Examining potential causes of neuronal dysfunction in Alzheimer’s disease

Abstract

Alzheimer’s disease (AD) is a debilitating neurodegenerative disease that is characterised by a number of intraneuronal hallmarks, which include the accumulation of autophagic vacuoles (AVs) within dystrophic neurites, and neurofibrillary tangles (NFTs) composed of both truncated and full-length forms of tau protein. Work presented in this thesis, outlines findings from three lines of investigation that set out to determine potential causative factors that contribute to AD-related neurodegeneration. For Aim 1, I investigated the role of impaired lysosomal digestion as a cause of AV accumulation in AD. Having developed a novel assay that utilised the detection of specific truncated forms of amyloid precursor protein C-terminal fragments (APP-CTFs), which preferentially accumulate when lysosomal digestion is impaired, findings from post-mortem human brain tissue at different Braak stages of AD (0 – VI), indicate that the accumulation of AVs in the AD brain is not caused by an impairment in lysosomal digestion. For Aim 2, I investigated the role of altered glucose availability as a cause of tau hyperphosphorylation in AD. To determine if excessive or insufficient amounts of glucose availability to neurons is a direct cause of tau hyperphosphorylation in the AD brain, I utilised a primary rat neuron culture system, to determine if hyperglycaemic or hypoglycaemic stress could lead to tau hyperphosphorylation. Despite finding high basal amounts of the AD-related tau phospho-epitope (PHF1), in both primary neurons and mouse brain, I did not report any change in levels of phospho-tau under glucose altering conditions, suggesting these changes are not directly responsible for inducing tau hyperphosphorylation in AD. For Aim 3, I investigated the role of dysfunctional neuron-glial interactions as a cause of truncation tau in AD. Having identified truncated forms of tau as early as Braak stage II in post-mortem human brain tissue, I subsequently found that neurons grown in co-cultures with glial cells, develop truncated forms of tau after two weeks in culture, which correlated with the progressive proliferation of astrocytes and microglia. I also found that certain excitatory stimuli, in particular glutamate and zinc, produced a rapid but transient increase in truncated tau, which was prevented by kynurenic acid (KynA). Concluding thoughts from all three investigations suggest that dysfunctional neuron-glial interactions are likely to occur early in AD pathogenesis and the therapeutic targeting of autonomous (neuronal) or non-autonomous (glial-mediated) factors that contribute to dysregulated neuronal excitation may prove to be beneficial in treating AD.