Antennal expression pattern of two olfactory receptors and an odorant binding protein implicated in host odor detection by the malaria vector Anopheles gambiae

Odor-detection in the malaria mosquito Anopheles gambiae involves large families of diverse proteins, including multiple odorant binding proteins (AgOBPs) and olfactory receptors (AgORs). The receptors AgOR1 and AgOR2, as well as the binding protein AgOBP1, have been implicated in the recognition of human host odors. In this study, we have explored the expression of these olfactory proteins, as well as the ubiquitous odorant receptor heteromerization partner AgOR7, in the thirteen flagellomeres (segments) of female and male antenna. Expressing cells were visualized by adapting a whole mount fluorescence in situ hybridization method. In female mosquitoes, AgOR1-expressing olfactory receptor neurons (ORNs) were almost exclusively segregated in segments 3 to 9, whereas AgOR2-expressing ORNs were distributed over flagellomeres 2 to 13. Different individuals comprised a similar number of cells expressing a distinct AgOR type, although their antennal topography and number per flagellomere varied. AgOBP1-expressing support cells were present in segments 3 to 13 of the female antenna, with increasing numbers towards the distal end. In male mosquitoes, total numbers of AgOR- and AgOBP1-expressing cells were much lower. While AgOR2-expressing cells were found on both terminal flagellomeres, AgOR1 cells were restricted to the most distal segment. High densities of AgOBP1-expressing cells were identified in segment 13, whereas segment 12 comprised very few. Altogether, the results demonstrate that both sexes express the two olfactory receptor types as well as the binding protein AgOBP1 but there is a significant sexual dimorphism concerning the number and distribution of these cells. This may suggest gender-specific differences in the ability to detect distinct odorants, specifically human host-derived volatiles.

Transfer vectors for maximal expression of passenger genes in the Bombyx mori nuclear polyhedrosis virus expression system

A series of Bombyx mori nuclear polyhedrosis virus (Bm-NPV) transfer vectors has been developed containing various lengths of the polyhedrin promoter, including sequences 3' of the initiation codon. The ATG initiation codon was mutated in some of these vectors to allow for the production of authentic nonfusion proteins. The ability of the various polyhedrin promoter constructs to direct expression of foreign gene sequences was assessed using two test genes, chloramphenicol acetyl transferase (cat), and human metallothionein II. Accumulation of cat mRNA and nonfused protein was low when only polyhedrin promoter sequences to -8 (relative to the translational start site of polyhedrin mRNA) were included in the transfer vector, but cat expression was comparable with that of the wild-type polyhedrin gene when promoter sequences to +5 were present. Further addition of polyhedrin gene sequences to +26 or +94 resulted in no further increase in expression. Similar results were obtained for expression of human metallothionein II, where constructs encoding polyhedrin-metallothionein fusion proteins containing polyhedrin sequences to at least +5 resulted in high levels of mRNA and protein accumulation. The expression vectors containing the +5, +26, or +94 BmNPV polyhedrin promoter can thus be used to direct maximal levels of production of nonfused proteins (when the polyedrin ATG has been mutated) or of fusion proteins, depending on which is more suitable for a particular application. These new vectors are a useful addition to those presently available and should increase the utility of the BmNPV expression system for large-scale protein production.

Assessment of species specificity of moulting accelerating compounds in Lepidoptera: comparison of activity between Bombyx mori and Spodoptera littoralis by in vitro reporter and in vivo toxicity assays

BACKGROUND

Dibenzoylhydrazine analogues have been developed successfully as a new group of insect growth regulators, called ecdysone agonists or moulting accelerating compounds. A notable feature is their high activity against lepidopteran insects, raising the question as to whether species-specific analogues can be isolated. In this study, the specificity of ecdysone agonists was addressed through a comparative analysis in two important lepidopterans, the silkworm Bombyx mori L. and the cotton leafworm Spodoptera littoralis (Boisd.).

RESULTS

When collections of non-steroidal ecdysone agonists containing different mother structures (dibenzoylhydrazine, acylaminoketone, tetrahydroquinoline) were tested, in vitro reporter assays showed minor differences using cell lines derived from both species. However, when compounds with high ecdysone agonist activity were examined in toxicity assays, larvicidal activity differed considerably. Of note was the identification of three dibenzoylhydrazine analogues with > 100-fold higher activity against Bombyx than against Spodoptera larvae.

CONCLUSION

The present study demonstrated that species-specific ecdysone-agonist-based insecticides can be developed, but their species specificity is not based on differences in the activation of the ecdysone receptor but rather on unidentified in vivo parameters such as permeability of the cuticle, uptake/excretion by the gut or metabolic detoxification.

BmCAP, a silkmoth gene encoding multiple protein isoforms characterized by SoHo and SH3 domains: expression analysis during ovarian follicular development

CAP/ArgBP2/vinexin family proteins, adaptor proteins characterized by three SH3 domains at their C-termini and a SoHo domain towards their N-termini, are known to regulate cell adhesion, cytoskeletal organization, and growth factor signaling. Here we present the isolation and ovarian expression of the BmCAP gene which encodes CAP/ArgBP2/vinexin family proteins in the silkmoth, Bombyx mori. Screening for full-length cDNA clones identified three mRNA isoforms, BmCAP-A1, BmCAP-A2 and BmCAP-B, which show expression throughout ovarian follicular development. Using an antibody raised against a unique region between the SoHo and SH3 domains, BmCAP-A protein isoforms were identified that show specific expression in different compartments of the ovarian follicles. Immunofluorescence staining of the cells of the follicular epithelium establishes a dynamic pattern of BmCAP-A protein localization during choriogenesis. During early choriogenesis, BmCAP-A has a diffuse localization in the cytoplasm but could also be found concentrated at the apical and basal sides at the cell-cell junctions. During late choriogenesis, the diffuse cytoplasmic staining of BmCAP-A disappears while the staining pattern at the apical side resembles a blueprint for the eggshell surface structure. We suggest that BmCAP-A isoforms have important functions during ovarian development, which involve not only the traditional roles in actin organization or cell-cell adhesion but also the regulation of secretion of chorion proteins and the sculpting of the chorion surface.

Ecdysteroid signaling in ecdysteroid-resistant cell lines from the polyphagous noctuid pest Spodoptera exigua

Although dibenzoylhydrazine-type non-steroidal ecdysone agonists such as methoxyfenozide (RH-2485) have an excellent performance record, the emergence of resistance could severely compromise the efficacy of these compounds in integrated pest management programs. To investigate possible mechanisms of resistance, cell lines derived from the polyphagous noctuid pest Spodoptera exigua (Se4 cells) were selected for continuous growth in the presence of high concentrations of 20-hydroxyecdysone (20E) or methoxyfenozide. Here we describe an analysis of ecdysteroid receptor signaling in the ecdysteroid-resistant Se4 cell lines. In contrast to other ecdysteroid-resistant cell lines described in literature, our data support the existence of a normal functioning ecdysteroid receptor complex in the resistant Se4 cell lines: (1) using a recombinant BmNPV baculovirus as a transduction tool, activation of an ecdysone-responsive luciferase cassette was demonstrated; (2) the early gene HR3 is constitutively expressed in the resistant cell lines that are grown in the presence of 20E or methoxyfenozide. Quantitative RT-PCR experiments indicated that expression levels of SeEcR mRNA were comparable among sensitive and resistant cell lines. Sequencing of PCR fragments also revealed the presence of SeEcR mRNA with a wild-type ligand-binding domain in resistant cells. Finally, a possible role for the gene FTZ-F1, whose expression correlates with the absence of circulating ecdysteroids during insect development, in the resistance mechanism was investigated. However, it was observed that FTZ-F1, in contrast to what is observed during insect development, is constitutively expressed in Se4 cells and that its expression is not regulated by the addition of ecdysteroid. It is proposed that the resistance mechanism in Se4 cells resides at the coupling between the conserved hierarchical cascade of early and early-late gene expression and the differentiation program in the Se4 cell line. The use of insect cell lines for the investigation of resistance against dibenzoylhydrazine ecdysone agonists and their relevance for uncovering resistance mechanisms in insects during pest control programs is discussed.

Percutaneous bronchial artery embolization in the management of massive hemoptysis in chronic lung diseases. Immediate and long-term outcomes

AIM

Bronchial artery embolization (BAE) is a well-established, non-surgical procedure in the emergency treatment of massive hemoptysis. This study aims to evaluate the immediate and long-term prognosis of BAE for the management of massive hemoptysis in our center.

METHODS

Twenty consecutive patients (mean age: 59+/-14 years) with massive hemoptysis, underwent BAE with microspheres (Embospheres BioSphere Medical SA, Paris, France), polyvinyl alcohol particles (PVA, Ivalon, Cathmed Science; Paris, France) or/and steel coils (Cook, Denmark) after thoracic aortography and diagnostic selective and superselective catheterization of bronchial arteries and systemic collateral vessels in the bleeding lung area. Hemoptysis was due to bronchiectasis (55%), non-operable aspergillomas (15%), active tuberculosis (15%), malignancy (10%) and cystic fibrosis (5%). Mean duration of follow-up was 29+/-18 months. The recurrent-free time was calculated with Kaplan-Meier analysis.

RESULTS

Immediate control of bleeding was achieved in all patients. Recurrent cases of hemoptysis were observed in 6/20 patients (30%) within 3 years and 4 of them (66.6%) occurred early in the first 3 months. Recurrent-free time was 9 months (standard error: 4) (95% confidence interval: 0-17). Repeated interventions were required in all early recurrences, due to either recanalization of the occluded arteries or non-bronchial systemic artery supply. Combined use of PVA and coils was proved effective in these cases. No serious complications were observed.

CONCLUSION

BAE is an effective and safe intervention in cases of massive hemoptysis. However, recurrences are common and long-term follow-up is considered important with a view to perform repeated interventions with combination of embolic materials.

Sex-biased expression of odorant receptors in antennae and palps of the African malaria vector Anopheles gambiae

At the heart of the odor recognition process in all animals are G-protein-coupled receptors, which are seven-transmembrane domain proteins that initiate G-protein-mediated signaling cascades when activated by their ligands. Odorant receptors (ORs) are a large, diverse family of proteins with some 80 members in the mosquito Anopheles gambiae. With the assumption that more sensilla on female antennae are tuned to human odors than on male antennae, comparison of specific OR mRNA levels in male and female antennae can provide an indication as to which receptors may be stimulated by host odors. We have used RT PCR and quantitative real-time PCR (qRT PCR) to investigate sex-biased expression levels of 80 A. gambiae ORs in male and female antennae and maxillary palps. On the basis of prevalence of expression in female antennae and on a strong female relative to male expression bias we identified a short list of ORs that are likely involved in host odor recognition by female mosquitoes.

Juvenile hormone analogs do not affect directly the activity of the ecdysteroid receptor complex in insect culture cell lines

During insect development, ecdysteroids and juvenile hormones (JHs) interact to regulate larval growth, metamorphosis and reproduction but the molecular mechanisms by which both hormones influence each other's activity remain unknown. Because of their ease of use and straightforward genetic manipulation, insect cell lines often have been used to clarify the actions and interactions of hormones at the molecular level. Here we report on the use of two insect culture cell lines, Drosophila melanogaster S2 and Bombyx mori Bm5 cells, to investigate two molecular processes in which ecdysteroids and JH have been shown to interact: (1) direct modulation of the activity of the ecdysteroid receptor transcription complex and (2) interference at the level of induction of the primary gene E75. Our data do not support JH analogs (JHAs) acting through the above processes: 'antagonism' of ecdysteroid receptor activity by JHAs correlated with cytotoxicity and induction of E75 expression by JHAs was not demonstrated. However, we confirm previous studies in which it was observed that methoprene can partially reverse the growth inhibition by 20E in S2 cells (but not Bm5 cells). Therefore, the molecular mechanism by which both hormones influence each other's activity to regulate cell growth in S2 cells remains unknown.

Prostaglandin signaling and ovarian follicle development in the silkmoth, Bombyx mori

Previous work on in vitro culturing of silkmoth (Bombyx mori) ovarian follicles has shown that starting from middle vitellogenesis, follicles develop according to an endogenous developmental program that does not require the presence of extra-ovarian factors. In this paper, we are reporting on our investigation for a possible involvement of autocrine/paracrine signaling by prostaglandins in the control of silkmoth ovarian follicle development. Using an initial rapid test that evaluates the formation of a protective eggshell around the oocyte, we are showing that aspirin and indomethacin, potent inhibitors of prostaglandin biosynthesis, block the transition of cultured vitellogenic follicles into choriogenesis. More detailed studies involving analyses of temporal expression patterns of genes known to be expressed in follicular epithelium cells at specific stages of ovarian development revealed that inhibition of prostaglandin biosynthesis arrests stages of follicle development from middle vitellogenesis to late choriogenesis. The arrest could be reversed by the addition of exogenous prostaglandins or cAMP into the culture media leading to the conclusion that the production of prostaglandins triggers cAMP-mediated intracellular signaling that allows the developmental progression of the follicles. Finally, because neither prostaglandins nor cAMP is capable of rescuing a developmental block effected at mid-vitellogenesis by the ecdysone agonist tebufenozide, we are proposing that prostaglandins have a role in the maintenance of normal physiological homeostasis in the ovarian follicles rather than a more specific role in developmental decision-making at distinct stages of follicle development.

Specific interactions among odorant-binding proteins of the African malaria vector Anopheles gambiae

In this report we present results from a comprehensive study undertaken toward the identification of proteins interacting with odourant-binding proteins (OBPs) of the African malaria vector Anopheles gambiae with a focus on the interactions among different OBPs. From an initial screen for proteins that interact with a member of the Plus-C group of OBPs, OBP48, which is primarily expressed in female antennae and downregulated after a blood meal, a number of interacting proteins were identified, which included five classic OBPs and OBP48 itself. The interacting OBPs as well as a number of other classic and Plus-C group OBPs that were not identified in the initial screen, were expressed in lepidopteran cells and subsequently examined for in vitro interactions in the absence of exogenously added ligands. Co-immunoprecipitation and chemical cross-linking studies suggest that OBP48 is capable of homodimerizing, heterodimerizing and forming higher order complexes with those examined examples of classical OBPs identified in the initial screen but not with other classical or Plus-C group OBPs that failed to appear in the screen. The latter OBPs are, however, also capable of forming homodimers in vitro and, at least in the case of two examined classic OBPs, heterodimers as well. These results suggest a previously unsuspected potential of nonrandom combinatorial complexity that may be crucial for odour discrimination by the mosquito.

Baculovirus-mediated gene delivery into Mammalian cells does not alter their transcriptional and differentiating potential but is accompanied by early viral gene expression

Gene delivery to neural cells is central to the development of transplantation therapies for neurological diseases. In this study, we used a baculovirus derived from the domesticated silk moth, Bombyx mori, as vector for transducing a human cell line (HEK293) and primary cultures of rat Schwann cells. Under optimal conditions of infection with a recombinant baculovirus containing the reporter green fluorescent protein gene under mammalian promoter control, the infected cells express the transgene with high efficiency. Toxicity assays and transcriptome analyses suggest that baculovirus infection is not cytotoxic and does not induce differential transcriptional responses in HEK293 cells. Infected Schwann cells retain their characteristic morphological and molecular phenotype as determined by immunocytochemistry for the marker proteins S-100, glial fibrillary acidic protein, and p75 nerve growth factor receptor. Moreover, baculovirus-infected Schwann cells are capable of differentiating in vitro and express the P0 myelination marker. However, transcripts for several immediate-early viral genes also accumulate in readily detectable levels in the transduced cells. This transcriptional activity raises concerns regarding the long-term safety of baculovirus vectors for gene therapy applications. Potential approaches for overcoming the identified problem are discussed.

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.

Transformed lepidopteran cells expressing a protein of the silkmoth fat body display enhanced susceptibility to baculovirus infection and produce high titers of budded virus in serum-free media

Baculovirus vectors constitute important tools for therapeutic protein production and mammalian cell transduction for gene therapy applications. A prerequisite for such applications is that the cell lines in which baculoviruses are propagated be maintained in serum-free media that are devoid of potential human pathogens. However, in serum-free media, the performance of baculovirus-based systems can be significantly reduced. In this report, we show that silkmoth-derived host cell lines for the Bombyx mori-nuclear polyhedrosis virus (BmNPV) that are transformed with the gene for the promoting protein (PP), a silkmoth-derived secreted factor containing a lipid-binding domain, display enhanced susceptibility to BmNPV infection and enhanced budded virus productivity in serum-free media. For transformed silkmoth cells maintained in serum-free media, the rate of BmNPV entry is enhanced by two orders of magnitude relative to the untransformed cells, while the rate of budded virus production is increased five-fold. The infectivity-enhancing effect can be also conferred to normal cells grown in serum-free media by addition of conditioned media from the transformed cells, which contain the secreted recombinant PP. Thus, PP substitutes for serum factors whose presence facilitates baculovirus entry into the cells. However, the effects of silkmoth-derived PP may be specific to the BmNPV-silkmoth system since little or no changes in viral infectivity are obtained by PP expression in Trichoplusia ni-derived High-Fivetrade mark cells grown in serum-free media and infected with a different baculovirus (AcNPV).

High-throughput screening of ecdysone agonists using a reporter gene assay followed by 3-D QSAR analysis of the molting hormonal activity

In this study, 172 diacylhydrazine analogs were examined for their ability to activate an ecdysone (molting hormone)-dependent reporter gene in a silkworm (Bombyx mori) cell-based high-throughput screening assay. The measured EC(50) values (concentration required to cause an effect in 50% of the cells) were used to construct a 3-D QSAR model that describes the ecdysone agonist activities of the diacylhydrazine analogs. Of these compounds, 14 exhibited no activity and were excluded from the 3-D QSAR analysis. The resulting equation described approximately 74% of the activity for 158 compounds. The final equation consisted of 42% electrostatic and 58% steric effects (r(2) = 0.74 and q(2) = 0.45). Comparative molecular field analysis (CoMFA) was used to visualize the steric and electrostatic potential fields that were favorable and unfavorable for biological activity. Of particular interest was the observation that the hydrophobic parameter (logP) was not necessary for describing the observed activities, although previous studies have cited the importance of hydrophobic parameters in both classical and 3-D QSAR analyses of these compounds. Modeling studies of the B. mori ecdysone receptor supported the observed physicochemical parameters required for activity reported by the CoMFA models. Comparison of the present analysis with those performed using other lepidopteran assay systems evidenced a high degree of correlation (r(2) = 0.81 for a Sf-9 cell-based assay and r(2) = 0.89 for a Chilo suppressalis integument-based assay), indicating that it is valid to compare the results generated with the B. mori cell-based system to those generated with previous lepidopteran assays. This novel assay system is amendable to a high-throughput screening format and should greatly increase our ability to discover novel agonists of molting hormone (ecdysone) activity.

A virus essential for insect host-parasite interactions encodes cystatins

Cotesia congregata is a parasitoid wasp that injects its eggs in the host caterpillar Manduca sexta. In this host-parasite interaction, successful parasitism is ensured by a third partner: a bracovirus. The relationship between parasitic wasps and bracoviruses constitutes one of the few known mutualisms between viruses and eukaryotes. The C. congregata bracovirus (CcBV) is injected at the same time as the wasp eggs in the host hemolymph. Expression of viral genes alters the caterpillar's immune defense responses and developmental program, resulting in the creation of a favorable environment for the survival and emergence of adult parasitoid wasps. Here, we describe the characterization of a CcBV multigene family which is highly expressed during parasitism and which encodes three proteins with homology to members of the cystatin superfamily. Cystatins are tightly binding, reversible inhibitors of cysteine proteases. Other cysteine protease inhibitors have been described for lepidopteran viruses; however, this is the first description of the presence of cystatins in a viral genome. The expression and purification of a recombinant form of one of the CcBV cystatins, cystatin 1, revealed that this viral cystatin is functional having potent inhibitory activity towards the cysteine proteases papain, human cathepsins L and B and Sarcophaga cathepsin B in assays in vitro. CcBV cystatins are, therefore, likely to play a role in host caterpillar physiological deregulation by inhibiting host target proteases in the course of the host-parasite interaction.

Functional expression of mammalian opioid receptors in insect cells and high-throughput screening platforms for receptor ligand mimetics

Lepidopteran cell lines have been engineered to constitutively express high levels of mouse delta opioid receptors either alone or in combination with human Galpha16 protein. Biochemical and pharmacological studies demonstrate that these lines contain all the mediator G proteins and downstream effectors required for opioid receptor function, including phospholipase C, and that expression of exogenous Galpha16 does not contribute significantly to increased receptor responses upon activation. The activation of the phospholipase C pathway in the transformed cells upon stimulation with known receptor ligands results in easily and quantitatively measurable increases in free intracellular calcium, which can be monitored by automated fluorescent methods, while the addition of specific antagonists blocks the agonist-induced responses. Therefore, the transformed lepidopteran cell lines can be used as sensitive high-throughput screening platforms for fast detection of delta opioid receptor ligand mimetics (agonists and antagonists) in collections of natural products and synthetic compounds.

Expression of a Toxoneuron nigriceps polydnavirus-encoded protein causes apoptosis-like programmed cell death in lepidopteran insect cells

The polydnavirus Toxoneuron nigriceps bracovirus (TnBV) is an obligate symbiont associated with the braconid wasp T. nigriceps, a parasitoid of Heliothis virescens larvae. Previously, to identify polydnavirus genes that allow parasitization by altering the host immune and endocrine systems, expression patterns of TnBV genes from parasitized H. virescens larvae were analysed and cDNAs were obtained. To study the function of the protein from one such cDNA, TnBV1, overexpression of the protein was attempted by using the baculovirus Autographa californica multicapsid nucleopolyhedrovirus. Recovery of stable recombinant virus was unsuccessful, with the exception of recombinants with deletions/mutations within the TnBV1 gene. It was hypothesized that TnBV1 expression was cytotoxic to the Spodoptera frugiperda (Sf21) insect cells that were used to produce the recombinants. Therefore, the Bac-to-Bac system was used to create recombinant baculoviruses maintained in Escherichia coli expressing either TnBV1 (Ac-TnBV1) or an initiator-methionine mutant [Ac-TnBV1(ATG−)]. Microscopy revealed substantial cell death of Sf21 and High Five cells from 48 h post-infection with Ac-TnBV1, but not with the Ac-TnBV1(ATG−) recombinant virus. Ac-TnBV1-infected Sf21 cells, but not those with parental virus infection, showed an increased caspase-3-like protease activity, as well as increased terminal deoxynucleotidyltransferase-mediated dUTP nick-end labelling (TUNEL) for breaks in host genomic DNA. Although indicative of apoptosis, blebbing and apoptotic bodies were not observed in infected cells. Transiently expressing TnBV1 alone caused TUNEL staining in High Five cells. These data suggest that TnBV1 expression alone can induce apoptosis-like programmed cell death in two insect cell lines. Injection of Ac-TnBV1 budded virus, compared with parental virus, did not result in an alteration of virulence in H. virescens larvae.

Transfected insect cells in suspension culture rapidly yield moderate quantities of recombinant proteins in protein-free culture medium

Methodology to rapidly express milligram quantities of recombinant proteins through the Lipofectin-mediated transfection of insect cells in small-scale, protein-free suspension culture is presented. The transfection phase in suspension culture was first optimized using the green fluorescence protein coupled with FACs analysis to examine the effect of variables such as the transfection media, duration, and cell density on transfection efficiency and expression level. The recombinant protein production phase was optimized using secreted alkaline phosphatase (SEAP) as a reporter protein to evaluate the cell seeding density and harvest time. Using this method, 5 secreted, 2 intracellular, and 1 chimeric protein were expressed at levels ranging from 6 to 50 mg/L. Furthermore, the ability to purify over 2 mg of His(6)-tagged SEAP by immobilized metal affinity chromatography from 50 mL insect cell culture medium to greater than 95% purity was also demonstrated. This method is suitable for scale-up and high-throughput applications.

The ecdysone regulatory cascade and ovarian development in lepidopteran insects: insights from the silkmoth paradigm

The developing ovariole of the silkmoth Bombyx mori represents an excellent model for studies on the changes that occur in gene expression during the execution of long-term developmental programs. All stages of follicle development, which differ from each other by 2-2.5 h of developmental time, are organized in a single array and can be isolated simultaneously for physiological, biochemical and gene expression studies from a single animal. Recently, significant progress has been made toward the understanding of the hormonal regulation and autonomous implementation of the developmental program that governs follicular cell differentiation during oogenesis in Bombyx. In this review, the developmental career of the ovarian follicle during pharate adult development is discussed in view of the new physiological, biochemical and gene expression data that have recently accumulated. An overview is presented of the changes in expression of structural and regulatory genes and their hormonal regulation in the developing follicle during the transitions among the broad developmental periods of previtellogenesis, vitellogenesis and choriogenesis. Ovarian development in silkmoth pharate adults is induced by 20-hydroxy-ecdysone (20E) through the activation of a regulatory cascade similar to the one observed during Drosophila development. The transition from previtellogenesis to vitellogenesis corresponds to a late response to the hormone and is characterized by the induction of the expression of a unique isoform of the nuclear receptor BmHR3 and the follicular cell-specific yolk protein ESP. The transition from early to middle and late vitellogenesis and choriogenesis, on the other hand, is regulated by positively and negatively acting intra- and extra-ovarian factors. In vitro cultures of developing ovarioles reveal the requirement for the presence of an as yet unidentified growth factor(s) in the hemolymph, while the follicle developmental arrest that is observed after treatment with the ecdysone agonist tebufenozide indicates the requirement for a decline in the level of 20E. The initiation of choriogenesis is characterized by the transcriptional activation of the gene BmGATAbeta that encodes GATA transcription factors, and the chorion genes in the follicular cells. Furthermore, modulation of the activity of BmGATAbeta at the posttranscriptional level is crucial for the stage-specific activation of chorion genes during late choriogenesis. The developing ovariole of the silkmoth is, therefore, emerging as an important model for the study of insect oogenesis and the action of the steroid hormone 20E at the molecular level.

A cell‐based high‐throughput screening system for detecting ecdysteroid agonists and antagonists in plant extracts and libraries of synthetic compounds

Screening systems for ecdysteroid mimetic or antiecdysteroid substances in plant extracts or libraries of synthetic compounds are commonly based on the observation of morphological and/or growth responses in insect cell lines. Because these responses are slow and require careful monitoring, existing screening systems are considered limited regarding their applicability to analysis in high-throughput (HT) formats. Here we describe the generation of transformed silkmoth (Bombyx mori) cell lines that respond to the addition of ecdysone-like substances through the expression of the green fluorescent protein (GFP) and the appearance of green fluorescence. Because tests consist of three simple steps, i.e., 1) distribution of transformed cells in microtiter plates; 2) addition of compounds/extracts at different concentrations; and 3) quantification of fluorescence intensity by a fluorescence plate reader, they can be performed quickly and be easily adapted to a HT format. The generated reporter cell lines are used for the screening of extracts from available plant collections for the presence of compounds with ecdysone mimetic or antagonistic activities as well as for monitoring subsequent activity during enrichment and purification steps. The same cell lines are also used here for the determination of structure-activity relationships among available synthetic dibenzoylhydrazine derivatives. Finally, for the identified agonists, we show that their activity as determined by the cell-based screening assays parallels their bioactivity in growth inhibition and toxicity assays carried out on live insects.

The orphan nuclear receptors BmE75A and BmE75C of the silkmoth Bombyx mori: hornmonal control and ovarian expression

The steroid hormone 20-hydroxyecdysone (20E) plays a key role in the stimulation of ovarian follicle development in the silkmoth, Bombyx mori. To understand better the mechanism by which 20E regulates silkmoth oogenesis, Bombyx homologs of the ecdysone-inducible orphan nuclear receptor E75 (BmE75) were cloned and their expression was analyzed in developing ovaries and staged follicles during metamorphosis. Of the two BmE75 isoforms isolated, only the A-isoform (BmE75A) has been identified previously in lepidopteran insects. BmE75C, on the other hand, shows significant sequence homology in its N-terminus to the Drosophila E75C isoform. Northern blot analysis shows unique expression patterns for each isoform mRNA during ovarian development. While the A-isoform seems to be mainly implicated in the earlier stages of the ecdysone response during previtellogenesis and vitellogenesis, expression of the C-isoform becomes strongly induced in an ecdysteroid-independent fashion at the transition from vitellogenesis to choriogenesis. Our data indicate a complex regulation of the expression of the BmE75 gene during oogenesis and postulate a new role for the BmE75C receptor at the end of vitellogenesis and the beginning of choriogenesis.

The BmE75 nuclear receptors function as dominant repressors of the nuclear receptor BmHR3A

The orphan nuclear receptors BmE75 and BmHR3 are induced by 20-hydroxyecdysone in the ovary of the silk moth Bombyx mori at the beginning of pupation and show stage-specific expression in ovarian follicles during pharate adult development. To analyze the function of these receptors, we have developed a transactivation assay based on the transcriptional stimulation of a retinoic acid receptor-related receptor response element (RORE)-linked promoter-reporter construct. Co-transfection of a Bombyx cell line with a BmHR3A expression construct results in constitutive activation of the reporter, whereas expression of BmE75 has no measurable effects on reporter expression. However, when the BmE75 receptors are co-introduced with BmHR3A into the cells, the BmHR3A-mediated transactivation is repressed. Repression of BmHR3A by BmE75 occurs by two distinct mechanisms. Increasing doses of BmE75 efficiently displace BmHR3A bound to the RORE target site in gel retardation assays, indicating that both receptors compete for common DNA target sites. However, analysis of the function of deletion mutants of BmE75 in the transactivation assay indicates that repression can also occur in the absence of the DNA-binding domain and that the C-terminal F domain is sufficient for repression. In gel retardation assays, the two receptor types form a ternary complex on a single RORE, suggesting that repression is also mediated by protein interactions on the DNA target site. Yeast two-hybrid assays show that BmHR3A interacts with BmE75 and that this interaction is dependent on the C terminus of BmHR3A and the F domain of BmE75. Because the C terminus of BmHR3A contains a strong activation domain, we predict that BmE75 blocks activation by BmHR3A through competition for co-activator binding sites located at the C terminus of BmHR3A. Our data also indicate that the transcriptional activities of BmHR3A and BmE75 are integrated in such a way that activation of RORE-linked target genes depends on the relative expression levels of the two receptor types.