Nemo like kinase negatively regulates NF-κB activation and coelomocytes apoptosis in Apostichopus japonicus

• First Nemo-like kinase (NLK) gene were identified in Apostichopus japonicus.
• AjNLK were ubiquitously expressed in all examined tissues.
• AjNLK mRNA in coelomocytes was depressed after Vibrio splendidus or LPS exposure.
• The expression level of Ajp105 and AjIκB were tightly correlated with AjNLK.
• AjNLK could negatively regulate coelomocytes apoptosis in vitro.

Nuclear factor kappa B (NF-κB) transcription factors are related to several physiological processes, including innate and acquired immunity. In this study, a novel negative regulator of the Nemo-like kinase (NLK) gene was identified from Apostichopus japonicus through PCR (denoted as AjNLK). The complete AjNLK cDNA was of 2335 bp, with a 5′-UTR of 315 bp, a 3′-UTR of 718 bp, and a putative ORF of 1302 bp, and encoded a polypeptide of 433 amino acid residues with a typical serine/threonine protein kinase domain. Blast analysis revealed that AjNLK shared a high degree of structural conservation with its counterparts from other invertebrates and vertebrates. Spatial expression analysis indicated that the expression of AjNLK mRNA transcripts was higher in the tentacles than that in coelomocytes. The expression of AjNLK mRNA in coelomocytes was suppressed after Vibrio splendidus challenge by 0.51-fold and 0.41-fold at 72 and 96 h, respectively, compared with that in the control group. Similarly, AjNLK expression was down-regulated in primary coelomocytes exposed to 1 μg mL−1 lipopolysaccharide (LPS). Functional investigation further revealed that the NF-κB factor p105 was induced at both mRNA and protein levels after AjNLK silencing in vitro. Meanwhile, the apoptosis of LPS-induced coelomocytes was significantly inhibited in AjNLK siRNA-transfected coelomocytes. These results supported that AjNLK negatively regulated NF-κB activation and cell apoptosis in sea cucumber.