Parkinson's Protein α-Synuclein Binds Efficiently and with a Novel Conformation to Two Natural Membrane Mimics

Structural insights into α-synuclein monomer-fibril interactions

The characteristic feature of Parkinson’s disease is the deposition of α-synuclein into insoluble amyloid fibrils. The so-called secondary nucleation mechanism appears to be key for the aggregation kinetics, because binding of monomers on the fibril surface can autocatalytically induce new amyloid seeds. We show by nuclear magnetic and electron paramagnetic resonance spectroscopy that α-synuclein monomer–fibril binding is primarily mediated by transient electrostatic interactions. These intermolecular contacts result in an unfolding of the loosely packed α-synuclein structures and expose the otherwise protected aggregation-prone non-amyloid-β component of the protein. Our data demonstrate that intramolecular unfolding of α-synuclein is a prerequisite for protein aggregation that leads to rapid multiplication of α-synuclein amyloid fibrils via the secondary nucleation process.

Protein aggregation into amyloid fibrils is associated with multiple neurodegenerative diseases, including Parkinson’s disease. Kinetic data and biophysical characterization have shown that the secondary nucleation pathway highly accelerates aggregation via the absorption of monomeric protein on the surface of amyloid fibrils. Here, we used NMR and electron paramagnetic resonance spectroscopy to investigate the interaction of monomeric α-synuclein (α-Syn) with its fibrillar form. We demonstrate that α-Syn monomers interact transiently via their positively charged N terminus with the negatively charged flexible C-terminal ends of the fibrils. These intermolecular interactions reduce intramolecular contacts in monomeric α-Syn, yielding further unfolding of the partially collapsed intrinsically disordered states of α-Syn along with a possible increase in the local concentration of soluble α-Syn and alignment of individual monomers on the fibril surface. Our data indicate that intramolecular unfolding critically contributes to the aggregation kinetics of α-Syn during secondary nucleation.

Probing Alpha-Synuclein Conformations by Electron Paramagnetic Resonance (EPR) Spectroscopy

Electron paramagnetic resonance (EPR) spectroscopy in combination with site-directed spin labeling is ideally suited to study structure, dynamics, and interactions of intrinsically disordered proteins as alpha-synuclein.Here we describe all steps required for a corresponding study: the spin labeling procedure, sample preparation, spectroscopic experimental procedure, and data analysis.

Structural characteristics and membrane interactions of tandem α-synuclein oligomers

Pre-fibrillar oligomers of α-synuclein are thought to be pathogenic molecules leading to neurotoxicity associated with Parkinson’s disease and other neurodegenerative disorders. However, small oligomers are difficult to isolate for study. To gain better insight into the properties of small α-synuclein oligomers, we investigated engineered oligomers of specific size (dimers, tetramers, and octamers) linked head-to-tail in tandem, comparing the behavior of the oligomers to monomeric α-synuclein. All oligomeric constructs remained largely disordered in solution, as determined from dynamic light scattering and size-exclusion chromatography. Electron microscopy revealed that each construct could aggregate to form fibrils similar to those formed by monomeric α-synuclein. The interactions with large unilamellar vesicles (LUVs) composed of negatively-charged lipids differed depending on size, with smaller oligomers forming more extensive helical structure as determined by CD spectroscopy. Monitoring the influx of a fluorescence bleaching agent into vesicles showed that larger oligomers were somewhat more effective at degrading vesicular integrity and inducing membrane permeabilization.

Involvement of Cellular Prion Protein in α-Synuclein Transport in Neurons

The cellular prion protein, encoded by the gene Prnp, has been reported to be a receptor of β-amyloid. Their interaction is mandatory for neurotoxic effects of β-amyloid oligomers. In this study, we aimed to explore whether the cellular prion protein participates in the spreading of α-synuclein. Results demonstrate that Prnp expression is not mandatory for α-synuclein spreading. However, although the pathological spreading of α-synuclein can take place in the absence of Prnp, α-synuclein expanded faster in PrP^C-overexpressing mice. In addition, α-synuclein binds strongly on PrP^C-expressing cells, suggesting a role in modulating the effect of α-synuclein fibrils.

A Bayesian approach to quantifying uncertainty from experimental noise in DEER spectroscopy

Double Electron-Electron Resonance (DEER) spectroscopy is a solid-state pulse Electron Paramagnetic Resonance (EPR) experiment that measures distances between unpaired electrons, most commonly between protein-bound spin labels separated by 1.5-8nm. From the experimental data, a distance distribution P(r) is extracted using Tikhonov regularization. The disadvantage of this method is that it does not directly provide error bars for the resulting P(r), rendering correct interpretation difficult. Here we introduce a Bayesian statistical approach that quantifies uncertainty in P(r) arising from noise and numerical regularization. This method provides credible intervals (error bars) of P(r) at each r. This allows practitioners to answer whether or not small features are significant, whether or not apparent shoulders are significant, and whether or not two distance distributions are significantly different from each other. In addition, the method quantifies uncertainty in the regularization parameter.