β-Defensin in Nile tilapia (Oreochromis niloticus): Sequence, tissue expression, and anti-bacterial activity of synthetic peptides

Beta-defensins (beta-defensins) are small cationic amphiphilic peptides that are widely distributed in plants, insects, and vertebrates, and are important for their antimicrobial properties. In this study, the beta-defensin (On beta-defensin) gene of the Nile tilapia (Oreochromis niloticus) was cloned from spleen tissue. On beta-defensin has a genomic DNA sequence of 674 bp and produces a cDNA of 454 bp. Sequence alignments showed that On beta-defensin contains three exons and two introns. Sequence analysis of the cDNA identified an open reading frame of 201 bp, encoding 66 amino acids. Bioinformatic analysis showed that On beta-defensin encodes a cytoplasmic protein molecule containing a signal peptide. The deduced amino acid sequence of this peptide contains six conserved cysteine residues and two conserved glycine residues, and shows 81.82% and 78.33% sequence similarities with beta-defensin-1 of fugu (Takifugu rubripes) and rainbow trout (Oncorhynchus mykiss), respectively. Real-time quantitative PCR showed that the level of On beta-defensin expression was highest in the skin (307.1-fold), followed by the spleen (773-fold), kidney (17.8-fold), and muscle (16.5-fold) compared to controls. By contrast, low levels of expression were found in the liver, heart, intestine, stomach, and gill (<3.0-fold). Artificial infection of tilapia with Streptococcus agalactiae (group B streptococcus [CBS] strain) resulted in a significantly upregulated expression of On beta-defensin in the skin, muscle, kidney, and gill. In vitro antimicrobial experiments showed that a synthetic On beta-defensin polypeptide had a certain degree of inhibitory effect on the growth of Escherichia coli DH5 alpha. and S. agalactiae. The results indicate that On beta-defensin plays a role in immune responses that suppress or kill pathogens. (C) 2015 Elsevier B.V. All rights reserved.