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Discovery of amyloid‐beta aggregation inhibitors using an engineered assay for intracellular protein folding and solubility

Genetic and biochemical studies suggest that Alzheimer's disease (AD) is caused by a series of events initiated by the production and subsequent aggregation of the Alzheimer's amyloid β peptide (Aβ), the so‐called amyloid cascade hypothesis. Thus, a logical approach to treating AD is the development of small molecule inhibitors that either block the proteases that generate Aβ from its precursor (β‐ and γ‐secretases) or interrupt and/or reverse Aβ aggregation. To identify potent inhibitors of Aβ aggregation, we have developed a high‐throughput screen based on an earlier selection that effectively paired the folding quality control feature of the Escherichia coli Tat protein export system with aggregation of the 42‐residue AD pathogenesis effecter Aβ42. Specifically, a tripartite fusion between the Tat‐dependent export signal ssTorA, the Aβ42 peptide and the β‐lactamase (Bla) reporter enzyme was found to be export incompetent due to aggregation of the Aβ42 moiety. Here, we reasoned that small, cell‐permeable molecules that inhibited Aβ42 aggregation would render the ssTorA‐Aβ42‐Bla chimera competent for Tat export to the periplasm where Bla is active against β‐lactam antibiotics such as ampicillin. Using a fluorescence‐based version of our assay, we screened a library of triazine derivatives and isolated four nontoxic, cell‐permeable compounds that promoted efficient Tat‐dependent export of ssTorA‐Aβ42‐Bla. Each of these was subsequently shown to be a bona fide inhibitor of Aβ42 aggregation using a standard thioflavin T fibrillization assay, thereby highlighting the utility of our bacterial assay as a useful screen for antiaggregation factors under physiological conditions.