Entry of Bombyx mori nucleopolyhedrovirus into BmN cells by cholesterol-dependent macropinocytic endocytosis

Bombyx mori nucleopolyhedrovirus (BmNPV) is a serious viral pathogen of silkworm, and no drug or specific protection against BmNPV infection is available at present time. Although functions of most BmNPV genes were depicted in recent years, knowledge on the mechanism of BmNPV entry into insect cells is still limited. Here BmNPV cell entry mechanism is investigated by different endocytic inhibitor application and subcellular analysis. Results indicated that BmNPV enters BmN cells by clathrin-independent macropinocytic endocytosis, which is mediated by cholesterol in a dose-dependent manner, and cholesterol replenishment rescued the BmNPV infection partially.     

Proteomic analysis of nucleopolyhedrovirus infection resistance in the silkworm, Bombyx mori (Lepidoptera: Bombycidae)

Silkworm hemolymph is an important defense tissue to resist bacteria and virus infections. To study the response of silkworm hemolymph in the resistance of Bombyx mori L. nucleopolyhedrovirus (BmNPV), we constructed a near-isogenic silkworm line with BmNPV resistance using highly resistant and highly susceptible parental strains. In this paper, two-dimensional gel electrophoresis (2-DE) and Matrix-Assisted Laser Desorption/Ionization (MALDI)-mass spectrometry were employed to investigate the differences of protein patterns in the hemolymph of the highly resistant, highly susceptible and near-isogenic silkworm strains after BmNPV was administrated to the larvae. A comparison between the proteomes of these three silkworm strains led us to identify two differentially expressed proteins, beta-N-acetylglucosaminidase 2 and aminoacylase. The expression levels of these proteins were higher in the BmNPV resistant strains.     

Engineering Silkworms for Resistance to Baculovirus Through Multigene RNA Interference

Bombyx mori nucleopolyhedrovirus (BmNPV) that infects the silkworm, B. mori, accounts for >50% of silk cocoon crop losses globally. We speculated that simultaneous targeting of several BmNPV essential genes in transgenic silkworm would elicit a stable defense against the virus. We introduced into the silkworm germline the vectors carrying short sequences of four essential BmNPV genes in tandem, either in sense or antisense or in inverted-repeat arrangement. The transgenic silkworms carrying the inverted repeat-containing transgene showed stable protection against high doses of baculovirus infection. Further, the antiviral trait was incorporated to a commercially productive silkworm strain highly susceptible to BmNPV. This led to combining the high-yielding cocoon and silk traits of the parental commercial strain and a very high level of refractoriness (>75% survival rate as compared to <15% in nontransgenic lines) to baculovirus infection conferred by the transgene. We also observed impaired infectivity of the occlusion bodies derived from the transgenic lines as compared to the wild-type ones. Currently, large-scale exploitation of these transgenic lines is underway to bring about economic transformation of sericulture.     

Molecular characterization and expression analysis of BmNOX in two strains of Bombyx mori with contrasting viral resistance phenotype

We recently documented the identification of a 26.5 kDa protein named BmNox in the gut fluid of Nistari strain of Bombyx mori, which possessed antiviral activity against BmNPV in vitro. In this report, we report the characterization of the full‐length gene encoding BmNOX and the levels of expression of this gene in select tissues of silkworm larvae from a BmNPV‐susceptible and a BmNPV‐resistant strain to the defense capability in Bombyx mori larvae challenged with BmNPV. We also evaluated the BmNox expression in various stages of larval life of a resistant and a susceptible strain of Bombyx mori selected from among a panel of strains of silkworm. Nistari, a multivoltine strain of silkworm, expressed BmNOX during all five larval stages, and were highly resistant to BmNPV infection. In sharp contrast, CSR₂, a bivoltine strain, showed weaker expression of BmNOX in the anterior midgut in larval life and was highly susceptible to BmNPV infection. BmNOX is a secretory protein with dual expression in gut fluid and mid gut tissue. BmNOX is expressed heavily in the posterior mid gut, with weaker expression in the fore‐ and mid‐gut regions. © 2010 Wiley Periodicals, Inc.     

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

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

A Hypothetical Model of Crossing Bombyx mori Nucleopolyhedrovirus through Its Host Midgut Physical Barrier

Bombyx mori nucleopolyhedrovirus (BmNPV) is a primary pathogen of silkworm (B. mori) that causes severe economic losses each year. However, the molecular mechanisms of silkworm-BmNPV interactions, especially the silkworm proteins that can interact with the virus, are still largely unknown. In this study, the total and membrane proteins of silkworm midguts were displayed using one- and two-dimensional electrophoresis. A virus overlay assay was used to detect B. mori proteins that specifically bind to BmNPV particles. Twelve proteins were located and identified using mass spectrometry, and the different expression of the corresponding genes in BmNPV susceptible and resistant silkworm strains also indicated their involvement in BmNPV infection. The 12 proteins are grouped based on their potential roles in viral infection, for example, endocytosis, intracellular transportation, and host responses. Based on these results, we hypothesize the following: I) vacuolar ATP synthase catalytic subunit A and subunit B may be implicated in the process of the membrane fusion of virus and the release of the nucleocapsid into cytoplasm; II) actin, enolase and phosphoglycerate kinase are cytoskeleton associated proteins and may play an important role in BmNPV intracellular transportation; III) mitochondrial prohibitin complex protein 2, ganglioside-induced differentiation-associated protein, calreticulin, regucalcin-like isoform X1 and 60 kDa heat shock protein are involved in cell apoptosis regulation during BmNPV infection in larvae midguts; IV) ribosomal P0 may be associated with BmNPV infection by regulating gene expression of BmNPV; V) arginine kinase has a role in the antiviral activities against BmNPV. Our work should prove informative by providing multiple protein targets and a novel direction to investigate the molecular mechanisms of the interactions between silkworms and BmNPV.     

BmNPV Resistance of Silkworm Larvae Resulting from the Ingestion of TiO2 Nanoparticles

Bombyx mori nucleopolyhedrovirus (BmNPV) causes infection in the silkworm that is often lethal. The infection is hard to prevent, partly because of the nature of the virus particles and partly because of the different strains of B. mori. Titanium dioxide nanoparticles (TiO₂ NPs) have been demonstrated to have antimicrobial properties. The present study investigated whether TiO₂ NPs added to an artificial diet can increase the resistance of B. mori larvae to BmNPV and examined the molecular mechanism behind any resistance shown. The results indicated that ingested TiO₂ NPs decreased reactive oxygen species and NO accumulation in B. mori larvae under BmNPV infection, which in turn led to a decrease in their growth inhibition and mortality. In addition, the TiO₂ NPs significantly promoted the expression of resistance-related genes, including those encoding superoxide dismutase, catalase, glutathione peroxidase, acetylcholine esterase, carboxylesterase, heat shock protein 21, glutathione S transferase o1, P53, and transferring and of genes encoding cytochrome p302 and nitric oxide synthase. These findings are a useful addition to the understanding of the mechanism of BmNPV resistance of B. mori larvae in response to TiO₂ NPs addition. Such information also provides a theoretical basis for the use of TiO₂ NPs in sericulture.     

家蚕CVDAR品系对BmNPV的抗性特征分析

【目的】家蚕核型多角体病毒(Bombyx mori nucleopolyhedrovirus,BmNPV)促使的血液型脓病是一种产业上非常严重的家蚕疾病,目前有效的防控方法较少。本研究以大造和CVDAR家蚕品系(对BmNPV有较强抗性的品系)为试验材料,通过分析CVDAR对BmNPV抗性特征,以期确定CVDAR对BmNPV的抗性机制。【方法】本研究通过半致死剂量分析,发现CVDAR品系比大造品系对BmNPV感染的半致死剂量提高10倍以上;进一步HE染色分析大造与CVDAR品系病毒感染前后的中肠组织的变化,具体解析抗性品系CVDAR的抗BmNPV机制。【结果】感染BmNPV 72 h后,大造中肠细胞细胞核明显膨大,着色变浅,到96h后,细胞核持续增大有脱落趋势;而CVDAR抗性品系只在感染96 h后有中肠部分细胞核膨大,但排列整齐;同时通过荧光定量分析大造与CVDAR品系病毒感染后的增殖情况,结合各个时期代表病毒基因的转录水平分析比较发现,感染BmNPV后0–12h也没有发现抗性品系CVDAR和大造之间的病毒拷贝数以及病毒基因转录水平的不同,但感染24h后发现抗性品系CVDAR无论是病毒拷贝数还是病毒基因的转录表达水平都明显低于对照大造。【结论】证明CVDAR口服添毒后中肠中病毒基因的转录在第一轮复制期间不受影响,之后转录水平降低。鉴定CVDAR品系抑制BmNPV增殖的关键时期是在感染BmNPV后的24 h,为解析抗性机制奠定基础。     

The role of N6‐methyladenosine modification on diapause in silkworm (Bombyx mori) strains that exhibit different voltinism

N6‐methyladenosine (m⁶A) plays a key role in regulating gene expression in myriad organisms. Diapause is an important plastic phenotype that allows insects to survive under specific environmental conditions. However, the diapause molecular mechanism remains unknown. In this study, we analyzed the phylogenetics of genes related to the m⁶A modification complex in the silkworm (Bombyx mori) based on identified sequences from other organisms. We detected the expression of these genes during different developmental phases from four strains with different voltinism. We also determined total m⁶A content in cells treated with different diapause hormone concentrations or eggs exposed to hydrochloric acid. Our data revealed that m⁶A‐modification‐related gene expression and m⁶A content were greater in diapause‐destinated compared to nondiapause‐destined strains. Our findings suggest that m⁶A modification may provide significant epigenetic regulation of diapause‐related genes in the silkworm.     

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.     

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.     

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.     

Standard method for detecting Bombyx mori nucleopolyhedrovirus disease-resistant silkworm varieties

Bombyx mori nucleopolyhedrovirus (BmNPV) disease is one of the most serious silkworm diseases, and it has caused great economic losses to the sericulture industry. So far, the disease has not been controlled effectively by therapeutic agents. Breeding resistant silkworm varieties breeding may be an effective way to improve resistance to BmNPV and reduce economic losses. A precise resistance-detection method will help to accelerate the breeding process. For this purpose, here we described the individual inoculation method (IIM). Details of the IIM include pathogen BmNPV preparation, mulberry leaf size, pathogen volume, rearing conditions, course of infection, and breeding conditions. Finally, a resistance comparison experiment was performed using the IIM and the traditional group inoculation method (GIM). The incidence of BmNPV infection and the within-group variance results showed that the IIM was more precise and reliable than the GIM.     

Identification of a PP2A gene in Bombyx mori with antiviral function against B. mori nucleopolyhedrovirus

Ser/Thr protein phosphatase 2A (PP2A) is one of the type 2 protein phosphatases, which is required for many intracellular physiological processes and pathogen infection. However, the function of PP2A is unclear in silkworm, Bombyx mori. Here, we cloned and identified BmPP2A, a PP2A gene from B. mori, which has two HEAT domains and a high similarity to PP2A from other organisms. Our results showed that BmPP2A is localized in the cytoplasm and highly expressed in silkworm epidermis and midgut, and that Bombyx mori nucleopolyhedrovirus (BmNPV) infection induces down‐regulation of BmPP2A expression. Furthermore, up‐regulation of BmPP2A via overexpression significantly inhibited BmNPV multiplication. In contrast, down‐regulation of BmPP2A via RNA interference and okadaic acid (a PP2A inhibitor) treatment allowed robust BmNPV replication. This is the first report of PP2A having an antiviral effect in silkworm and provides insights into the function of BmPP2A, a potential anti‐BmNPV mechanism, and a possible target for the breeding of silkworm‐resistant strains.     

METTL3 inhibition ameliorates liver damage in mouse with hepatitis B virus-associated acute-on-chronic liver failure by regulating miR-146a-5p maturation

Hepatitis B virus (HBV)-associated acute-on-chronic liver failure (ACLF) is a clinical syndrome of severe liver damage. HBV infection is affected by N6-methyladenosine (m⁶A) RNA modification. Here, we investigated whether methyltransferase-like 3 (METTL3)-mediated m⁶A methylation can affect ACLF. Human hepatic cells (THLE-2) were treated with lipopolysaccharide (LPS) to induce cell damage. Proliferation, apoptosis and m⁶A modification were measured by MTT assay, flow cytometry and Dot blot assay. Our results showed that HBV infection significantly enhanced the levels of m⁶A modification and elevated the expression of METTL3 and mature-miR-146a-5p in THLE-2 cells, which was repressed by cycloleucine (m⁶A inhibitor). METTL3 overexpression enhanced m⁶A modification and promoted mature-miR-146a-5p expression. METTL3 overexpression promoted HBV replication and apoptosis, enhanced the levels of pro-inflammatory cytokines, hepatitis B surface antigen (HBsAg) and hepatitis B e antigen (HBeAg), and repressed cell proliferation in THLE-2 cells, which attributed to repress miR-146a-5p maturation. Moreover, a severe liver failure mouse model was established by HBV infection to verify the impact of METTL3 knockdown on liver damage in vivo. HBV-infection led to a severe liver damage and increase of apoptosis in hepatic tissues of mice, which was abolished by METTL3 knockdown. METTL3 knockdown reduced METTL3 expression and impeded miR-146a-5p maturation in HBV-infected mice. In conclusion, this work demonstrates that METTL3 inhibition ameliorates liver damage in mouse with HBV-associated ACLF, which contributes to repress miR-146a-5p maturation. Thus, this article suggests a novel therapeutic avenue to prevent and treat HBV-associated ACLF.     

Effects of egt gene transfer on the development of Bombyx mori

This study investigated the effects of gain of ecdysteroid UDP-glucosyltransferase (EGT) gene function mutation on the development of the silkworm, Bombyx mori. A novel piggyBac-derived plasmid containing the egt gene from B. mori nucleopolyhedrovirus (BmNPV) driven by a heat-shock protein (hsp) 23.7 promoter, with a neomycin-resistance gene (neo) controlled by the BmNPV ie-1 promoter and a green fluorescent protein gene (gfp) under the control of the B. mori actin 3 (A3) promoter was constructed. The vector was transferred into silkworm eggs by sperm-mediated gene transfer. Transgenic silkworms were produced after screening for neo and gfp genes and gene transfer was verified by polymerase chain reaction, dot-blot hybridization and western blotting. The hatching rate of G1 generation silkworm eggs was about 60% lower than that of normal silkworm eggs. The duration of the G1 generation larval period was extended, and the G2 generation pupal stage lasted four days longer than that in non-transgenic silkworms. The ecdysone blood level in G2 silkworms in the third instar molting stage was reduced by up to 90%. These results show that EGT suppressed transgenic silkworm molting, and that egt expression in egt-transgenic silkworms resulted in arrest of metamorphosis from pupae to moths.