Stability of Aβ (1-42) peptide fibrils as consequence of environmental modifications

beta-Amyloid peptide (A beta) plays a key role in the pathogenesis of Alzheimer disease (AD). Monomeric A beta undergoes aggregation, forming oligomers and fibrils, resulting in the deposition of plaques in the brain of AD patients. A widely used protocol for fibril formation in vitro is based on incubation of the peptide at low pH and ionic strength, which generates A beta fibrils several microns long. What happens to such fibrils once they are brought to physiological pH and ionic strength for biological studies is not fully understood. In this investigation, we show that these changes strongly affect the morphology of fibrils, causing their fragmentation into smaller ones followed by their aggregation into disordered structures. We show that an increase in pH is responsible for fibril fragmentation, while increased ionic strength is responsible for the aggregation of fibril fragments. This behavior was confirmed on different batches of peptide either produced by the same company or of different origin. Similar aggregates of short fibrils are obtained when monomeric peptide is incubated under physiological conditions of pH and ionic strength, suggesting that fibril morphology is independent of the fibrillation protocol but depends on the final chemical environment. This was also confirmed by experiments with cell cultures showing that the toxicity of fibrils with different initial morphology is the same after addition to the medium. This information is of fundamental importance when A beta fibrils are prepared in vitro at acidic pH and then diluted into physiological buffer for biological investigations.