Identification and functional analysis of the novel ORF4 protein encoded by porcine circovirus type 2

Regulatory role of ASK1 in porcine circovirus type 2-induced apoptosis

Porcine circovirus type 2 (PCV2), a major causative agent of postweaning multisystemic wasting syndrome which is an emerging and important swine disease, can induce apoptotic responses in vitro and in vivo. However, the molecular mechanism of PCV2-induced apoptosis is not well understood. Apoptosis signal-regulating kinase 1 (ASK1), serves as an upstream enzyme that activates the JNK and p38 MAPK pathways, has been shown to play a target role in the regulation of apoptosis. Here, we showed that PCV2 infection induced ASK1 phosphorylation which preceded activation of JNK1/2 and p38 signaling in the cultured cells, and that the phosphatidylinostol 3-kinase (PI3K)/Akt signaling triggered by PCV2 infection limited the extent of JNK1/2 and p38 activation and thereby apoptotic cell death. Furthermore, inhibition of JNK and p38 activation is associated with PI3K-dependent negative-regulation of ASK1 in the PCV2-infected cells. These results indicate an important regulatory role of ASK1 in PCV2-induced apoptotic responses.

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

Porcine circovirus type 2 (PCV2) infection is associated with porcine circovirus-associated diseases in pigs, which is a serious threat to the swine industry worldwide. To date, only three open reading frames (ORFs) within the PCV2 genome have been reported: ORF1 codes for two replicase proteins (Rep and Rep'), ORF2 for the structural protein (Cap), and ORF3 for a protein implicated in cellular apoptosis. In this study, based on transcription analysis of ORF3 mRNA, a potential ORF4 mRNA was detected and characterized by real-time RT-PCR and rapid amplification of cDNA ends analysis. The results indicate that the ORF4 gene is expressed at the level of transcription in the PCV2-infected cells. In addition, a novel ORF3 associated (ORF3') mRNA was identified during virus replication in PK15 cells. Moreover, a 3' poly(A) addition signal sequence (AUUAAA, nt 258-263) was found 10-30 nucleotides upstream of the cleavage site in the novel ORF4 mRNA in the complementary-strand of the PCV2 genome. Furthermore, alternate trans-splicing was identified in the ORF3' mRNA between orientation diverse transcripts with typical GT-AG donor/acceptor junctions. Similar strategies as in this work can be applied to examine the transcription of other potential ORFs in PCV in the future.