Structures and Dynamics of Tau Assemblies from Solid-State NMR

Conspectus Aggregation of the microtubule-associated protein tau into β-sheet fibrils is a hallmark of many neurodegenerative diseases. Understanding the molecular mechanism of tau aggregation requires elucidating the structure and dynamics of fibrillar tau as the end product of aggregation, membrane-bound tau involved in nucleation and intercellular transmission of the aggregates, and microtubule-bound tau as the physiological state of the protein. Using solid-state NMR spectroscopy, we have obtained detailed information about these tau assemblies. Full-length tau fibrils formed in the presence of heparin adopt homogeneous structures that depend on the number of microtubule-binding repeats and that differ from ex vivo tau fibril structures. Phospho-mimetic mutations allowed heparin-free fibrillization, with mutation of the PHF1 epitope yielding a three-layered rigid core that mimics the fold of four-repeat (4R) tau in tauopathies. This three-layered structure is also adopted by other post-translational modification (PTM) mutants, indicating that tau’s PTM code contains redundancy and dominance. In all tau fibrils, the rigid core represents only a fifth of the protein. Truncation of the disordered regions accelerated fibrillization, but the fibril fold is sensitive to temperature, pH, and ionic condition of the environment. In a short tau construct, acidic pH stabilized a flat-ribbon dimer structure, whereas neutral pH stabilized the twisted C-shaped fold of Alzheimer’s disease (AD) tau. The reproducible in vitro reconstruction of AD-fold tau opens the path for studying small-molecule binding to AD tau for developing diagnostics and therapeutics. However, seeding experiments indicate that full-length tau adopting the AD fold lacks seeding potency; therefore, the rigid core structure alone does not ensure prion-like propagation of pathological tau. Instead, specific fuzzy coat dynamics may be required for serial amplification of tau aggregates. An essential property to replicate is likely the fuzzy coat dynamics. Solid-state NMR data show that tau dynamics depends on PTMs, and the most dynamic segments in three-layered tau lie in the proline-rich region, suggesting that separation of this region from the rigid core may be important for prion-like propagation of tau aggregates. Cholesterol-rich high-curvature lipid membranes alone induce tau fibrils and allow their insertion into the membrane, supporting the model that lipid membranes are involved in the nucleation and transmission of tau aggregates. Finally, solid-state NMR data show that the highest-affinity microtubule-binding domain of tau is the R′ segment that is N-terminal to the PHF1 epitope, suggesting that stabilizing this segment may inhibit aggregation. These solid-state NMR data of multiple types of tau assemblies have provided numerous insights into the structures and dynamics of tau in pathology and physiology, advancing the reconstruction of the aggregation mechanism of tau in neurodegeneration.