Evolution and Cell Physiology. 4. Why invent yet another protein complex to build junctions in epithelial cells?

The zonula adherens matura redefines the apical junction of intestinal epithelia

Cell-cell apical junctions of epithelia consist of multiprotein complexes that organize as belts regulating cell-cell adhesion, permeability, and mechanical tension: the tight junction (zonula occludens), the zonula adherens (ZA), and the macula adherens. The prevailing dogma is that at the ZA, E-cadherin and catenins are lined with F-actin bundles that support and transmit mechanical tension between cells. Using super-resolution microscopy on human intestinal biopsies and Caco-2 cells, we show that two distinct multiprotein belts are basal of the tight junctions as the intestinal epithelia mature. The most apical is populated with nectins/afadin and lined with F-actin; the second is populated with E-cad/catenins. We name this dual-belt architecture the zonula adherens matura. We find that the apical contraction apparatus and the dual-belt organization rely on afadin expression. Our study provides a revised description of epithelial cell-cell junctions and identifies a module regulating the mechanics of epithelia.

Cell polarity signalling at the birth of multicellularity: What can we learn from the first animals

The innovation of multicellularity has driven the unparalleled evolution of animals (Metazoa). But how is a multicellular organism formed and how is its architecture maintained faithfully? The defining properties and rules required for the establishment of the architecture of multicellular organisms include the development of adhesive cell interactions, orientation of division axis, and the ability to reposition daughter cells over long distances. Central to all these properties is the ability to generate asymmetry (polarity), coordinated by a highly conserved set of proteins known as cell polarity regulators. The cell polarity complexes, Scribble, Par and Crumbs, are considered to be a metazoan innovation with apicobasal polarity and adherens junctions both believed to be present in all animals. A better understanding of the fundamental mechanisms regulating cell polarity and tissue architecture should provide key insights into the development and regeneration of all animals including humans. Here we review what is currently known about cell polarity and its control in the most basal metazoans, and how these first examples of multicellular life can inform us about the core mechanisms of tissue organisation and repair, and ultimately diseases of tissue organisation, such as cancer.

Evolution of mechanisms controlling epithelial morphogenesis across animals: new insights from dissociation-reaggregation experiments in the sponge Oscarella lobularis

BACKGROUND

The ancestral presence of epithelia in Metazoa is no longer debated. Porifera seem to be one of the best candidates to be the sister group to all other Metazoa. This makes them a key taxon to explore cell-adhesion evolution on animals. For this reason, several transcriptomic, genomic, histological, physiological and biochemical studies focused on sponge epithelia. Nevertheless, the complete and precise protein composition of cell-cell junctions and mechanisms that regulate epithelial morphogenetic processes still remain at the center of attention.

RESULTS

To get insights into the early evolution of epithelial morphogenesis, we focused on morphogenic characteristics of the homoscleromorph sponge Oscarella lobularis. Homoscleromorpha are a sponge class with a typical basement membrane and adhaerens-like junctions unknown in other sponge classes. We took advantage of the dynamic context provided by cell dissociation-reaggregation experiments to explore morphogenetic processes in epithelial cells in a non-bilaterian lineage by combining fluorescent and electron microscopy observations and RNA sequencing approaches at key time-points of the dissociation and reaggregation processes.

CONCLUSIONS

Our results show that part of the molecular toolkit involved in the loss and restoration of epithelial features such as cell-cell and cell-matrix adhesion is conserved between Homoscleromorpha and Bilateria, suggesting their common role in the last common ancestor of animals. In addition, sponge-specific genes are differently expressed during the dissociation and reaggregation processes, calling for future functional characterization of these genes.

A putative stem-loop structure in Drosophila crumbs is required for mRNA localisation in epithelia and germline cells

Crumbs (Crb) is an evolutionarily conserved transmembrane protein localised to the apical membrane of epithelial cells. Loss or mislocalisation of Crb is often associated with disruption of apicobasal cell polarity. crb mRNA is also apically enriched in epithelial cells, and, as shown here, accumulates in the oocyte of developing egg chambers. We narrowed down the localisation element (LE) of crb mRNA to 47 nucleotides, which form a putative stem-loop structure that may be recognised by Egalitarian (Egl). Mutations in conserved nucleotides abrogate apical transport. crb mRNA enrichment in the oocyte is affected in egl mutant egg chambers. A CRISPR-based genomic deletion of the crb locus that includes the LE disrupts asymmetric crb mRNA localisation in epithelia and prevents its accumulation in the oocyte during early stages of oogenesis, but does not affect Crb protein localisation in embryonic and follicular epithelia. However, flies lacking the LE show ectopic Crb protein expression in the nurse cells. These data suggest an additional role for the Drosophila 3'-UTR in regulating translation in a tissue-specific manner.This article has an associated First Person interview with the first author of the paper.

Microridges are apical epithelial projections formed of F-actin networks that organize the glycan layer

Apical projections are integral functional units of epithelial cells. Microvilli and stereocilia are cylindrical apical projections that are formed of bundled actin. Microridges on the other hand, extend laterally, forming labyrinthine patterns on surfaces of various kinds of squamous epithelial cells. So far, the structural organization and functions of microridges have remained elusive. We have analyzed microridges on zebrafish epidermal cells using confocal and electron microscopy methods including electron tomography, to show that microridges are formed of F-actin networks and require the function of the Arp2/3 complex for their maintenance. During development, microridges begin as F-actin punctae showing signatures of branching and requiring an active Arp2/3 complex. Using inhibitors of actin polymerization and the Arp2/3 complex, we show that microridges organize the surface glycan layer. Our analyses have unraveled the F-actin organization supporting the most abundant and evolutionarily conserved apical projection, which functions in glycan organization.

New genomic data and analyses challenge the traditional vision of animal epithelium evolution

Background

The emergence of epithelia was the foundation of metazoan expansion. Epithelial tissues are a hallmark of metazoans deeply rooted in the evolution of their complex developmental morphogenesis processes. However, studies on the epithelial features of non-bilaterians are still sparse and it remains unclear whether the last common metazoan ancestor possessed a fully functional epithelial toolkit or if it was acquired later during metazoan evolution.

Results

To investigate the early evolution of animal epithelia, we sequenced the genome and transcriptomes of two new sponge species to characterize epithelial markers such as the E-cadherin complex and the polarity complexes for all classes (Calcarea, Demospongiae, Hexactinellida, Homoscleromorpha) of sponges (phylum Porifera) and compare them with their homologues in Placozoa and in Ctenophora. We found that Placozoa and most sponges possess orthologues of all essential genes encoding proteins characteristic of bilaterian epithelial cells, as well as their conserved interaction domains. In stark contrast, we found that ctenophores lack several major polarity complex components such as the Crumbs complex and Scribble. Furthermore, the E-cadherin ctenophore orthologue exhibits a divergent cytoplasmic domain making it unlikely to interact with its canonical cytoplasmic partners.

Conclusions

These unexpected findings challenge the current evolutionary paradigm on the emergence of epithelia. Altogether, our results raise doubt on the homology of protein complexes and structures involved in cell polarity and adhesive-type junctions between Ctenophora and Bilateria epithelia.

Electronic supplementary material

The online version of this article (10.1186/s12864-018-4715-9) contains supplementary material, which is available to authorized users.

A dual role of the extracellular domain of Drosophila Crumbs for morphogenesis of the embryonic neuroectoderm

Epithelia are highly polarised tissues and several highly conserved polarity protein complexes serve to establish and maintain polarity. The transmembrane protein Crumbs (Crb), the central component of the Crb protein complex, is required, among others, for the maintenance of polarity in most epithelia in the Drosophila embryo. However, different epithelia exhibit different phenotypic severity upon loss of crb. Using a transgenomic approach allowed us to more accurately define the role of crb in different epithelia. In particular, we provide evidence that the loss of epithelial tissue integrity in the ventral epidermis of crb mutant embryos is due to impaired actomyosin activity and an excess number of neuroblasts. We demonstrate that the intracellular domain of Crb could only partially rescue this phenotype, while it is able to completely restore tissue integrity in other epithelia. Based on these results we suggest a dual role of the extracellular domain of Crb in the ventral neuroectoderm. First, it is required for apical enrichment of the Crb protein, which in turn regulates actomyosin activity and thereby ensures tissue integrity; and second, the extracellular domain of Crb stabilises the Notch receptor and thereby ensures proper Notch signalling and specification of the correct number of neuroblasts.

Summary: Using a transgenomic approach we determine specific roles of the intra- and extracellular domain of the Crumbs protein for the maintenance of apico-basal epithelial polarity and epithelial morphogenesis in Drosophila embryos.

Crumbs, Moesin and Yurt regulate junctional stability and dynamics for a proper morphogenesis of the Drosophila pupal wing epithelium

The Crumbs (Crb) complex is a key epithelial determinant. To understand its role in morphogenesis, we examined its function in the Drosophila pupal wing, an epithelium undergoing hexagonal packing and formation of planar-oriented hairs. Crb distribution is dynamic, being stabilized to the subapical region just before hair formation. Lack of crb or stardust, but not DPatj, affects hexagonal packing and delays hair formation, without impairing epithelial polarities but with increased fluctuations in cell junctions and perimeter length, fragmentation of adherens junctions and the actomyosin cytoskeleton. Crb interacts with Moesin and Yurt, FERM proteins regulating the actomyosin network. We found that Moesin and Yurt distribution at the subapical region depends on Crb. In contrast to previous reports, yurt, but not moesin, mutants phenocopy crb junctional defects. Moreover, while unaffected in crb mutants, cell perimeter increases in yurt mutant cells and decreases in the absence of moesin function. Our data suggest that Crb coordinates proper hexagonal packing and hair formation, by modulating junction integrity via Yurt and stabilizing cell perimeter via both Yurt and Moesin. The Drosophila pupal wing thus appears as a useful system to investigate the functional diversification of the Crb complex during morphogenesis, independently of its role in polarity.

Role of the Crumbs proteins in ciliogenesis, cell migration and actin organization

Epithelial cell organization relies on a set of proteins that interact in an intricate way and which are called polarity complexes. These complexes are involved in the determination of the apico-basal axis and in the positioning and stability of the cell-cell junctions called adherens junctions at the apico-lateral border in invertebrates. Among the polarity complexes, two are present at the apical side of epithelial cells. These are the Par complex including aPKC, PAR3 and PAR6 and the Crumbs complex including, CRUMBS, PALS1 and PATJ/MUPP1. These two complexes interact directly and in addition to their already well described functions, they play a role in other cellular processes such as ciliogenesis and polarized cell migration. In this review, we will focus on these aspects that involve the apical Crumbs polarity complex and its relation with the cortical actin cytoskeleton which might provide a more comprehensive hypothesis to explain the many facets of Crumbs cell and tissue properties.

Paracellular transport as a strategy for energy conservation by multicellular organisms?

Paracellular transport of solutes and water accompanies transcellular transport across epithelial barriers and together they serve to maintain internal body composition. However, whether paracellular transport is necessary and why it evolved is unknown. In this commentary I discuss our recent studies to address this question in the proximal tubule of the kidney. Paracellular reabsorption of sodium occurs in the proximal tubule and is mediated by claudin-2. However, deletion of claudin-2 in mice does not affect whole kidney sodium excretion because it can be completely compensated by downtream transcellular transport mechanisms. This occurs at the expense of increased oxygen consumption, tissue hypoxia and increased susceptibility to ischemic injury. It is concluded that paracellular transport acts as an energy saving mechanism to increase transport without consuming additional oxygen. It is speculated that this might be why paracellular transport evolved in leaky epithelia with high transport needs.

The Crumbs_C isoform of Drosophila shows tissue- and stage-specific expression and prevents light-dependent retinal degeneration

Drosophila Crumbs (Crb) is a key regulator of epithelial polarity and fulfils a plethora of other functions, such as growth regulation, morphogenesis of photoreceptor cells and prevention of retinal degeneration. This raises the question how a single gene regulates such diverse functions, which in mammals are controlled by three different paralogs. Here, we show that in Drosophila different Crb protein isoforms are differentially expressed as a result of alternative splicing. All isoforms are transmembrane proteins that differ by just one EGF-like repeat in their extracellular portion. Unlike Crb_A, which is expressed in most embryonic epithelia from early stages onward, Crb_C is expressed later and only in a subset of embryonic epithelia. Flies specifically lacking Crb_C are homozygous viable and fertile. Strikingly, these flies undergo light-dependent photoreceptor degeneration despite the fact that the other isoforms are expressed and properly localised at the stalk membrane. This allele now provides an ideal possibility to further unravel the molecular mechanisms by which Drosophila crb protects photoreceptor cells from the detrimental consequences of light-induced cell stress.

Summary: Loss of Crb_C, one protein isoform encoded by Drosophila crumbs, results in light-dependent retinal degeneration, but does not affect any of the other crumbs-specific functions.

Crumbs 3b promotes tight junctions in an ezrin-dependent manner in mammalian cells

Crumbs 3 (CRB3) is a component of epithelial junctions, which has been implicated in apical-basal polarity, apical identity, apical stability, cell adhesion, and cell growth. CRB3 undergoes alternative splicing to yield two variants: CRB3a and CRB3b. Here, we describe novel data demonstrating that, as with previous studies on CRB3a, CRB3b also promotes the formation of tight junctions (TJs). However, significantly we demonstrate that the 4.1-ezrin-radixin-moesin-binding motif of CRB3b is required for CRB3b functionality and that ezrin binds to the FBM of CRB3b. Furthermore, we show that ezrin contributes to CRB3b functionality and the correct distribution of TJ proteins. We demonstrate that both CRB3 isoforms are required for the production of functionally mature TJs and also the localization of ezrin to the plasma membrane. Finally, we demonstrate that reduced CRB3b expression in head and neck squamous cell carcinoma (HNSCC) correlates with cytoplasmic ezrin, a biomarker for aggressive disease, and shows evidence that while CRB3a expression has no effect, low CRB3b and high cytoplasmic ezrin expression combined may be prognostic for HNSCC.

CRB2 completes a fully expressed Crumbs complex in the Retinal Pigment Epithelium

The CRB proteins CRB1, CRB2 and CRB3 are members of the cell polarity complex Crumbs in mammals that together with Scribble and Par complexes stablish the polarity of a variety of cell types. Although many members of the Crumbs complex proteins are expressed in the retinal pigment epithelium (RPE), and even though the mRNA of CRB2 has been detected in ARPE-19 cells and in the RPE/Choroid, to date no CRB protein has yet been found in this tissue. To investigate this possibility, we generated an antibody that specifically recognize the mouse CRB2 protein, and we demonstrate the expression of CRB2 in mouse RPE. Confocal analysis shows that CRB2 is restricted to the apicolateral membrane of RPE cells, and more precisely, in the tight junctions. Our study identified CRB2 as the member of the CRB protein family that is present together with the rest of the components of the Crumbs complex in the RPE apico-lateral cell membrane. Considering that the functions of CRB proteins are decisive in the establishment and maintenance of cell-cell junctions in several epithelial-derived cell types, we believe that these findings are a relevant starting point for unraveling the functions that CRB2 might perform in the RPE.