Lipids revert inert Aβ amyloid fibrils to neurotoxic protofibrils that affect learning in mice

Abstract

Although soluble oligomeric and protofibrillar assemblies of Aβ-amyloid peptide cause synaptotoxicity and potentially contribute to Alzheimer's disease (AD), the role of mature Aβ-fibrils in the amyloid plaques remains controversial. A widely held view in the field suggests that the fibrillization reaction proceeds ‘forward' in a near-irreversible manner from the monomeric Aβ peptide through toxic protofibrillar intermediates, which subsequently mature into biologically inert amyloid fibrils that are found in plaques. Here, we show that natural lipids destabilize and rapidly resolubilize mature Aβ amyloid fibers. Interestingly, the equilibrium is not reversed toward monomeric Aβ but rather toward soluble amyloid protofibrils. We characterized these ‘backward' Aβ protofibrils generated from mature Aβ fibers and compared them with previously identified ‘forward' Aβ protofibrils obtained from the aggregation of fresh Aβ monomers. We find that backward protofibrils are biochemically and biophysically very similar to forward protofibrils: they consist of a wide range of molecular masses, are toxic to primary neurons and cause memory impairment and tau phosphorylation in mouse. In addition, they diffuse rapidly through the brain into areas relevant to AD. Our findings imply that amyloid plaques are potentially major sources of soluble toxic Aβ-aggregates that could readily be activated by exposure to biological lipids.

Microglia convert aggregated amyloid-β into neurotoxic forms through the shedding of microvesicles

“…These data identify microglial MVs as an endogenous source of lipids able to shift the equilibrium toward toxic Ab species. This conclusion is in complete agreement with previous evidence that brain membrane lipids, including phosho-and (glycol)sphingolipids, favor formation of soluble forms, either promoting solubilization of inert fibrils, 13 or hindering their conversion to insoluble fibrils. 14 Interestingly, exosomes released by neurons have been found to promote rather than reduce Ab fibrillogenesis, 40 thus indicating that lipid composition of different EMVs generated by distinct cell types may have opposite effects on Ab extracellular assembly.…”

“…The evidence that natural lipids may shift the equilibrium between insoluble and soluble Ab toward highly toxic soluble species 13,34 prompted us to test whether MVs shed from microglial cells may promote Ab neurotoxicity. Ab 1-42 (4 mM) dissolved in dimethyl sulfoxide (DMSO) was incubated overnight with MVs derived from rat primary microglia (1 mg/100 ml) at 37 1C in neuronal medium and subsequently exposed to cultured hippocampal neurons for 1 h. Overnight pre-incubation of Ab 1-42 with MVs yielded a neurotoxic mixture that significantly increased the percentage of dead neurons, as assessed 24 h later by propidium iodide (PI) and calcein staining (Figures 1a and b; number of experiments ¼ 4).…”

“…Negative staining electron microscopic analysis revealed the presence of both globular structures of diameter between 4 and 8 and 5-8 nm wide Ab fibrils in samples of aggregated Ab 1-42 incubated overnight with MVs. After fractionation into two phases by centrifugation, To prove the capability of shed MVs to promote formation of soluble species, soluble amyloid forms generated in Ab 1-42/ MVs mixture were separated from insoluble species by sucrose gradient centrifugation at 100 000 Â g for 1 h 13 Binding of newly generated soluble Ab 1-42-488 to neurons is competed by PrP c . We next attempted to visualize soluble Ab 1-42 forms, generated in the presence of MVs, by imaging their binding to cultured neurons.…”

“…5,8,11 Recent biochemical studies indicated that natural sphingolipids and gangliosides, whose metabolism has been shown to be altered in AD patients, 12 destabilize and rapidly resolubilize long Ab fibrils to neurotoxic species. 13 These studies also showed that phospholipids stabilize toxic oligomers from monomeric peptides. 14 The toxicity of small soluble Ab species has been proposed to depend on the interaction with specific neuronal proteins, such as the NMDA receptor 15 or the prion protein (PrP C ), 16 which modulates NMDA receptors through Fyn kinase.…”

Direct three-dimensional visualization of membrane disruption by amyloid fibrils

“…Observations of ring-like structures formed from α-synuclein and prefibrillar assemblies of other amyloid-like systems, along with membrane leakage in the presence of prefibrillar oligomers, have led to the hypothesis of membrane pore formation, membrane thinning, and membrane destabilization by prefibrillar oligomers (8,9,12,34,35). In addition it has been shown that lipid-induced depolymerization of nontoxic Aβ fibrils leads to formation of so-called "reverse oligomers" that are cytotoxic, akin to the oligomers formed de novo during fibril assembly (36). Membrane-induced fibril depolymerization and membrane destabilization associated with fibril growth on lipid bilayers have also been proposed as alternative mechanisms of amyloid-mediated cytotoxicity (11, 13, 15).…”