The dual fates of exogenous tau seeds: Lysosomal clearance versus cytoplasmic amplification

Tau assembly movement from the extracellular to intracellular space may underlie transcellular propagation of neurodegenerative tauopathies. This begins with tau binding to cell surface heparan sulfate proteoglycans, which triggers macropinocytosis. Pathological tau assemblies are proposed then to exit the vesicular compartment as seeds for replication in the cytoplasm. Tau uptake is highly efficient, but only t o 10% of cells that endocytose aggregates exhibit seeding. Consequently, we studied fluorescently tagged full-length (FL) tau fibrils added to native U2OS cells or biosensor cells expressing FL tau or repeat domain. FL tau fibrils bound tubulin. Seeds triggered its aggregation in multiple locations simultaneously in the cytoplasm, generally independent of visible exogenous aggregates. Most exogenous tau trafficked to the lysosome, but fluorescence imaging revealed a small percentage that steadily accumulated in the cytosol. Intracellular expression of Gal3-mRuby, which binds intravesicular galactosides and forms puncta upon vesicle rupture, revealed no evidence of vesicle damage following tau exposure, and most seeded cells had no evidence of endolysosome rupture. However, live-cell imaging indicated that cells with pre-existing Gal3positive puncta were seeded at a slightly higher rate than the general population, suggesting a potential predisposing role for vesicle instability. Clearance of tau seeds occurred rapidly in both vesicular and cytosolic fractions. The lysosome/autophagy inhibitor bafilomycin inhibited vesicular clearance, whereas the proteasome inhibitor MG132 inhibited cytosolic clearance. Tau seeds that enter the cell thus have at least two fates: lysosomal clearance that degrades most tau, and entry into the cytosol, where seeds amplify, and are cleared by the proteasome.

Automated summary

Movement of Tau assemblies from extracellular to intracellular space may play a role in the transcellular propagation of neurodegenerative tauopathies. Tau binds to cell surface heparan sulfate proteoglycans, triggering macropinocytosis. Pathological tau assemblies may exit vesicles and replicate in the cytoplasm as seeds. Most exogenous tau is trafficked to the lysosome, but a small fraction accumulates in the cytosol. Tau seeds have two fates: lysosomal clearance or entry into the cytosol for amplification and clearance by the proteasome.