Capsid protein-encoding genes of hamster polyomavirus and properties of the viral capsid

Hamster Polyomavirus Research: Past, Present, and Future

Hamster polyomavirus (Mesocricetus auratus polyomavirus 1, HaPyV) was discovered as one of the first rodent polyomaviruses at the end of the 1960s in a colony of Syrian hamsters (Mesocricetus auratus) affected by skin tumors. Natural HaPyV infections have been recorded in Syrian hamster colonies due to the occurrence of skin tumors and lymphomas. HaPyV infections of Syrian hamsters represent an important and pioneering tumor model. Experimental infections of Syrian hamsters of different colonies are still serving as model systems (e.g., mesothelioma). The observed phylogenetic relationship of HaPyV to murine polyomaviruses within the genus Alphapolyomavirus, and the exclusive detection of other cricetid polyomaviruses, i.e., common vole (Microtus arvalis polyomavirus 1) and bank vole (Myodes glareolus polyomavirus 1) polyomaviruses, in the genus Betapolyomavirus, must be considered with caution, as knowledge of rodent-associated polyomaviruses is still limited. The genome of HaPyV shows the typical organization of polyomaviruses with an early and a late transcriptional region. The early region encodes three tumor (T) antigens including a middle T antigen; the late region encodes three capsid proteins. The major capsid protein VP1 of HaPyV was established as a carrier for the generation of autologous, chimeric, and mosaic virus-like particles (VLPs) with a broad range of applications, e.g., for the production of epitope-specific antibodies. Autologous VLPs have been applied for entry and maturation studies of dendritic cells. The generation of chimeric and mosaic VLPs indicated the high flexibility of the VP1 carrier protein for the insertion of foreign sequences. The generation of pseudotype VLPs of original VP1 and VP2–foreign protein fusion can further enhance the applicability of this system. Future investigations should evaluate the evolutionary origin of HaPyV, monitor its occurrence in wildlife and Syrian hamster breeding, and prove its value for the generation of potential vaccine candidates and as a gene therapy vehicle.

Codon usage patterns of LT-Ag genes in polyomaviruses from different host species

Background

Polyomaviruses (PyVs) have a wide range of hosts, from humans to fish, and their effects on hosts vary. The differences in the infection characteristics of PyV with respect to the host are assumed to be influenced by the biochemical function of the LT-Ag protein, which is related to the cytopathic effect and tumorigenesis mechanism via interaction with the host protein.

Methods

We carried out a comparative analysis of codon usage patterns of large T-antigens (LT-Ags) of PyVs isolated from various host species and their functional domains and sequence motifs. Parity rule 2 (PR2) and neutrality analysis were applied to evaluate the effects of mutation and selection pressure on codon usage bias. To investigate evolutionary relationships among PyVs, we carried out a phylogenetic analysis, and a correspondence analysis of relative synonymous codon usage (RSCU) values was performed.

Results

Nucleotide composition analysis using LT-Ag gene sequences showed that the GC and GC3 values of avian PyVs were higher than those of mammalian PyVs. The effective number of codon (ENC) analysis showed host-specific ENC distribution characteristics in both the LT-Ag gene and the coding sequences of its domain regions. In the avian and fish PyVs, the codon diversity was significant, whereas the mammalian PyVs tended to exhibit conservative and host-specific evolution of codon usage bias. The results of our PR2 and neutrality analysis revealed mutation bias or highly variable GC contents by showing a narrow GC12 distribution and wide GC3 distribution in all sequences. Furthermore, the calculated RSCU values revealed differences in the codon usage preference of the LT-AG gene according to the host group. A similar tendency was observed in the two functional domains used in the analysis.

Conclusions

Our study showed that specific domains or sequence motifs of various PyV LT-Ags have evolved so that each virus protein interacts with host cell targets. They have also adapted to thrive in specific host species and cell types. Functional domains of LT-Ag, which are known to interact with host proteins involved in cell proliferation and gene expression regulation, may provide important information, as they are significantly related to the host specificity of PyVs.

Survey of molecular chaperone requirement for the biosynthesis of hamster polyomavirus VP1 protein in Saccharomyces cerevisiae

A number of viruses utilize molecular chaperones during various stages of their life cycle. It has been shown that members of the heat-shock protein 70 (Hsp70) chaperone family assist polyomavirus capsids during infection. However, the molecular chaperones that assist the formation of recombinant capsid viral protein 1 (VP1)-derived virus-like particles (VLPs) in yeast remain unclear. A panel of yeast strains with single chaperone gene deletions were used to evaluate the chaperones required for biosynthesis of recombinant hamster polyomavirus capsid protein VP1. The impact of deletion or mild overexpression of chaperone genes was determined in live cells by flow cytometry using enhanced green fluorescent protein (EGFP) fused with VP1. Targeted genetic analysis demonstrated that VP1-EGFP fusion protein levels were significantly higher in yeast strains in which the SSZ1 or ZUO1 genes encoding ribosome-associated complex components were deleted. The results confirmed the participation of cytosolic Hsp70 chaperones and suggested the potential involvement of the Ydj1 and Caj1 co-chaperones and the endoplasmic reticulum chaperones in the biosynthesis of VP1 VLPs in yeast. Likewise, the markedly reduced levels of VP1-EGFP in Δhsc82 and Δhsp82 yeast strains indicated that both Hsp70 and Hsp90 chaperones might assist VP1 VLPs during protein biosynthesis.

Production of recombinant VP1-derived virus-like particles from novel human polyomaviruses in yeast

BACKGROUND

Eleven new human polyomaviruses (HPyVs) have been identified in the last decade. Serological studies show that these novel HPyVs sub-clinically infect humans at an early age. The routes of infection, entry pathways, and cell tropism of new HPyVs remain unknown. VP1 proteins of polyomaviruses can assembly into virus-like particles (VLPs). As cell culturing systems for HPyV are currently not available, VP1-derived VLPs may be useful tools in basic research and biotechnological applications.

RESULTS

Recombinant VP1-derived VLPs from 11 newly identified HPyVs were efficiently expressed in yeast. VP1 proteins derived from Merkel cell polyomavirus (MCPyV), trichodysplasia spinulosa-associated polyomavirus (TSPyV), and New Jersey polyomavirus (NJPyV) self-assembled into homogeneous similarly-sized VLPs. Karolinska Institutet polyomavirus (KIPyV), HPyV7, HPyV9, HPyV10, and St. Louis polyomavirus (STLPyV) VP1 proteins formed VLPs that varied in size with diameters ranging from 20 to 60 nm. Smaller-sized VLPs (25-35 nm in diameter) predominated in preparations from Washington University polyomavirus (WUPyV) and HPyV6. Attempts to express recombinant HPyV12 VP1-derived VLPs in yeast indicate that translation of VP1 might start at the second of two potential translation initiation sites in the VP1-encoding open reading frame (ORF). This translation resulted in a 364-amino acid-long VP1 protein, which efficiently self-assembled into typical PyV VLPs. MCPyV-, KIPyV-, TSPyV-, HPyV9-, HPyV10-, and HPyV12-derived VLPs showed hemagglutination (HA) assay activity in guinea pig erythrocytes, whereas WUPyV-, HPyV6-, HPyV7-, STLPyV- and NJPyV-derived VP1 VLPs did not.

CONCLUSIONS

The yeast expression system was successfully utilized for high-throughput production of recombinant VP1-derived VLPs from 11 newly identified HPyVs. HPyV12 VP1-derived VLPs were generated from the second of two potential translation initiation sites in the VP1-encoding ORF. Recombinant VLPs produced in yeast originated from different HPyVs demonstrated distinct HA activities and may be useful in virus diagnostics, capsid structure studies, or investigation of entry pathways and cell tropism of HPyVs until cell culture systems for new HPyVs are developed.

IgA antibody production by intrarectal immunization of mice using recombinant major capsid protein of hamster polyomavirus

Viral proteins are highly antigenic and known as potent stimulators of adaptive immune responses. This mechanism is often used for biotechnological applications in monoclonal antibody production resulting in high-affinity IgG antibodies in most cases. The aim of this study was to increase antigen-specific IgA antibody levels in mice in order to generate monoclonal IgA antibodies by hybridoma technology. For this purpose, hamster polyomavirus (HaPyV) major capsid protein VP1 was used to immunize mice by different routes in order to induce VP1-specific IgA titers. Recombinant HaPyV-VP1 was generated in Escherichia coli and administered intraperitoneally, orally, and intrarectally. VP1-specific antibodies were determined by ELISA in sera and organ culture supernatants. We found a significant increase of HaPyV-VP1-specific IgAs in spleen organ cultures after rectal immunization of mice but not in cultures of mesenteric lymph nodes, colon, or Peyer's patches. In contrast, oral and intraperitoneal immunization did not provide an appropriate specific IgA induction at all. These results show that specific IgA antibodies can be induced by intrarectal immunization in the spleen. The generation of monoclonal IgA antibodies with well-defined properties is a useful tool for the investigation of mucosal immune responses or autoimmune diseases and extends the spectrum of antibodies with specific effector functions.

Hamster polyomavirus-derived virus-like particles are able to transfer in vitro encapsidated plasmid DNA to mammalian cells

The authentic major capsid protein 1 (VP1) of hamster polyomavirus (HaPyV) consists of 384 amino acid (aa) residues (42 kDa). Expression from an additional in-frame initiation codon located upstream from the authentic VP1 open reading frame (at position -4) might result in the synthesis of a 388 aa-long, amino-terminally extended VP1 (aa -4 to aa 384; VP1(ext)). In a plasmid-mediated Drosophila Schneider (S2) cell expression system, both VP1 derivatives as well as a VP1(ext) variant with an amino acid exchange of the authentic Met1Gly (VP1(ext-M1)) were expressed to a similar high level. Although all three proteins were detected in nuclear as well as cytoplasmic fractions, formation of virus-like particles (VLPs) was observed exclusively in the nucleus as confirmed by negative staining electron microscopy. The use of a tryptophan promoter-driven Escherichia coli expression system resulted in the efficient synthesis of VP1 and VP1(ext) and formation of VLPs. In addition, establishment of an in vitro disassembly/reassembly system allowed the encapsidation of plasmid DNA into VLPs. Encapsidated DNA was found to be protected against the action of DNase I. Mammalian COS-7 and CHO cells were transfected with HaPyV-VP1-VLPs carrying a plasmid encoding enhanced green fluorescent protein (eGFP). In both cell lines eGFP expression was detected indicating successful transfer of the plasmid into the cells, though at a still low level. Cesium chloride gradient centrifugation allowed the separation of VLPs with encapsidated DNA from "empty" VLPs, which might be useful for further optimization of transfection. Therefore, heterologously expressed HaPyV-VP1 may represent a promising alternative carrier for foreign DNA in gene transfer applications.

Size and position of truncations in the carboxy-terminal region of major capsid protein VP1 of hamster polyomavirus expressed in yeast determine its assembly capacity

The hamster polyomavirus major capsid protein VP1 was modified in its carboxy-terminal region by consecutive truncations and single amino acid exchanges. The ability of yeast-expressed VP1 variants to form virus-like particles (VLPs) strongly depended on the size and position of the truncation. VP1 variants lacking 21, 69, and 79 amino acid (aa) residues in their carboxy-terminal region efficiently formed VLPs similar to those formed by the unmodified VP1 (diameter 40-45 nm). In contrast, VP1 derivatives with carboxy-terminal truncations of 35 to 56 aa residues failed to form VLPs. VP1 mutants with a single A336G aa exchange or internal deletions of aa 335 to aa 346 and aa 335 to aa 363 resulted in the formation of VLPs of a smaller size (diameter 20 nm). These data indicate that certain parts of the carboxy-terminal region of VP1 are not essential for pentamer-pentamer interactions in the capsid, at least in the yeast expression system used.

Segments of Puumala Hantavirus Nucleocapsid Protein Inserted into Chimeric Polyomavirus-Derived Virus-Like Particles Induce a Strong Immune Response in Mice

Insertion of a short-sized epitope at four different sites of yeast-expressed hamster polyomavirus major capsid protein VP1 has been found to result in the formation of chimeric virus-like particles. Here, we demonstrate that the insertion of 45 or 120 amino acid-long segments from the N-terminus of Puumala hantavirus nucleocapsid protein into sites 1 (amino acids 80-89) and 4 (amino acids 288-295) of VP1 allowed the highly efficient formation of virus-like particles. In contrast, expression level and assembly capacity of fusions to sites 2 (amino acids 222-225) and 3 (amino acids 243-247) were drastically reduced. Immunization of BALB/c mice with chimeric virus-like particles induced a high-titered antibody response against the hantavirus nucleocapsid protein, even in the absence of any adjuvant. The strongest response was observed in mice immunized with virus-like particles harboring 120 amino acids of hantavirus nucleocapsid protein. According to the immunoglobulin subclass distribution of nucleocapsid protein-specific antibodies a mixed Th1/Th2 response was detected. The VP1 carrier itself also induced a mixed Th1/Th2 response, which was found to be reduced in mice immunized with virus-like particles harboring 120 amino acid-long inserts. In conclusion, hamster polyomavirus VP1 represents a promising carrier moiety for future vaccine development.

Chimeric bacteriophage fr virus-like particles harboring the immunodominant C-terminal region of hamster polyomavirus VP1 induce a strong VP1-specific antibody response in rabbits and mice

The late region of the hamster polyomavirus (HaPyV, former HaPV) genome encodes three structural proteins VP1, VP2, and VP3, where VP1 represents the major capsid protein of 384 amino acids. Screening of sera from HaPyV-infected papilloma-bearing and papilloma-free hamsters demonstrated the immunodominant features of all three capsid proteins. For both groups of hamsters in the C-terminal region of VP1 immunodominant B-cell epitopes were identified in the regions between amino acids 305 and 351 and amino acids 351 and 384. The high flexibility of the C-terminal region of VP1 was confirmed by the formation of chimeric virus-like particles based on the coat protein of the RNA bacteriophage fr which was previously found to tolerate only very short-sized foreign insertions. Phage fr coat protein-derived virus-like particles tolerated the N-terminal fusion of amino acids 333-384, 351-384, 351-374, and 364-384, respectively, of VP1. The induction of VP1-specific antibodies in rabbits and mice by immunization with chimeric virus-like particles harboring amino acids 333-384, 351-384, and 364-384, respectively, of VP1 suggested the immunodominant nature of the C-terminal region of VP1.

The hamster polyomavirus--a brief review of recent knowledge

The hamster polyomavirus (HaPV) was first described in 1967 as a virus associated with skin epithelioma of the Syrian hamster. The tumors appear spontaneously in a hamster colony bred in Berlin-Buch (HaB). Virus particles isolated from skin epitheliomas cause lymphoma and leukemia when injected into newborn hamsters from a distinct colony bred in Potsdam, Germany (HaP). The viral genome has been totally sequenced and the overall genetic organization establishes HaPV as a member of the polyomaviruses. HaPV is a second example of an middle T (MT) antigen encoding polyomavirus and nucleotide sequence homologies designates the mouse polyomavirus (Py) as the closest relative. Lymphomas induced by HaPV in HaP hamsters do not contain virus particles but instead accumulate different amounts of nonrandomly deleted free and/or integrated viral genomes. Transgenic mice produced by microinjection of HaPV DNA into the pronucleus of fertilized eggs of Gat: NMRI mice developed both, epitheliomas and lymphomas. Both tumor types contain extrachromosomal DNA. HaPV DNA was found to replicate in hamster lymphoid and fibroblast cell lines. Fully reproductive cycles could be detected only in GD36 lymphoblastic leukemia cells. HaPV carries the full transforming properties of a polyomavirus in vitro. Immortalization of primary rat cells is essentially carried out by the HaPV large T (LT) antigen and coexpression of HaPV MT and HaPV small T (ST) antigen is required for full transformation of rat fibroblasts. The preferential binding of HaPV MT to c-Fyn, a Src family kinase, has been proposed as a mechanism leading to lymphoid malignancies. Heterologous expression of HaPV-VP1 allowed the formation of virus like particles (VLPs) resembling HaPV particles. The high flexibility of HaPV-VP1 for insertion of foreign peptides offers a broad range of potential applications, especially in vaccine development.

Formation of immunogenic virus-like particles by inserting epitopes into surface-exposed regions of hamster polyomavirus major capsid protein

We generated highly immunogenic virus-like particles that are based on the capsid protein VP1 of the hamster polyomavirus (HaPV-VP1) and harbor inserted foreign epitopes. The HaPV-VP1 regions spanning amino acids 81-88 (position 1), 222/223 (2), 244-246 (3), and 289-294 (4) were predicted to be surface exposed. An epitope of the pre-S1 region of the hepatitis B virus (designated S1; amino acid sequence DPAFR) was introduced into the predicted positions of VP1. All VP1/S1 fusion proteins were expressed in yeast and generated virus-like particles. Immunoassays using the S1-specific monoclonal antibody MA18/7 and immunization of C57Bl6 mice with different VP1/S1 constructs showed a pronounced reactivity and a strong S1-specific antibody response for particles carrying the insert in position 1, 2, 1+2, and 1+3. Our results suggest that HaPV-VP1 represents a highly flexible carrier moiety for the insertion of foreign sequences offering a broad range of potential uses, especially in vaccine development.

A novel polyomavirus (goose hemorrhagic polyomavirus) is the agent of hemorrhagic nephritis enteritis of geese

We have identified the etiological agent of hemorrhagic nephritis enteritis of geese (HNEG), a fatal disease of European geese. HNEG has been recognized in almost all goose breeding areas, with an epizootic pattern, and up to now, the infectious agent has remained unknown. In order to identify the causative agent, infected tissues from HNEG-affected geese were inoculated to 1-day-old goslings, which then developed clinical signs typical of HNEG. Tissue homogenates from these birds were subjected to Freon extraction followed by sucrose density gradient ultracentrifugation. The resulting main band was examined by electron microscopy and consisted of spherical, naked, papovavirus-like particles approximately 45 nm in diameter. The virus was isolated and propagated in goose kidney cell primary culture. Tissue- or culture-purified virus allowed the experimental reproduction of the disease in goslings. Random PCR amplification of viral nucleic acid produced a 1,175-bp fragment which was shown to be associated with field samples collected from geese affected by HNEG on commercial farms in France. Sequence analysis of the PCR product revealed a unique open reading frame, showing 63 to 72% amino acid similarity with the major capsid protein (VP1) of several polyomaviruses. Finally, based on phylogenetic analysis, we conclude that the causative agent of HNEG is closely related to but clearly distinct from other polyomaviruses; we thus have named this newly identified virus Goose hemorrhagic polyomavirus.