A conserved Oct4/POUV-dependent network links adhesion and migration to progenitor maintenance

The primitive endoderm supports lineage plasticity to enable regulative development

Mammalian blastocyst formation involves the specification of the trophectoderm followed by the differentiation of the inner cell mass into embryonic epiblast and extra-embryonic primitive endoderm (PrE). During this time, the embryo maintains a window of plasticity and can redirect its cellular fate when challenged experimentally. In this context, we found that the PrE alone was sufficient to regenerate a complete blastocyst and continue post-implantation development. We identify an in vitro population similar to the early PrE in vivo that exhibits the same embryonic and extra-embryonic potency and can form complete stem cell-based embryo models, termed blastoids. Commitment in the PrE is suppressed by JAK/STAT signaling, collaborating with OCT4 and the sustained expression of a subset of pluripotency-related transcription factors that safeguard an enhancer landscape permissive for multi-lineage differentiation. Our observations support the notion that transcription factor persistence underlies plasticity in regulative development and highlight the importance of the PrE in perturbed development.

The Oct4-related PouV gene, pou5f3, mediates isthmus development in zebrafish by directly and dynamically regulating pax2a

Using a transgenic zebrafish line harboring a heat-inducible dominant-interference pou5f3 gene (en-pou5f3), we reported that this PouV gene is involved in isthmus development at the midbrain-hindbrain boundary (MHB), which patterns the midbrain and cerebellum. Importantly, the functions of pou5f3 reportedly differ before and after the end of gastrulation. In the present study, we examined in detail the effects of en-pou5f3 induction on isthmus development during embryogenesis. When en-pou5f3 was induced around the end of gastrulation (bud stage), the isthmus was abrogated or deformed by the end of somitogenesis (24 hours post-fertilization). At this stage, the expression of MHB markers -- such as pax2a, fgf8a, wnt1, and gbx2 -- was absent in embryos lacking the isthmus structure, whereas it was present, although severely distorted, in embryos with a deformed isthmus. We further found that, after en-pou5f3 induction at late gastrulation, pax2a, fgf8a, and wnt1 were immediately and irreversibly downregulated, whereas the expression of en2a and gbx2 was reduced only weakly and slowly. Induction of en-pou5f3 at early somite stages also immediately downregulated MHB genes, particularly pax2a, but their expression was restored later. Overall, the data suggested that pou5f3 directly upregulates at least pax2a and possibly fgf8a and wnt1, which function in parallel in establishing the MHB, and that the role of pou5f3 dynamically changes around the end of gastrulation. We next examined the transcriptional regulation of pax2a using both in vitro and in vivo reporter analyses; the results showed that two upstream 1.0-kb regions with sequences conserved among vertebrates specifically drove transcription at the MHB. These reporter analyses confirmed that development of the isthmic organizer is regulated by PouV through direct regulation of pax2/pax2a in vertebrate embryos.

Shared features of blastula and neural crest stem cells evolved at the base of vertebrates

The neural crest is vertebrate-specific stem cell population that helped drive the origin and evolution of the vertebrate clade. A distinguishing feature of these stem cells is their multi-germ layer potential, which has drawn developmental and evolutionary parallels to another stem cell population-pluripotent embryonic stem cells (animal pole cells or ES cells) of the vertebrate blastula. Here, we investigate the evolutionary origins of neural crest potential by comparing neural crest and pluripotency gene regulatory networks (GRNs) in both jawed ( Xenopus ) and jawless (lamprey) vertebrates. Through comparative gene expression analysis and transcriptomics, we reveal an ancient evolutionary origin of shared regulatory factors between neural crest and pluripotency GRNs that dates back to the last common ancestor of extant vertebrates. Focusing on the key pluripotency factor pou5 (formerly oct4), we show that the lamprey genome encodes a pou5 ortholog that is expressed in animal pole cells, as in jawed vertebrates, but is absent from the neural crest. However, gain-of-function experiments show that both lamprey and Xenopus pou5 enhance neural crest formation, suggesting that pou5 was lost from the neural crest of jawless vertebrates. Finally, we show that pou5 is required for neural crest specification in jawed vertebrates and that it acquired novel neural crest-enhancing activity after evolving from an ancestral pou3 -like clade that lacks this functionality. We propose that a pluripotency-neural crest GRN was assembled in stem vertebrates and that the multi-germ layer potential of the neural crest evolved by deploying this regulatory program.

A single-cell, time-resolved profiling of Xenopus mucociliary epithelium reveals nonhierarchical model of development

The specialized cell types of the mucociliary epithelium (MCE) lining the respiratory tract enable continuous airway clearing, with its defects leading to chronic respiratory diseases. The molecular mechanisms driving cell fate acquisition and temporal specialization during mucociliary epithelial development remain largely unknown. Here, we profile the developing Xenopus MCE from pluripotent to mature stages by single-cell transcriptomics, identifying multipotent early epithelial progenitors that execute multilineage cues before specializing into late-stage ionocytes and goblet and basal cells. Combining in silico lineage inference, in situ hybridization, and single-cell multiplexed RNA imaging, we capture the initial bifurcation into early epithelial and multiciliated progenitors and chart cell type emergence and fate progression into specialized cell types. Comparative analysis of nine airway atlases reveals an evolutionary conserved transcriptional module in ciliated cells, whereas secretory and basal types execute distinct function-specific programs across vertebrates. We uncover a continuous nonhierarchical model of MCE development alongside a data resource for understanding respiratory biology.

Evolutionary origin of vertebrate OCT4/POU5 functions in supporting pluripotency

The support of pluripotent cells over time is an essential feature of development. In eutherian embryos, pluripotency is maintained from naïve states in peri-implantation to primed pluripotency at gastrulation. To understand how these states emerged, we reconstruct the evolutionary trajectory of the Pou5 gene family, which contains the central pluripotency factor OCT4. By coupling evolutionary sequence analysis with functional studies in mouse embryonic stem cells, we find that the ability of POU5 proteins to support pluripotency originated in the gnathostome lineage, prior to the generation of two paralogues, Pou5f1 and Pou5f3 via gene duplication. In osteichthyans, retaining both genes, the paralogues differ in their support of naïve and primed pluripotency. The specialization of these duplicates enables the diversification of function in self-renewal and differentiation. By integrating sequence evolution, cell phenotypes, developmental contexts and structural modelling, we pinpoint OCT4 regions sufficient for naïve pluripotency and describe their adaptation over evolutionary time.

Vertebrate Cell Differentiation, Evolution, and Diseases: The Vertebrate-Specific Developmental Potential Guardians VENTX/NANOG and POU5/OCT4 Enter the Stage

During vertebrate development, embryonic cells pass through a continuum of transitory pluripotent states that precede multi-lineage commitment and morphogenesis. Such states are referred to as “refractory/naïve” and “competent/formative” pluripotency. The molecular mechanisms maintaining refractory pluripotency or driving the transition to competent pluripotency, as well as the cues regulating multi-lineage commitment, are evolutionarily conserved. Vertebrate-specific “Developmental Potential Guardians” (vsDPGs; i.e., VENTX/NANOG, POU5/OCT4), together with MEK1 (MAP2K1), coordinate the pluripotency continuum, competence for multi-lineage commitment and morphogenesis in vivo. During neurulation, vsDPGs empower ectodermal cells of the neuro-epithelial border (NEB) with multipotency and ectomesenchyme potential through an “endogenous reprogramming” process, giving rise to the neural crest cells (NCCs). Furthermore, vsDPGs are expressed in undifferentiated-bipotent neuro-mesodermal progenitor cells (NMPs), which participate in posterior axis elongation and growth. Finally, vsDPGs are involved in carcinogenesis, whereby they confer selective advantage to cancer stem cells (CSCs) and therapeutic resistance. Intriguingly, the heterogenous distribution of vsDPGs in these cell types impact on cellular potential and features. Here, we summarize the findings about the role of vsDPGs during vertebrate development and their selective advantage in evolution. Our aim to present a holistic view regarding vsDPGs as facilitators of both cell plasticity/adaptability and morphological innovation/variation. Moreover, vsDPGs may also be at the heart of carcinogenesis by allowing malignant cells to escape from physiological constraints and surveillance mechanisms.

The functional diversity of the POUV-class proteins across vertebrates

POUV is a relatively newly emerged class of POU transcription factors present in jawed vertebrates (Gnathostomata). The function of POUV-class proteins is inextricably linked to zygotic genome activation (ZGA). A large body of evidence now extends the role of these proteins to subsequent developmental stages. While some functions resemble those of other POU-class proteins and are related to neuroectoderm development, others have emerged de novo. The most notable of the latter functions is pluripotency control by Oct4 in mammals. In this review, we focus on these de novo functions in the best-studied species harbouring POUV proteins-zebrafish, Xenopus (anamniotes) and mammals (amniotes). Despite the broad diversity of their biological functions in vertebrates, POUV proteins exert a common feature related to their role in safeguarding the undifferentiated state of cells. Here we summarize numerous pieces of evidence for these specific functions of the POUV-class proteins and recap available loss-of-function data.

Cadherins in early neural development

During early neural development, changes in signalling inform the expression of transcription factors that in turn instruct changes in cell identity. At the same time, switches in adhesion molecule expression result in cellular rearrangements that define the morphology of the emerging neural tube. It is becoming increasingly clear that these two processes influence each other; adhesion molecules do not simply operate downstream of or in parallel with changes in cell identity but rather actively feed into cell fate decisions. Why are differentiation and adhesion so tightly linked? It is now over 60 years since Conrad Waddington noted the remarkable "Constancy of the Wild Type" (Waddington in Nature 183: 1654-1655, 1959) yet we still do not fully understand the mechanisms that make development so reproducible. Conversely, we do not understand why directed differentiation of cells in a dish is sometimes unpredictable and difficult to control. It has long been suggested that cells make decisions as 'local cooperatives' rather than as individuals (Gurdon in Nature 336: 772-774, 1988; Lander in Cell 144: 955-969, 2011). Given that the cadherin family of adhesion molecules can simultaneously influence morphogenesis and signalling, it is tempting to speculate that they may help coordinate cell fate decisions between neighbouring cells in the embryo to ensure fidelity of patterning, and that the uncoupling of these processes in a culture dish might underlie some of the problems with controlling cell fate decisions ex-vivo. Here we review the expression and function of cadherins during early neural development and discuss how and why they might modulate signalling and differentiation as neural tissues are formed.

Understanding axial progenitor biology in vivo and in vitro

The generation of the components that make up the embryonic body axis, such as the spinal cord and vertebral column, takes place in an anterior-to-posterior (head-to-tail) direction. This process is driven by the coordinated production of various cell types from a pool of posteriorly-located axial progenitors. Here, we review the key features of this process and the biology of axial progenitors, including neuromesodermal progenitors, the common precursors of the spinal cord and trunk musculature. We discuss recent developments in the in vitro production of axial progenitors and their potential implications in disease modelling and regenerative medicine.

RNA-seq reveals tight junction-relevant erythropoietic fate induced by OCT4 in human hair follicle mesenchymal stem cells

BACKGROUND

Human hair follicle mesenchymal stem cells (hHFMSCs) isolated from hair follicles possess multilineage differentiation potential. OCT4 is a gene critically associated with pluripotency properties. The cell morphology and adhesion of hHFMSCs significantly changed after transduction of OCT4 and two subpopulations emerged, including adherent cells and floating cell. Floating cells cultured in hematopoietic induction medium and stimulated with erythropoetic growth factors could transdifferentiate into mature erythrocytes, whereas adherent cells formed negligible hematopoietic colonies. The aim of this study was to reveal the role of cell morphology and adhesion on erythropoiesis induced by OCT4 in hHFMSCs and to characterize the molecular mechanisms involved.

METHODS

Floating cell was separated from adherent cell by centrifugation of the upper medium during cell culture. Cell size was observed through flow cytometry and cell adhesion was tested by disassociation and adhesion assays. RNA sequencing was performed to detect genome-wide transcriptomes and identify differentially expressed genes. GO enrichment analysis and KEGG pathway analysis were performed to analysis the functions and pathways enriched by differentially expressed genes. The expression of tight junction core members was verified by qPCR and Western blot. A regulatory network was constructed to figure out the relationship between cell adhesin, cytoskeleton, pluripotency, and hematopoiesis.

RESULTS

The overexpression of OCT4 influenced the morphology and adhesion of hHFMSCs. Transcripts in floating cells and adherent cells are quite different. Data analysis showed that upregulated genes in floating cells were mainly related to pluripotency, germ layer development (including hematopoiesis lineage development), and downregulated genes were mainly related to cell adhesion, cell junctions, and the cytoskeleton. Most molecules of the tight junction (TJ) pathway were downregulated and molecular homeostasis of the TJ was disturbed, as CLDNs were disrupted, and JAMs and TJPs were upregulated. The dynamic expression of cell adhesion-related gene E-cadherin and cytoskeleton-related gene ACTN2 might cause different morphology and adhesion. Finally, a regulatory network centered to OCT4 was constructed, which elucidated that he TJ pathway critically bridges pluripotency and hematopoiesis in a TJP1-dependent way.

CONCLUSIONS

Regulations of cell morphology and adhesion via the TJ pathway conducted by OCT4 might modulate hematopoiesis in hHFMSCs, thus developing potential mechanism of erythropoiesis in vitro.

Roles of OCT4 in pathways of embryonic development and cancer progression

Somatic cells may be reprogrammed to pluripotent state by ectopic expression of certain transcription factors; namely, OCT4, SOX2, KLF4 and c-MYC. However, the molecular and cellular mechanisms are not adequately understood, especially for human embryonic development. Studies during the last five years implicated importance of OCT4 in human zygotic genome activation (ZGA), patterns of OCT4 protein folding and role of specialized sequences in binding to DNA for modulation of gene expression during development. Epigenetic modulation of OCT4 gene and post translational modifications of OCT4 protein activity in the context of multiple cancers are important issues. A consensus is emerging that chromatin organization and epigenetic landscape play crucial roles for the interactions of transcription factors, including OCT4 with the promoters and/or regulatory sequences of genes associated with human embryonic development (ZGA through lineage specification) and that when the epigenome niche is deregulated OCT4 helps in cancer progression, and how OCT4 silencing in somatic cells of adult organisms may impact ageing.

Chromatin accessibility and histone acetylation in the regulation of competence in early development

As development proceeds, inductive cues are interpreted by competent tissues in a spatially and temporally restricted manner. While key inductive signaling pathways within competent cells are well-described at a molecular level, the mechanisms by which tissues lose responsiveness to inductive signals are not well understood. Localized activation of Wnt signaling before zygotic gene activation in Xenopus laevis leads to dorsal development, but competence to induce dorsal genes in response to Wnts is lost by the late blastula stage. We hypothesize that loss of competence is mediated by changes in histone modifications leading to a loss of chromatin accessibility at the promoters of Wnt target genes. We use ATAC-seq to evaluate genome-wide changes in chromatin accessibility across several developmental stages. Based on overlap with p300 binding, we identify thousands of putative cis-regulatory elements at the gastrula stage, including sites that lose accessibility by the end of gastrulation and are enriched for pluripotency factor binding motifs. Dorsal Wnt target gene promoters are not accessible after the loss of competence in the early gastrula while genes involved in mesoderm and neural crest development maintain accessibility at their promoters. Inhibition of histone deacetylases increases acetylation at the promoters of dorsal Wnt target genes and extends competence for dorsal gene induction by Wnt signaling. Histone deacetylase inhibition, however, is not sufficient to extend competence for mesoderm or neural crest induction. These data suggest that chromatin state regulates the loss of competence to inductive signals in a context-dependent manner.

Acellular Biologic Nipple-Areolar Complex Graft: In Vivo Murine and Nonhuman Primate Host Response Evaluation

There are more than 3 million breast cancer survivors living in the United States of which a significant number have undergone mastectomy followed by breast and nipple-areolar complex (NAC) reconstruction. Current strategies for NAC reconstruction are dependent on nonliving or nonpermanent techniques, including tattooing, nipple prosthetics, or surgical nipple-like structures. Described herein is a tissue engineering approach demonstrating the feasibility of an allogeneic acellular graft for nipple reconstruction. Nonhuman primate (NHP)-derived NAC tissues were decellularized and their extracellular matrix components analyzed by both proteomic and histological analyses. Decellularized NHP nipple tissue showed the removal of intact cells and greatly diminished profiles for intracellular proteins, as compared with intact NHP nipple tissue. We further evaluated the biocompatibility of decellularized grafts and their potential to support host-mediated neovascularization against commercially available acellular dermal grafts by performing in vivo studies in a murine model. A follow-up NHP pilot study evaluated the host-mediated neovascularization and re-epithelialization of onlay engrafted decellularized NAC grafts. The murine model revealed greater neovascularization in the decellularized NAC than in the commercially available control grafts, with no observed biocompatibility issues. The in vivo NHP model confirmed that the decellularized NAC grafts encourage neovascularization as well as re-epithelialization. These results support the concept that a biologically derived acellular nipple graft is a feasible approach for nipple reconstruction, supporting neovascularization in the absence of adverse systemic responses. Impact statement Currently, women in the United States most often undergo a mastectomy, followed by reconstruction, after being diagnosed with breast cancer. These breast cancer survivors are often left with nipple-areolar complex (NAC) reconstructions that are subsatisfactory, nonliving, and/or nonpermanent. Utilizing an acellular biologically derived whole NAC graft would allow these patients a living and permanent tissue engineering solution to nipple reconstruction.

N-cadherin stabilises neural identity by dampening anti-neural signals

A switch from E- to N-cadherin regulates the transition from pluripotency to neural identity, but the mechanism by which cadherins regulate differentiation was previously unknown. Here, we show that the acquisition of N-cadherin stabilises neural identity by dampening anti-neural signals. We use quantitative image analysis to show that N-cadherin promotes neural differentiation independently of its effects on cell cohesiveness. We reveal that cadherin switching diminishes the level of nuclear β-catenin, and that N-cadherin also dampens FGF activity and consequently stabilises neural fate. Finally, we compare the timing of cadherin switching and differentiation in vivo and in vitro, and find that this process becomes dysregulated during in vitro differentiation. We propose that N-cadherin helps to propagate a stable neural identity throughout the emerging neuroepithelium, and that dysregulation of this process contributes to asynchronous differentiation in culture.

Summary: As pluripotent cells undergo neural differentiation they swap E-cadherin for N-cadherin. This switch in adhesion molecules modulates signalling in order to facilitate the differentiation process.

Lineage commitment of embryonic cells involves MEK1-dependent clearance of pluripotency regulator Ventx2

During early embryogenesis, cells must exit pluripotency and commit to multiple lineages in all germ-layers. How this transition is operated in vivo is poorly understood. Here, we report that MEK1 and the Nanog-related transcription factor Ventx2 coordinate this transition. MEK1 was required to make Xenopus pluripotent cells competent to respond to all cell fate inducers tested. Importantly, MEK1 activity was necessary to clear the pluripotency protein Ventx2 at the onset of gastrulation. Thus, concomitant MEK1 and Ventx2 knockdown restored the competence of embryonic cells to differentiate. Strikingly, MEK1 appeared to control the asymmetric inheritance of Ventx2 protein following cell division. Consistently, when Ventx2 lacked a functional PEST-destruction motif, it was stabilized, displayed symmetric distribution during cell division and could efficiently maintain pluripotency gene expression over time. We suggest that asymmetric clearance of pluripotency regulators may represent an important mechanism to ensure the progressive assembly of primitive embryonic tissues.

DOI:http://dx.doi.org/10.7554/eLife.21526.001

Different reprogramming propensities in plants and mammals: Are small variations in the core network wirings responsible?

Although the plant and animal kingdoms were separated more than 1,6 billion years ago, multicellular development is for both guided by similar transcriptional, epigenetic and posttranscriptional machinery. One may ask to what extent there are similarities and differences in the gene regulation circuits and their dynamics when it comes to important processes like stem cell regulation. The key players in mouse embryonic stem cells governing pluripotency versus differentiation are Oct4, Sox2 and Nanog. Correspondingly, the WUSCHEL and CLAVATA3 genes represent a core in the Shoot Apical Meristem regulation for plants. In addition, both systems have designated genes that turn on differentiation. There is very little molecular homology between mammals and plants for these core regulators. Here, we focus on functional homologies by performing a comparison between the circuitry connecting these players in plants and animals and find striking similarities, suggesting that comparable regulatory logics have been evolved for stem cell regulation in both kingdoms. From in silico simulations we find similar differentiation dynamics. Further when in the differentiated state, the cells are capable of regaining the stem cell state. We find that the propensity for this is higher for plants as compared to mammalians. Our investigation suggests that, despite similarity in core regulatory networks, the dynamics of these can contribute to plant cells being more plastic than mammalian cells, i.e. capable to reorganize from single differentiated cells to whole plants—reprogramming. The presence of an incoherent feed-forward loop in the mammalian core circuitry could be the origin of the different reprogramming behaviour.

MicroRNAs and ectodermal specification I. Identification of miRs and miR-targeted mRNAs in early anterior neural and epidermal ectoderm

The establishment of cell lineages occurs via a dynamic progression of gene regulatory networks (GRNs) that underlie developmental commitment and differentiation. To investigate how microRNAs (miRs) function in this process, we compared miRs and miR targets at the initiation of the two major ectodermal lineages in Xenopus. We used next-generation sequencing to identify over 170 miRs expressed in midgastrula ectoderm expressing either noggin or a constitutively active BMP receptor, reflecting anterior neural or epidermal ectoderm, respectively; 125 had not previously been identified in Xenopus. We identified the locations of the pre-miR sequences in the X. laevis genome. Neural and epidermal ectoderm express broadly similar sets of miRs. To identify targets of miR-dependent translational control, we co-immunoprecipitated Argonaute-Ribonucleoprotein (Ago-RNP) complexes from early neural and epidermal ectoderm and sequenced the associated RNA. The Ago-RNP RNAs from these tissues represent overlapping, yet distinct, subsets of genes. Moreover, the profile of Ago-RNP associated genes differs substantially from the profile of total RNAs in these tissues. We generated target predictions for the "high confidence" Ago-RNP RNAs using the identified ectodermal miRs; These RNAs generally had target sites for multiple miRs. Oct4 orthologues, as well as many of their previously identified transcriptional targets, are represented in the Ago-RNP pool in both tissues, suggesting that miR-dependent regulation contributes to the downregulation of the oct4 gene regulatory network and the reduction in ectodermal pluripotency.

Evolution and functions of Oct4 homologs in non-mammalian vertebrates

PouV class transcription factor Oct4/Pou5f1 is a central regulator of indefinite pluripotency in mammalian embryonic stem cells (ESCs) but also participates in cell lineage specification in mouse embryos and in differentiating cell cultures. The molecular basis for this versatility, which is shared between Oct4 and its non-mammalian homologs Pou5f1 and Pou5f3, is not yet completely understood. Here, I review the current understanding of the evolution of PouV class transcription factors and discuss equivalent and diverse roles of Oct4 homologs in pluripotency, differentiation, and cell behavior in different vertebrate embryos. This article is part of a Special Issue entitled: The Oct Transcription Factor Family, edited by Dr. Dean Tantin.

microRNA-103/107 Family Regulates Multiple Epithelial Stem Cell Characteristics

The stem cell niche is thought to affect cell cycle quiescence, proliferative capacity, and communication between stem cells and their neighbors. How these activities are controlled is not completely understood. Here we define a microRNA family (miRs-103/107) preferentially expressed in the stem cell-enriched limbal epithelium that regulates and integrates these stem cell characteristics. miRs-103/107 target the ribosomal kinase p90RSK2, thereby arresting cells in G0/G1 and contributing to a slow-cycling phenotype. Furthermore, miRs-103/107 increase the proliferative capacity of keratinocytes by targeting Wnt3a, which enhances Sox9 and YAP1 levels and thus promotes a stem cell phenotype. This miRNA family also regulates keratinocyte cell-cell communication by targeting: (a) the scaffolding protein NEDD9, preserving E-cadherin-mediated cell adhesion; and (b) the tyrosine phosphatase PTPRM, which negatively regulates connexin 43-based gap junctions. We propose that such regulation of cell communication and adhesion molecules maintains the integrity of the stem cell niche ultimately preserving self-renewal, a hallmark of epithelial stem cells.

A Regulatory Network Involving β-Catenin, e-Cadherin, PI3k/Akt, and Slug Balances Self-Renewal and Differentiation of Human Pluripotent Stem Cells In Response to Wnt Signaling

The mechanisms underlying disparate roles of the canonical Wnt signaling pathway in maintaining self‐renewal or inducing differentiation and lineage specification in embryonic stem cells (ESCs) are not clear. In this study, we provide the first demonstration that self‐renewal versus differentiation of human ESCs (hESCs) in response to Wnt signaling is predominantly determined by a two‐layer regulatory circuit involving β‐catenin, E‐cadherin, PI3K/Akt, and Slug in a time‐dependent manner. Short‐term upregulation of β‐catenin does not lead to the activation of T‐cell factor (TCF)‐eGFP Wnt reporter in hESCs. Instead, it enhances E‐cadherin expression on the cell membrane, thereby enhancing hESC self‐renewal through E‐cadherin‐associated PI3K/Akt signaling. Conversely, long‐term Wnt activation or loss of E‐cadherin intracellular β‐catenin binding domain induces TCF‐eGFP activity and promotes hESC differentiation through β‐catenin‐induced upregulation of Slug. Enhanced expression of Slug leads to a further reduction of E‐cadherin that serves as a β‐catenin “sink” sequestering free cytoplasmic β‐catenin. The formation of such a framework reinforces hESCs to switch from a state of temporal self‐renewal associated with short‐term Wnt/β‐catenin activation to definitive differentiation. Stem Cells2015;33:1419–1433

Zygotic Genome Activators, Developmental Timing, and Pluripotency

The transcription factors Pou5f1, Sox2, and Nanog are central regulators of pluripotency in mammalian ES and iPS cells. In vertebrate embryos, Pou5f1/3, SoxB1, and Nanog control zygotic genome activation and participate in lineage decisions. We review the current knowledge of the roles of these genes in developing vertebrate embryos from fish to mammals and suggest a model for pluripotency gene regulatory network functions in early development.

Intrinsic factors and the embryonic environment influence the formation of extragonadal teratomas during gestation

BACKGROUND

Pluripotent cells are present in early embryos until the levels of the pluripotency regulator Oct4 drop at the beginning of somitogenesis. Elevating Oct4 levels in explanted post-pluripotent cells in vitro restores their pluripotency. Cultured pluripotent cells can participate in normal development when introduced into host embryos up to the end of gastrulation. In contrast, pluripotent cells efficiently seed malignant teratocarcinomas in adult animals. In humans, extragonadal teratomas and teratocarcinomas are most frequently found in the sacrococcygeal region of neonates, suggesting that these tumours originate from cells in the posterior of the embryo that either reactivate or fail to switch off their pluripotent status. However, experimental models for the persistence or reactivation of pluripotency during embryonic development are lacking.

METHODS

We manually injected embryonic stem cells into conceptuses at E9.5 to test whether the presence of pluripotent cells at this stage correlates with teratocarcinoma formation. We then examined the effects of reactivating embryonic Oct4 expression ubiquitously or in combination with Nanog within the primitive streak (PS)/tail bud (TB) using a transgenic mouse line and embryo chimeras carrying a PS/TB-specific heterologous gene expression cassette respectively.

RESULTS

Here, we show that pluripotent cells seed teratomas in post-gastrulation embryos. However, at these stages, induced ubiquitous expression of Oct4 does not lead to restoration of pluripotency (indicated by Nanog expression) and tumour formation in utero, but instead causes a severe phenotype in the extending anteroposterior axis. Use of a more restricted T(Bra) promoter transgenic system enabling inducible ectopic expression of Oct4 and Nanog specifically in the posteriorly-located primitive streak (PS) and tail bud (TB) led to similar axial malformations to those induced by Oct4 alone. These cells underwent induction of pluripotency marker expression in Epiblast Stem Cell (EpiSC) explants derived from somitogenesis-stage embryos, but no teratocarcinoma formation was observed in vivo.

CONCLUSIONS

Our findings show that although pluripotent cells with teratocarcinogenic potential can be produced in vitro by the overexpression of pluripotency regulators in explanted somitogenesis-stage somatic cells, the in vivo induction of these genes does not yield tumours. This suggests a restrictive regulatory role of the embryonic microenvironment in the induction of pluripotency.

Primordial germ cells: the first cell lineage or the last cells standing?

Embryos of many animal models express germ line determinants that suppress transcription and mediate early germ line commitment, which occurs before the somatic cell lineages are established. However, not all animals segregate their germ line in this manner. The 'last cell standing' model describes primordial germ cell (PGC) development in axolotls, in which PGCs are maintained by an extracellular signalling niche, and germ line commitment occurs after gastrulation. Here, we propose that this 'stochastic' mode of PGC specification is conserved in vertebrates, including non-rodent mammals. We postulate that early germ line segregation liberates genetic regulatory networks for somatic development to evolve, and that it therefore emerged repeatedly in the animal kingdom in response to natural selection.

Myocyte enhancer factor 2D regulates ectoderm specification and adhesion properties of animal cap cells in the early Xenopus embryo

In Xenopus, animal cap (AC) cells give rise to ectoderm and its derivatives: epidermis and the central nervous system. Ectoderm has long been considered a default pathway of embryonic development, with cells that are not under the influence of vegetal Nodal signaling adopting an ectodermal program of gene expression. In the present study, we describe the involvement of the animally-localized maternal transcription factor myocyte enhancer factor (Mef) 2D in regulating the identity of AC cells. We find that Mef2D is required for the formation of both ectodermal lineages: neural and epidermis. Gain and loss of function experiments indicate that Mef2D regulates early gastrula expression of key ectodermal/epidermal genes in the animal region. Mef2D controls the activity of zygotic bone morphogenetic protein (BMP) signaling known to dictate the epidermal differentiation program. Exogenous expression of Mef2D in vegetal blastomeres was sufficient to induce ectopic expression of ectoderm/epidermal genes in the vegetal half of the embryo, when Nodal signaling was inhibited. Depletion of Mef2D caused a loss of AC cell adhesion that was rescued by the expression of E-cadherin or bone morphogenetic protein 4. In addition, expression of Mef2D in the prospective endoderm caused unusual aggregation of vegetal cells with animal cells in vitro and inappropriate segregation to other germ layers in vivo. Mef2D cooperates with another animally-expressed transcription factor, FoxI1e. Together, they regulate the expression of genes encoding signaling proteins and the transcription factors that control the regional identity of animal cells. Therefore, we describe a new role for the animally-localized Mef2D protein in early ectoderm specification, which is similar to that of the vegetally-localized VegT in endoderm and mesoderm formation.

Genome-wide view of TGFβ/Foxh1 regulation of the early mesendoderm program

Nodal/TGFβ signaling regulates diverse biological responses. By combining RNA-seq on Foxh1 and Nodal signaling loss-of-function embryos with ChIP-seq of Foxh1 and Smad2/3, we report a comprehensive genome-wide interaction between Foxh1 and Smad2/3 in mediating Nodal signaling during vertebrate mesendoderm development. This study significantly increases the total number of Nodal target genes regulated by Foxh1 and Smad2/3, and reinforces the notion that Foxh1-Smad2/3-mediated Nodal signaling directly coordinates the expression of a cohort of genes involved in the control of gene transcription, signaling pathway modulation and tissue morphogenesis during gastrulation. We also show that Foxh1 may function independently of Nodal signaling, in addition to its role as a transcription factor mediating Nodal signaling via Smad2/3. Finally, we propose an evolutionarily conserved interaction between Foxh1 and PouV, a mechanism observed in Pou5f1-mediated regulation of pluripotency in human embryonic stem and epiblast cells.

The POU-er of gene nomenclature

The pluripotency factor POU5F1 (OCT4) is well known as a key regulator of stem cell fate. Homologues of POU5F1 exist throughout vertebrates, but the evolutionary and functional relationships between the various family members have been unclear. The level to which function has been conserved within this family provides insight into the evolution of early embryonic potency. Here, we seek to clarify the relationship between POU5F1 homologues in the vertebrate lineage, both phylogenetically and functionally. We resolve the confusion over the identity of the zebrafish gene, which was originally named pou2, then changed to pou5f1 and again, more recently, to pou5f3. We argue that the use of correct nomenclature is crucial when discussing the degree to which the networks regulating early embryonic differentiation are conserved.

Spalt-like 4 promotes posterior neural fates via repression of pou5f3 family members in Xenopus

Amphibian neural development occurs as a two-step process: (1) induction specifies a neural fate in undifferentiated ectoderm; and (2) transformation induces posterior spinal cord and hindbrain. Signaling through the Fgf, retinoic acid (RA) and Wnt/β-catenin pathways is necessary and sufficient to induce posterior fates in the neural plate, yet a mechanistic understanding of the process is lacking. Here, we screened for factors enriched in posterior neural tissue and identify spalt-like 4 (sall4), which is induced by Fgf. Knockdown of Sall4 results in loss of spinal cord marker expression and increased expression of pou5f3.2 (oct25), pou5f3.3 (oct60) and pou5f3.1 (oct91) (collectively, pou5f3 genes), the closest Xenopus homologs of mammalian stem cell factor Pou5f1 (Oct4). Overexpression of the pou5f3 genes results in the loss of spinal cord identity and knockdown of pou5f3 function restores spinal cord marker expression in Sall4 morphants. Finally, knockdown of Sall4 blocks the posteriorizing effects of Fgf and RA signaling in the neurectoderm. These results suggest that Sall4, activated by posteriorizing signals, represses the pou5f3 genes to provide a permissive environment allowing for additional Wnt/Fgf/RA signals to posteriorize the neural plate.

Do all roads lead to Oct4? the emerging concepts of induced pluripotency

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Oct4 has unique and diverse functions in reprogramming.

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Oct4 is essential for lineage specification.

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Oct4 regulates multiple contrasting processes of cell identity change.

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Oct4 function may be regulated by cellular context and environment.

Pluripotent cells have the potential to differentiate into all of the cell types of an animal. This unique cell state is governed by an interconnected network of transcription factors. Among these, Oct4 plays an essential role both in the development of pluripotent cells in the embryo and in the self-renewal of its in vitro counterpart, embryonic stem (ES) cells. Furthermore, Oct4 is one of the four Yamanaka factors and its overexpression alone can generate induced pluripotent stem (iPS) cells. Recent reports underscore Oct4 as an essential regulator of opposing cell state transitions, such as pluripotency establishment and differentiation into embryonic germ lineages. Here we discuss these recent studies and the potential mechanisms underlying these contrasting functions of Oct4.

Ready, set, differentiate!

The expression of E-Cadherin, a protein best known for its role in cell adhesion, regulates the onset of embryonic differentiation.