Toll-like Receptor 3 in the Hybrid Yellow Catfish (Pelteobagrus fulvidraco ♀ × P. vachelli ♂): Protein Structure, Evolution and Immune Response to Exogenous Aeromonas hydrophila and Poly (I:C) Stimuli

Simple Summary Yellow catfish is an economically important fish that lives in the middle and upper reaches of the Yangtze River and the Pearl River Basin in China. Due to the intensive farming and overuse of drugs, viral and bacterial diseases have emerged and have caused serious problems in domestic fisheries during recent years and thus threaten the production of aquaculture. It has been proved that exploring disease-resistant varieties is the key way to raising the aquaculture production of yellow catfish. Previous research showed that TLR3 is involved in the antiviral response in mammals. However, the molecular mechanism of immune response against viruses in teleost fish remains unclear. In this study, the gene structure and evolutionary conservation of TLR3 in teleost fish, as well as the tissue distribution in hybrid yellow catfish and its response to Aeromonas hydrophila and poly (I:C) at different time points, were systematically investigated. Our results showed that TLR3 played a significant role in hybrid yellow catfish response to viral infection. Our study provides theoretical basis for further research on the molecular mechanism of antivirus response in fish and the cultivation of hybrid yellow catfish with high resistance. Abstract As a major mediator of cellular response to viral infection in mammals, Toll-like receptor 3 (TLR3) was proved to respond to double-stranded RNA (dsRNA). However, the molecular mechanism by which TLR3 functions in the viral infection response in teleosts remains to be investigated. In this study, the Toll-like receptor 3 gene of the hybrid yellow catfish was identified and characterized by comparative genomics. Furthermore, multiple sequence alignment, genomic synteny and phylogenetic analysis suggested that the homologous TLR3 genes were unique to teleosts. Gene structure analysis showed that five exons and four introns were common components of TLR3s in the 12 examined species, and interestingly the third exon in teleosts was the same length of 194 bp. Genomic synteny analysis indicated that TLR3s were highly conserved in various teleosts, with similar organizations of gene arrangement. De novo predictions showed that TLR3s were horseshoe-shaped in multiple taxa except for avian (with a round-shaped structure). Phylogenetic topology showed that the evolution of TLR3 was consistent with the evolution of the studied species. Selection analysis showed that the evolution rates of TLR3 proteins were usually higher than those of TLR3-TIR domains, indicating that the latter were more conserved. Tissue distribution analysis showed that TLR3s were widely distributed in the 12 tested tissues, with the highest transcriptions in liver and intestine. In addition, the transcription levels of TLR3 were significantly increased in immune-related tissues after infection of exogenous Aeromonas hydrophila and poly (I:C). Molecular docking showed that TLR3 in teleosts forms a complex with poly (I:C). In summary, our present results suggest that TLR3 is a pattern recognition receptor (PRR) gene in the immune response to pathogen infections in hybrid yellow catfish.