Structure and regulation of the salivary gland secretion protein gene Sgs-1 of Drosophila melanogaster

Gross morphology and adhesion-associated physical properties of Drosophila larval salivary gland glue secretion

One of the major functions of the larval salivary glands (SGs) of many Drosophila species is to produce a massive secretion during puparium formation. This so-called proteinaceous glue is exocytosed into the centrally located lumen, and subsequently expectorated, serving as an adhesive to attach the puparial case to a solid substrate during metamorphosis. Although this was first described almost 70 years ago, a detailed description of the morphology and mechanical properties of the glue is largely missing. Its main known physical property is that it is released as a watery liquid that quickly hardens into a solid cement. Here, we provide a detailed morphological and topological analysis of the solidified glue. We demonstrated that it forms a distinctive enamel-like plaque that is composed of a central fingerprint surrounded by a cascade of laterally layered terraces. The solidifying glue rapidly produces crystals of KCl on these alluvial-like terraces. Since the properties of the glue affect the adhesion of the puparium to its substrate, and so can influence the success of metamorphosis, we evaluated over 80 different materials for their ability to adhere to the glue to determine which properties favor strong adhesion. We found that the alkaline Sgs-glue adheres strongly to wettable and positively charged surfaces but not to neutral or negatively charged and hydrophobic surfaces. Puparia formed on unfavored materials can be removed easily without leaving fingerprints or cascading terraces. For successful adhesion of the Sgs-glue, the material surface must display a specific type of triboelectric charge. Interestingly, the expectorated glue can move upwards against gravity on the surface of freshly formed puparia via specific, unique and novel anatomical structures present in the puparial's lateral abdominal segments that we have named bidentia.

Higher evolutionary dynamics of gene copy number for Drosophila glue genes located near short repeat sequences

BACKGROUND

During evolution, genes can experience duplications, losses, inversions and gene conversions. Why certain genes are more dynamic than others is poorly understood. Here we examine how several Sgs genes encoding glue proteins, which make up a bioadhesive that sticks the animal during metamorphosis, have evolved in Drosophila species.

RESULTS

We examined high-quality genome assemblies of 24 Drosophila species to study the evolutionary dynamics of four glue genes that are present in D. melanogaster and are part of the same gene family - Sgs1, Sgs3, Sgs7 and Sgs8 - across approximately 30 millions of years. We annotated a total of 102 Sgs genes and grouped them into 4 subfamilies. We present here a new nomenclature for these Sgs genes based on protein sequence conservation, genomic location and presence/absence of internal repeats. Two types of glue genes were uncovered. The first category (Sgs1, Sgs3x, Sgs3e) showed a few gene losses but no duplication, no local inversion and no gene conversion. The second group (Sgs3b, Sgs7, Sgs8) exhibited multiple events of gene losses, gene duplications, local inversions and gene conversions. Our data suggest that the presence of short "new glue" genes near the genes of the latter group may have accelerated their dynamics.

CONCLUSIONS

Our comparative analysis suggests that the evolutionary dynamics of glue genes is influenced by genomic context. Our molecular, phylogenetic and comparative analysis of the four glue genes Sgs1, Sgs3, Sgs7 and Sgs8 provides the foundation for investigating the role of the various glue genes during Drosophila life.

Drosophila Glue: A Promising Model for Bioadhesion

The glue produced by Drosophila larvae to attach themselves to a substrate for several days and resist predation until the end of metamorphosis represents an attractive model to develop new adhesives for dry environments. The adhesive properties of this interesting material have been investigated recently, and it was found that it binds as well as strongly adhesive commercial tapes to various types of substrates. This glue hardens rapidly after excretion and is made of several proteins. In D. melanogaster, eight glue proteins have been identified: four are long glycosylated mucoproteins containing repeats rich in prolines, serines and threonines, and four others are shorter proteins rich in cysteines. This protein mix is produced by the salivary glands through a complex packaging process that is starting to be elucidated. Drosophila species have adapted to stick to various substrates in diverse environmental conditions and glue genes appear to evolve rapidly in terms of gene number, number of repeats and sequence of the repeat motifs. Interestingly, besides its adhesive properties, the glue may also have antimicrobial activities. We discuss future perspectives and avenues of research for the development of new bioadhesives mimicking Drosophila fly glue.

Knockdown of SLC38 Transporter Ortholog - CG13743 Reveals a Metabolic Relevance in Drosophila

Solute Carrier (SLC) is a cluster of families of membrane bound transporters, of which many members lack defined substrate profile, and many more are poorly characterized. Many play a vital role in regulating metabolic systems, protein synthesis, and post translational modifications. SLC38 is one of the families of SLCs, which are also known as sodium-coupled neutral amino acid transporters (SNATs). In mice, it has 11 members (SNAT1-11) but in Drosophila there are two homologs for the SLC38 family; CG13743 and CG30394. Here, we show characteristics of Drosophila CG13743 which closely resembles SLC38A11 in humans. SLC38A11 still remains an orphan member of the SLC38 family which has not been functionally well studied. We used the UAS-GAL4 system to investigate and control gene expression using RNAi lines for ubiquitous knockdown of the CG13743 gene. It was found to be expressed mainly in salivary gland and brain. Knockdown flies had reduced body weight and consumed less sugar compared with controls. The gene knockdown also affected stored energy pools (lipids and glycogen) and influenced feeding pattern and total activity. In all, this shows novel findings for the characterization of CG13743 in Drosophila and a possible role in maintaining general metabolic pathways and behavior of the fly.

Fine infrastructure of released and solidified Drosophila larval salivary secretory glue using SEM

The Golgi-derived large secretory granules of Drosophila salivary glands (SGs) constitute the components of the salivary glue secretion (Sgs). The Sgs represents a highly special and unique extracellular composite glue matrix that has not yet been identified outside of Cyclorrhaphous Dipterans. For over half a century, the only major and unambiguously documented function of the larval salivary glands was to produce a large amount of mucinous glue-containing secretory granules that, when released during pupariation, serves to affix the freshly formed puparia to a substrate. Besides initial biochemical characterization of the Sgs proteins and cloning of their corresponding Sgs genes, very little is known about other properties and functions of the Sgs glue. We report here observations on the fine SEM-ultrastructure of the Sgs glue released into to the lumen of SGs, and after it has been expectorated and solidified into the external environment. Surprisingly, in contrast to long held expectations, it appears to be a highly structured bioadhesive mass with an internal spongious to trabecular infrastructure, reflecting the state of its hydratation. We also found that in addition to its cementing properties, it is highly efficient at glueing and trapping microorganisms, and thus may serve a potentially very important immune and defense role. High hydration capacity, the speed by which this glue can dry, uniqueness of its protein composition and spongious infrastructure can provide inspiration for development of potential biomimetics that can attach completely different or incompatible surfaces with high efficiency and strength.

Evolution of salivary glue genes in Drosophila species

Background

At the very end of the larval stage Drosophila expectorate a glue secreted by their salivary glands to attach themselves to a substrate while pupariating. The glue is a mixture of apparently unrelated proteins, some of which are highly glycosylated and possess internal repeats. Because species adhere to distinct substrates (i.e. leaves, wood, rotten fruits), glue genes are expected to evolve rapidly.

Results

We used available genome sequences and PCR-sequencing of regions of interest to investigate the glue genes in 20 Drosophila species. We discovered a new gene in addition to the seven glue genes annotated in D. melanogaster. We also identified a phase 1 intron at a conserved position present in five of the eight glue genes of D. melanogaster, suggesting a common origin for those glue genes. A slightly significant rate of gene turnover was inferred. Both the number of repeats and the repeat sequence were found to diverge rapidly, even between closely related species. We also detected high repeat number variation at the intrapopulation level in D. melanogaster.

Conclusion

Most conspicuous signs of accelerated evolution are found in the repeat regions of several glue genes.

Electronic supplementary material

The online version of this article (10.1186/s12862-019-1364-9) contains supplementary material, which is available to authorized users.

Bombyx mori transcription factors FoxA and SAGE divergently regulate the expression of wing cuticle protein gene 4 during metamorphosis

Stage-specific gene expression governs metamorphosis of the silkworm, Bombyx mori. B. mori wing cuticle protein gene 4 (BmWCP4) is an essential gene for wing disc development expressed specifically during pupation. BmWCP4 transcription is suppressed at the larval stage by unknown mechanisms, which we sought to elucidate here. Bioinformatics analysis predicted seven potential Forkhead box (Fox) cis-regulatory elements (CREs) in the BmWCP4 promoter region, and we found that Fox CRE6 contributes to suppression of BmWCP4 expression. Electrophoretic mobility shift (EMSA) and DNA pull-down assays revealed that BmFoxA suppressed activity at the BmWCP4 promoter by specifically binding to the Fox CRE6. The expression level of BmFoxA in the wing discs was higher during the larval stage than at the pupal stage. In contrast, expression of another transcription factor, BmSAGE, increased over the course of development. Of note, the hormone 20-hydroxyecdysone (20E), which governs molting in insects, suppressed BmFoxA expression in the wing discs and up-regulated that of BmSage EMSA and cell co-transfection assays indicated that BmSAGE interacted with BmFoxA and suppressed its binding to the Fox CRE6, thereby releasing BmFoxA-mediated suppression of BmWCP4 In summary, higher BmFoxA expression during the larval stage suppresses BmWCP4 expression by binding to the Fox CRE6 on the BmWCP4 promoter. During metamorphosis, BmSAGE forms a complex with BmFoxA to relieve this repression, initiating BmWCP4 expression. Taken together, this study reveals a switchlike role for BmFoxA in regulating BmWCP4 expression and provides new insights into the regulatory regulation of wing disc development in insects.

Building and specializing epithelial tubular organs: the Drosophila salivary gland as a model system for revealing how epithelial organs are specified, form and specialize

The past two decades have witnessed incredible progress toward understanding the genetic and cellular mechanisms of organogenesis. Among the organs that have provided key insight into how patterning information is integrated to specify and build functional body parts is the Drosophila salivary gland, a relatively simple epithelial organ specialized for the synthesis and secretion of high levels of protein. Here, we discuss what the past couple of decades of research have revealed about organ specification, development, specialization, and death, and what general principles emerge from these studies.

Genomic analysis of the ecdysone steroid signal at metamorphosis onset using ecdysoneless and EcRnullDrosophila melanogaster mutants

Steroid hormone gene regulation is often depicted as a linear transduction of the signal, from molecule release to the gene level, by activation of a receptor protein after being bound by its steroid ligand. Such an action would require that the hormone be present and bound to the receptor in order to have target gene response. Here, we present data that presents a novel perspective of hormone gene regulation, where the hormone molecule and its receptor have exclusive target gene regulation function, in addition to the traditional direct target genes. Our study is the first genome-wide analysis of conditional mutants simultaneously modeling the steroid and steroid receptor gene expression regulation. We have integrated classical genetic mutant experiments with functional genomics techniques in the Drosophila melanogaster model organism, where we interrogate the 20-hydroxyecdysone signaling response at the onset of metamorphosis. Our novel catalog of ecdysone target genes illustrates the separable transcriptional responses among the hormone, the pre-hormone receptor and the post-hormone receptor. We successfully detected traditional ecdysone target genes as common targets and also identified novel sets of target genes which where exclusive to each mutant condition. Around 12 % of the genome responds to the ecdysone hormone signal at the onset of metamorphosis and over half of these are independent of the receptor. In addition, a significant portion of receptor regulated genes are differentially regulated by the receptor, depending on its ligand state. Gene ontology enrichment analyses confirm known ecdysone regulated biological functions and also validate implicated pathways that have been indirectly associated with ecdysone signaling.

AP-1 and clathrin are essential for secretory granule biogenesis in Drosophila

Regulated secretion of hormones, digestive enzymes, and other biologically active molecules requires the formation of secretory granules. Clathrin and the clathrin adaptor protein complex 1 (AP-1) are necessary for maturation of exocrine, endocrine, and neuroendocrine secretory granules. However, the initial steps of secretory granule biogenesis are only minimally understood. Powerful genetic approaches available in the fruit fly Drosophila melanogaster were used to investigate the molecular pathway for biogenesis of the mucin-containing "glue granules" that form within epithelial cells of the third-instar larval salivary gland. Clathrin and AP-1 colocalize at the trans-Golgi network (TGN) and clathrin recruitment requires AP-1. Furthermore, clathrin and AP-1 colocalize with secretory cargo at the TGN and on immature granules. Finally, loss of clathrin or AP-1 leads to a profound block in secretory granule formation. These findings establish a novel role for AP-1- and clathrin-dependent trafficking in the biogenesis of mucin-containing secretory granules.

A targeted gain-of-function screen identifies genes affecting salivary gland morphogenesis/tubulogenesis in Drosophila

During development individual cells in tissues undergo complex cell-shape changes to drive the morphogenetic movements required to form tissues. Cell shape is determined by the cytoskeleton and cell-shape changes critically depend on a tight spatial and temporal control of cytoskeletal behavior. We have used the formation of the salivary glands in the Drosophila embryo, a process of tubulogenesis, as an assay for identifying factors that impinge on cell shape and the cytoskeleton. To this end we have performed a gain-of-function screen in the salivary glands, using a collection of fly lines carrying EP-element insertions that allow the overexpression of downstream-located genes using the UAS-Gal4 system. We used a salivary-gland-specific fork head-Gal4 line to restrict expression to the salivary glands, in combination with reporters of cell shape and the cytoskeleton. We identified a number of genes known to affect salivary gland formation, confirming the effectiveness of the screen. In addition, we found many genes not implicated previously in this process, some having known functions in other tissues. We report the initial characterization of a subset of genes, including chickadee, rhomboid1, egalitarian, bitesize, and capricious, through comparison of gain- and loss-of-function phenotypes.

Genes and biological processes controlled by the Drosophila FOXA orthologue Fork head

The larval salivary glands of Drosophila express the FOXA transcription factor Fork head (Fkh) before, but not after, puparium formation. Forced expression of Fkh in late prepupae prevents the programmed destruction of the tissue, which normally occurs in the early pupa. Using Affymetrix GeneChips, we analysed changes in gene expression brought about by Fkh when expressed shortly before the normal time of salivary gland death. Genes identified as responsive to Fkh include not only cell death genes, but also genes involved in autophagy, phospholipid metabolism and hormone-controlled signalling pathways. In addition, Fkh changed the expression of genes involved in glucose and fatty acid metabolism that are known to be target genes of the FOXAs in vertebrates. Premature loss of fkh induced by RNAi and gain of Fkh by ectopic expression at earlier times of development confirmed that genes identified in the microarray study are under normal developmental control by Fkh. These genes include Eip63F-1, which is expressed in both salivary glands and Malpighian tubules, suggesting that Fkh controls common aspects of the secretory function of the two organs. Eip63F-1 is one of many genes controlled by the steroid hormone 20-hydroxyecdysone that appear to be co-regulated by Fkh.

A serial lectin approach to the mucin-typeO-glycoproteome ofDrosophila melanogaster S2 cells

Identification of mucin-type O-glycosylated proteins with known functions in model organisms like Drosophila could provide keys to elucidate functions of the O-glycan moiety and proteomic analyses of O-glycoproteins in higher eukaryotes remain a challenge due to structural heterogeneity and a lack of efficient tools for their specific isolation. Here we report a strategy to evaluate the O-glycosylation potential of the embryonal hemocyte-like Drosophila Schneider 2 (S2) cell line by expression of recombinant glycosylation probes derived from tandem repeats of the human mucin MUC1 or of the Drosophila salivary gland protein Sgs1. We obtained evidence that mucin-type O-glycosylation in S2 cells grown under serum-free conditions is restricted to the Tn-antigen (GalNAcalpha-Ser/Thr) and the T-antigen (Galbeta1-3GalNAcalpha-Ser/Thr) and this structural homogeneity enables unique glycoproteomic strategies. We present a label-free strategy for the isolation, profiling and analysis of O-glycosylated proteins consisting of serial lectin affinity capture, 2-DE-based glycoprotein analysis by O-glycan specific mAbs and protein identification by MALDI-MS. Protein identity and O-glycosylation was confirmed by ESI-MS/MS with detection of diagnostic sugar oxonium-ion fragments. Using this strategy, we established 2-D reference maps and identified 21 secreted and intracellular mucin-type O-glycoproteins. Our results show that Drosophila S2 cells express O-glycoproteins involved in a wide range of biological functions including proteins of the extracellular matrix (Laminin gamma-chain, Peroxidasin and Glutactin), pathogen recognition proteins (Gnbp1), stress response proteins (Glycoprotein 93), secreted proteases (Matrix-metalloprotease 1 and various trypsin-like serine proteases), protease inhibitors (Serpin 27 A) and proteins of unknown function.

Fork head controls the timing and tissue selectivity of steroid-induced developmental cell death

Cell death during Drosophila melanogaster metamorphosis is controlled by the steroid hormone 20-hydroxyecdysone (20E). Elements of the signaling pathway that triggers death are known, but it is not known why some tissues, and not others, die in response to a particular hormone pulse. We found that loss of the tissue-specific transcription factor Fork head (Fkh) is both required and sufficient to specify a death response to 20E in the larval salivary glands. Loss of fkh itself is a steroid-controlled event that is mediated by the 20E-induced BR-C gene, and that renders the key death regulators hid and reaper hormone responsive. These results implicate the D. melanogaster FOXA orthologue Fkh with a novel function as a competence factor for steroid-controlled cell death. They explain how a specific tissue is singled out for death, and why this tissue survives earlier hormone pulses. More generally, they suggest that cell identity factors like Fkh play a pivotal role in the normal control of developmental cell death.

Broad-Complex,E74, andE75early genes control DNA puffBhC4-1expression in prepupal salivary glands

The DNA puff BhC4-1 gene of the sciarid Bradysia hygida is induced in salivary glands prior to the pupal molt as a secondary response to the increase in ecdysone titers. Previous studies demonstrated that the BhC4-1 promoter is activated in transgenic Drosophila melanogaster salivary glands as a late response to the ecdysone peak that triggers metamorphosis, revealing that this aspect of BhC4-1 transcriptional regulation is conserved in the Drosophila background. To identify regulators of BhC4-1 expression, we utilized a candidate gene approach and tested the roles of the ecdysone-induced genes BR-C, E74, and E75. Our results reveal that the BR-C Z3 isoform is essential for BhC4-1-lacZ induction in prepupal salivary glands and constitute the first demonstration of the participation of early genes products on DNA puff genes regulation.

Making tubes in the Drosophila embryo

Epithelial and endothelial tubes come in various shapes and sizes and form the basic units of many tubular organs. During embryonic development, single unbranched tubes as well as highly branched networks of tubes form from simple sheets of cells by several morphogenic movements. Studies of tube formation in the Drosophila embryo have greatly advanced our understanding of the cellular and molecular mechanisms by which tubes are formed. This review highlights recent progress on formation of the hindgut, Malpighian tubules, proventriculus, salivary gland, and trachea of the Drosophila embryo, focusing on the cellular events that form each tube and their genetic requirements.

Nutritional regulation of vitellogenesis in mosquitoes: implications for anautogeny

Anautogeny is a successful reproductive strategy utilized by many mosquito species and other disease-transmitting arthropod vectors. Developing an understanding of the mechanisms underlying anautogeny in mosquitoes is very important because this reproductive strategy is the driving force behind the transmission of disease to millions of people. Information gained from mosquito studies may also be applicable to other blood feeding insect vectors. The conversion of protein from blood into yolk protein precursors for the developing oocytes is an essential part of the reproductive cycle, and understanding how this process is regulated could lead to safe, specific, and effective ways to block reproduction in blood feeding insects. Great gains have been made in elucidating the mechanisms that regulate vitellogenesis in mosquitoes, especially Ae. aegypti. However, a number of questions remain to be answered to make the picture more complete. In this review, we summarize what is currently known about the nutritional regulation of vitellogenesis in mosquitoes and the questions that remain to be answered about this important biological phenomenon.

Whole-genome analysis of temporal gene expression during foregut development

We have investigated the cis-regulatory network that mediates temporal gene expression during organogenesis. Previous studies demonstrated that the organ selector gene pha-4/FoxA is critical to establish the onset of transcription of Caenorhabditis elegans foregut (pharynx) genes. Here, we discover additional cis-regulatory elements that function in combination with PHA-4. We use a computational approach to identify candidate cis-regulatory sites for genes activated either early or late during pharyngeal development. Analysis of natural or synthetic promoters reveals that six of these sites function in vivo. The newly discovered temporal elements, together with predicted PHA-4 sites, account for the onset of expression of roughly half of the pharyngeal genes examined. Moreover, combinations of temporal elements and PHA-4 sites can be used in genome-wide searches to predict pharyngeal genes, with more than 85% accuracy for their onset of expression. These findings suggest a regulatory code for temporal gene expression during foregut development and provide a means to predict gene expression patterns based solely on genomic sequence.

The DNA binding of insect Fork head factors is strongly influenced by the negative cooperation of neighbouring bases

The Drosophila melanogaster Fork head and Bombyx mori SGF1/Fork head proteins are key regulators of tissue specific gene expression in the modified larval labial glands. Here we use the competitive electrophoretic mobility shift assay to create a detailed Fork head binding matrix and we investigate some unusual features of the Fork head interaction with DNA. We found that the Fork head-DNA interaction is context dependent--the binding specificity of the protein is partly determined by specific combinations of neighbouring bases. Although the total number of the sub-optimal dinucleotide steps is not high, the negative cooperation of neighbouring bases significantly contributes to the overall binding site specificity. Our results allow efficient recognition of insect Fork head binding sites and we show that the putative Fork head cognate elements preferentially accumulate in the near upstream region of genes abundantly expressed in the labial gland.

Constructing an organ: the Drosophila salivary gland as a model for tube formation

Tubes are required in metazoans to transport the liquids and gases that sustain life. The conservation of molecules and mechanisms involved in tube formation suggests that what we learn by studying simple systems will apply to related processes in higher animals. Studies over the past 10 years have revealed the molecules that specify cell fate in Drosophila salivary gland and the cellular events that mediate tube morphogenesis. Here, we discuss how anterior-posterior and dorsal-ventral patterning information specifies both the position of salivary-gland primordia and how many cells they contain. We examine the transformation of a polarized epithelial sheet into an elongated, unbranched tube, and the intrinsic and extrinsic factors that influence the final position of the salivary gland.

Functional analysis of regulatory elements controlling the expression of the ecdysone-regulated Drosophila ng-1 gene

The steroid hormone ecdysone controls multiple aspects of insect development, including larval moults and metamorphosis, and can induce specific genetic responses in different tissues. The definition of the molecular mechanisms able to mediate this tissue-specific responsiveness may greatly contribute to understanding how such an accurate genetic response is achieved. In this work we have identified, by transgenic analysis, the regulatory elements directing the expression of ng-1, an ecdysone-regulated Drosophila gene showing a highly specific developmental expression profile. Our results show that an ecdysone-responsive element located within the ng-1 coding region is necessary for high-level gene expression, whereas the gene's spatial and temporal expression profile is fully controlled by a distinct upstream regulatory region. This region binds a set of transcriptional factors, including the FKH regulatory protein, which can potentially modulate the ecdysone genetic regulated response.

The helix-loop-helix proteins dAP-4 and daughterless bind both in vitro and in vivo to SEBP3 sites required for transcriptional activation of the Drosophila gene Sgs-4

The expression of Sgs genes in the salivary gland of the third instar larva of Drosophila is a spatially restricted response to signalling by the steroid hormone 20-hydroxyecdysone. For Sgs-4, we have previously demonstrated that its strictly tissue and stage-specific expression is the result of combined action of the ecdysone receptor and secretion enhancer binding proteins (SEBPs). One of these SEBPs, SEBP2, was shown to be the product of the homeotic gene fork head. Together with SEBP3, SEBP2 appears to be responsible for the spatial restriction of the hormone response of Sgs-4. Here, we show that SEBP3 is a heterogeneous binding activity that consists of different helix-loop-helix (HLH) proteins. We cloned the Drosophila homologue of human transcription factor AP-4 (dAP-4) and identified it as one of these HLH proteins. The dAP-4 protein shows great similarity to its human and Caenorhabditis counterparts within the bHLHZip domain, the second leucine zipper dimerization motif, and a third region of unknown function. The expression pattern of dAP-4 indicates that it is a ubiquitously expressed HLH protein in Drosophila. As a second component of SEBP3 we identified the Daughterless (Da) protein, which is also ubiquitously expressed and binds to SEBP3 sites independent of dAP-4. Since both dAP-4 and Da can be detected in situ at transposed Sgs-4 transcriptional control elements in polytene salivary gland chromosomes, we propose that each of the two proteins contributes to the transcriptional control of Sgs-4.