Genome organization of the densovirus from Bombyx mori (BmDNV-1) and enzyme activity of its capsid

Effect of the Functional VP1 Unique Region of Human Parvovirus B19 in Causing Skin Fibrosis of Systemic Sclerosis

Human parvovirus B19 (B19V) is a single-stranded non-enveloped DNA virus of the family Parvoviridae that has been associated with various autoimmune disorders. Systemic sclerosis (SSc) is an autoimmune connective tissue disorder with high mortality and has been linked to B19V infection. However, the precise mechanism underlying the B19V contribution to the development of SSc remains uncertain. This study investigated the impacts of the functional B19V-VP1 unique region (VP1u) in macrophages and bleomycin (BLE)-induced SSc mice. Cell experimental data showed that significantly decreased viability and migration of both B19V-VP1u-treated U937 and THP-1 macrophages are detected in the presence of celastrol. Significantly increased MMP9 activity and elevated NF-kB, MMP9, IL-6, TNF-α, and IL-1β expressions were detected in both B19V-VP1u-treated U937 and THP-1 macrophages. Conversely, celastrol revealed an inhibitory effect on these molecules. Notably, celastrol intervened in this pathogenic process by suppressing the sPLA2 activity of B19V-VP1u and subsequently reducing the inflammatory response. Notably, the administration of B19V-VP1u exacerbated BLE-induced skin fibrosis in mice, with augmented expressions of TGF-β, IL-6, IL-17A, IL-18, and TNF-α, ultimately leading to α-SMA and collagen I deposits in the dermal regions of BLE-induced SSc mice. Altogether, this study sheds light on parvovirus B19 VP1u linked to scleroderma and aggravated dermal fibrosis.

Outbreak of densovirus with high mortality in a commercial mealworm (Tenebrio molitor) farm: A molecular, bright-field, and electron microscopic characterization

Mealworms are one of the most economically important insects in large-scale production for human and animal nutrition. Densoviruses are highly pathogenic for invertebrates and exhibit an extraordinary level of diversity which rivals that of their hosts. Molecular, clinical, histological, and electron microscopic characterization of novel densovirus infections is of utmost economic and ecological importance. Here, we describe an outbreak of densovirus with high mortality in a commercial mealworm (Tenebrio molitor) farm. Clinical signs included inability to prehend food, asymmetric locomotion evolving to nonambulation, dehydration, dark discoloration, and death. Upon gross examination, infected mealworms displayed underdevelopment, dark discoloration, larvae body curvature, and organ/tissue softness. Histologically, there was massive epithelial cell death, and cytomegaly and karyomegaly with intranuclear inclusion (InI) bodies in the epidermis, pharynx, esophagus, rectum, tracheae, and tracheoles. Ultrastructurally, these InIs represented a densovirus replication and assembly complex composed of virus particles ranging from 23.79 to 26.99 nm in diameter, as detected on transmission electron microscopy. Whole-genome sequencing identified a 5579-nucleotide-long densovirus containing 5 open reading frames. A phylogenetic analysis of the mealworm densovirus showed it to be closely related to several bird- and bat-associated densoviruses, sharing 97% to 98% identity. Meanwhile, the nucleotide similarity to a mosquito, cockroach, and cricket densovirus was 55%, 52%, and 41%, respectively. As this is the first described whole-genome characterization of a mealworm densovirus, we propose the name Tenebrio molitor densovirus (TmDNV). In contrast to polytropic densoviruses, this TmDNV is epitheliotropic, primarily affecting cuticle-producing cells.

Resistance mechanism of Nid-1, a dominant non-susceptibility gene, against Bombyx mori densovirus 1 infection

Bombyx mori densovirus 1 (BmDV1) is a pathogen that causes flacherie disease in mulberry silkworms (B. mori). The absolute resistance (non-susceptibility) to BmDV1 of certain silkworm strains is determined independently by two genes, nsd-1 and Nid-1. Previously, we investigated the expression of viral transcript in virus-inoculated silkworms carrying different nsd-1 and Nid-1 genotypes, and observed that nsd-1 and Nid-1 expression blocked the early and late steps of BmDV1 infection, respectively. In addition, we found that nsd-1 encoded a Bombyx-specific mucin-like membrane protein only present on the surface of the midgut, where BmDV1 could infect. In this study, we dissected the resistance mechanism by Nid-1 against BmDV1 infection by investigating the sequential changes in the accumulation of viral DNA, transcripts, and proteins derived from BmDV1 in susceptible strain (pxj) and Nid-1-carrying resistant strain (No. 908) after inoculation with BmDV1. Genomic PCR results showed that the BmDV1 DNA was detected immediately after the infection in both strains but rapidly decreased in the Nid-1-carrying strain No. 908 compared with the susceptible strain pxj. RT-PCR results also showed that the BmDV1 transcripts of Nid-1-carrying strain No. 908 were rapidly decreased after the infection. Moreover, BmDV1-derived proteins were not detected in No. 908 throughout the infection. These results suggest that Nid-1 expression might inhibit the accumulation of viral DNA and transcripts. As Nid-1 has not been molecularly characterized, its identification will contribute to the elucidation of the interactions between the silkworm and BmDV1.

The Diversity of Parvovirus Telomeres

Parvoviridae are small viruses composed of a 4–6 kb linear single-stranded DNA protected by an icosahedral capsid. The viral genes coding non-structural (NS), capsid, and accessory proteins are flanked by intriguing sequences, namely the telomeres. Telomeres are essential for parvovirus genome replication, encapsidation, and integration. Similar (homotelomeric) or different (heterotelomeric) at the two ends, they all contain imperfect palindromes that fold into hairpin structures. Up to 550 nucleotides in length, they harbor a wide variety of motifs and structures known to be recognized by host cell factors. Our study aims to comprehensively analyze parvovirus ends to better understand the role of these particular sequences in the virus life cycle. Forty Parvoviridae terminal repeats (TR) were publicly available in databases. The folding and specific DNA secondary structures, such as G4 and triplex, were systematically analyzed. A principal component analysis was carried out from the prediction data to determine variables signing parvovirus groups. A special focus will be put on adeno-associated virus (AAV) inverted terminal repeats (ITR), a member of the genus Dependoparvovirus used as vectors for gene therapy. This chapter highlights the diversity of the Parvoviridae telomeres regarding shape and secondary structures, providing information that could be relevant for virus-host interactions studies.

Transgenic genome editing-derived antiviral therapy to nucleopolyhedrovirus infection in the industrial strain of the silkworm

The silkworm (Bombyx mori) is a domesticated and economically important insect. During the whole growth period, silkworm suffers various pathogen infection. Nearly 80% of silk cocoon crop losses are attributed to viral diseases. The circular double-stranded DNA virus Bombyx mori nuclepolyhedrovirus (BmNPV) is the major viral pathogen responsible for massive silkworm death and huge economic losses in the sericulture industry. Up to now, almost all the new strategies for developing immunity against BmNPV are in laboratory strains because of the lack of transgenic technology in industrial silkworm strains. We previously demonstrated that modification of BmNPV genome DNA with the antivirus transgenic CRISPR/Cas9 system effectively improved the resistance of laboratory silkworm strains to viral pathogens. The industrial strains are monovoltine or bivoltine. It is very difficult to break the diapause before microinjection for genetic transformation. Here, we show that the anti-BmNPV transgenic CRISPR/Cas9 system also works in the industrial silkworm strain Jingsong. In this study, we successfully broke diapause and obtained generation-skipping embryos and constructed two TG Jingsong lines. Both lines exhibited significantly enhanced immunity to BmNPV without significant changes in most of the commercially important traits. These results demonstrate that the construction of TG silkworm lines carrying a heritable immune defense system against BmNPV could be an effective strategy to enhance the resistance of industrial silkworm strains to the most devastating DNA virus. The research opened a window for genetic modification of the important strains from laboratory strains to industrial strains.

The VP1u of Human Parvovirus B19: A Multifunctional Capsid Protein with Biotechnological Applications

The viral protein 1 unique region (VP1u) of human parvovirus B19 (B19V) is a multifunctional capsid protein with essential roles in virus tropism, uptake, and subcellular trafficking. These functions reside on hidden protein domains, which become accessible upon interaction with cell membrane receptors. A receptor-binding domain (RBD) in VP1u is responsible for the specific targeting and uptake of the virus exclusively into cells of the erythroid lineage in the bone marrow. A phospholipase A₂ domain promotes the endosomal escape of the incoming virus. The VP1u is also the immunodominant region of the capsid as it is the target of neutralizing antibodies. For all these reasons, the VP1u has raised great interest in antiviral research and vaccinology. Besides the essential functions in B19V infection, the remarkable erythroid specificity of the VP1u makes it a unique erythroid cell surface biomarker. Moreover, the demonstrated capacity of the VP1u to deliver diverse cargo specifically to cells around the proerythroblast differentiation stage, including erythroleukemic cells, offers novel therapeutic opportunities for erythroid-specific drug delivery. In this review, we focus on the multifunctional role of the VP1u in B19V infection and explore its potential in diagnostics and erythroid-specific therapeutics.

Molecular biology and structure of a novel penaeid shrimp densovirus elucidate convergent parvoviral host capsid evolution

The giant tiger prawn (Penaeus monodon) is a decapod crustacean widely reared for human consumption. Currently, viruses of two distinct lineages of parvoviruses (PVs, family Parvoviridae; subfamily Hamaparvovirinae) infect penaeid shrimp. Here, a PV was isolated and cloned from Vietnamese P. monodon specimens, designated Penaeus monodon metallodensovirus (PmMDV). This is the first member of a third divergent lineage shown to infect penaeid decapods. PmMDV has a transcription strategy unique among invertebrate PVs, using extensive alternative splicing and incorporating transcription elements characteristic of vertebrate-infecting PVs. The PmMDV proteins have no significant sequence similarity with other PVs, except for an SF3 helicase domain in its nonstructural protein. Its capsid structure, determined by cryoelectron microscopy to 3-Å resolution, has a similar surface morphology to Penaeus stylirostris densovirus, despite the lack of significant capsid viral protein (VP) sequence similarity. Unlike other PVs, PmMDV folds its VP without incorporating a βA strand and displayed unique multimer interactions, including the incorporation of a Ca²⁺ cation, attaching the N termini under the icosahedral fivefold symmetry axis, and forming a basket-like pentamer helix bundle. While the PmMDV VP sequence lacks a canonical phospholipase A2 domain, the structure of an EDTA-treated capsid, determined to 2.8-Å resolution, suggests an alternative membrane-penetrating cation-dependent mechanism in its N-terminal region. PmMDV is an observed example of convergent evolution among invertebrate PVs with respect to host-driven capsid structure and unique as a PV showing a cation-sensitive/dependent basket structure for an alternative endosomal egress.

Cytoplasmic Parvovirus Capsids Recruit Importin Beta for Nuclear Delivery

Parvoviruses are an important platform for gene and cancer therapy. Their cell entry and the following steps, including nuclear import, are inefficient, limiting their use in therapeutic applications. Two models exist on parvoviral nuclear entry: the classical import of the viral capsid using nuclear transport receptors of the importin (karyopherin) family or the direct attachment of the capsid to the nuclear pore complex leading to the local disintegration of the nuclear envelope. Here, by laser scanning confocal microscopy and in situ proximity ligation analyses combined with coimmunoprecipitation, we show that infection requires importin β-mediated access to the nuclear pore complex and nucleoporin 153-mediated interactions on the nuclear side. The importin β-capsid interaction continued within the nucleoplasm, which suggests a mixed model of nuclear entry in which the classical nuclear import across the nuclear pore complex is accompanied by transient ruptures of the nuclear envelope, also allowing the passive entry of importin β-capsid complexes into the nucleus.IMPORTANCE Parvoviruses are small DNA viruses that deliver their DNA into the postmitotic nuclei, which is an important step for parvoviral gene and cancer therapies. Limitations in virus-receptor interactions or endocytic entry do not fully explain the low transduction/infection efficiency, indicating a bottleneck after virus entry into the cytoplasm. We thus investigated the transfer of parvovirus capsids from the cytoplasm to the nucleus, showing that the nuclear import of the parvovirus capsid follows a unique strategy, which differs from classical nuclear import and those of other viruses.

Construction of a One-Vector Multiplex CRISPR/Cas9 Editing System to Inhibit Nucleopolyhedrovirus Replication in Silkworms

Recently the developed single guide (sg)RNA-guided clustered regularly interspaced short palindromic repeats/associated protein 9 nuclease (CRISPR/Cas9) technology has opened a new avenue for antiviral therapy. The CRISPR/Cas9 system uniquely allows targeting of multiple genome sites simultaneously. However, there are relatively few applications of CRISPR/Cas9 multigene editing to target insect viruses. To address the need for sustained delivery of a multiplex CRISPR/Cas9-based genome-editing vehicle against insect viruses, we developed a one-vector (pSL1180-Cas9-U6-sgRNA) system that expresses multiple sgRNA and Cas9 protein to excise Bombyx mori nucleopolyhedrovirus (BmNPV) in insect cells. We screened the immediate-early-1 gene (ie-1), the major envelope glycoprotein gene (gp64), and the late expression factor gene (lef-11), and identified multiple sgRNA editing sites through flow cytometry and viral DNA replication analysis. In addition, we constructed a multiplex editing vector (PSL1180-Cas9-sgIE1-sgLEF11-sgGP64, sgMultiple) to efficiently regulate multiplex gene-editing and inhibit BmNPV replication after viral infection. This is the first report of the application of a multiplex CRISPR/Cas9 system to inhibit insect virus replication. This multiplex system can significantly enhance the potential of CRISPR/Cas9-based multiplex genome engineering in insect virus.

CRISPR/Cas9-mediated disruption of the immediate early-0 and 2 as a therapeutic approach to Bombyx mori nucleopolyhedrovirus in transgenic silkworm

The CRISPR/Cas9 system is a powerful tool for the treatment of infectious diseases. In our previous study, we knocked out the Bombyx mori nucleopolyhedrovirus (BmNPV) key genes and BmNPV-dependent host factor to generate transgenic antiviral strains. To further expand the range of target genes for BmNPV and more effectively prevent and control pathogenic infections, we performed gene editing and antiviral analysis by constructing a target-directed baculovirus early transcriptional activator immediate early-0 (ie-0) and 2 (ie-2) transgenic silkworm line. We hybridized it with Cas9 transgenic line to produce a double-positive transgenic Cas9(+)/sgIE0-sgIE2(+) line that could activate the CRISPR gene editing system. We first demonstrated that the system is capable of efficiently editing target genes and resulting in fragment deletions in the BmNPV genome. Survival rate of the transgenic Cas9(+)/sgIE0-sgIE2(+) line reached 65% after inoculation with 1 × 10⁶ occlusion bodies/larva. Molecular analysis showed that BmNPV DNA replication and viral gene expression level in the transgenic Cas9(+)/sgIE0-sgIE2(+) line were significantly inhibited compared with the control Cas9(-)/sgIE0-sgIE2(-) line. These results indicated that IE-0 and IE-2, as baculovirus early transcriptional activators, can be used as target sites for gene therapy and that multigene editing could expand the range of target sites for research to create silkworm resistance breeds.

Intracellular Localization of Blattella germanica Densovirus (BgDV1) Capsid Proteins

Densovirus genome replication and capsid assembly take place in the nucleus of the infected cells. However, the mechanisms underlying such processes as the delivery of virus proteins to the nucleus and the export of progeny virus from the nucleus remain elusive. It is evident that nuclear transport signals should be involved in these processes. We performed an in silico search for the putative nuclear localization signal (NLS) and nuclear export signal (NES) motifs in the capsid proteins of the Blattella germanica Densovirus 1 (BgDV1) densovirus. A high probability NLS motif was found in the common C-terminal of capsid proteins together with a NES motif in the unique N-terminal of VP2. We also performed a global search for the nuclear traffic signals in the densoviruses belonging to five Densovirinae genera, which revealed high diversity in the patterns of NLSs and NESs. Using a heterologous system, the HeLa mammalian cell line expressing GFP-fused BgDV1 capsid proteins, we demonstrated that both signals are functionally active. We suggest that the NLS shared by all three BgDV1 capsid proteins drives the trafficking of the newly-synthesized proteins into the nucleus, while the NES may play a role in the export of the newly-assembled BgDV1 particles into the cytoplasm through nuclear pore complexes.

Discovery of anti-viral molecules and their vital functions in Bombyx mori

The silkworm Bombyx mori (B. mori), a lepidopteran model organism, has become an important model for molecular biology researches with its genome completely sequenced. Silkworms confront different types of virus diseases, mainly including those caused by Bombyx mori nucleopolyhedrovirus (BmNPV), Bombyx mori densovirus type 1 (BmDNV-1), Bombyx mori bidesovirus (BmBDV) which was termed as Bombyx mori densovirus type 2 (BmDNV-2) or Bombyx mori parvo-like virus (BmPLV) before in sericulture. B. mori offers excellent models to study the molecular mechanisms of insect innate immune responses to viruses. A variety of molecules and pathways have been identified to be involved in the immune responses in the silkworm to viruses, such as the antimicrobial peptides, prophenoloxidase-activating system, apoptosis, ROS, small RNA and related molecules. Here in this review, we summarize the current research advances in molecules involved in silkworm anti-virus pathways. Moreover, taking BmNPV as an example, we proposed a schematic model of molecules and pathways involved in silkworm immune responses against virus infection. We hope this review can facilitate further study of antiviral mechanisms in silkworm, and provide a reference for virus diseases in other organisms.

A new approach for detecting adventitious viruses shows Sf-rhabdovirus-negative Sf-RVN cells are suitable for safe biologicals production

Background

Adventitious viral contamination in cell substrates used for biologicals production is a major safety concern. A powerful new approach that can be used to identify adventitious viruses is a combination of bioinformatics tools with massively parallel sequencing technology. Typically, this involves mapping or BLASTN searching individual reads against viral nucleotide databases. Although extremely sensitive for known viruses, this approach can easily miss viruses that are too dissimilar to viruses in the database. Moreover, it is computationally intensive and requires reference cell genome databases. To avoid these drawbacks, we set out to develop an alternative approach. We reasoned that searching genome and transcriptome assemblies for adventitious viral contaminants using TBLASTN with a compact viral protein database covering extant viral diversity as the query could be fast and sensitive without a requirement for high performance computing hardware.

Results

We tested our approach on Spodoptera frugiperda Sf-RVN, a recently isolated insect cell line, to determine if it was contaminated with one or more adventitious viruses. We used Illumina reads to assemble the Sf-RVN genome and transcriptome and searched them for adventitious viral contaminants using TBLASTN with our viral protein database. We found no evidence of viral contamination, which was substantiated by the fact that our searches otherwise identified diverse sequences encoding virus-like proteins. These sequences included Maverick, R1 LINE, and errantivirus transposons, all of which are common in insect genomes. We also identified previously described as well as novel endogenous viral elements similar to ORFs encoded by diverse insect viruses.

Conclusions

Our results demonstrate TBLASTN searching massively parallel sequencing (MPS) assemblies with a compact, manually curated viral protein database is more sensitive for adventitious virus detection than BLASTN, as we identified various sequences that encoded virus-like proteins, but had no similarity to viral sequences at the nucleotide level. Moreover, searches were fast without requiring high performance computing hardware. Our study also documents the enhanced biosafety profile of Sf-RVN as compared to other Sf cell lines, and supports the notion that Sf-RVN is highly suitable for the production of safe biologicals.

Electronic supplementary material

The online version of this article (doi: 10.1186/s12896-017-0412-z) contains supplementary material, which is available to authorized users.

Viruses of insects reared for food and feed

The use of insects as food for humans or as feed for animals is an alternative for the increasing high demand for meat and has various environmental and social advantages over the traditional intensive production of livestock. Mass rearing of insects, under insect farming conditions or even in industrial settings, can be the key for a change in the way natural resources are utilized in order to produce meat, animal protein and a list of other valuable animal products. However, because insect mass rearing technology is relatively new, little is known about the different factors that determine the quality and yield of the production process. Obtaining such knowledge is crucial for the success of insect-based product development. One of the issues that is likely to compromise the success of insect rearing is the outbreak of insect diseases. In particular, viral diseases can be devastating for the productivity and the quality of mass rearing systems. Prevention and management of viral diseases imply the understanding of the different factors that interact in insect mass rearing. This publication provides an overview of the known viruses in insects most commonly reared for food and feed. Nowadays with large-scale sequencing techniques, new viruses are rapidly being discovered. We discuss factors affecting the emergence of viruses in mass rearing systems, along with virus transmission routes. Finally we provide an overview of the wide range of measures available to prevent and manage virus outbreaks in mass rearing systems, ranging from simple sanitation methods to highly sophisticated methods including RNAi and transgenics.

Structure and transcription of the Helicoverpa armigera densovirus (HaDV2) genome and its expression strategy in LD652 cells

Background

Densoviruses (DVs) are highly pathogenic to their hosts. However, we previously reported a mutualistic DV (HaDV2). Very little was known about the characteristics of this virus, so herein we undertook a series of experiments to explore the molecular biology of HaDV2 further.

Results

Phylogenetic analysis showed that HaDV2 was similar to members of the genus Iteradensovirus. However, compared to current members of the genus Iteradensovirus, the sequence identity of HaDV2 is less than 44% at the nucleotide-level, and lower than 36, 28 and 19% at the amino-acid-level of VP, NS1 and NS2 proteins, respectively. Moreover, NS1 and NS2 proteins from HaDV2 were smaller than those from other iteradensoviruses due to their shorter N-terminal sequences. Two transcripts of about 2.2 kb coding for the NS proteins and the VP proteins were identified by Northern Blot and RACE analysis. Using specific anti-NS1 and anti-NS2 antibodies, Western Blot analysis revealed a 78 kDa and a 48 kDa protein, respectively. Finally, the localization of both NS1 and NS2 proteins within the cell nucleus was determined by using Green Fluorescent Protein (GFP) labelling.

Conclusion

The genome organization, terminal hairpin structure, transcription and expression strategies as well as the mutualistic relationship with its host, suggested that HaDV2 was a novel member of the genus Iteradensovirus within the subfamily Densovirinae.

Electronic supplementary material

The online version of this article (doi:10.1186/s12985-017-0691-y) contains supplementary material, which is available to authorized users.

Genetic characterization of a densovirus isolated from great tit (Parus major) in China

During a study of ornithophilous viruses in China, a new densovirus (DNV) was isolated from the lung tissue of Parus major (PmDNV-JL). The complete genome of PmDNV-JL was cloned and sequenced. Five open reading frames (ORFs) were identified in the 5166nt sequence, on the basis of deduced amino acids. It was further shown that this virus caused cytopathic effects (CPE) in Feline kidney cells. The NS1 gene sequence of PmDNV-JL shares 70-99% nucleotide sequence identity with isolates of the Blattella germanica densovirus (BgDNV) and BgDNV-like virus. Phylogenetic analysis indicated that the predicted amino acid sequences of capsid (VP) and non-structural domain (NS1) of PmDNV-JL clustered with the BgDNV and were similar to BgDNV-HB within the genus Densovirus.

Down-regulation of inwardly rectifying Kir2.1 K+ channels by human parvovirus B19 capsid protein VP1

Parvovirus B19 (B19V) has previously been shown to cause endothelial dysfunction. B19V capsid protein VP1 harbors a lysophosphatidylcholine producing phospholipase A2 (PLA2). Lysophosphatidylcholine inhibits Na(+)/K(+) ATPase, which in turn may impact on the activity of inwardly rectifying K(+) channels. The present study explored whether VP1 modifies the activity of Kir2.1 K(+) channels. cRNA encoding Kir2.1 was injected into Xenopus oocytes without or with cRNA encoding VP1 isolated from a patient suffering from fatal B19V-induced inflammatory cardiomyopathy or the VP1 mutant (H153A)VP1 lacking a functional PLA2 activity. K(+) channel activity was determined by dual electrode voltage clamp. In addition, Na(+)/K(+)-ATPase activity was estimated from K(+)-induced pump current (I(pump)) and ouabain-inhibited current (I(ouabain)). Injection of cRNA encoding Kir2.1 into Xenopus oocytes was followed by appearance of inwardly rectifying K(+) channel activity (I(K)), which was significantly decreased by additional injection of cRNA encoding VP1, but not by additional injection of cRNA encoding (H153A)VP1. The effect of VP1 on I K was mimicked by lysophosphatidylcholine (1 μg/ml) and by inhibition of Na(+)/K(+)-ATPase with 0.1 mM ouabain. In the presence of lysophosphatidylcholine, I K was not further decreased by additional treatment with ouabain. The B19V capsid protein VP1 thus inhibits Kir2.1 channels, an effect at least partially due to PLA2-dependent formation of lysophosphatidylcholine with subsequent inhibition of Na(+)/K(+)-ATPase activity.

Gene expression of five different iteradensoviruses: Bombyx mori densovirus, Casphalia extranea densovirus, Papilio polyxenes densovirus, Sibine fusca densovirus, and Danaus plexippus densovirus

Iteradensoviruses are 5-kb parvoviruses with typical J-shaped inverted terminal repeats of about 250 nucleotides and terminal hairpins of about 165 nucleotides. The single-stranded DNA genome contains several open reading frames, but their expression strategy is still unknown. Here the transcription maps and expression of the viruses in this genus were explored. As for brevidensoviruses, the two nonstructural (NS) genes were expressed by overlapping promoters with alternate transcription starts at both sides of the NS1 start codon.

Expression analysis of Bombyx mori bidensovirus structural proteins and assembly of virus-like particles in insect cells

Bombyx mori bidensovirus (BmBDV) is a new designated species of the new genus Bidensovirus in the new family Bidnaviridae, which contains two single-stranded linear DNAs (VD1 and VD2) and causes the chronic densonucleosis disease of silkworm. Previous researches revealed that VD1-ORF3 encodes the major structural proteins VPs. In this work, through western blot, we found that VPs expressed from 48 h post-inoculation and kept increasing until 120 h post-inoculation in midgut of Bombyx mori. In order to further investigate the translation of vp gene, the ORFs (vp1 and vp2) of the VP started just up-stream of the first two candidate initiation codons were expressed in Sf9 cells by a baculovirus expression system. The expression products were purified by gradient density centrifugation and analyzed by Western blot and electron microscopy. The results showed that the expressions of vp1 yielded three proteins (VP1, VP1', and VP2), which are the same with the viral VPs expression in midgut of Bombyx mori, and vp2 generated two VPs with the molecular weights of about 51 kDa (VP2) and 37 kDa. The observation by electron microscopy indicated that these VPs can auto-assemble into virus-like particles that could not be distinguished from virus particles. These findings will provide materials for studying the structure of BmBDV and be helpful in the studies on BmBDV-based disease in silkworms.

A viral over-expression system for the major malaria mosquito Anopheles gambiae

Understanding pathogen/mosquito interactions is essential for developing novel strategies to control mosquito-borne diseases. Technical advances in reverse-genetics, such as RNA interference (RNAi), have facilitated elucidation of components of the mosquito immune system that are antagonistic to pathogen development, and host proteins essential for parasite development. Forward genetic approaches, however, are limited to generation of transgenic insects, and while powerful, mosquito transgenesis is a resource- and time-intensive technique that is not broadly available to most laboratories. The ability to easily "over-express" genes would enhance molecular studies in vector biology and expedite elucidation of pathogen-refractory genes without the need to make transgenic insects. We developed and characterized an efficient Anopheles gambiae densovirus (AgDNV) over-expression system for the major malaria vector Anopheles gambiae. High-levels of gene expression were detected at 3 days post-infection and increased over time, suggesting this is an effective system for gene induction. Strong expression was observed in the fat body and ovaries. We validated multiple short promoters for gene induction studies. Finally, we developed a polycistronic system to simultaneously express multiple genes of interest. This AgDNV-based toolset allows for consistent transduction of genes of interest and will be a powerful molecular tool for research in Anopheles gambiae mosquitoes.

Iteradensovirus from the Monarch Butterfly, Danaus plexippus plexippus

The 5,006-nucleotide (nt)-long genome of a new virus from monarch butterfly pupae was cloned and sequenced. It was flanked by inverted terminal repeats (ITRs) of 239 nt with 163-nt hairpins. The monosense genome with three open reading frames is typical of the genus Iteradensovirus in the subfamily Densovirinae of the family Parvoviridae.

A Novel Ambisense Densovirus, Acheta domesticus Mini Ambidensovirus, from Crickets

The genome structure of Acheta domesticus mini ambidensovirus, isolated from crickets, resembled that of ambisense densoviruses from Lepidoptera but was 20% smaller. It had the highest (<25%) protein sequence identity with the nonstructural protein 1 (NS1) of Iteravirus and VP of Densovirus members (both with 25% coverage) and smaller (0.2- versus 0.55-kb) Y-shaped inverted terminal repeats.

The structure and host entry of an invertebrate parvovirus

The 3.5-Å resolution X-ray crystal structure of mature cricket parvovirus (Acheta domesticus densovirus [AdDNV]) has been determined. Structural comparisons show that vertebrate and invertebrate parvoviruses have evolved independently, although there are common structural features among all parvovirus capsid proteins. It was shown that raising the temperature of the AdDNV particles caused a loss of their genomes. The structure of these emptied particles was determined by cryo-electron microscopy to 5.5-Å resolution, and the capsid structure was found to be the same as that for the full, mature virus except for the absence of the three ordered nucleotides observed in the crystal structure. The viral protein 1 (VP1) amino termini could be externalized without significant damage to the capsid. In vitro, this externalization of the VP1 amino termini is accompanied by the release of the viral genome.

The determinants for the enzyme activity of human parvovirus B19 phospholipase A2 (PLA2) and its influence on cultured cells

Human parvovirus B19 (B19V) is the causative agent of erythema infectiosum in humans. B19 infection also causes severe disease manifestations, such as chronic anemia in immunocompromised patients, aplastic crisis in patients with a high turnover rate of red blood cells, and hydrops fetalis in pregnant women. Although a secreted phospholipase A2 (PLA2) motif has been identified in the unique region of the B19V minor capsid protein VP1(VP1u), the determinants for its enzyme activity and its influences on host cells are not well understood. The purpose of this study was to investigate the contribution of the PLA2 motif and other regions of the VP1u to the PLA2 activity, to determine the cellular localization of the VP1u protein, and to examine the effects of VP1u on cellular cytokines. We found that in addition to the critical conserved and non-conserved amino acids within the VP1u PLA2 motif, amino acid residues outside the VP1u PLA2 motif are also important for the PLA2 activity. VP1u and various mutants all revealed a nucleo-cytoplasmic distribution. UT7-Epo cells treated with prokaryotic expressed VP1u or mutant proteins with PLA2 activity released a large amount of free fatty acid (FFA), and the cell morphological change occurred dramatically. However, neither free fatty acid nor cell morphology change occurred for cells treated with the mutants without PLA2 activity. The wild type and the VP1u mutants with the PLA2 activity also activated TNF-α promoter and upregulated the transcription activity of NF-κB in transfected cells. In addition, we found that the amino acids outside the PLA2 domain are critical for the viral PLA2 activity, and that these tested VP1u mutants did not affect the localization of the VP1u protein.

Analysis of the transcription strategy of the Junonia coenia densovirus (JcDNV) genome

The Junonia coenia densovirus (JcDNV) has an ambisense genome with the structural (VP) and nonstructural (NS) genes located in the 5' half on opposite strands. Northern blot analysis of Ld652 cells and Spodoptera littoralis larvae transfected with plasmid pBRJ encompassing an infectious sequence of the JcDNV genome revealed three transcripts, an unspliced 2.5 kb VP mRNA encoding capsid proteins and two NS mRNAs, one unspliced 2.5 kb mRNA encoding NS3, the other of 1.7 kb resulting from the splicing out of the NS3 coding sequence and expressing NS1 and NS2. Mapping of the transcriptional start sites revealed that VP and NS transcripts start both at 32 nt downsream of the P9 and P93 TATA boxes, respectively. The VP mRNA has a very short (3 nt) 5' untranslated region whereas the NS mRNAs have 83 nt (unspliced) and 86nt (Spliced) 5' UTR. The VP and NS transcripts co-terminate in the middle of their respective strand and possess an overlapping sequence of 61 nt at their 3' termini. Analysis of the in vivo and in vitro translation products of VP mRNA clearly showed that the 4 capsid proteins are generated by a leaky scanning mechanism.

Direct interactions between bidensovirus BmDNV-Z proteins and midgut proteins from the virus target Bombyx mori

In this study we aimed to identify the protein-protein interactions between Bombyx mori midgut and the bidensovirus BmDNV-Z via a yeast two-hybrid (Y2H) system. To achieve this we constructed a Gal4 activation domain fusion library that expresses the host genes and Gal4 DNA binding domain fusion bait vectors that express BmDNV-Z genes. Y2H assay revealed 15 potential interactions between host and viral proteins. To verify the interactions, we modified and reconstructed a pair of bimolecular fluorescence complementation (BiFC) vectors and achieved the co-expressions of the candidate host genes and viral genes in insect culture cells. The BiFC assay confirmed the specificity of the interactions including B. mori 35 kDa protease and two BmDNV-Z proteins encoded by VD1-ORF2 and VD2-ORF1; B. mori transgelin and BmDNV-Z protein encoded by VD2-ORF3; and B. mori serine protease precursor and BmDNV-Z encoded by VD2-ORF3 in vitro. Our findings revealed that the specific host midgut proteins are involved in the interactions between B. mori and BmDNV-Z, which will facilitate our understanding of the molecular mechanisms of BmDNV-Z infection.

Papilio polyxenes densovirus has an iteravirus-like genome organization

The genome of Papilio polyxenes densovirus was cloned and sequenced and contained 5,053 nucleotides (nt), including inverted terminal repeats (ITRs) of 271 nt with terminal hairpins of 175 nt. Its DNA sequence and monosense organization with 3 open reading frames (ORFs) are typical of the genus Iteravirus in the subfamily Densovirinae of the Parvoviridae.

Iteravirus-like genome organization of a densovirus from Sibine fusca Stoll

The complete genome of Sibine fusca densovirus was cloned and sequenced. The genome contained 5,012 nucleotides (nt), including inverted terminal repeats (ITRs) of 230 nt with terminal hairpins of 161 nt. Its DNA sequence and monosense organization with 3 open reading frames (ORFs) is typical of the genus Iteravirus in the subfamily Densovirinae of the Parvoviridae.

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

Densoviruses are insect parvoviruses that are orally infectious for Lepidoptera. To assess the mechanisms underlying their specificity and their virulence, we investigated the role of eight candidate residues in the densovirus capsid. We showed that the substitutions of four amino acids were associated with decreased virulence due to a decreased ability to cross the host midgut epithelium, without an effect on viral replication in other tissues.

Expression strategy of densonucleosis virus from the German cockroach, Blattella germanica

Blattella germanica densovirus (BgDNV) is an autonomous parvovirus that infects the German cockroach. BgDNV possesses three mRNAs for NS proteins, two of which are splice variants of the unspliced transcript. The unspliced variant encodes open reading frame 5 (ORF5) (NS3), while NSspl1 encodes ORF3 (NS1) and ORF4 (NS2) and NSspl2 encodes the C-proximal half of NS1. BgDNV possesses three VP transcripts, one of which (VP) is unspliced, while the other two (VPspl1 and VPspl2) are generated by alternative splicing. The unspliced VP transcript contains both ORF1 and ORF2, while in VPspl1, ORF1 and ORF2 are joined in frame. The transcription of NS genes begins at an earlier stage of the virus life cycle than the transcription of VP genes. NS and VP transcripts overlap by 48 nucleotides (nt). BgDNV is characterized by two additional NS transcripts overlapping by more than 1,650 nt with VP-coding transcripts. Four different bands (97, 85, 80, and 57 kDa) corresponding to three BgDNV capsid proteins were detected on SDS-PAGE. Mass spectrometry analysis showed that the amino acid composition of the 85-kDa and 80-kDa proteins is the same. Moreover, both of these proteins are ubiquitinated. The BgDNV PLA(2) domain, which is critical for cellular uptake of the virus, is located in ORF2 and is present only in VP1. In contrast to all of the parvoviruses studied in this respect, VP2 has a unique N terminus that is not contained within VP1 and VP3. In situ recognition with NS1- and VP-specific antibodies revealed an uneven pattern of NS1 expression resembling a halo within the nuclear membrane.

Structure of Bombyx mori densovirus 1, a silkworm pathogen

Bombyx mori densovirus 1 (BmDNV-1), a major pathogen of silkworms, causes significant losses to the silk industry. The structure of the recombinant BmDNV-1 virus-like particle has been determined at 3.1-Å resolution using X-ray crystallography. It is the first near-atomic-resolution structure of a virus-like particle within the genus Iteravirus. The particles consist of 60 copies of the 55-kDa VP3 coat protein. The capsid protein has a β-barrel "jelly roll" fold similar to that found in many diverse icosahedral viruses, including archaeal, bacterial, plant, and animal viruses, as well as other parvoviruses. Most of the surface loops have little structural resemblance to other known parvovirus capsid proteins. In contrast to vertebrate parvoviruses, the N-terminal β-strand of BmDNV-1 VP3 is positioned relative to the neighboring 2-fold related subunit in a "domain-swapped" conformation, similar to findings for other invertebrate parvoviruses, suggesting domain swapping is an evolutionarily conserved structural feature of the Densovirinae.

Susceptibility of North-American and European crickets to Acheta domesticus densovirus (AdDNV) and associated epizootics

The European house cricket, Acheta domesticus L., is highly susceptible to A. domesticus densovirus (AdDNV). Commercial rearings of crickets in Europe are frequently decimated by this pathogen. Mortality was predominant in the last larval stage and young adults. Infected A. domesticus were smaller, less active, did not jump as high, and the adult females seldom lived more than 10-14 days. The most obvious pathological change was the completely empty digestive caecae. Infected tissues included adipose tissue, midgut, epidermis, and Malpighian tubules. Sudden AdDNV epizootics have decimated commercial mass rearings in widely separated parts of North America since the autumn of 2009. Facilities that are producing disease-free crickets have avoided the importation of crickets and other non-cricket species (or nonliving material). Five isolates from different areas in North America contained identical sequences as did AdDNV present in non-cricket species collected from these facilities. The North American AdDNVs differed slightly from sequences of European AdDNV isolates obtained in 1977, 2004, 2006, 2007 and 2009 and an American isolate from 1988. The substitution rate of the 1977 AdDNV 5kb genome was about two nucleotides per year, about half of the substitutions being synonymous. The American and European AdDNV strains are estimated to have diverged in 2006. The lepidopterans Spodoptera littoralis and Galleria mellonella could not be infected with AdDNV. The Jamaican cricket, Gryllus assimilis, and the European field cricket, Gryllus bimaculatus, were also found to be resistant to AdDNV.

Intrinsic phospholipase A2 activity of adeno-associated virus is involved in endosomal escape of incoming particles

The unique region of the VP1 capsid protein of adeno-associated viruses (AAV) in common with autonomously replicating parvoviruses comprises a secreted phospholipase A2 (sPLA2) homology domain. While the sPLA2 domain of Minute Virus of Mice has recently been shown to mediate endosomal escape by lipolytic pore formation, experimental evidence for a similar function in AAV infection is still lacking. Here, we explored the function of the sPLA2 domain of AAV by making use of the serotype 2 mutant (76)HD/AN. The sPLA2 defect in (76)HD/AN, which severely impairs AAV's infectivity, could be complemented in trans by co-infection with wild-type AAV2. Furthermore, co-infection with endosomolytically active, but not with inactive adenoviral variants partially rescued (76)HD/AN, providing the first evidence for a function of this domain in endosomal escape of incoming AAV particles.

Structure of Penaeus stylirostris densovirus, a shrimp pathogen

Penaeus stylirostris densovirus (PstDNV), a pathogen of penaeid shrimp, causes significant damage to farmed and wild shrimp populations. In contrast to other parvoviruses, PstDNV probably has only one type of capsid protein that lacks the phospholipase A2 activity that has been implicated as a requirement during parvoviral host cell infection. The structure of recombinant virus-like particles, composed of 60 copies of the 37.5-kDa coat protein, the smallest parvoviral capsid protein reported thus far, was determined to 2.5-Å resolution by X-ray crystallography. The structure represents the first near-atomic resolution structure within the genus Brevidensovirus. The capsid protein has a β-barrel "jelly roll" motif similar to that found in many icosahedral viruses, including other parvoviruses. The N-terminal portion of the PstDNV coat protein adopts a "domain-swapped" conformation relative to its twofold-related neighbor similar to the insect parvovirus Galleria mellonella densovirus (GmDNV) but in stark contrast to vertebrate parvoviruses. However, most of the surface loops have little structural resemblance to any of the known parvoviral capsid proteins.

Expression of non-structural protein NS3 gene of Bombyx mori densovirus (China isolate)

The invertebrate parvovirus Bombyx mori Densonucleosis Virus type 3 (China isolate), named BmDNV-3, is a kind of bidensovirus. It is a new type of virus with unique replication mechanisms. To investigate the effects of the NS3 gene during viral DNA replication, a pair of primers was designed for amplifying NS3 gene of Bombyx mori densovirus (China isolate). Gene NS3 amplified was cloned into a prokaryotic expression vector pET-30a and the donor plasmid pFastBacHTe, respectively. The NS3 protein was expressed in Escherichia coli BL21. The pFastBacHTe-NS3 was transformed to E. coli DH10Bac. The recombinant bacmid baculoviruses (rBacmid-EGFP-NS3) isolated from the white colonies were transfected into BmN-4 cells using a transfection reagent. BmN-4 cells were infected with recombinant virus to express fusion proteins. The expression of fusion protein around 30 kDa in E. coli BL21 was identified by SDS-PAGE, Western blotting, and mass spectrometry. The expressed NS3 protein by B. mori nucleopolyhedrovirus bacmid system was confirmed by Western blotting using an anti-NS3 polyclonal antibody. And about 45 kDa protein was found. The expressed fusion protein was smaller than the expected size of EGFP-NS3, 55 kDa. Western blotting analysis indicated that EGFP-NS3 protein was expressed in infected larvae with smaller molecular size.

Structure and expression strategy of the genome of Culex pipiens densovirus, a mosquito densovirus with an ambisense organization

The genome of all densoviruses (DNVs) so far isolated from mosquitoes or mosquito cell lines consists of a 4-kb single-stranded DNA molecule with a monosense organization (genus Brevidensovirus, subfamily Densovirinae). We previously reported the isolation of a Culex pipiens DNV (CpDNV) that differs significantly from brevidensoviruses by (i) having a approximately 6-kb genome, (ii) lacking sequence homology, and (iii) lacking antigenic cross-reactivity with Brevidensovirus capsid polypeptides. We report here the sequence organization and transcription map of this virus. The cloned genome of CpDNV is 5,759 nucleotides (nt) long, and it possesses an inverted terminal repeat (ITR) of 285 nt and an ambisense organization of its genes. The nonstructural (NS) proteins NS-1, NS-2, and NS-3 are located in the 5' half of one strand and are organized into five open reading frames (ORFs) due to the split of both NS-1 and NS-2 into two ORFs. The ORF encoding capsid polypeptides is located in the 5' half of the complementary strand. The expression of NS proteins is controlled by two promoters, P7 and P17, driving the transcription of a 2.4-kb mRNA encoding NS-3 and of a 1.8-kb mRNA encoding NS-1 and NS-2, respectively. The two NS mRNAs species are spliced off a 53-nt sequence. Capsid proteins are translated from an unspliced 2.3-kb mRNA driven by the P88 promoter. CpDNV thus appears as a new type of mosquito DNV, and based on the overall organization and expression modalities of its genome, it may represent the prototype of a new genus of DNV.

Densovirus infectious pathway requires clathrin-mediated endocytosis followed by trafficking to the nucleus

Junonia coenia densovirus (JcDNV) is an ambisense insect parvovirus highly pathogenic for lepidopteran pests at larval stages. The potential use of DNVs as biological control agents prompted us to reinvestigate the host range and cellular mechanisms of infection. In order to understand the early events of infection, we set up a functional infection assay in a cell line of the pest Lymantria dispar to determine the intracellular pathway undertaken by JcDNV to infect a permissive lepidopteran cell line. Our results show that JcDNV particles are rapidly internalized into clathrin-coated vesicles and slowly traffic within early and late endocytic compartments. Blocking late-endocytic trafficking or neutralizing the pH with drugs inhibited infection. During internalization, disruption of the cytoskeleton, and inhibition of phosphatidylinositol 3-kinase blocked the movement of vesicles containing the virus to the nucleus and impaired infection. In summary, our results define for the first time the early endocytic steps required for a productive DNV infection.

Characterization of the genome structure of Bombyx mori densovirus (China isolate)

The genome of Bombyx mori densovirus (China isolate), termed as BmDNV-3, is composed of two kinds of different single-stranded linear DNA molecules (VD1 and VD2). In this study, the viral DNA molecules were purified and cloned into pUC119 vector, and the complete nucleotide sequence was determined. Sequence analysis showed that VD1 genome consisted of 6,543 nts including inverted terminal repeats (ITRs) of 224 nts, and VD2 genome consisted of 6,022 nts including ITRs of 524 nts. Comparison of the complete genome sequence between BmDNV-3 and BmDNV-2 (Yamanashi isolate) showed an identity of 98.4% in VD1 and 97.7% in VD2, with a total number of 228 bp substitutions, 11 bp deletions and 3 bp insertions found in BmDNV-3. A single nucleotide "A" deletion at nt 1589 in BmDNV-3 caused a frame shift mutation and brought about a premature stop codon, thus dividing VD2 of BmDNV-3 into two ORFs (named VD2 ORF1a and VD2 ORF1b) within that region, while there was only one ORF (named VD2 ORF1) in the corresponding region of BmDNV-2 (Yamanashi isolate). Comparative polymorphisms of ORFs and ITR regions of the two viral genomes showed that highly variable regions were mainly located in VD1 ORF3, VD1 ORF4, VD2 ORF2, and ITRs of BmDNV-3. Northern blots analysis revealed that VD1 had 1.1 kb and 1.5 kb transcripts from the left half of its plus strand, and one transcript about 3.3 kb from the right half of its minus strand. Sequencing of 3' and 5' RACE products showed that the 1.1 kb transcript started at nt 290 and ended at nt 1437, the 1.5 kb transcript started at nt 1423 and ended at nt 2931, and the 3.3 kb transcript started at nt 6287 and ended at nt 2922. These results help us to further understand the variation between different DNV genera and its possible causes, providing clues for studying the evolutionary history of densoviruses.

Conformational changes in the VP1-unique region of native human parvovirus B19 lead to exposure of internal sequences that play a role in virus neutralization and infectivity

The unique region of the capsid protein VP1 (VP1u) of human parvovirus B19 (B19) elicits a dominant immune response and has a phospholipase A(2) (PLA(2)) activity, which is necessary for the infection. In contrast to the rest of the parvoviruses, the VP1u of B19 is thought to occupy an external position in the virion, making this region a promising candidate for vaccine development. By using a monoclonal antibody against the most-N-terminal portion of VP1u, we revealed that this region rich in neutralizing epitopes is not accessible in native capsids. However, exposure of capsids to increasing temperatures or low pH led to its progressive accessibility without particle disassembly. Although unable to bind free virus or to block virus attachment to the cell, the anti-VP1u antibody was neutralizing, suggesting that the exposure of the epitope and the subsequent virus neutralization occur only after receptor attachment. The measurement of the VP1u-associated PLA(2) activity of B19 capsids revealed that this region is also internal but becomes exposed in heat- and in low-pH-treated particles. In sharp contrast to native virions, the VP1u of baculovirus-derived B19 capsids was readily accessible in the absence of any treatment. These results indicate that stretches of VP1u of native B19 capsids harboring neutralizing epitopes and essential functional motifs are not external to the capsid. However, a conformational change renders these regions accessible and triggers the PLA(2) potential of the virus. The results also emphasize major differences in the VP1u conformation between natural and recombinant particles.

Phospholipase A2 activity-dependent stimulation of Ca2+ entry by human parvovirus B19 capsid protein VP1

Recent reports demonstrated an association of human parvovirus B19 with inflammatory cardiomyopathy (iCMP), which is accompanied by endothelial dysfunction. As intracellular Ca(2+) activity is a key regulator of cell function and participates in mechanisms leading to endothelial dysfunction, the present experiments explored the effects of the B19 capsid proteins VP1 and VP2. A secreted phospholipase A2 (PLA2)-like activity has been located in the VP1 unique region of the B19 minor capsid protein. As PLA2 has recently been shown to activate the store-operated or capacitative Ca(2+) channel I(CRAC), we analyzed the impact of the viral PLA2 motif on Ca(2+) entry. We cloned the VP1 and VP2 genes isolated from a patient suffering from fatal B19 iCMP into eukaryotic expression vectors. We also generated a B19 replication-competent plasmid to demonstrate PLA2 activity under the control of the complete B19 genome. After the transfection of human endothelial cells (HMEC-1), cytosolic Ca(2+) activity was determined by utilizing Fura-2 fluorescence. VP1 and VP2 expression did not significantly modify basal cytosolic Ca(2+) activity or the decline of cytosolic Ca(2+) activity following the removal of extracellular Ca(2+). However, expression of VP1 and of the full-length B19 clone, but not of VP2, significantly accelerated the increase of cytosolic Ca(2+) activity following the readdition of extracellular Ca(2+) in the presence of thapsigargin, indicating an activation of I(CRAC.) The effect of VP1 was mimicked by the PLA2 product lysophosphatidylcholine and abolished by an inactivating mutation of the PLA2-encoding region of the VP1 gene. Our observations point to the activation of Ca(2+) entry by VP1 PLA2 activity, an effect likely participating in the pathophysiology of B19 infection.

Biochemical characterization of Periplaneta fuliginosa densovirus non-structural protein NS1

The non-structural (NS) proteins of parvoviruses are involved in essential steps of the viral life cycle. Various biochemical functions, such as ATP binding, ATPase, site-specific DNA binding and nicking, and helicase activities, have been assigned to the protein NS1. Compared with the non-structural proteins of the vertebrate parvoviruses, the NS proteins of the Densovirinae have not been well characterized. Here, we describe the biochemical properties of NS1 of Periplaneta fuliginosa densovirus (PfDNV). We have expressed and purified NS1 using a baculovirus system and analyzed its enzymatic activity. The purified recombinant NS1 protein possesses ATPase- and ATP- or dATP-dependent helicase activity requiring either Mg(2+) or Mn(2+) as a cofactor. The ATPase activity of NS1 can be efficiently stimulated by single-stranded DNA. The ATPase coupled helicase activity was detected on blunt-ended double-stranded oligonucleotide substrate. Using South-Western and Dot-spot assays, we identified a DNA fragment that is recognized specifically by the recombinant NS1 protein. The fragment consists of (CAC)(4) and is located on the hairpin region of the terminal palindrome. The domain for DNA binding was defined to the amino-terminal region (amino acids 1-250). In addition, we found that NS1 can form oligomeric complexes in vivo and in vitro. Mutagenesis analysis showed that ATP binding is necessary for oligomerization. Based on these results, it seems that PfDNV NS1, a multifunctional protein, plays an important role in viral DNA replication comparable to those of vertebrate parvovirus initiator proteins.

Mutational analysis of narrow pores at the fivefold symmetry axes of adeno-associated virus type 2 capsids reveals a dual role in genome packaging and activation of phospholipase A2 activity

Adeno-associated virus type 2 (AAV2) capsids show 12 pores at the fivefold axes of symmetry. We mutated amino acids which constitute these pores to investigate possible functions of these structures within the AAV2 life cycle. Mutants with alterations in conserved residues were impaired mainly in genome packaging or infectivity, whereas few mutants were affected in capsid assembly. The packaging phenotype was characterized by increased capsid-per-genome ratios. Analysis of capsid-associated DNA versus encapsidated DNA revealed that this observation was due to reduced and not partial DNA encapsidation. Most mutants with impaired infectivity showed a decreased capability to expose their VP1 N termini. As a consequence, the activation of phospholipase A2 (PLA2) activity, which is essential for efficient infection, was affected on intact capsids. In a few mutants, the exposure of VP1 N termini and the development of PLA2 activity were associated with enhanced capsid instability, which is obviously also deleterious for virus infection. Therefore, PLA2 activity seems to be required on intact capsids for efficient infection. In conclusion, these results suggest that the pores at the fivefold axes function not only as portals for AAV2 single-stranded DNA packaging but also as channels for presentation of the PLA2 domain on AAV2 virions during infection.

Nucleotide sequence and genomic organization of a newly isolated densovirus infecting Dendrolimus punctatus

The nucleotide sequence of a novel icosahedral DNA virus infecting Dendrolimus punctatus has been determined. The genome is 5039 nt long and includes inverted terminal repeats of 200 nt containing 131 nt long J-shaped terminal hairpins. The ‘plus' strand of the genome contains three large open reading frames (ORFs), the left and the mid-ORFs (within the left ORF) in the left-half encoding the non-structural proteins and the right ORF in the right-half encoding viral capsid proteins. NS1 protein contains conserved replication initiation and DNA-dependent ATPase/helicase domains. VP1 protein contains a conserved PGY and phospholipase A2 motifs and shows high identities with VPs of Casphalia extranea densovirus and Bombyx mori densovirus-1 belonging to the genus Iteravirus. Phylogenetic analysis also revealed that this virus is most closely related to Casphalia extranea densovirus and Bombyx mori densovirus-1. Consequently, this virus was considered as a new third member of the genus Iteravirus of the subfamily Densovirinae, and designated Dendrolimus punctatus densovirus.

A conformational change in the adeno-associated virus type 2 capsid leads to the exposure of hidden VP1 N termini

The complex infection process of parvoviruses is not well understood so far. An important role has been attributed to a phospholipase A2 domain which is located within the unique N terminus of the capsid protein VP1. Based on the structural difference between adeno-associated virus type 2 wild-type capsids and capsids lacking VP1 or VP2, we show via electron cryomicroscopy that the N termini of VP1 and VP2 are involved in forming globules inside the capsids of empty and full particles. Upon limited heat shock, VP1 and possibly VP2 become exposed on the outsides of full but not empty capsids, which is correlated with the disappearance of the globules in the inner surfaces of the capsids. Using molecular modeling, we discuss the constraints on the release of the globularly organized VP1-unique N termini through the channels at the fivefold symmetry axes outside of the capsid.

The ubiquitin-proteasome machinery is essential for nuclear translocation of incoming minute virus of mice

Minute virus of mice (MVM) infection is disrupted by proteasome inhibitors. Here, we show that inhibition of the ubiquitin-proteasome pathway did not affect viral entry and had influence neither on the natural proteolytic cleavage of VP2 to VP3 nor on the externalization of the N terminal of VP1. In both MG132-treated and untreated cells, MVM particles accumulated progressively in the perinuclear region. However, in MG132-treated cells, MVM was not able to penetrate into the nuclei, remaining blocked in the perinuclear region without capsid disassembly. MVM was similarly sensitive to MG132 in the two cell lines tested, A9 and NB324K. After releasing from the reversible MG132 block, MVM recovered the ability to translocate to the nuclei and replicate. Analysis of viral capsid proteins during internalization showed no evidence of capsid ubiquitination or degradation. We examined the effect of MG132 on two other parvoviruses, canine (CPV) and bovine parvovirus (BPV). Similarly to MVM, CPV infection was sensitive to MG132; however, BPV infection, as previously shown for adeno-associated viruses (AAVs), was not disturbed. These findings suggest that parvoviruses follow divergent strategies for nuclear transport, some of them requiring active proteasomes.

Expression strategy of densonucleosis virus from Mythimna loreyi

The genome of Mythimna loreyi densovirus (MlDNV) was cloned into the pEMBL(19)+ vector. This clone was infectious upon transfection, both in LD cells and larvae. The genome possessed ITRs of 543 nucleotides of which the distal 126 nucleotides could form a hairpin. The nonstructural (NS) and structural (VP) genes were located on the 5'-halves of the complementary strands and their transcripts started 27 nts downstream of the ITRs. These transcripts had an overlap of 57 nucleotides in middle of the genome. The NS cassette consisted of three genes with NS1 and the overlapping NS2 downstream of NS3. The NS3 gene was spliced out from a fraction of the NS transcripts to allow leaky scanning translation of the downstream bicistronic NS1 and NS2 genes. The four VPs were similarly generated by leaky scanning translation of unspliced mRNA. The 5'-untranslated region of the VP transcript was only seven nucleotides long.