Pathogen recognition receptors in channel catfish: II. Identification, phylogeny and expression of retinoic acid-inducible gene I (RIG-I)-like receptors (RLRs).

Vertebrates including teleost fish have evolved an array of pathogen recognition receptors (PRRs) for detecting and responding to various pathogen-associated molecular patterns (PAMPs), including Toll-like receptors (TLRs), nucleotide-binding domain, leucine-rich repeat containing receptors (NLRs), and the retinoic acid inducible gene I (RIG-I) like receptors (RLRs). As a part of the series of studies targeted to characterize catfish PRRs, we described 22 NLR receptors in the sister contribution. Here in this study, we focused on cytosolic PRRs recognizing nucleotide pathogen-associated molecular patterns (PAMPs) of invading viruses, the retinoic acid-inducible gene I (RIG-I)-like receptors (RLR receptors). Three RLRs with DExD/H domain containing RNA helicases, retinoic acid inducible gene-I (RIG-I), melanoma differentiation-associated gene 5 (MDA5) and laboratory of genetics and physiology 2 (LGP2), were identified from channel catfish, Ictalurus punctatus. The catfish RIG-I encodes 937 amino acids that contains two CARDs, a DExDc, a HELICc and a RD domains. MDA5 encodes 1005 amino acids with all the domains identified for RIG-I. LGP2 encodes 677 amino acids that contain other domains but not the CARD domain at the N-terminus. Phylogenetic analyses of the three genes of catfish showed close clustering with their counterparts from other teleost fish. All the genes were found to be constitutively expressed in various tissues of catfish with minor variations. Channel catfish ovarian cells when infected with channel catfish virus showed significant increase in the transcript abundance of all the three genes. Further, RLR genes showed significant increases in expression in the liver tissue collected at different time-points after bacterial infection as well. The results indicate that the catfish RLRs may play important roles in antiviral and anti-bacterial immune responses.