α-Synuclein Oligomers: an Amyloid Pore?

Abstract

In many human diseases, oligomeric species of amyloid proteins may play a pivotal role in cytotoxicity. Many lines of evidence indicate that permeabilization of cellular membranes by amyloid oligomers may be the key factor in disrupting cellular homeostasis. However, the exact mechanisms by which the membrane integrity is impaired remain elusive. One prevailing hypothesis, the so-called amyloid pore hypothesis, assumes that annular oligomeric species embed into lipid bilayers forming transbilayer protein channels. Alternatively, an increased membrane permeability could be caused by thinning of the hydrophobic core of the lipid bilayer due to the incorporation of the oligomers between the tightly packed lipids, which would facilitate the transport of small molecules across the membrane. In this review, we briefly recapitulate our findings on the structure of α-synuclein oligomers and the factors influencing their interaction with lipid bilayers. Our results, combined with work from other groups, suggest that α-synuclein oligomers do not necessarily form pore-like structures. The emerging consensus is that local structural rearrangements of the protein lead to insertion of specific regions into the hydrophobic core of the lipid bilayer, thereby disrupting the lipid packing.

Fluorescence as a Tool to Study Lipid-Protein Interactions: The Case of α-Synuclein

“…This process dependent on the presence of negatively charged lipids being in a liquid disordered phase, while the headgroup is only of minor importance [35,36]. This behavior, which is comparable to monomeric α-synuclein suggests that mainly electrostatic interactions between the positively charged core of the oligomers and negatively charged lipid headgroups cause membrane binding [37,38]. In order to elucidate the mechanisms by which the α-synuclein oligomers permeabilize lipid bilayers, Subramaniam group tested different lipid mixtures in leakage experiments either based on the release of calcein from large unilamellar vesicles (LUVs) containing self-quenching concentrations of the probe or on measuring the influx of dithionite into LUVs labeled with 1-palmitoyl-2-6-[(7-nitro-2-1,3-benzoxadiazol-4-yl) amino] hexanoyl-phosphatidylcholine (C 6 -NBD-PC).…”

Extracellular Alpha-Synuclein Oligomers Induce Parkin S-Nitrosylation: Relevance to Sporadic Parkinson’s Disease Etiopathology

“…Recently, it was demonstrated that the negative impact of aberrantly secreted ASN does not appear to involve internalization of this protein by the recipient neurons [40,41], but it depends on deregulation of various plasma membrane receptors most of which are Ca 2+ channels [28,42,43]. Alternatively, aberrant oligomeric structures of ASN were shown to evoke permeabilization of cellular membranes leading to uncontrolled Ca 2+ influx and synaptic vesicular depletion [27,[44][45][46][47]. In our studies, we generated an oligomeric species of ASN that are able to form annular pore-like structures and observed that treatment of PC12 cells with those oligomers leads to significant increase in cellular Ca 2+ concentration.…”

Fluorescence Methods for Unraveling Oligomeric Amyloid Intermediates

“…Instead, it became abundantly clear that, depending on the preparation protocol used, these oligomers exhibit a significant degree of structural and morphological heterogeneity, making it extremely challenging to identify a specific cytotoxic type of oligomer and to fully characterize these [12][13][14]. We, in particular, have extensively characterized the membrane binding and permeabilization capabilities of a specific, isolated, oligomeric species [15][16][17], and have focused on understanding the molecular composition and structural characteristics of this oligomeric species [10,18].…”