Beta-amyloid inhibits integrated mitochondrial respiration and key enzyme activities

The spirostenol (22R, 25R)-20alpha-spirost-5-en-3beta-yl hexanoate blocks mitochondrial uptake of Abeta in neuronal cells and prevents Abeta-induced impairment of mitochondrial function

Abeta(1-42) has been shown to uncouple the mitochondrial respiratory chain and promote the opening of the membrane permeability transition (MPT) pore, leading to cell death. We have previously reported that the spirostenol derivative (22R, 25R)-20alpha-spirost-5-en-3beta-yl hexanoate (SP-233) protects neuronal cells against Abeta(1-42) toxicity by binding to and inactivating the peptide. Picomolar concentrations of Abeta(1-42) decreased the mitochondrial respiratory coefficient in mitochondria isolated from the rat forebrain, and this decrease was partially reversed by SP-233. SP-233 abolished the uncoupling of oxidative phosphorylation induced by carbonyl cyanide 3-chlorophenylhydrazone on isolated mitochondria. These results are consistent with a direct effect of SP-233 on the MPT. Moreover, SP-233 displayed a neuroprotective effect on SK-N-AS human neuroblastoma cells treated with the MPT promoter, phenylarsine oxide. Treatment of SK-N-AS cells with Abeta(1-42) resulted in an accumulation of the peptide in the mitochondrial matrix; SP-233 completely scavenged Abeta(1-42) from the matrix. In addition, SP-233 protected the cells against mitochondrial toxins targeting complexes IV and V of the respiratory chain. These results indicate that Abeta(1-42) and SP-233 exert direct effects on mitochondrial function and SP-233 protects neuronal cells against Abeta-induced toxicity by targeting Abeta directly.

Huperzine A attenuates mitochondrial dysfunction in beta-amyloid-treated PC12 cells by reducing oxygen free radicals accumulation and improving mitochondrial energy metabolism

We observed previously that huperzine A (HupA), a selective acetylcholinesterase inhibitor, can counteract neuronal apoptosis and cell damage induced by several neurotoxic substances, and that this neuroprotective action somehow involves the mitochondria. We investigated the ability of HupA to reduce mitochondrial dysfunction in neuron-like rat pheochromocytoma (PC12) cells exposed in culture to the amyloid beta-peptide fragment 25-35 (Abeta(25-35)). After exposure to 1 microM Abeta(25-35) for various periods, cells exhibited a rapid decline of ATP levels and obvious disruption of mitochondrial membrane homeostasis and integrity as determined by characteristic morphologic alterations, reduced membrane potential, and decreased activity of ion transport proteins. In addition, Abeta(25-35) treatment also led to inhibition of key enzyme activities in the electron transport chain and the tricarboxylic acid cycle, as well as an increase of intracellular reactive oxygen species (ROS). Pre-incubation with HupA for 2 hr not only attenuated these signs of cellular stress caused by Abeta, but also enhanced ATP concentration and decreased ROS accumulation in unharmed normal cells. Those results indicate that HupA protects mitochondria against Abeta-induced damages, at least in part by inhibiting oxidative stress and improving energy metabolism, and that these protective effects reduce the apoptosis of neuronal cells exposed to this toxic peptide.

Conversion of MCI to dementia: Role of proton magnetic resonance spectroscopy

Mild cognitive impairment (MCI) represents a heterogeneous group of cognitive disturbances at high risk of dementia. The amnestic subtype (aMCI) might be a prodromal state of Alzheimer's disease (AD). The aim of this study is the identification, by proton magnetic resonance spectroscopy (1H MRS), of modifications in brain metabolites able to detect subjects with aMCI at risk of conversion towards AD. Twenty-five subjects with aMCI and 29 normal elderly were enrolled; they underwent a comprehensive clinical and instrumental assessment, a cerebral 1H MRS scan to measure N-acetyl aspartate (NAA), choline (Cho), myo-inositol (mI) and creatine (Cr) in the paratrigonal white matter, bilaterally. After 1 year, 5 MCI subjects became demented (progressive MCI, pMCI). Their baseline levels of metabolites were compared with those evaluated in stable MCI (sMCI) and in controls. We observed a significant difference of the NAA/Cr ratio between pMCI (1.48+/-0.08) and sMCI (1.65+/-0.12) and between pMCI and controls (1.63+/-0.16) in the left hemisphere, suggesting that this metabolic alteration can be detected before the clinical appearance of dementia.

Mitochondrial disease

Defects of mitochondrial metabolism cause a wide range of human diseases that include examples from all medical subspecialties. This review updates the topic of mitochondrial diseases by reviewing the most important recent advances in this area. The factors influencing inheritance, maintenance and replication of mtDNA are reviewed and the genotype-phenotype of mtDNA disorders has been expanded, with new insights into epidemiology, pathogenesis and its role in ageing. Recently identified nuclear gene mutations of mitochondrial proteins include mutations of frataxin causing Friedreich's ataxia, PINK1, DJ1 causing Parkinson's disease and POLG causing infantile mtDNA depletion syndrome, ophthalmoplegia, parkinsonism, male subfertility and, in a transgenic mouse model, premature senescence. Mitochondrial defects in neurodegenerative diseases include Parkinson's, Alzheimer's and Huntington's disease. Improved understanding of mtDNA inheritance and mutation penetrance patterns, and novel techniques for mtDNA modification offer significant prospects for more accurate genetic counselling and effective future therapies.

The role of an astrocytic NADPH oxidase in the neurotoxicity of amyloid beta peptides

Amyloid beta peptide (Abeta) accumulates in the CNS in Alzheimer's disease. Both the full peptide (1-42) or the 25-35 fragment are toxic to neurons in culture. We have used fluorescence imaging technology to explore the mechanism of neurotoxicity in mixed asytrocyte/neuronal cultures prepared from rat or mouse cortex or hippocampus, and have found that Abeta acts preferentially on astrocytes but causes neuronal death. Abeta causes sporadic transient increases in [Ca2+]c in astrocytes, associated with a calcium dependent increased generation of reactive oxygen species (ROS) and glutathione depletion. This caused a slow dissipation of mitochondrial potential on which abrupt calcium dependent transient depolarizations were superimposed. The mitochondrial depolarization was reversed by mitochondrial substrates glutamate, pyruvate or methyl succinate, and by NADPH oxidase (NOX) inhibitors, suggesting that it reflects oxidative damage to metabolic pathways upstream of mitochondrial complex I. The Abeta induced increase in ROS and the mitochondrial depolarization were absent in cells cultured from transgenic mice lacking the NOX component, gp91phox. Neuronal death after 24 h of Abeta exposure was dramatically reduced both by NOX inhibitors and in gp91phox knockout mice. Thus, by raising [Ca2+]c in astrocytes, Abeta activates NOX, generating oxidative stress that is transmitted to neurons, causing neuronal death.

Alpha-ketoglutarate dehydrogenase: a target and generator of oxidative stress

Alpha-ketoglutarate dehydrogenase (alpha-KGDH) is a highly regulated enzyme, which could determine the metabolic flux through the Krebs cycle. It catalyses the conversion of alpha-ketoglutarate to succinyl-CoA and produces NADH directly providing electrons for the respiratory chain. alpha-KGDH is sensitive to reactive oxygen species (ROS) and inhibition of this enzyme could be critical in the metabolic deficiency induced by oxidative stress. Aconitase in the Krebs cycle is more vulnerable than alpha-KGDH to ROS but as long as alpha-KGDH is functional NADH generation in the Krebs cycle is maintained. NADH supply to the respiratory chain is limited only when alpha-KGDH is also inhibited by ROS. In addition being a key target, alpha-KGDH is able to generate ROS during its catalytic function, which is regulated by the NADH/NAD+ ratio. The pathological relevance of these two features of alpha-KGDH is discussed in this review, particularly in relation to neurodegeneration, as an impaired function of this enzyme has been found to be characteristic for several neurodegenerative diseases.

Bis(7)-tacrine attenuates beta amyloid-induced neuronal apoptosis by regulating L-type calcium channels

Beta amyloid protein (Abeta) and acetylcholinesterase (AChE) have been shown to be closely implicated in the pathogenesis of Alzheimer's disease. In the current study, we investigated the effects of bis(7)-tacrine, a novel dimeric AChE inhibitor, on Abeta-induced neurotoxicity in primary cortical neurons. Bis(7)-tacrine, but not other AChE inhibitors, elicited a marked reduction of both fibrillar and soluble oligomeric forms of Abeta-induced apoptosis as evidenced by chromatin condensation and DNA specific fragmentation. Both nicotinic and muscarinic receptor antagonists failed to block the effects of bis(7)-tacrine. Instead, nimodipine, a blocker of L-type voltage-dependent Ca2+ channels (VDCCs), attenuated Abeta neurotoxicity, whereas N-, P/Q- or R-type VDCCs blockers and ionotropic glutamate receptor antagonists did not. Fluorescence Ca2+ imaging assay revealed that, similar to nimodipine, bis(7)-tacrine reversed Abeta-triggered intracellular Ca2+ increase, which was mainly contributed by the extracellular Ca2+ instead of endoplasmic reticulum and mitochondria Ca2+. Concurrently, using whole cell patch-clamping technique, it was found that bis(7)-tacrine significantly reduced the augmentation of high voltage-activated inward calcium currents induced by Abeta. These results suggest that bis(7)-tacrine attenuates Abeta-induced neuronal apoptosis by regulating L-type VDCCs, offers a novel modality as to how the agent exerts neuroprotective effects.

The role of mitochondria in inherited neurodegenerative diseases

In the past decade, the genetic causes underlying familial forms of many neurodegenerative disorders, such as Huntington's disease, Parkinson's disease, Alzheimer's disease, amyotrophic lateral sclerosis, Friedreich ataxia, hereditary spastic paraplegia, dominant optic atrophy, Charcot-Marie-Tooth type 2A, neuropathy ataxia and retinitis pigmentosa, and Leber's hereditary optic atrophy have been elucidated. However, the common pathogenic mechanisms of neuronal death are still largely unknown. Recently, mitochondrial dysfunction has emerged as a potential 'lowest common denominator' linking these disorders. In this review, we discuss the body of evidence supporting the role of mitochondria in the pathogenesis of hereditary neurodegenerative diseases. We summarize the principal features of genetic diseases caused by abnormalities of mitochondrial proteins encoded by the mitochondrial or the nuclear genomes. We then address genetic diseases where mutant proteins are localized in multiple cell compartments, including mitochondria and where mitochondrial defects are likely to be directly caused by the mutant proteins. Finally, we describe examples of neurodegenerative disorders where mitochondrial dysfunction may be 'secondary' and probably concomitant with degenerative events in other cell organelles, but may still play an important role in the neuronal decay. Understanding the contribution of mitochondrial dysfunction to neurodegeneration and its pathophysiological basis will significantly impact our ability to develop more effective therapies for neurodegenerative diseases.

Nitric oxide, cell bioenergetics and neurodegeneration

Following stimulation of NMDA receptors, neurons transiently synthesize nitric oxide (NO) in a calcium/calmodulin-dependent manner through the activation of neuronal NO synthase. Nitric oxide acts as a messenger, activating soluble guanylyl cyclase and participating in the transduction signalling pathways involving cyclic GMP. Nitric oxide also binds to cytochrome c oxidase, and is able to inhibit cell respiration in a process that is reversible and in competition with oxygen. This action can also lead to the release of superoxide anion from the mitochondrial respiratory chain. Here, we discuss recent evidence that this mitochondrial interaction represents a molecular switch for cell signalling pathways involved in the control of physiological functions. These include superoxide- or oxygen-dependent modulation of gene transcription, calcium-dependent cell signalling responses, changes in the mitochondrial membrane potential or AMP-activated protein kinase-dependent control of glycolysis. In pathophysiological conditions, such as brain ischaemia or neurological disorders, NO is formed excessively by NMDA receptor over-activation in neurons, or by inducible NO synthase from neighbouring glia (microglial cells and astrocytes). Elevated NO concentrations can then interact with superoxide anion, generated by the mitochondria or by other mechanisms, leading to the formation of the powerful oxidant species peroxynitrite. During pathological conditions activation of the NAD(+)-consuming enzyme poly(APD-ribose) polymerase-1 (PARP-1) is also a likely mechanism for NO-mediated energy failure and neurotoxicity. Activation of PARP-1 is, however, a repair process, which in milder forms of oxidative stress protects neurons from death. Thus, whilst NO plays a physiological role in neuronal cell signalling, its over-production may cause neuronal energy compromise leading to neurodegeneration.

Mitochondrial dysfunction in sporadic and genetic Alzheimer's disease

Increasing evidence suggests an important role of mitochondrial dysfunction in the pathogenesis of many common age-related neurodegenerative diseases, including Alzheimer's disease (AD). AD is the most common neurodegenerative disorder characterized by dementia, memory loss, neuronal apoptosis and eventually death of the affected individuals. AD is characterized by two pathologic hallmark lesions that consist of extracellular plaques of amyloid-beta peptides and intracellular neurofibrillary tangles composed of hyperphosphorylated microtubular protein tau. Even though the idea that amyloid beta peptide accumulation is the primary event in the pathogenesis of Alzheimer's disease has become the leading hypothesis, the causal link between aberrant amyloid precursor protein and tau alterations in this type of dementia remains controversial.

Identification of amyloid-beta 1–42 binding protein fragments by screening of a human brain cDNA library

Extracellular and intraneuronal formation of amyloid-beta (Abeta) deposits have been demonstrated to be involved in the pathogenesis of Alzheimer's disease (AD). However, the precise mechanism of Abeta neurotoxicity is not completely understood. Previous studies suggest that binding of Abeta with a number of targets have deleterious effects on cellular functions. It has been shown that Abeta directly interacted with intracellular protein ERAB (endoplasmic reticulum amyloid beta-peptide-binding protein) also known as ABAD (Abeta-binding alcohol dehydrogenase) resulting in mitochondrial dysfunction and cell death. In the present study we have identified another mitochondrial enzyme, ND3 of the human complex I, that binds to Abeta1-42 by the screening of a human brain cDNA library expressed on M13 phage. Our results indicated a strong interaction between Abeta and a phage-displayed 25 amino acid long peptide TTNLPLMVMSSLLLIIILALSLAYE corresponding to C-terminal peptide domain of NADH dehydrogenase, subunit 3 (MTND3) encoded by mitochondrial DNA (mtDNA). This interaction may explain, in part, the inhibition of complex I activity in astrocytes and neurons in the presence of Abeta, described recently. To our knowledge, the present study is the first demonstration of interaction between Abeta and one of the subunits of the human complex I.

Mitochondria are a direct site of A beta accumulation in Alzheimer's disease neurons: implications for free radical generation and oxidative damage in disease progression

Alzheimer's disease (AD) is a complex, neurodegenerative disease characterized by the impairment of cognitive function in elderly individuals. In a recent global gene expression study of APP transgenic mice, we found elevated expression of mitochondrial genes, which we hypothesize represents a compensatory response because of mitochondrial oxidative damage caused by the over-expression of mutant APP and/or amyloid beta (Abeta). We investigated this hypothesis in a series of experiments examining what forms of APP and Abeta localize to the mitochondria, and whether the presence of these species is associated with mitochondrial dysfunction and oxidative damage. Using immunoblotting, digitonin fractionation, immunofluorescence, and electron microscopy techniques, we found a relationship between mutant APP derivatives and mitochondria in brain slices from Tg2576 mice and in mouse neuroblastoma cells expressing mutant human APP. Further, to determine the functional relationship between mutant APP/Abeta and oxidative damage, we quantified Abeta levels, hydrogen peroxide production, cytochrome oxidase activity and carbonyl proteins in Tg2576 mice and age-matched wild-type (WT) littermates. Hydrogen peroxide levels were found to be significantly increased in Tg2576 mice when compared with age-matched WT littermates and directly correlated with levels of soluble Abeta in Tg2576 mice, suggesting that soluble Abeta may be responsible for the production of hydrogen peroxide in AD progression in Tg2576 mice. Cytochrome c oxidase activity was found to be decreased in Tg2576 mice when compared with age-matched WT littermates, suggesting that mutant APP and soluble Abeta impair mitochondrial metabolism in AD development and progression. An increase in hydrogen peroxide and a decrease in cytochrome oxidase activity were found in young Tg2576 mice, prior to the appearance of Abeta plaques. These findings suggest that early mitochondrially targeted therapeutic interventions may be effective in delaying AD progression in elderly individuals and in treating AD patients.

Mitochondrial dysfunction and apoptosis underlie the pathogenic process in alpha-B-crystallin desmin-related cardiomyopathy

BACKGROUND

Mitochondria and sarcomeres have a well-defined architectural relation that partially depends on the integrity of the cytoskeletal network. An R120G missense mutation in the small heat shock protein alpha-B-crystallin (CryAB) causes desmin-related cardiomyopathy. Desmin-related cardiomyopathy is characterized by the formation of intracellular aggregates containing CryAB and desmin that are amyloid positive, and disease can be recapitulated in transgenic mice by cardiac-specific expression of the mutant protein.

METHODS AND RESULTS

To understand the resultant pathology, we explored the acute effects of R120G expression both in vitro and in vivo. In vitro, transfection of adult cardiomyocytes with R120G-expressing adenovirus resulted in altered contractile mechanics. In vivo, as the cytoskeletal network is disturbed but before deficits in organ function can be detected, alterations in mitochondrial organization and architecture occur, leading to a reduction in the maximal rate of oxygen consumption with substrates that utilize complex I activity, alterations in the permeability transition pore, and compromised inner membrane potential. Apoptotic pathways are subsequently activated, which eventually results in cardiomyocyte death, dilation, and heart failure.

CONCLUSIONS

Cardiac chaperone dysfunction acutely leads to altered cardiomyocyte mechanics, perturbations in mitochondrial-sarcomere architecture, and deficits in mitochondrial function, which can result in activation of apoptosis and heart failure.

Mechanisms of hypoxic tolerance in presymptomatic APP23 transgenic mice

In the B6-Tg (ThylAPP)23Sdz (APP23tg) transgenic mouse model of Alzheimer's disease hypoxic tolerance is impaired prior to amyloid deposition. We therefore investigated mechanisms known to mediate resistance to hypoxic episodes in presymptomatic APP23tg and appropriate control strains. The mRNA expression levels in the hippocampus of adenosine receptor subtypes A1 and A3, estrogen receptors alpha and beta, progesterone receptor, and neuronal and endothelial nitric oxide synthase were investigated with semi-quantitative RT-PCR. Mice were pretreated in vivo with a low dose of 3-nitropropionate, an inhibitor of succinic dehydrogenase, known to mediate hypoxic tolerance within 1h. We found increased expression levels in presymptomatic, untreated APP23tg animals of adenosine A3 receptor mRNA and estrogen receptor alpha mRNA. In addition, we observed an increase in nNOS expression levels upon mild cellular hypoxia induced by 3-NP in transgenic but not in wild-type animals. We conclude that overexpression of human APP results in differential expression of receptors conferring hypoxic tolerance prior to amyloid deposition. Up-regulation of nNOS expression levels upon hypoxic challenge in APP23tg transgenic animals may therefore reflect a selective vulnerability in these animals even before amyloid deposition.

Amyloid precursor protein-mediated free radicals and oxidative damage: Implications for the development and progression of Alzheimer's disease

Alzheimer's disease (AD) is a late-onset dementia that is characterized by the loss of memory and an impairment of multiple cognitive functions. Advancements in molecular, cellular, and animal model studies have revealed that the formation of amyloid beta (Abeta) and other derivatives of the amyloid precursor protein (APP) are key factors in cellular changes in the AD brain, including the generation of free radicals, oxidative damage, and inflammation. Recent molecular, cellular, and gene expression studies have revealed that Abeta enters mitochondria, induces the generation of free radicals, and leads to oxidative damage in post-mortem brain neurons from AD patients and in brain neurons from cell models and transgenic mouse models of AD. In the last three decades, tremendous progress has been made in mitochondrial research and has provided significant findings to link mitochondrial oxidative damage and neurodegenerative diseases such as AD. Researchers in the AD field are beginning to recognize the possible involvement of a mutant APP and its derivatives in causing mitochondrial oxidative damage in AD. This article summarizes the latest research findings on the generation of free radicals in mitochondria and provides a possible model that links Abeta proteins, the generation of free radicals, and oxidative damage in AD development and progression.

Gradual alteration of mitochondrial structure and function by beta-amyloids: importance of membrane viscosity changes, energy deprivation, reactive oxygen species production, and cytochrome c release

Intracellular amyloid beta-peptide (A beta) accumulation is considered to be a key pathogenic factor in sporadic Alzheimer's disease (AD), but the mechanisms by which it triggers neuronal dysfunction remain unclear. We hypothesized that gradual mitochondrial dysfunction could play a central role in both initiation and progression of sporadic AD. Thus, we analyzed changes in mitochondrial structure and function following direct exposure to increasing concentrations of A beta(1--42) and A beta(25--35) in order to look more closely at the relationships between mitochondrial membrane viscosity, ATP synthesis, ROS production, and cytochrome c release. Our results show the accumulation of monomeric A beta within rat brain and muscle mitochondria. Subsequently, we observed four different and additive modes of action of A beta, which were concentration dependent: (i) an increase in mitochondrial membrane viscosity with a concomitant decrease in ATP/O, (ii) respiratory chain complexes inhibition, (iii) a potentialization of ROS production, and (iv) cytochrome c release.

Focally elevated creatine detected in amyloid precursor protein (APP) transgenic mice and Alzheimer disease brain tissue

The creatine/phosphocreatine system, regulated by creatine kinase, plays an important role in maintaining energy balance in the brain. Energy metabolism and the function of creatine kinase are known to be affected in Alzheimer diseased brain and in cells exposed to the beta-amyloid peptide. We used infrared microspectroscopy to examine hippocampal, cortical, and caudal tissue from 21-89-week-old transgenic mice expressing doubly mutant (K670N/M671L and V717F) amyloid precursor protein and displaying robust pathology from an early age. Microcrystalline deposits of creatine, suggestive of perturbed energetic status, were detected by infrared microspectroscopy in all animals with advanced plaque pathology. Relatively large creatine deposits were also found in hippocampal sections from post-mortem Alzheimer diseased human brain, compared with hippocampus from non-demented brain. We therefore speculate that this molecule is a marker of the disease process.

Mitochondrial Abeta: a potential focal point for neuronal metabolic dysfunction in Alzheimer's disease

Although amyloid-beta peptide (Abeta) is the neurotoxic species implicated in the pathogenesis of Alzheimer's disease (AD), mechanisms through which intracellular Abeta impairs cellular properties, resulting in neuronal dysfunction, remain to be clarified. Here we demonstrate that intracellular Abeta is present in mitochondria from brains of transgenic mice with targeted neuronal overexpression of mutant human amyloid precursor protein and AD patients. Abeta progressively accumulates in mitochondria and is associated with diminished enzymatic activity of respiratory chain complexes (III and IV) and a reduction in the rate of oxygen consumption. Importantly, mitochondria-associated Abeta, principally Abeta42, was detected as early as 4 months, before extensive extracellular Abeta deposits. Our studies delineate a new means through which Abeta potentially impairs neuronal energetics, contributing to cellular dysfunction in AD.

Production of reactive oxygen species in brain mitochondria: contribution by electron transport chain and non-electron transport chain sources

Overwhelming evidence has accumulated indicating that oxidative stress is a crucial factor in the pathogenesis of neurodegenerative diseases. The major site of production of superoxide, the primary reactive oxygen species (ROS), is considered to be the respiratory chain in the mitochondria, but the exact mechanism and the precise location of the physiologically relevant ROS generation within the respiratory chain have not been disclosed as yet. Studies performed with isolated mitochondria have located ROS generation on complex I and complex III, respectively, depending on the substrates or inhibitors used to fuel or inhibit respiration. A more "physiological" approach is to address ROS generation of in situ mitochondria, which are present in their normal cytosolic environment. Hydrogen peroxide formation in mitochondria in situ in isolated nerve terminals is enhanced when complex I, complex III, or complex IV is inhibited. However, to induce a significant increase in ROS production, complex III and complex IV have to be inhibited by >70%, which raises doubts as to the physiological importance of ROS generation by these complexes. In contrast, complex I inhibition to a small degree is sufficient to enhance ROS generation, indicating that inhibition of complex I by approximately 25-30% observed in postmortem samples of substantia nigra from patients suffering from Parkinson's disease could be important in inducing oxidative stress. Recently, it has been described that a key Krebs cycle enzyme, alpha-ketoglutarate dehydrogenase (alpha-KGDH), is also able to produce ROS. ROS formation by alpha-KGDH is regulated by the NADH/NAD+ ratio, suggesting that this enzyme could substantially contribute to generation of oxidative stress due to inhibition of complex I. As alpha-KGDH is not only a generator but also a target of ROS, it is proposed that alpha-KGDH is a key factor in a vicious cycle by which oxidative stress is induced and promoted in nerve terminals.

Copper-dependent inhibition of human cytochrome c oxidase by a dimeric conformer of amyloid-beta1-42

In studies of Alzheimer's disease pathogenesis there is an increasing focus on mechanisms of intracellular amyloid-beta (Abeta) generation and toxicity. Here we investigated the inhibitory potential of the 42 amino acid Abeta peptide (Abeta1-42) on activity of electron transport chain enzyme complexes in human mitochondria. We found that synthetic Abeta1-42 specifically inhibited the terminal complex cytochrome c oxidase (COX) in a dose-dependent manner that was dependent on the presence of Cu2+ and specific "aging" of the Abeta1-42 solution. Maximal COX inhibition occurred when using Abeta1-42 solutions aged for 3-6 h at 30 degrees C. The level of Abeta1-42-mediated COX inhibition increased with aging time up to approximately 6 h and then declined progressively with continued aging to 48 h. Photo-induced cross-linking of unmodified proteins followed by SDS-PAGE analysis revealed dimeric Abeta as the only Abeta species to provide significant temporal correlation with the observed COX inhibition. Analysis of brain and liver from an Alzheimer's model mouse (Tg2576) revealed abundant Abeta immunoreactivity within the brain mitochondria fraction. Our data indicate that endogenous Abeta is associated with brain mitochondria and that Abeta1-42, possibly in its dimeric conformation, is a potent inhibitor of COX, but only when in the presence of Cu2+. We conclude that Cu2+-dependent Abeta-mediated inhibition of COX may be an important contributor to the neurodegeneration process in Alzheimer's disease.

Mitochondrial dysfunction and Alzheimer's disease: role of amyloid-beta peptide alcohol dehydrogenase (ABAD)

An important means of determining how amyloid-beta peptide (Abeta) affects cells is to identify specific macromolecular targets and assess how Abeta interaction with such targets impacts on cellular functions. On the one hand, cell surface receptors interacting with extracellular Abeta have been identified, and their engagement by amyloid peptide can trigger intracellular signaling cascades. Recent evidence has indicated a potentially significant role for deposition of intracellular Abeta in cell stress associated with amyloidosis. Thus, specific intracellular targets of Abeta might also be of interest. Our review evaluates the potential significance of Abeta interaction with a mitochondrial enzyme termed Abeta-binding alcohol dehydrogenase (ABAD), a member of the short-chain dehydrogenase-reductase family concentrated in mitochondria of neurones. Binding of Abeta to ABAD distorts the enzyme's structure, rendering it inactive with respect to its metabolic properties, and promotes mitochondrial generation of free radicals. Double transgenic mice in which increased levels of ABAD are expressed in an Abeta-rich environment, the latter provided by a mutant amyloid precursor protein transgene, demonstrate accelerated decline in spatial learning/memory and pathologic changes. These data suggest that mitochondria ABAD, ordinarily a contributor to metabolic homeostasis, has the capacity to become a pathogenic factor in an Abeta-rich environment.

Mitochondrial abnormalities in Alzheimer brain: mechanistic implications

Reductions in cerebral metabolism sufficient to impair cognition in normal individuals also occur in Alzheimer's disease (AD). The degree of clinical disability in AD correlates closely to the magnitude of the reduction in brain metabolism. Therefore, we tested whether impairments in tricarboxylic acid (TCA) cycle enzymes of mitochondria correlate with disability. Brains were from patients with autopsy-confirmed AD and clinical dementia ratings (CDRs) before death. Significant (p < 0.01) decreases occurred in the activities of the pyruvate dehydrogenase complex (-41%), isocitrate dehydrogenase (-27%), and the alpha-ketoglutarate dehydrogenase complex (-57%). Activities of succinate dehydrogenase (complex II) (+44%) and malate dehydrogenase (+54%) were increased (p < 0.01). Activities of the other four TCA cycle enzymes were unchanged. All of the changes in TCA cycle activities correlated with the clinical state (p < 0.01), suggesting a coordinated mitochondrial alteration. The highest correlation was with pyruvate dehydrogenase complex (r = 0.77, r2= 0.59). Measures to improve TCA cycle metabolism might benefit AD patients.

Amyloid β peptide binds a novel death‐inducing protein, AB‐DIP

Amyloid beta-peptide (Abeta) plays a central role in the pathogenesis of Alzheimer's disease (AD). It is toxic to neurons, but the mechanism for its action remains largely unknown. Here, we have identified a novel death-inducing protein, Abeta-related DIP (AB-DIP), by two-hybrid screening of the human brain cDNA library and confirmed the binding of Abeta with AB-DIP by coimmunoprecipitation. Overexpression of AB-DIP-induced cell death and coexpression of Abeta enhanced the cell death. During apoptosis, the 97-kDa AB-DIP was cleaved to a 62-kDa protein (AB-DIP p62) at the caspase cleavage site, LEKD. It is more important that cotransfection of Abeta with AB-DIP produced the AB-DIP p62 fragment. Small interfering RNA-mediated knockdown of AB-DIP protein expression significantly protected neuroblastoma cells from Abeta-induced neurotoxicity. AB-DIP may mediate the neurotoxicity of Abeta, and therefore, AB-DIP may be a potential, therapeutic target for AD.

Effect of amyloid peptides on serum withdrawal-induced cell differentiation and cell viability

Abnormal deposition of amyloid-beta(A beta) peptides and formation of neuritic plaques are recognized as pathological processes in Alzheimer's disease (AD) brain. By using amyloid precursor protein (APP) transfected cells, this study aims to investigate the effect of overproduction of A beta on cell differentiation and cell viability. It was shown that after serum withdrawal, untransfected cell (N2a/Wt) and vector transfected cells (N2a/vector) extended long and branched cell processes, whereas no neurites was induced in wild type APP (N2a/APP695) and Swedish mutant APP (N2a/APPswe) transfected N2a cells. After differentiation by serum withdrawal, the localization of APP/A beta and neurofilament was extended to neurites, whereas those of APP-transfected cells were still restricted within the cell body. Levels of both APP and A beta were significantly higher in N2a/APP695 and N2a/APPswe than in N2a/Wt, as determined by Western blot and Sandwich ELISA, respectively. To further investigate the effect of A beta on the inhibition of cell differentiation, we added exogenously the similar level or about 10-times of the A beta level produced by N2a/APP695 and N2a/APPswe to the culture medium and co-cultured with N2a/Wt for 12 h, and we found that the inhibition of serum withdrawal-induced differentiation observed in N2a/APP695 and N2a/APPswe could not be reproduced by exogenous administration of A beta into N2a/Wt. We also observed that neither endogenous production nor exogenous addition of A beta 1-40 or A beta 1- 42, even to hundreds fold of the physiological concentration, affected obviously the cell viability. These results suggest that the overproduction of A beta could not arrest cell differentiation induced by serum deprivation and that, at least to a certain degree and in a limited time period, is not toxic to cell viability.

Proteomic and functional analyses reveal a mitochondrial dysfunction in P301L tau transgenic mice

Transgenic mice overexpressing the P301L mutant human tau protein exhibit an accumulation of hyperphosphorylated tau and develop neurofibrillary tangles. The consequences of tau pathology were investigated here by proteomics followed by functional analysis. Mainly metabolism-related proteins including mitochondrial respiratory chain complex components, antioxidant enzymes, and synaptic proteins were identified as modified in the proteome pattern of P301L tau mice. Significantly, the reduction in mitochondrial complex V levels in the P301L tau mice revealed using proteomics was also confirmed as decreased in human P301L FTDP-17 (frontotemporal dementia with parkinsonism linked to chromosome 17) brains. Functional analysis demonstrated a mitochondrial dysfunction in P301L tau mice together with reduced NADH-ubiquinone oxidoreductase activity and, with age, impaired mitochondrial respiration and ATP synthesis. Mitochondrial dys-function was associated with higher levels of reactive oxygen species in aged transgenic mice. Increased tau pathology as in aged homozygous P301L tau mice revealed modified lipid peroxidation levels and the up-regulation of antioxidant enzymes in response to oxidative stress. Furthermore, P301L tau mitochondria displayed increased vulnerability toward beta-amyloid (Abeta) peptide insult, suggesting a synergistic action of tau and Abeta pathology on the mitochondria. Taken together, we conclude that tau pathology involves a mitochondrial and oxidative stress disorder possibly distinct from that caused by Abeta.

The powerhouse takes control of the cell: is the mitochondrial permeability transition a viable therapeutic target against neuronal dysfunction and death?

Stroke and neurodegenerative disease exert an increasing large toll on human health at the levels both of the individual and of society. As an example of each, in the United States, stroke is the major single cause of overall morbidity and mortality, and the financial costs of Alzheimer's disease alone dwarfs the entire federal medical research budget. It has been long recognized that mitochondrial energy production is essential for the second to second functions of the central nervous system (CNS), and that severe mitochondrial impairment is incompatible with normal cerebral function. The last decade, however, has brought a growing understanding that mitochondria play an even greater role than previously suspected. Increased understanding of the role of mitochondria in antioxidant defense and calcium homeostasis further solidified the importance of mitochondria in CNS function--just as increased understanding of mitochondrial roles in calcium-mediated toxicity and production of reactive species further exemplified the Janus role of mitochondria--as mediators of CNS dysfunction. Perhaps most unexpected, however, was the evidence that mitochondria serve as the dominant integrators, checkpoints, and amplifiers of the cell death signals in the CNS. The mechanism of propagation of cell death cascades by mitochondria remains controversial. In this review, we focus on the evidence that supports the involvement of an event termed the mitochondrial permeability transition that (i) occurs (patho)physiologically; (ii) occurs in the CNS, and; (iii) is a potential target for pharmaceutical intervention against CNS dysfunction, injury, and cell loss resulting from stroke, trauma, and neurodegenerative disease.

Mitochondrial aging and dysfunction in Alzheimer's disease

Disruptions in energy metabolism have been suggested to be a prominent feature, perhaps even a fundamental component, of Alzheimer's disease (AD). These abnormalities in cerebral metabolism precede the onset of neurological dysfunction as well as gross neuropathology of AD. These changes may stem from inhibition of mitochondrial enzymes including pyruvate dehydrogenase, cytochrome c oxidase, and alpha-ketoglutarate dehydrogenase. Several lines of evidence also suggest a role for oxidative stress in the neuropathology associated with the disease state. Because mitochondria are the major site of free radical production in cells, they are also a primary target for oxidative damage and subsequent dysfunction. This link between mitochondrial dysfunction and the pathophysiology of AD is supported by several lines of evidence.

Mitochondria in health and disease: perspectives on a new mitochondrial biology

The integrity of mitochondrial function is fundamental to cell life. It follows that disturbances of mitochondrial function will lead to disruption of cell function, expressed as disease or even death. In this review, I consider recent developments in our knowledge of basic aspects of mitochondrial biology as an essential step in developing our understanding of the contributions of mitochondria to disease. The identification of novel mechanisms that govern mitochondrial biogenesis and replication, and the delicately poised signalling pathways that coordinate the mitochondrial and nuclear genomes are discussed. As fluorescence imaging has made the study of mitochondrial function within cells accessible, the application of that technology to the exploration of mitochondrial bioenergetics is reviewed. Mitochondrial calcium uptake plays a major role in influencing cell signalling and in the regulation of mitochondrial function, while excessive mitochondrial calcium accumulation has been extensively implicated in disease. Mitochondria are major producers of free radical species, possibly also of nitric oxide, and are also major targets of oxidative damage. Mechanisms of mitochondrial radical generation, targets of oxidative injury and the potential role of uncoupling proteins as regulators of radical generation are discussed. The role of mitochondria in apoptotic and necrotic cell death is seminal and is briefly reviewed. This background leads to a discussion of ways in which these processes combine to cause illness in the neurodegenerative diseases and in cardiac reperfusion injury. The demands of mitochondria and their complex integration into cell biology extends far beyond the provision of ATP, prompting a radical change in our perception of mitochondria and placing these organelles centre stage in many aspects of cell biology and medicine.

Mitochondrial failures in Alzheimer's disease

Mitochondrial dysfunction and free radical-induced oxidative damage have been implicated in the pathogenesis of several different neurodegenerative diseases such as Parkinson disease (PD), amyotrophic lateral sclerosis (ALS), Huntington's disease (HD), and Alzheimer's disease (AD). The defective adenosine triphosphate (ATP) production and increased oxygen radicals may induce mitochondria-dependent cell death because damaged mitochondria are unable to maintain the energy demands of the cell. The role of vascular hypoperfusion-induced mitochondria failure in the pathogenesis of AD now has been widely accepted. However, the exact cellular mechanisms behind vascular lesions and their relation to oxidative stress markers identified by RNA oxidation, lipid peroxidation, or mitochondrial DNA (mtDNA) deletion remain unknown. Future studies comparing the spectrum of mitochondrial damage and the relationship to oxidative stress-induced damage during the aging process or, more importantly, during the maturation of AD pathology are warranted.

Amyloid beta-induced changes in nitric oxide production and mitochondrial activity lead to apoptosis

Increasing evidence suggests an important role of mitochondrial dysfunction in the pathogenesis of Alzheimer's disease. Thus, we investigated the effects of acute and chronic exposure to increasing concentrations of amyloid beta (Abeta) on mitochondrial function and nitric oxide (NO) production in vitro and in vivo. Our data demonstrate that PC12 cells and human embryonic kidney cells bearing the Swedish double mutation in the amyloid precursor protein gene (APPsw), exhibiting substantial Abeta levels, have increased NO levels and reduced ATP levels. The inhibition of intracellular Abeta production by a functional gamma-secretase inhibitor normalizes NO and ATP levels, indicating a direct involvement of Abeta in these processes. Extracellular treatment of PC12 cells with comparable Abeta concentrations only leads to weak changes, demonstrating the important role of intracellular Abeta. In 3-month-old APP transgenic (tg) mice, which exhibit no plaques but already detectable Abeta levels in the brain, reduced ATP levels can also be observed showing the in vivo relevance of our findings. Moreover, we could demonstrate that APP is present in the mitochondria of APPsw PC12 cells. This presence might be directly involved in the impairment of cytochrome c oxidase activity and depletion of ATP levels in APPsw PC12 cells. In addition, APPsw human embryonic kidney cells, which produce 20-fold increased Abeta levels compared with APPsw PC12 cells, and APP tg mice already show a significantly decreased mitochondrial membrane potential under basal conditions. We suggest a hypothetical sequence of pathogenic steps linking mutant APP expression and amyloid production with enhanced NO production and mitochondrial dysfunction finally leading to cell death.

Molecular pathways to neurodegeneration

The molecular bases underlying the pathogenesis of neurodegenerative diseases are gradually being disclosed. One problem that investigators face is distinguishing primary from secondary events. Rare, inherited mutations causing familial forms of these disorders have provided important insights into the molecular networks implicated in disease pathogenesis. Increasing evidence indicates that accumulation of aberrant or misfolded proteins, protofibril formation, ubiquitin-proteasome system dysfunction, excitotoxic insult, oxidative and nitrosative stress, mitochondrial injury, synaptic failure, altered metal homeostasis and failure of axonal and dendritic transport represent unifying events in many slowly progressive neurodegenerative disorders.

Methionine 35 oxidation reduces toxic and pro-apoptotic effects of the amyloid beta-protein fragment (31-35) on isolated brain mitochondria

Amyloid beta-peptide (AbetaP), the central constituent of senile plaques in Alzheimer's disease (AD) brain, has been shown to be a source of free radical oxidative stress that may lead to neurodegeneration. In particular, it is well known that oxidation of methionine 35, is strongly related to the pathogenesis of AD, since it represents the residue in AbetaP most susceptible to oxidation in vivo. In the present study, we used the fragment 31-35 of AbetaP, which has a single methionine at residue 35, in order to investigate the influence of the oxidation state of methionine-35 on the toxic and pro-apoptotic effects induced by Abeta(31-35) on isolated brain mitochondria. The obtained results show that exposure of isolated mitochondria from rat brain to AbetaP(31-35) determines (i) a large release of cytochrome c (ii) a significant reduction in mitochondrial respiration and (iii) a slight drop in the mitochondrial membrane potential (deltapsi). In contrast, the amplitude of these events resulted attenuated or completely abrogated in isolated brain mitochondria exposed to the AbetaP(31-35)Met35OX, in which methionine 35 was oxidized to methionine sulfoxide. We have further characterized the action of AbetaP(31-35) and Abeta(31-35)Met35OX peptide on PC12 cells. Although these two peptides, compromised mitochondrial function at a different extent as assessed by MTT reduction, neither one of them decreased cell viability as measured by Trypan Blue exclusion assay. The results obtained in this study support the hypothesis that the oxidative state of Met-35 may play a critical role in the mechanisms responsible of neurotoxicity exerted by this peptide.

Toxicity of amyloid beta peptide: tales of calcium, mitochondria, and oxidative stress

Alzheimer's disease (AD) is characterized by the accumulation of amyloid-beta (Abeta) peptides. Although the disease undoubtedly reflects the interaction of complex multifactorial processes, Abeta itself is toxic to neurons in vitro and the load of Abeta in vivo correlates well with the degree of cognitive impairment. There has therefore been considerable interest in the mechanism(s) of Abeta neurotoxicity. We here review the basic biology of Abeta processing and consider some of the major areas of focus of this research. It is clear that both AD and Abeta toxicity are characterized by oxidative stress, alterations in the activity of enzymes of intermediary metabolism, and mitochondrial dysfunction, especially impaired activity of cytochrome c oxidase. Studies in vitro also show alterations in cellular calcium signaling. We consider the mechanisms proposed to mediate cell injury and explore evidence to indicate which of these many changes in function are primary and which secondary.

Mitochondrial dysfunction in neurodegenerative diseases associated with copper imbalance

Copper is an essential transition metal ion for the function of key metabolic enzymes, but its uncontrolled redox reactivity is source of reactive oxygen species. Therefore a network of transporters strictly controls the trafficking of copper in living systems. Deficit, excess, or aberrant coordination of copper are conditions that may be detrimental, especially for neuronal cells, which are particularly sensitive to oxidative stress. Indeed, the genetic disturbances of copper homeostasis, Menkes' and Wilson's diseases, are associated with neurodegeneration. Furthermore, copper interacts with the proteins that are the hallmarks of neurodegenerative disorders, such as Alzheimer's disease, Parkinson's disease, prion diseases, and familial amyotrophic lateral sclerosis. In all cases, copper-mediated oxidative stress is linked to mitochondrial dysfunction, which is a common feature of neurodegeneration. In particular we recently demonstrated that in copper deficiency, mitochondrial function is impaired due to decreased activity of cytochrome c oxidase, leading to production of reactive oxygen species, which in turn triggers mitochondria-mediated apoptotic neurodegeneration.

Beta-amyloid peptides induce mitochondrial dysfunction and oxidative stress in astrocytes and death of neurons through activation of NADPH oxidase

Beta-amyloid (betaA) peptide is strongly implicated in the neurodegeneration underlying Alzheimer's disease, but the mechanisms of neurotoxicity remain controversial. This study establishes a central role for oxidative stress by the activation of NADPH oxidase in astrocytes as the cause of betaA-induced neuronal death. betaA causes a loss of mitochondrial potential in astrocytes but not in neurons. The mitochondrial response consists of Ca2+-dependent transient depolarizations superimposed on a slow collapse of potential. The slow response is both prevented by antioxidants and, remarkably, reversed by provision of glutamate and other mitochondrial substrates to complexes I and II. These findings suggest that the depolarization reflects oxidative damage to metabolic pathways upstream of mitochondrial respiration. Inhibition of NADPH oxidase by diphenylene iodonium or 4-hydroxy-3-methoxy-acetophenone blocks betaA-induced reactive oxygen species generation, prevents the mitochondrial depolarization, prevents betaA-induced glutathione depletion in both neurons and astrocytes, and protects neurons from cell death, placing the astrocyte NADPH oxidase as a primary target of betaA-induced neurodegeneration.

Hypoxic remodelling of Ca2+ mobilization in type I cortical astrocytes: involvement of ROS and pro-amyloidogenic APP processing

Chronic hypoxia (CH) alters Ca2+ homeostasis in various cells and may contribute to disturbed Ca2+ homeostasis of Alzheimer's disease. Here, we have employed microfluorimetric measurements of [Ca2+]i to investigate the mechanism underlying augmentation of Ca2+ signalling by chronic hypoxia in type I cortical astrocytes. Application of bradykinin evoked significantly larger rises of [Ca2+]i in hypoxic cells as compared with control cells. This augmentation was prevented fully by either melatonin (150 micro m) or ascorbic acid (200 micro m), indicating the involvement of reactive oxygen species. Given the association between hypoxia and increased production of amyloid beta peptides (AbetaPs) of Alzheimer's disease, we performed immunofluorescence studies to show that hypoxia caused a marked and consistent increased staining for AbetaPs and presenilin-1 (PS-1). Western blot experiments also confirmed that hypoxia increased PS-1 protein levels. Hypoxic increases of AbetaP production was prevented with inhibitors of either gamma- or beta-secretase. These inhibitors also partially prevented the augmentation of Ca2+ signalling in astrocytes. Our results indicate that chronic hypoxia enhances agonist-evoked rises of [Ca2+]i in cortical astrocytes, and that this can be prevented by antioxidants and appears to be associated with increased AbetaP formation.

Cerebrometabolic abnormalities in Alzheimer's disease

Extensive, replicated evidence in patients in vivo and in Alzheimer (AD) tissues in vitro indicates that impaired brain metabolism is one of the cardinal and essentially invariable events in AD. The degree of impairment in brain metabolism is proportional to the degree of clinical disability, both in vivo and in vitro. The 'cerebrometabolic lesion' cannot be attributed to 'slower thinking' or 'brain atrophy', because of quantitative considerations and because the metabolic lesion precedes the development of neuropsychological abnormalities or decreases in brain mass detectable by modern imaging techniques. The causes of the cerebrometabolic lesion in AD are not well defined. Free radicals seem likely to be involved, including free radicals generated from Alzheimer amyloid. Thus, the importance of the cerebrometabolic lesion is entirely compatible with most versions of the widely accepted 'amyloid cascade hypothesis' of AD. A variety of plausible, redundantly documented mechanisms are compatible with the proposal that the cerebrometabolic lesion is a proximate cause of the clinical disability in AD. In agreement with these findings, recent attempts to treat the cerebrometabolic lesion in AD have given encouraging preliminary results. The cerebrometabolic lesion in AD deserves further study.

Mitochondrial dysfunction, apoptotic cell death, and Alzheimer’s disease

Being major sources of reactive oxygen species (ROS), mitochondrial structures are exposed to high concentrations of ROS and might therefore be particularly susceptible to oxidative injury. Mitochondrial damage may play a pivotal role in the cell death decision. Bolstered evidence indicates that mitochondrial abnormalities might be part of the spectrum of chronic oxidative stress occurring in Alzheimer's disease (AD) finally contributing to synaptic failure and neuronal degeneration. Accumulation and oligomerization of amyloid beta (Abeta) is also thought to play a central role in the pathogenesis of this disease by probably directly leading to mitochondrial dysfunction. Moreover, numerous lines of findings indicate increased susceptibility to apoptotic cell death and increased oxidative damage as common features in neurons from sporadic AD patients but also from familial AD (FAD) cases. Here we provide a summary of recent work demonstrating some key abnormalities that may initiate and promote pathological events in AD. Finally, we emphasize a hypothetical sequence of the pathogenic steps linking sporadic AD, FAD, and Abeta production with mitochondrial dysfunction, caspase pathway, and neuronal loss.

The application of functional genomics to Alzheimer's disease

Alzheimer's disease (AD) is a polygenic/complex disorder in which more than 50 genetic loci are involved. Primary and secondary loci are potentially responsible for the phenotypic expression of the disease under the influence of both environmental factors and epigenetic phenomena. The construction of haplotypes as genomic clusters integrating the different genotype combinations of AD-related genes is a suitable strategy to investigate functional genomics in AD. It appears that AD patients show about 3-5 times higher genetic variation than the control population. The analysis of genotype-phenotype correlations has revealed that the presence of the APOE-4 allele in AD, in conjunction with other loci distributed across the genome, influence disease onset, brain atrophy, cerebrovascular perfusion, blood pressure, beta-amyloid deposition, ApoE secretion, lipid metabolism, brain bioelectrical activity, cognition, apoptosis and treatment outcome. Pharmacogenomics studies also indicate that the therapeutic response in AD is genotype-specific and that approximately 15% of the cases with efficacy and/or safety problems are associated with a defective CYP2D6 gene. Consequently, the understanding of functional genomics in AD will foster productive pharmacogenomic studies in the search for effective medications and preventive strategies in AD.

Changes in intracellular calcium and glutathione in astrocytes as the primary mechanism of amyloid neurotoxicity

Although the accumulation of the neurotoxic peptide beta amyloid (betaA) in the CNS is a hallmark of Alzheimer's disease, the mechanism of betaA neurotoxicity remains controversial. In cultures of mixed neurons and astrocytes, we found that both the full-length peptide betaA (1-42) and the neurotoxic fragment (25-35) caused sporadic cytoplasmic calcium [intracellular calcium ([Ca2+]c)] signals in astrocytes that continued for hours, whereas adjacent neurons were completely unaffected. Nevertheless, after 24 hr, although astrocyte cell death was marginally increased, approximately 50% of the neurons had died. The [Ca2+]c signal was entirely dependent on Ca2+ influx and was blocked by zinc and by clioquinol, a heavy-metal chelator that is neuroprotective in models of Alzheimer's disease. Neuronal death was associated with Ca2+-dependent glutathione depletion in both astrocytes and neurons. Thus, astrocytes appear to be the primary target of betaA, whereas the neurotoxicity reflects the neuronal dependence on astrocytes for antioxidant support.

Decreased platelet cytochrome c oxidase activity is accompanied by increased blood lactate concentration during exercise in patients with Alzheimer disease

Increasing evidence indicates that mitochondrial dysfunction occurs in the central nervous system as well as in the peripheral tissues from Alzheimer's disease (AD) patients. We have recently shown that mitochondrial cytochrome c oxidase (COX) activity is significantly reduced in brain and platelets from AD patients compared to controls. In the present study we investigated whether impaired COX activity could have functional consequences on energy metabolism. Blood lactate concentration was monitored at rest and during incremental exercise in 22 AD patients in whom COX activity in platelets was decreased compared to controls (35.7 +/- 11.4 vs 48.4 +/- 1.4 nmol/min/mg, P < 0.01). In both resting and exercising conditions, blood lactate was significantly higher in AD patients than in controls. Although the magnitude of exercise-related lactate accumulation was not different between the two groups, an anticipated anaerobic lactate threshold during the incremental forearm exercise was found in AD patients (50% of maximal voluntary contraction MVC compared to 60% in controls). COX activity was inversely related to lactate at a significant level for resting condition (r = -0.65) and borderline for anaerobic threshold exercise level. These results support the hypothesis of a systemic impairment of the mitochondrial function in AD and indicate that decreased COX activity could have functional consequences on metabolism.

Regional brain cytochrome oxidase activity in beta-amyloid precursor protein transgenic mice with the Swedish mutation

Cytochrome oxidase activity was examined in a transgenic mouse model of Alzheimer's disease with overexpression of the 751 amino acid isoform of beta-amyloid precursor protein with the Swedish mutation under control of the murine thy-1 promoter. The neuritic plaques, abundantly localized in the hippocampus and anterior neocortical areas, showed a core devoid of enzymatic activity surrounded by higher cytochrome oxidase activity at the sites of the dystrophic neurites and activated glial cells. Quantitative measures, taken only in the healthy-appearing regional areas without neuritic plaques, were higher in numerous limbic and non-limbic regions of transgenic mice in comparison with controls. Enzymatic activity was higher in the dentate gyrus and CA2-CA3 region of the hippocampus, the anterior cingulate and primary visual cortex, two olfactory structures, the ventral part of the neostriatum, the parafascicularis nucleus of the thalamus, and the subthalamic nucleus. Brainstem regions anatomically related with altered forebrain regions were more heavily labeled as well, including the substantia nigra, the periaqueductal gray, the superior colliculus, the medial raphe, the locus coeruleus and the adjacent parabrachial nucleus, as well as the pontine nuclei, red nucleus, and trigeminal motor nucleus. Functional brain organization is discussed in the context of Alzheimer's disease. Although hypometabolism is generally observed in this pathology, the increased cytochrome oxidase activity obtained in these transgenic mice can be the result of a functional compensation on the surviving neurons, or of an early mitochondrial alteration related to increased oxidative damage.

A specific ELISA for measuring neurofilament heavy chain phosphoforms

Neurofilaments (Nf) are the major constitutents of the axoskeleton and body fluid Nf levels are an important tool for estimating axonal degeneration in vivo. This paper presents a new sandwich ELISA allowing quantification of the NfH(SMI35) phosphoform from CSF, brain tissue and cell culture homogenates. The sensitivity of the NfH(SMI35) ELISA is 0.2 ng/ml with a recovery of 119% and a mean within- and between-batch precision of 10.6% and 23%, respectively. CSF NfH(SMI35) was stable at 4 degrees C, is not influenced by freeze-thaw cycles, and proteolysis present at room temperature could be prevented by adding protease inhibitors. Aggregate formation was observed for HPLC-purified bovine NfH and could be resolved by sonication. The upper reference value for CSF NfH(SMI35) levels (0.73 ng/ml) was defined as the 95% cumulative frequency from 416 CSF samples. Based on this cutoff, a significantly higher proportion of patients with amyotrophic lateral sclerosis, space-occupying lesions, disc prolapse and subarachnoid haemorrhage had pathologically elevated NfH(SMI35) levels compared to patients with cluster headache or demyelinating disease.A new nomenclature is proposed to facilitate the comparison between ELISA, immunoblotting and immunocytochemistry.

Truncated product of the bifunctional DLST gene involved in biogenesis of the respiratory chain

Dihydrolipoamide succinyltransferase (DLST) is a subunit enzyme of the alpha-ketoglutarate dehydrogenase complex of the Krebs cycle. While studying how the DLST genotype contributes to the pathogenesis of Alzheimer's disease (AD), we found a novel mRNA that is transcribed starting from intron 7 in the DLST gene. The novel mRNA level in the brain of AD patients was significantly lower than that of controls. The truncated gene product (designated MIRTD) localized to the intermembrane space of mitochondria. To investigate the function of MIRTD, we established human neuroblastoma SH-SY5Y cells expressing a maxizyme, a kind of ribozyme, that specifically digests the MIRTD mRNA. The expression of the maxizyme specifically eliminated the MIRTD protein and the resultant MIRTD-deficient cells exhibited a marked decrease in the amounts of subunits of complexes I and IV of the mitochondrial respiratory chain, resulting in a decline of activity. A pulse-label experiment revealed that the loss of the subunits is a post-translational event. Thus, the DLST gene is bifunctional and MIRTD transcribed from the gene contributes to the biogenesis of the mitochondrial respiratory complexes.

Nitric oxide-induced mitochondrial dysfunction: implications for neurodegeneration

Excessive generation of nitric oxide (NO) has been implicated in the pathogenesis of several neurodegenerative disorders. Damage to the mitochondrial electron transport chain has also been implicated in these disorders. NO and its toxic metabolite peroxynitrite (ONOO(-)) can inhibit the mitochondrial respiratory chain, leading to energy failure and ultimately cell death. There appears to be a differential susceptibility of brain cell types to NO/ONOO(-), which may be influenced by factors including cellular antioxidant status and the ability to maintain energy requirements in the face of marked respiratory chain damage. Although formation of NO/ONOO(-) following cytokine exposure does not affect astrocyte survival, these molecules may diffuse out and cause mitochondrial damage to neighboring NO/ONOO(-)-sensitive cells such as neurons. Evidence suggests that NO/ONOO(-) causes release of neuronal glutamate, leading to glutamate-induced activation of neuronal NO synthase and generation of further damaging species. While neurons appear able to recover from short-term exposure to NO/ONOO(-), extending the period of exposure results in persistent damage to the respiratory chain and cell death ensues. These findings have important implications for acute infection vs. chronic neuroinflammatory disease states. The evidence for NO/ONOO(-)-mediated mitochondrial damage in neurodegenerative disorders is reviewed and potential therapeutic strategies are discussed.

Intracellular A-beta amyloid, a sign for worse things to come?

In this review the authors discuss the possible neuropathological role of intracellular amyloid-beta accumulation in Alzheimer's disease (AD) pathology. There is abundant evidence that at early stages of the disease, prior to A-beta amyloid plaque formation, A-beta peptides accumulate intraneuronally in the cerebral cortex and the hippocampus. The experimental evidence would indicate that intracellular amyloid-beta could originate both by intracellular biosynthesis and also from the uptake of amyloidogenic peptides from the extracellular milieu. Herein the aspects of the possible impact of intracellular amyloid-beta in human AD pathology are discussed, as well as recent observations from a rat transgenic model with a phenotype of intracellular accumulation of A-beta fragments in neurons of the hippocampus and cortex, without plaque formation. In this model, the intracellular amyloid-beta phenotype is accompanied by increased MAPK/ERK activity and tau hyperphosphorylation. Finally, the authors discuss the hypothesis that, prior to plaque formation, intracellular A-beta accumulation induces biochemical and pathological changes in the brain at the cellular level priming neurons to further cytotoxic attack of extracellular amyloidogenic peptides.