Four amino acids of an insect densovirus capsid determine midgut tropism and virulence

Cryo-EM-based discovery of a pathogenic parvovirus causing epidemic mortality by black wasting disease in farmed beetles

We use cryoelectron microscopy (cryo-EM) as a sequence- and culture-independent diagnostic tool to identify the etiological agent of an agricultural pandemic. For the past 4 years, American insect-rearing facilities have experienced a distinctive larval pathology and colony collapse of farmed Zophobas morio (superworm). By means of cryo-EM, we discovered the causative agent: a densovirus that we named Zophobas morio black wasting virus (ZmBWV). We confirmed the etiology of disease by fulfilling Koch's postulates and characterizing strains from across the United States. ZmBWV is a member of the family Parvoviridae with a 5,542 nt genome, and we describe intersubunit interactions explaining its expanded internal volume relative to human parvoviruses. Cryo-EM structures at resolutions up to 2.1 Å revealed single-strand DNA (ssDNA) ordering at the capsid inner surface pinned by base-binding pockets in the capsid inner surface. Also, we demonstrated the prophylactic potential of non-pathogenic strains to provide cross-protection in vivo.

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.

Capsid Proteins Are Necessary for Replication of a Parvovirus

Despite tight genetic compression, viral genomes are often organized into functional gene clusters, a modular structure that might favor their evolvability. This has greatly facilitated biotechnological developments such as the recombinant adeno-associated virus (AAV) systems for gene therapy. Following this lead, we endeavored to engineer the related insect parvovirus Junonia coenia densovirus (JcDV) to create addressable vectors for insect pest biocontrol. To enable safer manipulation of capsid mutants, we translocated the nonstructural (ns) gene cluster outside the viral genome. To our dismay, this yielded a virtually nonreplicable clone. We linked the replication defect to an unexpected modularity breach, as ns translocation truncated the overlapping 3' untranslated region (UTR) of the capsid transcript (vp). We found that the native vp 3' UTR is necessary for high-level VP production but that decreased expression does not adversely impact the expression of NS proteins, which are known replication effectors. As nonsense vp mutations recapitulate the replication defect, VP proteins appear to be directly implicated in the replication process. Our findings suggest intricate replication-encapsidation couplings that favor the maintenance of genetic integrity. We discuss possible connections with an intriguing cis-packaging phenomenon previously observed in parvoviruses whereby capsids preferentially package the genome from which they were expressed. IMPORTANCE Densoviruses could be used as biological control agents to manage insect pests. Such applications require an in-depth biological understanding and associated molecular tools. However, the genomes of these viruses remain difficult to manipulate due to poorly tractable secondary structures at their extremities. We devised a construction strategy that enables precise and efficient molecular modifications. Using this approach, we endeavored to create a split clone of Junonia coenia densovirus (JcDV) that can be used to safely study the impact of capsid mutations on host specificity. Our original construct proved to be nonfunctional. Fixing this defect led us to uncover that capsid proteins and their correct expression are essential for continued rolling-hairpin replication. This points to an intriguing link between replication and packaging, which might be shared with related viruses. This serendipitous discovery illustrates the power of synthetic biology approaches to advance our knowledge of biological systems.

Reorganizing the family Parvoviridae: a revised taxonomy independent of the canonical approach based on host association

Parvoviridae, a diverse family of small single-stranded DNA viruses was established in 1975. It was divided into two subfamilies, Parvovirinae and Densovirinae, in 1993 to accommodate parvoviruses that infect vertebrate and invertebrate animals, respectively. This relatively straightforward segregation, using host association as the prime criterion for subfamily-level classification, has recently been challenged by the discovery of divergent, vertebrate-infecting parvoviruses, dubbed "chapparvoviruses", which have proven to be more closely related to viruses in certain Densovirinae genera than to members of the Parvovirinae. Viruses belonging to these genera, namely Brevi-, Hepan- and Penstyldensovirus, are responsible for the unmatched heterogeneity of the subfamily Densovirinae when compared to the Parvovirinae in matters of genome organization, protein sequence homology, and phylogeny. Another genus of Densovirinae, Ambidensovirus, has challenged traditional parvovirus classification, as it includes all newly discovered densoviruses with an ambisense genome organization, which introduces genus-level paraphyly. Lastly, current taxon definition and virus inclusion criteria have significantly limited the classification of certain long-discovered parvoviruses and impedes the classification of some potential family members discovered using high-throughput sequencing methods. Here, we present a new and updated system for parvovirus classification, which includes the introduction of a third subfamily, Hamaparvovirinae, resolves the paraphyly within genus Ambidensovirus, and introduces new genera and species into the subfamily Parvovirinae. These proposals were accepted by the ICTV in 2020 March.

Transcytosis of Junonia coenia densovirus VP4 across the gut epithelium of Spodoptera frugiperda (Lepidoptera: Noctuidae)

The Junonia coenia densovirus rapidly traverses the gut epithelium of the host lepidopteran without replicating in the gut cells. The ability of this virus to transcytose across the gut epithelium is of interest for the potential use of virus structural proteins as delivery vehicles for insecticidal peptides that act within the insect hemocoel, rather than in the gut. In this study, we used fall armyworm, Spodoptera frugiperda to examine the binding of the virus to brush border membrane vesicle proteins by two-dimensional ligand blot analysis. We also assessed the rate of flux of the primary viral structural protein, VP4 fused to eGFP with a proline-rich linker (VP4-P-eGFP) through the gut epithelium ex vivo in an Ussing chamber. The mechanisms involved with transcytosis of VP4-P-eGFP were assessed by use of inhibitors. Bovine serum albumin (BSA) and eGFP were used as positive and negative control proteins, respectively. In contrast to BSA, which binds to multiple proteins on the brush border membrane, VP4-P-eGFP binding was specific to a protein of high molecular mass. Protein flux was significantly higher for VP4-P-eGFP after 2 h than for albumin or eGFP, with rapid transcytosis of VP4-P-eGFP within the first 30 min. In contrast to BSA which transcytosed following clathrin-mediated endocytosis, the movement of VP4-P-eGFP was vesicle-mediated but clathrin-independent. The specificity of binding combined with the efficiency of transport across the gut epithelium suggest that VP4 will provide a useful carrier for insecticidal peptides active within the hemocoel of key lepidopteran pests including S. frugiperda.

Evolution and phylogeography of Culex pipiens densovirus

Viruses of the Parvoviridae family infect a wide range of animals including vertebrates and invertebrates. So far, our understanding of parvovirus diversity is biased towards medically or economically important viruses mainly infecting vertebrate hosts, while invertebrate infecting parvoviruses—namely densoviruses—have been largely neglected. Here, we investigated the prevalence and the evolution of the only mosquito-infecting ambidensovirus, Culex pipiens densovirus (CpDV), from laboratory mosquito lines and natural populations collected worldwide. CpDV diversity generally grouped in two clades, here named CpDV-1 and -2. The incongruence of the different gene trees for some samples suggested the possibility of recombination events between strains from different clades. We further investigated the role of selection on the evolution of CpDV genome and detected many individual sites under purifying selection both in non-structural and structural genes. However, some sites in structural genes were under diversifying selection, especially during the divergence of CpDV-1 and -2 clades. These substitutions between CpDV-1 and -2 clades were mostly located in the capsid protein encoding region and might cause changes in host specificity or pathogenicity of CpDV strains from the two clades. However, additional functional and experimental studies are necessary to fully understand the protein conformations and the resulting phenotype of these substitutions between clades of CpDV.

Interaction of a Densovirus with Glycans of the Peritrophic Matrix Mediates Oral Infection of the Lepidopteran Pest Spodoptera frugiperda

The success of oral infection by viruses depends on their capacity to overcome the gut epithelial barrier of their host to crossing over apical, mucous extracellular matrices. As orally transmitted viruses, densoviruses, are also challenged by the complexity of the insect gut barriers, more specifically by the chitinous peritrophic matrix, that lines and protects the midgut epithelium; how capsids stick to and cross these barriers to reach their final cell destination where replication goes has been poorly studied in insects. Here, we analyzed the early interaction of the Junonia coenia densovirus (JcDV) with the midgut barriers of caterpillars from the pest Spodoptera frugiperda. Using combination of imaging, biochemical, proteomic and transcriptomic analyses, we examined in vitro, ex vivo and in vivo the early interaction of the capsids with the peritrophic matrix and the consequence of early oral infection on the overall gut function. We show that the JcDV particle rapidly adheres to the peritrophic matrix through interaction with different glycans including chitin and glycoproteins, and that these interactions are necessary for oral infection. Proteomic analyses of JcDV binding proteins of the peritrophic matrix revealed mucins and non-mucins proteins including enzymes already known to act as receptors for several insect pathogens. In addition, we show that JcDV early infection results in an arrest of N-Acetylglucosamine secretion and a disruption in the integrity of the peritrophic matrix, which may help viral particles to pass through. Finally, JcDV early infection induces changes in midgut genes expression favoring an increased metabolism including an increased translational activity. These dysregulations probably participate to the overall dysfunction of the gut barrier in the early steps of viral pathogenesis. A better understanding of early steps of densovirus infection process is crucial to build biocontrol strategies against major insect pests.

Viral Delivery of dsRNA for Control of Insect Agricultural Pests and Vectors of Human Disease: Prospects and Challenges

RNAi is applied as a new and safe method for pest control in agriculture but efficiency and specificity of delivery of dsRNA trigger remains a critical issue. Various agents have been proposed to augment dsRNA delivery, such as engineered micro-organisms and synthetic nanoparticles, but the use of viruses has received relatively little attention. Here we present a critical view of the potential of the use of recombinant viruses for efficient and specific delivery of dsRNA. First of all, it requires the availability of plasmid-based reverse genetics systems for virus production, of which an overview is presented. For RNA viruses, their application seems to be straightforward since dsRNA is produced as an intermediate molecule during viral replication, but DNA viruses also have potential through the production of RNA hairpins after transcription. However, application of recombinant virus for dsRNA delivery may not be straightforward in many cases, since viruses can encode RNAi suppressors, and virus-induced silencing effects can be determined by the properties of the encoded RNAi suppressor. An alternative is virus-like particles that retain the efficiency and specificity determinants of natural virions but have encapsidated non-replicating RNA. Finally, the use of viruses raises important safety issues which need to be addressed before application can proceed.

Densovirus crosses the insect midgut by transcytosis and disturbs the epithelial barrier function

Densoviruses are parvoviruses that can be lethal for insects of different orders at larval stages. Although the horizontal transmission mechanisms are poorly known, densoviral pathogenesis usually starts with the ingestion of contaminated food by the host. Depending on the virus, this leads to replication restricted to the midgut or excluding it. In both cases the success of infection depends on the virus capacity to enter the intestinal epithelium. Using the Junonia coenia densovirus (JcDNV) as the prototype virus and the lepidopteran host Spodoptera frugiperda as an interaction model, we focused on the early mechanisms of infection during which JcDNV crosses the intestinal epithelium to reach and replicate in underlying target tissues. We studied the kinetics of interaction of JcDNV with the midgut epithelium and the transport mechanisms involved. Using several approaches, in vivo, ex vivo, and in vitro, at molecular and cellular levels, we show that JcDNV is specifically internalized by endocytosis in absorptive cells and then crosses the epithelium by transcytosis. As a consequence, viral entry disturbs the midgut function. Finally, we showed that four mutations on the capsid of JcDNV affect specific recognition by the epithelial cells but not their binding.

Periplaneta fuliginosa densovirus nonstructural protein NS1 contains an endonuclease activity that is regulated by its phosphorylation

Periplaneta fuliginosa densovirus (PfDNV) is a single-stranded DNA virus, belonging to Densovirinae subfamily, Parvoviridae family. Parvovirus nonstructural protein 1 (NS1) contains various activities required for parvoviral DNA replication, like endonuclease, helicase and ATPase, which are regulated by serine/threonine phosphorylation. However, for PfDNV, NS1 endonuclease activity has not been determined. Moreover, for densoviruses, whether NS1 is phosphorylated, and if so, phosphorylation pattern and impact on NS1 activities have not been investigated. Here, we demonstrated that PfDNV NS1 possesses endonuclease activity, covalently attaches to 5'-end of nicking site, and includes an active-site tyrosine (Y178). Moreover, using different phosphatases, we uncovered that both serine/threonine and tyrosine phosphorylations are critical for NS1 endonuclease and helicase activities. Further mass-spec and mutational analyses revealed that Y345 is phosphorylated and functions as a critical regulatory site for NS1 activities. This study should foster our understanding of NS1 activities and regulations in PfDNV and other densoviruses.