Type 1 α-Syn Models: A Possible Toxic Pathway
Identification of which synuclein assemblies are essential and which are toxic may be vital for development of treatments. It is well established that an α-helical form of α-Syn binds to membrane surfaces and plays a vital role in the fusion of synaptic vesicles with the plasma membrane. However, interactions of α-Syn oligomers with membranes are not that simple: α-Syn adopts a multitude of membrane-bound conformations in unroofed cells and the Ct domain participates in membrane localization [61]. Using a microfluid setup that recapitulated the composition of the plasma membrane, Heo and Pinchet [23] found that α-Syn induced pores of discrete sizes and immobilized lipids on both the cis and trans sides of the membrane, indicating deep penetration inside the membrane. Quist et al. [21] used both electrical recording of membrane currents and atomic force microscopy to show that numerous amyloids, including α-Syn and its NAC domain, can form ion channels of multiple sizes in membranes. Kim et al. [22] also found that α-Syn oligomers formed well-defined conductance states in a variety of membranes and that the beta-structure of these membrane-bound oligomers differed from that of α-Syn fibrils. Parres-Gold et al. [24] used scanning ion conductance microscopy (SICM) to show that α-Syn oligomers induce morphological changes and formation of large transient pores in live SH-SY5Y neuroblastoma cells. Toxic α-Syn accumulation in SH-SY5Y is alleviated by membrane depolarization [62], suggesting that membrane interactions of α-Syn are related to toxicity in these cells. These interactions are affected by the lipid composition of the membranes. Binding of ganglioside and cholesterol appear to be involved in penetration of the membrane by α-Syn [26]. Based on results of nuclear magnet resonance spectroscopy Man et al. [63] concluded that cholesterol reduces the local affinity of the NAC domain to synaptic vesicles, supporting the role of disorder-to-order equilibrium of the NAC region. However, cholesterol-containing lipid nanodiscs accelerates oligomerization of α-Syn [64]. When the nanodiscs are made of DOPC, interaction of the NAC domain with the bilayer is increased; but when DOPC, DOPE, and DOPG all comprise the nanodiscs, binding of the NAC domain is not affected while binding of the Nt and Ct domains are inhibited.
Penetration of the alkyl phase of the membrane appears to depend upon the conformation of the α-Syn oligomer. Fusco et al. [65] analyzed two types of α-Syn oligomers of similar size that they called A and B. Type A interacts only with the surfaces of membranes, has little rigid secondary structure, and is not toxic. In contrast, segment 70-88 (red background in Figs. 18d and 19b) of the toxic Type B has a rigid β-secondary that penetrates the alkyl phase of the membrane. Most of this region is formed by Sy7 which is deleted in non-toxic β-Syn. Our working hypothesis is that their Type B oligomer is a Type 1 trimer in which the NAC domain forms a core six-stranded antiparallel β-barrel with the Sy7 segment in the turn region of the β-hairpin exposed.