β-Synuclein suppresses both the initiation and amplification steps of α-synuclein aggregation via competitive binding to surfaces

Abstract

α-Synuclein is an intrinsically disordered protein that is associated with the pathogenesis of Parkinson’s disease through the processes involved in the formation of amyloid fibrils. α and β-synuclein are homologous proteins found at comparable levels in presynaptic terminals but β-synuclein has a greatly reduced propensity to aggregate and indeed has been found to inhibit α-synuclein aggregation. In this paper, we describe how sequence differences between α- and β-synuclein affect individual microscopic processes in amyloid formation. In particular, we show that β-synuclein strongly suppresses both lipid-induced aggregation and secondary nucleation of α-synuclein by competing for binding sites at the surfaces of lipid vesicles and fibrils, respectively. These results suggest that β-synuclein can act as a natural inhibitor of α-synuclein aggregation by reducing both the initiation of its self-assembly and the proliferation of its aggregates.

A natural product inhibits the initiation of α-synuclein aggregation and suppresses its toxicity

“…The change in the relative rate of lipid-induced aggregation of α-synuclein with increasing concentration of squalamine is well described by a competitive binding model, with the binding constants determined here for squalamine (K D,S and L S ) and previously for α-synuclein (K D,α and L α ) (12). Taken together, these results are consistent with the conclusion that squalamine inhibits the lipid-induced aggregation of α-synuclein via competitive binding at the surfaces of the vesicles, as observed for β-synuclein (22). In addition, we probed the interaction of squalamine with α-synuclein fibrils by incubating squalamine in the presence of the fibrils.…”

“…The presence of squalamine progressively decreased the α-helical content of α-synuclein, again indicating that this compound displaces the protein from the vesicles, as recently observed for β-synuclein (22). To analyze these data, we thus used the same competitive binding model as the one describing the inhibitory effect of β-synculein on α-synuclein lipidinduced aggregation (22), where both α-synuclein and squalamine compete for binding sites at the surface of the DMPS vesicles, which validated the model in a quantitative manner.…”

“…We further explored the effects of squalamine on the binding and aggregation of α-synuclein in the presence of DMPS-containing vesicles, which have been shown to be particularly effective in enhancing the rate of aggregation and amyloid formation of α-synuclein (12). The presence of squalamine progressively decreased the α-helical content of α-synuclein, again indicating that this compound displaces the protein from the vesicles, as recently observed for β-synuclein (22). To analyze these data, we thus used the same competitive binding model as the one describing the inhibitory effect of β-synculein on α-synuclein lipidinduced aggregation (22), where both α-synuclein and squalamine compete for binding sites at the surface of the DMPS vesicles, which validated the model in a quantitative manner.…”

“…We thus used the same competitive binding model (Fig. 2B) as the one describing the displacement of α-synuclein from the vesicle by β-synuclein (22), where both α-synuclein and squalamine compete for binding sites at the surface of the DMPS vesicles, in order to analyze the CD data. This model, together with previously determined binding constants for the α-synuclein/DMPS system (12), described the data very closely and yielded values for both the binding constant and the stoichiometry of the binding of squalamine to DMPS vesicles, K D, S = 67 nM and L S = 7.3, respectively.…”

“…Indeed squalamine has been referred to as a "cationic lipid" (18) as it carries a net positive charge and shows a high affinity for anionic phospholipids (20) of the type that nucleates the aggregation of α-synuclein, thereby reducing the negative charge of the membrane surface to which it is bound (18,21) without significantly disrupting the integrity of lipid surfaces (18). In analogy, it has recently been shown that a homologous protein, β-synuclein, can inhibit α-synuclein lipid-induced aggregation via a competitive binding at the surface of lipid vesicles (22).…”