Pathological changes leading to neuronal degeneration in Alzheimer's disease

Alzheimer' s disease (AD) is the most common form of dementia. The pathological hallmarks of AD include ‚àövº-amyloid (A‚àövº) plaques, dystrophic neurites (DNs) and neurofibrillary tangles (NFTs). All of these pathological hallmarks involve abnormal insoluble proteinaceous aggregates that have the capacity to disturb normal cellular functioning. However, there is disagreement within the AD literature as to whether it is A‚àövº plaques, soluble A‚àövº or NFTs that are the primary causative agent of AD. Irrespective of the initial cause of AD, the burden of A‚àövº plaques and NFTs increases as AD progresses, eventually resulting in substantial brain atrophy, which is at least partially due to overt neuronal degeneration and death. Thus, a better understanding of the aetiology and progression of AD will enable more efficient therapeutics to be developed. This thesis investigates the role of apoptosis in AD with the aim of ascertaining whether apoptosis is involved in disease staging or progression or is a causative agent of AD. No increase in immunohistochemical labelling, or change in localisation that distinguished between control, preclinical AD and AD cases were present for a range of apoptosis-related proteins. In addition, mRNA levels of apoptosis-related proteins differed little between control, preclinical AD and AD cases when analysed by real time reverse transcriptase polymerase chain reaction. There was no difference in the percentage of apoptotic-like nuclei in the neocortex between control, preclinical AD and AD cases and very few of the nuclei associated with A‚àövº plaques or NFTs were abnormal. Cytochrome c (cyto c)-labelling was punctate in cortical neurons, including a subset of NFT-bearing neurons, but a subset of DNs demonstrated cytoplasmic cyto c-labelling. These data suggest that apoptosis may not play a major role in the pathogenesis or progression of AD and that activation of apoptotic pathways can occur in the absence of extensive terminal appptosis in the brain. As post-mortem human AD tissue provides only random time points in the dynamic process of disease progression, transgenic mouse models of AD are invaluable tools for investigating aspects of age-associated disease progression and also for testing potential therapeutics for AD. Although, for animal models to be used effectively, a detailed understanding of the pathology and disease processes that they model is required. Therefore, the A‚àövº plaque-associated neuronal pathology in two transgenic AD mouse models was investigated to determine whether the neuronal pathology in these mice more closely resembles human preclinical AD cases or clinically evident AD cases. Using immunohistochemistry the morphology and neurochemistry of the A‚àövº plaque-associated DNs present in the two lines of transgenic AD mice was demonstrated to be strikingly similar to that in human preclinical AD cases, but not AD cases. Importantly, quantitation demonstrated that the A‚àövº plaques in these transgenic AD mice were highly axonopathic, and were also associated with displaced or clipped apical dendrites in both transgenic mouse models. The results suggest that these mice represent an accurate and valuable model of preclinical AD that can be utilised as a platform for testing potential therapeutic agents for AD, to be administered prior to extensive neuronal loss. Finally, as current treatments for AD only treat the symptoms of the disease and do not slow or stop its progression, the potential of a novel therapeutic agent with zinc binding, neuroprotective and anti-oxidant properties, metallothionein isoform IIA (MTIIA), was investigated. Utilising immunohistochemistry, the A‚àövº and thioflavine s plaque loads and the A‚àövº plaque-associated neuronal pathology in 13 and 15 month old Tg2576 mice was investigated after three months of metallothionein IIA treatment. Although this pilot study did not produce any statistically significant results, there was a trend towards lower A‚àövº and thioflavine s plaque loads in MTIIA treated Tg2576 mice. Thus, MTIIA warrants further investigation as a potential therapeutic for AD in the future. The research in this thesis provides valuable new data on the staging of AD, with particular regard to the role of apoptosis in AD and A‚àövº plaque-associated neuronal pathology in transgenic AD mice and human AD. The current study indicates that apoptosis does not play a seminal role in the genesis or progression of AD pathology. This works has also clarified how two widely studied transgenic AD mice compare to the schema of disease progression that occurs in human AD, and strongly suggests that these transgenic AD mice mimic human preclinical AD. Finally, a pilot study of MTIIA administration to transgenic AD mice did not result in significant differences between the treatment and control groups.