Non-tau based neuronal degeneration in Alzheimer's disease — an immunocytochemical and quantitative study in the supragranular layers of the middle temporal neocortex

In Alzheimer's disease (AD), cortical neurons develop neurofibrillary tangles (NFTs) consisting of hyperphosphorylated tau. The neurons eventually die. There are some hints that cortical neurons may also degenerate without the development of cytoskeletal changes. We investigated this possibility by comparing changes in APP staining and neuronal size with respect to the presence or absence of hyperphosphorylated tau. Adjacent sections of the medial temporal neocortex (Brodmann's area 22) of 5 male AD patients aged 60–88 years (Braak V–VI) and 5 age-matched male non-demented control subjects were i) stained with a modified Bielschowsky silver method in order to reveal NFTs and ‘ghost’ tangles, ii) single-stained with anti-APP, and iii) double-labeled with anti-APP and AT8. Anti-APP is directed against the β-amyloid precursor protein and stains virtually all perikarya and proximal neurites of the cortical neurons. AT8 stains pre-tangles, NFTs and extracellular ‘ghost’ tangles due to the recognition of hyperphosphorylated tau. The study was focused on the supragranular cortical layers II–III, since these layers can be clearly delineated from the adjacent molecular and granular cell layers. The results showed that i) APP staining intensity in neurons was variable in the AD cortex, being clearly different from the invariably intense neuronal staining in all controls. Reduced cytoplasmic APP staining was observed, particular in small neurons, while lack of anti-APP staining in proximal neurites, too, was associated with AD. In addition, ii) cross-sectional area measurement on anti-APP-stained neurons revealed that in AD, as compared to controls, a clear decrease in the number of mainly large-sized neurons (> 150 µm2) was accompanied by a significant increase in the percentage of neurons in the smaller size classes, indicating that many large-sized neurons became smaller in AD. iii) Reduced APP staining and decreased neuronal size were not necessarily associated with the presence or absence of hyperphosphorylated tau in these cells. iv) Twenty-six percent of the neurons contained hyperphosphorylated tau, while the level of NFT-related neuronal loss was low in AD. The present study suggests that non-tau based neuronal degeneration is a major phenomenon in the AD neocortex.