Systemic and in vitro infection process of Bombyx mori nucleopolyhedrovirus

Bmo-miR-3351 modulates glutathione content and inhibits BmNPV proliferation by targeting BmGSTe6 in Bombyx mori

MicroRNAs (miRNAs) are small non-coding RNAs that play pivotal roles in the host response to invading pathogens. Among these pathogens, Bombyx mori nucleopolyhedrovirus (BmNPV) is one of the main causes of substantial economic losses in sericulture, and there are relatively few studies on the specific functions of miRNAs in the B. mori-BmNPV interaction. Therefore, we conducted transcriptome sequencing to identify differentially expressed (DE) messenger RNAs (mRNAs) and miRNAs in the midgut of 2 B. mori strains (BmNPV-susceptible strain P50 and BmNPV-resistant strain A35) after BmNPV infection. Through correlation analysis of the miRNA and mRNA data, we identified a comprehensive set of 21 miRNAs and 37 predicted target mRNAs. Notably, miR-3351, which has high expression in A35, exhibited remarkable efficacy in suppressing BmNPV proliferation. Additionally, we confirmed that miR-3351 binds to the 3' untranslated region (3' UTR) of B. mori glutathione S-transferase epsilon 6 (BmGSTe6), resulting in its downregulation. Conversely, BmGSTe6 displayed an opposite expression pattern to miR-3351, effectively promoting BmNPV proliferation. Notably, BmGSTe6 levels were positively correlated with glutathione S-transferase activity, consequently influencing intracellular glutathione content in the infected samples. Furthermore, our investigation revealed the protective role of glutathione against BmNPV infection in BmN cells. In summary, miR-3351 modulates glutathione content by downregulating BmGSTe6 to inhibit BmNPV proliferation in B. mori. Our findings enriched the research on the role of B. mori miRNAs in the defense against BmNPV infection, and suggests that the antiviral molecule, glutathione, offers a novel perspective on preventing viral infection in sericulture.

Bmo-miR-6498-5p suppresses Bombyx mori nucleopolyhedrovirus infection by down-regulating BmPLPP2 to modulate pyridoxal phosphate content in B. mori

The RNA interference pathway mediated by microRNAs (miRNAs) is one of the methods to defend against viruses in insects. Recent studies showed that miRNAs participate in viral infection by binding to target genes to regulate their expression. Here, we found that the Bombyx mori miRNA, miR-6498-5p was down-regulated, whereas its predicted target gene pyridoxal phosphate phosphatase PHOSPHO2 (BmPLPP2) was up-regulated upon Bombyx mori nucleopolyhedrovirus (BmNPV) infection. Both in vivo and in vitro experiments showed that miR-6498-5p targets BmPLPP2 and suppresses its expression. Furthermore, we found miR-6498-5p inhibits BmNPV genomic DNA (gDNA) replication, whereas BmPLPP2 promotes BmNPV gDNA replication. As a pyridoxal phosphate (PLP) phosphatase (PLPP), the overexpression of BmPLPP2 results in a reduction of PLP content, whereas the knockdown of BmPLPP2 leads to an increase in PLP content. In addition, exogenous PLP suppresses the replication of BmNPV gDNA; in contrast, the PLP inhibitor 4-deoxypyridoxine facilitates BmNPV gDNA replication. Taken together, we concluded that miR-6498-5p has a potential anti-BmNPV role by down-regulating BmPLPP2 to modulate PLP content, but BmNPV induces miR-6498-5p down-regulation to promote its proliferation. Our findings provide valuable insights into the role of host miRNA in B. mori-BmNPV interaction. Furthermore, the identification of the antiviral molecule PLP offers a novel perspective on strategies for preventing and managing viral infection in sericulture.

Transcriptome analysis indicates the mechanisms of BmNPV resistance in Bombyx mori midgut

Bombyx mori nucleopolyhedrovirus (BmNPV) caused serious economic losses in sericulture. Analyzing the molecular mechanism of silkworms (B. mori) resistance to BmNPV is of great significance for the prevention and control of silkworm virus diseases and the biological control of agricultural lepidopteran pests. In order to clarify the defense mechanisms of silkworms against BmNPV, we constructed a near isogenic line BC8 with high resistance to BmNPV through the highly BmNPV-resistant strain NB and the highly BmNPV-susceptible strain 306. In this study, RNA-Seq technique was used to analyze the transcriptome level differences in the midgut of BC8 and 306 following BmNPV infection. A total of 1350 DEGs were identified. Clustering analysis showed that these genes could be divided into 8 clusters with different expression patterns. Functional annotations based on GO and KEGG analysis indicated that they were involved in various metabolism pathways. Finally, 32 BmNPV defense responsive genes were screened. They were involved in metabolism, reactive oxygen species (ROS), signal transduction and immune response, and insect hormones. The further verification shows that HSP70 should participate in resistance responses of anti-BmNPV. These findings have paved the way in further functional characterization of candidate genes and subsequently can be used in breeding of BmNPV resistance dominant silkworms.

Recombinase-aided amplification coupled with lateral flow dipstick for efficient and accurate detection of Bombyx mori nucleopolyhedrovirus

The infection of Bombyx mori nucleopolyhedrovirus (BmNPV) is one of the main causes of economic losses in sericulture. Thus, it is essential to establish rapid and effective method for BmNPV detection. In the present study, we have developed a recombinase-aided amplification (RAA) to amplify the BmNPV genomic DNA at 37 °C within 30 min, and achieved a rapid detection method by coupling with a lateral flow dipstick (LFD). The RAA-LFD method had a satisfactory detection limit of 6 copies/μL of recombinant plasmid pMD19-T-IE1, and BmNPV infection of silkworm can be detected 12 h post-infection. This method was highly specific for BmNPV, and without cross-reactivity to other silkworm pathogens. In contrast to conventional polymerase chain reaction (PCR), the RAA-LFD assay showed higher sensitivity, cost-saving, and especially is apt to on-site detection of BmNPV infection in the sericulture production.

Haplotype determination of the Bombyx mori nucleopolyhedrovirus by Nanopore sequencing and linkage of single nucleotide variants

Naturally occurring isolates of baculoviruses, such as the Bombyx mori nucleopolyhedrovirus (BmNPV), usually consist of numerous genetically different haplotypes. Deciphering the different haplotypes of such isolates is hampered by the large size of the dsDNA genome, as well as the short read length of next generation sequencing (NGS) techniques that are widely applied for baculovirus isolate characterization. In this study, we addressed this challenge by combining the accuracy of NGS to determine single nucleotide variants (SNVs) as genetic markers with the long read length of Nanopore sequencing technique. This hybrid approach allowed the comprehensive analysis of genetically homogeneous and heterogeneous isolates of BmNPV. Specifically, this allowed the identification of two putative major haplotypes in the heterogeneous isolate BmNPV-Ja by SNV position linkage. SNV positions, which were determined based on NGS data, were linked by the long Nanopore reads in a Position Weight Matrix. Using a modified Expectation-Maximization algorithm, the Nanopore reads were assigned according to the occurrence of variable SNV positions by machine learning. The cohorts of reads were de novo assembled, which led to the identification of BmNPV haplotypes. The method demonstrated the strength of the combined approach of short- and long-read sequencing techniques to decipher the genetic diversity of baculovirus isolates.

Single-nucleus sequencing of silkworm larval midgut reveals the immune escape strategy of BmNPV in the midgut during the late stage of infection

The midgut is an important barrier against microorganism invasion and proliferation, yet is the first tissue encountered when a baculovirus naturally invades the host. However, only limited knowledge is available how different midgut cell types contribute to the immune response and the clearance or promotion of viral infection. Here, single-nucleus RNA sequencing (snRNA seq) was employed to analyze the responses of various cell subpopulations in the silkworm larval midgut to B. mori nucleopolyhedrovirus (BmNPV) infection. We identified 22 distinct clusters representing enteroendocrine cells (EEs), enterocytes (ECs), intestinal stem cells (ISCs), Goblet cell-like and muscle cell types in the BmNPV-infected and uninfected silkworm larvae midgut at 72 h post infection. Further, our results revealed that the strategies for immune escape of BmNPV in the midgut at the late stage of infection include (1) inhibiting the response of antiviral pathways; (2) inhibiting the expression of antiviral host factors; (3) stimulating expression levels of genes promoting BmNPV replication. These findings suggest that the midgut, as the first line of defense against the invasion of the baculovirus, has dual characteristics of "resistance" and "tolerance". Our single-cell dataset reveals the diversity of silkworm larval midgut cells, and the transcriptome analysis provides insights into the interaction between host and virus infection at the single-cell level.

The Bmtret1 Gene Family and Its Potential Role in Response to BmNPV Stress in Bombyx mori

Trehalose is a non-reducing disaccharide and participates in physiological activities such as organ formation, energy metabolism, and stress resistance in insects. The Bmtret1 gene family is mainly involved in in the sugar metabolism of silkworm. In the present study, phylogenetic analysis divided 21 Bmtret1 orthologs into three clades. These genes are equally distributed on the nine chromosomes. The cis-elements in the promoter regions of Bmtret1s indicated the possible function of Bmtret1s in response to hormones and environmental stimulus. The qPCR analysis showed the significantly different expression levels of Bmtret1s in different tissues and organs, indicating possible functional divergence. In addition, most Bmtret1s showed disturbed expression levels in response to silkworm nuclear polyhedrosis virus (BmNPV) stresses. Our results provide a clue for further functional dissection of the Tret1s in Bombyx mori and implicate them as potential regulators of antiviral responses.

Bombyx mori nucleopolyhedrovirus induces BmFABP1 downregulation to promote viral proliferation

Fatty acid binding proteins (FABPs) play an important role as endogenous cytoprotectants. However, studies on FABPs in invertebrates are scarce. Previously, we discovered Bombyx mori fatty acid binding protein 1 (BmFABP1) through co-immunoprecipitation. Here, we cloned and identified BmFABP1 from BmN cells. The results of immunofluorescence indicated that BmFABP1 was localized in the cytoplasm. The tissue expression profile of silkworms showed that BmFABP1 was expressed in all tissues except hemocytes. The expression level of BmFABP1 gradually decreases in BmN cells and B. mori larvae after infection with B. mori nucleopolyhedrovirus (BmNPV). Upregulation of BmFABP1 expression through overexpression or WY14643 treatment significantly inhibited the replication of BmNPV, while downregulation of BmFABP1 expression by RNA interference promoted the replication of BmNPV. The same results were obtained in experiments on silkworm larvae. These results suggest that BmNPV induces BmFABP1 downregulation to promote its proliferation and that BmFABP1 has a potential anti-BmNPV role. This is the first report on the antiviral effect of BmFABP1 in silkworms and provides new insights into the study of the FABP protein family. Also, it is important to study BmNPV resistance in silkworms to breed transgenic silkworms with BmNPV resistance.

Identification of Host Molecules Involved in the Proliferation of Nucleopolyhedrovirus in Bombyx mori

The Bombyx mori nucleopolyhedrovirus (BmNPV), a foodborne infectious virus, is the pathogen causing nuclear polyhedrosis and high lethality in the silkworm. In this study, we characterized the molecules involved in BmNPV-silkworm interaction by RNA sequencing of the fat body isolated from the virus-susceptible strain P50. Kyoto Encyclopedia of Genes and Genomes (KEGG) annotation showed that the upregulated differentially expressed genes (DEGs) were mainly involved in translation, signal transduction, folding, sorting, and degradation, as well as transport and catabolism, while the downregulated DEGs were predominantly enriched in the metabolism of carbohydrates, amino acids, and lipids at 72 h post BmNPV infection. Knockout of the upregulated somatomedin-B and thrombospondin type-1 domain-containing protein, probable allantoicase, trifunctional purine biosynthetic protein adenosine-3, and Psl and pyoverdine operon regulator inhibited the proliferation of BmNPV, while knockout of the downregulated clip domain serine protease 3 and carboxylesterase clade H, member 1 promoted it. The molecules herein identified provide a foundation for developing strategies and designing drugs against BmNPV.

Potential Proteins Interactions with Bombyx mori Nucleopolyhedrovirus Revealed by Co-Immunoprecipitation

Virus–host interactions are critical for virus replication, virulence, and pathogenicity. The Bombyx mori nucleopolyhedrovirus (BmNPV) is a typical model baculovirus, representing one of the most common and harmful pathogens in sericulture. Herein, we used co-immunoprecipitation to identify candidate proteins with potential interactions with BmNPV. First, a recombinant BV virus particle rBmBV-egfp-p64-3×flag-gp64sp was constructed using a MultiBac baculovirus multigene expression system. Co-immunoprecipitation experiments were then performed with the recombinant BV virus infected with BmN cells and Dazao silkworms. LC-MS/MS analysis revealed a total of 845 and 1368 candidate proteins were obtained from BmN cells and silkworm samples, respectively. Bioinformatics analysis (Gene Ontology, KEGG Pathway) was conducted for selection of proteins with significant enrichment for further confirmation of the effects on BmNPV replication. Overall, the results showed that SEC61 and PIC promoted the replication of BmNPV, while FABP1 inhibited the replication of BmNPV. In summary, this study reveals the potential proteins involved in BmNPV invasion and proliferation in the host and provides a platform for identifying the potential receptor proteins of BmNPV.

CRISPR/Cas9-Mediated Disruption of the lef8 and lef9 to Inhibit Nucleopolyhedrovirus Replication in Silkworms

Bombyx mori nucleopolyhedrovirus (BmNPV) is a pathogen that causes severe disease in silkworms. In a previous study, we demonstrated that by using the CRISPR/Cas9 system to disrupt the BmNPV ie-1 and me53 genes, transgenic silkworms showed resistance to BmNPV infection. Here, we used the same strategy to simultaneously target lef8 and lef9, which are essential for BmNPV replication. A PCR assay confirmed that double-stranded breaks were induced in viral DNA at targeted sequences in BmNPV-infected transgenic silkworms that expressed small guide RNAs (sgRNAs) and Cas9. Bioassays and qPCR showed that replication of BmNPV and mortality were significantly reduced in the transgenic silkworms in comparison with the control groups. Microscopy showed degradation of midgut cells in the BmNPV-infected wild type silkworms, but not in the transgenic silkworms. These results demonstrated that transgenic silkworms using the CRISPR/Cas9 system to disrupt BmNPV lef8 and lef9 genes could successfully prevent BmNPV infection. Our research not only provides more alternative targets for the CRISPR antiviral system, but also aims to provide new ideas for the application of virus infection research and the control of insect pests.

Identification and Characterization of Genes Related to Resistance of Autographa californica Nucleopolyhedrovirus Infection in Bombyx mori

Simple Summary

Autographa californica nucleopolyhedrovirus (AcMNPV) is a kind of baculovirus that was initially found and named for its host, but the previous study reveals several silkworm strains are preferentially susceptible to AcMNPV through intrahemocelical injection method. In the following study, genetics analysis showed that a set of potential genes which controlled resistance of AcMNPV was located on chromosome 3. In the present research, we performed Genome-Wide Association Studies to identify the gene that controls the resistance of AcMNPV, results show that the Niemann-Pick C1 (NPC-1) gene is strongly associated with this resistance. Then we found that there are several amino acid mutations in the protein sequence of BmNPC1 between two different resistance strains of Bombyx mori. RNAi results showed that BmNPC1 successfully suppressed virus infection ability and changed the expression pattern of viral genes.

Abstract

In Bombyx mori, as an important economic insect, it was first found that some strains were completely refractory to infection with Autographa californica nucleopolyhedrovirus (AcMNPV) through intrahemocelical injection; whereas almost all natural strains had difficulty resisting Bombyx mori nucleopolyhedrovirus (BmNPV), which is also a member of the family Baculoviridae. Previous genetics analysis research found that this trait was controlled by a potentially corresponding locus on chromosome 3, but the specific gene and mechanism was still unknown. With the help of the massive silkworm strain re-sequencing dataset, we performed the Genome-Wide Association Studies (GWAS) to identify the gene related to the resistance of AcMNPV in this study. The GWAS results showed that the Niemann-Pick type C1 (NPC-1) gene was the most associated with the trait. The knockdown experiments in BmN cells showed that BmNPC1 has a successful virus suppression infection ability. We found a small number of amino acid mutations among different resistant silkworms, which indicates that these mutations contributed to the resistance of AcMNPV. Furthermore, inhibition of the BmNPC1 gene also changed the viral gene expression of the AcMNPV, which is similar to the expression profile in the transcriptome data of p50 and C108 strains.

Transcriptome of miRNA during inhibition of Bombyx mori nuclear polyhedrosis virus by geldanamycin in BmN cells

Bombyx mori nuclear polyhedrosis virus (BmNPV) is one of several viruses that cause great harm to the sericulture industry, and its pathogenic mechanism is still being explored. Geldanamycin (GA), a kind of HSP90 inhibitor, has been verified to suppress BmNPV proliferation. However, the molecular mechanism by which GA inhibits BmNPV is unclear. MicroRNAs (miRNAs) have been shown to play a key role in regulating virus proliferation and host-pathogen interactions. In this study, BmN cells infected with BmNPV were treated by GA and DMSO for 72 h, respectively, then transcriptome analysis of miRNA was performed from the GA group and the control group. As a result, a total of 29 miRNAs were differentially expressed (DE), with 13 upregulated and 16 downregulated. Using bioinformatics analysis, it was found that the target genes of DEmiRNAs were involved in ubiquitin-mediated proteolysis, phagosome, proteasome, endocytosis pathways, and so on. Six DEmiRNAs were verified by quantitative reverse-transcription polymerase chain reaction. DElong noncoding RNA (DElncRNA)-DEmiRNA-messenger RNA (mRNA) regulatory networks involved in apoptosis and immune pathways were constructed in GA-treated BmN cells, which included 12 DEmiRNA, 132 DElncRNA, and 69 mRNAs. This regulatory network enriched the functional role of miRNA in the BmNPV-silkworm interactions and improved our understanding of the molecular mechanism of HSP90 inhibitors on BmNPV proliferation.

Assessing the Impact of a Viral Infection on the Expression of Transposable Elements in the Cabbage Looper Moth (Trichoplusia ni)

Most studies of stress-induced transposable element (TE) expression have so far focused on abiotic sources of stress. Here, we analyzed the impact of an infection by the AcMNPV baculovirus on TE expression in a cell line (Tnms42) and midgut tissues of the cabbage looper moth (Trichoplusia ni). We find that a large fraction of TE families (576/636 in Tnms42 cells and 503/612 in midgut) is lowly expressed or not expressed at all [≤ 4 transcripts per million (TPM)] in the uninfected condition (median TPM of 0.37 in Tnms42 and 0.46 in midgut cells). In the infected condition, a total of 62 and 187 TE families were differentially expressed (DE) in midgut and Tnms42 cells, respectively, with more up- (46) than downregulated (16) TE families in the former and as many up- (91) as downregulated (96) TE families in the latter. Expression log2 fold changes of DE TE families varied from -4.95 to 9.11 in Tnms42 cells and from -4.28 to 7.66 in midgut. Large variations in expression profiles of DE TEs were observed depending on the type of cells and on time after infection. Overall, the impact of AcMNPV on TE expression in T. ni is moderate but potentially sufficient to affect TE activity and genome architecture. Interestingly, one host-derived TE integrated into AcMNPV genomes is highly expressed in infected Tnms42 cells. This result shows that virus-borne TEs can be expressed, further suggesting that they may be able to transpose and that viruses may act as vectors of horizontal transfer of TEs in insects.

Comparative Transcriptome Analysis of Bombyx mori (Lepidoptera) Larval Hemolymph in Response to Autographa californica Nucleopolyhedrovirus in Differentially Resistant Strains

Bombyx mori nucleopolyhedrovirus (BmNPV) is a kind of pathogen that causes huge economic losses to silkworm production. Although Autographa californica nucleopolyhedrovirus (AcMNPV) and BmNPV are both baculoviruses, the host domains of these two viruses have almost no intersection in nature. Recently, it has been found that some silkworms could be infected by recombinant AcMNPV through a puncture, which provided valuable material for studying the infection mechanism of baculovirus to silkworm. In this study, comparative transcriptomics was used to analyse the hemolymph of two differentially resistant strains following AcMNPV inoculation. There were 678 DEGs in p50 and 515 DEGs in C108 following viral infection. Among them, the upregulation and downregulation of DEGs were similar in p50; however, the upregulated DEGs were nearly twice as numerous as the downregulated DEGs in C108. The DEGs in different resistant strains differed by GO enrichment. Based on KEGG enrichment, DEGs were mainly enriched in metabolic pathways in p50 and the apoptosis pathway in C108. Moreover, 13 genes involved in metabolic pathways and 11 genes involved in the apoptosis pathway were analysed. Among the DEGs involved in apoptosis, the function of BmTex261 in viral infection was analysed. The BmTex261 showed the highest expression in hemolymph and a significant response to viral infection in the hemolymph of C108, indicating that it is involved in anti-AcMNPV infection. This was further validated by the significantly decreased expression of viral gene lef3 after overexpression of BmTex261 in BmN cells. The results provide a theoretical reference for the molecular mechanism of resistance to BmNPV in silkworms.

BmNPV-induced hormone metabolic disorder in silkworm leads to enhanced locomotory behavior

Many parasites alter the host locomotory behaviors in a way that increases their fitness and progeny transmission. Baculoviruses can manipulate host physiology and alter the locomotory behavior by inducing 'hyperactivity' (increased locomotion) or 'tree-top disease' (climbing high up to the top before dying). However, the detailed molecular mechanism underlying virus-induced this hyperactive behavior remains elusive. In the present study, we showed that BmNPV invaded into silkworm brain tissue, resulting in severe brain damage. Moreover, BmNPV infection disturbed the insect hormone balance. The content of 20-hydroxyecdysone (20E) in hemolymph was much lower during the hyperactive stage, while the dopamine (DA) titer was higher than mock infection. Exogenous hormone treatment assays demonstrated that 20E inhibits virus-induced ELA (enhanced locomotory activity), while dopamine stimulates this behavior. More specificity, injection of dopamine or its agonist promote this hyperactive behavior in BmNPV-infected larvae. Taking together, our findings revealed the important role of hormone metabolism in BmNPV-induced ELA.

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.

Functional analysis of Dicer-2 gene in Bombyx mori resistance to BmNPV virus

Bombyx mori nucleopolyhedrovirus (BmNPV) is a primary pathogen in silkworm, and the molecular mechanism of B. mori defense to BmNPV infection is still unclear. RNA interference (RNAi) is well-known as an intracellular conserved mechanism that is critical in gene regulation and cell defense. The antiviral RNAi pathway processes viral double-stranded RNA (dsRNA) into viral small interfering RNAs that guide the recognition and cleavage of complementary viral target RNAs. In this study, a Dicer-2 (Dcr2) gene was identified in B. mori and its antiviral function was explored. Dcr2 messenger RNA (mRNA) expression was the highest in hemocytes and expressed in all stages of silkworm growth. After infection with BmNPV, the expression of Dcr2 mRNA was significantly increased after infection in midgut and hemocytes. The expression of Dcr2 was significantly upregulated by injecting dsRNA (dsBmSPH-1) into silkworm after 48 hr. Knocking down the expression level of Dcr2 using specific dsRNA in silkworm, which modestly enhanced the production of viral genomic DNA. Our results suggested that the Dcr2 gene in B. mori plays an important role in against BmNPV invasion.

Increased expression of Suppressor of cytokine signaling 2 (BmSOCS2) is correlated with suppression of Bombyx mori nucleopolyhedrovirus replication in silkworm larval tissues and cells

The resistance of silkworm to infection by Bombyx mori nuclear polyhedrosis virus (BmNPV) is a main focus of sericultural research. Previously, a BmNPV-resistant strain, NB, was identified among a collection of Chinese silkworm strains in our lab. To better understand the molecular mechanism of NB strain resistance, the patterns of host immune response gene transcription in resistant (NB) and susceptible (306) strains were examined. Quantative real-time PCR (qRT-PCR) revealed that multiple insect innate immune signaling pathways (Toll, Imd and JAK/STAT) were strongly activated upon infection with BmNPV. Notably, Suppressor of cytokine signaling 2 (BmSOCS2) mRNA expression was significantly up-regulated in midgut tissues of the resistant NB strain, suggesting that the BmSOCS2 gene product may be involved in host immune defense against BmNPV infection. A significant inhibition of BmNPV replication was also observed in BmN cells transfected with a vector encoding BmSOCS2. The results suggest that BmSOCS2 is a key gene involved in the resistance of the NB silkworm strain to BmNPV infection.

Cross-talking between baculoviruses and host insects towards a successful infection

Baculoviridae is a family of large DNA viruses that infect insects. They have been extensively used as safe and efficient biological agents for the control of insect pests. As a result of coevolution with their hosts, baculoviruses developed unique life cycles characterized by the production of two distinctive virion phenotypes, occlusion-derived virus and budded virus, which are responsible for mediating primary infection in insect midgut epithelia and spreading systemic infection within infected insects, respectively. In this article, advances associated with virus-host interactions during the baculovirus life cycle are reviewed. We mainly focus on how baculoviruses exploit versatile strategies to overcome diverse host barriers and establish successful infections. For example, in the midgut, baculoviruses encode enzymes to degrade peritrophic membranes and use a series of per os infectivity factors to initiate primary infection. A viral fibroblast growth factor is expressed to attract tracheoblasts that spread the virus for systemic infection. Baculoviruses use different strategies to suppress host defence systems, including apoptosis, melanization and RNA interference. Additionally, baculoviruses can manipulate host physiology and induce 'tree-top disease' for optimal virus replication and dispersal. These advances in our understanding of baculoviruses will greatly inform the development of more effective baculoviral pesticides. This article is part of the theme issue 'Biotic signalling sheds light on smart pest management'.

Bmserpin2 Is Involved in BmNPV Infection by Suppressing Melanization in Bombyx mori

Melanization, an important defense response, plays a vital role in arthropod immunity. It is mediated by serine proteases (SPs) that convert the inactive prophenoloxidase (PPO) to active phenoloxidase (PO) and is tightly regulated by serine protease inhibitors (serpins) which belong to a well distributed superfamily in invertebrates, participating in immune mechanisms and other important physiological processes. Here, we investigated the Bmserpin2 gene which was identified from a transcriptome database in response to Bombyx mori nucleopolyhedrovirus (BmNPV) infection. Quantitative real-time polymerase chain reaction (qRT-PCR) results showed that Bmserpin2 was expressed in all tissues, with maximum expression in fat body. Upon BmNPV infection, the expression of Bmserpin2 was up-regulated in P50 (susceptible strain) and BC9 (resistant strain) in haemocytes, fat body and the midgut. However, up-regulation was delayed in BC9 (48 or 72 h), in contrast to P50 (24 h), after BmNPV infection. Meanwhile, Bmserpin2 could delay or inhibit melanization in silkworm haemolymph. Significant increased PO activity can be observed in Bmserpin2-depleted haemolymph under NPV infection. Furthermore, the viral genomic DNA copy number was decreased in Bmserpin2-depleted haemolymph. We conclude that Bmserpin2 is an inducible gene which might be involved in the regulation of PPO activation and suppressed melanization, and have a potential role in the innate immune system of B. mori.

DNA methylomes and transcriptomes analysis reveal implication of host DNA methylation machinery in BmNPV proliferation in Bombyx mori

BACKGROUND

Bombyx mori nucleopolyhedrosis virus (BmNPV) is a major pathogen that threatens the sustainability of the sericultural industry. DNA methylation is a widespread gene regulation mode in epigenetics, which plays an important role in host immune response. Until now, little has been known about epigenetic regulation on virus diseases in insects. This study aims to explore the role of DNA methylation in BmNPV proliferation.

RESULTS

Inhibiting DNA methyltransferase (DNMT) activity of silkworm can suppress BmNPV replication. The integrated analysis of transcriptomes and DNA methylomes in silkworm midguts infected with or without BmNPV showed that both the expression pattern of transcriptome and DNA methylation pattern are changed significantly upon BmNPV infection. A total of 241 differentially methylated regions (DMRs) were observed in BmNPV infected midguts, among which, 126 DMRs were hyper-methylated and 115 DMRs were hypo-methylated. Significant differences in both mRNA transcript level and DNA methylated levels were found in 26 genes. BS-PCR validated the hypermethylation of BGIBMGA014008, a structural maintenance of chromosomes protein gene in the BmNPV-infected midgut. In addition, DNMT inhibition reduced the expression of inhibitor of apoptosis family genes, iap1 from BmNPV, Bmiap2, BmSurvivin1 and BmSurvivin2.

CONCLUSION

Our results indicate that DNA methylation plays positive roles in BmNPV proliferation and loss of DNMT activity could induce the apoptosis of infected cells to suppress BmNPV proliferation. Our results may provide a new idea and research direction for the molecular mechanism on insect-virus interaction.

Quantitative label-free proteomic analysis reveals differentially expressed proteins in the digestive juice of resistant versus susceptible silkworm strains and their predicted impacts on BmNPV infection

Bombyx mori nucleopolyhedrovirus (BmNPV) is a major pathogen causing severe economic loss. Previous studies have revealed that some proteins in silkworm digestive juice show antiviral activity. In this study, antiviral activity examination of different resistant strains showed that the digestive juice of the resistant strain (A35) had higher inhibition to virus than the susceptible strain (P50). Subsequently, the label-free quantitative proteomics was used to study the midgut digestive juice response to BmNPV infection in P50 and A35 strains. A total of 98 proteins were identified, of which 80 were differentially expressed proteins (DEPs) with 54 enzymes and 26 nonenzymatic proteins by comparing the proteomes of infected and non-infected P50 and A35 silkworms. These DEPs are mainly involved in metabolism, proteolysis, neuroactive ligand receptor interaction, starch and sucrose metabolism and glutathione metabolism. After removing the genetic background and individual immune stress response proteins, 9 DEPs were identified potentially involved in resistance to BmNPV. Further studies showed that a serine protease, an alkaline phosphatase and serine protease inhibitor 2 isoform X1 were differentially expressed in A35 compared to P50 or post BmNPV infection. Taken together, these results provide insights into the potential mechanisms for silkworm digestive juice to provide resistance to BmNPV infection. Signifcance: Bombyx mori nucleopolyhedrovirus (BmNPV) is highly pathogenic, which has a great impact on the sericulture. BmNPV entered the midgut lumen and exposed to digestive juices after oral infection. Previous studies have revealed that some proteins in silkworm digestive juice show antiviral activity, however, current information on the digestive juice proteome of high resistant silkworm strain after BmNPV challenge compared to susceptible strain is incomprehensive. Here, we combined label-free quantification method, bioinformatics, RT-qPCR and western blot analysis and found that BmNPV infection causes some protein changes in the silkworm midgut digestive juice. The DEPs were identified in the digestive juices of different resistant strains following BmNPV infection, and screened out some proteins potentially related to resistance to BmNPV. Three important differentially expression proteins were validated by independent approaches. These findings uncover the potential role of silkworm digestive juice in providing resistance to BmNPV and supplemented the profile of the proteome of the digestive juices in B. mori.

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.

BmAtg13 promotes the replication and proliferation of Bombyx mori nucleopolyhedrovirus

Autophagy is a cell adaptive response that involves the process of microbial infections. Our previous study has indicated that Bombyx mori nucleopolyhedrovirus (BmNPV) infection triggers the complete autophagic process in BmN-SWU1 cells, which is beneficial to the viral infection. Autophagy-related (ATG) protein ATG13, as part of the ULK complex (a serine-threonine kinase complex composed of ULK1, ULK2, ATG13, ATG101, and FIP200), is the most upstream component of the autophagy pathway, and how it affects virus infections will improve our understanding of the interaction between the virus and the host. This study has determined that the overexpression of the BmAtg13 gene promotes the expression of viral genes and increases viral production in BmN-SWU1 cells, whereas knocking down the BmAtg13 gene suppresses BmNPV replication. Moreover, the BmAtg13 overexpression transgenic line contributed to viral replication and increased mortality rate of BmNPV infection. In contrast, the BmAtg13 knockout transgenic line reduced viral replication 96 h post-infection. Furthermore, BmATG13 directly interacted with viral protein BRO-B, forming a protein complex. Taken together, the findings of this study suggest that BmATG13 may be utilized by the BRO-B protein to promote BmNPV replication and proliferation, which, in turn, provides important insights into the mechanism that autophagy influences viral infection.

Antioxidant Systems as a Response to Midgut Cellular of Bombyx mori Lineu, 1758 (Lepidoptera: Bombycidae) Infection for Baculoviruses

Bombyx mori nucleopolyhedrovirus (BmNPV) is a DNA virus that infects different tissues in Bombyx mori at immature stage. Caterpillars become infected after ingesting polyhedral occlusion bodies (POB) present in contaminated mulberry leaves and spread through the body after passing the epithelium of the midgut. As this organ is responsible for digestion, most absorption of nutrients requires an intact epithelium to maintain gastrointestinal physiology. Considering the importance of this organ in the feeding of caterpillars and in the production of quality silk threads, and because it is also the first barrier faced by the BmNPV, the study analyzed details of cytopathological events in the intestinal cells as well as evaluated the action of the antioxidant systems as a response to cellular infection. For this purpose, B. mori hybrid caterpillars of fifth instar were inoculated with a suspension of 7.8 × 107 POB ml-1 and, from the first to the eighth day post-inoculation (dpi), segments of the midgut were collected and processed for light and electronic microscopy. The nuclei of columnar cells showed polyhedric occlusion bodies in the seventh dpi and fragmentation of those cells, with peritrophic matrix disorganization. Analysis of antioxidant systems shows some moments of changes of the catalase enzymes and superoxide dismutase. Analysis of the cholinergic system revealed changes only at the beginning of the infection. Thus, the article acknowledges the antioxidant system as a barrier to stop viral infection, albeit it cannot stop infection from occurring, once a coevolutionary bond is maintained between virus and host.

Over expression of bmo-miR-2819 suppresses BmNPV replication by regulating the BmNPV ie-1 gene in Bombyx mori

Bombyx mori nucleopolyhedrovirus (BmNPV) is a major pathogen that threatens the growth and sustainability of the sericulture industry. Accumulating studies in recent years suggest that insect viruses infection can change the host microRNAs (miRNAs) expression profile and both cellular and viral miRNAs play roles in host-pathogen interactions. Until now, the functional analysis of miRNA encoded by silkworm for host-virus interaction is limited. In this study, we validate the down-regulation of bmo-miR-2819 upon BmNPV infection by qRT-PCR and confirm the BmNPV immediately early 1 gene, ie-1 is one of the targets for bmo-miR-2819 based on the results of dual luciferase report assay. Overexpression of bmo-miR-2819 can significantly decline the abundance of IE-1 protein level in BmNPV-infected silkworm larvae. Further, the expression level of polyhedrin gene and VP39 protein of BmNPV in the infected larvae after applying bmo-miR-2819 mimics was significantly decreased comparing with that of larvae with mimic control. Our results suggest that overexpression of bmo-miR-2819 could suppress BmNPV replication by down-regulating the expression of BmNPV ie-1 gene, which demonstrate that cellular miRNAs could affect virus infection by regulating the expression of virus genes.

Effects of Bombyx mori nuclear polyhedrosis virus on serpin and antibacterial peptide expression in B. mori

The silkworm (Bombyx mori) is a typical and economically important lepidopteran species, and research has resulted in the development and accumulation of breeding lines. Studies of immune-related silkworm genes not only promote our understanding of silkworm immune response mechanisms, but they also inform insect immune molecular diversity research. Here, silkworm proteins were screened using proteomics after Bombyx mori nuclear polyhedrosis virus (BmNPV) infection, and 2368 silkworm proteins were identified, including six antimicrobial peptides and 12 serpins. The mRNA expression levels of these 18 proteins were examined at different times. The results indicated that attacin had the highest expression level, while serpin-5 and cecropin-D exhibited a negative regulatory correlation. These results provide a significant step toward a deeper understanding of B. mori immunoregulation.

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.

Characterization of a Lipase From the Silkworm Intestinal Bacterium Bacillus pumilus With Antiviral Activity Against Bombyx mori (Lepidoptera: Bombycidae) Nucleopolyhedrovirus In Vitro

To investigate whether Bombyx mori Linnaeus (Lepidoptera: Bombycidae) intestinal microorganism play a role in the host defence system against viral pathogens, a lipase gene from the silkworm intestinal bacterium Bacillus pumilus SW41 was characterized, and antiviral activity of its protein against B. mori nucleopolyhedrovirus (BmNPV) was tested. The lipase gene has an open-reading frame of 648 bp, which encodes a 215-amino-acid enzyme with a 34-amino-acid signal peptide. The recombinant lipase (without signal peptide) was expressed and purified by using an Escherichia coli BL21 (DE3) expression system. The total enzyme activity of this recombinant lipase reached 277.40 U/mg at the optimum temperature of 25°C and optimum pH value of 8.0. The antiviral test showed that a relative high concentration of the recombinant lipase reduced BmNPV infectivity in vitro, which resulted in decreased viral DNA abundance and viral occlusion bodies. Besides, the preincubation method also suggested that the lipase probably directly acting on the budded virions. The results suggest that the lipase from intestinal bacterium B. pumilus SW41 is a potential antiviral factor for silkworm against BmNPV.

Inhibition of expression of BmNPV cg30 by bmo-miRNA-390 is a host response to baculovirus invasion

Bombyx mori larvae exhibit in vivo defensive reactions immediately after invasion by a virus. One of these defense systems is to express appropriate microRNAs (miRNAs) to respond to the infection. A novel Bombyx mori-encoded miRNA, bmo-miR-390, was identified previously by high-throughput sequencing. Based on bioinformatic predictions, the Bombyx mori nuclear polyhedrosis virus cg30 gene (BmNPV-cg30) is one of the target genes of bmo-miR-390. In this study, expression vectors with an enhanced green fluorescence protein (EGFP) or a luciferase (luc) reporter gene together with bm-miR-390 or the cg30 3' UTR were constructed and used to co-transfect BmN cells. Using a dual luciferase reporter (DLR) assay, we found that bmo-miR-390 significantly downregulates the expression of BmNPV-cg30 (P < 0.05) in vitro. Moreover, artificially synthesized bmo-miR-390 mimics enhanced the regulatory effect of bmo-miR-390, while an inhibitor eliminated the inhibitory effect. These results show for the first time that bmo-miR-390 can effectively downregulate the expression of BmNPV-cg30 in BmNPV-infected BmN cells.

Quantitative proteomics of Bombyx mori after BmNPV challenge

The domesticated silkworm is an ideal and economic insect model that plays crucial roles in sericulture and bioreactor. Bombyx mori nucleopolyhedrovirus (BmNPV) is not only an infectious pathogen to B. mori, but also an efficient vector expressing recombinant proteins. Although, the proteomics of silkworm and BmN cell membrane lipid raft towards BmNPV infection had been investigated, proteome results of BmN cells upon BmNPV challenge currently remain ambiguous. In order to explore the interaction between silkworm and BmNPV, we analyzed several pivotal processes of BmNPV infected BmN cell by quantitative mass spectrometry. Our study indicated that a total of 4205 identified proteins, among which 4194 were with quantitative level. Concretely, during BmNPV infection, several transcription factors and epigenetically modified proteins showed substantially different abundance levels. Especially, proteins with binding activity, displayed significant changes in their molecular functions. Disabled non-homologous end joining by BmNPV reflects irreversible breakage of DNA. Nevertheless, highly abundant superoxide dismutase suggests that the cellular defense system is persistently functional in maintaining biochemical homeostasis. Our comparative and quantitative proteomics will be helpful to unravel the dynamics of B.mori after BmNPV infection and could provide new insights to decipher the mechanism of interaction between BmN cell and BmNPV.

SWATH-based quantitative proteomics reveals the mechanism of enhanced Bombyx mori nucleopolyhedrovirus-resistance in silkworm reared on UV-B treated mulberry leaves

Bombyx mori nucleopolyhedrovirus (BmNPV) is one of the most acute infectious diseases in silkworm, which has led to great economic loss in sericulture. Previous study showed that the content of secondary metabolites in mulberry leaves, particularly for moracin N, was increased after UV-B irradiation. In this study, the BmNPV resistance of silkworms reared on UV-B treated and moracin N spread mulberry leaves was improved. To uncover the mechanism of enhanced BmNPV resistance, silkworm midguts from UV-B treated mulberry leaves (BUM) and moracin N (BNM) groups were analyzed by SWATH-based proteomic technique. Of note, the abundance of ribosomal proteins in BUM and BNM groups was significantly changed to maintain the synthesis of total protein levels and cell survival. While, cytochrome c oxidase subunit II, calcium ATPase and programmed cell death 4 involved in apoptotic process were up-regulated in BNM group. Expressions of lipase-1, serine protease precursor, Rab1 protein, and histone genes were increased significantly in BNM group. These results suggest that moracin N might be the main active component in UV-B treated mulberry leaves which could improve the BmNPV-resistance of silkworm through promoting apoptotic cell death, enhancing the organism immunity, and regulating the intercellular environment of cells in silkworm. It also presents an innovative process to reduce the mortality rate of silkworms infected with BmNPV.

Quantitative phosphoproteome on the silkworm (Bombyx mori) cells infected with baculovirus

Background

Bombyx mori has become an important model organism for many fundamental studies. Bombyx mori nucleopolyhedrovirus (BmNPV) is a significant pathogen to Bombyx mori, yet also an efficient vector for recombinant protein production. A previous study indicated that acetylation plays many vital roles in several cellular processes of Bombyx mori while global phosphorylation pattern upon BmNPV infection remains elusive.

Method

Employing tandem mass tag (TMT) labeling and phosphorylation affinity enrichment followed by high-resolution LC-MS/MS analysis and intensive bioinformatics analysis, the quantitative phosphoproteome in Bombyx mori cells infected by BmNPV at 24 hpi with an MOI of 10 was extensively examined.

Results

Totally, 6480 phosphorylation sites in 2112 protein groups were identified, among which 4764 sites in 1717 proteins were quantified. Among the quantified proteins, 81 up-regulated and 25 down-regulated sites were identified with significant criteria (the quantitative ratio above 1.3 was considered as up-regulation and below 0.77 was considered as down-regulation) and with significant p-value (p < 0.05). Some proteins of BmNPV were also hyperphosphorylated during infection, such as P6.9, 39 K, LEF-6, Ac58-like protein, Ac82-like protein and BRO-D.

Conclusion

The phosphorylated proteins were primary involved in several specific functions, out of which, we focused on the binding activity, protein synthesis, viral replication and apoptosis through kinase activity.

Comparative Subcellular Proteomics Analysis of Susceptible and Near-isogenic Resistant Bombyx mori (Lepidoptera) Larval Midgut Response to BmNPV infection

The molecular mechanism of silkworm resistance to Bombyx mori nucleopolyhedrovirus (BmNPV) infection remains largely unclear. Accumulating evidence suggests that subcellular fractionation combined with proteomics is an ideal technique to analyse host antiviral mechanisms. To clarify the anti-BmNPV mechanism of the silkworm, the near-isogenic line BC9 (resistant strain) and the recurrent parent P50 (susceptible strain) were used in a comparative subcellular proteomics study. Two-dimensional gel electrophoresis (2-DE) combined with mass spectrometry (MS) was conducted on proteins extracted from the cytosol, mitochondria, and microsomes of BmNPV-infected and control larval midguts. A total of 87 proteins were successfully identified from the three subcellular fractions. These proteins were primarily involved in energy metabolism, protein metabolism, signalling pathways, disease, and transport. In particular, disease-relevant proteins were especially changed in microsomes. After infection with BmNPV, differentially expressed proteins (DEPs) primarily appeared in the cytosolic and microsomal fractions, which indicated that these two fractions might play a more important role in the response to BmNPV infection. After removing genetic background and individual immune stress response proteins, 16 proteins were identified as potentially involved in repressing BmNPV infection. Of these proteins, the differential expression patterns of 8 proteins according to reverse transcription quantitative PCR (RT-qPCR) analyses were consistent with the 2-DE results.

Genome-Wide Analysis of Differentially Expressed microRNA in Bombyx mori Infected with Nucleopolyhedrosis Virus

Bombyx mori nucleopolyhedrosis virus (BmNPV) is a major pathogen that threatens the growth and sustainability of the sericulture industry. Since microRNAs (miRNAs) have been shown to play important roles in host-pathogen interactions, in this study we investigated the effects of BmNPV infection on silkworm microRNAs expression profile. To achieve this, we constructed and deep-sequenced two small RNA libraries generated from BmNPV infected and un-infected larvae. The results revealed that 38 silkworm miRNAs were differentially expressed after BmNPV infection. Based on the GO analysis, their predicted target genes were found to be involved in diverse functions such as binding, catalytic, virion and immune response to stimulus suggesting their potential roles in host-virus interactions. Using the dual-luciferase reporter assay, we confirmed that Bmo-miR-277-5p, up-regulated in BmNPV-infected larvae, targeted the B. mori DNA cytosine-5 methyltransferase (Dnmt2) gene which may play potential role in silkworm-BmNPV interaction. These results provide new insights into exploring the interaction mechanism between silkworm and BmNPV.

Gut immunity in Lepidopteran insects

Lepidopteran insects constitute one of the largest fractions of animals on earth, but are considered pests in their relationship with man. Key to the success of this order of insects is its ability to digest food and absorb nutrition, which takes place in the midgut. Because environmental microorganisms can easily enter Lepidopteran guts during feeding, the innate immune response guards against pathogenic bacteria, virus and microsporidia that can be devoured with food. Gut immune responses are complicated by both resident gut microbiota and the surrounding peritrophic membrane and are distinct from immune responses in the body cavity, which depend on the function of the fat body and hemocytes. Due to their relevance to agricultural production, studies of Lepidopteran insect midgut and immunity are receiving more attention, and here we summarize gut structures and functions, and discuss how these confer immunity against different microorganisms. It is expected that increased knowledge of Lepidopteran gut immunity may be utilized for pest biological control in the future.

Effects of Antiviral Agent, Acyclovir, on the Biological Fitness of Galleria mellonella (Lepidoptera: Pyralidae) Adults

The effects of a synthetic purine nucleoside analog, antiviral agent, acyclovir (ACV), on adult longevity, fecundity, and hatchability of a serious honeycomb pest, greater wax moth Galleria mellonella L. were investigated by adding 0.01, 0.1, 1.0, and 3.0% ACV into artificial and natural diets. Control larvae were reared on diet without ACV. The artificial diet containing the lowest level of ACV, 0.01%, raised egg production from a number of 12.9 ± 0.6 to 163.2 ± 1.3. The hatching rate of these eggs was increased from 49.2 ± 2.4% to 68.2 ± 3.2%. Higher concentrations of ACV in natural food significantly increased both egg production and egg hatching rate. Female reared on old dark combs as natural diet exposed to 1.0% of ACV produced 167.5 ± 5.8 eggs with 93.2 ± 6.8% hatched. This study emphasizes the importance of determining the dietary impact of an antimicrobial agent as a food additive to a particular species of insect before its using for dietary antimicrobial purpose.

Comparative Transcriptome Analysis of Bombyx mori (Lepidoptera) Larval Midgut Response to BmNPV in Susceptible and Near-Isogenic Resistant Strains

Bombyx mori nucleopolyhedrovirus (BmNPV) is one of the primary pathogens causing severe economic losses in sericulture. However, the molecular mechanism of silkworm resistance to BmNPV remains largely unknown. Here, the recurrent parent P50 (susceptible strain) and the near-isogenic line BC9 (resistance strain) were used in a comparative transcriptome study examining the response to infection with BmNPV. A total of 14,300 unigenes were obtained from two different resistant strains; of these, 869 differentially expressed genes (DEGs) were identified after comparing the four transcriptomes. Many DEGs associated with protein metabolism, cytoskeleton, and apoptosis may be involved in the host response to BmNPV infection. Moreover, some immunity related genes were also altered following BmNPV infection. Specifically, after removing genetic background and individual immune stress response genes, 22 genes were found to be potentially involved in repressing BmNPV infection. These genes were related to transport, virus replication, intracellular innate immune, and apoptosis. Our study provided an overview of the molecular mechanism of silkworm resistance to BmNPV infection and laid a foundation for controlling BmNPV in the future.