Thioflavin T triggers β amyloid peptide (1-40) fibrils formation.

Introduction A general characteristic of aggregation is the multiple interaction and cross-feedback among distinct mechanisms occurring at different hierarchical levels. The comprehension of the different species interconversion during aggregation is very important since emerging evidences indicate intermediate oligomeric aggregates as primary toxic species. In this context, Aβ amyloid peptide provides a challenging model for studying aggregation phenomena both for the complexity of its association process and for the direct implications in Alzheimer’s Disease. Aggregates growth conditions strongly affect the final morphology, the fibrillar molecular structure as well as the aggregation pathway which is characterized by the occurrence of multiple transient species. Methods The fluorescent dye Thioflavin T (ThT) is widely used to detect amyloid deposits and it is often used in situ to study aggregation kinetics, under the hypothesis that its presence does not affect the aggregation processes under study. Here we present an experimental study on Aβ(1-40) peptide fibrillation kinetics at pH 7.4. In the observed conditions, Aβ(1- 40) undergoes aggregation only if Thioflavin T is present in solution. This phenomenon was analyzed as a function of temperature, ThT and peptide concentrations in order to explore the underlying fibrillation mechanism. Light scattering, ThT fluorescence emission, two photon excitation fluorescence microscopy, were used in a kinetic fashion to highlight different sides and critical phases of the aggregation pathway. Circular Dichroism and FTIR measurements are used to characterize secondary structure of the aggregates. Results The selected approach gives detailed information on the time evolution of Aβ(1-40) fibrillation process highlighting structural changes at molecular level, different aggregate species growth and their morphologies. Our data show that Aβ(1-40) fibrillation process occurs only in the presence of ThT and that the observed aggregation involves at least three different aggregation mechanisms acting in competition. In the first step, small oligomers, which bind ThT, are formed via non nucleated polymerization mechanism and represent an activated state for following fibrils growth. This process appear to be a rate limiting step for two distinct fibril nucleation mechanisms probably affected by an high degree of spatial heterogeneity. Conclusions We demonstrated that in the selected experimental conditions ThT triggers the Aβ(1−40) fibrillation process (D’Amico et. al 2012). Sterical and chemical properties of ThT molecule may modulate the peptide conformation, with similar mechanisms to the ones that usually drive the binding of this dye to already formed amyloids. So, the presence of ThT in solution may change the thermodynamic equilibrium trapping specificmore ordered conformations prone to supramolecular assembly.