We have previously shown that overexpressing subunits of the iron-binding protein ferritin can rescue the toxicity of the amyloid ␤ (A␤) peptide in our Drosophila model system. These data point to an important pathogenic role for iron in Alzheimer disease. In this study, we have used an iron-selective chelating compound and RNAi-mediated knockdown of endogenous ferritin to further manipulate iron in the brain. We confirm that chelation of iron protects the fly from the harmful effects of A␤. To understand the pathogenic mechanisms, we have used biophysical techniques to see how iron affects A␤ aggregation. We find that iron slows the progression of the A␤ peptide from an unstructured conformation to the ordered cross-␤ fibrils that are characteristic of amyloid. Finally, using mammalian cell culture systems, we have shown that iron specifically enhances A␤ toxicity but only if the metal is present throughout the aggregation process. These data support the hypothesis that iron delays the formation of well ordered aggregates of A␤ and so promotes its toxicity in Alzheimer disease. AD3 remains the most common cause of dementia in the elderly; however, our incomplete understanding of its pathogenesis hinders therapeutic progress. It is widely believed that an important initiating factor is the accumulation of the amyloid ␤ peptide (A␤) within the brain (1, 2). A␤ is generated by the proteolytic processing of the amyloid precursor protein, and the behaviors of the resulting peptide isoforms, largely A␤ 1-40 and A␤ , are determined in large part by differences in their C-terminal residues. In the case of A␤ 1-40 , the peptide is 40 amino acids long and does not contain the final two hydrophobic residues that are present in the more aggregation-prone A␤ 1-42 . In some families, mutations in the APP gene result in amino acid substitutions within the A␤ peptide; a particular example of this is the Arctic mutation (E22G) that is linked to early onset autosomal dominant AD (3). In both sporadic and familial AD, however, it is the aggregation-prone A␤ peptide that comprises the amyloid plaques that are seen in the brain of individuals with AD (4). However, the mature amyloid fibrils that constitute these plaques are not thought to be the prime cytotoxic agent; rather there is strong evidence both in vitro and in vivo that the most toxic A␤ aggregates are small, soluble species that precede, but may also accompany, the appearance of mature fibrils (5).We have generated a model system in which A␤ peptides are secreted from neurons in the brain of Drosophila melanogaster (6). The consequent phenotypes correlate well with the propensity of a range of A␤ variants to form soluble aggregates (7). Using this system, we performed an unbiased genetic modifier screen that highlighted the importance of iron metabolism as a cofactor in mediating the A␤ toxicity (8). Indeed, we and others have shown that both genetic and pharmacological manipulation of iron metabolism in the fly (8) and mouse (9) brain can modify A␤ toxicity. Notably, w...

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