The role of insect antimicrobial peptides in immunity and mechanisms of action

Every living organism from prokaryotes to humans produces antimicrobial peptides (AMPs) as a component of innate immunity. AMPs less than 100 amino acid residues display broad-spectrum activity against pathogenic bacteria, fungi, and viruses. Particularly, insects that are the largest class within the animal kingdom due to a remarkable evolutionary success, have been continuously exposed to pathogenic microorganisms. Therefore, insects possess the potent antibacterial defense reactions and mainly rely on innate immunity rather than adaptive immunity because of energy efficiency. As a result, a single insect produces approximately 10-15 peptide antibiotics upon detection of invading the pathogens and the peptides are secreted into hemolymph. Insect AMPs are divided into five main classes: cecropins, insect defensins, glycine-rich/proline-rich peptides, and lysozymes. Although their groups and structures are diverse, insect AMPs are typically cationic and amphipathic structures, allowing them to interact with the anionic microbial surface and to insert into cytoplasmic membrane consisting of phospholipids. This action results in disruption of membrane integrity, like depolarization and pore formation. In addition to the membrane-active property, some insect AMPs have been recently reported to exert antimicrobial activity via different mechanisms including deactivation of bacterial protein and induction of yeast apoptosis. In case of targeting the cytosolic components by entering the cell without membrane perturbation, AMPs inhibit the synthesis of bacterial protein such as molecular chaperone DnaK and outer membrane protein or induce apoptotic death. During apoptosis induced by AMPs, intracellular reactive oxygen species accumulation and mitochondrial dysfunction play major roles in the pathway. Thereafter, various apoptotic phenotypes such as phosphatidylserine externalization, DNA and nuclei damage are shown in yeast cells. Insects have a highly successful immune system and the accurate understanding on the mechanisms can be used to design more potent AMPs.