Effect of solvents, buffer systems, and ionic strength on the fibrillation of beta-amyloid peptides

Abstract

Alzheimer's disease is the most common neurodegenerative disease. It has been widely accepted that neurodegeneration in Alzheimer's disease (AD) may be caused by deposition of beta-amyloid peptide (Aβ) plaques in brain tissue. According to the amyloid hypothesis, accumulation of Aβ in the brain is the primary influence driving AD pathogenesis. A majority of AD research and therapies are based on this hypothesis, and synthetic beta-amyloid peptides have been widely used to mimic the pathogenic process in vitro and used as target molecules for the therapies. Since the solubility and the initial states of the peptides play a key role in the aggregation of the peptides, the need to have suitable conditions in order to get the peptides into their expected state, mainly monomers is crucial. To obtain acceptable non-aggregated peptides, we tested and compared the solubility of the synthetic beta-amyloid (1-40) and (1-42) in commonly used solvents (NaOH, NH4OH, and HFIP) and their aggregation properties in different buffers. SEC-HPLC was used to analyze the dissolved Aβ peptides in selected solvents. Thioflovin T assay was applied to monitor the fibrillation kinetics of the peptides. Transmission electron microscopy (TEM) was employed to reveal the morphology of the fibrils. SEC-HPLC analysis showed that the beta-amyloid peptide dissolved in 1% NH4OH could get >99% monomeric form compared to 0.1 M NaOH (>98%) and hexafluoro-2-propanol (HFIP) (>95%). The fibrillation kinetics of the beta-amyloid peptide (1-42) showed no difference in HEPES, Tris-HCl, or phosphate buffer at neutral pH; all of which showed quick fibrillation without obvious lag time (peptide concentration 0.4 mg/ml, various buffers at pH 7.3, with agitation of 300 rpm, 2 mm diameter). TEM images of the fibrils showed no difference between the different solvents. With an increase in NaCl concentration at the range of 0-500 mM, the fibrillation rates increase in both nucleation and elongation phases. The solid synthetic beta-amyloid peptides can be dissolved in 1% NH4OH or 0.1 M NaOH to obtain monomeric peptide solutions without affecting the peptide fibrillation and aggregation.