The acquisition of competence to respond to ecdysone in Drosophila is transcript specific

Ecdysone response elements in the distal promoter of the Bombyx Broad-Complex gene, BmBR-C

The Bombyx mori silkworm's homologue of the Broad-Complex gene (BmBR-C) is transcribed from two promoters: a distal promoter (Pdist) and a proximal promoter (Pprox). As determined by a luciferase assay, the transcriptional activity of Pdist, but not Pprox, was activated by ecdysone. Further analyses using reporters driven by sequential deletion Pdist mutants indicated that two regions, ecdysone responsive element (EcRE)-D and EcRE-P, -4950 bp and -3480 bp upstream from the distal transcription start site, respectively, were important in the responsiveness of Pdist to 20-hydroxyecdysone (20E); however, no significant sequence similarities were found between the canonical EcRE and the EcRE-D or EcRE-P regions. Electrophoretic mobility shift assays showed that both the EcRE-D and -P sequences specifically bound to Bombyx protein(s). Sequence analyses and competition assays suggested that the protein(s) bound to EcRE-P might include components other than the ecdysone receptor (EcR), suggesting that BmBR-C transcription was indirectly activated by ecdysone through the EcRE-P. Remarkably, protein binding to the mid-region of the EcRE-D, EcRE-Db, was competitively inhibited by an oligonucleotide containing the Drosophila hsp27 EcRE sequence. Furthermore, an anti-EcR antibody interfered with the formation of the protein-EcRE-Db complex. These results indicated that a functional Bombyx ecdysone receptor binds to EcRE-D and activates the expression of BmBR-C.

Activation of BMWCP10 promoter and regulation by BR-C Z2 in wing disc of Bombyx mori

The cuticle protein gene BMWCP10 is transcriptionally upregulated by ecdysone during development. In the present study, using a transient reporter assay, the activity of various genomic segments at the 5'-flanking region of the BMWCP10 gene in driving gene expression and their involvement in ecdysone-mediated activation were assessed in the Bombyx wing disc. The promoter activity of BMWCP10 was responsive to 20-hydroxyecdysone (20E) in a dose-dependent manner, and the highest luciferase activity was observed in the presence of 2 microg/ml 20E. Furthermore, the upstream BMWCP10 promoter was activated by 20E in a stage-specific manner, and the 2.9-kb promoter contained essential elements for the temporal regulation of BMWCP10 in the Bombyx wing disc. Deletion studies revealed that the -598/-387 bp region was required for high-level transcription. In this region, a BR-C Z2 binding element was identified by electrophoretic mobility shift assay (EMSA). Site-directed mutagenesis of this element in the context of the 598-bp promoter fragment significantly decreased the reporter activity in response to ecdysone treatment. The results confirmed the role of BmBR-C Z2 in the transcription regulation of BMWCP10 and suggested the contribution of BmBR-C Z2 to BMWCP10 induction by 20E.

Drosophila Blimp-1 is a transient transcriptional repressor that controls timing of the ecdysone-induced developmental pathway

Regulatory mechanisms controlling the timing of developmental events are crucial for proper development to occur. ftz-f1 is expressed in a temporally regulated manner following pulses of ecdysteroid and this precise expression is necessary for the development of Drosophila melanogaster. To understand how insect hormone ecdysteroids regulate the timing of FTZ-F1 expression, we purified a DNA binding regulator of ftz-f1. Mass spectroscopy analysis revealed this protein to be a fly homolog of mammalian B lymphocyte-induced maturation protein 1 (Blimp-1). Drosophila Blimp-1 (dBlimp-1) is induced directly by 20-hydroxyecdysone, and its product exists during high-ecdysteroid periods and turns over rapidly. Forced expression of dBlimp-1 and RNA interference analysis indicate that dBlimp-1 acts as a repressor and controls the timing of FTZ-F1 expression. Furthermore, its prolonged expression results in delay of pupation timing. These results suggest that the transient transcriptional repressor dBlimp-1 is important for determining developmental timing in the ecdysone-induced pathway.

20-Hydroxyecdysone regulation of two isoforms of the Ets transcription factor E74 gene in programmed cell death in the silkworm anterior silk gland

Programmed cell death of larval-specific tissues in insects is under the control of 20-hydroxyecdysone (20E). The ecdysteroid-regulated early genes are conserved in the programmed cell death of anterior silk glands (ASGs) in Bombyx mori and salivary glands in Drosophila melanogaster. We identified and characterized two isoforms of the Ets transcription factor E74 gene in B. mori (BmE74). In ASGs of B. mori last instar larvae, the Bm74A mRNA level increased concomitantly with an increase in haemolymph ecdysteroid titre after gut purge. The optimal 20E concentration for stimulation of Bm74A in ASGs was 4 microM, a similar value to the peak haemolymph ecdysteroid concentration after gut purge. In contrast, BmE74B expression peaked on day 5 of the feeding period, after which it did not increase again. These findings suggest that the BmE74 isoforms play different roles in the regulation of programmed cell death in B. mori ASGs.

Farnesoid secretions of dipteran ring glands: what we do know and what we can know

Harnessing of the Drosophila genetic system toward ascertaining the molecular endocrinology of higher dipteran (cyclorrhaphan) larval development has been a goal for over 70 years, beginning with the data left to us by pioneer researchers from the classical endocrine era. The results of their experiments evidence numerous ring gland activities that are parsimoniously explained as arising from secretions of the larval corpora allatal cells. Utilization of those data toward an understanding of molecular endocrinology of cyclorrhaphan metamorphosis has not yet achieved its hoped for fruition, in part due to a perceived difficulty in identifying larval targets of the molecule "methyl epoxyfarnesoate" (=juvenile hormone III). However, as is reviewed here, it is important to maintain a conceptual distinction between "the target of JH III"Versus "the target(s) of products secreted by the larval corpora allatal cells of ring glands." Recent advances have been made on the identity, regulation and reception of ring gland farnesoid products. When these advances are evaluated together with the above data from the classical endocrine era, there is a new opportunity to frame experimental hypotheses so as to discern underlying mechanisms on cyclorrhaphan larval-pupal metamorphosis that have been heretofore intractable. This paper reconsiders a number of evidenced physiological targets of secretions of corpora allatal cells of the larval ring gland, and places them in the context of more recent biochemical and molecular advances in the field.

Specific transcriptional responses to juvenile hormone and ecdysone in Drosophila

Previous studies have shown that ecdysone (E), and its immediate downstream product 20-hydroxyecdysone (20E), can have different biological functions in insects, suggesting that E acts as a distinct hormone. Here, we use Drosophila larval organ culture in combination with microarray technology to identify genes that are transcriptionally regulated by E, but which show little or no response to 20E. These genes are coordinately expressed for a brief temporal interval at the onset of metamorphosis, suggesting that E acts together with 20E to direct puparium formation. We also show that E74B, pepck, and CG14949 can be induced by juvenile hormone III (JH III) in organ culture, and that CG14949 can be induced by JH independently of protein synthesis. In contrast, E74A and E75A show no response to JH in this system. These studies demonstrate that larval organ culture can be used to identify Drosophila genes that are regulated by hormones other than 20E, and provide a basis for studying crosstalk between multiple hormone signaling pathways.

The genomic response to 20-hydroxyecdysone at the onset of Drosophila metamorphosis

The genome-wide transcriptional response to 20-hydroxyecdisone at the onset of Drosophila metamorphosis, as well as its dependency on one of the ecdysone receptors is described.

Background

The steroid hormone 20-hydroxyecdysone (20E) triggers the major developmental transitions in Drosophila, including molting and metamorphosis, and provides a model system for defining the developmental and molecular mechanisms of steroid signaling. 20E acts via a heterodimer of two nuclear receptors, the ecdysone receptor (EcR) and Ultraspiracle, to directly regulate target gene transcription.

Results

Here we identify the genomic transcriptional response to 20E as well as those genes that are dependent on EcR for their proper regulation. We show that genes regulated by 20E, and dependent on EcR, account for many transcripts that are significantly up- or downregulated at puparium formation. We provide evidence that 20E and EcR participate in the regulation of genes involved in metabolism, stress, and immunity at the onset of metamorphosis. We also present an initial characterization of a 20E primary-response regulatory gene identified in this study, brain tumor (brat), showing that brat mutations lead to defects during metamorphosis and changes in the expression of key 20E-regulated genes.

Conclusion

This study provides a genome-wide basis for understanding how 20E and its receptor control metamorphosis, as well as a foundation for functional genomic analysis of key regulatory genes in the 20E signaling pathway during insect development.

Expression of nuclear receptor-transcription factor genes during Aedes aegypti midgut metamorphosis and the effect of methoprene on expression

Exposure of mosquito 4th instars to the juvenile hormone analogue methoprene prevents the emergence of adults by interfering with metamorphosis. One metamorphic processes that is disrupted is midgut remodeling. To investigate the molecular mechanisms by which this occurs, the pattern of transcription factor gene expression during the Aedes aegypti (L.) 4th instar was investigated by the method of real time PCR. The results indicate that in untreated larvae, expression of transcription factors genes AHR3 and AaE75B increases within 24h after the last larval-larval molt, transcription of AaEcR-B, AaUSP-a and AassFTZ-F1 increases approximately 24h later, and transcription of AaE75A increases just before the larval-pupal molt. There is uniform expression of AaUSP-b throughout the 4th instar. The effect of methoprene exposure on transcription factor gene expression during midgut remodeling was investigated. The results indicate that, in a dose and stage dependent manner, methoprene affects increases in expression that normally occur during midgut remodeling. The coincident effects of methoprene on metamorphic midgut remodeling and on transcription factor gene expression suggests that the two processes are related.

E74 exhibits stage-specific hormonal regulation in the epidermis of the tobacco hornworm, manduca sexta

The transcription factor E74 is one of the early genes induced by ecdysteroids during metamorphosis of Drosophila melanogaster. Here, we report the cloning and hormonal regulation of E74 from the tobacco hornworm, Manduca sexta (MsE74). MsE74 is 98% identical to that of D. melanogaster within the DNA-binding ETS domain of the protein. The 5'-isoform-specific regions of MsE74A and MsE74B share significantly lower sequence similarity (30-40%). Developmental expression by Northern blot analysis reveals that, during the 5th larval instar, MsE74B expression correlates with pupal commitment on day 3 and is induced to maximal levels within 12h by low levels of 20-hydroxyecdysone (20E) and repressed by physiologically relevant levels of juvenile hormone I (JH I). Immunocytochemical analysis shows that MsE74B appears in the epidermis before the 20E-induced Broad transcription factor that is correlated with pupal commitment (Zhou and Riddiford, 2001). In contrast, MsE74A is expressed late in the larval and the pupal molts when the ecdysteroid titer has declined to low levels and in the adult molt just as the ecdysteroid titer begins to decline. This change in timing during the adult molt appears not to be due to the absence of JH as there was no change during the pupal molt of allatectomized animals. When either 4th or 5th instar larval epidermis was explanted and subjected to hormonal manipulations, MsE74A induction occurred only after exposure to 20E followed by its removal. Thus, MsE74B appears to have a similar role at the onset of metamorphosis in Manduca as it does in Drosophila, whereas MsE74A is regulated differently at pupation in Manduca than at pupariation in Drosophila.

Activation of transcription through the ligand-binding pocket of the orphan nuclear receptor ultraspiracle

The invertebrate nuclear receptor, ultraspiracle (USP), an ortholog of the vertebrate RXR, is typically modelled as an orphan receptor that functions without a ligand-binding activity. The identification of a ligand that can transcriptionally activate USP would provide heuristic leads to the structure of potentially high affinity activating compounds, with which to detect unknown regulatory pathways in which this nuclear receptor participates. We show here that the application of the sesquiterpenoid methyl epoxyfarnesoate (juvenile hormone III) to Sf9 cells induces transcription from a transfected heterologous core promoter, through a 5'-placed DR12 enhancer to which the receptor ultraspiracle (USP) binds. Isolated, recombinant USP from Drosophila melanogaster specifically binds methyl epoxyfarnesoate, whereupon the receptor homodimerizes and changes tertiary conformation, including the movement of the ligand-binding domain alpha-helix 12. Ligand-binding pocket point mutants of USP that do not bind methyl epoxyfarnesoate act as dominant negative suppressors of methyl epoxyfarnesoate-activation of the reporter promoter, and addition of wild-type USP rescues this activation. These data establish a paradigm in which the USP ligand-binding pocket can productively bind ligand with a functional outcome of enhanced promoter activity, the first such demonstration for an invertebrate orphan nuclear receptor. USP thus establishes the precedent that invertebrate orphan receptors are viable targets for development of agonists and antagonists with which to discern and manipulate transcriptional pathways dependent on USP or other orphan receptors. The demonstration here of these functional capacities of USP in a transcriptional activation pathway has significant implications for current paradigms of USP action that do not include for USP a ligand-binding activity.

Isolation and characterization of novel mutations of the Broad-Complex, a key regulatory gene of ecdysone induction in Drosophila melanogaster

Seven new alleles of the Broad-Complex gene of Drosophila melanogaster, which encodes a family of four zinc finger protein isoforms BR-C Z1, Z2, Z3 and Z4, were generated by transposase-induced mobilization of a P[Zw] element inserted in either the first intron downstream from the P165 promoter or the exon encoding the Z2-specific zinc finger domain. They were characterized by genetic complementation tests, molecular mapping and cytogenetic analysis of their effect on ecdysone-induced puffing and BR-C proteins binding to polytene chromosomes. Four mutations that correspond to three overlapping deletions and one tandem insertion of the P[Zw] element are located in the intron. They provide evidence that regulatory elements essential for a correct expression of the BR-C Z2 and BR-C Z3 transcripts are located within the intron downstream from the P165 promoter. Three mutations correspond to internal deletions of the locus and exhibit a complete loss of all BR-C(+) genetic functions in the complementation and cytogenetic tests. They thus provide well characterized new amorphic reference alleles of the BR-C gene. The precise cytogenetic location of more than 300 binding sites of BR-C proteins on larval salivary gland polytene chromosomes was determined by immunostaining using specific antibodies. Sites were found in big ecdysone inducible puffs, constitutively active small puffs as well as interbands. A complete list of the major sites on all four salivary gland polytene chromosomes of BR-C(+) larvae is presented.

Krüppel-homolog, a stage-specific modulator of the prepupal ecdysone response, is essential for Drosophila metamorphosis

We have characterised a P-element-induced prepupal mutant of Drosophila melanogaster which after an apparently normal embryonic and larval development fails to complete head eversion, an essential step in metamorphosis. The P-element insertion disrupts an ecdysone-regulated transcript which, although expressed during embryonic and larval stages, appears critical for preparing the late prepupal response to ecdysone. By a combination of molecular and genetic studies, in which we recovered new alleles, we show that the locus is complex, containing at least two distinct promoters. Its transcripts contain a short region described previously by R. Schüh et al. (1986, Cell 47, 1025-1032), who screened for homologues of the Krüppel gene. Our studies on the corresponding gene, named Krüppel-homolog (Kr-h), add to a growing body of evidence that specific isoforms of a number of key genes are implicated in both embryogenesis and metamorphosis.