Biomedical Engineering ETDs

Juan Anaya

Publication Date

9-3-2013

Abstract

The misfolding and aggregation of proteins into fibrillar aggregates in the brain are linked to the pathogenesis of over 20 neurodegenerative diseases. Specifically, the toxicity and neurodegenerative symptoms of Alzheimers disease are directly related to the aggregation of the amyloid-β (Aβ) protein into β-sheet rich insoluble fibrils. However, the mechanism and driving forces of Aβ fibril formation in vivo are still unknown. It has been shown previously that Aβ's surface activity and favorable interaction with lipid membranes can induce the formation of fibrils, suggesting a possible membrane-based mechanism of Aβ aggregation in Alzheimer's disease. Unlike dilute solutions used for in vitro experiments, the cellular environment is highly crowded, with macromolecules and osmolytes occupying up to 40% of the cellular volume. The resulting molecular crowding and preferential exclusion modulate the thermodynamics of protein reactions to favor those that reduce total system volume and minimize solvent exposed surface area, respectively. To assess the effects of molecular crowding and preferential exclusion on interface-induced Aβ fibril formation, we investigate the effects of the osmolyte sucrose on Aβ surface activity and membrane interaction. We hypothesize that due to preferential exclusion, sucrose will favor the interface-partitioned states, i.e., air/water interface adsorbed and membrane associated, of Aβ. As such, sucrose is expected to enhance the surface activity and membrane interaction of Aβ. Adsorption isotherms of Aβ40 to the air/subphase interface confirm our hypothesis. With increasing sucrose concentration in the subphase, Aβ40 adsorbed to the air/subphase interface more readily, increasing the final adsorption surface pressure, decreasing the lag time before adsorption begins and increasing the rate of adsorption. Similarly, Aβ40 inserted into anionic DMPG and zwitterionic DPPC monolayers more readily in the presence of increasing sucrose concentrations. The amount of insertion increased, the lag time decreased, and the rate of insertion increased with increasing sucrose concentration. This increased interfacial activity in the presence of sucrose is important because association of Aβ in membranes has been associated with nucleation of fibril formation that leads to the neurodegenerative pathology of Alzheimer's disease. The effects of preferential exclusion and molecular crowding associated with sucrose on the interfacial dynamics of Aβ thus play an important role in formation of fibrils. The cellular environment is even more crowded and osmotically active than the dilute solutions investigated here. This suggests that the interactions of Aβ with membrane interfaces may be even more significant in the cellular environment and may serve as a nucleation site for the aggregation of Aβ in vivo.'

Language

English

Document Type

Thesis

Degree Name

Biomedical Engineering

Level of Degree

Masters

Department Name

Biomedical Engineering

Project Sponsors

Endowment Gift Agreement and Planned Estate Funding — Margaret Bell, Alzheimers Association NIRG-09-132478, Oak Ridge Associated Universities Ralph E. Powe Junior Faculty Enhancement Award, UNM Research Allocation Committee'

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