Preliminary analysis of the structural and functional differences between Squaliobarbus curriculus and Ctenopharyngodon idella TLR19s

In order to explore the structural and functional differences between a TLR19 homolog (designated as ScTLR19) from S. curriculus and the reported TLR19 (designated as CiTLR19) from C. idellus, we cloned the full-length cDNA sequence of ScTLR19 gene and analyzed its sequence and structural characteristics, investigated its expression patterns of various tissues and the responses to grass carp reovirus (GCRV) infection with real-time fluorescent quantitative PCR (qPCR) technique, and revealed its distinct regulation on the downstream functional genes of TLR19 signaling pathway by overexpression experiments in this study. The results showed that the ScTLR19 gene encoded 957 amino acids, whose sequence shared high similarity (94.04%) with that of CiTLR19. The extracellular region of ScTLR19 protein consisted of nine leucine-rich repeats (LRRs), and harbored one more LRR than that for CiTLR19 at the amino acid sites of 654~677. The ScTLR19 also owned two more α-helixes and six more β-folds than that of CiTLR19 for the three dimensional structure. The transcriptional expressions of ScTLR19 were widely detected in various tissues of S. curriculus, and significantly up-regulated in kidney after GCRV infection. When the S. curriculus fin cells overexpressed ScTLR19 and were then challenged with GCRV, the transcriptional expressions of interferon regulatory factor 7 (IRF7) did not significantly change (P>0.05), while the transcriptional expressions of interferon regulatory factor 3 (IRF3) and both MyD88 and TRIF that were reported as two important adaptor proteins in TLR signal pathways, were significantly up-regulated at 12 h and 24 h post GCRV infection, compared to the control group (P<0.05). According to the previous study on CiTLR19, ScTLR19, just like CiTLR19, only regulated IRF3, but not IRF7 signal pathway during GCRV infection, but unlike CiTLR19 only regulating TRIF signal pathway, ScTLR19 could regulate both MyD88 and TRIF signal pathways, which might enable ScTLR19 with more antiviral immune functions to enhance the resistance against GCRV for S. curriculus. These results shed light on the differences in the structure and function between two TLR19 genes from S. curriculus and C. idellus.