Evolution, expression, and characterisation of liver-expressed antimicrobial peptide genes in ancient chondrostean sturgeons

Liver-expressed antimicrobial peptide 2 (leap-2) is an evolutionarily ancient molecule that acts as the key component in vertebrate innate immunity against invading pathogens. Leap-2 has been identified and characterised in several teleosts, but not yet in chondrosteans. Herein, the complete coding sequences of leap-2b and leap-2c were identified from expressed sequence tags (ESTs) isolated from Dabry's sturgeon (Acipenser dabryanus) and Chinese sturgeon (A. sinensis), designated as adleap-2b, adleap-2c, asleap-2b, and asleap-2c, respectively. Adleap-2b and adleap-2c sequences share 98% and 100% sequence identity with asleap-2b, and asleap-2c, respectively. Sequence alignment revealed that all four genes contain four cysteine residues, conserved in all fish leap-2 homologs, that form two disulfide bonds. Comparative analysis of the exon-intron structure revealed a three exon/two intron structure for that leap-2 genes in animals, but intron 1 is much longer in sturgeons than in other species. The adleap-2c gene was expressed mainly in the liver of Dabry's sturgeon, and transcription of adleap-2c was significantly up-regulated (p < 0.05) in the liver and midkidney in response to Aeromonas hydrophila challenge. These results suggest adleap-2c may contribute to the defence against pathogenic bacterial invasion. The findings further our understanding of the function of adleap-2c and the molecular mechanism of innate immunity in chondrosteans.