Heparin and fluoride drive distinct tau (4R/1 N) aggregation pathways to fibrils and granular oligomers, as revealed by Raman spectroscopy.

The aggregation of the tau protein into pathological assemblies is a pivotal event in Alzheimer's disease and related tauopathies. Understanding how different cofactors influence the Tau aggregation pathway is crucial for elucidating disease mechanisms. This study directly compares the aggregation pathways of human Tau (hTau441, 4 R/1 N) induced by heparin with those induced by aluminum/sodium fluoride (AlF3/NaF). We employed time-resolved Raman spectroscopy, a technique uniquely suited for label-free, real-time secondary structure analysis of intrinsically disordered proteins in solution, to monitor structural transitions. Our results reveal two distinct trajectories: Heparin drives a classical pathway of progressive β-sheet enrichment, culminating in mature fibrils. In stark contrast, fluoride conditions suppress β-sheet formation and stabilize granular, nonfibrillar oligomers. These findings suggest that the neurotoxicity associated with fluoride may not arise from accelerating fibril formation but from diverting tau into an off-pathway oligomeric state. This work establishes Raman spectroscopy as a powerful tool for mechanistic studies of protein aggregation and identifies fluoride as a modulator of Tau misfolding with significant pathological implications.