An approved anticancer drug selectively targets the first step in the molecular cascade resulting in Alzheimer’s disease.

Alzheimer's disease is a neurodegenerative disorder associated with the aberrant aggregation of the amyloid-β peptide. Although increasing evidence implicates cholesterol in the pathogenesis of Alzheimer's disease, the detailed mechanistic link between this lipid molecule and the disease process remains to be fully established. To address this problem, we adopt a kinetics-based strategy that reveals a specific catalytic role of cholesterol in the aggregation of Aβ42 (the 42-residue form of the amyloid-β peptide). More specifically, we demonstrate that lipid membranes containing cholesterol promote Aβ42 aggregation by enhancing its primary nucleation rate by up to 20-fold through a heterogeneous nucleation pathway. We further show that this process occurs as a result of cooperativity in the interaction of multiple cholesterol molecules with Aβ42. These results identify a specific microscopic pathway by which cholesterol dramatically enhances the onset of Aβ42 aggregation, thereby helping rationalize the link between Alzheimer's disease and the impairment of cholesterol homeostasis.

The aggregation of the 42-residue form of the amyloid-β peptide (Aβ42) is a pivotal event in Alzheimer's disease (AD). The use of chemical kinetics has recently enabled highly accurate quantifications of the effects of small molecules on specific microscopic steps in Aβ42 aggregation. Here, we exploit this approach to develop a rational drug discovery strategy against Aβ42 aggregation that uses as a readout the changes in the nucleation and elongation rate constants caused by candidate small molecules. We thus identify a pool of compounds that target specific microscopic steps in Aβ42 aggregation. We then test further these small molecules in human cerebrospinal fluid and in a Caenorhabditis elegans model of AD. Our results show that this strategy represents a powerful approach to identify systematically small molecule lead compounds, thus offering an appealing opportunity to reduce the attrition problem in drug discovery.Alzheimer's disease | amyloid-β peptide | protein misfolding | drug discovery | protein aggregation A lzheimer's disease (AD) is, to date, an incurable neurodegenerative disorder that imposes substantial social and economic costs worldwide (1). According to the amyloid hypothesis, the aggregation of the amyloid-β peptide (Aβ) initiates a cascade of molecular events leading eventually to neuronal death (2-11). Because the presence of abnormal Aβ metabolism can be detected 10-20 years before the onset of AD (12, 13), early interventions may be possible before widespread and irreversible neurodegeneration has occurred. Although targeting Aβ accumulation has been pursued as a major potential therapeutic strategy against AD (14-17), no compound selected for this purpose has yet entered clinical use (18,19).Although these failures have raised doubts about the amyloid hypothesis (20), they can also be attributed to an incomplete knowledge of the molecular mechanisms by which the compounds tested so far affect the nucleation and growth of Aβ aggregates. Indeed, it has been shown that inhibiting Aβ aggregation without a detailed understanding of the underlying microscopic processes could affect the toxicity in unexpected ways (21,22). For example, the inhibition of nucleation events may delay or decrease toxicity, whereas the inhibition of elongation may lead to an overall increase in toxicity (21,22). Therefore, effective therapeutic strategies must be aimed at targeting precise microscopic steps during the Aβ aggregation process (21,(23)(24)(25).We describe here the development of a systematic pipeline based on chemical kinetics to identify a pool of candidate molecules directed against the aggregation of the 42-residue form of Aβ (Aβ42), and to understand the key chemical features responsible for their inhibitory activity. Results and DiscussionA Quasi-Structure-Based Drug Discovery Strategy. We introduce first a quasi-structure-based drug discovery (QSBDD) strategy, which builds on the recent finding that the small molecule bexarotene delays primary nucleation in Aβ42 aggregation (22) (Fig. 1A). Bec...

Transient oligomeric species formed during the aggregation process of the 42-residue form of the amyloid-β peptide (Aβ42) are key pathogenic agents in Alzheimer’s disease (AD). To investigate the relationship between Aβ42 aggregation and its cytotoxicity and the influence of a potential drug on both phenomena, we have studied the effects of trodusquemine. This aminosterol enhances the rate of aggregation by promoting monomer-dependent secondary nucleation, but significantly reduces the toxicity of the resulting oligomers to neuroblastoma cells by inhibiting their binding to the cellular membranes. When administered to a C. elegans model of AD, we again observe an increase in aggregate formation alongside the suppression of Aβ42-induced toxicity. In addition to oligomer displacement, the reduced toxicity could also point towards an increased rate of conversion of oligomers to less toxic fibrils. The ability of a small molecule to reduce the toxicity of oligomeric species represents a potential therapeutic strategy against AD.

Understanding how normally soluble peptides and proteins aggregate to form amyloid fibrils is central to many areas of modern biomolecular science, ranging from the development of functional biomaterials to the design of rational therapeutic strategies against increasingly prevalent medical conditions such as Alzheimer's and Parkinson's diseases. As such, there is a great need to develop models to mechanistically describe how amyloid fibrils are formed from precursor peptides and proteins. Here we review and discuss how ideas and concepts from chemical reaction kinetics can help to achieve this objective. In particular, we show how a combination of theory, experiments, and computer simulations, based on chemical kinetics, provides a general formalism for uncovering, at the molecular level, the mechanistic steps that underlie the phenomenon of amyloid fibril formation.