Cloning and expression analysis of the Mitochondrial Ubiquitin Ligase Activator of NF-κB (MULAN) in Atlantic salmon (Salmo salar)

Nuclear factor-κB (NF-κB) is a transcription factor involved in the regulation of a large number of genes including many involved in bacterial and viral infections. NF-κB is normally sequestered by inhibitory proteins (IκBs) in the cytoplasm of non-stimulated cells. The degradation of IκBs by the ubiquitin proteasome pathway leads to the rapid translocation of NF-κB to the nucleous where it regulates gene transcription. The Mitochondrial Ubiquitin Ligase Activator of NF-κB, (MULAN), is an E3 ubiquitin ligase believed to be central in controlling activation of NF-κB, and regulating the mitochondrial dynamics and apoptosis process. We report, for the first time in fish, the characterization of a MULAN cDNA in Atlantic salmon and rainbow trout. The salmonid MULAN sequences encode predicted proteins of 352 amino acids. The mRNA of MULAN was expressed in multiple tissues, with the highest abundance in brain and white muscle. An Aeromonas salmonicida bacterial challenge increased expression of this gene in head kidney, liver and gill both at 6 and at 24 h following the infection. In vitro experiments using the salmonid cell line RTG-2 indicated MULAN was increased in expression following 4 h stimulation with LPS and recombinant trout IL-1β. MULAN expression remained increased 24 h post-stimulation with both LPS and IL-1β, but was down regulated by PolyI:C at this time. These results suggest an active role of the MULAN gene in the activation of the NF-κB pathway during piscine immune responses.