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

Relationships between sterol/phospholipid composition and xenobiotic transport in nematodes

Therapeutic failure limits prophylaxis of nematode diseases and has been mainly attributed to mutations in cellular targets of anthelmintics. Besides these specific mechanisms, alterations of drug transport also occur in parasites resistant to anthelmintics and depend on both the presence of membrane pumps such as P-glycoprotein (Pgp) and on the lipid composition of membranes. We recently showed in the nematode Haemonchus contortus, using eggs as a model, that the total cholesterol (TC) concentration alters the transport of lipophilic molecules due to membrane pumps such as P-glycoprotein and the resistance to anthelmintics. The effect of TC may depend on the presence of other lipids interacting with TC. Therefore, we analysed the lipid composition and its relationship with Pgp and resistance to anthelmintics. Better correlations were found between Pgp and free cholesterol (FC) than with TC. We also showed that the relationships between lipid composition and resistance to anthelmintics or Pgp depended on the equilibrium between FC and phospholipids (PLs), mainly PLs known to be present primarily in either the external leaflets of cell membranes or the internal leaflets. The PLs phosphatidylcholine and phosphatidylethanolamine played the most significant role, but phosphatidic acid also influenced drug resistance.

Alzheimer’s Disease and Cholesterol: The Fat Connection

Since the discovery of the significance of the cholesterol-carrying apolipoprotein E and cholesterolaemia as major risk factors for Alzheimer's Disease (AD) there has been a mounting interest in the role of this lipid as a possible pathogenic agent. In this review we analyse the current evidence linking cholesterol metabolism and regulation in the CNS with the known mechanisms underlying the development of Alzheimer's Disease. Cholesterol is known to affect amyloid-beta generation and toxicity, although it must be considered that the results studies using the statin class of drugs to lower plasma cholesterol may be affected by other effects associated with these drugs. Finally, we report some of our results pointing at the interplay between neurons and astrocytes and NADPH oxidase activation as a new candidate mechanism linking cholesterol and AD pathology.

Autoinsertion of soluble oligomers of Alzheimer's Abeta(1-42) peptide into cholesterol-containing membranes is accompanied by relocation of the sterol towards the bilayer surface

Background

Soluble Alzheimer's Aβ oligomers autoinsert into neuronal cell membranes, contributing to the pathology of Alzheimer's Disease (AD), and elevated serum cholesterol is a risk factor for AD, but the reason is unknown. We investigated potential connections between these two observations at the membrane level by testing the hypothesis that Aβ(1–42) relocates membrane cholesterol.

Results

Oligomers of Aβ(1–42), but not the monomeric peptide, inserted into cholesterol-containing phosphatidylcholine monolayers with an anomalously low molecular insertion area, suggesting concurrent lipid rearrangement. Membrane neutron diffraction, including isomorphous replacement of specific lipid hydrogens with highly-scattering deuterium, showed that Aβ(1–42) insertion was accompanied by outward displacement of membrane cholesterol, towards the polar surfaces of the bilayer. Changes in the generalised polarisation of laurdan confirmed that the structural changes were associated with a functional alteration in membrane lipid order.

Conclusion

Cholesterol is known to regulate membrane lipid order, and this can affect a wide range of membrane mechanisms, including intercellular signalling. Previously unrecognised Aβ-dependent rearrangement of the membrane sterol could have an important role in AD.

The spirostenol (22R, 25R)-20alpha-spirost-5-en-3beta-yl hexanoate blocks mitochondrial uptake of Abeta in neuronal cells and prevents Abeta-induced impairment of mitochondrial function

Abeta(1-42) has been shown to uncouple the mitochondrial respiratory chain and promote the opening of the membrane permeability transition (MPT) pore, leading to cell death. We have previously reported that the spirostenol derivative (22R, 25R)-20alpha-spirost-5-en-3beta-yl hexanoate (SP-233) protects neuronal cells against Abeta(1-42) toxicity by binding to and inactivating the peptide. Picomolar concentrations of Abeta(1-42) decreased the mitochondrial respiratory coefficient in mitochondria isolated from the rat forebrain, and this decrease was partially reversed by SP-233. SP-233 abolished the uncoupling of oxidative phosphorylation induced by carbonyl cyanide 3-chlorophenylhydrazone on isolated mitochondria. These results are consistent with a direct effect of SP-233 on the MPT. Moreover, SP-233 displayed a neuroprotective effect on SK-N-AS human neuroblastoma cells treated with the MPT promoter, phenylarsine oxide. Treatment of SK-N-AS cells with Abeta(1-42) resulted in an accumulation of the peptide in the mitochondrial matrix; SP-233 completely scavenged Abeta(1-42) from the matrix. In addition, SP-233 protected the cells against mitochondrial toxins targeting complexes IV and V of the respiratory chain. These results indicate that Abeta(1-42) and SP-233 exert direct effects on mitochondrial function and SP-233 protects neuronal cells against Abeta-induced toxicity by targeting Abeta directly.

Sex-dependent Actions of Amyloid Beta Peptides on Hippocampal Choline Carriers of Postnatal Rats

It is suggested that amyloid beta peptides (Abeta) play a role in the pathogenesis of Alzheimer disease but their physiological function is still unknown. However, low pM-nM concentrations mediate a hypofunction of a basal forebrain cholinergic system without marked signs of neurotoxicity. In this study, we compared in vitro effects of soluble nonaggregated human Abeta 1-40 and 1-42 either on synaptosomal hemicholinium-3 sensitive choline carriers or on membrane fluidity in hippocampi of male and female Wistar rats aged 7 and 14 days or 2-3 months. The results indicate age- and sex-dependent effects mediated by peptides at nM concentrations but no significant differences between both fragments. Namely, opposite actions were observed in 14-day (the increase in the choline uptake and membrane fluidity) when compared to 7-day old and adult males (the mild drops). Lineweaver-Burk plot analysis revealed that the enhancement of the high-affinity choline transport in 14-day old males occurs via alterations in K (M )and the change was accompanied by a mild increase in the specific binding of [3H]hemicholinium-3. On the other hand, no age-dependent differences were found in females. Rat Abeta 1-40 mediated similar effects on 14-day old rats as the corresponding human fragment. Moreover, higher levels of soluble peptides were detected in immature when compared to mature male brains by means of competitive ELISA. Our study indicates that Abeta could play a role in postnatal sexual differentiation of hippocampal cholinergic system.

Membrane Associated Functions of Neurokinin B (NKB) on Aβ (25–35) Induced Toxicity in Aging Rat Brain Synaptosomes

The effect of different concentrations (0.1-5 microM) of neurokinin B (NKB) and Abeta (25-35) on acetylcholine esterase (AChE), Na(+)-K(+) ATPase and membrane fluidity (DPH anisotropy) were investigated in rat brain synaptosomes of 3, 9, 18 and 30 months old. An age dependent decrease was observed for all the three parameters studied. An in vitro incubation of isolated brain synaptosomes with Abeta (25-35) showed toxic effects on all the parameters studied and the peptide had concentration and age dependent effects, while NKB showed stimulating effect on the parameters and the combined NKB+Abeta (25-35) incubations showed a partial reversal effect as compared to the Abeta (25-35) alone. Thus, the results suggest a membrane mediated function for NKB and its role in neuromodulation, neuroprotection and antioxidant property against Abeta (25-35) induced toxicity in aging brain functions.

Ion channel formation by Alzheimer's disease amyloid beta-peptide (Abeta40) in unilamellar liposomes is determined by anionic phospholipids

Incorporation of Alzheimer's disease amyloid beta-proteins (AbetaPs) across natural and artificial bilayer membranes leads to the formation of cation-selective channels. To study the peptide-membrane interactions involved in channel formation, we used cation reporter dyes to measure AbetaP-induced influx of Na+, Ca2+, and K+ into liposomes formed from phosphatidylserine (PS), phosphatidylinositol (PI) and phosphatidylcholine (PC). We found that Abeta40, but not Abeta40-1 or Abeta28, caused a dose-dependent increase in the concentration of each cation in the lumen of liposomes formed from the acidic phospholipids PS and PI. The Abeta40-induced changes in cation concentration, which we attribute to ion entry through Abeta40 channels, were not observed when using liposomes formed from the neutral phospholipid PC. Using mixtures of phospholipids, the magnitude of the AbetaP40-induced ion entry increased with the acidic phospholipid content of the liposomes, with entry being observed with as little as 5% PS or PI. Thus, while negatively charged phospholipids are required for formation of cation-permeable channels by Abeta40, a small amount is sufficient to support the process. These results have implications for the mechanisms of AbetaP cytotoxicity, suggesting that even a small amount of externalized negative charge could render cells susceptible to the deleterious effects of unregulated ion influx through AbetaP channels.

Oxidative stress in the brain: novel cellular targets that govern survival during neurodegenerative disease

Despite our present knowledge of some of the cellular pathways that modulate central nervous system injury, complete therapeutic prevention or reversal of acute or chronic neuronal injury has not been achieved. The cellular mechanisms that precipitate these diseases are more involved than initially believed. As a result, identification of novel therapeutic targets for the treatment of cellular injury would be extremely beneficial to reduce or eliminate disability from nervous system disorders. Current studies have begun to focus on pathways of oxidative stress that involve a variety of cellular pathways. Here we discuss novel pathways that involve the generation of reactive oxygen species and oxidative stress, apoptotic injury that leads to nuclear degradation in both neuronal and vascular populations, and the early loss of cellular membrane asymmetry that mitigates inflammation and vascular occlusion. Current work has identified exciting pathways, such as the Wnt pathway and the serine-threonine kinase Akt, as central modulators that oversee cellular apoptosis and their downstream substrates that include Forkhead transcription factors, glycogen synthase kinase-3beta, mitochondrial dysfunction, Bad, and Bcl-x(L). Other closely integrated pathways control microglial activation, release of inflammatory cytokines, and caspase and calpain activation. New therapeutic avenues that are just open to exploration, such as with brain temperature regulation, nicotinamide adenine dinucleotide modulation, metabotropic glutamate system modulation, and erythropoietin targeted expression, may provide both attractive and viable alternatives to treat a variety of disorders that include stroke, Alzheimer's disease, and traumatic brain injury.

Insertion of Alzheimer's A beta 40 peptide into lipid monolayers

The amyloid beta (A beta) peptide is the major component found in the amyloid deposits in the brains of Alzheimer's disease patients. In vitro studies have demonstrated that the aggregation of A beta can take place at three orders of magnitude lower concentrations in the presence of phospholipid molecules compared to bulk peptide studies, suggesting that membrane lipids may mediate A beta toxicity. To understand the interaction of A beta with lipid membranes, we have examined A beta 40 with anionic dipalmitoylphosphatidylglycerol (DPPG), zwitterionic dipalmitoylphosphatidylcholine (DPPC), and cationic dipalmitoyltrimethylammonium propane (DPTAP) monolayers under different subphase conditions. We have used a constant surface pressure insertion assay to assess the degree of peptide insertion into the lipids. Simultaneously, we monitored the surface morphology of the monolayers with fluorescence microscopy. We have also performed dual-probe fluorescence measurements where both the peptide and lipid are tagged with chromophores. Isotherm measurements show that A beta inserts into both DPTAP and DPPG monolayers under physiologically relevant conditions. Insertion into DPPC occurs at lipid densities below that found in a bilayer. The level of insertion is inversely proportional to the lipid packing density. Our results indicate that lipids need not be anionic to interact with A beta. Electrostatic effects involved in A beta 40-lipid interaction are discussed.

Increased resistance to anthelmintics of Haemonchus contortus eggs associated with changes in membrane fluidity of eggshells during embryonation

The embryonation of nematode eggs has been shown to increase their resistance to anthelmintics when parasites are submitted to egg hatch assays. Nevertheless, no mechanism has been suggested to explain this phenomenon. Earlier observations by other authors showed that the biochemical composition of eggshells is altered during the embryonation of eggs. The functional consequences of these changes have not been identified. We studied the changes in membrane environment (eggshells) of Haemonchus contortus eggs during the embryonation by fluidity measurements and their effects on nonspecific mechanisms of resistance to anthelmintics. We previously demonstrated that these mechanisms imply P-glycoproteins (Pgp) belonging to the multi-drug resistance (MDR) system and that the Pgp activity is very susceptible to their lipidic environment. The results obtained here show that the embryonation induced a significant and gradual increase in eggshell fluidity which was associated with an increased resistance to anthelmintics. Differences were observed between H. contortus isolates with various levels of resistance which might result from their specific biology and/or membrane composition. The membrane environment changes could act both on the solubilization of anthelmintics into the eggs and on the efflux of these lipophilic molecules by Pgp.

Analysis of the effect of a peptide sequence of the E2 protein (HGV/GBV-C) on the physicochemical properties of zwitterionic and negatively charged bilayers

The membrane-interacting properties of a potential epitope of GB virus C/hepatitis G virus located at the region (99-118) of the E2 structural protein were investigated using several fluorescence techniques. SUV of DMPC:DPPC (1:1) or DMPG:DPPC (1:1) zwitterionic and anionic mixtures, respectively, were used as model membranes. FRET with NBD-PE as energy donor and Rho-PE as energy acceptor-labelled SUV indicated that the peptide was able to fuse both zwitterionic and anionic SUVs, the latter requiring lower peptide concentrations. However, the peptide increased the steady-state anisotropy of DPH embedded in the hydrophobic centre of the membrane with zwitterionic headgroups and to a lesser extent in anionic bilayers, suggesting that charge-charge interactions are not required for membrane interactions and also confirming the FRET results. No changes in anisotropy were observed with the probe TMA-DPH located at the surface of the bilayer. Finally, analysis of the intrinsic emission fluorescence of the tryptophan residue, upon incubation with SUV, showed a blue shift in the presence of anionic bilayers, both below and above the main transition temperature (T(m)) (gel to liquid-crystalline state) and, to a lesser extent, with the zwitterionic model membrane.

One-electron oxidation of beta-amyloid peptide: sequence modulation of reactivity

Amyloid beta peptide (Abeta) is a 39 to 43 amino-acid-long peptide implicated in Alzheimer's disease. One of its mechanisms of toxicity is related to its redox properties. Therefore we studied its one electron oxidation using azide free radicals produced in gamma and pulse radiolysis, and compared the results with those obtained with the reverse sequence Abeta(40-1). HPLC analysis combined with absorption, fluorescence, Raman spectroscopy, and MALDI-TOF MS were used for product identification. Met35 was shown to be the target in Abeta(1-40); oxidation leads to a major compound that is Abeta with methionine sulfoxide. Similarly, oxidation of fragment Abeta(29-40) also leads to methionine sulfoxide. For Abeta(40-1), Met35 is not reactive and Tyr10 is the target of azide radicals. The major products are peptide dimer linked by dityrosine and trimer. The lowering of the one-electron reduction potential of the MetS+/Met couple, which was proposed, is in agreement with our findings. To our knowledge, this is the first time that such a drastic effect of the primary sequence is observed in a small peptide. In addition, it is also the first experimental demonstration of the sensitivity of the one-electron reduction potential of methionine on neighboring groups.

An ellipsometry-based Alzheimer plaque mimic: Effect of β-amyloid, lipoprotein identity and apolipoprotein E isoform

Ca2+-induced deposition of low-density lipoprotein (LDL), intermediate-density lipoprotein (IDL), and high-density lipoprotein (HDL) at proteoheparan sulfate-modified surfaces was investigated as a function of beta-amyloid (Abeta) presence and apolipoprotein E isoform. Presence of beta-amyloid resulted in an increased deposition, as did the E4/E4 isoform compared to the corresponding E3/E3 isoform. The results are compatible with findings reported in literature on plaque formation in Alzheimer's disease, and suggest that, although simplistic, the present model system may have some potential in biosensor studies of Alzheimer plaque formation.

Identification of naturally occurring spirostenols preventing beta-amyloid-induced neurotoxicity

22R-Hydroxycholesterol is an intermediate in the steroid biosynthesis pathway shown to exhibit a neuroprotective property against beta-amyloid (1-42) (Abeta) toxicity in rat PCl2 and human NT2N neuronal cells by binding and inactivating Abeta. In search of potent 22R-hydroxycholesterol derivatives, we assessed the ability of a series of naturally occurring entities containing the 22R-hydroxycholesterol structure to protect PC12 cells against Abeta-induced neurotoxicity, determined by measuring changes in membrane potential, mitochondrial diaphorase activity, ATP levels and trypan blue uptake. 22R-Hydroxycholesterol derivatives sharing a common spirost-5-en-3-ol or a furost-5-en-3-ol structure were tested. Although some of these compounds were neuroprotective against 0.1 microM Abeta, only three protected against the 1-10 microM Abeta-induced toxicity and, in contrast to 22R-hydroxycholesterol, all were devoid of steroidogenic activity. These entities shared a common structural feature, a long chain ester in position 3 and common stereochemistry. The neuroprotective property of these compounds was coupled to their ability to displace radiolabeled 22R-hydroxycholesterol from Abeta, suggesting that the Abeta-22R-hydroxycholesterol physicochemical interaction contributes to their beneficial effect. In addition, a 22R-hydroxycholesterol derivative inhibited the formation of neurotoxic amyloid-derived diffusible ligands. Computational docking simulations of 22R-hydroxycholesterol and its derivatives on Abeta identified two binding sites. Chemical entities, as 22R-hydroxycholesterol, seem to bind preferentially only to one site. In contrast, the presence of the ester chain seems to confer the ability to bind to both sites on Abeta, leading to neuroprotection against high concentrations of Abeta. In conclusion, these results suggest that spirost-5-en-3-ol naturally occurring derivatives of 22R-hydroxycholesterol might offer a new approach for Alzheimer's disease therapy.

Dietary lipids in the aetiology of Alzheimer's disease: implications for therapy

Amyloid plaques and neurofibrillary tangles are the neuropathological hallmarks of Alzheimer's disease (AD), but no conclusive evidence has emerged showing that these hallmarks are the cause and not a product of the disease. Many studies have implicated oxidation and inflammation in the AD process, and there is growing evidence that abnormalities of lipid metabolism also play a role. Using epidemiology to elucidate risk factors and histological changes to suggest possible mechanisms, the hypothesis is advanced that dietary lipids are the principal risk factor for the development of late-onset sporadic AD. The degree of saturation of fatty acids and the position of the first double bond in essential fatty acids are the most critical factors determining the effect of dietary fats on the risk of AD, with unsaturated fats and n-3 double bonds conferring protection and an overabundance of saturated fats or n-6 double bonds increasing the risk. The interaction of dietary lipids and apolipoprotein E isoforms may determine the risk and rate of sustained autoperoxidation within cellular membranes and the efficacy of membrane repair. Interventions involving dietary lipids and lipid metabolism show great promise in slowing or possibly averting the development of AD, including dietary changes, cholesterol-modifying agents and antioxidants.

The distribution of lipid attached spin probes in bilayers: application to membrane protein topology

The distribution of the lipid-attached doxyl electron paramagnetic resonance (EPR) spin label in 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine membranes has been studied by (1)H and (13)C magic angle spinning nuclear magnetic resonance relaxation measurements. The doxyl spin label was covalently attached to the 5th, 10th, and 16th carbons of the sn-2 stearic acid chain of a 1-palmitoyl-2-stearoyl-(5/10/16-doxyl)-sn-glycero-3-phosphocholine analog. Due to the unpaired electron of the spin label, (1)H and (13)C lipid relaxation rates are enhanced by paramagnetic relaxation. For all lipid segments the influence of paramagnetic relaxation is observed even at low probe concentrations. Paramagnetic relaxation rates provide a measure for the interaction strength between lipid segments and the doxyl group. Plotted along the membrane director a transverse distribution profile of the EPR probe is obtained. The chain-attached spin labels are broadly distributed in the membrane with a maximum at the approximate chain position of the probe. Both (1)H and (13)C relaxation measurements show these broad distributions of the doxyl group in the membrane indicating that (1)H spin diffusion does not influence the relaxation measurements. The broad distributions of the EPR label result from the high degree of mobility and structural heterogeneity in liquid-crystalline membranes. Knowing the distribution profiles of the EPR probes, their influence on relaxation behavior of membrane inserted peptide and protein segments can be studied by (13)C magic angle spinning nuclear magnetic resonance. As an example, the location of Ala residues positioned at three sites of the transmembrane WALP-16 peptide was investigated. All three doxyl-labeled phospholipid analogs induce paramagnetic relaxation of the respective Ala site. However, for well ordered secondary structures the strongest relaxation enhancement is observed for that doxyl group in the closest proximity to the respective Ala. Thus, this approach allows study of membrane insertion of protein segments with respect to the high molecular mobility in liquid-crystalline membranes.

Kinetics of adsorption of beta-amyloid peptide Abeta(1-40) to lipid bilayers

The Alzheimer's disease-related peptide beta-amyloid (Abeta) is toxic to neurons. The toxicity of the peptide appears to require conversion of the monomeric form to an aggregated fibrillar species. The interaction of Abeta with cell membranes has attracted interest as one plausible mechanism by which the peptide exerts its toxic activity. We developed two methods to measure the adsorption of fresh (monomeric) and aged (aggregated) Abeta to lipid bilayers. In one method, the kinetics of Abeta adsorption and desorption to liposomes deposited onto a dextran-coated surface was measured using surface plasmon resonance. In the other method, Abeta was contacted with liposome-coated magnetic beads; adsorbed Abeta was separated from solution-phase peptide by use of a magnetic field. Monomeric Abeta adsorbed quickly but reversibly to lipid bilayers with low affinity, while aggregated Abeta adsorbed slowly but irreversibly. These two methods provide complementary means of quantifying the adsorption of aggregating proteins to membranes. The results correlate strongly with previous observations that fibrillar, but not monomeric, Abeta restricts the motion of acyl tails in phospholipid bilayers. The methods should be useful for further elucidation of the role of membrane adsorption in mediating Abeta toxicity, and in the search for inhibitors of toxicity.

Comparison of the interaction, positioning, structure induction and membrane perturbation of cell-penetrating peptides and non-translocating variants with phospholipid vesicles

Cell-penetrating peptides (CPPs) are able to translocate and carry cargo molecules across cell membranes. Using fluorescence techniques (polarization and quenching) and CD spectroscopy we studied the interaction, conformation and topology of two such peptides, transportan and 'penetratin' (pAntp), and two variants of differing translocating abilities, with small phospholipid vesicles of varying charge density. The induced structure of transportan is always helical independent of vesicle surface charge. pAntp and its two variants interact significantly only with negatively charged vesicles. The induced secondary structure depends on membrane charge and lipid/peptide ratio. The degree of membrane perturbation, evidenced by fluorescence polarization, of pAntp and its variants is related to their secondary structure. In the helical state, the peptides have little effect on the membrane. Under conditions where pAntp and its variants are converted into beta-structures, they cause membrane perturbation. Oriented CD suggests that the two CPPs (pAntp and transportan) in their helical state lie along the vesicle surface, while the two pAntp variants appear to penetrate deeper into the membrane.

A Kinetic Analysis of the Mechanism of beta-Amyloid Induced G Protein Activation

beta-Amyloid (A beta) is the primary protein component of senile plaques found in Alzheimer's disease. In an aggregated (amyloid fibril, protofibril, or low molecular weight oligomer) state, A beta has been consistently shown to be toxic to neurons, but the molecular mechanism of this toxicity is poorly understood. We have previously shown that A beta activates a G(i/o) protein, and that inhibition of this specific G protein activation attenuated A beta-induced cell toxicity. In the present study, we use a kinetic analysis to examine the mechanism of A beta-induced G protein activation. Using synthetic A beta(1-40) and phospholipid vesicles containing purified G(0)alpha subunits, we examined the relationship between A beta concentration, G(0)alpha subunit concentration, GTP concentration and rate of GTP hydrolysis experimentally. We found that at low concentrations of A beta (less than 10 microM), A beta increased the rate of GTP hydrolysis over the rate of hydrolysis in the absence of peptide, however, at high concentrations of A beta, significantly decreased rates of GTP hydrolysis were observed. We postulated several molecular level mechanisms for the observed rate behavior, from those mechanisms derived rate equations, and then tested the mechanisms against our experimental rate data. Based on our results, we identified a plausible mechanism for A beta-induced G protein activation which is consistent with available experimental data. This work demonstrates the utility of an engineering approach to examining steps in the mechanism of A beta-induced cell toxicity and could provide insight into our understanding of the mechanism of Alzheimer's disease.

Cholesterol is necessary both for the toxic effect of Abeta peptides on vascular smooth muscle cells and for Abeta binding to vascular smooth muscle cell membranes

Accumulation of beta amyloid (Abeta) in the brain is central to the pathogenesis of Alzheimer's disease. Abeta can bind to membrane lipids and this binding may have detrimental effects on cell function. In this study, surface plasmon resonance technology was used to study Abeta binding to membranes. Abeta peptides bound to synthetic lipid mixtures and to an intact plasma membrane preparation isolated from vascular smooth muscle cells. Abeta peptides were also toxic to vascular smooth muscle cells. There was a good correlation between the toxic effect of Abeta peptides and their membrane binding. 'Ageing' the Abeta peptides by incubation for 5 days increased the proportion of oligomeric species, and also increased toxicity and the amount of binding to lipids. The toxicities of various Abeta analogs correlated with their lipid binding. Significantly, binding was influenced by the concentration of cholesterol in the lipid mixture. Reduction of cholesterol in vascular smooth muscle cells not only reduced the binding of Abeta to purified plasma membrane preparations but also reduced Abeta toxicity. The results support the view that Abeta toxicity is a direct consequence of binding to lipids in the membrane. Reduction of membrane cholesterol using cholesterol-lowering drugs may be of therapeutic benefit because it reduces Abeta-membrane binding.

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.