Theresa Good, PhD 1996

Thesis Title: The aggregation state specific role of beta-amyloid peptide in neurotoxicity associated with Alzheimer's Disease

Alzheimer's Disease (AD) is the leading cause of dementia in the aging population, affecting more than 7 million people, and costing in excess of $80 billion annually. β-amyloid (Aβ) is the primary protein component of senile plaques, one of the defining pathological features of AD. Aβ has been implicated in neurotoxicity associated with AD but its role in the disease process is uncertain. In vitro neurotoxicity assays suggest that the degree of aggregation of Aβ is important in the neurotoxic mechanism, however the role of aggregation in toxicity is unclear.

Aβ block of a specific potassium current in hippocampal neurons was demonstrated at concentrations comparable to toxic concentrations of Aβ. Via a kinetic analysis of the current in the presence and absence of Aβ a mechanism of the Aβ current block was proposed.

A quantitative, mechanistic model of Aβ action on a neuron was developed in order to explain the impact of the potassium block on cell function. Model simulations showed that the Aβ block could cause adverse changes in intracellular calcium levels and cell excitability, both of which have been linked to neurotoxicity. Additionally, the model provides an explanation for differences in disease severity in different areas of the cortex.

The kinetics of Aβ assembly and disassembly were measured, and the reversibility of aggregation was demonstrated. In addition, the size of oligomers released from solid Aβ was determined. With the kinetic data, an existing model of Aβ assembly was modified to include the reversibility of aggregation.

The effects of Aβ aggregation on Aβ interactions with protein and membrane components of cortical tissue, and Aβ mediated changes in ion channel function were explored. Only Aβ binding to membranes was aggregation dependent. The role of phospholipid membranes in Aβ assembly was investigated. Finally, the toxicity of Aβ was assessed in an in vitro assay. Freshly prepared soluble Aβ was nontoxic, but both insoluble and soluble aged Aβ were toxic. Based on experimental data and the kinetic model of Aβ disassembly, the hypothesis was proposed that aggregated Aβ acts as a reservoir of soluble Aβ which is toxic to neurons.