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

In Silico Identification of Chemicals Capable of Binding to the Ecdysone Receptor

The process of molting, known alternatively as ecdysis, is a feature integral in the life cycles of species across the arthropod phylum. Regulation occurs as a function of the interaction of ecdysteroid hormones with the arthropod nuclear ecdysone receptor-a process preceding the triggering of a series of downstream events constituting an endocrine signaling pathway highly conserved throughout environmentally prevalent insect, crustacean, and myriapod organisms. Inappropriate ecdysone receptor binding and activation forms the essential molecular initiating event within possible adverse outcome pathways relating abnormal molting to mortality in arthropods. Definition of the characteristics of chemicals liable to stimulate such activity has the potential to be of great utility in mitigation of hazards posed toward vulnerable species. Thus the aim of the present study was to develop a series of rule-sets, derived from the key structural and physicochemical features associated with identified ecdysone receptor ligands, enabling construction of Konstanz Information Miner (KNIME) workflows permitting the flagging of compounds predisposed to binding at the site. Data describing the activities of 555 distinct chemicals were recovered from a variety of assays across 10 insect species, allowing for formulation of KNIME screens for potential binding activity at the molecular initiating event and adverse outcome level of biological organization. Environ Toxicol Chem 2020;39:1438-1450. © 2020 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC.

Impact of the Insect Growth Regulator Pyriproxyfen on Immature Development, Fecundity, and Fertility of Aedes albopictus

Aedes albopictus is a vector of several arboviruses; however, control of this day-active species is difficult with ultra-low-volume insecticide treatments applied at dusk/dawn periods. In the current laboratory study, blood-fed Ae. albopictus were exposed to Archer^® (insect growth regulator AI: pyriproxyfen) residue in glass bottles (to approximate barrier treatment) and allowed to oviposit. Control mosquitoes were exposed to clean bottles. To evaluate potential dilution effects of water volume, mosquitoes were allowed to oviposit in (relatively) small (59 ml water) or large (177 ml water) containers. The extent to which fecundity (number of eggs laid), fertility rate (number of larvae hatched/number of eggs laid × 100), and emergence rate (number of adults emerged/number of larvae hatched × 100) differed between groups was characterized. In the control group, 18-21 (82-95%) mosquitoes laid eggs, while only 10-11 (45-50%) of mosquitoes in the pyriproxyfen group laid eggs. These sample sizes should be considered when comparing results to other studies. Significantly lower (P = 0.0008) fecundity was observed in mosquitoes exposed to pyriproxyfen (mean ± SE) (small container: 25.2 ± 7.1, large container: 24.3 ± 7.1) compared to control mosquitoes (small container: 49.2 ± 7.8, large container: 52.7 ± 5.2). Regardless of treatment, no significant differences in fecundity were observed between mosquitoes allowed to oviposit in different-sized containers. Hatch rate was significantly lower in the pyriproxyfen group and was impacted by container size (P = 0.032) and treatment (P < 0.0001) (large, control: 61.9% ± 7.8; small, control: 38.0% ± 7.1; large, treated: 10.3% ± 2.4; small, treated: 2.9% ± 1.9). Adult emergence rates were not significantly impacted by treatment or container size. Pyriproxyfen applied as a barrier treatment may be an effective tool for controlling Ae. albopictus.

Culex quinquefasciatus Egg Membrane Alteration and Ovicidal Activity of Cipadessa baccifera (Roth) Plant Extracts Compared to Synthetic Insect Growth Regulators

Background

Insecticide resistance among mosquito vectors for synthetic insecticides still remains a major problem for control efforts. This study assessed the ovicidal potential of crude solvent extracts from the medicinal plant Cipadessa baccifera comparatively to standard registered synthetic insect growth regulators (IGR) on freshly laid eggs of Culex quinquefasciatus.

Method

Five plant extracts were prepared using different solvents. The batches of eggs were exposed to different concentrations of each solvent extract comparatively to synthetic IGR. The hatched eggs of Cx. quinquefasciatus were subjected to different concentrations. The first instars that emerged from the eggs were counted daily. The egg hatching inhibition was observed 24, 48 and 72 hrs post treatment. The desiccation median time (DT₅₀ and DT₉₀) was calculated.

Results

The percent egg hatching inhibition was inversely proportional to the concentration of extracts. The morphological damage to the eggs was observed. Among five solvent extracts, acetone extracts showed the highest ovicidal activity. The changes in eggshell morphology were observed. The maximum ovicidal activity was observed in acetone extracts with DT₅₀ value of 1.70 hrs (0.91–2.22). The methanol plant extract using gas chromatography-mass spectrometry identified 14 compounds.

Conclusion

These results suggest that the acetone extracts of C. baccifera have the potential to be used as an ovicidal agent for controlling mosquito populations in aquatic stages. The biodegradability of the extracts has the advantage of being eco-friendly.

Asymmetric synthesis of tetrahydroquinoline-type ecdysone agonists and QSAR for their binding affinity against Aedes albopictus ecdysone receptors

BACKGROUND

Tetrahydroquinolines (THQs) are a class of non-steroidal ecdysone agonists that specifically bind to mosquito ecdysone receptors (EcR). The THQ scaffold contains two chiral centers at the C-2 and C-4 positions, resulting in four stereoisomers. We have previously shown that the (2R,4S)-isomers are the most biologically active; however, the lack of a practical synthetic method for these isomers has hampered further structure-activity studies.

RESULTS

In this study, a chiral phosphoric acid-catalyzed Povarov reaction was employed to develop a facile asymmetric synthesis of THQs with a (2R,4S)-configuration, which allowed the preparation of a 40-compound library of enantiopure THQs. Evaluation of their binding affinity against Aedes albopictus EcR, followed by quantitative structure-activity relationship (QSAR) analyses, uncovered the physicochemical properties of THQs that are important for the ligand-receptor interaction. The most potent THQ derivative was twofold more active than the molting hormone, 20-hydroxyecdysone.

CONCLUSION

The QSAR results provide valuable information for the rational design of novel mosquito-specific ecdysone agonists. © 2018 Society of Chemical Industry.

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.

Exploration of the binding affinities between ecdysone agonists and EcR/USP by docking and MM-PB/GBSA approaches

Ecdysone receptor (EcR) is a significant target in the identification of new environmentally friendly pesticides. There are two types of ecdysone agonists: steroidal ecdysone agonists and dibenzoylhydrazines (DBHs). In this study, various modeling methods (homology modeling, molecular docking, MD simulation, binding free energy calculation, and per-residue binding free energy decomposition) were utilized to study the different binding mechanisms of two types of ecdysone agonists. Our theoretical results indicated that the relative binding potencies of DBHs can be ranked sufficiently accurately using the MOE docking method. However, MM/PBSA calculations more accurately predicted the binding affinities between steroidal ecdysone agonists and EcR-LBD. To identify the key residues involved in ecdysone agonist binding, the binding free energy (ΔG _Bind) was decomposed into the energy contributions of individual residues. The results revealed that nine residues-Ile339, Thr343, Met380, Met381, Tyr403, Tyr408, Asp419, Gln503, and Asn504-determined the binding affinities of the DBHs. Glu309, Met342, Arg383, Arg387, and Leu396 were important influences on the binding affinities of the steroidal ecdysone agonists. Graphical abstract The ecdysone receptor (EcR) is related to insect growth and has been shown to be a useful target for insecticides.

Methoxyfenozide resistance of the housefly, Musca domestica L. (Diptera: Muscidae): cross-resistance patterns, stability and associated fitness costs

BACKGROUND

The housefly, Musca domestica L. (Diptera: Muscidae), is an insect pest of public health and veterinary importance with the ability to develop resistance to insecticides. Methoxyfenozide, an ecdysone agonist, is a biorational insecticide used for the management of various insect pests, including houseflies. To design an effective resistance management strategy, life history traits based on laboratory observations were established for methoxyfenozide-resistant (MXY-SEL), unselected counterpart (UNSEL) and reciprocal cross-strains of housefly.

RESULTS

The MXY-SEL strain developed a resistance ratio of 160.99 after 30 generations of selection with methoxyfenozide by compared with the UNSEL strain. The MXY-SEL strain showed very low cross-resistance to cyromazine, fipronil and chlorpyrifos and no cross-resistance to spinosad and bifenthrin when compared with the Methoxy-Field population. Resistance to methoxyfenozide, cyromazine, fipronil, spinosad, chlorpyrifos and bifenthrin was unstable in the MXY-SEL strain. The MXY-SEL strain had a reduced relative fitness (0.31), with lower hatchability, a lower number of next-generation larvae, a lower intrinsic rate of natural increase and a lower biotic potential compared with the UNSEL strain.

CONCLUSIONS

The disadvantageous life history traits of the MXY-SEL strain suggest that development of resistance to methoxyfenozide has considerable fitness costs for this strain. Moreover, the unstable resistance to the tested chemicals provides useful information for preserving the efficacy of these chemicals. © 2016 Society of Chemical Industry.

Assessment of resistance risk in Musca domestica L. (Diptera: Muscidae) to methoxyfenozide

Methoxyfenozide, an ecdysone receptor agonist is an effective larvicide against many pests of public health and veterinary importance including house fly, Musca domestica L. (Diptera: Muscidae). Methoxyfenozide is a bio-rational insecticide having many environmentally friendly attributes that make it compatible with integrated pest management programs. This experiment was performed for the assessment of resistance evolution in M. domestica to methoxyfenozide. A field population of M. domestica, after 24 rounds of selection with methoxyfenozide, resulted in 64 fold and 915-fold increase in lethal concentration 50 (LC50) compared to field and susceptible strain, respectively. Realized heritability (h(2)) of resistance to methoxyfenozide was 0.17 in methoxyfenozide-selected strain of M. domestica. The projected rate of resistance development indicated that, if slope=1.71 and h(2)=0.17, then 13-5 generations are required for tenfold increase in LC50 at 50-95% selection intensity. These findings suggest that a risk for resistance development to methoxyfenozide occurred in M. domestica under continuous selection pressure.

Structure-activity relationship of imidazothiadiazole analogs for the binding to the ecdysone receptor of insect cells

Diacylhydrazines are the first non-steroidal ecdysone agonists, and five compounds are used as insecticides in agriculture. After the discovery of diacylhydrazine-type compounds, numerous non-steroidal structures were reported as ecdysone agonists. Among various ecdysone agonists, imidazothiadiazoles are reported to be very potent in vitro; however, the experimental detail for the structure identification and bioassays are not stated in the paper (Holmwood and Schindler, Bioorg. Med. Chem. 17, 4064-4070, 2009). In our present study, we synthesized 18 imidazothiadiazole-type compounds and confirmed the chemical structures by spectrometric analyses. The binding activity of the synthesized compounds to the ecdysone receptor was evaluated in terms of the concentration required for 50% inhibition of [(3)H]ponasterone A incorporation [IC50 (M)] into lepidopteran (Sf-9), coleopteran (BCRL-Lepd-SL1), and dipteran (NIAS-AeAl2) cells. 6-(2-Chlorophenyl)-2-(trifluoromethyl)imidazo[2,1-b] [1,3,4]-thiadiazol-5-yl)acrylamide analogs with CONHR (secondary amide) were very potent against Sf-9 cells, but further alkylation (tertiary amide: CONR2) decreased the activity dramatically. Additionally, a primary amide analog (CONH2) was inactive. The activity also decreased 150-fold by the saturation of olefin region of the acrylamide moiety. In addition, various substituents were introduced at the 2-position of the imidazothiadiazole ring to disclose the physicochemical properties of the substituents which are important for receptor binding. The activity increased by 7500-fold with the introduction of the CF2CF2CF3 group compared to the unsubstituted compound against Sf-9 cells. Quantitative structure-activity relationship analysis for these substituents indicated that hydrophobic and electron-withdrawing groups were favorable for binding. Some of the compounds with strong receptor binding activity showed good larvicidal activity against Spodoptera litura. In contrast, the binding affinity of imidazothiadiazole analogs was low or not observed against dipteran and coleopteran cells.

The insect ecdysone receptor is a good potential target for RNAi-based pest control

RNA interference (RNAi) has great potential for use in insect pest control. However, some significant challenges must be overcome before RNAi-based pest control can become a reality. One challenge is the proper selection of a good target gene for RNAi. Here, we report that the insect ecdysone receptor (EcR) is a good potential target for RNAi-based pest control in the brown planthopper Nilaparvata lugens, a serious insect pest of rice plants. We demonstrated that the use of a 360 bp fragment (NlEcR-c) that is common between NlEcR-A and NlEcR-B for feeding RNAi experiments significantly decreased the relative mRNA expression levels of NlEcR compared with those in the dsGFP control. Feeding RNAi also resulted in a significant reduction in the number of offspring per pair of N. lugens. Consequently, a transgenic rice line expressing NlEcR dsRNA was constructed by Agrobacterium- mediated transformation. The results of qRT-PCR showed that the total copy number of the target gene in all transgenic rice lines was 2. Northern blot analysis showed that the small RNA of the hairpin dsNlEcR-c was successfully expressed in the transgenic rice lines. After newly hatched nymphs of N. lugens fed on the transgenic rice lines, effective RNAi was observed. The NlEcR expression levels in all lines examined were decreased significantly compared with the control. In all lines, the survival rate of the nymphs was nearly 90%, and the average number of offspring per pair in the treated groups was significantly less than that observed in the control, with a decrease of 44.18-66.27%. These findings support an RNAi-based pest control strategy and are also important for the management of rice insect pests.

Modulation of the transcriptional response of innate immune and RNAi genes upon exposure to dsRNA and LPS in silkmoth-derived Bm5 cells overexpressing BmToll9-1 receptor

Injection or feeding of dsRNA is commonly used to induce specific gene silencing by RNAi in insects but very little research has been carried out to investigate non-specific effects on gene expression of dsRNA as pathogen-associated molecular pattern (PAMP). This study focuses on the potential role of the BmToll9-1 receptor to modulate the transcriptional response of innate immune and RNAi genes to dsRNA and lipopolysaccharide (LPS), which was used for comparison. To study this role, we took advantage of the silkmoth-derived Bm5 cell line, which does not express BmToll9-1 endogenously, and engineered a transformed cell line that permanently expresses BmToll9-1. Quantitative mRNA expression studies showed that BmToll9-1 can significantly alter the transcriptional response to dsRNA and LPS: (1) BmToll9-1 promotes the transcriptional response of Dicer2, encoding a key component of the RNAi machinery, and, to a lesser extent, that of transcription factors in the Jak-STAT and Toll pathways; and (2) BmToll9-1 represses the transcriptional induction of the IMD and Jak-STAT pathway genes, as well as the antimicrobial peptide (AMP) effector genes, by LPS. Thus, BmToll9-1 was identified as a modulator of innate immune and RNAi machinery gene expression that could be related to its preferential expression in the larval gut, the major barrier of pathogen entry. While BmToll9-1 was found to modulate RNAi-related gene expression, a reporter-based RNAi assay established no evidence for a direct interaction of BmToll9-1 with the intracellular RNAi machinery.

A cell-based reporter assay for screening for EcR agonist/antagonist activity of natural ecdysteroids in Lepidoptera (Bm5) and Diptera (S2) cell cultures, followed by modeling of ecdysteroid-EcR interactions and normal mode analysis

Ecdysteroid signal transduction is a key process in insect development and therefore an important target for insecticide development. We employed an in vitro cell-based reporter bioassay for the screening of potential ecdysone receptor (EcR) agonistic and antagonistic compounds. Natural ecdysteroids were assayed with ecdysteroid-responsive cell line cultures that were transiently transfected with the reporter plasmid ERE-b.act.luc. We used the dipteran Schneider S2 cells of Drosophila melanogaster and the lepidopteran Bm5 cells of Bombyx mori, representing important pest insects in medicine and agriculture. Measurements showed an EcR agonistic activity only for cyasterone both in S2 (EC50=3.3μM) and Bm5 cells (EC50=5.3μM), which was low compared to that of the commercial dibenzoylhydrazine-based insecticide tebufenozide (EC50=0.71μM and 0.00089μM, respectively). Interestingly, a strong antagonistic activity was found for castasterone in S2 cells with an IC50 of 0.039μM; in Bm5 cells this effect only became visible at much higher concentrations (IC50=18μM). To gain more insight in the EcR interaction, three-dimensional modeling of dipteran and lepidopteran EcR-LBD was performed. In conclusion, we showed that the EcR cell-based reporter bioassay tested here is a useful and practical tool for the screening of candidate EcR agonists and antagonists. The docking experiments as well as the normal mode analysis provided evidence that the antagonist activity of castasterone may be through direct binding with the receptor with specific changes in protein flexibility. The search for new ecdysteroid-like compounds may be particularly relevant for dipterans because the activity of dibenzoylhydrazines appears to be correlated with an extension of the EcR-LBD binding pocket that is prominent in lepidopteran receptors but less so in the modeled dipteran structure.

Ovicidal activity of three insect growth regulators against Aedes and Culex mosquitoes

Interspecific variations in the susceptibility of freshly and embryonated eggs of Aedes albopictus, Ae. aegypti, Ae. atropalpus and Culex pipiens were tested against three classes of insect growth regulators (IGRs) including ecdysone agonist (azadirachtin), chitin synthesis inhibitor (diflubenzuron) and juvenile hormone analog (pyriproxyfen) at 0.001, 0.01, 0.1 and 1.0ppm concentrations. Egg hatching inhibition was dose dependent, the highest being at 1.0ppm concentration for freshly laid eggs of Ae. albopictus (pyriproxyfen: 80.6%, azadirachtin: 42.9% and diflubenzuron: 35.8%). Aedes aegypti showed lower egg hatching inhibition when exposed to pyriproxyfen (47.3%), azadirachtin (15.7%) and diflubenzuron (25.5%). Freshly laid eggs of Cx. pipiens were most susceptible to diflubenzuron. Aedes atropalpus eggs were tolerant to all three classes of IGRs. Embryonated eggs of Ae. albopictus, Ae. aegypti, Ae. atropalpus and Cx. pipiens were resistant to pyriproxyfen, azadirachtin and diflubenzuron than freshly laid eggs. The median desiccation time (DT50) of Ae. atropalpus eggs was maximum (5.1h) as compared to Ae. aegypti (4.9h), Ae. albopictus (3.9h) or Cx. pipiens (1.7h) eggs. Insignificant relationship between the rates of desiccation and egg hatching inhibition suggests other factors than physical providing eggs the ability to tolerate exposures to various IGRs. Egg hatching inhibition was due to the alteration in embryonic development caused by IGRs. Changes in the egg shell morphology and abnormal egg hatching from the side of the egg wall instead of operculum, was observed at higher concentrations of diflubenzuron. Morphological and physiological variations in eggs may be the key factor to influence the ovicidal efficacy of IGRs. The present data provide a base line for the improvement of the ovicidal efficacy of the insecticide and its formulation.

Cloning and functional analysis of the ecdysteroid receptor complex in the opossum shrimp Neomysis integer (Leach, 1814)

In this paper, the non-target effects of tebufenozide were evaluated on the estuarine crustacean, the opposum shrimp Neomysis integer (Leach, 1814). Tebufenozide is a synthetic non-steroidal ecdysone agonist insecticide and regarded as potential endocrine-disrupting chemical (EDC). N. integer is the most used crustacean in ecotoxicological research in parallel to Daphnia sp. and has been proposed for the regulatory testing of potential EDCs in the US, Europe and Japan. Major results were: (i) cDNAs encoding the ecdysteroid receptor (EcR) and the retinoid-X-receptor (RXR), were cloned and sequenced, and subsequent molecular phylogenetic analysis (maximum likelihood and neighbor-joining) revealed that the amino acid sequence of the ligand binding domain (LBD) of N. integer EcR (NiEcR) clusters as an outgroup of the Crustacea, while NiRXR-LBD clusters in the Malacostracan clade (bootstrap percentage=75%). (ii) 3D-modeling of ligand binding to NiEcR-LBD demonstrated an incompatibility of the insecticide tebufenozide to fit into the NiEcR-ligand binding pocket. This was in great contrast to ponasterone A (PonA) that is the natural molting hormone in Crustacea and for which efficient docking was demonstrated. In addition, the heterodimerization of NiEcR-LBD with the common shrimp Crangon crangon (Linnaeus, 1758) RXR-LBD (CrcRXR-LBD) was also modeled in silico. (iii) With use of insect Hi5 cells, chimeric constructs of NiEcR-LBD and CrcRXR-LBD fused to either the yeast Gal4-DNA binding domain (DBD) or Gal4-activation domain (AD) were cloned into expression plasmids and co-transfected with a Gal4 reporter to quantify the protein-protein interactions of NiEcR-LBD with CrcRXR-LBD. Investigation of the ligand effect of PonA and tebufenozide revealed that only the presence of PonA could induce dimerization of this heterologous receptor complex. (iv) Finally, in an in vivo toxicity assay, N. integer juveniles were exposed to tebufenozide at a concentration of 100 μg/L, and no effects against the molting process and nymphal development were scored. In conclusion, the in vitro cell reporter assay, based on NiEcR-LBD/CrcRXR-LBD heterodimerization in Hi5 cells and validated with the natural ecdysteroid hormone PonA, represents a useful tool for the screening of putative EDCs. As a test example for non-steroidal ecdysone agonist insecticides, tebufenozide had no negative effects on NiEcR/RXR receptor dimerization in vitro, nor on the molting process and nymphal development of N. integer at the tested concentration (100 μg/L) in vivo.

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.

Structural changes under low evolutionary constraint may decrease the affinity of dibenzoylhydrazine insecticides for the ecdysone receptor in non-lepidopteran insects

Understanding how variations in genetic sequences are conveyed into structural and biochemical properties is of increasing interest in the field of molecular evolution. In order to gain insight into this process, we studied the ecdysone receptor (EcR), a transcription factor that controls moulting and metamorphosis in arthropods. Using an in silico homology model, we identified a region in the lepidopteran EcR that has no direct interaction with the natural hormone but is under strong evolutionary constraint. This region causes a small indentation in the three-dimensional structure of the protein which facilitates the binding of tebufenozide. Non-Mecopterida are considered much older, evolutionarily, than Lepidoptera and they do not have this extended cavity. This location shows differences in evolutionary constraint between Lepidoptera and other insects, where a much lower constraint is observed compared with the Lepidoptera. It is possible that the higher flexibility seen in the EcR of Lepidoptera is an entirely new trait and the higher constraint could then be an indication that this region does have another important function. Finally, we suggest that Try123, which is evolutionarily constrained and is up to now exclusively present in Lepidoptera EcRs, could play a critical role in discriminating between steroidal and non-steroidal ligands.

Identification of genes containing ecdysone response elements in the genome of Brugia malayi

Recent studies have demonstrated that filarial parasites contain a functional homologue of the insect ecdysone receptor (EcR). As a first step in deciphering the physiological role that ecdysteroids play in filarial parasites, adult female parasites cultured in the presence and absence of 20-OH ecdysone were metabolically labeled. Gel electrophoretic analysis of proteins extracted from the cultured parasites revealed changes in the level of expression of several proteins, indicating that adult female parasites contained an ecdysone-responsive gene network. A bioinformatic analysis was then conducted to identify putative ecdysone response elements (EcREs) in the Brugia malayi genome. A total of 18 genes were identified that contained putative EcREs located in the 4 kbp upstream from the start of their open reading frames. The most common functional classifications of the encoded proteins were factors involved in transcription and metabolism. These genes revealed a number of different developmental patterns of transcription. The promoter of one EcRE-containing gene was cloned into a luciferase reporter vector and transfected into B. malayi embryos. Reporter gene expression from embryos transfected with this construct was up-regulated by 20-OH ecdysone. Deletion and substitution mutations in the canonical EcRE resulted in a loss of the ecdysone response. These results demonstrate the presence of functional EcREs in the B. malayi genome.

Bombyx mori DNA/RNA non-specific nuclease: expression of isoforms in insect culture cells, subcellular localization and functional assays

A DNA/RNA non-specific alkaline nuclease (BmdsRNase) was isolated from the digestive juice of Bombyx mori. While originally reported to be produced by the midgut only, in this project it was found that the mRNA of this enzyme was also expressed in the epidermis, fat body, gut, thoracic muscles, Malpighian tubules, brain, and silk glands of 5th instar larvae, indicating additional functions to its reported role in nucleic acid digestion in the midgut. In order to study the functional properties of BmdsRNase, three pEA-BmdsRNase expression constructs were generated, characterized by presence or absence of a signal peptide and a propeptide, and used for expression in lepidopteran Hi5 tissue culture cells. Western blot indicated that these different forms of BmdsRNase protein were not secreted into the growth medium, while they were detected in the pellets and supernatants of Hi5 cell extracts. Nucleic acids cleavage experiments indicated that full-length BmdsRNase could digest dsRNA and that the processed form (absence of signal peptide and propeptide) of BmdsRNase could degrade both DNA and dsRNA in Hi5 cell culture. Using a reporter assay targeted by transfected homologous dsRNA, it was shown that the digestive property of the processed form could interfere with the RNAi response. Immunostaining of processed BmdsRNase protein showed asymmetric localization in the cellular cytoplasm and co-localization with Flag-tagged Dicer-2 was also observed. In conclusion, our in vitro studies indicated that intracellular protein isoforms of BmdsRNase can be functional and involved in the regulation of nucleic acid metabolism in the cytoplasm. In particular, because of its propensity to degrade dsRNA, the enzyme might be involved in the innate immune response against invading nucleic acids such as RNA viruses.

Sequencing and structural homology modeling of the ecdysone receptor in two chrysopids used in biological control of pest insects

In insects, the process of molting and metamorphosis are mainly regulated by a steroidal hormone 20-hydroxyecdysone (20E) and its analogs (ecdysteroids) that specifically bind to the ecdysone receptor ligand-binding domain (EcR-LBD). Currently, several synthetic non-steroidal ecdysone agonists, including tebufenozide, are commercially available as insecticides. Tebufenozide exerts its activity by binding to the 20E-binding site and thus activating EcR permanently. It appears that subtle differences in the architecture among LBDs may underpin the differential binding affinity of tebufenozide across taxonomic orders. In brief, first we demonstrated the harmlessness of tebufenozide towards Chrysoperla externa (Ce). Then, a molecular analysis of EcR-LBD of two neuropteran insects Chrysoperla carnea and Ce was presented. Finally, we constructed a chrysopid in silico homology model docked ponasterone A (PonA) and tebufenozide into the binding pocket and analyzed the amino acids indentified as critical for binding to PonA and tebufenozide. Due to a restrict extent in the cavity at the bottom of the ecdysone-binding pocket a steric clash occurred upon docking of tebufenozide. The absence of harm biological effect and the docking results suggest that tebufenozide is prevented of any deleterious effects on chrysopids.

Insect nuclear receptors

The nuclear receptors (NRs) of metazoans are an ancient family of transcription factors defined by conserved DNA- and ligand-binding domains (DBDs and LBDs, respectively). The Drosophila melanogaster genome project revealed 18 canonical NRs (with DBDs and LBDs both present) and 3 receptors with the DBD only. Annotation of subsequently sequenced insect genomes revealed only minor deviations from this pattern. A renewed focus on functional analysis of the isoforms of insect NRs is therefore required to understand the diverse roles of these transcription factors in embryogenesis, metamorphosis, reproduction, and homeostasis. One insect NR, ecdysone receptor (EcR), functions as a receptor for the ecdysteroid molting hormones of insects. Researchers have developed nonsteroidal ecdysteroid agonists for EcR that disrupt molting and can be used as safe pesticides. An exciting new technology allows EcR to be used in chimeric, ligand-inducible gene-switch systems with applications in pest management and medicine.

Search for limiting factors in the RNAi pathway in silkmoth tissues and the Bm5 cell line: the RNA-binding proteins R2D2 and Translin

RNA interference (RNAi), an RNA-dependent gene silencing process that is initiated by double-stranded RNA (dsRNA) molecules, has been applied with variable success in lepidopteran insects, in contrast to the high efficiency achieved in the coleopteran Tribolium castaneum. To gain insight into the factors that determine the efficiency of RNAi, a survey was carried out to check the expression of factors that constitute the machinery of the small interfering RNA (siRNA) and microRNA (miRNA) pathways in different tissues and stages of the silkmoth, Bombyx mori. It was found that the dsRNA-binding protein R2D2, an essential component in the siRNA pathway in Drosophila, was expressed at minimal levels in silkmoth tissues. The silkmoth-derived Bm5 cell line was also deficient in expression of mRNA encoding full-length BmTranslin, an RNA-binding factor that has been shown to stimulate the efficiency of RNAi. However, despite the lack of expression of the RNA-binding proteins, silencing of a luciferase reporter gene was observed by co-transfection of luc dsRNA using a lipophilic reagent. In contrast, gene silencing was not detected when the cells were soaked in culture medium supplemented with dsRNA. The introduction of an expression construct for Tribolium R2D2 (TcR2D2) did not influence the potency of luc dsRNA to silence the luciferase reporter. Immunostaining experiments further showed that both TcR2D2 and BmTranslin accumulated at defined locations within the cytoplasm of transfected cells. Our results offer a first evaluation of the expression of the RNAi machinery in silkmoth tissues and Bm5 cells and provide evidence for a functional RNAi response to intracellular dsRNA in the absence of R2D2 and Translin. The failure of TcR2D2 to stimulate the intracellular RNAi pathway in Bombyx cells is discussed.

The heterodimeric ecdysteroid receptor complex in the brown shrimp Crangon crangon: EcR and RXR isoform characteristics and sensitivity towards the marine pollutant tributyltin

Decapod crustaceans are characterized by multiple ecdysteroid receptor (EcR) and retinoid-X-receptor (RXR) isoforms, which likely exhibit variant dimerization and transactivation interactions. In the brown shrimp C. crangon we cloned C-terminally truncated CrcEcR and CrcRXR isoforms and isoforms exhibiting deletions within the hinge region. For the former, in silico modeling of the CrcEcR indicated that, where the conserved helices H10 and H11 of the ligand-binding domain (LBD) are missing, an alternative C-terminal α-helix repairs the ligand-binding pocket (LBP). The truncated CrcRXR isoforms lack a major part of the LBD (H4-H12), thereby compromising ligand binding and dimerization. Through an in vitro ecdysteroid responsive reporter assay, we showed that these natural receptor variations do not impair receptor functioning but probably alter the receptor dimerization preferences. By the same in vitro assay, using full-length CrcEcR and CrcRXR, the effect of tributyltin (TBT) on ecdysteroid-induced transactivation was evaluated. The transactivation by 10nM PonA was reduced with 64% by 20 nM TBT. In silico modeling confirmed that TBT fits in the full-length CrcRXR-LBD. Furthermore, semi-quantitative PCR indicated altered expression of CrcEcR and CrcRXR isoforms after in vivo acute exposure to TBT, especially in the ovaries.

Ecdysone signaling and transcript signature in Drosophila cells resistant against methoxyfenozide

Methoxyfenozide (RH-2485) is a non-steroidal ecdysteroid agonist with a dibenzoylhydrazine structure, representing a group used as novel biorational insecticides in the control of insect pests. Here we report on the selection of Drosophila melanogaster S2 cells for resistance to inhibition of cell proliferation by methoxyfenozide by ∼ 1000-fold over 4 months. Cells were exposed to gradually increasing concentrations of methoxyfenozide and selected out based on the ecdysteroid-sensitive response for cell proliferation. In the resistant cells, the ecdysteroid receptor (EcR/USP) complex was no longer active in the presence of methoxyfenozide. But when resistant cells were relaxed from pressure in methoxyfenozide-free medium, induction of the reporter construct was observed. In parallel, EcR/USP functionality was also restored when resistant cells were rescued by a Drosophila EcR plasmid. However, it was striking that in the resistant cells the ecdysteroid-sensitive response for cell proliferation was not restored upon methoxyfenozide withdrawal, indicating permanent changes in the physiology of the cells during selection. To investigate changes in gene expression caused by inactivation of the EcR/USP complex in resistant cells, Drosophila oligo 14kv1 microarrays were used and probed with cDNAs from resistant cells in the presence and absence of ecdysone agonist on one hand and from unselected sensitive cells on the other hand. A selection of 324 differentially expressed genes was assigned covering diverse functions as transport, enzyme activity, cytoskeleton organization, cell cycle machinery, transcription/translation and ecdysteroid signaling. Besides the identification of (primary and secondary) target genes of the EcR/USP signaling pathway, this analysis also allows to gain insights into the mechanism of resistance and on the crosstalk between ecdysteroid signaling and cell proliferation-linked processes.