Transcriptional response of immune-related genes after endogenous expression of VP1 and exogenous exposure to VP1-based VLPs and CPV virions in lepidopteran cell lines

Plant and insect virus-like particles: emerging nanoparticles for agricultural pest management

Virus-like particles (VLPs) represent a biodegradable, biocompatible nanomaterial made from viral coat proteins that can improve the delivery of antigens, drugs, nucleic acids, and other substances, with most applications in human and veterinary medicine. Regarding agricultural viruses, many insect and plant virus coat proteins have been shown to assemble into VLPs accurately. In addition, some plant virus-based VLPs have been used in medical studies. However, to our knowledge, the potential application of plant/insect virus-based VLPs in agriculture remains largely underexplored. This review focuses on why and how to engineer coat proteins of plant/insect viruses as functionalized VLPs, and how to exploit VLPs in agricultural pest control. The first part of the review describes four different engineering strategies for loading cargo at the inner or the outer surface of VLPs depending on the type of cargo and purpose. Second, the literature on plant and insect viruses the coat proteins of which have been confirmed to self-assemble into VLPs is reviewed. These VLPs are good candidates for developing VLP-based agricultural pest control strategies. Lastly, the concepts of plant/insect virus-based VLPs for delivering insecticidal and antiviral components (e.g., double-stranded RNA, peptides, and chemicals) are discussed, which provides future prospects of VLP application in agricultural pest control. In addition, some concerns are raised about VLP production on a large scale and the short-term resistance of hosts to VLP uptake. Overall, this review is expected to stimulate interest and research exploring plant/insect virus-based VLP applications in agricultural pest management. © 2023 Society of Chemical Industry.

Peptidoglycan Recognition Protein S5 of Bombyx mori Facilitates the Proliferation of Bombyx mori Cypovirus 1

Bombyx mori cypovirus 1 (BmCPV1), a primary pathogen of the silkworm, is a typical dsRNA virus belonging to the Reoviridae family. In this study, a total of 2520 differentially expressed genes (DEGs) were identified by RNA-seq analysis of the silkworm midgut after BmCPV1 infection and Gene Ontology (GO) functional annotation showed that the DEGs predominantly functioned in binding (molecular function), cell (cellular component), and cellular processes (biological process). Additionally, the Kyoto Encyclopedia of Genes and Genomes (KEGG) functional annotation revealed that the DEGs were mainly distributed in global and overview metabolism maps, translation, and signal transduction. Among the identified DEGs, BmPGRP-S5 belongs to the peptidoglycan recognition protein (PGRP) family. Previous studies have revealed that PGRPs were involved in the interactions between silkworm and BmCPV1. Here, we explored the effect of BmPGRP-S5 on BmCPV1 replication and demonstrated that BmPGRP-S5 promotes the proliferation of BmCPV1 in BmN cells through overexpression or knockdown experiments. Knocking down of BmPGRP-S5 in silkworm larvae similarly promoted the proliferation of BmCPV1. Through experimental validation, we therefore determined that BmPGRP-S5 acts as a proviral host factor for BmCPV1 infection. This study clarifies the proliferation mechanism of BmCPV1 and provides new insights into the functional role of BmPGRP-S5.

Analysis of luciferase dsRNA production during baculovirus infection of Hi5 cells: RNA hairpins expressed by very late promoters do not trigger gene silencing

The baculovirus expression vector system (BEVS) has become an important platform for the expression of recombinant proteins and is especially useful for the production of large protein complexes such as virus-like particles (VLPs). An important application for VLPs is their use as vehicles for targeted delivery of drugs or toxins which requires the development of methods for efficient loading with the intended cargo. Our research intends to employ the BEVS for the production of VLPs for the delivery of insecticidal dsRNA molecules to targeted insect pests (as "dsRNA-VLPs"). A convenient strategy would be the co-expression of long dsRNAs with viral capsid proteins and their simultaneous encapsulation during VLP assembly but the capacity of the BEVS for the production of long dsRNA has not been assessed so far. In this study, the efficiency of production of long RNA hairpins targeting the luciferase gene ("dsLuc") by the polyhedrin promoter during baculovirus infection was evaluated. However, RNAi reporter assays could not detect significant amounts of dsLuc in Hi5 cells infected with recombinant baculovirus, even in the presence of co-expressed dsRNA-binding protein B2-GFP or the employment of the MS2-MCP system. Nevertheless, dot blot analyses using anti-dsRNA antibody revealed that baculovirus-mediated expression of B2-GFP resulted in significant increases in dsRNA levels in infected cells that may correspond to hybridized complementary viral transcripts. Using B2-GFP as a genetically encoded sensor, dsRNA foci were detected in the nuclei that partially co-localized with DAPI staining, consistent with their localization at the virogenic stroma. Co-localization experiments with the baculovirus proteins vp39, Ac93, ODV-E25 and gp64 indicated limited overlap between B2-GFP and the ring zone compartment where assembly of nucleocapsids and virions occurs. Stability experiments showed that exogenous dsRNA is resistant to degradation in extracts of non-infected and infected Hi5 cells and it is proposed that strong unwinding activity at the virogenic stroma in the infected nuclei may neutralize the annealing of complementary RNA strands and block the production of long dsRNAs. Because the strong stability of exogenous dsRNA, transfection can be explored as an alternative method for delivery of cargo for dsRNA-VLPs during their assembly in baculovirus-infected Hi5 cells.

Characterization of virus-like particles assembled by co-expression of BmCPV capsid shell protein and large protrusion protein

Bombyx mori cytoplasmic polyhedrosis virus (BmCPV) is a typical single-layer capsid dsRNA virus belonging to the Reoviridae family of the Cypovirus genus. Previous studies have shown that the BmCPV major capsid shell protein (CSP) has the ability to self-assemble into virus-like particles (VLPs), and cryo-electron microscopy of the BmCPV virions has revealed a tight mutual binding region between CSP and another capsid protein known as the Large Protrusion Protein (LPP), which further stabilizes the capsid shell. In this study, the multi-gene baculovirus expression system, Ac-MultiBac, was used to produce both solely CSP-based and CSP-LPP co-assembled VLPs. Transmission electron microscopy (TEM) results showed that addition of LPP did not affect the assembly of VLPs resulting in almost identical structure in both cases. However, ex vivo administration of VLPs to silkworm midgut tissue showed that CSP-based VLPs did not induce a significant transcriptional response in the innate immunity and RNAi gene cascades, compared to the co-assembled CSP-LPP based VLPs and the natural BmCPV virions isolated from polyhedra. The experimental results indicate that CSP and LPP attach tightly ("Plug and Display" model with CSP acting as "catcher" and LPP as "tag") to form VLPs that have a structure similar to that of the native CPV virions. Moreover, our results showed that the formation of VLPs with the two BmCPV capsid proteins is feasible, which can form the basis for the production of BmCPV-based VLPs as a new type of biological material to display exogenous proteins.

Mechanisms of Cell Entry by dsRNA Viruses: Insights for Efficient Delivery of dsRNA and Tools for Improved RNAi-Based Pest Control

While RNAi is often heralded as a promising new strategy for insect pest control, a major obstacle that still remains is the efficient delivery of dsRNA molecules within the cells of the targeted insects. However, it seems overlooked that dsRNA viruses already have developed efficient strategies for transport of dsRNA molecules across tissue barriers and cellular membranes. Besides protecting their dsRNA genomes in a protective shell, dsRNA viruses also display outer capsid layers that incorporate sophisticated mechanisms to disrupt the plasma membrane layer and to translocate core particles (with linear dsRNA genome fragments) within the cytoplasm. Because of the perceived efficiency of the translocation mechanism, it is well worth analyzing in detail the molecular processes that are used to achieve this feat. In this review, the mechanism of cell entry by dsRNA viruses belonging to the Reoviridae family is discussed in detail. Because of the large amount of progress in mammalian versus insect models, the mechanism of infections of reoviruses in mammals (orthoreoviruses, rotaviruses, orbiviruses) will be treated as a point of reference against which infections of reoviruses in insects (orbiviruses in midges, plant viruses in hemipterans, insect-specific cypoviruses in lepidopterans) will be compared. The goal of this discussion is to uncover the basic principles by which dsRNA viruses cross tissue barriers and translocate their cargo to the cellular cytoplasm; such knowledge subsequently can be incorporated into the design of dsRNA virus-based viral-like particles for optimal delivery of RNAi triggers in targeted insect pests.

Effect of mutations in capsid shell protein on the assembly of BmCPV virus-like particles

Bombyx mori cytoplasmic polyhedrosis virus (BmCPV) is a typical single-layer capsid dsRNA virus belonging to the genus Cypovirus in the family Reoviridae. The results of cryo-electron microscopy showed that the BmCPV capsid consists of 60 asymmetric units, and each asymmetric unit contains one turret protein (TP), two large protrusion proteins (LPP) and two capsid shell proteins (CSP). CSP has the ability to self-assemble into virus-like particles (VLPs), and the small protrusion domain (SPD) in CSP may play an essential role in the assembly of viral capsids. In this study, three critical amino acid sites, D828, S829 and V945, in the SPD were efficiently mutated (point mutation) based on the principle of PCR circular mutagenesis. Moreover, a multi-gene expression system, Ac-MultiBac baculovirus, was used to produce eight different recombinant VLPs in vitro. Transmission electron microscopy showed that the single site and double site mutations had little effect on the efficiency and morphology of the assembly of VLPs. Still, the simultaneous mutation of the three sites had a significant impact. The experimental results demonstrate that the SPD of CSP plays an essential role in assembly of the viral capsid, which lays the foundation for further analysis of the molecular and structural mechanism of BmCPV capsid assembly.

Alkaline phosphatase can promote the replication of Bombyx mori cypovirus 1 by interaction with its turret protein

Bombyx mori cypovirus 1 (BmCPV1) is a member of the Reoviridae family which is characterized by its single-layered capsid. Similar with other turreted viruses in the Reoviridae, transcription of BmCPV1 occurs inside the capsid, and the nascent mRNA is released to the turret which consists of five turret proteins (TPs) and located at the 5-fold axis of the outer capsid, then the capping enzyme TP will guanylate and methylate the nascent viral mRNA to produce a matured mRNA. However, during these processes, how the BmCPV1 draws other cellular proteins to facilitate its replication is still lesser-known. Here we used an ELISA to investigate the interaction between ALP and BmCPV1. A co-immunoprecipitation technique was employed to detect the interaction of ALP with the Methylase domain of TP. We further studied whether ALP affects the replication of BmCPV1 inside the cell, results show that reducing the expression of ALP through RNAi reduced the transcription level of the BmCPV1 VP1 gene, which was increased by overexpression of ALP. In summary, our data demonstrate an interaction between ALP and BmCPV1 and that ALP promoted the replication of BmCPV1, and support our hypothesis of the ALP is an RTPase to facilitate the capping process of BmCPV1.

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.