Calorimetric studies of binary and ternary molecular interactions between transthyretin, Aβ peptides and small-molecule chaperones towards an alternative strategy for Alzheimer's disease drug discovery

 The  plasma  protein  transthyretin  (TTR)  modulates  Abeta  (Aβ)  peptides  deposition, processing and toxicity. We have shown that this physiological effect is enhanced in mice by treatment with compounds such as iododiflunisal (IDIF) which is a good TTR ligand  and tetrameric stabilizer. Isothermal Titration Calorimetry (ITC) is a powerful technique for the    quantitative      analysis      of   protein-protein      interactions.      Here,     we    describe     the  thermodynamics of the formation of binary and ternary complexes between recombinant  human TTR, Ab(1-42) peptides and small-molecule TTR tetramer stabilizers using ITC. It was found that a TTR/Aβ(1-42) (1:1) complex with a dissociation constant of Kd = 0.94 μM is formed. Also, in the presence of IDIF this constant improves up to a Kd = 0.32 μM, indicating the presence of a ternary complex TTR/IDIF/Ab(1-42). However, when IDIF was substituted by either the registered drug diflunisal or the orphan drug Tafamidis, an analogous chaperoning effect could not be observed. Similar phenomena could be recorded with the shorter and easy handling peptide Aβ(12-28). By using this small peptide we propose that a simple assay system could be devised for the search of other small molecule chaperones that behave like IDIF that may become potential candidate drugs for AD.