Impaired mitochondrial dynamics and function in the pathogenesis of Parkinson's disease

Evidence for the toxicity of bidirectional transcripts and mitochondrial dysfunction in blood associated with small CGG expansions in the FMR1 gene in patients with parkinsonism

PURPOSE

Our previous results showed that both gray zone and lower end premutation range (40-85 repeats) fragile X mental retardation 1 (FMR1) alleles were more common among males with parkinsonism than in the general population. This study aimed to determine whether these alleles have a significant role in the manifestations and pathogenesis of parkinsonian disorders.

METHODS

Detailed clinical assessment and genetic testing were performed in 14 male carriers of premutation and gray zone FMR1 alleles and in 24 noncarriers identified in a sample of males with parkinsonism.

RESULTS

The premutation + gray zone carriers presented with more severe symptoms than disease controls matched for age, diagnosis, disease duration, and treatment. The Parkinson disease (Unified Parkinson's Disease Rating Scale) motor score and the measures of cognitive decline (Mini-Mental State Examination and/or Addenbrooke's Cognitive Examination Final Revised Version A scores) were significantly correlated with the size of the CGG repeat and the (elevated) levels of antisense FMR1 and Cytochrome C1 mRNAs in blood leukocytes. In addition, the carriers showed a significant depletion of the nicotinamide adenine dinucleotide, reduced dehydrogenase subunit 1 mitochondrial gene in whole blood.

CONCLUSION

Small CGG expansion FMR1 alleles (gray zone and lower end premutation) play a significant role in the development of the parkinsonian phenotype, possibly through the cytotoxic effect of elevated sense and/or antisense FMR1 transcripts involving mitochondrial dysfunction and leading to progressive neurodegeneration.

CSF xanthine, homovanillic acid, and their ratio as biomarkers of Parkinson's disease

Diminished nigrostriatal dopaminergic neurotransmission is a biochemical hallmark of Parkinson's disease. Despite this, a reliable trait biomarker of sporadic Parkinson's disease has not emerged from measurements of cerebrospinal fluid dopamine metabolites. Previous studies have highlighted strong neurochemical relationships between dopamine and various purine compounds. In this study, we analyzed cerebrospinal fluid concentrations of homovanillic acid (the major catabolite of dopamine) and the purine compound xanthine for a comparison of 217 unmedicated Parkinson's disease subjects and 26 healthy controls. These compounds were highly correlated for both the Parkinson's disease subjects (r=0.68) and for controls (r=0.73; both groups, p<0.001). While neither homovanillic acid nor xanthine concentrations differentiated Parkinson's disease from controls, their ratio did. For controls, the mean [xanthine]/[homovanillic acid] quotient was 13.1±5.5 as compared to the Parkinson's disease value of 17.4±6.7 at an initial lumbar CSF collection (p=0.0017), and 19.7±8.7 (p<0.001) at a second CSF collection up to 24 months later. The [xanthine]/[homovanillic acid] ratio in the Parkinson's disease subjects differed as a function of disease severity, as measured by the sum of Unified Parkinson's Disease Rating Scale Activities of Daily Living and Motor Exam ratings. The [xanthine]/[homovanillic acid] ratio also increased between the first and second CSF collections, suggesting that this quotient provides both a state and trait biomarker of Parkinson's disease. These observations add to other neurochemical evidence that links purine metabolism to Parkinson's disease.

DJ-1 knock-down impairs astrocyte mitochondrial function

Mitochondrial dysfunction has long been implicated in the pathogenesis of Parkinson's disease (PD). PD brain tissues show evidence for mitochondrial respiratory chain Complex I deficiency. Pharmacological inhibitors of Complex I, such as rotenone, cause experimental parkinsonism. The cytoprotective protein DJ-1, whose deletion is sufficient to cause genetic PD, is also known to have mitochondria-stabilizing properties. We have previously shown that DJ-1 is over-expressed in PD astrocytes, and that DJ-1 deficiency impairs the capacity of astrocytes to protect co-cultured neurons against rotenone. Since DJ-1 modulated, astrocyte-mediated neuroprotection against rotenone may depend upon proper astrocytic mitochondrial functioning, we hypothesized that DJ-1 deficiency would impair astrocyte mitochondrial motility, fission/fusion dynamics, membrane potential maintenance, and respiration, both at baseline and as an enhancement of rotenone-induced mitochondrial dysfunction. In astrocyte-enriched cultures, we observed that DJ-1 knock-down reduced mitochondrial motility primarily in the cellular processes of both untreated and rotenone treated cells. In these same cultures, DJ-1 knock-down did not appreciably affect mitochondrial fission, fusion, or respiration, but did enhance rotenone-induced reductions in the mitochondrial membrane potential. In neuron-astrocyte co-cultures, astrocytic DJ-1 knock-down reduced astrocyte process mitochondrial motility in untreated cells, but this effect was not maintained in the presence of rotenone. In the same co-cultures, astrocytic DJ-1 knock-down significantly reduced mitochondrial fusion in the astrocyte cell bodies, but not the processes, under the same conditions of rotenone treatment in which DJ-1 deficiency is known to impair astrocyte-mediated neuroprotection. Our studies therefore demonstrated the following new findings: (i) DJ-1 deficiency can impair astrocyte mitochondrial physiology at multiple levels, (ii) astrocyte mitochondrial dynamics vary with sub-cellular region, and (iii) the physical presence of neurons can affect astrocyte mitochondrial behavior.

The parkinsonian mimetic, MPP+, specifically impairs mitochondrial transport in dopamine axons

Impaired axonal transport may play a key role in Parkinson's disease. To test this notion, a microchamber system was adapted to segregate axons from cell bodies using green fluorescent protein-labeled mouse dopamine (DA) neurons. Transport was examined in axons challenged with the DA neurotoxin, 1-methyl-4-phenylpyridinium ion (MPP+). MPP+ rapidly reduced overall mitochondrial motility in DA axons; among motile mitochondria, anterograde transport was slower yet retrograde transport was increased. Transport effects were specific for DA mitochondria, which were smaller and transported more slowly than their non-DA counterparts. MPP+ did not affect synaptophysin-tagged vesicles or any other measureable moving particle. Toxin effects on DA mitochondria were not dependent upon ATP, calcium, free radical species, JNK, or caspase3/PKC pathways but were completely blocked by the thiol-anti-oxidant N-acetyl-cysteine or membrane-permeable glutathione. Since these drugs also rescued processes from degeneration, these findings emphasize the need to develop therapeutics aimed at axons as well as cell bodies to preserve "normal" circuitry and function as long as possible.

Mitochondria, calcium, and endoplasmic reticulum stress in Parkinson's disease

Parkinson's disease (PD) is a progressive neurodegenerative disease characterized by a loss of dopaminergic neurons in the substantia nigra pars compacta (SNPC) and the presence of intracytoplasmatic inclusions known as Lewy bodies, largely composed of alpha-synuclein (α-syn). PD is a multifactorial disease and its etiology remains largely elusive. Although more than 90% of the cases are sporadic, mutations in several nuclear encoded genes have been linked to the development of autosomal recessive and dominant familial parkinsonian syndromes (Bogaerts et al. (2008) Genes Brain Behav 7, 129-151), enhancing our understanding of biochemical and cellular mechanisms contributing to the disease. Many cellular mechanisms are thought to be involved in the dopaminergic neuronal death in PD, including oxidative stress, intracellular Ca(2+) homeostasis impairment, and mitochondrial dysfunctions. Furthermore, endoplasmic reticulum (ER) stress together with abnormal protein degradation by the ubiquitin proteasome system is considered to contribute to the PD pathogenesis. This review covers all the aspects related to the molecular mechanisms underlying the interplay between mitochondria, ER, and proteasome system in PD-associated neurodegeneration.

Calcium dysregulation and homeostasis of neural calcium in the molecular mechanisms of neurodegenerative diseases provide multiple targets for neuroprotection

The intracellular free calcium concentration subserves complex signaling roles in brain. Calcium cations (Ca(2+)) regulate neuronal plasticity underlying learning and memory and neuronal survival. Homo- and heterocellular control of Ca(2+) homeostasis supports brain physiology maintaining neural integrity. Ca(2+) fluxes across the plasma membrane and between intracellular organelles and compartments integrate diverse cellular functions. A vast array of checkpoints controls Ca(2+), like G protein-coupled receptors, ion channels, Ca(2+) binding proteins, transcriptional networks, and ion exchangers, in both the plasma membrane and the membranes of mitochondria and endoplasmic reticulum. Interactions between Ca(2+) and reactive oxygen species signaling coordinate signaling, which can be either beneficial or detrimental. In neurodegenerative disorders, cellular Ca(2+)-regulating systems are compromised. Oxidative stress, perturbed energy metabolism, and alterations of disease-related proteins result in Ca(2+)-dependent synaptic dysfunction, impaired plasticity, and neuronal demise. We review Ca(2+) control processes relevant for physiological and pathophysiological conditions in brain tissue. Dysregulation of Ca(2+) is decisive for brain cell death and degeneration after ischemic stroke, long-term neurodegeneration in Alzheimer's disease, Parkinson's disease, Huntington's disease, inflammatory processes, such as in multiple sclerosis, epileptic sclerosis, and leucodystrophies. Understanding the underlying molecular processes is of critical importance for the development of novel therapeutic strategies to prevent neurodegeneration and confer neuroprotection.

Mitochondrial-nuclear epistasis: implications for human aging and longevity

There is substantial evidence that mitochondria are involved in the aging process. Mitochondrial function requires the coordinated expression of hundreds of nuclear genes and a few dozen mitochondrial genes, many of which have been associated with either extended or shortened life span. Impaired mitochondrial function resulting from mtDNA and nuclear DNA variation is likely to contribute to an imbalance in cellular energy homeostasis, increased vulnerability to oxidative stress, and an increased rate of cellular senescence and aging. The complex genetic architecture of mitochondria suggests that there may be an equally complex set of gene interactions (epistases) involving genetic variation in the nuclear and mitochondrial genomes. Results from Drosophila suggest that the effects of mtDNA haplotypes on longevity vary among different nuclear allelic backgrounds, which could account for the inconsistent associations that have been observed between mitochondrial DNA (mtDNA) haplogroups and survival in humans. A diversity of pathways may influence the way mitochondria and nuclear-mitochondrial interactions modulate longevity, including: oxidative phosphorylation; mitochondrial uncoupling; antioxidant defenses; mitochondrial fission and fusion; and sirtuin regulation of mitochondrial genes. We hypothesize that aging and longevity, as complex traits having a significant genetic component, are likely to be controlled by nuclear gene variants interacting with both inherited and somatic mtDNA variability.

Mitochondrial dynamics, cell death and the pathogenesis of Parkinson's disease

The structure and function of the mitochondrial network is regulated by mitochondrial biogenesis, fission, fusion, transport and degradation. A well-maintained balance of these processes (mitochondrial dynamics) is essential for neuronal signaling, plasticity and transmitter release. Core proteins of the mitochondrial dynamics machinery play important roles in the regulation of apoptosis, and mutations or abnormal expression of these factors are associated with inherited and age-dependent neurodegenerative disorders. In Parkinson's disease (PD), oxidative stress and mitochondrial dysfunction underlie the development of neuropathology. The recessive Parkinsonism-linked genes PTEN-induced kinase 1 (PINK1) and Parkin maintain mitochondrial integrity by regulating diverse aspects of mitochondrial function, including membrane potential, calcium homeostasis, cristae structure, respiratory activity, and mtDNA integrity. In addition, Parkin is crucial for autophagy-dependent clearance of dysfunctional mitochondria. In the absence of PINK1 or Parkin, cells often develop fragmented mitochondria. Whereas excessive fission may cause apoptosis, coordinated induction of fission and autophagy is believed to facilitate the removal of damaged mitochondria through mitophagy, and has been observed in some types of cells. Compensatory mechanisms may also occur in mice lacking PINK1 that, in contrast to cells and Drosophila, have only mild mitochondrial dysfunction and lack dopaminergic neuron loss. A better understanding of the relationship between the specific changes in mitochondrial dynamics/turnover and cell death will be instrumental to identify potentially neuroprotective pathways steering PINK1-deficient cells towards survival. Such pathways may be manipulated in the future by specific drugs to treat PD and perhaps other neurodegenerative disorders characterized by abnormal mitochondrial function and dynamics.

The Nrf2/ARE Pathway: A Promising Target to Counteract Mitochondrial Dysfunction in Parkinson's Disease

Mitochondrial dysfunction is a prominent feature of various neurodegenerative diseases as strict regulation of integrated mitochondrial functions is essential for neuronal signaling, plasticity, and transmitter release. Many lines of evidence suggest that mitochondrial dysfunction plays a central role in the pathogenesis of Parkinson's disease (PD). Several PD-associated genes interface with mitochondrial dynamics regulating the structure and function of the mitochondrial network. Mitochondrial dysfunction can induce neuron death through a plethora of mechanisms. Both mitochondrial dysfunction and neuroinflammation, a common denominator of PD, lead to an increased production of reactive oxygen species, which are detrimental to neurons. The transcription factor nuclear factor E2-related factor 2 (Nrf2, NFE2L2) is an emerging target to counteract mitochondrial dysfunction and its consequences in PD. Nrf2 activates the antioxidant response element (ARE) pathway, including a battery of cytoprotective genes such as antioxidants and anti-inflammatory genes and several transcription factors involved in mitochondrial biogenesis. Here, the current knowledge about the role of mitochondrial dysfunction in PD, Nrf2/ARE stress-response mechanisms, and the evidence for specific links between this pathway and PD are summarized. The neuroprotection of nigral dopaminergic neurons by the activation of Nrf2 through several inducers in PD is also emphasized as a promising therapeutic approach.

Lipid-mediated oxidative stress and inflammation in the pathogenesis of Parkinson's disease

Parkinson's disease (PD) is a neurodegenerative movement disorder of unknown etiology. PD is characterized by the progressive loss of dopaminergic neurons in the substantia nigra, depletion of dopamine in the striatum, abnormal mitochondrial and proteasomal functions, and accumulation of α-synuclein that may be closely associated with pathological and clinical abnormalities. Increasing evidence indicates that both oxidative stress and inflammation may play a fundamental role in the pathogenesis of PD. Oxidative stress is characterized by increase in reactive oxygen species (ROS) and depletion of glutathione. Lipid mediators for oxidative stress include 4-hydroxynonenal, isoprostanes, isofurans, isoketals, neuroprostanes, and neurofurans. Neuroinflammation is characterized by activated microglial cells that generate proinflammatory cytokines, such as TNF-α and IL-1β. Proinflammatory lipid mediators include prostaglandins and platelet activating factor, together with cytokines may play a prominent role in mediating the progressive neurodegeneration in PD.

The extending network of FOXO transcriptional target genes

The evolutionarily conserved Forkhead box O (FOXO) family of transcription factors regulates multiple transcriptional targets involved in various cellular processes, including proliferation, stress resistance, apoptosis, and metabolism. Target gene regulation appears to be controlled in a cell-type-specific manner due to association of FOXO isoforms with specific cofactors. Many of the cellular processes modulated by FOXO are themselves deregulated in tumorigenesis, and deletion of Foxo genes has demonstrated that these transcription factors function as tumor suppressors. Our understanding of the regulation of FOXO activity, and defining specific transcriptional targets, may provide clues to the molecular mechanisms controlling cell fate decisions. In this review we describe the functional consequences of FOXO activation based on our current knowledge of transcriptional targets.

Use of MRI to monitor Parkinson’s disease

SUMMARY Objective biological markers of Parkinson’s disease (PD) are pivotal for confirming diagnosis, monitoring disease progression, and evaluating therapeutic interventions and disease-modifying agents. Structural and functional MRI provide an in vivo means to investigate the cortical and subcortical regions known to be affected in PD. In this article, we summarize how several MRI techniques, namely conventional MRI, iron-based MRI, volume-based MRI, diffusion tensor imaging, magnetic resonance spectroscopy and functional MRI have been used to assess the neurobiological changes related to the motor features of PD. We also discuss promising new research in which multiple MRI techniques are combined to achieve greater sensitivity and specificity of disease detection. Longitudinal research is necessary to establish MRI techniques as viable disease-state biomarkers of PD.

The endotoxin-induced neuroinflammation model of Parkinson's disease

Parkinson's disease (PD) is a common neurodegenerative disorder characterized by the progressive loss of dopaminergic (DA) neurons in the substantia nigra. Although the exact cause of the dopaminergic neurodegeneration remains elusive, recent postmortem and experimental studies have revealed an essential role for neuroinflammation that is initiated and driven by activated microglial and infiltrated peripheral immune cells and their neurotoxic products (such as proinflammatory cytokines, reactive oxygen species, and nitric oxide) in the pathogenesis of PD. A bacterial endotoxin-based experimental model of PD has been established, representing a purely inflammation-driven animal model for the induction of nigrostriatal dopaminergic neurodegeneration. This model, by itself or together with genetic and toxin-based animal models, provides an important tool to delineate the precise mechanisms of neuroinflammation-mediated dopaminergic neuron loss. Here, we review the characteristics of this model and the contribution of neuroinflammatory processes, induced by the in vivo administration of bacterial endotoxin, to neurodegeneration. Furthermore, we summarize the recent experimental therapeutic strategies targeting endotoxin-induced neuroinflammation to elicit neuroprotection in the nigrostriatal dopaminergic system. The potential of the endotoxin-based PD model in the development of an early-stage specific diagnostic biomarker is also emphasized.

Role and treatment of mitochondrial DNA-related mitochondrial dysfunction in sporadic neurodegenerative diseases

Several sporadic neurodegenerative diseases display phenomena that directly or indirectly relate to mitochondrial function. Data suggesting altered mitochondrial function in these diseases could arise from mitochondrial DNA (mtDNA) are reviewed. Approaches for manipulating mitochondrial function and minimizing the downstream consequences of mitochondrial dysfunction are discussed.

Analysis on the susceptibility genes in two chinese pedigrees with familial Parkinson's disease

Objective. To screen the susceptibility genes in Chinese pedigrees with early-onset familial Parkinson's disease (FPD). Methods. Fifty-one genomic DNA samples extracted from two Chinese pedigrees with FPD, the alpha-synuclein genes (SNCA), the leucine-rich repeat kinase 2(LRRK2), PINK1(PTEN-induced putative kinase 1), PARK7(Protein DJ1), PARK2(Parkinson juvenile disease protein 2), the glucocerebrosidase (GBA), and ATP(Ezrin-binding protein PACE-1), were sequenced by the use of polymerase chain reaction (PCR) technique. The gene dose of SNCA was checked. Results. There were only two missense mutations observed, respectively, at exon 5 of LRRK2 and exon 10 of PARK2, and both were enrolled in SNPs. Conclusion. No meaningful mutations could be detected, and other susceptibility genes should be detected in FDP patients in China.

Proteasome and p97 mediate mitophagy and degradation of mitofusins induced by Parkin

Damage to mitochondria can lead to the depolarization of the inner mitochondrial membrane, thereby sensitizing impaired mitochondria for selective elimination by autophagy. However, fusion of uncoupled mitochondria with polarized mitochondria can compensate for damage, reverse membrane depolarization, and obviate mitophagy. Parkin, an E3 ubiquitin ligase that is mutated in monogenic forms of Parkinson's disease, was recently found to induce selective autophagy of damaged mitochondria. Here we show that ubiquitination of mitofusins Mfn1 and Mfn2, large GTPases that mediate mitochondrial fusion, is induced by Parkin upon membrane depolarization and leads to their degradation in a proteasome- and p97-dependent manner. p97, a AAA+ ATPase, accumulates on mitochondria upon uncoupling of Parkin-expressing cells, and both p97 and proteasome activity are required for Parkin-mediated mitophagy. After mitochondrial fission upon depolarization, Parkin prevents or delays refusion of mitochondria, likely by the elimination of mitofusins. Inhibition of Drp1-mediated mitochondrial fission, the proteasome, or p97 prevents Parkin-induced mitophagy.

Roles of mitochondria in human disease

The chapters throughout this volume illustrate the many contributions of mitochondria to the maintenance of normal cell and tissue function, experienced as the health of the individual. Mitochondria are essential for maintaining aspects of physiology as fundamental as cellular energy balance, the modulation of calcium signalling, in defining cellular redox balance, and they house significant biosynthetic pathways. Mitochondrial numbers and volume within cells are regulated and have an impact on their functional roles, while, especially in the CNS (central nervous system), mitochondrial trafficking is critical to ensure the cellular distribution and strategic localization of mitochondria, presumably driven by local energy demand. Maintenance of a healthy mitochondrial population involves a complex system of quality control, involving degrading misfolded proteins, while damaged mitochondria are renewed by fusion or removed by autophagy. It seems evident that mechanisms that impair any of these processes will impair mitochondrial function and cell signalling pathways, leading to disordered cell function which manifests as disease. As gatekeepers of cell life and cell death, mitochondria regulate both apoptotic and necrotic cell death, and so at its most extreme, disturbances involving these pathways may trigger untimely cell death. Conversely, the lack of appropriate cell death can lead to inappropriate tissue growth and development of tumours, which are also characterized by altered mitochondrial metabolism. The centrality of mitochondrial dysfunction to a surprisingly wide range of major human diseases is slowly becoming recognized, bringing with it the prospect of novel therapeutic approaches to treat a multitude of unpleasant and pervasive diseases.

Mitochondrial abnormalities in the putamen in Parkinson's disease dyskinesia

Prolonged treatment of Parkinson's disease (PD) with levodopa leads to disabling side effects collectively referred to as 'dyskinesias'. We hypothesized that bioenergetic function in the putamen might play a crucial role in the development of dyskinesias. To test this hypothesis, we used post mortem samples of the human putamen and applied real time-PCR approaches and gene expression microarrays. We found that mitochondrial DNA (mtDNA) levels are decreased in patients who have developed dyskinesias, and mtDNA damage is concomitantly increased. These pathologies were not observed in PD subjects without signs of dyskinesias. The group of nuclear mRNA transcripts coding for the proteins of the mitochondrial electron transfer chain was decreased in patients with dyskinesias to a larger extent than in patients who had not developed dyskinesias. To examine whether dopamine fluctuations affect mtDNA levels in dopaminoceptive neurons, rat striatal neurons in culture were repeatedly exposed to levodopa, dopamine or their metabolites. MtDNA levels were reduced after treatment with dopamine, but not after treatment with dopamine metabolites. Levodopa led to an increase in mtDNA levels. We conclude that mitochondrial susceptibility in the putamen plays a role in the development of dyskinesias.

Towards a unifying, systems biology understanding of large-scale cellular death and destruction caused by poorly liganded iron: Parkinson's, Huntington's, Alzheimer's, prions, bactericides, chemical toxicology and others as examples

Exposure to a variety of toxins and/or infectious agents leads to disease, degeneration and death, often characterised by circumstances in which cells or tissues do not merely die and cease to function but may be more or less entirely obliterated. It is then legitimate to ask the question as to whether, despite the many kinds of agent involved, there may be at least some unifying mechanisms of such cell death and destruction. I summarise the evidence that in a great many cases, one underlying mechanism, providing major stresses of this type, entails continuing and autocatalytic production (based on positive feedback mechanisms) of hydroxyl radicals via Fenton chemistry involving poorly liganded iron, leading to cell death via apoptosis (probably including via pathways induced by changes in the NF-κB system). While every pathway is in some sense connected to every other one, I highlight the literature evidence suggesting that the degenerative effects of many diseases and toxicological insults converge on iron dysregulation. This highlights specifically the role of iron metabolism, and the detailed speciation of iron, in chemical and other toxicology, and has significant implications for the use of iron chelating substances (probably in partnership with appropriate anti-oxidants) as nutritional or therapeutic agents in inhibiting both the progression of these mainly degenerative diseases and the sequelae of both chronic and acute toxin exposure. The complexity of biochemical networks, especially those involving autocatalytic behaviour and positive feedbacks, means that multiple interventions (e.g. of iron chelators plus antioxidants) are likely to prove most effective. A variety of systems biology approaches, that I summarise, can predict both the mechanisms involved in these cell death pathways and the optimal sites of action for nutritional or pharmacological interventions.

Balance is the challenge--the impact of mitochondrial dynamics in Parkinson's disease

Impaired mitochondrial function has been implicated in neurodegeneration in Parkinson's disease (PD) based on biochemical and pathoanatomical studies in brains of PD patients. This observation was further substantiated by the identification of exogenic toxins, i.e. complex I inhibitors that directly affect mitochondrial energy metabolism and cause Parkinsonism in humans and various animal models. Recently, insights into the underlying molecular signalling pathways leading to alterations in mitochondrial homeostasis were gained based on the functional characterization of mitoprotective genes identified in rare forms of inherited PD. Using in vitro and in vivo loss of function models of the Parkin, PINK1, DJ-1 and Omi/HtrA2 gene, the emerging field of mitochondrial dynamics in PD was established as being critical for the maintenance of mitochondrial function in neurons. This underscored the concept that mitochondria are highly dynamic organelles, which are tightly regulated to continuously adapt shape to functional and anatomical requirements during axonal transport, synaptic signalling, organelle degradation and cellular energy supply. The dissection of pathways involved in mitochondrial quality control clearly established the PINK1/Parkin-pathway in the clearance of dysfunctional mitochondria by autophagy and hints to a complex interplay between PD-associated proteins acting at the mitochondrial interface. The elucidation of this mitoprotective signalling network may help to define novel therapeutic targets for PD via molecular modelling of mitochondria and/or pharmacological modulation of mitochondrial dynamics.

Energy metabolism in amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is characterised by the progressive degeneration of upper and lower motor neurons. Besides motor neuron degeneration, ALS is associated with several defects in energy metabolism, including weight loss, hypermetabolism, and hyperlipidaemia. Most of these abnormalities correlate with duration of survival, and available clinical evidence supports a negative contribution of defective energy metabolism to the overall pathogenic process. Findings from animal models of ALS support this view and provide insights into the underlying mechanisms. Altogether, these results have clinical consequences for the management of defective energy metabolism in patients with ALS and pave the way for future therapeutic interventions.

Ubiquitination in postsynaptic function and plasticity

Neurons are highly specialized cells whose connectivity at synapses subserves rapid information transfer in the brain. Proper information processing, learning, and memory storage in the brain requires continuous remodeling of synaptic networks. Such remodeling includes synapse formation, elimination, synaptic protein turnover, and changes in synaptic transmission. An emergent mechanism for regulating synapse function is posttranslational modification through the ubiquitin pathway at the postsynaptic membrane. Here, we discuss recent findings implicating ubiquitination and protein degradation in postsynaptic function and plasticity. We describe postsynaptic ubiquitination pathways and their role in brain development, neuronal physiology, and brain disorders.

Gene therapy: a viable therapeutic strategy for Parkinson's disease?

Gene therapy represents a potentially useful additional technique to ameliorate the motor symptoms of Parkinson's disease (PD), and the motor complications of its treatment. The neurodegenerative process itself, as well as the non-motor symptoms of PD, both remain less amenable to most of the current gene therapy approaches. This review presents an overview of the four gene therapies in phase I/II clinical trials, outlines some of the challenges they face, and proposes additional alternative strategies that might improve the clinical prospects of gene therapy for PD. In so doing, we hope to highlight the issue of the current absence of effective treatment for non-motor symptoms of PD and the potential of further candidate targets for gene therapy intervention that might improve upon this, for both specific individuals with genetic forms of PD as well as "sporadic" PD patients.

Heat shock proteins and Drosophila aging

Since their discovery in Drosophila, the heat shock proteins (Hsps) have been shown to regulate both stress resistance and life-span. Aging is characterized by increased oxidative stress and the accumulation of abnormal (malfolded) proteins, and these stresses induce Hsp gene expression through the transcription factor HSF. In addition, a subset of Hsps is induced by oxidative stress through the JNK signaling pathway and the transcription factor Foxo. The Hsps counteract the toxicity of abnormal proteins by facilitating protein refolding and turnover, and through other mechanisms including inhibition of apoptosis. The Hsps are up-regulated in tissue-specific patterns during aging, and their expression correlates with, and sometimes predicts, life span, making them ideal biomarkers of aging. The tools available for experimentally manipulating gene function and assaying healthspan in Drosophila provides an unparalleled opportunity to further study the role of Hsps in aging.

Parkin is protective against proteotoxic stress in a transgenic zebrafish model

Background

Mutations in the gene encoding the E3 ubiquitin ligase parkin (PARK2) are responsible for the majority of autosomal recessive parkinsonism. Similarly to other knockout mouse models of PD-associated genes, parkin knockout mice do not show a substantial neuropathological or behavioral phenotype, while loss of parkin in Drosophila melanogaster leads to a severe phenotype, including reduced lifespan, apoptotic flight muscle degeneration and male sterility. In order to study the function of parkin in more detail and to address possible differences in its role in different species, we chose Danio rerio as a different vertebrate model system.

Methodology/Principal Findings

We first cloned zebrafish parkin to compare its biochemical and functional aspects with that of human parkin. By using an antisense knockdown strategy we generated a zebrafish model of parkin deficiency (knockdown efficiency between 50% and 60%) and found that the transient knockdown of parkin does not cause morphological or behavioral alterations. Specifically, we did not observe a loss of dopaminergic neurons in parkin-deficient zebrafish. In addition, we established transgenic zebrafish lines stably expressing parkin by using a Gal4/UAS-based bidirectional expression system. While parkin-deficient zebrafish are more vulnerable to proteotoxicity, increased parkin expression protected transgenic zebrafish from cell death induced by proteotoxic stress.

Conclusions/Significance

Similarly to human parkin, zebrafish parkin is a stress-responsive protein which protects cells from stress-induced cell death. Our transgenic zebrafish model is a novel tool to characterize the protective capacity of parkin in vivo.

Pantothenate kinase 1 is required to support the metabolic transition from the fed to the fasted state

Coenzyme A (CoA) biosynthesis is regulated by the pantothenate kinases (PanK), of which there are four active isoforms. The PanK1 isoform is selectively expressed in liver and accounted for 40% of the total PanK activity in this organ. CoA synthesis was limited using a Pank1(-/-) knockout mouse model to determine whether the regulation of CoA levels was critical to liver function. The elimination of PanK1 reduced hepatic CoA levels, and fasting triggered a substantial increase in total hepatic CoA in both Pank1(-/-) and wild-type mice. The increase in hepatic CoA during fasting was blunted in the Pank1(-/-) mouse, and resulted in reduced fatty acid oxidation as evidenced by abnormally high accumulation of long-chain acyl-CoAs, acyl-carnitines, and triglycerides in the form of lipid droplets. The Pank1(-/-) mice became hypoglycemic during a fast due to impaired gluconeogenesis, although ketogenesis was normal. These data illustrate the importance of PanK1 and elevated liver CoA levels during fasting to support the metabolic transition from glucose utilization and fatty acid synthesis to gluconeogenesis and fatty acid oxidation. The findings also suggest that PanK1 may be a suitable target for therapeutic intervention in metabolic disorders that feature hyperglycemia and hypertriglyceridemia.

Mitochondrial dysfunction: a potential link between neuroinflammation and neurodegeneration?

Dysfunctional mitochondria are thought to play a cardinal role in the pathogenesis of various neurological disorders, such as multiple sclerosis, Alzheimer's disease, Parkinson's disease and stroke. In addition, neuroinflammation is a common denominator of these diseases. Both mitochondrial dysfunction and neuroinflammatory processes lead to increased production of reactive oxygen species (ROS) which are detrimental to neurons. Therefore, neuroinflammation is increasingly recognized to contribute to processes underlying neurodegeneration. Here we describe the involvement of mitochondrial (dys)function in various neurological disorders and discuss the putative link between mitochondrial function and neuroinflammation.

Over-expression of alpha-synuclein in the nervous system enhances axonal degeneration after peripheral nerve lesion in a transgenic mouse strain

Wallerian degeneration in peripheral nerves occurs after a traumatic insult when the distal nerve part degenerates while peripheral macrophages enter the nerve stump and remove the accruing debris by phagozytosis. We used an experimental model to investigate the effect of either the absence or over-expression of alpha-synuclein (alpha-syn) after transecting the sciatic nerves of mice. alpha-Synuclein is a major component of Lewy bodies and its aggregation results in a premature destruction of nerve cells. It has also been found present in different peripheral nerves but its role in the axon remains still unclear. Following sciatic nerve transection in different mouse strains, we investigated the numbers of invading macrophages, the amounts of remaining myelin and axons 6 days after injury. All mice showed clear signs of Wallerian degeneration, but transgenic mice expressing human wild-type alpha-syn showed lower numbers of invading macrophages, less preserved myelin and significantly lower numbers of preserved axons in comparison with either knockout mice or a mouse strain with a spontaneous deletion of alpha-syn. The use of protein aggregation filtration blots and paraffin-embedded tissue blots displayed depositions of alpha-syn aggregates within sciatic nerve axons of transgenic mice. Thicker myelin sheaths and higher numbers of mitochondria were detected in old alpha-syn transgenic mice. In a human sural nerve, alpha-syn could also be identified within axons. Thus, alpha-syn and its aggregates are not only a component of Lewy bodies and synapses but also of axons and these aggregates might interfere with axonal transport. alpha-Synuclein transgenic mice represent an appropriate model for investigations on axonal transport in neurodegenerative diseases.

Extended lifespan of Drosophila parkin mutants through sequestration of redox-active metals and enhancement of anti-oxidative pathways

The mechanisms underlying neuron death in Parkinson's disease are unknown, but both genetic defects and environmental factors are implicated in its pathogenesis. Mutations in the parkin gene lead to autosomal recessive juvenile Parkinsonism (AR-JP). Here we report that compared to control flies, Drosophila lacking parkin show significantly reduced lifespan but no difference in dopamine neuron numbers when raised on food supplemented with environmental pesticides or mitochondrial toxins. Moreover, chelation of redox-active metals, anti-oxidants and overexpression of superoxide dismutase 1 all significantly reversed the reduced longevity of parkin-deficient flies. Finally, parkin deficiency exacerbated the rough eye phenotype of Drosophila caused by overexpression of the copper importer B (Ctr1B). Taken together, our results demonstrate an important function of parkin in the protection against redox-active metals and pesticides implicated in the etiology of Parkinson's disease. They also corroborate that oxidative stress, perhaps as a consequence of mitochondrial dysfunction, is a major determinant of morbidity in parkin mutant flies.

Zinc supplement greatly improves the condition of parkin mutant Drosophila

Parkinson's disease (PD) is a progressive neurodegenerative disorder in which oxidative stress is implicated as a major causative factor. Mutations in the gene encoding Parkin, a ubiquitin ligase, are responsible for a familial form of PD. In a Drosophila disease model lacking Parkin (park(25) null mutant), we tested the effect of zinc supplementation. Zinc is an essential trace metal and a component of many enzymes and transcriptional regulators. Unlike copper and iron, zinc is not redox-active and under most conditions serves as an antioxidant. We find that the condition of parkin mutants raised on zinc-supplemented food is greatly improved. At zinc concentrations where controls begin to show adverse effects as a result of the metal supplement, parkin mutants perform best, as manifested in a higher frequency of reaching adulthood, extended lifespan and improved motoric abilities.

The role of dopamine oxidation in mitochondrial dysfunction: implications for Parkinson's disease

The etiology of sporadic Parkinson's disease (PD) is unknown, although mitochondrial dysfunction and oxidative stress have been implicated in the mechanisms associated with PD pathogenesis. Dopamine (DA) neurons of the substantia nigra pars compacta have been shown to degenerate to a greater extent in PD than other neurons suggesting the possibility that DA itself may be contributing to the neurodegenerative process. This review discusses our work on the effects of DA oxidation and reactive DA quinones on mitochondrial function and protein modification and the potential for exacerbating toxicity associated with mitochondrial dysfunction in PD.

Mitochondrial quality control and neurological disease: an emerging connection

The human brain is a highly complex organ with remarkable energy demands. Although it represents only 2% of the total body weight, it accounts for 20% of all oxygen consumption, reflecting its high rate of metabolic activity. Mitochondria have a crucial role in the supply of energy to the brain. Consequently, their deterioration can have important detrimental consequences on the function and plasticity of neurons, and is thought to have a pivotal role in ageing and in the pathogenesis of several neurological disorders. Owing to their inherent physiological functions, mitochondria are subjected to particularly high levels of stress and have evolved specific molecular quality-control mechanisms to maintain the mitochondrial components. Here, we review some of the most recent advances in the understanding of mitochondrial stress-control pathways, with a particular focus on how defects in such pathways might contribute to neurodegenerative disease.

Nature and cause of mitochondrial dysfunction in Huntington's disease: focusing on huntingtin and the striatum

Polyglutamine expansion mutation in huntingtin causes Huntington's disease (HD). How mutant huntingtin (mHtt) preferentially kills striatal neurons remains unknown. The link between mitochondrial dysfunction and HD pathogenesis stemmed from postmortem brain data and mitochondrial toxin models. Current evidence from genetic models, containing mHtt, supports mitochondrial dysfunction with yet uncertain nature and cause. Because mitochondria composition and function varies across tissues and cell-types, mitochondrial dysfunction in HD vulnerable striatal neurons may have distinctive features. This review focuses on mHtt and the striatum, integrating experimental evidence from patients, mice, primary cultures and striatal cell-lines. I address the nature (specific deficits) and cause (mechanisms linked to mHtt) of HD mitochondrial dysfunction, considering limitations of isolated vs. in situ mitochondria approaches, and the complications introduced by glia and glycolysis in brain and cell-culture studies. Current evidence relegates respiratory chain impairment to a late secondary event. Upstream events include defective mitochondrial calcium handling, ATP production and trafficking. Also, transcription abnormalities affecting mitochondria composition, reduced mitochondria trafficking to synapses, and direct interference with mitochondrial structures enriched in striatal neurons, are possible mechanisms by which mHtt amplifies striatal vulnerability. Insights from common neurodegenerative disorders with selective vulnerability and mitochondrial dysfunction (Alzheimer's and Parkinson's diseases) are also addressed.

The primary role of glutathione against nuclear DNA damage of striatum induced by permethrin in rats

Pyrethroids are one of the most widely used class of insecticides and their toxicity is dominated by pharmacological actions upon the CNS. This study reports as the subchronic treatment (60 days) with permethrin (PERM) (1/10 of LD(50)) induced nuclear DNA damage in rat striatum cells. Comet assay outcomes showed that PERM produced single- and double-strand breaks in striatum cells, the DNA damage was not related to oxidation at pyrimidine and purine bases. Vitamin E (280 mg/kg body weight/day) and vitamin E+coenzyme Q(10) (10 mg/kg/3 ml) supplementation could protect PERM treated rats against nuclear DNA damage. With the aim to evaluate the cause of nuclear DNA damage observed in striatum of rat treated with PERM, in vitro studies on striatum submitochondrial particles (SMPs) and on striatum cells treated with 10 muM PERM alone or plus 16 or 32 nM GSH were performed. SMPs incubated with PERM showed a decrease in superoxide anion release from the electron transport chain by inhibition of mitochondrial complex I. The effect could be related to the decrease of membrane fluidity measured in the hydrophilic-hydrophobic region of the mitochondrial membrane. This result discarded the involvement of the mitochondrial reactive oxygen species in the nuclear DNA damage. On the contrary, GSH played a crucial role on striatum since it was able to protect the cells against nuclear DNA damage induced by PERM. In conclusion our outcomes suggested that nuclear DNA damage of striatum cells was directly related to GSH depletion due to PERM insecticide.

Dopamine neuron stimulating actions of a GDNF propeptide

BACKGROUND

Neurotrophic factors, such as glial cell line-derived neurotrophic factor (GDNF), have shown great promise for protection and restoration of damaged or dying dopamine neurons in animal models and in some Parkinson's disease (PD) clinical trials. However, the delivery of neurotrophic factors to the brain is difficult due to their large size and poor bio-distribution. In addition, developing more efficacious trophic factors is hampered by the difficulty of synthesis and structural modification. Small molecules with neurotrophic actions that are easy to synthesize and modify to improve bioavailability are needed.

METHODS AND FINDINGS

Here we present the neurobiological actions of dopamine neuron stimulating peptide-11 (DNSP-11), an 11-mer peptide from the proGDNF domain. In vitro, DNSP-11 supports the survival of fetal mesencephalic neurons, increasing both the number of surviving cells and neuritic outgrowth. In MN9D cells, DNSP-11 protects against dopaminergic neurotoxin 6-hydroxydopamine (6-OHDA)-induced cell death, significantly decreasing TUNEL-positive cells and levels of caspase-3 activity. In vivo, a single injection of DNSP-11 into the normal adult rat substantia nigra is taken up rapidly into neurons and increases resting levels of dopamine and its metabolites for up to 28 days. Of particular note, DNSP-11 significantly improves apomorphine-induced rotational behavior, and increases dopamine and dopamine metabolite tissue levels in the substantia nigra in a rat model of PD. Unlike GDNF, DNSP-11 was found to block staurosporine- and gramicidin-induced cytotoxicity in nutrient-deprived dopaminergic B65 cells, and its neuroprotective effects included preventing the release of cytochrome c from mitochondria.

CONCLUSIONS

Collectively, these data support that DNSP-11 exhibits potent neurotrophic actions analogous to GDNF, making it a viable candidate for a PD therapeutic. However, it likely signals through pathways that do not directly involve the GFRalpha1 receptor.

Minocycline protects dopaminergic neurons against long-term rotenone toxicity

BACKGROUND

In Parkinson's disease, most of current therapies only provide symptomatic treatment and so far there is no drug which directly affects the disease process.

OBJECTIVES

To investigate the neuroprotective effects of minocycline against long-term rotenone toxicity in primary dopaminergic cell cultures.

METHODS

Embryonic mice of 14-days-old were used for preparation of primary dopaminergic cell cultures. On the 6th day in vitro, prepared cultures were treated both with minocycline alone (1, 5, 10 and 20 microM) and concomitantly with rotenone (5 and 20 nM) and minocycline. Cultures were incubated at 37 degrees C for six consecutive days. On Day in vitro culture medium was aspirated and used for measuring lactate dehydrogenase. Cultured cells were fixed in 4% paraformaldhyde and stained immunohistochemically against tyrosine hydroxylase.

RESULTS

Treatment of cultures with 5 and 20 nM of rotenone significantly decreased the survival of tyrosine hydroxylase immunoreactive neurons by 27 and 31% and increased the release of lactate dehydrogenase into the culture medium by 31 and 236%, respectively compared to untreated controls. Minocycline (1, 5, 10 microM) significantly protected tyrosine hydroxylase immunoreactive neurons by 17, 15 and 19% and 13, 22 and 23% against 5 and 20 nM of rotenone, respectively compared to rotenone-treated cultures. Minocycline (only at 10 microM) significantly decreased the release of lactate dehydrogenase by 79% and 133% against 5 and 20 nM of rotenone, respectively.

CONCLUSION

Minocycline has neuroprotective potential against the progressive loss of tyrosine hydroxylase immunoreactive neurons induced by long-term rotenone toxicity in primary dopaminergic cultures.

Drosophila parkin requires PINK1 for mitochondrial translocation and ubiquitinates mitofusin

Loss of the E3 ubiquitin ligase Parkin causes early onset Parkinson's disease, a neurodegenerative disorder of unknown etiology. Parkin has been linked to multiple cellular processes including protein degradation, mitochondrial homeostasis, and autophagy; however, its precise role in pathogenesis is unclear. Recent evidence suggests that Parkin is recruited to damaged mitochondria, possibly affecting mitochondrial fission and/or fusion, to mediate their autophagic turnover. The precise mechanism of recruitment and the ubiquitination target are unclear. Here we show in Drosophila cells that PINK1 is required to recruit Parkin to dysfunctional mitochondria and promote their degradation. Furthermore, PINK1 and Parkin mediate the ubiquitination of the profusion factor Mfn on the outer surface of mitochondria. Loss of Drosophila PINK1 or parkin causes an increase in Mfn abundance in vivo and concomitant elongation of mitochondria. These findings provide a molecular mechanism by which the PINK1/Parkin pathway affects mitochondrial fission/fusion as suggested by previous genetic interaction studies. We hypothesize that Mfn ubiquitination may provide a mechanism by which terminally damaged mitochondria are labeled and sequestered for degradation by autophagy.

Basic mechanisms of neurodegeneration: a critical update

Neurodegenerative diseases are characterized by progressive dysfunction of specific populations of neurons, determining clinical presentation. Neuronal loss is associated with extra and intracellular accumulation of misfolded proteins, the hallmarks of many neurodegenerative proteinopathies. Major basic processes include abnormal protein dynamics due to deficiency of the ubiquitin-proteosome-autophagy system, oxidative stress and free radical formation, mitochondrial dysfunction, impaired bioenergetics, dysfunction of neurotrophins, 'neuroinflammatory' processes and (secondary) disruptions of neuronal Golgi apparatus and axonal transport. These interrelated mechanisms lead to programmed cell death is a long run over many years. Neurodegenerative disorders are classified according to known genetic mechanisms or to major components of protein deposits, but recent studies showed both overlap and intraindividual diversities between different phenotypes. Synergistic mechanisms between pathological proteins suggest common pathogenic mechanisms. Animal models and other studies have provided insight into the basic neurodegeneration and cell death programs, offering new ways for future prevention/treatment strategies.

Parkin-mediated selective mitochondrial autophagy, mitophagy: Parkin purges damaged organelles from the vital mitochondrial network

Cellular homeostasis is linked tightly to mitochondrial functions. Some damage to mitochondrial proteins and nucleic acids can lead to the depolarization of the inner mitochondrial membrane, thereby sensitizing impaired mitochondria for selective elimination by autophagy. Mitochondrial dysfunction is one of the key aspects of the pathobiology of neurodegenerative disease. Parkin, an E3 ligase located in the cytosol and originally discovered as mutated in monogenic forms of Parkinson's disease (PD), was found recently to translocate specifically to uncoupled mitochondria and to induce their autophagy.

Exercise protects against MPTP-induced neurotoxicity in mice

Exercise has been shown to be potently neuroprotective in several neurodegenerative models, including 1-methyl-4-phenyl-1, 2, 3, 6-tetrahydropyridine (MPTP) model of Parkinson's disease (PD). In order to determine the critical duration of exercise necessary for DA neuroprotection, mice were allowed to run for either 1, 2 or 3months prior to treatment with saline or MPTP. Quantification of DA neurons in the SNpc show that mice allowed to run unrestricted for 1 or 2months lost significant numbers of neurons following MPTP administration as compared to saline treated mice; however, 3months of exercise provided complete protection against MPTP-induced neurotoxicity. To determine the critical intensity of exercise for DA neuroprotection, mice were restricted in their running to either 1/3 or 2/3 that of the full running group for 3months prior to treatment with saline or MPTP. Quantification of DA neurons in the SNpc show that mice whose running was restricted lost significant numbers of DA neurons due to MPTP toxicity; however, the 2/3 running group demonstrated partial protection. Neurochemical analyses of DA and its metabolites DOPAC and HVA show that exercise also functionally protects neurons from MPTP-induced neurotoxicity. Proteomic analysis of SN and STR tissues indicates that 3months of exercise induces changes in proteins related to energy regulation, cellular metabolism, the cytoskeleton, and intracellular signaling events. Taken together, these data indicate that exercise potently protects DA neurons from acute MPTP toxicity, suggesting that this simple lifestyle element may also confer significant protection against developing PD in humans.

Pathogenesis of Parkinson's disease: emerging role of molecular chaperones

Several neurodegenerative diseases, including Parkinson's disease (PD) are associated with protein misfolding and the formation of distinct aggregates, resulting in a putative pathological protein load on the nervous system. A variety of factors cause proteins to aggregate, including aggregation-prone sequences, specific mutations, protein modifications and also dysregulation of the protein degradation machinery. Molecular chaperones are responsible for maintaining normal protein homeostasis within the cell by assisting protein folding and modulating protein-degrading pathways. Here, we review the fundamental mechanisms of neurodegeneration occurring in PD involving alpha-synuclein fibrillisation and aggregation, endoplasmic reticulum stress, ubiquitin proteasome systems, autophagy and lysosomal degradation. Molecular chaperones serve a neuroprotective role in many of these pathways, and we discuss recent evidence indicating that these proteins might provide the basis for new therapeutic approaches.

Mitochondrial Medicine and the Neurodegenerative Mitochondriopathies

Neurodegenerative diseases are a common late-life scourge for which disease-modifying treatments are sorely needed. Mitochondrial perturbation is commonly observed in these diseases, so pursuing treatment development strategies that target mitochondria or processes affected by mitochondria seems reasonable. This review discusses the rationale underlying past and current efforts to treat neurodegenerative diseases using mitochondrial medicine, and tries to predict how future efforts might proceed.

Nervous yeast: modeling neurotoxic cell death

Neurodegeneration is characterized by the disease-specific loss of neuronal activity, culminating in the irreversible destruction of neurons. Neuronal cell death can proceed via distinct subroutines such as apoptosis and necrosis, but the underlying molecular mechanisms remain poorly understood. Saccharomyces cerevisiae is an established model for programmed cell death, characterized by distinct cell death pathways conserved from yeast to mammals. Recently, yeast models for several major classes of neurodegeneration, namely alpha-synucleinopathies, polyglutamine disorders, beta-amyloid diseases, tauopathies, and TDP-43 proteinopathies, have been established. Heterologous expression of the human proteins implicated in these disorders has unraveled important insights in their detrimental function, pointing to ways in which yeast might advance the mechanistic dissection of cell death pathways relevant for human neurodegeneration.