Is complex II involved in the inhibition of mitochondrial respiration by N-methyl-4-phenylpyridinium cation (MMP+) and N-methyl-beta-carbolines?

Cytotoxicity of β-carbolines in dopamine transporter expressing cells: Structure–activity relationships

Some beta-carbolines (BC) are natural constituents in the human brain deriving from tryptophan, tryptamine, and serotonin. In vitro and animal experiments suggest that BC-cations may cause neurodegeneration with a higher vulnerability of dopaminergic than of other neurons. Despite the possible implication of the BC-cations in the pathogenesis of Parkinson's disease (PD), the underlying mechanisms are poorly understood. The present study further explores the structural requirements for the cytotoxic effects of BCs and searches for additional compounds involved in the pathogenesis of PD. Previous studies were now extended to serotonin-derived BCs, tetrahydro-BCs, a BC-dimer, and a BC-enantiomer to reveal possible stereoselectivity. Neutral, rather lipophilic BCs may pass the plasma membrane and the outer and inner mitochondrial membranes by diffusion whereas the cationic, more polar compounds, can be transported by the dopamine transporter (DAT). In the present study, 4 out of 17 BC-cations caused DAT-independent toxicity. This number is unexpected in view of previous findings that all BC-cations are transported by DAT. 3-Carboxylated and 6-methoxylated BCs were poor substrates. The size alone does not seem to be a limiting factor. A dimeric BC-cation was readily transported by the DAT despite its much larger structure compared to dopamine. Furthermore, (R)-enantiomers were preferentially transported. The neutral BCs were approximately one order of magnitude less toxic than the cationic BCs. There are considerable differences of the transport efficiency between the BCs. Potent cytotoxic tetrahydro-BCs were detected. Because precursor tetrahydro-BCs are present in the brain, the search for the occurrence of these compounds in human brain is warranted.

Dopamine transporter-mediated cytotoxicity of beta-carbolinium derivatives related to Parkinson's disease: relationship to transporter-dependent uptake

Endogenous or exogenous beta-carboline (betaC) derivatives structurally related to the selective dopaminergic neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and its active metabolite 1-methyl-4-phenylpyridinium (MPP(+)) may contribute to dopaminergic neurodegeneration in Parkinson's disease (PD). We addressed the importance of the dopamine transporter (DAT) for selective dopaminergic toxicity by testing the differential cytotoxicity and cellular uptake of 12 betaCs in human embryonic kidney HEK-293 cells ectopically expressing the DAT gene. Cell death was measured using [4,5-Dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide (MTT) and trypan blue exclusion assays, and uptake by a fluorescence-based uptake assay. All betaCs and MPP(+) showed general cytotoxicity in parental HEK-293 cells after 72 h with half-maximal toxic concentrations (TC(50) values) in the upper micromolar range. Besides MPP(+), only 2[N]-methylated compounds showed enhanced cytotoxicity in DAT expressing HEK-293 cells with 1.3- to 4.5-fold reduction of TC(50) values compared with parental cell line. The rank order of selectivity was: MPP(+) >> 2[N],9[N]-dimethyl-harminium > 2[N]-methyl-harminium > 2[N],9[N]-dimethyl-harmanium = 2[N]-methyl-norharmanium > 2[N]-methyl-harmanium > 2[N],9[N]-dimethyl-norharminium. Consistently, only 2[N]-methylated betaCs were transported into the cell through the DAT with up to five times greater K(m) and 12-220 times smaller V(max) values compared with dopamine and MPP(+). There was a weak relation of DAT-mediated selectivity with the affinity of betaCs at the DAT (K(m)), but not with V(max). Our data suggest that DAT-mediated cellular uptake of 2[N]-methylated betaCs represents a potential mechanism for selective toxicity towards dopaminergic neurons and may be relevant for the pathogenesis of Parkinson's disease.

Effects of enhancing mitochondrial oxidative phosphorylation with reducing equivalents and ubiquinone on 1-methyl-4-phenylpyridinium toxicity and complex I–IV damage in neuroblastoma cells

The effects of increasing mitochondrial oxidative phosphorylation (OXPHOS), by enhancing electron transport chain components, were evaluated on 1-methyl-4-phenylpyridinium (MPP+) toxicity in brain neuroblastoma cells. Although glucose is a direct energy source, ultimately nicotinamide and flavin reducing equivalents fuel ATP produced through OXPHOS. The findings indicate that cell respiration/mitochondrial O(2) consumption (MOC) (in cells not treated with MPP+) is not controlled by the supply of glucose, coenzyme Q(10) (Co-Q(10)), NADH+, NAD or nicotinic acid. In contrast, MOC in whole cells is highly regulated by the supply of flavins: riboflavin, flavin adenine dinucleotide (FAD) and flavin mononucleotide (FMN), where cell respiration reached up to 410% of controls. In isolated mitochondria, FAD and FMN drastically increased complex I rate of reaction (1300%) and (450%), respectively, having no effects on complex II or III. MPP+ reduced MOC in whole cells in a dose-dependent manner. In isolated mitochondria, MPP+ exerted mild inhibition at complex I, negligible effects on complexes II-III, and extensive inhibition of complex IV. Kinetic analysis of complex I revealed that MPP+ was competitive with NADH, and partially reversible by FAD and FMN. Co-Q(10) potentiated complex II ( approximately 200%), but not complex I or III. Despite positive influence of flavins and Co-Q(10) on complexes I-II function, neither protected against MPP+ toxicity, indicating inhibition of complex IV as the predominant target. The nicotinamides and glucose prevented MPP+ toxicity by fueling anaerobic glycolysis, evident by accumulation of lactate in the absence of MOC. The data also define a clear anomaly of neuroblastoma, indicating a preference for anaerobic conditions, and an adverse response to aerobic. An increase in CO(2), CO(2)/O(2) ratio, mitochondrial inhibition or O(2) deprivation was not directly toxic, but activated metabolism through glycolysis prompting depletion of glucose and starvation. In conclusion, the results of this study indicate that the mechanism of action for MPP+, involves the inhibition of complex I and and more specifically complex IV, leading to impaired OXPHOS and MOC. Moreover, flavin dervatives control the rate of complex I/cellular respiration and Co-Q10 augments complex II [corrected].

Alkaloids, alcohol and Parkinson's disease

Relatively early seminal investigations on 'mammalian alkaloid biosynthesis'-endogenous Pictet-Spengler condensations of catecholamines or indoleamines with aldehydes (such as acetaldehyde from ethanol metabolism) to form tetrahydroisoquinoline or beta-carboline alkaloids-and the roles of mammalian alkaloids in the CNS complications of chronic alcoholism were launched in Gerald Cohen's laboratory. While occasional studies on alcohol and the alkaloids continue today, the field of study has been expanded principally by others into Parkinson's disease. Certain mammalian or xenobiotic alkaloids have been examined by various laboratories as possible neurotoxic factors inducing mitochondrial energy depletion and/or oxidative stress in the nigrostriatum. In that regard, specific arguments for N-methylated 'MPP(+)-like' cationic alkaloids that can be generated centrally from beta-carbolines derived from the environment and diet are summarized.

MPP(+)-induced mitochondrial dysfunction is potentiated by dopamine

MPP(+), the major metabolite of the Parkinsonism-inducing compound MPTP, responsible for the destruction of the nigrostriatal pathway in primates and rodents, has been assayed in isolated rat liver mitochondria in the presence of physiological concentrations of dopamine or analogous concentrations of melanin-dopamine. 5 microM MPP(+) in the presence of 70 microM dopamine or melanin-dopamine, but not alone, decreased the heat production and oxygen consumption of a mitochondrial suspension activated with succinate and ADP. Both dopamine and oxidized dopamine plus MPP(+) also decreased the mitochondrial reductive power measured with MTT. Mitochondrial swelling was observed, associated with an increase in membrane mitochondrial potential, as a synergistic effect between low concentrations of MPP(+) and dopamine. It is suggested that cytosolic dopamine, by itself or via its autooxidation products, may play a relevant role in the mitochondrial toxicity of MPP(+). A failure in the regulation of the storage/release of dopamine could aggravate a mitochondrial damage and trigger the neurodegenerative process underlying MPTP toxicity and Parkinson's disease.

The role of iron in neurodegeneration: prospects for pharmacotherapy of Parkinson's disease

Although the aetiology of Parkinson's disease (PD) and related neurodegenerative disorders is still unknown, recent evidence from human and experimental animal models suggests that a misregulation of iron metabolism, iron-induced oxidative stress and free radical formation are major pathogenic factors. These factors trigger a cascade of deleterious events leading to neuronal death and the ensuing biochemical disturbances of clinical relevance. A review of the available data in PD provides the following evidence in support of this hypothesis: (i) an increase of iron in the brain, which in PD selectively involves neuromelanin in substantia nigra (SN) neurons; (ii) decreased availability of glutathione (GSH) and other antioxidant substances; (iii) increase of lipid peroxidation products and reactive oxygen (O2)species (ROS); and (iv) impaired mitochondrial electron transport mechanisms. Most of these changes appear to be closely related to interactions between iron and neuromelanin, which result in accumulation of iron and a continuous production of cytotoxic species leading to neuronal death. Some of these findings have been reproduced in animal models using 6-hydroxydopamine, N-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), iron loading and beta-carbolines, although none of them is an accurate model for PD in humans. Although it is not clear whether iron accumulation and oxidative stress are the initial events causing cell death or consequences of the disease process, therapeutic efforts aimed at preventing or at least delaying disease progression by reducing the overload of iron and generation of ROS may be beneficial in PD and related neurodegenerative disorders. Current pharmacotherapy of PD, in addition to symptomatic levodopa treatment, includes 'neuroprotective' strategies with dopamine agonists, monoamine oxidase-B inhibitors (MAO-B), glutamate antagonists, catechol O-methyltransferase inhibitors and other antioxidants or free radical scavengers. In the future, these agents could be used in combination with, or partly replaced by, iron chelators and lazaroids that prevent iron-induced generation of deleterious substances. Although experimental and preclinical data suggest the therapeutic potential of these drugs, their clinical applicability will be a major challenge for future research.

Mitochondrial dysfunction in neurodegeneration

Numerous toxins are known to interfere with mitochondrial respiratory chain functions. Use has been made of these in the development of pesticides and herbicides, and accidental use in man has led to the development of animal models for human disease. The propensity for mitochondrial toxins to induce neuronal cell death may well reflect not only their metabolic pathways but also the sensitivity of neurons to inhibition of oxidative phosphorylation. Thus, the accidental exposure of humans to 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine and to 3-nitropropionic acid had led to primate models of Parkinson's disease and Huntington's Disease, respectively. These models were made all the more remarkable when identical biochemical deficiencies were identified in relevant areas of human suffering from the respective idiopathic diseases. The place of complex I deficiency in Parkinson's disease remains undetermined, but there is recent evidence to suggest that, in some cases at least, it may play a primary role. The complex II/III deficiency in Huntington's disease is likely to be secondary and induced by other pathogenetic factors. The potential to intervene in the cascade of reactions involving mitochondrial dysfunction and cell death offers prospects for the development of new treatment strategies either for neuroprotection in prophylaxis or rescue.

Animal models of Parkinson's disease: an empirical comparison with the phenomenology of the disease in man

Animal models are an important aid in experimental medical science because they enable one to study the pathogenetic mechanisms and the therapeutic principles of treating the functional disturbances (symptoms) of human diseases. Once the causative mechanism is understood, animal models are also helpful in the development of therapeutic approaches exploiting this understanding. On the basis of experimental and clinical findings. Parkinson's disease (PD) became the first neurological disease to be treated palliatively by neurotransmitter replacement therapy. The pathological hallmark of PD is a specific degeneration of nigral and other pigmented brainstem nuclei, with a characteristic inclusion, the Lewy body, in remaining nerve cells. There is now a lot of evidence that degeneration of the dopaminergic nigral neurones and the resulting striatal dopamine-deficiency syndrome are responsible for its classic motor symptoms akinesia and bradykinesia. PD is one of many human diseases which do not appear to have spontaneously arisen in animals. The characteristic features of the disease can however be more or less faithfully imitated in animals through the administration of various neurotoxic agents and drugs disturbing the dopaminergic neurotransmission. The cause of chronic nigral cell death in PD and the underlying mechanisms remain elusive. The partial elucidation of the processes underlie the selective action of neurotoxic substances such as 6-hydroxydopamine (6-OHDA) or 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), has however revealed possible molecular mechanisms that give rise to neuronal death. Accordingly, hypotheses concerning the mechanisms of these neurotoxines have been related to the pathogenesis of nigral cell death in PD. The present contribution starts out by describing some of the clinical, pathological and neurochemical phenomena of PD. The currently most important animal models (e.g. the reserpine model, neuroleptic-induced catalepsy, tremor models, experimentally-induced degeneration of nigrostriatal dopaminergic neurons with 6-OHDA, methamphetamine, MPTP, MPP+, tetrahydroisoquinolines, beta-carbolines, and iron) critically reviewed next, and are compared with the characteristic features of the disease in man.

Differential cytotoxicities of N-methyl-beta-carbolinium analogues of MPP+ in PC12 cells: insights into potential neurotoxicants in Parkinson's disease

N-Methylated beta-carbolinium cations that can form in vivo from environmental or endogenous beta-carbolines are putative neurotoxic factors in Parkinson's disease. The cytotoxicities of 11 N-methylated beta-carbolinium cations and N-methyl-4-phenylpyridinium cation (MPP+), the experimental parkinsonian neurotoxicant which the carbolinium cations structurally resemble, were examined using rat pheochromocytoma (PC12) cells cultured in "low energy" N-5 medium; cell death was estimated by released lactate dehydrogenase activity and viable cell protein. Of the eight N2-monomethylated beta-carbolinium cations utilized, only 2-methyl-harmalinium (harmaline-2-methiodide) was as cytotoxic as MPP+. Also, three N2(beta), N9(indole)-dimethylated beta-carbolinium cations displayed cytotoxic effects, with the simplest, 2,9-dimethylnorharmanium, approaching the effectiveness of MPP+ in PC12 cells cultured in N-5 medium. However, when PC12 cells grown in higher energy Dulbecco's modified Eagle's medium were utilized with selected effective cations, it was observed that the cultures were relatively resistant to MPP+ and 2,9-dimethylnorharmanium, but remained vulnerable to 2-methylharmalinium. The results are interpreted to mean that different cytotoxic mechanisms exist for the two most potent beta-carbolinium cations--namely, a mechanism for the 2,9-dimethyl-beta-carbolinium species that, as with MPP+, is conditional on mitochondrial ATP depletion, but a different (or additional) mechanism for 2-methylharmalinium that is independent of mitochondrial inhibition. The possible accumulation of these cytotoxic cations in Parkinson's disease is discussed in the context of these findings.