Commentary
New ways and new hopes for IGR development
2021 Volume 46 Issue 1 Pages 3-6
Details
2021 Volume 46 Issue 1 Pages 3-6
Insect Growth Regulators (IGRs) represent advanced, bio-rational insecticides. This Special Issue reflects progress in IGR development that has been enabled by insight into the molecular principles of biosynthetic or hormone signaling pathways. The unifying principle is aiming at processes and molecular targets that are unique to arthropods and ideally to narrower insect taxa representing pests or disease vectors. While some strategies of obtaining the desired compounds for chemical intervention rely on rational, structure-based design or computational power, others exploit technologies allowing automated, high-throughput screening of large chemical libraries. All avenues leading to selective and environmentally safe pest control are valid as we face the imminent threat of the declining world insect population.
Just when conceiving this Commentary, I received an alert from the Proceedings of the National Academy of Sciences of the USA, announcing a release on January 11 of their Special Issue on the global decline in insect abundance and species diversity, including the threat to essential pollinators. The Introduction entitled “Insect decline in the Anthropocene: Death by a thousand cuts” explains that while the effects of environmental stressors including climate change, destruction of habitats, and chemical pollution combine, their relative contributions are hard to estimate.1) However, the overuse of neonicotinoids and other wide-spectrum pesticides is certainly one of the contributing stressors. Therefore, environmentally friendly insecticides, ideally ones that are harmless to non-target species, may (again) be in demand. The IGRs such as disruptors of insect-specific endocrine and biosynthetic processes hold new promises.
The reviews and original research featured in this Special Issue show examples of new efforts to find compounds with the desired properties through rational, structure-based or computational design, as well as high-throughput screening (HTS) strategies. For my lack of expertise on chitin metabolism, covered by three contributions to this issue,2–4) I will focus on those articles concerning endocrine signaling, either at the level of the juvenile hormone (JH) or ecdysteroid biosynthesis, transport, and receptor-mediated downstream effects. A large part of the progress presented in this Special Issue has relied on the atomic-level structure resolution of the EcR-Usp ecdysone receptor complex,5–7) and on the identification of the Methoprene-tolerant (Met) protein as an intracellular receptor for JH.8–10)
Picard and colleagues11) review the grounds establishing farnesyl diphosphate synthase (FPPS) as a promising target of chemical intervention with lepidopteran pests. Being part of the mevalonate pathway, FPPS contributes to biosynthesis of many essential terpenoids, which in insects include JH. FPPSs occur in all organisms. However, lepidopteran FPPSs display unique structural features, relating to the fact that unlike other insects possessing methyl-branched JH III, moths synthesize ethyl-branched JH homologs (such as JH I)12) from bulkier substrates. Data suggest that the unique structural features of lepidopteran FPPSs enable design of selective inhibitors not affecting other species. The ongoing target-specific inhibitor design is aided by ample X-ray structural information on FPPSs, including those from Lepidoptera.13)
The team of Ryusuke Niwa presents high-quality original research14) leading to inhibitors of a steroidogenic glutathione S-transferase that they had previously discovered as a product of the essential Drosophila noppera-bo (Nobo) gene.15) A HTS of a chemical library yielded five non-steroidal Nobo inhibitors acting at micromolar concentrations. Nobo was crystallized with each of the five compounds, revealing their distinct interactions with the enzyme. As Nobo has only been found in dipteran and lepidopteran insects, the identified compounds have a great potential to be developed into selective IGRs.
An entirely new level of potential intervention with ecdysteroid signaling is discussed in a review by Okamoto and Yamanaka.16) The idea is based on their discovery of an ecdysone importer (EcI)17) that is required in the target tissues to uptake the hormone and in the blood-brain barrier to deliver it to the CNS.18) Loss of EcI corresponds to ecdysteroid deficiency in Drosophila mutants. Besides altering the traditional tenet that steroid hormones cross the cell membranes via simple diffusion, this transmembrane protein offers a target for developing an arthropod-specific EcI blocker as a new type of IGR. The authors speculate that a prospective ecdysone antagonist acting at the cell-surface level of EcI would be less prone to intracellular metabolism and other forms of resistance.
Three-dimensional structure resolution has been the driving force to elucidate the interaction of non-steroidal ecdysone receptor agonists with the EcR-Usp receptor complex.19) The group led by Isabelle Billas presents receptor-ligand crystal structures explaining how dibenzoylhydrazine and imidazole-substituted EcR agonists utilize part of the ligand-binding pocket that is occupied with water molecules when binding the natural, active ecdysteroid agonist (20-hydroxyecdysone; 20E). The synthetic compounds cause a major conformational change to the EcR ligand-binding domain, illustrating its flexibility and adaptability to the disparate chemistries.
Ueno and colleagues20) present a solid study on non-steroidal EcR agonists of the tetrahydroquinoline (THQ) type that are interesting for their target preference of mosquito larvae. The authors have expanded the repertoire of existing THQ compounds through de novo synthesis and tested all 35 compounds for receptor binding in a mosquito cell line and in bioassays. The data positively correlate the agonists by their binding affinity, increasing hydrophobicity, and larvicidal potency in Aedes albopictus.
Cell-based reporter assays available for studies of both 20E and JH response are extensively reviewed by Ito-Harashima and Yagi.21) Various systems have been developed thus far to monitor 20E and JH receptor activities through the endogenous hormone receptors in insect cell lines. Alternative, heterologous mammalian or yeast cells expressing the recombinant insect receptor proteins have the advantage of eliminating endogenous target molecules. Most of the reporter assays measure the transcriptional activation of a reporter driven by ecdysteroid-response (EcRE) or JH-response (JHRE) DNA elements derived from known hormone-inducible genes. Two-hybrid assays report interaction between the dimerizing receptor subunits, either EcR and Usp for the ecdysteroid nuclear receptor or Met/Gce and Tai (aka SRC) for the complex of the bHLH-PAS JH receptor proteins. The authors explain how each reporter assay can aid discovery of new IGRs, as exemplified by the non-steroidal agonists of EcR. The potential of cell-based assays for HTS approaches is emphasized. Indeed, initial attempts to identify both inhibitors22,23) and novel activators (this issue)24) of JH signaling have been reported.
In their contribution, Kayukawa and colleagues24) exploit a JHRE-luciferase reporter assay based on a cell line from Bombyx mori to screen a large chemical library. The same HTS performed in an antagonistic mode has recently yielded an inhibitor of JH signaling, capable of downregulating the JH-response gene Kr-h1 and inducing precocious pupation in the silkworm larvae.23) Here, 10 novel activators of the JHRE reporter are presented, of which several share the 4-phenoxyphenoxymethyl structure of some existing juvenoid IGRs such as pyriproxyfen, while others have distinct chemistry,24) consistent with the great diversity of JH receptor agonists.25,26) When tested in final-instar Bombyx larvae, seven of the activators could delay metamorphosis although they were unable to prevent pupation and provoke an extra larval molt. Still, the obtained HTS hits could be promising lead compounds for further IGR development.
The article by Yokoi and colleagues27) sets the first example of a JH receptor agonist discovered through an in-silico approach. The virtual screening of five million compounds relied on a structural homology model of the Drosophila JH receptor protein Met. Compounds were computationally selected based on similarity to two potent juvenoid IGRs, fenoxycarb and pyriproxyfen, both agonist ligands of the JH receptors.8,25,26) In the following virtual screens, the hits were docked to the molecular model and the free energy of the ligand-receptor binding was calculated, finally yielding 11 candidates. Importantly, selected compounds were validated in a JHRE-based reporter assay in a human cell line expressing the Drosophila JH receptor proteins Met and Tai.28) One hit proved to activate the reporter with a sub-micromolar potency, providing a lead compound for further development. Interestingly, this piperazin-based compound is predicted to form a hydrogen bond with a conserved tyrosine residue, previously implicated in forming a hydrogen bond with the epoxide moiety of JH III within the ligand-binding pocket of the other Drosophila JH receptor, Gce.25)
Although the effect of JH on insect embryogenesis has far-reaching evolutionary implications,29) it remains subject to ongoing debate.30) Particularly embryos of hemimetabolous species are considered sensitive to ectopic JH treatments.29,31) However, this notion relies on defects inflicted by high doses of synthetic JH mimics or JH homologs foreign to the treated species. Naruse and colleagues32) subject the eggs of the bean bug, Riptortus pedestris, to the natural heteropteran JH, identified in this species as JH III skipped bisepoxide (JHSB3).33) They show that JHSB3 induced developmental malformations and arrest to the bean bug embryos at micromolar doses, whereas the common JH III or juvenoid IGRs such as pyriproxyfen had to be applied in doses at least thousand-fold higher.
Parthasarathy and Palli34) compile reported effects of commonly deployed JH mimicking insecticides (methoprene, hydroprene, and pyriproxyfen) on survival of three target species when treated at different stages. They emphasize the limited utility and rather partial success of current juvenoid IGRs in insect control. Their review in fact underscores the need of developing better means of intervention at the level of JH signaling.
Satoh and colleagues35) describe multistep chemical derivatization of existing insecticidal and lead compounds using pyrimidin substituents. Their synthesis yielded benzpyrimoxan, a new, currently registered insecticide active against the brown planthopper (Nilaparvata lugens), a major pest of rice. Low toxicity to pollinators and other beneficial arthropods is an advantage of this putative novel IGR. While the planthopper nymphs die during molting, the mode of action of benzpyrimoxan is unknown and its molecular target has not been reported.
