Sequence analysis of genome segment 10 encoding the major outer capsid protein (VP7) of genogroup B aquareovirus and its relationship with the VP7 protein of genogroup A aquareovirus

Using long, sequence-specific dsRNA to knockdown inducible protein expression and virus production via an RNAi-like mechanism

RNA interference (RNAi) is a powerful innate immune mechanism to knock down translation of specific proteins whose machinery is conserved from plants to mammals. The template used to determine which mRNA's translation is inhibited is dsRNA, whose origin can range from viruses (long dsRNA, ∼100-1000s bp) to host (micro(mi)RNA, ∼20mers). While miRNA-mediated RNAi is well described in vertebrates, the ability of long dsRNA to guide RNAi-mediated translation inhibition in vertebrates is controversial. Indeed, as long dsRNA is so effective at inducing type I interferons (IFNs), and IFNs down-regulate RNAi machinery, it is believed that IFN-competent cells are not capable of using long dsRNA for RNAi. In the present study the ability of long, sequence specific dsRNA to knock down both host protein expression and viral replication is investigated in IFN-competent rainbow trout cells. Before exploring RNAi effects, the optimal dsRNA concentration that would funnel into RNAi without triggering the IFN response was determined. After which, the ability of sequence specific long dsRNA to target knockdown via RNAi was evaluated in: (1) uninfected host cells using inducible luciferase gene expression and (2) host cells infected with chum salmon reovirus (CSV), frog virus 3 (FV3) or viral hemorrhagic septicemia virus genotype IVa (VHSV-IVa). Induced expression studies utilized RTG-P1, a luciferase reporter cell line, and dsRNA containing luciferase sequence (dsRNA-Luc) or a mis-matched sequence (dsRNA-GFP), and subsequent luminescence intensity was measured. Anti-CSV studies used dsRNA-CSVseg7 and dsRNA-CSVseg10 to target CSV segment 7 and CSV segment 10 respectively. Inhibition of virus replication was measured by viral titration and RT-qPCR. Taking advantage of the fact that long dsRNA can accommodate more sequences than miRNAs, the antiviral capability of dsRNA molecules containing both CSV segment 7 and segment 10 simultaneously was also measured. Target sequence appears important, as dsRNA-FV3MCP did not knock down FV3 titres, and while dsRNA-VHSV-N knocked down VHSV-IVa, dsRNA-VHSV-G and dsRNA-VHSV-M did not. This is the first study in fish to provide evidence that sequence specific long dsRNA induces potent gene expression silencing and antiviral responses in vitro via an RNAi-like mechanism instead of an IFN-dependent response.

Aquareovirus effects syncytiogenesis by using a novel member of the FAST protein family translated from a noncanonical translation start site

As nonenveloped viruses, the aquareoviruses and orthoreoviruses are unusual in their ability to induce cell-cell fusion and syncytium formation. While an extraordinary family of fusion-associated small transmembrane (FAST) proteins is responsible for orthoreovirus syncytiogenesis, the basis for aquareovirus-induced syncytiogenesis is unknown. We now report that the S7 genome segment of an Atlantic salmon reovirus is polycistronic and uses a noncanonical CUG translation start codon to produce a 22-kDa integral membrane protein responsible for syncytiogenesis. The aquareovirus p22 protein represents a fourth distinct member of the FAST family with a unique repertoire and arrangement of structural motifs.

Molecular characterization of a major outer capsid protein encoded by the Threadfin aquareovirus (TFV) gene segment 10 (S10)

Genome segment 10 (S10) of Threadfin aquareovirus (TFV) was cloned, sequenced, analyzed and found to be 987 bp long encoding a protein of 298 aa with a predicted molecular mass of 32.0 kDa. The TFV S10 gene possesses terminal motifs, (5' GTTTTA and ATTCATC 3') which are also conserved in the S6 and S11 TFV gene segments. Sequence comparison revealed that the TFV S10 gene was similar to the Striped bass reovirus (SBR) VP7 outer capsid protein (OCP). A conserved putative zinc-finger motif, CCHC, present in the mammalian reovirus (MRV) delta3 protein, was identified in TFV and other aquareovirus VP7 protein. Phylogenetic analysis of the TFV VP7 protein indicated that TFV is closely related to SBR and Chum salmon reovirus (CSV) and possibly belong to the same species Aquareovirus A as SBR and CSV. The TFV VP7 protein was expressed in E. coli, purified and injected into mice. Serum specific antibodies were generated, however, the serum showed weak neutralizing activity. In contrast, co-incubation of this serum with another serum obtained from mice immunized with another OCP encoded by the TFV S6 gene segment resulted in a highly elevated antibody neutralization titer.

Orthoreovirus and Aquareovirus core proteins: conserved enzymatic surfaces, but not protein-protein interfaces

Orthoreoviruses and Aquareoviruses constitute two respective genera in the family Reoviridae of double-stranded RNA viruses. Orthoreoviruses infect mammals, birds, and reptiles and have a genome comprising 10 RNA segments. Aquareoviruses infect fish and have a genome comprising 11 RNA segments. Despite these differences, recent structural and nucleotide sequence evidence indicate that the proteins of Orthoreoviruses and Aquareoviruses share many similarities. The focus of this review is on the structure and function of the Orthoreovirus core proteins lambda1, lambda2, lambda3, and sigma2, for which X-ray crystal structures have been recently reported. The homologous core proteins in Aquareoviruses are VP3, VP1, VP2, and VP6, respectively. By mapping the locations of conserved residues onto the Orthoreovirus crystal structures, we have found that enzymatic surfaces involved in mRNA synthesis are well conserved between these two groups of viruses, whereas several surfaces involved in protein-protein interactions are not well conserved. Other evidence indicates that the Orthoreovirus mu2 and Aquareovirus VP5 proteins are homologous, suggesting that VP5 is a core protein as mu2 is known to be. These findings provide further evidence that Orthoreoviruses and Aquareoviruses have diverged from a common ancestor and contribute to a growing understanding of the functions of the core proteins in viral mRNA synthesis.