Structure of the heterodimeric ecdysone receptor DNA-binding complex

Structural characterization of the DNA binding mechanism of retinoic acid-related orphan receptor gamma

Retinoic acid-related orphan receptor gamma (RORγ) plays critical roles in regulating various biological processes and has been linked to immunodeficiency disorders and cancers. DNA recognition is essential for RORγ to exert its functions. However, the underlying mechanism of the DNA binding by RORγ remains unclear. In this study, we present the crystal structure of the complex of RORγ1 DNA-binding domain (RORγ1-DBD)/direct repeat DNA element DR2 at 2.3 Å resolution. We demonstrate that RORγ1-DBD binds the DR2 motif as a homodimer, with the C-terminal extension (CTE) region of RORγ1-DBD contributing to the DNA recognition and the formation of dimeric interface. Further studies reveal that REV-ERB-DBD and RXR-DBD, also bind the DR2 site as a homodimer, while NR4A2-DBD binds DR2 as a monomer. Our research uncovers a binding mechanism of RORγ1 to the DR2 site and provides insights into the biological functions of RORγ1 and the broader RORs subfamily.

Molecular interplay between ecdysone receptor and retinoid X receptor in regulating the molting of the Chinese mitten crab, Eriocheir sinensis

Purpose

Molting is a pivotal biological process regulated by the ecdysteroid signaling pathway that requires molecular coordination of two transcription factors, Ecdysone receptor (EcR) and ultraspiracle (USP) in arthropods. However, the molecular interplay of EcR and Retinoid X receptor (RXR), the crustacean homolog of USP in the ecdysteroid signaling pathway, is not well understood.

Methods

In this study, we conducted temporal and spatial expression, co-immunoprecipitation (CO-IP), and luciferase reporter assay experiments to investigate the molecular function and interplay of EcR and RXR during the molting process of the Chinese mitten crab, Eriocheir sinensis.

Results

The results showed that the expression level of RXR was more stable and significantly higher than EcR during the entire molting process. However, the expression level of EcR fluctuated dynamically and increased sharply at the premolt stage. The CO-IP and luciferase reporter assay results confirmed the molecular interplay of EcR and RXR. The heterodimer complex formed by the two transcription factors significantly induced the transcription of E75, an essential gene in the ecdysteroid signaling pathway.

Conclusions

Our study unveiled the diverse molecular function and molecular interplay of EcR and RXR; RXR is possibly a "constitutive-type" gene, and EcR is possibly a vital speed-limiting gene while both EcR and RXR are required to initiate the ecdysteroid signaling cascade, which may be indispensable for molting regulation in E. sinensis. The results provide a theoretical basis for the endocrine control of molting in E. sinensis and novel insights into the molecular mechanism of molting mediated by the ecdysteroid signaling pathway in crustaceans.

Structural basis of the farnesoid X receptor/retinoid X receptor heterodimer on inverted repeat DNA

Farnesoid X receptor (FXR) is a ligand-activated transcription factor known as bile acid receptor (BAR). FXR plays critical roles in various biological processes, including metabolism, immune inflammation, liver regeneration and liver carcinogenesis. FXR forms a heterodimer with the retinoid X receptor (RXR) and binds to diverse FXR response elements (FXREs) to exert its various biological functions. However, the mechanism by which the FXR/RXR heterodimer binds the DNA elements remains unclear. In this study, we aimed to use structural, biochemical and bioinformatics analyses to study the mechanism of FXR binding to the typical FXRE, such as the IR1 site, and the heterodimer interactions in the FXR-DBD/RXR-DBD complex. Further biochemical assays showed that RAR, THR and NR4A2 do not form heterodimers with RXR when bound to the IR1 sites, which indicates that IR1 may be a unique binding site for the FXR/RXR heterodimer. Our studies may provide a further understanding of the dimerization specificity of nuclear receptors.

Structures of human TR4LBD-JAZF1 and TR4DBD-DNA complexes reveal the molecular basis of transcriptional regulation

Testicular nuclear receptor 4 (TR4) modulates the transcriptional activation of genes and plays important roles in many diseases. The regulation of TR4 on target genes involves direct interactions with DNA molecules via the DNA-binding domain (DBD) and recruitment of coregulators by the ligand-binding domain (LBD). However, their regulatory mechanisms are unclear. Here, we report high-resolution crystal structures of TR4DBD, TR4DBD-DNA complexes and the TR4LBD-JAZF1 complex. For DNA recognition, multiple factors come into play, and a specific mutual selectivity between TR4 and target genes is found. The coactivators SRC-1 and CREBBP can bind at the interface of TR4 originally occupied by the TR4 activation function region 2 (AF-2); however, JAZF1 suppresses the binding through a novel mechanism. JAZF1 binds to an unidentified surface of TR4 and stabilizes an α13 helix never reported in the nuclear receptor family. Moreover, the cancer-associated mutations affect the interactions and the transcriptional activation of TR4 in vitro and in vivo, respectively. Overall, our results highlight the crucial role of DNA recognition and a novel mechanism of how JAZF1 reinforces the autorepressed conformation and influences the transcriptional activation of TR4, laying out important structural bases for drug design for a variety of diseases, including diabetes and cancers.

Retinoic acid receptor structures: the journey from single domains to full-length complex

The retinoic acid receptors (RARα, β, and γ) are multi-domain polypeptides that heterodimerize with retinoid X receptors (RXRα, β, and γ) to form functional transcription factors. Understanding the three-dimensional molecular organization of these nuclear receptors (NRs) began with RAR and RXR DNA-binding domains (DBDs), and were followed with studies on isolated ligand-binding domains (LBDs). The more complete picture emerged in 2017 with the multi-domain crystal structure of RXRα-RARβ on its response element with retinoic acid molecules and coactivator segments on both proteins. The analysis of that structure and its complementary studies have clarified the direct communication pathways within RXR-RAR polypeptides, through which DNA binding, protein-ligand, and protein-protein interactions are integrated for overall functional responses. Understanding the molecular connections in the RXR-RAR complex has benefited from direct observations of the multi-domain structures of RXRα-PPARγ, RXRα-LXRβ, HNF-4α homodimer, and androgen receptor homodimer, each bound to its response element. These comprehensive NR structures show unique quaternary architectures, yet all have DBD-DBD, LBD-LBD, and DBD-LBD domain-domain contacts within them. These convergence zones allow signals from discrete domains of their polypeptides to be propagated and integrated across their entire complex, shaping their overall responses in an allosteric fashion.

Rapid Assessment of Insect Steroid Hormone Entry Into Cultured Cells

Steroid hormones control development and homeostasis in a wide variety of animals by interacting with intracellular nuclear receptors. Recent discoveries in the fruit fly Drosophila melanogaster revealed that insect steroid hormones or ecdysteroids are incorporated into cells through a membrane transporter named Ecdysone Importer (EcI), which may become a novel target for manipulating steroid hormone signaling in insects. In this study, we established an assay system that can rapidly assess EcI-mediated ecdysteroid entry into cultured cells. Using NanoLuc Binary Technology (NanoBiT), we first developed an assay to detect ligand-dependent heterodimerization of the ecdysone receptor (EcR) and retinoid X receptor (RXR) in human embryonic kidney (HEK) 293T cells. We also developed HEK293 cells that stably express EcI. By combining these tools, we can monitor ecdysteroid entry into the cells in real time, making it a reliable system to assess EcI-mediated steroid hormone incorporation into animal cells.

Cross-talk of insulin-like peptides, juvenile hormone, and 20-hydroxyecdysone in regulation of metabolism in the mosquito Aedes aegypti

Female mosquitoes feed sequentially on carbohydrates (nectar) and proteins (blood) during each gonadotrophic cycle to become reproductively competent and effective disease vectors. Accordingly, metabolism is synchronized to support this reproductive cyclicity. However, regulatory pathways linking metabolism to reproductive cycles are not fully understood. Two key hormones, juvenile hormone (JH) and ecdysteroids (20-hydroxyecdysone, 20E, is the most active form) govern female mosquito reproduction. Aedes aegypti genome codes for eight insulin-like peptides (ILPs) that are critical for controlling metabolism. We examined the effects of the JH and 20E pathways on mosquito ILP expression to decipher regulation of metabolism in a reproducing female mosquito. Chromatin immunoprecipitation assays showed genomic interactions between ilp genes and the JH receptor, methoprene-tolerant, a transcription factor, Krüppel homolog 1 (Kr-h1), and two isoforms of the ecdysone response early gene, E74. The luciferase reporter assays showed that Kr-h1 activates ilps 2, 6, and 7, but represses ilps 4 and 5 The 20E pathway displayed the opposite effect in the regulation of ilps E74B repressed ilps 2 and 6, while E74A activated ilps 4 and 5 Combining RNA interference, CRISPR gene tagging and enzyme-linked immunosorbent assay, we have shown that the JH and 20E regulate protein levels of all eight Ae. aegypti ILPs. Thus, we have established a regulatory axis between ILPs, JH, and 20E in coordination of metabolism during gonadotrophic cycles of Ae. aegypti.

Farnesoid X receptor (FXR): Structures and ligands

Farnesoid X receptor (FXR) is a bile acid activated nuclear receptor (BAR) and is mainly expressed in the liver and intestine. Upon ligand binding, FXR regulates key genes involved in the metabolic process of bile acid synthesis, transport and reabsorption and is also involved in the metabolism of carbohydrates and lipids. Because of its important functions, FXR is considered as a promising drug target for the therapy of bile acid-related liver diseases. With the approval of obeticholic acid (OCA) as the first small molecule to target FXR, many other small molecules are being evaluated in clinical trials. This review summarizes the structures of FXR, especially its ligand binding domain, and the development of small molecules (including agonists and antagonists) targeting FXR.

Post-Translational Modifications of FXR; Implications for Cholestasis and Obesity-Related Disorders

The Farnesoid X receptor (FXR) is a nuclear receptor which is activated by bile acids. Bile acids function in solubilization of dietary fats and vitamins in the intestine. In addition, bile acids have been increasingly recognized to act as signaling molecules involved in energy metabolism pathways, amongst others via activating FXR. Upon activation by bile acids, FXR controls the expression of many genes involved in bile acid, lipid, glucose and amino acid metabolism. An inability to properly use and store energy substrates may predispose to metabolic disorders, such as obesity, diabetes, cholestasis and non-alcoholic fatty liver disease. These diseases arise through a complex interplay between genetics, environment and nutrition. Due to its function in metabolism, FXR is an attractive treatment target for these disorders. The regulation of FXR expression and activity occurs both at the transcriptional and at the post-transcriptional level. It has been shown that FXR can be phosphorylated, SUMOylated and acetylated, amongst other modifications, and that these modifications have functional consequences for DNA and ligand binding, heterodimerization and subcellular localization of FXR. In addition, these post-translational modifications may selectively increase or decrease transcription of certain target genes. In this review, we provide an overview of the posttranslational modifications of FXR and discuss their potential involvement in cholestatic and metabolic disorders.

Modulation of nuclear receptor function: Targeting the protein-DNA interface

Nuclear receptors (NRs) are a superfamily of ligand-dependent transcription factors that modulate several biological processes. Traditionally, modulation of NRs has been focused on the development of ligands that recognize and bind to the ligand binding domain (LBD), resulting in activation or repression of transcription through the recruitment of coregulators. However, for more severe diseases, such as breast and prostate cancer, the conventional treatment addressing LBD modulation is not always successful, due to tumor resistance. To overcome these challenges and aiming to modulate NR activity by inhibiting the NR-DNA interaction, new studies focus on the development of molecules targeting alternative sites and domains on NRs. Here, we discuss two different approaches for this alternative NR modulation: one targeting the NR DNA binding domain (DBD); and the other targeting the DNA sites recognized by NRs. Our aim is to present the challenges and perspectives for developing specific inhibitors for each purpose, alongside with already reported examples.

Mode of Action of Farnesol, the "Noble Unknown" in Particular in Ca2+ Homeostasis, and Its Juvenile Hormone-Esters in Evolutionary Retrospect

Farnesol, the sesquiterpenoid precursor of insect juvenile hormones (JH) that itself has JH activity, existed already long before animals and their hormones came into being. Although it is omnipresent in all eukaryotes, this molecule remains a "noble unknown" in cell physiology. It is neither documented as a hormone nor as another type of signaling molecule. To date, its function as an intermediate in the synthesis of squalene-cholesterol-steroids in chordates/vertebrates, and of the insect/arthropod JHs, esters of farnesol, in the mevalonate biosynthetic pathway is assumed to be the only one. This assumption neglects that already two decades ago, farnesol has been shown to be a potent endogenous inhibitor of N-type voltage-gated Ca²⁺ channels in some mammalian cell types. The tandem mevalonate pathway and Ca²⁺ channels originated early in eukaryotic evolution, and has since been well conserved, "promoting" it as a ubiquitous player in Ca²⁺ homeostasis in all eukaryotes. This paper accentuates how this drastic change in thinking gained momentum after the discovery by Paroulek and Sláma that the huge amounts of JH I in male accessory glands of the Cecropia moth, are actually synthesized in these glands themselves and not in the corpora allata, the hitherto assumed unique synthesis site of such compounds. In addition, MAG-JHs have no hormonal- but an exocrine function. Here we hypothesize that MAG-JHs may function in protecting the spermatozoa against toxic Ca²⁺ concentrations, and in enabling their flagellum to undulate. They may do so by acting through membrane receptors. Our novel paradigm assigns to farnesol/JHs a function of flexible hydrophobic molecular valves for restricting untimely Ca²⁺-passage through some types of canonical Ca²⁺channels, using covalently bound farnesyl- or geranyl-geranyl group attachment as well as GPCRs-G proteins all containing a prenyl group. The high rotatable bond count, and their horseshoe-shape are instrumental to their valve function. In our paradigm, Met/Tai and Gce, to date generally thought to be the (only) functional (nuclear) receptors for JHs, are classified as probable Ca²⁺-sensitive transcription factors. Some theoretical and practical considerations for possible applications in a medical context will be discussed.

Identification of target genes for RNAi-mediated control of the Twospotted Spider Mite

RNA interference (RNAi) is being developed for the management of pests that destroy crops. The twospotted Spider Mite (TSSM), Tetranychus urticae is a worldwide pest due to its unique physiological and behavioral characteristics including extraordinary ability to detoxify a wide range of pesticides and feed on many host plants. In this study, we conducted experiments to identify target genes that could be used for the development of RNAi-based methods to control TSSM. Leaf disc feeding assays revealed that knockdown in the expression genes coding for proteins involved in the biosynthesis and action of juvenile hormone (JH) and action of ecdysteroids [Methoprene-tolerant (Met), retinoid X receptor β, farnesoic acid O-methyltransferase, and CREB-binding protein] caused 35-56% mortality. Transgenic tobacco plants expressing hairpin dsRNA targeting Met gene were generated and tested. About 48% mortality was observed in TSSM raised on transgenic tobacco plants expressing dsMet. These studies not only broaden our knowledge on understanding hormone action in TSSM but also identified target genes that could be used in RNAi-mediated control of TSSM.

Intrinsically disordered N-terminal domain of the Helicoverpa armigera Ultraspiracle stabilizes the dimeric form via a scorpion-like structure

Nuclear receptors (NRs) are a family of ligand-dependent transcription factors activated by lipophilic compounds. NRs share a common structure comprising three domains: a variable N-terminal domain (NTD), a highly conserved globular DNA-binding domain and a ligand-binding domain. There are numerous papers describing the molecular details of the latter two globular domains. However, very little is known about the structure-function relationship of the NTD, especially as an intrinsically disordered fragment of NRs that may influence the molecular properties and, in turn, the function of globular domains. Here, we investigated whether and how an intrinsically disordered NTD consisting of 58 amino acid residues affects the functions of the globular domains of the Ultraspiracle protein from Helicoverpa armigera (HaUsp). The role of the NTD was examined for two well-known and easily testable NR functions, i.e., interactions with specific DNA sequences and dimerization. Electrophoretic mobility shift assays showed that the intrinsically disordered NTD influences the interaction of HaUsp with specific DNA sequences, apparently by destabilization of HaUsp-DNA complexes. On the other hand, multi-angle light scattering and sedimentation velocity analytical ultracentrifugation revealed that the NTD acts as a structural element that stabilizes HaUsp homodimers. Molecular models based on small-angle X-ray scattering indicate that the intrinsically disordered NTD may exert its effects on the tested HaUsp functions by forming an unexpected scorpion-like structure, in which the NTD bends towards the ligand-binding domain in each subunit of the HaUsp homodimer. This structure may be crucial for specific NTD-dependent regulation of the functions of globular domains in NRs.

Identification of molt-inhibiting hormone and ecdysteroid receptor cDNA sequences in Gammarus pulex, and variations after endocrine disruptor exposures

In amphipods, growth, development and reproduction are mediated by the molt, which is a hormonally controlled process and which, therefore, could be impacted by endocrine disruption compounds (EDC). The molt process is controlled by both X-organ (XO) and Y-organ (YO) through a variety of hormones and receptors including the molt-inhibiting hormone (MIH) and the ecdysteroid receptor (EcR). However, although many studies were devoted to characterize MIH and EcR in crustaceans, only few works evaluated their variations under EDCs exposures. Consequently, the present work aimed to characterize MIH and EcR genes of the amphipod Gammarus pulex, as well as to study their relative expression variations after exposure to four EDCs, proved in vertebrates: ethinylestradiol (estrogen), 4-hydroxytamoxifen (anti-estrogen), 17α-methyltestosterone (androgen) and cyproterone acetate (anti-androgen). PCR amplification allowed to obtain 204 bp length and 255 bp length fragments, encoding for partial sequences of 68 amino acids and 85 amino acids, which correspond to EcR and MIH, respectively, and which are highly conserved in crustacean species. Results highlighted MIH and EcR expressions mainly in G. pulex head, which is the localization of XO and YO. Moreover, irrespective of the EDC exposure, increases of MIH and EcR relative expressions were observed, as it was observed after the exposure to 20-hydroxyecdysone (20HE), the natural molt hormone, used as positive control. Therefore, it appeared that tested EDCs behaved like 20HE, suggesting that their effects could occur through the ecdysteroids pathways, and so impact the molt process of G. pulex on the long term. Finally, the present study is a first step in the possibility of using MIH and EcR relative expressions as biomarkers of exposure for EDCs risk assessment. However additional studies must first be carried out to better characterize and understand their variations, and also better predicted consequences for the exposed amphipods.

Characterization and comparison of transcriptional activities of the retinoid X receptors by various organotin compounds in three prosobranch gastropods; Thais clavigera, Nucella lapillus and Babylonia japonica

Two cDNAs of RXR were isolated, for the first time, from the ivory shell, Babylonia japonica, and the transcriptional activities were tested in vitro to compare with other gastropod (Thais clavigera and Nucella lapillus) RXR isoforms. The transcriptional activities of all of these RXR isoforms were significantly induced by mammalian RXR agonist, 9-cis retinoic acid (9cRA). The transcriptional activity of T. clavigera RXR-1 was also examined by using 9cRA and 16 organotin compounds, and significant ligand-dependent transactivations were observed by 9cRA and 5 organotins (tributyltin (TBT), tetrabutyltin (TeBT), tripropyltin (TPrT), tricyclohexyltin (TcHT) and triphenyltin (TPhT)). These 5 organotins also induced significant transcriptional activities in N. lapillus and B. japonica RXR isoforms. These 4 organotins, except for TeBT, have been reported to promote the development of imposex after a month of a single injection each, using female T. clavigera. To investigate the function of gastropod RXR isoforms, the effects of mammalian specific pan-agonist, PA024, and pan-antagonist, HX531, were examined, and significant induction of transcriptional activity by PA024 was demonstrated in these gastropod RXR isoforms. The additions of HX531 significantly suppressed the transcriptional activities of these gastropod RXR isoforms by 9cRA and 5 organotins. Using the mammalian two retinoic acid response elements, the transcriptional activities by 2 agonists, 9cRA and PA024, were different among the RXR isoforms of each gastropod species. With retinoid X response element (RXRE), transcriptional activities of TcRXR-1, BjRXR-1, and NlRXRa were significantly higher than those of TcRXR-2, BjRXR-2, and NlRXRb. Transcriptional activities of TcRXR-2, BjRXR-2, and NlRXRb, however, were significantly higher than those of TcRXR-1, BjRXR-1, and NlRXRa with thyroid hormone response element, TREpal. Thus, induction of imposex in prosobranch gastropods is strongly suggested to be triggered by 9cRA and certain organotins, such as TBT and TPhT through the activation of RXRs. These gastropod RXRs might control the different gene transcription by forming homo- or heterodimer complex with their own isoforms. These findings will contribute to our understanding of the fundamentals of the endocrine system in molluscs, particularly on RXR signaling pathway.

Mutations in the nuclear bile acid receptor FXR cause progressive familial intrahepatic cholestasis

Neonatal cholestasis is a potentially life-threatening condition requiring prompt diagnosis. Mutations in several different genes can cause progressive familial intrahepatic cholestasis, but known genes cannot account for all familial cases. Here we report four individuals from two unrelated families with neonatal cholestasis and mutations in NR1H4, which encodes the farnesoid X receptor (FXR), a bile acid-activated nuclear hormone receptor that regulates bile acid metabolism. Clinical features of severe, persistent NR1H4-related cholestasis include neonatal onset with rapid progression to end-stage liver disease, vitamin K-independent coagulopathy, low-to-normal serum gamma-glutamyl transferase activity, elevated serum alpha-fetoprotein and undetectable liver bile salt export pump (ABCB11) expression. Our findings demonstrate a pivotal function for FXR in bile acid homeostasis and liver protection.

Neonatal cholestasis is a result of elevated bile acid levels, and is associated with mutations in genes regulating bile acid homeostasis. Here the authors identify mutations in the bile acid sensing farnesoid X receptor in four individuals with neonatal cholestasis from two unrelated families.

Characterization of a novel RXR receptor in the salmon louse (Lepeophtheirus salmonis, Copepoda) regulating growth and female reproduction

BACKGROUND

Nuclear receptors have crucial roles in all metazoan animals as regulators of gene transcription. A wide range of studies have elucidated molecular and biological significance of nuclear receptors but there are still a large number of animals where the knowledge is very limited. In the present study we have identified an RXR type of nuclear receptor in the salmon louse (Lepeophtheirus salmonis) (i.e. LsRXR). RXR is one of the two partners of the Ecdysteroid receptor in arthropods, the receptor for the main molting hormone 20-hydroxyecdysone (E20) with a wide array of effects in arthropods.

RESULTS

Five different LsRXR transcripts were identified by RACE showing large differences in domain structure. The largest isoforms contained complete DNA binding domain (DBD) and ligand binding domain (LBD), whereas some variants had incomplete or no DBD. LsRXR is transcribed in several tissues in the salmon louse including ovary, subcuticular tissue, intestine and glands. By using Q-PCR it is evident that the LsRXR mRNA levels vary throughout the L. salmonis life cycle. We also show that the truncated LsRXR transcript comprise about 50% in all examined samples. We used RNAi to knock-down the transcription in adult reproducing female lice. This resulted in close to zero viable offspring. We also assessed the LsRXR RNAi effects using a L. salmonis microarray and saw significant effects on transcription in the female lice. Transcription of the major yolk proteins was strongly reduced by knock-down of LsRXR. Genes involved in lipid metabolism and transport were also down regulated. Furthermore, different types of growth processes were up regulated and many cuticle proteins were present in this group.

CONCLUSIONS

The present study demonstrates the significance of LsRXR in adult female L. salmonis and discusses the functional aspects in relation to other arthropods. LsRXR has a unique structure that should be elucidated in the future.

Alternative splicing in the fiddler crab cognate ecdysteroid receptor: variation in receptor isoform expression and DNA binding properties in response to hormone

RXR cDNA cloning from three Uca species led to the identification of 4 conserved isoforms, indicative of alternative splicing in the hinge and ligand binding domains (LBD). Sequencing of overlapping clones from a Ucapugilator genomic library identified EcR isoforms matching previously identified cDNA variants; in addition, a cryptic exon in the LBD was detected and evidence for expression of this new isoform was obtained from next-generation sequencing. RNA-seq analysis also identified a new amino terminal EcR variant. EcR and RXR transcript abundance increases throughout ovarian maturation in U. pugilator, while cognate receptor transcript abundance remains constant in a related Indo-Pacific species with a different reproductive strategy. To examine if crab RXR LBD isoforms have different physical properties in vitro, electromobility shift assays were performed with different EcR isoforms. The cognate crab and fruit fly receptors differ in their responses to hormone. Ecdysteroids did not increase DNA binding for the crab heterodimers, while ecdysteroids stimulate binding for Drosophilamelanogaster EcR/USP heterodimers. In swapping experiments, UpEcR/USP heterodimers did not show ligand-responsive differences in DNA binding; both crab RXR LBD isoforms, however, conferred ligand-responsive increases in DNA binding with DmEcRs. These data indicate that both UpRXR LBD isoforms can heterodimerize with the heterologous DmEcR receptors and promote ligand and DNA binding. Unresponsiveness of the cognate receptors to ecdysteroid, however, suggest additional factors may be required to mediate endogenous, perhaps isoform-specific, differences in EcR conformation, consistent with previously reported effects of UpRXR isoforms on UpEcR ligand-binding affinities.

Ecdysone response elements in the distal promoter of the Bombyx Broad-Complex gene, BmBR-C

The Bombyx mori silkworm's homologue of the Broad-Complex gene (BmBR-C) is transcribed from two promoters: a distal promoter (Pdist) and a proximal promoter (Pprox). As determined by a luciferase assay, the transcriptional activity of Pdist, but not Pprox, was activated by ecdysone. Further analyses using reporters driven by sequential deletion Pdist mutants indicated that two regions, ecdysone responsive element (EcRE)-D and EcRE-P, -4950 bp and -3480 bp upstream from the distal transcription start site, respectively, were important in the responsiveness of Pdist to 20-hydroxyecdysone (20E); however, no significant sequence similarities were found between the canonical EcRE and the EcRE-D or EcRE-P regions. Electrophoretic mobility shift assays showed that both the EcRE-D and -P sequences specifically bound to Bombyx protein(s). Sequence analyses and competition assays suggested that the protein(s) bound to EcRE-P might include components other than the ecdysone receptor (EcR), suggesting that BmBR-C transcription was indirectly activated by ecdysone through the EcRE-P. Remarkably, protein binding to the mid-region of the EcRE-D, EcRE-Db, was competitively inhibited by an oligonucleotide containing the Drosophila hsp27 EcRE sequence. Furthermore, an anti-EcR antibody interfered with the formation of the protein-EcRE-Db complex. These results indicated that a functional Bombyx ecdysone receptor binds to EcRE-D and activates the expression of BmBR-C.

Phospholipase Cγ1 connects the cell membrane pathway to the nuclear receptor pathway in insect steroid hormone signaling

In addition to the classical nuclear receptor pathway, there is a nongenomic pathway in the cell membrane that regulates gene expression in animal steroid hormone signaling; however, this mechanism is unclear. Here, we report that the insect steroid hormone 20-hydroxyecdysone (20E) regulates calcium influx via phospholipase Cγ1 (PLCG1) to modulate the protein kinase C phosphorylation of the transcription factor ultraspiracle (USP1) in the lepidopteran insect Helicoverpa armigera. The PLCG1 mRNA levels are increased during the molting and metamorphic stages. The depletion of PLCG1 by RNA interference can block 20E-enhanced pupation, cause larvae death and pupation defects, and repress 20E-induced gene expression. 20E may induce the tyrosine phosphorylation of PLCG1 at the cytosolic tyrosine kinase (Src) homology 2 domains and then determine the migration of PLCG1 toward the plasma membrane. The G-protein-coupled receptor (GPCR) inhibitor suramin, Src family kinase inhibitor PP2, and the depletions of ecdysone-responsible GPCR (ErGPCR) and Gαq restrain the 20E-induced tyrosine phosphorylation of PLCG1. PLCG1 participates in the 20E-induced Ca(2+) influx. The inhibition of GPCR, PLC, inositol 1,4,5-trisphosphate receptor, and calcium channels represses the 20E-induced Ca(2+) influx. Through calcium signaling, PLCG1 mediates the transcriptional activation driven by the ecdysone-response element. Through PLCG1 and calcium signaling, 20E regulates PKC phosphorylation of USP1 at Ser-21 to determine its ecdysone-response element binding activity. These results suggest that 20E activates PLCG1 via the ErGPCR and Src family kinases to regulate Ca(2+) influx and PKC phosphorylation of USP1 to subsequently modulate gene transcription for metamorphosis.

Stage-specific activation of the E74B promoter by low ecdysone concentrations in the wing disc of Bombyx mori

To understand the transcriptional regulation of E74B by low concentrations of ecdysone, the promoter activity of Bombyx mori E74B was assessed in the B. mori wing disc using a transient reporter assay. We identified the transcription start sites of BmE74B and found that the core promoter region consists of initiator (Inr) and downstream promoter elements (DPE). The 3.6-kb upstream promoter region of BmE74B was responsive to 20-hydroxyecdysone (20E) in a dose-dependent manner, and the highest luciferase activity was observed in the presence of 0.2 μg/ml 20E. Moreover, the upstream BmE74B promoter activity was induced by 20E in a stage-specific and time-dependent manner, and the 3.6-kb promoter contained essential elements for the temporal regulation of BmE74B. Furthermore, we found a set of putative ecdysone response elements (EcREs). Five of these elements are highly conserved, capable of binding to the ecdysone receptor. Mutation of more than three putative EcREs, followed by introduction into the wing discs, abolished the activation of the BmE74B promoter by a low concentration of ecdysone. The results confirmed the role of ecdysone response elements in the transcription regulation of BmE74B and demonstrated that multiple putative EcREs were involved in the maximum response of BmE74B to low concentrations of ecdysone.

The molecular basis of conformational instability of the ecdysone receptor DNA binding domain studied by in silico and in vitro experiments

The heterodimer of the ecdysone receptor (EcR) and ultraspiracle (Usp), members of the nuclear receptors superfamily, regulates gene expression associated with molting and metamorphosis in insects. The DNA binding domains (DBDs) of the Usp and EcR play an important role in their DNA-dependent heterodimerization. Analysis of the crystal structure of the UspDBD/EcRDBD heterocomplex from Drosophila melanogaster on the hsp27 gene response element, suggested an appreciable similarity between both DBDs. However, the chemical denaturation experiments showed a categorically lower stability for the EcRDBD in contrast to the UspDBD. The aim of our study was an elucidation of the molecular basis of this intriguing instability. Toward this end, we mapped the EcRDBD amino acid sequence positions which have an impact on the stability of the EcRDBD. The computational protein design and in vitro analyses of the EcRDBD mutants indicate that non-conserved residues within the α-helix 2, forming the EcRDBD hydrophobic core, represent a specific structural element that contributes to instability. In particular, the L58 appears to be a key residue which differentiates the hydrophobic cores of UspDBD and EcRDBD and is the main reason for the low stability of the EcRDBD. Our results might serve as a benchmark for further studies of the intricate nature of the EcR molecule.

Understanding nuclear receptor form and function using structural biology

Nuclear receptors (NRs) are a major transcription factor family whose members selectively bind small-molecule lipophilic ligands and transduce those signals into specific changes in gene programs. For over two decades, structural biology efforts were focused exclusively on the individual ligand-binding domains (LBDs) or DNA-binding domains of NRs. These analyses revealed the basis for both ligand and DNA binding and also revealed receptor conformations representing both the activated and repressed states. Additionally, crystallographic studies explained how NR LBD surfaces recognize discrete portions of transcriptional coregulators. The many structural snapshots of LBDs have also guided the development of synthetic ligands with therapeutic potential. Yet, the exclusive structural focus on isolated NR domains has made it difficult to conceptualize how all the NR polypeptide segments are coordinated physically and functionally in the context of receptor quaternary architectures. Newly emerged crystal structures of the peroxisome proliferator-activated receptor-γ-retinoid X receptor α (PPARγ-RXRα) heterodimer and hepatocyte nuclear factor (HNF)-4α homodimer have recently revealed the higher order organizations of these receptor complexes on DNA, as well as the complexity and uniqueness of their domain-domain interfaces. These emerging structural advances promise to better explain how signals in one domain can be allosterically transmitted to distal receptor domains, also providing much better frameworks for guiding future drug discovery efforts.

Two adjacent cis-regulatory elements are required for ecdysone response of ecdysone receptor (EcR) B1 transcription

Three distinct classes of nuclear receptors, EcR, E75, and HR3, are key regulators in the ecdysone-inducible gene activation cascade in insects. The transcription of these genes is induced by ecdysone (20E) differently, although the detailed mechanisms underlying their responses to 20E are largely unknown. We identified ecdysone response elements (EcREs) present in the promoters of genes coding BmEcR-B1, BmE75-A, and BHR3-B isoforms from Bombyx mori employing luciferase reporter assays in an ecdysteroid-responsive cultured cell line, NIAS-Bm-aff3 (aff3). The EcRE of BmEcR-B1 at −2800 comprises of two adjacent elements separated by 5 bp, E1 (15 bp) and E2 (21 bp), both of which are required for the 20E response. Further analysis using electrophoretic mobility shift assays showed that E1 binds to the EcR/USP heterodimer and that E2 may bind to the E-box (CACGTG) binding factor such as bHLH protein. The unique E1+E2-type EcRE is also detected in the promoter upstream regions of EcR-B1 from seven lepidopteran species studied. In contrast, both a 20 bp EcRE identified in the promoter of BmE75-A and a 18 bp EcRE identified in the BHR3-B promoter, contained only E1-type EcR/USP binding element but the E2 type element was not in the promoter regions of these genes. The combination of presence of the E2 element or other cis-regulatory elements in promoter regions explains the different 20E response of each class of nuclear receptor genes. Furthermore, the E1+E2 structure for EcR-B1 can be involved in a possible cross-talk between ecdysteroid and other regulatory pathways.

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.

Retinoic acid receptors recognize the mouse genome through binding elements with diverse spacing and topology

Retinoic acid receptors (RARs) heterodimerize with retinoid X receptors (RXRs) and bind to RA response elements (RAREs) in the regulatory regions of their target genes. Although previous studies on limited sets of RA-regulated genes have defined canonical RAREs as direct repeats of the consensus RGKTCA separated by 1, 2, or 5 nucleotides (DR1, DR2, DR5), we show that in mouse embryoid bodies or F9 embryonal carcinoma cells, RARs occupy a large repertoire of sites with DR0, DR8, and IR0 (inverted repeat 0) elements. Recombinant RAR-RXR binds these non-canonical spacings in vitro with comparable affinities to DR2 and DR5. Most DR8 elements comprise three half-sites with DR2 and DR0 spacings. This specific half-site organization constitutes a previously unrecognized but frequent signature of RAR binding elements. In functional assays, DR8 and IR0 elements act as independent RAREs, whereas DR0 does not. Our results reveal an unexpected diversity in the spacing and topology of binding elements for the RAR-RXR heterodimer. The differential ability of RAR-RXR bound to DR0 compared to DR2, DR5, and DR8 to mediate RA-dependent transcriptional activation indicates that half-site spacing allosterically regulates RAR function.

Structural analysis of nuclear receptors: from isolated domains to integral proteins

Nuclear receptors (NRs) are ligand dependent transcription factors that regulate gene expression. A number of in depth structure-function relationship studies have been performed, in particular with drug design perspectives. Recent structural results concerning integral receptors in diverse functional states, obtained using a combination of different methods, now allow a better understanding of the mechanisms involved in molecular regulation. The structural data highlight the importance of DNA sequences for binding selectivity and the role of promoter response elements in the spatial organization of the protein domains into functional complexes. The solution structures of several heterodimer complexes reveal how the DNA directs the positioning of coactivators. In the case of PPARγ-RXRα the comparison with the crystal structure reveals two different conformational states that illustrate the flexibility of the receptors. The results shed light on the dynamics of the molecular recognition process.

Influence of hormone response elements (HREs) on ecdysteroid receptor concentration

Transcriptional activity of nuclear receptors is the result of transactivation capability and receptor protein concentration. The concentration of ecdysteroid receptor (EcR) constitutively expressed in vertebrate cells varies depending on the isoforms. Besides ligand binding and heterodimerization with ultraspiracle (USP), which stabilizes receptor protein concentration, degradation is regulated by interaction of the receptor complex with different ecdysteroid response elements (EcREs). Coexpression of EcREs significantly reduces ecdysteroid receptor concentration depending on the type of EcRE. Transcriptional activity and interaction with hormone response elements (HREs) as determined by Electrophoretic Mobility Shift Assay (EMSA) are often inversely related to receptor protein concentration. The complex regulation of receptor protein concentration offers an additional opportunity to regulate transcriptional activity in an isoform- and target cell-specific manner and allows the temporal limitation of hormone action.

The retinoid X receptors and their ligands

This chapter presents an overview of the current status of studies on the structural and molecular biology of the retinoid X receptor subtypes α, β, and γ (RXRs, NR2B1-3), their nuclear and cytoplasmic functions, post-transcriptional processing, and recently reported ligands. Points of interest are the different changes in the ligand-binding pocket induced by variously shaped agonists, the communication of the ligand-bound pocket with the coactivator binding surface and the heterodimerization interface, and recently identified ligands that are natural products, those that function as environmental toxins or drugs that had been originally designed to interact with other targets, as well as those that were deliberately designed as RXR-selective transcriptional agonists, synergists, or antagonists. Of these synthetic ligands, the general trend in design appears to be away from fully aromatic rigid structures to those containing partial elements of the flexible tetraene side chain of 9-cis-retinoic acid. This article is part of a Special Issue entitled Advances in High Density Lipoprotein Formation and Metabolism: A Tribute to John F. Oram (1945-2010).

Cloning and expression pattern of the ecdysone receptor and retinoid X receptor from the centipede Lithobius peregrinus (Chilopoda, Lithobiomorpha)

In arthropods, molting events are mediated by the binding of the ecdysone hormone to a heterodimer of two nuclear receptors: the ecdysone receptor (EcR) and the retinoid X receptor (RXR), a homolog of ultraspiracle (USP). We have cloned partial sequences of several isoforms for EcR and RXR genes from the centipede Lithobius peregrinus, and studied their expression profile during the second post-embryonic stage. LpEcR and LpRXR inferred amino acid sequences are very similar to other arthropod orthologs, especially to those of chelicerates and hemimetabolous insects, and their expression levels are significantly higher during the 48 h that precede the molt. Results obtained in this study represent the first data on the genetic basis of the ecdysone signal pathway for a myriapod, and in particular for an animal that, through a stereotyped developmental schedule paced by the molt cycle, completes trunk segmentation during post-embryonic life.

Molecular cloning and sequence of retinoid X receptor in the green crab Carcinus maenas: a possible role in female reproduction

Retinoid X receptor (RXR) belongs to an ancient superfamily of nuclear hormone receptors, and plays an important role in reproduction of vertebrates. However, the reproductive role of RXR has not been clarified in crustaceans. In this investigation, we first report the cloning of two alternative splice variants of RXR cDNA from green crab ovarian RNA. RXR mRNA levels were quantified in different vitellogenic stages of the crab hepatopancreas (HP) and ovary. The expression of RXR mRNA relative to the arginine kinase mRNA was significantly increased in the HP of vitellogenic crabs in a stage-dependent manner. The relative levels of RXR mRNA in the ovary were significantly lower in vitellogenic stage III crabs than in crabs in the other three stages. These data indicate that the HP and ovary of the crab are capable of expressing RXR, which may regulate, in part, vitellogenesis in the crab. We also examined the effects of methyl farnesoate (MF) and RXR-dsRNA treatments on vitellogenin and RXR gene expression. Vitellogenin and RXR mRNA levels in HP and ovarian fragments incubated in MF were significantly (P<0.001) higher than in control tissue fragments prepared from the same animal. Treatment of crabs with RXR-dsRNA significantly (P<0.001) reduced mRNA levels for RXR and for vitellogenin as well as MF levels in hemolymph. These results indicate that, MF and RXR form a complex (MF-RXR) directly and together stimulate ovarian development in these green crabs. This interaction of RXR, MF, and ovary development axis is a novel finding and is the first report to the best of our knowledge.

Ecdysone directly and indirectly regulates a cuticle protein gene, BMWCP10, in the wing disc of Bombyx mori

The hormonal regulation of cuticle protein genes is a good model to study the molecular mechanism of signaling by ecdysteroids, which initiates each of the major developmental transitions in insects. This study was conducted to clarify the regulation of the expression of an ecdysone-inducible cuticle protein gene, BMWCP10. Induction of the BMWCP10 transcript by ecdysone was partly inhibited in the presence of cycloheximide, which implies that the BMWCP10 promoter is directly and indirectly activated by ecdysone. Using electrophoretic mobility shift analysis and a competition experiment, we identified a putative ecdysone response element (EcRE1) located at positions -93 to -81 relative to the transcription start site. Site-directed mutagenesis of this site, followed by introduction into wing discs, dramatically abolished the reporter activity. This EcRE1 is necessary for the activation of the promoter by 20-hydroxyecdysone (20E) in the wing disc, since the mutation of EcRE1 caused loss of responsiveness to 20E. Collectively, the data obtained in our current and previous work indicate that ecdysone receptor and Broad-Complex Z2 (BR-Z2) are required for maximal BMWCP10 expression in wing disc.

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.

Retinoic acid regulates the human methionine sulfoxide reductase A (MSRA) gene via two distinct promoters

MSRAs (methionine sulfoxide reductases A) are enzymes that reverse the effects of oxidative damage by reducing methionine sulfoxide back to methionine and recovering protein function. In this study we demonstrate that the transcriptional regulation of the human MSRA gene is complex and driven by two distinct promoters. Both promoters demonstrate high expression in human brain and kidney tissues. The upstream (promoter 1) regulates the msrA1 transcript that codes for the mitochondrial form of MSRA and is highly active in a broad range of cell lines. The downstream promoter (promoter 2) regulates the msrA2/3 transcripts that code for the cytosolic/nuclear forms of MSRA and is generally less active. Promoter 2 contains a 65 bp putative enhancer region that is very active in the retinal pigment epithelium-derived D407 cell line. Both promoters are partially regulated by all-trans retinoic acid via RARA and other RARs.

Structural basis of natural promoter recognition by a unique nuclear receptor, HNF4alpha. Diabetes gene product

HNF4alpha (hepatocyte nuclear factor 4alpha) plays an essential role in the development and function of vertebrate organs, including hepatocytes and pancreatic beta-cells by regulating expression of multiple genes involved in organ development, nutrient transport, and diverse metabolic pathways. As such, HNF4alpha is a culprit gene product for a monogenic and dominantly inherited form of diabetes, known as maturity onset diabetes of the young (MODY). As a unique member of the nuclear receptor superfamily, HNF4alpha recognizes target genes containing two hexanucleotide direct repeat DNA-response elements separated by one base pair (DR1) by exclusively forming a cooperative homodimer. We describe here the 2.0 angstroms crystal structure of human HNF4alpha DNA binding domain in complex with a high affinity promoter element of another MODY gene, HNF1alpha, which reveals the molecular basis of unique target gene selection/recognition, DNA binding cooperativity, and dysfunction caused by diabetes-causing mutations. The predicted effects of MODY mutations have been tested by a set of biochemical and functional studies, which show that, in contrast to other MODY gene products, the subtle disruption of HNF4alpha molecular function can cause significant effects in afflicted MODY patients.

Switch of rhodopsin expression in terminally differentiated Drosophila sensory neurons

Specificity of sensory neurons requires restricted expression of one sensory receptor gene and the exclusion of all others within a given cell. In the Drosophila retina, functional identity of photoreceptors depends on light-sensitive Rhodopsins (Rhs). The much simpler larval eye (Bolwig organ) is composed of about 12 photoreceptors, eight of which are green-sensitive (Rh6) and four blue-sensitive (Rh5). The larval eye becomes the adult extraretinal 'eyelet' composed of four green-sensitive (Rh6) photoreceptors. Here we show that, during metamorphosis, all Rh6 photoreceptors die, whereas the Rh5 photoreceptors switch fate by turning off Rh5 and then turning on Rh6 expression. This switch occurs without apparent changes in the programme of transcription factors that specify larval photoreceptor subtypes. We also show that the transcription factor Senseless (Sens) mediates the very different cellular behaviours of Rh5 and Rh6 photoreceptors. Sens is restricted to Rh5 photoreceptors and must be excluded from Rh6 photoreceptors to allow them to die at metamorphosis. Finally, we show that Ecdysone receptor (EcR) functions autonomously both for the death of larval Rh6 photoreceptors and for the sensory switch of Rh5 photoreceptors to express Rh6. This fate switch of functioning, terminally differentiated neurons provides a novel, unexpected example of hard-wired sensory plasticity.

Impact of heterodimerization on intracellular localization of the ecdysteroid receptor (EcR)

Initially, nuclear import of the ecdysteroid receptor (EcR) in vertebrate cells (CHO-K1 and COS-7) does not afford a heterodimerization partner. Later on, EcR is retained in the nucleus only in the presence of a heterodimerization partner. Ultraspiracle (Usp) is more efficient compared to its vertebrate orthologue RXR and leads to an exclusively nuclear localization of EcR even in the absence of ligand. The DNA binding domain of the heterodimerization partner is important for retainment of EcR in the nucleus as shown by Usp4 (Usp(R130C)), which has lost its DNA binding capability. The C-terminal end of Usp (Usp(Delta205-508)) encompassing the C-terminal part of the D-domain and the E- and F-domains are essential for retainment of EcR in the nucleus. Nuclear localization is further influenced by cell-specific factors, since hormone and heterodimerization stabilizes the EcR protein in a cell-specific way.

Equilibrium analysis of the DNA binding domain of the ultraspiracle protein interaction with the response element from the hsp27 gene promoter--the application of molecular beacon technology

Ecdysteroids initiate molting and metamorphosis in insects via a receptor which belongs to the superfamily of nuclear receptors. The ecdysone receptor consists of two proteins: the ecdysone receptor (EcR) and the ultraspiracle (Usp). The EcR-Usp dimer conducts transcription through a hsp27(pal) response element. Usp acts as an anchor orienting the whole complex on the DNA. The molecular beacon methodology was applied to detect the sequence-specific DNA of a natural hsp27 (pal) or mutated protein interaction with the DNA binding domain from the Usp. The dissociation constant, K(d), of the UspDBD-hsp27 (pal) complex was determined to be 1.42+/-0.48 nM, whereas K(d) for UspDBD(DeltaA)-hsp27(pal) was 6.6+/-0.5 nM. Mutation of Val-71 for Ala blocks formation of the protein-DNA complex in contrast to Glu-19 mutation for Ala for which K(d)=4.31+/-1.01 nM. The results obtained with the molecular beacon technology are related to those obtained by fluorescence anisotropy titrations.

Influence of hormone on intracellular localization of the Drosophila melanogaster ecdysteroid receptor (EcR)

In the absence of hormone the ecdysteroid receptor (EcR) is distributed between the cytoplasm and the nucleus. Addition of the hormone muristerone A increases nuclear localization of wild type EcR within 5-10 min. Mutation of M504 to alanine, an amino acid, which is essential for ligand binding and which is situated in helix 5 of the ligand binding domain, abolishes hormone binding but still allows nuclear localization at only slightly reduced levels in the absence of hormone, whereas nuclear localization of EcR(M504R) is nearly abolished. Cotransfection with ultraspiracle (USP), the invertebrate ortholog of RXR, leads to exclusively nuclear localization of wild type EcR and EcR(M504A) indicating that basal heterodimerization in the absence of hormone is still possible. In the presence of Usp, EcR(M504R) is only partially localized in the nucleus. EMSA experiments show that the ligand muristerone A enhances binding of wild type EcR, but only slightly of mutated EcRs, to the canonical hsp 27 ecdysone response element. This is confirmed by transactivation studies. The results indicate that the architecture of the E-domain of EcR is important for nuclear localization even in the absence of a ligand.

Functional variants of the central bile acid sensor FXR identified in intrahepatic cholestasis of pregnancy

BACKGROUND AND AIMS

Intrahepatic cholestasis of pregnancy (ICP) is characterized by liver impairment, pruritus, and elevated maternal serum bile acids. It can cause premature delivery and intrauterine death. Bile acid synthesis, metabolism, and transport are regulated by the bile acid sensor FXR, and we hypothesized that genetic variation in FXR confers susceptibility to ICP.

METHODS

The coding regions and intron/exon boundaries of FXR were sequenced in 92 British ICP cases of mixed ethnicity. Subsequently, a case-control study of allele frequencies of these variants in 2 independent cohorts of Caucasian ICP patients and controls was performed. Variants were cloned into an FXR expression plasmid and tested in functional assays.

RESULTS

We identified 4 novel heterozygous FXR variants (-1g>t, M1V, W80R, M173T) in ICP. W80R was not present in Caucasians and M1V was detected uniquely in 1 British case. M173T and -1g>t occur both in Caucasian cases and controls, and we found a significant association of M173T with ICP (OR, 3.2; 95% confidence interval, 1.1-11.2; P = .02) when the allele frequencies of both Caucasian cohorts were analyzed together. We demonstrate functional defects in either translation efficiency or activity for 3 of the 4 variants (-1g>t, M1V, M173T).

CONCLUSIONS

This is the first report of functional variants in FXR. We propose that these variants may predispose to ICP, and because FXR has a central role in regulating bile and lipid homeostasis they may be associated with other cholestatic and dyslipidemic disorders.

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.

Analysis of transcriptional activity mediated by Drosophila melanogaster ecdysone receptor isoforms in a heterologous cell culture system

Ecdysteroid regulation of gene transcription in Drosophila melanogaster and other insects is mediated by a heterodimer comprised of Ultraspiracle (USP) and one of three ecdysone receptor (EcR) isoforms (A, B1 and B2). This study revealed that the EcR/USP heterodimer displays isoform-specific capabilities. EcRB1 is normally induced with a form of USP that is missing its DNA-binding domain (DBD), although potentiation by juvenile hormone (JH) III is reduced. The EcRA and B2 isoforms, however, display almost no response to ecdysteroids with the DBD(-) USP. A mutation, K497E, in the shared ligand-binding domain of the EcR isoforms caused elevated EcRB2-specific affinity for a canonical ecdysone response element. The effects of directed modification and mutagenesis offer a strategy for developing hypotheses and considerations for studying in vivo function.

The rapid divergence of the ecdysone receptor is a synapomorphy for Mecopterida that clarifies the Strepsiptera problem

In arthropods, the regulation by ecdysteroids is mediated by the heterodimer between the ecdysone receptor (ECR; NR1H1) and ultraspiracle (USP/RXR; NR2B4) nuclear receptors. Both ECR and USP/RXR ligand-binding domains experienced a strong acceleration of evolutionary rate in Diptera and Lepidoptera, which belong to the superorder Mecopterida. We performed a phylogenetic analysis of 28 ECR and 30 USP/RXR protein sequences from 36 arthropod species, including representatives from Trichoptera, Mecoptera and Siphonaptera. Our data show that the acceleration of ECR and USP/RXR was a unique event in the ancestor of Mecopterida. Our analysis shows further that Strepsiptera ECR and USP/RXR sequences are unambiguously placed outside of the Mecopterida clade. Protein alignments reveal that eight of 11 synapomorphies support an affinity between Strepsiptera and Coleoptera sequences. The affiliation of Strepsiptera to Diptera should therefore be rejected.

The DNA-binding domain of the ultraspiracle drives deformation of the response element whereas the DNA-binding domain of the ecdysone receptor is responsible for a slight additional change of the preformed structure

Ecdysteroids control molting and metamorphosis in insects via a heterodimeric complex of two nuclear receptors, the ecdysone receptor (EcR) and ultraspiracle protein (Usp). We used fluorescence resonance energy transfer (FRET) to study the topology of the natural pseudopalindromic element from the hsp27 gene (hsp27pal) in complex with the DNA-binding domains of Usp and EcR (UspDBD and EcRDBD, respectively). Steady-state data revealed shortening of the end-to-end distance of the hsp27pal-derived probe. For the 70.8 +/- 0.6 A distance obtained for the UspDBD-complexed DNA a bend of about 23.1 +/- 2.9 degrees was measured. Nearly the same value (23.0 +/- 3.4 degrees) was obtained for the DNA complexed with the UspDBD/EcRDBD heterodimer. The respective bend angles estimated using fluorescence decay measurements were 19.0 +/- 2.1 degrees and 20.9 +/- 3.6 degrees . Thus, the FRET data suggest for the first time that the UspDBD defines the architecture of the UspDBD/EcRDBD heterocomplex due to the significant deformation of the hsp27pal. This suggestion has been further reinforced using gel retardation experiments, which, in conjunction with high-resolution DNase I footprinting, indicate that the main contribution to the observed bend is given by the UspDBD itself, while binding of the EcRDBD molecule brings on a slight additional change of the preformed structure.

A steroid hormone affects sodium channel expression in Manduca central neurons

Neuronal differentiation is characterized by stereotypical sequences of membrane channel and receptor acquisition. This is regulated by the coordinated interactions of a variety of developmental mechanisms, one of which is the control by steroid hormones. We have used the metamorphosis of the holometabolous insect, Manduca sexta, as a model to study effects of 20-hydroxyecdysone on the maturation of thoracic neuron membrane channel expression. To test for direct hormone action, neurons were dissociated into primary cell culture on the first day of pupal life. In situ hybridization demonstrated that the amount of expression of the acetylcholine receptor alpha subunit, MARA1, was not affected by 20-hydroxyecdysone. Immunocytochemistry with an antibody directed against the SP19 segment of voltage-gated sodium channels revealed no effect of 20-hydroxyecdysone treatment during the first 6 days in culture. SP19 sodium channel protein was evenly distributed along all neurites. In contrast, after 8 days in culture, 20-hydroxyecdysone increased the amount of SP19 protein expression and strongly affected its distribution in differentiating neurons. In the presence of 20-hydroxyecdysone, patches of high densities of SP19 sodium channel protein were found in growth cones close to the base of filopodia. This is a further step toward unraveling the blend of membrane proteins under the control of steroids during the development of the central nervous system of postembryonic Manduca. Our results, taken together with previous studies, indicate that 20-hydroxyecdysone does not affect the expression of potassium membrane current or of the nicotinic acetylcholine receptor but instead regulates the amplitude of the calcium membrane current and the amount and distribution of SP19 sodium channel protein.

Sequence-specific deoxyribonucleic acid (DNA) recognition by steroidogenic factor 1: a helix at the carboxy terminus of the DNA binding domain is necessary for complex stability

Steroidogenic factor 1 (SF1) is a member of the NR5A subfamily of nuclear hormone receptors and is considered a master regulator of reproduction because it regulates a number of genes encoding reproductive hormones and enzymes involved in steroid hormone biosynthesis. Like other NR5A members, SF1 harbors a highly conserved approximately 30-residue segment called the FTZ-F1 box C-terminal to the core DNA binding domain (DBD) common to all nuclear receptors and binds to 9-bp DNA sequences as a monomer. Here we describe the solution structure of the SF1 DBD in complex with an atypical sequence in the proximal promoter region of the inhibin-alpha gene that encodes a subunit of a reproductive hormone. SF1 forms a specific complex with the DNA through a bipartite motif binding to the major and minor grooves through the core DBD and the N-terminal segment of the FTZ-F1 box, respectively, in a manner previously described for two other monomeric receptors, nerve growth factor-induced-B and estrogen-related receptor 2. However, unlike these receptors, SF1 harbors a helix in the C-terminal segment of the FTZ-F1 box that interacts with both the core DBD and DNA and serves as an important determinant of stability of the complex. We propose that the FTZ-F1 helix along with the core DBD serves as a platform for interactions with coactivators and other DNA-bound factors in the vicinity.