Unraveling the Structure and Dynamics of Ac-PHF6-NH₂ Tau Segment Oligomers

The aggregation of the proteins tau and amyloid-β is a salient feature of Alzheimer’s disease, the most common form of neurodegenerative disorders. Upon aggregation, proteins transition from their soluble, monomeric, and functional state into insoluble, fibrillar deposits through a complex process involving a variety of intermediate species of different morphologies, including monomers, toxic oligomers, and insoluble fibrils. To control and direct peptide aggregation, a complete characterization of all species present and an understanding of the molecular processes along the aggregation pathway are essential. However, this is extremely challenging due to the transient nature of oligomers and the complexity of the reaction networks. Therefore, we have employed a combined approach that allows us to probe the structure and kinetics of oligomeric species, following them over time as they form fibrillar structures. Targeting the tau protein peptide segment Ac-PHF6-NH₂, which is crucial for the aggregation of the full protein, soft nano-electrospray ionization combined with ion mobility mass spectrometry has been employed to study the kinetics of heparin-induced intact oligomer formation. The oligomers are identified and characterized using high-resolution ion mobility mass spectrometry, demonstrating that the addition of heparin does not alter the structure of the oligomeric species. The kinetics of fibril formation is monitored through a Thioflavin T fluorescence assay. Global fitting of the kinetic data indicates that secondary nucleation plays a key role in the aggregation of the Ac-PHF6-NH₂ tau segment, while the primary nucleation rate is greatly accelerated by heparin.