ERK- and JNK-dependent signaling pathways contribute to Bombyx mori nucleopolyhedrovirus infection

Codon Optimization-based Whole-gene Scanning Identifies Hidden Nucleotides Essential for Bombyx mori Nucleopolyhedrovirus polyhedrin Hyperexpression

During the late stage of infection, alphabaculoviruses produce many occlusion bodies (OBs) in the nuclei of the insect host's cells through the hyperexpression of polyhedrin (POLH), a major OB component encoded by polh. The strong polh promoter has been used to develop a baculovirus expression vector system for recombinant protein expression in cultured insect cells and larvae. However, the relationship between POLH accumulation and the polh coding sequence remains largely unelucidated. This study aimed to assess the importance of polh codon usage and/or nucleotide sequences in POLH accumulation by generating a baculovirus Bombyx mori nucleopolyhedrovirus (BmNPV) expressing mutant polh (co-polh) optimized according to the codon preference of its host insect. Although the deduced amino acid sequence of CO-POLH was the same as that of wild-type POLH, POLH accumulation was significantly lower in cells infected with the co-polh mutant. This reduction was due to decreased polh mRNA levels rather than translational repression. Analysis of mutant viruses with chimeric polh revealed that a 30 base-pair (bp) 5' proximal polh coding region was necessary for maintaining high polh mRNA levels. Sequence comparison of wild-type polh and co-polh identified five nucleotide differences in this region, indicating that these nucleotides were critical for polh hyperexpression. Furthermore, luciferase reporter assays showed that the 30 bp 5' coding region was sufficient for maintaining the polh promoter-driven high level of polh mRNA. Thus, our whole-gene scanning by codon optimization identified important hidden nucleotides for polh hyperexpression in alphabaculoviruses.

Enhanced baculoviral virulence by suppressing the degradation of an insect immune resolvin, epoxyoctadecamonoenoic acid, in three lepidopteran insects

Epoxyoctadecamonoenoic acids (EpOMEs) are produced from linoleic acid by a cytochrome P450 monooxygenase (CYP) and play a crucial role in terminating excessive and unnecessary immune responses during the late infection stage in insects. This suggests that an increase in the EpOME level may enhance the virulence of insect pathogens against pests. This study tested this hypothesis using a specific inhibitor against soluble epoxide hydrolase (sEH) to degrade EpOMEs, which leads to elevated endogenous EpOME levels. A baculovirus, Autographa californica multiple nucleopolyhedrovirus (AcMNPV), was used to infect three different lepidopteran insects (Spodoptera exigua, Maruca vitrata, and Plutella xylostella) by oral feeding or hemocoelic injection treatments. Within one hour, the viral infection induced the expression of three different phospholipase A2 (PLA2) genes and, after 12 h, up-regulated the expressions of CYP and sEH genes in Spodopera exigua. As expected, AcMNPV virulence was suppressed by the addition of arachidonic acid (a catalytic product of PLA2) but was enhanced by the addition of either of the EpOME regioisomers. In addition, treatment with a specific sEH inhibitor (AUDA) increased AcMNPV virulence against three different lepidopteran insects, presumably by increasing endogenous EpOME levels. This enhanced effect of EpOMEs on virulence was further supported by specific RNA interference (RNAi), in which RNAi specific to CYP expression decreased AcMNPV virulence while a specific RNAi against sEH expression significantly enhanced virulence. In response to AcMNPV infection, TUNEL assay results showed that S. exigua larvae exhibited apoptosis in the midgut, fat body, and epidermis. Inhibition of apoptosis by a pan-caspase inhibitor, Z-VAD-FMK, significantly increased virulence. Similarly, the addition of AUDA to the viral treatment suppressed the gene expression of five inducible caspases and cytochrome C to suppress apoptosis, which led to a significant increase in the tissue viral titers. These results indicate that EpOMEs play a role in terminating excessive and unnecessary immune responses against viral infection during the late stage by down-regulating antiviral apoptosis in lepidopteran insects.

Comprehensive transcriptome sequencing of silkworm Midguts: Uncovering extensive isoform diversity and alternative splicing in BmNPV-Sensitive and BmNPV-resistant strains

The silkworm, Bombyx mori, stands out as one of the few economically valuable insects within the realm of model organisms. However, Bombyx mori nucleopolyhedrovirus (BmNPV) poses a significant threat, decreasing the quality and quantity of silkworm cocoons. Over the past few decades, a multitude of researchers has delved into the mechanisms that underlie silkworm resistance to BmNPV, employing diverse methodologies and approaching the problem from various angles. Despite this extensive research, the role of alternative splicing (AS) in the silkworm's response to BmNPV infection has been largely unexplored. This study leveraged both third-generation (Oxford Nanopore Technologies) and second-generation (Illumina) high-throughput sequencing technologies to meticulously identify and analyze AS patterns in the context of BmNPV response, utilizing two distinct silkworm strains-the susceptible strain 306 and the resistant strain NB. Consequently, we identified five crucial genes (Dsclp, LOC692903, LOC101743583, LOC101742498, LOC101743809) that are linked to the response to BmNPV infection through AS and differential expression. Additionally, a thorough comparative analysis was conducted on their diverse transcriptomic expression profiles, including alternative polyadenylation, simple sequence repeats, and transcription factors.

The Bombyx mori G protein β subunit 1 (BmGNβ1) gene inhibits BmNPV infection

G protein β subunit 1 (GNβ1) has several functions, including cell growth regulation, the control of second messenger levels, and ion channel switching. Previous transcriptome analyses in our laboratory have shown that BmGNβ1 transcription is reduced following infection with Bombyx mori nucleopolyhedrovirus (BmNPV), but it is unknown what role this gene may have in the host response to BmNPV infection. In this study, the BmGNβ1 gene was cloned using the RACE method. After BmNPV infection, BmGNβ1 was downregulated in Baiyu strains in tissues such as the hemolymph and midgut. Indirect immunofluorescence showed that BmGNβ1 was localized to the cytoplasm. We further constructed a BmGNβ1-pIZ/V5-His-mCherry overexpression plasmid and designed siRNA to evaluate the role of BmGNβ1 in host response to infection. The results showed that BmGNβ1 overexpression inhibited BmNPV proliferation, while knockdown of BmGNβ1 was correlated with increased BmNPV proliferation. The siRNA-mediated reduction of BmGNβ1 was correlated with an increase in BmNPV infection of BmN cells, increased BmNPV vp39 transcription, and reduced survival time of BmNPV-infected B. mori. Overexpression of BmGNβ1 in BmN cells was also correlated with apoptosis and a modification in transcript levels of genes involved in host response to BmNPV infection (PI3K, AKT, Bmp53, BmFOXO, Caspase-1, Bmp21, BmPKN and BmCREB), suggesting that BmGNβ1 may influence the apoptotic host response of infected B. mori through the PI3K-AKT pathway. This study provides potential targets and theoretical support for breeding BmNPV-resistant silkworm varieties.

Ben-JNK signaling is required for host mortality during Periplaneta fuliginosa densovirus infection

BACKGROUND

Cockroaches are widely acknowledged as significant vectors of pathogenic microorganisms. The Periplaneta fuliginosa densovirus (PfDNV) infects the smoky-brown cockroach P. fuliginosa and causes host mortality, which identifies the PfDNV as a species-specific and environmentally friendly biopesticide. However, although the biochemical characterization of PfDNV has been extensively studied, the immune response against PfDNV remains largely unclear.

RESULTS

Here, we investigated the replication of PfDNV and its associated pathological phenotype in the foregut and hindgut. Consequently, we dissected and performed transcriptome sequencing on the foregut, midgut, and hindgut separately. We revealed the up-regulation of immune response signaling pathway c-Jun N-terminal kinase (JNK) and apoptosis in response to viral infection. Furthermore, knockdown of the JNK upstream gene Ben resulted in a decrease in virus titer and delayed host mortality.

CONCLUSION

Taken together, our findings provide evidence that the Ben-JNK signaling plays a crucial role in PfDNV infection, leading to excessive apoptosis in intestinal tissues and ultimately resulting in the death of the host. Our results indicated that the host response to PfDNV fosters viral infection, thereby increasing host lethality. This underscores the potential of PfDNV as a viable, environmentally friendly biopesticide. © 2024 Society of Chemical Industry.

Probenecid Inhibits Influenza A(H5N1) and A(H7N9) Viruses In Vitro and in Mice

Avian influenza (AI) viruses cause infection in birds and humans. Several H5N1 and H7N9 variants are highly pathogenic avian influenza (HPAI) viruses. H5N1 is a highly infectious bird virus infecting primarily poultry, but unlike other AIs, H5N1 also infects mammals and transmits to humans with a case fatality rate above 40%. Similarly, H7N9 can infect humans, with a case fatality rate of over 40%. Since 1996, there have been several HPAI outbreaks affecting humans, emphasizing the need for safe and effective antivirals. We show that probenecid potently inhibits H5N1 and H7N9 replication in prophylactically or therapeutically treated A549 cells and normal human broncho-epithelial (NHBE) cells, and H5N1 replication in VeroE6 cells and mice.

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

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

Advances in the extracellular signal-regulated kinase signaling pathway in silkworms, Bombyx mori (Lepidoptera)

Signaling pathways regulate the transmission of signals during organism growth and development, promoting the smooth and accurate completion of numerous physiological and biochemical reactions. The extracellular signal-regulated kinase (ERK) signaling pathway is an essential pathway involved in regulating various physiological processes, such as cell proliferation, differentiation, adhesion, migration, and more. This pathway also contributes to several important physiological processes in silkworms, including protein synthesis, reproduction, and immune defense against pathogens. Organizing related studies on the ERK signaling pathway in silkworms can provide a better understanding of its mechanism in Lepidopterans and develop a theoretical foundation for improving cocoon production and new strategies for pest biological control.

MicroRNA-Mediated Host Immune Genes Manipulation Benefits AcMNPV Proliferation in Spodoptera frugiperda

Spodoptera frugiperda is a highly destructive migratory pest that threatens various crops globally. Autographa californica multiple nucleopolyhedrovirus (AcMNPV) is an effective biocontrol agent against lepidopteran pests. Here, we explored the molecular mechanisms underlying the immune response to AcMNPV infection in S. frugiperda. RNA-seq and quantitative reverse transcription-polymerase chain reaction (qRT-PCR) analyses identified the Toll, IMD, and apoptosis pathways as primary immune responses. Investigation into AcMNPV-induced apoptosis in the S. frugiperda cell line (Sf9) revealed that the Toll pathway activated the JNK via the TRAF6 (TNF receptor-associated factor 6) adapter. In addition, AcMNPV-induced the differential expression of several host-encoded microRNAs (miRNAs), with significant negative regulatory effects, on S. frugiperda antiviral immune genes. RNAi and miRNA-mimic mediated silencing of these genes resulted in increased AcMNPV proliferation. Our findings reinforce the potential of AcMNPV as a potent biocontrol agent and further our understanding of developing biotechnology-based targeted pest control agents.

A seamless connection from the burst sequence to the start codon is essential for polyhedrin hyperexpression in alphabaculoviruses

Alphabaculoviruses produce a large number of occlusion bodies (OBs) in host cells during the late stage of infection. OBs are mainly composed of polyhedrin (POLH), and high-level transcription of the polh gene has been exploited to express foreign proteins in insect cells. While making Bombyx mori nucleopolyhedrovirus (BmNPV) polh mutants using a conventional transfer vector-based method, we noticed that a virus with a short sequence insertion just before the polh start codon produces fewer very small OBs. Detailed analysis of several BmNPV mutants revealed that insertions between the burst sequence and start codon markedly decrease POLH accumulation and polh transcription. We further confirmed this decrease using recombinant viruses expressing a reporter gene driven by the polh promoter. These findings underscore the critical importance of a seamless connection from the burst sequence to the start codon for baculovirus polh hyperexpression.

Symbiotic Bacteria Modulate Lymantria dispar Immunity by Altering Community Proportions after Infection with LdMNPV

The symbiotic bacteria-insect interaction is considered to be associated with immunity and drug resistance. However, the wide variety of insect species and habitats is thought to have a significant impact on the symbiotic community, leading to disparate results. Here, we demonstrated that symbiotic bacteria regulated the immune response by changing the proportion of the Gram-positive and the Gram-negative bacterial community in Lymantria dispar (L. dispar) after infection with its viral pathogen, L. dispar Nucleopolyhedrovirus (LdMNPV). After oral infection, the immune deficiency pathway was activated immediately, and the expression of Relish was up-regulated to promote the secretion of antimicrobial peptides. Meanwhile, the abundance of the Gram-negative bacterial community increased at the same time. Moreover, the Toll pathway was not regulated in the same way as the Imd pathway was after infection. However, the change in the Toll pathway's expression remained positively correlated to the abundance of Gram-positive bacteria. This finding implied that the ratio of Gram-negative to Gram-positive bacteria in the LdMNPV infected larvae had an effect on the immune response. Our findings revealed that the immune regulation of L. dispar was regulated by the relative abundance of its symbiotic bacteria at different infection times with LdMNPV, which provides a new way to understand symbiotic bacteria-insect interactions.

The JAK-STAT pathway promotes persistent viral infection by activating apoptosis in insect vectors

The Janus kinase-signal transducer and activator of transcription (JAK-STAT) pathway is an evolutionarily conserved signaling pathway that can regulate various biological processes. However, the role of JAK-STAT pathway in the persistent viral infection in insect vectors has rarely been investigated. Here, using a system that comprised two different plant viruses, Rice stripe virus (RSV) and Rice black-streaked dwarf virus (RBSDV), as well as their insect vector small brown planthopper, we elucidated the regulatory mechanism of JAK-STAT pathway in persistent viral infection. Both RSV and RBSDV infection activated the JAK-STAT pathway and promoted the accumulation of suppressor of cytokine signaling 5 (SOCS5), an E3 ubiquitin ligase regulated by the transcription factor STAT5B. Interestingly, the virus-induced SOCS5 directly interacted with the anti-apoptotic B-cell lymphoma-2 (BCL2) to accelerate the BCL2 degradation through the 26S proteasome pathway. As a result, the activation of apoptosis facilitated persistent viral infection in their vector. Furthermore, STAT5B activation promoted virus amplification, whereas STAT5B suppression inhibited apoptosis and reduced virus accumulation. In summary, our results reveal that virus-induced JAK-STAT pathway regulates apoptosis to promote viral infection, and uncover a new regulatory mechanism of the JAK-STAT pathway in the persistent plant virus transmission by arthropod vectors.

Reactive oxygen species-mediated phosphorylation of JNK is involved in the regulation of BmFerHCH on Bombyx mori nucleopolyhedrovirus proliferation

c-Jun N-terminal kinase (JNK) phosphorylation is widely observed during virus infection, modulating various aspects of the virus-host interaction. In our previous research, we have proved that B. mori ferritin heavy-chain homolog (BmFerHCH), an inhibitor of reactive oxygen species (ROS), facilitates B. mori nucleopolyhedrovirus (BmNPV) proliferation. However, one question remains: Which downstream signaling pathways does BmFerHCH regulate by inhibiting ROS? Here, we first determined that silencing BmFerHCH inhibits BmNPV proliferation, and this inhibition depends on ROS. Then, we substantiated that BmNPV infection activates the JNK signaling pathway. Interestingly, the JNK phosphorylation during BmNPV infection is activated by ROS. Further, we found that the enhanced nuclear translocation of phospho-JNK induced by BmNPV infection was dramatically reduced by pretreatment with the antioxidant N-acetylcysteine (NAC), whereas there was more detectable phospho-JNK in the cytoplasm. Next, we investigated how changes in BmFerHCH expression affect JNK phosphorylation. BmFerHCH overexpression suppressed the phosphorylation of JNK and nuclear translocation of phospho-JNK during BmNPV infection, whereas BmFerHCH knockdown facilitated phosphorylation of JNK and nuclear translocation of phospho-JNK. By measuring the viral load, we found the inhibitory effect of BmFerHCH knockdown on BmNPV infection depends on phosphorylated JNK. In addition, the JNK signaling pathway was involved in BmNPV-triggered apoptosis. Hence, we hypothesize that ROS-mediated JNK phosphorylation is involved in the regulation of BmFerHCH on BmNPV proliferation. These results elucidate the molecular mechanisms and signaling pathways of BmFerHCH-mediated response to 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.

Mutations in the polyhedrin NLS affect the assembly and polyhedral shape of alphabaculovirus occlusion bodies

Alphabaculoviruses produce occlusion bodies (OBs) in the nucleus of the infected cells at the late stage of infection. OBs are mainly composed of a single viral protein called polyhedrin (POLH). Autographa californica multiple nucleopolyhedrovirus (AcMNPV) POLH possesses a monopartite nuclear localization signal sequence (NLS), KRKK, from 32nd to 35th residues. However, the functions of POLH NLS of other alphabaculoviruses remain unknown. Here, POLH NLS mutants of Bombyx mori nucleopolyhedrovirus (BmNPV) were generated and NLS function as well as the relationship between NLS and OB localization or morphology was investigated. Deletion or mutation of BmNPV POLH NLS severely affected POLH and OB intracellular localization. Additionally, viruses in which the arginine residue at the 33rd position of POLH was mutated produced a lower number of OBs, which was presumably due to decreased POLH accumulation in the infected cells. Furthermore, cytoplasmic OBs were morphologically aberrant, even though nuclear OB morphology was normal in the same cell. These results indicate that NLS is required for nuclear localization and efficient accumulation of BmNPV POLH, which heavily affect the number and morphology of OBs.

Vector Specificity of Arbovirus Transmission

More than 25% of human infectious diseases are vector-borne diseases (VBDs). These diseases, caused by pathogens shared between animals and humans, are a growing threat to global health with more than 2.5 million annual deaths. Mosquitoes and ticks are the main vectors of arboviruses including flaviviruses, which greatly affect humans. However, all tick or mosquito species are not able to transmit all viruses, suggesting important molecular mechanisms regulating viral infection, dissemination, and transmission by vectors. Despite the large distribution of arthropods (mosquitoes and ticks) and arboviruses, only a few pairings of arthropods (family, genus, and population) and viruses (family, genus, and genotype) successfully transmit. Here, we review the factors that might limit pathogen transmission: internal (vector genetics, immune responses, microbiome including insect-specific viruses, and coinfections) and external, either biotic (adult and larvae nutrition) or abiotic (temperature, chemicals, and altitude). This review will demonstrate the dynamic nature and complexity of virus–vector interactions to help in designing appropriate practices in surveillance and prevention to reduce VBD threats.

The regulation landscape of MAPK signaling cascade for thwarting Bacillus thuringiensis infection in an insect host

Host-pathogen interactions are central components of ecological networks where the MAPK signaling pathways act as central hubs of these complex interactions. We have previously shown that an insect hormone modulated MAPK signaling cascade participates as a general switch to trans-regulate differential expression of diverse midgut genes in the diamondback moth, Plutella xylostella (L.) to cope with the insecticidal action of Cry1Ac toxin, produced by the entomopathogenic bacterium Bacillus thuringiensis (Bt). The relationship between topology and functions of this four-tiered phosphorylation signaling cascade, however, is an uncharted territory. Here, we carried out a genome-wide characterization of all the MAPK orthologs in P. xylostella to define their phylogenetic relationships and to confirm their evolutionary conserved modules. Results from quantitative phosphoproteomic analyses, combined with functional validations studies using specific inhibitors and dsRNAs lead us to establish a MAPK "road map", where p38 and ERK MAPK signaling pathways, in large part, mount a resistance response against Bt toxins through regulating the differential expression of multiple Cry toxin receptors and their non-receptor paralogs in P. xylostella midgut. These data not only advance our understanding of host-pathogen interactions in agricultural pests, but also inform the future development of biopesticides that could suppress Cry resistance phenotypes.

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

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

A novel JNK induces innate immune response by activating the expression of antimicrobial peptides in Chinese mitten crab Eriocheir sinensis

c-Jun NH₂-terminal kinase (JNK) is a member of mitogen-activated protein kinases (MAPKs) that participates in the regulation of various physiological and pathological processes. In this study, we identified a novel JNK (EsJNK) and determined the cDNA sequence of its isoform (EsJNK-a) from the Chinese mitten crab Eriocheir sinensis. The open reading frame (ORF) of EsJNK was predicted to encode 421 peptides with a serine/threonine protein kinase, a catalytic (S_TKc) domain, and a low complexity region. The ORF of EsJNK-a was 1380 bp encoding a protein with 459 amino acids, which was 38 amino acids more than that of EsJNK. The predicted tertiary structure of EsJNK was conserved and contained 15 α-helices and 10 β-sheets. Phylogenetic tree analysis revealed that EsJNK was clustered with the JNK homologs of other crustaceans. Quantitative real-time PCR assays showed that EsJNK was expressed in all the tissues examined, but it was relatively higher in hemocytes, muscles, and intestines. The expression of EsJNK mRNA in the hemocytes was upregulated by lipopolysaccharides and peptidoglycans, as well as by Staphylococcus aureus or Vibrio parahaemolyticus challenge. Functionally, after silencing EsJNK by siRNA in crabs, the expression levels of two antimicrobial peptides (AMPs), namely, anti-lipopolysaccharide factor and crustin, were significantly inhibited. The purified recombinant EsJNK protein with His-tag accelerated the elimination of the aforementioned bacteria in vivo. However, knockdown of EsJNK had an opposite effect. These findings suggested that EsJNK might be involved in the antibacterial immune defense of crabs by regulating the transcription of AMPs.

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

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

Laodelphax striatellus Atg8 facilitates Rice stripe virus infection in an autophagy-independent manner

Rice stripe virus (RSV) is the causative agent of rice stripe disease and is completely dependent on insect vectors for its plant-to-plant transmission. Laodelphax striatellus is the major insect vector for RSV. In this study, we explored the interactions between RSV infection and L. striatellus autophagy, a potential intrinsic antiviral mechanism in insects. We found that L. striatellus autophagic activity did not affect RSV infection; however, the autophagy-related-8 (Atg8) gene significantly enhanced virus infection. During RSV initial infection within the L. striatellus midgut, silencing of Atg8 expression significantly decreased the phosphorylation of c-Jun N-terminal kinase (p-JNK); however, when RSV infection is absent, silencing of Atg8 did not alter p-JNK levels. These results indicated that Atg8 might activate the JNK machinery by allowing more virus infection into cells. We further revealed that Atg8-deficiency significantly decreased RSV accumulation on the surface of the insect midgut epithelial cells, suggesting a receptor trafficking function of the γ-aminobutyric acid receptor-associated protein family. Using the RSV ovary entry as a model, in which vitellogenin receptor (VgR) mediates RSV cell entry, we clarified that Atg8-deficiency decreased the abundance of VgR localizing on the cytomembrane and disturbed the attachment of RSV in the germarium zones. Collectively, these results revealed an autophagy-independent function of L. striatellus Atg8 that enhances RSV initial infection by increasing virus attachment on the infection sites.

Loss of p24 from the Bombyx mori nucleopolyhedrovirus genome results in the formation of cuboidal occlusion bodies

Some insect viruses produce the occlusion body (OB), a large crystalline particle comprising a viral protein that occludes virions to protect them from harsh environments. The shapes and sizes of OBs are diverse depending on baculovirus species, but the detailed molecular mechanism determining them has yet to be totally clarified yet. Here we generated Bombyx mori nucleopolyhedrovirus (BmNPV) mutants of the p24 gene that encodes a viral capsid protein and found that p24-mutated BmNPVs produced cuboidal OBs with a slightly larger size than typical truncated octahedral OBs produced by wild-type BmNPVs. Meanwhile, p24 disruption has no significant impact on progeny virus production and viral pathogenicity. In addition, we experimentally demonstrated that a single amino acid substitution found in the P24 protein of the BmNPV Cubic isolate caused cuboidal OB production. These results suggest that p24 has a crucial role in generating the typical shape of OBs.

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.

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.

Hsp90 function is required for stable transcription of the baculovirus transactivator ie-1 gene

A molecular chaperone heat shock protein 90 (Hsp90) is required for efficient infection by several viruses. Hsp90 has been recently implicated in baculovirus infection, but its exact role remains obscure. This study investigated the effect of 17-N-allylamino-17-demethoxygeldanamycin (17-AAG), an Hsp90-specific inhibitor, on Bombyx mori nucleopolyhedrovirus (BmNPV) infection. The 17-AAG treatment significantly decreased the production of budded viruses and occlusion bodies in BmNPV-infected Bombyx mori cultured cells. Immunoblot and SDS-PAGE analyses showed that the expression of early and delayed early gene products, DBP and BRO, was delayed and dysregulated, and the very late gene product POLH was almost completely diminished. RT-qPCR experiments revealed that 17-AAG treatment did not affect initiation of the immediate early gene ie-1 expression, but the expression decreased by ∼50 % during the late stage of infection. 17-AAG treatment also decreased ie-1 promoter activity by ∼50 %. In addition, the expression of delayed early and late genes was dysregulated and inhibited, respectively. These results indicated that Hsp90 function is required for stable ie-1 transcription. Inhibiting Hsp90 function negatively affects ie-1 expression, resulting in dysregulation of delayed early genes and a severe decrease in late and very late gene expression.

Targeting genes involved in nucleopolyhedrovirus DNA multiplication through RNA interference technology to induce resistance against the virus in silkworms

RNA interference (RNAi) has become an efficient tool for inducing resistance to viruses in many organisms. In this study, Escherichia coli cells were engineered to produce stable double-stranded RNA (dsRNA) against the nucleopolyhedrosis virus to elicit RNAi in silkworms. The immediate-early-1 (ie-1) and late expression factor-1 (lef-1) genes of the Bombyx mori nucleopolyhedrovirus (BmNPV) involved in viral DNA multiplication were cloned in the plasmid L4440 under the influence of the double T7 promoter and transformed to E. coli HT115 DE3 host cells. On induction with isopropyl β-D-thiogalactopyranoside, these cells efficiently produced dsRNA of the cloned genes. The B. mori larvae were fed with 50 µL of E. coli cells expressing ie-1 and lef-1 dsRNAs (each approximately 25 µg) to elicit RNAi. The semi-quantitative and quantitative PCR analysis of RNA from the midgut of the dsRNA-fed larvae revealed a significant reduction in the expression of the target genes involved in BmNPV multiplication, which restricted virus copy numbers to 100 compared with 1.9 × 10⁵ in the infected controls. Furthermore, the dsRNA-fed infected larvae showed > 50% increased survivability compared with the infected controls. The study revealed the successful use of bacteria as vectors for efficiently delivering dsRNA to elicit RNAi against BmNPV in silkworms.

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

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

Fish c-Jun N-Terminal Kinase (JNK) Pathway Is Involved in Bacterial MDP-Induced Intestinal Inflammation

The c-Jun NH₂-terminal kinases (JNKs) are an evolutionarily conserved family of serine/threonine protein kinases that play critical roles in the pathological process in species ranging from insects to mammals. However, the function of JNKs in bacteria-induced intestinal inflammation is still poorly understood. In this study, a fish JNK (CiJNK) pathway was identified, and its potential roles in bacterial muramyl dipeptide (MDP)-induced intestinal inflammation were investigated in Ctenopharyngodon idella. The present CiJNK was found to possess a conserved dual phosphorylation motif (TPY) in a serine/threonine protein kinase (S_TKc) domain and to contain several potential immune-related transcription factor binding sites, including nuclear factor kappa B (NF-κB), activating protein 1 (AP-1), and signal transducer and activator of downstream transcription 3 (STAT3), in its 5′ flanking regions. Quantitative real-time PCR results revealed that the mRNA levels of the JNK pathway genes in the intestine were significantly upregulated after challenge with a bacterial pathogen (Aeromonas hydrophila) and MDP in a time-dependent manner. Additionally, the JNK pathway was found to be involved in regulating the MDP-induced expression levels of inflammatory cytokines (IL-6, IL-8, and TNF-α) in the intestine of grass carp. Moreover, the nutritional dipeptide carnosine and Ala–Gln could effectively alleviate MDP-induced intestinal inflammation by regulating the intestinal expression of JNK pathway genes and inflammatory cytokines in grass carp. Finally, fluorescence microscopy and dual-reporter assays indicated that CiJNK could associate with CiMKK4 and CiMKK7 involved in the regulation of the AP-1 signaling pathway. Overall, these results provide the first experimental demonstration that the JNK signaling pathway is involved in the intestinal immune response to MDP challenge in C. idella, which may provide new insight into the pathogenesis of inflammatory bowel disease.

Transcriptomic analysis of the interactions between the Spodoptera exigua midgut and nucleopolyhedrovirus

Spodoptera exigua nucleopolyhedrovirus (SeNPV) has been successfully applied as a bioinsecticide against S. exigua, one of the most devastating pests worldwide. However, due to limited information, the molecular mechanisms underlying interactions between S. exigua and SeNPV remain to be elucidated. In this study, RNA-Seq and differentially expressed gene (DEG) analysis of the S. exigua larva midgut were performed to explore molecular responses to SeNPV infection. A total of 1785 DEGs, including 935 upregulated and 850 downregulated genes, were identified in the midgut of SeNPV-infected S. exigua larvae. Ultrastructural observations showed that after SeNPV infection, the peritrophic matrix (PM) became a loose and highly porous surface with many clear ruptures; these changes were most likely associated with upregulation of chitin deacetylases. In addition, 124 putative innate immunity-related DEGs were identified and divided into several groups, including pattern recognition proteins, signaling pathways, signal modulation, antimicrobial peptides and detoxification. Interestingly, upregulation of some pattern recognition proteins, induction of the JAK/STAT signaling pathway and promotion of REPAT synthesis might be the main innate immunity responses occurring in the S. exigua larva midgut after SeNPV infection. According to quantitative real-time PCR, the expression profiles of 19 random DEGs were consistent with those obtained by RNA-Seq. These findings provide important basic information for understanding the molecular mechanisms of SeNPV invasion and the anti-SeNPV responses of the S. exigua midgut, promoting the utility of SeNPV as a bioinsecticide for the effective control of S. exigua and related pests.

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.

Baculoviral infection reduces the expression of four allergen proteins of silkworm pupa

Silkworm (Bombyx mori) larvae are widely used to express exogenous proteins. Moreover, some silkworm pupal proteins can be used as drug-loading materials for selfexpressed oral tolerance drugs. However, several proteins expressed in silkworm pupae cause severe allergic reactions in humans and animals. Interestingly, some baculovirus vectors have been shown to alter the host gene and its expression in insect cells, but this has not been confirmed in silkworm. Here, we analyzed the effects of infection with an empty B. mori baculovirus (BmNPV) vector on silkworm pupal protein expression. Using a proteomics approach, the allergens thiol peroxiredoxin (Jafrac1), 27-kDa glycoprotein (p27k), arginine kinase, and paramyosin as well as 32 additional differentially expressed proteins were identified. Downregulation of the messenger RNA expression of the four known allergens was observed after BmNPV infection; subsequent changes in protein expression were confirmed by the western blot analysis using polyclonal antibodies prepared with recombinant proteins of the four allergens. Collectively, these data indicate that the four known allergens of silkworm pupae can be reduced by infection ith an empty BmNPV vector to increase the safety of silkworm pupa-based exogenous protein expression and drug delivery of oral pharmaceuticals. In addition, the four recombinant allergen proteins may contribute to the diagnosis of allergic diseases of silkworm pupa.

Transcriptomic analysis of interactions between Hyphantria cunea larvae and nucleopolyhedrovirus

BACKGROUND

Hyphantria cunea is a destructive forest pest. To utilise H. cunea nucleopolyhedrovirus (HcNPV) for biological control, understanding insect-virus interactions is essential.

RESULTS

Four cDNA libraries were constructed from H. cunea larvae (two HcNPV-infected and two uninfected groups) and 76 004 expressed genes were obtained by next-generation sequencing. Compared with controls, 272 differentially expressed genes (DEGs) were identified in infected groups, including 162 up-regulated and 110 down-regulated genes. Transcription levels of 20 random DEGs were consistent with the Solexa expression profiles obtained by quantitative real-time PCR. DEGs associated with innate immunity were grouped into several categories, including pattern recognition proteins, heat-shock proteins, UDP-glycosyltransferases, cytochrome P450s, antimicrobial peptides and hormonal signalling proteins. Interestingly, up-regulated host genes included farnesoic acid O-methyltransferase, two juvenile hormone (JH) binding proteins, and a circadian clock-controlled protein related to JH regulation. Pathway enrichment analysis indicates that mitogen-activated protein kinase (MAPK) signalling pathways, key candidate genes and important biological pathways may be associated with molecular modification in H. cunea larvae in response to virus stress.

CONCLUSION

These findings provide insight for future research on the molecular mechanisms of HcNPV invasion and anti-HcNPV mechanisms in H. cunea. A better understanding of gene regulation following HcNPV invasion could help to develop the virus as a bio-insecticide. © 2018 Society of Chemical Industry.

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

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

An amino acid duplication/insertion in the Bm126 gene of Bombyx mori nucleopolyhedrovirus alters viral gene expression as shown by differential gene expression analysis

Open reading frame (ORF) 126 (Bm126) of Bombyx mori nucleopolyhedrovirus (BmNPV) is not essential for viral replication, and two subtypes of this gene have been identified in China. The Bm126-SX subtype encodes a protein with a simple amino acid duplication/insertion relative to the Bm126-GD subtype; however, significant differences in the cytopathic effect and infectivity of viruses carrying these variant genes have been observed. To elucidate the cause of these differences, differential gene expression analysis was performed at the early stage of infection with viruses harbouring variants of Bm126. Differential expression was observed for 103, 209, and 313 host genes and 9, 44, and 67 viral genes in vGD126 samples relative to the control samples (vSX126) at 6, 12, and 24 h postinfection, respectively. These results indicated that the duplication/insertion in Bm126 altered the viral expression pattern. The differentially expressed host genes were found to be related to ribosome, spliceosome, and proteasome pathways, and several factors involved in signal transduction were also identified. The differential expression of these viral and host genes was confirmed by qPCR. This study indicates that the amino acid duplication/insertion in the Bm126 gene has a biological function related to the regulation of viral gene expression and serves as a basis for further characterization of Bm126 gene function.

Identification of key genes and signaling pathways during Sendai virus infection in vitro

Sendai virus (SeV) has been used as a model strain to reveal molecular features of paramyxovirus biology. In this study, we comprehensively analyzed the gene profiling of murine macrophages and airway epithelial cells in response to SeV using gene expression data. The significantly differentially expressed genes (DEGs) were screened by GEO2R. Gene ontology (GO) and pathway enrichment analyses were performed by DAVID. The protein-protein interaction (PPI) map of DEGs was constructed by STRING. The modules of PPI network are produced by molecular complex detection (MCODE) plug-in of Cytoscape. In total, 241 up- and 83 downregulated DEGs were identified in airway epithelial cells while 130 up- and 148 downregulated in macrophage. Particularly, Tmem119 and Colla2 are significantly downregulated in airway epithelial cells and macrophages, respectively. Functional enrichment analysis showed that upregulated DEGs are clustered in innate immunity and inflammatory response in both cell types, whereas downregulated DEGs are involved in host metabolic pathway in airway epithelial cells. PI3K-AKT signaling pathway is downregulated in macrophages. PPI network analysis indicated that some high degree of nodes exist in both cell types, such as Stat1, Tnf, and Cxcl10. In conclusion, SeV infection can induce different host cell responses in airway epithelial cells and macrophages.

Molecular characterization of p38 MAPK from blunt snout bream (Megalobrama amblycephala) and its expression after ammonia stress, and lipopolysaccharide and bacterial challenge

p38 mitogen-activated protein kinase (MAPK) is an important protein which plays a key role in regulating the innate immunity, so exploring its molecular characterization is helpful in understanding the resistance against microbial infections in cultured fish. Here, a full-length cDNA of p38 MAPK was cloned from liver of blunt snout bream (Megalobrama amblycephala) which covered 2419 bp with an open reading frame of 1086 bp encoding 361 amino acids. p38 MAPK contained the characteristic structures of Thr-Gly-Tyr (TGY) motif and substrate binding site Ala-Thr-Arg-Trp (ATRW), which are conserved in MAPK family. To investigate p38 MAPK functions, two in vivo experiments were carried out to examine its expression following ammonia exposure and bacterial challenge. Also, an in vitro experiment was conducted to assess the role of p38 MAPK in inflammation of primary hepatocytes induced by lipopolysaccharide (LPS). The results showed the ubiquitous expression of p38 MAPK in all the tested tissues with varying levels. p38 MAPK mRNA expression was significantly up-regulated by ammonia stress and Aeromonas hydrophila challenge, and altered in a time-dependent manner. Moreover, the results indicated that the inflammatory response induced by LPS in hepatocytes is p38 MAPK dependent as knockdown of p38 MAPK using siRNA technology depressed the expression of IL-1β and IL-6. The findings in this study showed that p38 MAPK has anti-stress property, and plays key role in protection against bacterial infection and inflammation in blunt snout bream.

A novel JNK is involved in immune response by regulating IL expression in oyster Crassostrea gigas

The c-Jun N-terminal kinase (JNK) is a member of mitogen-activated protein kinases (MAPK) highly conserved from yeast to mammals and participates in regulating many physiological and pathological processes. In the present study, a novel JNK was identified from oyster Crassostrea gigas (designated as CgJNK) and its biological functions were investigated in response against lipopolysaccharide (LPS) stimulation. The CgJNK consists of 415 amino acids, which includes a serine/threonine protein kinase (S_TKc) domain with a conserved Thr-Pro-Tyr (TPY) motif. Phylogenetic analysis revealed that CgJNK shared high similarity with other members of the JNK subfamily. CgJNK mRNA was detected in all the tested tissues and CgJNK mRNA expression levels in hemocytes were significantly up-regulated from 6 to 72 h after LPS stimulation and reached the highest level (16.1-fold, p < 0.01) at 24 h. The phosphorylation level of CgJNK in C. gigas hemocytes was increased at 2 h after LPS stimulation. The subcellular localization of CgJNK phosphorylation in hemocytes was analyzed after LPS stimulation, and CgJNK phosphorylation could be detected in both cytoplasm and nucleus of oyster hemocytes at 2 h post LPS stimulation. Additionally, the interleukins (CgILs) were detected in hemocytes of CgJNK-knockdown oysters. CgIL17-1, CgIL17-2, CgIL17-4 and CgIL17-6 transcripts were decreased significantly in CgJNK-knockdown oysters at 24 h post LPS stimulation. In summary, these results suggested that CgJNK played an important role in the immune response of oysters by regulating IL expression.

Bombyx mori epidermal growth factor receptor is required for nucleopolyhedrovirus replication

Baculovirus-host interactions are important models for studying the biological control of lepidopteran pests. Research on baculovirus-host interactions has focussed on baculovirus manipulation of cellular signalling pathways, including the extracellular signal-regulated kinase (ERK) and phosphatidylinositol-3-kinases/protein kinase B (PI3K/Akt) signalling pathways. However, the mechanism underlying ERK and PI3K/Akt activation and function in response to baculovirus infection remains poorly understood. Here, we demonstrated that baculovirus activated the Bombyx mori ERK and PI3K/Akt signalling pathways via the B. mori epidermal growth factor receptor (BmEGFR). To further characterize the function of the BmEGFR/ERK signalling pathway in baculovirus replication, we calculated genome-wide changes in kinase-chromatin interactions for ERK after baculovirus infection using chromatin immunoprecipitation followed by high-throughput sequencing. A Gene Ontology analysis showed that virus infection had effects on the biological regulation, cellular process and metabolic process pathways. Moreover, ERK was shown to regulate the transcription of late viral genes. Taken together, our results suggest that baculoviruses manipulate components of the host cell machinery for replication via modulation of the BmEGFR signalling pathway.

Complex interactions between insect-borne rice viruses and their vectors

Insect-borne rice viral diseases are widespread and economically important in many rice-growing countries. Long-term associations between rice viruses and their insect vectors result in evolutionary trade-offs that maintain a balance between the fitness cost of the viral infection of insects and the persistent transmission of the virus by the insect. To promote optimal replication, rice viruses activate innate immune responses, such as autophagy, apoptosis, and stress-regulated signaling pathways in the vector; meanwhile, a conserved insect small interfering RNA antiviral pathway is activated to control excessive viral replication, guaranteeing persistent virus transmission. Furthermore, growing evidence has shown that rice viruses can manipulate their vectors either directly or by inducing changes in host plants to promote the spread of viral pathogens. Thus, understanding the plant-virus-insect relationships offers important insights into how disease epidemics occur and facilitates the design of powerful new strategies for disease control.

Essential function of VCP/p97 in infection cycle of the nucleopolyhedrovirus AcMNPV in Spodoptera frugiperda Sf9 cells

The protein VCP/p97 (also named CDC48 and TER94) belongs to a type II subfamily of the AAA+ATPases and controls cellular proteostasis by acting upstream of proteasomes in the ubiquitin-proteasome protein degradation pathway. The function of VCP/p97 in the baculovirus infection cycle in insect cells remains unknown. Here, we identified VCP/p97 in the fall armyworm Spodoptera frugiperda (Sf9) cells and analyzed the replication of the Autographa californica multiple nucleopolyhedrovirus, AcMNPV, in Sf9 cells in which the VCP/p97 function was inhibited. The specific allosteric inhibitor of the VCP/p97 ATPase activity, NMS-873, did not deplete VCP/p97 in infected cells but caused a dose-dependent inhibition of viral DNA synthesis and efficiently suppressed expression of viral proteins and production of budded virions. NMS-873 caused accumulation of ubiquitinated proteins in a manner similar to the inhibitor of proteasome activity, Bortezomib. This suggests the essential function of VCP/p97 in the baculovirus infection cycle might be associated, at least in part, with the ubiquitin-proteasome system.

The interaction between baculoviruses and their insect hosts

Baculoviruses are double-stranded circular DNA viruses that infect arthropods via the midgut. Because of their superiority as eukaryotic expression systems and their importance as biopesticides, extensive research on the functions of baculovirus genes as well as on the host response to baculovirus infection has been carried out, including transcriptomic and proteomic analyses of the midgut. The morphological and cellular changes caused by baculovirus infection are also important to better understand the infection pathway. Thanks to these previous studies, we now have a clearer picture of the mechanisms of action of the virus and of host immunity. In this paper, we systematically reviewed studies on the interaction between baculoviruses and their insect hosts. By better understanding these interactions, baculoviruses can be developed for use as more efficient biopesticides to improve agricultural development in the future.

Alimentary Tract Transcriptome Analysis of the Tea Geometrid, Ectropis oblique (Lepidoptera: Geometridae)

Ectropis oblique Prout (Lepidoptera: Geometridae) is one of the main pests that damages the tea crop in Southeast Asia. To understand the molecular mechanisms of its feeding biology, transcriptomes of the alimentary tract (AT) and of the body minus the AT of E. oblique were successfully sequenced and analyzed in this study. A total of 36,950 unigenes from de novo sequences were assembled. After analysis using six annotation databases (e.g., Gene Ontology, Kyoto Encyclopedia of Genes and Genome, and NCBI nr), a series of putative genes were found for this insect species that were related to digestion, detoxification, the immune system, and Bacillus thuringiensis (Bt) receptors. From this series of genes, 21 were randomly selected to verify the relative expression levels of transcripts using quantitative real-time polymerase chain reaction. These results will provide an invaluable genomic resource for future studies on the molecular mechanisms of E. oblique, which will be useful in developing biological control strategies for this pest.

Baculovirus PTP2 Functions as a Pro-Apoptotic Protein

The family Baculoviridae encompasses a large number of invertebrate viruses, mainly infecting caterpillars of the order Lepidoptera. The baculovirus Spodoptera exigua multiple nucleopolyhedrovirus (SeMNPV) induces physiological and behavioral changes in its host Spodoptera exigua, as well as immunological responses, which may affect virus transmission. Here we show that the SeMNPV-encoded protein tyrosine phosphatase 2 (PTP2) induces mild apoptosis in Spodoptera frugiperda (Sf) 21 cells upon transient expression. Transient expression of a catalytic-site mutant of ptp2 did not lead to apoptosis, indicating that the phosphatase activity of PTP2 is needed to induce apoptosis. We also found that the caspase level (indicator of apoptosis) was higher in cells transfected with the ptp2 gene than in cells transfected with the catalytic mutant. Adding a caspase inhibitor reduced the level of ptp2-induced apoptosis. Moreover, deletion of the ptp2 gene from the viral genome prevented the induction of apoptosis in S. exigua hemocytes. The virus titer and virulence indices (the viral infectivity and the time to death) were not affected by deletion of the ptp2 gene. However, the viral occlusion body yield from S. exigua larvae infected with the mutant virus lacking the ptp2 gene was much lower than the yield from larvae infected with the wild-type (WT) virus. We hypothesize that the observed pro-apoptotic effects of PTP2 are the result of PTP2-mediated immune suppression in larvae, which consequently leads to higher viral occlusion body yields.

Stimulation of JNK Phosphorylation by the PTTH in Prothoracic Glands of the Silkworm, Bombyx mori

In this study, phosphorylation of c-Jun N-terminal kinase (JNK) by the prothoracicotropic hormone (PTTH) was investigated in prothoracic glands (PGs) of the silkworm, Bombyx mori. Results showed that JNK phosphorylation was stimulated by the PTTH in time- and dose-dependent manners. In vitro activation of JNK phosphorylation in PGs by the PTTH was also confirmed in an in vivo experiment, in which a PTTH injection greatly increased JNK phosphorylation in PGs of day-6 last instar larvae. JNK phosphorylation caused by PTTH stimulation was greatly inhibited by U73122, a potent and specific inhibitor of phospholipase C (PLC) and an increase in JNK phosphorylation was also detected when PGs were treated with agents (either A23187 or thapsigargin) that directly elevated the intracellular Ca²⁺ concentration, thereby indicating involvement of PLC and Ca²⁺. Pretreatment with an inhibitor (U0126) of mitogen-activated protein kinase (MAPK)/extracellular signal-regulated kinase (ERK) kinase (MEK) and an inhibitor (LY294002) of phosphoinositide 3-kinase (PI3K) failed to significantly inhibit PTTH-stimulated JNK phosphorylation, indicating that ERK and PI3K were not related to JNK. We further investigated the effect of modulation of the redox state on JNK phosphorylation. In the presence of either an antioxidant (N-acetylcysteine, NAC) or diphenylene iodonium (DPI), PTTH-stimulated JNK phosphorylation was blocked. The JNK kinase inhibitor, SP600125, markedly inhibited PTTH-stimulated JNK phosphorylation and ecdysteroid synthesis. The kinase assay of JNK in PGs confirmed its stimulation by PTTH and inhibition by SP600125. Moreover, PTTH treatment did not affect JNK or Jun mRNA expressions. Based on these findings, we concluded that PTTH stimulates JNK phosphorylation in Ca²⁺- and PLC-dependent manners and that the redox-regulated JNK signaling pathway is involved in PTTH-stimulated ecdysteroid synthesis in B. mori PGs.

Transcriptome profiling of whitefly guts in response to Tomato yellow leaf curl virus infection

BACKGROUND

Plant viruses in agricultural crops are of great concern worldwide, and over 75% of them are transmitted from infected to healthy plants by insect vectors. Tomato yellow leaf curl virus (TYLCV) is a begomovirus, which is the largest and most economically important group of plant viruses, transmitted by the whitefly Bemisia tabaci. The circulation of TYLCV in the insect involves complex insect-virus interactions, whereas the molecular mechanisms of these interactions remain ambiguous. The insect gut as a barrier for viral entry and dissemination is thought to regulate the vector specificity. However, due to its tiny size, information for the responses of whitefly gut to virus infection is limited.

METHODS

We investigated the transcriptional response of the gut of B. tabaci Middle East-Asia Minor 1 species to TYLCV infection using Illumina sequencing.

RESULTS

A total of 5207 differentially expressed genes (DEGs) between viruliferous and non-viruliferous whitefly guts were identified. Enrichment analyses showed that cargo receptor and ATP-binding cassette (ABC) transporters were enriched in DEGs, and might help the virus to cross gut barrier. TYLCV could perturb cell cycle and DNA repair as a possible result of its replication in the whitefly. Our data also demonstrated that TYLCV can activate whitefly defense responses, such as antimicrobial peptides. Meanwhile, a number of genes involved in intracellular signaling were activated by TYLCV infection.

CONCLUSIONS

Our results reveal the complex insect-virus relationship in whitefly gut and provide substantial molecular information for the role of insect midguts in virus transmission.

Quantitative proteomics of Sf21 cells during Baculovirus infection reveals progressive host proteome changes and its regulation by viral miRNA

System level knowledge of alterations in host is crucial to elucidate the molecular events of viral pathogenesis and to develop strategies to block viral establishment and amplification. Here, we applied quantitative proteomics approach to study global proteome changes in the host; Spodoptera frugiperda upon infection by a baculovirus, Spodoptera litura NPV at two stages i.e. 12 h and 72 h post infection. At 12 hpi, >95% of host proteins remained stable, however at 72 hpi, 52% host proteins exhibited downregulation of 2-fold or more. Functional analysis revealed significant upregulation of transposition and proteasomal machinery while translation, transcription, protein export and oxidative phosphorylation pathways were adversely affected. An assessment of perturbed proteome after viral infection and viral miRNA expression led to the identification of 117 genes that are potential targets of 10 viral miRNAs. Using miRNA mimics, we confirmed the down regulation of 9 host genes. The results comprehensively show dynamics of host responses after viral infection.

The Role of p38 MAPK, JNK, and ERK in Antibacterial Responses of Chilo suppressalis (Lepidoptera: Crambidae)

The mitogen-activated protein kinases (MAPKs) are conserved signal transduction pathways and broadly responsible for bacterial infection from yeast to mammals, and virus, fungi, and bacteria, specifically Bacillus thuringiensis, to insects. But little is known about the MAPK pathways in antibacterial responses in Chilo suppressalis (Walker), an important lepidopteran pest of rice. In this study, we used the bacteria of Bacillus thuringiensis, Escherichia coli, and Staphyloccocus aureus to infect C. suppressalis larvae, and the responses of MAPK pathways were analyzed. The results showed that E. coli infection induced the up-regulated expression of Csp38 and CsERK1 at 24 h postinfection (pi). Meanwhile, injection of B. thuringiensis and S. aureus resulted in strong activation of CsJNK phosphorylation at 3 h pi. These results suggest that MAPK signaling pathways play important functional roles in antibacterial responses in C. suppressalis larvae.

The c-Jun N-terminal kinase pathway of a vector insect is activated by virus capsid protein and promotes viral replication

No evidence has shown whether insect-borne viruses manipulate the c-Jun N-terminal kinase (JNK) signaling pathway of vector insects. Using a system comprising the plant virus Rice stripe virus (RSV) and its vector insect, the small brown planthopper, we have studied the response of the vector insect’s JNK pathway to plant virus infection. We found that RSV increased the level of Tumor Necrosis Factor-α and decreased the level of G protein Pathway Suppressor 2 (GPS2) in the insect vector. The virus capsid protein competitively bound GPS2 to release it from inhibiting the JNK activation machinery. We confirmed that JNK activation promoted RSV replication in the vector, whereas JNK inhibition caused a significant reduction in virus production and thus delayed the disease incidence of plants. These findings suggest that inhibition of insect vector JNK may be a useful strategy for controling the transmission of plant viruses.

DOI:http://dx.doi.org/10.7554/eLife.26591.001

There are over a thousand different viruses that infect plants. Many plant viruses are transmitted by insects that feed on the plants, much as mosquitoes spread diseases between people when feeding on blood. Often the plant virus can replicate inside the cells of the insect. However, unlike in the plant hosts, the viruses do not seem to cause disease in the insects that carry them.

Rice stripe disease is a major viral disease of rice that can reduce the crop’s yield by more than 50% in some areas. An insect called the small brown planthopper spreads the rice stripe virus between plants. Like other animals, insects have an immune system that protects them against viral infections. This means that the rice stripe virus must manipulate the planthopper’s immune system in order to replicate inside the insect’s cells. It was not clear how the virus did this, but answering this question could provide important clues to help scientists develop new ways to protect crops against plant viruses.

Wang, Zhao, Li et al. now show that rice stripe virus manipulates its insect host to produce more of a protein called TNF-α and less of a protein called GPS2. Moreover, a protein that makes up part of the virus also binds to GPS2. This stops GPS2 from inhibiting a conserved signaling pathway that involves an enzyme known as JNK. When the JNK signaling pathway becomes active, replication of the rice stripe virus inside the insect is accelerated. Further experiments showed that inhibiting JNK made it harder for the virus to replicate, which meant that it took longer for the disease to develop in rice plants.

These findings uncover a host of proteins that could be manipulated in insects to benefit rice agriculture. Such alterations could possibly be achieved through breeding or otherwise genetically modifying the insects to make them less able to carry viruses and then releasing them into wild populations. Alternatively, if further studies can identify chemicals that cause insect cells to alter the levels of the proteins, such chemicals could be administered to farmland to reduce the spread of viruses.

DOI:http://dx.doi.org/10.7554/eLife.26591.002

iTRAQ-based quantitative proteomics analysis of molecular mechanisms associated with Bombyx mori (Lepidoptera) larval midgut response to BmNPV in susceptible and near-isogenic strains

Bombyx mori nucleopolyhedrovirus (BmNPV) has been identified as a major pathogen responsible for severe economic loss. Most silkworm strains are susceptible to BmNPV, with only a few highly resistant strains thus far identified. Here we investigated the molecular basis of silkworm resistance to BmNPV using susceptible (the recurrent parent P50) and resistant (near-isogenic line BC9) strains and a combination of iTRAQ-based quantitative proteomics, reverse-transcription quantitative PCR and Western blotting. By comparing the proteomes of infected and non-infected P50 and BC9 silkworms, we identified 793 differentially expressed proteins (DEPs). By gene ontology and KEGG enrichment analyses, we found that these DEPs are preferentially involved in metabolism, catalytic activity, amino sugar and nucleotide sugar metabolism and carbon metabolism. 114 (14.38%) DEPs were associated with the cytoskeleton, immune response, apoptosis, ubiquitination, translation, ion transport, endocytosis and endopeptidase activity. After removing the genetic background and individual immune stress response proteins, we identified 84 DEPs were found that are potentially involved in resistance to BmNPV. Further studies showed that a serine protease was down-regulated in P50 and up-regulated in BC9 after BmNPV infection. Taken together, these results provide insights into the molecular mechanism of silkworm response to BmNPV.

BIOLOGICAL SIGNIFICANCE

Bombyx mori nucleopolyhedrovirus (BmNPV) is highly pathogenic, causing serious losses in sericulture every year. However, the molecular mechanisms of BmNPV infection and host defence remain unclear. Here we combined quantitative proteomic, bioinformatics, RT-qPCR and Western blotting analyses and found that BmNPV invasion causes complex protein alterations in the larval midgut, and that these changes are related to cytoskeleton, immune response, apoptosis, ubiquitination, translation, ion transport, endocytosis and endopeptidase activity. Five important differentially expression proteins were validation by independent approaches. These finding will help address the molecular mechanisms of silkworm resistance to BmNPV and provide a molecular target for resisting BmNPV.