Context-dependent toxicity of human Tau isoforms in a Drosophila tauopathy model

Tauopathies are characterised by progressive deterioration of brain regions due to abnormal accumulation of the microtubule-associated protein tau (MAPT). Alternative splicing of MAPT pre-mRNA results in six tau isoforms, which are classified into two groups depending on the number of microtubule-binding domain repeats (3R vs 4R). Although many tauopathies are 3R or 4R-specific, the relative contributions of individual isoforms to neurotoxicity remain incompletely understood. To systematically characterise differences in tau isoform toxicity, we created a novel set of Drosophila lines expressing equivalent amounts of the six human tau isoforms (hTau) at levels sufficient to induce visible phenotypes. Using a variety of assays including survival, negative geotaxis and tissue-level or cell-type-specific degeneration, we found that hTau isoform toxicity is not uniform across different biological contexts. Despite generally higher toxicity of 4R isoforms compared to 3R, the effects of individual hTau isoforms varied with the temporal window of expression, tissue type, and neuronal identity. Restricting hTau expression to small homogeneous neuronal populations enabled detailed analysis of isoform-specific degeneration. Neurons previously observed to be vulnerable or resilient to hTau toxicity exhibited differences in the onset and progression of degeneration, suggesting that resilience may be an early and transitory state, with most or all neurons eventually succumbing to tau toxicity over time. Notably, these differences in toxicity were not readily explained by variations in hTau abundance and phosphorylation. Together, our findings demonstrate that tau toxicity is highly context-dependent, clearly isoform-specific, and shaped by interactions between tau and its cellular environment.