Mutations in mitochondrial cytochrome c oxidase genes segregate with late-onset Alzheimer disease

Review: Alzheimer's amyloid beta-peptide-associated free radical oxidative stress and neurotoxicity

Alzheimer's disease, the major dementing disorder of the elderly that affects over 4 million Americans, is related to amyloid beta-peptide, the principal component of senile plaques in Alzheimer's disease brain. Oxidative stress, manifested by protein oxidation and lipid peroxidation, among other alterations, is a characteristic of Alzheimer's disease brain. Our laboratory united these two observations in a model to account for neurodegeneration in Alzheimer's disease brain, the amyloid beta-peptide-associated oxidative stress model for neurotoxicity in Alzheimer's disease. Under this model, the aggregated peptide, perhaps in concert with bound redox metal ions, initiates free radical processes resulting in protein oxidation, lipid peroxidation, reactive oxygen species formation, cellular dysfunction leading to calcium ion accumulation, and subsequent neuronal death. Free radical antioxidants abrogate these findings. This review outlines the substantial evidence from multiidisciplinary approaches for amyloid beta-peptide-associated free radical oxidative stress and neurotoxicity and protection against these oxidative processes and cell death by free radical scavengers. In addition, we review the strong evidence supporting the notion that the single methionine residue of amyloid beta-peptide is vital to the oxidative stress and neurotoxicological properties of this peptide. Further, we discuss studies that support the hypothesis that aggregated soluble amyloid beta-peptide and not fibrils per se are necessary for oxidative stress and neurotoxicity associated with amyloid beta-peptide.

Insulin regulates soluble amyloid precursor protein release via phosphatidyl inositol 3 kinase-dependent pathway

Several lines of biochemical evidence correlate the presence of energy metabolic defects with the functional alterations associated with brain aging and with the pathogenesis of neurodegenerative disorders such as Alzheimer's disease. Within this context we tested the ability of insulin to regulate the amyloid precursor protein (APP) processing in SH-SY5Y neuroblastoma cells. Our findings show that insulin promotes APP metabolism by a glucose-independent mechanism. We demonstrate a novel intracellular pathway that increases the rate of secretion of soluble APP through the activity of phosphatidyl-inositol 3 kinase (PI3-K). This pathway, downstream of insulin receptor tyrosine kinase activity, does not involve either the activation of protein kinase C or the mitogen-activated protein kinase (MAP-K) pathway. Because of the physiological role of PI3-K in the translocation of glucose transporter-containing vesicles, we speculate that PI3-K involvement in APP metabolism may act at the level of vesicular trafficking.

Inherent abnormalities in energy metabolism in Alzheimer disease. Interaction with cerebrovascular compromise

Alzheimer disease (AD) is a form of the dementia syndrome. AD appears to have a variety of fundamental etiologies that lead to the neuropathological manifestations which define the disease. Patients who are at high risk to develop AD typically show impairments of cerebral metabolic rate in vivo even before they show any evidence of the clinical disease on neuropsychological, electrophysiological, and neuroimaging examinations. Therefore, impairment in energy metabolism in AD can not be attributed to loss of brain substance or to electrophysiological abnormalities. Among the characteristic abnormalities in the AD brain are deficiencies in several enzyme complexes which participate in the mitochondrial oxidation of substrates to yield energy. There include the pyruvate dehydrogenase complex (PDHC), the alpha-ketoglutarate dehydrogenase complex (KGDHC), and Complex IV of the electron transport chain (COX). The deficiency of KGDHC may be due to a mixture of causes including damage by free radicals and perhaps to genetic variation in the DLST gene encoding the core protein of this complex. Inherent impairment of glucose oxidation by the AD brain may reasonably be expected to interact synergistically with an impaired supply of oxygen and glucose to the AD brain, in causing brain damage. These considerations lead to the hypothesis that cerebrovascular compromise and inherent abnormalities in the brain's ability to oxidize substrates can interact to favor the development of AD, in individuals who are genetically predisposed to develop neuritic plaques.

Use of cytoplasmic hybrid cell lines for elucidating the role of mitochondrial dysfunction in Alzheimer's disease and Parkinson's disease

There is substantial evidence of mitochondrial defects in neurodegenerative disorders such as Alzheimer's and Parkinson's diseases (AD and PD). We have probed the molecular implications of mitochondrial dysfunction in these diseases by transferring mitochondria from platelets obtained from disease and control donors into mitochondrial DNA-depleted recipient neuron-based cells (rho 0 cells). This process creates cytoplasmic hybrid (cybrid) cells where the mitochondrial DNA (mtDNA) from the donor is expressed in the nuclear and cellular background of the host rho 0 cell. Differences in phenotype between disease and control groups can thus be attributed to the exogenous mitochondria and mtDNA. Key methodological issues relating to this approach were addressed by demonstrating that recipient rho 0 cells have < 1 mtDNA copy/cell, and that exclusive repopulation with donor mtDNA occurs in cybrid cells. Further, we describe that sampling of heterogeneous cell populations is a valid approach for cybrid analysis. Our studies show that the focal respiratory chain defects reported in platelets of AD and PD cybrids can be recapitulated in AD and PD cybrids. In addition, both AD and PD cybrids display increased oxidative stress and perturbations in calcium homeostasis. These data suggest that the transfer of a mtDNA defect from disease donor platelets is the likely cause of the cybrid biochemical phenotype, and highlight the potential value of these cell lines as cellular disease models.

Abnormal mitochondrial morphology in sporadic Parkinson's and Alzheimer's disease cybrid cell lines

Diseases linked to defective mitochondrial function are characterized by morphologically abnormal, swollen mitochondria with distorted cristae. Several lines of evidence now suggest that sporadic forms of Parkinson's disease (PD) and Alzheimer's disease (AD) are linked to mitochondrial dysfunction arising from defects in mitochondrial DNA (mtDNA). Human neuroblastoma (SH-SY5Y) cells that are deficient in mtDNA (Rho(0)) were repopulated with mitochondria from AD or PD patients or age-matched controls. These cytoplasmic hybrid (cybrid) cell lines differ only in the source of their mtDNA. Differences between cybrid cell lines therefore arise from differences in mtDNA and provide a model for the study of how impaired mitochondrial function alters the mitochondria themselves and how these changes adversely affect the neuronal cells they occupy. Cybrid cell mitochondria were labeled with the mitochondrial membrane potential-sensitive dye, JC-1. Analysis of these JC-1 labeled mitochondria by confocal microscopy revealed that mitochondrial membrane potential was significantly reduced in both PD and AD cybrid cells when compared with controls. Ultrastructural examination showed that control cybrid cells contained small, morphologically normal, round or oval mitochondria with a dark matrix and regular distribution of cristae. PD cybrid cells contained a significant and increased percentage of mitochondria that were enlarged or swollen and had a pale matrix with few remaining cristae (0.26-0.65 microm(2)). AD cybrid cells also contained a significantly increased percentage of enlarged or swollen mitochondria (0.25-5.0 microm(2)) that had a pale matrix and few remaining cristae. Other pathological features such as crystal-like intramitochondrial inclusions and cytoplasmic inclusion bodies were also found in PD and AD cybrids. These observations suggest that transfer of PD or AD mtDNA into Rho(0) cells was sufficient to produce pathological changes in mitochondrial ultrastructure that are similar to those seen in other mitochondrial disorders. These data were reported in abstract form (Trimmer et al., 1998, Soc. Neurosci. Abstr. 24: 476).

Oxidative stress and Alzheimer disease

Research in the field of molecular biology has helped to provide a better understanding of both the cascade of biochemical events that occurs with Alzheimer disease (AD) and the heterogeneous nature of the disease. One hypothesis that accounts for both the heterogeneous nature of AD and the fact that aging is the most obvious risk factor is that free radicals are involved. The probability of this involvement is supported by the fact that neurons are extremely sensitive to attacks by destructive free radicals. Furthermore, lesions are present in the brains of AD patients that are typically associated with attacks by free radicals (eg, damage to DNA, protein oxidation, lipid peroxidation, and advanced glycosylation end products), and metals (eg, iron, copper, zinc, and aluminum) are present that have catalytic activity that produce free radicals. beta-Amyloid is aggregated and produces more free radicals in the presence of free radicals; beta-amyloid toxicity is eliminated by free radical scavengers. Apolipoprotein E is subject to attacks by free radicals, and apolipoprotein E peroxidation has been correlated with AD. In contrast, apolipoprotein E can act as a free radical scavenger and this behavior is isoform dependent. AD has been linked to mitochondrial anomalies affecting cytochrome-c oxidase, and these anomalies may contribute to the abnormal production of free radicals. Finally, many free radical scavengers (eg, vitamin E, selegeline, and Ginkgo biloba extract EGb 761) have produced promising results in relation to AD, as has desferrioxamine-an iron-chelating agent-and antiinflammatory drugs and estrogens, which also have an antioxidant effect.

Vitamin E and Alzheimer disease: the basis for additional clinical trials

Many lines of evidence suggest that oxidative stress is important in the pathogenesis of Alzheimer disease. In particular, beta-amyloid, which is found abundantly in the brains of Alzheimer disease patients, is toxic in neuronal cell cultures through a mechanism involving free radicals. Vitamin E prevents the oxidative damage induced by beta-amyloid in cell culture and delays memory deficits in animal models. A placebo-controlled, clinical trial of vitamin E in patients with moderately advanced Alzheimer disease was conducted by the Alzheimer's Disease Cooperative Study. Subjects in the vitamin E group were treated with 2000 IU (1342 alpha-tocopherol equivalents) vitamin E/d. The results indicated that vitamin E may slow functional deterioration leading to nursing home placement. A new clinical trial is planned that will examine whether vitamin E can delay or prevent a clinical diagnosis of Alzheimer disease in elderly persons with mild cognitive impairment.

Mitochondrial respiratory chain disorders II: neurodegenerative disorders and nuclear gene defects

The first part of this review (Lancet 2000; 355: 299) covered primary disorders of mitochondrial DNA (mtDNA). This section will cover nuclear-encoded defects of the oxidative phosphorylation (OXPHOS) system, including mtDNA mutations that are secondary to nuclear gene mutations and nuclear gene defects responsible for secondary OXPHOS deficiency (panel). The latter group of diseases are predominantly neurodegenerative. The mitochondrion's role in apoptosis and its contribution to the pathogenesis of neurodegenerative diseases are also covered.

Causes of Alzheimer's disease

It is now understood that genetic factors play a crucial role in the risk of developing Alzheimer's disease (AD). Rare mutations in at least 3 genes are responsible for early-onset familial AD. A common polymorphism in the apolipoprotein E gene is the major determinant of risk in families with late-onset AD, as well as in the general population. Advanced age, however, remains the major established risk factor for AD, although environmental variables may also have some role in disease expression. Some pathogenic factors directly associated with aging include oxidative damage and mutations in messenger RNA. Other factors unrelated to the aging process may, in the future, be amenable to therapeutic intervention by way of estrogen replacement therapy for postmenopausal women, anti-inflammatory drug therapy and reducing vascular risk factors. Older theories, such as aluminum playing a role in the pathogenesis of AD, have been mostly discarded as our understanding of pathogenic mechanisms of AD has advanced.

Pharmacology of AMPA/kainate receptor ligands and their therapeutic potential in neurological and psychiatric disorders

It has been postulated, consistent with the ubiquitous presence of glutamatergic neurons in the brain, that defects in glutamatergic neurotransmission are associated with many human neurological and psychiatric disorders. This review evaluates the possible application of ligands acting on glutamate alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) and kainate (KA) receptors to minimise the pathology and/or symptoms of various diseases. Glutamate activation of AMPA receptors is thought to mediate most fast synaptic neurotransmission in the brain, while transmission via KA receptors contributes only a minor component. Variants of the protein subunits forming these receptors greatly extend the pharmacological and electrophysiological properties of AMPA/KA receptors. Disease and drug use can differentially affect the expression of the subunits and their variants. Ligands bind to AMPA receptors by competing with glutamate at the glutamate binding site, or non-competitively at other sites on the proteins (allosteric modulators). Ligands showing selective competitive antagonist actions at the AMPA/ KA class of glutamate receptors were first reported in 1988, and the systemically active antagonist 2,3-dihydroxy-6-nitro-7-sulphamoyl-benzo(F)quinoxaline (NBQX) was first shown to have useful therapeutic effects on animal models of neurological diseases in 1990. Since then, newer antagonists with increased potency, higher specificity, increased water solubility, and a longer duration of action in vivo have been developed. Negative allosteric modulators such as the prototype GYKI-52466 also block AMPA receptors but have little action at KA receptors. Positive allosteric modulators enhance glutamatergic neurotransmission at AMPA receptors. Polyamines and adamantane derivatives bind within the ion channel of calcium-permeable AMPA receptors. The latest developments include ligands selective for KA receptors containing Glu-R5 subunits. Evidence for advantages of AMPA receptor antagonists over N-methyl-D-aspartate (NMDA) receptor antagonists for symptomatic treatment of neurological and psychiatric conditions, and for minimising neuronal loss occurring after acute neurological diseases, such as physical trauma, ischaemia or status epilepticus, have been shown in animal models. However, as yet AMPA receptor antagonists have not been shown to be effective in clinical trials. On the other hand, a limited number of clinical trials have been reported for AMPA receptor ligands that enhance glutamatergic neurotransmission by extending the ion channel opening time (positive allosteric modulators). These acute studies demonstrate enhanced memory capability in both young and aged humans, without any apparent serious adverse effects. The use of these allosteric modulators as antipsychotic drugs is also possible. However, the long term use of both direct agonists and positive allosteric modulators must be approached with considerable caution because of potential adverse effects.

A unifying hypothesis of Alzheimer's disease. III. Risk factors

Normal ageing and Alzheimer's disease (AD) have many features in common and, in many respects, both conditions only differ by quantitative criteria. A variety of genetic, medical and environmental factors modulate the ageing-related processes leading the brain into the devastation of AD. In accordance with the concept that AD is a metabolic disease, these risk factors deteriorate the homeostasis of the Ca(2+)-energy-redox triangle and disrupt the cerebral reserve capacity under metabolic stress. The major genetic risk factors (APP and presenilin mutations, Down's syndrome, apolipoprotein E4) are associated with a compromise of the homeostatic triangle. The pathophysiological processes leading to this vulnerability remain elusive at present, while mitochondrial mutations can be plausibly integrated into the metabolic scenario. The metabolic leitmotif is particularly evident with medical risk factors which are associated with an impaired cerebral perfusion, such as cerebrovascular diseases including stroke, cardiovascular diseases, hypo- and hypertension. Traumatic brain injury represents another example due to the persistent metabolic stress following the acute event. Thyroid diseases have detrimental sequela for cerebral metabolism as well. Furthermore, major depression and presumably chronic stress endanger susceptible brain areas mediated by a host of hormonal imbalances, particularly the HPA-axis dysregulation. Sociocultural and lifestyle factors like education, physical activity, diet and smoking may also modulate the individual risk affecting both reserve capacity and vulnerability. The pathophysiological relevance of trace metals, including aluminum and iron, is highly controversial; at any rate, they may adversely affect cellular defences, antioxidant competence in particular. The relative contribution of these factors, however, is as individual as the pattern of the factors. In familial AD, the genetic factors clearly drive the sequence of events. A strong interaction of fat metabolism and apoE polymorphism is suggested by intercultural epidemiological findings. In cultures, less plagued by the 'blessings' of the 'cafeteria diet-sedentary' Western lifestyle, apoE4 appears to be not a risk factor for AD. This intriguing evidence suggests that, analogous to cardiovascular diseases, apoE4 requires a hyperlipidaemic lifestyle to manifest as AD risk factor. Overall, the etiology of AD is a key paradigm for a gene-environment interaction. Copyright 2000 John Wiley & Sons, Ltd.

Nitric oxide-induced damage to mtDNA and its subsequent repair

Mutations in mitochondrial DNA (mtDNA) have recently been associated with a variety of human diseases. One potential DNA-damaging agent to which cells are continually exposed that could be responsible for some of these mutations is nitric oxide (NO). To date, little information has been forthcoming concerning the damage caused by this gas to mtDNA. Therefore, this study was designed to investigate damage to mtDNA induced by NO and to evaluate its subsequent repair. Normal human fibroblasts were exposed to NO produced by the rapid decomposition of 1-propanamine, 3-(2-hydroxy-2-nitroso-1-propylhydrazino) (PAPA NONOate) and the resultant damage to mtDNA was determined by quantitative Southern blot analysis. This gas was found to cause damage to mtDNA that was alkali-sensitive. Treatment of the DNA with uracil-DNA glycosylase or 3-methyladenine DNA glycosylase failed to reveal additional damage, indicating that most of the lesions produced were caused by the deamination of guanine to xanthine. Studies using ligation-mediated PCR supported this finding. When a 200 bp sequence of mtDNA from cells exposed to NO was analyzed, guanine was found to be the predominantly damaged base. However, there also was damage to specific adenines. No lesions were observed at pyrimidine sites. The nucleotide pattern of damage induced by NO was different from that produced by either a reactive oxygen species generator or the methylating chemical, methylnitrosourea. Most of the lesions produced by NO were repaired rapidly. However, there appeared to be a subset of lesions which were repaired either slowly or not at all by the mitochondria.

Clinical and neurobiological correlates of D10S1423 genotype in Alzheimer's disease

BACKGROUND

In a previous genome survey, we detected associations of alleles at six microsatellite loci with typical-onset AD, including the 234bp allele of the D10S1423 locus. The goal of the current study was to explore the clinical, neuropathological, and neurochemical correlates of the D10S1423 234bp allele in a group of 50 autopsy-confirmed cases of Alzheimer's disease (AD) who lacked other brain diseases.

METHODS

Clinical assessments were performed as part of a longitudinal study of AD and related disorders. Autopsies were performed using standardized methods and diagnoses were made according to established criteria. Genotyping, morphometry, and neurochemical analyses were performed using postmortem brain tissue.

RESULTS

Patients with AD who carried the D10S1423 234bp allele manifested substantial reductions in dopamine levels in all six cortical regions examined. In contrast, carriers tended to have higher concentrations of cortical norepinephrine and revealed a dosage effect of the D10S1423 234bp allele.

CONCLUSIONS

These findings support the results of our genome survey and suggest that a novel susceptibility gene for AD resides near the D10S1423 locus. The characterization of biologically meaningful subtypes, including genotypic subtypes with particular neurobiological derangements, may be important for the advancement of experimental therapeutics in AD.

The genetics of Alzheimer disease and the application of molecular tests

Two general classes of genes are associated with the development of Alzheimer disease (AD). The first group consists of genes that appear to cause AD when mutated, and the second category is composed of genes that are statistically associated with AD, depending on the inheritance of specific alleles. This paper reviews the current state of knowledge about the genetics of AD, and we then discuss the two molecular tests that are currently commercially available. These include a genetic test for mutations in the presenilin 1 (PS1) gene that can diagnose or predict a subset of early onset familial AD with a high degree of certainty. The value of the genetic test for the apolipoprotein (APOE) allele status is far less clear. Inheritance of the epsilon 4 allele is associated with an increased risk of AD at a population level, but APOE genotyping is inappropriate for prediction of future disease in an individual and offers only a marginal increase in diagnostic certainty when symptomatic individuals are tested. In the future, genetic tests may become more broadly applicable to the diagnosis and prediction of AD. However, the utility of such tests is currently limited to a small subset of individuals because in the vast majority of AD cases no clear genetic or environmental cause has been defined.

Genetic epidemiological study of maternal and paternal transmission of Alzheimer's disease

Recent evidence for mitochondrial mutations associated with Alzheimers disease (AD) suggests the possibility of maternal transmission of this illness. We investigated this hypothesis by examining, in a variety of ways, the risk of a primary progressive dementia (PPD) in the parents (n = 650) and siblings (n = 1,220) of 325 AD probands. The results did not support maternal transmission in AD: The mothers of AD probands were not at greater risk of PPD than the fathers or the sisters of AD probands; the offspring of affected mothers were not at greater risk than the offspring of affected fathers or families with no affected parent; and, after selecting those proband families with evidence for increased familial loading, such families did not more frequently have affected mothers than fathers. In contrast, the cumulative risk of PPD in fathers of AD probands, while similar to that of mothers, was significantly increased over the brothers of AD probands. In addition, the cumulative risk curve of PPD in the offspring of affected fathers was significantly higher than the offspring of no affected parents. While no evidence for maternal transmission in AD was observed, unexpectedly, we did find evidence of increased paternal transmission.

A novel mitochondrial DNA-like sequence in the human nuclear genome

We describe here a nuclear mitochondrial DNA-like sequence (numtDNA) that is nearly identical in sequence to a continuous 5842 bp segment of human mitochondrial DNA (mtDNA) that spans nucleotide positions 3914 to 9755. On the basis of evolutionary divergence among modern primates, this numtDNA molecule appears to represent mtDNA from a hominid ancestor that has been translocated to the nuclear genome during the recent evolution of humans. This numtDNA sequence harbors synonymous and nonsynonymous nucleotide substitutions relative to the authentic human mtDNA sequence, including an array of substitutions that was previously found in the cytochrome c oxidase subunit 1 and 2 genes. These substitutions were previously reported to occur in human mtDNA, but subsequently contended to be present in a nuclear pseudogene sequence. We now demonstrate their exclusive association with this 5842-bp numtDNA, which we have characterized in its entirety. This numtDNA does not appear to be expressed as a mtDNA-encoded mRNA. It is present in nuclear DNA from human blood donors, in human SH-SY5Y and A431 cell lines, and in rho(0) SH-SY5Y and rho(0) A431 cell lines that were depleted of mtDNA. The existence of human numtDNA sequences with great similarities to human mtDNA renders the amplification of pure mtDNA from cellular DNA very difficult, thereby creating the potential for confounding studies of mitochondrial diseases and population genetics.

Mitochondrial DNA analysis: polymorphisms and pathogenicity

The investigation of mtDNA disease can be relatively straightforward if a person has a recognisable phenotype and if it is possible to identify a known pathogenic mtDNA mutation. The difficulties arise when no known mtDNA defect can be found, or when the clinical abnormalities are complex and not easily matched to those of the more common mitochondrial disorders. We will describe here the difficulties that can be encountered during the identification of pathogenic mtDNA mutations and the approaches that can be used to confirm, or eliminate, a likely pathogenic role, in either single gene diseases or in multifactorial disorders.

The expression of several mitochondrial and nuclear genes encoding the subunits of electron transport chain enzyme complexes, cytochrome c oxidase, and NADH dehydrogenase, in different brain regions in Alzheimer's disease

In this study, changes of the expression of two mitochondrial and two nuclear genes encoding the subunits of cytochrome c oxidase (CO) and NADH dehydrogenase (ND) were studied in the hippocampus, inferior parietal lobule, and cerebellum of 10 Alzheimer's disease (AD) and 10 age-matched control subjects. The altered proportion between CO II and CO IV mRNAs was observed in the AD brain. Changes of the proportion between CO II and CO IV transcripts may contribute to the kinetic perturbation of CO documented in AD. A coordinated decrease of ND4 and ND15 mRNAs was found in the AD hippocampus and inferior parietal lobule, but not in cerebellum. The decrease of ND4 gene expression may lead to the inhibition of normal ubiquinone oxidoreductase activity of ND. This study suggests that changes of the expression of mitochondrial and nuclear genes, encoding parts of ND and CO enzyme complexes, may contribute to alterations of oxidative metabolism in AD.

Metabolic consequences of the cytochrome c oxidase deficiency in brain of copper-deficient Mo(vbr) mice

Biochemical micromethods were used for the investigation of changes in mitochondrial oxidative phosphorylation associated with cytochrome c oxidase deficiency in brain cortex from Mo(vbr) (mottled viable brindled) mice, an animal model of Menkes' copper deficiency syndrome. Enzymatic analysis of cortex homogenates from Mo(vbr) mice showed an approximately twofold decrease in cytochrome c oxidase and a 1.4-fold decrease in NADH:cytochrome c reductase activities as compared with controls. Assessment of mitochondrial respiratory function was performed using digitonin-treated homogenates of the cortex, which exhibited the main characteristics of isolated brain mitochondria. Despite the substantial changes in respiratory chain enzyme activities, no significant differences were found in maximal pyruvate or succinate oxidation rates of brain cortex homogenates from Mo(vbr) and control mice. Inhibitor titrations were used to determine flux control coefficients of NADH:CoQ oxidoreductase and cytochrome c oxidase on the rate of mitochondrial respiration. Application of amobarbital to titrate the activity of NADH:CoQ oxidoreductase showed very similar flux control coefficients for control and mutant animals. Alternately, titration of respiration with azide revealed for Mo(vbr) mice significantly sharper inhibition curves than for controls, indicating a more than twofold elevated flux control coefficient of cytochrome c oxidase. Owing to the reserve capacity of respiratory chain enzymes, the reported changes in activities do not seem to affect whole-brain high-energy phosphates, as observed in a previous study using 31P NMR.

Energy metabolism inhibition impairs amyloid precursor protein secretion from Alzheimer's fibroblasts

The present study investigates the influence of aglycemia and sodium azide (a Cytochrome c Oxidase inhibitor) on sAPP secretion from skin fibroblasts derived from sporadic AD patients and control subjects. Aglycemia reduced sAPP release in the medium of both AD and control fibroblasts to a similar extent after 2 h incubation. Treatment for 2 h with increasing azide concentrations (1 microM-100 mM) under glucose deprivation did not significantly affect sAPP secretion from control fibroblasts, but was able to significantly inhibit sAPP secretion from AD fibroblasts (maximal inhibition 51%). The failure of antioxidants like glutathione (GSH) or N-acetylcysteine (NAC) to antagonize the azide effect on AD fibroblasts and lipoperoxidation data seemed to rule out the possibility that oxidative stress could mediate the sodium azide effect on sAPP release from AD fibroblasts.

Mitochondrial disorders: clinical and genetic features

Virtually all cells in humans depend on mitochondrial oxidative phosphorylation to generate energy, accounting for the remarkable diversity of clinical disorders associated with mitochondrial DNA mutations. However, certain tissues are particularly susceptible to mitochondrial dysfunction, resulting in recognizable clinical syndromes. Mitochondrial DNA mutations have been linked to seizures, strokes, optic atrophy, neuropathy, myopathy, cardiomyopathy, sensorineural hearing loss, diabetes mellitus, and other clinical features. Mitochondrial DNA mutations also may play an important role in aging, as well as in common age-related neurodegenerative disorders such as Parkinson's disease. Therefore, it is becoming increasingly important for clinicians to recognize the clinical syndromes suggestive of a mitochondrial disorder, and to understand the unique features of mitochondrial genetics that complicate diagnosis and genetic counseling.

Neurobiological correlates of a putative risk allele for Alzheimer's disease on chromosome 12q

OBJECTIVE

To explore the clinical, neuropathologic, and neurochemical correlates of the D12S1045 91 base pair (bp) allele in a group of 50 autopsy-confirmed cases of AD who lacked other concomitant brain diseases.

BACKGROUND

In a previous genome survey for novel risk loci for typical-onset (> or =60 years) AD conducted at 10 cM resolution, we detected associations of alleles at six microsatellite loci with AD. These included the 91bp allele of the D12S1045 locus that resides in the telomeric region of 12q.

METHODS

Clinical assessment was performed as part of a longitudinal study of AD and related disorders. Standardized pathologic methods, genotyping, morphometry, and neurochemical analyses were performed with postmortem brain tissue.

RESULTS

Patients with AD who carried the D12S1045 91bp allele manifested earlier ages at symptomatic onset and death, greater densities of cortical neurofibrillary tangles, and substantially greater reductions in cortical dopamine levels compared to noncarriers. A dosage effect of the number of D12S1045 91bp alleles on cortical dopamine levels was also observed.

CONCLUSIONS

Carrying the D12S1045 91bp allele was associated with greater clinical, neuropathologic, and neurochemical severity independent of sex and APOE genotype. These findings suggest that a novel susceptibility gene for AD resides at or in close proximity to the D12S1045 locus.

Functional integrity of mitochondrial genomes in human platelets and autopsied brain tissues from elderly patients with Alzheimer's disease

To determine whether pathogenic mutations in mtDNA are involved in phenotypic expression of Alzheimer's disease (AD), the transfer of mtDNA from elderly patients with AD into mtDNA-less (rho0) HeLa cells was carried out by fusion of platelets or synaptosomal fractions of autopsied brain tissues with rho0 HeLa cells. The results showed that mtDNA in postmortem brain tissue survives for a long time without degradation and could be rescued in rho0 HeLa cells. Next, the cybrid clones repopulated with exogenously imported mtDNA from patients with AD were used for examination of respiratory enzyme activity and transfer of mtDNA with the pathogenic mutations that induce mitochondrial dysfunction. The presence of the mutated mtDNA was restricted to brain tissues and their cybrid clones that formed with synaptosomes as mtDNA donors, whereas no cybrid clones that isolated with platelets as mtDNA donors had detectable mutated mtDNA. However, biochemical analyses showed that all cybrid clones with mtDNA imported from platelets or brain tissues of patients with AD restored mitochondrial respiration activity to almost the same levels as those of cybrid clones with mtDNA from age-matched normal controls, suggesting functional integrity of mtDNA in both platelets and brain tissues of elderly patients with AD. These observations warrant the reassessment of the conventional concept that the accumulation of pathogenic mutations in mtDNA throughout the aging process is responsible for the decrease of mitochondrial respiration capacity with age and with the development of age-associated neurodegenerative diseases.

Mitochondrial DNA--all things bad?

Mutations in mitochondrial DNA (mtDNA) are undoubtedly associated with a diverse spectrum of human disorders. More controversially, it has been claimed that they accumulate during ageing, and that they are responsible for an age-related decline in bioenergetic function and tissue viability. Here, we review the evidence for this assertion, concluding that claims for the age-accumulation of mtDNA mutations are based largely on non-quantitative methods, and that no clear, functional deficit of mitochondrial respiration has been shown to result from such lesions in aged individuals. The mitochondrial theory of ageing, however attractive in principle, is supported by very little hard evidence.

Mitochondrial involvement in Alzheimer's disease

The causes of most neurodegenerative diseases, including sporadic Alzheimer's disease (AD), remain enigmatic. There is, however, increasing evidence implicating mitochondrial dysfunction resulting from deafferentiation of disconnected neural circuits in the pathogenesis of energy deficit in AD. The patterns of reduced expression of both mitochondrial DNA (mtDNA) and nuclear DNA (nDNA) encoded genes is consistent with a physiological down-regulation of the mitochondrial respiratory chain in response to reduced neuronal activity. On the other hand, the role(s) of somatic cell or maternally inherited mtDNA mutations in the pathogenesis of mitochondrial dysfunction in AD are still controversial.

Reduced neuronal activity and reactivation in Alzheimer's disease

1. Alzheimer's disease is a multifactorial disease in which age and APOE-epsilon 4 are important risk factors. Various mutations and even viral infections such as herpes simplex (Itzhaki et al., 1997) may play an additional role. 2. The neuropathological hallmarks of Alzheimer's disease (AD), i.e. amorphous plaques, neuritic plaques (NPs), pretangles, neurofibrillary tangles (NFT) and cell death are not part of a single pathogenetic cascade but are basically independent phenomena. 3. Pretangles can occur in neurons from which the metabolic rate is not altered. However, in brain areas where classical AD changes, i.e. NPs and NFTs, are present, such as the CA1 area of the hippocampus, the nucleus basalis of Meynert and the tuberomamillary nucleus, a decreased metabolic rate is found. Decreased metabolic rate appears to be an independent phenomenon in Alzheimer's disease. It is not induced by the presence of pretangles, NFT or NPs. 4. Decreased metabolic rate may precede cognitive impairment and is thus an early occurring hallmark of Alzheimer's disease, which, in principle, may be reversible. The observation that the administration of glucose or insulin enhances memory in Alzheimer patients also supports the view that Alzheimer's disease is basically a metabolic disease. Moreover, several observations indicate that activated neurons are better able to withstand aging and AD, a phenomenon paraphrased by us as "use it or lose it". It is, therefore, attractive to direct the development of therapeutic strategies towards restimulation of neuronal metabolic rate in order to improve cognition and other symptoms in Alzheimer's disease. A number of pharmacological and non-pharmacological studies support the concept that activation of the brain indeed has beneficial effects on several aspects of cognition and other central functions.

Alzheimer's disease: genetic studies and transgenic models

Recent advances in a variety of areas of research, particularly in genetics and in transgenic (Tg)/gene targeting approaches, have had a substantial impact on our understanding of Alzheimer's disease (AD) and related disorders. After briefly reviewing the progress that has been made in diagnostic assessments of patients with senile dementia and in investigations of the neuropathology of AD, we discuss some of the genes/proteins that are causative or risk factors for this disease, including those encoding amyloid precursor protein, presenilin 1 and 2, and apolipoprotein E. In addition, we comment on several potential new candidate loci/genes. Subsequently, we review selected recent reports of analyses of a variety of lines of Tg mice that show several neuropathological features of AD, including A beta-amyloid deposits and dystrophic neurites. Finally, we discuss the several important issues in future investigations of Tg mice, with particular emphasis on the influences of genetic strains on phenotype, especially behavior, and strategies for making new models of neurodegenerative disorders. We believe that investigations of these Tg models will (a) enhance understanding of the relationships between impaired performance on memory tasks and the pathological/biochemical abnormalities in brain, (b) help to clarify pathogenic mechanisms in vivo, (c) lead to identification of new therapeutic targets, and (d) allow testing of new treatment strategies first in mice and then, if successful, in humans with AD.

Retrograde Ca2+ signaling in C2C12 skeletal myocytes in response to mitochondrial genetic and metabolic stress: a novel mode of inter-organelle crosstalk

We have investigated the mechanism of mitochondrial-nuclear crosstalk during cellular stress in mouse C2C12 myocytes. For this purpose, we used cells with reduced mitochondrial DNA (mtDNA) contents by ethidium bromide treatment or myocytes treated with known mitochondrial metabolic inhibitors, including carbonyl cyanide m-chlorophenylhydrazone (CCCP), antimycin, valinomycin and azide. Both genetic and metabolic stresses similarly affected mitochondrial membrane potential (Deltapsim) and electron transport-coupled ATP synthesis, which was also accompanied by an elevated steady-state cytosolic Ca2+ level ([Ca2+]i). The mitochondrial stress resulted in: (i) an enhanced expression of the sarcoplasmic reticular ryanodine receptor-1 (RyR-1), hence potentiating the Ca2+ release in response to its modulator, caffeine; (ii) enhanced levels of Ca2+-responsive factors calineurin, calcineurin-dependent NFATc (cytosolic counterpart of activated T-cell-specific nuclear factor) and c-Jun N-terminal kinase (JNK)-dependent ATF2 (activated transcription factor 2); (iii) reduced levels of transcription factor, NF-kappaB; and (iv) enhanced transcription of cytochrome oxidase Vb (COX Vb) subunit gene. These cellular changes, including the steady-state [Ca2+]i were normalized in genetically reverted cells which contain near-normal mtDNA levels. We propose that the mitochondria-to-nucleus stress signaling occurs through cytosolic [Ca2+]i changes, which are likely to be due to reduced ATP and Ca2+ efflux. Our results indicate that the mitochondrial stress signal affects a variety of cellular processes, in addition to mitochondrial membrane biogenesis.

Decreased brain protein levels of cytochrome oxidase subunits in Alzheimer's disease and in hereditary spinocerebellar ataxia disorders: a nonspecific change?

Controversy exists as to the clinical importance, cause, and disease specificity of the cytochrome oxidase (CO) activity reduction observed in some patients with Alzheimer's disease (AD). Although it is assumed that the enzyme is present in normal amount in AD, no direct measurements of specific CO protein subunits have been conducted. We measured protein levels of CO subunits encoded by mitochondrial (COX I, COX II) and nuclear (COX IV, COX VIc) DNA in autopsied brain of patients with AD whom we previously reported had decreased cerebral cortical CO activity. To assess disease specificity, groups of patients with spinocerebellar ataxia type I and Friedreich's ataxia were also included. As compared with the controls, mean protein concentrations of all four CO subunits were significantly decreased (-19 to -47%) in temporal and parietal cortices in the AD group but were not significantly reduced (-12 to -17%) in occipital cortex. The magnitude of the reduction in protein levels of the CO subunits encoded by mitochondrial DNA (-42 to -47%) generally exceeded that encoded by nuclear DNA (-19 to -43%). In the spinocerebellar ataxia disorders, COX I and COX II levels were significantly decreased in cerebellar cortex (-22 to -32%) but were normal or close to normal in cerebral cortex, an area relatively unaffected by neurodegeneration. We conclude that protein levels of mitochondrial- and nuclear-encoded CO subunits are moderately reduced in degenerating but not in relatively spared brain areas in AD and that the decrease is not specific to this disorder. The simplest explanation for our findings is that CO is decreased in human brain disorders as a secondary event in brain areas having reduced neuronal activity or neuronal/synaptic elements consequent to the primary neurodegenerative process.

Genes and susceptible loci of Alzheimer's disease

Alzheimer's disease (AD) is the most common and devastating neurodegenerative disease of the elderly. Many research findings on familial AD suggest that the mechanisms of the pathogenesis of the disorder is more complex although the overall neuropathology of all cases of AD is surprisingly very similar. Genetic studies on some families have shown that mutations in the genes encoding beta-amyloid precursor protein and presenilins 1 and 2 are responsible for early-onset AD. In addition, apolipoprotein E gene allele E4 and the bleomycin hydrolase locus are shown to be genetic risk factors for late-onset AD in certain sporadic cases. Mitochondrial dysfunctions and age-related oxidative stress may also contribute to degenerative processes in AD. Although several studies support the amyloid cascade hypothesis as the mechanism of the disease, transgenic experiments and recent findings on a variant form of an AD family suggest that A beta deposition may not be sufficient to cause AD. Identification in the future of other genetic, environmental, and age-related factors, may provide additional targets for therapies.

Anti-death properties of TNF against metabolic poisoning: mitochondrial stabilization by MnSOD

The cytokine tumor necrosis factor (TNF) is toxic to some mitotic cells, but protects cultured neurons from a variety of insults by mechanisms that are unclear. Pretreatment of neurons or astrocytes with TNF caused significant increases in MnSOD activity, and also significantly attenuated 3-nitropropionic acid (3-NP) induced superoxide accumulation and loss of mitochondrial transmembrane potential. In oligodendrocytes, however, MnSOD activity was not increased, and 3-NP toxicity was unaffected by TNF. Genetically engineered PC6 cells that overexpress MnSOD also were resistant to 3-NP-induced damage. TNF pretreatment and MnSOD overexpression prevented 3-NP induced apoptosis, and shifted the mode of death from necrosis to apoptosis in response to high levels of 3-NP. Mitochondria isolated from either MnSOD overexpressing PC6 cells or TNF-treated neurons maintained resistance to 3-NP-induced loss of transmembrane potential and calcium homeostasis, and showed attenuated release of caspase activators. Overall, these results indicate that MnSOD activity directly stabilizes mitochondrial transmembrane potential and calcium buffering ability, thereby increasing the threshold for lethal injury. Additional studies showed that levels of oxidative stress and striatal lesion size following 3-NP administration in vivo are increased in mice lacking TNF receptors.

Effect of oxidative stress on DNA damage and beta-amyloid precursor proteins in lymphoblastoid cell lines from a Nigerian population

The epsilon 4 allele of apolipoprotein E (APOE) is strongly associated with late-onset Alzheimer's disease (AD) in Caucasian populations, but our studies suggest that APOE epsilon 4 is not a risk factor for AD in Nigerian blacks and is a weak risk factor in African-Americans. The prevalence of AD is lower in Nigerians than in African-Americans. Increased oxidative damage to macromolecules in brain tissue by reactive oxygen species (ROS) has been reported in AD. Here we examined the effects of endogenous and induced oxidative stress on total (nuclear and mitochondrial) DNA damage in lymphoblastoid cell lines (5 probable AD and 3 controls) from Ibadan, Nigeria. Cells were exposed to 200 microM t-butyl peroxide (a generator of ROS) for 4 hours. Total DNA was isolated and digested with nuclease P1 and alkaline phosphatase. DNA fragments were separated by HPLC and the levels of 8-hydroxy-2'-deoxyguanosine (OH8dG, an indicator of DNA damage) and deoxyguanosine (dG) determined. We did not detect a significant difference in the OH8dG/dG ratio in untreated or treated cell lines in the two groups, and this was independent of APOE genotype. We also examined, by Western blotting, the level of beta-amyloid precursor protein (APP) which is involved in AD. The level of the heat shock protein (HSP-70) was examined as a control. There was a slight decrease in levels of APP and HSP-70 following treatment. Studies in cell lines from Caucasian subjects have shown an increase in mitochondrial DNA damage following oxidative challenge. Our preliminary results suggest that African populations are less vulnerable to chemical-induced oxidative DNA damage.

Genetic risk factors in Alzheimer's disease

Following a brief introduction and discussion of the pathological features of Alzheimer's disease, the main emphasis of this review article will be the genetic factors that have been implicated in this disease. These can be divided into two main categories. First, the three genes in which mutations are known to result in early onset autosomal dominant familial Alzheimer's disease will be discussed. These are well characterised but account for only a small proportion of Alzheimer's disease cases. Late onset, sporadic Alzheimer's disease is more common and evidence suggests that there is a genetic component to this type of disease. A number of genetic risk factors have been implicated that might increase the risk of developing sporadic disease. Many of these are controversial and studies have shown conflicting results, which are discussed in this section. Finally, a brief discussion of some of the mechanisms suggested to play a role in the pathogenesis of Alzheimer's disease is included. It is hoped that this will show why particular genes have been implicated in Alzheimer's disease and how they might be able to influence the development of the disease.

Overexpression of tau protein inhibits kinesin-dependent trafficking of vesicles, mitochondria, and endoplasmic reticulum: implications for Alzheimer's disease

The neuronal microtubule-associated protein tau plays an important role in establishing cell polarity by stabilizing axonal microtubules that serve as tracks for motor-protein-driven transport processes. To investigate the role of tau in intracellular transport, we studied the effects of tau expression in stably transfected CHO cells and differentiated neuroblastoma N2a cells. Tau causes a change in cell shape, retards cell growth, and dramatically alters the distribution of various organelles, known to be transported via microtubule-dependent motor proteins. Mitochondria fail to be transported to peripheral cell compartments and cluster in the vicinity of the microtubule-organizing center. The endoplasmic reticulum becomes less dense and no longer extends to the cell periphery. In differentiated N2a cells, the overexpression of tau leads to the disappearance of mitochondria from the neurites. These effects are caused by tau's binding to microtubules and slowing down intracellular transport by preferential impairment of plus-end-directed transport mediated by kinesin-like motor proteins. Since in Alzheimer's disease tau protein is elevated and mislocalized, these observations point to a possible cause for the gradual degeneration of neurons.

Diagnosis of mitochondrial disease: assessment of mitochondrial DNA heteroplasmy in blood

Mitochondrial DNA (mtDNA) mutations are an important cause of neurological disease. The identification of causative mtDNA mutations may be particularly troublesome in blood where there are often low levels of mutant mtDNA. This is evident from a recent study in which heteroplasmic mtDNA mutations in cytochrome c oxidase genes were incorrectly thought to be linked to Alzheimer's disease. We wished to explore whether analysis of blood mtDNA, prepared by a number of DNA extraction procedures, influenced the diagnosis of mtDNA disease. DNA was extracted by different procedures from 4 patients with heteroplasmic mtDNA mutations, and the level of heteroplasmy investigated by radioactive PCR-RFLP analysis. Whilst there was no consistent decrease in the level of mtDNA heteroplasmy, we observed the coamplification of a novel mtDNA pseudogene from DNA samples extracted by a simple 'boiling' procedure using primers designed to screen for the tRNALeu(UUR) A3243G mutation. This pseudogene was readily amplified from DNA extracted from rho degrees (mtDNA-less) cells, confirming its nuclear location. We believe that mtDNA pseudogenes may therefore present significant difficulties in the accurate identification of pathogenic heteroplasmic mtDNA mutations in blood.

Human mitochondrial diseases: answering questions and questioning answers

Since the first identification in 1988 of pathogenic mitochondrial DNA (mtDNA) mutations, the mitochondrial diseases have emerged as a major clinical entity. The most striking feature of these disorders is their marked heterogeneity, which extends to their clinical, biochemical, and genetic characteristics. The major mitochondrial encephalomyopathies include MELAS (mitochondrial encephalopathy with lactic acidosis and stroke-like episodes), MERRF (myoclonic epilepsy with ragged red fibers), KSS/CPEO (Kearns-Sayre syndrome/chronic progressive external ophthalmoplegia), and NARP/MILS (neuropathy, ataxia, and retinitis pigmentosum/maternally inherited Leigh syndrome) and they typically present highly variable multisystem defects that usually involve abnormalities of skeletal muscle and/or the CNS. The primary emphasis here is to review recent investigations of these mitochondrial diseases from the standpoint of how the complexities of mitochondrial genetics and biogenesis might determine their varied features. In addition, the mitochondrial encephalomyopathies are compared and contrasted to Leber hereditary optic neuropathy, a mitochondrial disease in which the pathogenic mtDNA mutations produce a more uniform and focal neuropathology. All of these disorders involve, at some level, a mitochondrial respiratory chain dysfunction. Because mitochondrial genetics differs so strikingly from the Mendelian inheritance of chromosomes, recent research on the origin and subsequent segregation and transmission of mtDNA mutations is reviewed.

Mitochondrial encephalomyopathies

It is nearly a decade since the discovery of the first mutations in mitochondrial DNA associated with mitochondrial encephalomyopathy, and the pace of discovery of new mitochondrial DNA mutations continues unabated. Nuclear gene defects in these disorders have been more difficult to identify; only one is known, but others have been mapped by linkage analysis. The rules governing transmission and segregation of mitochondrial DNA sequence variants are beginning to be unravelled and progress has been made in understanding genotype-phenotype relationships and elucidating mechanisms of pathogenesis.

Oxidative damage to DNA in plaques of MS brains

A major cause of clinical disability in multiple sclerosis (MS) is related to a degenerative process in the central nervous system (CNS) which ultimately develops from a potentially reversible inflammation and demyelination. The mechanism of this degenerative process within MS lesions is not completely understood. We hypothesize that oxidative damage to DNA secondary to inflammation may contribute to irreversible tissue alterations in a plaque. To test this assumption, we determined the level of a DNA oxidative marker, 8-hydroxy-deoxy-guanosine (8-OH-dG) in the normal appearing white matter (NAWM), plaque and cortical regions of cerebella from MS patients who suffered from severe cerebellar symptoms during the course of the disease, and in NAWM and cortical regions of cerebella from non-neurological controls. We found a significant increase in DNA oxidation within plaques compared to NAWM specimens in MS cerebella. A tendency for increase of oxidative markers in normal appearing cortical tissues located in the proximity of MS plaques was also observed when compared to those in control cortical specimens. Oxidative damage to DNA in MS lesions, and in neuron rich areas located in the proximity of these lesions is likely related to the release of reactive oxygen species (ROS) and nitric oxide (NO) during inflammation in the brain. This biochemical impairment of DNA and of other macromolecules may contribute to the development of severe clinical disability through the induction of degenerative changes within and outside of plaques in MS brains.

Catecholamines potentiate amyloid beta-peptide neurotoxicity: involvement of oxidative stress, mitochondrial dysfunction, and perturbed calcium homeostasis

Oxidative stress and mitochondrial dysfunction are implicated in the neuronal cell death that occurs in physiological settings and in neurodegenerative disorders. In Alzheimer's disease (AD) degenerating neurons are associated with deposits of amyloid beta-peptide (A beta), and there is evidence for increased membrane lipid peroxidation and protein oxidation in the degenerating neurons. Cell culture studies have shown that A beta can disrupt calcium homeostasis and induce apoptosis in neurons by a mechanism involving oxidative stress. We now report that catecholamines (norepinephrine, epinephrine, and dopamine) increase the vulnerability of cultured hippocampal neurons to A beta toxicity. The catecholamines were effective in potentiating A beta toxicity at concentrations of 10-200 microM, with the higher concentrations (100-200 microM) themselves inducing cell death. Serotonin and acetylcholine were not neurotoxic and did not modify A beta toxicity. Levels of membrane lipid peroxidation, and cytoplasmic and mitochondrial reactive oxygen species, were increased following exposure to neurons to A beta, and catecholamines exacerbated the oxidative stress. Subtoxic concentrations of catecholamines exacerbated decreases in mitochondrial energy charge and transmembrane potential caused by A beta, and higher concentrations of catecholamines alone induced mitochondrial dysfunction. Antioxidants (vitamin E, glutathione, and propyl gallate) protected neurons against the damaging effects of A beta and catecholamines, whereas the beta-adrenergic receptor antagonist propanolol and the dopamine (D1) receptor antagonist SCH23390 were ineffective. Measurements of intracellular free Ca2+ ([Ca2+]i) showed that A beta induced a slow elevation of [Ca2+]i which was greatly enhanced in cultures cotreated with catecholamines. Collectively, these data indicate a role for catecholamines in exacerbating A beta-mediated neuronal degeneration in AD and, when taken together with previous findings, suggest roles for oxidative stress induced by catecholamines in several different neurodegenerative conditions.