The first chordate big defensin: identification, expression and bioactivity

Area of Science:

• Immunology
• Marine Biology
• Genetics

Background:

• Defensins are crucial antimicrobial peptides found across diverse species.
• Amphioxus, a basal chordate, offers insights into early immune system evolution.
• Limited information exists on defensins within protochordates like amphioxus.

Purpose of the Study:

• To identify and characterize a big defensin gene in the amphioxus Branchiostoma japonicum.
• To investigate the expression patterns and antimicrobial functions of the identified defensin.
• To understand the evolutionary significance of defensins in early chordates.

Main Methods:

• cDNA identification and sequence analysis of the big defensin gene (Bjbd).
• Prediction of protein structure, including signal sequence, propeptide, and mature peptide domains.
• Quantitative real-time PCR (qPCR) to analyze gene expression in various tissues and after pathogen challenge.
• Recombinant protein expression and antimicrobial assays against bacterial pathogens.

Main Results:

• A novel big defensin cDNA, Bjbd, was identified in Branchiostoma japonicum, encoding a 117-amino acid protein.
• Bjbd exhibits characteristic defensin structural features, including disulfide bridges and alpha-helical/beta-sheet domains.
• Bjbd expression is constitutive across tissues and significantly upregulated upon challenge with bacterial components (LPS, LTA) and pathogens (Aeromonas hydrophila, Staphylococcus aureus).
• Recombinant BjBD demonstrated inhibitory activity against Staphylococcus aureus, Escherichia coli, and Aeromonas hydrophila.

Conclusions:

• Bjbd represents the first big defensin identified in the phylum Chordata.
• The findings suggest Bjbd plays a vital role in the innate immune response of amphioxus against bacterial infections.
• This study provides valuable insights into the evolution of defensins and innate immunity in chordates.