TBK1 regulates the induction of innate immune response against GCRV by phosphorylating IRF3 in rare minnow (Gobiocypris rarus).

Rare minnow (Gobiocypris rarus), a small cyprinid species that is highly sensitive to the grass carp reovirus (GCRV), is regarded as an ideal model to study the mechanisms of innate immunity in fish. In the present study, a TBK1 homologue from rare minnow (GrTBK1) was identified and its roles in defence against viral infection were investigated. Sequence analysis showed that GrTBK1 encoded a 727-amino acid peptide which shared 98% and 72% identity to the black carp (Mylopharyngodon piceus) and human (Homo sapiens) orthologues, respectively. The amino acid sequence analysis demonstrated that GrTBK1 contains a conserved Serine/Threonine protein kinases catalytic domain (S_TKc) at the N-terminus. Furthermore, cellular distribution proved that GrTBK1 was located in the cytoplasm region. Quantitative real-time PCR analysis revealed that GrTBK1 was ubiquitously expressed in all examined organs, but especially highly in liver. Temporal expression analysis in vivo showed that the expression levels of GrTBK1 were obviously up-regulated in response to GCRV infection. Meanwhile, qRT-PCR assay revealed that the levels of S7 RNA, an important segment of GCRV genome, were higher in the liver than in other tissues. This indicates that GrTBK1 might play a crucial role in responses to GCRV infection in fish. In addition, GrTBK1 activated several type I interferon (IFN) promoters and induced the expression of downstream type I IFN-stimulated genes (ISGs). Furthermore, GrTBK1 obviously phosphorylated the interferon regulatory factor 3 (IRF3). Furthermore, overexpression of GrTBK1 remarkably decreased the GCRV proliferation. In summary, we systematically characterized GrTBK1 and illustrated its role in the innate immune response to GCRV infections.