Sterol-activated amyloid beta fibril formation

Abstract

The metabolic processes that link Alzheimer’s disease (AD) to elevated cholesterol levels in the brain are not fully defined. Amyloid beta (Aβ) plaque accumulation is believed to begin decades prior to symptoms and to contribute significantly to the disease. Cholesterol and its metabolites accelerate plaque formation through as-yet undefined mechanisms. Here, the mechanism of cholesterol (CH) and cholesterol 3-sulfate (CS) induced acceleration of Aβ fibril formation is explored in quantitative ligand binding, Aβ fibril polymerization, and molecular dynamics studies. Equilibrium and presteady-state binding studies reveal that monomeric Aβ42•ligand complexes form and dissociate rapidly relative to oligomerization, that the ligand/peptide stoichiometry is 1-to-1 and that the peptide is likely saturated in vivo. Analysis of Aβ42 polymerization progress curves demonstrates that ligands accelerate polymer synthesis by catalyzing conversion of peptide monomers into dimers that nucleate the polymerization reaction. Nucleation is accelerated ∼49-fold by CH, and ∼13,000-fold by CS — a minor CH metabolite. Polymerization kinetic models predict that at presumed disease-relevant CS and CH concentrations, approximately half of polymerization nuclei will contain CS, small oligomers of neurotoxic dimensions (∼12-mers) will contain substantial CS, and fibril-formation lag times will decrease 13-fold relative to unliganded Aβ42. Molecular dynamics models, which quantitatively predict all experimental findings, indicate that the acceleration mechanism is rooted in ligand-induced stabilization of the peptide in non-helical conformations that readily form polymerization nuclei.