Diurnal rhythm and pathogens induced expression of toll-like receptor 9 (TLR9) in Pelteobagrus vachellii

Toll-like receptor 9 (TLR9) is activated by bacterial DNA and induces the production of inflammatory cytokines. In this study, the darkbarbel catfish Pelteobagrus vachellii TLR9 cDNA was cloned and sequenced. The daily expression pattern of TLR9 mRNA was investigated in various tissues. Furthermore, its expression was analyzed following exposure to the pathogen Aeromonas hydrophila. The 4249 bp cDNA includes a 3201 bp open reading frame (ORF) encoding 1067 amino acids. The predicted amino acid sequence comprises a leucine-rich domain (LRD), a toll/interleukin-1 receptor (TIR), and a transmembrane domain. P. vachellii TLR9 showed 42-87% amino acid sequence identity with TLR9 sequences of Ictalurus punctatus, Rhincodon typus, and Miichthys miiuy. The P. vachellii TLR9 mRNA was highly expressed in intestines, head kidney, and spleen in an apparently healthy fish. Following pathogen challenge, TLR9 expression increased significantly (P < 0.05) and peaked at 48 h post-exposure in the liver, at 24 in the head kidney, and at 12 h in the spleen. In addition, the pattern of TLR9 expression over a 24-h period showed a circadian rhythm in the head kidney, spleen, and intestine, with the acrophase at 20:34, 18:45, and 3:50, respectively. This result provided the basis for further study of the rhythm of innate immunity against bacteria in catfish.