Fluorescently-labelled amyloid paired helical filaments (PHF) in monitoring its fibrillation kinetics

The core of the tau fibrils in Alzheimer disease is a hexapeptide sequence organized in paired helical filaments (PHF). This sequence AcPHF6 can be used as tau fibrils model for the fast screening of potential therapeutic inhibitors of fibril formation or their disruption. The assay is usually performed by monitoring the fluorescence increase of Thioflavin T (ThT), well-known reporter dye for fibrillation. However, the ThT assay is not faultless, and here we present novel fluorescent dye, cyanine attached to amino acid side-chain (Cy-aa) that shows several advantages over ThT. The fibrillation kinetics of AcPHF6 was monitored via Cy-aa at twenty times lower concentration compared to ThT and successfully reported the presence of fibrillation inhibitor by Cy-aa fluorescence decrease. Additionally, spectral properties of Cy-aa are red-shifted in comparison to ThT allowing screening of a wider range of potential fibrillation inhibitors. Moreover, in the mixture with the pre-formed fibrils, Cy-aa shows strong fluorescence light-up proportional to fibrils concentration. We also successfully coupled this fluorescent amino acid to PHF in order to completely avoid the possibility of dye displacement with screening compound, and this newly designed conjugate showed to be a reliable intrinsic fluorescent probe for monitoring fibrillation kinetics of amyloid peptides.