Ion channel formation by Alzheimer's disease amyloid beta-peptide (Abeta40) in unilamellar liposomes is determined by anionic phospholipids

Nanomaterials that Aid in the Diagnosis and Treatment of Alzheimer's Disease, Resolving Blood-Brain Barrier Crossing Ability

As a form of dementia, Alzheimer's disease (AD) suffers from no efficacious cure, yet AD treatment is still imperative, as it ameliorates the symptoms or prevents it from deteriorating or maintains the current status to the longest extent. The human brain is the most sensitive and complex organ in the body, which is protected by the blood-brain barrier (BBB). This yet induces the difficulty in curing AD as the drugs or nanomaterials that are much inhibited from reaching the lesion site. Thus, BBB crossing capability of drug delivery system remains a significant challenge in the development of neurological therapeutics. Fortunately, nano-enabled delivery systems possess promising potential to achieve multifunctional diagnostics/therapeutics against various targets of AD owing to their intriguing advantages of nanocarriers, including easy multifunctionalization on surfaces, high surface-to-volume ratio with large payloads, and potential ability to cross the BBB, making them capable of conquering the limitations of conventional drug candidates. This review, which focuses on the BBB crossing ability of the multifunctional nanomaterials in AD diagnosis and treatment, will provide an insightful vision that is conducive to the development of AD-related nanomaterials.

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

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

Cryo-EM structures of lipidic fibrils of amyloid-β (1-40)

Alzheimer's disease (AD) is a progressive and incurable neurodegenerative disease characterized by the extracellular deposition of amyloid plaques. Investigation into the composition of these plaques revealed a high amount of amyloid-β (Aβ) fibrils and a high concentration of lipids, suggesting that fibril-lipid interactions may also be relevant for the pathogenesis of AD. Therefore, we grew Aβ40 fibrils in the presence of lipid vesicles and determined their structure by cryo-electron microscopy (cryo-EM) to high resolution. The fold of the major polymorph is similar to the structure of brain-seeded fibrils reported previously. The majority of the lipids are bound to the fibrils, as we show by cryo-EM and NMR spectroscopy. This apparent lipid extraction from vesicles observed here in vitro provides structural insights into potentially disease-relevant fibril-lipid interactions.

Macrocyclic Peptides Derived from Familial Alzheimer's Disease Mutants Show Charge-Dependent Oligomeric Assembly and Toxicity

This work probes the role of charge in the oligomeric assembly, toxicity, and membrane destabilization of a series of peptides derived from Aβ and the E22Q and E22K familial mutants. In the mutant Aβ peptides, an acidic residue (E) is replaced with either a neutral or basic residue (Q or K), thus altering the net charge of the peptide. Acetylation at peripheral positions permits modulation of charge of the peptides and allows investigation of the role of charge in their oligomeric assembly, cytotoxicity, and membrane disruption. Peptides with the same net charge generally behave similarly even if the amino acid residue at position 22 differs. As the net charge of the peptide decreases, so does the extent of assembly, cytotoxicity, and membrane destabilization, which were determined using sodium dodecyl sulfate-polyacrylamide gel electrophoresis, lactate dehydrogenase (LDH)-release assays with SH-SY5Y cells, and dye leakage assays using liposomes. These findings suggest that the charge of the amino acid side chain, rather than its size or hydrophobicity, accounts for the differences in the oligomeric assembly and toxicity of the E22 familial mutants of Aβ.

Membrane channel hypothesis of lysosomal permeabilization by beta-amyloid

Alzheimer's disease (AD) is the most common cause of dementia affecting millions of people. Neuronal death in AD is initiated by oligomeric amyloid-β (Aβ) peptides. Recently, we proposed the amyloid degradation toxicity hypothesis, which explains multiple major observations associated with AD including autophagy failure and a decreased metabolism. According to the hypothesis, the key event in the cellular toxicity of amyloid is the formation of non-selective membrane channels in lysosomal membranes by amyloid fragments that are produced by the digestion of Aβ previously absorbed by endocytosis. Electrophysiological data suggest that amyloid-formed channels have different sizes, which can be explained by the fact that channel creating barrel-shaped amyloid aggregates can consist of different number of monomers. To estimate the ability of channels to leak molecules of various molecular weights, we modeled the channels as saline-filled cylinders in non-conductive membranes that pass spheres with a density of average globular proteins. As a basis, we used the conductance distribution taken from the previously published experimental dataset, in which single channels with electrical conductance of up to one nanosiemens were registered. Our calculations show that channels with such a giant conductance can allow for passing macromolecules such as large as lysosomal cathepsins implicated in the activation of apoptosis. The formation of giant channels is disproportionally promoted in an acidic environment. Also, amyloid fragments leaking from permeabilized lysosomes can reach the internal leaflet of the plasma membrane and permeabilize it. We conclude that while dissipation of the proton gradient by any (even smallest) amyloid channels readily explains lysosomal failure, the relatively rare events of lysosomal permeabilization to large macromolecules can be an additional mechanism of cellular death induced by exposure to Aβ.

Intracellular ion changes induced by the exposure to beta-amyloid can be explained by the formation of channels in the lysosomal membranes

In this manuscript, we reassess the data on beta-amyloid-induced changes of intracellular ions concentrations published previously by Abramov et al. (2003, 2004). Their observations made using high-resolution confocal microscopy with fast temporal resolution of images formed by fluorescent ion-sensitive fluorescent probes in living cells represent an unequivocal support for the amyloid channel theory. However, closer look reveals multiple facts which cannot be explained by channel formation in plasma membrane. Recently proposed amyloid degradation toxicity hypothesis provides the interpretation to these facts by considering that channels are formed in the lysosomal membranes.

Mini-review: Amyloid degradation toxicity hypothesis of Alzheimer's disease

Alzheimer's disease (AD) is the most common cause of dementia affecting millions of people. Neuronal death in AD is initiated by oligomeric amyloid-β (Aβ) peptides. The amyloid channel hypothesis readily explains the primary molecular damage but does not address major observations associated with AD such as autophagy failure and decreased metabolism. The amyloid degradation toxicity hypothesis provides the interpretation as a sequence of molecular events. Aβ enters a cell by endocytosis, and the endocytic vesicle is merged with a lysosome. Lysosomal peptidases degrade the peptide. Fragments form membrane channels in lysosomal membranes that have a significant negative charge due to the presence of acidic phospholipids. Amyloid channels can transfer various ions (including protons) and even relatively large compounds, which explains lysosomal permeabilization. The neutralization of lysosomal content inactivates degradation enzymes, results in an accumulation of undigested amyloid, and stalls autophagy. Inadequate quality control of mitochondria is associated with an increased production of reactive oxygen species and decreased energy production. Also, the passage of lysosomal proteases through rare extremely large channels results in cell death. Proposed hypothesis identifies biochemical pathways involved in the initiation and progression of cellular damage induced by beta-amyloid and provides new potential pharmacological targets to treat Alzheimer's disease.

Degradation Products of Amyloid Protein: Are They The Culprits?

Objectives:

Beta-amyloid (Aβ) peptides are most toxic to cells in oligomeric form. It is commonly accepted that oligomers can form ion channels in cell membranes and allow calcium and other ions to enter cells. The activation of other mechanisms, such as apoptosis or lipid peroxidation, aggravates the toxicity, but it itself can result from the same initial point, that is, ion disturbance due to an increased permeability of membranes. However, experimental studies of membrane channels created by Aβ are surprisingly limited. Methods: Here, we report a novel flow cytometry technique which can be used to detect increased permeability of membranes to calcium induced by the exposure to amyloid peptides. Calcium entry into the liposome is monitored using calcium-sensitive fluorescent probe. Undamaged lipid membranes are not permeable to calcium. Liposomes that are prepared in a calcium-free medium become able to accumulate calcium in a calcium-containing medium only after the formation of channels. Using this technique, we demonstrated that the addition of short amyloid fragment Aβ, which is known for its extreme toxicity on cultured neurons, readily increased membrane permeability to calcium. However, neither similarly sized peptide Ab22-35 nor full-length peptide Ab1-42 were producing channels. The formation of channels was observed in the membranes made of phosphatidylserine, a negatively charged lipid, but not in membranes made of the neutral phosphatidylcholine.

Methods:

Here, we report a novel flow cytometry technique which can be used to detect increased permeability of membranes to calcium induced by the exposure to amyloid peptides. Calcium entry into the liposome is monitored using calcium-sensitive fluorescent probe. Undamaged lipid membranes are not permeable to calcium. Liposomes that are prepared in a calcium-free medium become able to accumulate calcium in a calcium-containing medium only after the formation of channels. Using this technique, we demonstrated that the addition of short amyloid fragment Aβ, which is known for its extreme toxicity on cultured neurons, readily increased membrane permeability to calcium. However, neither similarly sized peptide Ab22-35 nor full-length peptide Ab1-42 were producing channels. The formation of channels was observed in the membranes made of phosphatidylserine, a negatively charged lipid, but not in membranes made of the neutral phosphatidylcholine.

Results:

In the Discussion section, we have analyzed several issues which could be critical for understanding the pathogenesis of Alzheimer’s disease, specifically 1) the need for a negatively charged membrane to produce the ion channel; 2) the potential role of the aggregated form in cellular toxicity of Ab peptides; 3) channel-forming ability of multiple degradation products of amyloid; 4) non-specificity of ion channels formed by amyloid peptides. Potential targets of channel-forming oligomers appear to be intracellular and are organelles well-known for dysfunction in Alzheimer’s disease (mitochondria and lysosomes). In fact, lysosomes can also be the producers of degraded amyloid. Provided speculations support the hypothesis that neuronal toxicity can be caused by the degradation products of beta-amyloid.

Flow cytometry method to quantify the formation of beta-amyloid membrane ion channels

It is accepted that the cytotoxicity of beta-amyloid is mediated by its oligomers. Amyloid peptides can form ion channels in cell membranes and allow calcium and other ions to enter cells. In this project, we developed a technique to quantify the appearance of calcium in liposomes and applied this technique to study the effect of amyloid peptides on the permeability of membranes. Calcium influx was monitored in liposomes made of phosphatidylcholine (PC) or phosphatidylserine (PS) with an addition of a lipid-soluble dye DiD and containing fluorescent calcium-sensitive probe Fluo-3. The intensity of fluorescence of individual liposomes was measured using a flow cytometer. Calcium ionophore ionomycin served as a positive control. The addition of micromolar concentrations of short fragments of amyloid-beta (Aβ_25-35) permeabilized a significant number of PS liposomes. This effect was not observed in PC liposomes. Our data supports the hypothesis that the ion channel formation by amyloid peptide is dependent on electrostatic interactions. High concentrations of Aβ_25-35 (above 20 μM) increased signal intensity in a recording channel corresponding to the calcium-sensing probe. However, this phenomenon was also observed in Ca²⁺-free conditions and even in liposomes without Fluo-3, so we interpreted it as an artifact. Using the described technique, we were not able to detect the formation of calcium channels by several other amyloid peptides. Considering that liposomes appeared resistant to reasonable concentrations of solvents, we expect that described flowmetric technique can be used in high-throughput screening applications.

The Interaction between Amyloid Prefibrillar Oligomers of Salmon Calcitonin and a Lipid-Raft Model: Molecular Mechanisms Leading to Membrane Damage, Ca2+-Influx and Neurotoxicity

To investigate the interaction between amyloid assemblies and “lipid-rafts”, we performed functional and structural experiments on salmon calcitonin (sCT) solutions rich in prefibrillar oligomers, proto- and mature-fibers interacting with liposomes made of monosialoganglioside-GM1 (4%), DPPC (48%) and cholesterol (48%). To focus on the role played by electrostatic forces and considering that sCT is positive and GM1 is negative at physiologic pH, we compared results with those relative to GM1-free liposomes while, to assess membrane fluidity effects, with those relative to cholesterol-free liposomes. We investigated functional effects by evaluating Ca²⁺-influx in liposomes and viability of HT22-DIFF neurons. Only neurotoxic solutions rich in unstructured prefibrillar oligomers were able to induce Ca²⁺-influx in the “lipid-rafts” model, suggesting that the two phenomena were correlated. Thus, we investigated protein conformation and membrane modifications occurring during the interaction: circular dichroism showed that “lipid-rafts” fostered the formation of β-structures and energy filtered-transmission electron microscopy that prefibrillar oligomers formed pores, similar to Aβ did. We speculate that electrostatic forces between the positive prefibrillar oligomers and the negative GM1 drive the initial binding while the hydrophobic profile and flexibility of prefibrillar oligomers, together with the membrane fluidity, are responsible for the subsequent pore formation leading to Ca²⁺-influx and neurotoxicity.

Liposome delivery systems for the treatment of Alzheimer's disease

Alzheimer's disease (AD) will affect around 115 million people worldwide by the year 2050. It is associated with the accumulation of misfolded and aggregated proteins (β-amyloid and tau) in the senile plaques and neurofibrillary tangles found in the brain. Currently available drugs for AD only temporarily alleviate symptoms and do not slow the inevitable progression of this disease. New drugs are required that act on key pathologies in order to arrest or reverse cognitive decline. However, there has been a spectacular failure rate in clinical trials of conventional small molecule drugs or biological agents. Targeted nanoliposomes represent a viable and promising drug delivery system for AD that have not yet reached clinical trials. They are biocompatible, highly flexible, and have the potential to carry many different types of therapeutic molecules across the blood-brain barrier (BBB) and into brain cells. They can be tailored to extend blood circulation time and can be directed against individual or multiple pathological targets. Modifications so far have included the use of brain-penetrating peptides, together with Aβ-targeting ligands, such as phosphatidic acid, curcumin, and a retro-inverted peptide that inhibits Aβ aggregation. Combining several modifications together into multifunctional liposomes is currently a research area of great interest. This review focuses on recent liposomal approaches to AD therapy, including mechanisms involved in facilitating their passage across the BBB, and the evaluation of new therapeutic agents for blocking Aβ and/or tau aggregation.

Effect of saturation in phospholipid/fatty acid monolayers on interaction with amyloid β peptide

The effect of the saturation of fatty acid (FA) in 1,2-dimyristoyl-sn-glycero-3-phosphocoline (DMPC)/FA membrane on the interaction between lipid membrane and amyloid β monomer was investigated by using the Langmuir monolayer technique. The surface pressure (Π)-mean molecular area (A) isotherms and fluorescent measurements reveal that DMPC and octadecanoic acid (stearic acid, SA) molecules were somewhat miscible in the mixed membrane, which was maintained to homogeneous gel phase by enhance of the intermolecular hydrophobic interactions because of the all trans acyl chains. On the other hand, DMPC and 9Z,12Z-octadecadienoic acid (linoleic acid, LA) molecules were considered to be well miscible in the mixed membrane, where the membrane partially transferred from gel phase to liquid-crystalline phase. The Π-A isotherms of the monolayers on amyloid β-peptide (Aβ) solution indicated that Aβ monomers tend to be inserted into the saturated acyl chain region of monolayers at low surface pressure and that the Aβ monomers were then extruded from the monolayer at higher surface pressure. It was observed that behaviors of Aβ monomers at higher surface pressure depended on membrane microstructures. In the DMPC/SA monolayers, Aβ aggregated and then was extruded from monolayers at about 20 mN m^-1 of surface pressure irrespective of the SA proportion. On the other hand, in the DMPC/LA monolayers, Aβ, which favors to interact with DMPC, is dispersed in the monolayer even at high surface pressure because DMPC and LA molecules were well miscible in the monolayer.

Binding of protofibrillar Aβ trimers to lipid bilayer surface enhances Aβ structural stability and causes membrane thinning

Alzheimer's disease, a common neurodegenerative disease, is characterized by the aggregation of amyloid-β (Aβ) peptides. The interactions of Aβ with membranes cause changes in membrane morphology and ion permeation, which are responsible for its neurotoxicity and can accelerate fibril growth. However, the Aβ-lipid interactions and how these induce membrane perturbation and disruption at the atomic level and the consequences for the Aβ organization are not entirely understood. Here, we perform multiple atomistic molecular dynamics simulations on three protofibrillar Aβ9-40 trimers. Our simulations show that, regardless of the morphologies and the initial orientations of the three different protofibrillar Aβ9-40 trimers, the N-terminal β-sheet of all trimers preferentially binds to the membrane surface. The POPG lipid bilayers enhance the structural stability of protofibrillar Aβ trimers by stabilizing inter-peptide β-sheets and D23-K28 salt-bridges. The interaction causes local membrane thinning. We found that the trimer structure related to Alzheimer's disease brain tissue () is the most stable both in water solution and at membrane surface, and displays slightly stronger membrane perturbation capability. These results provide mechanistic insights into the membrane-enhanced structural stability of protofibrillar Aβ oligomers and the first step of Aβ-induced membrane disruption at the atomic level.

Amyloid β Ion Channels in a Membrane Comprising Brain Total Lipid Extracts

Amyloid β (Aβ) oligomers are the predominant toxic species in the pathology of Alzheimer's disease. The prevailing mechanism for toxicity by Aβ oligomers includes ionic homeostasis destabilization in neuronal cells by forming ion channels. These channel structures have been previously studied in model lipid bilayers. In order to gain further insight into the interaction of Aβ oligomers with natural membrane compositions, we have examined the structures and conductivities of Aβ oligomers in a membrane composed of brain total lipid extract (BTLE). We utilized two complementary techniques: atomic force microscopy (AFM) and black lipid membrane (BLM) electrical recording. Our results indicate that Aβ1-42 forms ion channel structures in BTLE membranes, accompanied by a heterogeneous population of ionic current fluctuations. Notably, the observed current events generated by Aβ1-42 peptides in BTLE membranes possess different characteristics compared to current events generated by the presence of Aβ1-42 in model membranes comprising a 1:1 mixture of DOPS and POPE lipids. Oligomers of the truncated Aβ fragment Aβ17-42 (p3) exhibited similar ion conductivity behavior as Aβ1-42 in BTLE membranes. However, the observed macroscopic ion flux across the BTLE membranes induced by Aβ1-42 pores was larger than for p3 pores. Our analysis of structure and conductance of oligomeric Aβ pores in a natural lipid membrane closely mimics the in vivo cellular environment suggesting that Aβ pores could potentially accelerate the loss of ionic homeostasis and cellular abnormalities. Hence, these pore structures may serve as a target for drug development and therapeutic strategies for AD treatment.

Amyloid beta modulation of neuronal network activity in vitro

In vitro assays offer a means of screening potential therapeutics and accelerating the drug development process. Here, we utilized neuronal cultures on planar microelectrode arrays (MEA) as a functional assay to assess the neurotoxicity of amyloid-β 1-42 (Aβ42), a biomolecule implicated in the Alzheimer׳s disease (AD). In this approach, neurons harvested from embryonic mice were seeded on the substrate-integrated microelectrode arrays. The cultured neurons form a spontaneously active network, and the spiking activity as a functional endpoint could be detected via the MEA. Aβ42 oligomer, but not monomer, significantly reduced network spike rate. In addition, we demonstrated that the ionotropic glutamate receptors, NMDA and AMPA/kainate, play a role in the effects of Aβ42 on neuronal activity in vitro. To examine the utility of the MEA-based assay for AD drug discovery, we tested two model therapeutics for AD, methylene blue (MB) and memantine. Our results show an almost full recovery in the activity within 24h after administration of Aβ42 in the cultures pre-treated with either MB or memantine. Our findings suggest that cultured neuronal networks may be a useful platform in screening potential therapeutics for Aβ induced changes in neurological function.

Amyloid pore-channel hypothesis: effect of ethanol on aggregation state using frog oocytes for an Alzheimer's disease study

Alzheimer's disease severely compromises cognitive function. One of the mechanisms to explain the pathology of Alzheimer’s disease has been the hypotheses of amyloid-pore/channel formation by complex Aβ-aggregates. Clinical studies suggested the moderate alcohol consumption can reduces probability developing neurodegenerative pathologies. A recent report explored the ability of ethanol to disrupt the generation of complex Aβ in vitro and reduce the toxicity in two cell lines. Molecular dynamics simulations were applied to understand how ethanol blocks the aggregation of amyloid. On the other hand, the in silico modeling showed ethanol effect over the dynamics assembling for complex Aβ-aggregates mediated by break the hydrosaline bridges between Asp 23 and Lys 28, was are key element for amyloid dimerization. The amyloid pore/channel hypothesis has been explored only in neuronal models, however recently experiments suggested the frog oocytes such an excellent model to explore the mechanism of the amyloid pore/channel hypothesis. So, the used of frog oocytes to explored the mechanism of amyloid aggregates is new, mainly for amyloid/pore hypothesis. Therefore, this experimental model is a powerful tool to explore the mechanism implicates in the Alzheimer’s disease pathology and also suggests a model to prevent the Alzheimer’s disease pathology. [BMB Reports 2015; 48(1): 13-18]

The involvement of lipids in Alzheimer's disease

It has been estimated that Alzheimer's disease (AD), the most common form of dementia, will affect approximately 81 million individuals by 2040. To date, the actual cause and cascade of events in the progression of this disease have not been fully determined. Furthermore, there is currently no definitive blood test or simple diagnostic method for AD. Considerable efforts have been put into proteomic approaches to develop a diagnostic blood test, but to date these efforts have not been successful. More recently, there has been a stronger focus on lipidomic studies in the hope of increasing our understanding of the underlying mechanisms leading to AD and developing an AD blood test. It is well known that the strongest genetic risk factor for AD is the ε4 variant of apolipoprotein E (APOE). Evidence suggests that the ApoE protein, a major lipid transporter, plays a key role in the pathogenesis of AD, and its role in both normal and aberrant lipid metabolism warrants further extensive investigation. Here, we review ApoE-lipid interactions, as well as the roles that lipids may play in the pathogenesis of AD.

Role of bilayer characteristics on the structural fate of aβ(1-40) and aβ(25-40)

The β-amyloid (Aβ) peptide is derived from the transmembrane (TM) helix of the amyloid precursor protein (APP) and has been shown to interact with membrane surfaces. To understand better the role of peptide-membrane interactions in cell death and ultimately in Alzheimer's disease, a better understanding of how membrane characteristics affect the binding, solvation, and secondary structure of Aβ is needed. Employing a combination of circular dichroism and deep-UV resonance Raman spectroscopies, Aβ(25-40) was found to fold spontaneously upon association with anionic lipid bilayers. The hydrophobic portion of the disease-related Aβ(1-40) peptide, Aβ(25-40), has often been used as a model for how its legacy TM region may behave structurally in aqueous solvents and during membrane encounters. The structure of the membrane-associated Aβ(25-40) peptide was found to depend on both the hydrophobic thickness of the bilayer and the duration of incubation. Similarly, the disease-related Aβ(1-40) peptide also spontaneously associates with anionic liposomes, where it initially adopts mixtures of disordered and helical structures. The partially disordered helical structures then convert to β-sheet structures over longer time frames. β-Sheet structure is formed prior to helical unwinding, implying a model in which β-sheet structure, formed initially from disordered regions, prompts the unwinding and destabilization of membrane-stabilized helical structure. A model is proposed to describe the mechanism of escape of Aβ(1-40) from the membrane surfaces following its formation by cleavage of APP within the membrane.

Free energy of lipid bilayer defects affected by Alzheimer's disease-associated amyloid-β42 monomers

Experimental evidence suggests that the amyloid β-peptide (Aβ) associated with Alzheimer's disease strongly disturbs the integrity of lipid bilayers and cell membranes, as a possible origin of the toxicity of this peptide. Here, we have used molecular dynamics simulations to compute the free energy of membrane pores in the presence and absence of Aβ. The validation of our approach included the calculation of lipid flip-flop waiting times, which were found to agree well with recent experiments, in contrast with an earlier simulation study that apparently overestimated these waiting times. We find that, compared with peptide-free lipid bilayers, attached Aβ42 peptides (i) increase the order parameters of the lipid tails but (ii) decrease the effective width of the hydrophobic region, (iii) reduce the free energy and thus enlarge the density of membrane pores, and (iv) increase the lifetime of pores. A detailed understanding of the interaction of Aβ42 with lipid bilayer membranes may assist in the design of therapeutical strategies against Alzheimer's disease.

The influence of cannabinoids on generic traits of neurodegeneration

In an increasingly ageing population, the incidence of neurodegenerative disorders such as Alzheimer's disease, Parkinson's disease and Huntington's disease are rising. While the aetiologies of these disorders are different, a number of common mechanisms that underlie their neurodegenerative components have been elucidated; namely neuroinflammation, excitotoxicity, mitochondrial dysfunction and reduced trophic support. Current therapies focus on treatment of the symptoms and attempt to delay the progression of these diseases but there is currently no cure. Modulation of the endogenous cannabinoid system is emerging as a potentially viable option in the treatment of neurodegeneration. Endocannabinoid signalling has been found to be altered in many neurodegenerative disorders. To this end, pharmacological manipulation of the endogenous cannabinoid system, as well as application of phytocannabinoids and synthetic cannabinoids have been investigated. Signalling from the CB1 and CB2 receptors are known to be involved in the regulation of Ca(2+) homeostasis, mitochondrial function, trophic support and inflammatory status, respectively, while other receptors gated by cannabinoids such as PPARγ, are gaining interest in their anti-inflammatory properties. Through multiple lines of evidence, this evolutionarily conserved neurosignalling system has shown neuroprotective capabilities and is therefore a potential target for neurodegenerative disorders. This review details the mechanisms of neurodegeneration and highlights the beneficial effects of cannabinoid treatment.

LINKED ARTICLES

This article is part of a themed section on Cannabinoids 2013. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2014.171.issue-6.

Cellular membrane fluidity in amyloid precursor protein processing

The senile plaque is a pathologic hallmark of Alzheimer's disease (AD). Amyloid-β peptide (Aβ), the main constituent of senile plaques, is neurotoxic especially in its oligomeric form. Aβ is derived from the sequential cleavage of amyloid precursor protein (APP) by β- and γ-secretases in the amyloidogenic pathway. Alternatively, APP can be cleaved by α-secretases within the Aβ domain to produce neurotrophic and neuroprotective α-secretase-cleaved soluble APP (sAPPα) in the nonamyloidogenic pathway. Since APP and α-, β-, and γ-secretases are membrane proteins, APP processing should be highly dependent on the membrane composition and the biophysical properties of cellular membrane. In this review, we discuss the role of the biophysical properties of cellular membrane in APP processing, especially the effects of phospholipases A(2) (PLA(2)s), fatty acids, cholesterol, and Aβ on membrane fluidity in relation to their effects on APP processing.

Divalent copper is a potent extracellular blocker for TRPM2 channel

Transient receptor potential melastatin 2 (TRPM2) is a Ca(2+)-permeable cationic channel in the TRP channel family. The channel activity can be regulated by reactive oxygen species (ROS) and cellular acidification, which has been implicated to the pathogenesis of diabetes and some neuronal disorders. However, little is known about the effect of redox-active metal ions, such as copper, on TRPM2 channels. Here we investigated the effect of divalent copper on TRPM2. TRPM2 channel was over-expressed in HEK-293 cells and the whole-cell current was recorded by patch clamp. We found the whole-cell current evoked by intracellular ADP-ribose was potently inhibited by Cu(2+) with a half maximal inhibitory concentration (IC(50)) of 2.59 μM. The inhibitory effect was irreversible. The single channel activity was abolished in the outside-out patches, and intracellular application of Cu(2+) did not prevent the channel activation, suggesting that the action site of Cu(2+) is located in the extracellular domains of the channel. TRPM2 current was also blocked by Hg(2+), Pb(2+), Fe(2+) and Se(2+). We concluded that Cu(2+) is a potent TRPM2 channel blocker. The sensitivity of TRPM2 channel to heavy metal ions could be a new mechanism for the pathogenesis of some metal ion-related diseases.

Aβ oligomer-induced synapse degeneration in Alzheimer's disease

Aβ oligomers cause a collection of molecular events associated with memory loss in Alzheimer's disease, centering on disrupting the maintenance of synapse structure and function. In this brief review of the synaptotoxic effects of Aβ oligomers, we focus on the neuronal properties governing oligomer targeting and toxicity-especially with respect to binding sites and mechanisms of binding. We also discuss ways in which mechanistic insights from other diseases offer clues in the pursuit of the molecular basis of Alzheimer's disease.

Impacts of membrane biophysics in Alzheimer's disease: from amyloid precursor protein processing to aβ Peptide-induced membrane changes

An increasing amount of evidence supports the notion that cytotoxic effects of amyloid-β peptide (Aβ), the main constituent of senile plaques in Alzheimer's disease (AD), are strongly associated with its ability to interact with membranes of neurons and other cerebral cells. Aβ is derived from amyloidogenic cleavage of amyloid precursor protein (AβPP) by β- and γ-secretase. In the nonamyloidogenic pathway, AβPP is cleaved by α-secretases. These two pathways compete with each other, and enhancing the non-amyloidogenic pathway has been suggested as a potential pharmacological approach for the treatment of AD. Since AβPP, α-, β-, and γ-secretases are membrane-associated proteins, AβPP processing and Aβ production can be affected by the membrane composition and properties. There is evidence that membrane composition and properties, in turn, play a critical role in Aβ cytotoxicity associated with its conformational changes and aggregation into oligomers and fibrils. Understanding the mechanisms leading to changes in a membrane's biophysical properties and how they affect AβPP processing and Aβ toxicity should prove to provide new therapeutic strategies for prevention and treatment of AD.

Models of membrane-bound Alzheimer's Abeta peptide assemblies

Although it is clear that amyloid beta (Aβ) peptides play a pivotal role in the development of Alzheimer's disease, the precise molecular model of action remains unclear. Aβ peptide forms assemble both in aqueous solution and in lipid membranes. It has been proposed that deleterious effects occur when the peptides interact with membranes, possibly by forming Ca(2+) permeant ion channels. In the accompanying manuscript, we propose models in which the C-terminus third of six Aβ42 peptides forms a six-stranded β-barrel in highly toxic soluble oligomers. Here we extend this hypothesis to membrane-bound assemblies. In these Aβ models, the hydrophobic β-barrel of a hexamer may either reside on the surface of the bilayer, or span the bilayer. Transmembrane pores are proposed to form between several hexamers. Once the β-barrels of six hexamers have spanned the bilayer, they may merge to form a more stable 36-stranded β-barrel. We favor models in which parallel β-barrels formed by N-terminus segments comprise the lining of the pores. These types of models explain why the channels are selective for cations and how metal ions, such as Zn(2+) , synthetic peptides that contain histidines, and some small organic cations may block channels or inhibit formation of channels. Our models were developed to be consistent with microscopy studies of Aβ assemblies in membranes, one of which is presented here for the first time.

Synaptotoxicity of Alzheimer beta amyloid can be explained by its membrane perforating property

The mechanisms that induce Alzheimer's disease (AD) are largely unknown thereby deterring the development of disease-modifying therapies. One working hypothesis of AD is that Aβ excess disrupts membranes causing pore formation leading to alterations in ionic homeostasis. However, it is largely unknown if this also occurs in native brain neuronal membranes. Here we show that similar to other pore forming toxins, Aβ induces perforation of neuronal membranes causing an increase in membrane conductance, intracellular calcium and ethidium bromide influx. These data reveal that the target of Aβ is not another membrane protein, but that Aβ itself is the cellular target thereby explaining the failure of current therapies to interfere with the course of AD. We propose that this novel effect of Aβ could be useful for the discovery of anti AD drugs capable of blocking these “Aβ perforates”. In addition, we demonstrate that peptides that block Aβ neurotoxicity also slow or prevent the membrane-perforating action of Aβ.

Simultaneous single-molecule fluorescence and conductivity studies reveal distinct classes of Abeta species on lipid bilayers

The extracellular senile plaques prevalent in brain tissue in Alzheimer's disease (AD) are composed of amyloid fibrils formed by the Abeta peptide. These fibrils have been traditionally believed to be featured in neurotoxicity; however, numerous recent studies provide evidence that cytotoxicity in AD may be associated with low-molecular weight oligomers of Abeta that associate with neuronal membranes and may lead to membrane permeabilization and disruption of the ion balance in the cell. The underlying mechanism leading to disruption of the membrane is the subject of many recent studies. Here we report the application of single-molecule optical detection, using fluorescently labeled human Alphabeta40, combined with membrane conductivity measurements, to monitor the interaction of single-oligomeric peptide structures with model planar black lipid membranes (BLMs). In a qualitative study, we show that the binding of Alphabeta to the membrane can be described by three distinctly different behaviors, depending on the Alphabeta monomer concentration. For concentrations much below 10 nM, there is uniform binding of monomers over the surface of the membrane with no evidence of oligomer formation or membrane permeabilization. Between 10 nM and a few hundred nanomolar, the uniform monomer binding is accompanied by the presence of peptide species ranging from dimers to small oligomers. The dimers are not found to permeabilize the membrane, but the larger oligomers lead to permeabilization with individual oligomers producing ion conductances of <10 pS/pore. At higher concentrations, perhaps beyond physiologically relevant concentrations, larger extended and dynamic structures are found with large conductances (hundreds of picosiemens), suggesting a major disruption of the membrane.

Amyloid peptide pores and the beta sheet conformation

Over 20 clinical syndromes have been described as amyloid diseases. Pathologically, these illnesses are characterized by the deposition in various tissues of amorphous, Congo red stainingdeposits, referred to as amyloid. Under polarizing light microscopy, these deposits exhibit characteristic green birefringence. X-ray diffraction reveals cross-beta structure of extended amyloid fibrils. Although there is always a major protein in amyloid deposits, the predominant protein differs in each ofthe clinical syndromes. All the proteins exhibit the characteristic nonnative beta-sheet state. These proteins aggregate spontaneously into extended fibrils and precipitate out of solution. At least a dozen of these peptides have been demonstrated to be capable of channel formation in lipid bilayers and it has been proposed that this represents a pathogenic mechanism. Remarkably, the channels formed by these various peptides exhibit a number of common properties including irreversible, spontaneous insertion into membranes, production oflarge, heterogeneous single-channel conductances, relatively poor ion selectivity, inhibition of channel formation by Congo red and related dyes and blockade of inserted channels by zinc. In vivo amyloid peptides have been shown to disrupt intracellular calcium regulation, plasma membrane potential, mitochondrial membrane potential and function and long-term potentiation in neurons. Amyloid peptides also cause cytotoxicity. Formation of the beta sheet conformation from native protein structures can be induced by high protein concentrations, metal binding, acidic pH, amino acid mutation and interaction with lipid membranes. Most amyloid peptides interact strongly with membranes and this interaction is enhanced by conditions which favor beta-sheet formation. Formation of pores in these illnesses appears to be a spontaneous process and available evidence suggests several steps are critical. First, destabilization of the native structure and formation of the beta-sheet conformation must occur. This may occur in solution or may be facilitated by contact with lipid membranes. Oligomerization of the amyloid protein is then mediated by the beta strands. Amyloid monomers and extended fibrils appear to have little potential for toxicity whereas there is much evidence implicating amyloid oligomers of intermediate size in the pathogenesis of amyloid disease. Insertion of the oligomer appears to take place spontaneously although there may be a contribution of acidic pH and/or membrane potential. Very little is known about the structure of amyloid pores, but given that the amyloid peptides must acquire beta-sheet conformation to aggregate and polymerize, it has been hypothesized that amyloid pores may in fact be beta-sheet barrels similar to the pores formed by alpha-latrotoxin, Staphylococcal alpha-hemolysin, anthrax toxin and clostridial perfringolysin.

Beta-amyloid causes depletion of synaptic vesicles leading to neurotransmission failure

Alzheimer disease is a progressive neurodegenerative brain disorder that leads to major debilitating cognitive deficits. It is believed that the alterations capable of causing brain circuitry dysfunctions have a slow onset and that the full blown disease may take several years to develop. Therefore, it is important to understand the early, asymptomatic, and possible reversible states of the disease with the aim of proposing preventive and disease-modifying therapeutic strategies. It is largely unknown how amyloid beta-peptide (A beta), a principal agent in Alzheimer disease, affects synapses in brain neurons. In this study, we found that similar to other pore-forming neurotoxins, A beta induced a rapid increase in intracellular calcium and miniature currents, indicating an enhancement in vesicular transmitter release. Significantly, blockade of these effects by low extracellular calcium and a peptide known to act as an inhibitor of the A beta-induced pore prevented the delayed failure, indicating that A beta blocks neurotransmission by causing vesicular depletion. This new mechanism for A beta synaptic toxicity should provide an alternative pathway to search for small molecules that can antagonize these effects of A beta.

Synthetic lipid vesicles recruit native-like aggregates and affect the aggregation process of the prion Ure2p: insights on vesicle permeabilization and charge selectivity

The yeast prion Ure2p polymerizes into native-like fibrils, retaining the overall structure and binding properties of the soluble protein. Recently we have shown that, similar to amyloid oligomers, the native-like Ure2p fibrils and their precursor oligomers are highly toxic to cultured mammalian cells when added to the culture medium, whereas Ure2p amyloid fibrils generated by heating the native-like fibrils are substantially harmless. We show here that, contrary to the nontoxic amyloid fibrils, the toxic, native-like Ure2p assemblies induce a significant calcein release from negatively charged phosphatidylserine vesicles. A minor and less-specific effect was observed with zwitterionic phosphatidylcholine vesicles, suggesting that the toxic aggregates preferentially bind to negatively charged sites on lipid membranes. We also found that cholesterol-enriched phospholipid membranes are protected against permeabilization by native-like Ure2p assemblies. Moreover, vesicle permeabilization appears charge-selective, allowing calcium, but not chloride, influx to be monitored. Finally, we found that the interaction with phosphatidylserine membranes speeds up Ure2p polymerization into oligomers and fibrils structurally and morphologically similar to the native-like Ure2p assemblies arising in free solution, although less cytotoxic. These data suggest that soluble Ure2p oligomers and native-like fibrils, but not amyloid fibrils, interact intimately with negatively charged lipid membranes, where they allow selective cation influx.

Amyloid-beta membrane binding and permeabilization are distinct processes influenced separately by membrane charge and fluidity

The 40 and 42 residue amyloid-beta (Abeta) peptides are major components of the proteinaceous plaques prevalent in the Alzheimer's disease-afflicted brain and have been shown to have an important role in instigating neuronal degeneration. Whereas it was previously thought that Abeta becomes cytotoxic upon forming large fibrillar aggregates, recent studies suggest that soluble intermediate-sized oligomeric species cause cell death through membrane permeabilization. The present study examines the interactions between Abeta40 and lipid membranes using liposomes as a model system to determine how changes in membrane composition influence the conversion of Abeta into these toxic species. Abeta40 membrane binding was monitored using fluorescence-based assays with a tryptophan-substituted peptide (Abeta40 [Y10W]). We extend previous observations that Abeta40 interacts preferentially with negatively charged membranes, and show that binding of nonfibrillar, low molecular mass oligomers of Abeta40 to anionic, but not neutral, membranes involves insertion of the peptide into the bilayer, as well as sequential conformational changes corresponding to the degree of oligomerization induced. Significantly, while anionic membranes in the gel, liquid crystalline, and liquid ordered phases induce these conformational changes equally, membrane permeabilization is reduced dramatically as the fluidity of the membrane is decreased. These findings demonstrate that binding alone is not sufficient for membrane permeabilization, and that the latter is also highly dependent on the fluidity and phase of the membrane. We conclude that binding and pore formation are two distinct steps. The differences in Abeta behavior induced by membrane composition may have significant implications on the development and progression of AD as neuronal membrane composition is altered with age.

Structure-function relationships of pre-fibrillar protein assemblies in Alzheimer's disease and related disorders

Several neurodegenerative diseases, including Alzheimer's, Parkinson's, Huntington's and prion diseases, are characterized pathognomonically by the presence of intra- and/or extracellular lesions containing proteinaceous aggregates, and by extensive neuronal loss in selective brain regions. Related non-neuropathic systemic diseases, e.g., light-chain and senile systemic amyloidoses, and other organ-specific diseases, such as dialysis-related amyloidosis and type-2 diabetes mellitus, also are characterized by deposition of aberrantly folded, insoluble proteins. It is debated whether the hallmark pathologic lesions are causative. Substantial evidence suggests that these aggregates are the end state of aberrant protein folding whereas the actual culprits likely are transient, pre-fibrillar assemblies preceding the aggregates. In the context of neurodegenerative amyloidoses, the proteinaceous aggregates may eventuate as potentially neuroprotective sinks for the neurotoxic, oligomeric protein assemblies. The pre-fibrillar, oligomeric assemblies are believed to initiate the pathogenic mechanisms that lead to synaptic dysfunction, neuronal loss, and disease-specific regional brain atrophy. The amyloid beta-protein (Abeta), which is believed to cause Alzheimer's disease (AD), is considered an archetypal amyloidogenic protein. Intense studies have led to nominal, functional, and structural descriptions of oligomeric Abeta assemblies. However, the dynamic and metastable nature of Abeta oligomers renders their study difficult. Different results generated using different methodologies under different experimental settings further complicate this complex area of research and identification of the exact pathogenic assemblies in vivo seems daunting. Here we review structural, functional, and biological experiments used to produce and study pre-fibrillar Abeta assemblies, and highlight similar studies of proteins involved in related diseases. We discuss challenges that contemporary researchers are facing and future research prospects in this demanding yet highly important field.

Positioning of the Alzheimer Abeta(1-40) peptide in SDS micelles using NMR and paramagnetic probes

NMR spectroscopy combined with paramagnetic relaxation agents was used to study the positioning of the 40-residue Alzheimer Amyloid beta-peptide Abeta(1-40) in SDS micelles. 5-Doxyl stearic acid incorporated into the micelle or Mn(2+) ions in the aqueous solvent were used to determine the position of the peptide relative to the micelle geometry. In SDS solvent, the two alpha-helices induced in Abeta(1-40), comprising residues 15-24, and 29-35, respectively, are surrounded by flexible unstructured regions. NMR signals from these unstructured regions are strongly attenuated in the presence of Mn(2+) showing that these regions are positioned mostly outside the micelle. The central helix (residues 15-24) is significantly affected by 5-doxyl stearic acid however somewhat less for residues 16, 20, 22 and 23. This alpha-helix therefore resides in the SDS headgroup region with the face with residues 16, 20, 22 and 23 directed away from the hydrophobic interior of the micelle. The C-terminal helix is protected both from 5-doxyl stearic acid and Mn(2+), and should be buried in the hydrophobic interior of the micelle. The SDS micelles were characterized by diffusion and (15)N-relaxation measurements. Comparison of experimentally determined translational diffusion coefficients for SDS and Abeta(1-40) show that the size of SDS micelle is not significantly changed by interaction with Abeta(1-40).

Abeta ion channels. Prospects for treating Alzheimer's disease with Abeta channel blockers

The main pathological features in the Alzheimer's brain are progressive depositions of amyloid protein plaques among nerve cells, and neurofibrillary tangles within the nerve cells. The major components of plaques are Abeta peptides. Numerous reports have provided evidence that Abeta peptides are cytotoxic and may play a role in the pathogenesis of AD. An increasing number of research reports support the concept that the Abeta-membrane interaction event may be followed by the insertion of Abeta into the membrane in a structural configuration which forms an ion channel. This review summarizes experimental procedures which have been designed to test the hypothesis that the interaction of Abeta with a variety of membranes, both artificial and natural, results in the subsequent formation of Abeta ion channels We describe experiments, by ourselves and others, that support the view that Abeta is cytotoxic largely due to the action of Abeta channels in the cell membrane. The interaction of Abeta with the surface of the cell membrane may results in the activation of a chain of processes that, when large enough, become cytotoxic and induce cell death by apoptosis. Remarkably, the blockage of Abeta ion channels at the surface of the cell absolutely prevents the activation of these processes at different intracellular levels, thereby preserving the life of the cells. As a prospect for therapy for Alzheimer's disease, our findings at cellular level may be testable on AD animal models to elucidate the potential role and the magnitude of the contribution of the Abeta channels for induction of the disease.

beta-Sheet structured beta-amyloid(1-40) perturbs phosphatidylcholine model membranes

The disruption of intracellular calcium homeostasis plays a central role in the pathology of Alzheimer's disease, which is also characterized by accumulation of the amyloid-beta peptides Abeta40 and Abeta42. These amphipathic peptides may become associated with neuronal membranes and affect their barrier function, resulting in the loss of calcium homeostasis. This suggestion has been extensively investigated by exposing protein-free model membranes, either vesicles or planar bilayers, to soluble Abeta. Primarily unstructured Abeta has been shown to undergo a membrane-induced conformational change to either primarily beta-structure or helical structure, depending, among other factors, on the model membrane composition. Association of Abeta renders lipid bilayers permeable to ions but there is dispute whether this is due to the formation of discrete transmembrane ion channels of Abeta peptides, or to a non-specific perturbation of bilayer integrity by lipid head group-associated Abeta. Here, we have attempted incorporation of Abeta in the hydrophobic core of zwitterionic bilayers, the most simple model membrane system, by preparing proteoliposomes by hydration of a mixed film of Abeta peptides and phosphatidylcholine (PC) lipids. Despite the use of a solvent mixture in which Abeta40 and Abeta42 are almost entirely helical, the Abeta analogs were beta-structured in the resulting vesicle dispersions. When Abeta40-containing vesicles were fused into a zwitterionic planar bilayer, the typical irregular "single channel-like" conductance of Abeta was observed. The maximum conductance increased with additional vesicle fusion, while still exhibiting single channel-like behavior. Supported bilayers formed from Abeta40/PC vesicles did not exhibit any channel-like topological features, but the bilayer destabilized in time. Abeta40 was present primarily as beta-sheets in supported multilayers formed from the same vesicles. The combined observations argue for a non-specific perturbation of zwitterionic bilayers by surface association of small amphipathic Abeta40 assemblies.

Natively folded HypF-N and its early amyloid aggregates interact with phospholipid monolayers and destabilize supported phospholipid bilayers

Recent data depict membranes as the main sites where proteins/peptides are recruited and concentrated, misfold, and nucleate amyloids; at the same time, membranes are considered key triggers of amyloid toxicity. The N-terminal domain of the prokaryotic hydrogenase maturation factor HypF (HypF-N) in 30% trifluoroethanol undergoes a complex path of fibrillation starting with initial 2-3-nm oligomers and culminating with the appearance of mature fibrils. Oligomers are highly cytotoxic and permeabilize lipid membranes, both biological and synthetic. In this article, we report an in-depth study aimed at providing information on the surface activity of HypF-N and its interaction with synthetic membranes of different lipid composition, either in the native conformation or as amyloid oligomers or fibrils. Like other amyloidogenic peptides, the natively folded HypF-N forms stable films at the air/water interface and inserts into synthetic phospholipid bilayers with efficiencies depending on the type of phospholipid. In addition, HypF-N prefibrillar aggregates interact with, insert into, and disassemble supported phospholipid bilayers similarly to other amyloidogenic peptides. These results support the idea that, at least in most cases, early amyloid aggregates of different peptides and proteins produce similar effects on the integrity of membrane assembly and hence on cell viability.