Heterodimerization of ecdysone receptor and ultraspiracle on symmetric and asymmetric response elements

Full-length nuclear receptor allosteric regulation

Nuclear receptors are a superfamily of transcription factors regulated by a wide range of lipids that include phospholipids, fatty acids, heme-based metabolites, and cholesterol-based steroids. Encoded as classic two-domain modular transcription factors, nuclear receptors possess a DNA-binding domain (DBD) and a lipid ligand-binding domain (LBD) containing a transcriptional activation function. Decades of structural studies on the isolated LBDs of nuclear receptors established that lipid-ligand binding allosterically regulates the conformation of the LBD, regulating transcriptional coregulator recruitment and thus nuclear receptor function. These structural studies have aided the development of several FDA-approved drugs, highlighting the importance of understanding the structure-function relationships between lipids and nuclear receptors. However, there are few published descriptions of full-length nuclear receptor structure and even fewer descriptions of how lipids might allosterically regulate full-length structure. Here, we examine multidomain interactions based on the published full-length nuclear receptor structures, evaluating the potential of interdomain interfaces within these nuclear receptors to act as inducible sites of allosteric regulation by lipids.

Reporter gene assays for screening and identification of novel molting hormone- and juvenile hormone-like chemicals

A reporter gene assay (RGA) is used to investigate the activity of synthetic chemicals mimicking the molting hormones (MHs) and juvenile hormones (JHs) of insects, so-called insect growth regulators (IGRs). The MH receptor, a heterodimer of the ecdysone receptor (EcR) and ultraspiracle (USP), and the JH receptor Methoprene-tolerant (Met) are ligand-dependent transcription factors. Ligand-bound EcR-USP and Met bind to specific cis-acting DNA elements, referred to as the ecdysone-responsive element (EcRE) and the JH-responsive element (JHRE), respectively, in order to transactivate target genes. Insect hormone-induced transactivation systems have been reconstituted by the introduction of reporter genes under the control of EcRE and JHRE, or two-hybrid reporter genes, into insect, mammalian, and yeast cells expressing receptor proteins. RGA is easy to use and convenient for examining the MH- and JH-like activities of synthetic chemicals and is suitable for the high-throughput screening of novel structural classes of chemicals targeting EcR-USP and Met.

Structure and function of the alternatively spliced isoforms of the ecdysone receptor gene in the Chinese mitten crab, Eriocheir sinensis

Alternative splicing is an essential molecular mechanism that increase the protein diversity of a species to regulate important biological processes. Ecdysone receptor (EcR), an essential nuclear receptor, is essential in the molting, growth, development, reproduction, and regeneration of crustaceans. In this study, the whole sequence of EcR gene from Eriocheir sinensis was obtained. The sequence was 45,481 bp in length with 9 exons. Moreover, four alternatively spliced EcR isoforms (Es-EcR-1, Es-EcR-2, Es-EcR-3 and Es-EcR-4) were identified. The four isoforms harbored a common A/B domain and a DNA-binding region but different D domains and ligand-binding regions. Three alternative splicing patterns (alternative 5′ splice site, exon skipping, and intron retention) were identified in the four isoforms. Functional studies indicated that the four isoforms have specific functions. Es-EcR-3 may play essential roles in regulating periodic molting. Es-EcR-2 may participate in the regulation of ovarian development. Our results indicated that Es-EcR has broad regulatory functions in molting and development and established the molecular basis for the investigation of ecdysteroid signaling related pathways in E. sinensis.

New reporter gene assays for detecting natural and synthetic molting hormone agonists using yeasts expressing ecdysone receptors of various insects

Synthetic nonsteroidal ecdysone agonists, a class of insect growth regulators (IGRs), target the ecdysone receptor (EcR), which forms a heterodimer with ultraspiracle (USP) to transactivate ecdysone response genes. These compounds have high binding affinities to the EcR–USP complexes of certain insects and their toxicity is selective for certain taxonomic orders. In the present study, we developed reporter gene assay (RGA) systems to detect molting hormone (ecdysone) activity by introducing EcR–USP cDNA and a bacterial lacZ reporter gene into yeast. EcR and USP were derived from the insect species of three different taxonomic orders: Drosophila melanogaster (Diptera), Chilo suppressalis (Lepidoptera), and Leptinotarsa decemlineata (Coleoptera). Transcriptional coactivator taiman (Tai) cDNA cloned from D. melanogaster was also used in this RGA system. This yeast RGA system responded to various EcR ligands in a dose‐dependent and ecdysteroid‐specific manner. Furthermore, the insect order‐selective ligand activities of synthetic nonsteroidal ecdysone agonists were linearly related to their binding activities, which were measured against in vitro translated EcR–USP complexes. Our newly established yeast RGA is useful for screening new molting hormone agonists that work selectively on target insects.

Ecdysone signaling opposes epidermal growth factor signaling in regulating cyst differentiation in the male gonad of Drosophila melanogaster

The development of stem cell daughters into the differentiated state normally requires a cascade of proliferation and differentiation steps that are typically regulated by external signals. The germline cells of most animals, in specific, are associated with somatic support cells and depend on them for normal development. In the male gonad of Drosophila melanogaster, germline cells are completely enclosed by cytoplasmic extensions of somatic cyst cells, and these cysts form a functional unit. Signaling from the germline to the cyst cells via the Epidermal Growth Factor Receptor (EGFR) is required for germline enclosure and has been proposed to provide a temporal signature promoting early steps of differentiation. A temperature-sensitive allele of the EGFR ligand Spitz (Spi) provides a powerful tool for probing the function of the EGRF pathway in this context and for identifying other pathways regulating cyst differentiation via genetic interaction studies. Using this tool, we show that signaling via the Ecdysone Receptor (EcR), a known regulator of developmental timing during larval and pupal development, opposes EGF signaling in testes. In spi mutant animals, reducing either Ecdysone synthesis or the expression of Ecdysone signal transducers or targets in the cyst cells resulted in a rescue of cyst formation and cyst differentiation. Despite of this striking effect in the spi mutant background and the expression of EcR signaling components within the cyst cells, activity of the EcR pathway appears to be dispensable in a wildtype background. We propose that EcR signaling modulates the effects of EGFR signaling by promoting an undifferentiated state in early stage cyst cells.

Steroid signaling promotes stem cell maintenance in the Drosophila testis

Stem cell regulation by local signals is intensely studied, but less is known about the effects of hormonal signals on stem cells. In Drosophila, the primary steroid twenty-hydroxyecdysone (20E) regulates ovarian germline stem cells (GSCs) but was considered dispensable for testis GSC maintenance. Male GSCs reside in a microenvironment (niche) generated by somatic hub cells and adjacent cyst stem cells (CySCs). Here, we show that depletion of 20E from adult males by overexpressing a dominant negative form of the Ecdysone receptor (EcR) or its heterodimeric partner ultraspiracle (usp) causes GSC and CySC loss that is rescued by 20E feeding, uncovering a requirement for 20E in stem cell maintenance. EcR and USP are expressed, activated and autonomously required in the CySC lineage to promote CySC maintenance, as are downstream genes ftz-f1 and E75. In contrast, GSCs non-autonomously require ecdysone signaling. Global inactivation of EcR increases cell death in the testis that is rescued by expression of EcR-B2 in the CySC lineage, indicating that ecdysone signaling supports stem cell viability primarily through a specific receptor isoform. Finally, EcR genetically interacts with the NURF chromatin-remodeling complex, which we previously showed maintains CySCs. Thus, although 20E levels are lower in males than females, ecdysone signaling acts through distinct cell types and effectors to ensure both ovarian and testis stem cell maintenance.

Ecdysone receptor gene from the freshwater prawn Macrobrachium nipponense: identification of different splice variants and sexually dimorphic expression, fluctuation of expression in the molt cycle and effect of eyestalk ablation

The full-length cDNA of an ecdysone receptor gene (MnEcR) from Macrobrachium nipponense was cloned and the expression of the gene was investigated. MnEcR maintained a relatively low expression level in the early stages of embryos, but from nauplius stage, a steady increase in MnEcR expression was detected, it had the highest expression level in zoea stage. MnEcR was highly expressed in the hepatopancreas and gills among ten different tissues examined. MnEcR was rapidly upregulated in the premolt stage and rapidly downregulated in the postmolt stage. The expression of MnEcR was remarkably downregulated after eyestalk ablation in M. nipponense. An 18-amino-acid insertion/deletion and a 49-amino-acid substitution were found in the coding region of MnEcR, resulting in four splice variants: MnEcR-L1, -L2, -S1 and-S2. The expression of four splice variants of MnEcR in gonads was investigated using RT-PCR. Interestingly, the expression patterns of these splice variants differed between males and females. The dominant splice variants in testis were MnEcR-S1 and -S2, while in ovary they were MnEcR-L1 and -S2, indicating specific roles for these splice variants in male and female individuals.

Steroid hormone signaling is involved in the age-dependent behavioral response to sex pheromone in the adult male moth Agrotis ipsilon

In most animals, including insects, male reproduction depends on the detection and processing of female-produced sex pheromones. In the male moth, Agrotis ipsilon, both behavioral response and neuronal sensitivity in the primary olfactory center, the antennal lobe (AL), to female sex pheromone are age- and hormone-dependent. In many animal species, steroids are known to act at the brain level to modulate the responsiveness to sexually relevant chemical cues. We aimed to address the hypothesis that the steroidal system and in particular 20-hydroxyecdysone (20E), the main insect steroid hormone, might also be involved in this olfactory plasticity. Therefore, we first cloned the nuclear ecdysteroid receptor EcR (AipsEcR) and its partner Ultraspiracle (AipsUSP) of A. ipsilon, the expression of which increased concomitantly with age in ALs. Injection of 20E into young sexually immature males led to an increase in both responsiveness to sex pheromone and amount of AipsEcR and AipsUSP in their ALs. Conversely, the behavioral response decreased in older, sexually mature males after injection of cucurbitacin B (CurB), an antagonist of the 20E/EcR/USP complex. Also, the amount of AipsEcR and AipsUSP significantly declined after treatment with CurB. These results suggest that 20E is involved in the expression of sexual behavior via the EcR/USP signaling pathway, probably acting on central pheromone processing in A. ipsilon.

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 N-terminus of ecdysteroid receptor isoforms and ultraspiracle interacts with different ecdysteroid response elements in a sequence specific manner to modulate transcriptional activity

The functional insect ecdysteroid receptor is comprised of two nuclear receptors, the ecdysteroid receptor (EcR) and the RXR homologue, ultraspiracle (USP), which form a heterodimer. The dimer recognizes various hormone response elements and the effect of these elements on transcriptional activity of EcR isoforms was determined in vertebrate cells transfected with EcR and USP. Only constitutive activity mediated by the core response elements was preserved after elimination of nonspecific binding sites on the DNA of the vector. The constitutive transcriptional activity was regulated in a complex manner by the N-termini of both EcR and USP, the DBD of USP and the type and number of hormone response elements (HRE). Cooperative effects at oligomeric response elements particularly DR1 depended on the type of ecdysteroid response element and the N-termini of EcR and USP. The DBD of USP abolishes or attenuates synergistic effects. The data show that in the absence of hormone, transcriptional activity is regulated in a complex manner that offers additional possibilities for ecdysteroid receptor mediated gene regulation during development.

Transcriptional activity of ecdysone receptor isoforms is regulated by modulation of receptor stability and interaction with Ab- and C-domains of the heterodimerization partner ultraspiracle

The stability of ecdysone receptor (EcR) expressed in a heterologous system is regulated in an isoform-specific manner and modified by ligand and heterodimerization partner. Transcriptional activities of various receptor complexes with Usp and ligand as determined by reporter assays are the result of two effects: change in receptor concentration and altered transcriptional capability. Transcriptional activity of EcR-A is low when compared to EcR-B1 independent of the absence or presence of Ultraspiracle (Usp). Ligand increased the concentration of EcR-A, but had no effect on the transcriptional capability, in contrast to EcR-B1, which is not stabilized by hormone or Usp, but the transcriptional capability is enhanced by heterodimerization and ligand. Exchange of the AB-domain of Usp by the activation domain (AD) of Vp16 revealed that the N-terminus of Usp inhibited transcriptional activity only with EcR-B isoforms, whereas the hexapeptide in the AB-domain of wild type Usp adjacent to the C-domain of Usp harbours an activating function. Deletion of the C-domain of Usp did not affect the stability of the receptor complex, but reduced the transcriptional capability of heterodimers with all EcR-isoforms, indicating that the stability of the receptor, which is important for termination of the hormone signal transduction, is regulated in a cooperative manner by the AB-domains of EcR and Usp, and ligand. We show the active role of Usp in modulation of the transcriptional activity of the heterodimer in an isoform-specific manner by the inhibitory N-terminus, the activating hexapeptide in the AB-domain, and the C-domain of Usp.

DNA-binding properties of Drosophila ecdysone receptor isoforms and their modification by the heterodimerization partner ultraspiracle

Transcriptional activity of ecdysone receptor (EcR) isoforms varies considerably and is modified further by the heterodimerization partner and hormone treatment. To investigate whether differences in DNA binding of receptor complexes are responsible for these variations in transcriptional activity, interaction of Drosophila EcR isoforms, and variants of Ultraspiracle (Usp), the orthologue of RXR, with the ecdysone response elements (EcRE) hsp 27, PAL-1, and DR-1, were determined by electrophoretic mobility shift assays. Receptor proteins were expressed in vertebrate cells (CHO-K1) in order to rule out an influence of endogenous receptor proteins. In the absence of a heterodimerization partner, weak DNA binding of EcR was detected even without hormone with EcR-A and -B1, but not EcR-B2. In the presence of hormone, all three isoforms show increased binding to the hsp 27 EcRE. The heterodimerization partner Usp increased DNA binding considerably. The hormone effect of heterodimers is more pronounced with both EcR-B isoforms compared to EcR-A. Two specific bands were obtained for EcR-A and B1 but only one band is visible with EcR-B2. Deletion of the C-domain of Usp still allows basal DNA binding of the heterodimer, but in contrast to full-length Usp, addition of hormone decreases the intensity of the retarded receptor band of all EcR isoforms and the EcREs hsp27 and DR-1 considerably, whereas interaction with the EcRE PAL-1 is only slightly affected. Synergistic effects on transcriptional activity are associated with the formation of different receptor DNA-complexes observed with 1xhsp27 and 3xhsp27. Comparison of DNA-binding properties of EcR isoforms and EcR/Usp heterodimers revealed that binding of receptor complexes to hsp 27 EcRE is dependent on the AB domain of EcR and the AB-, C-, and D-domains of the heterodimerization partner. Interaction with the hsp 27 EcRE correlates neither with ligand binding nor with transcriptional activity of the various receptor complexes. We, therefore, conclude that the different receptor functions are regulated separately, for example, by interaction with co-modulators or post-transcriptional modifications.

Molecular determinants of caste differentiation in the highly eusocial honeybee Apis mellifera

Background

In honeybees, differential feeding of female larvae promotes the occurrence of two different phenotypes, a queen and a worker, from identical genotypes, through incremental alterations, which affect general growth, and character state alterations that result in the presence or absence of specific structures. Although previous studies revealed a link between incremental alterations and differential expression of physiometabolic genes, the molecular changes accompanying character state alterations remain unknown.

Results

By using cDNA microarray analyses of >6,000 Apis mellifera ESTs, we found 240 differentially expressed genes (DEGs) between developing queens and workers. Many genes recorded as up-regulated in prospective workers appear to be unique to A. mellifera, suggesting that the workers' developmental pathway involves the participation of novel genes. Workers up-regulate more developmental genes than queens, whereas queens up-regulate a greater proportion of physiometabolic genes, including genes coding for metabolic enzymes and genes whose products are known to regulate the rate of mass-transforming processes and the general growth of the organism (e.g., tor). Many DEGs are likely to be involved in processes favoring the development of caste-biased structures, like brain, legs and ovaries, as well as genes that code for cytoskeleton constituents. Treatment of developing worker larvae with juvenile hormone (JH) revealed 52 JH responsive genes, specifically during the critical period of caste development. Using Gibbs sampling and Expectation Maximization algorithms, we discovered eight overrepresented cis-elements from four gene groups. Graph theory and complex networks concepts were adopted to attain powerful graphical representations of the interrelation between cis-elements and genes and objectively quantify the degree of relationship between these entities.

Conclusion

We suggest that clusters of functionally related DEGs are co-regulated during caste development in honeybees. This network of interactions is activated by nutrition-driven stimuli in early larval stages. Our data are consistent with the hypothesis that JH is a key component of the developmental determination of queen-like characters. Finally, we propose a conceptual model of caste differentiation in A. mellifera based on gene-regulatory networks.

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.

Caste development and reproduction: a genome-wide analysis of hallmarks of insect eusociality

The honey bee queen and worker castes are a model system for developmental plasticity. We used established expressed sequence tag information for a Gene Ontology based annotation of genes that are differentially expressed during caste development. Metabolic regulation emerged as a major theme, with a caste-specific difference in the expression of oxidoreductases vs. hydrolases. Motif searches in upstream regions revealed group-specific motifs, providing an entry point to cis-regulatory network studies on caste genes. For genes putatively involved in reproduction, meiosis-associated factors came out as highly conserved, whereas some determinants of embryonic axes either do not have clear orthologs (bag of marbles, gurken, torso), or appear to be lacking (trunk) in the bee genome. Our results are the outcome of a first genome-based initiative to provide an annotated framework for trends in gene regulation during female caste differentiation (representing developmental plasticity) and reproduction.