The interaction of amyloid Abeta(1-40) with lipid bilayers and ganglioside as studied by 31P solid-state NMR

NMR studies of amyloid interactions

Amyloid fibrils are insoluble, fibrous nanostructures that accumulate extracellularly in biological tissue during the progression of several human disorders, including Alzheimer's disease (AD) and type 2 diabetes. Fibrils are assembled from protein monomers via the transient formation of soluble, cytotoxic oligomers, and have a common molecular architecture consisting of a spinal core of hydrogen-bonded protein β-strands. For the past 25 years, NMR spectroscopy has been at the forefront of research into the structure and assembly mechanisms of amyloid aggregates. Until the recent boom in fibril structure analysis by cryo-electron microscopy, solid-state NMR was unrivalled in its ability to provide atomic-level models of amyloid fibril architecture. Solution-state NMR has also provided complementary information on the early stages in the amyloid assembly mechanism. Now, both NMR modalities are proving to be valuable in unravelling the complex interactions between amyloid species and a diverse range of physiological metal ions, molecules and surfaces that influence the assembly pathway, kinetics, morphology and clearance in vivo. Here, an overview is presented of the main applications of solid-state and solution-state NMR for studying the interactions between amyloid proteins and biomembranes, glycosaminoglycan polysaccharides, metal ions, polyphenols, synthetic therapeutics and diagnostics. Key NMR methodology is reviewed along with examples of how to overcome the challenges of detecting interactions with aggregating proteins. The review heralds this new role for NMR in providing a comprehensive and pathologically-relevant view of the interactions between protein and non-protein components of amyloid. Coverage of both solid- and solution-state NMR methods and applications herein will be informative and valuable to the broad communities that are interested in amyloid proteins.

A Small Molecule Impedes the Aβ1-42 Tetramer Neurotoxicity by Preserving Membrane Integrity: Microsecond Multiscale Simulations

Amyloid-β (Aβ1-42) peptides aggregated into plaques deposited in the brain are the main hallmark of Alzheimer's disease (AD), a social and economic burden worldwide. In this context, insoluble Aβ1-42 fibrils are the main components of plaques. The recent trials that used approved AD drugs show that they can remove the fibrils from AD patients' brains, but they did not halt the course of the disease. Mounting evidence envisages that the soluble Aβ1-42 oligomers' interactions with the neuronal membrane trigger higher cell death than Aβ1-42 fibril interactions. Developing a compound that can alleviate the oligomer's toxicity is one of the most demanding tasks for curing the disease. We performed two molecular dynamics (MD) simulations in an explicit solvent model. In the first case, 55-μs of multiscale all-atom (AA)/coarse-grained (CG) MD simulations were carried out to decipher the impact of a previously described small anti-Aβ molecule, termed M30 (2-octahydroisoquinolin-2(1H)-ylethanamine), on an Aβ1-42 tetramer structure in close contact with a DMPC bilayer. In the second case, 15-μs AA/CG MD simulations were performed to rationalize the dynamics between Aβ1-42 and Aβ1-42-M30 tetramer complexes embedded in DMPC. On the membrane bilayer, we found that the Aβ1-42 tetramer penetrates the bilayer surface due to unrestricted conformational flexibility and many contacts with the membrane phosphate groups. In contrast, no Aβ1-42-M30 tetramer penetration was observed during the entire course of the simulation. In the case of the membrane-embedded Aβ1-42 tetramer, the integrity of the bottom bilayer leaflet was severely affected by the interactions between the negatively charged phosphate groups and the positively charged residues of the Aβ1-42 tetramer, resulting in a deep tetramer penetration into the bilayer hydrophobic region. These contacts were not observed in the case of the membrane-embedded Aβ1-42-M30 tetramer. It was noted that M30 molecules bind to Aβ1-42 tetramer through hydrogen bonds, resulting in a conformational stable Aβ1-42-M30 complex. The associated complex has reduced conformational changes and an enhanced rigidity that prevents the tetramer dissociation by interfering with the tetramer-membrane contacts. Our findings suggest that the M30 molecules could bind to Aβ1-42 tetramer resulting in a rigid structure, and that such complexes do not significantly perturb the membrane bilayer organization. These observations support the in vitro and in vivo experimental evidence that the M30 molecules prevent synaptotocity, improving AD-affected mice memory.

Classification of binding property of amyloid β to lipid membranes: Membranomic research using quartz crystal microbalance combined with the immobilization of lipid planar membranes

A biomembrane-related fibrillogenesis of Amyloid β from Alzheimer' disease (Aβ) is closely related to its accumulation behavior. A binding property of Aβ peptides from Alzheimer' disease to lipid membranes was then classified by a quartz crystal microbalance (QCM) method combined with an immobilization technique using thiol self-assembled membrane. The accumulated amounts of Aβ, Δfmax, was determined from the measurement of the maximal frequency reduction using QCM. The plots of Δfmax to Aβ concentration gave the slope and saturated value of Δfmax, (Δfmax)sat that are the parameters for binding property of Aβ to lipid membranes. Therefore, the Aβ-binding property on lipid membranes was classified by the slope and (Δfmax)sat. The plural lipid system was described as X + Y where X = L1, L1/L2, and L1/L2/L3. The slope and (Δfmax)sat values plotted as a function of mixing ratio of Y to X was classified on a basis of the lever principle (LP). The LP violation observed in both parameters resulted from the formation of the crevice or pothole, as Aβ-specific binding site, generated at the boundary between ld and lo phases. The LP violation observed only in the slope resulted from glycolipid-rich domain acting as Aβ-specific binding site. Furthermore, lipid planar membranes indicating strong LP violation favored strong fibrillogenesis. Especially, lipid planar membranes indicating the LP violation only in the slope induced lateral aggregated and spherulitic fibrillar aggregates. Thus, the classification of Aβ binding property on lipid membranes appeared to be related to the fibrillogenesis with a certain morphology.

Amyloid β interaction with model cell membranes - What are the toxicity-defining properties of amyloid β?

Disruption of the neuronal membrane by toxic amyloid β oligomers is hypothesized to be the major event associated with Alzheimer's disease's neurotoxicity. Misfolding of amyloid β is followed by aggregation via different pathways in which structurally different amyloid β oligomers can be formed. The respective toxic actions of these structurally diverse oligomers can vary significantly. Linking a particular toxic action to a structurally unique kind of amyloid β oligomers and resolving their toxicity-determining feature remains challenging because of their transient stability and heterogeneity. Moreover, the lipids that make up the membrane affect amyloid β oligomers' behavior, thus adding to the problem's complexity. The present review compares and analyzes the latest results to improve understanding of amyloid β oligomers' interaction with lipid bilayers.

Exploring interactions between lipids and amyloid-forming proteins: A review on applying fluorescence and NMR techniques

A hallmark of Alzheimer's, Parkinson's, and other amyloid diseases is the assembly of amyloid proteins into amyloid aggregates or fibrils. In many cases, the formation and cytotoxicity of amyloid assemblies are associated with their interaction with cell membranes. Despite studied for many years, the characterization of the interaction is challenged for reasons on the multiple aggregation states of amyloid-forming proteins, transient and weak interactions in the complex system. Although several strategies such as computation biology, spectroscopy, and imaging methods have been performed, there is an urgent need to detail the molecular mechanism in different time scales and high resolutions. This review highlighted the recent applications of fluorescence, solution and solid-state NMR in exploring the interactions between amyloid protein and membranes attributing to their advantages of high sensitivity and atomic resolution.

Size-Dependent Interaction of Amyloid β Oligomers with Brain Total Lipid Extract Bilayer-Fibrillation Versus Membrane Destruction

Amyloid β, Aβ(1-42), is a component of senile plaques present in the brain of Alzheimer's disease patients and one of the main suspects responsible for pathological consequences of the disease. Herein, we directly visualize the Aβ activity toward a brain-like model membrane and demonstrate that this activity strongly depends on the Aβ oligomer size. PeakForce quantitative nanomechanical mapping mode of atomic force microscopy imaging revealed that the interaction of large-size (LS) Aβ oligomers, corresponding to high-molecular-weight Aβ oligomers, with the brain total lipid extract (BTLE) membrane resulted in accelerated Aβ fibrillogenesis on the membrane surface. Importantly, the fibrillogenesis did not affect integrity of the membrane. In contrast, small-size (SS) Aβ oligomers, corresponding to low-molecular-weight Aβ oligomers, created pores and then disintegrated the BTLE membrane. Both forms of the Aβ oligomers changed nanomechanical properties of the membrane by decreasing its Young's modulus by ∼45%. Our results demonstrated that both forms of Aβ oligomers induce the neurotoxic effect on the brain cells but their action toward the membrane differs significantly.

Cholesterol Changes the Mechanisms of Aβ Peptide Binding to the DMPC Bilayer

Using isobaric-isothermal all-atom replica-exchange molecular dynamics (REMD) simulations, we investigated the equilibrium binding of Aβ10-40 monomers to the zwitterionic dimyristoylphosphatidylcholine (DMPC) bilayer containing cholesterol. Our previous REMD simulations, which studied binding of the same peptide to the cholesterol-free DMPC bilayer, served as a control, against which we measured the impact of cholesterol. Our findings are as follows. First, addition of cholesterol to the DMPC bilayer partially expels the Aβ peptide from the hydrophobic core and promotes its binding to bilayer polar headgroups. Using thermodynamic and energetics analyses, we argued that Aβ partial expulsion is not related to cholesterol-induced changes in lateral pressure within the bilayer but is caused by binding energetics, which favors Aβ binding to the surface of the densely packed cholesterol-rich bilayer. Second, cholesterol has a protective effect on the DMPC bilayer structure against perturbations caused by Aβ binding. More specifically, cholesterol reduces bilayer thinning and overall depletion of bilayer density beneath the Aβ binding footprint. Third, we found that the Aβ peptide contains a single cholesterol binding site, which involves hydrophobic C-terminal amino acids (Ile31-Val36), Phe19, and Phe20 from the central hydrophobic cluster, and cationic Lys28 from the turn region. This binding site accounts for about 76% of all Aβ-cholesterol interactions. Because cholesterol binding site in the Aβ10-40 peptide does not contain the GXXXG motif featured in cholesterol interactions with the transmembrane domain C99 of the β-amyloid precursor protein, we argued that the binding mechanisms for Aβ and C99 are distinct reflecting their different conformations and positions in the lipid bilayer. Fourth, cholesterol sharply reduces the helical propensity in the bound Aβ peptide. As a result, cholesterol largely eliminates the emergence of helical structure observed upon Aβ transition from a water environment to the cholesterol-free DMPC bilayer. We explain this effect by the formation of hydrogen bonds between cholesterol and the Aβ backbone, which prevent helix formation. Taken together, we expect that our simulations will advance understanding of a molecular-level mechanism behind the role of cholesterol in Alzheimer's disease.

EGCG inhibits the oligomerization of amyloid beta (16-22) hexamer: Theoretical studies

An extensive replica exchange molecular dynamics (REMD) simulation was performed to investigate the progress patterns of the inhibition of (-)-epigallocatechin-3-gallate (EGCG) on the Aβ_16-22 hexamer. Structural variations of the oligomers without and with EGCG were monitored and analyzed in detail. It has been found that EGCG prevents the formation of Aβ oligomer through two different ways by either accelerating the Aβ oligomerization or reducing the β-content of the hexamer. It also decreases the potential "highly toxic" conformations of Aβ oligomer, which is related to the conformations having high order β-sheet sizes. Both electrostatic and van der Waals interaction energies are found to be involved to the binding process. Computed results using quantum chemical methods show that the π-π stacking is a critical factor of the interaction between EGCG and the peptides. As a result, the binding free energy of the EGCG to the Aβ peptides is slightly larger than that of the curcumin.

Reduced Lipid Bilayer Thickness Regulates the Aggregation and Cytotoxicity of Amyloid-β

The aggregation of amyloid-β (Aβ) on lipid bilayers has been implicated as a mechanism by which Aβ exerts its toxicity in Alzheimer's disease (AD). Lipid bilayer thinning has been observed during both oxidative stress and protein aggregation in AD, but whether these pathological modifications of the bilayer correlate with Aβ misfolding is unclear. Here, we studied peptide-lipid interactions in synthetic bilayers of the short-chain lipid dilauroyl phosphatidylcholine (DLPC) as a simplified model for diseased bilayers to determine their impact on Aβ aggregate, protofibril, and fibril formation. Aβ aggregation and fibril formation in membranes composed of dioleoyl phosphatidylcholine (DOPC) or 1- palmitoyl-2-oleoyl phosphatidylcholine mimicking normal bilayers served as controls. Differences in aggregate formation and stability were monitored by a combination of thioflavin-T fluorescence, circular dichroism, atomic force microscopy, transmission electron microscopy, and NMR. Despite the ability of all three lipid bilayers to catalyze aggregation, DLPC accelerates aggregation at much lower concentrations and prevents the fibrillation of Aβ at low micromolar concentrations. DLPC stabilized globular, membrane-associated oligomers, which could disrupt the bilayer integrity. DLPC bilayers also remodeled preformed amyloid fibrils into a pseudo-unfolded, molten globule state, which resembled on-pathway, protofibrillar aggregates. Whereas the stabilized, membrane-associated oligomers were found to be nontoxic, the remodeled species displayed toxicity similar to that of conventionally prepared aggregates. These results provide mechanistic insights into the roles that pathologically thin bilayers may play in Aβ aggregation on neuronal bilayers, and pathological lipid oxidation may contribute to Aβ misfolding.

Replica exchange molecular dynamics study of the amyloid beta (11–40) trimer penetrating a membrane

The transmembrane Aβ_11–40 trimer is investigated for the first time using REMD and FEP.

Replica exchange molecular dynamics study of the truncated amyloid beta (11–40) trimer in solution

The structure of the 3Aβ_11–40 oligomer is determined for the first time using T-REMD simulations.

Raman spectroscopy for detecting supported planar lipid bilayers composed of ganglioside-GM1/sphingomyelin/cholesterol in the presence of amyloid-β

The aggregation and fibril formation of amyloid β(Aβ) peptides onto a ganglioside-GM1-containing lipid membrane is a cause of neurodegenerative diseases. The mechanism of the initial binding and the conformational changes of Aβ on the membrane should be clarified. Fluorescence microscopy and Raman spectroscopy have been performed to investigate the supporting planar lipid bilayers (SPBs) composed of ganglioside-GM1, sphingomyelin and cholesterol. It is demonstrated that the SPBs are in a liquid-crystalline state when placed on mica, and increasing the amount of ganglioside-GM1 can decrease the lateral interaction between the acyl chains of the SPBs. It has been found that Aβ(1-40) initially interacts with the galactose ring of the ganglioside-GM1 head group, leading to its binding and gradual aggregation on the membrane surface. The obvious change observed in Raman spectroscopy in the ν(C-H) region confirms that the hydrophobic C-terminal of Aβ(1-40) inserts itself into the hydrophobic part of the SPBs. The Raman data indicate that α-helix and β-sheet structures of Aβ(1-40) increase and coexist over longer time frames. Based on these results, a model was proposed to describe the mechanism of the conformational changes and the aggregation of Aβ(1-40) that are mediated by ganglioside-GM1-containing SPBs.

Membrane-Induced Dichotomous Conformation of Amyloid β with the Disordered N-Terminal Segment Followed by the Stable C-Terminal β Structure

Various neurodegenerative disorders are ascribed to pathogenic molecular processes involving conformational transitions of amyloidogenic proteins into toxic aggregates characterized by their β structures. Accumulating evidence indicates that neuronal cell membranes provide platforms for such conformational transitions of pathogenic proteins as best exemplified by amyloid β (Aβ). Therefore, membrane-bound Aβ species can be promising targets for the development of novel drugs for Alzheimer’s disease. In the present study, solid-state nuclear magnetic resonance spectroscopy has elucidated the membrane-induced conformation of Aβ, in which the disordered N-terminal segment is followed by the stable C-terminal β strand. The data provides an insight into the molecular processes of the conformational transition of Aβ coupled with its assembly into parallel β structures.

Calcium enhances binding of Aβ monomer to DMPC lipid bilayer

Using isobaric-isothermal replica-exchange molecular dynamics and the all-atom explicit-solvent model, we studied the equilibrium binding of Aβ monomers to a zwitterionic dimyristoylphosphatidylcholine (DMPC) bilayer coincubated with calcium ions. Using our previous replica-exchange molecular dynamics calcium-free simulations as a control, we reached three conclusions. First, calcium ions change the tertiary structure of the bound Aβ monomer by destabilizing several long-range intrapeptide interactions, particularly the salt bridge Asp(23)-Lys(28). Second, calcium strengthens Aβ peptide binding to the DMPC bilayer by enhancing electrostatic interactions between charged amino acids and lipid polar headgroups. As a result, Aβ monomer penetrates deeper into the bilayer, making disorder in proximal lipids and bilayer thinning more pronounced. Third, because calcium ions demonstrate strong affinity to negatively charged amino acids, a considerable influx of calcium into the area proximal to the bound Aβ monomer is observed. Consequently, the localizations of negatively charged amino acids and calcium ions in the Aβ binding footprint overlap. Based on our data, we propose a mechanism by which calcium ions strengthen Aβ-bilayer interactions. This mechanism involves two factors: 1) calcium ions make the DMPC bilayer partially cationic and thus attractive to the anionic Aβ peptide; and 2) destabilization of the Asp(23)-Lys(28) salt bridge makes Lys(28) available for interactions with the bilayer. Finally, we conclude that a single Aβ monomer does not promote permeation of calcium ions through the zwitterionic bilayer.

Two-step mechanism of membrane disruption by Aβ through membrane fragmentation and pore formation

Disruption of cell membranes by Aβ is believed to be one of the key components of Aβ toxicity. However, the mechanism by which this occurs is not fully understood. Here, we demonstrate that membrane disruption by Aβ occurs by a two-step process, with the initial formation of ion-selective pores followed by nonspecific fragmentation of the lipid membrane during amyloid fiber formation. Immediately after the addition of freshly dissolved Aβ(1-40), defects form on the membrane that share many of the properties of Aβ channels originally reported from single-channel electrical recording, such as cation selectivity and the ability to be blockaded by zinc. By contrast, subsequent amyloid fiber formation on the surface of the membrane fragments the membrane in a way that is not cation selective and cannot be stopped by zinc ions. Moreover, we observed that the presence of ganglioside enhances both the initial pore formation and the fiber-dependent membrane fragmentation process. Whereas pore formation by freshly dissolved Aβ(1-40) is weakly observed in the absence of gangliosides, fiber-dependent membrane fragmentation can only be observed in their presence. These results provide insights into the toxicity of Aβ and may aid in the design of specific compounds to alleviate the neurodegeneration of Alzheimer's disease.

Cations as switches of amyloid-mediated membrane disruption mechanisms: calcium and IAPP

Disruption of the integrity of the plasma membrane by amyloidogenic proteins is linked to the pathogenesis of a number of common age-related diseases. Although accumulating evidence suggests that adverse environmental stressors such as unbalanced levels of metal ions may trigger amyloid-mediated membrane damage, many features of the molecular mechanisms underlying these events are unknown. Using human islet amyloid polypeptide (hIAPP, aka amylin), an amyloidogenic peptide associated with β-cell death in type 2 diabetes, we demonstrate that the presence of Ca(2+) ions inhibits membrane damage occurring immediately after the interaction of freshly dissolved hIAPP with the membrane, but significantly enhances fiber-dependent membrane disruption. In particular, dye leakage, quartz crystal microbalance, atomic force microscopy, and NMR experiments show that Ca(2+) ions promote a shallow membrane insertion of hIAPP, which leads to the removal of lipids from the bilayer through a detergent-like mechanism triggered by fiber growth. Because both types of membrane-damage mechanisms are common to amyloid toxicity by most amyloidogenic proteins, it is likely that unregulated ion homeostasis, amyloid aggregation, and membrane disruption are all parts of a self-perpetuating cycle that fuels amyloid cytotoxicity.

Biochemistry of amyloid β-protein and amyloid deposits in Alzheimer disease

Progressive cerebral deposition of the amyloid β-protein (Aβ) in brain regions serving memory and cognition is an invariant and defining feature of Alzheimer disease. A highly similar but less robust process accompanies brain aging in many nondemented humans, lower primates, and some other mammals. The discovery of Aβ as the subunit of the amyloid fibrils in meningocerebral blood vessels and parenchymal plaques has led to innumerable studies of its biochemistry and potential cytotoxic properties. Here we will review the discovery of Aβ, numerous aspects of its complex biochemistry, and current attempts to understand how a range of Aβ assemblies, including soluble oligomers and insoluble fibrils, may precipitate and promote neuronal and glial alterations that underlie the development of dementia. Although the role of Aβ as a key molecular factor in the etiology of Alzheimer disease remains controversial, clinical trials of amyloid-lowering agents, reviewed elsewhere in this book, are poised to resolve the question of its pathogenic primacy.

Surface chemistry of lipid raft and amyloid Aβ (1-40) Langmuir monolayer

Lipid rafts being rich in cholesterol and sphingolipids are considered to provide ordered lipid environment in the neuronal membranes, where it is hypothesized that the cleavage of amyloid precursor protein (APP) to Aβ (1-40) and Aβ (1-42) takes place. It is highly likely that the interaction of lipid raft components like cholesterol, sphingomylein or GM1 leads to nucleation of Aβ and results in aggregation or accumulation of amyloid plaques. One has investigated surface pressure-area isotherms of the lipid raft and Aβ (1-40) Langmuir monolayer. The compression-decompression cycles and the stability of the lipid raft Langmuir monolayer are crucial parameters for the investigation of interaction of Aβ (1-40) with the lipid raft Langmuir monolayer. It was revealed that GM1 provides instability to the lipid raft Langmuir monolayer. Adsorption of Aβ (1-40) onto the lipid raft Langmuir monolayer containing neutral (POPC) or negatively charged phospholipid (DPPG) was examined. The adsorption isotherms revealed that the concentration of cholesterol was important for adsorption of Aβ (1-40) onto the lipid raft Langmuir monolayer containing POPC whereas for the lipid raft Langmuir monolayer containing DPPG:cholesterol or GM1 did not play any role. In situ UV-vis absorption spectroscopy supported the interpretation of results for the adsorption isotherms.

The pathological roles of ganglioside metabolism in Alzheimer's disease: effects of gangliosides on neurogenesis

Conversion of the soluble, nontoxic amyloid β-protein (Aβ) into an aggregated, toxic form rich in β-sheets is a key step in the onset of Alzheimer's disease (AD). It has been suggested that Aβ induces changes in neuronal membrane fluidity as a result of its interactions with membrane components such as cholesterol, phospholipids, and gangliosides. Gangliosides are known to bind Aβ. A complex of GM1 and Aβ, termed “GAβ”, has been identified in AD brains. Abnormal ganglioside metabolism also may occur in AD brains. We have reported an increase of Chol-1α antigens, GQ1bα and GT1aα, in the brain of transgenic mouse AD model. GQ1bα and GT1aα exhibit high affinities to Aβs. The presence of Chol-1α gangliosides represents evidence for genesis of cholinergic neurons in AD brains. We evaluated the effects of GM1 and Aβ1–40 on mouse neuroepithelial cells. Treatment of these cells simultaneously with GM1 and Aβ1–40 caused a significant reduction of cell number, suggesting that Aβ1–40 and GM1 cooperatively exert a cytotoxic effect on neuroepithelial cells. An understanding of the mechanism on the interaction of GM1 and Aβs in AD may contribute to the development of new neuroregenerative therapies for this disorder.

DHPC strongly affects the structure and oligomerization propensity of Alzheimer's Aβ(1-40) peptide

Alzheimer's disease (AD) is thought to depend on the deleterious action of amyloid fibrils or oligomers derived from β-amyloid (Aβ) peptide. Out of various known Aβ alloforms, the 40-residue peptide Aβ(1-40) occurs at highest concentrations inside the brains of AD patients. Its aggregation properties critically depend on lipids, and it was thus proposed that lipids could play a major role in AD. To better understand their possible effects on the structure of Aβ and on the ability of this peptide to form potentially detrimental amyloid structures, we here analyze the interactions between Aβ(1-40) and 1,2-dihexanoyl-sn-glycero-3-phosphocholine (DHPC). DHPC has served, due to its controlled properties, as a major model system for studying general lipid properties. Here, we show that DHPC concentrations of 8 mM or higher exert dramatic effects on the conformation of soluble Aβ(1-40) peptide and induce the formation of β-sheet structure at high levels. By contrast, we find that DHPC concentrations well below the critical micelle concentration present no discernible effect on the conformation of soluble Aβ, although they substantially affect the peptide's oligomerization and fibrillation kinetics. These data imply that subtle lipid-peptide interactions suffice in controlling the overall aggregation properties and drastically accelerate, or delay, the fibrillation kinetics of Aβ peptide in near-physiological buffer solutions.

Roles for dysfunctional sphingolipid metabolism in Alzheimer's disease neuropathogenesis

Sphingolipids in the membranes of neurons play important roles in signal transduction, either by modulating the localization and activation of membrane-associated receptors or by acting as precursors of bioactive lipid mediators. Activation of cytokine and neurotrophic factor receptors coupled to sphingomyelinases results in the generation of ceramides and gangliosides, which in turn, modify the structural and functional plasticity of neurons. In aging and neurodegenerative conditions such as Alzheimer's disease (AD), there are increased membrane-associated oxidative stress and excessive production and accumulation of ceramides. Studies of brain tissue samples from human subjects, and of experimental models of the diseases, suggest that perturbed sphingomyelin metabolism is a pivotal event in the dysfunction and degeneration of neurons that occurs in AD and HIV dementia. Dietary and pharmacological interventions that target sphingolipid metabolism should be pursued for the prevention and treatment of neurodegenerative disorders.