Formation of Amyloids by Aβ-(1–42) on NGF-differentiated PC12 Cells: Roles of Gangliosides and Cholesterol

Amyloid β interferes with wound healing of brain microvascular endothelial cells by disorganizing the actin cytoskeleton

Cerebral amyloid angiopathy (CAA) is a disease in which amyloid β (Aβ) is deposited in the cerebral blood vessels, reducing compliance, tearing and weakening of vessel walls, leading to cerebral hemorrhage. The mechanisms by which Aβ leads to focal wall fragmentation and intimal damage are not well understood. We analyzed the motility of human brain microvascular endothelial cells (hBMECs) in real-time using a wound-healing assay. We observed the suppression of cell migration by visualizing Aβ aggregation using quantum dot (QD) nanoprobes. In addition, using QD nanoprobes and a SiR-actin probe, we simultaneously observed Aβ aggregation and F-actin organization in real-time for the first time. Aβ began to aggregate at the edge of endothelial cells, reducing cell motility. In addition, Aβ aggregation disorganized the actin cytoskeleton and induced abnormal actin aggregation. Aβ aggregated actively in the anterior group, where cell motility was active. Our findings may be a first step toward explaining the mechanism by which Aβ causes vascular wall fragility, bleeding, and rebleeding in CAA.

Real-time monitoring of the amyloid β1-42 monomer-to-oligomer channel transition using a lipid bilayer system

This study describes the observation of the transformation of monomeric amyloid β1-42 (Aβ42) into oligomers in a lipid membrane utilizing a lipid bilayer system for electrophysiological measurement. The relevance of oligomers and protofibrils in Alzheimer's disease (AD) is underscored given their significant neurotoxicity. By closely monitoring the shift of Aβ42 from its monomeric state to forming oligomeric channels in phospholipid membranes, we noted that this transformation transpired within a 2-h frame. We manipulated the lipid membrane's constitution with components such as glycerophospholipid, porcine brain total lipid extract, sphingomyelin (SM), and cholesterol (Chol.) to effectively imitate nerve cell membranes. Interesting findings showcased Chol.'s ability to foster stable oligomeric channel formation in the lipid membrane, with SM and GM1 lipids potentially enhancing channel formation as well. Additionally, the study identified the potential of a catechin derivative, epigallocatechin gallate (EGCG), in obstructing oligomerization. With EGCG present in the outer solution of the Aβ42-infused membrane, a noteworthy reduction in channel current was observed, suggesting the successful inhibition of oligomerization. This conclusion held true in both, prior and subsequent, stages of oligomerization. Our findings shed light on the toxicity of oligomers, promising invaluable information for future advancements in AD treatment strategies.

Aβ and Tau Interact with Metal Ions, Lipid Membranes and Peptide-Based Amyloid Inhibitors: Are These Common Features Relevant in Alzheimer’s Disease?

In the last two decades, the amyloid hypothesis, i.e., the abnormal accumulation of toxic Aβ assemblies in the brain, has been considered the mainstream concept sustaining research in Alzheimer’s Disease (AD). However, the course of cognitive decline and AD development better correlates with tau accumulation rather than amyloid peptide deposition. Moreover, all clinical trials of amyloid-targeting drug candidates have been unsuccessful, implicitly suggesting that the amyloid hypothesis needs significant amendments. Accumulating evidence supports the existence of a series of potentially dangerous relationships between Aβ oligomeric species and tau protein in AD. However, the molecular determinants underlying pathogenic Aβ/tau cross interactions are not fully understood. Here, we discuss the common features of Aβ and tau molecules, with special emphasis on: (i) the critical role played by metal dyshomeostasis in promoting both Aβ and tau aggregation and oxidative stress, in AD; (ii) the effects of lipid membranes on Aβ and tau (co)-aggregation at the membrane interface; (iii) the potential of small peptide-based inhibitors of Aβ and tau misfolding as therapeutic tools in AD. Although the molecular mechanism underlying the direct Aβ/tau interaction remains largely unknown, the arguments discussed in this review may help reinforcing the current view of a synergistic Aβ/tau molecular crosstalk in AD and stimulate further research to mechanism elucidation and next-generation AD therapeutics.

Elucidation of Complex Dynamic Intermolecular Interactions in Membranes

Biomembranes composed of various proteins and lipids play important roles in cellular functions, such as signal transduction and substance transport. In addition, some bioactive peptides and pathogenic proteins target membrane proteins and lipids to exert their effects. Therefore, an understanding of dynamic and complex intermolecular interactions among these membrane constituents is needed to elucidate their mechanisms. This review summarizes the major research carried out in the author's laboratory on how lipids and their inhomogeneous distributions regulate the structures and functions of antimicrobial peptides and Alzheimer's amyloid β-protein. Also, how to detect transmembrane helix-helix and membrane protein-protein interactions and how they are modulated by lipids are discussed.

The role of membranes in function and dysfunction of intrinsically disordered amyloidogenic proteins

Membrane-protein interactions play a major role in human physiology as well as in diseases pathology. Interaction of a protein with the membrane was previously thought to be dependent on well-defined three-dimensional structure of the protein. In recent decades, however, it has become evident that a large fraction of the proteome, particularly in eukaryotes, stays disordered in solution and these proteins are termed as intrinsically disordered proteins (IDPs). Also, a vast majority of human proteomes have been reported to contain substantially long disordered regions, called intrinsically disordered regions (IDRs), in addition to the structurally ordered regions. IDPs exist in an ensemble of conformations and the conformational flexibility enables IDPs to achieve functional diversity. IDPs (and IDRs) are found to be important players in cell signaling, where biological membranes act as anchors for signaling cascades. Therefore, IDPs modulate the membrane architectures, at the same time membrane composition also affects the binding of IDPs. Because of intrinsic disorders, misfolding of IDPs often leads to formation of oligomers, protofibrils and mature fibrils through progressive self-association. Accumulation of amyloid-like aggregates of some of the IDPs is a known causative agent for numerous diseases. In this chapter we highlight recent advances in understanding membrane interactions of some of the intrinsically disordered proteins involved in the pathogenesis of human diseases.

miRNA-124-3p targeting of LPIN1 attenuates inflammation and apoptosis in aged male rats cardiopulmonary bypass model of perioperative neurocognitive disorders

Perioperative neurocognitive disorder (PND) is recently recommended to define the cognitive decrease during the perioperative period. However, the disease's underlying mechanisms remain unclear. MicroRNAs (miRNAs) are noncoding RNAs that play a vital role in regulating neuroregeneration and neuronal apoptosis. In this study, miR-124-3p was significantly reduced in the PND rat model after a cardiopulmonary bypass (CPB) procedure. MicroRNA-124 (miR-124)-3p-overexpressed lentivirus was constructed and injected via the intracerebroventricular method before CPB. Morris Water Maze test (WMW) and the Open-Field test (OFT) were used to measure behavior changes, data shows decline of cognitive function of rats after CPB. PND rats expressed higher Aβ and p-Tau Protein by using immunohistochemistry (IHC) analyses and Enzyme-Linked Immune Sorbent Assay (ELISA). Moreover, the results of IHC, ELISA, Western Blot analysis (WB) and Terminal-deoxynucleotidyl Transferase Mediated Nick End Labeling Assay (TUNEL) showed CPB procedure induced inflammation and apoptosis in rats with PND. The data also revealed the protective function of miR-124-3p overexpression against PND in relieving inflammation, cell apoptosis, and alleviating repaired cognitive function. Moreover, miR-124-3p was predicted by directly targeting LPIN1. This study gives a novel viewpoint that miR-124-3p could improve the state of PND via modulating LPIN1, therefore providing a new strategy for preventing and treating PND in a preclinical application.

Real-Time Analysis of Specific Binding between Apolipoprotein E Isoforms and Amyloid β-Peptide by Dual Polarization Interferometry

One of the pathogenesis hypotheses of Alzheimer's disease (AD) is amyloid depositions and neurofibrillary tangles. Apolipoprotein E (Apo E) acts a vital part in the development of AD by affecting the aggregation and clearance of amyloid-β (Aβ). In this paper, a dual polarization interferometry (DPI) technique was employed for a real-time investigation toward the binding events of Apo E isoforms, for instance, Apo E2, Apo E3, and Apo E4, with Aβ_1-40. By evaluation of detailed binding information provided by DPI, the affinities between Apo E isoforms and Aβ_1-40 follow the order of E4 > E3 > E2, and the dissociation constants (K_D) of Aβ_1-40 with Apo E2, Apo E3, and Apo E4 were determined to be 251 ± 37, 40 ± 0.65, and 24.6 ± 2.42 nM, respectively. Our findings reveal the isoform-specific binding behaviors from a kinetics perspective, which can help us understand that Apo E4 has a higher risk of causing AD because of its promoting effect on Aβ aggregation and fibrillation and inefficient clearance of Aβ. Remarkably, this work provides a promising method for exploring the dynamics of interactions between biomolecules and expectantly contributes to the development of AD drugs and therapies targeting Apo E and Aβ.

Ginsenoside Rb1 protected PC12 cells from Aβ25-35-induced cytotoxicity via PPARγ activation and cholesterol reduction

Accumulating evidences suggest that amyloid β (Aβ)-peptide plays a key role in pathogenesis of Alzheimer's disease (AD) through aggregation and deposition into plaques in neuronal cells. Membrane components such as cholesterol and gangliosides not only enhance the production of amyloidogenic Aβ fragments, but also appear to strengthen Aβ-membrane interaction. Ginsenoside Rb1 (GRb1) is a major active component of Panax, which is widely used to improve learning and memory. In the present study, whether ginsenoside Rb1 could protect pheochromocytoma cells (PC12 cells) from Aβ_25-35-induced cytotoxicity including inhibiting cell growth, inducing apoptosis, producing reactive oxygen species (ROS), destroying the cytoskeleton and bringing about membrane toxicity was investigated. Our results indicated that ginsenoside Rb1 could serve as an agonist of peroxisom proliferator-activated receptor-γ (PPARγ) and reduce the level of cholesterol in AD model cells. Reduction of the Aβ_25-35-induced cytotoxicity by lowering cholesterol was evidenced by reduction of ROS production, lipid peroxidation, and protection of cytoskeleton and membrane surface rigidity. Most importantly, the viability of PC12 cells increased from 50.42 ± 5.51% for the AD group to 102.72 ± 4.34% for the 50 μM ginsenoside Rb1 group with cholesterol reduction. Our results suggested that ginsenoside Rb1 might function as an effective candidate to promote reverse cholesterol transport and lower ROS production, therefore providing a new insight into prevention and treatment of AD.

The Role of Lipid Environment in Ganglioside GM1-Induced Amyloid β Aggregation

Ganglioside GM1 is the most common brain ganglioside enriched in plasma membrane regions known as lipid rafts or membrane microdomains. GM1 participates in many modulatory and communication functions associated with the development, differentiation, and protection of neuronal tissue. It has, however, been demonstrated that GM1 plays a negative role in the pathophysiology of Alzheimer's disease (AD). The two features of AD are the formation of intracellular neurofibrillary bodies and the accumulation of extracellular amyloid β (Aβ). Aβ is a peptide characterized by intrinsic conformational flexibility. Depending on its partners, Aβ can adopt different spatial arrangements. GM1 has been shown to induce specific changes in the spatial organization of Aβ, which lead to enhanced peptide accumulation and deleterious effect especially on neuronal membranes containing clusters of this ganglioside. Changes in GM1 levels and distribution during the development of AD may contribute to the aggravation of the disease.

Three-dimensional real time imaging of amyloid β aggregation on living cells

Alzheimer’s disease (AD) is a progressive disorder of the brain that gradually decreases thinking, memory, and language abilities. The aggregation process of amyloid β (Aβ) is a key step in the expression of its neurocytotoxicity and development of AD because Aβ aggregation and accumulation around neuronal cells induces cell death. However, the molecular mechanism underlying the neurocytotoxicity and cell death by Aβ aggregation has not been clearly elucidated. In this study, we successfully visualized real-time process of Aβ₄₂ aggregation around living cells by applying our established QD imaging method. 3D observations using confocal laser microscopy revealed that Aβ₄₂ preferentially started to aggregate at the region where membrane protrusions frequently formed. Furthermore, we found that inhibition of actin polymerization using cytochalasin D reduced aggregation of Aβ₄₂ on the cell surface. These results indicate that actin polymerization-dependent cell motility is responsible for the promotion of Aβ₄₂ aggregation at the cell periphery. 3D observation also revealed that the aggregates around the cell remained in that location even if cell death occurred, implying that amyloid plaques found in the AD brain grew from the debris of dead cells that accumulated Aβ₄₂ aggregates.

Aβ-ganglioside interactions in the pathogenesis of Alzheimer's disease

It is widely accepted that the abnormal self-association of amyloid β-protein (Aβ) is central to the pathogenesis of Alzheimer's disease, the most common form of dementia. Accumulating evidence, both in vivo and in vitro, suggests that the binding of Aβ to gangliosides, especially monosialoganglioside GM1, plays an important role in the aggregation of Aβ. This review summarizes the molecular details of the binding of Aβ to ganglioside-containing membranes and subsequent structural changes, as revealed by liposomal and cellular studies. Furthermore, mechanisms of cytotoxicity by aggregated Aβ are also discussed.

The α-synuclein hereditary mutation E46K unlocks a more stable, pathogenic fibril structure

Aggregation of α-synuclein is a defining molecular feature of Parkinson's disease, Lewy body dementia, and multiple systems atrophy. Hereditary mutations in α-synuclein are linked to both Parkinson's disease and Lewy body dementia; in particular, patients bearing the E46K disease mutation manifest a clinical picture of parkinsonism and Lewy body dementia, and E46K creates more pathogenic fibrils in vitro. Understanding the effect of these hereditary mutations on α-synuclein fibril structure is fundamental to α-synuclein biology. We therefore determined the cryo-electron microscopy (cryo-EM) structure of α-synuclein fibrils containing the hereditary E46K mutation. The 2.5-Å structure reveals a symmetric double protofilament in which the molecules adopt a vastly rearranged, lower energy fold compared to wild-type fibrils. We propose that the E46K misfolding pathway avoids electrostatic repulsion between K46 and K80, a residue pair which form the E46-K80 salt bridge in the wild-type fibril structure. We hypothesize that, under our conditions, the wild-type fold does not reach this deeper energy well of the E46K fold because the E46-K80 salt bridge diverts α-synuclein into a kinetic trap-a shallower, more accessible energy minimum. The E46K mutation apparently unlocks a more stable and pathogenic fibril structure.

Structures of fibrils formed by α-synuclein hereditary disease mutant H50Q reveal new polymorphs

Deposits of amyloid fibrils of α-synuclein are the histological hallmarks of Parkinson’s disease, dementia with Lewy bodies, and multiple system atrophy, with hereditary mutations in α-synuclein linked to the first two of these conditions. Seeing the changes to the structures of amyloid fibrils bearing these mutations may help to understand these diseases. To this end, we determined the cryo-EM structures of α-synuclein fibrils containing the H50Q hereditary mutation. We find that the H50Q mutation results in two previously unobserved polymorphs of α-synuclein: Narrow and Wide Fibrils, formed from either one or two protofilaments, respectively. These structures recapitulate conserved features of the wild-type fold but reveal new structural elements including a previously unobserved hydrogen bond network and surprising new protofilament arrangements. The structures of the H50Q polymorphs help to rationalize the faster aggregation kinetics, higher seeding capacity in biosensor cells, and greater cytotoxicity we observe for H50Q compared to wild-type α-synuclein.

Toxic Amyloid Tape: A Novel Mixed Antiparallel/Parallel β-Sheet Structure Formed by Amyloid β-Protein on GM1 Clusters

The abnormal aggregation of amyloid β-protein (Aβ) is considered central in the pathogenesis of Alzheimer's disease. We focused on membrane-mediated amyloidogenesis and found that amyloid fibrils formed on monosialoganglioside GM1 clusters were more toxic than those formed in aqueous solution. In this study, we investigated the structure of the toxic fibrils by Aβ-(1-40) in detail in comparison with less-toxic fibrils formed in aqueous solution. The less-toxic fibrils contain in-resister parallel β-sheets, whereas the structure of the toxic fibrils is unknown. Atomic force microscopy revealed that the toxic fibrils had a flat, tape-like morphology composed of a single β-sheet layer. Isotope-edited infrared spectroscopy indicated that almost the entire sequence of Aβ is included in the β-sheet. Chemical cross-linking experiments using Cys-substituted Aβs suggested that the fibrils mainly contained both in-resister parallel and two-residue-shifted antiparallel β-sheet structures. Solid-state NMR experiments also supported this conclusion. Thus, the toxic fibrils were found to possess a novel unique structure.

Interactions between amyloid β peptide and lipid membranes

The presence of amyloid plaques in the brain is a typical characteristic of Alzheimer's disease (AD). Amyloid plaques are formed from the deposits of aggregated amyloid β peptide (Aβ). The toxicity induced by Aβ aggregates is correlated with Aβ-membrane interactions. The mutual influences between aggregation and membranes are complicated and unclear. In recent years advanced experiments and findings are emerging to give us more detailed information on Aβ-membrane interactions. In this review, we mainly focus on the Aβ-membrane interactions and membrane-induced Aβ structures. The mechanism of Aβ-membrane interactions is also summarized, which provides insights into the prevention and treatment of AD.

Interaction between amyloidogenic proteins and biomembranes in protein misfolding diseases: Mechanisms, contributors, and therapy

The toxic deposition of misfolded amyloidogenic proteins is associated with more than fifty protein misfolding diseases (PMDs), including Alzheimer's disease, Parkinson's disease and type 2 diabetes mellitus. Protein deposition is a multi-step process modulated by a variety of factors, in particular by membrane-protein interaction. The interaction results in permeabilization of biomembranes contributing to the cytotoxicity that leads to PMDs. Different biological and physiochemical factors, such as protein sequence, lipid composition, and chaperones, are known to affect the membrane-protein interaction. Here, we provide a comprehensive review of the mechanisms and contributing factors of the interaction between biomembranes and amyloidogenic proteins, and a summary of the therapeutic approaches to PMDs that target this interaction. This article is part of a Special Issue entitled: Protein Aggregation and Misfolding at the Cell Membrane Interface edited by Ayyalusamy Ramamoorthy.

Ganglioside-Mediated Assembly of Amyloid β-Protein: Roles in Alzheimer's Disease

Assembly and deposition of amyloid β-protein (Aβ) is an early and invariable pathological event of Alzheimer's disease (AD), a chronic neurodegenerative disease affecting the neurons in the brain of aging population. Thus, clarification of the molecular mechanism underlying Aβ assembly is crucial not only for understanding the pathogenesis of AD, but also for developing disease-modifying remedies. In 1995, ganglioside-bound Aβ (GAβ), with unique molecular characteristics, including its altered immunoreactivity and its conspicuous ability to accelerate Aβ assembly, was discovered in an autopsied brain showing early pathological changes of AD. Based on these findings, it was hypothesized that GAβ is an endogenous seed for amyloid fibril formation in the AD brain. A body of evidence that supports the GAβ hypothesis has been growing for over 20years as follows. First, the conformational changes of Aβ from a random coil to an α-helix, and then to a β-sheet in the presence of ganglioside were validated by several techniques. Second, the seed activity of GAβ to accelerate the assembly of soluble Aβ into amyloid fibrils was confirmed by various in vitro and in vivo experiments. Third, it was found that the Aβ binding to ganglioside to form GAβ occurs under limited conditions, which were provided by the lipid environment surrounding ganglioside. Fourth, the region-specific Aβ deposition in the brain appeared to be dependent on the presence of the lipid environment that was in favor of GAβ generation. In this chapter, further progress of the study of ganglioside-mediated Aβ assembly, especially from the aspects of physicochemistry, structural biology, and neuropathology, is reviewed.

Not Oligomers but Amyloids are Cytotoxic in the Membrane-Mediated Amyloidogenesis of Amyloid-β Peptides

The formation of neurotoxic aggregates by amyloid-β peptide (Aβ) is considered to be a key step in the onset of Alzheimer's disease. It is widely accepted that oligomers are more neurotoxic than amyloid fibrils in the aqueous-phase aggregation of Aβ. Membrane-mediated amyloidogenesis is also relevant to the pathology, although the relationship between the aggregate size and cytotoxicity has remained elusive. Here, aggregation processes of Aβ on living cells and cytotoxic events were monitored by fluorescence techniques. Aβ formed amyloids after forming oligomers composed of ≈10 Aβ molecules. The formation of amyloids was necessary to activate apoptotic caspase-3 and reduce the ability of the cell to proliferate; this indicated that amyloid formation is a key event in Aβ-induced cytotoxicity.

In situ fibrillizing amyloid-beta 1-42 induces neurite degeneration and apoptosis of differentiated SH-SY5Y cells

The progression of Alzheimer’s disease is causatively linked to the accumulation of amyloid-β aggregates in the brain, however, it is not clear how the amyloid aggregates initiate the death of neuronal cells. The in vitro toxic effects of amyloid peptides are most commonly examined using the human neuroblastoma derived SH-SY5Y cell line and here we show that differentiated neuron-like SH-SY5Y cells are more sensitive to amyloid peptides than non-differentiated cells, because the latter lack long neurites. Exogenous soluble amyloid-β 1–42 covered cell bodies and whole neurites in differentiated cells with dense fibrils, causing neurite beading and fragmentation, whereas preformed amyloid-β 1–42 fibrils had no toxic effects. Importantly, spontaneously fibrillizing amyloid-β 1–42 peptide exhibited substantially higher cellular toxicity than amyloid-β 1–40, which did not form fibrils under the experimental conditions. These results support the hypothesis that peptide toxicity is related to the active fibrillization process in the incubation mixture.

Fluorescent and luminescent fusion proteins for analyses of amyloid beta peptide aggregation

The amyloid beta (Aβ) peptide is regarded as a causative agent of Alzheimer's disease. In this study, fluorescent and luminescent fusion proteins were constructed to analyze Aβ aggregation. A system was developed to monitor changes in luminescence that provides information about Aβ aggregation. In the presence of monomeric Aβ, the fusion protein exhibits higher luminescence intensity, and the luminescence intensity is diminished after aggregation of the fusion protein and Aβ. In contrast, the fluorescence is sustained in the presence of Aβ. In the absence of Aβ, the fusion protein self-aggregates, and its luminescence and fluorescence are quenched, thus decreasing the background fluorescence and enhancing the detection of Aβ inside and outside the cells. The ratio of the luminescence intensity to the fluorescence intensity would allow the aggregation degrees of Aβ to be distinguished. This study would be a promising method for analyzing the aggregation state of a particular amyloid protein/peptide (monomer, oligomer, or fibril), as well as the distribution of the amyloid protein/peptide within and at the cell surface, by using a single fusion protein. Copyright © 2017 European Peptide Society and John Wiley & Sons, Ltd.

Endocytosed 2-Microglobulin Amyloid Fibrils Induce Necrosis and Apoptosis of Rabbit Synovial Fibroblasts by Disrupting Endosomal/Lysosomal Membranes: A Novel Mechanism on the Cytotoxicity of Amyloid Fibrils

Dialysis-related amyloidosis is a major complication in long-term hemodialysis patients. In dialysis-related amyloidosis, β2-microglobulin (β2-m) amyloid fibrils deposit in the osteoarticular tissue, leading to carpal tunnel syndrome and destructive arthropathy with cystic bone lesions, but the mechanism by which these amyloid fibrils destruct bone and joint tissue is not fully understood. In this study, we assessed the cytotoxic effect of β2-m amyloid fibrils on the cultured rabbit synovial fibroblasts. Under light microscopy, the cells treated with amyloid fibrils exhibited both necrotic and apoptotic changes, while the cells treated with β2-m monomers and vehicle buffer exhibited no morphological changes. As compared to β2-m monomers and vehicle buffer, β2-m amyloid fibrils significantly reduced cellular viability as measured by the lactate dehydrogenase release assay and the 3-(4,5-di-methylthiazol-2-yl)-2,5-diphenyltetrazolium bromide reduction assay and significantly increased the percentage of apoptotic cells as measured by the terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling method. β2-m amyloid fibrils added to the medium adhered to cell surfaces, but did not disrupt artificial plasma membranes as measured by the liposome dye release assay. Interestingly, when the cells were incubated with amyloid fibrils for several hours, many endosomes/lysosomes filled with amyloid fibrils were observed under confocal laser microscopy and electron microscopy, Moreover, some endosomal/lysosomal membranes were disrupted by intravesicular fibrils, leading to the leakage of the fibrils into the cytosol and adjacent to mitochondria. Inhibition of actin-dependent endocytosis by cytochalasin D attenuated the toxicity of amyloid fibrils. These results suggest that endocytosed β2-m amyloid fibrils induce necrosis and apoptosis by disrupting endosomal/lysosomal membranes, and this novel mechanism on the cytotoxicity of amyloid fibrils is described.

Differences between amyloid-β aggregation in solution and on the membrane: insights into elucidation of the mechanistic details of Alzheimer's disease

The association of the amyloid-β (Aβ) peptide with cellular membranes is hypothesized to be the underlying phenomenon of neurotoxicity in Alzheimer's disease. Misfolding of proteins and peptides, as is the case with Aβ, follows a progression from a monomeric state, through intermediates, ending at long, unbranched amyloid fibers. This tutorial review offers a perspective on the association of toxic Aβ structures with membranes as well as details of membrane-associated mechanisms of toxicity.

How do membranes initiate Alzheimer's Disease? Formation of toxic amyloid fibrils by the amyloid β-protein on ganglioside clusters

Alzheimer's disease (AD), a severe neurodegenerative disorder, causes more than half of dementia cases. According to the popular "Aβ hypothesis" to explain the mechanism of this disease, amyloid β-peptides (Aβ) of 39-43 amino acid residues aggregate and deposit onto neurons, igniting the neurotoxic cascade of the disease. Therefore, researchers studying AD would like to elucidate the mechanisms by which essentially water-soluble but hydrophobic Aβ aggregates under pathological conditions. Most researchers have investigated the aggregation of Aβ in aqueous solution, and they concluded that the final aggregation product, the amyloid fibrils, were less toxic than the component peptide oligomers. They consequently shifted their interests to more toxic "soluble oligomers", structures that form as intermediates or off-pathway products during the aggregation process. Some researchers have also investigated artificial oligomers prepared under nonphysiological conditions. In contrast to these "in solution" studies, we have focused on "membrane-mediated" amyloidogenesis. In an earlier study, other researchers identified a specific form of Aβ that was bound to monosialoganglioside GM1, a sugar lipid, in brains of patients who exhibited the early pathological changes associated with AD. This Account summarizes 15 years of our research on this topic. We have found that Aβ specifically binds to GM1 that occurs in clusters, but not when it is uniformly distributed. Clustering is facilitated by cholesterol. Upon binding, Aβ changes its conformation from a random coil to an α-helix-rich structure. A CH-π interaction between the aromatic side chains of Aβ and carbohydrate moieties appended to GM1 appears to be important for binding. In addition, as Aβ accumulates and reaches its first threshold concentration (Aβ/GM1 = ∼0.013), aggregated β-sheets of ∼15 molecules appear and coexist with the helical form. However, this β-structure is stable and does not form larger aggregates. When the disease progresses further and the Aβ/GM1 ratio exceeds ∼0.044, the β-structure converts to a second β-structure that can seed aggregates. The seed recruits monomers from the aqueous phase to form toxic amyloid fibrils that have larger surface hydrophobicity and can contain antiparallel β-sheets. In contrast, amyloid fibrils formed in aqueous solution are less toxic and have parallel β-sheets. The less polar environments of GM1 clusters play an important role in the formation of these toxic fibrils. Membranes that contain GM1 clusters not only accelerate the aggregation of Aβ by locally concentrating Aβ molecules but also generate amyloid fibrils with unique structures and significant cytotoxicity. The inhibition of this aggregation cascade could be a promising strategy for the development of AD-modulating therapies.

Attenuation of Aβ25-35-induced parallel autophagic and apoptotic cell death by gypenoside XVII through the estrogen receptor-dependent activation of Nrf2/ARE pathways

Amyloid-beta (Aβ) has a pivotal function in the pathogenesis of Alzheimer's disease. To investigate Aβ neurotoxicity, we used an in vitro model that involves Aβ25-35-induced cell death in the nerve growth factor-induced differentiation of PC12 cells. Aβ25-35 (20μM) treatment for 24h caused apoptotic cell death, as evidenced by significant cell viability reduction, LDH release, phosphatidylserine externalization, mitochondrial membrane potential disruption, cytochrome c release, caspase-3 activation, PARP cleavage, and DNA fragmentation in PC12 cells. Aβ25-35 treatment led to autophagic cell death, as evidenced by augmented GFP-LC3 puncta, conversion of LC3-I to LC3-II, and increased LC3-II/LC3-I ratio. Aβ25-35 treatment induced oxidative stress, as evidenced by intracellular ROS accumulation and increased production of mitochondrial superoxide, malondialdehyde, protein carbonyl, and 8-OHdG. Phytoestrogens have been proved to be protective against Aβ-induced neurotoxicity and regarded as relatively safe targets for AD drug development. Gypenoside XVII (GP-17) is a novel phytoestrogen isolated from Gynostemma pentaphyllum or Panax notoginseng. Pretreatment with GP-17 (10μM) for 12h increased estrogen response element reporter activity, activated PI3K/Akt pathways, inhibited GSK-3β, induced Nrf2 nuclear translocation, augmented antioxidant responsive element enhancer activity, upregulated heme oxygenase 1 (HO-1) expression and activity, and provided protective effects against Aβ25-35-induced neurotoxicity, including oxidative stress, apoptosis, and autophagic cell death. In conclusion, GP-17 conferred protection against Aβ25-35-induced neurotoxicity through estrogen receptor-dependent activation of PI3K/Akt pathways, inactivation of GSK-3β and activation of Nrf2/ARE/HO-1 pathways. This finding might provide novel insights into understanding the mechanism for neuroprotective effects of phytoestrogens or gypenosides.

Protection of SK-N-MC cells against β-amyloid peptide-induced degeneration using neuron growth factor-loaded liposomes with surface lactoferrin

A liposomal system with surface lactoferrin (Lf) was developed for delivering neuron growth factor (NGF) across the blood-brain barrier (BBB) and improving the viability of neuron-like SK-N-MC cells with deposited β-amyloid peptide (Aβ). The Lf-grafted liposomes carrying NGF (Lf/NGF-liposomes) were applied to a monolayer of human brain-microvascular endothelial cells (HBMECs) regulated by human astrocytes (HAs) and to fibrillar Aβ1-42-insulted SK-N-MC cells. An increase in cholesterol mole percentage enhanced the particle size, absolute value of zeta potential, and physical stability, however, reduced the entrapment efficiency and release rate of NGF. In addition, an increase in Lf concentration increased the particle size, surface nitrogen percentage, NGF permeability across the BBB, and viability of HBMECs, HAs, and SK-N-MC cells, however, decreased the absolute value of zeta potential, surface phosphorus percentage, and loading efficiency of Lf. After treating with Lf/NGF-liposomes, a higher Aβ concentration yielded a lower survival of SK-N-MC cells. The current Lf/NGF-liposomes are efficacious drug carriers to target the BBB and inhibit the Aβ-induced neurotoxicity as potential pharmacotherapy for Alzheimer's disease.

Characterization of large amyloid fibers and tapes with Fourier transform infrared (FT-IR) and Raman spectroscopy

Amyloids are self-assembled protein structures implicated in a host of neurodegenerative diseases. Organisms can also produce "functional amyloids" to perpetuate life, and these materials serve as models for robust biomaterials. Amyloids are typically studied using fluorescent dyes, Fourier transform infrared (FT-IR), or Raman spectroscopy analysis of the protein amide I region, and X-ray diffraction (XRD) because the self-assembled β-sheet secondary structure of the amyloid can be easily identified with these techniques. Here, FT-IR and Raman spectroscopy analyses are described to characterize amyloid structures beyond just identification of the β-sheet structure. It has been shown that peptide mixtures can self-assemble into nanometer-sized amyloid structures that then continue to self-assemble to the micrometer scale. The resulting structures are flat tapes of low rigidity or cylinders of high rigidity depending on the peptides in the mixture. By monitoring the aggregation of peptides in solution using FT-IR spectroscopy, it is possible to identify specific amino acids implicated in β-sheet formation and higher order self-assembly. It is also possible to predict the final tape or cylinder morphology and gain insight into the structure's physical properties based on observed intermolecular interactions during the self-assembly process. Tapes and cylinders are shown to both have a similar core self-assembled β-sheet structure. Soft tapes also have weak hydrophobic interactions between alanine, isoleucine, leucine, and valine that facilitate self-assembly. Rigid cylinders have similar hydrophobic interactions that facilitate self-assembly and also have extensive hydrogen bonding between glutamines. Raman spectroscopy performed on the dried tapes and fibers shows the persistence of these interactions. The spectroscopic analyses described could be generalized to other self-assembling amyloid systems to explain property and morphological differences.

The amyloid-cell membrane system. The interplay between the biophysical features of oligomers/fibrils and cell membrane defines amyloid toxicity

Amyloid cytotoxicity, structure and polymorphisms are themes of increasing importance. Present knowledge considers any peptide/protein able to undergo misfolding and aggregation generating intrinsically cytotoxic amyloids. It also describes growth and structure of amyloid fibrils and their possible disassembly, whereas reduced information is available on oligomer structure. Recent research has highlighted the importance of the environmental conditions as determinants of the amyloid polymorphisms and cytotoxicity. Another body of evidence describes chemical or biological surfaces as key sites of protein misfolding and aggregation or of interaction with amyloids and the resulting biochemical modifications inducing cell functional/viability impairment. In particular, the membrane lipid composition appears to modulate cell response to toxic amyloids, thus contributing to explain the variable vulnerability to the same amyloids of different cell types. Finally, a recent view describes amyloid toxicity as an emerging property dependent on a complex interplay between the biophysical features of early aggregates and the interacting cell membranes taken as a whole system.

Biophysical insights into how surfaces, including lipid membranes, modulate protein aggregation related to neurodegeneration

There are a vast number of neurodegenerative diseases, including Alzheimer’s disease (AD), Parkinson’s disease (PD), and Huntington’s disease (HD), associated with the rearrangement of specific proteins to non-native conformations that promotes aggregation and deposition within tissues and/or cellular compartments. These diseases are commonly classified as protein-misfolding or amyloid diseases. The interaction of these proteins with liquid/surface interfaces is a fundamental phenomenon with potential implications for protein-misfolding diseases. Kinetic and thermodynamic studies indicate that significant conformational changes can be induced in proteins encountering surfaces, which can play a critical role in nucleating aggregate formation or stabilizing specific aggregation states. Surfaces of particular interest in neurodegenerative diseases are cellular and subcellular membranes that are predominately comprised of lipid components. The two-dimensional liquid environments provided by lipid bilayers can profoundly alter protein structure and dynamics by both specific and non-specific interactions. Importantly for misfolding diseases, these bilayer properties can not only modulate protein conformation, but also exert influence on aggregation state. A detailed understanding of the influence of (sub)cellular surfaces in driving protein aggregation and/or stabilizing specific aggregate forms could provide new insights into toxic mechanisms associated with these diseases. Here, we review the influence of surfaces in driving and stabilizing protein aggregation with a specific emphasis on lipid membranes.

Lipid interaction triggering Septin2 to assembly into β-sheet structures investigated by Langmuir monolayers and PM-IRRAS

The molecular mechanisms responsible for protein structural changes in the central nervous system leading to Alzheimer's disease are unknown, but there is evidence that a family of proteins known as septins may be involved. Septins are a conserved group of GTP-binding proteins which participate in various cellular processes, including polarity determination and membrane dynamics. SEPT1, SEPT4, and SEPT2 have been found in deposits known as neurofibrillary tangles and glial fibrils in Alzheimer's disease. In this study, we provide molecular-level information for the interaction of SEPT2 with Langmuir monolayers at the air/water interface, which are used as simplified membrane models. The high surface activity of SEPT2 causes it to adsorb onto distinct types of lipid Langmuir monolayers, namely dipalmitoylphosphatidylcholine and PtdIns(4,5)P2. However, the interaction with PtdIns(4,5)P2 is much stronger, not only leading to a higher adsorption, but also to SEPT2 remaining inserted within the membrane at high surface pressures. Most importantly, in situ polarization-modulated infrared reflection absorption spectroscopy results indicated that the native secondary structure of SEPT2 is preserved upon interacting with PtdIns(4,5)P2, but not when dipalmitoylphosphatidylcholine is at the air/water interface. Taken together, the results presented here suggest that the interaction between SEPT2 and the cell membrane may play an important role in the assembly of SEPT2 into amyloid-like fibers.

Reciprocal regulation of cholesterol and beta amyloid at the subcellular level in Alzheimer's disease

Since the discovery that apolipoprotein E, a cholesterol transport protein, is a major risk factor for Alzheimer's disease (AD) development, there has been a remarkable interest in understanding the many facets of the relationship between cholesterol and AD. Several lines of evidence have demonstrated the importance of cholesterol in amyloid beta peptide (Aβ) production and metabolism, as well as the involvement of Aβ in cholesterol homeostasis. The emerging picture is complex and still incomplete. This review discusses findings that indicate that a reciprocal regulation exists between Aβ and cholesterol at the subcellular level. The pathological impact of such regulation is highlighted.

Membrane lipid composition and its physicochemical properties define cell vulnerability to aberrant protein oligomers

Increasing evidence suggests that the interaction of misfolded protein oligomers with cell membranes is a primary event resulting in the cytotoxicity associated with many protein-misfolding diseases, including neurodegenerative disorders. We describe here the results of a study on the relative contributions to toxicity of the physicochemical properties of protein oligomers and the cell membrane with which they interact. We altered the amount of cholesterol and the ganglioside GM1 in membranes of SH-SY5Y cells. We then exposed the cells to two types of oligomers of the prokaryotic protein HypF-N with different ultrastructural and cytotoxicity properties, and to oligomers formed by the amyloid-β peptide associated with Alzheimer's disease. We identified that the degree of toxicity of the oligomeric species is the result of a complex interplay between the structural and physicochemical features of both the oligomers and the cell membrane.

Islet amyloid polypeptide: aggregation and fibrillogenesis in vitro and its inhibition

The development of type 2 diabetes mellitus is associated with the dysfunction of b-cells which is correlated to the formation of deposits consisting of the islet amyloid polypeptide (IAPP). The process of human IAPP (hIAPP) self-association, the intermediate structures formed as well as the interaction of hIAPP with membrane systems seem to be responsible for the cytotoxicity. For monomeric hIAPP, a natively random coil conformation with transient a-helical parts could be determined in bulk solution, which rapidly converts to an amyloid structure consisting of cross b-sheets. By comparing the amyloidogenic propensities of hIAPP in the bulk and in the presence of various neutral and charged lipid bilayer systems as well as biological membranes, an enhancing effect of anionic and heterogeneous membranes to hIAPP fibril formation has been found. We also discuss the cross-interaction of hIAPP with other amyloidogenic peptides (e.g., insulin and Ab) and present first small-molecule inhibitors of the fibrillation process of hIAPP.

The modulating effect of mechanical changes in lipid bilayers caused by apoE-containing lipoproteins on Aβ induced membrane disruption

A major feature of Alzheimer's disease (AD), a late-onset neurodegenerative disorder, is the ordered aggregation of the β-amyloid peptide (Aβ) into fibrils that comprise extracellular neuritic plaques found in the disease brain. One of many potential pathways for Aβ toxicity may be modulation of lipid membrane function. Here, we show by in situ atomic force microscopy (AFM) that astrocyte secreted lipoprotein particles (ASLPs) containing different isoforms of apolipoprotein E (apoE), of which the apoE4 allele is a major risk factor for the development of AD, can protect total brain lipid extract bilayers from Aβ(1-40) induced disruption. The apoE4 allele was less effective in protecting lipid bilayers from disruption compared with apoE3. Size analysis of apoE-containing ASLPs and mechanical studies of bilayer properties revealed that apoE-containing ASLPs modulate the mechanical properties of bilayers by acquiring some bilayer components (most likely cholesterol and/or oxidatively damaged lipids). Measurement of bilayer mechanical properties was accomplished with scanning probe acceleration microscopy (SPAM). These measurements demonstrated that apoE4 was also less effective in modulating mechanical properties of bilayers in comparison with apoE3. This ability of apoE to alter the mechanical properties of lipid membranes may represent a potential mechanism for the suppression of Aβ(1-40) induced bilayer disruption.

Clustering and internalization of toxic amylin oligomers in pancreatic cells require plasma membrane cholesterol

Self-assembly of the human pancreatic hormone amylin into toxic oligomers and aggregates is linked to dysfunction of islet β-cells and pathogenesis of type 2 diabetes mellitus. Recent evidence suggests that cholesterol, an essential component of eukaryotic cells membranes, controls amylin aggregation on model membranes. However, the pathophysiological consequence of cholesterol-regulated amylin polymerization on membranes and biochemical mechanisms that protect β-cells from amylin toxicity are poorly understood. Here, we report that plasma membrane (PM) cholesterol plays a key role in molecular recognition, sorting, and internalization of toxic amylin oligomers but not monomers in pancreatic rat insulinoma and human islet cells. Depletion of PM cholesterol or the disruption of the cytoskeleton network inhibits internalization of amylin oligomers, which in turn enhances extracellular oligomer accumulation and potentiates amylin toxicity. Confocal microscopy reveals an increased nucleation of amylin oligomers across the plasma membrane in cholesterol-depleted cells, with a 2-fold increase in cell surface coverage and a 3-fold increase in their number on the PM. Biochemical studies confirm accumulation of amylin oligomers in the medium after depletion of PM cholesterol. Replenishment of PM cholesterol from intracellular cholesterol stores or by the addition of water-soluble cholesterol restores amylin oligomer clustering at the PM and internalization, which consequently diminishes cell surface coverage and toxicity of amylin oligomers. In contrast to oligomers, amylin monomers followed clathrin-dependent endocytosis, which is not sensitive to cholesterol depletion. Our studies identify an actin-mediated and cholesterol-dependent mechanism for selective uptake and clearance of amylin oligomers, impairment of which greatly potentiates amylin toxicity.

Oxidative stress and cell membranes in the pathogenesis of Alzheimer's disease

Amyloid β proteins and oxidative stress are believed to have central roles in the development of Alzheimer's disease. Lipid membranes are among the most vulnerable cellular components to oxidative stress, and membranes in susceptible regions of the brain are compositionally distinct from those in other tissues. This review considers the evidence that membranes are either a source of neurotoxic lipid oxidation products or the target of pathogenic processes involving amyloid β proteins that cause permeability changes or ion channel formation. Progress toward a comprehensive theory of Alzheimer's disease pathogenesis is discussed in which lipid membranes assume both roles and promote the conversion of monomeric amyloid β proteins into fibrils, the pathognomonic histopathological lesion of the disease.

Endogenous amyloidogenesis in long-term rat hippocampal cell cultures

BACKGROUND

Long-term primary neuronal cultures are a useful tool for the investigation of biochemical processes associated with neuronal senescence. Improvements in available technology make it possible to observe maturation of neural cells isolated from different regions of the rodent brain over a prolonged period in vitro. Existing experimental evidence suggests that cellular aging occurs in mature, long-term, primary neuronal cell cultures. However, detailed studies of neuronal development in vitro are needed to demonstrate the validity of long-term cell culture-based models for investigation of the biochemical mechanisms of in vitro neuronal development and senescence.

RESULTS

In the current study, neuron-enriched hippocampal cell cultures were used to analyze the differentiation and degeneration of hippocampal neurons over a two month time period. The expression of different neuronal and astroglial biomarkers was used to determine the cytochemical characteristics of hippocampal cells in long-term cultures of varying ages. It was observed that the expression of the intermediate filament nestin was absent from cultures older than 21 days in vitro (DIV), and the expression of neuronal or astrocytic markers appeared to replace nestin. Additionally, morphological evaluations of neuronal integrity and Hoescht staining were used to assess the cellular conditions in the process of hippocampal culture development and aging. It was found that there was an increase in endogenous production of Aβ(1-42) and an increase in the accumulation of Congo Red-binding amyloidal aggregates associated with the aging of neurons in primary culture. In vitro changes in the morphology of co-existing astrocytes and cell culture age-dependent degeneration of neurodendritic network resemble features of in vivo brain aging at the cellular level.

CONCLUSION

In conclusion, this study suggests that long-term primary CNS culture is a viable model for the study of basic mechanisms and effective methods to decelerate the process of neuronal senescence.

Aβ internalization by neurons and glia

In the brain, the amyloid β peptide (Aβ) exists extracellularly and inside neurons. The intracellular accumulation of Aβ in Alzheimer's disease brain has been questioned for a long time. However, there is now sufficient strong evidence indicating that accumulation of Aβ inside neurons plays an important role in the pathogenesis of Alzheimer's disease. Intraneuronal Aβ originates from intracellular cleavage of APP and from Aβ internalization from the extracellular milieu. We discuss here the different molecular mechanisms that are responsible for Aβ internalization in neurons and the links between Aβ internalization and neuronal dysfunction and death. A brief description of Aβ uptake by glia is also presented.

Formation of Toxic Amyloid Fibrils by Amyloid β-Protein on Ganglioside Clusters

It is widely accepted that the conversion of the soluble, nontoxic amyloid β-protein (Aβ) monomer to aggregated toxic Aβ rich in β-sheet structures is central to the development of Alzheimer's disease. However, the mechanism of the abnormal aggregation of Aβ in vivo is not well understood. Accumulating evidence suggests that lipid rafts (microdomains) in membranes mainly composed of sphingolipids (gangliosides and sphingomyelin) and cholesterol play a pivotal role in this process. This paper summarizes the molecular mechanisms by which Aβ aggregates on membranes containing ganglioside clusters, forming amyloid fibrils. Notably, the toxicity and physicochemical properties of the fibrils are different from those of Aβ amyloids formed in solution. Furthermore, differences between Aβ-(1–40) and Aβ-(1–42) in membrane interaction and amyloidogenesis are also emphasized.

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

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

Ganglioside-mediated aggregation of amyloid β-proteins (Aβ): comparison between Aβ-(1-42) and Aβ-(1-40)

Conversion of the soluble, non-toxic amyloid β-protein (Aβ) into an aggregated, toxic form rich in β-sheets is considered a key step in the development of Alzheimer's disease. Accumulating evidence suggests that lipid rafts in membranes play a pivotal role in this process. We have proposed that Aβ-(1-40) specifically bound to a ganglioside cluster forms cytotoxic fibrils via a conformational transition from an α-helix-rich structure to a β-sheet-rich one. In the present study, we compared the interaction of Aβ-(1-40) and Aβ-(1-42) with both model and living cell membranes. Aβ-(1-42) exhibited lipid specificity and affinity similar to Aβ-(1-40), though its amyloidogenic activity was more than 10-fold that of Aβ-(1-40). Antibody staining experiments, using the A11 antibody specific to Aβ oligomers, demonstrated that oligomers were not detected during the aggregation process, and cell death was observed only after significant accumulation of the proteins, suggesting that the fibril-induced disruption of cell membranes leads to the cytotoxicity. Furthermore, we succeeded in visualizing fibrils formed on cell membranes using total internal reflection fluorescence microscopy. Aβ-(1-40) formed long fibrils extruding to the aqueous phase, whereas Aβ-(1-42) fibrils appeared to be laterally co-assembled and short.

Measurement of the attachment and assembly of small amyloid-β oligomers on live cell membranes at physiological concentrations using single-molecule tools

It is thought that the pathological cascade in Alzheimer's disease is initiated by the formation of amyloid-β (Aβ) peptide complexes on cell membranes. However, there is considerable debate about the nature of these complexes and the type of solution-phase Aβ aggregates that may contribute to their formation. Also, it is yet to be shown that Aβ attaches strongly to living cell membranes, and that this can happen at low, physiologically relevant Aβ concentrations. Here, we simultaneously measure the aggregate size and fluorescence lifetime of fluorescently labeled Aβ(1-40) on and above the membrane of cultured PC12 cells at near-physiological concentrations. We find that at 350 nM Aβ concentration, large (>>10 nm average hydrodynamic radius) assemblies of codiffusing, membrane-attached Aβ molecules appear on the cell membrane together with a near-monomeric species. When the extracellular concentration is 150 nM, the membrane contains only the smaller species, but with a similar degree of attachment. At both concentrations, the extracellular solution contains only small (∼2.3 nm average hydrodynamic radius) Aβ oligomers or monomers. We conclude that at near-physiological concentrations only the small oligomeric Aβ species are relevant, they are capable of attaching to the cell membrane, and they assemble in situ to form much larger complexes.

Designed fluorescent probes reveal interactions between amyloid-beta(1-40) peptides and GM1 gangliosides in micelles and lipid vesicles

A hallmark of the common Alzheimer's disease (AD) is the pathological conversion of its amphiphatic amyloid-beta (Abeta) peptide into neurotoxic aggregates. In AD patients, these aggregates are often found to be tightly associated with neuronal G(M1) ganglioside lipids, suggesting an involvement of G(M1) not only in aggregate formation but also in neurotoxic events. Significant interactions were found between micelles made of newly synthesized fluorescent G(M1) gangliosides labeled in the polar headgroup or the hydrophobic chain and Abeta(1-40) peptide labeled with a BODIPY-FL-C1 fluorophore at positions 12 and 26, respectively. From an analysis of energy transfer between the different fluorescence labels and their location in the molecules, we were able to place the Abeta peptide inside G(M1) micelles, close to the hydrophobic-hydrophilic interface. Large unilamellar vesicles composed of a raftlike G(M1)/bSM/cholesterol lipid composition doped with labeled G(M1) at various positions also interact with labeled Abeta peptide tagged to amino acids 2 or 26. A faster energy transfer was observed from the Abeta peptide to bilayers doped with 581/591-BODIPY-C(11)-G(M1) in the nonpolar part of the lipid compared with 581/591-BODIPY-C(5)-G(M1) residing in the polar headgroup. These data are compatible with a clustering process of G(M1) molecules, an effect that not only increases the Abeta peptide affinity, but also causes a pronounced Abeta peptide penetration deeper into the lipid membrane; all these factors are potentially involved in Abeta peptide aggregate formation due to an altered ganglioside metabolism found in AD patients.

Biochemical and biophysical features of both oligomer/fibril and cell membrane in amyloid cytotoxicity

A great deal must still be learnt on the structural features of amyloid assemblies, particularly prefibrillar aggregates, and the relationship of the latter with amyloid cytotoxicity. Presently, it is recognized that the population of unstable, heterogeneous amyloid oligomers and protofibrils is mainly responsible for amyloid cytotoxicity. Conversely, mature fibrils are considered stable, harmless reservoirs of molecular species devoid of toxicity in the polymerized state. This view has been modified by recent reports showing that mature fibrils grown at different conditions can display different structural features and stabilities, possibly leading them to undergo disassembly with the leak of toxic oligomers. Fibril polymorphism is paralleled by oligomer polymorphism and both can be traced back to amyloid growth from differently destabilized monomers with distinct structural features at differing conditions. Recent research has started to unravel oligomer structural and biophysical features and the relationship between the latter and oligomer cytotoxicity. These data have led to the proposal that, together, both oligomer and membrane physical features determine the extent of oligomer-membrane interaction with the resulting disruption of membrane integrity and cell impairment. Such a view can help to explain the variable vulnerability of different cell types to the same amyloids and the lack of relationship between amyloid load and the severity of clinical symptoms. It also stresses the importance, for cell/tissue impairment, of the presence, in tissue, in addition to toxic oligomers, of fibrils conformers of reduced stability as a possible source of toxic oligomers, whose leakage can be favoured upon interaction with suitable surfaces or by other environmental conditions.

[Visualization of amyloid formation processes on cell membranes: gangliosides as key molecules for the onset of amyloidosis]

Deposition of insoluble amyloid fibrils in tissues is a common hallmark of a wide range of human diseases referred to as amyloidoses, including Alzheimer's disease, type II diabetes mellitus. The amyloid deposits cause cell dysfunction, death, and subsequently severe impairment in tissues. Elucidation of amyloid formation mechanisms is essential for prevention of the onset and development of amyloidoses. Accumulated experimental evidence demonstrates that membrane lipids enhance the fibril formation of amyloidogenic proteins. Our group demonstrated that amyloid formation by amyloid β-protein (Aβ) was facilitated by gangliosides in lipid raft-like model membranes. Phosphatidylserine and phosphatidylglycerol were also reported to trigger fibril formation by human islet amyloid polypeptide (hIAPP). However, it is not verified whether the proposed lipid-protein interactions can occur on plasma membranes of live cells. The author developed a method for visualizing amyloid fibrils on live cell membranes and investigated the roles of gangliosides and cholesterol in lipid rafts for amyloid formation. Congo red, an amyloid-specific dye, was found to be a promising compound for staining amyloids in live cells. Aβ was accumulated on cholesterol-dependent ganglioside-rich domains in PC12 neuronal cells in a time- and concentration-dependent manner, leading to cell death. Nerve growth factor-induced differentiation of PC12 cells increased both gangliosides and cholesterol and thereby greatly potentiated the accumulation and cytotoxic effect of Aβ. Amyloid formation by hIAPP was also facilitated by gangliosides in lipid rafts. Membrane lipid compositions, in this case, gangliosides in lipid rafts, actually caused striking change in amyloid formation on cell membranes.

Molecular insights into amyloid regulation by membrane cholesterol and sphingolipids: common mechanisms in neurodegenerative diseases

Alzheimer, Parkinson and other neurodegenerative diseases involve a series of brain proteins, referred to as ‘amyloidogenic proteins’, with exceptional conformational plasticity and a high propensity for self-aggregation. Although the mechanisms by which amyloidogenic proteins kill neural cells are not fully understood, a common feature is the concentration of unstructured amyloidogenic monomers on bidimensional membrane lattices. Membrane-bound monomers undergo a series of lipid-dependent conformational changes, leading to the formation of oligomers of varying toxicity rich in β-sheet structures (annular pores, amyloid fibrils) or in α-helix structures (transmembrane channels). Condensed membrane nano- or microdomains formed by sphingolipids and cholesterol are privileged sites for the binding and oligomerisation of amyloidogenic proteins. By controlling the balance between unstructured monomers and α or β conformers (the chaperone effect), sphingolipids can either inhibit or stimulate the oligomerisation of amyloidogenic proteins. Cholesterol has a dual role: regulation of protein–sphingolipid interactions through a fine tuning of sphingolipid conformation (indirect effect), and facilitation of pore (or channel) formation through direct binding to amyloidogenic proteins. Deciphering this complex network of molecular interactions in the context of age- and disease-related evolution of brain lipid expression will help understanding of how amyloidogenic proteins induce neural toxicity and will stimulate the development of innovative therapies for neurodegenerative diseases.