cDNA CLONING AND CHARACTERIZATION OF TOLL-LIKE RECEPTOR 3 IN BLUNTNOSE BLACK BREAM, MEGALOBRAMA AMBLYCEPHALA

Molecular characterization and immunological response analysis of toll-like receptors from the blunt snout bream (Megalobrama amblycephala)

The innate immunity serves as the primary line of defense against pathogen invasion. Toll-like receptors (TLRs) play a pivotal role in the innate immunity by sensing specific pathogen-associated molecular patterns and activating immune responses. In this study, 14 TLRs (TLR1, 2, 3, 4, 5, 7, 8a, 8b, 9, 18, 19, 20, 21 and 22) were identified and characterized in a cyprinid fish Megalobrama amblycephala. A majority of MaTLRs possessed the typical structural features of the TLR protein family: LRR domain, TM region and TIR domain. Phylogenetic analyses confirmed the existence of six TLR families and revealed close relationships with other cyprinid orthologs. All TLRs were constitutively expressed in all eight examined tissues. After Aeromonas hydrophila challenge, the expression profiles of MaTLR mRNAs were analyzed in liver, spleen and head kidney: MaTLR1, 2, 5, 9, 21 and 22 were up-regulated, MaTLR3, 7, 8a, 8b and 19 were down-regulated, and MaTLR4, 18 and 20 expression patterns varied among tissues. These results indicate that most of MaTLRs are likely to be involved in the immune responses against bacterial infection.

Isolation and analysis of a novel grass carp toll-like receptor 4 (tlr4) gene cluster involved in the response to grass carp reovirus

The mammalian response to lipopolysaccharide (LPS) is mainly mediated by Toll-Like Receptor 4 (TLR4). Fish and mammalian TLR4 vary; fish TLR4 ligands are unknown. Isolation of fish tlr4 genes is difficult due to their complex genomic structure. Three bacterial artificial chromosome (BAC) clones containing grass carp tlr4 were obtained. Four tlr4 genes, with a varied genomic structure and different protein domains were subsequently isolated by constructing a subcloned library and rapid amplification of cDNA ends (RACE). The four tlr4 genes were expressed during development from 12h post-fertilization, in all healthy adult fish tissues tested, and significantly increased in grass carp reovirus (GCRV)-infected liver and muscle, suggesting the tlr4 genes play a role in GCRV infection. This study effectively separated each gene in the tlr4 gene cluster, implies that grass carp TLR4 proteins have different ligand recognition specificities to mammalian TLRs, and provides information on the functional evolution of TLRs.