Alzheimer disease amyloid beta protein forms calcium channels in bilayer membranes: blockade by tromethamine and aluminum

Aβ(1-42) tetramer and octamer structures reveal edge conductivity pores as a mechanism for membrane damage

Formation of amyloid-beta (Aβ) oligomer pores in the membrane of neurons has been proposed to explain neurotoxicity in Alzheimer's disease (AD). Here, we present the three-dimensional structure of an Aβ oligomer formed in a membrane mimicking environment, namely an Aβ(1-42) tetramer, which comprises a six stranded β-sheet core. The two faces of the β-sheet core are hydrophobic and surrounded by the membrane-mimicking environment while the edges are hydrophilic and solvent-exposed. By increasing the concentration of Aβ(1-42) in the sample, Aβ(1-42) octamers are also formed, made by two Aβ(1-42) tetramers facing each other forming a β-sandwich structure. Notably, Aβ(1-42) tetramers and octamers inserted into lipid bilayers as well-defined pores. To establish oligomer structure-membrane activity relationships, molecular dynamics simulations were carried out. These studies revealed a mechanism of membrane disruption in which water permeation occurred through lipid-stabilized pores mediated by the hydrophilic residues located on the core β-sheets edges of the oligomers.

Annexin A5 prevents amyloid-β-induced toxicity in choroid plexus: implication for Alzheimer's disease

In Alzheimer’s disease (AD) amyloid-β (Aβ) deposits may cause impairments in choroid plexus, a specialised brain structure which forms the blood–cerebrospinal fluid (CSF) barrier. We previously carried out a mass proteomic-based study in choroid plexus from AD patients and we found several differentially regulated proteins compared with healthy subjects. One of these proteins, annexin A5, was previously demonstrated implicated in blocking Aβ-induced cytotoxicity in neuronal cell cultures. Here, we investigated the effects of annexin A5 on Aβ toxicity in choroid plexus. We used choroid plexus tissue samples and CSF from mild cognitive impairment (MCI) and AD patients to analyse Aβ accumulation, cell death and annexin A5 levels compared with control subjects. Choroid plexus cell cultures from rats were used to analyse annexin A5 effects on Aβ-induced cytotoxicity. AD choroid plexus exhibited progressive reduction of annexin A5 levels along with progressive increased Aβ accumulation and cell death as disease stage was higher. On the other hand, annexin A5 levels in CSF from patients were found progressively increased as the disease stage increased in severity. In choroid plexus primary cultures, Aβ administration reduced endogenous annexin A5 levels in a time-course dependent manner and simultaneously increased annexin A5 levels in extracellular medium. Annexin A5 addition to choroid plexus cell cultures restored the Aβ-induced impairments on autophagy flux and apoptosis in a calcium-dependent manner. We propose that annexin A5 would exert a protective role in choroid plexus and this protection is lost as Aβ accumulates with the disease progression. Then, brain protection against further toxic insults would be jeopardised.

Trehalose against Alzheimer's Disease: Insights into a Potential Therapy

Trehalose is a natural disaccharide with a remarkable ability to stabilize biomolecules. In recent years, trehalose has received growing attention as a neuroprotective molecule and has been tested in experimental models for different neurodegenerative diseases. Although the underlying neuroprotective mechanism of trehalose's action is unclear, one of the most important hypotheses is autophagy induction. The chaperone-like activity of trehalose and the ability to modulate inflammatory responses has also been reported. There is compelling evidence that the dysfunction of autophagy and aggregation of misfolded proteins contribute to the pathogenesis of Alzheimer's disease (AD) and other neurodegenerative disorders. Therefore, given the linking between trehalose and autophagy induction, it appears to be a promising therapy for AD. Herein, the published studies concerning the use of trehalose as a potential therapy for AD are summarized, providing a rationale for applying trehalose to reduce Alzheimer's pathology.

Insoluble tau aggregates induce neuronal death through modification of membrane ion conductance, activation of voltage-gated calcium channels and NADPH oxidase

Most neurodegenerative disorders are associated with aggregation and accumulation of misfolded proteins. One of these proteins, tau, is involved in a number of pathologies including Alzheimer's disease and frontotemporal dementia. Aggregation and phosphorylation of tau have been shown to be a trigger for abnormal signal transduction and disruption of cellular homeostasis. Here, we have studied the effect of extracellular tau at different stages of aggregation in cortical co-cultures of neurons and astrocytes, to understand how this process affects tau pathogenicity. We found that the species formed after prolonged in vitro aggregation of tau (longer than 1 day) are able to stimulate reactive oxygen species (ROS) production through the activation of NADPH oxidase without decreasing the level of the endogenous antioxidant glutathione. The same late insoluble aggregates of tau induced calcium signals in neurons and a gradual increase in the ionic current of artificial membranes. Both tau-induced calcium signals and ROS production in NADPH oxidase were reduced in the presence of the inhibitor of voltage-gated calcium channels (VGCC) nifedipine. This suggests that insoluble aggregates of tau incorporate into the membrane and modify ionic currents, changing plasma membrane potential and activating VGCCs, which induces a calcium influx that triggers ROS production in NADPH oxidase. The combination of all these effects likely leads to toxicity, as only the same insoluble tau aggregates which demonstrated membrane-active properties produced neuronal cell death.

Medin Oligomer Membrane Pore Formation: A Potential Mechanism of Vascular Dysfunction

Medin, a 50-amino-acid cleavage product of the milk fat globule-EGF factor 8 protein, is one of the most common forms of localized amyloid found in the vasculature of individuals older than 50 years. Medin induces endothelial dysfunction and vascular inflammation, yet despite its prevalence in the human aorta and multiple arterial beds, little is known about the nature of its pathology. Medin oligomers have been implicated in the pathology of aortic aneurysm, aortic dissection, and more recently, vascular dementia. Recent in vitro biomechanical measurements found increased oligomer levels in aneurysm patients with altered aortic wall integrity. Our results suggest an oligomer-mediated toxicity mechanism for medin pathology. Using lipid bilayer electrophysiology, we show that medin oligomers induce ionic membrane permeability by pore formation. Pore activity was primarily observed for preaggregated medin species from the growth-phase and rarely for lag-phase species. Atomic force microscopy (AFM) imaging of medin aggregates at different stages of aggregation revealed the gradual formation of flat domains resembling the morphology of supported lipid bilayers. Transmission electron microscopy images showed the coexistence of compact oligomers, largely consistent with the AFM data, and larger protofibrillar structures. Circular dichroism spectroscopy revealed the presence of largely disordered species and suggested the presence of β-sheets. This observation and the significantly lower thioflavin T fluorescence emitted by medin aggregates compared to amyloid-β fibrils, along with the absence of amyloid fibers in the AFM and transmission electron microscopy images, suggest that medin aggregation into pores follows a nonamyloidogenic pathway. In silico modeling by molecular dynamics simulations provides atomic-level structural detail of medin pores with the CNpNC barrel topology and diameters comparable to values estimated from experimental pore conductances.

Increased mitochondrial calcium levels associated with neuronal death in a mouse model of Alzheimer's disease

Mitochondria contribute to shape intraneuronal Ca2+ signals. Excessive Ca2+ taken up by mitochondria could lead to cell death. Amyloid beta (Aβ) causes cytosolic Ca2+ overload, but the effects of Aβ on mitochondrial Ca2+ levels in Alzheimer's disease (AD) remain unclear. Using a ratiometric Ca2+ indicator targeted to neuronal mitochondria and intravital multiphoton microscopy, we find increased mitochondrial Ca2+ levels associated with plaque deposition and neuronal death in a transgenic mouse model of cerebral β-amyloidosis. Naturally secreted soluble Aβ applied onto the healthy brain increases Ca2+ concentration in mitochondria, which is prevented by blockage of the mitochondrial calcium uniporter. RNA-sequencing from post-mortem AD human brains shows downregulation in the expression of mitochondrial influx Ca2+ transporter genes, but upregulation in the genes related to mitochondrial Ca2+ efflux pathways, suggesting a counteracting effect to avoid Ca2+ overload. We propose lowering neuronal mitochondrial Ca2+ by inhibiting the mitochondrial Ca2+ uniporter as a novel potential therapeutic target against AD.

A Spectroscopic Marker for Structural Transitions Associated with Amyloid-β Aggregation

An amyloid aggregate evolves through a series of intermediates that have different secondary structures and intra- and intermolecular contacts. The structural parameters of these intermediates are important determinants of their toxicity. For example, the early oligomeric species of the amyloid-β (Aβ) peptide have been implicated as the most cytotoxic species in Alzheimer's disease but are difficult to identify because of their dynamic and transitory nature. Conventional aggregation monitors such as the fluorescent dye thioflavin T report on only the overall transition of the soluble species to the final amyloid fibrillar aggregated state. Here, we show that the fluorescent dye bis(triphenylphosphonium) tetraphenylethene (TPE-TPP) identifies at least three distinct aggregation intermediates of Aβ. Some atomic-level features of these intermediates are known from solid state nuclear magnetic resonance spectroscopy. Hence, the TPE-TPP fluorescence data may be interpreted in terms of these Aβ structural transitions. Steady state fluorescence and lifetime characteristics of TPE-TPP distinguish between the small oligomeric species (emission wavelength maximum, λ_max = 465 nm; average fluorescence lifetime, τ_Fl measured at 420 nm = 3.58 ± 0.04 ns), the intermediate species (λ_max = 452 nm; τ_Fl = 3.00 ± 0.03 ns), and the fibrils (λ_max = 406 nm; τ_Fl = 5.19 ± 0.08 ns). Thus, TPE-TPP provides a ready diagnostic for differentiating between the various, including the toxic, Aβ aggregates and potentially can be utilized to screen for amyloid aggregation inhibitors.

Aβ42 Double Mutant Inhibits Aβ42-Induced Plasma and Mitochondrial Membrane Disruption in Artificial Membranes, Isolated Organs, and Intact Cells

Destabilization of plasma and inner mitochondrial membranes by extra- and intracellular amyloid β peptide (Aβ42) aggregates may lead to dysregulated calcium flux through the plasma membrane, mitochondrial-mediated apoptosis, and neuronal cell death in patients with Alzheimer's disease. In the current study, experiments performed with artificial membranes, isolated mitochondria, and neuronal cells allowed us to understand the mechanism by which a nonaggregating Aβ42 double mutant (designated Aβ42_DM) exerts its neuroprotective effects. Specifically, we showed that Aβ42_DM protected neuronal cells from Aβ42-induced accumulation of toxic intracellular levels of calcium and from apoptosis. Aβ42_DM also inhibited Aβ42-induced mitochondrial membrane potential depolarization in the cells and abolished the Aβ42-mediated decrease in cytochrome c oxidase activity in purified mitochondrial particles. These results can be explained in terms of the amelioration by Aβ42_DM of Aβ42-mediated changes in membrane fluidity in DOPC and cardiolipin/DOPC phospholipid vesicles, mimicking plasma and mitochondrial membranes, respectively. These observations are also in agreement with the inhibition by Aβ42_DM of phospholipid-induced conformational changes in Aβ42 and with the fact that, unlike Aβ42, the Aβ42-Aβ42_DM complex could not permeate into cells but instead remained attached to the cell membrane. Although most of the Aβ42_DM molecules were localized on the cell membrane, some penetrated into the cytosol in an Aβ42-independent process, and, unlike Aβ42, did not form intracellular inclusion bodies. Overall, we provide a mechanistic explanation for the inhibitory activity of Aβ42_DM against Aβ42-induced membrane permeability and cell toxicity and provide confirmatory evidence for its protective function in neuronal cells.

Amyloids: Regulators of Metal Homeostasis in the Synapse

Conformational changes in amyloidogenic proteins, such as β-amyloid protein, prion proteins, and α-synuclein, play a critical role in the pathogenesis of numerous neurodegenerative diseases, including Alzheimer’s disease, prion disease, and Lewy body disease. The disease-associated proteins possess several common characteristics, including the ability to form amyloid oligomers with β-pleated sheet structure, as well as cytotoxicity, although they differ in amino acid sequence. Interestingly, these amyloidogenic proteins all possess the ability to bind trace metals, can regulate metal homeostasis, and are co-localized at the synapse, where metals are abundantly present. In this review, we discuss the physiological roles of these amyloidogenic proteins in metal homeostasis, and we propose hypothetical models of their pathogenetic role in the neurodegenerative process as the loss of normal metal regulatory functions of amyloidogenic proteins. Notably, these amyloidogenic proteins have the capacity to form Ca²⁺-permeable pores in membranes, suggestive of a toxic gain of function. Therefore, we focus on their potential role in the disruption of Ca²⁺ homeostasis in amyloid-associated neurodegenerative diseases.

ATR-IR and EPR spectroscopy for detecting the alterations in cortical synaptosomes induced by aluminium stress

Aluminium (Al) is reported to promote free radical production, decrease the antioxidant enzyme status and disturb the enzyme activity involved in acetylcholine metabolism leading to cognitive dysfunction that are strongly associated with Alzheimer's disease (AD) pathogenesis. This work aimed at investigating the effect of Al-toxicity on synaptosomal membrane biophysical properties and lipid peroxidation during 65 days. We utilized ATR-IR spectroscopy to study the changes in membrane biochemical structure and biophysical properties of isolated rat cortical synaptosomes, and EPR spin trapping and labeling to follow NADPH oxidase activity and changes of membrane order parameter, respectively. The results showed increase in membrane fluidity and disorder in early 21d of AlCl₃ treatment, while after 42d the membrane rigidity, packing, and order increased. The late (65d) an increase in the amount of unsaturated fatty acids, the accumulation of lipid peroxide end products, and ROS production were detected in rat cortex synaptosomes mediated by Al toxicity and oxidative stress (OS). A dramatic increase was also detected in Ca²⁺ level, synaptic membrane polarity, and EPR-detected order S-parameter. These outcomes strongly suggest that the synaptosomal membrane phospholipids underwent free radical attacks mediated by AlCl₃ due to greater NOX activity, and the release of synaptic vesicles into synaptic cleft might be hindered. The adopted spectroscopic techniques have shed light on the biomolecular structure and membrane biophysical changes of isolated cortical synaptosomes for the first time, allowing researchers to move closer to a complete understanding of pathological tissues.

The Na+/Ca2+exchanger in Alzheimer's disease

As a pivotal player in regulating sodium (Na⁺) and calcium (Ca²⁺) homeostasis and signalling in excitable cells, the Na⁺/Ca²⁺ exchanger (NCX) is involved in many neurodegenerative disorders in which an imbalance of intracellular Ca²⁺ and/or Na⁺ concentrations occurs, including Alzheimer's disease (AD). Although NCX has been mainly implicated in neuroprotective mechanisms counteracting Ca²⁺ dysregulation, several studies highlighted its role in the neuronal responses to intracellular Na⁺ elevation occurring in several pathophysiological conditions. Since the alteration of Na⁺ and Ca²⁺ homeostasis significantly contributes to synaptic dysfunction and neuronal loss in AD, it is of crucial importance to analyze the contribution of NCX isoforms in the homeostatic responses at neuronal and synaptic levels. Some studies found that an increase of NCX activity in brains of AD patients was correlated with neuronal survival, while other research groups found that protein levels of two NCX subtypes, NCX2 and NCX3, were modulated in parietal cortex of late stage AD brains. In particular, NCX2 positive synaptic terminals were increased in AD cohort while the number of NCX3 positive terminals were reduced. In addition, NCX1, NCX2 and NCX3 isoforms were up-regulated in those synaptic terminals accumulating amyloid-beta (Aβ), the neurotoxic peptide responsible for AD neurodegeneration. More recently, the hyperfunction of a specific NCX subtype, NCX3, has been shown to delay endoplasmic reticulum stress and apoptotic neuronal death in hippocampal neurons exposed to Aβ insult. Despite some issues about the functional role of NCX in synaptic failure and neuronal loss require further studies, these findings highlight the putative neuroprotective role of NCX in AD and open new strategies to develop new druggable targets for AD therapy.

Intense Pulsed Electric Fields Denature Urease Protein

Background: This article describes the effects of nanosecond pulsed electric fields (nsPEFs) on the structure and enzyme activity of three types of proteins. Materials and Methods: Intense (up to 300 kV/cm) 5-ns-long electrical pulses were applied for 500 times at 3 Hz to solutions of lysozyme, albumin, and urease. We analyzed covalent bonds (peptide bonds and disulfide bonds) of lysozyme and albumin, and also the tertiary and quaternary structures of urease as well as urease activity. Results: The results indicated deformation of both the quaternary and tertiary structures of urease upon exposure to an electric field with an amplitude of 250 kV/cm or higher, whereas no structural changes were observed in lysozyme or albumin, even at 300 kV/cm. The enzyme activity of urease also decreased at field strengths of 250 kV/cm or higher. Conclusion: Our experiments demonstrated that intense nsPEFs physically affected the conformation and function of some types of proteins. Such intense electric fields often occur in cell membranes when exposed to a moderate pulsed electric field.

Remodeling of Intracellular Ca2+ Homeostasis in Rat Hippocampal Neurons Aged In Vitro

Aging is often associated with a cognitive decline and a susceptibility to neuronal damage. It is also the most important risk factor for neurodegenerative disorders, particularly Alzheimer's disease (AD). AD is related to an excess of neurotoxic oligomers of amyloid β peptide (Aβo); however, the molecular mechanisms are still highly controversial. Intracellular Ca²⁺ homeostasis plays an important role in the control of neuronal activity, including neurotransmitter release, synaptic plasticity, and memory storage, as well as neuron cell death. Recent evidence indicates that long-term cultures of rat hippocampal neurons, resembling aged neurons, undergo cell death after treatment with Aβo, whereas short-term cultures, resembling young neurons, do not. These in vitro changes are associated with the remodeling of intracellular Ca²⁺ homeostasis with aging, thus providing a simplistic model for investigating Ca²⁺ remodeling in aging. In vitro aged neurons show increased resting cytosolic Ca²⁺ concentration, enhanced Ca²⁺ store content, and Ca²⁺ release from the endoplasmic reticulum (ER). Ca²⁺ transfer from the endoplasmic reticulum (ER) to mitochondria is also enhanced. Aged neurons also show decreased store-operated Ca²⁺ entry (SOCE), a Ca²⁺ entry pathway related to memory storage. At the molecular level, in vitro remodeling is associated with changes in the expression of Ca²⁺ channels resembling in vivo aging, including changes in N-methyl-D-aspartate NMDA receptor and inositol 1,4,5-trisphosphate (IP₃) receptor isoforms, increased expression of the mitochondrial calcium uniporter (MCU), and decreased expression of Orai1/Stim1, the molecular players involved in SOCE. Additionally, Aβo treatment exacerbates most of the changes observed in aged neurons and enhances susceptibility to cell death. Conversely, the solely effect of Aβo in young neurons is to increase ER-mitochondria colocalization and enhance Ca²⁺ transfer from ER to mitochondria without inducing neuronal damage. We propose that cultured rat hippocampal neurons may be a useful model to investigate Ca²⁺ remodeling in aging and in age-related neurodegenerative disorders.

Oligomers Are Promising Targets for Drug Development in the Treatment of Proteinopathies

Currently, there is no effective treatment of proteinopathies, as well as their diagnosis in the early stages of the disease until the first clinical symptoms appear. The proposed model of fibrillation of the Aβ peptide and its fragments not only describes molecular rearrangements, but also offers models of processes that occur during the formation of amyloid aggregates. Since this model is also characteristic of other proteins and peptides, a new potential target for drug development in the treatment of Alzheimer’s disease (AD) and other proteinopathies is proposed on the basis of this model. In our opinion, it is oligomers that are promising targets for innovative developments in the treatment of these diseases.

Reversal of Calcium Dysregulation as Potential Approach for Treating Alzheimer's Disease

Despite decades of research and effort, there is still no effective disease-modifying treatment for Alzheimer's Disease (AD). Most of the recent AD clinical trials were targeting amyloid pathway, but all these trials failed. Although amyloid pathology is a hallmark and defining feature of AD, targeting the amyloid pathway has been very challenging due to low efficacy and serious side effects. Alternative approaches or mechanisms for our understanding of the major cause of memory loss in AD need to be considered as potential therapeutic targets. Increasing studies suggest that Ca2+ dysregulation in AD plays an important role in AD pathology and is associated with other AD abnormalities, such as excessive inflammation, increased ROS, impaired autophagy, neurodegeneration, synapse, and cognitive dysfunction. Ca2+ dysregulation in cytosolic space, Endoplasmic Reticulum (ER) and mitochondria have been reported in the context of various AD models. Drugs or strategies, to correct the Ca2+ dysregulation in AD, have been demonstrated to be promising as an approach for the treatment of AD in preclinical models. This review will discuss the mechanisms of Ca2+ dysregulation in AD and associated pathology and discuss potential approaches or strategies to develop novel drugs for the treatment of AD by targeting Ca2+ dysregulation.

Amyloid-β Peptide-Lipid Bilayer Interaction Investigated by Supercritical Angle Fluorescence

The understanding of the interaction between the membrane of neurons and amyloid-β peptides is of crucial importance to shed light on the mechanism of toxicity in Alzheimer's disease. This paper describes how supercritical angle fluorescence spectroscopy was applied to monitor in real-time the interaction between a supported lipid bilayer (SLB) and the peptide. Different forms of amyloid-β (40 and 42 amino acids composition) were tested, and the interfacial fluorescence was measured to get information about the lipid integrity and mobility. The results show a concentration-dependent damaging process of the lipid bilayer. Prolonged interaction with the peptide up to 48 h lead to an extraction and clustering of lipid molecules from the surface and a potential disruption of the bilayer, correlated with the formation of peptide aggregates. The natural diffusion of the lipid was slightly hindered by the interaction with amyloid-β(1-42) and closely related to the oligomerization of the peptide. The adsorption and desorption of Amyloid-β was also characterized in terms of affinity. Amyloid-β(1-42) exhibited a slightly higher affinity than amyloid-β(1-40). The former was also more prone to aggregate and to adsorb on the bilayer as oligomer.

Accurate prediction of protein beta-aggregation with generalized statistical potentials

Motivation

Protein beta-aggregation is an important but poorly understood phenomena involved in diseases as well as in beneficial physiological processes. However, while this task has been investigated for over 50 years, very little is known about its mechanisms of action. Moreover, the identification of regions involved in aggregation is still an open problem and the state-of-the-art methods are often inadequate in real case applications.

Results

In this article we present AgMata, an unsupervised tool for the identification of such regions from amino acidic sequence based on a generalized definition of statistical potentials that includes biophysical information. The tool outperforms the state-of-the-art methods on two different benchmarks. As case-study, we applied our tool to human ataxin-3, a protein involved in Machado–Joseph disease. Interestingly, AgMata identifies aggregation-prone residues that share the very same structural environment. Additionally, it successfully predicts the outcome of in vitro mutagenesis experiments, identifying point mutations that lead to an alteration of the aggregation propensity of the wild-type ataxin-3.

Availability and implementation

A python implementation of the tool is available at https://bitbucket.org/bio2byte/agmata.

Supplementary information

Supplementary data are available at Bioinformatics online.

Exploring the Role of P2X Receptors in Alzheimer's Disease

Several studies have pointed to soluble oligomers of beta amyloid peptide (SOAβ) as the principal neurotoxic agents responsible for the generation of synaptotoxic events that can explain the main symptoms of Alzheimer’s disease (AD). Among the toxic features associated with SOAβ, one of the most notorious is the formation of a non-selective pore-like structure in the plasma membrane, which may partly explain the overload of intracellular Ca²⁺. There is evidence that the pore causes leakage of key intracellular compounds, such as adenosine triphosphate (ATP), to the extracellular milieu. Extracellular ATP activates P2X receptors (P2XR), which are ligand-gated ion channels (LGICs) widely expressed in both neuron and glial cells and act as neuromodulators of synaptic activity by promoting Ca²⁺ entry and facilitating neurotransmitter release. There is abundant evidence correlating the overexpression of these receptors to neurodegenerative diseases, including AD, thus opening the possibility that P2XR could potentiate the toxic mechanisms induced by SOAβ and contribute to intracellular Ca²⁺ overload in neurons and other mechanisms related to glial activation and inflammation. In this review, we correlate scientific evidence related to the main toxic effects induced by SOAβ and those that are mediated by purinergic P2XR. The data suggest that these purinergic receptors participate in the deleterious cellular and molecular effects of SOAβ that lead to the pathogenesis of AD. This information sheds light on the participation of new components in SOAβ toxicity that could be interesting as pharmacological targets for the development of molecular or chemical compounds able to modulate them.

Mathematical Models to Shed Light on Amyloid-Beta and Tau Protein Dependent Pathologies in Alzheimer's Disease

The number of patients suffering from dementia due to Alzheimer's disease (AD) is constantly rising worldwide. This has accordingly resulted in huge burdens on the health systems and involved families. Lack of profound understanding of neural networking in normal brain and their interruption in AD makes the treatment of this neurodegenerative multifaceted disease a challenging issue. In recent years, mathematical and computational methods have paved the way towards a better understanding of the brain functional connectivity. Thus, much attention has been paid to this matter from both basic science researchers and clinicians with an interdisciplinary approach to determine what is not functioning properly in AD patients and how this malfunctioning can be addressed. In this review, a number of AD-related articles and well-studied pathophysiologic topics (e.g., amyloid-beta, neurofibrillary tangles, Ca²⁺ dysregulation, and synaptic plasticity alterations) has been literally surveyed from a computational and systems biology point of view. The neural networks were discussed from biological and mathematical point of views and their alterations in recent findings were further highlighted. Application of the graph theoretical analysis in the brain imaging was reviewed, depicting the relations between brain structure and function, without diving into mathematical details. Moreover, differential rate equations were briefly articulated, emphasizing the potential use of these equations in simplifying complex processes in relevance to pathologies of AD. Comprehensive insights were given into the AD progression from neural networks perspective, which may lead us towards potential strategies for early diagnosis and effective treatment of AD.

The small heat shock proteins, HSPB1 and HSPB5, interact differently with lipid membranes

Increasing evidence shows that heat shock proteins (hsp) escape the cytosol gaining access to the extracellular environment, acting as signaling agents. Since the majority of these proteins lack the information necessary for their export via the classical secretory pathway, attention has been focused on alternative releasing mechanisms. Crossing the plasma membrane is a major obstacle to the secretion of a cytosolic protein into the extracellular milieu. Several mechanisms have been proposed, including direct interaction with the plasma membrane or their release within extracellular vesicles (ECV). HSPB1 (Hsp27), which belongs to the small hsp family, was detected within the membrane of ECV released from stressed HepG2 cells. To further investigate this finding, we studied the interaction of HSPB1 with lipid membranes using liposomes. We found that HSPB1 interacted with liposomes made of palmitoyl oleoyl phosphatidylserine (POPS), palmitoyl oleoyl phosphatidylcholine (POPC), and palmitoyl oleoyl phosphatidylglycerol (POPG), with different characteristics. Another member of the small hsp family, HSPB5 (αB-crystallin), has also been detected within ECV released from HeLa cells transfected with this gene. This protein was found to interact with liposomes as well, but differently than HSPB1. To address the regions interacting with the membrane, proteoliposomes were digested with proteinase K and the protected domains within the liposomes were identified by mass spectroscopy. We observed that large parts of HSPB1 and HSPB5 were embedded within the liposomes, particularly the alpha-crystallin domain. These observations suggest that the interaction with lipid membranes may be part of the mechanisms of export of these proteins.

Accumulation of Amyloid Beta (Aβ) Peptide on Blood Vessel Walls in the Damaged Brain after Transient Middle Cerebral Artery Occlusion

It is well known that amyloid beta (Aβ) peptides are generated in blood vessels, released into the brain during thrombosis, and temporarily accumulate in this organ after injury. Here we demonstrate that 24 h after transient middle cerebral artery occlusion (tMCAO), one of the standard models of focal ischemic stroke, Aβ peptide accumulates in the brain, concentrating on the blood vessel walls. Because Aβ oligomers are known to induce significant damage to brain cells, they act as an additional damaging factor during ischemic stroke. Considering that they have been shown to form ion channels in cells, affecting osmotic balance, we used an Aβ peptide channel blocker, tromethamine (2-amino-2-(hydroxymethyl) propane-1,3-diol), to prevent this additional injury. Tromethamine injected 0.1 g/100 g body weight intraperitoneally at 5 min before tMCAO decreased water content in the damaged hemisphere, as measured by dry brain weight. Congo red staining, which binds only to Aβ oligomer plaques (amyloid), showed that there was no significant presence of plaques. Therefore, we suggest that Aβ peptide oligomers are responsible for some of the brain damage during stroke and that blockage of the ion channels that they form could be beneficial in treating this complex neurological syndrome.

Cytotoxic species in amyloid-associated diseases: Oligomers or mature fibrils

Amyloid diseases especially, Alzheimer's disease (AD), is characterized by an imbalance between the production and clearance of amyloid-β (Aβ) species. Amyloidogenic proteins or peptides can transform structurally from monomers into β-stranded fibrils via multiple oligomeric states. Among various amyloid species, structured oligomers are proposed to be more toxic than fibrils; however, the identification of amyloid oligomers has been challenging due to their heterogeneous and metastable nature. Multiple techniques have recently helped in better understanding of oligomer's assembly details and structural properties. Moreover, some progress on elucidating the mechanisms of oligomer-triggered toxicity has been made. Based on the collection of current findings, there is growing consensus that control of toxic amyloid oligomers could be a valid approach to regulate amyloid-associated toxicity, which could advance development of new diagnostics and therapeutics for amyloid-related diseases. In this review, we have described the recent scenario of amyloid diseases with a great deal of information about the recent understanding of oligomers' assembly, structural properties, and toxicity. Also comprehensive details have been provided to differentiate the degree of toxicity associated with prefibrillar aggregates.

Preparation of a Well-Defined and Stable β-Barrel Pore-Forming Aβ42 Oligomer

The formation of amyloid-β peptide (Aβ) oligomers at the cellular membrane is considered a crucial process that underlies neurotoxicity in Alzheimer's disease (AD). To obtain structural information on this type of oligomers, we were inspired by membrane protein approaches used to stabilize, characterize, and analyze the function of such proteins. Using these approaches, we developed conditions under which Aβ42, the Aβ variant most strongly linked to the aetiology of AD, assembles into an oligomer that inserts into lipid bilayers as a well-defined pore and adopts a specific structure with characteristics of a β-barrel arrangement. We named this oligomer β-barrel Pore-Forming Aβ42 Oligomer (βPFO_Aβ42). Here, we describe detailed protocols for its preparation and characterization. We expect βPFO_Aβ42 to be useful in establishing the involvement of membrane-associated Aβ oligomers in AD.

Alzheimer's Disease as a Membrane Disorder: Spatial Cross-Talk Among Beta-Amyloid Peptides, Nicotinic Acetylcholine Receptors and Lipid Rafts

Biological membranes show lateral and transverse asymmetric lipid distribution. Cholesterol (Chol) localizes in both hemilayers, but in the external one it is mostly condensed in lipid-ordered microdomains (raft domains), together with saturated phosphatidyl lipids and sphingolipids (including sphingomyelin and glycosphingolipids). Membrane asymmetries induce special membrane biophysical properties and behave as signals for several physiological and/or pathological processes. Alzheimer’s disease (AD) is associated with a perturbation in different membrane properties. Amyloid-β (Aβ) plaques and neurofibrillary tangles of tau protein together with neuroinflammation and neurodegeneration are the most characteristic cellular changes observed in this disease. The extracellular presence of Aβ peptides forming senile plaques, together with soluble oligomeric species of Aβ, are considered the major cause of the synaptic dysfunction of AD. The association between Aβ peptide and membrane lipids has been extensively studied. It has been postulated that Chol content and Chol distribution condition Aβ production and posterior accumulation in membranes and, hence, cell dysfunction. Several lines of evidence suggest that Aβ partitions in the cell membrane accumulate mostly in raft domains, the site where the cleavage of the precursor AβPP by β- and γ- secretase is also thought to occur. The main consequence of the pathogenesis of AD is the disruption of the cholinergic pathways in the cerebral cortex and in the basal forebrain. In parallel, the nicotinic acetylcholine receptor has been extensively linked to membrane properties. Since its transmembrane domain exhibits extensive contacts with the surrounding lipids, the acetylcholine receptor function is conditioned by its lipid microenvironment. The nicotinic acetylcholine receptor is present in high-density clusters in the cell membrane where it localizes mainly in lipid-ordered domains. Perturbations of sphingomyelin or cholesterol composition alter acetylcholine receptor location. Therefore, Aβ processing, Aβ partitioning, and acetylcholine receptor location and function can be manipulated by changes in membrane lipid biophysics. Understanding these mechanisms should provide insights into new therapeutic strategies for prevention and/or treatment of AD. Here, we discuss the implications of lipid-protein interactions at the cell membrane level in AD.

Alzheimer's Amyloid-β is an Antimicrobial Peptide: A Review of the Evidence

The amyloid-β (Aβ) peptide has long been considered to be the driving force behind Alzheimer's disease (AD). However, clinical trials that have successfully reduced Aβ burden in the brain have not slowed the cognitive decline, and in some instances, have resulted in adverse outcomes. While these results can be interpreted in different ways, a more nuanced picture of Aβ is emerging that takes into account the facts that the peptide is evolutionarily conserved and is present throughout life in cognitively normal individuals. Recent evidence indicates a role for Aβ as an antimicrobial peptide (AMP), a class of innate immune defense molecule that utilizes fibrillation to protect the host from a wide range of infectious agents. In humans and in animal models, infection of the brain frequently leads to increased amyloidogenic processing of the amyloid-β protein precursor (AβPP) and resultant fibrillary aggregates of Aβ. Evidence from in vitro and in vivo studies demonstrates that Aβ oligomers have potent, broad-spectrum antimicrobial properties by forming fibrils that entrap pathogens and disrupt cell membranes. Importantly, overexpression of Aβ confers increased resistance to infection from both bacteria and viruses. The antimicrobial role of Aβ may explain why increased rates of infection have been observed in some of the AD clinical trials that depleted Aβ. Perhaps progress toward a cure for AD will accelerate once treatment strategies begin to take into account the likely physiological functions of this enigmatic peptide.

In vivo synaptic activity-independent co-uptakes of amyloid β1-42 and Zn2+ into dentate granule cells in the normal brain

Neuronal amyloid β_1–42 (Aβ_1–42) accumulation is considered an upstream event in Alzheimer’s disease pathogenesis. Here we report the mechanism on synaptic activity-independent Aβ_1–42 uptake in vivo. When Aβ_1–42 uptake was compared in hippocampal slices after incubating with Aβ_1–42, In vitro Aβ_1–42 uptake was preferentially high in the dentate granule cell layer in the hippocampus. Because the rapid uptake of Aβ_1–42 with extracellular Zn²⁺ is essential for Aβ_1–42-induced cognitive decline in vivo, the uptake mechanism was tested in dentate granule cells in association with synaptic activity. In vivo rapid uptake of Aβ_1–42 was not modified in the dentate granule cell layer after co-injection of Aβ_1–42 and tetrodotoxin, a Na⁺ channel blocker, into the dentate gyrus. Both the rapid uptake of Aβ_1–42 and Zn²⁺ into the dentate granule cell layer was not modified after co-injection of CNQX, an AMPA receptor antagonist, which blocks extracellular Zn²⁺ influx, Both the rapid uptake of Aβ_1–42 and Zn²⁺ into the dentate granule cell layer was not also modified after either co-injection of chlorpromazine or genistein, an endocytic repressor. The present study suggests that Aβ_1–42 and Zn²⁺ are synaptic activity-independently co-taken up into dentate granule cells in the normal brain and the co-uptake is preferential in dentate granule cells in the hippocampus. We propose a hypothesis that Zn-Aβ_1–42 oligomers formed in the extracellular compartment are directly incorporated into neuronal plasma membranes and form Zn²⁺-permeable ion channels.

The diphenylpyrazole compound anle138b blocks Aβ channels and rescues disease phenotypes in a mouse model for amyloid pathology

Alzheimer's disease is a devastating neurodegenerative disease eventually leading to dementia. An effective treatment does not yet exist. Here we show that oral application of the compound anle138b restores hippocampal synaptic and transcriptional plasticity as well as spatial memory in a mouse model for Alzheimer's disease, when given orally before or after the onset of pathology. At the mechanistic level, we provide evidence that anle138b blocks the activity of conducting Aβ pores without changing the membrane embedded Aβ-oligomer structure. In conclusion, our data suggest that anle138b is a novel and promising compound to treat AD-related pathology that should be investigated further.

Azelnidipine Attenuates the Oxidative and NFκB Pathways in Amyloid-β-Stimulated Cerebral Endothelial Cells

Cerebral amyloid angiopathy (CAA), a condition depicting cerebrovascular accumulation of amyloid β-peptide (Aβ), is a common pathological manifestation in Alzheimer's disease (AD). In this study, we investigated the effects of Azelnidipine (ALP), a dihydropyridine calcium channel blocker known for its treatment of hypertension, on oligomeric Aβ (oAβ)-induced calcium influx and its downstream pathway in immortalized mouse cerebral endothelial cells (bEND3). We found that ALP attenuated oAβ-induced calcium influx, superoxide anion production, and phosphorylation of extracellular signal-regulated kinase 1/2 (ERK1/2) and calcium-dependent cytosolic phospholipase A₂ (cPLA₂). Both ALP and cPLA₂ inhibitor, methylarachidonyl fluorophosphate (MAFP), suppressed oAβ-induced translocation of NFκB p65 subunit to nuclei, suggesting that cPLA₂ activation and calcium influx are essential for oAβ-induced NFκB activation. In sum, our results suggest that calcium channel blocker could be a potential therapeutic strategy for suppressing oxidative stress and inflammatory responses in Aβ-stimulated microvasculature in AD.

Concentration-dependent effects of mercury and lead on Aβ42: possible implications for Alzheimer's disease

Mercury (Hg) and lead (Pb) are known to be toxic non-radioactive elements, with well-described neurotoxicology. Much evidence supports the implication of metals as potential risk cofactors in Alzheimer's disease (AD). Although the action mechanism of the two metals remains unclear, Hg and Pb toxicity in AD could depend on their ability to favour misfolding and aggregation of amyloid beta proteins (Aβs) that seem to have toxic properties, particularly in their aggregated state. In our study, we evaluated the effect of Hg and Pb both on the Aβ42 ion channel incorporated in a planar lipid membrane made up of phosphatidylcholine containing 30% cholesterol and on the secondary structure of Aβ42 in an aqueous environment. The effects of Hg and Pb on the Aβ42 peptide were observed for its channel incorporated into a membrane as well as for the peptide in solution. A decreasing Aβ42 channel frequency and the formation of large and amorphous aggregates in solution that are prone to precipitate were both dependent on metal concentration. These experimental data suggest that Hg and Pb interact directly with Aβs, strengthening the hypothesis that the two metals may be a risk factor in AD.

What are the links between hypoxia and Alzheimer's disease?

Alzheimer's disease (AD) is the most common neurodegenerative disease. Histological characterization of amyloid plaques and neurofibrillary tangles in the brains of AD patients, alongside genetic studies in individuals suffering the familial form of the disease, has fueled the accumulation of the amyloid-β protein as the initial pathological trigger of disease. Association studies have recently showed that cerebral hypoxia, via both genetic and epigenetic mechanisms, increase amyloid-β deposition by altering expression levels of enzymes involved in the production/degradation of the protein. Furthermore, hypoxia has also been linked to neuronal and glial-cell calcium dysregulation through formation of calcium-permeable pores, dysregulated glutamate signaling, and intracellular calcium-store dysfunction. Hypoxia has also been strongly linked to neuroinflammation; however, this relationship to AD has not been thoroughly discussed in the literature. Here, we highlight and organize critical research evidence showing that in both hypoxic and AD brains, there are similarities in terms of 1) the substances mediating/modulating the neuroinflammatory environment and 2) the immune cells that drive the formation of these substances.

Computational Investigation of Amyloid Peptide Channels in Alzheimer’s Disease

Aβ accumulation has been discovered to form large, relatively cation-permeable channels in the plasma membrane of a neuron. These channel formations in the membranes of a neuron could cause cell depolarisation, sodium and potassium dysregulation, depletion of neural energy stores and other types of cellular dysfunction. This study shows that the build-up of amyloid beta (Aβ) depositions during the onset of Alzheimer’s disease has profound effects on the activity of the local community of neurons in the central nervous system. These effects can include enhanced neural activity, spontaneous epileptiform activity and incidence of epileptic seizures. From the results in this area, it can be seen that the neurodegeneration observed in Alzheimer’s disease has been associated with the increase of toxicity of Aβ depositions. In this research paper, we examined this hypothesis in light of a computational model of a neuron.

Low Phytanic Acid-Concentrated DHA Prevents Cognitive Deficit and Regulates Alzheimer Disease Mediators in an ApoE-/- Mice Experimental Model

Alzheimer’s disease (AD) is the main cause of dementia and cognitive impairment. It has been associated with a significant diminution of omega-3 polyunsaturated fatty acid docosahexaenoic acid (DHA) levels in the brain. Clinical trials with DHA as a treatment in neurological diseases have shown inconsistent results. Previously, we reported that the presence of phytanic acid (PhA) in standard DHA compositions could be blunting DHA’s beneficial effects. Therefore, we aimed to analyze the effects of a low PhA-concentrated DHA and a standard PhA-concentrated DHA in Apolipoprotein E knockout (ApoE^−/−) mice. Behavioral tests and protein expression of pro-inflammatory, pro-oxidant, antioxidant factors, and AD-related mediators were evaluated. Low PhA-concentrated DHA decreased Aβ, ß-amyloid precursor protein (APP), p-tau, Ca²⁺/calmodulin-dependent protein kinase II (CAMKII), caspase 3, and catalase, and increased brain derived neurotrophic factor (BDNF) when compared to standard PhA-concentrated DHA. Low PhA-concentrated DHA decreased interleukin (IL)-6 and tumor necrosis factor alpha (TNF-α) protein expression in ApoE^−/− mice when compared to standard PhA-concentrated DHA. No significant differences were found in p22phox, inducible nitric oxide synthase (iNOS), glutathione peroxidase (GPx), superoxide dismutase 1 (SOD-1), and tau protein expression. The positive actions of a low PhA-concentrated DHA were functionally reflected by improving the cognitive deficit in the AD experimental model. Therefore, reduction of PhA content in DHA compositions could highlight a novel pathway for the neurodegeneration processes related to AD.

Cyanidin-3-glucoside inhibits amyloid β25-35-induced neuronal cell death in cultured rat hippocampal neurons

Increasing evidence implicates changes in [Ca²⁺]_i and oxidative stress as causative factors in amyloid beta (Aβ)-induced neuronal cell death. Cyanidin-3-glucoside (C3G), a component of anthocyanin, has been reported to protect against glutamate-induced neuronal cell death by inhibiting Ca²⁺ and Zn²⁺ signaling. The present study aimed to determine whether C3G exerts a protective effect against Aβ_25–35-induced neuronal cell death in cultured rat hippocampal neurons from embryonic day 17 fetal Sprague-Dawley rats using MTT assay for cell survival, and caspase-3 assay and digital imaging methods for Ca²⁺, Zn²⁺, MMP and ROS. Treatment with Aβ_25–35 (20 µM) for 48 h induced neuronal cell death in cultured rat pure hippocampal neurons. Treatment with C3G for 48 h significantly increased cell survival. Pretreatment with C3G for 30 min significantly inhibited Aβ_25–35-induced [Zn²⁺]_i increases as well as [Ca²⁺]_i increases in the cultured rat hippocampal neurons. C3G also significantly inhibited Aβ_25–35-induced mitochondrial depolarization. C3G also blocked the Aβ_25–35-induced formation of ROS. In addition, C3G significantly inhibited the Aβ_25–35-induced activation of caspase-3. These results suggest that cyanidin-3-glucoside protects against amyloid β-induced neuronal cell death by reducing multiple apoptotic signals.

Dysregulation of Intracellular Calcium Signaling in Alzheimer's Disease

SIGNIFICANCE

Calcium (Ca²⁺) hypothesis of Alzheimer's disease (AD) gains popularity. It points to new signaling pathways that may underlie AD pathogenesis. Based on calcium hypothesis, novel targets for the development of potential AD therapies are identified. Recent Advances: Recently, the key role of neuronal store-operated calcium entry (nSOCE) in the development of AD has been described. Correct regulation of nSOCE is necessary for the stability of postsynaptic contacts to preserve the memory formation. Molecular identity of hippocampal nSOCE is defined. Perspective nSOCE-activating molecule, prototype of future anti-AD drugs, is described.

CRITICAL ISSUES

Endoplasmic reticulum Ca²⁺ overload happens in many but not in all AD models. The nSOCE targeting therapy described in this review may not be universally applicable.

FUTURE DIRECTIONS

There is a need to determine whether AD is a syndrome with one critical signaling pathway that initiates pathology, or it is a disorder with many different signaling pathways that are disrupted simultaneously or one after each other. It is necessary to validate applicability of nSOCE-activating therapy for the development of anti-AD medication. There is an experimental correlation between downregulated nSOCE and disrupted postsynaptic contacts in AD mouse models. Signaling mechanisms downstream of nSOCE which are responsible for the regulation of stability of postsynaptic contacts have to be discovered. That will bring new targets for the development of AD-preventing therapies. Antioxid. Redox Signal. 29, 1176-1188.

Could blocking the formation of amyloid channels rescue Alzheimer's phenotype?

In a most simplified way, we can say that much of the symptomatology that characterizes Alzheimer's disease (AD) can be attributed to a cascade of toxic events initiated by the presence in the interstitial space of the brain of oligomers of the β‐amyloid peptide (Aβ) peptide, a cleavage by‐product of the Amyloid precursor protein (APP). Intuitively, it follows that the amyloid peptide is the ideal target to combat this disease. However, several anti‐Aβ therapies failed in clinical trials devoted to find a treatment for AD. However, last year, the results of a clinical trial prompted back the interests in this type of therapy. In this issue of EMBO Molecular Medicine, Martinez Hernandez and colleagues present encouraging results showing that the diphenylpyrazole compound Anle138b prevents and reduces the toxic effects of Aβ in a mouse model of AD (APPPS1ΔE9). Regarding the mechanisms of action, they present good evidence that Anle138b prevents the formation of conducting Aβ pores on artificial membranes and primary hippocampal neurons. While the data are encouraging, AD mouse models only represent part of the AD pathology and clinical trials are needed to determine the usefulness of Anle138b to treat AD patients.

Photoactive chlorin e6 is a multifunctional modulator of amyloid-β aggregation and toxicity via specific interactions with its histidine residues

Photoactive chlorin e6 selectively damage the histidine residues of amyloid-β and reduce its aggregation and toxicity even in the presence of Cu ions.

The self-assembly of Aβ to β-sheet-rich neurotoxic oligomers is a main pathological event leading to Alzheimer's disease (AD). Selective targeting of Aβ oligomers without affecting other functional proteins is therefore an attractive approach to prevent the disease and its progression. In this study, we report that photodynamic treatment of Aβ in the presence of catalytic amounts of chlorin e6 can selectively damage Aβ and inhibit its aggregation and toxicity. Chlorin e6 also reversed the amyloid aggregation process in the dark by binding its soluble and low molecular weight oligomers, as shown by thioflavin T (ThT) fluorescence and photoinduced cross-linking of unmodified protein (PICUP) methods. Using HSQC NMR spectroscopy, ThT assays, amino acid analysis, SDS/PAGE, and EPR spectroscopy, we show that catalytic amounts of photoexcited chlorin e6 selectively damage the Aβ histidine residues H6, H13, and H14, and induce Aβ cross-linking by generating singlet oxygen. In contrast, photoexcited chlorin e6 was unable to cross-link ubiquitin and α-synuclein, demonstrating its high selectivity for Aβ. By binding to the Aβ histidine residues, catalytic amounts of chlorin e6 can also inhibit the Cu²⁺-induced aggregation and toxicity in darkness, while at stoichiometric amounts it acts as a chelator to reduce the amount of free Cu²⁺. This study demonstrates the great potential of chlorin e6 as a multifunctional agent for treatment of AD, and shows that the three N-terminal Aβ histidine residues are a suitable target for Aβ-specific drugs.

Phospholipids Critical Micellar Concentrations Trigger Different Mechanisms of Intrinsically Disordered Proteins Interaction with Model Membranes

Amyloidogenic proteins are involved in many diseases, including Alzheimer's, Parkinson's, and type II diabetes. These proteins are thought to be toxic for cells because of their abnormal interaction with the cell membrane. Simpler model membranes (LUVs) have been used to study the early steps of membrane-protein interactions and their subsequent evolution. Phospholipid LUVs formed in water solution establish a chemical equilibrium between self-assembled LUVs and a small amount of phospholipids in water solution (CMC). Here, using both experimental and molecular dynamics simulations approach we demonstrate that the insertion of IAPP, an amyloidogenic peptide involved in diabetes, in membranes is driven by free lipids in solution in dynamic equilibrium with the self-assembled lipids of the bilayer. It is suggested that this could be a general mechanism lying at the root of membrane insertion processes of self-assembling peptides.

Calcium signaling in Alzheimer's disease & therapies

Alzheimer's disease (AD) is the most common type of dementia and is characterized by the accumulation of amyloid (Aβ) plaques and neurofibrillary tangles in the brain. Much attention has been given to develop AD treatments based on the amyloid cascade hypothesis; however, none of these drugs had good efficacy at improving cognitive functions in AD patients suggesting that Aβ might not be the disease origin. Thus, there are urgent needs for the development of new therapies that target on the proximal cause of AD. Cellular calcium (Ca²⁺) signals regulate important facets of neuronal physiology. An increasing body of evidence suggests that age-related dysregulation of neuronal Ca²⁺ homeostasis may play a proximal role in the pathogenesis of AD as disrupted Ca²⁺ could induce synaptic deficits and promote the accumulation of Aβ plaques and neurofibrillary tangles. Given that Ca²⁺ disruption is ubiquitously involved in all AD pathologies, it is likely that using chemical agents or small molecules specific to Ca²⁺ channels or handling proteins on the plasma membrane and membranes of intracellular organelles to correct neuronal Ca²⁺ dysregulation could open up a new approach to AD prevention and treatment. This review summarizes current knowledge on the molecular mechanisms linking Ca²⁺ dysregulation with AD pathologies and discusses the possibility of correcting neuronal Ca²⁺ disruption as a therapeutic approach for AD.

Cross talk between neurometals and amyloidogenic proteins at the synapse and the pathogenesis of neurodegenerative diseases

Increasing evidence suggests that disruption of metal homeostasis contributes to the pathogenesis of various neurodegenerative diseases, including Alzheimer's disease, prion diseases, Lewy body diseases, and vascular dementia. Conformational changes of disease-related proteins (amyloidogenic proteins), such as β-amyloid protein, prion proteins, and α-synuclein, are well-established contributors to neurotoxicity and to the pathogenesis of these diseases. Recent studies have demonstrated that these amyloidogenic proteins are metalloproteins that bind trace elements, including zinc, iron, copper, and manganese, and play significant roles in the maintenance of metal homeostasis. We present a current review of the role of trace elements in the functions and toxicity of amyloidogenic proteins, and propose a hypothesis integrating metal homeostasis and the pathogenesis of neurodegenerative diseases that is focused on the interactions among metals and between metals and amyloidogenic proteins at the synapse, considering that these amyloidogenic proteins and metals are co-localized at the synapse.

Amyloid Beta Peptide Is Released during Thrombosis in the Skin

While it is known that amyloid beta (Aβ) deposits are found in different tissues of both Alzheimer’s disease (AD) patients and healthy individuals, there remain questions about the physiological role of these deposits, the origin of the Aβ peptide, and the mechanisms of its localization to the tissues. Using immunostaining with specific antibodies, as well as enzyme-linked immunosorbent assay, this study demonstrated Aβ40 peptide accumulation in the skin during local experimental photothrombosis in mice. Specifically, Aβ peptide accumulation was concentrated near the dermal blood vessels in thrombotic skin. It was also studied whether the released peptide affects microorganisms. Application of Aβ40 (4 µM) to the external membrane of yeast cells significantly increased membrane conductance with no visible effect on mouse host cells. The results suggest that Aβ release in the skin is related to skin injury and thrombosis, and occurs along with clotting whenever skin is damaged. These results support the proposition that Aβ release during thrombosis serves as part of a natural defense against infection.

Soluble Oligomers Require a Ganglioside to Trigger Neuronal Calcium Overload

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

Evidence from ITIR-FCS Diffusion Studies that the Amyloid-Beta (Aβ) Peptide Does Not Perturb Plasma Membrane Fluidity in Neuronal Cells

The amyloid-beta (Aβ) peptide, commonly found in elevated levels in the brains of patients with Alzheimer's disease (AD) and in the cerebrospinal fluid of individuals presenting mild cognitive impairment, is thought to be one of the major factors resulting in the onset of AD. Although observed and studied at the molecular level for several decades, the exact disease pathology of AD is still not totally clear. One way in which Aβ is thought to affect neurons is by influencing cell membrane fluidity, which could result in abnormal synaptic or signaling function. The effects of Aβ on the fluidity of biological membranes have been studied using numerous membrane models such as artificial lipid bilayers and vesicles, living cells and membranes extracted from animal models of AD, yet there is still no consensus as to what effects Aβ has, if any, on membrane fluidity. As one of the most precise and accurate means of assaying membrane dynamics, we have thus chosen fluorescence correlation spectroscopy to investigate the issue, using fluorescent membrane-targeted probes on living cells treated with Aβ(1-42) oligomers and observing possible changes in membrane diffusion. Effects of Aβ on viability in different cell types varied from no detectable effect to extensive cell death by 72 h post-exposure. However, there was no change in the fluidity of either ordered membrane domains or the bulk membrane in any of these cells within this period. Our conclusion from these results is that perturbation of membrane fluidity is not likely to be a factor in acute Aβ-induced cytotoxicity.

Stabilization of a Membrane-Associated Amyloid-β Oligomer for Its Validation in Alzheimer's Disease

We have recently reported on the preparation of a membrane-associated β-barrel Pore-Forming Aβ42 Oligomer (βPFO_Aβ42). It corresponds to a stable and homogeneous Aβ42 oligomer that inserts into lipid bilayers as a well-defined pore and adopts a specific structure with characteristics of a β-barrel arrangement. As a follow-up of this work, we aim to establish βPFO_Aβ42's relevance in Alzheimer's disease (AD). However, βPFO_Aβ42 is formed under dodecyl phosphocholine (DPC) micelle conditions-intended to mimic the hydrophobic environment of membranes-which are dynamic. Consequently, dilution of the βPFO_Aβ42/DPC complex in a detergent-free buffer leads to dispersion of the DPC molecules from the oligomer surface, leaving the oligomer without the hydrophobic micelle belt that stabilizes it. Since dilution is required for any biological test, transfer of βPFO_Aβ42 from DPC micelles into another hydrophobic biomimetic membrane environment, that remains associated with βPFO_Aβ42 even under high dilution conditions, is a requisite for the validation of βPFO_Aβ42 in AD. Here we describe conditions for exchanging DPC micelles with amphipols (APols), which are amphipathic polymers designed to stabilize membrane proteins in aqueous solutions. APols bind in an irreversible but non-covalent manner to the hydrophobic surface of membrane proteins preserving their structure even under extreme dilution conditions. We tested three types of APols with distinct physical-chemical properties and found that the βPFO_Aβ42/DPC complex can only be trapped in non-ionic APols (NAPols). The characterization of the resulting βPFO_Aβ42/NAPol complex by biochemical tools and structural biology techniques allowed us to establish that the oligomer structure is maintained even under high dilution. Based on these findings, this work constitutes a first step towards the in vivo validation of βPFO_Aβ42 in AD.