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

Low GSK3β activity is required for insect diapause through responding to ROS/AKT signaling and down-regulation of Smad1/EcR/HR3 cascade

Glycogen synthase kinase 3β (GSK3β) plays important roles in gene transcription, metabolism, apoptosis, development, and signal transduction. However, its role in the regulation of pupal diapause remains unclear. In this paper, we find that low GSK3β activity in brains of diapause-destined pupae of Helicoverpa armigera is caused by elevated AKT activity. In response to ROS, AKT phosphorylates GSK3β to decrease its activity. In developing pupal brains, GSK3β can activate the transcription factor Smad1, which binds to the promoter region of the ecdysone receptor (EcR) gene and increases its expression. In the presence of 20-hydroxyecdysone (20E), EcR can bind to USP and increase the expression of 20E-response genes, including HR3, for pupal-adult development. In contrast, high levels of ROS in brains of diapause-destined pupae up-regulate p-AKT, which in turn decreases GSK3β activity. Low GSK3β activity causes low expression of EcR/HR3 via down-regulation of Smad1 activity, leading to diapause initiation. These results suggest that low GSK3β activity plays a key role in pupal diapause via ROS/AKT/GSK3β/Smad/EcR/HR3 signaling.

20-Hydroxyecdysone-responsive microRNAs of insects

20-Hydroxyecdysone (20-HE) plays essential roles in coordinating developmental transitions of insects through responsive protein-coding genes and microRNAs (miRNAs). The involvement of single miRNAs in the ecdysone-signalling pathways has been extensively explored, but the interplay between ecdysone and the majority of miRNAs still remains largely unknown. Here, by small RNA sequencing, we systematically investigated the genome-wide responses of miRNAs to 20-HE in the embryogenic cell lines of Bombyx mori and Drosophila melanogaster. Over 60 and 70 20-HE-responsive miRNAs were identified in the BmE cell line and S2 cell line, respectively. The response of miRNAs to ecdysone exhibited a time-dependent pattern, and the response intensity increased with extending exposure to 20-HE. The relationship between ecdysone and the miRNAs was further explored through knockdown of ecdysone-signalling pathway genes. Specifically, ecdysone regulated the cluster miR-275 and miR-305 through the coordination of BmEcR-B and downstream BmE75B, and the interaction between BmEcR and miR-275 cluster was strengthened by the feedback regulation of BmE75B. Ecdysone induced miR-275-3p and miR-305-5p through the ecdysone response effectors (EcREs) at the upstream of the pre-miR-275 cluster. Overall, the results might help us further understand the relationship between ecdysone signalling pathways and small RNAs in the development and metamorphosis of insects.

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.

Ecdysone receptor (EcR) and ultraspiracle (USP) genes from the cyclopoid copepod Paracyclopina nana: Identification and expression in response to water accommodated fractions (WAFs)

Ecdysteroid hormones are pivotal in the development, growth, and molting of arthropods, and the hormone pathway is triggered by binding ecdysteroid to a heterodimer of the two nuclear receptors; ecdysone receptors (EcR) and ultraspiracle (USP). We have characterized EcR and USP genes, and their 5'-untranslated region (5'-UTR) from the copepod Paracyclopina nana, and studied mRNA transcription levels in post-embryonic stages and in response to water accommodated fractions (WAFs) of crude oil. The open reading frames (ORF) of EcR and USP were 1470 and 1287bp that encoded 490 and 429 amino acids with molecular weight of 121.18 and 105.03kDa, respectively. Also, a well conserved DNA-binding domain (DBD) and ligand-binding domain (LBD) were identified which confirmed by phylogenetic analysis. Messenger RNA transcriptional levels of EcR and USP were developmental stage-specific in early post-embryonic stages (N3-4). However, an evoked expression of USP was observed throughout copepodid stage and in adult females. WAFs (40 and 80%) were acted as an ecdysone agonist in P. nana, and elicited the mRNA transcription levels in adults. Developmental stage-specific transcriptional activation of EcR and USP in response to WAFs was observed. USP gene was down-regulated in the nauplius in response to WAF, whereas up-regulation of USP was observed in the adults. This study represents the first data of molecular elucidation of EcR and USP genes and their regulatory elements from P. nana and the developmental stage specific expression in response to WAFs, which can be used as potential biomarkers for environmental stressors with ecotoxicological evaluations in copepods.

Molecular cloning, characterization, and expression analysis of an ecdysone receptor homolog in Teleogryllus emma (Orthoptera: Gryllidae)

Ecdysteroids are steroid hormones that play important roles in the regulation of Arthropoda animal growth development, larvae ecdysis, and reproduction. The effect of ecdysteroids is mediated by ecdysteroid receptor (EcR). The ecdysone receptor (EcR) belongs to the superfamily of nuclear receptors (NRs) that are ligand-dependent transcription factors. Ecdysone receptor is present only in invertebrates and plays a critical role in regulating the expression of a series of genes during development and reproduction. Here, we isolated and characterized cDNA of the cricket Teleopgryllus emma (Ohmachi & Matsuura) (Orthoptera: Gryllidae) and studied mRNA expression pattern using real time-polymerase chain reaction. The full-length cDNA of T. emma EcR, termed TeEcR, is 2,558 bp and contains a 5'-untranslated region of 555 bp and a 3'-untranslated region of 407 bp. The open reading frame of TeEcR encodes deduced 531-amino acid peptides with a predicted molecular mass of 60.7 kDa. The amino acid sequence of T. emma EcR was similar to that of known EcR especially in the ligand-binding domain of insect EcR. Real-time quantitative reverse transcription-polymerase chain reaction was performed to compare TeEcR mRNA expression level at the whole body and gonad during T. emma development. The data revealed that TeEcR mRNA is differentially expressed during T. emma development, with the highest expression level in late-instar larvae of the body and lowest in third instar. The levels of TeEcR transcripts also vary among gonads development, and levels in ovaries were higher than in testes at every developmental stage. These results suggest that TeEcR may have potential significance to regulate the morphological structure and gonad development of T. emma, due to its expression in different developmental periods.

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.

MicroRNA-281 regulates the expression of ecdysone receptor (EcR) isoform B in the silkworm, Bombyx mori

Insect development and metamorphosis are regulated by the coordination of ecdysone and juvenile hormones. Insect microRNAs (miRNAs) also act in insect development and metamorphosis by regulating genes in the ecdysone cascade. Although hundreds of insect miRNAs have been identified, the physiological functions of most remain poorly understood. Here, we report that a conserved insect miRNA, microRNA-281 (miR-281), regulates the ecdysone receptor (EcR), in an isoform-specific manner in the silkworm Bombyx mori. The B. mori EcR (BmEcR) gene encodes three isoforms: BmEcR-A, BmEcR-B1 and BmEcR-B2. The 3'UTR regions of A and B genes, which contain multiple potential microRNA targeting sites, are distinct. Target prediction revealed that miR-281 may specifically target the 3'UTR of BmEcR-B. Using a dual luciferase reporter assay in HEK293T cells, we confirmed that miR-281 suppressed transcription of BmEcR-B but not BmEcR-A. The expression of miR-281 and BmEcR-B are well coordinated in the Malpighian tubules from the fourth larval molt to pupation. In the Malpighian tubules of fifth instar larvae, BmEcR-B protein expression was down-regulated after injection of a miR-281 mimic while up-regulated after injection of a miR-281 inhibitor. miR-281 expression was suppressed by 20-hydroxyecdysone treatments but not affected by juvenile hormone treatments. Based on these findings, we propose that miR-281 participates in B. mori developmental regulation in the Malpighian tubules through suppression of BmEcR-B expression.

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.

Cloning and expression of ecdysone receptor (EcR) from the intertidal copepod, Tigriopus japonicus

Ecdysteroids are steroid hormones that play an important role in development, growth, molting of larva, and reproduction in the Arthropoda. The effect of ecdysteroids is mediated by its binding to ecdysteroid receptor (EcR). To investigate the role of EcR during development and the effect to environmental stressors on EcR expression in a copepod, we isolated and characterized cDNA and 5'-promoter region of the Tigriopus japonicus EcR (TJ-EcR), and studied mRNA expression pattern. The full-length TJ-EcR cDNA sequence was 1962bp in length and the open reading frame encoded 546 amino acids. The deduced TJ-EcR protein contained well-conserved DNA-binding domain and ligand-binding domain. Phylogenetic analysis revealed that TJ-EcR was clustered with the EcR of other crustaceans. TJ-EcR mRNA was expressed in a developmental stage-specific manner: high in early developmental stages and low in the adult stage. Significantly elevated expression of the TJ-EcR gene in adults was detected at hypersalinity (42ppt) and high temperature (35 degrees C) condition. The 5'-flanking region of TJ-EcR gene contains heat shock protein 70 response elements, implying that the environmental stressors may affect its expression via the stress-sensor. In addition, bisphenol A (100microg/L) repressed TJ-EcR expression. Our results suggest that TJ-EcR could be a biomarker for the monitoring of the impact of environmental stressors in copepods.

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.