Region-specific ups and downs in mitochondrial numerical densities during Alzheimer's disease progression : a pilot study in human brains

Background: Mitochondrial dysfunction is an important pathogenic factor in Alzheimer´s disease (AD) progression. Most studies analysed disturbances in the mitochondrial metabolism and oxidative stress or focussed on mitochondrial dynamics such as mitochondrial trafficking, fusion-fission and mitophagy. Objective: Very limited data exist regarding changes in the mitochondrial numerical density at different levels of AD neuropathologic changes (ADNC) in human brains. Methods: Mitochondrial numerical densities were analysed by morphometry using the marker protein ATP5B in sections of 13 brain areas of 8 patients with either low, mid or high ADNC, 6 patients with tauopathy and 10 control patients. Patient samples were classified according to the ABC score. Results: In comparison to control patients, we detected increases in mitochondrial densities at low (not in all cases), mid and high ADNC in neurons of the frontal (25%) and temporal (11%) neocortices, pontine nuclei (30%) and Purkinje neurons of the cerebellum (30%). Contrarily, mitochondrial densities decreased by 20% in hippocampal neurons of the entorhinal cortex and CA3 region at mid and high ADNC. Only minor changes occurred in other brain regions investigated (e.g., parietal and occipital neocortices, inferior olive, substantia nigra, striatum). In tauopathy patients, changes in mitochondrial densities were comparable to those in AD patients, except for a stronger decrease in the entorhinal cortex (40%) and a greater increase in the temporal neocortex (30%). Conclusions: In the neocortex, primarily affected by extracellular amyloid-β (Aβ) deposits, mitochondrial densities in neurons increased, whereas they decreased in the hippocampus, at first enriched in intracellular neurofibrillary tangles.