LIM-homeodomain transcription factor Awh is a key component activating all three fibroin genes, fibH, fibL and fhx, in the silk gland of the silkworm, Bombyx mori

Differential expression of fibroin-related genes in middle silk glands is induced by dietary differences in a strain-dependent manner in Bombyx mori

The silkworm (Bombyx mori) is a model organism for lepidopteran insects. It is an oligophagous insect that primarily feeds on mulberry leaves and has industrial use for the production of raw silk. The development of artificial diets has provided an alternative nutrient source for silkworms; however, one significant issue is that the production of cocoons is lower in silkworms reared on artificial diets compared with those reared on mulberry leaves. The differences in the silk gland in the late-stage fifth instar silkworm larvae, when silk synthesis is most active, between those raised on artificial diets and mulberry leaves, are unknown. In this study, we identified differences in the transcriptomes of the middle and posterior silk glands of fifth instar day five silkworm larvae reared on artificial diets compared with those reared on mulberry leaves using three strains: Daizo, Nichi01, and J137 × C146. We found that the silk-related genes fibrohexamerin (fhx), fibroin-light-chain (fibL), and fibroin-heavy-chain (fibH) in the middle silk gland, and ser1 in the posterior silk gland, were differentially expressed in a strain-dependent manner. In silkworms reared on artificial diets, fhx, fibL, and fibH in the middle silk gland were upregulated in Nichi01 and downregulated in J137 × C146, whereas ser1 in the posterior silk gland was upregulated in J137 × C146 compared with silkworms reared on mulberry leaves. Our results demonstrate that the diet and strain of silkworm larvae affect the expression of genes related to silk production in their silk glands during the late fifth instar stage.

Dynamic chromatin conformation and accessibility changes mediate the spatial-specific gene regulatory network in Bombyx mori

Silk gland genes of Bombyx mori can have strict spatial expression patterns, which impact their functions and silk quality; however, our understanding of their regulation mechanisms is currently insufficient. To address this, the middle silk gland (MSG) and posterior silk gland (PSG) of the silkworm were investigated. Gene ontology annotation showed that spatially specific expressed genes were involved in the formation of H3k9me and chromatin topology. Chromatin conformation data generated by Hi-C showed that the topologically associated domain boundaries around FibL and Sericin1 genes were significantly different between MSG and PSG. Changes in chromatin conformation led to changes in chromatin activity, which significantly affected the expression of nearby genes in silkworm. Chromatin accessibility regions of MSG and PSG were analyzed using FAIRE-seq, and 1006 transcription factor motifs were identified in open chromatin regions. Furthermore, the spatial-specific expression patterns of silk gland genes were mainly associated with homeobox-contained transcription factors, such as POU-M2, which was specifically bound and relatively highly expressed in the MSG. The regulatory network mediated by POU-M2 regulated most of the spatial-specific expressed genes in MSG, such as ADH1. These results can aid in improving silk performance, optimizing silkworm breeding, and improving the gene spatial regulatory model research for insects.

FibH Gene Complete Sequences (FibHome) Revealed Silkworm Pedigree

The highly repetitive and variable fibroin heavy chain (FibH) gene can be used as a silkworm identification; however, only a few complete FibH sequences are known. In this study, we extracted and examined 264 FibH gene complete sequences (FibHome) from a high-resolution silkworm pan-genome. The average FibH lengths of the wild silkworm, local, and improved strains were 19,698 bp, 16,427 bp, and 15,795 bp, respectively. All FibH sequences had a conserved 5' and 3' terminal non-repetitive (5' and 3' TNR, 99.74% and 99.99% identity, respectively) sequence and a variable repetitive core (RC). The RCs differed greatly, but they all shared the same motif. During domestication or breeding, the FibH gene mutated with hexanucleotide (GGTGCT) as the core unit. Numerous variations existed that were not unique to wild and domesticated silkworms. However, the transcriptional factor binding sites, such as fibroin modulator-binding protein, were highly conserved and had 100% identity in the FibH gene's intron and upstream sequences. The local and improved strains with the same FibH gene were divided into four families using this gene as a marker. Family I contained a maximum of 62 strains with the optional FibH (Opti-FibH, 15,960 bp) gene. This study provides new insights into FibH variations and silkworm breeding.

Dimmed gene knockout shortens larval growth and reduces silk yield in the silkworm, Bombyx mori

The bHLH domain transcription factor, Bombyx mori-derived dimmed (Bmdimm), is directly regulated by the JH-BmMet/BmSRC-BmKr-h1 pathway and plays a key role in regulating the expression of FibH, which codes the main component of silk protein. However, the other roles of Bmdimm in silk protein synthesis remain unclear. Here, we established a Bmdimm knockout (KO) line containing a 7-bp deletion via CRISPR/Cas9 system, which led to the absence of the bHLH domain. The expression level of silk protein genes and silk yield decreased significantly in the Bmdimm KO line. Moreover, knocking out Bmdimm led to shortened larval stages and significant weight loss in larvae and adults. Bmdimm was found to be highly expressed in the silk gland, but it was also expressed in the fat body. The expression level of Bmkr-h1 in the fat body was significantly downregulated in the Bmdimm KO line. Exogenous JHA treatment upregulated Bmkr-h1 and rescued the phenotype of larval growth in the Bmdimm KO line. In conclusion, knocking out Bmdimm led to a shortened larval stage via the inhibition of Bmkr-h1 expression, then reduced silk yield. These findings help to elucidate the regulatory mechanism of fibroin synthesis and larval development in silkworms.

A single-cell transcriptomic atlas characterizes the silk-producing organ in the silkworm

The silk gland of the domesticated silkworm Bombyx mori, is a remarkable organ that produces vast amounts of silk with exceptional properties. Little is known about which silk gland cells execute silk protein synthesis and its precise spatiotemporal control. Here, we use single-cell RNA sequencing to build a comprehensive cell atlas of the silkworm silk gland, consisting of 14,972 high-quality cells representing 10 distinct cell types, in three early developmental stages. We annotate all 10 cell types and determine their distributions in each region of the silk gland. Additionally, we decode the developmental trajectory and gene expression status of silk gland cells. Finally, we discover marker genes involved in the regulation of silk gland development and silk protein synthesis. Altogether, this work reveals the heterogeneity of silkworm silk gland cells and their gene expression dynamics, affording a deeper understanding of silk-producing organs at the single-cell level.

Co-Expression Network and Time-Course Expression Analyses to Identify Silk Protein Regulatory Factors in Bombyx mori

Simple Summary

Previous studies have reported how the silk production ability of Bombyx mori can be enhanced, but the mechanism that regulates silk protein genes remains unclear. We performed co-expression network analysis using networkz, an in-house program, which led to the identification of 91 transcription factors were co-expressed with silk protein genes. Of them, 13 transcripts were identified to be novel regulatory factors by time-course expression analysis during the fifth instar larvae stage. Their expression patterns were highly relevant to those of silk protein genes. Our results suggest that the two-step expression screening was robust and highly sensitive to screen relative genes, and a complex mechanism regulates silk protein production in B. mori. The novel candidates that were identified herein can serve as key genes to develop methods to enhance the silk protein production ability of B. mori.

Abstract

Bombyx mori is an important economic insect and an animal model in pharmacomedical research. Although its physiology has been studied for many years, the mechanism via which silk protein genes are regulated remains unclear. In this study, we performed two-step expression screening, namely co-expression network and time-course expression analyses to screen silk protein regulation factors. A co-expression network analysis using RNA-seq data that were obtained from various tissues, including the silk glands of B. mori, was performed to identify novel silk protein regulatory factors. Overall, 91 transcription factors, including some known ones, were found to be co-expressed with silk protein genes. Furthermore, time-course expression analysis during the fifth instar larvae stage revealed that the expression pattern of 13 novel transcription factors was highly relevant to that of silk protein genes and their known regulatory factor genes. In particular, the expression peak of several transcription factors (TFs) was detected before the expression of silk protein genes peak. These results indicated that a larger number of genes than expected may be involved in silk protein regulation in B. mori. Functional analyses of function-unknown transcription factors should enhance our understanding of this system.

New insights into the proteins interacting with the promoters of silkworm fibroin genes

The silkworm, Bombyx mori, is a silk-producing insect that has contributed greatly to human society. The silk gland of B. mori is a specialized organ responsible for synthesizing silk fibroin and sericin proteins under control of numerous factors. However, which factors are involved in direct silk protein synthesis regulation remains largely unknown. We report the identification of promoter-interacting proteins (PIPs) necessary for the regulation of genes encoding fibroin proteins, including the fibroin heavy chain (fibH), fibroin light chain (fibL), and a 25-kD polypeptide protein (P25). In the fourth larval molting stage (M4) or day 5 fifth-instar larvae (L5D5), a total of 198, 292, and 247 or 330, 305, and 460 proteins interacting with the promoter region of fibH, fibL and P25, respectively, were identified from the posterior silk gland by DNA pull-down combined with mass spectrometry. Many PIPs were particularly involved in ribosome- and metabolism-related pathways. Additionally, 135 and 212 proteins were identified as common PIPs of fibH, fibL and P25 in M4 and L5D5, respectively. Among all PIPs, we identified 31 potential transcription factors, such as Y-box and poly A-binding proteins, which play roles in nucleotide binding, ATP binding, or protein folding. This study provides the first in-depth profile of proteins interacting with fibroin gene promoters and contributes to a better understanding of silk protein synthesis regulation.

Transcriptomic analysis of the bagworm moth silk gland reveals a number of silk genes conserved within Lepidoptera

Lepidopteran insects produce cocoons with unique properties. The cocoons are made of silk produced in the larval tissue silk gland and our understanding of the silk genes is still very limited. Here, we investigated silk genes in the bagworm moth Eumeta variegata, a species that has recently been found to produce extraordinarily strong and tough silk. Using short-read transcriptomic analysis, we identified a partial sequence of the fibroin heavy chain gene and its product was found to have a C-terminal structure that is conserved within nonsaturniid species. This is in accordance with the presence of fibroin light chain/fibrohexamerin genes and it is suggested that the bagworm moth is producing silk composed of fibroin ternary complex. This indicates that the fibroin structure has been evolutionarily conserved longer than previously thought. Other than fibroins we identified candidates for sericin genes, expressed strongly in the middle region of the silk gland and encoding serine-rich proteins, and other silk genes, that are structurally conserved with other lepidopteran homologues. The bagworm moth is thus considered to be producing conventional lepidopteran type of silk. We further found a number of genes expressed in a specific region of the silk gland and some genes showed conserved expression with Bombyx mori counterparts. This is the first study allowing comprehensive silk gene identification and expression analysis in the lepidopteran Psychidae family and should contribute to the understanding of silk gene evolution as well as to the development of novel types of silk.

Reference Transcriptome Data in Silkworm Bombyx mori

Herein, we performed RNA-seq analysis of ten major tissues/subparts of silkworm larvae. The sequences were mapped onto the reference genome assembly and the reference transcriptome data were successfully constructed. The reference data provided a nearly complete sequence for sericin-1, a major silk gene with a complex structure. We also markedly improved the gene model for other genes. The transcriptomic expression was investigated in each tissue and a number of transcripts were identified that were exclusively expressed in tissues such as the testis. Transcripts strongly expressed in the midgut formed tight genomic clusters, suggesting that they originated from tandem gene duplication. Transcriptional factor genes expressed in specific tissues or the silk gland subparts were also identified. We successfully constructed reference transcriptome data in the silkworm and found that a number of transcripts showed unique expression profiles. These results will facilitate basic studies on the silkworm and accelerate its applications, which will contribute to further advances in lepidopteran and entomological research as well as the practical use of these insects.

Cell type innovation at the tips of the animal tree

Understanding how organs originate is challenging due to the twin problems of explaining how new cell types evolve and how collective interactions between cell types arise and become selectively advantageous. Animals are assemblages of organs and cell types of different antiquities, and among the most rapidly and convergently evolving are exocrine glands and their constituent secretory cell types. Such structures have arisen independently thousands of times across the Metazoa, impacting how animals chemically interact with their environments. The recurrent evolution of exocrine systems provides a paradigm for examining how qualitative phenotypic novelties arise from variation at the cellular level. Here, we take a hierarchical perspective, focusing on the evolutionary assembly of novel biosynthetic pathways and secretory cell types, and how both selection and non-adaptive molecular processes may combine to build the complex, modular architectures of many animal glands.

The biological characters of Bmelav-like genes in the development of Bombyx mori

The ELAV/Hu family is a conserved multigene family of pan-neuronal RNA-binding protein involved in post-transcriptional regulation in metazoans. In Drosophila, three members of this family, ELAV, RBP9 and FNE, are involved in neuronal differentiation, gene expression regulation and so on. This family is less well characterized in Bombyx mori. Two orthologs BmELAV-like-1 (BmEL-1) and BmELAV-like-2 (BmEL-2) share 55%-71% and 47%-62% identity with that of in Drosophila and humans, respectively. Bmel-1 is ubiquitously expressed while Bmel-2 is expressed in the head and ovaries specifically. Proteins encoded by both genes are localized in nuclear and cytoplasm. The weight of body, cocoon, pupae and cocoon shell are differently affected in Bmel-1^- /-2^- mutants created using CRISPR/Cas9 technology. Mutations of both genes increase the expression of four silk protein genes, Fib-L, Fib-H, P25 and Ser-1. In addition, the oviposition ability of Bmel-2^- females is decreased. This study not only provides valuable insights into the functional roles of Bmelav-like genes in the growth, cocoon characters and regulation of silk protein genes expression, but also provides useful information for silkworm variety breeding.

Comparative mRNA and LncRNA Analysis of the Molecular Mechanisms Associated With Low Silk Production in Bombyx mori

Naked pupa sericin and Naked pupa are two mutant strains of Bombyx mori with extremely low or no fibroin production compared to the Qiufeng and Baiyu strains, both of which exhibit very high silk fibroin production. However, the molecular mechanisms by which long non-coding RNAs regulate fibroin synthesis need further study. In this study, we performed high-throughput RNA-seq to investigate lncRNA and mRNA expression profiles in the posterior silk gland of Qiufeng, Baiyu, Nd-s^D, and Nd silkworms at the third day of the 5th instar. Our efforts yielded 26,767 novel lncRNAs and 6,009 novel mRNAs, the expression levels of silk protein genes and silk gland transcription factors were decreased in Qiufeng vs. Nd-s^D and Qiufeng vs. Nd, while those of many genes related to autophagy, apoptosis, RNA degradation, ubiquitin-mediated proteolysis and heat shock proteins were increased. Moreover, the expression of a large number of genes responsible for protein synthesis and secretion was significantly decreased in Nd. GO and KEGG analysis results showed that nucleotide excision repair, mRNA surveillance pathways, amino acid degradation, protein digestion and absorption, ER-associated degradation and proteasome pathways were significantly enriched for the Qiufeng vs. Nd-s^D and Qiufeng vs. Nd comparisons. In conclusion, our findings contribute to the lncRNA and mRNA database of Bombyx mori, and the identified differentially expressed mRNAs and lncRNAs help to reveal the molecular mechanisms of low silk production in Nd-s^D and Nd, providing new insights for improvement of silk yield and elucidation of silk mechanical properties.

Molecular evolution of gland cell types and chemical interactions in animals

Across the Metazoa, the emergence of new ecological interactions has been enabled by the repeated evolution of exocrine glands. Specialized glands have arisen recurrently and with great frequency, even in single genera or species, transforming how animals interact with their environment through trophic resource exploitation, pheromonal communication, chemical defense and parental care. The widespread convergent evolution of animal glands implies that exocrine secretory cells are a hotspot of metazoan cell type innovation. Each evolutionary origin of a novel gland involves a process of 'gland cell type assembly': the stitching together of unique biosynthesis pathways; coordinated changes in secretory systems to enable efficient chemical release; and transcriptional deployment of these machineries into cells constituting the gland. This molecular evolutionary process influences what types of compound a given species is capable of secreting, and, consequently, the kinds of ecological interactions that species can display. Here, we discuss what is known about the evolutionary assembly of gland cell types and propose a framework for how it may happen. We posit the existence of 'terminal selector' transcription factors that program gland function via regulatory recruitment of biosynthetic enzymes and secretory proteins. We suggest ancestral enzymes are initially co-opted into the novel gland, fostering pleiotropic conflict that drives enzyme duplication. This process has yielded the observed pattern of modular, gland-specific biosynthesis pathways optimized for manufacturing specific secretions. We anticipate that single-cell technologies and gene editing methods applicable in diverse species will transform the study of animal chemical interactions, revealing how gland cell types are assembled and functionally configured at a molecular level.

Deep Insight into the Transcriptome of the Single Silk Gland of Bombyx mori

The silk gland synthesizes and secretes a large amount of protein and stores liquid silk protein at an extremely high concentration. Interestingly, silk proteins and serine protease inhibitors are orderly arranged in the silk gland lumen and cocoon shells. Silk fiber formation and the spinning mechanism have not been fully elucidated. Therefore, we conducted a comparative transcriptome analysis of seven segments of the single silk gland to characterize internal changes in the silk gland during the 5th instar of mature larvae. In total, 3121 differentially expressed genes were identified in the seven segments. Genes highly expressed in the middle silk gland (MSG) were mainly involved in unsaturated fatty acid biosynthesis, fatty acid metabolism, apoptosis—fly, and lysosome pathways, whereas genes highly expressed in the posterior silk gland (PSG) were mainly involved in ribosome, proteasome, citrate cycle, and glycolysis/gluconeogenesis pathways. Thus, the MSG and PSG differ greatly in energy source use and function. Further, 773 gradually upregulated genes (from PSG to MSG) were involved in energy metabolism, silk protein synthesis, and secretion, suggesting that these genes play an important role in silk fiber formation. Our findings provide insights into the mechanism of silk protein synthesis and transport and silk fiber formation.

Insect Transcription Factors: A Landscape of Their Structures and Biological Functions in Drosophila and beyond

Transcription factors (TFs) play essential roles in the transcriptional regulation of functional genes, and are involved in diverse physiological processes in living organisms. The fruit fly Drosophila melanogaster, a simple and easily manipulated organismal model, has been extensively applied to study the biological functions of TFs and their related transcriptional regulation mechanisms. It is noteworthy that with the development of genetic tools such as CRISPR/Cas9 and the next-generation genome sequencing techniques in recent years, identification and dissection the complex genetic regulatory networks of TFs have also made great progress in other insects beyond Drosophila. However, unfortunately, there is no comprehensive review that systematically summarizes the structures and biological functions of TFs in both model and non-model insects. Here, we spend extensive effort in collecting vast related studies, and attempt to provide an impartial overview of the progress of the structure and biological functions of current documented TFs in insects, as well as the classical and emerging research methods for studying their regulatory functions. Consequently, considering the importance of versatile TFs in orchestrating diverse insect physiological processes, this review will assist a growing number of entomologists to interrogate this understudied field, and to propel the progress of their contributions to pest control and even human health.

New insight into the mechanism underlying the silk gland biological process by knocking out fibroin heavy chain in the silkworm

BACKGROUND

Exploring whether and how mutation of silk protein contributes to subsequent re-allocation of nitrogen, and impacts on the timing of silk gland degradation, is important to understand silk gland biology. Rapid development and wide application of genome editing approach in the silkworm provide us an opportunity to address these issues.

RESULTS

Using CRISPR/Cas9 system, we successfully performed genome editing of Bmfib-H. The loss-of-function mutations caused naked pupa and thin cocoon mutant phenotypes. Compared with the wild type, the posterior silk gland of mutant showed obviously degraded into fragments in advance of programmed cell death of silk gland cells. Comparative transcriptomic analyses of silk gland at the fourth day of the fifth instar larval stage(L5D4)identified 1456 differential expressed genes (DEGs) between posterior silk gland (PSG) and mid silk gland (MSG) and 1388 DEGs between the mutant and the wild type. Hierarchical clustering of all the DEGs indicated a remarkable down-regulated and an up-regulated gene clade in the mutant silk glands, respectively. Down-regulated genes were overrepresented in the pathways involved in cancer, DNA replication and cell proliferation. Intriguingly, up-regulated DEGs are significantly enriched in the proteasome. By further comparison on the transcriptome of MSG and PSG between the wild type and the mutant, we consistently observed that up-regulated DEGs in the mutant PSG were enriched in protein degrading activity and proteasome. Meantime, we observed a series of up-regulated genes involved in autophagy. Since these protein degradation processes would be normally occur after the spinning time, the results suggesting that these progresses were activated remarkably ahead of schedule in the mutant.

CONCLUSIONS

Accumulation of abnormal fib-H protein might arouse the activation of proteasomes as well as autophagy process, to promote the rapid degradation of such abnormal proteins and the silk gland cells. Our study therefore proposes a subsequent process of protein and partial cellular degradation caused by mutation of silk protein, which might be helpful for understanding its impact of the silk gland biological process, and further exploration the re-allocation of nitrogen in the silkworm.

Genome-wide open chromatin regions and their effects on the regulation of silk protein genes in Bombyx mori

Nucleosome-depleted open chromatin regions (OCRs) often harbor transcription factor (TF) binding sites that are associated with active DNA regulatory elements. To investigate the regulation of silk-protein genes, DNA molecules isolated from the silk glands of third-day fifth-instar silkworm larvae and embryo-derived (BmE) cells were subjected to formal dehyde-assisted isolation of regulatory elements (FAIRE) and high-throughput sequencing. In total, 68,000 OCRs were identified, and a number of TF-binding motifs were predicted. In particular, OCRs located near silk-protein genes contained potential binding sites for functional TFs. Moreover, many TFs were found to bind to clusters of OCRs upstream of silk-protein genes, and to regulate the expression of these genes. The expression of silk protein genes may be related not only to regulating TFs (such as fkh, Bmdimm, and Bmsage), but also to developmental and hormone-induced TFs (such as zen, eve, Br, and eip74ef). Elucidation of genome-wide OCRs and their regulatory motifs in silk protein genes will provide valuable data and clues for characterizing the mechanisms of transcriptional control of silk protein genes.

Gene expression analysis in the larval silk gland of the eri silkworm Samia ricini

Insects produce silk for a range of purposes. In the Lepidoptera, silk is utilized as a material for cocoon production and serves to protect larvae from adverse environmental conditions or predators. Species in the Saturniidae family produce an especially wide variety of cocoons, for example, large, golden colored cocoons and those with many small holes. Although gene expression in the silk gland of the domestic silkworm (Bombyx mori L.) has been extensively studied, considerably fewer investigations have focused on members of the saturniid family. Here, we established expression sequence tags from the silk gland of the eri silkworm (Samia ricini), a saturniid species, and used these to analyze gene expression. Although we identified the fibroin heavy chain gene in the established library, genes for other major silk proteins, such as fibroin light chain and fibrohexamerin, were absent. This finding is consistent with previous reports that these latter proteins are lacking in saturniid silk. Recently, a series of fibrohexamerin-like genes were identified in the Bombyx genome. We used this information to conduct a detailed analysis of the library established here. This analysis identified putative homologues of these genes. We also found several genes encoding small silk protein molecules that are also present in the silk of other Lepidoptera. Gene expression patterns were compared between eri and domestic silkworm, and both conserved and nonconserved expression patterns were identified for the tested genes. Such differential gene expression might be one of the major causes of the differences in silk properties between these species. We believe that our study can be of value as a basic catalogue for silk gland gene expression, which will yield to the further understanding of silk evolution.

Histomorphometric and transcriptomic features characterize silk glands' development during the molt to intermolt transition process in silkworm

The molt-intermolt cycle is an essential feature in holometabolous and hemimetabolous insects' development. In the silkworm, silk glands are under dramatic morphological and functional changes with fibroin genes' transcription being repeatedly turned off and on during the molt-intermolt cycles. However, the molecular mechanisms controlling it are still unknown. Here, silk gland's histomorphology and transcriptome analysis were used to characterize changes in its structure and gene expression patterns from molt to intermolt stages. By using section staining and transmission electron microscope, a renewable cell damage was detected in the silk gland at the molt stage, and an increased number of autophagosomes and lysosomes were found in silk gland cells' cytoplasm. Next, by using RNA sequencing, 54,578,413 reads were obtained, of which 85% were mapped to the silkworm reference genome. The expression level analysis of silk protein genes and silk gland transcription factors revealed that fibroin heavy chain, fibroin light chain, P25/fhx, sericin1, sericin3 and Dimm had consistent alteration trends in temporal expression. In addition, differentially expressed genes (DEGs) were identified, and most of the DEGs associated with ecdysone signal transduction, mRNA degradation, protein proteolysis, and autophagy were significantly down-regulated in the transition from molt to intermolt, suggesting that these pathways were activated for the silk gland renewal. These findings provide insights into the molecular mechanisms of silk gland development and silk protein genes transcriptional regulation during the molt to intermolt transition process.

Multiprotein bridging factor 2 regulates the expression of the fibroin heavy chain gene by interacting with Bmdimmed in the silkworm Bombyx mori

Multiprotein bridging factor 2 (MBF2) was first isolated from the posterior silk gland of Bombyx mori. However, its function in B. mori is still unknown. Herein, MBF2 transcripts were detected mainly in the posterior silk gland and Malpighian tubules of B. mori larvae via a quantitative PCR analysis. An analysis of temporal expression patterns showed that the expression pattern of MBF2 was the opposite of that of the fibroin heavy chain (fibH) gene, as its expression was high during the fourth-instar moulting stage, decreased gradually during the fifth-instar feeding stage and disappeared at the end of the fifth-instar phase. Furthermore, bimolecular fluorescent complementation and Far-Western blot assays showed that MBF2 interacted with the basic helix-loop-helix transcription factor Bmdimmed. Dual luciferase reporter assays showed that MBF2 down-regulated the promoter activity of fibH and inhibited the effect of Bmdimmed (Bmdimm) on fibH expression. MBF2 expression was induced in silk glands after treatment with 20-hydroxyecdysone in vivo and in vitro. These findings suggest that MBF2 is a transcriptional repressor that is involved in controlling the regulation of the fibH gene in the posterior silk gland by interacting with Bmdimm.

Nuclear hormone receptor BmFTZ-F1 is involved in regulating the fibroin heavy chain gene in the silkworm, Bombyx mori

BACKGROUND

The synthesis of silk protein is controlled by hormones. The expression of the nuclear hormone Bmftz-f1 in the posterior silk gland (PSG) is induced by 20-hydroxyecdysone in vivo and in vitro. However, whether Bmftz-f1 regulates silk protein expression is unknown.

METHODS

In our study, western blotting and quantitative polymerase chain reactions were conducted to detect the expression of FTZ-F1 in the PSG. Electrophoretic mobility shift, chromatin immunoprecipitation, far-western blotting, bimolecular fluorescence complementation, and dual luciferase reporter assays were performed to investigate the effect of FTZ-F1 on the fibH promoter.

RESULTS

(1) The expression of the hormone receptor BmFTZ-F1 was opposite to that of fibH. It was highly expressed in the PSG during the fourth molting stage and the beginning of the fifth instar, and then its expression decreased gradually until it disappeared at the end of the fifth instar and the wandering stage. (2) We identified a FTZ-F1 response element 390bp upstream of the transcription initiation site of the fibH promoter. (3) BmFTZ-F1 interacted with the basic helix-loop-helix transcription factor Bmdimm. (4) BmFTZ-F1 down-regulated fibH promoter activity and counteracted the effect of Bmdimm on fibH expression.

CONCLUSIONS

Integrating these results, we conclude that BmFTZ-F1 regulates the transcription of fibH by binding to the FTZ-F1 response element in the fibH promoter and counteracts the effect of Bmdimm on fibH expression.

GENERAL SIGNIFICANCE

These findings provide new insights into the mechanism of regulation of the silk protein gene.

Regulation of Silk Genes by Hox and Homeodomain Proteins in the Terminal Differentiated Silk Gland of the Silkworm Bombyx mori

The silk gland of the silkworm Bombyx mori is a long tubular organ that is divided into several subparts along its anteroposterior (AP) axis. As a trait of terminal differentiation of the silk gland, several silk protein genes are expressed with unique regional specificities. Most of the Hox and some of the homeobox genes are also expressed in the differentiated silk gland with regional specificities. The expression patterns of Hox genes in the silk gland roughly correspond to those in embryogenesis showing "colinearity". The central Hox class protein Antennapedia (Antp) directly regulates the expression of several middle silk gland-specific silk genes, whereas the Lin-1/Isl-1/Mec3 (LIM)-homeodomain transcriptional factor Arrowhead (Awh) regulates the expression of posterior silk gland-specific genes for silk fiber proteins. We summarize our results and discuss the usefulness of the silk gland of Bombyx mori for analyzing the function of Hox genes. Further analyses of the regulatory mechanisms underlying the region-specific expression of silk genes will provide novel insights into the molecular bases for target-gene selection and regulation by Hox and homeodomain proteins.

A Hox Gene, Antennapedia, Regulates Expression of Multiple Major Silk Protein Genes in the Silkworm Bombyx mori

Hoxgenes play a pivotal role in the determination of anteroposterior axis specificity during bilaterian animal development. They do so by acting as a master control and regulating the expression of genes important for development. Recently, however, we showed that Hoxgenes can also function in terminally differentiated tissue of the lepidopteranBombyx mori In this species,Antennapedia(Antp) regulates expression of sericin-1, a major silk protein gene, in the silk gland. Here, we investigated whether Antpcan regulate expression of multiple genes in this tissue. By means of proteomic, RT-PCR, and in situ hybridization analyses, we demonstrate that misexpression of Antpin the posterior silk gland induced ectopic expression of major silk protein genes such assericin-3,fhxh4, and fhxh5 These genes are normally expressed specifically in the middle silk gland as is Antp Therefore, the evidence strongly suggests that Antpactivates these silk protein genes in the middle silk gland. The putativesericin-1 activator complex (middle silk gland-intermolt-specific complex) can bind to the upstream regions of these genes, suggesting that Antpdirectly activates their expression. We also found that the pattern of gene expression was well conserved between B. moriand the wild species Bombyx mandarina, indicating that the gene regulation mechanism identified here is an evolutionarily conserved mechanism and not an artifact of the domestication of B. mori We suggest that Hoxgenes have a role as a master control in terminally differentiated tissues, possibly acting as a primary regulator for a range of physiological processes.