Molecular insights into amyloid regulation by membrane cholesterol and sphingolipids: common mechanisms in neurodegenerative diseases

Abstract

Alzheimer, Parkinson and other neurodegenerative diseases involve a series of brain proteins, referred to as 'amyloidogenic proteins', with exceptional conformational plasticity and a high propensity for self-aggregation. Although the mechanisms by which amyloidogenic proteins kill neural cells are not fully understood, a common feature is the concentration of unstructured amyloidogenic monomers on bidimensional membrane lattices. Membrane-bound monomers undergo a series of lipid-dependent conformational changes, leading to the formation of oligomers of varying toxicity rich in beta-sheet structures (annular pores, amyloid fibrils) or in alpha-helix structures (transmembrane channels). Condensed membrane nano- or microdomains formed by sphingolipids and cholesterol are privileged sites for the binding and oligomerisation of amyloidogenic proteins. By controlling the balance between unstructured monomers and alpha or beta conformers (the chaperone effect), sphingolipids can either inhibit or stimulate the oligomerisation of amyloidogenic proteins. Cholesterol has a dual role: regulation of protein-sphingolipid interactions through a fine tuning of sphingolipid conformation (indirect effect), and facilitation of pore (or channel) formation through direct binding to amyloidogenic proteins. Deciphering this complex network of molecular interactions in the context of age- and disease-related evolution of brain lipid expression will help understanding of how amyloidogenic proteins induce neural toxicity and will stimulate the development of innovative therapies for neurodegenerative diseases.

Figure 1

Different pathways of amyloidogenic protein oligomerisation and fibrillation on membrane surfaces. Upon interaction with neuronal membranes, unstructured soluble monomers of amyloidogenic proteins undergo an α-helix shift of their conformation (a). Further accumulation of the proteins on the surface of the membrane induces oligomerisation into β-sheet aggregates (b) or α-oligomers (c). Oligomers with a β-sheet structure can form protofibrils (d), amyloid fibrils (e) and amyloid pores (annular protofibrils) with ion-channel properties (f). In some instances (e.g. for α-synuclein), transmembrane channels can also be generated by oligomers with an α-helix structure (g). In all cases, the formation of functional ion channels requires the insertion and assembly of the oligomers in the neuronal membrane (green lipid bilayer). Note the possibility of conversion between α- and β-oligomers (h).

Figure 2

Neuronal membranes as concentration platforms and chaperones for amyloidogenic monomers: key role of sphingolipid–cholesterol domains. (a) Amyloidogenic monomers have a low affinity for the liquid-disordered (Ld) phase of plasma membranes enriched in phosphatidylcholine. (b) By contrast, the monomers have a high affinity for sphingolipid–cholesterol domains in the liquid-ordered (Lo) phase. In this case, monomers are not only concentrated on the membrane surface, but also undergo a major conformational change induced by the sphingolipids. This property is referred to as the chaperone effect of sphingolipids on amyloidogenic proteins.

Figure 3

Glycosphingolipids and amyloidogenic proteins: a common mechanism of interaction? Despite their high level of biochemical diversity, some glycosphingolipids share a common galactosyl ring with an orientation compatible with the establishment of stacking CH-π interactions between the sugar and an aromatic residue. Such galactosyl rings are indicated in shaded circles in the sugar moiety of ganglioside GM3 (a), globotriaosylceramide Gb₃ (b) and galactosylceramide GalCer (c). In these cases, the apolar side of the galactosyl ring fits particularly well with the aromatic-ring side chain of an aromatic residue (Phe, in this case) exposed at the surface of an amyloid protein (d). The electronic cloud of the π system stacks onto the CH groups of the galactosyl ring. This interaction is favoured by the common geometric structure of both rings, which allows a coordinated binding process between CH groups and π electrons. Cholesterol, which has a strong affinity for sphingolipids, can further improve the accessibility of the galactosyl group through fine tuning of glycosphingolipid conformation (e). This effect involves the establishment of an H-bond network between the OH group of cholesterol, the NH group of the sphingolipid and the oxygen atom of the glycosidic bond linking the ceramide and the sugar part of the sphingolipid. GalCer-NFA, GalCer with a nonhydroxylated fatty acid in the ceramide moiety.

Figure 4

A common sphingolipid-binding domain in amyloidogenic proteins. (a) Amyloidogenic proteins share a common sphingolipid-binding domain, which can be shown by superimposing the structure of micelle-bound Alzheimer Aβ peptide [PDB ID: 1BA4, in red] (Ref. 150) onto the structure of micelle-bound α-synuclein [PDB ID: 1XQ8, in blue] (Ref. 142). The motif corresponds to a helix–turn–helix structure displaying an aromatic residue (Tyr10 for Aβ; Tyr39 for α-synuclein) oriented towards the solvent and located at a similar position in the loop. The location of Glu residues associated with Glu to Lys mutations in genetic forms of Alzheimer and Parkinson diseases is indicated (Glu22 for Aβ and Glu46 for α-synuclein, respectively). The amino acid sequence of both proteins (upper panel) show very little homology, apart from the above-mentioned Tyr residues (asterisk) and a common Val residue (Val52 for α-synuclein and Val24 for Aβ). (b) Due to the high conformational plasticity of amyloidogenic proteins, the sphingolipid-binding domain can adopt several distinct conformations, as shown for the core amyloid-forming motif of amylin [NFGAILSS octapeptide, PDB ID: 1KUW] (Ref. 68). The α-carbon chain of the peptide is coloured in blue, and the Phe residue in red. Twenty conformers obtained by nuclear magnetic resonance (NMR) analysis are superposed. Once bound to the glycosphingolipid (GSL, coloured in green), the amyloid peptide is locked in a unique conformation. Cholesterol (Chol) is coloured in orange. This drawing is based on data in Ref. obtained with lactosylceramide (LacCer).

Figure 5

How glycosphingolipids could promote α-helix structures in amyloidogenic proteins. When bound to glycosphingolipids (GSLs) through the sphingolipid-binding domain, the aromatic residues of two vicinal amyloidogenic proteins cannot interact with each other and the protein is forced to adopt an α-helix conformation. The molecular mechanism of this interaction is the CH–π stacking between the galactosyl ring of GalCer and the aromatic residue of the protein. The α-helix structure of amyloidogenic proteins is not only present in membrane-bound monomers (as illustrated in Fig. 2, right panel), but also in oligomeric α-synuclein channels (Fig. 1g). The β-amyloid aggregation process is triggered when the glycosphingolipid–protein complex is destabilised, resulting in the release of the protein from its glycosphingolipid chaperone (a). In this new glycosphingolipid-free environment, the aromatic residues of two vicinal amyloidogenic proteins can interact together (b), inducing a drastic conformational change from an initial α-helix to a β-strand structure. Amyloid aggregation then results from the π–π-driven assembly of aromatic side chains (for a review, see Ref. 79), forming amyloid dimers that in turn associate to form amyloid fibrils (as illustrated in Fig. 1e).

Figure 6

Lipid-dependent formation of an α-synuclein oligomeric channel. Unfolded α-synuclein monomers (a) are attracted by the polar heads of glycosphingolipids and concentrated on the surface of sphingolipid–cholesterol membrane domains (b). Sphingolipids, complexed with cholesterol, induce a conformational change (unstructured to α-helix transition) of α-synuclein. The newly formed α-helices can insert into the plasma membrane (c) where they can further oligomerise under the control of cholesterol molecules (blue arrows) (d) and eventually form an oligomeric ion channel. Such channels are thought to disturb membrane permeability to calcium ions (red arrows), resulting in neuronal dysfunction and toxicity. The minus signs (in red) refer to the negative charges of α-synuclein.