Synergistic interactions of silkmoth chorion promoter-binding factors

Analysis of developmentally regulated chorion gene promoter architecture via electroporation of silk moth follicles

In the silk moth Bombyx mori, chorion genes of the same developmental specificity are organized in divergently transcribed α/β gene pairs, sharing a common 5' flanking promoter region. This bidirectional promoter contains a complete set of cis-elements responsible for developmentally accurate gene expression. In the present paper, based on the observation that Bombyx chorion gene promoters contain cis-elements for the same transcription factors without concrete evidence on which of them are essential, we address the question as to how promoter architecture (number, orientation and position of common factor binding sites) facilitates developmentally accurate chorion gene regulation. To this end, we constructed several mutated promoter regions of an early-middle gene pair and cloned them upstream of a reporter gene to introduce these plasmid constructs into silk moth follicle epithelial cells via electroporation as an efficient and quick method for transient expression. This is the first time that an ex vivo method had been applied to test the impact of systematic cis-element mutations on a chorion gene promoter. Our results confirmed the importance of the HMGA factor and the role of the GATA factor as an early repressor, and led to a more detailed understanding of which C/EBP sites participate in the regulation of early-middle chorion gene expression.

Chorion genes: a landscape of their evolution, structure, and regulation

Differential regulation at the level of transcription provides a means for controlling gene expression in eukaryotes, especially during development. Insect model systems have been extensively used to decipher the molecular basis of such regulatory cascades, and one of the oldest such model systems is the regulation of chorion gene expression during ovarian follicle maturation. Recent experimental and technological advances have shed new light onto the system, allowing us to revisit it. Thus, in this review we try to summarize almost 40 years' worth of studies on chorion gene regulation while-by comparing Bombyx mori and Drosophila melanogaster models-attempting to present a comprehensive, unified model of the various regulatory aspects of choriogenesis that takes into account the evolutionary conservation and divergence of the underlying mechanisms.

Silkmoth chorion gene regulation revisited: promoter architecture as a key player

Regulation of silkmoth chorion genes has long been used as a model system for studying differential gene expression. The large numbers of genes, their overlapping expression patterns and the overall complexity of the system hinted towards an elaborate mechanism for transcriptional control. Recent studies, however, offer evidence of a molecular pathway governed by the interplay between two general transcription factors, CCAAT enhancer binding proteins (C/EBP) and GATA, an architectural protein, high mobility group A and a chromatin remodeller, chromo-helicase/ATPase-DNA binding protein 1. In this review we present a parsimonious model that adequately describes regulation of transcription across all temporally regulated chorion genes, and propose a role for promoter architecture.

Architectural factor HMGA induces promoter bending and recruits C/EBP and GATA during silkmoth chorion gene regulation

A protein displaying significant similarity to mammalian HMGA (high-mobility group A) proteins, but also bearing unique structural features, was isolated from silkmoth (Bombyx mori) follicular cells. This factor, named BmHMGA, exhibits specific binding preference for chorion gene promoter elements and induces DNA bending thereon. BmHMGA deploys temporal-specific interaction with transcription factors BmC/EBP (C/EBP is CCAAT/enhancer-binding protein) and BmGATAβ during follicle maturation. The respective protein complexes can be detected on chorion gene promoters in vivo, with different developmental profiles each time. Analogous interaction takes place on the putative promoter of the BmC/EBP gene, hinting towards a transcriptional circuit that is responsible for the progress of choriogenesis as a whole. Finally, transient suppression of BmHMGA expression led to down-regulation of chorion genes and the BmC/EBP gene, and revealed recruitment of BmC/EBP, BmGATAβ and TFIID (transcription factor IID)/TBP (TATA-box-binding protein) by BmHMGA. A revised model for chorion gene regulation is discussed in view of these findings.

CHD1 assumes a central role during follicle development

During Bombyx mori follicle development, fine-tuning of chorion gene expression is under the control of bidirectional promoters. In this work, we show that the silkmoth chromo-helicase/ATPase-DNA binding protein 1 (CHD1) ortholog is responsible for repositioning of nucleosomes on chorion promoters, where the factor binds specifically. Chorion genes, occupying a single chromosomal locus, rely on an almost identical set of cis elements for their differential expression. As a direct consequence of remodeling, interaction of C/EBP and TFIID with promoter elements is facilitated and ultimately leads to initiation of transcription. Appending of methylation marks to H3K4 in a temporal-specific manner is dependent on CHD1 binding to cognate cis elements and signifies gene activation. Overall, CHD1 is a critical factor for proper development of the follicular epithelium in terms of whole-cell chromatin arrangement.

BmCbZ, an insect-specific factor featuring a composite DNA-binding domain, interacts with BmC/EBPγ

A novel factor featuring a composite AT hook/basic region-leucine zipper DNA-binding domain was isolated from Bombyx mori follicular cells. Screening of EST databases derived from a variety of metazoans revealed the exclusive presence of BmCbZ homologues in insect species. BmCbZ characteristic features and gene organization are discussed, in comparison to other known bZIP factors. We concordantly propose that this factor establishes a new insect-specific bZIP family. We further present the isolation of the silkmoth homologue of mammalian C/EBPgamma, BmC/EBPgamma, and in vitro evidence for its interaction with BmCbZ. The formation of a BmCbZ-BmC/EBPgamma heterodimer is a prerequisite for binding to specific C/EBP recognition sites on chorion gene promoters, most probably via both major and minor groove interactions.

A novel role for the Bombyx Slbo homologue, BmC/EBP, in insect choriogenesis

One previously unidentified cDNA clone coding for a C/EBP factor, BmC/EBP, was isolated from Bombyx mori follicular cells. This is the first time that a C/EBP factor has been isolated and characterized in Lepidoptera. We provide information concerning structural features and developmental specificity, as well as in vitro interaction properties with chorion gene promoter modules. BmC/EBP was capable of effectively recognizing homologous binding sites from chorion gene promoters derived from flies and other moths, despite significant diversity of chorion structure, gene organization, and gene expression profiles. We propose that the relative concentration of BmC/EBP, in relation to its differential binding affinity for promoter cis-elements, results in activation or repression of silkmoth chorion gene expression.

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

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

In vitro analysis of Bombyx mori early chorion gene regulation: stage specific expression involves interactions with C/EBP-like and GATA factors

This is the first attempt to identify regulatory elements that are involved in early choriogenesis of the silkworm Bombyx mori. A new cis element in the promoter region of five early chorion genes was identified. The consensus sequence of this element matches the consensus of the C/EBP DNA binding site. Moreover, this sequence interacts with a 70 kD protein (pX2) present in follicular nuclear extracts and complex formation exhibits early developmental specificity. There is strong evidence that this factor belongs to the C/EBP family. Surprisingly, the same protein binds with the same developmental specificity to a similar sequence of a late chorion gene promoter, which has been previously defined as the binding site for a putative late specific factor, BCFII. The possibility that pX2 and BCFII are isoforms or modifications of the same protein factor, which is presumably able to bind to the highly similar sequence elements of both early and late genes, is discussed. A hypothesis involving protein-protein interactions between C/EBP (pX2/BCFII) and GATA during choriogenesis is presented to explain the temporal specificity of chorion genes.

The biolistic method as a tool for testing the differential activity of putative silkmoth chorion gene promoters

Bombyx mori unpaired early chorion gene copies 6F6.1,.2 and.3 are exceptions to the typical organization and distribution pattern of known early ErA/ErB, middle A/B and late HcA/HcB divergently transcribed gene pairs. Contrary to such pairs, the boundaries of the 6F6 regulatory sequences are not easily defined; moreover, they share common sequence elements with the regulatory sequences of middle and late genes. In order to perform a functional study of the tissue and temporal specificity of the 6F6 putative promoter region, we decided to apply biolistics. In the present work, use of a region from the 6F6.2 5' untranslated sequence, spanning nucleotides -138 to the cap site, gave an expected expression pattern of a lacZ reporter gene. Temporal specificity was further verified by control experiments using the cloned intergenic sequence of the late gene pair HcA/B.12, which resulted in lacZ expression in late choriogenic follicles. At present, despite the recent successful germinal transgenesis of Bombyx mori, the biolistic transient expression system seems to be the most rapid technique to pursue the functional study of the promoter region of early chorion genes, including the three unconventional early 6F6 genes.

The orphan receptor BmHNF-4 of the silkmoth Bombyx mori: ovarian and zygotic expression of two mRNA isoforms encoding polypeptides with different activating domains

Two silkmoth nuclear receptor isoforms, BmHNF-4a and BmHNF-4b, that are related to the mammalian orphan receptor HNF-4, were characterized. Their characterization revealed that they differ from each other only in their 5' UTR and N-terminus of the predicted polypeptides. In ovarian tissue, the two receptors are expressed as a delayed response to 20-hydroxy-ecdysone and their expression increases during vitellogenesis. BmHNF-4 mRNA is localized in the cytoplasm of follicular cells and a binding activity that recognizes a mammalian HNF-4 response element is present in follicular cell nuclear extracts. BmHNF-4 mRNA is also present in the oocyte, the unfertilized egg and the early embryo, thus displaying a behavior reminiscent of maternal mRNA. Both mRNA isoforms are found in the embryo following fertilization and their abundance is modulated during ensuing embryogenesis. In contrast to the rather limited distribution of HNF-4 in mammalian tissues, BmHNF-4 is expressed in most larval and pharate adult tissues of the silkmoth.

Differential gene expression in insects: transcriptional control

Studies on transcriptional control of gene expression play a pivotal role in many areas of biology. In non-Drosophilid insects, the cuticle, chorion, immune response, silk gland, storage proteins, and vitellogenin are foci for advances in basic research on promoter elements and transcription factors. Insects offer other advantages for gene regulation studies, including the availability of applied problems. In non-Drosophilid insects, the most serious problem for transcriptional control studies is the lack of homologous in vivo expression systems. Once this deficiency is addressed, the full impact of research on transcription control will be realized throughout the field of entomology.

Bombyx EcR (BmEcR) and Bombyx USP (BmCF1) combine to form a functional ecdysone receptor

The Drosophila ecdysone receptor (DmEcR) is a member of the nuclear receptor superfamily; it functions as an obligate heterodimer with another nuclear receptor, DmUSP. EcR homologs have now been cloned from several other insects. We report here that one such homolog, BmEcR from the commercial silkmoth, Bombyx mori, is a functional ecdysone receptor. Upon dimerization with BmCF1, the silkmoth homology of DmUSP, BmEcR binds the radiolabeled steroid ligand 125I-iodoponasterone A with Kd = 1.1 nM, indistinguishable from that exhibited by DmEcR/DmUSP. BmEcR/BmCF1 forms a specific complex with an ecdysone response element (EcRE) derived from the heat shock protein 27 (hsp27) gene promoter of Drosophila; and, as with DmEcR/DmUSP, formation of this complex is stimulated by the presence of 20-hydroxyecdysone. Finally, BmEcR can substitute for DmEcR in an EcR-deficient Drosophila tissue culture line, stimulating trans-activation of an ecdysone-inducible reporter gene construct. Thus, BmEcR and BmCF1 are the functional counterparts of DmEcR and DmUSP, respectively and, despite considerable sequence divergence between the Drosophila and Bombyx proteins, the counterparts are--at least qualitatively--functionally equivalent.

The silkmoth homolog of the Drosophila ecdysone receptor (B1 isoform): cloning and analysis of expression during follicular cell differentiation

To understand the role that 20-hydroxy-ecdysone (20E) plays during ovarian development in Bombyx mori, we have undertaken the cloning of the silkworm ecdysone receptor (EcR) and a study of its expression during follicular cell differentiation. We have cloned a cDNA that contains a complete open reading frame for a 68.1 kDa polypeptide that shares extensive similarities with the B1 isoform of the Drosophila EcR. The presumed silkmoth EcR (BmEcR) is encoded by a single copy gene whose length is in excess of 23 kb. A portion of this gene encompassing seven exons that constitute the cloned BmEcR cDNA was also characterized. Employment of monoclonal antibodies, directed against the DNA binding domain of the Drosophila EcR, in Western blot analyses revealed the presence of a major 70 kDa polypeptide in extracts of follicular cells and other silkmoth tissues. The mRNA and protein encoded by BmEcR are present in constant amounts in follicular cells throughout vitellogenesis but disappear transiently at the onset of choriogenesis and reappear during the later stages of choriogenesis. The down-regulation of BmEcR in follicular cells during oogenesis suggest a complex relationship between 20E, the induction of the program of chorion gene expression in follicular cells during mid-vitellogenesis and the execution of this program at the end of vitellogenesis.

A baculovirus gp64 early promoter is activated by host transcription factor binding to CACGTG and GATA elements

The early promoter of the Orgyia pseudotsugata multicapsid nuclear polyhedrosis virus gp64 gene is active when transfected into several insect cell lines and does not require viral gene products for transcription in uninfected cells. Because previous studies have shown that the gp64 early promoter is activated above basal levels in uninfected cells, host transcription factors are likely to play a role in gp64 activation at early times postinfection. By using nuclear extracts from uninfected Sf9 cells for electrophoretic mobility shift analysis of gp64 regulatory regions, host nuclear proteins were shown to bind specifically to the upstream regulatory region of the gp64 early promoter. Host factor binding was mapped to a 24-bp sequence centered approximately 35 bp upstream of the TATA box. Two consensus eukaryotic transcription factor-binding site motifs, GATA and CACGTG, were identified within the 24-bp sequence. Competition assays using oligonucleotides containing either a GATA or a CACGTG motif and similar oligonucleotides with point mutations in these sites showed that each site is required for binding host transcription factors. To investigate the functional significance of host factor binding to GATA and CACGTG motifs, constructs containing point mutations in these motifs were examined in transient expression assays. Mutations in either or both GATA and CACGTG sites decreased reporter activity in transient expression assays, suggesting that binding of host transcription factors to these motifs is important in transcriptional regulation of the gp64 early promoter.

Early establishment and autonomous implementation of a developmental program controlling silkmoth chorion gene expression

To address the question of whether prechoriogenic follicles of the silkmoth Bombyx mori have the capacity to enter choriogenesis in organ culture and define the stage at which choriogenesis becomes established as a follicle-autonomous program, we have cultured immature ovarioles dissected from developing pupae and examined the protein synthetic profiles of follicular cells of individual follicles at the end of the culture period. The protein synthetic profiles of the cultured follicles were also correlated with corresponding profiles of chorion mRNA accumulation. Our results demonstrate that the last 17 (+/- 2) vitellogenic follicles of Day 5 to 7 pupae are capable of initiating choriogenesis in organ culture. The earliest vitellogenic stage to enter choriogenesis in vitro does so after 34 (+/- 4) hr in culture and follicles entering choriogenesis in vitro are capable of proceeding through all choriogenic stages at a speed comparable to that occurring in vivo. Therefore, once the choriogenic program becomes established in follicular cells, it can be implemented autonomously in the absence of extrafollicular factors. Earlier vitellogenic stages lack this capacity, presumably because they require additional hemolymph factors to establish the choriogenic potential. Our results demonstrate that the choriogenic potential of cultured vitellogenic follicles cannot be influenced by addition of 20-hydroxyecdysone to the culture medium.