Comparison of factors that may affect the inhibitory efficacy of transgenic RNAi targeting of baculoviral genes in silkworm, Bombyx mori

Fibroinase plays a vital role in silk gland degeneration by regulating autophagy and apoptosis in the silkworm, Bombyx mori

The silkworm is an incredibly valuable insect that produces silk through its silk gland. Within this organ, Fibroinase has been identified and named due to its ability to fibroin degradation. The expression of Fibroinase in the silk gland significantly increases during the larval-pupal stage, which might be associated with the degeneration of the silk gland. In this study, Fibroinase was overexpressed and knocked down specifically both in the middle and posterior silk glands, respectively, using transgenic technology. The investigation of silk gland development in these transgenic silkworms showed that Fibroinase plays a direct role in accelerating silk gland degeneration. The staining analyses performed in the silk glands of transgenic silkworms suggest that Fibroinase is involved in the processes of autophagy and apoptosis during silk gland degeneration. Further experiments demonstrated that Fibroinase, acting as a lysosomal regulator, negatively regulates autophagy via the mTOR (mechanistic target of rapamycin) pathway. Moreover, during apoptosis, Fibroinase could activate Caspase3 by increasing the activity of BmCaspase1, ultimately accelerating the apoptosis process. These findings enhance our understanding of the physiological role of Fibroinase in promoting silk gland degeneration, which plays a role in breaking down proteins in the silk gland and coordinating the regulation of autophagy and apoptosis.

Fatty Acid Desaturase Bmdesat5, Suppressed in the Salivary Glands by Domestication, is Involved in Regulation of Food Intake in Silkworms

Understanding the evolutionary genetics of food intake regulation in domesticated animals has relevance to evolutionary biology, animal improvement, and obesity treatment. Here, we observed that the fatty acid desaturase gene (Bmdesat5), which regulates food intake, is suppressed in domesticated silkworms, but expressed in the salivary glands of the wild silkworm Bombyx mandarina. The content of its catalytic product, cis-vaccenic acid, was related to the expression levels of Bmdesat5 in the salivary glands of domesticated and wild silkworm strains. These two strains also showed significant differences in food intake. Using orally administering cis-vaccenic acid and transgenic-mediated overexpression, we verified that cis-vaccenic acid functions as a satiation signal, regulating food intake and growth in silkworms. Selection analysis showed that Bmdesat5 experienced selection, especially in the potential promoter, 5'-untranslated, and intron regions. This study highlights the importance of the decrement of satiety in silkworm domestication and provides new insights into the potential involvement of salivary glands in the regulation of satiety in animals, by acting as a supplement to gut-brain nutrient signaling.

BmSPP is a virus resistance gene in Bombyx mori

Introduction

Signal peptide peptidase (SPP) is an intramembrane protease involved in a variety of biological processes, it participates in the processing of signal peptides after the release of the nascent protein to regulate the endoplasmic reticulum associated degradation (ERAD) pathway, binds misfolded membrane proteins, and aids in their clearance process. Additionally, it regulates normal immune surveillance and assists in the processing of viral proteins. Although SPP is essential for many viral infections, its role in silkworms remains unclear. Studying its role in the silkworm, Bombyx mori , may be helpful in breeding virus-resistant silkworms.

Methods

First, we performed RT-qPCR to analyze the expression pattern of BmSPP. Subsequently, we inhibited BmSPP using the SPP inhibitor 1,3-di-(N-carboxybenzoyl-L-leucyl-L-leucylaminopropanone ((Z-LL)2-ketone) and downregulated the expression of BmSPP using CRISPR/Cas9 gene editing. Furthermore, we assessed the impact of these interventions on the proliferation of Bombyx mori nucleopolyhedrovirus (BmNPV).

Results

We observed a decreased in the expression of BmSPP during viral proliferation. It was found that higher concentration of the inhibitor resulted in greater inhibition of BmNPV proliferation. The down-regulation of BmSPP in both in vivo and in vitro was found to affect the proliferation of BmNPV. In comparison to wild type silkworm, BmSPPKO silkworms exhibited a 12.4% reduction in mortality rate.

Discussion

Collectively, this work demonstrates that BmSPP plays a negative regulatory role in silkworm resistance to BmNPV infection and is involved in virus proliferation and replication processes. This finding suggests that BmSPP servers as a target gene for BmNPV virus resistance in silkworms and can be utilized in resistance breeding programs.

Transcription factor STAT enhanced antimicrobial activities in Bombyx mori

STAT, a transcription factor in the JAK/STAT signaling pathway, regulates immune response to pathogens. In the silkworm (Bombyx mori), STAT exists as two split-forms, STAT-S and STAT-L. However, the role of STAT in silkworm immunity remains unclear. Our purpose was to investigate the effect of STAT on the expression of antimicrobial peptide (AMP) genes and resistance against pathogens. The expression levels of STAT-S and STAT-L were significantly up-regulated after induction by pathogenic microorganisms. In BmE cells, lipopolysaccharide (LPS), peptidoglycan (PGN) and β-glucan stimulated STAT-S and STAT-L to transfer from the cytoplasm to the nucleus. We found that overexpression of STAT-S and STAT-L in cells could promote the expression of AMPs. We generated transgenic silkworm lines overexpressing STAT-L or STAT-S (OE-STAT-S; OE-STAT-L) or interfering with STAT (A4-dsSTAT). Overexpression of STAT-S and STAT-L upregulated the expression of AMP genes in the OE-STAT-S and OE-STAT-L, increased the survival rates of the OE-STAT-S silkworms and lowered the mortality of OE-STAT-L silkworms infected with S. aureus or Beauveria bassiana. By contrast, the death rate of A4-dsSTAT silkworms was higher after infection with these pathogenic microorganisms. These findings may provide insights into the role of STAT in the antimicrobial immune response of silkworms.

A Study on the Effect of Energy on the Development of Silkworm Embryos Using an Estrogen-Related Receptor

Energy metabolism is a fundamental process in all organisms. During silkworm (Bombyx mori) embryonic development, there is a high demand for energy due to continuous cell proliferation and differentiation. Estrogen-related receptors (ERRs) are transcriptional regulatory factors that play crucial roles in mammalian energy storage and expenditure. Although most insects have one ERR gene, it also participates in the regulation of energy metabolism, including carbohydrate metabolism in Drosophila, Aphid, and Silkworm. However, no study has reported the direct impact of energy metabolism on embryonic development in silkworms. In this study, we used transgenic technology to increase silkworm (B. mori; Bm) BmERR expression during embryonic development and explored the impact of energy on embryonic development. We found no significant change in the quality of silkworm eggs compared to that of wild-type silkworms. However, there was an increase in the consumption of vitellin, a major nutrient in embryos. This resulted in a decrease in glucose content and a significant increase in ATP content. These findings provide evidence that the acceleration of energy metabolism promotes embryonic development and enhances the motility of hatched silkworms. In addition, these results provide a novel perspective on the relationship between energy metabolism and embryonic development in other insects.

Common strategies in silkworm disease resistance breeding research

The silkworm, which is considered a model invertebrate organism, was the first insect used for silk production in human history and has been utilized extensively throughout its domestication. However, sericulture has been plagued by various pathogens that have  caused significant economic losses. To enhance the resistance of a host to its pathogens,numerous strategies have been developed. For instance, gene-editing techniques have been applied to a wide range of organisms, effectively solving a variety of experimental problems. This review focuses on several common silkworm pests and their pathogenic mechanisms, with a particular emphasis on breeding for disease resistance to control multiple types of silkworm diseases. The review also compares the advantages and disadvantages of transgenic technology and gene-editing systems. Finally, the paper provides a brief summary of current strategies used in breeding silkworm disease resistance, along with a discussion of the establishment of existing technologies and their future application prospects. © 2023 Society of Chemical Industry.

The development of single-chain antibody anchored on the BmE cell membrane to inhibit BmNPV infection

Bombyx mori nucleopolyhedrovirus (BmNPV) poses a significant threat to sericulture production, and traditional sanitation practices remain the main strategy for controlling BmNPV infection. Although RNAi targeting BmNPV genes engineered into transgenic silkworms has shown to be a promising approach in reducing viral infection, it cannot block viral entry into host cells. Therefore, there is an urgent need to develop new effective prevention and control measures. In this study, we screened a monoclonal antibody 6C5 that potently neutralizes BmNPV infection by clamping the internal fusion loop of the BmNPVglycoprotein64 (GP64). Furthermore, we cloned the VH and VL fragments of mAb-6C5 from the hybridoma cell, and the eukaryotic expression vector of scFv6C5 was constructed to anchor the antibody on the cell membrane. The GP64 fusion loop antibody-expressing cells exhibited a reduced capacity for BmNPV infection. The results from our study provide a novel BmNPV control strategy and lay the foundation for the future development of transgenic silkworms with improved antiviral efficacy.

The Mechanisms of Silkworm Resistance to the Baculovirus and Antiviral Breeding

Silkworm (Bombyx mori) is not only an economic insect but also a model organism for life science research. Bombyx mori nucleopolyhedrovirus (BmNPV) disease is a major infectious disease in the world's sericulture industry. The cocoon loss caused by this disease accounts for more than 60% of the total loss caused by all silkworm diseases. To date, there has been no effective solution for preventing and treating this disease. The most effective measure is to breed disease-resistant varieties. The quickest way to breed disease-resistant varieties is to apply genetic modification. However, this requires that we obtain disease resistance genes and know the mechanism of disease resistance. Since the discovery of disease-resistant resources in 1989, scholars in the sericulture industry around the world have been inspired to search for resistance genes. In the past two decades, with the help of multi-omics technologies, screening of resistance genes, gene localization, protein modification, virus-host interactions, etc., researchers have found some candidate genes that have been proposed to function at the cellular or individual level. Several disease-resistant varieties have been obtained and used in production through hybrid breeding, RNA interference, and genetic modification. This article summarizes and reviews the discovery of and research advances related to silkworm resistance to BmNPV. It is anticipated that the review will inspire scientific researchers to continue searching for disease resistance genes, clarify the molecular mechanism of silkworm disease resistance, and promote disease-resistant silkworm breeding.

Analysis of Silver Nanoparticles for the Treatment and Prevention of Nucleopolyhedrovirus Affecting Bombyx mori

Bombyx mori nucleopolyhedrovirus (BmNPV) causes major economic losses in sericulture. A number of agents have been employed to treat viral diseases. Silver nanoparticles (AgNPs) have wide applications in biomedical fields due to their unique properties. The anti-BmNPV effect of AgNPs has been evaluated, however, there are insufficient studies concerning its toxicity to other organisms and the environment. We chemically synthesized biocompatible BSA-AgNPs with a diameter range of 2–4 nm and characterized their physical properties. The toxicity of AgNPs towards cells and larvae with different concentrations was examined; the results indicated a biofriendly effect on cells and larvae within specific concentration ranges. The SEM observation of the surface of BmNPV after treatment with AgNPs suggested that AgNPs could destroy the polyhedral structure, and the same result was obtained by Coomassie blue staining. Further assays confirmed the weakened virulence of AgNPs-treated BmNPV toward cells and larvae. AgNPs also could effectively inhibit the replication of BmNPV in infected cells and larvae. In summary, our research provides valuable data for the further development of AgNPs as an antiviral drug for sericulture.

β-Arrestin 2 acts an adaptor protein that facilitates viral replication in silkworm

β-Arrestin 2 is known to be a widely distributed adaptor protein in mammals but its function has never been reported in Lepidoptera insects. Herein, the β-Arrestin 2 (BmArrestin 2) gene from silkworm was cloned and characterized. The spatiotemporal expression level of BmArrestin 2 was highest in the gonads at the 3rd day of 5th instar, whereas the highest and lowest abundance of BmArrestin 2 were identified in the tracheal and testis, respectively. BmArrestin 2 is mainly distributed in the cytoplasm. Furthermore, in BmN cells，overexpression of BmArrestin 2 promoted Bombyx mori nucleopolyhedrovirus (BmNPV) and B. mori cytoplasmic polyhedrosis virus (BmCPV) replication as the increment of the concentration of plasmid transfection, whereas silencing the gene with specific siRNA inhibited viral replication. Replication of BmNPV and BmCPV also was weakened using BmArrestin 2 antiserum as the increment of the concentration. Immunofluorescent staining revealed the invasion of recombinant BmNPV or BmCPV was decreased after blocking endogenous BmArrestin 2. On the other hand, BmArrestin 2 co-localizes with recombinant BmNPV and BmCPV virions in BmN cells. These results suggest that BmArrestin 2 may represent a novel target for antiviral strategies, as it is an adaptor protein that plays a key role in virus replication.

Transgenic genome editing-derived antiviral therapy to nucleopolyhedrovirus infection in the industrial strain of the silkworm

The silkworm (Bombyx mori) is a domesticated and economically important insect. During the whole growth period, silkworm suffers various pathogen infection. Nearly 80% of silk cocoon crop losses are attributed to viral diseases. The circular double-stranded DNA virus Bombyx mori nuclepolyhedrovirus (BmNPV) is the major viral pathogen responsible for massive silkworm death and huge economic losses in the sericulture industry. Up to now, almost all the new strategies for developing immunity against BmNPV are in laboratory strains because of the lack of transgenic technology in industrial silkworm strains. We previously demonstrated that modification of BmNPV genome DNA with the antivirus transgenic CRISPR/Cas9 system effectively improved the resistance of laboratory silkworm strains to viral pathogens. The industrial strains are monovoltine or bivoltine. It is very difficult to break the diapause before microinjection for genetic transformation. Here, we show that the anti-BmNPV transgenic CRISPR/Cas9 system also works in the industrial silkworm strain Jingsong. In this study, we successfully broke diapause and obtained generation-skipping embryos and constructed two TG Jingsong lines. Both lines exhibited significantly enhanced immunity to BmNPV without significant changes in most of the commercially important traits. These results demonstrate that the construction of TG silkworm lines carrying a heritable immune defense system against BmNPV could be an effective strategy to enhance the resistance of industrial silkworm strains to the most devastating DNA virus. The research opened a window for genetic modification of the important strains from laboratory strains to industrial strains.

Precocious Metamorphosis of Silkworm Larvae Infected by BmNPV in the Latter Half of the Fifth Instar

The mulberry silkworm (Bombyx mori) is a model organism, and BmNPV is a typical baculovirus. Together, these organisms form a useful model to investigate host-baculovirus interactions. Prothoracic glands (PGs) are also model organs, used to investigate the regulatory effect of synthetic ecdysone on insect growth and development. In this study, day-4 fifth instar silkworm larvae were infected with BmNPV. Wandering silkworms appeared in the infected groups 12 h earlier than in the control groups, and the ecdysone titer in infected larvae was significantly higher than that of the control larvae. We then used RNA sequencing (RNA-seq) to analyze silkworm PGs 48 h after BmNPV infection. We identified 15 differentially expressed genes (DEGs) that were classified as mainly being involved in metabolic processes and pathways. All 15 DEGs were expressed in the PGs, of which Novel01674, BmJing, and BmAryl were specifically expressed in the PGs. The transcripts of BmNGDN, BmTrypsin-1, BmACSS3, and BmJing were significantly increased, and BmPyd3, BmTitin, BmIGc2, Novel01674, and BmAryl were significantly decreased from 24 to 72 h in the PGs after BmNPV infection. The changes in the transcription of these nine genes were generally consistent with the transcriptome data. The upregulation of BmTrypsin-1 and BmACSS3 indicate that these DEGs may be involved in the maturation process in the latter half of the fifth instar of silkworm larvae. These findings further our understanding of silkworm larval development, the interaction between BmNPV infection and the host developmental response, and host-baculovirus interactions in general.

Insights Into the Antiviral Pathways of the Silkworm Bombyx mori

The lepidopteran model silkworm, Bombyx mori, is an important economic insect. Viruses cause serious economic losses in sericulture; thus, the economic importance of these viruses heightens the need to understand the antiviral pathways of silkworm to develop antiviral strategies. Insect innate immunity pathways play a critical role in the outcome of infection. The RNA interference (RNAi), NF-kB-mediated, immune deficiency (Imd), and stimulator of interferon gene (STING) pathways, and Janus kinase/signal transducer and activator of transcription (JAK/STAT) pathway are the major antiviral defense mechanisms, and these have been shown to play important roles in the antiviral immunity of silkworms. In contrast, viruses can modulate the prophenol oxidase (PPO), phosphatidylinositol 3-kinase (PI3K)/protein kinase B (Akt), and the extracellular signal-regulated kinase (ERK) signaling pathways of the host to elevate their proliferation in silkworms. In this review, we present an overview of the current understanding of the main immune pathways in response to viruses and the signaling pathways modulated by viruses in silkworms. Elucidation of these pathways involved in the antiviral mechanism of silkworms furnishes a theoretical basis for the enhancement of virus resistance in economic insects, such as upregulating antiviral immune pathways through transgenic overexpression, RNAi of virus genes, and targeting these virus-modulated pathways by gene editing or inhibitors.

Advances in the Arms Race Between Silkworm and Baculovirus

Insects are the largest group of animals. Nearly all organisms, including insects, have viral pathogens. An important domesticated economic insect is the silkworm moth Bombyx mori. B. mori nucleopolyhedrovirus (BmNPV) is a typical baculovirus and a primary silkworm pathogen. It causes major economic losses in sericulture. Baculoviruses are used in biological pest control and as a bioreactor. Silkworm and baculovirus comprise a well-established model of insect–virus interactions. Several recent studies have focused on this model and provided novel insights into viral infections and host defense. Here, we focus on baculovirus invasion, silkworm immune response, baculovirus evasion of host immunity, and enhancement of antiviral efficacy. We also discuss major issues remaining and future directions of research on silkworm antiviral immunity. Elucidation of the interaction between silkworm and baculovirus furnishes a theoretical basis for targeted pest control, enhanced pathogen resistance in economically important insects, and bioreactor improvement.

Heat shock protein 19.9 (Hsp19.9) from Bombyx mori is involved in host protection against viral infection

Adverse environmental conditions cause serious economic losses in sericulture; Bombyx mori nucleopolyhedrovirus (BmNPV) is the primary biotic stress and high temperature is the major abiotic stress in this industry. B. mori heat shock protein 19.9 (Bmhsp19.9) overexpression was previously demonstrated to protect transgenic silkworm H19.9 against extreme temperature. This study analyzed the role of Bmhsp19.9 in H19.9A and H19.9B silkworm lines and BmE cells infected with BmNPV at regular and high temperatures. qPCR results showed that Bmhsp19.9 expression was upregulated in BmE cells and silkworm after BmNPV challenge. Bmhsp19.9 overexpression significantly inhibited BmNPV proliferation in BmE cells. The viral DNA content was significantly decreased in transgenic H19.9 silkworm compared to the control. These results suggested that Bmhsp19.9 was involved in antiviral immunity against BmNPV. Furthermore, Bmhsp19.9 overexpression protected BmE cells against BmNPV under high temperature shock. This indicates that Bmhsp19.9 is a promising candidate for improving silkworm resistance to biotic and abiotic stresses, thereby reducing sericulture losses.

Toxicological evaluation of transgenic silkworms

Safety of transgenic silkworms must be evaluated before their commercial application. We assessed subacute toxicity using a 28-day feeding study in rats. Eighty rats were evenly allocated into four groups, with each group containing 10 male and 10 female rats. Rats of three groups were fed dried transgenic silkworm H19.9A pupae with overexpressed endogenous Bmhsp19.9, transgenic silkworm A4SOR pupae with overexpressed exogenous SOR, or normal silkworm pupae at a dose of 3.0 g/kg/day, respectively. The fourth group served as a normal feeding control. The body weight, feed consumption, hematology response variables, serum biochemical parameters, organ weights, gross necropsy, and histopathologic of animals were evaluated. No mortality, adverse effects, or major differences in the evaluated parameters were observed in the groups fed transgenic pupae in comparison with the control, suggesting that transgenic silkworms are toxicologically equivalent to normal silkworms and are safe for consumption in rats.

Characterization and potential application of an α-amylase (BmAmy1) selected during silkworm domestication

Efficient resource utilization plays a central role in the high productivity of domesticated plants and animals. Whether artificial selection acts on digestive enzymes in the domesticated silkworm (Bombyx mori), which is larger than its wild ancestor, Bombyx mandarina (B. mandarina), remains unknown. In this study, we present the characteristics of a novel alpha-amylase, BmAmy1, in B. mori. The activity of recombinant BmAmy1 was maximal at 35 °C and pH 9.0, and could be suppressed by amylase inhibitors from mulberry, the exclusive food source of silkworms. Three different transposable element fragments, which were independently inserted in the 5'-upstream regulatory region, might be responsible for the enhanced expression of BmAmy1 in different domesticated silkworm strains as revealed by dual-luciferase reporter assay. The BmAmy1 overexpression increased the weight of female and male B. mori by 11.9% and 6.8%, respectively, compared with non-transgenic controls. Our results emphasize that, by exploring the genetic mechanisms of human-selected traits, the domestication process could be further accelerated through genetic engineering and targeted breeding.

Potential of transferring transgenic DNA from silkworm to chicken

Safety assessment must be conducted before the commercial release of transgenic silkworms. This study was conducted to assess the potential of transferring transgenic DNA from silkworms to other organisms. One hundred hatched male chickens were evenly assigned into 4 groups (T1-4). Groups T1-3 were fed transgenic silkworms P3+5UI with enhanced green fluorescent protein DNA (EGFP) inserted, A4SOR with superoxide reductase DNA (SOR) inserted, and normal silkworm, respectively. Each chicken was fed one silkworm larva every day for 3 weeks. T4 was the normal feeding control. Twenty chickens were randomly selected from each treatment for sacrifice at 22 days of age. The serum was collected individually for biochemical examination, revealing no difference in the analyzed serum parameters between T4 and T1-3. DNA from the duodenum, jejunum, ileum, liver, kidney, and jejunal digesta was extracted for PCR analysis of EGFP, SOR, silkworm housekeeping gene TIF-4A, and chicken ovalbumin gene. No transgenic DNA or TIF-4A was detected in the digesta and tissues of chickens. The same results were observed in chicken upon increasing the amount and frequency of feeding transgenic silkworms, suggesting that the transgenic DNA from silkworms was degraded in the digestive tract and not transferred into the tissues of chicken. This study revealed that transferr recombinant DNA from transgenic silkworm to another organism is unlikely.

Spry is downregulated by multiple viruses to elevate ERK signaling and ensure viral reproduction in silkworm

Viral diseases of silkworm are mainly caused by Bombyx mori nucleopolyhedrovirus (BmNPV), B. mori cytoplasmic polyhedrosis virus (BmCPV) and B. mori bidensovirus (BmBDV). The virus alters host cellular pathways to facilitate its proliferation. It is still unclear whether the three silkworm viruses regulate a certain host pathway. Spry is a negative regulator upstream of ERK. In this study, we found that BmSpry was decreased and p-ERK was increased in silkworm after infection with each virus. A transgenic RNAi vector of BmSpry was constructed and used for embryo microinjection to generate the transgenic line Spry-I. The expression of BmSpry was significantly reduced in Spry-I compared to that in non-transgenic silkworm. The viral content and mortality in Spry-I were significantly higher than those in non-transgenic larvae after infection with the three viruses. p-ERK was increased in Spry-I compared to that in non-transgenic control after virus infection. These results suggest that BmSpry is downregulated by multiple different classes of viruses to elevate p-ERK and ensure viral reproduction in the silkworm.

Systemic disruption of the homeostasis of transfer RNA isopentenyltransferase causes growth and development abnormalities in Bombyx mori

Isopentenylation at A37 (i⁶ A37) of some transfer RNAs (tRNAs) plays a vital role in regulating the efficiency and fidelity of protein synthesis. However, whether insects, which are well known for their highly efficient protein synthesis machinery, employ this regulatory mechanism remains uninvestigated. In the current study, a candidate tRNA isopentenyltransferase (IPT) gene with three alternative splicing isoforms (BmIPT1-BmIPT3) was identified in Bombyx mori (silkworm). Only BmIPT1 could complement a yeast mutant lacking tRNA IPT. Phylogenetic analysis showed that silkworm tRNA IPT is conserved in the Lepidoptera. BmIPT was expressed in all B. mori tissues and organs that were investigated, but was expressed at a significantly higher level in silk glands of the fourth instar compared to the first day of the fifth instar. Interestingly, BmIPT was expressed at a significantly higher level in the domesticated silkworm, B. mori, than in wild Bombyx mandarina in multiple tissues and organs. Knock-down of BmIPT by RNA interference caused severe abnormalities in silk spinning and metamorphosis. Constitutive overexpression of BmIPT1 using a cytoplasmic actin 4 promoter in B. mori raised its messenger RNA level more than sixfold compared with nontransgenic insects and led to significant decreases in the body weight and cocoon shell ratio. Together, these results confirm the first functional tRNA IPT in insects and show that a suitable expression level of tRNA IPT is vital for silk spinning, normal growth, and metamorphosis. Thus, i⁶ A modification at position A37 in tRNA probably plays an important role in B. mori protein synthesis.

Distinct Functions of Bombyx mori Peptidoglycan Recognition Protein 2 in Immune Responses to Bacteria and Viruses

Peptidoglycan recognition protein (PGRP) is an important pattern recognition receptor in innate immunity that is vital for bacterial recognition and defense in insects. Few studies report the role of PGRP in viral infection. Here we cloned two forms of PGRP from the model lepidopteran Bombyx mori: BmPGRP2-1 is a transmembrane protein, whereas BmPGRP2-2 is an intracellular protein. BmPGRP2-1 bound to diaminopimelic acid (DAP)-type peptidoglycan (PGN) to activate the canonical immune deficiency (Imd) pathway. BmPGRP2-2 knockdown reduced B. mori nucleopolyhedrovirus (BmNPV) multiplication and mortality in cell lines and in silkworm larvae, while its overexpression increased viral replication. Transcriptome and quantitative PCR (qPCR) results confirmed that BmPGRP2 negatively regulated phosphatase and tensin homolog (PTEN). BmPGRP2-2 expression was induced by BmNPV, and the protein suppressed PTEN-phosphoinositide 3-kinase (PI3K)/Akt signaling to inhibit cell apoptosis, suggesting that BmNPV modulates BmPGRP2-2-PTEN-PI3K/Akt signaling to evade host antiviral defense. These results demonstrate that the two forms of BmPGRP2 have different functions in host responses to bacteria and viruses.

Screening of PI3K-Akt-targeting Drugs for Silkworm against Bombyx mori Nucleopolyhedrovirus

Bombyx mori nucleopolyhedrovirus (BmNPV) is the most prevalent threat to silkworms. Hence, there is a need for antiviral agents in sericulture. The PI3K-Akt pathway is essential for the efficient replication of the baculovirus. In an attempt to screen antiviral drugs against BmNPV, we summarized the commercial compounds targeting PI3K-Akt and selected the following seven oral drugs for further analyses: afuresertib, AZD8835, AMG319, HS173, AS605240, GDC0941, and BEZ235. Cell viability assay revealed that the cytotoxicity of these drugs at 10 µM concentration was not strong. Viral fluorescence observation and qPCR analysis showed that these candidate drugs significantly inhibited BmNPV in BmE cells. Only AMG319 and AZD8835 inhibited viral proliferation in silkworm larvae. The mortality of AZD8835-treated silkworms was lower than that of the control silkworms. Western blotting showed that AMG319 and AZD8835 decreased p-Akt expression after BmNPV infection. These results suggest that AZD8835 has application potential in sericulture.

Phosphoenolpyruvate carboxykinase is involved in antiviral immunity against Bombyx mori nucleopolyhedrovirus

Phosphoenolpyruvate carboxykinase (PEPCK) has cytoplasmic isoform (PEPCK-C) and a mitochondrial isoform (PEPCK-M). PEPCK-C plays an important role in gluconeogenesis, but the function of PEPCK-M is largely unknown. In this study, we cloned two isoforms of PEPCK (BmPEPCK-1 and BmPEPCK-2; both of PEPCK-M) from the lepidopteran model Bombyx mori. BmPEPCK-1 and BmPEPCK-2 were adjacently located in the silkworm genome, and both contained 13 exons. The main difference in the sequences was the 13th exon and 3'UTR. The expression of BmPEPCK-1 was higher than that of BmPEPCK-2, the overexpression of which did not affect BmNPV proliferation. The expression levels of BmPEPCK-2 and ATG6/7/8/13 decreased after BmNPV infection. Overexpression of BmPEPCK-2 increased the expression of ATG6/7/8 and significantly decreased viral fluorescence and content, suggesting that BmPEPCK-2 suppressed the multiplication of BmNPV by increasing ATGs expression. These results revealed that PEPCK-M has an important function in antiviral immunity.

CRISPR/Cas9-mediated disruption of the immediate early-0 and 2 as a therapeutic approach to Bombyx mori nucleopolyhedrovirus in transgenic silkworm

The CRISPR/Cas9 system is a powerful tool for the treatment of infectious diseases. In our previous study, we knocked out the Bombyx mori nucleopolyhedrovirus (BmNPV) key genes and BmNPV-dependent host factor to generate transgenic antiviral strains. To further expand the range of target genes for BmNPV and more effectively prevent and control pathogenic infections, we performed gene editing and antiviral analysis by constructing a target-directed baculovirus early transcriptional activator immediate early-0 (ie-0) and 2 (ie-2) transgenic silkworm line. We hybridized it with Cas9 transgenic line to produce a double-positive transgenic Cas9(+)/sgIE0-sgIE2(+) line that could activate the CRISPR gene editing system. We first demonstrated that the system is capable of efficiently editing target genes and resulting in fragment deletions in the BmNPV genome. Survival rate of the transgenic Cas9(+)/sgIE0-sgIE2(+) line reached 65% after inoculation with 1 × 10⁶ occlusion bodies/larva. Molecular analysis showed that BmNPV DNA replication and viral gene expression level in the transgenic Cas9(+)/sgIE0-sgIE2(+) line were significantly inhibited compared with the control Cas9(-)/sgIE0-sgIE2(-) line. These results indicated that IE-0 and IE-2, as baculovirus early transcriptional activators, can be used as target sites for gene therapy and that multigene editing could expand the range of target sites for research to create silkworm resistance breeds.

Increased antiviral capacity of transgenic silkworm via knockdown of multiple genes on Bombyx mori bidensovirus

Bombyx mori bidensovirus (BmBDV) causes fatal flacherie disease leading to severe economic losses in sericultures. The BmDNV-Z genome contains two single-stranded DNA molecules, VD1 and VD2. For generating silkworm lines with antiviral properties, two transgenic RNA interference (RNAi) vectors were constructed. Open reading frames (ORFs) 1-4 of VD1 were knockdown by vector pb-BDV1 while ORF1a, ORF1b, and ORF3 of VD2 were knockdown by vector pb-BDV2. Transgenic silkworm lines BDV1-I and BDV2-I were generated via RNAi microinjection. Mortality rates of BDV1-I and BDV2-I were reduced by 45% and 39%, respectively, and quantitative PCR showed that VD1 and VD2 contents in BDV1-I and BDV2-I were significantly lower than in the non-transgenic line. However, economic traits showed no obvious differences. Thus, knockdown of multiple BmDNV-Z genes provides strong resistance to BDV1-I and BDV2-I lines, and these can be used in sericulture without hampering silk production.

An efficient method for multigene co-interference by recombinant Bombyx mori nucleopolyhedrovirus

Bombyx mori Nucleopolyhedrovirus (BmNPV), which is a member of the Baculoviridae family, is a significant pathogen of the silkworm. The infection of BmNPV is often lethal and causes about 20% loss of cocoon in the silk industry annually. To explore the effects of different gene inhibition strategies on the replication cycle of baculovirus, we constructed the mutant virus to infect BmN cells directly and further identified ie0, ie1, and gp64 as the essential viral genes of BmNPV. To elucidate the significance of the inhibition effect of different interference strategies, we characterized and constructed the recombinant BmNPV that carried a single or multigene-interfering cassette. The results showed that the inhibition effect of dsie1 on target gene expression, virus titer, and silkworm mortality was significantly better than that of dsie0 and dsgp64. It also showed that the dsie1 interference produced fewer progeny virions and was less lethal, which indicates that ie1 played a more critical role in the BmNPV replication cycle. Furthermore, the inhibitory effect of the virus titer and mortality indicated that the multigene co-interference constructed by the baculovirus expression system was significantly better than the interference of any single-gene (p < 0.05). In summary, the strategy of multigene synergy can achieve the function of continuous interference and provide a new platform for the breeding of silkworm disease resistant. In addition, this strategy improves the various traits of the silkworm.

Discovery of anti-viral molecules and their vital functions in Bombyx mori

The silkworm Bombyx mori (B. mori), a lepidopteran model organism, has become an important model for molecular biology researches with its genome completely sequenced. Silkworms confront different types of virus diseases, mainly including those caused by Bombyx mori nucleopolyhedrovirus (BmNPV), Bombyx mori densovirus type 1 (BmDNV-1), Bombyx mori bidesovirus (BmBDV) which was termed as Bombyx mori densovirus type 2 (BmDNV-2) or Bombyx mori parvo-like virus (BmPLV) before in sericulture. B. mori offers excellent models to study the molecular mechanisms of insect innate immune responses to viruses. A variety of molecules and pathways have been identified to be involved in the immune responses in the silkworm to viruses, such as the antimicrobial peptides, prophenoloxidase-activating system, apoptosis, ROS, small RNA and related molecules. Here in this review, we summarize the current research advances in molecules involved in silkworm anti-virus pathways. Moreover, taking BmNPV as an example, we proposed a schematic model of molecules and pathways involved in silkworm immune responses against virus infection. We hope this review can facilitate further study of antiviral mechanisms in silkworm, and provide a reference for virus diseases in other organisms.

Enhancement of antiviral capacity of transgenic silkworms against cytoplasmic polyhedrosis virus via knockdown of multiple viral genes

Bombyx mori cytoplasmic polyhedrosis virus (BmCPV), a major pathogen of silkworms, causes serious economic losses in sericulture. The BmCPV genome contains 10 discrete dsRNA segments; among these, S1, S2, S3, S4, S6, and S7 encode virus structural proteins, whereas S5, S8, S9, and S10 encode nonstructural proteins. In an attempt to create an anti-BmCPV silkworm strain, we constructed transgenic RNAi vector pb-CNS for knockdown of S5, S8, S9, and S10, and pb-SNS targeting S1, S2, S4, S5, and S8. Transgenic silkworm line CNS and SNS were generated via microinjection of the practical diapause silkworm strain Furong. Following infection via the oral administration of a high dose of BmCPV, the mortality rates of the nontransgenic control, CNS, and SNS were 91%, 37%, and 41%, respectively. qPCR showed that the viral mRNA content in CNS and SNS was significantly lower than that in the nontransgenic line. The economic traits of CNS and SNS were not affected. These results suggest that the knockdown of multiple BmCPV genes significantly enhances the antiviral capacity of the silkworm.

Increasing the yield of middle silk gland expression system through transgenic knock-down of endogenous sericin-1

Various genetically modified bioreactor systems have been developed to meet the increasing demands of recombinant proteins. Silk gland of Bombyx mori holds great potential to be a cost-effective bioreactor for commercial-scale production of recombinant proteins. However, the actual yields of proteins obtained from the current silk gland expression systems are too low for the proteins to be dissolved and purified in a large scale. Here, we proposed a strategy that reducing endogenous sericin proteins would increase the expression yield of foreign proteins. Using transgenic RNA interference, we successfully reduced the expression of BmSer1 to 50%. A total 26 transgenic lines expressing Discosoma sp. red fluorescent protein (DsRed) in the middle silk gland (MSG) under the control of BmSer1 promoter were established to analyze the expression of recombinant. qRT-PCR and western blotting showed that in BmSer1 knock-down lines, the expression of DsRed had significantly increased both at mRNA and protein levels. We did an additional analysis of DsRed/BmSer1 distribution in cocoon and effect of DsRed protein accumulation on the silk fiber formation process. This study describes not only a novel method to enhance recombinant protein expression in MSG bioreactor, but also a strategy to optimize other bioreactor systems.

Transgenic Clustered Regularly Interspaced Short Palindromic Repeat/Cas9-Mediated Viral Gene Targeting for Antiviral Therapy of Bombyx mori Nucleopolyhedrovirus

We developed a novel antiviral strategy by combining transposon-based transgenesis and the clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated 9 (Cas9) system for the direct cleavage of Bombyx mori nucleopolyhedrovirus (BmNPV) genome DNA to promote virus clearance in silkworms. We demonstrate that transgenic silkworms constitutively expressing Cas9 and guide RNAs targeting the BmNPV immediate early-1 (ie-1) and me53 genes effectively induce target-specific cleavage and subsequent mutagenesis, especially large (∼7-kbp) segment deletions in BmNPV genomes, and thus exhibit robust suppression of BmNPV proliferation. Transgenic animals exhibited higher and inheritable resistance to BmNPV infection than wild-type animals. Our approach will not only contribute to modern sericulture but also shed light on future antiviral therapy.

IMPORTANCE Pathogen genome targeting has shown its potential in antiviral research. However, transgenic CRISPR/Cas9 system-mediated viral genome targeting has not been reported as an antiviral strategy in a natural animal host of a virus. Our data provide an effective approach against BmNPV infection in a real-world biological system and demonstrate the potential of transgenic CRISPR/Cas9 systems in antiviral research in other species.

Analysis of codon usage bias of envelope glycoprotein genes in nuclear polyhedrosis virus (NPV) and its relation to evolution

BACKGROUND

Analysis of codon usage bias is an extremely versatile method using in furthering understanding of the genetic and evolutionary paths of species. Codon usage bias of envelope glycoprotein genes in nuclear polyhedrosis virus (NPV) has remained largely unexplored at present. Hence, the codon usage bias of NPV envelope glycoprotein was analyzed here to reveal the genetic and evolutionary relationships between different viral species in baculovirus genus.

RESULTS

A total of 9236 codons from 18 different species of NPV of the baculovirus genera were used to perform this analysis. Glycoprotein of NPV exhibits weaker codon usage bias. Neutrality plot analysis and correlation analysis of effective number of codons (ENC) values indicate that natural selection is the main factor influencing codon usage bias, and that the impact of mutation pressure is relatively smaller. Another cluster analysis shows that the kinship or evolutionary relationships of these viral species can be divided into two broad categories despite all of these 18 species are from the same baculovirus genus.

CONCLUSIONS

There are many elements that can affect codon bias, such as the composition of amino acids, mutation pressure, natural selection, gene expression level, and etc. In the meantime, cluster analysis also illustrates that codon usage bias of virus envelope glycoprotein can serve as an effective means of evolutionary classification in baculovirus genus.

Transcriptome analysis of interactions between silkworm and cytoplasmic polyhedrosis virus

Bombyx mori cytoplasmic polyhedrosis virus (BmCPV) specifically infects silkworm midgut (MG) and multiplication occurs mainly in posterior midgut (PM). In this study, MG and fat body (FB) were extracted at 0, 3, 24, and 72 h after BmCPV infection. The total sequence reads of each sample were more than 1510000, and the mapping ratio exceeded 95.3%. Upregulated transcripts increased in MG during the infection process. Gene ontology (GO) categories showed that antioxidants were all upregulated in FB but not in MG. BGI001299, BGI014434, BGI012068, and BGI009201 were MG-specific genes with transmembrane transport function, the expression of which were induced by BmCPV. BGI001299, BGI014434, and BGI012068 expressed in entire MG and may be involved in BmCPV invasion. BGI009201 expressed only in PM and may be necessary for BmCPV proliferation. BmPGRP-S2 and BGI012452 (a putative serine protease) were induced by BmCPV and may be involved in immune defense against BmCPV. The expression level of BmCPV S1, S2, S3, S6, and S7 was high and there was no expression of S9 in MG 72 h, implying that the expression time of structural protein coding genes is earlier. These results provide insights into the mechanism of BmCPV infection and host defense.

Suppression of intestinal immunity through silencing of TCTP by RNAi in transgenic silkworm, Bombyx mori

Intestinal immune response is a front line of host defense. The host factors that participate in intestinal immunity response remain largely unknown. We recently reported that Translationally Controlled Tumor Protein (BmTCTP) was obtained by constructing a phage display cDNA library of the silkworm midgut and carrying out high throughput screening of pathogen binding molecules. To further address the function of BmTCTP in silkworm intestinal immunity, transgenic RNAi silkworms were constructed by microinjection piggBac plasmid to Dazao embryos. The antimicrobial capacity of transgenic silkworm decreased since the expression of gut antimicrobial peptide from transgenic silkworm was not sufficiently induced during oral microbial challenge. Moreover, dynamic ERK phosphorylation from transgenic silkworm midgut was disrupted. Taken together, the innate immunity of intestinal was suppressed through disruption of dynamic ERK phosphorylation after oral microbial infection as a result of RNAi-mediated knockdown of midgut TCTP in transgenic silkworm.

The 5'-UTR intron of the midgut-specific BmAPN4 gene affects the level and location of expression in transgenic silkworms

Introns are important for regulating gene expression. BmAPN4, which has a 5'-UTR upstream intron (5 UI), is specifically expressed in the entire silkworm midgut. In our previous study, the promoter region upstream of the 5 UI of BmAPN4 was cloned and identified as the P3 promoter (P3P) with activity only in the anterior midgut. In this study, the sequence consisting of the P3P and the 5 UI was cloned and named as P3P+5 UI. A transgenic vector was constructed in which EGFP was controlled by P3P+5 UI. Transgenic P3+5 UI silkworms were generated by embryo microinjection. RT-PCR showed P3P+5 UI activity throughout the larval stage. Intense green fluorescence was seen only in the entire midgut of P3+5 UI silkworms and expression was confirmed by RT-PCR. qPCR revealed that expression of EGFP in the anterior midgut of P3+5 UI silkworms was 64% higher than in P3 silkworms, indicating the 5 UI sustained intron-mediated enhancement of gene expression. These results suggested that the BmAPN4 5 UI affected the level and site of expression. The 5 UI was cloned and added behind P2P, another specific promoter with activity only in the anterior midgut of silkworm, to construct the P2P+5 UI and transgenic P2+5 UI silkworms. Expression patterns were the same for P2P+5 UI and P2P, suggesting that the 5UI of BmAPN4 did not affect P2P. This study found that the BmAPN4 5 UI affected the amount and location of gene expression. Its influence appeared to be dependent on a specific promoter.

dsRNA interference on expression of a RNA-dependent RNA polymerase gene of Bombyx mori cytoplasmic polyhedrosis virus

Bombyx mori cytoplasmic polyhedrosis virus (BmCPV) is one of the major viral pathogens in silkworm. Its infection often results in significant losses to sericulture. Studies have demonstrated that RNAi is one of the important anti-viral mechanisms in organisms. In this study, three dsRNAs targeting the RNA-dependent RNA polymerase (RDRP) gene of BmCPV were designed and synthesized with 2'-F modification to explore their interference effects on BmCPV replication in silkworm larvae. The results showed that injecting dsRNA in the dosage of 4-6 ng per mg body weight into the 5th instar larvae can interfere with the BmCPV-RDRP expression by 93% after virus infection and by 99.9% before virus infection. In addition, the expression of two viral structural protein genes (genome RNA segments 1 and 5) was also decreased with the decrease of RDRP expression, suggesting that RNAi interference of BmCPV-RDRP expression could affect viral replication. The study provides an effective method for investigating virus replication as well as the virus-host interactions in the silkworm larvae using dsRNA.

Small interfering (Si) RNA mediated baculovirus replication reduction without affecting target gene expression

The baculovirus expression vector system (BEVS) is widely used to produce large quantities of recombinant protein with posttranslational modification. Recombinant baculoviruses (such as Autographa californica multiple nuclear polyhedrosis virus) are especially useful in producing recombinant proteins and virus-like particles (VLPs) as biodrugs or candidate vaccines for the prevention of serious infectious diseases. However, during the bioprocessing of recombinant proteins in insect cells, baculovirus replication and viral budding are coincident. In some cases, residual baculovirus contaminants remain in the recombinant protein products, even though various purification processes are applied such as ion-exchange chromatography, ultracentrifugation, or gel filtration. To reduce unexpected contamination caused by replication and budding-out of the baculovirus, we designed short interfering (si) RNAs targeting glycoprotein 64 (GP64) or single-stranded DNA-binding protein (DBP) to inhibit baculovirus replication during overexpression of recombinant foreign genes. GP64 is known to be critical both for the entry of virions into cells and for the assembly of the budded virion at the cell surface. DBP is also essential for virus assembly by regulation of the capsid protein P39 and the polyhedrin protein. This study showed that GP64 expression was suppressed by GP64 siRNAs in Western blot experiments, while the expression of recombinant proteins was unaffected. In addition, transfection of GP64 siRNAs and DBP siRNAs reduced the level of baculovirus replication, compared with the treatment with scrambled siRNAs. However, DBP siRNA also suppressed the expression of recombinant proteins. In conclusion, our GP64 siRNAs showed that an interfering RNA system, such as siRNAs and short hairpin (sh) RNAs, can be applicable to reduce baculovirus contaminants during the bioprocessing of recombinant proteins in insect cells. Further investigation should be carried out to establish transformed insect cell lines with stable expression of corresponding interfering RNAs.

Overexpression of host plant urease in transgenic silkworms

Bombyx mori and mulberry constitute a model of insect-host plant interactions. Urease hydrolyzes urea to ammonia and is important for the nitrogen metabolism of silkworms because ammonia is assimilated into silk protein. Silkworms do not synthesize urease and acquire it from mulberry leaves. We synthesized the artificial DNA sequence ureas using the codon bias of B. mori to encode the signal peptide and mulberry urease protein. A transgenic vector that overexpresses ure-as under control of the silkworm midgut-specific P2 promoter was constructed. Transgenic silkworms were created via embryo microinjection. RT-PCR results showed that urease was expressed during the larval stage and qPCR revealed the expression only in the midgut of transgenic lines. Urea concentration in the midgut and hemolymph of transgenic silkworms was significantly lower than in a nontransgenic line when silkworms were fed an artificial diet. Analysis of the daily body weight and food conversion efficiency of the fourth and fifth instar larvae and economic characteristics indicated no differences between transgenic silkworms and the nontransgenic line. These results suggested that overexpression of host plant urease promoted nitrogen metabolism in silkworms.

Analysis of codon usage bias of mitochondrial genome in Bombyx mori and its relation to evolution

BACKGROUND

Synonymous codon usage bias (SCUB) is an inevitable phenomenon in organismic taxa, generally referring to differences in the occurrence frequency of codons across different species or within the genome of the same species. SCUB happens in various degrees under pressure from nature selection, mutation bias and other factors in different ways. It also attaches great significance to gene expression and species evolution, however, a systematic investigation towards the codon usage in Bombyx mori (B. mori) has not been reported yet. Moreover, it is still indistinct about the reasons contributing to the bias or the relationship between the bias and the evolution of B. mori.

RESULTS

The comparison of the codon usage pattern between the genomic DNA (gDNA) and the mitochondrial DNA (mtDNA) from B. mori suggests that mtDNA has a higher level of codon bias. Furthermore, the correspondence analysis suggests that natural selection, such as gene length, gene function and translational selection, dominates the codon preference of mtDNA, while the composition constraints for mutation bias only plays a minor role. Additionally, the clustering results of the silkworm superfamily suggest a lack of explicitness in the relationship between the codon usage of mitogenome and species evolution.

CONCLUSIONS

Among the complicated influence factors leading to codon bias, natural selection is found to play a major role in shaping the high bias in the mtDNA of B. mori from our current data. Although the cluster analysis reveals that codon bias correlates little with the species evolution, furthermore, a detailed analysis of codon usage of mitogenome provides better insight into the evolutionary relationships in Lepidoptera. However, more new methods and data are needed to investigate the relationship between the mtDNA bias and evolution.

Orthologs of human disease associated genes and RNAi analysis of silencing insulin receptor gene in Bombyx mori

The silkworm, Bombyx mori L., is an important economic insect that has been domesticated for thousands of years to produce silk. It is our great interest to investigate the possibility of developing the B. mori as human disease model. We searched the orthologs of human disease associated genes in the B. mori by bi-directional best hits of BLAST and confirmed by searching the OrthoDB. In total, 5006 genes corresponding to 1612 kinds of human diseases had orthologs in the B. mori, among which, there are 25 genes associated with diabetes mellitus. Of these, we selected the insulin receptor gene of the B. mori (Bm-INSR) to study its expression in different tissues and at different developmental stages and tissues. Quantitative PCR showed that Bm-INSR was highly expressed in the Malpighian tubules but expressed at low levels in the testis. It was highly expressed in the 3rd and 4th instar larvae, and adult. We knocked down Bm-INSR expression using RNA interference. The abundance of Bm-INSR transcripts were dramatically reduced to ~4% of the control level at 6 days after dsRNA injection and the RNAi-treated B. mori individuals showed apparent growth inhibition and malformation such as abnormal body color in black, which is the typical symptom of diabetic patients. Our results demonstrate that B. mori has potential use as an animal model for diabetic mellitus research.

Postintegration stability of the silkworm piggyBac transposon

The piggyBac transposon is the most widely used vector for generating transgenic silkworms. The silkworm genome contains multiple piggyBac-like sequences that might influence the genetic stability of transgenic lines. To investigate the postintegration stability of piggyBac in silkworms, we used random insertion of the piggyBac [3 × p3 EGFP afm] vector to generate a W chromosome-linked transgenic silkworm, named W-T. Results of Southern blot and inverse PCR revealed the insertion of a single copy in the W chromosome of W-T at a standard TTAA insertion site. Investigation of 11 successive generations showed that all W-T females were EGFP positive and all males were EGFP negative; PCR revealed that the insertion site was unchanged in W-T offspring. These results suggested that endogenous piggyBac-like elements did not affect the stability of piggyBac inserted into the silkworm genome.

The progress and future of enhancing antiviral capacity by transgenic technology in the silkworm Bombyx mori

Bombyx mori is a common lepidopteran model and an important economic insect for silk production. B. mori nucleopolyhedrovirus (BmNPV) is a typical pathogenic baculovirus that causes serious economic losses in sericulture. B. mori and BmNPV are a model of insect host and pathogen interaction including invasion of the host by the pathogen, host response, and enhancement of host resistance. The antiviral capacity of silkworms can be improved by transgenic technology such as overexpression of an endogenous or exogenous antiviral gene, RNA interference of the BmNPV gene, or regulation of the immune pathway to inhibit BmNPV at different stages of infection. Antiviral capacity could be further increased by combining different methods. We discuss the future of an antiviral strategy in silkworm, including possible improvement of anti-BmNPV, the feasibility of constructing transgenic silkworms with resistance to multiple viruses, and the safety of transgenic silkworms. The silkworm model could provide a reference for disease control in other organisms.

Inhibition of BmNPV replication in silkworm cells using inducible and regulated artificial microRNA precursors targeting the essential viral gene lef-11

Bombyx mori nucleopolyhedrovirus (BmNPV) is a major silkworm pathogen, causing substantial economic losses to the sericulture industry annually. We demonstrate a novel anti-BmNPV system expressing mature artificial microRNAs (amiRNAs) targeting the viral lef-11 gene. The mature amiRNAs inhibited the lef-11 gene in silkworm BmN-SWU1 cells. Antiviral assays demonstrated that mature amiRNAs silenced the gene and inhibited BmNPV proliferation efficiently. As constitutive overexpression of mature amiRNAs may induce acute cellular toxicity, we further developed a novel virus-induced amiRNA expression system. The amiRNA cassette is regulated by a baculovirus-induced fusion promoter. This baculovirus-induced RNA interference system is strictly regulated by virus infection, which functions in a negative feedback loop to activate the expression of mature amiRNAs against lef-11 and subsequently control inhibition of BmNPV replication. Our study advances the use of a regulatable amiRNA cassette as a safe and effective tool for research of basic insect biology and antiviral application.

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

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

Identification of a midgut-specific promoter in the silkworm Bombyx mori

The midgut is an important organ for digestion and absorption of nutrients and immune defense in the silkworm Bombyx mori. In an attempt to create a tool for midgut research, we cloned the 1080 bp P2 promoter sequence (P2P) of a highly expressed midgut-specific gene in the silkworm. The transgenic line (P2) was generated via embryo microinjection, in which the expression of EGFP was driven by P2P. There was strong green fluorescence only in the midgut of P2. RT-PCR and Western blot showed that P2P was a midgut-specific promoter with activity throughout the larval stage. A transgenic truncation experiment suggested that regions -305 to -214 and +107 to +181 were very important for P2P activity. The results of this study revealed that we have identified a midgut-specific promoter with a high level of activity in the silkworm that will aid future research and application of silkworm genes.

A transgenic animal with antiviral properties that might inhibit multiple stages of infection

Bombyx mori nucleopolyhedrovirus (BmNPV) is the primary pathogen of silkworms, causing severe economic losses in sericulture. To create antiviral silkworm strains, we constructed a transgenic vector in which the dsRNA for five tandem BmNPV genes was controlled by the BmNPV hr3 enhancer and IE1 promoter. The antivirus gene Bmlipase-1 was driven by B. mori midgut-specific promoter P2. Transgenic strains (SW-H) were generated via embryo microinjection using the practical silkworm strain SW. After infection with a high dose of BmNPV, the survival rates of SW-H and non-transgenic SW were 64% and 13%, respectively. SW-H could be the first transgenic animal that is highly antiviral and that might inhibit the virus at multiple stages of infection.