Degradation of rRNA in BM-N cells from the silkworm Bombyx mori during abortive infection with heterologous nucleopolyhedroviruses

The DEAD/H-box helicase DHX9 contributes to suppression of Bombyx mori nucleopolyhedrovirus propagation in B. mori cells

Antiviral immune responses are mainly triggered through the recognition of virus-derived nucleic acids by host-specific pattern recognition receptors (PRRs). Here, we identified and characterized homologs of human PRRs for virus-derived DNA in Bombyx mori upon infection with a nucleopolyhedrovirus (NPV), a member of the family Baculoviridae. We found that progeny virus production of B. mori NPV was promoted in B. mori cells silenced with B. mori homolog of DEAD/H box polypeptide 9 gene (Bm-DHX9), but not in cells silenced with the other examined genes. Silencing of Bm-DHX9 expression has no effect on apoptosis induction, one of the major antiviral responses in B. mori cells. We also showed that Bm-DHX9 has the ability to bind DNA containing unmethylated C-phosphate-G-motif, which are characteristic of microbial pathogens and contained in the NPV genome with high frequency. Our findings suggest that Bm-DHX9 has the potential for sensing NPV-derived DNA to induce antiviral immune responses.

NISES-AnPe-428 cell line derived from the Chinese oak silkworm Antheraea pernyi is permissive for multiple nucleopolyhedrovirus species from insects of four different families

The cell line NISES-AnPe-428 (AnPe), derived from the Chinese oak silkworm Antheraea pernyi, was characterized for its permissiveness and productivity for six different nucleopolyhedrovirus (NPV) species. These NPVs included homologous Antheraea pernyi NPV (AnpeNPV) and heterologous Autographa californica multiple NPV (AcMNPV), Bombyx mori NPV (BmNPV), Hyphantria cunea MNPV (HycuMNPV), Spodoptera exigua MNPV (SeMNPV), and Lymantria dispar MNPV (LdMNPV), representing viruses that had been isolated from insect species belonging to five different families (Saturniidae, Noctuidae, Bombycidae, Arctiidae, and Lymantriidae). We found that AnPe cells supported productive replication of AnpeNPV, AcMNPV, BmNPV, HycuMNPV, and SeMNPV to varying degrees. Upon infection with SeMNPV, a subset of AnPe cell population in the culture underwent apoptosis, while remaining cells produced limited amounts of progeny virions and polyhedra. AnPe cells were refractory to LdMNPV infection and failed to support replication of viral DNA, indicating that viral replication was restricted at or prior to the step of viral DNA replication. These results indicated that AnPe cells have the potential to provide excellent systems for studying the molecular mechanisms underlying cellular permissiveness for NPV replication and host-range determination of NPVs.

How Tupanvirus Degrades the Ribosomal RNA of Its Amoebal Host? The Ribonuclease T2 Track

Tupanviruses are giant viruses recently discovered in Brazil from extreme environments: Tupanvirus soda lake (TPV-SL) and Tupanvirus deep ocean (TPV-DO). Unexpected features in Tupanviruses is the cytotoxic effect observed during infection, where the virus degrades the ribosomal RNA (rRNA) of its amoebal host. Interestingly, only TPV-SL causes this rRNA shutdown. We performed a genomic comparison of the two strains to determine potential modifications explaining the absence of rRNA degradation by TPV-DO. Whole genome comparisons were performed as well as more in-depth analysis at the gene level. We also calculated selective pressure on the orthologous genes between the two viruses. Our computational and evolutionary investigations revealed a potential target: a ribonuclease T2. These enzymes are known to be involved in cellular RNA catabolism such as in lysosomal degradation of rRNA. Our results suggest a functional ribonuclease localized in acid compartment closely related to ribonuclease T2 from eukaryotes. Silencing of the RNAse T2 gene of TPV-SL abolished its rRNA shutdown ability thereby correlating in silico assumption to the experimental evidence. In conclusion, all our results pointed to RNAse T2 as a target for explaining the difference for rRNA degradation ability between both strains.

Identification of the minimal AcMNPV P143 protein region responsible for triggering apoptosis and rRNA degradation of Bombyx mori cells

Bombyx mori cells induce antiviral responses including global protein synthesis shutdown, rRNA degradation, and apoptosis upon infection with Autographa californica multiple nucleopolyhedrovirus (AcMNPV). We previously demonstrated that five and six amino acid residues located at positions between 514 and 599 of AcMNPV P143 (Ac-P143) protein are important for induction of apoptosis and rRNA degradation, respectively. However, it remains unexplored whether other residues of Ac-P143 protein also participate in antiviral immune responses. Here, we conducted transient expression analysis using a number of Ac-P143 protein deletion and truncation mutants and found that some of the N-terminal 413 residues (amino acids 1-413), besides previously identified residues between amino acids 514 and 599, are indispensable, whereas C-terminal 622 residues (amino acids 600-1221) are dispensable, for Ac-P143 protein to induce apoptosis or rRNA degradation. In addition, we found that the N-terminal 413 sequence (amino acids 1-413) of Ac-P143 protein can be substituted with corresponding BmNPV P143 (Bm-P143) protein sequence. Further analysis demonstrated that mutant Ac-P143 protein consisting of 275 residues (amino acids 325-599), but not 274 residues (amino acids 326-599) lacking glutamine residue at position 325 (Q325), is sufficient for triggering apoptosis and rRNA degradation of B. mori cells. These 275 residues are located outside the region of DNA helicase motifs of Ac-P143 protein, indicating that induction of apoptosis or rRNA degradation occurs independently of viral DNA replication-related function of the Ac-P143 protein. Moreover, Ac-P143(325-599/Q325A) and Ac-P143(1-599/Q325A) proteins harboring Q325A substitution retain the ability to induce apoptosis and rRNA degradation in B. mori cells. These findings suggest that the Ac-P143 protein needs minimal sequence length starting from the Q325 residue that contains a specific effector domain to induce apoptosis and rRNA degradation.

The Autographa californica Multiple Nucleopolyhedrovirus ac51 Gene Is Required for Efficient Nuclear Egress of Nucleocapsids and Is Essential for In Vivo Virulence

Alphabaculoviruses are lepidopteran-specific nucleopolyhedroviruses that replicate within the nucleus; however, the anterograde transport of the nucleocapsids of these viruses, which is an obligatory step for progeny virion production, is not well understood. In the present study, a unique Alphabaculovirus gene with unknown function, namely, the Autographa californica multiple nucleopolyhedrovirus (AcMNPV) ac51 gene, was found to be required for efficient nuclear egress of AcMNPV nucleocapsids. Our results indicate that ac51 is a late gene, and Ac51 protein was detectable from 24 to 72 h postinfection using an antibody raised against Ac51. Ac51 is distributed in both the cytoplasm and nuclei of infected cells. Upon ac51 deletion, budded virion (BV) production by 96 h posttransfection was reduced by approximately 1,000-fold compared with that of wild-type AcMNPV. Neither viral DNA synthesis nor viral gene expression was affected. Ac51 was demonstrated to be a nucleocapsid protein of BVs, and ac51 deletion did not interrupt nucleocapsid assembly and occlusion-derived virion (ODV) formation. However, BV production in the supernatants of transfected cells during a viral life cycle was substantially decreased when ac51 was deleted. Further analysis showed that, compared with wild-type AcMNPV, ac51 deletion decreased nucleocapsid egress, while the numbers of nucleocapsids in the nuclei were comparable. Deletion of ac51 also eliminated the virulence of AcMNPV in vivo Taken together, our results support the conclusion that ac51 plays an important role in the nuclear egress of nucleocapsids during BV formation and is essential for the in vivo virulence of AcMNPV.

Antiviral immune responses of Bombyx mori cells during abortive infection with Autographa californica multiple nucleopolyhedrovirus

Lepidopteran cells rely on multiple antiviral responses to defend against baculovirus infections, including apoptosis, global protein synthesis shutdown, and rRNA degradation. Here, we characterized apoptosis and rRNA degradation in Autographa californica multiple nucleopolyhedrovirus (AcMNPV)-infected Bombyx mori cells, a system resulting in abortive infection, in relation to viral DNA replication and viral late gene expression. RNAi-mediated silencing of viral DNA replication-related genes prevented apoptosis, but not rRNA degradation, in B. mori cells infected with p35-deficient AcMNPV. Additionally, AcMNPV, but not B. mori nucleopolyhedrovirus (BmNPV), drastically reduced B. mori cellular iap1 transcript levels and p35-deficient AcMNPV induced more prominent apoptosis than did p35-deficient BmNPV. These results, together with previous results that global protein synthesis shutdown follows viral DNA replication, demonstrate that rRNA degradation is the primary antiviral response that abolishes productive AcMNPV infection of B. mori cells. Our results also demonstrate that B. mori cells induce apoptosis to a different extent depending on NPV species.

Midgut-based resistance to oral infection by a nucleopolyhedrovirus in the laboratory-selected strain of the smaller tea tortrix, Adoxophyes honmai (Lepidoptera: Tortricidae)

A strain of Adoxophyes honmai resistant to Adoxophyes honmai nucleopolyhedrovirus (AdhoNPV) was established from a field-collected colony by repeated selection. Fifth-instar larvae of this resistant strain (R-strain) had over 66 666-fold greater resistance in terms of 50 % lethal concentration values to oral infection of AdhoNPV than non-selected strain larvae (susceptible for AdhoNPV; S2-strain). In this study, the mechanism of resistance to AdhoNPV was determined in R-strain larvae. An assessment of viral genome replication in AdhoNPV-infected S2- and R-strain larvae by quantitative PCR showed no viral genome replication occurring in R-strain larvae. Transcription of AdhoNPV ie-1, vp39 and polyhedrin genes was also not detected in R-strain midgut cells. Besides, a fluorescent brightener had no effect on AdhoNPV infection in either S2- or R-strain. However, binding and fusion of occlusion-derived virus with R-strain were significantly lower than those of S2-strain. These findings suggest that R-strain Adoxophyeshonmai larvae possess a midgut-based resistance to oral infection by AdhoNPV in which midgut epithelial cells are infected less efficiently.

A Role for the Anti-Viral Host Defense Mechanism in the Phylogenetic Divergence in Baculovirus Evolution

Although phylogenic analysis often suggests co-evolutionary relationships between viruses and host organisms, few examples have been reported at the microevolutionary level. Here, we show a possible example in which a species-specific anti-viral response may drive phylogenic divergence in insect virus evolution. Two baculoviruses, Autographa californica multiple nucleopolyhedrovirus (AcMNPV) and Bombyx mori nucleopolyhedrovirus (BmNPV), have a high degree of DNA sequence similarity, but exhibit non-overlapping host specificity. In our study of their host-range determination, we found that BmNPV replication in B. mori cells was prevented by AcMNPV-P143 (AcP143), but not BmNPV-P143 (BmP143) or a hybrid P143 protein from a host-range expanded phenotype. This suggests that AcMNPV resistance in B. mori cells depends on AcP143 recognition and that BmNPV uses BmP143 to escapes this recognition. Based on these data, we propose an insect-baculovirus co-evolution scenario in which an ancestor of silkworms exploited an AcMNPV-resistant mechanism; AcMNPV counteracted this resistance via P143 mutations, resulting in the birth of BmNPV.

Identification of amino acid residues of AcMNPV P143 protein involved in rRNA degradation and restricted viral replication in BM-N cells from the silkworm Bombyx mori

We previously demonstrated that rRNA undergoes rapid and extensive degradation in Bombyx mori BM-N cells upon infection with AcMNPV, which is triggered by AcMNPV P143 (Ac-P143) protein. Here, we showed that six amino acid residues of Ac-P143 protein, distributing between positions 514 and 599, are involved in rRNA degradation in BM-N cells. The six residues are highly conserved among P143 proteins from AcMNPV, HycuMNPV, SeMNPV and SpltMNPV, which trigger rRNA degradation in BM-N cells upon infection, but are only partially conserved in Bm-P143 protein, which does not induce rRNA degradation in BM-N cells. We also demonstrated that substitution of only two selected residues (N565S/L578F) of Bm-P143 protein with the corresponding Ac-P143 protein residues generates a mutant Bm-P143 protein that is capable of triggering rRNA degradation in BM-N cells. These results indicate that BmNPV evolved a unique P143 protein to evade the antiviral response and allow replication in B. mori cells.