Characterization of the Antimicrobial Peptides, Piscidins, in the Striped Bass (Morone saxatilis) and Hybrid Striped Bass (M. chrysops x M. saxatilis)

Prophylactic antibiotics and vaccination are currently the most commonly used methods to control disease outbreaks and mortality in the aquaculture industry. Unfortunately, these methods not only lead to the potential of antibiotic resistant strains of bacteria but may also have negative impacts on environmental and human health. The high stocking densities required to maximize profits in the aquaculture industry lead to crop mortality due to stress. How stress affects the immune system of fish grown in culture is not very well understood. Also, knowledge of the ontogeny of the immune system in fish may help to decrease these losses. Using the fishes' innate immune system to decrease disease related mortalities is a valuable alternative to drugs and chemicals. One component of the innate immune system are the antimicrobial peptides (AMPs). These are usually small amphipathic, a-helical peptides with a broad-spectrum of action against viral, bacterial, fungal, and protozoan pathogens. A family of AMPs have been found in the hybrid striped bass (HSB; white bass, Morone chrysops, x striped bass, M. saxatilis), named piscidins. We have identified four piscidins in the hybrid which were previously unknown, increasing our knowledge of known piscidins to 10, and have described the tissue distribution of the piscidins. We have also determined the parental contributions of each piscidin to the hybrids and devised a revised nomenclature in these species based on a three class system by length of the mature peptide. The Class I piscidins (22 AA in length) show broad-spectrum of activity against bacterial pathogens and ciliated protozoans and the Class III piscidins (55 AA in length) have activity mainly against protozoans, while the Class II piscidins (44-46 AA in length) have activities between the other two classes. In a crowding stress challenge, gene expression decreased for the majority of piscidins in the fish held in the high density tanks, versus those held in the low density tanks. Ms Piscidin IIA protein concentrations decreased, similar to the gene expression for this piscidin. High levels of variation were seen between individual fish sampled in each treatment. Using immunofluorescent histochemistry, we have determined the first presence and cellular and tissue distribution of three piscidins during the development of the striped bass. No staining was observed in fry prior to 16 days post hatch (dph). Staining was observed in many epithelial tissues such as the gill, digestive tissues, and skin, mainly in granular cells. Staining was also seen early on in what appear to be mononuclear and undifferentiated cells in the head and trunk kidney. Knowledge of piscidin function, activity, the effects of chronic stress on the piscidins, and when the larval fish gain immunocompetence, may lead to better methods of controlling mortalities from disease.