The identification of the first molluscan Akirin2 with immune defense function in the Hong Kong oyster Crassostrea hongkongensis

• Akirin2 with diversified roles in host defense and embryogenesis of oyster was characterized for the first time in mollusc.
• ChAkirin2 mRNA were ubiquitously expressed in all selected tissues and developmental stages.
• The mRNA transcript of ChAkirin2 could be significantly up-regulated by pathogens and PAMPs challenge.
• ChAkirin2 is a nuclear localization protein and could enhance NF-κB transactivation in HEK293 cell line.

The Akirin protein is a nuclear factor in the innate immune system that is highly conserved from insects to mammals and plays key roles in diverse biological processes, including immunity, myogenesis, development and the cellular stress response. However, the function of Akirins in mollusk, the second most diverse group of animals, is still poorly understood. In this study, we report the discovery of an Akirin2 gene homolog (ChAkirin2) and its biological functions in the Hong Kong oyster Crassostrea hongkongensis. ChAkirin2 is 189 amino acids in length and shares significant homology with invertebrate homologs. Phylogenetic analysis results revealed that ChAkirin2 is clustered with invertebrate Akirin2s. A sequence analysis of the 5' flanking regions of ChAkirin2 indicated that it harbors several potential PAMP-activated transcription factor binding sites (TFB), including sites for NF-κB, C/EBPα, AP-1, SRF, Oct-1 and GATA-1. An RT-PCR analysis showed that ChAkirin2 mRNA was ubiquitously expressed in various tissues and at different embryonic and larval stages. Additionally, upon infection by pathogens (Vibrio alginolyticus, Staphylococcus haemolyticus and Saccharomyces cerevisiae) and pathogen-associated molecular patterns (PAMPs: LPS, PGN and polyI:C), the expression of ChAkirin2 was significantly up-regulated. Moreover, fluorescence microscopy observations show that ChAkirin2 is located in the nuclei of HeLa cells, and the overexpression of ChAkirin2 activated the transcriptional activities of the NF-κB reporter gene in HEK293T cells. Altogether, this report provided the first experimental demonstration that mollusks possess a functional Akirin2 that is involved in the innate defense and embryogenesis processes of the oyster.