Innate immune system recognises
pathogen-associated molecular patterns (
PAMPs) by limited number of germline encoded and non-clonally developed pathogen recognition receptors (
PRRs).
Retinoic acid-inducible gene I (RIG-I)-like receptors (RLRs) are important cytosolic
PRRs for sensing viral RNAs. The receptor encoded by
melanoma differentiation associated gene 5 (MDA5), an RLR, recognises
viral RNA and enhances
antiviral response in host cells. The full-length MDA5
cDNA in Etroplus suratensis was cloned and found to have 3673
nucleotides encoding a
polypeptide of 978
amino acids. The deduced amino acid sequence contains four main structural domains: two CARD domains in the N-terminal region, a DExDc (DEAH/DEAD box helicase domain), HELICc (C-terminal helicase) domain and a C-terminal regulatory domain (RD). Phylogenetic analysis revealed a close relationship of E. suratensis MDA5 (EsMDA5) with MDA5 of Neolamprologus brichardi and Oreochromis niloticus, both belonging to Cichlidae family. EsMDA5 transcripts were ubiquitously expressed in all the 12 tissues tested in healthy fish. Although, transcript level was found to be the highest in muscle, high expression was also detected in the spleen, head kidney and hindgut. In
poly I:C-injected fish, EsMDA5 transcripts showed peak expression in the spleen, intestine and heart at 12h post-injection (hpi). However, in gill and kidney tissues, maximum up-regulation of EsMDA5 was observed at 6 and 48 hpi, respectively. Further, liver tissue showed an increasing trend in expression profile from 6 to 48 hpi.
Interferon promoter stimulator-1 (IPS-1) gene, an adaptor triggering RIG-I- and MDA5-mediated
type I interferon induction, also showed up-regulated expression at initial time-points in
poly I:C-injected E. suratensis. The constitutive expression and up-regulation of EsMDA5 and the IPS-1 genes in different tissues indicate that EsMDA5 may play an important role in sensing viral
PAMPs in conjunction with IPS-1.