Teleost fish rely heavily on their innate immunity for an adequate response against pathogens and environmental challenges, with the production of antimicrobial peptides being one of their first lines of defense. Among those is hepcidin, a small cysteine-rich antimicrobial peptide that is also the key regulator of iron metabolism. Although most mammals possess a single hepcidin gene, with a dual role in both iron metabolism regulation and antimicrobial response, many teleost fish present multiple copies of hepcidin, most likely because of genome duplications and positive Darwinian selection, suggesting that different hepcidins may perform different functions. To study the roles of hepcidin in teleost fish, we have isolated and characterized several genes in the European sea bass (Dicentrarchus labrax) and evaluated variations in their expression levels in response to different experimental conditions. Although several hepcidin genes were found, after phylogenetic analysis they could be clustered in two groups: hamp1-like, with a single isoform similar to mammalian hepcidins, and hamp2-like, with several isoforms. Under experimental conditions, hamp1 was upregulated in response to iron overload and infection and downregulated during anemia and hypoxic conditions. Hamp2 did not respond to either iron overload or anemia but was highly upregulated during infection and hypoxia. In addition, Hamp2 synthetic peptides exhibited a clear antimicrobial activity against several bacterial strains in vitro. In conclusion, teleost fish that present two hepcidin types show a degree of subfunctionalization of its functions, with hamp1 more involved in the regulation of iron metabolism and hamp2 mostly performing an antimicrobial role.

The current treatments applied in aquaculture to limit disease dissemination are mostly based on the use of antibiotics, either as prophylactic or therapeutic agents, with vaccines being available for a limited number of fish species and pathogens. Antimicrobial peptides are considered as promising novel substances to be used in aquaculture, due to their antimicrobial and immunomodulatory activities. Hepcidin, the major iron metabolism regulator, is found as a single gene in most mammals, but in certain fish species, including the European sea bass (Dicentrarchus labrax), two different hepcidin types are found, with specialized roles: the single type 1 hepcidin is involved in iron homeostasis trough the regulation of ferroportin, the only known iron exporter; and the various type 2 hepcidins present antimicrobial activity against a number of different pathogens. In this study, we tested the administration of sea bass derived hepcidins in models of infection and iron overload. Administration with hamp2 substantially reduced fish mortalities and bacterial loads, presenting itself as a viable alternative to the use of antibiotics. On the other hand, hamp1 seems to attenuate the effects of iron overload. Further studies are necessary to test the potential protective effects of hamp2 against other pathogens, as well as to understand how hamp2 stimulate the inflammatory responses, leading to an increased fish survival upon infection.