Echinoid mutants exhibit neurogenic phenotypes and show synergistic interactions with the Notch signaling pathway

The cell adhesion molecule Echinoid promotes tissue survival and separately restricts tissue overgrowth in Drosophila imaginal discs

The interactions that cells in Drosophila imaginal discs have with their neighbors are known to regulate their ability to survive. In a screen of genes encoding cell surface proteins for gene knockdowns that affect the size or shape of mutant clones, we found that clones of cells with reduced levels of echinoid (ed) are fewer, smaller, and can be eliminated during development. In contrast, discs composed mostly of ed mutant tissue are overgrown. We find that ed mutant tissue has lower levels of the anti-apoptotic protein Diap1 and has increased levels of apoptosis which is consistent with the observed underrepresentation of ed mutant clones and the slow growth of ed mutant tissue. The eventual overgrowth of ed mutant tissue results not from accelerated growth, but from prolonged growth resulting from a failure to arrest growth at the appropriate final size. Ed has previously been shown to physically interact with multiple Hippo-pathway components and it has been proposed to promote Hippo pathway signaling, to exclude Yorkie (Yki) from the nucleus, and restrain the expression of Yki-target genes. We did not observe changes in Yki localization in ed mutant tissue and found decreased levels of expression of several Yorkie-target genes, findings inconsistent with the proposed effect of Ed on Yki. We did, however, observe increased expression of several Yki-target genes in wild-type cells neighboring ed mutant cells, which may contribute to elimination of ed mutant clones. Thus, ed has two distinct functions: an anti-apoptotic function by maintaining Diap1 levels, and a function to arrest growth at the appropriate final size. Both of these are unlikely to be explained by a simple effect on the Hippo pathway.

Smurf Downregulates Echinoid in the Amnioserosa To Regulate Drosophila Dorsal Closure

Drosophila dorsal closure is a morphogenetic movement that involves flanking epidermal cells, assembling actomyosin cables, and migrating dorsally over the underlying amnioserosa to seal at the dorsal midline. Echinoid (Ed)-a cell adhesion molecule of adherens junctions (AJs)-participates in several developmental processes. The disappearance of Ed from the amnioserosa is required to define the epidermal leading edge for actomyosin cable assembly and coordinated cell migration. However, the mechanism by which Ed is cleared from amnioserosa is unknown. Here, we show that Ed is cleared in amnioserosa by both transcriptional and post-translational mechanisms. First, Ed mRNA transcription was repressed in amnioserosa prior to the onset of dorsal closure. Second, the ubiquitin ligase Smurf downregulated pretranslated Ed by binding to the PPXY motif of Ed. During dorsal closure, Smurf colocalized with Ed at AJs, and Smurf overexpression prematurely degraded Ed in the amnioserosa. Conversely, Ed persisted in the amnioserosa of Smurf mutant embryos, which, in turn, affected actomyosin cable formation. Together, our results demonstrate that transcriptional repression of Ed followed by Smurf-mediated downregulation of pretranslated Ed in amnioserosa regulates the establishment of a taut leading edge during dorsal closure.

A targeted in vivo RNAi screen reveals deubiquitinases as new regulators of Notch signaling

Notch signaling is highly conserved in all metazoan animals and plays critical roles in cell fate specification, cell proliferation, apoptosis, and stem cell maintenance. Although core components of the Notch signaling cascade have been identified, many gaps in the understanding of the Notch signaling pathway remain to be filled. One form of posttranslational regulation, which is controlled by the ubiquitin-proteasome system, is known to modulate Notch signaling. The ubiquitination pathway is a highly coordinated process in which the ubiquitin moiety is either conjugated to or removed from target proteins by opposing E3 ubiquitin ligases and deubiquitinases (DUBs). Several E3 ubiquitin ligases have been implicated in ubiquitin conjugation to the receptors and the ligands of the Notch signaling cascade. In contrast, little is known about a direct role of DUBs in Notch signaling in vivo. Here, we report an in vivo RNA interference screen in Drosophila melanogaster targeting all 45 DUBs that we annotated in the fly genome. We show that at least four DUBs function specifically in the formation of the fly wing margin and/or the specification of the scutellar sensory organ precursors, two processes that are strictly dependent on the balanced Notch signaling activity. Furthermore, we provide genetic evidence suggesting that these DUBs are necessary to positively modulate Notch signaling activity. Our study reveals a conserved molecular mechanism by which protein deubiquitination process contributes to the complex posttranslational regulation of Notch signaling in vivo.

Uif, a large transmembrane protein with EGF-like repeats, can antagonize Notch signaling in Drosophila

BACKGROUND

Notch signaling is a highly conserved pathway in multi-cellular organisms ranging from flies to humans. It controls a variety of developmental processes by stimulating the expression of its target genes in a highly specific manner both spatially and temporally. The diversity, specificity and sensitivity of the Notch signaling output are regulated at distinct levels, particularly at the level of ligand-receptor interactions.

METHODOLOGY/PRINCIPAL FINDINGS

Here, we report that the Drosophila gene uninflatable (uif), which encodes a large transmembrane protein with eighteen EGF-like repeats in its extracellular domain, can antagonize the canonical Notch signaling pathway. Overexpression of Uif or ectopic expression of a neomorphic form of Uif, Uif*, causes Notch signaling defects in both the wing and the sensory organ precursors. Further experiments suggest that ectopic expression of Uif* inhibits Notch signaling in cis and acts at a step that is dependent on the extracellular domain of Notch. Our results suggest that Uif can alter the accessibility of the Notch extracellular domain to its ligands during Notch activation.

CONCLUSIONS/SIGNIFICANCE

Our study shows that Uif can modulate Notch activity, illustrating the importance of a delicate regulation of this signaling pathway for normal patterning.

Sgt1 acts via an LKB1/AMPK pathway to establish cortical polarity in larval neuroblasts

Drosophila neuroblasts are a model system for studying stem cell self-renewal and the establishment of cortical polarity. Larval neuroblasts generate a large apical self-renewing neuroblast, and a small basal cell that differentiates. We performed a genetic screen to identify regulators of neuroblast self-renewal, and identified a mutation in sgt1 (suppressor-of-G2-allele-of-skp1) that had fewer neuroblasts. We found that sgt1 neuroblasts have two polarity phenotypes: failure to establish apical cortical polarity at prophase, and lack of cortical Scribble localization throughout the cell cycle. Apical cortical polarity was partially restored at metaphase by a microtubule-induced cortical polarity pathway. Double mutants lacking Sgt1 and Pins (a microtubule-induced polarity pathway component) resulted in neuroblasts without detectable cortical polarity and formation of "neuroblast tumors." Mutants in hsp83 (encoding the predicted Sgt1-binding protein Hsp90), LKB1, or AMPKα all show similar prophase apical cortical polarity defects (but no Scribble phenotype), and activated AMPKα rescued the sgt1 mutant phenotype. We propose that an Sgt1/Hsp90-LKB1-AMPK pathway acts redundantly with a microtubule-induced polarity pathway to generate neuroblast cortical polarity, and the absence of neuroblast cortical polarity can produce neuroblast tumors.

Immunoglobulin superfamily receptors and adherens junctions

The immunogroblin (Ig) superfamily proteins characterized by the presence of Ig-like domains are involved in various cellular functions. The properties of the Ig-like domains to form rod-like structures and to bind specifically to other proteins make them ideal for cell surface receptors and cell adhesion molecules (CAMs). Ig-CAMs, nectins in mammals and Echinoid in Drosophila, are crucial components of cadherin-based adherens junctions in the epithelium. Nectins form cell-cell adhesion by their trans-interactions and recruit cadherins to the nectin-initiated cell-cell adhesion site to establish adherens junctions. Thereafter junction adhesion molecules, occludin, and claudins, are recruited to the apical side of adherens junctions to establish tight junctions. The recruitment of these molecules by nectins is mediated both by the direct and indirect interactions of afadin with many proteins, such as catenins, and zonula occludens proteins, and by the nectin-induced reorganization of the actin cytoskeleton. Nectins contribute to the formation of both homotypic and heterotypic types of cell-cell junctions, such as synapses in the brain, contacts between pigment and non-pigment cell layers of the ciliary epithelium in the eye, Sertoli cell-spermatid junctions in the testis, and sensory cells and supporting cells in the sensory organs. In addition, cis- and trans-interactions of nectins with various cell surface proteins, such as integrins, growth factor receptors, and nectin-like molecules (Necls) play important roles in the regulation of many cellular functions, such as cell polarization, movement, proliferation, differentiation, survival, and cell sorting. Furthermore, the Ig-CAMs are implicated in many human diseases including viral infections, ectodermal dysplasia, cancers, and Alzheimer's disease.

Genetic characterization of ebi reveals its critical role in Drosophila wing growth

The ebi gene of Drosophila melanogaster has been implicated in diverse signalling pathways, cellular functions and developmental processes. However, a thorough genetic analysis of this gene has been lacking and the true extent of its biological roles is unclear. Here, we characterize eleven ebi mutations and find that ebi has a novel role in promoting growth of the wing imaginal disc: viable combinations of mutant alleles give rise to adults with small wings. Wing discs with reduced EBI levels are correspondingly small and exhibit down-regulation of Notch target genes. Furthermore, we show that EBI colocalizes on polytene chromosomes with Smrter (SMR), a transcriptional corepressor, and Suppressor of Hairless (SU(H)), the primary transcription factor involved in Notch signalling. Interestingly, the mammalian orthologs of ebi, transducin β-like 1 (TBL1) and TBL-related 1 (TBLR1), function as corepressor/coactivator exchange factors and are required for transcriptional activation of Notch target genes. We hypothesize that EBI acts to activate (de-repress) transcription of Notch target genes important for Drosophila wing growth by functioning as a corepressor/coactivator exchange factor for SU(H).

Echinoid regulates tracheal morphology and fusion cell fate in Drosophila

Morphogenesis of the Drosophila embryonic trachea involves a stereotyped pattern of epithelial tube branching and fusion. Here, we report unexpected phenotypes resulting from maternal and zygotic (M/Z) loss of the homophilic cell adhesion molecule Echinoid (Ed), as well as the subcellular localization of Ed in the trachea. ed(M/Z) embryos have convoluted trachea reminiscent of septate junction (SJ) and luminal matrix mutants. However, Ed does not localize to SJs, and ed(M/Z) embryos have intact SJs and show normal luminal accumulation of the matrix-modifying protein Vermiform. Surprisingly, tracheal length is not increased in ed(M/Z) mutants, but a previously undescribed combination of reduced intersegmental spacing and deep epidermal grooves produces a convoluted tracheal phenotype. In addition, ed(M/Z) mutants have unique fusion defects involving supernumerary fusion cells, ectopic fusion events and atypical branch breaks. Tracheal-specific expression of Ed rescues these fusion defects, indicating that Ed acts in trachea to control fusion cell fate.

Echinoid regulates Flamingo endocytosis to control ommatidial rotation in the Drosophila eye

Planar cell polarity (PCP) refers to a second polarity axis orthogonal to the apicobasal axis in the plane of the epithelium. The molecular link between apicobasal polarity and PCP is largely unknown. During Drosophila eye development, differentiated photoreceptors form clusters that rotate independently of the surrounding interommatidial cells (ICs). Here, we demonstrate that both Echinoid (Ed), an adherens junction-associated cell adhesion molecule, and Flamingo (Fmi), a PCP determinant, are endocytosed via a clathrin-mediated pathway in ICs. Interestingly, we found that Ed binds the AP-2 adaptor and is required for the internalization of Fmi into ICs. Loss of ed led to increased amounts of Fmi on the cell membrane of non-rotating ICs and also to the misrotation of photoreceptor clusters. Importantly, overexpression of fmi in ICs alone was sufficient to cause misrotation of the adjacent photoreceptor clusters. Together, we propose that Ed, when internalized by AP-2, undergoes co-endocytosis with, and thereby decreases, Fmi levels on non-rotating ICs to permit correct rotation of ommatidial clusters. Thus, co-endocytosis of Ed and Fmi provides a link between apicobasal polarity and PCP.

Sanpodo: a context-dependent activator and inhibitor of Notch signaling during asymmetric divisions

Asymmetric cell divisions generate sibling cells of distinct fates ('A', 'B') and constitute a fundamental mechanism that creates cell-type diversity in multicellular organisms. Antagonistic interactions between the Notch pathway and the intrinsic cell-fate determinant Numb appear to regulate asymmetric divisions in flies and vertebrates. During these divisions, productive Notch signaling requires sanpodo, which encodes a novel transmembrane protein. Here, we demonstrate that Drosophila sanpodo plays a dual role to regulate Notch signaling during asymmetric divisions - amplifying Notch signaling in the absence of Numb in the 'A' daughter cell and inhibiting Notch signaling in the presence of Numb in the 'B' daughter cell. In so doing, sanpodo ensures the asymmetry in Notch signaling levels necessary for the acquisition of distinct fates by the two daughter cells. These findings answer long-standing questions about the restricted ability of Numb and Sanpodo to inhibit and to promote, respectively, Notch signaling during asymmetric divisions.

Mutations in many genes affect aggressive behavior in Drosophila melanogaster

Background

Aggressive behavior in animals is important for survival and reproduction. Identifying the underlying genes and environmental contexts that affect aggressive behavior is important for understanding the evolutionary forces that maintain variation for aggressive behavior in natural populations, and to develop therapeutic interventions to modulate extreme levels of aggressive behavior in humans. While the role of neurotransmitters and a few other molecules in mediating and modulating levels of aggression is well established, it is likely that many additional genetic pathways remain undiscovered. Drosophila melanogaster has recently been established as an excellent model organism for studying the genetic basis of aggressive behavior. Here, we present the results of a screen of 170 Drosophila P-element insertional mutations for quantitative differences in aggressive behavior from their co-isogenic control line.

Results

We identified 59 mutations in 57 genes that affect aggressive behavior, none of which had been previously implicated to affect aggression. Thirty-two of these mutants exhibited increased aggression, while 27 lines were less aggressive than the control. Many of the genes affect the development and function of the nervous system, and are thus plausibly relevant to the execution of complex behaviors. Others affect basic cellular and metabolic processes, or are mutations in computationally predicted genes for which aggressive behavior is the first biological annotation. Most of the mutations had pleiotropic effects on other complex traits. We characterized nine of these mutations in greater detail by assessing transcript levels throughout development, morphological changes in the mushroom bodies, and restoration of control levels of aggression in revertant alleles. All of the P-element insertions affected the tagged genes, and had pleiotropic effects on brain morphology.

Conclusion

This study reveals that many more genes than previously suspected affect aggressive behavior, and that these genes have widespread pleiotropic effects. Given the conservation of aggressive behavior among different animal species, these are novel candidate genes for future study in other animals, including humans.

Signal integration during development: mechanisms of EGFR and Notch pathway function and cross-talk

Metazoan development relies on a highly regulated network of interactions between conserved signal transduction pathways to coordinate all aspects of cell fate specification, differentiation, and growth. In this review, we discuss the intricate interplay between the epidermal growth factor receptor (EGFR; Drosophila EGFR/DER) and the Notch signaling pathways as a paradigm for signal integration during development. First, we describe the current state of understanding of the molecular architecture of the EGFR and Notch signaling pathways that has resulted from synergistic studies in vertebrate, invertebrate, and cultured cell model systems. Then, focusing specifically on the Drosophila eye, we discuss how cooperative, sequential, and antagonistic relationships between these pathways mediate the spatially and temporally regulated processes that generate this sensory organ. The common themes underlying the coordination of the EGFR and Notch pathways appear to be broadly conserved and should, therefore, be directly applicable to elucidating mechanisms of information integration and signaling specificity in vertebrate systems.

Complex interaction of Drosophila GRIP PDZ domains and Echinoid during muscle morphogenesis

Glutamate receptor interacting protein (GRIP) homologues, initially characterized in synaptic glutamate receptor trafficking, consist of seven PDZ domains (PDZDs), whose conserved arrangement is of unknown significance. The Drosophila GRIP homologue (DGrip) is needed for proper guidance of embryonic somatic muscles towards epidermal attachment sites, with both excessive and reduced DGrip activity producing specific phenotypes in separate muscle groups. These phenotypes were utilized to analyze the molecular architecture underlying DGrip signaling function in vivo. Surprisingly, removing PDZDs 1-3 (DGripDelta1-3) or deleting ligand binding in PDZDs 1 or 2 convert DGrip to excessive in vivo activity mediated by ligand binding to PDZD 7. Yeast two-hybrid screening identifies the cell adhesion protein Echinoid's (Ed) type II PDZD-interaction motif as binding PDZDs 1, 2 and 7 of DGrip. ed loss-of-function alleles exhibit muscle defects, enhance defects caused by reduced DGrip activity and suppress the dominant DGripDelta1-3 effect during embryonic muscle formation. We propose that Ed and DGrip form a signaling complex, where competition between N-terminal and the C-terminal PDZDs of DGrip for Ed binding controls signaling function.

Localization and requirement for Myosin II at the dorsal-ventral compartment boundary of theDrosophila wing

As organisms develop, their tissues can become separated into distinct cell populations through the establishment of compartment boundaries. Compartment boundaries have been discovered in a wide variety of tissues, but in many cases the molecular mechanisms that separate cells remain poorly understood. In the Drosophila wing, a stripe of Notch activation maintains the dorsal-ventral compartment boundary, through a process that depends on the actin cytoskeleton. Here, we show that the dorsal-ventral boundary exhibits a distinct accumulation of Myosin II, and that this accumulation is regulated downstream of Notch signaling. Conversely, the dorsal-ventral boundary is depleted for the Par-3 homologue Bazooka. We further show that mutations in the Myosin heavy chain subunit encoded by zipper can impair dorsal-ventral compartmentalization without affecting anterior-posterior compartmentalization. These observations identify a distinct accumulation and requirement for Myosin activity in dorsal-ventral compartmentalization, and suggest a novel mechanism in which contractile tension along an F-actin cable at the compartment boundary contributes to compartmentalization.

MicroRNA1 influences cardiac differentiation in Drosophila and regulates Notch signaling

MicroRNAs (miRNAs) are genomically encoded small RNAs that hybridize with messenger RNAs, resulting in degradation or translational inhibition of targeted transcripts. The potential for miRNAs to regulate cell-lineage determination or differentiation from pluripotent progenitor or stem cells is unknown. Here, we show that microRNA1 (miR-1) is an ancient muscle-specific gene conserved in sequence and expression in Drosophila. Drosophila miR-1 (dmiR-1) is regulated through a serum response factor-like binding site in cardiac progenitor cells. Loss- and gain-of-function studies demonstrated a role for dmiR-1 in modulating cardiogenesis and in maintenance of muscle-gene expression. We provide in vivo evidence that dmiR-1 targets transcripts encoding the Notch ligand Delta, providing a potential mechanism for the expansion of cardiac and muscle progenitor cells and failure of progenitor cell differentiation in some dmiR-1 mutants. These findings demonstrate that dmiR-1 may "fine-tune" critical steps involved in differentiation of cardiac and somatic muscle progenitors and targets a pathway required for progenitor cell specification and asymmetric cell division.

Cell Adhesion: Sorting out Cell Mixing with Echinoid?

Cadherins control intercellular adhesion in epithelial cells. This property relies on the ability to recruit actin filaments at adherens junctions via beta-catenin and alpha-catenin. A recent study shows that Echinoid, a member of the immunoglobulin domain containing protein family, is a modulator of intercellular adhesion in Drosophila that controls cell sorting.

Echinoid Is a Component of Adherens Junctions That Cooperates with DE-Cadherin to Mediate Cell Adhesion

Echinoid is an immunoglobulin domain-containing transmembrane protein that modulates cell-cell signaling by Notch and the EGF receptors. We show that, in the Drosophila wing disc epithelium, Echinoid is a component of adherens junctions that cooperates with DE-Cadherin in cell adhesion. Echinoid and beta-catenin (a DE-Cadherin interacting protein) each possess a C-terminal PDZ domain binding motif that binds to Bazooka/PAR-3; these motifs redundantly position Bazooka to adherens junctions. Echinoid also links to actin filaments by binding to Canoe/AF-6/afadin. Moreover, interfaces between Echinoid- and Echinoid+ cells, like those between DE-Cadherin- and DE-Cadherin+ cells, are deficient in adherens junctions and form actin cables. These characteristics probably facilitate the strong sorting behavior of cells that lack either of these cell-adhesion molecules. Finally, cells lacking either Echinoid or DE-Cadherin accumulate a high density of the reciprocal protein, further suggesting that Echinoid and DE-Cadherin play similar and complementary roles in cell adhesion.

Echinoid synergizes with the Notch signaling pathway in Drosophila mesothorax bristle patterning

echinoid (ed) encodes an immunoglobulin domain-containing cell adhesion molecule that negatively regulates the Egfr signaling pathway during Drosophila photoreceptor development. We show a novel function of Ed, i.e. the restriction of the number of notum bristles that arise from a proneural cluster. Thus, loss-of-function conditions for ed give rise to the development of extra macrochaetae near the extant ones and increase the density of microchaetae. Analysis of ed mosaics indicates that extra sensory organ precursors (SOPs) arise from proneural clusters of achaete-scute expression in a cell-autonomous way. edembryos also exhibit a neurogenic phenotype. These phenotypes suggest a functional relation between ed and the Notch (N) pathway. Indeed,loss-of-function of ed reduces the expression of the N pathway effector E(spl)m8 in proneural clusters. Moreover, combinations of moderate loss-of-function conditions for ed and for different components of the N pathway show clear synergistic interactions manifested as strong neurogenic bristle phenotypes. We conclude that Ed is not essential for, but it facilitates, N signaling. It is known that the N and Egfr pathways act antagonistically in bristle development. Consistently, we find that Ed also antagonizes the bristle-promoting activity of the Egfr pathway, either by the enhancement of N signalling or, similar to the eye, by a more direct action on the Egfr pathway.