Purification and characterization of recombinant ligand-binding domains from the ecdysone receptors of four pest insects

Use of Differential Scanning Fluorimetry to Identify Nuclear Receptor Ligands

Identification of small molecules that interact specifically with the ligand-binding domains (LBDs) of nuclear receptors (NRs) can be accomplished using a variety of methodologies. Here, we describe the use of differential scanning fluorimetry to identify these ligands, a technique that requires no modification or derivatization of either the protein or the ligand, and uses an instrument that is becoming increasingly affordable and common in modern molecular biology laboratories, the quantitative, or real-time, PCR machine. Upon being introduced to specific ligands, nuclear receptors undergo structural and dynamic changes that tend to increase molecular stability, which can be measured by the resistance of the protein to heat denaturation. Differential scanning fluorimetry (DSF) uses a dielectric sensitive fluorescent dye to measure the thermal denaturation, or "melting" point (Tm) of a protein under different conditions, in this case in the absence and presence of a candidate ligand. Using DSF, multiple candidates can be screened at once, in numbers corresponding to plate size of the instrument used (e.g., 96- or 384-well), allowing significant throughput if a modest library of compounds needs to be tested.

Unprecedented conformational flexibility revealed in the ligand-binding domains of theBovicola ovisecdysone receptor (EcR) and ultraspiracle (USP) subunits

The heterodimeric ligand-binding region of theBovicola ovisecdysone receptor has been crystallized either in the presence of an ecdysteroid or a synthetic methylene lactam insecticide. Two X-ray crystallographic structures, determined at 2.7 Å resolution, show that the ligand-binding domains of both subunits of this receptor, like those of other nuclear receptors, can display significant conformational flexibility. Thermal melt experiments show that while ponasterone A stabilizes the higher order structure of the heterodimer in solution, the methylene lactam destabilizes it. The conformations of the EcR and USP subunits observed in the structure crystallized in the presence of the methylene lactam have not been seen previously in any ecdysone receptor structure and represent a new level of conformational flexibility for these important receptors. Interestingly, the new USP conformation presents an open, unoccupied ligand-binding pocket.

Silencing the ecdysone synthesis and signaling pathway genes disrupts nymphal development in the whitefly

Sap-sucking insects are important pests in agriculture and good models to study insect biology. The role of ecdysone pathway genes in the life history of this group of insects is largely unknown likely due to a lack of efficient gene silencing methods allowing functional genetic analyses. Here, we developed a new and high throughput method to silence whitefly genes using a leaf-mediated dsRNA feeding method. We have applied this method to explore the roles of genes within the molting hormone-ecdysone synthesis and signaling pathway for the survival, reproduction and development of whiteflies. Silencing of genes in the ecdysone pathway had a limited effect on the survival and fecundity of adult whiteflies. However, gene silencing reduced survival and delayed development of the whitefly during nymphal stages. These data suggest that the silencing method developed here provides a useful tool for functional gene discovery studies of sap-sucking insects, and further indicate the potential of regulating the ecdysone pathway in whitefly control.

Cloning, ligand-binding, and temporal expression of ecdysteroid receptors in the diamondback moth, Plutella xylostella

Background

The diamondback moth, Plutella xylostella (L.) (Lepidoptera: Plutellidae), is a devastating pest of cruciferous crops worldwide, and has developed resistance to a wide range of insecticides, including diacylhydrazine-based ecdysone agonists, a highly selective group of molt-accelerating biopesticides targeting the ecdysone receptors.

Result

In this study, we cloned and characterized the ecdysone receptors from P. xylostella, including the two isoforms of EcR and a USP. Sequence comparison and phylogenetic analysis showed striking conservations among insect ecdysone receptors, especially between P. xylostella and other lepidopterans. The binding affinity of ecdysteroids to in vitro-translated receptor proteins indicated that PxEcRB isoform bound specifically to ponasterone A, and the binding affinity was enhanced by co-incubation with PxUSP (K_d =3.0±1.7 nM). In contrast, PxEcRA did not bind to ponasterone A, even in the presence of PxUSP. The expression of PxEcRB were consistently higher than that of PxEcRA across each and every developmental stage, while the pattern of PxUSP expression is more or less ubiquitous.

Conclusions

Target site insensitivity, in which the altered binding of insecticides (ecdysone agonists) to their targets (ecdysone receptors) leads to an adaptive response (resistance), is one of the underlying mechanisms of diacylhydrazine resistance. Given the distinct differences at expression level and the ligand-binding capacity, we hypothesis that PxEcRB is the ecdysone receptor that controls the remodeling events during metamorphosis. More importantly, PxEcRB is the potential target site which is modified in the ecdysone agonist-resistant P. xylostella.

Molecular adaptation and resilience of the insect's nuclear receptor USP

Background

The maintenance of biological systems requires plasticity and robustness. The function of the ecdysone receptor, a heterodimer composed of the nuclear receptors ECR (NR1H1) and USP (NR2B4), was maintained in insects despite a dramatic divergence that occurred during the emergence of Mecopterida. This receptor is therefore a good model to study the evolution of plasticity. We tested the hypothesis that selection has shaped the Ligand-Binding Domain (LBD) of USP during evolution of Mecopterida.

Results

We isolated usp and cox1 in several species of Drosophilidae, Tenebrionidae and Blattaria and estimated non-synonymous/synonymous rate ratios using maximum-likelihood methods and codon-based substitution models. Although the usp sequences were mainly under negative selection, we detected relaxation at residues located on the surface of the LBD within Mecopterida families. Using branch-site models, we also detected changes in selective constraints along three successive branches of the Mecopterida evolution. Residues located at the bottom of the ligand-binding pocket (LBP) underwent strong positive selection during the emergence of Mecopterida. This change is correlated with the acquisition of a large LBP filled by phospholipids that probably allowed the stabilisation of the new Mecopterida structure. Later, when the two subgroups of Mecopterida (Amphiesmenoptera: Lepidoptera, Trichoptera; Antliophora: Diptera, Mecoptera, Siphonaptera) diverged, the same positions became under purifying selection. Similarly, several positions of the heterodimerisation interface experienced positive selection during the emergence of Mecopterida, rapidly followed by a phase of constrained evolution. An enlargement of the heterodimerisation surface is specific for Mecopterida and was associated with a reinforcement of the obligatory partnership between ECR and USP, at the expense of homodimerisation.

Conclusions

In order to explain the episodic mode of evolution of USP, we propose a model in which the molecular adaptation of this protein is seen as a process of resilience for the maintenance of the ecdysone receptor functionality.

Isoforms of the heteropteran Nezara viridula ecdysone receptor: protein characterisation, RH5992 insecticide binding and homology modelling

BACKGROUND

Certain bisacylhydrazine compounds such as tebufenozide (RH5992) have been shown to act as order-specific insecticides. Their compatibility with predatory Heteroptera, which are used as biological control agents, has also been demonstrated. However, the molecular mode of action of these ecdysone agonists has not been explored in a heteropteran, much less one that is a significant agricultural pest, such as Nezara viridula.

RESULTS

Alternatively spliced ligand-binding regions of the N. viridula ecdysone receptor were expressed, purified and characterised by 2D gel analysis, mass spectrometry, homology modelling and competitive binding of a bisacylhydrazine insecticidal compound (RH5992) and various ecdysteroids. Ligand binding by the two splice isoforms was indistinguishable, and relative affinities were found to occur in the order muristerone A > ponasterone A > 20-hydroxyecdysone > inokosterone > RH5992 > α-ecdysone.

CONCLUSION

The predicted difference in amino acid sequence between the ligand-binding domains of the N. viridula ecdysone receptor splice variants was verified by mass spectrometry. Both splice variant isoforms exhibit a greater affinity for the bisacylhydrazine insecticide RH5992 than do the other hemipteran ecdysone receptors characterised to date. Their affinities for a range of ecdysteroids also distinguish them from the ecdysone receptors of other Hemiptera characterised thus far. Homology models of both N. viridula receptor isoforms provide further insight into the bisacylhydrazine- and ecdysteroid-binding properties of these receptors, including their similar affinity for 20-hydroxyecdysone and the postulated pentatomomorphan moulting hormone makisterone A.

Production of protein complexes via co-expression

Multi-protein complexes are involved in essentially all cellular processes. A protein's function is defined by a combination of its own properties, its interacting partners, and the stoichiometry of each. Depending on binding partners, a transcription factor can function as an activator in one instance and a repressor in another. The study of protein function or malfunction is best performed in the relevant context. While many protein complexes can be reconstituted from individual component proteins after being produced individually, many others require co-expression of their native partners in the host cells for proper folding, stability, and activity. Protein co-expression has led to the production of a variety of biological active complexes in sufficient quantities for biochemical, biophysical, structural studies, and high throughput screens. This article summarizes examples of such cases and discusses critical considerations in selecting co-expression partners, and strategies to achieve successful production of protein complexes.

Ecdysteroid hormone action

Several reviews devoted to various aspects of ecdysone research have been published during the last few years. Therefore, this article concentrates mainly on the considerable progress in ecdysone research observed recently, and will cover the results obtained during the last 2 years. The main emphasis is put on the molecular mode of ecdysteroid receptor-mediated hormone action. Two examples of interaction with other hormonal signalling pathways are described, namely crosstalk with juvenile hormone and insulin. Some selected, recently investigated examples of the multitude of hormonal responses are described. Finally, ecological aspects and some practical applications are discussed.

Arthropod nuclear receptors and their role in molting

The molting process in arthropods is regulated by steroid hormones acting via nuclear receptor proteins. The most common molting hormone is the ecdysteroid, 20-hydroxyecdysone. The receptors of 20-hydroxyecdysone have also been identified in many arthropod species, and the amino acid sequences determined. The functional molting hormone receptors consist of two members of the nuclear receptor superfamily, namely the ecdysone receptor and the ultraspiracle, although the ecdysone receptor may be functional, in some instances, without the ultraspiracle. Generally, the ecdysone receptor/ultraspiracle heterodimer binds to a number of ecdysone response elements, sequence motifs that reside in the promoter of various ecdysteroid-responsive genes. In the ensuing transcriptional induction, the ecdysone receptor/ultraspiracle complex binds to 20-hydroxyecdysone or to a cognate ligand that, in turn, leads to the release of a corepressor and the recruitment of coactivators. 3D structures of the ligand-binding domains of the ecdysone receptor and the ultraspiracle have been solved for a few insect species. Ecdysone agonists bind to ecdysone receptors specifically, and ligand-ecdysone receptor binding is enhanced in the presence of the ultraspiracle in insects. The basic mode of ecdysteroid receptor action is highly conserved, but substantial functional differences exist among the receptors of individual species. Even though the transcriptional effects are apparently similar for ecdysteroids and nonsteroidal compounds such as diacylhydrazines, the binding shapes are different between them. The compounds having the strongest binding affinity to receptors ordinarily have strong molting hormone activity. The ability of the ecdysone receptor/ultraspiracle complex to manifest the effects of small lipophilic agonists has led to their use as gene switches for medical and agricultural applications.

Properties of ecdysteroid receptors from diverse insect species in a heterologous cell culture system--a basis for screening novel insecticidal candidates

Insect development is driven by the action of ecdysteroids on morphogenetic processes. The classic ecdysteroid receptor is a protein heterodimer composed of two nuclear receptors, the ecdysone receptor (EcR) and Ultraspiracle (USP), the insect ortholog of retinoid X receptor. The functional properties of EcR and USP vary among insect species, and provide a basis for identifying novel and species-specific insecticidal candidates that disrupt this receptor's normal activity. A heterologous mammalian cell culture assay was used to assess the transcriptional activity of the heterodimeric ecdysteroid receptor from species representing two major insect orders: the fruit fly, Drosophila melanogaster (Diptera), and the Colorado potato beetle, Leptinotarsa decemlineata (Coleoptera). Several nonsteroidal agonists evoked a strong response with the L. decemlineata heterodimer that was consistent with biochemical and in vivo evidence, whereas the D. melanogaster receptor's response was comparatively modest. Conversely, the phytoecdysteroid muristerone A was more potent with the D. melanogaster heterodimer. The additional presence of juvenile hormone III potentiated the inductive activity of muristerone A in the receptors from both species, but juvenile hormone III was unable to potentiate the inductive activity of the diacylhydrazine methoxyfenozide (RH2485) in the receptor of either species. The effects of USP on ecdysteroid-regulated transcriptional activity also varied between the two species. When it was tested with D. melanogaster EcR isoforms, basal activity was lower and ligand-dependent activity was higher with L. decemlineata USP than with D. melanogaster USP. Generally, the species-based differences validate the use of the cell culture assay screen for novel agonists and potentiators as species-targeted insecticidal candidates.

Structural and evolutionary innovation of the heterodimerization interface between USP and the ecdysone receptor ECR in insects

Understanding how the variability of protein structure arises during evolution and leads to new structure-function relationships ultimately promoting evolutionary novelties is a major goal of molecular evolution and is critical for interpreting genome sequences. We addressed this issue using the ecdysone receptor (ECR), a major developmental factor that controls development and reproduction of arthropods. The functional ECR is a heterodimer of two nuclear receptors: ECR, which binds ecdysteroids, and its obligatory partner ultraspirade (USP), which is orthologous to the retinoid X receptor of vertebrates. Both genes underwent a dramatic increase of evolutionary rate in Mecopterida, the major insect terminal group containing Dipteras and Lepidopteras. We therefore questioned the implication of this event in terms of coevolution of their dimerization interface. A structural comparison revealed a 30% larger ligand-binding domain (LBD) heterodimerization surface in the Lepidoptera Heliothis when compared with basal insects, associated with a symmetrization of the interface, which is exceptional for nuclear receptors. Reconstruction of ancestral sequences and homology modeling of the ancestral Mecopterida ECR-USP reveal that this enlarged dimerization surface is a synapomorphy for Mecopterida. Furthermore, we show that the residues implicated in the new dimerization surface underwent specific evolutionary constraints in Mecopterida indicative of their new and conserved role in the dimerization interface. Most of all, the novel surface originates from a 15 degrees torsion of a subdomain of USP LBD toward its partner ECR, which is a long-range consequence of the peculiar position of a Mecopterida-specific insertion in loop L1-3, located outside of the interaction surface, in a less crucial domain of the partner protein. These results indicate that the coevolution between ECR and USP occurred through a novel mechanism of intramolecular epistasis that will undoubtedly be generalized for other molecules because it uses flexibility of a less-constrained region of a protein to modify the structure of another, critical part of the molecule.

Ligand binding by recombinant domains from insect ecdysone receptors

The ligand binding domains (LBDs) from the EcR and USP proteins of four insect pests (Lucilia cuprina, Myzus persicae, Bemisia tabaci, Helicoverpa armigera) were purified as recombinant heterodimers. The K(d) values for [(3)H]-ponasterone A binding by LBD heterodimers that included the hinge regions (i.e., DE/F heterodimers) ranged 0.7-2.5 nM, with K(i) values for ecdysteroid and dibenzoylhydrazine ligands ranging from 0.1 nM to >448 microM. The K(d) and K(i) values for a recombinant H. armigera LBD heterodimer that lacked D-regions (i.e., an E/F heterodimer) were approximately 4 times higher than those for its DE/F counterpart. Rate constants were estimated for the L. cuprina LBD heterodimer. A fluorescein-inokosterone conjugate (K(i)~40 nM) was used to develop a novel binding assay based on fluorescence polarization. This assay, which ranked the affinity of competitor ecdysteroids in the same order as the [(3)H]-ponasterone A binding assay, is well suited to high-throughput screening. Ponasterone A had a higher affinity than muristerone A for the recombinant hemipteran LBD heterodimers, whereas the reverse was true for the recombinant dipteran one. The same preference was observed when these ligands were tested as inducers of ecdysone receptor-controlled gene expression in transfected mammalian cells. The binding data obtained in vitro using recombinant LBD heterodimers reflects the ability of agonists to induce transgene expression in recombinant mammalian cells, and can also reflect their efficacy as larvicides.