The multifaceted role of antimicrobial peptides in neurodegeneration: Insights from Drosophila and beyond.

Antimicrobial peptides (AMPs) are tiny proteins essential for innate immunity in various taxa, including mammals and insects. They provide defence against a wide range of pathogens, including bacteria, viruses, fungi, and parasites. Apart from their antimicrobial properties, new studies have revealed the roles of AMPs in brain ageing, neurodegeneration, and neuroinflammation. With an emphasis on their dysregulation in glial and neuronal tissues and their role in neuroinflammation, mitochondrial dysfunction, and neuronal loss, we reviewed the new function of AMPs beyond their antimicrobial activity. Findings from Drosophila models of Huntington's disease, Alzheimer's disease, Parkinson's disease, Amyotrophic lateral sclerosis, and Ataxia-telangiectasia show that immune pathways, like Toll and immune deficiency, drive persistent or ectopic AMP expression, which is similar to the inflammatory processes seen in human neurodegenerative diseases. Furthermore, the dual function of AMPs as mediators of sterile inflammation and protective immunological agents reveals a universal paradox. The translational relevance of these findings is further supported by comparisons with human AMPs, such as LL-37 and β-defensins. LL-37 and β-defensins levels were found to be increased in the cerebrospinal fluid of patients suffering from meningitis. LL-37 is released from neurons and activates glial cells, boosting the production of inflammatory cytokines and decreasing neuronal survival. This chapter redefines AMPs as not only sentinels of microbial defence but also as important participants in preserving or disturbing brain homeostasis by establishing them as a link between immunity and neurobiology.