Characterization of the regulatory elements controlling neuronal expression of the A-isoform of the ecdysone receptor gene of Drosophila melanogaster

Functional expression patterns of four ecdysteroid receptor isoforms indicate their different functions during vitellogenesis of Chinese mitten crab, Eriocheir sinensis

In arthropods, alternative splicing of ecdysteroid receptor gene (EcR) leads to multiple functions of different EcR isoforms during metamorphosis, growth and ovarian development via ecdysteroid signaling pathway. This study was conducted to investigate the expression patterns of four EcRs of Eriocheir sinensis (EsEcRs) and the changes of haemolymph ecdysteroid titer during the ovarian development. The results showed that four EsEcR isoforms had the tissue-specific expression among 12 examined tissues, and the highest transcript levels of the four EsEcR isoforms were detected in Y-organ or sinus gland. During the ovarian development, EsEcR1 showed the highest transcript abundance of the four EsEcR isoforms. The expression profiles of all the EsEcR isoforms were similar in the hepatopancreas during the ovarian maturation cycle of E. sinensis with a trend of "high-low-high-low". In ovary, the highest expression levels of EsEcR1 and EsEcR4 were both found at stage V ovary, while the peaks of EsEcR2 and EsEcR3 were found on stage III ovary and stage IV ovary, respectively. Meanwhile, the ecdysteroid titer in haemolymph decreased gradually during ovarian maturation cycle. Further regression analysis revealed significant negative correlations were found between the ovarian EsEcR3/ EsEcR4 expression levels and haemolymph ecdysteroid titer during part or whole ovarian development cycle. These results together indicated that four EsEcR isoforms may have different functions during ovary maturation of E. sinensis. All EcR isoforms and ecdysteroid seemed to have important roles in the hepatopancreas during early ovarian development stages, while EsEcR3 and EsEcR4 were closely related to the mid-late vitellogenesis stages.

Ligand-independent requirements of steroid receptors EcR and USP for cell survival

The active form of the Drosophila steroid hormone ecdysone, 20-hydroxyecdysone (20E), binds the heterodimer EcR/USP nuclear receptor to regulate target genes that elicit proliferation, cell death and differentiation during insect development. Although the 20E effects are relatively well known, the physiological relevance of its receptors remains poorly understood. We show here that the prothoracic gland (PG), the major steroid-producing organ of insect larvae, requires EcR and USP to survive in a critical period previous to metamorphosis, and that this requirement is 20E-independent. The cell death induced by the downregulation of these receptors involves the activation of the JNK-encoding basket gene and it can be rescued by upregulating EcR isoforms which are unable to respond to 20E. Also, while PG cell death prevents ecdysone production, blocking hormone synthesis or secretion in normal PG does not lead to cell death, demonstrating further the ecdysone-independent nature of the receptor-deprivation cell death. In contrast to PG cells, wing disc or salivary glands cells do not require these receptors for survival, revealing their cell and developmental time specificity. Exploring the potential use of this feature of steroid receptors in cancer, we assayed tumor overgrowth induced by altered yorkie signaling. This overgrowth is suppressed by EcR downregulation in PG, but not in wing disc, cells. The mechanism of all these cell death features is based on the transcriptional regulation of reaper. These novel and context-dependent functional properties for EcR and USP receptors may help to understand the heterogeneous responses to steroid-based therapies in human pathologies.

Cloning and characterization of two EcR isoforms from Japanese pine sawyer, Monochamus alternates

The ecdysone receptor (EcR) is the hormonal receptor of ecdysteroids, which regulates insect growth and development. In this study, we cloned and characterized two isoforms of EcR in Monochamus alternates named MaEcR A and MaEcR B. The cDNAs of MaEcR A and MaEcR B have open repeating frames of 1,695 and 1,392 bp, respectively. The deduced proteins have the same C-terminal sequence and varied in N-terminal, and are consistent with reports on other insect species, particularly with the receptor of another coleopteran, Tribolium castaneum. The isoform-specific developmental expression profile of EcR in the epidermis and the midgut were analyzed with quantitative real-time reverse-transcriptase polymerase chain reaction in the pupal stage. RNA interference (RNAi) with common or isoform-specific regions induced developmental stagnation. When treated in the later larval stage, RNAi with either the common sequence or an EcR A specific sequence caused more severe effects and most larvae died prior to adulthood. The EcR B specific sequence caused less severe effects and about half of the treated larvae became adults, but some showed developmental defects. RNAi with both isoforms at early pupal stage attenuated the expression of 20E-regulated genes E74, E75, and HR3. The study demonstrates the role of EcR in the transduction of ecdysteroid response in Monochamus alternatus.

Ecdysone-responsive transcription factors determine the expression region of target cuticular protein genes in the epidermis of Bombyx mori

In the present study, we found that different ecdysone-responsive transcription factors were expressed differentially in different regions of the epidermis at around pupation. βFTZ-F1 transcripts were strongly but E74A transcripts were barely observed in the thoracic region of the epidermis, and vice versa in the abdominal region. Transcripts of all the examined transcription factors were observed in wing disc. Transcript of a cuticular protein gene, BMWCP4, which does not have a βFTZ-F1 binding site in the 2-kb upstream region, was not observed in the thoracic region of the epidermis. Transcript of BMWCP9, which does not have an E74 binding site in the 2-kb upstream region, was not observed in the abdominal region of the epidermis. BMWCP2 has all the transcription factor binding sites examined and was expressed in the thoracic and abdominal region of the epidermis. Thus, it is suggested that ecdysone-responsive transcription factors determined the space where the cuticular protein genes were expressed, which, in turn, determined the character of the cuticle that was characterized by the combination of cuticular proteins.

Isoform-specific regulation of a steroid hormone nuclear receptor by an E3 ubiquitin ligase in Drosophila melanogaster

The steroid hormone 20-hydroxyecdysone (20E) regulates gene transcription through the heterodimeric nuclear receptor composed of ecdysone receptor (EcR) and Ultraspiracle (USP). The EcR gene encodes three protein isoforms--A, B1, and B2--with variant N-terminal domains that mediate tissue and developmental stage-specific responses to 20E. Ariadne-1a is a conserved member of the RING finger family of ubiquitin ligases first identified in Drosophila melanogaster. Loss-of-function mutations at key cysteines in either of the two RING finger motifs, as well as general overexpression of this enzyme, cause lethality in pupae, which suggests a requirement in metamorphosis. Here, we show that Ariadne-1a binds specifically the isoform A of EcR and ubiquitylates it. Co-immunoprecipitation experiments indicate that the full sequence of EcRA is required for this binding. Protein levels of EcRA and USP change in opposite directions when those of ARI-1a are genetically altered. This is an isoform-specific, E3-dependent regulatory mechanism for a steroid nuclear receptor. Further, qRT-PCR experiments show that the ARI-1a levels lead to the transcriptional regulation of Eip78C, Eip74EF, Eip75B, and Br-C, as well as that of EcR and usp genes. Thus, the activity of this enzyme results in the regulation of dimerizing receptors at the protein and gene transcription levels. This fine-tuned orchestration by a conserved ubiquitin ligase is required during insect metamorphosis and, likely, in other steroid hormone-controlled processes across species.

Characterization of core promoter elements for ecdysone receptor isoforms of the silkworm, Bombyx mori

Two ecdysone receptor (EcR) isoforms, EcR-A and EcR-B1, are expressed in a tissue- and stage-specific manner, although the details of their transcription mechanisms are unknown. We determined the transcription start sites of EcR-A and EcR-B1 isoforms of Bombyx mori and found that both core promoter regions consist of initiator (Inr) and downstream promoter elements (DPE) but not TATA boxes. Promoter truncation analysis performed using the luciferase reporter assays and BmN cells showed that, in both isoforms, the regions -296 to -74 for BmEcR-B1, -104 to -61 for BmEcR-A and downstream regions of +1 are essential for basal transcriptional activity. Mutation experiments revealed that both DPE and its 5'-flanking CGCGCG sequence are crucial but DPE of BmEcR-B1 is not important for BmEcR-A transcription. These results indicate that the basal promoter activities differ between the two BmEcR isoforms.

Stage- and cell-specific expression of ecdysone receptors and ecdysone-induced transcription factors during midgut remodeling in the yellow fever mosquito, Aedes aegypti

In insects, especially in mosquitoes that are adult blood feeders, midgut remodeling is an important event during metamorphosis. It involves two processes viz., programmed cell death (PCD) of larval cells, and proliferation and differentiation of imaginal cells to form pupal/adult midgut. These processes are regulated by 20-hydroxyecdysone (20E) and juvenile hormone (JH), but the signaling mechanisms, which trigger specific changes remain poorly understood. Here, we report stage- and cell-specific expression of ecydone receptor (EcR), ultraspiracle (USP), broad (Br), E75B and hormone receptor 3 (HR3) during midgut remodeling in Aedes aegypti. In Ae. aegypti both EcR and USP genes code for two isoforms each and the expression of mRNA for these isoforms showed both stage- and cell-specific regulation. In general, EcR-B and USP-A mRNAs were detected during larval stages in larval cells, and EcR-A and USP-B mRNAs were detected during pupal stages in imaginal cells. These data suggest that EcR-B/USP-A heterodimer is important for PCD of larval cells and EcR-A/USP-B heterodimer is important for formation of pupal/adult midgut. Broad Z1 mRNA was detected only in the larval cells suggesting its primary role in PCD. It is likely that E75B and HR3 are probably involved in both PCD and imaginal cell proliferation and differentiation as their mRNAs were expressed in the larval as well as in imaginal cells. Application of JH analog, methoprene, lowered or delayed the expression of all the genes studied. These data suggest that 20E plays a major role in midgut remodeling and coordinates this process through stage- and cell-specific expression of different isoforms of nuclear receptors and transcription factors in the target larval and imaginal cells.

Phenotypic analysis of EcR-A mutants suggests that EcR isoforms have unique functions during Drosophila development

The steroid hormone ecdysone triggers transitions between developmental stages in Drosophila by acting through a heterodimer consisting of the EcR and USP nuclear receptors. The EcR gene encodes three protein isoforms (EcR-A, EcR-B1, and EcR-B2) that have unique amino termini but that contain a common carboxy-terminal region including DNA-binding and ligand-binding domains. EcR-A and EcR-B1 are expressed in a spatially complementary pattern at the onset of metamorphosis, suggesting that specific responses to ecdysone involve distinct EcR isoforms. Here, we describe phenotypes of EcR-A specific deletion mutants isolated using transposon mutagenesis. Western blot analysis shows that each of these mutants completely lacks EcR-A protein, while the EcR-B1 protein is still present. The EcR(112) strain has a deletion of EcR-A specific non-coding and regulatory sequences but retains the coding exons, while the EcR(139) strain has a deletion of EcR-A specific protein coding exons but retains the regulatory region. In these mutants, the developmental progression of most internal tissues that normally express EcR-B1 is unaffected by the lack of EcR-A. Surprisingly, however, we found that one larval tissue, the salivary gland, fails to degenerate even though EcR-B1 is the predominant isoform. This result may indicate that the low levels of EcR-A in this tissue are in fact required. We identified yet another type of mutation, the EcR(94) deletion, that removes the EcR-A specific protein coding exons as well as the introns between the EcR-A and EcR-B transcription start sites. This deletion places the EcR-A regulatory region adjacent to the EcR-B transcription start site. While EcR(112) and EcR(139) mutant animals die during mid and late pupal development, respectively, EcR(94) mutants arrest prior to pupariation. EcR-A mutant phenotypes and lethal phases differ from those of EcR-B mutants, suggesting that the EcR isoforms have distinct developmental functions.

Creation of EcR isoform-specific mutations in Drosophila melanogaster via local P element transposition, imprecise P element excision, and male recombination

Collections of single P transposable-element insertion strains that currently inactivate more than 25% of essential Drosophila genes have proven to be a valuable tool for genome research in Drosophila melanogaster. For genes unrepresented in these collections, strategies including local P element transposition and transposase-induced imprecise excision can be used to inactivate or delete the gene of interest. Here we report our use of local P element transposition followed by imprecise P element excision and transposase-induced male recombination to generate two deficiencies specific for the EcR-A isoform of the ecdysone receptor ( EcR) gene, and four larger deficiencies likely to affect multiple EcR functions. We also report here the determination of sequences flanking six EcR-B deficiencies generated in a previous imprecise excision screen. EcR-A encodes one of a family of three related nuclear receptor proteins that, together with the heterodimer partner USP, mediate ecdysone signaling during Drosophila development. Our results delineate sequences required in vivo for EcR-A function, as well as identifying EcR-A intron 1 sequences that are not essential for EcR function.

Thinking globally, acting locally: steroid hormone regulation of the dendritic architecture, synaptic connectivity and death of an individual neuron

Steroid hormones act via evolutionarily conserved nuclear receptors to regulate neuronal phenotype during development, maturity and disease. Steroid hormones exert 'global' effects in organisms to produce coordinated physiological responses whereas, at the 'local' level, individual neurons can respond to a steroidal signal in highly specific ways. This review focuses on two phenomena-the loss of dendritic processes and the programmed cell death (PCD) of neurons-that can be regulated by steroid hormones (e.g. during sexual differentiation in vertebrates). In insects such as the moth, Manduca sexta, and fruit fly, Drosophila melanogaster, ecdysteroids orchestrate a reorganization of neural circuits during metamorphosis. In Manduca, accessory planta retractor (APR) motoneurons undergo dendritic loss at the end of larval life in response to a rise in 20-hydroxyecdysone (20E). Dendritic regression is associated with a decrease in the strength of monosynaptic inputs, a decrease in the number of contacts from pre-synaptic neurons, and the loss of a behavior mediated by these synapses. The APRs in different abdominal segments undergo segment-specific PCD at pupation and adult emergence that is triggered directly and cell-autonomously by a genomic action of 20E, as demonstrated in cell culture. The post-emergence death of APRs provides a model for steroid-mediated neuroprotection. APR death occurs by autophagy, not apoptosis, and involves caspase activation and the aggregation and ultracondensation of mitochondria. Manduca genes involved in segmental identity, 20E signaling and PCD are being sought by suppressive subtractive hybridization (SSH) and cDNA microarrays. Experiments utilizing Drosophila as a complementary system have been initiated. These insect model systems contribute toward understanding the causes and functional consequences of dendritic loss and neurodegeneration in human neurological disorders.