Zebrafish ANGPT4, member of fibrinogen-related proteins, is an LTA-, LPS- and PGN-binding protein with a bacteriolytic activity.

Fibrinogen-related proteins (FREPs) are a family of glycoproteins that contain a fibrinogen-like (FBG) domain. Many members of FREPs have been shown to play an important role in innate immune response in both vertebrates and invertebrates. Here we reported the immune functional characterization of ANGPT4, member of FREPs, in zebrafish Danio rerio. Quantitative real time PCR showed that the expression of zebrafish ANGPT4 gene is up-regulated by the challenge with lipoteichoic acid (LTA) or lipopolysaccharides (LPS), hinting its involvement in innate immune response. The recombinant ANGPT4 (rANGPT4) could bind to both gram-positive bacteria Staphylococcus aureus and Bacillus subtilis and the gram-negative bacteria Escherichia coli and Aeromonas hydrophila as well as the pathogen-associated molecular patterns (PAMPs) on the bacterial surfaces including LTA, LPS and peptidoglycan (PGN), suggesting it capable of identifying pathogens via LTA, LPS and PGN. In addition, rANGPT4 also displayed strong bacteriolytic activities against both gram-positive and -negative bacteria tested via inducing membrane depolarization and intracellular ROS production. Moreover, the bacterial clearance assay in vivo showed that the rANGPT4 could also accelerate the clearance of bacteria in zebrafish embryos/larvae. Finally, we showed that the eukaryotically expressed recombinant ANGPT4 maintained antibacterial activity and binding activity to bacteria and LTA, LPS and PGN. All these suggested that ANGPT4 could not only capable of recognizing pathogens via LTA, LPS and PGN, but also capable of killing the Gram-positive and Gram-negative bacteria, in innate immune response. This work also provides further information to understand the biological roles of FREPs and the innate immunity in vertebrates.