Use of high-affinity scFv antibody NUsc1 for isolation and sensitive detection of Alzheimer's-associated amyloid beta oligomers.

Alzheimer's disease (AD) is a progressive neurodegenerative disorder in which soluble amyloid-beta oligomers (AβOs) play a key role in disease onset and progression. This study further explores NUsc1, a single-chain variable fragment (scFv) antibody with high specificity for neurotoxic AβOs over monomers or fibrils, to selectively isolate and assist characterization of AβO species. NUsc1 binds AβOs with a kinetic dissociation constant under 10 nM, comparable to the affinity of Leqembi for protofibrils. NUsc1 inhibits aggregation of AβOs into protofibrillar and fibrillar structures and blocks AβO binding to cultured hippocampal neurons in a dose-dependent manner, with an IC50 of ∼100 nM in experiments using 2.25 ng/μl AβOs. Selectivity and high affinity of NUsc1 enabled immunoprecipitation of AβOs under non-denaturing conditions. Atomic force microscopy revealed globular NUsc1-AβO complexes with a z-height of ∼3 to 4 nm, while free NUsc1 measured ∼1.5 nm. Mass spectrometry revealed a heterogeneous population of NUsc1-isolated AβOs, consistent with a stepwise species assembly, while non-denaturing size-exclusion chromatography revealed species of approximately 70 kDa. In ELISA, phage-bound NUsc1 (pbNUsc1) detected ∼12 pM AβOs, a 33-fold increase in detection sensitivity compared to a commercial anti-Aβ antibody. NUsc1 also detected AβOs from 5xFAD mouse brain extracts, cerebrospinal fluid from AD rats, and, notably, from CSF of human donors, demonstrating direct detection of AD-relevant oligomers in a clinically relevant human biofluid. Collectively, these results highlight NUsc1's promise not only as a powerful research tool for structural and mechanistic studies of AβOs but also as a highly sensitive diagnostic tool for clinical use.