Gig1 and Gig2 homologs (CiGig1 and CiGig2) from grass carp (Ctenopharyngodon idella) display good antiviral activities in an IFN-independent pathway

The virus-induced genes, Gig1 and Gig2, were identified first as IFN-stimulated genes (ISGs) from CAB cells. Previous studies suggested that Gig protein may have some potential antiviral functions. In this study, we cloned and identified the full-length cDNA sequences of Gig1 and Gig2 homologs (designated as CiGig1 and CiGig2, respectively) from grass carp (Ctenopharyngodon idella). The complete cDNA sequences of Gig1 and Gig2 contain 1231 bp and 690 bp, encoding for a 194 amino acid protein and a 158 amino acid protein, respectively. Their structure characteristics of CiGig1 and CiGig2 are highly similar to the corresponding homologues in crucian carp. The tissue-specific expressions of CiGig1 and CiGig2 in liver, spleen, kidney, intestine, gill and heart were significantly up-regulated following GCHV challenge. The results indicated that CiGig1 and CiGig2 may be involved in the antiviral immune responses in cells. To better understand the antiviral functions of CiGig1 and CiGig2 in vivo, CiGig1 or CiGig2 ORF cDNA were inserted into the plasmid pcDNA3.1, respectively. Subsequently, the recombinant plasmids were transfected into C. idellus kidney (CIK) cells. The over-expressions of CiGig1 and CiGig2 were observed in the CIK cells after treatment with GCHV. Cells with pcDNA3.1-CiGig1 or pcDNA-CiGig2 exhibited a relatively higher survival rate of (70.84% or 69.24%) than non-transfection (22.16%) and mock-vehicle controls (24.38%) following the virus infection. Our data showed that both CiGig1 and CiGig2 could exert antiviral effects effectively in vivo. Cycloheximide blocking protein synthesis demonstrated that both CiGig1 and CiGig2 mRNA expression could be induced by GCHV rather than by recombinant grass carp IFN (rCiIFN) directly, suggesting that CiGig1 and CiGig2 may not be IFN-stimulated genes since they display their antivirus activities in an IFN-independent pathway.