Photodynamic studies reveal rapid formation and appreciable turnover of tau inclusions.

Accumulation of the tau protein in fibrillar intracellular aggregates is a defining feature of multiple neurodegenerative diseases collectively referred to as tauopathies. Despite intensive study of tau, there is limited information on the formation and clearance dynamics of tau inclusions. Using rAAV vectors to mediate expression of Dendra2-tagged human wild-type, P301L and pro-aggregant P301L/S320F tau proteins, with and without the addition of exogenous tau fibrillar seeds, we evaluated tau inclusion dynamics in organotypic brain slice culture (BSC) models using long-term optical pulse labeling methodology. Our studies reveal that tau inclusions typically form in 12-96 h in tauopathy BSC models. Unexpectedly, we demonstrate appreciable turnover of tau within inclusions with an average half-life of ~ 1 week when inclusions are newly formed. When BSCs with inclusions are aged in culture for extended periods, tau inclusions continue to turnover, but their half-lives increase to ~ 2 weeks and ~ 3 weeks after 1 and 2 months in culture, respectively. Individual tau inclusions can be long-lived structures that can persist for months in these BSC models and for even longer in the human brain. However, our data indicate that tau inclusions, are not 'tombstones', but dynamic structures with appreciable turnover. Understanding the cellular processes mediating this inclusion turnover may lead to new therapeutic strategies that could reverse pathological tau inclusion formation.