Polychaetoid controls patterning by modulating adhesion in the Drosophila pupal retina

Pten, PI3K, and PtdIns(3,4,5)P3 dynamics control pulsatile actin branching in Drosophila retina morphogenesis

Epithelial remodeling of the Drosophila retina depends on the pulsatile contraction and expansion of apical contacts between the cells that form its hexagonal lattice. Phosphoinositide PI(3,4,5)P3 (PIP3) accumulates around tricellular adherens junctions (tAJs) during contact expansion and dissipates during contraction, but with unknown function. Here, we found that manipulations of Pten or PI3-kinase (PI3K) that either decreased or increased PIP3 resulted in shortened contacts and a disordered lattice, indicating a requirement for PIP3 dynamics and turnover. These phenotypes are caused by a loss of branched actin, resulting from impaired activity of the Rac1 Rho GTPase and the WAVE regulatory complex (WRC). We additionally found that during contact expansion, PI3K moves into tAJs to promote the cyclical increase of PIP3 in a spatially and temporally precise manner. Thus, dynamic control of PIP3 by Pten and PI3K governs the protrusive phase of junctional remodeling, which is essential for planar epithelial morphogenesis.

Medioapical contractile pulses coordinated between cells regulate Drosophila eye morphogenesis

Lattice cells (LCs) in the developing Drosophila retina change shape before attaining final form. Previously, we showed that repeated contraction and expansion of apical cell contacts affect these dynamics. Here, we describe another factor, the assembly of a Rho1-dependent medioapical actomyosin ring formed by nodes linked by filaments that contract the apical cell area. Cell area contraction alternates with relaxation, generating pulsatile changes in cell area that exert force on neighboring LCs. Moreover, Rho1 signaling is sensitive to mechanical changes, becoming active when tension decreases and cells expand, while the negative regulator RhoGAP71E accumulates when tension increases and cells contract. This results in cycles of cell area contraction and relaxation that are reciprocally synchronized between adjacent LCs. Thus, mechanically sensitive Rho1 signaling controls pulsatile medioapical actomyosin contraction and coordinates cell behavior across the epithelium. Disrupting the kinetics of pulsing can lead to developmental errors, suggesting this process controls cell shape and tissue integrity during epithelial morphogenesis of the retina.

The Rap1 small GTPase affects cell fate or survival and morphogenetic patterning during Drosophila melanogaster eye development

The Drosophila melanogaster eye has been instrumental for determining both how cells communicate with one another to determine cell fate, as well as cell morphogenesis and patterning. Here, we describe the effects of the small GTPase Rap1 on the development of multiple cell types in the D. melanogaster eye. Although Rap1 has previously been linked to RTK-Ras-MAPK signaling in eye development, we demonstrate that manipulation of Rap1 activity is modified by increase or decrease of Delta/Notch signaling during several events of cell fate specification in eye development. In addition, we demonstrate that manipulating Rap1 function either in primary pigment cells or in interommatidial cells affects cone cell contact switching, primary pigment cell enwrapment of the ommatidial cluster, and sorting of secondary and tertiary pigment cells. These data suggest that Rap1 has roles in both ommatidial cell recruitment/survival and in ommatidial morphogenesis in the pupal stage. They lay groundwork for future experiments on the role of Rap1 in these events.

Polychaetoid/ZO-1 strengthens cell junctions under tension while localizing differently than core adherens junction proteins

During embryonic development, dramatic cell shape changes and movements reshape the embryonic body plan. These require robust but dynamic linkage between the cell-cell adherens junctions and the force-generating actomyosin cytoskeleton. Our view of this linkage has evolved, and we now realize linkage is mediated by mechanosensitive multiprotein complexes assembled via multivalent connections. Here we combine genetic, cell biological, and modeling approaches to define the mechanism of action and functions of an important player, Drosophila polychaetoid, homologue of mammalian ZO-1. Our data reveal that Pyd reinforces cell junctions under elevated tension, and facilitates cell rearrangements. Pyd is important to maintain junctional contractility and in its absence cell rearrangements stall. We next use structured illumination microscopy to define the molecular architecture of cell-cell junctions during these events. The cadherin-catenin complex and Cno both localize to puncta along the junctional membrane, but are differentially enriched in different puncta. Pyd, in contrast, exhibits a distinct localization to strands that extend out from the region occupied by core junction proteins. We then discuss the implications for the protein network at the junction-cytoskeletal interface, suggesting different proteins localize and function in distinct ways, perhaps in distinct subcomplexes, but combine to produce robust connections.

Medioapical contractile pulses coordinated between cells regulate Drosophila eye morphogenesis

Lattice cells (LCs) in the developing Drosophila retina constantly move and change shape before attaining final forms. Previously we showed that repeated contraction and expansion of apical cell contacts affect these dynamics. Here we describe a second contributing factor, the assembly of a medioapical actomyosin ring composed of nodes linked by filaments that attract each other, fuse, and contract the LCs' apical area. This medioapical actomyosin network is dependent on Rho1 and its known effectors. Apical cell area contraction alternates with relaxation, generating pulsatile changes in apical cell area. Strikingly, cycles of contraction and relaxation of cell area are reciprocally synchronized between adjacent LCs. Further, in a genetic screen, we identified RhoGEF2 as an activator of these Rho1 functions and RhoGAP71E/C-GAP as an inhibitor. Thus, Rho1 signaling regulates pulsatile medioapical actomyosin contraction exerting force on neighboring cells, coordinating cell behavior across the epithelium. This ultimately serves to control cell shape and maintain tissue integrity during epithelial morphogenesis of the retina.

Pten, Pi3K and PtdIns(3,4,5)P 3 dynamics modulate pulsatile actin branching in Drosophila retina morphogenesis

Epithelial remodeling of the Drosophila retina depends on the pulsatile contraction and expansion of apical contacts between the cells that form its hexagonal lattice. Phosphoinositide PI(3,4,5)P 3 (PIP 3 ) accumulates around tricellular adherens junctions (tAJs) during contact expansion and dissipates during contraction, but with unknown function. Here we found that manipulations of Pten or Pi3K that either decreased or increased PIP 3 resulted in shortened contacts and a disordered lattice, indicating a requirement for PIP 3 dynamics and turnover. These phenotypes are caused by a loss of protrusive branched actin, resulting from impaired activity of the Rac1 Rho GTPase and the WAVE regulatory complex (WRC). We additionally found that during contact expansion, Pi3K moves into tAJs to promote the cyclical increase of PIP 3 in a spatially and temporally precise manner. Thus, dynamic regulation of PIP 3 by Pten and Pi3K controls the protrusive phase of junctional remodeling, which is essential for planar epithelial morphogenesis.

Polychaetoid/ZO-1 strengthens cell junctions under tension while localizing differently than core adherens junction proteins

During embryonic development dramatic cell shape changes and movements re-shape the embryonic body plan. These require robust but dynamic linkage between the cell-cell adherens junctions and the force-generating actomyosin cytoskeleton. Our view of this linkage has evolved, and we now realize linkage is mediated by a mechanosensitive multiprotein complex assembled via multivalent connections. Here we combine genetic, cell biological and modeling approaches to define the mechanism of action and functions of an important player, Drosophila Polychaetoid, homolog of mammalian ZO-1. Our data reveal that Pyd reinforces cell junctions under elevated tension, and facilitates cell rearrangements. Pyd is important to maintain junctional contractility and in its absence cell rearrangements stall. We next use structured illumination microscopy to define the molecular architecture of cell-cell junctions during these events. The cadherin-catenin complex and Cno both localize to puncta along the junctional membrane, but are differentially enriched in different puncta. Pyd, in contrast, exhibits a distinct localization to strands that extend out from the region occupied by core junction proteins. We then discuss the implications for the protein network at the junction-cytoskeletal interface, suggesting different proteins localize and function in distinct ways but combine to produce robust connections.

Identification of the genes encoding candidate septate junction components expressed during early development of the sea urchin, Strongylocentrotus purpuratus, and evidence of a role for Mesh in the formation of the gut barrier

Septate junctions (SJs) evolved as cell-cell junctions that regulate the paracellular barrier and integrity of epithelia in invertebrates. Multiple morphological variants of SJs exist specific to different epithelia and/or phyla but the biological significance of varied SJ morphology is unclear because the knowledge of the SJ associated proteins and their functions in non-insect invertebrates remains largely unknown. Here we report cell-specific expression of nine candidate SJ genes in the early life stages of the sea urchin Strongylocentrotus purpuratus. By use of in situ RNA hybridization and single cell RNA-seq we found that the expression of selected genes encoding putatively SJ associated transmembrane and cytoplasmic scaffold molecules was dynamically regulated during epithelial development in the embryos and larvae with different epithelia expressing different cohorts of SJ genes. We focused a functional analysis on SpMesh, a homolog of the Drosophila smooth SJ component Mesh, which was highly enriched in the endodermal epithelium of the mid- and hindgut. Functional perturbation of SpMesh by both CRISPR/Cas9 mutagenesis and vivo morpholino-mediated knockdown shows that loss of SpMesh does not disrupt the formation of the gut epithelium during gastrulation. However, loss of SpMesh resulted in a severely reduced gut-paracellular barrier as quantitated by increased permeability to 3-5 ​kDa FITC-dextran. Together, these studies provide a first look at the molecular SJ physiology during the development of a marine organism and suggest a shared role for Mesh-homologous proteins in forming an intestinal barrier in invertebrates. Results have implications for consideration of the traits underlying species-specific sensitivity of marine larvae to climate driven ocean change.

Long-Term Memory Formation in Drosophila Depends on the 3'UTR of CPEB Gene orb2

Activation of local translation in neurites in response to stimulation is an important step in the formation of long-term memory (LTM). CPEB proteins are a family of translation factors involved in LTM formation. The Drosophila CPEB protein Orb2 plays an important role in the development and function of the nervous system. Mutations of the coding region of the orb2 gene have previously been shown to impair LTM formation. We found that a deletion of the 3'UTR of the orb2 gene similarly results in loss of LTM in Drosophila. As a result of the deletion, the content of the Orb2 protein remained the same in the neuron soma, but significantly decreased in synapses. Using RNA immunoprecipitation followed by high-throughput sequencing, we detected more than 6000 potential Orb2 mRNA targets expressed in the Drosophila brain. Importantly, deletion of the 3'UTR of orb2 mRNA also affected the localization of the Csp, Pyd, and Eya proteins, which are encoded by putative mRNA targets of Orb2. Therefore, the 3'UTR of the orb2 mRNA is important for the proper localization of Orb2 and other proteins in synapses of neurons and the brain as a whole, providing a molecular basis for LTM formation.

Sidekick dynamically rebalances contractile and protrusive forces to control tissue morphogenesis

Contractile actomyosin and protrusive branched F-actin networks interact in a dynamic balance, repeatedly contracting and expanding apical cell contacts to organize the epithelium of the developing fly retina. Previously we showed that the immunoglobulin superfamily protein Sidekick (Sdk) contributes to contraction by recruiting the actin binding protein Polychaetoid (Pyd) to vertices. Here we show that as tension increases during contraction, Sdk progressively accumulates at vertices, where it toggles to recruit the WAVE regulatory complex (WRC) to promote actin branching and protrusion. Sdk alternately interacts with the WRC and Pyd using the same C-terminal motif. With increasing protrusion, levels of Sdk and the WRC decrease at vertices while levels of Pyd increase paving the way for another round of contraction. Thus, by virtue of dynamic association with vertices and interchangeable associations with contractile and protrusive effectors, Sdk is central to controlling the balance between contraction and expansion that shapes this epithelium.

Circulative Transmission of Cileviruses in Brevipalpus Mites May Involve the Paracellular Movement of Virions

Plant viruses transmitted by mites of the genus Brevipalpus are members of the genera Cilevirus, family Kitaviridae, or Dichorhavirus, family Rhabdoviridae. They produce non-systemic infections that typically display necrotic and/or chlorotic lesions around the inoculation loci. The cilevirus citrus leprosis virus C (CiLV-C) causes citrus leprosis, rated as one of the most destructive diseases affecting this crop in the Americas. CiLV-C is vectored in a persistent manner by the flat mite Brevipalpus yothersi. Upon the ingestion of viral particles with the content of the infected plant cell, virions must pass through the midgut epithelium and the anterior podocephalic gland of the mites. Following the duct from this gland, virions reach the salivary canal before their inoculation into a new plant cell through the stylet canal. It is still unclear whether CiLV-C multiplies in mite cells and what mechanisms contribute to its movement through mite tissues. In this study, based on direct observation of histological sections from viruliferous mites using the transmission electron microscope, we posit the hypothesis of the paracellular movement of CiLV-C in mites which may involve the manipulation of septate junctions. We detail the presence of viral particles aligned in the intercellular spaces between cells and the gastrovascular system of Brevipalpus mites. Accordingly, we propose putative genes that could control either active or passive paracellular circulation of viral particles inside the mites.

Multivalent interactions make adherens junction-cytoskeletal linkage robust during morphogenesis

Embryogenesis requires cells to change shape and move without disrupting epithelial integrity. This requires robust, responsive linkage between adherens junctions and the actomyosin cytoskeleton. Using Drosophila morphogenesis, we define molecular mechanisms mediating junction-cytoskeletal linkage and explore the role of mechanosensing. We focus on the junction-cytoskeletal linker Canoe, a multidomain protein. We engineered the canoe locus to define how its domains mediate its mechanism of action. To our surprise, the PDZ and FAB domains, which we thought connected junctions and F-actin, are not required for viability or mechanosensitive recruitment to junctions under tension. The FAB domain stabilizes junctions experiencing elevated force, but in its absence, most cells recover, suggesting redundant interactions. In contrast, the Rap1-binding RA domains are critical for all Cno functions and enrichment at junctions under tension. This supports a model in which junctional robustness derives from a large protein network assembled via multivalent interactions, with proteins at network nodes and some node connections more critical than others.

The ZO-1 protein Polychaetoid as an upstream regulator of the Hippo pathway in Drosophila

The generation of a diversity of photoreceptor (PR) subtypes with different spectral sensitivities is essential for color vision in animals. In the Drosophila eye, the Hippo pathway has been implicated in blue- and green-sensitive PR subtype fate specification. Specifically, Hippo pathway activation promotes green-sensitive PR fate at the expense of blue-sensitive PRs. Here, using a sensitized triple heterozygote-based genetic screening approach, we report the identification of the single Drosophila zonula occludens-1 (ZO-1) protein Polychaetoid (Pyd) as a new regulator of the Hippo pathway during the blue- and green-sensitive PR subtype binary fate choice. We demonstrate that Pyd acts upstream of the core components and the upstream regulator Pez in the Hippo pathway. Furthermore, We found that Pyd represses the activity of Su(dx), a E3 ligase that negatively regulates Pez and can physically interact with Pyd, during PR subtype fate specification. Together, our results identify a new mechanism underlying the Hippo signaling pathway in post-mitotic neuronal fate specification.

The Hippo signaling pathway was originally discovered for its critical role in tissue growth and organ size control. Its evolutionarily conserved roles in various biological processes, including cell differentiation, stem cell regeneration and homeostasis, innate immune biology, as well as tumorigenesis, have been subsequently found in other species. During the development of the Drosophila eye, the Hippo pathway promotes green- and represses blue-sensitive photoreceptor (PR) subtype fate specification. Taking advantage of this binary PR fate choice, we screened Drosophila chromosomal deficiency lines to seek new regulators of the Hippo signaling pathway. We identified the Drosophila membrane-associated ZO-1 protein Pyd as an upstream regulator of the Hippo pathway to specify PR subtypes. Our results have demonstrated that Pyd represses Su(dx)’s activity in the Hippo pathway to specify PR subtypes. Our results demonstrate a new mechanism underlying the Hippo signaling pathway in post-mitotic neuronal fate specification.

Hexagonal patterning of the Drosophila eye

A complex network of transcription factor interactions propagates across the larval eye disc to establish columns of evenly-spaced R8 precursor cells, the founding cells of Drosophila ommatidia. After the recruitment of additional photoreceptors to each ommatidium, the surrounding cells are organized into their stereotypical pattern during pupal development. These support cells - comprised of pigment and cone cells - are patterned to encapsulate the photoreceptors and separate ommatidia with an hexagonal honeycomb lattice. Since the proteins and processes essential for correct eye patterning are conserved, elucidating how these function and change during Drosophila eye patterning can substantially advance our understanding of transcription factor and signaling networks, cytoskeletal structures, adhesion complexes, and the biophysical properties of complex tissues during their morphogenesis. Our understanding of many of these aspects of Drosophila eye patterning is largely descriptive. Many important questions, especially relating to the regulation and integration of cellular events, remain.

Transcriptomic analysis of feminizing somatic stem cells in the Drosophila testis reveals putative downstream effectors of the transcription factor Chinmo

One of the best examples of sexual dimorphism is the development and function of the gonads, ovaries and testes, which produce sex-specific gametes, oocytes, and spermatids, respectively. The development of these specialized germ cells requires sex-matched somatic support cells. The sexual identity of somatic gonadal cells is specified during development and must be actively maintained during adulthood. We previously showed that the transcription factor Chinmo is required to ensure the male sexual identity of somatic support cells in the Drosophila melanogaster testis. Loss of chinmo from male somatic gonadal cells results in feminization: they transform from squamous to epithelial-like cells that resemble somatic cells in the female gonad but fail to properly ensheath the male germline, causing infertility. To identify potential target genes of Chinmo, we purified somatic cells deficient for chinmo from the adult Drosophila testis and performed next-generation sequencing to compare their transcriptome to that of control somatic cells. Bioinformatics revealed 304 and 1549 differentially upregulated and downregulated genes, respectively, upon loss of chinmo in early somatic cells. Using a combination of methods, we validated several differentially expressed genes. These data sets will be useful resources to the community.

Comparative transcriptome analyses of the Drosophila pupal eye

Tissue function is dependent on correct cellular organization and behavior. As a result, the identification and study of genes that contribute to tissue morphogenesis is of paramount importance to the fields of cell and developmental biology. Many of the genes required for tissue patterning and organization are highly conserved between phyla. This has led to the emergence of several model organisms and developmental systems that are used to study tissue morphogenesis. One such model is the Drosophila melanogaster pupal eye that has a highly stereotyped arrangement of cells. In addition, the pupal eye is postmitotic that allows for the study of tissue morphogenesis independent from any effects of proliferation. While the changes in cell morphology and organization that occur throughout pupal eye development are well documented, less is known about the corresponding transcriptional changes that choreograph these processes. To identify these transcriptional changes, we dissected wild-type Canton S pupal eyes and performed RNA-sequencing. Our analyses identified differential expression of many loci that are documented regulators of pupal eye morphogenesis and contribute to multiple biological processes including signaling, axon projection, adhesion, and cell survival. We also identified differential expression of genes not previously implicated in pupal eye morphogenesis such as components of the Toll pathway, several non-classical cadherins, and components of the muscle sarcomere, which could suggest these loci function as novel patterning factors.

Mask, a component of the Hippo pathway, is required for Drosophila eye morphogenesis

Hippo signaling is an important regulator of tissue size, but it also has a lesser-known role in tissue morphogenesis. Here we use the Drosophila pupal eye to explore the role of the Hippo effector Yki and its cofactor Mask in morphogenesis. We found that Mask is required for the correct distribution and accumulation of adherens junctions and appropriate organization of the cytoskeleton. Accordingly, disrupting mask expression led to severe mis-patterning and similar defects were observed when yki was reduced or in response to ectopic wts. Further, the patterning defects generated by reducing mask expression were modified by Hippo pathway activity. RNA-sequencing revealed a requirement for Mask for appropriate expression of numerous genes during eye morphogenesis. These included genes implicated in cell adhesion and cytoskeletal organization, a comprehensive set of genes that promote cell survival, and numerous signal transduction genes. To validate our transcriptome analyses, we then considered two loci that were modified by Mask activity: FER and Vinc, which have established roles in regulating adhesion. Modulating the expression of either locus modified mask mis-patterning and adhesion phenotypes. Further, expression of FER and Vinc was modified by Yki. It is well-established that the Hippo pathway is responsive to changes in cell adhesion and the cytoskeleton, but our data indicate that Hippo signaling also regulates these structures.

Sidekick Is a Key Component of Tricellular Adherens Junctions that Acts to Resolve Cell Rearrangements

Tricellular adherens junctions are points of high tension that are central to the rearrangement of epithelial cells. However, the molecular composition of these junctions is unknown, making it difficult to assess their role in morphogenesis. Here, we show that Sidekick, an immunoglobulin family cell adhesion protein, is highly enriched at tricellular adherens junctions in Drosophila. This localization is modulated by tension, and Sidekick is itself necessary to maintain normal levels of cell bond tension. Loss of Sidekick causes defects in cell and junctional rearrangements in actively remodeling epithelial tissues like the retina and tracheal system. The adaptor proteins Polychaetoid and Canoe are enriched at tricellular adherens junctions in a Sidekick-dependent manner; Sidekick functionally interacts with both proteins and directly binds to Polychaetoid. We suggest that Polychaetoid and Canoe link Sidekick to the actin cytoskeleton to enable tricellular adherens junctions to maintain or transmit cell bond tension during epithelial cell rearrangements.

The Drosophila Afadin and ZO-1 homologues Canoe and Polychaetoid act in parallel to maintain epithelial integrity when challenged by adherens junction remodeling

During morphogenesis, cells must change shape and move without disrupting tissue integrity. This requires cell-cell junctions to allow dynamic remodeling while resisting forces generated by the actomyosin cytoskeleton. Multiple proteins play roles in junctional-cytoskeletal linkage, but the mechanisms by which they act remain unclear. Drosophila Canoe maintains adherens junction-cytoskeletal linkage during gastrulation. Canoe's mammalian homologue Afadin plays similar roles in cultured cells, working in parallel with ZO-1 proteins, particularly at multicellular junctions. We take these insights back to the fly embryo, exploring how cells maintain epithelial integrity when challenged by adherens junction remodeling during germband extension and dorsal closure. We found that Canoe helps cells maintain junctional-cytoskeletal linkage when challenged by the junctional remodeling inherent in mitosis, cell intercalation, and neuroblast invagination or by forces generated by the actomyosin cable at the leading edge. However, even in the absence of Canoe, many cells retain epithelial integrity. This is explained by a parallel role played by the ZO-1 homologue Polychaetoid. In embryos lacking both Canoe and Polychaetoid, cell junctions fail early, with multicellular junctions especially sensitive, leading to widespread loss of epithelial integrity. Our data suggest that Canoe and Polychaetoid stabilize Bazooka/Par3 at cell-cell junctions, helping maintain balanced apical contractility and tissue integrity.

A specific isoform of Pyd/ZO-1 mediates junctional remodeling and formation of slit diaphragms

Podocyte slit diaphragms are key components of the glomerular filtration barrier. Using Drosophila nephrocytes, Carrasco-Rando et al. propose a conserved role for Pyd/ZO-1 in triggering junctional remodeling leading to the formation of slit diaphragms.

The podocyte slit diaphragm (SD), responsible for blood filtration in vertebrates, is a major target of injury in chronic kidney disease. The damage includes severe morphological changes with destabilization of SDs and their replacement by junctional complexes between abnormally broadened foot processes. In Drosophila melanogaster, SDs are present in nephrocytes, which filter the fly's hemolymph. Here, we show that a specific isoform of Polychaetoid/ZO-1, Pyd-P, is essential for Drosophila SDs, since, in pyd mutants devoid of Pyd-P, SDs do not form and the SD component Dumbfounded accumulates at ectopic septate-like junctions between abnormally aggregated nephrocytes. Reintroduction of Pyd-P leads to junctional remodeling and their progressive normalization toward SDs. This transition requires the coiled-coil domain of Pyd-P and implies formation of nonclathrin vesicles containing SD components and their trafficking to the nephrocyte external membrane, where SDs assemble. Analyses in zebrafish suggest a conserved role for Tjp1a/ZO-1 in promoting junctional remodeling in podocytes.

The Drosophila ZO-1 protein Polychaetoid suppresses Deltex-regulated Notch activity to modulate germline stem cell niche formation

The developmental signalling protein Notch can be proteolytically activated following ligand-interaction at the cell surface, or can be activated independently of its ligands, following Deltex (Dx)-induced Notch endocytosis and trafficking to the lysosomal membrane. The means by which different pools of Notch are directed towards these alternative outcomes remains poorly understood. We found that the Drosophila ZO-1 protein Polychaetoid (Pyd) suppresses specifically the Dx-induced form of Notch activation both in vivo and in cell culture assays. In vivo we confirmed the physiological relevance and direction of the Pyd/Dx interaction by showing that the expanded ovary stem cell niche phenotypes of pyd mutants require the presence of functional Dx and other components that are specific to the Dx-induced Notch activation mechanism. In S2 cells we found that Pyd can form a complex with Dx and Notch at the cell surface and reduce Dx-induced Notch endocytosis. Similar to other known activities of ZO-1 family proteins, the action of Pyd on Dx-induced endocytosis and signalling was found to be cell density dependent. Thus, together, our results suggest an alternative means by which external cues can tune Notch signalling through Pyd regulation of Dx-induced Notch trafficking.

The LH/CG receptor activates canonical signaling pathway when expressed in Drosophila

G-protein coupled receptors (GPCRs) and their ligands provide precise tissue regulation and are therefore often restricted to specific animal phyla. For example, the gonadotropins and their receptors are crucial for vertebrate reproduction but absent from invertebrates. In mammals, LHR mainly couples to the PKA signaling pathway, and CREB is the major transcription factor of this pathway. Here we present the results of expressing elements of the human gonadotropin system in Drosophila. Specifically, we generated transgenic Drosophila expressing the human LH/CG receptor (denoted as LHR), a constitutively active form of LHR, and an hCG analog. We demonstrate activation-dependent signaling by LHR to direct Drosophila phenotypes including lethality and specific midline defects; these phenotypes were due to LHR activation of PKA/CREB pathway activity. That the LHR can act in an invertebrate demonstrates the conservation of factors required for GPCR function among phylogenetically distant organisms. This novel gonadotropin model may assist the identification of new modulators of mammalian fertility by exploiting the powerful genetic and pharmacological tools available in Drosophila.

Diet-Induced Podocyte Dysfunction in Drosophila and Mammals

Diabetic nephropathy is a major cause of end-stage kidney disease. Characterized by progressive microvascular disease, most efforts have focused on injury to the glomerular endothelium. Recent work has suggested a role for the podocyte, a highly specialized component of the glomerular filtration barrier. Here, we demonstrate that the Drosophila nephrocyte, a cell analogous to the mammalian podocyte, displays defects that phenocopy aspects of diabetic nephropathy in animals fed chronic high dietary sucrose. Through functional studies, we identify an OGT-Polycomb-Knot-Sns pathway that links dietary sucrose to loss of the Nephrin ortholog Sns. Reducing OGT through genetic or drug means is sufficient to rescue loss of Sns, leading to overall extension of lifespan. We demonstrate upregulation of the Knot ortholog EBF2 in glomeruli of human diabetic nephropathy patients and a mouse ob/ob diabetes model. Furthermore, we demonstrate rescue of Nephrin expression and cell viability in ebf2(-/-) primary podocytes cultured in high glucose.

A library of MiMICs allows tagging of genes and reversible, spatial and temporal knockdown of proteins in Drosophila

Here, we document a collection of ∼7434 MiMIC (Minos Mediated Integration Cassette) insertions of which 2854 are inserted in coding introns. They allowed us to create a library of 400 GFP-tagged genes. We show that 72% of internally tagged proteins are functional, and that more than 90% can be imaged in unfixed tissues. Moreover, the tagged mRNAs can be knocked down by RNAi against GFP (iGFPi), and the tagged proteins can be efficiently knocked down by deGradFP technology. The phenotypes associated with RNA and protein knockdown typically correspond to severe loss of function or null mutant phenotypes. Finally, we demonstrate reversible, spatial, and temporal knockdown of tagged proteins in larvae and adult flies. This new strategy and collection of strains allows unprecedented in vivo manipulations in flies for many genes. These strategies will likely extend to vertebrates.

DOI:http://dx.doi.org/10.7554/eLife.05338.001

In the last few decades, technical advances in altering the genes of organisms have led to many discoveries about how genes work. For example, it is now possible to add a specific DNA sequence to a gene so that the protein it makes will carry a ‘tag’ that enables us to track it in cells. One such tag is called green fluorescent protein (GFP) and it is often used to study other proteins in living cells because it produces green fluorescence that can be detected under a microscope.

It is labor intensive to add tags to individual genes, so this limits the number of proteins that can be studied in this way. In 2011, researchers developed a new method that can easily tag many genes in fruit flies. It makes use of small sections of DNA called transposons, which are able to move around the genome by ‘cutting’ themselves out of one location and ‘pasting’ themselves in somewhere else.

The researchers used a transposon called Minos, which is naturally found in fruit flies. When Minos inserts into a gene, it often disrupts the gene and stops it from working. However, the researchers could swap the inserted transposon for a gene encoding GFP by making use of a natural process that rearranges DNA in cells. This resulted in the protein encoded by the gene containing GFP and so it can be detected under a microscope. This method allowed the researchers to create a collection of fly lines that have the GFP tag on many different proteins.

Now, Nagarkar-Jaiswal et al. have greatly expanded this initial collection. More than 75% of GFP-tagged proteins worked normally and the flies producing these altered proteins remain healthy. It is possible to use a technique called RNA interference against the GFP to lower the production of the tagged proteins. Moreover, Nagarkar-Jaiswal et al. show that it is also possible to degrade the tagged proteins so that less protein is present. The removal of proteins is reversible and can be done in specific tissues during any phase in fly development. These techniques allow researchers to directly associate the loss of the protein with the consequences for the fly.

This collection of fruit fly lines is a useful resource that can help us understand how genes work. The method for tagging the proteins could also be modified to work in other animals.

DOI:http://dx.doi.org/10.7554/eLife.05338.002

Cell adhesion in the assembly of the Drosophila eye

Differential adhesion provides a mechanical force to drive cells into stable configurations during the assembly of tissues and organs. This is well illustrated in the Drosophila eye where differential adhesion plays a role in sequential recruitment of all support cells. Cell adhesion, on the other hand, is linked to the cytoskeleton and subject to regulation by cell signaling. The integration of cell adhesion with the cytoskeleton and cell signaling may provide a more thorough explanation for the diversity of forms and shapes seen in tissues and organs.

Drosophila Tel2 is expressed as a translational fusion with EpsinR and is a regulator of wingless signaling

Tel2, a protein conserved from yeast to vertebrates, is an essential regulator of diverse cellular processes including telomere maintenance, DNA damage checkpoints, DNA repair, biological clocks, and cell signaling. The Drosophila Tel2 protein is produced as a translational fusion with EpsinR, a Clathrin adapter that facilitates vesicle trafficking between the Golgi and endosomes. EpsinR and Tel2 are encoded by a Drosophila gene called lqfR. lqfR is required for viability, and its specific roles include cell growth, proliferation, and planar cell polarity. We find that all of these functions of lqfR are attributed entirely to Tel2, not EpsinR. In addition, we find that Drosophila LqfR/Tel2 is a component of one or more protein complexes that contain E-cadherin and Armadillo. Moreover, Tel2 modulates E-cadherin and Armadillo cellular dynamics. We propose that at least one of the functions of Drosophila Tel2 is regulation of Wingless signaling.

Cell adhesion in Drosophila: versatility of cadherin and integrin complexes during development

We highlight recent progress in understanding cadherin and integrin function in the model organism Drosophila. New functions for these adhesion receptors continue to be discovered in this system, emphasising the importance of cell adhesion within the developing organism and showing that the requirement for cell adhesion changes between cell types. New ways to control adhesion have been discovered, including controlling the expression and recruitment of adhesion components, their posttranslational modification, recycling and turnover. Importantly, even ubiquitous adhesion components can function differently in distinct cellular contexts.

Mutational analysis supports a core role for Drosophila α-catenin in adherens junction function

α-catenin associates the cadherin-catenin complex with the actin cytoskeleton. α-catenin binds to β-catenin, which links it to the cadherin cytoplasmic tail, and F-actin, but also to a multitude of actin-associated proteins. These interactions suggest a highly complex cadherin-actin interface. Moreover, mammalian αE-catenin has been implicated in a cadherin-independent cytoplasmic function in Arp2/3-dependent actin regulation, and in cell signaling. The function and regulation of individual molecular interactions of α-catenin, in particular during development, are not well understood. We have generated mutations in Drosophila α-Catenin (α-Cat) to investigate α-Catenin function in this model, and to establish a setup for testing α-Catenin-related constructs in α-Cat-null mutant cells in vivo. Our analysis of α-Cat mutants in embryogenesis, imaginal discs and oogenesis reveals defects consistent with a loss of cadherin function. Compromising components of the Arp2/3 complex or its regulator SCAR ameliorate the α-Cat loss-of-function phenotype in embryos but not in ovaries, suggesting negative regulatory interactions between α-Catenin and the Arp2/3 complex in some tissues. We also show that the α-Cat mutant phenotype can be rescued by the expression of a DE-cadherin::α-Catenin fusion protein, which argues against an essential cytosolic, cadherin-independent role of Drosophila α-Catenin.

Adherens junctions and cadherins in Drosophila development

Drosophila represents a paradigm for the analysis of the cellular, molecular and genetic mechanisms of development and is an ideal model system to study the contribution of Adherens Junctions (AJs) and their major components, cadherins, to morphogenesis. The combination of different techniques and approaches has allowed researchers to identify the requirements of these epithelial junctions in vivo in the context of a whole organism. The functional analysis of mutants for AJ core components, particularly for Drosophila DE-cadherin, has shown that AJs play critical roles in virtually all stages of development. For instance, AJs maintain tissue integrity while allowing the remodelling and homeostasis of many tissues. They control cell shape, contribute to cell polarity, facilitate cell-cell recognition during cell sorting, orient cell divisions, or regulate cell rearrangements, among other activities. Remarkably, these activities require a very fine control of the organisation and turnover of AJs during development. In addition, AJs engage in diverse and complex interactions with the cytoskeleton, signalling networks, intracellular trafficking machinery or polarity cues to perform these functions. Here, by summarising the requirements of AJs and cadherins during Drosophila morphogenesis, we illustrate the capital contribution of this model system to our knowledge of the mechanisms and biology of AJs.

Zonula occludens-1 and -2 regulate apical cell structure and the zonula adherens cytoskeleton in polarized epithelia

ETOC: Our study reveals that ZO proteins in fully polarized cells regulate the assembly and contractility of the perijunctional actomyosin ring associated with the adherens junction.

The structure and function of both adherens (AJ) and tight (TJ) junctions are dependent on the cortical actin cytoskeleton. The zonula occludens (ZO)-1 and -2 proteins have context-dependent interactions with both junction types and bind directly to F-actin and other cytoskeletal proteins, suggesting ZO-1 and -2 might regulate cytoskeletal activity at cell junctions. To address this hypothesis, we generated stable Madin-Darby canine kidney cell lines depleted of both ZO-1 and -2. Both paracellular permeability and the localization of TJ proteins are disrupted in ZO-1/-2–depleted cells. In addition, immunocytochemistry and electron microscopy revealed a significant expansion of the perijunctional actomyosin ring associated with the AJ. These structural changes are accompanied by a recruitment of 1-phosphomyosin light chain and Rho kinase 1, contraction of the actomyosin ring, and expansion of the apical domain. Despite these changes in the apical cytoskeleton, there are no detectable changes in cell polarity, localization of AJ proteins, or the organization of the basal and lateral actin cytoskeleton. We conclude that ZO proteins are required not only for TJ assembly but also for regulating the organization and functional activity of the apical cytoskeleton, particularly the perijunctional actomyosin ring, and we speculate that these activities are relevant both to cellular organization and epithelial morphogenesis.

Cooperative phosphoinositide and peptide binding by PSD-95/discs large/ZO-1 (PDZ) domain of polychaetoid, Drosophila zonulin

PDZ domains are well known protein-protein interaction modules that, as part of multidomain proteins, assemble molecular complexes. Some PDZ domains have been reported to interact with membrane lipids, in particular phosphatidylinositol phosphates, but few studies have been aimed at elucidating the prevalence or the molecular details of such interactions. We screened 46 Drosophila PDZ domains for phosphoinositide-dependent cellular localization and discovered that the second PDZ domain of polychaetoid (Pyd PDZ2) interacts with phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P(2)) at the plasma membrane. Surface plasmon resonance binding experiments with recombinant protein established that Pyd PDZ2 interacts with phosphatidylinositol phosphates with apparent affinities in the micromolar range. Electrostatic interactions involving an extended positively charged surface of Pyd PDZ2 are crucial for the PtdIns(4,5)P(2)-dependent membrane interactions as shown by a combination of three-dimensional modeling, mutagenesis, binding, and localization studies. In vivo localization studies further suggested that both lipid and peptide binding contribute to membrane localization. We identified the transmembrane protein Crumbs as a Pyd PDZ2 ligand and probed the relation between peptide and PtdIns(4,5)P(2) binding. Contrary to the prevalent view on PDZ/peptide/lipid binding, we did not find competition between peptide and lipid ligands. Instead, preloading the protein with the 10-mer Crb3 peptide increased the apparent affinity of Pyd PDZ2 for PtdIns(4,5)P(2) 6-fold. Our results suggest that membrane localization of Pyd PDZ2 may be driven by a combination of peptide and PtdIns(4,5)P(2) binding, which raises the intriguing possibility that the domain may coordinate protein- and phospholipid-mediated signals.

Role for a Cindr-Arf6 axis in patterning emerging epithelia

The fly pupal eye is used to explore dArf6 activity regulated by the Arf GTPase–activating proteins (ArfGAPs) dAsap and dArfGAP3 and Arf GTP exchange factors Schizo and dPsd, which promote cellular extensions that presage cell rearrangements. The adaptor protein Cindr bound to dArfGAP3 and dAsap to sequester ArfGAP function to Neph1/nephrin adhesion complexes, liberating active dArf6 elsewhere.

Patterning of the Drosophila pupal eye is characterized by precise cell movements. In this paper, we demonstrate that these movements require an Arf regulatory cycle that connects surface receptors to actin-based movement. dArf6 activity—regulated by the Arf GTPase–activating proteins (ArfGAPs) dAsap and dArfGAP3 and the Arf GTP exchange factors Schizo and dPsd—promoted large cellular extensions; time-lapse microscopy indicated that these extensions presage cell rearrangements into correct epithelial niches. During this process, the Drosophila eye also requires interactions between surface Neph1/nephrin adhesion receptors Roughest and Hibris, which bind the adaptor protein Cindr (CD2AP). We provide evidence that Cindr forms a physical complex with dArfGAP3 and dAsap. Our data suggest this interaction sequesters ArfGAP function to liberate active dArf6 elsewhere in the cell. We propose that a Neph1/nephrin–Cindr/ArfGAP complex accumulates to limit local Arf6 activity and stabilize adherens junctions. Our model therefore links surface adhesion via an Arf6 regulatory cascade to dynamic modeling of the cytoskeleton, accounting for precise cell movements that organize the functional retinal field. Further, we demonstrate a similar relationship between the mammalian Cindr orthologue CD2AP and Arf6 activity in cell motility assays. We propose that this Cindr/CD2AP-mediated regulation of Arf6 is a widely used mechanism in emerging epithelia.

The Drosophila nephrocyte

PURPOSE OF REVIEW

The functioning kidney requires proper organization in multiple cell types that mediate filtration and removal of wastes. Interest has increasingly focused on the podocyte as an important mediator of kidney function; defects in podocyte function likely mediate a broad palate of kidney dysfunctions. Here I explore recent work that establishes the Drosophila nephrocyte as a useful model for podocyte function and dysfunction.

RECENT FINDINGS

Although described many decades in the past, recent evidence has emphasized important similarities in the molecules that construct the 'nephrocyte diaphragm' and its vertebrate cousin the 'podocyte diaphragm'. For example, loss of Nephrin and its associated proteins lead to collapse of these structures and loss of proper filtration.

SUMMARY

These data emphasize both differences between the podocyte and nephrocyte and also useful similarities. These similarities provide the promise of bringing Drosophila genetics--strongly successful in other disciplines--to the complex problem of how podocyte dysfunction leads to disease. To further explore this point I discuss work on Nephrin in a better understood tissue, the Drosophila eye, in which the role of Nephrin and its connection to actin dynamics is under intense study.

ZO proteins and redox-dependent processes

SIGNIFICANCE

ZO-1, ZO-2, and ZO-3 are scaffold proteins of the tight junction (TJ) that belong to the MAGUK protein family characterized for exhibiting PDZ, SH3, and GuK domains. ZO proteins are present only in multicellular organisms, being the placozoa the first to have them. ZO proteins associate among themselves and with other integral and adaptor proteins of the TJ, of the ZA and of gap junctions, as with numerous signaling proteins and the actin cytoskeleton. ZO proteins are also present at the nucleus of proliferating cells.

RECENT ADVANCES

Oxidative stress disassembles the TJs of endothelial and epithelial cells.

CRITICAL ISSUES

Oxidative stress alters ZO proteins expression and localization, in conditions like hypoxia, bacterial and viral infections, vitamin deficiencies, age-related diseases, diabetes and inflammation, alcohol and tobacco consumption.

FUTURE DIRECTIONS

Molecules present in the signaling pathways triggered by oxidative stress can be targets for therapeutic intervention.

The lens in focus: a comparison of lens development in Drosophila and vertebrates

The evolution of the eye has been a major subject of study dating back centuries. The advent of molecular genetics offered the surprising finding that morphologically distinct eyes rely on conserved regulatory gene networks for their formation. While many of these advances often stemmed from studies of the compound eye of the fruit fly, Drosophila melanogaster, and later translated to discoveries in vertebrate systems, studies on vertebrate lens development far outnumber those in Drosophila. This may be largely historical, since Spemann and Mangold's paradigm of tissue induction was discovered in the amphibian lens. Recent studies on lens development in Drosophila have begun to define molecular commonalities with the vertebrate lens. Here, we provide an overview of Drosophila lens development, discussing intrinsic and extrinsic factors controlling lens cell specification and differentiation. We then summarize key morphological and molecular events in vertebrate lens development, emphasizing regulatory factors and networks strongly associated with both systems. Finally, we provide a comparative analysis that highlights areas of research that would help further clarify the degree of conservation between the formation of dioptric systems in invertebrates and vertebrates.

The BAR domain protein PICK1 regulates cell recognition and morphogenesis by interacting with Neph proteins

Neph proteins are evolutionarily conserved membrane proteins of the immunoglobulin superfamily that control the formation of specific intercellular contacts. Cell recognition through these proteins is essential in diverse cellular contexts such as patterning of the compound eye in Drosophila melanogaster, neuronal connectivity in Caenorhabditis elegans, and the formation of the kidney filtration barrier in mammals. Here we identify the PDZ and BAR domain protein PICK1 (protein interacting with C-kinase 1) as a Neph-interacting protein. Binding required dimerization of PICK1, was dependent on PDZ domain protein interactions, and mediated stabilization of Neph1 at the plasma membrane. Moreover, protein kinase C (PKCα) activity facilitated the interaction through releasing Neph proteins from their binding to the multidomain scaffolding protein zonula occludens 1 (ZO-1), another PDZ domain protein. In Drosophila, the Neph homologue Roughest is essential for sorting of interommatidial precursor cells and patterning of the compound eye. RNA interference-mediated knockdown of PICK1 in the Drosophila eye imaginal disc caused a Roughest destabilization at the plasma membrane and a phenotype that resembled rst mutation. These data indicate that Neph proteins and PICK1 synergistically regulate cell recognition and contact formation.

The single Drosophila ZO-1 protein Polychaetoid regulates embryonic morphogenesis in coordination with Canoe/afadin and Enabled

Analysis of the function of the fly ZO-1 homologue Polychaetoid shows that it is not essential for junctional assembly or maintenance but does play an important role in embryonic morphogenesis. The data suggest that it works with Canoe/afadin and the actin regulator Enabled to regulate actin anchoring at junctions.

Adherens and tight junctions play key roles in assembling epithelia and maintaining barriers. In cell culture zonula occludens (ZO)–family proteins are important for assembly/maturation of both tight and adherens junctions (AJs). Genetic studies suggest that ZO proteins are important during normal development, but interpretation of mouse and fly studies is limited by genetic redundancy and/or a lack of null alleles. We generated null alleles of the single Drosophila ZO protein Polychaetoid (Pyd). Most embryos lacking Pyd die with striking defects in morphogenesis of embryonic epithelia including the epidermis, segmental grooves, and tracheal system. Pyd loss does not dramatically affect AJ protein localization or initial localization of actin and myosin during dorsal closure. However, Pyd loss does affect several cell behaviors that drive dorsal closure. The defects, which include segmental grooves that fail to retract, a disrupted leading edge actin cable, and reduced zippering as leading edges meet, closely resemble defects in canoe zygotic null mutants and in embryos lacking the actin regulator Enabled (Ena), suggesting that these proteins act together. Canoe (Cno) and Pyd are required for proper Ena localization during dorsal closure, and strong genetic interactions suggest that Cno, Pyd, and Ena act together in regulating or anchoring the actin cytoskeleton during dorsal closure.

The Drosophila L1CAM homolog Neuroglian signals through distinct pathways to control different aspects of mushroom body axon development

The spatiotemporal integration of adhesion and signaling during neuritogenesis is an important prerequisite for the establishment of neuronal networks in the developing brain. In this study, we describe the role of the L1-type CAM Neuroglian protein (NRG) in different steps of Drosophila mushroom body (MB) neuron axonogenesis. Selective axon bundling in the peduncle requires both the extracellular and the intracellular domain of NRG. We uncover a novel role for the ZO-1 homolog Polychaetoid (PYD) in axon branching and in sister branch outgrowth and guidance downstream of the neuron-specific isoform NRG-180. Furthermore, genetic analyses show that the role of NRG in different aspects of MB axonal development not only involves PYD, but also TRIO, SEMA-1A and RAC1.

Su(dx) E3 ubiquitin ligase-dependent and -independent functions of polychaetoid, the Drosophila ZO-1 homologue

Zona occludens (ZO) proteins are molecular scaffolds localized to cell junctions, which regulate epithelial integrity in mammals. Using newly generated null alleles, we demonstrate that polychaetoid (pyd), the unique Drosophila melanogaster ZO homologue, regulates accumulation of adherens junction-localized receptors, such as Notch, although it is dispensable for epithelial polarization. Pyd positively regulates Notch signaling during sensory organ development but acts negatively on Notch to restrict the ovary germline stem cell niche. In both contexts, we identify a core antagonistic interaction between Pyd and the WW domain E3 ubiquitin ligase Su(dx). Pyd binds Su(dx) directly, in part through a noncanonical WW-binding motif. Pyd also restricts epithelial wing cell numbers to control adult wing shape, a function associated with the FERM protein Expanded and independent of Su(dx). As both Su(dx) and Expanded regulate trafficking, we propose that a conserved role of ZO proteins is to coordinate receptor trafficking and signaling with junctional organization.

Computer simulation of cellular patterning within the Drosophila pupal eye

We present a computer simulation and associated experimental validation of assembly of glial-like support cells into the interweaving hexagonal lattice that spans the Drosophila pupal eye. This process of cell movements organizes the ommatidial array into a functional pattern. Unlike earlier simulations that focused on the arrangements of cells within individual ommatidia, here we examine the local movements that lead to large-scale organization of the emerging eye field. Simulations based on our experimental observations of cell adhesion, cell death, and cell movement successfully patterned a tracing of an emerging wild-type pupal eye. Surprisingly, altering cell adhesion had only a mild effect on patterning, contradicting our previous hypothesis that the patterning was primarily the result of preferential adhesion between IRM-class surface proteins. Instead, our simulations highlighted the importance of programmed cell death (PCD) as well as a previously unappreciated variable: the expansion of cells' apical surface areas, which promoted rearrangement of neighboring cells. We tested this prediction experimentally by preventing expansion in the apical area of individual cells: patterning was disrupted in a manner predicted by our simulations. Our work demonstrates the value of combining computer simulation with in vivo experiments to uncover novel mechanisms that are perpetuated throughout the eye field. It also demonstrates the utility of the Glazier-Graner-Hogeweg model (GGH) for modeling the links between local cellular interactions and emergent properties of developing epithelia as well as predicting unanticipated results in vivo.

Two themes on the assembly of the Drosophila eye

Cells are sequentially recruited during formation of the Drosophila compound eye. A few simple rules are reiteratively utilized to control successive steps of eye assembly. Two themes emerge: the interplay between cell signaling and competence determines diversity of cell types and selective cell adhesion determines spatial patterns of cells. Cell signaling through competence creates signaling relays, which sequentially trigger differentiation of all cell types. Selective cell adhesion, on the other hand, provides forces to drive cells into energy-favored spatial configurations. Organ formation is nevertheless a complex process. The complexity lies in the spatial, temporal, and quantitative precision of gene expression. Many challenging questions remain.

Zonula occludens-1 and -2 are cytosolic scaffolds that regulate the assembly of cellular junctions

The integrity of the tight junction barrier in epithelial and endothelial cells is critical to human health, but we still lack a detailed mechanistic knowledge of how the barrier is formed during development or responds to pathological and pharmacological insults. This limits our understanding of barrier dysfunction in disease and slows the development of therapeutic strategies. Recent studies confirm the long-maintained but previously unsupported view that the zonula occludens (ZO) proteins ZO-1 and ZO-2 are critical determinants of barrier formation. However, ZO proteins can also be components of adherens junctions, and recent studies suggest that ZO proteins may also promote the assembly and function of these junctions during epithelial morphogenesis. We review these studies and outline several recent observations that suggest that one role of ZO proteins is to regulate cytoskeletal dynamics at cell junctions. Finally, we propose a model by which the functional activities of ZO proteins in the adherens junction and tight junction are differentiated by a novel regulatory motif known as the U6 or acidic motif.

Principles of Drosophila eye differentiation

The Drosophila eye is one of nature's most beautiful structures and one of its most useful. It has emerged as a favored model for understanding the processes that direct cell fate specification, patterning, and morphogenesis. Though composed of thousands of cells, each fly eye is a simple repeating pattern of perhaps a dozen cell types arranged in a hexagonal array that optimizes coverage of the visual field. This simple structure combined with powerful genetic tools make the fly eye an ideal model to explore the relationships between local cell fate specification and global tissue patterning. In this chapter, I discuss the basic principles that have emerged from three decades of close study. We now understand at a useful level some of the basic principles of cell fate selection and the importance of local cell-cell communication. We understand less of the processes by which signaling combines with morphogenesis and basic cell biology to create a correctly patterned neuroepithelium. Progress is being made on these fundamental issues, and in this chapter I discuss some of the principles that are beginning to emerge.