INTRODUCTION: Central to tau pathology and tangle formation in Alzheimer’s disease (AD) is the microtubule binding region (MTBR) of tau. MTBR fragments containing the first repeat (R1) harbour multiple phosphorylation sites that regulate tau-microtubule interactions and aggregation propensity. Among the three phospho-epitopes within the R1 domain (serine-258, serine-262, and threonine-263) serine-262 (p-tau262) stands out as critical epitope in both physiological brain development and pathological tangle formation.

AIM(S): To investigate the clinical utility of p-tau262 as a marker of tau pathology and tangle load in AD.

METHOD(S): We used mass spectrometry-based spatial proteomic profiling of laser-capture microdissected tangles from frozen hippocampal sections of AD patients. Tryptic peptides are analysed by LC-MS/MS, assessing peptide abundance changes in tangles versus adjacent tissue (Figure 1A). We further performed immunohistochemical staining of human brain to explore the colocalization of p-tau217 and p-tau262 across Braak stages. Finally, we developed a Simoa assay for p-tau262 based on the antibody used for the brain staining and two independent cohorts (n=542).

RESULTS: The untargeted proteomic analysis shows that certain tau posttranslational modifications are highly enriched in AT8-positive tangles compared to surrounding AT8-negative tissue, among these, phosphorylation at serine-262 was the main one present in the MTBR domain (Figure 1B). Moreover, cortical immunostaining revealed extensive co-localization between p-tau217 and p-tau262, with p-tau262 having a more granular and nuclear pattern compared to p-tau217 (Figure 2). In cohort-1, CSF p-tau262 showed a moderate correlation with CSF p-tau217 (r=0.76, p<0.0001) and significant increases in AD vs controls. In the AD group CSF p-tau262 decreased over time in those with rapid clinical progression, compared to stable or slow cognitive decline (Figure 3). In cohort-2, plasma p-tau262 concentrations were significantly higher in AD patients compared to non-AD individuals (p < 0.001).

CONCLUSIONS: Our results underscore the importance of phosphorylation at serine-262 in tau tangle formation. We also present the first highly sensitive immunoassay for p-tau262 in CSF and plasma. The longitudinal CSF data suggest that declining p-tau262 may track rapid clinical deterioration, consistent with dynamic changes in tangle-associated tau pathology. Together, these findings position CSF and plasma p-tau262 as a biologically grounded, translatable biomarker of tau pathology with potential utility for staging, and disease monitoring in AD.

FINANCIAL SUPPORT: FG-O is funded by Hjärnfonden (#PD2025-0459), Alzheimerfonden (#AF-1032222) and Demensfonden (#DF-1031511)