Interactions of amyloid beta-peptide (1-40) with ganglioside-containing membranes

Co-registration of MALDI-MSI and histology demonstrates gangliosides co-localize with amyloid beta plaques in Alzheimer's disease

Alzheimer's disease (AD) is a progressive neurological condition characterized by impaired cognitive function and behavioral alterations. While AD research historically centered around mis-folded proteins, advances in mass spectrometry techniques have triggered increased exploration of the AD lipidome with lipid dysregulation emerging as a critical player in AD pathogenesis. Gangliosides are a class of glycosphingolipids enriched within the central nervous system. Previous work has suggested a shift in a-series gangliosides from complex (GM1) to simple (GM2 and GM3) species may be related to the development of neurodegenerative disease. In addition, complex gangliosides with 20 carbon sphingosine chains have been shown to increase in the aging brain. In this study, we utilized matrix-assisted laser desorption ionization mass spectrometry imaging (MALDI-MSI) to interrogate the in situ relationship of a-series gangliosides with either 18 or 20 carbon sphingosine chains (d18:1 or d20:1, respectively) in the post-mortem human AD brain. Here, we expanded upon previous literature and demonstrated a significant decrease in the GM1 d20:1 to GM1 d18:1 ratio in regions of the dentate gyrus and entorhinal cortex in AD relative to control brain tissue. Then, we demonstrated that the MALDI-MSI profile of GM3 co-localizes with histologically confirmed amyloid beta (Aβ) plaques and found a significant increase in both GM1 and GM3 in proximity to Aβ plaques. Collectively, this study demonstrates a perturbation of the ganglioside profile in AD, and validates a pipeline for MALDI-MSI and classic histological staining in the same tissue sections. This demonstrates feasibility for integrating untargeted mass spectrometry imaging approaches into a digital pathology framework.

Lipids as Emerging Biomarkers in Neurodegenerative Diseases

Biomarkers are molecules that can be used to observe changes in an individual's biochemical or medical status and provide information to aid diagnosis or treatment decisions. Dysregulation in lipid metabolism in the brain is a major risk factor for many neurodegenerative disorders, including frontotemporal dementia, Alzheimer's disease, Parkinson's disease, and amyotrophic lateral sclerosis. Thus, there is a growing interest in using lipids as biomarkers in neurodegenerative diseases, with the anionic phospholipid bis(monoacylglycerol)phosphate and (glyco-)sphingolipids being the most promising lipid classes thus far. In this review, we provide a general overview of lipid biology, provide examples of abnormal lysosomal lipid metabolism in neurodegenerative diseases, and discuss how these insights might offer novel and promising opportunities in biomarker development and therapeutic discovery. Finally, we discuss the challenges and opportunities of lipid biomarkers and biomarker panels in diagnosis, prognosis, and/or treatment response in the clinic.

Brain Hydrophobic Peptides Antagonists of Neurotoxic Amyloid β Peptide Monomers/Oligomers-Protein Interactions

Amyloid β (Aβ) oligomers have been linked to Alzheimer's disease (AD) pathogenesis and are the main neurotoxic forms of Aβ. This review focuses on the following: (i) the Aβ(1-42):calmodulin interface as a model for the design of antagonist Aβ peptides and its limitations; (ii) proteolytic degradation as the major source of highly hydrophobic peptides in brain cells; and (iii) brain peptides that have been experimentally demonstrated to bind to Aβ monomers or oligomers, Aβ fibrils, or Aβ plaques. It is highlighted that the hydrophobic amino acid residues of the COOH-terminal segment of Aβ(1-42) play a key role in its interaction with intracellular protein partners linked to its neurotoxicity. The major source of highly hydrophobic endogenous peptides of 8-10 amino acids in neurons is the proteasome activity. Many canonical antigen peptides bound to the major histocompatibility complex class 1 are of this type. These highly hydrophobic peptides bind to Aβ and are likely to be efficient antagonists of the binding of Aβ monomers/oligomers concentrations in the nanomolar range with intracellular proteins. Also, their complexation with Aβ will protect them against endopeptidases, suggesting a putative chaperon-like physiological function for Aβ that has been overlooked until now. Remarkably, the hydrophobic amino acid residues of Aβ responsible for the binding of several neuropeptides partially overlap with those playing a key role in its interaction with intracellular protein partners that mediates its neurotoxicity. Therefore, these latter neuropeptides are also potential candidates to antagonize Aβ peptides binding to target proteins. In conclusion, the analysis performed in this review points out that hydrophobic endogenous brain neuropeptides could be valuable biomarkers to evaluate the risk of the onset of sporadic AD, as well as for the prognosis of AD.

Sugar distributions on gangliosides guide the formation and stability of amyloid-β oligomers

Aggregation of Aβ peptides is a key contributor to the etiology of Alzheimer's disease. Being intrinsically disordered, monomeric Aβ is susceptible to conformational excursions, especially in the presence of important interacting partners such as membrane lipids, to adopt specific aggregation pathways. Furthermore, components such as gangliosides in membranes and lipid rafts are known to play important roles in the adoption of pathways and the generation of discrete neurotoxic oligomers. Yet, what roles do carbohydrates on gangliosides play in this process remains unknown. Here, using GM1, GM3, and GD3 ganglioside micelles as models, we show that the sugar distributions and cationic amino acids within Aβ N-terminal region modulate oligomerization of Aβ temporally, and dictate the stability and maturation of oligomers. These results demonstrate the selectivity of sugar distributions on the membrane surface toward oligomerization of Aβ and thus implicate cell-selective enrichment of oligomers.

Sugar distributions on gangliosides guide the formation and stability of amyloid-β oligomers

Aggregation of Aβ peptides has been known as a key contributor to the etiology of Alzheimer's disease. Being intrinsically disordered, the monomeric Aβ is susceptible to conformational excursions, especially in the presence of key interacting partners such as membrane lipids, to adopt specific aggregation pathways. Furthermore, key components such as gangliosides in membranes and lipid rafts are known to play important roles in the adoption of pathways and the generation of discrete neurotoxic oligomers. Yet, what roles the carbohydrates on gangliosides play in this process remains unknown. Here, using GM1, GM3, and GD3 ganglioside micelles as models, we show that the sugar distributions and cationic amino acids within Aβ N-terminal region modulate oligomerization of Aβ temporally, and dictate the stability and maturation of oligomers.

Glucosylceramide synthase inhibition reduces ganglioside GM3 accumulation, alleviates amyloid neuropathology, and stabilizes remote contextual memory in a mouse model of Alzheimer’s disease

Background

Gangliosides are highly enriched in the brain and are critical for its normal development and function. However, in some rare neurometabolic diseases, a deficiency in lysosomal ganglioside hydrolysis is pathogenic and leads to early-onset neurodegeneration, neuroinflammation, demyelination, and dementia. Increasing evidence also suggests that more subtle ganglioside accumulation contributes to the pathogenesis of more common neurological disorders including Alzheimer’s disease (AD). Notably, ganglioside GM3 levels are elevated in the brains of AD patients and in several mouse models of AD, and plasma GM3 levels positively correlate with disease severity in AD patients.

Methods

Tg2576 AD model mice were fed chow formulated with a small molecule inhibitor of glucosylceramide synthase (GCSi) to determine whether reducing glycosphingolipid synthesis affected aberrant GM3 accumulation, amyloid burden, and disease manifestations in cognitive impairment. GM3 was measured with LC-MS, amyloid burden with ELISA and amyloid red staining, and memory was assessed using the contextual fear chamber test.

Results

GCSi mitigated soluble Aβ42 accumulation in the brains of AD model mice when treatment was started prophylactically. Remarkably, GCSi treatment also reduced soluble Aβ42 levels and amyloid plaque burden in aged (i.e., 70 weeks old) AD mice with preexisting neuropathology. Our analysis of contextual memory in Tg2576 mice showed that impairments in remote (cortical-dependent) memory consolidation preceded deficits in short-term (hippocampal-dependent) contextual memory, which was consistent with soluble Aβ42 accumulation occurring more rapidly in the cortex of AD mice compared to the hippocampus. Notably, GCSi treatment significantly stabilized remote memory consolidation in AD mice—especially in mice with enhanced cognitive training. This finding was consistent with GCSi treatment lowering aberrant GM3 accumulation in the cortex of AD mice.

Conclusions

Collectively, our results indicate that glycosphingolipids regulated by GCS are important modulators of Aβ neuropathology and that glycosphingolipid homeostasis plays a critical role in the consolidation of remote memories.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13195-022-00966-0.

Ganglioside-Enriched Phospholipid Vesicles Induce Cooperative Aβ Oligomerization and Membrane Disruption

A major hallmark of Alzheimer's disease (AD) is the accumulation of extracellular aggregates of amyloid-β (Aβ). Structural polymorphism observed among Aβ fibrils in AD brains seem to correlate with the clinical subtypes suggesting a link between fibril polymorphism and pathology. Since fibrils emerge from a templated growth of low-molecular-weight oligomers, understanding the factors affecting oligomer generation is important. Membrane lipids are key factors to influence early stages of Aβ aggregation and oligomer generation, which cause membrane disruption. We have previously demonstrated that conformationally discrete Aβ oligomers can be generated by modulating the charge, composition, and chain length of lipids and surfactants. Here, we extend our studies into liposomal models by investigating Aβ oligomerization on large unilamellar vesicles (LUVs) of total brain extracts (TBE), reconstituted lipid rafts (LRs), or 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC). Varying the vesicle composition by specifically increasing the amount of GM1 gangliosides as a constituent, we found that only GM1-enriched liposomes induce the formation of toxic, low-molecular-weight oligomers. Furthermore, we found that the aggregation on liposome surface and membrane disruption are highly cooperative and sensitive to membrane surface characteristics. Numerical simulations confirm such a cooperativity and reveal that GM1-enriched liposomes form twice as many pores as those formed in the absence GM1. Overall, this study uncovers mechanisms of cooperativity between oligomerization and membrane disruption under controlled lipid compositional bias, and refocuses the significance of the early stages of Aβ aggregation in polymorphism, propagation, and toxicity in AD.

Early-Stage β-Amyloid-Membrane Interactions Modulate Lipid Dynamics and Influence Structural Interfaces and Fibrillation

Molecular interactions between β-amyloid (Aβ) peptide and membranes contribute to the neuronal toxicity of Aβ and the pathology of Alzheimer's disease (AD). Neuronal plasma membranes serve as biologically relevant environments for the Aβ aggregation process as well as affect the structural polymorphisms of Aβ aggregates. However, the nature of these interactions is unknown. Here, we utilized solid-state NMR spectroscopy to explore the site-specific interactions between Aβ peptides and lipids in synaptic plasma membranes at the membrane-associated nucleation stage. The key results show that different segments in the hydrophobic sequence of Aβ initiate membrane binding and inter-strand assembling. We demonstrate early-stage Aβ-lipid interactions modulate lipid dynamics, leading to more rapid lipid headgroup motion and reduced lateral diffusive motion. These early events influence the structural polymorphisms of yielded membrane-associated Aβ fibrils with distinct C-terminal quaternary interface structure compared to fibrils grown in aqueous solutions. Based on our results, we propose a schematic mechanism by which Aβ-lipid interactions drive membrane-associated nucleation processes, providing molecular insights into the early events of fibrillation in biological environments.

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.

Intermolecular Interactions between a Membrane Protein and a Glycolipid Essential for Membrane Protein Integration

Inducing newly synthesized proteins to appropriate locations is an indispensable biological function in every organism. Integration of proteins into biomembranes in Escherichia coli is mediated by proteinaceous factors, such as Sec translocons and an insertase YidC. Additionally, a glycolipid named MPIase (membrane protein integrase), composed of a long sugar chain and pyrophospholipid, was proven essential for membrane protein integration. We reported that a synthesized minimal unit of MPIase possessing only one trisaccharide, mini-MPIase-3, involves an essential structure for the integration activity. Here, to elucidate integration mechanisms using MPIase, we analyzed intermolecular interactions of MPIase or its synthetic analogs with a model substrate, the Pf3 coat protein, using physicochemical methods. Surface plasmon resonance (SPR) analyses revealed the importance of a pyrophosphate for affinity to the Pf3 coat protein. Compared with mini-MPIase-3, natural MPIase showed faster association and dissociation due to its long sugar chain despite the slight difference in affinity. To focus on more detailed MPIase substructures, we performed docking simulations and saturation transfer difference-nuclear magnetic resonance. These experiments yielded that the 6-O-acetyl group on glucosamine and the phosphate of MPIase play important roles leading to interactions with the Pf3 coat protein. The high affinity of MPIase to the hydrophobic region and the basic amino acid residues of the protein was suggested by docking simulations and proven experimentally by SPR using protein mutants devoid of target regions. These results demonstrated the direct interactions of MPIase with a substrate protein and revealed detailed mechanisms of membrane protein integration.

The Progress of Label-Free Optical Imaging in Alzheimer's Disease Screening and Diagnosis

As the major neurodegenerative disease of dementia, Alzheimer's disease (AD) has caused an enormous social and economic burden on society. Currently, AD has neither clear pathogenesis nor effective treatments. Positron emission tomography (PET) and magnetic resonance imaging (MRI) have been verified as potential tools for diagnosing and monitoring Alzheimer's disease. However, the high costs, low spatial resolution, and long acquisition time limit their broad clinical utilization. The gold standard of AD diagnosis routinely used in research is imaging AD biomarkers with dyes or other reagents, which are unsuitable for in vivo studies owing to their potential toxicity and prolonged and costly process of the U.S. Food and Drug Administration (FDA) approval for human use. Furthermore, these exogenous reagents might bring unwarranted interference to mechanistic studies, causing unreliable results. Several label-free optical imaging techniques, such as infrared spectroscopic imaging (IRSI), Raman spectroscopic imaging (RSI), optical coherence tomography (OCT), autofluorescence imaging (AFI), optical harmonic generation imaging (OHGI), etc., have been developed to circumvent this issue and made it possible to offer an accurate and detailed analysis of AD biomarkers. In this review, we present the emerging label-free optical imaging techniques and their applications in AD, along with their potential and challenges in AD diagnosis.

Biophysical characteristics of lipid-induced Aβ oligomers correlate to distinctive phenotypes in transgenic mice

Alzheimer's disease (AD) is a progressive neurodegenerative disorder that affects cognition and memory. Recent advances have helped identify many clinical sub‐types in AD. Mounting evidence point toward structural polymorphism among fibrillar aggregates of amyloid‐β (Aβ) to being responsible for the phenotypes and clinical manifestations. In the emerging paradigm of polymorphism and prion‐like propagation of aggregates in AD, the role of low molecular weight soluble oligomers, which are long known to be the primary toxic agents, in effecting phenotypes remains inconspicuous. In this study, we present the characterization of three soluble oligomers of Aβ42, namely 14LPOs, 16LPOs, and GM1Os with discreet biophysical and biochemical properties generated using lysophosphatidyl glycerols and GM1 gangliosides. The results indicate that the oligomers share some biophysical similarities but display distinctive differences with GM1Os. Unlike the other two, GM1Os were observed to be complexed with the lipid upon isolation. It also differs mainly in detection by conformation‐sensitive dyes and conformation‐specific antibodies, temperature and enzymatic stability, and in the ability to propagate morphologically‐distinct fibrils. GM1Os also show distinguishable biochemical behavior with pronounced neuronal toxicity. Furthermore, all the oligomers induce cerebral amyloid angiopathy (CAA) and plaque burden in transgenic AD mice, which seems to be a consistent feature among all lipid‐derived oligomers, but 16LPOs and GM1Os displayed significantly higher effect than the others. These results establish a correlation between molecular features of Aβ42 oligomers and their distinguishable effects in transgenic AD mice attuned by lipid characteristics, and therefore help bridge the knowledge gap in understanding how oligomer conformers could elicit AD phenotypes.

Gangliosides in the Brain: Physiology, Pathophysiology and Therapeutic Applications

Gangliosides are glycosphingolipids highly abundant in the nervous system, and carry most of the sialic acid residues in the brain. Gangliosides are enriched in cell membrane microdomains ("lipid rafts") and play important roles in the modulation of membrane proteins and ion channels, in cell signaling and in the communication among cells. The importance of gangliosides in the brain is highlighted by the fact that loss of function mutations in ganglioside biosynthetic enzymes result in severe neurodegenerative disorders, often characterized by very early or childhood onset. In addition, changes in the ganglioside profile (i.e., in the relative abundance of specific gangliosides) were reported in healthy aging and in common neurological conditions, including Huntington's disease (HD), Alzheimer's disease (AD), Parkinson's disease (PD), amyotrophic lateral sclerosis (ALS), stroke, multiple sclerosis and epilepsy. At least in HD, PD and in some forms of epilepsy, experimental evidence strongly suggests a potential role of gangliosides in disease pathogenesis and potential treatment. In this review, we will summarize ganglioside functions that are crucial to maintain brain health, we will review changes in ganglioside levels that occur in major neurological conditions and we will discuss their contribution to cellular dysfunctions and disease pathogenesis. Finally, we will review evidence of the beneficial roles exerted by gangliosides, GM1 in particular, in disease models and in clinical trials.

Curcumin Inhibits the Primary Nucleation of Amyloid-Beta Peptide: A Molecular Dynamics Study

The amyloid plaques are a key hallmark of neurodegenerative diseases such as Alzheimer’s disease and Parkinson’s disease. Amyloidogenesis is a complex long-lasting multiphase process starting with the formation of nuclei of amyloid peptides: a process assigned as a primary nucleation. Curcumin (CU) is a well-known inhibitor of the aggregation of amyloid-beta (Aβ) peptides. Even more, CU is able to disintegrate preformed Aβ firbils and amyloid plaques. Here, we simulate by molecular dynamics the primary nucleation process of 12 Aβ peptides and investigate the effects of CU on the process. We found that CU molecules intercalate among the Aβ chains and bind tightly to them by hydrogen bonds, hydrophobic, π–π, and cation–π interactions. In the presence of CU, the Aβ peptides form a primary nucleus of a bigger size. The peptide chains in the nucleus become less flexible and more disordered, and the number of non-native contacts and hydrogen bonds between them decreases. For comparison, the effects of the weaker Aβ inhibitor ferulic acid (FA) on the primary nucleation are also examined. Our study is in good agreement with the observation that taken regularly, CU is able to prevent or at least delay the onset of neurodegenerative disorders.

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.

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

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

β-Amyloid aggregation and heterogeneous nucleation

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

Ganglioside GQ1b ameliorates cognitive impairments in an Alzheimer's disease mouse model, and causes reduction of amyloid precursor protein

Brain-derived neurotrophic factor (BDNF) plays crucial roles in memory impairments including Alzheimer’s disease (AD). Previous studies have reported that tetrasialoganglioside GQ1b is involved in long-term potentiation and cognitive functions as well as BDNF expression. However, in vitro and in vivo functions of GQ1b against AD has not investigated yet. Consequently, treatment of oligomeric Aβ followed by GQ1b significantly restores Aβ_1–42-induced cell death through BDNF up-regulation in primary cortical neurons. Bilateral infusion of GQ1b into the hippocampus ameliorates cognitive deficits in the triple-transgenic AD mouse model (3xTg-AD). GQ1b-infused 3xTg-AD mice had substantially increased BDNF levels compared with artificial cerebrospinal fluid (aCSF)-treated 3xTg-AD mice. Interestingly, we also found that GQ1b administration into hippocampus of 3xTg-AD mice reduces Aβ plaque deposition and tau phosphorylation, which correlate with APP protein reduction and phospho-GSK3β level increase, respectively. These findings demonstrate that the tetrasialoganglioside GQ1b may contribute to a potential strategy of AD treatment.

Multifunctional inhibitors of β-amyloid aggregation based on MoS2/AuNR nanocomposites with high near-infrared absorption

Recent advances in nanotechnology have developed a lot of opportunities for biological applications. In this work, multifunctional MoS2/AuNR nanocomposites with unique high NIR absorption were designed via combining MoS2 nanosheets and gold nanorods (AuNRs). The nanocomposites were synthesized through electrostatic self-assembly and showed high stability and good biocompatibility. Then they were used to modulate the aggregation of amyloid-β peptides, destabilize mature fibrils under NIR irradiation, and eliminate Aβ-induced ROS against neurotoxicity. The inhibition and destabilization effects were confirmed by Thioflavin T (ThT) fluorescence assay and transmission electron microscopy (TEM). Cell viability assay and ROS assay revealed that MoS2/AuNR nanocomposites could alleviate Aβ-induced oxidative stress and cell toxicity. More importantly, both MoS2 nanosheets and AuNRs can be used as NIR photothermal agents, MoS2/AuNR nanocomposites have enhanced ability of disrupting Aβ fibrils and improved cell viability by generating local heat under low power NIR irradiation. Our results provide new insights into the design of new multifunctional systems for the treatment of amyloid-related diseases.

Simulation Study on Complex Conformations of Aβ42 Peptides on a GM1 Ganglioside-Containing Lipid Membrane

Aggregation and complex formation of amyloid beta (Aβ) peptides on a neuronal cell membrane is a hallmark of neuro-disturbance diseases. In this work, we performed molecular dynamics (MD) simulations to investigate the initial stage of interactions of multiple Aβ₄₂ peptides on a GM1 ganglioside-containing membrane that mimics a micro-domain on the neuronal cell surface. Conformational changes of Aβs due to adhesion on the membrane and subsequent molecular interactions among the Aβs were monitored. It was suggested from results of the two 1.0 µs simulation trials that stable complexes of Aβ peptides were not rapidly generated but that a steady binding of two Aβs was gradually formed. Observation of two Aβs that will be a complex with steady binding revealed that one Aβ was bound to the membrane surface, while the other was attached to the first one without strong contact with the membrane. The motion of the first one was restricted and its conformational change was limited, with the basic side-chains of Arg5 and Lys28 working as anchors to hold the Aβ helix region on the membrane. In contrast, the second one had high flexibility and showed diversity in its conformation. The second Aβ can search for an energetically favorable binding position on the first one. A parallel β-sheet structure was formed between the C-terminal sides of the two Aβs. Ala30 was critically important to lead the stable β-sheet conformation at the C-terminal hydrophobic domains of Aβs. In the N-terminal sides, helix structures were kept in both Aβs.

Amyloid-β fibrils assembled on ganglioside-enriched membranes contain both parallel β-sheets and turns

Some protein and peptide aggregates, such as those of amyloid-β protein (Aβ), are neurotoxic and have been implicated in several neurodegenerative diseases. Aβ accumulates at nanoclusters enriched in neuronal lipids called gangliosides in the presynaptic neuronal membrane, and the resulting oligomeric and/or fibrous forms accelerate the development of Alzheimer's disease. Although the presence of Aβ deposits at such nanoclusters is known, the mechanism of their assembly and the relationship between Aβ secondary structure and topography are still unclear. Here, we first confirmed by atomic force microscopy that Aβ₄₀ fibrils can be obtained by incubating seed-free Aβ₄₀ monomers with a membrane composed of sphingomyelin, cholesterol, and the ganglioside GM1. Using Fourier transform infrared (FTIR) reflection-absorption spectroscopy, we then found that these lipid-associated fibrils contained parallel β-sheets, whereas self-assembled Aβ₄₀ molecules formed antiparallel β-sheets. We also found that the fibrils obtained at GM1-rich nanoclusters were generated from turn Aβ₄₀ Our findings indicate that Aβ generally self-assembles into antiparallel β-structures but can also form protofibrils with parallel β-sheets by interacting with ganglioside-bound Aβ. We concluded that by promoting the formation of parallel β-sheets, highly ganglioside-enriched nanoclusters help accelerate the elongation of Aβ fibrils. These results advance our understanding of ganglioside-induced Aβ fibril formation in neuronal membranes and may help inform the development of additional therapies for Alzheimer's disease.

Peptide Amyloids in the Origin of Life

How life can emerge from non-living matter is one of the fundamental mysteries of the universe. A bottom-up approach to this problem focuses on the potential chemical precursors of life, in particular the nature of the first replicative molecules. Such thinking has led to the currently most popular idea: that an RNA-like molecule played a central role as the first replicative and catalytic molecule. Here, we review an alternative hypothesis that has recently gained experimental support, focusing on the role of amyloidogenic peptides rather than nucleic acids, in what has been by some termed "the amyloid-world" hypothesis. Amyloids are well-ordered peptide aggregates that have a fibrillar morphology due to their underlying structure of a one-dimensional crystal-like array of peptides in a β-strand conformation. While they are notorious for their implication in several neurodegenerative diseases including Alzheimer's disease, amyloids also have many biological functions. In this review, we will elaborate on the following properties of amyloids in relation to their fitness as a prebiotic entity: they can be formed by very short peptides with simple amino acids sequences; as aggregates they are more chemically stable than their isolated component peptides; they can possess diverse catalytic activities; they can form spontaneously during the prebiotic condensation of amino acids; they can act as templates in their own chemical replication; they have a structurally repetitive nature that enables them to interact with other structurally repetitive biopolymers like RNA/DNA and polysaccharides, as well as with structurally repetitive surfaces like amphiphilic membranes and minerals.

Impact of membrane curvature on amyloid aggregation

The misfolding, amyloid aggregation, and fibril formation of intrinsically disordered proteins/peptides (or amyloid proteins) have been shown to cause a number of disorders. The underlying mechanisms of amyloid fibrillation and structural properties of amyloidogenic precursors, intermediates, and amyloid fibrils have been elucidated in detail; however, in-depth examinations on physiologically relevant contributing factors that induce amyloidogenesis and lead to cell death remain challenging. A large number of studies have attempted to characterize the roles of biomembranes on protein aggregation and membrane-mediated cell death by designing various membrane components, such as gangliosides, cholesterol, and other lipid compositions, and by using various membrane mimetics, including liposomes, bicelles, and different types of lipid-nanodiscs. We herein review the dynamic effects of membrane curvature on amyloid generation and the inhibition of amyloidogenic proteins and peptides, and also discuss how amyloid formation affects membrane curvature and integrity, which are key for understanding relationships with cell death. Small unilamellar vesicles with high curvature and large unilamellar vesicles with low curvature have been demonstrated to exhibit different capabilities to induce the nucleation, amyloid formation, and inhibition of amyloid-β peptides and α-synuclein. Polymorphic amyloidogenesis in small unilamellar vesicles was revealed and may be viewed as one of the generic properties of interprotein interaction-dominated amyloid formation. Several mechanical models and phase diagrams are comprehensively shown to better explain experimental findings. The negative membrane curvature-mediated mechanisms responsible for the toxicity of pancreatic β cells by the amyloid aggregation of human islet amyloid polypeptide (IAPP) and binding of the precursors of the semen-derived enhancer of viral infection (SEVI) are also described. The curvature-dependent binding modes of several types of islet amyloid polypeptides with high-resolution NMR structures are also discussed.

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.

The Pathogenic Role of Ganglioside Metabolism in Alzheimer's Disease-Cholinergic Neuron-Specific Gangliosides and Neurogenesis

Alzheimer's disease (AD) is the most common type of dementia with clinical symptoms that include deficits in memory, judgment, thinking, and behavior. Gangliosides are present on the outer surface of plasma membranes and are especially abundant in the nervous tissues of vertebrates. Ganglioside metabolism, especially the cholinergic neuron-specific gangliosides, GQ1bα and GT1aα, is altered in mouse model of AD and patients with AD. Thus, alterations in ganglioside metabolism may participate in several events related to the pathogenesis of AD. Increased expressions of GT1aα may reflect cholinergic neurogenesis. Most changes in ganglioside metabolism occur in the specific brain areas and their lipid rafts. Targeting ganglioside metabolism in lipid rafts may represent an underexploited opportunity to design novel therapeutic strategies for AD.

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.

Detection of β-Amyloid by Sialic Acid Coated Bovine Serum Albumin Magnetic Nanoparticles in a Mouse Model of Alzheimer's Disease

The accumulation and formation of β-amyloid (Aβ) plaques in the brain are distinctive pathological hallmarks of Alzheimer's disease (AD). Designing nanoparticle (NP) contrast agents capable of binding with Aβ highly selectively can potentially facilitate early detection of AD. However, a significant obstacle is the blood brain barrier (BBB), which can preclude the entrance of NPs into the brain for Aβ binding. In this work, bovine serum albumin (BSA) coated NPs are decorated with sialic acid (NP-BSAx -Sia) to overcome the challenges in Aβ imaging in vivo. The NP-BSAx -Sia is biocompatible with high magnetic relaxivities, suggesting that they are suitable contrast agents for magnetic resonance imaging (MRI). The NP-BSAx -Sia binds with Aβ in a sialic acid dependent manner with high selectivities toward Aβ deposited on brains and cross the BBB in an in vitro model. The abilities of these NPs to detect Aβ in vivo in human AD transgenic mice by MRI are evaluated without the need to coinject mannitol to increase BBB permeability. T2 *-weighted MRI shows that Aβ plaques in mouse brains can be detected as aided by NP-BSAx -Sia, which is confirmed by histological analysis. Thus, NP-BSAx -Sia is a promising new tool for noninvasive in vivo detection of Aβ plaques.

Size and Shape of Amyloid Fibrils Induced by Ganglioside Nanoclusters: Role of Sialyl Oligosaccharide in Fibril Formation

Ganglioside-enriched microdomains in the presynaptic neuronal membrane play a key role in the initiation of amyloid ß-protein (Aß) assembly related to Alzheimer's disease. We previously isolated lipids from a detergent-resistant membrane microdomain fraction of synaptosomes prepared from aged mouse brain and found that spherical Aß assemblies were formed on Aß-sensitive ganglioside nanoclusters (ASIGN) of reconstituted lipid bilayers in the synaptosomal fraction. In the present study, we investigated the role of oligosaccharides in Aß fibril formation induced by ganglioside-containing mixed lipid membranes that mimic the features of ASIGN. Ganglioside nanoclusters were constructed as ternary mixed lipid bilayers composed of ganglioside (GM1, GM2, GM3, GD1a, or GT1b), sphingomyelin, and cholesterol, and their surface topography was visualized by atomic force microscopy. Aß fibril formation on the nanocluster was strongly induced in the presence of 10 mol % ganglioside, and Aß-sensitive features were observed at cholesterol contents of 35-55 mol %. GM1-, GD1a-, and GT1b-containing membranes induced longer fibrils than those containing GD1b and GM2, indicating that the terminal galactose of GM1 along with N-acetylneuraminic acid accelerates protofibril elongation. These results demonstrate that Aß fibril formation is induced by ASIGN that are highly enriched ganglioside nanoclusters with a limited number of components and that the generation and elongation of Aß protofibrils are regulated by the oligosaccharide structure of gangliosides.

Impact of GM1 on Membrane-Mediated Aggregation/Oligomerization of β-Amyloid: Unifying View

In this perspective we summarize current knowledge of the effect of monosialoganglioside GM₁ on the membrane-mediated aggregation of the β-amyloid (Aβ) peptide. GM₁ has been suggested to be actively involved in the development of Alzheimer's disease due to its ability to seed the aggregation of Aβ. However, GM₁ is known to be neuroprotective against Aβ-induced toxicity. Here we suggest that the two scenarios are not mutually exclusive but rather complementary, and might depend on the organization of GM₁ in membranes. Improving our understanding of the molecular details behind the role of gangliosides in neurodegenerative amyloidoses might help in developing disease-modifying treatments.

Rational Design of a Stimuli-Responsive Polymer Electrode Interface Coupled with in Vivo Microdialysis for Measurement of Sialic Acid in Live Mouse Brain in Alzheimer's Disease

Sensitive and selective monitoring of sialic acid (SA) in cerebral nervous system is of great importance for studying the role that SA plays in the pathological process of Alzheimer's disease (AD). In this work, we first reported an electrochemical biosensor based on a novel stimuli-responsive copolymer for selective and sensitive detection of SA in mouse brain. Notably, through synergetic hydrogen-bonding interactions, the copolymer could translate the recognition of SA into their conformational transition and wettability switch, which facilitated the access and enrichment of redox labels and targets to the electrode surface, thus significantly improving the detection sensitivity with the detection limit down to 0.4 pM. Besides amplified sensing signals, the proposed method exhibited good selectivity toward SA in comparison to potential interference molecules coexisting in the complex brain system due to the combination of high affinity between phenylboronic acid (PBA) and SA and the directional hydrogen-bonding interactions in the copolymer. The electrochemical biosensor with remarkable analytical performance was successfully applied to evaluate the dynamic change of SA level in live mouse brain with AD combined with in vivo midrodialysis. The accurate concentration of SA in different brain regions of live mouse with AD has been reported for the first time, which is beneficial for progressing our understanding of the role that SA plays in physiological and pathological events in the brain.

GM1 Ganglioside and the Seeding of Amyloid in Alzheimer’s Disease: Endogenous Seed for Alzheimer Amyloid

A fundamental question about the pathogenesis of Alzheimer’s disease (AD) is how monomeric, nontoxic amyloid β-protein (Aβ) is converted to its toxic aggregates in the brain. The author previously identified a unique Aβ species in the AD brain, which is characterized by its binding to GM1 ganglioside (GM1). On the basis of the molecular characteristics of GM1-bound Aβ (GAβ), the author hypothesized that GM1 plays a critical role in the process. The author recently examined this possibility using a novel monoclonal antibody raised against purified GAβ and validated that GAβ is endogenously generated in the brain and accelerates Aβ assembly by acting as a seed. Furthermore, the author provided a possibility that aging and the expression of apolipoprotein E4 facilitate Aβ assembly in the brain through an increase in the GM1 content in the neuronal membranes, which likely induces GAβ generation. The author’s results imply a mechanism underlying the onset of AD and also provide a new insight into development of novel therapeutic strategy.

Synergistic Inhibitory Effect of Peptide-Organic Coassemblies on Amyloid Aggregation

Inhibition of amyloid aggregation is important for developing potential therapeutic strategies of amyloid-related diseases. Herein, we report that the inhibition effect of a pristine peptide motif (KLVFF) can be significantly improved by introducing a terminal regulatory moiety (terpyridine). The molecular-level observations by using scanning tunneling microscopy reveal stoichiometry-dependent polymorphism of the coassembly structures, which originates from the terminal interactions of peptide with organic modulator moieties and can be attributed to the secondary structures of peptides and conformations of the organic molecules. Furthermore, the polymorphism of the peptide-organic coassemblies is shown to be correlated to distinctively different inhibition effects on amyloid-β 42 (Aβ42) aggregations and cytotoxicity.

Biophysical insights into the membrane interaction of the core amyloid-forming Aβ40fragment K16–K28 and its role in the pathogenesis of Alzheimer's disease

Role of central hydrophobic region of Aβ40 in membrane interaction.

High Membrane Curvature Enhances Binding, Conformational Changes, and Fibrillation of Amyloid-β on Lipid Bilayer Surfaces

Aggregation of the amyloid-β (Aβ) protein and the formation of toxic aggregates are the possible pathogenic pathways in Alzheimer's disease. Accumulating evidence suggests that lipid membranes play key roles in protein aggregation, although the intermolecular forces that drive the interactions between Aβ-(1-40) and the membranes vary in different membrane systems. Here, we observed that a high positive curvature of lipid vesicles with diameters of ∼30 nm enhanced the association of Aβ with anionic phosphatidylglycerol membranes in the liquid-crystalline phase and with zwitterionic phosphatidylcholine membranes in the gel phase. The binding modes of Aβ to these membranes differ in terms of the location of the protein on the membrane and of the protein secondary structure. The fibrillation of Aβ was accelerated in the presence of the vesicles and at high protein-to-lipid ratios. Under these conditions, the protein accumulated on the surfaces, as demonstrated by a high (10(7) M(-1)) binding constant. Our findings suggest that packing defects on membranes with high curvatures, such as the intraluminal vesicles in multivesicular bodies and the exosomes, might result in the accumulation of toxic protein aggregates.

Alzheimer's disease is associated with disordered localization of ganglioside GM1 molecular species in the human dentate gyrus

Alzheimer's disease (AD) is a progressive dementia associated with loss of memory and cognitive dysfunction. In a previous study, we demonstrated a decrease in b-series gangliosides along with a change in ganglioside molecular species in the hippocampal grey matter of patients with AD. The present study demonstrates the use of imaging mass spectrometry for analyzing the spatial arrangement of ganglioside GM1 (GM1) molecular species in the hippocampus. In AD patients, we found a decrease in the ratio of GM1(d20:1/C18:0) to GM1 d18:1/C18:0) in the outer molecular layer (ML) of the dentate gyrus. Because the outer ML is the region of main input into the hippocampus, our findings may have a direct relationship to the mechanism of dysfunction in AD.

Diabetes mellitus accelerates Aβ pathology in brain accompanied by enhanced GAβ generation in nonhuman primates

Growing evidence suggests that diabetes mellitus (DM) is one of the strongest risk factors for developing Alzheimer’s disease (AD). However, it remains unclear why DM accelerates AD pathology. In cynomolgus monkeys older than 25 years, senile plaques (SPs) are spontaneously and consistently observed in their brains, and neurofibrillary tangles are present at 32 years of age and older. In laboratory-housed monkeys, obesity is occasionally observed and frequently leads to development of type 2 DM. In the present study, we performed histopathological and biochemical analyses of brain tissue in cynomolgus monkeys with type 2 DM to clarify the relationship between DM and AD pathology. Here, we provide the evidence that DM accelerates Aβ pathology in vivo in nonhuman primates who had not undergone any genetic manipulation. In DM-affected monkey brains, SPs were observed in frontal and temporal lobe cortices, even in monkeys younger than 20 years. Biochemical analyses of brain revealed that the amount of GM1-ganglioside-bound Aβ (GAβ)—the endogenous seed for Aβ fibril formation in the brain—was clearly elevated in DM-affected monkeys. Furthermore, the level of Rab GTPases was also significantly increased in the brains of adult monkeys with DM, almost to the same levels as in aged monkeys. Intraneuronal accumulation of enlarged endosomes was also observed in DM-affected monkeys, suggesting that exacerbated endocytic disturbance may underlie the acceleration of Aβ pathology due to DM.

Lipids in Amyloid-β Processing, Aggregation, and Toxicity

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

Small liposomes accelerate the fibrillation of amyloid β (1-40)

The deposition of amyloid β (Aβ) peptides is a pathological hallmark of Alzheimer disease. Aβ peptides were previously considered to interact specifically with ganglioside-containing membranes. Several studies have suggested that Aβ peptides also bind to phosphatidylcholine membranes, which lead to deformation of membranes and fibrillation of Aβ. Moreover, the role of membrane curvature, one type of deformation produced by binding of proteins to a membrane, in the binding and fibrillation of Aβ remains unclear. To clearly understand the relationship between the binding, consequent membrane deformation, and fibrillation of Aβ, we examined the amyloid fibrillation of Aβ-(1-40) in the presence of liposomes of various sizes. Membrane curvature increased with a decrease in the size of the liposomes. We used liposomes made of 1,2-dioleoyl-sn-glycero-3-phosphocholine to eliminate electrostatic effects. The results obtained showed that liposomes of smaller sizes (≤50 nm) significantly accelerated the nucleation step, thereby shortening the lag time of fibrillation. On the other hand, liposomes of larger sizes decreased the amount of fibrils but did not notably affect the lag time. The morphologies of fibrils, which were monitored by total internal reflection fluorescence microscopy, atomic force microscopy, and transmission electron microscopy, revealed that the length of Aβ-(1-40) fibrils became shorter and the amount of amorphous aggregates became larger as liposomes increased in size. These results suggest that the curvature of membranes coupled with an increase in water-accessible hydrophobic regions is important for binding and concentrating Aβ monomers, leading to amyloid nucleation. Furthermore, amyloid fibrillation on membranes may compete with non-productive binding to produce amorphous aggregates.

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.

An ALS-mutant TDP-43 neurotoxic peptide adopts an anti-parallel β-structure and induces TDP-43 redistribution

TDP-43 proteinopathies are clinically and genetically heterogeneous diseases that had been considered distinct from classical amyloid diseases. Here, we provide evidence for the structural similarity between TDP-43 peptides and other amyloid proteins. Atomic force microscopy and electron microscopy examination of peptides spanning a previously defined amyloidogenic fragment revealed a minimal core region that forms amyloid fibrils similar to the TDP-43 fibrils detected in FTLD-TDP brain tissues. An ALS-mutant A315E amyloidogenic TDP-43 peptide is capable of cross-seeding other TDP-43 peptides and an amyloid-β peptide. Sequential Nuclear Overhauser Effects and double-quantum-filtered correlation spectroscopy in nuclear magnetic resonance (NMR) analyses of the A315E-mutant TDP-43 peptide indicate that it adopts an anti-parallel β conformation. When added to cell cultures, the amyloidogenic TDP-43 peptides induce TDP-43 redistribution from the nucleus to the cytoplasm. Neuronal cultures in compartmentalized microfluidic-chambers demonstrate that the TDP-43 peptides can be taken up by axons and induce axonotoxicity and neuronal death, thus recapitulating key neuropathological features of TDP-43 proteinopathies. Importantly, a single amino acid change in the amyloidogenic TDP-43 peptide that disrupts fibril formation also eliminates neurotoxicity, supporting that amyloidogenesis is critical for TDP-43 neurotoxicity.

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

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

Role of lipid rafts and GM1 in the segregation and processing of prion protein

The prion protein (PrP^C) is highly expressed within the nervous system. Similar to other GPI-anchored proteins, PrP^C is found in lipid rafts, membrane domains enriched in cholesterol and sphingolipids. PrP^C raft association, together with raft lipid composition, appears essential for the conversion of PrP^C into the scrapie isoform PrP^Sc, and the development of prion disease. Controversial findings were reported on the nature of PrP^C-containing rafts, as well as on the distribution of PrP^C between rafts and non-raft membranes. We investigated PrP^C/ganglioside relationships and their influence on PrP^C localization in a neuronal cellular model, cerebellar granule cells. Our findings argue that in these cells at least two PrP^C conformations coexist: in lipid rafts PrP^C is present in the native folding (α-helical), stabilized by chemico-physical condition, while it is mainly present in other membrane compartments in a PrP^Sc-like conformation. We verified, by means of antibody reactivity and circular dichroism spectroscopy, that changes in lipid raft-ganglioside content alters PrP^C conformation and interaction with lipid bilayers, without modifying PrP^C distribution or cleavage. Our data provide new insights into the cellular mechanism of prion conversion and suggest that GM1-prion protein interaction at the cell surface could play a significant role in the mechanism predisposing to pathology.

A coarse-grained model for peptide aggregation on a membrane surface

The aggregation of peptides on a lipid bilayer is studied using coarse-grained molecular dynamics in implicit solvent. Peptides bind to and self-assemble on the membrane surface into β-rich fibrillar aggregates, even under conditions where only disordered oligomers form in bulk solution. Relative to a solid surface, the membrane surface facilitates peptide mobility and a more complex network of morphology transitions as aggregation proceeds. Additionally, final aggregate structures realized on the membrane surface are distinct from those observed on a comparable solid surface. The aggregated fibrils alter the local structure and material properties of the lipid bilayer in their immediate vicinity but have only a modest effect on the overall bending rigidity of the bilayer.