Plasmon Waveguide Resonance Shows Preferential Binding of Oligomeric alpha-Synuclein to Raft-Like Lipid Mixtures

Abstract

α-Synuclein is a presynaptic protein whose fibrillar and β-sheet rich aggregates are implicated in several neurodegenerative diseases such as Parkinson's disease (PD). Different lines of evidence suggest that oligomer intermediates rather than monomeric or mature fibrillar deposits constitute the toxic species, probably by membrane incorporation and pore formation [1]. We used plasmon waveguide resonance (PWR) spectroscopy [2] to characterize the binding of various α-Synuclein aggregates to planar lipid membranes. The binding isotherms yielded affinity constants for the membrane-active aggregation species of α-Synuclein. In addition, using different lipid mixtures, we studied the role of the lipid composition for membrane insertion of the α-Synuclein oligomers. To mimic the compositional and structural heterogeneity of neuronal membranes we used detergent-induced membrane fusion and employed raft-like mixtures of sphingomyelin, eggPC, and cholesterol. These mixtures exhibit stable lateral domain formation as revealed by solid-state 2H NMR [3]. The results show that binding and membrane insertion of α-Synuclein is highly dependent on the aggregation state of the protein. Our data suggest that the lateral segregation into lipid domains strongly promotes membrane insertion of the toxic aggregation species. Furthermore, we show that membrane lipids are able to dissolve pre-aggregated fibrils back into intermediate species. We therefore propose that the pathogenicity of α-Synuclein is highly dependent on the lipid composition of intracellular membranes, most notably the membranes of synaptic vesicles, and that the macroscopic aggregates found in PD patients act as a reservoir for toxic intermediates in vivo.[1] Beyer, K. (2007) Cell. Biochem. Biophys. 47, 285-299.[2] Salamon, Z. et al (1999) Trends Biochem. Sci. 24, 213-219.[3] Bartels, T. et al (2008) J. Am. Chem. Soc., in press.