The Role of Melatonin in Amyloid Interactions with the Model Neuronal Membranes

The amyloid cascade hypothesis has long directed research efforts in understanding the pathogenesis of Alzheimer’s disease (AD), ever since it was first proposed through clinical evidence by Hardy and Higgins in 1992. Currently, amyloid-β (Aβ) species have attracted significant interest, particularly as they demonstrate profound cellular membrane toxic effects. When there is no cure for AD, the current research direction shows there is a need to develop the protective strategies. One natural molecule, melatonin, has been shown to be protective against amyloid toxicity in animal and cellular studies, while the molecular mechanisms are not well understood. One of the hypothesis is that melatonin can alter the membrane structure and thus reduce amyloid toxicity to lipid membranes. In this thesis research, how the incorporation of melatonin to the lipid membrane would change the structure of the membranes and protect the membrane from amyloid toxicity was examined. This thesis focuses on investigations on the neuroprotective effects of melatonin against Aβ toxicity were achieved using various membrane models that mimic cellular membranes at different stages of AD. Utilizing a range of advanced nanotechnology tools including nuclear magnetic resonance (NMR), localized surface plasmon resonance (LSPR), atomic force microscopy (AFM), black lipid membrane (BLM) studies, complemented by neutron scattering, neutron reflectivity, and molecular dynamics (MD) simulations, the study explores the intricate molecular mechanisms behind melatonin’s interaction with Aβ peptides. Different membrane models representing various disease states were employed to assess melatonin’s multifaceted effects. The research uniquely contributes by combining these diverse techniques to offer a comprehensive understanding of the potential therapeutic role of melatonin in AD, paving the way for the development of more targeted and effective treatments. Future exploration into melatonin’s interactions with various forms of Aβ aggregates may further enhance therapeutic strategies. The research employs a range of neuronal membrane models — simple model (SPM) (DPPC/POPC/cholesterol) and complex (DPPC/POPC/cholesterol/SM/GM1) models —to elucidate the protective role of melatonin against Aβ toxicity. Each complex model represents a different disease state of membrane (healthy model (HM), early diseased model (EDM) and late diseased model (LDM)), allowing tests on multifaceted effects of melatonin in varying biological contexts and disease states. The studies demonstrated that melatonin changed the phase separation of lipid membranes, altered the amyloid binding to the membranes, and enhanced the resilience of membranes against amyloid-induced damage. Insights from the NMR study presented in Chapter 2 indicated that melatonin promoted DPPC/POPC phase separation in a simple lipid membrane model. The LSPR study, complemented with AFM in Chapter 3, showed that melatonin altered the amyloid binding to complex lipid membrane models. Interestingly, the intensity of this effect varied depending on the disease state, with the most pronounced protective effects of melatonin observed in EDM. The BLM study in Chapter 4 further investigated melatonin’s effect on different types of amyloid-induced membrane damages. The results clearly demonstrated melatonin’s ability to enhance the longevity of lipid membranes and increase the membrane’s resistance to rupturing. The advancement of knowledge on melatonin’s membrane protective effects can pave the way for the development of new preventive strategies and potentially open doors for innovative therapeutic approaches