Contrasting Effects of Ferric and Ferrous Ions on Oligomerization and Droplet Formation of Tau: Implications in Tauopathies and Neurodegeneration.

The dysregulation of metal homeostasis is reported to enhance the aggregation of tau, a key neuronal microtubule-associated protein. Herein, we found that ferric (Fe) ions enhanced tau aggregation. Fe and Al induced tau aggregation while several trivalent metal ions such as Cr, La, and V had no discernable effect on tau aggregation. Fe reduced the critical concentration of tau required for the liquid-liquid phase separation (LLPS); however, Cr, La, and V did not affect tau droplet formation. Dynamic light scattering, atomic force microscopic, and transmission electron microscopic analysis suggested that Fe significantly increased the formation of tau oligomers and fibrils. In contrast, Fe neither enhanced tau droplet formation nor increased the heparin-induced aggregation of tau. Using a tryptophan mutant (Y310W-tau) of tau, Fe was found to bind to tau with four times higher affinity than Fe. Acrylamide quenching of the tryptophan fluorescence of Y310W-tau, 1-anilino-8-naphthalene sulfonate (ANS) fluorescence experiment, and far-UV circular dichroism analysis indicated that Fe decreased the solvent exposure of the tryptophan residue, perturbed the hydrophobic surface arrangement, and disrupted the secondary structure of tau, respectively. The increase in the β-sheet content and a subsequent decrease in the disordered content of tau due to the binding of Fe may favor tau aggregation. Fe may enhance and stabilize the non-covalent interactions between disordered domains of tau molecules leading to tau aggregation. The data highlighted the relationship between the dysregulation of ferric ions and neurodegenerative disorders.