Alzheimer’s disease (AD) is the most common form of dementia, with more than 25 million people worldwide suffering this progressive intellectual failure. The disease was first described by the German psychiatrist, Alois Alzheimer in 1907, and is characterised by the appearance of proteinaceous depositions (first isolated in 1984), which are comprised of insoluble amyloid-ß (Aß)-aggregates. Aβ is derived from the β-amyloid precursor protein from which it is generated by the action of two proteases. Initially it was assumed that the insoluble amyloid fibrils, which were easily detectable, mediated the observed toxicity although it was recognised that amyloid plaque number did not correlate well with the severity of dementia. However, further studies with synthetic and human-derived Aß provided strong evidence that soluble prefibrillar aggregates of Aß mediated the synaptic failure and loss of cognitive performance. In 2008 genetic evidence showed that the presence of soluble Aß-oligomers is sufficient to cause an AD-like dementia, which centres the oligomeric Aβ as the probable effector of synapse loss. Although a variety of assemblies have been described their meta-stability and technical limitations caused a controversial debate about aggregate related pathogenesis. Thus, this study aimed to establish a structure-activity relationship comparing different synthetic Aß-aggregates using biophysical methods to follow aggregation and to assess morphology, absolute MW and meta-stability of monomeric, oligomeric, protofibrillar and fibrillar Aß. However, interference with the aggregate equilibrium, by changing the ionic environment, can cause structural conversion of Aß-aggregates. Therefore, different Aß-aggregates were only compared in short-termed physiological settings i.e. neuronal binding and hippocampal neurotransmission. Herein, only prefibrillar aggregates bound to neurons and differentially impaired hippocampal neurotransmission either by inhibition of basal neurotransmission or NMDA-dependent long-term potentiation. In addition, changing the ionic environment provoked a structural conversion, which also changed the pathogenic mode of action. This study provides experimental evidence that different soluble Aß-aggregates are highly potent synaptotoxins, impairing neurotransmission by different mechanisms. Furthermore, solution-based biophysical characterisation and acute biological paradigms are crucial for differential characterisation of Aß-aggregates revealing that virtually similar aggregates can have opponent pathogenic effects; thus, morphology only does not explain observed pathogenicity.

Thesis (Ph. D.)--University of Missouri-Columbia, 2006. / The entire dissertation/thesis text is included in the research.pdf file; the official abstract appears in the short.pdf file (which also appears in the research.pdf); a non-technical general description, or public abstract, appears in the public.pdf file. Title from title screen of research.pdf file viewed on (March 3, 2007) Vita. Includes bibliographical references.

Thesis (Ph.D.)--University of Central Florida, 2008. / Adviser: Sic L. Chan. Includes bibliographical references (p. 74-84).

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Alzheimer's disease (AD) is characterised by a series of biomolecular aggregation events, which include the formation of neurotoxic protein structures composed of the β-amyloid (Aβ) peptide. In this thesis, fluorescence self-quenching (FSQ) between fluorescently-labelled peptides is introduced as a strategy for detecting and characterizing Aβ aggregates in solution, and for overcoming limitations associated with conventional methods. Using a combination of steady-state, picosecond time-resolved fluorescence and transmission electron microscopy, the fluorescence response of HiLyte Fluor 555-labelled Aβ peptides is characterised to demonstrate that Aβ self-assembly organizes the covalently attached probes in close proximity to trigger the self-quenching sensing process over a broad range of conditions. Importantly, N-terminal tagging of β-amyloid peptides is shown to not alter the self-assembly kinetics or the resulting aggregated structures. When performed in Förster resonance energy transfer (FRET) format, this method becomes a ratiometric platform to gain insights into amyloid structure and for standardizing in vitro studies of amyloid self-assembly. The ability of FSQ-based methods to monitor the inhibition of Aβ aggregation by model test compounds including the small heat shock protein (Hsp), the amyloid-binding alcohol dehydrogenase protein (ABAD) and bovine serum albumin (BSA) is also demonstrated. Given that Aβ is formed within the cell membrane and is known to induce its disruption, sophisticated single-molecule fluorescence spectroscopy methods were developed to quantify membrane dynamics induced by the presence of disrupting agents, such as Aβ and detergents. The solubilisation dynamics of single liposomes induced by the non-ionic surfactant Triton-X 100 (TX-100) were studied in real-time. Using this approach, the swelling and permeabilization steps of the solubilisation process were unambiguously separated within single FRET trajectories, and their kinetic details as a function of Triton-X 100 and presence of cholesterol within the membrane structure were examined. Finally, single-molecule stepwise-photobleaching techniques were employed to study the effect of Aβ oligomers interacting with supported-lipid bilayers, establishing a platform from which to investigate how the presence of a membrane layer affects Aβ oligomerization at the level of individual molecules. Overall, the fluorescence-based strategies for amyloid- and liposome-sensing presented in this work bridges the gap between current morphology-specific techniques and highly-specialized single-molecule methods to provide a biophysical toolbox to investigate the changes in structure, size and molecular interactions accompanying the amyloid aggregation pathway and for the screening of novel therapeutic and diagnostic agents.

The entire dissertation/thesis text is included in the research.pdf file; the official abstract appears in the short.pdf file (which also appears in the research.pdf); a non-technical general description, or public abstract, appears in the public.pdf file. Title from PDF of title page (University of Missouri--Columbia, viewed on April 29, 2010). Vita. Thesis advisor: Grace Y. Sun. "May 2008" Includes bibliographical references.