Polarity-dependent distribution of angiomotin localizes Hippo signaling in preimplantation embryos

YAP-TEAD1 control of cytoskeleton dynamics and intracellular tension guides human pluripotent stem cell mesoderm specification

The tight regulation of cytoskeleton dynamics is required for a number of cellular processes, including migration, division and differentiation. YAP–TEAD respond to cell–cell interaction and to substrate mechanics and, among their downstream effects, prompt focal adhesion (FA) gene transcription, thus contributing to FA-cytoskeleton stability. This activity is key to the definition of adult cell mechanical properties and function. Its regulation and role in pluripotent stem cells are poorly understood. Human PSCs display a sustained basal YAP-driven transcriptional activity despite they grow in very dense colonies, indicating these cells are insensitive to contact inhibition. PSC inability to perceive cell–cell interactions can be restored by tampering with Tankyrase enzyme, thus favouring AMOT inhibition of YAP function. YAP–TEAD complex is promptly inactivated when germ layers are specified, and this event is needed to adjust PSC mechanical properties in response to physiological substrate stiffness. By providing evidence that YAP–TEAD1 complex targets key genes encoding for proteins involved in cytoskeleton dynamics, we suggest that substrate mechanics can direct PSC specification by influencing cytoskeleton arrangement and intracellular tension. We propose an aberrant activation of YAP–TEAD1 axis alters PSC potency by inhibiting cytoskeleton dynamics, thus paralyzing the changes in shape requested for the acquisition of the given phenotype.

Emerging roles for angiomotin in the nervous system

Angiomotins are a family of molecular scaffolding proteins that function to organize contact points (called tight junctions in vertebrates) between adjacent cells. Some angiomotin isoforms bind to the actin cytoskeleton and are part of signaling pathways that influence cell morphology and migration. Others cooperate with components of the Hippo signaling pathway and the associated networks to control organ growth. The 130-kDa isoform, AMOT-p130, has critical roles in neural stem cell differentiation, dendritic patterning, and synaptic maturation-attributes that are essential for normal brain development and are consistent with its association with autism. Here, we review and discuss the evidence that supports a role for AMOT-p130 in neuronal development in the central nervous system.

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'.

Establishment of a relationship between blastomere geometry and YAP localisation during compaction

Precise patterning within the three-dimensional context of tissues, organs and embryos implies that cells can sense their relative position. During preimplantation development, outside and inside cells rely on apicobasal polarity and the Hippo pathway to choose their fate. Despite recent findings suggesting that mechanosensing might be central to this process, the relationship between blastomere geometry (i.e. shape and position) and the Hippo pathway effector YAP remains unknown. We used a highly quantitative approach to analyse information on the geometry and YAP localisation of individual blastomeres of mouse and human embryos. We identified the proportion of exposed cell surface area as most closely correlating with the nuclear localisation of YAP. To test this relationship, we developed several hydrogel-based approaches to alter blastomere geometry in cultured embryos. Unbiased clustering analyses of blastomeres from such embryos revealed that this relationship emerged during compaction. Our results therefore pinpoint the time during early embryogenesis when cells acquire the ability to sense changes in geometry and provide a new framework for how cells might integrate signals from different membrane domains to assess their relative position within the embryo.

Highlighted Article: Localisation of YAP, a key factor during the first cell fate decision, is linked to individual blastomere geometry within the three-dimentional environment of the preimplantation embryo.

AMOTL2 inhibits JUN Thr239 dephosphorylation by binding PPP2R2A to suppress the proliferation in non-small cell lung cancer cells

Protein phosphatase 2A (PP2A) complex comprises an extended family of intracellular protein serine/threonine phosphatases, that participate in different signaling transduction pathways. Different functions of PP2As are determined by the variety of regulatory subunits. In this study, CRISPR/Cas9-mediated loss-of-function screen revealed that PPP2R2A downregulation suppressed cell growth in NSCLC cells. AMOTL2 was identified and confirmed as a novel binding partner of PPP2R2A in NSCLC cells by mass spectrometry, CO-IP, GST pull-down and immunofluorescence. Upregulation of AMOTL2 also led to cell proliferation delay in human and mouse lung tumor cells. The proto-oncogene JUN is a key subunit of activator protein-1 (AP-1) transcription factor which plays crucial role in regulating tumorigenesis and its activity is negatively regulated by the phosphorylation at T239. Our results showed that either AMOTL2 upregulation or PPP2R2A downregulation led to great increase in JUN T239 phosphorylation. AMOTL2 bound PPP2R2A in cytoplasm, which reduced nuclear localization of PPP2R2A. In conclusion, AMOTL2 and PPP2R2A act respectively as negative and positive regulator of cell growth in NSCLC cells and function in the AMOTL2-PPP2R2A-JUN axis, in which AMOTL2 inhibits the entry of PPP2R2A into the nucleus to dephosphorylate JUN at T239.

Yes-associated protein 1 translocation through actin cytoskeleton organization in trophectoderm cells

A mammalian embryo experiences the first cell segregation at the blastocyst stage, in which cells giving form to the embryo are sorted into two lineages; trophectoderm (TE) and inner cell mass (ICM). This first cell segregation process is governed by cell position-dependent Hippo signaling, which is a phosphorylation cascade determining whether Yes-associated protein 1 (YAP1), one of the key components of the Hippo signaling pathway, localizes within the nucleus or cytoplasm. YAP1 localization determines the transcriptional on/off switch of a key gene, Cdx2, required for TE differentiation. However, the control mechanisms involved in YAP1 nucleocytoplasmic shuttling post blastocyst formation remain unknown. This study focused on the mechanisms involved in YAP1 release from TE nuclei after blastocoel contraction in bovine blastocysts. The blastocysts contracted by blastocoel fluid aspiration showed that the YAP1 translocation from nucleus to cytoplasm in the TE cells was concomitant with the protruded actin cytoskeleton. This YAP1 release from TE nuclei in the contracted blastocysts was prevented by actin disruption and stabilization. In contrast, Y27632, which is a potent inhibitor of Rho-associated coiled-coil containing protein kinase 1/2 (ROCK) activity, was found to promote YAP1 nuclear localization in the TE cells of contracted blastocysts. Meanwhile, lambda protein phosphatase (LPP) treatment inducing protein dephosphorylation could not prevent YAP1 release from TE nuclei in the contracted blastocysts, indicating that YAP1 release from TE nuclei does not depend on the Hippo signaling pathway. These results suggested that blastocyst contraction causes YAP1 release from TE nuclei through actin cytoskeleton remodeling in a Hippo signaling-independent manner. Thus, the present study raised the possibility that YAP1 subcellular localization is controlled by actin cytoskeletal organization after the blastocyst formation. Our results demonstrate diverse regulatory mechanisms for YAP1 nucleocytoplasmic shuttling in TE cells.

Hippo Signaling in Embryogenesis and Development

Hippo pathway components are structurally and functionally conserved and are notable for their role in controlling organ size. More diverse functions of the Hippo pathway have been recognized, including development, tissue homeostasis, wound healing and regeneration, immunity, and tumorigenesis. During embryogenesis, different signaling pathways are repeatedly and cooperatively activated, leading to differential gene expression in specific developmental contexts. In this article, we present an overview on the regulation and function of the Hippo pathway in mammalian early development. We introduce the Hippo pathway components and major upstream signals that act through this pathway to influence embryogenesis. We also discuss the roles of Hippo pathway in tissue specification and organ development during organogenesis.

Keratins are asymmetrically inherited fate determinants in the mammalian embryo

To implant in the uterus, the mammalian embryo first specifies two cell lineages: the pluripotent inner cell mass that forms the fetus, and the outer trophectoderm layer that forms the placenta1. In many organisms, asymmetrically inherited fate determinants drive lineage specification2, but this is not thought to be the case during early mammalian development. Here we show that intermediate filaments assembled by keratins function as asymmetrically inherited fate determinants in the mammalian embryo. Unlike F-actin or microtubules, keratins are the first major components of the cytoskeleton that display prominent cell-to-cell variability, triggered by heterogeneities in the BAF chromatin-remodelling complex. Live-embryo imaging shows that keratins become asymmetrically inherited by outer daughter cells during cell division, where they stabilize the cortex to promote apical polarization and YAP-dependent expression of CDX2, thereby specifying the first trophectoderm cells of the embryo. Together, our data reveal a mechanism by which cell-to-cell heterogeneities that appear before the segregation of the trophectoderm and the inner cell mass influence lineage fate, via differential keratin regulation, and identify an early function for intermediate filaments in development.

From pluripotency to totipotency: an experimentalist's guide to cellular potency

Embryonic stem cells (ESCs) are derived from the pre-implantation mammalian blastocyst. At this point in time, the newly formed embryo is concerned with the generation and expansion of both the embryonic lineages required to build the embryo and the extra-embryonic lineages that support development. When used in grafting experiments, embryonic cells from early developmental stages can contribute to both embryonic and extra-embryonic lineages, but it is generally accepted that ESCs can give rise to only embryonic lineages. As a result, they are referred to as pluripotent, rather than totipotent. Here, we consider the experimental potential of various ESC populations and a number of recently identified in vitro culture systems producing states beyond pluripotency and reminiscent of those observed during pre-implantation development. We also consider the nature of totipotency and the extent to which cell populations in these culture systems exhibit this property.

Mechanical regulation of cell size, fate, and behavior during asymmetric cell division

Asymmetric cell division (ACD) is an evolutionary conserved mechanism used by prokaryotes and eukaryotes alike to generate cell diversity. ACD can be manifested in biased segregation of macromolecules or differential partitioning of cell organelles. Cells are also constantly subject to extrinsic or intrinsic mechanical forces, influencing cell behavior and fate. During ACD, cell intrinsic forces generated through the spatiotemporal regulation of the actomyosin cytoskeleton can influence sibling cell size. External mechanical stresses are further translated by transcriptional coactivators or mechanically gated ion channels. Here, we will discuss recent literature, exploring how mechanical cues influence various aspects of ACD and stem cell behavior, and how these mechanical cues contribute to cell fate decisions.

Common principles of early mammalian embryo self-organisation

Pre-implantation mammalian development unites extreme plasticity with a robust outcome: the formation of a blastocyst, an organised multi-layered structure ready for implantation. The process of blastocyst formation is one of the best-known examples of self-organisation. The first three cell lineages in mammalian development specify and arrange themselves during the morphogenic process based on cell-cell interactions. Despite decades of research, the unifying principles driving early mammalian development are still not fully defined. Here, we discuss the role of physical forces, and molecular and cellular mechanisms, in driving self-organisation and lineage formation that are shared between eutherian mammals.

Multiscale morphogenesis of the mouse blastocyst by actomyosin contractility

During preimplantation development, the mouse embryo forms the blastocyst, which consists of a squamous epithelium enveloping a fluid-filled lumen and a cluster of pluripotent cells. The shaping of the blastocyst into its specific architecture is a prerequisite to implantation and further development of the embryo. Recent studies identified the central role of the actomyosin cortex in generating the forces driving the successive steps of blastocyst morphogenesis. As seen in other developing animals, actomyosin functions across spatial scales from the subcellular to the tissue levels. In addition, the slow development of the mouse embryo reveals that actomyosin contractility operates at multiple timescales with periodic cortical waves of contraction every ∼80 s and tissue remodeling over hours.

Mechanisms of human embryo development: from cell fate to tissue shape and back

Gene regulatory networks and tissue morphogenetic events drive the emergence of shape and function: the pillars of embryo development. Although model systems offer a window into the molecular biology of cell fate and tissue shape, mechanistic studies of our own development have so far been technically and ethically challenging. However, recent technical developments provide the tools to describe, manipulate and mimic human embryos in a dish, thus opening a new avenue to exploring human development. Here, I discuss the evidence that supports a role for the crosstalk between cell fate and tissue shape during early human embryogenesis. This is a critical developmental period, when the body plan is laid out and many pregnancies fail. Dissecting the basic mechanisms that coordinate cell fate and tissue shape will generate an integrated understanding of early embryogenesis and new strategies for therapeutic intervention in early pregnancy loss.

Trophoblast lineage specification in the mammalian preimplantation embryo

BACKGROUND

The establishment of the trophectoderm (TE) and the inner cell mass (ICM) is the first cell lineage segregation that occurs in mammalian preimplantation development. TE will contribute to the placenta while ICM cells give rise to the epiblast (EPI) and primitive endoderm (PrE). There are two historical models for TE/ICM segregation: the positional (inside-outside) model and the polarity model, but both models alone cannot explain the mechanism of TE/ICM segregation.

METHODS

This article discusses a current possible model based on recent studies including the finding through live-cell imaging of the expression patterns of caudal type homeobox 2 (Cdx2), a key transcription factor of TE differentiation in the mouse embryo.

RESULTS

It was observed that a part of outer Cdx2-expressing blastomeres was internalized at the around 20- to 30-cell stage, downregulates Cdx2, ceases TE differentiation, and participates in ICM lineages.

CONCLUSION

The early blastomere, which starts differentiation toward the TE cell fate, still has plasticity and can change its fate. Differentiation potency of all blastomeres until approximately the 32-cell stage is presumably not irreversibly restricted even if they show heterogeneity in their epigenetic modifications or gene expression patterns.

The enigmatic morula: mechanisms of development, cell fate determination, self-correction and implications for ART

BACKGROUND

Assisted reproduction technology offers the opportunity to observe the very early stages of human development. However, due to practical constraints, for decades morphological examination of embryo development has been undertaken at a few isolated time points at the stages of fertilisation (Day 1), cleavage (Day 2-3) and blastocyst (Day 5-6). Rather surprisingly, the morula stage (Day 3-4) has been so far neglected, despite its involvement in crucial cellular processes and developmental decisions.

OBJECTIVE AND RATIONALE

The objective of this review is to collate novel and unsuspected insights into developmental processes occurring during formation of the morula, highlighting the key importance of this stage for a better understanding of preimplantation development and an improvement of ART.

SEARCH METHODS

PubMed was used to search the MEDLINE database for peer-reviewed English-language original articles and reviews concerning the morula stage in mammals. Searches were performed by adopting 'embryo', 'morula', 'compaction', 'cell fate' and 'IVF/assisted reproduction' as main terms, in association with other keywords expressing concepts relevant to the subject (e.g. cell polarity). The most relevant publications, i.e. those concerning major phenomena occurring during formation of the morula in established experimental models and the human species, were assessed and discussed critically.

OUTCOMES

Novel live cell imaging technologies and cell biology studies have extended our understanding of morula formation as a key stage for the development of the blastocyst and determination of the inner cell mass (ICM) and the trophectoderm (TE). Cellular processes, such as dynamic formation of filopodia and cytoskeleton-mediated zippering cell-to-cell interactions, intervene to allow cell compaction (a geometrical requisite essential for development) and formation of the blastocoel, respectively. At the same time, differential orientation of cleavage planes, cell polarity and cortical tensile forces interact and cooperate to position blastomeres either internally or externally, thereby influencing their cellular fate. Recent time lapse microscopy (TLM) observations also suggest that in the human the process of compaction may represent an important checkpoint for embryo viability, through which chromosomally abnormal blastomeres are sensed and eliminated by the embryo.

WIDER IMPLICATIONS

In clinical embryology, the morula stage has been always perceived as a 'black box' in the continuum of preimplantation development. This has dictated its virtual exclusion from mainstream ART procedures. Recent findings described in this review indicate that the morula, and the associated process of compaction, as a crucial stage not only for the formation of the blastocyst, but also for the health of the conceptus. This understanding may open new avenues for innovative approaches to embryo manipulation, assessment and treatment.

Regulation and functions of the Hippo pathway in stemness and differentiation

The Hippo pathway plays important roles in organ development, tissue regeneration, and human diseases, such as cancer. In the canonical Hippo pathway, the MST1/2-LATS1/2 kinase cascade phosphorylates and inhibits transcription coactivators Yes-associated protein and transcription coactivator with PDZ-binding motif and thus regulates transcription of genes important for cell proliferation and apoptosis. However, recent studies have depicted a much more complicate picture of the Hippo pathway with many new components and regulatory stimuli involving both chemical and mechanical signals. Furthermore, accumulating evidence indicates that the Hippo pathway also plays important roles in the determination of cell fates, such as self-renewal and differentiation. Here, we review regulations of the Hippo pathway and its functions in stemness and differentiation emphasizing recent discoveries.

Specification of the First Mammalian Cell Lineages In Vivo and In Vitro

Our understanding of how the first mammalian cell lineages arise has been shaped largely by studies of the preimplantation mouse embryo. Painstaking work over many decades has begun to reveal how a single totipotent cell is transformed into a multilayered structure representing the foundations of the body plan. Here, we review how the first lineage decision is initiated by epigenetic regulation but consolidated by the integration of morphological features and transcription factor activity. The establishment of pluripotent and multipotent stem cell lines has enabled deeper analysis of molecular and epigenetic regulation of cell fate decisions. The capability to assemble these stem cells into artificial embryos is an exciting new avenue of research that offers a long-awaited window into cell fate specification in the human embryo. Together, these approaches are poised to profoundly increase our understanding of how the first lineage decisions are made during mammalian embryonic development.

Effect of Downregulating the Hippo Pathway Members YAP1 and LATS2 Transcripts on Early Development and Gene Expression Involved in Differentiation in Porcine Embryos

In mouse development, differentiation of the inner cell mass (ICM) and trophectoderm (TE) during the transition from the morula to blastocyst stage is regulated by the Hippo pathway; however, the functions of the Hippo pathway in porcine embryogenesis have not been investigated. In the present study, we examined the gene expression patterns of the Hippo pathway members yes-associated protein 1 (YAP1) and large tumor suppressor 2 (LATS2) and the functions of these genes during porcine preimplantation development using RNA interference. Both YAP1 and LATS2 mRNA levels were shown high in the in vitro matured oocytes and 1-cell stage embryos and fell progressively with development. YAP1 nuclear localization was detected at the morula and blastocyst stages. Downregulation of either YAP1 or LATS2 inhibited porcine preimplantation development and affected the expression levels of POU class 5 homeobox 1 (OCT-4) and SRY-related HMG-box gene 2 (SOX2), transcription factors necessary for the ICM/TE differentiation. Taken together, YAP1 and LATS2 are essential for porcine preimplantation development, and it is possible that the Hippo pathway has important roles in porcine ICM/TE segregation.

ROCK and RHO Playlist for Preimplantation Development: Streaming to HIPPO Pathway and Apicobasal Polarity in the First Cell Differentiation

In placental mammalian development, the first cell differentiation produces two distinct lineages that emerge according to their position within the embryo: the trophectoderm (TE, placenta precursor) differentiates in the surface, while the inner cell mass (ICM, fetal body precursor) forms inside. Here, we discuss how such position-dependent lineage specifications are regulated by the RHOA subfamily of small GTPases and RHO-associated coiled-coil kinases (ROCK). Recent studies in mouse show that activities of RHO/ROCK are required to promote TE differentiation and to concomitantly suppress ICM formation. RHO/ROCK operate through the HIPPO signaling pathway, whose cell position-specific modulation is central to establishing unique gene expression profiles that confer cell fate. In particular, activities of RHO/ROCK are essential in outside cells to promote nuclear localization of transcriptional co-activators YAP/TAZ, the downstream effectors of HIPPO signaling. Nuclear localization of YAP/TAZ depends on the formation of apicobasal polarity in outside cells, which requires activities of RHO/ROCK. We propose models of how RHO/ROCK regulate lineage specification and lay out challenges for future investigations to deepen our understanding of the roles of RHO/ROCK in preimplantation development. Finally, as RHO/ROCK may be inhibited by certain pharmacological agents, we discuss their potential impact on human preimplantation development in relation to fertility preservation in women.

Rho-ROCK Signaling in Normal Physiology and as a Key Player in Shaping the Tumor Microenvironment

The Rho-ROCK signaling network has a range of specialized functions of key biological importance, including control of essential developmental processes such as morphogenesis and physiological processes including homeostasis, immunity, and wound healing. Deregulation of Rho-ROCK signaling actively contributes to multiple pathological conditions, and plays a major role in cancer development and progression. This dynamic network is critical in modulating the intricate communication between tumor cells, surrounding diverse stromal cells and the matrix, shaping the ever-changing microenvironment of aggressive tumors. In this chapter, we overview the complex regulation of the Rho-ROCK signaling axis, its role in health and disease, and analyze progress made with key approaches targeting the Rho-ROCK pathway for therapeutic benefit. Finally, we conclude by outlining likely future trends and key questions in the field of Rho-ROCK research, in particular surrounding Rho-ROCK signaling within the tumor microenvironment.

Angiomotin links ROCK and YAP signaling in mechanosensitive differentiation of neural stem cells

Mechanical cues regulate the function of a broad range of stem cells in culture and in tissue. For example, soft substrates promote the neuronal differentiation of neural stem cells (NSCs) by suppressing cytoskeletal contractility. However, the mechanisms that link cytoskeletal signaling to the transcriptional regulatory processes that ultimately govern stiffness-dependent NSC fate commitment are not fully understood. Here, we show that Angiomotin (AMOT), which can bind both F-actin and the neurosuppressive transcriptional coactivator Yes-associated protein (YAP), is critical for mechanotransduction in NSCs. On soft substrates, loss of AMOT substantially reduces neurogenesis, whereas on stiff substrates, loss of AMOT negates the rescue of neurogenesis normally induced by pharmacologic inhibition of myosin activity. Furthermore, overexpression of a phospho-mimetic S175E AMOT mutant, which has been established to enhance AMOT–YAP binding, increases β-catenin activity and rescues neurogenesis on stiff substrates. Together, our data identify AMOT as an important intermediate signal transducer that allows NSCs to sense and respond to extracellular stiffness cues.

Building an apical domain in the early mouse embryo: Lessons, challenges and perspectives

Cell polarization is critical for lineage segregation and morphogenesis during mammalian embryogenesis. However, the processes and mechanisms that establish cell polarity in the mammalian embryo are not well understood. Recent studies suggest that unique regulatory mechanisms are deployed by the mouse embryo to establish cell polarization. In this review, we discuss the current understanding of cell polarity establishment, focusing on the formation of the apical domain in the mouse embryo. We will also discuss outstanding questions and possible directions for future study.

Loss of Mob1a/b impairs the differentiation of mouse embryonic stem cells into the three germ layer lineages

The Hippo pathway plays a crucial role in cell proliferation and apoptosis and can regulate stem cell maintenance and embryonic development. MOB kinase activators 1A and 1B (Mob1a/b) are key components of the Hippo pathway, whose homozygous deletion in mice causes early embryonic lethality at the preimplantation stage. To investigate the role of Mob1a/b in stem cell maintenance and differentiation, an embryonic stem cell (ESC) clone in which Mob1a/b could be conditionally depleted was generated and characterized. Although Mob1a/b depletion did not affect the stemness or proliferation of mouse ESCs, this depletion caused defects in differentiation into the three germ layers. Yap knockdown rescued the in vitro and in vivo defects in differentiation caused by Mob1a/b depletion, suggesting that differentiation defects caused by Mob1a/b depletion were Yap-dependent. In teratoma experiments, Yap knockdown in Mob1a/b-depleted ESCs partially restored defects in differentiation, indicating that hyperactivation of Taz, another effector of the Hippo pathway, inhibited differentiation into the three germ layers. Taken together, these results suggest that Mob1a/b or Hippo signaling plays a critical role in the differentiation of mouse ESCs into the three germ layers, which is dependent on Yap. These close relationship of the Hippo pathway with the differentiation of stem cells supports its potential as a therapeutic target in regenerative medicine.

Novel approaches to link apicobasal polarity to cell fate specification

Understanding the development of apicobasal polarity (ABP) is a long-standing problem in biology. The molecular components involved in the development and maintenance of APB have been largely identified and are known to have ubiquitous roles across organisms. Our knowledge of the functional consequences of ABP establishment and maintenance is far less comprehensive. Recent studies using novel experimental approaches and cellular models have revealed a growing link between ABP and the genetic program of cell lineage. This mini-review describes some of the most recent advances in this new field, highlighting examples from Caenorhabditis elegans and mouse embryos, human pluripotent stem cells, and epithelial cells. We also speculate on the most interesting and challenging avenues that can be explored.

The emerging role of Hippo signaling in neurodegeneration

Neurodegeneration refers to the complex process of progressive degeneration or neuronal apoptosis leading to a set of incurable and debilitating conditions. Physiologically, apoptosis is important in proper growth and development. However, aberrant and unrestricted apoptosis can lead to a variety of degenerative conditions including neurodegenerative diseases. Although dysregulated apoptosis has been implicated in various neurodegenerative disorders, the triggers and molecular mechanisms underlying such untimely and faulty apoptosis are still unknown. Hippo signaling pathway is one such apoptosis-regulating mechanism that has remained evolutionarily conserved from Drosophila to mammals. This pathway has gained a lot of attention for its tumor-suppressing task, but recent studies have emphasized the soaring role of this pathway in inflaming neurodegeneration. In addition, strategies promoting inactivation of this pathway have aided in the rescue of neurons from anomalous apoptosis. So, a thorough understanding of the relationship between the Hippo pathway and neurodegeneration may serve as a guide for the development of therapy for various degenerative diseases. The current review focuses on the mechanism of the Hippo signaling pathway, its upstream and downstream regulatory molecules, and its role in the genesis of numerous neurodegenerative diseases. The recent efforts employing the Hippo pathway components as targets for checking neurodegeneration have also been highlighted.

Upstream regulation of the Hippo-Yap pathway in cardiomyocyte regeneration

The response of the adult mammalian heart to injury such as myocardial infarction has long been described as primarily fibrotic scarring and adverse remodeling with little to no regeneration of cardiomyocytes. Emerging studies have challenged this paradigm by demonstrating that, indeed, adult mammalian cardiomyocytes are capable of completing cytokinesis albeit at levels vastly insufficient to compensate for the loss of functional cardiomyocytes following ischemic injury. Thus, there is great interest in identifying mechanisms to guide adult cardiomyocyte cell cycle re-entry and facilitate endogenous heart regeneration. The Hippo signaling pathway is a core kinase cascade that functions to suppress the transcriptional co-activators Yap and Taz by phosphorylation and therefore cytoplasmic retention or phospho-degradation. This pathway has recently sparked interest in the field of cardiac regeneration as inhibition of Hippo kinase signaling or overdriving the transcriptional co-activator, Yap, significantly promotes proliferation of terminally differentiated adult mammalian cardiomyocytes and can restore function in failing mouse hearts. Thus, the Hippo pathway is an attractive therapeutic target for promoting cardiomyocyte renewal and cardiac regeneration. Although the core kinases and transcriptional activators of the Hippo pathway have been studied extensively over the last twenty years, the regulatory inputs of this pathway, particularly in vertebrates, are poorly understood. Recent studies have elucidated several upstream regulatory inputs to the Hippo pathway in adult mammalian cardiomyocytes that influence cell proliferation and heart regeneration. Considering upstream inputs to the Hippo pathway are thought to be context and cell type specific, targeting these various components could serve as a therapeutic approach for refining Hippo-Yap signaling in the heart. Here, we provide an overview of the emerging regulatory inputs to the Hippo pathway as they relate to mammalian cardiomyocytes and heart regeneration.

Control of cellular responses to mechanical cues through YAP/TAZ regulation

To perceive their three-dimensional environment, cells and tissues must be able to sense and interpret various physical forces like shear, tensile, and compression stress. These forces can be generated both internally and externally in response to physical properties, like substrate stiffness, cell contractility, and forces generated by adjacent cells. Mechanical cues have important roles in cell fate decisions regarding proliferation, survival, and differentiation as well as the processes of tissue regeneration and wound repair. Aberrant remodeling of the extracellular space and/or defects in properly responding to mechanical cues likely contributes to various disease states, such as fibrosis, muscle diseases, and cancer. Mechanotransduction involves the sensing and translation of mechanical forces into biochemical signals, like activation of specific genes and signaling cascades that enable cells to adapt to their physical environment. The signaling pathways involved in mechanical signaling are highly complex, but numerous studies have highlighted a central role for the Hippo pathway and other signaling networks in regulating the YAP and TAZ (YAP/TAZ) proteins to mediate the effects of mechanical stimuli on cellular behavior. How mechanical cues control YAP/TAZ has been poorly understood. However, rapid progress in the last few years is beginning to reveal a surprisingly diverse set of pathways for controlling YAP/TAZ. In this review, we will focus on how mechanical perturbations are sensed through changes in the actin cytoskeleton and mechanosensors at focal adhesions, adherens junctions, and the nuclear envelope to regulate YAP/TAZ.

Yap/Taz are required for establishing the cerebellar radial glia scaffold and proper foliation

Yap/Taz are well-established downstream effectors of the Hippo pathway, known to regulate organ size by directing proliferation and apoptosis. Although the functions of Yap/Taz have been extensively studied, little is known about their role in brain development. Here, through genetic ablation, we show that Yap/Taz are required for cerebellar morphogenesis. Yap/Taz deletion in neural progenitors causes defects in secondary fissure formation, leading to abnormal folia development. Although they seemed very likely to serve an important function in the development of cerebellar granule cell precursors, Yap/Taz are dispensable for their proliferation. Furthermore, Yap/Taz loss does not rescue the medulloblastoma phenotype caused by constitutively active Smoothened. Importantly, Yap/Taz are highly expressed in radial glia and play a crucial role in establishing the radial scaffold and cellular polarity of neural progenitors during embryogenesis. We found that Yap/Taz are necessary to establish and maintain junctional integrity of cerebellar neuroepithelium as prominent junction proteins are not maintained at the apical junction in the absence of Yap/Taz. Our study identifies a novel function of Yap/Taz in cerebellar foliation and finds that they are required to establish the radial glia scaffold and junctional stability.

TEAD4, YAP1 and WWTR1 prevent the premature onset of pluripotency prior to the 16-cell stage

In mice, pluripotent cells are thought to derive from cells buried inside the embryo around the 16-cell stage. Sox2 is the only pluripotency gene known to be expressed specifically within inside cells at this stage. To understand how pluripotency is established, we therefore investigated the mechanisms regulating the initial activation of Sox2 expression. Surprisingly, Sox2 expression initiated normally in the absence of both Nanog and Oct4 (Pou5f1), highlighting differences between embryo and stem cell models of pluripotency. However, we observed precocious ectopic expression of Sox2 prior to the 16-cell stage in the absence of Yap1, Wwtr1 and Tead4. Interestingly, the repression of premature Sox2 expression was sensitive to LATS kinase activity, even though LATS proteins normally do not limit activity of TEAD4, YAP1 and WWTR1 during these early stages. Finally, we present evidence for direct transcriptional repression of Sox2 by YAP1, WWTR1 and TEAD4. Taken together, our observations reveal that, while embryos are initially competent to express Sox2 as early as the four-cell stage, transcriptional repression prevents the premature expression of Sox2, thereby restricting the pluripotency program to the stage when inside cells are first created.

Highlighted Article: The pluripotency marker SOX2 is not initially regulated by OCT4 and NANOG, but by HIPPO pathway members during the first 2 days of mouse embryogenesis.

The Hippo Signaling Pathway in Development and Disease

The Hippo signaling pathway regulates diverse physiological processes, and its dysfunction has been implicated in an increasing number of human diseases, including cancer. Here, we provide an updated review of the Hippo pathway; discuss its roles in development, homeostasis, regeneration, and diseases; and highlight outstanding questions for future investigation and opportunities for Hippo-targeted therapies.

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.

Epiblast Formation by TEAD-YAP-Dependent Expression of Pluripotency Factors and Competitive Elimination of Unspecified Cells

The epiblast is a pluripotent cell population first formed in preimplantation embryos, and its quality is important for proper development. Here, we examined the mechanisms of epiblast formation and found that the Hippo pathway transcription factor TEAD and its coactivator YAP regulate expression of pluripotency factors. After specification of the inner cell mass, YAP accumulates in the nuclei and activates TEAD. TEAD activity is required for strong expression of pluripotency factors and is variable in the forming epiblast. Cells showing low TEAD activity are eliminated from the epiblast through cell competition. Pluripotency factor expression and MYC control cell competition downstream of TEAD activity. Cell competition eliminates unspecified cells and is required for proper organization of the epiblast. These results suggest that induction of pluripotency factors by TEAD activity and elimination of unspecified cells via cell competition ensure the production of an epiblast with naive pluripotency.

Dedifferentiation by adenovirus E1A due to inactivation of Hippo pathway effectors YAP and TAZ

In this study, Zemke et al. show that E1A inactivates the Hippo pathway-regulated TEAD coactivators YAP and TAZ by causing their sequestration in the cytoplasm. Their findings suggest that YAP/TAZ function in a developmental checkpoint controlled by signaling from the actin cytoskeleton that prevents differentiation of a progenitor cell until it is in the correct cellular and tissue environment.

Adenovirus transformed cells have a dedifferentiated phenotype. Eliminating E1A in transformed human embryonic kidney cells derepressed ∼2600 genes, generating a gene expression profile closely resembling mesenchymal stem cells (MSCs). This was associated with a dramatic change in cell morphology from one with scant cytoplasm and a globular nucleus to one with increased cytoplasm, extensive actin stress fibers, and actomyosin-dependent flattening against the substratum. E1A-induced hypoacetylation at histone H3 Lys27 and Lys18 (H3K27/18) was reversed. Most of the increase in H3K27/18ac was in enhancers near TEAD transcription factors bound by Hippo signaling-regulated coactivators YAP and TAZ. E1A causes YAP/TAZ cytoplasmic sequestration. After eliminating E1A, YAP/TAZ were transported into nuclei, where they associated with poised enhancers with DNA-bound TEAD4 and H3K4me1. This activation of YAP/TAZ required RHO family GTPase signaling and caused histone acetylation by p300/CBP, chromatin remodeling, and cohesin loading to establish MSC-associated enhancers and then superenhancers. Consistent results were also observed in primary rat embryo kidney cells, human fibroblasts, and human respiratory tract epithelial cells. These results together with earlier studies suggest that YAP/TAZ function in a developmental checkpoint controlled by signaling from the actin cytoskeleton that prevents differentiation of a progenitor cell until it is in the correct cellular and tissue environment.

Role of yes-associated protein 1, angiomotin, and mitogen-activated kinase kinase 1/2 in development of the bovine blastocyst

The morula-stage embryo is transformed into a blastocyst composed of epiblast, hypoblast, and trophectoderm (TE) through mechanisms that, in the mouse, involve the Hippo signaling and mitogen-activated kinase (MAPK) pathways. Using the cow as an additional model, we tested the hypotheses that TE and hypoblast differentiation were regulated by the Hippo pathway regulators, yes-associated protein 1 (YAP1) and angiomotin (AMOT), and MAPK kinase 1/2 (MAPK1/2). The presence of YAP1 and CDX2 in the nucleus and cytoplasm of MII oocytes and embryos was evaluated by immunofluorescence labeling. For both molecules, localization changed from cytoplasmic to nuclear as development advanced. Inhibition of YAP1 activity, either by verteporfin or a YAP1 targeting GapmeR, reduced the percent of zygotes that became blastocysts, the proportion of blastocysts that hatched and numbers of CDX2+ cells in blastocysts. Moreover, the YAP1-targeting GapmeR altered expression of 15 of 91 genes examined in the day 7.5 blastocyst. Treatment of embryos with an AMOT targeting GapmeR did not affect blastocyst development or hatching but altered expression of 16 of 91 genes examined at day 7.5 and reduced the number of CDX2+ nuclei and YAP1+ nuclei in blastocysts at day 8.5 of development. Inhibition of MAPK1/2 with PD0325901 did not affect blastocyst development but increased the number of epiblast cells. Results indicate a role for YAP1 and AMOT in function of TE in the bovine blastocyst. YAP1 can also affect function of the epiblast and hypoblast, and MAPK signaling is important for inner cell mass differentiation by reducing epiblast numbers.

Faulty oxygen sensing disrupts angiomotin function in trophoblast cell migration and predisposes to preeclampsia

Human placenta development and a successful pregnancy is incumbent upon precise oxygen-dependent control of trophoblast migration/invasion. Persistent low oxygen leading to failed trophoblast invasion promotes inadequate spiral artery remodeling, a characteristic of preeclampsia. Angiomotin (AMOT) is a multifaceted scaffolding protein involved in cell polarity and migration, yet its upstream regulation and significance in the human placenta remain unknown. Herein, we show that AMOT is primarily expressed in migratory extravillous trophoblast cells (EVTs) of the intermediate and distal anchoring column. Its expression increases after 10 weeks of gestation when oxygen tension rises and EVT migration/invasion peaks. Time-lapse imaging confirmed that the AMOT 80-kDa isoform promotes migration of trophoblastic JEG3 and HTR-8/SVneo cells. In preeclampsia, however, AMOT expression is decreased and its localization to migratory fetomaternal interface EVTs is disrupted. We demonstrate that Jumonji C domain-containing protein 6 (JMJD6), an oxygen sensor, positively regulates AMOT via oxygen-dependent lysyl hydroxylation. Furthermore, in vitro and ex vivo studies show that transforming growth factor-β (TGF-β) regulates AMOT expression, its interaction with polarity protein PAR6, and its subcellular redistribution from tight junctions to cytoskeleton. Our data reveal an oxygen- and TGF-β-driven migratory function for AMOT in the human placenta, and implicate its deficiency in impaired trophoblast migration that plagues preeclampsia.

Transitions in cell potency during early mouse development are driven by Notch

The Notch signalling pathway plays fundamental roles in diverse developmental processes in metazoans, where it is important in driving cell fate and directing differentiation of various cell types. However, we still have limited knowledge about the role of Notch in early preimplantation stages of mammalian development, or how it interacts with other signalling pathways active at these stages such as Hippo. By using genetic and pharmacological tools in vivo, together with image analysis of single embryos and pluripotent cell culture, we have found that Notch is active from the 4-cell stage. Transcriptomic analysis in single morula identified novel Notch targets, such as early naïve pluripotency markers or transcriptional repressors such as TLE4. Our results reveal a previously undescribed role for Notch in driving transitions during the gradual loss of potency that takes place in the early mouse embryo prior to the first lineage decisions.

We start life as a single cell, which immediately begins to divide to form an embryo that will eventually contain all the different kinds of cells found in the adult body. During the first few rounds of cell division, embryonic cells can become any type of adult cells, but also form the placenta, the organ that sustains the embryo while in the womb. As cells keep on dividing, they lose this ability, called potency, and they take on more specific and inflexible roles.

The first choice embryonic cells must make is whether to become part of the placenta or part of the future body. These types of decisions are controlled by molecular cascades known as signalling pathways, which relay information from the cells surface to its control centre. There, specific genes get turned on or off in response to an outside signal.

Previous research showed that two signalling pathways, Hippo and Notch, help separate placenta cells from those that will form the rest of the body. However, it was not known whether the two pathways worked independently, or if they were overlapping. Menchero et al. therefore wanted to find out when exactly the Notch pathway started to be active, and examine how it helped cells to either become the placenta or part of the future body.

Experiments with developing mouse embryos showed that the Notch pathway was activated after the very first two cell divisions, when the embryo consists of only four cells. Genetic manipulations combined with drug treatments that changed the activity of the Notch pathway confirmed that Notch and Hippo acted independently at this stage. Further, larger-scale analysis of gene activity in these embryos also revealed that Notch signalling was working in a previously unknown way: it turned off the genes that maintain potency, pushing the cells to become more specialised.

Ultimately, identifying this new mode of action for the Notch pathway in the early embryo may help to understand how the signalling cascade acts in other types of processes. This knowledge could be useful, for example, to push embryonic cells grown in the laboratory towards a desired fate.

Effects of DLC1 Deficiency on Endothelial Cell Contact Growth Inhibition and Angiosarcoma Progression

Background

Deleted in Liver Cancer 1 (DLC1) is a tumor suppressor gene frequently deleted in cancer. However, DLC1 is not known to be deleted in angiosarcoma, an aggressive malignancy of endothelial cell derivation. Additionally, the physiologic functions of DLC1 protein in endothelial cells are poorly defined.

Methods

We investigated the effects of shRNA-induced DLC1 depletion in endothelial cells. Cell growth was measured by 3H thymidine incorporation, IncuCyte imaging, and population doublings; cell death by cell cycle analysis; gene expression by Affimetrix arrays and quantitative polymerase chain reaction; NF-κB activity by reporter assays; and protein levels by immunoblotting and immunofluorescence staining. We tested Tanespimycin/17-AAG and Fasudil treatment in groups of nine to 10 mice bearing ISOS-1 angiosarcoma. All statistical tests were two-sided.

Results

We discovered that DLC1 is a critical regulator of cell contact inhibition of proliferation in endothelial cells, promoting statistically significant (P < .001) cell death when cells are confluent (mean [SD] % viability: control DLC1 = 15.6 [19.3]; shDLC1 = 73.4 [13.1]). This prosurvival phenotype of DLC1-depleted confluent endothelial cells is attributable to a statistically significant and sustained increase of NF-κB activity (day 5, P = .001; day 8, P = .03) associated with increased tumor necrosis factor alpha-induced protein 3 (TNFAIP3/A20) signaling. Consistently, we found that DLC1 is statistically significantly reduced (P < .001 in 5 of 6) and TNFAIP3/A20 is statistically significantly increased (P < .001 in 2 of 3 and P = 0.02 in 1 of 3) in human angiosarcoma compared with normal adjacent endothelium. Treatment with the NF-κB inhibitor Tanespimycin/17-AAG statistically significantly reduced angiosarcoma tumor growth in mice (treatment tumor weight vs control, 0.50 [0.19] g vs 0.91 [0.21] g, P = .001 experiment 1; 0.66 [0.26] g vs 1.10 [0.31] g, P = .01 experiment 2).

Conclusions

These results identify DLC1 as a previously unrecognized regulator of endothelial cell contact inhibition of proliferation that is depleted in angiosarcoma and support NF-κB targeting for the treatment of angiosarcoma where DLC1 is lost.

WW Domain-Containing Proteins YAP and TAZ in the Hippo Pathway as Key Regulators in Stemness Maintenance, Tissue Homeostasis, and Tumorigenesis

The Hippo pathway is a conserved signaling pathway originally defined in Drosophila melanogaster two decades ago. Deregulation of the Hippo pathway leads to significant overgrowth in phenotypes and ultimately initiation of tumorigenesis in various tissues. The major WW domain proteins in the Hippo pathway are YAP and TAZ, which regulate embryonic development, organ growth, tissue regeneration, stem cell pluripotency, and tumorigenesis. Recent reports reveal the novel roles of YAP/TAZ in establishing the precise balance of stem cell niches, promoting the production of induced pluripotent stem cells (iPSCs), and provoking signals for regeneration and cancer initiation. Activation of YAP/TAZ, for example, results in the expansion of progenitor cells, which promotes regeneration after tissue damage. YAP is highly expressed in self-renewing pluripotent stem cells. Overexpression of YAP halts stem cell differentiation and yet maintains the inherent stem cell properties. A success in reprograming iPSCs by the transfection of cells with Oct3/4, Sox2, and Yap expression constructs has recently been shown. In this review, we update the current knowledge and the latest progress in the WW domain proteins of the Hippo pathway in relevance to stem cell biology, and provide a thorough understanding in the tissue homeostasis and identification of potential targets to block tumor development. We also provide the regulatory role of tumor suppressor WWOX in the upstream of TGF-β, Hyal-2, and Wnt signaling that cross talks with the Hippo pathway.

RHOA activity in expanding blastocysts is essential to regulate HIPPO-YAP signaling and to maintain the trophectoderm-specific gene expression program in a ROCK/actin filament-independent manner

STUDY QUESTION

What molecular signals are required to maintain the functional trophectoderm (TE) during blastocyst expansion of the late stage of preimplantation development?

SUMMARY ANSWER

The activity of ras homology family member A (RHOA) GTPases is necessary to retain the expanded blastocyst cavity and also to sustain the gene expression program specific to TE.

WHAT IS KNOWN ALREADY

At the early stages of preimplantation development, the precursor of the TE lineage is generated through the molecular signals that integrate RHOA, RHO-associated coiled-coil containing protein kinase (ROCK), the apicobasal cell polarity, and the HIPPO-Yes-associated protein (YAP) signaling pathway. By contrast, molecular mechanisms regulating the maintenance of the TE characteristics at the later stage, which is crucial for blastocyst hatching and implantation, are scarcely understood.

STUDY DESIGN, SIZE, DURATION

Expanding mouse blastocysts, obtained from crosses of the F1 (C57BL6 × DBA/2) strain, were exposed to chemical agents that interfere with RHOA, ROCK, or the actin cytoskeleton for up to 8 h, and effects on the blastocyst cavity, HIPPO-YAP signaling, and cell lineage-specific gene expression profiles were examined.

PARTICIPANTS/MATERIALS, SETTING, METHODS

Mouse embryos at the embryonic stage E3.5 (expanding blastocysts) and E4.5 (fully expanded blastocysts) were treated with RHOA inhibitor (C3 exoenzyme), ROCK inhibitor (Y27632), or actin filament disruptors (cytochalasin B and latrunculin A). The integrity of the blastocyst cavity was evaluated based on the gross morphology. Effects on HIPPO-YAP signaling were assessed based on the presence of nuclearized YAP protein by immunofluorescence staining and the expression of YAP/TEA domain family member (TEAD) target genes by quantitative RT-PCR (qRT-PCR). The impact of these disruptors on cell lineages was evaluated based on expression of the TE-specific and inner cell mass-specific marker genes by qRT-PCR. The integrity of the apicobasal cell polarity was assessed by localization of protein kinase C zeta (PRKCZ; apical) and scribbled planar cell polarity (SCRIB; basal) proteins by immunofluorescence staining. For comparisons, cultured cell lines, NIH/3T3 (mouse fibroblast) and P19C5 (mouse embryonal carcinoma), were also treated with RHOA inhibitor, ROCK inhibitor, and actin filament disruptors for up to 8 h, and effects on HIPPO-YAP signaling were assessed based on expression of YAP/TEAD target genes by qRT-PCR. Each experiment was repeated using three independent batches of embryos (n = 40-80 per batch) or cell collections. Statistical analyses of data were performed, using one-way ANOVA and two-sample t-test.

MAIN RESULTS AND THE ROLE OF CHANCE

Inhibition of RHOA deflated the cavity, diminished nuclear YAP (P < 0.01), and down-regulated the YAP/TEAD target and TE-specific marker genes in both E3.5 and E4.5 blastocysts (P < 0.05), indicating that the maintenance of the key TE characteristics is dependent on RHOA activity. However, inhibition of ROCK or disruption of actin filament only deflated the blastocyst cavity, but did not alter HIPPO-YAP signaling or lineage-specific gene expressions, suggesting that the action of RHOA to sustain the TE-specific gene expression program is not mediated by ROCK or the actomyosin cytoskeleton. By contrast, ROCK inhibitor and actin filament disruptors diminished YAP/TEAD target gene expressions in cultured cells to a greater extent than RHOA inhibitor, implicating that the regulation of HIPPO-YAP signaling in expanding blastocysts is distinctly different from that in the cell lines. Furthermore, the apicobasal cell polarity proteins in the expanding blastocyst were mislocalized by ROCK inhibition but not by RHOA inhibition, indicating that cell polarity is not linked to regulation of HIPPO-YAP signaling. Taken together, our study suggests that RHOA activity is essential to maintain the TE lineage in the expanding blastocyst and it regulates HIPPO-YAP signaling and the lineage-specific gene expression program through mechanisms that are independent of ROCK or actomyosin cytoskeleton.

LARGE-SCALE DATA

Not applicable.

LIMITATIONS, REASONS FOR CAUTION

This study was conducted using one species, the mouse. Direct translation of the experiments and findings to human fertility preservation and ART requires further investigations.

WIDER IMPLICATIONS OF THE FINDINGS

The elucidation of the mechanisms of TE formation is highly pertinent to fertility preservation in women. Our findings may raise awareness among providers of ART that the TE is sensitive to disturbance even in the late stage of blastocyst expansion and that rational approaches should be devised to avoid conditions that may impair the TE and its function.

STUDY FUNDING/COMPETING INTEREST(S)

This study was funded by grants from the Ingeborg v.F. McKee Fund of the Hawaii Community Foundation (16ADVC-78882 to V.B.A.), and the National Institutes of Health (P20 GM103457 and R03 HD088839 to V.B.A.). The authors have no conflict of interest to declare.

Enhancement of connexin30.3 expression in mouse embryonic stem cell line EB3 in response to cell-cell contacts

To clarify the potential role of gap junction in cell-cell contact response, the expression of connexin30.3 gene (Cx30.3), a specifically expressed isoform in undifferentiated state of mouse embryonic stem (ES) cell line EB3 was investigated under different cell-cell contact conditions. ES cells were cultured by hanging drop culture method to increase cell-cell contact frequency. As control, a single cell culture was conducted. After culture for 12 h, the Cx30.3 expression level in hanging drop culture reached 1.73-fold that of the control (p < 0.001). By contrast, connexin43 gene (Cx43), a ubiquitously expressed gene, showed no difference between both cultures. The experiment of E-cadherin inhibition and β-catenin knockdown suggested the action of E-cadherin upstream of the Cx30.3 regulating pathway. The cell-cell contacts with different cell lines such as HeLa cells and B16/BL6 caused no effect on the Cx30.3 in ES cells. These suggest a potential role of Cx30.3 as a cell-cell contact signal mediator partially regulated by E-cadherin signaling.

Atypical protein kinase C isoforms differentially regulate directional keratinocyte migration during wound healing

BACKGROUND

The epidermis possesses regenerative properties that become apparent only after wounding. Atypical protein kinase C (aPKC) isoforms aPKCζ and aPKCλ form a ternary complex with Par3 and Par6, and play crucial roles in establishing and maintaining epithelial cell polarity. The epidermal loss of aPKCλ results in progressive depletion of hair follicle stem cells. However, it is unclear whether aPKCs have equivalent activities in epidermal regeneration.

OBJECTIVES

To clarify functional differences between aPKCζ and aPKCλ in cutaneous wound healing.

METHODS

We compared cutaneous wound healing processes in vivo using mutant mice with genetic deletion of each aPKC isoform. We also analyzed functional differences between aPKCζ and aPKCλ in cell proliferation, directional cell migration, and formation of microtubules in vitro using primary keratinocytes established from each mutant mouse.

RESULTS

Wound healing was significantly retarded in epidermis-specific aPKCλ knockout mice. In aPKCλ-deleted keratinocytes, the correct orientation of cell protrusions toward the wound was disrupted through the destabilization of Par6β. The elongation of stabilized β-tubulin was also deteriorated in aPKCλ-deleted keratinocytes, leading to defects in cell spreading. Conversely, wound healing and directional cell migration in aPKCζ-deleted mice were comparable to those in their control littermates.

CONCLUSIONS

aPKCs are not functionally equivalent; aPKCλ, but not aPKCζ, plays a primary role in cutaneous wound healing.

Visualizing HIPPO Signaling Components in Mouse Early Embryonic Development

The HIPPO signaling pathway plays an early and essential role in mammalian embryogenesis. The earliest known roles for HIPPO signaling during mouse development include segregating fetal and extraembryonic lineages and establishing the pluripotent progenitors of embryonic stem (ES) cells. In the mouse early embryo, HIPPO signaling responds to multiple cell biological inputs, including cell polarization, cytoskeleton, and cell environment, to influence gene expression and the first cell fate decisions in development. Methods to monitor and manipulate HIPPO signaling in the mouse early embryo are fundamental to discovering mechanisms regulating pluripotency in vivo, but properties of the early embryo, such as small cell number and spherical architecture, pose unique challenges for signaling pathway analysis. Here, we share approaches for visualizing HIPPO signaling in mouse early embryos. In addition, these methods can be applied to visualize HIPPO signaling in other spherical or cystic structures comprised of relatively few cells, such as organoids, or for the examination of other signaling pathways in these contexts.

Using the Predicted Structure of the Amot Coiled Coil Homology Domain to Understand Lipid Binding

Angiomotins (Amots) are a family of adapter proteins that modulate cellular polarity, differentiation, proliferation, and migration. Amot family members have a characteristic lipid-binding domain, the coiled coil homology (ACCH) domain that selectively targets the protein to membranes, which has been directly linked to its regulatory role in the cell. Several spot blot assays were used to validate the regions of the domain that participate in its membrane association, deformation, and vesicle fusion activity, which indicated the need for a structure to define the mechanism. Therefore, we sought to understand the structure-function relationship of this domain in order to find ways to modulate these signaling pathways. After many failed attempts to crystallize the ACCH domain of each Amot family member for structural analysis, we decided to pursue homologous models that could be refined using small angle x-ray scattering data. Theoretical models were produced using the homology software SWISS-MODEL and threading software I-TASSER and LOMETS, followed by comparison to SAXS data for model selection and refinement. We present a theoretical model of the domain that is driven by alpha helices and short random coil regions. These alpha helical regions form a classic dimer interface followed by two wide spread legs that we predict to be the lipid binding interface.

HIPPO signaling resolves embryonic cell fate conflicts during establishment of pluripotency in vivo

During mammalian development, the challenge for the embryo is to override intrinsic cellular plasticity to drive cells to distinct fates. Here, we unveil novel roles for the HIPPO signaling pathway in controlling cell positioning and expression of Sox2, the first marker of pluripotency in the mouse early embryo. We show that maternal and zygotic YAP1 and WWTR1 repress Sox2 while promoting expression of the trophectoderm gene Cdx2 in parallel. Yet, Sox2 is more sensitive than Cdx2 to Yap1/Wwtr1 dosage, leading cells to a state of conflicted cell fate when YAP1/WWTR1 activity is moderate. Remarkably, HIPPO signaling activity resolves conflicted cell fate by repositioning cells to the interior of the embryo, independent of its role in regulating Sox2 expression. Rather, HIPPO antagonizes apical localization of Par complex components PARD6B and aPKC. Thus, negative feedback between HIPPO and Par complex components ensure robust lineage segregation.

As an embryo develops, its cells divide, grow and migrate in specific patterns to build an organized collection of cells that go on to form our tissues and organs. One of the first steps – well before the embryo has implanted into the womb – is to allocate cells to make part of the placenta.

Once this process is complete, the remaining cells continue building the organism. These cells are pluripotent, meaning they can develop into any part of the body. Scientists think that the embryo manages to sort ‘placenta cells’ from pluripotent ones with the help of certain proteins, which the mother has packaged into her eggs.

To investigate this further, Frum et al. used genetic tools to track a specific gene called Sox2 that identifies pluripotent cells as soon as they are formed in mouse embryos. The experiments revealed that the mother places two closely related proteins known as YAP1 and WWTR1 within each egg, which help to make placenta cells different from pluripotent cells. Moreover, both proteins enable the embryo to segregate these two cell types to two different locations: placenta cells are moved to the outer layer of the embryo, while pluripotent cells are moved to the inside.

Current technologies allow researchers to create pluripotent cells in the laboratory. But these approaches often result in error, failing to replicate the embryo’s natural ability. By studying how embryos form and arrange pluripotent cells, scientists hope to advance stem cell technology (which emerge from pluripotent cells). This may help to find new ways to heal damaged tissues and organs, or to treat or even prevent many diseases.

The Hippo Signaling Network and Its Biological Functions

Hippo signaling is an evolutionarily conserved network that has a central role in regulating cell proliferation and cell fate to control organ growth and regeneration. It promotes activation of the LATS kinases, which control gene expression by inhibiting the activity of the transcriptional coactivator proteins YAP and TAZ in mammals and Yorkie in Drosophila. Diverse upstream inputs, including both biochemical cues and biomechanical cues, regulate Hippo signaling and enable it to have a key role as a sensor of cells' physical environment and an integrator of growth control signals. Several components of this pathway localize to cell-cell junctions and contribute to regulation of Hippo signaling by cell polarity, cell contacts, and the cytoskeleton. Downregulation of Hippo signaling promotes uncontrolled cell proliferation, impairs differentiation, and is associated with cancer. We review the current understanding of Hippo signaling and highlight progress in the elucidation of its regulatory mechanisms and biological functions.

The Hippo pathway effector Taz is required for cell morphogenesis and fertilization in zebrafish

Hippo signaling is a critical pathway that integrates extrinsic and intrinsic mechanical cues to regulate organ size. Despite its essential role in organogenesis, little is known about its role in cell fate specification and differentiation. Here, we unravel a novel and unexpected role of the Hippo pathway effector Taz (wwtr1) in controlling the size, shape and fate of a unique cell in the zebrafish ovary. We show that wwtr1 mutant females are infertile. In teleosts, fertilization occurs through the micropyle, a funnel-like opening in the chorion, formed by a unique enlarged follicle cell, the micropylar cell (MC). We describe here, for the first time, the mechanism that underlies the differentiation of the MC. Our genetic analyses show that Taz is essential for MC fate acquisition and subsequent micropyle formation in zebrafish. We identify Taz as the first bona fide MC marker and show that Taz is specifically and strongly enriched in the MC precursor. Altogether, we performed the first genetic and molecular characterization of the MC and propose that Taz is a key regulator of MC fate.This article has an associated 'The people behind the papers' interview.