Oyster TBK1/IKKε responds to bacterial and viral challenges and participates in the innate immune signaling

The Pacific oyster, Crassostrea gigas, is distributed worldwide and has important economic value. However, in recent years, oyster farming has been seriously plagued by diseases, and research on the innate immune system of oysters is scarce. The IKK (Inhibitor of NF-kappa B kinases)-related kinases TBK1 (TANK-binding kinase-1) and IKK epsilon are crucial signal transduction proteins, which play important roles in NF-kappa B and IRF3/7 activation. In this study, a TBK1/IKK epsilon homolog (termed CgTBK1/IKK epsilon) was obtained from the Pacific oyster. The open reading frame of CgTBK1/IKK epsilon is 2259-bp and encodes 752 amino acids. Quantitative real-time polymerase chain reaction results revealed that CgTBK1/IKK epsilon transcripts could be detected in all test tissues, and its expression was induced by lipopolysaccharide and polyinosinic-polycytidylic acid challenge. The results of co-immunoprecipitation assays showed that CgTBK1/IKK epsilon could interact with oyster mitochondria antiviral signaling (MAVS) and myeloid differential protein-88 (MyD88), which are the key adaptor proteins of the retinoic acid-inducible gene-I-like receptor and toll-like receptor signaling pathways, respectively. Additionally, CgTBK1/IKK epsilon, CgTBK1, and CgIKK epsilon-like can combine with each other and form a complex, which may be critical for the immune signaling transduction and downstream target protein activation. Finally, CgTBK1/IKK epsilon transfection into human cell lines inhibited the activation activity of interferon-stimulated response element (ISRE) and NF-kappa B reporter genes caused by CgIKK epsilon-like transfection. In summary, these results indicate that CgTBK1/IKK epsilon can respond to pathogen infection, participate in immune signaling, and regulate the activation of NF-kappa B and ISRE reporter genes. Therefore, CgTBK1/IKK epsilon may be a key signal transduction molecule and immune activity regulator, and may play an important role in the immune system of oysters.