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Alzheimer's Disease (AD) is a neuropathological disorder characterized by the progressive deposition of insoluble amyloid plaques and vascular deposits consisting primarily of 4.5 kDa amyloid beta peptides (Abeta). There is increasing evidence that the deposition of Abeta fibrils in the brain, an invariable feature of AD, and/or prefibrillar aggregates likely cause neurodegeneration in AD. While Abeta fibrils were a previous research focus, recent experiments implicate prefibrillar aggregates as the toxic species. The identification and characterization of prefibrillar aggregates is of great importance to understanding AD and the development of therapeutic strategies. Biophysical and spectroscopic techniques were used to examine the effects of electrostatic interactions on Abeta oligomerization. Experimental work demonstrated that, while salt bridges likely provide stability to preformed Abeta aggregates, these interactions are not essential for the early stages of aggregation. Abeta oligomerization is driven by the formation of pH-independent interactions and is impeded by electrostatic repulsion at pH values away from the isoelectric point. Diffuse plaques, containing only the 42-residue form of Abeta, are unstructured and non-toxic; they appear before toxic senile plaques containing both 40 and 42-residue forms. Through incubation, Abeta40 and Abeta42 were shown to co-incorporate into unstructured aggregates early during fibrillogenesis later leading to tightly packed aggregates with secondary structure. Previously, the stage at which the Abeta variants co-incorporated during the fibrillogenic process was unknown. After observing that the amyloid precursor protein transmembrane (APP-TM) domain contains two known dimerization motifs (GXXXG/A), oligomerization of the APP-TM domain was examined. A model system was developed to investigate the effects of familial AD mutations on the dimerization propensity of APP-TM domains. This work culminated in the first experimentally supported mechanism to explain how genetic mutations within the APP gene lead to the observed phenotype and predisposition to AD. Further experimentation led to the discovery of non-denaturing detergents that stabilize suspected on-pathway spherical Abeta aggregates. These detergent-stabilized Abeta oligomers share many of the structural features and biological activities of both membrane bound Abeta and spherical oligomers of Abeta formed in solution. Thus, these stabilizing detergents may prove useful in high-resolution structural analysis of spherical oligomers.
This thesis presents a method for reliably and robustly producing samples of amyloid-β (Aβ) by capturing them at various stages of aggregation, as well as the results of subsequent imaging with various atomic force microscopy (AFM) methods, all of which add value to the data gathered by collecting information on the peptide’s nanomechanical, elastic, thermal or spectroscopical properties. Amyloid-β (Aβ) undergoes a hierarchy of aggregation following a structural transition, making it an ideal subject of study using scanning probe microscopy (SPM), dynamic light scattering (DLS) and other physical techniques. By imaging samples of Aβ with Ultrasonic Force Microscopy, a detailed substructure to the morphology is revealed, which correlates well with the most advanced cryo-EM work. Early stage work in the area of thermal and spectroscopical AFM is also presented, and indicates the promise these techniques may hold for imaging sensitive and complex biological materials. This thesis demonstrates that physical techniques can be highly complementary when studying the aggregation of amyloid peptides, and allow the detection of subtle differences in their aggregation processes.
Alzheimer's disease is the most frequent type of dementia. With an exponentially growing number of cases, understanding the underlying molecular events leading to this devastating condition is of crucial importance. Much evidence points to a disequilibrium in the production and degradation of amyloid beta (Aß), a normally physiological 42 amino acid peptide, as an early key event in Alzheimer's etiology. Whether Aß is overproduced or poorly degraded, the overall result is an abnormally large pool of peptide that gradually aggregates forming extracellular deposits of fibrils, called amyloid plaques, in specific brain regions. Hence, modulation of Aß aggregation process is one of the suggested approaches to control the evolution of Alzheimer's disease. Universally conserved molecular chaperones have been intensively studied for their capacity to prevent aggregation of disease-related proteins, and many of them have proven to efficiently modulate Alzheimer's Aß aggregation. In a scenario where chaperones are overexpressed or directly administered into the affected tissue, the universal conservation and the relatively poor client-specificity of generic chaperones can become a downside because of the risk of interaction with proteins other than the targeted one is not dismissible, and thus the consequences unpredictable. In the first part of this work, we looked upon a bacterial chaperone call SecB with an unusually robust holdase activity (i.e. it prevents early protein folding) as a promising modulator of Alzheimer's Aß peptide aggregation. [...].
Alzheimer’s disease (AD) is the major cause of dementia and is characterized by neuronal death and brain atrophy. The amyloid [beta] (A[beta]) peptide is tightly associated with neuronal dysfunction during AD, but the molecular mechanism underlying the neurotoxic effect of A[beta] is poorly understood. Extracellular fibrillar deposits (plaques) of A[beta] were initially believed to be the cause of AD, but currently there is overwhelming evidence that the prefibrillar A[beta] oligomers are the major toxic entities. Structural characterization of A[beta] oligomers and fibrils is important for understanding the structural features determining the toxic potency of the peptide. This project has studied the aggregation and accompanying structural transitions of A[beta], a naturally occurring hypertoxic species, i.e. pyroglutamylated A[beta], and their combination, using biophysical approaches (circular dichroism, fluorescence, infrared spectroscopy). In addition, aggregation and structure of overlapping peptide fragments have been studied to identify the specific stretch of A[beta] that serves as seeding region initiating the aggregation and fibril formation by the full-length A[beta]peptide. These studies elucidate the structural features of A[beta] responsible for the peptide’s neurotoxic action.
A proven collection of readily reproducible techniques for studying amyloid proteins and their involvement in the etiology, pathogenesis, diagnosis, and therapy of amyloid diseases. The contributors provide methods for the preparation of amyloid and its precursors (oligomers and protofibrils), in vitro assays and analytical techniques for their study, and cell culture models and assays for the production of amyloid proteins. Additional chapters present readily reproducible techniques for amyloid extraction from tissue, its detection in vitro and in vivo, as well as nontransgenic methods for developing amyloid mouse models. The protocols follow the successful Methods in Molecular BiologyTM series format, each offering step-by-step laboratory instructions, an introduction outlining the principle behind the technique, lists of the necessary equipment and reagents, and tips on troubleshooting and avoiding known pitfalls.