Molecular cloning and characterization of a Toll receptor gene from Macrobrachium rosenbergii.

Toll receptors are cell surface molecules acting as pattern recognition receptors (PRRs) that have been implicated in the signaling pathway of innate immune responses. In this study, the full-length cDNA of a Toll receptor gene of Macrobrachium rosenbergii, designated MrToll, was successfully isolated using designed degenerate primers and the rapid amplification of cDNA ends (RACE). The MrToll gene sequence contained an open reading frame (ORF) of 2799 nucleotides encoding a protein of 932 amino acid residues. The protein contained distinct structural motifs of the Toll-like receptor (TLR) family, including an extracellular domain containing 15 leucine-rich repeats (LRRs), a transmembrane segment of 23 amino acids, and a cytoplasmic Toll/interleukin-1R (TIR) domain of 139 residues. Phylogenetic analysis revealed that MrToll and Toll receptor of Marsupenaeus japonicus (MjToll) evolved closely. However, the MrToll ORF demonstrated only 48-49% identity with shrimp Toll1, suggesting that MrToll isolated from a palaemonid shrimp might belong to a novel class of Toll receptors in shrimp. The transcripts of the MrToll gene were constitutively expressed in various tissues, with high levels in hemocytes, the stomach and muscle. A reverse transcriptase PCR assay demonstrated that the expression patterns of MrToll were distinctly modulated after Aeromonas caviae stimulation, with significant enhancement at 3-12 h post-challenge and a decline to basal levels at 24 h post-challenge. In addition, when MrToll-silenced shrimp were challenged with A. caviae, there was a significant increase in mortality and bacterial CFU counts. These results suggest that MrToll might be involved in host innate defense, especially against the pathogen A. caviae.