Identification and sequence analysis of chicken Toll-like receptors

Abstract

Toll-like receptors (TLRs) play an important role in the recognition of microbial components. Only chicken TLR2 and -4 have been reported in the literature. The objectives of this study were to identify new chicken TLRs and to evaluate evolutionary significance of these receptors. Searching chicken genomic databases and DNA sequencing revealed five new TLRs, TLR1 (type 1 and 2), -3, -5, and -7. No chicken orthologues of mammalian TLR8, -9, or -10 were found. As in mammals, all chicken TLRs (chTLRs) share identical protein secondary structure that consists of several leucine-rich domains, a transmembrane domain, and Toll/Interleukin-1 receptor domain(s). Phylogenetic analyses indicate that the identified chTLR genes are the orthologues of TLRs in mammals. Analyses of the number of synonymous substitutions per synonymous site and nonsynonymous substitutions per nonsynonymous site indicate that the nucleotide sequences coding for the leucine-rich repeats of chicken TLR1 type 1 and type 2 were significantly under positive Darwinian selection. In contrast, the sequences of other TLRs were under purifying selection. These results support the hypothesis that one of the major evolutionary strategies of the innate immune system is to recognize a few highly conserved microbial components with several conserved TLRs. The results also indicate that the sequence changes in the ligand-binding domains of TLR1 in chickens provide adaptive advantages during evolution.

The evolution of vertebrate Toll-like receptors

“…A polymorphic form of TLR5 could conceivably be secreted (34). Yilmaz et al (7) report expressed alternatively spliced forms of chicken TLR3 and TLR5 without membrane spanning or TIR domains. Lamprey have a humoral immune response possibly based on variable lymphocyte receptors (VLRs).…”

“…Yilmaz et al (7) predict the absence of a chicken ortholog to TLR9. In Xenopus, TLR9 lies between a cadherin and TRIP.…”

“…TLR genes have been recognized in a number of vertebrate genomes, and many partial and full-length sequences are available. Recent additions include draft predictions from the Japanese pufferfish Takifugu rubripes (3), the zebrafish Danio rerio (4)(5)(6), and the chicken Gallus gallus (7), and partially or fully sequenced mRNAs, including one from the goldfish Carassius auratus (8), several from the Japanese flounder Paralichthys olivaceus (9), and several from the rainbow trout Oncorhynchus mykiss (10). These papers provide incremental molecular phylogenetic analyses, and several reviews are available (11)(12)(13).…”

Combinational recognition of bacterial lipoproteins and peptidoglycan by chicken Toll-like receptor 2 subfamily

“…These four TLR2-related cDNAs were identified by molecular cloning, although the cDNA sequences were somewhat different form those expected from the genome. They are ubiquitously distributed in chicken organs (7). Further bioinformatics studies suggested the presence of additional chTLRs, TLR15 and TLR21, which might function in association with TLR2 (8).…”

“…They responded to mycoplasmal diacylated lipoprotein, namely the macrophage-activating lipopeptide 2 kDa (Malp-2) leading to NF-κB activation (5,6), though the response was very weak (4). Recently, Yilmaz et al, (7) have found from bioinformatics analysis of the chicken genome that chicken has a TLR2 subfamily consisting of chTLR1 type1 (1-1), chTLR1 type2 (1-2), chTLR2 type1 (2-1) and chTLR2 type2 (2-2). These four TLR2-related cDNAs were identified by molecular cloning, although the cDNA sequences were somewhat different form those expected from the genome.…”