotency-associated transcription factors NANOG, SOX2, OCT4, and KLF17 were maintained in the epiblast in the absence of NODAL signaling activity. Genome-wide transcriptional analysis showed that NODAL signaling inhibition did not decrease NANOG transcription or impact the wider pluripotency-associated gene regulatory network. These data suggest differences in the signaling requirements regulating pluripotency in the pre-implantation human epiblast compared with existing hESC culture.

Many mammals can temporally uncouple conception from parturition by pacing down their development around the blastocyst stage. In mice, this dormant state is achieved by decreasing the activity of the growth-regulating mTOR signaling pathway. It is unknown whether this ability is conserved in mammals in general and in humans in particular. Here, we show that decreasing the activity of the mTOR signaling pathway induces human pluripotent stem cells (hPSCs) and blastoids to enter a dormant state with limited proliferation, developmental progression, and capacity to attach to endometrial cells. These in vitro assays show that, similar to other species, the ability to enter dormancy is active in human cells around the blastocyst stage and is reversible at both functional and molecular levels. The pacing of human blastocyst development has potential implications for reproductive therapies.

Bone Morphogenic Protein (BMP) signaling plays an essential and highly conserved role in embryo axial patterning in animal species. However, in mammalian embryos, which develop inside the mother, early development includes a preimplantation stage, which does not occur in externally developing embryos. During preimplantation, the epiblast is segregated from extraembryonic lineages that enable implantation and development in utero. Yet, the requirement for BMP signaling in is imprecisely defined in mouse early embryos. Here, we show that, in contrast to prior reports, BMP signaling (SMAD1/5/9 phosphorylation) is not detectable until implantation when it is detected in the primitive endoderm - an extraembryonic lineage. Moreover, preimplantation development appears normal following deletion of maternal and zygotic Smad4, an essential effector of canonical BMP signaling. In fact, mice lacking maternal Smad4 are viable. Finally, we uncover a new requirement for zygotic Smad4 in epiblast scaling and cavitation immediately after implantation, via a mechanism involving FGFR/ERK attenuation. Altogether, our results demonstrate no role for BMP4/SMAD4 in the first lineage decisions during mouse development. Rather, multi-pathway signaling among embryonic and extraembryonic cell types drives epiblast morphogenesis post-implantation.

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Recent advances in single-cell omics have transformed characterisation of cell types in challenging-to-study biological contexts. In contexts with limited single-cell samples, such as the early human embryo inference of transcription factor-gene regulatory network (GRN) interactions is especially difficult. Here, we assessed application of different linear or non-linear GRN predictions to single-cell simulated and human embryo transcriptome datasets. We also compared how expression normalisation impacts on GRN predictions, finding that transcripts per million reads outperformed alternative methods. GRN inferences were more reproducible using a non-linear method based on mutual information (MI) applied to single-cell transcriptome datasets refined with chromatin accessibility (CA) (called MICA), compared with alternative network prediction methods tested. MICA captures complex non-monotonic dependencies and feedback loops. Using MICA, we generated the first GRN inferences in early human development. MICA predicted co-localisation of the AP-1 transcription factor subunit proto-oncogene JUND and the TFAP2C transcription factor AP-2γ in early human embryos. Overall, our comparative analysis of GRN prediction methods defines a pipeline that can be applied to single-cell multi-omics datasets in especially challenging contexts to infer interactions between transcription factor expression and target gene regulation.

The study of cellular and developmental processes in physiologically relevant three-dimensional (3D) systems facilitates an understanding of mechanisms underlying cell fate, disease and injury. While cutting-edge microscopy technologies permit the routine acquisition of 3D datasets, there is currently a limited number of open-source software packages to analyse such images. Here we describe GIANI (General Image Analysis of Nuclei-based Images; https://djpbarry.github.io/Giani), new software for the analysis of 3D images. The design primarily facilitates segmentation of nuclei and cells, followed by quantification of morphology and protein expression. GIANI enables routine and reproducible batch-processing of large numbers of images and comes with scripting and command line tools. We demonstrate the utility of GIANI by quantifying cell morphology and protein expression in confocal images of mouse early embryos and by segmenting nuclei from light sheet microscopy images of the flour beetle embryo. We also validate the performance of the software using simulated data. More generally, we anticipate that GIANI will be a useful tool for researchers in a variety of biomedical fields.

Current knowledge of the transcriptional regulation of human pluripotency is incomplete, with lack of interspecies conservation observed. Single-cell transcriptomics analysis of human embryos previously enabled us to identify transcription factors, including the zinc-finger protein KLF17, that are enriched in the human epiblast and naïve human embryonic stem cells (hESCs). Here, we show that KLF17 is expressed coincident with the known pluripotency-associated factors NANOG and SOX2 across human blastocyst development. We investigate the function of KLF17 using primed and naïve hESCs for gain- and loss-of-function analyses. We find that ectopic expression of KLF17 in primed hESCs is sufficient to induce a naïve-like transcriptome and that KLF17 can drive transgene-mediated resetting to naïve pluripotency. This implies a role for KLF17 in establishing naïve pluripotency. However, CRISPR-Cas9-mediated knockout studies reveal that KLF17 is not required for naïve pluripotency acquisition in vitro. Transcriptome analysis of naïve hESCs identifies subtle effects on metabolism and signalling pathways following KLF17 loss of function, and possible redundancy with other KLF paralogues. Overall, we show that KLF17 is sufficient, but not necessary, for naïve pluripotency under the given in vitro conditions.

The Human Developmental Cell Atlas (HDCA) initiative, which is part of the Human Cell Atlas, aims to create a comprehensive reference map of cells during development. This will be critical to understanding normal organogenesis, the effect of mutations, environmental factors and infectious agents on human development, congenital and childhood disorders, and the cellular basis of ageing, cancer and regenerative medicine. Here we outline the HDCA initiative and the challenges of mapping and modelling human development using state-of-the-art technologies to create a reference atlas across gestation. Similar to the Human Genome Project, the HDCA will integrate the output from a growing community of scientists who are mapping human development into a unified atlas. We describe the early milestones that have been achieved and the use of human stem-cell-derived cultures, organoids and animal models to inform the HDCA, especially for prenatal tissues that are hard to acquire. Finally, we provide a roadmap towards a complete atlas of human development.

The signalling pathways that maintain primed human pluripotent stem cells (hPSCs) have been well characterised, revealing a critical role for TGFβ/Activin/Nodal signalling. In contrast, the signalling requirements of naive human pluripotency have not been fully established. Here, we demonstrate that TGFβ signalling is required to maintain naive hPSCs. The downstream effector proteins - SMAD2/3 - bind common sites in naive and primed hPSCs, including shared pluripotency genes. In naive hPSCs, SMAD2/3 additionally bind to active regulatory regions near to naive pluripotency genes. Inhibiting TGFβ signalling in naive hPSCs causes the downregulation of SMAD2/3-target genes and pluripotency exit. Single-cell analyses reveal that naive and primed hPSCs follow different transcriptional trajectories after inhibition of TGFβ signalling. Primed hPSCs differentiate into neuroectoderm cells, whereas naive hPSCs transition into trophectoderm. These results establish that there is a continuum for TGFβ pathway function in human pluripotency spanning a developmental window from naive to primed states.

CRISPR-Cas9 mutagenesis facilitates the investigation of gene function in a number of developmental and cellular contexts. Human pluripotent stem cells (hPSCs), either embryonic or induced, are a tractable cellular model to investigate molecular mechanisms involved in early human development and cell fate decisions. hPSCs also have broad potential in regenerative medicine to model, investigate, and ameliorate diseases. Here, we provide an optimized protocol for efficient CRISPR-Cas9 genome editing of hPSCs to investigate the functional role of genes by engineering null mutations. We emphasize the importance of screening single guide RNAs (sgRNAs) to identify those with high targeting efficiency for generation of clonally derived null mutant hPSC lines. We provide important considerations for targeting genes that may have a role in hPSC maintenance. We also present methods to evaluate the on-target mutation spectrum and unintended karyotypic changes. © 2021 The Authors. Current Protocols published by Wiley Periodicals LLC. Basic Protocol 1: Selecting and ligating sgRNAs into expression plasmids Basic Protocol 2: Validation of sgRNA via in vitro transcription and cleavage assay Basic Protocol 3: Nucleofection of primed human embryonic stem cells Basic Protocol 4: MiSeq analysis of indel mutations Basic Protocol 5: Single cell cloning of targeted hPSCs Basic Protocol 6: Karyotyping of targeted hPSCs.

CRISPR-Cas9 genome editing is a promising technique for clinical applications, such as the correction of disease-associated alleles in somatic cells. The use of this approach has also been discussed in the context of heritable editing of the human germ line. However, studies assessing gene correction in early human embryos report low efficiency of mutation repair, high rates of mosaicism, and the possibility of unintended editing outcomes that may have pathologic consequences. We developed computational pipelines to assess single-cell genomics and transcriptomics datasets from OCT4 () CRISPR-Cas9-targeted and control human preimplantation embryos. This allowed us to evaluate on-target mutations that would be missed by more conventional genotyping techniques. We observed loss of heterozygosity in edited cells that spanned regions beyond the on-target locus, as well as segmental loss and gain of chromosome 6, on which the gene is located. Unintended genome editing outcomes were present in ∼16% of the human embryo cells analyzed and spanned 4-20 kb. Our observations are consistent with recent findings indicating complexity at on-target sites following CRISPR-Cas9 genome editing. Our work underscores the importance of further basic research to assess the safety of genome editing techniques in human embryos, which will inform debates about the potential clinical use of this technology.

The International Society for Stem Cell Research has updated its Guidelines for Stem Cell Research and Clinical Translation in order to address advances in stem cell science and other relevant fields, together with the associated ethical, social, and policy issues that have arisen since the last update in 2016. While growing to encompass the evolving science, clinical applications of stem cells, and the increasingly complex implications of stem cell research for society, the basic principles underlying the Guidelines remain unchanged, and they will continue to serve as the standard for the field and as a resource for scientists, regulators, funders, physicians, and members of the public, including patients. A summary of the key updates and issues is presented here.

The ISSCR Guidelines for Stem Cell Research and Clinical Translation were last revised in 2016. Since then, rapid progress has been made in research areas related to in vitro culture of human embryos, creation of stem cell-based embryo models, and in vitro gametogenesis. Therefore, a working group of international experts was convened to review the oversight process and provide an update to the guidelines. This report captures the discussion and summarizes the major recommendations made by this working group, with a specific emphasis on updating the categories of review and engagement with the specialized scientific and ethical oversight process.

Mammalian telomeres protect chromosome ends from aberrant DNA repair. TRF2, a component of the telomere-specific shelterin protein complex, facilitates end protection through sequestration of the terminal telomere repeat sequence within a lariat T-loop structure. Deleting TRF2 (also known as TERF2) in somatic cells abolishes T-loop formation, which coincides with telomere deprotection, chromosome end-to-end fusions and inviability. Here we establish that, by contrast, TRF2 is largely dispensable for telomere protection in mouse pluripotent embryonic stem (ES) and epiblast stem cells. ES cell telomeres devoid of TRF2 instead activate an attenuated telomeric DNA damage response that lacks accompanying telomere fusions, and propagate for multiple generations. The induction of telomere dysfunction in ES cells, consistent with somatic deletion of Trf2 (also known as Terf2), occurs only following the removal of the entire shelterin complex. Consistent with TRF2 being largely dispensable for telomere protection specifically during early embryonic development, cells exiting pluripotency rapidly switch to TRF2-dependent end protection. In addition, Trf2-null embryos arrest before implantation, with evidence of strong DNA damage response signalling and apoptosis specifically in the non-pluripotent compartment. Finally, we show that ES cells form T-loops independently of TRF2, which reveals why TRF2 is dispensable for end protection during pluripotency. Collectively, these data establish that telomere protection is solved by distinct mechanisms in pluripotent and somatic tissues.

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.

Current understandings of cell specification in early mammalian pre-implantation development are based mainly on mouse studies. The first lineage differentiation event occurs at the morula stage, with outer cells initiating a trophectoderm (TE) placental progenitor program. The inner cell mass arises from inner cells during subsequent developmental stages and comprises precursor cells of the embryo proper and yolk sac. Recent gene-expression analyses suggest that the mechanisms that regulate early lineage specification in the mouse may differ in other mammals, including human and cow. Here we show the evolutionary conservation of a molecular cascade that initiates TE segregation in human, cow and mouse embryos. At the morula stage, outer cells acquire an apical-basal cell polarity, with expression of atypical protein kinase C (aPKC) at the contact-free domain, nuclear expression of Hippo signalling pathway effectors and restricted expression of TE-associated factors such as GATA3, which suggests initiation of a TE program. Furthermore, we demonstrate that inhibition of aPKC by small-molecule pharmacological modulation or Trim-Away protein depletion impairs TE initiation at the morula stage. Our comparative embryology analysis provides insights into early lineage specification and suggests that a similar mechanism initiates a TE program in human, cow and mouse embryos.

Although somatic homologous pairing is common in Drosophila it is not generally observed in mammalian cells. However, a number of regions have recently been shown to come into close proximity with their homologous allele, and it has been proposed that pairing might be involved in the establishment or maintenance of monoallelic expression. Here, we investigate the pairing properties of various imprinted and non-imprinted regions in mouse tissues and ES cells. We find by allele-specific 4C-Seq and DNA FISH that the Kcnq1 imprinted region displays frequent pairing but that this is not dependent on monoallelic expression. We demonstrate that pairing involves larger chromosomal regions and that the two chromosome territories come close together. Frequent pairing is not associated with imprinted status or DNA repair, but is influenced by chromosomal location and transcription. We propose that homologous pairing is not exclusive to specialised regions or specific functional events, and speculate that it provides the cell with the opportunity of trans-allelic effects on gene regulation.

BMP is thought to induce hESC differentiation toward multiple lineages including mesoderm and trophoblast. The BMP-induced trophoblast phenotype is a long-standing paradox in stem cell biology. Here we readdressed BMP function in hESCs and mouse epiblast-derived cells. We found that BMP4 cooperates with FGF2 (via ERK) to induce mesoderm and to inhibit endoderm differentiation. These conditions induced cells with high levels of BRACHYURY (BRA) that coexpressed CDX2. BRA was necessary for and preceded CDX2 expression; both genes were essential for expression not only of mesodermal genes but also of trophoblast-associated genes. Maximal expression of the latter was seen in the absence of FGF but these cells coexpressed mesodermal genes and moreover they differed in cell surface and epigenetic properties from placental trophoblast. We conclude that BMP induces human and mouse pluripotent stem cells primarily to form mesoderm, rather than trophoblast, acting through BRA and CDX2.