Genome-wide identification, characterization, and expression of the Toll-like receptors in Japanese flounder (Paralichthys olivaceus)

Abstract Toll-like receptor (TLR) family is a key ancient component of pattern recognition receptors, conserved from insects to mammals, which plays significant roles in immune defense against pathogens invasion. Teleost-specific TLR family members have lost some of the mammalian TLR orthologues or produced duplicates, particularly all bacterial could recognize cell surface TLRs. Edwardsiella tarda is an important pathogen of Japanese flounder (Paralichthys olivaceus) resulting in great economic losses. However, there is little integrated information of the roles about TLRs in mediating immune response to E.tarda infection in Japanese flounder. Here, we performed data-mining on the P. olivaceus genome to extract eleven TLR members: TLR1, TLR2, TLR3, TLR5M, TLR5S, TLR7, TLR8, TLR9, TLR18, TLR21 and TLR22. The TLR family motifs, such as Toll/interleukin (IL)-1 receptor (TIR) domain and leucine rich repeat (LRR) are conserved in TLR1-3, 5M, 7-9, 18, 21 and 22. Phylogenetic analysis verified their identities and supported the classification of TLRs into six subfamilies as in other vertebrates. And then, the expression of P. olivaceus TLRs (PoTLRs) were widely detected in immune-related tissues including intestine, skin, spleen, kidney, liver, and gill. Subsequently, the PoTLRs were exhibited differential regulation in immune-related tissues, such as intestine, spleen, liver and kidney following bacterial challenge by infection with E.tarda, indicating their roles in the immune response. The syntenic analysis showed neighboring genes of TLR1, 7, 8 and 18 were relatively conserved among P.olivaceus, O.latipes, and C.semilaevis. Meanwhile, protein and protein interaction network (PPI) results explained that TLR genes interacted with immune-related mediating factors including Interleukin-related genes, e.g. Il1 and Il10, MyD88-dependent pathway, Sarm1 and NF-κB signaling pathway, which could bind to TIR domain. These analyses will contribute to the functional characterization of P. olivaceus TLRs in further study.