The role of antimicrobial skin peptides in defense of leopard frogs against chytridiomycosis

Abstract

Global amphibian populations, including the northern leopard frog (Rana pipiens), have been declining for the past four decades, and overwhelming evidence now links a pathogenic chytrid fungus, Batrachochytrium dendrobatidis (Bd) to declines across six continents. Because Bd colonizes the skin, antimicrobial peptides secreted onto the surface of the skin are thought to be an important component of the immune defenses against this pathogen. My studies showed that resting frogs constitutively secrete low levels of AMPs that increase following a natural activity such as movement to escape a predator. Using MALDI-TOF MS and growth inhibition assays, I showed that AMPs are present on the skin for at least two hours but are degraded in a time-dependent fashion. Injection of 40 nmol/g norepinephrine results in a long-term depletion of skin peptides. When skin peptides were depleted in juvenile frogs by norepinephrine injection, the frogs were more susceptible to Bd infection and died more rapidly than control frogs with an intact set of skin peptides. These studies support the hypothesis that AMPs are an important defense for this species against chytridiomycosis.