Protofibril Formation of Amyloid β-Protein at Low pH via a Non-cooperative Elongation Mechanism

Abstract

Deposition of the amyloid ␤-protein (A␤) in senile or diffuse plaques is a distinctive feature of Alzheimer's disease. The role of A␤ aggregates in the etiology of the disease is still controversial. The formation of linear aggregates, known as amyloid fibrils, has been proposed as the onset and the cause of pathological deposition. Yet, recent findings suggest that a more crucial role is played by prefibrillar oligomeric assemblies of A␤ that are highly toxic in the extracellular environment. In the present work, the mechanism of protofibril formation is studied at pH 3.1, starting from a solution of oligomeric precursors. By combining static light scattering and photon correlation spectroscopy, the growth of the mass and the size of aggregates are determined at different temperatures. Analysis and scaling of kinetic data reveal that under the studied conditions protofibrils are formed via a single non-cooperative elongation mechanism, not prompted by nucleation. This process is well described as a linear colloidal aggregation due to diffusion and coalescence of growing aggregates. The rate of elongation follows an Arrhenius law with an activation enthalpy of 15 kcal mol ؊1 . Such a value points to a conformational change of peptides or oligomers being involved in binding to protofibrils or in general to a local reorganization of each aggregate. These results contribute to establishing a clearer relation at the molecular level between the fibrillation mechanism and fibrillar precursors. The observation of a non-cooperative aggregation pathway supports the hypothesis that amyloid formation may represent an escape route from a dangerous condition, induced by the presence of toxic oligomeric species.A clear hallmark of Alzheimer disease is the presence in the brain of extracellular amyloid plaques containing a certain amount of cleavage products of the transmembrane amyloid ␤-protein precursor (1). These are small peptides of 39 -42 residues with a hydrophobic domain at the C terminus. They have been found in the core of senile plaques in typical linear aggregates, known as amyloid fibrils. Similar amyloid fibrils made from different proteins or peptides are associated with other neurodegenerative diseases, such as Parkinson and Huntington diseases, and transmissible spongiform encephalopathy (2). More generally, a large class of proteins is able to assemble into a fibrillar structure under appropriate conditions that typically favor misfolded or partially unfolded conformations (3, 4). These results support the hypothesis that fibril formation may be the onset if not the very cause of such pathologies (the "Amyloid Hypothesis") (5).In the case of Alzheimer disease, the relation between clinical symptoms and A␤ 1 production is well established (also confirmed by the known genotype-to-phenotype conversion in familial Alzheimer disease and Down syndrome) (6). However, the amount of amyloid deposits is poorly correlated with pathology, which seems to be primarily related to a synaptic dysfunction preceding neuronal d...

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