Nuclear factor 45 of tongue sole (Cynoglossus semilaevis): Evidence for functional differentiation between two isoforms in immune defense against viral and bacterial pathogens

Nuclear factor 45 (NF45) is known to play an important role in regulating interleukin-2 expression in mammals. The function of fish NF45 is largely unknown. In a previous study, we reported the identification of a NF45 (named CsNF45) from half smooth tongue sole (Cynoglossus semilaevis). In the present study, we identified an isoform of CsNF45 (named CsNF45i) from half smooth tongue sole and examined its biological properties in comparison with CsNF45. We found that CsNF45i is a truncated version of CsNF45 and lacks the N-terminal 38 residues of CsNF45. Genetic analysis showed that the CsNF45 gene consists of 14 exons and 13 introns, and that CsNF45 and CsNF45i are the products of alternative splicing. Constitutive expression of CsNF45 and CsNF45i occurred in multiple tissues but differed in patterns. Experimental infection with viral and bacterial pathogens upregulated the expression of both isoforms but to different degrees, with potent induction of CsNF45 being induced by bacterial pathogen, while dramatic induction of CsNF45i being induced by viral pathogen. Transient transfection analysis showed that both isoforms were localized in the nucleus and able to stimulate the activity of IL-2 promoter to comparable extents. To examine their in vivo effects, the two isoforms were overexpressed in tongue sole. Subsequent analysis showed that following viral and bacterial infection, the viral loads in CsNF45i-overexpressing fish were significantly lower than those in CsNF45-overexpressing fish, whereas the bacterial loads in CsNF45-overexpressing fish were significantly lower than those in CsNF45i-overexpressing fish. These results indicate that both CsNF45 and CsNF45i possess immunoregulatory properties, however, the two isoforms most likely participate in different aspects of host immune defense that target different pathogens.