Mitochondrial function in Parkinson's disease cybrids containing an nt2 neuron-like nuclear background

Mitochondrial metabolism in Parkinson's disease impairs quality control autophagy by hampering microtubule-dependent traffic

Abnormal presence of autophagic vacuoles is evident in brains of patients with Parkinson's disease (PD), in contrast to the rare detection of autophagosomes in a normal brain. However, the actual cause and pathological significance of these observations remain unknown. Here, we demonstrate a role for mitochondrial metabolism in the regulation of the autophagy-lysosomal pathway in ex vivo and in vitro models of PD. We show that transferring mitochondria from PD patients into cells previously depleted of mitochondrial DNA is sufficient to reproduce the alterations in the autophagic system observed in PD patient brains. Although the initial steps of this pathway are not compromised, there is an increased accumulation of autophagosomes associated with a defective autophagic activity. We prove that this functional decline was originated from a deficient mobilization of autophagosomes from their site of formation toward lysosomes due to disruption in microtubule-dependent trafficking. This contributed directly to a decreased proteolytic flux of α-synuclein and other autophagic substrates. Our results lend strong support for a direct impact of mitochondria in autophagy as defective autophagic clearance ability secondary to impaired microtubule trafficking is driven by dysfunctional mitochondria. We uncover mitochondria and mitochondria-dependent intracellular traffic as main players in the regulation of autophagy in PD.

Epigenetic engineering to reverse the Parkinson's expression state

Nature, initiation and ensuing cellular propagation mode of sporadic Parkinson's disease (comprising over 90% of all Parkinson's cases) remain open research questions. Accordingly, so does the best therapeutic avenue for addressing this debilitating disease that today affects an estimated 7-10 million people worldwide. Recently, we argued that sporadic Parkinson's be fundamentally characterized as a pathological deviation from normality in the expression program of a cell, the PD-state. Further, we suggested this generic cell state (not restricted to neurons) could be epigenetically locked-in. This raises the theoretical possibility of reverting a cell's PD-state to normality by appropriate epigenetic reprogramming. In here, we propose an in vitro relatively high throughput search for a cocktail of molecules that induces an epigenetic reversal of the PD-state. A generic multi-tissue PD-state phenotype appears to be a defect on mitochondrial bioenergetics. In the above search, we suggest utilizing a metabolic challenge as a preliminary screen for assessing, via improvement of energy metabolism, reversal of the PD-state.

Does mitochondrial DNA play a role in Parkinson's disease? A review of cybrid and other supportive evidence

SIGNIFICANCE

Mitochondria are currently believed to play an important role in the neurodysfunction and neurodegeneration that underlie Parkinson's disease (PD).

RECENT ADVANCES

While it increasingly appears that mitochondrial dysfunction in PD can have different causes, it has been proposed that mitochondrial DNA (mtDNA) may account for or drive mitochondrial dysfunction in the majority of the cases. If correct, the responsible mtDNA signatures could represent acquired mutations, inherited mutations, or population-distributed polymorphisms.

CRITICAL ISSUES AND FUTURE DIRECTIONS

This review discusses the case for mtDNA as a key mediator of PD, and especially focuses on data from studies of PD cytoplasmic hybrid (cybrid) cell lines.

Mitochondria in neurodegeneration

Many neurodegenerative diseases demonstrate abnormal mitochondrial morphology and biochemical dysfunction. Alterations are often systemic rather than brain-limited. Mitochondrial dysfunction may arise as a consequence of abnormal mitochondrial DNA, mutated nuclear proteins that interact directly or indirectly with mitochondria, or through unknown causes. In most cases it is unclear where mitochondria sit in relation to the overall disease cascades that ultimately causes neuronal dysfunction and death, and there is still controversy regarding the question of whether mitochondrial dysfunction is a necessary step in neurodegeneration. In this chapter we highlight and catalogue mitochondrial perturbations in some of the major neurodegenerative diseases including Alzheimer's disease (AD), Parkinson's disease (PD), amyotrophic lateral sclerosis (ALS), and Huntington's disease (HD). We consider data that suggest mitochondria may be critically involved in neurodegenerative disease neurodegeneration cascades.

The role of α-synuclein in neurodegeneration — An update

Genetic, neuropathological and biochemical evidence implicates α-synuclein, a 140 amino acid presynaptic neuronal protein, in the pathogenesis of Parkinson’s disease and other neurodegenerative disorders. The aggregated protein inclusions mainly containing aberrant α-synuclein are widely accepted as morphological hallmarks of α-synucleinopathies, but their composition and location vary between disorders along with neuronal networks affected. α-Synuclein exists physiologically in both soluble and membran-bound states, in unstructured and α-helical conformations, respectively, while posttranslational modifications due to proteostatic deficits are involved in β-pleated aggregation resulting in formation of typical inclusions. The physiological function of α-synuclein and its role linked to neurodegeneration, however, are incompletely understood. Soluble oligomeric, not fully fibrillar α-synuclein is thought to be neurotoxic, main targets might be the synapse, axons and glia. The effects of aberrant α-synuclein include alterations of calcium homeostasis, mitochondrial dysfunction, oxidative and nitric injuries, cytoskeletal effects, and neuroinflammation. Proteasomal dysfunction might be a common mechanism in the pathogenesis of neuronal degeneration in α-synucleinopathies. However, how α-synuclein induces neurodegeneration remains elusive as its physiological function. Genome wide association studies demonstrated the important role for genetic variants of the SNCA gene encoding α-synuclein in the etiology of Parkinson’s disease, possibly through effects on oxidation, mitochondria, autophagy, and lysosomal function. The neuropathology of synucleinopathies and the role of α-synuclein as a potential biomarker are briefly summarized. Although animal models provided new insights into the pathogenesis of Parkinson disease and multiple system atrophy, most of them do not adequately reproduce the cardinal features of these disorders. Emerging evidence, in addition to synergistic interactions of α-synuclein with various pathogenic proteins, suggests that prionlike induction and seeding of α-synuclein could lead to the spread of the pathology and disease progression. Intervention in the early aggregation pathway, aberrant cellular effects, or secretion of α-synuclein might be targets for neuroprotection and disease-modifying therapy.

Mitochondrial metabolic control of microtubule dynamics impairs the autophagic pathway in Parkinson's disease

BACKGROUND

Parkinson's disease (PD) is a progressive neurodegenerative disorder where the loss of dopaminergic neurons in the substantia nigra and the presence of Lewy bodies in surviving neurons are primary histopathological hallmarks. Recent evidence points to mitochondrial dysfunction as a common upstream event in PD etiopathology.

OBJECTIVE

In this overview, we will discuss some of our findings that provide support for the mitochondrial cascade hypothesis, whereas mitochondrial deficits trigger PD pathology through alterations in microtubule integrity and macroautophagy.

METHODS

Using, as a PD model, cells that have PD patients' mitochondrial DNA, cells without mitochondrial DNA and MPP(+)-treated cells, we showed that mitochondrial metabolism alteration may underlie changes in the microtubular net and in the autophagic-lysosomal pathway.

CONCLUSIONS

Finally, we will endow a potential new therapeutic target for PD pathology.

Mitochondrial and metabolic-based protective strategies in Huntington's disease: the case of creatine and coenzyme Q

Huntington's disease (HD) is a neurodegenerative genetic disorder caused by an expansion of CAG repeats in the HD gene encoding for huntingtin (Htt), resulting in progressive death of striatal neurons, with clinical symptoms of chorea, dementia and dramatic weight loss. Metabolic and mitochondrial dysfunction caused by the expanded polyglutamine sequence have been described along with other mechanisms of neurodegeneration previously described in human tissues and animal models of HD. In this review, we focus on mitochondrial and metabolic disturbances affecting both the central nervous system and peripheral cells, including mitochondrial DNA damage, mitochondrial complexes defects, loss of calcium homeostasis and transcriptional deregulation. Glucose abnormalities have also been described in peripheral tissues of HD patients and in HD animal and cellular models. Moreover, there are no effective neuroprotective treatments available in HD. Thus, we briefly discuss the role of creatine and coenzyme Q10 that target mitochondrial dysfunction and impaired bioenergetics and have been previously used in HD clinical trials.

Differentiation of SH-SY5Y cells to a neuronal phenotype changes cellular bioenergetics and the response to oxidative stress

Cell differentiation is associated with changes in metabolism and function. Understanding these changes during differentiation is important in the context of stem cell research, cancer, and neurodegenerative diseases. An early event in neurodegenerative diseases is the alteration of mitochondrial function and increased oxidative stress. Studies using both undifferentiated and differentiated SH-SY5Y neuroblastoma cells have shown distinct responses to cellular stressors; however, the mechanisms remain unclear. We hypothesized that because the regulation of glycolysis and oxidative phosphorylation is modulated during cellular differentiation, this would change bioenergetic function and the response to oxidative stress. To test this, we used retinoic acid (RA) to induce differentiation of SH-SY5Y cells and assessed changes in cellular bioenergetics using extracellular flux analysis. After exposure to RA, the SH-SY5Y cells had an increased mitochondrial membrane potential, without changing mitochondrial number. Differentiated cells exhibited greater stimulation of mitochondrial respiration with uncoupling and an increased bioenergetic reserve capacity. The increased reserve capacity in the differentiated cells was suppressed by the inhibitor of glycolysis 2-deoxy-d-glucose. Furthermore, we found that differentiated cells were substantially more resistant to cytotoxicity and mitochondrial dysfunction induced by the reactive lipid species 4-hydroxynonenal or the reactive oxygen species generator 2,3-dimethoxy-1,4-naphthoquinone. We then analyzed the levels of selected mitochondrial proteins and found an increase in complex IV subunits, which we propose contributes to the increase in reserve capacity in the differentiated cells. Furthermore, we found an increase in MnSOD that could, at least in part, account for the increased resistance to oxidative stress. Our findings suggest that profound changes in mitochondrial metabolism and antioxidant defenses occur upon differentiation of neuroblastoma cells to a neuron-like phenotype.

Do mtDNA Mutations Participate in the Pathogenesis of Sporadic Parkinson's Disease?

The pathogenesis of sporadic Parkinson’s disease (PD) remains enigmatic. Mitochondrial complex-I defects are known to occur in the substantia nigra (SN) of PD patients and are also debated in some extracerebral tissues. Early sequencing efforts of the mitochondrial DNA (mtDNA) did not reveal specific mutations, but a long lasting discussion was devoted to the issue of randomly distributed low level point mutations, caused by oxidative stress. However, a potential functional impact remained a matter of speculation, since heteroplasmy (mutational load) at any base position analyzed, remained far below the relevant functional threshold. A clearly age-dependent increase of the ‘common mtDNA deletion’ had been demonstrated in most brain regions by several authors since 1992. However, heteroplasmy did hardly exceed 1% of total mtDNA. It became necessary to exploit PCR techniques, which were able to detect any deletion in a few microdissected dopaminergic neurons of the SN. In 2006, two groups published biochemically relevant loads of somatic mtDNA deletions in these neurons. They seem to accumulate to relevant levels in the SN dopaminergic neurons of aged individuals in general, but faster in those developing PD. It is reasonable to assume that this accumulation causes mitochondrial dysfunction of the SN, although it cannot be taken as a final proof for an early pathogenetic role of this dysfunction. Recent studies demonstrate a distribution of deletion breakpoints, which does not differ between PD, aging and classical mitochondrial disorders, suggesting a common, but yet unknown mechanism.

An insight into ultrastructural and morphological alterations of platelets in neurodegenerative diseases

Platelets are evinced as a systemic tool in a variety of disorders, including neurodegenerative diseases. Evidence suggests that variations in the ultrastructure and morphology of platelets and related organelles are involved in the pathophysiology of diabetes, cancer, HIV/AIDS, cardiovascular and neurological diseases. Due to structural alterations of platelets in many diseases, it is informative to discuss the ultrastructural and morphological discrepancies of platelets in contemporary medical research. The present review reveals the usefulness of ultrastructural study in better understanding of the disease patterns and may help to improve the treatment regimes.

Mitochondrial fusion/fission, transport and autophagy in Parkinson's disease: when mitochondria get nasty

Understanding the molecular basis of Parkinson's disease (PD) has proven to be a major challenge in the field of neurodegenerative diseases. Although several hypotheses have been proposed to explain the molecular mechanisms underlying the pathogenesis of PD, a growing body of evidence has highlighted the role of mitochondrial dysfunction and the disruption of the mechanisms of mitochondrial dynamics in PD and other parkinsonian disorders. In this paper, we comment on the recent advances in how changes in the mitochondrial function and mitochondrial dynamics (fusion/fission, transport, and clearance) contribute to neurodegeneration, specifically focusing on PD. We also evaluate the current controversies in those issues and discuss the role of fusion/fission dynamics in the mitochondrial lifecycle and maintenance. We propose that cellular demise and neurodegeneration in PD are due to the interplay between mitochondrial dysfunction, mitochondrial trafficking disruption, and impaired autophagic clearance.

Mitochondrial Dysfunction: The Road to Alpha-Synuclein Oligomerization in PD

While the etiology of Parkinson's disease remains largely elusive, there is accumulating evidence suggesting that mitochondrial dysfunction occurs prior to the onset of symptoms in Parkinson's disease. Mitochondria are remarkably primed to play a vital role in neuronal cell survival since they are key regulators of energy metabolism (as ATP producers), of intracellular calcium homeostasis, of NAD(+)/NADH ratio, and of endogenous reactive oxygen species production and programmed cell death. In this paper, we focus on mitochondrial dysfunction-mediated alpha-synuclein aggregation. We highlight some of the findings that provide proof of evidence for a mitochondrial metabolism control in Parkinson's disease, namely, mitochondrial regulation of microtubule-dependent cellular traffic and autophagic lysosomal pathway. The knowledge that microtubule alterations may lead to autophagic deficiency and may compromise the cellular degradation mechanisms that culminate in the progressive accumulation of aberrant protein aggregates shields new insights to the way we address Parkinson's disease. In line with this knowledge, an innovative window for new therapeutic strategies aimed to restore microtubule network may be unlocked.

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.

Bifunctional phenolic-choline conjugates as anti-oxidants and acetylcholinesterase inhibitors

Because of the complex cascade of molecular events that can occur in the brain of an Alzheimer's disease (AD) patient, the therapy of this neurodegenerative disease seems more likely to be achieved by multifunctional drugs. Herein, a new series of dual-targeting ligands have been developed and in vitro bioevaluated. Their architecture is based on conjugating the acetylcholinesterase inhibition and anti-oxidant properties in one molecular entity. Specifically, a series of naturally occurring phenolic acids with recognized anti-oxidant properties (derivatives of caffeic acid, rosmarinic acid, and trolox) have been conjugated with choline to account for the recognition by acetylcholinesterase (AChE). The synthesized hybrid compounds evidenced AChE inhibitory capacity of micromolar range (rationalized by molecular modeling studies) and good antioxidant properties. Their effects on human neuroblastoma cells, previously treated with beta-amyloid peptides and 1-methyl-4-phenylpyridinium ion neurotoxins (to simulate AD and Parkinson's disease, respectively), also demonstrated a considerable capacity for protection against the cytotoxicity of these stressors.

Mitochondrial therapy for Parkinson's disease: neuroprotective pharmaconutrition may be disease-modifying

Progressive destruction of neurons that produce dopamine in the basal ganglia of the brain, particularly the substantia nigra, is a hallmark of Parkinson's disease. The syndrome of the Parkinsonian phenotype is caused by many etiologies, involving multiple contributing mechanisms. Characteristic findings are pathologic inclusions called Lewy bodies, which are protein aggregates inside nerve cells. Environmental insults are linked with the disease, and a number of associated genes have also been identified. Neuroinflammation, microglia activation, oxidative stress, and mitochondrial dysfunction are central processes producing nerve damage. In addition, protein misfolding, driven by accumulation and condensation of α-synuclein, compounded by inadequate elimination of defective protein through the ubiquitin- proteasome system, promote apoptosis. Current pharmacologic therapy is palliative rather than disease- modifying, and typically becomes unsatisfactory over time. Coenzyme Q10 and creatine, two agents involved in energy production, may be disease-modifying, and able to produce sufficient beneficial pathophysiologic changes in preclinical studies to warrant large studies now in progress. Use of long-chain omega-3 fatty acids and vitamin D in PD are also topics of current interest.

Paeoniflorin, a potent natural compound, protects PC12 cells from MPP+ and acidic damage via autophagic pathway

ETHNOPHARMACOLOGICAL RELEVANCE

Paeoniflorin (PF) is the principal bioactive component of Radix Paeoniae alba, which is widely used in Traditional Chinese Medicine for the treatment of neurodegenerative disorders such as Parkinson's disease (PD).

AIM OF THE STUDY

To evaluate the neuroprotective effects of PF on MPP(+)- or acid- (pH 5.0) induced injury in cultured PC12 cells and to investigate the activity of autophagy-lysosome pathway (ALP). Amiloride (Ami), a non-selective blocker of acid-sensing ion channels (ASICs), as a positive control drug, since it is neuroprotective in rodent models of PD.

MATERIALS AND METHODS

The cell viability was analyzed with MTT assay. The cell injury was assessed by lactate dehydrogenase (LDH) assay. Flow cytometry and Western blot analysis were used to study the apoptotic, calcium influx and autophagic mechanisms.

RESULTS

Ami (100 microM) and PF (50 microM) both protected PC12 cells against MPP(+)- or acid-induced injury as assessed by MTT assay, lactate dehydrogenase release, and apoptosis rate. The concentrations of cytosolic free Ca(2+) were raised after exposure to MPP(+) or acidosis, while Ami and PF both reduced the influx of Ca(2+). More importantly, we found that the mechanisms of neuroprotective effects of Ami and PF were closely associated with the upregulation of LC3-II protein, which is specifically associated with autophagic vacuole membranes. Furthermore, application of MPP(+) or acid induced the overexpression of LAMP2a, which is directly correlated with the activity of the chaperone-mediated autophagy pathway. However, Ami and PF inhibited the overexpression of LAMP2a.

CONCLUSIONS

Our data provide the first experimental evidence that PF modulates autophagy in models of neuron injury, as well as providing the first indication of a relationship between ASICs and ALP.

Cross-talk between mitochondria and proteasome in Parkinson's disease pathogenesis

Parkinson's disease (PD) is the most common progressive neurodegenerative movement disorder, characterized by the selective loss of nigrostriatal dopaminergic neurons, and the presence of intracellular insoluble proteinaceous inclusions, known as Lewy Bodies. Although PD etiopathogenesis remains elusive, the leading hypothesis for the death of specific groups of neurons establishes that mitochondrial dysfunction, alterations in the ubiquitin-proteasomal system (UPS), and oxidative stress are major events that act synergistically causing this devastating disease. In this review we will focus on mitochondrial impairment and its implications on proteasomal function and alpha-synuclein aggregation. We will address the role of mitochondria and proteasome cross-talk in the neuronal loss that leads to PD and discuss how this knowledge might further improve patient therapy.

Mitochondrial respiration and respiration-associated proteins in cell lines created through Parkinson's subject mitochondrial transfer

Parkinson's disease (PD) is associated with perturbed mitochondrial function. Studies of cytoplasmic hybrid (cybrid) cell lines containing mitochondria from PD subjects suggest complex I dysfunction in particular is a relatively upstream biochemical defect. To evaluate potential downstream consequences of PD mitochondrial dysfunction, we used a cybrid approach to model PD mitochondrial dysfunction; our cybrid cell lines were generated via transfer of PD or control subject platelet mitochondria to mtDNA-depleted NT2 cells. To confirm our PD cybrid mitochondria did indeed differ from control cybrid mitochondria we measured complex I V(max) activities. Consistent with other PD cybrid reports, relative to control cybrid cell lines the PD cybrid cell line mean complex I V(max) activity was reduced. In this validated model, we used an oxygen electrode to characterize PD cybrid mitochondrial respiration. Although whole cell basal oxygen consumption was comparable between the PD and control cybrid groups, the proton leak was increased and maximum respiratory capacity was decreased in the PD cybrids. PD cybrids also had reduced SIRT1 phosphorylation, reduced peroxisome proliferator-activated receptor-gamma coactivator-1alpha levels, and increased NF-kB activation. We conclude mitochondrial respiration and pathways influenced by aerobic metabolism are altered in NT2 cybrid cell lines generated through transfer of PD subject platelet mitochondria.

Mitochondrial medicine for neurodegenerative diseases

Mitochondrial dysfunction has been reported in a wide array of neurological disorders ranging from neuromuscular to neurodegenerative diseases. Recent studies on neurodegenerative diseases have revealed that mitochondrial pathology is generally found in inherited or sporadic neurodegenerative diseases and is believed to be involved in the pathophysiological process of these diseases. Commonly seen types of mitochondrial dysfunction in neurodegenerative diseases include excessive free radical generation, lowered ATP production, mitochondrial permeability transition, mitochondrial DNA lesions, perturbed mitochondrial dynamics and apoptosis. Mitochondrial medicine as an emerging therapeutic strategy targeted to mitochondrial dysfunction in neurodegenerative diseases has been proven to be of value, though this area of research is still at in its early stage. In this article, we report on recent progress in the development of several mitochondrial therapies including antioxidants, blockade of mitochondrial permeability transition, and mitochondrial gene therapy as evidence that mitochondrial medicine has promise in the treatment of neurodegenerative diseases.

Microtubule depolymerization potentiates alpha-synuclein oligomerization

Parkinson's disease (PD) is associated with perturbed mitochondria function and alpha-synuclein fibrillization. We evaluated potential mechanistic links between mitochondrial dysfunction and alpha-synuclein aggregation. We studied a PD cytoplasmic hybrid (cybrid) cell line in which platelet mitochondria from a PD subject were transferred to NT2 neuronal cells previously depleted of endogenous mitochondrial DNA. Compared to a control cybrid cell line, the PD line showed reduced ATP levels, an increased free/polymerized tubulin ratio, and alpha-synuclein oligomer accumulation. Taxol (which stabilizes microtubules) normalized the PD tubulin ratio and reduced alpha-synuclein oligomerization. A nexus exists between mitochondrial function, cytoskeleton homeostasis, and alpha-synuclein oligomerization. In our model, mitochondrial dysfunction triggers an increased free tubulin, which destabilizes the microtubular network and promotes alpha-synuclein oligomerization.

Insulin and Insulin-Sensitizing Drugs in Neurodegeneration: Mitochondria as Therapeutic Targets

Insulin, besides its glucose lowering effects, is involved in the modulation of lifespan, aging and memory and learning processes. As the population ages, neurodegenerative disorders become epidemic and a connection between insulin signaling dysregulation, cognitive decline and dementia has been established. Mitochondria are intracellular organelles that despite playing a critical role in cellular metabolism are also one of the major sources of reactive oxygen species. Mitochondrial dysfunction, oxidative stress and neuroinflammation, hallmarks of neurodegeneration, can result from impaired insulin signaling. Insulin-sensitizing drugs such as the thiazolidinediones are a new class of synthetic compounds that potentiate insulin action in the target tissues and act as specific agonists of the peroxisome proliferator-activated receptor gamma (PPAR-γ). Recently, several PPAR agonists have been proposed as novel and possible therapeutic agents for neurodegenerative disorders. Indeed, the literature shows that these agents are able to protect against mitochondrial dysfunction, oxidative damage, inflammation and apoptosis. This review discusses the role of mitochondria and insulin signaling in normal brain function and in neurodegeneration. Furthermore, the potential protective role of insulin and insulin sensitizers in Alzheimer´s, Parkinson´s and Huntington´s diseases and amyotrophic lateral sclerosis will be also discussed.

Dysfunctional mitochondria uphold calpain activation: contribution to Parkinson's disease pathology

Calpain is a ubiquitous calcium-sensitive protease that is essential for normal physiologic neuronal function. However, mitochondrial-mediated-calcium homeostasis alterations may lead to its pathologic activation that jeopardizes neuronal structure and function. Here, we provide evidence to support a role for the involvement of calpain 1 in mitochondrial-induced neurodegeneration in a Parkinson's disease (PD) cellular model. We show that dysfunctional mitochondria increases cytosolic calcium, thereby, inducing calpain activation. Interestingly, its inhibition significantly attenuated the accumulation of alpha-synuclein oligomers and contributed to an increase of insoluble alpha-synuclein aggregates, known to be cytoprotective. Moreover, our data corroborate that calpain-1 overactivation in our mitochondrial-deficient cells promote caspase-3 activation. Overall, our findings further clarify the crucial role of dysfunctional mitochondria in the control of molecular mechanisms occurring in PD brain cells, providing a potentially novel correlation between the degradation of calpain substrates suggesting a putative role of calpain and calpain inhibition as a therapeutic tool in PD.

Role of reactive oxygen species-elicited apoptosis in the pathophysiology of mitochondrial and neurodegenerative diseases associated with mitochondrial DNA mutations

A wide spectrum of pathogenic mutations of mitochondrial DNA (mtDNA) has been demonstrated to cause mitochondrial dysfunction and overproduction of reactive oxygen species (ROS), in relation to mitochondrial and neurodegenerative diseases. Our previous studies have shown that large-scale deletions of mtDNA not only serve as an indicator of oxidative damage, but also result in greater susceptibility of human cells to apoptosis triggered by UV irradiation and other apoptotic stimuli. In this review, we focus on the involvement of mtDNA-mutation-associated oxidative stress and susceptibility to apoptosis in the pathophysiology of mitochondrial and neurodegenerative diseases. Different lines of research have provided concordant data to suggest that the mtDNA-mutation-elicited energy insufficiency and enhanced oxidative stress and damage lead to cell dysfunction, and increase the susceptibility of affected cells to apoptosis in patients with these diseases. Moreover, accumulating experimental evidence has shown that antioxidant therapy is a good strategy for decreasing intracellular ROS and alleviating oxidative-stress-induced apoptosis in cells of patients that harbor pathogenic mtDNA mutations.

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.

Mitochondria: a therapeutic target in neurodegeneration

Mitochondrial dysfunction has long been associated with neurodegenerative disease. Therefore, mitochondrial protective agents represent a unique direction for the development of drug candidates that can modify the pathogenesis of neurodegeneration. This review discusses evidence showing that mitochondrial dysfunction has a central role in the pathogenesis of Alzheimer's, Parkinson's and Huntington's diseases and amyotrophic lateral sclerosis. We also debate the potential therapeutic efficacy of metabolic antioxidants, mitochondria-directed antioxidants and Szeto-Schiller (SS) peptides. Since these compounds preferentially target mitochondria, a major source of oxidative damage, they are promising therapeutic candidates for neurodegenerative diseases. Furthermore, we will briefly discuss the novel action of the antihistamine drug Dimebon on mitochondria.

Can cellular models revolutionize drug discovery in Parkinson's disease?

The study of mechanisms that underlie Parkinson's disease (PD), as well as translational drug development, has been hindered by the lack of appropriate models. Both cell culture systems and animal models have limitations, and to date none faithfully recapitulate all of the clinical and pathological phenotypes of the disease. In this review we examine the various cell culture model systems of PD, with a focus on different stem cell models that can be used for investigating disease mechanisms as well as drug discovery for PD. We conclude with a discussion of recent discoveries in the field of stem cell biology that have led to the ability to reprogram somatic cells to a pluripotent state via the use of a combination of genetic factors; these reprogrammed cells are termed "induced pluripotent stem cells" (iPSCs). This groundbreaking technique allows for the derivation of patient-specific cell lines from individuals with sporadic forms of PD and also those with known disease-causing mutations. Such cell lines have the potential to serve as a human cellular model of neurodegeneration and PD when differentiated into dopaminergic neurons. The hope is that these iPSC-derived dopaminergic neurons can be used to replicate the key molecular aspects of neural degeneration associated with PD. If so, this approach could lead to transformative new tools for the study of disease mechanisms. In addition, such cell lines can be potentially used for high-throughput drug screening. While not the focus of this review, ultimately it is envisioned that techniques for reprogramming of somatic cells may be optimized to a point sufficient to provide potential new avenues for stem cell-based restorative therapies.

The cybrid model of sporadic Parkinson's disease

Parkinson's disease (PD) is the eponym attached to the most prevalent neurodegenerative movement disorder of adults, derived from observations of an early nineteenth century physician and paleontologist, James Parkinson, and is now recognized to encompass much more than a movement disorder clinically or dopamine neuron death pathologically. Most PD ( approximately 90%) is sporadic (sPD), is associated with mitochondrial deficiencies and has been studied in cell and animal models arising from the use of mitochondrial toxins that unfortunately have not predicted clinical efficacy to slow disease progression in humans. We have extensively studied the cytoplasmic hybrid ("cybrid") model of sPD in which donor mtDNAs are introduced into and expressed in neural tumor cells with identical nuclear genetic and environmental backgrounds. sPD cybrids demonstrate many abnormalities in which increased oxidative stress drives downstream antioxidant response and cell death activating signaling pathways. sPD cybrids regulate mitochondrial ETC genes and gene ontology families like sPD brain. sPD cybrids spontaneously form Lewy bodies and Lewy neurites, linking mtDNA expression to neuropathology, and demonstrate impaired organelle transport in processes and reduced mitochondrial respiration. Our recent studies show that near-infrared laser light therapy normalizes mitochondrial movement and can stimulate respiration in sPD cybrid neurons, and mitochondrial gene therapy can restore respiration and stimulate mitochondrial ETC gene and protein expression. sPD cybrids have provided multiple lines of circumstantial evidence linking mtDNA to sPD pathogenesis and can serve as platforms for therapy development. sPD cybrid models can be improved by the use of non-tumor human stem cell-derived neural precursor cells and by an introduction of postmortem brain mtDNA to test its causality directly.

Reduced axonal transport in Parkinson's disease cybrid neurites is restored by light therapy

BACKGROUND

It has been hypothesized that reduced axonal transport contributes to the degeneration of neuronal processes in Parkinson's disease (PD). Mitochondria supply the adenosine triphosphate (ATP) needed to support axonal transport and contribute to many other cellular functions essential for the survival of neuronal cells. Furthermore, mitochondria in PD tissues are metabolically and functionally compromised. To address this hypothesis, we measured the velocity of mitochondrial movement in human transmitochondrial cybrid "cytoplasmic hybrid" neuronal cells bearing mitochondrial DNA from patients with sporadic PD and disease-free age-matched volunteer controls (CNT). The absorption of low level, near-infrared laser light by components of the mitochondrial electron transport chain (mtETC) enhances mitochondrial metabolism, stimulates oxidative phosphorylation and improves redox capacity. PD and CNT cybrid neuronal cells were exposed to near-infrared laser light to determine if the velocity of mitochondrial movement can be restored by low level light therapy (LLLT). Axonal transport of labeled mitochondria was documented by time lapse microscopy in dopaminergic PD and CNT cybrid neuronal cells before and after illumination with an 810 nm diode laser (50 mW/cm2) for 40 seconds. Oxygen utilization and assembly of mtETC complexes were also determined.

RESULTS

The velocity of mitochondrial movement in PD cybrid neuronal cells (0.175 +/- 0.005 SEM) was significantly reduced (p < 0.02) compared to mitochondrial movement in disease free CNT cybrid neuronal cells (0.232 +/- 0.017 SEM). For two hours after LLLT, the average velocity of mitochondrial movement in PD cybrid neurites was significantly (p < 0.003) increased (to 0.224 +/- 0.02 SEM) and restored to levels comparable to CNT. Mitochondrial movement in CNT cybrid neurites was unaltered by LLLT (0.232 +/- 0.017 SEM). Assembly of complexes in the mtETC was reduced and oxygen utilization was altered in PD cybrid neuronal cells. PD cybrid neuronal cell lines with the most dysfunctional mtETC assembly and oxygen utilization profiles were least responsive to LLLT.

CONCLUSION

The results from this study support our proposal that axonal transport is reduced in sporadic PD and that a single, brief treatment with near-infrared light can restore axonal transport to control levels. These results are the first demonstration that LLLT can increase axonal transport in model human dopaminergic neuronal cells and they suggest that LLLT could be developed as a novel treatment to improve neuronal function in patients with PD.

Endoplasmic reticulum and mitochondria interplay mediates apoptotic cell death: relevance to Parkinson's disease

Sporadic Parkinson's disease (PD) is a progressive neurodegenerative disease characterized by a loss of dopaminergic neurons in the substantia nigra pars compacta. Many cellular mechanisms are thought to be involved in the death of these specific neurons in PD, including oxidative stress, changes of intracellular calcium homeostasis, and mitochondrial dysfunction. Since recent studies have revealed that also endoplasmic reticulum (ER) stress in conjunction with abnormal protein degradation can contribute to the PD pathophysiology, we investigated here the molecular mechanisms underlying the interplay between ER and mitochondria and its relevance in the control of neuronal cell death in PD. We observed that MPP+ induced changes in the mitochondrial function, affecting mitochondrial membrane potential and electron transport chain function. Likewise, it was also evident the unfolded protein response activation by an overexpression of GRP78 protein. Moreover, stress stimuli caused the release of Ca2+ from the ER that consistently induced mitochondrial Ca2+ uptake, with a rise of mitochondrial matrix free Ca2+. Besides, Ca2+ release inhibition prevented MPP+ mediated mitochondria-dependent caspases activation. Our findings show that ER and mitochondria are in a close communication, establishing a dynamic ER-Ca2+-mitochondria interconnection that can play a prominent role in the neuronal cell death induction under particular stressful circumstances of PD pathology.

The search for biomarkers in Parkinson's disease: a critical review

Parkinson's disease (PD) is a progressive neurodegenerative disorder that primarily presents with features of bradykinesia, rigidity and tremor, and has, as part of its core pathology, the degeneration of dopaminergic neurons in the substantia nigra pars compacta. There is a great need for the development of a reliable diagnostic tool to improve promptness of diagnosis, definition of disease subtypes, and to monitor disease progression and demonstrate treatment efficacy in the case of disease modifying therapies. Current biomarkers range from objective clinical tools, to neuroimaging, to 'wet' markers involving blood and cerebrospinal fluid. To date, all candidate biomarkers for PD have failed to be developed into a clinically useful tool. Ideally, a combination of sensitive markers will be needed, not only to predict the onset of PD, but also to help in subtype classification and to follow progression. Here, we critically review various PD biomarker studies.

Oxidative stress involvement in alpha-synuclein oligomerization in Parkinson's disease cybrids

Mitochondrial dysfunction, oxidative stress, and alpha-synuclein oligomerization occur in Parkinson disease (PD). We used an in vitro PD cybrid approach that models these three phenomena specifically to evaluate the impact of mitochondria-derived oxidative stress on alpha-synuclein oligomerization. Compared with control cybrid cell lines, reactive oxygen species (ROS) production and protein oxidative stress markers were elevated in PD cybrids. The antioxidants CoQ(10) and GSH attenuated changes in PD cybrid peroxide, protein carbonyl, and protein sulfhydryl levels. Elevated PD cybrid alpha-synuclein oligomer levels were also attenuated by CoQ(10) and GSH. In PD cybrids, alpha-synuclein oligomerization was activated via a complex I-mediated increase in the free tubulin/polymerized tubulin ratio. CoQ(10) but not GSH increased complex I activity, restored ATP to control levels, and normalized the PD cybrid free tubulin/polymerized tubulin ratio. Overall, we conclude that two different antioxidants can decrease alpha-synuclein oligomerization whether by improving mitochondrial function or by preventing protein carbonylation or both. We conclude that mitochondrial dysfunction can induce alpha-synuclein oligomerization via ATP depletion-driven microtubule depolymerization and via ROS increase-driven protein oxidation.

The neurodegenerative mitochondriopathies

Mitochondria are physically or functionally altered in many neurodegenerative diseases. This is the case for very rare neurodegenerative disorders as well as extremely common age-related ones such as Alzheimer's disease and Parkinson's disease. In some disorders very specific patterns of altered mitochondrial function or systemic mitochondrial dysfunction are demonstrable. Some disorders arise from mitochondrial DNA mutation, some from nuclear gene mutation, and for some the etiology is not definitively known. This review classifies neurodegenerative diseases using mitochondrial dysfunction as a unifying feature, and in doing so defines a group of disorders called the neurodegenerative mitochondriopathies. It discusses what mitochondrial abnormalities have been identified in various neurodegenerative diseases, what is currently known about the mitochondria-neurodegeneration nexus, and speculates on the significance of mitochondrial function in some disorders not classically thought of as mitochondriopathies.

Relationships among molecular genetic and respiratory properties of Parkinson's disease cybrid cells show similarities to Parkinson's brain tissues

We have studied sporadic Parkinson's disease (sPD) from expression of patient mitochondrial DNA (mtDNA) in neural cells devoid of their own mtDNA, the "cybrid" model. In spite of reproducing several properties of sPD brain, it remains unclear whether sPD cybrid cells reflect more complex sPD brain bioenergetic pathophysiology. We characterized and correlated respiration of intact sPD cybrid cells with electron transport chain (ETC) protein assembly, complex I ETC gene expression and ETC protein levels in sPD brain. We also assayed expression for multiple ETC genes coded by mtDNA and nuclear DNA (nDNA) in sPD cybrid cells and brain. sPD cybrid cells have reduced levels of mtDNA genes, variable compensatory normalization of mitochondrial gene expression and show robust correlations with mitochondrial ETC gene expression in sPD brains. Relationships among ETC protein levels predict impaired complex I-mediated respiration in sPD brain. That sPD cybrid cells and sPD brain samples show very correlated regulation of nDNA and mtDNA ETC transcriptomes suggests similar bioenergetic physiologies. We propose that further insights into sPD pathogenesis will follow elucidation of mechanisms leading to reduced mtDNA gene levels in sPD cybrids. This will require characterization of the abnormalities and dynamics of mtDNA changes propagated through sPD cybrids over time.