Reconstruction of ligand-dependent transactivation of Choristoneura fumiferana ecdysone receptor in yeast

Reporter gene assays for screening and identification of novel molting hormone- and juvenile hormone-like chemicals

A reporter gene assay (RGA) is used to investigate the activity of synthetic chemicals mimicking the molting hormones (MHs) and juvenile hormones (JHs) of insects, so-called insect growth regulators (IGRs). The MH receptor, a heterodimer of the ecdysone receptor (EcR) and ultraspiracle (USP), and the JH receptor Methoprene-tolerant (Met) are ligand-dependent transcription factors. Ligand-bound EcR-USP and Met bind to specific cis-acting DNA elements, referred to as the ecdysone-responsive element (EcRE) and the JH-responsive element (JHRE), respectively, in order to transactivate target genes. Insect hormone-induced transactivation systems have been reconstituted by the introduction of reporter genes under the control of EcRE and JHRE, or two-hybrid reporter genes, into insect, mammalian, and yeast cells expressing receptor proteins. RGA is easy to use and convenient for examining the MH- and JH-like activities of synthetic chemicals and is suitable for the high-throughput screening of novel structural classes of chemicals targeting EcR-USP and Met.

Ecotoxicity of the nonsteroidal ecdysone mimic RH-5849 to Daphnia magna

The nonsteroidal ecdysone mimic 1,2-dibenzoyl-1-tert-butylhydrazine (RH-5849), a novel insect growth regulator, is mainly registered for use in rice fields. So far, its toxicity and ecological risks towards aquatic cladoceran invertebrates remain unclear. In this study, RH-5849 was evaluated for its acute and 21-day chronic toxicity towards Daphnia magna. The viability, morphology, growth, and reproduction of D. magna were observed to establish a concentration-toxicity relationship associated with the RH-5849 exposure. In addition, the relationship between the changes of physiological and biochemical indices and the chronic indices was analyzed in order to find potential early warning indicators in D. magna to the chronic risk of RH-5849 exposure. The results showed that the 48-h EC₅₀ of acute immobilization and EC₅₀ of 21-day survival of RH-5849 on D. magna were 45.3 and 1.34 mg/L, respectively. Chronic exposure to RH-5849 mainly affected the reproductive parameters of D. magna and the no observed effect concentration (NOEC) and the EC₅₀ were 0.050 and 0.5423 mg/L, respectively. The number of offspring per female reduced significantly after 21-day exposure to 0.10 mg/L of RH-5849. The morphological changes, manifested in head width and body length, the length of the helmet or apical spine, and the curvature and transparency of the body, were observed in RH-5849-treated groups. Moreover, it was found that the alkaline phosphatase activity in D. magna after 5-7-day exposure was positively correlated with the number of offspring per female after 21 days. These results indicate the potential risk of RH-5849 towards aquatic crustaceans should be taken into consideration when applied to rice fields.

Mortality and transcriptional effects of inorganic mercury in the marine copepod Calanus finmarchicus

Inorganic mercury (Hg) is highly toxic to organisms including crustaceans and displays multiple toxic modes of action (MoA). The main aim of this investigation was to assess the acute and sublethal toxicity mediated by mercury chloride (HgCl₂) in the marine copepod Calanus finmarchicus. A combination of short-term static studies to determine acute toxicity and a transcriptional investigation to characterize the sublethal MoA of HgCl₂ were conducted with an in-house continuous culture of C. finmarchicus. Transcriptional changes were determined by a custom 6.6 k C. finmarchicus Agilent oligonucleotide microarray and quantitative RT-PCR analysis. Data demonstrate that HgCl₂ produced a concentration- and time-dependent reduction in survival (NOEC_{48 h} = 6.9 μg/L [Hg²⁺] and LC₅₀ of 279, 73, 48, and 34 µg/L [Hg²⁺] after 24, 48, 72, and 96 h, respectively) and that exposure to sublethal concentrations of HgCl₂ (5 μg/L [Hg2⁺]) induced differential expression of 98 features (probes) on the microarray. Gene ontology (GO) and toxicological pathway analyses suggested that the main MOA were (1) uncoupling of mitochondrial oxidative phosphorylation (OXPHOS) and ATP production, (2) oxidative stress and macromolecular damage, (3) inactivation of cellular enzymes, (4) induction of cellular apoptosis and autophagocytosis, (5) over-excitation of glutamate receptors (neurotoxicity), (6) disruption of calcium homeostasis and signaling, and (7) modulation of nuclear receptor activity involved in vitamin D receptor signaling. Quantitative RT-PCR analysis verified that oligoarray performed reliably in terms of specificity and response, thus demonstrating that Hg²⁺ exerts multiple potential MoA in C. finmarchicus.

New reporter gene assays for detecting natural and synthetic molting hormone agonists using yeasts expressing ecdysone receptors of various insects

Synthetic nonsteroidal ecdysone agonists, a class of insect growth regulators (IGRs), target the ecdysone receptor (EcR), which forms a heterodimer with ultraspiracle (USP) to transactivate ecdysone response genes. These compounds have high binding affinities to the EcR–USP complexes of certain insects and their toxicity is selective for certain taxonomic orders. In the present study, we developed reporter gene assay (RGA) systems to detect molting hormone (ecdysone) activity by introducing EcR–USP cDNA and a bacterial lacZ reporter gene into yeast. EcR and USP were derived from the insect species of three different taxonomic orders: Drosophila melanogaster (Diptera), Chilo suppressalis (Lepidoptera), and Leptinotarsa decemlineata (Coleoptera). Transcriptional coactivator taiman (Tai) cDNA cloned from D. melanogaster was also used in this RGA system. This yeast RGA system responded to various EcR ligands in a dose‐dependent and ecdysteroid‐specific manner. Furthermore, the insect order‐selective ligand activities of synthetic nonsteroidal ecdysone agonists were linearly related to their binding activities, which were measured against in vitro translated EcR–USP complexes. Our newly established yeast RGA is useful for screening new molting hormone agonists that work selectively on target insects.

Assessment of species specificity of moulting accelerating compounds in Lepidoptera: comparison of activity between Bombyx mori and Spodoptera littoralis by in vitro reporter and in vivo toxicity assays

BACKGROUND

Dibenzoylhydrazine analogues have been developed successfully as a new group of insect growth regulators, called ecdysone agonists or moulting accelerating compounds. A notable feature is their high activity against lepidopteran insects, raising the question as to whether species-specific analogues can be isolated. In this study, the specificity of ecdysone agonists was addressed through a comparative analysis in two important lepidopterans, the silkworm Bombyx mori L. and the cotton leafworm Spodoptera littoralis (Boisd.).

RESULTS

When collections of non-steroidal ecdysone agonists containing different mother structures (dibenzoylhydrazine, acylaminoketone, tetrahydroquinoline) were tested, in vitro reporter assays showed minor differences using cell lines derived from both species. However, when compounds with high ecdysone agonist activity were examined in toxicity assays, larvicidal activity differed considerably. Of note was the identification of three dibenzoylhydrazine analogues with > 100-fold higher activity against Bombyx than against Spodoptera larvae.

CONCLUSION

The present study demonstrated that species-specific ecdysone-agonist-based insecticides can be developed, but their species specificity is not based on differences in the activation of the ecdysone receptor but rather on unidentified in vivo parameters such as permeability of the cuticle, uptake/excretion by the gut or metabolic detoxification.

Transcriptional activity of ecdysone receptor isoforms is regulated by modulation of receptor stability and interaction with Ab- and C-domains of the heterodimerization partner ultraspiracle

The stability of ecdysone receptor (EcR) expressed in a heterologous system is regulated in an isoform-specific manner and modified by ligand and heterodimerization partner. Transcriptional activities of various receptor complexes with Usp and ligand as determined by reporter assays are the result of two effects: change in receptor concentration and altered transcriptional capability. Transcriptional activity of EcR-A is low when compared to EcR-B1 independent of the absence or presence of Ultraspiracle (Usp). Ligand increased the concentration of EcR-A, but had no effect on the transcriptional capability, in contrast to EcR-B1, which is not stabilized by hormone or Usp, but the transcriptional capability is enhanced by heterodimerization and ligand. Exchange of the AB-domain of Usp by the activation domain (AD) of Vp16 revealed that the N-terminus of Usp inhibited transcriptional activity only with EcR-B isoforms, whereas the hexapeptide in the AB-domain of wild type Usp adjacent to the C-domain of Usp harbours an activating function. Deletion of the C-domain of Usp did not affect the stability of the receptor complex, but reduced the transcriptional capability of heterodimers with all EcR-isoforms, indicating that the stability of the receptor, which is important for termination of the hormone signal transduction, is regulated in a cooperative manner by the AB-domains of EcR and Usp, and ligand. We show the active role of Usp in modulation of the transcriptional activity of the heterodimer in an isoform-specific manner by the inhibitory N-terminus, the activating hexapeptide in the AB-domain, and the C-domain of Usp.

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.

Practical uses for ecdysteroids in mammals including humans: an update

Ecdysteroids are widely used as inducers for gene-switch systems based on insect ecdysteroid receptors and genes of interest placed under the control of ecdysteroid-response elements. We review here these systems, which are currently mainly used in vitro with cultured cells in order to analyse the role of a wide array of genes, but which are expected to represent the basis for future gene therapy strategies. Such developments raise several questions, which are addressed in detail. First, the metabolic fate of ecdysteroids in mammals, including humans, is only poorly known, and the rapid catabolism of ecdysteroids may impede their use as in vivo inducers. A second set of questions arose in fact much earlier with the pioneering "heterophylic" studies of Burdette in the early sixties on the pharmacological effects of ecdysteroids on mammals. These and subsequent studies showed a wide range of effects, most of them being beneficial for the organism (e.g. hypoglycaemic, hypocholesterolaemic, anabolic). These effects are reviewed and critically analysed, and some hypotheses are proposed to explain the putative mechanisms involved. All of these pharmacological effects have led to the development of a wide array of ecdysteroid-containing preparations, which are primarily used for their anabolic and/or "adaptogenic" properties on humans (or horses or dogs). In the same way, increasing numbers of patents have been deposited concerning various beneficial effects of ecdysteroids in many medical or cosmetic domains, which make ecdysteroids very attractive candidates for several practical uses. It may be questioned whether all these pharmacological actions are compatible with the development of ecdysteroid-inducible gene switches for gene therapy, and also if ecdysteroids should be classified among doping substances.

A single point mutation in ecdysone receptor leads to increased ligand specificity: implications for gene switch applications

The ecdysone receptor (EcR), a member of the nuclear receptor superfamily, plays an important role in regulating development and reproduction in insects. The EcR binds to ecdysteroids and regulates transcription of genes that contain ecdysone response elements. The EcR has been used to develop inducible gene switches for efficient regulation of foreign genes in applications such as gene therapy, protein production, and functional genomics. An EcR [Choristoneura fumiferana EcR (CfEcR)] homology model was constructed, and 17 amino acid residues were identified as critical for 20-hydroxyecdysone binding. Mutation of these amino acids followed by analysis of these mutants in transactivation (in insect and mammalian cells and in vivo in mice) and ligand-binding assays identified one particular mutant (A110P) that failed to respond to steroids, but its response to the diacylhydrazine nonsteroidal ligands RG-102240 (GS(TM)E) and RG-102317 was unaffected. This steroid-insensitive EcR mutant has potential gene switch applications in insects and plants that have endogenous ecdysteroids. In addition, this mutant would be also useful for developing orthogonal EcR-ligand pairs for simultaneous regulation of multiple genes in the same cell.

Ligand-induced heterodimerization between the ligand binding domains of the Drosophila ecdysteroid receptor and ultraspiracle

The insect ecdysteroid receptor consists of a heterodimer between EcR and the RXR-orthologue, USP. We addressed the question of whether this heterodimer, like all other RXR heterodimers, may be formed in the absence of ligand and whether ligand promotes dimerization. We found that C-terminal protein fragments that comprised the ligand binding, but not the DNA binding domain of EcR and USP and which were equipped with the activation or DNA binding region of GAL4, respectively, exhibit a weak ability to interact spontaneously with each other. Moreover, the heterodimer formation is greatly enhanced upon administration of active ecdysteroids in a dose-dependent manner. This was shown in vivo by a yeast two-hybrid system and in vitro by a modified electromobility shift assay. Furthermore, the EcR fragment expressed in yeast was functional and bound radioactively labelled ecdysteroid specifically. Ligand binding was greatly enhanced by the presence of a USP ligand binding domain. Therefore, ecdysteroids are capable of inducing heterodimer formation between EcR and USP, even when the binding of these receptor proteins to cognate DNA response elements does not occur. This capability may be a regulated aspect of ecdysteroid action during insect development.

Ecdysone-controlled expression of transgenes

Inducible expression systems show great potential for use in human gene therapy and systems based on insect ecdysone receptors are particularly promising candidates. This article describes such systems and reviews actual and potential uses of ecdysone-controlled transgenes in vitro and in vivo. The ligand specificity of ecdysone receptor-based systems is considered, along with the safety and efficacy of the ecdysteroid and non-steroidal compounds used to activate them.