The genome of the nematode Pristionchus pacificus encodes putative homologs of RXR/Usp and EcR

Compounds Inhibiting Noppera-bo, a Glutathione S-transferase Involved in Insect Ecdysteroid Biosynthesis: Novel Insect Growth Regulators

Glutathione S-transferases (GSTs) are conserved in a wide range of organisms, including insects. In 2014, an epsilon GST, known as Noppera-bo (Nobo), was shown to regulate the biosynthesis of ecdysteroid, the principal steroid hormone in insects. Studies on fruit flies, Drosophila melanogaster, and silkworms, Bombyx mori, demonstrated that loss-of-function mutants of nobo fail to synthesize ecdysteroid and die during development, consistent with the essential function of ecdysteroids in insect molting and metamorphosis. This genetic evidence suggests that chemical compounds that inhibit activity of Nobo could be insect growth regulators (IGRs) that kill insects by disrupting their molting and metamorphosis. In addition, because nobo is conserved only in Diptera and Lepidoptera, a Nobo inhibitor could be used to target IGRs in a narrow spectrum of insect taxa. Dipterans include mosquitoes, some of which are vectors of diseases such as malaria and dengue fever. Given that mosquito control is essential to reduce mosquito-borne diseases, new IGRs that specifically kill mosquito vectors are always in demand. We have addressed this issue by identifying and characterizing several chemical compounds that inhibit Nobo protein in both D. melanogaster and the yellow fever mosquito, Aedes aegypti. In this review, we summarize our findings from the search for Nobo inhibitors.

From Extrapolation to Precision Chemical Hazard Assessment: The Ecdysone Receptor Case Study

Hazard assessment strategies are often supported by extrapolation of damage probabilities, regarding chemical action and species susceptibilities. Yet, growing evidence suggests that an adequate sampling of physiological responses across a representative taxonomic scope is of paramount importance. This is particularly relevant for Nuclear Receptors (NR), a family of transcription factors, often triggered by ligands and thus, commonly exploited by environmental chemicals. Within NRs, the ligand-induced Ecdysone Receptor (EcR) provides a remarkable example. Long regarded as arthropod specific, this receptor has been extensively targeted by pesticides, seemingly innocuous to non-target organisms. Yet, current evidence clearly suggests a wider presence of EcR orthologues across metazoan lineages, with unknown physiological consequences. Here, we address the state-of-the-art regarding the phylogenetic distribution and functional characterization of metazoan EcRs and provide a critical analysis of the potential disruption of such EcRs by environmental chemical exposure. Using EcR as a case study, hazard assessment strategies are also discussed in view of the development of a novel "precision hazard assessment paradigm.

A putative UDP-glycosyltransferase from Heterorhabditis bacteriophora suppresses antimicrobial peptide gene expression and factors related to ecdysone signaling

Insect pathogens have adopted an array of mechanisms to subvert the immune pathways of their respective hosts. Suppression may occur directly at the level of host-pathogen interactions, for instance phagocytic capacity or phenoloxidase activation, or at the upstream signaling pathways that regulate these immune effectors. Insect pathogens of the family Baculoviridae, for example, are known to produce a UDP-glycosyltransferase (UGT) that negatively regulates ecdysone signaling. Normally, ecdysone positively regulates both molting and antimicrobial peptide production, so the inactivation of ecdysone by glycosylation results in a failure of host larvae to molt, and probably a reduced antimicrobial response. Here, we examine a putative ecdysteroid glycosyltransferase, Hba_07292 (Hb-ugt-1), which was previously identified in the hemolymph-activated transcriptome of the entomopathogenic nematode Heterorhabditis bacteriophora. Injection of recombinant Hb-ugt-1 (rHb-ugt-1) into Drosophila melanogaster flies resulted in diminished upregulation of antimicrobial peptides associated with both the Toll and Immune deficiency pathways. Ecdysone was implicated in this suppression by a reduction in Broad Complex expression and reduced pupation rates in r Hb-ugt-1-injected larvae. In addition to the finding that H. bacteriophora excreted-secreted products contain glycosyltransferase activity, these results demonstrate that Hb-ugt-1 is an immunosuppressive factor and that its activity likely involves the inactivation of ecdysone.

Halloween genes in panarthropods and the evolution of the early moulting pathway in Ecdysozoa

Moulting is a characteristic feature of Ecdysozoa-the clade of moulting animals that includes the hyperdiverse arthropods and less speciose groups, such as onychophorans, tardigrades and nematodes. Moulting has been best analysed in arthropods, specifically in insects and crustaceans, in which a complex neuroendocrine system acts at the genomic level and initiates the transcription of genes responsible for moulting. The key moulting hormones, ecdysone and 20-hydroxyecdysone, are subsequently synthesized from cholesterol ingested with food. Their biosynthesis is regulated by the Rieske-domain protein Neverland and cytochrome P450 enzymes encoded by the so-called 'Halloween' genes. Ecdysone is then released into the haemolymph and modified into 20-hydroxyecdysone, which binds to the nuclear receptor EcR/USP and initiates transcription of the Early genes. As little is known about the moulting pathway of other ecdysozoans, we examined the occurrence of genes involved in ecdysteroid biosynthesis and the early moulting cascade across ecdysozoan subgroups. Genomic and transcriptomic searches revealed no Halloween genes in cycloneuralians, whereas only shadow (CYP315A1) is present in onychophorans and tardigrades, suggesting that the Halloween genes evolved stepwise in panarthropods. These findings imply that the genes which were responsible for the ecdysteroid biosynthesis in the last common ancestor of Ecdysozoa are currently unknown.

CONSERVED AND EXAPTED FUNCTIONS OF NUCLEAR RECEPTORS IN ANIMAL DEVELOPMENT

The nuclear receptor gene family includes 18 members that are broadly conserved among multiple disparate animal phyla, indicating that they trace their evolutionary origins to the time at which animal life arose. Typical nuclear receptors contain two major domains: a DNA-binding domain and a C-terminal domain that may bind a lipophilic hormone. Many of these nuclear receptors play varied roles in animal development, including coordination of life cycle events and cellular differentiation. The well-studied genetic model systems of Drosophila, C. elegans, and mouse permit an evaluation of the extent to which nuclear receptor function in development is conserved or exapted (repurposed) over animal evolution. While there are some specific examples of conserved functions and pathways, there are many clear examples of exaptation. Overall, the evolutionary theme of exaptation appears to be favored over strict functional conservation. Despite strong conservation of DNA-binding domain sequences and activity, the nuclear receptors prove to be highly-flexible regulators of animal development.

Diet-delivered RNAi in Helicoverpa armigera--Progresses and challenges

Helicoverpa armigera (the cotton bollworm) is a significant agricultural pest endemic to Afro-Eurasia and Oceania. Gene suppression via RNA interference (RNAi) presents a potential avenue for management of the pest, which is highly resistant to traditional insecticide sprays. This article reviews current understanding on the fate of ingested double-stranded RNA in H. armigera. Existing in vivo studies on diet-delivered RNAi and their effects are summarized and followed by a discussion on the factors and hurdles affecting the efficacy of diet-delivered RNAi in H. armigera.

How Did Arthropod Sesquiterpenoids and Ecdysteroids Arise? Comparison of Hormonal Pathway Genes in Noninsect Arthropod Genomes

The phylum Arthropoda contains the largest number of described living animal species, with insects and crustaceans dominating the terrestrial and aquatic environments, respectively. Their successful radiations have long been linked to their rigid exoskeleton in conjunction with their specialized endocrine systems. In order to understand how hormones can contribute to the evolution of these animals, here, we have categorized the sesquiterpenoid and ecdysteroid pathway genes in the noninsect arthropod genomes, which are known to play important roles in the regulation of molting and metamorphosis in insects. In our analyses, the majority of gene homologs involved in the biosynthetic, degradative, and signaling pathways of sesquiterpenoids and ecdysteroids can be identified, implying these two hormonal systems were present in the last common ancestor of arthropods. Moreover, we found that the “Broad-Complex” was specifically gained in the Pancrustacea, and the innovation of juvenile hormone (JH) in the insect linage correlates with the gain of the JH epoxidase (CYP15A1/C1) and the key residue changes in the binding domain of JH receptor (“Methoprene-tolerant”). Furthermore, the gain of “Phantom” differentiates chelicerates from the other arthropods in using ponasterone A rather than 20-hydroxyecdysone as molting hormone. This study establishes a comprehensive framework for interpreting the evolution of these vital hormonal pathways in these most successful animals, the arthropods, for the first time.

Conserved behavioral and genetic mechanisms in the pre-hatching molt of the nematode Pristionchus pacificus

Background

During development, juvenile nematodes undergo four molts. Although the number of molts appears to be constant within the Nematoda, the timing of the first molt can occur either before or after hatching. A previous study indicates that, as in some parasitic nematode lineages, a pre-hatching juvenile stage also exists in Diplogastrid nematodes. A detailed description of these sequence of events has yet to be shown for any single species.

Findings

To delineate the timing of the pre-hatching molt in the beetle-associated Pristionchus pacificus, we tracked individual mid-J1 stage worms inside the eggshell through the J1-J2 transition and hatching. We found that active movement ended 21 hours after egg-laying, followed by lethargus and hatching. We inferred that lethargus behavior represents the onset of the first molt, which precedes each post-hatching molt in C. elegans and P. pacificus. The onset of the J1-J2 molt was also marked by the upregulation of the P. pacificus molting marker Ppa-pnhr-1. We further corroborated the pre-hatching molt with the isolation of two genetic mutants that exhibited aberrant molting both inside the egg and after hatching, as characterized by protracted and often-aborted shedding of the old cuticle.

Conclusion

Our results describe in detail the pre-hatching juvenile molt in P. pacificus, provide strong visual evidence of a pre-hatching molt, and show support for common genetic mechanisms regulating molting in the pre-hatching and post-hatching developmental stages. Our findings support the hypothesis that the evolution of pre-hatching development in Diplogastrid nematodes is likely due to a heterochronic shift between the timing of the first molt and hatching.

Nuclear receptors in nematode development: Natural experiments made by a phylum

The development of complex multicellular organisms is dependent on regulatory decisions that are necessary for the establishment of specific differentiation and metabolic cellular states. Nuclear receptors (NRs) form a large family of transcription factors that play critical roles in the regulation of development and metabolism of Metazoa. Based on their DNA binding and ligand binding domains, NRs are divided into eight NR subfamilies from which representatives of six subfamilies are present in both deuterostomes and protostomes indicating their early evolutionary origin. In some nematode species, especially in Caenorhabditis, the family of NRs expanded to a large number of genes strikingly exceeding the number of NR genes in vertebrates or insects. Nematode NRs, including the multiplied Caenorhabditis genes, show clear relation to vertebrate and insect homologues belonging to six of the eight main NR subfamilies. This review summarizes advances in research of nematode NRs and their developmental functions. Nematode NRs can reveal evolutionarily conserved mechanisms that regulate specific developmental and metabolic processes as well as new regulatory adaptations. They represent the results of a large number of natural experiments with structural and functional potential of NRs for the evolution of the phylum. The conserved and divergent character of nematode NRs adds a new dimension to our understanding of the general biology of regulation by NRs. This article is part of a Special Issue entitled: Nuclear receptors in animal development.

Ecdysone receptors: from the Ashburner model to structural biology

In 1974, Ashburner and colleagues postulated a model to explain the control of the puffing sequence on Drosophila polytene chromosomes initiated by the molting hormone 20-hydroxyecdysone. This model inspired a generation of molecular biologists to clone and characterize elements of the model, thereby providing insights into the control of gene networks by steroids, diatomic gases, and other small molecules. It led to the first cloning of the EcR subunit of the heterodimeric EcR-USP ecdysone receptor. X-ray diffraction studies of the ligand-binding domain of the receptor are elucidating the specificity of receptor-ecdysteroid interactions, the selectivity of some environmentally friendly insecticides, the evolution of the EcR-USP heterodimer, and indeed Ashburner's classical biochemical evidence for the central role of the ecdysone receptor in his model.

Analysis of C. elegans NR2E nuclear receptors defines three conserved clades and ligand-independent functions

BACKGROUND

The nuclear receptors (NRs) are an important class of transcription factors that are conserved across animal phyla. Canonical NRs consist of a DNA-binding domain (DBD) and ligand-binding domain (LBD). While most animals have 20-40 NRs, nematodes of the genus Caenorhabditis have experienced a spectacular proliferation and divergence of NR genes. The LBDs of evolutionarily-conserved Caenorhabditis NRs have diverged sharply from their Drosophila and vertebrate orthologs, while the DBDs have been strongly conserved. The NR2E family of NRs play critical roles in development, especially in the nervous system. In this study, we explore the phylogenetics and function of the NR2E family of Caenorhabditis elegans, using an in vivo assay to test LBD function.

RESULTS

Phylogenetic analysis reveals that the NR2E family of NRs consists of three broadly-conserved clades of orthologous NRs. In C. elegans, these clades are defined by nhr-67, fax-1 and nhr-239. The vertebrate orthologs of nhr-67 and fax-1 are Tlx and PNR, respectively. While the nhr-239 clade includes orthologs in insects (Hr83), an echinoderm, and a hemichordate, the gene appears to have been lost from vertebrate lineages. The C. elegans and C. briggsae nhr-239 genes have an apparently-truncated and highly-diverged LBD region. An additional C. elegans NR2E gene, nhr-111, appears to be a recently-evolved paralog of fax-1; it is present in C. elegans, but not C. briggsae or other animals with completely-sequenced genomes. Analysis of the relatively unstudied nhr-111 and nhr-239 genes demonstrates that they are both expressed--nhr-111 very broadly and nhr-239 in a small subset of neurons. Analysis of the FAX-1 LBD in an in vivo assay revealed that it is not required for at least some developmental functions.

CONCLUSIONS

Our analysis supports three conserved clades of NR2E receptors, only two of which are represented in vertebrates, indicating three ancestral NR2E genes in the urbilateria. The lack of a requirement for a FAX-1 LBD suggests that the relatively high level of sequence divergence for Caenorhabditis LBDs reflects relaxed selection on the primary sequence as opposed to divergent positive selection. This observation is consistent with a model in which divergence of some Caenorhabditis LBDs is allowed, at least in part, by the absence of a ligand requirement.

Improvement of pest resistance in transgenic tobacco plants expressing dsRNA of an insect-associated gene EcR

The adoption of pest-resistant transgenic plants to reduce yield loss and pesticide utilization has been successful in the past three decades. Recently, transgenic plant expressing double-stranded RNA (dsRNA) targeting pest genes emerges as a promising strategy for improving pest resistance in crops. The steroid hormone, 20-hydroxyecdysone (20E), predominately controls insect molting via its nuclear receptor complex, EcR-USP. Here we report that pest resistance is improved in transgenic tobacco plants expressing dsRNA of EcR from the cotton bollworm, Helicoverpa armigera, a serious lepidopteran pest for a variety of crops. When H. armigera larvae were fed with the whole transgenic tobacco plants expressing EcR dsRNA, resistance to H. armigera was significantly improved in transgenic plants. Meanwhile, when H. armigera larvae were fed with leaves of transgenic tobacco plants expressing EcR dsRNA, its EcR mRNA level was dramatically decreased causing molting defects and larval lethality. In addition, the transgenic tobacco plants expressing H. armigera EcR dsRNA were also resistant to another lepidopteran pest, the beet armyworm, Spodoptera exigua, due to the high similarity in the nucleotide sequences of their EcR genes. This study provides additional evidence that transgenic plant expressing dsRNA targeting insect-associated genes is able to improve pest resistance.

Target-based and whole-worm screening approaches to anthelmintic discovery

Experimental approaches for identifying new anthelmintics include target-based and whole-worm screening methods. The former involves basic research into characterising and validating new targets, mostly proteins, followed by identification of inhibitors or agonists through the use of target-based screening assays and/or in silico drug design. The latter experimental approach uses whole-worm assays to identify anthelmintic agents with unknown modes of action, or where the primary interest lies in whether analogues are able to kill (or disable) worms rather than in measuring their direct impact on their likely target. This paper focuses initially on the intestine and external layers of nematodes as potential drug targets. Specific anthelmintic agents targeting either tissue are discussed to illustrate the impact of disruption to these structures. In both cases, the activity of these agents against insects was known, and activity against nematodes was identified using whole worm screening assays. Recent literature identifying ecdysone signalling pathway receptors in nematodes is then used to provide an example of basic research into a specific target that may lead to the development of high-throughput target-based drug screening assays. Finally, the role of whole-worm screening approaches versus target-based screening is discussed briefly.

Ecdysone receptor homologs from mollusks, leeches and a polychaete worm

The genomes of the mollusk Lottia gigantea, the leech Helobdella robusta and the polychaete worm Capitella teleta each have a gene encoding an ecdysone receptor homolog. Publicly available genomic and EST sequences also contain evidence for ecdysone receptors in the seahare Aplysia californica, the bobtail squid Euprymna scolopes and the medicinal leech Hirudo medicinalis. Three-dimensional models of the ligand binding domains of these predicted ecdysone receptor homologs suggest that each of them could potentially bind an ecdysone-related steroid. Thus, ecdysone receptors are not limited to arthropods and nematodes.