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.

Cell death and morphogenesis during early mouse development: are they interconnected?

Shortly after implantation the embryonic lineage transforms from a coherent ball of cells into polarized cup shaped epithelium. Recently we elucidated a previously unknown apoptosis-independent morphogenic event that reorganizes the pluripotent lineage. Polarization cues from the surrounding basement membrane rearrange the epiblast into a polarized rosette-like structure, where subsequently a central lumen is established. Thus, we provided a new model revising the current concept of apoptosis-dependent epiblast morphogenesis. Cell death however has to be tightly regulated during embryogenesis to ensure developmental success. Here, we follow the stages of early mouse development and take a glimpse at the critical signaling and morphogenic events that determine cells destiny and reshape the embryonic lineage.

BMP signalling regulates the pre-implantation development of extra-embryonic cell lineages in the mouse embryo

Pre-implantation development requires the specification and organization of embryonic and extra-embryonic lineages. The separation of these lineages takes place when asymmetric divisions generate inside and outside cells that differ in polarity, position and fate. Here we assess the global transcriptional identities of these precursor cells to gain insight into the molecular mechanisms regulating lineage segregation. Unexpectedly, this reveals that complementary components of the BMP signalling pathway are already differentially expressed after the first wave of asymmetric divisions. We investigate the role of BMP signalling by expressing dominant negative forms of Smad4 and Bmpr2, by down-regulating the pathway using RNAi against BMP ligands and by applying three different BMP inhibitors at distinct stages. This reveals that BMP signalling regulates the correct development of both extra-embryonic lineages, primitive endoderm and trophectoderm, but not the embryonic lineage, prior to implantation. Together these findings indicate multiple roles of BMP signalling in the early mouse embryo.

Maternal-zygotic knockout reveals a critical role of Cdx2 in the morula to blastocyst transition

The first lineage segregation in the mouse embryo generates the inner cell mass (ICM), which gives rise to the pluripotent epiblast and therefore the future embryo, and the trophectoderm (TE), which will build the placenta. The TE lineage depends on the transcription factor Cdx2. However, when Cdx2 first starts to act remains unclear. Embryos with zygotic deletion of Cdx2 develop normally until the late blastocyst stage leading to the conclusion that Cdx2 is important for the maintenance but not specification of the TE. In contrast, down-regulation of Cdx2 transcripts from the early embryo stage results in defects in TE specification before the blastocyst stage. Here, to unambiguously address at which developmental stage Cdx2 becomes first required, we genetically deleted Cdx2 from the oocyte stage using a Zp3-Cre/loxP strategy. Careful assessment of a large cohort of Cdx2 maternal-zygotic null embryos, all individually filmed, examined and genotyped, reveals an earlier lethal phenotype than observed in Cdx2 zygotic null embryos that develop until the late blastocyst stage. The developmental failure of Cdx2 maternal-zygotic null embryos is associated with cell death and failure of TE specification, starting at the morula stage. These results indicate that Cdx2 is important for the correct specification of TE from the morula stage onwards and that both maternal and zygotic pools of Cdx2 are required for correct pre-implantation embryogenesis.

The basal position of nuclei is one pre-requisite for asymmetric cell divisions in the early mouse embryo

The early mouse embryo undertakes two types of cell division: symmetric that gives rise to the trophectoderm and then placenta or asymmetric that gives rise to inner cells that generate the embryo proper. Although cell division orientation is important, the mechanism regulating it has remained unclear. Here, we identify the relationship between the plane of cell division and the position of the nucleus and go towards identifying the mechanism behind it. We first find that as the 8-cell embryo progresses through the cell cycle, the nuclei of most - but not all - cells move from apical to more basal positions, in a microtubule- and kinesin-dependent manner. We then find that all asymmetric divisions happen when nuclei are located basally and, in contrast, all cells, in which nuclei remain apical, divide symmetrically. To understand the potential mechanism behind this, we determine the effects of modulating expression of Cdx2, a transcription factor key for trophectoderm formation and cell polarity. We find that increased expression of Cdx2 leads to an increase in a number of apical nuclei, whereas down-regulation of Cdx2 leads to more nuclei moving basally, which explains a previously identified relationship between Cdx2 and cell division orientation. Finally, we show that down-regulation of aPKC, involved in cell polarity, decreases the number of apical nuclei and doubles the number of asymmetric divisions. These results suggest a model in which the mutual interdependence of Cdx2 and cell polarity affects the cytoskeleton-dependent positioning of nuclei and, in consequence, the plane of cell division in the early mouse embryo.

Citrullination regulates pluripotency and histone H1 binding to chromatin

Citrullination is the post-translational conversion of an arginine residue within a protein to the non-coded amino acid citrulline. This modification leads to the loss of a positive charge and reduction in hydrogen-bonding ability. It is carried out by a small family of tissue-specific vertebrate enzymes called peptidylarginine deiminases (PADIs) and is associated with the development of diverse pathological states such as autoimmunity, cancer, neurodegenerative disorders, prion diseases and thrombosis. Nevertheless, the physiological functions of citrullination remain ill-defined, although citrullination of core histones has been linked to transcriptional regulation and the DNA damage response. PADI4 (also called PAD4 or PADV), the only PADI with a nuclear localization signal, was previously shown to act in myeloid cells where it mediates profound chromatin decondensation during the innate immune response to infection. Here we show that the expression and enzymatic activity of Padi4 are also induced under conditions of ground-state pluripotency and during reprogramming in mouse. Padi4 is part of the pluripotency transcriptional network, binding to regulatory elements of key stem-cell genes and activating their expression. Its inhibition lowers the percentage of pluripotent cells in the early mouse embryo and significantly reduces reprogramming efficiency. Using an unbiased proteomic approach we identify linker histone H1 variants, which are involved in the generation of compact chromatin, as novel PADI4 substrates. Citrullination of a single arginine residue within the DNA-binding site of H1 results in its displacement from chromatin and global chromatin decondensation. Together, these results uncover a role for citrullination in the regulation of pluripotency and provide new mechanistic insights into how citrullination regulates chromatin compaction.

Angiomotin prevents pluripotent lineage differentiation in mouse embryos via Hippo pathway-dependent and -independent mechanisms

Cell identity is specified in the early mammalian embryo by the generation of precursors for two cell lineages: the pluripotent inner cell mass and differentiating trophectoderm. Here we identify Angiomotin as a key regulator of this process. We show that the loss of Angiomotin, together with Angiomotin-like 2, leads to differentiation of inner cell mass cells and compromised peri-implantation development. We show that Angiomotin regulates localization of Yap, and Yap-binding motifs are required for full activity of Angiomotin. Importantly, we also show that Angiomotin function can compensate for the absence of Lats1/2 kinases, indicating the ability of Angiomotin to bypass the classical Hippo pathway for Yap regulation. In polarized outside cells, Angiomotin localizes apically, pointing to the importance of cell polarity in regulating Yap to promote differentiation. We propose that both Hippo pathway-dependent and Hippo pathway-independent mechanisms regulate Yap localization to set apart pluripotent and differentiated lineages in the pre-implantation mouse embryo.

Angiomotins retain the transcription co-activator YAP in the cytoplasm and thereby regulate the Hippo pathway in mammalian cultured cells. Here Leung and Zernicka-Goetz show that Angiomotin family members prevent the differentiation of inner cell mass cells in the mouse blastocyst, via both Hippo pathway-dependent and -independent mechanisms.

The differential response to Fgf signalling in cells internalized at different times influences lineage segregation in preimplantation mouse embryos

Lineage specification in the preimplantation mouse embryo is a regulative process. Thus, it has been difficult to ascertain whether segregation of the inner-cell-mass (ICM) into precursors of the pluripotent epiblast (EPI) and the differentiating primitive endoderm (PE) is random or influenced by developmental history. Here, our results lead to a unifying model for cell fate specification in which the time of internalization and the relative contribution of ICM cells generated by two waves of asymmetric divisions influence cell fate. We show that cells generated in the second wave express higher levels of Fgfr2 than those generated in the first, leading to ICM cells with varying Fgfr2 expression. To test whether such heterogeneity is enough to bias cell fate, we upregulate Fgfr2 and show it directs cells towards PE. Our results suggest that the strength of this bias is influenced by the number of cells generated in the first wave and, mostly likely, by the level of Fgf signalling in the ICM. Differences in the developmental potential of eight-cell- and 16-cell-stage outside blastomeres placed in the inside of chimaeric embryos further support this conclusion. These results unite previous findings demonstrating the importance of developmental history and Fgf signalling in determining cell fate.

Asymmetric localization of Cdx2 mRNA during the first cell-fate decision in early mouse development

A longstanding question in mammalian development is whether the divisions that segregate pluripotent progenitor cells for the future embryo from cells that differentiate into extraembryonic structures are asymmetric in cell-fate instructions. The transcription factor Cdx2 plays a key role in the first cell-fate decision. Here, using live-embryo imaging, we show that localization of Cdx2 transcripts becomes asymmetric during development, preceding cell lineage segregation. Cdx2 transcripts preferentially localize apically at the late eight-cell stage and become inherited asymmetrically during divisions that set apart pluripotent and differentiating cells. Asymmetric localization depends on a cis element within the coding region of Cdx2 and requires cell polarization as well as intact microtubule and actin cytoskeletons. Failure to enrich Cdx2 transcripts apically results in a significant decrease in the number of pluripotent cells. We discuss how the asymmetric localization and segregation of Cdx2 transcripts could contribute to multiple mechanisms that establish different cell fates in the mouse embryo.

Histone variant macroH2A marks embryonic differentiation in vivo and acts as an epigenetic barrier to induced pluripotency

How cell fate becomes restricted during somatic cell differentiation is a long-lasting question in biology. Epigenetic mechanisms not present in pluripotent cells and acquired during embryonic development are expected to stabilize the differentiated state of somatic cells and thereby restrict their ability to convert to another fate. The histone variant macroH2A acts as a component of an epigenetic multilayer that heritably maintains the silent X chromosome and has been shown to restrict tumor development. Here we show that macroH2A marks the differentiated cell state during mouse embryogenesis. MacroH2A.1 was found to be present at low levels upon the establishment of pluripotency in the inner cell mass and epiblast, but it was highly enriched in the trophectoderm and differentiated somatic cells later in mouse development. Chromatin immunoprecipitation revealed that macroH2A.1 is incorporated in the chromatin of regulatory regions of pluripotency genes in somatic cells such as mouse embryonic fibroblasts and adult neural stem cells, but not in embryonic stem cells. Removal of macroH2A.1, macroH2A.2 or both increased the efficiency of induced pluripotency up to 25-fold. The obtained induced pluripotent stem cells reactivated pluripotency genes, silenced retroviral transgenes and contributed to chimeras. In addition, overexpression of macroH2A isoforms prevented efficient reprogramming of epiblast stem cells to naïve pluripotency. In summary, our study identifies for the first time a link between an epigenetic mark and cell fate restriction during somatic cell differentiation, which helps to maintain cell identity and antagonizes induction of a pluripotent stem cell state.

Developmental plasticity is bound by pluripotency and the Fgf and Wnt signaling pathways

Plasticity is a well-known feature of mammalian development, and yet very little is known about its underlying mechanism. Here, we establish a model system to examine the extent and limitations of developmental plasticity in living mouse embryos. We show that halved embryos follow the same strict clock of developmental transitions as intact embryos, but their potential is not equal. We have determined that unless a minimum of four pluripotent cells is established before implantation, development will arrest. This failure can be rescued by modulating Fgf and Wnt signaling to enhance pluripotent cell number, allowing the generation of monozygotic twins, which is an otherwise rare phenomenon. Knowledge of the minimum pluripotent-cell number required for development to birth, as well as the different potentials of blastomeres, allowed us to establish a protocol for splitting an embryo into one part that develops to adulthood and another that provides embryonic stem cells for that individual.

Advances in embryo selection methods

Despite many recent advances in the field of reproductive biology and medicine, the efficiency of in vitro fertilization procedures remains relatively low. There is a need for a reliable and non-invasive method of embryo selection to ensure that only embryos with the highest developmental potential are chosen for transfer to mothers-to-be. Here, we compare various methods currently used for assessing embryonic viability, such as examination of embryonic morphology, quality of the genetic material, or metabolism. Additionally, we discuss novel procedures for embryonic assessment based on advanced time-lapse imaging techniques, which show great promise and may lead to increased in vitro fertilization efficiencies.

Protein arginine methyltransferase 6 regulates embryonic stem cell identity

Histone arginine methylation has emerged as an important histone modification involved in gene regulation. Protein arginine methyltransferase (PRMT) 4 and 5 have been shown to play essential roles in early embryonic development and in embryonic stem (ES) cells. Recently, it has been reported that PRMT6-mediated di-methylation of histone H3 at arginine 2 (H3R2me2) can antagonize tri-methylation of histone H3 at lysine 4 (H3K4me3), which marks active genes. However, whether PRMT6 and PRMT6-mediated H3R2me2 play crucial roles in early embryonic development and ES cell identity remain unclear. Here, we have investigated their roles using gain and loss of function studies with mouse ES cells as a model system. We report that Prmt6 and histone H3R2 methylation levels increased when ES cells are induced to differentiate. Consistently, we find that differentiation of ES cells upon upregulation of Prmt6 is associated with decreased expression of pluripotency genes and increased expression of differentiation markers. We also observe that elevation of Prmt6 increases the methylation level of histone H3R2 and decreases H3K4me, Chd1, and Wdr5 levels at the promoter regions of Oct4 and Nanog. Surprisingly, knockdown of Prmt6 also leads to downregulation of pluripotency genes and induction of expression of differentiation markers suggesting that Prmt6 is important for ES cell pluripotency and self-renewal. Our results indicate that a critical level of Prmt6 and histone H3R2me must be maintained in mouse ES cells to sustain their pluripotency.

Dynamics of anterior-posterior axis formation in the developing mouse embryo

The development of an anterior-posterior (AP) polarity is a crucial process that in the mouse has been very difficult to analyse, because it takes place as the embryo implants within the mother. To overcome this obstacle, we have established an in-vitro culture system that allows us to follow the step-wise development of anterior visceral endoderm (AVE), critical for establishing AP polarity. Here we use this system to show that the AVE originates in the implanting blastocyst, but that additional cells subsequently acquire AVE characteristics. These 'older' and 'younger' AVE domains coalesce as the egg cylinder emerges from the blastocyst structure. Importantly, we show that AVE migration is led by cells expressing the highest levels of AVE marker, highlighting that asymmetry within the AVE domain dictates the direction of its migration. Ablation of such leading cells prevents AVE migration, suggesting that these cells are important for correct establishment of the AP axis.

Phospholipase C-ζ-induced Ca2+ oscillations cause coincident cytoplasmic movements in human oocytes that failed to fertilize after intracytoplasmic sperm injection

Objective

To evaluate the imaging of cytoplasmic movements in human oocytes as a potential method to monitor the pattern of Ca²⁺ oscillations during activation.

Design

Test of a laboratory technique.

Setting

University medical school research laboratory.

Patient(s)

Donated unfertilized human oocytes from intracytoplasmic sperm injection (ICSI) cycles.

Intervention(s)

Microinjection of oocytes with phospholipase C (PLC) zeta (ζ) cRNA and a Ca²⁺-sensitive fluorescent dye.

Main Outcome Measure(s)

Simultaneous detection of oocyte cytoplasmic movements using particle image velocimetry (PIV) and of Ca²⁺ oscillations using a Ca²⁺-sensitive fluorescent dye.

Result(s)

Microinjection of PLCζ cRNA into human oocytes that had failed to fertilize after ICSI resulted in the appearance of prolonged Ca²⁺ oscillations. Each transient Ca²⁺ concentration change was accompanied by a small coordinated movement of the cytoplasm that could be detected using PIV analysis.

Conclusion(s)

The occurrence and frequency of cytoplasmic Ca²⁺ oscillations, a critical parameter in activating human zygotes, can be monitored by PIV analysis of cytoplasmic movements. This simple method provides a novel, noninvasive approach to determine in real time the occurrence and frequency of Ca²⁺ oscillations in human zygotes.

Formation of distinct cell types in the mouse blastocyst

Early development of the mouse comprises a sequence of cell fate decisions in which cells are guided along a pathway of restricted potential and increasing specialisation. The first choice faced by cells of the embryo is whether to become trophectoderm (TE) or inner cell mass (ICM); TE is an extra-embryonic tissue which will form the embryonic portion of the placenta, whilst ICM gives rise to cells responsible for generating the foetus. In the second cell fate decision, the ICM is further refined into pluripotent cells forming the future body of the embryo, epiblast (EPI) and extra-embryonic primitive endoderm (PE), a tissue essential for patterning the embryo and establishing the developmental circulation. Understanding this early lineage segregation is critical for informing attempts to capture pluripotency and direct cell fate in vitro. Unlike the predictability of nonmammalian cell fate, development of the mouse embryo retains the flexibility to adapt to changing circumstances during development. Here we describe these first cell fate decisions, how they can be biased whilst maintaining flexibility and, finally, some of the molecular circuitry underlying early fate choice.

The chromosome passenger complex is required for fidelity of chromosome transmission and cytokinesis in meiosis of mouse oocytes

The existence of two forms of the chromosome passenger complex (CPC) in the mammalian oocyte has meant that its role in female meiosis has remained unclear. Here we use loss- and gain-of function approaches to assess the meiotic functions of one of the shared components of these complexes, INCENP, and of the variable kinase subunits, Aurora B or Aurora C. We show that either the depletion of INCENP or the combined inhibition of Aurora kinases B and C activates the anaphase-promoting complex or cyclosome (APC/C) before chromosomes have properly congressed in meiosis I and also prevents cytokinesis and hence extrusion of the first polar body. Overexpression of Aurora C also advances APC/C activation and results in cytokinesis failure in a high proportion of oocytes, indicative of a dominant effect on CPC function. Together, this points to roles for the meiotic CPC in functions similar to the mitotic roles of the complex: correcting chromosome attachment to microtubules, facilitating the spindle-assembly checkpoint (SAC) function and enabling cytokinesis. Surprisingly, overexpression of Aurora B leads to a failure of APC/C activation, stabilization of securin and consequently a failure of chiasmate chromosomes to resolve - a dominant phenotype that is completely suppressed by depletion of INCENP. Taken together with the differential distribution of Aurora proteins B and C on chiasmate chromosomes, this points to differential functions of the two forms of CPC in regulating the separation of homologous chromosomes in meiosis I.

Epigenetic modification affecting expression of cell polarity and cell fate genes to regulate lineage specification in the early mouse embryo

Formation of inner and outer cells of the mouse embryo distinguishes pluripotent inner cell mass (ICM) from differentiating trophectoderm (TE). Carm1, which methylates histone H3R17 and R26, directs cells to ICM rather that TE. To understand the mechanism by which this epigenetic modification directs cell fate, we generated embryos with in vivo-labeled cells of different Carm1 levels, using time-lapse imaging to reveal dynamics of their behavior, and related this to cell polarization. This shows that Carm1 affects cell fate by promoting asymmetric divisions, that direct one daughter cell inside, and cell engulfment, where neighboring cells with lower Carm1 levels compete for outside positions. This is associated with changes to the expression pattern and spatial distribution of cell polarity proteins: Cells with higher Carm1 levels show reduced expression and apical localization of Par3 and a dramatic increase in expression of PKCII, antagonist of the apical protein aPKC. Expression and basolateral localization of the mouse Par1 homologue, EMK1, increases concomitantly. Increased Carm1 also reduces Cdx2 expression, a transcription factor key for TE differentiation. These results demonstrate how the extent of a specific epigenetic modification could affect expression of cell polarity and fate-determining genes to ensure lineage allocation in the mouse embryo.