MBF2 is a tissue- and stage-specific coactivator that Is regulated at the step of nuclear transport in the silkworm Bombyx mori

20-Hydroxyecdysone Regulates the Expression of 30 Genes Specifically Expressed in Larval Digestive Tube of the Silkworm, Bombyx mori

Nutrients needed to fuel the processes of growth, development, and reproduction, and to form the proteins in the cocoon, are derived from the larval stage. The amount of leaf ingested in the fifth instar silkworm larvae accounted for about 85% of the whole instars. Nutrients were absorbed following digestion in the digestive tube. We previously reported the successful identification of 227 silkworm larval digestive juice proteins, wherein most of the genes of the identified proteins had high expression features in the midgut, and 30 of them were midgut-specific genes. We chose 30 genes and detected their expression characteristics in the digestive tube, and investigated the developmental expression profiles in the midgut of silkworm larvae from the day-3 fourth instar to the wandering period, as well as the expression dynamics after the 20-hydroxyecdysone (20E) treatment. The genes were specifically expressed in the larval digestive tube. The expression of BMSK0013805 was activated, and that of the other 29 genes were suppressed by 20E. The ecdysteroid ingestion dramatically suppressed the protease activity in the larval midgut, which revealed that 20E may also directly affect digestive activity. The present study is the first to demonstrate that the expression control mechanism of these genes is primarily hormonal regulation, which contributes to furthering our understanding of the mechanism of the genes expressed in the larval digestive tube during development.

SfREPAT38, a pathogen response gene (REPAT), is involved in immune response of Spodoptera frugiperda larvae through mediating Toll signalling pathway

REPAT (response to pathogen) is an immune-associated gene family that plays important roles in insect immune response to pathogens. Although nine REPAT genes have been identified in Spodoptera frugiperda (Lepidoptera: Noctuidae) currently, their functions and mechanisms in the immune response to pathogens still remain unclear. Therefore, SfREPAT38, a pathogen response gene (REPAT) of S. frugiperda, was characterised and its function was analysed. The results showed that SfREPAT38 contains a signal peptide and a transcription activator MBF2 (multi-protein bridging factor 2) domain. Quantitative real-time polymerase chain reaction analysis showed that SfREPAT38 was highly expressed in the sixth-instar larvae (L6) and was the highest in expression in the midgut of L6. We found that the expression of SfREPAT38 could be activated by challenge with four microbial pathogens (Bacillus thuringiensis, Metarhizium anisopliae, Spodoptera exigua nuclearpolyhedrosis and Escherichia coli), except 12 h after E. coli infection. Furthermore, the SfREPAT38 expression levels significantly decreased at 24, 48 and 72 h after SfREPAT38 dsRNA injection or feeding. Feeding with SfREPAT38 dsRNA significantly decreased the weight gain of S. frugiperda, and continuous feeding led to the death of S. frugiperda larvae from the fourth day. Moreover, SfREPAT38 dsRNA injection resulted in a significant decrease of weight gain on the fifth day. Silencing SfREPAT38 gene down-regulated the expression levels of immune genes belonging to the Toll pathway, including SPZ, Myd88, DIF, Cactus, Pell and Toll18W. After treatment with SfREPAT38 dsRNA, S. frugiperda became extremely sensitive to the B. thuringiensis infection, and the survival rate dramatically increased, with 100% mortality by the eighth day. The weight of S. frugiperda larvae was also significantly lower than that of the control groups from the second day onwards. In addition, the genes involved in the Toll signalling pathway and a few antibacterial peptide related genes were down-regulated after treatment. These results showed that SfREPAT38 is involved in the immune response of S. frugiperda larvae through mediating Toll signalling pathway.

An effector of Phthorimaea absoluta oral secretions inhibits host plant defense

Insects have evolved effectors to regulate host defenses for efficient feeding, yet their impact on chewing insects, like the tomato leaf miner (Phthorimaea absoluta), a significant pest, is poorly understood. We used RNAi to target the REPAT38 gene in larvae, monitoring changes at 0.5, 1, 2, and 4 h in leaf stomata, plant hormone concentrations (jasmonic acid (JA), jasmonoyl-L-isoleucine (JA-Ile), salicylic acid (SA), ethylene (ET), and abscisic acid (ABA)), and 12 hormone-responsive genes to explore the molecular mechanism of REPAT38-mediated plant-insect interactions. The results showed that the effector induced stomatal closure at 0.5 h and inhibited the synthesis of JA, ET, and ABA at 1 h. Additionally, seven plant hormone-responsive genes-AOC, MYC2, ACS1A, PAL, PR1, EIL2, and SRK2E-were inhibited at various time points. Our data suggest that REPAT38, as an effector with conserved functions, can weaken tomato host defenses and conducive to insect adaptation to host plants.

A high-efficiency transient expression system mediated by Agrobacterium tumefaciens in Spinacia oleracea leaves

BACKGROUND

Optimization of a highly efficient transient expression system is critical for the study of gene function, particularly in those plants in which stable transformation methods are not widely available. Agrobacterium tumefaciens‑mediated transient transformation is a simple and low-cost method that has been developed and applied to a wide variety of plant species. However, the transient expression in spinach (Spinacia oleracea L.) is still not reported.

RESULTS

We developed a transient expression system in spinach leaves of the Sp75 and Sp73 varieties. Several factors influencing the transformation efficiency were optimized such as Agrobacterium strain, spinach seedling stage, leaf position, and the expression time after injection. Agrobacterium strain GV3101 (pSoup-p19) was more efficient than AGL1 in expressing recombinant protein in spinach leaves. In general, Sp75 leaves were more suitable than Sp73 leaves, regardless of grow stage. At four-leaf stage, higher intensity and efficiency of transient expression were observed in group 1 (G1) of Sp75 at 53 h after injection (HAI) and in G1 of Sp73 at 64 HAI. At six-leaf stage of Sp75, group 3 (G3) at 72 HAI were the most effective condition for transient expression. Using the optimized expression system, we detected the subcellular localization of a transcriptional co-activator SoMBF1c and a NADPH oxidase SoRbohF. We also detected the interaction of the protein kinase SoCRK10 and the NADPH oxidase SoRbohB.

CONCLUSION

This study established a method of highly efficient transient expression mediated by Agrobacterium in spinach leaves. The transient expression system will facilitate the analysis of gene function and lay a solid foundation for molecular design breeding of spinach.

The transcriptional coactivator CmMBF1c is required for waterlogging tolerance in Chrysanthemum morifolium

Waterlogging is one of the most serious abiotic stressors affecting Chrysanthemum morifolium during its lifespan. However, the molecular mechanisms underlying the waterlogging tolerance of chrysanthemum remain unclear. In this study, we discovered that the transcriptional coactivator MULTIPROTEIN BRIDGING FACTOR 1c (CmMBF1c) was significantly induced by waterlogging stress in chrysanthemums. Promoter sequence analysis and transient dual-luciferase assay using chrysanthemum protoplasts showed that the waterlogging-tolerant cultivar 'Nannongxuefeng' carried more response elements involved in waterlogging and hypoxia stress compared with the waterlogging-sensitive cultivar 'Qinglu', conferring on 'Nannongxuefeng' a stronger hypoxia responsive activity and higher CmMBF1c expression under waterlogging conditions. Subcellular localization and transcriptional activity assays showed that CmMBF1c protein was localized to the nucleus and had no transcriptional activation activity. Overexpression of CmMBF1c in 'Qinglu' enhanced its waterlogging tolerance by promoting its reactive oxygen species (ROS) scavenging ability and maintaining low ROS levels. However, RNAi-mediated knockdown of CmMBF1c in cultivar 'Nannongxuefeng' resulted in the opposite tendency. Yeast two-hybrid screening and tobacco bimolecular fluorescence complementation assays revealed that CmHRE2, a pivotal regulator of hypoxia response, could interact with CmMBF1c. In summary, this study demonstrates that CmMBF1c improves chrysanthemum waterlogging tolerance by regulating its ROS signaling pathway and interacting with CmHRE2. These findings together offer, to our knowledge, new mechanistic insights into chrysanthemum waterlogging tolerance and provide a rational foundation for future research on the genetic improvement of horticultural crops for waterlogging stress tolerance.

A transcriptomic atlas of Aedes aegypti reveals detailed functional organization of major body parts and gut regional specializations in sugar-fed and blood-fed adult females

Mosquitoes transmit numerous pathogens, but large gaps remain in our understanding of their physiology. To facilitate explorations of mosquito biology, we have created Aegypti-Atlas (http://aegyptiatlas.buchonlab.com/), an online resource hosting RNAseq profiles of Ae. aegypti body parts (head, thorax, abdomen, gut, Malpighian tubules, ovaries), gut regions (crop, proventriculus, anterior and posterior midgut, hindgut), and a gut time course of blood meal digestion. Using Aegypti-Atlas, we provide insights into regionalization of gut function, blood feeding response, and immune defenses. We find that the anterior and posterior midgut possess digestive specializations which are preserved in the blood-fed state. Blood feeding initiates the sequential induction and repression/depletion of multiple cohorts of peptidases. With respect to defense, immune signaling components, but not recognition or effector molecules, show enrichment in ovaries. Basal expression of antimicrobial peptides is dominated by holotricin and gambicin, which are expressed in carcass and digestive tissues, respectively, in a mutually exclusive manner. In the midgut, gambicin and other effectors are almost exclusively expressed in the anterior regions, while the posterior midgut exhibits hallmarks of immune tolerance. Finally, in a cross-species comparison between Ae. aegypti and Anopheles gambiae midguts, we observe that regional digestive and immune specializations are conserved, indicating that our dataset may be broadly relevant to multiple mosquito species. We demonstrate that the expression of orthologous genes is highly correlated, with the exception of a ‘species signature’ comprising a few highly/disparately expressed genes. With this work, we show the potential of Aegypti-Atlas to unlock a more complete understanding of mosquito biology.

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.

Repat33 Acts as a Downstream Component of Eicosanoid Signaling Pathway Mediating Immune Responses of Spodoptera exigua, a Lepidopteran Insect

Repat (=response to pathogen) is proposed for an immune-associated gene family from Spodoptera exigua, a lepidopteran insect. In this gene family, 46 members (Repat1-Repat46) have been identified. They show marked variations in their inducible expression patterns in response to infections by different microbial pathogens. However, their physiological functions in specific immune responses and their interactions with other immune signaling pathways remain unclear. Repat33 is a gene highly inducible by bacterial infections. The objective of this study was to analyze the physiological functions of Repat33 in mediating cellular and humoral immune responses. Results showed that Repat33 was expressed in all developmental stages and induced in immune-associated tissues such as hemocytes and the fat body. RNA interference (RNAi) of Repat33 expression inhibited the hemocyte-spreading behavior which impaired nodule formation of hemocytes against bacterial infections. Such RNAi treatment also down-regulated expression levels of some antimicrobial genes. Interestingly, Repat33 expression was controlled by eicosanoids. Inhibition of eicosanoid biosynthesis by RNAi against a phospholipase A₂ (PLA₂) gene suppressed Repat33 expression while an addition of arachidonic acid (a catalytic product of PLA₂) to RNAi treatment recovered such suppression of Repat33 expression. These results suggest that Repat33 is a downstream component of eicosanoids in mediating immune responses of S. exigua.

Sage controls silk gland development by regulating Dfd in Bombyx mori

Silk gland is an organ that produces and secretes silk proteins. The development of the silk gland is essential for high silk production yield and silk quality. Although Sage reportedly plays a pivotal role in embryonic silk gland development, the mechanism underlying its action remains unclear. Our study aimed to determine the genes downstream of Sage through which it regulates the development of the silk gland. After chromatin immunoprecipitation and sequencing, Dfd was identified as a downstream target gene of Sage and it was confirmed that Sage could inhibit Dfd expression by competing with SGF1. When Dfd was knocked down through RNA interference (RNAi), the number of cells in the middle silk gland decreased, and the posterior silk gland was straightened. Simultaneously, the expression of Ser1 and silk fibroin genes was no longer strictly regional. These changes eventually led to an alteration in the composition of the Dfd RNAi cocoon. In conclusion, our research contributes to a deeper understanding of the development of silk glands.

The plant MBF1 protein family: a bridge between stress and transcription

The Multiprotein Bridging Factor 1 (MBF1) proteins are transcription co-factors whose molecular function is to form a bridge between transcription factors and the basal machinery of transcription. MBF1s are present in most archaea and all eukaryotes, and numerous reports show that they are involved in developmental processes and in stress responses. In this review we summarize almost three decades of research on the plant MBF1 family, which has mainly focused on their role in abiotic stress responses, in particular the heat stress response. However, despite the amount of information available, there are still many questions that remain about how plant MBF1 genes, transcripts, and proteins respond to stress, and how they in turn modulate stress response transcriptional pathways.

Bmsage is involved in the determination of cell number in the silk gland of Bombyx mori

The number of cells in tissues is under strict genetic control, and research on the determination of cell number is of great importance to understand the growth and development of organs. Bmsage, a bHLH transcription factor, is involved in the development of the silk gland during the embryonic stage in Bombyx mori. However, the mechanism by which it influences silk gland development is unclear. In the present study, we determined via immunofluorescence staining during the embryonic stage of Bombyx mori that Bmsage is expressed in silk gland cells from the beginning of development of the silk gland until its complete formation. By comparing different silkworm strains, we found that Bmsage expression is positively correlated with the number of silk gland cells. Bmsage knockdown by RNAi resulted in shorter silk glands and lower cell numbers, especially in the posterior silk gland. The silk gland lumen also shriveled, and the silk protein content was significantly lower than that in the control. Further investigation revealed that all cyclins decreased after knock down of Bmsage, and cyclin B and cyclin 3 were significantly down-regulated. Bmsage may be involved in the regulation of the cyclin pathway to control silk gland development. Taken together, it can be concluded from our results that Bmsage is involved in the determination of cell number in silk glands. Our results help clarify the process of cell number determination in silk gland and identify a potential target for silkworm breeding.

Specific genes related to nucleopolyhedrovirus in Bombyx mori susceptible and near-isogenic resistant strains through comparative transcriptome analysis

Bombyx mori nucleopolyhedrovirus (BmNPV) is one of the primary pathogens that causes severe economic losses to sericulture. Comparative transcriptomics analysis has been widely applied to explore the antiviral mechanism in resistant strains. Here, to identify genes involved in BmNPV infection, we identified differentially expressed genes (DEGs) and performed weighted gene co-expression network analysis (WGCNA) between two Bombyx mori strains: strain 871 (susceptible to BmNPV infection) and the near-isogenic strain 871C (resistant to BmNPV). Our results showed that 400 genes were associated with resistance in strain 871C, and 76 genes were related to susceptibility in strain 871. In addition, the correlation analysis of DEGs and WGCNA showed that 40 genes related to resistance were highly expressed in the resistant strain. Among them, gene BGIBMGA004291 was the most noticeable. We further identified the effect of gene BGIBMGA004291, which encoded a multiprotein bridge factor 2 (MBF2) family member (MBF2-10), on viral infection in cells. Our data suggested that MBF2-10 inhibited viral infection. Taken together, this study showed specific module trait correlations related to viral infection in strains 871 and 871C, and we identified a resistance-related gene. These findings suggested promising candidate genes with antiviral activity, aiding in the analysis of the antiviral molecular mechanisms in resistant strains.

Transcriptional Responses of the Trichoplusia ni Midgut to Oral Infection by the Baculovirus Autographa californica Multiple Nucleopolyhedrovirus

Baculoviruses are large double-stranded DNA viruses that are virulent pathogens of certain insect species. In a natural host, Trichoplusia ni, infection by the model baculovirus Autographa californica multiple nucleopolyhedrovirus (AcMNPV) begins when the occluded form of the virus disassembles in the midgut and virions infect midgut epithelial cells to establish the primary phase of the infection. To better understand the primary phase of the AcMNPV infection cycle, newly molted 5th-instar T. ni larvae were orally infected with AcMNPV occlusion bodies and the transcriptional responses of the T. ni midgut were analyzed at various times from 0 to 72 h postinfection, using transcriptome sequencing analysis and a T. ni reference genome. The numbers of differentially expressed host genes increased as the infection progressed, and we identified a total of 3,372 differentially expressed T. ni transcripts in the AcMNPV-infected midgut. Genes encoding orthologs of HMG176, atlastin, and CPH43 were among the most dramatically upregulated in response to AcMNPV infection. A number of cytochrome P450 genes were downregulated in response to infection. We also identified the effects of AcMNPV infection on a large variety of genes associated with innate immunity. This analysis provides an abundance of new and detailed information on host responses to baculovirus infection during the primary phase of the infection in the midgut and will be important for understanding how baculoviruses establish productive infections in the organism.IMPORTANCE Baculoviruses are virulent pathogens of a number of important insect pest species. In the host Trichoplusia ni, infection begins in the midgut when infectious virions of the occlusion-derived virus (ODV) phenotype enter and subsequently replicate in cells of the midgut epithelium. A second virion phenotype (budded virus [BV]) is produced there, and BV mediates systemic infection of the animal. Most prior detailed studies of baculovirus infections have focused on BV infections of cultured cells. In this study, we examined the transcriptional responses of the T. ni midgut to infection by ODV of the baculovirus AcMNPV and identified a variety of host genes that respond dramatically to viral infection. Understanding the transcriptional responses of the host midgut to viral infection is critically important for understanding the biphasic infection in the animal as a whole.

Identification of MBF2 family genes in Bombyx mori and their expression in different tissues and stages and in response to Bacillus bombysepticus infection and starvation

The Multiprotein bridge factor 2 (MBF2) gene was first identified as a co-activator involved in BmFTZ-F1-mediated activation of the Fushi tarazu gene. Herein, nine homologous genes of MBF2 gene are identified. Evolutionary analysis showed that this gene family is insect-specific and that the family members are closely related to response to pathogens (REPAT) genes. Tissue distribution analysis revealed that these genes could be expressed in a tissue-specific manner. Developmental profiles analysis showed that the MBF2 gene family members were highly expressed in the different stages. Analysis of the expression patterns of nine MBF2 family genes showed that Bacillus bombysepticus treatment induced the up-regulation of several MBF2 family genes, including MBF2-4, -7, -9, -8. Furthermore, we found the MBF2 family genes were modulated by starvation and the expression of these genes recovered upon re-feeding, except for MBF2-5, -9. These findings suggested roles for these proteins in insect defense against pathogens and nutrient metabolism, which has an important guiding significance for designing pest control strategies.

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.

Transcription factor SGF1 is critical for the neurodevelopment in the silkworm, Bombyx mori

FoxA transcription factors play vital roles in regulating the expression of organ-specific genes. BmSGF1, the sole FoxA family member in Bombyx mori, is required for development of the silk gland. However, the function of BmSGF1 in development of the nervous system in the silkworm remains unknown. Here, we show that the amino acids sequence of BmSGF1 is evolutionarily conserved in its middle region from Trichoplax adhaerens to human and diverged from the homologues in most other species in its N-terminal region. BmSGF1 expresses in the nervous system at the embryonic stage. Knockdown of Bmsgf1 by RNA interference (RNAi) results in abnormal development of axons. Therefore, our results demonstrate that BmSGF1 is an indispensable regulator for neurodevelopment.

BmRobo1a and BmRobo1b control axon repulsion in the silkworm Bombyx mori

The development of the nervous system is based on the growth and connection of axons, and axon guidance molecules are the dominant regulators during this course. Robo, as the receptor of axon guidance molecule Slit, plays a key role as a conserved repellent cue for axon guidance during the development of the central nervous system. However, the function of Robo in the silkworm Bombyx mori is unknown. In this study, we cloned two novel robo genes in B. mori (Bmrobo1a and Bmrobo1b). BmRobo1a and BmRobo1b lack an Ig and a FNIII domain in the extracellular region and the CC0 and CC2 motifs in the intracellular region. BmRobo1a and BmRobo1b were colocalized with BmSlit in the neuropil. Knock-down of Bmrobo1a and Bmrobo1b by RNA interference (RNAi) resulted in abnormal development of axons. Our results suggest that BmRobo1a and BmRobo1b have repulsive function in axon guidance, even though their structures are different from Robo1 of other species.

Biochemical characterization and functional analysis of the POU transcription factor POU-M2 of Bombyx mori

POU-M2 is a homeodomain transcription factor which plays important roles in the development and silk synthesis of Bombyx mori. In this study, we expressed, purified and characterized POU-M2 and studied its transcription regulation on fibroin heavy chain gene of Bombyx mori. Gel filtration showed POU-M2 existed as a dimer in solution. Far-UV circular dichroism spectra indicated POU-M2 had a well-defined α-helix structure and the α-helix content was about 26.4%. The thermal unfolding transition of POU-M2 was a cooperative process. Tm, ΔH and ΔS were 45.15 ± 0.2 °C, 138.4 ± 0.5 KJ/mol and 0.4349 ± 0.04 KJ/(mol·K), respectively. Western blotting analysis indicated the expression level of POU-M2 increased slightly from day 3 to day 7 of the fifth instar larvae in the posterior silk gland. POU-M2 was positioned in the nucleus of cells. The luciferase reporter assay demonstrated POU-M2 could stimulate the promoter activity of fibroin heavy chain gene, and the activation effect was dependent on the amount of POU-M2. Our study suggested POU-M2 may be involved in the transcriptional regulation of fibroin heavy chain gene. These findings expand toward a better understanding of the structure of POU-M2 and its function in silk synthesis of Bombyx mori.

BmRobo2/3 is required for axon guidance in the silkworm Bombyx mori

Axon guidance is critical for proper wiring of the nervous system. During the neural development, the axon guidance molecules play a key role and direct axons to choose the correct way to reach the target. Robo, as the receptor of axon guidance molecule Slit, is evolutionarily conserved from planarians to humans. However, the function of Robo in the silkworm, Bombyx mori, remained unknown. In this study, we cloned robo2/3 from B. mori (Bmrobo2/3), a homologue of robo2/3 in Tribolium castaneum. Moreover, BmRobo2/3 was localized in the neuropil, and RNAi-mediated knockdown of Bmrobo2/3 resulted in the longitudinal connectives forming closer to the midline. These data demonstrate that BmRobo2/3 is required for axon guidance in the silkworm.

The homeodomain transcription factors antennapedia and POU-M2 regulate the transcription of the steroidogenic enzyme gene Phantom in the silkworm

The steroid hormone ecdysone, which controls insect molting and metamorphosis, is synthesized in the prothoracic gland (PG), and several steroidogenic enzymes that are expressed specifically in the PG are involved in ecdysteroidogenesis. In this study, we identified new regulators that are involved in the transcriptional control of the silkworm steroidogenic enzyme genes. In silico analysis predicted several potential cis-regulatory elements (CREs) for the homeodomain transcription factors Antennapedia (Antp) and POU-M2 in the proximal promoters of steroidogenic enzyme genes. Antp and POU-M2 are expressed dynamically in the PG during larval development, and their overexpression in silkworm embryo-derived (BmE) cells induced the expression of steroidogenic enzyme genes. Importantly, luciferase reporter analyses, electrophoretic mobility shift assays, and chromatin immunoprecipitation assays revealed that Antp and POU-M2 promote the transcription of the silkworm steroidogenic enzyme gene Phantom (Phm) by binding directly to specific motifs within overlapping CREs in the Phm promoter. Mutations of these CREs in the Phm promoter suppressed the transcriptional activities of both Antp and POU-M2 in BmE cells and decreased the activities of mutated Phm promoters in the silkworm PG. In addition, pulldown and co-immunoprecipitation assays demonstrated that Antp can interact with POU-M2. Moreover, RNA interference-mediated down-regulation of either Antp or POU-M2 during silkworm wandering not only decreased the ecdysone titer but also led to the failure of metamorphosis. In summary, our results suggest that Antp and POU-M2 coordinate the transcription of the silkworm Phm gene directly, indicating new roles for homeodomain proteins in regulating insect ecdysteroidogenesis.

A juvenile hormone transcription factor Bmdimm-fibroin H chain pathway is involved in the synthesis of silk protein in silkworm, Bombyx mori

The genes responsible for silk biosynthesis are switched on and off at particular times in the silk glands of Bombyx mori. This switch appears to be under the control of endogenous and exogenous hormones. However, the molecular mechanisms by which silk protein synthesis is regulated by the juvenile hormone (JH) are largely unknown. Here, we report a basic helix-loop-helix transcription factor, Bmdimm, its silk gland-specific expression, and its direct involvement in the regulation of fibroin H-chain (fib-H) by binding to an E-box (CAAATG) element of the fib-H gene promoter. Far-Western blots, enzyme-linked immunosorbent assays, and co-immunoprecipitation assays revealed that Bmdimm protein interacted with another basic helix-loop-helix transcription factor, Bmsage. Immunostaining revealed that Bmdimm and Bmsage proteins are co-localized in nuclei. Bmdimm expression was induced in larval silk glands in vivo, in silk glands cultured in vitro, and in B. mori cell lines after treatment with a JH analog. The JH effect on Bmdimm was mediated by the JH-Met-Kr-h1 signaling pathway, and Bmdimm expression did not respond to JH by RNA interference with double-stranded BmKr-h1 RNA. These data suggest that the JH regulatory pathway, the transcription factor Bmdimm, and the targeted fib-H gene contribute to the synthesis of fibroin H-chain protein in B. mori.

Evolutionarily conserved repulsive guidance role of slit in the silkworm Bombyx mori

Axon guidance molecule Slit is critical for the axon repulsion in neural tissues, which is evolutionarily conserved from planarians to humans. However, the function of Slit in the silkworm Bombyx mori was unknown. Here we showed that the structure of Bombyx mori Slit (BmSlit) was different from that in most other species in its C-terminal sequence. BmSlit was localized in the midline glial cell, the neuropil, the tendon cell, the muscle and the silk gland and colocalized with BmRobo1 in the neuropil, the muscle and the silk gland. Knock-down of Bmslit by RNA interference (RNAi) resulted in abnormal development of axons and muscles. Our results suggest that BmSlit has a repulsive role in axon guidance and muscle migration. Moreover, the localization of BmSlit in the silk gland argues for its important function in the development of the silk gland.

Basic helix-loop-helix transcription factor Bmsage is involved in regulation of fibroin H-chain gene via interaction with SGF1 in Bombyx mori

Silk glands are specialized in the synthesis of several secretory proteins. Expression of genes encoding the silk proteins in Bombyx mori silk glands with strict territorial and developmental specificities is regulated by many transcription factors. In this study, we have characterized B. mori sage, which is closely related to sage in the fruitfly Drosophila melanogaster. It is termed Bmsage; it encodes transcription factor Bmsage, which belongs to the Mesp subfamily, containing a basic helix-loop-helix motif. Bmsage transcripts were detected specifically in the silk glands of B. mori larvae through RT-PCR analysis. Immunoblotting analysis confirmed the Bmsage protein existed exclusively in B. mori middle and posterior silk gland cells. Bmsage has a low level of expression in the 4th instar molting stages, which increases gradually in the 5th instar feeding stages and then declines from the wandering to the pupation stages. Quantitative PCR analysis suggested the expression level of Bmsage in a high silk strain was higher compared to a lower silk strain on day 3 of the larval 5th instar. Furthermore, far western blotting and co-immunoprecipitation assays showed the Bmsage protein interacted with the fork head transcription factor silk gland factor 1 (SGF1). An electrophoretic mobility shift assay showed the complex of Bmsage and SGF1 proteins bound to the A and B elements in the promoter of fibroin H-chain gene(fib-H), respectively. Luciferase reporter gene assays confirmed the complex of Bmsage and SGF1 proteins increased the expression of fib-H. Together, these results suggest Bmsage is involved in the regulation of the expression of fib-H by being together with SGF1 in B. mori PSG cells.

betaFTZ-F1 and Broad-Complex positively regulate the transcription of the wing cuticle protein gene, BMWCP5, in wing discs of Bombyx mori

The present study was undertaken to clarify the mechanism regulating cuticle protein gene expression. Expression of BMWCP5 was strong at around pupation and weak at the mid-pupal stage in wing tissues of Bombyx mori. We analyzed the upstream region of the BMWCP5 gene using a transient reporter assay with a gene gun system to identify the regulatory elements responsible for its unique expression pattern. We identified two betaFTZ-F1 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. We also found that the regions carrying the binding sites for the ecdysone-responsive factor BR-C Z4 (BR-Z4) were responsible for the hormonal enhancement of the reporter gene activity in wing discs. Mutation of the BR-Z4 binding sites decreased the reporter activity. The nuclear proteins that bound to these betaFTZ-F1 and BR-Z4 sites were identified by an electrophoretic mobility shift assay (EMSA). The results demonstrate for the first time that the BR-Z4 isoform can bind to the upstream region of the cuticle protein gene, BMWCP5, and activate its expression. The results also suggest that the BMWCP5 transcription is primarily regulated by the ecdysone pulse through betaFTZ-F1, and the stage-specific enhancement is brought about through BR-Z4.

Role of multiprotein bridging factor 1 in archaea: bridging the domains?

MBF1 (multiprotein bridging factor 1) is a highly conserved protein in archaea and eukaryotes. It was originally identified as a mediator of the eukaryotic transcription regulator BmFTZ-F1 (Bombyx mori regulator of fushi tarazu). MBF1 was demonstrated to enhance transcription by forming a bridge between distinct regulatory DNA-binding proteins and the TATA-box-binding protein. MBF1 consists of two parts: a C-terminal part that contains a highly conserved helix-turn-helix, and an N-terminal part that shows a clear divergence: in eukaryotes, it is a weakly conserved flexible domain, whereas, in archaea, it is a conserved zinc-ribbon domain. Although its function in archaea remains elusive, its function as a transcriptional co-activator has been deduced from thorough studies of several eukaryotic proteins, often indicating a role in stress response. In addition, MBF1 was found to influence translation fidelity in yeast. Genome context analysis of mbf1 in archaea revealed conserved clustering in the crenarchaeal branch together with genes generally involved in gene expression. It points to a role of MBF1 in transcription and/or translation. Experimental data are required to allow comparison of the archaeal MBF1 with its eukaryotic counterpart.

Quantification of silkworm coactivator of MBF1 mRNA by SYBR Green I real-time RT-PCR reveals tissue- and stage-specific transcription levels

Transcriptional coactivators play a crucial role in gene transcription and expression. Multiprotein bridging factor 1 (MBF1) is a transcriptional coactivator necessary for transcriptional activation caused by DNA-binding activators, such as FTZ-F1 and GCN4. Until now, very few studies have been reported in the silkworm. We selected the Bombyx mori because it is a model insect and acts as an economic animal for silk industry. In this study, we conducted the quantitative analysis of MBF1 mRNA in silkworm B. mori L. with actin (A3) as internal standard by means of SYBR Green I real-time RT-PCR method. The total RNA was extracted from the silk gland, epidermis, fat body, and midguts of the fifth instar B. mori larvae. The mRNA was reverse transcripted, and the cDNA fragments of MBF1 mRNA and actin gene were amplified by RT-PCR using specific primers. MBF1 mRNA expression in different tissues of silkworm B. mori L. was quantified using standardized SYBR Green I RT-PCR. The results suggested MBF1 gene was expressed in all investigated organs but highly expressed in the silk gland, showing its relation to biosynthesis of silk proteins.

Development of an effective sample preparation approach for proteomic analysis of silkworm eggs using two-dimensional gel electrophoresis and mass spectrometry

Sample preparation is still the first and important step toward successful two-dimensional gel electrophoresis (2DE) and identification in proteomics study. The 2DE profiling of eggs of silkworm species by using conventional one-step extraction, however, is unsatisfactory because high-abundance proteins such as egg-specific protein (ESP) and No 30 family (30 KP) in the extract lead to difficulties in detecting most of biologically relevant proteins. Based on the tendency of these abundant proteins to be soluble in Tris-HCl buffer, we report herein a robust approach in which the extract enriched in ESP and 30 KP was fractionationed and mixed with the re-extract of residual pellet in an optimal proportion. In comparison with the one-step method, the 2DE pattern was improved by this new method with over one-third enhancement in spots. A total of 48 unique proteins obtained have been furthermore identified by mass spectrometry (MS) and MS/MS. The identified proteins are found to include heat shock proteins families, ribosomal proteins, disulfide isomerase proteins, Glutathione S-transferase, and elongation factor, etc., which are mainly involved in some important processes. To our knowledge, this is the first time that the several proteins have been detected in silkworm eggs by proteomics means. This simple and reproducible approach would raise the opportunity of discovering and identifying more biomarkers and determining their possible roles in further studies.

Transcriptional coactivator MBF1s from Arabidopsis predominantly localize in nucleolus

Multiprotein bridging factor 1 (MBF1) is a transcriptional coactivator. It has been reported that MBF1 changed its subcellular localization from cytoplasm into nuclei with a transcriptional activator for activation of a target gene expression in animals. We found that Arabidopsis MBF1s (AtMBF1s) predominantly localize in nucleolus. We previously reported that plant MBF1s were rapidly induced by several stresses, whereas animal MBF1s were not induced. Therefore, we suggest that MBF1-function in plants is controlled on the level of transcriptional induction but not by nuclear translocation, dissimilar from the case of MBF1s from animals.

Drosophila MBF1 is a co-activator for Tracheae Defective and contributes to the formation of tracheal and nervous systems

During gene activation, the effect of binding of transcription factors to cis-acting DNA sequences is transmitted to RNA polymerase by means of co-activators. Although co-activators contribute to the efficiency of transcription, their developmental roles are poorly understood. We used Drosophila to conduct molecular and genetic dissection of an evolutionarily conserved but unique co-activator, Multiprotein Bridging Factor 1 (MBF1), in a multicellular organism. Through immunoprecipitation, MBF1 was found to form a ternary complex including MBF1, TATA-binding protein (TBP) and the bZIP protein Tracheae Defective (TDF)/Apontic. We have isolated a Drosophila mutant that lacks the mbf1 gene in which no stable association between TBP and TDF is detectable, and transcription of a TDF-dependent reporter gene is reduced by 80%. Although the null mutants of mbf1 are viable, tdf becomes haploinsufficient in mbf1-deficient background, causing severe lesions in tracheae and the central nervous system, similar to those resulting from a complete loss of tdf function. These data demonstrate a crucial role of MBF1 in the development of tracheae and central nervous system.