Avian polyomavirus major capsid protein VP1 interacts with the minor capsid proteins and is transported into the cell nucleus but does not assemble into capsid-like particles when expressed in the baculovirus system

Genetic characterization of avian polyomaviruses identified from psittacine birds in South Korea

Budgerigar fledgling disease (BFD) is a contagious disease caused by avian polyomavirus (APV) in psittacine birds and causes high mortality rates. Here, eight APV-positive cases were confirmed from dead parrots or parrot tissue samples by polymerase chain reaction (PCR). Full-length genome sequencing showed high nucleotide identity (98.84-100%) between the APV strains. Phylogenetic analysis revealed that two genogroups were cocirculating in South Korea. The nucleotide sequences of five strains, collected from different parrot species, were identical; however, pathological lesions were observed in only two parrots, both aged 2 months. Pathology included necrotic spots in the liver, subcutaneous haemorrhage, hepatomegaly, ascites, intranuclear inclusion bodies, hepatocyte karyomegaly, hepatic necrosis, and bile duct proliferation. This suggests that the pathogenicity of APV might be host age-dependent regardless of the host species. This study improves our understanding of APV pathogenicity and provides a more detailed genetic characterization of APV strains.RESEARCH HIGHLIGHTS Eight APV strains were identified in South Korea from 2019 to 2021.By phylogenetic analysis, South Korean APV strains were classified into two clades.

Production and biomedical applications of virus-like particles derived from polyomaviruses

Virus-like particles (VLPs), aggregates of capsid proteins devoid of viral genetic material, show great promise in the fields of vaccine development and gene therapy. These particles spontaneously self-assemble after heterologous expression of viral structural proteins. This review will focus on the use of virus-like particles derived from polyomavirus capsid proteins. Since their first recombinant production 27 years ago these particles have been investigated for a myriad of biomedical applications. These virus-like particles are safe, easy to produce, can be loaded with a broad range of diverse cargoes and can be tailored for specific delivery or epitope presentation. We will highlight the structural characteristics of polyomavirus-derived VLPs and give an overview of their applications in diagnostics, vaccine development and gene delivery.

The structure of avian polyomavirus reveals variably sized capsids, non-conserved inter-capsomere interactions, and a possible location of the minor capsid protein VP4

Avian polyomavirus (APV) causes a fatal, multi-organ disease among several bird species. Using cryogenic electron microscopy and other biochemical techniques, we investigated the structure of APV and compared it to that of mammalian polyomaviruses, particularly JC polyomavirus and simian virus 40. The structure of the pentameric major capsid protein (VP1) is mostly conserved; however, APV VP1 has a unique, truncated C-terminus that eliminates an intercapsomere-connecting β-hairpin observed in other polyomaviruses. We postulate that the terminal β-hairpin locks other polyomavirus capsids in a stable conformation and that absence of the hairpin leads to the observed capsid size variation in APV. Plug-like density features were observed at the base of the VP1 pentamers, consistent with the known location of minor capsid proteins VP2 and VP3. However, the plug density is more prominent in APV and may include VP4, a minor capsid protein unique to bird polyomaviruses.

Recombinant subunit vaccine elicits protection against goose haemorrhagic nephritis and enteritis

Outbreaks of haemorrhagic nephritis and enteritis of geese (HNEG) have been reported in goose flocks in Hungary, Germany and France since 1969. HNEG is characterized by high morbidity and mortality rates in geese 3 to 10 weeks of age. The causative agent of HNEG is the goose haemorrhagic polyomavirus (GHPV), which has a circular double-stranded DNA genome encoding the structural proteins VP1, VP2 and VP3. In vitro culture of GHPV has been problematic, so the baculovirus system was used to construct a recombinant virus expressing the VP1 gene of GHPV under control of the polyhedrin promoter in Sf9 insect cells. The expression and the identity of recombinant goose polyomavirus VP1 in the crude Sf9 cell extracts were confirmed by mass spectrometry. Experimental oil-emulsion vaccines containing two different doses of antigen were prepared using this crude extract. Goslings were vaccinated either once at 1 day old or twice by boosting 18 days after the primary vaccination, and were challenged with a virulent polyomavirus isolate at 5 weeks of age. A single injection of either vaccine dose induced 95% protection against challenge. Using the booster vaccination regimen, 100% protection was achieved with either vaccine dose.

Generation of virus-like particles consisting of the major capsid protein VP1 of goose hemorrhagic polyomavirus and their application in serological tests

Goose hemorrhagic polyomavirus (GHPV) is the causative agent of hemorrhagic nephritis and enteritis of geese (HNEG), a fatal disease of young geese with high mortality rates. GHPV cannot be efficiently propagated in tissue culture. To provide antigens for diagnostic tests and vaccines, its major structural protein VP1 was recombinantly expressed in Sf9 insect cells and in the yeast Saccharomyces cerevisiae. As demonstrated by density gradient centrifugation and electron microscopy, GHPV-VP1 expressed in insect cells formed virus-like particles (VLPs) with a diameter of 45 nm indistinguishable from infectious polyomavirus particles. However, efficiency of VLP formation was low as compared to the monkey polyomavirus SV-40-VP1. In yeast cells, GHPV-VP1 alone formed smaller VLPs, 20 nm in diameter. Remarkably, co-expression of GHPV-VP2 resulted in VLPs with a diameter of 45 nm. All three types of GHPV-VLPs were shown to hemagglutinate chicken erythrocytes. ELISA and hemagglutination inhibition tests using the VLPs as antigen detected GHPV-specific antibodies in up to 85.7% of sera derived from flocks with HNEG but in none of the sera of a clinically healthy flock. However, GHPV-specific antibodies were also detected in sera from two other flocks without HNEG indicating a broad distribution of GHPV due to subclinical or unrecognised infections.

Nuclear localization of avian polyomavirus structural protein VP1 is a prerequisite for the formation of virus-like particles

Virions of polyomaviruses consist of the major structural protein VP1, the minor structural proteins VP2 and VP3, and the viral genome associated with histones. An additional structural protein, VP4, is present in avian polyomavirus (APV) particles. As it had been reported that expression of APV VP1 in insect cells did not result in the formation of virus-like particles (VLP), the prerequisites for particle formation were analyzed. To this end, recombinant influenza viruses were created to (co)express the structural proteins of APV in chicken embryo cells, permissive for APV replication. VP1 expressed individually or coexpressed with VP4 did not result in VLP formation; both proteins (co)localized in the cytoplasm. Transport of VP1, or the VP1-VP4 complex, into the nucleus was facilitated by the coexpression of VP3 and resulted in the formation of VLP. Accordingly, a mutant APV VP1 carrying the N-terminal nuclear localization signal of simian virus 40 VP1 was transported to the nucleus and assembled into VLP. These results support a model of APV capsid assembly in which complexes of the structural proteins VP1, VP3 (or VP2), and VP4, formed within the cytoplasm, are transported to the nucleus using the nuclear localization signal of VP3 (or VP2); there, capsid formation is induced by the nuclear environment.

Avian polyomavirus agnoprotein 1a is incorporated into the virus particle as a fourth structural protein, VP4

Agnoproteins, encoded by the 5'-region of the late bicistronic mRNA of some polyomaviruses, are small proteins with largely unknown functions. In avian polyomavirus (APV)-infected cells, mRNAs of seven putative agnoproteins have been observed. Recently, it has been shown that agnoprotein 1a and its truncated variant agnoprotein 1b, encoded by the predominant mRNA species, are essential for APV replication. Here, the presence of agnoprotein 1a is demonstrated in the nucleus of APV-infected cells and in purified APV particles. Interaction between agnoprotein 1a and the major structural protein, VP1, was demonstrated by co-immunoprecipitation experiments using lysates of recombinant baculovirus-infected insect cells. With proteins expressed in E. coli, binding to double-stranded DNA in a sequence-unspecific manner was shown for agnoprotein 1a, whereas agnoprotein 1b failed to bind. A leucine zipper-like motif present in agnoprotein 1a is considered to be involved in DNA binding. Due to the absence of any structural or functional homologies between APV agnoprotein 1a and the agnoproteins of mammalian polyomaviruses, it is suggested that this protein should be renamed VP4, indicating its function as a fourth structural protein of APV.