A calcium-sensitive antibody isolates soluble amyloid-beta aggregates and fibrils from Alzheimer's disease brain

Stern et al. develop a new monoclonal antibody specific to neurotoxic oligomers of amyloid-beta peptide. A unique calcium-sensitive property of this antibody enables isolation of soluble amyloid-beta aggregates from Alzheimer's disease brain without denaturation, some of which exhibit fibrillar morphology. Aqueously soluble oligomers of amyloid-beta peptide may be the principal neurotoxic forms of amyloid-beta in Alzheimer's disease, initiating downstream events that include tau hyperphosphorylation, neuritic/synaptic injury, microgliosis and neuron loss. Synthetic oligomeric amyloid-beta has been studied extensively, but little is known about the biochemistry of natural oligomeric amyloid-beta in human brain, even though it is more potent than simple synthetic peptides and comprises truncated and modified amyloid-beta monomers. We hypothesized that monoclonal antibodies specific to neurotoxic oligomeric amyloid-beta could be used to isolate it for further study. Here we report a unique human monoclonal antibody (B24) raised against synthetic oligomeric amyloid-beta that potently prevents Alzheimer's disease brain oligomeric amyloid-beta-induced impairment of hippocampal long-term potentiation. B24 binds natural and synthetic oligomeric amyloid-beta and a subset of amyloid plaques, but only in the presence of Ca2+. The amyloid-beta N terminus is required for B24 binding. Hydroxyapatite chromatography revealed that natural oligomeric amyloid-beta is highly avid for Ca2+. We took advantage of the reversible Ca2+-dependence of B24 binding to perform non-denaturing immunoaffinity isolation of oligomeric amyloid-beta from Alzheimer's disease brain-soluble extracts. Unexpectedly, the immunopurified material contained amyloid fibrils visualized by electron microscopy and amenable to further structural characterization. B24-purified human oligomeric amyloid-beta inhibited mouse hippocampal long-term potentiation. These findings identify a calcium-dependent method for purifying bioactive brain oligomeric amyloid-beta, at least some of which appears fibrillar.