Identification and characterization of a β-defensin gene involved in the immune defense response of channel catfish, Ictalurus punctatus

Antimicrobial peptides are small peptides that play important roles in a host's innate immune response. As an important antimicrobial peptide, β-defensin widely distribute in mammals, insects and plants with broad-spectrum antimicrobial activity. In this study, the β-defensin gene of the channel catfish, Ictalurus punctatus, was cloned, sequenced, and subjected to a bioinformatic analysis. The β-defensin gene of the channel catfish contains three exons and two introns, and encodes a precursor peptide consisting of two domains: a signal peptide of 24 amino acid residues and a mature peptide of 43 amino acid residues. The mature peptide is estimated to have a molecular mass of 7.1 kDa and a theoretical isoelectric point of 8.21. Channel catfish β-defensin (ccBD) has six conserved cysteine residues, forming three disulfide bridges at C1-C5, C2-C4, and C3-C6, and a β-sheet in the predicted three-dimensional structure. A phylogenetic analysis suggests that ccBD belongs to the type 1 β-defensins. Real-time quantitative PCR showed that channel catfish β-defensin transcripts are constitutively expressed in various tissues in healthy fish, with highest expression in the skin. The expression of ccBD in vivo increased significantly in the head kidney (2.9-fold), gill (2.2-fold), and skin (6.6-fold) at 48 h after bacterial (Edwardsiella ictaluri) challenge. In vitro, lipopolysaccharide (LPS), a bacterial mimic, induced significant changes in ccBD expression in leukocytes from the spleen (3.4-fold) and head kidney (3.9-fold) 24 h after stimulation. Chemically synthesized ccBD displayed marked inhibitory activity against a broad range of bacteria. These results suggest that ccBD is involved in the innate antibacterial defenses of the channel catfish.