Physicochemical Insights into Apolipoprotein E Isoform-Dependent Inhibition of α-Synuclein Fibrillation

Abstract

Apolipoprotein E (apoE) modulates amyloid deposits of disease-related proteins, and its genotype is a risk factor for multiple neurodegenerative disorders. We previously demonstrated that heparin induces α-synuclein (αS) fibrillation by shifting the elongation–depolymerization equilibrium toward elongation. Here, we examined the effect of apoE isoforms and their variants on the heparin-induced αS fibrillation, using a modified Finke–Watzky kinetic model. A thioflavin T fluorescence assay showed that apoE strongly inhibits αS fibril formation with decreased fibril production in an isoform-dependent manner in the order of apoE4 > apoE3 > apoE2. Atomic force microscopy demonstrated that apoE promotes formation of shorter fibrils, suggesting that apoE inhibits αS fibril growth. Size-exclusion chromatography revealed that apoE does not promote accumulation of stable oligomeric species during fibrillation. Based on these results, apoE appears to suppress elongation of αS fibrils, thereby shifting the equilibrium toward depolymerization. Consistently, kinetic modeling, which incorporates an apoE-dependent inhibition step, suggested that fibrillar αS rather than the monomeric form is likely involved in the inhibitory mechanism of apoE. Taken together with the finding that the K146E mutation in apoE3, which exhibits deficient heparin binding and increased solvent-exposure of hydrophobic patches, inhibited αS fibrillation more strongly than WT apoE3, we propose that solvent-exposed hydrophobic regions of apoE contribute to its inhibitory effect on αS fibrillation through a mechanism involving fibrillar αS. These findings provide mechanistic insights into how apoE modulates αS fibrillation and its potential role in disease pathogenesis.