Isolation, characterization and developmental expression of the ecdysteroid-induced E75 gene of the wax moth Galleria mellonella

František Sehnal: A project that worked out

František Sehnal was a prominent and inspiring figure in many areas of insect science, most notably endocrinology, developmental biology, silk research, and recently insect interactions with genetically modified crops. In this article, I will briefly overview Sehnal's research and other academic and educational activities. I would also like to share my personal experience with František Sehnal as a mentor who drafted, in 1990, a plan for my doctoral thesis: to identify a receptor for juvenile hormone. The project ended up taking more than two decades to complete. While František has passed away, his legacy stays.

Transcriptomic variation of eyestalk reveals the genes and biological processes associated with molting in Portunus trituberculatus

Background

Molting is an essential biological process throughout the life history of crustaceans, which is regulated by many neuropeptide hormones expressed in the eyestalk. To better understand the molting mechanism in Portunus trituberculatus, we used digital gene expression (DGE) to analyze single eyestalk samples during the molting cycle by high-throughput sequencing.

Results

We obtained 14,387,942, 12,631,508 and 13,060,062 clean sequence reads from inter-molt (InM), pre-molt (PrM) and post-molt (PoM) cDNA libraries, respectively. A total of 1,394 molt-related differentially expressed genes (DEGs) were identified. GO and KEGG enrichment analysis identified some important processes and pathways with key roles in molting regulation, such as chitin metabolism, peptidase inhibitor activity, and the ribosome. We first observed a pattern associated with the neuromodulator-related pathways during the molting cycle, which were up-regulated in PrM and down-regulated in PoM. Four categories of important molting-related transcripts were clustered and most of them had similar expression patterns, which suggests that there is a connection between these genes throughout the molt cycle.

Conclusion

Our work is the first molt-related investigation of P. trituberculatus focusing on the eyestalk at the whole transcriptome level. Together, our results, including DEGs, identification of molting-related biological processes and pathways, and observed expression patterns of important genes, provide a novel insight into the function of the eyestalk in molting regulation.

Identification of two juvenile hormone inducible transcription factors from the silkworm, Bombyx mori

Juvenile hormone (JH) regulates many physiological processes in insects. However, the signal cascades in which JH is active have not yet been fully elucidated, particularly in comparison to another major hormone ecdysteroid. Here we identified two JH inducible transcription factors as candidate components of JH signaling pathways in the silkworm, Bombyx mori. DNA microarray analysis showed that expression of two transcription factor genes, E75 and Enhancer of split mβ (E(spl)mβ), was induced by juvenile hormone I (JH I) in NIAS-Bm-aff3 cells. Real time RT-PCR analysis confirmed that expression of four E75 isoforms (E75A, E75B, E75C and E75D) and E(spl)mβ was 3-8 times greater after JH I addition. Addition of the protein synthesis inhibitor cycloheximide did not suppress JH-induced expression of the genes, indicating that they were directly induced by JH. JH-induced expression of E75 and E(spl)mβ was also observed in four other B. mori cell lines and in larval hemocytes of final instar larvae. Notably, E75A expression was induced very strongly in larval hemocytes by topical application of the JH analog fenoxycarb; the level of induced expression was comparable to that produced by feeding larvae with 20-hydroxyecdysone. These results suggest that E75 and E(spl)mβ are general and direct target genes of JH and that the transcription factors encoded by these genes play important roles in JH signaling.

Brain infection and activation of neuronal repair mechanisms by the human pathogen Listeria monocytogenes in the lepidopteran model host Galleria mellonella

Listeria monocytogenes the causative agent of the foodborne disease listeriosis in humans often involves fatal brainstem infections leading to meningitis and meningoencephalitis. We recently established the larvae of the greater wax moth (Galleria mellonella) as a model host for the investigation of L. monocytogenes pathogenesis and as a source of peptides exhibiting anti-Listeria-activity. Here we show that G. mellonella can be used to study brain infection and its impact on larval development as well as the activation of stress responses and neuronal repair mechanisms. The infection of G. mellonella larvae with L. monocytogenes elicits a cellular immune response involving the formation of melanized cellular aggregates (nodules) containing entrapped bacteria. These form under the integument and in the brain, resembling the symptoms found in human patients. We screened the G. mellonella transcriptome with marker genes representing stress responses and neuronal repair, and identified several modulated genes including those encoding heat shock proteins, growth factors, and regulators of neuronal stress. Remarkably, we discovered that L. monocytogenes infection leads to developmental shift in larvae and also modulates the expression of genes involved in the regulation of endocrine functions. We demonstrated that L. monocytogenes pathogenesis can be prevented by treating G. mellonella larvae with signaling inhibitors such as diclofenac, arachidonic acid, and rapamycin. Our data extend the utility of G. mellonella larvae as an ideal model for the high-throughput in vivo testing of potential compounds against listeriosis.

Distinct roles of isoforms of the heme-liganded nuclear receptor E75, an insect ortholog of the vertebrate Rev-erb, in mosquito reproduction

Mosquitoes are adapted to using vertebrate blood as a nutrient source to promote egg development and as a consequence serve as disease vectors. Blood-meal activated reproductive events in female mosquitoes are hormonally and nutritionally controlled with an insect steroid hormone 20-hydroxyecdysone (20E) playing a central role. The nuclear receptor E75 is an essential factor in the 20E genetic hierarchy, however functions of its three isoforms - E75A, E75B, and E75C - in mosquito reproduction are unclear. By means of specific RNA interference depletion of E75 isoforms, we identified their distinct roles in regulating the level and timing of expression of key genes involved in vitellogenesis in the fat body (an insect analog of vertebrate liver and adipose tissue) of the mosquito Aedes aegypti. Heme is required in a high level of expression of 20E-controlled genes in the fat body, and this heme action depends on E75. Thus, in mosquitoes, heme is an important signaling molecule, serving as a sensor of the availability of a protein meal for egg development. Disruption of this signaling pathway could be explored in the design of mosquito control approaches.

Expression and ecdysteroid responsiveness of the nuclear receptors HR3 and E75 in the crustacean Daphnia magna

Ecdysteroids initiate signaling along multiple pathways that regulate various aspects of development, maturation, and reproduction in arthropods. Signaling often involves the induction of downstream transcription factors that either positively or negatively regulate aspects of the pathway. We tested the hypothesis that crustaceans express the nuclear receptors HR3 (ortholog to vertebrate ROR) and E75 (ortholog to vertebrate rev-erb) in response to ecdysteroid signaling. HR3 and E75 cDNAs were cloned from the crustacean Daphnia magna. The DNA-binding domain and ligand-binding domain of the daphnid HR3 were 95% and 61% identical to those of Drosophila melanogaster. The DNA-binding domain and ligand-binding domain of the daphnid E75 were 100% and 71% identical to those of D. melanogaster. Both receptors exhibited structural characteristics of binding to DNA as a monomer. The expression of these receptor mRNAs was evaluated through the adult molt cycle and during embryo development. E75 levels were relatively constant throughout the adult molt cycle and through embryo development. HR3 levels were comparable to those of E75 during the initial phases of the adult molt cycle but were elevated approximately 30-fold at a time in the cycle co-incident with the pre-molt surge in ecdysteroid levels. HR3 mRNA levels in embryos also varied co-incident with ecdysteroid levels. To substantiate a role of ecdysteroids in the expression of HR3, daphnids were continuously exposed to 20-hydroxyecdysone and changes in gene expression were measured. HR3 levels were significantly induced by 20-hydroxyecdysone; while E75 levels were minimally affected. These results are consistent with the premise that transcription of HR3 is regulated by ecdysteroids in the crustacean D. magna and that HR3 likely serves as a mediator of ecdysteroid regulatory action in crustaceans. The marginal induction of E75 by 20-hydroxyecdysone may represent limited, tissue or cell-type-specific induction of this transcription factor.

Arthropod nuclear receptors and their role in molting

The molting process in arthropods is regulated by steroid hormones acting via nuclear receptor proteins. The most common molting hormone is the ecdysteroid, 20-hydroxyecdysone. The receptors of 20-hydroxyecdysone have also been identified in many arthropod species, and the amino acid sequences determined. The functional molting hormone receptors consist of two members of the nuclear receptor superfamily, namely the ecdysone receptor and the ultraspiracle, although the ecdysone receptor may be functional, in some instances, without the ultraspiracle. Generally, the ecdysone receptor/ultraspiracle heterodimer binds to a number of ecdysone response elements, sequence motifs that reside in the promoter of various ecdysteroid-responsive genes. In the ensuing transcriptional induction, the ecdysone receptor/ultraspiracle complex binds to 20-hydroxyecdysone or to a cognate ligand that, in turn, leads to the release of a corepressor and the recruitment of coactivators. 3D structures of the ligand-binding domains of the ecdysone receptor and the ultraspiracle have been solved for a few insect species. Ecdysone agonists bind to ecdysone receptors specifically, and ligand-ecdysone receptor binding is enhanced in the presence of the ultraspiracle in insects. The basic mode of ecdysteroid receptor action is highly conserved, but substantial functional differences exist among the receptors of individual species. Even though the transcriptional effects are apparently similar for ecdysteroids and nonsteroidal compounds such as diacylhydrazines, the binding shapes are different between them. The compounds having the strongest binding affinity to receptors ordinarily have strong molting hormone activity. The ability of the ecdysone receptor/ultraspiracle complex to manifest the effects of small lipophilic agonists has led to their use as gene switches for medical and agricultural applications.

Ecdysteroid-responsive genes, RXR and E75, in the tropical land crab, Gecarcinus lateralis: differential tissue expression of multiple RXR isoforms generated at three alternative splicing sites in the hinge and ligand-binding domains

In order to study the potential role of the steroid molting hormone (20-hydroxyecdysone) in regulating molt-induced claw muscle atrophy, full-length cDNAs encoding retinoid-X receptor (Gl-RXR) and E75 early ecdysone inducible gene (Gl-E75) were obtained from land crab (Gecarcinus lateralis) skeletal muscle mRNA using RT-PCR and 3' and 5' RACE. Gl-E75A (3528bp), which encoded a protein of 828 amino acids, had highest sequence identity to Me-E75A from a shrimp (Metapenaeus ensis). It was expressed in skeletal muscle and gonads. The deduced amino acid sequence of Gl-RXR was highly similar to that of the fiddler crab RXR (Up-RXR) and insect ultraspiracle (USP). Nine variant sequences occurred in Gl-RXR mRNAs at three alternative splicing sites, one in the "T box" in the linker D domain and two in the ligand-binding domain (LBD). The three T-box variants, termed T(+8), T(+7), and T(+12), contained insertions of 8, 7, or 12 amino acids, respectively. Four variants were generated at the first site in the LBD. Two of the LBD site 1 variants differed in the presence (+33) or absence (-33) of a 33-amino acid sequence; the other two were LBD truncations with or without the 33 amino acid sequence (+33DeltaE/F and -33DeltaE/F, respectively). Two variants differing in the presence (+35) or absence (-35) of a 35-amino acid sequence were generated at the second site in the LBD. The Gl-RXRa isoform (1516 bp) with the longest open reading frame (+12/+33/+35) encoded a protein of 436 amino acids. Thoracic muscle expressed only isoforms with the T(+12) sequence. In contrast, claw muscle expressed isoforms with T(+7) or T(+12) and fewer isoforms with T(+8). Ovary and testis expressed a greater number of RXR isoforms than skeletal muscle. All tissues expressed full-length and truncated RXR isoforms. These data suggest that differences in response of claw and thoracic muscles to elevated ecdysteroid are due in part to differences in the expression of RXR isoforms.

The Drosophila nuclear receptor e75 contains heme and is gas responsive

Nuclear receptors are a family of transcription factors with structurally conserved ligand binding domains that regulate their activity. Despite intensive efforts to identify ligands, most nuclear receptors are still "orphans." Here, we demonstrate that the ligand binding pocket of the Drosophila nuclear receptor E75 contains a heme prosthetic group. E75 absorption spectra, resistance to denaturants, and effects of site-directed mutagenesis indicate a single, coordinately bound heme molecule. A correlation between the levels of E75 expression and the levels of available heme suggest a possible role as a heme sensor. The oxidation state of the heme iron also determines whether E75 can interact with its heterodimer partner DHR3, suggesting an additional role as a redox sensor. Further, the E75-DHR3 interaction is also regulated by the binding of NO or CO to the heme center, suggesting that E75 may also function as a diatomic gas sensor. Possible mechanisms and roles for these interactions are discussed.

Up- and downregulated genes in muscles that undergo developmentally programmed cell death in the insectManduca sexta

This study was designed to investigate changes in gene expression associated with stage-specific programmed cell death (PCD) in intersegmental muscles (ISMs) of the moth, Manduca sexta. The technique of differential display reverse transcription PCR was applied to compare mRNA levels before and after the onset of PCD in ISMs. Expression of E75B transcription factor was repressed while another factor, betaFTZ-F1, stayed at a very low level. However, gene coding for a translation-initiation factor (eIF1A) was upregulated. Expression of these genes had not been previously reported to be altered in dying ISMs. An ecdysteroid agonist, RH-5849, that prevented PCD in ISMs also blocked these changes.

Juvenile hormone regulation of the E75 nuclear receptor is conserved in Diptera and Lepidoptera

Despite longstanding efforts, the juvenile hormone (JH) signaling pathway remains unknown. In Drosophila melanogaster (Diptera), JH activates expression of the E75A nuclear receptor. The E75 gene encodes a family of related proteins. A homologue of Drosophila E75 was previously identified and two isoforms, mE75A and mE75B, were reported in Manduca sexta (Lepidoptera). Here, we describe the identification of two additional isoforms, mE75C and mE75D, and the hormonal regulation of mE75 gene expression in Manduca CH1 cultured cells. mE75A and mE75B isoforms are specifically induced by ecdysone in CH1 cells. One isoform, mE75C, shows constitutive expression. The mE75D isoform exhibits dual hormonal regulation; it can be activated by either ecdysone or the JH analog, methoprene. E75-encoded proteins represent the first example of transcription factors directly induced by JH. E75 activation by JH, in both Diptera and Lepidoptera, suggests a conserved function in the JH signaling pathway.

Synchronization of Plodia interpunctella lepidopteran cells and effects of 20-hydroxyecdysone

We have investigated the molecular and cellular mechanisms involved in the control of insect cell cycle by 20-hydroxyecdysone (20E) using the IAL-PID2 cell line established from imaginal wing discs of Plodia interpunctella. We first defined conditions for use of hydroxyurea, a reversible inhibitor of DNA synthesis, in order to synchronize the IAL-PID2 cells in their division cycle. A high degree of synchrony was reached when cells were exposed to two consecutive hydroxyurea treatments at 1 mm for 36 h spaced 16 h apart. Under these conditions, flow cytometry analysis demonstrated that 20E at 10(-6) m induced an inhibition of cell growth by an arrest of 90% of the cells in G2/M phase. Using cDNA probes specifically designed from E75 and HR3 nuclear receptors of Plodia interpunctella, we showed that PiE75 and PHR3 were highly induced by 20E through S and G2 phases with maximal enhancement just before the G2/M arrest of cells. These findings suggest that PiE75 and PHR3 could be involved in a 20E-induced genetic cascade leading to G2/M arrest.

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.

A novel putative insect chitinase with multiple catalytic domains: hormonal regulation during metamorphosis

We have used differential display to identify genes that are regulated by juvenile hormone in the epidermis of the beetle Tenebrio molitor. One of the genes encodes T. molitor chitinase 5 (TmChit5), a chitinase possessing an unusual structure. Sequence analysis of TmChit5 identified five 'chitinase units' of approx. 480 amino acids with similarity to chitinase family 18. These units are separated by less conserved regions containing putative PEST (rich in proline, glutamic acid, serine and threonine) sequences, putative chitin-binding domains and mucin domains. Northern-blot analysis identified a single transcript of approx. 9 kb, whose abundance correlated with that of 20-hydroxyecdysone during metamorphosis. Injection of pupae with 20-hydroxyecdysone alone, or in combination with cycloheximide, indicated that TmChit5 expression is directly induced by the hormone. Further experiments indicated that methoprene (a juvenile hormone analogue) indirectly induced TmChit5 mRNA expression. On the basis of the present results and previous studies, we propose a molecular mechanism for cuticle digestion during the moulting process.

Cloning and development expression of Choristoneura hormone receptor 75: a homologue of the Drosophila E75A gene

Cloning and characterization of a cDNA of the spruce budworm, Choristoneura fumiferana, that showed high amino acid similarity with the deduced amino acid sequences of E75 cDNAs cloned from Manduca sexta, Galleria melonella, and Drosophila melanogaster are described. Initially, a cDNA fragment and then a full length cDNA were cloned from C. fumiferana. The longest open reading frame of this cDNA had 690 codons and its deduced amino acid sequence had all five domains typical of a steroid hormone nuclear receptor. The deduced amino acid sequence of this cDNA showed the highest identity with the deduced amino acid sequence of E75A cDNAs cloned from M. sexta, G. melonella, and D. melanogaster, and is therefore named Choristoneura hormone receptor 75A (CHR75A). The CHR75A cDNA probe detected a 2.6 kb mRNA that was abundant at the time of the ecdysteroid peaks during molting in the embryonic, larval and pupal stages. In the sixth instar larvae, CHR75 mRNA was detected in the epidermis, fat body and midgut, and maximum expression was observed during the prepupal peak of ecdysteroids in the hemolymph. CHR75 mRNA was induced in ecdysone treated CF-203 cells and in the midgut, fat body and epidermis of larvae that were fed the non-steroidal ecdysteroid agonist, RH-5992. In vitro transcription and translation of the CHR75A cDNA yielded a 79 kDa protein that bound to the retinoic acid receptor related orphan receptor response element (RORE).

E75 expression in mosquito ovary and fat body suggests reiterative use of ecdysone-regulated hierarchies in development and reproduction

The steroid hormone ecdysone controls genetic regulatory hierarchies underlying insect molting, metamorphosis and, in some insects, reproduction. Cytogenetic and molecular analysis of ecdysone response in Drosophila larval salivary glands has revealed regulatory hierarchies including early genes which encode transcription factors controlling late ecdysone response. In order to determine whether similar hierarchies control reproductive ecdysone response, we have investigated ecdysone-regulated gene expression in vitellogenic mosquito ovaries and fat bodies. Here, we identify the homologue of the Drosophila E75 early ecdysone inducible gene in the yellow fever mosquito Aedes aegypti, and show that, as in Drosophila, the mosquito homologue, AaE75, consists of three overlapping transcription units with three mRNA isoforms, AaE75A, AaE75B, and AaE75C, originating as a result of alternative splicing. All three AaE75 isoforms are induced at the onset of vitellogenesis by a blood meal-activated hormonal cascade, and highly expressed in the mosquito ovary and fat body, suggesting their involvement in the regulation of oogenesis and vitellogenesis, respectively. Furthermore, in vitro fat body culture experiments demonstrate that AaE75 isoforms are induced by 20-hydroxyecdysone, an active ecdysteroid in the mosquito. These findings suggest that related ecdysone-triggered regulatory hierarchies may be used reiteratively during developmental and reproductive ecdysone responses.

Cloning of a shrimp (Metapanaeus ensis) cDNA encoding a nuclear receptor superfamily member: an insect homologue of E75 gene

Degenerate primers were derived from the amino acid sequence in the DNA binding domain of the Drosophila ecdysone receptor (DmEcR). Several partial cDNAs were amplified from the shrimp epidermis by reverse transcription polymerase chain reaction (RT-PCR). One of these fragments shows the highest amino acid sequence homology to the insect ecdysone inducible gene E75. This partial cDNA was used as a probe to screen the swimming leg cDNA library of the shrimp, Metapenaeus ensis. A 3.6 kb cDNA clone was obtained. The longest open reading frame of this cDNA consists of 606 amino acids and its deduced amino acid sequence has all five domains typical of a nuclear receptor. The putative polyadenylation signal is located at about 400 bp 3' to the stop signal. The deduced amino acid sequence of this cDNA shows the highest identity to that of the E75A reported in Manduca sexta, Galleria melonella, Drosophila melanogaster, and Choristoneura fumiferana. Based on the amino acid sequence comparison, the shrimp nuclear receptor is considered the insect homologue of E75A. Northern blot analysis shows that the shrimp E75 is expressed in the epidermis, eyestalk and the nerve cord of the pre-molt shrimp. Moreover, E75 transcripts can be detected in the epidermal tissues of early pre-molt shrimp by in situ hybridization. To determine whether the shrimp could also express other E75s like the insects, 5' end RACE and RT-PCR were performed on epidermal cDNA of a single shrimp. Subcloning and DNA sequence determination of the PCR products confirmed the presence of two other forms of E75 (tentatively called E75C and E75D) in shrimp. By RT-PCR, different levels of E75 expression can be detected in the epidermis, nerve cord and the eyestalk of early pre-molt shrimp. In addition to the different levels of expression of the shrimp E75s in the epidermis, the pattern of their expression is also different during the molting cycle. This is the first report on the cloning of a shrimp nuclear receptor superfamily member.

Cloning and developmental expression of Choristoneura hormone receptor 3, an ecdysone-inducible gene and a member of the steroid hormone receptor superfamily

Degenerate oligonucleotides and cDNA converted from Choristoneura fumiferana embryonic RNA were used in a polymerase chain reaction (PCR) procedure to isolate a 683 bp cDNA fragment. Comparison of the deduced amino acid sequence of this cDNA fragment showed that it was a region of an MHR3-like gene from C. fumeferana; we therefore named it Choristoneura hormone receptor 3 (CHR3). This CHR3 cDNA fragment was used as a probe to screen a C. fumiferana embryonic cDNA library. Twenty clones were isolated and two overlapping clones were sequenced. The longest open reading frame of CHR3 cDNA codes for 546 amino acids. The deduced amino acid sequence of this open reading frame contained all five regions typical of a steroid hormone nuclear receptor. The C domain showed the highest identity to Manduca hormone receptor 3 (MHR3), Drosophila hormone receptor 3 (DHR3) and Galleria hormone receptor 3 (GHR3). The A/B, D and E domains also showed significant amino acid similarity with MHR3, DHR3 and GHR3. The 683 bp CHR3 cDNA probe detected two mRNAs of 3.8 and 4.5 kb present during the ecdysteroid peaks for embryonic, larval, pupal and adult molts but were not detected during the intermolt periods. In sixth instar larvae, the 3.8 and 4.5 kb mRNA were detected in the epidermis, fat body and midgut tissues and the maximum expression was observed during the prepupal peak of ecdysteroids in the hemolymph. CHR3 mRNA was induced in 20-hydroxyecdysone treated CF-203 cells as well as in the midgut, fat body and epidermis of larvae that were fed the non-steroidal molting hormone agonist, RH-5992. In vitro transcription and translation of the CHR3 cDNA yielded a 61 kDa protein that bound to the retinoid related orphan receptor response element.

Insect nuclear receptors: a developmental and comparative perspective

The appearance of puffs on the polytene chromosomes of insect salivary glands incubated with 20-hydroxyecdysone provided the first demonstration that steroids act directly at the gene transcriptional level to bring about subsequent cellular changes (Becker, 1959; Clever and Karlson, 1960). Despite that auspicious beginning, learning about the molecular mechanisms that underlie the hormonal regulation of insect development was impeded for many years by the difficulty associated with isolating and identifying rare regulatory factors from limited tissue sources. The advent of recombinant DNA methodology and powerful techniques such as the polymerase chain reaction (PCR) along with the recognition that many important endocrine factors are structurally conserved across a wide range of species has, however, all but eliminated the technical obstacles once facing the insect endocrinologist trying to isolate and study these regulatory molecules. This review will discuss recent progress and recall some earlier experiments concerning the molecular basis of hormonal action in insects focusing primarily on the members of the nuclear hormone receptor superfamily in Drosophila melanogaster. Two members of this family comprise the functional ecdysteroid receptor and at least a dozen other "orphans" have been identified in Drosophila for which no cognate ligand has yet been found. Many of these orphans are regulated by ecdysteroids. A discussion of juvenile hormone binding proteins that are not family members has been included because of their potential impact on nuclear receptor function. As receptor homologues have been identified in other insects, several general ideas concerning insect hormonal regulation have begun to emerge and these will be examined from a comparative point of view.

Cloning of an ecdysone receptor homolog from Manduca sexta and the developmental profile of its mRNA in wings

Using the Drosophila melanogaster ecdysone receptor (DmEcR) B1 cDNA clone, we isolated three genomic clones for EcR from the tobacco hornworm, Manduca sexta. Subsequent isolation and sequencing of several cDNAs yielded a homolog of the B1 isoform with 50, 95 and 70% amino acid identities with DmEcR in the N-terminal A/B, the DNA binding and the ligand binding domains respectively. Unlike Drosophila, an intron occurs between the exons encoding the two zinc fingers of Manduca EcR (MsEcR). A 6.0 kb mRNA encoding MsEcR was found in both larval wing discs and prothoracic glands and in pupal wings. During the final larval instar, the mRNA was maximal in the wing discs at one day after wandering (W1), whereas in the prothoracic gland EcR mRNA increased rapidly to high levels on day 2 and remained high thereafter. During the onset of adult development, two peaks of EcR mRNA were observed in wings from day 3 to 5 and on day 8 after pupal ecdysis. These two peaks correlated with the time of increasing titers of ecdysone (E) and 20-hydroxyecdysone (20E), respectively. The EcR mRNA peaks always preceded the large ecdysteroid peak, suggesting that the transcription of the EcR gene is induced by a low concentration of ecdysteroid in vivo.

Analysis of ecdysteroid action in Malacosoma disstria cells: cloning selected regions of E75- and MHR3-like genes

IPRI-MD-66 (MD-66) cells respond to 20-hydroxyecdysone (20E, 4 x 10(-6) M) in the medium by producing cytoplasmic extensions, clumping and attaching themselves to the substrate. These morphological changes are at a maximum by 6 days post treatment. Degenerate oligonucleotides, designed on the basis of conserved amino acid sequences in the DNA and ligand binding regions of the members of the steroid hormone receptor superfamily, were used in RNA-PCR to isolate two cDNA fragments, Malacosoma disstria hormone receptor 2 (MdHR2) and Malacosoma disstria hormone receptor 3 (MdHR3) from the MD-66 cells. Comparison of deduced amino acid sequences of these cDNA fragments with the members of the steroid hormone receptor superfamily showed that MdHR2 is most closely related to E75 proteins of Manduca sexta, Galleria mellonella and Drosophila melanogaster. The MdHR3 is most closely related to Manduca hormone receptor 3 (MHR3), Galleria hormone receptor 3 (GHR3) and Drosophila hormone receptor 3 (DHR3) proteins. At a concentration of 4 x 10(-6) M, 20E induces the expression of MdHR2 and MdHR3 beginning at 3 h, reaching maximum levels in 12 h and declining in 24 h. MdHR2 binds to a 2.5 kb mRNA, whereas MdHR3 binds to a 4.5 kb mRNA. Based on sequence similarity, RNA size and ecdysone inducibility, we conclude that these cDNA fragments, cloned from MD-66 cells, are regions of E75- (MdHR2) and MHR3- (MdHR3) like genes.

Expression cloning and characterization of a pupal cuticle protein cDNA of Galleria mellonella L

Epidermal mRNA of freshly ecdysed pupae of Galleria mellonella was used to establish a cDNA library in phage lambda gt11. A cDNA clone was isolated by means of a pupal cuticle protein (PCP) specific antibody. Nucleic acid sequence data show an insert of 1212 bp with an open reading frame of 1062 bp. The presence of start, stop, and polyadenylation sites suggests, that this insert represents a full length transcript. Conceptual translation resulted in a protein of 353 amino acids including a signal peptide. The final processed protein product is rich in alanine and charged amino acids like glutamic acid. It has a calculated pI of 4.19 and a molecular mass of 34.272 kDa. In hybrid selection/in vitro translation and in vitro transcription/translation experiments a translational product of 54 kDa was synthesized. The difference between apparent and calculated molecular mass is thought to be due to the relatively high amount of glutamic acid residues clustered in two regions. The developmental profile of expression of the corresponding gene was analyzed by northern blot hybridization using a cDNA probe. The time course of expression is coincident with developmentally regulated metamorphic changes in the integument.

Isolation and developmental expression of the ecdysteroid-induced GHR3 gene of the wax moth Galleria mellonella

Three degenerate primers were designed to match the most conserved regions within the DNA-binding domains of several selected members of the steroid hormone receptor family. Use of these primers in the polymerase chain reaction with cDNA from Galleria mellonella prepupae detected a 177 bp fragment that had 87% identity to the Manduca sexta gene MHR3 and 75% to the Drosophila melanogaster DHR3 gene, and therefore was named "GHR3". Screening of a Galleria penultimate instar cDNA library with this fragment yielded a cDNA clone that contained a 557 codon open reading frame, predicting a 62.3 kDa protein. The deduced amino acid sequence of GHR3 showed 92% overall identity with the MHR3 protein and 97 and 70% identity with DHR3 in the putative DNA- and ligand-binding domains, respectively. Hybridization of whole body RNA revealed high GHR3 mRNA levels during both the larval and pupal molts, coincident with the molt-inducing ecdysteroid pulses, and low or undetectable levels during the first half of the last instar. During the larval-pupal transformation, no GHR3 mRNA was found at the beginning of the stemmatal pigment retraction at the onset of the ecdysteroid rise; maximal levels were observed 4 h later, coincident with the peak ecdysteroid titer (over 2.3 micrograms 20E equivalents/ml hemolymph). Two mRNAs (4.6 and 3.6 kb) were detected when the ecdysteroid titer was high. Injection of 2 micrograms/gm 20E into isolated final instar larval abdomens induced the appearance of the 4.6 kb mRNA within 1.5 h; the mRNA level then reached maximum by 3 h and declined by 6 h. No 3.6 kb mRNA was detectable during that time. A 10-fold lower 20E dose caused only trace induction by 3 h.