Loss of mitochondrial complex I activity potentiates dopamine neuron death induced by microtubule dysfunction in a Parkinson's disease model

Mitochondrial complex I inhibitors and forced oxidative phosphorylation synergize in inducing cancer cell death

Cancer cells generally rely mostly on glycolysis rather than oxidative phosphorylation (OXPHOS) for ATP production. In fact, they are particularly sensitive to glycolysis inhibition and glucose depletion. On the other hand mitochondrial dysfunctions, involved in the onset of the Warburg effect, are sometimes also associated with the resistance to apoptosis that characterizes cancer cells. Therefore, combined treatments targeting both glycolysis and mitochondria function, exploiting peculiar tumor features, might be lethal for cancer cells. In this study, we show that glucose deprivation and mitochondrial Complex I inhibitors synergize in inducing cancer cell death. In particular, our results reveal that low doses of Complex I inhibitors, ineffective on immortalized cells and in high glucose growth, become specifically cytotoxic on cancer cells deprived of glucose. Importantly, the cytotoxic effect of the inhibitors on cancer cells is strongly enhanced by forskolin, a PKA pathway activator, that we have previously shown to stimulate OXPHOS. Taken together, we demonstrate that induction in cancer cells of a switch from a glycolytic to a more respirative metabolism, obtained by glucose depletion or mitochondrial activity stimulation, strongly increases their sensitivity to low doses of mitochondrial Complex I inhibitors. Our findings might be a valuable approach to eradicate cancer cells.

[Neurotoxicity of pesticides: its relationship with neurodegenerative diseases]

Several epidemiological studies suggest that pesticides could lead to neurodegenerative diseases such as Parkinson's and Alzheimer's diseases. Among pesticides, insecticides appear more neurotoxic than others but the neurotoxic mechanisms leading to adverse health effects remain unclear. The currently used pesticides such as rotenone and paraquat could disrupt mitochondrial bioenergetic function, reactive oxygen metabolism, redox function and promote α-synuclein aggregation. In addition, recent studies demonstrate that genetic susceptibility to Parkinson's disease could monitor pesticide susceptibility, as demonstrated for polymorphisms in pesticide metabolizing enzymes that are involved in organophosphorus sensitivity.

Cellular and animal models for mitochondrial complex I deficiency: a focus on the NDUFS4 subunit

To allow the rational design of effective treatment strategies for human mitochondrial disorders, a proper understanding of their biochemical and pathophysiological aspects is required. The development and evaluation of these strategies require suitable model systems. In humans, inherited complex I (CI) deficiency is one of the most common deficiencies of the mitochondrial oxidative phosphorylation system. During the last decade, various cellular and animal models of CI deficiency have been presented involving mutations and/or deletion of the Ndufs4 gene, which encodes the NDUFS4 subunit of CI. In this review, we discuss these models and their validity for studying human CI deficiency.

Complex I deficiencies in neurological disorders

Complex I is the point of entry in the mitochondrial electron transport chain for NADH reducing equivalents, and it behaves as a regulatable pacemaker of respiratory ATP production in human cells. Defects in complex I are associated with several human neurological disorders, including primary mitochondrial diseases, Parkinson disease (PD), and Down syndrome, and understanding the activity and regulation of complex I may reveal aspects of the underlying pathogenic mechanisms. Complex I is regulated by cyclic AMP (cAMP) and the protein kinase A (PKA) signal transduction pathway, and elucidating the role of the cAMP/PKA system in regulating complex I and oxygen free radical production provides new perspectives for devising therapeutic strategies for neurological diseases.

The mechanism of alternative splicing of the X-linked NDUFB11 gene of the respiratory chain complex I, impact of rotenone treatment in neuroblastoma cells

A study is presented on the regulation of alternative splicing (AS) of the Ndufb11 gene of complex I of the mitochondrial respiratory chain and the impact on this process of rotenone treatment in neuroblastoma cells. In physiological conditions the Ndufb11 gene produces at high level a short transcript isoform encoding for a 153 aa protein. This subunit is essential for the assembly of a functional and stable mammalian complex I. The gene produces also, at low level, a longer transcript isoform encoding for a 163 aa protein whose role is unknown. Evidence is presented here showing that the level of the two isoforms is regulated by three DGGGD ESS elements located in exon 2 which can bind the hnRNPH1 protein. In neuronal cells rotenone treatment affects the Ndufb11 alternative splicing pathway, with the increase of the 163/153 mRNAs ratio. This effect appears to be due to the down-regulation of the hnRNPH1 protein. Since rotenone induces apoptosis in neuronal cells, the post-transcriptional regulation of the Ndufb11 gene can be involved in the programmed cell death process.

AIF, reactive oxygen species, and neurodegeneration: a "complex" problem

Apoptosis-inducing factor (AIF) is a flavin-binding mitochondrial intermembrane space protein that is implicated in diverse but intertwined processes that include maintenance of electron transport chain function, reactive oxygen species regulation, cell death, and neurodegeneration. In acute brain injury, AIF acquires a pro-death role upon translocation from the mitochondria to the nucleus, where it initiates chromatin condensation and large-scale DNA fragmentation. Although harlequin mice exhibiting an 80-90% global reduction in AIF protein are resistant to numerous forms of acute brain injury, they paradoxically undergo slow, progressive neurodegeneration beginning at three months of age. Brain deterioration, accompanied by markers of oxidative stress, is most pronounced in the cerebellum and retina, although it also occurs in the cortex, striatum, and thalamus. Loss of an AIF pro-survival function linked to assembly or stabilization of electron transport chain complex I underlies chronic neurodegeneration. To date, most studies of neurodegeneration have failed to adequately separate the relative importance of the mitochondrial and nuclear functions of AIF in determining the extent of injury, or whether oxidative stress plays a causative role. This review explores the complicated relationship among AIF, complex I, and the regulation of mitochondrial reactive oxygen species levels. It also discusses the controversial role of complex I deficiency in Parkinson's disease, and what can be learned from the AIF- and complex I-depleted harlequin mouse.

Primary fibroblasts of NDUFS4(-/-) mice display increased ROS levels and aberrant mitochondrial morphology

The human NDUFS4 gene encodes an accessory subunit of the first mitochondrial oxidative phosphorylation complex (CI) and, when mutated, is associated with progressive neurological disorders. Here we analyzed primary muscle and skin fibroblasts from NDUFS4(-/-) mice with respect to reactive oxygen species (ROS) levels and mitochondrial morphology. NDUFS4(-/-) fibroblasts displayed an inactive CI subcomplex on native gels but proliferated normally and showed no obvious signs of apoptosis. Oxidation of the ROS sensor hydroethidium was increased and mitochondria were less branched and/or shorter in NDUFS4(-/-) fibroblasts. We discuss the relevance of these findings with respect to previous results and therapy development.

Environmental toxins trigger PD-like progression via increased alpha-synuclein release from enteric neurons in mice

Pathological studies on Parkinson's disease (PD) patients suggest that PD pathology progresses from the enteric nervous system (ENS) and the olfactory bulb into the central nervous system. We have previously shown that environmental toxins acting locally on the ENS mimic this PD-like pathology progression pattern in mice. Here, we show for the first time that the resection of the autonomic nerves stops this progression. Moreover, our results show that an environmental toxin (i.e. rotenone) promotes the release of alpha-synuclein by enteric neurons and that released enteric alpha-synuclein is up-taken by presynaptic sympathetic neurites and retrogradely transported to the soma, where it accumulates. These results strongly suggest that pesticides can initiate the progression of PD pathology and that this progression is based on the transneuronal and retrograde axonal transport of alpha-synuclein. If confirmed in patients, this study would have crucial implications in the strategies used to prevent and treat PD.

Transcriptome analysis of a rotenone model of parkinsonism reveals complex I-tied and -untied toxicity mechanisms common to neurodegenerative diseases

The pesticide rotenone, a neurotoxin that inhibits the mitochondrial complex I, and destabilizes microtubules (MT) has been linked to Parkinson disease (PD) etiology and is often used to model this neurodegenerative disease (ND). Many of the mechanisms of action of rotenone are posited mechanisms of neurodegeneration; however, they are not fully understood. Therefore, the study of rotenone-affected functional pathways is pertinent to the understanding of NDs pathogenesis. This report describes the transcriptome analysis of a neuroblastoma (NB) cell line chronically exposed to marginally toxic and moderately toxic doses of rotenone. The results revealed a complex pleiotropic response to rotenone that impacts a variety of cellular events, including cell cycle, DNA damage response, proliferation, differentiation, senescence and cell death, which could lead to survival or neurodegeneration depending on the dose and time of exposure and cell phenotype. The response encompasses an array of physiological pathways, modulated by transcriptional and epigenetic regulatory networks, likely activated by homeostatic alterations. Pathways that incorporate the contribution of MT destabilization to rotenone toxicity are suggested to explain complex I-independent rotenone-induced alterations of metabolism and redox homeostasis. The postulated mechanisms involve the blockage of mitochondrial voltage-dependent anions channels (VDACs) by tubulin, which coupled with other rotenone-induced organelle dysfunctions may underlie many presumed neurodegeneration mechanisms associated with pathophysiological aspects of various NDs including PD, AD and their variant forms. Thus, further investigation of such pathways may help identify novel therapeutic paths for these NDs.

Biosignatures for Parkinson's disease and atypical parkinsonian disorders patients

Diagnosis of Parkinson' disease (PD) carries a high misdiagnosis rate due to failure to recognize atypical parkinsonian disorders (APD). Usually by the time of diagnosis greater than 60% of the neurons in the substantia nigra are dead. Therefore, early detection would be beneficial so that therapeutic intervention may be initiated early in the disease process. We used splice variant-specific microarrays to identify mRNAs whose expression is altered in peripheral blood of early-stage PD patients compared to healthy and neurodegenerative disease controls. Quantitative polymerase chain reaction assays were used to validate splice variant transcripts in independent sample sets. Here we report a PD signature used to classify blinded samples with 90% sensitivity and 94% specificity and an APD signature that resulted in a diagnosis with 95% sensitivity and 94% specificity. This study provides the first discriminant functions with coherent diagnostic signatures for PD and APD. Analysis of the PD biomarkers identified a regulatory network with nodes centered on the transcription factors HNF4A and TNF, which have been implicated in insulin regulation.

Mitochondrial dysfunction and oxidative stress promote apoptotic cell death in the striatum via cytochrome c/caspase-3 signaling cascade following chronic rotenone intoxication in rats

Parkinson's disease (PD) is a progressive neurological disorder marked by nigrostriatal dopaminergic degeneration. Evidence suggests that mitochondrial dysfunction may be linked to PD through a variety of different pathways, including free-radical generation and dysfunction of the mitochondrial Complex I activity. In Lewis rats, chronic systemic administration of a specific mitochondrial Complex I inhibitor, rotenone (3 mg/kg/day) produced parkinsonism-like symptoms. Increased oxidized proteins and peroxynitrite, and mitochondrial or cytosol translocation of Bim, Bax or cytochrome c in the striatum was observed after 2-4 weeks of rotenone infusion. After 28 days of systemic rotenone exposure, imunohistochemical staining for tyrosine hydroxylase indicated nigrostriatal dopaminergic neuronal cell degeneration. Characteristic histochemical (TUNEL or activated caspase-3 staining) or ultrastructural (electron microscopy) features of apoptotic cell death were present in the striatal neuronal cell after chronic rotenone intoxication. We conclude that chronic rotenone intoxication may enhance oxidative and nitrosative stress that induces mitochondrial dysfunction and ultrastructural damage, resulting in translocation of Bim and Bax from cytosol to mitochondria that contributes to apoptotic cell death in the striatum via cytochrome c/caspase-3 signaling cascade.

Mitochondrial dysfunction in Parkinson's disease: molecular mechanisms and pathophysiological consequences

Neurons are critically dependent on mitochondrial integrity based on specific morphological, biochemical, and physiological features. They are characterized by high rates of metabolic activity and need to respond promptly to activity-dependent fluctuations in bioenergetic demand. The dimensions and polarity of neurons require efficient transport of mitochondria to hot spots of energy consumption, such as presynaptic and postsynaptic sites. Moreover, the postmitotic state of neurons in combination with their exposure to intrinsic and extrinsic neuronal stress factors call for a high fidelity of mitochondrial quality control systems. Consequently, it is not surprising that mitochondrial alterations can promote neuronal dysfunction and degeneration. In particular, mitochondrial dysfunction has long been implicated in the etiopathogenesis of Parkinson's disease (PD), based on the observation that mitochondrial toxins can cause parkinsonism in humans and animal models. Substantial progress towards understanding the role of mitochondria in the disease process has been made by the identification and characterization of genes causing familial variants of PD. Studies on the function and dysfunction of these genes revealed that various aspects of mitochondrial biology appear to be affected in PD, comprising mitochondrial biogenesis, bioenergetics, dynamics, transport, and quality control.

Reactive oxygen species regulated mitochondria-mediated apoptosis in PC12 cells exposed to chlorpyrifos

Reactive oxidative species (ROS) generated by environmental toxicants including pesticides could be one of the factors underlying the neuronal cell damage in neurodegenerative diseases. In this study we found that chlorpyrifos (CPF) induced apoptosis in dopaminergic neuronal components of PC12 cells as demonstrated by the activation of caspases and nuclear condensation. Furthermore, CPF also reduced the tyrosine hydroxylase-positive immunoreactivity in substantia nigra of the rat. In addition, CPF induced inhibition of mitochondrial complex I activity. Importantly, N-acetyl cysteine (NAC) treatment effectively blocked apoptosis via the caspase-9 and caspase-3 pathways while NAC attenuated the inhibition of mitochondrial complex I activity as well as the oxidative metabolism of dopamine (DA). These results demonstrated that CPF-induced apoptosis was involved in mitochondrial dysfunction through the production of ROS. In the response of cellular antioxidant systems to CPF, we found that CPF treatment increased HO-1 expression while the expression of CuZnSOD and MnSOD was reduced. In addition, we found that CPF treatment activated MAPK pathways, including ERK 1/2, the JNK, and the p38 MAP kinase in a time-dependent manner. NAC treatment abolished MAPK phosphorylation caused by CPF, indicating that ROS are upstream signals of MAPK. Interestingly, MAPK inhibitors abolished cytotoxicity and reduced ROS generation by CPF treatment. Our results demonstrate that CPF induced neuronal cell death in part through MAPK activation via ROS generation, suggesting its potential to generate oxidative stress via mitochondrial damage and its involvement in oxidative stress-related neurodegenerative disease.

The interplay of neuronal mitochondrial dynamics and bioenergetics: implications for Parkinson's disease

The dynamic properties of mitochondria (mitochondrial fission, fusion, transport biogenesis and degradation) are critical for neuronal function and health, and dysregulation of mitochondrial dynamics has been increasingly linked to the pathogenesis of Parkinson's disease (PD). Mitochondrial dynamics and bioenergetics are interconnected, and this is of particular importance in neurons, which have a unique bioenergetic profile due to their energetic dependence on mitochondria and specialized, compartmentalized energetic needs. In this review, we summarize the interplay of mitochondrial dynamics and bioenergetics, and its particular relevance for neurodegeneration. Evidence linking dysregulation of mitochondrial dynamics to PD is presented from both toxin and genetic models, including newly emerging details of how PD-relevant genes PTEN-induced kinase 1 (PINK1) and Parkin regulate fission, fusion, mitophagy and transport. Finally, we discuss how neuronal bioenergetics may impact PD-relevant regulation of mitochondrial dynamics, and possible implications for understanding the role of mitochondrial dynamics in PD.

Impaired mitochondrial biogenesis contributes to depletion of functional mitochondria in chronic MPP+ toxicity: dual roles for ERK1/2

The regulation of mitochondrial quality has emerged as a central issue in neurodegeneration, diabetes, and cancer. We utilized repeated low-dose applications of the complex I inhibitor 1-methyl-4-phenylpyridinium (MPP⁺) over 2 weeks to study cellular responses to chronic mitochondrial stress. Chronic MPP⁺ triggered depletion of functional mitochondria resulting in diminished capacities for aerobic respiration. Inhibiting autophagy/mitophagy only partially restored mitochondrial content. In contrast, inhibiting activation of extracellular signal-regulated protein kinases conferred complete cytoprotection with full restoration of mitochondrial functional and morphological parameters, enhancing spare respiratory capacity in MPP⁺ co-treated cells above that of control cells. Reversal of mitochondrial injury occurred when U0126 was added 1 week after MPP⁺, implicating enhanced repair mechanisms. Chronic MPP⁺ caused a >90% decrease in complex I subunits, along with decreases in complex III and IV subunits. Decreases in respiratory complex subunits were reversed by co-treatment with U0126, ERK1/2 RNAi or transfection of dominant-negative MEK1, but only partially restored by degradation inhibitors. Chronic MPP⁺ also suppressed the de novo synthesis of mitochondrial DNA-encoded proteins, accompanied by decreased expression of the mitochondrial transcription factor TFAM. U0126 completely reversed each of these deficits in mitochondrial translation and protein expression. These data indicate a key, limiting role for mitochondrial biogenesis in determining the outcome of injuries associated with elevated mitophagy.

Microtubule destabilization is shared by genetic and idiopathic Parkinson's disease patient fibroblasts

Data from both toxin-based and gene-based models suggest that dysfunction of the microtubule system contributes to the pathogenesis of Parkinson's disease, even if, at present, no evidence of alterations of microtubules in vivo or in patients is available. Here we analyze cytoskeleton organization in primary fibroblasts deriving from patients with idiopathic or genetic Parkinson's disease, focusing on mutations in parkin and leucine-rich repeat kinase 2. Our analyses reveal that genetic and likely idiopathic pathology affects cytoskeletal organization and stability, without any activation of autophagy or apoptosis. All parkinsonian fibroblasts have a reduced microtubule mass, represented by a higher fraction of unpolymerized tubulin in respect to control cells, and display significant changes in microtubule stability-related signaling pathways. Furthermore, we show that the reduction of microtubule mass is so closely related to the alteration of cell morphology and behavior that both pharmacological treatment with microtubule-targeted drugs, and genetic approaches, by transfecting the wild type parkin or leucine-rich repeat kinase 2, restore the proper microtubule stability and are able to rescue cell architecture. Taken together, our results suggest that microtubule destabilization is a point of convergence of genetic and idiopathic forms of parkinsonism and highlight, for the first time, that microtubule dysfunction occurs in patients and not only in experimental models of Parkinson's disease. Therefore, these data contribute to the knowledge on molecular and cellular events underlying Parkinson's disease and, revealing that correction of microtubule defects restores control phenotype, may offer a new therapeutic target for the management of the disease.

Administration of rotenone enhanced voluntary alcohol drinking behavior in C57BL/6J mice

Rotenone, a commonly used lipophic pesticide, is a high-affinity mitochondrial complex I inhibitor. The aim of this project is to study the causal relationship between changes of brain monoamine levels and drinking behavior in rotenone-treated mice. In the first experiment, we investigated the effects of acute exposure to rotenone (20 mg/kg, p.o.) on the 8-h time limited-access alcohol drinking behavior and brain monoamine levels in C57BL/6J mice at 0, 2, 8 and 24 h. Dopamine (DA), 3,4-dihydroxyphenylacetic acid (DOPAC) and 5-hydroxyindoleacetic acid (5HIAA) levels in the nucleus accumbens (ACC), caudate-putamen (C/P) and lateral hypothalamus (LH) of rotenone-treated mice were decreased at 2 and/or 8 h. Rotenone-exposed mice showed a suppression of voluntary alcohol intake at 4 and 8 h, but total daily alcohol intake did not differ significantly between the two groups. The effects of chronic exposure to rotenone (1, 5, 10 and 20 mg/kg, p.o. for 30 days) on the alcohol drinking behavior and monoamine levels of rotenone-exposed mice (10 mg/kg, p.o.) were investigated in the second experiment. The mice treated with rotenone showed increases in alcohol drinking behavior. Levels of DA and 5-HT in the ACC and C/P of chronic rotenone-treated mice were decreased, while the ratios of DOPAC to DA in the ACC and C/P and of 5HIAA to 5-HT in the ACC, C/P and DRN were increased significantly. Tyrosine hydroxylase immunoreactivity of chronic rotenone-treated mice (10 mg/kg, p.o.) slightly were decreased in both the striatum and the substantia nigra. Ethanol and acetaldehyde metabolism was not significantly different between mice treated with rotenone (10 mg/kg, p.o.) and controls. It was suggested that rotenone-treated mice had increased alcohol drinking behavior associated with increases in the DA turnover ratios of ACC and striatum to compensate for the neural degeneration.

Rotenone-induced apoptosis and role of calcium: a study on Neuro-2a cells

Rotenone causes cytotoxicity in astrocytic cell culture by glial activation, which is linked to free radical generation. The present study is an investigation to explore whether rotenone could also cause cellular toxicity in mouse neuroblastoma cells (Neuro-2a) under treatment similar to astroglial cells. The effect of rotenone (0.1, 1, and 10 μM) on mitochondrial dehydrogenase enzyme activity by MTT reduction assay, PI uptake, total reactive oxygen species (ROS)/superoxide levels, nitrite levels, extent of DNA damage (by comet assay), and nuclear morphological alteration by Hoechst staining was studied. Caspase-3 and Ca⁺²/calmodulin-dependent protein kinase II (CaMKIIα) gene expression was determined to evaluate the apoptotic cell death and calcium kinase, respectively. Calcium level was estimated fluorometrically using fura-2A stain. Rotenone decreased mitochondrial dehydrogenase enzyme activity and generated ROS, superoxide, and nitrite. Rotenone treatment impaired cell intactness and nuclear morphology as depicted by PI uptake and chromosomal condensation of Neuro-2a cells, respectively. In addition, rotenone resulted in increased intracellular Ca⁺² level, caspase-3, and CaMKIIα expression. Furthermore, co-exposure of melatonin (300 μM), an antioxidant to cell culture, significantly suppressed the rotenone-induced decreased mitochondrial dehydrogenase enzyme activity, elevated ROS and RNS. However, melatonin was found ineffective to counteract rotenone-induced increased PI uptake, altered morphological changes, DNA damage, elevated Ca⁺², and increased expression of caspase-3 and CaMKIIα. The study indicates that intracellular calcium rather than oxidative stress is a major factor for rotenone-induced apoptosis in neuronal cells.

Nicotinamide, NAD(P)(H), and Methyl-Group Homeostasis Evolved and Became a Determinant of Ageing Diseases: Hypotheses and Lessons from Pellagra

Compartmentalized redox faults are common to ageing diseases. Dietary constituents are catabolized to NAD(H) donating electrons producing proton-based bioenergy in coevolved, cross-species and cross-organ networks. Nicotinamide and NAD deficiency from poor diet or high expenditure causes pellagra, an ageing and dementing disorder with lost robustness to infection and stress. Nicotinamide and stress induce Nicotinamide-N-methyltransferase (NNMT) improving choline retention but consume methyl groups. High NNMT activity is linked to Parkinson's, cancers, and diseases of affluence. Optimising nicotinamide and choline/methyl group availability is important for brain development and increased during our evolution raising metabolic and methylome ceilings through dietary/metabolic symbiotic means but strict energy constraints remain and life-history tradeoffs are the rule. An optimal energy, NAD and methyl group supply, avoiding hypo and hyper-vitaminoses nicotinamide and choline, is important to healthy ageing and avoids utilising double-edged symbionts or uncontrolled autophagy or reversions to fermentation reactions in inflammatory and cancerous tissue that all redistribute NAD(P)(H), but incur high allostatic costs.

Effect of pesticides on the aggregation of mutant huntingtin protein

The classical reports on neurodegeneration concentrate on studying disruption of signalling cascades. Although it is now well recognized that misfolding and aggregation of specific proteins are associated with a majority of these diseases, their role in aggravating the symptoms is not so well understood. Huntington's disease (HD) is a neurodegenerative disorder that results from damage to complex II of mitochondria. In this work, we have studied the effect of mitochondrial complex I inhibitors, viz. 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine and rotenone, and complex II inhibitor, viz. 3-nitropropionic acid, on the aggregation of mutant huntingtin (mthtt) protein, whose misfolding and aggregation results in cellular abnormalities characteristic of HD. All three inhibitors were found to accelerate the aggregation of mthtt in vitro, although the amounts of aggregates formed were different in all cases. Thus, apart from their effect on mitochondrial viability, these neurotoxins are capable of interfering with the protein aggregation process and thus, hastening the onset of the disease.

Axon degeneration in Parkinson's disease

Parkinson's disease (PD) is the most common neurodegenerative disease of the basal ganglia. Like other adult-onset neurodegenerative disorders, it is without a treatment that forestalls its chronic progression. Efforts to develop disease-modifying therapies to date have largely focused on the prevention of degeneration of the neuron soma, with the tacit assumption that such approaches will forestall axon degeneration as well. We herein propose that future efforts to develop neuroprotection for PD may benefit from a shift in focus to the distinct mechanisms that underlie axon degeneration. We review evidence from human post-mortem studies, functional neuroimaging, genetic causes of the disease and neurotoxin models that axon degeneration may be the earliest feature of the disease, and it may therefore be the most appropriate target for early intervention. In addition, we present evidence that the molecular mechanisms of degeneration of axons are separate and distinct from those of neuron soma. Progress is being made in understanding these mechanisms, and they provide possible new targets for therapeutic intervention. We also suggest that the potential for axon re-growth in the adult central nervous system has perhaps been underestimated, and it offers new avenues for neurorestoration. In conclusion, we propose that a new focus on the neurobiology of axons, their molecular pathways of degeneration and growth, will offer novel opportunities for neuroprotection and restoration in the treatment of PD and other neurodegenerative diseases.

Modelling of Parkinson's disease in mice

Although progress has been made in the symptomatic treatment of Parkinson's disease since the discovery of L-dopa in the 1960s, no neuroprotective therapy is yet available. Absence of adequate animal models of the disease that enable prediction of clinical success of potential treatments is often cited as a major impediment to progress and discourages researchers and pharmaceutical companies from investing resources to develop such treatments. Classic models are still widely used, but have been disappointing, and development of genetic models has given new hope. However, can a human disease be faithfully reproduced in a mouse? In this Review we summarise evidence that some genetic mouse models do reproduce key features of Parkinson's disease and show that much can be learned from even imperfect models. The hope is that this information could be used to advance the search for neuroprotective therapies for Parkinson's disease.

Anticancer drugs targeting the mitochondrial electron transport chain

SIGNIFICANCE

Mitochondria are emerging as highly intriguing organelles showing promise but that are yet to be fully exploited as targets for anticancer drugs.

RECENT ADVANCES

A group of compounds that induce mitochondrial destabilization, thereby affecting the physiology of cancer cells, has been defined and termed 'mitocans.' Based on their mode of action of targeting in and around mitochondria, we have placed these agents into several groups including hexokinase inhibitors, compounds targeting Bcl-2 family proteins, thiol redox inhibitors, VDAC/ANT targeting drugs, electron transport chain-targeting drugs, lipophilic cations targeting the inner membrane, agents affecting the tricarboxylic acid cycle, drugs targeting mtDNA, and agents targeting other presently unknown sites.

CRITICAL ISSUES

Mitocans have a potential to prove highly efficient in suppressing various malignant diseases in a selective manner. They include compounds that are currently in clinical trial and offer substantial promise to become clinically applied drugs. Here we update and redefine the individual classes of mitocans, providing examples of the various members of these groups with a particular focus on agents targeting the electron transport chain, and indicate their potential application in clinical practice.

FUTURE DIRECTIONS

Even though reactive oxygen species induction is important for the anticancer activity of many mitocans, the precise sequence of events preceding and following this pivotal event are not yet fully clarified, and warrant further investigation. This is imperative for effective deployment of these compounds in the clinic.

Altered dopamine metabolism and increased vulnerability to MPTP in mice with partial deficiency of mitochondrial complex I in dopamine neurons

A variety of observations support the hypothesis that deficiency of complex I [reduced nicotinamide-adenine dinucleotide (NADH):ubiquinone oxidoreductase] of the mitochondrial respiratory chain plays a role in the pathophysiology of Parkinson's disease (PD). However, recent data from a study using mice with knockout of the complex I subunit NADH:ubiquinone oxidoreductase iron-sulfur protein 4 (Ndufs4) has challenged this concept as these mice show degeneration of non-dopamine neurons. In addition, primary dopamine (DA) neurons derived from such mice, reported to lack complex I activity, remain sensitive to toxins believed to act through inhibition of complex I. We tissue-specifically disrupted the Ndufs4 gene in mouse heart and found an apparent severe deficiency of complex I activity in disrupted mitochondria, whereas oxidation of substrates that result in entry of electrons at the level of complex I was only mildly reduced in intact isolated heart mitochondria. Further analyses of detergent-solubilized mitochondria showed the mutant complex I to be unstable but capable of forming supercomplexes with complex I enzyme activity. The loss of Ndufs4 thus causes only a mild complex I deficiency in vivo. We proceeded to disrupt Ndufs4 in midbrain DA neurons and found no overt neurodegeneration, no loss of striatal innervation and no symptoms of Parkinsonism in tissue-specific knockout animals. However, DA homeostasis was abnormal with impaired DA release and increased levels of DA metabolites. Furthermore, Ndufs4 DA neuron knockouts were more vulnerable to the neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine. Taken together, these findings lend in vivo support to the hypothesis that complex I deficiency can contribute to the pathophysiology of PD.

The wrong suspect?

Parkinson's disease may be caused by microtubule, rather than mitochondrial complex I, dysfunction.