Alzheimer’s disease is an increasingly prevalent neurodegenerative disorder whose pathogenesis has been associated with aggregation of the amyloid-β peptide (Aβ42). Recent studies have revealed that once Aβ42 fibrils are generated, their surfaces strongly catalyse the formation of neurotoxic oligomers. Here we show that a molecular chaperone, a Brichos domain, can specifically inhibit this catalytic cycle and limit Aβ42 toxicity. We demonstrate in vitro that Brichos achieves this inhibition by binding to the surfaces of fibrils, thereby redirecting the aggregation reaction to a pathway that involves minimal formation of toxic oligomeric intermediates. We verify that this mechanism occurs in living brain tissue by means of cytotoxicity and electrophysiology experiments. These results reveal that molecular chaperones can help maintain protein homeostasis by selectively suppressing critical microscopic steps within the complex reaction pathways responsible for the toxic effects of protein misfolding and aggregation.

Soluble oligomeric aggregates of the amyloid-β peptide (Aβ) have been implicated in the pathogenesis of Alzheimer's disease (AD). Although the conformation adopted by Aβ within these aggregates is not known, a β-hairpin conformation is known to be accessible to monomeric Aβ. Here we show that this β-hairpin is a building block of toxic Aβ oligomers by engineering a doublecysteine mutant (called AβCC) in which the β-hairpin is stabilized by an intramolecular disulfide bond. Aβ 40 CC and Aβ 42 CC both spontaneously form stable oligomeric species with distinct molecular weights and secondary-structure content, but both are unable to convert into amyloid fibrils. Biochemical and biophysical experiments and assays with conformation-specific antibodies used to detect Aβ aggregates in vivo indicate that the wild-type oligomer structure is preserved and stabilized in AβCC oligomers. Stable oligomers are expected to become highly toxic and, accordingly, we find that β-sheet-containing Aβ 42 CC oligomers or protofibrillar species formed by these oligomers are 50 times more potent inducers of neuronal apoptosis than amyloid fibrils or samples of monomeric wild-type Aβ 42 , in which toxic aggregates are only transiently formed. The possibility of obtaining completely stable and physiologically relevant neurotoxic Aβ oligomer preparations will facilitate studies of their structure and role in the pathogenesis of AD. For example, here we show how kinetic partitioning into different aggregation pathways can explain why Aβ 42 is more toxic than the shorter Aβ 40 , and why certain inherited mutations are linked to protofibril formation and early-onset AD.amyloid-β peptide | protein aggregation | protein structure | protofibril | β-hairpin conformation

Protein misfolding and aggregation is increasingly being recognized as a cause of disease. In Alzheimer’s disease the amyloid-β peptide (Aβ) misfolds into neurotoxic oligomers and assembles into amyloid fibrils. The Bri2 protein associated with Familial British and Danish dementias contains a BRICHOS domain, which reduces Aβ fibrillization as well as neurotoxicity in vitro and in a Drosophila model, but also rescues proteins from irreversible non-fibrillar aggregation. How these different activities are mediated is not known. Here we show that Bri2 BRICHOS monomers potently prevent neuronal network toxicity of Aβ, while dimers strongly suppress Aβ fibril formation. The dimers assemble into high-molecular-weight oligomers with an apparent two-fold symmetry, which are efficient inhibitors of non-fibrillar protein aggregation. These results indicate that Bri2 BRICHOS affects qualitatively different aspects of protein misfolding and toxicity via different quaternary structures, suggesting a means to generate molecular chaperone diversity.

Background: Alzheimer disease (AD) is associated with A␤ protein misfolding and aggregation into fibrils rich in ␤-sheet structure. Results: BRICHOS domains prevent fibril formation of A␤ far below the stoichiometric ratio. Conclusion: A␤ is maintained as an unstructured monomer in the presence of BRICHOS. Significance: BRICHOS domain can have a natural protective role against A␤ aggregation, which may open new routes toward AD therapy.