The Effects of N-terminal Mutations on ß-amyloid Peptide Aggregation and Toxicity.

ABSTRACT

Human amyloid ß1-42 (hAß1-42) peptides are known to self-aggregate into oligomers that contribute to the degeneration of neurons and development of Alzheimer's disease (AD) pathology. Unlike humans, rodents do not develop AD, possibly due to differences in three amino acids (R5G, Y10F and H13R) within the hydrophilic N-terminal domain of Aß1-42. This is partly supported by evidence that hAß1-42 is more prone to fibrillization and has a higher cellular toxicity than rodent Aß1-42 (rAß1-42). Mutagenesis studies, however, have shown that correlation between fibrillization potential and toxicity is not always direct. Thus, to understand better how N-terminal mutations can affect hAß1-42 toxicity through oligomerization, we evaluated fibrillization kinetics, oligomer sizes and toxicity profiles of double mutant (human toward rodent) Aß1-42. Additionally, we tested the mutant peptides in combination with hAß1-42, to assess effects on hAß1-42 aggregation/toxicity. Our results clearly show that double mutations to humanize rAß1-42 result in a significantly reduced efficiency of fibril formation, as determined by Thioflavin-T aggregation assays and confirmed with electron micrographic studies. Interestingly, the mutants are still able to aggregate into oligomers, which are predominantly larger than those comprised of hAß1-42. Our cell viability experiments further showed a rank order of oligomer toxicity of hAß1-42 > rAß1-42 ≫ mutant Aß1-42, suggesting that toxicity can be influenced by N-terminal Aß1-42 mutations via reduction of fibril formation and/or alteration of oligomer size. These results, taken together, confirm that N-terminal mutations can affect Aß fibril and oligomer formation with reduced toxicity despite lying outside the core amyloid region of Aß peptide.