Density of GM1 in Nanoclusters Is a Critical Factor in the Formation of a Spherical Assembly of Amyloid β-Protein on Synaptic Plasma Membranes

Abstract

The deposition of amyloid β-protein (Aβ) is a pathological hallmark of Alzheimer's disease (AD). We previously found that the ganglioside-enriched microdomains (ganglioside clusters) in presynaptic neuronal membranes play a key role in the initiation of the Aβ assembly process. However, not all ganglioside clusters accelerate Aβ assembly. In the present study, we directly observed a spherical Aβ in an atomic force microscopic study on the morphology of a reconstituted lipid bilayer composed of lipids that were extracted from a detergent-resistant membrane microdomain (DRM) fraction of synaptosomes prepared from aged mouse brain. The Aβ assembly was generated on a distinctive GM1 domain, which was characterized as the Aβ-sensitive ganglioside nanocluster (ASIGN). By using an artificial GM1 cluster-binding peptide, ASIGN was found to have a high density of GM1; therefore, there would be a critical density of GM1 in nanoclusters to induce Aβ binding and assembly. These results suggest that ganglioside-bound Aβ (GAβ), which acts as an endogenous seed for Aβ fibril formation in AD brains, is generated on ASIGN on synaptosomal membranes.

Interaction of toxic and non-toxic HypF-N oligomers with lipid bilayers investigated at high resolution with atomic force microscopy

“…These data add to, and complement, those previously reported showing the key importance of GM1 and its clusters in the neuronal membrane not only as promoters of Aβ aggregation [27, 28], but also as binders of Aβ oligomers [29, 61], In the past years, the observation of annular amyloid structures on model membranes, as well as of an increased membrane permeability to Ca 2+ ions in aggregate-exposed synthetic bilayers and cultured cells, led to the formulation of the “channel hypothesis”. This states that the altered intracellular calcium concentration in cells treated with protein misfolded oligomers is related to uncontrolled calcium fluxes resulting from the organization of non-specific pores in aggregate-exposed bilayers [34, 62–64].…”

“…Overall, these results indicated that oligomer toxicity depends not only on specific structural properties of the oligomers themselves but also on the biochemical and biophysical properties of the membrane they interact with, in a delicate and complex interplay between the structural and physicochemical features of both [26]. Furthermore, these considerations add to, and confirm, previous and recent data indicating that GM1 is a pivotal player in Aβ peptide neurotoxicity both by contributing to Aβ oligomer nucleation and growth into toxic fibrils [27, 28] and by sequestering Aβ oligomers from brain interstitial fluid onto neuronal membranes [29]. …”

Soluble Oligomers Require a Ganglioside to Trigger Neuronal Calcium Overload

“…In addition, we have recently shown the ability of GM1 to recruit oligomers grown from the 42-residue form of the amyloid-␤ peptide (A␤ 42 ) to lipid raft domains of the cell membrane [4]. These data add to, and complement, those previously reported showing the key importance of GM1 and its clusters in the neuronal membrane, not only as promoters of A␤ aggregation [5,6], but also as binding sites of A␤ oligomers [7,8].…”

Dynamic Morphological Changes Induced By GM1 and Protein Interactions on the Surface of Cell-Sized Liposomes

“…Furthermore, molecular dynamic simulations suggested that there appeared to be numerous hydrogen bonds among glycan portions of the GM1 clusters due to condensation [18]. Such kinds of clusters might provide a hydrophobic environment which mediates the formation of toxic amyloid β fibrils [19,20]. These recent reports may be related to our results using model membrane systems.…”