Correlations between cortical and subcortical tau pathology

Recent studies indicate that tau pathology in Alzheimer's disease (AD) does not initially manifest in the cerebral cortex but in selected subcortical nuclei, in particular the locus ceruleus (LC). In this study we correlate both olfactory and brainstem tau pathology with neuritic Braak stages.

We examined 239 unselected autopsy cases (57.3% female, 42.7% male; aged 55-102, mean 82.8 ± 9.7 SD years; AD, 44.8%; non-demented controls, 31.8%; Parkinson's disease, 5.0%; dementia with Lewy bodies, 2.5%; AD+Lewy body disease, 15.9%). Neuropathological examination according to standardized methods included immunohistochemistry and semiquantitative assessment of tau lesions in LC, substantia nigra (SN), dorsal motor nucleus of nervus vagus (dmX), and olfactory bulb (OB).

In Braak stage 0, tau pathology (usually very sparse pretangle material) was seen in the OB in 52.9% and in the SN/LC in 44%. The prevalence of OB and subcortical tau pathology increased with increasing Braak stages and reached 100% in OB, SN and LC and 95.2% in dmX in Braak stage VI, respectively. The severity of tau pathology in OB and subcortical nuclei significantly (P < 0.001) correlated with Braak stages and these correlations remained statistically significant when controlling for concomitant α-synuclein pathology in the respective regions.

Our finding of an increase in both prevalence and severity of OB, LC, SN and dmX tau pathology in AD with increasing Braak stages suggests that these regions become increasingly involved during AD progression rather than representing sites initially affected by AD-associated tau pathology.