Local Ionic Conditions Modulate the Aggregation Propensity and Influence the Structural Polymorphism of α‑Synuclein

Parkinson’s disease (PD) is linked to the aggregation of
the intrinsically disordered protein α-synuclein (aSyn), but
the precise triggers and mechanisms driving this process remain unclear.
Local environmental factors, such as ion concentrations, can influence
aSyn’s conformational ensemble and its tendency to aggregate.
In this study, we explore how physiologically relevant ions, mainly
Ca2+ and Na+, affect aSyn aggregation, monomer
structural dynamics, and fibril polymorphism. ThT fluorescence assays
show that all ions speed up aggregation, with Ca2+ having
the strongest effect. Using heteronuclear single quantum correlation
nuclear magnetic resonance (1H–15N HSQC
NMR) spectroscopy, we validate that Ca2+ binds at the C-terminus
while Na+ interacts nonspecifically across the sequence.
Small-angle neutron scattering (SANS) and hydrogen–deuterium
exchange mass spectrometry (HDX-MS) show that Na+ leads
to more extended aSyn structures, while Ca2+ results in
moderate extension. Molecular dynamics (MD) simulations support this,
showing Na+ increases extension between the NAC region
and C-terminus, whereas Ca2+ biases the ensemble toward
a moderately elongated structure. MD also shows that Ca2+ increases water persistence times in the hydration shell, indicating
that aSyn aggregation propensity is due to a combination of conformational
bias of the monomer and solvent mobility. Atomic force microscopy
(AFM) points toward the formation of distinct fibril polymorphs under
different ionic conditions, suggesting ion-induced monomer changes
contribute to the diversity of fibril structures. These findings underscore
the pivotal influence of the local ionic milieu in shaping the structure
and aggregation propensity of aSyn, offering insights into the molecular
underpinnings of PD and potential therapeutic strategies targeting
aSyn dynamics.