Non-genomic ecdysone effects and the invertebrate nuclear steroid hormone receptor EcR--new role for an "old" receptor?

Evolution of non-genomic nuclear receptor function

Nuclear receptors (NRs) are responsible for the regulation of diverse developmental and physiological systems in metazoans. NR actions can be the result of genomic and non-genomic mechanisms depending on whether they act inside or outside of the nucleus respectively. While the actions of both mechanisms have been shown to be crucial to NR functions, non-genomic actions are considered less frequently than genomic actions. Furthermore, hypotheses on the origin and evolution of non-genomic NR signaling pathways are rarely discussed in the literature. Here we summarize non-genomic NR signaling mechanisms in the context of NR protein family evolution and animal phyla. We find that NRs across groups and phyla act via calcium flux as well as protein phosphorylation cascades (MAPK/PI3K/PKC). We hypothesize and discuss a possible synapomorphy of NRs in the NR1 and NR3 families, including the thyroid hormone receptor, vitamin D receptor, ecdysone receptor, retinoic acid receptor, steroid receptors, and others. In conclusion, we propose that the advent of non-genomic NR signaling may have been a driving force behind the expansion of NR diversity in Cnidarians, Placozoans, and Bilaterians.

The ecdysteroid receptor regulates salivary gland degeneration through apoptosis in Rhipicephalus haemaphysaloides

Background

It is well established that ecdysteroid hormones play an important role in arthropod development and reproduction, mediated by ecdysteroid receptors. Ticks are obligate hematophagous arthropods and vectors of pathogens. The salivary gland plays an essential role in tick growth and reproduction and in the transmission of pathogens to vertebrate hosts. During tick development, the salivary gland undergoes degeneration triggered by ecdysteroid hormones and activated by apoptosis. However, it is unknown how the ecdysteroid receptor and apoptosis regulate salivary gland degeneration. Here, we report the functional ecdysteroid receptor (a heterodimer of the ecdysone receptor [EcR] and ultraspiracle [USP]) isolated from the salivary gland of the tick Rhipicephalus haemaphysaloides and explore the molecular mechanism of ecdysteroid receptor regulation of salivary gland degeneration.

Methods

The full length of RhEcR and RhUSP open reading frames (ORFs) was obtained from the transcriptome. The RhEcR and RhUSP proteins were expressed in a bacterial heterologous system, Escherichia coli. Polyclonal antibodies were produced against synthetic peptides and were able to recognize recombinant and native proteins. Quantitative real-time PCR and western blot were used to detect the distribution of RhEcR, RhUSP, and RhCaspases in the R. haemaphysaloides organs. A proteomics approach was used to analyze the expression profiles of the ecdysteroid receptors, RhCaspases, and other proteins. To analyze the function of the ecdysteroid receptor, RNA interference (RNAi) was used to silence the genes in adult female ticks. Finally, the interaction of RhEcR and RhUSP was identified by heterologous co-expression assays in HEK293T cells.

Results

We identified the functional ecdysone receptor (RhEcR/RhUSP) of 20-hydroxyecdysone from the salivary gland of the tick R. haemaphysaloides. The RhEcR and RhUSP genes have three and two isoforms, respectively, and belong to a nuclear receptor family but with variable N-terminal A/B domains. The RhEcR gene silencing inhibited blood-feeding, blocked engorgement, and restrained salivary gland degeneration, showing the biological role of the RhEcR gene in ticks. In the ecdysteroid signaling pathway, RhEcR silencing inhibited salivary gland degeneration by suppressing caspase-dependent apoptosis. The heterologous expression in mammalian HEK293T cells showed that RhEcR1 interacts with RhUSP1 and induces caspase-dependent apoptosis.

Conclusions

These data show that RhEcR has an essential role in tick physiology and represents a putative target for the control of ticks and tick-borne diseases.

Graphical Abstract

Supplementary Information

The online version contains supplementary material available at 10.1186/s13071-021-05052-2.

Molecular dynamics and free energy studies on the Drosophila melanogaster and Leptinotarsa decemlineata ecdysone receptor complexed with agonists: Mechanism for binding and selectivity

The ecdysone receptor is a nuclear hormone receptor that plays a pivotal role in the insect metamorphosis and development. To address the molecular mechanisms of binding and selectivity, the interactions of two typical agonists Ponasterone A and 20-Hydroxyecdysone with Drosophila melanogaster (DME) and Leptinotarsa decemlineata ecdysone (LDE) receptors were investigated by homology modeling, molecular docking, molecular dynamic simulation, and thermodynamic analysis. We discover that 1) the L5-loop, L11-loop, and H12 helix for DME, L7-loop, and L11-loop for LDE are more flexible, which affect the global dynamics of the ligand-binding pocket, thus facilitating the ligand recognition of ecdysone receptor; 2) several key residues (Thr55/Thr37, Phe109/Phe91, Arg95/Arg77, Arg99/Arg81, Phe108/Leu90, and Ala110/Val92) are responsible for the binding of the proteins; 3) the binding-free energy is mainly contributed by the van der Waals forces as well as the electrostatic interactions of ligand and receptor; 4) the computed binding-free energy difference between DME-C1 and LDE-C1 is -4.65 kcal/mol, explains that C1 can form many more interactions with the DME; 5) residues Phe108/Leu90 and Ala110/Val92 have relatively position and orientation difference in the two receptors, accounting most likely for the ligand selectivity of ecdysone receptor from different orders of insects. This study underscores the expectation that different insect pests should be able to discriminate among compounds from different as yet undiscovered compounds, and the results firstly show a structural and functional relay between the agonists and receptors (DME and LDE), which can provide an avenue for the development of target-specific insecticides. Communicated by Ramaswamy H. Sarma.

Modulation of neuronal activity in the Drosophila mushroom body by DopEcR, a unique dual receptor for ecdysone and dopamine

G-protein-coupled receptors (GPCRs) for steroid hormones mediate unconventional steroid signaling and play a significant role in the rapid actions of steroids in a variety of biological processes, including those in the nervous system. However, the effects of these GPCRs on overall neuronal activity remain largely elusive. Drosophila DopEcR is a GPCR that responds to both ecdysone (the major steroid hormone in insects) and dopamine, regulating multiple second messenger systems. Recent studies have revealed that DopEcR is preferentially expressed in the nervous system and involved in behavioral regulation. Here we utilized the bioluminescent Ca²⁺-indicator GFP-aequorin to monitor the nicotine-induced Ca²⁺-response within the mushroom bodies (MB), a higher-order brain center in flies, and examined how DopEcR modulates these Ca²⁺-dynamics. Our results show that in DopEcR knockdown flies, the nicotine-induced Ca²⁺-response in the MB was significantly enhanced selectively in the medial lobes. We then reveal that application of DopEcR's ligands, ecdysone and dopamine, had different effects on nicotine-induced Ca²⁺-responses in the MB: ecdysone enhanced activity in the calyx and cell body region in a DopEcR-dependent manner, whereas dopamine reduced activity in the medial lobes independently of DopEcR. Finally, we show that flies with reduced DopEcR function in the MB display decreased locomotor activity. This behavioral phenotype of DopEcR-deficient flies may be partly due to their enhanced MB activity, since the MB have been implicated in the suppression of locomotor activity. Overall, these data suggest that DopEcR is involved in region-specific modulation of Ca²⁺ dynamics within the MB, which may play a role in behavioral modulation.

Mitochondria and the insect steroid hormone receptor (EcR): A complex relationship

The actions of the insect steroid molting hormones, ecdysteroids, on the genome of target cells has been well studied, but little is known of their extranuclear actions. We previously showed in Rhodnius prolixus that much of the ecdysteroid receptor (EcR) resides in the cytoplasm of various cell types and undergoes shuttling between nucleus and cytoplasm with circadian periodicity, possibly using microtubules as tracks for translocation to the nucleus. Here we report that cytoplasmic EcR appears to be also involved in extranuclear actions of ecdysteroids by association with the mitochondria. Western blots of subcellular fractions of brain lysates revealed that EcR is localized in the mitochondrial fraction, indicating an intimate association of EcR with mitochondria. Confocal laser microscopy and immunohistochemistry using anti-EcR revealed abundant co-localization of EcR with mitochondria in brain neurons and their axons, especially intense in the subplasmalemmal region, raising the possibility of EcR involvement in mitochondrial functions in subplasmalemmal microdomains. When mitochondria are dispersed by disruption of microtubules with colchicine, EcR remains associated with mitochondria showing strong receptor association with mitochondria. Treatment in vitro with ecdysteroids of brains of developmentally arrested R. prolixus (containing neither ecdysteroids nor EcR) induces EcR and abundant co-localization with mitochondria in neurons, concurrently with a sharp increase of the mitochondrial protein COX 1, suggesting involvement of EcR in mitochondrial function. These findings align EcR with various vertebrate steroid receptors, where actions of steroid receptors on mitochondria are widely known and suggest that steroid receptors across distant phyla share similar functional attributes.

G-protein αq participates in the steroid hormone 20-hydroxyecdysone nongenomic signal transduction

The nuclear receptor-mediated genomic pathways of the animal steroid hormones are well known. However, the cell membrane receptor-mediated nongenomic pathways of the animal steroid hormones are little understood. In this study, we report the participation of a G-protein alpha q (Gαq)(1) subunit in the 20E nongenomic pathway in the cell membrane and regulating gene expression during molting and metamorphosis in a lepidopteran insect, Helicoverpa armigera. 20E-induced phosphorylation of Gαq was detected using two-dimensional electrophoresis techniques. Knockdown of Gαq by injecting double-stranded RNA suppressed the development of larvae, delayed metamorphosis, and inhibited 20E-induced gene expression. Gαq was distributed throughout the cell, and migrated toward the plasma membrane upon 20E induction. Gαq was necessary in the 20E-induced intracellular Ca(2+) release and extracellular Ca(2+) influx. The protein kinase C (PKC) inhibitor could repress 20E-induced phosphorylation of cyclin-dependent kinase 10 (CDK10) and transcription factor ultraspiracle (USP1). PKC inhibitor could repress the Gαq phosphorylation and membrane trafficking. These results suggest that Gαq participates in 20E signaling in the cell membrane at the pre-genomic stage by modulating the increase of the intracellular Ca(2+) and phosphorylation of CDK10 and USP1 in 20E transcription complex to regulate gene transcription.

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.

Ecdysone signaling in adult Drosophila melanogaster

The steroid hormone 20-hydroxyecdysone and its EcR/USP receptor are vital during arthropod development for coordinating molting and metamorphosis. Traditionally, little attention has been given to potential post-developmental functions for this hormone signaling system. However, recent studies in Drosophila melanogaster indicate that the hormone and receptor are present and active in adults and that mutations decreasing hormone or receptor levels affect diverse processes such as reproduction, behavior, stress resistance, and lifespan. We review the current state of knowledge regarding adult hormone production and titers and discuss receptor expression and activity in order to identify potential mechanisms which explain the observed mutant phenotypes. Finally, we describe future research directions focused on identifying isoform-specific functions of EcR, distinguishing effects from EcR/USP gene activation and repression, and determining how ecdysone signaling impacts different tissue types.

Regulation of crustacean molting: a review and our perspectives

Molting is a highly complex process that requires precise coordination to be successful. We describe the early classical endocrinological experiments that elucidated the hormones and glands responsible for this process. We then describe the more recent experiments that have provided information on the cellular and molecular aspects of molting. In addition to providing a review of the scientific literature, we have also included our perspectives.

Ecdysteroids elicit a rapid Ca2+ flux leading to Akt activation and increased protein synthesis in skeletal muscle cells

Phytoecdysteroids, structurally similar to insect molting hormones, produce a range of effects in mammals, including increasing growth and physical performance. In skeletal muscle cells, phytoecdysteroids increase protein synthesis. In this study we show that in a mouse skeletal muscle cell line, C(2)C(12), 20-hydroxyecdysone (20HE), a common phytoecdysteroid in both insects and plants, elicited a rapid elevation in intracellular calcium, followed by sustained Akt activation and increased protein synthesis. The effect was inhibited by a G-protein coupled receptor (GPCR) inhibitor, a phospholipase C (PLC) inhibitor, and a phosphoinositide kinase-3 (PI3K) inhibitor.

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.

Ecdysteroid receptor (EcR) is associated with microtubules and with mitochondria in the cytoplasm of prothoracic gland cells of Rhodnius prolixus (Hemiptera)

We have shown previously that EcR in larval Rhodnius is present in the cytoplasm of various cell types and undergoes daily cycling in abundance in the cytoplasm (Vafopoulou and Steel, 2006. Cell Tissue Res 323:443-455). It is unknown which organelles are associated with EcR. Here, we report that cytoplasmic EcR in prothoracic gland cells is associated with both microtubules and mitochondria, and discuss the implications for both nuclear and non-genomic actions of EcR. EcR was localized immunohistochemically using several antibodies to EcR of Manduca and Drosophila and a confocal laser scanning microscope. Double labels were made to visualize EcR and (1) microtubules (using an antibody to tyrosylated alpha-tubulin) and (2) mitochondria (using a fluorescent MitoTracker probe), both after stabilization of microtubules with taxol. EcR co-localized with both tubulin and mitochondria. All the different EcR antibodies produced similar co-localization patterns. EcR was seen in the perinuclear aggregation of mitochondria, indicating that mitochondria are targets of ecdysone, which could influence mitochondrial gene transcription. EcR was also distributed throughout the microtubule network. Co-localization of EcR with tubulin or mitochondria was maintained after depolymerization of microtubules with colchicine. Treatment with taxol resulted in accumulation of EcR in the cytoplasm and simultaneous depletion of EcR from the nucleus, suggesting that microtubules may be involved in targeted intracellular transport of EcR to the nucleus (genomic action) or may play a role in rapid ecdysone signal transduction in the extranuclear compartment, i.e., in non-genomic actions of ecdysone. These findings align EcR more closely with steroid hormone receptors in vertebrates.

The variety of complexes formed by EcR and Usp nuclear receptors in the nuclei of living cells

The heterodimer of the ecdysone receptor (EcR) and ultraspiracle (Usp), members of the nuclear receptor superfamily, is considered to be functional receptor for the ecdysteroids that coordinate essential biological processes in insects. In this work we have applied a bimolecular fluorescence complementation (BiFC) method to directly analyze the formation of the EcR/Usp complex. The BiFC experiments were carried out in mammalian cells which are routinely used for heterologous studies of the EcR/Usp complex, including experiments on EcR-based artificial molecular gene switches. BiFC analysis, supported by flow cytometry, revealed that EcR-Usp interaction is nuclei-restricted. If expressed separately, Usp and EcR are able to form nuclear complexes in the absence of the cognate dimerization partner. We have observed that Muristerone A that is widely used for the induction of ecdysteroid-dependent transcription in mammalian cells, does not significantly change the number of EcR/Usp and EcR/EcR complexes, and it does not influence their subcellular localization.

Ecdysteroids, juvenile hormone and insect neuropeptides: Recent successes and remaining major challenges

In the recent decade, tremendous progress has been realized in insect endocrinology as the result of the application of a variety of advanced methods in neuropeptidome- and receptor research. Hormones of which the existence had been shown by bioassays four decades ago, e.g. bursicon (a member of the glycoprotein hormone family) and pupariation factor (Neb-pyrokinin 2, a myotropin), could be identified, along with their respective receptors. In control of diurnal rhythms, clock genes got company from the neuropeptide Pigment Dispersing Factor (PDF), of which the receptor could also be identified. The discovery of Inka cells and their function in metamorphosis was a true hallmark. Analysis of the genomes of Caenorhabditis elegans, Drosophila melanogaster and Apis mellifera yielded about 75, 100 and 200 genes coding for putative signaling peptides, respectively, corresponding to approximately 57, 100 and 100 peptides of which the expression could already be proven by means of mass spectrometry. The comparative approach invertebrates-vertebrates recently yielded indications for the existence of counterparts in insects for prolactin, atrial natriuretic hormone and Growth Hormone Releasing Hormone (GRH). Substantial progress has been realized in identifying the Halloween genes, a membrane receptor(s) for ecdysteroids, a nuclear receptor for methylfarnesoate, and dozens of GPCRs for insect neuropeptides. The major remaining challenges concern the making match numerous orphan GPCRs with orphan peptidic ligands, and elucidating their functions. Furthermore, the endocrine control of growth, feeding-digestion, and of sexual differentiation, in particular of males, is still poorly understood. The finding that the prothoracic glands produce an autocrine factor with growth factor-like properties and secrete proteins necessitates a reevaluation of their role in development.

Novel DNA-binding element within the C-terminal extension of the nuclear receptor DNA-binding domain

The heterodimer of the ecdysone receptor (EcR) and ultraspiracle (Usp), members of the nuclear receptors superfamily, is considered as the functional receptor for ecdysteroids initiating molting and metamorphosis in insects. Here we report the 1.95 A structure of the complex formed by the DNA-binding domains (DBDs) the EcR and the Usp, bound to the natural pseudopalindromic response element. Comparison of the structure with that obtained previously, using an idealized response element, shows how the EcRDBD, which has been previously reported to possess extraordinary flexibility, accommodates DNA-induced structural changes. Part of the C-terminal extension (CTE) of the EcRDBD folds into an alpha-helix whose location in the minor groove does not match any of the locations previously observed for nuclear receptors. Mutational analyses suggest that the alpha-helix is a component of EcR-box, a novel element indispensable for DNA-binding and located within the nuclear receptor CTE. This element seems to be a general feature of all known EcRs.

Chemical biology of steroid and nuclear hormone receptors

The nuclear hormone receptors are ligand-gated transcription factors that modulate gene expression by directly acting upon genomic DNA, and have been of profound interest across all biological disciplines. Recent advancements in this area have included the expansion of transgene activation through ligand-receptor engineering, drug development from structural design and the exploitation of innate ligand-specific associations towards developing novel conditional protein-based recombinant and diagnostic tools. These advancements come on the heels of exciting new modes of hormone action that challenge and expand upon the classic paradigms of hormone receptor function.

EcR expression in the prothoracicotropic hormone-producing neurosecretory cells of the Bombyx mori brain

The steroid hormone 20-hydroxyecdysone (20E) initiates insect molting and metamorphosis through binding with a heterodimer of two nuclear receptors, the ecdysone receptor (EcR) and ultraspiracle (USP). Expression of the specific isoforms EcR-A and EcR-B1 governs steroid-induced responses in the developing cells of the silkworm Bombyx mori. Here, analysis of EcR-A and EcR-B1 expression during larval-pupal development showed that both genes were up-regulated by 20E in the B. mori brain. Whole-mount in situ hybridization and immunohistochemistry revealed that EcR-A and EcR-B1 mRNAs and proteins were exclusively located in two pairs of lateral neurosecretory cells in the larval brain known as the prothoracicotropic hormone (PTTH)- producing cells (PTPCs). In the pupal brain, EcR-A and EcR-B1 expression was detected in tritocerebral cells and optic lobe cells in addition to PTPCs. As PTTH controls ecdysone secretion by the prothoracic gland, these results indicate that 20E-responsive PTPCs are the master cells of insect metamorphosis.

EcR and Usp, components of the ecdysteroid nuclear receptor complex, exhibit differential distribution of molecular determinants directing subcellular trafficking

Ecdysteroids coordinate development, reproduction and other essential biological processes in insects and other arthropods through the receptor which is a heterodimer of two members of the nuclear receptors superfamily, the ecdysteroid receptor (EcR) and the Ultraspiracle (Usp). Although the transcriptionally active EcR/Usp heterocomplex is believed to be the only functional form of the receptor, there are data indicating that EcR may be involved in the mediation of the non-genomic effects outside of the nucleus. Since the nucleocytoplasmic shuttling could be a key element determining participation of the single nuclear receptor molecule both in the genomic and non-genomic functions we have analyzed nuclear import and export properties of the EcR and Usp from Drosophila melanogaster. We show for the first time that both receptors exhibit differential distribution of the nuclear localization and nuclear export signals (NLSs and NESs). In particular, the Usp which exhibits exclusively nuclear localization in all cell types analyzed, contains apparently only NLS activity within the DNA-binding domain. In contrast, the three known EcR isoforms (A, B1 and B2) are mosaics of elements which can potentially mediate their nucleocytoplasmic shuttling. We have found two active NESs in ligand binding domain and NLS activity within the DNA-binding domain of all isoforms. Simultaneously we demonstrate that B1 and A isoforms possess an additional NLS activity localized in AB regions. We speculate that this characteristic, along with the previously reported structural pliability of the EcR molecule, allows the single receptor to evoke many different genomic as well as non-genomic ecdysteroid-dependent responses.