Cholesterol in Membranes Facilitates Aggregation of Amyloid β Protein at Physiologically Relevant Concentrations

Receptor modulated assembly and drug induced disassembly of amyloid beta aggregates at asymmetric neuronal model biomembranes

Amyloid peptide non-fibrillar oligomers cause neurotoxicity and may contribute to Alzheimer's disease (AD) pathogenesis. Mounting evidence indicates that Aβ1-42 oligomers disrupt and remodel neuronal membrane, causing neuronal cell death. The involvement of individual neuronal membrane constituents in real-time Aβ1-42 aggregate assembly is unclear due to complexity of neuronal cell membrane. We used non-Faradaic electrochemical impedance spectroscopy (EIS) to track monomeric Aβ1-42 peptide binding and aggregation pathways in real-time in asymmetric micropore suspended lipid bilayer membranes with anionic phospholipids and glycosphingolipids. Anionic DOPC:PIP2 pore suspended membrane showed pore-formation within 2 h of incubation, but peptide insertion occurred over 6 h, with an onset time of ∼6-8 h for peptide aggregation at the membrane surface. Among different gangliosides, peptide binding to GM1- and GM3-containing membranes did not result pore development, but receptor mediated peptide aggregation formation caused membrane admittance to decrease within 2 h. In contrast, partial peptide insertion in the membrane surface increases membrane admittance at GD1a and mixed GSL membranes, arresting aggregation. Time-lapsed AFM imaging at asymmetric solid supported lipid bilayers (aSLBs) corroborated EIS findings, capturing pore-formation and receptor mediated peptide assembly routes. Fluorescence lifetime imaging (FLIM) imaging and spatial resolved single-point fluorescence correlation spectroscopy (FCS) at aSLBs revealed membrane-peptide interaction, assembly, and peptide induced membrane reorganization. Treatment with antidepressants fluoxetine and imipramine therapeutics, in synergy, which are cost-effective and capable of crossing the central nervous system (CNS+), resulted in the disassembly of membrane mediated Aβ1-42 aggregates, but not fibrils. Overall, the data suggests that membrane-mediated aggregate disassembly at the correct timing of AD progression may halt or reverse amyloid assembly through the use of repurposed drugs.

Cholesterol Metabolism in CNS Diseases: The Potential of SREBP2 and LXR as Therapeutic Targets

The brain is the organ with the highest cholesterol content in the body. Cholesterol in the brain plays a crucial role in maintaining the integrity of synapses and myelin sheaths to ensure normal brain function. Disruptions in cholesterol metabolism are closely associated with various central nervous system (CNS) diseases, including Alzheimer's disease (AD), Huntington's disease (HD), and multiple sclerosis (MS). In this review, we explore the synthesis, regulation, transport, and functional roles of cholesterol in the CNS. We discuss in detail the associations between cholesterol homeostasis imbalance and CNS diseases including AD, HD, and MS, highlighting the significant role of cholesterol metabolism abnormalities in the development of these diseases. Sterol regulatory element binding protein-2 (SREBP2) and liver X receptor (LXR) are two critical transcription factors that play central roles in cholesterol synthesis and reverse transport, respectively. Their cooperative interaction finely tunes the balance of brain cholesterol metabolism, presenting potential therapeutic value for preventing and treating CNS diseases. We particularly emphasize the alterations in SREBP2 and LXR under pathological conditions and their impacts on disease progression. This review summarizes current therapeutic agents targeting these two pathways, with the hope of broadening the perspectives of CNS drug developers and encouraging further study into SREBP2 and LXR-related therapies for CNS diseases.

Uncovering Amyloid-β Interactions: Gray versus White Matter

Alzheimer's disease is characterized by the accumulation of amyloid plaques in the brain. Recent studies suggest that amyloid-β (Aβ) peptides interact with cell membranes, potentially catalyzing plaque formation. However, the effect of varying cell membrane compositions on this catalytic process requires further investigation. Using molecular dynamics simulations, we demonstrate that a model gray matter membrane significantly influences the secondary structure of β-amyloid peptides. Notably, residues Asp1 and Glu22 play crucial roles in the membrane interaction. Glutamic acid at position 22, located in the middle of the peptide chain, appears to promote the formation of β-hairpin conformations, which are critical for aggregation. Additionally, our simulations reveal that the model white matter membrane allows a spontaneous insertion of segments of the peptide into the membrane, suggesting that membrane interaction not only alters the peptide structure but may also compromise membrane integrity. Our results show that the different membrane compositions in the brain may play different roles when interacting with β-amyloid peptides.

Membrane-assisted Aβ40 aggregation pathways

Alzheimer's disease (AD) is caused by the assembly of amyloid-beta (Aβ) peptides into oligomers and fibrils. Endogenous Aβ aggregation may be assisted by cell membranes, which can accelerate the nucleation step enormously, but knowledge of membrane-assisted aggregation is still very limited. Here, we used extensive molecular dynamics (MD) simulations to structurally and energetically characterize key intermediates along the membrane-assisted aggregation pathways of Aβ40. Reinforcing experimental observations, the simulations reveal unique roles of GM1 ganglioside and cholesterol in stabilizing membrane-embedded β sheets and of Y10 and K28 in the ordered release of a small oligomeric seed into solution. The same seed leads to either an open-shaped or R-shaped fibril, with significant stabilization provided by inter- or intra-subunit interfaces between a straight β sheet (residues Q15-D23) and a bent β sheet (residues A30-V36). This work presents a comprehensive picture of membrane-assisted aggregation of Aβ40, with broad implications for developing AD therapies and rationalizing disease-specific polymorphisms of amyloidogenic proteins.

Changes in Adsorption, Aggregation, and Diffusion Nature of Amyloid β on a Lipid Membrane in an Open System

The aggregation and accumulation of amyloid β 42 (Aβ42) peptides on the surface of brain cells is associated with Alzheimer's disease (AD); however, the underlying molecular mechanisms remain unclear. Herein, we used a unique brain-mimetic open system that continuously flows Aβ42 solution to analyze the initial aggregation and adsorptive nature of Aβ42 at physiological concentrations on the lipid membrane. The open system accelerated the adsorption and dimerization kinetics. Upon the addition of Aβ42, monomeric Aβ42 was dominant on the lipid bilayer surface in the closed system with no flow, whereas dimers and high-order oligomers were dominant in the open system. Closed and open systems exhibited different oligomerization kinetics, lipid-Aβ42 interactions, and diffusive properties of monomers and oligomers. These results indicate the specific adsorptive and diffusive nature of Aβ42 on the cell membrane. The open system may help in elucidating the molecular mechanisms underlying AD progression.

Amyloid beta (Aβ) fibrillation kinetics and its impact on membrane polarity

Fibrillation of the amyloid beta (Aβ) peptide has often been associated with neurodegenerative pathologies such as Alzheimer's disease. In this study we examined the influence of several potential compositions of the lipid membrane on Aβ fibrillation by using liposomes as a basic model membrane. Firstly, it was revealed that Aβ fibrillation kinetics were enhanced and had the potential to occur at a faster rate on more fluid membranes compared to solid membranes. Next, the extent of fibril-related damage to membranes was examined with analysis of membrane polarity via the steady-state emission spectra of 6-dodecanoyl-2-dimethylaminonaphthalene (Laurdan). It was revealed that there was slight hydration behavior of the membrane during the lag phase (tlag) of the kinetic process, possibly coinciding with Aβ monomer binding. However, as the fibrillation kinetic process continued the membrane gradually dehydrated. Hydration states of membranes during and after Aβ fibrillation processes were further examined via deconvolution analysis of the obtained Laurdan spectra. This allows a mapping of membrane hydration from the interior to exterior regions of the lipid membrane. Results revealed slight but definitive variations in deeper region membrane polarity during the time course of Aβ fibrillation, suggesting Aβ aggregation impacts not only the surface level aggregating region but also the inner regions of the membrane. These results can ultimately contribute to the future investigations of the nature of the membrane damage caused by Aβ aggregation.

Opposite Roles of Cholesterol and Lanosterol in Lipid Membrane on Amyloid-Beta 42 Peptide Nucleation and Fibril Formation

Molecular self-assembly of amyloid-beta peptides to form fibrillar aggregates is a known cause of Alzheimer's disease. Although homogeneous nucleation of amyloid-beta is unfavorable, heterogeneous nucleation of amyloid-beta in cell membranes plays a key role in fibril formation. We observed these opposite roles in the effects of cholesterol and lanosterol, the precursor of cholesterol in the brain, on nucleation. As previously reported, cholesterol accelerated nucleation, whereas lanosterol decelerated it when mixed with dioleoyl-phosphatidylcholine at 20%. The observed opposite effects of cholesterol and lanosterol on nucleation do not correlate with the differences in the mechanical and thermodynamic nature of mixed membranes. However, the affinity of amyloid-beta to the inner membrane seems to be related to the opposite effects on nucleation kinetics. Cholesterol reduced the insertion of amyloid-beta into the lipid membrane, whereas lanosterol promoted the insertion of amyloid-beta into the membrane, which would make amyloid-beta more tightly bound by lipid molecules and reduce its diffusivity in the membrane and consequently inhibit nucleation. Our study provides insights into the effects of sterol compounds other than the well-investigated cholesterol on the self-assembly of amyloid-beta to clarify the molecular basis underlying Alzheimer's disease pathology and to develop targeted therapeutic strategies.

Inhibition of Aβ Aggregation by Cholesterol-End-Modified PEG Vesicles and Micelles

Background/Objectives: This study aimed to design and evaluate Chol-PEG2000 micelles and Chol-PEG500 vesicles as drug delivery system (DDS) carriers and inhibitors of amyloid-β (Aβ) aggregation, a key factor in Alzheimer's disease (AD). Methods: The physical properties of Chol-PEG assemblies were characterized using dynamic light scattering (DLS), electrophoretic light scattering (ELS), and transmission electron microscopy (TEM). Inhibitory effects on Aβ aggregation were assessed via thioflavin T (ThT) assay, circular dichroism (CD) spectroscopy, and native polyacrylamide gel electrophoresis (native-PAGE). Results: Chol-PEG2000 micelles and Chol-PEG500 vesicles were found to exhibit diameters of 20-30 nm and 70-80 nm, respectively, with neutral surface charges and those physical properties indicated the high affinity for Aβ. At a 10-fold molar ratio, thioflavin T (ThT) assay revealed that Chol-PEG2000 delayed Aβ fibril elongation by 20 hours, while Chol-PEG500 delayed it by 40 hours against Aβ peptide. At a 50-fold molar ratio, both Chol-PEG2000 and Chol-PEG500 significantly inhibited Aβ aggregation, as indicated by minimal fluorescence intensity increases over 48 hours. CD spectroscopy indicated that Aβ maintained its random coil structure in the presence of Chol-PEG assemblies at a 50-fold molar ratio. Native-PAGE analysis demonstrated a retardation in Aβ migration immediately after mixing with Chol-PEG assemblies, suggesting complex formation. However, this retardation disappeared within 5 min, implying rapid dissociation of the complexes. Conclusions: This study demonstrated that Chol-PEG500 vesicles more effectively inhibit Aβ aggregation than Chol-PEG2000 micelles. Chol-PEG assemblies perform as DDS carriers to be capable of inhibiting Aβ aggregation. Chol-PEG assemblies can deliver additional therapeutics targeting other aspects of AD pathology. This dual-function platform shows promise as both a DDS carrier and a therapeutic agent, potentially contributing to a fundamental cure for AD.

Near-infrared light therapy normalizes amyloid load, neuronal lipid membrane order, rafts and cholesterol level in Alzheimer's disease

Cholesterol dysregulation, disorder of neuronal membrane lipid packing, and lipid rafts lead to the synthesis and accumulation of toxic amyloid-β (Aβ), contributing to the development of Alzheimer's disease (AD). Our study shows that near-infrared (NIR) transcranial photobiomodulation therapy (tPBMT) can reduce Aβ load and restore the properties of neuronal plasma membrane, including Aβ production, bilayer order, rafts, lipid content, and Ca2+ channels during AD. Mice in the experiments were exposed to 808-nm LED for 1 h daily over 3 months. In the APOE transgenic model with cholesterol dysregulation, the cholesterol levels increased by 22 times, causing healthy neurons to produce toxic Aβ three times faster, increasing its load by five times. Consequently, Aβ disrupts the membrane bilayer and prompts the formation of lipid rafts and pores. NIR-tPBMT can nearly half attenuate Aβ load, restore membrane lipid order and rigidity, reduce the number of lipid rafts, modulate cholesterol synthesis, normalize Ca2+ influx by activated endocytosis, and reduce neuronal death. The Ca2+ influx induced by light does not cause excitotoxicity but modulates Ca2+/calmodulin signaling involved in AD mechanisms and cell viability. The transcriptome analysis of the brain cortex and hippocampus shows that light can downregulate Ca2+/calmodulin-dependent AD-risk genes BACE, PSEN, and APP, and normalize cholesterol homeostasis via the HMGCR, DHCR7, and INSIG1 genes. Additionally, light enhances neuron resistance to the endoplasmic reticulum stress via activating transcription factors of the unfolded protein response. Thus, red/NIR light could be promising in combating AD, restoring synaptic plasticity in degenerating neurons and reducing Aβ load.

Damage of the Phospholipid Bilayer by Aβ42 at Physiologically Relevant Peptide Concentrations

Amyloid β (Aβ) aggregates are implicated in the pathology of several neurodegenerative diseases such as Alzheimer's disease, Huntington's disease, and Parkinson's disease, and damage to membranes is considered one of the pathology-related effects of Aβ. Experiments in vitro indicate that Aβ can damage these membranes; however, such experiments were performed at Aβ concentrations in the micromolar range, several orders above the physiologically relevant conditions. Our studies with Aβ42 in the low nanomolar concentrations did not reveal any damage to the supported lipid bilayer, questioning this membrane damage mechanism of Aβ. However, the phospholipid composition can be a factor contributing to the interaction of Aβ with the membrane. Therefore, in this study, we investigated the interaction of 50 nM Aβ42 with supported lipid bilayers composed of equimolar ratios of POPS and POPC at phospholipid concentrations of 0.1 and 0.25 mg/mL. Using atomic force microscopy (AFM), we observed that Aβ42 induced damage to bilayers at 0.1 mg/mL, characterized by forming defects that grew in size and number over time. The defects penetrate only the upper leaflet of the bilayer, but no such defects were observed at 0.25 mg/mL phospholipid concentrations. We additionally determined Young's modulus of these bilayers as a measure of stiffness, and these values were 6.9 ± 3.6 MPa and 16.6 ± 5.3 MPa for the 0.1 mg/mL and the 0.25 mg/mL bilayers, respectively. These findings suggest that Aβ42's ability to induce bilayer damage depends on membrane stiffness, with softer bilayers (0.1 mg/mL) being more susceptible to Aβ42-induced damage. The results are discussed and compared with models in which Aβ42 oligomers create localized membrane damage. The implication of the results to the mechanisms of the Aβ42 oligomer pathology is discussed.

Lipid-polymer hybrid-vesicles interrupt nucleation of amyloid fibrillation

Solubility and aggregation of proteins are crucial factors for their functional and further biological roles. Aggregation of proteins in vivo, such as the amyloid beta (Aβ1-40) peptide into fibrils, is significantly modulated by membrane lipids, abundantly present in cells. We developed a model membrane system, composed of lipid hybrid-vesicles bearing embedded hydrophilic polymers to in vitro study the aggregation of the Aβ1-40 peptide. Focus is to understand and inhibit the primordial, nucleation stages of their fibrillation by added hybrid-vesicles, composed of a natural lipid and amphiphilic polymers. These designed hybrid-vesicles are based on 1-palmitoyl-2-oleoyl-glycero-3-phosphocholine (POPC), displaying embedded hydrophilic (EO) m P n A_EG polymers (m = 2 or 3; P n = 10 to 52 with M n = 2800-9950 gmol-1) in amounts ranging from 5-20 mol%, anchored to the POPC vesicles via hydrophobic hexadecyl-, glyceryl- and cholesteryl-moieties, affixed to the polymers as end-groups. All investigated hybrid-vesicles significantly delay fibrillation of the Aβ1-40 peptide as determined by thioflavin T (ThT) assays. We observed that the hybrid-vesicles interacted with early aggregating species of Aβ1-40 peptide, irrespective of their composition or size. A substantial perturbation of both primary (k + k n ) and secondary (k + k 2) nucleation rates of Aβ1-40 by the POPC-polymer vesicles compared to POPC vesicles was observed, particularly for the cholesteryl-anchored polymers, interfering with the fragmentation and elongation steps of Aβ1-40. Furthermore, morphological differences of the aggregates were revealed by transmission electron microscopy (TEM) images supported the inhibitory kinetic signatures.

Modulation of Biological Membranes Using Small-Molecule Compounds to Counter Toxicity Caused by Amyloidogenic Proteins

The transition of peptides or proteins along a misfolding continuum from soluble functional states to pathological aggregates, to ultimately deposit as amyloid fibrils, is a process that underlies an expanding group of human diseases-collectively known as protein-misfolding disorders (PMDs). These include common and debilitating conditions, such as Alzheimer's disease, Parkinson's disease, and type-2 diabetes. Compelling evidence has emerged that the complex interplay between the misfolded proteins and biological membranes is a key determinant of the pathogenic mechanisms by which harmful amyloid entities are formed and exert their cytotoxicity. Most efforts thus far to develop disease-modifying treatments for PMDs have largely focused on anti-aggregation strategies: to neutralise, or prevent the formation of, toxic amyloid species. Herein, we review the critical role of the phospholipid membrane in mediating and enabling amyloid pathogenicity. We consequently propose that the development of small molecules, which have the potential to uniquely modify the physicochemical properties of the membrane and make it more resilient against damage by misfolded proteins, could provide a novel therapeutic approach in PMDs. By way of an example, natural compounds shown to intercalate into lipid bilayers and inhibit amyloid-lipid interactions, such as the aminosterols, squalamine and trodusquamine, cholesterol, ubiquinone, and select polyphenols, are discussed. Such a strategy would provide a novel approach to counter a wide range of toxic biomolecules implicit in numerous human amyloid pathologies.

Pore Formation by Amyloid-like Peptides: Effects of the Nonpolar-Polar Sequence Pattern

One of the mechanisms accounting for the toxicity of amyloid peptides in diseases like Alzheimer's and Parkinson's is the formation of pores on the plasma membrane of neurons. Here, we perform unbiased all-atom simulations of the full membrane damaging pathway, which includes adsorption, aggregation, and perforation of the lipid bilayer accounting for pore-like structures. Simulations are performed using four peptides made with the same amino acids. Differences in the nonpolar-polar sequence pattern of these peptides prompt them to adsorb into the membrane with the extended conformations oriented either parallel [peptide labeled F1, Ac-(FKFE)2-NH2], perpendicular (F4, Ac-FFFFKKEE-NH2), or with an intermediate orientation (F2, Ac-FFKKFFEE-NH2, and F3, Ac-FFFKFEKE-NH2) in regard to the membrane surface. At the water-lipid interface, only F1 fully self-assembles into β-sheets, and F2 peptides partially fold into an α-helical structure. The β-sheets of F1 emerge as electrostatic interactions attract neighboring peptides to intermediate distances where nonpolar side chains can interact within the dry core of the bilayer. This complex interplay between electrostatic and nonpolar interactions is not observed for the other peptides. Although β-sheets of F1 peptides are mostly parallel to the membrane, some of their edges penetrate deep inside the bilayer, dragging water molecules with them. This precedes pore formation, which starts with the flow of two water layers through the membrane that expand into a stable cylindrical pore delimited by polar faces of β-sheets spanning both leaflets of the bilayer.

Membrane-assisted Aβ40 aggregation pathways

Alzheimer's disease (AD) is caused by the assembly of amyloid-beta (Aβ) peptides into oligomers and fibrils. Endogenous Aβ aggregation may be assisted by cell membranes, which can accelerate the nucleation step enormously, but knowledge of membrane-assisted aggregation is still very limited. Here we used extensive MD simulations to structurally and energetically characterize key intermediates along the membrane-assisted aggregation pathways of Aβ40. Reinforcing experimental observations, the simulations reveal unique roles of GM1 ganglioside and cholesterol in stabilizing membrane-embedded β-sheets and of Y10 and K28 in the ordered release of a small oligomeric seed into solution. The same seed leads to either an open-shaped or R-shaped fibril, with significant stabilization provided by inter- or intra-subunit interfaces between a straight β-sheet (residues Q15-D23) and a bent β-sheet (residues A30-V36). This work presents the first comprehensive picture of membrane-assisted aggregation of Aβ40, with broad implications for developing AD therapies and rationalizing disease-specific polymorphisms of amyloidogenic proteins.

Strategies for measuring concentrations and forms of amyloid-β peptides

Alzheimer's disease (AD) is affecting more and more people worldwide without the effective treatment, while the existed pathological mechanism has been confirmed barely useful in the treatment. Amyloid-β peptide (Aβ), a main component of senile plaque, is regarded as the most promising target in AD treatment. Aβ clearance from AD brain seems to be a reliably therapeutic strategy, as the two exited drugs, GV-971 and aducanumab, are both developed based on it. However, doubt still exists. To exhaustive expound on the pathological mechanism of Aβ, rigorous analyses on the concentrations and aggregation forms are essential. Thus, it is attracting broad attention these years. However, most of the sensors have not been used in pathological studies, as the lack of the bridge between analytical chemist and pathologists. In this review, we made a brief introduce on Aβ-related pathological mechanism included in β-amyloid hypothesis to elucidate the detection conditions of sensor methods. Furthermore, a summary of the sensor methods was made, which were based on Aβ concentrations and form detections that have been developed in the past 10 years. As the greatest number of the sensors were built on fluorescent spectroscopy, electrochemistry, and Roman spectroscopy, detailed elucidation on them was made. Notably, the aggregation process is another important factor in revealing the progress of AD and developing the treatment methods, so the sensors on monitoring Aβ aggregation processes were also summarized.

Aggregation Behavior of Amyloid Beta Peptide Depends Upon the Membrane Lipid Composition

Protein aggregation plays a crucial role in the development of several neurodegenerative diseases. It is important to understand the aggregation process for the detection of the onset of these diseases. Alzheimer's Disease (AD) is one of the most prevalent neurodegenerative diseases caused by the aggregation of Aβ-40 and Aβ-42 peptides. The smaller oligomers lead to the formation of protein plaque at the neural membranes leading to memory loss and other disorders. Interestingly, aggregation takes place at the neural membranes, therefore the membrane composition seems to play an important role in the aggregation process. Despite a large number of literatures on the effect of lipid composition on protein aggregation, there are very few concise reviews that highlight the role of membrane composition in protein aggregation. In this review, we have discussed the implication of membrane composition on the aggregation of amyloid beta peptide with a special emphasis on cholesterol. We have further discussed the role of the degree of unsaturation of fatty acids and the participation of apolipoprotein E4 (ApoE4) in the onset of AD.

Fabricating Multiphasic Angiogenic Scaffolds Using Amyloid/Roxadustat-Assisted High-Temperature Protein Printing

Repairing multiphasic defects is cumbersome. This study presents new soft and hard scaffold designs aimed at facilitating the regeneration of multiphasic defects by enhancing angiogenesis and improving cell attachment. Here, the nonimmunogenic, nontoxic, and cost-effective human serum albumin (HSA) fibril (HSA-F) was used to fabricate thermostable (up to 90 °C) and hard printable polymers. Additionally, using a 10.0 mg/mL HSA-F, an innovative hydrogel was synthesized in a mixture with 2.0% chitosan-conjugated arginine, which can gel in a cell-friendly and pH physiological environment (pH 7.4). The presence of HSA-F in both hard and soft scaffolds led to an increase in significant attachment of the scaffolds to the human periodontal ligament fibroblast (PDLF), human umbilical vein endothelial cell (HUVEC), and human osteoblast. Further studies showed that migration (up to 157%), proliferation (up to 400%), and metabolism (up to 210%) of these cells have also improved in the direction of tissue repair. By examining different in vitro and ex ovo experiments, we observed that the final multiphasic scaffold can increase blood vessel density in the process of per-vascularization as well as angiogenesis. By providing a coculture environment including PDLF and HUVEC, important cross-talk between these two cells prevails in the presence of roxadustat drug, a proangiogenic in this study. In vitro and ex ovo results demonstrated significant enhancements in the angiogenic response and cell attachment, indicating the effectiveness of the proposed design. This approach holds promise for the regeneration of complex tissue defects by providing a conducive environment for vascularization and cellular integration, thus promoting tissue healing.

Protective effect of trehalose sugar on amyloid-membrane interactions using BLM electrophysiology

Alzheimer's disease (AD) is a neurodegenerative disease characterized by dementia and memory loss in the elderly population. The amyloid-β peptide (Aβ) is one of the main pathogenic factors in AD and is known to cause damage to neuronal cellular membranes. There is no cure currently available for AD, and new approaches, including preventive strategies, are highly desirable. In this work, we explore the possibility of protecting neuronal membranes from amyloid-induced damage with naturally existing sugar trehalose. Trehalose has been shown to protect plant cellular membranes in extreme conditions and modify Aβ misfolding. We hypothesize that trehalose can protect the neuronal membrane from amyloid toxicity. In this work, we studied the protective effect of trehalose against Aβ1-42-induced damage in model lipid membranes (DPPC/POPC/cholesterol) using atomic force microscopy and black lipid membrane electrophysiology. Our results demonstrate that Aβ1-42 damaged membranes and led to ionic current leakage across these membranes due to the formation of various defects and pores. The presence of trehalose reduced the ion current across membranes caused by Aβ1-42 peptide damage, thus efficiently protecting the membranes. These findings suggest that the trehalose sugar can potentially be useful in protecting neuronal membranes against amyloid toxicity in AD.

Localized surface plasmon resonance and atomic force microscopy study of model lipid membranes and their interactions with amyloid and melatonin

Alzheimer's disease (AD) is a progressive neurodegenerative disorder characterized by the accumulation of amyloid plaques in the brain. The toxicity of amyloid to neuronal cell surfaces arises from interactions between small intermediate aggregates, namely amyloid oligomers, and the cell membrane. The nature of these interactions changes with age and disease progression. In our previous work, we demonstrated that both membrane composition and nanoscale structure play crucial roles in amyloid toxicity, and that membrane models mimicking healthy neuron were less affected by amyloid than model membranes mimicking AD neuronal membranes. This understanding introduces the possibility of modifying membrane properties with membrane-active molecules, such as melatonin, to protect them from amyloid-induced damage. In this study, we employed atomic force microscopy and localized surface plasmon resonance to investigate the protective effects of melatonin. We utilized synthetic lipid membranes that mimic the neuronal cellular membrane at various stages of AD and explored their interactions with amyloid-β(1-42) in the presence of melatonin. Our findings reveal that the early diseased membrane model is particularly vulnerable to amyloid binding and subsequent damage. However, melatonin exerts its most potent protective effect on this early-stage membrane. These results suggest that melatonin could act at the membrane level to alleviate amyloid toxicity, offering the most protection during the initial stages of AD.

Structural characterisation of α-synuclein-membrane interactions and the resulting aggregation using small angle scattering

The presence of amyloid fibrils is a hallmark of several neurodegenerative diseases. Some amyloidogenic proteins, such as α-synuclein and amyloid β, interact with lipids, and this interaction can strongly favour the formation of amyloid fibrils. In particular the primary nucleation step, i.e. the de novo formation of amyloid fibrils, has been shown to be accelerated by lipids. However, the exact mechanism of this acceleration is still mostly unclear. Here we use a range of scattering methods, such as dynamic light scattering (DLS) and small angle X-ray and neutron scattering (SAXS and SANS) to obtain structural information on the binding of α-synuclein to model membranes formed from negatively charged lipids and their co-assembly into amyloid fibrils. We find that the model membranes take an active role in the reaction. The binding of α synuclein to the model membranes immediately induces a major structural change in the lipid assembly, which leads to a break-up into small and mostly disc- or rod-like lipid-protein particles. This transition can be reversed by temperature changes or proteolytic protein removal. Incubation of the small lipid-α-synuclein particles for several hours, however, leads to amyloid fibril formation, whereby the lipids are incorporated into the amyloid fibrils.

The influence of zwitterionic and anionic phospholipids on protein aggregation

The progressive aggregation of misfolded proteins is the underlying molecular cause of numerous pathologies including Parkinson's disease and injection and transthyretin amyloidosis. A growing body of evidence indicates that protein deposits detected in organs and tissues of patients diagnosed with such pathologies contain fragments of lipid membranes. In vitro experiments also showed that lipid membranes could strongly change the aggregation rate of amyloidogenic proteins, as well as alter the secondary structure and toxicity of oligomers and fibrils formed in their presence. In this review, the effect of large unilamellar vesicles (LUVs) composed of zwitterionic and anionic phospholipids on the aggregation rate of insulin, lysozyme, transthyretin (TTR) and α- synuclein (α-syn) will be discussed. The manuscript will also critically review the most recent findings on the lipid-induced changes in the secondary structure of protein oligomers and fibrils, as well as reveal the extent to which lipids could alter the toxicity of protein aggregates formed in their presence.

Binding mechanism of full-length Aβ40 peptide to a mixed lipid bilayer

The destructive effect of Aβ peptides on membranes is an important source of its cytotoxicity in the pathogenesis of Alzheimer's disease. We have investigated the binding mechanism between the Aβ42 peptide and bilayer in our former work. However, as another abundant form of Aβ peptides in the physiological environment, the binding mechanism between Aβ40 peptide and the lipid bilayer still remains ambiguous. Hence, we performed all-atom simulations on the Aβ40 peptides with the lipid bilayer herein using replica exchange with the solute tempering 2 method. We obtained four major binding models with the hydrophobic C-terminus as the most preferable binding region. Hydrophobic residues and positively charged residues are the principal residues involved in the peptide-bilayer interactions. Aβ40 peptides in our simulation mainly adopt a β-rich conformation in both bound and unbound states. Besides, we determined peptide-water interactions and found that bound peptides prefer forming hydrogen bonds with water molecules than unbound peptides. Our findings herein may provide new insights for the in-depth understanding of the membrane-destructive mechanism of Aβ peptides.

Lipid rafts and human diseases: why we need to target gangliosides

Gangliosides are functional components of membrane lipid rafts that control critical functions in cell communication. Many pathologies involve raft gangliosides, which therefore represent an approach of choice for developing innovative therapeutic strategies. Beginning with a discussion of what a disease is (and is not), this review lists the major human pathologies that involve gangliosides, which includes cancer, diabetes, and infectious and neurodegenerative diseases. In most cases, the problem is due to a protein whose binding to gangliosides either creates a pathological condition or impairs a physiological function. Then, I draw up an inventory of the different molecular mechanisms of protein-ganglioside interactions. I propose to classify the ganglioside-binding domains of proteins into four categories, which I name GBD-1, GBD-2, GBD-3, and GBD-4. This structural and functional classification could help to rationalize the design of innovative molecules capable of disrupting the binding of selected proteins to gangliosides without generating undesirable effects. The biochemical specificities of gangliosides expressed in the human brain must also be taken into account to improve the reliability of animal models (or any animal-free alternative) of Alzheimer's and Parkinson's diseases.

Cholesterol Metabolism in Aging and Age-Related Disorders

All mammalian cell membranes contain cholesterol to maintain membrane integrity. The transport of this hydrophobic lipid is mediated by lipoproteins. Cholesterol is especially enriched in the brain, particularly in synaptic and myelin membranes. Aging involves changes in sterol metabolism in peripheral organs and also in the brain. Some of those alterations have the potential to promote or to counteract the development of neurodegenerative diseases during aging. Here, we summarize the current knowledge of general principles of sterol metabolism in humans and mice, the most widely used model organism in biomedical research. We discuss changes in sterol metabolism that occur in the aged brain and highlight recent developments in cell type-specific cholesterol metabolism in the fast-growing research field of aging and age-related diseases, focusing on Alzheimer's disease. We propose that cell type-specific cholesterol handling and the interplay between cell types critically influence age-related disease processes.

Effect of the Lipid Landscape on the Efficacy of Cell-Penetrating Peptides

Every cell biological textbook teaches us that the main role of the plasma membrane is to separate cells from their neighborhood to allow for a controlled composition of the intracellular space. The mostly hydrophobic nature of the cell membrane presents an impenetrable barrier for most hydrophilic molecules larger than 1 kDa. On the other hand, cell-penetrating peptides (CPPs) are capable of traversing this barrier without compromising membrane integrity, and they can do so on their own or coupled to cargos. Coupling biologically and medically relevant cargos to CPPs holds great promise of delivering membrane-impermeable drugs into cells. If the cargo is able to interact with certain cell types, uptake of the CPP-drug complex can be tailored to be cell-type-specific. Besides outlining the major membrane penetration pathways of CPPs, this review is aimed at deciphering how properties of the membrane influence the uptake mechanisms of CPPs. By summarizing an extensive body of experimental evidence, we argue that a more ordered, less flexible membrane structure, often present in the very diseases planned to be treated with CPPs, decreases their cellular uptake. These correlations are not only relevant for understanding the cellular biology of CPPs, but also for rationally improving their value in translational or clinical applications.

A review: Exploring the metabolic and structural characterisation of beta pleated amyloid fibril in human tissue using Raman spectrometry and SAXS

Amyloidosis is a deleterious condition caused by abnormal amyloid fibril build-up in living tissues. To date, 42 proteins that are linked to amyloid fibrils have been discovered. Amyloid fibril structure variation can affect the severity, progression rate, or clinical symptoms of amyloidosis. Since amyloid fibril build-up is the primary pathological basis for various neurodegenerative illnesses, characterization of these deadly proteins, particularly utilising optical techniques have been a focus. Spectroscopy techniques provide significant non-invasive platforms for the investigation of the structure and conformation of amyloid fibrils, offering a wide spectrum of analyses ranging from nanometric to micrometric size scales. Even though this area of study has been intensively explored, there still remain aspects of amyloid fibrillization that are not fully known, a matter hindering progress in treating and curing amyloidosis. This review aims to provide recent updates and comprehensive information on optical techniques for metabolic and proteomic characterization of β-pleated amyloid fibrils found in human tissue with thorough literature analysis of publications. Raman spectroscopy and SAXS are well established experimental methods for study of structural properties of biomaterials. With suitable models, they offer extended information for valid proteomic analysis under physiologically relevant conditions. This review points to evidence that despite limitations, these techniques are able to provide for the necessary output and proteomics indication in order to extrapolate the aetiology of amyloid fibrils for reliable diagnostic purposes. Our metabolic database may also contribute to elucidating the nature and function of the amyloid proteome in development and clearance of amyloid diseases.

Does statin use affect amyloid beta deposition and brain metabolism?

BACKGROUND

There are contradictory findings regarding the effect of statin drugs on amyloid β (Aβ) deposition as one of the main hallmarks of Alzheimer's disease (AD), along with tau pathology. We aimed to longitudinally investigate the therapeutic and preventive role of statin drugs by examining the brain Aβ deposition and metabolism rate in AD, mild cognitive impairment (MCI), and healthy controls (HC).

METHODS

The data of 828 subjects including 178 HC, 492 MCI, and 158 AD individuals were obtained from ADNI. The baseline and longitudinal [18 F] AV45 and 18-fluorodeoxyglucose (FDG) PET standard uptake value ratio (SUVR) measures were investigated among statin users and non-users.

RESULTS

Our results showed that there is no significant difference in baseline Aβ deposition and metabolism rate between statin users and non-users among HC, MCI, and AD subjects. While there was no significant effect of statin on metabolism rate, there was a significant difference in Aβ deposition change after 4 years (from baseline) between statin users and non-users within HC subjects (p = 0.011). The change of Aβ deposition at 4 years from baseline was -2.0 ± 6.3% for statin users and 1.4 ± 4.7% for non-users. There was no significant association between statin duration use with baseline and longitudinal Aβ deposition and metabolism rate. However, statin dosage was significantly associated with Aβ deposition in 2 years (r = -0.412, p = 0.021) in the HC group. Moreover, our analysis showed a significant correlation between total statin exposure (duration×dosage) and Aβ deposition in 2 years visit (r = -0.198, p = 0.037) in HC subjects. Furthermore, we investigated the longitudinal changes within each group of statin users and non-users separately in linear mixed models. Our findings showed that there are no significant changes in AV45 and FDG SUVR among both groups.

CONCLUSION

The present longitudinal analysis revealed that using statins might be beneficial in slowing down or stabilizing the Aβ deposition due to aging in subjects without cognitive impairment. However, once the clinical symptoms of cognitive impairment appear, statins fail to slow down Aβ deposition. Overall, our findings revealed that statin users might have slower Aβ aggregation than non-users.

Role of phosphatidylserine in amyloid-beta oligomerization at asymmetric phospholipid bilayers

Amyloid-beta (Aβ_1-42) aggregation triggers neurotoxicity and is linked to Alzheimer's disease. Aβ_1-42 oligomers, rather than extended fibrils, adhere to the cell membrane, causing cell death. Phosphatidylserine (PS), an anionic phospholipid, is prevalent in neuronal membranes (< 20 molar percentage) and, while isolated to the cytoplasmic leaflet of the membrane in healthy cells, its exposure in apoptotic cells and migration to exoplasmic leaflet is triggered by oxidative damage to the membrane. It is widely believed that PS plays a crucial role in the Aβ peptide interaction in the membranes of neuronal cells. However, due to the complexity of the cell membrane, it can be challenging to address molecular level understanding of the PS-Aβ binding and oligomerization processes. Herein, we use microcavity supported lipid bilayers (MSLBs) to analyse PS and Aβ_1-42 binding, oligomer formation, and membrane damage. MSLBs are a useful model to evaluate protein-membrane interactions because of their cell-like dual aspect fluidity, their addressability and compositional versatility. We used electrochemical impedance spectroscopy (EIS) and confocal fluorescence microscopy to compare the impact of Aβ_1-42 on simple zwitterioinic membrane, dioleoylphosphatidylcholine (DOPC), with MSLBs comprised of transversally asymmetric binary DOPC and dioleoylphosphatidylserine (DOPS). Monomeric Aβ_1-42 adsorbs weakly to the pristine zwitterionic DOPC membrane without aggregation. Using a membrane integrity test, with pyranine trapped within the cavities beneath the membrane, Aβ_1-42 exposure did not result in pyranine leakage, indicating that DOPC membranes were intact. When 10 mol% DOPS was doped asymmetrically into the membrane's outer leaflet, oligomerization of Aβ_1-42 monomer was evident in EIS and atomic force microscopy (AFM), and confocal imaging revealed that membrane damage, resulted in extensive pyranine leakage from the pores. The effects were time, and DOPS and Aβ_1-42 concentration-dependent. Membrane pore formation was visible within 30 minutes, and oligomerization, membrane-oligomer multilayer, and Aβ_1-42 fibril formation evident over 3 to 18 hours. In asymmetric membranes with DOPS localized to the lower leaflet, optothermally (laser induced) damage increased local DOPS concentrations at the distal leaflet, promoting Aβ_1-42 aggregation.

Protein Self-Assembly at the Liquid-Surface Interface. Surface-Mediated Aggregation Catalysis

Protein self-assembly into aggregates of various morphologies is a ubiquitous phenomenon in physical chemistry and biophysics. The critical role of amyloid assemblies in the development of diseases, neurodegenerative diseases especially, highlights the importance of understanding the mechanistic picture of the self-assembly process. The translation of this knowledge to the development of efficient preventions and treatments for diseases requires designing experiments at conditions mimicking those in vivo. This Perspective reviews data satisfying two major requirements: membrane environment and physiologically low concentrations of proteins. Recent progress in experiments and computational modeling resulted in a novel model for the amyloid aggregation process at the membrane-liquid interface. The self-assembly under such conditions has a number of critical features, further understanding of which can lead to the development of efficient preventive means and treatments for Alzheimer's and other devastating neurodegenerative disorders.

Probing the interactions between amyloidogenic proteins and bio-membranes

Protein misfolding diseases (PMDs) in humans are characterized by the deposition of protein aggregates in tissues, including Alzheimer's disease, Parkinson's disease, type 2 diabetes, and amyotrophic lateral sclerosis. Misfolding and aggregation of amyloidogenic proteins play a central role in the onset and progression of PMDs, and these processes are regulated by multiple factors, especially the interaction between proteins and bio-membranes. Bio-membranes induce conformational changes in amyloidogenic proteins and affect their aggregation; on the other hand, the aggregates of amyloidogenic proteins may cause membrane damage or dysfunction leading to cytotoxicity. In this review, we summarize the factors that affect the binding of amyloidogenic proteins and membranes, the effects of bio-membranes on the aggregation of amyloidogenic proteins, mechanisms of membrane disruption by amyloidogenic aggregates, technical approaches for detecting these interactions, and finally therapeutic strategies targeting membrane damage caused by amyloidogenic proteins.

White matter injury, cholesterol dysmetabolism, and APP/Abeta dysmetabolism interact to produce Alzheimer's disease (AD) neuropathology: A hypothesis and review

We postulate that myelin injury contributes to cholesterol release from myelin and cholesterol dysmetabolism which contributes to Abeta dysmetabolism, and combined with genetic and AD risk factors, leads to increased Abeta and amyloid plaques. Increased Abeta damages myelin to form a vicious injury cycle. Thus, white matter injury, cholesterol dysmetabolism and Abeta dysmetabolism interact to produce or worsen AD neuropathology. The amyloid cascade is the leading hypothesis for the cause of Alzheimer's disease (AD). The failure of clinical trials based on this hypothesis has raised other possibilities. Even with a possible new success (Lecanemab), it is not clear whether this is a cause or a result of the disease. With the discovery in 1993 that the apolipoprotein E type 4 allele (APOE4) was the major risk factor for sporadic, late-onset AD (LOAD), there has been increasing interest in cholesterol in AD since APOE is a major cholesterol transporter. Recent studies show that cholesterol metabolism is intricately involved with Abeta (Aβ)/amyloid transport and metabolism, with cholesterol down-regulating the Aβ LRP1 transporter and upregulating the Aβ RAGE receptor, both of which would increase brain Aβ. Moreover, manipulating cholesterol transport and metabolism in rodent AD models can ameliorate pathology and cognitive deficits, or worsen them depending upon the manipulation. Though white matter (WM) injury has been noted in AD brain since Alzheimer's initial observations, recent studies have shown abnormal white matter in every AD brain. Moreover, there is age-related WM injury in normal individuals that occurs earlier and is worse with the APOE4 genotype. Moreover, WM injury precedes formation of plaques and tangles in human Familial Alzheimer's disease (FAD) and precedes plaque formation in rodent AD models. Restoring WM in rodent AD models improves cognition without affecting AD pathology. Thus, we postulate that the amyloid cascade, cholesterol dysmetabolism and white matter injury interact to produce and/or worsen AD pathology. We further postulate that the primary initiating event could be related to any of the three, with age a major factor for WM injury, diet and APOE4 and other genes a factor for cholesterol dysmetabolism, and FAD and other genes for Abeta dysmetabolism.

Perphenazine-Macrocycle Conjugates Rapidly Sequester the Aβ42 Monomer and Prevent Formation of Toxic Oligomers and Amyloid

Alzheimer's disease is imposing a growing social and economic burden worldwide, and effective therapies are urgently required. One possible approach to modulation of the disease outcome is to use small molecules to limit the conversion of monomeric amyloid (Aβ42) to cytotoxic amyloid oligomers and fibrils. We have synthesized modulators of amyloid assembly that are unlike others studied to date: these compounds act primarily by sequestering the Aβ42 monomer. We provide kinetic and nuclear magnetic resonance data showing that these perphenazine conjugates divert the Aβ42 monomer into amorphous aggregates that are not cytotoxic. Rapid monomer sequestration by the compounds reduces fibril assembly, even in the presence of pre-formed fibrillar seeds. The compounds are therefore also able to disrupt monomer-dependent secondary nucleation, the autocatalytic process that generates the majority of toxic oligomers. The inhibitors have a modular design that is easily varied, aiding future exploration and use of these tools to probe the impact of distinct Aβ42 species populated during amyloid assembly.

Binding Models of Aβ42 Peptide with Membranes Explored by Molecular Simulations

One of the factors contributing to the toxicity of amyloid-β (Aβ) peptides is the destruction of membrane integrity through Aβ peptide-membrane interactions. The binding of Aβ peptides to membranes has been studied by experiments and theoretical simulations extensively. The exact binding mechanism, however, still remains elusive. In the present study, the molecular basis of the peptide-bilayer binding mechanism of the full-length Aβ42 monomer with POPC/POPS/CHOL bilayers is investigated by all-atom (AA) simulations. Three main binding models in coil, bend, and turn structures are obtained. Model 1 of the three models with the central hydrophobic core (CHC) buried inside the membrane is the dominant binding model. The structural features of the peptide, the peptide-bilayer interacting regions, the intrapeptide interactions, and peptide-water interactions are studied. The binding of the Aβ42 monomer to the POPC/POPS/CHOL bilayer is also explored by coarse-grained (CG) simulations as a complement. Both the AA and CG simulations show that residues in CHC prefer forming interactions with the bilayer, indicating the crucial role of CHC in peptide-bilayer binding. Our results can provide new insights for the investigation of the peptide-bilayer binding mechanism of the Aβ peptide.

Exploring the binding kinetics and behaviors of self-aggregated beta-amyloid oligomers to phase-separated lipid rafts with or without ganglioside-clusters

Lipid binding kinetics and energetics of self-aggregated and disordered beta-amyloid oligomers of various sizes, from solution to lipid raft surfaces, were investigated using MD simulations. Our systems include small (monomers to tetramers) and larger (octamers and dodecamers) oligomers. Our lipid rafts contain saturated and unsaturated phosphatidylcholine (PC), cholesterol, and with or without asymmetrically distributed monosialotetrahexosylganglioside (GM1). All rafts exhibited dynamic and structurally diversified domains including liquid-ordered (Lo), liquid-disordered (Ld), and interfacial Lod domains. For rafts without GM1, all oligomers bound to the Lod domain. For GM1-containing rafts, all small oligomers and most larger oligomers bound specifically to the GM1-clusters embedded in the Lo domain. Lipid-protein binding energies followed an order of GM1 >> unsaturated PC > saturated PC > cholesterol for all rafts. In addition, protein-induced membrane structural disruption increased progressively with the size of the oligomer for the annular lipids surrounding the membrane-bound protein in non-GM1-containing rafts. We propose that the tight binding of beta-amyloid oligomers to the GM1-clusters and the structural perturbation of lipids surrounding the membrane-bound proteins at the Lod domain are early molecular events of the beta-amyloid aggregation process on neuronal membrane surfaces that trigger the onset of Alzheimer's.

Atomic Insights into Amyloid-Induced Membrane Damage

Amphipathic peptides can cause biological membranes to leak either by dissolving their lipid content via a detergent-like mechanism or by forming pores on the membrane surface. These modes of membrane damage have been related to the toxicity of amyloid peptides and to the activity of antimicrobial peptides. Here, we perform the first all-atom simulations in which membrane-bound amphipathic peptides self-assemble into β-sheets that subsequently either form stable pores inside the bilayer or drag lipids out of the membrane surface. An analysis of these simulations shows that the acyl tail of lipids interact strongly with non-polar side chains of peptides deposited on the membrane. These strong interactions enable lipids to be dragged out of the bilayer by oligomeric structures accounting for detergent-like damage. They also disturb the orientation of lipid tails in the vicinity of peptides. These distortions are minimized around pore structures. We also show that membrane-bound β-sheets become twisted with one of their extremities partially penetrating the lipid bilayer. This allows peptides on opposite leaflets to interact and form a long transmembrane β-sheet, which initiates poration. In simulations, where peptides are deposited on a single leaflet, the twist in β-sheets allows them to penetrate the membrane and form pores. In addition, our simulations show that fibril-like structures produce little damage to lipid membranes, as non-polar side chains in these structures are unavailable to interact with the acyl tail of lipids.

Concentration-Dependent Effects of Cholesterol on the Dimerization of Amyloid-β Peptides in Lipid Bilayers

Membrane disruption mediated by the accumulation of amyloid-β (Aβ) on cell membranes is central to the pathogenesis of Alzheimer's disease (AD). Cholesterol, an important component of membranes, is well-recognized as a risk factor in AD. It can affect the aggregation and pore formation of Aβ on membranes whereas the specific effects are rather complex, particularly regarding the non-linear response to cholesterol concentrations. Yet, the mechanistic understanding of the role of cholesterol in Aβ-membrane interactions remains incomplete. Herein, we employed microsecond-scale molecular dynamics simulations to investigate the effects of cholesterol on Aβ dimerization in a lipid bilayer containing different molar ratios of cholesterol (0, 20, and 40 mol %). Cholesterol reduces the time required for the formation of stable dimers and exerts dual effects on Aβ-membrane interactions. First, cholesterol promotes the extraction of the C-terminal region from the membrane to water. Consequently, at the ratios of 0 and 20 mol %, peptides are anchored at the membrane-water interface, but they are repelled to water at a ratio of 40 mol % with high structural flexibility. Second, cholesterol weakens Aβ-membrane interactions, thereby enhancing inter-peptide interactions. The former is favorable for dimerization while the latter is not. The balance between two factors eventually leads to a non-monotonic effect on the degree of dimerization, whereby the number of inter-peptide contacts is the largest at a cholesterol ratio of 20 mol %. These results provide atomistic insights into the regulation mechanism of Aβ42 aggregation by cholesterol and help to understand the pathological link between cholesterol and AD.

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.

The intriguing dose-dependent effect of selected amphiphilic compounds on insulin amyloid aggregation: Focus on a cholesterol-based detergent, Chobimalt

The amyloidogenic self-assembly of many peptides and proteins largely depends on external conditions. Among amyloid-prone proteins, insulin attracts attention because of its physiological and therapeutic importance. In the present work, the amyloid aggregation of insulin is studied in the presence of cholesterol-based detergent, Chobimalt. The strategy to elucidate the Chobimalt-induced effect on insulin fibrillogenesis is based on performing the concentration- and time-dependent analysis using a combination of different experimental techniques, such as ThT fluorescence assay, CD, AFM, SANS, and SAXS. While at the lowest Chobimalt concentration (0.1 µM; insulin to Chobimalt molar ratio of 1:0.004) the formation of insulin fibrils was not affected, the gradual increase of Chobimalt concentration (up to 100 µM; molar ratio of 1:4) led to a significant increase in ThT fluorescence, and the maximal ThT fluorescence was 3-4-fold higher than the control insulin fibril's ThT fluorescence intensity. Kinetic studies confirm the dose-dependent experimental results. Depending on the concentration of Chobimalt, either (i) no effect is observed, or (ii) significantly, ∼10-times prolonged lag-phases accompanied by the substantial, ∼ 3-fold higher relative ThT fluorescence intensities at the steady-state phase are recorded. In addition, at certain concentrations of Chobimalt, changes in the elongation-phase are noticed. An increase in the Chobimalt concentrations also triggers the formation of insulin fibrils with sharply altered morphological appearance. The fibrils appear to be more flexible and wavy-like with a tendency to form circles. SANS and SAXS data also revealed the morphology changes of amyloid fibrils in the presence of Chobimalt. Amyloid aggregation requires the formation of unfolded intermediates, which subsequently generate amyloidogenic nuclei. We hypothesize that the different morphology of the formed insulin fibrils is the result of the gradual binding of Chobimalt to different binding sites on unfolded insulin. A similar explanation and the existence of such binding sites with different binding energies was shown previously for the nonionic detergent. Thus, the data also emphasize the importance of a protein partially-unfolded state which undergoes the process of fibrils formation; i.e., certain experimental conditions or the presence of additives may dramatically change not only kinetics but also the morphology of fibrillar aggregates.

Physical Mechanism of Amyloid-β Peptide Chain Aggregation on Fluidic Lipid Nanotubules

The question of how peptide chain aggregation is influenced by lipid membranes with varying shapes and structures is crucial for a detailed understanding of the neurotoxicity effect of the peptide chains. Not like the more usual spherical liposomes and planar lipid membranes, herein, we use lipid nanotubules as a model of important neuron synapse nanowire structures and devote particular attention to the effect of nanotubule fluidity on amyloid-β peptide (Aβ) chain aggregation. We apply single-molecule tracking (SMT) to elucidate how Aβ chains diffuse and aggregate on lipid nanotubules with different fluidities. The physical mechanism implies that fluidic lipid nanotubules facilitate the super-diffusion of two-dimensional (2D)-mobile precursor Aβ chains and promote their aggregation. This aggregation mechanism is retarded on less fluidic lipid nanotubules where the super-diffusion of 2D-mobile precursor Aβ chains is restricted by "frozen" lipids with less mobility. This work provides a mechanistic explanation for Aβ chain aggregation on fluidic lipid nanotubules.

Free Cholesterol Accelerates Aβ Self-Assembly on Membranes at Physiological Concentration

The effects of membranes on the early-stage aggregation of amyloid β (Aβ) have come to light as potential mechanisms by which neurotoxic species are formed in Alzheimer’s disease. We have shown that direct Aβ-membrane interactions dramatically enhance the Aβ aggregation, allowing for oligomer assembly at physiologically low concentrations of the monomer. Membrane composition is also a crucial factor in this process. Our results showed that apart from phospholipids composition, cholesterol in membranes significantly enhances the aggregation kinetics. It has been reported that free cholesterol is present in plaques. Here we report that free cholesterol, along with its presence inside the membrane, further accelerate the aggregation process by producing aggregates more rapidly and of significantly larger sizes. These aggregates, which are formed on the lipid bilayer, are able to dissociate from the surface and accumulate in the bulk solution; the presence of free cholesterol accelerates this dissociation as well. All-atom molecular dynamics simulations show that cholesterol binds Aβ monomers and significantly changes the conformational sampling of Aβ monomer; more than doubling the fraction of low-energy conformations compared to those in the absence of cholesterol, which can contribute to the aggregation process. The results indicate that Aβ-lipid interaction is an important factor in the disease prone amyloid assembly process.

Effect of Cholesterol Molecules on Aβ1-42 Wild-Type and Mutants Trimers

Alzheimer’s disease displays aggregates of the amyloid-beta (Aβ) peptide in the brain, and there is increasing evidence that cholesterol may contribute to the pathogenesis of the disease. Though many experimental and theoretical studies have focused on the interactions of Aβ oligomers with membrane models containing cholesterol, an understanding of the effect of free cholesterol on small Aβ42 oligomers is not fully established. To address this question, we report on replica exchange with a solute tempering simulation of an Aβ42 trimer with cholesterol and compare it with a previous replica exchange molecular dynamics simulation. We show that the binding hot spots of cholesterol are rather complex, involving hydrophobic residues L17–F20 and L30–M35 with a non-negligible contribution of loop residues D22–K28 and N-terminus residues. We also examine the effects of cholesterol on the trimers of the disease-causing A21G and disease-protective A2T mutations by molecular dynamics simulations. We show that these two mutations moderately impact cholesterol-binding modes. In our REST2 simulations, we find that cholesterol is rarely inserted into aggregates but rather attached as dimers and trimers at the surface of Aβ42 oligomers. We propose that cholesterol acts as a glue to speed up the formation of larger aggregates; this provides a mechanistic link between cholesterol and Alzheimer’s disease.

Synapses, Microglia, and Lipids in Alzheimer's Disease

Alzheimer's disease (AD) is characterised by synaptic dysfunction accompanied by the microscopically visible accumulation of pathological protein deposits and cellular dystrophy involving both neurons and glia. Late-stage AD shows pronounced loss of synapses and neurons across several differentially affected brain regions. Recent studies of advanced AD using post-mortem brain samples have demonstrated the direct involvement of microglia in synaptic changes. Variants of the Apolipoprotein E and Triggering Receptors Expressed on Myeloid Cells gene represent important determinants of microglial activity but also of lipid metabolism in cells of the central nervous system. Here we review evidence that may help to explain how abnormal lipid metabolism, microglial activation, and synaptic pathophysiology are inter-related in AD.

Topographically smooth and stable supported lipid bilayer for high-resolution AFM studies

The cellular membrane has been identified to play a critical role in various biological processes including the assembly of biological systems. Membranes are complex, primarily two-dimensional assemblies with varied lipid compositions depending on the particular region of the cell. Supported lipid bilayers are considered as appropriate models for physio-chemical studies of membranes including numerous single molecule techniques. Atomic force microscopy (AFM) as a topographic technique is a fully appropriate single molecule technique capable of direct observation of molecular processes on membranes. However, reliable experimental AFM studies require the preparation of the bilayer with a sub-nanometer smooth morphology, which remains stable over long-time observation. Here we present the methodology, which allows one to prepare a smooth, stable, structurally homogeneous lipid bilayer without the presence of any trapped vesicles. We described the application of such lipid bilayers to probe time-dependent early stages of aggregation of monomeric amyloid proteins. Importantly, the proposed methodology can be extended to bilayers with various compositions, by incorporating different lipids for on-membrane aggregation study including cholesterol. Furthermore, this methodology development allowed us to monitor the aggregation of amyloid protein at its physiologically relevant low protein concentration. The flexibility of altering the membrane composition allows to identify the specific role of a particular lipid towards the aggregation kinetics, revealing the plausible mechanism of disease development.

Mechanistic Understanding from Molecular Dynamics in Pharmaceutical Research 2: Lipid Membrane in Drug Design

We review the use of molecular dynamics (MD) simulation as a drug design tool in the context of the role that the lipid membrane can play in drug action, i.e., the interaction between candidate drug molecules and lipid membranes. In the standard "lock and key" paradigm, only the interaction between the drug and a specific active site of a specific protein is considered; the environment in which the drug acts is, from a biophysical perspective, far more complex than this. The possible mechanisms though which a drug can be designed to tinker with physiological processes are significantly broader than merely fitting to a single active site of a single protein. In this paper, we focus on the role of the lipid membrane, arguably the most important element outside the proteins themselves, as a case study. We discuss work that has been carried out, using MD simulation, concerning the transfection of drugs through membranes that act as biological barriers in the path of the drugs, the behavior of drug molecules within membranes, how their collective behavior can affect the structure and properties of the membrane and, finally, the role lipid membranes, to which the vast majority of drug target proteins are associated, can play in mediating the interaction between drug and target protein. This review paper is the second in a two-part series covering MD simulation as a tool in pharmaceutical research; both are designed as pedagogical review papers aimed at both pharmaceutical scientists interested in exploring how the tool of MD simulation can be applied to their research and computational scientists interested in exploring the possibility of a pharmaceutical context for their research.

An Overview of Several Inhibitors for Alzheimer's Disease: Characterization and Failure

Amyloid beta (Aβ) oligomers are the most neurotoxic aggregates causing neuronal death and cognitive damage. A detailed elucidation of the aggregation pathways from oligomers to fibril formation is crucial to develop therapeutic strategies for Alzheimer's disease (AD). Although experimental techniques rely on the measure of time- and space-average properties, they face severe difficulties in the investigation of Aβ peptide aggregation due to their intrinsically disorder character. Computer simulation is a tool that allows tracing the molecular motion of molecules; hence it complements Aβ experiments, as it allows to explore the binding mechanism between metal ions and Aβ oligomers close to the cellular membrane at the atomic resolution. In this context, integrated studies of experiments and computer simulations can assist in mapping the complete pathways of aggregation and toxicity of Aβ peptides. Aβ oligomers are disordered proteins, and due to a rapid exploration of their intrinsic conformational space in real-time, they are challenging therapeutic targets. Therefore, no good drug candidate could have been identified for clinical use. Our previous investigations identified two small molecules, M30 (2-Octahydroisoquinolin-2(1H)-ylethanamine) and Gabapentin, capable of Aβ binding and inhibiting molecular aggregation, synaptotoxicity, intracellular calcium signaling, cellular toxicity and memory losses induced by Aβ. Thus, we recommend these molecules as novel candidates to assist anti-AD drug discovery in the near future. This review discusses the most recent research investigations about the Aβ dynamics in water, close contact with cell membranes, and several therapeutic strategies to remove plaque formation.

Enhancing the binding of the β-sheet breaker peptide LPFFD to the amyloid-β fibrils by aromatic modifications: A molecular dynamics simulation study

Alzheimer's is a fatal neurodegenerative disease for which there is no cure at present. The disease is characterized by the presence of plaques in the brains of a patient, which are composed mainly of aggregates of the amyloid-β peptide in the form of β-sheet fibrils. Here, we investigated the possibility of exploiting the superior binding ability of aromatic amino acids to a particular model of the amyloid-β fibrils. which is a difficult target for drug design. The β-sheet breaker peptide LPFFD was modified with aromatic amino acids and its binding to these fibrils was studied. We found that the orientation and the electrostatic complementarity of the modified peptide with respect to the fibrils played a crucial role in determining whether its binding was improved by the aromatic amino acids. The modified LPFFD peptides were able to bind to those fibril residues. which are important in the aggregation of amyloid-β peptides and thus can potentially inhibit the further aggregation of the amyloid-beta peptides by blocking their interactions. We found that the tryptophan modified LPFFD peptides had the best binding affinities. In most cases, the aromatic amino acids in the N-terminus of the modified peptides made more contacts with the fibrils than those in the C-terminus. We also found that increasing the aromatic content did not significantly improve the binding of the LPFFD peptide to the fibrils. Our study can serve as a basis for the design of novel peptide-based drugs for Alzheimer's disease in which aromatic interactions play an important role.