Study on anti-BmNPV mechanism of branched-chain amino acid aminotransferases in silkworm

Bombyx mori nucleopolyhedrovirus (BmNPV) is the most important virus that threatens sericulture industry. At present, there is no effective treatment for BmNPV infection in silkworms, and lncRNA plays an important role in biological immune response and host-virus interaction, but there are relatively few studies in silkworms. In this study, the four midgut tissue samples of the resistance strain NB (NB) and susceptible strain 306 (306) and the NB and 306 continuously infected with BmNPV for 96 h are used for whole transcriptome sequencing to analyze the differences in the genetic background of NB and 306 and the differences after inoculation of BmNPV, and the significantly different mRNA, miRNA and lnRNA between NB and 306 after BmNPV inoculation were screened. By comparing NB and 306, 2651 significantly different mRNAs, 57 significantly different miRNAs and 198 significantly different lncRNAs were screened. By comparing NB and 306 after BmNPV inoculation, 2684 significantly different mRNAs, 39 significantly different miRNAs and 125 significantly different lncRNAs were screened. According to the significantly different mRNA, miRNA and lncRNA screened from NB and 306 and NB and 306 after virus inoculation, the mRNA-miRNA-lncRNA regulatory network was constructed before and after virus inoculation, and the BmBCAT-Bomo_chr7_8305-MSTRG.3236.2 regulatory axis was screened from them, and it was found that BmBCAT was not Bomo_chr7_8305 regulated in the genetic background, after viral infection, MSTRG.3236.2 competes for binding Bomo_chr7_8305 regulates BmBCAT. The whole transcriptome sequencing results were verified by qPCR and the time-series expression analysis was performed to prove the reliability of the regulatory network. The BmBCAT-Bomo_chr7_8305-MSTRG.3236.2 regulatory axis may play a potential role in the interaction between silkworms and BmNPV. These results provide new insights into the interaction mechanism between silkworms and BmNPV.

Bombyx mori triose-phosphate transporter protein inhibits Bombyx mori nucleopolyhedrovirus infection by reducing the cell glycolysis pathway

Bombyx mori triose-phosphate transporter protein (BmTPT) is a member of the solute carrier (SLC) family. Its main function is to transport triose phosphate between intracellular and extracellular. In this study, BmTPT was cloned and characterised from the fat body of the silkworm Bombyx mori, resulting in an open reading frame (ORF) with a full length of 936 bp, which can encode 311 amino acid residues and has eight transmembrane structural domains. BmTPT was distributed throughout the cell and deposited the most in the nucleus, and is expressed in all tissues of Bombyx mori. Bombyx mori nucleopolyhedrovirus (BmNPV) infection significantly up-regulated BmTPT expression in immune tissue fat bodies. In addition, overexpression of BmTPT significantly inhibited BmNPV infection and markedly reduced the expression of enzymes related to the cellular glycolytic pathway; on the contrary, down-regulation of BmTPT expression by RNA interference resulted in robust replication of BmNPV and a significant increase in the expression of enzymes related to the cellular glycolytic pathway. This is the first report that BmTPT has antiviral effect in silkworm, and also could result in a lack of energy and raw materials for BmNPV replication and infection through down-regulation of the cellular glycolytic pathway.

Single-Nucleus Sequencing of Silkworm Larval Brain Reveals the Key Role of Lysozyme in the Antiviral Immune Response in Brain Hemocytes

INTRODUCTION

The brain is considered as an immune-privileged organ, yet innate immune reactions can occur in the central nervous system of vertebrates and invertebrates. Silkworm (Bombyx mori) is an economically important insect and a lepidopteran model species. The diversity of cell types in the silkworm brain, and how these cell subsets produce an immune response to virus infection, remains largely unknown.

METHODS

Single-nucleus RNA sequencing (snRNA-seq), bioinformatics analysis, RNAi, and other methods were mainly used to analyze the cell types and gene functions of the silkworm brain.

RESULTS

We used snRNA-seq to identify 19 distinct clusters representing Kenyon cell, glial cell, olfactory projection neuron, optic lobes neuron, hemocyte-like cell, and muscle cell types in the B. mori nucleopolyhedrovirus (BmNPV)-infected and BmNPV-uninfected silkworm larvae brain at the late stage of infection. Further, we found that the cell subset that exerts an antiviral function in the silkworm larvae brain corresponds to hemocytes. Specifically, antimicrobial peptides were significantly induced by BmNPV infection in the hemocytes, especially lysozyme, exerting antiviral effects.

CONCLUSION

Our single-cell dataset reveals the diversity of silkworm larvae brain cells, and the transcriptome analysis provides insights into the immune response following virus infection at the single-cell level.

SUMOylation of Translationally Regulated Tumor Protein Modulates Its Immune Function

Translationally controlled tumor protein (TCTP) is a highly conserved protein possessing numerous biological functions and molecular interactions, ranging from cell growth to immune responses. However, the molecular mechanism by which TCTP regulates immune function is largely unknown. Here, we found that knockdown of Bombyx mori translationally controlled tumor protein (BmTCTP) led to the increased susceptibility of silkworm cells to virus infection, whereas overexpression of BmTCTP significantly decreased the virus replication. We further demonstrated that BmTCTP could be modified by SUMOylation molecular BmSMT3 at the lysine 164 via the conjugating enzyme BmUBC9, and the stable SUMOylation of BmTCTP by expressing BmTCTP-BmSMT3 fusion protein exhibited strong antiviral activity, which confirmed that the SUMOylation of BmTCTP would contribute to its immune responses. Further work indicated that BmTCTP is able to physically interact with interleukin enhancer binding factor (ILF), one immune molecular, involved in antivirus, and also induce the expression of BmILF in response to virus infection, which in turn enhanced antiviral activity of BmTCTP. Altogether, our present study has provided a novel insight into defending against virus via BmTCTP SUMOylation signaling pathway and interacting with key immune molecular in silkworm.

Bombyx mori Akirin hijacks a viral peptide vSP27 encoded by BmCPV circRNA and activates the ROS-NF-κB pathway against viral infection

Bombyx mori cypovirus (BmCPV), a member of the family Reoviridae, is a model of Cypovirus, has a 10 segmented double-stranded RNA genome. However, so far, only one viral small peptide vSP27 with negative regulation on viral infection was identified; the mechanisms underlying host-BmCPV interaction are still unknown. Here, we identified that vSP27 was translated from a BmCPV derived circular RNA (circRNA-vSP27). Subsequently, results showed that vSP27 induced generation of ROS activated the NF-κB signaling pathway, induced the expression of antimicrobial peptides, and suppressed BmCPV infection. On the other hand, we identified a nuclear protein Akirin that could hijack vSP27, positively regulate the NF-κB pathway, and lead to inhibiting the viral infection. Altogether, our data suggested that BmCPV derived circRNA-vSP27 with small peptide translation activity may be employed by the host immunity in defense against the BmCPV infection.

Construction of Baculovirus-Inducible CRISPR/Cas9 Antiviral System Targeting BmNPV in Bombyx mori

The silkworm Bombyx mori is an economically important insect. The sericulture industry is seriously affected by pathogen infections. Of these pathogens, Bombyx mori nucleopolyhedrovirus (BmNPV) causes approximately 80% of the total economic losses due to pathogen infections. We previously constructed a BmNPV-specific CRISPR/Cas9 silkworm line with significantly enhanced resistance to BmNPV. In order to optimize the resistance properties and minimize its impact on economic traits, we constructed an inducible CRISPR/Cas9 system for use in transgenic silkworms. We used the 39k promoter, which is induced by viral infection, to express Cas9 and the U6 promoter to express four small guide RNA targeting the genes encoding BmNPV late expression factors 1 and 3 (lef-1 and lef-3, respectively), which are essential for viral DNA replication. The system was rapidly activated when the silkworm was infected and showed considerably higher resistance to BmNPV infection than the wild-type silkworm. The inducible system significantly reduced the development effects due to the constitutive expression of Cas9. No obvious differences in developmental processes or economically important characteristics were observed between the resulting transgenic silkworms and wild-type silkworms. Adoption of this accurate and highly efficient inducible CRISPR/Cas9 system targeting BmNPV DNA replication will result in enhanced antivirus measures during sericulture, and our work also provides insights into the broader application of the CRISPR/Cas9 system in the control of infectious diseases and insect pests.

Unraveling the innate immune responses of Bombyx mori hemolymph, fat body, and midgut to Bombyx mori nucleopolyhedrovirus oral infection by metabolomic analysis

Bombyx mori nucleopolyhedrovirus (BmNPV) infection causes a series of physiological and pathological changes in Bombyx mori (B. mori). Here, a metabolomic study of the innate immunity organs including hemolymph, fat body, and midgut of the silkworm strain Dazao following BmNPV challenge was conducted to reveal the metabolic variations in B. mori. Compared to the control, 4964 and 4942 features with 4077 and 4327 high-quality features were generated under positive and negative modes, respectively, from BmNPV-infected larvae. The principal component analysis and supervised learning method using partial least squares discrimination analysis demonstrated good analytical stability and experimental reproducibility of the metabolic profiles. Based on database annotations, a total of 296, 108, and 215 differential expressed metabolites (DEMs) were identified from BmNPV-infected group of hemolymph, fat body, and midgut, respectively, which were all mainly grouped into carboxylic acids and derivatives, fatty acyls, and glycerophospholipids. Kyoto Encyclopedia of Genes and Genomes Database enrichment analysis of the DEMs showed that amino acid metabolism was increased at 24 h after BmNPV infection. BmNPV induction was adopted to significantly alter a series of immune-related pathways including phospholipase D signaling pathway, FoxO signaling pathway, metabolism of xenobiotics by cytochrome P450, melanogenesis, membrane transport, carbohydrate metabolism, and lipid metabolism. The different levels of expression of several DEMs including l-glutamate, naphthalene, 3-succinoylpyridine 1-acyl-sn-glycerol 3-phosphate, and l-tyrosine which were involved in those pathways exhibited the immune responses of B. mori to BmNPV infection. Our findings are valuable for a better understanding of the antiviral mechanism of B. mori underlying the interaction between the silkworm and BmNPV.

Response to Bombyx mori nucleopolyhedrovirus infection in silkworm: Gut metabolites and microbiota

The diversity of microbiota and metabolites in the digestive tract gut is important in physiology and homeostasis, nutrient uptake and virus infection. In lepidopteran insect model silkworms, little is known about how microbiota and metabolites are altered after oral infection with BmNPV. Herein, we used 16S rDNA sequencing and metabolomics to show that the gut microbiota and metabolites of silkworm midgut are significantly altered after BmNPV infection. Kyoto Encyclopedia of Genes and Genomes analysis revealed enrichment of flavone and flavonol biosynthesis, glycosyltransferases, and electron transfer carriers signaling pathways, suggesting potential roles in viral infection. Infection also changed the abundance of metabolites in the digestive tract gut, where most pathways were related to metabolism of amino acids, fatty acids and other pathways (e.g., citrate cycle). In addition, a correlation was observed between digestive tract gut microbiota and metabolites. These results shed light on the interaction between digestive tract gut microbiota, metabolites and host-virus interaction, and enhance our understanding of viral infection links to midgut microbiota and metabolic activity in the silkworm.

Functional analysis of a putative Bombyx mori cypovirus miRNA BmCPV-miR-10 and its effect on virus replication

Bombyx mori cypovirus (BmCPV) is an important pathogen of silkworm (B. mori), the economically beneficial insect. The mechanism of its interaction with host immune defence system in the process of infection is still not yet completely clear. Researches have demonstrated that virus-encoded microRNAs (miRNA) play a crucial role in regulating host-pathogen interaction, but few reports are available so far on miRNAs encoded by insect viruses, especially the RNA viruses. In this study, a putative miRNA encoded by the 10th segment of BmCPV genomic RNA, BmCPV-miR-10, was identified and functionally analysed. The expression of the putative BmCPV-miR-10 could be detected via stem-loop RT-PCR (reverse transcription-Polymerase Chain Reaction) in the midgut of silkworm larvae infected with BmCPV. BmCSDE1 (B. mori cold shock domain E1 protein) gene was predicted to be a candidate target gene for BmCPV-miR-10 with the miRNA binding site located in 3' untranslated region of its mRNA. The regulation effect of the putative BmCPV-miR-10 on BmCSDE1 was verified in HEK293 cells by lentiviral expression system, in BmN cells by transfecting BmCPV-miR-10 mimics. The qRT-PCR (quantitative real-time PCR) results showed that the putative BmCPV-miR-10 could suppress the expression of BmCSDE1. By injection of BmCPV-miR-10 mimics into the silkworm larvae infected with BmCPV, it was further proved that the putative BmCPV-miR-10 could suppress the expression of BmCSDE1 in vivo, then inhibit the expression of BmApaf-1 (B. mori apoptotic protease activating factor 1), while enhance the replication of BmCPV genomic RNAs to a certain extent. These results implied that the putative BmCPV-miR-10 could down-regulate the expression of BmCSDE1, then suppress the expression of BmApaf-1, thereby created a favourable intracellular environment for virus replication and proliferation.

Deacetylation of BmAda3 is required for cell apoptosis caused by Bombyx mori nucleopolyhedrovirus infection

Silkworm is not only an ideal insect model with a biological significance, but it is also crucially important in sericulture and bioreactors. Bombyx mori nucleopolyhedrovirus (BmNPV) is a principal pathogen of silkworm. However, the molecular mechanism underlying BmNPV invasion is still unclear. Based on our previous acetylome research findings of B. mori after BmNPV infection, here, we focused on silkworm alteration/deficiency in activation-3 (BmAda3). The acetylation of K124 and K128 were significantly reduced (0.66-fold) upon the virus challenge. Due to the interaction between Ada3 and P53, acetylation-mimic K124Q/K128Q and deacetylation-mimic K124R/K128R mutants of BmAda3 were constructed to explore the roles exerted by the acetylation modification of BmAda3 on P53. Yeast two-hybrid and IP results revealed that both BmAda3 and its deacetylation mutants (K124R/K128R) interacted with P53. Interestingly, we found that the deacetylation mutants (K124R/K128R) of BmAda3 significantly promoted the stability of P53. Since P53 is a proapoptotic factor, cell apoptosis was detected. We established that the deacetylation of BmAda3 at K124/K128 facilitated cellular apoptosis during BmNPV infection. Finally, viral proliferation was analyzed, and the results indicated that virus generation was reduced by K124/K128 deacetylation. Our report, based on the deacetylation of two lysine sites 124/128 of BmAda3, shows possible regulatory pathways of BmNPV proliferation and provides novel insights into the development of antiviral agents.

The effects of quercetin combined with nucleopolyhedrovirus on the growth and immune response in the silkworm (Bombyx mori)

Flavonoids are secondary metabolites that help plants resist insect attack. It can resist insect attack by inhibiting insect immune defense, and pathogens can also inhibit insect immune defense. It is speculated that the combination of flavonoids and pathogens may inhibit the immune defense and have stronger toxicity to silkworm. In this study, the combined treatment of quercetin with Bombyx mori nuclear polyhedrosis virus (BmNPV) had significant negative effects on the growth and survival of silkworm compared with BmNPV group. The detoxifying enzyme activity of BmNPV group was significantly increased at 96 h, while the activity of the combined treatment group was significantly decreased with the increase of quercetin exposure time (72 or 96 h). The activity of antioxidant enzymes also showed a similar trend, that was, the activity of antioxidant enzymes in the combined treatment group also decreased significantly with the increase of quercetin exposure time, which led to the increase of reactive oxygen species content. The silkworm cells would produce lipid peroxidation, malondialdehyde content was significantly increased, so that the expression of immune-related genes (the antimicrobial peptide, Toll pathway, IMD pathway, JAK-STAT pathway, and melanin genes) were decreased, leading to the damage of the immune system of silkworm. These results indicated that quercetin combined with BmNPV could inhibit the activities of protective enzymes and lead to oxidative damage to silkworm. It can also affect the immune response of the silkworm, and thus resulting in abnormal growth. This study provides the novel conclusion that quercetin accumulation will increase the susceptibility of silkworm to pathogens.

The hemolymph melanization response is related to defence against the AcMNPV infection in Bombyx mori

Melanization is mediated by the prophenoloxidase (proPO) activation cascade and plays an important role in the arthropods immune system. Previously, we found that the hemolymph of the p50 strain does not perform melanization after infection with Autographa californica multiple nucleopolyhedrovirus (AcMNPV). However, this mechanism is still unclear. In this study, the underlying mechanism of the inhibition of hemolymph melanization was investigated by analysing the AcMNPV-susceptible or -resistant silkworm strains after inoculation with AcMNPV. The results showed that the level of hemolymph melanization was higher in resistant strain C108 than in susceptible strain p50 at the late stage (72 to 120 h postinoculation). The PO activity decreased significantly at the late stage of infection (72 to 120 hpi), and the expression of BmPPO1 and BmPPO2 was downregulated in p50. However, the PO activity increased in the resistant strain C108, while the expression level of BmPPO1 and BmPPO2 displayed no significant changes. The expression of the BmPPAE gene was upregulated in two strains during viral infection. In addition, the hemolymph melanization can weaken the viral activity in vitro. Our results suggested that the silkworm hemolymph melanization response is related to defence against the AcMNPV infection.

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.

Bmcas-1 plays an important role in response against BmNPV infection in vitro

Apoptosis, as one kind of innate immune system, is involved in host response against pathogens innovation. Caspases play a vital role in the execution stage of host cell apoptosis. It has been reported that Bmcaspase-1 (Bmcas-1) has a close relationship with Bombyx mori nucleopolyhedrovirus (BmNPV) infection for its differentially expressed patterns after viral infection. However, its underlying response mechanism is still unclear. The significant differential expression of Bmcas-1 in different tissues of differentially resistant strains revealed its vital role in BmNPV infection. To further validate its role in BmNPV infection, budded virus (BV)-eGFP was analyzed after knockdown and overexpression of Bmcas-1 by small interfering RNA and the pIZT-mCherry vector, respectively. The reproduction of BV-eGFP obviously increased at 72 h after knockdown of Bmcas-1, and decreased after overexpression in BmN cells. Moreover, the conserved functional domain of Cas-1 among different species and the closed evolutionary relationship of Cas-1 in Lepidoptera hinted that Bmcas-1 might be associated with apoptosis, and this was also validated by the apoptosis inducer, Silvestrol, and the inhibitor, Z-DEVD-FMK. Therefore, Bmcas-1 plays an essential antiviral role by activating apoptosis, and this result lays a fundament for clarifying the molecular mechanism of silkworm in response against BmNPV infection and breeding of resistant strains.

The piRNA response to BmNPV infection in the silkworm fat body and midgut

Bombyx mori nucleopolyhedrovirus (BmNPV) is a DNA virus that causes huge losses to the silkworm industry but the piRNA responses during BmNPV infection in the silkworm remain uninvestigated. Here, silkworm piRNA profiles of uninfected and BmNPV-infected fat body and midgut were determined by high-through sequencing in the early stages of BmNPV infection. A total of 2675 and 3396 genome-derived piRNAs were identified from fat body and midgut, respectively. These genome-derived piRNAs mainly originated from unannotated instead of transposon regions in the silkworm genome. In total, 572 piRNAs were associated with 280 putative target genes in fat body and 805 piRNAs with 380 target genes in midgut. Compared to uninfected tissues, 322 and 129 piRNAs were significantly upregulated in BmNPV-infected fat body and midgut, respectively. In addition, 276 and 117 piRNAs were significantly downregulated. Moreover, differentially expressed (DE) piRNAs during BmNPV infection differed significantly between fat body and midgut. Putative DE piRNA-targeted genes were associated with "response to stimulus" and "environmental information processing" in fat body after infection with BmNPV, which may indicate an active piRNA response to BmNPV infection in fat body. This study may lay the foundation for future research of the potential roles of the piRNA pathway and specific piRNAs in BmNPV pathogenesis.

The digestive proteinase trypsin, alkaline A contributes to anti-BmNPV activity in silkworm (Bombyx mori)

Bombyx mori nucleopolyhedrovirus (BmNPV) is a serious pathogenic microorganism that causes tremendous loss to sericulture. Previous studies have found that some proteins of serine protease family in the digestive juice of B. mori larvae have anti-BmNPV activity. In our previous publication about proteome analysis of the digestive juice of B. mori larvae, the digestive enzyme trypsin, alkaline A (BmTA) was filtered as a differentially expressed protein possibly involved in BmNPV resistance. Here, the biological characteristics and anti-BmNPV functions of BmTA were comprehensively analysed. The cDNA sequence of BmTA had an ORF of 768 nucleotides encoding 255 amino acid residues. Domain architecture analysis showed that BmTA contained a signal peptide and a typical Tryp_SPc domain. Quantitative real-time PCR analysis showed that BmTA was highly expressed in the larval stages and specifically expressed in the midgut of B. mori larvae. The expression level of BmTA in BmNPV resistant strain A35 was higher than that in susceptible strain P50. After BmNPV infection, the expression of BmTA increased in both strains from 24 to 72 h. Virus amplification analysis showed that the relative levels of VP39 in B. mori larvae and BmN cells infected with the appropriate concentration of recombinant-BmTA-treated BmNPV were significantly lower than in the control groups. Moreover, overexpression of BmTA in BmN cells significantly inhibited the amplification of BmNPV. Taken together, the results of this study indicated that BmTA possessed anti-BmNPV activity in B. mori, which broadens the horizon for virus-resistant breeding of silkworms.

Bombyx mori nucleopolyhedrovirus ptp and egt genes are dispensable for triggering enhanced locomotory activity and climbing behavior in Bombyx mandarina larvae

Baculoviruses are classic pathogens that alter host behavior to enhance their dispersal and transmission. While viral protein tyrosine phosphatase (ptp) has been considered as a critical factor for inducing enhanced locomotory activity, preceding investigations have reported that viral ecdysteroid UDP-glucosyltransferase (egt) contributes to triggering climbing behavior in some virus and host species. Here we found that both egt and ptp were dispensable for these abnormal behaviors in Bombyx mandarina larvae induced by Bombyx mori nucleopolyhedrovirus, thus implying that there is an unknown core mechanism of baculovirus-induced alteration of host behaviors.

Global Metabolic Profiling of Baculovirus Infection in Silkworm Hemolymph Shows the Importance of Amino-Acid Metabolism

Viruses rely on host cell metabolism to provide the necessary energy and biosynthetic precursors for successful viral replication. Infection of the silkworm, Bombyx mori, by Bombyx mori nucleopolyhedrovirus (BmNPV), has been studied extensively in the past to unravel interactions between baculoviruses and their lepidopteran hosts. To understand the interaction between the host metabolic responses and BmNPV infection, we analyzed global metabolic changes associated with BmNPV infection in silkworm hemolymph. Our metabolic profiling data suggests that amino acid metabolism is strikingly altered during a time course of BmNPV infection. Amino acid consumption is increased during BmNPV infection at 24 h post infection (hpi), but their abundance recovered at 72 hpi. Central carbon metabolism, on the other hand, particularly glycolysis and glutaminolysis, did not show obvious changes during BmNPV infection. Pharmacologically inhibiting the glycolytic pathway and glutaminolysis also failed to reduce BmNPV replication, revealing that glycolysis and glutaminolysis are not essential during BmNPV infection. This study reveals a unique amino acid utilization process that is implemented during BmNPV infection. Our metabolomic analysis of BmNPV-infected silkworm provides insights as to how baculoviruses induce alterations in host metabolism during systemic infection.

Identification of long noncoding RNAs in silkworm larvae infected with Bombyx mori cypovirus

Bombyx mori cypovirus (BmCPV) is one of the most important pathogens causing severe disease to silkworm. Emerging evidence indicates that long noncoding RNAs (lncRNAs) play importantly regulatory roles in virus infection and host immune response. To better understand the interaction between silkworm, Bombyx mori and BmCPV, we performed a comparative transcriptome analysis on lncRNAs and mRNAs between the virus-infected and noninfected silkworm larvae midgut at two time points postinoculation. A total of 16,753 genes and 1845 candidate lncRNAs were identified, among which 356 messenger RNA (mRNAs) and 41 lncRNAs were differentially expressed (DE). Target gene prediction revealed that most of DEmRNAs (123) were coexpressed with 28 DElncRNAs, suggesting that the expression of mRNA is mainly affected through trans- regulation by BmCPV-induced lncRNAs, and a regulatory network of DElncRNAs and DEmRNAs was then constructed. According to the network, many genes involved in apoptosis, autophagy, and antiviral response, such as ATG3, PDCD6, IBP2, and MFB1, could be targeted by different DElncRNAs, implying the essential roles of these genes and lncRNAs in BmCPV infection. In all, our studies revealed for the first time the alteration of lncRNA expression in BmCPV-infected larvae and its potential influence on BmCPV replication, providing a new perspective for host-cypovirus interaction studies.

Bombyx mori nucleopolyhedrovirus downregulates transcription factor BmFoxO to elevate virus infection

Forkhead-box O (FoxO) is the primary transcriptional effector of the insulin-like signaling pathway that enhances gluconeogenesis through transcriptional activation of PEPCK and G6Pase in mammals. We have previously demonstrated the involvement of phosphoenolpyruvate carboxykinase (BmPEPCK-2) in antiviral immunity against the multiplication of Bombyx mori nuclearpolyhedrosisvirus (BmNPV) in silkworm. Therefore, we speculated that BmFoxO might suppress BmNPV by regulating the expression of PEPCK in silkworm. In the present study, we found that the expression of BmFoxO decreased after BmNPV infection in Bombyx mori; this finding was consistent with BmPEPCK-2 expression. In addition, the expression of BmFoxO was altered, and it was found that reduced expression of BmFoxO (dsBmFoxO) downregulated the expression of BmPEPCK-2 and increased the viral fluorescence and content in silkworm embryonic cell line BmE cells, and vice versa. BmFoxO could upregulate the expression of BmPEPCK-2 by binding to the BmPEPCK-2 promoter. Moreover, overexpression of BmFoxO significantly increased the expression of autophagy genes ATG6/7/8 after infection with BmNPV, consistent with BmPEPCK-2. These results indicate that BmNPV downregulates transcription factor BmFoxO to elevate virus infection, and BmFoxO overexpression upregulates BmPEPCK-2 expression and enhances silkworm antiviral resistance.

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.

The validation of the role of several genes related to Bombyx mori nucleopolyhedrovirus infection in vivo

Bombyx mori nucleopolyhedrovirus (BmNPV) is one of primary silkworm pathogens and causes a serious damage of cocoon losses every year. Recent years, many works have been done to clarify the silkworm anti-BmNPV mechanism, and a significant progress has been made in screening and studying of genes and proteins related to BmNPV infection, but several of them lacked the proofs in vivo. In this study, to further validate the function of seven newly reported genes in vivo, including BmAtlatin-n, Bmferritin-heavy chain (BmFerHCH), Bmthymosin (BmTHY), Bmseroin1, Bmseroin2, Bmnuclear hormone receptors 96 (BmNHR96), and BmE3 ubiquitin-protein ligase SINA-like 10 (BmSINAL10), the response of them in the midgut, fat body, and hemolymph of differentially resistant strains (resistant strain YeA and susceptible strain YeB) at 48 h following BmNPV infection were analyzed. The results showed that the relative stable or upregulated expression level of BmAtlatin-n, BmTHY, Bmseroin1, and Bmseroin2 in YeA resistant strain following BmNPV infection further indicated their antiviral role in vivo, compared with susceptible YeB strain. Moreover, the significant downregulation of BmFerHCH, BmNHR96, and BmSINAL10 in both strains following BmNPV infection revealed their role in benefiting virus infection, as well as the upregulation of BmFerHCH in YeB midgut and BmSINAL10 in YeB hemolymph. These data could be used to complementary the proofs of the function of these genes in response to BmNPV infection.

Potential for small RNA production against Bombyx mori latent virus in Bombyx mori ovaries

Bombyx mori latent virus (BmLV) is a positive, single-stranded insect RNA virus closely related to plant maculaviruses. BmLV was first isolated from Bombyx mori ovary-derived cell line BmN-4, and this virus has already infected most B. mori-derived cultured cell lines. We previously reported that small interfering RNA (siRNA) and PIWI-interacting RNA (piRNA) pathways function cooperatively to maintain the amount of BmLV RNA for normal BmN-4 cell growth. On the other hand, BmLV does not propagate in B. mori larvae. Here we conducted BmLV injection into the larval body cavities of B. mori, and examined BmLV accumulation in larval ovaries where siRNA and piRNA pathways are both active, to investigate whether this in vivo resistance is governed by small RNA pathways. Expression levels of RNA-dependent RNA polymerase, coat protein, and p15 genes in BmLV-injected larval ovaries were extremely low compared with those in B. mori cultured cells, indicating that B. mori larval ovaries are more resistant to BmLV than B. mori cultured cells. We also sequenced small RNAs prepared from BmLV-injected larval ovaries and mapped them onto the BmLV genome. Although their amounts were very small, we were able to detect BmLV-derived small RNAs in the ovaries. According to their length distribution and nucleotide bias, they were likely to be siRNAs and piRNAs. These results suggest that B. mori ovaries can potentially produce small RNAs against BmLV, but the resistance of larval ovaries against BmLV is not dependent on RNA silencing pathways.

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.

Characterization of the lipidomic profile of BmN cells in response to Bombyx mori cytoplasmic polyhedrosis virus infection

Bombyx mori cytoplasmic polyhedrosis virus (BmCPV)that belongs to the genus Cypovirus in the family of Reoviridae is one of the problematic pathogens in sericulture. In our previous study, we have found that lipid-related constituents in the host cellular membrane are associated with the BmCPV life cycle. It is important to note that the lipids not only affect the cellular biological processes, they also impact the virus life cycle. However, the intracellular lipid homeostasis in BmN cells after BmCPV infection remains unclear. Here, the lipid metabolism in BmCPV-infected BmN cells was studied by lipidomics analysis. Our results revealed that the intracellular lipid homeostasis was disturbed in BmN cells upon BmCPV infection. Major lipids constituents in cellular membrane were found to be significantly induced upon BmCPV infection, which included triglycerides, phosphatidylcholine, phosphatidylethanolamine, sphingomyelin, phospholipids, glucoside ceramide, monoetherphosphatidylcholin, ceramide, ceramide phosphoethanolamine and cardiolipin. Further analysis of the pathways related to these altered lipids (such as PE and PC) showed that glycerophospholipid metabolism was one of the most enriched pathways. These results suggested that BmCPV may manipulate the lipid metabolism of cells for their own interest. The findings may facilitate a better understanding of the roles of lipid metabolic changes during virus infection in future studies.

Expression profile analysis of circular RNAs in BmN cells (Bombyx mori) upon BmNPV infection

The disease caused by Bombyx mori nucleopolyhedrovirus (BmNPV) has always been difficult to control, resulting in tremendous economic losses in the sericulture industry. Although much has been learned about the impact of noncoding RNAs on pathogenesis, the role of circular RNA (circRNA) in insect immunity remains unclear. To explore circRNA regulation involved in BmNPV infection, we used transcriptome analysis of BmN cells with or without BmNPV infection to generate circRNA data set. A total of 444 novel circRNAs were identified in BmN cells, with 198 pervasively distributed both in the control group and BmNPV-infection group. The host genes were enriched inMAPK signaling pathway, dorso-ventral axis formation, and ECM-receptor interaction, which were required for the normal larval growth. A total of 75 circRNAs were differentially expressed (DE) on BmNPV infection. Six downregulated circRNAs were validated by Sanger sequencing and qRT-PCR. DEcircRNA-miRNA-DEmRNA network was constructed based on the six validated circRNAs. Pathway analysis indicated that the predicted target genes were mainly enriched in the metabolic pathway and immune-related signaling pathway. Our results may provide a basis for further studies on circRNA function in BmN cells challenged by BmNPV infection and offer an insight into the molecular mechanism on silkworm-virus interaction.

Identification and effect of Zf-AD-containing C2H2 zinc finger genes on BmNPV replication in the silkworm (Bombyx mori)

Zf-AD-containing C2H2 zinc -finger genes (ZAD) are uniquely present and have lineage-specific expansion in arthropods. Arthropods are also the hosts of Baculoviruses. We studied the possible relationship between the lineage-specific expansion of ZAD genes and arthropod-Baculovirus co-evolution. We used the silkworm (Bombyx mori) as a model. We identified 73 ZAD genes (BmZAD) in the silkworm. Sequence-based similarity analysis showed that nine clusters involving 28 BmZADs may have undergone species-specific expansion in the silkworm. Expression pattern analysis showed that the BmZADs were divided into five groups. Group I comprised 10 genes with high expression in multiple tissues, suggesting that BmZADs may play roles in the development of various tissues. We identified six BmZADs that could be induced by the Nucleopolyhedrovirus (BmNPV). Among them, BmZAD69 expression is capable of responding to BmNPV infection, and the ZAD domain is indispensable for the function of BmZAD69 in BmNPV replication. We also detected a 3 bp deletion at 1.7 kb upstream of BmZAD69, which may make it more sensitive to BmNPV infection, and thus elevate the BmNPV resistance in Qiufeng_N, a strain with strong virus resistance. These data suggest that BmZADs may be involved in BmNPV infection and that ZAD genes may play a role in arthropod-Baculovirus co-evolution.

Identification of A functional region in Bombyx mori nucleopolyhedrovirus VP39 that is essential for nuclear actin polymerization

Nuclear actin polymerization plays an indispensable role in the nuclear assembly of baculovirus nucleocapsid, but the underlying viral infection-mediated mechanism remains unclear. VP39 is the major protein in baculovirus capsid, which builds the skeleton of the capsid tubular structure. VP39 is suggested in previous studies to interact with cellular actin and mediate actin polymerization. However, it is unclear about the role of VP39 in mediating nuclear actin polymerization. Results in this study indicated that vp39 deletion abolished nuclear actin polymerization, which was recovered after vp39 repair, revealing the essential part of VP39 in nuclear actin polymerization. Furthermore, a series of mutants with vp39 deletions were constructed to analyze the important region responsible for nuclear actin polymerization. In addition, intracellular localization analysis demonstrated that the amino acids 192-286 in VP39 C-terminal are responsible for nuclear actin polymerization.

Identification of Peritrophins and Antiviral Effect of Bm01504 against BmNPV in the Silkworm, Bombyx mori

The insect midgut secretes a semi-permeable, acellular peritrophic membrane (PM) that maintains intestinal structure, promotes digestion, and protects the midgut from food particles and pathogenic microorganisms. Peritrophin is an important PM protein (PMP) in the PM. Here, we identified 11 peritrophins with 1–16 chitin binding domains (CBDs) comprising 50–56 amino acid residues. Multiple CBDs in the same peritrophin clustered together, rather than by species. The CBD contained six highly conserved cysteine residues, with the key feature of amino acids between them being CX_11-15CX₅CX_9-14CX_11-12CX_6-7C. Peritrophins with 2 and 4 CBDs (Bm09641 and Bm01504, respectively), and with 1, 8, and 16 CBDs (Bm11851, Bm00185, and Bm01491, respectively) were mainly expressed in the anterior midgut, and throughout the midgut, respectively. Survival rates of transgenic silkworms with Bm01504 overexpression (Bm01504-OE) and knockout (Bm01504-KO) infected with B. morinucleopolyhedrovirus (BmNPV) were significantly higher and lower, whereas expression of the key viral gene, p10, were lower and higher, respectively, compared with wild type (WT). Therefore, Bm01504-OE and Bm01504-KO transgenic silkworms were more and less resistant, respectively, to BmNPV. Bm01504 plays important roles in resisting BmNPV invasion. We provide a new perspective for studying PM function, and reveal how the silkworm midgut resists invasive exogenous pathogenic microorganisms.

Screening of Bombyx mori brain proteins interacting with protein tyrosine phosphatase of BmNPV

In this study, glutathione-S-transferase pull-down combined with mass spectrometry techniques were used to identify the candidate proteins interacting with protein tyrosine phosphatase of the Bombyx Mori nucleopolyhedrovirus in the B. mori (BmNPV-PTP) brain. A total of 36 proteins were identified from BmNPV-PTP coprecipitate samples by searching the NCBI_Bombyx Mori database with the original mass spectrum data. Among those proteins, the interaction between BmNPV-PTP and B. mori cyclophilin A may accelerate the apoptosis of certain nerve cells involved in regulating behavior, and thus may be an inducer of enhanced locomotor activity (ELA). After the BmNPV invasion, BmNPV-PTP binding to peripheral-type benzodiazepine receptors may initiate a series of abnormal cascades of the nervous system, which results in abnormal hyperactive behavior in B. mori. Besides this, vacuolar ATP synthase catalytic subunit A, annexin, and several enzymes for energy conversion were identified, which may play a role in enhancing viral entry and infectivity and provide energy for enhancing the locomotor activity of B. mori. In general, the results of this study will facilitate the understanding of the molecular mechanisms underlying the ELA of B. mori larva induced by BmNPV.

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.

Dynamic chromatin accessibility profiling reveals changes in host genome organization in response to baculovirus infection

DNA viruses can hijack and manipulate the host chromatin state to facilitate their infection. Multiple lines of evidences reveal that DNA virus infection results in the host chromatin relocation, yet there is little known about the effects of viral infection on the architecture of host chromatin. Here, a combination of epigenomic, transcriptomic and biochemical assays were conducted to investigate the temporal dynamics of chromatin accessibility in response to Bombyx mori nucleopolyhedrovirus (BmNPV) infection. The high-quality ATAC-seq data indicated that progressive chromatin remodeling took place following BmNPV infection. Viral infection resulted in a more open chromatin architecture, along with the marginalization of host genome and nucleosome disassembly. Moreover, our results revealed that chromatin accessibility in uninfected cells was regulated by euchromatic modifications, whereas the viral-induced highly accessible chromatin regions were originally associated with facultative heterochromatic modification. Overall, our findings illustrate for the first time the organization and accessibility of host chromatin in BmNPV-infected cells, which lay the foundation for future studies on epigenomic regulation mediated by DNA viruses.

As a well-studied arthropod-specific double-stranded DNA virus, Bombyx mori nucleopolyhedrovirus (BmNPV) is a representative member of baculoviruses. BmNPV infection results in significant host chromatin marginalization, which has also been found in most DNA viruses. However, the effects of baculovirus infection on the organization and accessibility of host chromatin are poorly understood. Here, by using ATAC-seq, we show that DNA virus BmNPV infection gradually remodels the accessibility of host chromatin. ATAC-seq data reveal that the marginalized host chromatin is a more accessible architecture along with the depletion of multi-nucleosome depositions. Moreover, our findings suggest the increased accessibility regions are regulated by the facultative heterochromatic modification. Overall, we provide a novel understanding of molecular mechanisms by which baculovirus and DNA viruses alter the organization of host chromatin in epigenomic regulation.

Evolutionary and functional analyses of the interaction between the Bombyx mori inhibitor of apoptosis (IAP) and nucleopolyhedrovirus IAPs

As an important insect immune response, apoptosis plays a critical role in the interaction between baculoviruses and insect hosts. Previous reports have identified inhibitor of apoptosis (IAP) proteins in both insects and baculoviruses, but the relationship between these proteins is still not clearly understood. Here, we found that insect IAP proteins were clustered with baculovirus IAP3, suggesting that the baculovirus iap3 gene might be derived from the Lepidoptera or Diptera. We demonstrated that Bombyx mori inhibitor of apoptosis (Bmiap) gene had an inhibitory effect on apoptosis in silkworm cells. Further analysis of the effects of Bmiap genes on the proliferation of B. mori nucleopolyhedrovirus (BmNPV) showed that both the Bmiap and BmNPV iap genes increased BmNPV proliferation after BmNPV infected silkworm cells. Our results also indicated that BmNPV IAP1 and IAP2 directly interacted with BmIAP in silkworm cells, implying that the Bmiap gene might be hijacked by BmNPV iap genes during BmNPV infection. Taken together, our results provide important insights into the functional relationships of iap genes, and improve our knowledge of apoptosis in baculoviruses and insect hosts.

Antigenic properties of VP15 from white spot syndrome virus in kuruma shrimp Marsupenaeus japonicus

White spot syndrome virus (WSSV) is known as one of the most lethal pathogenic viruses in shrimp causing massive damage to shrimp aquaculture industries. To date, no effective treatment or prevention has been found. In this study, five recombinant viral proteins VP15, VP19, VP24, VP26, and VP28 were expressed and purified in E. coli, which were employed as candidates against WSSV in Kuruma shrimp Marsupenaeus japonicus. In vivo antiviral assay in this study newly revealed that VP15 of major nucleocapsid protein, being known as a DNA-binding protein provided the substantial protection against the viral infection when pre-injected into shrimps. Furthermore, we also verified the immunogenic effects of purified VP15 and VP19 proteins produced in a silkworm-bacmid expression system. Taken together, our study identified VP15 as an effective candidate against WSSV infection in the Kuruma shrimp. It is interesting to uncover why and how VP15 is involved in the immune memory in shrimp in the future study.

Functional analysis of a miRNA-like small RNA derived from Bombyx mori cytoplasmic polyhedrosis virus

Bombyx mori cytoplasmic polyhedrosis virus (BmCPV) is a major pathogen of the economic insect silkworm, Bombyx mori. Virus-encoded microRNAs (miRNAs) have been proven to play important roles in host-pathogen interactions. In this study we identified a BmCPV-derived miRNA-like 21 nt small RNA, BmCPV-miR-1, from the small RNA deep sequencing of BmCPV-infected silkworm larvae by stem-loop quantitative real-time PCR (qPCR) and investigated its functions with qPCR and lentiviral expression systems. Bombyx mori inhibitor of apoptosis protein (BmIAP) gene was predicted by both target prediction software miRanda and Targetscan to be one of its target genes with a binding site for BmCPV-miR-1 at the 5' untranslated region. It was found that the expression of BmCPV-miR-1 and its target gene BmIAP were both up-regulated in BmCPV-infected larvae. At the same time, it was confirmed that BmCPV-miR-1 could up-regulate the expression of BmIAP gene in HEK293T cells with lentiviral expression systems and in BmN cells by transfecting mimics. Furthermore, BmCPV-miR-1 mimics could up-regulate the expression level of BmIAP gene in midgut and fat body in the silkworm. In the midgut of BmCPV-infected larvae, BmCPV-miR-1 mimics could be further up-regulated and inhibitors could lower the virus-mediated expression of BmIAP gene. With the viral genomic RNA segments S1 and S10 as indicators, BmCPV-miR-1 mimics could up-regulate and inhibitors down-regulate their replication in the infected silkworm. These results implied that BmCPV-miR-1 could inhibit cell apoptosis in the infected silkworm through up-regulating BmIAP expression, providing the virus with a better cell circumstance for its replication.

Transcriptome analysis of the immune response of silkworm at the early stage of Bombyx mori bidensovirus infection

Bombyx mori bidensovirus (BmBDV) infects silkworm midgut and causes chronic flacherie disease; however, the interaction between BmBDV and silkworm is unclear. Twenty-four hours after BmBDV infection, the midgut was extracted for RNA-seq to analyze the factors associated with BmBDV-invasion and the early antiviral immune response in silkworms. The total reads from each sample were more than 16100000 and the number of expressed genes exceeded 8200. There were 334 upregulated and 272 downregulated differentially expressed genes (DEGs). Gene ontology analysis of DEGs showed that structural constituents of cuticle, antioxidant, and immune system processes were upregulated. Further analysis revealed BmBDV-mediated induction of BmorCPR23 and BmorCPR44, suggesting possible involvement in viral invasion. Antioxidant genes that protect host cells from virus-induced oxidative stress, were significantly upregulated after BmBDV infection. Several genes related to peroxisomes, apoptosis, and autophagy-which may be involved in antiviral immunity-were induced by BmBDV. These results provide insights into the mechanism of BmBDV infection and host defense.

Genome-wide CRISPR screening reveals genes essential for cell viability and resistance to abiotic and biotic stresses in Bombyx mori

High-throughput genetic screens are powerful methods to interrogate gene function on a genome-wide scale and identify genes responsible to certain stresses. Here, we developed a piggyBac strategy to deliver pooled sgRNA libraries stably into cell lines. We used this strategy to conduct a screen based on genome-wide clustered regularly interspaced short palindromic repeat technology (CRISPR)-Cas9 in Bombyx mori cells. We first constructed a single guide RNA (sgRNA) library containing 94,000 sgRNAs, which targeted 16,571 protein-coding genes. We then generated knockout collections in BmE cells using the piggyBac transposon. We identified 1006 genes that are essential for cell viability under normal growth conditions. Of the identified genes, 82.4% (829 genes) were homologous to essential genes in seven animal species. We also identified 838 genes whose loss facilitated cell growth. Next, we performed context-specific positive screens for resistance to biotic or nonbiotic stresses using temperature and baculovirus separately, which identified several key genes and pathways from each screen. Collectively, our results provide a novel and versatile platform for functional annotations of B. mori genomes and deciphering key genes responsible for various conditions. This study also shows the effectiveness, practicality, and convenience of genome-wide CRISPR screens in nonmodel organisms.

Inhibition of heat shock protein 90 suppresses Bombyx mori nucleopolyhedrovirus replication in B. mori

Heat shock protein 90 (Hsp90) plays a very important role in facilitating the replication of many viruses. Until now, little has been known about the role of Hsp90 in Bombyx mori virus infection. In this study, we explored the role of BmHsp90 in B. mori nucleopolyhedrovirus (BmNPV) replication. We found that BmHsp90 inhibition by geldanamycin (GA) significantly reduced the BmNPV titre, the protein expression level of BmNPV nucleocapsid protein 39 (VP39) and the transcript level of BmNPV genes. Silencing the hsp90 gene in BmN cells by small interfering RNA suppressed BmNPV replication whereas overexpression of hsp90 promoted the replication of BmNPV. After inhibition of Hsp90, the expression of three key genes [signal transducing activator of transcription (stat), suppressor of cytokine signalling protein 2 (socs2), socs6] involved in the Janus kinase/STAT pathway significantly changed, with up-regulation of stat and down-regulation of socs2 and socs6. In addition, the expression of two antiapoptosis genes, BmNPV inhibitor of apoptosis protein1 (BmNPV-iap1) and Bmiap2, was greatly decreased in GA-treated cells, whereas their expression was significantly increased in hsp90-overexpressed silkworm larvae. Our results indicated that inhibition of Hsp90 can suppress BmNPV proliferation in B. mori. Our findings may provide new clues to elucidate the molecular mechanisms of silkworm-virus interactions.

Metabolomic Analysis of Cricket paralysis virus Infection in Drosophila S2 Cells Reveals Divergent Effects on Central Carbon Metabolism as Compared with Silkworm Bm5 Cells

High-throughput approaches have opened new opportunities for understanding biological processes such as persistent virus infections, which are widespread. However, the potential of persistent infections to develop towards pathogenesis remains to be investigated, particularly with respect to the role of host metabolism. To explore the interactions between cellular metabolism and persistent/pathogenic virus infection, we performed untargeted and targeted metabolomic analysis to examine the effects of Cricket paralysis virus (CrPV, Dicistroviridae) in persistently infected silkworm Bm5 cells and acutely infected Drosophila S2 cells. Our previous study (Viruses 2019, 11, 861) established that both glucose and glutamine levels significantly increased during the persistent period of CrPV infection of Bm5 cells, while they decreased steeply during the pathogenic stages. Strikingly, in this study, an almost opposite pattern in change of metabolites was observed during different stages of acute infection of S2 cells. More specifically, a significant decrease in amino acids and carbohydrates was observed prior to pathogenesis, while their abundance significantly increased again during pathogenesis. Our study illustrates the occurrence of diametrically opposite changes in central carbon mechanisms during CrPV infection of S2 and Bm5 cells that is possibly related to the type of infection (acute or persistent) that is triggered by the virus.

Sensitivity of Polyvoltine Thai Strains of Bombyx mori to a BmNPV Isolate From Mahasarakham

Virus infection by the Bombyx mori nucleopolyhedrovirus (BmNPV) is the most severe disease in Thai sericultural practice of polyvoltine silkworms. Here, we characterized a newly isolated BmNPV isolated from the Mahasarakham province in Thailand (BmNPV-MSU). The purity and morphology of BmNPV-MSU were examined using light microscopy and scanning electron microscopy. The polyhedral inclusion bodies (PIBs) of BmNPV-MSU appeared in tetragonal, hexagonal, octagonal, and globular forms. The virions were both single and multiple embedded as observed by transmission electron microscopy. We also determined the virulence of BmNPV-MSU for six different Thai polyvoltine strains by LC50 and time to death after infection. The LC50 values of Nang Lai, NK04, and Sam Rong strains were 5.05-1.52 × 107 PIBs per ml and mortality peaked 7- to 8-d after inoculation. For Nang Noi, SP2, and RE05 strains the LC50 values were 7.91-1.82 × 106 PIBs/ml and mortality peaked 4-5 d after inoculation, thus having lower chance of survival to infection by BmNPV-MSU.

Antiviral defense against Cypovirus 1 (Reoviridae) infection in the silkworm, Bombyx mori

Recent years have shown a large increase in studies of infection of the silkworm (Bombyx mori) with Cypovirus 1 (previously designated as B. mori cytoplasmic polyhedrosis virus), that causes serious damage in sericulture. Cypovirus 1 has a single-layered capsid that encapsulates a segmented double-strand RNA (dsRNA) genome which are attractive features for the establishment of a biotechnological platform for the production of specialized gene silencing agents, either as recombinant viruses or as viral-like particles with nonreplicative dsRNA cargo. For both combatting viral disease and application of Cypovirus-based pest control, however, a better understanding is needed of the innate immune response caused by Cypovirus infection of the midgut of lepidopteran larvae. Studies of deep sequencing of viral small RNAs have indicated the importance of the RNA interference pathway in the control of Cypovirus infection although many functional aspects still need to be elucidated and conclusive evidence is lacking. A considerable number of transcriptome studies were carried out that revealed a complex response that hitherto remains uncharacterized because of a dearth in functional studies. Also, the uptake mechanism of Cypovirus by the midgut cells remains unclarified because of contrasting mechanisms revealed by electron microscopy and functional studies. The field will benefit from an increase in functional studies that will depend on transgenic silkworm technology and reverse genetics systems for Cypovirus 1.

Phosphorylation of nucleopolyhedrovirus 39K is essential for the regulation of viral gene transcription in silkworm cells

39K protein was proven to be the most notable hyperphosphorylated protein of Bombyx mori nucleopolyhedrovirus (BmNPV), since its amino acid 136 has the highest phosphorylation ratio (16.683) among all of BmNPV phosphorylated proteins (Shobahah et al., 2017). Therefore, we inserted positive (mimicking phosphorylation) and negative (mimicking dephosphorylation) mutants of the highest phosphorylated site of 39K and the wild type 39k into the 39k-ko-Bacmid using the Bac-to-Bac system. These three kinds of recombinant Bacmids along with wild type and knocked-out Bacmid were then transfected into the Bombyx mori cells (BmN) and further investigated by qPCR analysis. The results of the qPCR showed that 39K phosphorylation had no significant effect on viral genome replication, unlike the positive mutation that reduced the viral gene transcription drastically compared to the wild type. Thus, phosphorylation of BmNPV 39K protein does not appear to be the essential mechanism for viral genome replication, even though it has an important role in the viral gene transcription. Keywords: BmNPV; 39K; phosphorylation; virus replication; virus transcription.

Antiviral activity of seselin from Aegle marmelos against nuclear polyhedrosis virus infection in the larvae of silkworm, Bombyx mori

ETHNOPHARMACOLOGICAL RELEVANCE

Traditional Indian medicine has utilized Aeglemarmelos (L.) Corr. commonly called as bael in several indigenous systems against various diseases. Bioactive components isolated from various plant parts of A. marmelos were used in ethno-medicine. More precisely they are known for its antiviral property against various human and animal viruses.

AIM OF THE STUDY

The study was conducted to investigate the antiviral activity of A.marmelos against Bombyx mori nucleopolyhedrovirus (BmNPV).

MATERIALS AND METHODS

Among the various crude extracts tested, hexane extracts of leaves of A. marmelos with promising anti-BmNPV activity was subjected to bioactivity guided fractionation based on column chromatography. Out of 40 fractions obtained from the fractionation, fractions showing similar TLC profiles were pooled into 14 fractions. A fraction with potential activity was used to purify a molecule with anti-BmNPV activity. This molecule was characterized through structural and functional analyses.

RESULTS

The functionally and structurally characterized molecule in the fraction with prospective anti-BmNPV activity revealed a single crystal compound 'seselin' (8, 8-dimethyl pyrido oxazine-2-one).

CONCLUSION

It is therefore understood that this seselin compound could be used as a natural medicine for the management of NPV infection in the silkworm larvae under commercial conditions after suitable field evaluations.

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.

A Metabolomics Approach to Unravel Cricket Paralysis Virus Infection in Silkworm Bm5 Cells

How a host metabolism responds to infection with insect viruses and how it relates to pathogenesis, is little investigated. Our previous study observed that Cricket paralysis virus (CrPV, Dicistroviridae) causes short term persistence in silkworm Bm5 cells before proceeding to acute infection. In this study, a metabolomics approach based on high resolution mass spectrometry was applied to investigate how a host metabolism is altered during the course of CrPV infection in Bm5 cells and which changes are characteristic for the transition from persistence to pathogenicity. We observed that CrPV infection led to significant and stage-specific metabolic changes in Bm5 cells. Differential metabolites abundance and pathway analysis further identified specific metabolic features at different stages in the viral life cycle. Notably, both glucose and glutamine levels significantly increased during CrPV persistent infection followed by a steep decrease during the pathogenic stages, suggesting that the central carbon metabolism was significantly modified during CrPV infection in Bm5 cells. In addition, dynamic changes in levels of polyamines were detected. Taken together, this study characterized for the first time the metabolic dynamics of CrPV infection in insect cells, proposing a central role for the regulation of both amino acid and carbohydrate metabolism during the period of persistent infection of CrPV in Bm5 cells.

Study on the Role of Cytc in Response to BmNPV Infection in Silkworm, Bombyx mori (Lepidoptera)

Bombyx mori nucleopolyhedrovirus (BmNPV) is one of the primary pathogens of the silkworm. Cytochrome c (cytc) showed a significant response to BmNPV infection in our previous transcriptome study. However, little is known about the role of Bombyx mori cytc (Bmcytc) in resistance to BmNPV infection. In this study, the expression levels analysis of Bmcytc showed stable expression levels in selected tissues of the resistant strain AN following BmNPV infection, while there was downregulation in the susceptible strain p50, except in the malpighian tubule. To further study the role of Bmcytc in viral infection, Bmcytc was knocked down with siRNA in vitro, resulting in significant downregulation of selected downstream genes of the mitochondrial pathway, including Bmapaf, Bmcaspase-Nc, and Bmcaspase-1; this was also confirmed by overexpression of Bmcytc using the pIZT/V5-His-mCherry insect vector, except Bmcaspase-1. Moreover, knockdown of Bmcytc significantly promoted the infection process of BmNPV in vitro, while the infection was inhibited by overexpression of Bmcytc at the early stage and subsequently increased rapidly. Based on these results, we concluded that Bmcytc plays a vital role in BmNPV infection by regulating the mitochondrial apoptosis pathway. Our work provides valuable data for the clarification of the mechanism of silkworm resistance to BmNPV infection.

BmNPV infection correlates with the enhancement of the resistance of Bombyx mori cells to UV radiation

At present, the effect of ultraviolet (UV) radiation on the interaction between Bombyx mori nucleopolyhedrovirus (BmNPV) and host remains unclear. In the current study, UV treatment significantly reduced the activity of BmNPV budded viruses (BVs), and UV-damaged BmN cells were not conducive to BmNPV proliferation. BmNPV infection significantly reduced the viability of host cells, but increased the viability of high-dose UV-treated host cells. Furthermore, the quantitative reverse-transcription PCR (qPCR) results suggested that BmNPV and Bombyx mori might mutually use the same DNA repair proteins for repairing UV-induced damage and BmNPV infection promote the ability of host cells to repair UV-induced damage.

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.

A 1H NMR based study of hemolymph metabonomics in different resistant silkworms, Bombyx mori (Lepidotera), after BmNPV inoculation

Bombyx mori nucleopolyhedrovirus (BmNPV) is a primary silkworm pathogen, and the molecular mechanism of silkworm defense to BmNPV infection is still unclear. Herein, comparative metabolomics was adopted to analyze the variations in the hemolymph metabolites of different resistant silkworm strains following BmNPV inoculation using a ¹H NMR method. Trehalose, as an instant source of energy, plays a crucial role in the response to pathogen infections in insects. The level of trehalose was persistently upregulated in the hemolymph of the resistant silkworm strain YeA following infection with BmNPV, compared to that of the susceptible strain YeB, indicating that trehalose metabolism plays a vital role in the response to BmNPV infection. The significant upregulation of TCA cycle relevant metabolites, including malate, fumarate, citrate, succinate, and α-ketoglutarate, was identified at 0 h, 12 h, 48 h, and 96 h post-infection in YeA hemolymph, whereas a significant upregulation in YeB hemolymph was only detected at an early stage of infection (0 h-24 h). The expression level of selected key metabolic enzymes, determined using RT-qPCR, validated the differences in trehalose and TCA cycle relevant metabolite levels. The variations in branched-chain amino acid (BCAA) pathway relevant metabolites in resistant silkworm strains following BmNPV infection showed a regular undulation at different times after infection. A significant accumulation of phenylalanine and tyrosine was observed in YeA following BmNPV infection compared to YeB. The glycolysis and gluconeogenesis pathways showed a relatively low activity in YeA following BmNPV infection. Moreover, the levels of other metabolites related to fat metabolism, transamination, energy metabolism, and glycometabolism, such as glycine, threonine, glutamine, and glutamate, were unstable in the two silkworm strains following BmNPV infection. Thus, our study provides an overview of the metabolic response of the silkworm in response to BmNPV infection, which lays the foundation for clarifying the mechanism of silkworm resistance to BmNPV infection.

Bombyx mori cypovirus encoded small peptide inhibits viral multiplication

Bombyx mori cypovirus (BmCPV) is one of the most infectious pathogen in sericulture and a member of the family Reoviridae. It specifically infects the midgut of silkworm. The BmCPV genome consists of 10 dsRNAs segments (S1-S10), which have generally been assumed to be monocistronic. In this study, a small open reading frame encoding the peptide S5-sORF, containing 27 amino acid residues, was predicted in a region of the negative (-) strand of BmCPV segment S5. An immunofluorescence assay detected S5-sORF in the cytoplasm and nuclei of BmCPV-infected cells, and it was also detected in the virion with western blotting, suggesting that S5-sORF may be assembled into the BmCPV virion. Viral gene expression was inhibited by overexpressed S5-sORF, and viral multiplication was dose-dependently suppressed by the S5-sORF peptide. A viable recombinant virus, BmCPV-S5-sORF^mut, in which the start codon (ATG) of S5-sORF was mutated to a stop codon (TGA), was generated with reverse genetics. The proliferation of BmCPV was increased by the abolition of S5-sORF expression. Furthermore, the RNA transcript of S5-sORF and small peptide of S5-sORF were involved in BmCPV replication. The expression of genes related to the innate immune pathways and apoptosis in the silkworm were not significantly affected by S5-sORF overexpression. Our results suggest that a viral nucleotide sequence is utilized by the host to generate an antiviral peptide, which may be a novel strategy protecting the host from viral infection.