Ecdysteroid-dependent regulation of genes in mammalian cells by a Drosophila ecdysone receptor and chimeric transactivators

Essential functions of mosquito ecdysone importers in development and reproduction

Steroid hormones control sexual maturation and reproduction in insects and humans alike. The insect steroid hormone ecdysone uses a membrane transporter named Ecdysone Importer (EcI) to enter cells and promote these physiological processes, but EcI is unexpectedly missing in mosquito genomes. Using the yellow fever mosquito Aedes aegypti, here we show that mosquitoes use alternative ecdysone importers to facilitate ecdysone-dependent development and reproduction. These transporters are also present in other insects, including fruit flies, but they are dispensable for fly development and reproduction likely due to their limited expression patterns. Our results thus indicate that differential expression of steroid hormone importers enables tissue- and stage-specific hormone responses, and some importers can obtain critical physiological functions only in certain species.

The primary insect steroid hormone ecdysone requires a membrane transporter to enter its target cells. Although an organic anion-transporting polypeptide (OATP) named Ecdysone Importer (EcI) serves this role in the fruit fly Drosophila melanogaster and most likely in other arthropod species, this highly conserved transporter is apparently missing in mosquitoes. Here we report three additional OATPs that facilitate cellular incorporation of ecdysone in Drosophila and the yellow fever mosquito Aedes aegypti. These additional ecdysone importers (EcI-2, -3, and -4) are dispensable for development and reproduction in Drosophila, consistent with the predominant role of EcI. In contrast, in Aedes, EcI-2 is indispensable for ecdysone-mediated development, whereas EcI-4 is critical for vitellogenesis induced by ecdysone in adult females. Altogether, our results indicate unique and essential functions of these additional ecdysone importers in mosquito development and reproduction, making them attractive molecular targets for species- and stage-specific control of ecdysone signaling in mosquitoes.

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.

Transporter-mediated ecdysteroid trafficking across cell membranes: A novel target for insect growth regulators

Ecdysteroids are a class of steroid hormones in arthropods that control molting and metamorphosis through interaction with intracellular nuclear receptors. In contrast to the extensive literature describing their biosynthetic pathways and signaling components, little has been known about how these hormones are traveling into and out of the cells through lipid bilayers of the cell membranes. Recently, a series of studies conducted in the fruit fly Drosophila melanogaster revealed that membrane transporters have critical functions in trafficking ecdysteroids across cell membranes, challenging the classical simple diffusion model of steroid hormone transport. Here we summarize recent advances in our understanding of membrane transporters involved in ecdysteroid signaling in Drosophila, with particular focus on Ecdysone Importer (EcI) that is involved in ecdysteroid uptake in peripheral tissues. We then discuss the potential advantage of EcI blockers as a novel pest management tool as compared to classical insect growth regulators.

The evolutionary road to invertebrate thyroid hormone signaling: Perspectives for endocrine disruption processes

Thyroid hormones (THs) are the only iodine-containing hormones that play fundamental roles in chordates and non-chordates. The chemical nature, mode of action and the synthesis of THs are well established in mammals and other vertebrates. Although thyroid-like hormones have been detected in protostomes and non-chordate deuterostomes, TH signaling is poorly understood as compared to vertebrates, particularly in protostomes. Therefore, the central objective of this article is to review TH system components and TH-induced effects in non-vertebrate chordates, non-chordate deuterostomes and protostomes based on available genomes and functional information. To accomplish this task, we integrate here the available knowledge on the THs signaling across non-vertebrate chordates, non-chordate deuterostomes and protostomes by considering studies encompassing TH system components and physiological actions of THs. We also address the possible interactions of thyroid disrupting chemicals and their effects in protostomes and non-chordate deuterostomes. Finally, the perspectives on current and future challenges are discussed.

Mechanisms of transcriptional regulation of ecdysone response

The mechanisms of ecdysone-dependent expression have been studied for many decades. Initially, the activation of individual genes under the influence of ecdysone was studied on the model of polythene chromosomes from salivary glands of Drosophila melanogaster. These works helped to investigate the many aspects of the Drosophila development. They also revealed plenty of valuable information regarding the fundamental mechanisms controlling the genes’ work. Many years ago, a model describing the process of gene activation by ecdysone, named after the author – Ashburner model – was proposed. This model is still considered an excellent description of the ecdysone cascade, which is implemented in the salivary glands during the formation of the Drosophila pupa. However, these days there is an opinion that the response of cells to the hormone ecdysone can develop with significant differences, depending on the type of cells. The same genes can be activated or repressed under the influence of ecdysone in different tissues. Likely, certain DNA-binding transcription factors that are involved in the ecdysonedependent response together with the EcR/Usp heterodimer are responsible for cell-type specificity. A number of transcriptional regulators involved in the ecdysone response have been described. Among them are several complexes responsible for chromatin remodeling and modification. It has been shown by various methods that ecdysone-dependent activation/repression of gene transcription develops with significant structural changes of chromatin on regulatory elements. The description of the molecular mechanism of this process, in particular, the role of individual proteins in it, as well as structural interactions between various regulatory elements is a matter of the future. This review is aimed to discuss the available information regarding the main regulators that interact with the ecdysone receptor. We provide a brief description of the regulator’s participation in the ecdysone response and links to the corresponding study. We also discuss general aspects of the mechanism of ecdysone-dependent regulation and highlight the most promising points for further research.

A Membrane Transporter Is Required for Steroid Hormone Uptake in Drosophila

Steroid hormones are a group of lipophilic hormones that are believed to enter cells by simple diffusion to regulate diverse physiological processes through intracellular nuclear receptors. Here, we challenge this model in Drosophila by demonstrating that Ecdysone Importer (EcI), a membrane transporter identified from two independent genetic screens, is involved in cellular uptake of the steroid hormone ecdysone. EcI encodes an organic anion transporting polypeptide of the evolutionarily conserved solute carrier organic anion superfamily. In vivo, EcI loss of function causes phenotypes indistinguishable from ecdysone- or ecdysone receptor (EcR)-deficient animals, and EcI knockdown inhibits cellular uptake of ecdysone. Furthermore, EcI regulates ecdysone signaling in a cell-autonomous manner and is both necessary and sufficient for inducing ecdysone-dependent gene expression in culture cells expressing EcR. Altogether, our results challenge the simple diffusion model for cellular uptake of ecdysone and may have wide implications for basic and medical aspects of steroid hormone studies.

Design principles for generating robust gene expression patterns in dynamic engineered tissues

Recapitulating native tissue organization is a central challenge in regenerative medicine as it is critical for generating functional tissues. One strategy to generate engineered tissues with predictable and appropriate organization is to mimic the gene expression patterning process that organizes tissues in the developing embryo. In a developing embryo, correct organization is accomplished by tissue patterning via the generation of temporal and spatial patterns of gene expression coupled with, and leading to, extensive cellular re-organization. Methods to pattern gene expression in vitro could therefore provide both better models for understanding the cellular and molecular events taking place during tissue morphogenesis and novel strategies for engineering tissues with more realistic and complex architectures. While a few attempts have been made to genetically pattern tissues in vitro, these do not produce sharp predictable patterning. In both the embryo and an in vitro tissue, patterning often occurs during extensive cell re-organization but how the dynamics of gene induction and cell re-distribution interact to impact the final outcome of patterning and ultimately tissue organization is not known. Understanding this relationship and the system parameters that dictate robust pattern formation is critical for engineering genetic patterning in vitro to organize artificial tissues. We set out to identify key requirements for pattern formation by patterning gene expression in vitro in sheets of re-distributing cells using a drug-inducible gene expression system and patterned drug delivery to mimic morphogen gene induction. Based on our experimental observations, we develop a mathematical model that allows us to identify and experimentally verify the conditions under which generation of sharp gene expression patterns is possible in vitro. Our results highlight the importance of coordinating gene induction dynamics and cellular movement in order to achieve robust pattern formation.

Regulated protein expression for in vivo gene therapy for neurological disorders: progress, strategies, and issues

The field of in vivo gene therapy has matured to the point where there are numerous clinical trials underway including late-stage clinical trials. Several viral vectors are especially efficient and support lifetime protein expression in the brain and a number of clinical trials are underway for various progressive or chronic neurological disorders including Parkinson's disease, Alzheimer's disease, and Batten's disease. To date, however, none of the vectors in clinical use have any direct way to reverse or control their transgene product in the event continued protein expression should become problematic. Several schemes that use elements within the vector design have been developed that allow an external drug or pro-drug to alter ongoing protein expression after in vivo gene transfer. The most promising and most studied regulated protein expression methods for in vivo gene transfer are reviewed. In addition, potential scientific and clinical advantages of transgene regulation for gene therapy are discussed.

Isoforms of the heteropteran Nezara viridula ecdysone receptor: protein characterisation, RH5992 insecticide binding and homology modelling

BACKGROUND

Certain bisacylhydrazine compounds such as tebufenozide (RH5992) have been shown to act as order-specific insecticides. Their compatibility with predatory Heteroptera, which are used as biological control agents, has also been demonstrated. However, the molecular mode of action of these ecdysone agonists has not been explored in a heteropteran, much less one that is a significant agricultural pest, such as Nezara viridula.

RESULTS

Alternatively spliced ligand-binding regions of the N. viridula ecdysone receptor were expressed, purified and characterised by 2D gel analysis, mass spectrometry, homology modelling and competitive binding of a bisacylhydrazine insecticidal compound (RH5992) and various ecdysteroids. Ligand binding by the two splice isoforms was indistinguishable, and relative affinities were found to occur in the order muristerone A > ponasterone A > 20-hydroxyecdysone > inokosterone > RH5992 > α-ecdysone.

CONCLUSION

The predicted difference in amino acid sequence between the ligand-binding domains of the N. viridula ecdysone receptor splice variants was verified by mass spectrometry. Both splice variant isoforms exhibit a greater affinity for the bisacylhydrazine insecticide RH5992 than do the other hemipteran ecdysone receptors characterised to date. Their affinities for a range of ecdysteroids also distinguish them from the ecdysone receptors of other Hemiptera characterised thus far. Homology models of both N. viridula receptor isoforms provide further insight into the bisacylhydrazine- and ecdysteroid-binding properties of these receptors, including their similar affinity for 20-hydroxyecdysone and the postulated pentatomomorphan moulting hormone makisterone A.

Post-transcriptionally regulated expression system in human xenogeneic transplantation models

Cells have developed a mechanism to discriminate between premature termination codons (PTCs) and normal stop codons during translation, sparking vigorous research to develop drugs promoting readthrough at PTCs to treat genetic disorders caused by PTCs. It was posed that this concept could also be applied to regulated gene therapy protocols by incorporating a PTC into a therapeutic gene, so active protein would only be made after administration of a readthrough agent. The strengths of the system are highlighted here by results demonstrating: (i) background expression levels were reduced to 0.01% to 0.0005% of wild type in unselected mass populations of cells depending upon the specific stop codon utilized and its position within the gene; (ii) expression levels responded well to multiple "On" and "Off" regulation cycles in vivo in human xenograft systems; (iii) the level of induction approached three logs using aminoglycoside activators including NB54, a newly synthesized aminoglycoside with significantly reduced toxicity; and (iv) expression levels could be appreciably altered when employing different promoters in a variety of cell types. These results strongly support the contention that this system should have important clinical applications when tight control of gene expression is required.

Synthetic strategies for studying embryonic development

Developmental biology has evolved from a descriptive science to one based on genetic principles and molecular mechanisms. Although molecular biology and genetic technologies have been the primary drivers of this transformation, synthetic strategies have been increasingly utilized to interrogate the mechanisms of embryonic patterning with spatial and temporal precision. In this review, we survey how chemical tools and engineered proteins have been used to perturb developmental processes at the DNA, RNA, protein, and cellular levels. We discuss the design principles, experimental capabilities, and limitations of each method, as well as future challenges for the chemical and developmental biology communities.

Characterization of a heat resistant beta-glucosidase as a new reporter in cells and mice

BACKGROUND

Reporter genes are widely used in biology and only a limited number are available. We present a new reporter gene for the localization of mammalian cells and transgenic tissues based on detection of the bglA (SYNbglA) gene of Caldocellum saccharolyticum that encodes a thermophilic beta-glucosidase.

RESULTS

SYNbglA was generated by introducing codon substitutions to remove CpG motifs as these are associated with gene silencing in mammalian cells. SYNbglA expression can be localized in situ or detected quantitatively in colorimetric assays and can be co-localized with E. coli beta-galactosidase. Further, we have generated a Cre-reporter mouse in which SYNbglA is expressed following recombination to demonstrate the general utility of SYNbglA for in vivo analyses. SYNbglA can be detected in tissue wholemounts and in frozen and wax embedded sections.

CONCLUSIONS

SYNbglA will have general applicability to developmental and molecular studies in vitro and in vivo.

Animal transgenesis: an overview

Transgenic animals are extensively used to study in vivo gene function as well as to model human diseases. The technology for producing transgenic animals exists for a variety of vertebrate and invertebrate species. The mouse is the most utilized organism for research in neurodegenerative diseases. The most commonly used techniques for producing transgenic mice involves either the pronuclear injection of transgenes into fertilized oocytes or embryonic stem cell-mediated gene targeting. Embryonic stem cell technology has been most often used to produce null mutants (gene knockouts) but may also be used to introduce subtle genetic modifications down to the level of making single nucleotide changes in endogenous mouse genes. Methods are also available for inducing conditional gene knockouts as well as inducible control of transgene expression. Here, we review the main strategies for introducing genetic modifications into the mouse, as well as in other vertebrate and invertebrate species. We also review a number of recent methodologies for the production of transgenic animals including retrovirus-mediated gene transfer, RNAi-mediated gene knockdown and somatic cell mutagenesis combined with nuclear transfer, methods that may be more broadly applicable to species where both pronuclear injection and ES cell technology have proven less practical.

Influence of cell cycle on ecdysteroid receptor in CHO-K1 cells

CHO-K1 cells are routinely used for characterization of ecdysone receptor (EcR) function, because these vertebrate cells are devoid of endogenous ecdysone receptor protein. Moreover, the endogenous expression of RXR, the vertebrate orthologue of Ultraspiracle (Usp), the most important heterodimerization partner, is neglectable. In contrast to insect cells, there is also no influence of moulting hormone on CHO-K1 cells on cell proliferation either in the absence or presence of transiently expressed EcR. In contrast to Usp, which is exclusively found in nuclei, EcR is heterogeneously distributed between cytoplasm and nuclei in non-synchronized cells. Synchronization of CHO-K1 cells by nocodazole revealed that the cell cycle influences receptor concentration with lowest amounts in late S-phase and G2/M phase and intracellular distribution of the receptor protein showing a minimum of receptors present in nuclei during S-phase. EcR, but not Usp reduces cyclin D1 expression and cyclin D1 concentration is impaired by cyclin D1. Coimmunoprecipitation studies reveal physical interaction of EcR and cyclin D1.

Pb2+: an endocrine disruptor in Drosophila?

Environmental exposure to Pb(2+) affects hormone-mediated responses in vertebrates. To help establish the fruit fly, Drosophila melanogaster, as a model system for studying such disruption, we describe effects of Pb(2+) on hormonally regulated traits. These include duration of development, longevity, females' willingness to mate, fecundity and adult locomotor activity. Developmental Pb(2+) exposure has been shown to affect gene expression in a specific region of the Drosophila genome (approximately 122 genes) involved in lead-induced changes in adult locomotion and to affect regulation of intracellular calcium levels associated with neuronal activity at identified synapses in the larval neuromuscular junction. We suggest ways in which Drosophila could become a new model system for the study of endocrine disruptors at genetic, neural and behavioral levels of analysis, particularly by use of genomic methods. This will facilitate efforts to distinguish between behavioral effects of Pb(2+) caused by direct action on neural mechanisms versus effects of Pb(+2) on behavior mediated through endocrine disruption.

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

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

Improvement of a monopartite ecdysone receptor gene switch and demonstration of its utility in regulation of transgene expression in plants

In plants, regulation of transgene expression is typically accomplished through the use of inducible promoter systems. The ecdysone receptor (EcR) gene switch is one of the best inducible systems available to regulate transgene expression in plants. However, the monopartite EcR gene switches developed to date require micromolar concentrations of ligand for activation. We tested several EcR mutants that were generated by changing one or two amino acid residues in the highly flexible ligand-binding domain of Choristoneura fumiferana EcR (CfEcR). Based on the transient expression assays, we selected a double mutant, V395I + Y415E (VY), of CfEcR (CfEcR(VY)) for further testing in stable transformation experiments. The CfEcR(VY) mutant only slightly improved the induction characteristics of the two-hybrid gene switch, whereas the CfEcR(VY) mutant significantly improved the induction characteristics of the monopartite gene switch (VGCfE(VY)). The ligand sensitivity of the VGCfE(VY) switch was improved by 125-15 625-fold in different transgenic lines analyzed, compared to the VGCfE(Wt) switch. The utility of the VGCfE(VY) switch was tested by regulating the expression of an Arabidopsis zinc finger protein gene (AtZFP11) in both tobacco and Arabidopsis plants. These data showed that the VGCfE(VY) switch efficiently regulated the expression of AtZFP11 and that the phenotype of AtZFP11 could be induced by the application of ligand. In addition, the affected plants recovered after withdrawal of the ligand, demonstrating the utility of this gene switch in regulating the expression of critical transgenes in plants.

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.

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.

Switching on the lights for gene therapy

Strategies for non-invasive and quantitative imaging of gene expression in vivo have been developed over the past decade. Non-invasive assessment of the dynamics of gene regulation is of interest for the detection of endogenous disease-specific biological alterations (e.g., signal transduction) and for monitoring the induction and regulation of therapeutic genes (e.g., gene therapy). To demonstrate that non-invasive imaging of regulated expression of any type of gene after in vivo transduction by versatile vectors is feasible, we generated regulatable herpes simplex virus type 1 (HSV-1) amplicon vectors carrying hormone (mifepristone) or antibiotic (tetracycline) regulated promoters driving the proportional co-expression of two marker genes. Regulated gene expression was monitored by fluorescence microscopy in culture and by positron emission tomography (PET) or bioluminescence (BLI) in vivo. The induction levels evaluated in glioma models varied depending on the dose of inductor. With fluorescence microscopy and BLI being the tools for assessing gene expression in culture and animal models, and with PET being the technology for possible application in humans, the generated vectors may serve to non-invasively monitor the dynamics of any gene of interest which is proportionally co-expressed with the respective imaging marker gene in research applications aiming towards translation into clinical application.

Ligand binding by recombinant domains from insect ecdysone receptors

The ligand binding domains (LBDs) from the EcR and USP proteins of four insect pests (Lucilia cuprina, Myzus persicae, Bemisia tabaci, Helicoverpa armigera) were purified as recombinant heterodimers. The K(d) values for [(3)H]-ponasterone A binding by LBD heterodimers that included the hinge regions (i.e., DE/F heterodimers) ranged 0.7-2.5 nM, with K(i) values for ecdysteroid and dibenzoylhydrazine ligands ranging from 0.1 nM to >448 microM. The K(d) and K(i) values for a recombinant H. armigera LBD heterodimer that lacked D-regions (i.e., an E/F heterodimer) were approximately 4 times higher than those for its DE/F counterpart. Rate constants were estimated for the L. cuprina LBD heterodimer. A fluorescein-inokosterone conjugate (K(i)~40 nM) was used to develop a novel binding assay based on fluorescence polarization. This assay, which ranked the affinity of competitor ecdysteroids in the same order as the [(3)H]-ponasterone A binding assay, is well suited to high-throughput screening. Ponasterone A had a higher affinity than muristerone A for the recombinant hemipteran LBD heterodimers, whereas the reverse was true for the recombinant dipteran one. The same preference was observed when these ligands were tested as inducers of ecdysone receptor-controlled gene expression in transfected mammalian cells. The binding data obtained in vitro using recombinant LBD heterodimers reflects the ability of agonists to induce transgene expression in recombinant mammalian cells, and can also reflect their efficacy as larvicides.

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.

Functional characterization of ecdysone receptor gene switches in mammalian cells

Regulated expression of transgene is essential in basic research as well as for many therapeutic applications. The main purpose of the present study is to understand the functioning of the ecdysone receptor (EcR)-based gene switch in mammalian cells and to develop improved versions of EcR gene switches. We utilized EcR mutants to develop new EcR gene switches that showed higher ligand sensitivity and higher magnitude of induction of reporter gene expression in the presence of ligand. We also developed monopartite versions of EcR gene switches with reduced size of the components that are accommodated into viral vectors. Ligand binding assays revealed that EcR alone could not bind to the nonsteroidal ligand, RH-2485. The EcR's heterodimeric partner, ultraspiracle, is required for efficient binding of EcR to the ligand. The essential role of retinoid X receptor (RXR) or its insect homolog, ultraspiracle, in EcR function is shown by RXR knockdown experiments using RNAi. Chromatin immunoprecipitation assays demonstrated that VP16 (activation domain, AD):GAL4(DNA binding domain, DBD):EcR(ligand binding domain, LBD) or GAL4(DBD):EcR(LBD) fusion proteins can bind to GAL4 response elements in the absence of ligand. The VP16(AD) fusion protein of a chimera between human and locust RXR could heterodimerize with GAL4(DBD):EcR(LBD) in the absence of ligand but the VP16(AD) fusion protein of Homo sapiens RXR requires ligand for its heterodimerization with GAL4(DBD):EcR(LBD).

Optimization of the Tet-On system for inducible expression of RAGE

We have optimized a two-plasmid Tet-On system, the regulatory plasmid and the response plasmid, to produce tightly controlled inducible expression of the gene RAGE in cell-culture models. Two sets of plasmids were constructed: set 1 (universal; for broad range of cell types) and set 2 (neuron specific). For the response plasmid, the gene RAGE was cloned in pIRES2-EGFP plasmid (Clontech) and the CMV promoter replaced with TREtight (modified seven copies of Tet-operon fused with CMVm promoter). For the regulatory plasmid, rtTA (reverse tetracycline transactivator) was placed under either the CMV promoter or the cell-specific promoter neuronal specific enolase. Both plasmids have the mammalian selection marker neomycine; the EGFP reporter gene is only in the response plasmid and IRES is between the gene and EGFP. Following induction with doxycycline, cells expressing RAGE showed neomycine resistance and green fluorescence (EGFP). Our system has been tested in two different cell lines and showed negligible basal leakiness, high induction of the gene RAGE (142-fold), dose-dependent response to doxycycline, and strict cell-type specificity. This system is highly suitable for cell-specific expression of any gene of interest in primary cultures and mixed cell populations.

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

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

Flow cytometry-based method to analyze the change in Tau phosphorylation in a hGSK-3β and hTau over-expressing EcR-293 cell line

Neurofibrillary tangles are composed of insoluble aggregates of microtubule-associated protein Tau. In the pathology of Alzheimer's disease (AD), accumulation of hyperphosphorylated Tau results in formation of paired helical filaments. One of the main candidate to hyperphosphorylate Tau in AD is glycogen synthase kinase 3beta (GSK-3beta). Here we introduce a non-neuronal cell line, stably co-expressing human Tau and GSK-3beta proteins, where the effect of potential kinase inhibitors on Tau phosphorylation can be monitored. The aim of our study was to establish a new flow-cytometry-based method to quantitatively analyze the changing of Tau phosphorylation, which is a suitable alternative to the well-accepted but non-quantitative Western blot technique. Our results demonstrate that the flow cytometry-based method is a convenient tool to analyze the effect of GSK-3beta inhibitors on Tau phosphorylation. This new approach provides appropriate throughput for screening purposes in preclinical research for characterization of GSK-3beta inhibitors, as potential drug candidate to cure Alzheimer's disease.

Development of a methoxyfenozide-responsive gene switch for applications in plants

The ecdysone receptor (EcR) has been used to develop gene switches for conditional regulation of transgene expression in plants and humans. All EcR-based gene switches developed to date for use in plants are monopartate and require micromolar concentrations of ligand for activation of the transgene; this has limited the use of these gene switches. We have developed a Choristoneura fumiferana ecdysone receptor (CfEcR)-based two-hybrid gene switch that works through the formation of a functional heterodimer between EcR and the retinoid X receptor (RXR) upon application of the chemical ligand methoxyfenozide. Methoxyfenozide is already registered for field use with an excellent safety profile, and it has potential as a gene switch ligand for applications in the field. The receptor constructs were prepared by fusing DEF domains (hinge region plus ligand-binding domain) of CfEcR to the GAL4 DNA-binding domain and EF domains (ligand-binding domain) of ultraspiracle from Choristoneura fumiferana (CfUSP) or RXR from Locusta migratoria (LmRXR), Mus musculus (MmRXR) or Homo sapiens (HsRXR) to the VP16 activation domain. These receptor constructs were tested for their ability to induce expression of the luciferase gene placed under the control of 5x GAL4 response elements and -46 35S minimal promoter in tobacco, corn and soybean protoplasts and in transgenic Arabidopsis and tobacco plants. By adopting the two-hybrid format, the sensitivity of the CfEcR gene switch has been improved from micromolar to nanomolar concentrations of methoxyfenozide. The sensitivity of the CfEcR + LmRXR two-hybrid switch was 25 to 625 times greater than the monopartate gene switch, depending on the plant species tested.

Stably Transformed Insect Cell Lines: Tools for Expression of Secreted and Membrane‐anchored Proteins and High‐throughput Screening Platforms for Drug and Insecticide Discovery

Insect cell-based expression systems are prominent amongst current expression platforms for their ability to express virtually all types of heterologous recombinant proteins. Stably transformed insect cell lines represent an attractive alternative to the baculovirus expression system, particularly for the production of secreted and membrane-anchored proteins. For this reason, transformed insect cell systems are receiving increased attention from the research community and the biotechnology industry. In this article, we review recent developments in the field of insect cell-based expression from two main perspectives, the production of secreted and membrane-anchored proteins and the establishment of novel methodological tools for the identification of bioactive compounds that can be used as research reagents and leads for new pharmaceuticals and insecticides.

The influence of heterodimer partner ultraspiracle/retinoid X receptor on the function of ecdysone receptor

A pair of nuclear receptors, ecdysone receptor (EcR) and ultraspiracle (USP), heterodimerize and transduce ecdysteroid signals. The EcR and its nonsteroidal ligands are being developed for regulation of transgene expression in humans, animals and plants. In mammalian cells, EcR:USP heterodimers can function in the absence of ligand, but EcR/retinoid X receptor (EcR:RXR) heterodimers require the presence of ligand for activation. The heterodimer partner of EcR can influence ligand sensitivity of EcR so that the EcR/Locusta migratoria RXR (EcR:LmRXR) heterodimers are activated at lower concentrations of ligand when compared with the concentrations of ligand required for the activation of EcR/Homo sapiens RXR (EcR:HsRXR) heterodimers. Analysis of chimeric RXRs containing regions of LmRXR and HsRXR and point mutants of HsRXR showed that the amino acid residues present in helix 9 and in the two loops on either end of helix 9 are responsible for improved activity of LmRXR. The EcR:Lm-HsRXR chimera heterodimer induced reporter genes with nanomolar concentration of ligand compared with the micromolar concentration of ligand required for activating the EcR:HsRXR heterodimer. The EcR:Lm-HsRXR chimera heterodimer, but not the EcR:HsRXR heterodimer, supported ligand-dependent induction of reporter gene in a C57BL/6 mouse model.

Tight transcriptional regulation of foreign genes in insect cells using an ecdysone receptor-based inducible system

The use of insect cells has been highly successful for the expression of foreign proteins from baculoviruses or plasmid vectors. Here, we describe a tight transcriptional regulation of foreign genes in insect cells using an ecdysone receptor-based inducible system. The system includes the DEF domains of the spruce budworm (Choristoneura fumiferana) EcR (CfEcR) fused to the Saccharomyces cerevisiae GAL4 DNA-binding domain and the EF domains of mammalian Mus musculus retinoid X receptor (MmRXR) fused to the acidic activation domains (AADs) of the baculovirus transactivators IE1 and IE0. Using a GAL4 response element in reporter constructs, both transient and stable expression in insect lepidopteran cells showed that the chimeric MmRXR and CfEcR only activated the reporter genes in the presence of inducer; no gene expression was detectable in the absence of inducer. Characterization of heterogenous activation domains in insect cells showed that the AADs from Autographa californica multiple nucleopolyhedrovirus (MNPV) IE1 and Orgyia pseudotsugata MNPV IE0 consistently exhibited higher inducible levels than the archetype AAD from herpesvirus VP16 in insect cells. To confirm the tight regulation of this system the highly toxic protein, diphtheria toxin (DT), was used. In the absence of an inducer no cytotoxic effect was observed in insect cells that had been transiently transformed with DT expressing plasmids. This system will therefore be a very useful tool for biotechnology applications expressing highly toxic proteins in insect cells and for studying the functional genomics of insects and microorganisms that infect them.

Inducible expression and pharmacological characterization of recombinant rat NR1a/NR2A NMDA receptors

In this study, we have established a non-neuronal cell line stably and inducibly expressing recombinant NMDA receptors (NRs) composed of rat NR1a/NR2A subunits. EcR-293 cells were transfected with rat NR1a and NR2A cDNAs using the inducible mammalian expression vector pIND. Cell colonies resistant for the selecting agents were picked and tested for NR2A mRNA as well as protein expression using quantitative RT-PCR and flow cytometry based immunocytochemistry. Clonal cells expressing functional NMDA receptors were identified by measuring NMDA-evoked ion currents, and NMDA-induced increase in cytosolic free calcium concentration in whole-cell patch-clamp and fluorimetric calcium measurements, respectively. One clone named D5/H3, which exhibited the highest response to NMDA, was chosen to examine inducibility of the expression and for pharmacological profiling of recombinant NR1a/NR2A NMDA receptors. To check inducibility, NR2A subunit expression in D5/H3 cells treated with the inducing agent muristerone A (MuA) was compared with that in non-induced cells. Both NR2A mRNA and protein expression was several folds higher in cells treated with the inducing agent. As part of the pharmacological characterization, we examined the activation of the expressed NR1a/NR2A receptors as a function of increasing concentration of NMDA. NMDA-evoked concentration-dependent increases in cytosolic [Ca2+] with an EC50 value of 41 +/- 1 microM. In addition, whereas the NMDA response was concentration-dependently inhibited by the channel blocker MK-801 (IC50 = 58 +/- 6 nM), NR2B subunit selective NMDA receptor antagonists were ineffective. Thus, this cell line, which stably and inducibly expresses recombinant NR1a/NR2A NMDA receptors, can be a useful tool for testing NMDA receptor antagonists and studying their subunit selectivity.

Regulated and constitutive expression of anti-inflammatory cytokines by nontransforming herpesvirus saimiri vectors

Herpesviral saimiri-(HVS) mediated expression of bovine growth hormone was one of the first applications of an episomal viral vector for gene therapy. Meanwhile, the long-term persistence of HVS vectors has been confirmed in a broad spectrum of infectable target cells in vitro and in vivo. Regulated gene expression is useful for many applications of gene therapy. Therefore, we inserted the Mifepristone-antiprogestin-inducible expression system (GeneSwitchtrade mark) into HVS viral vectors to regulate the combined expression of anti-inflammatory cytokines, IL-10 and IL-1RA. Constitutive CMV-promoter/enhancer-driven and Mifepristone-inducible cytokine expression was compared in the viral context in transduced primary human fibroblasts and rheumatoid arthritis (RA) fibroblast-like cells (RASF). Long-term persistence of vector genomes was shown for both construct types. Constitutive expression was efficient and more rapid in onset than in the inducible system, in which the selective induction of interleukin expression along with low background levels was obtained by Mifepristone concentrations that were more than 1000-fold below those required for endogenous Progesterone antagonism. Furthermore, transgene expression corresponded to vector doses. Global patterns of cytokine secretion were not significantly changed due to viral transduction, indicating a rather inert behavior of the viral vector itself. In an attempt to emulate the inflammatory cytokine-enriched environment in rheumatoid arthritic joints, the function of the vectors could be demonstrated in vitro by the successful blockade of IL-1beta-stimulated matrix-metalloproteinase (MMP)-3 expression from RASF cells. Evaluation of this system in future studies, in suitable long-term SCID models of RA or in non-human primate models, will exploit the possible in vivo benefits of nontransforming HVS vectors in gene therapy.

Ligand-binding pocket of the ecdysone receptor

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 central role in regulating the expression of a vast array of genes during development and reproduction. The functional entity is a heterodimer composed of EcR and the ultraspiracle protein (USP)-the orthologue of the vertebrate retinoid X receptor (RXR). Ecdysone receptor is the molecular target of ecdysteroids-the endogenous steroidal molting hormones found in arthropods and nonarthropod invertebrates. In addition, EcR is the target of the environmentally safe bisacylhydrazine insecticides used against pests, such as caterpillars, that cause severe damage to agriculture. The crystal structures of the ligand-binding domains (LBDs) of the EcR/USP heterodimer, complexed to the ecdysteroid ponasterone A (ponA) and to the lepidopteran specific bisacylhydrazine BYI06830 used in the agrochemical pest control, provide the first insight at atomic level for these important functional complexes. The EcR/USP heterodimer has a shape similar to that seen for the known vertebrate heterodimer complexes with a conserved main interface, but with features, that are specific to this invertebrate heterodimer. The two EcR-LBD structures in complex with steroidal and nonsteroidal ligands reveal substantial differences. The adaptability of EcR to its ligand results in two radically different and only partially overlapping ligand-binding pockets with different residues involved in ligand recognition. The concept brought by these structural studies of a ligand-dependent binding pocket has potential applications for other NRs.

Nonsteroidal ecdysone agonists

Nonsteroidal ecdysone agonists are novel compounds that have become attractive candidates not only as pest control agents in agriculture but also as tools for research. Their narrow spectrum of activity makes them relatively safe as pesticides, and their mode of action as ligands for gene expression has found application in gene therapy and inducing transgenic gene expression in plants. These diacylhydrazines (DAHs) are potent nonsteroidal ecdysone agonists, and four of them, tebufenozide, methoxyfenozide, chromafenozide, and halofenozide, have been developed as insecticides. Although these compounds are very toxic to insects, they are safe for mammals and are environmentally benign. Their action on insects is also selective, the first three are effective against Lepidoptera but weakly active or inactive on Diptera and Coleoptera. On the other hand, halofenozide is effective on Coleoptera but mildly active on Lepidoptera. Previous reviews on ecdysone agonists have concentrated on the biological response of some DAHs and their effects on pests. In this review, the chemistry, biological effects and their modes of action at the molecular level will be covered. In addition, a few studies on other nonsteroidal ecdysone agonists, such as 3,5-di-tert-butyl-4-hydroxy-N-iso-butylbenzamide, acylaminoketones, and benzoyl-1,2,3,4-tetrahydroquinolines, will be briefly reviewed.

Differential target gene activation by TBX2 and TBX2VP16: evidence for activation domain-dependent modulation of gene target specificity

The determinants of in vivo target site selectivity by transcription factors are poorly understood. To find targets for the developmentally regulated transcription factor TBX2, we generated stable transfectants of human embryonic kidney cells (293) that express a TBX2-ecdysone receptor (EcR) chimeric protein. While constitutive expression of TBX2 is toxic to 293 cells, clones expressing TBX2EcR are viable in the absence of an EcR ligand. Using cDNA arrays and quantitative PCR, we discovered nine genes whose expression was increased, but no genes whose expression was reduced, following 24 h of induction with Ponasterone A (PonA), a ligand for EcR. Since TBX2 was reported previously to be a transcriptional repressor, we also generated cell lines expressing a TBX2VP16EcR protein which we showed was a potent conditional transcriptional activator in transient transfection assays. Treatment of these cells with PonA induced the expression of five genes, none of which were affected in TBX2EcR-expressing cells. This discordance between TBX2- and TBX2VP16-regulated genes strongly suggests that specific transactivation domains can be a major determinant of gene target site selectivity by transcription factors that possess the same DNA-binding domain.

Regulated expression of sFRP-1 protein by the GeneSwitch system

The GeneSwitch system is a mifepristone-inducible expression system that provides exceptionally low uninduced and high-induced protein expression in mammalian cells. We have developed an adenovirus recombinant containing GeneSwitch protein driven by the GAL4-tk promoter, as well as recombinants containing sFRP-1 and luciferase reporter under the control of the GAL4-E1b promoter. Luciferase activity in A549 cells infected with the GeneSwitch and Luciferase viruses is very low in ethanol-treated cells, while the level of luciferase activity increases 200-fold in cells treated with mifepristone. Conditional expression of functional sFRP-1 is demonstrated in A549, human osteoblast, and CHO cell lines by either the co-infection of cells with sFRP-1 and GeneSwitch viruses or the infection of GeneSwitch expressing cell lines with sFRP-1 virus and subsequent treatment with mifepristone. The expression of sFRP-1 is seen as early as 4 h post-mifepristone treatment, reaching the highest levels at 20 h. The sFRP-1 protein is present in conditioned media, and the protein is functional based upon its ability to inhibit the Wnt-mediated activation of TCF-Luciferase reporter activity. The regulated expression of sFRP-1 utilizing adenovirus vectors provides an opportunity to address the contribution of sFRP-1 in the regulation of stem cell differentiation, maturation, and their function by modulating the Wnt signaling.

Optimization of the Tet-on system to regulate interleukin 12 expression in the liver for the treatment of hepatic tumors

Interleukin 12 (IL-12) is a potent antitumoral cytokine, but it can be toxic at high doses. Therapy of liver tumors might benefit from the use of vectors enabling tight control of IL-12 expression in hepatic tissue for long periods of time. To this aim, we have improved the Tet-on system by modifying the minimal region of the inducible promoter and adjusting the level of the trans-activator using liver-specific promoters with graded activities. The resulting vectors allowed hepato-specific gene regulation with lower basal activity and higher inducibility compared with the original system in the absence of repressor molecules. The basal and final protein levels depend on the strength of the promoter that directs the transcripcional activator as well as the relative orientation of the two genes in the same plasmid. We have selected the construct combining minimal leakage with higher level of induced gene expression to regulate IL-12 after DNA transfer to mouse liver. Administration of doxycycline (Dox) enhanced IL-12 expression in a dose-dependent manner, whereas it was undetectable in serum in the noninduced state. Gene activation could be repeated several times, and sustained levels of IL-12 were achieved by daily administration of Dox. The antitumor effect of IL-12 was evaluated in a mouse model of metastatic colon cancer to the liver. Complete eradication of liver metastasis and prolonged survival was observed in all mice receiving Dox for 10 days. These data demonstrate the potential of a naked DNA gene therapy strategy to achieve tight control of IL-12 within the liver for the treatment of cancer.

Two new ecdysteroids from Serratula wolffii

11alpha-Hydroxypoststerone (1) and herkesterone (2), two new natural ecdysteroids, were isolated from the herb Serratula wolffii. The former compound is the first 11-hydroxylated C21 ecdysteroid, while the latter is a new ecdysteroid with a 7,9(11)-dien-6-one chromophore. Their structures were determined using a combination of spectroscopic techniques.

Functional and comparative analysis of two distinct ecdysteroid-responsive gene expression constructs in Drosophila S2 cells

Inducible expression systems have proven to be of major interest when analysing the function of specific genes or when expressing cytotoxic proteins. In an effort to develop inducible switches allowing for flexible fine-tuning of gene expression levels in insect cells, we have compared the induction capacities of two Drosophila minimal promoters when linked to four consecutive ecdysone response elements. These minimal promoters, either containing a TATA-box or a downstream promoter element, drove the expression of a luciferase reporter gene. Potent induction capacities were observed with the insect moulting hormone, 20-hydroxyecdysone, and with ponasterone A, a plant ecdysteroid. The developed inducible switches further expand the repertoire of molecular tools for functional expression of proteins of interest in insect cells. In addition, the combination of an ecdysone switch with promoters that possess different structural elements can provide novel insights into ecdysteroid-induced transcription in an insect cell line.

A gene expression system offering multiple levels of regulation: the Dual Drug Control (DDC) system

Background

Whether for cell culture studies of protein function, construction of mouse models to enable in vivo analysis of disease epidemiology, or ultimately gene therapy of human diseases, a critical enabling step is the ability to achieve finely controlled regulation of gene expression. Previous efforts to achieve this goal have explored inducible drug regulation of gene expression, and construction of synthetic promoters based on two-hybrid paradigms, among others.

Results

In this report, we describe the combination of dimerizer-regulated two-hybrid and tetracycline regulatory elements in an ordered cascade, placing expression of endpoint reporters under the control of two distinct drugs. In this Dual Drug Control (DDC) system, a first plasmid expresses fusion proteins to DBD and AD, which interact only in the presence of a small molecule dimerizer; a second plasmid encodes a cassette transcriptionally responsive to the first DBD, directing expression of the Tet-OFF protein; and a third plasmid encodes a reporter gene transcriptionally responsive to binding by Tet-OFF. We evaluate the dynamic range and specificity of this system in comparison to other available systems.

Conclusion

This study demonstrates the feasibility of combining two discrete drug-regulated expression systems in a temporally sequential cascade, without loss of dynamic range of signal induction. The efficient layering of control levels allowed by this combination of elements provides the potential for the generation of complex control circuitry that may advance ability to regulate gene expression in vivo.

Highly flexible ligand binding pocket of ecdysone receptor: a single amino acid change leads to discrimination between two groups of nonsteroidal ecdysone agonists

The insect steroid hormone 20-hydroxyecdysone works through a ligand-activated nuclear receptor, the ecdysone receptor (EcR), which plays critical roles in insect development and reproduction. The EcR has been exploited to develop insecticides to control pests and gene switches for gene regulation. Recently reported crystal structures of the EcR protein show different but partially overlapping binding cavities for ecdysteroid (ECD) and diacylhydrazine (DAH) ligands, providing an explanation for the differential activity of DAH ligands in insects. 1-Aroyl-4-(arylamino)-1,2,3,4-tetrahydroquinoline (THQ) ligands were recently discovered as ecdysone agonists. Mutagenesis of the EcR (from Choristoneura fumiferana, CfEcR) ligand binding domain followed by screening in a reporter assay led to the identification of CfEcR mutants, which responded well to THQ ligands but poorly to both ECD and DAH ligands. These mutants were further improved by introducing a second mutation, A110P, which was previously reported to cause ECD insensitivity. Testing of these V128F/A110P and V128Y/A110P mutants in a C57BL/6 mouse model coactivator interaction assay and in insect cells showed that this mutant EcR is activated by THQ ligands but not by ECD or DAH ligands. The CfEcR and its V128F/A110P mutant were used to demonstrate simultaneous regulation of two reporter genes using THQ and DAH ligands.

Glucocorticoid-inducible gene expression vectors for use in Drosophila melanogaster Schneider 2 cells

Inducible, vector-based, expression systems that allow fine control of transgene expression are gaining more and more use in fundamental research as well as in therapeutic applications. In an effort to develop a tightly regulated heterologous expression system for Drosophila Schneider 2 cells, three different inducible reporter constructs were compared. These comprised six copies of the glucocorticoid response element fused to one of three distinct types of Drosophila gene promoters: (1) a TATA-box containing, (2) a TATA-less and (3) a bidirectional core sequence. These were fused to a luciferase reporter gene. The promoter constructs displayed different basal as well as agonist-induced activities. The implications of the observations made are discussed in the context of promoter properties and of induction of genes that may be studied in Drosophila.

Ecdysone agonist-inducible expression of a coat protein gene from tobacco mosaic virus confers viral resistance in transgenic Arabidopsis

Constitutive expression of a gene encoding tobacco mosaic virus (TMV) coat protein (CP) in transgenic plants confers resistance to infection by TMV and related tobamoviruses. Here, we examined resistance to TMV by temporal and quantitative control of TMV Cg CP (CgCP) gene expression using a simple, methoxyfenozide-inducible system in Arabidopsis plants. By soil drenching with a commercial ecdysone agonist (Intrepid-2F/methoxyfenozide), most transgenic lines were induced from undetectable levels of gene expression to protein levels from 0.05 to 0.8% (w/w) of CgCP. This corresponds to up to four times the amount of CP produced by the constitutive cauliflower mosaic virus (CaMV) double 35S promoter. CgCP transcripts were induced by 700-fold, without changing the expression patterns of pathogenesis-related (PR) genes. The high level of accumulation of CgCP was sufficient to produce large amounts of virus-like particles that accumulate in large aggregates throughout the cells. In virus challenge assays, treatment with Intrepid-2F prior to TMV infection resulted in high levels of viral resistance, while no treatment or treatment with the inducer following infection did not confer resistance. This report demonstrates chemically controlled disease resistance and confirms the utility of the ecdysone agonist-inducible system under greenhouse conditions.