Many lines of evidence suggest that mitochondria have a central role in ageing-related neurodegenerative diseases. Mitochondria are critical regulators of cell death, a key feature of neurodegeneration. Mutations in mitochondrial DNA and oxidative stress both contribute to ageing, which is the greatest risk factor for neurodegenerative diseases. In all major examples of these diseases there is strong evidence that mitochondrial dysfunction occurs early and acts causally in disease pathogenesis. Moreover, an impressive number of disease-specific proteins interact with mitochondria. Thus, therapies targeting basic mitochondrial processes, such as energy metabolism or free-radical generation, or specific interactions of disease-related proteins with mitochondria, hold great promise.

Mitochondrial dysfunction and oxidative stress in neurodegenerative diseases

The cause of Parkinson's disease (PD) is unknown, but epidemiological studies suggest an association with pesticides and other environmental toxins, and biochemical studies implicate a systemic defect in mitochondrial complex I. We report that chronic, systemic inhibition of complex I by the lipophilic pesticide, rotenone, causes highly selective nigrostriatal dopaminergic degeneration that is associated behaviorally with hypokinesia and rigidity. Nigral neurons in rotenone-treated rats accumulate fibrillar cytoplasmic inclusions that contain ubiquitin and alpha-synuclein. These results indicate that chronic exposure to a common pesticide can reproduce the anatomical, neurochemical, behavioral and neuropathological features of PD.

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Chronic systemic pesticide exposure reproduces features of Parkinson's disease

Oxidative stress has been implicated in the progression of Alzheimer's disease, Parkinson's disease and amyotrophic lateral sclerosis. Oxygen is vital for life but is also potentially dangerous, and a complex system of checks and balances exists for utilizing this essential element. Oxidative stress is the result of an imbalance in pro-oxidant/antioxidant homeostasis that leads to the generation of toxic reactive oxygen species. The systems in place to cope with the biochemistry of oxygen are complex, and many questions about the mechanisms of oxygen regulation remain unanswered. However, this same complexity provides a number of therapeutic targets, and different strategies, including novel metal-protein attenuating compounds, aimed at a variety of targets have shown promise in clinical studies.

Neurodegenerative diseases and oxidative stress.

Radicals are species containing one or more unpaired electrons. The oxygen radical superoxide (O 2 - ) and the non-radical oxidants hydrogen peroxide (H2O2) and hypochlorous acid (HOCl) are produced during normal metabolism and perform several useful functions. Excessive production of O 2 - and H2O2 can result in tissue damage, which often involves generation of highly reactive hydroxy 1 radical (· OH) and other oxidants in the presence of “catalytic” iron or copper ions. A major form of antioxidant defence is the storage and transport of iron or copper ions in forms that will not catalyze formation of reactive radicals. Tissue injury, e. g., by ischaemia or trauma, can cause increased iron availability and accelerate free radical reactions. This may be especially important in the brain, since areas of this organ are rich in iron and cerebrospinal fluid cannot bind released iron ions. Oxidative stress upon nervous tissue can produce damage by several interacting mechanisms, including rises in intracellular free Ca2+ and, possibly, release of excitatory amino acids. Recent suggestions that free radical reactions are involved in the neurotoxicity of aluminium and in damage to the substantia nigra in Parkinson’s disease are reviewed. Finally, the nature of antioxidants is discussed, with a suggestion that antioxidant enzymes and chelators of iron ions may be more generally useful protective agents than chain-breaking antioxidants. Careful precautions must be taken in the design of antioxidants for therapeutic use.

Reactive Oxygen Species and the Central Nervous System

Oxidative DNA damage is an inevitable consequence of cellular metabolism, with a propensity for increased levels following toxic insult. Although more than 20 base lesions have been identified, only a fraction of these have received appreciable study, most notably 8-oxo-2'deoxyguanosine. This lesion has been the focus of intense research interest and been ascribed much importance, largely to the detriment of other lesions. The present work reviews the basis for the biological significance of oxidative DNA damage, drawing attention to the multiplicity of proteins with repair activities along with a number of poorly considered effects of damage. Given the plethora of (often contradictory) reports describing pathological conditions in which levels of oxidative DNA damage have been measured, this review critically addresses the extent to which the in vitro significance of such damage has relevance for the pathogenesis of disease. It is suggested that some shortcomings associated with biomarkers, along with gaps in our knowledge, may be responsible for the failure to produce consistent and definitive results when applied to understanding the role of DNA damage in disease, highlighting the need for further studies.

Oxidative DNA damage: mechanisms, mutation, and disease

Mitochondrial complex I deficiency in Parkinson's disease.

Polyunsaturated fatty acid (PUFA) levels (an index of the amount of substrate available for lipid peroxidation) were measured in several brain regions from patients who died with Parkinson's disease and age-matched control human postmortem brains. PUFA levels were reduced in parkinsonian substantia nigra compared to other brain regions and to control tissue. However, basal malondialdehyde (MDA; an intermediate in the lipid peroxidation process) levels were increased in parkinsonian nigra compared with other parkinsonian brain regions and control tissue. Expressing basal MDA levels in terms of PUFA content, the difference between parkinsonian and control substantia nigra was even more pronounced. Stimulating MDA production by incubating tissue with FeSO4 plus ascorbic acid, FeSO4 plus H2O2, or air alone produced lower MDA levels in the parkinsonian substantia nigra, probably reflecting the lower PUFA content. These results may indicate that an increased level of lipid peroxidation continues to occur in the parkinsonian nigra up to the time of death, perhaps because of continued exposure to excess free radicals derived from some endogenous or exogenous neurotoxic species.

Basal Lipid Peroxidation in Substantia Nigra Is Increased in Parkinson's Disease

Levels of iron, copper, zinc, manganese, and lead were measured by inductively coupled plasma spectroscopy in parkinsonian and age-matched control brain tissue There was 31-35% increase in the total iron content of the parkinsonian substantia nigra when compared to control tissue In contrast, in the globus pallidus total iron levels were decreased by 29% in Parkinson's disease There was no change in the total iron levels in any other region of the parkinsonian brain Total copper levels were reduced by 34-45% in the substantia nigra in Parkinson's disease; no difference was found in the other brain areas examined Zinc levels were increased in substantia nigra in Parkinson's disease by 50-54%, and the zinc content of the caudate nucleus and lateral putamen was also raised by 18-35% Levels of manganese and lead were unchanged in all areas of the parkinsonian brain studied when compared to control brains, except for a small decrease (20%) in manganese content of the medial putamen Increased levels of total iron in the substantia nigra may cause the excessive formation of toxic oxygen radicals, leading to dopamine cell death

Increased Nigral Iron Content and Alterations in Other Metal Ions Occurring in Brain in Parkinson's Disease

Increased nigral iron content in postmortem parkinsonian brain.

1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) is thought to produce parkinsonism in humans and other primates through its inhibition of complex I The recent discovery of mitochondrial complex I deficiency in the substantia nigra of patients with Parkinson's disease has provided a remarkable link between the idiopathic disease and the action of the neurotoxin MPTP This article shows that complex I deficiency in Parkinson's disease is anatomically specific for the substantia nigra, and is not present in another neurodegenerative disorder involving the substantia nigra Evidence is also provided to show that there is no correlation between L-3,4-dihydroxyphenylalanine therapy and complex I deficiency These results suggest that complex I deficiency may be the underlying cause of dopaminergic cell death in Parkinson's disease

D. Dexter

Papers

Mitochondrial complex I deficiency in Parkinson's disease.

Basal Lipid Peroxidation in Substantia Nigra Is Increased in Parkinson's Disease

Increased Nigral Iron Content and Alterations in Other Metal Ions Occurring in Brain in Parkinson's Disease

Increased nigral iron content in postmortem parkinsonian brain.

Anatomic and Disease Specificity of NADH CoQ1 Reductase (Complex I) Deficiency in Parkinson's Disease