Molecular characterization and expression analysis of TLR 7 and TLR 8 homologs in large yellow croaker (Pseudosciaena crocea)

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The two toll-like receptor (TLR) genes, LycTLR7 and LycTLR8, were cloned from large yellow croaker (Pseudosciaena crocea), an economically important marine fish in China. The full-length cDNAs of LycTLR7 and LycTLR8 are 3544 and 3593 bp, with an open reading frame (ORF) of 3165 and 3093 bp, encoding 1053 and 1030 amino acids, respectively. The TLR family motifs, such as leucine rich repeat (LRR) and Toll/interleukin (IL)-1 receptor (TIR) domain, are conserved in the LycTLR7 and LycTLR8, with 17 and 14 LRRs, and with a TIR domain, respectively. It is also noted that an LRR N-terminal domain (LRR-NT, residues 24–60) is present in the LycTLR7 but not in the LycTLR8. Both LycTLR7 and LycTLR8 contain a conserved extracellular CxRCxxxxxPCxxC motif, which was found in TLR7/TLR8 of other species and required for stimulus-induced signal transduction. Homology comparison shows that LycTLR7 has 79%, 71.9%, 65.9% and 65.8% identity to fugu, rainbow trout, carp and catfish TLR7, while LycTLR8 has 67.1%, 60.7%, 60.6%, 52.4%, and 51.5% identity to fugu TLR8, rainbow trout TLR8a1, rainbow trout TLR8a2, catfish TLR8-2, and catfish TLR8-1, respectively. Subcellular localization analysis revealed that both LycTLR7 and LycTLR8 are located in the endoplasmic reticulum of epithelioma papulosum cyprini (EPC) cells, which is similar to TLR7/TLR8 in mammals. The two TLRs were constitutively expressed in all tissues tested, especially in immune-related tissues such as spleen, head kidney and gills. An increased expression of LycTLR7 and LycTLR8 was observed in head kidney and spleen of large yellow croakers stimulated by poly (I:C), a viral mimic. In head kidney, their mRNA expression was up-regulated more than 10 times compared to the controls at 12 h after poly (I:C) stimulation. These results suggested that LycTLR7 and LycTLR8 may play a role in the defense against viral infection like their mammalian homologs.

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