Dermal cytolytic peptides and the system of innate immunity in anurans

Cationic peptides with the propensity to adopt an amphipathic alpha-helical conformation in a membrane-mimetic environment are synthesized in the skin of many species of frogs. These peptides frequently display potent cytolytic activities against a range of pathogenic bacteria and fungi, consistent with the hypothesis that they play a role in host defense. However, the importance of the peptides in the survival strategy of the animal is not clearly understood. At this time, antimicrobial peptides have been identified in the skin of frogs from species belonging to the Bombinatoridae, Hylidae, Hyperoliidae, Leiopelmatidae, Leptodactylidae, Myobatrachidae, Pipidae, and Ranidae families, but several well-studied species from the Bufonidae, Ceratophryidae, Dicroglossidae, Microhylidae, Pelobatidae, Pyxicephalidae, Rhacophoridae, and Scaphiopodidae families do not appear to synthesize these peptides. Although cytolytic activity against the chytrid fungus Batrachochytrium dendrobatidis, responsible for anuran population declines worldwide, has been demonstrated in vitro, the ability of frog skin antimicrobial peptides to protect the animal in the wild appears to be limited. While the production of dermal cytolytic peptides may offer definite evolutionary advantage to anurans, their precise biological function, for example during metamorphosis, may need to be re-evaluated.