Cholinergic nucleus basalis tauopathy emerges early in the aging‐MCI‐AD continuum

The cholinergic denervation in Alzheimer's disease (AD) provides the rationale for treatments with anticholinesterases. The presence of this cholinergic lesion is solidly established in advanced AD. Whether it also exists in early disease remains unsettled. This question was addressed with thioflavin-S histofluorescence to identify neurofibrillary tangles (NFT) and two tau antibodies (AT8, Alz-50) to identify pre-tangle cytopathology in the nucleus basalis, the source of cortical cholinergic innervation. Methods for the concurrent visualization of tauopathy and choline acetyltransferase were used to determine if the cytopathology was selectively located within cholinergic neurons. Five elderly index cases who had died at the stage of mild cognitive impairment (MCI) or early AD were identified by longitudinal neuropsychological and behavioral assessments. They were compared to 7 age-matched cognitively normal subjects. NFT and AT8 (or Alz-50) immunostaining in cholinergic nucleus basalis neurons existed even in the cognitively normal subjects. The percentage of tauopathy-containing nucleus basalis neurons was greater in the cognitively impaired and showed a significant correlation with memory scores obtained 1-18 months prior to death. These results show that cytopathology in cortical cholinergic pathways is a very early event in the course of the continuum that leads from advanced age to MCI and AD.