Stage‐Dependent Signatures of Tau K18 Fibril Growth and Maturation Revealed by Tip‐Enhanced Raman Spectroscopy

Abstract

Tau protein aggregation is one of the key pathological processes driving the global burden of dementia, with Alzheimer's disease being its leading cause. Research on the aggregation pathways and fibrillar structures could provide key insights into the pathogenesis and potential therapeutic approaches for dementia‐related disorders. Despite extensive studies of tau aggregation, the nanoscale chemical and structural heterogeneity of tau fibrils during their maturation remains poorly understood. In particular, early aggregation intermediates, fibril‐to‐fibril variability, secondary nucleation processes, and differences at the molecular level between truncated and full‐length tau fibrils have not been directly resolved due to limitations in spatial resolution and chemical sensitivity of conventional analytical techniques. In this study, we focus on the stage‐dependent fibrillization of the tau K18 fragment using an extremely sensitive nanospectroscopic technique, tip‐enhanced Raman spectroscopy (TERS) that provides chemical information with nanometer spatial resolution. Single‐point TER spectra provide insights into the structural transitions related to tau K18 maturation at the level of individual fibrils. Our findings indicate that TERS directly differentiates K18 from full‐length tau fibrils at early fibrillization stages, revealing the impact of the fuzzy coat on aggregation pathways. Additionally, we show that TER hyperspectral imaging enables direct visualization of the ongoing secondary nucleation process. Finally, our findings reveal the stage‐specific roles of amino‐acid side chains, indicating that histidine is involved in the initial aggregation and secondary nucleation processes, whereas phenylalanine and tyrosine contribute to the stabilization of β‐sheet architecture.