The study investigates the mechanisms by which tau seeds spread within cells, particularly in astrocytes and neurons. It finds that endocytosis of tau seeds leads to their accumulation in lysosomes, causing lysosomal swelling, deacidification, and recruitment of ESCRT proteins but not Galectin-3. This results in nanoscale damage to the lysosomal membrane, leading to the nucleation of cytosolic tau aggregates primarily at the lysosome surface. Live cell imaging and STORM superresolution microscopy confirm that tau aggregates form on the surface of tau-burdened lysosomes, suggesting that tau seeds escape from lysosomes through this nanoscale damage rather than complete rupture. The findings provide new insights into the prion-like spread of tau inclusions in Alzheimer's disease and other tauopathies.The study investigates the mechanisms by which tau seeds spread within cells, particularly in astrocytes and neurons. It finds that endocytosis of tau seeds leads to their accumulation in lysosomes, causing lysosomal swelling, deacidification, and recruitment of ESCRT proteins but not Galectin-3. This results in nanoscale damage to the lysosomal membrane, leading to the nucleation of cytosolic tau aggregates primarily at the lysosome surface. Live cell imaging and STORM superresolution microscopy confirm that tau aggregates form on the surface of tau-burdened lysosomes, suggesting that tau seeds escape from lysosomes through this nanoscale damage rather than complete rupture. The findings provide new insights into the prion-like spread of tau inclusions in Alzheimer's disease and other tauopathies.