Multiple domains of Stardust differentially mediate localisation of the Crumbs-Stardust complex during photoreceptor development in Drosophila

Drosophila Stardust (Sdt), a member of the MAGUK family of scaffolding proteins, is a constituent of the evolutionarily conserved Crumbs-Stardust (Crb-Sdt) complex that controls epithelial cell polarity in the embryo and morphogenesis of photoreceptor cells. Although apical localisation is a hallmark of the complex in all cell types and in all organisms analysed, only little is known about how individual components are targeted to the apical membrane. We have performed a structure-function analysis of Sdt by constructing transgenic flies that express altered forms of Sdt to determine the roles of individual domains for localisation and function in photoreceptor cells. The results corroborate the observation that the organisation of the Crb-Sdt complex is differentially regulated in pupal and adult photoreceptors. In pupal photoreceptors, only the PDZ domain of Sdt – the binding site of Crb – is required for apical targeting. In adult photoreceptors, by contrast, targeting of Sdt to the stalk membrane, a distinct compartment of the apical membrane between the rhabdomere and the zonula adherens, depends on several domains, and seems to be a two-step process. The N-terminus, including the two ECR domains and a divergent N-terminal L27 domain that binds the multi-PDZ domain protein PATJ in vitro, is necessary for targeting the protein to the apical pole of the cell. The PDZ-, the SH3- and the GUK-domains are required to restrict the protein to the stalk membrane. Drosophila PATJ or Drosophila Lin-7 are stabilised whenever a Sdt variant that contains the respective binding site is present, independently of where the variant is localised. By contrast, only full-length Sdt, confined to the stalk membrane, stabilises and localises Crb, although only in reduced amounts. The amount of Crumbs recruited to the stalk membrane correlates with its length. Our results highlight the importance of the different Sdt domains and point to a more intricate regulation of the Crb-Sdt complex in adult photoreceptor cells.

On the role of the MAGUK proteins encoded by Drosophila varicose during embryonic and postembryonic development

BACKGROUND

Membrane-associated guanylate kinases (MAGUKs) form a family of scaffolding proteins, which are often associated with cellular junctions, such as the vertebrate tight junction, the Drosophila septate junction or the neuromuscular junction. Their capacity to serve as platforms for organising larger protein assemblies results from the presence of several protein-protein interaction domains. They often appear in different variants suggesting that they also mediate dynamic changes in the composition of the complexes.

RESULTS

Here we show by electron microscopic analysis that Drosophila embryos lacking varicose function fail to develop septate junctions in the tracheae and the epidermis. In the embryo and in imaginal discs varicose expresses two protein isoforms, which belong to the MAGUK family. The two isoforms can be distinguished by the presence or absence of two L27 domains and are differentially affected in different varicose alleles. While the short isoform is essential for viability, the long isoform seems to have a supportive function. Varicose proteins co-localise with Neurexin IV in pleated septate junctions and are necessary, but not sufficient for its recruitment. The two proteins interact in vitro by the PDZ domain of Varicose and the four C-terminal amino acids of Neurexin IV. Postembryonic reduction of varicose function by expressing double-stranded RNA affects pattern formation and morphogenesis of the wing and the development of normal-shaped and -sized eyes.

CONCLUSION

Expression of two Varicose isoforms in embryonic epithelia and imaginal discs suggests that the composition of Varicose-mediated protein scaffolds at septate junctions is dynamic, which may have important implications for the modulation of their function.

Drosophila Lin-7 is a component of the Crumbs complex in epithelia and photoreceptor cells and prevents light-induced retinal degeneration

The Drosophila Crumbs protein complex is required to maintain epithelial cell polarity in the embryo, to ensure proper morphogenesis of photoreceptor cells and to prevent light-dependent retinal degeneration. In Drosophila, the core components of the complex are the transmembrane protein Crumbs, the membrane-associated guanylate kinase (MAGUK) Stardust and the scaffolding protein DPATJ. The composition of the complex and some of its functions are conserved in mammalian epithelial and photoreceptor cells. Here, we report that Drosophila Lin-7, a scaffolding protein with one Lin-2/Lin-7 (L27) domain and one PSD-95/Dlg/ZO-1 (PDZ) domain, is associated with the Crumbs complex in the subapical region of embryonic and follicle epithelia and at the stalk membrane of adult photoreceptor cells. DLin-7 loss-of-function mutants are viable and fertile. While DLin-7 localization depends on Crumbs, neither Crumbs, Stardust nor DPATJ require DLin-7 for proper accumulation in the subapical region. Unlike other components of the Crumbs complex, DLin-7 is also enriched in the first optic ganglion, the lamina, where it co-localizes with Discs large, another member of the MAGUK family. In contrast to crumbs mutant photoreceptor cells, those mutant for DLin-7 do not display any morphogenetic abnormalities. Similar to crumbs mutant eyes, however, DLin-7 mutant photoreceptors undergo progressive, light-dependent degeneration. These results support the previous conclusions that the function of the Crumbs complex in cell survival is independent from its function in photoreceptor morphogenesis.

The use of P-element transposons to generate transgenic flies

The development of a technique to stably integrate exogenous DNA into the germline of Drosophila melanogaster marked a milestone in the ability to study gene function in the fly. On the molecular level germline transformation mainly relies on a particular transposable element, the D. melanogaster P-element. Based on certain features of the P-element, vectors have been designed for diverse applications like gene disruption, chromosome engineering, gene tagging, and inducible gene expression/repression. Despite the fact that an increasing number of other transposons have been utilized for germline transformation of Drosophila most transformation vectors are still P-element based. Technically, microinjection serves as the method of choice to physically introduce transgenes into preblastoderm Drosophila embryos. Besides an appropriate technical equipment including suitable microcapillaries in conjunction with a micromanipulator, a microinjector, and a microscope, proper handling of the Drosophila embryos before and after microinjection is the key step to the generation of transgenic flies. Pioneer work in Drosophila also served as a general guideline for the transformation of other insect species including those with medical and agricultural importance.

Unraveling the genetic complexity of Drosophila stardust during photoreceptor morphogenesis and prevention of light-induced degeneration

Drosophila Stardust, a membrane-associated guanylate kinase (MAGUK), recruits the transmembrane protein Crumbs and the cytoplasmic proteins DPATJ and DLin-7 into an apically localized protein scaffold. This evolutionarily conserved complex is required for epithelial cell polarity in Drosophila embryos and mammalian cells in culture. In addition, mutations in Drosophila crumbs and DPATJ impair morphogenesis of photoreceptor cells (PRCs) and result in light-dependent retinal degeneration. Here we show that stardust is a genetically complex locus. While all alleles tested perturb epithelial cell polarity in the embryo, only a subset of them affects morphogenesis of PRCs or induces light-dependent retinal degeneration. Alleles retaining particular postembryonic functions still express some Stardust protein in pupal and/or adult eyes. The phenotypic complexity is reflected by the expression of distinct splice variants at different developmental stages. All proteins expressed in the retina contain the PSD95, Discs Large, ZO-1 (PDZ), Src homology 3 (SH3), and guanylate kinase (GUK) domain, but lack a large region in the N terminus encoded by one exon. These results suggest that Stardust-based protein scaffolds are dynamic, which is not only mediated by multiple interaction partners, but in addition by various forms of the Stardust protein itself.

Cell polarity from cell division

Apical-basal polarity of epithelial cells is critical for their symmetric versus asymmetric division and commonly thought to be established in interphase. In a novel type of cell division termed "mirror-symmetric", apical cell constituents accumulate during M-phase at the cleavage furrow, resulting in epithelial daughter cells with opposite apical-basal polarity.

Coordinated control of cell adhesion, polarity, and cytoskeleton underlies Hox-induced organogenesis in Drosophila

BACKGROUND

Hox genes control animal body plans by directing the morphogenesis of segment-specific structures. As transcription factors, HOX proteins achieve this through the activation of downstream target genes. Much research has been devoted to the search for these targets and the characterization of their roles in organogenesis. This has shown that the direct targets of Hox activation are often transcription factors or signaling molecules, which form hierarchical genetic networks directing the morphogenesis of particular organs. Importantly, very few of the direct Hox targets known are "realizator" genes involved directly in the cellular processes of organogenesis.

RESULTS

Here, we describe for the first time a complete network linking the Hox gene Abdominal-B to the realizator genes it controls during the organogenesis of the external respiratory organ of the larva. In this process, Abdominal-B induces the expression of four intermediate signaling molecules and transcription factors, and this expression results in the mosaic activation of several realizator genes. The ABD-B spiracle realizators include at least five cell-adhesion proteins, cell-polarity proteins, and GAP and GEF cytoskeleton regulators. Simultaneous ectopic expression of the Abd-B downstream targets can induce spiracle-like structure formation in the absence of ABD-B protein.

CONCLUSION

Hox realizators include cytoskeletal regulators and molecules required for the apico-basal cell organization. HOX-coordinated activation of these realizators in mosaic patterns confers to the organ primordium its assembling properties. We propose that during animal development, Hox-controlled genetic cascades coordinate the local cell-specific behaviors that result in organogenesis of segment-specific structures.

Isoform-specific interaction of Flamingo/Starry Night with excess Bazooka affects planar cell polarity in theDrosophila wing

Epithelia display two types of polarity, apical-basal and planar cell polarity (PCP), and both are crucial for morphogenesis and organogenesis. PCP signaling pathways comprise transmembrane proteins, such as Flamingo/Starry Night, and cytoplasmic, membrane-associated proteins such as Dishevelled. During establishment of PCP in the Drosophila wing, PCP proteins accumulate apically in distinct "cortical domains" on proximal and distal plasma membranes. This finding suggests that their localized function depends on prior definition of apicobasal polarity. Here, we show that overexpression of Bazooka, a PDZ-domain protein essential for apicobasal polarity in the embryo, perturbs development of PCP, but has no effect on apicobasal polarity. The PCP phenotype is associated with a failure to restrict Flamingo/Starry night to the proximal and distal plasma membranes of the wing epithelium. We further demonstrate that flamingo expresses two differentially spliced RNAs in wing imaginal discs, which encode two isoforms of the atypical cadherin Flamingo. The predominant Starry night-type form contains a PDZ-binding motif, which mediates binding to Bazooka in vitro. Pull-down assays support the occurrence of such an interaction in wing imaginal discs. The results suggest that interaction between the apicobasal and planar cell polarity systems has to be tightly coordinated to ensure proper morphogenesis of the wing disc epithelium.

Towards understanding CRUMBS function in retinal dystrophies

Mutations in the Crumbs homologue 1 (CRB1) gene cause autosomal recessive retinitis pigmentosa (arRP) and autosomal Leber congenital amaurosis (arLCA). The crumbs (crb) gene was originally identified in Drosophila and encodes a large transmembrane protein required for maintenance of apico-basal cell polarity and adherens junction in embryonic epithelia. Human CRB1 and its two paralogues, CRB2 and CRB3, are highly conserved throughout the animal kingdom. Both in Drosophila and in vertebrates, the short intracellular domain of Crb/CRB organizes an evolutionary conserved protein scaffold. Several lines of evidence, obtained both in Drosophila and in mouse, show that loss-of-function of crb/CRB1 or some of its intracellular interactors lead to morphological defects and light-induced degeneration of photoreceptor cells, features comparable to those observed in patients lacking CRB1 function. In this review, we describe how understanding Crb complex function in fly and vertebrate retina enhances our knowledge of basic cell biological processes and might lead to new therapeutic approaches for patients affected with retinal dystrophies caused by mutations in the CRB1 gene.

Computer modelling in combination with in vitro studies reveals similar binding affinities of Drosophila Crumbs for the PDZ domains of Stardust and DmPar-6

Formation of multiprotein complexes is a common theme to pattern a cell, thereby generating spatially and functionally distinct entities at specialised regions. Central components of these complexes are scaffold proteins, which contain several protein-protein interaction domains and provide a platform to recruit a variety of additional components. There is increasing evidence that protein complexes are dynamic structures and that their components can undergo various interactions depending on the cellular context. However, little is known so far about the factors regulating this behaviour. One evolutionarily conserved protein complex, which can be found both in Drosophila and mammalian epithelial cells, is composed of the transmembrane protein Crumbs/Crb3 and the scaffolding proteins Stardust/Pals1 and DPATJ/PATJ, respectively, and localises apically to the zonula adherens. Here we show by in vitro analysis that, similar as in vertebrates, the single PDZ domain of Drosophila DmPar-6 can bind to the four C-terminal amino acids (ERLI) of the transmembrane protein Crumbs. To further evaluate the binding capability of Crumbs to DmPar-6 and the MAGUK protein Stardust, analysis of the PDZ structural database and modelling of the interactions between the C-terminus of Crumbs and the PDZ domains of these two proteins were performed. The results suggest that both PDZ domains bind Crumbs with similar affinities. These data are supported by quantitative yeast two-hybrid interactions. In vivo analysis performed in cell cultures and in the Drosophila embryo show that the cytoplasmic domain of Crumbs can recruit DmPar-6 and DaPKC to the plasma membrane. The data presented here are discussed with respect to possible dynamic interactions between these proteins.

DPATJ plays a role in retinal morphogenesis and protects against light-dependent degeneration of photoreceptor cells in theDrosophila eye

The establishment of apicobasal polarity in epithelial cells is a prerequisite for their function. Drosophila photoreceptor cells derive from epithelial cells, and their apical membranes undergo elaborate differentiation during pupal development, forming photosensitive rhabdomeres and associated stalk membranes. Crumbs (Crb), a transmembrane protein involved in the maintenance of epithelial polarity in the embryo, defines the stalk as a subdomain of the apical membrane. Crb organizes a complex composed of several PDZ domain-containing proteins, including DPATJ (formerly known as Discs lost). Taking advantage of a DPATJ mutant line in which only a truncated form of the protein is synthesized, we demonstrate that DPATJ is necessary for the stability of the Crb complex at the stalk membrane and is crucial for stalk membrane development and rhabdomere maintenance during late pupal stages. Moreover, DPATJ protects against light-induced photoreceptor degeneration.

Are dendrites in Drosophila homologous to vertebrate dendrites?

Dendrites represent arborising neurites in both vertebrates and invertebrates. However, in vertebrates, dendrites develop on neuronal cell bodies, whereas in higher invertebrates, they arise from very different neuronal structures, the primary neurites, which also form the axons. Is this anatomical difference paralleled by principal developmental and/or physiological differences? We address this question by focussing on one cellular model, motorneurons of Drosophila and characterise the compartmentalisation of these cells. We find that motorneuronal dendrites of Drosophila share with typical vertebrate dendrites that they lack presynaptic but harbour postsynaptic proteins, display calcium elevation upon excitation, have distinct cytoskeletal features, develop later than axons and are preceded by restricted localisation of Par6-complex proteins. Furthermore, we demonstrate in situ and culture that Drosophila dendrites can be shifted from the primary neurite to their soma, i.e. into vertebrate-like positions. Integrating these different lines of argumentation, we propose that dendrites in vertebrates and higher invertebrates have a common origin, and differences in dendrite location can be explained through translocation of neuronal cell bodies introduced during the evolutionary process by which arthropods and vertebrates diverged from a common urbilaterian ancestor. Implications of these findings for studies of dendrite development, neuronal polarity, transport and evolution are discussed.

IrreC/rst-mediated cell sorting duringDrosophilapupal eye development depends on proper localisation of DE-cadherin

Remodelling of tissues depends on the coordinated regulation of multiple cellular processes, such as cell-cell communication, differential cell adhesion and programmed cell death. During pupal development, interommatidial cells (IOCs) of the Drosophila eye initially form two or three cell rows between individual ommatidia, but then rearrange into a single row of cells. The surplus cells are eliminated by programmed cell death, and the definitive hexagonal array of cells is formed, which is the basis for the regular pattern of ommatidia visible in the adult eye. Here, we show that this cell-sorting process depends on the presence of a continuous belt of the homophilic cell adhesion protein DE-cadherin at the apical end of the IOCs. Elimination of this adhesion belt by mutations in shotgun, which encodes DE-cadherin, or its disruption by overexpression of DE-cadherin, the intracellular domain of Crumbs, or by a dominant version of the monomeric GTPase Rho1 prevents localisation of the transmembrane protein IrreC-rst to the border between primary pigment cells and IOCs. As a consequence, the IOCs are not properly sorted and supernumerary cells survive. During the sorting process, Notch-mediated signalling in IOCs acts downstream of DE-cadherin to restrict IrreC-rst to this border. The data are discussed in relation to the roles of selective cell adhesion and cell signalling during tissue reorganisation.

Cell type-specific recruitment of Drosophila Lin-7 to distinct MAGUK-based protein complexes defines novel roles for Sdt and Dlg-S97

Stardust (Sdt) and Discs-Large (Dlg) are membrane-associated guanylate kinases (MAGUKs) involved in the organization of supramolecular protein complexes at distinct epithelial membrane compartments in Drosophila. Loss of either Sdt or Dlg affects epithelial development with severe effects on apico-basal polarity. Moreover, Dlg is required for the structural and functional integrity of synaptic junctions. Recent biochemical and cell culture studies have revealed that various mammalian MAGUKs can interact with mLin-7/Veli/MALS, a small PDZ-domain protein. To substantiate these findings for their in vivo significance with regard to Sdt- and Dlg-based protein complexes, we analyzed the subcellular distribution of Drosophila Lin-7 (DLin-7) and performed genetic and biochemical assays to characterize its interaction with either of the two MAGUKs. In epithelia, Sdt mediates the recruitment of DLin-7 to the subapical region, while at larval neuromuscular junctions, a particular isoform of Dlg, Dlg-S97, is required for postsynaptic localization of DLin-7. Ectopic expression of Dlg-S97 in epithelia, however, was not sufficient to induce a redistribution of DLin-7. These results imply that the recruitment of DLin-7 to MAGUK-based protein complexes is defined by cell-type specific mechanisms and that DLin-7 acts downstream of Sdt in epithelia and downstream of Dlg at synapses.

Composition and formation of intercellular junctions in epithelial cells

The polarized nature of epithelial cells is manifested by the nonrandom partitioning of organelles within the cells, the concentration of intercellular junctions at one pole, and the asymmetric distribution of proteins and lipids within the plasma membrane. These features allow epithelia to fulfill their specific tasks, such as targeted uptake and secretion of molecules and the segregation of different tissue compartments. The accessibility of Drosophila melanogaster and Caenorhabditis elegans to genetic and cell biological analyses, combined with the study of mammalian cells in culture, provides an ideal basis for understanding the mechanisms that control the establishment and maintenance of epithelial cell polarity and tissue integrity. Here, we focus on some of the best-studied junctions and membrane-associated protein complexes and their relation to cell polarity. Comparisons between fly, worm, and vertebrate epithelia reveal marked similarities with respect to the molecules used, and pronounced differences in the organization of the junctions themselves.

Drosophila crumbs is required to inhibit light-induced photoreceptor degeneration

Mutations in the human transmembrane protein CRB1 are associated with severe forms of retinal dystrophy, retinitis pigmentosa 12 (RP12), and Leber's congenital amaurosis (LCA). The Drosophila homolog, crumbs, is required for polarity and adhesion in embryonic epithelia and for correct formation of adherens junctions and proper morphogenesis of photoreceptor cells. Here, we show that mutations in Drosophila crumbs result in progressive, light-induced retinal degeneration. Degeneration is prevented by expression of p35, an inhibitor of apoptosis, or by reduction of rhodopsin levels through a vitamin A-deficient diet. In the dark, rhabdomeres survive but exhibit morphogenetic defects. We demonstrate that it is the extracellular portion of the Crumbs protein that is essential to suppress light-induced programmed cell death, while proper morphogenesis depends on the intracellular part. We conclude that human and Drosophila Crumbs proteins are functionally conserved to prevent light-dependent photoreceptor degeneration. This experimental system is now ideally suited to study the genetic and molecular basis of RP12- and LCA-related retinal degeneration.

Regulation of epithelial cell shape and polarity by cell-cell adhesion (Review)

Among all cell types that exhibit a polarized phenotype, epithelial cells are unique in that their polarity depends on the integration of the cell into a tissue, the epithelium. In recent years, the analysis of epithelial cell polarity in different epithelia and organisms has contributed to an understanding of the components involved and has further demonstrated that cell polarity and cell adhesion are intimately related to each other. Therefore, processes that mediate and modulate cell adhesion and coordinate adhesion and cell shape are fundamental for the function of epithelia. Recent results obtained in Drosophila melanogaster and Caenorhabditis elegans have provided further insight into the complex circuits regulating these processes, and have laid the direction for future analysis.

Drosophila Stardust is a partner of Crumbs in the control of epithelial cell polarity

The polarized architecture of epithelial cells depends on the highly stereotypic distribution of cellular junctions and other membrane-associated protein complexes. In epithelial cells of the Drosophila embryo, three distinct domains subdivide the lateral plasma membrane. The most apical one comprises the subapical complex (SAC). It is followed by the zonula adherens (ZA) and, further basally, by the septate junction. A core component of the SAC is the transmembrane protein Crumbs, the cytoplasmic domain of which recruits the PDZ-protein Discs Lost into the complex. Cells lacking crumbs or the functionally related gene stardust fail to organize a continuous ZA and to maintain cell polarity. Here we show that stardust provides an essential component of the SAC. Stardust proteins colocalize with Crumbs and bind to the carboxy-terminal amino acids of its cytoplasmic tail. We introduce two different Stardust proteins here: one MAGUK protein, characterized by a PDZ domain, an SH3 domain and a guanylate kinase domain; and a second isoform comprising only the guanylate kinase domain. The Stardust proteins represent versatile candidates as structural and possibly regulatory constituents of the SAC, a crucial element in the control of epithelial cell polarity.

CRB1 has a cytoplasmic domain that is functionally conserved between human and Drosophila

Mutations in the human Crumbs homologue 1 (CRB1) gene cause severe retinal dystrophies, ranging from retinitis pigmentosa to Leber congenital amaurosis. The CRB1 gene is expressed specifically in human retina and brain and encodes a protein homologous to the Drosophila Crumbs protein. In crumbs mutant embryos apico-basal polarity of epithelial cells is lost, leading to widespread epidermal cell death. The small cytoplasmic domain of Crumbs organizes an intracellular protein scaffold that defines the assembly of a continuous zonula adherens. The crumbs mutant phenotype can be partially rescued by expression of just the membrane-bound cytoplasmic domain, and overexpression of this domain in a wild-type background results in a multilayered epidermis. A striking difference between CRB1 and Crumbs was that the latter contains a transmembrane region and a 37 amino acid cytoplasmic domain. Here we describe an alternative splice variant of human CRB1 that encodes a cytoplasmic domain 72% similar to that of Drosophila Crumbs. Two intracellular subdomains that are necessary for function in Drosophila are absolutely conserved. Rescuing and overexpression studies in Drosophila show that the cytoplasmic domains are functionally related between these distant species. This suggests that CRB1 organizes an intracellular protein scaffold in the human retina. Human homologues of proteins binding to Crumbs may be part of this complex and represent candidate genes for retinal dystrophies.

G protein signaling and asymmetric cell division

Asymmetric cell division depends on the polarization of the dividing cell for the correct alignment of the mitotic spindle and the localization of cytoplasmic determinants. Receptor-independent activation of heterotrimeric G proteins by the Drosophila GoLoco protein Partner of Inscuteable seems to represent a novel mechanism to control these events.

Zonula adherens formation in Caenorhabditis elegans requires dlg-1, the homologue of the Drosophila gene discs large

The correct assembly of junction components, such as E-cadherin and beta-catenin, into the zonula adherens is fundamental for the function of epithelia, both in flies and in vertebrates. In C. elegans, however, the cadherin-catenin system is not essential for general adhesion, raising the question as to the genetic basis controlling junction morphogenesis in nematodes. Here we show that dlg-1, the C. elegans homologue of the Drosophila tumour-suppressor gene discs-large, plays a crucial role in epithelial development. DLG-1 is restricted to adherens junctions of all embryonic epithelia, which contrasts with the localisation of the Drosophila and vertebrate homologues in septate and tight junctions, respectively. Proper localisation of DLG-1 requires the basolateral LET-413 protein, but is independent of the cadherin-catenin system. Embryos in which dlg-1 activity was eliminated by RNA-mediated interference fail to form a continuous belt of junction-associated antigens and arrest development. Loss of dlg-1 activity differentially affects localisation of proteins normally enriched apically to the zonula adherens. While the distribution of an atypical protein kinase C (PKC-3) and other cytoplasmic proteins (PAR-3, PAR-6) is not affected in dlg-1 (RNAi) embryos, the transmembrane protein encoded by crb-1, the C. elegans homologue of Drosophila crumbs, is no longer concentrated in this domain. In contrast to Drosophila, however, crb-1 and a second crb-like gene are not essential for epithelial development in C. elegans. Together the data indicate that several aspects of the spatial organisation of epithelial cells and its genetic control differ between flies, worms, and vertebrates, while others are conserved. The molecular nature of DLG-1 makes it a likely candidate to participate in the organisation of a protein scaffold that controls the assembly of junction components into the zonula adherens.

Epithelial morphogenesis: filopodia at work

Spreading and fusion of epithelial sheets are conserved morphogenetic mechanisms that help shape embryos and tissues. Recent findings suggest that the formation of dynamic filopodia at the leading front of the epithelia plays a critical role in regulating cell movement and recognition during these processes.

Control of epithelial cell shape and polarity

The polarised character of a cell is often obvious from its shape and is largely dependent on the actin cytoskeleton and the membrane-associated cell cortex---a dense network comprising spectrin and other related proteins. Spatially and functionally distinct protein scaffolds, assembled from transmembrane and cytoplasmic proteins, provide the cues for polarisation. Recent data have provided new insights into the molecular nature of these cues and the mechanisms by which they may be translated into a polarised phenotype.