Targeted autophagic clearance of Tau protects against Alzheimer's disease through amelioration of Tau-mediated lysosomal stress

<b>Background:</b> Lysosomal dysfunction could be an underlying cause of Alzheimer's disease, with Tau oligomer being an important inducer or amplifier of lysosomal stress associated with the disease. Tau oligomer is a well-known substrate of autophagy, and selective degradation of Tau with Tau-specific autophagy degrader might be feasible. <b>Methods:</b> Tau-specific autophagic degraders were synthesized by combining leucomethylene blue, linkers and a lysosomal degradation tag (Autac). Tau clearance and changes of Tau-mediated lysosomal stress by these degraders were studied <i>in vitro</i>. <i>In vivo</i> effects of a Tau-specific degrader were investigated employing a combined Tau/Aβ mutant mouse model characterized by an accelerated onset of neurological deficits. Human relevance was investigated using induced pluripotent stem cell (iPSC)-derived neuronal cells from an Alzheimer's disease patient. <b>Results</b>: Among Tau-specific Autac degraders, TauAutac-3 (TA-3) efficiently degraded Tau oligomer and monomer, an effect inhibited by bafilomycin A1, suggesting lysosomal Tau degradation. TA-3 treatment induced LC3, K63, OPTN or NDP52 puncta, which was partially colocalized with Tau oligomer. Signs of lysosomal stress, such as galectin-3 puncta, pHluorin fluorescence, altered lysosomal pH and CHMP2B recruitment, induced by Tau expression were reversed by TA-3. Autophagy impairment by Tau expression <i>in vitro</i>, likely due to lysosomal stress, was also reversed by TA-3. <i>In vivo</i>, TA-3 administration markedly reduced the accumulation of both Tau and Aβ in 6xTg mice, which was associated with amelioration of Tau-mediated lysosomal stress and autophagy impairment. Neuroinflammation characterized by increased numbers of GFAP⁺ glial cells and Iba1⁺ microglial cells, was also reduced following TA-3 administration. TA-3 remarkably improved neurologic deficits in 6xTg mice, such as impaired memory and reduced exploratory behavior. TA-3 reduced Tau and phospho-Tau accumulation in iPSC-derived neuronal cells from an Alzheimer's disease patient. <b>Conclusion:</b> These results suggest that Tau-specific autophagic (Autac) degraders could serve as novel therapeutic agents for Alzheimer's disease through reduction of Tau-mediated lysosomal stress.