Thionine modulates tau phosphorylation in an Alzheimer's disease cell culture model.

Background/aim: Tau protein, which is crucial for sustaining the cytoskeletal network by assisting microtubule construction, contributes significantly to the pathophysiology of Alzheimer's disease (AD). The hyperphosphorylation of tau causes it to detach from microtubules (MTs), leading to the formation of neurofibrillary tangles (NFTs) in neurons, which ultimately results in cell death. Thionine (TH), a cationic phenothiazine-structured compound, has been the topic of extensive research due to its interesting physicochemical properties. It is a common biological dye, especially useful in histology due to its strong affinity for biological membranes. Furthermore, TH serves as a photosensitizer in phototherapy. It has a phenothiazine pharmacophore, which makes it selective against microbial and tumor cells. Our prior studies demonstrated that TH inhibits human plasma butyrylcholinesterase (BChE) by acting as a nonlinear inhibitor and also affects amyloid precursor protein (APP) metabolism in PS70 cells. In the current research, we investigated whether TH modulates the phosphorylation of tau in N2a/APPSwe cells. Materials and methods: Using flow cytometry, we identified the dose range and treatment time of TH that did not affect the viability of N2a/APPSwe cells. The western blot method was used to investigate the effects of TH on total tau and four key tau phosphorylation sites. Results: The results indicated that TH reduces tau phosphorylation at residues Ser202/Thr205, Ser396, Ser396/Ser404, and Thr181, which contribute to NFT formation. Conclusion: When all these findings are evaluated together, TH may have a therapeutic potential against AD.