The relationship between the aggregational state of the amyloid-beta peptides and free radical generation by the peptides

Current biomarkers and treatment strategies in Alzheimer disease: An overview and future perspectives

Alzheimer's disease (AD), a progressive degenerative disorder first identified by Alois Alzheimer in 1907, poses a significant public health challenge. Despite its prevalence and impact, there is currently no definitive ante mortem diagnosis for AD pathogenesis. By 2050, the United States may face a staggering 13.8 million AD patients. This review provides a concise summary of current AD biomarkers, available treatments, and potential future therapeutic approaches. The review begins by outlining existing drug targets and mechanisms in AD, along with a discussion of current treatment options. We explore various approaches targeting Amyloid β (Aβ), Tau Protein aggregation, Tau Kinases, Glycogen Synthase kinase-3β, CDK-5 inhibitors, Heat Shock Proteins (HSP), oxidative stress, inflammation, metals, Apolipoprotein E (ApoE) modulators, and Notch signaling. Additionally, we examine the historical use of Estradiol (E2) as an AD therapy, as well as the outcomes of Randomized Controlled Trials (RCTs) that evaluated antioxidants (e.g., vitamin E) and omega-3 polyunsaturated fatty acids as alternative treatment options. Notably, positive effects of docosahexaenoic acid nutriment in older adults with cognitive impairment or AD are highlighted. Furthermore, this review offers insights into ongoing clinical trials and potential therapies, shedding light on the dynamic research landscape in AD treatment.

Acetylcholinesterase Inhibitory and Antioxidant Activity of the Compounds Isolated from Vanda roxburghii

Vanda roxburghii has been used in traditional medicine to treat nervous system disorders including Alzheimer's disease (AD). We reported earlier a high acetylcholinesterase inhibitory and antioxidant activity in the chloroform fraction of this plant. Therefore, this study was designed to explore the compounds with acetylcholinesterase inhibitory and antioxidant activities from the chloroform fraction of Vanda roxburghii. Phytochemical investigation led to the isolation for the first time of a fatty acid ester: methyl linoleate (1), and three phenolics: syringaldehyde (2), vanillin (3), and dihydroconiferyl dihydro-p-coumarate (4) along with the previously reported compound gigantol (5). Among the isolates, vanillin (3) and dihydroconiferyl dihydro-p-coumarate (4) were found to significantly inhibit the activity of acetylcholinesterase, scavenge the free radicals, exhibit the reducing power and total antioxidant activity, and effectively reduce the peroxidation of lipid. Gigantol (5) and syringaldehyde (2), despite lacking the activity against acetylcholinesterase, exhibited antioxidant activity. Among the compounds, gigantol (5) appeared to be the most potent antioxidant. These findings revealed that V. roxburghii contained compounds with potential acetylcholinesterase inhibitory and antioxidant activity, which support its traditional use in the treatment of AD.

Plant isoquinoline alkaloids as potential neurodrugs: A comparative study of the effects of benzo[c]phenanthridine and berberine-based compounds on β-amyloid aggregation

Herein we present a comparative study of the effects of isoquinoline alkaloids belonging to benzo[c]phenanthridine and berberine families on β-amyloid aggregation. Results obtained using a Thioflavine T (ThT) fluorescence assay and circular dichroism (CD) spectroscopy suggested that the benzo[c]phenanthridine nucleus, present in both sanguinarine and chelerythrine molecules, was directly involved in an inhibitory effect of Aβ_1-42 aggregation. Conversely, coralyne, that contains the isomeric berberine nucleus, significantly increased propensity for Aβ_1-42 to aggregate. Surface Plasmon Resonance (SPR) experiments provided quantitative estimation of these interactions: coralyne bound to Aβ_1-42 with an affinity (K_D = 11.6 μM) higher than benzo[c]phenanthridines. Molecular docking studies confirmed that all three compounds are able to recognize Aβ_1-42 in different aggregation forms suggesting their effective capacity to modulate the Aβ_1-42 self-recognition mechanism. Molecular dynamics simulations indicated that coralyne increased the β-content of Aβ_1-42, in early stages of aggregation, consistent with fluorescence-based promotion of the Aβ_1-42 self-recognition mechanism by this alkaloid. At the same time, sanguinarine induced Aβ_1-42 helical conformation corroborating its ability to delay aggregation as experimentally proved in vitro. The investigated compounds were shown to interfere with aggregation of Aβ_1-42 demonstrating their potential as starting leads for the development of therapeutic strategies in neurodegenerative diseases.

Nanoparticles as contrast agents for brain nuclear magnetic resonance imaging in Alzheimer's disease diagnosis

Nuclear Magnetic Resonance Imaging (MRI) of amyloid plaques is a powerful non-invasive approach for the early and accurate diagnosis of Alzheimer's disease (AD) along with clinical observations of behavioral changes and cognitive impairment. The present article aims at giving a critical and comprehensive review of recent advances in the development of nanoparticle-based contrast agents for brain MRI. Nanoparticles considered for the MRI of AD must comply with a highly stringent set of requirements including low toxicity and the ability to cross the blood-brain-barrier. In addition, to reach an optimal signal-to-noise ratio, they must exhibit a specific ability to target amyloid plaques, which can be achieved by grafting antibodies, peptides or small molecules. Finally, we propose to consider new directions for the future of MRI in the context of Alzheimer's disease, in particular by enhancing the performances of contrast agents and by including therapeutic functionalities following a theranostic strategy.

Ferrous iron formation following the co-aggregation of ferric iron and the Alzheimer's disease peptide β-amyloid (1-42)

For decades, a link between increased levels of iron and areas of Alzheimer's disease (AD) pathology has been recognized, including AD lesions comprised of the peptide β-amyloid (Aβ). Despite many observations of this association, the relationship between Aβ and iron is poorly understood. Using X-ray microspectroscopy, X-ray absorption spectroscopy, electron microscopy and spectrophotometric iron(II) quantification techniques, we examine the interaction between Aβ(1–42) and synthetic iron(III), reminiscent of ferric iron stores in the brain. We report Aβ to be capable of accumulating iron(III) within amyloid aggregates, with this process resulting in Aβ-mediated reduction of iron(III) to a redox-active iron(II) phase. Additionally, we show that the presence of aluminium increases the reductive capacity of Aβ, enabling the redox cycling of the iron. These results demonstrate the ability of Aβ to accumulate iron, offering an explanation for previously observed local increases in iron concentration associated with AD lesions. Furthermore, the ability of iron to form redox-active iron phases from ferric precursors provides an origin both for the redox-active iron previously witnessed in AD tissue, and the increased levels of oxidative stress characteristic of AD. These interactions between Aβ and iron deliver valuable insights into the process of AD progression, which may ultimately provide targets for disease therapies.

Aβ association inhibition by transferrin

The iron-transport glycoprotein transferrin has recently been shown to serve as a potent inhibitor of Aβ self-association. Although this novel, to our knowledge, inhibitory function of transferrin is of potential therapeutic interest for the treatment of Alzheimer's disease, the underlying mechanism is still not fully understood. Although it has been shown that the Fe(III) sequestration by transferrin reduces oxidative damage and Aβ aggregation, it is not clear whether transferrin is also able to inhibit Aβ self-association through direct binding of Aβ. Here, using saturation transfer and off-resonance relaxation NMR spectroscopy, we show that transferrin inhibits Aβ aggregation also by preferentially binding Aβ oligomers and outcompeting Aβ monomers that would otherwise cause the growth of the Aβ oligomers into larger assemblies. This inhibitory mechanism is different from the iron-sequestration model, but it is qualitatively similar to a mechanism previously proposed for the inhibition of Aβ self-association by another plasma and cerebrospinal fluid protein, i.e., human serum albumin. These results suggest that Aβ monomer competition through direct Aβ oligomer binding might be a general strategy adopted by proteins in plasma and cerebrospinal fluid to prevent Aβ aggregation.

Overexpression of Hsp27 ameliorates symptoms of Alzheimer's disease in APP/PS1 mice

Hsp27 belongs to the small heat shock protein family, which are ATP-independent chaperones. The most important function of Hsp27 is based on its ability to bind non-native proteins and inhibit the aggregation of incorrectly folded proteins maintaining them in a refolding-competent state. Additionally, it has anti-apoptotic and antioxidant activities. To study the effect of Hsp27 on memory and synaptic functions, amyloid-β (Aβ) accumulation, and neurodegeneration, we generated transgenic mice overexpressing human Hsp27 protein and crossed with APPswe/PS1dE9 mouse strain, a mouse model of Alzheimer's disease (AD). Using different behavioral tests, we found that spatial learning was impaired in AD model mice and was rescued by Hsp27 overexpression. Electrophysiological recordings have revealed that excitability of neurons was significantly increased, and long-term potentiation (LTP) was impaired in AD model mice, whereas they were normalized in Hsp27 overexpressing AD model mice. Using anti-amyloid antibody, we counted significantly less amyloid plaques in the brain of APPswe/PS1dE9/Hsp27 animals compared to AD model mice. These results suggest that overexpression of Hsp27 protein might ameliorate certain symptoms of AD.

Decursin Isolated from Angelica gigas Nakai Rescues PC12 Cells from Amyloid β-Protein-Induced Neurotoxicity through Nrf2-Mediated Upregulation of Heme Oxygenase-1: Potential Roles of MAPK

Decursin (D), purified from Angelica gigas Nakai, has been proven to exert neuroprotective property. Previous study revealed that D reduced A β 25 ‒ 35-induced cytotoxicity in PC12 cells. Our study explored the underlying mechanisms by which D mediates its therapeutic effects in vitro. Pretreatment of cells with D diminished intracellular generation of ROS in response to A β 25 ‒ 35. Western blot revealed that D significantly increased the expression and activity of HO-1, which was correlated with its protection against A β 25 ‒ 35-induced injury. Addition of ZnPP, an HO-1 competitive inhibitor, significantly attenuated its protective effect in A β 25 ‒ 35-treated cells, indicating the vital role of HO-1 resistance to oxidative injury. Moreover, D induced Nrf2 nuclear translocation, the upstream of HO-1 expression. While investigating the signaling pathways responsible for HO-1 induction, D activated ERK and dephosphorylated p38 in PC12 cells. Addition of U0126, a selective inhibitor of ERK, blocked D-induced Nrf2 activation and HO-1 induction and meanwhile reversed the protection of D against A β 25 ‒ 35-induced cell death. These findings suggest D augments cellular antioxidant defense capacity through both intrinsic free radical scavenging activity and activation of MAPK signal pathways that leads to Nrf2 activation, and subsequently HO-1 induction, thereby protecting the PC12 cells from A β 25 ‒ 35-induced oxidative cytotoxicity.

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

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

Protective effects of walnut extract against amyloid beta peptide-induced cell death and oxidative stress in PC12 cells

Amyloid beta-protein (Aβ) is the major component of senile plaques and cerebrovascular amyloid deposits in individuals with Alzheimer’s disease. Aβ is known to increase free radical production in neuronal cells, leading to oxidative stress and cell death. Recently, considerable attention has been focused on dietary antioxidants that are able to scavenge reactive oxygen species (ROS), thereby offering protection against oxidative stress. Walnuts are rich in components that have anti-oxidant and anti-inflammatory properties. The inhibition of in vitro fibrillization of synthetic Aβ, and solubilization of preformed fibrillar Aβ by walnut extract was previously reported. The present study was designed to investigate whether walnut extract can protect against Aβ-induced oxidative damage and cytotoxicity. The effect of walnut extract on Aβ-induced cellular damage, ROS generation and apoptosis in PC12 pheochromocytoma cells was studied. Walnut extract reduced Aβ-mediated cell death assessed by MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) reduction, and release of lactate dehydrogenase (membrane damage), DNA damage (apoptosis) and generation of ROS in a concentration-dependent manner. These results suggest that walnut extract can counteract Aβ-induced oxidative stress and associated cell death.

Neuroprotective effects of salidroside against beta-amyloid-induced oxidative stress in SH-SY5Y human neuroblastoma cells

Beta-amyloid (Abeta) peptide, the hallmark of Alzheimer's disease (AD), invokes a cascade of oxidative damages to neurons and eventually leads to neuronal death. In this study, salidroside (Sald), an active compound isolated from a traditional Chinese medicinal plant, Rhodiola rosea L., was investigated to assess its protective effects and the underlying mechanisms against Abeta-induced oxidative stress in SH-SY5Y human neuroblastoma cells. Abeta(25-35)-induced neuronal toxicity was characterized by the decrease of cell viability, the release of lactate dehydrogenase (LDH), morphological alterations, neuronal DNA condensation, and the cleavage of poly(ADP-ribose) polymerase (PARP) by activated caspase-3. Pretreatment with salidroside markedly attenuated Abeta(25-35)-induced loss of cell viability and apoptosis in a dose-dependent manner. The mechanisms of salidroside protected neurons from oxidative stress included the induction of antioxidant enzymes, thioredoxin (Trx), heme oxygenase-1 (HO-1), and peroxiredoxin-I (PrxI); the downregulation of pro-apoptotic protein Bax and the upregulation of anti-apoptotic protein Bcl-X(L). Furthermore, salidroside dose-dependently restored Abeta(25-35)-induced loss of mitochondrial membrane potential (MMP) as well as suppressed the elevation of intracellular reactive oxygen species (ROS) level. It was also observed that Abeta(25-35) stimulated the phosphorylation of mitogen-activated protein (MAP) kinases, including c-Jun NH(2)-terminal kinase (JNK) and p38 MAP kinase, but not extracellular signal-regulated kinase1/2 (ERK1/2). Salidroside inhibited Abeta(25-35)-induced phosphorylation of JNK and p38 MAP kinase, but not ERK1/2. These results suggest that salidroside has protective effects against Abeta(25-35)-induced oxidative stress, which might be a potential therapeutic agent for treating or preventing neurodegenerative diseases.

Protective effects of Ginkgo biloba extract (EGb761) and its constituents quercetin and ginkgolide B against beta-amyloid peptide-induced toxicity in SH-SY5Y cells

Ginkgo biloba extract EGb761 has been shown to protect against beta-amyloid peptide (Abeta)-induced neurotoxicity but the specific mechanisms remain unclear. In the present study, effects of EGb761 and two of its constituents, quercetin and ginkgolide B, on the cytotoxic action of Abeta (1-42) were tested with human neuroblastoma SH-SY5Y cells. We found that EGb761 was able to block Abeta (1-42)-induced cell apoptosis, reactive oxygen species (ROS) accumulation, mitochondrial dysfunction and activation of c-jun N-terminal kinase (JNK), extracellular signal-regulated kinase 1/2 (ERK1/2) and Akt signaling pathways. Both quercetin and ginkgolide B may be involved in the inhibitory effects of EGb761 on JNK, ERK1/2 and Akt signaling pathways. Ginkgolide B also helped to improve mitochondrial functions but quercetin failed to show this effect. Additional experiments suggest that, protective effects of EGb761 against Abeta toxicity may be associated with its antioxidant and platelet activating factor (PAF) antagonist activities. Quercetin but not ginkgolide B is one of the constituents responsible for the antioxidant action of EGb761. Both quercetin and ginkgolide B may be involved in the PAF antagonist activity of EGb761. Overall, actions of individual EGb761 components provide further insights into direct mechanisms underlying the neuroprotective effects of EGb761.

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.

Chemical chaperone and inhibitor discovery: potential treatments for protein conformational diseases

Protein misfolding and aggregation cause a large number of neurodegenerative diseases in humans due to (i) gain of function as observed in Alzheimer’s disease, Huntington’s disease, Parkinson’s disease, and Prion’s disease or (ii) loss of function as observed in cystic fibrosis and α1-antitrypsin deficiency. These misfolded proteins could either lead to the formation of harmful amyloids that become toxic for the cells or to be recognized and prematurely degraded by the protein quality control system. An increasing number of studies has indicated that some low-molecular-weight compounds named as chemical chaperones can reverse the mislocalization and/or aggregation of proteins associated with human conformational diseases. These small molecules are thought to non-selectively stabilize proteins and facilitate their folding. In this review, we summarize the probable mechanisms of protein conformational diseases in humans and the use of chemical chaperones and inhibitors as potential therapeutic agents against these diseases. Furthermore, recent advanced experimental and theoretical approaches underlying the detailed mechanisms of protein conformational changes and current structure-based drug designs towards protein conformational diseases are also discussed. It is believed that a better understanding of the mechanisms of conformational changes as well as the biological functions of these proteins will lead to the development and design of potential interfering compounds against amyloid formation associated with protein conformational diseases.

Phosphatidylserine and phosphatidylcholine-containing liposomes inhibit amyloid beta and interferon-gamma-induced microglial activation

There is increasing evidence that microglial activation is one of the major pathogenic factors for Alzheimer's disease (AD) and the inhibition of the inflammatory activation of the microglia thus appears to be neuroprotective and a potentially useful treatment for AD. Phospholipids such as phosphatidylserine (PS) and phosphatidylcholine (PC) have been reported to modulate the immune function of phagocytes. In addition, PS has been reported to be a nootropics that can be used as nonprescription memory or cognitive enhancers. We therefore evaluated the effects of liposomes, which comprise both PS and PC (PS/PC liposomes), on the microglial production of tumor necrosis factor-alpha (TNF-alpha), nitric oxide (NO), and superoxide (*O(2)-) induced by amyloid beta (Abeta) and interferon-gamma (IFN-gamma). Pretreatment of microglia with PS/PC liposomes considerably inhibited the TNF-alpha, NO and *O(2)- production induced by Abeta/IFN-gamma. These results suggest that PS/PC liposomes have both neuroprotective and antioxidative properties through the inhibition of microglial activation, thus supporting the nootropic and antidementia effect of PS.

Inhibitor Discovery Targeting the Intermediate Structure of β-Amyloid Peptide on the Conformational Transition Pathway: Implications in the Aggregation Mechanism of β-Amyloid Peptide

Abeta peptides cleaved from the amyloid precursor protein are the main components of senile plaques in Alzheimer's disease. Abeta peptides adopt a conformation mixture of random coil, beta-sheet, and alpha-helix in solution, which makes it difficult to design inhibitors based on the 3D structures of Abeta peptides. By targeting the C-terminal beta-sheet region of an Abeta intermediate structure extracted from molecular dynamics simulations of Abeta conformational transition, a new inhibitor that abolishes Abeta fibrillation was discovered using virtual screening in conjunction with thioflavin T fluorescence assay and atomic force microscopy determination. Circular dichroism spectroscopy demonstrated that the binding of the inhibitor increased the beta-sheet content of Abeta peptides either by stabilizing the C-terminal beta-sheet conformation or by inducing the intermolecular beta-sheet formation. It was proposed that the inhibitor prevented fibrillation by blocking interstrand hydrogen bond formation of the pleated beta-sheet structure commonly found in amyloid fibrils. The study not only provided a strategy for inhibitor design based on the flexible structures of amyloid peptides but also revealed some clues to understanding the molecular events involved in Abeta aggregation.

Induction of vascular amyloidosis-β by oxidative stress depends on APOE genotype

The reduced antioxidant defense in apolipoprotein E epsilon4/epsilon4 carriers may contribute to beta-amyloidosis. Previously we found that Fe(2+)-induced oxidative stress caused greater protein oxidation in epsilon4/epsilon4 than in epsilon3/epsilon3 human brain vascular smooth muscle cells. Moreover, Fe(2+) induced lysosomal accumulation of endogenous Abeta and APOE in cultured cells, and Abeta deposition in vascular tunica media in organotypic cultures of brain vessels. Here we demonstrated that Fe(2+) enhanced an uptake of exogenous Abeta 1-40 and its deposition together with APOE in lysosomes in myocytes. Abeta deposits were associated with lipid-peroxidation and protein ubiquitination, and were more abundant and stable in epsilon4/epsilon4 than in epsilon3/epsilon3 cells. In organotypic cultures of brain vessels Fe(2+) induced deposition of non-fibrillar and fibrillar Abeta 1-40 in vascular tunica media. We hypothesize that locally increased concentrations of iron induce accumulation of exogenous and endogenous Abeta in SMCs, triggering beta-amyloid angiopathy. The greater susceptibility of epsilon4 carriers to Fe(2+) ions may result in an increased risk of beta-amyloidosis.

Redox cycling of iron by Abeta42

The amyloid cascade hypothesis and oxidative damage have been inextricably linked in the neurodegeneration that is characteristic of Alzheimer's disease. We have investigated this link and sought to suggest a mechanism whereby the precipitation of Abeta42 might contribute to the redox cycling of iron and hence the generation of reactive oxygen species via Fenton-like chemistry. We have shown that the critical step in the auto-oxidation of Fe(II) under the near-physiological conditions of our study involved the generation of H2O2 via O2.- and that Abeta42 influenced Fenton chemistry through aggregation state-specific binding of both Fe(II) and Fe(III). The net result of these interactions was the delayed precipitation of kinetically redox-inactive Fe(OH)3(s) such that Fe(II)/Fe(III) were cycled in redox-active forms over a substantially longer time period than if peptide had been absent from preparations. The addition of physiologically significant concentrations of either Cu(II) or Zn(II) reduced the role played by Abeta42 in the Fe(II)/Fe(III) redox cycle whereas a pathophysiologically significant concentration of Al(III) potentiated the redox cycle in favour of Fe(II) whether or not Cu(II) or Zn(II) was additionally present. The results support the notion that oxidative damage in the immediate vicinity of, for example, senile plaques, may be the result of Fenton chemistry catalysed by the codeposition of Abeta42 with metals such as Fe(II)/Fe(III) and Al(III).

Metals accelerate the formation and direct the structure of amyloid fibrils of NAC

Non-beta amyloid component of Alzheimer's disease amyloid or NAC is a highly amyloidogenic peptide consisting of 35 amino acids which was first identified associated with senile plaques in the Alzheimer's disease brain. It is a fragment of the presynaptic protein alpha-synuclein and, as such, it is implicated in the aetiologies of both Alzheimer's (AD) and Parkinson's (PD) disease. Metals are involved in the aggregation of amyloidogenic peptides such as beta amyloid (Abeta), British amyloid peptide (ABri) and alpha-synuclein though nothing is yet known about how they might influence the aggregation of NAC. We show herein that NAC will form beta-pleated conformers at a peptide concentration of only 2.0 microM and that metals, and Zn(II) and Cu(II) in particular, accelerate the formation of these fibrils. Cu(II) and Zn(II) did not influence the diameter or general structure of the fibrils which were formed though many more shorter fibrils were observed in their presence and these shorter fibrils were highly thioflavin T positive and they were efficient catalysts of the redox cycling of added Fe(II). By way of contrast, beta-pleated conformers of NAC which were formed in the presence of Al(III) showed much lower levels of thioflavin T fluorescence and were poorer catalysts of the redox cycling of added Fe(II) and these properties were commensurate with an increased abundance of a novel amyloid morphology which consisted of twisted fibrils with a periodicity of about 100 nm. These spirals of twisted fibrils were especially abundant in the presence of added Al(III) and it is speculated that NAC binding of Al(III) may be important in their formation and subsequent stability.

Employing new cellular therapeutic targets for Alzheimer's disease: a change for the better?

Alzheimer's disease is a progressive disorder that results in the loss of cognitive function and memory. Although traditionally defined by the presence of extracellular plaques of amyloid-beta peptide aggregates and intracellular neurofibrillary tangles in the brain, more recent work has begun to focus on elucidating the complexities of Alzheimer's disease that involve the generation of reactive oxygen species and oxidative stress. Apoptotic processes that are incurred as a function of oxidative stress affect neuronal, vascular, and monocyte derived cell populations. In particular, it is the early apoptotic induction of cellular membrane asymmetry loss that drives inflammatory microglial activation and subsequent neuronal and vascular injury. In this article, we discuss the role of novel cellular pathways that are invoked during oxidative stress and may potentially mediate apoptotic injury in Alzheimer's disease. Ultimately, targeting new avenues for the development of therapeutic strategies linked to mechanisms that involve inflammatory microglial activation, cellular metabolism, cell-cycle regulation, G-protein regulated receptors, and cytokine modulation may provide fruitful gains for both the prevention and treatment of Alzheimer's disease.

Erythropoietin requires NF-kappaB and its nuclear translocation to prevent early and late apoptotic neuronal injury during beta-amyloid toxicity

No longer considered exclusive for the function of the hematopoietic system, erythropoietin (EPO) is now considered as a viable agent to address central nervous system injury in a variety of cellular systems that involve neuronal, vascular, and inflammatory cells. Yet, it remains unclear whether the protective capacity of EPO may be effective for chronic neurodegenerative disorders such as Alzheimer's disease (AD) that involve beta-amyloid (Abeta) apoptotic injury to hippocampal neurons. We therefore investigated whether EPO could prevent both early and late apoptotic injury during Abeta exposure in primary hippocampal neurons and assessed potential cellular pathways responsible for this protection. Primary hippocampal neuronal injury was evaluated by trypan blue dye exclusion, DNA fragmentation, membrane phosphatidylserine (PS) exposure, and nuclear factor-kappaB (NF-kappaB) expression with subcellular translocation. We show that EPO, in a concentration specific manner, is able to prevent the loss of both apoptotic genomic DNA integrity and cellular membrane asymmetry during Abeta exposure. This blockade of Abeta generated neuronal apoptosis by EPO is both necessary and sufficient, since protection by EPO is completely abolished by co-treatment with an anti-EPO neutralizing antibody. Furthermore, neuroprotection by EPO is closely linked to the expression of NF-kappaB p65 by preventing the degradation of this protein by Abeta and fostering the subcellular translocation of NF-kappaB p65 from the cytoplasm to the nucleus to allow the initiation of an anti-apoptotic program. In addition, EPO intimately relies upon NF-kappaB p65 to promote neuronal survival, since gene silencing of NF-kappaB p65 by RNA interference removes the protective capacity of EPO during Abeta exposure. Our work illustrates that EPO is an effective entity at the neuronal cellular level against Abeta toxicity and requires the close modulation of the NF-kappaB p65 pathway, suggesting that either EPO or NF-kappaB may be used as future potential therapeutic strategies for the management of chronic neurodegenerative disorders, such as AD.

Lack of glutathione peroxidase-1 exacerbates Abeta-mediated neurotoxicity in cortical neurons

The aetiologies of Alzheimer's disease (AD) are complex and multifactorial. Current therapies are largely ineffective, as the pathophysiological pathways are poorly understood. Observations in AD autopsies, as well as in vivo and in vitro observations in transgenic mice, have implicated oxidative stress as pathogenic in AD. This study used the Glutathione Peroxidase-1 knockout mouse (Gpx1--/--) model to investigate the role of antioxidant disparity in neuropathologies. Cultured neurons from control and Gpx1--/-- embryos were treated with AD-related peptides and the degree of cell loss compared. Results show that antioxidant disparity makes Gpx1--/-- cells more susceptible to Abeta toxicity. Surrogate replacement of Gpx1 with the reactive oxygen species scavenger N-acetyl cysteine and the Gpx1 mimetic ebselen, reverses the Gpx1--/-- increased susceptibility to Abeta toxicity. Such results support a role for oxidative stress in AD-related neuronal loss. This study is the first to report such findings using the Gpx1--/-- model, and supports a role for oxidative stress as one of the contributing factors, in development of AD-like pathologies.

Amyloid-β fibril formation is not necessarily required for microglial activation by the peptides

There is increasing evidence that microglial activation has pathogenic influence on Alzheimer's disease. According to in vitro studies, microglia activated by amyloid-beta (Abeta) peptides have been reported to damage or kill neurons by the release of neurotoxic molecules such as tumor necrosis factor-alpha (TNF-alpha), interleukin-1beta, nitric oxide or reactive oxygen species. Although the relationship between the aggregational state of Abeta peptides and their neurotoxic activities has been well investigated, little is known about the relationship between the aggregational state of Abeta peptides and their ability to induce microglial activation. In the present study, we thus performed both structural and biochemical studies to clarify the relationship between the aggregational state of Abeta peptides and their ability to activate microglia. Our results have shown that, in the presence of interferon-gamma, the Abeta25-35(M(35)Nle) peptide had almost the same potency of activating microglia and producing TNF-alpha as the Abeta25-35 peptide on both protein and mRNA levels, in spite of the fact that former peptide represented much less amyloid fibril formation than the latter in a thioflavine-T fluorometric assay. These results suggest that Abeta fibril formation is not necessarily required for microglial activation by the peptides.

Oxidative stress in the brain: novel cellular targets that govern survival during neurodegenerative disease

Despite our present knowledge of some of the cellular pathways that modulate central nervous system injury, complete therapeutic prevention or reversal of acute or chronic neuronal injury has not been achieved. The cellular mechanisms that precipitate these diseases are more involved than initially believed. As a result, identification of novel therapeutic targets for the treatment of cellular injury would be extremely beneficial to reduce or eliminate disability from nervous system disorders. Current studies have begun to focus on pathways of oxidative stress that involve a variety of cellular pathways. Here we discuss novel pathways that involve the generation of reactive oxygen species and oxidative stress, apoptotic injury that leads to nuclear degradation in both neuronal and vascular populations, and the early loss of cellular membrane asymmetry that mitigates inflammation and vascular occlusion. Current work has identified exciting pathways, such as the Wnt pathway and the serine-threonine kinase Akt, as central modulators that oversee cellular apoptosis and their downstream substrates that include Forkhead transcription factors, glycogen synthase kinase-3beta, mitochondrial dysfunction, Bad, and Bcl-x(L). Other closely integrated pathways control microglial activation, release of inflammatory cytokines, and caspase and calpain activation. New therapeutic avenues that are just open to exploration, such as with brain temperature regulation, nicotinamide adenine dinucleotide modulation, metabotropic glutamate system modulation, and erythropoietin targeted expression, may provide both attractive and viable alternatives to treat a variety of disorders that include stroke, Alzheimer's disease, and traumatic brain injury.

The amyloid paradox: amyloid-beta-metal complexes can be neurotoxic and neuroprotective

Senile plaques in the brains of people with Alzheimer disease (AD) are primarily composed of the amyloid-beta (Abeta) peptide and contain substantially elevated levels of iron, copper and zinc. These metals bind to Abeta and have been reported to increase the toxicity of Abeta to cultured neurones. Other reports have demonstrated that Abeta can reduce the neurotoxicity of metal ions, suggesting that the interaction can, under some circumstances, be protective. To investigate these apparently conflicting results, human Abeta1-42 was co-injected with iron, copper or zinc (at the concentrations found in plaques) into rat cerebral cortex, and the resulting numbers of dying neurones were compared. It was found that Abeta complexed with either iron or zinc was more toxic than Abeta alone. In contrast, Abeta-copper complexes were not neurotoxic. Surprisingly, we observed that when iron or copper were combined with Abeta, the neurotoxicity of these metals was substantially reduced, suggesting that Abeta may help to limit the toxicity of redox-active metal ions, thereby assisting the antioxidant defence of the brain. Thus paradoxical effects occur when Abeta complexes with metal ions, where Abeta-metal complexes are capable of being neurotoxic and neuroprotective.

Stress in the brain: novel cellular mechanisms of injury linked to Alzheimer's disease

More than a century has elapsed since the description of Alois Alzheimer's patient Auguste D. Yet, the well-documented generation of beta-amyloid aggregates and neurofibrillary tangles that define Alzheimer's disease is believed to represent only a portion of the cellular processes that can determine the course of Alzheimer's disease. Understanding of the complex nature of this disorder has evolved with an increased appreciation for pathways that involve the generation of reactive oxygen species and oxidative stress, apoptotic injury that leads to nuclear degradation in both neuronal and vascular populations, and the early loss of cellular membrane asymmetry that mitigates inflammation and vascular occlusion. Recent work has identified novel pathways, such as the Wnt pathway and the serine-threonine kinase Akt, as central modulators that oversee cellular apoptosis and the formation of neurofibrillary tangles through their downstream substrates that include glycogen synthase kinase-3beta, Bad, and Bcl-xL. Other closely integrated pathways control microglial activation, release of inflammatory cytokines, and caspase and calpain activation for the processing of amyloid precursor protein, tau protein cleavage, and presenilin disposal. New therapeutic avenues that are just open to exploration, such as with nicotinamide adenine dinucleotide modulation, cell cycle modulation, metabotropic glutamate system modulation, and erythropoietin targeted expression, may provide both attractive and viable alternatives to treat Alzheimer's disease.

Amyloid-beta metal interaction and metal chelation

Alzheimer's disease (AD) is associated with the abnormal aggregation of amyloid-beta (Abeta) protein. Abeta and its precursor protein (APP) interact with metal ions such as zinc, copper and iron. Evidence shows that these metals play a role in the precipitation and cytotoxicity of Abeta. Despite recent advances in AD research, there is a lack of therapeutic agents to hinder the apparent aggregation and toxicity of Abeta. Recent studies show that drugs with metal chelating properties could produce a significant reversal of amyloid-beta plaque deposition in vitro and in vivo. Here we discuss the interaction of Abeta with metals, metal dyshomeostasis in the CNS of patients with AD, and the potential therapeutic effects of metal chelators.

Transferrin neutralization of amyloid β 25–35 cytotoxicity

BACKGROUND

Fibrillar aggregates of amyloid beta 25-35 (Abeta(25-35)) form rapidly in vitro able to lyse human red blood cells (RBCs). Human sera, albumin, and apolipoprotein E (ApoE) each limit fibrillation and cytotoxicity. Potentially, these substances protect neurons from Abeta(1-40/42) aggregates. Transferrin (TF) is investigated in this study.

METHODS

The Mattson red blood cells model was employed to determine whether co-incubation of transferrin and Abeta(25-35) prevented lysis. The formation of fibrillar Abeta(25-35) in the presence of transferrin was investigated using Congo red staining and spectrophotometric studies.

RESULTS

We found that incubation of 20 muM Abeta(25-35) with physiologic levels of transferrin prevented red blood cells lysis and the formation of macro-aggregates.

CONCLUSIONS

These in vitro results suggest that transferrin may limit fibrillar beta amyloid formation in vivo and cytotoxicity.

Essential cellular regulatory elements of oxidative stress in early and late phases of apoptosis in the central nervous system

The generation of reactive oxygen species and subsequent oxidative stress in the central nervous system is now considered to be one of the primary etiologies of a host of neurodegenerative disorders, such as Alzheimer disease, Parkinson disease, and cerebral ischemia. On a cellular level, oxidative stress leads to an apoptotic early phase that involves cellular membrane phosphatidylserine (PS) exposure and a late phase that pertains to the degradation of genomic DNA. The translocation of membrane PS from the inner cellular membrane to the surface is a critical component for both microglial activation and cellular disposal of injured cells. During oxidative stress, this early phase of apoptosis is intimately controlled by neuronal PS exposure and microglial PS receptor expression. The late phase of apoptosis that involves a loss of genomic DNA integrity can result as a function of an ill-fated attempt to enter the cell cycle in postmitotic neurons. By using a cascade of pathways that involve cysteine proteases to modulate programmed cell death, protein kinase B (Akt) surfaces as a key regulatory element of both extrinsic pathways of inflammation and intrinsic pathways of cellular integrity. Further understanding of the cellular mechanisms modulating neuronal cellular integrity and phagocytic cell disposal during oxidative stress may form the basis for the future development of cytoprotective strategies in the nervous system.

Age-related myelin breakdown: a developmental model of cognitive decline and Alzheimer's disease

A hypothetical model of Alzheimer's disease (AD) as a uniquely human brain disorder rooted in its exceptional process of myelination is presented. Cortical regions with the most protracted development are most vulnerable to AD pathology, and this protracted development is driven by oligodendrocytes, which continue to differentiate into myelin producing cells late into the fifth decade of life. The unique metabolic demands of producing and maintaining their vast myelin sheaths and synthesizing the brain's cholesterol supply make oligodendrocytes especially susceptible to a variety of insults. Their vulnerability increases with increasing age at differentiation as later-differentiating cells myelinate increasing numbers of axonal segments. These vulnerable late-differentiating cells drive the protracted process of intracortical myelination and by increasing local cholesterol and iron levels, progressively increase the toxicity of the intracortical environment forming the basis for the age risk factor for AD. At older ages, the roughly bilaterally symmetrical continuum of oligodendrocyte vulnerability manifests as a progressive pattern of myelin breakdown that recapitulates the developmental process of myelination in reverse. The ensuing homeostatic responses to myelin breakdown further increase intracortical toxicity and results in the relentless progression and non-random anatomical distribution of AD lesions that eventually cause neuronal dysfunction and degeneration. This process causes a slowly progressive disruption of neural impulse transmission that degrades the temporal synchrony of widely distributed neural networks underlying normal brain function. The resulting network "disconnections" first impact functions that are most dependent on large-scale synchronization including higher cognitive functions and formation of new memories. Multiple genetic and environmental risk factors (e.g. amyloid beta-peptide and free radical toxicity, head trauma, anoxia, cholesterol levels, etc.) can contribute to the cognitive deficits observed in aging and AD through their impact on the life-long trajectory of myelin development and breakdown. This development-to-degeneration model is testable through imaging and post mortem methods and highlights the vital role of myelin in impulse transmission and synchronous brain function. The model offers a framework that explains the anatomical distribution and progressive course of AD pathology, some of the failures of promising therapeutic interventions, and suggests further testable hypotheses as well as novel approaches for intervention efforts.

Neurotoxicity and physicochemical properties of Abeta mutant peptides from cerebral amyloid angiopathy: implication for the pathogenesis of cerebral amyloid angiopathy and Alzheimer's disease

Cerebral amyloid angiopathy (CAA) due to beta-amyloid (Abeta) is one of the specific pathological features of familial Alzheimer's disease. Abeta mainly consisting of 40- and 42-mer peptides (Abeta40 and Abeta42) exhibits neurotoxicity and aggregative abilities. All of the variants of Abeta40 and Abeta42 found in CAA were synthesized in a highly pure form and examined for neurotoxicity in PC12 cells and aggregative ability. All of the Abeta40 mutants at positions 22 and 23 showed stronger neurotoxicity than wild-type Abeta40. Similar tendency was observed for Abeta42 mutants at positions 22 and 23 whose neurotoxicity was 50-200 times stronger than that of the corresponding Abeta40 mutants, suggesting that these Abeta42 mutants are mainly involved in the pathogenesis of CAA. Although the aggregation of E22G-Abeta42 and D23N-Abeta42 was similar to that of wild-type Abeta42, E22Q-Abeta42 and E22K-Abeta42 aggregated extensively, supporting the clinical evidence that Dutch and Italian patients are diagnosed as hereditary cerebral hemorrhage with amyloidosis. In contrast, A21G mutation needs alternative explanation with the exception of physicochemical properties of Abeta mutants. Attenuated total reflection-Fourier transform infrared spectroscopy spectra suggested that beta-sheet content of the Abeta mutants correlates with their aggregation. However, beta-turn is also a critical secondary structure because residues at positions 22 and 23 that preferably form two-residue beta-turn significantly enhanced the aggregative ability.

Iron: a pathological mediator of Alzheimer disease?

Brains from patients with Alzheimer disease (AD) show a disruption in the metabolism of iron, such that there is an accumulation of iron in senile plaques, and an altered distribution of iron transport and storage proteins. One of the earliest events in AD is the generation of oxidative stress, which may be related to the generation of free radicals by the excess iron that is observed in the disease. Iron has also been shown to mediate the in vitro toxicity of amyloid-beta peptide, and the presence of iron in most in vitro systems could underlie the toxicity that is normally attributed to amyloid-beta in these studies. In contrast, several recent studies have suggested that amyloid-beta may decrease oxidative stress and decrease the toxicity of iron. Continued examination of the complex interactions that occur between iron and amyloid-beta may assist in the elucidation of the mechanisms that underlie the neurodegeneration that leads to dementia in AD.

Zinc takes the center stage: its paradoxical role in Alzheimer's disease

There is compelling evidence that the etiology of Alzheimer's disease (AD) involves characteristic amyloid-beta (Abeta) deposition, oxidative stress, and anomalous metal-Abeta protein interaction. New studies have implicated redox active metals such as copper, iron, and zinc as key mediating factors in the pathophysiology of Alzheimer's disease. There is also evidence that drugs with metal chelating properties could produce a significant reversal of amyloid-beta plaque deposition in vitro and in vivo. This paper reviews current observations on the etiologic role of zinc in AD. We also discuss the interactions of zinc and copper with Abeta, a factor that purportedly facilitates disease processes. Finally, we review the protective role of zinc against Abeta cytotoxicity and hypothesize how the apparent effect of zinc on AD pathology may be paradoxical, The Zinc Paradox. Indeed, complex pathologic stressors inherent to the Alzheimer's diseased brain dictate whether or not zinc will be neuroprotective or neurodegenerative. Further research on the zinc paradox in AD is needed in order to elucidate the exact role zinc plays in AD pathogenesis.

Inactivation of the human brain muscarinic acetylcholine receptor by oxidative damage catalyzed by a low molecular weight endogenous inhibitor from Alzheimer's brain is prevented by pyrophosphate analogs, bioflavonoids and other antioxidants

Oxidative stress has been implicated as a contributing factor to neurodegeneration in Alzheimer's disease. An endogenous, low molecular weight (LMW) inhibitor from Alzheimer's brain inactivates the human brain muscarinic acetylcholine receptor (mAChR). The inhibitor prevents agonist and antagonist binding to the mAChR as assessed by radioligand binding studies. The LMW endogenous inhibitor, which has components with molecular weights between 100 and 1000 Da, requires dissolved oxygen and glutathione. Prevention of inactivation of the mAChR with peroxidase suggests that the LMW endogenous inhibitor generates peroxide. Heme, previously shown to be present in the LMW endogenous inhibitor, also inactivates the mAChR in the presence of peroxide. Free radical damage to the muscarinic receptor by the endogenous inhibitor can be prevented through the use of naturally occurring antioxidants including bilirubin, biliverdin, carnosol, myricetin and quericetin. In addition, pyrophosphate, imidodiphosphate, bisphosphonates and related compounds also protect the muscarinic receptor from free radical damage. Inactivation of the mAChR by the LMW endogenous inhibitor is likely to be a factor in the continual decline of Alzheimer's patients, even those taking acetylcholinesterase inhibitors. Natural antioxidants and pyrophosphate analogs may improve the effectiveness of acetylcholinesterase inhibitors and prove useful in the treatment and prevention of Alzheimer's disease since the muscarinic acetylcholine receptor is required for memory, and decreased cholinergic function is a critical deficit in Alzheimer's disease.

A novel function of monomeric amyloid beta-protein serving as an antioxidant molecule against metal-induced oxidative damage

Aggregated and oligomeric amyloid beta-protein (Abeta) is known to exhibit neurotoxicity. However, the action of Abeta monomers on neurons is not fully understood. We have studied aggregation state-dependent actions of Abeta and found an oligomer-specific effect of Abeta on lipid metabolism in neurons (Michikawa et al., 2001). Here, we show a novel function of monomeric Abeta1-40, which is the major species found in physiological fluid, as a natural antioxidant molecule that prevents neuronal death caused by transition metal-induced oxidative damage. Monomeric Abeta1-40, which is demonstrated by SDS-PAGE after treatment with glutaraldehyde, protects neurons cultured in a medium containing 1.5 microm Fe(II) without antioxidant molecules. Metal ion chelators such as EDTA, CDTA (trans-1,2-diaminocyclohexane-N,N,N',N'-tetraacetic acid), and DTPA (diethylenetriamine-N,N,N',N",N"-penta-acetic acid, an iron-binding protein, transferrin, and antioxidant scavengers such as catalase, glutathione, and vitamin E also inhibit neuronal death under the same conditions. Monomeric Abeta1-40 inhibits neuronal death caused by Cu(II), Fe(II), and Fe(III) but does not protect neurons against H2O2-induced damage. Monomeric Abeta1-40 inhibits the reduction of Fe(III) induced by vitamin C and the generation of superoxides and prevents lipid peroxidation induced by Fe(II). Abeta1-42 remaining as a monomer also exhibits antioxidant and neuroprotective effects. In contrast, oligomeric and aggregated Abeta1-40 and Abeta1-42 lose their neuroprotective activity. These results indicate that monomeric Abeta protects neurons by quenching metal-inducible oxygen radical generation and thereby inhibiting neurotoxicity. Because aggregated Abeta is known to be an oxygen radical generator, our results provide a novel concept that the aggregation-dependent biological effects of Abeta are dualistic, being either an oxygen radical generator or its inhibitor.

Formation of hydrogen peroxide and hydroxyl radicals from A(beta) and alpha-synuclein as a possible mechanism of cell death in Alzheimer's disease and Parkinson's disease

The formation of extracellular or intracellular deposits of amyloid-like protein fibrils is a prominent pathological feature of many different neurodegenerative diseases, including Alzheimer's disease (AD) and Parkinson's disease (PD). In AD, the beta-amyloid peptide (A(beta)) accumulates mainly extracellularly at the center of senile plaques, whereas, in PD, the alpha-synuclein protein accumulates within neurons inside the Lewy bodies and Lewy neurites. We have shown recently that solutions of A(beta) 1-40, A(beta) 1-42, A(beta) 25-35, alpha-synuclein and non-A(beta) component (NAC; residues 61-95 of alpha-synuclein) all liberate hydroxyl radicals upon incubation in vitro followed by the addition of small amounts of Fe(II). These hydroxyl radicals were readily detected by means of electron spin resonance spectroscopy, employing 5,5-dimethyl-1-pyrroline N-oxide (DMPO) as a spin trapping agent. Hydroxyl radical formation was inhibited by the inclusion of catalase or metal-chelators during A(beta) or alpha-synuclein incubation. Our results suggest that hydrogen peroxide accumulates during the incubation of A(beta) or alpha-synuclein, by a metal-dependent mechanism, and that this is subsequently converted to hydroxyl radicals, on addition of Fe (II), by Fenton's reaction. Consequently, one of the fundamental molecular mechanisms underlying the pathogenesis of cell death in AD and PD, and possibly other neurodegenerative or amyloid diseases, could be the direct production of hydrogen peroxide during formation of the abnormal protein aggregates.