Soluble amyloid-beta oligomers and synaptic dysfunction in Alzheimer's disease

Alzheimer's disease (AD) is characterized by the insidious loss of memory and cognitive function. Histopathologic analysis of post-mortem brain tissue from AD patients reveals two characteristic lesions: (1) intraneuronal neurofibrillary tangles consisting of hyperphosphorylated tau and (2) extracellular amyloid plaques consisting of the amyloid-β (Aβ) peptide. Considerable data have emerged to suggest that Aβ plays a central role in initiating Alzheimer's disease. While insoluble amyloid plaque density correlates weakly with the severity of AD, the extent of the dementia is more robustly gauged by the concentration of soluble Aβ species. This work focuses on defining which of these soluble Aβ species actively contribute to synaptic dysfunction in AD. We first used a cell line that stably overexpresses amyloid precursor protein (7PA2 cells), which secretes a range of soluble Aβ species. The conditioned medium (CM) from 7PA2 cells was subjected to size exclusion chromatography (SEC) to separate soluble Aβ monomers from oligomers. In vivo field recordings demonstrated that Aβ oligomers inhibit long term potentation (LTP), whereas monomers did not. Furthermore, rats receiving intracerebroventricular administration of Aβ oligomers committed significantly more errors on the alternating lever cycle ratio test. Severity of dementia strongly correlates with synapse loss. Although considerable evidence supports a causal role for Aβ in AD, a direct link between a specific form of Aβ and synapse loss has not been established. Here, we demonstrate the loss of dendritic spines and excitatory synapses in pyramidal neurons from rat organotypic slices following exposure to soluble Aβ oligomers. Aβ-mediated spine loss required activity of NMDA-type glutamate receptors (NMDARs) and occurred through a pathway involving cofilin and calcineurin. Lastly, soluble Aβ dimers were extracted from the cerebral cortex of patients with AD. Soluble dimers inhibited LTP, enhanced long term depression (LTD), and reduced dendritic spine density in normal rodent hippocampus. Importantly, Aβ dimers disrupted the memory of a learned behavior in normal rats. Insoluble amyloid plaque cores isolated from AD cortex did not impair LTP unless solubilized to release Aβ dimers. We conclude that soluble dimers are the minimal Aβ aggregate sufficient to impairs the structure and function of hippocampal synapses.