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

Exploring the effects of three-finger toxins from Naja ashei venom on neuronal and immunological cancer cell membranes

Three-finger proteins are the most abundant toxins in the venom of Naja ashei, a snake species from the Elapidae family. This research aimed to describe the effects of varying charges of these proteins, isolated from Naja ashei venom using SEC and IEX chromatography. The study examined how differently charged three-finger toxin fractions interact with and affect neuroblastoma (SK-N-SH) and promyeloblast (HL-60) cells, as well as model Langmuir membranes and liposomes designed to mimic cellular lipid composition. Findings revealed that protein surface charges significantly impact cell survival (MTT assay), membrane damage (lactate dehydrogenase release, malondialdehyde formation), and the structural and electrochemical properties of model membranes (Langmuir membranes and zeta potential for liposomes and cancer cell lines). Results indicated that SK-N-SH cells, characterized by a higher negative charge on their cell membranes, interacted more effectively with positively charged toxins than HL-60 cells. However, the mechanism of these electrostatic interactions is complex. The research demonstrated that electrostatic and mechanical membrane modifications induced by venom proteins can significantly affect cell metabolism. Additionally, the total charge of the membrane, influenced by polar lipid components and phospholipid saturation, plays a decisive role in toxin interaction.

Curcumin's membrane localization and disruptive effects on cellular processes - insights from neuroblastoma, leukemic cells, and Langmuir monolayers

In therapies, curcumin is now commonly formulated in liposomal form, administered through injections or creams. This enhances its concentration at the cellular level compared to its natural form ingestion. Due to its hydrophobic nature, curcumin is situated in the lipid part of the membrane, thereby modifying its properties and influencing processes The aim of the research was to investigate whether the toxicity of specific concentrations of curcumin, assessed through biochemical tests for the SK-N-SH and H-60 cell lines, is related to structural changes in the membranes of these cells, caused by the localization of curcumin in their hydrophobic regions. Biochemical tests were performed using spectrophotometric methods. Langmuir technique were used to evaluate the interaction of the curcumin with the studied lipids. Direct introduction of curcumin into the membranes alters their physicochemical parameters. The extent of these changes depends on the initial properties of the membrane. In the conducted research, it has been demonstrated that curcumin may exhibit toxicity to human cells. The mechanism of this toxicity is related to its localization in cell membranes, leading to their dysfunction. The sensitivity of cells to curcumin presence depends on the saturation level of their membranes; the more rigid the membrane, the lower the concentration of curcumin causes its disruption.

Quantitative NMR-Based Lipoprotein Analysis Identifies Elevated HDL-4 and Triglycerides in the Serum of Alzheimer’s Disease Patients

Alzheimer’s disease (AD) is the most common form of dementia in the elderly and has been associated with changes in lipoprotein metabolism. We performed quantitative lipoprotein analysis in a local cohort of cognitively impaired elderly and control subjects using standardized nuclear magnetic resonance (NMR) spectroscopy. A commercially available quantitative NMR-based assay covering 112 lipoprotein main and subtype variables was used to investigate blood serum samples from a moderate cohort size of 161 persons (71 female, 90 male), including measures of quality control. Additionally, clinical metadata and cerebrospinal fluid AD biomarkers were collected and used for analysis. High-density lipoprotein (HDL) HDL-4 subfraction levels were mostly high in female individuals with mild cognitive impairment (MCI), followed by AD. Low-density lipoprotein (LDL) LDL-2 cholesterol was slightly elevated in male AD patients. HDL-2 apolipoprotein Apo-A1, HDL-2 phospholipids, and HDL-3 triglycerides were highly abundant in AD and MCI women compared to men. When considering clinical biomarkers (Aβ, tau), very low-density lipoprotein (VLDL) VLDL-1 and intermediate-density lipoprotein (IDL) triglycerides were substantially higher in AD compared to MCI. In addition, triglyceride levels correlated positively with dementia. Different lipoprotein serum patterns were identified for AD, MCI, and control subjects. Interestingly, HDL-4 and LDL-2 cholesterol parameters revealed strong gender-specific changes in the context of AD-driven dementia. As gender-based comparisons were based on smaller sub-groups with a low n-number, several statistical findings did not meet the significance threshold for multiple comparisons testing. Still, our finding suggests that serum HDL-4 parameters and various triglycerides correlate positively with AD pathology which could be a read-out of extended lipids traveling through the blood-brain barrier, supporting amyloid plaque formation processes. Thereof, we see herein a proof of concept that this quantitative NMR-based lipoprotein assay can generate important and highly interesting data for refined AD diagnosis and patient stratification, especially when larger cohorts are available.

Cholesterol as a key player in amyloid β-mediated toxicity in Alzheimer’s disease

Alzheimer’s disease (AD) is a neurodegenerative disorder that is one of the most devastating and widespread diseases worldwide, mainly affecting the aging population. One of the key factors contributing to AD-related neurotoxicity is the production and aggregation of amyloid β (Aβ). Many studies have shown the ability of Aβ to bind to the cell membrane and disrupt its structure, leading to cell death. Because amyloid damage affects different parts of the brain differently, it seems likely that not only Aβ but also the nature of the membrane interface with which the amyloid interacts, helps determine the final neurotoxic effect. Because cholesterol is the dominant component of the plasma membrane, it plays an important role in Aβ-induced toxicity. Elevated cholesterol levels and their regulation by statins have been shown to be important factors influencing the progression of neurodegeneration. However, data from many studies have shown that cholesterol has both neuroprotective and aggravating effects in relation to the development of AD. In this review, we attempt to summarize recent findings on the role of cholesterol in Aβ toxicity mediated by membrane binding in the pathogenesis of AD and to consider it in the broader context of the lipid composition of cell membranes.

Astrocytes and oligodendrocytes undergo subtype-specific transcriptional changes in Alzheimer's disease

Resolving glial contributions to Alzheimer's disease (AD) is necessary because changes in neuronal function, such as reduced synaptic density, altered electrophysiological properties, and degeneration, are not entirely cell autonomous. To improve understanding of transcriptomic heterogeneity in glia during AD, we used single-nuclei RNA sequencing (snRNA-seq) to characterize astrocytes and oligodendrocytes from apolipoprotein (APOE) Ɛ2/3 human AD and age- and genotype-matched non-symptomatic (NS) brains. We enriched astrocytes before sequencing and characterized pathology from the same location as the sequenced material. We characterized baseline heterogeneity in both astrocytes and oligodendrocytes and identified global and subtype-specific transcriptomic changes between AD and NS astrocytes and oligodendrocytes. We also took advantage of recent human and mouse spatial transcriptomics resources to localize heterogeneous astrocyte subtypes to specific regions in the healthy and inflamed brain. Finally, we integrated our data with published AD snRNA-seq datasets, highlighting the power of combining datasets to resolve previously unidentifiable astrocyte subpopulations.

Membrane elasticity modulated by cholesterol in model of porcine eye lens-lipid membrane

Experimental evidence shows that the eye lens loses its elasticity dramatically with age. It has also been reported that the cholesterol (Chol) content in the eye lens fiber cell plasma membrane increases significantly with age. High Chol content leads to the formation of cholesterol bilayer domains (CBDs) in the lens membrane. The role of high Chol associated with lens elasticity is unclear. The purpose of this research is to investigate the membrane elasticity of the model of porcine lens-lipid (MPLL) membrane with increasing Chol content to elucidate the role of high Chol in lens membrane elasticity. In this study, we used atomic force microscopy (AFM) to study the mechanical properties (breakthrough force and area compressibility modulus (K_A)) of the MPLL membrane with increasing Chol content where K_A is the measure of membrane elasticity. We varied Chol concentration in Chol/MPLL membrane from 0 to ∼71 mol%. Supported Chol/MPLL membranes were prepared by fusion of small unilamellar vesicles (SUVs) on top of a flat mica surface. SUVs of the Chol/MPLL lipid mixture were prepared with the rapid solvent exchange method followed by probe-tip sonication. For the Chol/MPLL mixing ratio of 0, AFM image showed the formation of two distinct phases of the membrane, i.e., liquid-disordered phase (l_d) and solid-ordered phase (s_o) membrane. However, with Chol/MPLL mixing ratio of 0.5 and above, only liquid-ordered phase (l_o) membrane was formed. Also, two distinct breakthrough forces corresponding to l_d and s_o were observed for Chol/MPLL mixing ratio of 0, whereas only one breakthrough force was observed for membranes with Chol/MPLL mixing ratio of 0.5 and above. No significant difference in the membrane surface roughness was measured with increasing Chol content for these membranes; however, breakthrough force and K_A for l_o membrane increased when Chol/MPLL mixing ratio was increased from 0.5 to 1. Interestingly above the Chol/MPLL mixing ratio of 1, both breakthrough force and K_A decreased, indicating the formation of CBDs. Furthermore, these results showed that membrane elasticity increases at high Chol content, suggesting that high Chol content in lens membrane might be responsible for maintaining lens membrane elasticity.

Intracellular Lipid Accumulation and Mitochondrial Dysfunction Accompanies Endoplasmic Reticulum Stress Caused by Loss of the Co-chaperone DNAJC3

Recessive mutations in DNAJC3, an endoplasmic reticulum (ER)-resident BiP co-chaperone, have been identified in patients with multisystemic neurodegeneration and diabetes mellitus. To further unravel these pathomechanisms, we employed a non-biased proteomic approach and identified dysregulation of several key cellular pathways, suggesting a pathophysiological interplay of perturbed lipid metabolism, mitochondrial bioenergetics, ER-Golgi function, and amyloid-beta processing. Further functional investigations in fibroblasts of patients with DNAJC3 mutations detected cellular accumulation of lipids and an increased sensitivity to cholesterol stress, which led to activation of the unfolded protein response (UPR), alterations of the ER-Golgi machinery, and a defect of amyloid precursor protein. In line with the results of previous studies, we describe here alterations in mitochondrial morphology and function, as a major contributor to the DNAJC3 pathophysiology. Hence, we propose that the loss of DNAJC3 affects lipid/cholesterol homeostasis, leading to UPR activation, β-amyloid accumulation, and impairment of mitochondrial oxidative phosphorylation.

Advantageous/Unfavorable Effect of Quercetin on the Membranes of SK-N-SH Neuroblastoma Cells

Quercetin is a polyphenolic compound, the effects of which raise scientists’ doubts. The results of many experiments show that it has anticancer, antiinflammatory, and antioxidant properties, while other studies indicate its pro-oxidative and cytotoxic action. This compound can react with reactive oxygen species, and due to its chemical properties, it can be found in the hydrophobic-hydrophilic area of cells. These features of quercetin indicate that its action in cells will be associated with the modification of membranes and its participation in maintaining the redox balance. Therefore, this study distinguishes these two mechanisms and determines whether they are important for cell function. We check: (1) Whether the selected concentrations of quercetin are cytotoxic and destructive for SK-N-SH cell membranes (MTT, LDH, MDA tests) in situations with and without the applied oxidative stress; (2) what is the level of changes in the structural/mechanical properties of the lipid part of the membranes of these cells due to the presence of polyphenol molecules; and (3) whether the antioxidative action of quercetin protects the membrane against its modification. Our results show that changes in the stiffness/elasticity of the lipid part of the membrane constitute the decisive mechanism of action of quercetin, potentially influencing cellular processes whose initial stages are associated with membranes (e.g., reception of signals from the environment, transport).

Nanodroplet Oligomers (NanDOs) of Aβ40

Using atomic force microscopy (AFM) and nuclear magnetic resonance (NMR), we describe small Aβ40 oligomers, termed nanodroplet oligomers (NanDOs), which form rapidly and at Aβ40 concentrations too low for fibril formation. NanDOs were observed in putatively monomeric solutions of Aβ40 (e.g., by size exclusion chromatography). Video-rate scanning AFM shows rapid fusion and dissolution of small oligomer-sized particles, of which the median size increases with peptide concentration. In NMR (¹³C HSQC), a small number of chemical shifts changed with a change in peptide concentration. Paramagnetic relaxation enhancement NMR experiments also support the formation of NanDOs and suggest prominent interactions in hydrophobic domains of Aβ40. Addition of Zn²⁺ to Aβ40 solutions caused flocculation of NanDO-containing solutions, and selective loss of signal intensity in NMR spectra from residues in the N-terminal domain of Aβ40. NanDOs may represent the earliest aggregated form of Aβ40 in the aggregation pathway and are akin to premicelles in solutions of amphiphilies.

Mechanical properties of the high cholesterol-containing membrane: An AFM study

Cholesterol (Chol) content in most cellular membranes does not exceed 50 mol%, only in the eye lens's fiber cell plasma membrane, its content surpasses 50 mol%. At this high concentration, Chol induces the formation of pure cholesterol bilayer domains (CBDs), which coexist with the surrounding phospholipid-cholesterol domain (PCD). Here, we applied atomic force microscopy to study the mechanical properties of Chol/phosphatidylcholine membranes where the Chol content was increased from 0 to 75 mol%, relevant to eye lens membranes. The surface roughness of the membrane decreases with an increase of Chol content until it reaches 60 mol%, and roughness increases with a further increment in Chol content. We propose that the increased roughness at higher Chol content results from the formation of CBDs. Force spectroscopy on the membrane with Chol content of 50 mol% or lesser exhibited single breakthrough events, whereas two distinct puncture events were observed for membranes with the Chol content greater than 50 mol%. We propose that the first puncture force corresponds to the membranes containing coexisting PCD and CBDs. In contrast, the second puncture force corresponds to the "CBD water pocket" formed due to coexisting CBDs and PCD. Membrane area compressibility modulus (K_A) increases with an increase in Chol content until it reaches 60 mol%, and with further increment in Chol content, CBDs are formed, and K_A starts to decrease. Our results report the increase in membrane roughness and decrease K_A at very high Chol content (>60 mol%) relevant to the eye lens membrane.

Defining the Neuropathological Aggresome across in Silico, in Vitro, and ex Vivo Experiments

The loss of proteostasis over the life course is associated with a wide range of debilitating degenerative diseases and is a central hallmark of human aging. When left unchecked, proteins that are intrinsically disordered can pathologically aggregate into highly ordered fibrils, plaques, and tangles (termed amyloids), which are associated with countless disorders such as Alzheimer's disease, Parkinson's disease, type II diabetes, cancer, and even certain viral infections. However, despite significant advances in protein folding and solution biophysics techniques, determining the molecular cause of these conditions in humans has remained elusive. This has been due, in part, to recent discoveries showing that soluble protein oligomers, not insoluble fibrils or plaques, drive the majority of pathological processes. This has subsequently led researchers to focus instead on heterogeneous and often promiscuous protein oligomers. Unfortunately, significant gaps remain in how to prepare, model, experimentally corroborate, and extract amyloid oligomers relevant to human disease in a systematic manner. This Review will report on each of these techniques and their successes and shortcomings in an attempt to standardize comparisons between protein oligomers across disciplines, especially in the context of neurodegeneration. By standardizing multiple techniques and identifying their common overlap, a clearer picture of the soluble neuropathological aggresome can be constructed and used as a baseline for studying human disease and aging.

Therapeutic Strategies to Reduce the Toxicity of Misfolded Protein Oligomers

The aberrant aggregation of proteins is implicated in the onset and pathogenesis of a wide range of neurodegenerative disorders, including Alzheimer’s and Parkinson’s diseases. Mounting evidence indicates that misfolded protein oligomers produced as intermediates in the aggregation process are potent neurotoxic agents in these diseases. Because of the transient and heterogeneous nature of these elusive aggregates, however, it has proven challenging to develop therapeutics that can effectively target them. Here, we review approaches aimed at reducing oligomer toxicity, including (1) modulating the oligomer populations (e.g., by altering the kinetics of aggregation by inhibiting, enhancing, or redirecting the process), (2) modulating the oligomer properties (e.g., through the size–hydrophobicity–toxicity relationship), (3) modulating the oligomer interactions (e.g., by protecting cell membranes by displacing oligomers), and (4) reducing oligomer toxicity by potentiating the protein homeostasis system. We analyze examples of these complementary approaches, which may lead to the development of compounds capable of preventing or treating neurodegenerative disorders associated with protein aggregation.

Melatonin Alters Fluid Phase Coexistence in POPC/DPPC/Cholesterol Membranes

The structure and biophysical properties of lipid membranes are important for cellular functions in health and disease. In Alzheimer's disease, the neuronal membrane is a target for toxic amyloid-β (Aβ). Melatonin is an important pineal gland hormone that has been shown to protect against Aβ toxicity in cellular and animal studies, but the molecular mechanism of this protection is not fully understood. Melatonin is a small membrane-active molecule that has been shown to interact with model lipid membranes and alter the membrane biophysical properties, such as membrane molecular order and dynamics. This effect of melatonin has been previously studied in simple model bilayers with one or two lipid components. To make it more relevant to neuronal membranes, we used a more complex ternary lipid mixture as our membrane model. In this study, we used ²H-NMR to investigate the effect of melatonin on the phase behavior of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), and cholesterol lipid membranes. We used deuterium-labeled POPC-d₃₁ and DPPC-d₆₂,separately to probe the changes in hydrocarbon chain order as a function of temperature and melatonin concentration. We find that POPC/DPPC/cholesterol at molar proportions of 3:3:2 is close to liquid-disordered/liquid-ordered phase separation and that melatonin can induce phase separation in these ternary mixtures by preferentially incorporating into the disordered phase and increasing its level of disorder. At 5 mol% melatonin, we observed phase separation in samples with POPC-d₃₁, but not with DPPC-d₆₂, whereas at 10 mol% melatonin, phase separation was observed in both samples with either POPC-d₃₁ or DPPC-d₆₂. These results indicate that melatonin can have a strong effect on membrane structure and physical properties, which may provide some clues to understanding how melatonin protects against Aβ, and that choice of chain perdeuteration is an important consideration from a technical point of view.

Complementary experimental and computational analysis of the effects of non-ionic detergents and phospholipids on insulin amyloid aggregation

Amphiphilic compounds, both detergents and lipids, are important tools for in vitro analysis of water-soluble and integral membrane proteins. A key question is whether these two groups of amphiphilic molecules use the same pathway to affect structural and functional integrity of proteins. In the present study, we tested the effect of non-ionic detergent dodecyl maltoside (DDM), two phospholipids, 1,2-dimyristoyl-sn-glycero-3- phosphocholine (DMPC), 1,2-dihexanoyl-sn-glycero-3-phosphocholine (DHPC), and the detergent-phospholipid mixtures on insulin amyloidogenesis in vitro. Amyloidogenesis of insulin is significantly affected by DDM in a time-and dose-dependent manner, but only slightly affected by either of phospholipids. Addition of DHPC or DMPC to detergent does not alter the inhibiting pattern, suggesting that DDM preferably binds to insulin. The molecular modeling revealed that DDM and the phospholipids occupy equivalent binding sites. DDM, due to the presence of maltose with several oxygen atoms (hydroxylic, glycosidic and ring) is involved in more hydrogen bonds than DHPC or DMPC. Hydrophobic interactions are important factors to stabilize both, DDM and phospholipids in their binding sites. Our results indicate that certain detergents (applying DDM as an example) and selected phospholipids are not always interchangeable in their use to investigate the effect of amphiphilic compounds on the behavior of amyloid-prone proteins.

Cholesterol Interaction with the Trimeric HIV Fusion Protein gp41 in Lipid Bilayers Investigated by Solid-State NMR Spectroscopy and Molecular Dynamics Simulations

HIV-1 entry into cells is mediated by the fusion protein gp41. Cholesterol plays an important role in this virus-cell fusion, but molecular structural information about cholesterol-gp41 interaction is so far absent. Here, we present experimental and computational data about cholesterol complexation with gp41 in lipid bilayers. We focus on the C-terminal region of the protein, which comprises a membrane-proximal external region (MPER) and the transmembrane domain (TMD). We measured peptide-cholesterol contacts in virus-mimetic lipid bilayers using solid-state NMR spectroscopy, and augmented these experimental data with all-atom molecular dynamics simulations. 2D 19F NMR spectra show correlation peaks between MPER residues and the cholesterol isooctyl tail, indicating that cholesterol is in molecular contact with the MPER-TMD trimer. 19F-13C distance measurements between the peptide and 13C-labeled cholesterol show that C17 on the D ring and C9 at the intersection of B and C rings are ~7.0 Å from the F673 side-chain 4-19F. At high peptide concentrations in the membrane, the 19F-13C distance data indicate three cholesterol molecules bound near F673 in each trimer. Mutation of a cholesterol recognition amino acid consensus motif did not change these distances, indicating that cholesterol binding does not require this sequence motif. Molecular dynamics simulations further identify two hotspots for cholesterol interactions. Taken together, these experimental data and simulations indicate that the helix-turn-helix conformation of the MPER-TMD is responsible for sequestering cholesterol. We propose that this gp41-cholesterol interaction mediates virus-cell fusion by recruiting gp41 to the boundary of the liquid-disordered and liquid-ordered phases to incur membrane curvature.

A Novel Aβ40 Assembly at Physiological Concentration

Aggregates of amyloid-β (Aβ) are characteristic of Alzheimer's disease, but there is no consensus as to either the nature of the toxic molecular complex or the mechanism by which toxic aggregates are produced. We report on a novel feature of amyloid-lipid interactions where discontinuities in the lipid continuum can serve as catalytic centers for a previously unseen microscale aggregation phenomenon. We show that specific lipid membrane conditions rapidly produce long contours of lipid-bound peptide, even at sub-physiological concentrations of Aβ. Using single molecule fluorescence, time-lapse TIRF microscopy and AFM imaging we characterize this phenomenon and identify some exceptional properties of the aggregation pathway which make it a likely contributor to early oligomer and fibril formation, and thus a potential critical mechanism in the etiology of AD. We infer that these amyloidogenic events occur only at areas of high membrane curvature, which suggests a range of possible mechanisms by which accumulated physiological changes may lead to their inception. The speed of the formation is in hours to days, even at 1 nM peptide concentrations. Lipid features of this type may act like an assembly line for monomeric and small oligomeric subunits of Aβ to increase their aggregation states. We conclude that under lipid environmental conditions, where catalytic centers of the observed type are common, key pathological features of AD may arise on a very short timescale under physiological concentration.

Cholesterol Modulates the Formation of the Aβ Ion Channel in Lipid Bilayers

The misfolding of amyloid beta (Aβ) is one of the predominant hallmarks in the pathology of Alzheimer's disease (AD). In this study, we showed that the formation of the Aβ ion channel on the membrane depended on the cholesterol concentration. From a mechanical aspect, we found that cholesterol levels affected the stability and assembly of lipid bilayers. Measurements on planar lipid bilayers indicated that a small amount of cholesterol interacted with Aβ proteins and promoted the insertion process. Conversely, high cholesterol integrated the lipid bilayer and exerted an opposite effect on Aβ insertion. The Aβ ion channel was then detected by graphene-based field-effect transistors. Results demonstrated that the Aβ ion channel promoted a Ca²⁺ flux in the presence of 15% cholesterol but prevented a Ca²⁺ flux in high cholesterol. Thus, cholesterol had a complex impact on the Aβ ion channel that can be described as two different effects. First, a small amount of cholesterol interacted with Aβ and facilitated the Aβ ion channel formation in the membrane. Second, a large amount of cholesterol did not induce the ion flux in the membrane, which can be explained by the cholesterol damage to the regular distribution of the lipid bilayer. Overall, this study suggested a possible approach to consider cholesterol levels for the treatment of AD patients.

Membrane domain modulation of Aβ1-42 oligomer interactions with supported lipid bilayers: an atomic force microscopy investigation

Alzheimer's disease is a devastating pathology affecting an increasing number of individuals following the general rise in life expectancy. Amyloid peptide Aβ_1-42 has been identified as one of the main culprits of the disease. The peptide has been shown to have major effects on lipid membranes, including membrane fragmentation. The membrane composition has been identified as a factor that plays a pivotal role in regulating peptide/membrane interactions and several results suggest that lipid domains, or rafts, can promote peptide-induced membrane damage. In this work, we examined the effects of lipid segregation on the membrane-perturbing ability of Aβ_1-42 and an oligomeric mutant (G37C), a peptide that shares common features with the suspected toxic intermediates involved in the neurodegeneration process. Atomic force microscopy (AFM) was used to determine the impact of these peptides on the supported lipid bilayers of various compositions. In 1,2-dioleoyl-sn-glycero-3-phosphocholine/1,2-dipalmitoyl-sn-glycero-3-phosphocholine/cholesterol (DOPC/DPPC/cholesterol) and DOPC/sphingomyelin/cholesterol ternary mixtures, two systems exhibiting liquid-liquid phase separations, it was shown that Aβ_1-42 and G37C exclusively aggregated on liquid-disordered-phase domains, creating large deposits and even causing membrane fragmentation for the latter composition. Cholesterol and ganglioside GM1, the two most documented lipids in the context of Alzheimer's disease, are also considered to play a crucial role in promoting detrimental interactions with amyloid peptides. We show that, in model 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) membranes, the presence of either cholesterol or GM1 in a proportion of 10 mol%, a content supposed to lead to domain formation, favoured the association of both Aβ_1-42 and G37C, leading to a harmful membrane fragmentation. The AFM results established that the presence of domains favoured membrane perturbations induced by the amyloid peptides. It is proposed that lipid packing defects at the domain interface could act as adsorption and nucleation sites for the amyloid peptides. The more extensive bilayer perturbations induced by G37C compared to Aβ_1-42 supported this hypothesis, indicating that oligomers that cannot mature to the fibril state can present considerable toxicity.

β-Amyloid aggregation and heterogeneous nucleation

In this article, we consider the role of heterogeneous nucleation in β-amyloid aggregation. Heterogeneous nucleation is more common and occurs at lower levels of supersaturation than homogeneous nucleation. The nucleation period is also the stage at which most of the polymorphism of amyloids arises, this being one of the defining features of amyloids. We focus on several well-known heterogeneous nucleators of β-amyloid, including lipid surfaces, especially those enriched in gangliosides and cholesterol, and divalent metal ions. These two broad classes of nucleators affect β-amyloid particularly in light of the amphiphilicity of these peptides: the N-terminal region, which is largely polar and charged, contains the metal binding site, whereas the C-terminal region is aliphatic and is important in lipid binding. Notably, these two classes of nucleators can interact cooperatively, aggregation begetting greater aggregation.

Effects of in vivo conditions on amyloid aggregation

One of the grand challenges of biophysical chemistry is to understand the principles that govern protein misfolding and aggregation, which is a highly complex process that is sensitive to initial conditions, operates on a huge range of length- and timescales, and has products that range from protein dimers to macroscopic amyloid fibrils. Aberrant aggregation is associated with more than 25 diseases, which include Alzheimer's, Parkinson's, Huntington's, and type II diabetes. Amyloid aggregation has been extensively studied in the test tube, therefore under conditions that are far from physiological relevance. Hence, there is dire need to extend these investigations to in vivo conditions where amyloid formation is affected by a myriad of biochemical interactions. As a hallmark of neurodegenerative diseases, these interactions need to be understood in detail to develop novel therapeutic interventions, as millions of people globally suffer from neurodegenerative disorders and type II diabetes. The aim of this review is to document the progress in the research on amyloid formation from a physicochemical perspective with a special focus on the physiological factors influencing the aggregation of the amyloid-β peptide, the islet amyloid polypeptide, α-synuclein, and the hungingtin protein.

Hydrogen Peroxide Modifies Aβ-Membrane Interactions with Implications for Aβ40 Aggregation

Alzheimer's disease (AD) is pathologically characterized by the formation of extracellular senile plaques, predominately comprised of aggregated β-amyloid (Aβ), deposited in the brain. Aβ aggregation can result in a myriad of distinct aggregate species, from soluble oligomers to insoluble fibrils. Aβ strongly interacts with membranes, which can be linked to a variety of potential toxic mechanisms associated with AD. Oxidative damage accompanies the formation of Aβ aggregates, with a 10-50% proportion of Aβ aggregates being oxidized in vivo. Hydrogen peroxide (H₂O₂) is a reactive oxygen species implicated in a number of neurodegenerative diseases. Recent evidence has demonstrated that the H₂O₂ concentration fluctuates rapidly in the brain, resulting in large concentration spikes, especially in the synaptic cleft. Here, the impact of environmental H₂O₂ on Aβ aggregation in the presence and absence of lipid membranes is investigated. Aβ₄₀ was exposed to H₂O₂, resulting in the selective oxidation of methionine 35 (Met35) to produce Aβ₄₀Met35[O]. While oxidation mildly reduced the rate of Aβ aggregation and produced a distinct fibril morphology at high H₂O₂ concentrations, H₂O₂ had a much more pronounced impact on Aβ aggregation in the presence of total brain lipid extract vesicles. The impact of H₂O₂ on Aβ aggregation in the presence of lipids was associated with a reduced affinity of Aβ for the vesicle surface. However, this reduced vesicle affinity was predominately associated with lipid peroxidation rather than Aβ oxidation.

Imaging Artificial Membranes Using High-Speed Atomic Force Microscopy

Supported lipid bilayers represent a very attractive way to mimic biological membranes, especially to investigate molecular mechanisms associated with the lateral segregation of membrane components. Observation of these model membranes with high-speed atomic force microscopy (HS-AFM) allows the capture of both topography and dynamics of membrane components, with a spatial resolution in the nanometer range and image capture time of less than 1 s. In this context, we have developed new protocols adapted for HS-AFM to form supported lipid bilayers on small mica disks using the vesicle fusion or Langmuir-Blodgett methods. In this chapter we describe in detail the protocols to fabricate supported artificial bilayers as well as the main guidelines for HS-AFM imaging of such samples.

Effect of Cholesterol on Membrane Fluidity and Association of Aβ Oligomers and Subsequent Neuronal Damage: A Double-Edged Sword

Background: The beta-amyloid peptide (Aβ) involved in Alzheimer's disease (AD) has been described to associate/aggregate on the cell surface disrupting the membrane through pore formation and breakage. However, molecular determinants involved for this interaction (e.g., some physicochemical properties of the cell membrane) are largely unknown. Since cholesterol is an important molecule for membrane structure and fluidity, we examined the effect of varying cholesterol content with the association and membrane perforation by Aβ in cultured hippocampal neurons. Methods: To decrease or increase the levels of cholesterol in the membrane we used methyl-β-cyclodextrin (MβCD) and MβCD/cholesterol, respectively. We analyzed if membrane fluidity was affected using generalized polarization (GP) imaging and the fluorescent dye di-4-ANEPPDHQ. Additionally membrane association and perforation was assessed using immunocytochemistry and electrophysiological techniques, respectively. Results: The results showed that cholesterol removal decreased the macroscopic association of Aβ to neuronal membranes (fluorescent-puncta/20 μm: control = 18 ± 2 vs. MβCD = 10 ± 1, p < 0.05) and induced a facilitation of the membrane perforation by Aβ with respect to control cells (half-time for maximal charge transferred: control = 7.2 vs. MβCD = 4.4). Under this condition, we found an increase in membrane fluidity (46 ± 3.3% decrease in GP value, p < 0.001). On the contrary, increasing cholesterol levels incremented membrane rigidity (38 ± 2.7% increase in GP value, p < 0.001) and enhanced the association and clustering of Aβ (fluorescent-puncta/20 μm: control = 18 ± 2 vs. MβCD = 10 ± 1, p < 0.01), but inhibited membrane disruption. Conclusion: Our results strongly support the significance of plasma membrane organization in the toxic effects of Aβ in hippocampal neurons, since fluidity can regulate distribution and insertion of the Aβ peptide in the neuronal membrane.

Soluble Oligomers Require a Ganglioside to Trigger Neuronal Calcium Overload

An altered distribution of membrane gangliosides (GM), including GM1, has recently been reported in the brains of Alzheimer's disease (AD) patients. Moreover, amyloid-positive synaptosomes obtained from AD brains were found to contain high-density GM1 clusters, suggesting a pathological significance of GM1 increase at presynaptic neuritic terminals in AD. Here, we show that membrane GM1 specifically recruits small soluble oligomers of the 42-residue form of amyloid-β peptide (Aβ42), with intracellular flux of Ca2+ ions in primary rat hippocampal neurons and in human neuroblastoma cells. Specific membrane proteins appear to be involved in the early and transient influx of Ca2+ ions induced by Aβ42 oligomers with high solvent-exposed hydrophobicity (A+), but not in the sustained late influx of the same oligomers and in that induced by Aβ42 oligomers with low solvent-exposed hydrophobicity (A-) in GM1-enriched cells. In addition, A+ oligomers accumulate in proximity of membrane NMDA and AMPA receptors, inducing the early and transient Ca2+ influx, although FRET shows that the interaction is not direct. These results suggest that age-dependent clustering of GM1 within neuronal membranes could induce neurodegeneration in elderly people as a consequence of an increased ability of the lipid bilayers to recruit membrane-permeabilizing oligomers. We also show that both lipid and protein components of the plasma membrane can contribute to neuronal dysfunction, thus expanding the molecular targets for therapeutic intervention in AD.

Sphingomyelin and GM1 Influence Huntingtin Binding to, Disruption of, and Aggregation on Lipid Membranes

Huntington disease (HD) is an inherited neurodegenerative disease caused by the expansion beyond a critical threshold of a polyglutamine (polyQ) tract near the N-terminus of the huntingtin (htt) protein. Expanded polyQ promotes the formation of a variety of oligomeric and fibrillar aggregates of htt that accumulate into the hallmark proteinaceous inclusion bodies associated with HD. htt is also highly associated with numerous cellular and subcellular membranes that contain a variety of lipids. As lipid homeostasis and metabolism abnormalities are observed in HD patients, we investigated how varying both the sphingomyelin (SM) and ganglioside (GM1) contents modifies the interactions between htt and lipid membranes. SM composition is altered in HD, and GM1 has been shown to have protective effects in animal models of HD. A combination of Langmuir trough monolayer techniques, vesicle permeability and binding assays, and in situ atomic force microscopy (AFM) were used to directly monitor the interaction of a model, synthetic htt peptide and a full-length htt-exon1 recombinant protein with model membranes comprised of total brain lipid extract (TBLE) and varying amounts of exogenously added SM or GM1. The addition of either SM or GM1 decreased htt insertion into the lipid monolayers. However, TBLE vesicles with an increased SM content were more susceptible to htt-induced permeabilization, whereas GM1 had no effect on permeablization. Pure TBLE bilayers and TBLE bilayers enriched with GM1 developed regions of roughened, granular morphologies upon exposure to htt-exon1, but plateau-like domains with a smoother appearance formed in bilayers enriched with SM. Oligomeric aggregates were observed on all bilayer systems regardless of induced morphology. Collectively, these observations suggest that the lipid composition and its subsequent effects on membrane material properties strongly influence htt binding and aggregation on lipid membranes.

Effect of Cholesterol on Membrane Fluidity and Association of Aβ Oligomers and Subsequent Neuronal Damage: A Double-Edged Sword

Background: The beta-amyloid peptide (Aβ) involved in Alzheimer's disease (AD) has been described to associate/aggregate on the cell surface disrupting the membrane through pore formation and breakage. However, molecular determinants involved for this interaction (e.g., some physicochemical properties of the cell membrane) are largely unknown. Since cholesterol is an important molecule for membrane structure and fluidity, we examined the effect of varying cholesterol content with the association and membrane perforation by Aβ in cultured hippocampal neurons. Methods: To decrease or increase the levels of cholesterol in the membrane we used methyl-β-cyclodextrin (MβCD) and MβCD/cholesterol, respectively. We analyzed if membrane fluidity was affected using generalized polarization (GP) imaging and the fluorescent dye di-4-ANEPPDHQ. Additionally membrane association and perforation was assessed using immunocytochemistry and electrophysiological techniques, respectively. Results: The results showed that cholesterol removal decreased the macroscopic association of Aβ to neuronal membranes (fluorescent-puncta/20 μm: control = 18 ± 2 vs. MβCD = 10 ± 1, p < 0.05) and induced a facilitation of the membrane perforation by Aβ with respect to control cells (half-time for maximal charge transferred: control = 7.2 vs. MβCD = 4.4). Under this condition, we found an increase in membrane fluidity (46 ± 3.3% decrease in GP value, p < 0.001). On the contrary, increasing cholesterol levels incremented membrane rigidity (38 ± 2.7% increase in GP value, p < 0.001) and enhanced the association and clustering of Aβ (fluorescent-puncta/20 μm: control = 18 ± 2 vs. MβCD = 10 ± 1, p < 0.01), but inhibited membrane disruption. Conclusion: Our results strongly support the significance of plasma membrane organization in the toxic effects of Aβ in hippocampal neurons, since fluidity can regulate distribution and insertion of the Aβ peptide in the neuronal membrane.

Amyloid and membrane complexity: The toxic interplay revealed by AFM

Lipid membranes play a fundamental role in the pathological development of protein misfolding diseases. Several pieces of evidence suggest that the lipid membrane could act as a catalytic surface for protein aggregation. Furthermore, a leading theory indicates the interaction between the cell membrane and misfolded oligomer species as the responsible for cytotoxicity, hence, for neurodegeneration in disorders such as Alzheimer's and Parkinson's disease. The definition of the mechanisms that drive the interaction between pathological protein aggregates and plasma membrane is fundamental for the development of effective therapies for a large class of diseases. Atomic force microscopy (AFM) has been employed to study how amyloid aggregates affect the cell physiological properties. Considerable efforts were spent to characterize the interaction with model systems, i.e., planar supported lipid bilayers, but some works also addressed the problem directly on living cells. Here, an overview of the main works involving the use of the AFM on both model system and living cells will be provided. Different kind of approaches will be presented, as well as the main results derived from the AFM analysis.

Bacoside-A, an Indian Traditional-Medicine Substance, Inhibits β-Amyloid Cytotoxicity, Fibrillation, and Membrane Interactions

Bacoside-A, a family of compounds extracted from the Bacopa monniera plant, is a folk-medicinal substance believed to exhibit therapeutic properties, particularly enhancing cognitive functions and improving memory. We show that bacoside-A exerted significant inhibitory effects upon cytotoxicity, fibrillation, and particularly membrane interactions of amyloid-beta (1-42) (Aβ42), the peptide playing a prominent role in Alzeheimer's disease progression and toxicity. Specifically, preincubation of bacoside-A with Aβ42 significantly reduced cell toxicity and inhibited fibril formation both in buffer solution and, more significantly, in the presence of membrane vesicles. In parallel, spectroscopic and microscopic analyses reveal that bacoside-A blocked membrane interactions of Aβ42, while formation of Aβ42 oligomers was not disrupted. These interesting phenomena suggest that inhibition of Aβ42 oligomer assembly into mature fibrils, and blocking membrane interactions of the oligomers are likely the underlying factors for ameliorating amyloid toxicity by bacoside-A and its putative physiological benefits.

The pleiotropic effects of omega-3 docosahexaenoic acid on the hallmarks of Alzheimer's disease

Among omega-3 polyunsaturated fatty acids (PUFAs), docosahexaenoic acid (DHA, 22:6n-3) is important for adequate brain development and cognition. DHA is highly concentrated in the brain and plays an essential role in brain functioning. DHA, one of the major constituents in fish fats, readily crosses the blood-brain barrier from blood to the brain. Its critical role was further supported by its reduced levels in the brain of Alzheimer's disease (AD) patients. This agrees with a potential role of DHA in memory, learning and cognitive processes. Since there is yet no cure for dementia such as AD, there is growing interest in the role of DHA-supplemented diet in the prevention of AD pathogenesis. Accordingly, animal, epidemiological, preclinical and clinical studies indicated that DHA has neuroprotective effects in a number of neurodegenerative conditions including AD. The beneficial effects of this key omega-3 fatty acid supplementation may depend on the stage of disease progression, other dietary mediators and the apolipoprotein ApoE genotype. Herein, our review investigates, from animal and cell culture studies, the molecular mechanisms involved in the neuroprotective potential of DHA with emphasis on AD.

Effects of Charged Cholesterol Derivatives on Aβ40 Amyloid Formation

Understanding of the mechanistic progess of amyloid-β peptide (Aβ) aggregation is critical for elucidating the underlying pathogenesis of Alzheimer's disease (AD). Herein, we report for the first time the effects of two cholesterol derivatives, negatively charged cholesterol sulfate (cholesterol-SO4) and positively charged 3β-[N-(dimethylaminoethane)carbamoyl]-cholesterol (DC-cholesterol), on the fibrillization of Aβ40. Our results demonstrate that both of the nonvesicular forms of cholesterol-SO4 and DC-cholesterol moderately accelerate the aggregation rate of Aβ40. This effect is similar to that observed for unmodified cholesterol, indicating the importance of hydrophobic interactions in binding of Aβ40 to these steroid molecules. Furthermore, we show that the vesicles formed at higher concentrations of anionic cholesterol-SO4 facilitate Aβ40 aggregation rate markedly. In contrast, the cationic DC-cholesterol vesicles show the ability to inhibit Aβ40 fibril formation under appropriate experimental conditions. The results suggest that the electrostatic interactions between Aβ40 and the charged vesicles can be of great importance in regulating Aβ40-vesicle interaction. Our results also indicate that the structural properties of the aggregates of the cholesterol derivatives, including the surface charge and the size of the vesicles, are critical in regulating the effects of these vesicles on Aβ40 aggregation kinetics.

Cholesterol Modifies Huntingtin Binding to, Disruption of, and Aggregation on Lipid Membranes

Huntington's disease (HD) is an inherited neurodegenerative disease caused by abnormally long CAG-repeats in the huntingtin gene that encode an expanded polyglutamine (polyQ) domain near the N-terminus of the huntingtin (htt) protein. Expanded polyQ domains are directly correlated to disease-related htt aggregation. Htt is found highly associated with a variety of cellular and subcellular membranes that are predominantly comprised of lipids. Since cholesterol homeostasis is altered in HD, we investigated how varying cholesterol content modifies the interactions between htt and lipid membranes. A combination of Langmuir trough monolayer techniques, vesicle permeability and binding assays, and in situ atomic force microscopy were used to directly monitor the interaction of a model, synthetic htt peptide and a full-length htt-exon1 recombinant protein with model membranes comprised of total brain lipid extract (TBLE) and varying amounts of exogenously added cholesterol. As the cholesterol content of the membrane increased, the extent of htt insertion decreased. Vesicles containing extra cholesterol were resistant to htt-induced permeabilization. Morphological and mechanical changes in the bilayer associated with exposure to htt were also drastically altered by the presence of cholesterol. Disrupted regions of pure TBLE bilayers were grainy in appearance and associated with a large number of globular aggregates. In contrast, morphological changes induced by htt in bilayers enriched in cholesterol were plateau-like with a smooth appearance. Collectively, these observations suggest that the presence and amount of cholesterol in lipid membranes play a critical role in htt binding and aggregation on lipid membranes.

Detergent-type membrane fragmentation by MSI-78, MSI-367, MSI-594, and MSI-843 antimicrobial peptides and inhibition by cholesterol: a solid-state nuclear magnetic resonance study

Multidrug resistance against the existing antibiotics is becoming a global threat, and any potential drug that can be designed using cationic antimicrobial peptides (AMP) could be an alternate solution to alleviate this existing problem. The mechanism of action of killing bacteria by an AMP differs drastically in comparison to that of small molecule antibiotics. The main target of AMPs is to interact with the lipid bilayer of the cell membrane and disrupt it to kill bacteria. Consequently, the modes of membrane interaction that lead to the selectivity of an AMP are very important to understand. Here, we have used different membrane compositions, such as negatively charged, zwitterionic, or mixed large unilamellar vesicles (LUVs), to study the interaction of four different synthetically designed cationic, linear antimicrobial peptides: MSI-78 (commercially known as pexiganan), MSI-367, MSI-594, and MSI-843. Our solid-state nuclear magnetic resonance (NMR) experiments confirmed that the MSI peptides fragmented LUVs through a detergent-like carpet mechanism depending on the amino acid sequence of the MSI peptide and/or the membrane composition of LUVs. Interestingly, the fragmented lipid aggregates such as SUVs or micelles are sufficiently small to produce an isotropic peak in the (31)P NMR spectrum. These fragmented lipid aggregates contain only MSI peptides bestowed with lipid molecules as confirmed by NMR in conjunction with circular dichroism spectroscopy. Our results also demonstrate that cholesterol, which is present only in the eukaryotic cell membrane, inhibits the MSI-induced fragmentation of LUVs, suggesting that the MSI peptides can discriminate the bacteria and the eukaryotic cell membranes, and this selectivity could be used for further development of novel antibiotics.

Investigation of temperature induced mechanical changes in supported bilayers by variants of tapping mode atomic force microscopy

Tapping mode atomic force microscopy (AFM) is an invaluable technique for examining topographical features of biological materials in solution, and there has been a growing interest in developing techniques to provide further compositional contrast and information concerning surface mechanical properties. Phase shifts, cantilever response at higher harmonic frequencies of the drive, and time-resolved tip/sample force reconstruction have all been shown to provide additional compositional contrast of surfaces, as compared to basic tapping mode AFM imaging. This study aimed to demonstrate the relative ability of these different imaging techniques to detect temperature induced changes in the elastic modulus of supported total brain lipid extract (TBLE) bilayer patches on mica. To aid in direct comparison between the different imaging techniques, all required data was obtained simultaneously while capturing traditional tapping mode AFM topography images. While all of the techniques were able to provide compositional contrast consistent with known temperature-induced changes in the bilayer patch, interpretation of the resulting contrast was not always straightforward. Phase imaging suffered from contrast inversion. Individual harmonics responded in a variety of ways to the temperature-induced changes in elastic modulus of the bilayer. The maximum tapping force (or peak force) associated with imaging the bilayer correctly reflected the changes in elastic modulus of the lipid bilayer. Importantly, as the required data can be obtained simultaneously, combining these different imaging techniques can lead to a more complete understanding of a sample's mechanical features.

Protein seeding in Alzheimer’s disease and Parkinson’s disease: Similarities and differences

Neurodegenerative pathology can be seeded by introduction of misfolded proteins and peptides into the nervous system. Models of Alzheimer’s disease (AD) and Parkinson’s disease (PD) have both demonstrated susceptibility to this seeding mechanism, emphasizing the role of misfolded conformations of disease-specific proteins and peptides in disease progression. Thinking of the amyloidogenic amyloid-beta peptide (Aβ) and alpha-synuclein (α-syn), of AD and PD, respectively, as prionoids requires a comparison of these molecules and the mechanisms underlying the progression of disease. Aβ and α-syn, despite their size differences, are both natively unstructured and misfold into β-structured conformers. Additionally, several studies implicate the significant role of membrane interactions, such as those with lipid rafts in the plasma membrane, in mediating protein aggregation and transfer of Aβ and α-syn between cells that may be common to both AD and PD. Examination of inter-neuronal transfer of proteins/peptides provides evidence into the core mechanism of neuropathological propagation. Specifically, uptake of aggregates likely occurs by the endocytic pathway, possibly in response to their formation of membrane pores via a mechanism shared with pore-forming toxins. Failure of cellular clearance machinery to degrade misfolded proteins favours their release into the extracellular space, where they can be taken up by directly connected, nearby neurons. Although similarities between AD and PD are frequent and include mechanistically similar transfer processes, what differentiates these diseases, in terms of temporal and spatial patterns of propagation, may be in part due to the differing kinetics of protein misfolding. Several examples of animal models demonstrating seeding and propagation by exogenous treatment with Aβ and α-syn highlight the importance of both the environment in which these seeds are formed as well as the environment into which the seeds are propagated. Although these studies suggest potent seeding effects by both Aβ and α-syn, they emphasize the need for future studies to thoroughly characterize “seeds” as well as analyze changes in the nervous system in response to exogenous insults.

Preparation protocols of aβ(1-40) promote the formation of polymorphic aggregates and altered interactions with lipid bilayers

The appearance of neuritic amyloid plaques comprised of β-amyloid peptide (Aβ) in the brain is a predominant feature in Alzheimer's disease (AD). In the aggregation process, Aβ samples a variety of potentially toxic aggregate species, ranging from small oligomers to fibrils. Aβ has the ability to form a variety of morphologically distinct and stable amyloid fibrils. Commonly termed polymorphs, such distinct aggregate species may play a role in variations of AD pathology. It has been well documented that polymorphic aggregates of Aβ can be produced by changes in the chemical environment and peptide preparations. As Aβ and several of its aggregated forms are known to interact directly with lipid membranes and this interaction may play a role in a variety of potential toxic mechanisms associated with AD, we determine how different Aβ(1-40) preparation protocols that lead to distinct polymorphic fibril aggregates influence the interaction of Aβ(1-40) with model lipid membranes. Using three distinct protocols for preparing Aβ(1-40), the aggregate species formed in the absence and presence of a lipid bilayers were investigated using a variety of scanning probe microscopy techniques. The three preparations of Aβ(1-40) promoted distinct oligomeric and fibrillar aggregates in the absence of bilayers that formed at different rates. Despite these differences in aggregation properties, all Aβ(1-40) preparations were able to disrupt supported total brain lipid extract bilayers, altering the bilayer's morphological and mechanical properties.

Molecular understanding of a potential functional link between antimicrobial and amyloid peptides

Antimicrobial and amyloid peptides do not share common sequences, typical secondary structures, or normal biological activity but both the classes of peptides exhibit membrane-disruption ability to induce cell toxicity. Different membrane-disruption mechanisms have been proposed for antimicrobial and amyloid peptides, individually, some of which are not exclusive to either peptide type, implying that certain common principles may govern the folding and functions of different cytolytic peptides and associated membrane disruption mechanisms. Particularly, some antimicrobial and amyloid peptides have been identified to have dual complementary amyloid and antimicrobial properties, suggesting a potential functional link between amyloid and antimicrobial peptides. Given that some similar structural and membrane-disruption characteristics exist between the two classes of peptides, this review summarizes major findings, recent advances, and future challenges related to antimicrobial and amyloid peptides and strives to illustrate the similarities, differences, and relationships in the sequences, structures, and membrane interaction modes between amyloid and antimicrobial peptides, with a special focus on direct interactions of the peptides with the membranes. We hope that this review will stimulate further research at the interface of antimicrobial and amyloid peptides - which has been studied less intensively than either type of peptides - to decipher a possible link between both amyloid pathology and antimicrobial activity, which can guide drug design and peptide engineering to influence peptide-membrane interactions important in human health and diseases.

Interaction of Alzheimer's β-amyloid peptides with cholesterol: mechanistic insights into amyloid pore formation

Brain cholesterol plays a critical role in Alzheimer's disease and other neurodegenerative diseases. The molecular mechanisms linking cholesterol to neurotoxicity have remained elusive for a long time, but recent data have allowed the identification of functional cholesterol-binding domains in several amyloidogenic proteins involved in neurodegenerative diseases, including Alzheimer's disease. In this review, we analyze the cholesterol binding properties of β-amyloid (Aβ) peptides and the impact of these interactions on amyloid pore formation. We show that although the cholesterol-binding domains of Aβ peptides and of transmembrane precursor C99 are partially overlapping, they involve distinct amino acid residues, so that cholesterol has a greater affinity for Aβ than for C99. Synthetic 22-35 and 25-35 fragments of Aβ retained the ability of the full-length peptide 1-42 to bind cholesterol and to form zinc-sensitive, calcium-permeable amyloid pores in cultured neural cells. Studies with mutant peptides allowed the identification of key residues involved in cholesterol binding and channel formation. Cholesterol promoted the insertion of Aβ in the plasma membrane, induced α-helical structuration, and forced the peptide to adopt a tilted topology that favored the oligomerization process. Bexarotene, an amphipathic drug currently considered as a potential candidate medication for the treatment of neurodegenerative diseases, competed with cholesterol for binding to Aβ and prevented oligomeric channel formation. These studies indicate that it is possible to prevent the generation of neurotoxic oligomers by targeting the cholesterol-binding domain of Aβ peptides. This original strategy could be used for the treatment of Alzheimer's and other neurodegenerative diseases that involve cholesterol-dependent toxic oligomers.

Role of the fast kinetics of pyroglutamate-modified amyloid-β oligomers in membrane binding and membrane permeability

Membrane permeability to ions and small molecules is believed to be a critical step in the pathology of Alzheimer’s disease (AD). Interactions of oligomers formed by amyloid-β (Aβ) peptides with the plasma cell membrane are believed to play a fundamental role in the processes leading to membrane permeability. Among the family of Aβs, pyroglutamate (pE)-modified Aβ peptides constitute the most abundant oligomeric species in the brains of AD patients. Although membrane permeability mechanisms have been studied for full-length Aβ_1–40/42 peptides, these have not been sufficiently characterized for the more abundant Aβ_pE3–42 fragment. Here we have compared the adsorbed and membrane-inserted oligomeric species of Aβ_pE3–42 and Aβ_1–42 peptides. We find lower concentrations and larger dimensions for both species of membrane-associated Aβ_pE3–42 oligomers. The larger dimensions are attributed to the faster self-assembly kinetics of Aβ_pE3–42, and the lower concentrations are attributed to weaker interactions with zwitterionic lipid headgroups. While adsorbed oligomers produced little or no significant membrane structural damage, increased membrane permeabilization to ionic species is understood in terms of enlarged membrane-inserted oligomers. Membrane-inserted Aβ_pE3–42 oligomers were also found to modify the mechanical properties of the membrane. Taken together, our results suggest that membrane-inserted oligomers are the primary species responsible for membrane permeability.

Platelets, a reliable source for peripheral Alzheimer's disease biomarkers?

Peripheral biomarkers play an indispensable role in quick and reliable diagnoses of any kind of disease. With the population ageing, the number of people suffering from age-related diseases is expected to rise dramatically over the coming decades. In particular, all types of cognitive deficits, such as Alzheimer's disease, will increase. Alzheimer's disease is characterised mainly by coexistence of amyloid plaques and neurofibrillary tangles in brain. Reliable identification of such molecular characteristics antemortem, however, is problematic due to restricted availability of appropriate sample material and definitive diagnosis is only possible postmortem. Currently, the best molecular biomarkers available for antemortem diagnosis originate from cerebrospinal fluid. Though, this is not convenient for routine diagnosis because of the required invasive lumbar puncture. As a consequence, there is a growing demand for additional peripheral biomarkers in a more readily accessible sample material. Blood platelets, due to shared biochemical properties with neurons, can constitute an attractive alternative as discussed here. This review summarises potential platelet Alzheimer's disease biomarkers, their role, implication, and alteration in the disease. For easy comparison of their performance, the Hedge effect size was calculated whenever data were available.

Metals and cholesterol: two sides of the same coin in Alzheimer's disease pathology

Alzheimer's disease (AD) is a multifactorial neurodegenerative disease. It begins years prior to the onset of clinical symptoms, such as memory loss and cognitive decline. Pathological hallmarks of AD include the accumulation of β-amyloid in plaques and hyperphosphorylated tau in neurofibrillary tangles. Copper, iron, and zinc are abnormally accumulated and distributed in the aging brain. These metal ions can adversely contribute to the progression of AD. Dysregulation of cholesterol metabolism has also been implicated in the development of AD pathology. To date, large bodies of research have been carried out independently to elucidate the role of metals or cholesterol on AD pathology. Interestingly, metals and cholesterol affect parallel molecular and biochemical pathways involved in AD pathology. The possible links between metal dyshomeostasis and altered brain cholesterol metabolism in AD are reviewed.

Endoplasmic reticulum stress mediates amyloid β neurotoxicity via mitochondrial cholesterol trafficking

Disrupted cholesterol homeostasis has been reported in Alzheimer disease and is thought to contribute to disease progression by promoting amyloid β (Aβ) accumulation. In particular, mitochondrial cholesterol enrichment has been shown to sensitize to Aβ-induced neurotoxicity. However, the molecular mechanisms responsible for the increased cholesterol levels and its trafficking to mitochondria in Alzheimer disease remain poorly understood. Here, we show that endoplasmic reticulum (ER) stress triggered by Aβ promotes cholesterol synthesis and mitochondrial cholesterol influx, resulting in mitochondrial glutathione (mGSH) depletion in older age amyloid precursor protein/presenilin-1 (APP/PS1) mice. Mitochondrial cholesterol accumulation was associated with increased expression of mitochondrial-associated ER membrane proteins, which favor cholesterol translocation from ER to mitochondria along with specific cholesterol carriers, particularly the steroidogenic acute regulatory protein. In vivo treatment with the ER stress inhibitor 4-phenylbutyric acid prevented mitochondrial cholesterol loading and mGSH depletion, thereby protecting APP/PS1 mice against Aβ-induced neurotoxicity. Similar protection was observed with GSH ethyl ester administration, which replenishes mGSH without affecting the unfolded protein response, thus positioning mGSH depletion downstream of ER stress. Overall, these results indicate that Aβ-mediated ER stress and increased mitochondrial cholesterol trafficking contribute to the pathologic progression observed in old APP/PS1 mice, and that ER stress inhibitors may be explored as therapeutic agents for Alzheimer disease.

The mechanism of membrane disruption by cytotoxic amyloid oligomers formed by prion protein(106-126) is dependent on bilayer composition

The formation of fibrillar aggregates has long been associated with neurodegenerative disorders such as Alzheimer and Parkinson diseases. Although fibrils are still considered important to the pathology of these disorders, it is now widely understood that smaller amyloid oligomers are the toxic entities along the misfolding pathway. One characteristic shared by the majority of amyloid oligomers is the ability to disrupt membranes, a commonality proposed to be responsible for their toxicity, although the mechanisms linking this to cell death are poorly understood. Here, we describe the physical basis for the cytotoxicity of oligomers formed by the prion protein (PrP)-derived amyloid peptide PrP(106-126). We show that oligomers of this peptide kill several mammalian cells lines, as well as mouse cerebellar organotypic cultures, and we also show that they exhibit antimicrobial activity. Physical perturbation of model membranes mimicking bacterial or mammalian cells was investigated using atomic force microscopy, polarized total internal reflection fluorescence microscopy, and NMR spectroscopy. Disruption of anionic membranes proceeds through a carpet or detergent model as proposed for other antimicrobial peptides. By contrast, when added to zwitterionic membranes containing cholesterol-rich ordered domains, PrP(106-126) oligomers induce a loss of domain separation and decreased membrane disorder. Loss of raft-like domains may lead to activation of apoptotic pathways, resulting in cell death. This work sheds new light on the physical mechanisms of amyloid cytotoxicity and is the first to clearly show membrane type-specific modes of action for a cytotoxic peptide.