EFFECT OF TREHALOSE AND LITHIUM IN MOLECULAR MECHANISM OF NEUROPROTECTION IN ALZHEIMER’S DISEASE

Abstract

Alzheimer's Disease (AD) is still a challenging issue for humans since its first case was identified by Alois Alzheimer over one hundred years ago. Approximately thirty years ago, the "Amyloid cascade hypothesis" was proposed, which is a milestone that began to reveal the mystery of AD. The aggregation and deposition of endogenous amyloid-beta (A-beta) proteins in brains are known to be one of the main pathogenic factors of AD. One of the pathways to neurodegeneration driven by A-beta proteins involves A-beta damage to neuronal membranes, which may result in neuron impairment and death. On the other hand, A-beta proteins have antimicrobial properties, suggesting they may serve functionally in the brain. This could be one of the reasons to explain the severe side effects seen in clinical anti-A-beta treatment for AD.

Instead of focusing on anti-A-beta, I aim to explore a therapeutic strategy that focuses on membrane protection. The goal of my work is to investigate the potential of membrane-targeted agents, trehalose and lithium, to protect lipid membranes against A-beta toxicity.

Trehalose, a natural-derived sugar, is explored as a potential treatment for neurodegenerative Parkinson's Disease. Lithium, as a mood stabilizer, is commonly used for treating bipolar disorder. Both of the agents are investigated for neurological disorders and can interact with cellular membranes with distinct mechanisms. In this thesis, I ask whether their interaction with lipid membranes can protect membranes from A-beta-induced damage, thereby lowering A-beta neurotoxicity.

Hence, the entire thesis addresses two main questions. 1. How does trehalose/lithium affect membrane properties? 2. Can trehalose/lithium protect membranes from A-beta toxicity?

To explore the two questions for trehalose and lithium, respectively, the thesis is divided into two parts: Part 1 - trehalose (Chapters 3-8) and Part 2 - lithium (Chapters 9-11).

In Part 1, I used Langmuir-Blodgett (LB) Trough, atomic force microscopy (AFM), and Kelvin probe force Microscopy (KPFM) to explore the influence of trehalose on the mechanical and electrostatic properties of model lipid monolayers composed of DPPC, POPC lipids, and cholesterol. The study found that trehalose can enhance the fluidity and alter the electrostatic properties of lipid monolayers, with modulation by NaCl.

To assess whether trehalose can protect lipid membranes from A-beta damage, I utilized black lipid membrane (BLM) electrophysiological techniques to evaluate the quality and permeability of membranes exposed to trehalose and A-beta. Results from BLM experiments demonstrated trehalose alleviates A-beta-induced membrane disruption. Furthermore, I explored the binding of A-beta to lipid membranes in the presence of trehalose solutions by localized surface plasmon resonance (LSPR) spectroscopy and found that trehalose can reduce A-beta binding to lipid membranes. Finally, I confirmed the unique neuroprotection of trehalose in cell studies, where trehalose decreased the cell mortality rate caused by toxic A-beta proteins.

Part 2 explored the potential of lithium in mitigating A-beta toxicity on lipid membranes. Similarly, I used LB trough, AFM, and KFPM to compare the influence of LiCl and KCl on lipid membranes. The results demonstrated the distinct contribution of Li+ and K+ on the mechanical and electrostatic properties of DPPC-POPC-Chol lipid monolayers. Li+ has a pronounced effect on reducing the lipid molecular area, increasing monolayer fluidity, and strongly competing with K+ in interacting with lipid monolayers. Lastly, BLM was employed to evaluate the membrane permeability in exposure to A-beta and LiCl. The membrane conductance results obtained by BLM suggested that the modulation of LiCl at the therapeutic level enhances membrane resilience to A-beta-induced damage.

This research exposes the modulation of membrane-active trehalose and lithium on lipid membrane properties and their protective role in AD. It contributes to exploring a new therapeutic approach against A-beta toxicity that focuses on membrane protection, which may aid in developing prevention and treatment strategies for AD.