Tau Folding and Assembly Triggered by Electroreduction of Its Protonated Amino Acids

Abstract

The physiological function of the microtubule-binding protein tau is regulated by charge-neutralizing phosphorylation, while hyperphosphorylation drives its irreversible assembly into amyloid fibrils associated with Alzheimer’s disease and other tauopathies. In vitro, tau is remarkably stable and typically requires the use of polyanions such as heparin or RNA to induce fibrillation. Here, we describe an alternative method that uses electroreduction as a surrogate for charge-neutralization by hyperphosphorylation. To follow the kinetics of tau conformational change in real time, we combined our electrochemical method with in situcircular dichroism and UV-absorption spectroscopies. These spectroelectrochemical techniques demonstrate that electroreduction of positively charged amino acids in freely diffusing tau triggers its assembly, with formation of β-rich fibrillar structures observed at reductive potentials sufficient to neutralize lysine residues. Analyses suggest that the negatively charged electrode might provide a templating function analogous to that seen with heparin and other polyanions (heterogeneous coacervation) but without those polymers’ permanent binding to tau fibrils. Taking advantage of our method to rapidly form electro-assemblies in as quickly as 15 min, we demonstrate its applicability for fast screening of small molecules such as epigallocatechin 3-gallate (EGCG) for tau fibril disassembly. This study demonstrates the unique advantage of our spectroelectrochemical method as a rapid and facile approach to induce cofactor free tau assembly, with the ability to observe and kinetically resolve conformational transitions as a function of time, providing a platform for rapid screening of tau-aggregation modulators.