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 Biosynthesis Halloween Gene Spook Plays an Important Role in the Oviposition Process of Spider Mite, Tetranychus urticae

In insects, the ecdysteroid hormone regulates development and reproduction. However, its function in the reproduction process of spider mites is still unclear. In this study, we investigated the effect of the Halloween gene Spook on the oviposition of the reproduction process in a spider mite, Tetranychus urticae. The expression patterns of the ecdysteroid biosynthesis and signaling pathway genes, as analyzed by RT-qPCR, showed that the expression pattern of the Halloween genes was similar to the oviposition pattern of the female mite and the expression patterns of the vitellogenesis-related genes TuVg and TuVgR, suggesting that the Halloween genes are involved in the oviposition of spider mites. To investigate the function of the ecdysteroid hormone on the oviposition of the reproduction process, we carried out an RNAi assay against the Halloween gene Spook by injection in female mites. Effective silencing of TuSpo led to a significant reduction of oviposition. In summary, these results provide an initial study on the effect of Halloween genes on the reproduction in T. urticae and may be a foundation for a new strategy to control spider mites.

A luciferase reporter assay for ecdysone agonists using HEK293T cells

Ecdysone agonists are a class of insecticides that activate the ecdysone receptor (EcR) heterodimerized with the ultraspiracle (USP). Here, we report a new luciferase reporter assay for ecdysone agonists. The assay employs mammalian HEK293T cells transiently transfected with the EcR and USP genes of Chilo suppressalis, along with the taiman (Tai) gene of Drosophila melanogaster that encodes a steroid receptor coactivator. This assay system gave results consistent with those of radioligand binding assays and showed sensitivity superior to that of the existing in vitro methods. In addition, use of the heterologous host cells precludes perturbation from intrinsic players of the ecdysone signaling, which is a potential drawback of insect cell-based methods. This reporter system is suitable for detailed structure-activity analysis of ecdysone agonists and will serve as a valuable tool for the rational design of novel insect growth regulators.

Invertebrates facing environmental contamination by endocrine disruptors: Novel evidences and recent insights

The crisis of biodiversity we currently experience raises the question of the impact of anthropogenic chemicals on wild life health. Endocrine disruptors are notably incriminated because of their possible effects on development and reproduction, including at very low doses. As commonly recorded in the field, the burden they impose on wild species also concerns invertebrates, with possible specificities linked with the specific physiology of these animals. A better understanding of chemically-mediated endocrine disruption in these species has clearly gained from knowledge accumulated on vertebrate models. But the molecular pathways specific to invertebrates also need to be reckoned, which implies dedicated research efforts to decipher their basic functioning in order to be able to assess its possible disruption. The recent rising of omics technologies opens the way to an intensification of these efforts on both aspects, even in species almost uninvestigated so far.

Compared activity of agonist molecules towards ecdysone receptor in insect cell-based screening system

ABSTRACT: The ecdysone receptor, naturally activated by steroidal hormones, is a key protein for molting and reproduction processes of insects. Artificial activation of such receptor by specific pesticides induces an anomalous process of ecdysis, causing death of insects by desiccation and starvation. In this paper, we established a protocol for screening agonistic molecules towards ecdysone receptor of insect cells line S2 (Diptera) and Sf9 (Lepidoptera), transfected with the reporter plasmid ere.b.act.luc. Therefore, we set dose-response curves with the ecdysteroid 20-hydroxyecdysone, the phytoecdysteroid ponasterone-A, and tebufenozide, a pesticide belonging to the class of diacylhydrazines. In both cell lines, the median effective concentration values on reporter gene induction (EC50) of ponasterone-A was the smallest, meaning the most active agonist molecule. In Sf9 cells, tebufenozide had as smaller EC50 than 20-hydroxyecdysone, indicating the high agonistic capability and lepidopteran specificity. The protocol established in this study can be useful for a quick screening and rational research of site-specific pesticides.

Cucurbitacin B acts a potential insect growth regulator by antagonizing 20-hydroxyecdysone activity

BACKGROUND

20-Hydroxyecdysone (20E), a crucial insect steroid hormone, can bind to its cognate nuclear receptor composed of ecdysone receptor (EcR) and ultraspiracle (USP) to activate expression of 20E-response genes, enabling subsequent metamorphosis. In this study, we tried to find out which steroid-like compounds can block insect metamorphosis effectively and provide useful information for biopesticide study. For this purpose, we screened 126 steroid-like compounds for possible 20E antagonists using a dual-luciferase reporter assay with Drosophila melanogaster Kc and Bombyx mori Bm12 cells.

RESULTS

Among 126 steroid-like compounds, three cucurbitacins (CucB, D and E) were identified as 20E antagonists in both Kc and Bm12 cells. Notably, CucB caused significant molting defects and mortality in both B. mori and D. melanogaster larvae, and dramatically hindered larval growth of Helicoverpa armigera by its anti-feeding activity.

CONCLUSION

In vivo and in vitro experiments demonstrate that CucB acts as a potential insect growth regulator by antagonizing 20E activity and thus blocking molting and metamorphosis induced by 20E signaling. © 2017 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.

Flexibility and extracellular opening determine the interaction between ligands and insect sulfakinin receptors

Despite their fundamental importance for growth, the mechanisms that regulate food intake are poorly understood. Our previous work demonstrated that insect sulfakinin (SK) signaling is involved in inhibiting feeding in an important model and pest insect, the red flour beetle Tribolium castaneum. Because the interaction of SK peptide and SK receptors (SKR) initiates the SK signaling, we have special interest on the structural factors that influence the SK-SKR interaction. First, the three-dimensional structures of the two T. castaneum SKRs (TcSKR1 and TcSKR2) were generated from molecular modeling and they displayed significance in terms of the outer opening of the cavity and protein flexibility. TcSKR1 contained a larger outer opening of the cavity than that in TcSKR2, which allows ligands a deep access into the cavity through cell membrane. Second, normal mode analysis revealed that TcSKR1 was more flexible than TcSKR2 during receptor-ligand interaction. Third, the sulfated SK (sSK) and sSK-related peptides were more potent than the nonsulfated SK, suggesting the importance of the sulfate moiety.

Establishment of a cell line from the ash and privet borer beetle Tylonotus bimaculatus Haldeman and assessment of its sensitivity to diacylhydrazine insecticides

A novel cell line, NRCAN-Tb521, was developed from larvae of the longhorn beetle Tylonotus bimaculatus (Coleoptera: Cerambycidae), a pest of North American ash trees. The cell line has been successfully passaged more than 50 times and displayed very strong attachment to the substrate and a modal chromosomal count distribution of 19. Sequencing of a 649 bp fragment of the mitochondrial cytochrome oxidase I gene confirmed the identity of NRCAN-Tb521 as T. bimaculatus. The response of the cell line to 20-hydroxyecdysone and diacylhydrazine ecdysone agonist insecticides was also studied. At 10(-6) M, 20-hydroxyecdysone, tebufenozide, methoxyfenozide and halofenozide triggered the production of numerous filamentous cytoplasmic extensions, and the cells tended to form aggregates, indicative of a cell differentiation response. This response was followed by a strong decrease in viability after 4 d. Reverse transcription polymerase chain reaction (PCR) experiments and sequencing of PCR fragments showed that the 20E receptor gene EcR is expressed in the cells and that 20E, tebufenozide, methoxyfenozide and halofenozide also induce the expression of the nuclear hormone receptor gene HR3. This report establishes that NRCAN-Tb521 is a valuable in vitro model to study effects of ecdysone agonists in wood-boring cerambycids.

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.

Insect spontaneous ultraweak photon emission as an indicator of insecticidal compounds

The influence of beta-cypermethrin, a commercial insecticide, and Cicuta virosa L. var. latisecta Celak (Umbelliferae:Cicutal), an insecticidal plant, on the spontaneous ultraweak photon emissions from larvae of Spodoptera litura Fabricius and Zophobas morio Fabricius were studied. The increased percentages of spontaneous photon emission intensities from S. litura treated with 0.1 and 1 μg/ml beta-cypermethrin were both lower than those of the control in the 24 post-treatment hours, remarkable difference could also be observed during the same period from Z. morio treated with beta-cypermethrin at 0.156, 0.313 and 0.625 μg/ml. The increased percentages of spontaneous photon emission intensities from the two mentioned insects treated with 10,100 and 1000 μg/ml petroleum ether fraction of C. virosa L. var. latisecta, which displayed little activity against whole insects, could also be changed noticeably. The present study indicated that change in the intensity of spontaneous ultraweak photon emission from insect could be used as a novel method for screening insecticidal compounds with very low content in plant.