Effects of beta-amyloid peptides on the fluidity of membranes from frontal and parietal lobes of human brain. High potencies of A beta 1-42 and A beta 1-43

Protective Role of a Donepezil-Huprine Hybrid against the β-Amyloid (1-42) Effect on Human Erythrocytes

Aβ(1-42) peptide is a neurotoxic agent strongly associated with the etiology of Alzheimer's disease (AD). Current treatments are still of very low effectiveness, and deaths from AD are increasing worldwide. Huprine-derived molecules have a high affinity towards the enzyme acetylcholinesterase (AChE), act as potent Aβ(1-42) peptide aggregation inhibitors, and improve the behavior of experimental animals. AVCRI104P4 is a multitarget donepezil-huprine hybrid that improves short-term memory in a mouse model of AD and exerts protective effects in transgenic Caenorhabditis elegans that express Aβ(1-42) peptide. At present, there is no information about the effects of this compound on human erythrocytes. Thus, we considered it important to study its effects on the cell membrane and erythrocyte models, and to examine its protective effect against the toxic insult induced by Aβ(1-42) peptide in this cell and models. This research was developed using X-ray diffraction and differential scanning calorimetry (DSC) on molecular models of the human erythrocyte membrane constituted by lipid bilayers built of dimyristoylphosphatidylcholine (DMPC) and dimyristoylphosphatidylethanolamine (DMPE). They correspond to phospholipids representative of those present in the external and internal monolayers, respectively, of most plasma and neuronal membranes. The effect of AVCRI104P4 on human erythrocyte morphology was studied by scanning electron microscopy (SEM). The experimental results showed a protective effect of AVCRI104P4 against the toxicity induced by Aβ(1-42) peptide in human erythrocytes and molecular models.

Molecular rotors report on changes in live cell plasma membrane microviscosity upon interaction with beta-amyloid aggregates

Amyloid deposits of aggregated beta-amyloid Aβ(1-42) peptides are a pathological hallmark of Alzheimer's disease. Aβ(1-42) aggregates are known to induce biophysical alterations in cells, including disruption of plasma membranes. We investigated the microviscosity of plasma membranes upon interaction with oligomeric and fibrillar forms of Aβ(1-42). Viscosity-sensing fluorophores termed molecular rotors were utilised to directly measure the microviscosities of giant plasma membrane vesicles (GPMVs) and plasma membranes of live SH-SY5Y and HeLa cells. The fluorescence lifetimes of membrane-inserting BODIPY-based molecular rotors revealed a decrease in bilayer microviscosity upon incubation with Aβ(1-42) oligomers, while fibrillar Aβ(1-42) did not significantly affect the microviscosity of the bilayer. In addition, we demonstrate that the neuroprotective peptide H3 counteracts the microviscosity change induced by Aβ(1-42) oligomers, suggesting the utility of H3 as a neuroprotective therapeutic agent in neurodegenerative disorders and indicating that ligand-induced membrane stabilisation may be a possible mechanism of neuroprotection during neurodegenerative disorders such as Alzheimer's disease.

Cause and consequence of Aβ - Lipid interactions in Alzheimer disease pathogenesis

Self-templating propagation of protein aggregate conformations is increasingly becoming a significant factor in many neurological diseases. In Alzheimer disease (AD), intrinsically disordered amyloid-β (Aβ) peptides undergo aggregation that is sensitive to environmental conditions. High-molecular weight aggregates of Aβ that form insoluble fibrils are deposited as senile plaques in AD brains. However, low-molecular weight aggregates called soluble oligomers are known to be the primary toxic agents responsible for neuronal dysfunction. The aggregation process is highly stochastic involving both homotypic (Aβ-Aβ) and heterotypic (Aβ with interacting partners) interactions. Two of the important members of interacting partners are membrane lipids and surfactants, to which Aβ shows a perpetual association. Aβ-membrane interactions have been widely investigated for more than two decades, and this research has provided a wealth of information. Although this has greatly enriched our understanding, the objective of this review is to consolidate the information from the literature that collectively showcases the unique phenomenon of lipid-mediated Aβ oligomer generation, which has largely remained inconspicuous. This is especially important because Aβ aggregate "strains" are increasingly becoming relevant in light of the correlations between the structure of aggregates and AD phenotypes. Here, we will focus on aspects of Aβ-lipid interactions specifically from the context of how lipid modulation generates a wide variety of biophysically and biochemically distinct oligomer sub-types. This, we believe, will refocus our thinking on the influence of lipids and open new approaches in delineating the mechanisms of AD pathogenesis. This article is part of a Special Issue entitled: Protein Aggregation and Misfolding at the Cell Membrane Interface edited by Ayyalusamy Ramamoorthy.

Evidence from ITIR-FCS Diffusion Studies that the Amyloid-Beta (Aβ) Peptide Does Not Perturb Plasma Membrane Fluidity in Neuronal Cells

The amyloid-beta (Aβ) peptide, commonly found in elevated levels in the brains of patients with Alzheimer's disease (AD) and in the cerebrospinal fluid of individuals presenting mild cognitive impairment, is thought to be one of the major factors resulting in the onset of AD. Although observed and studied at the molecular level for several decades, the exact disease pathology of AD is still not totally clear. One way in which Aβ is thought to affect neurons is by influencing cell membrane fluidity, which could result in abnormal synaptic or signaling function. The effects of Aβ on the fluidity of biological membranes have been studied using numerous membrane models such as artificial lipid bilayers and vesicles, living cells and membranes extracted from animal models of AD, yet there is still no consensus as to what effects Aβ has, if any, on membrane fluidity. As one of the most precise and accurate means of assaying membrane dynamics, we have thus chosen fluorescence correlation spectroscopy to investigate the issue, using fluorescent membrane-targeted probes on living cells treated with Aβ(1-42) oligomers and observing possible changes in membrane diffusion. Effects of Aβ on viability in different cell types varied from no detectable effect to extensive cell death by 72 h post-exposure. However, there was no change in the fluidity of either ordered membrane domains or the bulk membrane in any of these cells within this period. Our conclusion from these results is that perturbation of membrane fluidity is not likely to be a factor in acute Aβ-induced cytotoxicity.

Membrane Aging as the Real Culprit of Alzheimer's Disease: Modification of a Hypothesis

Our previous studies proposed that Alzheimer's disease (AD) is a metabolic disorder and hypothesized that abnormal brain glucose metabolism inducing multiple pathophysiological cascades contributes to AD pathogenesis. Aging is one of the great significant risk factors for AD. Membrane aging is first prone to affect the function and structure of the brain by impairing glucose metabolism. We presume that risk factors of AD, including genetic factors (e.g., the apolipoprotein E ε4 allele and genetic mutations) and non-genetic factors (such as fat, diabetes, and cardiac failure) accelerate biomembrane aging and lead to the onset and development of the disease. In this review, we further modify our previous hypothesis to demonstrate "membrane aging" as an initial pathogenic factor that results in functional and structural alterations of membranes and, consequently, glucose hypometabolism and multiple pathophysiological cascades.

Real-time investigation of cytochrome c release profiles in living neuronal cells undergoing amyloid beta oligomer-induced apoptosis

Intracellular Cyt c release profiles in living human neuroblastoma undergoing amyloid β oligomer (AβO)-induced apoptosis, as a model Alzheimer's disease-associated pathogenic molecule, were analysed in a real-time manner using plasmon resonance energy transfer (PRET)-based spectroscopy.

Lipids in Amyloid-β Processing, Aggregation, and Toxicity

Aggregation of amyloid-beta (Aβ) peptide is the major event underlying neuronal damage in Alzheimer's disease (AD). Specific lipids and their homeostasis play important roles in this and other neurodegenerative disorders. The complex interplay between the lipids and the generation, clearance or deposition of Aβ has been intensively investigated and is reviewed in this chapter. Membrane lipids can have an important influence on the biogenesis of Aβ from its precursor protein. In particular, increased cholesterol in the plasma membrane augments Aβ generation and shows a strong positive correlation with AD progression. Furthermore, apolipoprotein E, which transports cholesterol in the cerebrospinal fluid and is known to interact with Aβ or compete with it for the lipoprotein receptor binding, significantly influences Aβ clearance in an isoform-specific manner and is the major genetic risk factor for AD. Aβ is an amphiphilic peptide that interacts with various lipids, proteins and their assemblies, which can lead to variation in Aβ aggregation in vitro and in vivo. Upon interaction with the lipid raft components, such as cholesterol, gangliosides and phospholipids, Aβ can aggregate on the cell membrane and thereby disrupt it, perhaps by forming channel-like pores. This leads to perturbed cellular calcium homeostasis, suggesting that Aβ-lipid interactions at the cell membrane probably trigger the neurotoxic cascade in AD. Here, we overview the roles of specific lipids, lipid assemblies and apolipoprotein E in Aβ processing, clearance and aggregation, and discuss the contribution of these factors to the neurotoxicity in AD.

Atomic force microscopy to study molecular mechanisms of amyloid fibril formation and toxicity in Alzheimer's disease

Alzheimer's disease (AD) is a devastating neurodegenerative disease characterized by dementia and memory loss for which no cure or effective prevention is currently available. Neurodegeneration in AD is linked to formation of amyloid plaques found in brain tissues of Alzheimer's patients during post-mortem examination. Amyloid plaques are composed of amyloid fibrils and small oligomers - insoluble protein aggregates. Although amyloid plaques are found on the neuronal cell surfaces, the mechanism of amyloid toxicity is still not well understood. Currently, it is believed that the cytotoxicity is a result of the nonspecific interaction of small soluble amyloid oligomers (rather than longer fibrils) with the plasma membrane. In recent years, nanotechnology has contributed significantly to understanding the structure and function of lipid membranes and to the study of the molecular mechanisms of membrane-associated diseases. We review the current state of research, including applications of the latest nanotechnology approaches, on the interaction of lipid membranes with the amyloid-β (Aβ) peptide in relation to amyloid toxicity. We discuss the interactions of Aβ with model lipid membranes with a focus to demonstrate that composition, charge and phase of the lipid membrane, as well as lipid domains and rafts, affect the binding of Aβ to the membrane and contribute to toxicity. Understanding the role of the lipid membrane in AD at the nanoscale and molecular level will contribute to the understanding of the molecular mechanism of amyloid toxicity and may aid into the development of novel preventive strategies to combat AD.

Fluorescence study on aggregated lysozyme and lipid bilayer interactions

Fluorescent probes 1,6-diphenyl-1,3,5-hexatriene (DPH), pyrene, 4-dimethylaminochalcone (DMC) and 4-p-(dimethylaminostyryl)-1-dodecylpyridinium (DSP-12) have been utilized to monitor the impact of lysozyme (Lz) oligomers on physicochemical properties of phosphatidylcholine/cardiolipin (PC/CL) membranes. Analysis of spectral responses of the employed probes revealed the reduction of membrane free volume and dehydration of lipid bilayer surface upon incorporation of Lz self-assemblies. Hydrophobic interactions were found to control the binding of Lz oligomers to the lipid bilayer. Comparison of the effects of Lz monomers, oligomers and fibrils showed that soluble oligomeric intermediates exert the most destructive influence on membrane properties.

Unsaturated fatty acids drive disintegrin and metalloproteinase (ADAM)-dependent cell adhesion, proliferation, and migration by modulating membrane fluidity

The disintegrin-metalloproteinases ADAM10 and ADAM17 mediate the release of several cell signaling molecules and cell adhesion molecules such as vascular endothelial cadherin or L-selectin affecting endothelial permeability and leukocyte transmigration. Dysregulation of ADAM activity may contribute to the pathogenesis of vascular diseases, but the mechanisms underlying the control of ADAM functions are still incompletely understood. Atherosclerosis is characterized by lipid plaque formation and local accumulation of unsaturated free fatty acids (FFA). Here, we show that unsaturated FFA increase ADAM-mediated substrate cleavage. We demonstrate that these alterations are not due to genuine changes in enzyme activity, but correlate with changes in membrane fluidity as revealed by measurement of 1,6-diphenyl-1,3,5-hexatriene fluorescence anisotropy and fluorescence recovery after photobleaching analyses. ELISA and immunoblot experiments conducted with granulocytes, endothelial cells, and keratinocytes revealed rapid increase of ectodomain shedding of ADAM10 and ADAM17 substrates upon membrane fluidization. Large amounts of unsaturated FFA may be liberated from cholesteryl esters in LDL that is entrapped in atherosclerotic lesions. Incubation of cells with thus modified LDL resulted in rapid cleavage of ADAM substrates with corresponding functional consequences on cell proliferation, cell migration, and endothelial permeability, events of high significance in atherogenesis. We propose that FFA represent critical regulators of ADAM function that may assume relevance in many biological settings through their influence on mobility of enzyme and substrate in lipid bilayers.

Calcium dyshomeostasis and neurotoxicity of Alzheimer's beta-amyloid protein

Neurotoxicity of Alzheimer's beta-amyloid protein (AbetaP) is central to the pathogenesis of Alzheimer's disease (AD). Recent approaches have emphasized the importance of AbetaP oligomerization, which causes synaptic degeneration and neuronal loss, finally leading to the pathogenesis of AD. Although the precise molecular mechanism of AbetaP neurotoxicity remains elusive, our and other numerous findings have demonstrated that AbetaP directly incorporated into neuronal membranes formed calcium-permeable ion channels (amyloid channels) and resulted in an abnormal elevation of the intracellular calcium levels. The formation of amyloid channels and the abnormal increase of intracellular Ca(2+) have also been commonly observed in other neurodegenerative diseases, including conformational diseases such as prion disease or dementia with Lewy bodies. This article reviews the current understanding of the pathology of AD based on the hypothesis that the disruption of calcium homeostasis through amyloid channels may be the molecular basis of AbetaP neurotoxicity. The potential development of preventive agents is also discussed.

Amyloid-beta peptide induces temporal membrane biphasic changes in astrocytes through cytosolic phospholipase A2

Oligomeric amyloid-beta peptide (Abeta) is known to induce cytotoxic effects and to damage cell functions in Alzheimer's disease. However, mechanisms underlying the effects of Abeta on cell membranes have yet to be fully elucidated. In this study, Abeta 1-42 (Abeta(42)) was shown to cause a temporal biphasic change in membranes of astrocytic DITNC cells using fluorescence microscopy of Laurdan. Abeta(42) made astrocyte membranes became more molecularly-disordered within the first 30 min to 1 h, but gradually changed to more molecularly-ordered after 3 h. However, Abeta(42) caused artificial membranes of vesicles made of rat whole brain lipid extract to become more disordered only. The trend for more molecularly-ordered membranes in astrocytes induced by Abeta(42) was abrogated by either an NADPH oxidase inhibitor, apocynin, or an inhibitor of cytosolic phospholipase A(2) (cPLA(2)), but not by an inhibitor of calcium-independent PLA(2) (iPLA(2)). Apocynin also suppressed the increased production of superoxide anions (O(2)(-)) and phosphorylation of cPLA(2) induced by Abeta(42). In addition, hydrolyzed products of cPLA(2), arachidonic acid (AA), but not lysophosphatidylcholine (LPC) caused astrocyte membranes to become more molecularly-ordered. These results suggest (1) a direct interaction of Abeta(42) with cell membranes making them more molecularly-disordered, and (2) Abeta(42) also indirectly makes membranes become more molecularly-ordered by triggering the signaling pathway involving NADPH oxidase and cPLA(2) in astrocytes.

Membrane disordering effects of beta-amyloid peptides

The interaction of Abeta with synaptosomal plasma membranes decreases membrane fluidity. Using model membrane/liposome systems the interaction of Abeta with specific lipids (e.g. phospholipids, gangliosides, cholesterol) has been defined. The formation of the beta-sheet structure of Abeta when undergoing peptide aggregation is important for Abeta's membrane perturbing properties. This effect can be correlated with the peptide length of Abeta, the longer Abeta1-42 having the greatest effect on membrane fluidity and on neurotoxicity.

Copper-dependent inhibition of cytochrome c oxidase by A?1?42requires reduced methionine at residue 35 of the A? peptide

By altering key amino acid residues of the Alzheimer's disease-associated amyloid-beta peptide, we investigated the mechanism through which amyloid-beta inhibits cytochrome c oxidase (EC 1.9.3.1). Native amyloid-beta inhibited cytochrome oxidase by up to 65%, and the level of inhibition was determined by the period of amyloid-beta ageing before the cytochrome oxidase assay. Substituting tyrosine-10 with alanine did not affect maximal enzyme inhibition, but the altered peptide required a longer period of ageing. By contrast, oxidizing the sulfur of methionine-35 to a sulfoxide, or substituting methionine-35 with valine, completely abrogated the peptide's inhibitory potential towards cytochrome oxidase. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis revealed that the loss of inhibitory potential towards cytochrome oxidase with the methionine-35-altered peptides did not correlate with a substantially different distribution of amyloid-beta oligomeric species. Although the amyloid-beta-mediated inhibition of cytochrome oxidase was completely dependent on the presence of divalent Cu2+, it was not supported by monovalent Cu+, and experiments with catalase and H2O2 indicated that the mechanism of cytochrome oxidase inhibition does not involve amyloid-beta-mediated H2O2 production. We propose that amyloid-beta-mediated inhibition of cytochrome oxidase is dependent on the peptide's capacity to bind, then reduce Cu2+, and that it may involve the formation of a redox active amyloid-beta-methionine radical.

Hexapeptides HLDF-6 and PEDF-6 restore memory in rats after chronic intracerebroventricular treatment with β-amyloid peptide Aβ(25–35)

Effects of homologous peptides HLDF-6 and PEDF-6 on behavior of animals with experimental Alzheimer's disease induced by chronic intracerebroventricular administration of beta-amyloid peptide Abeta(25-35) were studied in the zoosocial recognition test and Morris water maze. Peptides HLDF-6 and PEDF-6 possessed neuroprotective activity and counteracted the toxic effect of Abeta(25-35). Peptides HLDF-6 and PEDF-6 mainly improved long-term memory and working memory, respectively.

Alzheimer's amyloid beta-peptide (1-42) induces cell death in human neuroblastoma via bax/bcl-2 ratio increase: an intriguing role for methionine 35

The beta amyloid (Abeta), the major protein component of brain senile plaques in Alzheimer's disease, is known to be directly responsible for the production of free radicals toxic to brain tissue and the redox state of Met-35 residue seems to play a particular and critical role in peptide's neurotoxic actions. In this study, we investigated, in human neuroblastoma cells (IMR-32), the relationship between the oxidative state of methionine, and both neurotoxic and pro-apoptotic actions induced by Abeta-peptide, comparing the effects of native peptide, in which the Met-35 is present in the reduced state, with those of a modified peptide with oxidized Met-35 (Abeta(1-42)(35Met-ox)), as well as an Abeta-derivative with Met-35 substituted with norleucine (Abeta(1-42)(35Nle)). The obtained results show that Abeta induces a time-dependent decrease in cell viability; Abeta(1-42)(35Met-ox) was significantly less potent, though inducing a remarkable decrease in cell viability compared to control. On the contrary, no toxic effects were observed after treatment with Abeta(1-42)(35Nle). Abeta-peptide as well as the amyloid modified peptide with oxidized Met-35 induced the pro-apoptotic gene bax over-expression after 24 h, whereas Abeta(1-42)(35Nle) had no effect. Conversely, bcl-2, an anti-apoptotic gene, became highly down-regulated by Abeta peptide treatment, in contrast to that evidenced by the Abeta(1-42)(35Met-ox) peptide. Finally, Abeta caused an increase in caspase-3 activity to be higher with respect to that shown by Abeta(1-42)(35Met-ox) while Abeta(1-42)(35Nle) had no effect. These results support the hypothesis that Abeta-induced neurotoxicity occurs via bax over-expression, bcl-2 down-regulation, and caspase-3 activation, first indicating that methionine 35 redox state may alter this cell death pathway.

Aβ(31-35) and Aβ(25-35) fragments of amyloid beta-protein induce cellular death through apoptotic signals: Role of the redox state of methionine-35

In order to clarify the basis of neuronal toxicity exerted by the shortest active peptides of amyloid beta-protein (Abeta), the toxic effects of Abeta(31-35) and Abeta(25-35) peptides on isolated rat brain mitochondria were investigated. The results show that exposure of isolated rat brain mitochondria to Abeta(31-35) and Abeta(25-35) peptides determines: (i) release of cytochrome c; (ii) mitochondrial swelling and (iii) a significant reduction in mitochondrial oxygen consumption. In contrast, the amplitude of these events resulted attenuated in isolated brain mitochondria exposed to the Abeta(31-35)Met35(OX) in which methionine-35 was oxidized to methionine sulfoxide. The Abeta peptide derivative with norleucine substituting Met-35, i.e., Abeta(31-35)Nle-35, had not effect on any of the biochemical parameters tested. We have further characterized the action of Abeta(31-35) and Abeta(25-35) peptides on neuronal cells. Taken together our result indicate that Abeta(31-35) and Abeta(25-35) peptides in non-aggregated form, i.e., predominantly monomeric, are strongly neurotoxic, having the ability to enter within the cells, determining mitochondrial damage with an evident trigger of apoptotic signals. Such a mechanism of toxicity seems to be dependent by the redox state of methionine-35.

Synaptic plasma membrane-bound acetylcholinesterase activity is not affected by docosahexaenoic acid-induced decrease in membrane order

We investigated the effect of administration of docosahexaenoic acid (C22:6, n-3; 300 mg/kg.day, for 12 weeks) on the degree of membrane order and membrane-bound acetylcholinesterase activity of the cerebral cortex synaptic plasma membrane in male Wistar rats. Docosahexaenoic acid levels in the synaptic plasma membrane increased significantly by 16% over levels in control rats concomitant with an increase in the molar ratio of docosahexaenoic acid to arachidonic acid. Synaptic plasma membrane order, assessed by 1,6-diphenyl-1,3,5-hexatriene, which measures order of the bulk internal hydrophobic lipid core, decreased significantly in the docosahexaenoic acid-fed rats. Lateral mobility of both global and annular lipids measured by pyrene also increased. Acetylcholinesterase activity of the synaptic plasma membrane was unaffected, and synaptic plasma membrane phospholipid contents increased in the docosahexaenoic acid-fed rats, with a concomitant decrease in the cholesterol/phospholipid molar ratio. Lipid peroxide and reactive oxygen species, indicators of tissue oxidative stress, decreased in both the cerebral cortex synaptosome and homogenate of the docosahexaenoic acid-fed rats. Arrhenius plot showed a break point in acetylcholinesterase activity at 22 degrees C and 24 degrees C in plasma membranes from docosahexaenoic acid-fed and control rats, respectively. The present experiment indicates that chronic administration of docosahexaenoic acid does not affect synaptic acetylcholinesterase activity and evoke oxidative stress, although it increases the disorder of the global and annular lipids of rat synaptic plasma membranes.

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.

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.

Aluminum accumulation and membrane fluidity alteration in synaptosomes isolated from rat brain cortex following aluminum ingestion: effect of cholesterol

In the present work, we studied the effect of cholesterol/phospholipid (CH/PL) molar ratio on aluminum accumulation and aluminum-induced alteration of membrane fluidity in rat brain cortex synaptosomes. We observed that sub-acute (daily supply of 1.00 g of AlCl(3) during 10 days) and chronic (daily supply of 0.03 g of AlCl(3) during 4 months) exposure to dietary aluminum leads to a synaptosomal aluminum enrichment of 45 and 59%, respectively. During chronic exposure to AlCl(3), the enhancement of aluminum content was prevented by administration of colestipol (0.31 g/day), which decreased the synaptosomal membrane CH/PL molar ratio (nmol/nmol) from 1.2 to 0.4. Fluorescence anisotropy analysis, using 1,6-diphenyl-1,3,5-hexatriene (DPH) and 1-(4-(trimethylamino)phenyl)-6-phenylhexa-1,3,5-triene (TMA-DPH), showed that after treatment with colestipol a decrease in membrane order occurs at the level of hydrophilic lipid-water surface and deeper hydrophobic region of the synaptosomal membrane. When the rats were exposed to aluminum, it was observed a significant enhancement of membrane fluidity, which was more pronounced at the level of the membrane hydrophilic regions. Meanwhile, when chronic exposure to dietary AlCl(3) was accompanied by treatment with colestipol, the aluminum-induced decrease in membrane order was negligible when compared to TMA-DPH and DPH anisotropy values measured upon colestipol treatment. In contrast, in vitro incubation of synaptosomes (isolated from control rats) with AlCl(3) induced a concentration-dependent rigidification of this more hydrophilic membrane region. The opposite action of aluminum on synaptosomal membrane fluidity, during in vivo and in vitro experiments, appears to be explained by alteration of synaptosomal CH/PL molar ratio, since a significant reduction (approximately 80%) of this parameter occurs during in vivo exposure to aluminum. In conclusion, during in vivo exposure to aluminum, fluidification of hydrophilic regions and reduction of CH/PL molar ratio of presynaptic membranes accompany the accumulation of this cation, which appear to restrict aluminum retention in brain cortex nerve terminals.

Plasma membrane cholesterol controls the cytotoxicity of Alzheimer's disease AbetaP (1-40) and (1-42) peptides

Cell degeneration in Alzheimer's disease is mediated by a toxic mechanism that involves interaction of the AbetaP peptide with the plasma membrane of the target cell. We report here that PC12 cells become resistant to the cytotoxic action of AbetaP when incubated in a medium that enriches cholesterol levels of the surface membrane. On the other hand, making cholesterol-deficient membranes by either cholesterol extraction with cyclodextrin or by inhibiting de novo synthesis of cholesterol makes PC12 cells more vulnerable to the action of AbetaP. Increasing cholesterol content of PS liposomes also suppresses AbetaP-dependent liposome aggregation. We suggest that by modifying the fluidity of the neuronal membranes, cholesterol modulates the incorporation and pore formation of AbetaP into cell membranes. This idea is supported by our finding that the enhanced cytotoxicity generated by lowering the membrane cholesterol content can be reversed by AbetaP calcium channel blockers Zn2+ and tromethamine.

Cholesterol, a modulator of membrane-associated Abeta-fibrillogenesis and neurotoxicity

Recent studies have suggested that cholesterol, an important determinant of the physical state of biological membranes, plays a significant role in the development of Alzheimer's disease. We have employed in situ scanning probe microscopy, fluorescence anisotropy, and electron microscopy to investigate how cholesterol levels within total brain lipid bilayers effect amyloid beta-peptide (Abeta)-assembly. Fluorescence anisotropy measurements revealed that the relative fluidity of the total brain lipid membranes was influenced by the level of cholesterol and the addition of Abeta40 resulted in a decrease in the overall vesicle fluidity. In situ scanning probe microscopy performed on supported planar bilayers of total brain lipid revealed a correlation between membrane fluidity, as influenced by cholesterol level, and the extent of Abeta-insertion and subsequent fibrillogenesis. These observations were consistent with fluorescence microscopy studies of PC-12 and SH-SY5Y cell lines exposed to exogenous Abeta, which revealed an inverse correlation between membrane cholesterol level, and Abeta-cell surface binding and subsequent cell death. These results collectively suggest that Abeta-cell surface interactions are mediated by cellular cholesterol levels, the distribution of cholesterol throughout the cell, and membrane fluidity.

Diversity in antistaphylococcal mechanisms among membrane-targeting antimicrobial peptides

Many antimicrobial peptides permeabilize the bacterial cytoplasmic membrane. However, it is unclear how membrane permeabilization and antimicrobial activity are related for distinct peptides. This study investigated the relationship between Staphylococcus aureus membrane permeabilization and cell death due to the following antistaphylococcal peptides: thrombin-induced platelet microbicidal protein 1 (tPMP-1), gramicidin D, and protamine. Isogenic S. aureus strains ISP479C and ISP479R (tPMP-1 susceptible and resistant, respectively), were loaded with the fluorochrome calcein and exposed to a range of concentrations of each peptide. Flow cytometry was then used to monitor membrane permeabilization by quantifying the release of preloaded calcein. Killing was determined by quantitative culture at time points simultaneous to measurement of membrane permeabilization. Membrane permeabilization and killing caused by tPMP-1 occurred in a time- and concentration-dependent manner, reflecting the intrinsic tPMP-1 susceptibilities of ISP479C and ISP479R. In comparison, gramicidin D killed both S. aureus strains to equivalent extents in a concentration-dependent manner between 0.5 to 50 microg/ml, but cell permeabilization only occurred at the higher peptide concentrations (25 and 50 microg/ml). Protamine permeabilized, but did not kill, either strain at concentrations up to 10 mg/ml. Regression analyses revealed different relationships between membrane permeabilization and staphylocidal activity for the distinct antimicrobial peptides. Taken together, these findings demonstrate that permeabilization, per se, does not invariably result in staphylococcal death due to distinct antimicrobial peptides. Thus, although each of these peptides interacts with the S. aureus cytoplasmic membrane, diversity exists in their mechanisms of action with respect to the relationship between membrane permeabilization and staphylocidal activity.

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.

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.

Membrane fluidity effects of estratrienes

Estrogens have demonstrable neuroprotective effects. This fact has lead to the proposed use of estrogens for the prevention and/or treatment of Alzheimer's disease. The exact protective mechanism estrogens provide is not fully understood. In this report, a potential non-genomic mechanism for estratrienes involving alterations in membrane fluidity was studied. Steroids, such as estrogen, are known to be membrane-active and can alter the lipid packing. In this study we used fluorescent methodologies to address the effect of naturally occurring steroids (17alpha and 17beta-estradiol, testosterone, and progesterone) and new estratriene analogs on membrane fluidity using liposomes and HT-22 hippocampal cells. The study's results indicate steroids, based on the estratriene nucleus, can modulate lipid packing as evidenced by (1) decreased membrane fusion events and (2) decreased membrane fluidity. The effects on the membrane were both time and concentration dependent. It was also demonstrated through rational design estratriene analogs can be synthesized with enhanced membrane effects. Finally, in a glutamate-induced toxicity HT-22 model, we also demonstrated cellular protection with the estratriene-based molecules and analogs. The data suggest the plethora of cellular actions of estrogens may relate to or be influenced by membrane effects of the steroid.

A nonfibrillar form of the fusogenic prion protein fragment [118-135] induces apoptotic cell death in rat cortical neurons

Neuronal loss is a salient feature of prion diseases. However, its cause and mechanism, particularly its relationship with the accumulation and precipitation of the pathogenic, protease-resistant isoform PrP(Sc) of the cellular prion protein PrP(C), are still an enigma. Several studies suggest that neuronal loss could occur through a process of programmed cell death, which is consistent with the lack of inflammation in these conditions. By analogy with the pathological events occurring during the development of Alzheimer's disease, controversies still exist regarding the relationship between amyloidogenesis, prion aggregation, and neuronal loss. We recently demonstrated that a prion protein fragment (118-135) displayed membrane-destabilizing properties and was able to induce, in a nonfibrillar form, the fusion of unilamellar liposomes. To unravel the mechanism of prion protein neurotoxicity, we characterize the effects of the human Pr[118-135] peptide on rat cortical neurons. We demonstrate that low concentrations of the Pr[118-135] peptide, in a nonfibrillar form, induce a time- and dose- dependent apoptotic cell death, including caspase activation, DNA condensation, and fragmentation. This toxicity might involve oxidative stress, because antioxidant molecules, such as probucol and propyl gallate, protect neurons against prion peptide toxicity. By contrast, a nonfusogenic variant Pr[118-135, 0 degrees ] peptide, which displays the same amino acid composition but several amino acid permutations, is not toxic to cortical neurons, which emphasizes the critical role of the fusogenic properties of the prion peptide in its neurotoxicity. Taken together, our results suggest that the interaction between the Pr[118-135] peptide and the plasma membrane of neurons might represent an early event in a cascade leading to neurodegeneration.

Apoptosis induced in neuronal cells by C-terminal amyloid beta-fragments is correlated with their aggregation properties in phospholipid membranes

A number of findings suggest that lipophilic monomeric Abeta peptides can interact with the cellular lipid membranes. These interactions can affect the membrane integrity and result in the initiation of apoptotic cell death. The secondary structure of C-terminal Abeta peptides (29-40) and the longer (29-42) variant have been investigated in solution by circular dichroism measurements. The secondary structure of lipid bound Abeta (29-40) and (29-42) peptides prepared at different lipid/peptide ratio's, was investigated by ATR-FTIR spectroscopy. Finally, the changes in secondary structure (i.e. the transition of alpha-helix to beta-sheet) of the lipid bound peptides were correlated with the induction of neurotoxic and apoptotic effects in neuronal cells. The data suggest that the C-terminal fragments of the Abeta peptide induce a significant apoptotic cell death, as demonstrated by caspase-3 measurements and DNA laddering, with consistently a stronger effect of the longer Abeta (29-42) variant. Moreover, the induction of apoptotic death induced by these peptides can be correlated with the secondary structure of the lipid bound amyloid beta peptides. Based on these observations, it is proposed that membrane bound aggregated Abeta peptides (produced locally as the result of gamma-secretase cleavage) can accumulate and aggregate in the membrane. These membrane bound beta-sheet aggregated amyloid peptides induce neuronal apoptotic cell death.

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.

Abnormal erythrocyte anion exchange in Alzheimer disease

CONTEXT

Several abnormalities have been described in red blood cells of patients with Alzheimer disease (AD), but to date none of these has been confirmed by a second, independent study. Erythrocyte anion exchange has been reported to be abnormal in AD; we have developed a new technique for measuring anion exchange.

OBJECTIVES

To confirm the abnormality of erythrocyte anion exchange in AD and to determine whether the phenomenon has potential for clinical utility.

DESIGN

Comparison of patients with probable AD to age-matched controls.

SETTING

University hospital and ambulatory clinic.

METHODS

Chloride-bicarbonate exchange was measured in erythrocyte ghosts resealed with a fluorescent probe of chloride concentration.

RESULTS

Erythrocyte anion exchange is abnormal in AD. This difference appears in citrate but not EDTA anticoagulant. Mahalanobis's generalized distance between the 2 populations is 1.7, and a discriminant function derived from our technique classifies 82% of the study population in accordance with the National Institute of Neurological and Communicative Disorders and Stroke-Alzheimer's Disease and Related Disorders Association criteria. Receiver operating characteristic analysis demonstrates the possibility of choosing cutoffs with high sensitivity and specificity.

CONCLUSIONS

Measurement of red blood cell anion exchange may be useful in classifying patients with AD. The dependence of this phenomenon on anticoagulant suggests the involvement of platelet activation or complement fixation.

Alzheimer's beta-amyloid, human islet amylin, and prion protein fragment evoke intracellular free calcium elevations by a common mechanism in a hypothalamic GnRH neuronal cell line

A growing number of reports suggest that elevated levels of extracellular Alzheimer's beta-amyloid protein alter the homeostasis of free [Ca(2+)](i) in different cell types of the mammalian brain. In line with these results, we have previously shown that AbetaP[1-40] forms cation-selective channels (Ca(2+) included) across artificial planar bilayers formed from acidic phospholipids and across excised membrane patches from immortalized hypothalamic GnRH neurons (GT1-7 cells), suggesting that the nonregulated Ca(2+)-influx through these spontaneously formed "amyloid channels" may provide a mechanism to explain its toxicity (1). We have now found and report here that the application of AbetaP[1-40] to GT1-7 neurons consistently elevates [Ca(2+)](i) levels. We also found that human islet amylin and the prion protein fragment (PrP106-126), peptides that acquire beta-pleated sheet conformation in water solutions and have been reported to form ion channels across planar bilayer membranes, also increase cytosolic free calcium in GT1-7 neurons. Searching for protective agents, we found that soluble cholesterol, known to decrease the fluidity of the cell membrane, inhibits AbetaP[1-40]-evoked [Ca(2+)](i) rise. These results suggest that unregulated Ca(2+) entry across amyloid channels may be a common mechanism causing cell death, not only in diseases of the third age, including Alzheimer's disease and type 2 diabetes mellitus, but also in prion-induced diseases.

Brain biochemistry using magnetic resonance spectroscopy: relevance to psychiatric illness in the elderly

Magnetic resonance spectroscopy (MRS) allows for the noninvasive study of cerebral biochemistry. It has been used to investigate cerebral metabolic changes associated with mental illness in vivo and in vitro. In this review, we will discuss the application of MRS to psychiatric illness in the elderly. Following a brief description of the basic principles of MRS, the use of phosphorus (31P) and proton (1H) MRS to enable a better understanding of normal brain aging, dementia (Alzheimer's disease, multiple subcortical infarct dementia, Down syndrome, frontotemporal dementia, vascular dementia, age-associated memory impairment, and other dementias), major depression, and electroconvulsive therapy is detailed.

Interaction of amyloid beta-protein with anionic phospholipids: possible involvement of Lys28 and C-terminus aliphatic amino acids

Fibrillar amyloid beta-protein (Abeta) is the major protein of amyloid plaques in the brains of patients with Alzheimer's disease (AD). The mechanism by which normally produced soluble Abeta gets fibrillized in AD is not clear. We studied the effect of neutral, zwitterionic, and anionic lipids on the fibrillization of Abeta 1-40. We report here that acidic phospholipids such as phosphatidic acid, phosphatidylserine, phosphatidylinositol (PI), PI 4-phosphate, PI 4,5-P2 and cardiolipin can increase the fibrillization of Abeta, while the neutral lipids (diacylglycerol, cholesterol, cerebrosides), zwitterionic lipids (phosphatidylcholine, phosphatidylethanolamine, sphingomyelin) and anionic lipids lacking phosphate groups (sulfatides, gangliosides) do not affect Abeta fibrillization. Abeta was found to increase the fluorescence of 1-acyl-2-[12-[(7-nitro-2-1, 3-benzoxadiazol-4-yl) amino] dodecanoyl]-sn-glycero-3-phosphate (NBD-PA) in a concentration-dependent manner, while no change was observed with 1-acyl-2- [12-[(7-nitro-2-1, 3-benzoxadiazol-4-yl) amino] dodecanoyl]-sn-glycero-3-phosphoethanolamine (NBD-PE). Under similar conditions, other proteins such as apolipoprotein E, gelsolin and polyglutamic acid did not interact with NBD-PA. The order of interaction of amyloid beta-peptides with NBD-PA was Abeta 1-43 = Abeta 1-42 = Abeta 17-42 > Abeta 1-40 = Abeta 17-40. Other Abeta peptides such as Abeta 1-11, Abeta 1-16, Abeta 1-28, Abeta 1-38, Abeta 12-28, Abeta 22-35, Abeta 25-35, and Abeta 31-35 did not increase the NBD-PA fluorescence. These results suggest that phosphate groups, fatty acids, and aliphatic amino acids at the C-terminus end of Abeta 1-40/Abeta 1-42 are essential for the interaction of Abeta with anionic phospholipids, while hydrophilic Abeta segment from 1-16 amino acids does not participate in this interaction. Since positively charged amino acids in Abeta are necessary for the interaction with negatively charged phosphate groups of phospholipids, it is suggested that Lys28 of Abeta may provide anchor for the phosphate groups of lipids, while aliphatic amino acids (Val-Val-Ile-Ala) at the C-terminus of Abeta interact with fatty acids of phospholipids.

Aggregated beta amyloid peptide 1-40 decreases Ca2+- and cholinergic receptor-mediated phosphoinositide degradation by alteration of membrane and cytosolic phospholipase C in brain cortex

The effects of full-length amyloid beta protein, A(beta) (1-40), on phosphoinositide-specific phospholipase C (PLC) were investigated in synaptic plasma membranes (SPM) and cytosol prepared from the cerebral cortex of adult rats. Moreover, the role of A(beta) (1-40) on the activation of lipid peroxidation was evaluated. The activity of phospholipase C (PLC) acting on phosphatidylinositol (PI) and phosphatidylinositol-4,5-bisphosphate (PIP2) was determined using exogenous labeled substrates. The subcellular fractions were the source of enzyme(s). The radioactivity of lipid messengers derived from degradation of [14C- arachidonoyl] PI was also determined. The stable aggregated form of beta-amyloid peptide (1-40) at 25 microM concentration exerted reproducible effects. The aggregated form of A(beta) (1-40) inhibited Ca(2+)-regulated PI and PIP2 degradation by SPM and cytosolic enzymes. Aggregated A(beta) also decreased significantly the level of diacylglycerol, the product of PLC. This additionally supports the inhibitory effect of A(beta) on membrane-bound and cytosolic PLC. Moreover, A(beta) (1-40) significantly decreased the basal activity of the PIP2-PLC in SPM and the enzyme activity regulated through cholinergic receptors. However, in spite of the lower enzyme activity, the percentage distribution of inositol (1,4,5) P3 radioactivity (IP3) in the total pool of inositol metabolites was not significantly changed. The aggregated neurotoxic fragment, A(beta) (25-35), mimicked the effect of full-length A(beta) (1-40). A(beta) (1-40) enhanced the level of malondialdehyde indicating an activation of free radical stimulated membrane lipid peroxidation that may be involved in alteration of phospholipase(s) activity. Our results indicated that aggregated A(beta) (1-40) alters Ca(2+)-dependent phosphoinositide degradation affecting synaptic plasma membrane and cytosolic phospholipase(s) activity. Moreover, this peptide significantly decreased the phosphoinositide-dependent signal transduction mediated by cholinergic receptors. The effect of aggregated A(beta) (1-40) is more pronounced than that of the neurotoxic fragment A(beta) (25-35). Our study suggests that the deposition of aggregated A(beta) may alter phosphoinositide signaling in brain.

Amyloid beta peptide impaired carbachol but not glutamate-mediated phosphoinositide pathways in cultured rat cortical neurons

Signal transduction systems, including cholinergic pathways, which are likely to be of pathophysiological significance are altered in Alzheimer's disease (AD). Muscarinic cholinergic receptors are linked to the hydrolysis of phosphoinositide, involving the production of inositol 1,4,5-trisphosphate [Ins(1,4,5)P3] and the mobilization of cytosolic free calcium concentrations ([Ca2+]i). Effects of amyloid peptide (A(beta)) on these signals prior to neuronal degeneration were examined in cultured rat cortical cells. A(beta) increased the release of lactate dehydrogenase (LDH) in a concentration-dependent manner, however, it was blocked by B27 supplement. Prolonged exposure to a sublethal dose of A(beta) 25-35 or 1-42 disrupted carbachol-mediated release of Ins(1,4,5)P3 and [Ca2+]i, which was inhibited in media supplemented with B27 or the antioxidant vitamin E. In order to determine the specificity of the effect of A(beta), various agonists glutamate or KCl but not bradykinin which utilize the phosphoinositide cascade were investigated. Our results indicated that A(beta) did not affect the stimulation of glutamate or KCl-mediated production of Ins(1,4,5)P3 or cause elevation in [Ca2+]i. Furthermore, metabotropic agonist trans-1-amino-cyclopentane-1,3,-dicarboxylate (ACPD) elevated calcium level was not inhibited by A(beta) pre-treatment. Taken together, the results demonstrate that a sublethal dose of A(beta) selectively impaired cholinergic receptor-mediated signal transduction pathways, and antioxidant or B27 supplement attenuated this effect of A(beta). Alterations of cholinergic signaling by prolonged exposure to A(beta) could be involved in cortical neurodegeneration that occurs in AD. Because functional loss of cholinergic pathways is an important aspect of AD, the differences in susceptibility of these two types of receptors prior to other signs of A(beta) action is important and requires further investigation.

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.

Plasma membrane ordering agent pluronic F-68 (PF-68) reduces neurotransmitter uptake and release and produces learning and memory deficits in rats

A substantial body of evidence indicates that aged-related changes in the fluidity and lipid composition of the plasma membrane contribute to cellular dysfunction in humans and other mammalian species. In the CNS, reductions in neuronal plasma membrane order (PMO) (i.e., increased plasma membrane fluidity) have been attributed to age as well as the presence of the beta-amyloid peptide-25-35, known to play an important role in the neuropathology of Alzheimer's disease (AD). These PMO increases may influence neurotransmitter synthesis, receptor binding, and second messenger systems as well as signal transduction pathways. The effects of neuronal PMO on learning and memory processes have not been adequately investigated, however. Based on the hypothesis that an increase in PMO may alter a number of aspects of synaptic transmission, we investigated several neurochemical and behavioral effects of the membrane ordering agent, PF-68. In cell culture, PF-68 (nmoles/mg SDS extractable protein) reduced [3H]norepinephrine (NE) uptake into differentiated PC-12 cells as well as reduced nicotine stimulated [3H]NE release. The compound (800-2400 microg/kg, i.p., resulting in nmoles/mg SDS extractable protein in the brain) decreased step-through latencies and increased the frequencies of crossing into the unsafe side of the chamber in inhibitory avoidance training. In the Morris water maze, PF-68 increased the latencies and swim distances required to locate a hidden platform and reduced the time spent and distance swam in the previous target quadrant during transfer (probe) trials. PF-68 did not impair performance of a well-learned working memory task, the rat delayed stimulus discrimination task (DSDT), however. Studies with 14C-labeled PF-68 indicated that significant (pmoles/mg wet tissue) levels of the compound entered the brain from peripheral (i.p.) injection. No PF-68 related changes were observed in swim speeds or in visual acuity tests in water maze experiments, rotorod performance, or in tests of general locomotor activity. Furthermore, latencies to select a lever in the DSDT were not affected. These results suggest that PF-68 induced deficits in learning and memory without confounding peripheral motor, sensory, or motivational effects at the tested doses. Furthermore, none of the doses induced a conditioned taste aversion to a novel 0.1% saccharin solution indicating a lack of nausea or gastrointestinal malaise induced by the compound. The data indicate that increases in neuronal plasma membrane order may have significant effects on neurotransmitter function as well as learning and memory processes. Furthermore, compounds such as PF-68 may also offer novel tools for studying the role of neuronal PMO in mnemonic processes and changes in PMO resulting from age-related disorders such as AD.

Laminin 1 attenuates beta-amyloid peptide Abeta(1-40) neurotoxicity of cultured fetal rat cortical neurons

A growing amount of evidence indicates the involvement of extracellular matrix components, especially laminins, in the development of Alzheimer's disease, although their role remains unclear. In this study, we clearly demonstrate that laminin 1 inhibits beta-amyloid peptide (Abeta)-induced neuronal cell death by preventing the fibril formation and interaction of the Abeta peptide with cell membranes. The presence of laminin at a laminin/Abeta peptide molar ratio of 1:800 significantly inhibits the Abeta-induced apoptotic events, together with inhibition of amyloid fibril formation. The inhibitory effects of laminin 1 were time- and dose-dependent, whereas laminin 2 had less effect on Abeta neurotoxicity. A preincubation of laminin and Abeta was not required to observe the protective effect of laminin, suggesting a direct interaction between laminin 1 and Abeta. Moreover, laminin had no effect on the toxicity of the fibrillar Abeta peptide, suggesting an interaction of laminin with nonfibrillar species of the Abeta peptide, sequestering the peptide in a soluble form. These data extend our understanding of laminin-dependent binding of Abeta and highlight the possible modulation role of laminin regarding Abeta aggregation and neurotoxicity in vivo.

Internalization and resistance to degradation of Alzheimer's A beta 1-42 at nanomolar concentrations in THP-1 human monocytic cell line

Microglial cell involvement in Alzheimer's disease has been related to amyloid beta (A beta) internalization, the release of inflammatory cytokines and the development of neuritic plaques. The human monocyte/macrophage THP-1 cell line has been widely used as a model of human microglial cells. We used THP-1 cells to study the adsorption, internalization and resistance to degradation of A beta1-40 and A beta1-42 isoforms offered at nanomolar concentrations and free of large aggregates, conditions that may mimic a pre-fibrillar stage of A beta in the brain. Under these conditions, A betas did not induce THP-1 activation, as assessed by interleukin-1beta expression. A beta1-42 showed a preferential adsorption and intracellular accumulation as compared to A beta1-40, supporting that competent nuclei for A beta1-42 ordered aggregation may be formed inside microglial cells. In light of the possible neurotoxicity of soluble A beta1-42, we propose that amyloid formation within brain phagocytic cells may be a protective mechanism in early stages of the disease.