Molecular Cloning and Characterization of tlr1, tlr5s, tlr5m, and tlr14 Genes in the Swamp Eel, Monopterus albus

Abstract

Toll-like receptors (TLRs) are a critical family of pattern recognition receptors (PRRs) that are tightly regulated by complex mechanisms involving many molecules to ensure a good response to foreign invaders. In this study, we identified and analyzed the sequence characteristics and homology of four TLR genes (tlr1, tlr5s, tlr5m, and tlr14) from the swamp eel. Sequence similarity analysis, functional domain prediction, and phylogenetic analysis supported their annotation and orthologies. Their relative expression levels in different tissues were assessed. The four TLRs were expressed in all tested tissues. tlr5m was highly expressed in the spleen, whereas tlr5s was highly expressed in the kidney and spleen. High expression levels of tlr1 and tlr14 were detected in the spleen and muscle. These results indicated that the TLRs are related to host immunity. Moreover, the differential expression of TLRs was examined after Aeromonas veronii infection, which showed that all the TLR genes were induced with diverse patterns. tlr1 was significantly downregulated in the spleen after A. veronii challenge. In the kidneys and intestines, tlr1 expression decreased initially and then increased, with its lowest level at 4 h. tlr5s expression was upregulated significantly in three tissues at 1, 4, and 12 h, with the maximum expression at 1 h, indicating that tlr5s actively responded to bacterial invasion in the early stage of the challenge. tlr5m showed tissue specific expression: it was slightly upregulated in the intestines and spleen and downregulated in the kidneys. The expression pattern of tlr14 was similar to that of tlr5s, and both reached maximum expression at 1 h after infection. Collectively, our results indicated that TLRs might play important roles in the innate immune response against Gram negative bacteria in the swamp eel.