Travels with tau prions

Tau was originally identified as a microtubule associated protein, and subsequently recognized to constitute the fibrillar assemblies found in Alzheimer disease and related neurodegenerative tauopathies. Point mutations in the microtubule associated protein tau (MAPT) gene cause dominantly inherited tauopathies, and most predispose it to aggregate. This indicates tau aggregation underlies pathogenesis of tauopathies. Our work has suggested that tau functions as a prion, forming unique intracellular pathological assemblies that subsequently move to other cells, inducing further aggregation that underlies disease progression. Remarkably, in simple cells tau forms stably propagating aggregates of distinct conformation, termed strains. Each strain induces a unique and, in some cases, transmissible, neuropathological phenotype upon inoculation into a mouse model. After binding heparan sulfate proteoglycans on the plasma membrane, tau assemblies enter cells via macropinocytosis. From within a vesicle, if not trafficked to the endolysosomal system, tau subsequently enters the cytoplasm, where it becomes a template for its own replication, apparently after processing by valosin containing protein. The smallest seed unit is a stable monomer, which suggests that initial folding events in tau presage subsequent pathological aggregation. The study of tau prions has raised important questions about basic cell biological processes that underlie their replication and propagation, with implications for therapy of tauopathies.