Urea modulation of -amyloid fibril growth: Experimental studies and kinetic models

Aggregation of -amyloid (A) into fibrillar deposits is widely believed to initiate a cascade of adverse biological responses associated with Alzheimer’s disease. Although it was once assumed that the mature fibril was the toxic form of A, recent evidence supports the hypothesis that A oligomers, intermediates in the fibrillogenic pathway, are the dominant toxic species. In this work we used urea to reduce the driving force for A aggregation, in an effort to isolate stable intermediate species. The effect of urea on secondary structure, size distribution, aggregation kinetics, and aggregate morphology was examined. With increasing urea concentration, -sheet content and the fraction of aggregated peptide decreased, the average size of aggregates was reduced, and the morphology of aggregates changed from linear to a globular/linear mixture and then to globular. The data were analyzed using a previously published model of A aggregation kinetics. The model and data were consistent with the hypothesis that the globular aggregates were inter-mediates in the amyloidogenesis pathway rather than alternatively aggregated species. Increasing the urea concentration from 0.4 M to 2 M decreased the rate of filament initiation the most; between 2 M and 4 M urea the largest change was in partitioning between the nonamyloid and amyloid pathways, and between 4 M and 6 M urea, the most significant change was a reduction in the rate of filament elongation