Biophysical Studies of Lipid Membranes and their Interactions with Amyloid Peptides
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Amyloid beta peptides are known to form amyloid fibrils which are implicated in more than 20 currently incurable neurodegenerative diseases, including Alzheimer’s, Huntington’s and Parkinson’s. The proposed mechanism of amyloid fibril formation involves protein unfolding and formation of amyloid β-sheet aggregates. Although fibril plaque formation is associated with biological membranes in vivo, the role of membrane heterogeneity - and especially the effect of cholesterol and lipid rafts - in the process of amyloid fibril formation and toxicity is not well understood. Therefore, research in this area is of great interest and necessity. Cholesterol is a well-known sterol, which is found in eukaryotic membranes and is important for membrane structure and function. It has been shown that an increased level of cholesterol may lead to various disorders. It is my hypothesis that cholesterol may alter the interaction of plasma membrane with membrane-interacting biomolecules such as amyloid beta and may play an important role in amyloid toxicity. In this thesis, I used multiple methods of investigation, including neutron scattering, atomic force microscopy, the Langmuir-Blodgett trough, and frequency modulated-Kelvin probe force microscopy, to study both simple and complex lipid systems. The thesis is organized in such a way that it begins with looking at the simple systems of a single to a few lipids and proceed to examine more complex systems, with multiple constituents. Through these methods, it was found that cholesterol and melatonin have opposite effects on altering membrane thickness. Lipid properties like head group charge and lipid phase were shown to define the size and the amount of amyloid clusters when incubated on single lipid systems, and the presence of cholesterol resulted in cholesterol-induced electrostatic domains that can cause targeted binding of amyloid. It was also shown that cholesterol has measureable effects on membrane properties even in systems more complex than just a single lipid. Finally, through the development of specific membrane models, it was shown that the models differed in properties and also had differential interactions with amyloid in terms of electrophysiology and amyloid accumulation. The goal of this thesis is to investigate the nanoscale effects of both cholesterol, and the interactions of lipids in complex mixtures on the physical and electrical properties of model lipid membranes, especially nanoscale heterogeneity of the membrane and membrane domains, and how these altered properties affect binding of Aβ and amyloid fibril formation on the surface of lipid membranes. The results will help to understand the role of membrane nanoscale heterogeneity in the molecular mechanism of amyloid toxicity and therefore will help towards the development of new approaches for the prevention and treatment of neurodegenerative diseases.