PKC signalling regulates tight junction membrane assembly in the pre-implantation mouse embryo

Overview of junctional complexes during mammalian early embryonic development

Cell-cell junctions form strong intercellular connections and mediate communication between blastomeres during preimplantation embryonic development and thus are crucial for cell integrity, polarity, cell fate specification and morphogenesis. Together with cell adhesion molecules and cytoskeletal elements, intercellular junctions orchestrate mechanotransduction, morphokinetics and signaling networks during the development of early embryos. This review focuses on the structure, organization, function and expressional pattern of the cell-cell junction complexes during early embryonic development. Understanding the importance of dynamic junction formation and maturation processes will shed light on the molecular mechanism behind developmental abnormalities of early embryos during the preimplantation period.

From Mice to Men: Generation of Human Blastocyst-Like Structures In Vitro

Advances in the field of stem cell-based models have in recent years lead to the development of blastocyst-like structures termed blastoids. Blastoids can be used to study key events in mammalian pre-implantation development, as they mimic the blastocyst morphologically and transcriptionally, can progress to the post-implantation stage and can be generated in large numbers. Blastoids were originally developed using mouse pluripotent stem cells, and since several groups have successfully generated blastocyst models of the human system. Here we provide a comparison of the mouse and human protocols with the aim of deriving the core requirements for blastoid formation, discuss the models’ current ability to mimic blastocysts and give an outlook on potential future applications.

Principles of Self-Organization of the Mammalian Embryo

Early embryogenesis is a conserved and self-organized process. In the mammalian embryo, the potential for self-organization is manifested in its extraordinary developmental plasticity, allowing a correctly patterned embryo to arise despite experimental perturbation. The underlying mechanisms enabling such regulative development have long been a topic of study. In this Review, we summarize our current understanding of the self-organizing principles behind the regulative nature of the early mammalian embryo. We argue that geometrical constraints, feedback between mechanical and biochemical factors, and cellular heterogeneity are all required to ensure the developmental plasticity of mammalian embryo development.

Blastocyst-like structures generated from human pluripotent stem cells

Limited access to embryos has hampered the study of human embryogenesis and disorders that occur during early pregnancy. Human pluripotent stem cells provide an alternative means to study human development in a dish^1-7. Recent advances in partial embryo models derived from human pluripotent stem cells have enabled human development to be examined at early post-implantation stages^8-14. However, models of the pre-implantation human blastocyst are lacking. Starting from naive human pluripotent stem cells, here we developed an effective three-dimensional culture strategy with successive lineage differentiation and self-organization to generate blastocyst-like structures in vitro. These structures-which we term 'human blastoids'-resemble human blastocysts in terms of their morphology, size, cell number, and composition and allocation of different cell lineages. Single-cell RNA-sequencing analyses also reveal the transcriptomic similarity of blastoids to blastocysts. Human blastoids are amenable to embryonic and extra-embryonic stem cell derivation and can further develop into peri-implantation embryo-like structures in vitro. Using chemical perturbations, we show that specific isozymes of protein kinase C have a critical function in the formation of the blastoid cavity. Human blastoids provide a readily accessible, scalable, versatile and perturbable alternative to blastocysts for studying early human development, understanding early pregnancy loss and gaining insights into early developmental defects.

Cell-cell junctions: structure and regulation in physiology and pathology

Epithelial and endothelial cell-cell contacts are established and maintained by several intercellular junctional complexes. These structurally and biochemically differentiated regions on the plasma membrane primarily include tight junctions (TJs), and anchoring junctions. While the adherens junctions (AJs) provide essential adhesive and mechanical properties, TJs hold the cells together and form a near leak-proof intercellular seal by the fusion of adjacent cell membranes. AJs and TJs play essential roles in vascular permeability. Considering their involvement in several key cellular functions such as barrier formation, proliferation, migration, survival, and differentiation, further research is warranted on the composition and signaling pathways regulating cell-cell junctions to develop novel therapeutics for diseases such as organ injuries. The current review article presents our current state of knowledge on various cell-cell junctions, their molecular composition, and mechanisms regulating their expression and function in endothelial and epithelial cells.

Coordination between patterning and morphogenesis ensures robustness during mouse development

The mammalian preimplantation embryo is a highly tractable, self-organizing developmental system in which three cell types are consistently specified without the need for maternal factors or external signals. Studies in the mouse over the past decades have greatly improved our understanding of the cues that trigger symmetry breaking in the embryo, the transcription factors that control lineage specification and commitment, and the mechanical forces that drive morphogenesis and inform cell fate decisions. These studies have also uncovered how these multiple inputs are integrated to allocate the right number of cells to each lineage despite inherent biological noise, and as a response to perturbations. In this review, we summarize our current understanding of how these processes are coordinated to ensure a robust and precise developmental outcome during early mouse development. This article is part of a discussion meeting issue 'Contemporary morphogenesis'.

Self-Organization of Mouse Stem Cells into an Extended Potential Blastoid

Mammalian blastocysts comprise three distinct cell lineages essential for development beyond implantation: the pluripotent epiblast, which generates the future embryo, and surrounding it the extra-embryonic primitive endoderm and the trophectoderm tissues. Embryonic stem cells can reintegrate into embryogenesis but contribute primarily to epiblast lineages. Here, we show that mouse embryonic stem cells cultured under extended pluripotent conditions (EPSCs) can be partnered with trophoblast stem cells to self-organize into blastocyst-like structures with all three embryonic and extra-embryonic lineages. Morphogenetic and transcriptome profiling analyses reveal that these blastocyst-like structures show distinct embryonic-abembryonic axes and primitive endoderm differentiation and can initiate the transition from the pre- to post-implantation egg cylinder morphology in vitro.

Activation of the Ca2+ sensing receptor and the PKC/WNK4 downstream signaling cascade induces incorporation of ZO-2 to tight junctions and its separation from 14-3-3

Zonula occludens-2 (ZO-2) is a tight junction (TJ) cytoplasmic protein, whose localization varies according to cell density and Ca²⁺ in the media. In cells cultured in low calcium (LC), ZO-2 displays a diffuse cytoplasmic distribution, but activation of the Ca²⁺ sensing receptor (CaSR) with Gd³⁺ triggers the appearance of ZO-2 at the cell borders. CaSR downstream signaling involves activation of protein kinase C, which phosphorylates and activates with no lysine kinase-4 that phosphorylates ZO-2 inducing its concentration at TJs. In LC, ZO-2 is protected from degradation by association to 14-3-3 proteins. When monolayers are transferred to normal calcium, the complexes ZO-2/14-3-3ζ and ZO-2/14-3-3σ move to the cell borders and dissociate. The 14-3-3 proteins are then degraded in proteosomes, whereas ZO-2 integrates to TJs. From the plasma membrane residual ZO-2 is endocyted and degradaded in lysosomes. The unique region 2 of ZO-2, and S261 located within a nuclear localization signal, are critical for the interaction with 14-3-3 ζ and σ and for the efficient nuclear importation of ZO-2. These results explain the molecular mechanism through which extracellular Ca²⁺ triggers the appearance of ZO-2 at TJs in epithelial cells and reveal the novel interaction between ZO-2 and 14-3-3 proteins, which is critical for ZO-2 protection and intracellular traffic.

Binding of FGF2 to FGFR2 in an autocrine mode in trophectoderm cells is indispensable for mouse blastocyst formation through PKC-p38 pathway

Fibroblast growth factors (FGF1, FGF2 and FGF4) and fibroblast growth factor receptors (FGFR1, FGFR2, FGFR3 and FGFR4) have been reported to be expressed in preimplantation embryos and be required for their development. However, the functions of these molecules in trophectoderm cells (TEs) that lead to the formation of the blastocyst as well as the underlying mechanism have not been elucidated. The present study has demonstrated for the first time that endogenous FGF2 secreted by TEs can regulate protein expression and distribution in TEs via the FGFR2-mediated activation of PKC and p38, which are important for the development of expanded blastocysts. This finding provides the first explanation for the long-observed phenomenon that only high concentrations of exogenous FGFs have effects on embryonic development, but in vivo the amount of endogenous FGFs are trace. Besides, the present results suggest that FGF2/FGFR2 may act in an autocrine fashion and activate the downstream PKC/p38 pathway in TEs during expanded blastocyst formation.

Making the first decision: lessons from the mouse

Pre-implantation development encompasses a period of 3-4 days over which the mammalian embryo has to make its first decision: to separate the pluripotent inner cell mass (ICM) from the extra-embryonic epithelial tissue, the trophectoderm (TE). The ICM gives rise to tissues mainly building the body of the future organism, while the TE contributes to the extra-embryonic tissues that support embryo development after implantation. This review provides an overview of the cellular and molecular mechanisms that control the critical aspects of this first decision, and highlights the role of critical events, namely zytotic genome activation, compaction, polarization, asymmetric cell divisions, formation of the blastocyst cavity and expression of key transcription factors.

Emerging Role of the Hippo Signaling Pathway in Position Sensing and Lineage Specification in Mammalian Preimplantation Embryos

In preimplantation mouse embryos, the first lineage differentiation takes place in the 8- to 16-cell-stage embryo and results in formation of the trophectoderm (TE) and inner cell mass (ICM), which will give rise to the trophoblast of the placenta and the embryo proper, respectively. Although, it is widely accepted that positioning of a cell within the embryo influences lineage differentiation, the mechanism underlying differential lineage differentiation and how it involves cell position are largely unknown. Interestingly, novel cues from the Hippo pathway have been recently demonstrated in the preimplantation mouse embryo. Unlike the mechanisms reported from epithelium-cultured cells, the Hippo pathway was found to be responsible for translating positional information to lineage specification through a position-sensing mechanism. Disruption of Hippo pathway-component genes in early embryos results in failure of lineage specification and failure of postimplantation development. In this review, we discuss the unique role of the Hippo signaling pathway in early embryo development and its role in lineage specification. Understanding the activity and regulation of the Hippo pathway may offer new insights into other areas of developmental biology that evolve from understanding of this cell-fate specification in the early embryonic cell.

aPKC phosphorylates p27Xic1, providing a mechanistic link between apicobasal polarity and cell-cycle control

During the development of the nervous system, apicobasally polarized stem cells are characterized by a shorter cell cycle than nonpolar progenitors, leading to a lower differentiation potential of these cells. However, how polarization might be directly linked to the kinetics of the cell cycle is not understood. Here, we report that apicobasally polarized neuroepithelial cells in Xenopus laevis have a shorter cell cycle than nonpolar progenitors, consistent with mammalian systems. We show that the apically localized serine/threonine kinase aPKC directly phosphorylates an N-terminal site of the cell-cycle inhibitor p27Xic1 and reduces its ability to inhibit the cyclin-dependent kinase 2 (Cdk2), leading to shortening of G1 and S phases. Overexpression of activated aPKC blocks the neuronal differentiation-promoting activity of p27Xic1. These findings provide a direct mechanistic link between apicobasal polarity and the cell cycle, which may explain how proliferation is favored over differentiation in polarized neural stem cells.

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aPKC shortens G1 and S phases of cell cycle by phosphorylating p27Xic1

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Phosphorylated p27Xic1 exhibits weaker binding to and inhibition of Cdk2

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p27Xic1 promotes neuronal differentiation and elongates cell cycle via G1 phase

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Effects of p27Xic1 on neuronal differentiation are rescued by activated aPKC

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.

Actin nucleator Arp2/3 complex is essential for mouse preimplantation embryo development

The Arp2/3 complex is a critical actin nucleator, which promotes actin assembly and is widely involved in a diverse range of actin-related processes such as cell locomotion, phagocytosis and the establishment of cell polarity. Previous studies showed that the Arp2/3 complex regulates spindle migration and asymmetric division during mouse oocyte maturation; however, the role of the Arp2/3 complex in early mouse embryo development is still unknown. The results of the present study show that the Arp2/3 complex is critical for cytokinesis during mouse embryo development. The Arp2/3 complex was concentrated at the cortex of each cell at the 2- to 8-cell stage and the peripheral areas of the morula and blastocyst. Inhibition of the Arp2/3 complex by the specific inhibitor CK666 at the zygote stage caused a failure in cell division; mouse embryos failed to undergo compaction and lost apical-basal polarity. The actin level decreased in the CK666-treated group, and two or more nuclei were observed within a single cell, indicating a failure of cell division. Addition of CK666 at the 8-cell stage caused a failure of blastocyst formation, and CDX2 staining confirmed the loss of embryo polarity and the failure of trophectoderm and inner cell mass formation. Taken together, these data suggest that the Arp2/3 complex may regulate mouse embryo development via its effect on cell division.

Atypical protein kinase C couples cell sorting with primitive endoderm maturation in the mouse blastocyst

During mouse pre-implantation development, extra-embryonic primitive endoderm (PrE) and pluripotent epiblast precursors are specified in the inner cell mass (ICM) of the early blastocyst in a ‘salt and pepper’ manner, and are subsequently sorted into two distinct layers. Positional cues provided by the blastocyst cavity are thought to be instrumental for cell sorting; however, the sequence of events and the mechanisms that control this segregation remain unknown. Here, we show that atypical protein kinase C (aPKC), a protein associated with apicobasal polarity, is specifically enriched in PrE precursors in the ICM prior to cell sorting and prior to overt signs of cell polarisation. aPKC adopts a polarised localisation in PrE cells only after they reach the blastocyst cavity and form a mature epithelium, in a process that is dependent on FGF signalling. To assess the role of aPKC in PrE formation, we interfered with its activity using either chemical inhibition or RNAi knockdown. We show that inhibition of aPKC from the mid blastocyst stage not only prevents sorting of PrE precursors into a polarised monolayer but concomitantly affects the maturation of PrE precursors. Our results suggest that the processes of PrE and epiblast segregation, and cell fate progression are interdependent, and place aPKC as a central player in the segregation of epiblast and PrE progenitors in the mouse blastocyst.

Regulation of paracellular permeability: factors and mechanisms

Epithelial permeability is composed of transcellular permeability and paracellular permeability. Paracellular permeability is controlled by tight junctions (TJs). Claudins and occludin are two major transmembrane proteins in TJs, which directly determine the paracellular permeability to different ions or large molecules. Intracellular signaling pathways including Rho/Rho-associated protein kinase, protein kinase Cs, and mitogen-activated protein kinase, modulate the TJ proteins to affect paracellular permeability in response for diverse stimuli. Cytokines, growth factors and hormones in organism can regulate the paracellular permeability via signaling pathway. The transcellular transporters such as Na-K-ATPase, Na(+)-coupled transporters and chloride channels, can interact with paracellular transport and regulate the TJs. In this review, we summarized the factors affecting paracellular permeability and new progressions of the related mechanism in recent studies, and pointed out further research areas.

Insights into the role of cell-cell junctions in physiology and disease

Contacting cells establish different classes of intricate structures at the cell-cell junctions. These structures are of increasing research interest as they regulate a broad variety of processes in development and disease. Further, in vitro studies are revealing that various cell-cell interaction proteins are involved not only in cell-cell processes but also in many additional aspects of physiology, such as migration and apoptosis. This chapter reviews the basic classification of cell-cell junctional structures and some of their representative proteins. Their roles in development and disease are briefly outlined, followed by a section on contemporary methods for probing cell-cell interactions and some recent developments. This chapter concludes with a few suggestions for potential research directions to further develop this promising area of study.

Intercellular interactions, position, and polarity in establishing blastocyst cell lineages and embryonic axes

The formation of the three lineages of the mouse blastocyst provides a powerful model system to study interactions among cell behavior, cell signaling, and lineage development. Hippo signaling differences between the inner and outer cells of the early cleavage stages, combined with establishment of a stably polarized outer epithelium, lead to the establishment of the inner cell mass and the trophectoderm, whereas FGF signaling differences among the individual cells of the ICM lead to gradual separation and segregation of the epiblast and primitive endoderm lineages. Events in the late blastocyst lead to the formation of a special subset of cells from the primitive endoderm that are key sources for the signals that establish the subsequent body axis. The slow pace of mouse early development, the ability to culture embryos over this time period, the increasing availability of live cell imaging tools, and the ability to modify gene expression at will are providing increasing insights into the cell biology of early cell fate decisions.

Isolated mouse inner cell mass is unable to reconstruct trophectoderm

The ability of ICM to differentiate into TE is still a controversial issue. Many of authors have showed the reconstruction of TE from isolated ICMs. We showed that immunosurgical method is not 100% efficient and that the original TE cells very often remain on the surface of isolated ICMs. We also found that isolated ICM cells cultured in vitro do not express Cdx2, and that the TE is reconstituted from TE cells which have survived immunosurgery. This indicates that very soon after the formation of TE in the blastocyst, the cells of ICM lose the potency to differentiate into trophectoderm.

An unexpected role for the clock protein timeless in developmental apoptosis

Background

Programmed cell death is critical not only in adult tissue homeostasis but for embryogenesis as well. One of the earliest steps in development, formation of the proamniotic cavity, involves coordinated apoptosis of embryonic cells. Recent work from our group demonstrated that c-Src protein-tyrosine kinase activity triggers differentiation of mouse embryonic stem (mES) cells to primitive ectoderm-like cells. In this report, we identified Timeless (Tim), the mammalian ortholog of a Drosophila circadian rhythm protein, as a binding partner and substrate for c-Src and probed its role in the differentiation of mES cells.

Methodology/Principal Findings

To determine whether Tim is involved in ES cell differentiation, Tim protein levels were stably suppressed using shRNA. Tim-defective ES cell lines were then tested for embryoid body (EB) formation, which models early mammalian development. Remarkably, confocal microscopy revealed that EBs formed from the Tim-knockdown ES cells failed to cavitate. Cells retained within the centers of the failed cavities strongly expressed the pluripotency marker Oct4, suggesting that further development is arrested without Tim. Immunoblots revealed reduced basal Caspase activity in the Tim-defective EBs compared to wild-type controls. Furthermore, EBs formed from Tim-knockdown cells demonstrated resistance to staurosporine-induced apoptosis, consistent with a link between Tim and programmed cell death during cavitation.

Conclusions/Significance

Our data demonstrate a novel function for the clock protein Tim during a key stage of early development. Specifically, EBs formed from ES cells lacking Tim showed reduced caspase activity and failed to cavitate. As a consequence, further development was halted, and the cells present in the failed cavity remained pluripotent. These findings reveal a new function for Tim in the coordination of ES cell differentiation, and raise the intriguing possibility that circadian rhythms and early development may be intimately linked.

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.

Regulation of primary cilia formation by ceramide

The primary cilium is an important sensory organelle, the regulation of which is not fully understood. We found that in polarized Madin-Darby Canine Kidney cells, the sphingolipid ceramide is specifically distributed to a cis-Golgi compartment at the base of the primary cilium. This compartment immunostained for the centrosome marker gamma-tubulin, the Rho type GTPase cell division cycle 42 (Cdc42), and atypical protein kinase Czeta/lambda (aPKC), a kinase activated by ceramide and associated with a polarity protein complex consisting of partitioning defective (Par)6 and Cdc42. Inhibition of ceramide biosynthesis with Fumonisin B1 prevented codistribution of aPKC and Cdc42 in the centrosomal/pericentriolar compartment and severely impaired ciliogenesis. Cilium formation and codistribution of aPKC and Cdc42 were restored by incubation with N-acetyl or N-palmitoyl sphingosine (C2 or C16 ceramide), or the ceramide analog N-oleoyl serinol (S18). Cilium formation was also restored by the glycogen synthase kinase-3beta (GSK-3beta) inhibitor indirubin-3-monoxime, suggesting that regulation of ciliogenesis depends on the inhibition of GSK-3beta by ceramide-activated aPKC. Consistently, inhibition of aPKC with a pseudosubstrate inhibitor prevented restoration of ciliogenesis by C2 ceramide or S18. Our data show for the first time that ceramide is required for primary cilium formation.

Tight junction protein ZO-2 expression and relative function of ZO-1 and ZO-2 during mouse blastocyst formation

Apicolateral tight junctions (TJs) between epithelial cells are multiprotein complexes regulating membrane polarity and paracellular transport and also contribute to signalling pathways affecting cell proliferation and gene expression. ZO-2 and other ZO family members form a sub-membranous scaffold for binding TJ constituents. We investigated ZO-2 contribution to TJ biogenesis and function during trophectoderm epithelium differentiation in mouse preimplantation embryos. Our data indicate that ZO-2 is expressed from maternal and embryonic genomes with maternal ZO-2 protein associated with nuclei in zygotes and particularly early cleavage stages. Embryonic ZO-2 assembled at outer blastomere apicolateral junctional sites from the late 16-cell stage. Junctional ZO-2 first co-localised with E-cadherin in a transient complex comprising adherens junction and TJ constituents before segregating to TJs after their separation from the blastocyst stage (32-cell onwards). ZO-2 siRNA microinjection into zygotes or 2-cell embryos resulted in specific knockdown of ZO-2 mRNA and protein within blastocysts. Embryos lacking ZO-2 protein at trophectoderm TJs exhibited delayed blastocoel cavity formation but underwent normal cell proliferation and outgrowth morphogenesis. Quantitative analysis of trophectoderm TJs in ZO-2-deficient embryos revealed increased assembly of ZO-1 but not occludin, indicating ZO protein redundancy as a compensatory mechanism contributing to the mild phenotype observed. In contrast, ZO-1 knockdown, or combined ZO-1 and ZO-2 knockdown, generated a more severe inhibition of blastocoel formation indicating distinct roles for ZO proteins in blastocyst morphogenesis.

Tight junction and polarity interaction in the transporting epithelial phenotype

Development of tight junctions and cell polarity in epithelial cells requires a complex cellular machinery to execute an internal program in response to ambient cues. Tight junctions, a product of this machinery, can act as gates of the paracellular pathway, fences that keep the identity of plasma membrane domains, bridges that communicate neighboring cells. The polarization internal program and machinery are conserved in yeast, worms, flies and mammals, and in cell types as different as epithelia, neurons and lymphocytes. Polarization and tight junctions are dynamic features that change during development, in response to physiological and pharmacological challenges and in pathological situations like infection.

Crosstalk of tight junction components with signaling pathways

Tight junctions (TJs) regulate the passage of ions and molecules through the paracellular pathway in epithelial and endothelial cells. TJs are highly dynamic structures whose degree of sealing varies according to external stimuli, physiological and pathological conditions. In this review we analyze how the crosstalk of protein kinase C, protein kinase A, myosin light chain kinase, mitogen-activated protein kinases, phosphoinositide 3-kinase and Rho signaling pathways is involved in TJ regulation triggered by diverse stimuli. We also report how the phosphorylation of the main TJ components, claudins, occludin and ZO proteins, impacts epithelial and endothelial cell function.

Phosphorylation of claudin-4 by PKCepsilon regulates tight junction barrier function in ovarian cancer cells

Claudin proteins belong to a large family of transmembrane proteins essential to the formation and maintenance of tight junctions (TJs). In ovarian cancer, TJ protein claudin-4 is frequently overexpressed and may have roles in survival and invasion, but the molecular mechanisms underlying its regulation are poorly understood. In this report, we show that claudin-4 can be phosphorylated by protein kinase C (PKC) at Thr189 and Ser194 in ovarian cancer cells and overexpression of a claudin-4 mutant protein mimicking the phosphorylated state results in the disruption of the barrier function. Furthermore, upon phorbol ester-mediated PKC activation of OVCA433 cells, TJ strength is decreased and claudin-4 localization is altered. Analyses using PKC inhibitors and siRNA suggest that PKCepsilon, an isoform typically expressed in ovarian cancer cells, may be important in the TPA-mediated claudin-4 phosphorylation and weakening of the TJs. Furthermore, immunofluorescence studies showed that claudin-4 and PKCepsilon are co-localized at the TJs in these cells. The modulation of claudin-4 activity by PKCepsilon may not only provide a mechanism for disrupting TJ function in ovarian cancer, but may also be important in the regulation of TJ function in normal epithelial cells.

Human embryos developing in vitro are susceptible to impaired epithelial junction biogenesis correlating with abnormal metabolic activity

BACKGROUND

Blastocyst biogenesis occurs over several cell cycles during the preimplantation period comprising the gradual expression and membrane assembly of junctional protein complexes which distinguish the outer epithelial trophectoderm (TE) cells from the inner cell mass (ICM). In the human, TE integrity and the formation of a junctional seal can often be impaired. Embryos likely to result in a successful pregnancy after transfer are mostly selected according to morphological criteria. Recent data suggest that non-invasive measurement of amino acid turnover may be useful to complement such morphological scores. Whether morphological and metabolic criteria can be linked to poor TE differentiation thereby underpinning developmental predictions mechanistically remains unknown.

METHODS

We examined TE intercellular junction formation in human embryos by immunofluorescence and confocal microscopy and correlated this process with morphological criteria and amino acid turnover during late cleavage.

RESULTS

Our results show that TE differentiation may be compromised by failure of membrane assembly of specific junction constituents. This abnormality relates more closely to metabolic profiles than morphological criteria.

CONCLUSION

Our data identify that amino acid turnover can predict TE differentiation. These findings are the first to link two mechanisms, metabolism and junction membrane assembly, which contribute to early embryo development.

Imprinted gene expression in the rat embryo-fetal axis is altered in response to periconceptional maternal low protein diet

In our previous study, we have shown that maternal low protein diet (LPD, 9% casein vs 18% casein control) fed exclusively during the rat preimplantation period (0-4.25 day postcoitum) induced low birth weight, altered postnatal growth and hypertension in a gender-specific manner. In this study, we investigated the effect of maternal LPD restricted only to the preimplantation period (switched diet) or provided throughout gestation on fetal growth and imprinted gene expression in blastocyst and fetal stages of development. Male, but not female, blastocysts collected from LPD dams displayed a significant reduction (30%) in H19 mRNA level. A significant reduction in H19 (9.4%) and Igf2 (10.9%) mRNA was also observed in male, but not in female, fetal liver at day 20 postcoitum in response to maternal LPD restricted to the preimplantation period. No effect on the blastocyst expression of Igf2R was observed in relation to maternal diet. The reduction in H19 mRNA expression did not correlate with an observed alteration in DNA methylation at the H19 differentially methylated region in fetal liver. In contrast, maternal LPD throughout 20 days of gestation did not affect male or female H19 and Igf2 imprinted gene expression in fetal liver. Neither LPD nor switched diet treatments affected H19 and Igf2 imprinted gene expression in day 20 placenta. Our findings demonstrate that one contributor to the alteration in postnatal growth induced by periconceptional maternal LPD may derive from a gender-specific programming of imprinted gene expression originating within the preimplantation embryo itself.

Expression profile of protein kinase C isozymes in preimplantation mouse development

In the preimplantation mouse embryo, the protein kinase C (PKC) family has been implicated in regulation of egg activation, progression of meiotic and mitotic cell cycles, embryo compaction, and blastulation, but the involvement of the individual isozymes is largely unknown. Here, using semiquantitative immunocytochemistry and confocal microscopy we analyze the relative amount and subcellular distribution of ten isozymes of PKC (alpha, betaI, betaII, gamma, delta, epsilon, eta, theta, zeta, iota/lambda) and a PKC-anchoring protein, receptor for activated C-kinase 1 (RACK1). Our results show that all of these isoforms of PKC are present between the two-cell and blastocyst stages of mouse preimplantation development, and that each has a distinct, dynamic pattern and level of expression. The data suggest that different complements of the isozymes are involved in various steps of preimplantation development, and will serve as a framework for further functional studies of the individual isozymes. In particular, there was a transient increase in the nuclear concentration of several isozymes at the early four-cell stage, suggesting that some of the PKC isozymes might be involved in regulation of nuclear organization and function in the early mouse embryo.

Na+/K+-ATPase regulates tight junction formation and function during mouse preimplantation development

Research applied to the early embryo is required to effectively treat human infertility and to understand the primary mechanisms controlling development to the blastocyst stage. The present study investigated whether the Na(+)/K(+)-ATPase regulates tight junction formation and function during blastocyst formation. To investigate this hypothesis, three experimental series were conducted. The first experiments defined the optimal dose and treatment time intervals for ouabain (a potent and specific inhibitor of the Na(+)/K(+)-ATPase) treatment. The results demonstrated that mouse embryos maintained a normal development to the blastocyst stage following a 6-h ouabain treatment. The second experiments investigated the effects of ouabain treatment on the distribution of ZO-1 and occludin (tight junction associated proteins). Ouabain treatment (up to 6 h) or culture in K(+)-free medium (up to 6 h) resulted in the appearance of a discontinuous ZO-1 protein distribution and a loss of occludin immunofluorescence. The third set of experiments examined the influence of ouabain treatment on tight junction function. Ouabain treatment or culture in K(+)-free medium affected tight junction permeability as indicated by an increase in the proportion of treated embryos accumulating both 4 kDa and 40 kDa fluorescein isothiocyanate (FITC)-dextran into their blastocyst cavities. The results indicate that the Na(+)/K(+)-ATPase is a potent regulator of tight junction formation and function during mouse preimplantation development.

Relative contribution of cell contact pattern, specific PKC isoforms and gap junctional communication in tight junction assembly in the mouse early embryo

In mouse early development, cell contact patterns regulate the spatial organization and segregation of inner cell mass (ICM) and trophectoderm epithelium (TE) during blastocyst morphogenesis. Progressive membrane assembly of tight junctional (TJ) proteins in the differentiating TE during cleavage is upregulated by cell contact asymmetry (outside position) and suppressed within the ICM by cell contact symmetry (inside position). This is reversible, and immunosurgical isolation of the ICM induces upregulation of TJ assembly in a sequence that broadly mimics that occurring during blastocyst formation. The mechanism relating cell contact pattern and TJ assembly was investigated in the ICM model with respect to PKC-mediated signaling and gap junctional communication. Our results indicate that complete cell contact asymmetry is required for TJ biogenesis and acts upstream of PKC-mediated signaling. Specific inhibition of two PKC isoforms, PKCdelta and zeta, revealed that both PKC activities are required for membrane assembly of ZO-2 TJ protein, while only PKCzeta activity is involved in regulating ZO-1alpha+ membrane assembly, suggesting different mechanisms for individual TJ proteins. Gap junctional communication had no apparent influence on either TJ formation or PKC signaling but was itself affected by changes of cell contact patterns. Our data suggest that the dynamics of cell contact patterns coordinate the spatial organization of TJ formation via specific PKC signaling pathways during blastocyst biogenesis.

Endocytosis of the apical junctional complex: mechanisms and possible roles in regulation of epithelial barriers

Tight junctions (TJ) and adherens junctions (AJ) regulate cell-cell adhesion and barrier function of simple polarized epithelia. These junctions are positioned in the apical end of the lateral plasma membrane and form the so-called apical junctional complex (AJC). Although initially seen as purely structural features, the AJC is now known to play important roles in cell differentiation and proliferation. The AJC is a highly dynamic entity, undergoing rapid remodeling during normal epithelial morphogenesis and under pathologic conditions. There is growing evidence that remodeling of the AJC is mediated by internalization of junctional proteins. This review summarizes what is known about endocytic pathways, intracellular destinations and signaling cascades involved in internalization of AJC proteins. Potential biological roles for AJC endocytosis in maintaining functional apical junctions, reversible opening of epithelial barrier and disruption of intercellular adhesion are also discussed.

Phosphorylation of claudin-3 at threonine 192 by cAMP-dependent protein kinase regulates tight junction barrier function in ovarian cancer cells

Claudins are integral membrane proteins essential in the formation and function of tight junctions (TJs). Disruption of TJs, which have essential roles in cell permeability and polarity, is thought to contribute to epithelial tumorigenesis. Claudin-3 and -4 are frequently overexpressed in ovarian cancer, but the molecular pathways involved in the regulation of these proteins are unclear. Interestingly, several studies have demonstrated a role for phosphorylation in the regulation of TJ complexes, although evidence for claudin phosphorylation is scarce. Here, we showed that claudin-3 and -4 can be phosphorylated in ovarian cancer cells. In vitro phosphorylation assays using glutathione S-transferase fusion constructs demonstrated that the C terminus of claudin-3 is an excellent substrate for cAMP-dependent protein kinase (PKA). Using site-directed mutagenesis, we identified a PKA phosphorylation site at amino acid 192 in the C terminus of claudin-3. Overexpression of the protein containing a T192D mutation, mimicking the phosphorylated state, resulted in a decrease in TJ strength in ovarian cancer cell line OVCA433. Our results suggest that claudin-3 phosphorylation by PKA, a kinase frequently activated in ovarian cancer, may provide a mechanism for the disruption of TJs in this cancer. In addition, our findings may have general implications for the regulation of TJs in normal epithelial cells.

Downregulation of Par3 and aPKC function directs cells towards the ICM in the preimplantation mouse embryo

Generation of inside cells that develop into inner cell mass (ICM) and outside cells that develop into trophectoderm is central to the development of the early mouse embryo. Critical to this decision is the development of cell polarity and the associated asymmetric (differentiative) divisions of the 8-cell-stage blastomeres. The underlying molecular mechanisms for these events are not understood. As the Par3/aPKC complex has a role in establishing cellular polarity and division orientation in other systems, we explored its potential function in the developing mouse embryo. We show that both Par3 and aPKC adopt a polarized localization from the 8-cell stage onwards and that manipulating their function re-directs cell positioning and consequently influences cell fate. Injection of dsRNA against Par3 or mRNA for a dominant negative form of aPKC into a random blastomere at the 4-cell stage directs progeny of the injected cell into the inside part of the embryo. This appears to result from both an increased frequency by which such cells undertake differentiative divisions and their decreased probability of retaining outside positions. Thus, the natural spatial allocation of blastomere progeny can be over-ridden by downregulation of Par3 or aPKC, leading to a deceased tendency for them to remain outside and so develop into trophectoderm. In addition, this experimental approach illustrates a powerful means of manipulating gene expression in a specific clonal population of cells in the preimplantation embryo.

Contribution of JAM-1 to epithelial differentiation and tight-junction biogenesis in the mouse preimplantation embryo

We have investigated the contribution of the tight junction (TJ) transmembrane protein junction-adhesion-molecule 1 (JAM-1) to trophectoderm epithelial differentiation in the mouse embryo. JAM-1-encoding mRNA is expressed early from the embryonic genome and is detectable as protein from the eight-cell stage. Immunofluorescence confocal analysis of staged embryos and synchronized cell clusters revealed JAM-1 recruitment to cell contact sites occurred predominantly during the first hour after division to the eight-cell stage, earlier than any other TJ protein analysed to date in this model and before E-cadherin adhesion and cell polarization. During embryo compaction later in the fourth cell cycle, JAM-1 localized transiently yet precisely to the apical microvillous pole, where protein kinase Cζ (PKCζ) and PKCδ are also found, indicating a role in cell surface reorganization and polarization. Subsequently, in morulae and blastocysts, JAM-1 is distributed ubiquitously at cell contact sites within the embryo but is concentrated within the trophectoderm apicolateral junctional complex, a pattern resembling that of E-cadherin and nectin-2. However, treatment of embryos with anti-JAM-1-neutralizing antibodies indicated that JAM-1 did not contribute to global embryo compaction and adhesion but rather regulated the timing of blastocoel cavity formation dependent upon establishment of the trophectoderm TJ paracellular seal.

Specific PKC isoforms regulate blastocoel formation during mouse preimplantation development

During early mammalian development, blastocyst morphogenesis is achieved by epithelial differentiation of trophectoderm (TE) and its segregation from the inner cell mass (ICM). Two major interrelated features of TE differentiation required for blastocoel formation include intercellular junction biogenesis and a directed ion transport system, mediated by Na+/K+ ATPase. We have examined the relative contribution of intercellular signalling mediated by protein kinase C (PKC) and gap junctional communication in TE differentiation and blastocyst cavitation. The distribution pattern of four (delta, theta, iota/lambda, zeta) PKC isoforms and PKCmicro/PKD1 showed partial colocalisation with the tight junction marker ZO-1alpha+ in TE and all four PKCs (delta, theta, iota/lambda, zeta) showed distinct TE/ICM staining patterns (predominantly at the cell membrane within the TE and cytoplasmic within the ICM), indicating their potential contribution to TE differentiation and blastocyst morphogenesis. Specific inhibition of PKCdelta and zeta activity significantly delayed blastocyst formation. Although modulation of these PKC isoforms failed to influence the already established programme of epithelial junctional differentiation within the TE, Na+/K+ ATPase alpha1 subunit was internalised from membrane to cytoplasm. Inhibition of gap junctional communication, in contrast, had no influence on any of these processes. Our results demonstrate for the first time that distinct PKC isotypes contribute to the regulation of cavitation in preimplantation embryos via target proteins including Na+/K+ ATPase.