Molecular characterization and expression of AjNLRP3 in the antibacterial host defense of the sea cucumber (Apostichopus japonicus)

Abstract

The NOD-like receptor proteins (NLRP) is an important genes primarily involved in innate immunity. In this study, we identified a novel NLRP gene designated as AjNLRP3 (GenBank accession number MF663701) in Apostichopus japonicus using transcriptome sequencing and the rapid amplification of cDNA ends approach. The full-length of AjNLRP3 is 3642 bp and a putative open reading frame of 2424 bp encodes a polypeptide with 807 amino acid residues. The predicted molecular mass of the protein sequence predicted for AjNLRP3 is 93.03 kDa and its theoretical pI is 6.14. AjNLRP3 contains two low complexity regions, two internal repeat regions, and a NACHT domain. Spatial distribution expression analysis detected AjNLRP3 in all of the tissues tested, with greater expression levels in the body wall, moderate expression in the respiratory tree, and weaker levels in the intestine, tube feet, celomocytes, and longitudinal muscle. The expression levels of AjNLRP3 increased by 2.60-fold in the celomocytes and decreased by 0.87-fold in the respiratory tree of diseased sea cucumbers compared with those in healthy sea cucumbers. Time-course expression analysis under bacterial challenge in celomocytes showed that the expression of AjNLRP3 was significantly decreased by 0.23-fold at 8 h and by 0.20-fold at 24 h. In conclusion, this study showed that the AjNLRP3 protein found in the sea cucumber has a similar structure and biological function to that in other organisms, where it appears to be involved with the innate immune responses against bacterial infection.