Shikonin impedes phase separation and aggregation of tau and protects SH-SY5Y cells from the toxic effects of tau oligomers

Tauopathies such as Alzheimer's and Parkinson's diseases involve the abnormal deposition of tau aggregates in the brain and neuronal tissues. We report that a natural naphthoquinone, shikonin, impeded the oligomerization and fibrillization of tau. The compound strongly inhibited heparin, arachidonic acid, and RNA-induced tau aggregation. Atomic force microscopy, dynamic light scattering, SDS-PAGE, and dot blot assays revealed that shikonin diminished tau oligomerization and decreased the mean size of tau oligomers. Transmission electron microscopy and atomic force microscopy analysis further showed that shikonin could suppress tau fibrillization and shorten the tau filaments. Shikonin inhibited tau droplet formation. The compound significantly reduced the aggregation rate of a tryptophan mutant (Y310W-tau) of tau. In addition, shikonin disaggregated preformed tau filaments with a half-maximal disaggregation concentration (DC50) of 6.3 ± 0.4 μM. Pre-treatment of neuroblastoma cells (SH-SY5Y) with shikonin protected the cells from the toxicity induced by tau oligomers and increased their viability. The findings imply that shikonin inhibited several steps in the tau aggregation pathways, especially the early stages, such as liquid-liquid phase separation. Therefore, shikonin is an attractive candidate for developing a therapy against tauopathy.