Lipid Extraction by α-Synuclein Generates Semi-Transmembrane Defects and Lipoprotein Nanoparticles

Modulations of synaptic membranes play an essential role in the physiological and pathological functions of the presynaptic protein alpha-synuclein (alpha Syn). Here we used solution atomic force microscopy (AFM) and electron paramagnetic resonance (EPR) spectroscopy to investigate membrane modulations caused by alpha Syn. We used several lipid bilayers to explore how different lipid species may regulate alpha Syn-membrane interactions. We found that at a protein-to-lipid ratio of similar to 1/9, alpha Syn perturbed lipid bilayers by generating semi-transmembrane defects that only span one leaflet. In addition, alpha Syn coaggregates with lipid molecules to produce similar to 10 nm-sized lipoprotein nanoparticles. The obtained AFM data are consistent with the apolipoprotein characteristic of alpha Syn. The role of anionic lipids was elucidated by comparing results from zwitterionic and anionic lipid bilayers. Specifically, our AFM measurements showed that anionic bilayers had a larger tendency of forming bilayer defects; similarly, our EPR measurements revealed that anionic bilayers exhibited more substantial changes in lipid chain mobility and bilayer polarity. We also studied the effect of cholesterol. We found that cholesterol increased the capability of alpha Syn in inducing bilayer defects and altering lipid chain mobility and bilayer polarity. These data can be explained by an increase in the lipid headgroup-headgroup spacing and/or specific cholesterol-alpha Syn interactions. Interestingly, we found an inhibitory effect of the cone-shaped phosphatidylethanolamine lipids on alpha Syn-induced bilayer remodeling. We explained our data by considering interlipid hydrogen-bonding that can stabilize bilayer organization and suppress lipid extraction. Our results of lipid-dependent membrane modulations are likely relevant to alpha Syn functioning.