Cathelicidins are an important family of antimicrobial peptides involved in the innate immune system and have been discovered quite recently in teleost. Differently from the well-studied mammal cathelicidins, information about their structural and functional characteristics and the tissue expression and localization are yet scattered or inconsistent in fish and in particular in salmonids.
The present work highlights some characteristics of salmonid cathelicidins. Results indicate that fragments of the C-terminal domain own a wide-spectrum, salt-sensitive antibacterial activity especially against Aeromonas salmonicida and Lactococcus garvieae fish pathogens. Even if these peptides adopt a random-coiled structure, rather than a more usual organised conformation, they are able to highly permeabilize bacteria cell membranes within 15 minutes, resulting highly selective for prokaryotic targets and with no cytotoxic effect towards eukaryotic cells. Importantly, we have shown that the antibacterial activity is directly correlated with the presence of the KIRT cationic motif at the N-terminus of the antimicrobial domain and with the length of the peptides.
Regarding the basal expression of endogenous salmonid cath genes, rtcath_1 has been found to be constitutively present at higher level than rtcath_2 in all the analysed tissues both using RNA-seq and real-time PCR methods. I.p. inoculum of inactivated bacteria in trout was able to stimulate rtcath_1 and rtcath_2 expression together with other genes encoding for the acute phase proteins already within 24 hours from the bacterial cells injection. Among them, rtcath_2 was one of the most highly up-regulated genes. Interestingly, rtcath genes remained up-regulated at least 4 days after the challenge, with differences in expression level among tissues and depending on the diverse pathogen used as stimulus.
Endogenous Cath protein expression in tissue was also evaluated using a specific antibody raised against a full length recombinant cathelicidin of rainbow trout that was produced in bacteria. Preliminary data identify some bands consistent with this protein in head kidney and spleen, but additional work has to be done to confirm the results.
These findings indicate that Cath peptides are endogenous antibiotics and an important element of salmonid innate immune response. Cathelicidin up-regulation might be exploited in the protection of salmonids from infection and at the same time they may be used as antibiotics for fighting fish pathogens in aquaculture.
