Site-specific modification of Alzheimer's peptides by cholesterol oxidation products enhances aggregation energetics and neurotoxicity

Accumulation of amyloid beta-peptide (A beta) and tau aggregates, possibly linked to age-associated deficiencies in protein homeostasis, appear to cause Alzheimer's disease. Schiff-base formation between A beta and the aldehyde-bearing cholesterol oxidation product 3-beta-hydroxy-5-oxo-5,6-secocholestan-6-al is known to increase A beta amyloidogenicity. Here, we synthesized A beta variants site-specifically modified with the cholesterol aldehyde at Asp-1, Lys-16, or Lys-28, rather than studying mixtures. These distinct modifications have a similar effect on the thermodynamic propensity for aggregation, enabling aggregation at low concentrations. In contrast, the modification site differentially influences the aggregation kinetics; Lys-16-modified A beta formed amorphous aggregates fastest and at the lowest concentration (within 2 h at a concentration of 20 nM), followed by the Lys-28 and Asp-1 conjugates. Also, the aggregates resulting from A beta Lys-16 cholesterol aldehyde conjugation were more toxic to primary rat cortical neurons than treatment with unmodified A beta under identical conditions and at the same concentration. Our results show that A beta modification by cholesterol derivatives, especially at Lys-16, renders it kinetically and thermodynamically competent to form neurotoxic aggregates at concentrations approaching the physiologic concentration of A beta.