Membrane disruption by Alzheimer beta-amyloid peptides mediated through specific binding to either phospholipids or gangliosides. Implications for neurotoxicity

Amyloid beta-protein interactions with membranes and cholesterol: causes or casualties of Alzheimer's disease

Amyloid beta-protein (Abeta) is thought to be one of the primary factors causing neurodegeneration in Alzheimer's disease (AD). This protein is an amphipathic molecule that perturbs membranes, binds lipids and alters cell function. Several studies have reported that Abeta alters membrane fluidity but the direction of this effect has not been consistently observed and explanations for this lack of consistency are proposed. Cholesterol is a key component of membranes and cholesterol interacts with Abeta in a reciprocal manner. Abeta impacts on cholesterol homeostasis and modification of cholesterol levels alters Abeta expression. In addition, certain cholesterol lowering drugs (statins) appear to reduce the risk of AD in human subjects. However, the role of changes in the total amount of brain cholesterol in AD and the mechanisms of action of statins in lowering the risk of AD are unclear. Here we discuss data on membranes, cholesterol, Abeta and AD, and propose that modification of the transbilayer distribution of cholesterol in contrast to a change in the total amount of cholesterol provides a cooperative environment for Abeta synthesis and accumulation in membranes leading to cell dysfunction including disruption in cholesterol homeostasis.

Metal ions, pH, and cholesterol regulate the interactions of Alzheimer's disease amyloid-beta peptide with membrane lipid

The interaction of A beta peptides with the lipid matrix of neuronal cell membranes plays an important role in the pathogenesis of Alzheimer's disease. By using EPR and CD spectroscopy, we found that in the presence of Cu(2+) or Zn(2+), pH, cholesterol, and the length of the peptide chain influenced the interaction of these peptides with lipid bilayers. In the presence of Zn(2+), A beta 40 and A beta 42 both inserted into the bilayer over the pH range 5.5-7.5, as did A beta 42 in the presence of Cu(2+). However, A beta 40 only penetrated the lipid bilayer in the presence of Cu(2+) at pH 5.5-6.5; at higher pH there was a change in the Cu(2+) coordination sphere that inhibited membrane insertion. In the absence of the metals, insertion of both peptides only occurred at pH < 5.5. Raising cholesterol to 0.2 mol fraction of the total lipid inhibited insertion of both peptides under all conditions investigated. Membrane insertion was accompanied by the formation of alpha-helical structures. The nature of these structures was the same irrespective of the conditions used, indicating a single low energy structure for A beta in membranes. Peptides that did not insert into the membrane formed beta-sheet structures on the surface of the lipid.

Piracetam inhibits the lipid-destabilising effect of the amyloid peptide Abeta C-terminal fragment

Amyloid peptide (Abeta) is a 40/42-residue proteolytic fragment of a precursor protein (APP), implicated in the pathogenesis of Alzheimer's disease. The hypothesis that interactions between Abeta aggregates and neuronal membranes play an important role in toxicity has gained some acceptance. Previously, we showed that the C-terminal domain (e.g. amino acids 29-42) of Abeta induces membrane permeabilisation and fusion, an effect which is related to the appearance of non-bilayer structures. Conformational studies showed that this peptide has properties similar to those of the fusion peptide of viral proteins i.e. a tilted penetration into membranes. Since piracetam interacts with lipids and has beneficial effects on several symptoms of Alzheimer's disease, we investigated in model membranes the ability of piracetam to hinder the destabilising effect of the Abeta 29-42 peptide. Using fluorescence studies and 31P and 2H NMR spectroscopy, we have shown that piracetam was able to significantly decrease the fusogenic and destabilising effect of Abeta 29-42, in a concentration-dependent manner. While the peptide induced lipid disorganisation and subsequent negative curvature at the membrane-water interface, the conformational analysis showed that piracetam, when preincubated with lipids, coats the phospholipid headgroups. Calculations suggest that this prevents appearance of the peptide-induced curvature. In addition, insertion of molecules with an inverted cone shape, like piracetam, into the outer membrane leaflet should make the formation of such structures energetically less favourable and therefore decrease the likelihood of membrane fusion.

Membrane destabilization induced by beta-amyloid peptide 29-42: importance of the amino-terminus

Increasing evidence implicates interactions between Abeta-peptides and membrane lipids in Alzheimer's disease. To gain insight into the potential role of the free amino group of the N-terminus of Abeta29-42 fragment in these processes, we have investigated the ability of Abeta29-42 unprotected and Abeta29-42 N-protected to interact with negatively-charged liposomes and have calculated the interaction with membrane lipids by conformational analysis. Using vesicles mimicking the composition of neuronal membranes, we show that both peptides have a similar capacity to induce membrane fusion and permeabilization. The fusogenic effect is related to the appearance of non-bilayer structures where isotropic motions occur as shown by 31P and 2H NMR studies. The molecular modeling calculations confirm the experimental observations and suggest that lipid destabilization could be due to the ability of both peptides to adopt metastable positions in the presence of lipids. In conclusion, the presence of a free or protected (acetylated) amino group in the N-terminus of Abeta29-42 is therefore probably not crucial for destabilizing properties of the C-terminal fragment of Abeta peptides.

Beta-amyloid 25 to 35 is intercalated in anionic and zwitterionic lipid membranes to different extents

Neuronal plasma membranes are thought to be the primary target of the neurotoxic beta-amyloid peptides (Abeta) in the pathogenesis of the Alzheimer's disease. Histologically, Abeta peptides are observed as extracellular macroscopic senile plaques, and most biophysical techniques have indicated the presence of Abeta close to the lipid headgroup region but not in the core of the membrane bilayers. The focus of this study is an investigation of the interaction between Abeta and lipid bilayers from a structural point of view. Neutron diffraction with the use of selectively deuterated amino acids has allowed us to determine unambiguously the position of the neurotoxic fragment Abeta (25-35) in the membrane. Two populations of the peptide are detected, one in the aqueous vicinity of the membrane surface and the second inside the hydrophobic core of the lipid membrane. The location of the C terminus was studied in two different lipid compositions and was found to be dependent on the surface charge of the membrane. The localization of beta-amyloid peptides in cell membranes will offer new insights on their mechanism in the neurodegenerative process associated with Alzheimer's disease and might provide clues for therapeutic developments.

Amyloid beta-protein affects cholesterol metabolism in cultured neurons: implications for pivotal role of cholesterol in the amyloid cascade

Recently, we have found that alterations in cellular cholesterol metabolism are involved in promotion of tau phosphorylation (Fan et al. [2001] J. Neurochem. 76: 391-400; Sawamura et al. [2001] J. Biol. Chem. 276:10314-10319). In addition, we have shown that amyloid beta-protein (A beta) promotes cholesterol release to form A beta-lipid particles (Michikawa et al. [2001] J. Neurosci. 21:7226-7235). These lines of evidence inspired us to conduct further studies on whether A beta affects cholesterol metabolism in neurons, which might lead to tau phosphorylation. Here, we report the effect of A beta1-40 on cholesterol metabolism in cultured neurons prepared from rat cerebral cortex. Oligomeric A beta1-40 inhibited cholesterol synthesis and reduced cellular cholesterol levels in a dose- and time-dependent manner, while freshly dissolved A beta had no effect on cholesterol metabolism. However, oligomeric A beta had no effect on the proteolysis of sterol regulatory element binding protein-2 (SREBP-2) or protein synthesis in cultured neurons. Oligomeric A beta did not enhance lactate dehydrogenase (LDH) release from neuronal cells or decrease signals in the cultures reactive to 3,3'-Bis[N,N-bis(carboxymethyl)aminomethyl]fluorescein, hexaacetoxymethyl ester (calcein AM) staining, indicating that A beta used in this experiment did not cause neuronal death during the time course of our experiments. Since alterations in cholesterol metabolism induce tau phosphorylation, our findings that oligomeric A beta alters cellular cholesterol homeostasis may provide new insight into the mechanism underlying the amyloid cascade hypothesis.

Annexin 5 and apolipoprotein E2 protect against Alzheimer's amyloid-beta-peptide cytotoxicity by competitive inhibition at a common phosphatidylserine interaction site

Amyloid-beta-protein (betaA/4, AbetaP) accumulates in the brains of patients with Alzheimer's disease (AD), regardless of genetic etiology, and is thought to be the toxic principle responsible for neuronal cell death. The varepsilon4 allele of apoE has been linked closely to earlier onset of AD and increased deposition of the amyloid peptide, regardless of the clinical status of AD, while the apoE varepsilon2 allele is generally protective. We have previously hypothesized that the cell target for amyloid peptide might be the apoptotic signal molecule phosphatidylserine (PS). We report here that annexin 5, a specific ligand for PS, not only blocks amyloid peptide AbetaP[1-40] cytotoxicity, but competitively inhibits AbetaP[1-40]-dependent aggregation of PS liposomes. In addition, we find that apoE2, but not apoE4, can not only perform the same protective effect on cells exposed to AbetaP[1-40], but can also competitively inhibit PS liposome aggregation and fusion by the amyloid peptide. Altogether, the in vivo and in vitro results reported here provide fundamental insight to the process by which amyloid targets specific neurons for destruction, and suggest that PS may be a surface "receptor" site for AbetaP binding. These results also provide a biochemical mechanism by which the apoE varepsilon2 allele, but not apoE varepsilon4, can be protective towards the incidence and progression of Alzheimer's disease.

Interactions of amyloid beta-protein with various gangliosides in raft-like membranes: importance of GM1 ganglioside-bound form as an endogenous seed for Alzheimer amyloid

GM1 ganglioside-bound amyloid beta-protein (GM1-Abeta), found in brains exhibiting early pathological changes of Alzheimer's disease (AD) plaques, has been suggested to accelerate amyloid fibril formation by acting as a seed. We have previously found using dye-labeled Abeta that Abeta recognizes a GM1 cluster, the formation of which is facilitated by cholesterol [Kakio, A., Nishimoto, S., Yanagisawa, K., Kozutsumi, Y., and Matsuzaki, K. (2001) J. Biol. Chem. 276, 24985-24990]. In this study, we investigated the ganglioside species-specificity in its potency to induce a conformational change of Abeta, by which ganglioside-bound Abeta acts as a seed for Abeta fibrillogenesis, using a major ganglioside occurring in brains (GM1, GD1a, GD1b, and GT1b) in raft-like membranes composed of cholesterol and sphingomyelin. Abeta recognized ganglioside clusters, the density of which increased with the number of sialic acid residues. Interestingly, however, mixing of gangliosides inhibited cluster formation. In contrast, the affinities of the protein for the clusters were similar irrespective of lipid composition and of the order of 10(6) M(-)(1) at 37 degrees C. Abeta underwent a conformational transition from an alpha-helix-rich structure to a beta-sheet-rich structure with the increase in protein density on the membrane. Ganglioside-bound Abeta proteins exhibited seeding abilities for amyloid formation. GM1-Abeta exhibited the strongest seeding potential, especially under beta-sheet-forming conditions. This study suggested that lipid composition including gangliosides and cholesterol strictly controls amyloid formation.

Physiopathological modulators of amyloid aggregation and novel pharmacological approaches in Alzheimer's disease

The biological mechanisms underlying the neuropathology of Alzheimer's disease (AD) are complex, as several factors likely contribute to the development of the disease. Therefore, it is not surprising that a number of different possible therapeutic approaches addressing distinct aspects of this disease are currently being investigated. Among these are ways to prevent amyloid aggregation and/or deposition, to prevent neuronal degeneration, and to increase brain neurotransmitter levels. Here, we discuss possible roles of endogenous modulators of Abeta aggregation in the physiopathology of AD and some of the strategies currently under consideration to interfere with brain levels of beta-amyloid, its aggregation and neurotoxicity.

Abeta42-peptide assembly on lipid bilayers

One of the major pathological features of Alzheimer's disease (AD) is the presence of extracellular amyloid plaques that are composed predominantly of the amyloid-beta peptide (Abeta). Diffuse plaques associated with AD are composed predominantly of Abeta42, whereas senile plaques contain both Abeta40 and Abeta42. Recently, it has been suggested that diffuse plaque formation is initiated as a plasma membrane-bound Abeta species and that Abeta42 is the critical component. In order to investigate this hypothesis, we have examined Abeta42-membrane interactions using in situ atomic force microscopy and fluorescence spectroscopy. Our studies demonstrate the association of Abeta42 with planar bilayers composed of total brain lipids, which results initially in peptide aggregation and then fibre formation. Modulation of the cholesterol content is correlated with the extent of Abeta42-assembly on the bilayer surface. Although Abeta42 was not visualized directly on cholesterol-depleted bilayers, fluorescence anisotropy and fluorimetry demonstrate Abeta42-induced membrane changes. Our results demonstrate that the composition of the lipid bilayer governs the outcome of Abeta interactions.

Role of p75 neurotrophin receptor in the neurotoxicity by beta-amyloid peptides and synergistic effect of inflammatory cytokines

The neurodegenerative changes in Alzheimer's disease (AD) are elicited by the accumulation of beta-amyloid peptides (Abeta), which damage neurons either directly by interacting with components of the cell surface to trigger cell death signaling or indirectly by activating astrocytes and microglia to produce inflammatory mediators. It has been recently proposed that the p75 neurotrophin receptor (p75(NTR)) is responsible for neuronal damage by interacting with Abeta. By using neuroblastoma cell clones lacking the expression of all neurotrophin receptors or engineered to express full-length or various truncated forms of p75(NTR), we could show that p75(NTR) is involved in the direct signaling of cell death by Abeta via the function of its death domain. This signaling leads to the activation of caspases-8 and -3, the production of reactive oxygen intermediates and the induction of an oxidative stress. We also found that the direct and indirect (inflammatory) mechanisms of neuronal damage by Abeta could act synergistically. In fact, TNF-alpha and IL-1beta, cytokines produced by Abeta-activated microglia, could potentiate the neurotoxic action of Abeta mediated by p75(NTR) signaling. Together, our results indicate that neurons expressing p75(NTR), mostly if expressing also proinflammatory cytokine receptors, might be preferential targets of the cytotoxic action of Abeta in AD.

The neuroprotective activities of melatonin against the Alzheimer beta-protein are not mediated by melatonin membrane receptors

Exposure of neuronal cells to the Alzheimer's amyloid beta protein (Abeta) results in extensive oxidative damage of bio-molecules that are profoundly harmful to neuronal homeostasis. It has been demonstrated that melatonin protects neurons against Abeta-mediated neurotoxicity, including cell death and a spectrum of oxidative lesions. We undertook the current study to determine whether melatonin membrane receptors are involved in the mechanism of neuroprotection against Abeta neurotoxicity. For this purpose, we characterized the free-radical scavenging potency of several compounds exhibiting various affinities for melatonin membrane receptors (MLT 1a and 1b). Abeta-mediated neurotoxicity was assessed in human neuroblastoma cells and in primary hippocampal neurons. In sharp contrast with melatonin, no neuroprotection against Abeta toxicity was observed when we used melatonin membrane receptor agonists that were devoid of antioxidant activity. In contrast, the cells were fully protected in parallel control experiments when either melatonin, or the structurally unrelated free-radical scavenger phenyl-N-t-butyl nitrone (PBN), were added to Abeta-containing culture media. This study demonstrates that the neuroprotective properties of melatonin against Abeta-mediated toxicity does not require binding of melatonin to a membrane receptor and is likely the result of the antioxidant and antiamyloidogenic features of the agent.

Cholesterol is an important factor affecting the membrane insertion of beta-amyloid peptide (A beta 1-40), which may potentially inhibit the fibril formation

beta-Amyloid peptide (A beta), a normal constituent of neuronal and non-neuronal cells, has been proven to be the major component of extracellular plaque of Alzheimer's disease. Interactions between A beta and neuronal membranes have been postulated to play an important role in the neuropathology of Alzheimer's disease. Here we show that A beta is able to insert into lipid bilayer. The membrane insertion ability of A beta is critically controlled by the ratio of cholesterol to phospholipids. In a low concentration of cholesterol A beta prefers to stay in membrane surface region mainly in a beta-sheet structure. In contrast, as the ratio of cholesterol to phospholipids rises above 30 mol%, A beta can insert spontaneously into lipid bilayer by its C terminus. During membrane insertion A beta generates about 60% alpha-helix and removes almost all beta-sheet structure. Fibril formation experiments show that such membrane insertion can reduce fibril formation. Our findings reveal a possible pathway by which A beta prevents itself from aggregation and fibril formation by membrane insertion.

Reduction in cholesterol and sialic acid content protects cells from the toxic effects of beta-amyloid peptides

beta-Amyloid (Abeta) is the primary protein component of senile plaques associated with Alzheimer's disease and has been implicated in the neurotoxicity associated with the disease. A variety of evidence points to the importance of Abeta-membrane interactions in the mechanism of Abeta neurotoxicity and indicates that cholesterol and gangliosides are particularly important for Abeta aggregation and binding to membranes. We investigated the effects of cholesterol and sialic acid depletion on Abeta-induced GTPase activity in cells, a step implicated in the mechanism of Abeta toxicity, and Abeta-induced cell toxicity. Cholesterol reduction and depletion of membrane-associated sialic acid residues both significantly reduced the Abeta-induced GTPase activity. In addition, cholesterol and membrane-associated sialic acid residue depletion or inhibition of cholesterol and ganglioside synthesis protected PC12 cells from Abeta-induced toxicity. These results indicate the importance of Abeta-membrane interactions in the mechanism of Abeta toxicity. In addition, these results suggest that control of cellular cholesterol and/or ganglioside content may prove useful in the prevention or treatment of Alzheimer's disease.

A novel action of alzheimer's amyloid beta-protein (Abeta): oligomeric Abeta promotes lipid release

Interactions between amyloid beta-protein (Abeta) and lipids have been suggested to play important roles in the pathogenesis of Alzheimer's disease. However, the molecular mechanism underlying these interactions has not been fully understood. We examined the effect of Abeta on lipid metabolism in cultured neurons and astrocytes and found that oligomeric Abeta, but not monomeric or fibrillar Abeta, promoted lipid release from both types of cells in a dose- and time-dependent manner. The main components of lipids released after the addition of Abeta were cholesterol, phospholipids, and monosialoganglioside (GM1). Density-gradient and electron microscopic analyses of the conditioned media demonstrated that these Abeta and lipids formed particles and were recovered from the fractions at densities of approximately 1.08-1.18 g/ml, which were similar to those of high-density lipoprotein (HDL) generated by apolipoproteins. The lipid release mediated by Abeta was abolished by concomitant treatment with Congo red and the PKC inhibitor, H7, whereas it was not inhibited with N-acetyl-l-cysteine. These Abeta-lipid particles were not internalized into neurons, whereas HDL-like particles produced by apolipoprotein E were internalized. Our findings indicate that oligomeric Abeta promotes lipid release from neuronal membrane, which may lead to the disruption of neuronal lipid homeostasis and the loss of neuronal function.

Cellular membrane composition defines A beta-lipid interactions

Alzheimer's disease pathology has demonstrated amyloid plaque formation associated with plasma membranes and the presence of intracellular amyloid-beta (A beta) accumulation in specific vesicular compartments. This suggests that lipid composition in different compartments may play a role in A beta aggregation. To test this hypothesis, we have isolated cellular membranes from human brain to evaluate A beta 40/42-lipid interactions. Plasma, endosomal, lysosomal, and Golgi membranes were isolated using sucrose gradients. Electron microscopy demonstrated that A beta fibrillogenesis is accelerated in the presence of plasma and endosomal and lysosomal membranes with plasma membranes inducing an enhanced surface organization. Alternatively, interaction of A beta with Golgi membranes fails to progress to fibril formation, suggesting that A beta-Golgi head group interaction stabilizes A beta. Fluorescence spectroscopy using the environment-sensitive probes 1,6-diphenyl-1,3,5-hexatriene, laurdan, N-epsilon-dansyl-L-lysine, and merocyanine 540 demonstrated variations in the inherent lipid properties at the level of the fatty acyl chains, glycerol backbone, and head groups, respectively. Addition of A beta 40/42 to the plasma and endosomal and lysosomal membranes decreases the fluidity not only of the fatty acyl chains but also the head group space, consistent with A beta insertion into the bilayer. In contrast, the Golgi bilayer fluidity is increased by A beta 40/42 binding which appears to result from lipid head group interactions and the production of interfacial packing defects.

Computational study of lipid-destabilizing protein fragments: towards a comprehensive view of tilted peptides

Tilted peptides are short sequence fragments (10-20 residues long) that possess an asymmetric hydrophobicity gradient along their sequence when they are helical. Due to this gradient, they adopt a tilted orientation towards a single lipid/water interface and destabilize the lipids. We have detected those peptides in many different proteins with various functions. While being all tilted-oriented at a single lipid/water interface, no consensus sequence can be evidenced. In order to better understand the relationships between their lipid-destabilizing activity and their properties, we used IMPALA to classify the tilted peptides. This method allows the study of interactions between a peptide and a modeled lipid bilayer using simple restraint functions designed to mimic some of the membrane properties. We predict that tilted peptides have access to a wide conformational space in membranes, in contrast to transmembrane and amphipathic helices. In agreement with previous studies, we suggest that those metastable configurations could lead to the perturbation of the acyl chains organization and could be a general mechanism for lipid destabilization. Our results further suggest that tilted peptides fall into two classes: those from proteins acting on membrane behave differently than destabilizing fragments from interfacial proteins. While the former have equal access to the two layers of the membrane, the latter are confined within a single lipid layer. This could be in relation with the organization of lipid substrate on which the peptides physiologically act.

Profile of changes in lipid bilayer structure caused by beta-amyloid peptide

beta-Amyloid peptide (A beta) is the primary constituent of senile plaques, a defining feature of Alzheimer's disease. Aggregated A beta is toxic to neurons, but the mechanism of toxicity is uncertain. One hypothesis is that interactions between A beta aggregates and cell membranes mediate A beta toxicity. Previously, we described a positive correlation between the A beta aggregation state and surface hydrophobicity, and the ability of the peptide to decrease fluidity in the center of the membrane bilayer [Kremer, J. J., et al. (2000) Biochemistry 39, 10309--10318]. In this work, we report that A beta aggregates increased the steady-state anisotropy of 1,6-diphenyl-1,3,5-hexatriene (DPH) embedded in the hydrophobic center of the membrane in phospholipids with anionic, cationic, and zwitterionic headgroups, suggesting that specific charge--charge interactions are not required for A beta--membrane interactions. A beta did not affect the fluorescence lifetime of DPH, indicating that the increase in anisotropy is due to increased ordering of the phospholipid acyl chains rather than changes in water penetration into the bilayer interior. A beta aggregates affected membrane fluidity above, but not below, the lipid phase-transition temperature and did not alter the temperature or enthalpy of the phospholipid phase transition. A beta induced little to no change in membrane structure or water penetration near the bilayer surface. Overall, these results suggest that exposed hydrophobic patches on the A beta aggregates interact with the hydrophobic core of the lipid bilayer, leading to a reduction in membrane fluidity. Decreases in membrane fluidity could hamper functioning of cell surface receptors and ion channel proteins; such decreases have been associated with cellular toxicity.

Cholesterol-dependent formation of GM1 ganglioside-bound amyloid beta-protein, an endogenous seed for Alzheimer amyloid

GM1 ganglioside-bound amyloid beta-protein (GM1/Abeta), found in brains exhibiting early pathological changes of Alzheimer's disease (AD) including diffuse plaques, has been suggested to be involved in the initiation of amyloid fibril formation in vivo by acting as a seed. To elucidate the molecular mechanism underlying GM1/Abeta formation, the effects of lipid composition on the binding of Abeta to GM1-containing lipid bilayers were examined in detail using fluorescent dye-labeled human Abeta-(1-40). Increases in not only GM1 but also cholesterol contents in the lipid bilayers facilitated the binding of Abeta to the membranes by altering the binding capacity but not the binding affinity. An increase in membrane-bound Abeta concentration triggered its conformational transition from helix-rich to beta-sheet-rich structures. Excimer formation of fluorescent dye-labeled GM1 suggested that Abeta recognizes a GM1 "cluster" in membranes, the formation of which is facilitated by cholesterol. The results of the present study strongly suggested that increases in intramembrane cholesterol content, which are likely to occur during aging, appear to be a risk factor for amyloid fibril formation.

Neuropeptides interact with glycolipid receptors: a surface plasmon resonance study

Using Surface Plasmon Resonance (SPR) we investigated the interaction of seven neuropeptides with different characteristics and beta-amyloid (Abeta42) peptide, with membranes containing gangliosides. A wide range of affinities characterized the bindings (K(D) = 10(-3)- 10(-7) M), following the scheme: for GM1, Abeta42 > DYN > SP = GAL = SOM = BRD > OXY = ENK; for GD1a, Abeta42 = DYN = GAL > SP = SOM = BRD = OXY > ENK and for GT1b, Abeta42 > DYN > SP = GAL > SOM = BRD = OXY > ENK. The ganglioside sugar moiety, specifically the sialic acid, had an important role in the interactions. In general the affinities were higher with polysialo, than with monosialo gangliosides. The sensorgrams describing the interactions of Abeta42 and SP with gangliosides differed from the interactions of the other studied peptides. Ca(2+) promoted changes in peptide-glycolipid interactions.

Alzheimer's disease amyloid-beta binds copper and zinc to generate an allosterically ordered membrane-penetrating structure containing superoxide dismutase-like subunits

Amyloid beta peptide (Abeta) is the major constituent of extracellular plaques and perivascular amyloid deposits, the pathognomonic neuropathological lesions of Alzheimer's disease. Cu(2+) and Zn(2+) bind Abeta, inducing aggregation and giving rise to reactive oxygen species. These reactions may play a deleterious role in the disease state, because high concentrations of iron, copper, and zinc have been located in amyloid in diseased brains. Here we show that coordination of metal ions to Abeta is the same in both aqueous solution and lipid environments, with His(6), His(13), and His(14) all involved. At Cu(2+)/peptide molar ratios >0.3, Abeta coordinated a second Cu(2+) atom in a highly cooperative manner. This effect was abolished if the histidine residues were methylated at N(epsilon)2, indicating the presence of bridging histidine residues, as found in the active site of superoxide dismutase. Addition of Cu(2+) or Zn(2+) to Abeta in a negatively charged lipid environment caused a conformational change from beta-sheet to alpha-helix, accompanied by peptide oligomerization and membrane penetration. These results suggest that metal binding to Abeta generated an allosterically ordered membrane-penetrating oligomer linked by superoxide dismutase-like bridging histidine residues.

The amyloid-beta peptide and its role in Alzheimer's disease

Amyloid formation plays a central role in the cause and progression of Alzheimer's disease. The major component of this amyloid is the amyloid-beta (A beta) peptide, which is currently the subject of intense study. This review discusses some recent studies in the area of A beta synthesis, purification and structural analysis. Also discussed are proposed mechanisms for A beta-induced neurotoxicity and some recent advances in the development of A beta-related therapeutic strategies.

Characterization of High-Affinity Binding between Gangliosides and Amyloid β-Protein

The binding specificities of amyloid beta-protein (A beta) such as A beta 1-40, A beta 1-42, A beta 40-1, A beta 1-38, A beta 25-35, and amyloid beta precursor protein (beta-APP) analogues for different glycosphingolipids were determined by surface plasmon resonance (SPR) using a liposome capture method. A beta 1-42, A beta 1-40, A beta 40-1, and A beta 1-38, but not A beta 25-35, bound to GM1 ganglioside in the following rank order: A beta 1-42 > A beta 40-1 > A beta 1-40 > A beta 1-38. The beta-APP analogues bound to GM1 ganglioside with a relatively lower affinity. Aged derivatives of A beta were found to have higher affinity to GM1 ganglioside than fresh or soluble derivatives. A beta 1-40 bound to a number of gangliosides with the following order of binding strength: GQ1b alpha > GT1a alpha > GQ1b > GT1b > GD3 > GD1a = GD1b > LM1 > GM1 > GM2 = GM3 > GM4. Neutral glycosphingolipids had a lower affinity for A beta 1-40 than gangliosides with the following order of binding strength: Gb4 > asialo-GM1 (GA1) > Gb3 > asialo-GM2 (GA2) = LacCer. The results seem to indicate that an alpha2,3NeuAc residue on the neutral oligosaccharide core is required for binding. In addition, the alpha2-6NeuAc residue linked to GalNAc contributes significantly to binding affinity for A beta.

Amyloid-beta interactions with chondroitin sulfate-derived monosaccharides and disaccharides. implications for drug development

In Alzheimer's disease, the major pathological features are diffuse and senile plaques that are primarily composed of the amyloid-beta (A beta) peptide. It has been proposed that proteoglycans and glycosaminoglycans (GAG) facilitate amyloid fibril formation and/or stabilize the plaque aggregates. To develop effective therapeutics based on A beta-GAG interactions, understanding the A beta binding motif on the GAG chain is imperative. Using electron microscopy, fluorescence spectroscopy, and competitive inhibition ELISAs, we have evaluated the ability of chondroitin sulfate-derived monosaccharides and disaccharides to induce the structural changes in A beta that are associated with GAG interactions. Our results demonstrate that the disaccharides GalNAc-4-sulfate(4S), Delta UA-GalNAc-6-sulfate(6S), and Delta UA-GalNAc-4,6-sulfate(4S,6S), the iduronic acid-2-sulfate analogues, and the monosaccharides d-GalNAc-4S, d-GalNAc-6S, and d-GalNAc-4S,6S, but not d-GalNAc, d-GlcNAc, or Delta UA-GalNAc, induce the fibrillar features of A beta-GAG interactions. The binding affinities of all chondroitin sulfate-derived saccharides mimic those of the intact GAG chains. The sulfated monosaccharides and disaccharides compete with the intact chondroitin sulfate and heparin GAGs for A beta binding, as illustrated by competitive inhibition ELISAs. Therefore, the development of therapeutics based on the model of A beta-chondroitin sulfate binding may lead to effective inhibitors of the GAG-induced amyloid formation that is observed in vitro.

Amyloid-beta peptide assembly: a critical step in fibrillogenesis and membrane disruption

Identifying the mechanisms responsible for the assembly of proteins into higher-order structures is fundamental to structural biology and understanding specific disease pathways. The amyloid-beta (Abeta) peptide is illustrative in this regard as fibrillar deposits of Abeta are characteristic of Alzheimer's disease. Because Abeta includes portions of the extracellular and transmembrane domains of the amyloid precursor protein, it is crucial to understand how this peptide interacts with cell membranes and specifically the role of membrane structure and composition on Abeta assembly and cytotoxicity. We describe the results of a combined circular dichroism spectroscopy, electron microscopy, and in situ tapping mode atomic force microscopy (TMAFM) study of the interaction of soluble monomeric Abeta with planar bilayers of total brain lipid extract. In situ extended-duration TMAFM provided evidence of membrane disruption via fibril growth of initially monomeric Abeta1-40 peptide within the total brain lipid bilayers. In contrast, the truncated Abeta1-28 peptide, which lacks the anchoring transmembrane domain found in Abeta1-40, self-associates within the lipid headgroups but does not undergo fibrillogenesis. These observations suggest that the fibrillogenic properties of Abeta peptide are in part a consequence of membrane composition, peptide sequence, and mode of assembly within the membrane.

The role of G protein activation in the toxicity of amyloidogenic Abeta-(1-40), Abeta-(25-35), and bovine calcitonin

More than 16 different proteins have been identified as amyloid in clinical diseases; among these, beta-amyloid (Abeta) of Alzheimer's disease is the best characterized. In the present study, we performed experiments with Abeta and calcitonin, another amyloid-forming peptide, to examine the role of G protein activation in amyloid toxicity. We demonstrated that the peptides, when prepared under conditions that promoted beta-sheet and amyloid fibril (or protofibril) formation, increased high affinity GTPase activity, but the nonamyloidogenic peptides had no discernible effects on GTP hydrolysis. These increases in GTPase activity were correlated to toxicity. In addition, G protein inhibitors significantly reduced the toxic effects of the amyloidogenic Abeta and calcitonin peptides. Our results further indicated that the amyloidogenic peptides significantly increased GTPase activity of purified Galpha(o) and Galpha(i) subunits and that the effect was not receptor-mediated. Collectively, these results imply that the amyloidogenic structure, regardless of the actual peptide or protein sequence, may be sufficient to cause toxicity and that toxicity is mediated, at least partially, through G protein activation. Our abilities to manipulate G protein activity may lead to novel treatments for Alzheimer's disease and the other amyloidoses.

Molecular mechanism of neurodegeneration induced by Alzheimer's beta-amyloid protein: channel formation and disruption of calcium homeostasis

The etiology of Alzheimer's disease has been suggested to be linked to the neurodegeneration induced by beta-amyloid protein (AbetaP), however, the mechanism underlying the latter remains unknown. We have previously shown the direct incorporation of AbetaP into neuronal membranes of immortalized hypothalamic neurons (GT1-7 cells) associated with the formation of calcium-permeable pores, and the elevation of the intracellular calcium concentrations in the GT1-7 cells. Taking together our results and those of numerous other studies, we hypothesize that the disruption of calcium homeostasis by AbetaP-channels may be the molecular basis of the neurotoxicity of AbetaP, and the development of Alzheimer's disease. It is also proposed that the constituents of membrane lipids may play important roles in the process of this channel formation. Our hypothesis may also explain the mechanism of development of other 'conformational diseases', such as prion disease or type 2 diabetes mellitus, which share some common features with Alzheimer's disease.

Correlation of beta-amyloid aggregate size and hydrophobicity with decreased bilayer fluidity of model membranes

beta-amyloid peptide (Abeta) is the primary constituent of senile plaques, a defining feature of Alzheimer's disease. Aggregated Abeta is toxic to neurons, but the mechanism of toxicity remains unproven. One proposal is that Abeta toxicity results from relatively nonspecific Abeta-membrane interactions. We hypothesized that Abeta perturbs membrane structure as a function of the aggregation state of Abeta. Toward exploring this hypothesis, Abeta aggregate size and hydrophobicity were characterized using dynamic and static light scattering and 1,1-bis(4-anilino)naphthalene-5,5-disulfonic acid (bis-ANS) fluorescence. The effect of Abeta aggregation state on the membrane fluidity of unilamellar liposomes was assessed by monitoring the anisotropy of the membrane-embedded fluorescent dye, 1,6-diphenyl-1,3,5-hexatriene (DPH). Unaggregated Abeta at pH 7 did not bind bis-ANS and had little to no effect on membrane fluidity. More significantly, Abeta aggregated at pH 6 or 7 decreased membrane fluidity in a time- and dose-dependent manner. Aggregation rate and surface hydrophobicity were considerably greater for Abeta aggregated at pH 6 than at neutral pH and were strongly correlated with the extent of decrease in membrane fluidity. Prolonged (7 days) Abeta aggregation resulted in a return to near-baseline levels in both bis-ANS fluorescence and DPH anisotropy at pH 7 but not at pH 6. The addition of gangliosides to the liposomes significantly increased the DPH anisotropy response. Hence, self-association of Abeta monomers into aggregates exposes hydrophobic sites and induces a decrease in membrane fluidity. Abeta aggregate-induced changes in membrane physical properties may have deleterious consequences on cellular functioning.

Inositol stereoisomers stabilize an oligomeric aggregate of Alzheimer amyloid beta peptide and inhibit abeta -induced toxicity

Inositol has 8 stereoisomers, four of which are physiologically active. myo-Inositol is the most abundant isomer in the brain and more recently shown that epi- and scyllo-inositol are also present. myo-Inositol complexes with Abeta42 in vitro to form a small stable micelle. The ability of inositol stereoisomers to interact with and stabilize small Abeta complexes was addressed. Circular dichroism spectroscopy demonstrated that epi- and scyllo- but not chiro-inositol were able to induce a structural transition from random to beta-structure in Abeta42. Alternatively, none of the stereoisomers were able to induce a structural transition in Abeta40. Electron microscopy demonstrated that inositol stabilizes small aggregates of Abeta42. We demonstrate that inositol-Abeta interactions result in a complex that is non-toxic to nerve growth factor-differentiated PC-12 cells and primary human neuronal cultures. The attenuation of toxicity is the result of Abeta-inositol interaction, as inositol uptake inhibitors had no effect on neuronal survival. The use of inositol stereoisomers allowed us to elucidate an important structure-activity relationship between Abeta and inositol. Inositol stereoisomers are naturally occurring molecules that readily cross the blood-brain barrier and may represent a viable treatment for AD through the complexation of Abeta and attenuation of Abeta neurotoxic effects.

Review: modulating factors in amyloid-beta fibril formation

Amyloid formation is a key pathological feature of Alzheimer's disease and is considered to be a major contributing factor to neurodegeneration and clinical dementia. Amyloid is found as both diffuse and senile plaques in the parenchyma of the brain and is composed primarily of the 40- to 42-residue amyloid-beta (Abeta) peptides. The characteristic amyloid fiber exhibits a high beta-sheet content and may be generated in vitro by the nucleation-dependent self-association of the Abeta peptide and an associated conformational transition from random to beta-conformation. Growth of the fibrils occurs by assembly of the Abeta seeds into intermediate protofibrils, which in turn self-associate to form mature fibers. This multistep process may be influenced at various stages by factors that either promote or inhibit Abeta fiber formation and aggregation. Identification of these factors and understanding the driving forces behind these interactions as well as the structural motifs necessary for these interactions will help to elucidate potential sites that may be targeted to prevent amyloid formation and its associated toxicity. This review will discuss some of the modulating factors that have been identified to date and their role in fibrillogenesis.

Interactions of Alzheimer amyloid-beta peptides with glycosaminoglycans effects on fibril nucleation and growth

Proteoglycans and their constituent glycosaminoglycans are associated with all amyloid deposits and may be involved in the amyloidogenic pathway. In Alzheimer's disease, plaques are composed of the amyloid-beta peptide and are associated with at least four different proteoglycans. Using CD spectroscopy, fluorescence spectroscopy and electron microscopy, we examined glycosaminoglycan interaction with the amyloid-beta peptides 1-40 (Abeta40) and 1-42 (Abeta42) to determine the effects on peptide conformation and fibril formation. Monomeric amyloid-beta peptides in trifluoroethanol, when diluted in aqueous buffer, undergo a slow random to amyloidogenic beta sheet transition. In the presence of heparin, heparan sulfate, keratan sulfate or chondroitin sulfates, this transition was accelerated with Abeta42 rapidly adopting a beta-sheet conformation. This was accompanied by the appearance of well-defined amyloid fibrils indicating an enhanced nucleation of Abeta42. Incubation of preformed Abeta42 fibrils with glycosaminoglycans resulted in extensive lateral aggregation and precipitation of the fibrils. The glycosaminoglycans differed in their relative activities with the chondroitin sulfates producing the most pronounced effects. The less amyloidogenic Abeta40 isoform did not show an immediate structural transition that was dependent upon the shielding effect by the phosphate counter ion. Removal or substitution of phosphate resulted in similar glycosaminoglycan-induced conformational and aggregation changes. These findings clearly demonstrate that glycosaminoglycans act at the earliest stage of fibril formation, namely amyloid-beta nucleation, and are not simply involved in the lateral aggregation of preformed fibrils or nonspecific adhesion to plaques. The identification of a structure-activity relationship between amyloid-beta and the different glycosaminoglycans, as well as the condition dependence for glycosaminoglycan binding, are important for the successful development and evaluation of glycosaminoglycan-specific therapeutic interventions.

A sulfated proteoglycan aggregation factor mediates amyloid-beta peptide fibril formation and neurotoxicity

Proteoglycans are associated with senile plaques in Alzheimer's disease and may be involved in A beta fibril formation and plaque formation. In vitro, glycosaminoglycans have been shown to inhibit the proteolysis of A beta fibrils, accelerate formation and maintain their stability. To model their interaction, we investigated the binding of a sulfated proteoglycan derived from a natural source; marine sponge Microciona prolifera aggregation factor (MAF). This species-specific re-aggregation of sponge cells has two functional properties, a Ca2+ independent cell binding activity and a Ca2+ dependent self-aggregation. It has been shown that a novel sulfated disaccharide and a pyruvylated trisaccharide are important in the Ca(2+)-dependent MAF aggregation. Aggregation demonstrated by homophilic binding of MAF subunits may be chemically distinct from other heterotypic binding effects. We investigated A beta-MAF interactions and show that MAF induces a structural transition in A beta 40 and A beta 42 from random to beta-structure as detected by circular dichroism spectroscopy. Electron microscopy revealed that the structural transition correlated with an increase in the number of A beta 40 and A beta 42 aggregated that have a truncated fibrillar morphology. Finally, MAF increased A beta-induced toxicity of nerve growth factor (NGF)-differentiated PC-12 cells in the absence of Ca2+. The addition of Ca2+ to MAF-A beta incubations resulted in a moderate attenuation of toxicity possibly due to a reduction in A beta-cell interactions caused by extensive lateral aggregation of the MAF-A beta complexes. Our results indicate that A beta is generally susceptible to proteoglycan-mediated aggregation and fibril formation. We also propose that the MAF model system may be useful in delineating these interactions and represent a means to develop and examine potential inhibitors of the proteoglycan effects.

Amyloid beta peptide membrane perturbation is the basis for its biological effects

Experimental studies have indicated that the mechanisms offered for explaining the neurotoxicity of amyloid beta peptide (AbetaP) are diverse, and include altered enzyme activities, disrupted calcium homeostasis, and increased free radical formation. AbetaP appears to interact at the cell membrane with a multitude of receptor sites and also inserts physically into the membrane matrix. This membrane insertion affects the membrane fluidity and potentially influences the function of resident membrane proteins. We propose a unifying hypothesis to explain the experimental observations of the diverse cellular responses to AbetaP. The indiscriminate physical insertion of AbetaP into the cell membrane unspecifically activates a host of membrane processes by perturbation of the membrane proteins. This recurrent activation of membrane processes eventually culminates in neuronal cell death. We recommend that successful therapeutic interventions should be directed at reducing or preventing the interaction of AbetaP with neuronal cell membranes.

Structure of the Alzheimer beta-amyloid peptide (25-35) and its interaction with negatively charged phospholipid vesicles

The secondary structure of amyloid betaAP(25-35) peptide was studied in pure form and in the presence of different phospholipid vesicles, by using Fourier transform infrared spectroscopy (FT-IR). Pure peptide aggregated with time, forming fibrils with beta-structure. Phospholipid vesicles formed by negatively charged phospholipids such as 1,2-dimyristoyl-sn-glycerol-3-phospho-L-serine (Myr2PtdSer), 1,2-dimyristoyl-sn-glycerol-3-phospho-rac-1-glycerol (Myr2PtdGro) and 1,2-dimyristoyl-sn-glycerol 3-phosphate (Myr2PtdH), greatly accelerated the aggregation of the peptide. However, the presence of vesicles formed by the zwitterionic phospholipid, 1, 2-dimyristoyl-sn-glycerol-3-phosphocholine (Myr2PtdCho), slowed down the aggregation process. Differential scanning calorimetry (DSC) measurements showed that the effect of betaAP(25-35) on the gel to crystal liquid phase transition was small at neutral pH for negatively charged phospholipids and practically nil for Myr2PtdCho. In the case of Myr2PtdSer the effect was also zero at pH 9 but the effect was large at pH 3. The effect on Myr2PtdH was not, however, very dependent on pH. These results were fully confirmed by the observation through FT-IR of the change with temperature of the CH2 antisymmetric stretching vibration. The case of Myr2PtdGro was special as this phospholipid presents polymorphism giving solid quasicrystalline phases when it is not sufficiently hydrated, and it is remarkable that betaAP(25-35) was able to induce the formation of crystalline phases in samples prepared through a method which ensure a good hydration of phospholipid. These results show that the interaction of amyloid betaAP(25-35) peptide with phospholipids is based on electrostatic interactions, that these interactions favour the aggregation of the peptides, and that the presence of the aggregates may disturb the lipid-water interphase of the membrane.

The nonfibrillar amyloid beta-peptide induces apoptotic neuronal cell death: involvement of its C-terminal fusogenic domain

The toxicity of the nonaggregated amyloid beta-peptide (1-40) [A beta(1-40)] on the viability of rat cortical neurons in primary culture was investigated. We demonstrated that low concentrations of A beta peptide, in a nonfibrillar form, induced a time- and dose-dependent apoptotic cell death, including DNA condensation and fragmentation. We compared the neurotoxicity of the A beta(1-40) peptide with those of several A beta-peptide domains, comprising the membrane-destabilizing C-terminal domain of A beta peptide (e.g., amino acids 29-40 and 29-42). These peptides reproduced the effects of the (1-40) peptide, whereas mutant nonfusogenic A beta peptides and the central region of the A beta peptide (e.g., amino acids 13-28) had no effect on cell viability. We further demonstrated that the neurotoxicity of the nonaggregated A beta peptide paralleled a rapid and stable interaction between the A beta peptide and the plasma membrane of neurons, preceding apoptosis and DNA fragmentation. By contrast, the peptide in a fibrillar form induced a rapid and dramatic neuronal death mainly through a necrotic pathway, under our conditions. Taken together, our results suggest that A beta induces neuronal cell death by either apoptosis and necrosis and that an interaction between the nonfibrillar C-terminal domain of the A beta peptide and the plasma membrane of cortical neurons might represent an early event in a cascade leading to neurodegeneration.

Ultrastructural characterization of peptide-induced membrane fusion and peptide self-assembly in the lipid bilayer

The peptide sequence B18, derived from the membrane-associated sea urchin sperm protein bindin, triggers fusion between lipid vesicles. It exhibits many similarities to viral fusion peptides and may have a corresponding function in fertilization. The lipid-peptide and peptide-peptide interactions of B18 are investigated here at the ultrastructural level by electron microscopy and x-ray diffraction. The histidine-rich peptide is shown to self-associate into two distinctly different supramolecular structures, depending on the presence of Zn(2+), which controls its fusogenic activity. In aqueous buffer the peptide per se assembles into beta-sheet amyloid fibrils, whereas in the presence of Zn(2+) it forms smooth globular clusters. When B18 per se is added to uncharged large unilamellar vesicles, they become visibly disrupted by the fibrils, but no genuine fusion is observed. Only in the presence of Zn(2+) does the peptide induce extensive fusion of vesicles, which is evident from their dramatic increase in size. Besides these morphological changes, we observed distinct fibrillar and particulate structures in the bilayer, which are attributed to B18 in either of its two self-assembled forms. We conclude that membrane fusion involves an alpha-helical peptide conformation, which can oligomerize further in the membrane. The role of Zn(2+) is to promote this local helical structure in B18 and to prevent its inactivation as beta-sheet fibrils.

Contribution of cysteine 158, the glycosylation site threonine 194, the amino- and carboxy-terminal domains of apolipoprotein E in the binding to amyloid peptide beta (1-40)

Recent studies have shown that at physiological conditions (pH 7.6, 37 degrees C), the reactivity of recombinant apoE isoforms secreted by mammalian cells toward amyloid peptide beta (Abeta40) follows the order apoE2 > apoE3 > apoE4 for the apoE monomer and apoE2 > apoE3 for apoE dimer that is formed via that intramolecular disulfide bridges. Different Abeta binding properties have been reported for the plasma-derived apoE and commercially available apoE preparations that differ from the native apoE forms in the degree of their O-glycosylation. To define structural elements of apoE involved in the interaction with Abeta, we have introduced point mutations as well as amino- and carboxy-terminal deletions in the apoE structure. The mutant apoE forms were expressed transiently using the Semliki Forest Virus system, and the culture medium was utilized to study the reactivity of the mutated proteins with Abeta 40. This analysis showed that a mutation in the O-glycosylation site of apoE2 (Thr194-Ala) did not affect the SDS-stable binding of apoE to Abeta. In contrast, introduction of cysteine at position 158 of apoE4 (Arg112, Cys158) increased the SDS-stable binding of apoE to Abeta to the levels similar to those observed in apoE2. Similar analysis showed that apoE truncated at residues 259, 249, 239, and 229 retains the SDS-stable binding to Abeta40, whereas apoE truncated at residues 185 and 165 does not bind to Abeta. The deletion of aminoterminal residues 2-19 reduced the SDS-stable binding of apoE2 to Abeta and deletion of residues 2-81 abolished binding to Abeta. It is also noteworthy that the (Delta2-81) apoE mutant exists predominantly as a dimer, suggesting that removal of residues 2-81 promoted dimerization of apoE. These findings suggest that the amino- and carboxy-terminal residues of apoE are required for SDS-stable binding of apoE to Abeta and that the presence of at least one cysteine contributes to the efficient Abeta binding.

Distribution and fluidizing action of soluble and aggregated amyloid beta-peptide in rat synaptic plasma membranes

The effects of soluble and aggregated amyloid beta-peptide (Abeta) on cortical synaptic plasma membrane (SPM) structure were examined using small angle x-ray diffraction and fluorescence spectroscopy approaches. Electron density profiles generated from the x-ray diffraction data demonstrated that soluble and aggregated Abeta1-40 peptides associated with distinct regions of the SPM. The width of the SPM samples, including surface hydration, was 84 A at 10 degrees C. Following addition of soluble Abeta1-40, there was a broad increase in electron density in the SPM hydrocarbon core +/-0-15 A from the membrane center, and a reduction in hydrocarbon core width by 6 A. By contrast, aggregated Abeta1-40 contributed electron density to the phospholipid headgroup/hydrated surface of the SPM +/-24-37 A from the membrane center, concomitant with an increase in molecular volume in the hydrocarbon core. The SPM interactions observed for Abeta1-40 were reproduced in a brain lipid membrane system. In contrast to Abeta1-40, aggregated Abeta1-42 intercalated into the lipid bilayer hydrocarbon core +/-0-12 A from the membrane center. Fluorescence experiments showed that both soluble and aggregated Abeta1-40 significantly increased SPM bulk and protein annular fluidity. Physico-chemical interactions of Abeta with the neuronal membrane may contribute to mechanisms of neurotoxicity, independent of specific receptor binding.

beta-amyloid activates the O-2 forming NADPH oxidase in microglia, monocytes, and neutrophils. A possible inflammatory mechanism of neuronal damage in Alzheimer's disease

The deposition of beta-amyloid in the brain is the key pathogenetic event in Alzheimer's disease. Among the various mechanisms proposed to explain the neurotoxicity of beta-amyloid deposits, a new one, recently identified in our and other laboratories, suggests that beta-amyloid is indirectly neurotoxic by activating microglia to produce toxic inflammatory mediators such as cytokines, nitric oxide, and oxygen free radicals. Three findings presented here support this mechanism, showing that beta-amyloid peptides (25-35), (1-39), and (1-42) activated the classical NADPH oxidase in rat primary culture of microglial cells and human phagocytes: 1) The exposure of the cells to beta-amyloid peptides stimulates the production of reactive oxygen intermediates; 2) the stimulation is associated with the assembly of the cytosolic components of NADPH oxidase on the plasma membrane, the process that corresponds to the activation of the enzyme; 3) neutrophils and monocytes of chronic granulomatous disease patients do not respond to beta-amyloid peptides with the stimulation of reactive oxygen intermediate production. Data are also presented that the activation of NADPH oxidase requires that beta-amyloid peptides be in fibrillary state, is inhibited by inhibitors of tyrosine kinases or phosphatidylinositol 3-kinase and by dibutyryl cyclic AMP, and is potentiated by interferon-gamma or tumor necrosis factor-alpha.

Cholesterol-dependent generation of a seeding amyloid beta-protein in cell culture

Deposition of aggregated amyloid beta-protein (Abeta), a proteolytic cleavage product of the amyloid precursor protein (Abeta ), is a critical step in the development of Alzheimer's disease(Abeta++). However, we are far from understanding the molecular mechanisms underlying the initiation of Abeta polymerization in vivo. Here, we report that a seeding Abeta, which catalyzes the fibrillogenesis of soluble Abeta, is generated from the apically missorted amyloid precursor protein in cultured epithelial cells. Furthermore, the generation of this Abeta depends exclusively on the presence of cholesterol in the cells. Taken together with mass spectrometric analysis of this novel Abeta and our recent study (3), it is suggested that a conformationally altered form of Abeta, which acts as a "seed" for amyloid fibril formation, is generated in intracellular cholesterol-rich microdomains.

Interactions of amyloid beta-peptide (1-40) with ganglioside-containing membranes

Interactions between amyloid beta-peptides (Abeta) and neuronal membranes have been postulated to play an important role in the neuropathology of Alzheimer's disease. To gain insight into the molecular details of this association, we investigated the interactions of Abeta (1-40) with ganglioside-containing membranes by circular dichroism (CD) and Fourier transform infrared-polarized attenuated total reflection (FTIR-PATR) spectroscopy. The CD study revealed that at physiological ionic strength Abeta (1-40) specifically binds to ganglioside-containing membranes inducing a two-state, unordered --> beta-sheet transition above a threshold intramembrane ganglioside concentration, which depends on the host lipid bilayers used. Furthermore, differences in the number and position of sialic acid residues of the carbohydrate backbone significantly affected the conformational transition of the peptide. FTIR-PATR spectroscopy experiments demonstrated that Abeta (1-40) forms an antiparallel beta-sheet, the plane of which lies parallel to the membrane surface, inducing dehydration of lipid interfacial groups and perturbation of acyl chain orientation. These results suggest that Abeta (1-40) imposes negative curvature strain on ganglioside-containing lipid bilayers, disturbing the structure and function of the membranes.

Solution structures of micelle-bound amyloid beta-(1-40) and beta-(1-42) peptides of Alzheimer's disease

The amyloid beta-peptide is the major protein constituent of neuritic plaques in Alzheimer's disease. The beta-peptide varies slightly in length and exists in two predominant forms: (1) the shorter, 40 residue beta-(1-40), found mainly in cerebrovascular amyloid; and (2) the longer, 42 residue beta-(1-42), which is the major component in amyloid plaque core deposits. We report here that the sodium dodecyl sulphate (SDS) micelle, a membrane-mimicking system for biophysical studies, prevents aggregation of the beta-(1-40) and the beta-(1-42) into the neurotoxic amyloid-like, beta-pleated sheet structure, and instead encourages folding into predominantly alpha-helical structures at pH 7.2. Analysis of the nuclear Overhauser enhancement (NOE) and the alphaH NMR chemical shift data revealed no significant structural differences between the beta-(1-40) and the beta-(1-42). The NMR-derived, three-dimensional structure of the beta-(1-42) consists of an extended chain (Asp1-Gly9), two alpha-helices (Tyr10-Val24 and Lys28-Ala42), and a looped region (Gly25-Ser26-Asn27). The most stable alpha-helical regions reside at Gln15-Val24 and Lys28-Val36. The majority of the amide (NH) temperature coefficients were less than 5, indicative of predominately strong NH backbone bonding. The lack of a persistent region with consistently low NH coefficients, together with the rapid NH exchange rates in deuterated water and spin-labeled studies, suggests that the beta-peptide is located at the lipid-water interface of the micelle and does not become inbedded within the hydrophobic interior. This result has implications for the circulation of membrane-bound beta-peptide in biological fluids, and may also facilitate the design of amyloid inhibitors to prevent an alpha-helix-->beta-sheet conversion in Alzheimer's disease.

Enhanced efficiency of a targeted fusogenic peptide

Membrane targeting was investigated as a potential strategy to increase the fusogenic activity of an isolated fusion peptide. This was achieved by coupling the fusogenic carboxy-terminal part of the beta-amyloid peptide (Abeta, amino acids 29-40), involved in Alzheimer's disease, to a positively charged peptide (PIP2-binding peptide, PBP) interacting specifically with a naturally occurring negatively charged phospholipid, phosphatidylinositol 4, 5-bisphosphate (PIP2). Peptide-induced vesicle fusion was spectroscopically evidenced by: (i) mixing of membrane lipids, (ii) mixing of aqueous vesicular contents, and (iii) an irreversible increase in vesicle size, at concentrations five to six times lower than the Abeta(29-40) peptide. In contrast, at these concentrations the PBP-Abeta(29-40) peptide did not display any significant activity on neutral vesicles, indicating that negatively charged phospholipids included as targets in the membranes, are required to compensate for the lower hydrophobicity of this peptide. When the alpha-helical structure of the chimeric peptide was induced by dissolving it in trifluoroethanol, an increase of the fusogenic potential of the peptide was observed, supporting the hypothesis that the alpha-helical conformation of the peptide is crucial to trigger the lipid-peptide interaction. The specificity of the interaction between PIP2 and the PBP moiety, was shown by the less efficient targeting of the chimeric peptide to membranes charged with phosphatidylserine. These data thus demonstrate that the specific properties of both the Abeta(29-40) and the PBP peptide are conserved in the chimeric peptide, and that a synergetic effect is reached through chemical linkage of these two fragments.

Evidence that glypican is a receptor mediating beta-amyloid neurotoxicity in PC12 cells

Docking of beta-amyloid fibrils to neuronal or glial cell membranes may be an early, necessary and intervenable step during the progression of Alzheimer's disease. Formation of neurofibrillary tangles and amyloid plaques as well as neurotoxicity and inflammation may be direct or indirect consequences. In an attempt to find a receptor that mediates those effects, we assessed rat pheochromocytoma PC12 cell 3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyl tetrazolium bromide (MTT) reduction after addition of beta-amyloid to the culture medium. Presence of competitive substances in the medium, cell-surface treatment and specific block of cellular synthesis pathways helped to identify the heparan sulphate moiety of a glycosylphosphatidylinositol-anchored protein likely to represent glypican as a possible receptor mediating beta-amyloid neurotoxicity.

Phosphatidylinositol and inositol involvement in Alzheimer amyloid-beta fibril growth and arrest

A key pathological feature of Alzheimer's disease is the formation and accumulation of amyloid fibres. The major component is the 39 to 42 residue amyloid-beta peptide (Abeta) which is an internal proteolytic fragment of the integral membrane amyloid precursor protein. Aggregation of Abeta into insoluble amyloid fibres is a nucleation-dependent event that may be modulated by the presence of amyloid-associated molecules. Fibril formation is also associated with neurotoxicity which may be the result of specific Abeta interactions with membrane proteins and/or lipids. Using circular dichroism spectroscopy, tyrosine fluorescence spectroscopy and electron microscopy, we have examined the binding of Abeta peptides 1-40 (Abeta40) and 1-42 (Abeta42) to the glycolipid, phosphatidylinositol (PI), and different inositol headgroups. At pH 6.0 and in the presence of PI vesicles, both Abeta40 and Abeta42 adopted an amyloidogenic beta-structure. In contrast, at neutral pH only Abeta42 folded into a beta-structure in the presence of PI vesicles. To determine whether the induction of beta-structure stemmed from interactions with the headgroup of PI, the effects of inositol derivatives on Abeta were also examined. At pH 7.0, myo-inositol was sufficient to induce beta-structure in Abeta42 but had no effect on the conformation of Abeta40. Myo-inositol may promote beta-structure as a result of its ability to be both a hydrogen-bond donor and acceptor. Mono-, di- and triphosphorylated forms of inositol had reduced ability to induce beta-structure in both peptides. The results from this study indicate that interaction of Abeta40 and Abeta42 with PI acts as a seed for fibril formation while myo-inositol stabilizes a soluble Abeta42 micelle.

Folding of beta-sheet membrane proteins: a hydrophobic hexapeptide model

Beta-sheets, in the form of the beta-barrel folding motif, are found in several constitutive membrane proteins (porins) and in several microbial toxins that assemble on membranes to form oligomeric transmembrane channels. We report here a first step towards understanding the principles of beta-sheet formation in membranes. In particular, we describe the properties of a simple hydrophobic hexapeptide, acetyl-Trp-Leu5 (AcWL5), that assembles cooperatively into beta-sheet aggregates upon partitioning into lipid bilayer membranes from the aqueous phase where the peptide is strictly monomeric and random coil. The aggregates, containing 10 to 20 monomers, undergo a relatively sharp and reversible thermal unfolding at approximately 60 degreesC. No pores are formed by the aggregates, but they do induce graded leakage of vesicle contents at very high peptide to lipid ratios. Because beta-sheet structure is not observed when the peptide is dissolved in n-octanol, trifluoroethanol or sodium dodecyl sulfate micelles, aggregation into beta-sheets appears to be an exclusive property of the peptide in the bilayer membrane interface. This is an expected consequence of the hypothesis that a reduction in the free energy of partitioning of peptide bonds caused by hydrogen bonding drives secondary structure formation in membrane interfaces. But, other features of interfacial partitioning, such as side-chain interactions and reduction of dimensionality, must also contribute. We estimate from our partitioning data that the free energy reduction per residue for aggregation is about 0.5 kcal mol-1. Although modest, its aggregate effect on the free energy of assembling beta-sheet proteins can be huge. This surprising finding, that a simple hydrophobic hexapeptide readily assembles into oligomeric beta-sheets in membranes, reveals the potent ability of membranes to promote secondary structure in peptides, and shows that the formation of beta-sheets in membranes is more facile than expected. Furthermore, it provides a basis for understanding the observation that membranes promote self-association of beta-amyloid peptides. AcWL5 and related peptides thus provide a good starting point for designing peptide models for exploring the principles of beta-sheet formation in membranes.