Osmolytes suppress liquid‒liquid phase separation and the aggregation of Tau.

Tau liquid-liquid phase separation (LLPS) is increasingly recognized as an early event that promotes pathological aggregation in Alzheimer's disease and other tauopathies. Osmolytes are natural chemical chaperones that stabilize protein conformations, yet their effects on Tau phase behavior remain poorly understood. Here, we show that sucrose, trehalose, glucose, maltose, and betaine suppress Tau LLPS and dissolve preformed Tau droplets, shifting the equilibrium toward soluble monomers in vitro. Among the five osmolytes, trehalose and glucose exhibited the strongest inhibitory effects on Tau LLPS. They also suppressed the liquid-to-solid phase transition of Tau droplets, as evidenced by reduced Thioflavin T (ThT) fluorescence, inhibition of fibril formation observed by transmission electron microscopy (TEM), and improved condensate dynamics in fluorescence recovery after photobleaching (FRAP) assays performed in okadaic acid-treated cells. These results indicate that osmolytes interfere with multiple stages of Tau phase transition-from initial condensate formation to pathological maturation and fibrillization. Our findings establish osmolytes as effective modulators of Tau phase behavior and provide mechanistic insights into the physicochemical regulation of Tau phase transitions.