An ATR–FTIR Sensor Unraveling the Drug Intervention of Methylene Blue, Congo Red, and Berberine on Human Tau and Aβ

Alzheimer’s disease affects millions of human beings worldwide. The disease progression is characterized by the formation of plaques and neurofibrillary tangles in the brain, which are based on aggregation processes of the Aβ peptide and tau protein. Today there is no cure and even no <i>in vitro</i> assay available for the identification of drug candidates, which provides direct information concerning the protein secondary structure label-free. Therefore, we developed an attenuated total reflection Fourier transform infrared spectroscopy (ATR–FTIR) sensor, which uses surface bound antibodies to immobilize a desired target protein. The secondary structure of the protein can be evaluated based on the secondary structure sensitive frequency of the amide I band. Direct information about the effect of a drug candidate on the secondary structure distribution of the total target protein fraction within the respective body fluid can be detected by a frequency shift of the amide I band. Thereby, the extent of the amide I shift is indicative for the compound efficiency. The functionality of this approach was demonstrated by the quantification of the effect of the drug candidate methylene blue on the pathogenic misfolded tau protein as extracted from cerebrospinal fluid (CSF). Methylene blue induces a shift from pathogenic folded β-sheet dominated to the healthy monomeric state. A similar effect was observed for congo red on pathogenic Aβ isoforms from CSF. In addition, the effect of berberine on synthetic Aβ_1–42 is studied. Berberine seems to decelerate the aggregation process of synthetic Aβ_1–42 peptides