there are two main mechanisms ensuring the efficient transfer of amyloidogenic proteins 
           from the cytosolic or extracellular milieu to neuronal plasma membranes .
the first one is 
           the reduction of dimensionality ( from 3d to 2d ) that occurs when a soluble protein 
           dissolved in the bulk solvent binds to a membrane surface ( ref .
the underlying idea is that the membrane 2d space can 
           concentrate proteins far more efficiently than the 3d space of the bulk solvent . as a 
           consequence ,
the average distance between two proteins is shorter in 2d than in 3d , and 
           this effect is more pronounced for higher protein concentrations ( ref .
this phenomenon is particularly important for amyloidogenic 
           proteins that tend to self - aggregate after a conformational transition ( fig .
2 ) . thus , the simple fact of the concentration of 
           monomers in a restricted membrane area is , by itself , a crucial step in the complex 
           process of amyloid oligomerisation / aggregation .
figure 2neuronal membranes as concentration platforms and chaperones for 
                   amyloidogenic monomers : key role of sphingolipid 
( 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 .
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 . the second mechanism explaining the affinity of amyloidogenic proteins for neuronal 
           membranes is lipid specificity .
converging data obtained from various laboratories with a 
           broad range of experimental approaches have shown that amyloidogenic proteins interact 
           with the lipids found in the lo phase  that is , sphingolipids and cholesterol 
             ( fig .
alzheimer a peptides 
           recognise several glycosphingolipids , including neutral species such as asialo - gm1 or 
           galactosylceramide ( galcer ) as well as gangliosides such as gm1 ( refs 26 , 27 , 
             28 , 29 ) ; -synuclein binds to gangliosides gm1 and gm3 ( refs 30 , 31 ) ; prp 
           interacts with sphingomyelin , galcer , gm1 and gm3 ( refs 27 , 32 , 33 ) and is associated with sphingolipid signalling platforms ( ref . 
            
overall , this high affinity for 
           sphingolipids determines the concentration of amyloid proteins in lipid raft areas of the 
           extracellular leaflet of the plasma membrane .
several amyloidogenic proteins can also bind 
           to negatively charged glycerophospholipids ( refs 35 , 36 ) , which are enriched in the 
           cytoplasmic leaflet of the plasma membrane ( ref .
this type of interaction has been extensively studied for 
           -synuclein , which recognises anionic phosphatidylserine and phosphatidylglycerol 
           ( refs 36 , 37 , 38 , 39 ) . in this review ,
we restrict our analysis to the role of cholesterol and 
           sphingolipids in the interaction of amyloidogenic proteins and membranes .
as we will see , 
           these lipids play a more active role in the conformation , oligomerisation and aggregation 
           of amyloidogenic proteins than just acting as a concentration platform .
that the same amyloidogenic protein could interact with several distinct sphingolipids 
           ( refs 26 , 27 , 31 ) could be viewed as paradoxical 
           and difficult to reconcile with a real specificity of interaction .
a biochemical analysis 
           of sphingolipid structures is necessary to get a clearer view on this issue .
sphingolipids 
           are formed by the condensation of a fatty acid chain with a sphingoid base ( which is 
           generally sphingosine although five different sphingoid bases have been detected in 
           mammalian cells ) .
the biochemical diversity of sphingolipids is first generated by the 
           numerous fatty acids ( more than 20 , varying in chain length , degree of saturation and 
           degree of hydroxylation ) that can be attached to the sphingoid base to form a ceramide 
           ( ref .
sphingolipids are then classified into 
           sphingomyelin and glycosphingolipids , according to the biochemical nature of the polar 
           part attached to ceramide : phosphorylcholine for sphingomyelin , and a glycan moiety for 
           glycosphingolipids .
several hundred different carbohydrate structures have been described 
           in glycosphingolipids , with various combinations of neutral sugars , sulfated sugars and 
           sialic acids ( ref .
this is 
           the case for the glycosphingolipids galcer , globotriaosylceramide ( gb3 ) and 
           gm3 , which have a distinct sugar moiety but a common galactosyl residue with the same 
           orientation ( fig .
c ) . as a logical 
           consequence of this common structural feature , a protein that recognises one of these 
           glycosphingolipids ( e.g. galcer ) , through binding to this galactose residue , might also 
           interact with the other two ( in this case , gb3 and gm3 ) .
this means that the 
           biochemical code governing glycosphingolipid  protein interactions is degenerate . 
          
an example of this striking property is the v3 loop domain of the human immunodeficiency 
           virus 1 ( hiv-1 ) surface - envelope glycoprotein gp120 , which was shown to recognise first 
           galcer ( refs 41 , 42 , 43 ) , and then , several 
           years later , both gb3 and gm3 ( refs 44 , 45 ) .
however , there is a unique 
           molecular mechanism that accounts for the interaction of each of these glycosphingolipids 
           with gp120 ( ref .
figure 3glycosphingolipids 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- 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  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  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 .
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- 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  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  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 .
let us first consider the way in which proteins bind to galcer , which is both the major 
           lipid of myelin sheaths ( ref .
47 ) and the 
           simplest possible brain - expressed glycosphingolipid , consisting of a ceramide linked to a 
           single galactosyl group ( i.e. a ceramide monohexoside ) .
most of the galcer molecule is 
           dipped into the membrane , so that the galactosyl moiety is the principal region of the 
           glycolipid available for protein binding . in this respect
, there is a functional 
           similarity between proteins that recognise galcer and galactose - specific lectins .
however , 
           the local concentration of galactosyl groups in a nanodomain of galcer is extremely high 
           compared with the maximal concentration of free galactose in biological fluids ( another 
           illustration of the reduction - in - dimensionality concept discussed above ) .
moreover , the 
           conformation of the galactose head group in the glycolipid is restricted by the ceramide 
           backbone , which allows only limited motion of the sugar ( ref .
by contrast , galactose in water solution is free to adopt a wide 
           range of conformations . finally ,
the most polar part of ceramide , which includes both 
           amide and hydroxyl ( oh ) groups , as well as the -glycosidic bond linking ceramide 
           to galactose , can also participate in the binding reaction and further stabilise the 
           galcer  protein complex . taken together , these biochemical properties of the 
           galactosyl head group of galcer
protein interactions are 
           distinct from galactose  lectin interactions ( ref . 49 ) . at the molecular level , numerous proteins that bind to galcer
have a solvent - exposed 
           aromatic residue that provides a complementary surface for a stacking interaction with the 
           galactose ring ( fig .
3d ) . as a result of the 
           distribution of the oh groups linked to the pyranosyl ring of galactose
, this sugar has 
           two chemically distinct faces : one apolar with hydrocarbyl ( ch ) groups and the other polar 
           with oh groups .
therefore , the apolar face of galactose is particularly suited for a 
           stacking interaction with an aromatic structure ( ref .
50 ) . this type of interaction between ch groups and a -electron cloud 
           system has been described and referred to as ch interaction by nishio 
           and co - workers ( ref .
the 
           ch stacking mechanism is involved in the interaction between 
           glycosphingolipids and several amyloidogenic proteins , including prp ( ref .
, the apolar side of the central galactosyl group has a similar 
           orientation to that in galcer ( fig . 
          
3a  c ) ; thus , galcer - binding proteins are predicted to also bind to 
             gb3 and gm3 through ch stacking .
note that the 
           conformation of the sugar moiety of glycosphingolipids is restricted by a network of 
           h - bonds ( ref .
52 ) that involves either an oh 
           group in the acyl chain of the ceramide ( intramolecular network ) or the oh group of 
           cholesterol ( intermolecular network , fig .
3e ) , as 
           further discussed below ( see the section  the outside and inside effects of 
           cholesterol ) .
the notion that unrelated proteins , displaying a solvent - accessible aromatic residue in a 
           hairpin domain , might bind these glycosphingolipids through a similar stacking mechanism 
           has motivated the search for v3-like domains in various proteins , including amyloidogenic 
           proteins ( ref .
this is the concept of the 
           glycosphingolipid - binding domain or , more generally , of the sphingolipid - binding domain 
           ( ref .
such domains are characterised by the 
           presence , in a flexible region of the surface of a protein , of an aromatic residue with 
           its side chain oriented towards the solvent , as illustrated in figure 4a with the superimposition of the glycosphingolipid - binding 
           domains of a and -synuclein ( ref .
the recognition of the glycosphingolipid by this region of the protein is 
           thought to proceed through an induced - fit mechanism involving the conformational mobility 
           of the aromatic side chain and a concomitant adjustment of the orientation of the 
           galactosyl head group ( fig .
( a ) 
                 amyloidogenic proteins share a common sphingolipid - binding domain , which can be 
                 shown by superimposing the structure of micelle - bound alzheimer a peptide 
                 [ pdb i d : 1ba4 , in red ] ( ref .
150 ) onto the 
                 structure of micelle - bound -synuclein [ pdb i d : 1xq8 , in blue ] ( ref . 142 ) .
the motif corresponds to a 
                 helix  turn  helix structure displaying an aromatic residue ( tyr10 
                 for a ; tyr39 for -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 a and glu46 for -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 -synuclein and val24 for a ) .
( 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 i d : 1kuw ] ( ref .
the -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 )
53 obtained with lactosylceramide 
                 ( laccer ) . 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 a peptide 
                 [ pdb i d : 1ba4 , in red ] ( ref .
150 ) onto the 
                 structure of micelle - bound -synuclein [ pdb i d : 1xq8 , in blue ] ( ref .
the motif corresponds to a 
                 helix  turn  helix structure displaying an aromatic residue ( tyr10 
                 for a ; tyr39 for -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 a and glu46 for -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 -synuclein and val24 for a ) .
( 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 i d : 1kuw ] ( ref .
68 ) . the -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 )
53 obtained with lactosylceramide 
                 ( laccer ) . in summary , a glycosphingolipid - binding domain is a flexible domain containing 
           turn - inducing ( gly , pro ) , basic ( arg , lys , his ) and aromatic residues ( phe , tyr , trp ) 
           ( refs 27 , 54 , 55 , 56 , 57 ) .
the critical involvement of 
           aromatic residues in the interaction between proteins and glycosphingolipids has been 
           convincingly demonstrated by using synthetic peptides in which these residues were 
           replaced by ala or leu ( refs 53 , 54 , 55 ) : 
           correspondingly , the interaction with the glycosphingolipid has been dramatically 
           decreased or even totally lost .
reciprocally , conservative aromatic substitutions ( e.g. 
           phe to trp ) did not affect binding to glycosphingolipids ( ref .
glycosphingolipid interactions driven by 
           ch stacking of a sugar and aromatic residue ( refs 52 , 58 ) .
one important consequence of protein  sphingolipid interactions is that the 
           conformation of the protein can be significantly affected on binding to sphingolipids 
             ( fig .
this chaperone effect of 
           sphingolipids is a direct consequence of the unique modalities of 
           protein  sphingolipid interactions ( ref .
as soon as binding begins , the structure of the sphingolipid changes and the 
           protein gradually adapts its shape to this moving target .
a stable anchoring of the 
           protein to the sphingolipids can be obtained by optimising protein 
the 
           flexibility of the sphingolipid - binding domain is particularly well suited to trigger 
           structural rearrangements in the protein ( ref .
. this unexpected consequence of protein  sphingolipid interactions 
           has been extensively studied with hiv-1 .
briefly , on binding to raft glycosphingolipids 
           through its v3 domain , hiv-1 gp120 undergoes a series of conformational changes that are 
           required for virus fusion ( refs 19 , 45 ) .
because the sphingolipid - binding domain is 
           evolutionarily conserved ( it has been found in virus , bacteria , insect and mammalian 
           proteins ) , it is likely that sphingolipids can modulate the conformation and regulate the 
           function of a wide range of proteins ( refs 52 , 
             53 , 54 , 55 , 56 , 57 ) .
how could this concept of a sphingolipid - mediated chaperone effect apply to 
           membrane - bound amyloids ?
seminal work by matsuzaki , yanagasiwa and co - workers has shown 
           that lipid rafts act as surface catalysts able to accelerate the aggregation of 
           a ( refs 29 , 59 , 60 , 61 , 62 , 
             63 ) .
most interestingly , gm1-bound a 
           has been identified as a pre - amyloid form acting as an endogenous seed for the formation 
           of neurotoxic amyloid fibrils in alzheimer disease ( refs 29 , 61 , 62 , 63 ) .
these data have not 
           only strengthened the role of lipid rafts as preferential binding sites for unstructured 
           amyloidogenic proteins on neuronal membranes but have also suggested that these membrane 
           domains behave as conformational catalysts for amyloid formation . in other words , lipid 
           rafts can control the conformation of membrane - bound amyloid through a chaperone effect 
           ( refs 49 , 59 , 64 ) . yet
, conflicting data have 
           suggested that binding of a to gm1 induces a conformational transition from 
           random coil to a protective -helical structure ( ref .
30 ) , or to a fibrillation - prone -structure 
           ( refs 61 , 66 ) .
the structure of amyloidogenic proteins bound to gm1 depends on several 
           physicochemical parameters , including ph , membrane fluidity , gm1-carrier lipid ratios , 
           protein concentration and the absence or presence of cholesterol , which could explain the 
           discrepancies reported in the literature . in any case
, the available data suggest that 
           lipid rafts could ( 1 ) modulate the conformational changes of unstructured monomers from 
           the unfolded state to more - ordered  or  structures , and ( 2 ) control 
           the balance between  and  structures , which in turn determine the 
           oligomerisation / aggregation status of amyloidogenic proteins .
59 ) have shown that the 
           oligomerisation and aggregation of a peptides occur in ganglioside - enriched 
           domains of the plasma membrane .
a possible sequence of events could be as follows : ( 1 ) 
           soluble a monomers bind to the sugar head group of gangliosides in 
           cholesterol  sphingolipid - enriched domains such as lipid rafts ; ( 2 ) on binding to 
           gangliosides , a is constrained to adopt an -helical structure ; ( 3 ) as 
           the protein density on the membrane increases , a undergoes a conformational 
           transition from an -helix - rich structure to a -strand - rich one ; and 
           ( 4 ) the ganglioside - bound a complex with acquired secondary structures serves as 
           a seed for amyloid fibril formation .
indeed , a fibrils with high toxicity have 
           been successfully generated in an acellular system using gm1-containing raft - like 
           liposomes ( ref .
67 ) . altogether , these data 
           strongly support the concept that the ganglioside cluster can act as a chaperone able to 
           lower the activation energy for the conformational changes of a ( ref .
this scenario is also consistent with the 
           involvement of the glycosphingolipid - binding domain in membrane 
unstructured amyloidogenic proteins can adopt a wide range of conformations in water and 
           in biological membranes ( fig .
the 
           conformational flexibility of the amyloid core peptide of amylin ( nfgailss ) has been 
           studied by nuclear magnetic resonance ( nmr ) ( ref .
, this peptide can adopt three 
           distinct classes of conformations owing to the wide flexibility of the peptide chain 
             ( fig .
4b ) . correspondingly , the unique phe 
           residue of the motif has three main possible orientations ( ref .
53 ) . on binding to a glycosphingolipid bearing an appropriately 
           oriented galactosyl group ( lactosylceramide in fig
. 
           4b ) , the phe residue stacks onto the sugar so that only one of the numerous peptide 
           conformations is selected and locked .
this illustrates the potency of ch stacking interactions to 
           stabilise a thermodynamically unstable conformation at a minimal energetic cost , just like 
           a wedge can efficiently block a car on a sloping street ( ref .
this simple but efficient mechanism could allow raft 
           glycosphingolipids to act as molecular chaperones inducing -helix - rich 
           structures in amyloidogenic proteins ( fig .
the stabilisation of an -helix structure is beneficial , because 
           this can prevent protein misfolding and aggregation ( ref .
49 ) . however , the -helix conformation could also facilitate 
           membrane insertion and channel / pore formation , leading to dramatic perturbations of 
           membrane permeability , as shown for -synuclein ( ref .
in addition , the chaperone effect of glycosphingolipids is not 
           irreversible . changing the physicochemical properties of the raft 
protein 
           complex , for example through oxidative damage to the proteins or lipids ( refs 70 , 71 ) , 
           could theoretically induce the dissociation of the amyloid peptides from 
           glycosphingolipids such as galcer .
then , the sudden exposure of aromatic side chains would 
           allow a tight packing ( through  interactions ) between 
           adjacent helices .
this would lead to the formation of a dimer , a key step in the 
           generation of amyloid fibrils ( refs 72 , 73 ) , which is thought to proceed through an ordered 
           polymerisation of  structures ( ref .
when bound to glycosphingolipids ( gsls ) through the 
                 sphingolipid - binding domain , the aromatic residues of two vicinal amyloidogenic 
                 proteins can not interact with each other and the protein is forced to adopt an 
                 -helix conformation .
the molecular mechanism of this interaction is the 
                 ch stacking between the galactosyl ring of galcer and the 
                 aromatic residue of the protein .
the -helix structure of amyloidogenic 
                 proteins is not only present in membrane - bound monomers ( as illustrated in fig .
2 , right panel ) , but also in oligomeric 
                 -synuclein channels ( fig .
the 
                 -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 -helix to a -strand structure .
amyloid 
                 aggregation then results from the -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 .
when bound to glycosphingolipids ( gsls ) through the 
                 sphingolipid - binding domain , the aromatic residues of two vicinal amyloidogenic 
                 proteins can not interact with each other and the protein is forced to adopt an 
                 -helix conformation .
the molecular mechanism of this interaction is the 
                 ch stacking between the galactosyl ring of galcer and the 
                 aromatic residue of the protein .
the -helix structure of amyloidogenic 
                 proteins is not only present in membrane - bound monomers ( as illustrated in fig .
2 , right panel ) , but also in oligomeric 
                 -synuclein channels ( fig .
the 
                 -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 -helix to a -strand structure .
amyloid 
                 aggregation then results from the -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 .
, we would like to underscore ( 1 ) the key role of aromatic residues 
           in the self - assembly of amyloid fibrils and ( 2 ) the chaperone effect of raft 
           glycosphingolipids that could induce otherwise thermodynamically unstable -helix 
           structures of amyloid fibril - forming proteins . indeed , most amyloidogenic proteins 
           ( including a and prp ) have an amino acid sequence that is expected to form 
            but not  structures ( ref .
-helix/-strand discordant. lipid rafts can 
           prevent those proteins from adopting the conformation they prefer , stabilising their 
           structure in a  forced  conformation .
this stabilisation 
           can occur in the early biosynthetic pathway as demonstrated for prp ( ref .
inhibition of sphingolipid biosynthesis 
           logically increases prp misfolding into disease - associated  structures ( ref . 
            
this mechanism seems to also apply for 
           several neurotransmitter receptors ( e.g. the nicotinic acetylcholine receptor ) whose 
           transport and assembly in the plasma membrane require the assistance of sphingolipids 
           ( ref .
mutations in amyloidogenic proteins 
           that are associated with inherited forms of neurodegenerative diseases have been shown to 
           directly affect sphingolipid binding .
this is the case for the 
           creutzfeldt  jakob - linked e200k mutation of prp , which affects 
           prp  sphingomyelin interactions ( ref .
, by providing a complementary surface for amyloidogenic proteins , cellular 
           glycosphingolipids might constitutively stabilise their  conformation and 
           inhibit their aggregation , as recently demonstrated for amylin ( ref .
protein 
            interactions ( refs 18 , 50 ) would then trigger amyloid growth 
           and deposition ( refs 49 , 79 ) .
to what extent glycosphingolipids could also stimulate 
           -strand - mediated amyloid aggregation ( ref .
the mutual 
           effect of gm1 and a on their respective conformations is likely to displace the 
           equilibrium of the reaction towards the formation of amyloid aggregates , as observed in 
           vitro ( ref .
finally , it should be noted that noncovalent interactions distinct from 
           ch stacking might also contribute to sphingolipid 
81 ) , h - bonds and electrostatic bridges ( especially for sulfated 
           glycosphingolipids , gangliosides and sphingomyelin ) ( ref . 82 ) .
indeed , in addition to aromatic side chains ( tyr10 ) , several 
           aliphatic ( ref .
81 ) and basic residues such as 
           his13 have been implicated in gm1a interactions ( ref .
anionic phospholipids such as 
           phosphatidylserine , which bind to -synuclein through electrostatic interactions 
           ( ref .
84 ) , could , in the cytoplasmic 
           leaflet of the plasma membrane , have a role equivalent to the one played by gangliosides 
           in the external leaflet .
high cholesterol has been identified as a major risk factor for both alzheimer ( ref . 
            
85 ) and parkinson ( ref . 86 ) diseases , and dysregulation of cholesterol homeostasis is a 
           hallmark of neurodegenerative diseases ( ref .
unfortunately , the effects of cholesterol on amyloid fibrillogenesis and toxicity are not 
           well understood and the results reported so far are controversial ( ref .
89 ) and stimulates its insertion into phospholipid 
           monolayers ( ref . 90 ) .
92 ) or 
           decreases ( refs 93 , 94 ) a polymerisation is still uncertain .
moreover , the 
           generation of a peptides through app proteolysis occurs within lipid rafts and 
           is sensitive to inhibitors of cholesterol biosynthesis ( ref .
95 ) , so that the involvement of cholesterol homeostasis in alzheimer 
           disease can not be simply ascribed to the regulation of a fibrillogenesis . in the case of parkinson disease , data obtained with both cultured neuronal cells and 
           transgenic mice have shown that the cholesterol - depleting agent 
           methyl--cyclodextrin decreased the level of -synuclein in membrane 
           fractions ( ref .
96 ) . moreover , metabolic 
           inhibition of cholesterol biosynthesis with statins reduced the levels of 
           -synuclein accumulation in neuronal membranes , whereas cholesterol 
           supplementation of cultured neurons increased -synuclein aggregation ( ref .
consistent with these studies , it was recently 
           reported that lovastatin treatment of -synuclein transgenic mice was associated 
           with a marked reduction of -synuclein aggregation and abrogation of neuronal 
           pathology ( ref .
this suggests that treatment 
           with cholesterol - lowering agents might be beneficial for patients with parkinson disease . 
          
it has been suggested that oxidised cholesterol metabolites could accelerate 
           -synuclein aggregation ( ref .
71 ) , which 
           provides a mechanistic link between oxidative stress and parkinson disease .
yet the 
           molecular mechanisms by which cholesterol and/or its metabolites could affect the 
           oligomerisation / aggregation status of amyloidogenic proteins have not been fully 
           deciphered .
first , cholesterol has a major impact on the conformation of sphingolipids . the apolar 
           part of sphingolipids ( i.e. the most important part of the ceramide ) interacts with the 
           smooth face of cholesterol , whereas the oh group of cholesterol is accessible to the polar 
           part of the sphingolipid ( ref .
this oh group is involved in an h - bond network that restricts the 
           conformation of the sugar moiety in a parallel orientation with respect to the membrane . 
          
this effect is particularly important when the ceramide contains a nonhydroxylated fatty 
           acid ( nfa , fig .
because this conformation of 
           the sphingolipid is particularly suited for a sphingolipid - binding domain , cholesterol 
           usually accelerates protein binding to sphingolipid ( ref .
by contrast , when the sphingolipid contains a 
           hydroxylated fatty acid ( hfa ) , the oh group of cholesterol is excluded from the h - bond 
           network and it can not exert its conformational effect on the sphingolipid . in this case , 
           cholesterol does not improve but rather perturbs the organisation of sphingolipids and can 
           even inhibit glycosphingolipid binding to amyloidogenic proteins ( ref .
thus , according to the distribution of hfas 
           versus nfas in sphingolipids , an increase of membrane cholesterol can lead to opposite 
           effects on sphingolipid - mediated amyloidogenic protein binding and aggregation . in any 
           case , these effects are due to a fine tuning of sphingolipid conformation induced by 
           cholesterol , and do not involve any kind of physical interaction between amyloidogenic 
           proteins and cholesterol .
by contrast , the second mechanism involves a direct interaction between the amyloidogenic 
           protein and cholesterol .
therefore , this effect requires the insertion of a part of the 
           amyloid protein in the membrane ; otherwise the protein would not be in physical contact 
           with cholesterol .
100 ) 
           have shown that a peptide aggregation on the bilayer surface requires a 
           sphingomyelin - rich environment but can occur in the absence of cholesterol .
this is in 
           line with our hypothesis that sphingolipids are fully responsible for the initial binding 
           of amyloidogenic proteins to lipid rafts , and that cholesterol is not directly involved at 
           this stage .
however , at the second step the presence of cholesterol may greatly facilitate 
           peptide insertion into the bilayer .
the preferential interaction of amyloidogenic proteins 
           with lipid raft domains ensures that cholesterol is indeed present underneath the 
           sphingolipids that have attracted the unstructured monomer ( fig .
once inserted in the membrane , the amyloidogenic protein 
           will immediately find cholesterol and interact with its free side  that is , the 
           face that is not in contact with sphingolipids ( for a detailed explanation of cholesterol 
           topology , see ref .
figure 6lipid - dependent formation of an -synuclein oligomeric 
                 channel . unfolded -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 -helix 
                 transition ) of -synuclein .
the newly formed -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 -synuclein .
unfolded -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 -helix 
                 transition ) of -synuclein .
the newly formed -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 -synuclein . by stimulating the oligomerisation of membrane - inserted amyloidogenic proteins , 
          
cholesterol could facilitate the formation of a pore - like assembly displaying ion channel 
           properties ( fig .
10 ) have hypothesised that amyloid pore / channels 
           provide the most direct pathway for inducing neurodegenerative effects , through loss of 
           ionic homeostasis increasing cell calcium to toxic levels .
the structure of amyloid pores , 
           which have been observed by atomic force microscopy ( refs 9 , 10 ) , remains to be experimentally 
           elucidated at the atomic level .
computational models have suggested that most amyloid 
           pores are probably formed by a complex assembly of -rich protofibrils or 
           oligomers ( ref .
130 ) , as shown in figure 1 . however , in the case of 
           -synuclein , tsigelny et al .
101 ) 
           have elegantly modelled a realistic oligomeric channel formed by the assembly of 
           -helical monomers .
the possible events leading to the formation of an 
           -synuclein oligomeric channel under the dual control of sphingolipids and 
           cholesterol are summarised in figure 6 .
there is a 
           striking similarity between -synuclein ( refs 69 , 31 ) and colicin e1 , a bacterial 
           toxin that also inserts into anionic areas of the plasma membrane , forming channel - like 
           structures consisting of an -helix bundle ( ref .
moreover , it has been hypothesised that amyloid pores might be 
           similar to -barrel pore - forming bacterial toxins ( ref .
103 ) . since cholesterol controls the oligomerisation and insertion 
           of these bacterial toxins ( ref .
104 ) in the 
           target membrane , it is likely that it is also required for the assembly of amyloid pores .
how could cholesterol control the oligomerisation process of membrane - inserted 
           amyloidogenic proteins and facilitate the formation of amyloid pores ?
the recent 
           high - resolution crystal structure of an engineered human adrenergic 2 receptor , 
           suggesting a molecular mechanism by which cholesterol mediates receptor dimerisation ( ref . 
            
on the basis of these structural data , one could hypothesise that 
           cholesterol binds to membrane - embedded fragments of amyloidogenic proteins , facilitates 
           their recruitment and coordinates their oligomerisation .
overall , this would mean that 
           amyloid pore formation and ion channel function are under the control of both 
           sphingolipids and cholesterol , in agreement with recent data obtained in model membranes 
           with -synuclein ( ref .
finally , it has been reported that 
           spherical amyloid oligomers or protofibrils can also increase membrane conductance without 
           forming ion channels or pores ( ref .
it is 
           not known if cholesterol and/or sphingolipids are involved in this nonspecific 
           permeabilising activity .
the involvement of gangliosides in several neurodegenerative diseases is not totally 
         surprising if one considers that these sphingolipids are critical for neuronal integrity and 
         plasticity ( ref .
since the probability of acquiring these 
         diseases gradually increases with age , it is important to evaluate how ganglioside 
         expression varies during the lifetime of individuals , and whether these diseases can alter 
         this process ( ref .
the 
         differentiation of pc12 cells into neuron - like cells caused a marked increase in both 
         gangliosides and cholesterol , and thereby greatly potentiated the accumulation and 
         cytotoxicity of a ( ref .
gm1 seems 
         to control the oligomerisation and aggregation process of a and 
         -synuclein ( refs 29 , 30 , 31 ) .
gm3 
         has been shown to control -synuclein channel formation and to correct the 
         channelopathy induced in planar lipid membranes by the parkinson - disease - linked e46k mutant 
         of the protein ( ref .
gm3 
         is a minor ganglioside , which , in marked contrast with the major ganglioside species gm1 and 
         gd1a , shows a regular increase with age ( ref .
the expression of gm3 is highly regulated during brain development ( ref . 113 ) , and this ganglioside has been implicated in the 
         regulation of neuronal cell death ( ref .
114 ) . 
         moreover , the homozygous loss - of - function mutation of gm3 synthase , which totally suppresses 
         the expression of gm3 and all gm3-derived gangliosides , has been linked to an infantile 
         symptomatic epilepsy syndrome ( ref .
these data indicate that ganglioside gm3 probably plays a more important role in 
         brain physiology and pathology than its low expression levels in adult brain could have 
         suggested in the past .
several studies support the view that deregulation of lipid metabolism is an important 
         feature of neurodegenerative diseases .
the three main lipid categories  
         glycerophospholipids , sphingolipids and cholesterol  are affected ( refs 116 , 117 , 
           118 , 119 , 120 , 121 ) .
a large body of data has established a link between cholesterol 
         homeostasis , apolipoprotein e polymorphism and app processing ( ref .
correspondingly , cholesterol - lowering strategies with statins 
         have acquired potential therapeutic importance in treating alzheimer disease ( ref .
statins have also proved to be efficient in 
         decreasing -synuclein aggregation and neuronal toxicity in animal models of 
         parkinson disease ( ref .
specific changes in 
         ganglioside content have been detected in the brains of patients with alzheimer disease 
         ( refs 116 , 125 ) : a decrease or even loss of the major gangliosides gm1 , gd1a , gd1b and gt1b , 
         and an increase in gm2 , gm3 and gd3 .
similar alterations of ganglioside expression were 
         observed in the brains of transgenic mouse models of alzheimer disease ( ref .
moreover , the balance between nfa and hfa ceramides is altered in animal models ( ref .
interestingly , the authors of the latter study 
         showed that there is a gender - dependent accumulation of ceramides in the cerebral cortex : 
         female mice exhibited a strong increase in hfa species , and males in nfa species .
this 
         observation could be linked to the increased risk of women versus men for developing 
         alzheimer disease ( ref .
129 ) . because cholesterol 
         can exert distinct effects on amembrane interactions according to the 
         nfa : hfa content of brain sphingolipids ( ref .
52 ) , 
         it will be interesting to determine how cholesterol impacts on 
         aglycosphingolipid interaction in men and women suffering from 
         alzheimer disease .
how amyloidogenic proteins kill neurons is still a mystery . in particular , as recently 
         discussed by butterfield and lashuel ( ref .
130 ) , 
          there remains a knowledge gap regarding the molecular - level details by which 
         amyloid - forming proteins act on the membrane and induce membrane 
         permeabilization. deciphering the complex interplay between amyloidogenic 
         proteins and membrane lipids , especially sphingolipids and cholesterol , will certainly help 
         to achieve a clearer view on this enigma .
several milestones have already been reached , 
         which have inspired an important part of today 's research efforts worldwide . despite their 
         lack of amino acid sequence homology ,
amyloidogenic proteins share a number of intriguing 
         properties : ( 1 ) an important conformational plasticity , allowing the same protein to remain 
         unordered or to form highly ordered  and  structures ( refs 4 , 5 ) ; ( 2 ) 
         self - oligomerising capacities that revealed unexpected common antigenic properties ( ref . 
          
7 ) ; ( 3 ) the ability to form pore - like structures 
         with ion channel properties ( refs 9 , 10 ) ; ( 4 ) self - aggregating activity leading to 
         fibrillation and plaque deposition ( ref .
131 ) ; and 
         ( 5 ) common structural motifs allowing specific interactions with sphingolipids ( ref .
49 ) and cholesterol ( yet uncharacterised ) in lipid raft 
         domains of the plasma membrane .
it is intriguing that hiv-1 , which interacts with 
         glycosphingolipids through the v3 domain of its surface glycoprotein gp120 , can induce major 
         neurological disabilities , identified as hiv-1-associated dementia ( ref .
110 ) . in this respect , it is also worth noting that 
         the protein cln3 , involved in batten disease ( the juvenile form of neuronal ceroid 
         lipofuscinosis ) , also binds to galcer through a v3-like sphingolipid - binding domain ( ref . 
          
this neural disease is caused by a 
         mutation in this domain , namely e295k ( ref .
133 ) , 
         which is analogous to mutations e22k in a , e46k in -synuclein and e200k 
         in prp , all associated with inherited forms of neurodegenerative disease and located in the 
         sphingolipid - binding domains of these proteins . at least for two of these proteins ( prp and 
         -synuclein ) , the substitution of an anionic glutamic acid side chain by a cationic 
         lysine resulted in altered binding to membrane sphingolipids ( refs 27 , 31 ) .
this further 
         emphasises the key role of the sphingolipid - binding domain in neuronal diseases .
future studies will be necessary to better evaluate the neurotoxicity of the various forms 
         of amyloidogenic proteins , including monomers , oligomers and aggregates , and to understand 
         how these different species cooperate to accelerate ( or slow down ) the onset of 
         neurodegenerative symptoms . this will help to decide on a rational basis which therapeutic 
         strategy should be used at the different stages of the diseases .
this will be possible with 
         the finding of nontoxic drugs specifically affecting amyloid channels ( ref .
135 ) . combining molecular ( refs 31 , 52 ) , biophysical ( refs 
           14 , 31 , 
           69 , 136 , 137 ) , cellular ( refs 93 , 114 , 
           138 ) and animal ( refs 126 , 128 , 139 ) studies will allow a better characterisation of 
         the sphingolipids that regulate amyloid oligomerisation / aggregation .
biophysical properties 
         of the membrane such as membrane curvature should be taken into consideration ( ref .
130 ) . indeed , depending on the surface curvature of 
         the model membrane ,
interestingly , gm1 and gm3 are 
         concentrated in membrane areas that markedly differ in membrane curvature ( ref .
143 ) , so that on binding to distinct gangliosides , the 
         same amyloid protein could adopt distinct conformations
. this could be the case for 
         -synuclein , which recognises both gm1 ( ref .
antiganglioside 
         antibodies could interfere with normal ganglioside function and could thus play a role in 
         disease pathogenesis , as anti - gm1 antibodies probably do in some parkinson patients ( refs 
           110 , 144 ) .
careful determinations of the titres of these antibodies in various biological 
         fluids , including cerebrospinal fluid , will be particularly informative
. beneficial effects 
         of ganglioside supplementation have been reported in animal models of parkinson disease 
         ( ref .
enzymes involved in glycosphingolipid metabolism might represent targets that inhibit both 
         the production and amyloid aggregation of alzheimer a peptides ( ref .
correspondingly , lipid raft disruption has been 
         shown to protect neurons against amyloid oligomer toxicity in vitro ( ref .
we need to better understand how cholesterol 
         interacts with amyloidogenic proteins , regulates their supramolecular structures and is 
         involved in the pathophysiology of neurodegenerative diseases ( refs 86 , 87 ) .
we should also 
         determine how mutations of amyloidogenic proteins affect their interaction with neural 
         membranes ( refs 27 , 69 , 148 ) .
a decisive 
         breakthrough will be to understand why neurodegenerative diseases involve specific brain 
         areas , and how local lipid composition may account for such geographic restrictions ( refs 
           127 , 135 ) .
finally , we have to identify the environmental molecules ( such as food 
         contaminants , pesticides and mycotoxins ) that could modulate amyloid formation through 
         direct binding to amyloid proteins and could represent important risk factors for 
         neurodegenerative diseases ( ref .
 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 -sheet structures ( annular pores , amyloid fibrils ) or in 
           -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  or  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 . 
Mechanisms of concentration of amyloidogenic proteins on neuronal membranes
Molecular mechanisms of sphingolipidamyloidogenic-protein interactions
Glycosphingolipids exert a chaperone effect on amyloidogenic proteins
The outside and inside effects of cholesterol
Changes in lipid content of the brain: what impact on neurodegenerative diseases?
Perspectives