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

iTRAQ Proteomic Analysis of Interactions Between 20E and Phospholipase C in Apolygus lucorum (Meyer-Dür)

Polyphagous Apolygus lucorum has become the dominant insect in Bacillus thuringiensis (Bt) cotton fields. Hormone 20-hydroxyecdysone (20E) regulates multiple insect development and physiology events. 20E responses are controlled by pathways triggered by phospholipase C (PLC)-associated proteins. However, 20E-modulated genes and related proteins that can be affected by PLC still remain unknown. Here, isobaric tag for relative and absolute quantitation (iTRAQ) and immunoblotting techniques were used to compare differentially expressed proteins (DEPs) in A. lucorum in response to the treatment of 20E and the PLC inhibitor U73122 as well as their combination. A total of 1,624 non-redundant proteins and 97, 248, 266 DEPs were identified in the 20E/control, U73122/control, and 20E + U73122/control groups, respectively. Only 8 DEPs, including pathogenesis-related protein 5-like, cuticle protein 19.8, trans-sialidase, larval cuticle protein A2B-like, cathepsin L1, hemolymph juvenile hormone-binding protein, ATP-dependent RNA helicase p62-like, and myosin-9 isoform X1, were detected in all three groups. Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analysis showed that the DEPs were involved in diverse signaling pathways. The results were validated by immunoblotting, which highlighted the reliability of proteomics analysis. These findings provided novel insights into the function of PLC in 20E signaling pathway in A. lucorum.

Cuticular protein genes showing peaks at different stages are probably regulated by different ecdysone responsive transcription factors during larval-pupal transformation

We aimed to explain the reason and function of the successive expression of ecdysone-responsive transcription factors (ERTFs) and related cuticular protein (CP) genes during transformation from larva to pupa. The regulation of the expression of CP genes by ERTFs was examined by in vitro wing disc culture and reporter assay using a gene gun transduction system. Two CP genes that showed expression peaks at different stages-BmorCPG12 at W3L and BmorCPH2 at P0 stage-were selected and examined. Reporter constructs conveying putative BHR3, ßFTZ-F1, BHR39, and E74A binding sites of BmorCPG12 and BmorCPH2 showed promoter activity when introduced into wing discs. In the present study, we showed the functioning of the putative BHR3 and E74A binding sites, together with putative ßFTZ-F1 binding sites, on the activation of CP genes, and different ERTF binding sites functioned in one CP gene. From these, we conclude that BHR3, ßFTZ-F1, and E74A that are successively expressed bring about the successive expression of CP genes, resulting in insect metamorphosis. In addition to this, reporter constructs conveying putative BHR39 binding sites of BmorCPG12 and BmorCPH2 showed negative regulation.

Genome-wide identification of target genes for transcription factor BR-C in the silkworm, Bombyx mori

Transcription factor Broad Complex (BR-C) is an ecdysone primary response gene in insects and participates in the regulation of insect growth and development. In this study, we performed a genome-wide identification of BR-C target genes in silkworm (Bombyx mori) using chromatin immunoprecipitation followed by high-throughput sequencing (ChIP-seq). As a result, a total of 1006 BR-C ChIP peaks were identified, and 15% of peaks were located in the promoter regions of 133 protein-coding genes. Functional annotation revealed that these ChIP peak-associated genes, as potential BR-C targets, were enriched in pathways related to biosynthetic process, metabolic process, and development. Transcriptome analysis and quantitative real-time polymerase chain reaction (PCR) examination revealed that developmental changes in expression patterns of a portion of potential BR-C targets, including HR96 and GC-α1, were similar to those of BR-C. ChIP-PCR examination confirmed that BR-C could directly bind to the promoters of potential targets. Further, dual luciferase assays demonstrated that HR96 promoter activity was significantly upregulated following BR-C overexpression, and this upregulation was abolished when the binding motif in the promoter was truncated. This study will be helpful for deciphering the regulatory roles of BR-C during insect growth and development.

Transgenic Ectopic Overexpression of Broad Complex (BrC-Z2) in the Silk Gland Inhibits the Expression of Silk Fibroin Genes of Bombyx mori

Bombyx mori silk protein genes are strictly turned on and off in different developmental stages under the hormone periodically change. The broad complex (BrC) is a transcription factor mediating 20-hydroxyecdysone action, which plays important roles during metamorphosis. Here, we observed that two isoforms of BmBrC (BmBrC-Z2 and BmBrC-Z4) exhibited contrasting expression patterns with fibroin genes (FibH, FibL and P25) in the posterior silk gland (PSG), suggesting that BmBrC may negatively regulate fibroin genes. Transgenic lines were constructed to ectopically overexpress BmBrC-Z2 in the PSG. The silk protein genes in the transgenic line were decreased to almost half of that in the wild type. The silk yield was decreased significantly. In addition, the expression levels of regulatory factors (BmKr-h1 and BmDimm) response to juvenile hormone (JH) signal were inhibited significantly. Then exogenous JH in the BmBrC-Z2 overexpressed lines can inhibit the expression of BmBrC-Z2 and activate the expression of silk protein genes and restore the silk yield to the level of the wild type. These results indicated that BmBrC may inhibit fibroin genes by repressing the JH signal pathway, which would assist in deciphering the comprehensive regulation mechanism of silk protein genes.

Isoform specific roles of Broad-Complex in larval development in Leptinotarsa decemlineata

Broad-Complex (BrC) is a downstream target of both 20-hydroxyecdysone and juvenile hormone signalling. BrC regulates morphogenetic changes between nymphal instars in hemimetabolans, whereas it controls pupal commitment, pupal morphogenesis and inhibits adult differentiation in holometabolans. Among five BrC cDNAs (Z1-Z4 and Z6) identified in the Colorado potato beetle, we found in this work that Z1, Z2 and Z6 were mainly expressed at the last (fourth) instar and prepupal stages, whereas the levels of Z3 and Z4 increased during the penultimate (third) instar stage, peaked at the last instar larval phase and gradually decreased at the prepupal and pupal periods. When knocking down all BrC isoforms by RNA interference (RNAi) at the penultimate instar stage, around 20% of the resultant larvae remained as moribund beetles. These moribund BrC RNAi larvae were completely or partially wrapped in old cuticle. Likewise, a portion of larvae treated for a single double-stranded RNA of Z3, Z4 or Z6 displayed a degree of similar aberrancies, increasing in the order of isoforms Z6 < Z3 < Z4. When silencing all BrC isoforms at the last instar period, most of the RNAi larvae did not normally pupate or emerge as adults. Separately silencing each of the five zinc finger domains revealed that approximately 70% of the Z1 RNAi larvae remained as prepupae, around 60% of the Z6 RNAi specimens formed aberrant prepupae or pupae and about 60% of the Z2 RNAi beetles became deformed pupae. After removal of the old exuviae, these deformed larvae in which either Z1, Z2 or Z6 was depleted possessed adult prothorax and mesothorax, developing antenna, mouthparts and wing discs. Moreover, less than 50% of the resultant pupae finally emerged as adults when either of Z1, Z2 or Z6 was knocked down. Therefore, our findings reveal, for the first time, that the two roles of BrC in insect groups (ie directing morphogenetic changes during juvenile development and regulating larval-pupal-adult metamorphosis) are played by different BrC isoforms in Leptinotarsa decemlineata.

20-Hydroxyecdysone regulates the transcription of the lysozyme via Broad-Complex Z2 gene in silkworm, Bombyx mori

Broad-Complex Z2 (Br-C Z2) is an ecdysone inducible transcription factor that regulates physiological, innate immune and developmental events in insects. Here, we identified an orthologue of Br-C Z2 from silkworm, Bombyx mori (BmBr-C Z2) to study its involvement in immune responses. The quantitative real-time PCR analysis revealed that BmBr-C Z2 was expressed ubiquitously in all tested tissues under normal physiological conditions. Further, developmental profile displayed that BmBr-C Z2 expression was detectable in different developmental stages, however the gene's expression was highest in the molting and pre-pupal stages. Administration of 20-hydroxyecdysone (20E) enhanced the expression levels of BmBr-C Z2 in hemocytes. The challenge with pathogens and pathogen associated molecular patterns (PAMPs) also upregulated the mRNA levels of BmBr-C Z2 in hemocytes when compared with the control. By contrast, the ectopic expression of BmBr-C Z2 remarkably increased the production of antimicrobial peptides, while the knock-down of this gene by double stranded RNA decreased their production. Dual-luciferase assay exhibited that BmBr-C Z2 induced the expression of lysozyme by directly binding to its promoter region. The treatment of Escherichia coli following the knock-down of BmBr-C Z2 strongly reduced the survival rate of silkworm larvae. These results suggest that BmBr-C Z2 plays an important biological role in the innate immune responses of silkworm by regulating immune-related genes.

Analysis of cysteine protease inhibitor gene (BmCPI) promoter activity in silkworms using bac-to-bac baculovirus systems

A cysteine protease inhibitor of Bombyx mori (BmCPI) plays an important role in pupation, molting, and dissociation of tissues. The present study identified and analyzed the BmCPI promoter region to better understand its functional regulatory mechanisms. Eight promoter fragments of different lengths were analyzed using an improved Bac-to-Bac expression system. Luciferase activities were investigated both in BmE cells and larval organisms after infection with the Bac-to-Bac system, and similar changes in activity were observed in both models. Strong activity was detected in the longest promoter (2005 bp, −1969 to +36), and activity changed significantly with truncation of promoter length. An electrophoretic mobility shift assay showed that the promoter region from −32 to +6 bp played a critical role in activating the downstream gene promoter element, where some potential elements were also predicted by informatics tools. The findings offer a basic reference for the mechanism of transcriptional regulation of BmCPI.

Protein kinase A-mediated phosphorylation of the Broad-Complex transcription factor in silkworm suppresses its transcriptional activity

The insect-specific transcription factor Broad-Complex (BR-C) is transcriptionally activated by the steroid 20-hydroxyecdysone (20E) and regulates the expression of many target genes involved in insect growth and development. However, although the transcriptional regulation of BR-C proteins has been well studied, how BR-C is regulated at post-transcription and -translation levels is poorly understood. To this end, using liquid chromatography-tandem mass spectrometry analysis, we identified residue Ser-186 as a phosphorylation site of BR-C in silkworm. Site-directed mutagenesis and treatment with specific kinase activators and inhibitors indicated that the Ser-186 residue in silkworm BR-C is phosphorylated by protein kinase A (PKA). Immunostaining assays disclosed that PKA-mediated phosphorylation of silkworm BR-C has no effect on its nuclear import. However, luciferase reporter analysis, electrophoretic mobility shift assays, and chromatin immunoprecipitation revealed that the PKA phosphorylation event suppresses the transcriptional activation of silkworm BR-C target genes and that this inhibition was caused by repression of BR-C binding to its DNA targets. Of note, both in vitro and ex vivo experiments disclosed that a continuous 20E signal inhibits the PKA-mediated BR-C phosphorylation and also the cAMP/PKA pathway, indicating that 20E's inhibitory effect on PKA-mediated phosphorylation of silkworm BR-C contributes to maintaining BR-C transcriptional activity. In conclusion, our findings indicate that PKA-mediated phosphorylation inhibits silkworm BR-C activity by interfering with its binding to DNA and that 20E signaling relieves PKA-mediated phosphorylation of BR-C, thereby maintaining its transcriptional activity.

Mutation of a Cuticle Protein Gene, BmCPG10, Is Responsible for Silkworm Non-Moulting in the 2nd Instar Mutant

In the silkworm, metamorphosis and moulting are regulated by ecdysone hormone and juvenile hormone. The subject in the present study is a silkworm mutant that does not moult in the 2^nd instar (nm2). Genetic analysis indicated that the nm2 mutation is controlled by a recessive gene and is homozygous lethal. Based on positional cloning, nm2 was located in a region approximately 275 kb on the 5^th linkage group by eleven SSR polymorphism markers. In this specific range, according to the transcriptional expression of thirteen genes and cloning, the relative expression level of the BmCPG10 gene that encodes a cuticle protein was lower than the expression level of the wild-type gene. Moreover, this gene’s structure differs from that of the wild-type gene: there is a deletion of 217 bp in its open reading frame, which resulted in a change in the protein it encoded. The BmCPG10 mRNA was detectable throughout silkworm development from the egg to the moth. This mRNA was low in the pre-moulting and moulting stages of each instar but was high in the gluttonous stage and in newly exuviated larvae. The BmCPG10 mRNA showed high expression levels in the epidermis, head and trachea, while the expression levels were low in the midgut, Malpighian tubule, prothoracic gland, haemolymph and ventral nerve cord. The ecdysone titre was determined by ELISA, and the results demonstrated that the ecdysone titre of nm2 larvae was lower than that of the wild-type larvae. The nm2 mutant could be rescued by feeding 20-hydroxyecdysone, cholesterol and 7—dehydrocholesterol (7dC), but the rescued nm2 only developed to the 4^th instar and subsequently died. The moulting time of silkworms could be delayed by BmCPG10 RNAi. Thus, we speculated that the mutation of BmCPG10 was responsible for the silkworm mutant that did not moult in the 2^nd instar.

Cuticular protein and transcription factor genes expressed during prepupal-pupal transition and by ecdysone pulse treatment in wing discs of Bombyx mori

We aimed to understand the underlying mechanism that regulates successively expressed cuticular protein (CP) genes around pupation in Bombyx mori. Quantitative PCR was conducted to clarify the expression profile of CP genes and ecdysone-responsive transcription factor (ERTF) genes around pupation. Ecdysone pulse treatment was also conducted to compare the developmental profiles and the ecdysone induction of the CP and ERTF genes. Fifty-two CP genes (RR-1 13, RR-2 18, CPG 8, CPT 3, CPFL 2, CPH 8) in wing discs of B. mori were examined. Different expression profiles were found, which suggests the existence of a mechanism that regulates CP genes. We divided the genes into five groups according to their peak stages of expression. RR-2 genes were expressed until the day of pupation and RR-1 genes were expressed before and after pupation and for longer than RR-2 genes; this suggests different construction of exo- and endocuticular layers. CPG, CPT, CPFL and CPH genes were expressed before and after pupation, which implies their involvement in both cuticular layers. Expression profiles of ERTFs corresponded with previous reports. Ecdysone pulse treatment showed that the induction of CP and ERTF genes in vitro reflected developmental expression, from which we speculated that ERTFs regulate CP gene expression around pupation.

20-Hydroxyecdysone (20E) Primary Response Gene E93 Modulates 20E Signaling to Promote Bombyx Larval-Pupal Metamorphosis

As revealed in a previous microarray study to identify genes regulated by 20-hydroxyecdysone (20E) and juvenile hormone (JH) in the silkworm, Bombyx mori, E93 expression in the fat body was markedly low prior to the wandering stage but abundant during larval-pupal metamorphosis. Induced by 20E and suppressed by JH, E93 expression follows this developmental profile in multiple silkworm alleles. The reduction of E93 expression by RNAi disrupted 20E signaling and the 20E-induced autophagy, caspase activity, and cell dissociation in the fat body. Reducing E93 expression also decreased the expression of the 20E-induced pupal-specific cuticle protein genes and prevented growth and differentiation of the wing discs. Importantly, the two HTH domains in E93 are critical for inducing the expression of a subset of 20E response genes, including EcR, USP, E74, Br-C, and Atg1. By contrast, the LLQHLL and PLDLSAK motifs in E93 inhibit its transcriptional activity. E93 binds to the EcR-USP complex via a physical association with USP through its LLQHLL motif; and this association is enhanced by 20E-induced EcR-USP interaction, which attenuates the transcriptional activity of E93. E93 acts through the two HTH domains to bind to GAGA-containing motifs present in the Atg1 promoter region for inducing gene expression. In conclusion, E93 transcriptionally modulates 20E signaling to promote Bombyx larval-pupal metamorphosis.

Transcriptional regulation of cuticular protein glycine-rich13 gene expression in wing disc of Bombyx mori, Lepidoptera

Cuticular protein genes are good models to study the molecular mechanisms of signaling by ecdysteroids, which regulate molting and metamorphosis in insects. The present research demonstrates on hormonal regulation and analysis of the regulatory sequences and transcription factors important for Bombyx mori cuticular protein glycine-rich13 (CPG13) gene expression. Expression of CPG13 was strong at prepupal stage in wing tissues of B. mori. CPG13 expression was induced by the addition of 20E, which was inhibited by cycloheximide in the wing disc. The upstream region of the CPG13 gene was analyzed using a transient reporter assay with a gene gun system and identified two BR-Z2 binding sites to be important cis-acting elements for the transcription activation of the luciferase reporter gene by an ecdysone pulse. Site-directed mutagenesis of these sites, followed by introduction into wing discs, significantly decreased the reporter activity. It was found that the regions carrying the binding sites for the ecdysone-responsive transcription factor BR-Z2 were responsible for the hormonal enhancement of the reporter gene activity in wing discs. Mutation of the BR-Z2 binding sites decreased the reporter activity suggesting that the BR-Z2 isoform can bind to the upstream region of the cuticle protein gene, CPG13 and activates its expression.

Involvement of FTZ-F1 in the regulation of pupation in Leptinotarsa decemlineata (Say)

During the final instar larvae of holometabolous insects, a pulse of 20-hydroxyecdysone (20E) and a drop in juvenile hormone (JH) trigger larval-pupal metamorphosis. In this study, two LdFTZ-F1 cDNAs (LdFTZ-F1-1 and LdFTZ-F1-2) were cloned in Leptinotarsa decemlineata. Both LdFTZ-F1-1 and LdFTZ-F1-2 were highly expressed just before or right after each molt, similar to the expression pattern of an ecdysteroidogenesis gene LdSHD. Ingestion of an ecdysteroid agonist halofenozide (Hal) enhanced LdFTZ-F1-1 and LdFTZ-F1-2 expression in the final larval instar. Conversely, a decrease in 20E by feeding a double-stranded RNA (dsRNA) against LdSHD repressed the expression. Moreover, Hal rescued the expression levels in LdSHD-silenced larvae. Thus, 20E peaks seem to induce the transcription of LdFTZ-F1s. Furthermore, ingesting dsLdFTZ-F1 from a common fragment of LdFTZ-F1-1 and LdFTZ-F1-2 successfully knocked down both LdFTZ-F1s, and impaired pupation. Finally, knocking down LdFTZ-F1s significantly repressed the transcription of three ecdysteroidogenesis genes, lowered 20E titer, and reduced the expression of two 20E receptor genes. Silencing LdFTZ-F1s also induced the expression of a JH biosynthesis gene, increased JH titer, but decreased the mRNA level of a JH early-inducible gene. Thus, LdFTZ-F1s are involved in the regulation of pupation by modulating 20E and JH titers and mediating their signaling pathways.

Nuclear import of transcription factor BR-C is mediated by its interaction with RACK1

The transcription factor Broad Complex (BR-C) is an early ecdysone response gene in insects and contains two types of domains: two zinc finger domains for the activation of gene transcription and a Bric-a-brac/Tramtrack/Broad complex (BTB) domain for protein-protein interaction. Although the mechanism of zinc finger-mediated gene transcription is well studied, the partners interacting with the BTB domain of BR-C has not been elucidated until now. Here, we performed a yeast two-hybrid screen using the BTB domain of silkworm BR-C as bait and identified the receptor for activated C-kinase 1 (RACK1), a scaffolding/anchoring protein, as the novel partner capable of interacting with BR-C. The interaction between BR-C and RACK1 was further confirmed by far-western blotting and pull-down assays. Importantly, the disruption of this interaction, via RNAi against the endogenous RACK1 gene or deletion of the BTB domain, abolished the nuclear import of BR-C in BmN4 cells. In addition, RNAi against the endogenous PKC gene as well as phosphorylation-deficient mutation of the predicted PKC phosphorylation sites at either Ser373 or Thr406 in BR-C phenocopied RACK1 RNAi and altered the nuclear localization of BR-C. However, when BTB domain was deleted, phosphorylation mimics of either Ser373 or Thr406 had no effect on the nuclear import of BR-C. Moreover, mutating the PKC phosphorylation sites at Ser373 and Thr406 or deleting the BTB domain significantly decreased the transcriptional activation of a BR-C target gene. Given that RACK1 is necessary for recruiting PKC to close and phosphorylate target proteins, we suggest that the PKC-mediated phosphorylation and nuclear import of BR-C is determined by its interaction with RACK1. This novel finding will be helpful for further deciphering the mechanism underlying the role of BR-C proteins during insect development.

Cloning, expression patterns, and preliminary characterization of AccCPR24, a novel RR-1 type cuticle protein gene from Apis cerana cerana

Cuticular proteins (CPs) are key components of insect cuticle, a structure that plays a pivotal role in insect development and defense. In this study, we cloned the full-length cDNA of a CP gene from Apis cerana cerana (AccCPR24). An amino acid sequence alignment indicated that AccCPR24 contains the conserved Rebers and Riddiford consensus sequence and shares high similarity with the genes from other hymenopteran insects. We then isolated the genomic DNA and found that the first intron, which is present in other CP genes, is absent in AccCPR24. Real-time quantitative polymerase chain reaction (qPCR) analysis revealed that AccCPR24 is highly expressed in the late pupal stage and midgut. Expression was inhibited by an exogenous ecdysteroid in vitro but was enhanced by this hormone in vivo; environmental stressors, such as heavy metals and pesticides, also influenced gene expression. In addition, a disc diffusion assay showed that AccCPR24 enhanced the ability of bacterial cells to resist multiple stresses. We infer from our results that AccCPR24 acts in honeybee development and in protecting these insects from abiotic stresses.

Advances in silkworm studies accelerated by the genome sequencing of Bombyx mori

Significant progress has been achieved in silkworm (Bombyx mori) research since the last review on this insect was published in this journal in 2005. In this article, we review the new and exciting progress and discoveries that have been made in B. mori during the past 10 years, which include the construction of a fine genome sequence and a genetic variation map, the evolution of genomes, the advent of functional genomics, the genetic basis of silk production, metamorphic development, immune response, and the advances in genetic manipulation. These advances, which were accelerated by the genome sequencing project, have promoted B. mori as a model organism not only for lepidopterans but also for general biology.

Genes involved in thoracic exoskeleton formation during the pupal-to-adult molt in a social insect model, Apis mellifera

Background

The insect exoskeleton provides shape, waterproofing, and locomotion via attached somatic muscles. The exoskeleton is renewed during molting, a process regulated by ecdysteroid hormones. The holometabolous pupa transforms into an adult during the imaginal molt, when the epidermis synthe3sizes the definitive exoskeleton that then differentiates progressively. An important issue in insect development concerns how the exoskeletal regions are constructed to provide their morphological, physiological and mechanical functions. We used whole-genome oligonucleotide microarrays to screen for genes involved in exoskeletal formation in the honeybee thoracic dorsum. Our analysis included three sampling times during the pupal-to-adult molt, i.e., before, during and after the ecdysteroid-induced apolysis that triggers synthesis of the adult exoskeleton.

Results

Gene ontology annotation based on orthologous relationships with Drosophila melanogaster genes placed the honeybee differentially expressed genes (DEGs) into distinct categories of Biological Process and Molecular Function, depending on developmental time, revealing the functional elements required for adult exoskeleton formation. Of the 1,253 unique DEGs, 547 were upregulated in the thoracic dorsum after apolysis, suggesting induction by the ecdysteroid pulse. The upregulated gene set included 20 of the 47 cuticular protein (CP) genes that were previously identified in the honeybee genome, and three novel putative CP genes that do not belong to a known CP family. In situ hybridization showed that two of the novel genes were abundantly expressed in the epidermis during adult exoskeleton formation, strongly implicating them as genuine CP genes. Conserved sequence motifs identified the CP genes as members of the CPR, Tweedle, Apidermin, CPF, CPLCP1 and Analogous-to-Peritrophins families. Furthermore, 28 of the 36 muscle-related DEGs were upregulated during the de novo formation of striated fibers attached to the exoskeleton. A search for cis-regulatory motifs in the 5′-untranslated region of the DEGs revealed potential binding sites for known transcription factors. Construction of a regulatory network showed that various upregulated CP- and muscle-related genes (15 and 21 genes, respectively) share common elements, suggesting co-regulation during thoracic exoskeleton formation.

Conclusions

These findings help reveal molecular aspects of rigid thoracic exoskeleton formation during the ecdysteroid-coordinated pupal-to-adult molt in the honeybee.

Homeodomain POU and Abd-A proteins regulate the transcription of pupal genes during metamorphosis of the silkworm, Bombyx mori

A cascade of 20-hydroxyecdysone-mediated gene expression and repression initiates larva-to-pupa metamorphosis. We recently showed that two transcription factors, BmPOUM2 and BmβFTZ-F1, bind to the cis-regulatory elements in the promoter of the gene coding for cuticle protein, BmWCP4, and regulate its expression during Bombyx mori metamorphosis. Here we show that down-regulation of BmPOUM2 expression by RNA interference during the wandering stage resulted in failure to complete metamorphosis. The thorax epidermis of RNA interference-treated larvae became transparent, wing disc growth and differentiation were arrested, and the larvae failed to spin cocoons. Quantitative real-time PCR analysis showed that expression of the genes coding for pupal-specific wing cuticle proteins BmWCP1, BmWCP2, BmWCP3, BmWCP4, BmWCP5, BmWCP6, BmWCP8, and BmWCP9 were down-regulated in BmPOUM2 dsRNA-treated animals, whereas overexpression of BmPOUM2 protein increased the expression of BmWCP4, BmWCP5, BmWCP6, BmWCP7, and BmWCP8. Pull-down assays, far-Western blot, and electrophoretic mobility shift assay showed that the BmPOUM2 protein interacted with another homeodomain transcription factor, BmAbd-A, to induce the expression of BmWCP4. Immunohistochemical localization of BmPOUM2, BmAbd-A, and BmWCP4 proteins revealed that BmAbd-A and BmPOUM2 proteins are colocalized in the wing disc cell nuclei, whereas BmWCP4 protein is localized in the cytoplasm. Together these data suggest that BmPOUM2 interacts with the homeodomain transcription factor BmAbd-A and regulates the expression of BmWCP4 and probably other BmWCPs to complete the larva-to-pupa transformation. Although homeodomain proteins are known to regulate embryonic development, this study showed that these proteins also regulate metamorphosis.

Transcription factors BmPOUM2 and BmβFTZ-F1 are involved in regulation of the expression of the wing cuticle protein gene BmWCP4 in the silkworm, Bombyx mori

In Bombyx mori, the wing cuticle protein gene BmWCP4 is expressed specifically in the epidermis at the onset and mid-stage of pupation and is responsible for the formation of the pupal cuticle during the larval-pupal metamorphosis. The gene consists of four exons and three introns and is present as a single copy in the genome. Its expression was up-regulated by 20-hydroxyecdysone (20E) and the 20E-induced expression was suppressed by juvenile hormone (JH) III. The upstream regulatory sequence region of the BmWCP4 gene was cloned and the regulatory elements responsible for 20E induction were identified. Two cis-regulatory elements (CREs) bound by the transcription factors BmPOUM2 and BmβFTZ-F1 were identified that mediated 20E-regulated expression of this gene. An electrophoretic mobility shift assay detected two nuclear proteins isolated from the epidermis and the BmN cell line that specifically bound to the POU and βFTZ-F1 CREs, respectively. BmPOUM2 recombinant protein explicitly bound to the POU CRE. Developmental and 20E-induced expression of the BmWCP4, BmPOUM2 and BmβFTZ-F1 genes showed that BmPOUM2 and BmβFTZ-F1 were initially expressed, followed by BmWCP4. These data suggest that the 20E-induced expression of BmWCP4 is mediated by the transcription factors BmPOUM2 and BmβFTZ-F1 binding to their CREs in the regulatory sequence region of the BmWCP4 gene.

Ecdysone directly and indirectly regulates a cuticle protein gene, BMWCP10, in the wing disc of Bombyx mori

The hormonal regulation of cuticle protein genes is a good model to study the molecular mechanism of signaling by ecdysteroids, which initiates each of the major developmental transitions in insects. This study was conducted to clarify the regulation of the expression of an ecdysone-inducible cuticle protein gene, BMWCP10. Induction of the BMWCP10 transcript by ecdysone was partly inhibited in the presence of cycloheximide, which implies that the BMWCP10 promoter is directly and indirectly activated by ecdysone. Using electrophoretic mobility shift analysis and a competition experiment, we identified a putative ecdysone response element (EcRE1) located at positions -93 to -81 relative to the transcription start site. Site-directed mutagenesis of this site, followed by introduction into wing discs, dramatically abolished the reporter activity. This EcRE1 is necessary for the activation of the promoter by 20-hydroxyecdysone (20E) in the wing disc, since the mutation of EcRE1 caused loss of responsiveness to 20E. Collectively, the data obtained in our current and previous work indicate that ecdysone receptor and Broad-Complex Z2 (BR-Z2) are required for maximal BMWCP10 expression in wing disc.

The regulation of expression of insect cuticle protein genes

The exoskeleton of insects (cuticle) is an assembly of chitin and cuticle proteins. Its physical properties are determined largely by the proteins it contains, and vary widely with developmental stages and body regions. The genes encoding cuticle proteins are therefore good models to study the molecular mechanisms of signalling by ecdysteroids and juvenile hormones, which regulate molting and metamorphosis in insects. This review summarizes the studies of hormonal regulation of insect cuticle protein genes, and the recent progress in the analysis of the regulatory sequences and transcription factors important for their expression.