Fresh and nonfibrillar amyloid beta protein(1-40) induces rapid cellular degeneration in aged human fibroblasts: evidence for AbetaP-channel-mediated cellular toxicity

Leveraging Plasma Membrane Repair Therapeutics for Treating Neurodegenerative Diseases

Plasma membrane repair is an essential cellular mechanism that reseals membrane disruptions after a variety of insults, and compromised repair capacity can contribute to the progression of many diseases. Neurodegenerative diseases are marked by membrane damage from many sources, reduced membrane integrity, elevated intracellular calcium concentrations, enhanced reactive oxygen species production, mitochondrial dysfunction, and widespread neuronal death. While the toxic intracellular effects of these changes in cellular physiology have been defined, the specific mechanism of neuronal death in certain neurodegenerative diseases remains unclear. An abundance of recent evidence indicates that neuronal membrane damage and pore formation in the membrane are key contributors to neurodegenerative disease pathogenesis. In this review, we have outlined evidence supporting the hypothesis that membrane damage is a contributor to neurodegenerative diseases and that therapeutically enhancing membrane repair can potentially combat neuronal death.

Current understanding of metal-dependent amyloid-β aggregation and toxicity

The discovery of effective therapeutics targeting amyloid-β (Aβ) aggregates for Alzheimer's disease (AD) has been very challenging, which suggests its complicated etiology associated with multiple pathogenic elements. In AD-affected brains, highly concentrated metals, such as copper and zinc, are found in senile plaques mainly composed of Aβ aggregates. These metal ions are coordinated to Aβ and affect its aggregation and toxicity profiles. In this review, we illustrate the current view on molecular insights into the assembly of Aβ peptides in the absence and presence of metal ions as well as the effect of metal ions on their toxicity.

A Comprehensive Insight into the Mechanisms of Dopamine in Disrupting Aβ Protofibrils and Inhibiting Aβ Aggregation

Fibrillary aggregates of amyloid-β (Aβ) are the pathological hallmark of Alzheimer's disease (AD). Clearing Aβ deposition or inhibiting Aβ aggregation is a promising approach to treat AD. Experimental studies reported that dopamine (DA), an important neurotransmitter, can inhibit Aβ aggregation and disrupt Aβ fibrils in a dose-dependent manner. However, the underlying molecular mechanisms still remain mostly elusive. Herein, we investigated the effect of DA on Aβ₄₂ protofibrils at three different DA-to-Aβ molar ratios (1:1, 2:1, and 10:1) using all-atom molecular dynamics simulations. Our simulations demonstrate that protonated DA at a DA-to-Aβ ratio of 2:1 exhibits stronger Aβ protofibril disruptive capacity than that at a molar-ratio of 1:1 by mostly disrupting the F4-L34-V36 hydrophobic core. When the ratio of DA-to-Aβ increases to 10:1, DA has a high probability to bind to the outer surface of protofibril and has negligible effect on the protofibril structure. Interestingly, at the same DA-to-Aβ ratio (10:1), a mixture of protonated (DA⁺) and deprotonated (DA⁰) DA molecules significantly disrupts Aβ protofibrils by the binding of DA⁰ to the F4-L34-V36 hydrophobic core. Replica-exchange molecular dynamics simulations of Aβ₄₂ dimer show that DA⁺ inhibits the formation of β-sheets, K28-A42/K28-D23 salt-bridges, and interpeptide hydrophobic interactions and results in disordered coil-rich Aβ dimers, which would inhibit the subsequent fibrillization of Aβ. Further analyses reveal that DA disrupts Aβ protofibril and prevents Aβ dimerization mostly through π-π stacking interactions with residues F4, H6, and H13, hydrogen bonding interactions with negatively charged residues D7, E11, E22 and D23, and cation-π interactions with residues R5. This study provides a complete picture of the molecular mechanisms of DA in disrupting Aβ protofibril and inhibiting Aβ aggregation, which could be helpful for the design of potent drug candidates for the treatment/intervention of AD.

The Cytotoxicity and Clearance of Mutant Huntingtin and Other Misfolded Proteins

Protein misfolding and aggregation are implicated in many neurodegenerative diseases. One of these diseases is Huntington's, which is caused by increased glutamine-encoding trinucleotide repeats within the Huntingtin gene. Like other misfolded proteins, mutated Huntingtin proteins with polyglutamine expansions are prone to aggregation. Misfolded proteins exist as soluble monomers, small aggregates, or as large insoluble inclusion bodies. Misfolded protein aggregates are believed to be cytotoxic by stressing the protein degradation machinery, disrupting membrane structure, or sequestering other proteins. We recently showed that expression of misfolded proteins lowers cellular free ubiquitin levels, which compromises the protein degradation machinery. Therefore, the efficient degradation of misfolded proteins is critical to preserve cell health. Cells employ two major mechanisms to degrade misfolded proteins. The first is the ubiquitin-proteasome system (UPS), which ubiquitinates and degrades misfolded proteins with the assistance of segregase Cdc48/p97. The UPS pathway is mainly responsible for the clearance of misfolded proteins present as monomers or smaller aggregates. The second pathway is macroautophagy/autophagy, in which protein aggregates or inclusion bodies are recruited into an autophagosome before transport to the vacuole/lysosome for degradation. This review is focused on the current understanding of the cytotoxicity of misfolded proteins as well as their clearance pathways, with a particular emphasis on mutant Huntingtin.

Formaldehyde-Crosslinked Nontoxic Aβ Monomers to Form Toxic Aβ Dimers and Aggregates: Pathogenicity and Therapeutic Perspectives

Alzheimer's disease (AD) is characterized by the presence of senile plaques in the brain. However, medicines targeting amyloid-beta (Aβ) have not achieved the expected clinical effects. This review focuses on the formation mechanism of the Aβ dimer (the basic unit of oligomers and fibrils) and its tremendous potential as a drug target. Recently, age-associated formaldehyde and Aβ-derived formaldehyde have been found to crosslink the nontoxic Aβ monomer to form the toxic dimers, oligomers and fibrils. Particularly, Aβ-induced formaldehyde accumulation and formaldehyde-promoted Aβ aggregation form a vicious cycle. Subsequently, formaldehyde initiates Aβ toxicity in both the early-and late-onset AD. These facts also explain why AD drugs targeting only Aβ do not have the desired therapeutic effects. Development of the nanoparticle-based medicines targeting both formaldehyde and Aβ dimer is a promising strategy for improving the drug efficacy by penetrating blood-brain barrier and extracellular space into the cortical neurons in AD patients.

An Unbalanced Synaptic Transmission: Cause or Consequence of the Amyloid Oligomers Neurotoxicity?

Amyloid-β (Aβ) 1-40 and 1-42 peptides are key mediators of synaptic and cognitive dysfunction in Alzheimer's disease (AD). Whereas in AD, Aβ is found to act as a pro-epileptogenic factor even before plaque formation, amyloid pathology has been detected among patients with epilepsy with increased risk of developing AD. Among Aβ aggregated species, soluble oligomers are suggested to be responsible for most of Aβ's toxic effects. Aβ oligomers exert extracellular and intracellular toxicity through different mechanisms, including interaction with membrane receptors and the formation of ion-permeable channels in cellular membranes. These damages, linked to an unbalance between excitatory and inhibitory neurotransmission, often result in neuronal hyperexcitability and neural circuit dysfunction, which in turn increase Aβ deposition and facilitate neurodegeneration, resulting in an Aβ-driven vicious loop. In this review, we summarize the most representative literature on the effects that oligomeric Aβ induces on synaptic dysfunction and network disorganization.

Single Point Mutation from E22-to-K in Aβ Initiates Early-Onset Alzheimer's Disease by Binding with Catalase

Amyloid-beta (Aβ) is a critical etiological factor for late-onset familial Alzheimer's disease (AD). However, an early-onset AD has been found to be related with an Aβ mutation in glutamic acid 22-to-lysine (Italian type E22K). Why only one single point mutation at E22 residue induces AD remains unclear. Here, we report that a Chinese familial AD pedigree with E22K mutation was associated with higher levels of serum hydrogen peroxide (H₂O₂) and lower activity of catalase (a H₂O₂ degrading enzyme) than controls. Further, we found that E22K binding with catalase caused more severe H₂O₂ accumulation in the brains of E22K-injected rats than Aβ-injected rats. Unexpectedly, H₂O₂ bound with the mutation site 22K residue of E22K and elicited more rapid aggregation of E22K than Aβ in vitro. Moreover, H₂O₂ acted with E22K synergistically to induce higher cellular toxicity than with Aβ. Notably, intrahippocampal infusion of E22K led to more severe plaque deposition, neuron death, and more rapid memory decline than Aβ-injected rats. However, L-cysteine, a H₂O₂ scavenger, not only prevented self-aggregation of E22K but also reduced H₂O₂-promoted E22K assembly in vitro; subsequently, it alleviated Alzheimer-related phenotypes. Hence, E22K binding with catalase promotes the early onset of familial AD, and L-cys may reverse this disease.

A missense mutation sheds light on a novel structure-function relationship of RANKL

The tumor necrosis factor (TNF)-like core domain of receptor activator of nuclear factor-κB ligand (RANKL) is a functional domain critical for osteoclast differentiation. One of the missense mutations identified in patients with osteoclast-poor autosomal recessive osteopetrosis (ARO) is located in residue methionine 199 that is replaced with lysine (M199K) amid the TNF-like core domain. However, the structure-function relationship of this mutation is not clear. Sequence-based alignment revealed that the fragment containing human M199 is highly conserved and equivalent to M200 in rat. Using site-directed mutagenesis, we generated three recombinant RANKL mutants M200K/A/E (M200s) by replacing the methionine 200 with lysine (M200K), alanine (M200A), and glutamic acid (M200E), representative of distinct physical properties. TRAcP staining and bone pit assay showed that M200s failed to support osteoclast formation and bone resorption, accompanied by impaired osteoclast-related signal transduction. However, no antagonistic effect was found in M200s against wild-type rat RANKL. Analysis of the crystal structure of RANKL predicted that this methionine residue is located within the hydrophobic core of the protein, thus, likely to be crucial for protein folding and stability. Consistently, differential scanning fluorimetry analysis suggested that M200s were less stable. Western blot analysis analyses further revealed impaired RANKL trimerization by M200s. Furthermore, receptor-ligand binding assay displayed interrupted interaction of M200s to its intrinsic receptors. Collectively, our studies revealed the molecular basis of human M199-induced ARO and elucidated the indispensable role of rodent residue M200 (equivalent to human M199) for the RANKL function.

Aggregation and Cellular Toxicity of Pathogenic or Non-pathogenic Proteins

More than 20 unique diseases such as diabetes, Alzheimer’s disease, Parkinson’s disease are caused by the abnormal aggregations of pathogenic proteins such as amylin, β-amyloid (Aβ), and α-synuclein. All pathogenic proteins differ from each other in biological function, primary sequences, and morphologies; however, the proteins are toxic when aggregated. Here, we investigated the cellular toxicity of pathogenic or non-pathogenic protein aggregates. In this study, six proteins were selected and they were incubated at acid pH and high temperature. The aggregation kinetic and cellular toxicity of protein species with time were characterized. Three non-pathogenic proteins, bovine serum albumin (BSA), catalase, and pepsin at pH 2 and 65 °C were stable in protein structure and non-toxic at a lower concentration of 1 mg/mL. They formed aggregates at a higher concentration of 20 mg/mL with time and they induced the toxicity in short incubation time points, 10 min and 20 min only and they became non-toxic after 30 min. Other three pathogenic proteins, lysozyme, superoxide dismutase (SOD), and insulin, also produced the aggregates with time and they caused cytotoxicity at both 1 mg/mL and 20 mg/mL after 10 min. TEM images and DSC analysis demonstrated that fibrils or aggregates at 1 mg/mL induced cellular toxicity due to low thermal stability. In DSC data, fibrils or aggregates of pathogenic proteins had low thermal transition compared to fresh samples. The results provide useful information to understand the aggregation and cellular toxicity of pathogenic and non-pathogenic proteins.

Cholesterol Modulates the Formation of the Aβ Ion Channel in Lipid Bilayers

The misfolding of amyloid beta (Aβ) is one of the predominant hallmarks in the pathology of Alzheimer's disease (AD). In this study, we showed that the formation of the Aβ ion channel on the membrane depended on the cholesterol concentration. From a mechanical aspect, we found that cholesterol levels affected the stability and assembly of lipid bilayers. Measurements on planar lipid bilayers indicated that a small amount of cholesterol interacted with Aβ proteins and promoted the insertion process. Conversely, high cholesterol integrated the lipid bilayer and exerted an opposite effect on Aβ insertion. The Aβ ion channel was then detected by graphene-based field-effect transistors. Results demonstrated that the Aβ ion channel promoted a Ca²⁺ flux in the presence of 15% cholesterol but prevented a Ca²⁺ flux in high cholesterol. Thus, cholesterol had a complex impact on the Aβ ion channel that can be described as two different effects. First, a small amount of cholesterol interacted with Aβ and facilitated the Aβ ion channel formation in the membrane. Second, a large amount of cholesterol did not induce the ion flux in the membrane, which can be explained by the cholesterol damage to the regular distribution of the lipid bilayer. Overall, this study suggested a possible approach to consider cholesterol levels for the treatment of AD patients.

Metal binding to the amyloid-β peptides in the presence of biomembranes: potential mechanisms of cell toxicity

The amyloid-β (Aβ) peptides are key molecules in Alzheimer's disease (AD) pathology. They interact with cellular membranes, and can bind metal ions outside the membrane. Certain oligomeric Aβ aggregates are known to induce membrane perturbations and the structure of these oligomers-and their membrane-perturbing effects-can be modulated by metal ion binding. If the bound metal ions are redox active, as e.g., Cu and Fe ions are, they will generate harmful reactive oxygen species (ROS) just outside the membrane surface. Thus, the membrane damage incurred by toxic Aβ oligomers is likely aggravated when redox-active metal ions are present. The combined interactions between Aβ oligomers, metal ions, and biomembranes may be responsible for at least some of the neuronal death in AD patients.

Zn2+ Interaction with Amyloid-Β: Affinity and Speciation

Conflicting values, obtained by different techniques and often under different experimental conditions have been reported on the affinity of Zn²⁺ for amyloid-β, that is recognized as the major interaction responsible for Alzheimer’s disease. Here, we compare the approaches employed so far, i.e., the evaluation of K_d and the determination of the stability constants to quantitatively express the affinity of Zn²⁺ for the amyloid-β peptide, evidencing the pros and cons of the two approaches. We also comment on the different techniques and conditions employed that may lead to divergent data. Through the analysis of the species distribution obtained for two selected examples, we show the implications that the speciation, based on stoichiometric constants rather than on K_d, may have on data interpretation. The paper also demonstrates that the problem is further complicated by the occurrence of multiple equilibria over a relatively narrow pH range.

Journey of cystatins from being mere thiol protease inhibitors to at heart of many pathological conditions

Cystatins are thiol proteinase inhibitors (TPI), present ubiquitously in animals, plants and micro-organisms. These are not merely inhibitors rather they are at heart of many pathological conditions ranging from diabetes to renal failure. These are essential for maintenance of protein balance of the cell; once this balance gets disturbed, it may lead to cell death. Thus, cystatins cannot be merely regarded as TPI's as these have been found to play a pivotal role in tumorigenesis and neurodegenerative diseases. Many studies have reported the variation in cystatin level in incidences of different types of cancer; highlighting an important role played by these inhibitors in cancer development and progression. Cystatin C is increasingly replacing creatinine as a biomarker of glomerular filtration rate (GFR) thereby highlighting the importance of this important inhibitor. Some recent studies have also reported the interaction pattern of various anti-cancer drugs with cystatins in a bid to find how these drugs affect this important inhibitors and whether these drugs have any side effect on cystatins. Thus, in this growing disease era it can be said that cystatins are no more just inhibitors blocking the activity of thiol proteases rather they play a pivotal role in variety of pathological conditions.

The Pathogenesis of Alzheimers Disease—Is It a Lifelong “Calciumopathy”?

Alzheimer's disease (AD) is a fatal neurodegenerative disorder that has no known cure, nor is there a clear mechanistic understanding of the disease process itself. Although amyloid plaques, neurofibrillary tangles, and cognitive decline are late-stage markers of the disease, it is unclear how they are initially generated, and if they represent a cause, effect, or end phase in the pathology process. Recent studies in AD models have identified marked dysregulations in calcium signaling and related downstream pathways, which occur long before the diagnostic histopathological or cognitive changes. Under normal conditions, intracellular calcium signals are coupled to effectors that maintain a healthy physiological state. Consequently, sustained up-regulation of calcium may have pathophysiological consequences. Indeed, upon reviewing the current body of literature, increased calcium levels are functionally linked to the major features and risk factors of AD: ApoE4 expression, presenilin and APP mutations, beta amyloid plaques, hyperphosphorylation of tau, apoptosis, and synaptic dysfunction. In turn, the histopathological features of AD, once formed, are capable of further increasing calcium levels, leading to a rapid feed-forward acceleration once the disease process has taken hold. The views proposed here consider that AD pathogenesis reflects long-term calcium dysregulations that ultimately serve an enabling role in the disease process. Therefore, “Calcinists” do not necessarily reject βAptist or Tauist doctrine, but rather believe that their genesis is associated with earlier calcium signaling dysregulations. NEUROSCIENTIST 13(5):546—559, 2007.

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]

Single-cell screening of cytosolic [Ca(2+)] reveals cell-selective action by the Alzheimer's Aβ peptide ion channel

Interaction of the Alzheimer's Aβ peptides with the plasma membrane of cells in culture results in chronic increases in cytosolic [Ca(2+)]. Such increases can cause a variety of secondary effects leading to impaired cell growth or cell degeneration. In this investigation, we made a comprehensive study of the changes in cytosolic [Ca(2+)] in single PC12 cells and human neurons stressed by continuous exposure to a medium containing Aβ42 for several days. The differential timing and magnitude of the Aβ42-induced increase in [Ca(2+)] reveal subpopulations of cells with differential sensitivity to Aβ42. These results suggest that the effect produced by Aβ on the level of cytosolic [Ca(2+)] depends on the type of cell being monitored. Moreover, the results obtained of using potent inhibitors of Aβ cation channels such as Zn(2+) and the small peptide NA7 add further proof to the suggestion that the long-term increases in cytosolic [Ca(2+)] in cells stressed by continuous exposure to Aβ is the result of Aβ ion channel activity.

Neurotoxic amyloid beta oligomeric assemblies recreated in microfluidic platform with interstitial level of slow flow

Alzheimer's disease is accompanied by progressive, time-dependent changes of three moieties of amyloid beta. In vitro models therefore should provide same conditions for more physiologic studies. Here we observed changes in the number of fibrils over time and studied the correlation between amyloid beta moieties and neurotoxicity. Although the number of fibrils increased dramatically, the change in neurotoxicity with time was small, suggesting that fibrils make little contribution to neurotoxicity. To study the neurotoxicity of diffusible moieties by regulating microenvironments, we created a bio-mimetic microfluidic system generating spatial gradients of diffusible oligomeric assemblies and assessed their effects on cultured neurons. We found amyloid beta exposure produced an atrophy effect and observed neurite extension during the differentiation of neural progenitor cells increased when cells were cultured with continuous flow. The results demonstrate the potential neurotoxicity of oligomeric assemblies and establish a prospective microfluidic platform for studying the neurotoxicity of amyloid beta.

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

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

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

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

Long-term Aβ exposure augments mCa2+-independent mROS-mediated depletion of cardiolipin for the shift of a lethal transient mitochondrial permeability transition to its permanent mode in NARP cybrids: a protective targeting of melatonin

Mitochondrial dysfunction is a hallmark of amyloid β-peptide (Aβ)-induced neurodegeneration of Alzheimer's disease (AD). This study investigated whether mtDNA T8993G mutation-induced complex V inhibition, clinically associated with neurological muscle weakness, ataxia, and retinitis pigmentosa (NARP), is a potential risk factor for AD and the pathological link for long-term exposure of Aβ-induced mitochondrial toxicity and apoptosis in NARP cybrids. Using noninvasive fluorescence probe-coupled laser scanning imaging microscopy and NARP cybrids harboring 98% mutant genes along with its parental 143B osteosarcoma cells, we demonstrated that Aβ-augmented mitochondrial Ca(2+) (mCa(2+))-independent mitochondrial reactive oxygen species (mROS) formation for a cardiolipin (CL, a major mitochondrial protective phospholipid)-dependent lethal modulation of the mitochondrial permeability transition (MPT). Aβ augmented not only the amount but also the propagation rate of mROS-induced mROS formation to significantly depolarize mitochondrial membrane potential (∆Ψ(m)) and reduce mCa(2+) stress. Aβ-augmented mROS oxidized and depleted CL, thereby enhances mitochondrial fission and movement retardation, which promoted the NARP-augmented lethal transient-MPT (t-MPT) to switch to its irreversible mode of permanent-MPT (p-MPT). Interestingly, melatonin, a multiple mitochondrial protector, markedly reduced Aβ-augmented mROS formation and therefore significantly reduced mROS-mediated depolarization of ∆Ψ(m), fission of mitochondria and retardation of mitochondrial movement to stabilize CL and hence the MPT. In the presence of melatonin, Aβ-promoted p-MPT was reversed to a protective t-MPT, which preserved ∆Ψ(m) and lowered elevated mCa(2+) to sublethal levels for an enhanced mCa(2+)-dependent O(2) consumption. Thus, melatonin may potentially rescue AD patients associated with NARP symptoms.

Aggregation and fibril morphology of the Arctic mutation of Alzheimer's Aβ peptide by CD, TEM, STEM and in situ AFM

Morphology of aggregation intermediates, polymorphism of amyloid fibrils and aggregation kinetics of the "Arctic" mutant of the Alzheimer's amyloid β-peptide, Aβ((1-40))(E22G), in a physiologically relevant Tris buffer (pH 7.4) were thoroughly explored in comparison with the human wild type Alzheimer's amyloid peptide, wt-Aβ((1-40)), using both in situ atomic force and electron microscopy, circular dichroism and thioflavin T fluorescence assays. For arc-Aβ((1-40)) at the end of the 'lag'-period of fibrillization an abrupt appearance of ≈ 3 nm size 'spherical aggregates' with a homogeneous morphology, was identified. Then, the aggregation proceeds with a rapid growth of amyloid fibrils with a variety of morphologies, while the spherical aggregates eventually disappeared during in situ measurements. Arc-Aβ((1-40)) was also shown to form fibrils at much lower concentrations than wt-Aβ((1-40)): ≤ 2.5 μM and 12.5 μM, respectively. Moreover, at the same concentration, 50 μM, the aggregation process proceeds more rapidly for arc-Aβ((1-40)): the first amyloid fibrils were observed after c.a. 72 h from the onset of incubation as compared to approximately 7 days for wt-Aβ((1-40)). Amyloid fibrils of arc-Aβ((1-40)) exhibit a large variety of polymorphs, at least five, both coiled and non-coiled distinct fibril structures were recognized by AFM, while at least four types of arc-Aβ((1-40)) fibrils were identified by TEM and STEM and their mass-per-length statistics were collected suggesting supramolecular structures with two, four and six β-sheet laminae. Our results suggest a pathway of fibrillogenesis for full-length Alzheimer's peptides with small and structurally ordered transient spherical aggregates as on-pathway immediate precursors of amyloid fibrils.

Toxic fibrillar oligomers of amyloid-β have cross-β structure

Although amyloid fibers are found in neurodegenerative diseases, evidence points to soluble oligomers of amyloid-forming proteins as the cytotoxic species. Here, we establish that our preparation of toxic amyloid-β(1-42) (Abeta42) fibrillar oligomers (TABFOs) shares with mature amyloid fibrils the cross-β structure, in which adjacent β-sheets adhere by interpenetration of protein side chains. We study the structure and properties of TABFOs by powder X-ray diffraction, EM, circular dichroism, FTIR spectroscopy, chromatography, conformational antibodies, and celluar toxicity. In TABFOs, Abeta42 molecules stack into short protofilaments consisting of pairs of helical β-sheets that wrap around each other to form a superhelix. Wrapping results in a hole along the superhelix axis, providing insight into how Abeta may form pathogenic amyloid pores. Our model is consistent with numerous properties of Abeta42 fibrillar oligomers, including heterogenous size, ability to seed new populations of fibrillar oligomers, and fiber-like morphology.

Probing structural features of Alzheimer's amyloid-β pores in bilayers using site-specific amino acid substitutions

A current hypothesis for the pathology of Alzheimer's disease (AD) proposes that amyloid-β (Aβ) peptides induce uncontrolled, neurotoxic ion flux across cellular membranes. The mechanism of ion flux is not fully understood because no experiment-based Aβ channel structures at atomic resolution are currently available (only a few polymorphic states have been predicted by computational models). Structural models and experimental evidence lend support to the view that the Aβ channel is an assembly of loosely associated mobile β-sheet subunits. Here, using planar lipid bilayers and molecular dynamics (MD) simulations, we show that amino acid substitutions can be used to infer which residues are essential for channel structure. We created two Aβ(1-42) peptides with point mutations: F19P and F20C. The substitution of Phe19 with Pro inhibited channel conductance. MD simulation suggests a collapsed pore of F19P channels at the lower bilayer leaflet. The kinks at the Pro residues in the pore-lining β-strands induce blockage of the solvated pore by the N-termini of the chains. The cysteine mutant is capable of forming channels, and the conductance behavior of F20C channels is similar to that of the wild type. Overall, the mutational analysis of the channel activity performed in this work tests the proposition that the channels consist of a β-sheet rich organization, with the charged/polar central strand containing the mutation sites lining the pore, and the C-terminal strands facing the hydrophobic lipid tails. A detailed understanding of channel formation and its structure should aid studies of drug design aiming to control unregulated Aβ-dependent ion fluxes.

Membrane pores in the pathogenesis of neurodegenerative disease

The neurodegenerative diseases described in this volume, as well as many nonneurodegenerative diseases, are characterized by deposits known as amyloid. Amyloid has long been associated with these various diseases as a pathological marker and has been implicated directly in the molecular pathogenesis of disease. However, increasing evidence suggests that these proteinaceous Congo red staining deposits may not be toxic or destructive of tissue. Recent studies strongly implicate smaller aggregates of amyloid proteins as the toxic species underlying these neurodegenerative diseases. Despite the outward obvious differences among these clinical syndromes, there are some striking similarities in their molecular pathologies. These include dysregulation of intracellular calcium levels, impairment of mitochondrial function, and the ability of virtually all amyloid peptides to form ion-permeable pores in lipid membranes. Pore formation is enhanced by environmental factors that promote protein aggregation and is inhibited by agents, such as Congo red, which prevent aggregation. Remarkably, the pores formed by a variety of amyloid peptides from neurodegenerative and other diseases share a common set of physiologic properties. These include irreversible insertion of the pores in lipid membranes, formation of heterodisperse pore sizes, inhibition by Congo red of pore formation, blockade of pores by zinc, and a relative lack of ion selectivity and voltage dependence. Although there exists some information about the physical structure of these pores, molecular modeling suggests that 4-6-mer amyloid subunits may assemble into 24-mer pore-forming aggregates. The molecular structure of these pores may resemble the β-barrel structure of the toxics pore formed by bacterial toxins, such as staphylococcal α-hemolysin, anthrax toxin, and Clostridium perfringolysin.

Changes in the physiology of CA1 hippocampal pyramidal neurons in preplaque CRND8 mice

Amyloid-β protein (Aβ) is thought to play a central pathogenic role in Alzheimer's disease. Aβ can impair synaptic transmission, but little is known about the effects of Aβ on intrinsic cellular properties. Here we compared the cellular properties of CA1 hippocampal pyramidal neurons in acute slices from preplaque transgenic (Tg+) CRND8 mice and wild-type (Tg-) littermates. CA1 pyramidal neurons from Tg+ mice had narrower action potentials with faster decays than neurons from Tg- littermates. Action potential-evoked intracellular Ca(2+) transients in the apical dendrite were smaller in Tg+ than in Tg- neurons. Resting calcium concentration was higher in Tg+ than in Tg- neurons. The difference in action potential waveform was eliminated by low concentrations of tetraethylammonium ions and of 4-aminopyridine, implicating a fast delayed-rectifier potassium current. Consistent with this suggestion, there was a small increase in immunoreactivity for Kv3.1b in stratum radiatum in Tg+ mice. These changes in intrinsic properties may affect information flow through the hippocampus and contribute to the behavioral deficits observed in mouse models and patients with early-stage Alzheimer's disease.

Synaptic failure and adenosine triphosphate imbalance induced by amyloid-β aggregates are prevented by blueberry-enriched polyphenols extract

The potential neuroprotective properties of fruits have been widely recognized. In this study, we evaluated the protective properties of a blueberry extract (BB-4), rich in polyphenols, in a neurodegenerative model induced by amyloid-β peptide (Aβ). Chronic treatment with Aβ drastically reduced synaptic transmission and the extent of secretory vesicles, which were recovered partially with BB-4. Also, the extract recovered Ca(2+) transients in hippocampal neurons preincubated with Aβ (0.5 and 5 μM) by about 25% ± 3% and 30% ± 2, respectively. In this work, we demonstrate a novel effect of the BB-4 extract on Aβ-induced ATP leakage, in which this extract was able to antagonize the acute ATP leakage but not chronic ATP depletion. On the other hand, BB-4 prevented the uncoupling of mitochondrial function induced by FCCP by about 85%, but it was unable to modify the uncoupling induced by Aβ. The present results strongly indicate that BB-4 plays a role in the process of Aβ aggregation by reducing the toxic species (i.e., 40 kDa). These findings suggest that a blueberry extract can protect neuronal tissue from Aβ toxicity mainly through its antiaggregation property, and its antioxidant properties and mitochondrial membrane potential capacities are secondary mechanisms important in chronic stages. Our work suggests that BB-4 could be an important nutritional complement to neuronal health as well as a potential nutraceutical formulation useful as a dietary supplement in the elderly.

Alzheimer's disease and metals: a review of the involvement of cellular membrane receptors in metallosignalling

Alzheimer's disease (AD) is a debilitating form of dementia. The hallmark protein associated with the disease is the amyloid beta (Aβ) peptide. Aggregation of Aβ has been shown to depend on interactions with metals. The recent studies now demonstrate that metals also play additional important roles in the disease process. Consequently, there may be benefit from modulating metal homeostasis. However, the role and subcellular location of metals within neurons is not well understood. There is growing evidence to suggest that metals can act at the site of cellular membrane receptors and affect cellular signaling by modulating the signal transduction of those receptors. The glutamatergic and cholinergic receptor systems, both well-known neurotransmitter systems affected in AD, have well-documented metal interactions, as do the tropomyosin-receptor kinase (Trk) family of receptors and the epidermal growth factor (EGF) receptor. In this paper, the metal interactions with these membrane receptor systems will be explored and thus the potential for membrane receptors as an intervention point in AD will be assessed.

Polyhistidine peptide inhibitor of the Abeta calcium channel potently blocks the Abeta-induced calcium response in cells. Theoretical modeling suggests a cooperative binding process

On the basis of the consistent demonstrations that the Abeta peptide of Alzheimer's disease forms calcium permeant channels in artificial membranes, we have proposed that the intracellular calcium increase observed in cells exposed to Abeta is initiated by calcium fluxes through Abeta channels. We have found that a small four-histidine peptide, NAHis04, potently inhibits the Abeta-induced calcium channel currents in artificial lipid membranes. Here we report that NaHis04 also potently blocks the intracellular calcium increase which is observed in cells exposed to Abeta. PC12 cells loaded with Fura-2AM show a rapid increase in fluorescence and a rapid return to baseline after Abeta is added to the medium. This fluorescence change occurs even when the medium contains nitrendipine, a voltage-gated calcium channel blocker, but fails to occur when application of Abeta is preceded by addition of NAHis04. Steep dose-response curves of the percentage of responding cells and cell viability show that NAHis04 inhibits in the micromolar range in an apparently cooperative manner. We have developed numerous models of Abeta pores in which the first part of the Abeta sequence forms a large beta-barrel ending at His 13. We have modeled how up to four NAHis04 peptides may block these types of pores by binding to side chains of Abeta residues Glu 11, His 13, and His 14.

β-Barrel topology of Alzheimer's β-amyloid ion channels

Emerging evidence supports the ion channel mechanism for Alzheimer's disease pathophysiology wherein small β-amyloid (Aβ) oligomers insert into the cell membrane, forming toxic ion channels and destabilizing the cellular ionic homeostasis. Solid-state NMR-based data of amyloid oligomers in solution indicate that they consist of a double-layered β-sheets where each monomer folds into β-strand-turn-β-strand and the monomers are stacked atop each other. In the membrane, Aβ peptides are proposed to be β-type structures. Experimental structural data available from atomic force microscopy (AFM) imaging of Aβ oligomers in membranes reveal heterogeneous channel morphologies. Previously, we modeled the channels in a non-tilted organization, parallel with the cross-membrane normal. Here, we modeled a β-barrel-like organization. β-Barrels are common in transmembrane toxin pores, typically consisting of a monomeric chain forming a pore, organized in a single-layered β-sheet with antiparallel β-strands and a right-handed twist. Our explicit solvent molecular dynamics simulations of a range of channel sizes and polymorphic turns and comparisons of these with AFM image dimensions support a β-barrel channel organization. Different from the transmembrane β-barrels where the monomers are folded into a circular β-sheet with antiparallel β-strands stabilized by the connecting loops, these Aβ barrels consist of multimeric chains forming double β-sheets with parallel β-strands, where the strands of each monomer are connected by a turn. Although the Aβ barrels adopt the right-handed β-sheet twist, the barrels still break into heterogeneous, loosely attached subunits, in good agreement with AFM images and previous modeling. The subunits appear mobile, allowing unregulated, hence toxic, ion flux.

Aberrant subcellular neuronal calcium regulation in aging and Alzheimer's disease

In this mini-review/opinion article we describe evidence that multiple cellular and molecular alterations in Alzheimer's disease (AD) pathogenesis involve perturbed cellular calcium regulation, and that alterations in synaptic calcium handling may be early and pivotal events in the disease process. With advancing age neurons encounter increased oxidative stress and impaired energy metabolism, which compromise the function of proteins that control membrane excitability and subcellular calcium dynamics. Altered proteolytic cleavage of the β-amyloid precursor protein (APP) in response to the aging process in combination with genetic and environmental factors results in the production and accumulation of neurotoxic forms of amyloid β-peptide (Aβ). Aβ undergoes a self-aggregation process and concomitantly generates reactive oxygen species that can trigger membrane-associated oxidative stress which, in turn, impairs the functions of ion-motive ATPases and glutamate and glucose transporters thereby rendering neurons vulnerable to excitotoxicity and apoptosis. Mutations in presenilin-1 that cause early-onset AD increase Aβ production, but also result in an abnormal increase in the size of endoplasmic reticulum calcium stores. Some of the events in the neurodegenerative cascade can be counteracted in animal models by manipulations that stabilize neuronal calcium homeostasis including dietary energy restriction, agonists of glucagon-like peptide 1 receptors and drugs that activate mitochondrial potassium channels. Emerging knowledge of the actions of calcium upstream and downstream of Aβ provides opportunities to develop novel preventative and therapeutic interventions for AD. This article is part of a Special Issue entitled: 11th European Symposium on Calcium.

Structural properties and dynamic behavior of nonfibrillar oligomers formed by PrP(106-126)

The formation of nonfibrillar oligomers has been proposed as a common element of the aggregation pathway of proteins and peptides associated with neurodegenerative diseases such as Alzheimer's and Creutzfeldt-Jakob disease. While fibrillar structures have long been considered indicators of diseases linked with the accumulation of amyloid plaques, it has more recently been proposed that amyloid oligomers are in fact the cytotoxic form. Here we describe the local structure and dynamics of stable oligomers formed by a peptide comprising residues 106-126 of the human prion protein (PrP). Structural constraints from solid-state NMR reveal quaternary packing interactions within the hydrophobic core, similar to those previously reported for amyloid fibrils formed by this peptide, and consistent with structural studies of oligomers formed by the Alzheimer's beta-amyloid peptide. However, a hydration-dependent increase in disorder is observed for nonfibrillar oligomers of PrP(106-126). In solution NMR spectra we observe narrow (1)H and (13)C resonances corresponding to a monomer in exchange with the approximately 30 nm diameter nonfibrillar oligomers, giving additional information on the molecular structure of these species. Taken together, our data support a model in which the local structure of the oligomers contains the basic elements of amyloid fibrils, but with long-range disorder and local mobility that distinguishes these assemblies from the fibrillar form of PrP(106-126). These characteristics may provide a basis for the differing biological activities of amyloid fibrils and oligomers.

Amyloid-beta-induced neuronal dysfunction in Alzheimer's disease: from synapses toward neural networks

Alzheimer's disease is the most frequent neurodegenerative disorder and the most common cause of dementia in the elderly. Diverse lines of evidence suggest that amyloid-beta (Abeta) peptides have a causal role in its pathogenesis, but the underlying mechanisms remain uncertain. Here we discuss recent evidence that Abeta may be part of a mechanism controlling synaptic activity, acting as a positive regulator presynaptically and a negative regulator postsynaptically. The pathological accumulation of oligomeric Abeta assemblies depresses excitatory transmission at the synaptic level, but also triggers aberrant patterns of neuronal circuit activity and epileptiform discharges at the network level. Abeta-induced dysfunction of inhibitory interneurons likely increases synchrony among excitatory principal cells and contributes to the destabilization of neuronal networks. Strategies that block these Abeta effects may prevent cognitive decline in Alzheimer's disease. Potential obstacles and next steps toward this goal are discussed.

Structural convergence among diverse, toxic beta-sheet ion channels

Recent studies show that an array of beta-sheet peptides, including N-terminally truncated Abeta peptides (Abeta(11-42/17-42)), K3 (a beta(2)-microglobulin fragment), and protegrin-1 (PG-1) peptides form ion channel-like structures and elicit single channel ion conductance when reconstituted in lipid bilayers and induce cell damage through cell calcium overload. Striking similarities are observed in the dimensions of these toxic channels irrespective of their amino acid sequences. However, the intriguing question of preferred channel sizes is still unresolved. Here, exploiting ssNMR-based, U-shaped, beta-strand-turn-beta-strand coordinates, we modeled truncated Abeta peptide (p3) channels with different sizes (12- to 36-mer). Molecular dynamics (MD) simulations show that optimal channel sizes of the ion channels presenting toxic ionic flux range between 16- and 24-mer. This observation is in good agreement with channel dimensions imaged by AFM for Abeta(9-42), K3 fragment, and PG-1 channels and highlights the bilayer-supported preferred toxic beta-channel sizes and organization, regardless of the peptide sequence.

The functional neurophysiology of the amyloid precursor protein (APP) processing pathway

Amyloid beta (Abeta) peptides derived from proteolytic cleavage of amyloid precursor protein (APP) are thought to be a pivotal toxic species in the pathogenesis of Alzheimer's disease (AD). Furthermore, evidence has been accumulating that components of APP processing pathway are involved in non-pathological normal function of the CNS. In this review we aim to cover the extensive body of research aimed at understanding how components of this pathway contribute to neurophysiological function of the CNS in health and disease. We briefly outline changes to clinical neurophysiology seen in AD patients before discussing functional changes in mouse models of AD which range from changes to basal synaptic transmission and synaptic plasticity through to abnormal synchronous network activity. We then describe the various neurophysiological actions that are produced by application of exogenous Abeta in various forms, and finally discuss a number or other neurophysiological aspects of the APP pathway, including functional activities of components of secretase complexes other than Abeta production.

Misfolded amyloid ion channels present mobile beta-sheet subunits in contrast to conventional ion channels

In Alzheimer's disease, calcium permeability through cellular membranes appears to underlie neuronal cell death. It is increasingly accepted that calcium permeability involves toxic ion channels. We modeled Alzheimer's disease ion channels of different sizes (12-mer to 36-mer) in the lipid bilayer using molecular dynamics simulations. Our Abeta channels consist of the solid-state NMR-based U-shaped beta-strand-turn-beta-strand motif. In the simulations we obtain ion-permeable channels whose subunit morphologies and shapes are consistent with electron microscopy/atomic force microscopy. In agreement with imaged channels, the simulations indicate that beta-sheet channels break into loosely associated mobile beta-sheet subunits. The preferred channel sizes (16- to 24-mer) are compatible with electron microscopy/atomic force microscopy-derived dimensions. Mobile subunits were also observed for beta-sheet channels formed by cytolytic PG-1 beta-hairpins. The emerging picture from our large-scale simulations is that toxic ion channels formed by beta-sheets spontaneously break into loosely interacting dynamic units that associate and dissociate leading to toxic ionic flux. This sharply contrasts intact conventional gated ion channels that consist of tightly interacting alpha-helices that robustly prevent ion leakage, rather than hydrogen-bonded beta-strands. The simulations suggest why conventional gated channels evolved to consist of interacting alpha-helices rather than hydrogen-bonded beta-strands that tend to break in fluidic bilayers. Nature designs folded channels but not misfolded toxic channels.

Metal loading capacity of Abeta N-terminus: a combined potentiometric and spectroscopic study of zinc(II) complexes with Abeta(1-16), its short or mutated peptide fragments and its polyethylene glycol-ylated analogue

Aggregation of the amyloid beta-peptide (Abeta) into insoluble fibrils is a key pathological event in Alzheimer's Disease (AD). There is now compelling evidence that metal binding to Abeta is involved in AD pathogenesis. The amino acid region 1-16 is widely considered as the metal binding domain of Abeta. In this work, we used a combined potentiometric, NMR, and electrospray ionization mass spectrometry (ESI-MS) approach to study the zinc(II) binding to a new polyethylene glycol (PEG)-conjugated peptide fragment encompassing the 1-16 amino acid sequence of Abeta (Abeta(1-16)PEG). Our results demonstrate for the first time that the Abeta(1-16) is able to coordinate up to three zinc ions, all the histidyl residues acting as independent anchor sites. The study was complemented by systematically investigating the zinc(II) complexes of a series of shorter peptide fragments related to the Abeta(1-16) sequence, namely, Abeta(1-4), Abeta(1-6), AcAbeta(1-6), AcAbeta(8-16)Y10A. The comparison of the whole results allowed the identification of the zinc(II) preferred binding sites within the longer Abeta(1-16) amino acid sequence. Unlike copper(II) that prefers the N-terminal amino group as the main binding site, the zinc(II) is preferentially placed in the 8-16 amino acidic region of Abeta(1-16).

Protein aggregation diseases: toxicity of soluble prefibrillar aggregates and their clinical significance

Amyloid diseases, the most clinically relevant protein misfolding pathologies due to the high prevalence of some of them in the population, are characterized by the presence, in specific tissues and organs, of fibrillar deposits of specific peptides or proteins. Increasing efforts are presently dedicated at investigating the structural features and the structure-toxicity relation of the soluble oligomeric precursors arising in the path of fibril formation. In fact, it is increasingly recognised that these unstable, dynamic assemblies are remarkably toxic to cells thus featuring these as the main factor responsible for cell impairment in amyloid diseases. This chapter will review shortly the data presently available on the structural and biochemical features of these assemblies, as well as on their biological and clinical significance.

Amyloid-beta-induced ion flux in artificial lipid bilayers and neuronal cells: resolving a controversy

Understanding the pathogenicity of amyloid-beta (Abeta) peptides constitutes a major goal in research on Alzheimer's disease (AD). One hypothesis entails that Abeta peptides induce uncontrolled, neurotoxic ion flux through cellular membranes. The exact biophysical mechanism of this ion flux is, however, a subject of an ongoing controversy which has attenuated progress toward understanding the importance of Abeta-induced ion flux in AD. The work presented here addresses two prevalent controversies regarding the nature of transmembrane ion flux induced by Alphabeta peptides. First, the results clarify that Alphabeta can induce stepwise ion flux across planar lipid bilayers as opposed to a gradual increase in transmembrane current; they show that the previously reported gradual thinning of membranes with concomitant increase in transmembrane current arises from residues of the solvent hexafluoroisopropanol, which is commonly used for the preparation of amyloid samples. Second, the results provide additional evidence suggesting that Abeta peptides can induce ion channel-like ion flux in cellular membranes that is independent from the postulated ability of Alphabeta to modulate intrinsic cellular ion channels or transporter proteins.

Protein folding and misfolding on surfaces

Protein folding, misfolding and aggregation, as well as the way misfolded and aggregated proteins affects cell viability are emerging as key themes in molecular and structural biology and in molecular medicine. Recent advances in the knowledge of the biophysical basis of protein folding have led to propose the energy landscape theory which provides a consistent framework to better understand how a protein folds rapidly and efficiently to the compact, biologically active structure. The increased knowledge on protein folding has highlighted its strict relation to protein misfolding and aggregation, either process being in close competition with the other, both relying on the same physicochemical basis. The theory has also provided information to better understand the structural and environmental factors affecting protein folding resulting in protein misfolding and aggregation into ordered or disordered polymeric assemblies. Among these, particular importance is given to the effects of surfaces. The latter, in some cases make possible rapid and efficient protein folding but most often recruit proteins/peptides increasing their local concentration thus favouring misfolding and accelerating the rate of nucleation. It is also emerging that surfaces can modify the path of protein misfolding and aggregation generating oligomers and polymers structurally different from those arising in the bulk solution and endowed with different physical properties and cytotoxicities.

The polyphenol piceid destabilizes preformed amyloid fibrils and oligomers in vitro: hypothesis on possible molecular mechanisms

Alzheimer's disease (AD) is characterized by deposits of amyloid in various tissues. The neuronal cytotoxicity of Abeta peptides is attributed not only to various mechanisms but also to amyloid fibrils and soluble oligomeric intermediates. Consequently, finding molecules to prevent or reverse the oligomerization and fibrillization of Abeta could be of therapeutic value in the treatment of AD. We show that piceid, a polyphenol of the stilbene family, destabilized fibrils and oligomers to give back monomers that are not neurotoxic molecules. The mechanism of this destabilization could be a dynamic interaction between the polyphenol and the Abeta that could open the hydrophobic zipper and shift the reversible equilibrium "random coil<-->beta-sheet" to the disordered structure.

Efficiency of histidine-associating compounds for blocking the alzheimer's Abeta channel activity and cytotoxicity

The opening of the Alzheimer's Abeta channel permits the flux of calcium into the cell, thus critically disturbing intracellular ion homeostasis. Peptide segments that include the characteristic histidine (His) diad, His(13) and His(14), efficiently block the Abeta channel activity, blocking Abeta cytotoxicity. We hypothesize that the vicinal His-His peptides coordinate with the rings of His in the mouth of the pore, thus blocking the flow of calcium ions through the channel, with consequent blocking of Abeta cytotoxicity. To test this hypothesis, we studied Abeta ion channel activity and cytotoxicity after the addition of compounds that are known to have His association capacity, such as Ni(2+), imidazole, His, and a series of His-related compounds. All compounds were effective at blocking both Abeta channel and preventing Abeta cytotoxicity. The efficiency of protection of His-related compounds was correlated with the number of imidazole side chains in the blocker compounds. These data reinforce the premise that His residues within the Abeta channel sequence are in the pathway of ion flow. Additionally, the data confirm the contribution of the Abeta channel to the cytotoxicity of exogenous Abeta.