Phosphorylated tau fluid biomarker sites recognize earlier neurofibrillary tangle maturity levels in the postmortem Alzheimer’s disease brain

Alzheimer’s disease (AD) biomarkers have become increasingly more reliable in predicting AD pathology. While phosphorylated tau fluid biomarkers have been studied for over 20 years, there is a lack of deep characterization of these sites in the postmortem brain. Neurofibrillary tangle-bearing neurons, one of the major neuropathologic hallmarks of AD, undergo morphologic changes that mature along a continuum as hyperphosphorylated tau aggregates. To facilitate interpretation of phosphorylated tau sites as an early fluid biomarker, our goal was to characterize which neurofibrillary tangle maturity levels (pretangle, intermediary 1, mature tangle, intermediary 2, and ghost tangle) they recognize. We queried the Florida Autopsied Multi-Ethnic (FLAME) cohort for cases from Braak stages I-VI. We excluded non-AD pathologies and tauopathies. A total of 24 cases, 2 males and 2 females for each Braak stage, were selected. We performed immunohistochemistry on the posterior hippocampus using antibodies directed towards phospho (p) threonine (T) 181, pT205, pT217, and pT231. Slides were digitized to enable quantification of tau burden. To examine differences in regional vulnerability between CA1 and subiculum, we developed a semi-quantitative system to rank the frequency of each neurofibrillary tangle maturity level. We identified all neurofibrillary tangle maturity levels at least once for each phosphorylated tau site. Primarily earlier neurofibrillary tangle maturity levels (pretangle, intermediary 1, mature tangle) were recognized for all phosphorylated tau sites. There was an increase in tau burden in the subiculum compared to CA1; however, this was attenuated compared to thioflavin-S positive tangle counts. On a global scale, tau burden generally increased with each Braak stage. These results provide neurobiologic evidence that these phosphorylated tau fluid biomarker sites are present during earlier neurofibrillary tangle maturity levels. This may help explain why these phosphorylated tau biomarker sites are observed before symptom onset in fluids.