Characterization of NLRP3-like gene from Apostichopus japonicus provides new evidence on inflammation response in invertebrates

Inflammatory/defensive response after pathogen invasion is considered a local defense reaction in vertebrates. Inflammation response in Apostichopus japonicus was hardly determined due to scarce information available for nucleotide binding domain-like receptor family, pyrin domain-containing (NLRP) family. In the present study, invertebrate NLRP homologue was identified from A. japonicus (designated as AjNLRP3-like) by rapid amplification of cDNA ends. Full-length cDNA of AjNLRP3-like measured 2970 bp with 2265 bp open reading frame encoding a 754-amino acid (aa) residue protein. Structural analysis revealed that AjNLRP3-like processed characteristic domains of pyrin (32-102aa) and NACHT (183-339aa). Multiple sequence alignment and phylogenetic analysis supported that AjNLRP3-like belongs to a new member of NLRP3 protein subfamily. Spatial expression analysis revealed that AjNLRP3-like was ubiquitously expressed in all examined tissues with larger magnitude in coelomocytes. Both Vibrio splendidus challenge in vivo and lipopolysaccharide stimulation in vitro significantly upregulated mRNA expression of AjNLRP3-like when compared with the control group. NLRP3-mediated inflammation response depended on release of lysosomal cathepsin B (CTSB) and subsequent activation of high-mobility group box (HMGB) in vertebrates. We investigated expression profiles of AjNLRP3-like and AjHMGB after AjCTSB knock-down and discovered that AjNLRP3-like was depressed by 0.66-fold and 0.47-fold, whereas AjHMGB was depressed by 0.70-fold and 0.50-fold at 24 and 48 h in AjCTSB-silenced group, respectively. Similarly, down-regulation of AjHMGB was also observed after AjNLRP3-like knock-down. This study therefore suggests that A. japonicus feature similar inflammatory events as those in vertebrates, and activation of AjNLRP3-like depends on AjCTSB expression and release of AjHMGB.