A functional role of the extracellular domain of Crumbs in cell architecture and apicobasal polarity

The dynamics of tubulogenesis in development and disease

Tubes are crucial for the function of many organs in animals given their fundamental roles in transporting and exchanging substances to maintain homeostasis within an organism. Therefore, the development and maintenance of these tube-like structures within organs is a vital process. Tubes can form in diverse ways, and advances in our understanding of the molecular and cellular mechanisms underpinning these different modes of tubulogenesis have significant impacts in many biological contexts, including development and disease. This Review discusses recent progress in understanding developmental mechanisms underlying tube formation.

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.

How cells tell up from down and stick together to construct multicellular tissues - interplay between apicobasal polarity and cell-cell adhesion

Polarized epithelia define a topological inside and outside, and hence constitute a key evolutionary innovation that enabled the construction of complex multicellular animal life. Over time, this basic function has been elaborated upon to yield the complex architectures of many of the organs that make up the human body. The two processes necessary to yield a polarized epithelium, namely regulated adhesion between cells and the definition of the apicobasal (top-bottom) axis, have likewise undergone extensive evolutionary elaboration, resulting in multiple sophisticated protein complexes that contribute to both functions. Understanding how these components function in combination to yield the basic architecture of a polarized cell-cell junction remains a major challenge. In this Review, we introduce the main components of apicobasal polarity and cell-cell adhesion complexes, and outline what is known about their regulation and assembly in epithelia. In addition, we highlight studies that investigate the interdependence between these two networks. We conclude with an overview of strategies to address the largest and arguably most fundamental unresolved question in the field, namely how a polarized junction arises as the sum of its molecular parts.

Crumbs and the apical spectrin cytoskeleton regulate R8 cell fate in the Drosophila eye

The Hippo pathway is an important regulator of organ growth and cell fate. In the R8 photoreceptor cells of the Drosophila melanogaster eye, the Hippo pathway controls the fate choice between one of two subtypes that express either the blue light-sensitive Rhodopsin 5 (Hippo inactive R8 subtype) or the green light-sensitive Rhodopsin 6 (Hippo active R8 subtype). The degree to which the mechanism of Hippo signal transduction and the proteins that mediate it are conserved in organ growth and R8 cell fate choice is currently unclear. Here, we identify Crumbs and the apical spectrin cytoskeleton as regulators of R8 cell fate. By contrast, other proteins that influence Hippo-dependent organ growth, such as the basolateral spectrin cytoskeleton and Ajuba, are dispensable for the R8 cell fate choice. Surprisingly, Crumbs promotes the Rhodopsin 5 cell fate, which is driven by Yorkie, rather than the Rhodopsin 6 cell fate, which is driven by Warts and the Hippo pathway, which contrasts with its impact on Hippo activity in organ growth. Furthermore, neither the apical spectrin cytoskeleton nor Crumbs appear to regulate the Hippo pathway through mechanisms that have been observed in growing organs. Together, these results show that only a subset of Hippo pathway proteins regulate the R8 binary cell fate decision and that aspects of Hippo signalling differ between growing organs and post-mitotic R8 cells.

Zebrafish Crb1, Localizing Uniquely to the Cell Membranes around Cone Photoreceptor Axonemes, Alleviates Light Damage to Photoreceptors and Modulates Cones' Light Responsiveness

The crumbs (crb) apical polarity genes are essential for the development and functions of epithelia. Adult zebrafish retinal neuroepithelium expresses three crb genes (crb1, crb2a, and crb2b); however, it is unknown whether and how Crb1 differs from other Crb proteins in expression, localization, and functions. Here, we show that, unlike zebrafish Crb2a and Crb2b as well as mammalian Crb1 and Crb2, zebrafish Crb1 does not localize to the subapical regions of photoreceptors and Müller glial cells; rather, it localizes to a small region of cone outer segments: the cell membranes surrounding the axonemes. Moreover, zebrafish Crb1 is not required for retinal morphogenesis and photoreceptor patterning. Interestingly, Crb1 promotes rod survival under strong white light irradiation in a previously unreported non--cell-autonomous fashion; in addition, Crb1 delays UV and blue cones' chromatin condensation caused by UV light irradiation. Finally, Crb1 plays a role in cones' responsiveness to light through an arrestin-translocation-independent mechanism. The localization of Crb1 and its functions do not differ between male and female fish. We conclude that zebrafish Crb1 has diverged from other vertebrate Crb proteins, representing a neofunctionalization in Crb biology during evolution.SIGNIFICANCE STATEMENT Apicobasal polarity of epithelia is an important property that underlies the morphogenesis and functions of epithelial tissues. Epithelial apicobasal polarity is controlled by many polarity genes, including the crb genes. In vertebrates, multiple crb genes have been identified, but the differences in their expression patterns and functions are not fully understood. Here, we report a novel subcellular localization of zebrafish Crb1 in retinal cone photoreceptors and evidence for its new functions in photoreceptor maintenance and light responsiveness. This study expands our understanding of the biology of the crb genes in epithelia, including retinal neuroepithelium.

The cytoskeletal motor proteins Dynein and MyoV direct apical transport of Crumbs

Crumbs (Crb in Drosophila; CRB1-3 in mammals) is a transmembrane determinant of epithelial cell polarity and a regulator of Hippo signalling. Crb is normally localized to apical cell-cell contacts, just above adherens junctions, but how apical trafficking of Crb is regulated in epithelial cells remains unclear. We use the Drosophila follicular epithelium to demonstrate that polarized trafficking of Crb is mediated by transport along microtubules by the motor protein Dynein and along actin filaments by the motor protein Myosin-V (MyoV). Blocking transport of Crb-containing vesicles by Dynein or MyoV leads to accumulation of Crb within Rab11 endosomes, rather than apical delivery. The final steps of Crb delivery and stabilisation at the plasma membrane requires the exocyst complex and three apical FERM domain proteins - Merlin, Moesin and Expanded - whose simultaneous loss disrupts apical localization of Crb. Accordingly, a knock-in deletion of the Crb FERM-binding motif (FBM) also impairs apical localization. Finally, overexpression of Crb challenges this system, creating a sensitized background to identify components involved in cytoskeletal polarization, apical membrane trafficking and stabilisation of Crb at the apical domain.

Crumbs2 mediates ventricular layer remodelling to form the spinal cord central canal

In the spinal cord, the central canal forms through a poorly understood process termed dorsal collapse that involves attrition and remodelling of pseudostratified ventricular layer (VL) cells. Here, we use mouse and chick models to show that dorsal ventricular layer (dVL) cells adjacent to dorsal midline Nestin(+) radial glia (dmNes+RG) down-regulate apical polarity proteins, including Crumbs2 (CRB2) and delaminate in a stepwise manner; live imaging shows that as one cell delaminates, the next cell ratchets up, the dmNes+RG endfoot ratchets down, and the process repeats. We show that dmNes+RG secrete a factor that promotes loss of cell polarity and delamination. This activity is mimicked by a secreted variant of Crumbs2 (CRB2S) which is specifically expressed by dmNes+RG. In cultured MDCK cells, CRB2S associates with apical membranes and decreases cell cohesion. Analysis of Crb2F/F/Nestin-Cre+/- mice, and targeted reduction of Crb2/CRB2S in slice cultures reveal essential roles for transmembrane CRB2 (CRB2TM) and CRB2S on VL cells and dmNes+RG, respectively. We propose a model in which a CRB2S-CRB2TM interaction promotes the progressive attrition of the dVL without loss of overall VL integrity. This novel mechanism may operate more widely to promote orderly progenitor delamination.

The extracellular and intracellular regions of Crb2a play distinct roles in guiding the formation of the apical zonula adherens

The transmembrane protein Crumbs (Crb), a key regulator of apical polarity, has a known involvement in establishment of the apical zonula adherens in epithelia, although the precise mechanism remains elusive. The zonula adherens are required to maintain the integrity and orderly arrangement of epithelia. Loss of the zonula adherens leads to morphogenetic defects in the tissues derived from epithelium. In this study, we revealed that the intracellular tail of Crb2a promoted the apical distribution of adherens junctions (AJs) in zebrafish retinal and lens epithelia, but caused assembly into unstable punctum adherens-like adhesion plaques. The extracellular region of Crb2a guided the transformation of AJs from the punctum adherens into stable zonula adherens. Accordingly, a truncated form of Crb2a lacking the extracellular region (Crb2a^ΔEX) could only partially rescue the retinal patterning defects in crb2a null mutant zebrafish (crb2a^m289). By contrast, constitutive over-expression of Crb2a^ΔEX disrupted the integrity of the outer limiting membrane in photoreceptors, which is derived from the zonula adherens of the retinal neuroepithelium. This study demonstrated that both the extracellular region and the intracellular tail of Crb2a are required to guide the formation of the apical zonula adherens.

Targeted deletion of Crb1/Crb2 in the optic vesicle models key features of leber congenital amaurosis 8

The Crb1 and 2 (Crumbs homolog 1 & 2) genes encode large, single-pass transmembrane proteins essential for the apicobasal polarity and adhesion of epithelial cells. Crb1 mutations cause degenerative retinal diseases in humans, including Leber congenital amaurosis type 8 (LCA8) and retinitis pigmentosa type 12 (RP12). In LCA8, impaired photoreceptor development and/or survival is thought to cause blindness during early infancy, whereas, in RP12, progressive photoreceptor degeneration damages peripheral vision later in life. There are multiple animal models of RP12 pathology, but no experimental model of LCA8 recapitulates the full spectrum of its pathological features. To generate a mouse model of LCA8 and identify the functions of Crb1/2 in developing ocular tissues, we used an mRx-Cre driver to generate allelic combinations that enabled conditional gene ablation from the optic vesicle stage. In this series only Crb1/2 double knockout (dKO) mice exhibited characteristics of human LCA8 disease: locally thickened retina with spots devoid of cells, aberrant positioning of retinal cells, severely disrupted lamination, and depigmented retinal-pigmented epithelium. Retinal defects antedated E12.5, which is far earlier than the stage at which photoreceptor cells mainly differentiate. Most remarkably, Crb1/Crb2 dKO showed a severely attenuated electroretinogram at the eye opening stage. These results suggest that human LCA8 can be modeled in the mouse by simultaneously ablating Crb1/2 from the beginning of eye development. Importantly, they also indicate that LCA8 is caused by malfunction of retinal progenitor cells during early ocular development rather than by defective photoreceptor-Muller glial interaction, a mechanism proposed for RP12.

The apical protein Apnoia interacts with Crumbs to regulate tracheal growth and inflation

Most organs of multicellular organisms are built from epithelial tubes. To exert their functions, tubes rely on apico-basal polarity, on junctions, which form a barrier to separate the inside from the outside, and on a proper lumen, required for gas or liquid transport. Here we identify apnoia (apn), a novel Drosophila gene required for tracheal tube elongation and lumen stability at larval stages. Larvae lacking Apn show abnormal tracheal inflation and twisted airway tubes, but no obvious defects in early steps of tracheal maturation. apn encodes a transmembrane protein, primarily expressed in the tracheae, which exerts its function by controlling the localization of Crumbs (Crb), an evolutionarily conserved apical determinant. Apn physically interacts with Crb to control its localization and maintenance at the apical membrane of developing airways. In apn mutant tracheal cells, Crb fails to localize apically and is trapped in retromer-positive vesicles. Consistent with the role of Crb in apical membrane growth, RNAi-mediated knockdown of Crb results in decreased apical surface growth of tracheal cells and impaired axial elongation of the dorsal trunk. We conclude that Apn is a novel regulator of tracheal tube expansion in larval tracheae, the function of which is mediated by Crb.

The transmembrane protein Crb2a regulates cardiomyocyte apicobasal polarity and adhesion in zebrafish

Tissue morphogenesis requires changes in cell-cell adhesion as well as in cell shape and polarity. Cardiac trabeculation is a morphogenetic process essential to form a functional ventricular wall. Here we show that zebrafish hearts lacking Crb2a, a component of the Crumbs polarity complex, display compact wall integrity defects and fail to form trabeculae. Crb2a localization is very dynamic at a time when other cardiomyocyte junctional proteins also relocalize. Before the initiation of cardiomyocyte delamination to form the trabecular layer, Crb2a is expressed in all ventricular cardiomyocytes and colocalizes with the junctional protein ZO-1. Subsequently, Crb2a becomes localized all along the apical membrane of compact layer cardiomyocytes and is downregulated in the delaminating cardiomyocytes. We show that blood flow and Nrg/ErbB2 signaling regulate Crb2a localization dynamics. crb2a−/− display a multilayered wall with polarized cardiomyocytes, a unique phenotype. Our data further indicate that Crb2a regulates cardiac trabeculation by controlling the localization of tight and adherens junction proteins in cardiomyocytes. Importantly, transplantation data show that Crb2a controls CM behavior in a cell-autonomous manner in the sense that crb2a−/− cardiomyocytes transplanted into wild-type animals were always found in the trabecular layer. Altogether, our study reveals a critical role for Crb2a during cardiac development.

PAR-Complex and Crumbs Function During Photoreceptor Morphogenesis and Retinal Degeneration

The fly photoreceptor has long been used as a model to study sensory neuron morphogenesis and retinal degeneration. In particular, elucidating how these cells are built continues to help further our understanding of the mechanisms of polarized cell morphogenesis, intracellular trafficking and the causes of human retinal pathologies. The conserved PAR complex, which in flies consists of Cdc42-PAR6-aPKC-Bazooka, and the transmembrane protein Crumbs (Crb) are key players during photoreceptor morphogenesis. While the PAR complex regulates polarity in many cell types, Crb function in polarity is relatively specific to epithelial cells. Together Cdc42-PAR6-aPKC-Bazooka and Crb orchestrate the differentiation of the photoreceptor apical membrane (AM) and zonula adherens (ZA), thus allowing these cells to assemble into a neuro-epithelial lattice. In addition to its function in epithelial polarity, Crb has also been shown to protect fly photoreceptors from light-induced degeneration, a process linked to Rhodopsin expression and trafficking. Remarkably, mutations in the human Crumbs1 (CRB1) gene lead to retinal degeneration, making the fly photoreceptor a powerful disease model system.

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.

Distinct functions of Crumbs regulating slit diaphragms and endocytosis in Drosophila nephrocytes

Mammalian podocytes, the key determinants of the kidney's filtration barrier, differentiate from columnar epithelial cells and several key determinants of apical-basal polarity in the conventional epithelia have been shown to regulate podocyte morphogenesis and function. However, little is known about the role of Crumbs, a conserved polarity regulator in many epithelia, for slit-diaphragm formation and podocyte function. In this study, we used Drosophila nephrocytes as model system for mammalian podocytes and identified a conserved function of Crumbs proteins for cellular morphogenesis, nephrocyte diaphragm assembly/maintenance, and endocytosis. Nephrocyte-specific knock-down of Crumbs results in disturbed nephrocyte diaphragm assembly/maintenance and decreased endocytosis, which can be rescued by Drosophila Crumbs as well as human Crumbs2 and Crumbs3, which were both expressed in human podocytes. In contrast to the extracellular domain, which facilitates nephrocyte diaphragm assembly/maintenance, the intracellular FERM-interaction motif of Crumbs is essential for regulating endocytosis. Moreover, Moesin, which binds to the FERM-binding domain of Crumbs, is essential for efficient endocytosis. Thus, we describe here a new mechanism of nephrocyte development and function, which is likely to be conserved in mammalian podocytes.

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.

Drosophila Crumbs prevents ectopic Notch activation in developing wings by inhibiting ligand-independent endocytosis

Many signalling components are apically restricted in epithelial cells, and receptor localisation and abundance is key for morphogenesis and tissue homeostasis. Hence, controlling apicobasal epithelial polarity is crucial for proper signalling. Notch is a ubiquitously expressed, apically localised receptor, which performs a plethora of functions; therefore, its activity has to be tightly regulated. Here, we show that Drosophila Crumbs, an evolutionarily conserved polarity determinant, prevents Notch endocytosis in developing wings through direct interaction between the two proteins. Notch endocytosis in the absence of Crumbs results in the activation of the ligand-independent, Deltex-dependent Notch signalling pathway, and does not require the ligands Delta and Serrate or γ-secretase activity. This function of Crumbs is not due to general defects in apicobasal polarity, as localisation of other apical proteins is unaffected. Our data reveal a mechanism to explain how Crumbs directly controls localisation and trafficking of the potent Notch receptor, and adds yet another aspect of Crumbs regulation in Notch pathway activity. Furthermore, our data highlight a close link between the apical determinant Crumbs, receptor trafficking and tissue homeostasis.

Crumbs2 promotes cell ingression during the epithelial-to-mesenchymal transition at gastrulation

During gastrulation of the mouse embryo, individual cells ingress in an apparently stochastic pattern during the epithelial-to-mesenchymal transition (EMT). Here we define a critical role of the apical protein Crumbs2 (CRB2) in the gastrulation EMT. Static and live imaging show that ingressing cells in Crumbs2 mutant embryos become trapped at the primitive streak, where they continue to express the epiblast transcription factor SOX2 and retain thin E-cadherin-containing connections to the epiblast surface that trap them at the streak. CRB2 is distributed in a complex anisotropic pattern on apical cell edges, and the level of CRB2 on a cell edge is inversely correlated with the level of myosin IIB. The data suggest that the distributions of CRB2 and myosin IIB define which cells will ingress, and we propose that cells with high apical CRB2 are basally extruded from the epiblast by neighbouring cells with high levels of apical myosin.

Domain-specific functions of Stardust in Drosophila embryonic development

In Drosophila, the adaptor protein Stardust is essential for the stabilization of the polarity determinant Crumbs in various epithelial tissues, including the embryonic epidermis, the follicular epithelium and photoreceptor cells of the compound eye. In turn, Stardust recruits another adaptor protein, PATJ, to the subapical region to support adherens junction formation and morphogenetic events. Moreover, Stardust binds to Lin-7, which is dispensable in epithelial cells but functions in postsynaptic vesicle fusion. Finally, Stardust has been reported to bind directly to PAR-6, thereby linking the Crumbs-Stardust-PATJ complex to the PAR-6/aPKC complex. PAR-6 and aPKC are also capable of directly binding Bazooka (the Drosophila homologue of PAR-3) to form the PAR/aPKC complex, which is essential for apical-basal polarity and cell-cell contact formation in most epithelia. However, little is known about the physiological relevance of these interactions in the embryonic epidermis of Drosophila in vivo. Thus, we performed a structure-function analysis of the annotated domains with GFP-tagged Stardust and evaluated the localization and function of the mutant proteins in epithelial cells of the embryonic epidermis. The data presented here confirm a crucial role of the PDZ domain in binding Crumbs and recruiting the protein to the subapical region. However, the isolated PDZ domain is not capable of being recruited to the cortex, and the SH3 domain is essential to support the binding to Crumbs. Notably, the conserved N-terminal regions (ECR1 and ECR2) are not crucial for epithelial polarity. Finally, the GUK domain plays an important role for the protein's function, which is not directly linked to Crumbs stabilization, and the L27N domain is essential for epithelial polarization independently of recruiting PATJ.

Ebi modulates wing growth by ubiquitin-dependent downregulation of Crumbs in Drosophila

Notch signaling at the dorsoventral (DV) boundary is essential for patterning and growth of wings in Drosophila The WD40 domain protein Ebi has been implicated in the regulation of Notch signaling at the DV boundary. Here we show that Ebi regulates wing growth by antagonizing the function of the transmembrane protein Crumbs (Crb). Ebi physically binds to the extracellular domain of Crb (Crb^ext), and this interaction is specifically mediated by WD40 repeats 7-8 of Ebi and a laminin G domain of Crb^ext Wing notching resulting from reduced levels of Ebi is suppressed by decreasing the Crb function. Consistent with this antagonistic genetic relationship, Ebi knockdown in the DV boundary elevates the Crb protein level. Furthermore, we show that Ebi is required for downregulation of Crb by ubiquitylation. Taken together, we propose that the interplay of Crb expression in the DV boundary and ubiquitin-dependent Crb downregulation by Ebi provides a mechanism for the maintenance of Notch signaling during wing development.

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.

The Ski2-family helicase Obelus regulates Crumbs alternative splicing and cell polarity

The conserved Ski2-family helicase Obelus regulates alternative splicing of the Crumbs polarity protein to control epithelial polarity and junctional organization in Drosophila.

Alternative splicing can have profound consequences for protein activity, but the functions of most alternative splicing regulators are not known. We show that Obelus, a conserved Ski2-family helicase, is required for cell polarity and adherens junction organization in the Drosophila melanogaster embryo. In obelus mutants, epithelial cells display an expanded apical domain, aggregation of adherens junctions at the cell membrane, and microtubule-dependent defects in centrosome positioning. Through whole-genome transcriptome analysis, we found that Obelus is required for the alternative splicing of a small number of transcripts in the early embryo, including the pre-mRNA that encodes the apical polarity protein Crumbs. In obelus mutants, inclusion of an alternative exon results in increased expression of a Crumbs isoform that contains an additional epidermal growth factor–like repeat in the extracellular domain. Overexpression of this alternative Crumbs isoform recapitulates the junctional aggregation and centrosome positioning defects of obelus mutants. These results indicate that regulation of Crumbs alternative splicing by the Obelus helicase modulates epithelial polarity during development.

The Hippo signalling pathway maintains quiescence in Drosophila neural stem cells

Stem cells control their mitotic activity to decide whether to proliferate or to stay in quiescence. Drosophila neural stem cells (NSCs) are quiescent at early larval stages, when they are reactivated in response to metabolic changes. Here we report that cell-contact inhibition of growth through the canonical Hippo signalling pathway maintains NSC quiescence. Loss of the core kinases hippo or warts leads to premature nuclear localization of the transcriptional co-activator Yorkie and initiation of growth and proliferation in NSCs. Yorkie is necessary and sufficient for NSC reactivation, growth and proliferation. The Hippo pathway activity is modulated via inter-cellular transmembrane proteins Crumbs and Echinoid that are both expressed in a nutrient-dependent way in niche glial cells and NSCs. Loss of crumbs or echinoid in the niche only is sufficient to reactivate NSCs. Finally, we provide evidence that the Hippo pathway activity discriminates quiescent from non-quiescent NSCs in the Drosophila nervous system.

Drosophila neural stem cells (NSCs) are quiescent at early larval stages but how this is regulated is unclear. Here, Ding et al. show that quiescence of NSCs is mediated by cell-contact inhibition via the Hippo pathway transmembrane proteins Crumbs and Echinoid, which in turn are regulated by nutrient levels.

The apical complex protein Pals1 is required to maintain cerebellar progenitor cells in a proliferative state

Through their biased localization and function within the cell, polarity complex proteins are necessary to establish the cellular asymmetry required for tissue organization. Well-characterized germinal zones, mitogenic signals and cell types make the cerebellum an excellent model for addressing the crucial function of polarity complex proteins in the generation and organization of neural tissues. Deletion of the apical polarity complex protein Pals1 in the developing cerebellum results in a remarkably undersized cerebellum with disrupted layers in poorly formed folia and strikingly reduced granule cell production. We demonstrate that Pals1 is not only essential for cerebellum organogenesis, but also for preventing premature differentiation and thus maintaining progenitor pools in cerebellar germinal zones, including cerebellar granule neuron precursors in the external granule layer. In the Pals1 mouse mutants, the expression of genes that regulate the cell cycle was diminished, correlating with the loss of the proliferating cell population of germinal zones. Furthermore, enhanced Shh signaling through activated Smo cannot overcome impaired cerebellar cell generation, arguing for an epistatic role of Pals1 in proliferation capacity. Our study identifies Pals1 as a novel intrinsic factor that regulates the generation of cerebellar cells and Pals1 deficiency as a potential inhibitor of overactive mitogenic signaling.

AP-2-complex-mediated endocytosis of Drosophila Crumbs regulates polarity by antagonizing Stardust

Maintenance of epithelial polarity depends on the correct localization and levels of polarity determinants. The evolutionarily conserved transmembrane protein Crumbs is crucial for the size and identity of the apical membrane, yet little is known about the molecular mechanisms controlling the amount of Crumbs at the surface. Here, we show that Crumbs levels on the apical membrane depend on a well-balanced state of endocytosis and stabilization. The adaptor protein 2 (AP-2) complex binds to a motif in the cytoplasmic tail of Crumbs that overlaps with the binding site of Stardust, a protein known to stabilize Crumbs on the surface. Preventing endocytosis by mutation of AP-2 causes expansion of the Crumbs-positive plasma membrane domain and polarity defects, which can be partially rescued by removing one copy of crumbs. Strikingly, knocking down both AP-2 and Stardust leads to the retention of Crumbs on the membrane. This study provides evidence for a molecular mechanism, based on stabilization and endocytosis, to adjust surface levels of Crumbs, which are essential for maintaining epithelial polarity.

The transmembrane protein Crumbs displays complex dynamics during follicular morphogenesis and is regulated competitively by Moesin and aPKC

The transmembrane protein Crumbs (Crb) functions in apical polarity and epithelial integrity. To better understand its role in epithelial morphogenesis, we examined Crb localization and dynamics in the late follicular epithelium of Drosophila. Crb was unexpectedly dynamic during middle-to-late stages of egg chamber development, being lost from the marginal zone (MZ) in stage 9 before abruptly returning at the end of stage 10b, then undergoing a pulse of endocytosis in stage 12. The reappearance of MZ Crb is necessary to maintain an intact adherens junction and MZ. Although Crb has been proposed to interact through its juxtamembrane domain with Moesin (Moe), a FERM domain protein that regulates the cortical actin cytoskeleton, the functional significance of this interaction is poorly understood. We found that whereas the Crb juxtamembrane domain was not required for adherens junction integrity, it was necessary for MZ localization of Moe, aPKC and F-actin. Furthermore, Moe and aPKC functioned antagonistically, suggesting that Moe limits Crb levels by reducing its interactions with the apical Par network. Additionally, Moe mutant cells lost Crb from the apical membrane and accumulated excess Crb at the MZ, suggesting that Moe regulates Crb distribution at the membrane. Together, these studies reveal reciprocal interactions between Crb, Moe and aPKC during cellular morphogenesis.

Crumbs promotes expanded recognition and degradation by the SCF(Slimb/β-TrCP) ubiquitin ligase

In epithelial tissues, growth control depends on the maintenance of proper architecture through apicobasal polarity and cell-cell contacts. The Hippo signaling pathway has been proposed to sense tissue architecture and cell density via an intimate coupling with the polarity and cell contact machineries. The apical polarity protein Crumbs (Crb) controls the activity of Yorkie (Yki)/Yes-activated protein, the progrowth target of the Hippo pathway core kinase cassette, both in flies and mammals. The apically localized Four-point-one, Ezrin, Radixin, Moesin domain protein Expanded (Ex) regulates Yki by promoting activation of the kinase cascade and by directly tethering Yki to the plasma membrane. Crb interacts with Ex and promotes its apical localization, thereby linking cell polarity with Hippo signaling. We show that, as well as repressing Yki by recruiting Ex to the apical membrane, Crb promotes phosphorylation-dependent ubiquitin-mediated degradation of Ex. We identify Skp/Cullin/F-box(Slimb/β-transducin repeats-containing protein) (SCF(Slimb/β-TrCP)) as the E3 ubiquitin ligase complex responsible for Ex degradation. Thus, Crb is part of a homeostatic mechanism that promotes Ex inhibition of Yki, but also limits Ex activity by inducing its degradation, allowing precise tuning of Yki function.

Apical localisation of crumbs in the boundary cells of the Drosophila hindgut is independent of its canonical interaction partner stardust

The transmembrane protein Crumbs/Crb is a key regulator of apico-basal epithelial cell polarity, both in Drosophila and in vertebrates. In most cases studied so far, the apical localisation of Drosophila Crumbs depends on the interaction of its C-terminal amino acids with the scaffolding protein Stardust. Consequently, embryos lacking either Crumbs or Stardust develop a very similar phenotype, characterised by the loss of epithelial tissue integrity and cell polarity in many epithelia. An exception is the hindgut, which is not affected by the loss of either gene. The hindgut is a single layered epithelial tube composed of two cell populations, the boundary cells and the principal cells. Here we show that Crumbs localisation in the principal cells depends on Stardust, similarly to other embryonic epithelia. In contrast, localisation of Crumbs in the boundary cells does not require Stardust and is independent of its PDZ domain- and FERM-domain binding motifs. In line with this, the considerable upregulation of Crumbs in boundary cells is not followed by a corresponding upregulation of its canonical binding partners. Our data are the first to suggest a mechanism controlling apical Crumbs localisation, which is independent of its conserved FERM- and PDZ-domain binding motifs.

Controlling cell shape changes during salivary gland tube formation in Drosophila

Any type of tubulogenesis is a process that is highly coordinated between large numbers of cells. Like other morphogenetic processes, it is driven to a great extent by complex cell shape changes and cell rearrangements. The formation of the salivary glands in the fly embryo provides an ideal model system to study these changes and rearrangements, because upon specification of the cells that are destined to form the tube, there is no further cell division or cell death. Thus, morphogenesis of the salivary gland tubes is entirely driven by cell shape changes and rearrangements. In this review, we will discuss and distill from the literature what is known about the control of cell shape during the early invagination process and whilst the tubes extend in the fly embryo at later stages.

Sticking together the Crumbs - an unexpected function for an old friend

Cell polarity and cell-cell junctions have pivotal roles in organizing cells into tissues and in mediating cell-cell communication. The transmembrane protein Crumbs has a well-established role in the maintenance of epithelial polarity, and it can also regulate signalling via the Notch and Hippo pathways to influence tissue growth. The functions of Crumbs in epithelial polarity and Hippo-mediated growth depend on its short intracellular domain. Recent evidence now points to a conserved and fundamental role for the extracellular domain of Crumbs in mediating homophilic Crumbs-Crumbs interactions at cell-cell junctions.