Grouper (Epinephelus coioides) IRAK-4 regulates activation of NF-κB and expression of inflammatory cytokines in grouper spleen cells.

IRAK-4 is a serine/threonine kinase that can bind to interleukin-1 receptor induced by interleukin-1. It plays a key role in the Toll-like receptor signaling pathway and is involved in innate and adaptive immune responses. In this study, piscine IRAK-4 significantly activated nuclear factor (NF)-κB signaling in grouper spleen cells. Grouper (Epinephelus coioides) IRAK-4 (EcIRAK-4) co-localized with EcMyD88 and did not impair EcMyD88-dependent NF-κB activation. Different doses of EcIRAK-4 caused different degrees of nuclear translocation of the transcription factor NF-κB p65 subunit, and it induced transcription of multiple pro-inflammatory cytokines. Using expression vectors of deletion domains or mutations at important sites of EcIRAK-4, we found that the EcIRAK-4 kinase domain is necessary for its signal transduction function. The conserved amino acid sites performed functions similar to those in mammals, and grouper-specific amino acids such as E339 also played important roles. These findings provide information about the functional characteristics of IRAK-4 in lower vertebrates.