Identification of a retinoic acid-inducible gene I from grass carp (Ctenopharyngodon idella) and expression analysis in vivo and in vitro

RIG-I (retinoic acid inducible gene-I) is a key mediator of antiviral immunity, able to couple detection of infection by RNA and DNA viruses to the induction of interferons. In the present study, a RIG-I gene from grass carp Ctenopharyngodon idella (CiRIG-I) was isolated and characterized. The full-length cDNA of CiRIG-I was of 3198 bp and encoded a polypeptide of 947 amino acids with an estimated molecular mass of 108,730 Da and a predicted isoelectric point of 5.85, including six main overlapping structural domains: two CARDs (caspase activation and recruitment domain), one ResIII (conserved restriction domain of bacterial type III restriction enzyme), one DEXDc (DEAD/DEAH box helicase domain), one HELICc (helicase superfamily c-terminal domain) and one RD (regulatory domain). The CiRIG-I mRNA was widespread expression in the tested 15 tissues by semi-quantitative RT-PCR (sqRT-PCR) assay. The CiRIG-I expressions in spleen and liver were significantly induced following grass carp reovirus (GCRV) infection. CiRIG-I mRNA expression was rapidly and significantly up-regulated in vitro after GCRV infection, and the CiRIG-I transcripts were also significantly enhanced in vitro post the synthetic double stranded RNA polyinosinic–polycytidylic potassium salt (poly(I:C)) stimulation. These results collectively suggested that CiRIG-I was an inducible protein, involved in the antiviral innate immune defense to GCRV in grass carp, and laid the foundation for the further mechanism research of RIG-I in fishes.