Concentration-dependent effects of proteasomal inhibition on tau processing in a cellular model of tauopathy

Tauopathies are characterized by accumulation of filamentous tau aggregates. These aggregates can be recapitulated in transfectant M1C overproducing wild-type human brain tau 4R0N via the tetracycline off (TetOff) inducible expression mechanism. To determine the contribution of proteasomes to tau degradation and aggregation, we exposed M1C cells to epoxomicin (Epx; 2-50 nM) or MG132 (0.5 μM) on the 3rd or 4th day of a 5-day TetOff induction and demonstrated a reduction of proteasomal activity. Cultures treated with 2 nM Exp showed accumulation of full-length tau without affecting ubiquitin and β-catenin immunoblotting profiles. In contrast, cells treated with 10, 50 nM Epx or MG132 displayed changes in ubiquitin or β-catenin immunoblotting profiles and extensive tau degradation/truncation. The increase of tau degradation/truncation was accompanied with accumulation of oligomers and sarkosyl-insoluble aggregates of tau, augmented thioflavin-binding and activation of caspases and calpains. Truncated, oligomeric and sarkosyl-insoluble tau derivatives appeared with caspase-specific cleavage and their production was diminished when pretreated with a pan-caspase inhibitor. The results demonstrate (i) a dose-dependent, opposite effect of proteasome inhibition on tau processing, (ii) the participation of proteasome-dependent, ubiquitination-independent mechanisms in tau degradation and aggregation, and (iii) the promotion of tau aggregation by caspase-mediated tau degradation/truncation.