Binding Sites of a PET Ligand in Tau Fibrils with the Alzheimer’s Disease Fold from ¹⁹F and ¹³C Solid-State NMR

Aggregation of the tau protein into cross-β amyloid fibrils is a hallmark of Alzheimer's disease (AD) and many other neurodegenerative disorders. Developing small molecules that bind these tau fibrils is important for the diagnosis and treatment of tauopathies. Here, we report the binding sites of a positron emission tomography (PET) ligand, PI-2620, to a recombinant tau construct that adopts the C-shaped AD fold. Using solid-state NMR ¹³C-¹⁹F rotational-echo double-resonance (REDOR) experiments, we measured the proximity of protein residues to the fluorine atom of the ligand. These data indicate that PI-2620 binds at two main locations in the concave interior of the C-shaped structure. Molecular docking simulations constrained by these REDOR data identified five binding poses at these two locations. In addition, 2D ¹³C-¹³C correlation NMR spectra indicate that PI-2620 decreased the intensities of residues at the protofilament interfaces, indicating that the ligand disordered the filament packing. Quantitative analysis of the ¹⁹F NMR spectra indicates that PI-2620 binds these AD-fold tau fibrils with a stoichiometry of ∼20 mol %, in which 10 mol % are immobilized and the rest are mobile. These results provide experimental constraints to the interaction of this second-generation PET tracer with tau fibrils adopting the AD fold and should be useful for the development of future imaging agents with improved stoichiometry and specificity for AD tau.