Determination of alpha-sheet secondary structure as an early biomarker in the pathogenesis of Alzheimer's disease

Abstract

Alzheimer’s disease (AD) is one of over 50 amyloid diseases discovered to-date and is now the sixth leading cause of death in the United States, affecting over 5.8 million Americans and projected to cost upwards of $1.1 trillion in paid and unpaid care by 2050. It is characterized by the deposition of beta-sheet-rich, insoluble amyloid beta-peptide (Abeta) plaques; however, the presence of toxic soluble oligomers – not plaque burden – is correlated with cognitive impairment in AD patients. Additionally, plaque deposition is a late-stage indicator of disease, while the early-stage oligomers mediate toxicity 10-20 years before symptoms present. Most existing treatments for AD target and alleviate symptoms of the disease, rather than the early stages of pathogenesis. Additionally, current diagnostics either detect late-stage indicators of disease progression or utilize biomarker levels that are not directly correlated with toxicity in AD pathogenesis. Here, we show that Abeta oligomers adopt a nonstandard secondary structure, termed alpha-sheet, in the early stages of aggregation that is strongly correlated with toxicity. De novo designed alpha-sheet peptides specifically and tightly bind the toxic oligomers over monomeric and fibrillar forms of Abeta. Alpha-sheet peptides inhibit aggregation in vitro, neurotoxicity in cell culture, and remove toxic oligomers from the brains in vivo in transgenic (Tg) mice. We further developed and characterized a novel diagnostic agent, termed the soluble oligomer binding assay (SOBA), that specifically detects alpha-sheet containing Abeta oligomers in solution. SOBA detected toxic Abeta oligomers in the brains of Tg mice long before plaques deposited or symptoms presented, and correctly identified all mild cognitive impairment (MCI) and AD patients in a cohort using plasma samples. SOBA also correctly identified all control (CO) patients (including some with non-AD-related MCI) with an overall sensitivity of 100% and specificity of 100%. Additionally, 8 pre-symptomatic CO patients that converted to MCI within 1-5 years were correctly identified by SOBA before clinical assessments or biomarker characterizations could. The alpha-sheet hypothesis proposes the earliest mechanism through which AD pathology initiates and propagates, presenting the possibility for novel therapeutic agents for efficacious treatment of AD.