In vivo functional analysis of ezrin during mouse blastocyst formation

Proteomic Analysis of Domestic Cat Blastocysts and Their Secretome Produced in an In Vitro Culture System without the Presence of the Zona Pellucida

Domestic cat blastocysts cultured without the zona pellucida exhibit reduced implantation capacity. However, the protein expression profile has not been evaluated in these embryos. The objective of this study was to evaluate the protein expression profile of domestic cat blastocysts cultured without the zona pellucida. Two experimental groups were generated: (1) domestic cat embryos generated by IVF and cultured in vitro (zona intact, (ZI)) and (2) domestic cat embryos cultured in vitro without the zona pellucida (zona-free (ZF group)). The cleavage, morula, and blastocyst rates were estimated at days 2, 5 and 7, respectively. Day 7 blastocysts and their culture media were subjected to liquid chromatography-tandem mass spectrometry (LC-MS/MS). The UniProt Felis catus database was used to identify the standard proteome. No significant differences were found in the cleavage, morula, or blastocyst rates between the ZI and ZF groups (p > 0.05). Proteomic analysis revealed 22 upregulated and 20 downregulated proteins in the ZF blastocysts. Furthermore, 14 proteins involved in embryo development and implantation were present exclusively in the culture medium of the ZI blastocysts. In conclusion, embryo culture without the zona pellucida did not affect in vitro development, but altered the protein expression profile and release of domestic cat blastocysts.

Ezrin gone rogue in cancer progression and metastasis: An enticing therapeutic target

Cancer metastasis is the primary cause of morbidity and mortality in cancer as it remains the most complicated, devastating, and enigmatic aspect of cancer. Several decades of extensive research have identified several key players closely associated with metastasis. Among these players, cytoskeletal linker Ezrin (the founding member of the ERM (Ezrin-Radixin-Moesin) family) was identified as a critical promoter of metastasis in pediatric cancers in the early 21st century. Ezrin was discovered 40 years ago as a aminor component of intestinal epithelial microvillus core protein, which is enriched in actin-containing cell surface structures. It controls gastric acid secretion and plays diverse physiological roles including maintaining cell polarity, regulating cell adhesion, cell motility and morphogenesis. Extensive research for more than two decades evinces that Ezrin is frequently dysregulated in several human cancers. Overexpression, altered subcellular localization and/or aberrant activation of Ezrin are closely associated with higher metastatic incidence and patient mortality, thereby justifying Ezrin as a valuable prognostic biomarker in cancer. Ezrin plays multifaceted role in multiple aspects of cancer, with its significant contribution in the complex metastatic cascade, through reorganizing the cytoskeleton and deregulating various cellular signaling pathways. Current preclinical studies using genetic and/or pharmacological approaches reveal that inactivation of Ezrin results in significant inhibition of Ezrin-mediated tumor growth and metastasis as well as increase in the sensitivity of cancer cells to various chemotherapeutic drugs. In this review, we discuss the recent advances illuminating the molecular mechanisms responsible for Ezrin dysregulation in cancer and its pleiotropic role in cancer progression and metastasis. We also highlight its potential as a prognostic biomarker and therapeutic target in various cancers. More importantly, we put forward some potential questions, which we strongly believe, will stimulate both basic and translational research to better understand Ezrin-mediated malignancy, ultimately leading to the development of Ezrin-targeted cancer therapy for the betterment of human life.

Stain-free detection of embryo polarization using deep learning

Polarization of the mammalian embryo at the right developmental time is critical for its development to term and would be valuable in assessing the potential of human embryos. However, tracking polarization requires invasive fluorescence staining, impermissible in the in vitro fertilization clinic. Here, we report the use of artificial intelligence to detect polarization from unstained time-lapse movies of mouse embryos. We assembled a dataset of bright-field movie frames from 8-cell-stage embryos, side-by-side with corresponding images of fluorescent markers of cell polarization. We then used an ensemble learning model to detect whether any bright-field frame showed an embryo before or after onset of polarization. Our resulting model has an accuracy of 85% for detecting polarization, significantly outperforming human volunteers trained on the same data (61% accuracy). We discovered that our self-learning model focuses upon the angle between cells as one known cue for compaction, which precedes polarization, but it outperforms the use of this cue alone. By compressing three-dimensional time-lapsed image data into two-dimensions, we are able to reduce data to an easily manageable size for deep learning processing. In conclusion, we describe a method for detecting a key developmental feature of embryo development that avoids clinically impermissible fluorescence staining.

An ex vivo system to study cellular dynamics underlying mouse peri-implantation development

Upon implantation, mammalian embryos undergo major morphogenesis and key developmental processes such as body axis specification and gastrulation. However, limited accessibility obscures the study of these crucial processes. Here, we develop an ex vivo Matrigel-collagen-based culture to recapitulate mouse development from E4.5 to E6.0. Our system not only recapitulates embryonic growth, axis initiation, and overall 3D architecture in 49% of the cases, but its compatibility with light-sheet microscopy also enables the study of cellular dynamics through automatic cell segmentation. We find that, upon implantation, release of the increasing tension in the polar trophectoderm is necessary for its constriction and invagination. The resulting extra-embryonic ectoderm plays a key role in growth, morphogenesis, and patterning of the neighboring epiblast, which subsequently gives rise to all embryonic tissues. This 3D ex vivo system thus offers unprecedented access to peri-implantation development for in toto monitoring, measurement, and spatiotemporally controlled perturbation, revealing a mechano-chemical interplay between extra-embryonic and embryonic tissues.

Ca2+-activated cleavage of ezrin visualised dynamically in living myeloid cells during cell surface area expansion

The intracellular events underlying phagocytosis, a crucial event for innate immunity, are still unresolved. In order to test whether the reservoir of membrane required for the formation of the phagocytic pseudopodia is maintained by cortical ezrin, and that its cleavage is a key step in releasing this membrane, the cleavage of cortical ezrin was monitored within living phagocytes (the phagocytically competent cell line RAW264.7) through expressing two ezrin constructs with fluorescent protein tags located either inside the FERM or at the actin-binding domains. When ezrin is cleaved in the linker region by the Ca2+-activated protease calpain, separation of the two fluorophores would result. Experimentally induced Ca2+ influx triggered cleavage of peripherally located ezrin, which was temporally associated with cell expansion. Ezrin cleavage was also observed in the phagocytic pseudopodia during phagocytosis. Thus, our data demonstrates that peripheral ezrin is cleaved during Ca2+-influx-induced membrane expansion and locally within the extending pseudopodia during phagocytosis. This is consistent with a role for intact ezrin in maintaining folded membrane on the cell surface, which then becomes available for cell spreading and phagocytosis.

No evidence of involvement of E-cadherin in cell fate specification or the segregation of Epi and PrE in mouse blastocysts

During preimplantation mouse development stages, emerging pluripotent epiblast (Epi) and extraembryonic primitive endoderm (PrE) cells are first distributed in the blastocyst in a "salt-and-pepper" manner before they segregate into separate layers. As a result of segregation, PrE cells become localised on the surface of the inner cell mass (ICM), and the Epi is enclosed by the PrE on one side and by the trophectoderm on the other. During later development, a subpopulation of PrE cells migrates away from the ICM and forms the parietal endoderm (PE), while cells remaining in contact with the Epi form the visceral endoderm (VE). Here, we asked: what are the mechanisms mediating Epi and PrE cell segregation and the subsequent VE vs PE specification? Differences in cell adhesion have been proposed; however, we demonstrate that the levels of plasma membrane-bound E-cadherin (CDH1, cadherin 1) in Epi and PrE cells only differ after the segregation of these lineages within the ICM. Moreover, manipulating E-cadherin levels did not affect lineage specification or segregation, thus failing to confirm its role during these processes. Rather, we report changes in E-cadherin localisation during later PrE-to-PE transition which are accompanied by the presence of Vimentin and Twist, supporting the hypothesis that an epithelial-to-mesenchymal transition process occurs in the mouse peri-implantation blastocyst.

The Apical Domain Is Required and Sufficient for the First Lineage Segregation in the Mouse Embryo

Mammalian development begins with segregation of the extra-embryonic trophectoderm from the embryonic lineage in the blastocyst. While cell polarity and adhesion play key roles, the decisive cue driving this lineage segregation remains elusive. Here, to study symmetry breaking, we use a reduced system in which isolated blastomeres recapitulate the first lineage segregation. We find that in the 8-cell stage embryo, the apical domain recruits a spindle pole to ensure its differential distribution upon division. Daughter cells that inherit the apical domain adopt trophectoderm fate. However, the fate of apolar daughter cells depends on whether their position within the embryo facilitates apical domain formation by Cdh1-independent cell contact. Finally, we develop methods for transplanting apical domains and show that acquisition of this domain is not only required but also sufficient for the first lineage segregation. Thus, we provide mechanistic understanding that reconciles previous models for symmetry breaking in mouse development.

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A reduced system was established to study symmetry breaking in mouse development

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8-cell stage blastomeres acquire the capacity to self-organize the apical domain

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The apical domain is required and sufficient for the first lineage segregation

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Contact asymmetry specifies cell fate, leading to self-organized embryo patterning

Mouse blastomeres acquire ability to divide asymmetrically before compaction

The mouse preimplantation embryo generates the precursors of trophectoderm (TE) and inner cell mass (ICM) during the 8- to 16-cell stage transition, when the apico-basal polarized blastomeres undergo divisions that give rise to cells with different fate. Asymmetric segregation of polar domain at 8–16 cell division generate two cell types, polar cells which adopt an outer position and develop in TE and apolar cells which are allocated to inner position as the precursors of ICM. It is still not know when the blastomeres of 8-cell stage start to be determined to undergo asymmetric division. Here, we analyze the frequency of symmetric and asymmetric divisions of blastomeres isolated from 8-cell stage embryo before and after compaction. Using p-Ezrin as the polarity marker we found that size of blastomeres in 2/16 pairs cannot be used as a criterion for distinguishing symmetric and asymmetric divisions. Our results showed that at early 8-cell stage, before any visible signs of cortical polarity, a subset of blastomeres had been already predestined to divide asymmetrically. We also showed that almost all of 8-cell stage blastomeres isolated from compacted embryo divide asymmetrically, whereas in intact embryos, the frequency of asymmetric divisions is significantly lower. Therefore we conclude that in intact embryo the frequency of symmetric and asymmetric division is regulated by cell-cell interactions.

ERM proteins in cancer progression

Members of the ezrin-radixin-moesin (ERM) family of proteins are involved in multiple aspects of cell migration by acting both as crosslinkers between the membrane, receptors and the actin cytoskeleton, and as regulators of signalling molecules that are implicated in cell adhesion, cell polarity and migration. Increasing evidence suggests that the regulation of cell signalling and the cytoskeleton by ERM proteins is crucial during cancer progression. Thus, both their expression levels and subcellular localisation would affect tumour progression. High expression of ERM proteins has been shown in a variety of cancers. Mislocalisation of ERM proteins reduces the ability of cells to form cell-cell contacts and, therefore, promotes an invasive phenotype. Similarly, mislocalisation of ERM proteins impairs the formation of receptor complexes and alters the transmission of signals in response to growth factors, thereby facilitating tumour progression. In this Commentary, we address the structure, function and regulation of ERM proteins under normal physiological conditions as well as in cancer progression, with particular emphasis on cancers of epithelial origin, such as those from breast, lung and prostate. We also discuss any recent developments that have added to the understanding of the underlying molecular mechanisms and signalling pathways these proteins are involved in during cancer progression.

Cell fate regulation during preimplantation development: a view of adhesion-linked molecular interactions

In the developmental process of the early mammalian embryo, it is crucial to understand how the identical cells in the early embryo later develop different fates. Along with existing models, many recently discovered molecular, cellular and developmental factors play roles in cell position, cell polarity and transcriptional networks in cell fate regulation during preimplantation. A structuring process known as compaction provides the "start signal" for cells to differentiate and orchestrates the developmental cascade. The proper intercellular junctional complexes assembled between blastomeres act as a conducting mechanism governing cellular diversification. Here, we provide an overview of the diversification process during preimplantation development as it relates to intercellular junctional complexes. We also evaluate transcriptional differences between embryonic lineages according to cell- cell adhesion and the contributions of adhesion to lineage commitment. These series of processes indicate that proper cell fate specification in the early mammalian embryo depends on junctional interactions and communication, which play essential roles during early morphogenesis.

Initiation of Hippo signaling is linked to polarity rather than to cell position in the pre-implantation mouse embryo

In the mouse embryo, asymmetric divisions during the 8-16 cell division generate two cell types, polar and apolar cells, that are allocated to outer and inner positions, respectively. This outer/inner configuration is the first sign of the formation of the first two cell lineages: trophectoderm (TE) and inner cell mass (ICM). Outer polar cells become TE and give rise to the placenta, whereas inner apolar cells become ICM and give rise to the embryo proper and yolk sac. Here, we analyze the frequency of asymmetric divisions during the 8-16 cell division and assess the relationships between cell polarity, cell and nuclear position, and Hippo signaling activation, the pathway that initiates lineage-specific gene expression in 16-cell embryos. Although the frequency of asymmetric divisions varied in each embryo, we found that more than six blastomeres divided asymmetrically in most embryos. Interestingly, many apolar cells in 16-cell embryos were located at outer positions, whereas only one or two apolar cells were located at inner positions. Live imaging analysis showed that outer apolar cells were eventually internalized by surrounding polar cells. Using isolated 8-cell blastomeres, we carefully analyzed the internalization process of apolar cells and found indications of higher cortical tension in apolar cells than in polar cells. Last, we found that apolar cells activate Hippo signaling prior to taking inner positions. Our results suggest that polar and apolar cells have intrinsic differences that establish outer/inner configuration and differentially regulate Hippo signaling to activate lineage-specific gene expression programs.

Long-term live-cell imaging of mammalian preimplantation development and derivation process of pluripotent stem cells from the embryos

Mammalian fertilization is a process in which two highly specialized haploid gametes unite and endow totipotency to the resulting diploid zygote. This is followed by cell proliferation and the onset of differentiation during the brief period leading up to implantation. In these processes, a number of cellular components and structures are regulated spatially and temporally, as seen in repeated cell division, cell cycle progression, and epigenetic reprogramming. In mammals, the numbers of oocytes and embryos that can be collected are very limited. Therefore, analyses of molecular mechanisms are hampered because of difficulties in conducting biochemical analyses on such limited material. Furthermore, immunostaining methods require cell fixation and are insufficient for understanding ontogeny, because the processes observed in fertilization and early embryonic development progress in time-dependent manners and each phenomenon is connected with others by cause-and-effect relationships. Consequently, it is important to develop an experimental system that enables molecular imaging without affecting embryonic development. To achieve the above advantages, especially retrospective and prospective analyses, we have established a live-cell imaging system that enables observations under minimally invasive conditions. Using this approach, we have succeeded in visualizing and predicting the developmental potential of embryos after various perturbations. We also succeeded in imaging embryonic stem (ES) cell derivation in natural conditions. In this review, we describe a brief history of embryonic imaging and detailed protocols. We also discuss promising aspects of imaging in the fields of developmental and stem cell biology.

Anatomy of a blastocyst: cell behaviors driving cell fate choice and morphogenesis in the early mouse embryo

The preimplantation period of mouse early embryonic development is devoted to the specification of two extraembryonic tissues and their spatial segregation from the pluripotent epiblast. During this period two cell fate decisions are made while cells gradually lose their totipotency. The first fate decision involves the segregation of the extraembryonic trophectoderm (TE) lineage from the inner cell mass (ICM); the second occurs within the ICM and involves the segregation of the extraembryonic primitive endoderm (PrE) lineage from the pluripotent epiblast (EPI) lineage, which eventually gives rise to the embryo proper. Multiple determinants, such as differential cellular properties, signaling cues and the activity of transcriptional regulators, influence lineage choice in the early embryo. Here, we provide an overview of our current understanding of the mechanisms governing these cell fate decisions ensuring proper lineage allocation and segregation, while at the same time providing the embryo with an inherent flexibility to adjust when perturbed.

Local phosphocycling mediated by LOK/SLK restricts ezrin function to the apical aspect of epithelial cells

Local cycling of LOK/SLK-dependent phosphorylation of ezrin is required for its apical localization and for microvillus formation.

In this paper, we describe how a dynamic regulatory process is necessary to restrict microvilli to the apical aspect of polarized epithelial cells. We found that local phosphocycling regulation of ezrin, a critical plasma membrane–cytoskeletal linker of microvilli, was required to restrict its function to the apical membrane. Proteomic approaches and ribonucleic acid interference knockdown identified lymphocyte-oriented kinase (LOK) and SLK as the relevant kinases. Using drug-resistant LOK and SLK variants showed that these kinases were sufficient to restrict ezrin function to the apical domain. Both kinases were enriched in microvilli and locally activated there. Unregulated kinase activity caused ezrin mislocalization toward the basolateral domain, whereas expression of the kinase regulatory regions of LOK or SLK resulted in local inhibition of ezrin phosphorylation by the endogenous kinases. Thus, the domain-specific presence of microvilli is a dynamic process requiring a localized kinase driving the phosphocycling of ezrin to continually bias its function to the apical membrane.

Gene expression profiling reveals distinct cocaine-responsive genes in human fetal CNS cell types

OBJECTIVES

Prenatal exposure to cocaine causes cytoarchitectural alterations in the developing neocortex. Previously, we reported that cocaine inhibits neural progenitor cell proliferation through oxidative endoplasmic reticulum stress and consequent down-regulation of cyclin A, whereas cyclin A expression was increased in astrocytes. In the present study, cell type-specific responses to cocaine were further explored.

METHODS

Gene expression profiles were examined in five types of cells obtained from the human fetal cerebral cortex at 20 weeks gestation. Cells were treated with 100 µM cocaine in vitro for 24 hr, followed by gene expression analysis using a human neural/stem cell/drug abuse-focused cDNA array, with verification by quantitative real-time RT-PCR.

RESULTS

Cocaine influenced transcription of distinct categories of genes in a cell type-specific manner. Cocaine down-regulated cytoskeleton-related genes including ezrin, γ2 actin, α3d tubulin and α8 tubulin in neural and/or A2B5+ progenitor cells. In contrast, cocaine modulated immune and cell death-related genes in microglia and astrocytes. In microglia, cocaine up-regulated the immunoregulatory and pro-apoptotic genes IL-1β and BAX. In astrocytes, cocaine down-regulated the immune response gene glucocorticoid receptor and up-regulated the anti-apoptotic genes 14-3-3 ε and HVEM. Therefore, cell types comprising the developing neocortex show differential responses to cocaine.

CONCLUSIONS

These data suggest that cocaine causes cytoskeletal abnormalities leading to disturbances in neural differentiation and migration in progenitor cells, while altering immune and apoptotic responses in glia. Understanding the mechanisms of cocaine's effects on human CNS cells may help in the development of therapeutic strategies to prevent or ameliorate cocaine-induced impairments in fetal brain development.

Understanding paternal genome demethylation through live-cell imaging and siRNA

Identification of a DNA demethylase responsible for zygotic paternal DNA demethylation has been one of the most challenging goals in the field of epigenetics. Several candidate molecules have been proposed, but their involvement in the demethylation remains controversial, partly due to the difficulty of preparing a sufficient quantity of materials for biochemical analysis. In this review, we utilize a recently developed method for live-cell imaging of mouse zygotes combined with RNA interference (RNAi) to search for factors that affect zygotic paternal DNA demethylation. The combined use of various fluorescent probes and RNAi is a useful approach for the study of not only DNA demethylation but also the spatiotemporal dynamics of histone depositions in zygotes, although it is not appropriate for large-scale screening or knockdown of genes that are abundantly expressed before fertilization. This new technique enables us to understand the epigenetic hierarchy during cellular response and differentiation in preimplantation embryos.

Emerging role for ERM proteins in cell adhesion and migration

The highly related ERM (Ezrin, Radixin, Moesin) proteins provide a regulated linkage between the membrane and the underlying actin cytoskeleton. They also provide a platform for the transmission of signals in responses to extracellular cues. Studies in different model organisms and in cultured cells have highlighted the importance of ERM proteins in the generation and maintenance of specific domains of the plasma membrane. A central question is how do ERM proteins coordinate actin filament organization and membrane protein transport/stability with signal transduction pathways to build up complex structures? Through their interaction with numerous partners including membrane proteins, actin cytoskeleton and signaling molecules, ERM proteins have the ability to organize multiprotein complexes in specific cellular compartments. Likewise, ERM proteins participate in diverse functions including cell morphogenesis, endocytosis/exocytosis, adhesion and migration. This review focuses on aspects still poorly understood related to the function of ERM proteins in epithelial cell adhesion and migration.

Ezrin promotes actin assembly at the phagosome membrane and regulates phago-lysosomal fusion

Phagosome maturation is defined as the process by which phagosomes fuse sequentially with endosomes and lysosomes to acquire an acidic pH and hydrolases that degrade ingested particles. While the essential role of actin cytoskeleton remodeling during particle internalization is well established, its role during the later stages of phagosome maturation remains largely unknown. We have previously shown that purified mature phagosomes assemble F-actin at their membrane, and that the ezrin-radixin-moesin (ERM) proteins ezrin and moesin participate in this process. Moreover, we provided evidence that actin assembly on purified phagosomes stimulates their fusion with late endocytic compartments in vitro. In this study, we further investigated the role of ezrin in phagosome maturation. We engineered a structurally open form of ezrin and demonstrated that ezrin binds directly to the actin assembly promoting factor N-WASP (Neural Wiskott-Aldrich Syndrome Protein) by its FERM domain. Using a cell-free system, we found that ezrin stimulates F-actin assembly on purified phagosomes by recruiting the N-WASP-Arp2/3 machinery. Accordingly, we showed that the down-regulation of ezrin activity in macrophages by a dominant-negative approach caused reduced F-actin accumulation on maturing phagosomes. Furthermore, using fluorescence and electron microscopy, we found that ezrin is required for the efficient fusion between phagosomes and lysosomes. Live-cell imaging analysis supported the notion that ezrin is necessary for the fusogenic process itself, promoting the transfer of the lysosome content into the phagosomal lumen.

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.

Orientation of mitotic spindles during the 8- to 16-cell stage transition in mouse embryos

BACKGROUND

Asymmetric cell divisions are involved in the divergence of the first two lineages of the pre-implantation mouse embryo. They first take place after cell polarization (during compaction) at the 8-cell stage. It is thought that, in contrast to many species, spindle orientation is random, although there is no direct evidence for this.

METHODOLOGY/PRINCIPAL FINDINGS

Tubulin-GFP and live imaging with a spinning disk confocal microscope were used to directly study spindle orientation in whole embryos undergoing the 8- to 16-cell stage transition. This approach allowed us to determine that there is no predetermined cleavage pattern in 8-cell compacted mouse embryos and that mitotic spindle orientation in live embryo is only modulated by the extent of cell rounding up during mitosis.

CONCLUSIONS

These results clearly demonstrate that spindle orientation is not controlled at the 8- to 16-cell transition, but influenced by cell bulging during mitosis, thus reinforcing the idea that pre-implantation development is highly regulative and not pre-patterned.

Inactivation of aPKClambda reveals a context dependent allocation of cell lineages in preimplantation mouse embryos

Background

During mammalian preimplantation development, lineage divergence seems to be controlled by the interplay between asymmetric cell division (once cells are polarized) and positional information. In the mouse embryo, two distinct cell populations are first observed at the 16-cell stage and can be distinguished by both their position (outside or inside) and their phenotype (polarized or non-polarized). Many efforts have been made during the last decade to characterize the molecular mechanisms driving lineage divergence.

Methodology/Principal Findings

In order to evaluate the importance of cell polarity in the determination of cell fate we have disturbed the activity of the apical complex aPKC/PAR6 using siRNA to down-regulate aPKCλ expression. Here we show that depletion of aPKCλ results in an absence of tight junctions and in severe polarity defects at the 16-cell stage. Importantly, we found that, in absence of aPKCλ, cell fate depends on the cellular context: depletion of aPKCλ in all cells results in a strong reduction of inner cells at the 16-cell stage, while inhibition of aPKCλ in only half of the embryo biases the progeny of aPKCλ defective blastomeres towards the inner cell mass. Finally, our study points to a role of cell shape in controlling cell position and thus lineage allocation.

Conclusion

Our data show that aPKCλ is dispensable for the establishment of polarity at the 8-cell stage but is essential for the stabilization of cell polarity at the 16-cell stage and for cell positioning. Moreover, this study reveals that in addition to positional information and asymmetric cell divisions, cell shape plays an important role for the control of lineage divergence during mouse preimplantation development. Cell shape is able to influence both the type of division (symmetric or asymmetric) and the position of the blastomeres within the embryo.

A subcortical maternal complex essential for preimplantation mouse embryogenesis

We have identified a subcortical maternal complex (SCMC) that assembles during oocyte growth and is essential for zygotes to progress beyond the first embryonic cell divisions. At least four maternally encoded proteins contribute to this MDa complex: FLOPED, MATER, and TLE6 interact with each other while Filia binds independently to MATER. Although the transcripts encoding these proteins are degraded during meiotic maturation and ovulation, the SCMC proteins persist in the early embryo. The SCMC, located in the subcortex of eggs, is excluded from regions of cell-cell contact in the cleavage-stage embryo and segregates to the outer cells of the morulae and blastocyst. Floped(tm/tm) and/or Mater(tm/tm) eggs lack the SCMC but can be fertilized. However, these embryos do not progress beyond cleavage stage development and female mice are sterile. The proteins are conserved in humans, and similar maternal effect mutations may result in recurrent embryonic loss.

Maternally derived FILIA-MATER complex localizes asymmetrically in cleavage-stage mouse embryos

Initial cell lineages that presage the inner cell mass and extra-embryonic trophectoderm are established when eight blastomeres compact to form polarized morulae in preimplantation mouse development. FILIA has been identified as a binding partner to MATER (maternal antigen that embryos require; also known as NLRP5), which is encoded by a maternal effect gene. Products of each gene are detected in growing oocytes and, although transcripts are degraded before fertilization, the cognate proteins persist in early blastocysts. The two proteins co-localize to the cytocortex of ovulated eggs, where the stability of FILIA is dependent on the presence of MATER. After fertilization,FILIA-MATER complexes become asymmetrically restricted in the apical cytocortex of two-cell embryos due to their absence in regions of cell-cell contact. This asymmetry is reversible upon disaggregation of blastomeres of the two- and four-cell embryo. Each protein persists in cells of the preimplantation embryo, but the continuous cell-cell contact of `inner' cells of the morulae seemingly precludes formation of the subcortical FILIA-MATER complex and results in cell populations that are marked by its presence(`outer') or absence (`inner'). Thus, the FILIA-MATER complex provides a molecular marker of embryonic cell lineages, but it remains to be determined if the molecular asymmetry established after the first cell division plays a role in cell fate determinations in the early mouse embryo. If so, the plasticity of the FILIA-MATER complex localization may reflect the regulative nature of preimplantation mouse development.

Cell shape and cell division

The correlation between cell shape elongation and the orientation of the division axis described by early cell biologists is still used as a paradigm in developmental studies. However, analysis of early embryo development and tissue morphogenesis has highlighted the role of the spatial distribution of cortical cues able to guide spindle orientation. In vitro studies of cell division have revealed similar mechanisms. Recent data support the possibility that the orientation of cell division in mammalian cells is dominated by cell adhesion and the associated traction forces developed in interphase. Cell shape is a manifestation of these adhesive and tensional patterns. These patterns control the spatial distribution of cortical signals and thereby guide spindle orientation and daughter cell positioning. From these data, cell division appears to be a continuous transformation ensuring the maintenance of tissue mechanical integrity.

DCC regulates cell adhesion in human colon cancer derived HT-29 cells and associates with ezrin

The deleted in colorectal cancer (DCC) gene encodes a 170- to 190-kDa protein of the Immunoglobulin superfamily. Firstly identified as a tumor suppressor gene in human colorectal carcinomas, the main function for DCC has been described in the nervous system as part of a receptor complex for netrin-1. Moreover, roles in mucosecretory cell differentiation and as inducer of apoptosis have also been reported. DCC knockout mice supported a crucial role for this gene in axonal migration, yet questioned its implication in tumor suppression and mucosecretory differentiation. The work presented here demonstrates that a DCC-transfected HT-29 colonic human cell line (HT-29/DCC) displays an increase in cell-cell adhesion to the detriment of cell-matrix interactions: HT-29/DCC cells exhibit more and better-structured desmosomes while focal adhesions and hemidesmosomes are disrupted. HT-29/DCC cells show no changes in adherent junctions but upon treatment with TPA, HT-29/DCC cells show resistance to scattering, and maintain E-cadherin in the membrane. In addition, the actin cytoskeleton is affected in HT-29/DCC cells: stress fibers are disrupted while cortical actin remains intact. We identified a putative ERM-M (ezrin/radixin/moesin and merlin) binding domain in the juxtamembrane region of the DCC protein. In vitro pull-down assays demonstrate the interaction of the DCC cytoplasmic domain with the N-terminal region of ezrin and merlin, and co-immunoprecipitation assays in transiently DCC-transfected COS-1 cells showed that the interaction between DCC and ezrin also takes place in vivo. Altogether, our results suggest that DCC could regulate cell adhesion and migration through its association with ERM-M proteins.

Identification of regulators of germ layer morphogenesis using proteomics in zebrafish

During vertebrate gastrulation, a well-orchestrated series of morphogenetic changes leads to the formation of the three germ layers: the ectoderm, mesoderm and endoderm. The analysis of gene expression patterns during gastrulation has been central to the identification of genes involved in germ layer formation. However, many proteins are regulated on a translational or post-translational level and are thus undetectable by gene expression analysis. Therefore, we developed a 2D-gel-based comparative proteomic approach to target proteins involved in germ layer morphogenesis during zebrafish gastrulation. Proteomes of ectodermal and mesendodermal progenitor cells were compared and 35 significantly regulated proteins were identified by mass spectrometry, including several proteins with predicted functions in cytoskeletal organization. A comparison of our proteomic results with data obtained in an accompanying microarray-based gene expression analysis revealed no significant overlap, confirming the complementary nature of proteomics and transcriptomics. The regulation of ezrin2, which was identified based on a reduction in spot intensity in mesendodermal cells, was independently validated. Furthermore, we show that ezrin2 is activated by phosphorylation in mesendodermal cells and is required for proper germ layer morphogenesis. We demonstrate the feasibility of proteomics in zebrafish, concluding that proteomics is a valuable tool for analysis of early development.

Trophoblast and hypoblast in the monotreme, marsupial and eutherian mammal: evolution and origins

The pregastrula stage mammalian conceptus consists of both embryonic and non-embryonic components. The latter forms the bulk of the tissues, provides nutrition for the developing embryo and also contributes developmental signals that influence events within the embryo itself. Understanding the origins and relationships between the embryonic and extraembryonic cell lineages is thus central to understanding development in mammals. Despite the apparent gross differences in early developmental strategy and form, the conceptuses of eutherian, marsupial and monotreme mammals show some remarkable similarities in the lineage allocation to trophoblast and hypoblast and in the emergent properties of the two cell types. We suggest that the gross differences can be explained by two relatively small evolutionary timing changes affecting cell adhesion patterns and the polarisation of developmentally significant information. These changes result in the conversion of a unilaminar blastocyst to a morula form composed of blastomeres with increased regulatory capacity.

Mitotic Spindles and Cleavage Planes Are Oriented Randomly in the Two-Cell Mouse Embryo

Most experimental embryological studies performed on the early mouse embryo have led to the conclusion that there are no mosaically distributed developmental determinants in the zygote and early embryo (for example see [1-6]). It has been suggested recently that "the cleavage pattern of the early mouse embryo is not random and that the three-dimensional body plan is pre-patterned in the egg" (in [7] for review see [8-10]). Two major spatial cues influencing the pattern of cleavage divisions have been proposed: the site of the second meiotic division [11, 12] and the sperm entry point [13-14], although the latter is controversial [15-17]. An implication of this hypothesis is that the orientations of the first few cleavage divisions are stereotyped. Such a define cleavage pattern, leading to the segregation of developmental determinants, is observed in many species [18]. Recently, it was shown that the first cleavage plane is not predetermined but defined by the topology of the two apposing pronuclei [19]. Because the position of the female pronucleus is dependent upon the site of polar body extrusion and the position of the male pronuclei is dependent upon the sperm entry point [19-20], this observation leaves open the possibility that the sperm may provide some kind of directionality [7]. But, even if asymmetries were set up only after fertilization, a stereotyped cleavage pattern should take place during the following cleavage divisions. Thus, we studied the cleavage pattern of two-cell embryos by videomicroscopy to distinguish between the two hypotheses. After the mitotic spindle formed, its orientation did not change until cleavage. During late metaphase and anaphase, the spindle poles appear to be anchored to the cortex through astral microtubules and PARD6a. Only at the time of cleavage, during late anaphase, do the forming daughter cells change their relative positions. These studies show that cleavage planes are oriented randomly in two-cell embryos. This argues against a prepatterning of the mouse embryo before compaction.

Embryotrophic factor-3 from human oviductal cells enhances proliferation, suppresses apoptosis and stimulates the expression of the β1 subunit of sodium–potassium ATPase in mouse embryos

BACKGROUND

Embrytrophic factor-3 (ETF-3) from human oviductal cells enhanced the development of mouse preimplantation embryos. This report studied the embryotrophic mechanisms of the molecule.

METHODS AND RESULTS

Mouse embryos were incubated with ETF-3 for 24 h at different stages of development. ETF-3 treatment between 96 and 120 h post-HCG increased the cell count of blastocysts, whilst treatment between 72 and 96 h post-HCG enhanced the expansion and hatching of the blastocysts. ETF-3 increased the cell number of the embryos by suppressing apoptosis and increasing proliferation as determined by TUNEL and bromodeoxyuridine uptake assays, respectively. Real-time quantitative PCR showed that the in vivo developed and ETF-3-treated blastocysts had a significantly higher mRNA copy number of Na/K-ATPase-beta1, but not of hepsin, than that of blastocysts cultured in medium alone. The former gene was associated with cavitation of blastocysts while the latter was related to hatching of blastocyst. The beneficial effect of ETF-3 on blastocyst hatching was also seen when ETF-3-supplemented commercially available sequential culture medium for human embryo culture was used to culture mouse embryos.

CONCLUSIONS

ETF-3 improves embryo development by enhancing proliferation, suppressing apoptosis and stimulating expression of genes related to blastocyst cavitation. Supplementating human embryo culture medium with ETF-3 may improve the success rate in clinical assisted reproduction.

Restricted expression domains of Ezrin in developing epithelia of the chick

Ezrin is a member of the ERM- (Ezrin-Radixin-Moesin-) family of actin binding proteins, which function as linkers of the cortical cytoskeleton to components of the plasma membrane. Additional roles for Ezrin in intracellular signalling and ion channel regulation were suggested. We found Ezrin mRNA in the anterior endo- and mesoderm of chick gastrula stage embryos. In these tissues Ezrin message is strongly expressed throughout early development of the foregut (pharynx) and heart tube. During later stages of development, highly restricted expression domains of Ezrin mRNA were detected in the endodermal lining of the pharyngeal pouches, the mesonephric duct and tubuli, and in the ectodermal placodes giving rise to the inner ear, eye lens and olfactory epithelium.

Lineage allocation and cell polarity during mouse embryogenesis

The first developmental lineage allocation during the generation of the mouse blastocyst is to outer trophoblast or to inner pluriblast (inner cell mass; ICM) cells. This allocation seems to be initiated at the 8-cell stage, when blastomeres polarise. Polarisation is followed by differentiative divisions at the subsequent two cleavage divisions to generate polar outer and non-polar inner 16- and 32-cells. The key events in polarisation are regulated post-translationally through a cell contact-mediated pathway, which imposes a heritable determinant-like organisation on the blastomere cortex. Two proteins in particular, E-cadherin and ezrin, are intimately involved in the generation and stabilisation of developmentally significant information. Transcriptional differences between lineages appear to follow and may coincide with the lineage commitment of cells.

Phosphorylation of ezrin on threonine T567 plays a crucial role during compaction in the mouse early embryo

The preimplantation development of the mouse embryo leads to the divergence of the first two cell lineages, the inner cell mass and the trophectoderm. The formation of a microvillus pole during compaction at the eight-cell stage and its asymmetric inheritance during mitosis are key events in the emergence of these two cell populations. Ezrin, a member of the ERM protein family, seems to be involved in the formation and stabilization of this apical microvillus pole. To further characterize its function in early development, we mutated the key residue T567, which was reported to be essential for regulation of ezrin function through phosphorylation. Here, we show that expression of ezrin mutants in which the COOH-terminal threonine T567 was replaced by an aspartate (to mimic a phosphorylated residue; T567D) or by an alanine (to avoid phosphorylation; T567A) interferes with E-cadherin function and disrupts the first morphogenetic events of development: compaction and cavitation. The active mutant ezrin-T567D induces the formation of numerous and abnormally long microvilli at the surface of blastomeres. Moreover, it localizes all around the cell cortex and inhibits cell-cell adhesion and cell polarization at the eight-cell stage. During the following stages, only half of the embryos are able to compact and finally to cavitate. In those embryos, the amount of ezrin-T567D decreases in the basolateral areas, while the proportion of adherens junctions increases. The reverse inactive mutant ezrin-T567A is mainly cytoplasmic and does not perturb compaction at the eight-cell stage. However, at the 16-cell stage, it relocalizes at the basolateral cortex, leading to a strong decrease in the surface of adherens junctions, and finally, embryos abort development. Our results show that ezrin is directly involved in the formation of microvilli in the early mouse embryo. Moreover, they indicate that maintenance of ezrin in basolateral areas prevents microvilli breakdown and inhibits the formation of normal cell-cell contacts mediated by E-cadherin, thereby impairing blastomeres polarization and morphogenesis of the blastocyst.

Society for Reproductive Biology Founders' Lecture 2003.The making of an embryo: short-term goals and long-term implications

During early development, the eutherian mammalian embryo forms a blastocyst comprising an outer trophectoderm epithelium and enclosed inner cell mass (ICM). The short-term goal of blastocyst morphogenesis, including epithelial differentiation and segregation of the ICM, is mainly regulated autonomously and comprises a combination of temporally controlled gene expression, cell polarisation, differentiative cell divisions and cell–cell interactions. This aspect of blastocyst biogenesis is reviewed, focusing, in particular, on the maturation and role of cell adhesion systems. Early embryos are also sensitive to their environment, which can affect their developmental potential in diverse ways and may lead to long-term consequences relating to fetal or postnatal growth and physiology. Some current concepts of embryo–environment interactions, which may impact on future health, are also reviewed.

Expression and role of the ether-à-go-go-related (MERG1A) potassium-channel protein during preimplantation mouse development

Potassium channels play important roles in many cellular processes, including cell-cycle progression and cell differentiation. In the present study, we investigated the pattern of expression of the mouse ether-à-go-go-related (KCNH2; MERG1A) potassium channel during mouse embryogenic development. Analysis by reverse transcription-polymerase chain reaction revealed maternal MERG1A transcripts until the late 2-cell stage of development, after which MERG1A expression from the zygotic genome was low until the 8-cell stage, then rose in the morula, but was low in trophoblast compared to inner cell mass cells. A trophoblast stem cell line also was shown to express MERG1A mRNA. Immunoblotting of oocytes, blastocysts, and the trophoblast stem cell line revealed different posttranslationally processed forms of MERG1A. Immunofluorescence analysis showed that the subcellular localization of MERG1A varied at different stages of the embryogenic cell cycle. In addition, MERG1A protein levels increased following compaction at the 8-cell stage, and its distribution became polarized. This relocalization of MERG1A was affected by treatment with specific inhibitors of ether-à-go-go-related gene (ERG)-channel function and of actin polymerization. Puromycin treatment of morulae indicated that membrane-associated MERG1A had a half-life of greater than 24 h. The ERG-specific inhibitor E-4031 reduced the incidence of blastocyst formation and the number of cells per blastocyst. These results show that MERG1A is developmentally regulated and suggest that it might play a role in early mouse embryogenic development.

Embryotrophic factor-3 from human oviductal cells affects the messenger RNA expression of mouse blastocyst

Our previous results showed that embryotrophic factor-3 (ETF-3) from human oviductal cells increased the size and hatching rate of mouse blastocysts in vitro. The present study investigated the production of ETF-3 by an immortalized human oviductal cell line (OE-E6/E7) and the effects of ETF-3 on the mRNA expression of mouse embryos. The ETF-3 was purified from primary oviductal cell conditioned media using sequential liquid chromatographic systems, and antiserum against ETF-3 was raised. The ETF-3-supplemented Chatot-Ziomek-Bavister medium was used to culture Day 1 MF1 x BALB/c mouse embryos for 4 days. The ETF-3 treatment significantly enhanced the mouse embryo blastulation and hatching rate. The antiserum, at concentrations of 0.03-3%, abolished the embryotrophic effect of ETF-3. Positive ETF-3 immunoreactivity was detected in the primary oviductal cells, OE-E6/E7, and blastocysts derived from ETF-3 treatment. Vero cells (African Green Monkey kidney cell line), fibroblasts, and embryos cultured in control medium did not possess ETF-3 immunoreactivity. The mRNA expression patterns of the treated embryos were studied at the blastocyst stage by mRNA differential display reverse transcription-polymerase chain reaction (DDRT-PCR). The DDRT-PCR showed that some of the mRNAs were differentially expressed after ETF-3 treatment. Twelve of the differentially expressed mRNAs that had high homology with cDNA sequences in the GenBank were selected for further characterization. The differential expression of seven of these mRNAs (ezrin, heat shock 70-kDa protein, cytochrome c oxidase subunit VIIa-L precursor, proteinase-activated receptor 2, eukaryotic translation initiation factor 2beta, cullin 1, and proliferating cell nuclear antigen) was confirmed by semiquantitative RT-PCR. In conclusion, immortalized oviductal cells produce ETF-3, which influences mRNA expression of mouse blastocyst.

Visualization of molecular and cellular events with green fluorescent proteins in developing embryos: a review

During the past 5 years, green fluorescent protein (GFP) has become one of the most widely used in vivo protein markers for studying a number of different molecular processes during development, such as promoter activation, gene expression, protein trafficking and cell lineage determination. GFP fluorescence allows observation of dynamic developmental processes in real time, in both transiently and stably transformed cells, as well as in live embryos. In this review, we include the most up-to-date use of GFP during embryonic development and point out the unique contribution of GFP visualization, which resulted in novel discoveries.

Molecular analysis of microscopic ezrin dynamics by two-photon FRAP

Ezrin plays a key role in coupling signal transduction to cortical cell organization. This actin-membrane linker undergoes a series of conformational changes that modulate its interactions with various partners and its localization in membrane or cytosolic pools. Its mobility and exchange rates within and between these two pools were assessed by two-photon fluorescence recovery after photobleaching in epithelial cell microvilli. Analysis of ezrin mutants with an altered actin-binding site revealed three ezrin membrane states of different mobilities and exchange properties, reflecting sequential association with membrane components and F-actin in the context of a fast overall turnover.

ERM proteins and NF2 tumor suppressor: the Yin and Yang of cortical actin organization and cell growth signaling

The ERM (ezrin, radixin and moesin) family of proteins are linkers that tether actin microfilaments to the plasma membrane. Merlin, the NF2 tumor suppressor gene product, is highly homologous to ERM proteins. In ERM proteins and merlin, interdomain binding promotes auto-inhibition and homo-oligomerization or hetero-oligomerization. Recent studies have revealed that ERM proteins transduce growth signals, and have shed new light on how merlin links cell growth to the cytoskeleton.