Alzheimer's Aβ peptides containing an isostructural backbone mutation afford distinct aggregate morphologies but analogous cytotoxicity.: Evidence for a common low-abundance toxic Structure(s)?

Amyloid beta (A beta) peptide amyloidogenesis, involving the formation of numerous distinct quaternary structures, appears to cause Alzheimer's disease. However, the precise identification of the toxic structure(s) and the neurotoxicity mechanism(s) remains elusive. Mutating the A beta 1-40 Phe19-Phe20 backbone amide bond to an isostructural E-olefin bond enables formation of spherical aggregates to the exclusion of detectable amyloid fibrils. Herein, the fibrillization and toxicity of amide-to-ester mutants of A beta 1-40 at the 19-20 position and surrounding backbone amide bonds are compared to the fibrillization and toxicity of the 19-20 E-olefin A beta analogue and wild type A beta. Whereas isostructural amide-to-E-olefin mutations eliminate both the H-bond donor and acceptor capabilities, isostructural amide-to-ester mutations eliminate the donor while retaining the ester carbonyl as a weakened acceptor. None of the amide-to-ester A beta 1-40 mutants prevent fibrillization; in fact several exhibit hastened amyloidogenesis. The 18-19 amide-to-ester substitution is the only backbone mutation within the hydrophobic core region of the fibril (residues 17-21) that significantly slows fibrillization. Despite forming different morphologies, the 19-20 E-olefin mutant, the 18-19 amide-to-ester mutant, and WT A beta 1-40 fibrils all exhibit similar toxicities when applied to PC12 cells at 18 h into the aggregation reactions, as assessed by MTT metabolic activity measurements. This result suggests that a common but low abundance aggregate morphology, that is accessible to these A beta analogues, mediates toxicity, or that several different aggregate morphologies are similarly toxic.