CBD and PSP cell-passaged Tau Seeds Generate Heterogeneous Fibrils with A Subpopulation Adopting Disease Folds

Abstract

The recent discovery by cryo-electron microscopy (cryo-EM) that the neuropathological hallmarks of different tauopathies, including Alzheimers disease, corticobasal degeneration (CBD), and progressive supranuclear palsy (PSP), are caused by unique misfolded conformations of the protein tau is among the most profound developments in neurodegenerative disease research. To capitalize on these discoveries for therapeutic development, one must achieve in vitro replication of tau fibrils that adopt the representative tauopathy disease folds, which represents a grand challenge for the field. An widely used approach is seeded propagation using pathological tau fibrils derived from post-mortem patients in biosensor cells that expresses a fragment of the tau protein, known as K18 or Tau4RD, containing mainly the microtubule-binding repeat domain of tau as the substrate. The new insight from cryo-EM raised the question whether the Tau4RD fragment is capable of adopting the specific tau folds found in CBD and PSP patient fibrils, and whether cell-passaged and amplified tau fibrils can be used as seeds to achieve cell-free assembly of recombinant 4R tau into fibrils without the addition of any cofactors. Using Double Electron Electron Resonance (DEER) spectroscopy, we discovered that cell-passaged pathological seeds generate heterogeneous fibrils that are distinct between the CBD and PSP lysate-seeded fibrils. These fibrils are also distinct from heparin-induced tau fibril populations. Moreover, the lysate-seeded fibrils contain a characteristic sub-population that resembles the disease fold corresponding to the respective starting patient sample. These findings indicate that templated propagation using CBD and PSP patient-derived fibrils is possible with a tau fragment that does not contain the entire pathological fibril core, and that additional mechanisms must be tuned to converge to a homogeneous fibril population.