Ets2-dependent trophoblast signalling is required for gastrulation progression after primitive streak initiation

New findings on the orientation of the mouse anterior-posterior (A-P) axis before and during the initiation of gastrulation using a more refined embryo staging

A clinically significant event of early mammalian embryogenesis is the generation and early development of the anterior-posterior (A-P) axis, the imaginary line along which the structures from head to tail will form. This axis not only appears before gastrulation but is also oriented in a specific way in relation to the long and short diameters of the bilaterally symmetric epiblast. In mice, the most widely used mammalian in vivo model of early embryogenesis, the A-P axis is normally aligned with the long epiblast diameter by the early streak (ES) stage, a time during early gastrulation around embryonic day 6.5 (E6.5). Incorrect orientation of the A-P axis by the ES stage, that is, being aligned with the short epiblast diameter, leads to failure in completing gastrulation and results in embryo death soon after. Knowing the orientation of this axis from when it forms before gastrulation (around E5.5) until just before the ES stage is crucial for: (a) understanding the ill-defined factors involved in its formation and early development since they must be spatially related to it, and (b) providing explanations for the underlying mechanism when it is incorrectly orientated. However, the orientation of the A-P axis in pre-ES embryos of the E5.5-E6.5 period remains unclear. Specifically, although it is thought that this axis initially aligns with the short epiblast diameter and subsequently changes its orientation to become aligned with the long diameter by an unidentified pre-gastrulation stage before the ES stage, this proposition remains unresolved. This is largely due to the lack of clearly defined morphological criteria for staging certain periods of pre-ES mouse embryos (especially when the A-P axis initiates and when gastrulation begins prior to the ES stage), which are a prerequisite for identifying A-P axis orientation at specific pre-ES stages. Furthermore, although the orientation of an extraembryonic trophoblast asymmetry, specifically the tilt of the ectoplacental cone (EPC), coincides with that of the A-P axis by the ES stage, it is unknown whether such an association also exists at pre-gastrulation stages during A-P axis formation. Knowing this would exclude or implicate this trophoblast asymmetry as an upstream factor in orientating the A-P axis when it forms. To address these issues, we established a more refined embryo staging for the E5.5-E6.5 period using a novel combination of live morphological criteria and used it to examine the orientation of the A-P axis and that of the EPC tilt at specific stages. First, contrary to current thinking, we show that when the A-P axis first appears at our newly described anterior visceral endoderm-1 (AVE-1) and AVE-2 stages, it aligns with the long epiblast diameter in all embryos. This orientation is maintained in most embryos at all subsequent pre-gastrulation stages, specifically at our AVE-3 and pre-streak stages (the remaining embryos of these stages had this axis aligned with the short epiblast diameter). Second, we identified for the first time the pre-ES stage when gastrulation initiates, which we named the nascent streak (NS) stage, and further subdivided it into NS-1 and NS-2. At variance with current belief, we provide evidence that the earliest stage just before the ES stage when all embryos align their A-P axis with the long epiblast diameter is not a pre-gastrulation stage, but the NS-2 stage (at NS-1, most but not all embryos had this A-P axis orientation). Third, we implicate the EPC tilt as a possible extraembryonic factor in promoting correct A-P axis orientation, as this tilt exists before the AVE-1 stage and its orientation coincided with that of the A-P axis in all embryos at AVE-1, AVE-2 and ES stages and almost all embryos at AVE-3, pre-streak and NS stages. Overall, our work: (a) identified the previously unresolved orientation of the mouse A-P axis within the epiblast before the ES stage during the E5.5-E6.5 period; (b) provides an alternative explanation for when this axis is incorrectly oriented by the ES stage, namely, its defective alignment with the short epiblast diameter by this stage could be due to its failure to align with the long epiblast diameter from the time of its formation; and (c) implicates the pre-existing orientation of the EPC tilt as a possible factor in orientating the newly formed A-P axis.

The beginning of becoming a human

According to birth certificates, the life of a child begins once their body comes out of the mother's womb. But when does their organismal life begin? Science holds a palette of answers-depending on how one defines a human life. In 1984, a commission on the regulatory framework for human embryo experimentation opted not to answer this question, instead setting a boundary, 14 days post-fertilization, beyond which any experiments were forbidden. Recently, as the reproductive technologies developed and the demand for experimentation grew stronger, this boundary may be set aside leaving the ultimate decision to local oversight committees. While science has not come closer to setting a zero point for human life, there has been significant progress in our understanding of early mammalian embryogenesis. It has become clear that the 14-day stage does in fact possess features, which make it a foundational time point for a developing human. Importantly, this stage defines the separation of soma from the germline and marks the boundary between rejuvenation and aging. We explore how different levels of life organization emerge during human development and suggest a new meaning for the 14-day stage in organismal life that is grounded in recent mechanistic advances and insights from aging studies.

Development of innovative marker detection methods for high-fertility ducks (Anas plastyrhynchos)

We previously analyzed the genome-wide gene expression at the transcription level in pre-hierarchical ovarian follicles (approximate 5 mm in diameter) between two groups of ducks representing high and low fertility. Orthodenticle homeobox 2 (otx2) was identified with significantly differential expression in the high-fertility group versus the low-fertility group. To identify the relationships between genotypes and phenotypes, we recorded the reproductive performance in advance, including fertility, hatchability, and fertile period of female ducks. To ensure coverage of the entire duration of the fertile period, we extended the egg collection period after artificial insemination. Naturally, sperm cannot survive after a certain period of time in the female reproductive tract (sperm is not immortal); therefore, lower average values for fertility were observed in this study than that observed after a normal egg collection period, i.e., the lower average values of fertility (18 days after artificial insemination), were not due to the effect of otx2. The otx2 genomic sequence of Tsaiya ducks was firstly amplified with a primer pair of i3F and i3R for polymerase chain reaction based on Pekin duck sequence and a resultant 444-base pair fragment was obtained for DNA sequencing. Using multiple sequence alignment, new single-nucleotide polymorphisms g.366T > C and g.182G > T were discovered in the otx2 gene. With respect to g.366T > C, ducks were classified into CC, CT, and TT genotypes. For g.182G > T, three genotypes (GG, GT, and TT) were identified. Ducks were genotyped using novel specific primers and probes to rapidly screen their single-nucleotide polymorphisms. The results indicated that ducks with the CC genotype of g.366T > C exhibited the highest fertility among the CC, CT, and TT genotypes (p < 0.05). No significant difference was found in the fertile period and hatchability among three genotypes of g.366T > C. Moreover, no association was found between g.182G > T genotypes and the three reproductive phenotypes examined in this study. Collectively, the otx2 g.366T > C genotype is associated with duck females, and can be used as a marker for farming a flock of ducks with high fertility, as well as for genetic selection of breeders.

Construction of a mammalian embryo model from stem cells organized by a morphogen signalling centre

Generating properly differentiated embryonic structures in vitro from pluripotent stem cells remains a challenge. Here we show that instruction of aggregates of mouse embryonic stem cells with an experimentally engineered morphogen signalling centre, that functions as an organizer, results in the development of embryo-like entities (embryoids). In situ hybridization, immunolabelling, cell tracking and transcriptomic analyses show that these embryoids form the three germ layers through a gastrulation process and that they exhibit a wide range of developmental structures, highly similar to neurula-stage mouse embryos. Embryoids are organized around an axial chordamesoderm, with a dorsal neural plate that displays histological properties similar to the murine embryo neuroepithelium and that folds into a neural tube patterned antero-posteriorly from the posterior midbrain to the tip of the tail. Lateral to the chordamesoderm, embryoids display somitic and intermediate mesoderm, with beating cardiac tissue anteriorly and formation of a vasculature network. Ventrally, embryoids differentiate a primitive gut tube, which is patterned both antero-posteriorly and dorso-ventrally. Altogether, embryoids provide an in vitro model of mammalian embryo that displays extensive development of germ layer derivatives and that promises to be a powerful tool for in vitro studies and disease modelling.

EGF suppresses the expression of miR-124a in pancreatic β cell lines via ETS2 activation through the MEK and PI3K signaling pathways

Diabetes mellitus is characterized by pancreatic β cell dysfunction. Previous studies have indicated that epidermal growth factor (EGF) and microRNA-124a (miR-124a) play opposite roles in insulin biosynthesis and secretion by beta cells. However, the underlying mechanisms remain poorly understood. In the present study, we demonstrated that EGF could inhibit miR-124a expression in beta cell lines through downstream signaling pathways, including mitogen-activated protein kinase kinase (MEK) and phosphatidylinositol 3-kinase (PI3K) cascades. Further, the transcription factor ETS2, a member of the ETS (E26 transformation-specific) family, was identified to be responsible for the EGF-mediated suppression of miR-124a expression, which was dependent on ETS2 phosphorylation at threonine 72. Activation of ETS2 decreased miR-124a promoter transcriptional activity through the putative conserved binding sites AGGAANA/TN in three miR-124a promoters located in different chromosomes. Of note, ETS2 played a positive role in regulating beta cell function-related genes, including miR-124a targets, Forkhead box a2 (FOXA2) and Neurogenic differentiation 1 (NEUROD1), which may have partly been through the inhibition of miR-124 expression. Knockdown and overexpression of ETS2 led to the prevention and promotion of insulin biosynthesis respectively, while barely affecting the secretion ability. These results suggest that EGF may induce the activation of ETS2 to inhibit miR-124a expression to maintain proper beta cell functions and that ETS2, as a novel regulator of insulin production, is a potential therapeutic target for diabetes mellitus treatment.

Human stem cells alter the invasive properties of somatic cells via paracrine activation of mTORC1

Controlled invasion is essential during many physiological processes, whereas its deregulation is a hallmark of cancer. Here we demonstrate that embryonic, induced pluripotent and amniotic fluid stem cells share the property to induce the invasion of primary somatic cells of various origins through insulin-like growth factor I (IGF-I)- or II (IGF-II)-mediated paracrine activation of mechanistic target of rapamycin complex 1 (mTORC1). We propose a model in which downstream of mTORC1 this stem cell-induced invasion is mediated by hypoxia-inducible factor 1-alpha (HIF-1α)-regulated matrix metalloproteinases. Manipulating the IGF signalling pathway in the context of teratoma formation experiments demonstrates that human stem cells use this mechanism to induce invasion and thereby attract cells from the microenvironment in vivo. In this study we have identified a so far unknown feature of human stem cells, which might play a role for the development of stem cell-derived tumours.Cell invasion is required for several physiological processes but it is unknown if stem cells induce invasiveness in other cells. Here, the authors show that human stem cells secrete insulin-like growth factor, which in turn activates the mTORC1 pathway, initiating invasive behaviour and attracting other cells.

The epigenetic modifier Fam208a is required to maintain epiblast cell fitness

Gastrulation initiates with the formation of the primitive streak, during which, cells of the epiblast delaminate to form the mesoderm and definitive endoderm. At this stage, the pluripotent cell population of the epiblast undergoes very rapid proliferation and extensive epigenetic programming. Here we show that Fam208a, a new epigenetic modifier, is essential for early post-implantation development. We show that Fam208a mutation leads to impaired primitive streak elongation and delayed epithelial-to-mesenchymal transition. Fam208a mutant epiblasts had increased expression of p53 pathway genes as well as several pluripotency-associated long non-coding RNAs. Fam208a mutants exhibited an increase in p53-driven apoptosis and complete removal of p53 could partially rescue their gastrulation block. This data demonstrates a new in vivo function of Fam208a in maintaining epiblast fitness, establishing it as an important factor at the onset of gastrulation when cells are exiting pluripotency.

EMT: 2016

The significant parallels between cell plasticity during embryonic development and carcinoma progression have helped us understand the importance of the epithelial-mesenchymal transition (EMT) in human disease. Our expanding knowledge of EMT has led to a clarification of the EMT program as a set of multiple and dynamic transitional states between the epithelial and mesenchymal phenotypes, as opposed to a process involving a single binary decision. EMT and its intermediate states have recently been identified as crucial drivers of organ fibrosis and tumor progression, although there is some need for caution when interpreting its contribution to metastatic colonization. Here, we discuss the current state-of-the-art and latest findings regarding the concept of cellular plasticity and heterogeneity in EMT. We raise some of the questions pending and identify the challenges faced in this fast-moving field.

ELF5 transcription factor expression during gestation in humans and rats - an immunohistochemical analysis

OBJECTIVE

The purpose of this study was to measure immunohistochemically the expression of ELF5 protein in term human and rat placentas and in human placentas associated with gestational diabetes (GD) and intrauterine growth restriction (IUGR).

METHODS

The results were quantitated stereologically using the stereological variable of volume density. A semiquantitative analysis was performed independently by a certified pathologist.

RESULTS

Total expression of ELF5 protein was higher in pathological pregnancies than in corresponding control term placentas, with both methods of quantifications showing similar results. In addition, ELF5 expression was also higher in connective tissue and blood vessels in chorionic villi in IUGR placentas (but not in GD placentas) compared to healthy controls. ELF5 is higher in placenta as a whole and in most of its components in both pathologies. The two exceptions are chorionic plates in IUGR and decidua in GD, where its expression is lower than in healthy controls.

CONCLUSIONS

We have shown that IUGR and GD are associated with significantly increased levels of ELF5 protein in placentas, which suggests that ELF5 may play an important role in normal placentation. However, these are term placentas and to study ELF5 in premature births would give better insight into human placentation in health and disease.

ETS transcription factors in embryonic vascular development

At least thirteen ETS-domain transcription factors are expressed during embryonic hematopoietic or vascular development and potentially function in the formation and maintenance of the embryonic vasculature or blood lineages. This review summarizes our current understanding of the specific roles played by ETS factors in vasculogenesis and angiogenesis and the implications of functional redundancies between them.

Synergistic action of master transcription factors controls epithelial-to-mesenchymal transition

Epithelial-to-mesenchymal transition (EMT) is a complex multistep process in which phenotype switches are mediated by a network of transcription factors (TFs). Systematic characterization of all dynamic TFs controlling EMT state transitions, especially for the intermediate partial-EMT state, represents a highly relevant yet largely unexplored task. Here, we performed a computational analysis that integrated time-course EMT transcriptomic data with public cistromic data and identified three synergistic master TFs (ETS2, HNF4A and JUNB) that regulate the transition through the partial-EMT state. Overexpression of these regulators predicted a poor clinical outcome, and their elimination readily abolished TGF-β-induced EMT. Importantly, these factors utilized a clique motif, physically interact and their cumulative binding generally characterized EMT-associated genes. Furthermore, analyses of H3K27ac ChIP-seq data revealed that ETS2, HNF4A and JUNB are associated with super-enhancers and the administration of BRD4 inhibitor readily abolished TGF-β-induced EMT. These findings have implications for systematic discovery of master EMT regulators and super-enhancers as novel targets for controlling metastasis.

The Efficient Derivation of Trophoblast Cells from Porcine In Vitro Fertilized and Parthenogenetic Blastocysts and Culture with ROCK Inhibitor Y-27632

Trophoblasts (TR) are specialized cells of the placenta and play an important role in embryo implantation. The in vitro culture of trophoblasts provided an important tool to investigate the mechanisms of implantation. In the present study, porcine trophoblast cells were derived from pig in vitro fertilized (IVF) and parthenogenetically activated (PA) blastocysts via culturing in medium supplemented with KnockOut serum replacement (KOSR) and basic fibroblast growth factor (bFGF) on STO feeder layers, and the effect of ROCK (Rho-associated coiled-coil protein kinases) inhibiter Y-27632 on the cell lines culture was tested. 5 PA blastocyst derived cell lines and 2 IVF blastocyst derived cell lines have been cultured more than 20 passages; one PA cell lines reached 110 passages without obvious morphological alteration. The derived trophoblast cells exhibited epithelium-like morphology, rich in lipid droplets, and had obvious defined boundaries with the feeder cells. The cells were histochemically stained positive for alkaline phosphatase. The expression of TR lineage markers, such as CDX2, KRT7, KRT18, TEAD4, ELF5 and HAND1, imprinted genes such as IGF2, PEG1 and PEG10, and telomerase activity related genes TERC and TERF2 were detected by immunofluorescence staining, reverse transcription PCR and quantitative real-time PCR analyses. Both PA and IVF blastocysts derived trophoblast cells possessed the ability to differentiate into mature trophoblast cells in vitro. The addition of Y-27632 improved the growth of both PA and IVF blastocyst derived cell lines and increased the expression of trophoblast genes. This study has provided an alternative highly efficient method to establish trophoblast for research focused on peri-implantation and placenta development in IVF and PA embryos.

Developmental plasticity, cell fate specification and morphogenesis in the early mouse embryo

A critical point in mammalian development is when the early embryo implants into its mother's uterus. This event has historically been difficult to study due to the fact that it occurs within the maternal tissue and therefore is hidden from view. In this review, we discuss how the mouse embryo is prepared for implantation and the molecular mechanisms involved in directing and coordinating this crucial event. Prior to implantation, the cells of the embryo are specified as precursors of future embryonic and extra-embryonic lineages. These preimplantation cell fate decisions rely on a combination of factors including cell polarity, position and cell–cell signalling and are influenced by the heterogeneity between early embryo cells. At the point of implantation, signalling events between the embryo and mother, and between the embryonic and extraembryonic compartments of the embryo itself, orchestrate a total reorganization of the embryo, coupled with a burst of cell proliferation. New developments in embryo culture and imaging techniques have recently revealed the growth and morphogenesis of the embryo at the time of implantation, leading to a new model for the blastocyst to egg cylinder transition. In this model, pluripotent cells that will give rise to the fetus self-organize into a polarized three-dimensional rosette-like structure that initiates egg cylinder formation.

Sonic hedgehog through Gli2 and Gli3 is required for the proper development of placental labyrinth

Sonic hedgehog (Shh) functions as a conserved morphogen in the development of various organs in metazoans ranging from Drosophila to humans. Here, we have investigated the potential roles and underlying mechanisms of Shh signaling in murine placentation. Immunostaining revealed the abundant expression of the main components of Shh pathway in both the trophectoderm of blastocysts and developing placentas. Disruption of Shh led to impaired vascularogenesis of yolk sac, less branching and malformation of placental labyrinth, thereby leading to a robust decrease in capacity of transplacental passages. Moreover, placenta-specific gene incorporation by lentiviral transduction of mouse blastocysts and blastocyst transplantation robustly knocked down the expression of Gli3 and Gli2 in placenta but not in embryos. Finally, Gli3 knockdown in Shh^−/− placentas partially rescued the defects of both yolk sac and placental labyrinth, and robustly restored the capacity of transplacental passages. Gli2 knockdown in Shh^+/− placentas affected neither the capacity of tranplacental passages nor the vascularogenesis of yolk sac, however, it partially phenocopied the labyrinthine defects of Shh^−/− placentas. Taken together, these results uncover that both Shh/Gli2 and Shh/Gli3 signals are required for proper development of murine placentas and are possibly essential for pregnant maintenance.

Elf5 and Ets2 maintain the mouse extraembryonic ectoderm in a dosage dependent synergistic manner

The ETS superfamily transcription factors Elf5 and Ets2 have both been implicated in the maintenance of the extraembryonic ectoderm (ExE) of the mouse embryo. While homozygous mutants of either gene result in various degrees of ExE tissue loss, heterozygotes are without phenotype. We show here that compound heterozygous mutants exhibit a phenotype intermediate to that of the more severe Elf5-/- and the milder Ets2-/- mutants. Functional redundancy is shown via commonalities in expression patterns, in target gene expression, and by partial rescue of Elf5-/- mutants through overexpressing Ets2 in an Elf5-like fashion. A model is presented suggesting the functional division of the ExE region into a proximal and distal domain based on gene expression patterns and the proximal to distal increasing sensitivity to threshold levels of combined Elf5 and Ets2 activity.

Transcriptional regulators of the trophoblast lineage in mammals with hemochorial placentation

Mammalian reproduction is critically dependent on the trophoblast cell lineage, which assures proper establishment of maternal-fetal interactions during pregnancy. Specification of trophoblast cell lineage begins with the development of the trophectoderm (TE) in preimplantation embryos. Subsequently, other trophoblast cell types arise with the progression of pregnancy. Studies with transgenic animal models as well as trophoblast stem/progenitor cells have implicated distinct transcriptional and epigenetic regulators in trophoblast lineage development. This review focuses on our current understanding of transcriptional and epigenetic mechanisms regulating specification, determination, maintenance and differentiation of trophoblast cells.