Acceleration of amyloid beta-peptide aggregation by physiological concentrations of calcium

Ca2+ enhances Aβ polymerization rate and fibrillar stability in a dynamic manner

Identifying factors that affect the self-assembly of Aβ (amyloid-β peptide) is of utmost importance in the quest to understand the molecular mechanisms causing AD (Alzheimer's disease). Ca2+ has previously been shown to accelerate both Aβ fibril nucleation and maturation, and dysregulated Ca2+ homoeostasis frequently correlates with development of AD. The mechanisms regarding Ca2+ binding, as well as its effect on fibril kinetics, are not fully understood. Using a polymerization assay we show that Ca2+ in a dynamic and reversible manner enhances both the elongation rate and fibrillar stability, where specifically the ‘dock and lock’ phase mechanism is enhanced. Through NMR analysis we found that Ca2+ affects the fibrillar architecture. In addition, and unexpectedly, we found that Ca2+ does not bind the free Aβ monomer. This implies that Ca2+ binding requires an architecture adopted by assembled peptides, and consequently is mediated through intermolecular interactions between adjacent peptides. This gives a mechanistic explanation to the enhancing effect on fibril maturation and indicates structural similarities between prefibrillar structures and mature amyloid. Taken together we show how Ca2+ levels affect the delicate equilibrium between the monomeric and assembled Aβ and how fluctuations in vivo may contribute to development and progression of the disease.

S-nitrosylated protein disulfide isomerase contributes to mutant SOD1 aggregates in amyotrophic lateral sclerosis

A major hallmark of mutant superoxide dismutase (SOD1)-linked familial amyotrophic lateral sclerosis is SOD1-immunopositive inclusions found within motor neurons. The mechanism by which SOD1 becomes aggregated, however, remains unclear. In this study, we aimed to investigate the role of nitrosative stress and S-nitrosylation of protein disulfide isomerase (PDI) in the formation of SOD1 aggregates. Our data show that with disease progression inducible nitric oxide synthase (iNOS) was up-regulated, which generated high levels of nitric oxide (NO) and subsequently induced S-nitrosylation of PDI in the spinal cord of mutant SOD1 transgenic mice. This was further confirmed by in vitro observation that treating SH-SY5Y cells with NO donor S-nitrosocysteine triggered a dose-dependent formation of S-nitrosylated PDI. When mutant SOD1 was over-expressed in SH-SY5Y cells, the iNOS expression was up-regulated, and NO generation was consequently increased. Furthermore, both S-nitrosylation of PDI and the formation of mutant SOD1 aggregates were detected in the cells expressing mutant SOD1(G93A). Blocking NO generation with the NOS inhibitor N-nitro-L-arginine attenuated the S-nitrosylation of PDI and inhibited the formation of mutant SOD1 aggregates. We conclude that NO-mediated S-nitrosylation of PDI is a contributing factor to the accumulation of mutant SOD1 aggregates in amyotrophic lateral sclerosis.

Redox regulation of protein misfolding, mitochondrial dysfunction, synaptic damage, and cell death in neurodegenerative diseases

The loss or injury of neurons associated with oxidative and nitrosative redox stress plays an important role in the onset of various neurodegenerative diseases. Specifically, nitric oxide (NO), can affect neuronal survival through a process called S-nitrosylation, by which the NO group undergoes a redox reaction with specific protein thiols. This in turn can lead to the accumulation of misfolded proteins, which generally form aggregates in Alzheimer's, Parkinson's, and other neurodegenerative diseases. Evidence suggests that S-nitrosylation can also impair mitochondrial function and lead to excessive fission of mitochondria and consequent bioenergetic compromise via effects on the activity of the fission protein dynamin-related protein 1 (Drp1). This insult leads to synaptic dysfunction and loss. Additionally, high levels of NO can S-nitrosylate a number of aberrant targets involved in neuronal survival pathways, including the antiapoptotic protein XIAP, inhibiting its ability to prevent apoptosis.

Inhaled essential oil from Chamaecyparis obtuse ameliorates the impairments of cognitive function induced by injection of β-amyloid in rats

CONTEXT

Chamaecyparis obtusa Sieb. & Zucc., Endlicher (Cupressaceae) forest bathing or aromatherapy has been shown in various studies to have biological functions such as anticancer, antiallergies, antiinflammatory, and antioxidant activity. However, no reports exist on the pharmacological or biological activities of the essential oil of C. obtusa (EOCO) or its effects on central nervous system.

OBJECTIVE

The aggregation and formation of β-amyloid peptides (Aβ) into fibrils are central events in the pathogenesis of Alzheimer's disease (AD), and overproduction and aggregation of Aβ into oligomers have been known to trigger neurotoxicity. In this study, we investigated the effects of inhaled EOCO on cognitive function and neuronal apoptosis in rats intrahippocampally injected with Aβ.

MATERIALS AND METHODS

To model AD, 4 μg of aggregated Aβ was injected into the hippocampus. To test the effects of EOCO, behavioral performance in the Morris water maze was tested 4 days after injection. After behavioral testing, brain sections were prepared for TTC staining and TUNEL assay.

RESULTS

Inhaled EOCO protected spatial learning and memory from the impairments induced by Aβ(1-40) injection. In addition, the behavioral deficits accompanying Aβ(1-40)-induced AD were attenuated by inhalation of EOCO. Furthermore, acetylcholinesterase (AChE) activity and neuronal apoptosis were significantly inhibited in rats treated with Aβ(1-40) and EOCO compared to rats treated only with Aβ(1-40).

DISCUSSION AND CONCLUSION

EOCO suppressed both AD-related neuronal cell apoptosis and AD-related dysfunction of the memory system. Thus, the results of this study support EOCO as a candidate drug for the treatment of AD.

Common benzothiazole and benzoxazole fluorescent DNA intercalators for studying Alzheimer Aβ1-42 and prion amyloid peptides

Amyloids are fibrillar protein aggregates associated with a number of neurodegenerative pathologies including Alzheimer and Creutzfeldt—Jakob disease. The study of amyloids is usually based on fluorescence with the dye thioflavin-T. Although a number of amyloid binding compounds have been synthesized, many are nonfluorescent or not readily available for research use. Here we report on a class of commercial benzothiazole/benzoxazole containing fluorescent DNA intercalators from Invitrogen that possess the ability to bind amyloid Aβ1-42 peptide and hamster prion. These dyes fluoresce from 500–750 nm and are available as dimers or monomers. We demonstrate that these dyes can be used as acceptors for thioflavin-T fluorescence resonance energy transfer as well as reporter groups for binding studies with Congo red and chrysamine G. As more potential therapeutic compounds for these diseases are generated, there is a need for simple and inexpensive methods to monitor their interactions with amyloids. The fluorescent dyes reported here are readily available and can be used as tools for biochemical studies of amyloid structures and in vitro screening of potential therapeutics.

Zn2+-Aβ40 complexes form metastable quasi-spherical oligomers that are cytotoxic to cultured hippocampal neurons

The roles of metal ions in promoting amyloid β-protein (Aβ) oligomerization associated with Alzheimer disease are increasingly recognized. However, the detailed structures dictating toxicity remain elusive for Aβ oligomers stabilized by metal ions. Here, we show that small Zn(2+)-bound Aβ1-40 (Zn(2+)-Aβ40) oligomers formed in cell culture medium exhibit quasi-spherical structures similar to native amylospheroids isolated recently from Alzheimer disease patients. These quasi-spherical Zn(2+)-Aβ40 oligomers irreversibly inhibit spontaneous neuronal activity and cause massive cell death in primary hippocampal neurons. Spectroscopic and x-ray diffraction structural analyses indicate that despite their non-fibrillar morphology, the metastable Zn(2+)-Aβ40 oligomers are rich in β-sheet and cross-β structures. Thus, Zn(2+) promotes Aβ40 neurotoxicity by structural organization mechanisms mediated by coordination chemistry.

Alzheimer's disease: pathological mechanisms and the beneficial role of melatonin

Alzheimer's disease (AD) is a highly complex neurodegenerative disorder of the aged that has multiple factors which contribute to its etiology in terms of initiation and progression. This review summarizes these diverse aspects of this form of dementia. Several hypotheses, often with overlapping features, have been formulated to explain this debilitating condition. Perhaps the best-known hypothesis to explain AD is that which involves the role of the accumulation of amyloid-β peptide in the brain. Other theories that have been invoked to explain AD and summarized in this review include the cholinergic hypothesis, the role of neuroinflammation, the calcium hypothesis, the insulin resistance hypothesis, and the association of AD with peroxidation of brain lipids. In addition to summarizing each of the theories that have been used to explain the structural neural changes and the pathophysiology of AD, the potential role of melatonin in influencing each of the theoretical processes involved is discussed. Melatonin is an endogenously produced and multifunctioning molecule that could theoretically intervene at any of a number of sites to abate the changes associated with the development of AD. Production of this indoleamine diminishes with increasing age, coincident with the onset of AD. In addition to its potent antioxidant and anti-inflammatory activities, melatonin has a multitude of other functions that could assist in explaining each of the hypotheses summarized above. The intent of this review is to stimulate interest in melatonin as a potentially useful agent in attenuating and/or delaying AD.

Cryptotanshinione inhibits β-amyloid aggregation and protects damage from β-amyloid in SH-SY5Y cells

The deposition of amyloid β-protein (Aβ) fibrils into plaques within the brain parenchyma and along cerebral blood vessels is a hallmark of Alzheimer's disease (AD). Aβ42 oligomers and fibrils cause the breakdown of neural circuits, neuronal death and eventually dementia. Drugs that inhibit Aβ42 aggregation may be a novel direction in AD drug discovery. Cryptotanshinone (CTS), an active component of the medicinal herb Salvia miltiorrhiza, has been shown to improve learning and memory in several pharmacological models of AD. However, the effects of CTS on the Aβ aggregation and toxicity are unclear. The current work shows the effectiveness of CTS on the inhibition of Aβ42 aggregation and toxicity to human neuroblastoma cells. In this study, we demonstrated that CTS can inhibit Aβ42 spontaneous aggregation using thioflavin T fluorescence assay and transmission electron microscopy. Furthermore, we investigated the effects of CTS on Aβ-induced oxidative cell death in cultured SH-SY5Y cells. MTT and lactate dehydrogenase assays showed that CTS reduced the cytotoxicity induced by Aβ42. CTS also dramatically reduced Aβ42-induced cellular apoptosis and increased level of reactive oxygen species in these cells. Our study suggests that CTS may be useful in the inhibition or prevention of AD development and progression.

Self-assembly of tissue transglutaminase into amyloid-like fibrils using physiological concentration of Ca2+

Tissue transglutaminase (tTG or TG2) is a member of the transglutaminase family that catalyzes calcium dependent formation of isopeptide bonds. It has been shown that the expression of TG2 is elevated in neurodegenerative diseases such as Parkinson's, Huntington's, and Alzheimer's. We have investigated the self-assembly of TG2 in vitro. First, using software, hot spots, which are prone for aggregation, were identified in domain 2 of the enzyme. Next we expressed and purified recombinant TG2 and its truncated version that contains only the catalytic domain, and examined their amyloidogenic behavior in various conditions including different temperatures and pHs, in the presence of metal ions and Guanosine triphosphate (GTP). To analyze various stages leading to TG2 fibrillation, we employed various techniques including Thioflavin T (ThT) binding assay, Congo-Red, birefringence, Circular Dichroism (CD), 8-anilino-1-naphthalene sulfonic acid (ANS) binding, Transmission Electron Microscopy (TEM) and Atomic Force Microscopy (AFM). Our results indicated that using low concentrations of Ca(2+), TG2 self-assembled into amyloid-like fibrils; this self-assembly occurred at the physiological temperature (37 °C) and at a higher temperature (57 °C). The truncated version of TG2 (domain 2) also forms amyloid-like fibrils only in the presence of Ca(2+). Because amyloid formation has occurred with domain 2 alone where no enzymatic activity was shown, self-cross-linking by the enzyme was ruled out as a mechanism of amyloid induction. The self-assembly of TG2 was not significant with magnesium and zinc ions, indicating specificity of the self-assembly for calcium ions. The calcium role in self-assembly of TG2 into amyloid may be extended to other proteins with similar biophysical properties to produce novel biomaterials.

Endoplasmic reticulum Ca(2+) handling in excitable cells in health and disease

The endoplasmic reticulum (ER) is a morphologically and functionally diverse organelle capable of integrating multiple extracellular and internal signals and generating adaptive cellular responses. It plays fundamental roles in protein synthesis and folding and in cellular responses to metabolic and proteotoxic stress. In addition, the ER stores and releases Ca(2+) in sophisticated scenarios that regulate a range of processes in excitable cells throughout the body, including muscle contraction and relaxation, endocrine regulation of metabolism, learning and memory, and cell death. One or more Ca(2+) ATPases and two types of ER membrane Ca(2+) channels (inositol trisphosphate and ryanodine receptors) are the major proteins involved in ER Ca(2+) uptake and release, respectively. There are also direct and indirect interactions of ER Ca(2+) stores with plasma membrane and mitochondrial Ca(2+)-regulating systems. Pharmacological agents that selectively modify ER Ca(2+) release or uptake have enabled studies that revealed many different physiological roles for ER Ca(2+) signaling. Several inherited diseases are caused by mutations in ER Ca(2+)-regulating proteins, and perturbed ER Ca(2+) homeostasis is implicated in a range of acquired disorders. Preclinical investigations suggest a therapeutic potential for use of agents that target ER Ca(2+) handling systems of excitable cells in disorders ranging from cardiac arrhythmias and skeletal muscle myopathies to Alzheimer disease.

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.

S-nitrosylation of critical protein thiols mediates protein misfolding and mitochondrial dysfunction in neurodegenerative diseases

Excessive nitrosative and oxidative stress is thought to trigger cellular signaling pathways leading to neurodegenerative conditions. Such redox dysregulation can result from many cellular events, including hyperactivation of the N-methyl-D-aspartate-type glutamate receptor, mitochondrial dysfunction, and cellular aging. Recently, we and our colleagues have shown that excessive generation of free radicals and related molecules, in particular nitric oxide species (NO), can trigger pathological production of misfolded proteins, abnormal mitochondrial dynamics (comprised of mitochondrial fission and fusion events), and apoptotic pathways in neuronal cells. Emerging evidence suggests that excessive NO production can contribute to these pathological processes, specifically by S-nitrosylation of specific target proteins. Here, we highlight examples of S-nitrosylated proteins that regulate misfolded protein accumulation and mitochondrial dynamics. For instance, in models of Parkinson's disease, these S-nitrosylation targets include parkin, a ubiquitin E3 ligase and neuroprotective molecule, and protein-disulfide isomerase, a chaperone enzyme for nascent protein folding. S-Nitrosylation of protein-disulfide isomerase may also be associated with mutant Cu/Zn superoxide dismutase toxicity in amyotrophic lateral sclerosis. Additionally, in models of Alzheimer's disease, excessive NO generation leads to the formation of S-nitrosylated dynamin-related protein 1 (forming SNO-Drp1), which contributes to abnormal mitochondrial fragmentation and resultant synaptic damage.

Calcium ions promote formation of amyloid β-peptide (1-40) oligomers causally implicated in neuronal toxicity of Alzheimer's disease

Amyloid β-peptide (Aβ) is directly linked to Alzheimer's disease (AD). In its monomeric form, Aβ aggregates to produce fibrils and a range of oligomers, the latter being the most neurotoxic. Dysregulation of Ca(2+) homeostasis in aging brains and in neurodegenerative disorders plays a crucial role in numerous processes and contributes to cell dysfunction and death. Here we postulated that calcium may enable or accelerate the aggregation of Aβ. We compared the aggregation pattern of Aβ(1-40) and that of Aβ(1-40)E22G, an amyloid peptide carrying the Arctic mutation that causes early onset of the disease. We found that in the presence of Ca(2+), Aβ(1-40) preferentially formed oligomers similar to those formed by Aβ(1-40)E22G with or without added Ca(2+), whereas in the absence of added Ca(2+) the Aβ(1-40) aggregated to form fibrils. Morphological similarities of the oligomers were confirmed by contact mode atomic force microscopy imaging. The distribution of oligomeric and fibrillar species in different samples was detected by gel electrophoresis and Western blot analysis, the results of which were further supported by thioflavin T fluorescence experiments. In the samples without Ca(2+), Fourier transform infrared spectroscopy revealed conversion of oligomers from an anti-parallel β-sheet to the parallel β-sheet conformation characteristic of fibrils. Overall, these results led us to conclude that calcium ions stimulate the formation of oligomers of Aβ(1-40), that have been implicated in the pathogenesis of AD.

Redox regulation of mitochondrial fission, protein misfolding, synaptic damage, and neuronal cell death: potential implications for Alzheimer's and Parkinson's diseases

Normal mitochondrial dynamics consist of fission and fusion events giving rise to new mitochondria, a process termed mitochondrial biogenesis. However, several neurodegenerative disorders manifest aberrant mitochondrial dynamics, resulting in morphological abnormalities often associated with deficits in mitochondrial mobility and cell bioenergetics. Rarely, dysfunctional mitochondrial occur in a familial pattern due to genetic mutations, but much more commonly patients manifest sporadic forms of mitochondrial disability presumably related to a complex set of interactions of multiple genes (or their products) with environmental factors (G × E). Recent studies have shown that generation of excessive nitric oxide (NO), in part due to generation of oligomers of amyloid-β (Aβ) protein or overactivity of the NMDA-subtype of glutamate receptor, can augment mitochondrial fission, leading to frank fragmentation of the mitochondria. S-Nitrosylation, a covalent redox reaction of NO with specific protein thiol groups, represents one mechanism contributing to NO-induced mitochondrial fragmentation, bioenergetic failure, synaptic damage, and eventually neuronal apoptosis. Here, we summarize our evidence in Alzheimer’s disease (AD) patients and animal models showing that NO contributes to mitochondrial fragmentation via S-nitrosylation of dynamin-related protein 1 (Drp1), a protein involved in mitochondrial fission. These findings may provide a new target for drug development in AD. Additionally, we review emerging evidence that redox reactions triggered by excessive levels of NO can contribute to protein misfolding, the hallmark of a number of neurodegenerative disorders, including AD and Parkinson’s disease. For example, S-nitrosylation of parkin disrupts its E3 ubiquitin ligase activity, and thereby affects Lewy body formation and neuronal cell death.

Diverse roles of extracellular calcium-sensing receptor in the central nervous system

The G-protein-coupled calcium-sensing receptor (CaSR), upon activation by Ca(2+) or other physiologically relevant polycationic molecules, performs diverse functions in the brain. The CaSR is widely expressed in the central nervous system (CNS) and is characterized by a robust increase in its expression during postnatal brain development over adult levels throughout the CNS. Developmental increases in CaSR levels in brain correlate with myelinogenesis. Indeed, neural stem cells differentiating to the oligodendrocyte lineage exhibit the highest CaSR expression compared with those differentiating to astrocytic or neuronal lineages. In adult CNS, CaSR has broad relevance in maintaining local ionic homeostasis. CaSR shares an evolutionary relationship with the metabotropic glutamate receptor and forms heteromeric complexes with the type B-aminobutyric acid receptor subunits that affects its cell surface expression, activation, signaling, and functions. In normal physiology as well as in pathologic conditions, CaSR is activated by signals arising from mineral ions, amino acids, polyamines, glutathione, and amyloid-beta in conjunction with Ca(2+) and other divalent cationic ligands. CaSR activation regulates membrane excitability of neurons and glia and affects myelination, olfactory and gustatory signal integration, axonal and dendritic growth, and gonadotrophin-releasing hormonal-neuronal migration. Insofar as the CaSR is a clinically important therapeutic target for parathyroid disorders, development of its agonists or antagonists as therapeutics for CNS disorder could be a major breakthrough.

The galanin receptor 2/3 agonist Gal2-11 protects the SN56 cells against beta-amyloid 25-35 toxicity

The neuropeptide galanin is a modulator of cholinergic function and may play a role in A beta peptide-induced degeneration of cholinergic forebrain neurons. We have studied the effect of galanin and its galanin receptor subtype 2/3 agonist Gal2-11on toxicity induced by freshly-prepared beta-amyloid(25-35) in the cholinergic cell line SN56. Both nuclear fragmentation and caspase-3 expression were analysed. beta-amyloid(25-35)-exposure induced a significant increase in caspase-3 mRNA expression after 30, 60, 90 or 150 min of beta-amyloid(25-35) exposure. These effects were abolished in the presence of Gal2-11 (10 nM). Similarly, beta-amyloid(25-35)-induced nuclear fragmentation was prevented by the galanin agonist at all time points studied. These findings indicate that the galanin 2/3 agonist Gal2-11 protects SN56 cholinergic cells from beta-amyloid(25-35)-induced cell death and that this action is mediated by an early reduction of caspase-3 expression.

Membrane biophysics and mechanics in Alzheimer's disease

Alzheimer's disease is a chronic neurodegenerative disorder characterized by neuronal loss, cerebrovascular inflammation, and accumulation of senile plaques in the brain parenchyma and cerebral blood vessels. Amyloid-beta peptide (Abeta), a major component of senile plaques, has been shown to exert multiple toxic effects to neurons, astrocytes, glial cells, and brain endothelium. Oligomeric Abeta can disturb the structure and function of cell membranes and alter membrane mechanical properties, such as membrane fluidity and molecular order. Much of these effects are attributed to their capability to trigger oxidative stress and inflammation. In this review, we discuss the effects of Abeta on neuronal cells, astrocytes, and cerebral endothelial cells with special emphasis on cell membrane properties and cell functions.

Calcium signaling and amyloid toxicity in Alzheimer disease

Intracellular Ca(2+) signaling is fundamental to neuronal physiology and viability. Because of its ubiquitous roles, disruptions in Ca(2+) homeostasis are implicated in diverse disease processes and have become a major focus of study in multifactorial neurodegenerative diseases such as Alzheimer disease (AD). A hallmark of AD is the excessive production of beta-amyloid (Abeta) and its massive accumulation in amyloid plaques. In this minireview, we highlight the pathogenic interactions between altered cellular Ca(2+) signaling and Abeta in its different aggregation states and how these elements coalesce to alter the course of the neurodegenerative disease. Ca(2+) and Abeta intersect at several functional levels and temporal stages of AD, thereby altering neurotransmitter receptor properties, disrupting membrane integrity, and initiating apoptotic signaling cascades. Notably, there are reciprocal interactions between Ca(2+) pathways and amyloid pathology; altered Ca(2+) signaling accelerates Abeta formation, whereas Abeta peptides, particularly in soluble oligomeric forms, induce Ca(2+) disruptions. A degenerative feed-forward cycle of toxic Abeta generation and Ca(2+) perturbations results, which in turn can spin off to accelerate more global neuropathological cascades, ultimately leading to synaptic breakdown, cell death, and devastating memory loss. Although no cause or cure is currently known, targeting Ca(2+) dyshomeostasis as an underlying and integral component of AD pathology may result in novel and effective treatments for AD.

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

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

Redox reactions induced by nitrosative stress mediate protein misfolding and mitochondrial dysfunction in neurodegenerative diseases

Overstimulation of N-methyl-D-aspartate (NMDA)-type glutamate receptors accounts, at least in part, for excitotoxic neuronal damage, potentially contributing to a wide range of acute and chronic neurologic diseases. Neurodegenerative disorders including Alzheimer's disease (AD) and Parkinson's disease (PD), manifest deposits of misfolded or aggregated proteins, and result from synaptic injury and neuronal death. Recent studies have suggested that nitrosative stress due to generation of excessive nitric oxide (NO) can mediate excitotoxicity in part by triggering protein misfolding and aggregation, and mitochondrial fragmentation in the absence of genetic predisposition. S-Nitrosylation, or covalent reaction of NO with specific protein thiol groups, represents a convergent signal pathway contributing to NO-induced protein misfolding and aggregation, compromised dynamics of mitochondrial fission-fusion process, thus leading to neurotoxicity. Here, we review the effect of S-nitrosylation on protein function under excitotoxic conditions, and present evidence suggesting that NO contributes to protein misfolding and aggregation via S-nitrosylating protein-disulfide isomerase or the E3 ubiquitin ligase parkin, and mitochondrial fragmentation through beta-amyloid-related S-nitrosylation of dynamin-related protein-1. Moreover, we also discuss that inhibition of excessive NMDA receptor activity by memantine, an uncompetitive/fast off-rate (UFO) drug can ameliorate excessive production of NO, protein misfolding and aggregation, mitochondrial fragmentation, and neurodegeneration.

Preventing Ca2+-mediated nitrosative stress in neurodegenerative diseases: possible pharmacological strategies

Overactivation of the NMDA-subtype of glutamate receptor is known to trigger excessive calcium influx, contributing to neurodegenerative conditions. Such dysregulation of calcium signaling results in generation of excessive free radicals, including reactive oxygen and nitrogen species (ROS/RNS), including nitric oxide (NO). In turn, we and our colleagues have shown that these free radicals trigger pathological production of misfolded proteins, mitochondrial dysfunction, and apoptotic pathways in neuronal cells. Here, we discuss emerging evidence that excessive calcium-induced NO production can contribute to the accumulation of misfolded proteins, specifically by S-nitrosylation of the ubiquitin E3 ligase, parkin, and the chaperone enzyme for nascent protein folding, protein-disulfide isomerase. Additionally, excessive calcium-induced NO generation leads to the formation of S-nitrosylated dynamin-related protein 1, which causes abnormal mitochondrial fragmentation and resultant synaptic damage. In this review, we also discuss how two novel classes of pharmacological agents hold promise to interrupt these pathological processes. Firstly, the NMDA receptor antagonists, Memantine and NitroMemantine, block excessive extrasynaptic glutamate excitation while maintaining synaptic transmission, thereby limiting excessive calcium influx and production of ROS/RNS. Secondly, therapeutic pro-electrophiles are activated in the face of oxidative insult, thus protecting cells from calcium-induced oxidative stress via the Keap1/Nrf2 transcriptional pathway.

Cellular stress and protein misfolding during aging

Cells are under constant onslaught from several intrinsic and extrinsic stressors, which lead to the occurrence and accumulation of molecular damage, functional impairment, aging, and eventual death. Protein misfolding is both a cause and a consequence of increased cellular stress. An age-related failure of the complex systems for handling protein misfolding results in the accumulation of misfolded and aggregated proteins, and consequent conformational diseases. However, some misfolded proteins have been found to be both toxic and, in some cases, protective, highlighting the various complex, dynamic, and interdependent mechanisms at play. Molecular mechanisms are being elucidated for the occurrence of protein misfolding and for its prevention by chaperones and various pathways of degradation. Insights from the knowledge about proteodynamics are likely to impact future interventional strategies to counter stress and to promote healthy aging by preventing and/or treatment of protein conformational diseases.

Alzheimer's disease: synaptic dysfunction and Abeta

Synapse loss is an early and invariant feature of Alzheimer's disease (AD) and there is a strong correlation between the extent of synapse loss and the severity of dementia. Accordingly, it has been proposed that synapse loss underlies the memory impairment evident in the early phase of AD and that since plasticity is important for neuronal viability, persistent disruption of plasticity may account for the frank cell loss typical of later phases of the disease. Extensive multi-disciplinary research has implicated the amyloid β-protein (Aβ) in the aetiology of AD and here we review the evidence that non-fibrillar soluble forms of Aβ are mediators of synaptic compromise. We also discuss the possible mechanisms of Aβ synaptotoxicity and potential targets for therapeutic intervention.

N-terminal truncated pyroglutamyl beta amyloid peptide Abetapy3-42 shows a faster aggregation kinetics than the full-length Abeta1-42

We tested directly the differences in the aggregation kinetics of three important beta amyloid peptides, the full-length Abeta1-42, and the two N-terminal truncated and pyroglutamil modified Abetapy3-42 and Abetapy11-42 found in different relative concentrations in the brains in normal aging and in Alzheimer disease. By following the circular dichroism signal and the ThT fluorescence of the solution in phosphate buffer, we found substantially faster aggregation kinetics for Abetapy3-42. This behavior is due to the particular sequence of this peptide, which is also responsible for the specific oligomeric aggregation states, found by TEM, during the fibrillization process, which are very different from those of Abeta1-42, more prone to fibril formation. In addition, Abetapy3-42 is found here to have an inhibitory effect on Abeta1-42 fibrillogenesis, coherently with its known greater infective power. This is an indication of the important role of this peptide in the aggregation process of beta-peptides in Alzheimer disease.

Decreased brain-derived neurotrophic factor depends on amyloid aggregation state in transgenic mouse models of Alzheimer's disease

Downregulation of brain-derived neurotrophic factor (BDNF) in the cortex occurs early in the progression of Alzheimer's disease (AD). Since BDNF plays a critical role in neuronal survival, synaptic plasticity, and memory, BDNF reduction may contribute to synaptic and cellular loss and memory deficits characteristic of AD. In vitro evidence suggests that amyloid-beta (A beta) contributes to BDNF downregulation in AD, but the specific A beta aggregation state responsible for this downregulation in vivo is unknown. In the present study, we examined cortical levels of BDNF mRNA in three different transgenic AD mouse models harboring mutations in APP resulting in A beta overproduction, and in a genetic mouse model of Down syndrome. Two of the three A beta transgenic strains (APP(NLh) and TgCRND8) exhibited significantly decreased cortical BDNF mRNA levels compared with wild-type mice, whereas neither the other strain (APP(swe)/PS-1) nor the Down syndrome mouse model (Ts65Dn) was affected. Only APP(NLh) and TgCRND8 mice expressed high A beta(42)/A beta(40) ratios and larger SDS-stable A beta oligomers (approximately 115 kDa). TgCRND8 mice exhibited downregulation of BDNF transcripts III and IV; transcript IV is also downregulated in AD. Furthermore, in all transgenic mouse strains, there was a correlation between levels of large oligomers, A beta(42)/A beta(40), and severity of BDNF decrease. These data show that the amount and species of A beta vary among transgenic mouse models of AD and are negatively correlated with BDNF levels. These findings also suggest that the effect of A beta on decreased BDNF expression is specific to the aggregation state of A beta and is dependent on large oligomers.

Ca(2+), within the physiological concentrations, selectively accelerates Abeta42 fibril formation and not Abeta40 in vitro

Alzheimer's disease (AD) in humans is a common progressive neurodegenerative disease, associated with cognitive dysfunction, memory loss and neuronal loss. Alzheimer peptides Abeta40 and Abeta42 are precursors of the amyloid fibers that accumulate in the brain of patients. These peptides misfold and the monomers aggregate to neurotoxic oligomers and fibrils. Thus, the aggregation kinetics of these peptides is central to understanding the etiology of AD. Using size exclusion chromatography as well as filtration methods, we report here that Ca(2+) ions at physiological concentrations greatly accelerate the rate of aggregation of Abeta42 to form intermediate soluble associated species and fibrils. In the presence of 1 or 2 mM Ca(2+), CD spectra indicated that the secondary structure of Abeta42 changed from an unfolded to a predominantly beta-sheet conformation. These concentrations of Ca(2+) greatly decreased the lag time for Abeta42 fibril formation, measured with thioflavin T. However, the elongation rate was apparently unaffected. Ca(2+) appears to predominantly accelerate the nucleation stage of Abeta42 on pathway to the Alzheimer's fibril formation. Unlike Abeta42, Ca(2+) was not observed to trigger similar effect at any stage during the study of fibrillation kinetics of Abeta40 by any techniques. Abeta40 and Abeta42 seem to have distinct aggregation pathways.

Evidence supporting a role for the calcium-sensing receptor in Alzheimer disease

The calcium-sensing receptor (CASR) is a G-protein coupled, transmembrane receptor that responds to changes in Ca(2+) levels. We hypothesized that the CASR could have a role in Alzheimer disease (AD) given expression of the CASR in brain, knowledge that calcium dysregulation promotes susceptibility to neuronal cell damage, the important role that the CASR plays in calcium regulation, and the fact that systemic calcium homeostasis and G-protein signal transduction are altered in AD patients. To investigate the association of CASR variation in AD susceptibility, we genotyped a polymorphic dinucleotide repeat marker within intron 4, one SNP within the promoter region and three non-synonymous SNPs within exon 7 of the CASR gene and tested for association analysis, using a well-characterized cohort of AD cases (n = 692) and controls (n = 435). The dinucleotide repeat polymorphism was significantly associated with AD status (OR = 1.62; 95% CI: 1.27-2.07, P = 0.00037, Bonferroni corrected P = 0.0011) and the three non-synonymous SNP haplotype was boarderline associated with AD status (P = 0.032, Bonferroni corrected P = 0.096). Stratifying by APOE4 allele carrier status revealed that the significant association was only in non-APOE4 carriers (OR of 1.90; 95% CI: 1.37-2.62, P = 0.0001). We also investigated whether apoE or beta amyloid could activate the calcium-sensing receptor. The receptor activation assays revealed that apoE as well as beta amyloid activated the CASR and that the level of activation appeared to be isoform dependent for apoE. These data support our hypothesis that the CASR has a role in AD susceptibility, particularly in individuals without an APOE4 allele.

Hybrid peptides attenuate cytotoxicity of beta-amyloid by inhibiting its oligomerization: implication from solvent effects

Abnormal assembly of monomeric beta-amyloid (Abeta) in Alzheimer's disease leads to the formation of most neurotoxic oligomers in vivo. In this study, we explored a linking strategy to design hybrid peptides, by combining the Abeta recognition motif and the solvent disruptive sequences. We found that in vitro all synthetic peptides with the recognition motif can affect Abeta fibrillization and alter the morphology of Abeta aggregates variously, different from those without the recognition motif. The effects of peptides containing recognition motif on Abeta aggregation correlate with their abilities to change the surface tension of solutions. In addition, compounds with the recognition motif, not those without such motif, can inhibit cytotoxicity of Abeta in cell culture probably by decreasing the amount of toxic Abeta oligomers. These results indicate that recognition domain and solvent effect should be considered as important factors when designing molecules to target Abeta aggregation.

Quenched hydrogen/deuterium exchange NMR characterization of amyloid-beta peptide aggregates formed in the presence of Cu2+ or Zn2+

Alzheimer's disease, a neurodegenerative disorder causing synaptic impairment and neuronal cell death, is strongly correlated with aggregation of the amyloid-beta peptide (Abeta). Divalent metal ions such as Cu(2+) and Zn(2+) are known to significantly affect the rate of aggregation and morphology of Abeta assemblies in vitro and are also found at elevated levels within cerebral plaques in vivo. The present investigation characterized the architecture of the aggregated forms of Abeta(1-40) and Abeta(1-42) in the presence or absence of either Cu(2+) or Zn(2+) using quenched hydrogen/deuterium exchange combined with solution NMR spectroscopy. The NMR analyses provide a quantitative and residue-specific structural characterization of metal-induced Abeta aggregates, showing that both the peptide sequence and the type of metal ion exert an impact on the final architecture. Common features among the metal-complexed peptide aggregates are two solvent-protected regions with an intervening minimum centered at Asn27, and a solvent-accessible N-terminal region, Asp1-Lys16. Our results suggest that Abeta in complex with either Cu(2+) or Zn(2+) can attain an aggregation-prone beta-strand-turn-beta-strand motif, similar to the motif found in fibrils, but where the metal binding to the N-terminal region guides the peptide into an assembly distinctly different from the fibril form.

The role of abnormal mitochondrial dynamics in the pathogenesis of Alzheimer's disease

Mitochondria play critical roles in neuronal function and almost all aspects of mitochondrial function are altered in Alzheimer neurons. Emerging evidence shows that mitochondria are dynamic organelles that undergo continuous fission and fusion, the balance of which not only controls mitochondrial morphology and number, but also regulates mitochondrial function and distribution. In this review, after a brief overview of the basic mechanisms involved in the regulation of mitochondrial fission and fusion and how mitochondrial dynamics affects mitochondrial function, we will discuss in detail our and others' recent work demonstrating abnormal mitochondrial morphology and distribution in Alzheimer's disease (AD) models and how these abnormalities may contribute to mitochondrial and synaptic dysfunction in AD. We propose that abnormal mitochondrial dynamics plays a key role in causing the dysfunction of mitochondria that ultimately damage AD neurons.

Cell death: protein misfolding and neurodegenerative diseases

Several chronic neurodegenerative disorders manifest deposits of misfolded or aggregated proteins. Genetic mutations are the root cause for protein misfolding in rare families, but the majority of patients have sporadic forms possibly related to environmental factors. In some cases, the ubiquitin-proteasome system or molecular chaperones can prevent accumulation of aberrantly folded proteins. Recent studies suggest that generation of excessive nitric oxide (NO) and reactive oxygen species (ROS), in part due to overactivity of the NMDA-subtype of glutamate receptor, can mediate protein misfolding in the absence of genetic predisposition. S-Nitrosylation, or covalent reaction of NO with specific protein thiol groups, represents one mechanism contributing to NO-induced protein misfolding and neurotoxicity. Here, we present evidence suggesting that NO contributes to protein misfolding via S-nitrosylating protein-disulfide isomerase or the E3 ubiquitin ligase parkin. We discuss how memantine/NitroMemantine can inhibit excessive NMDA receptor activity to ameliorate NO production, protein misfolding, and neurodegeneration.

Amyloid β peptide promotes differentiation of pro-inflammatory human myeloid dendritic cells

A key event of Alzheimer's disease (AD) pathogenesis is the production of amyloid beta peptides (A beta), which are hypothesized to lead to neurodegeneration by still unclear mechanisms, including a chronic inflammatory response characterized by innate immune cell activation and pro-inflammatory molecule release. Since dendritic cells (DCs) are central players of innate immune response and brain dendritic-like cells may have a crucial role in AD pathogenesis, this study investigates the effects of A beta on human DC functions. Myeloid DCs differentiated in the presence of A beta 42 showed an increase in survival and soluble antigen uptake, a reduction in HLA molecule expression and in IL-10 and IL-12 production. Accordingly, A beta 42-treated DCs were impaired in inducing T cell proliferation and IL-2 production. On the other hand, A beta 42 treatment provided DCs with the ability to release higher levels of IL-1 beta, IL-6 and IL-18, than control DCs. These results demonstrate that A beta 42 can modulate the immune system by inducing pro-inflammatory DC differentiation, thus gaining new insights into AD pathogenesis and immune-based therapeutic intervention.

Current Management of the Cognitive Dysfunction in Parkinson’s Disease: How Far Have We Come?

Parkinson’s disease (PD) clinical features comprise both motor and nonmotor manifestations. Among the nonmotor complications, dementia is the most important. Approximately 40% of PD patients are affected by cognitive impairment. Remarkably, in addition to age, dementia is an independent predictor of mortality, whereas age at onset of PD and severity of neurological symptoms are not. In this review, I summarize the current knowledge of the pathogenesis of the PD cognitive impairment in relation to the therapies presently accessible and those that could become strategic in the near future. It is hypothesized that patients with PD show two components of cognitive dysfunction (CD): a generalized profile of subcortical dementia (PDsCD), and an overlapped pattern suggesting specific prefrontal damage with CD (PDpFCD). PDsCD is associated with structural neocortical/subcortical changes in the brain (in frontal, parietal, limbic, and temporal lobes, as well as in midbrain structures). In PDpFCD cognitive deficits comprise impairments in neuropsychological tests sensitive for frontal lobe function (discrete elements of episodic and working memory for instance), which are considered to be the consequence of dysfunction in neuronal loops connecting the prefrontal cortex and basal ganglia. Drugs reviewed for targeting PDsCD include: cholinesterase inhibitors, agents with mixed cholinergic and dopaminergic properties, antiglutamatergic drugs, mixed antiglutamatergic/dopaminergic agents; antioxidants and enhancers of mitochondrial functions, and anti-COX-2, as well as other anti-inflammatory mediators. Preliminary studies with vehicles that may target PDpFCD include piribedil, tolcapone, amantadine, and farampator. Additional agents (citicoline and neuroimmuniphilines, among others) will be outlined. A brief overview on neuroprotection and promising new biological advances in PD (deep brain stimulation, stem cells, gene therapy) also will be summarized.

Etazolate, a neuroprotective drug linking GABA(A) receptor pharmacology to amyloid precursor protein processing

Pharmacological modulation of the GABA(A) receptor has gained increasing attention as a potential treatment for central processes affected in Alzheimer disease (AD), including neuronal survival and cognition. The proteolytic cleavage of the amyloid precursor protein (APP) through the alpha-secretase pathway decreases in AD, concurrent with cognitive impairment. This APP cleavage occurs within the beta-amyloid peptide (Abeta) sequence, precluding formation of amyloidogenic peptides and leading to the release of the soluble N-terminal APP fragment (sAPPalpha) which is neurotrophic and procognitive. In this study, we show that at nanomolar-low micromolar concentrations, etazolate, a selective GABA(A) receptor modulator, stimulates sAPPalpha production in rat cortical neurons and in guinea pig brains. Etazolate (20 nM-2 microM) dose-dependently protected rat cortical neurons against Abeta-induced toxicity. The neuroprotective effects of etazolate were fully blocked by GABA(A) receptor antagonists indicating that this neuroprotection was due to GABA(A) receptor signalling. Baclofen, a GABA(B) receptor agonist failed to inhibit the Abeta-induced neuronal death. Furthermore, both pharmacological alpha-secretase pathway inhibition and sAPPalpha immunoneutralization approaches prevented etazolate neuroprotection against Abeta, indicating that etazolate exerts its neuroprotective effect via sAPPalpha induction. Our findings therefore indicate a relationship between GABA(A) receptor signalling, the alpha-secretase pathway and neuroprotection, documenting a new therapeutic approach for AD treatment.

Site-specific blockade of RAGE-Vd prevents amyloid-beta oligomer neurotoxicity

In the genesis of Alzheimer's disease (AD), converging lines of evidence suggest that amyloid-beta peptide (Abeta) triggers a pathogenic cascade leading to neuronal loss. It was long assumed that Abeta had to be assembled into extracellular amyloid fibrils or aggregates to exert its cytotoxic effects. Over the past decade, characterization of soluble oligomeric Abeta species in the brains of AD patients and in transgenic models has raised the possibility that different conformations of Abeta may contribute to AD pathology via different mechanisms. The receptor for advanced glycation end products (RAGE), a member of the Ig superfamily, is a cellular binding site for Abeta. Here, we investigate the role of RAGE in apoptosis induced by distinct well characterized Abeta conformations: Abeta oligomers (AbetaOs), Abeta fibrils (AbetaFs), and Abeta aggregates (AbetaAs). In our in vitro system, treatment with polyclonal anti-RAGE antibodies significantly improves SHSY-5Y cell and neuronal survival exposed to either AbetaOs or AbetaAs but does not affect AbetaF toxicity. Interestingly, using site-specific antibodies, we demonstrate that targeting of the V(d) domain of RAGE attenuates AbetaO-induced toxicity in both SHSY-5Y cells and rat cortical neurons, whereas inhibition of AbetaA-induced apoptosis requires the neutralization of the C(1d) domain of the receptor. Thus, our data indicate that distinct regions of RAGE are involved in Abeta-induced cellular and neuronal toxicity with respect to the Abeta aggregation state, and they suggest the blockage of particular sites of the receptor as a potential therapeutic strategy to attenuate neuronal death.

Rifampicin does not prevent amyloid fibril formation by human islet amyloid polypeptide but does inhibit fibril thioflavin-T interactions: implications for mechanistic studies of beta-cell death

Amyloid formation has been implicated in more than 20 different human diseases, including Alzheimer's disease, Parkinson's disease, and type 2 diabetes. The development of inhibitors of amyloid is a topic of considerable interest, both because of their potential therapeutic applications and because they are useful mechanistic probes. Recent studies have highlighted the potential use of rifampicin as an inhibitor of amyloid formation by a variety of polypeptides; however, there are conflicting reports on its ability to inhibit amyloid formation by islet amyloid polypeptide (IAPP). IAPP is the cause of islet amyloid in type 2 diabetes. We show that rifampicin does not prevent amyloid formation by IAPP and does not disaggregate preformed IAPP amyloid fibrils;, instead, it interferes with standard fluorescence-based assays of amyloid formation. Rifampicin is unstable in aqueous solution and is readily oxidized. However, the effects of oxidized and reduced rifampicin are similar, in that neither prevents amyloid formation by IAPP. Furthermore, use of a novel p-cyanoPhe analogue of IAPP shows that rifampicin does not significantly affect the kinetics of IAPP amyloid formation. The implications for the development of amyloid inhibitors are discussed as are the implications for studies of the toxicity of islet amyloid. The work also demonstrates the utility of p-cyanoPhe IAPP for the screening of inhibitors. The data indicate that rifampicin cannot be used to test the relative toxicity of IAPP fibrils and prefibril aggregates of IAPP.

Coaccumulation of calcium and beta-amyloid in the thalamus after transient middle cerebral artery occlusion in rats

Transient occlusion of the middle cerebral artery (MCAO) in rats leads to abnormal accumulation of beta-amyloid (Abeta) peptides in the thalamus. This study investigated the chemical composition of these deposits. Adult male human beta-amyloid precursor protein (APP) overexpressing (hAPP695) rats and their wild-type littermates were subjected to transient MCAO for 2 h or sham operation. After 26-week survival time, histological examination revealed an overlapping distribution pattern for rodent and human Abeta in the thalamus of hAPP695 rats subjected to MCAO. X-ray microanalysis showed that the deposits did not contain significant amount of iron, zinc, or copper typical to senile plaques. In contrast, the deposit both in hAPP695 and non-transgenic rats contained calcium and phosphorus in a ratio (1.28+/-0.15) characteristic to hydroxyapatites. Alizarin red staining confirmed that calcium coaccumulated in these Abeta deposits. It is suggested that APP expression is induced by ischemic insult in cortical neurons adjacent to infarct, which in turn is reflected as increased release of Abeta peptides by their corticothalamic axon endings. This together with insufficient clearance or atypical degradation of Abeta peptides lead to dysregulation of calcium homeostatis and coaccumulation in the thalamus.

Mixed macromolecular crowding inhibits amyloid formation of hen egg white lysozyme

The effects of two single macromolecular crowding agents, Ficoll 70 and bovine serum albumin (BSA), and one mixed macromolecular crowding agent containing both BSA and Ficoll 70, on amyloid formation of hen egg white lysozyme have been examined by thioflavin T binding, Congo red binding, transmission electron microscopy, and activity assay, as a function of crowder concentration and composition. Both the mixed crowding agent and the protein crowding agent BSA at 100 g/l almost completely inhibit amyloid formation of lysozyme and stabilize lysozyme activity on the investigated time scale, but Ficoll 70 at the same concentration neither impedes amyloid formation of lysozyme effectively nor stabilizes lysozyme activity. Further kinetic and isothermal titration calorimetry analyses indicate that a mixture of 5 g/l BSA and 95 g/l Ficoll 70 inhibits amyloid formation of lysozyme and maintains lysozyme activity via mixed macromolecular crowding as well as weak, nonspecific interactions between BSA and nonnative lysozyme. Our data demonstrate that BSA and Ficoll 70 cooperatively contribute to both the inhibitory effect and the stabilization effect of the mixed crowding agent, suggesting that mixed macromolecular crowding inside the cell may play a role in posttranslational quality control mechanism.

Emerging roles of S-nitrosylation in protein misfolding and neurodegenerative diseases

Overactivation of N-methyl-D-aspartate (NMDA)-type glutamate receptors accounts, at least in part, for excitotoxic neuronal damage, potentially contributing to a wide range of acute and chronic neurologic disorders. Recent studies have suggested that generation of excessive nitric oxide (NO) and reactive oxygen species (ROS) can mediate excitotoxicity, in part by triggering protein misfolding. S-Nitrosylation, which is a covalent reaction of a NO group with a cysteine thiol, represents one such mechanism that can contribute to NO-induced neurotoxicity. The ubiquitin-proteasome system (UPS), in conjunction with molecular chaperones, can prevent accumulation of aberrantly-folded proteins. For example, protein disulfide isomerase (PDI) can provide neuroprotection from misfolded proteins or endoplasmic reticulum stress through its molecular chaperone and thiol-disulfide oxidoreductase activities. Here, the authors present recent evidence suggesting that NO contributes to degenerative conditions by S-nitrosylating PDI (forming SNO-PDI) and the ubiquitin protein ligase, parkin (forming SNO-parkin). Moreover, it is demonstrated for the first time that inhibition of excessive NMDA receptor activity by memantine, via a mechanism of uncompetitive open-channel block with a relatively rapid off-rate, can ameliorate excessive production of NO, protein misfolding, and neurodegeneration.

Insights into amyloid-beta-induced mitochondrial dysfunction in Alzheimer disease

Amyloid-beta has long been implicated in the pathogenesis of Alzheimer disease. The focus was initially on the extracellular fibrillar deposits of amyloid-beta but more recently has shifted to intracellular oligomeric forms of amyloid-beta. Unfortunately, the mechanism(s) by which either extracellular or intracellular amyloid-beta induces neuronal toxicity remains unclear. That said, a number of recent studies indicate that mitochondria might be an important target of amyloid-beta. Neurons rely heavily on mitochondria for energy and it is well established that mitochondrial dysfunction might be an important target of amyloid-beta. Mechanistically, amyloid-beta aggregates in mitochondria to impair function, leading to energy hypometabolism and elevated reactive oxygen species production. Additionally, amyloid-beta affects the balance of mitochondrial fission/fusion and mitochondrial transport, negatively impacting a host of cellular functions of neurons. Here, we review the role that amyloid-beta plays in mitochondrial structure and function of neurons and the importance of this in the pathogenesis of Alzheimer disease.