Isolation and expression of four Megalobrama amblycephala toll-like receptor genes in response to a bacterial infection

Toll-like receptor 21 in Labeo rohita recognizes double-stranded RNA and lipopolysaccharides by engaging the critical motifs in the LRR domain and gets activated against bacterial assaults

TLR (Toll-like receptor)-21 is a non-mammalian TLR and exhibits a unique function within the innate immune systems of fishes, birds, and amphibians. Despite its important role as PRR (pattern recognition receptor), research on TLR21 in many fish species, as well as in rohu (Labeo rohita), remains relatively limited. This article describes the molecular cloning of LrTLR21 (TLR21 in L. rohita), 3D (3-dimensional) modelling of its LRR (leucine-rich repeat)-regions, prediction of LRR18 to LRR20 as the LPS (lipopolysaccharide)-binding site, and LRR1 to LRR5 and LRR21 to LRR23 as the poly I:C (polyinosinic-polycytidylic acid) binding sites. It also describes the response of LrTLR21 in response to Aeromonas hydrophila and Edwardsiella tarda infections and PAMPs (pathogen-associated molecular patterns) (LPS and poly I:C)-stimulations. The ORF (open reading frame) of the LrTLR21 comprises 2955 nucleotides, encoding 984 aa (amino acid) residues with molecular weight and isoelectric point of 113.791 kDa and ∼8.79, respectively. Domain analysis of the deduced LrTLR21 displayed the existence of a signal peptide (residues 1-24), 26 LRR regions (residues 61-685), a TM (transmembrane) domain (residues 736-758), and a TIR (Toll/interleukin-1 receptor) domain (residues 790-937). The 3D model of LrTLR21-LRR regions has parallel β-sheets and few α-helices. Phylogenetically, LrTLR21 is closely related to the Onychostoma macrolepis and Carassius gibelio TLR21, and during ontogenesis, it is expressed in most of the developmental stages. In rohu fingerlings, it is consistently expressed in all examined tissues viz., skin, liver, heart, blood, eye, muscle, kidney, intestine, brain, gill, and spleen. Upon exposure to E. tarda and A. hydrophila infections, as well as LPS and poly I:C stimulations, the expression of LrTLR21 is significantly (p < 0.05) increased in the blood, kidney, liver, and gill. In the RBCs (red blood cells), PBLs (peripheral blood leukocytes), and kidney macrophages, LrTLR21 is also significantly induced following in-vivo and in-vitro LPS and poly I: C-stimulations. These findings on LrTLR21 are the first ones showing its structural insights and PAMPs binding motifs and its key role in recognizing pathogens and their PAMPs in RBCs, PBLs, and macrophages.

TLR21 is involved in the NF-κB and IFN-β pathways in largemouth bass (Micropterus salmoides) and interacts with TRIF but not with the Myd88 adaptor

Toll-like receptors (TLRs) are pattern recognition receptors that trigger host immune responses against various pathogens by detecting evolutionarily conserved pathogen-associated molecular patterns (PAMPs). TLR21 is a member of the Toll-like receptor family, and emerging data suggest that it recognises unmethylated CpG DNA and is considered a functional homologue of mammalian TLR9. However, little is known regarding the role of TLR21 in the fish immune response. In the present study, we isolated the cDNA sequence of TLR21 from the largemouth bass (Micropterus salmoides) and termed it MsTLR21. The MsTLR21 gene contained an open reading frame (ORF) of 2931 bp and encodes a polypeptide of 976 amino acids. The predicted MsTLR21 protein has two conserved domains, a conserved leucine-rich repeats (LRR) domain and a C-terminal Toll-interleukin (IL) receptor (TIR) domain, similar to those of other fish and mammals. In healthy largemouth bass, the TLR21 transcript was broadly expressed in all the examined tissues, with the highest expression levels in the gills. After challenge with Nocardia seriolae and polyinosinic polycytidylic acid (Poly[I:C]), the expression of TLR21 mRNA was upregulated or downregulated in all tissues tested. Overexpression of TLR21 in 293T cells showed that it has a positive regulatory effect on nuclear factor-kappaB (NF-κB) and interferons-β (IFN-β) activity. Subcellular localisation analysis showed that TLR21 was expressed in the cytoplasm. We performed pull-down assays and determined that TLR21 did not interact with myeloid differentiation primary response gene 88 (Myd88); however, it interacted with TIR domain-containing adaptor inducing interferon-β (TRIF). Taken together, these findings suggest that MsTLR21 plays important roles in TLR/IL-1R signalling pathways and the immune response to pathogen invasion.

Genome-wide identification and characterization of toll-like receptor 5 (TLR5) in fishes

Toll-like receptors 5 (TLR5), a member of the toll-like receptors (TLRs) family, is a class of pattern recognition receptors (PRRs) that recognize pathogen-associated molecular patterns (PAMPs). It responds to vertebrate recognition of bacterial flagellin and participates in innate immune responses. However, genome-wide identification and characterization of TLR5 in fishes have not been investigated. Here, three TLR5M isotypes (TLR5Ma, TLR5Mb1, and TLR5Mb2) and a TLR5S are all extracted from fish genomes on the basis of phylogenetic and synteny analyses. We confirmed that the non-teleost fishes have one TLR5M gene, as well as additional TLR5 genes (TLR5M and TLR5S) in teleost fishes. In addition, some special teleost fishes possess two to three TLR5 genes, which have undergone the fourth whole-genome duplication (WGD). According to our results, we inferred that the diversity of TLR5 genes in fishes seems to be the result of combinations of WGD and gene loss. Furthermore, TLR5 isoforms displayed differences at the flagellin interaction sites and viral binding sites, and showed lineage-specific, which indicated that TLR5 duplicates may generate functional divergence. Bacterial experiments also supported the idea that CiTLR5Ma and CiTLR5Mb are subfunctionalized to sense bacterial flagellin. In summary, our present comparative genomic survey will benefit for further functional investigations of TLR5 genes in fish.