Graduation Year

2004

Document Type

Dissertation

Degree

Ph.D.

Degree Granting Department

Pharmacology and Therapeutics

Major Professor

David Morgan

Keywords

memory, gene expression, amyloid plaques, immediate early genes, synaptic plasticity

Abstract

Alzheimer's disease (AD) is characterized by anterograde amnesia followed by a progressive decline in cognitive function. Post mortem examination of forebrain tissue from sufferers reveals the presence of extracellular amyloid-beta plaques, intracellular neurofibrillary tangles, activation of glial cells and massive neuron loss. Transgenic mice expressing mutated forms of the amyloid precursor protein (APP) and presenilin-1 (PS1) genes develop neuritic amyloid plaques, glial cell activation and memory deficits, without the formation of intracellular tangles and neurodegeneration. The mechanisms by which these transgenic mice develop mnemonic deficiencies are unclear.

Gene expression of aged memory-deficient APP+PS1 mice compared with non-transgenic littermates measured by microarray and subsequent quantitative real-time PCR (qRT-PCR) analysis revealed 6 inducible immediated-early genes (IEGs) and 5 other more stably expressed plasticity-related genes (PRGs) that were significantly down-regulated in amyloid-containing hippocampus, but not down-regulated in amyloid-free cerebellum. Other genes linked to memory remained stably expressed in both regions. Analysis of forebrain AD tissue revealed that all genes measured were down-regulated presumably due to neurodegeneration, while the amyloid-free region maintained stable expression. IEG expression in APP+PS1 mice was sensitive to lower levels of amyloid. However, only in the presence of a substantially larger amyloid burden, when memory deficits reliably persist, were both PRGs and IEGs down-regulated.

Importantly, we found that IEG expression was decreased in APP+PS1 mice following exposure to a novel environment, indicating that the induction of these IEGs was impaired, rather than the basal expression of resting mice. Na+/K+ ATPase, an enzyme critical for the maintenance of membrane potential, was identified as a down-regulated PRG. We found that activity of this enzyme was both impaired in the hippocampi of APP+PS1 mice and specifically inhibited by high concentrations of amyloid-beta. Na+/K+ ATPase immunostaining revealed decreased protein in the area surrounding the amyloid plaque, where dystrophic neurites were visible, indicating amyloid may disrupt ionic gradients resulting in neuritic dystrophia. These findings suggest that amyloid accumulation may result in the impairment of IEG induction and disruption of the Na+/K+ ATPase, possibly eliciting the memory loss developed in APP+PS1 mice.

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