BmNHR96 participate BV entry of BmN-SWU1 cells via affecting the cellular cholesterol level

Silkworm glycosaminoglycans bind to Bombyx mori nuclear polyhedrosis virus and facilitate its entry

Interacting with cell surface attachment factors or receptors is the first step for virus infection. Glycans cover a thick layer on eukaryotic cells and are potential targets of various viruses. Bombyx mori nuclear polyhedrosis viruses (BmNPV) is a baculovirus that causes huge economic loss to the sericulture industry but the mechanism of infection is unclear. Looking for potential host receptors for the virus is an important task. In this study, we investigated the role of glycosaminoglycan (GAG) modifications, including heparan sulfate (HS) and chondroitin sulfate (CS), during BmNPV infection. Enzymatic removal of cell surface HS and CS effectively inhibited BmNPV infection and replication. Exogenous HS and CS can directly bind to BmNPV virion in solution and act as neutralizers for viral infection. Furthermore, the expression of enzymes involved in GAG biosynthesis was upregulated in the BmNPV susceptible silkworm after virus administration, but down-regulated in the resistant strain after virus treatment, suggesting that BmNPV was able to utilize host cell machinery to promote the biosynthesis of GAGs. This study demonstrated HS and CS as important attachment factors that facilitate the viral entry process, and targeting HS and CS can be an effective means of inhibiting BmNPV infection.

The Mechanisms of Silkworm Resistance to the Baculovirus and Antiviral Breeding

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

Apolipoprotein D facilitate the proliferation of BmNPV

The silkworm, Bombyx mori, a model Lepidopteran specie, is an important economic insect. It is specifically infected by Bombyx mori nucleopolyhedrovirus (BmNPV), causing huge losses to the sericulture industry. Therefore, the understandings of the interaction mechanism between BmNPV and the host will help to provide the theoretical basis for the sericulture industry to control BmNPV. Apolipoprotein D (ApoD) is a member of lipid transport family and capable of binding to a variety of lipophilic ligands. ApoD is mainly used in neurodegenerative disease research in mammals, and there is little research on ApoD against viruses. Here, we explored the effects of Bombyx mori Apolipoprotein D (BmApoD) on BmNPV replication. We knocked out and overexpressed BmApoD in BmN cells and infected them with Bombyx mori nucleopolyhedrovirus (BmNPV). The results showed that BmApoD promote the replication of BmNPV in BmN cells. It was also confirmed that BmApoD promote the replication of BmNPV after knocking down BmApoD in silkworm larvae. This study is the first to explore the role of ApoD in insect-virus interactions, providing new insights into the functional role of ApoD.

Overexpression Bombyx mori HEXIM1 Facilitates Immune Escape of Bombyx mori Nucleopolyhedrovirus by Suppressing BmRelish-Driven Immune Responses

Bombyx mori nucleopolyhedrovirus (BmNPV), a typical arthropod-specific enveloped DNA virus, is one of the most serious pathogens in silkworm farming, but the potential mechanisms of the evasion of innate immune responses from BmNPV infection are still poorly understood. HEXIM1 is an RNA-binding protein, best known as an inhibitor of positive transcription elongation factor b (P-TEFb), which controls transcription elongation by RNA polymerase II. In this study, Bombyx mori HEXIM1 (BmHEXIM1) was cloned and characterized, and its expression was found to be remarkably upregulated after BmNPV infection. Furthermore, BmHEXIM1 was detected to increase the proliferation of BmNPV, and its full length is essential for assisting BmNPV immune escape by suppressing BmRelish-driven immune responses. This study brought new insights into the mechanisms of immune escape of BmNPV and provided theoretical guidance for the breeding of BmNPV-resistant silkworm varieties.

Functional analysis of nuclear receptor HR96 gene in Bombyx mori exposed to phoxim

The nuclear receptor (NRs) gene family functions as ligand-dependent transcription factors in a variety of animals, which participates in a variety of biological processes, such as cell differentiation, metabolic regulation, reproduction, development, insect metamorphosis. In this study, a nuclear receptor HR96 gene in silkworm Bombyx mori (BmHR96) was identified, and the responses of BmHR96 gene to 20-hydroxyecdysone (20E), three insecticides, and two disinfectants were analyzed and its function in phoxim exposure was explored.  Quantitative real-time polymerase chain reaction indicated that the expression of BmHR96 mRNA was the highest in ovary of 5th instar Day 3 silkworm larvae and in silk gland of the wandering stage. The expression patterns of BmHR96 gene in ovary, head, testis, and midgut of different stages were different. After injecting 20E into B. mori, the expression of BmHR96 mRNA had no significant difference compared with control. Three insecticides and two disinfectants were used to treat B. mori, respectively, and it was found that they had different influence patterns on the expression level of BmHR96. siRNA of BmHR96 was injected into silkworm larvae and the expression of BmHR96 was decreased significantly after injecting 72 h. After silencing of BmHR96, B. mori was fed with phoxim-treated leaves. The results showed that the mortality of B. mori after silencing of BmHR96 was significantly higher than the control. Our results indicated that HR96 plays an important role in regulating the stress response of phoxim.

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.

Two Cholesterol Recognition Amino Acid Consensus Motifs of GP64 with Uncleaved Signal Peptide Are Required for Bombyx mori Nucleopolyhedrovirus Infection

The signal peptide (SP) of integrated membrane proteins is removed cotranslationally or posttranslationally in the endoplasmic reticulum, while GP64, a membrane fusion protein of Bombyx mori nucleopolyhedrovirus (BmNPV), retains its SP in the mature protein and virion. In this study, we revealed that uncleaved SP is a key determinant with additional functions in infection. First, uncleaved SP endows BmNPV with strong virulence; second, SP retention-induced BmNPV infection depends on cholesterol recognition amino acid consensus domain 1 (CRAC1) and CRAC2. In contrast, the recombinant virus with SP-cleaved GP64 has reduced infectivity, and only CRAC2 is required for BmNPV infection. Furthermore, we showed that cholesterol in the plasma membrane is an important fusion receptor that interacts with CRAC2 of GP64. Our study suggested that BmNPV GP64 is a key cholesterol-binding protein and uncleaved SP determines GP64's unique dependence on the CRAC domains. IMPORTANCE BmNPV is a severe pathogen that mainly infects silkworms. GP64 is the key membrane fusion protein that mediates BmNPV infection, and some studies have indicated that cholesterol and lipids are involved in BmNPV infection. A remarkable difference from other membrane fusion proteins is that BmNPV GP64 retains its SP in the mature protein, but the cause is still unclear. In this study, we investigated the reason why BmNPV retains this SP, and its effects on protein targeting, virulence, and CRAC dependence were revealed by comparison of recombinant viruses harboring SP-cleaved or uncleaved GP64. Our study provides a basis for understanding the dependence of BmNPV infection on cholesterol/lipids and host specificity.

Bombyx mori cell division cycle protein 37 promotes the proliferation of BmNPV

Cell division cycle protein 37 (Cdc37) is a molecular chaperone that actively participates in many intracellular physiological and biochemical processes as well as pathogen infection. However, the function of Cdc37 in silkworm cells under Bombyx mori nucleopolyhedrovirus (BmNPV) infection is unknown. We cloned and identified BmCdc37, a Cdc37 gene from B. mori, which is highly conserved among other species. After BmNPV infection, the expression level of the BmCdc37 gene was up-regulated and showed an expression pattern similar to the BmHsp90 gene, which relies on Cdc37 to stabilize and activate specific protein kinases. The immunofluorescence, bimolecular fluorescence complementation (BiFC), and co-immunoprecipitation (Co-IP) assays all indicated that BmCdc37 interacts with BmHsp90 in silkworm cells. Both BmCdc37 and BmHsp90 promote the reproduction of BmNPV. Co-expression of BmCdc37 and BmHsp90 was better at promoting virus proliferation than overexpression alone. These findings all indicate that BmCdc37 plays an active role in the proliferation of BmNPV.

Heat Shock Protein 75 (TRAP1) facilitate the proliferation of the Bombyx mori nucleopolyhedrovirus

The viruses utilize multiple cellular proteins to facilitate their proliferation. The Heat Shock Protein (HSP), the highly conserved protein in eukaryotes and prokaryotes, plays a critical role in facilitating viral proliferation. However, less is known about the role of the HSPs in the life cycles of the Baculoviruses. We constructed recombinant Bombyx mori nucleopolyhedrovirus and discovered the Heat Shock Protein 75 (TRAP1) in the B. mori ovary (BmN) cells by the co-immunoprecipitation experiment using the GP64 (glycoprotein 64) as the bait protein. Tissue expression profile analysis of B. mori indicated that the TRAP1 gene has higher expression levels in the ovary, midgut, and hemolymph. Down-regulation of TRAP1 via RNA interference (RNAi) and geldanamycin (GA, a TRAP1 inhibitor) treatment can reduce the expression level of the major capsid protein VP39 (viral protein 39) of BmNPV. In contrast, the up-regulation of TRAP1 via overexpression can increase the expression level of the VP39. These results indicated that the TRAP1 of B. mori could facilitate the proliferation of the BmNPV. This study provided new insights into the function of TRAP1, and the basic mechanisms of the baculoviruses life cycle for disease prevention.

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.

Gene editing the BmNPV inhibitor of apoptosis protein 2 (iap2) as an antiviral strategy in transgenic silkworm

Apoptosis is a cellular defense mechanism used for the elimination of host cells infected by viruses. Viruses have evolved corresponding inhibitors of apoptosis genes to promote their replication. Anti-apoptosis-related genes, involved in baculovirus proliferation, have been proposed but it is unclear whether these genes can be manipulated in gene therapy. We constructed a transgenic silkworm, using the CRISPR/Cas9 system to knock out the BmNPV inhibitor of apoptosis 2 (iap2). The sequencing results showed that all the sequences could edit the target site of BmNPV iap2 gene. There were no differences in economic traits and growth tests between the BmNPV iap2 knockout strain transgenic silkworm lines and the control groups. However, the mortality rate was significantly reduced, the median lethal dose (LD50) was about 100 times higher than the control group, and the onset time was prolonged by 1-2 days after knocking out BmNPV iap2. In addition, the expression levels of apoptotic-related genes Bmiap2, BmICE and BmDreed were significantly affected and the activity of caspase 9 was increased after BmNPV iap2 being edited in transgenic silkworm. These results demonstrated that gene editing BmNPV iap2 could significantly inhibit BmNPV replication and proliferation. This approach provides a new strategy for antiviral research.

Isolation of ferritin and its interaction with BmNPV in the silkworm, Bombyx mori

Ferritin is a ubiquitous iron storage protein that plays an important role in host defence against pathogen infections. In the present study, native ferritin was isolated from the hemolymph of Bombyx mori using native-polyacrylamide gel electrophoresis (native-PAGE) and sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). The results revealed that ferritin consisted of two subunits, designated as BmFerHCH and BmFerLCH. Previously integrated previous transcriptome and iTRAQ data showed that the two subunits were down-regulated in resistant silkworm strain BC9 and there was no obvious change in the expression levels of the subunits in susceptible silkworm strain P50 after BmNPV infection. Virus overlay assays revealed that B. mori ferritin as the form of heteropolymer had an interaction with B. mori nucleopolyhedrovirus (BmNPV), but it can't interact with BmNPV after depolymerisation. What's more, reverse transcription quantitative PCR (RT-qPCR) analysis suggested that BmFerHCH and BmFerLCH could be induced by bacteria, virus and iron. This is the first study to extract B. mori ferritin successfully and confirms their roles in the process of BmNPV infection. All these results will lay a foundation for further research the function of B. mori ferritin.

A Novel Gene Bombyx mori Carotenoid Oxygenases and Retinal Isomerase (BmCORI) Related to β-Carotene Depletion

Carotenoids are the precursors of Vitamin A. They are cleaved by carotenoid oxygenase and then isomerized by retinoid isomerase. In this study, we identified a gene, Bombyx mori Carotenoid Oxygenases and Retinal Isomerase (BmCORI), which was the homolog of β-carotene 15,15'-monooxygenase and the retinal pigment epithelium protein of 65 kD. Further analysis indicated that the expression of BmCORI in silkworms was significantly higher in females than in males. We also found that the β-carotene content in BmCORI-expressed human embryonic kidney 293 cells was significantly lower than in the controls, while the lutein content showed a slight difference. These results suggested that BmCORI is related to carotenoid depletion, especially β-carotene depletion. Our research provides new insight into the study of BmCORI function.