Ecdysone triggers the expression of Golgi genes in Drosophila imaginal discs via broad-complex

Silencing Br-C impairs larval development and chitin synthesis in Lymantria dispar larvae

In insects, 20-hydroxyecdysone (20E) mediates developmental transitions and regulates molting processes through activation of a series of transcription factors. Broad-Complex (Br-C), a vital gene in the 20E signalling pathway, plays crucial roles during insect growth processes. However, whether Br-C affects chitin synthesis in insects remains unclear. In the present study, the Br-C gene from Lymantria dispar, a notorious defoliator of forestry, was identified based on transcriptome data, and subjected to bioinformatic analysis. The regulatory functions of LdBr-C in chitin synthesis and metabolism in L. dispar larvae were analysed by RNA interference (RNAi). The full-length LdBr-C gene (1431 bp) encodes a 477 amino acid (aa) polypeptide containing a common BRcore region (391 aa) at the N-terminus and a C-terminal Zinc finger domain (56 aa) harbouring two characteristic C2H2 motifs (CXXC and HXXXXH). Phylogenetic analyses showed that LdBr-C shares highest homology and identity with Br-C isoform 7 (83.12%) of Helicoverpa armigera. Expression profiles indicate that LdBr-C was expressed throughout larval and pupal stages, and highly expressed in prepupal and pupal stages. Furthermore, LdBr-C expression was strongly induced by exogenous 20E, and suppressed dramatically after application of dsLdBr-C. Bioassay results showed that knockdown of LdBr-C caused larval developmental deformity, significant weight loss, and a mortality rate of 67.18%. Knockdown of LdBr-C significantly down-regulated transcription levels of eight critical genes (LdTre1, LdTre2, LdG6PI, LdUAP, LdCHS1, LdCHS2, LdTPS and LdCHT) related to chitin synthesis and metabolism, thereby lowering the chitin content in the midgut and epidermis. Our findings demonstrate that Br-C knockdown impairs larval development and chitin synthesis in L. dispar.

Genome-wide identification of alternative splicing events that regulate protein transport across the secretory pathway

Alternative splicing (AS) strongly increases proteome diversity and functionality in eukaryotic cells. Protein secretion is a tightly-controlled process, especially in a tissue-specific and differentiation-dependent manner. While previous work has focussed on transcriptional and post-translational regulatory mechanisms, the impact of AS on the secretory pathway remains largely unexplored. Here we integrate a published screen for modulators of protein transport and RNA-Seq analyses to identify over 200 AS events as secretion regulators. We confirm that splicing events along all stages of the secretory pathway regulate the efficiency of membrane trafficking using Morpholinos and CRISPR/Cas9. We furthermore show that these events are highly tissue-specific and adapt the secretory pathway during T-cell activation and adipocyte differentiation. Our data substantially advance the understanding of AS functionality, add a new regulatory layer to a fundamental cell biological process and provide a resource of alternative isoforms that control the secretory pathway.

Knockdown of Broad-Complex Gene Expression of Bombyx mori by Oligopyrrole Carboxamides Enhances Silk Production

Bombyx mori (B. mori) is important due to its major role in the silk production. Though DNA binding ligands often influence gene expression, no attempt has been made to exploit their use in sericulture. The telomeric heterochromatin of B. mori is enriched with 5'-TTAGG-3' sequences. These sequences were also found to be present in several genes in the euchromatic regions. We examined three synthetic oligopyrrole carboxamides that target 5'-TTAGG-3' sequences in controlling the gene expression in B. mori. The ligands did not show any defect or feeding difference in the larval stage, crucial for silk production. The ligands caused silencing of various isoforms of the broad-complex transcription factor and cuticle proteins which resulted in late pupal developmental defects. Furthermore, treatment with such drugs resulted in statistically enhanced cocoon weight, shell weight, and silk yield. This study shows for the first time use of oligopyrrole carboxamide drugs in controlling gene expression in B. mori and their long term use in enhancing silk production.

An RNAi Screen for Genes Involved in Nanoscale Protrusion Formation on Corneal Lens in Drosophila melanogaster

The "moth-eye" structure, which is observed on the surface of corneal lens in several insects, supports anti-reflective and self-cleaning functions due to nanoscale protrusions known as corneal nipples. Although the morphology and function of the "moth-eye" structure, are relatively well studied, the mechanism of protrusion formation from cell-secreted substances is unknown. In Drosophila melanogaster, a compound eye consists of approximately 800 facets, the surface of which is formed by the corneal lens with nanoscale protrusions. In the present study, we sought to identify genes involved in "moth-eye" structure, formation in order to elucidate the developmental mechanism of the protrusions in Drosophila. We re-examined the aberrant patterns in classical glossy-eye mutants by scanning electron microscope and classified the aberrant patterns into groups. Next, we screened genes encoding putative structural cuticular proteins and genes involved in cuticular formation using eye specific RNAi silencing methods combined with the Gal4/UAS expression system. We identified 12 of 100 candidate genes, such as cuticular proteins family genes (Cuticular protein 23B and Cuticular protein 49Ah), cuticle secretion-related genes (Syntaxin 1A and Sec61 ββ subunit), ecdysone signaling and biosynthesis-related genes (Ecdysone receptor, Blimp-1, and shroud), and genes involved in cell polarity/cell architecture (Actin 5C, shotgun, armadillo, discs large1, and coracle). Although some of the genes we identified may affect corneal protrusion formation indirectly through general patterning defects in eye formation, these initial findings have encouraged us to more systematically explore the precise mechanisms underlying the formation of nanoscale protrusions in Drosophila.

Trafficking along the secretory pathway in Drosophila cell line and tissues: a light and electron microscopy approach

In the past, Drosophila has been used for molecular and developmental biology studies that have led to many important conceptual advances. In the last decade, this model organism has also been utilized to address cell biology issues, in particular those related to membrane traffic through the secretory pathway. This has confirmed that the functional organization of the secretory pathway is conserved and it allowed further integrating secretion to signaling and development. Furthermore, Drosophila tissue culture S2 cells have been the basis of many RNAi screens, some addressing aspects of the functional organization of the secretory pathway and others identifying proteins of the secretory pathway in seemingly unrelated processes. Taken together, studying the protein trafficking and the organization of the secretory pathway both in S2 cells and in tissues has become important. Here, we review light and electron microscopy techniques applied to Drosophila that allow gaining insight into the secretory pathway, and can easily be extended to other cell biology-related fields.

Signalling to and from the secretory pathway

For growth, survival, communication and homeostasis, cells transport a large number of proteins to the plasma membrane and the extracellular medium by using the secretory pathway. Consequently, to adapt to the surrounding environment and the different intracellular contexts, the secretory pathway needs to accommodate and respond to a plethora of endogenous and exogenous stimuli. It is now well established that several kinases, known to be activated by environmental stimuli, signal from the plasma membrane to the secretory pathway in order to remodel its architecture and modulate the cellular secretion capacity. By contrast, membranes of the early secretory pathway, similar to the endosomal system, can also initiate and modulate signalling cascades, thereby spatially organising cellular signalling and eliciting a different cellular outcome than when signalling is localised to the plasma membrane. This Commentary highlights recent contributions to our understanding of the mutual regulation of the secretory pathway and cellular signalling.

The Golgi apparatus: lessons from Drosophila

Historically, Drosophila has been a model organism for studying molecular and developmental biology leading to many important discoveries in this field. More recently, the fruit fly has started to be used to address cell biology issues including studies of the secretory pathway, and more specifically on the functional integrity of the Golgi apparatus. A number of advances have been made that are reviewed below. Furthermore, with the development of RNAi technology, Drosophila tissue culture cells have been used to perform genome-wide screens addressing similar issues. Last, the Golgi function has been involved in specific developmental processes, thus shedding new light on the functions of a number of Golgi proteins.

Integrins mediate their unconventional, mechanical-stress-induced secretion via RhoA and PINCH in Drosophila

During the epithelium remodelling such as the flattening of the Drosophila follicular epithelium, the alpha-integrin subunits are unconventionally secreted through a dGRASP-dependent route that is built de novo. The biogenetic process starts with the upregulation of a small subset of targeted mRNAs, including dgrasp. Here, we show that dgrasp mRNA upregulation is triggered by the tension of the underlying oocyte and by applied external forces at the basal side of the follicular epithelium. We show that integrins are also involved in dgrasp mRNA upregulation and the epithelium remodelling. Tension leads to the recruitment of RhoA to the plasma membrane, where it participates in its remodelling. The LIM protein PINCH can cycle to the nucleus and is involved in dgrasp mRNA upregulation. We propose that integrins are involved in triggering the biogenesis of their own unconventional secretion route that they use to strengthen adhesion and ensure epithelial integrity at the next stages of development, perhaps by acting as mechanosensors of the underlying tension through RhoA and PINCH.

dGRASP-mediated noncanonical integrin secretion is required for Drosophila epithelial remodeling

Integral plasma membrane proteins are typically transported in the secretory pathway from the endoplasmic reticulum and the Golgi complex. Here we show that at specific stages of Drosophila development corresponding to morphological changes in epithelia, apposed basolateral membranes separate slightly, allowing new plasma membrane contacts with basal extracellular matrix. At these sites, newly synthesized integrin alpha subunits are deposited via a mechanism that appears to bypass the Golgi. We show that the Drosophila Golgi resident protein dGRASP localizes to these membrane domains and that, in the absence of dGRASP, the integrin subunit is retained intracellularly in both follicular and wing epithelia that are found disrupted. We propose that this dGRASP-mediated noncanonical secretion route allows for developmental regulation of integrin function upon epithelial remodeling. We speculate that this mechanism might be used during development as a means of targeting a specific subset of transmembrane proteins to the plasma membrane.

The cis-Golgi Drosophila GMAP has a role in anterograde transport and Golgi organization in vivo, similar to its mammalian ortholog in tissue culture cells

The Golgi microtubule-associated protein of 210kDa (GMAP-210) has been shown to play a role in the assembly and maintenance of the Golgi apparatus in mammalian tissue culture cells. To develop a genetic model to study the function of GMAP-210 in vivo, we identified its closest relative in Drosophila melanogaster, dGMAP. We show that the dGMAP gene encodes two alternatively spliced transcripts, only one of which is translated into a protein product. To gain insight into the role of dGMAP, we generated a polyclonal antibody and investigated the protein distribution during development. This gene is ubiquitously expressed during embryonic and larval development with the highest level in polar cells, gut and salivary glands. We further show that dGMAP is present in the Golgi apparatus, and using electron microscopy of salivary glands, we observed a preferential localization at the cis side and at the rims of the Golgi stacks. Finally, we demonstrate that overexpression of dGMAP in salivary glands impairs Golgi architecture and function, whereas RNAi-mediated depletion of dGMAP does not induce any structural modification of the Golgi apparatus, and anterograde transport is seemingly unaffected. Altogether our results suggest that dGMAP is the ortholog of mammalian GMAP-210.

Expression of COPI components during development of Drosophila melanogaster

In a P{lArB} enhancer detector collection, a line was found that showed upregulated expression within centrally to posteriorly located germarial cysts. It was inserted in the gammaCOP locus on chromosome 3R. GammaCOP is a component of the COPI coatomer involved in membrane traffic. Most of the other known components of the COPI coatomer also showed higher expression in the posterior half of the germarium. Not only meiotic germline cysts but also migrating follicle cells upregulate the COPI subunits. During embryonic and larval development, the COPI subunits are expressed ubiquitously as expected for genes required for cell viability. In addition, they are strongly expressed in the salivary glands and the proventriculus. Whether tissue-specific transcriptional upregulation of COPI subunits is required for the reorganization of membranous compartments that are needed for the developmental processes that confer cyst polarity and follicle maturation will have to be addressed in a genetic study.

Transcription of Myocyte enhancer factor-2 in adult Drosophila myoblasts is induced by the steroid hormone ecdysone

The steroid hormone 20-hydroxyecdysone (ecdysone) activates a relatively small number of immediate-early genes during Drosophila pupal development, yet is able to orchestrate distinct differentiation events in a wide variety of tissues. Here, we demonstrate that expression of the muscle differentiation gene Myocyte enhancer factor-2 (Mef2) is normally delayed in twist-expressing adult myoblasts until the end of the third larval instar. The late up-regulation of Mef2 transcription in larval myoblasts is an ecdysone-dependent event which acts upon an identified Mef2 enhancer, and we identify enhancer sequences required for up-regulation. We also present evidence that the ecdysone-induced Broad Complex of zinc finger transcription factor genes is required for full activation of the myogenic program in these cells. Since forced early expression of Mef2 in adult myoblasts leads to premature muscle differentiation, our results explain how and why the adult muscle differentiation program is attenuated prior to pupal development. We propose a mechanism for the initiation of adult myogenesis, whereby twist expression in myoblasts provides a cellular context upon which an extrinsic signal builds to control muscle-specific differentiation events, and we discuss the general relevance of this model for gene regulation in animals.

Interaction between hormonal signaling pathways in Drosophila melanogaster as revealed by genetic interaction between methoprene-tolerant and broad-complex

Juvenile hormone (JH) regulates insect development by a poorly understood mechanism. Application of JH agonist insecticides to Drosophila melanogaster during the ecdysone-driven onset of metamorphosis results in lethality and specific morphogenetic defects, some of which resemble those in mutants of the ecdysone-regulated Broad-Complex (BR-C). The Methoprene-tolerant (Met) bHLH-PAS gene mediates JH action, and Met mutations protect against the lethality and defects. To explore relationships among these two genes and JH, double mutants were constructed between Met alleles and alleles of each of the BR-C complementation groups: broad (br), reduced bristles on palpus (rbp), and 2Bc. Defects in viability and oogenesis were consistently more severe in rbp Met or br Met double mutants than would be expected if these genes act independently. Additionally, complementation between BR-C mutant alleles often failed when MET was absent. Patterns of BRC protein accumulation during metamorphosis revealed essentially no difference between wild-type and Met-null individuals. JH agonist treatment did not block accumulation of BRC proteins. We propose that MET and BRC interact to control transcription of one or more downstream effector genes, which can be disrupted either by mutations in Met or BR-C or by application of JH/JH agonist, which alters MET interaction with BRC.

The steroid hormone-regulated geneBroad Complex is required for dendritic growth of motoneurons during metamorphosis ofDrosophila

Dendrites are subject to subtle modifications as well as extensive remodeling during the assembly and maturation of neural circuits in a wide variety of organisms. During metamorphosis, Drosophila flight motoneurons MN1-MN4 undergo dendritic regression, followed by regrowth, whereas MN5 differentiates de novo (Consoulas et al. [2002] J. Neurosci. 22:4906-4917). Many cellular changes during metamorphosis are triggered and orchestrated by the steroid hormone 20-hydroxyecdysone, which initiates a cascade of coordinated gene expression. Broad Complex (BRC), a primary response gene in the ecdysone cascade, encodes a family of transcription factors (BRC-Z1-Z4) that are essential for metamorphic reorganization of the central nervous system (CNS). Using neuron-filling techniques that reveal cellular morphology with very high resolution, we tested the hypothesis that BRC is required for metamorphic development of MN1-MN5. Through a combination of loss-of-function mutant analyses, genetic mapping, and transgenic rescue experiments, we found that 2Bc function, mediated by BRC-Z3, is required selectively for motoneuron dendritic regrowth (MN1-MN4) and de novo outgrowth (MN5), as well as for soma expansion of MN5. In contrast, larval development and dendritic regression of MN1-MN4 are BRC-independent. Surprisingly, BRC proteins are not expressed in the motoneurons, suggesting that BRC-Z3 exerts its effect in a non-cell-autonomous manner. The 2Bc mutants display no gross defects in overall thoracic CNS structure, or in peripheral structures such as target muscles or sensory neurons. Candidates for mediating the effect of BRC-Z3 on dendritic growth of MN1-MN5 include their synaptic inputs and non-neuronal CNS cells that interact with them through direct contact or diffusible factors.

Genetic analysis of soluble N-ethylmaleimide-sensitive factor attachment protein function in Drosophila reveals positive and negative secretory roles

The N-ethylmaleimide-sensitive factor (NSF) and soluble NSF attachment protein (SNAP) are cytosolic factors that promote vesicle fusion with a target membrane in both the constitutive and regulated secretory pathways. NSF and SNAP are thought to function by catalyzing the disassembly of a SNAP receptor (SNARE) complex consisting of membrane proteins of the secretory vesicle and target membrane. Although studies of NSF function have provided strong support for this model, the precise biochemical role of SNAP remains controversial. To further explore the function of SNAP, we have used mutational and transgenic approaches in Drosophila to investigate the effect of altered SNAP dosage on neurotransmitter release and SNARE complex metabolism. Our results indicate that reduced SNAP activity results in diminished neurotransmitter release and accumulation of a neural SNARE complex. Increased SNAP dosage results in defective synapse formation and a variety of tissue morphological defects without detectably altering the abundance of neural SNARE complexes. The SNAP overexpression phenotypes are enhanced by mutations in other secretory components and are at least partially overcome by co-overexpression of NSF, suggesting that these phenotypes derive from a specific perturbation of the secretory pathway. Our results indicate that SNAP promotes neurotransmitter release and SNARE complex disassembly but inhibits secretion when present at high abundance relative to NSF.

MicroRNA targets in Drosophila

A computational method for whole-genome prediction of microRNA target genes is presented. Application of this method to the Drosophila melanogaster, Drosophila pseudoobscura and Anopheles gambiae genomes identifies several hundred target genes potentially regulated by one or more known microRNAs.

Background

The recent discoveries of microRNA (miRNA) genes and characterization of the first few target genes regulated by miRNAs in Caenorhabditis elegans and Drosophila melanogaster have set the stage for elucidation of a novel network of regulatory control. We present a computational method for whole-genome prediction of miRNA target genes. The method is validated using known examples. For each miRNA, target genes are selected on the basis of three properties: sequence complementarity using a position-weighted local alignment algorithm, free energies of RNA-RNA duplexes, and conservation of target sites in related genomes. Application to the D. melanogaster, Drosophila pseudoobscura and Anopheles gambiae genomes identifies several hundred target genes potentially regulated by one or more known miRNAs.

Results

These potential targets are rich in genes that are expressed at specific developmental stages and that are involved in cell fate specification, morphogenesis and the coordination of developmental processes, as well as genes that are active in the mature nervous system. High-ranking target genes are enriched in transcription factors two-fold and include genes already known to be under translational regulation. Our results reaffirm the thesis that miRNAs have an important role in establishing the complex spatial and temporal patterns of gene activity necessary for the orderly progression of development and suggest additional roles in the function of the mature organism. In addition the results point the way to directed experiments to determine miRNA functions.

Conclusions

The emerging combinatorics of miRNA target sites in the 3' untranslated regions of messenger RNAs are reminiscent of transcriptional regulation in promoter regions of DNA, with both one-to-many and many-to-one relationships between regulator and target. Typically, more than one miRNA regulates one message, indicative of cooperative translational control. Conversely, one miRNA may have several target genes, reflecting target multiplicity. As a guide to focused experiments, we provide detailed online information about likely target genes and binding sites in their untranslated regions, organized by miRNA or by gene and ranked by likelihood of match. The target prediction algorithm is freely available and can be applied to whole genome sequences using identified miRNA sequences.

Evolution of the Drosophila broad locus: the Manduca sexta broad Z4 isoform has biological activity in Drosophila

The Drosophila melanogaster broad locus is essential for normal metamorphic development. Broad encodes three genetically distinct functions (rbp, br, and 2Bc) and a family of four zinc-finger DNA-binding proteins (Z1-Z4). The Z1, Z2, and Z3 protein isoforms are primarily associated with the rbp, br, and 2Bc genetic functions respectively. The Z4 protein isoform also provides some rbp genetic function, however an essential function for the Z4 isoform in metamorphosis has not been identified. To determine the degree of conservation of Z4 function between the tobacco hornworm Manduca sexta and Drosophila we generated transgenic Drosophila expressing the Manduca broad Z4 isoform and used this transgene to rescue rbp mutant lethality during Drosophila metamorphosis. We find that the Manduca Z4 protein has significant biological activity in Drosophila with respect to rescue of rbp-associated lethality. There was also some overlap in effects on cuticle gene expression between the Manduca Z4 and Drosophila Z1 isoforms that was not shared with the Drosophila Z4 isoform. Our findings show that Z4 function has been conserved over the 260-million-year period since the divergence of Diptera and Lepidoptera, and are consistent with the hypothesis that the Drosophila Z4 and Manduca Z4 isoforms have essential roles in metamorphosis.

A novel role for dp115 in the organization of tER sites in Drosophila

Here, we describe that depletion of the Drosophila homologue of p115 (dp115) by RNA interference in Drosophila S2 cells led to important morphological changes in the Golgi stack morphology and the transitional ER (tER) organization. Using conventional and immunoelectron microscopy and confocal immunofluorescence microscopy, we show that Golgi stacks were converted into clusters of vesicles and tubules, and that the tERs (marked by Sec23p) lost their focused organization and were now dispersed throughout the cytoplasm. However, we found that this morphologically altered exocytic pathway was nevertheless largely competent in anterograde protein transport using two different assays. The effects were specific for dp115. Depletion of the Drosophila homologues of GM130 and syntaxin 5 (dSed5p) did not lead to an effect on the tER organization, though the Golgi stacks were greatly vesiculated in the cells depleted of dSed5p. Taken together, these studies suggest that dp115 could be implicated in the architecture of both the Golgi stacks and the tER sites.