Diffusible, highly bioactive oligomers represent a critical minority of soluble Aβ in Alzheimer’s disease brain

Abstract

Significant data suggest that soluble Aβ oligomers play an important role in Alzheimer's disease (AD), but there is great confusion over what exactly constitutes an Aβ oligomer and which oligomers are toxic. Most studies have utilized synthetic Aβ peptides, but the relevance of these test tube experiments to the conditions that prevail in AD is uncertain. A few groups have studied Aβ extracted from human brain, but they employed vigorous tissue homogenization which is likely to release insoluble Aβ that was sequestered in plaques during life. Several studies have found such extracts to possess disease-relevant activity and considerable efforts are being made to purify and better understand the forms of Aβ therein. Here, we compared the abundance of Aβ in AD extracts prepared by traditional homogenization versus using a far gentler extraction, and assessed their bioactivity via real-time imaging of iPSC-derived human neurons plus the sensitive functional assay of long-term potentiation. Surprisingly, the amount of Aβ retrieved by gentle extraction constituted only a small portion of that released by traditional homogenization, but this readily diffusible fraction retained all of the Aβ-dependent neurotoxic activity. Thus, the bulk of Aβ extractable from AD brain was innocuous, and only the small portion that was aqueously diffusible caused toxicity. This unexpected finding predicts that generic anti-oligomer therapies, including Aβ antibodies now in trials, may be bound up by the large pool of inactive oligomers, whereas agents that specifically target the small pool of diffusible, bioactive Aβ would be more useful. Furthermore, our results indicate that efforts to purify and target toxic Aβ must employ assays of disease-relevant activity. The approaches described here should enable these efforts, and may assist the study of other disease-associated aggregation-prone proteins.

PrP is a central player in toxicity mediated by soluble aggregates of neurodegeneration-causing proteins

“…The YZ1 iPSC line [95] was used to prepare neurogenin 2 (Ngn2)-induced human neurons [96] as described previously [41,45]. On iN day 4, cells were plated at 4000 cells/ well on Matrigel (Corning Life Sciences, Corning, NY)coated plates (#655090; Greiner Bio-One, Monroe, NC) and maintained until iN day 21, a time point when iNs are fully mature (Supplementary Figure 5A).…”

“…Given the pleomorphic nature of Aβ assemblies formed in vitro [89] and those in extracts of human brain [25,41,92], it is reasonable to expect that toxicity is mediated by more than one Aβ assembly and that this could involve interactions between specific Aβ assemblies and specific receptors, such as PrP C and rather non-specific membrane perturbations [87]. PrP C is a membrane-associated protein best known for its role in transmissible spongiform encephalopathies [71].…”

Amyloid‐β immunotherapy for alzheimer disease: Is it now a long shot?

“…Conversely, other work indicates that the large oligomer Aβ*56 induces specific changes in neuronal signaling leading to tau phosphorylation and aggregation, whereas dimers and trimers do not . Recent studies have shown that most Aβ extracted from AD brain is innocuous and only some species cause neurotoxicity . This suggests that generic antioligomer therapies, including Aβ antibodies, may be largely bound by the extensive pool of inactive oligomers, whereas targeting the small pool of diffusible, bioactive Aβ may be more useful.…”

PrP-grafted antibodies bind certain amyloid β-protein aggregates, but do not prevent toxicity

“…While Aβ aggregation is widely acknowledged to be central to AD pathogenesis, an increasing body of evidence indicates that only a sub-population of Aβ aggregates are bioactive (Brinkmalm et al, 2018;Hong et al, 2018;Lesne, 2013). As with PrP and prion disease Sandberg et al, 2011;Sandberg et al, 2014), the form or forms of Aβ which mediate toxicity are not yet defined.…”