Role of mitochondria in the etiology and pathogenesis of Parkinson's disease

The alpha-ketoglutarate dehydrogenase complex in neurodegeneration

Altered energy metabolism is characteristic of many neurodegenerative disorders. Reductions in the key mitochondrial enzyme complex, the alpha-ketoglutarate dehydrogenase complex (KGDHC), occur in a number of neurodegenerative disorders including Alzheimer's Disease (AD). The reductions in KGDHC activity may be responsible for the decreases in brain metabolism, which occur in these disorders. KGDHC can be inactivated by several mechanisms, including the actions of free radicals (Reactive Oxygen Species, ROS). Other studies have associated specific forms of one of the genes encoding KGDHC (namely the DLST gene) with AD, Parkinson's disease, as well as other neurodegenerative diseases. Reductions in KGDHC activity can be plausibly linked to several aspects of brain dysfunction and neuropathology in a number of neurodegenerative diseases. Further studies are needed to assess mechanisms underlying the sensitivity of KGDHC to oxidative stress and the relation of KGDHC deficiency to selective vulnerability in neurodegenerative diseases.

Thiol oxidation and loss of mitochondrial complex I precede excitatory amino acid-mediated neurodegeneration

Human ingestion of "chickling peas" from the plant Lathyrus sativus, which contains an excitatory amino acid, L-BOAA (L-beta-N-oxalylamino-L-alanine), leads to a progressive corticospinal neurodegenerative disorder, neurolathyrism. Exposure to L-BOAA, but not its optical enantiomer D-BOAA, causes mitochondrial dysfunction as evidenced by loss of complex I activity in vitro in male mouse brain slices and in vivo in selected regions of mouse CNS (lumbosacral cord and motor cortex). Loss of complex I activity in lumbosacral cord after L-BOAA administration to mice was accompanied by concurrent loss of glutathione. The inhibited complex I activity in mitochondria isolated from lumbosacral cord of animals treated with L-BOAA rebounded after incubation with the thiol-reducing agent dithiothreitol, indicating that oxidation of protein thiols to disulfides was responsible for enzyme inhibition. The inhibition of complex I could be abolished by pretreatment with antioxidant thiols such as glutathione ester and alpha-lipoic acid. Chronic treatment of male mice, but not female mice, with L-BOAA resulted in loss of complex I activity and vacuolation and dendritic swelling of neurons in the motor cortex and lumbar cord, paralleling the regionality of the aforementioned biochemical effects on CNS mitochondria. These results support the view that thiol oxidation and concomitant mitochondrial dysfunction (also implicated in other neurodegenerative disorders), occurring downstream of glutamate receptor activation by L-BOAA, are primary events leading to neurodegeneration. Maintenance of protein thiol homeostasis by thiol delivery agents could potentially offer protection against excitotoxic insults such as those seen with L-BOAA.

Alpha-ketoglutarate dehydrogenase in Alzheimer brains bearing the APP670/671 mutation

Alzheimer's disease (AD) is associated with a striking reduction in the activity of the alpha-ketoglutarate dehydrogenase complex (KGDHC). The deficiency occurs in brains from AD patients of undefined etiology, and in fibroblasts from both sporadic and familial AD cases. To further assess the nature of the abnormality of KGDHC in AD, KGDHC activities and immunoreactivities were analyzed in brains from AD patients bearing the Swedish APP670/671 mutation. This gene defect causes overproduction of the amyloid beta peptide. KGDHC activities were reduced by 55 to 57% compared with control values in the mutation-bearing AD cases in the medial temporal and superior frontal cortices. The immunochemical levels of KGDHC subunits Elk (-51%) and E2k (-76%) declined, whereas E3 concentrations were unchanged. The results suggest that mitochondrial dysfunction is a part of the pathophysiological process in AD even when the primary pathogenic cause is nonmitochondrial.

Protective effect of talipexole on MPTP-treated planarian, a unique parkinsonian worm model

The planarian, a flatworm, has a high potential for regeneration, and dopamine plays a key role in its behavior. Planarians treated with MPTP underwent autolysis and individual death in a concentration-dependent manner. When the planarian body was cut into anterior, middle and posterior pieces, each piece subsequently regenerated and reorganized to form a new individual within approximately 10 days. The anterior piece was significantly more sensitive than the middle and posterior pieces to MPTP cytotoxicity. Concomitant treatment with talipexole, an anti-parkinsonian drug, inhibited MPTP-induced autolysis and individual death in a concentration-dependent manner. Pramipexole showed a similar protective effect. In addition, post-treatment with talipexole at 1 hr after MPTP completely inhibited MPTP-induced individual death. Although MPTP treatment caused 30% of the planarians to undergo autolysis and individual death within 12 hr, post-treatment with talipexole even at 12 hr completely rescued the remaining 70% of the planarians from death. These results suggest that the MPTP-treated planarian may be useful as a novel parkinsonian model in which talipexole has a protective effect even in the case of post-treatment.

Transgenic mice expressing human Bcl-2 in their neurons are resistant to 6-hydroxydopamine and 1-methyl-4-phenyl-1,2,3,6- tetrahydropyridine neurotoxicity

The protooncogene bcl-2 inhibits neuronal apoptosis during normal brain development as well as that induced by cytotoxic drugs or growth factor deprivation. We have previously demonstrated that neurons of mice deficient in Bcl-2 are more susceptible to neurotoxins and that the dopamine (DA) level in the striatum after systemic 1-methyl-4-phenyl-1,2,3,6 tetrahydropyridine (MPTP) administration was significantly lower than in wild-type mice. In the present study we have used transgenic mice overexpressing human Bcl-2 under the control of neuron-specific enolase promoter (NSE-hbcl-2) to test the effects of the neurotoxins 6-hydroxydopamine (6-OHDA) and MPTP on neuronal survival in these mice. Primary cultures of neocortical neurons from normal and transgenic mice were exposed to these dopaminergic neurotoxins. Addition of 6-OHDA resulted in cell death of essentially all neurons from normal mice. In contrast, in cultures generated from heterozygous NSE-hbcl-2 transgenic mice, only 69% of the cells died while those generated from homozygous transgenic mice were highly resistant and exhibited only 34% cell death. A similar effect was observed with neurons treated with MPP+. Moreover, while the striatal dopamine level after MPTP injections was reduced by 32% in the wild type, the concentration remained unchanged in the NSE-hbcl-2 heterozygous mice. In contrast levels of glutathione-related enzymes were unchanged. In conclusion, overexpression of Bcl-2 in the neurons provided protection, in a dose-dependent manner, against neurotoxins known to selectively damage dopaminergic neurons. This study provides ideas for inhibition of neuronal cell death in neurodegenerative diseases and for the development of efficient neuroprotective gene therapy.

Dose-related inversion of cinnarizine and flunarizine effects on mitochondrial permeability transition

We investigated the effects of cinnarizine and flunarizine on mitochondrial permeability transition, ATP synthesis, membrane potential and NAD(P)H oxidation. Both drugs were effective in inhibiting the mitochondrial permeability transition induced either by Ca2+ alone or in the presence of tert-butylhydroperoxide. This protective effect occurred at low concentrations (< 50 microM) of these drugs and was accompanied by the inhibition of NAD(P)H oxidation and the restoration of the mitochondrial membrane potential decreased by a high concentration of Ca2+ (25 microM). However, at higher concentrations (> 50 microM) of cinnarizine and flunarizine and in the absence of both tert-butylhydroperoxide and Ca2+, their effects on the mitochondria were reversed as follows: mitochondrial permeability transition was generated, mitochondrial NAD(P)H was oxidized and membrane potential collapsed. These deleterious effects were not antagonized by cyclosporine A, the most potent inhibitor of the mitochondrial permeability transition, but by 2,6-di-tert-butyl-4-methylphenol, a known antioxidant agent. This mitochondrial effect was neither accompanied by an increase in malondialdehyde production nor by an increase in H2O2 generation, which attested that the effect of both drugs was not due to an increase in reactive oxygen species production. The dual effects of both cinnarizine and flunarizine on mitochondrial functions is discussed with regard to both the protective effect afforded by these drugs against ischemia-reperfusion injury and their side effect observed in some therapeutic situations where an overdosage seems likely.

Mutations in the parkin gene cause autosomal recessive juvenile parkinsonism

Parkinson's disease is a common neurodegenerative disease with complex clinical features. Autosomal recessive juvenile parkinsonism (AR-JP) maps to the long arm of chromosome 6 (6q25.2-q27) and is linked strongly to the markers D6S305 and D6S253; the former is deleted in one Japanese AR-JP patient. By positional cloning within this microdeletion, we have now isolated a complementary DNA done of 2,960 base pairs with a 1,395-base-pair open reading frame, encoding a protein of 465 amino acids with moderate similarity to ubiquitin at the amino terminus and a RING-finger motif at the carboxy terminus. The gene spans more than 500 kilobases and has 12 exons, five of which (exons 3-7) are deleted in the patient. Four other AR-JP patients from three unrelated families have a deletion affecting exon 4 alone. A 4.5-kilobase transcript that is expressed in many human tissues but is abundant in the brain, including the substantia nigra, is shorter in brain tissue from one of the groups of exon-4-deleted patients. Mutations in the newly identified gene appear to be responsible for the pathogenesis of AR-JP, and we have therefore named the protein product 'Parkin'.

Genotype in the 24-kDa subunit gene (NDUFV2) of mitochondrial complex I and susceptibility to Parkinson disease

We analyzed the gene encoding the 24-kDa subunit of mitochondrial complex I, which has been implicated in the pathogenesis of Parkinson disease (PD). We set out to identify a polymorphism in the 24-kDa subunit gene (NDUFV2) in patients with PD and determine whether genetic polymorphism of this gene is associated with a higher risk of PD. The subjects comprised 126 patients with PD, and the control group comprised 113 unrelated individuals without neurodegenerative disorders. A novel polymorphism (Ala29Val) in the mitochondrial targeting sequence of NDUFV2 was found in patients with PD. The distribution of the three genotypes was significantly different between the two groups (chi 2 = 7.53, df = 2, P = 0.023). The frequency of homozygotes for the mutation was significantly higher in PD patients (23.8%) than in control subjects (11.5%, Fisher's exact test, P = 0.0099 < 0.01). The risk of developing PD associated with homozygosity for this mutation was calculated as 2.40 (95% CI: 1.18-4.88). NDUFV2 constitutes one genetic risk factor for PD, and the mutation may well be a cause of complex I deficiency in this disease.

Role of mitochondrial Ca2+ regulation in neuronal and glial cell signalling

It is becoming increasingly clear that mitochondrial Ca2+ uptake from and release into the cytosol has important consequences for neuronal and glial activity. Ca2+ regulates mitochondrial metabolism, and mitochondrial Ca2+ uptake and release modulate physiological and pathophysiological cytosolic responses. In glial cells, inositol 1,4,5-trisphosphate-dependent Ca2+ responses are faithfully translated into elevations in mitochondrial Ca2+ levels, which modifies cytosolic Ca2+ wave propagation and may activate mitochondrial enzymes. The location of mitochondria within neurones may partially determine their role in Ca2+ signalling. Neuronal death due to NMDA-evoked Ca2+ entry can be delayed by an inhibitor of the mitochondrial permeability transition pore, and mitochondrial dysfunction is being increasingly implicated in a number of neurodegenerative conditions. These findings are illustrative of an emerging realization by neuroscientists of the importance of mitochondrial Ca2+ regulation as a modulator of cellular energetics, endoplasmic reticulum Ca2+ release and neurotoxicity.

Plasma thiamine deficiency associated with Alzheimer's disease but not Parkinson's disease

In this study we compared plasma and erythrocyte thiamine levels in a group of patients with idiopathic Parkinson's Disease (iPD) to a group of patients with probable Alzheimer's Disease (pAD). pAD patients had significantly lower plasma thiamine levels (raw and z-score) than iPD patients. A significantly higher number of pAD patients had plasma thiamine deficiencies than iPD patients. The demographics of our patient groups were similar to those reported by other investigators, making age, sex and nutritional status unlikely explanations for our findings. These results suggest that plasma thiamine deficiency is associated with pAD but not with iPD.

Paraquat-induced free radical reaction in mouse brain microsomes

Paraquat has been implicated as an environmental toxin which may induce the syndrome of Parkinson's disease after exposure to this agent. However, the biochemical mechanism by which paraquat causes cell death and neurodegeneration has not been extensively studied. Paraquat was rapidly taken up by nerve terminals isolated from mouse cerebral cortices. It induced lipid peroxidation in a concentration dependent manner in the presence of NADPH and ferrous ion. The maximal stimulation effect was obtained at a paraquat concentration around 100 microM and the Km value for paraquat was 46.7 microM. The lipid peroxidation required microsomal enzymes. Antioxidants, such as superoxide dismutase, catalase and promethazine significantly inhibited paraquat-induced lipid peroxidation. Due to its structural similarity to the pyridinium compound MPP+ (N-methyl-4-phenyl pyridium ion), it may be taken up by dopamine neurons and cause lipid peroxidation and cell death resulting in the manifestation of Parkinsonian syndrome.

Inhibition of α-ketoglutarate-and pyruvate dehydrogenase complexes in E. coli by a glutathione S-transferase containing a pathological length poly-Q domain: A possible role of energy deficit in neurological diseases associated with poly-Q expansions?

At least seven adult-onset neurodegenerative diseases, including Huntington's disease (HD), are caused by genes containing expanded CAG triplets within their coding regions. The expanded CAG repeats give rise to extended stretches of polyglutamines (Qn) in the proteins expressed by the affected genes. Generally, n ≥40 in affected individuals and ≤36 in clinically unaffected individuals. The expansion has been proposed to confer a "toxic gain of function" to the mutated protein. Poly-Q domains have recently been shown to be excellent substrates of tissue transglutaminase. We investigated the effects of expression of glutathione S-transferase constructs containing poly-Q inserts of various lengths (GSTQn where n = 0, 10, 62 or 81) on the activity of some key metabolic enzymes in the host Escherischia coil-an organism not known to have transglutaminase activity. E. coil carrying the GSTQ62 construct exhibited statistically significant decreases in the specific activities of α-ketoglutarate dehydrogenase complex (KGDHC) and pyruvate dehydrogenase complex (PDHC). Previous work has shown that KGDHC and PDHC activities are reduced in the brains of Alzheimer's disease (AD) patients. Our results suggest that KGDHC and PDHC may be particularly susceptible to the effects of a number of disparate insults, including those associated with AD and HD.

Mitochondrial toxins in models of neurodegenerative diseases. I: In vivo brain hydroxyl radical production during systemic MPTP treatment or following microdialysis infusion of methylpyridinium or azide ions

Mitochondrial electron transport chain (ETC) function is selectively reduced in multiple tissues, including brain, from patients with Parkinson's disease (PD) and Alzheimer's disease (AD). The ETC defects are specific to each illness, involve complex I in PD and complex IV in AD, are transferable with mitochondrial DNA (mtDNA) and lead to increased production of reactive oxygen species (ROS) in mtDNA-deficient clonal neuronal cells hybridized with mtDNA ('cybrids') from PD or AD patients. C57BL/6 mice treated with MPTP developed elevated tissue hydroxyl radical ('OH) levels in striatum and ventral midbrain but not cerebellum. In brain microdialysis in awake rats, striatal 'OH output increased 3-5-fold after infusion of methylpyridinium ion (MPP+), a complex I inhibitor, or sodium azide, a complex IV inhibitor. Elevated 'OH after MPP+ was blocked stereospecifically by infusion of the nitric oxide synthase (NOS) inhibitor nitro-L-arginine or by the NMDA channel blocker MK801. Neither NOS inhibition nor NMDA blockade altered azide-induced 'OH production. ETC inhibition in vivo increases production of toxic 'OH, but the underlying mechanisms vary as a function of which ETC complex is inhibited. These results support the concept of developing oxygen free radical scavengers for both AD and PD and further suggest that inhibition of NOS and blockade of NMDA receptor function may alter progression of idiopathic PD.

Apoptosis, neurotrophic factors and neurodegeneration

Apoptosis is an active process of cell death characterized by distinct morphological features, and is often the end result of a genetic programme of events, i.e. programmed cell death (PCD). There is growing evidence supporting a role for apoptosis in some neurodegenerative diseases. This conclusion is based on DNA fragmentation studies and findings of increased levels of pro-apoptotic genes in human brain and in in vivo and in vitro model systems. Additionally, there is some evidence for a loss of neurotrophin support in neurodegenerative diseases. In Alzheimer's disease, in particular, there is strong evidence from human brain studies, transgenic models and in vitro models to suggest that the mode of nerve cell death is apoptotic. In this review we describe the evidence implicating apoptosis in neurodegenerative diseases with a particular emphasis on Alzheimer's disease.

Fragile mitochondrial DNA: the missing link in the apoptotic neuronal cell death in Parkinson's disease

The oxidative stress theory, the mitochondrial (mt) hypothesis, and the apoptosis hypothesis are proposed as the cause of neuronal cell death in Parkinson's disease (PD). However, the direct link between them has remained unknown. Recently, the mt control of nuclear apoptosis is documented that collapse of mt transmembrane potential due to energy crisis leads to release of apoptotic protease activating-factors into cytosol and subsequently nuclear DNA fragmentation. However, an endogenous factor responsible for the energy crisis under physiological conditions is missing. Here we report the missing factor as that mtDNA in the striatum of a parkinsonian patient fragments into 134 types of deleted pieces, being detected by the total detection system for mtDNA deletion. The system has documented that the mtDNA is extremely susceptible to hydroxyl radical damage, hence to oxidative stress, enough to cause the cellular energy crisis. The extensive fragility of mtDNA in brain stem could link the oxidative stress up with the apoptotic neuronal cell-death of PD.

Parkinson disease: a new link between monoamine oxidase and mitochondrial electron flow

Two factors that contribute to the progression of Parkinson disease are a brain defect in mitochondrial respiration and the generation of hydrogen peroxide (H2O2) by monoamine oxidase (MAO). Here we show that the two are linked. Metabolism of the neurotransmitter dopamine, or other monoamines (benzylamine, tyramine), by intact rat brain mitochondria suppresses pyruvate- and succinate-dependent electron transport. MAO inhibitors prevent this action. Mitochondrial damage is also reversed during electron flow. A probable explanation is that MAO-generated H2O2 oxidizes glutathione to glutathione disulfide (GSSG), which undergoes thiol-disulfide interchange to form protein mixed disulfides, thereby interfering reversibly with thiol-dependent enzymatic function. In agreement with this premise, direct addition of GSSG to mitochondria resulted in similar reversible inhibition of electron transport. In addition, the monoamines induced an elevation in protein mixed disulfides within mitochondria. These observations imply that (i) heightened activity and metabolism of neurotransmitter by monoamine neurons may affect neuronal function, and (ii) apparent defects in mitochondrial respiration associated with Parkinson disease may reflect, in part, an established increase in dopamine turnover. The experimental results also target mitochondrial repair mechanisms for further investigation and may, in time, lead to newer forms of therapy.

Apoptosis in neurodegenerative disorders

Although the exact mechanism of nigral cell death in Parkinson's disease (PD) is not known, increasing evidence suggests the presence of apoptotic cell death in PD. When we applied the TUNEL method to detect DNA fragmentation, four out of seven late onset sporadic patients with PD showed TUNEL-positive neurons. The percentages of those neurons among the remaining melanin containing neurons were 0.6 to 4.8% (average 2.1%). But TUNEL-positive neurons could not be detected in control subjects as well as four patients with young onset (under 40 years of the age) PD. Numbers of nigral toxins such as MPTP, complex I inhibitors, and mitochondrial respiratory inhibitors have been reported to induced apoptotic cell death. These findings suggest that apoptosis is involved in nigral cell cleath in PD at least in part and warrant further studies on apoptosis-related substances in PD.

Mesencephalic neuron death induced by congeners of nitrogen monoxide is prevented by the lazaroid U-83836E

We explored the effects of congeners of nitrogen monoxide (NO) on cultured mesencephalic neurons. Sodium nitroprusside (SNP) was used as a donor of NO, the congeners of which have been found to exert either neurotoxic or neuroprotective effects depending on the surrounding redox milieu. In contrast to a previous report that suggests that the nitrosonium ion (NO+) is neuroprotective to cultured cortical neurons, we found that the nitrosonium ion reduces the survival of cultured dopamine neurons to 32% of control. There was a trend for further impairment of dopamine neuron survival, to only 7% of untreated control, when the cultures were treated with SNP plus ascorbate, i.e. when the nitric oxide radical (NO.) had presumably been formed. We also evaluated the effects of an inhibitor of lipid peroxidation, the lazaroid U-83836E, against SNP toxicity. U-83836E exerted marked neuroprotective effects in both insult models. More than twice as many dopamine neurons (75% of control) survived when the lazaroid was added to SNP-treated cultures and the survival was increased eight-fold (to 55% of control) when U-83836E was added to cultures treated with SNP plus ascorbate. We conclude that the congeners of NO released by SNP are toxic to mesencephalic neurons in vitro and that the lazaroid U-83836E significantly increases the survival of dopamine neurons in situations where congeners of NO are generated.

Decreased cytochrome c oxidase activity but unchanged superoxide dismutase and glutathione peroxidase activities in the spinal cords of patients with amyotrophic lateral sclerosis

The cause of selective degeneration of motor neurons in the ventral horn of the spinal cord associated with amyotrophic lateral sclerosis (ALS) has still not been elucidated. Recently, so-called oxidative stress has been suggested to be a significant factor in the pathogenesis of this disease. We measured the antioxidant actions of superoxide dismutase (SOD), glutathione peroxidase (GSH-Px), and cytochrome c oxidase (CO) of the human spinal cord in patients with ALS in comparison with those in control patients. Total SOD activity in spinal cord transections from patients with sporadic ALS was not significantly different from the controls in ventral, lateral, or dorsal regions, although enzymic activity was relatively higher in the ventral compared with the dorsal region. GSH-Px activity in the spinal cord of ALS patients was not very different from that in the control tissue. In contrast, CO activity was significantly reduced in all three regions of the spinal cord in patients with ALS, although the reduction was more marked in the ventral region. These results suggest that reactive oxygen species may attack the mitochondrial respiratory chain, leading eventually to the degeneration of vulnerable motor neurons in the spinal cord, even though no obvious changes in the activity of antioxidant enzymes are detectable.