Noncoordinate expression of Drosophila glue genes: Sgs-4 is expressed at many stages and in two different tissues

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.

A novel ecdysone receptor mediates steroid-regulated developmental events during the mid-third instar of Drosophila

The larval salivary gland of Drosophila melanogaster synthesizes and secretes glue glycoproteins that cement developing animals to a solid surface during metamorphosis. The steroid hormone 20-hydroxyecdysone (20E) is an essential signaling molecule that modulates most of the physiological functions of the larval gland. At the end of larval development, it is known that 20E--signaling through a nuclear receptor heterodimer consisting of EcR and USP--induces the early and late puffing cascade of the polytene chromosomes and causes the exocytosis of stored glue granules into the lumen of the gland. It has also been reported that an earlier pulse of hormone induces the temporally and spatially specific transcriptional activation of the glue genes; however, the receptor responsible for triggering this response has not been characterized. Here we show that the coordinated expression of the glue genes midway through the third instar is mediated by 20E acting to induce genes of the Broad Complex (BRC) through a receptor that is not an EcR/USP heterodimer. This result is novel because it demonstrates for the first time that at least some 20E-mediated, mid-larval, developmental responses are controlled by an uncharacterized receptor that does not contain an RXR-like component.

Characterization of mucin-type core-1 beta1-3 galactosyltransferase homologous enzymes in Drosophila melanogaster

Mucin type O-glycosylation is a widespread modification of eukaryotic proteins. The transfer of N-acetylgalactosamine to selected serine or threonine residues is catalyzed by a family of polypeptide N-acetylgalactosaminyltransferases localized in the Golgi apparatus. The most abundant elongation of O-glycans is the addition of a beta1-3 linked galactose by the core-1 beta1-3 galactosyltransferase (core-1 beta3GalT), thereby building the T-antigen or core-1 structure Gal(beta1-3)GalNAc(alpha1-O). We have isolated four Drosophila melanogaster cDNAs encoding proteins structurally similar to the human core-1 beta3GalT enzyme and expressed them as FLAG-tagged proteins in Sf9 insect cells. The identity of these D. melanogasterbeta3GalT enzymes with a core-1 beta3GalT activity was confirmed by utilization of MUC5AC mucin derived O-glycopeptide acceptors. In addition to the core-1 beta3GalT activity toward O-glycoprotein substrates, one member of this enzyme family showed a strong activity towards glycolipid acceptors, thereby building the core-1 terminated Nz6 glycosphingolipid. Transcripts of the embryonically expressed core-1 beta3GalTs were found in the maternally deposited mRNA, in salivary glands and in the amnioserosa. The presence of multiple core-1 beta3GalT genes in D. melanogaster suggests an increased complexity of core-1 O-glycan expression, which is possibly related to multiple developmental and physiological functions attributable to this class of glycans.

Discriminant analysis to evaluate clustering of gene expression data

In this work we present a procedure that combines classical statistical methods to assess the confidence of gene clusters identified by hierarchical clustering of expression data. This approach was applied to a publicly released Drosophila metamorphosis data set [White et al., Science 286 (1999) 2179-2184]. We have been able to produce reliable classifications of gene groups and genes within the groups by applying unsupervised (cluster analysis), dimension reduction (principal component analysis) and supervised methods (linear discriminant analysis) in a sequential form. This procedure provides a means to select relevant information from microarray data, reducing the number of genes and clusters that require further biological analysis.

Microarray analysis of Drosophila development during metamorphosis

Metamorphosis is an integrated set of developmental processes controlled by a transcriptional hierarchy that coordinates the action of hundreds of genes. In order to identify and analyze the expression of these genes, high-density DNA microarrays containing several thousand Drosophila melanogaster gene sequences were constructed. Many differentially expressed genes can be assigned to developmental pathways known to be active during metamorphosis, whereas others can be assigned to pathways not previously associated with metamorphosis. Additionally, many genes of unknown function were identified that may be involved in the control and execution of metamorphosis. The utility of this genome-based approach is demonstrated for studying a set of complex biological processes in a multicellular organism.

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

The Drosophila melanogaster gene Sgs-1 belongs to the secretion protein genes, which are coordinately expressed in salivary glands of third instar larvae. Earlier analysis had implied that Sgs-1 is located at the 25B2-3 puff. We cloned Sgs-1 from a YAC covering 25B2-3. Despite using a variety of vectors and Escherichia coli strains, subcloning from the YAC led to deletions within the Sgs-1 coding region. Analysis of clonable and unclonable sequences revealed that Sgs-1 mainly consists of 48-bp tandem repeats encoding a threonine-rich protein. The Sgs-1 inserts from single lambda clones are heterogeneous in length, indicating that repeats are eliminated. By analyzing the expression of Sgs-1/lacZ fusions in transgenic flies, cis-regulatory elements of Sgs-1 were mapped to lie within 1 kb upstream of the transcriptional start site. Band shift assays revealed binding sites for the transcription factor fork head (FKH) and the factor secretion enhancer binding protein 3 (SEBP3) at positions that are functionally relevant. FKH and SEBP3 have been shown previously to be involved in the regulation of Sgs-3 and Sgs-4. Comparison of the levels of steady state RNA and of the transcription rates for Sgs-1 and Sgs-1/lacZ reporter genes indicates that Sgs-1 RNA is 100-fold more stable than Sgs-1/lacZ RNA. This has implications for the model of how Sgs transcripts accumulate in late third instar larvae.

Dual requirement for the EcR/USP nuclear receptor and the dGATAb factor in an ecdysone response in Drosophila melanogaster

The EcR/USP nuclear receptor controls Drosophila metamorphosis by activating complex cascades of gene transcription in response to pulses of the steroid hormone ecdysone at the end of larval development. Ecdysone release provides a ubiquitous signal for the activation of the receptor, but a number of its target genes are induced in a tissue- and stage-specific manner. Little is known about the molecular mechanisms involved in this developmental modulation of the EcR/USP-mediated pathway. Fbp1 is a good model of primary ecdysone response gene expressed in the fat body for addressing this question. We show here that the dGATAb factor binds to three target sites flanking an EcR/USP binding site in a 70-bp enhancer that controls the tissue and stage specificity of Fbp1 transcription. We demonstrate that one of these sites and proper expression of dGATAb are required for specific activation of the enhancer in the fat body. In addition, we provide further evidence that EcR/USP plays an essential role as a hormonal timer. Our study provides a striking example of the integration of molecular pathways at the level of a tissue-specific hormone response unit.

Ecdysone-regulation of synthesis and processing of fat body protein 1, the larval serum protein receptor of Drosophila melanogaster

At the end of the third larval instar of Drosophila melanogaster, larval serum proteins 1 and 2 (LSP-1 and -2) are taken up by cells of the fat body. Here, we show that the product of the ecdysteroid-inducible gene Fbp-1 (Fat Body Protein 1) is the receptor that binds LSP-1. Transcription and translation of Fbp-1 is stage-specifically restricted to the end of the third larval instar, starting around 99 h after egg laying. Expression of Fbp-1 is induced by a low level of 20-hydroxy-ecdysone (>/= 10-7 m). After translation, the FBP-1 protein is thought to be proteolytically cleaved in three subsequent steps. The final cleavage step is delayed by 6 h and relies on a higher concentration of ecdysone (>/= 10-5 m). Therefore, 20-hydroxy-ecdysone regulates Fbp-1 expression and function at two different levels. To the best of our knowledge, this study is the first to date to demonstrate two distinct functions for different concentrations of a steroid hormone on a single biochemical process.

Promoter-directed expression of recombinant fire-fly luciferase in the salivary glands of Hermes-transformed Aedes aegypti

Molecular genetic analyses of biological properties characteristic of insect vectors of disease, such as hematophagy and competence for pathogens, require the ability to isolate and characterize genes involved in these processes. We have been working to develop molecular approaches for studying the promoter function of genes that are expressed specifically in the adult salivary glands of the yellow fever mosquito, Aedes aegypti. Genomic DNA fragments containing cis-acting promoter elements from the Maltase-like I (MalI) and Apyrase (Apy) genes were cloned so as to direct the expression of the reporter gene, luciferase (luc). The function of the promoters was assayed transiently in cultured insect cells and by germ-line transformation of Ae. aegypti. MalI and Apy DNA fragments consisting of at least 650 nucleotides (nt) of DNA immediately adjacent to the 5'-end of the initiation codon of the mosquito genes directed constitutive expression of the luc reporter gene in cultured cells. When introduced into Ae. aegypti chromosomes, approximately 1.5 kilobases (kb) of each promoter were able to direct the predicted developmental-, sex- and tissue-specific expression of the reporter gene in patterns identical to those determined for the respective endogenous genes.

Relationships between protein isoforms and genetic functions demonstrate functional redundancy at the Broad-Complex during Drosophila metamorphosis

Metamorphosis in holometabolous insects is an ecdysone-dependent process by which the larval form is replaced by a reproductive, adult form. At the onset of metamorphosis ecdysone induces a set of early genes which coordinate tissue-specific responses to hormone. The Broad-Complex (BR-C) early gene, which acts as a global regulator of tissue-specific responses to ecdysone, encodes a family of zinc-finger DNA binding proteins known as Z1, Z2, Z3, and Z4. Genetically the BR-C encodes three complementing functions, br, rbp, and 2Bc, and a class of npr1 alleles that fail to complement any of the other genetic functions. The effects of BR-C mutations on metamorphic development are highly pleiotropic, yet little is known about the roles of individual BR-C proteins in directing the required responses to ecdysone. Because the BR-C is a vital regulator of metamorphosis it is essential to establish the relationships between BR-C genetic functions and protein products. We present here the first general and definitive study of these relationships. Using heat-inducible transgenes we have rescued lethality associated with each of the complementing genetic functions and have restored transcriptional activity of tissue-specific BR-C(+)-dependent target genes. Our data lead us to conclude that br+ function is only provided by the Z2 isoform. We find that Z1 transgenes provide full rbp+ function, while Z4 provides partial function. Likewise, while Z3 provides full 2Bc+ function, Z2 also provides partial function. These results indicate possible functional redundancy or regulatory dependence (via autoregulation) associated with the rbp+ and 2Bc+ functions. The establishment of these relationships between BR-C genetic functions and protein isoforms is an important step toward understanding the roles of BR-C proteins in directing metamorphic responses to ecdysone.

The Drosophila 63F early puff contains E63-1, an ecdysone-inducible gene that encodes a novel Ca(2+)-binding protein

Pulses of ecdysone at the end of Drosophila larval development dramatically reprogram gene expression as they signal the onset of metamorphosis. Ecdysone directly induces several early puffs in the salivary gland polytene chromosomes that, in turn, activate many late puffs. Three early puffs, at 2B5, 74EF, and 75B, have been studied at the molecular level. Each contains a single ecdysone primary-response gene that encodes a family of widely expressed transcription factors. We report here a molecular characterization of the 63F early puff. Unexpectedly, we have found this locus to be significantly different from the previously characterized early puff loci. First, the 63F puff contains a pair of ecdysone-inducible genes that are transcribed in the larval salivary glands: E63-1 and E63-2. Second, E63-1 induction in late third instar larvae appears to be highly tissue-specific, restricted to the salivary gland. Third, E63-1 encodes a novel Ca(2+)-binding protein related to calmodulin. The discovery of an ecdysone-inducible Ca(2+)-binding protein provides a foundation for integrating steroid hormone and calcium second messenger signaling pathways and generates an additional level for potential regulation of the ecdysone response.

A transcriptional switch between the Pig-1 and Sgs-4 genes of Drosophila melanogaster

Pig-1 and Sgs-4 are a pair of closely linked and divergently transcribed Drosophila melanogaster genes, which are both expressed in larval salivary glands but at different times during development. While Sgs-4 is expressed at high levels only at the end of the third instar, Pig-1 exhibits a major peak of expression during late second and early third instar. Thus, Pig-1 expression declines as Sgs-4 expression is induced. In this paper, we show that three adjacent elements located within the short region between these genes can account for the switch from Pig-1 to Sgs-4 expression. A 170-bp segment acts as an enhancer to direct Sgs-4 expression in late-third-instar salivary glands. A 64-bp sequence located just upstream from the enhancer can modify its temporal specificity so that it works throughout the third instar. Expression induced at mid-third instar by a combination of these two elements can be repressed by a negative regulatory sequence located still further upstream. We present evidence suggesting that the changing interactions between these regulatory elements and the Sgs-4 and Pig-1 promoters lead to the correct pattern of expression of the two genes.

A transcriptional switch between the Pig-1 and Sgs-4 genes of Drosophila melanogaster

Pig-1 and Sgs-4 are a pair of closely linked and divergently transcribed Drosophila melanogaster genes, which are both expressed in larval salivary glands but at different times during development. While Sgs-4 is expressed at high levels only at the end of the third instar, Pig-1 exhibits a major peak of expression during late second and early third instar. Thus, Pig-1 expression declines as Sgs-4 expression is induced. In this paper, we show that three adjacent elements located within the short region between these genes can account for the switch from Pig-1 to Sgs-4 expression. A 170-bp segment acts as an enhancer to direct Sgs-4 expression in late-third-instar salivary glands. A 64-bp sequence located just upstream from the enhancer can modify its temporal specificity so that it works throughout the third instar. Expression induced at mid-third instar by a combination of these two elements can be repressed by a negative regulatory sequence located still further upstream. We present evidence suggesting that the changing interactions between these regulatory elements and the Sgs-4 and Pig-1 promoters lead to the correct pattern of expression of the two genes.