Maternal beta-catenin and E-cadherin in mouse development

Embryonic stem cells require Wnt proteins to prevent differentiation to epiblast stem cells

Pluripotent stem cells exist in naive and primed states, epitomized by mouse embryonic stem cells (ESCs) and the developmentally more advanced epiblast stem cells (EpiSCs; ref. 1). In the naive state of ESCs, the genome has an unusual open conformation and possesses a minimum of repressive epigenetic marks. In contrast, EpiSCs have activated the epigenetic machinery that supports differentiation towards the embryonic cell types. The transition from naive to primed pluripotency therefore represents a pivotal event in cellular differentiation. But the signals that control this fundamental differentiation step remain unclear. We show here that paracrine and autocrine Wnt signals are essential self-renewal factors for ESCs, and are required to inhibit their differentiation into EpiSCs. Moreover, we find that Wnt proteins in combination with the cytokine LIF are sufficient to support ESC self-renewal in the absence of any undefined factors, and support the derivation of new ESC lines, including ones from non-permissive mouse strains. Our results not only demonstrate that Wnt signals regulate the naive-to-primed pluripotency transition, but also identify Wnt as an essential and limiting ESC self-renewal factor.

Differential requirement for the dual functions of β-catenin in embryonic stem cell self-renewal and germ layer formation

Canonical Wnt-signalling has been implicated in mouse and human embryonic stem cell (ESC) maintenance, however its requirement is controversial. β-catenin is the key component in this highly conserved Wnt pathway, acting as a transcriptional transactivator. Yet, β-catenin has additional roles at the plasma membrane regulating cell-cell adhesion, complicating the analyses of cells/tissues lacking β-catenin. We report here the generation of a β-catenin deficient mouse ESC (mESC) line and show that self-renewal is maintained in absence of β-catenin. Cell-adhesion is partially rescued by plakoglobin up-regulation, but fails to be maintained during differentiation. When differentiated as aggregates, wild-type mESCs form descendents of all three germ layers, while mesendodermal germ layer formation and neuronal differentiation are defective in β-catenin deficient mESCs. A Tcf/Lef-signalling defective β-catenin variant, which re-establishes cadherin-mediated cell-adhesion, rescues definitive endoderm and neuroepithelial formation, suggesting that β-catenin cell-adhesion function is more important than its signalling function for these processes.

Requirement for N-cadherin-catenin complex in heart development

Cell adhesion, mediated by N-cadherin, is critical for embryogenesis since N-cadherin-null embryos die during mid-gestation with multiple developmental defects. To investigate the role of N-cadherin in heart muscle development, N-cadherin was specifically deleted from myocardial cells in mice. The structural integrity of the myocardial cell wall was compromised in the N-cadherin mutant embryos, leading to a malformed heart and a delay in embryonic development. In contrast, cardiac-specific deletion of αE-catenin, found in adherens junctions, or β-catenin, did not cause any morphological defects in the embryonic heart, presumably due to compensation by αT-catenin that is normally found in intercalated disks and γ-catenin (plakoglobin), respectively. Embryos lacking β-catenin in the heart also exhibited a cardiac defect, but only later in development resulting in partial lethality. These genetic studies underscore the importance of the N-cadherin/catenin complex in cardiogenesis.

Dynamics of zonula occludens-2 expression during preimplantation embryonic development in the hamster

The objective was to study the expression of zonula occludens-2, a tight junction protein, during preimplantation hamster embryonic development, to predict its possible localization, source, and roles in trophectoderm differentiation and blastocyst formation in this species. Comparison of zonula occludens-2 expression pattern between the hamster and mouse preimplantation embryos from the zygote up to the blastocyst stage was also an objective of this study. Zonula occludens-2 localization was noted in nuclei of blastomeres in all stages of hamster and mouse embryonic development. Compared to mice, where zonula occludens-2 was first localized in the interblastomere membrane at the morula stage, hamster embryos had membranous zonula occludens-2 localization from the 2-cell stage onwards. Based on combined results of immunolocalization study in parthenogenic embryos and ovarian and epididymal sections, and quantitative PCR done in oocytes and all developmental stages of preimplantation embryos, perhaps there was a carry-over of zonula occludens-2 proteins or mRNA from the dam to the embryo. Based on these findings, we inferred that maternally derived zonula occludens-2 was involved in nuclear functions, as well as differentiation of blastomeres and blastocoel formation during preimplantation embryonic development in the hamster.

Stroma regulates increased epithelial lateral cell adhesion in 3D culture: a role for actin/cadherin dynamics

BACKGROUND

Cell shape and tissue architecture are controlled by changes to junctional proteins and the cytoskeleton. How tissues control the dynamics of adhesion and cytoskeletal tension is unclear. We have studied epithelial tissue architecture using 3D culture models and found that adult primary prostate epithelial cells grow into hollow acinus-like spheroids. Importantly, when co-cultured with stroma the epithelia show increased lateral cell adhesions. To investigate this mechanism further we aimed to: identify a cell line model to allow repeatable and robust experiments; determine whether or not epithelial adhesion molecules were affected by stromal culture; and determine which stromal signalling molecules may influence cell adhesion in 3D epithelial cell cultures.

METHODOLOGY/PRINCIPAL FINDINGS

The prostate cell line, BPH-1, showed increased lateral cell adhesion in response to stroma, when grown as 3D spheroids. Electron microscopy showed that 9.4% of lateral membranes were within 20 nm of each other and that this increased to 54% in the presence of stroma, after 7 days in culture. Stromal signalling did not influence E-cadherin or desmosome RNA or protein expression, but increased E-cadherin/actin co-localisation on the basolateral membranes, and decreased paracellular permeability. Microarray analysis identified several growth factors and pathways that were differentially expressed in stroma in response to 3D epithelial culture. The upregulated growth factors TGFβ2, CXCL12 and FGF10 were selected for further analysis because of previous associations with morphology. Small molecule inhibition of TGFβ2 signalling but not of CXCL12 and FGF10 signalling led to a decrease in actin and E-cadherin co-localisation and increased paracellular permeability.

CONCLUSIONS/SIGNIFICANCE

In 3D culture models, paracrine stromal signals increase epithelial cell adhesion via adhesion/cytoskeleton interactions and TGFβ2-dependent mechanisms may play a key role. These findings indicate a role for stroma in maintaining adult epithelial tissue morphology and integrity.

Lipid rafts enriched in monosialylGb5Cer carrying the stage-specific embryonic antigen-4 epitope are involved in development of mouse preimplantation embryos at cleavage stage

Background

Lipid rafts enriched in glycosphingolipids (GSLs), cholesterol and signaling molecules play an essential role not only for signal transduction started by ligand binding, but for intracellular events such as organization of actin, intracellular traffic and cell polarity, but their functions in cleavage division of preimplantation embryos are not well known.

Results

Here we show that monosialylGb5Cer (MSGb5Cer)-enriched raft domains are involved in development during the cleavage stage of mouse preimplantation embryos. MSGb5Cer preferentially localizes at the interfaces between blastomeres in mouse preimplantation embryos. Live-imaging analysis revealed that MSGb5Cer localizes in cleavage furrows during cytokinesis, and that by accumulating at the interfaces, it thickens them. Depletion of cholesterol from the cell membrane with methyl-beta-cyclodextrin (MbCD) reduced the expression of MSGb5Cer and stopped cleavage. Extensive accumulation of MSGb5Cer at the interfaces by cross-linking with anti-MSGb5Cer Mab (6E2) caused F-actin to aggregate at the interfaces and suppressed the localization of E-cadherin at the interfaces, which resulted in the cessation of cleavage. In addition, suppression of actin polymerization with cytochalasin D (CCD) decreased the accumulation of MSGb5Cer at the interfaces. In E-cadherin-targeted embryos, the MSGb5Cer-enriched raft membrane domains accumulated heterotopically.

Conclusions

These results indicate that MSGb5Cer-enriched raft membrane domains participate in cytokinesis in a close cooperation with the cortical actin network and the distribution of E-cadherin.

Noninvasive monitoring of placenta-specific transgene expression by bioluminescence imaging

Background

Placental dysfunction underlies numerous complications of pregnancy. A major obstacle to understanding the roles of potential mediators of placental pathology has been the absence of suitable methods for tissue-specific gene manipulation and sensitive assays for studying gene functions in the placentas of intact animals. We describe a sensitive and noninvasive method of repetitively tracking placenta-specific gene expression throughout pregnancy using lentivirus-mediated transduction of optical reporter genes in mouse blastocysts.

Methodology/Principal Findings

Zona-free blastocysts were incubated with lentivirus expressing firefly luciferase (Fluc) and Tomato fluorescent fusion protein for trophectoderm-specific infection and transplanted into day 3 pseudopregnant recipients (GD3). Animals were examined for Fluc expression by live bioluminescence imaging (BLI) at different points during pregnancy, and the placentas were examined for tomato expression in different cell types on GD18. In another set of experiments, blastocysts with maximum photon fluxes in the range of 2.0E+4 to 6.0E+4 p/s/cm²/sr were transferred. Fluc expression was detectable in all surrogate dams by day 5 of pregnancy by live imaging, and the signal increased dramatically thereafter each day until GD12, reaching a peak at GD16 and maintaining that level through GD18. All of the placentas, but none of the fetuses, analyzed on GD18 by BLI showed different degrees of Fluc expression. However, only placentas of dams transferred with selected blastocysts showed uniform photon distribution with no significant variability of photon intensity among placentas of the same litter. Tomato expression in the placentas was limited to only trophoblast cell lineages.

Conclusions/Significance

These results, for the first time, demonstrate the feasibility of selecting lentivirally-transduced blastocysts for uniform gene expression in all placentas of the same litter and early detection and quantitative analysis of gene expression throughout pregnancy by live BLI. This method may be useful for a wide range of applications involving trophoblast-specific gene manipulations in utero.

Dynamic distribution of spindlin in nucleoli, nucleoplasm and spindle from primary oocytes to mature eggs and its critical function for oocyte-to-embryo transition in gibel carp

Spindlin (Spin) was thought as a maternal-effect factor associated with meiotic spindle. Its role for the oocyte-to-embryo transition was suggested in mouse, but its direct evidence for the function had been not obtained in other vertebrates. In this study, we used the CagSpin-specific antibody to investigate CagSpin expression pattern and distribution during oogenesis of gibel carp (Carassius auratus gibelio). First, the oocyte-specific expression pattern and dynamic distribution was revealed in nucleoli, nucleoplasm, and spindle from primary oocytes to mature eggs by immunofluorescence localization. In primary oocytes and growth stage oocytes, CagSpin accumulates in nucleoli in increasing numbers along with the oocyte growth, and its disassembly occurs in vitellogenic oocytes, which implicates that CagSpin may be a major component of a large number of nucleoli in fish growth oocytes. Then, co-localization of CagSpin and β-tubulin was revealed in meiotic spindle of mature egg, indicating that CagSpin is one spindle-associated factor. Moreover, microinjection of CagSpin-specific antibody into the fertilized eggs blocked the first cleavage, and found that the CagSpin depletion resulted in spindle assembly disturbance. Thereby, our study provided the first direct evidence for the critical oocyte-to-embryo transition function of Spin in vertebrates, and confirmed that Spin is one important maternal-effect factor that participates in oocyte growth, oocyte maturation, and oocyte-to-embryo transition.

Negative regulation of EGFR-Vav2 signaling axis by Cbl ubiquitin ligase controls EGF receptor-mediated epithelial cell adherens junction dynamics and cell migration

The E3 ubiquitin ligase Casitas B lymphoma protein (Cbl) controls the ubiquitin-dependent degradation of EGF receptor (EGFR), but its role in regulating downstream signaling elements with which it associates and its impact on biological outcomes of EGFR signaling are less clear. Here, we demonstrate that stimulation of EGFR on human mammary epithelial cells disrupts adherens junctions (AJs) through Vav2 and Rac1/Cdc42 activation. In EGF-stimulated cells, Cbl regulates the levels of phosphorylated Vav2 thereby attenuating Rac1/Cdc42 activity. Knockdown of Cbl and Cbl-b enhanced the EGF-induced disruption of AJs and cell motility. Overexpression of constitutively active Vav2 activated Rac1/Cdc42 and reorganized junctional actin cytoskeleton; these effects were suppressed by WT Cbl and enhanced by a ubiquitin ligase-deficient Cbl mutant. Cbl forms a complex with phospho-EGFR and phospho-Vav2 and facilitates phospho-Vav2 ubiquitinylation. Cbl can also interact with Vav2 directly in a Cbl Tyr-700-dependent manner. A ubiquitin ligase-deficient Cbl mutant enhanced the morphological transformation of mammary epithelial cells induced by constitutively active Vav2; this effect requires an intact Cbl Tyr-700. These results indicate that Cbl ubiquitin ligase plays a critical role in the maintenance of AJs and suppression of cell migration through down-regulation of EGFR-Vav2 signaling.

Intercellular junction assembly, dynamics, and homeostasis

Intercellular anchoring junctions are highly specialized regions of the plasma membrane where members of the cadherin family of transmembrane adhesion molecules on opposing cells interact through their extracellular domains, and through their cytoplasmic domains serve as a platform for organizing cytoskeletal anchors and remodelers. Here we focus on assembly of so-called "anchoring" or "adhering" junctions-adherens junctions (AJs) and desmosomes (DSMs), which associate with actin and intermediate filaments, respectively. We will examine how the assembly and function of AJs and DSMs are intimately connected during embryogenesis and in adult cells and tissues, and in some cases even form specialized "mixed" junctions. We will explore signaling and trafficking machineries that drive assembly and remodeling and how these mechanisms are co-opted in human disease.

Molecular bases of cell-cell junctions stability and dynamics

Epithelial cell-cell junctions are formed by apical adherens junctions (AJs), which are composed of cadherin adhesion molecules interacting in a dynamic way with the cortical actin cytoskeleton. Regulation of cell-cell junction stability and dynamics is crucial to maintain tissue integrity and allow tissue remodeling throughout development. Actin filament turnover and organization are tightly controlled together with myosin-II activity to produce mechanical forces that drive the assembly, maintenance, and remodeling of AJs. In this review, we will discuss these three distinct stages in the lifespan of cell-cell junctions, using several developmental contexts, which illustrate how mechanical forces are generated and transmitted at junctions, and how they impact on the integrity and the remodeling of cell-cell junctions.

Disorganized epithelial polarity and excess trophectoderm cell fate in preimplantation embryos lacking E-cadherin

The first two cell lineages in the mouse, the surface trophectoderm (TE) and inner cell mass (ICM), are morphologically distinguishable by E3.5, with the outer TE forming a polarized epithelial layer enclosing the apolar ICM. We show here that in mouse embryos completely lacking both maternal and zygotic E-cadherin (cadherin 1), the normal epithelial morphology of outside cells is disrupted, but individual cells still initiate TE- and ICM-like fates. A larger proportion of cells than normal showed expression of TE markers such as Cdx2, suggesting that formation of an organized epithelium is not necessary for TE-specific gene expression. Individual cells in such embryos still generated an apical domain that correlated with elevated Cdx2 expression. We also show that repolarization can occur in isolated early ICMs from both wild-type and Cdx2 mutant embryos, indicating that Cdx2 is not required for initiating polarity. The results demonstrate that epithelial integrity mediated by E-cadherin is not required for Cdx2 expression, but is essential for the normal allocation of TE and ICM cells. They also show that Cdx2 expression is strongly linked to apical membrane polarization.

Inhibition of GSK3beta enhances both adhesive and signalling activities of beta-catenin in mouse embryonic stem cells

BACKGROUND

GSK3beta (glycogen synthase kinase 3beta) regulates the expression level and activity of various target proteins, including beta-catenin. beta-Catenin is a co-activator of Wnt-dependent genes as well as a partner for transmembrane cadherins to mediate cell-to-cell adhesion. In some cases, inhibition of GSK3beta activity was shown to promote self-renewal of ESCs (embryonic stem cells), but immediate effects of GSK3beta inhibitors in these cells still remain elusive.

RESULTS

Here, we address the effects of GSK3beta inhibitors BIO (6-bromoindirubin-3'-oxime) and CHIR99021 on mESCs (mouse ESCs), focusing on modulation of beta-catenin activities. We found that, upon GSK3beta inhibition, the colonies of undifferentiated mESCs acquire a more compact morphology. This change is paralleled by two somewhat polar effects: (i) the accumulation of the beta-catenin, which is co-localized with E-cadherin at the plasma membrane, and the cytoplasmic, tyrosine unphosphorylated beta-catenin, which is able to bind the GST (glutathione transferase)-fused cytoplasmic domain of E-cadherin; and (ii) the accumulation of the tyrosine phosphorylated beta-catenin and its nuclear translocation that is accompanied by activation of the Tcf (T-cell factor)/beta-catenin-dependent transcription of Top-Flash reporter. The Tcf-mediated activation, however, does not affect most of the analysed Wnt-responsive genes involved in EMT (epithelial-mesenchymal transition) or cell-cycle progression, suggesting that the adhesive function of beta-catenin is dominant over transcription in undifferentiated mESCs. Treatment with BIO decreases proliferation rates of mESCs. This is not due to apoptosis, but rather to accumulation of cells in G1 phase of the cell cycle and is accompanied by down-regulation of the c-myc mRNA content.

CONCLUSION

Our results suggest that inhibition of GSK3beta activity in mESCs enhances both the beta-catenin/E-cadherin-mediated adhesion and the Tcf/beta-catenin-dependent transcription, but does not activate transcription in most of the examined genes involved in EMT and cell cycle progression.

Wnt signalling in implantation, decidualisation and placental differentiation--review

The family of secreted Wingless ligands plays major roles in embryonic development, stem cell maintenance, differentiation and tissue homeostasis. Accumulating evidence suggests that the canonical Wnt pathway involving nuclear recruitment of β-catenin and activation of Wnt-dependent transcription factors is also critically involved in development and differentiation of the diverse reproductive tissues. Here, we summarise our present knowledge about expression, regulation and function of Wnt ligands and their frizzled receptors in murine and human endometrial and placental cell types. In mice, Wnt signalling promotes early trophoblast lineage development, blastocyst activation, implantation and chorion-allantois fusion. Moreover, different Wnt ligands play essential roles in the development of the murine uterine tract, in cycling endometrial cells and during decidualisation. In humans, estrogen-dependent endometrial cell proliferation, decidualisation, trophoblast attachment and invasion were shown to be controlled by the particular signalling pathway. Failures in Wnt signalling are associated with infertility, endometriosis, endometrial cancer and gestational diseases such as complete mole placentae and choriocarcinomas. However, our present knowledge is still scarce due to the complexity of the Wnt network involving numerous ligands, receptors and non-canonical pathways. Hence, much remains to be learned about the role of different Wnt signalling cascades in reproductive cell types and their changes under pathological conditions.

Preferential adhesion maintains separation of ommatidia in the Drosophila eye

In the Drosophila eye, neighboring ommatidia are separated by inter-ommatidial cells (IOCs). How this ommatidial spacing emerges during eye development is not clear. Here we demonstrate that four adhesion molecules of the Irre cell recognition module (IRM) family play a redundant role in maintaining separation of ommatidia. The four IRM proteins are divided into two groups: Kirre and Rst are expressed in IOCs, and Hbs and Sns in primary pigment cells (1 degrees s). Kirre binds Hbs and Sns in vivo and in vitro. Reducing activity of either Rst or Kirre alone had minimal effects on ommatidial spacing, but reducing both together led to direct ommatidium:ommatidium contact. A similar phenotype was also observed when reducing both Hbs and Sns. Consistent with the role of these factors in sorting ommatidia, mis-expression of Hbs plus Sns within a single IOC led to complete separation of the cell from neighboring ommatidia. Our results indicate mutual preferential adhesion between ommatidia and IOCs mediated by four IRM proteins is both necessary and sufficient to maintain separation of ommatidia.

SAX-7/L1CAM and HMR-1/cadherin function redundantly in blastomere compaction and non-muscle myosin accumulation during Caenorhabditis elegans gastrulation

Gastrulation is the first major morphogenetic movement in development and requires dynamic regulation of cell adhesion and the cytoskeleton. Caenorhabditis elegans gastrulation begins with the migration of the two endodermal precursors, Ea and Ep, from the surface of the embryo into the interior. Ea/Ep migration provides a relatively simple system to examine the intersection of cell adhesion, cell signaling, and cell movement. Ea/Ep ingression depends on correct cell fate specification and polarization, apical myosin accumulation, and Wnt activated actomyosin contraction that drives apical constriction and ingression (Lee et al., 2006; Nance et al., 2005). Here, we show that Ea/Ep ingression also requires the function of either HMR-1/cadherin or SAX-7/L1CAM. Both cadherin complex components and L1CAM are localized at all sites of cell-cell contact during gastrulation. Either system is sufficient for Ea/Ep ingression, but loss of both together leads to a failure of apical constriction and ingression. Similar results are seen with isolated blastomeres. Ea/Ep are properly specified and appear to display correct apical-basal polarity in sax-7(eq1);hmr-1(RNAi) embryos. Significantly, in sax-7(eq1);hmr-1(RNAi) embryos, Ea and Ep fail to accumulate myosin (NMY-2Colon, two colonsGFP) at their apical surfaces, but in either sax-7(eq1) or hmr-1(RNAi) embryos, apical myosin accumulation is comparable to wild type. Thus, the cadherin and L1CAM adhesion systems are redundantly required for localized myosin accumulation and hence for actomyosin contractility during gastrulation. We also show that sax-7 and hmr-1 function are redundantly required for Wnt-dependent spindle polarization during division of the ABar blastomere, indicating that these cell surface proteins redundantly regulate multiple developmental events in early embryos.

Maternal control of early mouse development

The hiatus between oocyte and embryonic gene transcription dictates a role for stored maternal factors in early mammalian development. Encoded by maternal-effect genes, these factors accumulate during oogenesis and enable the activation of the embryonic genome, the subsequent cleavage stages of embryogenesis and the initial establishment of embryonic cell lineages. Recent studies in mice have yielded new findings on the role of maternally provided proteins and multi-component complexes in preimplantation development. Nevertheless, significant gaps remain in our mechanistic understanding of the networks that regulate early mammalian embryogenesis, which provide an impetus and opportunities for future investigations.

Making the blastocyst: lessons from the mouse

Mammalian preimplantation development, which is the period extending from fertilization to implantation, results in the formation of a blastocyst with three distinct cell lineages. Only one of these lineages, the epiblast, contributes to the embryo itself, while the other two lineages, the trophectoderm and the primitive endoderm, become extra-embryonic tissues. Significant gains have been made in our understanding of the major events of mouse preimplantation development, and recent discoveries have shed new light on the establishment of the three blastocyst lineages. What is less clear, however, is how closely human preimplantation development mimics that in the mouse. A greater understanding of the similarities and differences between mouse and human preimplantation development has implications for improving assisted reproductive technologies and for deriving human embryonic stem cells.

FGF signal-dependent segregation of primitive endoderm and epiblast in the mouse blastocyst

Primitive endoderm (PE) and epiblast (EPI) are two lineages derived from the inner cell mass (ICM) of the E3.5 blastocyst. Recent studies showed that EPI and PE progenitors expressing the lineage-specific transcriptional factors Nanog and Gata6, respectively, arise progressively as the ICM develops. Subsequent sorting of the two progenitors during blastocyst maturation results in the ormation of morphologically distinct EPI and PE layers at E4.5. It is, however, unknown how the initial differences between the two populations become established in the E3.5 blastocyst. Because the ICM cells are derived from two distinct rounds of polarized cell divisions during cleavage, a possible role for cell lineage history in promoting EPI versus PE fate has been proposed. We followed cell lineage from the eight-cell stage by live cell tracing and could find no clear linkage between developmental history of individual ICM cells and later cell fate. However, modulating FGF signaling levels by inhibition of the receptor/MAP kinase pathway or by addition of exogenous FGF shifted the fate of ICM cells to become either EPI or PE, respectively. Nanog- or Gata6-expressing progenitors could still be shifted towards the alternative fate by modulating FGF signaling during blastocyst maturation, suggesting that the ICM progenitors are not fully committed to their final fate at the time that initial segregation of gene expression occurs. In conclusion, we propose a model in which stochastic and progressive specification of EPI and PE lineages occurs during maturation of the blastocyst in an FGF/MAP kinase signal-dependent manner.

Establishment of trophectoderm and inner cell mass lineages in the mouse embryo

The first cell lineage specification in mouse embryo development is the formation of trophectoderm (TE) and inner cell mass (ICM) of the blastocyst. This article is to review and discuss the current knowledge on the cellular and molecular mechanisms of this particular event. Several transcription factors have been identified as the critical regulators of the formation or maintenance of the two cell lineages. The establishment of TE manifests as the formation of epithelium, and is dependent on many structural and regulatory components that are commonly found and that function in many epithelial tissues. Distinct epithelial features start to emerge at the late 8-cell stage, but the fates of blastomeres are not fixed as TE or ICM until around 32-cell stage. The location of blastomeres at this stage, that is, external or internal of the embryo, in effect defines the commitment towards the TE or ICM lineage, respectively. Some studies implicate the presence of a developmental bias among blastomeres at 2- or 4-cell stage, although it is unlikely to play a decisive role in the establishment of TE and ICM. The unique mode of cell lineage specification in the mouse embryo is further discussed in comparison with the formation of initial cell lineages, namely the three germ layers, in non-mammalian embryos.

From mouse egg to mouse embryo: polarities, axes, and tissues

This review describes the three classical models (mosaic, positional, and polarization) proposed to explain blastocyst formation and summarizes the evidence concerning them. It concludes that the polarization model incorporates elements of the other two models and best explains most known information. I discuss key requirements of a molecular basis for the generation and stabilization of polarity and identify ezrin/E-cadherin, PAR proteins, and Cdx2 as plausible key molecular players. I also discuss the idea of a network process operating to build cell allocations progressively into committed differences. Finally, this review critically considers the possibility of developmental information being encoded within the oocyte and zygote. No final decision can be reached on a mechanism of action underlying any encoded information, but a cell interaction process model is preferred over one that relies solely on differential inheritance.

Involvement of E-cadherin in early in vitro development of adult and juvenile sheep embryos

The oocyte-to-embryo transition in mammals depends on maternal proteins and transcripts, which accumulate during oocyte differentiation. The aim of the present study was to examine the role of the junctional proteins β-catenin and E-cadherin during preimplantation in vitro embryo development in sheep, comparing the competence of adult and prepubertal oocytes. We analysed the concentration of β-catenin and E-cadherin in immature and in vitro-matured oocytes. There was a significant increase in E-cadherin concentration after 24 h of in vitro maturation and this was lower in prepubertal oocytes than in adult ones. We therefore studied the expression and distribution of E-cadherin during the major transition from maternal to embryonic genome. E-cadherin distribution and localisation in sheep was age- and developmental-stage dependent and was related to developmental kinetics. In fact, in adults, the majority of embryos showed the proper distribution of E-cadherin just beneath the membrane surfaces of all blastomeres and the percentage of embryos with this distribution increased with the increase in cell number during development. On the contrary, and regardless of their developmental stage, the majority of prepubertal embryos showed an uneven distribution of the protein, often associated with the occurrence of cellular fragmentation. In conclusion, our results suggest that E-cadherin plays a pivotal role during preimplantation embryo growth in sheep and may be one of the possible cytoplasmic factors involved in the reduced developmental competence of prepubertal female gametes.

Two themes on the assembly of the Drosophila eye

Cells are sequentially recruited during formation of the Drosophila compound eye. A few simple rules are reiteratively utilized to control successive steps of eye assembly. Two themes emerge: the interplay between cell signaling and competence determines diversity of cell types and selective cell adhesion determines spatial patterns of cells. Cell signaling through competence creates signaling relays, which sequentially trigger differentiation of all cell types. Selective cell adhesion, on the other hand, provides forces to drive cells into energy-favored spatial configurations. Organ formation is nevertheless a complex process. The complexity lies in the spatial, temporal, and quantitative precision of gene expression. Many challenging questions remain.

Excessive ovarian stimulation up-regulates the Wnt-signaling molecule DKK1 in human endometrium and may affect implantation: an in vitro co-culture study

BACKGROUND

High serum estradiol (E2) levels following ovarian stimulation lead to reduced implantation and pregnancy rates, yet the underlying mechanisms remain unknown. We investigated if aberrant expression of genes in the Wnt-signaling pathway may be involved.

METHODS

Microarray and real-time PCR analysis were performed to analyze gene expression profiles of endometrial samples taken at day hCG + 7 in stimulated cycles, and days LH + 7 and LH + 10 in natural cycles. Expression of several Wnt-signaling transcripts, including Dickkopf homolog 1 (DKK1), DKK2 and secreted frizzled-related protein 4 (sFRP4), was analyzed throughout the menstrual cycle. JAr spheroid/Ishikawa endometrial cell co-culture experiments were established to study effects of DKK1 on spheroid attachment in vitro.

RESULTS

We identified 351 differentially expressed genes. Endometrial samples taken at hCG + 7 had similar expression profiles to those at LH + 10. DKK1 transcripts were up-regulated and DKK2 and sFRP4 were down-regulated in the stimulated compared with LH + 7 group (all P < 0.05). DKK1 transcripts were low in proliferative phase (PS) and increased in late-secretory phase (LS, P < 0.05), although DKK2 peaked in mid-secretory phase (P < 0.05). sFRP4 transcripts were high in PS. Treatment of spheroid with recombinant human DKK-1 protein dose-dependently suppressed (P < 0.05 versus control) spheroids attachment onto endometrial cells (associated with decreased beta-catenin protein): this suppression was nullified by anti-DKK1 antibody.

CONCLUSION

Gene expression patterns in stimulated cycles resembled those of LS in natural cycles, when the implantation window is about to close, suggesting high serum E2 and/or progesterone concentrations may advance endometrial development, altering the implantation window and possibly decreasing pregnancy rate. Aberrant expression of DKK1 might impair embryo attachment and implantation in vivo.

Adhesive and signaling functions of cadherins and catenins in vertebrate development

Properly regulated intercellular adhesion is critical for normal development of all metazoan organisms. Adherens junctions play an especially prominent role in development because they link the adhesive function of cadherin-catenin protein complexes to the dynamic forces of the actin cytoskeleton, which helps to orchestrate a spatially confined and very dynamic assembly of intercellular connections. Intriguingly, in addition to maintaining intercellular adhesion, cadherin-catenin proteins are linked to several major developmental signaling pathways crucial for normal morphogenesis. In this article we will highlight the key genetic studies that uncovered the role of cadherin-catenin proteins in vertebrate development and discuss the potential role of these proteins as molecular biosensors of external cellular microenvironment that may spatially confine signaling molecules and polarity cues to orchestrate cellular behavior throughout the complex process of normal morphogenesis.

Beta-catenin/Tcf determines the outcome of thymic selection in response to alphabetaTCR signaling

Thymic maturation of T cells depends on the intracellular interpretation of alphabetaTCR signals by processes that are poorly understood. In this study, we report that beta-catenin/Tcf signaling was activated in double-positive thymocytes in response to alphabetaTCR engagement and impacted thymocyte selection. TCR engagement combined with activation of beta-catenin signaled thymocyte deletion, whereas Tcf-1 deficiency rescued from negative selection. Survival/apoptotis mediators including Bim, Bcl-2, and Bcl-x(L) were alternatively influenced by stabilization of beta-catenin or ablation of Tcf-1, and Bim-mediated beta-catenin induced thymocyte deletion. TCR activation in double-positive cells with stabilized beta-catenin triggered signaling associated with negative selection, including sustained overactivation of Lat and Jnk and a transient activation of Erk. These observations are consistent with beta-catenin/Tcf signaling acting as a switch that determines the outcome of thymic selection downstream the alphabetaTCR cascade.

E-cadherin, beta-catenin, and ZEB1 in malignant progression of cancer

The embryonic program 'epithelial-mesenchymal transition' (EMT) is activated during tumor invasion in disseminating cancer cells. Characteristic to these cells is a loss of E-cadherin expression, which can be mediated by EMT-inducing transcriptional repressors, e.g. ZEB1. Consequences of a loss of E-cadherin are an impairment of cell-cell adhesion, which allows detachment of cells, and nuclear localization of beta-catenin. In addition to an accumulation of cancer stem cells, nuclear beta-catenin induces a gene expression pattern favoring tumor invasion, and mounting evidence indicates multiple reciprocal interactions of E-cadherin and beta-catenin with EMT-inducing transcriptional repressors to stabilize an invasive mesenchymal phenotype of epithelial tumor cells.

5-AZA-2'-deoxycytidine (5-AZA-CdR) leads to down-regulation of Dnmt1o and gene expression in preimplantation mouse embryos

5-AZA-2'-deoxycytidine (5-AZA-CdR) is a demethylating, teratogenic agent and a mutagen, which causes defects in the developing mouse and rat after implantation. Our previous data indicated that 5-AZA-CdR (0.2 and 1.0 muM) inhibited the development of mouse preimplantation embryos. Pronuclear embryos exposed to 5-AZA-CdR at the pronuclear stage were unable to form 8-cell embryos, while 2-cell-stage embryos exposed to 5-AZA-CdR only developed into uncompacted 8-cell-stage embryos. And there was no formation of blastocysts when 4-cell embryos cultured in 5-AZA-CdR. In our present study, we detected Dnmt1o protein and some developmental gene expression in order to find the reasons for the developmental arrest. Dnmt1o could not traffic to 8-cell nuclei as control when embryos were exposed to 5-AZA-CdR. Dnmt1o was in cytoplasm at 2-cell and 4-cell stages before and after treated with 5-AZA-CdR. Gene expression changes were also detected in this research. Our data indicated that connexin 31 (Cx31), connexin 43 (Cx43), connexin 45 (Cx45), E-cadherin (Cdh1) and beta-catenin (Ctnnb1) were all downregulated by 5-AZA-CdR. Cx31, Cx43 and Cx45 are members of connexins family, which have a central role in gap junctions. Cdh1 and Ctnnb1 are necessary for the foundation of tight junctions. Therefore, developmental arrest induced by 5-AZA-CdR may be caused by the failure of Dnmt1o cytoplasmic-nuclear traffic and the down-regulation of developmental gene expression. Normal compaction and blastocoel cavitation need Dnmt1o traffic to 8-cell nuclei and the right gene expression, especially the correlative genes in gap junctions and tight junctions.

Embryo-uterine cross-talk during implantation: the role of Wnt signaling

During mammalian pregnancy, it has been demonstrated that the quality of embryo implantation determines the quality of ongoing pregnancy and fetal development. Recent studies have provided increasing evidence that differential Wnt signaling plays diverse roles in multiple peri-implantation events. This review focuses on recent progress on various aspects of Wnt signaling in preimplantation embryo development, blastocyst activation for implantation and uterine decidualization. Future studies with conditional deletion of Wnt family members are hoped to provide deeper insight on the pathophysiological significance of Wnt proteins on early pregnancy events.

Reprogramming and differentiation in mammals: motifs and mechanisms

The natural reprogramming of the mammalian egg and sperm genomes is an efficient process that takes place in less than 24 hours and gives rise to a totipotent zygote. Transfer of somatic nuclei to mammalian oocytes also leads to their reprogramming and formation of totipotent embryos, albeit very inefficiently and requiring an activation step. Reprogramming of differentiated cells to induced pluripotent stem (iPS) cells takes place during a period of time substantially longer than reprogramming of the egg and sperm nuclei and is significantly less efficient. The stochastic expression of endogenous proteins during this process would imply that controlled expression of specific proteins is crucial for reprogramming to take place. The fact that OCT4, NANOG, and SOX2 form the core components of the pluripotency circuitry would imply that control at the transcriptional level is important for reprogramming to iPS cells. In contradistinction, the much more efficient reprogramming of the mammalian egg and sperm genomes implies that other levels of control are necessary, such as chromatin remodeling, translational regulation, and efficient degradation of no longer needed proteins and RNAs.

Maternal Oct-4 is a potential key regulator of the developmental competence of mouse oocytes

Background

The maternal contribution of transcripts and proteins supplied to the zygote is crucial for the progression from a gametic to an embryonic control of preimplantation development. Here we compared the transcriptional profiles of two types of mouse MII oocytes, one which is developmentally competent (MII^SN oocyte), the other that ceases development at the 2-cell stage (MII^NSN oocyte), with the aim of identifying genes and gene expression networks whose misregulated expression would contribute to a reduced developmental competence.

Results

We report that: 1) the transcription factor Oct-4 is absent in MII^NSN oocytes, accounting for 2) the down-regulation of Stella, a maternal-effect factor required for the oocyte-to-embryo transition and of which Oct-4 is a positive regulator; 3) eighteen Oct-4-regulated genes are up-regulated in MII^NSN oocytes and are part of gene expression networks implicated in the activation of adverse biochemical pathways such as oxidative phosphorylation, mitochondrial dysfunction and apoptosis.

Conclusion

The down-regulation of Oct-4 plays a crucial function in a sequence of molecular processes that leads to the developmental arrest of MII^NSN oocytes. The use of a model study in which the MII oocyte ceases development consistently at the 2-cell stage has allowed to attribute a role to the maternal Oct-4 that has never been described before. Oct-4 emerges as a key regulator of the molecular events that govern the establishment of the developmental competence of mouse oocytes.

Complex N-glycans or core 1-derived O-glycans are not required for the expression of stage-specific antigens SSEA-1, SSEA-3, SSEA-4, or Le(Y) in the preimplantation mouse embryo

The glycan epitopes termed stage-specific embryonic antigens (SSEA) occur on glycoproteins and glycolipids in mammals. However, it is not known whether these epitopes are attached to N- or O-glycans on glycoproteins and/or on glycolipids in the developing mouse embryo. In this paper the expression of the antigens SSEA-1, SSEA-3, SSEA-4 and Le(Y) was examined on ovulated eggs, early embryos and blastocysts lacking either complex and hybrid N-glycans or core-1 derived O-glycans. In all cases, antigen expression determined by fluorescence microscopy of bound monoclonal antibodies to embryos at the stage of development of maximal expression was similar in mutant and control embryos. Thus, none of these developmental antigens are expressed solely on either complex N- or core 1-derived O-glycans attached to glycoproteins in the preimplantation mouse embryo. Furthermore, neither of these classes of glycan is essential for the expression of SSEA-1, SSEA-3, SSEA-4 or Le(Y) on mouse embryos.

The canonical Wnt signaling pathway plays an important role in lymphopoiesis and hematopoiesis

The evolutionarily conserved canonical Wnt-beta-catenin-T cell factor (TCF)/lymphocyte enhancer binding factor (LEF) signaling pathway regulates key checkpoints in the development of various tissues. Therefore, it is not surprising that a large body of gain-of-function and loss-of-function studies implicate Wnt-beta-catenin signaling in lymphopoiesis and hematopoiesis. In contrast, recent papers have reported that Mx-Cre-mediated conditional deletion of beta-catenin and/or its homolog gamma-catenin (plakoglobin) did not impair hematopoiesis or lymphopoiesis. However, these studies also report that TCF reporter activity remains active in beta-catenin- and gamma-catenin-deficient hematopoietic stem cells and all cells derived from these precursors, indicating that the canonical Wnt signaling pathway was not abrogated. Therefore, these studies in fact show that the canonical Wnt signaling pathway is important in hematopoiesis and lymphopoiesis, even though the molecular basis for the induction of the reporter activity is currently unknown. In this perspective, we provide a broad background to the field with a discussion of the available data and create a framework within which the available and future studies may be evaluated.

Members of the WNT signaling pathways are widely expressed in mouse ovaries, oocytes, and cleavage stage embryos

The mammalian oocyte-to-embryo transition, characterized by a period of transcriptional silence, is dependent on maternal RNAs and proteins produced during the growth phase of the oocyte. Signaling pathways control timely transcription and translation of RNA, as well as post-translational modification of proteins. The WNT/beta-catenin pathway is clearly not active during preimplantation embryo development. However, alternative Wnt signaling pathways may play a role during early embryo development. This study describes the extensive expression, at the transcript and protein level, of receptors, ligands, and intracellular molecules known to play a role in WNT signaling, as well as those known to negatively regulate the canonical WNT/beta-catenin pathway in developing oocytes and preimplantation embryos. This expression of a wide array of molecules involved in WNT signaling suggests that the alternative WNT pathways may be active during oogenesis and the oocyte-to-embryo transition.

Maternal depletion of CTCF reveals multiple functions during oocyte and preimplantation embryo development

CTCF is a multifunctional nuclear factor involved in epigenetic regulation. Despite recent advances that include the systematic discovery of CTCF-binding sites throughout the mammalian genome, the in vivo roles of CTCF in adult tissues and during embryonic development are largely unknown. Using transgenic RNAi, we depleted maternal stores of CTCF from growing mouse oocytes, and identified hundreds of misregulated genes. Moreover, our analysis suggests that CTCF predominantly activates or derepresses transcription in oocytes. CTCF depletion causes meiotic defects in the egg, and mitotic defects in the embryo that are accompanied by defects in zygotic gene expression, and culminate in apoptosis. Maternal pronuclear transfer and CTCF mRNA microinjection experiments indicate that CTCF is a mammalian maternal effect gene, and that persistent transcriptional defects rather than persistent chromosomal defects perturb early embryonic development. This is the first study detailing a global and essential role for CTCF in mouse oocytes and preimplantation embryos.

Ras-MAPK signaling promotes trophectoderm formation from embryonic stem cells and mouse embryos

In blastocyst chimeras, embryonic stem (ES) cells contribute to embryonic tissues but not extraembryonic trophectoderm. Conditional activation of HRas1(Q61L) in ES cells in vitro induces the trophectoderm marker Cdx2 and enables derivation of trophoblast stem (TS) cell lines that, when injected into blastocysts, chimerize placental tissues. Erk2, the downstream effector of Ras-mitogen-activated protein kinase (MAPK) signaling, is asymmetrically expressed in the apical membranes of the 8-cell-stage embryo just before morula compaction. Inhibition of MAPK signaling in cultured mouse embryos compromises Cdx2 expression, delays blastocyst development and reduces trophectoderm outgrowth from embryo explants. These data show that ectopic Ras activation can divert ES cells toward extraembryonic trophoblastic fates and implicate Ras-MAPK signaling in promoting trophectoderm formation from mouse embryos.

Cadherins in development and cancer

Proper embryonic development is guaranteed under conditions of regulated cell-cell and cell-matrix adhesion. The cells of an embryo have to be able to distinguish their neighbours as being alike or different. Cadherins, single-pass transmembrane, Ca(2+)-dependent adhesion molecules that mainly interact in a homophilic manner, are major contributors to cell-cell adhesion. Cadherins play pivotal roles in important morphogenetic and differentiation processes during development, and in maintaining tissue integrity and homeostasis. Changes in cadherin expression throughout development enable differentiation and the formation of various organs. In addition to these functions, cadherins have strong implications in tumourigenesis, since frequently tumour cells show deregulated cadherin expression and inappropriate switching among family members. In this review, I focus on E- and N-cadherin, giving an overview of their structure, cellular function, importance during development, role in cancer, and of the complexity of Ecadherin gene regulation.

Morphogenesis of the mammalian blastocyst

The first 4 days of mouse pre-implantation development are characterized by a period of segmentation, including morphogenetic events that are required for the divergence of embryonic and extra-embryonic lineages. These extra-embryonic tissues are essential for the implantation into the maternal uterus and for the development of the foetus. In this review, we first discuss data showing unambiguously that no essential axis of development is set up before the late blastocyst stage, and explain why the pre-patterning described during the early phases (segmentation) of development in other vertebrates cannot apply to mammalian pre-implantation period. Then, we describe important cellular and molecular events that are required for the morphogenesis of the blastocyst.

Inactivation of nuclear Wnt-beta-catenin signaling limits blastocyst competency for implantation

The activation of the blastocyst, a process by which it gains competency to attach with the receptive uterus, is a prerequisite for successful implantation. However, the molecular basis of blastocyst activation remains largely unexplored. Combining molecular, pharmacological and physiological approaches, we show here that silencing of Wnt-beta-catenin signaling in mice does not adversely affect the development of preimplantation embryos to blastocysts and uterine preparation for receptivity, but, remarkably, blocks blastocyst competency to implantation. Using the physiologically relevant delayed implantation model and trophoblast stem cells in culture, we further demonstrate that a coordinated activation of canonical Wnt-beta-catenin signaling with attenuation of the non-canonical Wnt-RhoA signaling pathway ensures blastocyst competency to implantation. These findings constitute novel evidence that Wnt signaling is at least one pathway that determines blastocyst competency for implantation.

Constitutive PtdIns(3,4,5)P3 synthesis promotes the development and survival of early mammalian embryos

Mammalian preimplantation embryos develop in the oviduct as individual entities, and can develop and survive in vitro, in defined culture media lacking exogenous growth factors or serum. Therefore, early embryos must generate intrinsic signals that promote their development and survival. In other cells, activation of class I phosphoinositide 3-kinase (PI3K) is a universal mechanism to promote cell proliferation and survival. Here, we examined whether PI3K is intrinsically activated during preimplantation development. Using GFP-tagged pleckstrin homology domains to monitor PtdIns(3,4,5)P(3) synthesis, we show that PI3K is constitutively activated in mouse preimplantation embryos. E-cadherin ligation promotes PtdIns(3,4,5)P(3) synthesis at sites of blastomere adhesion at all cleavage stages. In addition, in culture conditions that promote autocrine signalling, a second pool of PtdIns(3,4,5)P(3) is generated in the apical membrane of early stage blastomeres. We show that constitutive PtdIns(3,4,5)P(3) synthesis is necessary for optimal development to blastocyst and to prevent large-scale apoptosis at the time of cavitation.

Cell surface mechanics and the control of cell shape, tissue patterns and morphogenesis

Embryonic morphogenesis requires the execution of complex mechanisms that regulate the local behaviour of groups of cells. The orchestration of such mechanisms has been mainly deciphered through the identification of conserved families of signalling pathways that spatially and temporally control cell behaviour. However, how this information is processed to control cell shape and cell dynamics is an open area of investigation. The framework that emerges from diverse disciplines such as cell biology, physics and developmental biology points to adhesion and cortical actin networks as regulators of cell surface mechanics. In this context, a range of developmental phenomena can be explained by the regulation of cell surface tension.

Lentivirus as a tool for lineage-specific gene manipulations

The trophectoderm (TE) of blastocysts, the first epithelium established in mammalian development, (1) plays signaling, supportive, and patterning functions during preimplantation development, (2) ensures embryo implantation into the uterine wall, and (3) gives rise to extraembryonic tissues essential for embryo patterning and growth after implantation. We show that mouse TE, itself permissive to lentiviral (LV) infection, represents a robust nonpermeable physical barrier to the virus particles, thereby shielding the cells of the inner cell mass from viral infection. This LV feature will allow modulations of gene expression in a lineage-specific manner, thus having significant applications in mouse functional genetics.

Ovarian gene expression in the absence of FIGLA, an oocyte-specific transcription factor

Background

Ovarian folliculogenesis in mammals is a complex process involving interactions between germ and somatic cells. Carefully orchestrated expression of transcription factors, cell adhesion molecules and growth factors are required for success. We have identified a germ-cell specific, basic helix-loop-helix transcription factor, FIGLA (Factor In the GermLine, Alpha) and demonstrated its involvement in two independent developmental processes: formation of the primordial follicle and coordinate expression of zona pellucida genes.

Results

Taking advantage of Figla null mouse lines, we have used a combined approach of microarray and Serial Analysis of Gene Expression (SAGE) to identify potential downstream target genes. Using high stringent cutoffs, we find that FIGLA functions as a key regulatory molecule in coordinating expression of the NALP family of genes, genes of known oocyte-specific expression and a set of functionally un-annotated genes. FIGLA also inhibits expression of male germ cell specific genes that might otherwise disrupt normal oogenesis.

Conclusion

These data implicate FIGLA as a central regulator of oocyte-specific genes that play roles in folliculogenesis, fertilization and early development.