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Extracellular senile plaques are a central pathological feature of Alzheimer's disease. At the core of these plaques are fibrillar deposits of β-amyloid peptide (Aβ). In vitro, Aβ spontaneously assembles into amyloid fibrils of cross-β sheet structure. Although it was once believed that the fibrils themselves were toxic, more recent data supports the hypothesis that aggregation intermediates, rather than fully formed fibrils, are the most damaging to neuronal tissue. In previously published work, we identified several small peptides that interact with Aβ and increase its aggregation rate while decreasing its toxicity. In this work, we examined in detail the interaction between Aβ and one of these peptides. Using a mathematical model of Aβ aggregation kinetics, we show that the dominant effect of the peptide is to accelerate lateral association of Aβ filaments into fibrils.