Molecular characterization and functional analysis of a Rel gene in the Pacific oyster

The nuclear factor-κB (NF-κB) signaling pathway plays a crucial role in regulating many physiological processes such as development, inflammation, apoptosis, cell proliferation, differentiation and immune responses. And the NF-κB/Rel family members were considered as the most important transcription factors in the NF-κB signaling pathway. In this study, we cloned a Rel homolog gene (named as CgRel2) from the Pacific oyster, Crassostrea gigas. The 2115-bp open reading frame (ORF) encodes 704 amino acids and CgRel2 possesses a conserved Rel Homology Domain (RHD) at the N-terminus. Phylogenetic analysis revealed that CgRel2 is most closely related to Pinctada fucata dorsal protein. CgRel2 transcripts are widely expressed in all tested tissues, with the highest expression observed in the labial palp and the gill. Moreover, the expression of CgRel2 is significantly upregulated after lipopolysaccharide (LPS), peptidoglycan (PGN), and polyinosinic-polycytidylic acid [poly(I:C)] challenge. CgRel2 transfection into human cell lines activated NF-κB, TNFα and oyster IL-17 (CgIL-17) reporter genes in a dose-dependent manner, while CgRel2 overexpression cannot induce ISRE (Interferon stimulation response element) reporter gene's transcriptional activity. Additionally, the results of co-immunoprecipitation showed that CgRel2 or CgRel1 could interact with oyster IκB1, IκB2 and IκB3 proteins strongly, which may be critical for the immune signaling transduction and the regulation of its immune functions. Together, these results suggest that CgRel2 could respond to pathogenic infection, participate in the immune signal transduction and activate NF-κB, TNFα and CgIL-17 reporter genes. Thus, CgRel2 could play an important role in the oyster immune system.