Aggregation and catabolism of disease-associated intra-Aβ mutations:: reduced proteolysis of AβA21G by neprilysin

Five point mutations within the amyloid beta-protein (A beta) sequence of the APP gene are associated with hereditary diseases which are similar or identical to Alzheimer's disease and encode: the A21G (Flemish), E22G (Arctic), E22K (Italian), E22Q (Dutch) and the D23N (Iowa) amino acid substitutions. Although a substantial body of data exists on the effects of these mutations on A beta production, whether or not intra-A beta mutations alter degradation and how this relates to their aggregation state remain unclear. Here we report that the E22G, E22Q and the D23N substitutions significantly increase fibril nucleation and extension, whereas the E22K substitution exhibits only an increased rate of extension and the A21G substitution actually causes a decrease in the extension rate. These substantial differences in aggregation together with our observation that aggregated wild type A beta(1-0) was much less well degraded than monomeric wild type A beta(1-40), prompted us to assess whether or not disease-associated intra-A beta mutations alter proteolysis independent of their effects on aggregation. Neprilysin (NEP), insulin degrading enzyme (IDE) and plasmin play a major role in A beta catabolism, therefore we compared the ability of these enzymes to degrade wild type and mutant monomeric A beta peptides. Experiments investigating proteolysis revealed that all monomeric peptides are degraded similarly by IDE and plasmin, but that the Flemish peptide was degraded significantly more slowly by NEP than wild type A beta or any of the other mutant peptides. This finding suggests that resistance to NEP-mediated proteolysis may underlie the pathogenicity associated with the A21G mutation. (C) 2008 Elsevier Inc. All rights reserved.