Identification and characterization of two novel transcription units of porcine circovirus 2

Therapeutic applications of trans-splicing

BACKGROUND

RNA trans-splicing joins exons from different pre-mRNA transcripts to generate a chimeric product. Trans-splicing can also occur at the protein level, with split inteins mediating the ligation of separate gene products to generate a mature protein.

SOURCES OF DATA

Comprehensive literature search of published research papers and reviews using Pubmed.

AREAS OF AGREEMENT

Trans-splicing techniques have been used to target a wide range of diseases in both in vitro and in vivo models, resulting in RNA, protein and functional correction.

AREAS OF CONTROVERSY

Off-target effects can lead to therapeutically undesirable consequences. In vivo efficacy is typically low, and delivery issues remain a challenge.

GROWING POINTS

Trans-splicing provides a promising avenue for developing novel therapeutic approaches. However, much more research needs to be done before developing towards preclinical studies.

AREAS TIMELY FOR DEVELOPING RESEARCH

Increasing trans-splicing efficacy and specificity by rational design, screening and competitive inhibition of endogenous cis-splicing.

Genomic sequence and virulence of a novel NADC30-like porcine reproductive and respiratory syndrome virus isolate from the Hebei province of China

Porcine reproductive and respiratory syndrome virus (PRRSV) is the causative agent of porcine reproductive and respiratory syndrome (PRRS), which results in immense economic losses in the swine industry. Outbreaks of disease caused by NADC30-like PRRSV are of great concern in China. Here, a novel variant, NADC30-like PRRSV strain HB17A, was analyzed and its pathogenicity in pigs was examined. The full-length genome sequence of HB17A shared 83.6-95.1% nucleotide similarity with NADC30-like and NADC30 PRRSV without any gene insertions, but with a unique 2-amino acid deletion in Nsp2. A phylogenetic analysis showed that HB17A clustered with NADC30 strains. Different degrees of variation in the signal peptide, transmembrane region (TM), primary neutralizing epitope (PNE), non-neutral epitopes, and N-glycosylation sites were observed in GP5. Challenge experiments showed that HB17A infection resulted in persistent fever, moderate respiratory clinical signs, low levels of viremia and viral loads in serum, and mild gross and microscopic lung lesions. Moreover, IFN-γ, IL-6, and IL-10 cytokine levels were significantly elevated in serum, but the levels of IFN-α and IL-2 were similar to those of the negative controls. HB17A was less pathogenic but was secreted longer in nasal discharge than HP-PRRSV FZ06A. Our findings indicate that HB17A is a novel NADC30-like strain with certain deletions and mutations but with no evidence of genomic recombination. This strain exhibits intermediate virulence in pigs. This research will be help define the evolutionary characteristics of Chinese NADC30-like PRRSV.

Mouse models of porcine circovirus 2 infection

PCV2 is considered the main pathogen of porcine circovirus diseases and porcine circovirus-associated diseases (PCVD/PCVAD). However, the exact mechanism underlying PCVD/PCVAD is currently unknown. Mouse models of PCV2 are valuable experimental tools that can shed light on the pathogenesis of infection and will enable the evaluation of antiviral agents and vaccine candidates. In this review, we discuss the current state of knowledge of mouse models used in PCV2 research that has been performed to date, highlighting their strengths and limitations, as well as prospects for future PCV2 studies.

Peptides mimicking viral proteins of porcine circovirus type 2 were profiled by the spectrum of mouse anti-PCV2 antibodies

BACKGROUND

Porcine circovirus 2 (PCV2) is a small, non-enveloped DNA virus causing swine lymphocyte depletion and severe impact on the swine industry. The aim of this study was to evaluate the antigenicity and immunogenicity of specific peptides, and seeking the potential candidate of PCV2 peptide-based vaccine. It's initiating from peptides reacting with PCV2-infected pig sera and peptide-immunized mouse sera.

RESULTS

The data showed that the sera from PCV2-infected pigs could react with the N-terminal (C1), middle region (C2), and C-terminal peptide (C3) of PCV2 capsid protein (CP), ORF3 protein (N1), ORF6 protein (N2) and ORF9 protein (N3). This study demonstrated that anti-PCV2 mouse antisera could be generated by specific synthetic peptides (C3 and N2) and recognized PCV2 viral protein. We found that the tertiary or linear form C-terminal sequence (C3) of PCV2 capsid peptide only appeared a local distribution in the nucleus of PCV2-infected PK cells, virus-like particles of PCV2 major appeared a local distribution in the cytoplasm, and ORF 6 protein of PCV2 were shown unusually in cytoplasm. Furthermore, most residues of the C1 and the C3 were presented on the surface of PCV2 CP, in the view of 3-D structure of the CP. Our data demonstrated that PCV2-infected pigs had higher OD₄₀₅ value of anti-C3 IgG on Day 1, Month 3 and Month 6 than in Month 1. These pigs had higher anti-C3 IgM level in Month 3 and Month 6 than on Day 1 (P < 0.01).

CONCLUSIONS

We demonstrated that the key peptide (C3) mimic the C-terminal of PCV2 capsid protein which were capable of inducing antibodies. The specific antibody against the C3 were confirmed as the serological marker in PCV2-infected pigs.

Porcine circovirus type 2 (PCV2) evolution before and after the vaccination introduction: A large scale epidemiological study

Since their commercialization, vaccines against Porcine circovirus type 2 (PCV2) have been the cornerstone control strategy. Nevertheless, the periodic emergence of new genotype waves and the recent reports of vaccine failure outbreaks have raised the question if widespread vaccination strategies could have driven viral evolution and affected different genotype fitness. To investigate this issue an in-deep analysis, based on a bioinformatics and biostatistics approach, has been implemented. ORF2 sequences from vaccinated and non-vaccinated populations (i.e. domestic pigs before and after vaccine introduction and wild boars) were considered. The action of selective forces on PCV2 strains has been analyzed and compared among groups. Remarkable differences were found in the selective forces acting on viral populations circulating in different “immune environments”. Particularly for PCV2a, a directional selection promoting a change in the viral capsid away from the vaccine specific antigenic determinants has been detected after vaccine introduction. Involved amino acids were previously reported to be part of viral epitopes whose variability is responsible of immune escape. Our findings support a change in PCV2 evolutionary pattern after widespread vaccination introduction and stress once more the compulsoriness of a continuous monitoring of PCV2 epidemiology to promptly act in response to the emergence of possible vaccine-escaping mutants.

Evolutionary Insights into RNA trans-Splicing in Vertebrates

Pre-RNA splicing is an essential step in generating mature mRNA. RNA trans-splicing combines two separate pre-mRNA molecules to form a chimeric non-co-linear RNA, which may exert a function distinct from its original molecules. Trans-spliced RNAs may encode novel proteins or serve as noncoding or regulatory RNAs. These novel RNAs not only increase the complexity of the proteome but also provide new regulatory mechanisms for gene expression. An increasing amount of evidence indicates that trans-splicing occurs frequently in both physiological and pathological processes. In addition, mRNA reprogramming based on trans-splicing has been successfully applied in RNA-based therapies for human genetic diseases. Nevertheless, clarifying the extent and evolution of trans-splicing in vertebrates and developing detection methods for trans-splicing remain challenging. In this review, we summarize previous research, highlight recent advances in trans-splicing, and discuss possible splicing mechanisms and functions from an evolutionary viewpoint.

Transcriptional analysis of porcine circovirus-like virus P1

Background

Recently identified porcine circovirus-like virus P1 has the smallest DNA viral genome. In this study, we identified the viral genes and their corresponding mRNA transcripts.

Results

The RNAs of P1, synthesized in porcine kidney cells, were examined with northern blotting and PCR analyses.

Eight virus-specific RNAs were detected. Four mRNAs (open reading frames (ORFs) 1, 2, 4, and 5) are encoded by the viral (−) strand and four (ORFs 3, 6, 7, and 8) are encoded by the viral (+) strand. All proteins encoded by the ORFs of the P1 virus are less than 50 amino acids in length, except that encoded by ORF1 (113 amino acids).

Conclusions

We show a very complex viral transcription pattern in P1-infected cells.

ORF4-protein deficient PCV2 mutants enhance virus-induced apoptosis and show differential expression of mRNAs in vitro

Porcine circovirus type 2 (PCV2) is the essential infectious agent of PCV associated disease (PCVAD). During previous in vitro studies, 11 RNAs and four viral proteins have been detected in PCV2-infected cells. Open reading frame (ORF) 4 is 180bp in length and has been identified at the transcription and the translation level. It overlaps completely with ORF3, which has a role in virus-induced apoptosis. In this study, start codon mutations (M1-PCV2) or in-frame termination mutations (M2-PCV2) were utilized to construct two ORF4-protein deficient viruses aiming to investigate its role in viral infection. The abilities of M1-PCV2 and M2-PCV2 to replicate, transcribe, express viral proteins, and to cause cellular apoptosis were evaluated. Viral DNA replication curves supported that the ORF4 protein is not essential for viral replication, but inhibits viral replication in the early stage of infection. Comparison of the expression level of ORF3 mRNA among wild-type and ORF4-deficient viruses in infected PK-15 cell demonstrated enhanced ORF3 transcription of both ORF4 mutants suggesting that the ORF4 protein may play an important role by restricting ORF3 transcription thereby preventing virus-induced apoptosis. This is further confirmed by the significantly higher caspase 3 and 8 activities in M1-PCV2 and M2-PCV2 compared to wild-type PCV2. Furthermore, the role of ORF4 in cell apoptosis and a possible interaction with the ORF1 associated Rep protein could perhaps explain the rapid viral growth in the early stage of infection and the higher expression level of ORF1 mRNA in ORF4 protein deficient PCV2 mutants.