Toll-like receptor 4 (TLR4) is critical for LPS recognition and cellular responses.
It also recognizes some viral envelope proteins. Detection mostly results in the inflammation
rather than specific antiviral responses. However, it's unclear in fish. In this report,
a TLR4 gene (named as GrTLR4b) was cloned and characterized from rare minnow Gobiocypris
rarus. The full length of GrTLR4b cDNA consists of 2766 nucleotides and encodes a
polypeptide of 818 amino acids with an estimated molecular mass of 94,518Da and a
predicted isoelectric point of 8.41. The predicted amino acid sequence comprises a
signal peptide, six leucine-rich repeat (LRR) motifs, one leucine-rich repeat C-terminal
(LRRCT) motif, followed by a transmembrane segment of 23 amino acids, and a cytoplasmic
region of 167 amino acids containing one Toll--interleukin 1--receptor (TIR) motif.
It's closely similar to the zebrafish (Danio rerio) TLR4b amino acid sequence with
an identity of 77%. Quantitative RT-PCR analysis showed GrTLR4b mRNA was constitutive
expression in gill, heart, intestine, kidney, liver, muscle and spleen tissues in
healthy animals and up-regulated by viruses and bacteria. After being infected by
grass carp reovirus or Aeromonas hydrophila, GrTLR4b expressions were up-regulated
from 24h post-injection and lasted until the fish became moribund (P<0.05). These
data implied that TLR4 signaling pathway could be activated by both viral and bacterial
infection in rare minnow.