Importance of Endobiotics in Defense against Protozoan Ectoparasites of Fish

Infectious disease is one of the most serious causes of economic loss in all sectors of aquaculture. There is a critical need to understand the molecular basis for protection against infectious disease so that safer, more reliable and more cost-effective strategies can be designed for their control. As part of this effort, the major goal of our BARD project was to determine the importance of endobiotics as a defense against protozoan ectoparasites in fish. Endobiotics, or antimicrobial polypeptides, are peptides and small proteins that are increasingly recognized as having a vital role in the innate defense of virtually all animals. One objective of our BARD project was to determine the antiparasitic potency of one specific group of endobiotics that were isolated from hybrid striped bass (Morone saxatilis x M chrysops). We found that these endobiotics, which we had previously named histone-like proteins (HLPs), exhibited potent activity against Amyloodinium and that the putative levels of HLPs in the skin were well within the levels that we found to be lethal to the parasite in vitro. We also found evidence for the presence of similar antibiotics in sea bream (Sparus aurata) and Mediterranean sea bass (Dicentrarchus labrax). We also examined the effect of chronic stress on the expression of HLP in fish and found that HLP levels were dramatically decreased after only one week of a crowding/high ammonia sublethal stress. We also began to explore the feasibility of upregulating endobiotics via immunostimulation. However, we did not pursue this objective as fully as we originally intended because we spent a much larger effort than originally anticipated on the last objective, the attempted isolation of novel endobiotics from hybrid striped bass. In this regard, we purified and identified four new peptide endobiotics. These endobiotics, which we have named piscidins (from "Pisces" meaning fish), have potent, broad-spectrum activity against a number of both fish and human pathogens. This includes not only parasites but also bacteria. We also demonstrated that these peptides are present in the mast cell. This was the first time that the mast cell, the most common tissue granulocyte in vertebrates, was shown to possess any type of endobiotic. This finding has important implications in explaining the possible function of mast cells in the immune response of vertebrates. In summary, the research we have accomplished in this BARD project has demonstrated that endobiotics in fish have potent activity against many serious pathogens in aquaculture and that there is considerable potential to use these compounds as stress indicators in aquaculture. There is also considerable potential to use some of these compounds in other areas of medicine, including treatment of serious infectious diseases of humans and animals.