Nadia El Mammeri
MIT, Department of Chemistry
Invited by Antoine Loquet, IECB
Understanding Tau’s Chemical code: microtubules, lipid membranes, anionic co-factors, and phosphorylation
The protein tau associates and stabilizes microtubules (MT) to maintain neuronal health. Post-translational modifications that lead to tau aggregation in brains with neurodegenerative diseases. To understand the mechanism of the early misfolding events in AD, we are using magic-angle-spinning (MAS) solid-state NMR spectroscopy and cryo-electron microscopy to investigate the structure and dynamics of tau bound to microtubules , lipid bilayers , as well as amyloid fibrils formed with anionic cofactors  or co-factor-free pseudophosphorylation mutations.
When complexed with MTs, tau exhibits well-resolved dipolar 2D correlation spectra that can be assigned to a 45-residue segment spanning the R’ and the C-terminal half of the R4 repeat. This result changes the prevailing model that all 5 MT-binding regions are immobilized. Moreover, the NMR assigned immobilized residues of R’ dock well into cryo-EM densities, leading to a revised paradigm of MT-binding.
When complexed to lipid membranes, tau converted from a disordered random coil to an ordered β-sheet. Small unilamellar vesicles (SUVs) and large unilamellar vesicles (LUVs) cause different equilibrium conformations. SUV-bound tau developed amyloid fibrils while LUV-bound tau does not but has a different β-sheet assembly that contains the R’ repeat. Removing cholesterol from the SUV abolished fibril formation. Thus, high membrane curvature and cholesterol are both required for fibril formation.
In the presence of heparin, P2R tau assembles into amyloid fibrils. We assigned the rigid β-sheet core to the R2 and R3 repeats. Unexpectedly, the folds differ between 24˚C and 12˚C: R2 forms a β-arch at 24˚C but a continuous β-strand at 12˚C, which dimerizes with another protofilament. This indicates that R2 has enhanced conformational plasticity than R3, similarly to in brain-extracted 4R tau aggregates.
Pseudo-phosphorylated mutants of tau can self-assemble in the absence of any co-factors. Using SSNMR and cryo-EM, we found that specific disease-related phosphorylation patterns induce specific amyloid structures. These results have implications for the functional and pathological state of tau in diseases. They also demonstrate the power of solid-state NMR to reveal both the structure and dynamics of complex biomolecular assemblies in neurodegenerative diseases.