A novel scavenger receptor (EcSRECII) as a lipopolysaccharide recognition molecule involved in regulating NF-κB activation through extracellular EGF-like cysteine-rich repeat domains with lysosomes in Epinephelus coioides

Scavenger receptors (SRs), as multifunctional pattern recognition receptors, play an important role in innate immunity in mammals, however, their function in fish is limited. Herein, scavenger receptor F2 in Epinephelus coioides (EcSRECII) induced an innate immune response to LPS in GS cells. EcSRECII markedly enhanced LPS-induced NF-kappa B and IFN-ss signaling pathways, whereas knockdown of EcSRECII significantly inhibited LPS-induced NF-kappa B and IFN-ss promoter activation. Interestingly, only retain of epidermal growth factor (EGF)/EGF-like domain in EcSRECII resulted in a punctate cytoplasmic distribution, while the C-terminal domain exhibited a distinct cytoskeletal cytoplasmic distribution. Moreover, devoid of this EGF/EGF-like domain fragment more sharply impaired its ability to activate EcSRECII-induced NF-kappa B activation than deletion of the C-terminal domain region, but both domains significantly induced IFN-ss promoter activation. Full-length EcSRECII and the deletion mutant of C-terminal domain could partly colocalize with lysosomes by LPS derived from V. parahaemolyticus (V.p. LPS) in GS cells, but there was no similar distribution in the deletion mutant of EGF/EGF-like domain. This finding firstly suggested that the N-terminal EGF/EGF-like domain was necessary for the NF-kappa B signaling pathway to trigger resistance to vibrio infection and its functional exertion may be associated with lysosomes, thus providing insights into the regulation of resistance to vibrio infection in teleosts.