Tau4RD fibril polymorphism is imprinted during early aggregation

Microtubule-associated protein tau forms characteristic fibrillar species in many neurodegenerative diseases. Neurofibrillary tangles, tau deposits observed in Alzheimer's disease (AD), contain a mixture of amyloid-type polymorphic fibrils called paired helical filaments (PHFs) and straight filaments. The formation of heterogenous fibril populations is observed in other diseases and when tau aggregation is induced in vitro with polyanionic species. This suggests that tau's structural transition from a conformational ensemble to various amyloid morphologies is a controlled and, therefore, controllable process. Despite many years of work toward describing aggregation intermediates that could address open questions such as whether fibril polymorphism is imprinted at the start of aggregation or arises due to conformational conversions, our understanding of amyloid structure remains predominantly based on observations of mature fibrils. It is unclear whether these processes are mutually exclusive and to what extent we can bias intermediate conformations toward less toxic states. Here to address the challenge of studying aggregation intermediates and tau's structural conversion, we apply pulsed hydrogen-deuterium exchange with mass spectrometry (pulsed HDX-MS), which revealed differences in the subpopulations formed by tau4RD (a truncated tau construct) within seconds of initiating aggregation with polyphosphate and within hours of heparin-induction. This work begins to address the gap in knowledge regarding whether amyloid polymorphism is directly imprinted during nucleation or results from structural rearrangement during later stages of aggregation.