Arthropod nuclear receptors and their role in molting

N- and C-terminal degradation of ecdysteroid receptor isoforms, when transiently expressed in mammalian CHO cells, is regulated by the proteasome and cysteine and threonine proteases

Transcriptional activity of nuclear receptors is the result of transactivation capability and the concentration of the receptor protein. The concentration of ecdysteroid receptor (EcR) isoforms, constitutively expressed in mammalian CHO cells, is dependent on a number of factors. As shown previously, ligand binding stabilizes receptor protein concentration. In this paper, we investigate the degradation of EcR isoforms and provide evidence that N-terminal degradation is modulated by isoform-specific ubiquitination sites present in the A/B domains of EcR-A and -B1. This was demonstrated by the increase in EcR concentration by treatment with carbobenzoxy-L-leucyl-L-leucyl-L-leucinal (MG132), an inhibitor of ubiquitin-mediated proteasomal degradation and by deletion of ubiquitination sites. In addition, EcR is degraded by the peptidyl-dipeptidase cathepsin B (CatB) and the endopeptidase cathepsin S (CatS) at the C-terminus in an isoform-specific manner, despite identical C-termini. Ubiquitin-proteasome-mediated degradation and the proteolytic action are modulated by heterodimerization with Ultraspiracle (USP). The complex regulation of receptor protein concentration offers an additional opportunity to regulate transcriptional activity in an isoform- and target cell-specific way and allows the temporal limitation of hormone action.

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.

Cytoplasmic travels of the ecdysteroid receptor in target cells: pathways for both genomic and non-genomic actions

Signal transduction of the insect steroid hormones, ecdysteroids, is mediated by the ecdysteroid receptor, EcR. In various cells of the insect Rhodnius prolixus, EcR is present in both the nucleus and the cytoplasm, where it undergoes daily cycling in abundance and cellular location at particular developmental times of the last larval instar that are specific to different cell types. EcR favors a cytoplasmic location in the day and a nuclear location in the night. This study is the first to examine the potential mechanisms of intracellular transport of EcR and reveals close similarities with some of its mammalian counterparts. In double and triple labels using several antibodies, immunohistochemistry, and confocal laser scanning microscopy, we observed co-localization of EcR with the microtubules (MTs). Treatments with either the MT-stabilizing agent taxol or with colchicine, which depolymerizes MTs, resulted in considerable reduction in nuclear EcR with a concomitant increase in cytoplasmic EcR suggesting that MT disruption inhibits receptor accumulation in the nucleus. EcR also co-localizes with the chaperone Hsp90, the immunophilin FKBP52, and the light chain 1 of the motor protein dynein. All these factors also co-localize with MTs. We propose that in Rhodnius, EcR exerts its genomic effects by forming a complex with Hsp90 and FKBP52, which uses dynein on MTs as a mechanism for daily nucleocytoplasmic shuttling. The complex is transported intact to the nucleus and dissociates within it. We propose that EcR utilizes the cytoskeletal tracks for movement in a manner closely similar to that used by the glucocorticoid receptor. We also observed co-localization of EcR with mitochondria which suggests that EcR, like its mammalian counterparts, may be involved in the coordination of non-genomic responses of ecdysteroids in mitochondria.

Distinct roles of isoforms of the heme-liganded nuclear receptor E75, an insect ortholog of the vertebrate Rev-erb, in mosquito reproduction

Mosquitoes are adapted to using vertebrate blood as a nutrient source to promote egg development and as a consequence serve as disease vectors. Blood-meal activated reproductive events in female mosquitoes are hormonally and nutritionally controlled with an insect steroid hormone 20-hydroxyecdysone (20E) playing a central role. The nuclear receptor E75 is an essential factor in the 20E genetic hierarchy, however functions of its three isoforms - E75A, E75B, and E75C - in mosquito reproduction are unclear. By means of specific RNA interference depletion of E75 isoforms, we identified their distinct roles in regulating the level and timing of expression of key genes involved in vitellogenesis in the fat body (an insect analog of vertebrate liver and adipose tissue) of the mosquito Aedes aegypti. Heme is required in a high level of expression of 20E-controlled genes in the fat body, and this heme action depends on E75. Thus, in mosquitoes, heme is an important signaling molecule, serving as a sensor of the availability of a protein meal for egg development. Disruption of this signaling pathway could be explored in the design of mosquito control approaches.

Control of target gene specificity during metamorphosis by the steroid response gene E93

Hormonal control of sexual maturation is a common feature in animal development. A particularly dramatic example is the metamorphosis of insects, in which pulses of the steroid hormone ecdysone drive the wholesale transformation of the larva into an adult. The mechanisms responsible for this transformation are not well understood. Work in Drosophila indicates that the larval and adult forms are patterned by the same underlying sets of developmental regulators, but it is not understood how the same regulators pattern two distinct forms. Recent studies indicate that this ability is facilitated by a global change in the responsiveness of target genes during metamorphosis. Here we show that this shift is controlled in part by the ecdysone-induced transcription factor E93. Although long considered a dedicated regulator of larval cell death, we find that E93 is expressed widely in adult cells at the pupal stage and is required for many patterning processes at this time. To understand the role of E93 in adult patterning, we focused on a simple E93-dependent process, the induction of the Dll gene within bract cells of the pupal leg by EGF receptor signaling. In this system, we show that E93 functions to cause Dll to become responsive to EGF receptor signaling. We demonstrate that E93 is both necessary and sufficient for directing this switch. E93 likely controls the responsiveness of many other target genes because it is required broadly for patterning during metamorphosis. The wide conservation of E93 orthologs suggests that similar mechanisms control life-cycle transitions in other organisms, including vertebrates.

Identification and expression of the ecdysone receptor in the harpacticoid copepod, Amphiascus tenuiremis, in response to fipronil

The marine copepod, Amphiascus tenuiremis (A. tenuiremis), is a well characterized invertebrate model for the screening and evaluation of endocrine and reproductive toxins using life-cycle assays. These tests evaluate phenotypic endpoints related to development and reproduction, which are utilized to predict population outcomes. Some of these endpoints in arthropods, including sexual maturation and molting, are controlled by the hormone ecdysone which acts through its cognate receptor, the ecdysone receptor. The purpose of this research was to obtain and characterize sequence information for the A. tenuiremis ecdysone receptor and investigate modulation of expression levels by fipronil, an insecticide that causes infertility in males and reduced egg extrusion in female copepods, and ponasterone, a natural ecdysone receptor agonist. Results show successful cloning and phylogenetic analysis of the ecdysone receptor for A. tenuiremis, providing the first genetic information for a hormone receptor in this species. Exposure of copepodites to fipronil for 1, 2, 4, 18 and 30 h caused a significant increase in ecdysone receptor transcriptional expression at 30 h compared to control unexposed animals. This work illustrates a potential mechanism whereby exposure to fipronil, and potentially other endocrine disrupting compounds, results in impacted reproduction. Furthermore, this exemplifies the potential utility of ecdysone receptor transcriptional measurement as a sensitive and rapid biomarker of ecological relevance when linked to traditional A. tenuiremis bioassays.

LC/MS/MS identification of 20-hydroxyecdysone in a scorpion (Liocheles australasiae) and its binding affinity to in vitro-translated molting hormone receptors

Recent advances in mass spectrometry (MS) technology have facilitated the detection and quantification of minor components in organisms and the environment. In this study, we successfully identified 20-hydroxyecdysone (20E) in first instar nymphs (7 days after hatching) of the scorpion Liocheles australasiae, using tandem mass spectrometry combined with high-performance liquid chromatography (LC/MS/MS). This substance was not found in adults after the fifth stage. Other possible molting hormone candidates such as makisterone A (MaA) and ponasterone A (PoA), both of which are reported to be the molting hormones of a few arthropod species, were not detected in this scorpion. The ligand-receptor binding of 20E and its analogs was quantitatively evaluated against the in vitro-translated molting hormone receptor, the heterodimer of ecdysone receptor (EcR) and the retinoid X receptor (RXR) of L. australasiae (LaEcR/LaRXR). The concentrations of ecdysone (E), MaA, 20E, and PoA that are required to inhibit 50% of [(3)H]PoA binding to the LaEcR/LaRXR complex were determined to be 1.9, 0.69, 0.05, and 0.017 μM, respectively. The activity profiles of these 4 ecdysteroids are consistent with those obtained for the molting hormone receptors of several insects. The binding of a non-steroidal E agonist, tebufenozide, to EcR was not observed even at high concentrations, indicating that the structure of the ligand-binding pocket of LaEcR is not favorable for interaction with tebufenozide.

Juvenile hormone action through a defined enhancer motif to modulate ecdysteroid-activation of natural core promoters

We have established a model system of hormone action, in an Sf9 cell transfection system, using defined enhancer motifs and natural core promoters of metamorphosis-associated genes. The DR1 enhancer, that is an established DNA binding site for the ecdysone receptor/ultraspiracle heterodimer, was necessary for transcriptional activation by 20-OH ecdysone. For this activated transcription, a natural sequence closely 5' to the TATA box is necessary. Cotreatment with juvenile hormone III strongly suppressed the steroid activation of transcription. However, in the absence of the sequence located closely 5' to the TATA box, cotreatment with juvenile hormone instead increased transcription over that occurring due to 20-hydroxy-ecdysone alone. This sensitivity to activation by cotreatment with juvenile hormone could be transferred to a related, but otherwise unresponsive, hexamerin core promoter simply by transferring to the unresponsive promoter the five base transcription start site (ACAGT) from the responsive hexamerin gene. These are the first reports that the direction of JH action on 20-OH ecdysone-activated transcription can be reversed by removal of a sequence at the core promoter, and that modulatory action of juvenile hormone can be transferred to a different gene by transferring the transcription start site motif.

Conserved repressive function of Krüppel homolog 1 on insect metamorphosis in hemimetabolous and holometabolous species

Insect metamorphosis is regulated by ecdysteroids, which induce molts, and juvenile hormone (JH), which inhibits metamorphic changes. The molecular action of ecdysteroids has been thoroughly studied, but that of JH is poorly understood, with data currently only being available for holometabolous species, like Drosophila melanogaster and Tribolium castaneum. We studied the function of Krüppel homolog 1 (Kr-h1) in Blattella germanica, a hemimetabolous model. Kr-h1 is a Zn finger transcription factor whose function as transductor of the antimetamorphic action of JH has recently been demonstrated in D. melanogaster and T. castaneum. The RNAi experiments reported herein indicated that Kr-h1 transduces the antimetamorphic action of JH also in B. germanica, thereby suggesting that this role is an ancestral condition that has been conserved in insect evolution from hemimetabolous to holometabolous species.

Ecdysteroid metabolism in crustaceans

The molting gland, or Y-organ (YO), is the primary site for ecdysteroid synthesis in decapod crustaceans. Ecdysteroid biosynthesis is divided into two stages: (1) conversion of cholesterol to 5β-diketol and (2) conversion of 5β-diketol to secreted products. Stage 1 involves the conversion of cholesterol to 7-dehydrocholesterol (7DC) by 7,8-dehydrogenase, the "Black Box" reactions involving 3-oxo-Δ(4) intermediates, and the conversion of Δ(4)-diketol to 5β-diketol by 5β[H]-reductase. The stage 2 reactions generate four major products, depending on species: ecdysone, 3-dehydroecdysone (3DE), 25-deoxyecdysone (25dE), and 3-dehydro-25-deoxyecdysone (3D25dE). Peripheral tissues convert these compounds to the active hormones 20-hydroxyecdysone (20E) and ponasterone A (25-deoxy-20-hydroxyecdysone or 25d20E). The hydroxylations at C25, C22, C2, and C20 are catalyzed by cytochrome P-450 mono-oxygenases, which are encoded by the Halloween genes Phantom, Disembodied, Shadow, and Shade, respectively, in insects. Orthologs of these genes are present in the Daphnia genome and a cDNA encoding Phantom has been cloned from prawn. Inactivation involves conversion of ecdysteroids to polar metabolites and/or conjugates, which are eliminated in the urine and feces. The antennal gland is the major route for excretion of ecdysteroids synthesized by the YO. The hepatopancreas eliminates ingested ecdysteroids by forming apolar conjugates. The concentrations of ecdysteroids vary over the molt cycle and are determined by the combined effects biosynthesis, metabolism, and excretion.

Ecdysteroid receptor from the American lobster Homarus americanus: EcR/RXR isoform cloning and ligand-binding properties

In arthropods, ecdysteroids regulate molting by activating a heterodimer formed by the ecdysone receptor (EcR) and retinoid X receptor (RXR). While this mechanism is similar in insects and crustaceans, variation in receptor splicing, dimerization and ligand affinity adds specificity to molting processes. This study reports the EcR and RXR sequences from American lobster, a commercially and ecologically important crustacean. We cloned two EcR splice variants, both of which specifically bind ponasterone A, and two RXR variants, both of which enhance binding of ponasterone A to the EcR. Lobster EcR has high affinity for ponasterone A and muristerone and moderately high affinity for the insecticide tebufenozide. Bisphenol A, diethyl phthalate, and two polychlorinated biphenyls (PCB 29 and PCB 30), environmental chemicals shown to interfere with crustacean molting, showed little or no affinity for lobster EcR. These studies establish the molecular basis for investigation of lobster ecdysteroid signaling and signal disruption by environmental chemicals.

Effects of crustacean hyperglycemic hormone (CHH) on the transcript expression of carbohydrate metabolism-related enzyme genes in the kuruma prawn, Marsupenaeus japonicus

Crustacean hyperglycemic hormone (CHH), a member of a neuropeptide family present only in arthropods, plays a pivotal role in the modulation of hemolymph glucose levels, molting, reproduction, and the stress response. Although it has been determined that hepatopancreas and muscle are the major tissues in which CHH regulates hyperglycemic activity, the molecular mechanism by which CHH regulates carbohydrate metabolism remains unclear. In this study, we analyzed the mRNA expression levels of enzymes involved in glycogen metabolism and gluconeogenesis in order to determine how CHH regulates hemolymph glucose levels. We first cloned cDNAs encoding four carbohydrate metabolism-related enzymes from the kuruma prawn, Marsupenaeus japonicus, glycogen phosphorylase (MjGP), glycogen synthase (MjGS), fructose 1,6-bisphosphatase (MjFBPase), and phosphoenolpyruvate carboxykinase (MjPEPCK). RT-PCR analysis showed that eyestalk ablation remarkably decreased MjGP and increased MjGS transcript levels in the hepatopancreas, but not in muscle. Considering the fact that various eyestalk factors, including MIH, are removed by eyestalk ablation, these results indicate that after eyestalk ablation the metabolic state proceeds towards glycogen accumulation in the specific tissues related to molting. In contrast, MjFBPase and MjPEPCK transcript levels were not significantly changed by eyestalk ablation, indicating that CHH and other eyestalk-derived factors might not induce gluconeogenesis. Quantitative real-time PCR analysis showed that exposure of hepatopancreas to recombinant CHH significantly changed the expression levels of MjGP and MjGS, but not MjFBPase and MjPEPCK. Collectively, these results indicate that CHH is involved in glycogen metabolism in hepatopancreas.

Ecdysteroid-dependent expression of the tweedle and peroxidase genes during adult cuticle formation in the honey bee, Apis mellifera

Cuticle renewal is a complex biological process that depends on the cross talk between hormone levels and gene expression. This study characterized the expression of two genes encoding cuticle proteins sharing the four conserved amino acid blocks of the Tweedle family, AmelTwdl1 and AmelTwdl2, and a gene encoding a cuticle peroxidase containing the Animal haem peroxidase domain, Ampxd, in the honey bee. Gene sequencing and annotation validated the formerly predicted tweedle genes, and revealed a novel gene, Ampxd, in the honey bee genome. Expression of these genes was studied in the context of the ecdysteroid-coordinated pupal-to-adult molt, and in different tissues. Higher transcript levels were detected in the integument after the ecdysteroid peak that induces apolysis, coinciding with the synthesis and deposition of the adult exoskeleton and its early differentiation. The effect of this hormone was confirmed in vivo by tying a ligature between the thorax and abdomen of early pupae to prevent the abdominal integument from coming in contact with ecdysteroids released from the prothoracic gland. This procedure impaired the natural increase in transcript levels in the abdominal integument. Both tweedle genes were expressed at higher levels in the empty gut than in the thoracic integument and trachea of pharate adults. In contrast, Ampxd transcripts were found in higher levels in the thoracic integument and trachea than in the gut. Together, the data strongly suggest that these three genes play roles in ecdysteroid-dependent exoskeleton construction and differentiation and also point to a possible role for the two tweedle genes in the formation of the cuticle (peritrophic membrane) that internally lines the gut.

Virtual screening for ligands of the insect molting hormone receptor

Insect growth is regulated by the orchestrated event of ecdysteroids and their receptor proteins. Agonists/antagonists of ecdysteroid receptor are predicted to disrupt normal growth, providing good candidates of new insecticides. A database of over 2 million compounds was subjected to a shape-based virtual screening cascade to identify novel nonsteroidal hits similar to the known EcR ligand ponasterone A. Testing revealed micromolar hits against two strains of insect cells. Docking experiments against EcR were used to support the predicted binding mode of these ligands based on their overlay to ponasterone A.

Comparative endocrinology of aging and longevity regulation

Hormones regulate growth, development, metabolism, and other complex processes in multicellular animals. For many years it has been suggested that hormones may also influence the rate of the aging process. Aging is a multifactorial process that causes biological systems to break down and cease to function in adult organisms as time passes, eventually leading to death. The exact underlying causes of the aging process remain a topic for debate, and clues that may shed light on these causes are eagerly sought after. In the last two decades, gene mutations that result in delayed aging and extended longevity have been discovered, and many of the affected genes have been components of endocrine signaling pathways. In this review we summarize the current knowledge on the roles of endocrine signaling in the regulation of aging and longevity in various animals. We begin by discussing the notion that conserved systems, including endocrine signaling pathways, "regulate" the aging process. Findings from the major model organisms: worms, flies, and rodents, are then outlined. Unique lessons from studies of non-traditional models: bees, salmon, and naked mole rats, are also discussed. Finally, we summarize the endocrinology of aging in humans, including changes in hormone levels with age, and the involvement of hormones in aging-related diseases. The most well studied and widely conserved endocrine pathway that affects aging is the insulin/insulin-like growth factor system. Mutations in genes of this pathway increase the lifespan of worms, flies, and mice. Population genetic evidence also suggests this pathway's involvement in human aging. Other hormones including steroids have been linked to aging only in a subset of the models studied. Because of the value of comparative studies, it is suggested that the aging field could benefit from adoption of additional model organisms.

Effects of seasonality and moult cycle on the proliferation of nerve cells and on the labelling of ecdysone receptors in an estuarine crab

Decapod crustaceans show proliferation of the nerve cells in the olfactory lobe throughout their lives. However, the regulation of this process is still poorly understood, since it may vary with endogenous and exogenous factors. The objective of the present investigation was to quantify the proliferation of nerve cells and number of nerve cells with ecdysone receptors in the clusters of the central olfactory system in Neohelice granulata, according to moult stages and in different seasons (summer and winter). Three injections of bromodeoxyuridine (BrdU) were administered to the crabs. Brains were sectioned by microtome and fixed on slides for immunohistochemistry with anti-BrdU and anti-EcR antibodies. The proliferation of nerve cells was higher in winter than in summer, probably because in winter the crabs do not breed and the premoult and postmoult periods are longer. Crabs in postmoult exhibited more BrdU-labelled cells than crabs in premoult or intermoult in winter, because of a greater number of mitoses related to an increase in body size and addition of olfactory receptor neurons. The number of EcR-labelled cells was higher in premoult than in postmoult or intermoult in winter. The proliferation of nerve cells is regulated seasonally and according to moult stages.

Ecdysone and the cell cycle: investigations in a mosquito cell line

Cell lines provide a tool for investigating basic biological processes that underlie the complex interactions among the tissues and organs of an intact organism. We compare the evolution of insect and mammalian populations as they progress from diploid cell strains to continuous cell lines, and review the history of the well-characterized Aedes albopictus mosquito cell line, C7-10. Like Kc and S3 cells from Drosophila melanogaster, C7-10 cells are sensitive to the insect steroid hormone, 20-hydroxyecdysone (20E), and express 20E-inducible proteins as well as the EcR and USP components of the ecdysteroid receptor. The decrease in growth associated with 20E treatment results in an accumulation of cells in the G1 phase of the cycle, and a concomitant decrease in levels of cyclin A. In contrast, 20E induces a G2 arrest in a well-studied imaginal disc cell line from the moth, Plodia interpunctella. We hypothesize that 20E-mediated events associated with molting and metamorphosis include effects on regulatory proteins that modulate the mitotic cell cycle and that differences between the 20E response in diverse insect cell lines reflect an interplay between classical receptor-mediated effects on gene expression and non-classical effects on signaling pathways similar to those recently described for the vertebrate steroid hormone, estrogen.

dDOR is an EcR coactivator that forms a feed-forward loop connecting insulin and ecdysone signaling

BACKGROUND

Mammalian DOR was discovered as a gene whose expression is misregulated in muscle of Zucker diabetic rats. Because no DOR loss-of-function mammalian models are available, we analyze here the in vivo function of DOR by studying flies mutant for Drosophila DOR (dDOR).

RESULTS

We show that dDOR is a novel coactivator of ecdysone receptor (EcR) that is needed during metamorphosis. dDOR binds EcR and is required for maximal EcR transcriptional activity. In the absence of dDOR, flies display a number of ecdysone loss-of-function phenotypes such as impaired spiracle eversion, impaired salivary gland degradation, and pupal lethality. Furthermore, dDOR knockout flies are lean. We find that dDOR expression is inhibited by insulin signaling via FOXO.

CONCLUSION

This work uncovers dDOR as a novel EcR coactivator. It also establishes a mutual antagonistic relationship between ecdysone and insulin signaling in the fly fat body. Furthermore, because ecdysone signaling inhibits insulin signaling in the fat body, this also uncovers a feed-forward mechanism whereby ecdysone potentiates its own signaling via dDOR.