Getting to know your neighbor: cell polarization in early embryos

Cytoskeleton-modulating nanomaterials and their therapeutic potentials

The cytoskeleton, an intricate network of protein fibers within cells, plays a pivotal role in maintaining cell shape, enabling movement, and facilitating intracellular transport. Its involvement in various pathological states, ranging from cancer proliferation and metastasis to the progression of neurodegenerative disorders, underscores its potential as a target for therapeutic intervention. The exploration of nanotechnology in this realm, particularly the use of nanomaterials for cytoskeletal modulation, represents a cutting-edge approach with the promise of novel treatments. Inorganic nanomaterials, including those derived from gold, metal oxides, carbon, and black phosphorus, alongside organic variants such as peptides and proteins, are at the forefront of this research. These materials offer diverse mechanisms of action, either by directly interacting with cytoskeletal components or by influencing cellular signaling pathways that, in turn, modulate the cytoskeleton. Recent advancements have introduced magnetic field-responsive and light-responsive nanomaterials, which allow for targeted and controlled manipulation of the cytoskeleton. Such precision is crucial in minimizing off-target effects and enhancing therapeutic efficacy. This review explores the importance of research into cytoskeleton-targeting nanomaterials for developing therapeutic interventions for a range of diseases. It also addresses the progress made in this field, the challenges encountered, and future directions for using nanomaterials to modulate the cytoskeleton. The continued exploration of nanomaterials for cytoskeleton modulation holds great promise for advancing therapeutic strategies against a broad spectrum of diseases, marking a significant step forward in the intersection of nanotechnology and medicine.

Comparative transcriptome profile of embryos at different developmental stages derived from somatic cell nuclear transfer (SCNT) and in-vitro fertilization (IVF) in riverine buffalo (Bubalus bubalis)

Somatic cell nuclear transfer (SCNT) is a very important reproductive technology with many diverse applications, such as fast multiplication of elite animals, the production of transgenic animals and embryonic stem (ES) cells. However, low cloning efficiency, a low live birth rate and the abnormally high incidence of abnormalities in the offspring born are attributed to incomplete or aberrant nuclear reprogramming. In SCNT embryos, the aberrant expression pattern of the genes throughout embryonic development is responsible for the incomplete nuclear reprogramming. The present study was carried out to identify the differential gene expression (DEGs) profile and molecular pathways of the SCNT and IVF embryos at different developmental stages (2 cell, 8 cell and blastocyst stages). In the present study, 1164 (2 cell), 1004 (8 cell) and 530 (blastocyst stage) DEGs were identified in the SCNT embryos as compared to IVF embryos. In addition, several genes such as ZEB1, GDF1, HSF5, PDE3B, VIM, TNNC, HSD3B1, TAGLN, ITGA4 and AGMAT were affecting the development of SCNT embryos as compared to IVF embryos. Further, Gene Ontology (GO) and molecular pathways analysis suggested, SCNT embryos exhibit variations compared to their IVF counterparts and affected the development of embryos throughout the different developmental stages. Apart from this, q-PCR analysis of the GDF1, TMEM114, and IGSF22 genes were utilized to validate the RNA-seq data. These findings contribute valuable insights about the different genes and molecular pathways underlying SCNT embryo development and offer crucial information for improving SCNT efficiency.

SimuCell3D: three-dimensional simulation of tissue mechanics with cell polarization

The three-dimensional (3D) organization of cells determines tissue function and integrity, and changes markedly in development and disease. Cell-based simulations have long been used to define the underlying mechanical principles. However, high computational costs have so far limited simulations to either simplified cell geometries or small tissue patches. Here, we present SimuCell3D, an efficient open-source program to simulate large tissues in three dimensions with subcellular resolution, growth, proliferation, extracellular matrix, fluid cavities, nuclei and non-uniform mechanical properties, as found in polarized epithelia. Spheroids, vesicles, sheets, tubes and other tissue geometries can readily be imported from microscopy images and simulated to infer biomechanical parameters. Doing so, we show that 3D cell shapes in layered and pseudostratified epithelia are largely governed by a competition between surface tension and intercellular adhesion. SimuCell3D enables the large-scale in silico study of 3D tissue organization in development and disease at a great level of detail.

Origin and development of primary animal epithelia

Epithelia are the first organized tissues that appear during development. In many animal embryos, early divisions give rise to a polarized monolayer, the primary epithelium, rather than a random aggregate of cells. Here, we review the mechanisms by which cells organize into primary epithelia in various developmental contexts. We discuss how cells acquire polarity while undergoing early divisions. We describe cases where oriented divisions constrain cell arrangement to monolayers including organization on top of yolk surfaces. We finally discuss how epithelia emerge in embryos from animals that branched early during evolution and provide examples of epithelia-like arrangements encountered in single-celled eukaryotes. Although divergent and context-dependent mechanisms give rise to primary epithelia, here we trace the unifying principles underlying their formation.

Morphology of 16-cell embryo in bovine: Inside cells, compaction, fragmentation and effects of X-sorted semen

In mouse embryos, inside cells are allocated in 16-cell embryos through a well-orchestrated sequence of events involving compaction and polarization. The emergence of inside cells is of great importance as itl later gives rise to the inner cell mass and epiblast. In this study, we report the sequence of critical events in embryology (compaction, inside cells allocation and fragmentation) in bovine 72 h.p.i. 9-16 cell embryos, while also investigating the effects of X-sorted semen on these events. We found a wide distribution of total cell numbers among embryos, attributed to an asynchronous cleavage pattern and blastomere death. Additionally, 13% of embryos displayed irregular shapes. The establishment of the inside cell compartment increased (p < 0.01) in embryos with more cells. However, only 53.8% of 16-cell embryos presented inside cells. Compaction was present in 32.4% embryos and was positively correlated (p = 0.03, OR 3.02) with the establishment of inside cells, occurring independently of cell number. Fragmentation was present in 36% embryos, being more frequent (p = 0.01) in embryos with lower cell numbers. A possible association between irregular shape and fragmentation was considered (p = 0.06). The use of X-sorted semen had no effect on most evaluated parameters. However, it did have a marked effect on cleavage rate (p < 0.01) and the arrest of 2- and 4- cell embryos. In conclusion, bovine embryos exhibit an asynchronous cleavage pattern, high levels of fragmentation, and demonstrate compaction and inside cell allocation later in development compared to mouse embryos. Semen X-sorting has major effects on cleavage and embryo arrest. Further studies are needed to elucidate the association between irregularly shaped embryos and fragmentation, as well as the effects of sex on inside cell allocation.

Adhesion and Polarity protein distribution-regulates hexagon dominated plasma membrane organization in Drosophila blastoderm embryos

Epithelial cells contain polarity complexes on the lateral membrane and are organized in a hexagon-dominated polygonal array. The mechanisms regulating the organization of polygonal architecture in metazoan embryogenesis are not completely understood. Drosophila embryogenesis enables mechanistic analysis of epithelial polarity formation and its impact on polygonal organization. The plasma membrane (PM) of syncytial Drosophila blastoderm embryos is organized as a polygonal array with pseudocleavage furrow formation during the almost synchronous cortical division cycles. We find that polygonal (PM) organization arises in the metaphase (MP) of division cycle 11, and hexagon dominance occurs with an increase in furrow length in the metaphase of cycle 12. There is a decrease in cell shape index in metaphase from cycles 11 to 13. This coincides with Drosophila E-cad (DE-cadherin) and Bazooka enrichment at the edges and the septin, Peanut at the vertices of the furrow. We further assess the role of polarity and adhesion proteins in pseudocleavage furrow formation and its organization as a polygonal array. We find that DE-cadherin depletion leads to decreased furrow length, loss of hexagon dominance, and increased cell shape index. Bazooka and Peanut depletion lead to decreased furrow length, delay in onset of hexagon dominance from cycle 12 to 13, and increased cell shape index. Hexagon dominance occurs with an increase in furrow length in cycle 13 and increased DE-cadherin, possibly due to the inhibition of endocytosis. We conclude that polarity protein recruitment and regulation of endocytic pathways enable pseudocleavage furrow stability and the formation of a hexagon-dominated polygon array.

Cell polarity: Adapting the PAR cascade to diverse cellular contexts

Two new studies shed light on the intricacies of Caenorhabditis elegans embryo patterning, revealing how the conserved interaction and crosstalk of PAR proteins are adapted to perceive distinct cues, ultimately shaping unique asymmetries in form and function.

Inflammation and oxidative stress impair preimplantation embryonic morphogenesis in allergic asthma model

The incidence of allergic asthma has been increasing worldwide in recent decades. Also, an increasing number of women are suffering from poor pregnancy outcome. However, the causal relationship between allergic asthma and embryonic growth in terms of cell morphogenesis has not been well elucidated. Here, we investigated the impact of allergic asthma on the morphogenesis of preimplantation embryos. Twenty-four female BALB/c were randomly divided into control (PBS), 50-μg (OVA1), 100-μg (OVA2) and 150-μg (OVA3). On Days-0 and -14, mice were induced intraperitoneally (i.p) with ovalbumin (OVA). On Days-21 until -23, mice were challenged with OVA via intranasal instillation (i.n). Control animals were sensitized and challenged with PBS. At the end of treatment (Day-25), 2-cell embryos were retrieved and cultured in vitro until the blastocysts hatched. Results showed reduced number of preimplantation embryos at all developing stages in all treated groups (p ≤ 0.0001). Uneven blastomere size, partial compaction- and cavitation-activity, low formation of trophectoderm (TE), as well as cell fragmentation were noted in all the treated groups. Maternal serum interleukin (IL)-4, immunoglobulin (Ig)-E and 8-hydroxydeoxyguanosine (8-OHdG) were notably high (p ≤ 0.0001, p ≤ 0.01) in contrast with low total antioxidant capacity (TAOC) (p ≤ 0.0001). Our findings indicated that OVA-induced allergic asthma had compromised cell morphogenesis through reduced blastomere cleavage division, partial compaction and cavitation-activity, impairment of TE production, and cell fragmentation leading to embryonic cell death via OS mechanism.

Context matters: Lessons in epithelial polarity from the Caenorhabditis elegans intestine and other tissues

Epithelia are tissues with diverse morphologies and functions across metazoans, ranging from vast cell sheets encasing internal organs to internal tubes facilitating nutrient uptake, all of which require establishment of apical-basolateral polarity axes. While all epithelia tend to polarize the same components, how these components are deployed to drive polarization is largely context-dependent and likely shaped by tissue-specific differences in development and ultimate functions of polarizing primordia. The nematode Caenorhabditis elegans (C. elegans) offers exceptional imaging and genetic tools and possesses unique epithelia with well-described origins and roles, making it an excellent model to investigate polarity mechanisms. In this review, we highlight the interplay between epithelial polarization, development, and function by describing symmetry breaking and polarity establishment in a particularly well-characterized epithelium, the C. elegans intestine. We compare intestinal polarization to polarity programs in two other C. elegans epithelia, the pharynx and epidermis, correlating divergent mechanisms with tissue-specific differences in geometry, embryonic environment, and function. Together, we emphasize the importance of investigating polarization mechanisms against the backdrop of tissue-specific contexts, while also underscoring the benefits of cross-tissue comparisons of polarity.

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.

Separable mechanisms drive local and global polarity establishment in the Caenorhabditiselegans intestinal epithelium

Apico-basolateral polarization is essential for epithelial cells to function as selective barriers and transporters, and to provide mechanical resilience to organs. Epithelial polarity is established locally, within individual cells to establish distinct apical, junctional and basolateral domains, and globally, within a tissue where cells coordinately orient their apico-basolateral axes. Using live imaging of endogenously tagged proteins and tissue-specific protein depletion in the Caenorhabditiselegans embryonic intestine, we found that local and global polarity establishment are temporally and genetically separable. Local polarity is initiated prior to global polarity and is robust to perturbation. PAR-3 is required for global polarization across the intestine but local polarity can arise in its absence, as small groups of cells eventually established polarized domains in PAR-3-depleted intestines in a HMR-1 (E-cadherin)-dependent manner. Despite the role of PAR-3 in localizing PKC-3 to the apical surface, we additionally found that PAR-3 and PKC-3/aPKC have distinct roles in the establishment and maintenance of local and global polarity. Taken together, our results indicate that different mechanisms are required for local and global polarity establishment in vivo.

Mouse Trophoblast Cells Can Provide IFN-Based Antiviral Protection to Embryonic Stem Cells via Paracrine Signaling

The blastocyst is the preimplantation stage embryo that consists of two major components: the inner cell mass (ICM) and the trophectoderm (TE). The ICM gives rise to the fetus and some extraembryonic tissues whereas the TE contributes to development of the placenta. Previous studies have demonstrated that both human and mouse embryonic stem cells (ESCs) derived from the ICM are deficient in expressing type I IFNs in response to viral infection. In this study, we investigated the IFN response in mouse trophoblast stem cells (TSCs) and their in vitro differentiated trophoblasts (TSC-TBs). In this study, we report that, unlike ESCs, TSCs have a functional IFN system. They can express type I IFNs in response to viral stimuli and express IFN-stimulated genes in response to type I IFNs. TSC-TBs have a further developed IFN system and acquired the ability to express specialized type III IFN-λ. Furthermore, TSCs and TSC-TBs can provide ESCs with antiviral activity against Chikungunya, West Nile, and Zika virus infection, as demonstrated with a novel coculture model that simulates the temporal and spatial relationship between the ICM and the TE in a blastocyst. Taken together, our data demonstrate that mouse ESCs can respond to type I IFNs and gain IFN-based antiviral protection from TSCs and TSC-TBs via paracrine signaling mechanisms even though they themselves are unable to express type I IFNs.

Global MicroRNA Expression Profiling of Buffalo (Bubalus bubalis) Embryos at Different Developmental Stages Produced by Somatic Cell Nuclear Transfer and In-Vitro Fertilization Using RNA Sequencing

Despite the success of cloning technology in the production of offspring across several species, its application on a wide scale is severely limited by the very low offspring rate obtained with cloned embryos. The expression profile of microRNAs (miRNAs) in cloned embryos throughout embryonic development is reported to deviate from regular patterns. The present study is aimed at determining the dynamics of the global expression of miRNA profile in cloned and in-vitro fertilization (IVF) pre-implantation embryos at different developmental stages, i.e., the two-cell, eight-cell, and blastocyst stages, using next-generation sequencing. The results of this study suggest that there is a profound difference in global miRNA profile between cloned and IVF embryos. These differences are manifested throughout the course of embryonic development. The cloned embryos differ from their IVF counterparts in enriched Gene Ontology (GO) terms of biological process, molecular function, cellular component, and protein class categories in terms of the targets of differentially expressed miRNAs. The major pathways related to embryonic development, such as the Wnt signaling pathway, the apoptosis signaling pathway, the FGF signaling pathway, the p53 pathway, etc., were found to be affected in cloned relative to IVF embryos. Overall, these data reveal the distinct miRNA profile of cloned relative to IVF embryos, suggesting that the molecules or pathways affected may play an important role in cloned embryo development.

Thinning or Opening: A Randomized Sibling-Embryo Pilot Trial on the Efficacy of Two Laser-Assisted Hatching Modes During the Extended Culture of Highly Fragmented Cleavage Embryos

A randomized sibling-embryo pilot trial investigated whether two ways of laser-assisted hatching result in different blastulation and clinical outcomes after extended in vitro culture process of highly fragmented day-3 cleavage embryos. From 92 couples, a total of 315 highly fragmented day-3 embryos (the fragmentation >25%) were recruited and randomized into laser-assisted zona thinning (LAT, n=157) and opening (LAO, n=158) groups, and then underwent a blastocyst culture in vitro. The main endpoint measurements including blastocyst formation and grading as well as the clinical pregnancy after blastocyst transfer were obtained during the treatment procedure of in vitro fertilization and embryo transfer, and then analyzed with generalized estimating equation (GEE) and/or time-to blastocyst analysis models. A total of 166 day-3 embryos developed into blastocyst stage (52.70%), of which 97 were viable blastocysts (30.79%), and 42 top-quality ones (13.33%). LAT did not have any inferior or superior to LAO in the endpoints of either total, viable, top-quality or hatched blastocyst formation, with the ORs (95%CI) from GEE model as 0.89 (0.55-1.45), 0.71 (0.42-1.21), 1.12 (0.56-2.25) and 0.68 (0.42-1.12) respectively for LAT treatment. And the time-to-blastocyst analysis showed a similar result. Additionally, no difference in clinical outcomes after blastocyst transfer was found between the two groups. The author concluded that when applying the LAHs during the extended culture of highly fragmented embryos, both LAT and LAO can generate a promising clinical outcome, and the LAT operation be equivalent to the LAO. Future well-designed, multiple-center, larger-sample investigations are required to ascertain above conclusion.

Polarization of Stem Cells Directed by Magnetic Field-Manipulated Supramolecular Polymeric Nanofibers

Precise assembly of the cytoskeleton (e.g., actin, tubulin, and intermediate filaments) is of great importance for stem cell polarization and tissue regeneration. Recently, artificial manipulation of cytoskeleton assembly for remodeling stem cell polarization and ultimate cell fates attracts more and more interest of both chemists and biologists. Herein, we report the magnetic field-directed formation of biocompatible supramolecular polymeric nanofibers composed of two subunits: a β-cyclodextrin-bearing hyaluronic acid host polymer (HACD) and magnetic nanoparticles modified with actin-binding peptide and adamantane (MS-ABPAda). Transmission electron microscopy indicated that when HACD and MS-ABPAda were exposed to a magnetic field, they self-assembled into long nanofibers along the direction of the magnetic field, and the rate of nanofiber formation was linearly correlated with the strength of the magnetic field. Interestingly, when incubated with dental pulp stem cells, the nanofibers specifically drove tip extension and polarization of the cells, a phenomenon that can be attributed to targeting of actin-binding peptide to the actin cytoskeleton and subsequent polarization of the nanofibers. The successful application of these magnetic field-responsive supramolecular polymers on accurately driving polarization of mammalian cells is expected to be of great value for artificially manipulating cell fate and developing intelligent responsive materials in regenerative medicine.

A Comparative Analysis of Hippo Signaling Pathway Components during Murine and Bovine Early Mammalian Embryogenesis

The time required for successful blastocyst formation varies among multiple species. The formation of a blastocyst is governed by numerous molecular cell signaling pathways, such as the Hippo signaling pathway. The Hippo signaling pathway is initiated by increased cell-cell contact and via apical polarity proteins (AMOT, PARD6, and NF2) during the period of preimplantation embryogenesis. Cell-cell contact and cell polarity activate (phosphorylates) the core cascade components of the pathway (mammalian sterile twenty like 1 and 2 (MST1/2) and large tumor suppressor 1 and 2 (LATS1/2)), which in turn phosphorylate the downstream effectors of the pathway (YAP1/TAZ). The Hippo pathway remains inactive with YAP1 (Yes Associated protein 1) present inside the nucleus in the trophectoderm (TE) cells (polar blastomeres) of the mouse blastocyst. In the inner cell mass (ICM) cells (apolar blastomeres), the pathway is activated with p-YAP1 present in the cytoplasm. On the contrary, during bovine embryogenesis, p-YAP1 is exclusively present in the nucleus in both TE and ICM cells. Contrary to mouse embryos, transcription co activator with PDZ-binding motif (TAZ) (also known as WWTR1) is also predominantly present in the cytoplasm in all the blastomeres during bovine embryogenesis. This review outlines the major differences in the localization and function of Hippo signaling pathway components of murine and bovine preimplantation embryos, suggesting significant differences in the regulation of this pathway in between the two species. The variance observed in the Hippo signaling pathway between murine and bovine embryos confirms that both of these early embryonic models are quite distinct. Moreover, based on the similarity of the Hippo signaling pathway between bovine and human early embryo development, bovine embryos could be an alternate model for understanding the regulation of the Hippo signaling pathway in human embryos.

Polarity Establishment and Maintenance in Ascidian Notochord

Cell and tissue polarity due to the extracellular signaling and intracellular gene cascades, in turn, signals the directed cell behaviors and asymmetric tissue architectures that play a crucial role in organogenesis and embryogenesis. The notochord is a characteristic midline organ in chordate embryos that supports the body structure and produces positioning signaling. This review summarizes cellular and tissue-level polarities during notochord development in ascidians. At the early stage, planar cell polarity (PCP) is initialized, which drives cell convergence extension and migration to form a rod-like structure. Subsequently, the notochord undergoes a mesenchymal-epithelial transition, becoming an unusual epithelium in which cells have two opposing apical domains facing the extracellular lumen deposited between adjacent notochord cells controlled by apical-basal (AB) polarity. Cytoskeleton distribution is one of the main downstream events of cell polarity. Some cytoskeleton polarity patterns are a consequence of PCP: however, an additional polarized cytoskeleton, together with Rho signaling, might serve as a guide for correct AB polarity initiation in the notochord. In addition, the notochord's mechanical properties are associated with polarity establishment and transformation, which bridge signaling regulation and tissue mechanical properties that enable the coordinated organogenesis during embryo development.

Interplay of mesoscale physics and agent-like behaviors in the parallel evolution of aggregative multicellularity

Myxobacteria and dictyostelids are prokaryotic and eukaryotic multicellular lineages, respectively, that after nutrient depletion aggregate and develop into structures called fruiting bodies. The developmental processes and resulting morphological outcomes resemble one another to a remarkable extent despite their independent origins, the evolutionary distance between them and the lack of traceable homology in molecular mechanisms. We hypothesize that the morphological parallelism between the two lineages arises as the consequence of the interplay within multicellular aggregates between generic processes, physical and physicochemical processes operating similarly in living and non-living matter at the mesoscale (~10^-3-10^-1 m) and agent-like behaviors, unique to living systems and characteristic of the constituent cells, considered as autonomous entities acting according to internal rules in a shared environment. Here, we analyze the contributions of generic and agent-like determinants in myxobacteria and dictyostelid development and their roles in the generation of their common traits. Consequent to aggregation, collective cell-cell contacts mediate the emergence of liquid-like properties, making nascent multicellular masses subject to novel patterning and morphogenetic processes. In both lineages, this leads to behaviors such as streaming, rippling, and rounding-up, as seen in non-living fluids. Later the aggregates solidify, leading them to exhibit additional generic properties and motifs. Computational models suggest that the morphological phenotypes of the multicellular masses deviate from the predictions of generic physics due to the contribution of agent-like behaviors of cells such as directed migration, quiescence, and oscillatory signal transduction mediated by responses to external cues. These employ signaling mechanisms that reflect the evolutionary histories of the respective organisms. We propose that the similar developmental trajectories of myxobacteria and dictyostelids are more due to shared generic physical processes in coordination with analogous agent-type behaviors than to convergent evolution under parallel selection regimes. Insights from the biology of these aggregative forms may enable a unified understanding of developmental evolution, including that of animals and plants.

Par protein localization during the early development of Mnemiopsis leidyi suggests different modes of epithelial organization in the metazoa

In bilaterians and cnidarians, epithelial cell-polarity is regulated by the interactions between Par proteins, Wnt/PCP signaling pathway, and cell-cell adhesion. Par proteins are highly conserved across Metazoa, including ctenophores. But strikingly, ctenophore genomes lack components of the Wnt/PCP pathway and cell-cell adhesion complexes raising the question if ctenophore cells are polarized by mechanisms involving Par proteins. Here, by using immunohistochemistry and live-cell imaging of specific mRNAs, we describe for the first time the subcellular localization of selected Par proteins in blastomeres and epithelial cells during the embryogenesis of the ctenophore Mnemiopsis leidyi. We show that these proteins distribute differently compared to what has been described for other animals, even though they segregate in a host-specific fashion when expressed in cnidarian embryos. This differential localization might be related to the emergence of different junctional complexes during metazoan evolution.

Tight Junctions in Cell Proliferation

Tight junction (TJ) proteins form a continuous intercellular network creating a barrier with selective regulation of water, ion, and solutes across endothelial, epithelial, and glial tissues. TJ proteins include the claudin family that confers barrier properties, members of the MARVEL family that contribute to barrier regulation, and JAM molecules, which regulate junction organization and diapedesis. In addition, the membrane-associated proteins such as MAGUK family members, i.e., zonula occludens, form the scaffold linking the transmembrane proteins to both cell signaling molecules and the cytoskeleton. Most studies of TJ have focused on the contribution to cell-cell adhesion and tissue barrier properties. However, recent studies reveal that, similar to adherens junction proteins, TJ proteins contribute to the control of cell proliferation. In this review, we will summarize and discuss the specific role of TJ proteins in the control of epithelial and endothelial cell proliferation. In some cases, the TJ proteins act as a reservoir of critical cell cycle modulators, by binding and regulating their nuclear access, while in other cases, junctional proteins are located at cellular organelles, regulating transcription and proliferation. Collectively, these studies reveal that TJ proteins contribute to the control of cell proliferation and differentiation required for forming and maintaining a tissue barrier.

Switching states: dynamic remodelling of polarity complexes as a toolkit for cell polarization

Polarity is defined by the segregation of cellular components along a defined axis. To polarize robustly, cells must be able to break symmetry and subsequently amplify these nascent asymmetries. Finally, asymmetric localization of signaling molecules must be translated into functional regulation of downstream effector pathways. Central to these behaviors are a diverse set of cell polarity networks. Within these networks, molecules exhibit varied behaviors, dynamically switching among different complexes and states, active versus inactive, bound versus unbound, immobile versus diffusive. This ability to switch dynamically between states is intimately connected to the ability of molecules to generate asymmetric patterns within cells. Focusing primarily on polarity pathways governed by the conserved PAR proteins, we discuss strategies enabled by these dynamic behaviors that are used by cells to polarize. We highlight not only how switching between states is linked to the ability of polarity proteins to localize asymmetrically, but also how cells take advantage of 'state switching' to regulate polarity in time and space.

A Polarizing Issue: Diversity in the Mechanisms Underlying Apico-Basolateral Polarization In Vivo

Polarization along an apico-basolateral axis is a hallmark of epithelial cells and is essential for their selective barrier and transporter functions, as well as for their ability to provide mechanical resiliency to organs. Loss of polarity along this axis perturbs development and is associated with a wide number of diseases. We describe three steps involved in polarization: symmetry breaking, polarity establishment, and polarity maintenance. While the proteins involved in these processes are highly conserved among epithelial tissues and species, the execution of these steps varies widely and is context dependent. We review both theoretical principles underlying these steps and recent work demonstrating how apico-basolateral polarity is established in vivo in different tissues, highlighting how developmental and physiological contexts play major roles in the execution of the epithelial polarity program.

Polarity signaling ensures epidermal homeostasis by coupling cellular mechanics and genomic integrity

Epithelial homeostasis requires balanced progenitor cell proliferation and differentiation, whereas disrupting this equilibrium fosters degeneration or cancer. Here we studied how cell polarity signaling orchestrates epidermal self-renewal and differentiation. Using genetic ablation, quantitative imaging, mechanochemical reconstitution and atomic force microscopy, we find that mammalian Par3 couples genome integrity and epidermal fate through shaping keratinocyte mechanics, rather than mitotic spindle orientation. Par3 inactivation impairs RhoA activity, actomyosin contractility and viscoelasticity, eliciting mitotic failures that trigger aneuploidy, mitosis-dependent DNA damage responses, p53 stabilization and premature differentiation. Importantly, reconstituting myosin activity is sufficient to restore mitotic fidelity, genome integrity, and balanced differentiation and stratification. Collectively, this study deciphers a mechanical signaling network in which Par3 acts upstream of Rho/actomyosin contractility to promote intrinsic force generation, thereby maintaining mitotic accuracy and cellular fitness at the genomic level. Disturbing this network may compromise not only epidermal homeostasis but potentially also that of other self-renewing epithelia.

Many developing tissues require Par-driven polarization, but its role in mammalian tissue maintenance is unclear. Here, the authors show that in mouse epidermis, Par3 governs tissue homeostasis not via orientation of cell division but by coupling cell mechanics with mitotic accuracy and genome integrity.

Identification of a novel embryo-prevalent gene, Gm11545, involved in preimplantation embryogenesis in mice

In mammals, the early embryo travels down the oviduct to the uterus and prepares for implantation. The unique features of preimplantation development include compaction followed by blastocyst formation. This first cell lineage specification involves various proteins including cell polarity regulators, kinases, and transcription factors. In this study, a novel gene named predicted gene 11545 (Gm11545) expressed predominantly in mouse early embryos was identified and characterized at the transcript, protein, cellular, and functional levels. The Gm11545 protein localized to both cytoplasmic and membrane regions of preimplantation embryos. Remarkably, knockdown of Gm11545 led to arrest of mouse embryos at the morula stage and consequent impairment of blastocyst formation. Expression patterns of the key transcription factors critical for early lineage specification, octamer-binding transcription factor 4 and caudal type homeobox 2, were affected by Gm11545 depletion. Based on the collective findings, we propose that the novel protein identified in this study, Gm11545, is implicated in cell proliferation and cell lineage specification critical for blastocyst formation.-Kim, J., Kim, J., Jeong, J., Hong, S. H., Kim, D., Choi, S., Choi, I., Oh, J. S., Cho, C. Identification of a novel embryo-prevalent gene, Gm11545, involved in preimplantation embryogenesis in mice.

The Canonical but Not the Noncanonical Wnt Pathway Inhibits the Development of Allergic Airway Disease

Asthma is a syndrome with multifactorial causes, resulting in a variety of different phenotypes. Current treatment options are not curative and are sometimes ineffective in certain disease phenotypes. Therefore, novel therapeutic approaches are required. Recent findings have shown that activation of the canonical Wnt signaling pathway suppresses the development of allergic airway disease. In contrast, the effect of the noncanonical Wnt signaling pathway activation on allergic airway disease is not well described. The aim of this study was to validate the therapeutic effectiveness of Wnt-1-driven canonical Wnt signaling compared with Wnt-5a-driven noncanonical signaling in murine models. In vitro, both ligands were capable of attenuating allergen-specific T cell activation in a dendritic cell-dependent manner. In addition, the therapeutic effects of Wnt ligands were assessed in two different models of allergic airway disease. Application of Wnt-1 resulted in suppression of airway inflammation as well as airway hyperresponsiveness and mucus production. In contrast, administration of Wnt-5a was less effective in reducing airway inflammation or goblet cell metaplasia. These results suggest an immune modulating function for canonical as well as noncanonical Wnt signaling, but canonical Wnt pathway activation appears to be more effective in suppressing allergic airway disease than noncanonical Wnt activation.

Differential integrin expression in pre-implantation embryos developing under in vivo and in vitro conditions

Implantation failure is a major problem in human assisted reproduction, which persists regardless the optimization of endometrial receptivity and selection of genetically and morphologically healthy embryos. Since embryo-endometrium interaction depends on cell junctional, cell adhesion and cell-substratum adhesion molecules, the present study inquired whether in vitro growing murine embryos display similar to the in vivo growing embryos patterns of adhesion molecules. To this extend aVb3 expression and distribution in zygotes and 2-cell stage embryos were studied. The results demonstrated that only the in vivo growing embryos displayed specifically polarized aVb3 distribution, indicating their potential successful interaction with endometrium. Based on previous studies showing that L-carnitine (L-Cn) could affect embryonic development, it was demonstrated that the addition of L-Cn to the culture medium, could lead the in vitro growing embryos to acquire aVb3 expression and distribution similar to the in vivo growing embryos. Visualization of the effect of L-Cn using third harmonic generation imaging showed decreased lipid droplet levels in 2-cell-stage embryos, observation that correlates with an active energetic state of the growing embryos. Thus, the application of L-Cn to the culture medium could assist pre-implantation-state embryos to acquire aVb3 expression and distribution similar to the in vivo developing conditions.

Cell polarity: having and making sense of direction-on the evolutionary significance of the primary cilium/centrosome organ in Metazoa

Cell-autonomous polarity in Metazoans is evolutionarily conserved. I assume that permanent polarity in unicellular eukaryotes is required for cell motion and sensory reception, integration of these two activities being an evolutionarily constrained function. Metazoans are unique in making cohesive multicellular organisms through complete cell divisions. They evolved a primary cilium/centrosome (PC/C) organ, ensuring similar functions to the basal body/flagellum of unicellular eukaryotes, but in different cells, or in the same cell at different moments. The possibility that this innovation contributed to the evolution of individuality, in being instrumental in the early specification of the germ line during development, is further discussed. Then, using the example of highly regenerative organisms like planarians, which have lost PC/C organ in dividing cells, I discuss the possibility that part of the remodelling necessary to reach a new higher-level unit of selection in multi-cellular organisms has been triggered by conflicts among individual cell polarities to reach an organismic polarity. Finally, I briefly consider organisms with a sensorimotor organ like the brain that requires exceedingly elongated polarized cells for its activity. I conclude that beyond critical consequences for embryo development, the conservation of cell-autonomous polarity in Metazoans had far-reaching implications for the evolution of individuality.

Membrane trafficking in morphogenesis and planar polarity

Our understanding of how membrane trafficking pathways function to direct morphogenetic movements and the planar polarization of developing tissues is a new and emerging field. While a central focus of developmental biology has been on how protein asymmetries and cytoskeletal force generation direct cell shaping, the role of membrane trafficking in these processes has been less clear. Here, we review recent advances in Drosophila and vertebrate systems in our understanding of how trafficking events are coordinated with planar cytoskeletal function to drive lasting changes in cell and tissue topologies. We additionally explore the function of trafficking pathways in guiding the complex interactions that initiate and maintain core PCP (planar cell polarity) asymmetries and drive the generation of systematically oriented cellular projections during development.

Tracing the origin of heterogeneity and symmetry breaking in the early mammalian embryo

A fundamental question in developmental and stem cell biology concerns the origin and nature of signals that initiate asymmetry leading to pattern formation and self-organization. Instead of having prominent pre-patterning determinants as present in model organisms (worms, sea urchin, frog), we propose that the mammalian embryo takes advantage of more subtle cues such as compartmentalized intracellular reactions that generate micro-scale inhomogeneity, which is gradually amplified over several cellular generations to drive pattern formation while keeping developmental plasticity. It is therefore possible that by making use of compartmentalized information followed by its amplification, mammalian embryos would follow general principle of development found in other organisms in which the spatial cue is more robustly presented.

Dynamics of cortical domains in early Drosophila development

Underlying the plasma membrane of eukaryotic cells is an actin cortex that includes actin filaments and associated proteins. A special feature of all polarized and epithelial cells are cortical domains, each of which is characterized by specific sets of proteins. Typically, an epithelial cell contains apical, subapical, lateral and basal domains. The domain-specific protein sets contain evolutionarily conserved proteins, as well as cell-type-specific factors. Among the conserved proteins are, the Par proteins, Crumbs complex and the lateral proteins Scribbled and Discs large 1. Organization of the plasma membrane into cortical domains is dynamic and depends on cell type, differentiation and developmental stage. The dynamics of cortical organization is strikingly visible in early Drosophila embryos, which increase the number of distinct cortical domains from one, during the pre-blastoderm stage, to two in syncytial blastoderm embryos, before finally acquiring the four domains that are typical for epithelial cells during cellularization. In this Review, we will describe the dynamics of cortical organization in early Drosophila embryos and discuss the processes and mechanisms underlying cortical remodeling.

Actomyosin polarisation through PLC-PKC triggers symmetry breaking of the mouse embryo

Establishment of cell polarity in the mammalian embryo is fundamental for the first cell fate decision that sets aside progenitor cells for both the new organism and the placenta. Yet the sequence of events and molecular mechanism that trigger this process remain unknown. Here, we show that de novo polarisation of the mouse embryo occurs in two distinct phases at the 8-cell stage. In the first phase, an apical actomyosin network is formed. This is a pre-requisite for the second phase, in which the Par complex localises to the apical domain, excluding actomyosin and forming a mature apical cap. Using a variety of approaches, we also show that phospholipase C-mediated PIP₂ hydrolysis is necessary and sufficient to trigger the polarisation of actomyosin through the Rho-mediated recruitment of myosin II to the apical cortex. Together, these results reveal the molecular framework that triggers de novo polarisation of the mouse embryo.

The molecular trigger that establishes cell polarity in the mammalian embryo is unclear. Here, the authors show that de novo polarisation of the mouse embryo at the 8-cell stage is directed by Phospholipase C and Protein kinase C and occurs in two phases: polarisation of actomyosin followed by the Par complex.

ELMO and Sponge specify subapical restriction of Canoe and formation of the subapical domain in early Drosophila embryos

Canoe/Afadin and the GTPase Rap1 specify the subapical domain during cellularization in Drosophila embryos. The timing of domain formation is unclear. The subapical domain may gradually mature or emerge synchronously with basal and lateral domain. The mechanism for activation of Rap1 by potential guanyl nucleotide exchange factors (GEF) or GTPase activating proteins (GAP) is unknown. Here, we retraced the emergence of the subapical domain at the onset of cellularization by in vivo imaging with CanoeYFP in comparison to the lateral and basal markers, ScribbledGFP and CherrySlam. CanoeYFP accumulates at a subapical position at about the same time as the lateral marker ScribbledGFP but a few minutes prior to basal CherrySlam. Furthermore, we show that the unconventional GEF complex ELMO-Sponge is subapically enriched and is required for subapical restriction of Canoe. The localization dynamics of ELMO-Sponge suggests a patterning mechanism for positioning the subapical region adjacent to the apical region. While marking the disc-like apical regions before cellularization, ELMO-Sponge redistributes to a ring-like pattern surrounding the apical region at the onset of cellularization.

Transcriptional Control of Developmental Cell Behaviors

Tissue-specific transcription regulators emerged as key developmental control genes, which operate in the context of complex gene regulatory networks (GRNs) to coordinate progressive cell fate specification and tissue morphogenesis. We discuss how GRNs control the individual cell behaviors underlying complex morphogenetic events. Cell behaviors classically range from mesenchymal cell motility to cell shape changes in epithelial sheets. These behaviors emerge from the tissue-specific, multiscale integration of the local activities of universal and pleiotropic effectors, which underlie modular subcellular processes including cytoskeletal dynamics, cell-cell and cell-matrix adhesion, signaling, polarity, and vesicle trafficking. Extrinsic cues and intrinsic cell competence determine the subcellular spatiotemporal patterns of effector activities. GRNs influence most subcellular activities by controlling only a fraction of the effector-coding genes, which we argue is enriched in effectors involved in reading and processing the extrinsic cues to contextualize intrinsic subcellular processes and canalize developmental cell behaviors. The properties of the transcription-cell behavior interface have profound implications for evolution and disease.

Mollusc models I. The snail Ilyanassa

Ilyanassa obsoleta has been a model system for experimental embryology for over a century. Here we highlight new insight into early cell lineage specification in Ilyanassa. As in all molluscs and other spiralians, stereotyped cleavage patterns establish a homunculus of regional founder cells. Ongoing studies are beginning to dissect mechanisms of asymmetric cell division that specify these cells' fates. This is only part of the story: overlaid on intrinsic cell identities is a graded 'organizer' signal, and emerging evidence suggests wider roles for short-range intercellular signaling. Modern methods, combined with the intrinsic experimental advantages of Ilyanassa, offer attractive opportunities for studying basic developmental cell biology as well as its evolution over a wide range of phylogenetic scales.

Unequal distribution of 16S mtrRNA at the 2-cell stage regulates cell lineage allocations in mouse embryos

Cell lineage determination during early embryogenesis has profound effects on adult animal development. Pre-patterning of embryos, such as that of Drosophila and Caenorhabditis elegans, is driven by asymmetrically localized maternal or zygotic factors, including mRNA species and RNA binding proteins. However, it is not clear how mammalian early embryogenesis is regulated and what the early cell fate determinants are. Here we show that, in mouse, mitochondrial ribosomal RNAs (mtrRNAs) are differentially distributed between 2-cell sister blastomeres. This distribution pattern is not related to the overall quantity or activity of mitochondria which appears equal between 2-cell sister blastomeres. Like in lower species, 16S mtrRNA is found to localize in the cytoplasm outside of mitochondria in mouse 2-cell embryos. Alterations of 16S mtrRNA levels in one of the 2-cell sister blastomere via microinjection of either sense or anti-sense RNAs drive its progeny into different cell lineages in blastocyst. These results indicate that mtrRNAs are differentially distributed among embryonic cells at the beginning of embryogenesis in mouse and they are functionally involved in the regulation of cell lineage allocations in blastocyst, suggesting an underlying molecular mechanism that regulates pre-implantation embryogenesis in mouse.

Jeremy Nance: Charting gastrulation's gyrations

Nance surveys the cell movements that orchestrate early embryogenesis.

Getting to the core of cadherin complex function in Caenorhabditis elegans

The classic cadherin-catenin complex (CCC) mediates cell-cell adhesion in metazoans. Although substantial insights have been gained by studying the CCC in vertebrate tissue culture, analyzing requirements for and regulation of the CCC in vertebrates remains challenging. Caenorhabditis elegans is a powerful system for connecting the molecular details of CCC function with functional requirements in a living organism. Recent data, using an “angstroms to embryos” approach, have elucidated functions for key residues, conserved across all metazoans, that mediate cadherin/β-catenin binding. Other recent work reveals a novel, potentially ancestral, role for the C. elegans p120ctn homologue in regulating polarization of blastomeres in the early embryo via Cdc42 and the partitioning-defective (PAR)/atypical protein kinase C (aPKC) complex. Finally, recent work suggests that the CCC is trafficked to the cell surface via the clathrin adaptor protein complex 1 (AP-1) in surprising ways. These studies continue to underscore the value of C. elegans as a model system for identifying conserved molecular mechanisms involving the CCC.

Polarity and cell division orientation in the cleavage embryo: from worm to human

Cleavage is a period after fertilization, when a 1-cell embryo starts developing into a multicellular organism. Due to a series of mitotic divisions, the large volume of a fertilized egg is divided into numerous smaller, nucleated cells—blastomeres. Embryos of different phyla divide according to different patterns, but molecular mechanism of these early divisions remains surprisingly conserved. In the present paper, we describe how polarity cues, cytoskeleton and cell-to-cell communication interact with each other to regulate orientation of the early embryonic division planes in model animals such as Caenorhabditis elegans, Drosophila and mouse. We focus particularly on the Par pathway and the actin-driven cytoplasmic flows that accompany it. We also describe a unique interplay between Par proteins and the Hippo pathway in cleavage mammalian embryos. Moreover, we discuss the potential meaning of polarity, cytoplasmic dynamics and cell-to-cell communication as quality biomarkers of human embryos.

Cell density regulates cancer metastasis via the Hippo pathway

Metastatic spread of cancer cells from the primary tumor site to distant organs is the major cause of death in cancer patients. To disseminate, cancer cells detach from the primary tumor, enter the blood stream and extravasate at distant organ sites such as the liver, lung, bone or brain. While cancer cells are known to evade contact inhibition during growth in culture, we found that cell density is still sensed and can signal through the Hippo pathway effectors LATS1 and YAP. These effectors control cancer cell invasive behavior into stromal tissues, expression of cytokines that recruit inflammatory cells and progression toward metastatic spread. In this perspective, we discuss the drivers and the significance of pathways controlled by cell growth density.

Par system components are asymmetrically localized in ectodermal epithelia, but not during early development in the sea anemone Nematostella vectensis

Background

The evolutionary origins of cell polarity in metazoan embryos are unclear. In most bilaterian animals, embryonic and cell polarity are set up during embryogenesis with the same molecules being utilized to regulate tissue polarity at different life stages. Atypical protein kinase C (aPKC), lethal giant larvae (Lgl), and Partitioning-defective (Par) proteins are conserved components of cellular polarization, and their role in establishing embryonic asymmetry and tissue polarity have been widely studied in model bilaterian groups. However, the deployment and role of these proteins in animals outside Bilateria has not been studied. We address this by characterizing the localization of different components of the Par system during early development of the sea anemone Nematostella vectensis, a member of the clade Cnidaria, the sister group to bilaterian animals.

Results

Immunostaining using specific N. vectensis antibodies and the overexpression of mRNA-reporter constructs show that components of the N. vectensis Par system (NvPar-1, NvPar-3, NvPar-6, NvaPKC, and NvLgl) distribute throughout the microtubule cytoskeleton of eggs and early embryos without clear polarization along any embryonic axis. However, they become asymmetrically distributed at later stages, when the embryo forms an ectodermal epithelial layer. NvLgl and NvPar-1 localize in the basolateral cortex, and NvaPKC, NvPar-6, and NvPar-3 at the apical zone of the cell in a manner seen in bilaterian animals.

Conclusions

The cnidarian N. vectensis exhibits clear polarity at all stages of early embryonic development, which appears to be established independent of the Par system reported in many bilaterian embryos. However, in N. vectensis, using multiple immunohistochemical and fluorescently labeled markers in vivo, components of this system are deployed to organize epithelial cell polarity at later stages of development. This suggests that Par system proteins were co-opted to organize early embryonic cell polarity at the base of the Bilateria and that, therefore, different molecular mechanisms operate in early cnidarian embryogenesis.

Electronic supplementary material

The online version of this article (doi:10.1186/s13227-015-0014-6) contains supplementary material, which is available to authorized users.

Cell growth density modulates cancer cell vascular invasion via Hippo pathway activity and CXCR2 signaling

Metastasis of cancer cells involves multiple steps, including their dissociation from the primary tumor and invasion through the endothelial cell barrier to enter the circulation and finding their way to distant organ sites where they extravasate and establish metastatic lesions. Deficient contact inhibition is a hallmark of invasive cancer cells, yet surprisingly the vascular invasiveness of commonly studied cancer cell lines is regulated by the density at which cells are propagated in culture. Cells grown at high density were less effective at invading an endothelial monolayer than cells grown at low density. This phenotypic difference was also observed in a zebrafish model of vascular invasion of cancer cells after injection into the yolk sac and extravasation of cancer cells into tissues from the vasculature. The vascular invasive phenotypes were reversible. A kinome-wide RNAi screen was used to identify drivers of vascular invasion by panning shRNA library transduced non-invasive cancer cell populations on endothelial monolayers. The selection of invasive subpopulations showed enrichment of shRNAs targeting the LATS1 (large tumor suppressor 1) kinase that inhibits the activity of the transcriptional coactivator YAP in the Hippo pathway. Depletion of LATS1 from non-invasive cancer cells restored the invasive phenotype. Complementary to this, inhibition or depletion of YAP inhibited invasion in vitro and in vivo. The vascular invasive phenotype was associated with a YAP-dependent up-regulation of the cytokines IL6, IL8, and CXCL1, 2, and 3. Antibody blockade of cytokine receptors inhibited invasion and confirmed that they are rate-limiting drivers that promote cancer cell vascular invasiveness and could provide therapeutic targets.

MiRNA-320 in the human follicular fluid is associated with embryo quality in vivo and affects mouse embryonic development in vitro

Previous work from our laboratory demonstrated the existence of miRNAs in human follicular fluid. In the current study, we have sought to identify miRNAs that might affect oocyte/embryo quality in patients undergoing intracytoplasmic sperm injection and to investigate their roles in in vitro fertilization outcomes in mouse oocytes. 53 samples were classified as Group 1 (high quality) if the day-3 embryos had seven and more cells or as Group 2 (low quality) if the embryos had six and fewer cells. TaqMan Human microRNAs cards and qRT-PCR were performed to verify differently expressed miRNAs. The function of the corresponding miRNA was investigated in mouse oocytes by injecting them with miRNA-inhibitor oligonucleotides. We found that hsa-miR-320a and hsa-miR-197 had significantly higher expression levels in the Group 1 follicular fluids than in Group 2 (p = 0.0073 and p = 0.008, respectively). Knockdown of mmu-miR-320 in mouse oocytes strongly decreased the proportions of MII oocytes that developed into two-cell and blastocyst stage embryos (p = 0.0048 and p = 0.0069, respectively). Wnt signaling pathway components had abnormal expression level in miR-320 inhibitor-injected oocytes. This study provides the first evidence that miRNAs in human follicular fluid are indicative of and can influence embryo quality.