Insights into alpha-synuclein oligomer interactions with model membranes

This thesis gives insights into the biophysical mechanisms of α-synuclein (αS) oligomer-membrane interactions. Model membranes of different compositions are used to mimic physiological membranes. We have systematically investigated how αS oligomers bind and permeabilize model membranes and how these activities are modulated by membrane charge and lipid composition. We have addressed oligomer binding to complex model membranes. Our studies show that at least 25% of negatively charged DOPG is necessary for αS oligomer binding. The membranes that contain physiologically relevant cholesterol and sphingomyelin content are not damaged by oligomers. A focus is given on mitochondrial inner membrane and plasma membrane inner leaflet mimics because these two membranes are thought to be the primary sites of αS oligomer-induced damage. Mitochondrial model membranes are more prone to oligomer-induced damage at longer timescales while the more complex plasma membrane model systems do not show a concentration dependent permeabilization on the same time scale. We attribute this result to the complexity of the composition of the model plasma membranes. SAXS data showed that all αS oligomers studied (WT, and disease mutants A30P, E46K, A53T, H50Q and G51D) contain approximately 30 monomers. We have found that the binding affinity of the monomeric protein and the aggregation number of the oligomers formed under our specific protocol are comparable for WT, H50Q and G51D. However, G51D oligomers were not able to disrupt negatively-charged and physiologically-relevant model membranes. We have proven that the toxicity of the externally added oligomers tested here depend on the availability and toxicity of additional compounds rather than on the oligomers themselves. We wanted to gain more details about toxicity of αS oligomers and we suggested that oligomer-induced hemifusion could be connected to the vesicle docking and fusion problems observed in vivo. Multivalent oligomers may be directly involved in disturbing the well-controlled membrane fusion machinery of the cell. We make the conceptual step of considering αS oligomers as small multivalent biological nanoparticles that cause formation of holes in the membrane or membrane thinning. The observed uncontrolled and efficient hemifusion of vesicles sheds a new light on the possible toxicity of oligomers in Parkinson’s disease.