Functional and comparative analysis of two distinct ecdysteroid-responsive gene expression constructs in Drosophila S2 cells

Development of an inducible platform for intercellular protein delivery

A challenge to protein based therapies is the ability to produce biologically active proteins and their ensured delivery. Various approaches have been utilised including fusion of protein transduction domains with a protein or biomolecule of interest. A compounding issue is lack of specificity, efficiency and indeed whether the protein fusions are actually translocated into the cell and not merely an artefact of the fixation process. Here we present a novel platform, allowing the inducible export and uptake of a protein of interest. The system utilises a combination of the Tetracyline repressor system, combined with a fusion protein containing the N-terminal signal peptide from human chorionic gonadotropin beta-subunit, and a C-terminal poly-arginine domain for efficient uptake by target cells. This novel platform was validated using enhanced green fluorescent protein as the gene of interest. Doxycycline efficiently induced expression of the fusion protein. The human chorionic gonadotropin beta-subunit facilitated the export of the fusion protein into the cell culture media. Finally, the fusion protein was able to efficiently enter into neighbouring cells (target cells), mediated by the poly-arginine cell penetrating peptide. Importantly we have addressed the issue of whether the observed uptake is an artefact of the fixation process or indeed genuine translocation. In addition this platform provides a number of potential applications in diverse areas such as stem cell biology, immune therapy and cancer targeting therapies.

Membrane-bound transporter controls the circadian transcription of clock genes in Drosophila

Little is known about molecular mechanisms that control the Drosophila circadian clock beyond the transcriptional-translational feedback regulation of clock genes as an intracellular process. In this study, Early gene at 23 (E23) was identified as a novel clock gene that encodes the membrane-bound ABC transporter that is induced by the molting hormone ecdysone. E23 expresses in pacemaker neurons in fly head, and its knockdown flies lengthened circadian period with an increased expression of the clock gene vrille. E23 and vrille responded to both ecdysone and clock signals, whereas E23 protein specifically suppressed the ecdysone response and is necessary for rhythmicity. Thus, E23 forms its own feedback loop in the ecdysone response to control circadian oscillation through ecdysone-mediated vrille expression. The ecdysone signaling pathway with E23 is essential not only in developmental stage but also for the circadian behavior in adult fly.

Comparison of the activity of non-steroidal ecdysone agonists between dipteran and lepidopteran insects, using cell-based EcR reporter assays

BACKGROUND

Diacylhydrazine (DAH) analogues have been developed successfully as a new group of insect growth regulators, called ecdysone agonists or moulting accelerating compounds. These DAHs have been shown to manifest their toxicity via interaction with the ecdysone receptor (EcR) in susceptible insects, as does the natural insect moulting hormone 20-hydroxyecdysone (20E). A notable feature is their high activity and specificity, particularly against lepidopteran insects, raising the question as to whether non-lepidopteran-specific analogues can be isolated. However, for the discovery of ecdysone agonists that target other important insect groups such as Diptera, efficient screening systems that are based on the activation of the EcR are needed.

RESULTS

In this study, a dipteran-specific reporter-based screening system with transfected S2 cells of Drosophila melanogaster Meig. was developed in order to discover and evaluate compounds that have ecdysone agonistic or antagonistic activity. A library of non-steroidal ecdysone agonists containing different mother structures with DAH and other related analogues such as acylaminoketone (AAK) and tetrahydroquinoline (THQ) was tested. None of the compounds tested was as active as 20E. This is in contrast to the very high activity of several DAH and AAK congeners in lepidopteran cells (Bombyx mori L.-derived Bm5 cells). The latter agrees with a successful docking of a DAH, tebufenozide, in the binding pocket of the lepidopteran EcR (B. mori), while this was not the case with the dipteran EcR (D. melanogaster). Of note was the identification of two THQ compounds with activity in S2 but not in Bm5 cells. Although marked differences in activity exist with respect to the activation of EcR between dipterans and lepidopterans, there exists a positive correlation (R = 0.724) between the pLC(50) values in S2 and Bm5 cells. In addition, it was found through protein modelling that a second lobe was present in the ligand-binding pocket of lepidopteran BmEcR but was lacking in the dipteran DmEcR protein, suggesting that this difference in structure of the binding pocket is a major factor for preferential activation of the lepidopteran over the dipteran receptors by DAH ligands.

CONCLUSIONS

The present study confirmed the marked specificity of DAH and AAK analogues towards EcRs from lepidopteran insects. THQ compounds did not show this specificity, indicating that dipteran-specific ecdysone-agonist-based insecticides based on the THQ mother structure can be developed. The differences in activity of ecdysone agonists in dipteran and lepidopteran ecdysone-reporter-based screening systems are discussed.

Recombinational micro-evolution of functionally different metallothionein promoter alleles from Orchesella cincta

Background

Metallothionein (mt) transcription is elevated in heavy metal tolerant field populations of Orchesella cincta (Collembola). This suggests that natural selection acts on transcriptional regulation of mt in springtails at sites where cadmium (Cd) levels in soil reach toxic values This study investigates the nature and the evolutionary origin of polymorphisms in the metallothionein promoter (pmt) and their functional significance for mt expression.

Results

We sequenced approximately 1600 bp upstream the mt coding region by genome walking. Nine pmt alleles were discovered in NW-European populations. They differ in the number of some indels, consensus transcription factor binding sites and core promoter elements. Extensive recombination events between some of the alleles can be inferred from the alignment. A deviation from neutral expectations was detected in a cadmium tolerant population, pointing towards balancing selection on some promoter stretches. Luciferase constructs were made from the most abundant alleles, and responses to Cd, paraquat (oxidative stress inducer) and moulting hormone were studied in cell lines. By using paraquat we were able to dissect the effect of oxidative stress from the Cd specific effect, and extensive differences in mt induction levels between these two stressors were observed.

Conclusion

The pmt alleles evolved by a number of recombination events, and exhibited differential inducibilities by Cd, paraquat and molting hormone. In a tolerant population from a metal contaminated site, promoter allele frequencies differed significantly from a reference site and nucleotide polymorphisms in some promoter stretches deviated from neutral expectations, revealing a signature of balancing selection. Our results suggest that the structural differences in the Orchesella cincta metallothionein promoter alleles contribute to the metallothionein -over-expresser phenotype in cadmium tolerant populations.

Organization and repression by juvenile hormone of a vitellogenin gene cluster in the crustacean, Daphnia magna

Two Daphnia magna vitellogenin (VTG) genes in neighboring but opposite orientations were identified. One was the gene for DmagVTG1, a previously characterized VTG polypeptide with a superoxide dismutase (SOD)-like domain at its NH(2)-terminus [Kato et al., Gene 334 (2004) 157-165]. Both genes had a 17-exon and 16-intron structure in the same configuration. DmagVTG2, a polypeptide encoded by the other gene, also had a SOD-like domain at its NH(2)-terminus. The amino acid sequences of the two VTG domains were highly homologous (95.5% identity), while those of the SOD-like domains were less homologous (62.4% identity). The VTG domains are phylogenetically related to insect VTGs while the SOD-like domains are related to viral and bacterial SODs. The intergenic region of 2.6kb between the two genes contains sequences resembling known juvenile hormone (JH)-responsive and ecdysone-responsive elements. JH agonists, pyriproxyfen and fenoxycarb, strongly repressed the expression of VTG genes in neonate daphnids.

Tight transcriptional regulation of foreign genes in insect cells using an ecdysone receptor-based inducible system

The use of insect cells has been highly successful for the expression of foreign proteins from baculoviruses or plasmid vectors. Here, we describe a tight transcriptional regulation of foreign genes in insect cells using an ecdysone receptor-based inducible system. The system includes the DEF domains of the spruce budworm (Choristoneura fumiferana) EcR (CfEcR) fused to the Saccharomyces cerevisiae GAL4 DNA-binding domain and the EF domains of mammalian Mus musculus retinoid X receptor (MmRXR) fused to the acidic activation domains (AADs) of the baculovirus transactivators IE1 and IE0. Using a GAL4 response element in reporter constructs, both transient and stable expression in insect lepidopteran cells showed that the chimeric MmRXR and CfEcR only activated the reporter genes in the presence of inducer; no gene expression was detectable in the absence of inducer. Characterization of heterogenous activation domains in insect cells showed that the AADs from Autographa californica multiple nucleopolyhedrovirus (MNPV) IE1 and Orgyia pseudotsugata MNPV IE0 consistently exhibited higher inducible levels than the archetype AAD from herpesvirus VP16 in insect cells. To confirm the tight regulation of this system the highly toxic protein, diphtheria toxin (DT), was used. In the absence of an inducer no cytotoxic effect was observed in insect cells that had been transiently transformed with DT expressing plasmids. This system will therefore be a very useful tool for biotechnology applications expressing highly toxic proteins in insect cells and for studying the functional genomics of insects and microorganisms that infect them.