G37V mutation of Aß42 induces a nontoxic ellipse-like aggregate: An in vitro and in silico study.

The glycine zipper motif at the C-terminus of the ß-amyloid (Aß) peptide have been shown to strongly influence the formation of neurotoxic aggregates. A previous study showed that the G37L mutation dramatically reduces the Aß toxicity in vivo and in vitro. However, the primary cause and mechanism of the glycine zipper motif on Aß properties remain unknown. To gain molecular insights into the impact of glycine zipper on Aß properties, we substituted the residue 37 of Glycine by Valine and studied the structural and biochemical properties of G37V mutation, Aß42(37V), by using in vitro and in silico approaches. Unlike G37L mutation, the G37V mutation reduced toxicity substantially but did not significantly accelerate the aggregation rate or change the content of secondary structures. Further TEM analyses showed that the G37V mutation formed an ellipse-like aggregate rather than a network-like fibril as wild type or G37L mutation of Aß42 form. This different aggregation morphology may be highly linked with the reduction of toxicity. To gain the insight for the different properties of Aß42(37V), we studied the structure of Aß42 and G37V mutation using the replica exchange molecular dynamics simulation. Our results demonstrate that although the overall secondary structure population is similar with Aß42 and Aß42(G37V), Aß42(G37V) shows an increase in the ß-turn and ß-hairpin at residues 36-37 and the flexibility of the Asp23-Lys28 salt bridge. These unique structural features may be the possible reason to account for the ellipse-like morphology.

Aggregation rate of amyloid beta peptide is controlled by beta-content in monomeric state.

Understanding the key factors that govern the rate of protein aggregation is of immense interest since protein aggregation is associated with a number of neurodegenerative diseases. Previous experimental and theoretical studies have revealed that the hydrophobicity, charge, and population of the fibril-prone monomeric state control the fibril formation rate. Because the fibril structures consist of cross beta sheets, it is widely believed that those sequences that have a high beta content (ß) in the monomeric state should have high aggregation rates as the monomer can serve as a template for fibril growth. However, this important fact has never been explicitly proven, motivating us to carry out this study. Using replica exchange molecular dynamics simulation with implicit water, we have computed ß of 19 mutations of amyloid beta peptide of 42 residues (Aß42) for which the aggregation rate κ has been measured experimentally. We have found that κ depends on ß in such a way that the higher the propensity to aggregation, the higher the beta content in the monomeric state. Thus, we have solved a long-standing problem of the dependence of fibril formation time of the ß-structure on a quantitative level.

Aß41 Aggregates More Like Aß40 than Like Aß42: In Silico and in Vitro Study.

Formation of intracellular plaques and small oligomeric species of amyloid ß (Aß) peptides inside neurons is a hallmark of Alzheimer's disease. The most abundant Aß species in the brain are Aß1-40 and Aß1-42, which are composed, respectively, of 40 and 42 residues. Aß1-42 differs from Aß1-40 only in two residues, Ile41 and Ala42, yet it shows remarkably faster aggregation and greater neurotoxicity than Aß1-40. Thus, it is crucial to understand the relative contributions of Ile41 and Ala42 to these distinct behaviors. To achieve this, secondary structures of the Aß1-41 monomer, which contribute to aggregation propensity, were studied by all-atom molecular dynamics simulation in an implicit solvent and compared to those of Aß1-40 and Aß1-42. We find that the secondary structure populations of Aß1-41 are much closer to those of Aß1-40 than to those of Aß1-42, suggesting that Aß1-41 and Aß1-40 are likely to have similar aggregation properties. This prediction was confirmed through a thioflavin-T aggregation assay. Thus, our finding indicates that the hydrophobic residue at position 42 is the major contributor to the increased fibril formation rates and consequently neurotoxicity of Aß peptides.