The extent of protein hydration dictates the preference for heterogeneous or homogeneous nucleation generating either parallel or antiparallel β-sheet α-synuclein aggregates

Abstract

α-Synuclein amyloid self-assembly is the hallmark of a number of neurodegenerative disorders, including Parkinson’s disease, although there is still very limited understanding about the factors and mechanisms that trigger this process. Primary nucleation has been observed to be initiated in vitro at hydrophobic/hydrophilic interfaces by heterogeneous nucleation generating parallel β-sheet aggregates, although no such interfaces have yet been identified in vivo. In this work, we have discovered that α-synuclein can self-assemble into amyloid aggregates by homogeneous nucleation, without the need of an active surface, and with a preference for an antiparallel β-sheet arrangement. This particular structure has been previously proposed to be distinctive of stable toxic oligomers and we here demonstrate that it indeed represents the most stable structure of the preferred amyloid pathway triggered by homogeneous nucleation under limited hydration conditions, including those encountered inside α-synuclein droplets generated by liquid-liquid phase separation. In addition, our results highlight the key role that water plays not only in modulating the transition free energy of amyloid nucleation, and thus governing the initiation of the process, but also in dictating the type of preferred primary nucleation and the type of amyloid polymorph generated depending on the extent of protein hydration. These findings are particularly relevant in the context of in vivo α-synuclein aggregation where the protein can encounter a variety of hydration conditions in different cellular microenvironments, including the vicinity of lipid membranes or the interior of membraneless compartments, which could lead to the formation of remarkably different amyloid polymorphs by either heterogeneous or homogeneous nucleation.

Naringenin-Functionalized Gold Nanoparticles and Their Role in α-Synuclein Stabilization

“…19,20 In vitro analysis also showed that α-synuclein can self-assemble at room temperature to produce amyloid-like fibrils under several solution conditions such as low pH, 21 orbital shaking, 22 an air−water interface, etc. 23,24 The protein secondary structure and morphological features of these aggregates are quite similar to those found in the patients' samples. Therefore, the prevention of aggregation of α-synuclein and other such proteins and peptides may be one of the possible treatment options for PD, AD, and other similar amyloid diseases which are becoming a huge burden for society.…”

Characterization of Pairs of Toxic and Nontoxic Misfolded Protein Oligomers Elucidates the Structural Determinants of Oligomer Toxicity in Protein Misfolding Diseases

“…3 Alternatively, incubation of αSyn under limited hydration conditions (either PBS with moderate concentrations of alcohols or high concentrations of salts, or upon protein lyophilization in water) promoted αSyn aggregation into structures with an antiparallel intermolecular βsheet structure and particularly slow conversion to fibrils. 28,29 These oligomeric forms, particularly those generated upon αSyn lyophilization and resuspension at high protein concentrations (typically 12 mg•mL −1 ), were morphologically and biologically analogous to the type B oligomers, and were thus named type B* oligomers. 28 Importantly, similar oligomeric species are formed inside neuronal cells, 30 and analogous levels of stress and dysfunction were observed in PD-related cellular models of αSyn aggregation.…”

Human cerebral vascular amyloid contains both antiparallel and parallel in-register Aβ40 fibrils

“…Recently, Aβ40-WT fibrils with mixed anti-parallel and parallel structures have been observed in the presence of the ganglioside GM1 (41). Moreover, studies on -synuclein have indicated that more hydrophobic environments favor anti-parallel fibril structure over parallel structure (42).…”