Cloning and expression of Toll-like receptors 1 and 2 from a teleost fish, the orange-spotted grouper Epinephelus coioides

The two Toll-like receptors, TLR1 and TLR2 were cloned from orange-spotted grouper, Epinephelus coioides, an important teleost fish in mariculture of Asia. The cDNA sequences of TLR1 and TLR2 are 3195 and 3439 bp long, with an open reading frame (ORF) of 2406 and 2466 bp, encoding 801 and 821 amino acids, respectively. The TLR family motifs, i.e. leucine-rich repeat (LRR) domains and Toll/interleukin (IL)-1 receptor (TIR) domains are conserved in the TLR1 and TLR2, with nine and ten leucine-rich repeat (LRR) domains, and with one TIR domain, respectively. The TLR1 and TLR2 had a constitutive expression in examined organs/tissues of naïve orange-spotted grouper, and an increased expression of TLR1 and TLR2 at mRNA level was observed in immune organs such as in spleen of LPS and Poly(I:C) stimulated fish. An increased expression of TLR1 and TLR2 was also recorded in immune organs of the fish injected with the bacterial pathogen, Vibrio alginolyticus. Similarly, a significant rise in the expression of MyD88, an adaptor molecule which forms signalling complex with intracellular TIR domain, thus leading to the production of pro-inflammatory cytokines, such as IL-1β, was also observed in the LPS- and Poly(I:C)-stimulated, and V. alginolyticus-infected fish, indicating the possible role of TLR1 and TLR2 in the MyD88 signalling pathway. However, the mechanism involved in the increased expression of TLR1 and TLR2 following LPS and Poly(I:C) stimulation is at present unknown in fish, and further research should be carried out to identify ligands of fish TLR1 and TLR2 in order to understand the function of these receptors.