The 'toxic Aβ oligomer' hypothesis has attracted considerable attention among Alzheimer's disease researchers as a way of resolving the lack of correlation between deposited amyloid-β (Aβ) in amyloid plaques-in terms of both amount and location-and cognitive impairment or neurodegeneration. However, the lack of a common, agreed-upon experimental description of the toxic Aβ oligomer makes interpretation and direct comparison of data between different research groups impossible. Here we critically review the evidence supporting toxic Aβ oligomers as drivers of neurodegeneration and make some suggestions that might facilitate progress in this complex field.

The mechanism of γ-Secretase dysfunction in familial Alzheimer diseaseMutations in presenilin (PSEN) and amyloid precursor protein (APP) cause dominant early-onset Alzheimer's disease (AD), but the mechanism involved is debated. Here, such mutations are shown to alter γ-secretase activity, leading to changes in Aβ peptide cleavage patterns.

The amyloid peptides Ab 40 and Ab 42 of Alzheimer's disease are thought to contribute differentially to the disease process. Although Ab 42 seems more pathogenic than Ab 40 , the reason for this is not well understood. We show here that small alterations in the Ab 42 :Ab 40 ratio dramatically affect the biophysical and biological properties of the Ab mixtures reflected in their aggregation kinetics, the morphology of the resulting amyloid fibrils and synaptic function tested in vitro and in vivo. A minor increase in the Ab 42 :Ab 40 ratio stabilizes toxic oligomeric species with intermediate conformations. The initial toxic impact of these Ab species is synaptic in nature, but this can spread into the cells leading to neuronal cell death. The fact that the relative ratio of Ab peptides is more crucial than the absolute amounts of peptides for the induction of neurotoxic conformations has important implications for anti-amyloid therapy. Our work also suggests the dynamic nature of the equilibrium between toxic and non-toxic intermediates.

The blood-CSF barrier (BCSFB) consists of a monolayer of choroid plexus epithelial (CPE) cells that maintain CNS homeostasis by producing CSF and restricting the passage of undesirable molecules and pathogens into the brain. Alzheimer's disease is the most common progressive neurodegenerative disorder and is characterized by the presence of amyloid ␤ (A␤) plaques and neurofibrillary tangles in the brain. Recent research shows that Alzheimer's disease is associated with morphological changes in CPE cells and compromised production of CSF. Here, we studied the direct effects of A␤ on the functionality of the BCSFB. Intracerebroventricular injection of A␤1-42 oligomers into the cerebral ventricles of mice, a validated Alzheimer's disease model, caused induction of a cascade of detrimental events, including increased inflammatory gene expression in CPE cells and increased levels of proinflammatory cytokines and chemokines in the CSF. It also rapidly affected CPE cell morphology and tight junction protein levels. These changes were associated with loss of BCSFB integrity, as shown by an increase in BCSFB leakage. A␤1-42 oligomers also increased matrix metalloproteinase (MMP) gene expression in the CPE and its activity in CSF. Interestingly, BCSFB disruption induced by A␤1-42 oligomers did not occur in the presence of a broad-spectrum MMP inhibitor or in MMP3-deficient mice. These data provide evidence that MMPs are essential for the BCSFB leakage induced by A␤1-42 oligomers. Our results reveal that Alzheimer's disease-associated soluble A␤1-42 oligomers induce BCSFB dysfunction and suggest MMPs as a possible therapeutic target.

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Neuronal and synaptic degeneration are the best pathological correlates for memory decline in Alzheimer's disease (AD). Although the accumulation of soluble low-molecular-weight amyloid-␤ (A␤) oligomers has been suggested to trigger neurodegeneration in AD, animal models overexpressing or infused with A␤ lack neuronal loss at the onset of memory deficits. Using a novel in vivo approach, we found that repeated hippocampal injections of small soluble A␤ 1-42 oligomers in awake, freely moving mice were able to induce marked neuronal loss, tau hyperphosphorylation, and deficits in hippocampus-dependent memory. The neurotoxicity of small A␤ 1-42 species was observed in vivo as well as in vitro in association with increased caspase-3 activity and reduced levels of the NMDA receptor subunit NR2B. We found that the sequestering agent transthyretin is able to bind the toxic A␤ 1-42 species and attenuated the loss of neurons and memory deficits. Our novel mouse model provides evidence that small, soluble A␤ 1-42 oligomers are able to induce extensive neuronal loss in vivo and initiate a cascade of events that mimic the key neuropathological hallmarks of AD.

BACE1 cleaves the amyloid precursor protein (APP) at the β-cleavage site (Met671–Asp672) to initiate the generation of amyloid peptide Aβ. BACE1 is also known to cleave APP at a much less well-characterized β′-cleavage site (Tyr681–Glu682). We describe here the identification of a novel APP mutation E682K located at this β′-site in an early onset Alzheimer's disease (AD) case. Functional analysis revealed that this E682K mutation blocked the β′-site and shifted cleavage of APP to the β-site, causing increased Aβ production. This work demonstrates the functional importance of APP processing at the β′-site and shows how disruption of the balance between β- and β′-site cleavage may enhance the amyloidogenic processing and consequentially risk for AD. Increasing exon- and exome-based sequencing efforts will identify many more putative pathogenic mutations without conclusive segregation-based evidence in a single family. Our study shows how functional analysis of such mutations allows to determine the potential pathogenic nature of these mutations. We propose to classify the E682K mutation as probable pathogenic awaiting further independent confirmation of its association with AD in other patients.

Structural and biochemical studies of the aggregation of the amyloid-β peptide (Aβ) are important to understand the mechanisms of Alzheimer's disease, but research is complicated by aggregate inhomogeneity and instability. We previously engineered a hairpin form of Aβ called Aβcc, which forms stable protofibrils that do not convert into amyloid fibrils. Here we provide a detailed characterization of Aβ42 cc protofibrils. Like wild type Aβ they appear as smooth rod-like particles with a diameter of 3.1 (±0.2) nm and typical lengths in the range 60 to 220 nm when observed by atomic force microscopy. Non-perturbing analytical ultracentrifugation and nanoparticle tracking analyses are consistent with such rod-like protofibrils. Aβ42 cc protofibrils bind the ANS dye indicating that they, like other toxic protein aggregates, expose hydrophobic surface. Assays with the OC/A11 pair of oligomer specific antibodies put Aβ42 cc protofibrils into the same class of species as fibrillar oligomers of wild type Aβ. Aβ42 cc protofibrils may be used to extract binding proteins in biological fluids and apolipoprotein E is readily detected as a binder in human serum. Finally, Aβ42 cc protofibrils act to attenuate spontaneous synaptic activity in mouse hippocampal neurons. The experiments indicate considerable structural and chemical similarities between protofibrils formed by Aβ42 cc and aggregates of wild type Aβ42. We suggest that Aβ42 cc protofibrils may be used in research and applications that require stable preparations of protofibrillar Aβ.