Context-dependent wiring of Sox2 regulatory networks for self-renewal of embryonic and trophoblast stem cells

Characterization of spontaneous spheroids from oral mucosa-derived cells and their direct comparison with spheroids from skin-derived cells

Background

Our group has developed a novel method for spontaneous spheroid formation using a specific low-adherence culture plate with around 90° water contact angle. In this study, this method was applied for oral mucosa-derived cells. First, the feasibility of spontaneous spheroid formation was tested. Next, the characteristics of spontaneous spheroids from oral mucosa- and skin-derived cells were compared with special focus on the stemness and neuronal differentiation capability.

Methods

Oral mucosal cells were obtained from the palate and buccal mucosa of C57BL/6J mice. Similarly, skin cells were obtained from the back of the same mouse strain. Passage 2–3 cells were inoculated into the specific low-adherence culture plates to form spontaneous spheroids. The effect of basic fibroblast growth factor (bFGF), epidermal growth factor (EGF), and B27 supplement on spheroid formation and maintenance was assessed. Immunofluorescence and quantitative reverse transcription polymerase chain reaction (qRT-PCR) were performed to investigate the expression of pluripotency markers, cell proliferation and apoptosis markers, and neurogenic differentiation markers.

Results

Using this culture plate, spontaneous spheroid formation was feasible. This process depended on the presence of serum but was independent of the additives such as bFGF, EGF, and B27 supplement, although they improved the efficiency and were essential for spheroid maintenance. This result was confirmed by the higher expression of Caspase7 in the spheroids cultured without the additives than that with the additives. The spheroids from oral mucosa-derived cells expressed stem cell markers, such as Sox2, SSEA1, Oct4, Nanog, and Nestin. The expression of Sox2 in spheroids from oral mucosal cells was higher than that in spheroids from skin-derived cells. Both spheroid-forming cell types had the ability to differentiate into neural and Schwann cells after neurogenic induction, although significantly higher MAP 2, MBP, Nestin, and Nurr1 gene expression was noted in the cells from oral mucosa-derived spheroids.

Conclusions

The results showed that spontaneous spheroids from oral mucosa-derived cells contain highly potent stem cells, which were as good as skin-derived stem cells. The high expression of certain neuronal marker genes suggests an advantage of these cells for regeneration therapy for neuronal disorders.

Electronic supplementary material

The online version of this article (10.1186/s13287-019-1283-0) contains supplementary material, which is available to authorized users.

Genome-Scale CRISPRa Screen Identifies Novel Factors for Cellular Reprogramming

Primed epiblast stem cells (EpiSCs) can be reverted to a pluripotent embryonic stem cell (ESC)-like state by expression of single reprogramming factor. We used CRISPR activation to perform a genome-scale, reprogramming screen in EpiSCs and identified 142 candidate genes. Our screen validated a total of 50 genes, previously not known to contribute to reprogramming, of which we chose Sall1 for further investigation. We show that Sall1 augments reprogramming of mouse EpiSCs and embryonic fibroblasts and that these induced pluripotent stem cells are indeed fully pluripotent including formation of chimeric mice. We also demonstrate that Sall1 synergizes with Nanog in reprogramming and that overexpression in ESCs delays their conversion back to EpiSCs. Lastly, using RNA sequencing, we identify and validate Klf5 and Fam189a2 as new downstream targets of Sall1 and Nanog. In summary, our work demonstrates the power of using CRISPR technology in understanding molecular mechanisms that mediate complex cellular processes such as reprogramming.

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Genome-scale CRISPRa screen in mouse EpiSCs identifies novel reprogramming factors

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50 novel genes, including Sall1 and Fam189a2, identified to mediate reprogramming

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Sall1 synergizes with Nanog to increase reprogramming efficiency in EpiSCs and MEFs

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RNA-seq provides insight into downstream pathways of Sall1 and Nanog-mediated reprogramming

Pulmonary carcinosarcoma characterized by small round cells with neuroendocrine, myogenic, and chondrogenic differentiation: An extremely rare case

Carcinosarcoma is a clonal tumor developed through sarcomatoid changes in a carcinoma via the epithelial-mesenchymal transition (EMT). Here, we present an extremely rare case of pulmonary carcinosarcoma characterized by components suggesting pluripotency, namely neuroendocrine, myogenic, and chondrogenic differentiation, based on immunohistochemical analysis. A 42-year-old Japanese man was admitted to our hospital. Analysis of tumor tissue after right upper lobe lobectomy revealed a transition between carcinomatous and sarcomatous components. Immunohistochemical analysis suggested immortality owing to complete loss of p53 and diffuse expression of p16 in both the carcinomatous and sarcomatous components. There were also scattered cell groups expressing aldehyde dehydrogenase 1 family member A1, SOX2, CD133, and c-kit, suggesting the possible presence of cancer stem cells. Our findings in this case suggested that the EMT may play a key role in mediating the immortality of tumor cells in carcinosarcoma and facilitating the pluripotency of cancer stem cells.

Bioinformatics Analysis Makes Revelation to Potential Properties on Regulation and Functions of Human Sox2

Sex determining region Y-box 2 (Sox2) is a transcription factor that is essential for maintaining self-renewal or pluripotency of undifferentiated embryonic stem cells. The expression and distribution of Sox2 in tumor tissues have been extensively recorded, which are related to the progression and metastasis of tumor. However, a complete mechanistic understanding of Sox2 regulation and function remains to be studied. Herein, we show new potential properties of Sox2 regulation and functions from bioinformatics analysis. We use numerous algorithms to characterize the Sox2 gene promoter elements and the Sox2 protein structure, physio-chemical, localization properties and its evolutionary relationships. The expression of Sox2 is regulated by a diverse set of transcription factors and associated with the levels of methylation of CpG Islands in promoters. The structural properties of Sox2 indicate that Sox2 expresses as a stem cell marker in a variety of stem cells. Sox2 together with other transcription factors or proteins regulate the expression of downstream target genes, which makes a great difference to the biological function of stem cells. Not only stem cells, Sox2 also play an important role in tumor cells. In conclusion, this information from bioinformatics analysis will help to understand Sox2 regulation and functions better in future attempts.

The Differentiation Potency of Trophoblast Stem Cells from Mouse Androgenetic Embryos

In mice, trophoblast stem (TS) cells are derived from the polar trophectoderm of blastocysts. TS cells cultured in the presence of fibroblast growth factor 4 (Fgf4) are in an undifferentiated state and express undifferentiated marker genes such as Cdx2. After removing Fgf4 from the culture medium, TS cells drastically reduce the expression of undifferentiated marker genes, stop cell proliferation, and differentiate into all trophoblast cell subtypes. To clarify the roles of the parental genomes in placentation, we previously established TS cells from androgenetic embryos (AGTS cells). AGTS cells are in the undifferentiated state when cultured with Fgf4 and express undifferentiated marker genes. After removing Fgf4, AGTS cells differentiate into trophoblast giant cells (TGCs), but not into spongiotrophoblast cells, and some of the AGTS cells continue to proliferate. In this study, we investigated the differentiation potency of AGTS cells by analyzing the expression of undifferentiated marker genes and all trophoblast cell subtype-specific genes. After removing Fgf4, some undifferentiated marker genes (Cdx2, Eomes and Elf5) continued to be expressed. Interestingly, TGCs differentiated from AGTS cells also expressed Cdx2, but not Prl3d1. Moreover, the expression of Gcm1 and Synb was induced after the differentiation, indicating that AGTS cells preferentially differentiated into labyrinth progenitor cells. Cdx2 knockdown resulted in increased Prl3d1 expression, suggesting that Fgf4-independent Cdx2 expression inhibited the functional TGCs. Moreover, Fgf4-independent Cdx2 expression was activated by Gab1, one of the paternally expressed imprinted genes via the mitogen-activated protein kinase kinase (MEK)-extracellular signal regulated protein kinase (ERK) pathway. These results suggested that the paternal genome activates the MEK-ERK pathway without the Fgf4 signal, accelerates the differentiation into labyrinth progenitor cells and controls the function of TGCs.

Esrrb extinction triggers dismantling of naïve pluripotency and marks commitment to differentiation

Self-renewal of embryonic stem cells (ESCs) cultured in LIF/fetal calf serum (FCS) is incomplete with some cells initiating differentiation. While this is reflected in heterogeneous expression of naive pluripotency transcription factors (TFs), the link between TF heterogeneity and differentiation is not fully understood. Here, we purify ESCs with distinct TF expression levels from LIF/FCS cultures to uncover early events during commitment from naïve pluripotency. ESCs carrying fluorescent Nanog and Esrrb reporters show Esrrb downregulation only in Nanoglow cells. Independent Esrrb reporter lines demonstrate that Esrrbnegative ESCs cannot effectively self-renew. Upon Esrrb loss, pre-implantation pluripotency gene expression collapses. ChIP-Seq identifies different regulatory element classes that bind both OCT4 and NANOG in Esrrbpositive cells. Class I elements lose NANOG and OCT4 binding in Esrrbnegative ESCs and associate with genes expressed preferentially in naïve ESCs. In contrast, Class II elements retain OCT4 but not NANOG binding in ESRRB-negative cells and associate with more broadly expressed genes. Therefore, mechanistic differences in TF function act cumulatively to restrict potency during exit from naïve pluripotency.

Extracellular glucose levels in cultures of undifferentiated mouse trophoblast stem cells affect gene expression during subsequent differentiation with replicable cell line-dependent variation

Mouse trophoblast stem cells (TSCs) have been established and maintained using hyperglycemic conditions (11 mM glucose) for no apparent good reason. Because glucose metabolites are used as resources for cellular energy production, biosynthesis, and epigenetic modifications, differences in extracellular glucose levels may widely affect cellular function. Since the hyperglycemic culture conditions used for TSC culture have not been fully validated, the effect of extracellular glucose levels on the properties of TSCs remains unclear. To address this issue, we investigated the gene expression of stemness-related transcription factors in TSCs cultured in the undifferentiated state under various glucose concentrations. We also examined the expression of trophoblast subtype markers during differentiation, after returning the glucose concentration to the conventional culture concentration (11 mM). As a result, it appeared that the extracellular glucose conditions in the stem state not only affected the gene expression of stemness-related transcription factors before differentiation but also affected the expression of marker genes after differentiation, with some line-to-line variation. In the TS4 cell line, which showed the largest glucose concentration-dependent fluctuations in gene expression among all the lines examined, low glucose (1 mM glucose, LG) augmented H3K27me3 levels. An Ezh2 inhibitor prevented these LG-induced changes in gene expression, suggesting the possible involvement of H3K27me3 in the changes in gene expression seen in LG. These results collectively indicate that the response of the TSCs to the change in the extracellular glucose concentration is cell line-dependent and a part of which may be epigenetically memorized.

Divergent wiring of repressive and active chromatin interactions between mouse embryonic and trophoblast lineages

The establishment of the embryonic and trophoblast lineages is a developmental decision underpinned by dramatic differences in the epigenetic landscape of the two compartments. However, it remains unknown how epigenetic information and transcription factor networks map to the 3D arrangement of the genome, which in turn may mediate transcriptional divergence between the two cell lineages. Here, we perform promoter capture Hi-C experiments in mouse trophoblast (TSC) and embryonic (ESC) stem cells to understand how chromatin conformation relates to cell-specific transcriptional programmes. We find that key TSC genes that are kept repressed in ESCs exhibit interactions between H3K27me3-marked regions in ESCs that depend on Polycomb repressive complex 1. Interactions that are prominent in TSCs are enriched for enhancer-gene contacts involving key TSC transcription factors, as well as TET1, which helps to maintain the expression of TSC-relevant genes. Our work shows that the first developmental cell fate decision results in distinct chromatin conformation patterns establishing lineage-specific contexts involving both repressive and active interactions.

A molecular network for functional versatility of HECATE transcription factors

During the plant life cycle, diverse signaling inputs are continuously integrated and engage specific genetic programs depending on the cellular or developmental context. Consistent with an important role in this process, HECATE (HEC) basic helix-loop-helix transcription factors display diverse functions, from photomorphogenesis to the control of shoot meristem dynamics and gynoecium patterning. However, the molecular mechanisms underlying their functional versatility and the deployment of specific HEC subprograms remain elusive. To address this issue, we systematically identified proteins with the capacity to interact with HEC1, the best-characterized member of the family, and integrated this information with our data set of direct HEC1 target genes. The resulting core genetic modules were consistent with specific developmental functions of HEC1, including its described activities in light signaling, gynoecium development and auxin homeostasis. Importantly, we found that HEC genes also play a role in the modulation of flowering time, and uncovered that their role in gynoecium development may involve the direct transcriptional regulation of NGATHA1 (NGA1) and NGA2 genes. NGA factors were previously shown to contribute to fruit development, but our data now show that they also modulate stem cell homeostasis in the shoot apical meristem. Taken together, our results delineate a molecular network underlying the functional versatility of HEC transcription factors. Our analyses have not only allowed us to identify relevant target genes controlling shoot stem cell activity and a so far undescribed biological function of HEC1, but also provide a rich resource for the mechanistic elucidation of further context-dependent HEC activities.

Esrrb Unlocks Silenced Enhancers for Reprogramming to Naive Pluripotency

Transcription factor (TF)-mediated reprogramming to pluripotency is a slow and inefficient process, because most pluripotency TFs fail to access relevant target sites in a refractory chromatin environment. It is still unclear how TFs actually orchestrate the opening of repressive chromatin during the long latency period of reprogramming. Here, we show that the orphan nuclear receptor Esrrb plays a pioneering role in recruiting the core pluripotency factors Oct4, Sox2, and Nanog to inactive enhancers in closed chromatin during the reprogramming of epiblast stem cells. Esrrb binds to silenced enhancers containing stable nucleosomes and hypermethylated DNA, which are inaccessible to the core factors. Esrrb binding is accompanied by local loss of DNA methylation, LIF-dependent engagement of p300, and nucleosome displacement, leading to the recruitment of core factors within approximately 2 days. These results suggest that TFs can drive rapid remodeling of the local chromatin structure, highlighting the remarkable plasticity of stable epigenetic information.

The Histone Deacetylase Inhibitor, CI994, Improves Nuclear Reprogramming and In Vitro Developmental Potential of Cloned Pig Embryos

Epigenetic reprogramming and somatic cell nuclear transfer (SCNT) cloning efficiency were recently enhanced using histone deacetylase inhibitors (HDACis). In this study, we investigated the time effect of CI994, an HDACi, on the blastocyst formation rate, acetylation levels of H3K9 and H4K12, DNA methylation levels of anti-5-methylcytosine (5mC), and some mRNA expression of pluripotency-related genes in pig SCNT embryos. Treatment with 10 μM CI994 for 24 hours significantly improved the blastocyst formation rate of SCNT embryos in comparison with the untreated group (p < 0.05). Moreover, average fluorescence intensities of H3K9 and H4K12 in CI994-treated embryos were remarkably increased at the pseudo-pronuclear stage, but not at the blastocyst stage. The intensity of POU5F1 was higher in CI994-treated blastocysts than in control blastocysts, whereas that of 5mC did not differ between the two groups. The percentage of apoptotic cells in blastocysts was significantly higher in the untreated group than in the CI994-treated group. mRNA levels of POU5F1 and SOX2 were significantly increased in the CI994-treated group. These observations suggest that optimum exposure (10 μM for 24 hours) to CI994 after activation elevates the level of histone acetylation and subsequently improves the in vitro development of pig SCNT embryos.

The principles that govern transcription factor network functions in stem cells

Tissue-specific transcription factors primarily act to define the phenotype of the cell. The power of a single transcription factor to alter cell fate is often minimal, as seen in gain-of-function analyses, but when multiple transcription factors cooperate synergistically it potentiates their ability to induce changes in cell fate. By contrast, transcription factor function is often dispensable in the maintenance of cell phenotype, as is evident in loss-of-function assays. Why does this phenomenon, commonly known as redundancy, occur? Here, I discuss the role that transcription factor networks play in collaboratively regulating stem cell fate and differentiation by providing multiple explanations for their functional redundancy.

Derivation of Human Trophoblast Stem Cells

Trophoblast cells play an essential role in the interactions between the fetus and mother. Mouse trophoblast stem (TS) cells have been derived and used as the best in vitro model for molecular and functional analysis of mouse trophoblast lineages, but attempts to derive human TS cells have so far been unsuccessful. Here we show that activation of Wingless/Integrated (Wnt) and EGF and inhibition of TGF-β, histone deacetylase (HDAC), and Rho-associated protein kinase (ROCK) enable long-term culture of human villous cytotrophoblast (CT) cells. The resulting cell lines have the capacity to give rise to the three major trophoblast lineages, which show transcriptomes similar to those of the corresponding primary trophoblast cells. Importantly, equivalent cell lines can be derived from human blastocysts. Our data strongly suggest that the CT- and blastocyst-derived cell lines are human TS cells, which will provide a powerful tool to study human trophoblast development and function.

Single-cell gene expression of the bovine blastocyst

The first two differentiation events in the embryo result in three cell types - epiblast, trophectoderm (TE) and hypoblast. The purpose here was to identify molecular markers for each cell type in the bovine and evaluate the differences in gene expression among individual cells of each lineage. The cDNA from 67 individual cells of dissociated blastocysts was used to determine transcript abundance for 93 genes implicated as cell lineage markers in other species or potentially involved in developmental processes. Clustering analysis indicated that the cells belonged to two major populations (clades A and B) with two subpopulations of clade A and four of clade B. Use of lineage-specific markers from other species indicated that the two subpopulations of clade A represented epiblast and hypoblast respectively while the four subpopulations of clade B were TE. Among the genes upregulated in epiblast were AJAP1, DNMT3A, FGF4, H2AFZ, KDM2B, NANOG, POU5F1, SAV1 and SLIT2 Genes overexpressed in hypoblast included ALPL, FGFR2, FN1, GATA6, GJA1, HDAC1, MBNL3, PDGFRA and SOX17, while genes overexpressed in all four TE populations were ACTA2, CDX2, CYP11A1, GATA2, GATA3, IFNT, KRT8, RAC1 and SFN The subpopulations of TE varied among each other for multiple genes including the prototypical TE marker IFNT. New markers for each cell type in the bovine blastocyst were identified. Results also indicate heterogeneity in gene expression among TE cells. Further studies are needed to confirm whether subpopulations of TE cells represent different stages in the development of a committed TE phenotype.

From the stem of the placental tree: trophoblast stem cells and their progeny

Trophoblast stem cells (TSCs) retain the capacity to self-renew indefinitely and harbour the potential to differentiate into all trophoblast subtypes of the placenta. Recent studies have shown how signalling cascades integrate with transcription factor circuits to govern the fine balance between TSC self-renewal and differentiation. In addition, breakthroughs in reprogramming strategies have enabled the generation of TSCs from fibroblasts, opening up exciting new avenues that may allow the isolation of this stem cell type from other species, notably humans. Here, we review these recent advances in light of their importance for understanding placental pathologies and developing personalised medicine approaches for pregnancy complications.

Mapping the chromatin landscape and Blimp1 transcriptional targets that regulate trophoblast differentiation

Trophoblast stem cells (TSCs) give rise to specialized cell types within the placenta. However, the regulatory mechanisms that guide trophoblast cell fate decisions during placenta development remain ill defined. Here we exploited ATAC-seq and transcriptional profiling strategies to describe dynamic changes in gene expression and chromatin accessibility during TSC differentiation. We detect significantly increased chromatin accessibility at key genes upregulated as TSCs exit from the stem cell state. However, downregulated gene expression is not simply due to the loss of chromatin accessibility in proximal regions. Additionally, transcriptional targets recognized by the zinc finger transcriptional repressor Prdm1/Blimp1, an essential regulator of placenta development, were identified in ChIP-seq experiments. Comparisons with previously reported ChIP-seq datasets for primordial germ cell-like cells and E18.5 small intestine, combined with functional annotation analysis revealed that Blimp1 has broadly shared as well as cell type-specific functional activities unique to the trophoblast lineage. Importantly, Blimp1 not only silences TSC gene expression but also prevents aberrant activation of divergent developmental programmes. Overall the present study provides new insights into the chromatin landscape and Blimp1-dependent regulatory networks governing trophoblast gene expression.

SOX2 and Rb1 in esophageal small-cell carcinoma: their possible involvement in pathogenesis

Clinicopathological features and pathogenesis of esophageal small-cell carcinoma remain unclear. We hypothesized common cellular origin and pathogenesis in small-cell carcinoma of esophagus and lung associated with SOX2 overexpression and loss of Rb1. Expression of squamous-basal markers (CK5/6 and p40), glandular markers (CK18 and CEA), SOX2, and Rb1 were evaluated in 15 esophageal small-cell carcinomas, 46 poorly differentiated squamous cell carcinomas, and 88 small-cell lung carcinoma, as well as 16 embryonic esophagus. Esophageal small-cell carcinoma expressed higher levels of glandular markers and lower levels of squamous-basal markers than poorly differentiated squamous cell carcinoma. No significant differences were observed in immunohistochemistry profiles between small-cell carcinoma of the esophagus and the lung. SOX2 expression was high in esophageal small-cell carcinoma (70%±33% of nuclei), small-cell lung carcinoma (70%±26%), and the embryonic esophagus (75%±4%), and it was significantly lower in poorly differentiated squamous cell carcinoma (29%±28%). Rb1 expression was significantly lower in esophageal small-cell carcinoma (0.3%±1%), small-cell lung carcinoma (2%±6%), and the embryonic esophagus (7%±5%), and it was significantly higher in poorly differentiated squamous cell carcinoma (51%±24%). The immunohistochemistry profiles of small-cell carcinoma of the esophagus and the lung are highly similar. The loss of Rb1 function is a key contributor to the pathogenesis of both neoplasms. In addition, SOX2 overexpression observed in esophageal and lung small-cell carcinoma as well as in the embryonic esophagus indicated that esophageal small-cell carcinoma may arise from embryonic-like stem cells in the esophageal epithelium. The two distinct differentiation patterns (neuroendocrine and glandular) of esophageal small-cell carcinoma further support the fact that SOX2 has a pivotal role in the differentiation of pluripotent stem cells into esophageal small-cell carcinoma cells.

Nephronectin plays critical roles in Sox2 expression and proliferation in dental epithelial stem cells via EGF-like repeat domains

Tooth development is initiated by epithelial-mesenchymal interactions via basement membrane (BM) and growth factors. In the present study, we found that nephronectin (Npnt), a component of the BM, is highly expressed in the developing tooth. Npnt localizes in the BM on the buccal side of the tooth germ and shows an expression pattern opposite that of the dental epithelial stem cell marker Sox2. To identify the roles of Npnt during tooth development, we performed knockdown and overexpression experiments using ex vivo organ and dental epithelial cell cultures. Our findings showed that loss of Npnt induced ectopic Sox2-positive cells and reduced tooth germ size. Over expression of Npnt showed increased proliferation, whereas the number of Sox2-positive cells was decreased in dental epithelial cells. Npnt contains 5 EGF-like repeat domains, as well as an RGD sequence and MAM domain. We found that the EGF-like repeats are critical for Sox2 expression and cell proliferation. Furthermore, Npnt activated the EGF receptor (EGFR) via the EGF-like repeat domains and induced the PI3K-Akt signaling pathway. Our results indicate that Npnt plays a critical scaffold role in dental epithelial stem cell differentiation and proliferation, and regulates Sox2 expression during tooth development.

The pioneer factor OCT4 requires the chromatin remodeller BRG1 to support gene regulatory element function in mouse embryonic stem cells

Pioneer transcription factors recognise and bind their target sequences in inaccessible chromatin to establish new transcriptional networks throughout development and cellular reprogramming. During this process, pioneer factors establish an accessible chromatin state to facilitate additional transcription factor binding, yet it remains unclear how different pioneer factors achieve this. Here, we discover that the pluripotency-associated pioneer factor OCT4 binds chromatin to shape accessibility, transcription factor co-binding, and regulatory element function in mouse embryonic stem cells. Chromatin accessibility at OCT4-bound sites requires the chromatin remodeller BRG1, which is recruited to these sites by OCT4 to support additional transcription factor binding and expression of the pluripotency-associated transcriptome. Furthermore, the requirement for BRG1 in shaping OCT4 binding reflects how these target sites are used during cellular reprogramming and early mouse development. Together this reveals a distinct requirement for a chromatin remodeller in promoting the activity of the pioneer factor OCT4 and regulating the pluripotency network.

DOI:http://dx.doi.org/10.7554/eLife.22631.001

All cells in your body contain the same genetic information in the form of genes encoded within DNA. Yet, cells use this information in different ways so that the activities of individual genes within that DNA can vary from cell to cell. This allows identical cells to become different to each other and to adapt to changing circumstances.

A group of proteins called transcription factors control the activity of certain genes by binding to specific sites on DNA. However, this isn’t a straightforward process because DNA in human and other animal cells is usually associated with structures called nucleosomes that can block access to the DNA. Pioneer transcription factors, such as OCT4, are a specific group of transcription factors that can attach to DNA in spite of the nucleosomes, but it’s not clear how this is possible. Once pioneer transcription factors attach to DNA they can help other transcription factors to bind alongside them.

King et al. studied OCT4 in stem cells from mouse embryos to investigate how it is able to act as a pioneer transcription factor and control gene activity. The experiments show that several other transcription factors lose the ability to bind to DNA when OCT4 is absent. This leads to widespread changes in gene activity in the cells, which seems to be due to other transcription factors being unable to get past the nucleosomes to attach to the DNA.

Further experiments showed that OCT4 needs a protein called BRG1 in order to act as a pioneer transcription factor. BRG1 is an enzyme that is able to move and remove (remodel) nucleosomes attached to DNA, suggesting that normal transcription factor binding requires this activity. The next challenge is to investigate whether BRG1, or similar enzymes, are also needed by other pioneer transcription factors that are required for normal gene activity and cell identity. This will be important because many enzymes that remodel nucleosomes are disrupted in human diseases like cancer where cells lose their normal identity.

DOI:http://dx.doi.org/10.7554/eLife.22631.002

Esrrb directly binds to Gata6 promoter and regulates its expression with Dax1 and Ncoa3

Estrogen-related receptor beta (Esrrb) is expressed in embryonic stem (ES) cells and is involved in self-renewal ability and pluripotency. Previously, we found that Dax1 is associated with Esrrb and represses its transcriptional activity. Further, the disruption of the Dax1-Esrrb interaction increases the expression of the extra-embryonic endoderm marker Gata6 in ES cells. Here, we investigated the influences of Esrrb and Dax1 on Gata6 expression. Esrrb overexpression in ES cells induced endogenous Gata6 mRNA and Gata6 promoter activity. In addition, the Gata6 promoter was found to contain the Esrrb recognition motifs ERRE1 and ERRE2, and the latter was the responsive element of Esrrb. Associations between ERRE2 and Esrrb were then confirmed by biotin DNA pulldown and chromatin immunoprecipitation assays. Subsequently, we showed that Esrrb activity at the Gata6 promoter was repressed by Dax1, and although Dax1 did not bind to ERRE2, it was associated with Esrrb, which directly binds to ERRE2. In addition, the transcriptional activity of Esrrb was enhanced by nuclear receptor co-activator 3 (Ncoa3), which has recently been shown to be a binding partner of Esrrb. Finally, we showed that Dax1 was associated with Ncoa3 and repressed its transcriptional activity. Taken together, the present study indicates that the Gata6 promoter is activated by Esrrb in association with Ncoa3, and Dax1 inhibited activities of Esrrb and Ncoa3, resulting maintenance of the undifferentiated status of ES cells.

Stem cell insights into human trophoblast lineage differentiation

BACKGROUND

The human placenta is vital for fetal development, yet little is understood about how it forms successfully to ensure a healthy pregnancy or why this process is inadequate in 1 in 10 pregnancies, leading to miscarriage, intrauterine growth restriction or preeclampsia. Trophoblasts are placenta-specific epithelial cells that maximize nutrient exchange. All trophoblast lineages are thought to arise from a population of trophoblast stem cells (TSCs). However, whilst the isolation of murine TSC has led to an explosion in understanding murine placentation, the isolation of an analogous human TSC has proved more difficult. Consequently, alternative methods of studying human trophoblast lineage development have been employed, including human embryonic stem cells (hESCs), induced pluripotent stem cells (iPS) and transformed cell lines; but what do these proxy models tell us about what is happening during early placental development?

OBJECTIVE AND RATIONALE

In this systematic review, we evaluate current approaches to understanding human trophoblast lineage development in order to collate and refine these models and inform future approaches aimed at establishing human TSC lines.

SEARCH METHODS

To ensure all relevant articles were analysed, an unfiltered search of Pubmed, Embase, Scopus and Web of Science was conducted for 25 key terms on the 13th May 2016. In total, 47 313 articles were retrieved and manually filtered based on non-human, non-English, non-full text, non-original article and off-topic subject matter. This resulted in a total of 71 articles deemed relevant for review in this article.

OUTCOMES

Candidate human TSC populations have been identified in, and isolated from, both the chorionic membrane and villous tissue of the placenta, but further investigation is required to validate these as 'true' human TSCs. Isolating human TSCs from blastocyst trophectoderm has not been successful in humans as it was in mice, although recently the first reported TSC line (USFB6) was isolated from an eight-cell morula. In lieu of human TSC lines, trophoblast-like cells have been induced to differentiate from hESCs and iPS. However, differentiation in these model systems is difficult to control, culture conditions employed are highly variable, and the extent to which they accurately convey the biology of 'true' human TSCs remains unclear, particularly as a consensus has not been met among the scientific community regarding which characteristics a human TSC must possess.

WIDER IMPLICATIONS

Human TSC models have the potential to revolutionize our understanding of trophoblast differentiation, allowing us to make significant gains in understanding the underlying pathology of pregnancy disorders and to test potential therapeutic interventions on cell function in vitro. In order to do this, a collaborative effort is required to establish the criteria that define a human TSC to confirm the presence of human TSCs in both primary isolates and to determine how accurately trophoblast-like cells derived from current model systems reflect trophoblast from primary tissue. The in vitro systems currently used to model early trophoblast lineage formation have provided insights into early human placental formation but it is unclear whether these trophoblast-like cells are truly representative of primary human trophoblast. Consequently, continued refinement of current models, and standardization of culture protocols is essential to aid our ability to identify, isolate and propagate 'true' human TSCs from primary tissue.

The evolutionally-conserved function of group B1 Sox family members confers the unique role of Sox2 in mouse ES cells

BACKGROUND

In mouse ES cells, the function of Sox2 is essential for the maintenance of pluripotency. Since the Sox-family of transcription factors are well conserved in the animal kingdom, addressing the evolutionary origin of Sox2 function in pluripotent stem cells is intriguing from the perspective of understanding the origin of pluripotency.

RESULTS

Here we approach this question using a functional complementation assay in inducible Sox2-null ES cells. Assaying mouse Sox proteins from different Groups, we found that only Group B1 and Group G proteins were able to support pluripotency. Interestingly, invertebrate homologs of mammalian Group B1 Sox proteins were able to replace the pluripotency-associated function of mouse Sox2. Moreover, the mouse ES cells rescued by the Drosophila SoxNeuro protein are able to contribute to chimeric embryos.

CONCLUSIONS

These data indicate that the function of mouse Sox2 supporting pluripotency is based on an evolutionally conserved activity of the Group B1 Sox family. Since pluripotent stem cell population in developmental process could be regarded as the evolutional novelty in vertebrates, it could be regarded as a co-optional use of their evolutionally conserved function.

Distinct mechanisms regulate Cdx2 expression in the blastocyst and in trophoblast stem cells

The first intercellular differences during mammalian embryogenesis arise in the blastocyst, producing the inner cell mass and the trophectoderm. The trophectoderm is the first extraembryonic tissue and does not contribute to the embryo proper, its differentiation instead forming tissues that sustain embryonic development. Crucial roles in extraembryonic differentiation have been identified for certain transcription factors, but a comprehensive picture of the regulation of this early specification is still lacking. Here, we investigated whether the regulatory mechanisms involved in Cdx2 expression in the blastocyst are also utilized in the postimplantation embryo. We analyzed an enhancer that is regulated through Hippo and Notch in the blastocyst trophectoderm, unexpectedly finding that it is inactive in the extraembryonic structures at postimplantation stages. Further analysis identified other Cdx2 regulatory elements including a stem-cell specific regulatory sequence and an element that drives reporter expression in the trophectoderm, a subset of cells in the extraembryonic region of the postimplantation embryo and in trophoblast stem cells. The cross-comparison in this study of cis-regulatory elements employed in the blastocyst, stem cell populations and the postimplantation embryo provides new insights into early mammalian development and suggests a two-step mechanism in Cdx2 regulation.

Cellular Dynamics of Mouse Trophoblast Stem Cells: Identification of a Persistent Stem Cell Type

Mouse trophoblast stem cells (TSCs) proliferate indefinitely in vitro, despite their highly heterogeneous nature. In this study, we sought to characterize TSC colony types by using methods based on cell biology and biochemistry for a better understanding of how TSCs are maintained over multiple passages. Colonies of TSCs could be classified into four major types: type 1 is compact and dome-shaped, type 4 is flattened but with a large multilayered cell cluster, and types 2 and 3 are their intermediates. A time-lapse analysis indicated that type 1 colonies predominantly appeared after passaging, and a single type 1 colony gave rise to all other types. These colony transitions were irreversible, but at least some type 1 colonies persisted throughout culture. The typical cells comprising type 1 colonies were small and highly motile, and they aggregated together to form primary colonies. A hierarchical clustering based on global gene expression profiles suggested that a TSC line containing more type 1 colony cells was similar to in vivo extraembryonic tissues. Among the known TSC genes examined, Elf5 showed a differential expression pattern according to colony type, indicating that this gene might be a reliable marker of undifferentiated TSCs. When aggregated with fertilized embryos, cells from types 1 and 2, but not from type 4, distributed to the polar trophectoderm in blastocysts. These findings indicate that cells typically found in type 1 colonies can persist indefinitely as stem cells and are responsible for the maintenance of TSC lines. They may provide key information for future improvements in the quality of TSC lines.

Zscan4 Is Activated after Telomere Shortening in Mouse Embryonic Stem Cells

ZSCAN4 is a DNA-binding protein that functions for telomere elongation and genomic stability. In vivo, it is specifically expressed at the two-cell stage during mouse development. In vitro, it is transiently expressed in mouse embryonic stem cells (ESCs), only in 5% of the population at one time. Here we attempted to elucidate when, under what circumstances, Zscan4 is activated in ESCs. Using live cell imaging, we monitored the activity of Zscan4 together with the pluripotency marker Rex1. The lengths of the cell cycles in ESCs were diverse. Longer cell cycles were accompanied by shorter telomeres and higher activation of Zscan4. Since activation of Zscan4 is involved in telomere elongation, we speculate that the extended cell cycles accompanied by Zscan4 activation reflect the time for telomere recovery. Rex1 and Zscan4 did not show any correlation. Taken together, we propose that Zscan4 is activated to recover shortened telomeres during extended cell cycles, irrespective of the pluripotent status.

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At longer cell cycles, telomeres are shorter

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Zscan4 is activated when the cell cycles become long

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After the activation of Zscan4, the next cell cycle becomes short

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We propose Zscan4 is activated for telomere maintenance irrespective of pluripotency

Developmental Competence for Primordial Germ Cell Fate

During mammalian embryonic development, the trophectoderm and primitive endoderm give rise to extraembryonic tissues, while the epiblast differentiates into all somatic lineages and the germline. Remarkably, only a few classes of signaling pathways induce the differentiation of these progenitor cells into diverse lineages. Accordingly, the functional outcome of a particular signal depends on the developmental competence of the target cells. Thus, developmental competence can be defined as the ability of a cell to integrate intrinsic and extrinsic cues to execute a specific developmental program toward a specific cell fate. Downstream of signaling, there is the combinatorial activity of transcription factors and their cofactors, which is modulated by the chromatin state of the target cells. Here, we discuss the concept of developmental competence, and the factors that regulate this state with reference to the specification of mammalian primordial germ cells.

Selection of accurate reference genes in mouse trophoblast stem cells for reverse transcription-quantitative polymerase chain reaction

Mouse trophoblast stem cells (TSCs) form colonies of different sizes and morphologies, which might reflect their degrees of differentiation. Therefore, each colony type can have a characteristic gene expression profile; however, the expression levels of internal reference genes may also change, causing fluctuations in their estimated gene expression levels. In this study, we validated seven housekeeping genes by using a geometric averaging method and identified Gapdh as the most stable gene across different colony types. Indeed, when Gapdh was used as the reference, expression levels of Elf5, a TSC marker gene, stringently classified TSC colonies into two groups: a high expression groups consisting of type 1 and 2 colonies, and a lower expression group consisting of type 3 and 4 colonies. This clustering was consistent with our putative classification of undifferentiated/differentiated colonies based on their time-dependent colony transitions. By contrast, use of an unstable reference gene (Rn18s) allowed no such clear classification. Cdx2, another TSC marker, did not show any significant colony type-specific expression pattern irrespective of the reference gene. Selection of stable reference genes for quantitative gene expression analysis might be critical, especially when cell lines consisting of heterogeneous cell populations are used.

A Resource for the Transcriptional Signature of Bona Fide Trophoblast Stem Cells and Analysis of Their Embryonic Persistence

Trophoblast stem cells (TSCs) represent the multipotent progenitors that give rise to the different cells of the embryonic portion of the placenta. Here, we analysed the expression of key TSC transcription factors Cdx2, Eomes, and Elf5 in the early developing placenta of mouse embryos and in cultured TSCs and reveal surprising heterogeneity in protein levels. We analysed persistence of TSCs in the early placenta and find that TSCs remain in the chorionic hinge until E9.5 and are lost shortly afterwards. To define the transcriptional signature of bona fide TSCs, we used inducible gain- and loss-of-function alleles of Eomes or Cdx2, and Eomes^GFP, to manipulate and monitor the core maintenance factors of TSCs, followed by genome-wide expression profiling. Combinatorial analysis of resulting expression profiles allowed for defining novel TSC marker genes that might functionally contribute to the maintenance of the TSC state. Analyses by qRT-PCR and in situ hybridisation validated novel TSC- and chorion-specific marker genes, such as Bok/Mtd, Cldn26, Duox2, Duoxa2, Nr0b1, and Sox21. Thus, these expression data provide a valuable resource for the transcriptional signature of bona fide and early differentiating TSCs and may contribute to an increased understanding of the transcriptional circuitries that maintain and/or establish stemness of TSCs.

NANOG alone induces germ cells in primed epiblast in vitro by activation of enhancers

Nanog, a core pluripotency factor in the inner cell mass of blastocysts, is also expressed in unipotent primordial germ cells (PGCs) in mice, where its precise role is yet unclear. We investigated this in an in vitro model, in which naive pluripotent embryonic stem (ES) cells cultured in basic fibroblast growth factor (bFGF) and activin A develop as epiblast-like cells (EpiLCs) and gain competence for a PGC-like fate. Consequently, bone morphogenetic protein 4 (BMP4), or ectopic expression of key germline transcription factors Prdm1, Prdm14 and Tfap2c, directly induce PGC-like cells (PGCLCs) in EpiLCs, but not in ES cells. Here we report an unexpected discovery that Nanog alone can induce PGCLCs in EpiLCs, independently of BMP4. We propose that after the dissolution of the naive ES-cell pluripotency network during establishment of EpiLCs, the epigenome is reset for cell fate determination. Indeed, we found genome-wide changes in NANOG-binding patterns between ES cells and EpiLCs, indicating epigenetic resetting of regulatory elements. Accordingly, we show that NANOG can bind and activate enhancers of Prdm1 and Prdm14 in EpiLCs in vitro; BLIMP1 (encoded by Prdm1) then directly induces Tfap2c. Furthermore, while SOX2 and NANOG promote the pluripotent state in ES cells, they show contrasting roles in EpiLCs, as Sox2 specifically represses PGCLC induction by Nanog. This study demonstrates a broadly applicable mechanistic principle for how cells acquire competence for cell fate determination, resulting in the context-dependent roles of key transcription factors during development.

Elf5-centered transcription factor hub controls trophoblast stem cell self-renewal and differentiation through stoichiometry-sensitive shifts in target gene networks

Latos et al. demonstrate that precise levels of Elf5 are critical for normal expansion of the trophoblast stem cell (TSC) compartment and embryonic survival. Their data place Elf5 at the center of a stoichiometry-sensitive transcriptional network, where it acts as a molecular switch governing the balance between TSC proliferation and differentiation.

Elf5 is a transcription factor with pivotal roles in the trophoblast compartment, where it reinforces a trophoblast stem cell (TSC)-specific transcriptional circuit. However, Elf5 is also present in differentiating trophoblast cells that have ceased to express other TSC genes such as Cdx2 and Eomes. In the present study, we aimed to elucidate the context-dependent role of Elf5 at the interface between TSC self-renewal and the onset of differentiation. We demonstrate that precise levels of Elf5 are critical for normal expansion of the TSC compartment and embryonic survival, as Elf5 overexpression triggers precocious trophoblast differentiation. Through integration of protein interactome, transcriptome, and genome-wide chromatin immunoprecipitation data, we reveal that this abundance-dependent function is mediated through a shift in preferred Elf5-binding partners; in TSCs, Elf5 interaction with Eomes recruits Tfap2c to triply occupied sites at TSC-specific genes, driving their expression. In contrast, the Elf5 and Tfap2c interaction becomes predominant as their protein levels increase. This triggers binding to double- and single-occupancy sites that harbor the cognate Tfap2c motif, causing activation of the associated differentiation-promoting genes. These data place Elf5 at the center of a stoichiometry-sensitive transcriptional network, where it acts as a molecular switch governing the balance between TSC proliferation and differentiation.

Extensive Nuclear Reprogramming Underlies Lineage Conversion into Functional Trophoblast Stem-like Cells

Induced pluripotent stem cells (iPSCs) undergo extensive nuclear reprogramming and are generally indistinguishable from embryonic stem cells (ESCs) in their functional capacity and transcriptome and DNA methylation profiles. However, direct conversion of cells from one lineage to another often yields incompletely reprogrammed, functionally compromised cells, raising the question of whether pluripotency is required to achieve a high degree of nuclear reprogramming. Here, we show that transient expression of Gata3, Eomes, and Tfap2c in mouse fibroblasts induces stable, transgene-independent trophoblast stem-like cells (iTSCs). iTSCs possess transcriptional profiles highly similar to blastocyst-derived TSCs, with comparable methylation and H3K27ac patterns and genome-wide H2A.X deposition. iTSCs generate trophoectodermal lineages upon differentiation, form hemorrhagic lesions, and contribute to developing placentas in chimera assays, indicating a high degree of nuclear reprogramming, with no evidence of passage through a transient pluripotent state. Together, these data demonstrate that extensive nuclear reprogramming can be achieved independently of pluripotency.

Gene Expression Profiling Reveals a Novel Regulatory Role for Sox21 Protein in Mouse Trophoblast Stem Cell Differentiation

Appropriate self-renewal and differentiation of trophoblast stem cells (TSCs) are key factors for proper placental development and function and, in turn, for appropriate in utero fetal growth. To identify novel TSC-specific genes, we performed genome-wide expression profiling of TSCs, embryonic stem cells, epiblast stem cells, and mouse embryo fibroblasts, derived from mice of the same genetic background. Our analysis revealed a high expression of Sox21 in TSCs compared with other cell types. Sox21 levels were high in undifferentiated TSCs and were dramatically reduced upon differentiation. In addition, modulation of Sox21 expression in TSCs affected lineage-specific differentiation, based on both marker analysis and functional assessment. Our results implicate Sox21 specifically in the promotion of spongiotrophoblast and giant cell differentiation and establish a new mechanism through which trophoblast sublineages are specified.

Ube2s regulates Sox2 stability and mouse ES cell maintenance

Sox2 has a critical role in embryonic stem (ES) cell maintenance and differentiation. Interestingly, its activity is highly dosage-dependent. Although transcriptional regulation of Sox2 has been extensively studied, the mechanisms orchestrating its degradation remain unclear. In this study, we identified ubiquitin-conjugating enzyme E2S (Ube2s) as a novel effector for Sox2 protein degradation. Ube2s mediates K11-linked polyubiquitin chain formation at the Sox2-K123 residue, thus marking it for proteasome-mediated degradation. Besides its role in fine-tuning the precise level of Sox2, Ube2s reinforces the self-renewing and pluripotent state of ES cells. Importantly, it also represses Sox2-mediated ES cell differentiation toward the neural ectodermal lineage.

Activin Regulates Self-renewal and Differentiation of Trophoblast Stem Cells by Down-regulating the X Chromosome Gene Bcor

The development of a functional placenta is largely dependent upon proper proliferation and differentiation of trophoblast stem cells (TSCs). Activin signaling has long been regarded to play important roles during this process, but the exact mechanism is largely unknown. Here, we demonstrate that the X chromosome gene BCL-6 corepressor (Bcor) is a critical downstream effector of activin to fine-tune mouse TSC fate decision. Bcor was specifically down-regulated by activin A in TSCs in a dose-dependent manner, and immediately up-regulated upon TSC differentiation. Knockdown of Bcor partially compensated for the absence of activin A in maintaining the self-renewal of TSCs together with FGF4, while promoting syncytiotrophoblast differentiation in the absence of FGF4. Moreover, the impaired trophoblast giant cell and spongiotrophoblast differentiation upon Bcor knockdown also resembled the function of activin. Reporter analysis showed that BCOR inhibited the expression of the key trophoblast regulator genes Eomes and Cebpa by binding to their promoter regions. Our findings provide us with a better understanding of placental development and placenta-related diseases.

Fgf and Esrrb integrate epigenetic and transcriptional networks that regulate self-renewal of trophoblast stem cells

Esrrb (oestrogen-related receptor beta) is a transcription factor implicated in embryonic stem (ES) cell self-renewal, yet its knockout causes intrauterine lethality due to defects in trophoblast development. Here we show that in trophoblast stem (TS) cells, Esrrb is a downstream target of fibroblast growth factor (Fgf) signalling and is critical to drive TS cell self-renewal. In contrast to its occupancy of pluripotency-associated loci in ES cells, Esrrb sustains the stemness of TS cells by direct binding and regulation of TS cell-specific transcription factors including Elf5 and Eomes. To elucidate the mechanisms whereby Esrrb controls the expression of its targets, we characterized its TS cell-specific interactome using mass spectrometry. Unlike in ES cells, Esrrb interacts in TS cells with the histone demethylase Lsd1 and with the RNA Polymerase II-associated Integrator complex. Our findings provide new insights into both the general and context-dependent wiring of transcription factor networks in stem cells by master transcription factors.

SOX2 Expression in Gastrointestinal Cancers of Iranian Patients

Purpose

Gastrointestinal (GI) malignancies are among the 5 most common cancers in Iran, and their high associated mortality rates are attributable to late diagnosis and poor treatment options. SOX2, a transcription factor necessary for maintenance and induction of pluripotency and self-renewal, has been identified as a lineage-survival oncogene in several cancers. In the present study, we examined SOX2 expression in esophageal squamous cell carcinoma (ESCC), gastric adenocarcinoma and colon squamous cell carcinoma (SCC), as well as normal GI tissues, in Iranian patients.

Methods

To elucidate the role of SOX2 in GI carcinogenesis, formalin-fixed tissues were analyzed using immunohistochemistry (IHC), while frozen ESCC samples were studied by quantitative reverse transcription polymerase chain reaction (qRT-PCR).

Results

IHC studies indicated presence of SOX2+ cells in a subset of cancerous and normal tissues of stomach and colon, while no significant difference was observed between groups, and no correlation was found between SOX2 expression and tumors grades. Nevertheless, studying ESCC samples with IHC and qRT-PCR revealed overexpression of SOX2 in comparison with normal adjacent tissues.

Conclusions

The present results are in line with other studies and indicate SOX2 up-regulation in ESCC; however, due to our small sample size and contradictory reports, more research is needed to determine the importance of SOX2 in GI cancers.

From blastocyst to gastrula: gene regulatory networks of embryonic stem cells and early mouse embryogenesis

To date, many regulatory genes and signalling events coordinating mammalian development from blastocyst to gastrulation stages have been identified by mutational analyses and reverse-genetic approaches, typically on a gene-by-gene basis. More recent studies have applied bioinformatic approaches to generate regulatory network models of gene interactions on a genome-wide scale. Such models have provided insights into the gene networks regulating pluripotency in embryonic and epiblast stem cells, as well as cell-lineage determination in vivo. Here, we review how regulatory networks constructed for different stem cell types relate to corresponding networks in vivo and provide insights into understanding the molecular regulation of the blastocyst-gastrula transition.

The nature of embryonic stem cells

Mouse embryonic stem (ES) cells perpetuate in vitro the broad developmental potential of naïve founder cells in the preimplantation embryo. ES cells self-renew relentlessly in culture but can reenter embryonic development seamlessly, differentiating on schedule to form all elements of the fetus. Here we review the properties of these remarkable cells. Arising from the stability, homogeneity, and equipotency of ES cells, we consider the concept of a pluripotent ground state. We evaluate the authenticity of ES cells in relation to cells in the embryo and examine their utility for dissecting mechanisms that confer pluripotency and that execute fate choice. We summarize current knowledge of the transcription factor circuitry that governs the ES cell state and discuss the opportunity to expose molecular logic further through iterative computational modeling and experimentation. Finally, we present a perspective on unresolved questions, including the challenge of deriving ground state pluripotent stem cells from non-rodent species.

Integrative Analysis of the Acquisition of Pluripotency in PGCs Reveals the Mutually Exclusive Roles of Blimp-1 and AKT Signaling

Primordial germ cells (PGCs) are lineage-restricted unipotent cells that can dedifferentiate into pluripotent embryonic germ cells (EGCs). Here we performed whole-transcriptome analysis during the conversion of PGCs into EGCs, a process by which cells acquire pluripotency. To examine the molecular mechanism underlying this conversion, we focused on Blimp-1 and Akt, which are involved in PGC specification and dedifferentiation, respectively. Blimp-1 overexpression in embryonic stem cells suppressed the expression of downstream targets of the pluripotency network. Conversely, Blimp-1 deletion in PGCs accelerated their dedifferentiation into pluripotent EGCs, illustrating that Blimp-1 is a pluripotency gatekeeper protein in PGCs. AKT signaling showed a synergistic effect with basic fibroblast growth factor plus 2i+A83 treatment on EGC formation. AKT played a major role in suppressing genes regulated by MBD3. From these results, we defined the distinct functions of Blimp-1 and Akt and provided mechanistic insights into the acquisition of pluripotency in PGCs.

Transcription factor AP-2γ induces early Cdx2 expression and represses HIPPO signaling to specify the trophectoderm lineage

Cell fate decisions are fundamental to the development of multicellular organisms. In mammals the first cell fate decision involves segregation of the pluripotent inner cell mass and the trophectoderm, a process regulated by cell polarity proteins, HIPPO signaling and lineage-specific transcription factors such as CDX2. However, the regulatory mechanisms that operate upstream to specify the trophectoderm lineage have not been established. Here we report that transcription factor AP-2γ (TFAP2C) functions as a novel upstream regulator of Cdx2 expression and position-dependent HIPPO signaling in mice. Loss- and gain-of-function studies and promoter analysis revealed that TFAP2C binding to an intronic enhancer is required for activation of Cdx2 expression during early development. During the 8-cell to morula transition TFAP2C potentiates cell polarity to suppress HIPPO signaling in the outside blastomeres. TFAP2C depletion triggered downregulation of PARD6B, loss of apical cell polarity, disorganization of F-actin, and activation of HIPPO signaling in the outside blastomeres. Rescue experiments using Pard6b mRNA restored cell polarity but only partially corrected position-dependent HIPPO signaling, suggesting that TFAP2C negatively regulates HIPPO signaling via multiple pathways. Several genes involved in regulation of the actin cytoskeleton (including Rock1, Rock2) were downregulated in TFAP2C-depleted embryos. Inhibition of ROCK1 and ROCK2 activity during the 8-cell to morula transition phenocopied TFAP2C knockdown, triggering a loss of position-dependent HIPPO signaling and decrease in Cdx2 expression. Altogether, these results demonstrate that TFAP2C facilitates trophectoderm lineage specification by functioning as a key regulator of Cdx2 transcription, cell polarity and position-dependent HIPPO signaling.

Adaptive mechanisms controlling uterine spiral artery remodeling during the establishment of pregnancy

Implantation of the embryo into the uterus triggers the initiation of hemochorial placentation. The hemochorial placenta facilitates the acquisition of maternal resources required for embryo/fetal growth. Uterine spiral arteries form the nutrient supply line for the placenta and fetus. This vascular conduit undergoes gestation stage-specific remodeling directed by maternal natural killer cells and embryo-derived invasive trophoblast lineages. The placentation site, including remodeling of the uterine spiral arteries, is shaped by environmental challenges. In this review, we discuss the cellular participants controlling pregnancy-dependent uterine spiral artery remodeling and mechanisms responsible for their development and function.

A Sox2 distal enhancer cluster regulates embryonic stem cell differentiation potential

The Sox2 transcription factor must be robustly transcribed in embryonic stem (ES) cells to maintain pluripotency. Zhou et al. identify three novel enhancers that, through the formation of chromatin loops, form a chromatin complex with the Sox2 promoter in ES cells. The distal cluster containing SRR107 and SRR111, located >100 kb downstream from Sox2, is required for cis-regulation of Sox2 in ES cells.

The Sox2 transcription factor must be robustly transcribed in embryonic stem (ES) cells to maintain pluripotency. Two gene-proximal enhancers, Sox2 regulatory region 1 (SRR1) and SRR2, display activity in reporter assays, but deleting SRR1 has no effect on pluripotency. We identified and functionally validated the sequences required for Sox2 transcription based on a computational model that predicted transcriptional enhancer elements within 130 kb of Sox2. Our reporter assays revealed three novel enhancers—SRR18, SRR107, and SRR111—that, through the formation of chromatin loops, form a chromatin complex with the Sox2 promoter in ES cells. Using the CRISPR/Cas9 system and F1 ES cells (Mus musculus¹²⁹ × Mus castaneus), we generated heterozygous deletions of each enhancer region, revealing that only the distal cluster containing SRR107 and SRR111, located >100 kb downstream from Sox2, is required for cis-regulation of Sox2 in ES cells. Furthermore, homozygous deletion of this distal Sox2 control region (SCR) caused significant reduction in Sox2 mRNA and protein levels, loss of ES cell colony morphology, genome-wide changes in gene expression, and impaired neuroectodermal formation upon spontaneous differentiation to embryoid bodies. Together, these data identify a distal control region essential for Sox2 transcription in ES cells.

Molecular basis of embryonic stem cell self-renewal: from signaling pathways to pluripotency network

Embryonic stem cells (ESCs) can be maintained in culture indefinitely while retaining the capacity to generate any type of cell in the body, and therefore not only hold great promise for tissue repair and regeneration, but also provide a powerful tool for modeling human disease and understanding biological development. In order to fulfill the full potential of ESCs, it is critical to understand how ESC fate, whether to self-renew or to differentiate into specialized cells, is regulated. On the molecular level, ESC fate is controlled by the intracellular transcriptional regulatory networks that respond to various extrinsic signaling stimuli. In this review, we discuss and compare important signaling pathways in the self-renewal and differentiation of mouse, rat, and human ESCs with an emphasis on how these pathways integrate into ESC-specific transcription circuitries. This will be beneficial for understanding the common and conserved mechanisms that govern self-renewal, and for developing novel culture conditions that support ESC derivation and maintenance.

Molecular biology of the stress response in the early embryo and its stem cells

Stress is normal during early embryogenesis and transient, elevated stress is commonplace. Stress in the milieu of the peri-implantation embryo is a summation of maternal hormones, and other elements of the maternal milieu, that signal preparedness for development and implantation. Examples discussed here are leptin, adrenaline, cortisol, and progesterone. These hormones signal maternal nutritional status and provide energy, but also signal stress that diverts maternal and embryonic energy from an optimal embryonic developmental trajectory. These hormones communicate endocrine maternal effects and local embryonic effects although signaling mechanisms are not well understood. Other in vivo stresses affect the embryo such as local infection and inflammation, hypoxia, environmental toxins such as benzopyrene, dioxin, or metals, heat shock, and hyperosmotic stress due to dehydration or diabetes. In vitro, stresses include shear during handling, improper culture media and oxygen levels, cryopreservation, and manipulations of the embryo to introduce sperm or mitochondria. We define stress as any stimulus that slows stem cell accumulation or diminishes the ability of cells to produce normal and sufficient parenchymal products upon differentiation. Thus stress deflects downwards the normal trajectories of development, growth and differentiation. Typically stress is inversely proportional to embryonic developmental and proliferative rates, but can be proportional to induction of differentiation of stem cells in the peri-implantation embryo. When modeling stress it is most interesting to produce a 'runting model' where stress exposures slow accumulation but do not create excessive apoptosis or morbidity. Windows of stress sensitivity may occur when major new embryonic developmental programs require large amounts of energy and are exacerbated if nutritional flow decreases and removes energy from the normal developmental programs and stress responses. These windows correspond to zygotic genome activation, the large mRNA program initiated at compaction, ion pumping required for cavitation, the differentiation of the first lineages, integration with the uterine environment at implantation, rapid proliferation of stem cells, and production of certain lineages which require the highest energy and are most sensitive to mitochondrial inhibition. Stress response mechanisms insure that stem cells for the early embryo and placenta survive at lower stress exposures, and that the organism survives through compensatory and prioritized stem cell differentiation, at higher stress exposures. These servomechanisms include a small set of stress enzymes from the 500 protein kinases in the kinome; the part of the genome coding for protein kinases that hierarchically regulate the activity of other proteins and enzymes. Important protein kinases that mediate the stress response of embryos and their stem cells are SAPK, p38MAPK, AMPK, PI3K, Akt, MEK1/2, MEKK4, PKA, IRE1 and PERK. These stress enzymes have cytosolic function in cell survival at low stress exposures and nuclear function in modifying transcription factor activity at higher stress exposures. Some of the transcription factors (TFs) that are most important in the stress response are JunC, JunB, MAPKAPs, ATF4, XBP1, Oct1, Oct4, HIFs, Nrf2/KEAP, NFKB, MT1, Nfat5, HSF1/2 and potency-maintaining factors Id2, Cdx2, Eomes, Sox2, Nanog, Rex1, and Oct4. Clearly the stress enzymes have a large number of cytosolic and nuclear substrates and the TFs regulate large numbers of genes. The interaction of stress enzymes and TFs in the early embryo and its stem cells are a continuing central focus of research. In vitro regulation of TFs by stress enzymes leads to reprogramming of the stem cell when stress diminishes stem cell accumulation. Since more differentiated product is produced by fewer cells, the process compensates for fewer cells. Coupled with stress-induced compensatory differentiation of stem cells is a tendency to prioritize differentiation by increasing the first essential lineage and decreasing later lineages. These mechanisms include stress enzymes that regulate TFs and provide stress-specific, shared homeostatic cellular and organismal responses of prioritized differentiation.

Ex Uno Plures: Molecular Designs for Embryonic Pluripotency

Pluripotent cells in embryos are situated near the apex of the hierarchy of developmental potential. They are capable of generating all cell types of the mammalian body proper. Therefore, they are the exemplar of stem cells. In vivo, pluripotent cells exist transiently and become expended within a few days of their establishment. Yet, when explanted into artificial culture conditions, they can be indefinitely propagated in vitro as pluripotent stem cell lines. A host of transcription factors and regulatory genes are now known to underpin the pluripotent state. Nonetheless, how pluripotent cells are equipped with their vast multilineage differentiation potential remains elusive. Consensus holds that pluripotency transcription factors prevent differentiation by inhibiting the expression of differentiation genes. However, this does not explain the developmental potential of pluripotent cells. We have presented another emergent perspective, namely, that pluripotency factors function as lineage specifiers that enable pluripotent cells to differentiate into specific lineages, therefore endowing pluripotent cells with their multilineage potential. Here we provide a comprehensive overview of the developmental biology, transcription factors, and extrinsic signaling associated with pluripotent cells, and their accompanying subtypes, in vitro heterogeneity and chromatin states. Although much has been learned since the appreciation of mammalian pluripotency in the 1950s and the derivation of embryonic stem cell lines in 1981, we will specifically emphasize what currently remains unclear. However, the view that pluripotency factors capacitate differentiation, recently corroborated by experimental evidence, might perhaps address the long-standing question of how pluripotent cells are endowed with their multilineage differentiation potential.