The Drosophila EcR gene encodes an ecdysone receptor, a new member of the steroid receptor superfamily

Activation and repression of Drosophila alcohol dehydrogenase distal transcription by two steroid hormone receptor superfamily members binding to a common response element

Developmental activation of the Drosophila alcohol dehydrogenase (Adh) distal promoter is controlled by the Adh adult enhancer (AAE). Within this 150 bp, complex enhancer is a small (12 bp) positive cis-acting element that is required for high levels of distal transcription in adult flies and ADH-expressing tissue culture cells. We previously reported that the steroid receptor superfamily member FTZ-F1 binds to this site. We have identified a second steroid receptor superfamily member, DHR39, which also binds to this site. DHR39 is expressed throughout development in transcripts of several sizes. In situ hybridization to embryos has shown that DHR39 RNA is found primarily in the central nervous system, and not in embryonic tissues that express ADH. FTZ-F1 RNA, however, shows temporal-specific patterns similar to those of the distal promoter. FTZ-F1 and DHR39 have identical amino acids in the 'P-box' of the DNA binding domain, suggesting that they have identical DNA recognition characteristics. By electrophoretic mobility shift analysis we show that a DHR39 fusion protein binds specifically to two FTZ-F1 binding sites. By over expressing the full length DHR39 protein in a transient co-transfection assay we have shown that it represses distal Adh expression in a dosage- and binding site-dependent manner. Over expression of an alternative DHR39 open reading frame that lacks part of the putative ligand binding domain does not alter Adh expression. In contrast, over expression of FTZ-F1 specifically activates distal Adh expression.

Heterodimerization of the Drosophila ecdysone receptor with retinoid X receptor and ultraspiracle

Ecdysone in Drosophila has been a paradigm for steroid hormones since its ability to induce gene activity directly was demonstrated by its effects on moulting and polytene chromosome puffing. The ecdysone receptor (EcR) was recently confirmed as a member of the nuclear receptor superfamily by cloning and characterization in a Drosophila cell line. Here we show that EcR needs to heterodimerize with either the retinoid X receptor (RXR) or its Drosophila homologue, ultraspiracle (USP), for DNA binding and transactivation. These results place the ecdysone receptor in the heterodimerizing class of the nuclear receptor superfamily and demonstrate that the role of RXR/USP as a central and promiscuous partner in mediating the activity of these receptors is highly conserved. Whereas EcR-USP DNA-binding activity is unaffected by hormone, EcR-RXR DNA-binding activity is stimulated by either ecdysteroid or 9-cis-retinoic acid, demonstrating that hormone can play a role in heterodimer stabilization.

Different TBP-associated factors are required for mediating the stimulation of transcription in vitro by the acidic transactivator GAL-VP16 and the two nonacidic activation functions of the estrogen receptor

The estrogen receptor (ER) contains two nonacidic transcriptional activation functions, AF-1 and AF-2 (formerly TAF-1 and TAF-2). In this study we show that AF-1 and AF-2 are able to stimulate transcription in vitro in a HeLa cell system when fused to the DNA binding domain of the yeast activator GAL4. We also demonstrate that a factor(s) required for the function of the ER AFs is chromatographically separable from a factor(s) necessary for the activity of the acidic activation domain of VP16. Moreover, immunoprecipitation experiments using a monoclonal antibody directed against the TATA box binding protein (TBP) indicate, that these different factors are associated with TBP in distinct TFIID complexes.

The E75 gene of Manduca sexta and comparison with its Drosophila homolog

The ecdysone-inducible E75 gene responsible for the 75B puff of Drosophila melanogaster encodes a family of proteins which are members of the steroid receptor superfamily. These proteins are believed to be involved in the regulation of ecdysone response. In order to investigate the evolutionary conservation of E75, we have identified the E75 gene of Manduca sexta. We show here the structure of a cDNA believed to encode the Manduca homolog of the E75B protein, and demonstrate that the putative DNA binding, hormone binding and amino and carboxy terminal flanking domains are conserved. However, due to a relative reduction in intron size and number and the absence of homopolymeric amino acid repeats, the E75 B transcription unit and protein are considerably smaller in M. sexta than in D. melanogaster. These findings have implications for the identification of critical structural features of E75 and also suggest that E75 has a conserved function and a shared ligand in Lepidoptera.

The reaction with polytene chromosomes of antibodies raised against Drosophila E75A protein

The steroid insect molting hormone ecdysone rapidly induces a small number of polytene chromosome puffs in Drosophila. The Ashburner model proposes that the corresponding early genes encode proteins involved in both the induction of the late genes and the repression of the early genes. The Drosophila E75 early gene has been isolated and two of its products, E75A and E75B, have been shown to be members of the steroid receptor superfamily. We have now prepared antisera directed against A- and B-specific regions of the E75 proteins. Antisera and a monoclonal antibody raised against E75A, the major larval protein product of the E75 gene, bind to discrete sites in native salivary gland chromosomes. These sites are closely correlated with early and late ecdysone responsive loci.

Enhancer and silencer elements within the first intron mediate the transcriptional regulation of the beta 3 tubulin gene by 20-hydroxyecdysone in Drosophila Kc cells

We have studied the transcriptional regulation of the beta 3 tubulin gene by the steroid hormone 20-hydroxyecdysone (20-E) in Drosophila Kc cells. A series of hybrid genes, with different fragments of the beta 3 tubulin gene driving the bacterial chloramphenicol acetyl transferase (CAT) gene were constructed. The promoter activity was assayed after transient expression in Kc cells, in the presence and the absence of 20-E. Constructs with 0.91 kb upstream from the transcription start site and 360 bp from the first large intron allowed the hormonal regulation, i.e. a repression in the absence of 20-E and a derepression-activation in the presence of the hormone. This 360 bp fragment contains several enhancers and silencer(s) sequences. The regulation of the expression of the beta 3 tubulin gene results from the combined activity of all the positive and negative regulatory sequences of the first intron, and a dialogue with the promoter sequences. The nucleotide sequence of this intronic regulatory-fragment has been established and we have identified several EcRE (ecdysone responsive element) consensus sequences.

Structural features critical to the activity of an ecdysone receptor binding site

Two ecdysone-response elements from the hsp27 (hsp27 EcRE) and the Fbp1 (D EcRE) genes of Drosophila melanogaster were used as probes in a gel shift assay to investigate the interactions of the ecdysone receptor (EcR) with its cognate DNA response element. The source of EcR was a nuclear extract from the late third-larval instar fat body. The hsp27 and D EcREs share a sequence similarity at 12 positions over a 15bp region including an imperfect palindromic structure consisting of two pentamer half-sites separated by a single intervening nucleotide. We have shown that a short oligonucleotide containing this 11bp imperfect palindrome of the hsp27 EcRE and three flanking bp on each side is an efficient EcR binding site. Mutational analysis confirms that the integrity of both these half-sites as well as their 1bp spacing are critical for binding of the ecdysone receptor. The D EcRE behaved as a much weaker EcR binding site than the hsp27 EcRE but a single bp substitution was sufficient to confer upon it a binding capacity equivalent to that of the hsp27 EcRE. These results have led us to propose the sequence PuG(G/T)T(C/G)A(N)TG(C/A)(C/A)(C/t)Py as a revised version of a previously proposed EcRE consensus sequence.

FTZ-F1 beta, a novel member of the Drosophila nuclear receptor family

The Drosophila melanogaster gene FTZ-F1 beta, encoding a novel member of the steroid/thyroid hormone receptor gene superfamily, was isolated by cross-hybridization with a complementary DNA for the Drosophila nuclear receptor, FTZ-F1 (Lavorgna et al., 1991). The cDNA deduced protein sequence for FTZ-F1 beta displays significant amino acid identity with other vertebrate and invertebrate nuclear receptors, most notably with FTZ-F1. Also, bacterially expressed FTZ-F1 beta protein binds to a FTZ-F1 binding site found in the zebra stripe promoter element of the segmentation gene fushi tarazu (ftz). Northern blot analysis detected FTZ-F1 beta expression at all stages of the Drosophila life cycle including a possible maternal component. In situ hybridization in whole-mounted embryos localized transcripts for FTZ-F1 beta evenly expressed throughout the blastodermal layer in early embryos. At later stages of development strong FTZ-F1 beta expression is observed in both the brain and ventral chord structures as well as in the hindgut. Temporal and spatial expression patterns of the FTZ-F1 beta gene suggest that it may have multiple roles in early embryogenesis, neurogenesis, and in the adult. Furthermore, the identification of FTZ-F1 beta as a nuclear receptor family member suggests that an as yet undiscovered FTZ-F1 beta specific ligand is involved in Drosophila development.

Effects of a protein synthesis inhibitor on the hormonally mediated regression and death of motoneurons in the tobacco hornworm, Manduca sexta

The larval-pupal transformation of Manduca sexta is accompanied by the loss of the abdominal prolegs. The proleg muscles degenerate, the dendritic arbors of proleg motoneurons regress, and a subset of the proleg motoneurons dies. The regression and death of proleg motoneurons are triggered by the prepupal peak of ecdysteroids in the hemolymph. To investigate the possible involvement of protein synthesis in these events, we gave insects repeated injections of the protein synthesis inhibitor, cycloheximide (CHX), during the prepupal peak. Examination of insects 3-5 days following CHX treatment showed that CHX inhibited the death of proleg motoneurons and the production of pupal cuticle in a dose-dependent fashion. When insects were allowed to survive for 10 days after the final CHX injection, motoneuron death and pupal cuticle production sometimes occurred belatedly, apparently in response to the ecdysteroid rise that normally triggers adult development. CHX treatments that inhibited motoneuron death were less effective in inhibiting dendritic regression in the same neurons. In another set of experiments, abdomens were isolated from the ecdysteroid-secreting glands prior to the prepupal peak, and infused with 20-hydroxyecdysone (20-HE). Single injections of CHX delivered just prior to the start of the 20-HE infusion inhibited motoneuron death and pupal cuticle production, but in the range of doses tested, did not prevent dendritic regression. Our findings suggest that protein synthesis is a required step in the steroid-mediated death of proleg motoneurons, and that dendritic regression is less susceptible to inhibition by CHX than is motoneuron death.

Nucleotide sequence of an adult-specific cuticular protein gene from the beetle Tenebrio molitor: effects of 20-hydroxyecdysone on mRNA accumulation

The accumulation of transcripts from two adult-specific cuticular genes (ACP-20 and ACP-22) is shown to be modified after addition of exogenous 20-hydroxyecdysone. In the continuous presence of high levels of the hormone, the expression of ACP-20 gene is significantly weaker than that of untreated controls, while ACP-22 expression is 2.5-fold increased. During active synthesis of the ACP messages, a 0.5 microg 20-hydroxyecdysone injection causes a rapid 2-fold increase in ACP-22 mRNA and is not able to repress ACP-20 mRNA accumulation. We conclude that these genes whose transcripts appear in an almost coordinated manner in epidermal cells during the moulting cycle are regulated by ecdysteroids in a different way. In order to undertake a functional dissection of the promoter regions of ACP-22 gene, we have isolated and sequenced a genomic clone. The sequence similarities with other cuticular protein genes are discussed.

Developmental requirements for the ecdysoneless (ecd) locus in Drosophila melanogaster

The ecdysoneless locus in Drosophila melanogaster has been defined previously by a single conditional mutation, I(3)ecd1, that causes an ecdysteroid deficit and larval death at the restrictive temperature, 29 degrees C, although the primary role of the mutation in developmental processes has been unclear. Gene dosage and complementation studies reported here for ecd1 and five nonconditional lethal alleles indicate that the ecd locus plays prezygotic and postzygotic roles essential for normal embryonic development, the successful completion of each larval molt, adult eclosion, and female fertility. The ecd locus is also required for normal macrochaete differentiation. For each observed phenotype, the severity of mutational effects was correlated with ecd mutant genotypes. In all cases, ecd1 homozygotes were least affected. Mutants heteroallelic for ecd1 and any one of four nonconditional recessive mutations were more severely affected than ecd1 homozygotes, revealing these as hypomorphic alleles. For all phenotypic effects, mutants heteroallelic for ecd1 and a dominant mutation (ecd3D) were most severely affected. These individuals died during embryogenesis at 29 degrees C and developed no macrochaetes on the dorsal thorax when transferred to 29 degrees C during the white prepupal stage. The ecd3D mutation also caused female semisterility in heterozygotes. Ecdysteroid regulation has been implicated previously in all the developmental processes disrupted by these ecd mutations except for macrochaete differentiation.

Cloning of a Chironomus tentans cDNA encoding a protein (cEcRH) homologous to the Drosophila melanogaster ecdysteroid receptor (dEcR)

We have cloned a cDNA sequence coding for a Chironomus tentans steroid hormone receptor homologue which exhibits extensive amino acid sequence co-linearity with the ecdysteroid receptor of Drosophila melanogaster (dEcR; cell 67, 59-77). The DNA-binding domain has 95% and the hormone-binding domain 75% amino acid sequence identity with the cloned dEcR. The gene for this C. tentans protein is located on chromosome II, region 17C, as determined by in situ hybridization to polytene chromosomes of salivary glands. On Northern blots cDNA probes of the cloned gene hybridize to polyadenylated RNA of ca 4.2 kb. The expression of the cloned gene seems to be developmentally regulated and correlates to changes in ecdysteroid titer. Transfection of this C. tentans protein into D. melanogaster Schneider's line 2 cells leads to transcriptional interference with endogenous dEcR on an ecdysteroid-regulated promoter.

The IVth Karlson Lecture: ecdysone-responsive genes

Those of us who study ecdysone action share at least two important long-range goals: (i) to understand the developmental specificity of steroid action in full molecular detail, by integrating ecdysone action with our rapidly expanding knowledge of the molecular biology of insect development, and (ii) to better understand the nature of the steroid response and its evolution by taking advantage of the unparalleled opportunities for both genetic and comparative study afforded by the diversity of the "ecdysone world". However, until recently, the molecular fundamentals of the ecdysone system were unknown and our efforts have, of necessity, been devoted to their elucidation. Now that the situation has changed: we have a small but varied catalog of ecdysone-responsive genes for study and it is clear that some of these are tissue- and stage-specific in their expression. The ecdysone receptor (EcR), like other steroid receptors a member of the nuclear receptor family, is now accessible to molecular study, and we have a preliminary understanding of the DNA sequences (EcREs) that bind receptor and specify a gene as ecdysone-responsive. With these tools in hand and with the opportunity to turn to larger questions, it is a propitious moment to consider the nature of those questions and how ecdysone can contribute to the answers.

Magnetic DNA affinity purification of ecdysteroid receptor

A new method for rapid purification to near homogeneity of the ecdysteroid receptor (EcdR) from Drosophila melanogaster nuclear extract is presented. In the first step of the purification procedure the EcdR molecules were radiolabelled with [3H]ponasterone A and the [3H]ponasterone A-EcdR complexes were chromatographed under very mild conditions on Fractogel EMD TMAE(s) ion-exchanger. A 23-fold purified receptor was obtained which can be stored in liquid N2 without loss of activity. The second step involved the use of a magnetic DNA affinity technique where the double stranded hsp 27 oligonucleotide containing EcdR binding sequence was biotin 5'-end labelled and bound to monodisperse superparamagnetic particles coated with streptavidin (Dynabeads M-280 Streptavidin) giving magnetic DNA affinity beads. The chromatographed EcdR-ponasterone A complexes were bound to the magnetic DNA affinity beads and by magnetic separation, wash and elution, a 29,000-fold enriched EcdR preparation was obtained within 1.5 h. This procedure can be applied for other EcdR sources with minor modifications.

Effects of the steroid hormone, 20-hydroxyecdysone, on the growth of neurites by identified insect motoneurons in vitro

During metamorphosis in the hawkmoth, Manduca sexta, identified larval leg motoneurons survive the degeneration of their larval targets to innervate new muscles of the adult legs. The dendrites and axon terminals of these motoneurons regress at the end of the larval stage and then regrow during adult development. Previous studies have implicated the insect steroid, 20-hydroxyecdysone (20-HE), in similar examples of dendritic reorganization during metamorphosis. The present studies were undertaken to test whether 20-HE acts directly on the leg motoneurons to regulate dendritic growth. Larval leg motoneurons were labeled with a fluorescent dye to permit their identification in culture following the dissociation of thoracic ganglia at later stages of development. Leg motoneurons isolated from early pupal stage animals (just before the normal onset of dendritic regrowth) survived in vitro and grew processes regardless of whether 20-HE was added to the culture medium. The extent of process outgrowth, however, as measured by the total length of all processes and the number of branches, was significantly greater for motoneurons maintained in the presence of 20-HE. The enhancement could be blocked by the addition of a juvenile hormone analog. By contrast, larval leg motoneurons that were isolated just before the normal period of dendritic regression did not show enhanced growth of neurites in the presence of 20-HE. The results suggest that 20-HE acts directly on the leg motoneurons to regulate the growth of processes during metamorphosis.

Tissue-specific ecdysone responses: regulation of the Drosophila genes Eip28/29 and Eip40 during larval development

The Drosophila genes Eip28/29 and Eip40 are expressed in Kc cells and are rapidly induced by the steroid hormone ecdysone. The molecular basis for Eip28/29's regulation in those cells has been studied in some detail. To determine how this regulation relates to normal development, we have examined the expression of both genes throughout Drosophila development, with special attention to Eip28/29 and the final larval instar. Eip28/29 expression is complex; there are tissues in which it is never expressed, others in which it is continuously expressed at a low level and tissues in which its expression is regulated without obvious relationship to endocrine events. However high-level Eip28/29 expression always correlates with the presence of ecdysone and there is good evidence that Eip28/29 is directly regulated by the hormone in some tissues and at some stages. Most striking are the induction of Eip28/29 transcripts in numerous tissues at the last larval molt, their induction in the epidermis at the time of the ‘late 3rd transition’, their extinction in the same tissue by the premetamorphic ecdysone peak, and their induction by that peak in the lymph gland, hemocytes and proventriculus. These contrasting regulatory behaviors provide a well-defined model for studying the developmental specificity of steroid responses. Eip40 appears to be ecdysone-inducible only in the lymph gland and there only at the premetamorphic peak. The similarities been Eip28/29 and Eip40 regulation in the lymph gland and Kc cells support the idea that Kc cells are derived from a hematopoietic ancestor.

Past and present studies with ponasterones, the first insect molting hormones from plants

The search for antitumor compounds from Southeast Asian plants led to ponasterones, the first phytoecdysteroids, just a year after structure determination of ecdysone and 20-hydroxyecdysone. An independent study of Chinese herb constituents by Takemoto et al. at Tohoku University led to the simultaneous and totally independent discovery of phytoecdysteroids. These findings greatly facilitated research in insect and crustacean physiology. The original structural studies on various phytoecdysteroids have led to interdisciplinary bioorganic studies in the area related to ecdysone receptor, ecdysone biosynthetic precursor (or its storage form), crustacean molt inhibitory factors, and so on.

Programmed neuronal death in insect development

Programmed death in the developing nervous system of insects serves to remove obsolete neurons, generate segmental specializations and sexual dimorphism, as well as adjust neuronal number. This diversity is also reflected in the mechanisms which control the death of these neurons. In general, but not without exception, these deaths occur independent of target fate, while endocrine cues, segmental identity, and neural signalling often play critical roles. In addition, the programmed death of at least some neurons can be delayed by behavioral feedback. The study of neuronal death in Drosophila and the cloning of an ecdysteroid receptor bring the promise of understanding the genetic factors and molecular events that regulate this phenomenon.

Drosophila ultraspiracle modulates ecdysone receptor function via heterodimer formation

The vertebrate retinoid X receptor (RXR) has been implicated in the regulation of multiple hormonal signaling pathways through the formation of heteromeric receptor complexes that bind DNA with high affinity. We now demonstrate that ultraspiracle (usp), a Drosophila RXR homolog, can substitute for RXR in stimulating the DNA binding of receptors for retinoic acid, T3, vitamin D, and peroxisome proliferator activators. These observations led to the search and ultimate identification of the ecdysone receptor (EcR) as a Drosophila partner of usp. Together, usp and EcR bind DNA in a highly cooperative fashion. Cotransfection of both EcR and usp expression vectors is required to render cultured mammalian cells ecdysone responsive. These results implicate usp as an integral component of the functional EcR. By demonstrating that receptor heterodimer formation precedes the divergence of vertebrate and invertebrate lineages, these data underscore a central role for RXR and its homolog usp in the evolution and control of the nuclear receptor-based endocrine system.

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

Steroid receptors are members of a large family of transcription factors whose activity is tightly regulated by the binding of their cognate steroid ligand. Mammalian steroid hormone receptors have been exploited to obtain the regulated expression of heterologous genes in mammalian cells. However, the utility of these systems in cultured cells and transgenic animals is limited by the presence of endogenous steroids and their receptors. We show that a Drosophila ecdysone receptor can function in cultured mammalian cells as an ecdysteroid-dependent transcription factor. The activity of the ecdysone receptor was not induced by any of the mammalian steroid hormones tested. The DNA-binding and transactivation activities of viral, mammalian, or bacterial proteins were rendered ecdysteroid-dependent when fused to the ligand-binding domain of the ecdysone receptor. The ecdysone receptor may prove useful in selectively regulating the expression of endogenous or heterologous genes in mammalian cells.

Elk-1 protein domains required for direct and SRF-assisted DNA-binding

The Ets-related Elk-1 protein can bind to purine-rich DNA target sites in a sequence specific fashion and, in addition, can form a ternary complex with the c-fos serum response element (SRE) and the serum response factor (SRF). We demonstrate that Elk-1 can readily interchange between its different interaction partners. The amino terminal ETS-domain of Elk-1 was shown to be necessary and sufficient for direct DNA-binding activity. For ternary complex formation with the SRE and SRF, both the Elk-1 ETS-domain as well as flanking sequences up to amino acid 169 were required. Removal of sequences between the ETS-domain and amino acids 137-169 did not abolish ternary complex formation. This suggests the Elk-1 region spanning amino acids 137-169 to contain a protein-protein interaction domain. Furthermore, we have shown that a single amino acid exchange introduced into the ETS-domain can drastically alter the direct DNA-binding affinity of Elk-1 without severely affecting SRF-assisted binding to the SRE. Thus, Elk-1 requires different propensities of the ETS-domain to exert its different modes of DNA sequence recognition.

DHR3: a Drosophila steroid receptor homolog

In Drosophila the steroid hormone ecdysone triggers a genetic regulatory hierarchy in which ecdysone combines with a receptor protein to form a complex that induces the transcription of a small class of "early" genes, which encode transcription factors that regulate other genes. We previously reported that one of the early genes, E75, encodes members of the steroid receptor superfamily. Using an E75 hybridization probe, we have identified two additional Drosophila genes that encode members of this superfamily. One of these is the ecdysone receptor gene, EcR, as previously reported. In this work, we examine the sequence, genomic organization, and developmental expression of the other gene, DHR3, which, like E75, encodes one of a growing number of "orphan" receptors for which ligands have not yet been identified. The structure of the DHR3 protein is strikingly similar to that of the MHR3 protein (e.g., 97% amino acid identity for the DNA binding domains), another orphan receptor encoded by an ecdysone-inducible early gene of another insect, Manduca sexta. The temporal developmental profile for DHR3 expression closely parallels that for the ecdysone titer and for the ecdysone-inducible E75 and E74 Drosophila early genes. The structural similarity to a Manduca early gene and the expression similarities to Drosophila early genes suggest that the DHR3 gene may also belong to the early gene class.

The juvenile hormone analogue, methoprene, inhibits ecdysterone induction of small heat shock protein gene expression

The small heat shock protein (hsp) genes of Drosophila are expressed in cultured cells in response to the moulting hormone, ecdysterone. We show here that juvenile hormone (JHIII) and the juvenile hormone analogue, methoprene, inhibit that induction in a dose-dependent manner. Heat shock induction is not inhibited. In transient expression studies using S3 line cells transfected with EcRE-CAT constructs, methoprene inhibition was found to require a 2-hr pretreatment (before ecdysterone addition), and methoprene's continued presence was essential. Farnesol, farnesyl acetate, and retinoic acid did not cause inhibition. Several models of methoprene inhibition are discussed.

The dioxin and peroxisome proliferator-activated receptors: nuclear receptors in search of endogenous ligands

Dioxins and peroxisome proliferators represent two diverse classes of xenobiotic compounds that induce transcription of specific genes encoding cytochrome P-450 drug-metabolizing enzymes. Signal transduction by these chemicals is mediated by two distinct nuclear receptors, one of which has recently been demonstrated to be a member of the steroid hormone receptor superfamily of ligand-activated transcription factors. However, no endogenous ligand has so far been identified for either of these nuclear receptors. Lorenz Poellinger, Martin Göttlicher and Jan-Ake Gustafsson review properties of both these xenobiotic receptor systems and discuss how the molecular details in the receptor activation pathways compare with those of nuclear hormone receptors.

An ecdysteroid-inducible Manduca gene similar to the Drosophila DHR3 gene, a member of the steroid hormone receptor superfamily

Using cDNAs for the human retinoic acid receptor alpha (hRAR alpha) and Drosophila hormone receptor 3 (DHR3), we isolated a cDNA encoding a member of the steroid hormone receptor superfamily from the tobacco hornworm, Manduca sexta. Sequencing showed that this cDNA is most closely related to DHR3 (97 and 68% amino acid identity in the DNA and ligand binding regions, respectively) followed by hRAR alpha (65 and 20% identity, respectively) and therefore is named MHR3. The cDNA hybridized to two mRNAs (3.8 and 4.5 kb) found in the epidermis during the ecdysteroid rises for the embryonic, larval, and pupal molts. Culture of fourth instar larval epidermis with 4 microM 20-hydroxyecdysone (2 micrograms/ml 20HE) caused the appearance of MHR3 mRNA within 3 hr and maximal expression by 6 hr; after 12 hr continuous exposure to 20HE, the mRNA level declined. The 4.5-kb mRNA appeared first, both were present in equal amounts by 12 hr, and by 20 hr the predominant transcript was 3.8 kb. Similar 20HE-induced expression was seen in epidermis explanted 1 day after the onset of wandering, although with a slower time course. The induction was largely independent of protein synthesis, but the subsequent decline required protein synthesis as is typical of the "early" puffs in Drosophila. Continuous exposure to 20HE was necessary for MHR3 expression; in its absence, the mRNA declined with a half-life of 2 hr. Thus, MHR3 is an ecdysteroid-inducible DNA binding protein that likely is a transcription factor involved in the cascade of gene activation and inactivation caused by ecdysteroids during the insect molt.

The Drosophila nuclear receptors: new insight into the actions of nuclear receptors in development

In Drosophila melanogaster, an increasing number of members of the steroid hormone receptor superfamily are being identified and characterized. Molecular and genetic analysis of receptor function provides evidence for a set of functions underlying the determination of pattern formation, metamorphosis, eye development, and reproduction. Many of the Drosophila receptor genes show striking homologies to mammalian receptor genes. This suggests that genetic analysis in flies could facilitate the generation of biological models that pertain to complex hormonal responses in development and which are relevant to both vertebrate and invertebrate systems.

Evolution of the nuclear receptor gene superfamily

Nuclear receptor genes represent a large family of genes encoding receptors for various hydrophobic ligands such as steroids, vitamin D, retinoic acid and thyroid hormones. This family also contains genes encoding putative receptors for unknown ligands. Nuclear receptor gene products are composed of several domains important for transcriptional activation, DNA binding (C domain), hormone binding and dimerization (E domain). It is not known whether these genes have evolved through gene duplication from a common ancestor or if their different domains came from different independent sources. To test these possibilities we have constructed and compared the phylogenetic trees derived from two different domains of 30 nuclear receptor genes. The tree built from the DNA binding C domain clearly shows a common progeny of all nuclear receptors, which can be grouped into three subfamilies: (i) thyroid hormone and retinoic acid receptors, (ii) orphan receptors and (iii) steroid hormone receptors. The tree constructed from the central part of the E domain which is implicated in transcriptional regulation and dimerization shows the same distribution in three subfamilies but two groups of receptors are in a different position from that in the C domain tree: (i) the Drosophila knirps family genes have acquired very different E domains during evolution, and (ii) the vitamin D and ecdysone receptors, as well as the FTZ-F1 and the NGF1B genes, seem to have DNA binding and hormone binding domains belonging to different classes. These data suggest a complex evolutionary history for nuclear receptor genes in which gene duplication events and swapping between domains of different origins took place.

The binding site of a steroid hormone receptor-like protein within the Drosophila Adh adult enhancer is required for high levels of tissue-specific alcohol dehydrogenase expression

Developmental and tissue-specific transcription from the Adh distal promoter is regulated in part by the Adh adult enhancer, located 450 to 600 bp upstream from the distal RNA start site. We have characterized four proteins (DEP1 to DEP4), present in Drosophila tissue culture cell nuclear extracts, which bind to this enhancer. DEP1 and DEP2 bind to a positive cis-acting element (-492 to -481) and share nucleotide contacts. A small linker replacement deletion mutation, which disrupts the overlapping DEP1- and DEP2-binding sites, reduces Adh distal transcription in an alcohol dehydrogenase (ADH)-expressing cultured cell line, in the adult fat body (the major tissue of ADH expression), as well as in some but not all adult tissues where ADH is normally expressed. This enhancer element contains an imperfect palindromic sequence similar to steroid hormone receptor superfamily response elements. Binding-site screening of a lambda gt11 expression library has identified the steroid receptor superfamily member fushi tarazu factor 1 (FTZ-F1) as a protein that binds to this site. Anti-FTZ-F1 antibodies have identified DEP1 as FTZ-F1. DEP2 also binds to the FTZ-F1 site from the fushi tarazu zebra element, suggesting that DEP2 may also be a steroid receptor superfamily member. Our results raise the possibility that Adh regulation in certain adult tissues involves a hormone-mediated pathway. Because DEP1 (FTZ-F1) and DEP2 contact some of the same nucleotides within the positive cis element, it is unlikely that they can bind simultaneously. Such alternative binding may play a role in the tissue-specific and developmental transcription of Adh.

The binding site of a steroid hormone receptor-like protein within the Drosophila Adh adult enhancer is required for high levels of tissue-specific alcohol dehydrogenase expression

Developmental and tissue-specific transcription from the Adh distal promoter is regulated in part by the Adh adult enhancer, located 450 to 600 bp upstream from the distal RNA start site. We have characterized four proteins (DEP1 to DEP4), present in Drosophila tissue culture cell nuclear extracts, which bind to this enhancer. DEP1 and DEP2 bind to a positive cis-acting element (-492 to -481) and share nucleotide contacts. A small linker replacement deletion mutation, which disrupts the overlapping DEP1- and DEP2-binding sites, reduces Adh distal transcription in an alcohol dehydrogenase (ADH)-expressing cultured cell line, in the adult fat body (the major tissue of ADH expression), as well as in some but not all adult tissues where ADH is normally expressed. This enhancer element contains an imperfect palindromic sequence similar to steroid hormone receptor superfamily response elements. Binding-site screening of a lambda gt11 expression library has identified the steroid receptor superfamily member fushi tarazu factor 1 (FTZ-F1) as a protein that binds to this site. Anti-FTZ-F1 antibodies have identified DEP1 as FTZ-F1. DEP2 also binds to the FTZ-F1 site from the fushi tarazu zebra element, suggesting that DEP2 may also be a steroid receptor superfamily member. Our results raise the possibility that Adh regulation in certain adult tissues involves a hormone-mediated pathway. Because DEP1 (FTZ-F1) and DEP2 contact some of the same nucleotides within the positive cis element, it is unlikely that they can bind simultaneously. Such alternative binding may play a role in the tissue-specific and developmental transcription of Adh.

Purification, cloning, and RXR identity of the HeLa cell factor with which RAR or TR heterodimerizes to bind target sequences efficiently

We have purified and cloned a HeLa cell nuclear protein that strongly stimulates binding of retinoic acid and thyroid hormone receptors (RARs and TRs) to response elements. The purified protein is a human retinoid X receptor beta (hRXR beta). Three murine members of the RXR family (mRXR alpha, beta, and gamma) have also been cloned, and their interactions with RARs and TRs have been investigated. Under conditions where RAR, RXR, and TR bound poorly as homodimers to various response elements, strongly cooperative RAR-RXR and TR-RXR binding was observed. The binding efficiency was dependent on the sequence, relative orientation, and spacing of the repeated motifs of response elements. We show also that unstable RAR-RXR heterodimers were formed in solution, and that C-terminal sequences and the DNA-binding domains of both receptors were required for efficient formation of stable heterodimers on response elements. These findings suggest a convergence of the signaling pathways of some members of the nuclear receptor superfamily.

Sequence and expression of IMP-L1, an ecdysone-inducible gene expressed during Drosophila imaginal disc morphogenesis

Drosophila imaginal discs are induced by the steroid hormone 20-hydroxyecdysone to initiate morphogenesis leading to formation of the adult appendages and thoracic epidermis at the end of the third larval instar. Ecdysone-dependent transcriptional activation of a set of genes that encode imaginal disc transcripts found on membrane-bound polysomes precedes and may be responsible for some aspects of the cellular changes that mediate epithelial morphogenesis in this system. A 1.35 kb transcript from one of these genes, IMP-L1, is first observed in vivo at or just prior to pupariation, as ecdysone titers are peaking and beginning to decline. Expression is initiated in proximal areas of the antennal disc, later spreading to a more widespread but nonuniform distribution throughout other thoracic imaginal discs. IMP-L1 is not, however, expressed in other ecdysone target tissues such as salivary glands or fat body. The IMP-L1 gene encodes a novel protein product containing a signal peptide, a possible transmembrane domain, two highly charged domains and a proline rich C-terminal domain. We suggest that the delayed timing of expression of this secondary response gene is necessary for proper ordering of cellular events associated with disc morphogenesis.