2B and 3C Proteins of Senecavirus A Antagonize the Antiviral Activity of DDX21 via the Caspase-Dependent Degradation of DDX21.

Senecavirus A (SVA), also known as Seneca Valley virus, is a recently discovered picornavirus that can cause swine vesicular disease, posing a great threat to the global swine industry. It can replicate efficiently in cells, but the molecular mechanism remains poorly understood. This study determined the host's differentially expressed proteins (DEPs) during SVA infection using dimethyl labeling based on quantitative proteomics. Among the DE proteins, DDX21, a member of the DEAD (Asp-Glu-Ala-Asp)-box RNA helicase (DDX) family, was downregulated and demonstrated inhibiting SVA replication by overexpression and knockdown experiment. To antagonize this antiviral effect of DDX21, SVA infection induces the degradation of DDX21 by 2B and 3C proteins. The Co-IP results showed that 2B and 3C did not interact with DDX21, suggesting that the degradation of DDX21 did not depend on their interaction. Moreover, the 3C protein protease activity was necessary for the degradation of DDX21. Furthermore, our study revealed that the degradation of DDX21 by 2B and 3C proteins of SVA was achieved through the caspase pathway. These findings suggest that DDX21 was an effective antiviral factor for suppressing SVA infection and that SVA antagonized its antiviral effect by degrading DDX21, which will be useful to guide further studies into the mechanism of mutual regulation between SVA and the host.

The Cellular and Viral circRNAs Induced by Fowl Adenovirus Serotype 4 Infection.

Circular RNAs (circRNAs) are a new class of noncoding RNAs that play vital roles in many biological processes. Virus infection induces modifications in cellular circRNA transcriptomes and expresses viral circRNAs. The outbreaks of Hydropericardium-hepatitis syndrome (HHS) caused by fowl adenovirus serotype 4 (FAdV-4) have resulted in huge economic losses to the poultry industry worldwide. To investigate the expression of circRNAs during FAdV-4 infection, we performed transcriptome analysis of FAdV-4-infected leghorn male hepatoma (LMH) cells. In total, 19,154 cellular circRNAs and 135 differentially expressed (DE) cellular circRNAs were identified. The characteristics of the DE cellular circRNAs were analyzed and most of them were related to multiple biological processes according to GO and KEGG enrichment analysis. The accuracy of 10 cellular circRNAs were verified by semiquantitative RT-PCR and sequencing. The change trend was consistent with the RNA sequencing results. Moreover, 2014 viral circRNAs were identified and 10 circRNAs were verified by the same methods. Our analysis showed that seven circRNAs with the same 3' terminal and variable 5' terminal regions were located at pTP protein and DNA pol protein of FAdV-4, which may be generated via alternative splicing events. Moreover, the expression level of viral circRNAs was closely related to the replication efficiency of the virus and partial of the viral circRNAs promoted the replication of FAdV-4. Competing endogenous RNA analysis further showed that the effects of cellular and viral circRNAs on host or viral genes may act via miRNAs. Collectively, our findings first indicate that FAdV-4 infection induced the differential expression of cellular circRNAs and FAdV-4 also expressed viral circRNAs, some of which affected FAdV-4 replication. These findings will provide new clues for further understanding FAdV-4 and provide a basis for investigating host-virus interactions.

Characterization of two novel porcine reproductive and respiratory syndrome virus isolates with deletions in the GP2 gene.

Two newly emerged, porcine reproductive and respiratory syndrome virus (PRRSV) strains (Henan-A10 and A11) were isolated from the sera of aborting sows. Interestingly, both of the isolates could replicate in primary porcine alveolar macrophage (PAM) cells but not in MARC-145 cells. A phylogenetic tree based on the complete genome was constructed and the results showed that Henan-A10 and A11 were most closely related to other highly pathogenic PRRSV (HP-PRRSV) strains. However, genomic sequence analysis showed that Henan-A10 and A11 shared only 96.8-97.8% nucleotide identity with the representative HP-PRRSV strain JXA1. Notably, a 10 amino acids deletion in the GP2 endodomain was identified for the first time. A full-length, infectious cDNA clone of HuN4-F112 (attenuated strain from a HP-PRRSV) was used to construct a chimeric clone with the corresponding deletion in GP2. We found that the deletion did not affect viral growth in MARC-145 cells, indicating that the endodomain of PRRSV GP2 may be variable.

Characterization of two newly emerged isolates of porcine reproductive and respiratory syndrome virus from Northeast China in 2013.

A newly emerged porcine reproductive and respiratory syndrome virus (PRRSV) that has caused severe reproductive losses in sows appeared in some regions of China in 2013. To explore the biology of this new PRRSV and understand more fully genetic diversity in PRRSV isolates from China, the complete genome of the two 2013 Chinese isolates, designated HLJA1 and HLJB1, were analyzed. Genomic sequence analysis showed that HLJA1 and HLJB1 shared 88.6-98.3% nucleotide identity with genotype 2 (North American type, NA-type) isolates, but only 61.1% with the genotype 1 (European type, EU-type) isolate of Lelystad virus, indicating that both these isolates belong to the NA-type PRRSV genotype. Phylogenetic analysis showed that the NA-type PRRSV isolates formed three subgroups (1, 2 and 3); representatives of these subgroups are VR-2332, CH-1a and HUN4, respectively. HLJA1 and HLJB1 belong to subgroup 2. Analysis of NSP2 revealed that HLJA1 has a 48-amino acid deletion at positions 473-480 and 482-521, unlike other HP-PRRSV isolates, while HLJB1 has only a 1-amino acid deletion at position 481 compared with CH-1a. Interestingly, HLJA1 replicated in PAM cells but not in MARC-145 cells, whereas HLJB1 replicated in both cell types. The neutralizing antibody titer of pig hyperimmune sera against HUN4 was significantly higher than that of HLJA1 or HLJB1. Additionally, genetic variability in GP5 and GP3 proteins and in the novel ORF5a protein was evident. In addition to elucidating the genetic relationships between PRRSV isolates, our results suggest that Chinese PRRSV will remain a pandemic virus.